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A

TEXT-BOOK OF HUMAN PHYSIOLOGY,

BRUBAKER

TEXT-BOOK

OF

HUMAN PHYSIOLOGY

INCLUDING A SECTION ON

PHYSIOLOGIC APPARATUS.

BY
ALBERT P. BRUBAKER, A.M., M.D.,

PROFESSOR OF PHYSIOLOGY AND HYGIENE IN THE JEFFERSON MEDICAL COLLEGE; PROFESSOR

OF PHYSIOLOGY IN THE PENNSYLVANIA COLLEGE OF DENTAL SURGERY;

LECTURER ON PHYSIOLOGY AND HYGIENE IN THE DREXEL

INSTITUTE OF ART, SCIENCE, AND INDUSTRY.

THIRD EDITION. REVISED AND ENLARGED.
WITH COLORED PLATES AND 383 ILLUSTRATIONS.

PHILADELPHIA:

P. BLAKISTON'S SON & CO.,

1012 WALNUT STREET.
1908.

Copyright, 1908, by P. Blakiston's Son & Co.

Printed by

The Maple Press

York Pa.

TO

HENRY C. CHAPMAN, M.D.,

PROFESSOR OF INSTITUTES OF MEDICINE AND MEDICAL JURISPRUDENCE IN
THE JEFFERSON MEDICAL COLLEGE.

IN GRATEFUL RECOGNITION OF THE MANY KINDNESSES RECEIVED FROM

HIM, DURING A PERIOD OF TWENTY-FIVE YEARS, THIS

VOLUME IS RESPECTFULLY DEDICATED BY

THE AUTHOR.

PREFACE TO THIRD EDITION

The publishers having requested the preparation of a Third Edition
of the Text-Book of Physiology, an attempt has been made to still
further increase its value to those for •whom it was primarily intended
by subjecting the whole text to a careful revision, by the insertion of
a number of new diagrams and by the incorporation of new material.
In this attempt the author has been assisted by the suggestions con-
tained in reviews and in letters received from teachers and students.
Altogether some fifty additional pages have been added to the body
of the text. This material will be found in the chapters on the
chemistry of the proteids, the physiology of muscle tissue, absorption,
the physiology of the heart and vascular apparatus, the nerve system
and vision.

Once again I wish to express my sincere thanks to those teachers
and students who have used and recommended this book and for the
generous praise they have bestowed on it verbally and by letter and
trust that in its present form it will still further meet their approval.

I am indebted to Dr. George Bachmann for the new diagrams in
the text to which his name is appended.

To Mr. L A. Hagy my thanks are due for invaluable assistance in
seeing the work through the press.

To Messrs. P. Blakiston's Son & Co. I am greatly indebted for
their encouragement and generosity in the promotion of every thing
that pertains to the material value of this work.

A. P. B.

vii

PREFACE

The object in view in the preparation of this volume was the selec-
tion and presentation of the more important facts of physiology, in a
form which it is believed will be helpful to students and to practi-
tioners of medicine. Inasmuch as the majority of students in a
medical college are preparing for the practical duties of professional
life, such facts have been selected as will not only elucidate the
normal functions of the tissues and organs of the body, but which
will be of assistance in understanding their abnormal manifestations
as they present themselves in hospital and private work. Both in
the selection of facts and in the method of presentation the author
has been guided by an experience gained during twenty years of
active teaching.

The description of physiologic apparatus and the methods of
investigation, other than those having a clinical interest, have been
largely excluded from the text, for the reason that both are more
appropriately considered in works devoted to laboratory methods
and laboratory instruction, and for the further reason that the student
receives this information while engaged in the practical study of
physiology in the laboratory, now an established feature in the
curriculum of the majority of medical colleges. For those who have
not had laboratory opportunities a brief account of some essential
forms of apparatus and the purposes for which they are intended will
be found in an appendix.

I wish to acknowledge my indebtedness to Professor Colin C.
Stewart for many valuable suggestions in the preparation of different
sections of the volume; to Dr. Carl Weiland for assistance in the
chapter on vision; to Dr. Joseph P. Bolton for excellent suggestions
on questions relating to physiologic chemistry.

TABLE OF CONTENTS.

CHAPTER I. page

Introduction i

CHAPTER II.
Chemic Composition of the Human Body 7

CHAPTER III.
Physiology of the Cell „ 26

CHAPTER IV. .
Histology of the Epithelial and Connective Tissues 33

CHAPTER V.
The Physiology of Movement 43

CHAPTER VI.
The Physiology of the Skeleton 48

CHAPTER VII.
General Physiology of Muscle-tissue 53

CHAPTER VIII.
The General Physiology of Nerve-tissue 96

CHAPTER IX.
Foods 127

CHAPTER X.
Digestion 145

CHAPTER XI.
Absorption 213

CHAPTER XII.
The Blood ' 236

CHAPTER XIII.
The Circulation of the Blood 271

CHAPTER XIV.
The Circulation of the Blood (Continued) 326

CHAPTER XV.
Respiration 3S2

CHAPTER XVI.

Animal Heat 436

xi

xii TABLE OF CONTENTS.

CHAPTER XVII. page

Secretion 446

CHAPTER XVIII.
Excretion 472

CHAPTER XIX.
The Central Organs of the Nerve System and their Nerves 491

CHAPTER XX.

The Medulla Oblongata; the Isthmus of the Encephalon; the Basal

Ganglia 519

CHAPTER XXI.
The Cerebrum 537

CHAPTER XXII.
The Cerebellum 569

CHAPTER XXIII.
The Cranial Nerves 577

CHAPTER XXIV.
The Sympathetic Nerve System 616

CHAPTER XXV.
Phonation; Articulate Speech 626

CHAPTER XXVI.
The Special Senses 638

CHAPTER XXVII.
The Sense of Sight 650

CHAPTER XXVIII.
The Sense of Hearing 689

CHAPTER XXIX.
Reproduction 701

APPENDIX.
Physiologic Apparatus 721

Index 745

TEXT-BOOK OF PHYSIOLOGY.

CHAPTER I.
INTRODUCTION.

An animal organism in the living condition exhibits a series of
phenomena which relate to growth, movement, mentality, and re-
production. During the period preceding birth, as well as during
the period included between birth and adult life, the individual grows
in size and complexity from the introduction and assimilation of
material from without. Throughout its life the animal exhibits
a series of movements, in virtue of which it not only changes the relation
of one part of its body to another, but also changes its position relatively
to its environment. If, in the execution of these movements, the parts
are directed to the overcoming of opposing forces, such as gravity,
friction, cohesion, elasticity, etc., the animal may be said to be doing
work. The result of normal growth is the attainment of a physical
development that will enable the animal, and, more especially, man,
to perform the work necessitated by the nature of its environment and
the character of its organization. In man, and probably in lower
animals as well, mentality manifests itself as intellect, feeling, and
volition. At a definite period in the life of the animal it reproduces
itself, in consequence of which the species to which it belongs is per-
petuated.

The study of the phenomena of growth, movement, mentality,
and reproduction constitutes the science of animal physiology.
But as these general activities are the resultant of and dependent
on the special activities of the individual structures of which an animal
body is composed, physiology in its more restricted and generally
accepted sense is the science which investigates the actions or functions
of the individual organs and tissues of the body and the physical and
chemic conditions which underlie and determine them.

This may naturally be divided into:
i. Special physiology, the object of which is a study of the vital

phenomena or functions exhibited by the organs of any individual

animal.

2 TEXT-BOOK OF PHYSIOLOGY.

2. Comparative physiology, the object of which is a comparison of
the vital phenomena or functions exhibited by the organs of
two or more animals of different species, with a view to unfold-
ing their points of resemblance or dissimilarity.

Human physiology is that department of physiologic science
which has for its object the study of the functions of the organs of
the human body in a state of health.

Inasmuch as the study of function, or physiology, is associated
with and dependent on a knowledge of structure, or anatomy, it is
essential that the student should have a general acquaintance not
only with the structure of man, but with that of typical forms of lower
animal life as well.

If the body of any animal be dissected, it will be found to be com-
posed of a number of well-defined structures, such as heart, lungs,
stomach, brain, eye, etc., to which the term organ was originally ap-
plied, for the reason that they were supposed to be instruments capable
of performing some important act or function in the general activities
of the body. Though the term organ is usually employed to designate
the larger and more familiar structures just mentioned, it is equally
applicable to a large number of other structures which, though possibly
less obvious, are equally important in maintaining the life of the in-
dividual— e. g., bones, muscles, nerves, skin, teeth, glands, blood-
vessels, etc. Indeed, any complexly organized structure capable of
performing a given function may be described as an organ. A descrip-
tion of the various organs which make up the body of an animal, their
external form, their internal arrangement, their relations to one another,
constitutes the science of animal anatomy.

This may naturally be divided into :
i. Special anatomy, the object of which is the investigation of the
construction, form, and arrangement of the organs of any in-
dividual animal.
2. Comparative anatomy, the object of which is a comparison of the
organs of two or more animals of different species, with a view
to determining their points of resemblance or dissimilarity.
If the organs, however, are subjected to a further analysis, they
can be resolved into simple structures, apparently homogeneous, to
which the name tissue has been given — e. g., epithelial, connective,
muscle, and nerve tissue. When the tissues are subjected to a micro-
scopic analysis, it is found that they are not homogeneous in structure,
but composed of still simpler elements, termed cells and fibers. The
investigation of the internal structure of the organs, the physical prop-
erties and structure of the tissues, as well as the structure of their
component elements, the cells and fibers, constitutes a department of
anatomic science known as histology, or as it is prosecuted largely
with the microscope, microscopic anatomy.

INTRODUCTION. 3

Human anatomy is that department of anatomic science which
has for its object the investigation of the construction of the human
body.

GENERAL STRUCTURE OF THE ANIMAL BODY.

The body of every animal, from fish to man, may be divided into — ■

1. An axial portion, consisting of the head, neck, and trunk; and —

2. An appendicular portion, consisting of the anterior and posterior

limbs or extremities.

The axial portion of all mammals, to which class man zoologic-
ally belongs, as well as of all birds, reptiles, amphibians, and osseous
fish, is characterized by the presence of a bony, segmented axis, which
extends in a longitudinal direction from before backward, and which
is known as the vertebral column or backbone. In virtue of the ex-
istence of this column all the classes of animals just mentioned form
one great division of the animal kingdom, the Vertebraia.

Each segment, or vertebra, of this axis consists of —

1. A solid portion, known as the body or centrum, and

2. A bony arch arising from the dorsal aspect and surmounted by a

spine-like process.

At the anterior extremity of the body of the animal the vertebra;
are variously modified and expanded, and, with the addition of new
elements, form the skull; at the posterior extremity they rapidly di-
minish in size, and terminate in man in a short, tail-like process. In
many animals, however, the vertebral column extends for a consider-
able distance beyond the trunk into the tail. The vertebral column
may be regarded as the foundation element in the plan of organization
of all the higher animals and the center around which the rest of the
body is developed and arranged with a certain degree of conformity.
In all vertebrate animals the bodies of the segments of the vertebral
column form a partition which serves to divide the trunk of the body
into two cavities — viz., the dorsal and the ventral. (See Fig. 1.)

The dorsal cavity is found not only in the trunk, but also in the
head. Its walls are formed partly by the arches which arise from the
posterior or dorsal surface of the vertebras and partly by the bones of
the skull. If a longitudinal section be made through the center of the
vertebral column, and including the head, the dorsal cavity will be
observed running through its entire extent. Though for the most
part it is quite narrow, at the anterior extremity it is enlarged and
forms the cavity of the skull. This cavity is lined by a membranous
canal, the neural canal, in which are contained the brain and the neural
or spinal cord. Through openings in the sides of the dorsal cavitv
nerves pass out which connect the brain and spinal cord with all the
structures of the body.

The ventral cavity is confined mainly to the trunk of the body.
Its walls are formed bv muscles and skin, strengthened in most animals

TEXT-BOOK OF PHYSIOLOGY.

by bony arches, the ribs. Within the ventral cavity is contained a
musculo-membranous tube or canal known as the alimentary or food
canal, which begins at the mouth on the ventral side of the head, and,

after passing through the neck and
trunk, terminates at the posterior ex-
tremity of the trunk at the anus. It
may be divided into mouth, pharynx,
esophagus, stomach, small and large
intestines.

In all mammals the ventral cavity
is divided by a musculo-membranous
partition into two smaller cavities, the
thorax and abdomen. The former
contains the lungs, heart and its great
blood-vessels, and the anterior part of
the alimentary canal, the gullet or
esophagus; the latter contains the
continuation of the alimentary canal
— that is, the stomach and intestines
— and the glands in connection with
it, the liver and pancreas. In the
posterior portion of the abdominal
cavity are found the kidneys, ureters,
and bladder, and in the female the
organs of reproduction. The thoracic
and abdominal cavities are each lined
by a thin serous membrane, known,
respectively, as the pleural and perit-
oneal membranes, which, in addition,
are reflected over the surfaces of the
organs contained within them. The
alimentary canal and the various cav-
ities connected with it are lined
throughout by a mucous membrane.

The surface of the body is
covered by the skin. This is com-
posed of an inner portion, the derma,
and an outer portion, the epidermis.
The former consists of fibers, blood-
vessels, nerves, etc.; the latter of layers
of scales or cells. Embedded within
the skin are numbers of glands; which
exude, in the different classes of ani-
mals, sweat, oily matter, etc. Project-
ing from the surface of the skin are
Beneath the skin are found muscles,

Fig. i. — Diagrammatic Longit-
udinal Section of the Body. V,
V. Bodies of the vertebrae which
divide the body into the dorsal and
ventral cavities, a, a'. The dorsal
cavity. C, p'. The abdominal and
thoracic divisions of the ventral cavity,
separated from each other by a trans-
verse muscular partition, the dia-
phragm d. B. The brain. Sp.C.
The spinal cord. e. The esophagus.
S. The stomach, from which con-
tinues the intestine to the opening at
the posterior portion of the body.
/. The liver, p'. The pancreas, k.
The kidney, o. The bladder. I'. The
lungs, h. The heart.

hairs, bristles, feathers, claws,
bones, blood-vessels, nerves, etc.

INTRODUCTION. 5

The appendicular portion of the body consists of two pairs of
symmetric limbs, which project from the sides of the trunk, and
which bear a determinate relation to the vertebral column. They
consist fundamentally of bones, surrounded by muscles, blood-vessels,
nerves and lymphatics. The limbs, though having a common plan
of organization, are modified in form and adapted for prehension
and locomotion in accordance with the needs of the animal.

Anatomic Systems. — All the organs of the body which have
certain peculiarities of structure in common are classified by anato-
mists into systems — e. g., the bones, collectively, constitute the bony
or osseous system; the muscles, the nerves, the skin, constitute, respec-
tively, the muscle, the nerve, and the tegumentary systems.

Physiologic Apparatus. — More important from a physiologic
point of view than a classification of organs based on similarities of
structure is the natural association of two or more organs acting together
for the accomplishment of some definite object, and to which the term
physiologic apparatus has been applied. While in the community of
organs which together constitute the animal body each one performs
some definite function, and the harmonious cooperation of all is neces-
sary to the life of the individual, everywhere it is found that two or
more organs, though performing totally distinct functions, are cooperat-
ing for the accomplishment of some larger or compound function in
which their individual functions are blended — e. g., the mouth, stom-
ach, and intestines, with the glands connected with them, constitute the
digestive apparatus, the object or function of which is the complete
digestion of the food. The capillary blood-vessels and lymphatic
vessels of the body, and especially those in relation to the villi of the
small intestine, constitute the absorptive apparatus, the function of
which is the introduction of new material into the blood. The heart
and blood-vessels constitute the circulatory apparatus, the function of
which is the distribution of blood to all portions of the body. The
lungs and trachea, together with the diaphragm and the walls of the
chest, constitute the respiratory apparatus, the function of which is the
introduction of oxygen into the blood and the elimination from it of
carbon dioxid and other injurious products. The kidneys, the ureters,
and the bladder constitute the urinary apparatus. The skin, with its
sweat-glands, constitutes the perspiratory apparatus, the functions of
both being the excretion of waste products from the body. The liver,
the pancreas, the mammary glands, as well as other glands, each form
a secretory apparatus which elaborates some specific material necessary
to the nutrition of the individual. The functions of these different
physiologic apparatus — e. g., digestion, absorption of food, elaboration
of blood, circulation of blood, respiration, production of heat, secretion,
and excretion — are classified as nutritive junctions, and have for their
final object the preservation of the individual.

6 TEXT-BOOK OF PHYSIOLOGY.

The nerves and muscles constitute the nervo-muscle apparatus,
the function of which is the production of motion. The eye, the ear,
the nose, the tongue, and the skin, with their related structures, con-
stitute, respectively, the visual, auditory, olfactory, gustatory, and
tactile apparatus, the function of which, as a whole, is the reception
of impressions and the transmission of nerve impulses to the brain,
where they give rise to visual, auditory, olfactory, gustatory, and tactile
sensations and volitional impulses.

The brain, in association with the sense organs, forms an appa-
ratus related to mental processes. The larynx and its accessory organs
— the lungs, trachea, respiratory muscles, the mouth and resonant
cavities of the face — form the vocal and articulating apparatus, by
means of which voice and articulate speech are produced. The func-
tions exhibited by the apparatus just mentioned — viz., motion, sensa-
tion, language, mental and moral manifestations — are classified as
functions of relation, as they serve to bring the individual into con-
scious relationship with the external world.

The ovaries and the testes are the essential reproductive organs,
the former producing the germ-cell, the latter the sperm element.
Together with their related structures — the fallopian tubes, uterus,
and vagina in the female, and the urogenital canal in the male —
they constitute the reproductive apparatus characteristic of the two
sexes. Their cooperation results in the union of the germ-cell and
sperm element and the consequent development of a new being. The
function of reproduction serves to perpetuate the species to which the
individual belongs.

The animal body is therefore not a homogeneous organism, but
one composed of a large number of widely dissimilar but related organs.
As all vertebrate animals have the same general plan of organization,
there is a marked similarity both in form and structure among corre-
sponding parts of different animals. Hence it is that in the study of
human anatomy a knowledge of the form, construction, and arrange-
ment of the organs in different types of animal life is essential to its
correct interpretation; it follows also that in the investigation and com-
prehension of the complex problems of human physiology a knowledge
of the functions of the organs as they manifest themselves in the differ-
ent types of animal life is indispensable. As many of the functions of
the human body are not only complex, but the organs exhibiting them
are practically inaccessible to investigation, we must supplement our
knowledge and judge of their functions by analogy, by attributing to
them, within certain limits, the functions revealed by experimentation
upon the corresponding organs of lower animals. This experimental
knowledge, corrected by a study of the clinical phenomena of disease
and the results of post-mortem investigations, forms the basis of modern
human physiology.

CHAPTER II.
CHEMIC COMPOSITION OF THE HUMAN BODY.

Since it has been demonstrated that every exhibition of functional
activity is associated with changes of structure, it has been apparent
that a knowledge of the chemic composition of the body, not only
when in a state of rest, but to a far greater degree when in a state of
activity, is necessary to a correct understanding of the intimate nature
of physiologic processes. Though the analysis of the dead body is
comparatively easy, the determination of the successive changes in com-
position of the living body is attended with many difficulties. The
living material, the bioplasm, is not only complex and unstable in com-
position, but extremely sensitive to all physical and chemic influences.
The methods, therefore, which are employed for analysis destroy its
composition and vitality, and the products which are obtained are
peculiar to dead rather than to living material.

Chemic analysis, therefore, may be directed —
i. To the determination of the composition of the dead body.
2. To the determination of the successive changes in composition
which the living bioplasm undergoes during functional activity.

A chemic analysis of the dead body, with a view to disclosing the
substances of which it is composed, their properties, their intimate
structure, their relationship to one another, constitutes what might
be termed chemic anatomy. An investigation of the living ma-
terial and of the successive changes it undergoes in the performance
of its functions constitutes what has been termed chemic physiology
or physiologic chemistry.

By chemic analysis the animal body can be reduced to a number
of liquid and solid compounds which belong to both the inorganic
and organic worlds. These compounds, resulting from a proximate
analysis, have been termed proximate principles. That they may
merit this term, however, they must be obtained in the form under
which they exist in the living condition. The organic compounds
consist of representatives of the carbohydrate, fatty, and protein groups
of organic bodies; the inorganic compounds consist of water, various
acids, and inorganic salts.

The compounds or proximate principles thus obtained can be
further resolved by an ultimate analysis into a small number of chemic
elements which are identical with elements found in many other organic
as well as inorganic compounds. The different chemic elements
which are thus obtained, and the percentages in which they exist in the
body, are as follows — viz., oxygen, 72 percent.; hydrogen, 9.10; nitrogen,

8 TEXT-BOOK OF PHYSIOLOGY.

2.5; carbon, 13.50; phosphorus, 1.15; calcium, 1.30; sulphur, 0.147;
sodium, 0.10; potassium, 0.026; chlorin, 0.085; fluorin, iron, silicon,
magnesium, in small and variable amounts.

THE CARBOHYDRATES.

The carbohydrate compounds, which enter into the composition
of the animal body, are mainly starches and sugar. In many respects
they are closely related, and by appropriate means are readily con-
verted into one another. In composition they consist of the elements
carbon, hydrogen, and oxygen. As their name implies, the hydrogen
and oxygen are present in these compounds in the proportion in which
they exist in water, or as 2 : 1. The molecule of the carbohydrates
above mentioned consists of either six atoms of carbon or a multiple
of six; in the latter case the quantity of hydrogen and oxygen taken up
by the carbon is increased, though the ratio remains unchanged.

The carbohydrates may be divided into three groups — viz.: (1)
amyloses, including starch, dextrin, glycogen, and cellulose; (2) dex-
troses, including dextrose, levulose, galactose; (3) saccharoses, including
saccharose, lactose, and maltose. According to the number of carbon
atoms entering into the second group (six), they are frequently termed
monosaccharids; those of the third group, disaccharids — twice six;
those of the first group, polysaccharids — multiples of six.

Though but few of the members of the carbohydrate group are
constituents of the human body, many are constituents of the foods;
on account of their importance in this respect, and their relation to
one another, the chemic features of the more generally consumed car-
bohydrates will be stated in this connection.

1. AMYLOSES, (C6H10O5)n.

Starch is widely distributed in the vegetable world, being abundant
in the seeds of the cereals, leguminous plants, and in the tubers and
roots of many vegetables. It occurs in the form of microscopic granules
which vary in size, shape, and appearance, according to the plant from
which they are obtained. Each granule presents a nucleus, or hilum,
around which is arranged a series of eccentric rings, alternately light and
dark. The granule consists of an envelope and stroma of cellulose,
containing in its meshes the true starch material — granulose. Starch
is insoluble in cold water and alcohol. When heated with water up to
70 ° C, the granules swell, rupture, and liberate the granulose, which
forms an apparent solution; if present in sufficient quantity, it forms a
gelatinous mass termed starch paste. On the addition of iodin, starch
strikes a characteristic deep blue color; the compound formed — iodid
of starch — is weak, the color disappearing on heating, but reappearing
on cooling.

Boiling starch with dilute sulphuric acid (twenty-five per cent.)
converts it into dextrose. In the presence of vegetable diastase or

CHEMIC COMPOSITION OF THE HUMAN BODY. 9

animal ferments, starch is converted into maltose and dextrose, two
forms of sugar.

Dextrin is a substance formed as an intermediate product in the
transformation of starch into sugar. There are at least two principal
varieties — erythrodextrin, which strikes a red color with iodin, and
achr 00 dextrin, which is without color when treated with this reagent.
In the pure state dextrin is a yellow-white powder, soluble in water.
In the presence of vegetable ferments erythrodextrin is converted into
maltose.

Glycogen is a constituent of the animal liver, and, to a slight ex-
tent, of muscles and of tissues generally. In the tissues of the embryo
it is especially abundant. When obtained in a pure state it is an
amorphous, white powder. It is soluble in water, forming an opales-
cent solution. With iodin it strikes a port- wine color. In some respects
it resembles starch, in others dextrin. Like vegetable starch, glycogen
or animal starch can be converted by dilute acids and ferments into
sugar (dextrose).

Cellulose is the basic material of the more or less solid framework
of plants. It is soluble in ammoniacal solution of cupric oxid, from
which it can be precipitated by acids. It is an amorphous powder;
dilute acids can convert it into dextrose.

2. DEXTROSES, C6H12Oe.

Dextrose, glucose, or grape-sugar is found in grapes, most
sweet fruits, and honey, and as a normal constituent of liver, blood,
muscles, and other animal tissues. In the disease diabetes mellitus
it is found also in the urine.

When obtained from any source, it is soluble in wTater and in hot
alcohol, from which it crystallizes in six-sided tables or prisms. As
usually met with, it is in the form of irregular, warty masses. It is
sweet to the taste. When examined with the polariscope, dextrose
turns the plane of polarized light to the right. It is therefore termed
dextro-rotatory. It has for a long time been known that when sugar,
cupric hydroxid, and an alkali — e. g., sodium or potassium — are
present in solution, the sugar will abstract from the cupric hydroxid
a portion of its oxygen, thus reducing it to a lower stage of oxidation
giving rise to cuprous oxid. Sugar has a similar action on both silver
and bismuth. On this property of sugar a standard solution of
cupric hydroxid was suggested by Fehling which may be employed
for both qualitative and quantitative tests for the presence of sugar in
solution.

Fehling s Test Solution. — This is a solution of cupric hydroxid
made alkaline by an excess of sodium or potassium hydroxid with
the addition of sodium and potassium tartrate. It is made by dissolv-
ing cupric sulphate 34.64 grams, potassium hydroxid 125 grams, so-
dium and potassium tartrate 173 grams, in distilled water sufficient to
make one liter.

io TEXT-BOOK OF PHYSIOLOGY.

The reaction is expressed by the following equation :
CuS04 + 2KOH = Cu(OH)2 + K2S04.

The object of the sodium and potassium tartrate is to dissolve
the cupric hydroxid and hold it in solution.

For qualitative analysis it is only necessary to boil a few cubic centi-
meters of this solution in a test-tube; then add the suspected solution
and again heat to the boiling-point. If sugar be present, the cupric
hydroxid is reduced to the condition of a cuprous oxid, which shows
itself as a red or orange-yellow precipitate. The color of the precipitate
depends on the relative excess of either copper or sugar, being red with
the former, orange or yellow with the latter. The delicacy of this test is
shown by the fact that a few minims of this solution will detect in 1 c.c.
of water the TV of a milligram of sugar.

For quantitative analysis, 10 c.c. of Fehling's solution, diluted with
40 c.c. of water, are heated in a porcelain capsule, to which the sus-
pected solution is cautiously added from a buret until the blue color
entirely disappears. The strength of this solution is such that 1 c.c.
is decolorized by 5 milligrams of sugar {dextrose), from which the per-
centage of sugar in any solution can be determined.

All the sugars, with the exception of chemically pure saccharose,
may be tested for with this solution.

The Fermentation Test. — All the sugars with the exception of
lactose undergo reduction to simpler compounds, mainly alcohol and
carbon dioxid, under the action of the yeast plant, Saccharomyces
cerevisice. The change with dextrose is expressed in the following
equation : .

C6HI206 = 2C2H60 + 2C02.
Dextrose = Alcohol + Carbon Dioxid.

About 95 per cent, of the dextrose is so changed, the remaining
5 per cent, yielding secondary products — succinic acid, glycerin, etc.
As a means of testing any solution for the presence of sugar this method
may be adopted. It is generally very satisfactory. From the quantity
of carbon dioxid and alcohol thus produced the quantity of sugar in the
solution may be determined.

Levulose, or fruit-sugar, is found in association with dextrose
as a constituent of many fruits. It is sweeter than dextrose and more
soluble in both water and dilute alcohol. From alcoholic solutions it
crystallizes in fine, silky needles, though it usually occurs in the form
of a syrup.

Levulose is distinguished from dextrose by its property of turning
the plane of polarized light to the left; the extent to which it does so,
however, varies with the temperature and concentration of the solution.
For this reason it is turned levulo-rotatory.

Under the influence of the yeast plant it slowly undergoes fermen-
tation, yielding the same products as dextrose. It also has a reducing
action on cupric hydroxid.

CHEMIC COMPOSITION OF THE HUMAN BODY. n

Galactose is obtained by boiling milk-sugar (lactose) with dilute
sulphuric acid. In many chemic relations it resembles dextrose. It
is less soluble in water, however, crystallizes more easily, and has a
greater dextro-rotatory power. It also undergoes fermentation with
the yeast plant.

3. SACCHAROSES, C12H22Ou.

Saccharose, or cane-sugar, is widely distributed throughout
the vegetable world, but is especially abundant in sugar-cane, sor-
ghum cane, sugar-beet, Indian corn, etc. It crystallizes in large mono-
clinic prisms. It is soluble in water and in dilute alcohol. Saccharose
has no reducing power on cupric hydroxid, and hence its presence can
not be detected by Fehling's solution. It is dextro-rotatory. Boiled
with dilute mineral, as well as with organic acids, saccharose combines
with water and undergoes a change in virtue of which it rotates the
plane of polarized light to the left, and hence the product was termed
invert sugar. This latter has been shown to be a mixture of equal
quantities of levulose and dextrose. This inversion of saccharose
through hydration and decomposition is expressed in the following
equation :

CI2H22OXI + H20 = C6HI206 + C6HI206
Saccharose + Water = Levulose + Dextrose

Invert Sugar.

Saccharose is not directly fermentable by yeast, but through the
specific action of a ferment, invertin or invertase, secreted by the yeast
plant, or the inverting ferment of the small intestine, it undergoes in-
version, as previously stated, after which it is readily fermented, yield-
ing alcohol and carbon dioxid.

Lactose is the form of sugar found exclusively in the milk of the
mammalia, from which it can be obtained in the form of hard, white,
rhombic prisms united with one molecule of water. It is soluble in
water, insoluble in alcohol and ether. It is dextro-rotatory. It reduces
cupric hydroxid, but to a less extent than dextrose. Dilute acids
decompose it into equal quantities of dextrose and galactose. Lactose
is not fermentable with yeast, but in the presence of the lactic acid
.bacillus it is decomposed into lactic acid, and finally into butyric acid,
as expressed in the following equation:

CI2H220„ + H20 = 4C3H6O3
Lactose + Water = Lactic Acid.

2C3H60., = C4H802 + 2C02 + 2H,
Lactic Acid = Butyric Acid -r Carbon + Free

Dioxid Hydrogen.

Maltose is a transformation product of starch, and arises when-
ever the latter is acted on by malt extract or the diastatic ferments
in saliva and pancreatic juice. The change is expressed by the fol-
lowing equation :

2C6HioOs + H.O = CI2H220„
Starch. Water. Maltose.

12 TEXT-BOOK OF PHYSIOLOGY.

Maltose crystallizes in the form of white needles, which are soluble
in water and in dilute alcohol. It is dextro-rotatory. In the presence
of ferments and dilute acids maltose undergoes hydration and decom-
position, giving rise to two molecules of dextrose. It has a reducing
action on cupric hydroxid. Fermentation is readily caused by yeast,
but whether directly or indirectly by inversion is somewhat uncertain.

Osazones. — All the sugars which possess the power of reducing
cupric hydroxid are capable of combining with phenyl-hydrazin, with
the formation of compounds termed osazones. The osazones so
formed are crystalline in structure, but have different melting-points,
varying degrees of solubility and optic properties, all of which serve to
detect the various sugars and to distinguish one from the other. Of
the different osazones, phenyl-glucosazone is the most characteristic,
and occurs in the form of long, yellow needles. It may be obtained
from dextrose by the following method : To 50 c.c. of a dextrose solu-
tion add 2 gm. of phenyl-hydrazin and 2 gm. of sodium acetate, and
boil for an hour. On cooling, the osazone crystallizes in the form of
long, yellow needles.

THE FATS.

The fats constitute a group of organic bodies found in the tissues
of both vegetables and animals. In the vegetable world they are
largely found in fruits, seeds, and nuts, where they probably originate
from a transformation of the carbohydrates. In the animal body
the fats are found largely in the subcutaneous tissue, in the marrow
of bones, in and around various internal organs and in milk. In these
situations fat is contained in small, round or polygon-shaped vesicles,
which are united by areolar tissue and surrounded by blood-vessels.
At the temperature of the body the fat is liquid, but after death it soon
solidifies from the loss of heat.

The fats are compounds consisting of carbon, hydrogen, and oxygen,
of which the first is the chief ingredient, forming by weight about 75
per cent., while the last is present only in small quantity. The fat
found in animals is a mixture, in varying proportions in different ani-
mals, of three neutral fats — stearin, palmitin, and olein. Each fat is a
derivative of glycerin and the particular acid indicated by its name —
e. g., stearic acid, in the case of stearin, etc. The reaction which takes
place in the combination of glycerin and the acid is expressed in the
following equation:

C,H5(HO)3 + (HCl8H3502)3 = C3Hs(Cl8H3SOfl)3 + 3H.O.
Glycerin. Stearic Acid. Stearin. Water.

Hence, strictly speaking, the fats are compound ethers, in which
the hydrogen of the organic acid is replaced by the trivalent radicle,
tritenyl, C3HS.

Stearin, C3Hs(Cl8H3S02)3, is the chief constituent of the more
solid fats. It is solid at ordinary temperatures, melting at 55 ° C,

CHEMIC COMPOSITION OF THE HUMAN BODY. 13

then solidifying again as the temperature rises, until at 71 ° C. it melts
permanently. It crystallizes in square tables.

Palmitin, C3H5(Cl6H3102)3 is a semifluid fat, solid at 45 ° C.
and melting at 62 ° C. It crystallizes in fine needles, and is soluble
in ether.

Olein, C3H.(Cl8H3302)3, is a colorless, transparent fluid, liquid at
ordinary temperatures, only solidifying at o° C. It possesses marked
solvent powers, and holds stearin and palmitin in solution at the tem-
perature of the body.

Saponification. — When subjected to the action of superheated
steam, a neutral fat is saponified — i. e., decomposed into glycerin and
the particular acid indicated by the name of the fat used: e. g., stearic,
palmitic, or oleic. The reaction is expressed as follows:

C3Hs(Cl8H330?)3 + 3H20 = C3H5(HO)3 + 3(ClSH3402)
Olein. Water. Glycerin. Oleic Acid.

The fat acids thus obtained are characterized by certain chemic
features, as follows:

Stearic acid is a firm, white solid, fusible at 69 ° C. It is soluble
in ether and alcohol, but not in water.

Palmitic acid occurs in the form of white, glistening scales or
needles, melting at 62 ° C.

Oleic acid is a clear, colorless liquid, tasteless and odorless when
pure. It crystallizes in white needles at o° C.

If this saponification takes place in the presence of an alkali —
e. g., potassium hydro xid or sodium hydroxid — the acid produced
combines at once with the alkali to form a salt known as a soap, while
the glycerin remains in solution. The reaction is as follows:

3KHO + (ClSH3402)3 = 3(KCl8H3302) + 3HaO
Potassium. Oleic Acid. Potassium Oleate, Water.

All soaps are, therefore, salts formed by the union of alkalies and
fat acids. The sodium soaps are generally hard, while the potas-
sium soaps are soft. Those made with stearin and palmitin are harder
than those made with olein. If the soap is composed of lead, zinc,
copper, etc., it is insoluble in water.

Emulsification. — When a neutral oil is vigorously shaken with
water or other fluid, it is broken up into minute globules that are more
or less permanently suspended; the permanency depending on the
nature of the liquid. The most permanent emulsions are those made
with soap solutions. The process of emulsification and the part
played by soap can be readily observed by placing on a few cubic centi-
meters of a solution of sodium carbonate (0.25 per cent.) a small quant-
ity of a perfectly neutral oil to which has been added 2 or 3 per cent,
of a fat acid. The combination of the acid and the alkali at once
forms a soap. The energy set free by this combination rapidly divides
the oil into extremely minute globules. A spontaneous emulsion is
thus formed.

i4 TEXT-BOOK OF PHYSIOLOGY.

THE PROTEINS.

The proteins constitute a group of organic bodies which are found
in both vegetable and animal tissues. Though present in all animal
tissues, they are especially abundant in muscles and bones, where
they constitute 20 per cent, and 30 per cent, respectively. Though
genetically related, and possessing many features in common, the
different members of the protein group are distinguished by character-
istic physical and chemic properties which serve not only for their
identification, but for their classification into more or less well-defined
groups.

Chemic Composition. — A chemic analysis of proteins shows that
they consist of carbon, hydrogen, oxygen, nitrogen and sulphur, though
the percentage of each of these elements varies somewhat in the
different proteins.

A certain number of proteins contain phosphorus while almost
all of them contain different inorganic salts in varying amounts. The
average percentage composition of several proteins is shown in the
following analyses:

C. H. N. O. S.

Egg-albumin, 52.9 7.2 15.6 23.9 0.4 (Wiirtz).

Serum-albumin, 53.0 6.8 16.0 22.29 1.77 (Hammersten).

Casein, 52.3 7.07 15.91 22.03 0.82 (Chittenden and Painter).

Myosin, 52.82 7. 11 16.77 21.90 1.27 (Chittenden and Cummins).

The molecular composition of the proteins is not definitely known
and the formulas which have been suggested are therefore only approxi-
mative. Leow assigns to albumin the formula C72HII2Nl8022S,
while Schiitzenberger raises the numbers to C240H392N6s07SS3, either
of which shows that the proteid molecule is extremely complex.

Structure of the Protein Molecule. — From the large size of the
protein molecule as indicated by its chemic composition it might be
inferred that its structure was equally complex. This modern inves-
tigation has shown to be the case.

When any one of the typical proteins, found in animal or vegetable
tissues, is hydrolysed by acids, alkalies and animal ferments under
appropriate conditions, it can be resolved through a series of descending
stages into relatively simple nitrogen-holding bodies termed amino-
acids and diamino- acids, of which somewhat more than twenty have
been isolated and their properties determined. The principal amino-
acids are as follows: Glycocoll, alanin, leucin, isoleucin, amino-iso-
valerianic acid, serin, aspartic acid, glutamic acid, phenylalanin,
tyrosin, prolin, tryptophan. The principal diamino-acids are as
follows: Ornithin, lysin, histidin, arginin, cystin.

The protein molecule is therefore structurally complex. The
manner in which these elementary compounds are arranged, united

CHEMIC COMPOSITION OF THE HUMAN BODY. 15

or grouped in any given protein, is practically unknown. More or less
successful attempts have been made at the reconstruction of the
protein molecule by synthetic methods, by the union of two or more of
the amino-acids. A number of such compounds have been formed
by the union of from two to ten or more amino-acids, all of them
exhibiting many of the protein reactions. Such bodies are termed
polypeptids.

Physical Properties. — As a class the proteins are characterized
by the following properties:

1 . Indiffusibility. — None of the proteins normally assume the

crystalline form, and hence they are not capable of diffusing
through parchment or an animal membrane. Peptone, a product
of the digestion of proteins, is an exception as regards its diffu-
sibility. As met with in the body, all proteins are amorphous, but
vary in consistence from the liquid to the solid state. The colloid
character of the proteins permits of their separation and purifi-
cation from crystalloid diffusible compounds by the process of
dialysis.

2. Solubility. — Some of the proteids are soluble in water, others in

solutions of the neutral salts of varying degrees of concentration,
in strong acids and alkalies. All are insoluble in alcohol and
ether.

3. Coagulability. — Under the influence of heat and animal ferments,

some of the proteins readily pass from the soluble liquid state to
the insoluble solid state, attended by a permanent alteration in
their chemic composition. To this change the term coagulation
has been given. The various proteins, however, coagulate at
different temperatures. Proteins are capable of precipitation
without losing their solubility by ammonium sulphate, sodium
chlorid, and magnesium sulphate.

4. Fermentability. — In the presence of specific microorganisms-

bacteria — the proteins, owing to their complexity and instability,
are prone to undergo disintegration and reduction to simpler
compounds. This decomposition or putrefaction occurs most
readily when the conditions most favorable to the growth of
bacteria are present — viz., a temperature varying from 250 C.
to 400 C, moisture, and oxygen. The intermediate as well as
the terminal products of the decomposition of the proteins are
numerous, and vary with the composition of the protein and
the specific physiologic action of the bacteria. Among the
intermediate products is a series of alkaloid bodies, some of which
possess marked toxic properties, known as ptomains. The toxic
symptoms which frequently follow7 the ingestion of foods in
various stages of putrefaction are to be attributed to these com-
pounds. The terminal products are represented by hydrogen
sulphid, ammonia, carbon dioxid, fats, phosphates, nitrates, etc.

16 TEXT-BOOK OF PHYSIOLOGY.

Classification. — The protein compounds by virtue of their struc-
tural composition, their physical and chemic properties, permit of a
provisional arrangement into groups as follows:

PROTAMINS.

These proteins are derived for the most part from the heads
of the spermatozoa of fish. They take their names from the
species of fish from which they are obtained, e. g., salmin (salmon),
sturin (sturgeon), scombrom, (mackerel), etc. Inasmuch as they
respond to Piotrowski's test in a characteristic way they are regarded
as true proteins. When subjected to hydrolysis they can be resolved
into the diamino bodies, lysin, arginin and histidin, of which they con-
stitute about 90 per cent., and a small number of the mono-amino-acids.
Because of the fact that the diamino bodies, lysin, histidin and arginin
contain 6 atoms of carbon they are known as the hexone bases.
Inasmuch as the protamins contain practically but these three bodies,
they are regarded as the simplest of all the proteins. Since a typical
protein always yields on hydrolysis the hexone bases, in addition to a
variable number of mono-amino-acids, it is believed that the usual
protein is composed of a nucleus of the hexone bases to which is attached
a variable number of mono-amino-acids. The proportions in which
the bases exist in the nucleus and the proportions in which the amino -
acids are united to the nucleus, vary in different proteins.

HISTONS.

The proteins embraced in this class comprise a series of compounds
which are somewhat more complex than the protamins and less com-
plex than the typical proteins; for on hydrolysis they not only yield
the hexone bases but in addition a certain number of amino-acids.
They are, therefore, intermediate in structural composition between
the protamins and the usual proteins. Their protein character is
indicated by their reaction to Millon's reagent and to Piotrowski's
test. The histons are usually found in combination with nucleic acid,
in the spermatozoa of most animals and especially in fish, and in the
coloring matter (the hemoglobin) of the red corpuscles. The proteins,
of the tissues usually contain from 25 to 30 per cent, of histons.

ALBUMINS.

The members of this group are soluble in water, in dilute saline
solutions, and in saturated solutions of sodium chlorid and mag-
nesium sulphate. They are coagulated by heat, and when dried form
an amber-colored mass.

(a) Serum-albumin. — This most important protein is found in
blood, lymph, chyle, and some tissue fluids. It is obtained
readily by precipitation from blood-serum, after the other
proteins have been removed, on the addition of ammonium
sulphate. When freed from saline constituents, it presents
itself as a pale, amorphous substance, soluble in water and in

CHEMIC COMPOSITION OF THE HUMAN BODY. 17

strong nitric acid. It is coagulated at a temperature of 73 °
C, as well as by various acids — e.g., citric, picric, nitric, etc.
It has a rotatory power of — 62. 6°.

(b) Egg-albumin. — Though not a constituent of the human
body, egg-albumin resembles the foregoing in many respects.
When obtained in the solid form from the white of the egg,
it is a yellow mass without taste or odor. Though similar
to serum-albumin, it differs from it in being precipitated by
ether, in coagulating at 540 C, and in having a lower rotatory
power,— 35. 50.

(c) Lact-albumin. — As its name implies, this protein is found
in milk. It can be precipitated from milk-plasma by sodium
sulphate after the precipitation of the other proteids by half
saturation with ammonium sulphate. It slowly coagulates at
77° C.

(d) Myo-albumin. — This protein is found in muscle-plasma from
which it subjects the plasma to fractional heat coagulation.
At 730 C. myo-albumin coagulates.

GLOBULINS.

(a) Serum-globulin or Paraglobulin. — This protein, as its name
implies, is found in blood-serum, though it is present in other
animal fluids. When precipitated by magnesium sulphate
or carbon dioxid, it presents itself as a flocculent substance,
insoluble in water, soluble in dilute acids and alkalies, and
coagulating at 75 ° C.

(b) Fibrinogen. — This protein is found in blood-plasma in asso-
ciation with serum-globulin and serum-albumin. It is also
present in lymph-tissue fluids and in pathologic transudates.
It can be obtained from blood-plasma which has been pre-
viously treated with magnesium sulphate on the addition of
a saturated solution of sodium chlorid. It is soluble in dilute
acids and alkalies, and coagulates at 56 ° C.

(c) Para-myosinogen. — This protein is a constituent of the
muscle-plasma from which it can be precipitated by a tem-
perature of 470 C.

(d) Myosinogen. — This protein is the chief constituent of the
muscle-plasma and is of great nutritive value. During the
living condition it is liquid, but after death it readily undergoes
a chemic change and contributes to the formation of an insoluble
protein known as myosin. It is soluble in dilute hydrochloric
acid and dilute alkalies. It coagulates at 56 ° C.

(e) Crystallin or Globulin. — This is obtained by passing a stream
of C02 through a watery extract of the crystalline lens.

SCLERO-PROTEINS (ALBUMINOIDS).

The sclero-proteins constitute a group of substances similar to the

18 TEXT-BOOK OF PHYSIOLOGY.

proteins in many respects, though differing from them in others.
When obtained from the tissues, in which they form an organic basis,
they are found to be amorphous, colloid, and when decomposed yield
products similar to those of the true proteins. The principal members
of this group are as follows:

(a) Collagen, Ossein. — These are two closely allied, if not identical,
substances, found respectively in the white fibrous connective
tissue and in bone. When the tendons of muscles, the liga-
ments, or decalcified bone are boiled for several hours, the
collagen and ossein are converted into soluble gelatin, which,
when the solution cools becomes solid.

(b) Chondrigen. — This is supposed to be the organic basis of the
more permanent cartilages. When the latter are boiled, they
yield a substance which gelatinizes on cooling, and to which
the name chondrin has been given. Chondrin, however, is
not a pure gelatin, but has associated with it a compound
proteid known as chrondro-mucoid.

(c) Elastin is the name given to the substance composing the fibers
of the yellow, elastic connective tissue.

(d) Keratin is the substance found in all horny and epidermic
tissues, such as hairs, nails, scales, etc. It differs from most
proteids in containing a high percentage of sulphur.

PHOSPHO-PROTEINS.

The two members of this group are distinguished by yielding on
decomposition a protein which contains phosphorus. It was formerly
regarded as a nuclein.

(a) Caseinogen. — This is the principal protein of milk, in which
it exists in association with an alkali, and hence was formerly
regarded as an alkali-albumin. It is precipitated by acetic
acid and by magnesium sulphate. It is coagulated by rennet —
that is, separated into an insoluble protein, casein or tyrein,
and a soluble albumin. Calcium phosphate seems to be the
natural alkali necessary to this process, for if it be removed
by dialysis, or precipitated by the addition of potassium oxalate,
coagulation does not take place.

(b) Vitellin. — Vitellin is a constituent of the vitellis or yolk of eggs.
It differs from other proteins in the fact that it is semicrystalline
in character. Though usually regarded as a nucleo-protein
it is not definitely known whether or not it contains phosphorus
in its composition.

CONJUGATED OR COMBINED PROTEINS.

The different members of this group are capable of being decom-
posed by chemic methods into a protein and a non-protein substance;
e. g., a coloring matter, a carbohydrate, or a nuclein. The chemic
character of the non-protein substance furnishes the basis for the
following classification :

CHEMIC COMPOSITION OF THE HUMAN BODY. 19

CHROMO-PROTEINS.

(a) Hemoglobin. — Hemoglobin is the coloring matter of the red
corpuscles, of which it constitutes about 94 per cent. It
possesses the power of absorbing oxygen as it passes through
the lung capillaries and of yielding it up to the tissues as it
passes through the tissue capillaries. In the arterial blood
it is known as oxyhemoglobin, and in the venous blood as
deoxy- or reduced-hemoglobin. When hydrolysed by acids
or alkalies, hemoglobin undergoes a cleavage into a protein,
globin, and a pigment hematin.

(b) Myohematin.— Myohematin is a protein supposed to be
present in muscle. It has never been isolated, hence its chemic
features are unknown. Spectroscopic examination indicates
that it is capable of absorbing and again yielding up oxygen.
For this reason it is believed to be a derivative of hemoglobin.

GLUCO-PROTEINS.

(a) Mucin.— Mucin is the proteid which gives the mucus, secreted
by the epithelial cells of the mucous membranes and related
glands, its viscid, tenacious character. It is also a constituent
of the intercellular substance of the connective tissues. It
is readily precipitated by acetic acid. When heated with
dilute acids, mucin undergoes a cleavage into a simpler proteid
and a carbohydrate termed mucose, which is capable of reducing
Fehling's solution.

(b) Mucoids. — The mucoids resemble the mucins though differ-
ing from them in solubility and in not being precipitable from
alkaline solutions by acetic acid. They are found in the
vitreous humor, white of egg, cartilage, and in other situations.
They differ slightly one from the other in properties and chemic
composition. They yield on decomposition a carbohydrate.

NUCLEO-PROTEINS.

The nucleo-proteins are obtained from the nuclei and cell-sub-
stance of tissue-cells. Chemically they are characterized by
the presence of phosphorus in relatively large amounts. When
hydrolysed, they separate into a protein and a nuclein. The
nucleins derived from cell nuclei can be still further separated
into a simpler protein and nucleic acid, which latter in turn
yields phosphoric acid and the so-called purin bases, xanthin,
hypoxanthin, adenin, and guanin. All nucleins which yield
the purin bases are termed true nucleins.

PROTEIN DERIVATIVES.

The proteins of this group are derived from both albumins and
globulins by the gradual action of dilute acids and alkalies, and may
be regarded as compounds of a proteid with an acid or an alkali.
Thev have been designated.

20 TEXT-BOOK OF PHYSIOLOGY.

INFRA-PROTEINS.

(a) Acid-albumin. — This is formed when a native albumin is
digested with dilute hydrochloric acid (0.2 per cent.) or dilute
sulphuric acid for some minutes. It is precipitated by neu-
tralization with sodium hydroxid (0.1 per cent, solution).
After the precipitate is washed, it is found to be insoluble
in distilled water and in neutral saline solutions. In acid
solutions it is not coagulated by heat.

(b) Alkali-albumin. — This is formed when a native albumin is
treated with a dilute alkali — e. g., 0.1 per cent, of sodium
hydroxid — for five or ten minutes. On careful neutralization
with dilute hydrochloric acid, it is precipitated. It is also
insoluble in distilled water and in saline solutions; it is not
coagulable by heat.

PROTEOSES AND PEPTONES.

During the progress of the digestive process, as it takes place in
the stomach and intestines, there is produced by the action of the
gastric and pancreatic juices, out of the proteins of the food, a series
of new proteins, known as proteoses and peptones. The chemic
properties of these substances will be considered in connection with
the process of digestion.

COAGULATED PROTEINS.

Although these proteins are not found as constituents of the animal
organism, they possess much interest on account of their relation to
prepared foods and to the digestive process. They are produced
when solutions of egg-albumin, serum-albumin, or globulins are
subjected to a temperature of 100 ° C. or to the prolonged action of
alcohol. They are insoluble in water, in dilute acids, and in neutral
saline solutions.

In this same group may be included also those coagulated pro-
teins which are produced by the action of animal ferments on soluble
proteins — e. g., fibrin, myosin, casein.

(a) Fibrin. — Fibrin is derived from a soluble protein — fibrinogen
— by the action of a special ferment. It is not present under
normal circumstances in the circulating blood, but makes its
appearance after the blood is withdrawn from the vessels
and at the time of coagulation. It can also be obtained by
whipping the blood with a bundle of twigs, on which it accu-
mulates. When freed from blood by washing under water,
it is seen to consist of bundles of white elastic fibers or threads.
It is insoluble in water, in alcohol, and ether. In dilute acids
it swells, becomes transparent, and finally is converted into
acid-albumin. In dilute alkalies a similar change takes place,
but the resulting product is an alkali-albumin. Fibrin pos-
sesses the property of decomposing hydrogen dioxid, H202

CHEMIC COMPOSITION OF THE HUMAN BODY. 21

— i. e., liberating oxygen, which accumulates in the form of
bubbles on the fibrin. On incineration fibrin yields an ash
which contains calcium phosphate and magnesium phosphate.

(b) Myosin. — Myosin develops in muscles after death and is the
cause of the stiffening of the muscles. It has been regarded
as a derivative of the soluble protein myosinogen alone, but
there is evidence that in its formation both paramyosinogen
and myosinogen take part. It is not definitely known whether
this is the result of the action of a special ferment or not.

(c) Casein. — Casein is derived from the chief protein of milk —
caseinogen — by the action of a special ferment known as
rennin or chymosin. This ferment is a constituent of gas-
tric juice.

The Color Reactions of Proteins. — When proteins are present in
solution, they may be detected by the following color reactions — viz. :

1. Xanthoproteic. The solution is boiled with nitric acid for several

minutes, when the proteid assumes a light yellow color. After
the solution has cooled, the addition of ammonia changes the
color to an orange or amber-red due to the presence of phenyl-
alanin and tyrosin.

2. The rose-red reaction. The solution is boiled with acid nitrate

of mercury (Millon's reagent) for a few minutes, when the co-
agulated proteid turns a purple-red color. This color is attributed
to the presence of tyrosin.

3. The blue- violet reaction. A few drops of copper sulphate solution

arc first added to the protein solution, and then an excess of
sodium hydroxid. A blue- violet color is produced, which deepens
somewhat on heating, but no further change ensues. This is
also known as Piotrowski's test: As this same color is developed
with the substance biuret, it is also known as the biuret reaction.
Biuret is formed by heating urea and driving off ammonia.
Precipitation Tests. — Proteins in solution may be precipitated by
nitric acid, acetic acid and potassium ferrocyanid, picric acid, copper
sulphate, tannin, alcohol, etc. As stated in a foregoing paragraph,
certain of the proteins, e. g., fibrinogen, caseinogen and myosinogen,
will undergo, by the action of an animal ferment, a cleavage into a
solid and a soluble portion. To this process the term ferment coagu-
lation is applied. The solidification of proteins by the action of heat,
is designated heat coagulation.

INORGANIC CONSTITUENTS.

The inorganic compounds and mineral constituents obtained
from the solids and fluids of the body are very numerous, and, in
some instances, quite abundant. Though many of the compounds
thus obtained are undoubtedly derivatives of the tissues and necessary
to their physical and physiologic activity, others, in all probability,

22 TEXT-BOOK OF PHYSIOLOGY.

are decomposition products, or transitory constituents introduced
with the food. Of the inorganic compounds, the following are the
most important:

WATER.

Water is the most important of the inorganic constituents, as it
is indispensable to life. It is present in all the tissues and fluids
without exception, varying from 99 per cent, in the saliva to 80 per
cent, in the blood, 75 per cent, in the muscles to 2 per cent in the
enamel of the teeth. The total quantity contained in a body weigh-
ing 75 kilograms (165 pounds) is 52.5 kilograms (115 pounds). Much
of the water exists in a free condition, and forms the chief part of the
fluids, giving to them their characteristic degree of fluidity. Possess-
ing the capability of holding in solution a large number of inorganic
as well as some organic compounds, and being at the same time diffus-
ible, it renders an interchange of materials between all portions of the
body possible. It aids in the absorption of new material into the
blood and tissues, and at the same time it transfers waste products
from the tissues to the blood, from which they are finally eliminated,
along with the water in which they are dissolved. A portion of the
water is chemically combined with other tissue constituents and gives
to the tissues their characteristic physical properties. The consistency,
elasticity, and pliability are, to a large extent, conditioned by the
amount of water they contain. The total quantity of water eliminated
by the kidneys, lungs, and skin amounts to about 3 kilograms (6|
pounds).

CALCIUM COMPOUNDS.

Calcium phosphate, Ca3(P04)2, has a very extensive distribu-
tion throughout the body. It exists largely in the bones, teeth, and
to a slight extent in cartilage, blood, and other tissues. Milk con-
tains 0.27 per cent. The solidity of the bones and teeth is almost
entirely due to the presence of this salt, and is, therefore, to be re-
garded as necessary to their structure. It enters into chemic union
with the organic matter, as shown by the fact that it can not be separated
from it except by chemic means, such as hydrochloric acid. Though
insoluble in water, it is held in solution in the blood and milk by the
protein constituents, and in the urine by the acid phosphate of soda.
The total quantity of calcium phosphate which enters into the forma-
tion of the body has been estimated at 2.5 kilograms. The amount
eliminated daily from the body has been estimated at 0.4 gm., a fact
which indicates that nutritive changes do not take place with much
rapidity in those tissues in which it is contained.

Calcium carbonate, CaC03, is present in practically the same
situations in the body as the phosphate, and plays essentially the
same role. It is, however, found in the crystalline form, aggregated
in small masses in the internal ear, forming the otoliths, or ear stones.

CHEMIC COMPOSITION OF THE HUMAN BODY. 23

Though insoluble, it is held in solution by the carbonic acid diffused
through the fluids.

Calcium fluorid, CaF2, is found in bones and teeth.

SODIUM COMPOUNDS.

Sodium chlorid, NaCl, is present in all the tissues and fluids
of the body, but especially in the blood, 0.6 per cent., lymph, 0.5,
and pancreatic juice, 0.25 per cent. The entire quantity in the body
has been estimated at about 200 gm. Sodium chlorid is of much
importance in the body as it determines and regulates to a large
extent the phenomena of diffusion which are there constantly taking
place. This is illustrated by the fact that a solution of albumin
placed in the rectum without the addition of this salt will not be
absorbed. When the salt is added, absorption takes place. The
ingested water is absorbed into the blood largely in consequence of
the percentage of this salt which it contains. The normal percentage
of sodium chlorid in the blood-plasma assists in maintaining the shape
and structure of the red blood- corpuscles by determining the amount
of water entering into their composition. The same is true of other
tissue elements.

Sodium chlorid also influences the general nutritive process, in-
creasing the disintegration of the proteins, as shown by the increased
amount of urea excreted. During its existence in the body it under-
goes chemic transformations or decompositions, yielding its chlorid
to form the potassium chlorid of the blood-corpuscles and muscles
and to form the hydrochloric acid of the gastric juice.

Sodium phosphate, Na2HP04, is found in all solids and fluids
of the body, to which, with but few exceptions, it imparts an alkaline
reaction. This is especially true of blood, lymph, and tissue fluids
generally. It is essential to physiologic action that all tissue elements
should be bathed by an alkaline medium.

Sodium carbonate, Na2C03, is generally found in association
with the preceding salt. As it is an alkaline compound, it also assists
in giving to the blood and lymph their characteristic alkalinity. In
carnivorous animals the sodium phosphate is the more abundant,
while in the herbivorous animals the sodium carbonate is the more
abundant.

Sodium sulphate, Na2S04, is present in many of the tissues and
fluids, especially in the urine. Though introduced in the food, it is
also, in all probability, formed in the body from the decomposition
and oxidation of the proteids.

POTASSIUM COMPOUNDS.

Potassium chlorid, KC1, is met with in association with sodium
chlorid in almost all situations in the body. It preponderates, how-
ever, in the tissue elements, especially in the muscle tissue, nerve
tissue, and red corpuscles. The plasma with which these structure?

24 TEXT-BOOK OF PHYSIOLOGY.

are bathed contains but a very small amount of this salt, but, as
previously stated, a relatively large quantity of sodium chlorid. Though
introduced to some extent in the food, it is very likely that it is also
formed through the decomposition of the sodium chlorid.

Potassium phosphate, K2HP04, is found in association with
sodium phosphate in all the fluids and solids. As it has similar
chemic properties, its functions are practically the same.

Potassium carbonate, K2C03, is generally found with the pre-
ceding salt.

MAGNESIUM COMPOUNDS.

Magnesium phosphate, Mg3(P04)2, is found in all tissues, in
association with calcium phosphate, though in much smaller quantity.

Magnesium carbonate, MgC03, occurs only in traces in the
blood.

Both of these compounds have functions similar to the calcium
compounds, and exist, in all probability, under similar conditions.

IRON COMPOUNDS.

Iron is a constituent of the coloring-matter of the blood. Traces,
however, are also found in lymph, bile, gastric juice, and in the pig-
ment of the eyes, skin and hair. The amount of iron contained in a
body weighing 75 kilograms is about 3 gm. It exists under various
forms — e. g., ferric oxid, and in combination with organic compounds.

Chemic analysis thus shows that the chemic elements into which
the compounds may be resolved by an ultimate analysis do not exist
in the body in a free state, but only in combination, and in char-
acteristic proportions, to form compounds whose properties are the
resultant of those of the elements. Of the four principal elements
which make up 97 per cent, of the body, O, H, N are extremely mobile,
elastic, and possessed of great atomic heat. C, H, N are distinguished
for the narrow range of their affinities, and for their chemic inertia.
C possesses the great atomic cohesion. O is noted for the number and
intensity of its combinations.

As the properties of the compounds formed by the union of ele-
ments must be the resultants of the properties of the elements them-
selves, it follows that the ternary compounds, starches, sugars, and
fats must possess more or less inertia, and at the same time instability;
while in the more complex proteids, in which sulphur and phosphorus
are frequently combined with the four principal elements, molecular
instability attains its maximum. As all the foregoing compounds
possess in varying degrees the properties of inertia and instability,
it follows that living matter must possess corresponding properties,
and the capability of undergoing unceasingly a series of chemic changes,
both of composition and decomposition, in response to the chemic and
physical influences by which it is surrounded, and which underlie all
the phenomena of life.

CHEMICAL COMPOSITION OF THE HUMAN BODY.

25

PRINCIPLES OF DISSIMILATION.

In addition to the previously mentioned compounds— viz., carbo-
hydrates, fats, proteids, and inorganic salts— there is obtained bv
chemic analysis from the tissues and fluids of the body:

1. A number of organic acids, such as acetic, lactic, oxalic, butyric,

propionic, etc., in combination with alkaline and earthy bases.

2. Organic compounds, such as alcohol, glycerin, cholesterin.

3. Pigments, such as those found in bile and urine.

4. Crystallizable nitrogenized bodies, such as urea, uric acid, xanthin,

hippuric acid, creatin, creatinin, etc.
While some few of these compounds may possibly be regarded as
necessary to the physiologic integrity of the tissues and fluids, the
majority of them are to be regarded as products of dissimilation of
the tissues and foods in consequence of functional activity, and rep-
resent stages in their reduction to simpler forms previous to bein<r
eliminated from the bod v.

CHAPTER III.
PHYSIOLOGY OF THE CELL.

A microscopic analysis of the tissues shows that they can be re-
solved into ultimate elements, termed cells, which may, therefore,
be regarded as the primary units of structure. Though cells vary
considerably in shape, size, and chemic composition in the different
tissues of the adult body, they are, nevertheless, descendants from
typical cells, known as embryonic or undifferentiated cells, examples
of which are the leukocytes of the blood and lymph and the first
offspring of the fertilized ovum. Ascending the fine of embryonic
development, it will be found that every organized body originates
in a single cell — the. ovum. As the cell is the elementary unit of all
tissues, the function of each tissue must be referred to the function
of the cell. Hence the cell may be defined as the primary anatomic
and physiologic unit of the organic world, to which every exhibition
of life, whether normal or abnormal, is to be referred.

Structure of Cells. — Though cells vary in shape and size and
internal structure in different portions of the body, a typical cell may
be said to consist mainly of a gelatinous substance forming the body
of the cell, termed protoplasm or bioplasm, in which is embedded a
smaller spheric body, the nucleus. The shape of the adult cell varies
according to the tissue in which it is found; when young and free
to move in a fluid medium, the cell assumes a spheric form, but when
subjected to pressure, may become cylindric, fusiform, polygonal,
or stellate. Cells vary in size within wide limits, ranging from 7.7^
(3TF0" °f an inch, the diameter of a red blood-corpuscle), to 135/* (3-3-5-
of an inch, the diameter of the large cells in the gray matter of the
spinal cord). (See Fig. 2.)

The cell protoplasm consists of a soft, semifluid, gelatinous
material, varying somewhat in appearance in different tissues.
Though frequently homogeneous, it often exhibits a finely granular
appearance under medium powers of the microscope. Young cells
consist almost entirely of clear protoplasm. Mature cells contain,
according to the tissue in which they are found, material of an en-
tirely different character — e. g., small globules of fat, granules of
glycogen, mucigen, pigments, digestive ferments, etc. Under high
powers of the microscope the cell protoplasm is found to be pervaded
by a network of fibers, termed spongioplasm, in the meshes of which
is contained a clearer and more fluent substance, the hyaloplasm.
The relative amount of these two constituents varies in different
cells, the proportion of hyaloplasm being usually greater in young

26

PHYSIOLOGY OF THE CELL.

27

cells. The arrangement of the fibers forming the spongioplasm also
varies, the fibers having sometimes a radial direction, in others a
concentric disposition, but most frequently being distributed evenly
in all directions. In many cells the outer portion of the cell proto-
plasm undergoes chemic changes and is transformed into a thin,
transparent, homogeneous membrane— the cell membrane — which
completely incloses the cell substance. The cell membrane is per-
meable to water and watery solutions of various inorganic and organic
substances. It is, however, not an essential part of the cell.

The nucleus is a small vesicular body embedded in the proto-
plasm near the center of the cell. In the resting condition of the cell
it consists of a distinct membrane, composed of amphipyrenin, in-
closing the nuclear contents. The latter consists of a homogeneous

Nuclear membrane. ,

Linin.

Nuclear fluid (matrix). -

Nucleolus.

Chromatin-cords
(nuclear network).

Nodal enlargements _
o£ the chromatin.

" — Cell membrane.
Exoplasm.

Microsomes.
Centrosoma.

~ " Spongioplasm.
— - Hyaloplasm.

Foreign inclosures.

Fig. 2. — Diagram of a Cell. Microsomes and spongioplasm are onlv partly drawn.
—{Stohr.)

amorphous substance — the nuclear matrix — in which is embedded
the nuclear network. It can often be seen that a portion of one
side of the nucleus, called the pole, is free from this network. The
main cords of the network are arranged as V-shaped loops about it.
These main cords send out secondary branches or twigs, which,
uniting with one another, complete the network. The nuclear cords
are composed of granules of chromatin — so called because of its
affinity for certain staining materials — held together by an achromatin
substance known as linin. Besides the nuclear network, there are
embedded in the nuclear matrix one or more small bodies composed
of pyrenin, known as nucleoli. At the pole of the nucleus, either
within or just without in the protoplasm, is a small body, the centro-
som-e, or pole corpuscle.

Chemic Composition of the Cell. — The composition of living

28 TEXT-BOOK OF PHYSIOLOGY.

bioplasm is difficult of determination, for the reason that all chemic
and physical methods employed for its analysis destroy its vitality,
and the products obtained are peculiar to dead rather than to living
matter. Moreover, as bioplasm is the seat of extensive chemic
changes, it is not easy to determine whether the products of analysis
are crude food constituents or cleavage or disintegration products.
Nevertheless, chemic investigations have shown that even in the living
condition bioplasm is a highly complex compound — the resultant
of the intimate union of many different substances. About 75 per
cent, of bioplasm consists of water and 25 per cent, of solids, of which the
more important compounds are various nucleo-proteins (characterized
by their large percentage of phosphorus), globulins, traces of lecithin,
cholesterin, and possibly fat and carbohydrates. Inorganic salts, es-
pecially the potassium, sodium, and calcium chlorids and phosphates,
are almost invariable and essential constituents.

MANIFESTATIONS OF CELL LIFE.

Growth, Nutrition, and Reproduction. — All cells exhibit the
three fundamental properties of life — viz., growth, nutrition, repro-
duction. Growth is an increase in size. When newly reproduced all
cells are extremely small, but in consequence of their organization and
the character of their surrounding medium, they gradually grow
until they attain the size characteristic of the adult state.

Nutrition is the maintenance of the physiologic condition of the
cell and includes both growth and repair. So long as this is accom-
plished, the cells and the tissues which are formed by them continue
to exhibit their functions or their characteristic modes of activity.
Both growth and nutrition are dependent on the power which living
material possesses of not only absorbing nutritive material from the
surrounding medium, the lymph, but of subsequently assimilating it,
organizing it, transforming it into material like itself and endowing
it with its own physiologic properties.

In the physiologic condition the living material of the cell, the
bioplasm, is the seat of a series of chemic changes which vary in
degree from moment to moment in accordance with the degree of
functional activity, and on the continuance of which all life phenomena
depend. Some of these chemic changes are related to or connected
with the molecules of the living material, while others are connected
with the food material supplied to them. Of the chemic changes
occurring within the molecules some are destructive, dissimilative or
disintegrative in character, whereby the molecule is in part eventually
reduced through a series of descending chemic stages to simpler com-
pounds which, apparently of no use in the cell, are eliminated from
it. It is, therefore, said that the living material undergoes molecular
disintegration as a result of functional activity. To these changes
the term katabolism is also applied. Other of these changes are con-

PHYSIOLOGY OF THE CELL. 29

structive, assimilative or integrative in character, whereby a part at
least of the food material is transformed through a series of ascending
chemic stages into living material, and whereby it is repaired and its
former physiologic condition restored. It is, therefore, said that the
living material undergoes molecular integration as a preparation for
functional activity. To these changes the term anabolism is also
applied. The sum total of all chemic changes which go on in the cell,
both anabolic and katabolic, is embraced in the general term metab-
olism. During the course of its physiologic activities the cell bioplasm
produces materials of an entirely different character which vary with
the cell, such as fat, glycogen, mucigen, pigments, ferments, etc.,
which are generally spoken of as metabolic products.

The food materials, sugar, fat and proteins, which are constituents
of the fluids circulating in the tissue spaces, as above stated, are also
reduced to simpler forms and the energy which they contain liberated
in the form of heat. It is, therefore, said that the food also undergoes
metabolism.

There is, however, much difference of opinion as to the extent to
which the living material is metabolized and to the actual disposition of
the food materials, and especially the proteins. Thus Voit contends
that the tissue molecules are comparatively stable in composition and
under ordinary conditions of nutrition do not undergo any material
change during either rest or activity, and that metabolism is confined
to the food materials occupying spaces in and around the living cell.
The cause which initiates this metabolism is unknown, but is supposed
to reside in the cell, if it is not a property of the cell itself. Because
of the fact that but a very small amount, if any, of sugar or fat enters
into the composition of bioplasm it is generally admitted that these
foods are metabolized in the tissue spaces and in the manner just
alluded to. The problem, however, is different in the case of the
proteins. Voit contends, as previously stated, that the proteins of
the tissue molecules, which he distinguishes as tissue proteins, do
not metabolize and confines all protein metabolism to the food pro-
teins circulating in the tissue spaces and which he distinguishes as
circulating proteins. Even in starvation the tissue proteins, as such,
do not metabolize until they have been dissolved in consequence of
chemic changes and transformed into circulating protein.

Pfltiger, however, asserts that the circulating proteins can not
be metabolized but that they must first be built up into tissue proteins.
The metabolism of protein is, therefore, confined, in this view, to the
molecules of the living material. It is possible, however, that both
views are correct and that in the physiologic condition the activity of
the tissues is attended by a partial destruction of the tissue molecules
which is followed in turn by construction during the subsequent rest,
but that the greater part of the protein metabolism takes place out-
side the cell, though in contact with it.

Though the cell is, therefore, the seat of two opposing processes,

3o TEXT-BOOK OF PHYSIOLOGY.

assimilation and dissimilation, it retains under normal conditions an
average physiologic state, and so long as this is the case it is in a con-
dition of nutritive equilibrium and capable of performing its various
functions.

Though the foregoing statements are applied to the individual cell
they are equally applicable to the body as a whole, inasmuch as the
organs and tissues of which it consists are composed of cells. The
body grows in size and maintains its nutrition, by the introduction
of food materials which are utilized in part, for the repair of the tissues
which have undergone molecular disintegration in consequence of
activity, and in part for the liberation of energy. As a result of the
disintegration or the metabolism of tissue and food materials, products
such as carbon dioxid, urea, etc., are formed which, apparently of no
further use, are discharged from the body by eliminating organs as
the kidney, lungs, skin, etc. Assimilation and dissimilation are
constantly taking place. If the food assimilated and metabolized
exactly replaces the tissues dissimilated and the food metabolized the
body will retain a condition of nutritive equilibrium.

Physiologic Properties of Bioplasm. — All living bioplasm
possesses properties winch serve to distinguish and characterize it —
viz., irritability, conductivity, and motility.

Irritability, or the power of reacting in a definite manner to some
form of external excitation, whether mechanic, chemic, or electric,
is a fundamental property of all living bioplasm. The character
and extent of the reaction will vary, and will depend both on the
nature of the bioplasm and the character and strength of the stimulus.
If the bioplasm be muscle, the response will be a contraction ; if it be
gland, the response will be a secretion; if it be nerve, the response will
be a sensation or some other form of nerve activity.

Conductivity, or the power of transmitting molecular disturbances
arising at one point to all portions of the irritable material, is also
a characteristic feature of all bioplasm. This power, however, is
best developed in that form of bioplasm found in nerves, which serves
to transmit, with extreme rapidity, molecular disturbances arising at
the periphery to the brain, as well as from the brain to the periphery.
Muscle bioplasm also possesses the same power in a high degree.

Motility, or the power of executing apparently spontaneous move-
ments, is exhibited by many forms of cell bioplasm. In addition to
the molecular movements which take place in certain cells, other forms
of movement are exhibited, more or less constantly, by many cells
in the animal body — e. g., the waving of cilia, the ameboid movements
and migrations of white blood-corpuscles, the activities of spermato-
zooids, the projection of pseudopodia, etc. These movements,
arising without any recognizable cause, are frequently spoken of as
spontaneous. Strictly speaking, however, all protoplasmic movement
is the resultant of natural causes, the true nature of which is beyond
the reach of present methods of investigation.

PHYSIOLOGY OF THE CELL.

3i

Reproduction. — Cells reproduce themselves in the higher ani-
mals in two ways — by direct division and by indirect division, or
karyokinesis. In the former the nucleus becomes constricted, and
divides without any special grouping of the nuclear elements. It is
probable that this occurs only in disintegrating cells, and never in
a physiologic multiplication. In division by karyokinesis (Fig. 3)
there is a progressive rearranging and definite grouping of the nucleus,
the result of which changes is the division of the centrosome, the
chromatin, and the rest of the nucleus into two equal portions, which

Close Skein Loose Skein (viewed

(viewed from from above — i. e., from

the side). the pole). Mother Stars (viewed from the side).

Polar field.

Spindle.

Mother Star (viewed Daughter Star
from above).

?-r>v.

IjPPJP

Beginning Completed

Division of the Protoplasm.

Fig. 3. — Karyokinetic Figures Observed in the Epithelium of the Oral
Cavity of a Salamander. The picture in the upper right-hand corner is from a section
through 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. —
(Stohr.)

form the nuclei. Following the division of the nuclei, the protoplasm
divides. The process may be divided into three phases:
1. Prophase. — The centrosome, at first small and lying within the
nucleus, increases in size and moves into the protoplasm, where
it lies near the nucleus, surrounded by a clear zone, from winch
delicate threads radiate through an area known as the attraction
sphere. The nucleus enlarges and becomes richer in chromatin.
The lateral twigs of the chromatin cords are drawn in, while
the main cords become much contorted. These cords have a
general direction transverse to the long axis of the cell, and
parallel to the plane of future cleavage. They are seen as V-
shaped segments or loops, chromosomes, having their closed ends
directed toward a common center, the polar field, while the other

32 TEXT-BOOK OF PHYSIOLOGY.

ends interdigitate on the opposite side of the nucleus — the anti-
pole. The polar field corresponds to the area occupied by the cen-
trosome. This arrangement is known as the close skein; but as
the process goes on, the chromosomes become thicker, shorter
and less contorted, producing a much looser arrangement, known
as the loose skein. During the formation of the loose skein, the
centrosome divides into two portions, which move apart to posi-
tions at the opposite ends of the long axis of the nucleus. At the
same time delicate achromatin fibers make their appearance,
arranged in the form of a double cone, the apices of which corre-
spond in position to the centrosomes. This is known as the
nuclear spindle. During the prophase the nuclear membrane and
the nucleoli disappear.

2. The Metaphase. — The two centrosomes are at opposite ends of

the long axis of the nucleus, each surrounded by an attraction
sphere, now called the polar radiation. The chromosomes
become yet shorter and thicker, and move toward the equator
of the nucleus, where they lie with their closed ends toward the
axis, presenting the appearance, when seen from the poles, of
a star — the so-called mother star, or monaster. While moving
toward the equator of the nucleus, and often earlier, each chromo-
some undergoes longitudinal cleavage, the sister loops remaining
together for a time. Upon the completion of the monaster, one loop
of each pair passes to each pole of the nucleus, guided, and per-
haps drawn by the threads of the nuclear spindle. The separation
of the sister segments begins at their apices, and as the open ends
are drawn apart they remain connected by delicate achromatin
filaments drawn out from the chromosomes. This separation of the
daughter chromosomes, and their movement toward the daughter
centrosomes, is called metakinesis. As they approach their des-
tination, we have the appearance of two stars in the nucleus — the
daughter stars, or diasters.

3. Anaphase. — The daughter stars undergo, in reverse order, much

the same changes that the mother star passed through. The chro-
mosomes become much convoluted, and perhaps united to one
another, the lateral twigs appear, and the chromatin resumes the
appearance of the resting nucleus. The nuclear spindle, with
most of the polar radiation, disappears, and the nucleoli and the
nuclear membrane reappear, thus forming two complete daughter
nuclei. Meanwhile the protoplasm becomes constricted midway
between the young nuclei. This constriction gradually deepens
until the original cell is divided, with the formation of two complete
cells.

CHAPTER IV.

HISTOLOGY OF THE EPITHELIAL AND CONNECTIVE

TISSUES.

i. EPITHELIAL TISSUE.

The epithelial tissue consists of one or more layers of cells resting
on a homogeneous membrane, the other side of which is abundantly
supplied with blood-vessels and nerves. The form of the epithelial
cell varies in different situations, and may be flattened, cuboid, sphe-
roid, or columnar. (See Figs. 4, 5, and 6.) The form of the cell in
all instances is related to some specific function. When arranged in
layers or strata, the cells are cemented together by an intercellular
substance.

Fig. 4. — Epithelial Cells of Rabbit, Isolated. X 560 1. Squamous cells
(mucous membrane of mouth). 2. Columnar cells (corneal epithelium). 3. Colum-
nar cells, with cuticular border, 5 (intestinal epithelium). 4. Ciliated cells; h, cilia
(bronchial epithelium). — (Stohr.)

The epithelial tissue forms a continuous covering for the surfaces of
the body. The external investment (the skin) and the internal
investment (the mucous membrane, which lines the entire alimentary
canal as well as associated body cavities) are both formed, in all
situations, by the homogeneous basement membrane, covered with
one or more layers of cells. The glands of the skin, the lungs and the
glands in connection with the alimentary canal and the uro-genital
apparatus are formed of the same elemental structures. All materials,
therefore, whether nutritive, secretory, or excretory, must pass through
epithelial cells before they can enter into the formation of the blood
or be eliminated from it. The nutrition of the epithelial tissue is
maintained by the nutritive material derived from the blood diffusing
itself into and through the basement membrane. Chemically, the
epithelial cells of the epidermis — hair, nails, etc. — are composed 'of

3 33

34

TEXT-BOOK OF PHYSIOLOGY.

an albuminoid material (keratin), a small quantity of water, and
inorganic salts. In other situations, especially on the mucous mem-
branes, the cells consist largely of mucin, in association with other
proteins. The consistency of epithelium varies in accordance with
external influences, such as the presence or absence of moisture, pres-
sure, friction, etc. This is well seen in the skin of the palms of the
hands and the soles of the feet — situations where it acquires its greatest
density. In the alimentary canal, in the lungs, and in other cavities,
where the reverse conditions prevail, the epithelium is extremely soft.
Epithelial tissues also possess varying degrees of cohesion and elasticity
— physical properties which enable them to resist considerable pressure
and distention without having their physiologic integrity destroyed.
Inasmuch as these tissues are poor conductors of heat, they assist in

preventing too rapid radiation of
H heat from the body, and cooperate

with other mechanisms in main-
~ taining the normal temperature.

0 ®_;

Fig. 5. — Stratified Squamous
Epithelium (Larynx of Man).
X 240. 1. Columnar cells. 2. Prickle-
cells. 3. Squamous cells. — (Stohr.)

Fig. 6. — Stratified Ciliated
Epithelium. X 560. From the res-
piratory nasal mucous membrane
of man. 1. Oval cells. 2. Spindle-
shaped cells. 3. Columnar cells. —
(Stohr.)

The physiologic activity of all epithelial tissue depends on a due supply of
nutritive material derived from the blood, which not only maintains
its nutrition, but affords those materials out of which are formed the
secretions of the glands, whether of the skin or mucous membrane.

The cells lining the blood-vessels, the lymph- vessels, the peritoneal,
pleural, pericardial, and other closed cavities are usually termed
endothelial cells. These cells are flat, irregular in shape, with borders
more or less wavy or sinuous in outline.

Functions of Epithelial Tissue. — In succeeding chapters the
form, chemic composition, and functions of epithelial cells will be
considered in connection with the functions of the organs of which
they constitute a part. In this connection it may be stated in a general
way that the functions of the epithelial tissues are:
1 . To serve on the surface of the body as a protective covering to the

THE CONNECTIVE TISSUES. 35

underlying structures which collectively form the true skin, thus
protecting them from the injurious influences of moisture, air,
dust, microorganisms, etc., which would otherwise impair their
vitality. Wherever continuous pressure is applied to the skin,
as on the palms of the hands and soles of the feet, the epithelium
increases in thickness and density, and thus prevents undue
pressure on the nerves of the true skin. The density of the
epidermis enables it to resist, within limits, the injurious influence
of acids, alkalies, and poisons.

2. To promote absorption. Inasmuch as the skin and mucous mem-

branes cover the surfaces of the body, it is obvious that all nutritive
material entering the body must first traverse the epithelial
tissue. Owing to their density, however, the epithelial cells
covering the skin play but a feeble role as absorbing agents in man
and the higher animals. The epithelium of the mucous mem-
brane of the alimentary canal, particularly that of the small
intestine, is especially adapted, from its situation, consistency,
and properties, to play the chief role in the absorption of new
materials from the canal. The epithelium lining the air-vesicles
of the lungs is engaged in promoting the absorption of oxygen and
the exhalation of carbon dioxid.

3. To form secretions and excretions. Each secretory gland con-

nected with the surfaces of the body is lined by epithelial cells,
which are actively concerned in the formation of the secretion
peculiar to the gland. Each excretory organ is similarly provided
with epithelial cells, which are engaged either in the production
of the constituents of the excretion or in their removal from the
blood.

2. THE CONNECTIVE TISSUES.

The connective tissues, in their collective capacity, constitute
a framework which pervades the body in all directions, and, as the
name implies, serve as a bond of connection between the individual
parts, at the same time affording a basis of support for the muscle,
nerve, and gland tissues. The connective-tissue group includes a
number of varieties, among which may be mentioned the areolar,
adipose, retiform, white fibrous, yellow elastic, cartilaginous and
osseous. Notwithstanding their apparent diversity, they possess
many points of similarity. They have a common origin, developing
from the same embryonic material; they have much the same struc-
ture, passing imperceptibly into one another, and perform practically
the same functions.

Areolar Tissue. — This variety is found widely distributed through-
out the body. It serves to unite the skin and mucous membrane to
the structures on which they rest; to form sheaths for the support
of blood-vessels, nerves, and lymphatics; to unite into compact masses
the muscular tissue of the body, etc. Examined with the naked eye,

36 TEXT-BOOK OF PHYSIOLOGY.

it presents the appearance of being composed of bundles of fine fibers
interlacing in every direction. In the embryonic state the elements
of this form of connective tissue are united by a ground substance,
gelatinous in character. In the adult state this substance shrinks
and largely disappears, leaving intercommunicating spaces of varying
size and shape, from which the tissue takes its name. When subjected
to the action of various reagents, and examined microscopically, the
bundles can be shown to consist of extremely delicate, colorless,
transparent, wavy fibers, which are cemented together by a ground
substance composed largely of mucin. Other fibers are also observed,
which are distinguished by a straight course, a sharp, well-defined
outline, a tendency to branch and unite with adjoining fibers, and to

super- c\55@^^5S^T1^1 FlG" 8' — Fai'-cells FROM THE

posed — ^W^S&^/\}^yTir\ Axilla of Man. i. The equator

M^&vQ^^C/ik of the cell in focus. 2. The ob-

jective somewhat elevated. 3, 4.
Forms changed by pressure, p.
Traces of protoplasm in the vicinity

Fig. 7. — Adipose Tissue. — (Stdhr.) of the flat nucleus k. — (Stdhr.)

curl up at their extremities when torn. From their color and elasticity
they are known as yellow elastic fibers. Distributed throughout the
meshes of the areolar tissue are found flattened, irregularly branched,
or stellate corpuscles, connective-tissue corpuscles, plasma cells, and
granule cells.

Adipose Tissue.- — This tissue, which exists very generally through-
out the body, though found most abundantly beneath the skin, around
the kidneys, and in the bones, is practically but a modification of
areolar tissue. In these situations it presents itself in small masses
or lobules of varying size and shape, surrounded and penetrated by the
fibers of connective tissue. (See Fig. 7.) Microscopic examination
shows that these masses consist of small vesicles or cells, round, ellip-
tical or polyhedral in shape, depending somewhat on pressure. (See Fig.
8.) Each vesicle consists of a thin, colorless, protoplasmic membrane,
thickened at one point, in which a nucleus can usually be detected.
This membrane incloses a globule of fat, which during life is in the
liquid state. It is composed of olein, stearin, and palmitin. The
origin of the fat is to be referred to a retrograde change in the proto-

THE CONNECTIVE TISSUES.

37

plasmic material of the connective-tissue cells. When this protoplasm
becomes rich in carbon and hydrogen, it is speedily converted into fat,
which makes its appearance in the form of minute drops in different
portions of the cell. As the drops accumulate, at the expense of the
cell protoplasm, they gradually coalesce, until there remains but a thin
stratum of the protoplasm, which forms the wall of the vesicle. Adi-
pose tissue may, therefore, be regarded as areolar tissue, in which,
and at the expense of some of its elements, fat is stored for the future
needs of the organism. A diminution of food, especially of fat and
carbohydrates, is promptly followed by an absorption of fat by the
blood-vessels and by its transference to the tissues, where it is either
utilized for tissue construction or for oxidation purposes. In the
situations in which adipose tissue is found it serves, by its chemic
and physical properties, to assist in the
prevention of a too rapid radiation of
heat from the body, to give form and
roundness, and to diminish angularities,
etc.

Retiform and adenoid tissue are
also modifications of areolar tissue. The
meshes of the former contain but little
ground substance, its place being taken
by fluids; the meshes of the latter contain
large numbers of lymph corpuscles.

Fibrous Tissue. — This variety of
connective tissue is widely distributed
throughout the body. It constitutes
almost entirely the ligaments around the
joints, the tendons of the muscles, the
membranes covering organs such as the
heart, liver, nerve system, bones, etc.

All fibrous tissue, wherever found, can be resolved into elementary
bundles, which on microscopic examination are seen to consist of
delicate, wavy, transparent, homogeneous fibers, which pursue an
independent course, neither branching nor uniting with adjoining
fibers. (See Fig. 9.) A small amount of ground substance serves to
hold them together. Fibrous tissue is tough and inextensible, and in
consequence is admirably adapted to fulfil various mechanical func-
tions in the body. It is, however, quite pliant, bending easily in all
directions. When boiled, fibrous tissue yields gelatin, a derivative
of collagen.

Elastic Tissue. — The fibers of elastic tissue are usually associated
in varying proportions with the white fibrous tissue; but in some
structures — as the ligamentum nuchas, the ligamenta subflava, the
middle coat of the larger blood-vessels — the elastic fibers are almost
the only elements present, and give to these structures a distinctly
yellow appearance. The fibers throughout their course give off

Fig. 9. — Connective-tissue
Bundles of Various Thick-
nesses of the Intermuscular
Connective Tissue of Man.
X 240. — (Stohr.)

38

TEXT-BOOK OF PHYSIOLOGY.

many branches, which unite with adjoining branches to form a more
or less close network. As the name implies, these fibers are highly
elastic, and are capable of being extended as much as 60 per cent,
before breaking. (See Fig. 10.)

Cartilaginous Tissue. — This form of connective tissue differs
from the preceding varieties chiefly in its density. As a rule, it is
firm in consistency, though somewhat elastic. ' It is opaque, bluish-
white in color, though in thin sections translucent. All cartilaginous
tissues consist of connective-tissue cells embedded in a solid ground

Fig. 10. — Elastic Fibers. X 560. A. Fine elastic fibers, /, from intermuscular
connective tissue of man; b, connective-tissue bundles swelled by treatment with acetic
acid- B. Very thick elastic fibers, /, from ligamentum nuchae of ox; b, connective-tissue
bundles. C. From a cross-section of the ligamentum nuchae of ox; /, elastic fibers;
b, connective-tissue bundles. — (Stohr.)

substance. According to the amount and texture of the ground
substance, three principal varieties may be distinguished:
1. Hyaline cartilage, in which the cells, relatively few in number, are
embedded in an abundant quantity of ground substance- (Fig. n).
The body of the cells is in many instances distinctly marked off
from the surrounding substance by concentric fines or fibers,
which form a capsule for the cell. Repeated division of the cell
substance takes place, until the whole capsule is completely
occupied by daughter cells. The ground substance is pervaded
by minute channels, which communicate on one hand with the
spaces around the cells, and on the other with lymph-spaces in
the connective tissue surrounding the cartilage. By means of
these channels, nutritive fluid can permeate the entire structure.
Hyaline cartilage is found on the ends of the long bones, where
it enters into the formation of the joints; between the ribs and
sternum, forming the costal cartilage, as well as in the nose and
larynx.

THE CONNECTIVE TISSUES. 39

White fibro-cartilage, the ground substance of which is pervaded
by white fibers, arranged in bundles or layers, between which
are scattered the usual encapsulated cells. (See Fig. 12.) White
fibro-cartilage is tough, resistant, but flexible, and is found in
joints where strength and fixedness are required. Hence it is
present between the vertebras, forming the intervertebral discs,
between the condyle of the lower jaw and the glenoid fossa, in
the knee-joint, around the margins of the joint cavities, etc. In
these situations it assists in maintaining the apposition of the

it

s-

Fig. 11. — Hyaline Cartilage. X 240. A. Surface view of the ensiform process
of frog, fresh; p, protoplasm of cartilage-cell, which entirely fills the lacuna; k, nucleus;
g, hyaline matrix. B. Portion of cross-section of human rib-cartilage several days
after' death; examined in water: the protoplasm, z, of the cartilage-cells has withdrawn
from the walls of the lacuna?, h; the nuclei are invisible. 1. Two cells within one capsule,
k; x, a developing partition. 2. Five cartilage-cells within one capsule; the lowest cell
has fallen out, and here only the empty space is seen. 3. Capsule cut obliquely, and
apparently thicker on one side. 4. Capsule not cut, but showing the cell within, g.
Hyaline matrix transformed into rigid fibers, /• — (Stohr.)

bones, in giving a certain degree of mobility to the joints, and in
diminishing the effects of shock and pressure imparted to the
bones.

3. Yellow fibro-cartilage, the ground substance of which is pervaded
by opaque, yellow elastic fibers, which form, by the interlacing
of their branches, a complicated network, in the meshes of which
are to be found the usual corpuscles. (See Fig. 13.) As these
fibers are elastic, they impart to the cartilage a very considerable
degree of elasticity. Yellow fibro-cartilage is well adapted, there-
fore, for entering into the formation of the external ear, epiglottis,
Eustachian tube, etc. — structures which require for their func-
tional activity a certain degree of flexibility and elasticity.
Osseous Tissue. — Osseous tissue, as distinguished from bone,

is a member of the connective-tissue group, the ground substance of

40

TEXT-BOOK OF PHYSIOLOGY.

which is permeated with insoluble lime salts, of which the phosphate
and carbonate are the most abundant. Immersed in dilute solutions
of hydrochloric acid, they can be converted into soluble salts and dis-
solved out. The osseous matrix left
behind is soft and pliable. When
boiled, it yields gelatin.

A thin, transverse section of a
decalcified bone, when examined
microscopically, reveals a number of
small, round, or oval openings, which
represent transverse sections of canals
which run through the bone, for the
most part in a longitudinal direction,
though frequently anastomosing with
one another. These so-called Haver-
sian canals in the living state contain
blood-vessels and lymphatics. (See
Fig. 14.)

Around each Haversian canal is a
series of concentric lamina;, composed
of white fibers. Between every two
laminae are found small cavities
(lacunas), from which radiate in all directions small canals (canaliculi),
which communicate freely with one another. The Haversian canals,
with their associated lacunas and canaliculi, form a system of inter-
communicating passages, which circulate lymph destined for the

Fig. 12. — From a Horizontal
Section of the Intervertebral
Disc of Man. g. Fibrillar connec-
tive tissue. 2. Cartilage-cell (nucleus
invisible), k. Capsule surrounded
bv calcareous granules. X 240. —
(Stohr.)

til^m-^''^*^1

Fig. 13. — Elastic Cartilage. X 240. 1. Portion of section of vocal process (ante-
rior angle) of arytenoid cartilage of a woman thirty years old; the elastic substance in the
form of granules. 2 and 3. Portions of sections of epiglottis of a woman sixty years old;
a fine network of elastic fibers in 2, a coarser network in 3. z. Cartilage-cell, nucleus
not visible; k, capsule. — (Stohr.)

nourishment of bone. Each lacuna contains the bone corpuscle, which
bears a close resemblance to the usual branched connective-tissue cor-
puscle, and whose function appears to be the maintenance of the
nutrition of the bone.

THE CONNECTIVE TISSUES.

4i

The surface of every bone in the living state is invested with a
fibrous membrane, the periosteum, except where it is covered with
cartilage. The inner surface of this membrane is loose in texture,
and supports a fine plexus of capillary blood-vessels and numerous
protoplasmic cells — the osteoblasts. As this layer is directly con-
cerned in the formation of bone, it is spoken of as the osteo genetic
layer.

A section of a bone shows that it is composed of two kinds of
tissue — compact and cancellated. The compact is dense, resembling
ivory, and is found on the outer portion of the bone; the cancellated

Periosteum.

Outer ground lamellae.

Haversian canals.

Haversian lamella?.

Interstitial lamella?.
Inner ground lamella?.

Marrow.

Fig. 14. — From a Cross-section of a Metacarp of Man. X 5°. The Haver-
sian canals contain a little marrow (fat-cells). Resorption line at h. — (Stohr.)

is spongy, and appears to be made up of thin, bony plates, winch
intersect one another in all directions, and is found in greatest abun-
dance in the interior of the bones. The shaft of a long bone is hollow.
This central cavity, which extends from one end of the bone to the
other, as well as the interstices of the cancellated tissue, is filled in the
living state with marrow. The marrow or medulla is composed of a
connective-tissue framework supporting blood-vessels. In its meshes
are to be found characteristic bone cells or osteoblasts, the function
of which is supposed to be the formation of bone. In the long bones
the marrow is yellow, from the presence in the connective-tissue
corpuscle of fat globules, which arise through the transformation of
the cell protoplasm. In the cancellated tissue, near the extremities
of the long bones, this fatty transformation does not take place to the
same extent, and the marrow appears red. The cells of the red
marrow arc believed to give birth indirectly to the red blood-cor-
puscles.

Physical and Physiologic Properties of Connective Tissues. —
Among the physical properties may be mentioned consistency, cohesion,
and elasticity. Their consistency varies from the semiliquid to the
solid state, and depends on the quantity of water which enters into

42 TEXT-BOOK OF PHYSIOLOGY.

their composition. Their cohesion, except in the softer varieties, is
very considerable, and offers great resistance to traction, pressure,
torsion, etc. In all the movements of the body, in the contraction of
muscles, in the performance of work, the consistence and cohesion of
these tissues play most important roles. Wherever the various forms
of connective tissue are found, their chemic composition and structure
are in relation to their functions. If traction be the preponderating
force, the structure becomes fibrous, as in ligaments and tendons, and
the cohesion greatest in the longitudinal direction. If pressure be
exerted in all directions, as upon membranes, the fibers interlace and
offer a uniform resistance. When pressure is exerted in a definite
direction, as on the extremities of the long bones, the tissue becomes
expanded and cancellated. The lamellae of the cancellated tissue
arrange themselves in curves which correspond to the direction of the
greatest pressure or traction. Extensibility is not a characteristic
feature, except in those forms containing an abundance of yellow
elastic fibers. The elasticity is an essential factor in many physiologic
actions. It not only opposes and limits forces of traction, pressure,
torsion, etc., but on their cessation returns the tissues or organs to their
original condition. Elasticity thus assists in maintaining the natural
form and position of the organs by counterbalancing and opposing
temporarily acting forces.

The Skeleton. — The connective tissues in their entirety con-
stitute a framework which presents itself under two aspects: (i)
As a solid, bony skeleton, situated in the trunk and limbs, affording
attachment for muscles and viscera; (2) as a fine, fibrous skeleton,
found everywhere throughout the body, connecting the various viscera
and affording support for the epithelial, muscle, and nerve tissues.

CHAPTER V.
THE PHYSIOLOGY OF MOVEMENT.

The Animal Body as a Machine for Doing Work. — Of the four
phenomena presented by an animal, that which more immediately
interests the physiologist is movement, for the reason that it is not
only its most characteristic form of activity, and that which serves
to distinguish it in the main from forms of vegetable life, but its solution
affords an explanation of many physiologic processes occurring within
the human body- It is also for this reason that movement constitutes
for the most part the subject matter of physiologic experimentation.

The movements may be general as when the animal changes its
position relatively to its environment as in the various acts of loco-
motion, or special as in the changes of relation of one part of the body
with reference to another.

In the execution of these movements the animal, of necessity, meets
with various forms of resistance, viz. : gravity, cohesion, friction, etc.
When its different parts are applied or directed, either volitionally
and in a determinate manner, or non-volitionally and in an indeter-
minate or reflex manner to the overcoming of these opposing forces
in the environment, the animal may be said to be doing work.

In addition to these obvious external movements, a series of less
obvious though no less characteristic internal movements are being
exhibited by the various organs of the animal, e. g., heart, lungs, stomach,
intestines, bladder, etc.; and when these organs are applied to the
overcoming of opposing forces or resistances, as they are from moment
to moment in the performance of their functions, it may be said that
they also are doing work. The cooperation of both external and
internal organs is necessary, not only for the maintenance of the life
of the animal but also for the accomplishment of work. In the con-
ception of the animal body as a machine for doing work, the skeletal,
the muscle and nerve tissues constitute a primary mechanism in which
each bears a certain definite relation to the other. The internal organs
collectively constitute a secondary mechanism in which each bears
not only a definite relation to the other, but to the primary mechanism
as well. The relation of skeletal, muscle and nerve tissue, are shown
in Fig. 15.

General Considerations. — The skeleton is the passive framework
of the body, the axial portions of which (the vertebral column, ribs,
sternum and skull) impart more or less fixity and rigidity to the
mechanism, while the appendicular portions (the bones of the arms
and legs) impart extreme mobility. The bones of the arms and legs,

43

44

TEXT-BOOK OF PHYSIOLOGY.

c.s.c.

Fig. 15. — Diagram Showing the Relaton of Skeletal, Muscle and Nerve
Tissues. (G. Bachman.) j.a. Bones of the forearm representing the skeletal tissue; e.j.
the elbow joint, the fulcrum of the lever formed by the bones of the forearm; W. a weight
acting in a downward direction and representing the passive force of gravity; sk.m. a
skeletal muscle acting in an upward direction and the source of the active power to be ap-
plied to the lever; sp.c. transection of the spinal cord showing the relation of the white and
the gray matter: m.c. a motor cell in the anterior horn of the gray matter; ej.n. an effer-
ent nerve-fiber connecting the motor cell from which it arises with the skeletal muscle and
contained in the ventral roots of the spinal nerves; af.n. an afferent nerve-fiber arising from
the ganglion cell along its course and connecting the skin, s., on the one hand with the spinal
cord on the other hand and contained in the dorsal roots of the spinal nerves; c.s.c.
coronal section of the cerebrum showing the relation of the gray to the white matter; v.c.
a volitional or motor cell; d.a. a descending axon or nerve-fiber connecting the volitional
cell from which it arises with the motor cell in the spinal cord; s.c. a sensor cell; a.a. an
ascending axon or nerve-fiber connecting a receptive cell from which it arises (not shown in
the diagram) with the sensor cell in the gray matter of the cerebrum. The nerve-fibers
which pass outward from the spinal cord to the glands, blood-vessels, and the muscle
walls of the viscera, have for the sake of simplicity been omitted from the diagram.

THE PHYSIOLOGY OF MOVEMENT. 45

more especially, may be looked upon as constituting a system of
levers, the fulcra of which, the points around which they move, lie in
the joints.

That a lever may be effective as an instrument for the accomplish-
ment of work, it must not only be capable of moving around its ful-
crum, but it must, at the same time, be acted on by two opposing
forces, one passive, the other active.

In the movements of the bony levers of the animal body, the
passive forces to be overcome are largely those connected with the
environment, e. g., gravity, cohesion, friction, elasticity, etc. The
active forces by which these are opposed and overcome through the
mediation of the bony levers are found in the muscles attached to
them.

The muscle tissue may therefore be regarded as the seat of
those energies that impart movement to the levers. This tissue
is arranged in masses of irregular shape and size, termed muscles.
The majority of the muscles of the body are connected with the bones
of the body in such a manner that by an alteration in their form, they
can change not only the position of the bones with reference to one
another, but also the individual's relation to surrounding objects.
When the lever is applied to the overcoming of an opposing force, the
muscle is said to be doing work.

Though consisting of a highly active tissue, muscles in themselves
do not possess spontaneity of action, but require for the manifestation
of their energy the stimulating influence of nerve energy or nerve
impulses developed in, and transmitted by, nerve tissue to the muscles.

The nerve tissue is the seat of origin of those energies or impulses
necessary to the physiologic excitation of the muscles. This tissue is
partly arranged in masses contained in the cavities of the head and
spinal column (the brain and spinal cord), forming the central organs
of the nerve system, and partly in the form of cords or nerves, forming
the peripheral organs of the nerve system; the latter connects the for-
mer, not only with the muscles, but with glands, blood-vessels, skin,
mucous membranes, etc., as well.

A transection of the spinal cord shows that it is composed externally
of white matter and internally of grey matter. The grey matter is
arranged in the form of two crescents united in the median line by a
transverse band or commissure forming a figure resembling the letter
H. Though varying in shape in different regions of the cord, the grey
matter in all situations presents on either side an anterior or ventral
and a posterior or dorsal horn.

In the ventral horns of the grey matter are located nerve cells
which not only generate, but under appropriate circumstances discharge,
a form of energy termed a nerve impulse, which is transmitted by
efferent nerve fibers arising from the cells, and by way of the ventral
roots of the spinal nerves, to the muscles, glands, blood-vessels, and
walls of the viscera with which they are directly or indirectly connected.

46 TEXT-BOOK OF PHYSIOLOGY.

The arrival of the nerve impulse at once calls forth the form of activity
characteristic of the structure excited.

Thus the muscle, for example, passes from the passive to the active
state, that is, the muscle becomes shorter and thicker, and the bone
to which it is attached is moved. This is at once followed by a
return of the muscle to the passive state; that is, it lengthens, be-
comes narrower, and resumes its original form; the bone at the same
time returns to its former position.

Coincident with this change of shape there is a liberation of heat
and electricity. The nerve impulse which occasions this transforma-
tion of potential into kinetic energy is the normal or the physiologic
stimulus. The muscle responding to the stimulus is said to be irrit-
able or to possess irritability. The glands in response to the nerve
impulse pour out a secretion. The blood-vessels and viscera change
their caliber. These tissues, too, responding to the nerve impulse, are
said to be irritable. The nerve cells of the spinal cord, however, do
not possess spontaneity of action but require for their excitation the
arrival of other nerve impulses. These may come, (i) from the per-
iphery through afferent nerve fibers by way of the dorsal roots of the
spinal nerves; and (2) from the brain as a result of an act of volition
through descending axons or nerve fibers. In the first instance, the
resulting movement taking place independently of volition, and in re-
sponse to a peripheral or surface excitation, is termed a reflex move-
ment; in the second instance, the resulting movement taking place in
response to an act of volition is termed conventionally a volitional or
voluntary movement.

In the case of reflex movements, the nerve impulses are primarily
developed in specialized organs located in the skin or mucous membranes
and as a result of the impact of various external agents, which for
this reason are termed stimuli. The nerve impulses thus developed
are transmitted by the afferent nerves to the nerve cells which are in
turn excited to activity.

In the case of volitional muscle movements, the nerve impulses
which cause the movement are discharged from certain motor or
volitional nerve cells in the grey matter of the cerebrum and trans-
mitted by descending axons or nerve fibers direct to the nerve cells in
the spinal cord, by which they in turn are excited to activity.

The volitional movements are however the immediate or the more
or kss remote effects of sensations which have been evoked in the
sense areas of the brain, by the arrival of nerve impulses coming through
ascending axons or nerve fibers from peripheral sense organs, e. g., skin,
ar, nose, tongue, and which have been developed by the impact of
objects in the external world.

The nerve cells and their related nerve fibers, responding by the
development and conduction of nerve impulses are also said to be
irritable. The transformation of energy, however, manifests itself
mainlv as electricitv and molecular motion. The animal bodv in its

THE PHYSIOLOGY OF MOVEMENT. 47

entirety may therefore be regarded as a machine for the transformation
of potential energy into kinetic energy, viz. : heat and electricity, move-
ments of muscles and bony levers, secretion, sensation and other forms
of nerve activity. When muscles and bones are applied to the over-
coming of opposing forces, mechanic work is accomplished. In the
following chapters some of the problems connected with the activities
of the skeletal, muscle and nerve tissues will be considered.

CHAPTER VI.
THE PHYSIOLOGY OF THE SKELETON.

The skeleton in its entirety determines the plan of organization
of the animal body. Its axial portion is the foundation element
and the center around which the appendicular portions are developed
and arranged with a certain degree of conformity. The character and
the arrangement of the bones of the axial portion endow the animal
mechanism with a certain degree of fixity, combined with slight mo-
bility, while the character and arrangement of the bones of the append-
icular portions endow it with extreme mobility. The bones collectively
constitute a system of levers, the fulcra of which lie in the points of
union of the bones, and with which the animal is enabled to execute a
variety of movements, to change its position relatively to its environ-
ment and overcome opposing forces. The structure and the chemic
composition of the bones, consisting as they do of inorganic matter
67 per cent, and of organic matter 33 per cent, endow them with both
rigidity and elasticity, physical properties which admirably adapt them
to the character of the work necessitated by the environment and the
organization of the animal. The rigidity of bone is considerable as
compared with other hard and rigid materials. The breaking limit,
in terms of the weight in kilos required to tear across a rod one square
millimeter in cross-section of various materials is as follows : Cast iron
13; bone 12; oak 6.5; granite 1.9. The elasticity is about one-sixth
that of wrought iron and twice that of oak parallel to the grain
(MacAlister). In youth bones are quite elastic; in old age they are
fragile because of a diminution of tissue and an increased porosity, and,
therefore, at both periods less capable of functionating as effectively
as in the middle period of life. The skeleton also serves for the
attachment of muscles and affords support and protection to viscera.

For the manifestation of the activities of the animal it is essential
that the relation of the various portions of the bony skeleton to one
another shall be such as to permit of movement while yet retaining
close apposition. This is accomplished by the mechanical conditions
which have been evolved at the points of union of bones, and which
are technically known as articulations or joints.

A consideration of the body movements involves an account of
d) the static conditions, or those states of equilibrium in which the
body is at rest — e. g., standing, sitting; (2) the dynamic conditions,
or those states of activity characterized by movement — e. g., walking,
running, etc. Jn this connection, however, only those physical and
physiologic peculiarities of the skeleton, especially in its relation to

THE PHYSIOLOGY OF THE SKELETON. 49

joints, will be referred to, which underlie and determine both the
static and dynamic states of the body.

Structure of Joints. — The structures entering into the formation
of joints are:

1. Bones, the articulating surfaces of which are often more or less

expanded, especially in the case of long bones, and at the same
time variously modified and adapted to one another in accordance
with the character and extent of the movements which there
take place.

2. Hyaline cartilage, which is closely applied to the articulating end

of each bone. The smoothness of this form of cartilage facili-
tates the movements of the opposing surfaces, while its elasticity
diminishes the force of shocks and jars imparted to the bones
during various muscular acts. In a number of joints, plates
or discs of white fibro-cartilage are inserted between the surfaces
of the bones.

3. A synovial membrane, which is attached to the edge of the hyaline

cartilage, entirely inclosing the cavity of the joint. This mem-
brane is composed largely of connective tissue, the inner surface
of which is lined by endothelial cells, which secrete a clear, color-
less, viscid fluid — the synovia. This fluid not only fills up the
joint-cavity, but, flowing over the articulating surfaces, diminishes
or prevents friction.

4. Ligaments — tough, inelastic bands, composed of white fibrous

tissue — which pass from bone to bone in various directions on
the different aspects of the joint. As white fibrous tissue is in-
extensible but pliant, ligaments assist in keeping the bones in
apposition, and prevent displacement while yet permitting of
free and easy movements.
Classification of Joints. — All joints may be divided, according

to the extent and kind of movements permitted by them, into (1)

diarthroses; (2) amphiarthroses; (3) synarthroses.

1. Diarthroses. — In this division of the joints are included all those
which permit of free movement. In the majority of instances
the articulating surfaces are mutually adapted to each other.
If the articulating surface of one bone is convex, the opposing
but corresponding surface is concave. Each surface, therefore,
represents a section of a sphere or a cylinder, which latter arises
by rotation of a line around an axis in space. According to the
number of axes around which the movements take place all
diarthrodial joints may be divided into :

1. Uniaxial Joints. — In tins group the convex articulating surface is
a segment of a cylinder or cone, to which the opposing surface
more or less completely corresponds. In such a joint the single
axis of rotation, though nearly, is not exactly at right angles
to the long axis of the bone, and hence the movements — flexion
and extension — which take place are not confined to one plane.

5o TEXT-BOOK OF PHYSIOLOGY.

Joints of this character — e. g., the elbow, knee, ankle, the pha-
■ langeal joints of the fingers and toes — are, therefore, termed
ginglymi, or hinge-joints. Owing to the obliquity of their ar-
ticulating surfaces, the elbow and ankle are cochleoid or screw-
ginglymi. Inasmuch as the axes of these joints on the opposite
sides of the body are not coincident, the right elbow and left
ankle are right-handed screws; the left elbow and right ankle,
left-handed screws. In the knee-joint the form and arrangement
of the articulating surfaces are such as to produce that modifica-
tion of a simple hinge known as a spiral hinge, or helicoid. As
the articulating surfaces of the condyles of the femur increase in
convexity from before backward, and as the inner condyle is
longer than the outer, and, therefore, represents a spiral surface,
the line of translation or the movement of the leg is also a spiral
movement. During flexion of the leg there is a simultaneous
inward rotation around a vertical axis passing through the outer
condyle of the femur; during extension a reverse movement takes
place. Moreover, the slightly concave articulating surfaces of
the tibia do not revolve around a single fixed transverse axis, as
in the elbow-joint, for during flexion they slide backward, during
extension forward, around a shifting axis, which varies in posi-
tion with the point of contact.

In some few instances the axis of rotation of the articulating
surface is parallel with rather than transverse to the long axis of
the bone, and as the movement then takes place around a more
or less conic surface, the joint is termed a trochoid or pulley —
e. g., the odonto-atlantal and the radio-ulnar. In the former the
collar formed by the atlas and its transverse ligament rotates
around the vertical odontoid process of the axis. In the latter the
head of the radius revolves around its own long axis upon the ulna,
giving rise to the movements of pronation and supination of the
hand. The axis around which these two movements take place
is continued through the head of the radius to the styloid process
of the ulna. .
2. Biaxial Joints. — In this group the articulating surfaces are un-
equally curved, though intersecting each other. When the sur-
faces lie in the same direction, the joint is termed an ovoid joint
— e. g., the radio-carpal and the atlanto-occipital. As the axes
of these surfaces are vertical to each other, the movements per-
mitted by the former joint are flexion, extension, adduction, and
abduction, combined with a slight amount of circumduction;
the latter joint permits of flexion and extension of the head, with
inclination to either side.' When the surfaces do not take the
same direction, the joint, from its resemblance to the surfaces of
a saddle, is termed a saddle-joint — e. g., the trapezio-metacarpal.
The movements permitted by this joint are also flexion, exten-
sion, adduction, abduction, and circumduction.

THE PHYSIOLOGY OF THE SKELETON. 51

3. Polyaxial Joints. — In this group the convex articulating surface
is a segment of a sphere, which is received by a socket formed
by the opposing articulating surface. In such a joint, termed an
enarthrodial or ball-and-socket joint — e. g., the shoulder-joint,
hip-joint — the distal bone revolves around an indefinite number
of axes, all of which intersect one another at the center of rotation.
For simplicity, however, the movement may be described as
taking place around axes in the three ordinal planes— viz., a
transverse, a sagittal, and a vertical axis. The movements around
the transverse axis are termed flexion and extension; around the
sagittal axis, adduction and abduction; around the vertical
axis, rotation. When the bone revolves around the surface of an
imaginary cone, the apex of which is the center of rotation and
the base the curve described by the hand, the movement is
termed circumduction.

2. Amphiarthroses. — In this division are included all those joints

which permit of but slight movement — e. g., the intervertebral,
the interpubic, and the sacro-iliac joints. The surfaces of the
opposing bones are united and held in position largely by the
intervention of a firm, elastic disc of fibro-cartilage. Each joint
is also strengthened by ligaments.

3. Synarthroses. — In this division are included all those joints in

which the opposing surfaces of the bones are immovably united,
and hence do not permit of any movement — e. g., the joints
between the bones of the skull.
The Vertebral Column. — In all static and dynamic states of the
body the vertebral column plays a most essential role. Situated in
the middle of the back of the trunk, it forms the foundation of the
entire skeleton. It is composed of a series of superimposed bones,
termed vertebrae, which increase in size from above downward as
far as the brim of the pelvic cavity. Superiorly, it supports the skull ;
laterally, it affords attachment for the ribs, which in turn support the
weight of the upper extremities; below, it rests upon the pelvic bones,
which transmit the weight of the body to the inferior extremities.
The bodies of the vertebras are united one to another by tough elastic
discs of fibro-cartilage, which, collectively, constitute about one-
quarter of the length of the vertebral column. The vertebrae are held
together by ligaments situated on the anterior and posterior surfaces
of their bodies, and by short, elastic ligaments between the neural
arches and processes. These structures combine to render the verte-
bral column elastic and flexible, and enable it to resist and diminish
the force of shocks communicated to it.

The amphiarthrodial character of the intervertebral joints endows
the entire column with certain forms of movement which are neces-
sary to the performance of many body activities. While the range
of movement between any two vertebrae is slight, the sum total of move-
ment of the entire series of vertebrae is considerable. In different

52 TEXT-BOOK OF PHYSIOLOGY.

regions of the column the character, as well as the range of move-
ment, varies in accordance with the form of the vertebrae and the
inclination of their articular processes. In the cervical and lumbar
regions extension and flexion are freely permitted, though the former
is greater in the cervical, the latter in the lumbar region, especially
between the fourth and fifth vertebras. Lateral flexion takes place
in all portions of the column, but is particularly marked in the cer-
vical region. A rotatory movement of the column as a whole takes
place through an angle of about twenty-eight degrees. This is most
evident in the lower cervical and dorsal regions.

CHAPTER VII.
GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

The muscle-tissue, which closely invests the bones of the body
and which is familiar to all as the flesh of animals, is the immediate
cause of the active movements of the body. This tissue is grouped
in masses of varying size and shape, which are technically known as
muscles. The majority of the muscles of the body are connected
with the bones of the skeleton in such a manner that, by an alteration
in their form, they can change not only the position of the bones with
reference to one another, but can also change the individual's relation
to surrounding objects. They are, therefore, the active organs of
both motion and locomotion, in contradistinction to the bones and
joints, which are but passive agents in the performance of the corre-
sponding movements. In addition to the muscle masses which are
attached to the skeleton, there are also other collections of muscle-
tissue surrounding cavities such as the stomach, intestine, blood-
vessels, etc., which impart to their walls motility, and so influence the
passage of material through them.

Muscles produce movement of the structures to which they are
attached by the property with which they are endowed of changing
their shape, shortening or contracting under the influence of a stimulus
transmitted to them from the nervous system. Muscles are divided
into:
i. Voluntary muscles, comprising those the activity of which is

called forth by an act or effort of volition.
2. Involuntary muscles, comprising those the activity of which is
entirely independent of the volition.

The voluntary muscles are also known from their attachment to
the skeleton as skeletal, and from their microscopic appearance as
striped or striated muscles. Though for the most part these muscles
are red, there are certain muscles in man and other animals winch
are pale in color and in many muscles pale fibers are extensively dis-
tributed among the red fibers. The involuntary muscles, from their
relation to the viscera of the body, are known also as visceral, and from
their microscopic appearance as plain, smooth, or non-striated muscles.

THE VOLUNTARY OR SKELETAL MUSCLE.

x\ll skeletal muscles consist of a central fleshy portion, the bodv
■or belly, provided at either extremity with a tendon in the form of a
cord or membrane. The body is the active, contractile region, the

53

54

TEXT-BOOK OF PHYSIOLOGY.

source of the movement ; the tendon is the inactive region, the passive
transmitter of the movement to the bones.

A skeletal muscle is a complex organ consisting of a framework of
connective tissue, supporting muscle-fibers, blood-vessels, nerves, and
lymphatics. The general body of the muscle is covered by a dense
layer of connective tissue, the epi-mysium, which blends with and
partly forms the tendon. From the under surface of this covering,
septa of connective tissue pass inward, dividing and grouping the
fibers into larger and smaller bundles, termed fasciculi. The fasciculi,

invested by a special sheath, the
r^ll^sLsffla peri-mysium, are prismatic in

shape and on cross-section pre-
sent an irregular outline. The
muscle-fibers composing the fas-
ciculi are separated one from
another and supported by a very
delicate connective tissue, the
endo-mysium. The connective
tissue thus surrounding and
penetrating the muscle binds the
fibers into a distinct organ and
affords support to all remaining
structures (Fig. 16).

Histology of the Skeletal
Muscle -fiber. — The muscle-
fiber is the ultimate anatomic
units of the muscle system. The
fibers for the most part are ar-
ranged parallel one to another
and in a direction corresponding
to the long axis of the muscle.
They vary in length from 30 to
40 millimeters and in breadth
from 20 to 30 mi cromilli meters.
There are exceptional fibers, however, which have a much greater
length. As the fibers have but a limited length in the vast majority
of muscles, each end, more or less pointed or beveled, is united to
adjoining fibers by cement. In this way a muscle is increased in
length.

When examined with the microscope, the muscle-fiber is seen to be
cylindric or prismatic in shape and to consist of a thin transparent
membrane, the sarcolemma, in which is contained the true muscle
or sarcous substance. The sarcolemma is elastic and adapts itself
to all changes of form the sarcous substance undergoes. Beneath
the sarcolemma there are several nuclei surrounded by granular
material. Each fiber also presents a series of transverse bands alter-
nately dim and bright which give to it a striated appearance. If the

t#

Fig. 16. — From a Cross-section of the
Adductor Muscle of a Rabbit. P. Peri-
mysium, containing two blood-vessels, at g;
m, muscle-fibers; many are shrunken and be-
tween them the endomysium, p, can be seen;
at x the section of muscle-fiber has fallen out.
X 60.— {Stohr.)

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

55

Fig. 17. — Muscle-fiber
of a Rabbit. a. Dark
band. b. Light band. c. In-
termediate line, n. Nucleus.
— {Landois and Stirling. —
Ranvier.)

bright bands are examined with high magnifying powers, each one is
seen to be crossed by a fine dark line which at the time of its discovery
was regarded as the optic expression of a mem-
brane attached laterally to the sarcolemma.
It has since been resolved into a row of gran-
ules (Fig. 17).

The muscle-fiber also presents a longit-
udinal striation which indicates that it is
composed of finer elements placed side by side,
termed fibrillae. The fibrillae extend through-
out the entire length of the fiber, though they
are not of uniform thickness (Fig. 18). That
portion of the fibril corresponding in position
to the dim band is thick, prismatic, or rod-like
in shape, and termed a sarcostyle; that por-
tion corresponding in position to the bright
band is extremely thin and narrow and pre-
sents at its middle a slight enlargement or
granule. The fibrillae are embedded in a clear
transparent fluid which, from its supposed nutritive character, is termed
sarcoplasm. The diminution in caliber of the fibrillae at different levels
permits of the accumulation and storage of a
larger amount of nutritive material than could
otherwise be the case. It is for this reason
that the fiber at these points presents a
brighter appearance.

When the muscle-fiber is examined under
crossed Nichol prisms, the dim band appears
bright and the bright band appears dim against
a dark background, indicating that the former
is doubly refracting or anisotropic, the latter
singly refracting or isotropic.

The pale muscle fiber is histologically
similar to the red. It is, however, somewhat
larger, paler in color and does not contain so
much muscle material. The transverse striation
is closer and the nuclei are not so abundant.
The Blood-supply. — Muscles in the
physiologic condition require for the main-
tenance of their activity a large amount of
nutritive material. Tins is obtained di recti v
from the lymph and indirectly from the blood
furnished by the blood-vessels. The vascular
supply to the muscles is very great and the
disposition of the capillary vessels with refer-
ence to the muscle-fiber is very characteristic. The arterial vessels,
after entering the muscle, are supported by the peri-mysium; in this

Fig. 18 — .4. Diagram of
arrangement of the contrac-
tile substance according to
the view of Rollett; the
granular figures represent
the contractile elements, the
intervening light areas the
sarcoplasm. B. Small
muscle-fiber of man, the
corresponding parts in the
two figures are indicated;
t, i, I, respectively the trans-
verse, the intermediate, and
lateral discs, n. Muscle
nu clei . — (P iersol. )

56

TEXT-BOOK OF PHYSIOLOGY.

LYMPH SPACE

MUSCLE
FIBER

-CAPILLARY BLOOD
VESSEL

situation they give off short, transverse branches, which immediately
break up into a capillary network of rectangular shape within which
the muscle-fibers are contained. The relation of the capillary vessel
to the muscle-fiber is shown in Fig. 19.

The muscle-fiber, in intimate relation with the capillary, is bathed
with lymph derived from it. Its contractile substance, however, is
separated from the lymph by its own investing membrane, through
which all interchange of nutritive and waste material must take place.
The nutritive material passes through the capillary wall into the
lymph-space, then through the sarcolemma into the interior of the

fiber, where it comes into re-
lation with the living muscle
material. The waste prod-
ucts arising in the muscle
as a result of nutritive changes
pass in the reverse direction
first into the lymph and then
into the blood, by which they
are carried away to eliminat-
ing organs. Lymphatics are
present in muscle, but confined
to the connective tissue, in the
spaces of which they take their
origin.

The Nerve-supply. — The
nerves which carry the stimuli
to a muscle enter near its
geometric center. Many of
the fibers pass directly to the
muscle-fibers with which they
are connected; others are dis-
tributed to blood-vessels.
Every muscle-fiber is supplied with a special nerve-fiber except in
those instances where the nerve-trunks entering a muscle do not con-
tain as many fibers as the muscle. In such cases the nerve-fibers divide-
near their termination until the number of branches equals the number
of muscle-fibers. The individual muscle-fiber is penetrated near its
center by the nerve where it terminates; the ends being practically
free from nerve influence. The stimulus that comes to the muscle-
fiber acts primarily upon its center, the effect of which then travels in
both directions to the ends. The manner in which the nerve-fibers
terminate in muscle will be more fully described in connection with
the histology of the nerve tissue.

CHEMIC COMPOSITION OF MUSCLE.

The chemic composition of living muscle is but imperfectly under-
stood owing to the fact that shortly after death some of its constituents

Fig. 19. — Relation of the Blood-ves-
sel to the Muscle-fiber.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 57

undergo a spontaneous coagulation and for the reason that the methods
employed for analysis also tend to alter its composition. To human
muscle, the following average percentage composition has been given:

Water, 73.5

Proteins, including those of sarcolemma, connective-
tissue, pigments, 18.02

Gelatin, 1.99

Fat, 2.27

Extractives, 0.22

Inorganic salts, 3. 12. (Halliburton.)

(The composition of muscles of different animals, consumed as
foods, will be found in the chapter on Foods.)

When fresh muscle is freed from fat and connective tissue, frozen,
rubbed up in a mortar, and expressed through linen, a slightly yellow
syrupy alkaline or neutral liquid is obtained which has been termed
muscle-plasma. This fluid at normal temperatures coagulates
spontaneously, the phenomena resembling in many respects those
observed in the coagulation of blood-plasma. The coagulum subse-
quently contracts and squeezes out an acid muscle-serum. The
coagulated protein is known as myosin and belongs to the class of
globulins. Inasmuch as it is not present in living muscle and only
makes its appearance under conditions not strictly physiologic, it is
regarded as a derivative of a pre-existing protein which has been
termed myosinogen. According to Halliburton, the proteins of living
muscle are four in number, distinguished by their varying solubilities
in different salts and by the varying temperatures at which they
coagulate. From muscle-plasma may then be obtained: (1) Para-
myosinogen and (2) myosinogen, the former coagulating at 47 ° C, the
latter at 56 ° C. It is myosinogen which is converted into myosin
under the influence of some special ferment, though both enter into
the formation of the muscle-clot. From the muscle-serum may also
be obtained at 68° C. a globulin body termed myoglobulin and a
small quantity of myoalbumin. Among the proteids may be men-
tioned hemoglobin, which gives the color to the muscles. Spectro-
scopic investigation reveals the presence of a special pigment, myo-
hematin, which is supposed to have a respiratory function, inasmuch
as its absorption bands change by oxidation and reduction.

Among the extractives containing nitrogen may be mentioned
creatin, creatinin, xanthin, carnin, urea, uric acid, carnic acid, etc.
Among the extractives free of nitrogen, glycogen, dextrose, inosite,
lactic acid, fat, are the most important. Inorganic salts are relatively
abundant, of which potassium is the most abundant among the bases,
and phosphoric acid among the acids.

THE PHYSICAL AND PHYSIOLOGIC PROPERTIES OF MUSCLE-
TISSUE.

Consistency. — The consistency of muscle-tissue during life
varies considerablv in accordance with different states of the muscle.

58

TEXT-BOOK OF PHYSIOLOGY.

Fig. 20. — Extension Curve of
Muscle. — (Gad.)

In a state of tension it is hard and resistant; in the absence of tension
it is soft and fluctuating to the sense of touch. Tension alone gives
rise to hardness.

Cohesion. — The cohesion of a muscle is largely dependent on the

quantity of connective tissue it contains.
A band of fresh human muscle one square
centimeter in cross-section was able to
resist a weight of 14 kilograms without
rupture (MacAlister). Cohesion resists
the forces of traction and pressure.

Elasticity. — Muscle, in common with
many other organic as well as inorganic
substances, is capable of being extended
beyond the normal length through the
action of external forces and of resuming
the normal length when these forces
cease to act. All such bodies are said to
be elastic; and the greater the variations
between the natural and acquired lengths,
the greater is their elasticity said to be.
Muscle, therefore, possesses extensibility
and elasticity.* If the muscle of a frog,
preferably the sartorius, the fibers of
which are arranged in a practically parallel manner, be fastened at one
extremity by a clamp, and then extended by a series of successive
weights which differ by a common increment, it will be found that the
extensibility of muscle does not follow the law of elasticity as deter-
mined for inorganic bodies; i. e., directly
proportional to the weight and to the length
of the body extended; but that while in-
creasing in length with each successive
weight, the increase is always in a diminish-
ing ratio. Thus, for example, as shown in
Fig. 20: The extension produced by 5
grams is 5 millimeters, that produced by 10
grams is only 4 millimeters more, and so on
with additional weights until the increase in
passing from 25 to 30 grams is only 1 milli-
meter. The extensibility is thus shown to
be proportionately greater with small than

with larger weights. It is, however, actually greater with the larger
weights. The extension curve A B formed by joining the ends of the
muscle approximates that of a parabola. The muscle in returning to
its original length also shows a variation from the behavior of inorganic
bodies. With the successive removal of the weights, the elasticity of the

* By this latter term is here meant the power by virtue of which the muscle returns
to its original length and is used synonymously with perfect retractibility.

Fig. 21. — Curve of Elas-
ticity Produced by Continu-
ous Extension and Recoil
of a Frog's Muscle. 0 x. Ab-
scissa before; x', after exten-
sion.— (Landois and Stirling.)

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 59

muscle asserts itself with gradually increasing energy until its normal
length is nearly, if not entirely, regained (Fig. 21). Though it is
usually stated that the elasticity of muscle is incomplete, it must be
borne in mind that the experiments have usually been made on muscles
removed from the body, deprived of blood and nerve influences, and
hence under abnormal conditions. It is highly probable that in the
living body muscles possess perfect elasticity which enables them to
completely return to their normal length after extension. The extension
and elastic recoil of muscle depends on the maintenance of physiologic
conditions. If the nutrition is impaired by fatigue, deficient blood-
supply, or any pathologic condition, the elasticity is at once impaired.

Tonicity. — This is a property possessed by all muscles in the
body in consequence of being stretched to a slight extent beyond their
normal length. This may be due to the action of antagonistic muscles
or to their mode of growth, the muscles growing somewhat more
slowly than the bones to which they are attached. That muscles are
so stretched is shown by the shortening which at once takes place
when their tendons are divided. This muscle tonus or tension is
closely connected with the elasticity and plays an important role in
muscle contraction; being always on the stretch, the muscle loses no
time in acquiring that degree of tension necessary to immediate action
on the bone to which it is attached. The working power of a muscle
is also increased by the presence, within limits, of some resistance to
the act of contraction. According to Marey, the amount of work is
considerably increased when the muscle energy is transmitted by an
elastic body to the mass to be moved, while at the same time the shock
of the contraction is lessened. The position of a passive limb is the
resultant also of the elastic tension of antagonistic groups of muscles.

Another explanation for the tonicity of muscle is found in the fact
that the skeletal muscles of the body receive continuously nerve im-
pulses from the nerve cells of the spinal cord as a consequence of the
arrival of nerve impulses reflected through afferent nerves from the
tendons and muscles themselves. The stimulus here being the slight
degree of extension and variations in extension to which the muscles
are being subjected from moment to moment. That this is a con-
siderable factor in the production of the tonus is shown by the effects
which follow division of the afferent or dorsal roots of those spinal
nerves coming from any given muscle group. With the division of
the nerves the muscles relax and lose their usual tone. As a result of
this slight but constant stimulation from the spinal cord, the metabolic
changes of the muscle material are maintained at a certain level with
a corresponding liberation of heat. The chief function of the tonicity
would thus be the production of heat, other functions which the tone
subserves being merely secondary.

Irritability, Contractility. — These are terms employed to denote
that property of muscle-tissue by virtue of which it responds by a
change of form, becoming shorter and thicker on the application of

60 TEXT-BOOK OF PHYSIOLOGY.

a stimulus. On the withdrawal of the stimulus the muscle again
undergoes a reverse change of form, becoming longer and narrower,
and returning to its original condition. All muscles which possess this
capability are said to be irritable and contractile; and all agents which
call forth this response of the muscle are termed stimuli. The rapid
change of form which a highly irritable muscle undergoes in response
to the action of a stimulus of short duration is usually termed a twitch
or pulsation. With appropriate apparatus it can be shown that the
muscle at the time of the twitch becomes warmer and exhibits electric
phenomena. The muscle is therefore an apparatus for the conversion
of potential into kinetic energy: viz., heat, electricity, and mechanic
motion.

Though usually associated with the activity of the nerve system,
and to some extent dependent on it, irritability is nevertheless an
independent endowment of the muscle and persists for a longer or
shorter period, as shown by many experiments, after all nerve con-
nections have been destroyed. Among the proofs which may be
presented in support of this view are the following : The introduction
of the drug, curara, into the body of an animal produces in a short
time complete paralysis. Experiment has shown that curara sus-
pends the conductivity of the intramuscular terminations of the nerve-
fiber and thus separates the muscle entirely from the nerve. Though
the animal is incapable of executing a single movement, its muscles
respond promptly on the application of a stimulus. Moreover,
portions of muscles exhibit irritability in which there is no trace of
nerve structure. This is the case with the ends of the sartorius muscle
of the frog and the anterior end of the retractor muscle of the eyeball
of the cat. These and other facts demonstrate the independence
of muscle irritability.

In the living body irritability and nutritive activity, with which
it is closely associated, are maintained by a due supply of oxygen,
of nutritive material, the removal of waste products, and a normal
temperature. The muscles of the cold-blooded animals, and especially
the frog, retain their irritability for a much longer period after death
than the muscles of the warm-blooded animals. This is the case also
with the individual muscles after removal from the body of the animal.
The reason for this is found in all probability in the difference in the
rate of their nutritive activities and in the quantity of nutritive material
stored up in their cells. The duration of the irritability of isolated
muscles can be considerably prolonged by keeping them in a moist
atmosphere.

Muscle Stimuli. — Though consisting of a highly irritable tissue,
muscles do not possess spontaneity of action. They require for the
manifestation of their characteristic activity the application of a
stimulus. In the living body all contractions, at least of the skeletal
muscles, occurring under normal or physiologic conditions are caused
by the action of "nerve impulses" transmitted by the nerves from the

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 61

central nerve system to the muscles. The nerve impulse is the normal
or physiologic stimulus. After removal from the body and freed from
nerve connections muscles can be excited to action by various agents
of a mechanic, chemic, thermic, or electric nature. These are artificial
or non-physiologic stimuli.

i. Mechanic Stimuli. — Cutting, pinching, sharply tapping the muscle
will cause it to contract, providing the stimulus has sufficient
intensity. With each stimulation a short, fleeting contraction
ensues. If repeated with sufficient rapidity, a series of con-
tinuous but irregular pulsations are produced.

2. Chemic Stimuli. — Various chemic substances in solution will

excite single or continuous pulsations if the strength of the solu-
tion is not such as to destroy at once the irritability. They owe
their efficiency as stimuli to the rapidity with which they alter
the composition of the muscle-substance. Among these may be
mentioned solutions of potassium and sodium, weak solutions of
the mineral and organic acids, ammonium vapor, distilled water,
glycerin, and sugar.

3. Thermic Stimuli. — The application of a heated object, such as a

hot wire, causes the muscle to rapidly contract.'

4. Electric Stimuli. — The most efficient stimulus and the one least

injurious to the tissue is the electric current. Either the con-
stant or the induced current may be used.*

The Constant Current. — If the ends of the wires in connection with
an electric cell be provided with non-polarizable electrodes and the
latter placed on opposite ends of a freshly prepared sartorius muscle
of a frog which has been previously curarized, it will be found on
closing or making the circuit that the muscle will exhibit a short quick
pulsation. During the actual passage of the current, especially if
it is weak, there may be no apparent change in the muscle. If the
current is strong, the muscle may, on the contrary, remain in a state
of continuous contraction. With the opening or breaking of the
current the muscle at once relaxes, or perhaps again contracts and
then relaxes. The extent of the contraction depends mainly on the
strength of the current, being greater with strong, less with weak
currents. When the current is sufficiently strong to elicit both making
and breaking contractions, it is found that the contraction occurring
on the make or closure of the circuit is always greater than that occur-
ring on the break or opening of the circuit. Moreover, it has been
shown in many ways that the contraction occurring on the closure of
the circuit has its origin at the point where the current is leaving the
muscle — i. e., in the immediate neighborhood of the negative pole or

* Since the study of the physiologic properties of both muscle-tissue and nerve-tissue
involves the employment of electricity as a stimulus, it becomes necessary for the student
to familiarize himself with certain forms of apparatus by which it is generated, controlled,
and applied. For the purpose of not interrupting the continuity of the text this inform-
ation is embodied in an appendix. The facts therein contained should be mastered at
this time by the student.

62 TEXT-BOOK OF PHYSIOLOGY.

cathode — and propagates itself to the opposite extremity; while the
contraction occurring on the opening of the circuit has its origin at the
point where the current is entering the muscle, i. e., in the neighborhood
of the positive pole or anode.

These facts can be readily demonstrated by destroying the ir-
ritability and contractility of one extremity of a muscle with parallel
fibers such as the sartorius. On applying non-polarizable electrodes
to the muscle as in Fig. 22, A, it will be found that when the circuit
is made a contraction occurs which must, of course, have developed at
the irritable cathodic region, for on the break of the circuit the muscle
remains at rest. When the electrodes are applied as in Fig. 22, B, and
the circuit made the muscle remains at rest, but on the break of the
circuit a contraction occurs which must have developed at the irritable
anodic region.

The Induced Current. — If the primary spiral of the inductorium
be connected with an electric cell and the secondary spiral be con-
nected with a muscle, it will be found that the current induced in the

A. B.

Fig. 22. — Diagram to Show the Effect of Local Injury on the Irritability of
A Muscle (after Starling). C Z an electric cell from which wires pass to non-polarizable
electrodes, anode and kathode, in contact with a muscle, the injured end of which is more
deeply shaded. The arrows indicate the direction of the current.

secondary circuit, both on the make and break of the primary, will
also cause the muscle to sharply and rapidly pulsate if the two spirals
are sufficiently near each other. Observation, however, makes it
evident that the pulsation occurring with the break of the primary
circuit is more energetic than that occurring with the make, a result
the opposite of that obtained with the constant current. This is
not due to any difference in the electricity, however, but to peculiarities
in the construction of the inductorium. When the primary circuit
is interrupted with sufficient frequency, as it can be by throwing into
the circuit Neef's hammer or some other form of interrupter, the con-
tractions excited by the induced currents may be made to succeed one
another so rapidly that they become fused together, producing a
spasm or tetanus of the muscle. The rapidity with which the induced
current appears and disappears, its brief duration, the ease with which
its strength can be regulated, combine to render it a most efficient
stimulus for either muscle or nerve.

Conductivity. — All muscle protoplasm possesses conductivity.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 63

The change excited in a muscle-fiber by the arrival of a nerve impulse
is at once conducted with great rapidity in opposite directions to
the end of the fibers; the advance of the excitation process is im-
mediately succeeded by the contraction process, the change of form
which constitutes the contraction. With the disappearance of the
former, the latter also disappears and the muscle resumes its pre-
vious passive condition. There is no evidence, however, that the
excitation process travels transversely — that is, into adjoining fibers
— being prevented from doing so by the presence of the limiting
membranes, the sarcolemmata. The fact that each muscle-fiber
receives its own, or at least a branch of a nerve-fiber, and hence its
own nerve impulse or stimulus, would also indicate that the excitation
process can not be conducted longitudinally into adjoining fibers,
or at least with sufficient rapidity for the purposes of ordinary muscle
actions. Nevertheless if a long muscle, such as the sartorius, from a
curarized frog be stimulated at one end with an induced electric cur-
rent, the excitation and the contraction processes will be conducted
with extreme rapidity to the opposite end of the muscle. The rapidity
of conduction in human muscles has been estimated at from 10 to 13
meters per second, and in frog's muscle at from 3 to 4.5 meters per
second. The contraction process, the thickening of the muscle, is
termed the contraction wave. As it is the result of the excitation
process and immediately succeeds it, its rate of conduction must be
the same as that given above. With appropriate apparatus the du-
ration of the wave at any given point has been shown to be, in the
frog's muscle, one-tenth of a second and its length three-tenths of a
meter.

PHENOMENA ATTENDING A MUSCLE CONTRACTION.

PHYSICAL PHENOMENA.

The most obvious change in a muscle during the contraction is
that relating to its form. The muscle not only becomes shorter, but
at the same time thicker. The extent to which it may shorten when
unopposed may amount to 30 per cent, or more of its original length.
The increase in thickness practically compensates for the diminution
in length, for there is no observable diminution in volume. The
change in form of the entire muscle results from a corresponding
change of form of its individual fibers as determined by microscopic
examination, each of which becomes shorter and thicker. The
successive changes in both the muscle and the individual fibers are
represented in Fig. 23.

When the contraction begins, the dim band increases and the
bright band diminishes in width. This Engelmann attributes to the
passage of fluid material from the bright into the dim band. At the
time of relaxation there is a return of this material and the bands
assume their original shape and volume. As the contraction wave

64

TEXT-BOOK OF PHYSIOLOGY.

reaches its maximum the optic properties of the bright and dim bands
change. The former now becomes darker and less transparent
until at the crest of the wave it assumes the appearance of a distinct
dark band; the latter now becomes clear and bright in comparison.
This change in the appearance of the fiber is due to an increase in
refrangibility of the bright, and a decrease in the refrangibility of the
dim band, coincident with the passage of the fluid from the former
into the latter. There is at the height of the contraction a complete
reversal in the positions of the striations. At a certain stage between
the beginning and the crest of the wave the striae almost entirely dis-
appear, giving to the fiber an appearance of homogeneity. There is,

Fig. 23. — Showing the Changes in a Muscle and Muscle-fiber during

Contraction.

however, no change in refractive power as shown by the polarizing
apparatus. When the contraction wave has reached the stage of
greatest intensity, there is a reversal of the above phenomena as the
fiber returns to its former condition, that of relaxation.

Elasticity. — During the contraction of a muscle there is a greater
or less alteration in its elasticity, as shown by the fact that it is ex-
tended to a greater degree by the same weight in the active than in
the passive condition. The degree to which the extensibility is in-
creased and the elasticity decreased is dependent on the amount of
the resisting force. These facts, as determined experimentally, are
represented in Fig. 24. Let A B and A b represent the length of the
normal unweighted muscle, passive and active states respectively; the
line B B', the extension curve of the passive muscle produced by
successive weights, 5, 10, 15, 20, 25, 30 grams, differing by a com-
mon increment; the line b B', the extension curve of the active con-
tracted muscle when weighted with the same weights; A' B' the length
of the muscle when the weight is sufficiently great to prevent shorten-
ing. It will be observed from these facts that while the muscle is
extended in both the passive and active states by corresponding

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

65

weights, the extension during the latter is progressively greater, until
with a given weight the length of the muscle is the same. Under
such circumstances, there being no shortening of the muscle, the
energy of its contraction manifests itself physically merely as tension.
In the successive actions of the muscle represented in the same figure
there is to be observed also a combination of a change of length and
a change of tension, the ratio of the one to the other being deter-

Fig. 24. — Extension Curves: B B', of the resting; b B', of the contracting muscle.

mined by the amount of the supported weights. When the weight
is slight in amount, the shortening of the muscle reaches a maximum
and the tension a minimum; when the weight is large in amount, the
reverse conditions obtain.

THE CONTRACTION PROCESS. METHODS OF INVESTIGATION.

The contraction of a muscle as it takes place in the living body and
under normal physiologic conditions is a complex act persisting for
a variable length of time in accordance with the number of stimuli
transmitted to it in a given unit of time, and as determined experi-
mentally is the resultant of the fusion of a greater or less number of
separate and individual contractions or pulsations. To this enduring
contraction the term tetanus has been given. With the aid of ap-
propriate apparatus it has become possible to obtain and record
single muscle contractions, to analyze and decompose them into their
constituent elements, or to combine them in such a manner as to pro-
duce practically a normal physiologic tetanus. As in the experi-
mental study of the phenomena of a muscle contraction it frequently
becomes necessary to remove the muscle from the body of the animal,
the muscles of warm-blooded animals are not well adapted for this
purpose, owing to the rapid alteration in composition they undergo,
with a consequent loss of irritability, when deprived of their normal
blood-supply. The excised muscles of cold-blooded animals, par-
ticularly of the frog — in which, owing to the relatively slow rate of

66

TEXT-BOOK OF PHYSIOLOGY.

the nutritive activities, the irritability and contractility endure for a
relatively long period of time, even though deprived of blood— are
particularly valuable for experimental studies. The muscles generally
employed are the gastrocnemius, the sartorius, and the hyoglossus.
If kept at a normal temperature and moistened with 0.6 per cent.
solution of sodium chlorid, such a muscle will contract for a long period
of time on the application of any form of stimulus, but especially the
electric.

Graphic Record of a Muscle Contraction. — Inasmuch as the
changes in the form of a muscle during a single contraction take place
with extreme rapidity, their succession, peculiarities, and time re-
lations cannot be determined with any degree of accuracy by the
unaided eye. This difficulty can largely be overcome by the employ-
ment of the graphic method, the principle of which consists in record-

Fig. 25. — Myograph. K. Recording cylinder. M. Moist chamber. L.
Recording lever. W. Weight. I. Induction coil.

ing the movements by means of a pen on some appropriate moving
and receiving surface. To accomplish this object the muscle is at-
tached at one extremity by a clamp to a firm support, and at the other
extremity to a weighted lever, which is, however, sufficiently light to
enable it to take up, reproduce, and magnify its movements. The
end of the lever provided with a pen is applied to a smooth surface,
such as glazed paper on a cylinder or plate, and covered with lamp-
black. If the surface is stationary, the contraction is recorded as a
vertical line; if i+ is placed in movement at a uniform rate by clock-
work, the contraction is recorded in the form of a curve, the width of
the arms of which will depend on the rate of movement. The time
relations of the phases of the contraction can be obtained by placing
beneath the lever a pen attached to an electro-magnet thrown into

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 67

action by a tuning-fork vibrating in hundredths of a second. In
order to determine the rapidity with which the contraction follows
the stimulation, it is essential that the movement of the latter be also
recorded. This is accomplished by an automatic key, the opening
or closing of which develops the stimulus which excites the muscle.
A combination of these different appliances constitutes a myograph
and the curve of contraction a myogram. (See Fig. 25.)

The Isotonic Myogram. — With the object of obtaining a curve
of successive changes in the length of a muscle during a single con-
traction and at the same time avoiding changes in tension, the weight
attached to the lever should be applied close to its axis, a mechanic

Fig. 26. — The Isotonic Myogram.

■condition which practically maintains a uniform tension throughout
the contraction. To this method the term "isotonic" has been given
and the curve so obtained an isotonic myogram.*

The Character of an Isotonic Myogram. — With the muscle
arranged as previously described and stimulated directly with a single
induction shock, the contraction will be recorded in the form of a
curve similar to that represented in Fig. 26, in which the horizontal line
represents the abscissa of time; a, the moment of stimulation; and bed,
the degree of shortening at each successive moment. The undulating
line shows the time relations, the distance from crest to crest represent-
ing hundredths of a second. The curve may be divided into three
portions:

1. A short but measurable portion between the point of stimulation
and the first evidence of the shortening, a b, known as the "latent
period." The duration of this period- for the skeletal muscle
of the frog was originally determined to be 0.01 second, but with
the employment of more accurate apparatus it has been reduced
to 0.0025 to 0.004 second. During this period it is supposed
that certain chemic changes are taking place preparatory to the

* In the ordinary method of recording a muscular movement, i. e., with the weight
attached to the lever immediately beneath the muscle and known as the "loaded method."
a certain momentum is imparted to the weight, which continues after the muscle has
ceased to act, both when shortening and relaxing, and so imparts to the recording lever
additional movements which vitiate the true character of the curve

68 TEXT-BOOK OF PHYSIOLOGY.

exhibition of the movement. The duration of the latent period
is influenced by a variety of conditions, e. g., temperature, fatigue,
strength of stimulus, etc.

2. An ascending portion, b c, the contraction or period of increasing

energy. The contraction as shown by the character of the curve
begins slowly, then proceeds rapidly, and again slowly as the
shortening reaches its maximum. The contraction may be said
to end when the tangent to the curve becomes parallel with the
abscissa.

3. A descending portion, c d, the relaxation or period of decreasing

energy. The relaxation as shown by the character of the curve
begins slowly, then proceeds rapidly, and again slowly as the
muscle attains its original length. The termination of the re-
laxation is at the point where the curve cuts the abscissa. The
curve beyond this point may be complicated by the presence
of one or more residual or after-vibrations, which are probably
due to the inertia of the lever as well as to changes in the muscle
elasticity.
The duration of the period of shortening is about 0.04 second, and
of the period of relaxation 0.05 second. A single pulsation of the
isolated muscle of the frog therefore occupies, from the moment of
stimulation to termination, the tenth of a second. Muscles of many
other animals have a contraction period the duration of which varies
considerably from this. Thus, in man the time of a single contrac-
tion is one-twentieth of a second, in some insects one three-hundredth
of a second, and in the turtle one second. Pale muscles have a shorter
period than the red.

Influences Modifying the Contraction Process. — The con-
traction process in its entirety as well as in its individual parts is
considerably modified by both external and internal conditions, among
which may be mentioned the following:

1. Stimulus. — As the contraction is the response of the muscle to
a stimulus, the vigor of the former is proportional, within limits,
to the strength of the latter. Thus using as a stimulus the single
induced current, it has been found that if the strength of the
current is progressively increased, the height of the contraction
will correspondingly increase until a certain maximum height is
attained (Fig. 27, A); then notwithstanding a continued increase
in the strength of the stimulus, this height will not be exceeded
for some time. But if the strength of the stimulus be yet further
increased, there comes a moment when the contractions again
increase in vigor and a second maximum height is attained (Fig.
27, B). Beyond this no further increase in height is observed.
The second maximum has been attributed to the presence in the
muscle of two different substances differently affected by changes
in temperature, by fatigue and by various drugs.

The rate at which the muscle is stimulated with a given stimulus

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 69

of uniform strength will also influence the character of the contrac-
tion process. If the intervals between the successive stimulations
be such as permit the muscle to recover from the effects of the
contraction, it may contract as many as a thousand times without
showing any particular variation from the normal form; but if
the intervals are shorter than that just stated it is found that from
the beginning of the stimulation each succeeding contraction
slightly exceeds in height the preceding contraction, until a cer-
tain maximum is reached and maintained, indicating that for
some reason the irritability and the energy of the contraction have
been increased. This gradual increase in the height of the con-
traction has been termed the stair case effect, or the treppe. In

A. B.

Fig. 27. — Tracing Showing the Effects of a Gradual Increase in the Strength
of the Stimulus on the Height of the Contraction, a. Minimal contraction; ab.
progressive increase in the height; b c. first maximum (a number of contractions have been
omitted for economy of space); de. second maximum.

the beginning of the period of stimulation there is sometimes ob-
served a decrease in the height of the contraction following
several stimulations before the stair case effect develops, indi-
cating a temporary decrease in the irritability. These stair case
contractions have been observed in the muscle of both warm-
blooded and cold-blooded animals. The cause for this increase
in irritability upon which the effect depends is attributed to the
presence of certain chemic substances in the muscle arising as a
result of its katabolism, such as carbon dioxid, mono-potassium
phosphate, and paralactic acid. These compounds when pres-
ent in small amounts or in larger amounts for a short time, aug-
ment the action of the muscle and give rise to the treppe effect.
(Lee.) In time, however, if the stimulation be continued, the
irritability declines, the height of the contraction diminishes and
finally the muscle ceases to respond to any stimulus.
2. Temperature. — The temperature at winch all phases of the con-
traction process, as represented by the myogram, attain their
physiologic maximum value is about 30 ° C. If the tempera-

yo TEXT-BOOK OF PHYSIOLOGY.

ture of the muscle falls to 20 ° C. there is a corresponding decline
in activity, as shown by an increase of the latent period, a de-
crease in the height of curve — i. e., in the shortening of the mus-
cle— an increase both in the contraction and relaxation periods.
As the temperature approaches o° C. the height of the curve
again suddenly increases, indicating, for some unknown reason,
an increase in the irritability. This is, however, scarcely a physio-

Fig. 28. — Single Contractions of the Gastrocnemius Muscle at Different
Temperatures. Time tracing 200 per second. — (Brodie.)

logic condition. At a temperature of 40 ° C. to 50 ° C. the muscle
suddenly contracts and passes into the condition of heat rigor.
The proteid constituents of the muscle are coagulated and the
irritability destroyed. (Fig. 28.)
The Load.- — The extent to which a muscle is loaded or weighted
will not only determine the height of the contraction, but also the
time relations of all its phases. This is apparent from an ex-

Fig. 29.— Contractions of a Gastrocnemius Muscle
with Different Loads. — (Brodie.)

amination of Fig. 29, in which it is shown that with an increase
in load there is a decrease in the height of the contraction, an
increase in the latent period, and a general increase in the dura-
tion of both the periods of rising and falling energy.
Continuous Stimulation. — Prolonged or excessive activity of our own
muscles is accompanied by a feeling of stiffness or soreness and
lassitude. There is at the same time a diminution in the speed

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 71

and vigor of the contractions and the power of doing work. To
this condition the term fatigue has been given. The cause of the
fatigue is attributed to a diminution in the amount of the energy-
vielding compounds as well as to the production and accumulation
of waste products resulting from katabolic activity. Among the
waste products mono-potassium phosphate, paralactic acid, and
carbon dioxid are the most important. These substances, when
present in small amounts or in larger amounts for a short time,
increase the irritability of 'the muscle, but when they accumulate
more rapidly than they are removed, as is the case during excessive
activity, they exert a depressive influence on the irritability of
the muscle and thus diminish its contractile power and its capacity
for doing work. The more rapidly they are removed, the sooner
is a fatigued muscle restored to its normal condition. The condi-
tion of fatigue with its attendant phenomena is shown by stimulat-

Fig. 30. — Fatigue Curves. Every Twentieth Contraction Recorded.

ing through its nerve an excised frog muscle with induced electric
currents at intervals of one second. In a variable period of time
the muscle shows an increase in the duration of the latent period, a
diminution of the height of the contraction, in the power of doing
work, and an increase in the time required for relaxation. (Fig. 30.)
If the stimulation is continued the contractions gradually decline as
the muscle becomes exhausted. When a muscle will no longer re-
spond to stimulation through its related nerve, it can be made to
respond to direct stimulation with the electric current. This
taken in connection with the fact that stimulation of a nerve-trunk
even for several hours does not fatigue it, leads to the inference
that the cause of the cessation of contraction does not wholly lie in
the muscle but partly in the nerve endings in the muscle. These
structures it is believed fatigue more readily than the muscle
structures, and hence fail to conduct the nerve impulse to the
muscle. By this means it is protected from absolute exhaustion.
Nutrition. — The irritability of a muscle which conditions the con-
traction process is dependent on the maintenance of its nutrition ;
hence a continuous and sufficient supply of nutritive material
and a rapid removal of waste products are essential conditions
for the exhibition of normal contractions. A diminution of blood
supply or an accumulation of waste products sooner or later im-
pairs the irritability and diminishes the vigor and extent of the

72 TEXT-BOOK OF PHYSIOLOGY.

contraction. Various drugs — e. g., veratrin, barium, etc. — in-
troduced into the circulation and finding their way into the muscle
modify the contraction process, as shown by a very great increase
in the duration of the relaxation period.

The Isometric Myogram. — With the object of obtaining a curve
of the increase and decrease in the tension of a muscle during a single
contraction, with the exclusion as far as possible of a change in length,
the muscle may be made to contract against a strong spring or similar
resistance sufficient to practically though not absolutely prevent short-
ening. To this method the term isometric has been given, and the
curve so obtained an isometric myogram or a tonogram. The record-
ing Jportion of the lever is prolonged some distance so that the very
slight upward movement at its axis, close to which the muscle is at-
tached, will be considerably magnified. That the ordinate value of

an isometric curve may be known,
the apparatus must be graduated
by subjecting the spring to a series
of weights playing over a pulley
supported by the muscle clamp.
The curve of the variation
in tension obtained by the iso-
metric method is shown in Fig.
31, b, in which the two curves
are contrasted. The form of
Fig. 31.— a. Diagram of Isotonic; b, the curve indicates that the mus-
of Isometric Muscle Curves.— (Landois c\e attains its maximum of ten-
and Stirling.) sion m0Te rapidly than its max-

imum of shortening; that the
tension endures for a certain period of time unchanged ; that the fall in
tension takes place more rapidly than the muscle relaxes.

The Myogram Due to the Make and the Break of a Galvanic
Current. — The contraction of the muscle which has heretofore been
recorded has been caused by the momentary action of an induced
current. The contraction of the muscle which is caused by the action
of a constant or galvanic current presents features which are some-
what different and as it serves to illustrate the difference in the effects
of a constant or galvanic and an induced or interrupted current, a
myogram of a contraction due to the make and break of a galvanic
current is introduced at this place. The effects which are observed in
a muscle during the passage of both feeble and strong currents have been
alluded to in a previous section. (See page 61.) In Figure 32 these
effects are graphically represented. It will be observed that on the
closure of the circuit at c the muscle at once contracted and so long as
the current was flowing, the muscle remained in a more or less con-
tracted state known as galvanotonus; on opening the circuit at o the
muscle again contracted, after which it gradually relaxed and returned
to its original condition. The record shows also that during the

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

73

actual passage of the current the muscle substance was being stimu-
lated by it.

The Work Accomplished by a Muscle during the Time of a
Single Contraction. — By work is meant the overcoming of opposing
forces. In the physiologic activities of the body the muscles at each
contraction not only overcome the resistances of antagonistic muscles,

Fig. 32. — A Myogram Due to the Action of a Galvanic Current, Applied Di-
rectly to a Muscle, when the Circuit was Closed (c) and when it was Opened (0).

the weight of the limbs, the friction of joints, etc., but in addition over-
come various external resistances connected with the environment —
e. g., gravity, cohesion, friction, elasticity, etc. The muscles may
therefore be regarded as machines for the accomplishment of work.
Experimentally the work done by an isolated muscle may be calcu-
lated when first the height
of the contraction is ob-
tained and then multiply-
ing it by the weight raised.
The influence of the
weight on the height of
the contraction is shown
in Fig. 33. From this
tracing it will be observed
that the extent to which
a muscle will shorten in
response to a maximal
stimulus is greatest when
it is unweighted; but as
weights differing by a
common increment are
added, the height of the
contraction diminishes until with a given weight it is nil.

A careful study of the results of this experiment will show that the
work done gradually increased as the load was increased from o to 70
grams, when it amounted to 210 milligrammeters; but that after this,
even though the weight lifted was greater, the height to which it was
lifted was less, and hence the work done gradually decreased, until
it amounted to nothing.

Fig. 33. — Tracing Showing the Gradual
Diminution in the Height of the Contrac-
tion as the Weight was Increased by a Com-
mon Increment of 10 Grams from o to 180
Grams. Magnification of the Lever, 4.

74 TEXT-BOOK OF PHYSIOLOGY.

The following table will also show the work done by a frog's muscle
according to Rosenthal.

Weight.

Height.

Work

Done.

o grams

14 mm.

0

grarn-

millimeters

5° "

9 "

45°

"

IOO "

7 "

700

150 "

5 "

75°

<£

200 "

2 "

400

a

250 o o

From the preceding figures it is evident that the mechanical work of a
muscle increases with increasing weights up to a certain maximum,
and then declines to zero. Equally when the muscle contracts to its
maximum without being weighted, and when it does not contract at
all from being overweighted, no work is done. Between these two
extremes the muscle performs varying amounts of work.

The maximum amount of force which a muscle puts forth during a
contraction is naturally measured by the amount of work done; but
as this varies with the degree to which the muscle is weighted, another
measure has been adopted, to which the term absolute muscle force
or static force has been given. The absolute force is measured by
the weight which is sufficient to prevent the muscle from shortening.
This is best determined by the method of after-loading in which the
muscle is not extended by the weight previous to the contraction.
It has been found that the absolute force of a muscle is directly de-
pendent on the number and not the length of the fibers it contains
and proportional to the physiologic transverse section of the muscle.
The transverse section of a muscle is obtained by dividing its volume
(obtained by dividing its actual weight by the specific weight of mus-
cle-tissue, 1 .058) by the average length of the fibers. Assuming that the
muscle weighs 622 grams, its volume would be 576 c.c; and if it be
further assumed that the fibers have an average length of 4 centimeters
the transverse section would contain 144 sq. centimeters each of which
would have a length of 4 centimeters.

For purposes of comparison it is customary to refer the absolute
force to these units of diameter — viz., one square centimeter. Rosen-
thal estimates the force for the square centimeter of the muscle of the
frog at from 2 to 8 kilograms ; for the muscles of man at 6 to 8 kilograms;
Koster at about 10 kilograms for the muscles of the leg and 7 or 8
kilograms for the muscles of the arm.

Summation Effects. — If a series of successive stimuli be applied
to a muscle, the effect will vary according to the rapidity with which
they follow one another. As previously stated, if the interval preced-
ing each stimulus be sufficiently long to enable the muscle to recover
from the effects of the previous contraction, there will be no change
in the form or the character of the contraction for a long time except a
slight increase, in the early period, of the irritability as shown by the in-
creased height of the curve or shortening of the muscle. If, however, a
second stimulus be applied to a muscle during the period of relaxation,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

75

a second contraction immediately follows which is added to or super-
posed on the first; the effect produced will be greater than that pro-
duced by either stimulus separately. See Fig. 34.

A third stimulus applied during the relaxation of the second con-
traction produces a third contraction which adds itself to the second,
and so on. The increment of increase in the extent of the success-
ive contractions gradually di-
minishes, however, until the
muscle reaches a maximum of
contraction. The superposition
of the second contraction on the
first, the third on the second, and
so on, is termed summation 0}
contractions or effects. Experi-
ment has shown that the greatest
effect of a second stimulus — that
is, the highest contraction— is
produced when the stimulus is
applied during the last third of
the period of rising energy, when
the sum of the two contractions
is almost twice as great as the
first contraction (Fig. 34). The
effects following both maximal
and submaximal stimuli indicate
that the muscle cannot attain its
maximum of shortening except
through a summation of several
stimuli. If a second maximal
stimulus enter a muscle during
the latent period following the
first, the effect produced will be
no greater than that produced by
a single stimulus. The muscle
during this period is said to be
refractory or non-responsive to a
second stimulus. If, however,
the stimuli are submaximal they read from below uPward-
add themselves together, and

though the effect is but a single contraction, it is larger than either would
have produced separately. This is termed the summation of stimuli.

Still further, if a series of subminimal stimuli, each of which is
alone insufficient to produce a contraction of the muscle, be applied
in rapid succession, a contraction frequently results. This is termed
the summation 0} subminimal stimuli.

Tetanus. — Tetanus may be defined as a more or less continuous
contraction of a muscle which arises when the time intervals between

Fig. 34. — Tracing Showing the Ef-
fects of Two Successive Stimuli, a. a'
with Gradually Diminishing Inter-
val on a Muscle Contraction. To be

76 TEXT-BOOK OF PHYSIOLOGY.

the stimuli are shorter than the time of the contraction process.
Tetanus will be incomplete or complete according to the number of
stimuli that fall into the muscle in a second of time. When a muscle
is stimulated directly or, better, indirectly through its related nerve by
a series of induced currents at the rate of four or six per second, it
undergoes a corresponding number of contractions of about equal ex-
tent. If the rate of stimulation is increased up to the point when the
interval between each stimulus is less than the duration of the entire
contraction process, the muscle does not have time to completely relax
before the arrival of the succeeding stimulus, and hence remains in a
more or less contracted state, during which it exhibits a series of
alternate partial contractions and relaxations. To this condition of
muscle activity the term incomplete tetanus or clonus is applied. A
graphic record of an incomplete tetanus is given in Fig. 35.

Fig. 35. — Curves Showing the Analysis of Tetanus of a Frog's Muscle
(Gastrocnemius). The numbers under the curves indicate the number of shocks per
second applied to the muscle. There is almost complete tetanus with twenty-five per
second, and it is a little lower than the previous one because the muscle was slightly
fatigued. — (Stirling.)

In such a tracing it is observed that the second stimulation, occurring
before the muscle had time to relax, gave rise to a second contraction,
which was superposed on the first; the same result followed the third
stimulus, the fourth, the fifth, and so on. Owing largely to this sum-
mation of the contractions there is a gradual rise in the height of the con-
traction curve. This condition of the muscle, viz., continued contrac-
tion, combined with diminished power of relaxation, is termed con-
tracture. The tracing also shows that as the stimulus continues, the
base line, that connecting the lowest points of the contractions, grad-
ually rises and takes the form of a curve which increases in height with
the stimulation. The apex line, that connecting the highest points
of the contractions, also rises at the same time, indicating a continuous
increase in the height of the contractions. The length of time a muscle
will exhibit incomplete tetanus depends on a variety of circumstances,
e. g., character of muscle, rate and strength of stimulation, etc., but
mainly on the rapidity with which the muscle becomes fatigued. With
the oncoming of fatigue the muscle begins to relax, and ultimately
returns to its normal condition, notwithstanding the continued stimu-

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 77

lation. If the stimulation be withdrawn, the muscle does not at once
return to its original length but remains more or less contracted for a
variable time. This contraction after stimulation is known as the con-
traction-remainder.

If the stimulation be still further increased in frequency, the in-
dividual contractions become fused together and the curve described
by the lever becomes a continuous line. (See Fig. 35.) Notwith-
standing the fact that the individual contractions are no longer visible,
it can be shown by other methods that the muscle is undergoing a
series of slight alternate contractions and relaxations or vibrations
at least. After a varying length of time the muscle becomes fatigued,
relaxes, and returns to its natural condition even though the stimu-
lation be^continued. The number of stimuli per second necessary to

Fig. 36. — Development of Fatigue and Contraction. Muscle stimulated once a
second by a strong induced current.

develop complete tetanus will depend under normal circumstances on
the period of duration of the individual contractions. The longer this
period, the less the number of stimuli required, and the reverse. Hence
the number of stimuli will vary for different classes of animals and for
different muscles in the same animal, e. g., 2 or 3 for the muscles of
the tortoise, 10 for the muscles of the rabbit, 15 to 20 for the frog, 70
to 80 for birds, 330 to 340 for insects.

An effect which follows frequent stimulation of a muscle, e. g., 50 to
60 times per minute, and especially when the muscle is somewhat
fatigued or cold is shown in Fig. 36. It is evidently a combination of
contracture and fatigue. It will be observed that at the beginning
of the stimulation there is a stair case effect, a-b, combined with dim-
inished relaxation. This in turn is followed by a decline in the height
of the contractions, b-c, and a fall of the base line which may be attributed
to fatigue conditions. After a short time there is a second rise of the
base line, d, and a rapid development of contracture. The muscle at
this period is in a condition of incomplete tetanus which gradually
passes into complete tetanus attended by fatigue.

78 TEXT-BOOK OF PHYSIOLOGY.

The tetani of muscles may be classified in accordance with their
causes as .follows : —

-r,, . , . ! Volitional,
i. Phvsiologic

Reflex.

2. Experimental.

3. Pharmacologic.

t, ,ii • Bacterial.

4. Pathologic

[ Reflex.

1. Physiologic Tetanus. — 1. Volitional.- — Because of the fact that
during the continuance of a volitional movement the muscle is in a
state of continuous contraction, it may be accepted that volitional con-
tractions are states of tetanus, more or less complete; for the shortest
possible volitional contraction, however quickly it takes place, has al-
ways a longer duration than a single contraction caused by an induced
electric current. As the volitional contraction is similar to that observed
when a muscle or its related nerve is stimulated by rapidly repeated
induced currents, it is assumed that the nerve-cells in the spinal cord
are discharging in a rhythmic manner a certain number of nerve im-
pulses per second in consequence of the arrival of nerve impulses com-
ing from the cerebral cortex, the result of volitional acts. In other
words the volitional tetanus is the result of a discontinuous stimulation.
The number of stimuli transmitted to a muscle during a volitional
tetanus has been estimated by the employment of the graphic method
at from 8 to 13 per second, 10 being about the average. When a voli-
tional contraction is recorded the myogram not infrequently exhibits a
series of small wave-like elevations which indicate that the muscle is
not in a state of complete tetanus but is undergoing slight alternate
contractions and relaxations. Unless the contraction process in human
muscle differs from that of frogs it is difficult to see how 10 or even
20 stimuli per second can give rise to even an incomplete tetanus
when the single contraction is -^ of a second in duration.

2. Reflex. — A tetanus of muscle, physiologic in character, arises
during the performance of many muscle movements in consequence
of peripherally acting causes and may therefore be termed a reflex
tetanus. The duration of a tetanus thus induced, like the duration of
a volitional tetanus, will vary with the duration of the exciting cause.
Reflex tetani are presented by the muscles of the lower jaw during
mastication, by the intercostal muscles during breathing, by the
muscles of the limbs during walking, etc. In these and other in-
stances there are reasons for believing that for a variable period of
time the muscles are in a state of continuous contraction from the dis-
charge of nerve impulses from the nerve cells in the spinal cord as the
result of the arrival of nerve impulses coming from a peripheral sur-
face.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 79

2. Experimental Tetanus. — The tetanus of muscle developed
in accordance with the method described in foregoing paragraphs, i. e.,
by the employment of instrumental procedures, may be termed ex-
perimental tetanus. Its mode of development serves to illustrate and
explain the method by which individual contractions are summated
and continuous contractions made possible for the performance of
volitional acts.

3. Pharmacologic Tetanus. — The administration of certain
drugs, e. g., strychnin, in sufficient amounts, is followed in a short time
by a series of intermittent spasms in which all the muscles of the body
are involved. At the beginning of the spasms the muscles are thrown
into tonic or complete tetanus, during the continuance of which the
muscles are hard and firm. In a short time this tonic state begins
to subside, giving way to tremors or a series of irregular contractions
resembling incomplete tetanus or clonus. A tetanus of this char-
acter may be termed pharmacologic. Though the onset of the
tetanus is occasioned largely by peripheral stimulation, the seat of
action of strychnin is central and for the most part focalized in the
spinal cord. The exact seat of its action is not definitely determined
but there are reasons for believing that it is on the end-tufts of afferent
nerves in the spinal cord or on the intercalated neuron between them
and the nerve-cells in the anterior horns of the gray matter, the irrita-
bility of which is raised and the resistance to the transmission of nerve
impulses coming from the periphery diminished. As a result the nerve
impulses are transmitted to the nerve-cells more readily, not only in a
horizontal but also in a longitudinal direction, and the effects they pro-
duce enormously increased.

4. Pathologic Tetanus.— 1. Bacterial. — The introduction of a
specific bacillus into a wound in any region of the body is followed
after a period of incubation of from three or four days to a week by a
tetanus in which nearly all the muscles of the body are involved, char-
acterized by a tonic contraction and clonic exacerbations. A tetanus
of this character may be termed pathologic. The persistent tonic
contraction is the result of a more or less continuous discharge of nerve ■
impulses from the nerve- cells of the spinal cord which have been
rendered abnormally irritable by the action of a toxin, produced by the
bacilli, and which has a selective action on these structures. The
clonic exacerbations are evoked from time to time by various forms of
peripheral stimulation.

2. Keflex — A tetanus of individual muscles more or less continu-
ous in character is occasionally the result of peripheral irritations of
a pathologic character. A tonic contraction of the masseter muscles,
for example, firmly closing the jaws for weeks and months at a time
is caused in some instances by an impacted wisdom tooth or an ulcera-
tive condition of the mouth. Since the removal of the cause is followed
by a relaxation of the muscle, this form of tetanus, known as trismus,
may be regarded as pathologic in character and reflex in origin.

8o TEXT-BOOK OF PHYSIOLOGY.

The Muscle Sound. — If a stethoscope or a myophone with tele-
phone connections be placed on a muscle while in a condition of voli-
tional tetanus and at the same time kept in a certain degree of tension,
there will be developed in the observer a sensation of sound or tone
which is spoken of as a muscle sound or tone. It is also readily heard
in the masseter muscle when the side of the face is placed on a receiv-
ing body such as a pillow, and the masseter muscles made to con-
tract volitionally. This tone is attributed to a vibration or an alternate
contraction or relaxation of the muscle or to an intermittent rhythmic
variation in tension, the result of the rate of stimulation. This tone
corresponds to a vibration frequency of from 1 8 to 20 per second and is
accepted as one of the proofs that the physiologic volitional tetanus is
not continuous but discontinuous in character. If a muscle is tetanized
with induced currents, the tone increases in pitch for a limited time
as the frequency of the current per second increases up to a certain
maximum, which for frogs is about 200 and for mammals about 1000.

CHEMIC PHENOMENA.

The chemic changes which underlie the transformation of energy
in the living muscle even when in a state of rest are active and com-
plex, though but little is known as to their exact character. As shown
by an analysis of the blood flowing to and from the resting muscle,
it has, while flowing through the capillaries, lost oxygen and gained
carbon dioxid. The amount of oxygen absorbed by the muscle (9
per cent.) is greater than the amount of carbon dioxid (6.7 per cent.)
given off. Notwithstanding the relation of the oxygen absorbed to
the carbon dioxid produced, there is no parallelism between these two
processes, as the carbon dioxid will be given off in the absence of free
oxygen or in an atmosphere of nitrogen.

In the active or contracting muscle all the chemic changes are
increased, as shown both by an increased absorption of oxygen and
an increased production of carbon dioxid, though the ratio existing
between them differs considerably from that of the resting muscle.
Thus, according to Ludwig, an active muscle absorbs 12.26 per cent.
of oxygen and gives off 10.8 per cent, carbon dioxid. During the
activity of a muscle its tissue changes from a neutral to an acid reac-
tion, from the development of sarcolactic acid and possibly phos-
phoric acid. The degree of the acidity depends to some extent on the
duration of the contraction periods. Chemic analysis of a tetanized
muscle shows that it contains less glycogen than a resting muscle, and
that it contains a larger amount of water. Coincident with muscular
contraction, the blood-vessels become widely dilated, leading to a
large increase in the blood-supply and a rapid removal of the products
of decomposition.

Rigor Mortis.— A short time after death the muscles pass into a
condition of extreme rigidity or contraction known as death stiffening

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 81

or rigor mortis, which lasts from one to five days. In this state they
offer great resistance to extension. At the same time their tonicity dis-
appears, their cohesion diminishes, and their irritability ceases. The
time of the appearance of this post-mortem rigidity varies from a quarter
of an hour to seven hours. Its onset and duration are influenced by the
condition of the muscle irritability at the time of death. When the ir-
ritability is impaired from any cause, such as. chronic disease or defec-
tive blood-supply, the rigidity appears promptly but is of short duration.
After death from acute diseases it is apt to be delayed, but will continue
for a longer period. The rigidity first appears in the muscles of the lower
jaw and neck; next in the muscles of the abdomen and upper extrem-
ities; finally in the trunk and lower extremities. It disappears in prac-
tically the same order. Chemic changes of a marked character ac-
company this process. The muscle becomes acid in reaction from
the development of sarcolactic acid and there is a large increase in
the amount of carbon dioxid given off. The immediate cause of the
rigidity appears to be coagulation of the myosinogen within the sarco-
lemma with the formation of an insoluble protein, myosin. In the
early stages of the coagulation restitution is possible by the circula-
tion of arterial blood through the vessels. The final disappearance
of this post-mortem rigidity is due probably to the action of acids which
render the myosin soluble, and possibly to the action of various micro-
organisms which give rise to putrefactive changes.

Source of the Muscle Energy.— Notwithstanding many in-
vestigations, the nature of the materials which are the immediate source
of the muscle energy is not known. The absence of any noticeable
increase in the quantity of urea or other nitrogen-holding compounds
excreted renders it probable that the energy does not come from the
metabolism of proteid materials. The marked production of carbon
dioxid and sarcolactic acid points to the decomposition of some un-
stable compound, of a carbohydrate character, rich in carbon and
oxygen. It has been suggested that glycogen furnishes the energy,
inasmuch as this substance, generally present in muscle, disappears
during activity. A muscle which has been tetanized contains less
glycogen than the corresponding muscle at rest. A muscle which
has been separated from the nervous system by division of its nerves
and thus prevented from contracting accumulates glycogen. Bunge
is of the opinion that though the carbohydrates are the main, they are
not the only sources of muscle energy. If there is a deficiency or ab-
sence of carbohydrate food, the muscle will utilize fat and proteid,
for experiment has shown that the available glycogen is entirely con-
sumed the second or third day. The mechanism by which the energy
is liberated, whether by decomposition or direct oxidation, is unknown.
The fact that muscle will contract in an atmosphere free of oxygen, that
no free oxygen can be obtained from muscle, would support the idea
that the mechanism is one of decomposition. Hermann suggests that
the energy of a contraction is liberated by the splitting and subsequent
6

82 TEXT-BOOK OF PHYSIOLOGY.

re-formation of a complex body belonging neither to the carbohydrates
nor fats, but to the proteids — to this hypothetic body the term inogen is
given. This complex molecule, the product of the nutritive activity of
the muscle-cell in undergoing decomposition, would yield carbon dioxid,
sarcolactic acid, and a proteid residue resembling myosin. On the
cessation of the contraction the muscle-cell recombines the proteid
residue with oxygen, carbohydrates, and fats, and again forms the
energy-holding compound, inogen. The phenomena of rigor mortis
support this view. At the moment of this contraction the muscle
gives off C02 in large amount, develops sarcolactic acid and myosin.
There is thus a close analogy between the two processes; in other words,
a contraction is a partial death of the muscle. If this view is correct,
then the oxygen is required mainly for heat production through oxida-
tion processes.

THERMIC PHENOMENA.

The potential energy liberated during a contraction is transformed
into kinetic energy — viz., heat and mechanic motion. Though heat
production is taking place even during the passive condition, prob-
ably through oxidation processes, it is largely increased by muscle
activity. The skeletal muscle of the frog, the gastrocnemius, shows
after tetanization an increase in temperature from 0.140 C. to 0.180
C, and after a single contraction from 0.0010 C. to 0.005 ° C. The
amount of heat thus produced will vary with a variety of conditions,
as strength of stimulus, tension, work done, etc.

Stimulus. — It has been experimentally determined that an in-
crease in the strength of the stimulus from a minimal to a maximal
value increases the amount of heat liberated. This is the direct result
of increased chemic change naturally following increased stimulation.

Tension. — The greater the tension of a muscle, the greater, other
conditions being the same, is the amount of heat liberated. If the
muscle is securely fastened at both extremities so that shortening is
practically impossible during the stimulation, the maximum of heat
production is reached. In the tetanic state the great increase in tem-
perature is due to the tension of antagonistic and strongly contracted
muscles. In both instances, mechanic motion being prevented, the
liberated energy is transformed into heat.

Mechanic Work. — If the muscle is permitted to shorten and
raise a weight, some of the energy liberated takes the form of mechanic
motion. If the weight is removed at the height of the contraction,
external work is accomplished. The greater the weight raised, within
limits, the greater is the percentage of energy which takes the direction
of mechanic motion. The percentage of the total energy liberated
which is thus utilized, has been estimated at from 25 to 40 per cent. In
accordance with the law of the conservation of energy, the heat produced,
stated in calories, plus the energy required in the raising of the weight,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 83

expressed in kilogrammeters of work, must equal the potential energy
transformed.

A muscle during a tetanic contraction of short duration accom-
plishes more work than during a single contraction; the weight in each
case being the same. In the former condition the height of contrac-
tion through summation, and hence the work done, is greater than in
the latter. The work done by a short tetanic contraction may be two
or three times that of a single contraction, but after the muscle reaches
its maximum degree of shortening and then continues in a state of tet-
anus, no further work is done. Internal work is done, however, as
shown by an increase in the temperature.

When a weight which is lifted by a muscle during a single con-
traction is allowed to act on the muscle during the relaxation, no ex-
ternal work is accomplished. All the energy set free manifests itself
as heat. Internal work is done, as shown by the fact that the muscle
becomes fatigued.

Work Done Daily. — The muscle system in its entirety is to be
regarded as a machine for the transformation of potential into kinetic
energy, and in so doing accomplishes work. Through the interme-
diations of the bones of the sketelon which play the part of levers the
individual not only changes his position in space, but overcomes to
some extent the resistances offered by the environment. The em-
ployment of artificial levers, tools, as distinguished from natural levers,
bones, materially adds to the effectiveness of the muscle machine.
The amount of work which a man of average physical development
weighing 72 kilos can perform in eight hours has been variously esti-
mated. It will naturally vary according to the character of the occupa-
tion. If the work done be calculated from the number of kilograms
raised one meter, the average laboring-man performs about 300,000
kilogrammeters.

ELECTRIC PHENOMENA.

Electric Currents from Injured Muscles. — The energy liber-
ated during a muscle contraction is not only transformed into heat
and mechanic motion, but to some extent also into electric energy.
The presence of points of different potential on the surface of the
muscle,, the necessary condition for the development of electric currents,
is tested by means of non-polarizable electrodes connected by wires
with a sensitive galvanometer or capillary electrometer. When such
electrodes are brought in contact with a muscle properly prepared,
there is at once developed and conducted to the galvanometer an electric
current the intensity and direction of which are indicated by the de-
flection of the galvanometer needle. The existence of this current is
most conveniently demonstrated with single muscles the fibers of which
are parallel — e. g., the sartorius, or the semimembranosus of the frog.
If the tendinous ends of either of these muscles be removed bv a section

84

TEXT-BOOK OF PHYSIOLOGY.

made at right angles to the long axis, a muscle prism is obtained which
presents a natural longitudinal surface and two artificial transverse
surfaces. A line drawn around the surface of such a muscle prism at a
point midway between the two transverse sections constitutes the equator.
"When the natural longitudinal and artificial transverse surfaces are
connected with the wires of a galvanometer the terminals of which are
provided with non-polarizable electrodes, an electric current is at once
developed. In all instances the current, as shown by the deflection

of the needle, originates at the transverse
surface, passes through the muscle to the
longitudinal surface, thence through the
galvanometer to the transverse surface.
The longitudinal surface is, therefore,
electropositive, the transverse surface
electronegative. The two points exhibit-
ing the greatest difference of potential,
and hence the most powerful current, lie
in the equator and in the center of the
transverse surface. Currents of gradually
diminishing intensity are obtained when
the electrode placed on the longitudinal
surface is removed toward either end.
Feeble currents are developed when two
points situated at unequal distances,
either on corresponding or opposite sides
of the equator, are connected; in either
case the current flows from the point
lying nearest the equator to the point
farthest from it. Similar currents are
obtained when two points on the cross-
section situated at unequal distances from
the central axis are connected, in which
case the direction of the currents will be
from the point lying nearest the periphery
toward the center. On the contrary, no
current is developed when two points on the longitudinal surface
equally distant from the equator, or two points on the transverse sur-
face equally distant from the central axis, are connected. Such points
are said to be isoelectric. These facts are shown in Fig. 37. The
natural ends of the muscle, enclosed by sarcolemma and tendon, do not
exhibit, if carefully preserved from injury, the negativity characteristic
of the artificial transverse ends.

Similar electric conditions are exhibited by the muscles of man
and other mammals, by the muscles of birds, reptiles, amphibia, etc.
The currents developed by connecting the equator on the longitu-
dinal surface with the axis of the transverse surface have an electromo-
tive force in the frog muscle of from 0.037 to °-°75 °f a Daniell cell.

Fig. 37. — Diagram to Illus-
trate the Current in Muscle.
The arrowheads indicate the direc-
tion; the thickness of the lines in-
dicates the strength of the currents.
— (Landois and Stirling.)

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

85

The electric currents in the muscle are intimately associated with
the chemic changes underlying its nutrition, and hence their intensity
rises and falls with all the conditions which maintain or impair mus-
cle nutrition and irritability. The currents observed in the injured
muscle during the inactive state have been termed currents 0} rest.
du Bois-Reymond regarded them as pre-existent, intimately connected
with the living condition of the muscle, and essential to the performance
of its functions, and to be explained by the view that the entire muscle
is composed of molecules each of which exhibits the same difference
of potential on its longitudinal and transverse surfaces as the muscle
prism itself. Hermann denies the existence of currents in normal
resting muscle and attributes them to injuries of the surface, due to
methods of preparation, in consequence of which the tissue dies and
becomes electronegative to the uninjured area, which remains electro-
positive. These currents Hermann terms "demarcation currents."

Negative Variation of the Muscle Current. — If a muscle ex-
hibiting a current of injury
be excited to activity by
tetanizing induced currents
applied to the opposite end
of the muscle, it will be ob-
served that as the contrac-
tion wave passes over the
muscle there is a movement
of the galvanometer needle
toward the zero point, in-
dicating a diminution of the
potential on the longitudinal
surface. To this diminu-
tion in the strength of the
current the term negative

variation was given. On the withdrawal of the stimulus the needle
again returns in a short time to its former position. The diminution
of potential on the longitudinal surface of the muscle is now attributed
to the passage of the excitation and contraction processes, to a tem-
porary disintegration of the muscle substance (Fig. 38). With their
disappearance and the subsequent restoration of the nutrition of the
muscle, the former electric condition returns.

The primary deflection of the galvanometer needle is due to the
demarcation current which arises as a result of the difference in
electric potential produced by the destructive chemic changes taking
place at the cut end of the muscle. The negative variation is caused
by the fact that the activity of the muscle, with its attendant chemic
changes, will always be greater in the uninjured equatorial region,
and hence will always tend to counterbalance the original source of
difference in electric potential.

Electric Currents from Non-injured Muscles. — Though per-

Fig. 38. — The Negative Variation of the
Demarcation Current. A. The contraction
wave, which as it passes beneath the electrode at B
causes a diminution of potential-

86

TEXT-BOOK OF PHYSIOLOGY.

fectly normal resting muscle, according to Hermann, is isoelectric,
nevertheless electric currents are developed during activity to which
he has given the term action currents, and which are attributed to
the propagation of the contraction wave.

Action Currents. — When two isoelectric points on the longitu-
dinal surface of a muscle are connected with a galvanometer and a
single stimulus applied directly to one extremity, it can be shown that
as the contraction wave passes beneath A, Fig. 39, the muscle-tissue
at that point becomes electronegative toward B and a current at once
passes through the galvanometer from B to A, as shown by the deflec-
tion of the needle toward A. As the contraction wave passes beneath
B it in turn becomes electronegative, and a temporary condition of

Fig.

39. — The Condition Leading to the Development of the First Action

Current.

equal potential is established when the needle returns to the zero point.
In a very short time the nutrition of A is restored and becomes elec-
tropositive toward B, when a current will pass through the galvan-
ometer in the opposite direction from A to B, as shown by the movement
of the needle toward B, Fig. 40. As the contraction wave passes beyond
B its nutrition is restored and becomes of equal potential with A. The
term phasic is applied to these currents. The first current flows in
the muscle in the direction of progress of the contraction wave — first
phase; the second current flows in the reverse direction — second phase;
the current is therefore diphasic. When a muscle is tetanized, there
is but a single current observed, which, however, endures so long as
the tetanic contraction is maintained. . To this current the term de-
cremential is given. When a muscle is excited to action by the nerve
impulse which enters at its center, two contraction waves are developed,
one in each half of the muscle, and hence there are two sets of diphasic
action currents.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

87

The presence of action currents in the muscle of the living body
during a single contraction was demonstrated by Hermann in the mus-
cles of the forearm. The arrangement of the experiment was, briefly,
as follows : The forearm was surrounded by two twine electrodes sat-
urated with zinc solution, one being placed at the physiologic middle
— the nervous equator — the other at the wrist. Both electrodes were
then connected with the galvanometer. When the brachial plexus was
stimulated in the axillary space, the deflections of the galvanometer
needle, when analyzed with the repeating rheotome, indicated phasic
currents with a single contraction. In the first phase — atterminal — '■

Fig. 40. — The Condition Leading to the Development of the Second Action

Current.

the wrist became positive and the current passed in the muscle toward
its termination; and in the second — abterminal — it became negative
and the current now passed in the reverse direction. The action cur-
rents which are observed in the frog's muscle were thus shown to be
present in the living human muscle, with this difference, however:
that the second phase — abterminal — instead of being weaker in man,
is equally strong with the atterminal. This experiment also revealed
the fact that the rapidity of propagation of the excitation wave was
much greater in man, amounting to about twelve meters per second.
Hermann therefore denies the pre-existence of electric currents and
regards them as due to localized temporary disintegration of the mus-
cle in consequence of activity, as they disappear on the restoration of
the muscle to its normal condition.

SPECIAL ACTION OF MUSCLE GROUPS.

The individual muscles of the axial and appendicular portions of
the body are named with reference to their shape, action, structure,
etc.; e. g., deltoid, flexor, penniform, etc. In different localities a

88 TEXT-BOOK OF PHYSIOLOGY.

group of muscles having a common function is named in accordance
with the kind of motion it produces or to which it gives rise: e. g.,
groups of muscles which alternately diminish or increase the angular
distance between two bones are known respectively as flexors and
extensors; such muscle groups are usually found in association with
ginglymus joints. Muscles which rotate the bone to which they are
attached around its own axis without producing any great change of
position are known as rotators, and are found in association with en-
arthrodial or ball-and-socket joints. Muscles which impart an angu-
lar movement to the extremities to and from
| the median line of the body are termed ad-

r^ — j5-(i) ductors and abductors respectively.

a In addition to the actions of individual

F ® £ . . groups of muscles in producing special move-

A W a P ments, in some regions of the body, several

• | f groups of muscles are coordinated for the ac-

W p a complishment of certain definite functions;

Pig. 41.— The Theee'oe- e- g-> tne functions of respiration, mastication,
ders of Levers. etc. The coordination of axial and appen-

dicular muscles enables the individual to as-
sume certain postures, such as standing, sitting, and lying; to engage
in various acts of locomotion, as walking, running, dancing, swimming.
Levers. — The function or special mode of action of individual
muscles can be understood only when the bones with which they are
connected are regarded as levers whose fulcra or fixed points lie in
the joints where the movement takes place, and the muscles as sources
of power for imparting movement to the levers with the object of over-
coming resistance.

In mechanics levers of three kinds or orders are recognized ac-
cording to the relative positions of the fulcrum or axis of motion,
the applied power, and the weight to be moved. (See Fig. 41.)

In levers of the first order the fulcrum, F, lies between the weight
or resistance, W, and the power or moving force, P. The distance
P F is known as the power arm and the distance W F as the weight
arm. As examples of this form of lever found in the human body may
be mentioned :

1. The elevation of the trunk from the flexed position. The axis

of movement, the fulcrum, lies in the hip-joint; the weight, that
of the trunk, acting as if concentrated at the center of gravity,
which lies close to the tenth dorsal vertebra; the power, the mus-
cles attached to the tuberosity of the ischium. The opposite
movement is equally one of the first order, but the relative posi-
tions of P and W are reversed.

2. The head in its movement backward and forward on the atlas.

In levers of the second order the weight lies between the power
and the fulcrum. As illustration of this form of lever may be men-
tioned:

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 89

1. The depression of the lower jaw, in which movement the fulcrum is

the temporomaxillary articulation; the resistance, the tension
of the elevator muscles; the power, the contraction of the de-
pressor muscles.

2. The raising of the body on the toes, in which movement the ful-

crum is the toes, the weight that of the body acting through the
ankle, the power the gastrocnemius muscle applied to the heel
bone.
In levers of the third order the power is applied at a point lying

between the fulcrum and the weight. As examples of this form of

lever may be mentioned :

1. The flexion of the forearm, in which the fulcrum is the elbow-

joint, the power the biceps and brachialis anticus muscles ap-
plied at their points of insertion, the weight that of the forearm
and hand.

2. The extension of the leg on the thigh.

When levers are employed in mechanic operations, the object
aimed at is the overcoming of a great resistance by the application of
a small force acting through a great distance, so as to obtain mechanic
advantage. In the mechanism of the human body the reverse gener-
ally obtains, viz,, the overcoming of a small resistance by the applic-
ation of a large force acting through a short distance. As a result
there is a gain in the extent and rapidity of the movement of the
lever. The power, however, owing to its point of application, acts
at a great mechanic disadvantage in many instances, especially in
levers of the third order.

Postures. — Owing to its system of joints, levers, and muscles the
human body can assume a series of positions of equilibrium, such as
standing and sitting, to which the term posture has been given. In
order that the body may. remain in a state of stable equilibrium in
any posture, it is essential that the vertical line passing through its
center of gravity shall fall within the base of support.

Standing is that position of equilibrium in which a line drawn
through the center of gravity of the entire body falls within the base
of support. This position is maintained largely by the mechanical
conditions of the joints, apparently for the purpose of reducing to a
minimum muscular action, so that it can be prolonged for some time
without giving rise to fatigue. In the military position, which may
be assumed as the normal position, all the joints must be in such a
condition of extension and fixation that the body will represent a
rigid column resting on the astragalus and supported by the arch of
the foot. This is accomplished:

1. By balancing the head on the apex of the vertebral column. This
is done by the action of the muscles on the back of the neck.
The muscular effort is, however, very slight, as the center of
gravity of the head lies but a short distance in front of the ar-
ticulation.

go TEXT-BOOK OF PHYSIOLOGY.

2. By making the vertebral column erect and rigid. This is brought
about by the action of the common extensor muscles of the trunk.
In this condition the center of gravity lies just in front of the
tenth dorsal vertebra. The head, trunk, and upper extremities
are now supported by the hip-joints; and in order that this sup-
port may give to the body a certain degree of stable equilibrium,
independent of muscular action, the line of gravity falls behind the
line uniting the center of rotation of the two joints. In conse-
quence the body would fall backward were it not prevented by
the tension of the iliofemoral ligament and the fascia lata.
The line of gravity, continued downward, passes through the knee-
joint posterior to the axis of rotation, and hence the body would now
fall backward were it not prevented by the tension of the lateral
ligaments and the contraction of the quadriceps femoris muscle.
Though the body is supported by the astragalus, the line of grav-
ity does not pass through the line uniting the two joints, for in so
doing constant muscular effort would be required to maintain stable
equilibrium; passing a short distance in advance of this line, there
would be a tendency of the body to fall forward, which is prevented
by the extensor muscles of the foot. When the body is in the erect
or military position, the center of gravity lies between the sacrum
and last lumbar vertebra. Standing is thus an act of balancing, and
requires not only the static conditions of joints, but the dynamic
conditions of various groups of muscles, and hence is not a position
of absolute ease and cannot be maintained for any length of time
without experiencing discomfort and fatigue. Sitting erect is an
attitude of equilibrium in which the body is balanced on the tubera
ischii, when the head and trunk together form a rigid column.

Locomotion is the act of transferring the body as a whole through
space, and is accomplished by the combined action of its own muscles.
The acts involved consist of walking, running, jumping, etc.

Walking is a complicated act involving almost all the voluntary
muscles of the body either for purposes of progression or for bal-
ancing the head and trunk, and may be defined as a progression in
a forward horizontal direction due to the alternate action of both
legs. In walking one leg becomes for the time being the active or
supporting leg, carrying the trunk and head; the other the passive
but progressing leg, to become in turn the active leg when the foot
touches the ground. Each leg is therefore alternately in an active
and passive state.

Running is distinguished from walking by the fact that at a given
moment both feet are off the ground and the body is raised in the air.

THE VISCERAL MUSCLE.

The visceral muscle, as the name implies, is found in the walls
of hollow viscera, where it is arranged in the form of a membrane

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 91

or sheet. It is present in the walls of the alimentary canal, blood-
vessels, respiratory tract, ureter, bladder, vas deferens, uterus, fallopian
tubes, iris, etc. In some situations it is especially thick and well
developed — e. g., uterus and pyloric end of the stomach; in others it is
thin and slightly developed.

The Histology of the Visceral Muscle-fiber. — When examined
with the microscope, the muscle sheet is seen to be composed of
fibers, narrow, elongated, and fusiform in shape. As a rule, they

Fig. 42. — Two Smooth Muscle-fibers from Small Intestine of Frog. X 240.
Isolated with 35 per cent, potash-lye. The nuclei have lost their characteristic form
through the action of the lye. — (Stohr.)

are extremely small, measuring only from 40 to 250 micromillimeters
in length and from 4 to 8 micromilHmeters in breadth. The center
of each fiber presents a narrow, elongated nucleus. The muscle-
protoplasm which makes up the body of the fiber appears to be en-
closed by a delicate elastic membrane resembling in some respects
the sarcolemma of the skeletal muscle. In some animals the visceral
fiber presents a longitudinal striation suggesting the existence of
fibrillse surrounded by sarcoplasm (Fig. 42). The fibers are united
longitudinally and transversely by a cement material. The muscle
is increased in thickness by the
superposition of successive layers.
At varying intervals the fibers are

I ■ . 1 ii r t Connective-tissue.

grouped into bundles or fasciculi septum.

by septa of connective tissue (Fig.

43.) Blood-vessels ramify in the

connective tissue and furnish the „

, .j. , . 1 Smooth muscie-fiber-

necessary nutritive material. in transverse section.

The visceral muscle receives
stimuli from the sninal rord not Fig. 43-— Section of the Circular

stimuli irom me spinal cora, not layer of the Muscular Coat of the

directly, however, as in the case of Human Intestine.— (Stohr.)
the skeletal muscle, but indirectly

through the intermediation of ganglion cells, which may be located
at some distance from the muscle or near the walls of the viscera.
Non-medullated fibers from the ganglion pass directly into the muscle,
where they frequently unite to form a general plexus. From this
plexus fine branches take their origin and ultimately become phys-
iologically associated with the muscle-fiber.

Physiologic Properties. — The visceral muscles which have
been subjected to experiment are mainly those of the stomach, in-
testine, bladder, ureter, and iris. From the results of the experiments
which have been published, it is evident that all visceral muscles pos-
sess elasticity, tonicity, irritability, and conductivity.

Nucleus.

92 TEXT-BOOK OF PHYSIOLOGY.

The elasticity of the bladder muscle of the cat was strikingly
shown in the experiments published by Dr. Colin C. Stewart. When
this muscle was weighted with weights differing by a common incre-
ment, it was extended on the addition of each weight, though to a
progressively less extent. On the removal of the weights the muscle
eventually returned to its former length. The records of the extension
were similar to, if not identical with, those of the skeletal muscle.

Tonicity is a property common to all visceral muscles. Each
muscle is continuously in a state of contraction intermediate between
that of complete contraction and that of relaxation. In how far this
is due to local and inherent causes or to stimuli reflected from the
nervous system as a result of peripherally acting causes is not in
individual instances readily determinable. From time to time the
tonicity varies, increasing and decreasing in response to these various
stimuli and in accordance with the functional activities of the organs
in which the muscle is found.

The irritability manifests itself by a change of form, and doubt-
less by the liberation of heat on the application of any form of stimulus
— mechanic, chemic, thermic, electric.

The conductivity is less marked in the visceral than in the skeletal
muscle, and, contrary to what is observed in the latter, the conduction
extends laterally as well as longitudinally from fiber to fiber. This
is shown by stimulation of the exposed intestine. Shortly after the
stimulus is applied the muscle contracts longitudinally — i. e., in a di-
rection at right angles to the long axis of the intestine, partially
obliterating its lumen. From this point the conduction process indi-
cated by the contraction wave passes in opposite directions for some
distance along the canal. As to whether this is accomplished by
protoplasmic processes extending from fiber to fiber, or whether the
uniting membrane, differs in conducting power from the sarcolemma,
is as yet a matter of doubt. From the fact that the upper two-thirds
of the ureter, though free of nerve-cells, exhibits lateral conduction,
it is evident that it may take place independent of the nervous
system.

The Contraction of the Visceral Muscle. — The general character
of the contraction may be witnessed on opening the abdomen of a
recently killed animal, especially the rabbit. Shortly after exposure
to the air the walls of the intestine begin to contract in a most vig-
orous manner. The contraction wave beginning at various points
is propagated in both directions, running along the intestinal wall
for a variable distance. A succession of similar waves may be ob-
served for some minutes. To the alternate contraction and relaxa-
tion of the muscle-fibers, which are circularly arranged, the term
peristalsis is usually given. The excised stomach of a dog kept
under suitable conditions will exhibit similar movements. The
same holds true of the bladder muscle of the cat, the muscle of the
ureter, etc. Careful observation shows a certain periodicity in the

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

93

movements. Inasmuch as the cause is not apparent, these contrac-
tions are termed spontaneous or automatic.

Graphic Record of the Contraction. — For experimental pur-
poses narrow transverse sections of the stomach of the frog or the
entire bladder muscle of the cat, excised or in situ, according to the
method of Prof. Colin C. Stewart, may be employed. If kept moist,
they will retain their irritability for some hours. The changes of
form may be recorded with the usual muscle lever. When thus pre-
pared, the muscle may exhibit for several hours a series of pulsa-
tions, rhythmic in character. With spontaneously acting mammalian
muscle the contraction and relaxation periods are of equal duration.
With the amphibian muscle they are of unequal
duration, as a rule. In both classes of animals
the character of the record, a succession of large
and small contractions, would indicate that the
general rhythmic movement is compounded of
two or three secondary rhythms which differ in
rate and character. A single pulsation may be
recorded by stimulating the bladder muscle with
the induced or the make and break of the con-
stant current. A curve of such a contraction is
shown in Fig. 44. The contraction takes place
more rapidly than the relaxation; the two phases
occupying five and thirty-five seconds respec-
tively. The latent period covered 0.25 second.
With other muscles the time relations are slightly
different. Tetanization of the bladder muscle of
the cat occurred when the stimuli succeeded each
other with a certain rapidity; the interval between
stimuli approximating a period somewhat less
than two seconds. This muscle responds to

variations in temperature, to strength of stimulus, to the load, in a
manner similar to, if not identical with, the skeletal muscle.

The Function of the Visceral Muscle. — In a general way it
may be said that the visceral muscle determines and regulates the pas-
sage through the viscus or organ of the material contained within it.
The food in the stomach and intestines is subjected to a churning proc-
ess by the muscles, in consequence of which the digestive fluids
are more thoroughly incorporated and their characteristic action in-
creased. At the same time the food is carried through the canal, the
absorption of the nutritive material promoted, and the indigestible
residue removed from the body. The blood is delivered in larger or
smaller volumes according to the needs of the tissues through a re-
laxation or contraction of the muscle-fibers of the blood-vessels.
The urine is forced through the ureter and from the bladder by the
contraction of their respective muscles. The mode of action of the
individual muscles will be described in successive chapters.

itimmmiiiiimwimmimmiiiili

Fig. 44. — The Curve
of Contraction of the
Bladder Muscle at
Body-temperature in
Response to a Single
Induction Current.
The time 'is indicated
in seconds. — (Stewart.)

94 TEXT-BOOK OF PHYSIOLOGY.

Ciliary Movement. — The free surface of the epithelium cover-
ing the mucous membrane in certain regions of the body is charac-
terized by the presence of delicate filamentous processes termed
cilia. (See Fig. 45.) Ciliated epithelium is found in man and mam-
mals generally, in the nose, Eustachian tube, larynx, with the ex-
ception of the vocal membranes, trachea and bronchial tubes as far
as the pulmonary lobules, Fallopian tubes, uterus, and epididymis.
The lumen of the central canal of the spinal cord and the cavities of
the brain are lined, especially in childhood, by cells provided with
similar cilia. Ciliated epithelium is also found in all classes of ani-
mals, and especially in the invertebrates.

The cilia found in the human body vary in length from 0.003 mm-
to 0.005 mm. They are apparently structureless and colorless, and
appear to have their origin in and to be a pro-
longation of a transparent material on the outer
surface of the cell material. The number of cilia
present on the surface of any individual cell
varies approximately from five to twenty-five.
When ciliated epithelial cells, freshly removed
from the mucous membrane and moistened with
normal saline, are examined with the microscope,
it will be found that the cilia are in continuous
Fig 4 s-— Ciliated Epi- anc' rapid vibratile movement, so much so that
thelium. the individual cilium can not be distinguished.

In time, however, their vitality declines and the
rapidity of movement diminishes. When the movement of the in-
dividual cilium falls to about eight or ten per second, its character can
be readily determined. It will then be seen that the movement is, as a
rule, alternately a backward and a forward one, the cilium lowering
and then raising itself, the latter taking place more quickly and ener-
getically than the former. As the cilium raises itself it becomes some-
what flexed in a direction corresponding to that of the general move-
ment. The movement, however, varies in character in different
situations and in different animals. The cause of the movements and
the mechanism of their coordination are unknown. They are, as far
as known, independent of the nerve system. The force of ciliary
motion is very great. A load of twenty grams can be supported and
carried forward by the cilia on the mucous membrane of the mouth
and esophagus of the frog. The activity of the cilia is associated
with the nutrition of the cell of which they are a part and rises and
falls with it. Experimentally it has been found that the rate and
energy of the movement are greatest at a temperature of about 35 ° to
40 ° C., especially if they arc bathed with normal saline, rendered
slightly alkaline. Low temperatures, acids, alkalies, carbon dioxid,
etc., retard the movement.

The function of the cilia, though not always apparent, is asso-
ciated with the function of the passages in which they are found. As

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 95

the surfaces of these passages are swept by a current of considerable
power, it is probable that they assist in the passage of the materials
which ordinarily traverse them. Mucus and particles of dust are
carried upward through the air-passages; the ovarian cell is carried
from the ovary toward the uterus; the spermatozoa, as well as the
fluid in which they are contained, are carried forward through the
epididymis ducts.

CHAPTER VIII.

THE GENERAL PHYSIOLOGY OF NERVE-TISSUE.

The nerve-tissue, which unites and coordinates the various
organs and tissues of the body and brings the individual into relation-
ship with the external world, is arranged in two systems, termed
the encephalo spinal or cerebrospinal and the sympathetic.

The encephalospinal system consists of:
i. The brain and spinal cord, contained within the cavities of the

cranium and the spinal column respectively, and
2. The cranial and spinal nerves.

The sympathetic system consists of:
i. A double chain of ganglia situated on each side of the spinal column
and extending from the base of the skull to the tip of the coccyx.
2. Various collections of ganglia situated in the head, face, thorax,
abdomen, and pelvis. All these ganglia are united by an elab-
orate system of intercommunicating nerves, many of which are
connected with the cerebrospinal system.

HISTOLOGY OF NERVE-TISSUE.

The Neuron. — The nerve-tissue has been resolved by the in-
vestigations of modern histologists into a single morphologic unit, to
which the term neuron has been applied. The entire nerve system
has been shown to be but an aggregate of an infinite number of neurons,
each of which is histologically distinct and independent. Though
having a common origin, as shown by embryologic investigations,
they have acquired a variety of forms in different parts of the nervous
system in the course of development. The old conception that the
nerve system consisted of two distinct histologic elements, nerve-
cells and nerve-fibers, which differed not only in their mode of origin,
but also in their properties, their relation to each other, and their
functions, has been entirely disproved.

The neuron, or neurologic unit, is histologically a nerve-cell, the
surface of which presents a greater or less number of processes in
varying degrees of differentiation. As represented in Figure 46, A, the
neuron may be said to consist of: (1) The nerve-cell, neurocyte, or
corpus; (2) the axon, or nerve process; (3) the end-tufts, or terminal
branches. Though these three main histologic features are every-
where recognizable, they exhibit a variety of secondary features in
different situations in accordance with peculiarities of function.

96

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

97

Though the nerve-cell and the nerve-fiber are but part of the same
neuron, it is convenient at present to describe them separately.

The Nerve-cell. — The nerve-cell, or body of the neuron, presents
a variety of shapes and sizes in different portions of the nervous
system. Originally ovoid in shape, it has acquired, in course of de-
velopment, peculiarities of form which are described as pyramidal,
stellate, pear-shaped, spindle-shaped, etc. The size of the cell varies
considerably, the smallest having a diameter of not more than 3 0\ 0 of an
inch, the largest not more than T^ of an inch. Each cell consists of
granular, striated protoplasm, containing a distinct vesicular nucleus

Dendrites

Nerve-cell.

Nerve process
or axon.

Neurilemma.

Medulla

Neurilemma.

.Nerve-cell.

Fig. 46. — A. Efferent neuron. B. Afferent neuron.

and a well-defined nucleolus. A cell membrane has not been observed.
From the surface of the adult cell portions of the protoplasm are pro-
jected in various directions, which portions, rapidly dividing and sub-
dividing, form a series of branches, termed dendrites or dendrons.
In some situations the ultimate branches of the dendrites present short
lateral processes, known as lateral buds, or gemmules, which impart to
the branches a feathery appearance. This characteristic is common
to the cells of the cortex of the cerebrum and of the cerebellum. The
ultimate branches of the dendrites, though forming an intricate felt-
work, never anastomose with one another nor unite with dendrites
of adjoining cells. According to the number of axons, nerve-cells are
classified as monaxonic, diaxonic, polyaxonic. Most of the cells of
7

9S TEXT-BOOK OF PHYSIOLOGY.

the nervous system of the higher vertebrates are monaxonic. In the
ganglia of the posterior or dorsal roots of the spinal and cranial nerves,
however, they are diaxonic. In this situation the axons, emerging
from opposite poles of the cell, either remain separate and pursue oppo-
site directions, or unite to form a common stem, which subsequently
divides into two branches, which then pursue opposite directions.
(See Fig. 46, B.) The nerve-cell maintains its own nutrition, and
presides over that of the dendrites and the axon as well. If the latter
be separated in any part of its course from the cell, it speedily degener-
ates and dies.

The axon, or nerve process, arises from a cone-shaped projection
from the surface of the cell, and is the first outgrowth from its pro-
toplasm. At a short distance from its origin it becomes markedly
differentiated from the dendrites which subsequently develop. It
is characterized by a sharp, regular outline, a uniform diameter, and
a hyaline appearance. In structure, the axon appears to consist of
fine fibrillae embedded in a clear, protoplasmic substance. Shafer
advocates the view that the fibrillae are exceedingly fine tubes filled
with fluid. The axon varies in length from a few millimeters to one
meter. In the former instance the axon, at a short distance from its
origin, divides into a number of branches, which form an intricate
feltwork in the neighborhood of the cell. In the latter instance
the axon continues for an indefinite distance as an individual struc-
ture. In its course, however, especially in the brain and spinal cord,
it gives off a number of collateral branches, which possess all its his-
tologic features. The long axons serve to bring the body of the cell
into direct relation with peripheral organs, or with more or less re-
mote portions of the nerve system, thus constituting association or
commissural fibers.

The more or less elongated axon becomes invested, as a rule, at a
short distance from the cell with nucleated oblong cells, which subse-
quently become modified and constitute the medullary or myelin
sheath. This is invested by a thin, cellular membrane — the neu-
rilemma. These three structures thus constitute what is known as a
medullated nerve-fiber. In the brain and spinal cord the outer sheath,
however, is frequently absent. In the sympathetic system the myelin
is also frequently absent, though the axon is inclosed by the neurilemma,
thus constituting a non-medullaled nerve-fiber.

The end-tujls or terminal organs are formed by the splitting of the
axon into a number of filaments, which remain independent of one
another and are free from the medullary investment. The histologic
peculiarities of the terminal organs vary in different situations, and in
many instances are quite complex and characteristic. In peripheral
organs, as muscles, glands, blood-vessels, skin, mucous membrane,
the tufts are in direct histologic and physiologic connection with their
cellular elements. In the brain and spinal cord the tufts are in more
or less intimate relation with the dendrites of adjacent neurons.

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 99

The neurons in their totality constitute the neuron or nerve tissue.
From the fact that they are arranged both serially and collaterally into
a regular and connected whole, they collectively constitute a system
known as the neuron or nerve system.

Neurons, moreover, are grouped into more or less completely
organized masses, termed organs, which in accordance with their
locations may for convenience be divided into central and peripheral
organs. The central organs of the nerve system consist of the en-
cephalon or brain and the spinal cord ; the peripheral organs consist of
the cranial nerves, the spinal nerves, the sympathetic ganglia and their
branches.

Nerve-fibers. — The nerve-fibers which constitute by far the
larger part of both the peripheral and central organs of the nerve
system, are simply the axonic processes of neurons with their secon-
dary investments, the myelin and neurilemma; according as they
possess or do not possess the medullary sheath, they may be divided
into two groups — viz., mcdullated and non-medullated fibers.

Medullated Nerve-fibers. — These consist for the most part of
three distinct structures:

1 . An external investing sheath, tubular in shape, termed the neuril-

emma.

2. An intermediate semifluid substance — the medulla or myelin.

3. An internal dark thread — the axis- cylinder.

The neurilemma is a thin, transparent, homogeneous membrane
closely adherent to the medulla. Owing to its colorless appearance,
it can be seen only with difficulty in fresh tissue. When treated
with various reagents, it becomes distinct. Physically, it is quite
resistant and elastic. Its function is doubtless that of a protective
agent to the structures within.

The medulla, myelin, or white substance 0} Schwann completely
fills the neurilemma and closely invests the axis-cylinder or axon. In
fresh tissue the medulla is clear, homogeneous, semifluid, and highly
refracting. When the nerve is treated with various reagents which
alter its composition, the medulla becomes opaque and imparts a
white, glistening appearance. The function of the medulla is quite
unknown.

At intervals of about seventy-five times its diameter the medul-
lated nerve-fiber undergoes a remarkable diminution in size, due to
an interruption of the medullary substance, so that the neurilemma
lies directly on the axis-cylinder. These constrictions, or nodes of
Ranvier, taking their name from their discoverer, occur at regular
intervals along the course of the nerve, separating it into a series of
segments. The portion between the nodes is termed the internodal
segment. It has been suggested that in consequence of the absence
of the myelin at these nodes, a free exchange of nutritive material
and decomposition products can take place between tbe axis-cylinder
and the surrounding plasma. Beneath the neurilemma in each inter-

ioo TEXT-BOOK OF PHYSIOLOGY.

nodal segment there is a large nucleus surrounded by a small amount
of granular protoplasm.

The axis-cylinder, or axon, the direct outgrowth of the nerve-cell,
is the most essential element of the nerve-fiber, as it alone is uni-
formly continuous throughout. In the natural condition it is trans-
parent and invisible; but when treated with proper reagents, it presents
itself as a pale, granular, flattened band, more or less solid and some-
what elastic. It is albuminous in composition. With high magnifi-
cation the axis presents a longitudinal striation, indicating a fibrillar
structure. The fibrillar appear to be embedded in an intervening
semifluid substance, the neuroplasm.

Non-Medullated Nerve-fibers. — These consist, for the most
part, only of the axis-cylinder, though in some portions of the nerve
system a neurilemma is also present. Though much less abundant
than the former variety, they are distributed largely throughout the
nerve system, but are particularly abundant in the sympathetic.
Owing to the absence of a medulla, they present a rather pale or
grayish appearance.

Sympathetic Ganglia. — A sympathetic ganglion consists essen-
tially of a connective-tissue capsule with an interior framework.
The meshes of this framework contain nerve-cells provided with
dendrites and branching axons. The majority of the axons are non-
medullated. In all instances, with the exception of the ganglion cells
of the heart, the axons are distributed to non-striated muscle tissue
and to the epithelium of glands.

The nerve-cells of the ganglia are also in histologic connection
with the terminal branches of fine medullated nerve-fibers which
leave the spinal cord by way of the anterior roots of the spinal nerves.
These nerve-fibers are designated autonomic or pre- ganglionic fibers,
while those emerging from the cells are designated p ost- ganglionic
fibers. (See Sympathetic System.)

The Peripheral Organs of the Nerve System. — These consist
of the cranial and spinal nerves and the sympathetic ganglia. Each
nerve consists of a variable number of nerve-fibers united into firm
bundles by connective tissue which supports blood-vessels and lym-
phatics. The bundles are technically known as nerve-trunks or nerves.

The nerve-trunks connect the brain and cord with all the re-
maining structures of the body. Each nerve is invested by a thick
layer of lamellated connective tissue, known as the epineurium.
A transverse section of a nerve shows (see Fig. 47) that it is made
up of a number of small bundles of libers, each of which possesses
a separate investment of connective tissue — the perineurium. With-
in this membrane the nerve-fibers are supported by a fine stroma—
the endoneurium. After pursuing a longer or shorter course, the
nerve-trunk gives off branches, which interlace very freely with neigh-
boring branches, forming plexuses, the fibers of which are distributed
to associated organs and regions of the body. From their origin to

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 101

their termination, however, nerve-fibers retain their individuality, and
never become blended with adjoining fibers.

As nerves pass from their origin to their peripheral terminations,
they give off a number of branches, each of which becomes invested
with a lamellated sheath— an offshoot from that investing the parent
trunk. This division of nerve-bundles and sheath continues through-
out all the branchings down to the ultimate nerve-fibers, each of
which is surrounded by a sheath of its own, consisting of a single
layer of endothelial cells. This delicate transparent membrane, the
sheath of Henle, is separated from the nerve-fiber by a considerable

^%ii^5>

Fig. 47. — Transverse Section of a Nerve (Median), ep. Epineurium.
pe. Perineurium, ed. Endoneurium. — (Landois and Stirling.)

space, in which is contained lymph destined for the nutrition of the
fiber. Near their ultimate terminations the nerve-fibers themselves
undergo division, so that a single fiber may give origin to a number
of branches, each of which contains a portion of the parent axis-
cylinder and myelin.

Blood-supply. — Nerves being parts of living cells require for
the maintenance of their nutrition a certain amount of blood. This
is furnished by the blood-vessels ramifying in and supported by the
connective-tissue framework. Here as elsewhere there is a constant
exchange, through the capillary wall and the neurilemma, of nutritive
material to the nerve proper and of waste materials to the blood.

The Chemic Composition and Metabolism. — Chemic analysis
of nerve-tissue has shown the. presence of water, proteins (two glob-
ulins and a nucleo-protein), neurokeratin and nuclein, two phos-
phorized bodies (protagon and lecithin), several cerebrosides (nitro-

102 TEXT-BOOK OF PHYSIOLOGY.

gen-holding bodies of a glucoside character, as shown by their yielding
the reducing carbohydrate galactose), inorganic salts, and a series of
nitrogen-holding bodies such as creatin, xanthin, urea, leucin, etc. As
to the metabolism that is taking place in nerve-cells and fibers, practic-
ally nothing definite is known. That such changes, however, are
taking place would be indicated first by the blood-supply, and second
by the fact that withdrawal of the blood-supply is followed by a loss
of irritability. The metabolism of the central organs of the nerve sys-
tem is more active and extensive. In this situation any withdrawal
of blood from compression or occlusion of blood-vessels is followed
by impairment of nutrition and loss of function.

THE RELATION OF THE PERIPHERAL ORGANS OF THE NERVE
SYSTEM TO THE CENTRAL ORGANS.

Spinal Nerves. — The nerves in connection with the spinal cord
are thirty-one in number on each side. If traced toward the spinal
column, it will be found that the nerve-trunk passes through an in-
tervertebral foramen. Near the outer limits of the foramina each
nerve-trunk divides into two branches, generally termed roots, one
of which, curving slightly forward and upward, enters the spinal
cord on its anterior or ventral surface, while the other, curving back-
ward and upward, enters the spinal cord on its posterior or dorsal
surface. The former is termed the anterior or ventral root ; the latter,
the posterior or dorsal root. Each dorsal root presents near its union
with the ventral root a small ovoid grayish enlargement known as
a ganglion. Both roots previous to entering the cord subdivide into
from four to six fasciculi.

A microscopic examination of a cross-section of the spinal cord
shows that the fibers of the ventral roots can be traced directly into
the body of the nerve-cells in the ventral horns of the gray matter.
The fibers of the dorsal roots are not so easily traced, for they diverge
in several directions shortly after entering the cord. In their course
they give off collateral branches which, in common with the main
fiber, end in tufts which become associated with nerve-cells in both
the ventral and dorsal horns of the gray matter.

Cranial Nerves. — The nerves in connection with the base of the
brain are known as cranial nerves; some of these nerves present a
similar ganglionic enlargement, and therefore may be regarded as
dorsal nerves, while others may be regarded as ventral nerves. Their
relations within the medulla oblongata are similar to those within
the spinal cord.

Efferent and Afferent Nerves. — Nerves are channels of com-
munication between the brain and spinal cord, on the one hand, and
the muscles, glands, blood-vessels, skin, mucous membrane, viscera,
etc., on the other. Some of the nerve-fibers serve for the transmission
of nerve energy from the brain and spinal cord to certain peripheral

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

103

organs, and so increase or retard their activities; others serve for the
transmission of nerve energy from certain peripheral organs to the
brain and spinal cord which gives rise to sensation or other modes of
nerve activity. The former are termed efferent or centrifugal, the
latter afferent or centripetal nerves. Experimentally it has been de-
termined that the anterior or ventral roots contain all the efferent
fibers, the posterior or dorsal roots all the afferent fibers.

THE PERIPHERAL ENDINGS OF NERVES.

The efferent nerves as they approach their ultimate terminations
lose both the neurilemma and myelin sheath. The axon or axis-
cylinder then divides into a number of branches which become directly

Motor plate.

Fig. 48. — Motor Nerve-endings of Intercostal Muscle-fibers of a Rabbit.

X 150. — (Stohr.)

and intimately associated with tissue-cells. The particular mode of
termination varies in different situations. These terminations are
generally spoken of as end-organs, terminal organs, or end-tufts.

In the skeletal muscle the nerve-fiber loses both neurilemma and
myelin sheath at the point where it comes in contact with the muscle-
fiber. After penetrating the sarcolemma, the axon or axis-cylinder
divides into a number of small branches which appear to be embedded
in a relatively large mass of sarcoplasm and nuclei, the whole form-
ing the so-called "motor plate." Each muscle-fiber possesses one
such plate or end-organ in mammalia, several in the frog. (Fig. 48.)

In the visceral muscle the terminal nerve-fibers derived from
sympathetic or peripheral neurons are primarily non-medullated.
The axons divide and subdivide and form plexuses which surround
the muscle-cell bundles. Fine fibers from the plexuses are given off

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TEXT-BOOK OF PHYSIOLOGY.

Fig. 49. — Terminations of Nerve-
fibers in the Gland-cells. A. Cell
of the parotid gland of a rabbit. B.
Cells of the mammary gland of a cat in
gestation. — (Doyon and Morat.)

which ultimately come into relation with each individual cell, on the
surface of which they terminate in the form of one or more granular
masses.

In the glands, taking as an illustration the parotid and mammary
glands, the nerve-fibers, also derived from sympathetic or peripheral
neurons, pass into the body of the gland and ultimately reach the
acini, on the outer surface of which they ramify and form a plexus.
From this plexus fine fibers penetrate the acinus wall and end on
the gland-cell. The fibers present a varicose appearance (Fig. 49).
The afferent nerves as they approach their ultimate terminations
undergo similar changes. The end-tufts become associated, in some
situations, with specialized end-organs which are extremely complex;

e. g., the retina in the eye, the
organ of Corti in the ear, the taste-
beakers in the tongue, the olfactory
cells in the nose.

In the skin and mucous mem-
branes the mode of termination
varies considerably. The follow-
ing are some of the principal
modes:

1. Free endings in the epithelium.

2. Tactile cells of Merkel.

3. Tactile corpuscles in the papillae of the true skin.

4. Pacinian corpuscles found attached to the nerves of the hand and

feet, to the intercostal nerves, and to nerves in other situations.

5. End-bulbs of Krause in the conjunctiva, clitoris, penis, etc.

(A consideration of these end-organs will be found in the chapters
devoted to the organs of which they form a part.)

In the skeletal muscles afferent fibers become associated with small
spindle-shaped structures known as muscle-spindles or neuromuscle
end-organs. These spindles vary in length from 1 mm. to 4 mm.
They consist of a capsule of fibrous tissue containing from five to
twenty muscle-fibers. After penetrating the several layers of the
capsule, the nerve-fibers lose the neurilemma and myelin sheaths.
The axons or axis-cylinders then divide into several long narrow
branches which wind themselves in a spiral manner around the con-
tained muscle-fiber and terminate in small oval-shaped discs. Similar
endings have been observed in the tendons of muscles.

Development and Nutrition of Nerves. — The efferent nerve-
fibers, which constitute some of the cranial nerves and all the ventral
roots of the spinal nerves, have their origin in cells located in the gray
matter beneath the aqueduct of Sylvius, beneath the floor of the fourth
ventricle, and in the anterior horns of the gray matter of the spinal
cord. These cells are the modified descendants of independent, oval,
pear-shaped cells — the neuroblasts — which migrate from the medullary
tube. As they approach the surface of the cord their axons are

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

105

Posterior

Ganglion/

directed toward the ventral surface, which eventually they pierce.
Emerging from the cord, the axons continue to grow, and become
invested with the myelin sheath and neurilemma, thus constituting
the ventral roots.

The afferent nerve-fibers, which constitute some of the cranial
nerves and all the dorsal roots of the spinal nerves, develop outside
of the central nervous system and only subsequently become con-
nected with it. (See Fig. 50.) At the time of the closure of the
medullary tube a band or ridge of epithelial tissue develops near the
dorsal surface, which, becoming segmented, moves outward and forms
the rudimentary spinal gang-
lia. The cells in this situa-
tion develop two axons, one
from each end of the cell,
which pass in opposite direc-
tions, one toward the spinal
cord, the other toward the
periphery. In the adult con-
dition the two axons shift
their position, unite, and form
a T shaped process, after
which a division into two
branches again takes place.
In the ganglia of all the
sensori-cranial and sensori-
spinal nerves the cells have
this histologic peculiarity.

The efferent fibers are
therefore to be regarded as
outgrowths from the nerve-
cells in the ventral horns of the gray matter, and serve to bring the
cells into anatomic and physiologic relationship directly with the
skeletal muscles and indirectly, through the intermediation of ganglia
(see sympathetic nervous system), with visceral muscles and glands.

The afferent fibers are to be regarded as outgrowths from the
cells of the dorsal nerve ganglia, and serve to bring the skin, mucous
membrane, and certain visceral structures into relation with special-
ized centers in the central nerve system.

Nerve Degeneration. — If any one of the cranial or spinal nerves
be divided in any portion of its course, the part in connection with
the periphery in a 'short time exhibits certain structural changes, to
which the term degeneration is applied. The portion in connection
with the brain or cord retains its normal condition with the exception
of a few millimeters at its peripheral end. The degenerative process
begins simultaneously throughout the entire course of the nerve, and
consists in a disintegration and reduction of the myelin and axis-
cylinder into nuclei, drops of myelin, and fat, which in time disappear

ntercor
£oot

Fig. 50. — Diagram Showing the Mode
of Origin of the Ventral and Dorsal
Roots.— (Edinger, ajter His)

io6

TEXT-BOOK OF PHYSIOLOGY.

through absorption, leaving the neurilemma intact. Coincident with
these structural changes there is a progressive alteration and diminu-
tion in the excitability of the nerve. Inasmuch as the central portion
of the nerve, which retains its connection with the nerve-cell, remains
histologically normal, it has been assumed that the nerve-cells exert
over the entire course of the nerve-fibers a nutritive or a trophic in-
fluence. This idea has been greatly strengthened since the discovery
that the axis-cylinder, or the axon, has its origin in and is a direct
outgrowth of the cell. When separated from the parent cell, the fiber
appears to be incapable in itself of maintaining its nutrition.

The relation of the nerve-cells to the nerve-fibers, in reference to
their nutrition, is demonstrated by the results which follow section
of the ventral and dorsal roots of the spinal nerves. If the anterior

Fig. 51. — Degeneration of Spinal Nerves and Nerve-roots after Section.
A. Section of nerve-trunk beyond the ganglion. B. Section of anterior root. C. Sec-
tion of posterior root. D. Excision of ganglion, a. Anterior root. p. Posterior root.
g. Ganglion. — (Dalton.)

root alone be divided, the degenerative process is confined to the
peripheral portion, the central portion remaining normal. If the
posterior root be divided on the peripheral side of the ganglion, de-
generation takes place only in the peripheral portion of the nerve.
(See Fig. 51.) If the root be divided between the ganglion and the
cord, degeneration takes place only in the central portion of the root.
From these facts it is evident that the trophic centers for the ventral
and dorsal roots lie in the spinal cord and spinal nerve ganglia, re-
spectively, or, in other words, in the cells of which they are an integral
part. The structural changes which nerves undergo after separation
from their centers are degenerative in character, and the process is
usually spoken of, after its discoverer, as the Wallerian degeneration.
When the nerve-cells from which the nerve-fibers arise, whether
efferent or afferent, undergo degeneration from any cause whatever, the
nerve-fiber becomes involved in the degenerative process and when it is
completed the structures to which they are distributed, especially
the muscles, undergo an atrophic or fatty degeneration, with a change
or loss of their irritability. This is, apparently, not to be attributed
merely to inactivity, but rather to a loss of nerve influences, inasmuch
as inactivity merely leads to atrophy and not to degeneration.

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 107

Reunion and Regeneration. — When a nerve-trunk is divided
there is a loss of function of the parts to which it is distributed, and
usually involves both motion and sensation. This, however, is not
necessarily permanent, for after a variable period of time it not in-
frequently happens, that the functions are restored because of a reunion
of the separated ends and a regeneration of the peripheral portion.
A histologic study of the nerve-fibers after separation from the nerve-
cells shows that coincidently with the degenerative process there
occurs a regenerative process, consisting in a multiplication of the nuclei
lying just beneath the neurilemma and an accumulation around them
of a granular protoplasm which in due time completely fill the neuril-
emma. At this stage the fiber is known as a band-fiber. If now the
physical conditions are such as to permit of a reunion of the nerve,
this takes place, and under the nutritive influence of the cell the axis-
cylinder grows into the band-fiber and the protoplasm becomes trans-
formed into myelin as in the original fiber. The axis-cylinder con-
tinues to grow and extend itself forward until it reaches its ultimate
termination.

CLASSIFICATION OF NERVES.

The efferent nerves may be classified, in accordance with the
characteristic form of activity to which they give rise, into several
groups, as follows:

1. Skeletal-muscle or motor nerves, those which convey nerve energy

or nerve impulses to skeletal-muscles and excite them to activity.

2. Gland or secretor nerves, those which convey nerve impulses to

glands, and cause the formation and discharge of the secretion
peculiar to the gland.

3. Vascular or vaso-motor nerves, those which convey nerve impulses

to the muscle-fibers of the blood-vessels and change in one direction
or the other the degree of their natural contraction. Those which
increase the contraction are known as vaso-constrictors or vaso-
augmentors; those which decrease the contraction are known
as vaso-dilatators or vaso-inhibitors. The nerves which pass
to that specialized part of the vascular apparatus, the heart,
transmit nerve impulses which on the one hand accelerate its
rate or augment its force, and on the other hand inhibit or retard
its rate and diminish its force. For this reason they are termed
cardio-motor nerves, one set of which is known as accelerator
and augmentor, the other as inhibitor nerves.

4. Visceral or viscero-motor nerves, those which transmit nerve im-

pulses to the muscle walls of the viscera and change in one direc-
tion or another the degree of their contraction. Those Avhich
increase or augment the contraction are known as viscero-aug-
mentor, while those which decrease or inhibit the contraction,
are known as viscero-inhibitor nerves.

108 TEXT-BOOK OF PHYSIOLOGY.

5. Hair bulb or pilo-motor nerves, those which transmit nerve im-
pulses to the muscle-fibers which cause an erection of the hairs.
Of the foregoing nerves the skeletal-muscle or motor nerves alone
pass directly to the muscle. The gland, the vascular and the visceral
nerves, all terminate at a variable distance from the peripheral organ
around a local sympathetic ganglion, which in turn is connected with
the peripheral organ. The former are termed pre-ganglionic. The
latter post-ganglionic fibers. (See Fig. 61.)

The afferent nerves may also be classified, in accordance with
their distribution and the character of the sensations or other modes
of nerve activity to which they give rise, into several groups, as fol-
lows:

1. Tegumentary nerves, comprising those distributed to skin, mucous
membranes and sense organs and which transmit nerve impulses
from the periphery to the nerve centers. They may be divided
into sensorifacient and reflex nerves.

A. Sensorifacient nerves, those which transmit nerve impulses
to the brain where they give rise to conscious sensations.
They may be subdivided into:

1. Nerves of special sense — e. g., olfactory, optic, auditory,
gustatory, tactile, thermal, pain, pressure — which give
rise to correspondingly named sensations.

2. Nerves of general sense — e. g., the visceral afferent
nerves — those which give rise normally to vague and
scarcely perceptible sensations, such as the general sen-
sations of well-being or discomfort, hunger, thirst, fatigue,
sex, want of air, etc.

B. Reflex nerves, those which transmit nerve impulses to the
spinal cord and medulla oblongata, where they give rise to
different modes of nerve activity. They may be divided into :

1. Reflex excitator nerves, which transmit nerve impulses
which cause an excitation of nerve centers and in conse-
quence increased activity of peripheral organs, e. g.,
skeletal muscles, glands, blood-vessels and viscera.

2. Reflex inhibitor nerves, which transmit nerve impulses
which cause an inhibition of nerve centers and in conse-
quence, decreased activity of the peripheral organs. It
is quite probable that one and the same nerve may sub-
serve both sensation and reflex action, owing to the col-
lateral branches which are given off from the afferent
roots as they ascend the posterior column of the cord.

Muscle nerves, comprising those distributed to muscles and
tendons and which transmit nerve impulses from muscle and
tendons to the brain where they give rise to the so-called muscle
sensations, e. g., the direction and the duration of a movement,
the resistance offered and the posture of the body or of its indi-
vidual parts.

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 109

PHYSIOLOGIC PROPERTIES OF NERVES.

Nerve Irritability or Excitability and Conductivity. — These
terms are employed to express that condition of a nerve which enables
it to develop and to conduct nerve impulses from the center to the
periphery, or from the periphery to the center, in response to the action
of stimuli. A nerve is said to be excitable or irritable so long as it
possesses these capabilities or properties. For the manifestation of
these properties the nerve must retain a state of physical and chemic
integrity; it must undergo no change in structure or chemic composi-
tion. The irritability of an efferent nerve is demonstrated by the
contraction of a muscle, by the secretion of a gland, or by a change
in the caliber of a blood-vessel, whenever a corresponding nerve is
stimulated. The irritability of an afferent nerve is demonstrated by
the production of a sensation or a reflex action whenever it is stimu-
lated. The irritability of nerves continues for a certain period of
time after separation from the nerve-centers and even after the death
of the animal, the time varying in different classes of animals. In
the warm-blooded animals, in which the nutritive changes take place
with great rapidity, the irritability soon disappears — a result due to
disintegrative changes in the nerve, caused by the withdrawal of the
blood-supply and other non-physiologic conditions. In cold-blooded
animals, on the contrary, in which the nutritive changes take place
relatively slowly, the irritability lasts, under favorable conditions, for
a considerable time. Other tissues besides nerves possess irritability,
that is, the property of responding to the action of stimuli — e. g.,
glands and muscles, which respond by the production of a secretion
or a contraction.

Independence of Tissue Irritability. — The irritability of nerves
is distinct and independent of the irritability of muscles and glands,
as shown by the fact that it persists in each a variable length of time
after their histologic connections have been impaired or destroyed by
the introduction of various chemic agents into the circulation. Curara,
for example, induces a state of complete paralysis by modifying or
depressing the conductivity of the end-organs of the nerves just where
they come in contact with the muscles, without impairing the irrita-
bility of either nerve-trunks or muscles. Atropin induces complete
suspension of gland activity by impairing the terminal organs of the
secretor nerves just where they come into relation with the gland-
cells, without destroying the irritability of either gland-cell or nerve.

Nerve Stimuli. — Nerves do not possess the power of spontaneously
generating and propagating nerve impulses; they can be aroused to
activity only by the action of an external stimulus. In the physiologic
condition the stimuli capable of throwing the nerve into an active
condition act for the most part on either the central or peripheral
end of the nerve. In the case of motor nerves the stimulus to the
excitation, originating in some molecular disturbance in the nerve-

no TEXT-BOOK OF PHYSIOLOGY.

cells, acts upon the nerve-fibers in connection with them. In the
case of sensor or afferent nerves the stimuli act upon the peculiar
end-organs with which the sensor nerves are in connection, which in
turn excite the nerve-fibers. Experimentally, it can be demonstrated
that nerves can be excited by a sufficiently powerful stimulus applied
in any part of their extent.

Nerves respond to stimulation according to their habitual function;
thus, stimulation of a sensor nerve, if sufficiently strong, results in
the sensation of pain; of the optic nerve, in the sensation of light;
of a motor nerve, in contraction of the muscle to which it is distrib-
uted; of a secretor nerve, in the activity of the related gland, etc.
It is, therefore, evident that peculiarity of nerve function depends
neither upon any special construction or activity of the nerve itself
nor upon the nature of the stimulus, but entirely upon the pecul-
iarities of its central and peripheral end-organs.

Nerve stimuli may be divided into —
i. General stimuli, comprising those agents which are capable of

exciting a nerve in any part of its course.
2. Special stimuli, comprising those agents which act upon nerves
only through the intermediation of the end-organs.

The end-organs are specialized highly irritable structures placed
between the nerve-fibers and the surface. They are especially adapted
for the reception of special stimuli and for the liberation of energy,
which in turn excites the nerve-fiber to activity.

General stimuli:
i. Mechanic: Sharp taps, sudden pressure, cutting, etc.

2. Thermic: Sudden application of heated object.

3. Chemic: Contact of various substances which alter their chemic

composition quickly, e. g., strong acids or alkalies, sol. sodium
chlorid 15 per cent., sugar, urea, etc.

4. Electric: Either the constant or induced current.
Special stimuli:

For afferent nerves —

1. Light or ethereal vibrations acting upon the end-organs of the

optic nerve in the retina.

2. Sound or atmospheric undulations acting upon the end-organs of

the auditory nerve.

3. Heat or vibrations of the air acting upon the end-organs in the skin.

4. Chemic agencies acting upon the end-organs of the olfactory and

gustatory nerves.

For efferent nerves —

A molecular disturbance in the central nerve-cells from which
they arise, the nature of which is unknown.

Nature of the Nerve Impulse. — As to the nature of the nerve
impulse generated by any of the foregoing stimuli, either general or
special, but little is known. It has been supposed to partake of the
nature of a molecular disturbance, a combination of physical and

GENERAL PHYSIOLOGY OF NERVE-TISSUE. in

chemic processes attended by the liberation of energy, which propag-
ates itself from molecule to molecule. The passage of the nerve
impulse is accompanied by changes of electric tension, the extent of
which is an indication of the intensity of the molecular disturbance.
Judging from the deflections of the galvanometer needle it is probable
that when the nerve impulse makes its appearance at any given point
it is at first feeble, but soon reaches a maximum development, after
which it speedily declines and disappears. It may, therefore, be
graphically represented as a wave-like movement with a definite
length and time duration. Under strictly physiologic conditions the
nerve impulse passes in one direction only; in efferent nerves from
the center to the periphery, in afferent nerves from the periphery to
the center. Experimentally, however, it can be demonstrated that
when a nerve impulse is aroused in the course of a nerve by an ade-
quate stimulus it travels equally well in both directions from the point
of stimulation. When once started, the impulse is confined to the
single fiber and does not diffuse itself to fibers adjacent to it in the
same nerve-trunk.

Rapidity of Conduction of the Nerve Impulse. — The passage
of a nerve impulse, either from the brain to the periphery or in the
reverse direction, requires an appreciable period of time. The
velocity with which the impulse travels in human sensory nerves has
been estimated at about 50 meters a second, and for motor nerves at
from 28 to 33 meters a second. The rate of movement is, however,
somewhat modified by temperature, cold lessening and heat increasing
the rapidity; it is also modified by electric conditions, by the action
of drugs, the strength of the stimulus, etc. The rate of transmission
through the spinal cord is considerably slower than in nerves, the
average velocity for voluntary motor impulses being only n meters
a second, for sensory impulses 12 meters, and for tactile impulses 40
meters a second.

Nerve Fatigue. — Inasmuch as nerves are parts of living cells,
the seat of nutritive changes, it might be supposed that the passage of
nerve impulses would be attended by the disruption of energy-holding
compounds, the production of waste products, the liberation of heat,
and in time by the phenomena of fatigue. Though it is probable that
changes of this character occur, yet no reliable experimental data
have been obtained which afford a clue as to the nature or extent of
any such changes. Stimulation of motor nerves with the induced
electric current for four hours appears to be without influence either
on the intensity of the nerve impulse or the rate of its conduction.

Identity of Efferent and Afferent Nerves and Nerve Impulses.
— Notwithstanding the classification of nerve-fibers based on differ-
ences of physiologic actions, there are no characters, either histologic
or chemic, which serve to distinguish them from one another. More-
over, as the nerve impulse is conducted through a nerve-fiber equally
well in both directions, as determined by experiments, it is probable

TEXT-BOOK OF PHYSIOLOGY.

that it does not differ in character in the two classes of nerves. That
the efferent fibers conduct the nerve impulses from the nerve-centers
to the periphery, and the afferent nerves from the periphery to the
centers, is because of the fact that they receive their stimulus physio-
logically only in the centers or at the periphery. The fundamental
reason for difference of effects produced by stimulation of different
nerves is the character of the organ to which the nerve impulse is
conducted. A nerve is merely the transmitter of the nerve impulse,
which if conducted to a muscle excites contraction; to a gland, secre-
tion; to a blood-vessel, variation in caliber; to
special areas in the brain, sensations of light,
sound, pain, etc.

Electric Excitation of Nerves. — For the pur-
pose of studying the physiologic activities of nerves
it has been found convenient to employ the nerve-
muscle preparation (the gastrocnemius muscle and
sciatic nerve) and to use as a stimulus the induced
electric current. (See Fig. 52.) When kept moist,
this preparation is extremely sensitive to either the
galvanic or the induced current.

Though the development and conduction of a
nerve impulse may be demonstrated by the deflec-
tion of the galvanometer needle or the movement
of the mercury in the capillary electrometer, it is
more conveniently demonstrated by the contraction
of a muscle, the vigor of which, within limits, may
be taken as a measure of the intensity of the im-
pulse. The preparation should be enclosed in a
moist chamber and the nerve connected with the inductorium through
the intervention of non-polarizable electrodes. The muscle may be
attached to the muscle-lever and its contractions recorded.

A single shock of an induced current develops, it is believed,
a single nerve impulse followed by a single muscle contraction. A
minimal contraction following a minimal electric stimulus presupposes
the development of a nerve impulse of low intensity. Within certain
limits a maximal contraction following a maximal electric stimulus
presupposes the development of a nerve impulse of high intensity.
Intermediate contractions indicate nerve impulses of corresponding
intensity.

Tetanization of a muscle indicates that the nerve impulses arrive
at the muscle with a frequency so great that the muscle does not
succeed in relaxing from the effect of one stimulus before the next
arrives. Complete as well as incomplete tetanus may be developed
by gradually increasing the frequency of the stimulus. The character
of the contraction caused by indirect stimulation — i. e., through the
nerve — does not differ in any essential respect from that due to direct
stimulation.

Fig. 52. — Nerve-
muscle Prepara-
tion oe a Frog. F.
Femur. S. Sciatic
nerve. I. Tendo
AchilHs. — (Landois
and Stirling.)

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

ELECTRIC PHENOMENA OF NERVES.

Electric Currents from Injured Nerves. — It was discovered
by du Bois-Reymond that electric currents can be obtained from
nerves as well as from muscles, and that the electric properties of
the former correspond in most respects to those of the latter. The
laws governing the development and mode of action of the currents
derived from muscles are equally applicable to the currents derived
from nerves.

A nerve-cylinder obtained by making two transverse sections of
any given nerve presents, as in the case of muscles, a natural and
two artificial transverse surfaces. A line
drawn around the cylinder at a point
lying midway between the two end sur-
faces constitutes the equator. From
such a cylinder strong currents are ob-
tained when the natural longitudinal
surface and the transverse surface are
connected with the electrodes of the
galvanometer circuit. The strength of
the current thus obtained will diminish
or increase according as the electrode on
the longitudinal surface is removed from
or brought near to the equator. If two
symmetric points on the longitudinal sur-
face equidistant from the equator are
united, no current is obtainable. When
asymmetric points on the longitudinal
surface are connected, weak currents are
obtained, in which case the point lying
nearer the equator becomes positive to
the point more distant, which becomes
negative. From these facts it is evident
that all points on the longitudinal surface
are electrically positive to the transverse
surface and that the point of greatest
positive tension is situated near the
equator (Fig. 53).

The electromotive force of the nerve current varies in strength
with the length and thickness of the nerve. The strongest current
obtained from the nerve of the frog is equal to the 0.002 of a Daniell
cell; that obtained from the nerve of the rabbit, 0.026 of a Daniell.
The existence of the nerve current, its strength, duration, etc., depend
largely on the maintenance of physiologic conditions. All influences
which impair the nutrition of the nerve diminish the current. With
the death of the nerve all electric phenomena disappear.

Negative Variation of the Nerve Current. — During the pas-

Fig. 53. — Diagram to Illus-
trate the Currents in Nerves.
The arrowheads indicate the direc-
tion; the thickness of the lines in-
dicates the strength of the currents.
— (Landois and Stirling.)

ii4 TEXT-BOOK OF PHYSIOLOGY.

sage of the nerve impulse the resting nerve current, or the demarca-
tion current, diminishes more or less completely in intensity, undergoes
a negative variation, as shown by the return of the galvanometer
needle, due to a change in its electromotive condition or to a diminu-
tion of the difference in potential between the positive longitudinal
and negative transverse sections. This negative variation of the de-
marcation current is observed equally well from either the central or
peripheral end of the nerve. If the two ends of the nerve are con-
nected with galvanometers and the nerve stimulated in the middle,
the demarcation currents simultaneously undergo a negative variation.
This may be taken as a proof that the excitation process propagates
itself equally well in both directions. The negative variation is inti-
mately connected with changes in the molecular condition of the nerve
and is not due to any extraneous electric or other influence. And
du Bois-Reymond was also enabled to obtain a negative variation of
the current in the nerves of a living frog which were yet in connection
with the spinal cord. In this experiment the sciatic nerve was divided
at the knee and freed from its connections up to the spinal column; the
transverse and longitudinal surfaces were then placed in connection
with the electrodes of the galvanometer wires and the current per-
mitted to influence the needle. The animal was then subjected to
the action of strychnin. Upon the appearance of the muscle spasms
the needle was observed to swing backward toward the zero point to
the extent of from i to 4 degrees, and upon the cessation of the spasms
to return to its previous position. In an experiment of this nature
it is obvious that the negative variation was the result of a physiologic
stimulation of the nerve arising within the spinal cord.

The question also here arises as to whether the negative variation
is due to a steady, continuous decrease of the natural current, or
whether it is due to successive and rapidly following variations in its
intensity, similar to that observed in muscles. Though this can not
be demonstrated with the physiologic rheoscope, as was the case with
the muscle, there can be no doubt, both from experimentation and
analogy, that the latter supposition is the correct one. It has been
shown that when non-polarizable electrodes connected with Siemen's
telephone are placed in connection with the longitudinal and trans-
verse sections of a nerve, low, sonorous vibrations are perceived
during tetanic stimulation — a proof that the active state of the nerve
is connected with the production of discontinuous electric currents.
The oscillations of the mercurial column of the capillary electrometer
also reveal similar electric changes. It was also demonstrated by
Bernstein with a specially devised apparatus, the repeating rheotome,
that the negative variation is composed of a large number of single
variations which succeed each other in rapid succession and sum-
marize themselves in their effect on the needle.

Electric Currents from Uninjured Nerves. — The pre-existence
of electric currents in living and wholly uninjured nerves while at

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 115

rest has also been denied by Hermann, who regards all portions of
the nerve as isoelectric, any difference of potential being the result of
some injury to its surface.

Action Currents. — For reasons to be stated below, it is very diffi-
cult to determine the presence of diphasic action currents during the
passage of an excitatory impulse through the nerve-fiber. The so-
called negative variation of the resting nerve current — the demarca-
tion current — which is occasioned by tetanic stimulation, Hermann
regards as the expression of an action current which flows in the nerve
in a direction opposite to the demarcation current. The origin of this
action current is to be sought for in the continuous negativity of that
portion of the longitudinal surface of the nerve in contact with the
diverting electrode, while the dying substance of the transverse surface
takes no part in the excitation. This tetanic action current, or nega-
tive variation, was discovered by du Bois-Reymond, and Bernstein
later succeeded in obtaining this action current during the passage
of a single excitation process. That the return of the galvanometer
needle toward the zero point is not due to an annulment of the demarc-
ation current itself, but to the appearance of an action current, is
shown by the fact that if the former be compensated by a battery
current until the needle rests on the zero point the appearance of the
latter current will cause the needle to swing in a direction the opposite
of that caused by the demarcation current. The negative variation
and action current may therefore be regarded as one and the same
thing. It is the expression of the change the nerve is undergoing
during the passage of the nerve impulse. The rapidity with which
the negative variation or action current travels, the variation in its
intensity from moment to moment, the time required for it to pass
a given point, would express the change in the nerve to which the
term nerve impulse is given. From experiments made with the
differential rheotome, Bernstein calculated that the speed of the
negative variation is about 28 meters a second; that it is at first feeble,
soon rises to a maximum, and then declines; that it requires 0.0006
to 0.0008 of a second to pass a given point. From these data it is
evident that the negative variation or action current has a space value
of 0.0006 of 28 meters or about 18 mm. Transferring these statements
to the nerve impulse, it may be said that it is a molecular disturbance,
traveling at the rate of about 28 meters a second, is wave-like in char-
acter, the wave being 18 millimeters in length, and occupying from
0.0006 to 0.0008 of a second in passing any given point.

Absence of Diphasic Action Currents. — When any two points on
the longitudinal surface which do not exhibit a current are connected
with the galvanometer and a single wave of excitation passes beneath
the electrodes, it might be expected that, as in the case of the muscle,
a diphasic action current would be observed, from the fact that the
portions of the nerve beneath the electrodes become alternately neg-
ative with reference to all the rest of the nerve. This, however, is

n6

TEXT-BOOK OF PHYSIOLOGY.

not the case, the absence of the two opposing phases of the action
current being explained on the supposition that the negativity of the
two led-off points is of equal amount, and that, owing to the great
rapidity with which the excitation wave travels, the two phases fall
together too closely in time to alternately influence the galvanometer
needle. During stimulation of the nerve, when two currentless or iso-
electric points are connected, there is also an absence of the action
current, as was observed first by du Bois-Reymond, and which is to
be explained on similar grounds. It is true that an apparent action
current is sometimes seen when the stimulating current is very power-
ful or the seat of stimulation too near the diverting electrodes. This,
however, must be attributed to an electrotonic state of the nerve.

The Effects of a Galvanic Current on a Nerve. — When a con-
stant galvanic current of medium strength is made to pass through a
portion of a nerve, several distinct effects are produced:

* POLARIZING j
<? CURRENT j

GALVANOME.TER

ANELECTROTONIC
CURRENTS

KATELECTROTONIC
CURRENTS

Fig. 54. — Electrotonic Currents.

1. The development of a nerve impulse at the moment the current
enters and at the moment the current leaves the nerve, i. e., at the
moment the circuit is made and at the moment it is broken. The
development of the nerve impulse is made evident by the contraction
of the muscle if the nerve-muscle preparation be used. If the current
be either very weak, or very strong, the muscle contraction may not
always take place.

2. The development 0} electric currents on each side of the positive
pole or anode, and the negative pole or kathode (see Fig. 54), which
can be led off by means of wires into a galvanometer circuit from
either the artificial transverse and longitudinal surfaces, or from any
two points on the longitudinal surface as shown by the deflection
of the galvanometer needle. The direction of these electric cur-
rents in the nerve coincides with that of the galvanic or "polarizing
current." The "natural nerve currents," the currents of injury or
demarcation currents, as they are variously termed, are at the same
time increased and decreased at opposite extremities of the nerve
according to the direction of the polarizing current.

To this changed condition of the electromotive forces in a nerve

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 117

the term electrotonus was given (du Bois-Reymond). The currents
themselves are known as electrotonic currents; from their relation
to the anode and kathode, they are termed anelectrotonic and kat-
electrotonic currents. The condition of the nerve around the poles
both in the intra-polar and extra-polar regions is known as anelectro-
tonus and katelectrotonus.

The electrotonic currents vary considerably in strength and ex-
tent, according to the intensity of the polarizing current, increasing
steadily with the intensity of the latter up to the point at which the
polarizing current begins to destroy the physical and chemic integrity
of the nerve. The electrotonic currents are strongest in the imme-
diate neighborhood of the electrodes, but gradually diminish in strength
as the distance between the polarized and led-off portions is increased.
The distance to which the electrotonic currents extend along the nerve
will depend very largely upon the strength of the polarizing current,
though it is conditioned by the physical state of the nerve; for if it be
ligated or injured beyond the polarized portion, the electrotonic cur-
rents are abolished. The electrotonic currents have no necessarv
connection with the natural nerve currents, nor are they to be regarded
as branchings of the galvanic current. They are in all probability of
artificial origin, due to an inner positive and negative polarization of
the nerve which extends for a variable distance on each side of the
poles, and due to the action of the polarizing or the galvanic current.

3. An alteration in the excitability and conductivity of the nerve
in the neighborhood of the poles, whereby the results of nerve stimu-
lation— that is, muscle contraction, sensation, and inhibition — are
increased or decreased according to the strength and direction of the
current. To this condition the term electrotonus was also given
(Pfluger). This word has thus been employed to express two distinct
series of effects exhibited by a nerve through a portion of which a con-
stant galvanic current is passing. It appears desirable, for the sake of
clearness, to limit the term electrotonus to the electric or electrotonic
currents which can be led off from either extremity of the nerve, and
to apply to the modifications of irritability which accompany electro-
tonus the expression, electrotonic alteration of excitability and con-
ductivity.

During the passage of the current the excitability of the intra-
polar as well as the extra-polar regions undergoes a change which,
as shown on examination, is found to be diminished in the neigh-
borhood of the anode or positive pole and increased in the neighbor-
hood of the kathode or negative pole. These alterations in the excita-
bility are most marked in the immediate vicinity of the electrodes-;
though they extend for some distance into both the extra-polar and
intra-polar regions, though with gradually diminishing intensity,
until they finally disappear. Between the electrodes there is a point
where the excitability is unchanged and known as the neutral or
indifferent point (Fig. 55). The extent to which the excitability is

nS

TEXT-BOOK OF PHYSIOLOGY.

modified as well as the position of the neutral point will depend largely
on the strength of the polarizing or galvanic current.

The electrotonic alterations of excitability and conductivity can
be experimentally demonstrated on the muscle-nerve preparation in
the following manner:
i. With a descending current of medium strength. Previous to the

N f

Fig. 55. — Sch&me of the Electrotonic Excitability. — (Landois and Stirling.)

closure of the polarizing current, the nerve is stimulated first
in the extra-polar anodic region and the extra-polar kathodic
region with an induction shock of medium intensity and the
height of the contraction recorded. On repeating the stimulation
after closure of the polarizing current the contraction resulting

'Miimmiii,

ANODE

^.REGION OF

J\ INCREASED EXCITABILITY

KATHODE

SECONDARY COIL

Fig. 56. — Diagram Showing the Region of Increased Excitability Caused
j-.y the Passage of a Galvanic Current, Stimulation of which Gtves Rise to In-
creased Contraction.

from .stimulation of the anodic region will be enfeebled or may be
entirely wanting, while the contraction from stimulation of the
kathodic region will be decidedly increased. (See Fig. 56.)
With an ascending current of the same strength. After prelim-
inary testing of the excitability and the subsequent closure of

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

119

the polarizing current, it will be found that stimulation of the
extra-polar anodic region will provoke a much less energetic
contraction or perhaps none at all. Stimulation of the extra-
kathodic region, though of increased excitability, as shown by
the previous experiment, may also fail to provoke a contraction,
owing to the diminished conductivity of the region in the neighbor-
hood of the anode. The impulse on reaching this region is
blocked in its passage. A similar if not more marked decrease
in the conductivity may be developed in the region of the kathode
if the current strength be very great. (See Fig. 57.)
The Law of Contraction; Polar Stimulation. — It was stated
in a previous paragraph that when a galvanic current of medium
strength is made to enter a nerve, and when it is withdrawn from the
nerve, there is a contraction of its related muscle. These are generally

REGION OF
DECREASED EXCITABILITY

Fig. 57. — Diagram Showing the Region of Decreased Excitability Caused by
the Passage of a Galvanic Current, Stimulation of which Gives Rise to De-
creased Contraction.

known as the make and break effects. During the actual passage
of the current no effect is observed so long as its strength remains
uniform. Any sudden variation in the strength of the current at
once arouses the nerve to activity, as shown by a muscle contraction.

The muscle response to the make and break of the constant current
is more or less variable unless the direction of the current as well as
its strength be taken into consideration. If the current is made to
flow from the central toward the peripheral end of the nerve it is
termed a direct, descending, or centrifugal current; if it is made to
flow in the reverse direction, it is termed an indirect, ascending, or
centripetal current. The strength of the current is determined and
regulated by means of a rheocord.

The make and break of currents of different but known strengths
and directions give rise to contractions which occur with more or less
regularitv. The order in which thev occur under these van-ins:

TEXT-BOOK OF PHYSIOLOGY,

conditions of experimentation has been determined and tabulated
as follows by Pfluger, and is termed the law of contraction:

Ascending Current.

Descending Current.

Current Intensity.

Make.

Break.

Make.

Break.

Ccii rraction.
Cc'1 ili action.
Res-

Rest.

Contraction.
Contraction.

Contraction.
Contraction.
Contraction.

Rest.

Contraction.

Rest or weak

contraction.

The results as above tabulated are sometimes complicated on the
opening of the circuit by a series of irregular pulsations of the muscle,
an apparent tetanus, and long known as the opening tetanus of Ritter,
which is attributed to rapid changes in the irritability of the nerve,
in the region of the anode. A similar tetanic contraction of the
muscle is sometimes observed on the closure of the circuit due to
continued excitation in the region of the kathode. This is known
as the closing tetanus of Wundt or of Pfluger. All the phenomena
of the law of contraction were explained by Pfluger on the assumption
that the current stimulates the nerve only at the one electrode, at the
kathode on closing, and at the anode on opening; or, in other words, by
the appearance of katelectrotonus or by the disappearance of anelectro-
tonus, both conditions being attended by a rise of excitability — not,
however, by the opposite changes. It is further assumed that the
appearance of katelectrotonus is more effective as a stimulus than the
disappearance of anelectrotonus. For these reasons the term polar
stimulation is generally employed in discussing the make and break
effects of the galvanic current. The law of contraction may then be
explained as follows: Very feeble currents, either ascending or de-
scending, produce contraction only upon the closure of the circuit, the
sudden increase of the excitability in the katelectrotonic area being
alone sufficient to generate an impulse. The contraction which
follows the closing of the weak ascending current depends upon the
fact that the decrease of excitability and conductivity at the anode is
insufficient to interfere with the conduction of the kathodal stimulus.
Medium currents, either ascending or descending, produce contrac-
tion both on closing and opening the circuit. The appearance of
katelectrotonus and the disappearance of anelectrotonus are both
sufficiently powerful to generate an impulse without, however, seri-
ously impairing the conductivity of the nerve.

Very strong currents produce contraction only upon the opening
of the ascending and closure of the descending currents, or upon the
passage of the excitability in the former from the marked anelectro-
tonic decrease to the normal condition, and in the latter from the
normal to that of katelectrotonic increase. The absence of contraction
upon the closure of the ascending current is dependent upon the

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 121

blocking of the kathodal stimulus by the decrease of the excitability
and conductivity at the anode. With the opening of the descending
current the disappearance of anelectrotonus should also be followed
by contraction, which would indeed be the case if the stimulus so
generated was not blocked by the decrease of the conductivity at the
kathode in consequence of the fall of a high state of katelectrotonus to
the normal condition.

The order in which the contractions occur may be tabulated as
follows :

With Ascending Current. With Descending Current.

Weak, 1. K. C. C* . . K. C. C. .

Medium, 2. K. C. C. A. O. C.f K. C. C. A. O. C.

Strong, 3. .. A. O. C. K. C. C. A. O. C.(?)

Polar Stimulation of Human Nerves. — The preceding state-
ments as to changes in the excitability caused by the passage of a
constant current, as well as to the law of contraction, are based en-
tirely on experiments made with the isolated nerve of the frog. It
is probable, however, that the same phenomena would have been
observed had the nerve of a mammal been used and its excitability
been maintained.

If the electrodes connected with the wires of a sufficiently strong
galvanic battery be applied to the skin over the course of a superficially
lying nerve, e. g., the brachial, it will be found that there occurs on
the closure of the circuit an increase in the excitability in the extra-
polar anelectrotonic region and a decrease in the excitability in the
extra-polar katelectrotonic region, as shown by stimulating the nerve
in the extra-polar regions with the induced current — results which are
in apparent contradiction to those obtained with the isolated nerve.
This want of accordance in the results of the two classes of experi-
ments arises from a failure to recognize the fact that the physiologic
anode and kathode do not coincide with the physical anode and
kathode.

It has been experimentally demonstrated that owing to the large
amount of readily conducting tissue by which the nerve is surrounded,
the current density, though great immediately under the electrode,
quickly decreases at a short distance from it, so that for the nerve it
becomes almost nil. The current, therefore, shortly after entering,
again leaves the nerve at various points which become physiologic
kathodes. Stimulation of this physiologic kathode with the induced
current gives rise, therefore, to the phenomenon of increased excita-
bility in the region of the anode. If, however, the galvanic and
stimulating current be combined in one circuit and both be applied
to the same tract of nerve, results will be obtained which harmonize
with those obtained with the frog's nerve.

The changes in the excitability of a nerve of a living man and the

*K. C. C, kathodal closing contraction. fA. O. C. anodal opening contraction.

122 TEXT-BOOK OF PHYSIOLOGY.

contractions which follow the closing and opening of the constant
current have been thoroughly studied by Waller and de Watteville.
These observers employed a method similar to that of Erb, conjoin-
ing in one circuit the testing and polarizing currents. By the graphic
method they recorded first the contraction produced by an induc-
tion shock alone; and, secondly, the contraction produced by the
same stimulus under the influence of the polarizing current. As a
result of many experiments, they also demonstrated an increase of
the excitability in the polar region when it is cathodic, and a decrease
when it is anodic. Following the suggestion of Helmholtz, that the
current density quickly decreases with the distance from the elec-
trodes, they recognize, at the point of entrance and exit of the current
from the nerve, two regions — a polar, having the same sign as the
electrode, and a peripolar, having the opposite sign (Figs. 58 and 59).

Fig. 58. — Anode of Battery.
Polar region of nerve is anodic. Peri-
polar region of nerve is cathodic.

Fig. 59. — Cathode of Battery.
Polar region of nerve is cathodic. Peri-
polar region of nerve is anodic. — (Walk)-.)

The peripolar regions also experience similar alterations of excita-
bility, though less in degree, according as they are kathodic or anodic.

As it is impossible to confine the current to the trunk of the nerve
when surrounded by living tissues, as is easily the case when experi-
menting with the frog's nerves, it is incorrect to speak of either as-
cending or descending currents. Waller,* who has thoroughly
studied the electrotonic effects of the galvanic current from this point
of view, sums up his conclusions in the following words: "We must
apply one electrode only to the nerve and attend to its effects alone,
completing the circuit through a second electrode, which is applied
according to convenience to some other part of the body.

"Confining our attention to the first electrode, let us see what
will happen according as it is anode or kathode of a galvanic current
(Figs. 58 and 59). If this electrode be the anode of a current, the
latter enters the nerve by a scries of points and leaves it by a second
series of points; the former, or proximal series of points, collectively
constitutes the polar zone or region; the latter, or distal scries of
points, collectively constitutes the peripolar zone or region. In such
case the polar region is the seat of entrance of current into the nerve —

*" Human Physiology," p. 363, 1891.

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 123

i. e., is anodic; the peripolar region is the seat of exit of current from
the nerve — i. e., is kathodic. If, on the contrary, the electrode under
observation be the kathode of a current, the latter enters the nerve
by a series of points which collectively constitute a 'peripolar' region,
and it leaves the nerve by a series of points which collectively con-
stitute a 'polar' region. The current, at its entrance into the body,
diffuses widely, and at its exit it concentrates; its 'density' is greatest
close to the electrode, and, the greater the distance of any point from
the electrode, the less the current density at that point; hence it is
obvious that the current density is greater in the polar than in the
peripolar region. These conditions having been recognized, we may
apply to them the principles learned by study of frogs' nerves under
simpler conditions.

" Seeing that, with either pole of the battery, whether anode or kath-
ode, the nerve has in each case points of entrance (constituting a
collective anode) and points of exit to the current (constituting a col-
lective kathode), and admitting as proved that make excitation is
kathodic, break excitation anodic, we may, with a sufficiently strong
current, expect to obtain a contraction at make and at break with
either anode or kathode applied to the nerve; and we do so, in fact.
When the kathode is applied, and the current is made and broken, we
obtain a kathodic make contraction and a kathodic break contraction;
when the anode is applied, and the current is made and broken, Ave
obtain an anodic make contraction and an anodic break contraction.
These four contractions are, however, of very different strengths; the
kathodic make contraction is by far the strongest; the kathodic break
contraction is by far the weakest; the kathodic make contraction is
stronger than the anodic make contraction; the anodic break con-
traction is stronger than the kathodic break contraction. Or, other-
wise regarded, if, instead of comparing the contractions obtained with
a sufficiently strong current, we observe the order of their appearance
with currents gradually increased from weak to strong, we shall find
that the kathodic make contraction appears first, that the kathodic
break contraction appears last, and the formula of contraction for man
reads as follows:

Weak current, . . .

. K. C. C.

Medium current, .

. K. C. C.

A. C C.

A. 0. C.

Strong current, . .

. K. C. C.

A. C. C.

A. O. C.

K. O. C."

The constant or the galvanic current is frequently used for thera-
peutic and diagnostic purposes. In accordance with the statements
above quoted, one electrode should be applied to the part to be in-
vestigated, the other to some indifferent region. The electrode con-
veying the current to or from this part should be of a size sufficient
to localize the current and to increase its density. It was discovered
by Duchenne that there are certain points all over the body stimula-
tion of which is more quickly followed by muscle contraction than
others. It was subsequently discovered by Remak that these points

124

TEXT-BOOK OF PHYSIOLOGY.

coincide with the entrance of the nerve into the muscle. It is to
these motor points that the one electrode should be applied. The
position of some of these points on the forearm is shown in Fig. 60.
Reactions of Degeneration. — In consequence of the degen-
eration and changes in irritability which occur in nerves when separ-
ated from their centers and in muscles when separated from their
related nerves, either experimentally or as the result of disease, the
response of these structures to the induced, and the make and break
of the constant current, differs from that observed in the physiologic
condition. The facts observed under the application of these two

M. biceps brachii.

brach. anticus.
N. medianus.

pronator teres.
M. flex, digitor. commun. profund.
M. flex, carpi radialis.

M. flex, digitor. sublim.

M. flex. dig. subl. (dig. ind. et min.)
tM. flex. poll,
longus.
N.med
ianus.

N. ulnaris.

M. flexor carpi ulnaris.

N. ulnaris.

Fig. 60. — Motor Points of the Median and Ulnar Nerves, with the Muscles
Supplied by Them. — (Landois and Stirling.)

forms of electricity, are of importance in the diagnosis and thera-
peutics of the precedent lesions. The principal difference of behavior
is observed in the muscles, which exhibit diminished or abolished
excitability to the induced current, while at the same time manifesting
an increased excitability to the constant current; so much so is this the
case that a closing contraction is just as likely to occur at the positive
as at the negative pole. This peculiarity of the muscle response is
termed the reaction 0} degeneration. The synchronous diminished
excitability of the nerves is the same for either current. The term
"partial reaction of degeneration" is used when there is a normal
reaction of the nerves, with the degenerative reaction of the muscles.
This condition is observed in progressive muscular atrophy.

Reflex Action. — Inasmuch as many of the muscle movements of
the body, as well as the formation and discharge of secretions from
glands, variations in the caliber of blood-vessels, inhibition and ac-

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

125

celeration in the activity of various organs, are the result of stimu-
lations of the terminal organs of afferent nerves, they are termed, for
convenience, reflex actions, and, as they take place for the most part
through the spinal cord and medulla oblongata and independently
of the brain or of volitional influences, they are also termed involun-
tary actions. A reflex action of skeletal muscles, glands, or non-
striated muscles of blood-vessels or of viscera, therefore, may be defined
as an action which takes place independent of volition and in response to
peripheral stimulation. As many of the processes to be described in

sp.c

Fig. 61. — Diagpaai Showing the Structures Involved in the Production
of Reflex Actions. {G. Bachman.) r.s. Receptive surface; a).n. afferent nerve; ex.
emissive or motor cells in the anterior horn of the gray matter of the spinal cord, sp.c;
ej. n. efferent nerves distributed to responsive organs, e.g., directly to skeletal muscles, sk.n.,
and indirectly through the intermediation of sympathetic ganglia, sym.g., to blood-vessels,
b.v., and to glands, g. The nerves distributed to viscera are not represented.

succeeding chapters are of this character, requiring for their per-
formance the cooperation of several organs and tissues associated
through the intermediation of the nerve system, it seems advisable to
consider briefly, in this connection, the parts involved in a reflex
action, as well as their mode of action. As shown in Fig. 61, the
necessary structures are as follows :

1. A receptive surface, skin, mucous membrane, sense-organ, etc.

2. An afferent nerve-fiber and cell.

3. An emissive cell, from which arises —

4. An efferent nerve, distributed to a responsive organ, as

5. Skeletal muscle, gland, blood-vessel, etc.

Such a combination of structures constitutes a reflex mechanism
or arc, the nerve portion of which, in the case of skeletal muscles, is
composed of but two neurons — an afferent and an efferent. In the

126

TEXT-BOOK OF PHYSIOLOGY.

case of glands and non-striated muscles, whether of blood-vessels or
viscera, the efferent neuron instead of passing direct to the responsive
organ, arborizes around the nerve-cells of a peripheral sympathetic
ganglion. The reflex arc is then continued by the processes of the
ganglion cells. An arc of this simplicity would of necessity sub-
serve but a simple movement. The majority of reflex activities,
however, are extremely complex, and involve the cooperation and
coordination of a number of nerve centers situated at different
levels of the spinal cord on the same and opposite side, and of re-
sponsive organs frequently situated at distances more or less remote
from one another. This implies that a number of neurons are as-
sociated in function. The transference
of nerve impulses coming from a localized
area of a sentient surface to emissive cells
situated at different levels is accom-
plished by the intercalation of a third
neuron situated in the gray matter which
is in connection, on the one hand, with
the central terminals of the afferent
neuron, and, on the other hand, through
its collateral branches with the dendrites
of the efferent neurons situated at differ-
ent levels of the cord. (Fig. 62.)

For the excitation of a reflex action
it is essential that the stimulus applied
to the sentient surface be of an intensity
sufficient to develop in the terminals of
the afferent nerve a series of nerve im-
pulses, which, traveling inward, will be
distributed to and received by the den-
drites of the emissive or motor cell. With
the reception of these impulses there is
apparently a disturbance of the equilib-
rium of its molecules, a liberation of
energy, and, in consequence, a transmis-
sion outward of impulses through the efferent nerve to muscle, gland, or
blood-vessel, separately or collectively, with the production of muscle
contraction, a secretion, vascular dilatation or contraction, etc. The
reflex actions take place, for the most part, through the spinal cord
and medulla oblongata, which, by virtue of their contained centers,
coordinate the various organs and tissues concerned in the performance
of the organic functions. The movements of mastication; the secretion
of saliva; the muscle, gland, and vascular phenomena of gastric and
intestinal digestion; the vascular and respiratory movements; the
mechanism of micturition, etc., arc illustrations of reflex activity.

Fig. 62. — Diagram Showing
the) Relation of the Third
Neuron a, to the Afferent
Neuron b, and to the Efferent
Neurons c, c, c. — {After Kolliker.)

CHAPTER IX.
FOODS.

The functional activity of every organ and tissue of the body is
accompanied by a more or less active disintegration of the living
material, the bioplasm, of which it is composed, as well as of the food
materials circulating in its interstices. The complex molecules of
the living material and of the non-living food materials are continually
undergoing disruption and falling into less complex and more stable
compounds; these, through oxidative processes, are eventually re-
duced through a series of descending chemic stages to a small number
of simpler compounds which, being of no further value to the organ-
ism, are eliminated by the various eliminating or excretory organs,
the lungs, skin, kidney, and liver. Among these excreted compounds
derived from tissue and from food metabolism the more important
are urea, uric acid, and carbon dioxid. Many other compounds,
organic as well as inorganic, are also eliminated from the body in the
various excretions, though they are present in but small amounts.
Coincident with this metabolic process, there is a transformation of
potential into kinetic energy, which manifests itself for the most part
as heat and mechanic motion.

In order that the organs and tissues may continue in the per-
formance of their functions, it is essential that they be supplied with
nutritive materials similar to those which enter into their own com-
position: viz., proteids, fat, carbohydrates, water, and inorganic
salts. These compounds, though originally derived from the food,
are immediately derived from the blood as it flows through the capil-
lary blood-vessels. The blood is therefore to be regarded as a reser-
voir of nutritive material in a condition to be absorbed and trans-
formed into utilizable and living material. Inasmuch as the materials
lost to the body daily, through disintegration and oxidation, though
considerable, are supplied by the blood, it is evident that this fluid
would diminish rapidly in volume, with a corresponding decline
in functional activity, were it not restored by the introduction into
the body of new material in the food. With the diminution of the
volume of the blood and an insufficient supply to the tissues, there
arise the sensations of hunger and thirst, which lead to the consump-
tion of food and the subsequent restoration of the physiologic condi-
tion of the tissues. These two sensations are also partially dependent
on the empty condition of the stomach and the dryness of the mucous
membrane of the mouth and throat.

127

i28 TEXT-BOOK OF PHYSIOLOGY.

The foods which are consumed daily in response to the sensations
of hunger and thirst are complex in composition and contain, though
in varying amounts, proteins, fats, carbohydrates, water and inor-
ganic salts, which, in contradistinction to foods, are termed food
principles or nutritive principles. In these compounds is also to be
found the potential energy necessary to maintain the dynamic equi-
librium of the body and which will become manifest as heat and
mechanic motion in the transformations of the material underlying
the nutritive processes.

The animal body may be therefore regarded as a machine capable
each day of performing a certain amount of work by the expendi-
ture of a definite amount of energy. In the performance of its work,
whether it be the raising of weights against gravity, the overcoming
of friction, cohesion, or elasticity, the machine suffers disintegration
and metabolizes a portion of the food materials and loses a portion of
its available energy. Unlike other machines, however, it possesses the
power, within limits, of self-renewal, self-adjustment, when supplied
with foods in proper quantity and quality.

QUANTITIES OF FOOD PRINCIPLES REQUIRED DAILY.

In order that the body may continue in the performance of its
work and yet retain a given weight, it is essential that the loss to the
body daily shall be exactly compensated by the introduction and
assimilation of a corresponding amount of food principles. If this
condition is realized, the body neither gains nor loses, but remains in
a condition of nutritive equilibrium. The determination of the exact
quantities of the different food principles required daily and their
ratio one to another is made from an examination of the quantity and
composition of the daily excretions. Since the proteins disintegrated
are represented in the excretions by urea and similar nitrogen-holding
compounds and the fats and carbohydrates by carbon dioxid, it
becomes possible to determine from them the quantities required to
restore equilibrium under any given condition. But as the activity
of the nutritive changes will vary in accordance with climatic condi-
tions, work done, etc., and as the excreted products will vary in the
same ratio, it is obvious that the required amounts of food will vary
in accordance with these varying conditions, if equilibrium is to be
maintained.

Various estimates have been made by different investigators as
to the amounts of the excreted products and the food principles re-
quired daily, which, though differing to some extent, have, neverthe-
less, an average nutritive and energy-producing value. The follow-
ing table shows the diet scale of Vierordt and the excretions to which
it would give rise. As the income and outgo practically balance,
there would be no change in the weight.

FOODS.
COMPARISON OF THE INCOME AND OUTGO.

129

Income.

Protein,

Fat

Carbohydrates,

Salts,

Water,

Oxygen,

Grams.

Ounces.

120

4-25

90

3-!7

33°

11.64

32

i-i3

2818
756

99-3°
26.66

4146

146.13

Outgo.

Grams. Ounces.

Water, 2818

Urea, 40

Feces, dry, 38

Salts, 32

Carbon dioxid, 922

Water formed in body, . 296

99-3°
1.40

1.60

"3

32-37
i°-33

4146 146.15

Other estimates as to the amounts of the organic food principles
required daily are as follows:

Ranke. Voit. Moleschott. Atwater. Hultgren.

Grams. Grams. Grams. Grams. Grams.

Protein, 100 118 130 125 134

Fat, 100 56 84 125 79

Starch, 250 500 550 400 522

In arranging tables showing the relation between the income and
the outgo, it is generally customary to state merely the amounts by
weight of the nitrogen and carbon each contains. This method
furnishes sufficiently accurate information regarding the metabolism
of the body, for the reason that the nitrogen represents the protein,
and the carbon, with the exception of that contained in the protein,
the fat and carbohydrates which have undergone disintegration or
metabolism.

The following balance table, as given by Ranke, shows the rela-
tion of the nitrogen to the carbon in the average mixed diet and in the
excretions of a man weighing 70 kilograms, in a condition of nutritive
equilibrium :

Income.

Grams.

Protein, I 100

Fat, : 100

Carbohydrates, ; 250

15-5

53-°

79.0

93-o

--
15-5

225.0

Outgo.

Grams.

N.

c.

Urea,

3i-5 \
°-5J

14.4

1.1

6.16

Feces,

10. S4

co2,

J5-5

225.00

i3o TEXT-BOOK OF PHYSIOLOGY.

From the above it will be observed that the daily discharge for
each kilogram of body-weight is 0.21 gram nitrogen and 3.03 grams
of carbon; the relation of the two being ^ = 14.5. On a diet in
which there is an excess of either proteid or carbohydrates this ratio
necessarily changes.

CLASSIFICATION OF FOOD PRINCIPLES.

Though the food principles are grouped as proteids, fats, carbo-
hydrates, etc., the members of each group differ somewhat in chemic
composition, digestibility, and nutritive value. These groups are as
follows :

1. Proteins.

Principle. Where found.

Myosin, Flesh of animals.

Albumin, vitellin, White of egg, yolk of egg.

Caseinogen, Milk.

Serum-albumin, fibrin, Blood contained in meat.

Glutin, Grain of wheat and some other cereals.

Vegetable albumin, Soft -growing vegetables.

Legumin, Peas, beans, lentils, etc.

2. Fats.

Animal fats, In adipose tissue of animals.

Vegetable oils, In seeds, grains, nuts, fruits, and other

vegetable tissues.

3. Carbohydrates.

Dextrose or grape-sugar, \ In fmits>

Levulose or fruit-sugar, /

Lactose or milk-sugar, . Milk.

Saccharose or cane-sugar, Sugar-cane, beet roots.

Maltose, Malt and malted foods.

c , 1 Cereals, tuberous roots, and legumin-

btarch' ] ous plants.

Glycogen, Liver, muscles.

4. Inorganic.

Water, ]

Sodium and potassium chlorid, I T , „ , , . , ,

„ ,. ' j ■ 1 ■ In nearly all animal and vegetable

Sodium, potassium, and calcium \ ? ,

phosphates and carbonates, |

Iron, J

5. Vegetable Acids.

Citric, tartaric, acetic, malic, In fruit and vegetables.

6. Accessory Foods.
Coffee, Tea, Cocoa, Alcohol.

Disposition of Food. — The protein principles of the food, after
undergoing all those changes which are embraced under the term di-

FOODS. 131

gestion, are absorbed from the intestine. During the act of absorption
they are transformed into the forms of protein characteristic of blood.
After being distributed by the blood-stream to the tissues, they are
brought into relation with the living cells. The disposition made of
the protein material by the bioplasm of the cell has not been definitely
determined. According to Voit, of the protein thus brought into con-
tact with the living tissues, only a small percentage is utilized and assim-
ilated for tissue repair. This he terms tissue or organ protein. The
remaining large percentage circulating in the interstices of the tissues,
though not forming an integral part of them, is acted on directly by
them, merely in virtue of contact— split up, oxidized, and reduced to
simpler compounds. This he terms circulating protein.

According to Pfluger and others, this view is not tenable. Pfliiger
asserts that, as material changes or metabolism can only take place
within living cells, all the protein must first be assimilated and organ-
ized by the cells before it can undergo metabolic changes. Metab-
olism by contact action is denied, and the division of proteins into
organ and circulating protein is not justifiable.

In the process of metabolism the protein suffers disintegration, giv-
ing rise through oxidation to some carbon-holding compound, possibly
fat, and to some nitrogen-holding compounds, which eventually give
rise to urea. The intermediate stages, however, are not definitely
known; the immediate antecedents of urea are probably carbamate and
carbonate of ammonia. The disintegration of the proteins is attended
by the disengagement of heat, thus contributing to the general store
of the energy of the body.

The fat principles, after digestion, are absorbed by the lymphatic
vessels and discharged by the thoracic duct into the blood, from which
they rapidly disappear. Though it is possible that a portion of the
fat enters directly into the formation of the living material, it is gener-
ally believed that it is at once oxidized and reduced to carbon dioxid
and water with the liberation of energy. The natural supposition
that a portion of the ingested fat was directly stored up in the cells of
the areolar connective tissue, thus giving rise to adipose tissue, has
been a subject of much controversy, though modern experimentation
renders this very probable. The body-fat, under physiologic con-
ditions, is also a product of the metabolic activity of connective-tissue
cells and is a derivative of both proteids and carbohydrates.

The carbohydrate principles, after digestion, are absorbed into
the blood as dextrose. This compound is then stored up in the liver
and muscles as glycogen. The intermediate stages which glycogen
passes through before it is reduced to carbon dioxid and water are
only imperfectly known. Though a large part of the carbohydrate
material is at once oxidized, it is now well established that another
portion contributes to the formation of, if it is not directly converted
into, fat. As the carbohydrates form a large portion of the food, they
contribute materially to the production of energy.

i32 TEXT-BOOK OF PHYSIOLOGY.

The inorganic principles, though not playing apparently as active
a part in the metabolism of the body as the organic, are nevertheless
essential to its physiologic activity.

Water is promptly absorbed after ingestion and becomes a part
of the circulating fluids- — blood and lymph. In the digestive appa-
ratus it favors the occurrence of those chemic changes in the food
necessary for their absorption, it promotes absorption of the food,
holds various constituents of the blood and other fluids in solution,
hastens the general metabolism of the body, holds in solution various
products of metabolic activity, and, leaving the body through the
excretory organs, promotes their elimination.

Sodium chlorid is absorbed into the blood and, unless taken in
excess, is utilized in replacing that which is lost to the organism daily.
The exact role which sodium chlorid plays in the nutritive process
is unknown; but, as it is present as a necessary constituent in all the
fluids and solids of the body, and as it is instinctively employed as
a condiment, it may be assumed to have a more or less important
function.

When taken as a condiment, it imparts sapidity to the food and
excites the flow of the digestive fluids; it ultimately furnishes the
chlorin for the hydrochloric acid of the gastric juice. Judging from
the impairment of the nutrition as observed in animals after depriva-
tion of salt for a long period of time, if favorably influences the growth
and functional activity of all tissues.

It is well known that herbivorous animals, races of men as well as
individuals who live largely on vegetable foods, require a larger addi-
tional amount of sodium chlorid than carnivorous animals, or human
beings who live largely on animal foods, even though the two classes
of foods contain relatively the same amounts. The explanation is
that the vegetable foods contain potassium salts which, meeting in
the blood with sodium chlorid, undergo decomposition into potassium
chlorid and sodium carbonate or phosphate, all of which, when in
excess, are at once eliminated by the kidneys. The blood, therefore,
becomes poorer in sodium chlorid, one of its necessary constituents.

Potassium phosphate and carbonate are also essential to the nor-
mal composition of the solids and fluids. They impart a certain
degree of alkalinity to the blood and lymph, one of the conditions
necessary to the life and activity of the tissue-cells bathed by them.
When administered in small doses, they increase the force of the
heart, raise the arterial pressure, and increase the activity of the
circulation.

Calcium phosphate and carbonate are partly utilized in maintain-
ing the solidity of the bones and teeth, replacing the amount metab-
olized daily. Inasmuch as the metabolism of these two tissues is
slight, there is not much need in the adult for lime as an article of
food. In young animals lime is essential to the solidification and
development of bone. When deprived of it, the skeleton undergoes

FOODS. 133

a defective development similar to the pathologic condition known
as rickets. Lime is present in milk to the extent of 0.15 per cent., as
well as in eggs and peas in relatively large quantities.

Iron is contained in both animal and vegetable foods, not, how-
ever, in the form of inorganic iron, nor in the form of an organic salt,
but as a compound with nuclein, thus forming an integral part of the
proteid molecule. After absorption the iron is utilized in the forma-
tion of the coloring-matter of the blood-corpuscles — hemoglobin.
The organic compounds of iron and the nucleins have been termed
hematogens. The amount of iron ingested has been estimated at
from 10 to 90 milligrams, the larger part of which is eliminated in
the feces. The relatively small part eliminated by the kidneys and
liver is usually taken as the amount metabolized, though it is probable
that this is not wholly true, as there is evidence that iron can be re-
tained in the body and utilized again in the formation of newr hemo-
globin. Contrary to what might be expected, milk contains but a
very small quantity of iron, not more than 3 or 4 milligrams in 1000
grams (human milk) — an amount insufficient for the development of
the necessary hemoglobin. This is compensated for, however, by
the accumulation of iron in the liver during intrauterine life. Ac-
cording to Bunge, the liver of a newly born rabbit contains as much
as 18.2 milligrams per 100 grams of body-weight, while at the end
of twenty-four days it only contains 3.2 milligrams per 100 grams of
body-weight.

Vegetable acids increase the secretions of the alimentary canal, and
are apt, in large amounts, to produce flatulence and diarrhea. After
entering into combination with bases to form salts, they stimulate the
action of the kidneys and promote a greater elimination of all the
urinary constituents. In some unknown way they influence nutrition;
when deprived of these acids, the individual becomes scorbutic.

The accessory foods — coffee, tea, and cocoa — when taken in
moderation have a stimulating influence on the nervous system, as
shown by the removal of both mental and physical fatigue, by an
increased capacity for sustained mental work, by the persistent wake-
fulness among those unaccustomed to their use. Coffee more especially
increases the frequency and force of the heart-beat, raises the arterial
pressure, and hastens the general blood-flow. It has no influence
either in the way of increasing or decreasing proteid metabolism.

Tea frequently acts as an astringent on the alimentary canal on
account of the tannin which passes into the water when the infusion
is made. Inasmuch as tannin also coagulates peptones, the excessive
use of tea as a beverage is apt to derange the digestive organs and
the general process of digestion.

Cocoa is more nutritive than either coffee or tea, on account of the
large amount of fat and proteid it contains. It is, however, less
stimulating.

The active principles in coffee, tea, and cocoa, and to which their

i34 TEXT-BOOK OF PHYSIOLOGY.

effects are to be attributed, are caffein, thein, and theobromin respec-
tively. These alkaloids are chemically closely related one to the
other and to the compound xanthin. They are present in the coffee
seeds, the tea leaves, and the cocoa bean to the extent of 1.7 per cent.,
1.4 per cent., and 1.6 per cent, respectively. When prepared as a
beverage, however, there is three times as much caffein in coffee as thein
in tea.

Alcohol when taken in small quantities stimulates the digestive
glands to increased activity and thus promotes digestive power. Its
absorption into the blood is followed by increased action of the heart,
dilatation of the cutaneous blood-vessels, a sensation of warmth,
and an excitation of the brain. In large quantities it acts as a paraly-
zant, depressing more especially the vaso-constrictor nerve-centers and
certain areas of the brain, as shown by an impairment in the power of
sustained attention, clearness of judgment, and muscle coordination.

Alcohol is undoubtedly oxidized in the body, as only about 2
per cent, can be obtained from the urine and expired air. It thus
contributes to the store of the body-energy. As to whether for this
reason it can be regarded as a food — that is, whether it can be sub-
stituted in part at least for fat or carbohydrate material without im-
pairing the proteid metabolism — is at present a subject of experimen-
tation and discussion. According to some investigators, alcohol does
not retard proteid metabolism, for when it is introduced into the body
in amounts equivalent to the carbohydrates withdrawn from the food
there is at once a rise in the amount of nitrogen excreted. Hence it
cannot be regarded as a food. According to other investigators,
alcohol retards or protects proteid metabolism just as effectually as
an equivalent amount of starch or sugar. Many more experiments
are required to decide this question. When taken habitually in large
quantities, alcohol deranges the activities of the digestive organs,
lowers the body temperature, impairs muscle power, lessens the
resistance to depressing external conditions, diminishes the capacity
for sustained mental work, and leads to the development of structural
changes in the connective tissues of the brain, spinal cord, and other
organs. In zymotic diseases and in cases of depression of the vital
powers it is most useful as a restorative agent.

THE ENERGY OR HEAT VALUE OF FOOD PRINCIPLES.

The food consumed not only restores the material metabolized
and discharged from the body, but also the energy which has been
expended as heat and mechanic motion. The food principles are
products of the constructive processes taking place in the vegetable
world during the period of growth and activity. At the time of their
formation there is an absorption and storing of the sun's energy which
then exists in a potential condition. During the metabolism of the
animal body these compounds are reduced through oxidation to rela-

FOODS. 135

tively simple bodies, such as carbon dioxid, water, urea, etc., with the
liberation of their contained energy. All of the energy of the body,
whatever its manifestations may be, can be traced to chemic changes
going on in the tissues, and more particularly to those changes in-
volved in the oxidation of the food principles.

The amount of heat or energy which any given food principle will
yield can be determined by burning a definite amount (e. g., 1 gram)
to carbon dioxid and water and ascertaining the extent to which the
heat thus liberated will raise the temperature of a given amount of
water (e. g., 1 kilogram). The amount of heat may be expressed in
gram or kilogram degrees or calories, a gram calorie or kilogram
calorie being the amount of heat required to raise the temperature of
a gram or a kilogram (1000 grams) of water i° C. The apparatus
employed for this purpose is termed a calorimeter, and consists es-
sentially of a closed chamber in which the oxidation takes place,
surrounded by a water jacket, the rise in temperature of the water
indicating the amount of heat produced.

The results obtained by investigators employing different calorim-
eters and different food principles of the same group vary, though
within certain limits: e. g., 1 gram of casein yields 5.867 kilogram
calories; 1 gram of lean beef, 5.656 calories; 1 gram of fat yields
9.353, 9.423, 9.686 calories; 1 gram of carbohydrate, 4.182, 4.479,
etc., calories. These numbers represent the physical heat values of
these food principles.

In the human body as determined by calorimetric methods the
oxidation of the food principles yields practically the same amount
of heat they yield when oxidized outside the body, with the excep-
tion of the proteins, which are oxidized only to the stage of urea. As
this compound is capable of further reduction in the calorimeter to
carbon dioxid and water with the liberation of heat, the quantity of
heat it contains must therefore be deducted from the calorimetric
heat value of the protein. According to Rubner, 1 gram of urea will
yield 2.523 kilogram calories. As the urea which results from the
oxidation of 1 gram of protein is about -J of a gram, the amount of
heat to be deducted from the heat value of the protein is ^ of 2.523, or
0.841 calories. It has also been shown that some of the ingested
protein escapes in the feces, the heat value of which must also be
determined and deducted. This having been done, the physiologic
heat value becomes 4.124 calories.

The following estimates give approximately the number of kilo-
gram calories produced when the food is burned to carbon dioxid,
water, and urea in the body:

1 gram of protein yields,, 4-124 calories.

1 "fat " 9.353

1 " " carbohydrate yields, 4. 116 "

The total number of kilogram calories or kilogram degrees of heat
yielded by any of the previously given diet scales can be readily deter-

136 TEXT-BOOK OF PHYSIOLOGY.

mined by multiplying the quantities of food principles consumed by
the above-mentioned factors. The diet scale of Vierordt, for example,
yields the following:

120 grams of protein yields, 494.88 calories.

90 " "fat " 841.77 "

330 " " starch " 1358.28 "

2694.93

The total calories obtained from other diet scales would be as follows :
Ranke, 2335; Voit, 3387; Moleschott, 2984; Atwater, 3331; Hultgren,

3436-

Starvation. — The relation of the .different food principles to the
general nutritive process becomes more apparent from an examination
of the excretions from the body during the process of starvation com-
bined with an examination of the organs and tissues after death. If
an animal be deprived entirely of food, a decline in body-weight at
once sets in, which continues until about 40 per cent, of the weight
has been lost, when death generally ensues. This results from the
fact that the active tissue cells consume, for the purpose of maintain-
ing the normal temperature of the body, not only their own reserve
food material, but that of the less active or storage tissues as well; and,
in consequence, there is a progressive diminution in weight.

The phenomena which characterize this non-physiologic con-
dition are as follows: hunger, intense thirst, gastric and intestinal
uneasiness and pain, diminished pulse-rate, and respiration, muscular
weakness and emaciation, a lessening in the amount of urine and its
constituents, diminished exhalation of carbon dioxid, an exhalation
of a fetid odor from the body, vertigo, stupor, delirium, at times con-
vulsions, a sudden fall in body-temperature, and finally death. The
duration of life after complete deprivation of food varies from eight to
thirteen days or more, though this period can be prolonged if the ani-
mal be supplied with water, this being more essential under the cir-
cumstances than the organic materials which can be supplied by the
organism itself. The duration of the starvation period will vary in
accordance with the previous condition of the animal and the amount
of reserved food the body contains. The excretion of urea declines
very rapidly during the first two days — a fact which has been attrib-
uted to a rapid consumation of the surplus proteid food. After this
period, when the tissues begin to metabolize their own proteid, the
excretion remains fairly constant until toward the close, when the
am6unt eliminated falls very rapidly. As proteids contain about 16
per cent, of nitrogen, 1 part of nitrogen equals 6.25 parts of protein.
Hence, for every 1 gram of nitrogen or 2.14 grams urea excreted,
it may be assumed that 6.25 grams of protein or, according to Voit, 30
grams of flesh have been metabolized. The daily excretion of urea,
therefore, indicates the extent of the protein metabolism.

It has been observed also that there is a steady diminution in the

FOODS.

137

excretion of carbon dioxid, though this is greatest in the last few days.
As fat contains about 76 per cent, of carbon, 1 part of carbon equals 1.3 1
parts of fat. Hence, for every 1 gram of carbon or 3.66 grams carbon
dioxid excreted it may be assumed that 1.3 1 grams of fat have been
metabolized. The daily excretion of carbon, therefore, indicates the
extent of fat metabolism. The carbohydrates are here left out of con-
sideration, as they constitute only about 1 per cent, of the body-weight.
It must be borne in mind, however, that in the metabolism of protein a
certain quantity of fat is produced which also undergoes oxidation.
The amount of the carbon or the fat that the protein would give rise to,
as previously determined, must therefore be subtracted from that elim-
inated by the lungs, etc., in order to determine the amount of body-
fat metabolized. Observations of human beings in the fasting con-
dition show that for a period of ten days there is a daily excretion of
about 21 grams of urea, equivalent to about 70 grams of protein. This
amount, however, may be reduced to from 50 to 60 per cent, if the in-
dividual has a surplus of body-fat. . Human beings under similar
circumstances may lose during the first few days 200 grams of fat
daily.

The following table shows the excretion of nitrogen and carbon and
the calculated amounts of protein and fat metabolized from an experi-
ment made by Ranke on himself during a fast of twenty-hours, be-
ginning; twentv-four hours after the last meal:

Disintegration of Tissue.
(Calculated.)

Expenditure.

Nitrogen.

Carbon.

Nitrogen.

Carbon.

7.8
O.O

26.5

157-5

7.2
0.0

Protein, 50 gm.,

Fat, 199.6 gm.,

Uric acid, 0.2 gm., . . . J
Carbon dioxid,

3-4
180.6

7.8

184.0

7.2

184.0

Coincidently with these losses to the body there is also a gradual
loss of inorganic salts, and toward the termination of the period a
sudden fall in temperature of several degrees centigrade, in conse-
quence of the final consumption of all available foods, when death
ensues, in all probability, from a cessation in the action of the heart.

Post-mortem Appearances. — It has been experimentally determined
that animals die when the body-weight has declined to about 40 per
cent. Post-mortem examination shows that the loss of material,
though very generally distributed throughout the body, is greatest
in organs and tissues least essential to life.

The results of an analysis of the organs and tissues of a cat after
a thirteen-day period of starvation, during which the animal lost 1017

138

TEXT-BOOK OF PHYSIOLOGY.

grams in weight, are given in the following table, based on data fur-
nished by Voit:

Organ.

Adipose tissue, .

Spleen,

Liver,

Testes.

Muscles,

Blood,

Kidneys,

Skin and hair,

Lungs,

Intestines,

Pancreas,

Bones,

Heart,

Nervous system

Percentage.

Actual Loss
of Tissue.

Grams.

97

267

67

6

54

49

40

1

3i

429

27

37

26

7

21

S9

18

3

18

21

17

1

14

55

3

0

3

1

It will be observed from this table that the adipose tissue suffers
the greatest loss, the entire amount disappearing with the exception
of a small portion in the posterior part of the orbital cavity and around
the kidneys. The muscles, though only losing 31 per cent, of their
weight, yet furnish 429 grams of presumably proteid material, for
nutritive purposes. The heart and nervous system experience but
slight loss.

Mixed Diet. — The chemic composition of the tissues, taken in
connection with their metabolism during starvation, implies that no
one article of food is sufficient for tissue repair and heat production;
but that all classes of foods — in other words, a mixed diet — are essential
to the maintenance of a normal nutrition. Experimental investiga-
tion has also conclusively established this fact. Moreover, the amounts
of nitrogen and carbon eliminated daily, and the ratio existing between
them, indicate the amounts of proteid, fat, and carbohydrate which
are required to cover the loss.

Metabolism on a Purely Protein Diet. — Notwithstanding
the chemic composition of the proteins and the possibility of their
giving rise to both fat and a carbohydrate during their metabolism
it has been found extremely difficult to maintain the normal nutrition
for any length of time on a pure protein or fat-free flesh diet. This,
however, has been accomplished with dogs. It was found, however,
that, in order to maintain the equilibrium, it was necessary to increase
the proteins from two to three times the usual amount. Thus, a dog
weighing 30 to 35 kilograms required from 1500 to 1800 grams of
flesh daily in order to get the requisite amount of carbon to prevent
consumption of its own adipose tissue. Under similar circumstances,
a human being weighing 70 kilograms would require more than 2000
grams of lean beef — an amount which, from the nature of the digestive

FOODS.

!39

apparatus, it would be practically impossible to digest and assimilate
for any length of time. Even the slight habitual excess beyond the
amount normally required is imperfectly assimilated and gives rise
to the production of nitrogen-holding compounds which, on account
of the difficulty with which they are eliminated by the kidneys, ac-
cumulate within the body and develop the gouty diathesis, with all
its protean manifestations.

Metabolism on a Fat and Carbohydrate Diet. — As nitrogen
is an indispensable constituent of the tissues, it is evident that neither
fat nor carbohydrates can maintain nutritive equilibrium except for
very short periods. On such a diet the tissues consume their own
proteids, as shown by the continuous excretion of urea, though the
amount is less than during starvation. An excess of fat retards the
metabolism of proteids. The same holds true for the carbohydrates.

Thus, in any well-arranged dietary there should be a combina-
tion of proteins, fats, and carbohydrates in amounts sufficient to main-
tain nutritive equilibrium; in other words, to repair the loss of tissue
and to furnish the requisite amount of heat in accordance with work
done, as well as with climatic and seasonal variations.

COMPOSITION OF FOODS.

The food principles essential to the maintenance of the nutrition
of the body are contained in varying proportions in compound sub-
stances termed foods; e. g., meat, milk, wheat, potatoes, etc. Their
nutritive value depends partly on the amounts of their contained food
principles and partly on their digestibility. The dietary of civilized
man embraces foods derived from both the animal and vegetable
worlds.

Composition of Animal Foods. — The following table shows the
average percentage composition of various kinds of meats, cow's
milk, and eggs:

In ioo Parts

Beef.

Water, [ 76.25

Protein, 20.24

Fat, 1. 68

Carbohydrates, 0.50

Salts, 1.38

Veal.

Mut-
ton.

77.S2

19.86

0.82

0.80

0.70

75-59
17. 11

5-47
0.60

i-23

Pork. Fowl. Fish.

72-57

I9-3I

5.82

0.60

1.70

70.80

22.70

4.10

1.20

1.20

Cow's

Milk.

79-3°

S6.S7

18.30

4-75

0.70

3-5°

0.90

4.00

0.80

0.17

Eggs.

73.67

12.55
12. 11

°-55
i-i3

Meats. — It will be observed from these analyses that the meats
contain from 18 to 20 per cent, of a protein which belongs in virtue
of its chemic relations to the group of globulins. In the living con-
dition this body, known as myosinogen, is in a semifluid condition,
but shortly after death undergoes coagulation, giving rise to solid
myosin and a soluble albumin. There are also present in meat small

140 TEXT-BOOK OF PHYSIOLOGY.

percentages of other forms of protein; e. g., myoalbumin, myoglob-
ulin, paramyosinogen, etc. After being subjected to the cooking
process, meats contain the albuminoid body gelatin, a product of the
transformation of the proteids of the connective tissue.

The percentage of fat, contained within the meat substance, is
very small except in mutton and pork, where it rises to 5.4 per cent,
and 5.8 per cent, respectively. The fat-globules in these meats are
packed closely between the muscle-fibers, and prevent the easy entrance
of the digestive fluids, and hence they are more difficult of digestion
than beef.

The carbohydrates vary from 0.5 to 1 per cent., and are represented
by glycogen. The principal inorganic salts are potassium phosphate
and sodium chlorid.

Cooking, when properly done, not only makes the meat more
palatable and appetizing from the development of agreeable flavors,
but converts the connective tissue, which, in old animals especially,
is tough and resisting, into gelatin, thus rendering it more easy of
mastication and digestion. At the same time parasitic organisms,
such as the embyronic forms of tenia or tapeworm, trichina spiralis,
as well as bacterial growths, which frequently infest the bodies of an-
imals, are destroyed and made harmless.

Milk is the natural food of the young of all mammals, and is usu-
ally regarded as typical on account of the ratio existing among its nu-
tritive principles. The analysis given above is that of cow's milk.
Examined microscopically, milk is seen to consist of a clear fluid, the
milk plasma, holding in suspension an enormous number of small,
highly refractive oil- globules, which measure on the average about TT}F¥
of an inch in diameter. Each globule is supposed by some observers
to be surrounded by a thin albuminous envelope, which enables it to
maintain the discrete form. Others deny the existence of such a mem-
brane. The chief protein constituent of milk, caseinogen, is held in
solution by the presence of phosphate of lime. On the addition of
acetic acid or sodium chlorid up to the point of saturation the casein-
ogen is precipitated as such and may be collected by appropriate chemic
methods. When taken into the stomach, caseinogen is coagulated;
that is, it is separated into casein or tyrein and a small quantity of a
new soluble proteid. This change is brought about by the presence in
the gastric juice of a special ferment known as rennin or pexin.

The fat of milk is more or less solid at ordinary temperatures.
It is a combination of olein, palmitin, and stearin, with a small quan-
tity of butyrin and caproin. When milk is allowed to stand for some
time, the fat-globules rise to the surface and form a thick layer known
as cream. When subjected to the churning process, fat-globules
run together and form a coherent mass — butter.

Lactose is the particular form of sugar found in milk. In the
presence of Bacillus acidi lactici, the lactose is decomposed into lactic
acid and carbon dioxid, the former of which not only imparts a sour

FOODS.

141

taste to the milk, but causes a precipitation of the caseinogen. The
chief salt found in milk is phosphate of lime, and this is the chief source
of this agent in the formation of bones.

Eggs are also to be regarded as complete natural foods, inasmuch
as they contain all the necessary food principles. The analysis given
in the above table represents the composition of the entire egg. The
white of the egg contains 12 per cent, of proteid and 2 per cent, of fat.
The yolk, however, contains 15 per cent, of proteid and 30 per cent,
of fat.

Composition of Cereal Foods. — The average composition of
the principal cereals is shown in the following table :

In 100 Parts.

Water,

Protein,

Fat,

Carbohydrate
Cellulose, . . .
Salts,

Wheat.

Rye.

Barley.

Oats.

Corn.

Rice.

I3-56

12.65

13-77

12-37

13.10

13.12

12-35

12-55

11. 14

10.41

9-«5

7.88

i-75

I.97

2.16

5-23

4-57

0.85

67.90

67-95

64-93

57-78

68.42

76-55

2.63

3.00

5-3i

II. 19

2-5°

°-55

1.81

1.88

2.69

3.02

1.56

1.05

Buck-
wheat.

12.62

10.02

2.24

64-43
8.67
2.02

That the cereals are most important and useful articles of diet is
evident from their composition, consisting, as they do, of proteins
and carbohydrates in large proportion. Owing to the cellulose or
woody fiber which envelops and penetrates the grain, they are some-
what difficult of digestion. A section of a grain of wheat shows the
external cellulose envelope, the husk, beneath which is a layer of large
cells containing the chief protein — the gluten. The interior of the
grain consists of small cavities, the walls of which are formed of cellu-
lose and which contain the granules of starch, fat, small quantities of
proteid, and inorganic salts. All other cereals have a similar structure.

In the preparation of white flour from wheat it is customary to
remove the husk, a process which involves the removal also of a por-
tion, if not all, of the gluten cells, so that such flour contains less nitrog-
enized material than the original grain. It is possible, however, in
the milling of wheat, to remove only the husk and retain the gluten in
the flour, as in the preparation of whole wheat flour.

Bread is an artificially prepared food made either of wheat or
rye. Owing to the fact that the proteids of the other cereals do not
possess the same adhesive properties when kneaded with water, they
can not be used for bread-making purposes. In the making of bread,
the flour is kneaded with water until a glutinous mass — dough — is
formed. During this process, salt, sugar, and yeast are added. It
is then placed in a temperature of about ioo° F. In the presence of
heat and moisture the natural ferment of the flour — diastase — con-
verts a portion of the starch into sugar, which in turn is split up into

142

TEXT-BOOK OF PHYSIOLOGY.

carbon dioxid and alcohol by the yeast plant. The bubbles of carbon
dioxid, becoming entangled in the dough, cause it to swell or rise and
subsequently give the porous or spongy character to the bread. When
baked at a temperature of 400 ° F., the alcohol is driven off; yeast cells
and other organisms are destroyed; the starch, particularly that on the
surface, is dextrinized. Thus prepared, white bread consists of water,
32 per cent.; protein, 8.8 per cent.; fat, 1.7 per cent.; carbohydrate,
56.3 per cent.; salts, 0.9 per cent. The principal salts are potassium
and magnesium phosphate. Whole wheat bread consists of water, 40
per cent.; protein, 12.2 per cent.; fat, 1.2 per cent.; carbohydrate, 43.5
per cent.; salts, 1.3 per cent.; cellulose, 1.8 per cent.

Composition of Vegetable Foods. —The average composition
of some of the principal vegetables is shown in the following table :

In 100 Parts.

Water,

Protein,

Fat,

Carbohydrates,

Cellulose,

Salts,

Beans,

^3-74

23.21

2.14

53-67

3-69

3-55

Peas.

14.99

22.85

1.79

52-36

5-43
2.58

Pota-
toes.

75-47
i-95
0.15

20.69
0.76
0.98

Beets,

0.30

13.00

1.60

Tur-
nips.

89.42

1-35
0.18

7-36
0.94
0.75

Toma-
toes.

96.30
0.90

0.50
2.80

0.40

Aspa-
ragus.

93-75
1.79
0.25
2.63
1.04
0-54

Cab-
bage.

89.97
1.89
0.20
4.87
1.84
1.23

The vegetable foods, as a class, vary considerably in nutritive
value and digestibility, the latter depending on the amount of cellu-
lose they contain. A section of a vegetable shows not only the pres-
ence of an external cellulose envelope, but also an inner framework
which penetrates its substance in all directions. The nutritive prin-
ciples are contained in small cavities, the walls of which are formed
by the framework. Nearly all vegetables require cooking before
being eaten. When subjected to heat and moisture, not only is the
texture of the vegetable softened and disintegrated, but the starch
grains are hydrated and partially prepared for conversion into dextrin
and sugar. At the same time various savory substances are set free,
which make the food more palatable.

Beans and peas contain large quantities of a proteid, legumin,
and starch, and hence are especially valuable as nutritive foods. The
presence of the cellulose envelope, especially in ripe beans and peas,
combined with rather a dense texture, renders them somewhat difficult
of digestion. Potatoes, though largely employed as food, are ex-
tremely poor in protein, 2 per cent., and carbohydrates, 20 per cent.
When sufficiently cooked they are easily digested, owing to the small
amount of cellulose they contain.

Green vegetables, — e. g., lettuce, spinach, tomatoes, asparagus,
onions, etc., though containing food principles in small amounts, are,
nevertheless, valuable adjuncts to the dietary, for the reason that they
contain inorganic as well as organic salts, which appear to be necessary

FOODS.

M3

to the maintenance of the normal nutrition. The want of green veget-
ables has been supposed to be the cause of scurvy.

Ripe fruits, grapes, cherries, apples, pears, peaches, strawberries,
lemons, oranges, etc., though consumed largely, possess but little nutri-
tive value. They consist largely of water, 75 to 85 per cent., proteins
a trace, sugar from 5 to 13 per cent., organic acids (citric, malic, tar-
taric), pectose, and various inorganic salts.

Relative Value of Animal and Vegetable Foods. — Though
both animal and vegetable foods contain the different classes of food
principles, it is not a matter of entire indifference as to which are con-
sumed. It has been found by experiment that animal proteids are
more easily and completely digested and absorbed than vegetable
proteids; that cellulose is not only highly indigestible, but by its pres-
ence in large quantities retards the digestive process and impairs the
activity of the entire digestive mechanism, though in moderate quan-
tity it undoubtedly aids digeston indirectly by mechanically promot-
ing peristalsis. The following table shows the relative digestibility
of the two classes of foods:

Vegetable.

Animal.

Digested.

Undigested.

Digested.

Undigested.

Of 100 parts of solids,

Of 100 parts of protein,

Of 100 parts of fats or carbohy-
drates

75-5
46.6

9°-3

24-5
53-4

9-7

89.9
81.2

96.9

II. 1
18.S

3-1

Construction of Dietaries. — Inasmuch as neither animal nor
vegetable foods contain the food principles in proper quantities and
proportions, the instinctive choice of mankind has led to a combina-
tion of the two classes of foods. From the analyses tabulated above
it becomes comparatively easy to construct a suitable dietary, com-
posed of different articles of food, in which the food principles shall
bear the proper ratio one to the other — a ratio based on the total quan-
tity of nitrogen (15 to 20 grams) and carbon (225 to 300 grams) elim-
inated from the body daily.

It is only necessary, therefore, to combine two or more foods, the
composition of which is known, in quantities sufficient to furnish
the requisite amount of nitrogen and carbon, or their equivalents in
proteid, fat, and carbohydrates. As illustrations of such combina-
tions the following examples are given:

Foods.

Meat, 250 gm., 8.8 oz.

Bread, 400 gm., 14.2 oz.

Fat, 100 gm., 3.5 oz.

Sugar, 70 gm., 2.5 oz.

Food Principles.
Protein, 100 gm.

Fat, 100 gm.

Carbohydrates, 2^0 gm.

N. C.

144 TEXT-BOOK OF PHYSIOLOGY.

Foods. N. C.

Meat, 225 gm., % lb. 7.5 gm. 34 gm.

Bread 450 gm., 1 lb. 5.5 gm. 117 gm.

Fats, 113 gm., J lb. ... 84 gm.

Potatoes, 450 gm., 1 lb. 1.3 gm. 45 gm.

Milk, 225 gm., J pint 1.7 gm. 20 gm.

Eggs, 113 gm., 1 lb. 2.0 gm. 15 gm.

Cheese, 56 gm., J lb. 3.0 gm. 20 gm.

2i-o 335

Waller.
DAILY RATION OF THE UNITED STATES SOLDIER.

Fresh beef, 20 oz.

or pork, 12 oz.

or bacon, 12 oz.

Flour, 18 oz.

or soft bread, 18 oz.

or hard bread, 16 oz.

Potatoes, 16 oz.

or potatoes ni and tomatoes 45 16 oz.

Beans or peas, 2.4 oz.

Rice or hominy, 1.6 oz.

Coffee, 1 . 6 oz.

Sugar, 2.00 oz.

Vinegar, 0.32 gill

Salt, 0.60 gill

CHAPTER X.
DIGESTION.

Foods are heterogeneous compounds consisting of organic and
inorganic nutritive principles associated with a varying amount of
non-nutritive material, such as the dense parts of the connective tissue
of the animal foods and the woody fiber or cellulose of the vegetable
foods. Before the nutritive principles can be utilized they must be
dissociated from the non-nutritive material. Even when consumed
in the free state, the food principles are seldom in a condition to be
absorbed into the blood and assimilated by the tissues. When foods
are consumed in their natural state or after they have been subjected
to the cooking process, they are subjected while in the food canal to the
solvent action of various fluids by which they are disintegrated and
reduced to the liquid condition. The nutritive principles freed from
their combinations are changed in chemic composition and transformed
into substances capable of absorption. To all the physical and chemic
changes which foods undergo in the food canal the term digestion has
been given.

The digestive apparatus comprises the entire alimentary or food
canal and its various appendages: the teeth, the tongue, the mouth,
the gastric and intestinal glands, the pancreas, and the liver (Fig. 63).

The canal itself is a musculo-membranous tube about thirty-two
feet in length, and extends from the mouth to the anus. It may be
subdivided into several distinct portions, as mouth, pharynx, esoph-
agus, stomach, small and large intestines. The mouth is provided
(1) with teeth, by which the food is divided, (2) with the tongue, and
(3) with glands, by which a solvent fluid, the saliva, is secreted. The
glands, though situated for the most part outside the mouth, are con-
nected with it by means of ducts. Posteriorly the mouth opens into
the pharynx or throat, a somewhat pyramidal-shaped structure about
five inches in length, which in turn is followed bv the esophagus or
gullet, a tube about nine inches in length. As the esophagus passes
through the diaphragm it expands into the stomach, a curved pyri-
form organ, which serves as a reservoir for the reception and retention
of the food for a varying length of time. The small intestine is that
portion of the alimentary canal extending from the end of the stomach
to the beginning of the large intestine; owing to its length, about twenty-
two feet, it presents a very convoluted appearance in the abdominal
cavity. Embedded in its walls arc the intestinal glands which open on
its surface and secrete the intestinal fluid. In the upper portion of

10 M5

146

TEXT-BOOK OF PHYSIOLOGY.

the small intestine, within five inches of the stomach, there are generally
two orifices, the terminations of the ducts of the liver and pancreas,
organs which secrete the bile and pancreatic juice respectively. The
large intestine is from five to six feet in length and extends from the
end of the small intestine to the anus. Its walls contain a large num-
ber of glands.

■U'A I Salivary Gland

Vermiform Appendix

Fjg. 63. — Diagram of the Alimentary Canal. — (Modified from Landois.)

The general process of digestion is largely accomplished by the
chemic action of the digestive fluids : the saliva, the gastric, intestinal,
and pancreatic juices, and the bile. Though taking place through-
out a large portion of the food canal, the process may be subdivided
into several stages: viz., mouth digestion, gastric digestion, and intesti-
nal digestion.

DIGESTION. 147

As a result of the action of these fluids the nutritive principles are
prepared for absorption into the blood; the non-nutritive principles,
along with certain waste products, pass into the large intestine to be
finallv extruded from the bodv.

MOUTH DIGESTION.

The digestion of the food as it takes place in the mouth comprises
a series of physical and chemic changes, the result of the action of
the teeth, the tongue, and the saliva. The mechanic division of the
food and the incorporation of the saliva with it are termed respectively
mastication and insalivation.

MASTICATION.

Mastication is the mechanic division of the food, and is accom-
plished by the teeth and the movements of the lower jaw under the
influence of muscle contractions. Complete mechanic disintegra-
tion of the food is essential to its subsequent solution and chemic
transformation; for when finely divided it presents a larger surface
to the action of the digestive fluids and thus enables them to exert
their respective actions more effectively and in a shorter period of
time.

The Teeth. — In man passing from childhood to adult life two
sets of teeth make their appearance. The first set constitute the tem-
porary, deciduous, or milk teeth; the second set constitute the per-
manent teeth, which should last with proper care through life or to an
advanced age.

The temporary teeth, twenty in number, ten in each jaw, though
smaller than the permanent teeth, have the same general conforma-
tion. They are divided into four incisors, two cuspids or canines,
and four molars for each jaw.

The permanent teeth, thirty-two in number, sixteen in each jaw,
are divided into four incisors, two cuspids or canines, four bicuspids or
premolars, and six molars for each jaw.

Each tooth may be said to consist of three portions : (i) the crown,
or that portion which projects above the gums; (2) the root or fang,
that portion embedded in the alveolar socket; (3) the constricted por-
tion or neck, which is surrounded by the free margin of the gum. The
teeth are firmly secured in their sockets by a fibrous membrane, the
peridental membrane, which is attached, on the one hand, to the alveo-
lar process, and, on the other, to the cementum.

A vertical section of a tooth shows that it consists of three distinct
solid structures, the enamel, the dentine, and the cementum, which
have the anatomic relationship as represented in Fig. 64. In the
center of the dentine there is a cavity the general shape of which varies

TEXT-BOOK OF PHYSIOLOGY.

in different teeth, and which is occupied during the living condition by
the tooth pulp.

Microscopic examination of the tooth reveals the presence of ir-
regular stellate spaces, the interglobular spaces, between the dentine
and the cementum, which are occupied by connective-tissue cells.
Clefts of varying size are also observed at the junction of the dentine
and the enamel, and which extend for some distance into the latter.
The enamel is composed of dense hard cylinders which, on account
of their small size and close relationship, appear to be hexagonal in
shape. These cylinders are held together by cement substance. The

free border of the enamel is covered,
in early life at least, by a thin mem-
brane known as the cuticle or mem-
brane of Nasmyth.

The dentine is somewhat less dense
than the enamel. It is composed of
connective-tissue fibers embedded in a
ground-substance, both of which have
undergone calcification in the course
of development. The dentine is pen-
etrated by a series of fine canals, the
dentine canals or tubules, which begin
by open mouths on the pulp side.
From this point the tubules pass out-
ward to the cementum and enamel,
where their terminal branches com-
municate with and terminate in the
interglobular spaces and clefts. In
their course the tubules give off a
series of branches which communicate
freely with one another. The dentine
bordering the tubule is somewhat
more dense than the intertubular por-
tion and constitutes what is known as
the dentinal sheath or Neumann's
sheath.

The cementum resembles bone be-
cause it contains both lacunae and
canaliculi, though it is, as a rule, devoid of Haversian canals.

The pulp consists of a framework of connective tissue which affords
support to blood-vessels and nerves, both of which enter the pulp
chamber through a small foramen at the apex of the root. The outer
surface of the pulp is covered with a layer of large spheric cells, the
odontoblasts. Each cell presents on its inner surface short processes
which pass into the pulp; on its outer surface it presents a long process
which enters a dentine tubule and extends as far as its ultimate ter-
minations. Collectively these processes are known as the dentine

Fig
Tooth
Dentine. P.
brane. P.C.

ment.
Vein.
ling.)

64. — Vertical Section of
in Jaw. E. Enamel. D.
M. Periodontal mem-
Pulp cavity. C. Ce-

Bone of lower jaw. V.
a. Artery. N. Nerve. — {Stir-

DIGESTION. 149

fibers. Inasmuch as the fibers do not completely occupy the lumen
of the tubule, it is probable that there is a free circulation of lymph
from the pulp chamber through the dentine tubules into the enamel
clefts, into the interglobular spaces, and possibly into the lacunas of the
cementum.

The peridental membrane is composed of connective-tissue fibers
abundantly supplied with blood-vessels, and nerves.

Movements of the Lower Jaw. — The lower jaw is capable of a
downward and upward, an antero-posterior, and a lateral movement,
all dependent on the peculiar construction of the —

Temporo-maxillary Articulation. — This articulation is formed by
the anterior portion of the glenoid cavity, the eminentia articularis
and the condyle of the inferior maxilla, all of which are united by
means of ligaments. Situated between the glenoid cavity and the
condyle is a plate of fibro-cartilage oval in shape and biconcave. This
cartilage divides the joint into two cavities — one above, the other
below — each of which is provided with a synovial membrane. The
function of the cartilage is to present constantly an articulating surface
to the condyle in the various movements of the lower jaw, which it is
enabled to do by virtue of its mobility.

In the downward movement of the lower jaw each condyle glides
forward, carrying with it the interarticular fibro-cartilage the upper
concave surface of which is applied to the convex surface of the emin-
entia articularis. In the upward movement of the jaw both the con-
dyles and the cartilages pass backward and resume their normal posi-
tion. The movements of depression and elevation are made possible
by the transverse direction of the condyle. In the carnivorous ani-
mals, whose food requires considerable cutting, these movements are
especially well developed. In the antero-posterior movement the jaw
moves in a horizontal direction and the condyles and the articular car-
tilages glide forward and backward in the glenoid fossae. In the rodent
animals the long axis of the condyle runs in the antero-posterior direc-
tion, which allows of a considerable gliding movement. When the
jaw performs a lateral movement, the condyle and cartilage of one side
remain in their normal position, while the opposite condyle and cartil-
age glide forward in the glenoid fossa, directing the symphysis of the
jaw to the opposite side of the median line. The lateral movements
are well exhibited by the herbivorous animals, in which they are quite
extensive, and made possible by the small size of the condyle and the
large extent of articulating surface. In man the structure of the joint
is such as to admit of all these possibilities, and the lower jaw acquires
in consequence great freedom of movement.

The Functions of the Muscles of Mastication. —The move-
ments of the lower jaw are caused by the action of numerous muscles,
which, having a fixed point of origin, are attached to various points on
its surface. The muscles concerned in the movements of mastication
are presented in the following table :

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TEXT-BOOK OF PHYSIOLOGY.

Anterior belly of digastic
Mylohyoid
Geniohyoid
Temporal

Internal portion of masseter
Internal pterygoid
External pterygoids
External portion of masseter
Anterior fibers of temporal
Posterior fibers of temporal
Internal portion of masseter
Digastric, mylohyoid, and genio-
hyoid
Internal pterygoids
External pterygoids
Pterygoids, external and internal
Temporal
Masseter

Depress the lower jaw and open the
mouth.

Elevate the lower jaw and close the

mouth.

Draw the lower jaw forward and
cause the lower teeth to project
beyond the upper.

Draw the lower jaw back to its nor
mal position.

Contracting alternately, draw the
jaw to the opposite side.

Produce grinding movements of the
lower jaw.

The action of the depressor muscles becomes apparent when their
points of origin and insertion are considered. The anterior belly
of the digastric, the mylohyoid, the geniohyoid muscles, agree in having
a similarity of origin— the hyoid bone — and a common area of inser-
tion, the anterior portion of the inferior maxillary. Their anatomic
relation is such that their combined action will depress the lower jaw
and open the mouth.

The elevator muscles arise from various points on the side of the
head, and are inserted into the coronoid process, ramus, and internal
surface of the angle of the lower jaw. When the mouth has been
opened, the simultaneous contraction of these muscles elevates the jaw
and closes the mouth with considerable force. The power of these
muscles is very great, and depends on the shortness and thickness of
the muscle-bundles.

The action of the rotator muscles, those which give rise to the lateral
movements of the jaw, depends in like manner on their origin and in-
sertion. Arising from the superior maxillary and sphenoid bones,
they are inserted into the neck of the condyle and angle of the lower
jaw respectively. When they contract, the condyle on the correspond-
ing side is drawn forward, while the opposite condyle remains stationary.
As a result, the symphysis of the jaw is directed to the opposite side.
The grinding movements of the jaw are produced by the coordinated
action of all the groups of muscles acting more or less successively.

For the proper mastication of the food it is essential that it be kept
between the opposing surfaces of the teeth. This is accomplished by
the contraction of the orbicularis oris and buccinator muscles from
without and the tongue muscles from within.

The Nerve Mechanism of Mastication.*— The movements of
mastication, though originating in efforts of the will and under its
control, are for the most part of an automatic or reflex character; for

* By this term is meant a combination of nerves, afferent and efferent, and nerve
centers which when stimulated coordinates the actions of the organs with which it is
associated.

DIGESTION. 151

when once initiated by a voluntary effort they continue for an indef-
inite period — so long, in fact, as the impressions which the food makes
upon the afferent nerves are received by the nerve-centers which
regulate and control them. That the masticatory movements are of
this reflex nature is shown by the fact that they will be maintained
even though the voluntary effort which called them forth has sub-
sided and the attention has been directed to some entirely different
subject. It would appear that all that is necessary under such con-
ditions is the exciting action of the food upon the periphery of the
afferent nerves distributed to the tongue and mouth.

The nerves involved in this reflex are shown in the following table :

Afferent Nerves. Efferent Nerves,

i. Lingual branch of fifth nerve. 1. Inferior maxillary division of fifth nerve.

2. Glossopharyngeal. 2. Hypoglossal or sublingual.

3. Facial or portio dura.

The nerve-center coordinating the movements of mastication is
situated in the medulla oblongata. The afferent or excitor nerves
which receive the impressions of the food are distributed largely to
the mucous membrane of the tongue. When these impressions are
received by the center in the medulla oblongata, it discharges nerve
impulses, which, passing outward through motor nerves, excite con-
traction in the masticatory muscles. The motor nerves innervating
the muscles are : (1) The small root of the fifth nerve, which, after emerg-
ing from the cavity of the cranium through the foramen ovale, joins
the inferior maxillary division of the large sensor root. It then is dis-
tributed to the masseter, temporal, internal, and external pterygoids,
anterior belly of the digastric, and mylohyoid muscles, and controls
their movements. (2) The hypoglossal nerve, which, after emerging
through the anterior condyloid foramen, passes downward and forward
to be distributed to the extrinsic and intrinsic muscles of the tongue.
(3) The facial or portio dura, which, after emerging from the stylo-
mastoid foramen, is distributed to the muscles of the face. Irritation
of any one of these nerves produces convulsive movements in the mus-
cles to which it is distributed, while their division is followed by paral-
ysis of these muscles. The medulla not only generates the impulses
which are directly responsible for the movements of mastication, but
also coordinates them in such a manner that the movements of mastica-
tion may be directed toward the accomplishment of a definite purpose.

INSALIVATION.

Insalivation is the incorporation of the saliva with the food, and
takes place for the most part during mastication. The saliva ordi-
narily present in the mouth is a complex fluid composed of the various
secretions of the parotid, submaxillary, and sublingual glands and
the muciparous follicles of the mouth, which collectively constitute
the salivary apparatus (Fig. 65).

i5'

TEXT-BOOK OF PHYSIOLOGY.

The parotid gland is situated in front of and partly below the
external ear, where it is held in position by the fascia and skin. From
the anterior border of the gland there emerges a duct (Stensen's),
which, after crossing the masseter muscle to its anterior border, turns
inward, pierces the buccinator, and opens on the surface of the cheek
opposite the second upper molar tooth.

The submaxillary gland is situated below the jaw in the anterior
part of the submaxillary triangle. From the gland there emerges a

duct (Wharton's) which opens
into the mouth by a minute
orifice on the surface of a
papilla by the side of the
tongue.

The sublingual gland is
situated just beneath the mu-
cous membrane in the anterior
part of the mouth, where it
forms a projection between
the gums and tongue. The
posterior part of the gland
gives origin to a duct (the
duct of Rivinus, described also
by Bartholin) which opens
into the mouth with or very
near to the duct of Wharton.
The anterior part of the gland
gives origin to a varying num-
ber of ducts (Walthers) which
open separately along the edge
of the sublingual plica of the
mucous membrane.

Histologic Structure of
the Salivary Glands.— In
their ultimate structure the
salivary glands bear a close
resemblance to one another.
They are compound tubulo-alveolar glands composed of small
irregularly shaped lobules united by areolar tissue and connected
by branches of the salivary ducts. Each lobule is made up of a
number of small alveoli or acini more or less tubular in shape which
are the terminal expansions of the smallest ducts. (See Fig. 66.)
The wall of the acinus is formed by a reticulated basement mem-
brane, surrounded externally by blood-vessels, the spaces between
which constitute lymph-spaces or channels. The inner surface of
the acinus membrane supports a single layer of irregular spheric
or polygonic epithelial cells. The cells do not entirely fill up the
cavity of the acinus, but leave an intercellular space, the lumen,

Fig. 65. — Salivary Glands, i, 2. Parotid.
3. DuctofSteno. 4. Submaxillary. 5. Sublin-
gual. 6. Mylohyoid muscle. 7. Lingual branch
of the fifth nerve. 8. Duct of Wharton. 9. Di-
gastric muscle. 10. Sternomastoid muscle. 11.
External jugular vein. 12. Facial vein. 13.
Temporal vein. 14, 15. Internal jugular vein.
16. Branch of the cervical plexus. 17. Sublin-
gual nerve. — (Le Bon.)

DIGESTION.

i53

which constitutes the beginning of the duct for the transmission of
the secretion to the mouth. From each acinus there passes a narrow
intercalary duct lined by a layer of flattened cells. The common
excretory duct— formed by the union of the intralobular and inter-
lobular ducts— consists also of a basement membrane, lined, however,
by tall columnar epithelial cells. The salivary glands are abundantly
supplied with blood-vessels and nerves which are closely related to
their activity.

Based partly on the character of their secretions and partly on the
microscopic appearance of their secreting cells, the salivary glands
have been divided by Heidenhain into two classes: viz., serous or al-
buminous, and mucous glands. To the first class belong the parotid,
a portion of the submaxillary, and a
portion of the glands of the tongue.
To the second class belong a portion
of the submaxillary gland, the sub-
lingual, a portion of the glands of the
tongue, the glands of the cheeks, lips,
palate, and pharynx. It is possible
that a single alveolus of any gland may
contain both albuminous and mucous
cells.

In the serous glands the cells are
more or less spheric in shape, nu-
cleated, and almost completely filled
with dark granular material (Fig. 67).
In the mucous glands the cells are
large, clear in appearance, and loaded
with a highly refracting material re-
sembling mucin (Fig. 68). Between
the basement membrane and the clear
cells are to be found in the acini of

the submaxillary glands small crescentic shaped cells filled with
granular material which stains deeply with various coloring-matters.
These are known as the demilunes of Heidenhain. At one time it
was supposed that they were young cells destined to take the place
of the clear cells which were believed to be exhausted and to have
undergone disintegration. At the present time they are regarded as
albuminous or serous cells which exhibit changes similar to the cells
of the parotid gland.

The glands embedded in the mucous membrane covering the
tongue, lips, cheek, palate, and pharynx are for the most part lined
with epithelial cells which contain a highly refracting matter similar
to, if not identical with, that found in the cells of the submaxillary
and sublingual glands.

Nerve-supply. — Histologic investigation has demonstrated that
the cells and blood-vessels of the salivary glands are supplied with

Secretory
tubule.

Intercalated
pieces.

Acini.

Fig. 66. — Scheme of the Human
Submaxillary Gland. — (Stdhr.)

154

TEXT-BOOK OF PHYSIOLOGY.

nerve-fibers directly from ganglion cells situated in their immediate
neighborhood. Thus the cells and blood-vessels of the submaxillary
and sublingual glands receive nerve-fibers from the submaxillary,
sublingual and superior cervical ganglia, while the cells and blood-
vessels of the parotid gland receive nerve-fibers from the otic and the
superior cervical ganglia. From their ultimate distribution it may be
inferred that some of the ganglion cells and fibers influence the pro-
duction of the secretions (secretor nerves), while others influence the
caliber of the blood-vessels causing either constriction or dilatation
(vaso-constrictor and vaso-dilatator nerves). (Fig. 69.) The secretor
fibers penetrate the basement membrane enclosing the gland acinus
and finally terminate between and on the surface of the secretory cells.
The vaso-motor fibers terminate between and on the muscle cells in
the walls of the blood-vessels.

The local ganglion cells, however, are in anatomic relation with
nerve-trunks coming directly from the medulla oblongata and the spinal

Fig. 67. — Section of Human Par-
otid Gland Including Several Acini.
d. Cut intralobular duct.— (Piersol.)

Fig. 68. — Section of Human Sub-
lingual Gland. Among the clear cells
lining the mucous acini are nests (g, g)
of granular elements which constitute the
demilunes of Heidenhain. — (Piersol.)

cord. As they approach the ganglia, their terminal branches arborize
around and closely invest the cells of the ganglia and come into in-
timate histologic and physiologic connection with them. The nerve-
fibers coming from the central nerve system are known as pre-ganglionic
fibers, while those coming from the ganglia are known as post-gangli-
onic fibers. Through the intermediation, therefore, of the ganglion
cells, the secretor cells of the salivary glands and the blood-vessels
surrounding them, are brought into relation with the central organs of
the nerve system and become susceptible of being influenced by them.
The Parotid Saliva. — The parotid saliva, as it flows from the
orifice of Stensen's duct, is clear, limpid, free from viscidity, distinctly
alkaline in reaction, with a specific gravity of 1.003. Chemic analysis
shows that it consists of water, a small quantity of proteid matter, a
trace of a sulphocyanogen compound, and inorganic salts. The secre-
tion is increased during mastication, and especially on the side engaged
in mastication. Dry food causes a larger flow than moist food. The
situation of the orifice of the parotid duct is such that as the secretion

DIGESTION.

i55

is poured into the mouth it is at once incorporated with the food by
the movements of the lower jaw, and thus fulfils the physical function
of softening and moistening it.

The Submaxillary Saliva.— The submaxillary saliva is clear,
slightlv viscid, alkaline in reaction, and has a specific gravity of 1.002.
It consists of water, proteid matter (mucin), and inorganic salts.

The Sublingual Saliva.— The sublingual saliva is clear, extremely
viscid, and strongly alkaline in reaction. It consists of water, proteid
matter (chiefly mucin), and inorganic salts.

5-Aeroe
7tJiMeney

Glosso-Pharyngtal

Otic Ganglion
rarotid Gland

Jacoiseris jVen/e

Sub-Maxillary Ganglion^ ~?£

dub-Maxillary Glancfr—

CAorJaTympaniMw Sympathetic Nerves

Fig. 69. — Scheme of the Nerves Involved em the Secretion of Saliva.

The small racemose glands embedded in the mucous membrane
on the inner surface of the cheeks and lips, on the hard and soft
palate, on the tongue and pharynx, secrete a fluid which is grayish
in color, extremely viscid and ropy. It contains a large amount of
mucin.

Mixed Saliva.— The saliva of the mouth is a complex fluid com-
posed of the secretions of all the salivary glands. As obtained from
the mouth it is frothy, opalescent, slightly turbid, and somewhat viscid.
The specific gravity is low, ranging from 1.003 t0 1-006. The reac-
tion is usually distinctly alkaline. It may, however, be neutral or
even acid in reaction if there is any fermentation of food particles in
the mouth or certain disorders of the aliment an* canal. When ex-

156 TEXT-BOOK OF PHYSIOLOGY.

amined with the microscope, the saliva is seen to contain epithelial
cells, salivary corpuscles resembling leukocytes, particles of food,
various microorganisms, and especially Leptothrix buccalis.

The chemic composition of the saliva is shown in the following
table:

COMPOSITION OF HUMAN SALIVA.

Water, 995-!6 994.20

Epithelium, 1.62 2.20

Soluble organic matter, 1.34 1.40

Potassium sulphocyanid, 0.06 0.04

Inorganic salts, 1.82 2.20

1000.00 1000.04
(Jacubowitsch.) (Hammerbacher.)

Water constitutes the main ingredient, amounting to 99.5 per cent.
The soluble organic matter is proteid in character and is a mixture
of mucin, globulin, and serum-albumin. The potassium sulpho-
cyanid is mainly derived from the parotid gland. Its presence can be
demonstrated by the addition of a few minims of a dilute solution of
slightly acidulated ferric chlorid, when a characteristic red color is
developed. The inorganic constituents comprise the sodium, calcium
and magnesium phosphates, sodium carbonate, sodium and potassium
chlorids.

The relative amounts of the different constituents of the saliva
will depend on the relative degree of activity of the different glands,
and this in turn will -be determined by the character of the food.
When the food is dry, there will be an excess of the parotid secretion;
when it partakes of the consistence of meat, there will be a larger
secretion of the submaxillary saliva. The glands apparently adapt
their activity to the character of the food.

Quantity of Saliva. — The estimation of the total quantity of
mixed saliva secreted in twenty-four hours is exceedingly difficult, and
the results obtained must be only approximative. It is, of course,
subject to considerable variation, depending upon habit, the nature of
the food, etc. The experiments of Professor Dalton and the results
obtained by him are eminently trustworthy, and in all probability
represent as nearly as possible the exact amount secreted. He found
that without any artificial stimulus he was enabled to collect from
the mouth about 36 grams (540 grains) of saliva per hour, but that
upon the introduction of any stimulating substance into the mouth
the quantity could be greatly increased. During mastication the saliva
was poured out in greater abundance, the amount depending upon
the relative dryness of the food. He found that wheaten bread ab-
sorbed 55 per cent, of its weight, and fresh cooked meat 48 per cent.
If, therefore, the average quantity of bread and meat required daily
by a man of ordinary physical development and activity be assumed
to be 540 grams (19 oz.) of the former and 450 grams (16 oz.) of the

DIGESTION.

i57

latter, these two substances would absorb respectively 297 grams
(4573.8 grains) and 216 grams (3326.4 grains), making a total of 513
grams (7900 grains). If, therefore, the amount secreted and mixed
with the food during an estimated two hours of mastication be added
to the amount secreted during the remaining twenty-two hours, sup-
posing that it continues at the rate of 36 grams per hour, we have a
total amount of 513 -f- 792 grams, or 1305 grams (19,780 grains),
or about 2.8 pounds.

Histologic Changes in the Salivary Glands during Secretion.
— During and after secretion very remarkable changes take place in
the cells lining the acini, which are in some way connected with the
production of the essential constituents of the salivary fluids. In the
case of the parotid gland, which may be regarded as the type of a
serous or albuminous gland, the following changes have been observed
by Langley (Fig. 70). During the period of rest and just previous to
secretory activity, the epithelial cells are enlarged and swollen, and

Fig. 70. — Cells of the Alveoli of a Serous or Watery Salivary Gland. A
After rest. B. After a short period of activity- C. After a prolonged period of activitv.
— (Yeo's "Text-Book of Physiology.")

encroach on the lumen of the acinus. The protoplasm of the cells
is so completely filled with dark fine granules as not only to obscure
the nucleus, but to almost obliterate the line of union of the cells.
As soon as secretion becomes active, however, the granules begin to
disappear from the outer region of the cell and move toward the inner
border and into the lumen of the acinus. From these observations
it might be inferred that during rest the protoplasm of the cells gives
rise to granular material, and that during and after secretory activity
there is an absorption of new material from the lymph and a recon-
struction of the granular material.

In the submaxillary gland, a portion of which may be taken as
a type of a mucous gland, similar changes have been observed (Fig.
71). During rest the epithelial cells are large, clear in appearance,
highly refractive, and loaded with small globules resembling mucin.
The nucleus, surrounded by a small quantity of protoplasm, lies near
the margin of the cell. That the granules are not protoplasmic in
character is shown by the fact that they do not stain on the addition
of carmine. When treated with water or dilute acids, the globules
swell up, coalesce, and form a uniform mass. The chemic relations

i58 TEXT-BOOK OF PHYSIOLOGY.

of this substance indicate that it is the precursor of mucin — namely,
mucigen. During secretory activity the cells discharge these mucigen
granules into the lumen of the acinus where they are transformed into
mucin. Though the appearance of the gland-cell appears to in-
dicate it, there is no evidence for the view that the cell itself undergoes
disintegration in the process.

The Physiologic Actions of Saliva. — The constant presence of
salivary glands in the different classes of animals and the large amount
of secretion they pour daily into the alimentary canal point to the
conclusion that this mixed fluid»plays an important role in the general
digestive process. Experiment has demonstrated that it has a two-
fold action, physical and chemical.

Physically, saliva softens and moistens the food, unites its par-
ticles into a consistent mass by
means of its contained mucin,
and thus facilitates swallowing.
Chemically it converts starch
into sugar. This action is more
marked with boiled than with
raw starch, a fact which depends
on the physical structure of the
starch grain. In the natural
condition each starch grain con-
Fig. 71.— The Appearance Presented s}sts 0f a cellulose envelope or
by the Cells of the Submaxillary Gland , +1 1 f ■>• 1

of the Dog after Prolonged Secretion. Stroma in tlie mesnes 01 wnicn
— (Modified from Landois and Stirling.) is contained the true starch ma-

terial, the granulose. When
boiled for some minutes, the starch grain absorbs water, the granulose
swells and ruptures the cellulose envelope, after which it passes into
an imperfect opalescent solution more or less viscid, depending on
the relative amounts of water and starch. This is the change largely
brought about by the process of cooking. If a portion of this hydrated
starch be kept in the mouth for a few minutes it will be converted into
sugar, a fact made evident by the sense of taste.

The chemic action of saliva in converting starch into sugar, as
well as the intermediate stages, can be experimentally shown in the
following manner: To 5 volumes of a thin starch solution in a test-
tube add two volumes of filtered saliva. Place the mixture in a
water-bath at a temperature of 35 ° C. In a few minutes the starch
passes into a soluble condition and the fluid becomes clear and trans-
parent. On testing the solution from time to time with iodin the
characteristic blue reaction will be found to gradually disappear, the
color passing from blue to violet, to red, to yellow. If now the solu-
tion be boiled with a solution of cupric hydroxid (Fehling's solution)
a ( opious red or yellow precipitate of cuprous oxid is formed, which
indicates the presence of sugar. The polariscope shows that this
sugar is maltose, CI2H220„. During the conversion of the starch

DIGESTION. 159

intermediate substances are formed to which the term dextrin is
applied. After the starch has been rendered soluble it undergoes
a cleavage into maltose and a dextrin, which, as it gives rise to a red
color with iodin, is termed erythrodextrin. At a later stage this
erythrodextrin also undergoes a cleavage into maltose and a second
variety of dextrin, which, as it does not give rise to any color with
iodin, is termed achroodextrin. It is claimed by some investigators
that this form can also in time be transformed into sugar. It is pos-
sible that a small quantity of dextrose is also formed.

The successive stages of the conversion of starch into sugar may
be represented by the following schema:

, -r, ... , . f Achroodextrin.
Starch = Soluble Starch = {^0™ I MaltOSe-

This change consists in the assumption by the starch of a molecule
of water, and for this reason the process is termed hydrolysis. The
nature of the chemic change is shown in the following formula:

3(C6HI005) + H20 = CI2H22On + C6HioOs
Starch + Water = Maltose + Dextrin.

The amylolytic or starch-changing action of saliva depends on
the presence of an unorganized ferment or enzyme known as -ptyalin.
This enzyme is present in the secretion of each of the salivary glands.
The chemic character of ptyalin is unknown, though there are reasons
for believing that it partakes of the nature of a proteid. It is a prod-
uct in all probability of the katabolic activity of the secretory cells.
According to Latimer and Warren, ptyalin is a derivative of the
zymogen, ptyalogen. This latter compound has been shown to be
present in the glands of the dog, cat, and sheep. Ptyalin effects the
transformation of starch merely by its presence, and undergoes no
perceptible consumption in the process. The activity of this enzyme
is very great, and unless interfered with by an excess of sugar and
dextrin, it acts practically indefinitely.

The activity of ptyalin is modified by various external conditions,
among which may be mentioned the chemic reaction of the medium
in which it is placed. It is most active when the medium is moder-
ately alkaline. Its activity is arrested either by strong alkalies or
acids, though the presence of a small percentage of an acid does not
appear to have any effect in either hastening or retarding the process.
This fact has a bearing upon the question as to whether the action of
the saliva is interfered with in the stomach by the presence of the
gastric juice. At present it is a disputed matter, but the weight of
authority is in favor of the view that the transforming action may
continue for almost half an hour during the early stages of gastric
digestion. The temperature also influences the rapidity with which
the transformation of the starch is effected. At a temperature of
950 to 1060 F. the ptyalin acts most energetically, while its activity

i6o TEXT-BOOK OF PHYSIOLOGY.

is entirely arrested by reducing the temperature to the freezing-point
or raising it to the boiling-point.

The Nerve Mechanism of the Secretion of Saliva. — The
secretion of the saliva is a complex act and involves the cooperation
of gland-cells, blood-vessels, and nerves. During the intervals of
mastication the glands are practically at rest as far as the discharge
of saliva is concerned. The cells, however, are actively engaged in
absorbing from the surrounding lymph-spaces materials derived
from the blood from which they construct their characteristic con-
tents. The blood-vessels possess that degree of dilatation necessary
for nutritive purposes.

With the beginning of mastication the blood-vessels suddenly
dilate, the blood-supply is increased, and the gland-cells begin to dis-
charge water, inorganic salts, and their organic constituents into the
lumen of the acinus. This continues until mastication ceases, when
all the structures return to their former condition of relative inactivity.
The entire process is reflex in character and takes place through the
medulla oblongata. It requires the usual mechanism necessary for
all reflex acts — viz., a sentient surface, afferent nerves, emissive
cells, efferent nerves, and the responsive organs.

With the introduction of food into the mouth impressions are
made on the terminal branches of the afferent nerves distributed in
the mucous membrane. Nerve impulses developed by the mechanic
and chemic action of the food are then transmitted to the medulla
oblongata and received by emissive cells. These in turn discharge
nerve impulses which are transmitted through efferent nerves to the
structures, producing the vascular and secretory effects already stated.

The nerves and nerve-centers which constitute the reflex mechan-
ism for the secretion of saliva are shown in the following table :

Afferent Nerves. Nerve-centers. Efferent Nerves.

i. Lingual branch of fifth Medulla oblongata. Chorda tympani for the submax-
nerve. illary and sublingual glands,

auriculo-temporal branch of the

2. Taste fibers in the chorda fifth nerve for the parotid gland.

tympani.

3. Glossopharyngeal. Sympathetic nerve for all glands.

That the secretion of the saliva is regulated by the above mechan-
ism, and that the lingual branches of the fifth nerves and the glosso-
pharyngeal are the afferent nerves, can be demonstrated by exposing
the glands and their nerve connections and subjecting them to ex-
periment. Under such circumstances, if a cannula be placed in the
duct of the submaxillary gland, and the lingual nerve stimulated by
an induced electric current of moderate strength, a copious flow of
saliva at once takes place. If now the glossopharyngeal nerve be
stimulated in a similar manner, the effect on the secretion will be the
same. Division of these two nerves in an animal, in such a wav as to

DIGESTION. 161

prevent the nerve impulses from reaching the medulla oblongata, is
followed by a marked diminution in the amount of saliva secreted.
The reflex centers, however, may receive impulses and be excited to
activity by impulses coming through other nerves — e. g., the pneumo-
gastric, when the mucous membrane of the stomach is stimulated;
the sciatic, when after division its central end is stimulated.

The central mechanism which causes through its efferent nerves the
discharge of saliva is also capable of being excited to activity by psychic
influences. It is well known that ideas and emotional states developed
by the sight and the odor of foods, especially after long abstinence,
cause a discharge of saliva into the mouth. The watering of the
mouth under such circumstances is a demonstration of the influence
of such psychic states. This fact was experimentally demonstrated
on dogs by Pawlow. This investigator caused the ducts of the glands
to be brought to the surface in such a manner that they healed into the
edges of the skin wounds. By means of suitable receivers applied
over the orifices of the ducts, the saliva could be readily collected.
When the dog, under such circumstances, was tempted by the sight
of food there was at once a free discharge of saliva.

Whenever the central mechanism is stimulated, either by nerve
impulses coming through afferent nerves, from the periphery or from
the brain, impulses are generated which pass outward through efferent
nerves — the chorda tympani nerve to the submaxillary and sublingual
glands, the auriculo-temporal nerve to the parotid gland, and the
sympathetic nerve to all three.

The Chorda Tympani.— The chorda tympani nerve is a branch
of the facial, the trunk of which it leaves in the aqueduct of Fallopius.
It then crosses the tympanic cavity, emerges through the Glaserian
fissure, and joins the lingual branch of the inferior maxillary division
of the fifth nerve. After passing forward as far as the sublingual
gland, nearly all of the original fibers leave the lingual nerve by four
or five strands to become connected by terminal branches with nerve-
ganglion cells in relation with the submaxillary and sublingual glands.
(See Fig. 69.)

The effects on the secretion and flow of saliva from the submaxil-
lary gland which follow division and stimulation of the chorda tym-
pani nerve are shown in the following way: a cannula is inserted into
Wharton's duct and the rate of flow estimated; the nerve is then
divided, after which the flow ceases. The peripheral end of the
nerve is then stimulated with the induced electric current, when a
copious secretion of a thin saliva takes place, accompanied by a
marked dilatation of the blood-vessels of the gland. The quantity of
blood passing through the vessels is so great as to give to the venous
blood an arterial hue and to the small veins a distinct pulsation. It
would appear from these effects that the chorda contains two sets of
fibers, one of which inhibits the action of a local vaso-motor mechan-
ism permitting the blood-vessels to dilate (vaso-dilatator fibers), the

i62 TEXT-BOOK OF PHYSIOLOGY.

other of which stimulates the secretor cells to activity, either directly
or through the intermediation of local ganglia. That local ganglia
are involved is shown by the effects which follow the injection of
nicotin into the circulation. After a sufficient dose — 10 milligrams
for the cat — stimulation of the chorda has no effect. Stimulation of
the nerve-plexus beyond the ganglion, however, is at once followed
by vascular dilatation and secretion.

It might be inferred that the increase in the flow of saliva is due
to filtration, the result of the increased blood-supply to the gland, and
not to the influence of any true secretor fibers stimulating the activities
of the secretor cells. That this is not the case, however, can be
demonstrated in several ways: First, the pressure in the duct of the
submaxillary gland, as shown by the mercurial manometer, rises,
when the gland is secreting, considerably above the pressure in the
carotid artery, which could not be the case if it were clue to a mere
filtration; for if pressure alone were the cause, the flow of saliva would
cease as soon as the pressure in the tube equaled the pressure in the
blood-vessels. Second, even in the absence of blood the gland can
be made to yield a secretion, as shown by stimulating the nerve in a
recently killed animal. Third, after the injection of atropin into the
circulation the secretion is abolished, but the local vaso-motor mechan-
ism is unimpaired, for stimulation of the nerve, as in the previous
instance, gives rise to a dilatation of the vessels and an increased
blood-supply. There is thus abundant proof that the chorda tym-
pani contains two sets of fibers — one regulating the blood-supply to
the gland, the other stimulating the secretor cells.

The Auriculo-temporal Nerve. — The nerve-fibers which conduct
nerve impulses outward from the medulla to the parotid gland are
believed to pass through the glossopharyngeal nerve, through the
tympanic branch or nerve of Jacobson, to the otic ganglion, with which
they become connected. From this ganglion new nerve-fibers arise
which pass into the fifth nerve and reach the secretor cells of the
parotid gland through the auriculo-temporal nerve.

The influence of the auriculo-temporal branch of the fifth nerve
on the parotid gland is similar to the action of the chorda tympani
on the submaxillary gland. The active fibers of this nerve are prob-
ably derived from the ninth nerve or glossopharyngeal. If the nerve
be stimulated by the induced electric current, there follows a dilata-
tion of the blood-vessels and an abundant discharge of a thin saliva,
rich in water and salts, but containing a small amount of organic
matter. Division of the nerve, extirpation of the otic ganglion, or
division of Jacobson's nerve, is followed by a loss of reflex secretion.
Stimulation of Jacobson's nerve, as shown by Hcidenhain, gives rise
to the secretion.

The Sympathetic Nerves. — The sympathetic fibers which in-
fluence the .salivary secretion emerge from the spinal cord mainly
through the second, third, and fourth thoracic nerves. After passing

DIGESTION. 163

into the sympathetic chain they ascend to the superior cervical ganglion,
with the cells of which they become connected through the interme-
diation of fine terminal branches. From this point non-medullated
nerve-fibers follow the branches of the external carotid artery to the
different glands. There is no evidence that these fibers have any
connection, anatomic or physiologic, with local ganglia at or near
the submaxillary, sublingual or parotid glands. If the sympathetic
nerve in the neck, especially in the dog, be divided and the peripheral
end stimulated with the induced electric current, there is at once a
contraction of the smaller blood-vessels of the submaxillary and sub-
lingual glands and a diminution of the blood-supply, a result showing
the presence of vaso-constrictor fibers. Nevertheless both the sub-
maxillary and sublingual glands pour out a saliva which is different
from that poured out when the chorda tympani is stimulated. The
quantity is less, it is more viscid, richer in organic matter, of a higher
specific gravity, and more active in the transformation of starch into
sugar.

Stimulation of the sympathetic fibers passing to the parotid gland
is followed by contraction of the vessels and an abolition of the secre-
tion; but at the same time there is an increased activity of the secretor
cells, for subsequent stimulation of the auriculo-temporal nerve not
only causes an increase in the amount of water and inorganic salts,
but an increase also in the amount of organic matter far beyond thai
produced when the auriculo-temporal has alone been stimulated.
Histologic examination shows that the small ducts of the gland are
filled with thick organic matter after stimulation of the cervical sym
pathetic.

The foregoing facts led Heidenhain to the conclusion that there
are two physiologically distinct efferent nerve-fibers distributed to the
glands, viz., trophic nerves, derived from the sympathetic which
stimulate the cells to the production of organic constituents; and
secretor nerves, derived from the cranial nerves, which stimulate the
cells to the production of water and inorganic salts. This view has
however, been controverted by Langley who regards the secretor
fibers to the glands as essentially the same, and considers the differ-
ences in the character of the secretion to be dependent on differences
in the quantity of the blood-supply induced by the simultaneous
stimulation of the vaso-motor nerves.

DEGLUTITION.

Deglutition is that part of the digestive process which is concerned
in the transference of the food from the mouth through the pharynx
and esophagus into the stomach. This is an extremely complex act
and involves the action of a large number of structures, all of which
are made to act in proper sequence under the coordinating influence
of the nervous system. The deglutitory canai consists of the mouth.

164

TEXT-BOOK OF PHYSIOLOGY.

pharynx, and esophagus, each of which presents certain anatomic
features on which its physiologic action depends.

The cavity of the mouth communicates posteriorly with the pharynx
by a narrow orifice, the isthmus of the fauces. This orifice is bounded
above by the soft palate, laterally by the anterior and posterior half
arches, and below by the tongue.

The pharynx is an oval-shaped cavity extending from the base of

Fig. 72. — Vertical Section of the Nasal Fossa and Mouth, i. Left nares.
2. Lateral cartilage of the nose. 3. Portion of the internal alar cartilage forming
the skeleton of the lower part. 4. Superior meatus. 5. Middle meatus. 6. Inferior
meatus. 7. Sphenoidal sinuses. 8. External boundary of the posterior nares. q.
Internal elliptical opening of the Eustachian tube. 10. Soft palate, n. Vestibule
of the mouth. 12. Vault of palate. 13. Genioglossus muscle. 14. Geniohyoid
muscle. 15. Cut margin of the mylohyoid muscle. 16. Anterior pillar of the palate
(anterior half-arch), presenting a triangular figure with the base inferiorly, covering
partly the tonsil. 17. Posterior pillar (posterior half-arch) of the palate. 18. Tonsil.
19. Follicular (mucous) glands at the base of the tongue. 20. Cavity of the larynx.
21. Ventricle of the larynx. 22. Epiglottis. 23. Cut os hyoides, 24. Cut thyroid
cartilage. 25. Thyrohyoid membrane. 26. Section of posterior portion of the cricoid
cartilage. 27. Section of the anterior portion of the same cartilage. 28. Crico-thyroid
membrane. — (Sappey.)

the skull to the lower border of the cricoid cartilage, a distance of
about 12 centimeters. (See Fig. 72.) Its walls are formed mainly by
three pairs of muscles — the superior, middle, and inferior constrictors
—each consisting of red, striated muscle-fibers, and hence capable
of rapid and- energetic contractions. Superiorly the pharynx is
attached to and supported by the basilar process of the occipital
bone; inferiorly it becomes continuous with the esophagus. The
anterior wall of the pharynx is imperfect and presents openings which

DIGESTION. 165

communicate with the nasal chambers, the mouth, and the larynx.
The lateral wall on either side presents the opening of the Eustachian
tube which leads directly into the cavity of the middle ear. The
interior of the pharynx is lined by mucous membrane. The pharynx
is partially separated from the mouth by the velum pendulum palati,
a muscular structure attached above to the hard palate; its lower
edge or border is directed downward and backward and presents in
the middle line a conical process, the uvula. On either side the palate
presents two curved arches, the anterior and posterior, formed re-
spectively by the palato-glossei and palato-pharyngei muscles. The
laryngeal orifice or glottis is placed just beneath the base of the tongue.
It is triangular in shape, wide in front, narrow behind, and directed
downward and backward. It is bounded above by a thin plate of
cartilage, the epiglottis, placed just behind the tongue and so arranged
that it can easily be depressed and elevated.

The esophagus, the continuation of the- deglutitory canal, extends
downward from the lower border of the cricoid cartilage for a dis-
tance of from 22 to 25 centimeters, to a point opposite the ninth
thoracic vertebra, where it expands into the stomach. Its walls are
composed of an internal or mucous and an external or muscular coat,
united by areolar tissue. The muscular coat consists of an external
layer of longitudinal fibers arranged in three bands and of an internal
layer composed of fibers arranged circularly in the upper part and
obliquely in the lower part of the esophagus. In the upper third
the fibers are striated; in the middle third they are a mixture of both
striated and non-striated; in the lower third they are entirely non-
striated.

The muscle fibers surrounding the esophago-gastric orifice are
arranged in the form of and play the part of a sphincter muscle, and
for this reason may be termed the sphincter cardial muscle. By its
action it prevents a return under normal conditions of food into the
esophagus.

The deglutitive act may be for convenience divided into three
stages, viz. :

1. The passage of the food from the mouth into the pharynx.

2. The passage of the food through the pharynx into the esophagus.

3. The passage of the food through the esophagus into the stomach.

In the first stage the bolus of food is placed on the superior surface
of the tongue. The mouth is then closed and respiration is momen-
tarily suspended. The tip of the tongue is placed against the pos-
terior surfaces of the teeth. The tongue, because of its intrinsic
musculature, then arches from before backward against the roof of
the mouth and pushes the bolus of food through the isthmus of the
fauces into the pharynx. This completes the first stage. It is a
voluntary effort and accomplished partly by the tongue, though, as
shown by Meltzer, mainly by the mylohyoid muscles.

The second and third stages, or the passage of the food through

166 TEXT-BOOK OF PHYSIOLOGY.

the pharynx and esophagus into the stomach, have been attributed
until quite recently entirely to peristaltic movements of their muscu-
lature.* It has been stated that with the passage of the food through
the isthmus of the fauces the posterior wall of the pharynx advances
and seizes the food, and in consequence of a rapid peristaltic move-
ment running through its constrictor muscles from above downward
is transferred to the esophagus; that with the entrance of the food
into the esophagus a similar peristalsis, varying in rapidity in different
sections in consequence of a change in the character of its muscula-
ture, gradually transfers the food into the stomach. There can be
but slight doubt that by this method the bolus of food, especially if
it is of firm consistence and of a size sufficient to distend the esoph-
agus, is transferred into the stomach, but that it is the exceptional
rather than the usual method has been demonstrated by Kronecker,
Falk, and Meltzer.

In 1880 the first of these experimenters made the observation that
the sensation in the stomach following the swallowing of a mouthful
of cold water occurred too quickly to be explained by the prevalent
belief that its transference was caused by ordinary peristalsis, the rate
of progression of which was known to be slow. Falk then discovered
the fact, by introducing through the mouth into the pharynx a tube
connected externally with a water manometer, that during the act of
swallowing there is a sudden rise of pressure equal to about twenty
centimeters of water.

These experiments demonstrated that at the beginning of degluti-
tion there is a sudden rise of pressure, the result of a quickly acting
force resident in the mouth or pharynx, in consequence of which the
food is rapidly thrown down into the stomach, peristalsis playing no
part in the process. The proof, however, of these statements was
furnished by Meltzer. This observer introduced into the pharynx
and esophagus rubber tubes, the ends of which were provided with
thin-walled rubber balloons which could be distended with air. The
outer ends of the tubes were connected with Marey's recording tam-
bours. Any compression of the balloon would be followed by the
passage of the air into the tambour and an elevation of the lever.
With one balloon in the pharynx and the other in the esophagus at
varying depths, and the recording levers of the tambours applied
against the surface of a revolving cylinder, it became possible, with
the addition of a chronogram, to obtain a graphic representation of
the time relations of simultaneous and successive compressions of
the two balloons.

It was found as the result of many experiments that no matter

* Peristalsis may be defined as a progressive wave-like movement which passes over
different portions of the walls of the alimentary canal. Its effect physiologically is the
propulsion of its solid and semisolid contents. It is characterized by a contraction of
the muscle-fibers behind the object and an inhibition or relaxation of the muscle-fibers in
front of it. (Bayliss and Starling.)

DIGESTION.

167

how deep the position of the esophageal balloon, it was compressed
simultaneously with the pharyngeal balloon, as shown by the rise of
the levers on swallowing a mouthful of water. The interval of time
between the rise of the two levers did not amount to more than the
tenth of a second. The inference was that the water was projected
or shot down the pharynx and esophagus in this period of time, and
in its passage compressed both balloons practically at the same in-
stant. The same was found to be true when small masses of more
consistent food were swallowed.

The curves of the entire deglutitive act recorded by the two levers
are, however, different in form. (See Fig. 73.) The pharyngeal curve,
1, presents two crests, the first, A, being due to the compression caused
by the passage of the bolus, the second, B, due to the compression

Fig. 73. — Tracing of the Act of Deglutition, i. A indicates the compression of
the elastic bag caused by the bolus projected by the contraction of the mylohyoid muscles.
B. Contraction of the pharynx. 2. Line marking seconds. 3. Tracing of the bag
in the. esophagus 12 cm. from the teeth. C. Compression of the bag by the bolus corre-
sponding to A. D. Compression by the residues of the bolus carried on by the contraction
of the pharynx, B. E. Contraction of the esophagus. — (Landois and Stirling.)

exerted by the contraction of the pharyngeal muscles. The interval
of time between these two crests amounts to not more than 0.3 second.
In the esophageal curve, 3, the elevation, C, corresponds to the eleva-
tion, A, and is likewise due to the compression exerted by the bolus.
The interval of time between the beginning of the first and second
curves was not more than 0.1 second, regardless of the depth to which
the esophageal balloon was plunged. At a later period a second rise
of the lever was recorded; the time of its appearance, height, duration,
etc., were found to increase with the depth of the balloon.

These facts demonstrate that deglutition consists of two phases:
(1) a rapid rise of pressure in the pharynx, as a result of which the
bolus is suddenly shot down to the lower end of the esophagus; (2)
a peristaltic contraction of the musculature of the canal, which, acting
as a supplementary force, carries onward any particles of food in the
canal and forces the bolus through the closed sphincter cardicr at the
end of the esophagus.

1 68 TEXT-BOOK OF PHYSIOLOGY.

The immediate cause of the sudden rise of pressure was shown by
Meltzer to be the contraction of the mylohyoid muscles. When
the nerves going to these muscles were divided in a dog, deglutition
was practically abolished. These muscles are probably assisted in
their action by the contraction of the hyoglossus muscles as well as
the tongue itself.

It was also demonstrated in these experiments that the contrac-
tion of the esophagus did not partake of the character of ordinary
peristalsis. It was found that the esophagus contracted in three
distinct segments, corresponding in all probability to the difference in
the character of their muscular fibers. The first segment, about six
centimeters in length, was found to begin to contract about 1.2 seconds
after the beginning of the first curve and lasting 2 seconds; the second
segment, about twelve centimeters in length, beginning to contract
about 1.8 seconds or 3 seconds after the beginning of the first section,
and lasting for from 5 to 7 seconds; the third segment, six centimeters
in length, contracting from 6 to 7 seconds. The beginning and the
end of the contraction for each segment occurred simultaneously
throughout its entire extent. If, however, a series of deglutitory acts
follow each other in quick succession, there is an inhibition of the
peristaltic contractions until after the final swallow.

An examination of the action of the esophagus during degluti-
tion, made by Cannon and Moser with x-rays and the fluoroscope,
disclosed the fact that the method of food transmission varied in
different animals. In the cat and dog the transmission was effected
by peristalsis alone. The time required for the food to reach the
stomach varied in the cat from nine to twelve seconds and in the
dog from four to five seconds. The descent of the bolus was more
rapid in the upper than in the lower part of the esophagus. In man,
liquids descended rapidly, at the rate of several feet a second, in con-
sequence of the rapid and energetic contraction of the mylohyoid mus-
cles. A peristaltic contraction, passing over the entire esophagus,
was necessary to the passage of solid and semisolid food through it,

Closure of the Posterior Nares and Larynx. — Notwithstand-
ing the rise of pressure in the pharynx during the act of swallowing,
it is seldom under normal circumstances that any portion of the
bolus ever finds its way either into the larynx or nasal chambers,
for the reason that the openings of these cavities are fully closed by
appropriate means.

At the moment the food passes into the pharynx the posterior
nasal openings are closed against the entrance of the food by a septum
formed by the pendulous veil of the palate and the posterior half
arches. The palate is drawn upward and backward until it meets
the posterior wall of the pharynx, and at the same time is made tense,
by the action of the levator palati and tensor palati muscles respec-
tively (Fig. 74). This septum is completed by the advance toward
the middle line of the posterior half arches caused by the contraction

DIGESTION.

169

of the muscles which compose them — the palato-pharyngei. When
these structures are impaired in their functional activity, as in diph-
theritic paralysis and ulcerations, there is not infrequently a regurgita-
tion of food, especially liquids, into the nose.

The larynx is equally protected against the entrance of food during
deglutition under normal circumstances. That this accident occasion-
ally happens, giving rise to severe spasmodic coughing, and even in
extreme cases to suffocation, is abundantly shown by the records
of clinical medicine. Usually it does not occur, for the following rea-
sons: Just preceding and during the act of deglutition there is a com-
plete suspension of the act of
inspiration by which particles of
food might otherwise be drawn
into the larynx; at the same time
the larynx is always drawn well
up under the base of the tongue
and its entrance closed by the
downward and backward move-
ment of the epiglottis.

The action here attributed
to the epiglottis has been denied
by Stuart and McCormick.
These observers had the oppor-
tunity of looking into a naso-
pharynx which had been laid
open by a surgical operation for
the removal of a morbid growth.
In this patient, the epiglottis, at
the time of deglutition, was
always more or less erect and
closely applied to the base of the
tongue. So complete was this
that the food passed over its
posterior or inferior surface for a certain distance,
it ever observed to fold backward like a lid.

Because of the possibility that this position of the epiglottis was
due to pathologic causes, Kanthack and Anderson instituted a new
series of experiments with a view of determining the action of the
epiglottis. As a result of many experiments on animals and of ob-
servations on themselves, these observers reaffirm the generally
accepted view, that under normal conditions, the entrance of the
larynx is always closed by the epiglottis after the manner of a lid.

In addition to the downward and backward movement of the epi-
glottis and the ascent of the larynx under the base of the tongue, it is
also certain from the observations of Meltzer that the larynx is protected
from the entrance of food, in the rabbit at least, by the closure of the
glottis itself. This experimenter noticed, while observing the interior

TrcocJiea/

Fig. 74. — Diagram Showing the Man-
ner of Closure of the Posterior Nares
and Larynx during Deglutition. — (Lan-
dois and Stirling.)

In no instance was

i7o TEXT-BOOK OF PHYSIOLOGY.

of the larynx, both from above, through an opening in the hyothyreoid
membrane, and from below, through an opening in the trachea that
when an act of deglutition was excited by touching the soft palate with
a sound, there was simultaneously with the contraction of the mylo-
hyoid muscles, a firm closure of the glottis. This was accomplished
by an approximation of the true vocal bands, a close approximation and
a downward and forward movement of the arytenoid cartilages, until
they almost touched the anterior wall of the thyreoid cartilage. This
movement preceded the ascent of the larynx. When the larynx was
separated from all surrounding structures with the exception of the
laryngeal nerves, a touch of the palate excited the same phenomena.
Under such circumstances the closure of the glottis must have been
due to the contraction of its own intrinsic muscles and in consequence
of a reflex action through the inferior laryngeal nerves.

The Nerve Mechanism of Deglutition. — Deglutition is almost
exclusively a reflex act throughout its entire extent, and requires for
its inauguration merely a stimulus to some portion of the mucous
membrane of the deglutitory canal. The first stage is primarily
voluntary, but from inattention to the process may become secon-
darily reflex. The origin and course of the afferent nerves, stimu-
lation of which excite reflexly the movements of the pharynx and
esophagus, however, are practically unknown. In the rabbit deg-
lutition can be excited by stimulating the anterior central part of
the soft palate; in man it has not yet been possible to locate an area
stimulation of which will give rise to a reflex deglutitory act. Though
electric stimulation of the superior laryngeal nerve will cause reflex
deglutitory movements, it is obvious that the terminals of this nerve
can not be the source of the natural afferent impulses. Stimulation
of the glossopharyngeal nerve causes an inhibition of the movements.

The center from which emanate nerve impulses which excite the
various muscles to action has been located experimentally in the
medulla oblongata just above the alse cinereae. The efferent nerves
comprise branches of the facial, hypoglossal, motor filaments of the
third division of the fifth nerve, motor filaments of the glossopharyn-
geal and vagus derived in all probability directly from the medulla
oblongata. Inasmuch as the different mechanisms of this reflex, act
not only in a coordinate but sequential manner, it would appear as if
the deglutition center sent out, in response to the nerve impulses com-
ing from a single peripheral area,' a series of nerve impulses successively
to succeeding portions of the canal, through the groups of nerve-cells
corresponding to the origins of the efferent nerves. That this orderly
and progressive peristalsis usually observed is due to a sequence of
changes in the central nerve system is shown by the fact, that if the
esophagus is divided or a ring of it excised, the extremity in connection
with the stomach will exhibit a well-marked peristalsis after a short
interval, when an act of deglutition is excited in the customary manner.
The efferent nerve fibers, which stimulate the esophageal muscles to

DIGESTION. 171

action are contained in the trunk of the vagi nerves for after their
division the peristalsis is abolished.

In addition to this primary reflex mechanism, the esophagus appears
to possess a secondary reflex mechanism consisting of a series of reflex
arcs, whose afferent and efferent paths are found in the trunk of the
vagus and both connected with successive portions of the esophagus.
The first mechanism is temporarily suspended during deep anesthesia
while the second persists. (Meltzer.)

Though the peristalsis of the esophagus is excited by nerve impulses
coming through the vagus nerves and is abolished by their division,
Cannon has shown by means of the Rontgen rays that this effect
for the lower portion of the esophagus, at least in the cat and monkey,
is of a temporary duration only, lasting from one to several days, after
which a peristalsis again develops and of sufficient vigor to force food
through the cardiac orifice into the stomach. The muscle coat of this
portion of the esophagus is composed of non-striated muscle-fibers, is
supplied with a myenteric nerve plexus and resembles lower portions
of the alimentary canal. It is capable of developing a peristalsis,
solely in response to the pressure of food within and independent of
extrinsic nerves.

GASTRIC DIGESTION.

After the food has passed through the esophagus it is received by
the stomach, where it is retained for a variable length of time, during
which important changes are induced in its physical and chemic com-
position. The disintegration of the food inaugurated by mastication
and insalivation is still further carried on in the stomach by the sol-
vent action of the acid fluid there present, until the entire mass is
reduced to a liquid or semiliquid condition.

The stomach is dilated and highly specialized portion of the
alimentary canal intervening between the esophagus and small intes-
tine. When moderately distended with food, it is somewhat conical
or pyriform in shape and slightly curved on itself. It is situated
obliquely and in some individuals almost vertically in the upper part
of the abdominal cavity, extending from the left hypochondrium to
the right of the epigastrium. The dimensions and capacity of the
stomach undergo considerable periodic variation according to the
extent to which it is distended by food. In the average condition it
measures in its long diameter from 25 to 35 centimeters, in its vertical
diameter at the cardia 15 centimeters, in its antero-posterior diameter
from 11 to 12 centimeters. The capacity of the stomach varies
from 1500 to 1700 c.c. In the empty condition its walls are con-
tracted and partly in contact, and the entire organ is drawn up
into the upper part of the abdominal cavity. The opening through
which the food passes into the stomach is known as the esoph-
ago-gastric orilice or the cardia. The opening through which it

172

TEXT-BOOK OF PHYSIOLOGY.

passes into the intestine is known as the pylorus, the pyloric or gas tro-
duodenal orifice. Between these two orifices the stomach along its
upper border presents a curve and along its lower border a much larger
curve, known as the lesser and greater curvatures respectively. The
left end of the stomach is termed the fundus or cardiac end; the right,
the pyloric end. Passing from the fundus toward the pylorus, the
stomach gradually narrows, and at a point situated about 5 cm. from
the pyloric opening it frequently presents a constriction which divides
the general cavity into two portions: viz., the fundus and the antrum
of the pylorus.

The walls of the stomach are formed by four distinct coats united

Fig. 75. — Fibers Seen with the Stomach Everted. 1,1. Esophagus. 2.
Circular fibers at the esophageal opening. 3, 3. Circular fibers at the lesser curvature.
4, 4. Circular fibers at the pylorus. 5, 5, 6, 7, 8. Oblique fibers. 0, 10. Fibers of
this layer covering the greater pouch. 11. Portion of the stomach from which these
fibers have been removed to show the subjacent circular fibers. — (Sappey.)

by areolar tissue and named, from without inward, as the serous,
muscle-, submucous, and mucous.

The external or serous coat is thin and transparent and formed by a
reduplication of the general peritoneal membrane.

The middle or muscle-coat consists of three layers of non-striated
muscle-fibers, named from their direction the longitudinal, circular,
and oblique (Fig. 75). The longitudinal fibers are most abundant
along the lesser curvature and are a continuation of those of the
esophagus; over the remainder of the stomach they are thinly scat-
tered, but toward the pyloric orifice they are more numerous and
form a tolerably thick layer which becomes continuous with the
fibers of the small intestine. The circular fibers form a complete

DIGESTION.

173

Mucosa.

layer encircling the entire organ, with the exception, perhaps, of a
portion of the fundus. The fibers of this coat cross the longitudinal
fibers at right angles. At the lower end of the esophagus and sur-
rounding the cardia the circular muscle fibers form a true sphincter
which is known as sphincter cardia. At the junction of the fundus
with the pyloric antrum the circular fibers are arranged in a well-de-
fined bundle termed the sphincter antri pylorici. In the pyloric region
the circular fibers are more closely arranged, forming thick well-
defined rings. At the pyloric opening the circular fibers are again
crowded together and form
a distinct muscle band — the f Epithelium.

sphincter pylori — which
projects for some distance
into the interior of the
stomach. It has been
stated by Riidinger that
the inner fibers of the lon-
gitudinal coat become con-
nected with this circular
band and constitute a dis-
tinct muscle, the dilatator
pylori. The oblique fibers
are most distinct over the
cardiac portion of the
stomach, but extend from
left to right as far as the
junction of the middle and
last thirds of the stomach.
They are continuations of
the circular fibers of the
esophagus.

The submucous coat con-
sists of loose areolar tissue
carrying blood-vessels,
nerves, and lymphatics. It
serves to unite the muscle

to the mucous coat. Its inner surface bears a thin layer of muscular
tissue, the muscularis mucosa, which supports the mucous membranes.
The internal or mucous coat is loosely attached to the muscular
coat. In the empty and contracted state of the stomach it is thrown
into longitudinal folds, or ruga?, which are, however, obliterated when
the organ is distended with food. The mucous membrane in adult
life is smooth and velvety in appearance, gray in color, and covered
with a layer of mucus. Its average thickness is about one millimeter.
The surface of the membrane is covered with a layer of columnar
epithelial cells, each of which possesses a nucleus and nucleolus. At
the pylorus there is a circular involution of the mucous membrane

Serosa

Fig. 76. — Transverse Section of the Wall
of A Human Stomach. X 15- The tunica pro-
pria contains glands standing so close together that
its tissue is visible only at the base of the glands
toward the muscularis mucosa*. — (Stolir.)

J74

TEXT-BOOK OF PHYSIOLOGY.

which is known as the pyloric valve. This is strengthened by fibrous
tissue and embraced by the sphincter muscle previously described.
Gastric Glands. — The surface of the mucous membrane when
examined with a low magnifying power presents throughout innumer-
able depressions polygonal in shape and separated by slightly elevated

ridges. At the bottom of these
spaces are to be seen small orifices,
which are the mouths of the glands
embedded in the mucous mem-
brane. A vertical section of the
gastric walls (Fig. 76) shows not
only the position and appearance of
the glands, but the relation of the
various tissues which enter into the
formation of these walls. An ex-
amination of the mucous membrane
in different regions of the stomach
reveals two distinct types of glands,
cardiac or fundic, and pyloric, which

d-i

Fig. 77. — Peptic Gland from
Stomach of Dog. a. Wide
mouth and duct which receive
the terminal divisions of the
gland, b, c. Neck and fundus of
the tubes, e. Central or chief
cells, d. Parietal or acid cells.
— (After Pier sol.)

u

v&*ri

fe

1

L.v

''~'&\ '

Cj

.Lumen.

Secretory
capillaries.

•-.';'

Fig. 78. — Section of Fundus Gland
OF Mouse. Left upper half drawn after
an alcohol preparation, right upper half
after a Golgi preparation. The entire
lower portion is a diagrammatic combi-
nation of both preparations. — (Stohr.)

differ not only in histologic structure, but also in function. Both types
extend through the entire thickness of the mucosa.

The cardiac or fundic glands are formed by an involution of the
basement membrane of the mucosa and lined by epithelial cells.
Each gland may be said to consist of a short duct, or neck, and a body,
or fundus (Fig. 77). The latter portion is wavy or tortuous and fre-
quently subdivided into as many as 8 to 10 distinct and separate
tubules. The duct is lined by columnar epithelial cells similar to

DIGESTION.

i75

those covering the surface of the mucosa. The lumen of the fundus
is bordered by epithelial cells, cuboid in shape, and consisting of a
granular protoplasm containing a distinct spherical nucleus. These cells
are generally spoken of as the chief or central cells. In addition to the
chief cells, the fundus contains a second variety of cell, which is of a
larger size, of a triangular or oval shape, and consisting of a finely
granular protoplasm. From their situation in and just beneath the
gland wall they have been termed parietal or border cells. Each parie-
tal cell appears to be surrounded and penetrated by a system of pas-
sages which open into the lumen of the gland by means of a delicate
cleft or canaliculus (Fig. 78). Glands with these histologic features
are most abundant in the middle zone of the stomach.

The pyloric glands are also formed by
an involution of the mucous membrane
and lined by epithelial cells (Fig. 79).
The ducts are much longer than the
ducts of the fundic glands. At its ex-
tremity each duct becomes branched,
giving rise to a number, from 2 to 16, of
short tubes, each of which has a large
lumen and communicates with the duct
by a narrow short neck. The ducts are
lined throughout by columnar epithelium.
According to Mall, the total number of
openings on the surface of the mucous
membrane of the dog's stomach is some-
what over 1,000,000, and the total num-
ber of blind tubes opposite the muscularis
mucosa exceeds 16,500,000. According
to Sappey, the surface of the mucous
membrane of the human stomach pre-
sents over 5,000,000 orifices of gastric
glands.

Blood-vessels, Nerves, and Lymphatics. — The blood-vessels
of the stomach after entering the mucosa break up into a number of
branches which are distributed to the muscular and mucous coats.
The branches to the latter soon form a capillary network with oblong
meshes which not only surround the tubules but form a network just
beneath the surface of the mucosa. Veins gradually arise from the
capillaries which empty into the larger veins of the mucosa. The
glands are also supported by processes of smooth muscle-fibers passing
up from the muscularis mucosa.

The nerve-fibers distributed to the stomach are derived from the
vagus and the sympathetic branches of the solar plexus. After pierc-
ing the serous coat the fibers form or unite with a plexus of fibers sit-
uated between the circular and longitudinal layers of the muscle-coat.
At 1 he nodal points of this plexus large nerve-ganglion cells are to be

Fig. 79. — Section of Pylo-
ric Glands from Human Stom-
ach, a. Mouth of gland leading
into long, wide duct (6): into
which open the terminal divisions.
c. Connective tissue of the mu-
cosa.— (After Piersol.)

176 TEXT-BOOK OF PHYSIOLOGY.

found, the whole forming the mechanism known as Auerbach's
plexus. A similar plexus of cells and fibers in more or less intimate
anatomic connection with the foregoing is found between the muscle
and submucous coats, and is known as Meissner's plexus. From
this plexus fine nerve filaments are distributed to muscle-fibers,
blood-vessels, and glands. In the latter structure terminal arbori-
zations have been detected in close contact with the secreting cells
themselves.

The lymphatics, which are quite numerous, originate in the meshes
of the mucosa. The larger trunks enter lymph-glands lying along the
greater and lesser curvatures of the stomach.

Gastric Fistulae. — The general process of digestion, as it takes
place in the stomach, has been studied in human beings and animals
with a fistula in the walls of the stomach and abdomen, the result
either of accident or of necessary surgical or experimental procedures.

The earliest observations on gastric digestion were made by Dr.
Beaumont on Alexis St. Martin, who, as the result of a gunshot
wound, was left with a permanent fistulous opening into the fundus
of the stomach. This opening two years after the accident was about
two and a half inches in circumference and usually closed from within
by a fold of mucous membrane which prevented the escape of the food.
This valve could be readily displaced by the finger and the interior
of the stomach exposed to view. After the complete recovery of St.
Martin, Dr. Beaumont during the years between 1825 and 183 1 at
intervals made numerous experiments on the nature of gastric diges-
tion. As the result of an admirable series of investigations it was
established that the digestion of the food is largely a chemic act, due
to the presence of an acid fluid secreted by the mucous membrane;
that this fluid is secreted most abundantly after the introduction of
food into the stomach; that different articles of food possess varying
degrees of digestibility; that the duration of digestion varies according
to the nature of the food, exercise, mental states, etc., and that the
process is aided by continuous movements of the muscular walls.

Since Dr. Beaumont's time the establishing of a gastric fistula in
human beings has been necessitated by pathologic conditions of the
esophagus. After recovery these cases offered fair facilities for the
study of the process when the food was introduced through the opening.
Similar fistulae have been established in both carnivorous and her-
bivorous animals with a view of studying the process as it takes place
in them. The results obtained in these instances in many respects
corroborate those obtained by Dr. Beaumont, though many new facts,
unobserved by him, have been brought to light.

Much additional information as to the mode of secretion and the
characteristics of the gastric juice has been obtained, since the intro-
duction of two new procedures by Pawlow. The first consists in
establishing a gastric fistula and subsequently dividing the esophagus
in the neck, and then so adjusting the divided ends that they heal

DIGESTION.

177

separately into an angle of the skin incision. The second procedure
consists in forming a diverticulum or pouch out of the cardiac end of
the stomach which opens on the surface of the abdomen but is sepa-
rated from the rest of the stomach by a thin septum formed of two
layers of mucous membrane. (Fig. 80.) The serous and muscle-coats
of this pouch are in direct continuity with the large stomach and all
possess the same vascular and nerve connections. Because of this fact
this miniature stomach, about one-tenth the size of the natural stomach,
exhibits the same phenomena, so far as the secretion of the gastric
juice is concerned, that the large
stomach does. The phenomena which
are observed in it may be taken as an
indication as to the phenomena which
are taking place in the natural stomach.

By the first procedure it is possible
to feed an animal with different kinds
of food and to observe the effects of
psychic states on the secretion of gas-
tric juice. As the swallowed food is
discharged from the lower end of the
divided esophagus the appetite con-
tinues, and hence the animal will eat
for several hours. By the second
procedure it is possible to collect gas-
tric juice from the miniature stomach
and to study the effects on its quantity
and quality produced by psychic
states, mastication, different articles of
food, and by the process of digestion
itself as it goes on in the large stomach.
In both instances the juice is obtained
free from admixture with saliva or food.

Gastric Juice. — The gastric juice obtained from the human
stomach free from mucus and other impurities is a clear, colorless
fluid with a constant acid reaction, a slightly saline and acid taste,
and a specific gravity varying from 1.002 to 1.005. The juice ob-
tained from the dog's stomach possesses essentially the same char-
acteristics, though its acidity as well as its specific gravity are slightly
greater. When kept from atmospheric influences, it resists putre-
factive change for a long period of* time, undergoes no apparent
change in composition, and loses none of its digestive power. It
will also prevent and even arrest putrefactive change in organic
matter. The chemic composition of the gastric juice has never been
satisfactorily determined, owing to the fact that the secretion as
obtained from fistulous openings has not been absolutely normal.
The following analyses represent the composition of a sample obtained
by Schmidt from the stomach of a woman who had a fistula, but who

Fig. So. — Diagram Showing
the Relation of the Natural
Stomach to the Miniature
Stomach or Pouch made Ac-
cording to the Procedure De-
vised by Pawlow. V. The nat-
ural stomach. 5. The minature
stomach, e, e. The septum formed
by the mucous membrane. A, A.
The abdominal walls.

178 TEXT-BOOK OF PHYSIOLOGY.

was nevertheless in good health; also the composition of the juice from
a dog:

COMPOSITION OF GASTRIC JUICE.

Human. Dog.

Water, 994-4° 973-°6

Organic matter, 3.19 I7-13

Hydrochloric acid, 0.20 ? 3.34

Calcium chlorid, 0.06 0.26

Sodium chlorid, 1.46 2.50

Potassium chlorid, 0.55 1.12

Calcium phosphate] 1.73

Magnesium " [ 0.12 0.23

Ferric " 0.08

Ammonium chlorid, 0.47

The organic matter present in gastric juice is a mixture of mucin
and a proteid, products of the metabolic activity of the epithelial
cells on the surface of the mucous membrane and of the chief or
central cells of the gastric glands respectively. Associated with the
proteid material are two ferment or enzyme bodies, termed pepsin
and rennin. As is the case with other enzymes, their true chemic
nature is practically unknown.

Pepsin, though present in gastric juice, is not present as such in
the chief cells of the glands, but is derived from a zymogen, propepsin
or pepsinogen, when the latter is treated with hydrochloric acid.
This antecedent compound is related to the granules observed in and
produced by the cell protoplasm during the period of rest. Though
pepsin is largely produced by the central cells of the fundic glands,
it is also produced, though in less amount, by the cells of the pyloric
glands. Pepsin is the chief proteolytic agent of the gastric juice and
exerts its influence most energetically in the presence of hydrochloric
acid and at a temperature of about 400 C. Other acids — e. g., phos-
phoric, nitric, lactic, etc. — are also capable of exciting it to activity,
though with less intensity.

Rennin or pexin is present in the gastric juice not only of man and
all the mammalia, but also of birds and even fish. In its origin from a
zymogen substance, in its relation to an acid medium and an optimum
temperature, it bears a close resemblance to pepsin. Its specific
action is the coagulation of milk, a condition due to the cleavage of
caseinogen into a solid flaky body, casein, and a soluble, albumin.

Hydrochloric acid is the agent which gives to the gastric juice its
normal acidity. Though the juice frequently contains lactic, acetic,
and even phosphoric acids, it is generally believed that they are the
result of fermentation changes occurring in the food, the result of
bacterial action. The percentage of hydrochloric acid has been the
subject of much discussion. The analysis of human gastric juice
made by Schmidt shows a percentage of 0.02, while that of the dog
is 0.34. it i. probable, however, that the low percentage of HC1 in
human gastric juice was due to the admixture with saliva. At present

DIGESTION. i79

it is believed from analyses made for clinical purposes that the acid is
present to the extent of at least 0.2 per cent. This degree of acidity is
not constant during the entire process of digestion. In the earlier as
well as in the later stages it is much less.

The immediate origin of the hydrochloric acid is difficult of ex-
planation. That it is derived, however, primarily from the chlorids
of the food and secondarily from the blood-plasma has been established
by direct experiment. If all the chlorids be removed from the food
and all chlorids be withdrawn from the animal tissue by the adminis-
tration of various diuretics — e. g., potassium nitrate — there will be
a total disappearance of hydrochloric acid from the stomach. On
the addition of sodium or potassium chlorids to the food, there is at
once a reappearance of the acid.

As to the nature of the process by which the acid is formed, noth-
ing definite is known. Various theories of a chemic and physical
character have been offered, all of which are more or less unsatis-
factory. As no hydrochloric acid is found either in the blood or
lymph, the most plausible view as to its origin is that which regards
it as one of the products of the metabolism of the gland-cells, and more
particularly of the parietal or border cells, and which for this reason
have been termed acid-producing or oxyntic cells. From the chlorids
furnished by the blood the chlorin is derived, which, uniting with
hydrogen, forms the HC1. The base set free returns to the blood,
which in part accounts for its increased alkalinity during digestion
as well as the diminished acidity of the urine. The acid thus formed
passes through the canaliculi, which penetrate and surround the cells,
into the lumen of the gland.

Hydrochloric acid exerts its influence in a variety of ways. It is the
main agent in the derivation of pepsin and rennin or pexin from their
antecedent zymogen compounds, pepsinogen and pexinogen (Warren) ;
it imparts activity to these ferments; it prevents and even arrests fer-
mentative and putrefactive changes in the food by destroying micro-
organisms; it softens connective tissue, it dissolves proteids and acid-
ifies the proteids, thus making possible the subsequent action of pepsin.

The inorganic salts of the gastric juice are probably only inci-
dental and play no part in the digestive process.

Mode of Secretion. — The observations of Dr. Beaumont and
the experiments of many physiologists have made it certain that the
secretion of the gastric juice is intermittent and not continuous, that
it is only on the introduction and digestion of the food that the normal
amount is poured out. During the intervals of digestive activity the
stomach is practically free from all traces of the juice. The mucous
membrane is pale and covered with a layer of mucus having an alka-
line or neutral reaction. The introduction, however, of small por-
tions of food or irritation with a glass rod causes a change in the ap-
pearance of the mucous membrane. At the points of irritation the
membrane becomes red and vascular and in a few minutes small

1S0 TEXT-BOOK OF PHYSIOLOGY.

drops of a secretion make their appearance; these coalesce and run
down the sides of the stomach.

The statements of Beaumont and many subsequent investigators
that the secretion thus obtained is gastric juice have been apparently-
disproved by Pawlow, who asserts that it is only an alkaline mucous
the function of which is protective in character. Mechanic stimula-
tion is incapable of exciting the secretion.

The primary stimulus to gastric secretion, according to Pawlow,
is a psychic state induced, on the one hand, by the sight or the odor of
food especially if the animal is hungry and the food appetizing; and
on the other hand by the mastication of food which is agreeable to the
animal. Thus when a dog was tempted by the sight of food, the
secretion made its appearance at the end of six minutes and during
the time of the experiment, which lasted for an hour and a half, 80
cubic centimeters of the juice were obtained. This is known as
psychic or appetite juice. The character of a psychic state, however,
greatly influences the amount of the juice secreted. Agreeable emo-
tions increase, depressing emotions inhibit it. Again when a dog
with a divided esophagus and a gastric fistula was subjected to sham
feeding, mastication continued for five or six hours during which
time 700 cubic centimeters of juice were obtained from the stomach.
Similar results have been obtained in human beings with an occluded
esophagus and a gastric fistula. It is evident from these facts that the
secretion of gastric juice is favorably influenced by the sight and odor
of appetizing food, by exhilarating emotional states and thorough
mastication.

Though the secretion of the gastric juice can be initiated by these
means, the amount secreted is but small compared with the quantity
secreted after digestion has begun. Then it is that the blood-vessels
dilate to their full capacity and furnish for several hours the requisite
materials for the production of the juice on a relatively large scale.
That some factor is active and keeping up the secretion in the large
stomach, is apparent from the increase in the quantity and the change
in the quality of the juice secreted by the miniature stomach. This
secondary stimulus to the gastric secretion is in all probability chemic
in character and developed in the stomach or in its walls during diges-
tive activity, inasmuch as the secretion takes place independent of nerve
influences and after division of all afferent and efferent nerves that
pass from and to the stomach.

On the assumption that this factor might be developed in the walls
of the stomach itself, Edkins conducted a series of experiments, the re-
sults of which lead to the inference that there is developed in the pyloric
mucous membrane, by the action of the first products of digestive
activity a (hemic agent which, absorbed by the blood, is carried to
the glands throughout the stomach and which stimulates their cells in a
specific manner. For this reason it has been called the gastric hor-
mone or the gastric secretin. Whatever the agent or the mechanism

DIGESTION. 181

may be, there is not only an increase in the quantity but a change in the
quality of the juice in accordance with the character of the food; in
other words, there is an adaptation of the juice to the kind of food to
be digested. Thus the protein of bread causes a secretion of five times
more pepsin than the same amount of the protein of milk, while the
protein of meat causes secretion of 25 percent, more pepsin than milk.
Meat extract and bouillon have a very stimulating effect on the quan-
tity of juice produced, while alkalies have an inhibitor effect.

Histologic Changes in the Gastric Cells during Secretion. —
During the periods of rest and secretor activity the cells of the gas-

.....h

>pA 9 a * ■*99»ze^*f?>\

,pr

^fef®» S@®^

*■"■',

Fig. 81. — Sections of Deep Ends of Fundus Glands of the Cat in Different
Secretive Phases. X 1000. — (Benstey.) A. From a fasting stomach. The chief cells
are filled with large zymogen granules; nuclei near the outer ends of cells. Gentian-
violet preparation, b b b. Border cells. B. Six hours after an abundant meal of raw
flesh. The chief cells exhibit two zones, the inner occupied by large zymogen granules,
the outer by a deeply staining, obscurely fibrillar element, prozymogen; the nuclei lie at the
junction of the two zones, bbb. Border cells, pr. Prozymogen. c. Mucin-secreting
cell, similar to those found in the neck of the gland. Gentian-violet preparation. —
(Hcmmeter after Bensley.)

trie glands undergo changes in histologic structure which are believed
to be connected with the production of the enzymes, pepsin and rennin,
and the acid. In the resting period the protoplasm of the chief or central
cells of the fundus glands becomes crowded with large and well-
defined granules, which during the period of secretory activity largely
disappear, so much so, that only the luminal border of the cell is
occupied by them, the outer border being clear and hyaline in appear-
ance. The parietal cells during rest are large and finely granular,
but after secretion they are smaller in size though still granular. (See
Fig. 81, A and B.)

The cells of the pyloric glands, though containing granules, do not
show any marked difference between the resting and active condition.
According to some observers, they contain pepsinogen; according to
others, mucin. The epithelial cells lining the ducts of the pylorus

i82 TEXT-BOOK OF PHYSIOLOGY.

and fundus glands, if not identical with the epithelial cells on the sur-
face of the mucous membrane, pass by transitional forms into them.
Among these cells are found many goblet cells which secrete a portion
of the mucus found in the stomach and gastric juice. In the period
of rest the protoplasm of the epithelial cells absorbs and assimilates
from the surrounding lymph-spaces material which eventually makes
its reappearance as a product of metabolism in the form of granules
and hydrochloric acid. With the onset of digestive activity there is a
dilatation of the blood-vessels, an increase in the blood-supply, a
stimulation through the nerve-supply of the cells, and an output of a
fluid to which the name gastric juice is given.

Influence of the Nerve System. — The primary secretion of gas-
tric juice is a reflex act and under the control and influence of the
central nerve system. Experimental investigations render it probable
that the central mechanism is located in the medulla oblongata and
that the efferent path lies in the trunk of the vagus nerve. Though
this nerve has been the subject of much experimentation, the results
which have been obtained have not been uniform. The investigations
of Pawlow seem to be the most reliable. He found that after division
of the nerve, secretion was arrested, and that stimulation of the per-
ipheral ends with induced electric currents at the rate of one or two
per second, after a latent period of several minutes' duration caused a
flow of gastric juice. This center is excited to activity by nerve im-
pulses descending from the cerebrum as shown by the effects which
follow the development of psychic states caused by the sight or the
odor of food, as well as by the mastication of agreeable food; for with
their development there is a dilatation of blood-vessels and a copious
discharge of the juice in a few minutes, showing the cooperation of
both vaso-motor and secretor nerves. It is uncertain if the medullary
center can be influenced by nerve impulses reflected from the stomach.

The Physiologic Action of Gastric Juice. — In the study of the
physiology of gastric digestion as it takes place under normal con-
ditions it is important to bear in mind that the foods introduced into
the stomach are heterogeneous compounds consisting of both nutritive
and non-nutritive materials, and that before the former can be digested
and utilized for nutritive purposes they must be freed from their
combinations with the latter. This is accomplished by the solvent
action of the gastric juice, which in virtue of the chemic activity of its
constituents cm proteids, gradually disintegrates the food and reduces
it to the liquid or semiliquid condition.

The nature of this change and the respective influence which the
acid and pepsin exert can be studied with almost any form of proteid.
The most suitable form, however, is coagulated fibrin obtained from
blood by whipping and thoroughly freed from blood by washing
under a stream of water. The chemic features of proteins, as well as
the typical forms contained in the different articles of food, have
been considered in connection with the chemic composition of the body

DIGESTION. 183

and the composition of foods (see pages 16 and 139). For purposes of
experimentation artificial gastric juice may be employed. This is as
effective as the normal secretion and in no essential respect differs
from it. A glycerin extract of the mucous membrane acidulated
with 0.2 per cent, hydrochloric acid is probably the best.

If the small pieces of fibrin be suspended in clear gastric juice and
kept at a temperature of 1040 F. (400 C.) for an hour or two, they will
be dissolved and will entirely disappear, giving rise to a slightly opales-
cent mixture. In the early stages of the process the fibrin becomes
swollen and transparent and partly dissolved. If at this time the
solution be carefully neutralized, the dissolved portion can be regained
in the form of acid-albumin or syntonin — a fact which indicates that
the first effect of the gastric juice is the acidification of the proteids.
This having been accomplished, the pepsin becomes operative, and
in a varying length of time transforms the acid-albumin into a new
form of proteid, termed peptone. This form of proteid differs from
all other forms of proteid in being soluble in both acids and alkalies
and non-coagulable by heat. In the transformation of acid-albumin
into peptone it is possible to isolate by the addition of magnesium
sulphate and ammonium sulphate intermediate bodies to which the
term albumoses or proteoses has been given, and which differ somewhat
in their solubility. The proteoses are termed, from the order in which
they make their appearance, primary and secondary. The primary
proteoses are precipitated by magnesium sulphate, the secondary by
ammonium sulphate. As some of the primary proteoses are soluble
in water while others require in addition sodium chlorid for their solu-
tion, they have been divided into two groups — viz. : proto- and hetero-
albumoses. The secondary proteoses or deutero-albumoses are solu-
ble in water. Though in the subjoined scheme two forms of deutero-
albumose are represented and two forms of peptone developed out of
them, the results of chemic investigation would indicate that there is
but one form of deutero-albumose and hence but one form of peptone.
This supposed change produced by gastric juice is represented by the
following scheme:

Albumin
Acid-albumin

Proto-albumose = (p -jeose ) = Hetero-albumose

Deutero-albumose(SpC°f dary) = Deutero-albumose
V Proteoses'

Peptone (Ampho-peptones) Peptone.

From the fact that when peptones are subjected to the prolonged
action of pancreatic juice there arise compounds such as leucin,

i84 TEXT-BOOK OF PHYSIOLOGY.

tyrosin, aspartic acid, arginin, etc., it was believed that two kind
of peptones were formed out of a simple protein one of which suc-
cumbed to the destructive action of pancreatic juice, while the other
resisted it; for this reason the latter was termed anti- and the former
hemi- peptone. The two were included under the term ampho- pep-
tone. It is generally admitted now, however, that the body termed
anti-peptone is not a peptone at all, but a compound termed carnic
acid and which is also separable into leucin, tyrosin, etc. Hemi-
peptone has never been isolated. The probabilities are, therefore,
that but one form of peptone is developed from any given simple pro-
tein.

Nearly all forms of protein are in a similar manner transformed
into peptones by gastric juice. Beyond this stage, however, there
does not seem to be any further change, peptones apparently being
the final products of gastric digestion. The intimate nature of this
change is practically unknown, but there are reasons for thinking
that it is a process of hydration, attended by cleavage, with increasing
solubility of the resulting products.

Characters of Peptones. — The peptones resulting from the diges-
tion of different proteins, though resembling each other in many re-
spects, yet possess different chemic characteristics, as shown by their
reaction to various chemic reagents. Though having some resem-
blance to the proteids from which they are derived, they are to be
distinguished from them by the following general characteristics:
i. They are not coagulable either by heat or by nitric acid.

2. They are soluble in water, either hot or cold, and in acid and

alkaline solutions.

3. They are diffusible, passing through animal membranes with

great rapidity. It has been demonstrated that peptones diffuse
about twelve times as rapidly as the proteids from which they
are derived.
Neither on fat nor starch has gastric juice any appreciable effect,
and when these substances are introduced into the stomach they
pass into the intestine unchanged. It has been stated, however, that
the gastric mucosa produces a fat-splitting enzyme, but that its action
is prevented by the presence of the hydrochloric acid. Notwithstanding
the fact that dilute solutions of hydrochloric acid (0.3 per cent.) will
promptly invert cane-sugar into dextrose and levulose, and that gastric
juice will accomplish the same result in test-tubes, there is no strong
evidence for the belief that the inversion of cane-sugar takes place to
any marked extent in the stomach under normal conditions. The
starch, however, which has been subjected to the action of the saliva,
still continues to be converted into maltose during the first fifteen to
thirty minutes or even longer. Even though gastric juice is being
secreted and though hydrochloric acid solutions with a strength of 0.3
per cent, will arrest the action of ptyalin, starch digestion continues for
the reason that the acid combines with the proteids and is thus rendered

DIGESTION. 185

inoperative and for the further reason that the food is largely retained
in the extreme cardiac end of the stomach where the gastric juice is not
abundant. After the above-mentioned period, free acid makes its
appearance when salivary digestion ceases.

Action of Gastric Juice on Foods. — The action of gastric juice
on proteids affords a key to its influence in the reduction of foods to
the liquid or semiliquid condition. It is evident that it will be most
active in the digestion of food consisting largely of proteid materials,
such as meat, eggs, milk, etc. Meat is disintegrated first by the con-
version of the proteids of the connective tissue, which have been more
or less gelatinized by cooking, into peptones. The sarcolemma of
the muscle-fibers which have been thus separated is in a similar man-
ner attacked and converted into peptones. The true muscle or sar-
coid substance, consisting largely of myosin, undergoes a correspond-
ing change. If the quantity of meat be not too large and the gastric
juice be secreted in proper amount, it is possible that all the meat will
be digested in the stomach. It is quite probable, however, that this
is not the case and that a portion of the semidigested meat passes into
the intestine, where its final solution is effected.

The white of egg, especially when slightly boiled, is much more
readily digested than when raw or firmly coagulated by prolonged
boiling. In either condition, however, the connective tissue is dis-
solved and peptonized, after which the native albumin undergoes the
same change. The yolk of the egg consists largely of fat held in sus-
pension by a proteid substance, vitellin, which is also capable of trans-
formation into peptone.

Adipose tissue is similarly reduced. The proteids of the con-
nective tissue and of the fat vesicles are dissolved and peptonized
and the fat-drops set free.

Milk undergoes a peculiar change in composition before its proteid
constituents can be transformed into peptones. The caseinogen
in the presence of calcium salts is always in the soluble state. When
acted on by the gastric juice, the caseinogen undergoes coagulation
which consists in the formation of a solid compound, casein, and a
soluble albumin. This change is due to the presence and activity
of the ferment, rennin. The necessity for this change, however, is
not apparent. The coagulated casein presents itself in the form of a
flocculent curd, which is finer in human than in cow's milk, and hence
more easily digestible. The casein is acidified by the hydrochloric
acid and then converted by the pepsin into peptone.

Vegetables, though consisting of a woody or cellulose framework,
undergo a partial disintegration in the stomach. When boiled and
physically disintegrated by the teeth, the gastric juice is enabled to
penetrate the framework and dissolve and peptonize the various
proteid constituents. As a general rule, the vegetable proteids are
more difficult of digestion than the animal proteids.

Duration of Gastric Digestion.— The length of time the food

iS6

TEXT-BOOK OF PHYSIOLOGY.

remains in the stomach and the relative digestibility of different
articles of food were carefully studied by Dr. Beaumont on St. Martin,
and though the results obtained by him may not be absolutely correct,
viewed in the light of recent knowledge of the digestive process, yet
in the main they have been corroborated in various ways. As a
result of many observations Dr. Beaumont came to the conclusion
that the average length of time an ordinary meal consisting of meat,
bread, potatoes, etc., remained in the stomach undergoing digestion
was about three and a half hours, the duration of the process, how-
ever, being increased when an excessive quantity of food was taken or
the quantity and quality of the gastric juice impaired by abnormal
conditions of the system. As soon as the food is liquefied by the
gastric juice that portion not absorbed by the gastric vessels passes
into the intestines, this continuing for two to three hours until the
stomach is completely emptied. The relative digestibility of the differ-
ent foods was also made the subject of many experiments by Dr.
Beaumont. After repeating and verifying his observations made
under varying conditions, he summed up his results in a table, of
which the following is an abstract, in which the mode of preparation
and the time required for the digestion of different foods are exhibited :

TABLE SHOWING DIGESTIBILITY QF VARIOUS ARTICLES OF FOOD

Hours Minutes.

Eggs, whipped, i 20

" soft boiled, 3

" hard boiled, 3 30

Oysters, raw, 2 55

" stewed, 3 30

Lamb, broiled, 2 30

Veal, broiled, 4

Pork, roasted, 5

Beefsteak, broiled, 3

Turkey, roasted, 2

Chicken, boiled, 4

" fricasseed, 2 45

Duck, roasted, 4

Soup, barley, boiled, 1 30

bean, " 3

" chicken, " 3

" mutton, " 3 30

Liver, beef, broiled, 2

Sausage, 3 20

Green corn, boiled, 3 45

Beans, " 2 30

Potatoes, roasted, 2 30

boiler], 3 30

Cabbage, " 4 3°

Turnips, " 3 30

Beets, " 3 45

Parsnips, " 2 30

Movements of the Stomach. — During the period of gastric
digestion the muscle walls of the stomach become the seat of a series
of movements, peristaltic in character, which not only incorporate

DIGESTION.

187

the gastric juice with the food, but also serve to eject the liquefied

portions of the food into the small intestine.

The movements of the human stomach as described by Beaumont,

as well as the movements of the dog's stomach as stated by different

observers, are not in agreement in
all respects, and are, moreover,
open to question for the reason
that they were not observed under
strictly physiologic conditions. The
more recent investigations of Can-
non have thrown new light on this
subject. By means of the Rontgen
rays he has been enabled to study
the movements in the living animal
and under normal conditions. The
animal (the cat) was fed with bread
and milk, to which was added sub-
nitrate of bismuth. This sub-

Left

Fig. 82. — Shadow Sketches
of the Outlines of the
Stomach of a cat Immedi-
ately after a Meal (ii.o),
and at Various Intervals
Afterward (at 12.0, at 2.0,
3.30, 4.30). — (IF. B. Cannon.)

Post

Fig. S3. — The cardiac portion is all that
part to the left, as the stomach lies in the
body, of WX. The cardia is at C. The
pylorus is at P, and the pyloric portion
is the part between P and WX. This
has two divisions: the antrum, between
P and YZ, and the pre-antral part, between
WX and YZ. The lesser curvature is on the
top of the outline between C and P, and the
greater curvature between the same points
along the lower border. — (Amer. Jour, oj
Physiology, Cannon.')

stance, being opaque, rendered the movements of the stomach walls
visible on the fluorescent screen. With paper placed over the screen
it was possible to sketch the changes in shape that the stomach under-
goes at different periods of the digestive act. Some of these changes
are represented in Fig. 82. The anatomic features of the cat stomach
of interest in this connection are represented in Fig. 83.

These investigations show that different portions of the stomach
walls exhibit different forms of activity, which for convenience of
description are separately described by Cannon as follows :

1. The Movements oj the Pyloric Part. — Within five minutes after

188 TEXT-BOOK OF PHYSIOLOGY.

a cat has finished a meal of bread there is visible near the duodenal
end of the antrum a slight annular contraction which moves peristal-
tically to the pylorus; this is followed by several waves recurring at
regular intervals. Two or three minutes after the first movement
is seen, very slight constrictions appear near the middle of the stomach,
and, pressing deeper into the greater curvature, course slowly toward
the pyloric end. As new regions enter into constriction, the fibers
just previously contracted become relaxed, so that there is a true moving
wave, with a trough between two crests. When a wave swings round
the bend in the pyloric part, the indentation made by it deepens;
and as digestion goes on the antrum elongates and the constrictions
running over it grow stronger, but, until the stomach is nearly empty,
they do not entirely divide the cavity. After the antrum has length-
ened, a wave takes about thirty-six seconds to move from the middle
of the stomach to the pylorus. At all periods of digestion the waves
recur at intervals of almost exactly ten seconds. It results from this
rhythm that when one wave is just beginning several others are already
running in order before it. Between the rings of constriction the
stomach is bulged out, as shown in the various outlines in Fig. 82.

Movements of the Pyloric Sphincter.- — During the first ten or fifteen
minutes after the first constriction of the antrum the pylorus is tightly
closed. After this period it opens at irregular intervals to permit
the passage of liquefied food which is ejected by peristaltic waves for
a distance of two or three centimeters into the duodenum. The fre-
quency with which the pylorus opens depends apparently on the degree
to which the food is softened. When the food is hard, the pylorus
closes more tightly and remains closed a longer period than when it is
soft.

The Activity of the Cardiac Portion. — As digestion proceeds, the
pre-antral part of the stomach elongates and assumes the shape of a
tube, which becomes the seat also of peristaltic constriction waves.
As a result, some of the food is gradually forced into the antrum to
succeed that which has been prepared and ejected into the duodenum.
As the pre-antral tube is emptied of its contents the longitudinal and
circular fibers of the fundus steadily contract and gradually force its
contents into the tubular portion. This continues until the fundus is
completely emptied. The changes in shape which the cardiac portion
undergoes during digestion are represented in Fig. 82. The fundus
acts as a reservoir for the food and forces out its contents a little at a
time as the antral mechanism is ready to receive them. Since peristal-
tic movements are absent from the cardiac portion the food is not
mixed with gastric juice, and therefore salivary digestion can continue
for a considerable period. There is no evidence of a circulation of
food in the stomach as usually described. On the contrary, the move-
ment through the pre-antral tube and antrum is in general a progres-
sive though an oscillating one. As the constriction waves rapidly
pass over the food it is advanced toward the pyloric opening, but as

DIGESTION. 189

this is closed the food is forced backward through the advancing
constricted ring for a variable distance.

The effect of the constriction waves is to mix the food with the
gastric juice, triturate and soften it. So soon as this is effected, the
pylorus relaxes, when the advancing constriction wave expels it into
the intestine. With its expulsion room is afforded for an additional
quantity of food, and hence there is a general advance of the food mass
toward the pylorus.

Though these observations were made on the cat, evidence is
accumulating which goes to show that in human beings the walls
of the stomach exhibit constriction waves which are similar in all
respects to those above described.

The Nerve Mechanism of the Stomach. — In preceding para-
graphs it was stated that during the period of gastric digestion the
food is retained in the stomach because of the closure of the cardia
(the esophago-gastric orifice) and of the pylorus (the gastro-duode-
nal orifice) both orifices being tightly closed by the tonic contraction
of sphincter muscles; that both sphincters relax from time to time,
the one to permit the entrance of food into the stomach for further
digestion, the other to permit the exit of food into the intestine after
its more or less complete digestion, after which in both instances the
sphincters again contract and close the orifices; that the pyloric or
antral muscles are vigorously active throughout the digestive period,
triturating the food, mixing it with gastric juice, and finally driving it
through the temporarily open pylorus into the intestine.

These separate but related groups of muscle-fibers, by reason of
their endowments, and possibly by virtue of the presence of local
nerve mechanisms, exhibit activities which are independent of the
central nerve system. Thus the isolated stomach of the dog and of
other animals as well, if kept warm and moist, will exhibit rhythmic
movements for a period of time varying from an hour to an hour and
a half. Though nerve-cells and nerve-fibers (Auerbach's plexus)
are present in the walls of the stomach between the layers of muscle-
fibers, it is not believed that they are the immediate sources of the
stimulus to the contraction, though they may act as a coordinating
mechanism. The stimulus in all probability develops in the muscle-
fiber itself and is therefore myogenic in origin.

The sphincter car dire muscle surrounding the esophago-gastric
orifice is always under normal conditions, tonically contracted and the
orifice closed. This contraction is partly due to inherent causes as
shown by the fact that it persists for from 24 hours to several days
after division of all nerves distributed to it. The contraction may be
so pronounced as to offer considerable resistance to the passage not
only of food but even to the introduction of a sound into the stomach.
(Cannon.) That the normal contraction is under the influence of the
central nerve system is shown by the effects which follow stimulation
of the peripheral end of the divided vagus. If it is stimulated with

i9o TEXT-BOOK OF PHYSIOLOGY.

weak induced currents, the contraction is somewhat inhibited and the
orifice enlarged ; if it is stimulated with strong currents the contraction
is markedly increased. Apparently there are in the vagus two sets
of efferent nerve-fibers, one of which augments, while the other inhibits
the contraction, and corresponding to the nerves there must be in the
medulla oblongata two centers from which they arise, an augmentor
and an inhibitor.

Observation has shown that at the beginning of each act of degluti-.
tion, there is an inhibition of the sphincter muscle, and if the acts fol-
low each other in quick succession, the inhibition and relaxation are
increased. With the passage of the food into the stomach the tonic con-
traction again supervenes. These effects also follow stimulation of the
glossopharyngeal nerve which evidently carries nerve impulses to
these centers. Whether the sphincter inhibition is the result of an
inhibition of the center which maintains the tonus, or a stimulation
of the inhibitor center, is uncertain.

The degree of activity also of both the pyloric sphincter and the antral
muscles is modified by the central nerve system either in the way of
inhibition or augmentation and in response to gastric stimulation.
The nerves more especially concerned in the maintenance and regula-
tion of the gastric contractions are the vagi and the splanchnics.
The afferent fibers through which nerve impulses pass to the nerve
centers are in all probability contained in the trunk of the vagus nerve;
the efferent fibers through which nerve impulses from the centers reach
the stomach, are contained partly in the trunk of the vagus and partly
in the trunk of the splanchnic nerve.

If the vagus nerves are divided in the neck, there is a loss of muscle
tonus though the contractions do not wholly disappear. Stimulation
of the peripheral end of one divided vagus is followed by an augmen-
tation in the vigor of the contraction of the antral muscles, an increase
in the tone of the fundus muscles, as well as an increase in the con-
traction of the sphincter pylori and sphincter cardiae. Though this
is the usual result there may be a primary relaxation or inhibition of
short duration of one or all of these structures before the augmen-
tation occurs. May states that this was always the case in his experi-
ments. A similar inhibition may be brought about reflexly by stimu-
lation of the central end of a divided vagus. This result will not be
produced if the opposite vagus has previously been divided. The
vagi, therefore, contain both inhibitor and augmentor nerve-fibers
for the gastric musculature.

Stimulation of the peripheral end of a divided splanchnic is followed
by an inhibition of the peristalsis and a loss of tone. Morat, however,
has observed a primary opposite effect. From these facts it would
appear that the gastric muscles receive both inhibitor and augmentor
fibers from two different sources.

The excitatory cause for the activity of this mechanism is doubtless
connected with the chemic and mechanic stimulation by the gastric

DIGESTION. 191

contents. The relaxation or inhibition of the sphincter pylori appears
to be caused by the presence of free acid at the pylorus; its contrac-
tion, by the presence of acids in the duodenum. With their neutral-
ization in the duodenum, their influence on the sphincter muscle
is weakened after which it again becomes susceptible to the inhibitor
influence of the acid within the stomach. It is probably for this
reason that carbohydrates which do not absorb the acid are discharged
from the stomach early; that the proteids which postpone the appear-
ance of free acid are retained longer and that fats which check the
secretion of gastric juice are discharged slowly (Cannon). It should be
emphasized, however, that the relaxation and contraction of the pyloric
sphincter, due to the action of free acid on the gastric and duodenal
sides, respectively, can take place independently of the nerve system.

INTESTINAL DIGESTION.

The physical and chemic changes which the alimentary principles
undergo in the small intestine, and which collectively constitute in-
testinal digestion, are probably more important and complex than
those taking place in the stomach, for the food is, in this situation,
subjected to the solvent action of the pancreatic and intestinal juices,
as well as to the action of the bile, each of which exerts a transforming
influence on one or more substances and still further prepares them
for absorption into the blood.

To rightly appreciate the physiologic actions of the digestive
juices poured into the intestine, the nature of the partially digested
food as it comes from the stomach must be kept in mind. This
consists of water, inorganic salts, acidified proteids, albumoses, pep-
tones, starch, maltose, liquefied fat, saccharose, lactose, dextrose,
cellulose, and the indigestible portion of meats, cereals, and fruits.
Collectively they are known as chyme. As this acidified mass passes
through the duodenum its contained acids excite a reflex secretion
and discharge of the intestinal fluids: e. g.} pancreatic juice, bile, and
intestinal juice. Inasmuch as these fluids are alkaline in reaction
they exert a neutralizing and precipitating influence on various con-
stituents of the chyme. As soon as this has taken place, gastric
digestion ceases and those chemic changes are inaugurated which
eventuate in the transformation of all the remaining undigested
nutritive materials into absorbable and assimilable compounds which
collectively constitute intestinal digestion.

THE SMALL INTESTINE.

The small intestine, in which this stage of digestion takes place,
is a convoluted tube, measuring about seven meters in length and 3.5
cm. in diameter, and extending from the pyloric orifice of the stomach
to the beginning of the large intestine.

The intestine consists of four coats: viz., serous, muscular, sub-
mucous, and mucous.

i92 TEXT-BOOK OF PHYSIOLOGY.

The serous coat is the most external and is formed by a reflection
of the general peritoneal membrane. It is, however, wanting in the
duodenal portion.

The muscle coat, situated just beneath the former, surrounds
the entire intestine. It is composed of non-striated fibers which are
more abundant and better developed in the upper than in the lower
portions of the intestine. The muscle coat consists of two layers
of fibers: (i) an external or longitudinal, and (2) an internal or circular
layer. The longitudinal fibers are most marked at that border of the
intestine free from peritoneal attachment, though they form a thin
layer all over the intestine. The circular fibers are much more numer-
ous, and completely encircle the intestine throughout its entire extent.
It has been demonstrated that at the junction of the ileum and colon,
and surrounding the orifice, the ileo-colic, common to both, the muscle-
fibers are arranged in the form of, and play the part of, a sphincter
muscle, which has been termed the ileo-colic sphincter. It is usually
in a state of tonic contraction and regulates the passage of materials
from the small into the large intestine, and possibly also in the reverse
direction under special circumstances.

The submucous coat consists of areolar tissue and serves to unite
the muscular with the mucous coat. A thin layer of muscle-fibers,
the muscularis mucosa, is placed on its inner surface.

The mucous coat is soft and velvety in appearance and covered
by a single layer of columnar epithelium. Its entire surface is covered
with small conical projections termed villi. Throughout its entire
extent, with the exception of the lower portion of the ileum and the
duodenum, the mucous membrane presents a series of transverse
folds — the valvules conniventes, or valves of Kirkring. These folds
vary from one-fourth to half an inch in width and extend one-half
to two-thirds of the distance around the interior of the bowel. Each
valve consists of two layers of the mucous membrane permanently
united by fibrous tissue. It is believed that the valves retard to some
extent the passage of the food through the intestine and present a
greater surface for absorption.

Blood-vessels, Nerves, and Lymphatics. — The blood-vessels,
of the small intestine, which are very numerous, are derived mainly
from the superior mesenteric artery. After penetrating the intestinal
walls the smaller vessels ramify in the submucous coat and send
branches to the muscle and mucous coats, supplying all their struc-
tures with blood. After circulating through the capillary vessels the
blood is returned by small veins which subsequently unite to form
the superior mesenteric vein, which, uniting with the splenic and gas-
tric veins, forms the portal vein. The nerves are derived from the
lower part of the solar plexus. The branches follow the blood-vessels
and become associated with two plexuses, one (Auerbach's) lying
between the muscle coats, the other (Meissner's) lying in the sub-
mucous coat. The lymphatics, which originate in the mucous and

DIGESTION. 193

muscle coats, are very abundant. They unite to form those vessels
seen in the mesentery and empty into the thoracic duct.

Intestinal Glands. — -The gland apparatus of the intestine by
which the intestinal juice is secreted consists of the duodenal (Brun-
ner's) and the intestinal (Lieberkiihn's) glands.

The duodenal glands are situated beneath the mucous membrane
and open by a short wide duct on its free surface. They are racemose
glands lined by nucleated epithelium. The secretion of these glands
is clear, slightly viscid, and alkaline. Its chemic composition and
function are unknown. %

The intestinal glands or follicles are distributed throughout the
entire mucous membrane in enormous numbers. They are formed
mainly by an inversion of the mucous membrane and hence open on
its free surface. Each tubule consists of a thin basement membrane
lined by a layer of spheric epithelial cells, some of which undergo
distention by mucin and become converted into mucous or goblet
cells. The epithelial secreting cells consist of granular protoplasm
containing a well-defined nucleus. The intestinal follicles constitute
the apparatus which secretes the chief portion of the intestinal juice.

Intestinal Juice.— Owing to its admixture with other secretions
and to the profound disturbance of the digestive function, caused by
the establishment of intestinal fistula?, this fluid has rarely been ob-
tained in a state of purity or in quantities sufficient for accurate analyses
or for experimental purposes. Its physiologic properties and functions
are therefore imperfectly known. Various attempts have been made
by physiologists, by the employment of different methods, to obtain
this secretion. The method usually employed is that of Thiry and
Vella. This consists in dividing the intestine at two places, about
eight or ten inches apart, restoring the continuity of the intestine,
and then uniting the two ends of the resected portion to the edges of
two openings in the abdominal walls. The resected portion, being
supplied with blood-vessels and nerves, maintains its nutrition and
secretes a more or less normal juice.

When obtained from a dog under these circumstances the intestinal
juice is watery in consistence, slightly opalescent, light yellow in color,
alkaline in reaction, with a specific gravity of 1.010. Chemic analysis
reveals the presence of proteids, mucin, and sodium carbonate.

The intestinal juice obtained by Tubbey and Manning from a
small portion of the human intestine (ileum) was opalescent, occa-
sionally brownish in color, alkaline, and had a specific gravity of 1.006.
On the addition of hydrochloric acid, carbonic acid was given off,
showing the presence of carbonates. It contained proteids and
mucins.

PANCREAS.

The pancreas is a long flattened gland, situated deep in the
abdominal cavity, lying just behind the stomach. It measures from
13

194

TEXT-BOOK OF PHYSIOLOGY.

six to eight inches in length, two and a half in breadth, and one in
thickness. It is usually divided into a head, body, and tail. The
head is directed to the right side and is embraced by the curved portion

Pancreatic ducts

Common bile-duct-

Fig. 84. — Pancreas .and Duodenum Removed from the Body and Seen from
Behind. The Gland is Cut to Show the Ducts. — (Landois and Stirling.)

of the duodenum; the tail is directed to the left side and extends as
far as the spleen (Fig. 84.) The pancreas communicates with the
intestine by means of a duct. This duct commences at the tail and
runs transversely through the body of the gland. As it approaches

the head of the gland it
gradually increases in
size until it measures
about one-tenth of an
inch in diameter. It
then curves downward
and forward and opens
into the duodenum. In
its course through the
gland it receives
branches which enter it
nearly at right angles.
The pancreas is richly
supplied with blood-
vessels and nerves, the
latter coming from the
solar plexus.

Histologic Struc-
ture.— In its structure
the pancreas resembles
the salivary glands. It consists of a connective-tissue framework
which divides the gland tissue into lobules. Each lobule is com-
posed of a number of acini or alveoli, more or less elongated or tubular
in shape. Each acinus gives origin to a small duct which, uniting

Fig. 85.

Fig. 86.

One Saccule of the Pancreas of the Rabbit
in Different States of Activity. Fig. 85. — 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) next the lumen (c) — is broad and filled with
fine granules. Fig. 86. — 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. — (Yeo's "Text-book of Physiology,"
after Kiihne and Lea.)

DIGESTION.

i95

with adjoining ducts, forms the lobular duct, which becomes tributary
to the main duct. The acinus is lined by a layer of cylindric epithelial
cells characterized by a difference in structure between the central
and peripheral ends (Fig. 85). The central end, that bordering the
lumen of the acinus, is dark in appearance and filled with dark granules,
while the peripheral end is clear and homogeneous. The relative
depth of these two zones varies according to the functional activity
of the gland. During the intervals of digestion the granular layer
is very deep and occupies almost the entire cell; after active digestion
the granular layer is very narrow, while the clear zone is largely in-
creased in depth. The blood-vessels of the pancreas are arranged
around the acini in a manner similar to that observed in the salivary

Fig. 87. — Section of Human
Pancreas, including Several Acini
and Two Ducts. The Cells Pre-
sent a Central Granular and a
Peripheral Clear Zone. — (Piersol.)

Fig. 88. — Section of Human Pan-
creas showing, a, a, Islands of
Langerhans, and b, the Usual Acini. —
(Piersol):

glands. The ultimate terminations of the nerves in the epithelium
are probably by means of the usual end-tufts.

The Islands of Langerhans. — Throughout the body of the pan-
creas and especially in the outer extremity there are found between
and among the acini collections of globular cells arranged in the
form of rods or columns, separated from the acini and from one another
by layers of connective tissue in which ramify large tortuous capillary
blood-vessels. These columnar bodies, seen in cross-section in Fig.
88, have been named, after their discoverer, the islands of Langerhans.

Embryologic investigations have shown that these cells are out-
growths from the primitive acini, to which they remain attached for
some time by means of a foot-stalk. This subsequently becomes
constricted by the connective tissue and the cells become completely
detached. The cells then assume the columnar arrangement, after
which vascularization takes place.

From the fact that complete extirpation of the pancreas as well as
its various diseases is followed by serious disturbances of the carbohy-

196 TEXT-BOOK OF PHYSIOLOGY.

drate metabolism it has been suggested that the islands of Langerhans
have a function separate and distinct from that of the glandular portion
of the pancreas ; that they secrete a specific material which partakes of
the nature of an internal secretion which is absorbed by the blood
circulating around them and carried to different tissues. The effect
on the metabolism of the body which follows extirpation of the pan-
creas will be referred to in a subsequent chapter.

Pancreatic Juice. — The pancreatic juice may be obtained by
introducing a silver cannula, through an opening in the abdominal
wall, into the duct, and securing it by a ligature. In a short time
the juice flows from the distal end of the cannula, when it can be
collected. According to Bernard, normal juice can only be obtained
during the first twenty-four hours. The juice obtained from a tem-
porary fistula is clear, slightly opalescent, viscid, of a decidedly alkaline
reaction, and has a specific gravity in the dog of 1.040. When cooled
to o° C, it assumes a gelatinous consistence. At ioo° C. it com-
pletely coagulates. When obtained from a permanent fistula, the juice is
watery and the solid constituents are very much diminished in amount.

The chemic composition of the pancreatic juice of the dog as deter-
mined by Schmidt is as follows: water, 900.76; organic matter, 90.44;
inorganic salts, 8.80. Of the inorganic salts, sodium carbonate is
probably the most essential, as it is this salt which gives to the juice its
alkaline reaction.

Mode of Secretion. — The secretion of the juice is, in the rabbit
and dog at least, almost continuous during a period of twenty-four
hours after a single average meal, though the rate of flow varies con-
siderably during this period. As soon as food enters the stomach
'the flow of the pancreatic juice begins and steadily increases in amount
until about the third hour, when it reaches its maximum; after this
period the flow diminishes until the sixth hour, when it again increases
for about an hour. It then gradually diminishes until it ceases entirely.
During the period of secretory activity the gland becomes red and
vascular from a dilatation of the blood-vessels. .

The discharge of the juice associated with the introduction of
food into the stomach is brought about in all probability through
the agency of the nerve system, though the exact mechanism is im-
perfectly understood. It is probable that impressions made on the
terminal filaments of the pneumogastric nerve ascend to the medulla,
whence impulses pass outward through vaso-motor and secretor
nerves to the blood-vessels and secreting cells of the glands. Stim-
ulation of the peripheral end of the divided vagus gives rise to increased
secretion. Inasmuch as various agents, such as mineral and organic
acids, placed on the duodenal mucous membrane excite the flow,
it is quite probable that the passage of the acid contents of the stomach
through the duodenum also acts as a powerful stimulus to the dis-
charge of the juice. But as the secretion and discharge of the juice
is excited by the same conditions after the division of all related nerves,

DIGESTION. 197

other explanations were sought for. Bayliss and Starling made the
discovery that the secretory activity of the pancreas is initiated and
maintained by the action of a specific substance to which they have
given the term secretin. This substance is developed in the duodenal
glands out of a precursor, prosecretin, in consequence of the action
of the acids in the chyme, after which it is carried by the blood-stream
to the pancreas. An extract of the duodenal mucous membrane made
with hydrochloric acid 0.4 per cent, and presumably containing
secretin, when injected into the blood will evoke a profuse discharge
of pancreatic juice. Hydrochloric acid alone will not have this effect.
The total amount of pancreatic juice secreted in twenty-four hours
has been only approximately determined; the estimates based upon
the amount obtained from dogs vary from 175 to 800 grams.

Histologic Changes in the Cells during Secretory Activity. —
Reference has already been made to the fact that the cells lining the
acini consist of two zones: an outer one, clear and homogeneous;
and an inner one, dark and granular. The position of the nucleus
of the cell varies, being at one time in the outer, at another time in the
inner, zone. If the pancreas be examined microscopically during
the intervals of digestion, it will be observed that the inner zone is
broad, highly granular, occupying nearly the entire cell, while the
outer zone is narrow and clear. If, however, the gland be examined
shortly after a period of active secretion, the reverse conditions will
be observed; that is, the inner zone will be narrow, containing rela-
tively few granules, while the outer zone will be clear and wide. This
change in the cell has been witnessed in the pancreas of the living
animal — rabbit— by Kiihne and Lea. They observed that as soon
as digestion set in, the granules of the broad inner zone began to pass
toward the lumen of the acinus and to gradually disappear as the
secretion was poured out, while the outer zone increased in width
until almost the entire cell became clear and homogeneous. (See
Fig. 86.) After secretion ceased the granules again made their
appearance, the result, in all probability, of metabolic activity.

Physiologic Action of Pancreatic Juice. — Experimental in-
vestigations have demonstrated the fact that pancreatic juice is the
most complex in its physiologic action of all the digestive fluids. In
virtue of its contained enzymes, pancreatic juice acts:

1. On starch. When normal pancreatic juice or a glycerin
extract of the gland is added to a solution of hydrated starch, the
latter is speedily transformed into maltose, passing through the inter-
mediate stage of dextrin. The process is in all respects similar to that
observed in the digestion of starch by saliva. Pancreatic juice, how-
ever, is more energetic in this respect than saliva. The enzyme which
effects this change is termed amylopsin. When the starch which
escapes salivary digestion passes into the small intestine and mingles
with pancreatic juice, it is very promptly converted into maltose by
the action or in the presence of this enzyme.

i9S TEXT-BOOK OF PHYSIOLOGY.

2. On protein. When protein compounds are subjected to the ac-
tion of pancreatic juice, they are transformed into peptones which do
not differ in essential respects from those formed by gastric juice. The
intermediate stages, however, are believed to be somewhat different.
The enzyme which effects this change is termed trypsin.

When fibrin, for example, is added to trypsin in a solution rendered
alkaline by sodium carbonate, it does not swell up and become trans-
lucent, as it does when treated with hydrochloric acid and pepsin.
On the contrary, it becomes corroded on the surface, fragile, and in a
short time undergoes solution. The first product is a compound
termed alkali-albumin. After solution has taken place, various
chemic changes are initiated which eventuate in the production of
peptone and certain nitrogenized bodies, leucin, tyrosin, aspartic acid,
etc. The intermediate stages in this process have not been satis-
factorily determined. At no time during artificial pancreatic digestion
is there any evidence of the presence of the primary proteoses (proto-
albumose and hetero-albumose). The secondary proteoses (deutero-
albumose) are usually present. It will be -recalled that when the
peptone of peptic digestion is subjected to the action of trypsin a por-
tion of it is decomposed into leucin and tyrosin, while another portion
presumably is not so decomposed, for which reason the latter was
called anti- and the former, /zemi-peptone. It is now believed that
anti-peptone is not a peptone at all, but a compound termed carnic acid,
which can be decomposed into simpler nitrogen-holding bodies such
as leucin, tyrosin, arginin, etc.

The action of trypsin on proteins in an alkaline medium may be
illustrated by the following scheme:

Protein

I
Alkali-albumin

I
Deutero-proteose or deutero-albumose

Peptone

Leucin Tyrosin Aspartic acid Arginin Ammonia

When the proteins which have escaped digestion in the stomach
pass into the small intestine and mingle with the pancreatic juice,
they are doubtless digested in the course of the intestinal canal, passing
through the stages just described. As leucin and tyrosin are found
in the intestine during digestion, it is probable that a portion of the
peptone undergoes decomposition into these bodies; but as to the
extent to which this takes place or in how far it is a necessary process
under normal conditions is yet a subject of investigation. It is certain
that it takes place when there is an excess of protein food or when for
any reason digestion is prolonged or absorption is delayed.

While the view that the final stage in the digestion of proteins is

DIGESTION. i99

the formation of peptones, which in due time are absorbed and syn-
thetized into blood albumin, is generally accepted, there is some
evidence that it is not wholly true, and that the final stage may be
the formation of the nitrogen-holding compounds above mentioned;
in other words, that the cleavage of the proteins is far more com-
plete than has heretofore been assumed. Indeed it has been asserted
that they are reduced, if not to their ultimate constituents, the amino-
and diamino-acids, at least to one or more of the different polypeptid
stages. Ever since the discovery by Cohnheim of the existence in the
intestinal juice of a substance termed by him erepsin, which is capable
of splitting proteoses and peptones into simple nitrogen-holding com-
pounds, there has been slowly developing the idea that normally
during intestinal digestion the proteoses and peptones are reduced by
this agent to leucin, tyrosin, histidin, arginin, aspartic acid, etc., which
in turn are absorbed and synthetized to blood or tissue albumin.
The discovery by Vernon of erepsin in pancreatic juice lends further
support to this view. Until more convincing evidence is furnished,
however, it may be assumed that peptone represents the final stage in
the digestion of proteins.

3. On fat. If pancreatic juice be added to a perfectly neutral
fat — olein, palmitin, or stearin — and kept at a temperature of about
ioo° F. (380 C), it will at the end of an hour or two be partially de-
composed into glycerin and the particular fatty acid indicated by
the name of the fat used — e. g., oleic, palmitic, stearic. The oil will
then exhibit an acid reaction. The reaction is represented in the
following formula:

C3H;(ClSH3302)3 + 3H20 = (Cl8H3402\3 + C3H=(HO)3
Triolein. Water. Oleic Acid. Glycerin.

If to this acidified oil there be added an alkali, e. g., potassium or
sodium carbonate, the latter will at once combine with the fatty acid
to form a salt known as a soap. The reaction is expressed in the
following equation:

Sodium Carbonate. Oleic Acid. Sodium Oleate. Carbonic Acid.

Na2C03 + ClSH3402 = Na2OCl8H3202 + H2C03

Coincident with the formation of the soap the remaining neutral oil
undergoes diviion into drops of microscopic size, which float in the
soap solution, forming what has been termed an emulsion, which is
white and creamy in appearance. The action of the pancreatic juice
may then be said to consist in the cleavage of the neutral fats into
fatty acids and glycerin, after which the formation of the soap and
the division of the fat takes place spontaneously. The enzyme which
produces the cleavage of the neutral fats has been termed steapsin
or lipase. The extent to which the cleavage of the fat takes place in
the intestine has not been definitely determined. There are some
who think the amount is relatively small, while others consider that
it i-; large, practically all of the fat undergoing this decomposition,
with the formation of soap and glycerin prior to their absorption.

200 TEXT-BOOK OF PHYSIOLOGY.

According to Pawlow, the relative amounts of the pancreatic
enzymes produced are conditioned by the character and amounts of
the food principles consumed. Thus, if chyme contains an excess
of either starch, proteid, or fat, there is a corresponding increase in
the amount of either amylopsin, trypsin, or steapsin produced. The
pancreas apparently adapts its activities to the character of the food.
Though it is probable that each enzyme is a derivative of a special
zymogen, it is only positively known that this is the case with trypsin.
This enzyme is a derivative of the zymogen, trypsinogen, the pro-
duction of which is thought to be the special function of secretin.
The pancreatic juice at the moment of its discharge into the intestine
does not contain trypsin but trypsinogen. The transformation of
the latter into the former is accomplished, according to Pawlow, by
a special activating ferment secreted by the epithelium of the small
intestine and termed enter okinase.

The rapidity with which pancreatic juice in the presence of bile
and hydrochloric acid (under conditions such as are present in the
duodenum) can develop sufficient fatty acid to form an emulsion was
determined by Rachford to be two minutes. The activity of steapsin
is thus shown to be very great.

Physiologic Action of the Intestinal Juice.— The part played
by the intestinal juice in the digestive process is yet a subject of dis-
cussion, as the results obtained by different observers are in some
respects contradictory, due to the fact that animals, including human
beings, have been the subjects of experimentation. Notwithstanding
the actions of saliva, gastric and pancreatic juice, there yet remain
in the food saccharose, maltose, and lactose, three forms of sugar which
are believed by most observers to be non-assimilable and therefore
require some change before they can be absorbed and assimilated.
An extract of the intestinal mucous membrane or the intestinal juice
of a dog, added to a solution of saccharose, will in a very short time
convert it into dextrose and levulose, which together constitute invert
sugar. The enzyme by which this inversion is produced, though
nothing definite is known as to its nature, has been termed invertin
or invertase. Tubbey and Manning state that the human intestinal
juice as obtained by them has the same action. In the case of intestinal
fistulae reported by Busch, which were supposed to be located in the
upper third of the intestine, it was found that when saccharose was
introduced into the lower opening, it was not inverted but appeared
in the feces unchanged.

Maltose is also rapidly transformed into dextrose. Lactose
appears to be unaffected by the pure juice. As it is non-assimilable
it has been supposed to undergo conversion into dextrose and galac-
tose while passing through the epithelial cells of the intestinal mu-
cosa. In either case the transformation is brought about by two
ferments known respectively as mallase and lactase.

Intestinal juice also has a slight diastatic action on starch.

DIGESTION. 201

THE LIVER.

The liver is a highly vascular conglomerate gland situated in the
right hypochondriac region and connected with the intestine by a duct.

Inasmuch as the liver performs several functions related to both
secretion and excretion, a consideration of its structure and its vari-
ous functions will be deferred to a subsequent chapter. In this con-
nection the bile, its physical properties and chemic composition in
relation to the digestive process, will only be considered.

The bile is a product of the secretory activity of the liver cells.
As it is poured into the intestine in man and most mammals at a point

"1 29

Fig. 89. — Gall-bladder, Hepatic, Cystic, and Common Ducts, i, 2, 3. Duode-
num 4, 4, 5, 6, 7, 7, 8. Pancreas and pancreatic ducts. 9, 10, 11, 12, 13. Liver. 14.
Gall-bladder. 15. Hepatic duct. 16. Cystic duct. 17. Common duct. 18. Portal
vein. 19. Branch from the celiac axis. 20. Hepatic artery. 21. Coronary artery
of the stomach. 22. Cardiac portion of the stomach. 23. Splenic artery. 24.
Spleen. 25. Left kidney. 26. Right kidney. 27. Superior mesenteric artery and vein.
28. Inferior vena cava. — (Sappey.)

corresponding to the orifice of the pancreatic duct, and most abundantly
at the time the food is passing through the duodenum, it is usually
regarded as a digestive fluid possessing an influence favorable if not
necessary to the completion of the general digestive process.

Anatomic Relations of the Biliary Passages. — After its forma-
tion by the liver cells the bile is conveyed from the liver by the bile
capillaries, which uniting finally from the main hepatic duct. This
duct emerges from the liver at the transverse fissure. At a distance
of about 5 centimeters it is joined by the cystic duct, the distal ex-
tremity of which expands into a pear-shaped reservoir, the gall-bladder
in which the bile is temporarily stored (Fig. 89). The duct formed
by the union of the hepatic and cystic ducts, the common bile-duct,

202 TEXT-BOOK OF PHYSIOLOGY.

passes downward and forward for a distance of about 7 centimeters,
pierces the walls of the intestine and passes obliquely through its coats
for about a centimeter and opens on the surface of a papilla in con-
junction with the pancreatic duct. The walls of the biliary passages
are composed of a mucous membrane internally, a fibrous and mus-
cular coat externally. The termination of the common bile-duct is
provided with a distinct band of circularly disposed muscle-fibers,
which when in action completely close the orifice and prevent the dis-
charge of bile. It may therefore be regarded as a true sphincter
muscle. Small racemose glands are embedded in the mucous mem-
brane of the main ducts.

Physical Properties and Chemic Composition of Bile. — The
bile obtained directly from the liver through a fistulous opening in
the hepatic duct is always thin and watery, while that obtained from
the gall-bladder is more or less viscid from admixture with mucin,
the degree of this viscidity depending on the length of time it remains
in this reservoir. The specific gravity of human bile varies within
normal limits from 1.010 to 1.020. The reaction is invariably alka-
line in the human subject when first discharged from the liver, but
may become neutral in the gall-bladder. The alkalinity depends on the
presence of sodium carbonate and sodium phosphate. When fresh,
it is inodorous; but it readily undergoes putrefactive changes, and
soon becomes offensive. Its taste is decidedly bitter. When shaken
with water, it becomes frothy — a condition which lasts for some time
and which is due to the presence of mucin. In ox bile the mucin is
replaced by a nucleo-proteid.

The color of bile obtained from the hepatic duct is variable, usually
a shade between a greenish-yellow and a brownish-red. In different
animals the color varies. In the herbivorous animals it is usually
green; in the carnivorous animals it is orange or brown. In man it is
green or a golden yellow. The colors are due to. the presence of pig-
ments. Microscopic examination does not show the presence of struc-
tural elements.

Human bile obtained from an accidental biliary fistula was shown
by Jacobson to contain the following ingredients, viz.:

COMPOSITION OF HUMAN BILE.

Water, 977-4©

Sodium glycocholate, 9.94

Sodium taurocholate, a trace

Cholesterin, 0.54

Free fat, o. 10

, Sodium palmitate and stearate, 1.36

Lecithin, 0.04

Other organic matters, 2.26

Sodium chlorid, 5.45

Potassium chlorid, 0.28

Sodium phosphate. 1.33

Calcium phosphate, 0.37

Sodium 1 arbonate, 0.93

1000.00

DIGESTION. 203

In this analysis the solid ingredients constitute 22.5 parts per 1000,
of which two-thirds are organic and one-third inorganic. The amount
of solid varies according to the animal from which the bile is obtained.
Sodium Glycocholate and Taurocholate. — Of the various in-
gredients of the bile none are more important than these two salts,
usually known as the bile salts. The sodium glycocholate is found
most abundantly in the bile of herbivora, the sodium taurocholate
in the bile of the carnivora. These salts are compounds of sodium
and glycocholic and taurocholic acids. When separated from the
sodium, the acids will crystallize in the form
of fine acicular needles. Under the influ-
ence of hydrating agents, such as dilute
acids and alkalies, both acids will undergo
cleavage into their respective components —
e. g., glycocoll and cholalic acid, taurine
and cholalic acid. Glycocoll and taurine
are crystallizable nitrogenized compounds

known chemicallv as amido-acetic and FlG- 9°- — Chol ester in
• j • ,1 • -j 1 rpi Crystals. — (Landois and

amido-isotnionic acids respectively. I he Stirling.)

bile salts are produced in the liver by a

true act of secretion, as they are not found in any of the tissues and

fluids of the body. After being discharged into the intestine they

undergo chemic changes, after which they can no longer be recognized.

In all probability they are reabsorbed into the blood and play some

ulterior part in the nutrition of the body.

Cholesterin. — Cholesterin is a constant ingredient of bile, though
it is not confined to this fluid, as its presence has been determined in
the crystalline lens, blood-corpuscles, nerve-tissue, and various patho-
logic fluids. It is an organic non-nitrogenized substance resembling
the fats in some particulars, but differing from them in not being ca-
pable of saponification with alkalies. It presents itself in the form
of thin transparent rectangular crystals, insoluble in water but soluble
in ether and boiling alcohol (Fig. 90) . It is held in solution in bile by
the bile salts. If they are deficient in amount, the cholesterin may
pass out of solution, collect around some foreign matter, and form
a gall-stone. Cholesterin is a product of the metabolism largely of
nerve-tissue, from which it is absorbed by the blood, carried to the
liver, and excreted. In the intestine it is converted into stercorin
and discharged from the body in the feces.

Bilirubin, Biliverdin. — These two pigments impart to the bile its
red and green colors respectively. Bilirubin is present in the bile of
human beings and the carnivora, biliverdin in the bile of the herbivora.
As the former pigment readily undergoes oxidation in the gall-bladder,
giving rise to the latter pigment, almost any specimen of bile may
present any shade of color between red and green. Bilirubin is re-
garded as a derivative of hematin,one of the cleavage products of hem-
oglobin, the coloring-matter of the blood. In the liver the hematin

2o4 TEXT-BOOK OF PHYSIOLOGY.

combines with water, loses its iron, and is changed to bilirubin. By
continuous oxidation there are formed biliverdin, bilicyanin, and
choletelin. After their discharge into the intestine the bile pigments
are finally reduced to hydrobilirubin, which becomes one of the con-
stituents of the feces. An oxidation of the bilirubin can be produced
by nitroso-nitric acid. If this agent is added to a thin layer of bile on a
porcelain surface, a series of colors will rapidly succeed one another,
commencing with green and passing to blue, orange, purple, and yellow.
This is the basis of the well-known test for bile pigments suggested by
Gmelin.

Lecithin.— Lecithin is regarded, because of its physical properties
and chemic composition, as a complex fat. When pure it presents
itself generally as a white crystalline powder, though very frequently
as a white waxy mass which is soluble in ether and alcohol. Its chemic
formula is C44H90NP09. Lecithin is widely distributed throughout
the body, being found in blood, lymph, red and white corpuscles, nerve-
tissue, yolk of egg, semen, milk and bile. It is readily decomposed,
yielding with various reagents gly co-phosphoric acid, a fat acid (stearic),
and the alkaloid, cholin. Lecithin has been regarded as one of the
decomposition products of nerve-tissue, removed from the blood by
the liver and thus becoming one of the constituents of the bile in
which it is held in solution by the bile salts.

The Mode of Secretion and Discharge of Bile. — The manner
in which the bile flows from the liver into the main hepatic ducts,
the variations in the rate of its discharge into the intestine, as well
as the total quantity secreted daily, have been approximately deter-
mined by fistulous openings either in the hepatic ducts or in the gall-
bladder. Although the liver presents some physiologic peculiarities,
there is no reason to believe that the conditions of secretion therein are
different from those in any other secretory organ, or that any other
structure than the cell is engaged in this process. As shown by chemic
analysis, the bile consists of compounds, some of which, like the bile
salts, are formed in the liver cells, out of material furnished by the blood
by a true act of secretion, while others, such as cholesterin and lecithin,
principles of waste, are merely excreted from the blood to be finally
eliminated from the body. The bile is thus a compound of both
secretory and excretory principles.

The flow of bile from the liver is continuous but subject to con-
siderable variation during the twenty-four hours. The introduction
of food into the stomach at once causes a slight increase in the flow,
but it is not until about two hours later that the amount secreted reaches
its maximum; after this period it gradually decreases up to the eighth
hour, but never entirely ceases. During the intervals of digestion
though a small quantity passes into the intestine, the main portion is
diverted into the gall-bladder, because of the closure of the common
bile-duct by the sphincter muscle near its termination, where it is re-
tained until required for digestive purposes. When acidulated food

DIGESTION. 205

passes over the surface of the duodenum there is excited, through re-
flex action, a contraction of the muscle walls of the gall-bladder and
ducts, a relaxation of the sphincter, and a gush of bile into the intestine,
the discharge continuing intermittently until digestion ceases and the
intestine is emptied of its contents.

The storage and the discharge of bile, brought about by the alter-
nate contraction and relaxation of the muscle walls of the gall-bladder
and of the sphincter is regulated by the nerve system. As the result
of his experiments Doyon concludes, that during the intervals of intes-
tinal digestion the vagus nerve is carrying nerve impulses which on
the one hand augment the contraction of the sphincter and inhibit
the contraction of the walls of the gall-bladder, thus establishing the
conditions for the storage of bile; but when intestinal digestion is in-
augurated the splanchnic nerve carries nerve impulses which inhibit
the sphincter and augment the contraction of the walls of the gall-
bladder, thus establishing the condition for the discharge of the bile.

The total quantity of bile secreted daily has been estimated to be
from 500 to 800 grams.

Physiologic Action of Bile.— Notwithstanding our knowledge
of the complex composition of bile, the quantity discharged daily, and
the time and place of its discharge, its exact relation to the digestive
process has not been fully determined. No specific action can be
attributed to it. It has but a slight, if any, diastatic action on starch.
It is without influence on proteins or on fats directly. But indirectly
and by virtue of the bile salts it contains, it plays an important part
in increasing the action of the pancreatic enzymes. Thus the anxiolytic
power of the pancreatic juice is almost doubled and the same is true
for its proteolytic power, while its fat-splitting power is tripled. The
bile salts also dissolve insoluble soaps which may be formed during
digestion.

Bile thus favors the digestion of fat. If it be excluded from the in-
testine there is found in the feces from 22 to 58 per cent, of the ingested
fats. At the same time the chyle, instead of presenting the usual white
creamy appearance, is thin and slightly yellow. The manner in which
the bile promotes fat digestion is yet a subject of investigation. If
all the fat is converted into fatty acid and glycerin, with the formation
of soaps, as seems probable, the action of the bile becomes more ap-
parent from the fact, already stated, that it dissolves and holds in so-
lution the soaps so formed which would be necessary to their absorp-
tion by the epithelial cells. As an aid to digestion the bile has been
regarded as important, for the reason that its entrance into the intes-
tine is attended by a neutralization and precipitation of the proteins
which have not been fully digested and are yet in the stage of acid-
albumin. In this way gastric digestion is arrested and the foods are
prepared for intestinal digestion.

Though bile possesses no antiseptic properties outside the bodv,
itself undergoing putrefactive changes very rapidly, it has been believed

2o6 TEXT-BOOK OF PHYSIOLOGY.

that in the intestine it in some way prevents or retards putrefactive
changes in the food. There can be no doubt that if the bile be pre-
vented from entering the intestine there is an increase in the formation
of gases and other products which impart to the feces certain char-
acteristics which are indicative of putrefaction. As to the manner
in which bile retards this process nothing definite can be stated.

Bile has been supposed to be a stimulant to the peristaltic move-
ments of the intestine, inasmuch as these movements diminish when
bile is diverted from the intestine.

Though no definite nor specific action on any of the different classes
of food principles can be attributed to the bile, there is abundant evi-
dence to show that its presence in the alimentary canal during digestion
is essential to the maintenance of the nutrition of the body. That
the bile as a whole, or at least part of its constituents, favorably in-
fluences digestion and general nutrition is evident from the phenomena
which follow its total exclusion from the intestine, as when the common
bile-duct is ligated and a fistula of the gall-bladder is established.
The following phenomena were observed in a young dog so prepared
by Professor Flint. During the first five days succeeding the opera-
tion the abdomen was tumid and there was some rumbling in the bowels.
Though the animal ate every day, the discharge of fecal matter became
infrequent, the matter passed being grayish in color and highly offen-
sive. After two weeks the alvine discharges took place three and four
times daily. For four days the weight remained normal; afterward
it began to diminish, and from this time the animal continued to lose
strength and weight until its death, thirty-eight days after the opera-
tion. Ten days after the operation the appetite, which had been very
good, increased, but did not become ravenous until a few days before
death. The animal usually ate from a pound to a pound and a half
of beef-heart daily, always refusing fat. There was an absence at
all times of jaundice, fetor of the breath, and falling of the hair. Post-
mortem examination showed that the bile-duct was obliterated, and
there was no evidence that any bile could have passed into the intes-
tine. The results of this and similar cases go to show that that portion
of the bile which is secretory in character is essential to digestion and
the nutrition of the body- — that, though large quantities of food are con-
sumed, progressive diminution of weight takes place until nearly 40
per cent, of the body is consumed. In some instances the breath be-
comes fetid and there is a falling of the hair, showing some profound
disturbance of the general nutritive process.

The Movements of the Intestine. — The movements of the in-
testine have been studied by means of the Rontgen rays by Cannon.
The method adopted was to mix with the food, subnitrate of bismuth,
which being opaque rendered the movements of the intestinal con-
tents and thereby the movements of the intestinal walls visible, on the
fluorescent screen. These investigations revealed the presence of
two forms of activity, one of which is more or less stationary and due

DIGESTION.

207

to rhythmic contraction of circular muscle-fibers, the other progres-
sive, passing from above downward and due to the contraction of
circular and longitudinal muscle-fibers. The former activity, which
is by far the more common, results in a division of the intestinal con-
tents into small segments and for this reason was termed by Cannon,
rhythmic segmentation; the latter activity is the well-known peristaltic
wave.

After bands of circular muscle-fibers, situated at variable dis-
tances one from another, contract and divide a mass of food into seg-

B

/\/\/\/\/\

\/\/\/\/w

D

Fig. qi. — The Divisive or Segmenting Movements of the Small Intestine.
A, surface view of a portion of the intestine, showing six constrictions which divide the
contents into five segments, as shown in B; as these constrictions pass away new ones
come in between them and divide each segment of the contents into two, the adjoining
halves of neighboring segments fusing to make the new segments shown in C. Repetition
of this process results in the condition shown in D. {Modified, After Hougli and Sedge
wick, "The Human Mechanism.")

ments, they at once relax and are followed by contraction of other
bands in the segments of the intestines overlying the segments of food.
The result is again a division of the food into two new segments (Fig.
91). The lower half of each segment then unites with the upper half of
the segment below to commingle with it and expose new surfaces of
the food mass to contact with the actively absorbing mucosa. The
continual repetition of this process results in a thorough mixing of the
food with the digestive juices. From the manner in which these con-
tractions make their appearance it would seem that the mere presence
of a segment in the lumen of the intestine is sufficient to excite the
overlying fibers to activity.

In certain regions of the intestine rhythmic segmentation may
continue for half to three-quarters of an hour without moving the

2oS TEXT-BOOK OF PHYSIOLOGY.

food forward to any marked extent. In the cat the segmentation
may proceed at the rate of thirty divisions a minute.

Bayliss and Starling state, from observations made on the exposed
intestine of a dog, that in addition to the usual peristaltic movement
the intestinal coils exhibit a swaying or pendulum movement accom-
panied by slight waves of contraction which may arise apparently at
any point and pass down the intestine. These contractions may
occur from ten to twelve times a minute and travel at a rate varying
from two to five centimeters a second. In how far this movement
represents the normal movement as it takes place under physiologic
conditions and as observed by Cannon, remains for further investiga-
tors to decide.

After the food has been prepared by the process above described,
it is then slowly carried downward by what is known as the vermicular
or peristaltic wave. This wave is characterized by a contraction of
the circular fibers behind a bolus and a relaxation of the fibers in ad-
vance of it. The result is a movement forward of the bolus, and as
it moves it is followed by a ring of constriction and preceded by a
ring of relaxation or inhibition. The rate of movement of the peris-
taltic wave is extremely slow.

The Nerve Mechanism of the Intestine. — The causes of these
two forms of intestinal activity, rhythmic segmentation or pendulum
movement and peristalsis, have been the subject of much investiga-
tion. Because of the presence of a network or plexus (Auerbach's
and Meissner's) of nerve-cells and nerve-fibers in the walls of the intes-
tines and in close relation to the muscle cells, and because of the fact
that the intestines will contract for some time after removal from the
body of the animal, it has been difficult to decide whether the contrac-
tions are of myogenic or neurogenic origin.

As the rhythmic contractions continue, though the peristaltic are
abolished by the introduction of nicotin into the blood, an agent which
temporarily paralyses peripheral nerve-cells, it was concluded by
Bayliss and Starling that the rhythmic contractions are of myogenic
origin and propagated from fiber to fiber and that the peristaltic con-
tractions are reflex in character, the coordination being carried out by
the local nerve mechanisms and initiated by stimulation of the intes-
tine. Whether this is the case or not, the general contractions of the
intestine are augmented and inhibited by the central nerve system
through the vagus and splanchnic nerves.

Stimulation of the vagus is followed by an augmentation of the
contraction, though not infrequently there is a primary inhibition of
short duration. Stimulation of the splanchnic is followed by a relax-
ation or inhibition of the contraction, though according to some observ-
ers there is at times an opposite effect.

The Large Intestine. — The large intestine is that portion of the
alimentary canal situated between the termination of the ileum and
the anus. It varies in length from four and a half to five feet, in di-

DIGESTION. 209

ameter from one and a half to two and a half inches. It is divided
into the cecum, the colon (subdivided into an ascending, transverse,
and descending portion, including the sigmoid flexure), and the rectum.

The cecum is situated in the right iliac fossa. It is that dilated
portion of the large intestine below the orifice of the small intestine.
The posterior and inner wall presents a small opening which leads
into a narrow round process about four inches in length — the vermi-
form appendix. The opening of the small intestine into the cecum is
narrow and elongated and bordered by two folds of mucous membrane
strengthened by fibrous and muscle-tissue. These folds constitute the
so-called ileo-cecal valve. When the cecum is distended the margins
of these folds are approximated and effectually prevent the return of
material into the small intestine.

The closure of this opening is now attributed to a sphincter muscle
— the ileo-colic — the action of which is regulated by the nerve system.

The colon ascends to the under surface of the liver, where it bends
at a right angle, crosses the abdominal cavity to the spleen, bends
again, and descends to the left iliac fossa. At this point it turns upon
itself to form the sigmoid flexure. The rectum is a dilated pouch,
situated within the true pelvis. It measures from 15 to 18 centimeters
in length. Within an inch of its termination at the anus it presents a
constriction formed by a circular band of muscle-fibers known as the
internal sphincter. The margin of the anus is also surrounded by
bands of muscle-fibers known collectively as the external sphincter.

The walls of the large intestine consist of three coats: viz., serous,
muscular, and mucous.

The serous is a reflection of the general peritoneal membrane.

The muscle is composed of both longitudinal and circular fibers.
The longitudinal fibers are collected into three narrow bands which
are situated at points equidistant from one another. At the rectum
they spread out so as to completely surround it. As the longitudinal
bands are shorter than the intestine itself, its surface becomes saccu-
lated, each sac being partially separated from adjoining sacs by nar-
row constrictions. The circular fibers are arranged in the form of a
thin layer over the entire intestine. Between the sacculi, however,
they are more closely arranged. In the rectum they are well developed,
and at a point an inch above the anus they form, as stated above, the
internal sphincter.

The mucous membrane of the large intestine possesses neither villi
nor valvulse conniventes. It contains a large number of tubules consist-
ing of a basement membrane lined by columnar epithelium. They
resemble the follicles of Lieberkuhn. The secretion of these glands
is thick and viscid and contains a large quantity of mucin.

Contents of the Large Intestine. — As a result of the actions

of saliva, of gastric, intestinal, and pancreatic juice, and of the bile,

the food is disintegrated and liquefied. The nutritive principles, pro-

teid, starches, sugars, and fats, undergo chemic changes and are trans-

14

210 TEXT-BOOK OF PHYSIOLOGY.

formed into peptones, dextrose, soap and glycerin, fatty acids, under
which forms they are absorbed. After the more or less complete di-
gestion and absorption of these nutritive substances the residue of the
food, comprising the indigestible and undigested matter, passes out
of the small intestine into the large intestine and forms a portion of its
contents. This residue consists of the hard parts of the cereals, veget-
able seeds, cellulose, etc., the quantity and variety of which depend on
the nature of the food. These substances, passing into the large intes-
tine along with the excrementitious matter of the bile, become incor-
porated with the mucous secretions and assist in the formation of the
feces. Under the influence of a peristaltic movement similar to that
witnessed in the small intestine, all this excrementitious matter,- de-
prived by absorption of the excess of its contained water and nutritive
material, is gradually carried downward to the sigmoid flexure, where
it accumulates prior to its extrusion from the body.

The effects of the peristaltic waves are to some extent interfered
with by anti-peristaltic waves which beginning in the transverse colon
run toward and to the cecum. An anti-peristaltic wave occurs in the
cat about every fifteen minutes and lasts for about five minutes. The
intestinal contents are thereby driven back toward the cecum. The
effect is a still further admixture with the secretions and exposure to
the absorbing mucosa. There is some evidence also that the anti-per-
istaltic wave may force some of the liquefied contents through the ileo-
colic opening into the small intestine because of the relaxation of the
sphincter muscle.

Intestinal Fermentation.— Owing to the favorable conditions
in the intestine for fermentative and putrefactive processes — e. g.,
heat, moisture, oxygen, and the presence of various microorganisms
— the food, when consumed in excessive quantity or when acted on
by defective secretions, undergoes a series of decomposition changes
which are attended by the production of gases and various chemic
compounds. Dextrose and maltose are partially reduced to lactic
acid; this to butyric acid, carbon dioxid, and hydrogen. Fats are
reduced to glycerol and fatty acids, the glycerol, according to the or-
ganisms present, yields succinic acid, carbon dioxid, and hydrogen.
The proteids under the prolonged action of the pancreatic juice are
decomposed, with the production of leucin and tyrosin. These crys-
talline compounds are in turn reduced to simpler forms. The former
yields valerianic acid, ammonia, and carbon dioxid; the latter gives
rise to indol, which is the antecedent of indican, found in the urine.
Skatol, another derivative of the proteid molecule, due to bacterial
action, gives the characteristic odor to the feces.

Feces. — The feces consist of water, mucin, the indigestible resi-
due of the food, decomposition products, and inorganic salts. The
consistency of the fecal matter varies from fluid to semifluid, depend-
ing largely on the length of time it remains in the intestine and the
extent to which absorption of its watery portion has taken place. The

DIGESTION. 211

odor is due to the presence of sulphuretted hydrogen and skatol. The
color is due partly to the altered coloring-matter of the bile, hydro-
bilirubin or 'stercobilin, and partly to the character of the food. The
total quantity discharged daily varies from four to six ounces.

Defecation. — Defecation is the final act of the digestive process
and consists in the expulsion of the indigestible residue of the food
from the intestine. This act usually takes place in the human being
but once in twenty-four hours, as the diet contains but a minimum
quantity of indigestible matter. Previous to their expulsion the feces
which have accumulated in the sigmoid flexure must pass downward
into the rectum. In so doing they develop the sensation which leads
to the act of defecation. The descent of the feces is accomplished by
the peristaltic contraction of the intestinal wall. Coincident with the
passage of the feces into the rectum there is a relaxation of the sphinc-
ter muscles and a contraction of the longitudinal and circular mus-
cular fibers, in consequence of which the feces are expelled. These
complex muscular actions are also aided by the voluntary contrac-
tions of the diaphragm and abdominal muscles.

Nerve Mechanism of Defecation. — The act of defecation is
primarily reflex, though somewhat influenced by voluntary efforts.
Under normal conditions the sphincter muscles governing the anal
orifice are firmly contracted, thus preventing the escape of gases or
semisolid matter. This tonic contraction is maintained by a nerve-
center located in the lumbar region of the spinal cord. The circu-
lar and longitudinal muscle-fibers of the rectum are at the same time
in a relaxed condition. When the desire to evacuate the bowels is ex-
perienced, the impressions made by the feces on the afferent nerves of
the rectal mucous membrane develop nerve impulses, which trans-
mitted to the rectal center and to the brain, influence in one direction
or another, their activities. If the act of defecation is to take place
there is an inhibition of the contraction of the sphincter ani muscles
and an augmentation of the contraction of the rectal muscles. In
their expulsive efforts, these latter muscles are assisted by the con-
traction of the diaphragm, abdominal and other muscles. After the
expulsion of the feces there is a return to the former condition, viz., a
relaxation of the rectal muscles and a contraction of the sphincters.

If the act is to be suppressed, the controlling influence, of the rectal
or sphincter center is strengthened and the reflex mechanism for a
while held in abeyance.

The exact course of the afferent and efferent nerves concerned
in this reflex is yet a subject of investigation.

The nerve-supply for the circular and longitudinal muscles of*
the lower part of the colon and rectum varies somewhat in different
animals, though it is usually derived from the second, third, and fourth
lumbar and the second and third sacral or pelvic nerves.

The lumbar nerves pass into and through the sympathetic chain
and thence to the inferior mesenteric ganglion, around the cells of which

212 TEXT-BOOK OF PHYSIOLOGY.

most of the nerves arborize. From this ganglion nerve-fibers pass to
both the circular and longitudinal fibers.

The sacral or pelvic nerves pass to the hypogastric plexus and
are ultimately connected with ganglion cells, which in turn send fibers
to both the longitudinal and circular fibers. The explanation of the
action of this complex mechanism is a subject of discussion because
of the want of agreement in the results that follow stimulation of these
nerves.

CHAPTER XL
ABSORPTION.

Absorption is the process by which nutritive material is trans-
ferred from the tissues, from the serous cavities, and from the mucous
surfaces of the body, into the blood. The most important of these
surfaces, especially in its relation to the formation of blood, is the
mucous surface of the alimentary canal, for it is from this organ that
the new materials are derived which maintain the quantity and quality
of the blood. The absorption of material from the interstices of the
tissues and from the serous cavities may be regarded as an act of re-
sorption, or a return to the blood of liquid nutritive material which has
escaped through the walls of the capillary blood-vessels for purposes
of nutrition, and which, if not returned, would lead to an accumulation
and the development of edematous conditions.

The anatomic mechanisms involved in the absorptive process
are, primarily, the tissue or lymph-spaces, the lymph- and blood-cap-
illaries; secondarily, the lymph-vessels and the veins.

Tissue or Lymph-spaces; Lymph-capillaries. — Everywhere
throughout the body, in the connective-tissue system and in the inter-
stices of the several structures of which an organ is composed, are
found spaces or clefts of irregular shape and size, determined largely
by the structure of the organ in which they are found, which have
been termed tissue or lymph-spaces, from the fact that they contain
a clear fluid, the lymph. These spaces are devoid for the most part of
any endothelial lining, but as they communicate more or less freely
one with another, there is a circulation of lymph through them and
around the islets of tissue (Fig. 93). In addition to the connective-
tissue lymph-spaces, different observers have described special spaces
or clefts in organs such as the kidney, liver, spleen, testicle, and in
all secreting glands between their basement membrane and the sur-
rounding blood-vessels, all of which contain a greater or less quan-
tity of lymph. Within the brain, spinal cord, bone, and other tissues
it has been shown that the smallest blood-vessels and capillaries are
bounded and limited by a cylindrical sheath containing lymph, which
is known as a perivascular lymph-space. A similar sheath surrounds
the smallest nerve-bundles and fibers, enclosing a perineural lymph-
space. The large serous cavities of the body, pleural, peritoneal,
pericardial, etc., are also to be regarded as lymph-spaces. The sur-
faces of these cavities, however, are covered with a layer of endo-
thelial cells with sinuous margins. At intervals between these cells are
to be found small free openings which have received the name of stomata.

213

214

TEXT-BOOK OF PHYSIOLOGY.

The lymph-capillaries in which the lymph-vessels proper take
their origin are arranged in the form of plexuses of quite irregular
shape. In most situations they are intimately interwoven with the
blood-vessels, from which they can be readily distinguished by their
larger caliber and irregular expansions. The wall of the lymph-cap-
illary is formed by a single layer of endothelial cells with characteristic
sinuous outlines. These capillaries anastomose very freely one with
another and communicate, on the one hand, with the lymph-spaces
and on the other with the lymph-vessels proper. As the shape, size,
etc., of .both lymph-spaces and capillaries are determined largely by

the nature of the tissue in
which they are found, it is
not always possible to
separate one from the
other. Their function,
however, may be regarded
as similar: viz., the recep-
tion and collection of the
lymph which has transuded
through the walls of the
blood-vessels and its trans-
mission onward into the
regular lymph-vessels.

The blood-capillaries
not only permit of a transu-
dation of the liquid nutri-
tive material from the blood
through their delicate walls,
but are also engaged, if not
in the reabsorption of a
portion of this transudate,
at least in the absorption of
waste products resulting
from tissue metabolism.
Lymph-vessels.- — The lymph- vessels constitute a system of minute,
delicate, transparent vessels found in nearly all the organs and tissues
of the body, and take their origin from the lymph-capillaries and spaces
above described (Figs. 94 and 95.) From their origin they gradually con-
verge toward the trunk of the body, and finally empty into the thoracic
duct. In their course they anastomose very freely with adjoining ves-
sels. The diameter of a lymph-vessel varies from 1 to 2 mm. After the
lymphatics have emerged from the lymph-capillaries they acquire
three distinct coats, each of which possesses definite histologic features.
The internal coal is composed of a delicate lamina of longitudinally
disposed elastic fibers covered with a layer of flattened nucleated
endothelial cells with wavy outlines.

The middle coat consists of white fibrous tissue arranged longit-

Fig. 93. — Origin of Lymphatics from the
Central Tendon of the Diaphragm Stained
with Nitrate of Silver. 5. The lymph-spaces
and lymph-canals communicating at x with the
lymphatics, a. Origin of the lymphatics by the
confluence of several juice canals. B. Capillary
blood-vessel. — (Landois and Stirling.)

ABSORPTION.

215

udinally and of non-striated muscle and elastic fibers arranged trans-
versely.

The external coat consists of practically the same structures, though
the muscle-fibers are longitudinally disposed.

The lymphatics are provided with valves which are so numerous
and located at such short intervals as to give the vessels a beaded ap-
pearance. These valves are arranged in pairs and consist of two semi-
lunar folds with their concavities directed toward the larger vessels.

Fig. 94. — Lymph-vessels and Lymph-glands of the Head and Neck. — {From
Gould's Dictionary.)

They are formed by a reduplication of the lining membrane, which is
strengthened by fibrous tissue derived from the middle coat.

Lymph-glands.— In their course toward the thoracic duct the
lymph-vessels pass through a number of small lenticular bodies termed
lymph-glands. These are exceedingly abundant in some situations,
as the cervical, axillary, and inguinal regions, and the abdominal
cavity. As the lymph-vessels approach a gland they divide into a
number of branches before entering it, known as the afferent vessels.
From the opposite side of the gland the lymphatics again emerge as
efferent vessels to unite to form larger trunks. A section of a gland
shows that it consists of an outer dense cortical and an inner soft pulpy

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TEXT-BOOK OF PHYSIOLOGY.

medullary portion. Each gland is covered externally by a dense mem-
brane of fibrous tissue containing in its meshes non-striated muscle-
fibers. From the inner surface of this membrane there pass inward

septa of connective tissue which, as
they converge toward the center of the
gland, divide the outer zone of the
gland into small conical compartments
or alveoli. When the septa reach the
medullary portion, they subdivide and
form bands or cords which interlace
in every direction and constitute a
loose meshwork the spaces of which
communicate with one another and
with the alveoli (Fig. 96). Within
the meshes of this framework the
proper gland substance is contained.
In the cortical compartments it is
moulded into pear-shaped masses; in
the medullary meshwork it assumes
the form of rounded cords which are
connected with one another. In both
regions, however, it is separated from
the septa by a space termed a lymph
sinus, through which the lymph flows
as it passes through the gland. The
lymph sinus is crossed by a network
of retiform connective tissue which
offers considerable resistance to the
passage of the lymph. The gland sub-
stance consists also of a framework of
retiform connective tissue in the
meshes of which large numbers of
lymph-corpuscles are contained. The
gland substance is separated from the
lymph sinus by a dense layer of a
reticulum, which, however, does not
prevent lymph and even corpuscles
from passing through it into the lymph
sinus.

The lymph-glands are abundantly

supplied with blood-vessels. The

arteries enter the gland at the hilum,

penetrate into the medullary substance, and terminate in a fine capillary

plexus which is supported by the connective tissue. The veins arising

from this plexus leave the gland also at the hilum.

The lymph-vessels which enter a gland first ramify in the invest-
ing membrane and then open directly into the lymph sinus. The

Fig. 95. — Lymph-vessels of the
Arm. — (Deaver.)

ABSORPTION.

217

vessels which leave the gland are also in communication with the
sinus. After the lymphatics enter the gland they lose their external
and middle coats, retaining only the internal or endothelial coat, which
lines the inner surface of the lymph sinus. The current of lymph,
therefore, is from the afferent vessels through the lymph sinus into
the efferent vessels. In addition to this primary current, there
is a secondary current flowing from the capillary blood-vessels outward
and into the sinus, which carries with it large numbers of lymph-cor-
puscles. It is quite probable that the movement of the lymph through
this complicated system of passages is aided by the contraction of the
muscle-fibers in the capsule of the gland.

The lymph-corpuscles or lymphocytes originate for the most part

a.l.

Fig. 96. — Diagrammatic Section of a Lymph-gland, a.l., Afferent, e. I., efferent
lymphatics. C. Cortical substance. M. Reticular cords of medulla, l.s. Lymph sinus.
c. Capsule, with trabecular, tr. — {Landois and Stirling.)

in the gland substance of the cortical alveoli. In this situation there
are groups of cells, so-called germ centers, which divide very rapidly
by mitosis and give rise constantly to groups of young cells which
soon find their way into the lymph stream.

The Thoracic Duct. — The thoracic duct is the general trunk of
the lymph system, into which the vessels of the lower extremities,
of the abdominal organs, of the trunk, of the left arm, and of the left
side of the head empty their contents. It is about fifty centimeters
in length and four millimeters in diameter. It extends upward
from the third lumbar vertebra along the vertebral column to the
seventh cervical vertebra, where it empties into the venous system
at the function of the internal jugular and subclavian veins on the left
side. The thoracic duct wall has the same general structure as the
wall of the lymph- vessel: viz., an internal or endothelial; a middle

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TEXT-BOOK OF PHYSIOLOGY.

It is also provided

elastic and muscular; an external or fibrous,
with numerous valves.

The lymph-vessels of the right side of the head, of the right arm,
and a portion of the right ride of the trunk terminate in the right

Fig. 97. — 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 (RC), 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 arm
to the left (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). — (Yeo's "Text-book 0} Physiology.")

thoracic duct, which is about 25 to 30 mm. in length and which emp-
ties into the venous system at the junction of the internal jugular and
subclavian veins on the right side. The general arrangement of the
lymphatic system is diagrammatically shown in Fig. 07.

LYMPH.

Lymph is the clear fluid found within the tissue spaces and with-
in the lymph-vessels. Inasmuch as there are reasons for the view

ABSORPTION. 219

that lymph varies in composition, as well as in function, in these
different regions it will be found conducive to clearness to designate
the lymph found in the tissue spaces as intercellular lymph, and that
found in the lymph-vessels as intravascular lymph.

The Physical Properties of Lymph. — Whether obtained from
tissue spaces or from lymph-vessels, the lymph presents practically
the same physical properties. The lymph obtained from the tho-
racic duct during the intervals of digestion or from one of the large
trunks of the leg is a clear, colorless or slightly opalescent fluid hav-
ing an alkaline reaction and a specific gravity of 1.020 to 1.040. Ex-
amined microscopically it is seen to hold in suspension a large number
of corpuscles similar to those seen in the lymph-glands and to the
white corpuscles of the blood. Their number has been estimated
at about 8200 per cubic millimeter, though this count will vary within
wide limits according as the lymph examined has passed through a
larger or smaller number of glands. The lymph-corpuscle consists
of a small quantity of protoplasm in which is embedded a distinct
nucleus. Some of these lymphocytes contain distinct granules,
more or less refractive, which impart to the corpuscle a distinctly
granular appearance. When withdrawn from the vessels lymph
undergoes a spontaneous coagulation, though the coagulum is never
as firm as that observed in the coagulation of the blood. The cause
of the coagulation is the appearance of fibrin. After a variable length
of time the coagulum separates into a liquid and a solid portion, the
serum and the clot.

The Chemic Composition of Lymph. — Although the lymph
obtained from the tissue spaces, from the lymph-vessels, as well as
from the so-called serous cavities has the same general chemic char-
acteristics, there is reason for the view that it varies in its ultimate
composition according as it is derived from one region of the body
or from another. The needs of any individual tissue as well as the
character of its metabolic products will in all probability not only
change its normal composition, but also the relative amounts of its
normal constituents.

Chemic analysis has shown that the lymph from the thoracic duct
contains from 3.4 to 4.1 per cent, of proteins (serum-albumin, fibrin-
ogen), 0.046 to 0.13 per cent, of substances soluble in ether (probably
fat), 0.1 per cent, of sugar, and from 0.8 to 0.9 per cent, of inorganic
salts, of which sodium chlorid (0.55 per cent.) and sodium carbonate
(0.24 per cent.) are the most abundant (Munk). There are usually
in most specimens small quantities of potassium, calcium, and mag-
nesium salts. Fibrinogen is seldom present beyond 0.1 per cent.,
which will account for the feeble and slow coagulation. Lymph
contains both free oxygen and carbon dioxid. Of the former, how-
ever, there is but a small percentage; of the latter, about 45 vols, per
cent., partially in the free state and partially combined with sodium.
Urea is also present in very small amounts. This analysis indicates

220 TEXT-BOOK OF PHYSIOLOGY.

that lymph resembles blood-plasma in the character of its constitu-
ents, though their relative quantities vary considerably. With the
exception that it contains no red corpuscles, lymph may be regarded
as a diluted blood.

The Production of Lymph. — Though blood is the common res-
ervoir of nutritive material, the latter is not available for nutritive
purposes as long as it is confined within the blood-vessels. The
capillary wall, thin as it is, and composed of but a single layer of
endothelial cells, would be sufficient to prevent its utilization by the
tissues, if it were not permeable to the liquid portion of the blood.
As this is the case, however, it is found that as the blood flows through
the capillary vessels a portion of the blood-plasma passes through the
capillary wall and is received into the tissue spaces, where it comes into
intimate contact with the tissue-cells.

The forces concerned in the passage of the constituents of the
blood-plasma across the capillary wall have been the subject of much
investigation. According to some investigators, diffusion, osmosis
and filtration are sufficient to account for all the phenomena. For
a consideration of the phenomena of diffusion, osmosis and filtration
the reader is referred to paragraphs at the end of this chapter. It is
assumed that the capillary wall, being an animal membrane is freely
permeable to water and crystalloid bodies generally; less so, however,
to colloid bodies, such as the proteids of the blood-plasma; moreover,
it is further assumed that the physiologic conditions of the capillary
walls are such as not only to permit of the passage of the constituents
of the blood into the tissue spaces, but also the passage of the con-
stituents of the intercellular lymph into the blood, according to laws
similar at least to those determining the passage of substances through
animal membranes as determined experimentally. The force giving
rise to filtration is the difference of pressure between that exerted by
the blood within the capillary vessels and that exerted by the fluid in
the tissue spaces; hence any increase or decrease of this difference of
pressure is attended by an increase or decrease in the production of
lymph. Thus compression of the veins of a part which interferes with
the outflow of blood from the capillaries, or a dilatation of the arterioles
which increases the inflow of blood to them will increase the capillary
pressure and therefore the production of lymph. The reverse con-
ditions will, of course, diminish the intracapillary pressure and
lymph production. Hemorrhages which lower the general blood-
pressure may so lower the capillary pressure as not only to stop the
flow of lymph to the tissues, but may give rise to a diffusion current
from the tissues into the blood.

The quantitative composition of the lymph compared with that
of the blood indicates that it is produced by diffusion, osmosis, and
filtration. In the lymph the concentration of the inorganic salts is
practically the same as in the blood; the concentration of the pro-
teids, however, is somewhat less. These facts are in accordance with

ABSORPTION. 221

what is known regarding the diffusibility of both crystalloids and col-
loids through animal membranes.

According to other investigators, the production of lymph is not
so much due to intracapillary pressure as it is to the specialized ac-
tivities of the endothelial cells, activities which indicate that lymph
is a secretion the composition of which varies in different situations
by virtue of a difference in the molecular structure of the endothelial
cells. As is the case with many of the secreting cells of the body, the
injection of various substances into the blood apparently increases
the activity of the endothelial cells, as shown by an increased lymph
production without any appreciable increase of intracapillary pressure.
Thus it has been shown that after the injection into the blood of
sugar, sodium chlorid, sodium sulphate, urea, etc., there is an increase
in the flow of lymph from the thoracic duct. The lymph, however,
under these circumstances is richer in water than is normally the
case. As the blood at the same time increases its percentage of
water, it is assumed that the water is extracted from the tissues, by
reason of an increased percentage of salts in the tissue spaces due to
increased activity of the endothelial cells. A higher percentage of
salts in the lymph than in the blood is difficult to account for on the
diffusion-filtration theory. The injection of peptones, albumin, the
extract of the muscles of the leech, crab, mussel, etc., is also followed
by an increase in the amount of lymph discharged from the thoracic
duct; but in this instance the lymph possesses a higher degree of con-
centration, being richer not only in inorganic but also in organic constit-
uents. The cause of this increase in both the quantity and quality
of the lymph is believed to be an increased activity in the secreting
power of the endothelial cells. The more recent experiments of
Starling indicate that in addition to the difference of pressure be-
tween the blood in the capillaries and the lymph in the tissue spaces,
a new factor must be considered and that is, the permeability of the
capillary wall. This he finds to vary considerably in different parts
of the vascular apparatus, being greatest in the capillaries of the liver,
less in the capillaries of the intestines and least in the capillaries of
the extremities. It also varies doubtless in all other situations. The
increase in the production of lymph by the injection of peptones,
extract of muscles of the leech, the crab, etc., Starling explains by the
assumption that these substances alter the properties of the capillary
wall and thus increase its permeability. The difference of pressure,
therefore, between blood and lymph taken in connection with the de-
gree of permeability of the capillary wall will account for the pro-
duction of lymph in all regions of the body. It is possible that all
these facts may be otherwise interpreted; the subject is yet a matter
of investigation.

The Functions of Intercellular Lymph. — The origin and
composition of lymph, its situation and relation to the tissue cells
indicate that its function is to provide the tissue cells with those nutri-

222 TEXT-BOOK OF PHYSIOLOGY.

tive materials necessary to their growth, repair, and functional activ-
ities, and to receive from the tissue cells their waste products prior
to their removal by the blood- and lymph-vessels.

The necessity for the production of lymph becomes apparent
when the chemic changes which the tissues undergo at all times are
considered. Thus whether in a state of relative rest or in a state of
activity, disintegrative changes are constantly taking place and al-
ways in direct proportion to the degree and continuance of the activity.
If the tissues are to continue in the performance of their customary
activities, it is essential that repair and restoration be at once established.
This is made possible by the presence of lymph, and by the power which
living material possesses of absorbing from the lymph the necessary
nutritive materials, of assimilating them and transforming them into
material like unto itself and endowing them with their own physiologic
properties.

Coincidently with the loss of nutritive material, the lymph receives
the waste products of the tissues and hence changes in composition.
Should this change in composition continue for any length of time,
the lymph would lose its restorative character and become destruc-
tive to tissue vitality. Therefore it is essential that the nutritive
material be renewed as rapidly as consumed and the waste products
be carried away as rapidly as produced. Both these conditions
are fulfilled by the blood- and lymph-vessels.

The Absorption of Intercellular Lymph. — From the fact that
lymph is being discharged from the thoracic duct into the blood,
more or less continually, it is evident that lymph is being absorbed
from the intercellular spaces; from which fact it may be inferred
that the production of lymph is a continuous process and that
it is passing through the capillaries in amounts greater than is
necessary for the immediate needs of the tissues. Should this excess
accumulate there would soon arise the condition of edema and an inter-
ference with the functional activities of the tissues. Therefore so soon
as the accumulation attains a certain volume it is absorbed in large
measure by the lymph-capillaries and transmitted to the lymph-
vessels and thoracic duct. Because of the general belief that the
lymph-capillaries were in open communication with the tissue spaces
it was assumed that the absorption of lymph and its flow through
the lymph-vessels was the result of a difference of pressure between
the lymph in the tissue spaces and the blood in the innominate veins.
But if the lymph-capillaries are closed vessels, as recent investiga-
tions indicate, then additional factors, in explanation of lymph ab-
sorption, must be sought for.

It is quite possible under even normal conditions of pressure in
the tissue spaces that some of the more diffusible constituents of the
lymph should be absorbed by the capillary blood-vessels. As to
whether the relatively feebly diffusible colloids should be so resorbed
is as yet a matter of investigation.

ABSORPTION. 223

ABSORPTION OF FOODS.

The most important of the absorbing surfaces, especially in its
relation to the absorption of new material, is the mucous membrane
of the alimentary canal, and more particularly that portion lining the
small intestine, provided as it is with specialized absorbing structures
— the villi. Though certain substances can be absorbed from the
mouth, it is not probable that any food is so absorbed. From the
changes which the food principles undergo in the stomach it might
naturally be inferred that their absorption would promptly follow.
Experimental researches have demonstrated, however, that this takes
place, if at all, but to a slight extent. If, however, solutions of inor-
ganic salts, sugars, and peptones possessing a concentration of at
least 5 per cent. — a degree of concentration seldom realized under
normal conditions — are introduced into the stomach, their absorption
will be effected, the rate of. absorption following in a general way the
increase, within limits, in concentration. Water is practically not
absorbed from the stomach. The absorption of the products of
digestion — i. <?., dextrose, levulose, peptones, soaps, glycerin, fatty
acids, salts, along with water, in which for the most part they are
held in solution — is therefore limited very largely to the small intes-
tine, and is accomplished by the villous processes projecting from the
surface of the mucous membrane.

Structure of the Villi. — The villi are small filiform or conical
processes, from 0.5 to 1 mm. in length, and from 0.2 to 0.5 mm. in
breadth, covering the surface of the mucous membrane from the
pyloric orifice to the upper surface of the ileo-cecal valve. Each
villus consists of a basement membrane (see Fig. 98; supporting tall
columnar epithelial cells. Each cell is composed of granular bio-
plasm containing a distinct nucleus. At its free extremity a narrow
border of the cell presents a striated appearance, as if it were com
posed of small rods embedded in some cement substance. Goblet
or mucin-holding cells are also to be found among the columnar
cells. The body of the villus, that portion within the basement
membrane, consists of a reticulated connective tissue supporting arte-
ries, capillaries, veins, and lymphoid corpuscles. In the center of the
villus there is usually a single though at times a double club-shaped
lymph-capillary, the walls of which are composed of epithelioid cells
with sinuous margins. This capillary probably begins by a blind ex-
tremity and opens at the base of the villus into the subjacent lymph-
vessels. The communicating orifice is guarded by a valve. It is also
surrounded by a layer of non-striated muscle-ribers, arranged longitu-
dinally, derived from the muscularis mucosae and attached to the apex
of the body of the villus.

The arteries which penetrate the villi are derived from those
of the submucous coat of the intestine, which are the ultimate branches
of the intestinal artery, and serve the purpose of delivering nutritive

224

TEXT-BOOK OF PHYSIOLOGY.

material to the capillary plexus (Fig. 99). While passing through
the latter a portion of the blood-plasma transudes through the capil-
lary walls into the spaces of the reticulated tissue, constituting lymph.
At the same time products of tissue metabolism pass through the capil-
lary walls into the blood. The blood then passes into the venules,
which, leaving the villus at its base, unite with the veins of the sub-

Fig. 98. — Longitudinal Sec-
tion of a Villus from In-
testine of the Dog, Highly
Magnified, a. Columnar epi-
thelium containing goblet-cclls (b)
and migratory leukocytes (h). c.
Basement membrane, d. Plate-
like connective-tissue elements of
core, e, e. Blood-vessels. /. Ab-
sorbent radical or lacteal.—
{Pier sol.)

Fig. 99. — Section of Injected Small
Intestine of Cat. a, b. Mucosa, g.
Villi, i. Their absorbent vessels, h. Sim-
ple follicles, c. Muscularis mucosae, j. Sub-
mucosa. g, e'. Circular and longitudinal
layers of muscle. /. Fibrous coat. All the
dark lines represent blood-vessels filled with
the injection mass. — (Piersol.)

mucous coat to form the intestinal veins. These finally unite with the
gastric and splenic veins to form the portal vein, which enters the liver
at the transverse fissure (Fig. 100). The excess of lymph within the
villus passes into the club-shaped lymph-capillary, to be finally carried
by the lymphatics of the mesentery into the thoracic duct. During
the intervals of digestion and in the absence of food from the intestine
there is, of course, no absorption of food nor the removal from the villus
of anything but the excess of lymph and metabolic products.

Function of the Villi. — The villi, and especially the epithelial

ABSORPTION.

225

cells covering them, are the essential agents in the absorption of the
products of digestion. It is by the activity of these cells that the
new materials are taken out of the alimentary canal and transferred
into the lymph-spaces, in the body of the villi, from which they are
subsequently removed by the blood-vessels and lymphatics. As to
the mechanism by which the epithelial cells accomplish this result,
nothing definite can be asserted.
Inasmuch as the absorption of
food does not take place in ac-
cordance with the laws of osmosis
as at present understood, it has
been suggested that the cells
possess a "selective action" de-
pendent on their organization
and living condition, an action
which is to a great extent con-
ditioned and limited by the de-
gree of diffusibility of the sub-
stances to be absorbed.

Absorption of Water and Inor-
ganic Salts. — Water and inor-
ganic salts after their absorption
from the intestine and transfer-
ence into the lymph-spaces of
the villi pass through the walls
of the capillary blood-vessels and
are carried by the way of the
portal vein into the liver. Unless
water be present in excessive
amounts, there is no appreciable
absorption of water by the lym-
phatics.

Absorption of Sugar. — As previously stated, all the carbohydrates,
with the exception possibly of lactose, are transformed by the. diges-
tive fluids into either dextrose or levulose, under which forms they
are absorbed by the epithelial cells. It is possible, however, that
soluble dextrin may also be absorbed. Whatever the form under
which the carbohydrates are absorbed, they never leave the epithe-
lial cells except as dextrose and levulose. Direct experimentation
has shown that the sugars are taken up by the capillary blood-vessels
and carried direct to the liver. Analysis of the blood of the portal
vein after the ingestion of large quantities of sugar may reveal an
increase of 0.25 per cent.; Avhile after the injection of sugar into the
intestine the percentage may rise as high as 0.4. As chemic analysis
of lymph obtained from the thoracic duct shows no increase in the
percentage of sugar beyond that normally present (0.1 per cent.), it
is assumed that sugar is not removed from the villi by the lymphatics.

Fig. 100. — Diagram of the Portal
Vein (pv) arising in the Alimentary
Tract and Spleen (s), and Carrying the
Blood from These Organs to the Liver.
— (Yeo's "Text-book 0} Physiology.'')

226 TEXT-BOOK OF PHYSIOLOGY.

Absorption of Proteins. — Since most of the proteins are transformed
through hydration and cleavage by the action of the gastric and pan-
creatic enzymes into peptones, there is reason to believe that this
change is necessary to their complete and rapid absorption. Never-
theless it has been shown by the results of experimentation that un-
changed native proteins, such as egg-albumin, and partially digested
proteins, such as acid and alkali-albumin, albumoses, may likewise be
absorbed from the small intestine, though in far less amounts. It
has also been demonstrated that native proteins can be absorbed from
the large intestine. Inasmuch as chemic analysis has failed to detect
more than a trace of either peptone or native protein in the portal blood
or in the lymph of the thoracic duct, it must be assumed that the
epithelium after absorbing must also synthetize them into some form
of coagulable protein (serum-albumin) which is readily assimilable by
the blood. That such a reconversion is necessary would appear from
the fact that the introduction of peptones even in small amounts into
the blood is followed by their elimination unchanged in the urine.
When injected into the blood in large amounts, they act as toxic
agents, giving rise to a fall of blood-pressure, a diminished coagula-
bility of the blood, coma, and death.

After passing through the epithelium into the spaces of the villi
they are removed by the blood-vessels and carried direct to the liver.
Even though there is no appreciable increase in the amount of pro-
tein in the portal blood during digestion, there is every reason to
think that this is the route by which it reaches the general circulation.
Ligation of the thoracic duct does not interfere with protein absorp-
tion nor with the normal elimination of urea nor with the weight of
the animal.

The foregoing statements are based on the view that the final stage in
the digestion of proteins is the formation of peptones. There are reasons
however for believing that the change is more far reaching and com-
plete, and that the peptones in turn are disintegrated and reduced to
still less complex bodies represented by polypeptids, peptids and even
amino-acids. The extent to which this disintegration proceeds will
doubtless depend on the quantity and variety of proteins consumed.

If the reduction of the protein molecule to this fragmentary condi-
tion is the outcome of protein digestion, as recent investigations in-
dicate, then the problem of absorption is transferred to these fragment-
ary bodies rather than to the peptone molecule. Inasmuch as the
presence of the peptids and the amino-acids in the blood of the portal
vein has not been demonstrated beyond question, the supposition is that
after their absorption by the intestinal epithelium, they are synthesized
and a protein molecule constructed, similar to if not identical with, the
albumin of the blood-plasma. This view renders it much easier to
understand how out of the different proteins, varying widely in their
composition, the specific proteins of the blood are constructed. It is
only necessary to assume that the epithelial cell selects from the variety

ABSORPTION. 227

of fragments presented to it, those which are necessary to the formation
of the blood albumin and the blood globulin, and to synthesize them.
The fate of the unused fragments is various. Some are carried to the
liver and transformed into urea; others are acted on by intestinal bac-
teria, after which they may be eliminated in the feces or absorbed and
carried to the liver where they undergo other changes and eventually
appear in the urine.

Absorption of Fat. — As previously stated, there are two views as
to the changes which fats undergo during digestion. According as
the one or the other is accepted will depend the view as to the nature
of the absorptive process. If it be assumed that the final stage in
the digestion of fat is a purely physical one, the production of an
emulsion in which the fats present themselves as fine granules, it is
difficult to give any satisfactory explanation of the mechanism by
which the epithelial cells take them up. Various theories have been
advanced to explain the process, but none are free from serious ob-
jections. This view of fat absorption has largely been based on the
observation that during digestion fatty granules can be seen in all
portions of the cell apparently passing toward the interior of the
villus. If, on the contrary, it be admitted that the final stage in the
digestion of fats is the formation of soaps and glycerin, both of which
are soluble, their absorption can more readily be accounted for.
According to this view, the soaps and glycerin are again synthesized
by a process the reverse of that which is produced by the pancreatic
enzyme, with the appearance of minute granules of fat. That this
is the more probable view as to the mechanism of fat absorption is
evident from the fact that when animals are fed with alkaline soaps
and glycerin, or with fatty acids alone, globules of fat are found in
the epithelial cells and in the interior of the villus.

With the passage of the fat-granules into the interior of the villus
they at once enter the lymph-radicle and become constituents of the
lymph-stream, to which they impart a white, milky appearance. If
the abdomen of an animal in full digestion be opened, the lymph-ves-
sels of the mesentery present themselves as distinct white threads.
An examination of the fluid they contain, known as chyle, shows the
presence of fat-granules of microscopic size. With the passage of
the chyle into the thoracic duct it also presents the same milky ap-
pearance. For this reason the lymphatics of the mesentery were
erroneously termed lacteals. The chyle as obtained from these
lymph-vessels possesses the same qualitative though not quantitative
composition as lymph, the difference being mainly in the large excess
of fat in the former. Indeed, chyle may be regarded as lymph plus fat.

Routes for the Absorbed Food. — Physiologic experiments have

demonstrated that the agents concerned in the removal of the products

of digestion after their absorption from the interior of the villus are:

1. The veins of the gastro-intestinal tract, which converge to form the

portal vein.

228 TEXT-BOOK OF PHYSIOLOGY.

2. The lymph- vessels of the small intestine, which converge to empty
into the thoracic duct.

The products of digestion find their way into the general circu-
lation by these two routes, as follows: (See Fig. 101).

The water, inorganic salts, proteins, and sugar after entering the
blood-vessels of the villus are carried by the blood of the intestinal
veins directly into the liver by the portal vein; after circulating through
the capillaries of the liver and being influenced by the liver cells,
they are discharged by the hepatic veins into the inferior or ascending
vena cava.

The fats after entering the lymph-radicle of the villus are carried
bv the lymph-stream of the intestinal lymph-vessels and emptied into
the receptaculum chyli from which they ascend into the thoracic duct,
and by which they are discharged into the blood at the junction of the
left subclavian and internal jugular veins.

Forces Aiding the Movement of Lymph and Chyle. — The
force which primarily determines the movement of the lymph has its
origin in the beginnings of the lymph-vessels, the lymph-spaces,
and depends on a difference in pressure here and at the termination
of the thoracic duct. The rise of pressure in the lymph-spaces is
due to the continual production of lymph, either by filtration or se-
cretory activity of the capillary walls. As soon as the pressure rises
above that in the thoracic duct a forward movement of lymph takes
place. Other things being equal, the rate of movement will be pro-
portional to the difference of pressure. The first movement of the
chyle, its passage from the lymph-capillary in the villus into the sub-
jacent lymph-vessel, has been attributed to a shortening of the villus
and a compression of the capillary by the contraction of the non-
striated muscle-fibers by which it is surrounded. With the entrance
of the chyle into the subjacent lymph-vessel there is a distention of the
vessel and a rise in pressure. When the muscle-fibers relax, regurgita-
tion is prevented by the closure of the valves at the base of the villus.
The elastic tissue of the lymph-vessel now recoils and forces the chyle
toward the thoracic duct. After the emptying of the lymph-capillary
the conditions as far as pressure is concerned are favorable to the
absorption of new material. The rhythmic contractions of the in-
testinal wall undoubtedly aid in the movement of lymph and chyle.

It is quite possible that the walls of the general lymphatic system
aid the forward movement of lymph by more or less rhythmic con-
tractions of their contained muscle-fibers.

Inasmuch as the lymph-vessels lie in situations in which they
are subject to compression by muscles during contraction, it is prob-
able that the fluid in the vessels will be forced onward toward the
thoracic duct at each compression, a backward movement being
prevented by the closure of the valves which are everywhere present
in the vessels. Experimental observations have demonstrated the
truth of this supposition. Alternate contraction and relaxation of

ABSORPTION.

229

Fig. ioi. — Diagram Showing the Routes by which the Absorbed Foods Reach
the Blood of the General Circulation (G. Bachman). I. i., Loop of small intestine;
int., v., intestinal veins converging to form in part, p. u., the portal vein, which enters the
liver and by repeated branchings assists in the formation of the hepatic capillary plexus;
h. u. the hepatic veins carrying blood from the liver and discharging it into, inf. v. c. the
inferior vena cava; int. I. v., the intestinal lymph vessels converging to discharge their
contents, chyle, into rec. c. the receptaculum chyli, the lower expanded part of the thoracic
duct; th. d., the thoracic duct discharging lymph and chyle into the blood at the juncdon
of the internal jugular and subclavian veins; sup. v. c, the superior vena cava.

230 TEXT-BOOK OF PHYSIOLOGY.

the muscles of the leg will, in an animal at least, increase considerably
the flow as well as the production of lymph from the thoracic duct.
Massage has a similar influence. The respiratory movements also
aid the flow of both lymph and chyle from the thoracic duct and
larger lymph-vessels into the venous blood. During inspiration the
negative pressure of the thorax is increased, the increase being pro-
portional to the extent of the inspiration. The positive pressure of
the air within the lungs on the thoracic structures, venae cavse, thoracic
duct, etc., being at the same time diminished, there is an expansion of
and a fall of pressure in the thoracic duct and vena? cavas. As the lymph
in the abdominal portion of the thoracic duct is subjected to the higher
intra-abdominal pressure, its contents are forced energetically toward
the end of the thoracic duct. During expiration the reverse conditions
obtain. As the negative pressure diminishes and the positive intra-
pulmonary pressure increases the upper part of the thoracic duct is
compressed and the lymph is forced into the subclavian vein at its
junction with the internal jugular. Regurgitation here is prevented
by a closure of the valves.

DIFFUSION. OSMOSIS. FILTRATION.

As these three factors are believed to play an important part in many
physiologic processes, it is essential to a better understanding of these proc-
esses, that certain elementary facts relating to these three factors be known.

Diffusion. — By diffusion is meant the gradual and spontaneous mixture
of the molecules of two or more liquids, or of two or more gases, when brought
into contact with each other, without the application of an external force.
The reason for both processes lies in the fact that the molecules of a liquid
and of a gas are in constant motion, in consequence of which a mutual inter-
penetration of the molecules takes place, which continues until a condition
of homogeneity is established.

Again, when a soluble substance, inorganic or organic, is placed in water,
the molecules of the substance will at once begin to separate themselves and
to diffuse throughout the water until the solution becomes homogeneous,
and notwithstanding the fact that the dissolved substance possesses weight,
the solution remains homogeneous. The force of gravity is overcome by
the force of diffusion.

The velocity with which the molecules of a substance will diffuse through
a solvent like water, varies considerably. The experiments of Graham
show that if the molecules of a given weight of hydrochloric acid diffuse
completely in a unit of time, the molecules of the same weight of sodium
chlorid, cane-sugar, albumin and caramel, will require for their diffusion
2.33, 7, 48, and 98 units of time respectively.

Osmosis. — Osmosis may be defined as the passage of the molecules of
water across an intervening membrane. If the water on one side of the
membrane, parchment for example, contains in solution substances such
inorganic salts, their molecules will also pass across the membrane though
the time required for this to take place may be much longer than in the case
of the water molecules. The passage of the dissolved substance across
the membrane though usually included under the term osmosis is more
properly termed dialysis.

ABSORPTION. 231

If the two volumes of water on opposite sides of the membrane are the
same in amount, and if the one volume contains a salt in solution, the salt
molecules will continue to pass across the membrane until the water on both
sides contains the same number of molecules, or, in other words, until it is
homogeneous in composition. The time required for their passage being
longer than the time required for the passage of the water molecules, there
will be (owing to factors which will be explained later), a temporary increase
in the volume of the water originally containing the salt, but in time the two
volumes will again become equal. Certain other substances which may be
in solution, such as albumin, starch, etc., will not pass across a membrane,
because of the large size of their molecules. Graham termed all those sub-
stances which by virtue of the small size of their molecules pass across
membranes, crystalloids, and all those which by virtue of the large size of
their molecules do not pass across membranes or to a very slight extent,
colloids.

It was stated in the foregoing paragraph that if two equal volumes of
water are separated by a parchment septum, one of which contains in solu-
tion an inorganic salt, the molecules of the salt-free water will osmose across
the septum into salt-containing water, more rapidly than they will in the
opposite direction, and as a result, there will be a temporary increase in the
volume of the water containing the salt. If the membrane were impermeable
to the salt molecules, the difference in the two volumes of the water would
be far more permanent and striking. The reason assigned for this, is that
the molecules of the salt exert a pressure against the outer layer of the water
molecules and these in turn against the membrane, in consequence of which
there is a more rapid osmosis of the water molecules towards the salt than in
the reverse direction. To this pressure is applied the term —

Osmotic Pressure. — Osmotic pressure may be defined as the pressure
exerted by the molecules of the substance in solution against an enclosing
wall, in consequence of which there is an osmosis of the surrounding solvent
towards it. The reason for this pressure lies in the fact that, when the
molecules of a substance are separated a certain distance, as they are
when in solution, they repulse one another as do the molecules of a gas
and in their flight strike against the outer layer of the solvent. The pressure
of the molecules of a substance in solution is therefore comparable to the
pressure of the molecules of a gas.

Three methods may be employed for measuring the force of the osmotic
pressure of different substances, viz.: 1. Physical. 2. The determination of
the freezing point. 3. By calculation.

1. Physical Method. — For the purpose of measuring osmotic pressure
by physical methods, it is customary to make use of an apparatus similar to
that represented in Fig. 102, which consists of an earthenware vessel (a),
into the upper open end of which a tall vertical glass tube has been hermet-
ically sealed. The pores of the earthenware vessel have been filled by a
membrane made by precipitating ferro-cyanid of copper within them.
This membrane is freely permeable to water, but impermeable to certain
substances in solution, e. g., cane-sugar. Such a membrane, which permits
the passage of the molecules of the solvent but not the molecules of the dis-
solved substance, is termed a semipermeable membrane, and its use is
absolutely necessitated when it is desired to obtain the actual pressure
exerted by any given substance in solution. An apparatus of this character
is termed an osmometer.

232

TEXT-BOOK OF PHYSIOLOGY.

When, therefore, the osmometer containing a solution of cane-sugar is
placed in the vessel (b) containing water, the following phenomena arise,
viz.: an ascent of the cane-sugar solution in the vertical glass tube, and a
descent of the level of the water in the vessel b. These phenomena con-
tinue until the level of the fluid in the glass tube reaches a certain height
when it becomes stationary, and no further effect takes place.

In explanation of the foregoing phenomena it may be said that the
molecules of the sugar strike or press against the outer layer of the molecules
of the solvent, which at all points are in contact
with the rigid walls of the earthenware vessel, ex-
cept at the open extremity of the vertical glass
tube. Inasmuch as the rigid walls of the osmom-
eter prevent any outward displacement of the
molecules of the water, the force of the impact of
the sugar molecule is directed against the mole-
cules at the extremity of the vertical tube which
are in consequence pressed or pushed upward
a certain distance. Because of the loss of energy
due to the impact, the sugar molecule does not re-
bound with the same velocity, and hence time is
permitted for the molecules of the water to pass
into the sugar solution, to occupy the space, and
thus maintain the level of the fluid in the vertical
tube. (For the reason that the osmometer is per-
meable to water, the molecules will pass outward
as well as inward though more will pass in a unit
of time in the latter, than in the former direction,
until equilibrium is established.) The pressure
of the sugar molecules continuing, the level of the
fluid in the glass tube continues to rise and the
level of the fluid in the vessel, b, continues to fall
until the force of gravity prevents any further up-
ward movement of the molecules of sugar against
the outer film of the molecules of the water. The
difference in the level of the two fluids expressed
in millimeters of mercury is taken as a measure
of, and equal to, the pressure of the sugar in so-
lution. A i per cent, solution of cane-sugar at
a temperature of from 130 C. to 160 C, as de-
termined by this method, exerts an osmotic pres-
sure of about 535 mm. Hg. ; a 2 per cent, solution
exerts an osmotic pressure approximately twice
this amount.
Experiments made with this and similar osmometers show —
That the osmotic pressure of any substance in solution is proportional

to the concentration, providing the temperature is constant.
That when the concentration is constant the osmotic pressure rises with,

and is proportional to, the temperature.
That when different substances are present in the same solvent the os-
motic pressure is equal to the sum of the individual or partial pressures.
That whatever the nature of the substance in solution it will exert the
same osmotic pressure, providing always, the same number of molecules

Solution
Cane Sugar

-a

b-

Wate,

Fig. 102. — An Osmometer.

ABSORPTION. 233

are present; hence the molecular weights in grams per liter of different
substances, exert the same osmotic pressure at the same temperature.
Because of the fact that when certain substances, e. g., many inorganic
salts, many acids and bases, are dissolved, some of their molecules undergo
ionization, i. e., separation into parts which are charged with electricity, and
hence the two together, molecules and ions, exert a greater osmotic pressure
than would otherwise be the case; and because of the further fact, that it is
extremely difficult to obtain absolutely semipermeable membranes, uniform
results are not obtained by the employment of the three methods; therefore,
the osmometric methods as well as the calculation or arithmetric method
have been largely discarded and the method based on the determination of
the freezing point has been adopted.

2. The Determination of the Freezing Point. — Because of the difficulty in
obtaining the exact osmotic pressure by means of the osmometer as stated
above, reliance is now placed on the mathematic relation known to exist
between osmotic pressure and the freezing point. Thus the freezing point
of water holding any substance in solution is lower than water itself and is
indeed proportional to the number of molecules dissolved. As a standard
of comparison it is customary to employ a gram-molecule of a substance dis-
solved in one liter of water. (A gram-molecule is the quantity of a substance
expressed in grams equal to its molecular weight.) The lowering of the
freezing point of a gram-molecule solution below that of water is constant,
viz., 1.870 C. The osmotic pressure therefore of such a solution, as de-
termined by calculation (see below), is equal to 22.38 atmospheres, or
17,008 mm. of Hg.

Therefore it is only necessary to determine by means of a differential
thermometer the lowering of the freezing point in degrees centigrade, which
is usually expressed by the symbol A- Then the osmotic pressure is equal
to A divided by 1.870 C, and the quotient multiplied by 22.38 atmospheres,
or 17,008 mm. of Hg. Thus if the freezing point of any solution was found
to be 0.830 C. lower than water, its osmotic pressure would be 0.83 -=- 1.87 X
22.38 atmospheres or 9.847 atmospheres = 7,483 mm. Hg. If any two
solutions have the same freezing point they contain the same number of
molecules and hence have the same osmotic pressure. Blood plasma has
a freezing point of 0.560 C. Experimentally it has been determined that the
freezing point of water is lowered to the same level, when it contains sodium
chlorid to the extent of 0.95 per cent. Hence these two fluids have the same
osmotic pressure and are isotonic; each exerts a pressure of 6.696 atmospheres.

For this reason the sodium chlorid solution can be employed for preserv-
ing, for a time at least, the form of blood corpuscles or other living mammalian
cells, from which it may be inferred, that the contents of the cells have an
osmotic pressure approximately equal to that of the blood plasma or the
salt solution. If the salt solution has a lower concentration and hence a
lower osmotic pressure, water will osmose into the corpuscle and cause a
discharge of its hemoglobin content. Such a fluid is said to be hypoisotonic.
If on the contrary, the salt solution has a higher concentration and hence a
higher osmotic pressure, water will osmose from the corpuscle causing a
shrinkage and crenation of the corpuscle. Such a fluid is said to be hyper-
isotonic.

3. By Calculation. — The osmotic pressure may also be obtained by
calculation based on the known pressure exerted by a gram-molecule of
hydrogen — 2 grams — when compressed to a volume of one liter. It is well

234 TEXT-BOOK OF PHYSIOLOGY.

known that i gram of hydrogen at o° C. and at an atmospheric pressure of 760
mm. Hg. occupies a volume of 11. 19 liters, and that 2 grams under the same
conditions will occupy a volume of 22.38 liters, and that when the two grams,
that is, the gram-molecules are compressed to a volume of 1 liter the molecules
will exert a pressure equal to that of 22.38 liters or 22.38 atmospheres or
17,008 mm. of Hg. Since a gram-molecule of any substance dissolved in 1
liter of water contains the same number of molecules as a gram-molecule of
hydrogen, they have the same osmotic pressure.

From this it is possible to calculate the osmotic pressure of an electrolyte,
if the percentage composition of the substance in solution be known. Let
it be supposed, for example, that it is desirable to know the osmotic pressure
of a 1 per cent, solution of cane-sugar. The procedure is as follows: A
gram-molecule of cane-sugar (C12H22Ou) contains 342 grams; a 1 per cent.
solution contains 10 grams to the liter; hence its osmotic pressure is 10 -f-
342 X 22.38 atmospheres or 0.65 atmosphere which is equal to 494 mm.
of Hg.

Filtration. — Filtration may be defined as the passage of water and of
all substances dissolved in it, across a membrane as a result of a difference
Df hydrostatic pressure on the two sides. The difference between the two
pressures constitutes the force of filtration, and hence the greater the differ-
ence, the greater will be the amount of fluid filtered.

With any given artificially prepared animal membrane the quantities of
water and crystalloids in general which pass across the membrane are pro-
portional to the filtration force, and hence the filtrate will have a concen-
tration similar to, if not identical with, that of the original solution. The
passage of colloids in solution will be proportional to the permeability of the
membrane and an increase in the filtration force. The filtrate, however,
will have a lower degree of concentration than the original solution for the
reason that as the pressure rises the quantity of water filtered increases in a
greater ratio than the quantity of colloid filtered.

Physiologic Applications. — In the animal body the fluids are separated
by delicate membranes across which the constituents of the fluids, inorganic
and organic, are continually passing. Thus prepared foods in the intestine
pass across the intestinal wall into blood- and lymph-vessels; the constituents
of the blood pass across the wall of the capillary vessel into the tissue spaces
from which they pass (a) through the walls of various glands to take part
in the formation of their secretion; (b) across the sarcolemma into the inte-
rior of the muscle fiber; (c) across the limiting surface of, and into the
interior of all tissue cells. The waste products, the result of tissue and food
metabolism, pass from the interior of cells across their limiting membranes
or surfaces into the tissue spaces; thence across the wall of the capillary
vessel into the blood and finally across the wall of the capillary vessel and the
epithelium of the lung, the kidney, the liver, etc., to take part in the forma-
tion of the excretions. These and other processes are believed to be accom-
plished by the factors, diffusion, osmosis and filtration.

The statements that have been made in foregoing paragraphs in refer-
ence to diffusion, osmosis and filtration have been based on the results of
experiments which have been made with non-living membranes, and under
conditions purely physical; and though it is quite true that in the animal
body the fluids are separated by membranes more or less permeable to all
their constituents, and that all pass across these membranes, it is possible
that the facts which have been obtained experimentally are not strictly

ABSORPTION. 235

paralleled in the living body, and hence not strictly applicable to the elucida-
tion of physiologic processes. Nevertheless there are reasons for thinking
that a thorough understanding of these factors will eventually throw much
light on the intimate nature of the process by which organic as well as
inorganic substances in solution pass across animal membranes in the living
condition.

CHAPTER XII.
THE BLOOD.

The blood is a highly complex nutritive fluid, the presence and
proper circulation of which in the living organism are essential to the
maintenance and activity of all physiologic processes. The escape
of the blood from the vessels, especially in the higher animals, is
.followed by a loss of the physiologic properties of all the tissues within
a short period of time. The immediate dependence of the functional
activities of the tissues and organs on the presence of the blood can be
demonstrated by the following experiment : If the nozzle of a syringe,
adapted to the size of the animal, be introduced through the jugular
vein into the right side of the heart and the blood be suddenly with-
drawn, there is an immediate cessation in the activity of all the organs;
the return of the blood to the vessels within a limited period of time is
promptly followed by a renewal of their activity.

Though contained within a practically closed system of vessels,
the blood is brought into intimate relation with all the tissue elements
through the intermediation of the capillaries. As the blood flows
through these delicate vessels, portions of its soluble nutritive con-
stituents, including oxygen, are given up to the tissues, by which they
are utilized for growth, repair, and functional activity. At the same
time the tissues yield up to the blood a series of decomposition prod-
ucts, resulting from their activity, which vary in quantity and quality
according as the blood traverses the muscles, nerves, glands, or other
tissues.

The blood may be regarded, therefore, as a reservoir of nutritive
materials prepared by the digestive apparatus and absorbed from
the food canal; of oxygen, absorbed from the respiratory surface of
the lungs; of decomposition products, produced by and absorbed from
the tissues. Though the blood varies in composition in different
parts of the body in consequence of the introduction of both nutritive
material and decomposition products, it nevertheless presents certain
average physical, morphologic, and chemic properties which distin-
guish it as an individual tissue.

Constituents of Blood. — A microscopic examination of the blood
as it flows through the capillary vessels of the web of the frog or the
mesentery of the rabbit shows that it is not a homogeneous fluid, but
that it consists of two distinct portions: viz., (i) a clear, transparent,
slightly yellow fluid, the plasma or liquor sanguinis; (2) small par-
ticles termed corpuscles floating in it, of which there are two varieties,
the red or the erythrocytes and the white or the leukocytes. By

236

THE BLOOD. 237

appropriate methods it can be shown that a third corpuscle, color-
less in appearance and smaller in size than the ordinary white corpuscle,
is present in the blood-stream and known as the blood-plate or plaque.
The different constituents can be roughly separated by appropriate
means when the blood is withdrawn from the body. If the blood of
the horse is allowed to flow directly into a tall cylindric glass vessel,
surrounded by ice, it separates in the course of a few hours into three
distinct layers in accordance with their specific gravities. The lower
layer is dark red and consists of the red corpuscles; the middle layer is
grayish in color and consists of the white corpuscles; the upper layer
is clear and transparent and consists of the plasma. The red corpus-
cles occupy almost one-half, the white one-fortieth, the plasma a trifle
more than one-half of the height of the entire blood-column, which
indicates approximately the different volumes of each. The same re-
sult can be obtained with human blood by the use of the centrifuge or
hematocrit.

PHYSICAL PROPERTIES OF BLOOD.

1. Color. — Within the blood-vessels two kinds of blood are dis-
tinguished— the arterial, the color of which is a bright scarlet, and
the venous, the color of which is a dark bluish-red or purple. The
cause of the color as well as the difference in color is the presence in the
red corpuscles of a coloring-matter, hemoglobin, in different degrees of
combination with oxygen. The intensity of the color in either kind
of blood is dependent on the thickness of the blood-stream, for in the
finest capillaries, as seen under the microscope, there is an almost
total absence of color. As the arterial blood passes into and through
the systemic capillaries, the hemoglobin yields up a portion of its
oxygen to the tissues and changes in color, though the change is
not appreciable by the eye. On passing into the veins, however,
the blood-stream soon presents its characteristic dark bluish color,
which deepens as it approaches the lungs. On passing into and through
the capillary vessels of the lungs the hemoglobin absorbs a new volume
of oxygen, changes in color, and on emerging from the lungs the blood
presents its characteristic scarlet color.

2. Opacity. — Owing to the fact that the corpuscles have a re-
fracting power different from the plasma, the blood, even in thin
layers, is opaque. The repeated refractions and reflections which
light undergoes in passing through plasma and corpuscles is attended
by such a dissipation that it is impossible to see printed matter through
it. That the opacity is due to the shape of the corpuscles rather than
to their contained coloring-matter is evident from the fact that when
the hemoglobin is caused to separate from the corpuscles by the
addition of chemic reagents, the blood, though it deepens in color,
becomes at once transparent.

3. Odor. — When freshly drawn the blood possesses a peculiar

238 TEXT-BOOK OF PHYSIOLOGY.

characteristic odor which has been attributed to the presence of a
volatile fatty acid in combination with an alkaline base. The in-
tensity of the odor may be increased by the addition of concentrated
sulphuric acid, by means of which the volatile acid is set free.

4. Specific Gravity.- — The specific gravity of blood lies within
the limits of 1.05 1 and 1.059, averaging in man 1.056 and in woman
1.053. Normally, variations from these values are only temporary and
are connected with variations in physiologic processes. The specific
gravity is diminished by the ingestion of liquids and abstinence from
solid food. It is increased by abstinence from liquids, by the inges-
tion of dry food, and by the elimination of large quantities of water
by the lungs, skin, and kidneys.

5. Alkalinity.— The reaction of the blood is alkaline from the
presence of the disodium phosphate (Na2HP04) and the sodium car-
bonate, Na2C03. The alkalinity can be readily shown by allowing
the blood to remain for a few seconds on slightly reddened glazed
litmus paper. On washing off the blood a distinct blue color pre-
sents itself against a red or violet background. The alkalinity varies
within narrow limits in consequence of variations in physiologic
processes. It is increased in the early stages of digestion and de-
creased in the later stages. It is decreased after muscular exercise
in consequence of the increased production and absorption of acids.
According to v. Jaksch, the alkalinity corresponds to from 260 to 300
milligrams of sodium hydrate, NaOH, for every 100 c.c. of blood;
according to Lowy, from 300 to 325 milligrams. The hitherto un-
avoidable error in these estimates is about 30 milligrams.

6. Temperature.— The temperature varies from 36.780 C.
(98.2 ° F.) in the superior vena cava to 39.7 ° C. (103.4 ° F.) in the
hepatic vein, the mean being about 38 ° C. (100 ° F.).

Coagulation of the Blood. — Within a few minutes after the
blood is withdrawn from the vessels of a living animal it begins to
lose its fluidity, becomes somewhat viscid, and if left undisturbed
passes rapidly into a semisolid or jelly-like state. To this change
in the physical condition of the blood the term coagulation has been
applied. The blood, during the progress of coagulation, not only
assumes the shape of the vessel in which it is contained, but becomes
so firmly adherent to its walls that it may be inverted without the
coagulum becoming dislodged. If a portion of such a jelly-like mass
be examined microscopically, it will be found to be penetrated in all
directions by a felt-work of extremely fine delicate fibrils, which,
having made their appearance before the corpuscles had time to
settle to the bottom of the fluid, have entangled them in the meshes
so that the entire mass retains its characteristic color. These fibrils
are collectively known as fibrin (Fig. 103).

If the coagulated blood be allowed to remain undisturbed, a
clear, transparent, slightly yellowish fluid makes its appearance on
the surface of the mass, which as it accumulates forms a layer of

THE BLOOD.

239

varying degrees of thickness. Within a few hours the blood-mass
detaches itself from the sides of the vessel in consequence of the re-
traction of the fibrils, while at the same time the clear fluid increases
in amount and accumulates along the sides and bottom of the vessel.
The shrinkage in the volume of the red coagulum and the increase
of the volume of the clear fluid which is expressed from its meshes
continue for a period varying from ten to fifteen hours, according
to certain external conditions. The blood has now become separated
into two distinct portions: viz., a solid contracted red mass, termed
clot, and a clear fluid, termed serum. The clot consists of the fibrin
containing in its meshes the red and white corpuscles; the serum
consists of all the constituents of the plasma except the antecedents
of the fibrin. The stages of coagulation are shown in Fig. 103.

Fig. 103. — Diagram to Illustrate the Process of Coagulation, i. Fresh blood,
plasma, and corpuscles. 2. Coagulating blood (birth of fibrin). 3. Coagulated blood
(clot and serum).— {Waller.)

If the blood coagulates slowly the red corpuscles, owing to their
greater specific gravity, subside to the bottom of the blood-mass,
giving to it a deeper color; the white corpuscles, owing to their lesser
specific gravity, remain near the surface of the clot and give to it a
more or less whitish appearance, producing the so-called bujjy coat.
In certain inflammatory conditions the coagulating power of the blood
is much diminished, and the corpuscles, having time to subside, a well-
developed buffy coat is formed which at one time had much interest
for pathologists. As the contraction of the fibrin takes place more
actively in the center, there being here less resistance than at the
sides of the coagulum, the upper surface usually becomes depressed
or cupped.

Coagulation of Plasma. — Clear plasma may be obtained by
means of the centrifuge from blood to which sufficient magnesium
sulphate has been added to prevent coagulation, or from horse's
blood which has been allowed to flow into a tall vessel surrounded by
a cooling mixture so as to prevent coagulation and thus permit the
red corpuscles to subside. If such plasma be subjected to room-tem-
perature, it very shortly undergoes coagulation, exhibiting practically
the same phenomena as blood itself. After a variable length of time
it also separates into a soft, colorless coagulum or clot consisting of
fibrin, and a clear fluid, the serum. The presence of the red cor-
puscles is therefore not essential to the process of coagulation.

24o TEXT-BOOK OF PHYSIOLOGY.

Rapidity of Coagulation. — The rapidity with which the blood
coagulates varies in different classes of animals under the same con-
ditions: e. g., the blood of the pigeon coagulates immediately; that of
the dog, in from one to three minutes; that of the horse, in from five
to thirteen minutes; that of man, in from four to seven minutes.
The time, however, can be lengthened or shortened by either chang-
ing the external conditions or by altering temporarily the normal
composition of the blood.

Coagulation is retarded or prevented by the following agents,
viz.: (i) A low temperature, especially that of melting ice. (2) The
addition of magnesium sulphate (1 volume of a 25 per cent, solution
to 3 volumes of blood); of sodium sulphate (1 volume of a saturated
solution to 7 volumes of blood). (3) The addition of potassium
oxalate (1 volume of a 1 per cent, solution to 3 volumes of blood).
(4) The injection into the blood of commercial peptone. (5) The
mouth secretion of the leech.

Coagulation is hastened by the following agents, viz.: (1)
gradually increasing temperature from 38 ° C. to 500 C. (2) The ad-
dition of water in not too large amounts. (3) The presence of foreign
bodies. (4) Agitation of the blood — e. g., stirring.

Fibrin and Defibrinated Blood. — If freshly drawn blood is
stirred with a bundle of twigs or glass rods for a few minutes, the
fibrin collects on them in the form of thick bundles or strands; after
washing it with water the entangled corpuscles are removed, when
the fibrin assumes its natural white appearance. The strands can
be resolved into a large number of delicate fibers which possess ex-
tensibility and retractibility, and are therefore elastic. The chemic
features of fibrin have already been considered (see page 20). The
remaining fluid, similar in its physical appearance to the blood, is
termed defibrinated blood. When such blood is allowed to remain
at rest for a few days, the remaining red corpuscles gradually sink
to the bottom of the fluid, above which will be found the clear serum.

COMPOSITION OF PLASMA AND SERUM.

Plasma. — The plasma obtained by any of the methods previously
described is a clear, colorless, transparent, slightly viscid fluid, with
a specific gravity of 1.026 to 1.029. It is composed largely of water
holding in solution proteids, sugar, fatty matter, inorganic salts, urea,
cholesterin, lecithin, etc. In composition it is quite complex, con-
taining as it does not only the nutritive materials derived from the
digestion of the food, but also the substances resulting from the dis-
integration of the tissues consequent on their functional activity.

Serum. — The scrum is the clear, transparent, slightly yellow
fluid expressed from the coagulated blood during the contraction of
the fibrin. It consists practically of the ingredients of the plasma,
with the exception of those substances which entered into the for-

THE BLOOD. 241

mation of fibrin. The average composition of plasma is shown in
the following table:

COMPOSITION OF PLASMA.

Water, ; . . 90. 00

f Serum-albumin, 4.50

Proteids < Paraglobulin, 3.40

[ Fibrinogen, 0.30

Fatty matters, 0.25

Sugar, o. 10

Extractives, 0.60

Inorganic salts, 0.85

100.00

Serum-albumin. — Of the proteid constituents of the blood,
serum-albumin is the most abundant, existing to the extent of from
4 to 5 per cent. From its similarity to egg-albumin it is regarded as
holding an important position as a nutritive agent, for it is out of this
common proteid that in all probability each individual tissue elabor-
ates the special proteid characteristic of it, since during starvation
the albumin steadily diminishes in amount. As it passes through
the walls of the capillary vessels it is found in the lymph, pericardial
fluid, and similar secretions in various parts of the body, as well as in
various pathologic transudates. It is also present in serum. While
circulating in the lymph-spaces the serum-albumin is utilized in
replacing the proteids which have undergone disintegration during
tissue metabolism. Its supply in the blood is maintained by the
absorption of peptones which are formed from the proteids of the food
and which during the time of absorption are changed in some unknown
way into serum-albumin. It is readily obtained from plasma or
serum by saturating either of these fluids with magnesium sulphate,
when all the proteids except serum-albumin are precipitated. After
their removal the remaining fluid is subjected to a temperature of
from 70 ° to 750 C, when the serum-albumin is precipitated in a
coagulable form, after which it can be removed and its chemic features
determined.

Paraglobulin. — -This proteid, though present in plasma, is best
obtained from serum when this fluid is saturated with magnesium
sulphate. As the line of saturation is approached the fluid becomes
turbid, and after a few minutes a fine white precipitate occurs. It
can then be collected on a filter, dried, and its chemic properties
determined. In its reactions it resembles the various members of the
globulin class. The amount varies from 2 to 4 per cent, in the blood
of man. As to the physiologic importance or antecedents of para-
globulin nothing is definitely known. Its constant presence in the
blood would indicate that it plays an equally important, though per-
haps different, part with serum-albumin in the nutrition of the body.

Fibrinogen. — This proteid can be obtained from plasma, lymph,
pericardial, and peritoneal fluids, as well as from hydrocele fluid.
16

242 TEXT-BOOK OF PHYSIOLOGY.

It is, however, not to be obtained from serum, as it is removed from
the blood during the formation of solid fibrin. It is normally present
in the blood in very small quantity, amounting to not more than 2.2
to 3.3 parts per thousand. Fibrinogen may be obtained from plasma
which has been prevented from coagulating, by the addition of mag-
nesium sulphate in certain quantities or by the addition of a satu-
rated solution of sodium chlorid. In a few minutes a flaky precipitate
occurs. By repeated washing and precipitation with sodium-chlorid
solutions of varying strength the fibrinogen may be obtained in a
pure state. The history of fibrinogen is unknown. Beyond the fact
that it contributes to the formation of fibrin there is no positive knowl-
edge either as to its origin, its nutritive value, or its final disposition
in the blood under normal conditions.

Fat. — The plasma as well as the serum contains a very small
quantity of fat in the form of microscopic globules. Though the
percentage is normally not above 0.25, yet just after a meal rich in
fatty matter the amount may be so great as to give to the blood a
milky or opalescent appearance. Within a few hours, however,
this excess of fat disappears from the blood, though its immediate
disposition is unknown. Soaps or alkaline salts of the fatty acids,
though formed during the digestion of fats, are not present in the
blood. Lecithin and cholesterin are present in very small quantities.

Sugar. — Sugar is present in the blood in the form of dextrose,
and is now regarded as a normal constituent. The amount is about
1 part per thousand, though it may be present to the extent of 3 parts
per thousand. Beyond this, the excess soon appears in the urine.

Extractives. — The blood contains a series of nitrogenized bodies,
such as urea, uric acid, creatin, creatinin, xanthin, etc., which result
from the katabolic changes in nerve- and muscle-tissues as well as
from subsequent chemic combinations and decompositions. Though
constantly absorbed from the tissues, they seldom accumulate beyond
a small amount, since they are constantly being eliminated from the
blood by the various excretory organs.

Inorganic Salts. — The inorganic salts of the plasma are chiefly
sodium and potassium chlorids, sulphates, and phosphates, together
with calcium and magnesium phosphates. Of the salts, sodium
chlorid is the most abundant, amounting to 5.5 parts per thousand.
Some of the salts are alkaline and impart to the blood its alkalinity.
Calcium phosphate is present in small quantity — 2 parts per 1000.
This salt is wanting in serum for the reason that it became a constitu-
ent of fibrin at the time of coagulation. In other respects serum
differs but slightly from plasma in the proportions of its saline con-
stituents.

HISTOLOGY OF THE RED CORPUSCLES OR ERYTHROCYTES.

The histologic features of the red corpuscles are readily observed
in a drop of freshly drawn blood when examined microscopically.

THE BLOOD.

243

The field of the microscope will be seen to be crowded with red cor-
puscles floating in a clear transparent fluid — the plasma. Here and
there will also be seen a white corpuscle, round or irregular in shape,
and granular in appearance. Within a short time a characteristic
phenomenon takes place: viz., the arranging of the corpuscles in the
form of columns of varying lengths, resembling rolls of coins. These
rolls interlace with each other
at all angles and form a net-
work in the meshes of which
lie individual red and white
corpuscles. (See (Fig. 104.J
The cause of this tendency of
the corpuscles to adhere to
one another is not definitely
known. Since it does not
take place in circulating blood,
and since it is to a great ex-
tent prevented by defibrinating
the blood, it has been supposed
to be dependent on the forma-
tion of some adhesive sub-
stance connected with the
formation of fibrin.

Color. — When viewed by
transmitted light, a single
corpuscle is slightly yellow or

greenish in color; but wThen a number are grouped together, the color
deepens and the corpuscles appear red. In either case the color is
due to the presence in the corpuscle of a specific coloring-matter,
hemoglobin.

Shape. — The red corpuscle is a circular, flattened disk with
rounded edges. Each surface is perfectly smooth and presents a

shallow depression in its center, so
that it is also biconcave. A longit-
udinal section of a corpuscle would
present, when viewed edgewise, an
outline similar to that of Fig. 105.
This difference in the thickness of the
peripheral and central portions of the
corpuscle gives rise to differences in
optical appearances when examined
microscopically. At a certain distance of the object-glass the
corpuscle presents in its peripheral portion a bright rim, and in its
central portion a dark spot. If the objective be brought nearer
and the center accurately focused, the reverse appearance obtains;
the central portion becomes bright and the peripheral portion dark.
The cause of this difference in optical appearance is the unequal

Fig. 104. — Corpuscles from Human
Subject. A few colorless corpuscles axe
seen among the colored discs, many of
which are arranged in rouleaux. — (Funke.)

Fig. 105. — Ideal Transverse
Section of a Human Red Corpus-
cle. (Magnified 5000 times.) a, b.
Diameter, c, d. Thickness.

244 TEXT-BOOK OF PHYSIOLOGY.

distribution of the transmitted light in consequence of the shape of the
corpuscle.

Size. — The diameter of a typical well-developed red corpuscle
under normal conditions is 0.0075 mm-> its greatest thickness is 0.0019
mm. Though this may be assumed as the average diameter, there
is a small percentage of distinctly smaller and a small percentage
of distinctly larger corpuscles. The following table shows the results
of measurement made by different observers:

Normal Limits. Average Diameter.

Welcker, diameter 0.0045-0.0095 mm 0.0070 mm.

Hayem, " 0.0060-0.0088 " 0-0075 "

Gram, " 0.0067-0.0093 " 0.0078 "

Melassez, " 0.0076 "

0.00747
(32V0 inch)

Structure. — The red corpuscle of man as well as of all other mam-
mals possesses neither a nucleus nor a limiting membrane, but appears
to consist of a homogeneous substance more or less semisolid in con-
sistence. Under the in-
fluence of certain re-
agents the corpuscle
separates into two dis-
-y tinct portions: viz., a

colorless protoplasmic
stroma and a coloring-
lb. matter which diffuses
into the surrounding

I

5

r> ^\\ liquid. The presence of

the former can be dem-
onstrated by the addi-
-^ tion of iodin, which im-

onstrated by

' w tinn nf iodin

parts to it a faint yellow

c- d- color. The stroma is

Fig. io6.-The Shape of the Red Corpuscle elastic and determines

in Different Mammals. (Wetdenreich .) a. Man. , , , ,. , ,

b. Dog. c. Pig. d. Rabbit. not only the shape of the

corpuscle but gives to it
the properties of extensibility and retractibility.

The foregoing is the classic and generally accepted view as to the
shape, size, and structure of the red corpuscle. Nevertheless recent in-
vestigations render it probable that the statements were based on ob-
servations of the corpuscles under artificial rather than natural condi-
tions, and therefore not strictly true. For many years histologists
from time to time have stated that the red corpuscle is not circular and
biconcave in shape, in the circulating blood, but bell shaped, similar to
that shown in Fig. 106. It was not until 1902, after the publication of
Weidenreich's investigations that this view began to receive more at-
tention than bad hitherto been accorded it. Weidenreich preserved

THE BLOOD.

245

in a moist chamber a hanging drop of human blood, and on examina-
tion found that the red corpuscles were bell shaped though the dq3th
of the bell cavity varied considerably. An examination of the capil-
lary circulation in the omentum of the
rabbit revealed the fact that the cor-
puscles in their natural medium were
also bell shaped. The circular bicon-
cave shape they ordinarily present
when a drop of blood is examined
microscopically he attributes to cool-
ing, evaporation and concentration of
the drawn blood. Experimentally it
was shown that when blood was added
to 0.6 to 0.65 per cent, solution of
sodium chlorid all the corpuscles were
bell shaped; but if the solution was
increased or decreased in strength,
this form was at once changed.

The dimensions of the bell-shaped cell according to Weidenreich
are as follows: —

Fig. 107. — Red Corpuscles
Sketched while Circulating in
the Vessels of the Omentum of a
Guinea-pig. (F. T. Lewis in Stohr's
Histology.)

Greatest diameter 7 microns 0.007 mm.

Diameter of cavity 3 " 0.003

Height of cell 4 " 0.004

" " cavity 2.5 " 0.0025

Thickness of wall at apex 2 " 0.002

" " " " base 1.5 " 0.0015

The foregoing observations have been confirmed by many subse-
quent investigators. Thus Lewis states that if a drop of blood is placed
immediately on a warm slide and examined, the corpuscles exhibit
the bell shape, but as the slide cools they gradually become biconcave
disks of the conventional form. He also observed that the corpuscles
in the capillary blood-vessels of the omentum of the guinea-pig were
bell shaped and presenting an appearance similar to that shown in
Fig. 107. Radasch found on examination of fetal tissues such as the
spleen, kidney, liver, placenta, etc., that the great majority of the cor-
puscles in all situations presented the bell shape rather than the circu-
lar biconcave shape. This observer is of the opinion that the bell
shape can not be due to the action of the fixatives employed in the
preparation of the tissues.

The structure of the corpuscle, according to Weidenreich, differs
also from that usually stated. He asserts that the corpuscle is sur-
rounded by a structureless, colorless membrane enclosing a colored but
not nucleated semi-fluid mass, which consists chemically of protein
material, inorganic salts and hemoglobin. There is no evidence of
the existence of a stroma in the adult state.

Number of Red Corpuscles. — In any given specimen of blood

246

TEXT-BOOK OF PHYSIOLOGY.

the corpuscles are so numerous and the spaces between them so
small that it seems almost impossible to determine their number.
This, however, has been accomplished for a cubic millimeter of
blood by various observers employing different methods with compara-
tively uniform results. The average normal number of corpuscles in
one cubic millimeter of blood is, for men, 5,000,000; and for women,
4,500,000. This value, however, will vary within slight limits, with
variations in the activity of physiologic processes and to a large extent
at times in pathologic states of the blood or body. The number is
increased in the cutaneous veins by all influences which cause a dim-
inution in the quantity of water in the blood — e. g., copious sweating,
acute watery diarrhea, fasting, abstinence from liquids; the number
is diminished by influences which dilute the blood — e. g., the ingestion
of liquids, the absorption of fluids from the tissue spaces, etc. But
it is well to remember that these influences which produce changes in
the number of corpuscles per cubic millimeter do not necessarily pro-
duce corresponding changes in the total number of red corpuscles in
the body. In women lactation, menstruation, and the act of parturi-
tion diminish the number. High altitudes apparently increase the
number of corpuscles, as shown by their increase in the blood of the
peripheral vessels. Whether this is an indication that there is a cor-
responding increase of the total number in the general volume of the
blood is uncertain. The following table will show the increase in the
count per cubic millimeter at different altitudes:

Place.

Height Above Sea-level.

Red Cells.

Author.

Christiania

Gottingen,

Tubingen,

Zurich,

Auerbach,

Reibaldsgrlin,

Arosa,

0 meter
148 meters
3i4 "
414

425

700 "
1800 "
4392 "

4,974,000
5,225,000
5,322,000
5,752,000
5,748,000
5,900,000
7,000,000
8,000,000

Laache.

Schaper.

Reinert.

Steirlin.

Koppe.
it

Egger.
Viault.

(Koppe.)

The Cordilleras,

Moro cocha.

This increase in the number of corpuscles takes place, according
to Viault's observations, within two or three weeks, and is apparently
not connected with either diet or mode of life, but rather with dimin-
ished atmospheric, if not oxygen, pressure. On returning to sea-
level there is a gradual reduction, without any apparent destruction
of the corpuscles, to their normal number. The reason for these
variations is not clear.

The method of counting corpuscles introduced by Vierordt and
Welcker has been modified by different observers, and especially by
Thoma. On account of the great number of corpuscles in 1 cubic
millimeter of blood, it becomes necessary for purposes of enumeration
that the blood be diluted a definite number of times and that the di-

THE BLOOD.

247

luted mixture be placed in a counting chamber possessing a definite
capacity. By means of the pipette or melangeur of Potain and the
counting chamber of Thoma both these objects are attained.

The pipette consists of a capillary tube (Fig. 108) provided with an en-
largement containing a freely movable small glass ball, a. One end of the
tube is pointed, while to the other end is attached a
rubber tube for the purpose of facilitating the introduc- A

tion of the blood and the diluting fluid. The capillary
tube, which is accurately calibrated, carries marks, %, 1.
101, which signify that if the tube be filled with blood
up to the mark 1 and the diluting fluid be sucked into
the tube up to the mark 101, the blood will be diluted
100 times. If the blood be sucked up to the mark ^ and
the diluting fluid to 101, then the blood will be diluted
200 times. In using the pipette the point is introduced
into a drop of blood derived from a small wound in the
skin of the lobe of the ear or finger and sucked into
the tube by introducing the rubber tube into the mouth.
The tube is then quickly inserted into a solution, which
will preserve the shape and size of the corpuscles, such
as Gowers's sodium sulphate solution, sp. gr. 1.025, or
a 3 per cent, sodium chlorid solution,* and the fluid
sucked into the tube up to the mark 101. On shaking
the pipette for a few minutes, the admixture will take
place, aided by the movements of the glass ball.

Fig. 109 shows both a section view, A, and a surface
view, C, of the counting chamber. This consists of an
oblong glass plate on which are cemented two small
pieces of glass, one of which has in the center a circular
opening in which is placed the other, a circular disc or
stage. Their relation is such that a narrow groove or
moat separates the one from the other, the floor of which
is formed by the glass plate. The surface of the circular
stage is exactly 0.1 mm. lower than that of the cover-
glass, a. On the surface of the glass stage a series of
small squares is engraved, each one of which has a side length of -^ mm.
and an area of ±\-$ square mm., B. To facilitate counting, a group of 16
squares is surrounded by a heavy dark line. This group is separated from
adjoining groups, also enclosed by dark lines, by an intermediate light line,

Fig. 10S. — Me-
langeur or P 1-
p E T T E . — (Landois
and Stirling.)

* Various solutions have been devised for dilutin
employed, e. g.:

Hayem's Fluid:

Hydrarg. bichlor., 0.5 gm

Sodium sulphate, 5.0 "

Sodium chlorid, 2.0 "

Aqua? destillat. , 200.0 "

blood, anv one of which may be

Toisson's Fluid:

Aqua? destillat., . ... 160.00 parts.

Glycerinar, 30.00 li

Sodium sulphate, . . 8.00 "

Sodium chlorid, .... 1.00 "

Methyl-violet, 0.025 part..

Gowers's Fluid:

Sodium sulphate, gr. 104

Acid, acetic, 5j

Aquae dest, q. s. ad oiv.

TEXT-BOOK OF PHYSIOLOGY.

which serves as a guide in passing from one group to another. When a
cover-glass is accurately applied to the glass, b, each one of the small squares

will have a cubic capacity of
iw X o.i, or T7TVo cubic
millimeter, and every four
such squares will have there-
fore a capacity of 1()1()0 cubic
millimeter.

Before placing the diluted
blood on the counting stage,
the fluid in the tube of the
pipette should be blown out
and discarded, as it contains
no portion of the blood. A
small drop is then placed on
the glass stage and covered
with the cover-glass. After
a few minutes the corpuscles
settle over the ruled spaces
and are ready for counting.
The number of corpuscles in
at least five series of sixteen
small squares is then counted;
this number is then multiplied
by the degree of dilution (ioo
or 200 as the case may be)
and this by the cubic contents
of each small square (4000) ;
the product is then divided
by the number of squares
counted (80 in the instance
given above): e. g., five series
of sixteen small squares con-
tain 500 corpuscles

a

h

mmm

»"•

."

':

1 0 °

Q.«

' ° ' 0

1 - ■

,

■ .'],•",

- : : s 0

'

•»lh" ■•"

". ,%•,

'-

o°o°

v«

' f °°

B -

'.'• •

•

"-:

\"o

.-.

°» ° »«

°. " ',

"»"/«

;*.

•

y;

-,'-",

°o°"

.••'.:

;, ••.

;;

::

0°

° o°

0 0°

0 °° ° \°

\ l°°:

0 V

0

%* ■-

;

-: ~ .v

_

-

'«

- 0':

.'

' ' ' '!

.- ■■ »,

° I «

%°

{

°,°°

'."«

° „ °

0 ° °

'I '' o° •

\

Fig. 109. — Apparatus of Thoma and Zeiss
foe Counting the Corpuscles. A. In section.
C. Surface view without cover-glass. B. Micro-
scopic appearance with the blood-corpuscles. —
(Landois and Stirling.)

or,

■500X200X4000 ,1 .

g — - — = 5,000,000 erythrocytes per c.mm.,

500X10,000 = 5,000,000, erythrocytes per c.mm.

The accuracy of the counting is proportional to the number of squares
counted. If 200 squares are counted with each of two different drops, and
the average taken the probable limit of error will be less than 2 per cent.

The Effects of Reagents on the Red Corpuscles. — Within the
blood-vessels the physical conditions and chemic composition of the
plasma are such that both the form and the composition of the cor-
puscle or the relation of the hemoglobin to the stroma, are maintained
in the normal or physiologic condition. The plasma is preservative
of jhe structure and function of the corpuscle. The reason assigned
for this is that the osmotic pressure of the salts in the plasma and
of the salts in the corpuscle exactly balance one another so that there
is neither an absorption of water from, nor a yielding of water to, the

THE BLOOD. 249

plasma on the part of the corpuscle. The plasma having an osmotic
pressure equal to that within the corpuscle is said to be isotonic with it.

When blood is to be prepared for microscopic examination with a
view of determining the histologic features of the corpuscles or for pur-
poses of enumeration, it must be diluted, and unless special precau-
tions are observed the condition of equal osmotic pressure will be dis-
turbed by the diluting agent and the corpuscles will lose their charac-
teristic form and structure from either an absorption or loss of water,
as the case may be.

If distilled water is employed for this purpose, the osmotic pressure
of the plasma is of course diminished, and in consequence the osmotic
pressure of the constituents of the corpuscles (particularly sodium
chlorid) causes an inflow of water. The corpuscle therefore swells
and assumes a more or less spheric form; the hemoglobin is dissociated
and discharged into the surrounding fluid throughout which it diffuses.
Such an environment having an osmotic pressure less than that of the
corpuscle is said to be hypotonic, hypisotonic, or hypoisotonic to it.

If on the contrary, water containing inorganic salts (particularly
sodium chlorid) in amounts which impart to the plasma an osmotic
pressure greater than that within the corpuscle, there will be an outflow
of water from the corpuscle, a shrinkage of the volume and a crenation
of its surface. Such an environment having an osmotic pressure
greater than that of the corpuscle is said to be hypertonic, or hyperisotonic
to it. It becomes essential therefore in diluting the plasma with water,
that the latter contains inorganic salts in such amounts that the result-
ing mixture (plasma and water) possesses an osmotic pressure equal
to that of the original plasma or to that of the corpuscle. A diluting
agent well adapted for this purpose is the well-known Ringer's mixture.
Other solutions which preserve the form of the corpuscles during the
time required for their enumeration are the solutions devised by Hayem,
Toisson and Gower alluded to on a preceding page. Because of the
fact that sodium chlorid is the chief inorganic constituent of the plasma
and of the corpuscle it is common in laboratory work to dilute the
plasma of mammalian blood and of frog's blood with solutions of so-
dium chlorid of 0.9 per cent, and 0.6 per cent, respectively, which
though not absolutely are sufficiently isotonic for the purpose desired.

Many other saline solutions with an osmotic pressure greater or less
than normal plasma, dilute solutions of acids and alkalies, bile salts,
chloroform, ether, ammonium sulphocyanid, electricity, etc., also
destroy the physical and chemic integrity of the corpuscle and cause
the hemoglobin to separate from the stroma and diffuse into the plasma
without itself undergoing any appreciable change in composition.
With the escape and diffusion of the hemoglobin the blood becomes trans-
parent and changes to a dark red color to which the term "lake color"
has been given. The mechanism by which the hemoglobin becomes
dissociated and discharged from the corpuscle by these agents is
unknown.

250

TEXT-BOOK OF PHYSIOLOGY.

The Corpuscles of Other Vertebrated Animals. — In all mam-
mals, with the exception of the camel, llama, and dromedary, the
red corpuscles present the same shape and structure as the corpuscles
of man, and may be described as circular, flattened, biconcave disks.
In the animals excepted the corpuscles are oval. The size, however,
varies in different animals from 0.0092 mm. (2-7V5 mcn) m the- ele-
phant to 0.0023 mm. (i^ inch) in the musk-deer, while in most
animals the average lies between 0.0084 mm- and 0.0050 mm. Inas-
much as the question may arise as to whether the corpuscles of any
given specimen of blood are those of a human being or of some other
mammal, a knowledge of the size of the corpuscles becomes a matter
of medicolegal as well as of physiologic interest. Though the dif-
ferences in size are slight, yet it is possible for skilled microscopists,
when examining fresh blood, to make a diagnosis between the cor-
puscles of man and those of the domesticated animals, with the ex-
ception, perhaps, of the guinea-pig. The diagnosis of the corpuscles
of dried blood which have been altered by the action of various ex-
ternal agents, even though capable of a certain degree of restoration,
is most difficult, and should not be attempted in criminal cases with-
out large experience in microscopy, in measurements and methods
of preparation of all kinds of blood-corpuscles, and a proper per-
ception of corpuscular forms and sizes. In the following table the
average results of the measurements of the corpuscles in different
classes of animals are given (abstracted from Formad's compilation) :

Man ,

Guinea pig.

Dog

Rabbit, . . .

Ox

Pig

Horse, . . .

Cat

Sheep,
Goat

Gulliver.

Inch. Mm.

1.3200
I-3S38
1-3532
1.3607
1.4267
1.4230
1.4600
1.4404
1.5300
1.6366

0.0079
0.0071
0.0071
0.0070
0.0060
0.0060
0.0057
0.0058
0.0048
0.0040

WORMLEY.

Inch. Mm

1-3250
1.3223
1-3561
1-3653
1. 4219
1.4268
1.4243
1-4372
1. 4912
1. 6189

0.0078
0.0079
0.0071
0.0070
0.0060
0.0059
0.0059
0.0058
0.0031
0.0041

C Schmidt.
Mallinin.

Inch. Mm

3300
3300*
3636
3968
4354
4098
4464
4545
5649
6369

0.0077
0.0077
0.0070
0.0064
0.0058
0.0062
0.0057
0.0056
0.0045
0.0040

French Medi-
colegal Soc.
Welcker.

Inch. Mm

1-3257
1-3213!
1.3485
1-3653
1-4545
1.4098
1.4545
1.3922
1.5076
1-5525

0.0078
0.0079
0.0073
0.0069
0.0056
0.0062
0.0056
0.0065
0.0059
0.0046

FORMAD.

Inch. Mm.

1.3200
1.3400
1.3580
1.3662
1.4200
1.4250
1. 4310

1.5000
1. 6100

0.0079
0.0075
0.0071
0.0069
0.0060
0.0060
0.0059

0.0051
0.0042

In birds, reptiles, and amphibians the corpuscles are larger than
in mammals, are oval in shape, and nucleated. (See Figs, no and
in.) As the scale of animal life is descended the corpuscles increase
in size, until in the proteus and amphiuma the long diameter attains
an average length of 0.058 mm. and 0.077 mm- respectively. In fish
the corpuscles are smaller, oval, and nucleated, with the exception of
the lamprey eels, in which they are circular, biconcave, and nucleated,
though the nucleus is generally concealed in the peripheral portion of
the corpuscle. As in these animals, the corpuscles are almost twice
the size of the human red corpuscles, they can, notwithstanding the
similarity of shape, be readily distinguished from them.

* Masson. t Woodward.

III.

THE BLOOD. 251

The Function of the Red Corpuscles. — The red corpuscles, by
virtue of the capacity of their contained hemoglobin for oxygen ab-
sorption, may be regarded as carriers of oxygen from the lungs to
the tissues, and therefore important factors in the general respiratory
process. The size as well as the number of the corpuscles in different
classes of animals appears to be directly related to the activity of the
respiratory process. In those animals in which the corpuscles are
small and numerous and the total superficial area large, respiration
is active, the quantity of oxygen absorbed is
large, and the energy evolved through oxida-
tion great. In those animals, on the contrary,
in which the corpuscles are large and relatively
few in number, the reverse conditions obtain.
This is in accordance with the fact that the
superficial area of any given volume of sub-
stance is increased in proportion to the extent
to which it is subdivided. FIG. no. Fio.

The superficial area of a single human red Amphibian Colored

corpuscle has been estimated at 0.000128 sq. Blood-corpuscles. Fig.

-rr xi -Li! 1 !_• no, on the flat; Fig. in,

mm. If the number of corpuscles in 1 cubic on ecjge _ (Landots and
millimeter of blood averages 5,000,000, the Stirling.)
superficial area would amount to 640 square

millimeters; and if the amount of blood in the body of a man weigh-
ing 75 kilos is taken as one- thirteenth of this weight — that is, 5769
grams (5463 c.c.) — the total area of the corpuscular surface will amount
to 3496 square meters.

Life-history of Red Corpuscles. — In the performance of their
functions the red corpuscles undergo more or less disintegration and
finally destruction; but as the average number is maintained under
normal physiologic conditions, there must be a constant renewal of
corpuscles from day to day. The evidence of destruction of red
corpuscles is furnished by the presence in the blood, in various situ-
ations of the body, of a pigment containing iron and the presence of
pigments in the bile and urine, all of which are believed to be deriv-
atives of effete hemoglobin. The blood-pigment (hematin), which
contains the iron of the hemoglobin, is found in the capillaries of
the liver, in the cells of the splenic pulp, and in the marrow of the
bones. Whether the presence of the pigment in these organs is a
proof that the corpuscles are destroyed here, or whether they are to
be regarded merely as agents concerned in the further reduction
and elimination of the hematin, is uncertain. The genetic rela-
tionship between bile-pigment and hemoglobin is shown by the fact
that any artificial destruction of hemoglobin or its injection into the
blood is attended by an increase in the quantity of bile-pigment elim-
inated. It appears also from chemic considerations that the hemo-
globin will undergo cleavage into a globulin body and hematin, which
by the loss of its iron is readily converted into the bile-pigment, bili-

252 TEXT-BOOK OF PHYSIOLOGY.

rubin. The amount of this latter pigment may therefore be taken as
an index of the extent of corpuscular destruction.

This gradual decay of corpuscles as well as the losses occasioned
by hemorrhages necessitate a continuous formation of new corpuscles,
so that the normal number may be maintained. The rapidity with
which corpuscles may be renewed, in the woman at least, is shown
by a computation of Mr. Charles L. Mix. A woman loses during a
menstrual period 150 c.c. of blood. At the end of twenty-eight or
thirty days this volume is restored, so that in one day 5 c.c, or 5000
c.mm., of blood must be formed, or 208 c.mm. per hour and 3^ c.mm.
per minute. That is, during a certain number of years 15,750,000
corpuscles must be formed every minute, and this independent of the
daily loss due to functional activity.

At the present time there is a general agreement among histolo-
gists that in adult life the red corpuscles are derived from embryonic
forms, the so-called erythroblasts, which are found chiefly in the
red marrow of the long bones.* In this situation both arterial and
venous capillaries are relatively large and the blood is separated from
the surrounding marrow by extremely thin walls. In the passages of
this capillary network the erythroblasts make their appearance most
probably by a transformation of pre-existing marrow cells. At first
they are large, homogeneous, colorless, perhaps slightly tinged with
hemoglobin and distinctly nucleated. They increase in number by
karyokinesis and at the same time increase in their hemoglobin con-
tent. The nucleus is finally extruded, earrying with it a portion of
the perinuclear cytoplasm, after which the remainder of the cor-
puscle assumes the shape and size of the adult corpuscle and is carried
out into the general circulation. After severe hemorrhage the forma-
tive processes in the marrow may become so active that erythroblasts
make their appearance in the blood-stream before the extrusion of the
nucleus has taken place.

CHEMIC COMPOSITION OF RED CORPUSCLES.

Hemoglobin. — The red corpuscle consists of a stroma and a
coloring-matter, hemoglobin. The amount of hemoglobin in the
normal corpuscle is about 30 per cent, of the total weight, the re-
maining 70 per cent, consisting of 68 per cent, of water and 2 per
cent, of solid matter, e. g., lecithin, cholesterin and inorganic salts.
When the corpuscle is freed from water the hemoglobin constitutes
about 94 per cent, of the total solids, 32 per cent. In the normal
condition of the corpuscle the hemoglobin is in an amorphous con-
dition and is combined in some unknown way with the stroma.

If blood Which has been rendered laky, by water or any other

* For an admirable resume of the various views regarding the origin and formation
of red corpuscles see the paper of Mr. Charles L. Mix, Boston Med. and Surg. Journal,
1892, Nos. n and 12; also paper by Prof. W. H. Howell, Journal of Morphology, vol.
iv, 1802.

THE BLOOD.

253

of the known agencies, be allowed so slowly evaporate, the dissolved
hemoglobin undergoes crystallization. The rapidity with which the
crystals form varies in the blood of different animals under similar con-
ditions. According to the ease with which crystallization takes place,
Preyer has classified various animals
as follows: (i) Very difficult — calf,
pigeon, pig, frog; (2) difficult — man,
monkey, rabbit, sheep; (3) easy —
cat, dog, mouse, horse; (4) very easy
— guinea-pig, rat.

The hemoglobin crystals vary in
shape according to the blood from
which they are obtained (Fig. 112).
Those obtained from the guinea-pig
are tetrahedral; those from man
and most mammals are prismatic
rhombs; those from the squirrel are
in the form of hexagonal plates.
Notwithstanding these slight differ-
ences, all forms belong to the same
crystal system, with the exception
of those from the squirrel.

A simple but very effective
method of obtaining blood-crystals
suggested by Reichert is to lake
denbrinated blood, especially that
of the dog, rat, guinea-pig, and
horse, with acetic or ethylic ether
and then add a solution, 1 to 5 per
cent., of ammonium oxalate. A
drop of this mixture placed under
the microscope will show crystal
formation in a very few minutes.

Chemic Composition of Hemoglobin. — By appropriate methods
hemoglobin can be obtained in a practically pure form, and when
subjected to a temperature of 100 ° C. its water of crystallization is
driven off, after which it can be analyzed. In the subjoined table
the results of several analyses are given for 100 parts of hemoglobin.

Fig. 112. — Crystallized Hemo-
globin, a, b. Crystals from venous
blood of man. c. From blood of cat.
d. Of guinea-pig. e. Of marmot. /.
Of squirrel. — (Gaulier).

C,
O,
H,
N,
S,.
Fe,

Dog.

Horse.

Dog.

Guinea-pig.

53-91

22.62

6.62

15.98

0.54

°-33
Jaquet.

5"5

23-43
6.76

17.94
o-39

. °-33

Zinoffsky.

53-85
21.84

7-32

16.17

°-39
0.43

54-12

20.6S

7-36
16.78

0.58
0.48

Hoppe

Seyler.

254 TEXT-BOOK OF PHYSIOLOGY.

The elementary composition of hemoglobin is thus seen to vary
slightly in different animals, suggesting that there may be different kinds
of hemoglobin. The rational molecular formula is not known. On
the assumption that each molecule contains one atom of iron, Preyer
suggested the following empirical formula: C6ooH96oNIS40179S3Fe,
with a molecular weight of 13,332; Jaquet has suggested a different
formula: viz., C7s8HI203NI9502l8S3Fe,with a molecular weight of 16.669.
It is very evident from this that the molecule is of enormous size and
exceedingly complex.

Quantity of Hemoglobin.— The quantity of hemoglobin in blood
as determined by chemic, chromometric, and spectro-photometric
methods amounts to about 14 per cent, in man and 13 per cent, in
woman. Of the chemic methods, that based on the amount of iron
is the most familiar. Chemic analysis has shown that hemoglobin
contains 0.43 per cent, and blood 0.056 per cent, of iron; with these
two factors the quantity of hemoglobin can be determind by thee
following formula: x=iooox°2°56 = 13.33 per cent. The total quantity
of hemoglobin in the blood, assuming the latter to be about 5769
grams (one-thirteenth of the body- weight, 75 kilos) will therefore
amount to 769 grams; e.g., x = 5769**3"33 = 769. The total amount
of iron in the blood is obtained by the following formula: viz.,
^=^7^=3.23 grams.

Under normal physiologic conditions the percentage of hemo-
globin undergoes but slight variation. In pathologic states there is
frequently a great diminution in the amount, especially in chlorosis,
splenic leukemia, and pernicious anemia, diseases in which it dimin-
ishes to 2 \ per cent, in many instances. For the determination of
these variations in the hemoglobin for clinical purposes two chromo-
metric methods are at present largely employed, that of Gowers and
v. Fleischl. All chromometric methods are based on the principle
that if two equally thick and equally well-illuminated solutions pre-
sent the same intensity of color, their richness in coloring-matter is
the same. There are two methods by which this can be done: (1) By
diluting the blood to be examined with water until the shade of color
corresponds to that of a solution of hemoglobin of known strength
(Gowers). (2) Diluting a given quantity of blood with a given
quantity of water and then finding an identical color which repre-
sents a previously determined quantity of hemoglobin (v. Fleischl).

Gowers' hemoglobinometer consists (Fig. 113) of two glass tubes
of exactly the same size. One, D, contains glycerin jelly colored
with picro-carmine the shade of which corresponds to that of normal
blood diluted 100 times, 20 c.mm. in 2000 c.mm. of water repre-
senting a 1 per cent, solution. The other tube, C, is ascendingly
graduated with 120 divisions, each one of which corresponds to 20
c.mm. With a graduated pipette B 20 cubic millimeters of blood are
accurately measured and blown into the bottom of the tube C, in

THE BLOOD.

255

which a few drops of distilled water have been placed so as to prevent
coagulation. Water is then added drop by drop until the color of
the diluted blood is exactly that of the standard. The division of
the scale reached by the dilution will represent the relative percentage
of hemoglobin. If this tint is not obtained until the dilution reaches
100 divisions, the quantity of hemoglobin is normal. If more water
must be added, it is in excess; if less, it is diminished. If, for example,
the 20 cubic millimeters of blood from an anemic patient gave the
standard tint at 60 divisions, the blood contained but 60 per cent,
of the normal amount of hemoglobin.

Fig. 113. — Gowers' Hemoglobinometer. A. Pipette bottle for distilled water.
B. Capillary pipette. C. Graduated tube. D. Tube with standard dilution. F.
Lancet for pricking the finger. — [Landois and Stirling.)

Von Fleischl's hemometer consists of a metallic cell divided into
two compartments, a and a', by a vertical partition (Fig. 114). In
the former a definite quantity of blood is placed and diluted with a
known quantity of water. Beneath the compartment a' is placed
a glass wedge stained with the golden purple of Cassius or simi-
lar pigment, the color of which passes from a deep red at one end to
clear glass at the other (Fig. 115). To the side of this wedge is placed
a scale ranging from o to 120. By means of the screw, R T, the glass
wedge is moved until the color of the glass and diluted blood is identical.
The illumination of the blood and glass wedge is accompanied by lamp-
light reflected from the white reflecting surface beneath. The depth
of color of the glass opposite 100 on the scale corresponds to that of
normal blood. If, therefore, the colors are identical at 75 divisions,
the blood contains but 75 per cent, of hemoglobin.

Very frequently the diminution of corpuscles and hemoglobin
proceeds along parallel lines, in which case the amount of hemoglobin

256

TEXT-BOOK OF PHYSIOLOGY.

per corpuscle is supposed to be normal and the color-index = 1.
If the hemoglobin diminution is greater than the corpuscles, as is
the case in many pathologic conditions, the color-index is less than
unity. If the percentage of corpuscles is determined by the method

Fig. 114. — Von Fleischl's Hemometee. K. Red colored wedge of glass moved
by R. G. Mixing vessel with two compartments, a and a'. M. Table with hole to read
off the percentage of hemoglobin on the scale P. T. To move K. S. Mirror of plaster-
of-Paris.

of counting to be 80 per cent. (4,000,000 per cubic millimeter) and the
percentage of hemoglobin 60, the color-index is obtained by dividing
the latter by the former; e. g., -§-$■ = 0.75. In other words, each cor-
puscle has but 0.75 per cent, of the normal amount of hemoglobin.
Absorption Spectra. — Both oxyhemoglobin and reduced hemo-
globin, like other soluble pig-
ments, have an absorbing influ-
ence on certain waves of light,
and hence give rise to absorption
bands which can be studied with
the spectroscope, and which are
so characteristic as to serve for
their identification.

In principle a spectroscope
consists of a prism which decom-
poses the light from a narrow slit into a band of all the spectral colors.
A form of spectroscope in common use is that shown in Fig. 116.
It consists of a tube, B, which has at one end a slit that can be nar-
rowed or widened by means of a screw. The light, having passed
through it, falls on an achromatic convex lens (called the collimator)

Fig. 115. — Tinted Glass Wedge of
the von Fleischl Hemometer. — (Da
Costa's Hematology.)

{Triacid Stain.)

i, 2, 3, 4. Small Lymphocytes.

Contrast the faintly colored protoplasm of these cells in the triple stained specimen,
with their intensely basic protoplasm in the film stained with eosin and methylene-
blue, 17 and 18. The cell body of 1 is invisible. Note the kidney -shaped nucleus
in 4.

5, 6. Large Lymphocytes.

With this stain the nucleus reacts more strongly than the protoplasm; with eosin
and methvlene-blue (19, 20), on the contrary, the protoplasm is so deeply stained
that the nucleus appears pale by contrast. This peculiarity is also observed in
the smaller forms of lymphocytes.

7, 8. Transitional Forms.

Note the moderately basic and indented nucleus, and the almost hyaline non-granular
protoplasm. Compare 8 with the myelocyte, 7, Plate I, these cells differing chiefly
in that the myelocyte contains neutrophile granules.

9, 10, 11. Polynuclear Neutrophils.

These cells are characterized by a polymorphous or polynuclear nucleus, surrounded
bv a cell-body filled with fine neutrophile granules. In 11 the nuclear structure is
obviously separated into four parts; in 9 it is moderately, and in 10 markedly, poly-
morphous.

12, 13. Eosinophiles.

The nuclei are not unlike those of the polynuclear neutrophile, except that they are
somewhat less convoluted, and poorer in chromatin, staining less intensely. The
protoplasm is filled with coarse eosinophile granules, the characteristics of which
are clearly illustrated by 13, a "fractured" eosinophile.

14. Eosinophilic Myelocyte.
Compare with 15.

15, 16. Myelocytes. {Neutrophilic.)

These cells are morphologically similar to 14, except that they contain neutrophile
instead of eosinophile granules. Note that the granules of the myelocyte are identical
with those of the polynuclear neutrophile. A dwarf form of myelocyte is repre-
sented by 16.

(Eosin and Methylene -blue.)

17, 18. Small Lymphocytes.

Note the narrow rim of pseudo-granular basic protoplasm surrounding the nucleus,
and the pale appearance of the latter.

19, 20. Large Lymphocytes.

Budding of the basic zone of protoplasm is represented by 20. Both of these cells
belong to the same type as 3 and 6.

21, 22. Large Mononuclear Leukocytes.

Compared with 19 and 20, these cells have a decidedly less basic protoplasm, but
a somewhat more basic nucleus. In the triple stained film these differences can-
not be detected, so that they must be classed as large lymphocytes.

23. Transitional Form.

The distinction between this cell and 24 is not marked; the nucleus of the latter
simply being somewhat more basic and convoluted.

24, 25, 26, 27. Polynuclear Neutrophiles.

With this stain these cells show a feebly acid protoplasm, and lack granules. Note
that the more twisted the nucleus the deeper it is stained. Compare with 9, 10,
and 1 r.
28, 29. Eosinophiles.

Compare with 12 and 13.

30. Eosinophilic Myelocyte.
Compare with 14

31. Basophile. (Finely granular.)

This (ell is characterized by the presence of exceedingly fine 5-granuIes, staining
the- pure color of the basic dye. The nucleus is markedly convoluted and deficient
in chromatin. The cell here shown was found in normal blood.

32. t,t,, 34, 35, 36. Mast Cells.

The granules take a modified basic color, as shown by their royal-purple tint in this
illustration. Note their unusually large size and ovoid shape in t,^, their peculiar
distribution in 35 and 36, and their irregularity in size in 32 and 36. With the triacid
mixture these granules, as well as those of the finely granular basophile, 31, remain
unstained, showing as dull-white stippled areas in the cell-body. The nuclear chro-
matin of the mast cell is so delicate and so feebly stained that it is barely visible.
These cells were found in the blood of a case of splcnomcdullary leukemia.

PLATE I.

1 Wk

.»> 0|e°o i^

■fog j?it'->>sP

f,4*&%

"*>%%*?

%p

• •si'V-''!-".

^SSsjs* \hJ?

The Leukocytes.

(2-16, Triacid Stain; 17-36, Eosin and Methylene-blue.)

(E. F. Faber, /ec.)

(From DaCosla's "Clinical Hematology.")

THE BLOOD.

257

at the opposite end of the tube which renders the divergent rays of
light parallel. These parallel rays subsequently fall on the prism, by
which they are dispersed and directed into the tube, A, which is
nothing more than a small telescope. On looking into it at the ocular
end the spectral colors are seen. If the light has been derived from
the sun, the spectrum will present vertical dark lines, the so-called
Fraunhofer's lines. They are given from A to F in Fig. 117. If a
colored medium be held in front of the slit so that the light has to pass

Fig. 116. — The Spectroscope. A. Telescope. B. Tube for the admission of
light and carrying the collimator. C. Tube containing a scale, the image of which when
illuminated is reflected above the spectrum. D. The fluid examined. — (Landois and
Stirling.)

through it first, certain dark bands will appear in the spectrum, owing
to the absorption of certain rays.

Dilute solutions of arterial blood show two absorption bands
between the Fraunhofer lines, D and E, in the green and yellow
portion of the spectrum. (See Fig. 117.) The band nearest D
frequently designated as alpha is dark in the center and sharply
defined. The band which lies toward E is broader and less sharply
defined.

As the amount of light absorbed varies with the concentration of
the solution as well as its thickness, and gives rise to absorption bands
of different widths and intensities, it becomes necessary, in order to
obtain the characteristic bands, to employ only dilute solutions.

The absorption spectra, as seen with different strengths of solution
one centimeter thick, are shown graphically in Fig. 118. It will be
observed that solutions varying in strength from 0.1 per cent, to 0.6
17

258

TEXT-BOOK OF PHYSIOLOGY.

per cent, give rise to the two characteristic bands, but with gradually
increasing breadths. With a percentage greater than 0.65 per cent,
the light between D and E, the yellow-green, becomes extinguished
and the two bands fuse together, forming a single band overlapping
slightlv the lines D and E. At the same time there is a progressive
darkening of the violet end of the spectrum. At 0.85 per cent., all
the light is absorbed with the exception of a small amount of the red.

Red. Orange. Yellow.

Green.

Cyan Blue.

f

\ a
,,1,1,

B

50
.Y..I....I-.

c

fao
nil..'.

1

70
1 1 n 1 1 11

E 1

80

linil

)

Mill

00

III MM

100 1

in il 1 1 1 1 1 in 1

D

L

II

HIM

2

II

1 1

|

•

1

Hi J

I

4.

J

1

1

Oxy

hemoglobin

0.8%.

Oxy-

hemoglocin
0.28%.

Carbon *
Monoxid
Hemoglobin

Reduced
Hemoglobin

FiGi 117. — Spectra of Hemoglobin and Some of its Compounds.
— (Landois and Stirling.)

Solutions less than 0.01 per cent, to 0.003 Per cent- show but a single
absorption band — that nearest D.

A solution of venous blood or of reduced hemoglobin shows but a
single absorption band (see Fig. 117), frequently designated as gamma,
broader and less marked between the lines D and E, but extending
slightly beyond D. Fig. 117 shows in the same graphic manner the
increasing breadth of the absorption band with increasing strengths
of solution, as well as the simultaneous absorption of light at both
the red and violet ends of the spectrum.

Compounds of Hemoglobin.— The coloring-matter of the
blood is characterized by the property of combining with and of again
yielding up oxygen. The union is a chemic one, taking place under
certain pressure conditions. It therefore may exist in two states of
oxidation, distinguished by a difference in color and their absorption
spectra. If hemoglobin either in blood or in solution be shaken with
air, it at once combines with oxygen and is converted into oxyhemo-
globin, which imparts to the blood or solution a bright red or scarlet
color. If the blood or solution be now deprived of oxygen, the oxy-

THE BLOOD.

259

hemoglobin is converted into reduced hemoglobin, which imparts to
the blood or solution a dark bluish or purple color.

The quantity of oxygen absorbed by 1 gram of hemoglobin is
estimated at 1.56 c.c. measured at o° C. and 760 mm. of mercury.
The compound formed by the union of oxygen and hemoglobin is a
very feeble one; for when the pressure is lowered the union becomes
less stable, and as the zero point is approached, as in the Torricellian
vacuum, a rapid dissociation of the oxygen takes place. This, how-
ever, is not due entirely to a fall of pressure but partly to the dis-
sociating force of heat, which increases in power as the pressure falls.

Fig. 118. — Graphic Representa-
tion of the Absorption of Light in
a Spectrum by Solutions of Oxy-
hemoglobin of Different Strengths.
The shading indicates the amount of
absorption of the spectrum, and the
numbers at the side the strength of the
solution.

Fig. 119 — Graphic Representation
of the Absorption of Light in a
Spectrum by Solutions of Hemoglobin
of Different Strengths. The shad-
ing indicates the amount of absorption
of the spectrum, and the numbers at
the side the strength of the solution.

The same dissociation of oxygen can be brought about by passing
through blood indifferent gases, such as hydrogen, nitrogen, carbon
dioxid, which lower oxygen pressure, or by the addition of reducing
agents, such as ammonium sulphid or Stokes' fluid.

These experimental determinations of the relation of oxygen to
hemoglobin partly explain the oxidation and deoxidation of the
hemoglobin in the lungs and tissues. As the blood passes through
the lungs and is subjected to the oxygen pressure there, the hemoglo-
bin combines with a definite quantity of oxygen, and on emerging
from the lungs exhibits a bright red or scarlet color; as the blood
passes through the systemic capillaries where the oxygen pressure
in the surrounding tissues is low, the oxyhemoglobin yields up a por-
tion of its oxygen, becoming deoxidized or reduced, and the blood
on emerging from the tissues exhibits a dark bluish color. The portion
of oxygen given up to the tissues is termed respiratory oxygen. In
100 parts of arterial blood the coloring-matter presents itself almost
exclusively in the form of oxyhemoglobin. In passing through the

26o TEXT-BOOK OF PHYSIOLOGY.

capillaries about 5 per cent, only gives up its oxygen and becomes
reduced, so that both kinds are present in venous blood. In asphyx-
iated blood only reduced hemoglobin is present. It is this capa-
bility of combining with and of again yielding up oxygen, that enables
hemoglobin to become the carrier of oxygen from the lungs to the
tissues.

Carbon Monoxid Hemoglobin. — Carbon monoxid is a con-
stituent of both coal-gas and water-gas. From either source it is
likely to accumulate in the air, and if inspired for any length of time
produces a series of effects which may eventuate in death. If blood
be brought into contact with this gas, it assumes a bright cherry-red
color, which is quite persistent and due to the displacement of the
loosely combined oxygen and the union of the carbon monoxid with
the hemoglobin. The compound thus formed is very stable and resists
the action of various reducing agents. The passage of air or of some
neutral gas through the blood for a long period of time will gradually
displace the carbon monoxid and enable the hemoglobin to again
absorb oxygen. It is for this reason that partial poisoning with the
gas is not fatal. It is to its power of displacing oxygen and form-
ing a stable compound with hemoglobin and thus interfering with
its respiratory function that carbon monoxid owes its poisonous
properties. Examined spectroscopically, solutions of carbon monoxid
hemoglobin exhibit two absorption bands closely resembling in position
and extent those of oxyhemoglobin; but careful examination shows
that they are slightly nearer the violet end of the spectrum and closer
together. (See Fig. 117.) A useful test for CO blood is the addition
of caustic soda, which produces a cinnabar red precipitate.

Methemoglobin. — This is a pigment, closely related to oxy-
hemoglobin, found in the blood after the administration of various
drugs, in cysts and in the urine in hematuria and hemoglobinuria.
It is also produced when a solution of hemoglobin is exposed to the
air and becomes acid in reaction and brown in color. The spectrum
shows two absorption bands similar to oxyhemoglobin, but in addition
a new and quite distinct band near the line C, in the red. If the
acid solution be rendered alkaline by the addition of ammonia, this
band disappears and another makes its appearance near the line D.
The addition of ammonium sulphid develops reduced hemoglobin,
which, on the absorption of oxygen, produces again oxyhemoglobin,
as shown by the spectroscope.

Hematin. — Boiling hemoglobin or adding to it acids or alka-
lies decomposes it and develops one or more proteid bodies to which
the general term globulin has been given, and an iron-holding pig-
ment termed hematin. This is regarded as an oxidation product of
hemoglobin and constitutes about 4 per cent, of its composition.
When obtained in a pure state, it is a non-crystallizable blue-black
powder with a metallic luster. According as it is treated with acidskor
alkalies, two forms of hematin can be obtained (acid and alkaline),

THE BLOOD. 261

each of which has special properties, giving rise to different absorp-
tion bands.

Hemochromogen. — This pigment is derived from hemoglobin,
of which it constitutes about 96 per cent., during decomposition in
the absence of oxygen. In solution it produces a purple color, but
soon absorbs oxygen and is converted into hematin.

Hemin. — This pigment is a derivative of hematin, presenting
itself in the form of microscopic rhombic plates or rods (Teichmann's
crystals), which are so characteristic as to serve as tests for blood-
stains in medicolegal inquiries. These crystals are readily obtained
by adding to a small quantity of dried blood on a glass slide a few
drops of glacial acetic acid and a crystal of sodium chlorid; after
heating gently for a few minutes over a spirit lamp and then allowing
the mixture to cool, crystallization of the hemin soon takes place.

Hematoidin. — This term has been applied to a pigment which
occurs in the form of yellow crystals in old blood-clots or in blood
which has been extravasated into the tissues. In its chemic com-
position and in its reactions it closely resembles bilirubin, the pigment
of the bile, exhibiting the same characteristic play of colors on the
addition of nitric acid.

The Stroma. — The stroma of the red corpuscles obtained by the
methods which dissolve out the hemoglobin has been shown by analysis
to consist of from 60 to 70 per cent, of water and 40 to 30 per cent,
of solid material, containing a proteid resembling cell-globulin, lecithin,
cholesterin, and inorganic salts, among which potassium phosphate
is especially abundant.

HISTOLOGY OF THE WHITE CORPUSCLES OR LEUKOCYTES.

The presence of white corpuscles in the blood can be readily
observed under the same conditions as the red corpuscles are observed.
Thus when the mesentery of the frog or the guinea-pig is examined
with the microscope the white corpuscles are seen adhering to the walls
of the blood-vessels; in a drop of freshly drawn blood they are found
in the spaces between red corpuscles (Fig. 104).

Shape and Size. — In the resting condition, whether seen in the
vessel or on the stage of the microscope, the white corpuscle, as its
name implies, is grayish in color, round or globular in form, though
often presenting a more or less irregular surface. Its diameter varies
from 0.0004 to 0.0013 rnm., though the average is about 0.0011 mm.
or about -5-5V0 inch-
Structure. — A typical white corpuscle consists of a ground-
substance uniformly transparent and apparently homogeneous, in
which are embedded a number of granules of varying size, some of
which are very fine, while others are larger. By various reagents it
has been demonstrated that the granules are fatty, proteid, and
carbohydrate (glycogen) in character. In the fresh cells the existence

262 TEXT-BOOK OF PHYSIOLOGY.

of a nucleus is difficult of detection, though its presence can be demon-
strated by the addition of acetic acid, which renders the perinuclear
cytoplasm more transparent and makes the nucleus conspicuous and
sharply defined. From its structure it is apparent that the white
corpuscle belongs to the group of undifferentiated tissues and re-
sembles the cells of the embryo in its earliest stages as well as the
unicellular organism, the amoeba.

Chemic Composition. — The chemic composition of the white
corpuscles has been inferred from an analysis of pus-corpuscles,
with which they are practically identical, and of lymph-corpuscles
from the lymph-glands. Of the corpuscle about go per cent, is
water and the remainder solid matter consisting mainly of proteids,
of which nuclein, nucleo-albumin, and cell globulin are the most
abundant. The two former are characterized by the presence of a
considerable quantity of phosphorus, amounting to as much as 10
per cent. Lecithin, fat, glycogen, and earthy and alkaline phosphates
are also present.

Number of White Corpuscles. — The number of white cor-
puscles per cubic millimeter of blood is much less than the number
of red corpuscles, the ratio being in the neighborhood of i white to
700 red. This ratio, however, varies within wide limits in different
portions of the body and under normal variations in physiologic
conditions. In the blood of the splenic artery there is but 1 white
to 2260 red, while in the splenic vein there is 1 white to every 60 red;
or about thirty-eight times as many as in the artery. In the portal
vein there is 1 white to 740 red, while in the hepatic vein there is 1
white to 170 red.

The total number of white corpuscles per cubic millimeter has
been estimated at from 5000 to 10,000, though the average is about
7500. The number, however, is influenced by a variety of physio-
logic conditions. The ingestion of food rich in proteid material
raises the count from 30 to 40 per cent., as compared with the count
before the meal. Fasting for a few days always lowers the count,
and in a case of total abstinence of food for a week, reported by Luciani,
the count fell to 861 per cubic millimeter, after which it rose to 1530,
where it practically remained for the succeeding three weeks of the
fasting period. In the new-born the number is greater than in adults
—17,000 to 20,000 per cubic millimeter. Cabot states that 30,000
is never a high count after a meal in infants under two years. In the
later months of pregnancy, especially in primiparas, the number in-
creases to 16,000 to 18,000. Many pathologic conditions of the body
also influence the count very considerably.

The method for counting the white corpuscles is similar to that
used in counting the red. The given volume of blood should, however,
be diluted with 10 or 20 volumes of a one per cent, solution of acetic
acid, which disintegrates the red corpuscles and thus facilitates the
counting of the white. The pipette should have a larger bore than

THE BLOOD.

263

that used for the red, and a much greater number of squares in the
counting chamber should be counted, so as to diminish the percent-
age of error.

Physiologic Properties. — The white corpuscles possess the
characteristic property of exhibiting movements similar to those
observed in the amoeba, and are therefore termed amoeboid. These
movements consist in alternate protrusions and retractions of portions
of the cell body, as a result, of which they exhibit a great variety of
forms. (See Fig. 120.) The protruded process can also attach itself
to some point of the surface on which it rests, and then draw the body
of the corpuscle after it.
By a repetition of this ..... )

process the corpuscle can
slowly creep about and
change its position in
space. In virtue of these
ameboid movements the
corpuscle can appropri-
ate small particles of pig-
ment, such as indigo or
carmine, and after a
short time eliminate
them from various parts
of the surface. It is also
capable of thrusting a
process into and through
the wall of the capillary
vessel, after which the
remainder of the corpuscle follows (Fig. 121). This continues until
the corpuscle is outside the vessel and in the lymph-space, where
it resumes its original shape and movement. This process is best
observed in inflammatory conditions, when the blood has come to
rest and the vessels are occluded with both red and white corpuscles.
To this passage of the white blood-corpuscles through the capillary
wall the term diapedesis is given. The movements of the white
corpuscles are increased by a rise in temperature up to 40 ° C, beyond
which they cease, owing to the coagulation of the cell-substance. A
low temperature also arrests the movements. Induced electric cur-
rents also cause contraction and death of the cell. Moisture and
oxygen are necessary to their activity. From their similarity to lower
organisms the white corpuscles may be regarded as independent
organisms living in the animal fluids, just as the amoeba lives in its
natural liquid medium.

Varieties of Leukocytes. — A detailed study of the blood with the
aid of the triacid staining fluid of Ehrlich or any of the various eosin
and methylene-blue stains, reveals the presence of five distinct varieties
of leukocvtes and transitional forms which may be classified as follows:

Fig. 120-
of Shape
Bachman.)

-Human Leukocytes Showing Changes
Due to Amceboid Movements. (G.

264

TEXT-BOOK OF PHYSIOLOGY.

1. Small lymphocytes, so called from their resemblance to the cor-

puscles of the lymph-glands, consisting of a deeply staining and
relatively large round nucleus, encircled by a narrow rim of
cytoplasm. Found in from 20 to 25 per cent, of all leukocytes.

2. Large lymphocytes or hyaline cells, which are believed by some to

represent the preceding type at a later stage of development, by
others to have an independent origin, are distinguished by a
round or ovoid nucleus staining faintly n
and surrounded by a relatively larger
layer of cytoplasm than is seen in the
small lymphocyte. The large lymphocyte
is present to the extent of from 4 to 8 per
cent. The transitional forms are very
much like the large lymphocyte in appear-
ance and size, with the exception, how-
ever, that they possess a crescentic or in-
dented nucleus having a somewhat greater
affinity for basic dyes. They are usually
counted with the large lymphocytes.

Both varieties of lymphocytes are char-
acterized by a cytoplasm which is devoid
of granules. Rarely, basophilic granules
may be present.

3. Polymorphonuclear leukocytes or neutro-

philes. The nucleus of this cell is irreg-
ular and assumes a great variety of shapes
in different cells, a feature which has
suggested the name given to the cell.
The perinuclear cytoplasm contains a
large number of fine granules which are
neutrophilic or faintly acidophilic in their
staining reaction. They make up about
60 to 70 per cent, of the whole number
of the white blood-cells.

4. Eosinophile cells. The nucleus resembles

in many respects that of the preceding
variety; it is, however, less apt to stain
so deeply. It is also very irregular in
possess several apparently distinct nuclei.

defined but its presence is easily revealed through the large, in-
tensely acidophilic granules which it possesses.
It is present to the extent of 0.5 to 2 per cent.

5. Basophile cells, the nucleus of which is round or slightly irregular.

The granules which may be large or small are basophilic and
stain more deeply than the nucleus, though they have the same
color. It is rare for this cell to be present above 0.5 per cent,
of all leukocytes.

Fig. 121. — Small Ves-
sel showing Various
Stages in the Diapedesis
of Leukocytes. {G.Bach-
man.)

shape and many cells
The cytoplasm is ill-

THE BLOOD. 265

Origin of the White Corpuscles. — Of the various theories ad-
vanced to explain the origin of leukocytes, that formulated by Ehrlich
has found the most credence. According to this theory the leukocytes
may genetically be classed into two groups. In the first group are
the large and small lymphocytes which take their origin entirely from
the lymph-adenoid tissues of the body, e.g., the lymph-glands, solitary
and agminated follicles of the intestines, etc. As the lymph flows
through these structures the lymph-corpuscles, as the future lymph-
ocytes of the blood are called in these situations, are washed out and
carried by way of the lymph-stream into the general circulation.

In the second group are the transitional forms, the polymorpho-
nuclear, eosinophile and basophile leukocytes which originate from
the bone-marrow only. The immediate ancestors of these cells are
known as myelocytes and are normally found in the red bone-marrow.
These cells, through transitional stages, assume the characteristics
of the leukocytes just mentioned and pass directly into the capillaries
of the marrow whence they are distributed throughout the body.

Several attempts have been made by different investigators to trace
all varieties of leukocytes to a common mother cell. While this is
believed to take place during embryonal life, the proofs of such an
origin of leukocytes in the normal adult are insufficient and uncon-
vincing.

After an unknown period of life the leukocytes undergo dissolution
and disappear.

Functions. — The functions of the white corpuscles are but im-
perfectly known, and at present no positive statements can be made.
It has been suggested that wherever found in the body, whether in
blood or tissues, they are engaged in the removal of more or less in-
soluble particles of disintegrated tissues, in attacking and destroying
more or less effectively various forms of invading bacteria and thus
protecting the body against their deleterious activity. This they do
by surrounding, enveloping, and incorporating either the tissue par-
ticle or bacterium and digesting it. On account of this swallowing
action these cells were termed by Metchnikoff phagocytes and the
process phagocytosis. The cells engaged in this process are the
polymorphonuclear leukocytes and the large and the small lymphocytes.
He regards them as the general scavengers of the body. It has been
suggested that they are also engaged in the absorption of fat from
the lymphoid tissue of the intestine. In their dissolution they con-
tribute to the blood-plasma certain proteid materials which assist
under favorable circumstances in the coagulation of the blood.

HISTOLOGY OF THE BLOOD-PLATES.

The blood-plates or plaques are small histologic elements circu-
lating in the blood-plasma. They were discovered by Hayem, who
applied to them the term hematoblasts, on the supposition that they

266 TEXT-BOOK OF PHYSIOLOGY.

were the early stages in the development of the red corpuscles. This
is now known to be erroneous. On account of their specific, distinct
characters, and their constant presence in the blood of living animals
(guinea-pig and bat), they are now regarded as normal constituents
of the blood and designated as the third corpuscle. When blood is
freshly drawn from the body, the plaques rapidly undergo disintegra-
tion and disappear; but by treating the blood with osmic acid, the
form and structure of the plaque may be retained.

The blood-plaque may be defined as a colorless, grayish-white,
homogeneous or finely granular protoplasmic disk, varying in diam-
eter from 1.5 to 3.5 micro-millimeters. The edges are rounded and
well defined, but it is not certain whether they are only flattened or
are slightly biconcave. There is, however, no nucleus. The ratio of
the plaques to the red corpuscles is 1 to 18 or 20, and the total number
per cubic millimeter has been estimated to be 250,000 to 300,000.

When blood is shed they tend to adhere to each other and form
irregular masses known as Schultze's granular masses. If threads
are suspended in blood, the plaques accumulate in enormous numbers
upon them and appear to form a center from which fibrin filaments
radiate as coagulation proceeds. The white thrombi which form in
blood-vessels in consequence of diseased states — e. g., endocarditis,
atheromatous ulceration, etc. — are composed very largely of blood-
plaques and fibrin threads. The function of the blood-plaques is
unknown, but it has been surmised that in some way they are, like
the leukocytes, concerned in the coagulation of the blood. When-
ever they are diminished in number, as in purpura and hemophilia,
coagulation takes place very slowly.

The blood-plaques can be seen with high powers of the micro-
scope in the blood-vessels of the omentum of the guinea-pig and rat,
especially when the blood-stream begins to slow. They are also
readily seen in the blood-vessels of subcutaneous connective tissue
of various animals, and especially in that of the new-born rat. A
small quantity of this tissue moistened with normal saline and exam-
ined microscopically with suitable powers will show large numbers
of plaques within the blood-vessels.

THE TOTAL QUANTITY OF THE BLOOD; ITS GENERAL
COMPOSITION.

The determination of the total quantity of the blood in an animal
is best made by the chromometric method, somewhat modified at
present, of Welcker. This consists, first, in bleeding an animal,
collecting all the blood it yields, and weighing it; second, in washing
out the vessels with a normal saline solution until the fluid comes
from the veins clear and free from blood; third, in mincing the tissues
of the body, after removal of the contents of the alimentary canal,
soaking them in water for twenty-four hours, and then expressing
them. All the washings are collected and weighed. A given volume

THE BLOOD. 267

of the normal defibrinated blood, treated with carbon monoxid so as
to give it uniform color, is then diluted with water until its tint is
identical with that of the washings similarly treated with carbon
monoxid. From the quantity of water necessary to dilute the blood
the quantity of blood in the washings is readily determined. The
animal having been previously weighed and the weight of the contents
of the alimentary canal deducted, the ratio of the total weight of the
blood to the weight of the body at once becomes apparent. By this
method it has been shown that the ratio of blood to body-weight in a
human adult is 1:13; in an infant, 1:19; in a dog, 1:13; in a cat,
1:21. Thus an adult man of 75 kilos weight would have 5769 grams
of blood.

The amount of blood in the different organs has been determined
by ligating the blood-vessels in the living animal, removing the organ,
and after allowing the blood to escape subjecting the tissues to the
chromometric methods described above. According to Ranke, the
volume of the blood is distributed as follows: Heart, lungs, arteries,
and veins, \; liver, \\ muscles, \; other organs, -J.

General Composition. — The results of the analyses of the blood
will vary with the animal and the methods employed. The following
table, taken from Gad, shows the average composition, expressed in
whole numbers, of horse's blood. In essential respects the ratio of
the constituents in human blood would not be materially different.

One thousand parts of blood contain:

f Water, 200 200

Cells, 328 \ f Hemoglobin, 116

[ Solids, 128 < Other organic matter, 10

[ Salts, 2

f Water, 604 604

plasma> 6?2 f Sbumin;.' :::::.:::::::::::::::::: $1

*- Solids, 68 •{ .~ .,'

] Other organic matter, 3

] Potassium and sodium salts, 4

[ Calcium and magnesium salts. 1

CHEMISTRY OF COAGULATION.

The changes which eventuate in the formation of fibrin, and
hence all the subsequent phenomena of coagulation, are chemic in
character; but as these changes take place in organic compounds the
composition of which is but imperfectly known, the intimate nature
of the process is quite obscure: All the theories which have been
advanced in explanation, though approximating the truth, are more
or less incomplete and in some respects contradictory. Since the
coagulation is coincident with the appearance of the fibrin, the ante-
cedents of this substance, the physical and chemic conditions which
condition its development, and the succession of chemic changes in-
volved must be determined, before any consistent theory can be
established.

268 TEXT-BOOK OF PHYSIOLOGY.

Extra-vascular Coagulation. — At present it is generally be-
lieved that the immediate factors concerned in extra-vascular coagu-
lation are fibrinogen, a calcium salt, and a ferment-body. As to the
manner in which these three bodies react one with another there is a
diversity of opinion. At least five different theories are current at
the present time, all of which have some features in common, though
presenting points of difference.

Alexander Schmidt long contended that fibrin was the result of
a union of fibrinogen and paraglobulin; that the union was brought
about by a ferment-body; that the presence of the neutral salts of the
plasma was necessary to the activity of the ferment. Previous to his
death in 1893 Schmidt modified his view as follows: The insoluble
fibrin is developed out of a soluble fibrin derived from paraglobulin,
which in turn is a product of general cell disintegration; the conver-
sion of the fibrinogen into fibrin is due to the activity of a ferment,
thrombin, a derivative of pro-thrombin, a product of the disintegra-
tion of leukocytes, lymph-cells, etc.; that the production of thrombin
is conditioned by the presence of the neutral salts of the plasma in
normal percentages; that no one of these salts, calcium included, acts
in a specific manner; finally, that fibrin is not a compound of a pro-
teid and calcium.

Hammersten, as a result of many years of investigation, believes
that paraglobulin is not necessary to the process, fibrinogen alone
being transformed into fibrin under the influence of the ferment, in the
presence of a neutral salt, especially- calcium, which acts specifically
in a manner different from the sodium salts. Inasmuch as the quan-
tity of fibrin produced is always less than the quantity of fibrinogen
previously present, Hammersten concludes that the latter substance,
under the influence of the ferment, undergoes a cleavage into two
unequal portions, one of which remains in solution, the other solidify-
ing as fibrin. While admitting that the calcium salts act specifically,
he believes that they are concerned rather with the production of the
ferment than the fibrin, for if the ferment is present in sufficient
quantity coagulation takes place in a typical manner even in the total
absence of calcium.

Arthus and Pages conclude that for the transformation of fibrin-
ogen into fibrin the calcium salts are absolutely essential and act in a
specific manner; that the ferment causes a cleavage of fibrinogen into
two substances, one of which remains in solution, the other com-
bines with calcium to form fibrin. They offer in support of this
view the fact that if a 1 per cent, solution of potassium oxalate be
added to blood in quantity sufficient to precipitate the calcium, coagu-
lation will not take place; but if calcium is restored coagulation pro-
ceeds in the usual manner. They transfer the sphere of influence of
calcium to the formation of the fibrin rather than to the formation
of the ferment.

Pekelharing's researches led him to the conclusion that there

THE BLOOD. 269

arises from the disintegration of the leukocytes a nucleo-proteid,
pro-thrombin, which combining with the calcium salt forms the
ferment thrombin. This compound then transfers the calcium to
the fibrinogen, which in turn becomes fibrin; the latter is therefore
a proteid-calcium compound.

Lilienfeld asserts that fibrin formation is a cleavage process by
which fibrinogen is separated into two bodies, one an albumose which
remains in solution, the other a proteid to which he has given the
name thrombosin. This cleavage is attributed to the action of the
usual ferment, a product of the disintegration of leukocytes. Throm-
bosin combines, according to Lilienfeld, with calcium to form fibrin.

In a critical examination of these different theories Hammersten
denies that fibrin is a compound of a proteid and calcium; for chemic
analysis of both fibrinogen and fibrin shows that the former contains
as much calcium as the latter, and that therefore the view of coagu-
lation according to which fibrinogen unites with calcium to form
fibrin is without foundation. On the contrary, he maintains that the
specific influence of the calcium is directed toward the production of
the ferment, for if this be present in sufficient quantity coagulation
takes place in a typical manner, no matter whether the blood has
been decalcified by potassium oxalate or not.

Intra-vascular Coagulation. — So long as the relations of the
blood and the vascular system remain physiologic no coagulation
occurs in the vessels. The reason assigned for this is that the fer-
ment, though continually being produced, is as rapidly being de-
stroyed, and hence never accumulates in amount sufficient to develop
fibrin. This view is supported by the fact that if a solution of cell-
protoplasm, leukocytes, lymph-corpuscles, etc., presumably contain-
ing a large amount of the ferment, be injected into the blood-vessels,
extensive intra-vascular coagulation promptly follows. It is also
believed that the lining of the blood-vessel in some unknown way
restrains the coagulation process even though the circulation has
come to rest.

Under pathologic conditions of the circulatory apparatus, espe-
cially of the internal lining, intra-vascular coagulation frequently
arises, though the process can not be considered as identical with
extra-vascular coagulation. Many pathologists assert that in its
origin, mode of formation, and structure the intra-vascular coagulum
or thrombus is not a true coagulum as ordinarily understood, but
rather a conglutination of blood-plaques and leukocytes. Whenever
the integrity of the internal wall of the vessel is impaired by disease
or by the introduction of foreign bodies, there is primarily a deposition
and accumulation of blood-plaques at the injured area or on the
foreign body which constitutes to a large extent the mass of the throm-
bus which at once forms. The thrombi which form on the surface
of atheromatous ulcers, on the valves of the heart, and in the veins
in consequence of diseased states, on threads or needles passed through

27o TEXT-BOOK OF PHYSIOLOGY.

the vessels, at the orifices of torn blood-vessels, consist largely of
blood-plaques. A thrombus so formed may contain a number of
delicate fibrin threads, which, however, present a different appearance
from the fibrin of the extra-vascular clot. In the thrombi which
form around foreign bodies there is always a larger quantity of fibrin
than in those originating from causes wholly within the vessel.

CHAPTER XIII.
THE CIRCULATION OF THE BLOOD.

Each organ and tissue of the body is the seat of a more or less
active metabolism, the maintenance of which is essential to its physio-
logic activity. This metabolism is characterized by the assimilation
of food materials and the production of waste products; that it may
be maintained it is imperative that there shall be a continuous supply
of the former and a continuous removal of the latter. Both condi-
tions are subserved by the blood. In order, however, that this fluid
may fulfil these functions it must be kept in continuous movement,
must flow into and out of the tissues in volumes varying with their
activity, under a given pressure and with a certain velocity.

The apparatus by which these results are attained is termed
the circulatory apparatus. This consists of a central organ, the
heart; a series of branching diverging tubes, the arteries; a network
of minute passageways with extremely delicate walls, the capillaries;
a series of converging tubes, the veins. These structures are so
arranged as to form a closed system of vessels within which the blood
is kept in continuous movement mainly by the pressure produced by
the. pumping action of the heart, though aided by other forces. (See
Fig. 122.)

In this system a particle of blood which passes any given point
will eventually return to the same point, no matter how intricate or
tortuous the route may be through which it in the meanwhile travels;
for this reason the blood is said to move in a circle, and the movement
itself is termed the circulation.

In order to understand the reasons for the movement of the blood
in one direction only, as well as for many other phenomena connected
with the circulation, a knowledge of the structure of the heart and
its internal mechanism is of primary importance.

THE PHYSIOLOGIC ANATOMY OF THE HEART.

The heart is a cone or pyramid-shaped hollow muscular organ
situated in the thorax just behind the sternum. The base is directed
upward and to the right side; the apex downward and to the left side,
extending as far as the space between the cartilages of the fifth and
sixth ribs. In this situation the heart is enclosed and suspended
in a fibroserous sac, the pericardium, attached to the great vessels at
its base.

The heart is a hollow, double organ, consisting of a right and a

272

TEXT-BOOK OF PHYSIOLOGY.

left half, separated by a musculo-membranous septum. The general
cavity of each side is subdivided by an incomplete transverse fibrous

septum into two smaller cavities, an upper
and a lower, known respectively as the
auricle and the ventricle. The heart
may therefore be said to consist of four
cavities, the walls of which are composed
of muscle-tissue. Of these four cavities,
the right auricle and the right ventricle
constitute the venous heart; the left
auricle and the left ventricle, the arterial
heart.

The right auricle is quadrangular in
shape and presents on its posterior aspect
two large openings, the terminations of
the two final trunks of the venous system,
the superior and inferior vena cava (Fig.
123). Below, the auricle communicates
with the ventricle by a large opening
which, from its position, is termed the
auriculo-ventricular opening. The walls
of the auricle are extremely thin, not
measuring more than two millimeters in
thickness.

The right ventricle, as shown on
cross-section, is crescentic in shape owing
to the projection of the ventricular sep-
tum. It presents at its upper left angle
a cone-shaped prolongation, the conus
arteriosus. From this prolongation, and
continuous with it, arises the pulmonary
artery. The wall of the ventricle meas-
ures in the middle about four millimeters
in thickness. The inner surfaces of the
ventricle show: (1) a complicated system
of muscle ridges and bands, the columnce
carnea (fleshy columns), and (2) a set of
muscle projections, the musculi papillares
(papillary muscles), which arise by a
broad base from the walls of the ventricle
and project upward toward the auriculo-
ventricular opening. From the apex of
each papillary muscle there are given off
fine tendinous cords, the chorda tendinece,
which .become attached above to the
under surface of the auriculo-ventricular valve.

The left auricle, similar in general shape to the right, presents

Fig. 122. — Diagram of Cir-
culation. 1. Heart. 2. Lungs.

3. Head and upper extremities.

4. Spleen. 5. Intestine. 6. Kid-
ney. 7. Lower extremities. 8. Liver.
— (Dalton.)

THE CIRCULATION OF THE BLOOD.

273

posteriorly four openings, the terminations of the four final trunks
of the venous system of the lungs, the pulmonary veins. Below is
found the corresponding auriculo-ventricular opening. The wall
of the auricle measures about 3 mm. in thickness. The left ventricle
(Fig. 124) is conic in shape from above downward and oval or cir-

Fig. 123. — The Right Auricle and Ventricle Opened, and a Part of Their
Right and Anterior Walls Removed, so as to show Their Interior, h — 1. Supe-
rior vena cava. 2. Inferior vena cava. 2'. Hepatic veins cut short. 3. Right auricle.
3'. Placed in the fossa ovalis, below which is the Eustachian valve. 3". Is placed close
to the aperture of the coronary vein. + +. Placed in the auriculo-ventricular groove,
where a narrow portion of the adjacent walls of the auricle and ventricle has been preserved.
4, 4. Cavity of the right ventricle; the upper figure is immediately below the semilunar
valves. 4'. Large columna carnea or musculus papillaris. 5, 5', 5". Tricuspid valve.
6. Placed in the interior of the pulmonary artery, a part of the anterior wall of that vessel
having been removed, and a narrow portion of it preserved at its commencement, where
the semilunar valves are attached. 7. Concavity of the aortic arch close to the cord of
the ductus arteriosus. 8. Ascending part or sinus of the arch covered at its commence-
ment by the auricular appendix and pulmonary artery. 9. Placed between the innom-
inate and left carotid arteries. 10. Appendix of the left auricle. 11, n. The outside of
the left ventricle, the lower figure near the apex. — (Alien Thomson.)

cular in shape on cross-section. At its upper right angle it presents
a circular orifice, the margins of which give attachment to the walls
of the aorta, the main arterial trunk of the systemic circulation. The
inner surfaces of the ventricle show a similar though better devel-
oped system of columnae carnea?, musculi papillares, chorda? tendinea?,
18

274

TEXT-BOOK OF PHYSIOLOGY.

etc. The wall of the left ventricle measures about 11.5 mm. in thick-
ness in the middle.

The Endocardium. — The cavities of both the right and left sides
of the heart are lined by a thin firm connective-tissue membrane,

\ 1 \ wn"1"""" wmmm^.

■ ■■■■■ T Jnw *#

l ,tJ,*inP :/

Fig. 124. — The Left Auricle and Ventricle Opened and a Part of Their
Anterior and Left Walls Removed. \. — The pulmonary artery has been divided
at its commencement; the opening into the left ventricle is carried a short distance into the
aorta between two of the segments of the semilunar valves; and the left part of the auricle
with its appendix has been removed. The right auricle is out of view. i. The two
right pulmonary veins cut short; their openings are seen within the auricle, i'. Placed
within the cavity of the auricle on the left side of the septum and on the part which forms
the remains of the valve of the foramen ovale, of which the crescentic fold is seen toward
the left hand of i'. 2. A narrow portion of the wall of the auricle and ventricle preserved
round the auriculo-ventricular orifice. 3, 3'. The cut surface of the walls of the ventricle,
seen to become very much thinner toward 3", at the apex. 4. A small part of the anterior
wall of the left ventricle which has been preserved with the principal anterior columna
carnea or musculus papillaris attached to it. 5, 5. Musculi papillares. 5'. The left side
of the septum, between the two ventricles, within the cavity of the left ventricle. 6, 6'.
The mitral valve. 7. Placed in the interior of the aorta, near its commencement and above
the three segments of its semilunar valve which are hanging loosely together. 7'. The
exterior of the great aortic sinus. 8. The root of the pulmonary artery and its semilunar
valves. 8'. The separated portion of the pulmonary artery remaining attached to the
aorta by 9, the cord of the ductus arteriosus. 10. The arteries rising from the summit
of the aortic arch. — {Allen Thomson?)

THE CIRCULATION OF THE BLOOD.

275

closely adherent to the muscle-tissue, termed the endocardium. It
also contains elastic fibers and smooth muscle-fibers. Its entire surface
is covered with a layer of polygonal endothelial cells. This membrane
partially serves to resist undue distention of the heart during contrac-
tion and to prevent separation of the muscle-fibers. The endocar-
dium is continuous with the lining membrane of the blood-vessels.

The inter-auricular septum is quite thin and composed of the two
layers of the endocardium between which is a layer of muscle-fibers.
It' presents at its lower portion an oval depression, the fossa ovalis.

The inter-ventricular septum
is quite thick and well developed,
and composed of the two layers
of the endocardium enclosing the
muscle-fibers. In the upper and
central portion of the septum,
there is, however, a small region
which is thin owing to the ab-
sence of muscle-tissue and com-
posed of endocardium only.
This region is known as the pars
membranacea septi.

The Cardio- pulmonary
Vessels. — Though the two sides
of the heart are separated from
each other by the auriculo-ven-
tricular septum, they are anatom-
ically and physiologically con-
nected by the intermediation of
the pulmonary system of vessels:
viz., the pulmonary artery, capil-
laries, and veins (Fig. 125).

The pulmonary artery arises
from the conus arteriosus of the
right ventricle. After a short upward course it divides into a right and
a left branch, which enter corresponding lungs. The vessel at once
divides and subdivides into a number of branches, which, after fol-
lowing the bronchial tubes to their termination, give origin to capil-
laries that surround the air-cells of the pulmonary lobules.

The capillaries in this situation are extremely abundant and well
developed. They lie close to the inner surfaces of the air-cells. The
blood is thus brought into intimate relationship with the intra-pul-
monary air, and the exchange of gases — the excretion of carbon dioxid
and the absorption of oxygen — for which the cardio-pulmonary vessels
exist, is readily accomplished.

The pulmonary veins which return the blood to the heart are
formed by the convergence and union of the small veins which emerge
from the capillary system. The final trunks thus formed, the four

Fig. 125. — Diagram of the Cardio-
pulmonary Vessels, a. Right auricle, b.
Right ventricle, c. Pulmonary artery, d.
Lungs, e. Pulmonary vein. /. Left auricle.
g. Left ventricle, h. Aorta, i. Vena cava.
—(Datton.)

276

TEXT-BOOK OF PHYSIOLOGY.

pulmonary veins- — two from each lung — enter the posterior wall of
the left auricle.

The Course of the Blood through the Heart.— There is thus
established a pathway between the venae cavae on the right side and the
aorta on the left side, by way of the right side of the heart, the cardio-
pulmonary vessels, and the left side of the heart.

The venous blood flowing toward the heart is emptied by the supe-
rior and inferior venae cavae
into the right auricle, from
which it passes through the
auriculo-ventricular open-
ing into the right ventricle
(Fig. 126); thence into and
through the pulmonary
artery and its branches to
the pulmonary capillaries,
where it is arterialized by
the exchange of gases — the
giving up of a portion of
carbon dioxid to the lungs
and the absorption of oxy-
gen— and changed in color
from bluish-red to scarlet-
red . The arterialized
blood, flowing toward the
heart, is emptied by the
pulmonary veins into the
left auricle, from which it
passes through the auriculo-
ventricular opening into the
left ventricle; thence into
the aorta and its branches
to the systemic capillaries,
where it is de-arterialized
by a second but opposite
exchange of gases — the giv-
ing up of a portion of its
oxygen to the tissues and
the absorption of carbon dioxid from the tissues — and changed in color
from scarlet to bluish-red. The venous blood is again returned by
the systemic veins to the venae cavae. Though the blood is thus
described as flowing first through the right side and then through the
left side, it must be kept in mind that the two sides fill synchronously;
that while the blood is flowing into the right side from the venae cavae,
it is also flowing from the pulmonary veins into the left side in equal
quantities and velocities.

Though there is but one set of capillaries, as a rule, between

Fig. 126. — Diagram of Course of Blood
through the Heart, i, 2. Superior and inferior
venae cavae, 3. Right auricle. 4. Right ventricle.
5, 5, 5. Pulmonary artery and branches. 6, 6.
Pulmonary veins. 7. Left auricle. 8. Left ven-
tricle. 9. Aorta. 10. Innominate artery- 11.
Left carotid artery. 12. Left subclavian artery. —
(After Moral and Doyon.)

THE CIRCULATION OF THE BLOOD.

277

arteries and veins, there is an exception in the case of the arteries and
veins of some of the abdominal
viscera. Thus the veins emerg-
ing from the capillaries of the
stomach, intestines, pancreas,
and spleen, instead of passing
directly to the inferior vena cava,
unite to form a large vein — the
portal vein — which enters the
liver. In this organ the portal
vein divides to form a second
capillary system which is in close
relation to the liver cells and
from which arise the veins which
unite to form the hepatic veins.
These latter vessels empty and
discharge the blood into the in-
ferior vena cava just below the
diaphragm.

From the fore^oin^ facts FlG' 127'~ RlGHT Cavities of the

rrom me I0reb0ino IdClS Heart Auriculo-ventricular valves open.

physiologists frequently divide arterial valves closed.— (Dalton.)

the general circulation into:

1. The pulmonary circulation, which includes the course of the blood

from the right side of the
heart through the lungs,
to the left side of the
heart.

2. The systemic circulation,
which includes the course
of the blood from the left
side of the heart through
the aorta and its branches,
through the capillaries
and veins to the right side
of the heart.

3. The portal circulation,
which includes the course
of the blood from the
capillaries of the stomach,
intestines, pancreas, and
spleen through the portal
vein to the liver.

Orifices and Valves. —
The movement of the blood
along the path of the circle
above 'outlined is accomplished by the alternate contraction and relax-
ation of the muscle walls of the heart. That the movement mav be a

Fig. 12S. — Right Cavities of the Heart.
Auriculo-ventricular valves closed, semilunar
valves open. — {Dalton.)

!78

TEXT-BOOK OF PHYSIOLOGY.

Fig. 129. — Valves of the Heart, i.
Right auriculo-ventricular orifice, closed by the
tricuspid valve. 2. Fibrinous ring. 3. Left
auriculo-ventricular orifice, closed by the mitral
valve. 4. Fibrinous ring. 5. Aortic orifice and
valves. 6. Pulmonic orifice and valves. 7, 8,
9. Muscular fibers. — (Bonamy and Beau.)

progressive one, that there shall be no regurgitation during either the
contraction or the relaxation, it is essential that some of the orifices
of the heart be closed during each of these periods. This is accom-
plished by the heart valves.

The right auriculo-ven-
tricular opening is surrounded
and strengthened by a ring of
fibrous tissue to which is at-
tached a membrane partially
subdivided into three portions
or cusps, which during the
period of relaxation are di-
rected into the ventricle (Fig.
127); during the period of
contraction they are raised
and placed in complete appo-
sition, when they act as a
valve preventing a backward
flow into the auricle (Fig.
128). In the former position
the valve is open; in the latter,
shut. For these reasons this
structure is known as the tri-
cuspid valve. This valve is formed of fibrous tissue derived from the
fibrous ring, some muscle-fibers, covered over by a reduplication of
the endocardium. To the under surface and to the edges of this valve
the tendinous cords of the papillary muscles are firmly and intricately
attached. These cords are just sufficiently long
to permit closure of the valve and to prevent
their being floated into the auricle.

The orifice of the pulmonary artery is also
surrounded by a ring of fibrous tissue to which
are attached three semilunar or pocket-shaped
membranes, the semilunar valves. Each valve
is formed by a reduplication of the endocardium
strengthened by fibrous tissue. In the center of
the free edge of the valve there is a small nodule of
fibro-cartilage (the corpus Aurantius) . The outer
edge of the valve is strengthened by a delicate
fibrous band. A similar band strengthens the
convex attached portion of the valve just where
it is joined to the fibrous ring. A third set of

fibers pass toward the nodule, interlacing in all directions. Two
narrow crescentic-shaped areas (the lunulae) near the free edge are
devoid of these fibers. During the period of relaxation of the heart
the edges of the valves are in close apposition and prevent a return
of the blood into the ventricle (Fig. 127); during the contraction they

Fig. 130. — Muscle-
fibers from the
Heart of a Mammal. —
(Landois and Stirling.)

THE CIRCULATION OF THE BLOOD.

79

are directed into the artery (Fig. 128). In the former position they
are shut; in the latter, they are open.

The left auriculo-ventricular opening is provided .with a similar
though better developed fibrous ring and membranous valve. It is,
however, subdivided into but two portions or cusps, and is therefore
termed the bicuspid valve, or, from its fancied resemblance to a bishop's
mitre, the mitral valve. The general arrangement, connections,
and mode of action of this
valve are similar in all re-
spects to those of the tri-
cuspid valve. The orifice
of the aorta is also sur-
rounded by a ring of fibrous
tissue to which are attached
three semilunar or pocket-
shaped valves (Fig. 124),
which in their arrangement,
connections, and mode of
action are similar in all re-
spects to those at the orifice
of the pulmonary artery.
The anatomic relations of
the cardiac orifices one to
the other and the appear-
ance presented by the valves
when closed are represented
in Fig. 129.

The Heart Muscle-
fibers and Their Arrange-
ment.— The muscle-fibers
of the heart, though trans-
versely striated and nu-
cleated, differ in shape and
arrangement from those
found in any other situa-
tion. The individual fiber
is short and broad and

usually divided at one or both ends. By this means the fibers are
united not only longitudinally, but laterally. (See Fig. 130.) The
fibers are devoid of a sarcolemma and united one to the other by a
cement material. The entire musculature is permeated and supported
by connective tissue which is so arranged as to group the fibers in
bundles or fasciculi of varying size.

The arrangement of the muscle-bundles is quite complicated and
in accordance with the functions of the individual portions of the
heart. In the auricles the bundles are arranged in two sets: an outer
transverse set, which pass from auricle to auricle, and an inner longit-

Fig. 131. — Muscle-fibers of the Ventricles.

1. Superficial fibers, common to both ventricles.

2. Fibers of the left ventricle. 3. Deep fibers,
passing upward toward the base of the heart. 4.
Fibers penetrating the left ventricle. — {Sappcy, after
Bonamy and Beau.)

28o TEXT-BOOK OF PHYSIOLOGY.

udinal set, which pass over the auricles and are attached anteriorly
and posteriorly to the connective tissue of the transverse auriculo-
ventricular septum. The longitudinal fibers of each auricle are
practically independent of each other. Circularly arranged fibers are
present near the terminations of the venae cavae and pulmonary veins.
In the ventricles the muscle-bundles are also arranged in two
sets, a superficial longitudinal and a deep transverse, though their
arrangement is somewhat more complicated than that observed in
the auricles. In a general way it may be said that the superficial
longitudinal fibers on both the anterior and posterior surfaces take
their origin in the connective tissue of the auriculo-ventricular septum.
The superficial fibers on the anterior surface of the heart pass obliquely
downward and forward from right to left toward the apex, where they
turn backward and inward in a vortex manner after which they ascend
to terminate in the wall of the septum, the columnse carneae and musculi
papillares. The superficial fibers of the posterior surface of the heart
pass obliquely downward from left to right, wind around the apex,
turn upward and end in the same structures as do the fibers from
the anterior surface. The fibers from the base of the right ventricle
terminate in the structures of the left ventricle while those from the
left ventricle terminate in the structures of the right ventricle. Longit-
udinal fibers are also found on the inner surface. The transverse
fibers are very abundant and surround each ventricle separately though
they are continuous with each other across the septum. Between the
superficial longitudinal and deep transverse fibers there are several
layers of fibers which possess varying degrees of obliquity. The
general arrangement of the fibers is such as to ensure a complete and
simultaneous discharge of blood from both auricles and ventricles

(Fig- 131)-

THE MUSCLE CONNECTION BETWEEN THE AURICLES
AND VENTRICLES.

The Muscle Band of His, or the Auriculo-ventricular Bundle.

— In the mammalian heart there is no continuity of the muscle-fibers
across the auriculo-ventricular groove, uniting auricles and ventricles
such as exists in the frog or turtle heart. The muscle-fibers of the
auricles and ventricles are completely separated from each other by
the transverse fibrous septum to which they are attached. This fact
has for a long time made it difficult to understand how the contraction
process which begins in the auricles (to which there will be occasion
to refer in subsequent paragraphs) is conducted to the ventricles.
The physiologic necessity for the existence of a muscle connection
between the auricles and ventricles led to a series of investigations
which have resulted in the discovery of an elaborate system of muscle-
fibers by which they are united both anatomicly and physiologicly.
In 1893 Wilhelm His, Jr., discovered the existence of a band or bundle

THE CIRCULATION OF THE BLOOD. 281

of muscle-fibers which apparently took its origin from the posterior
part of the right side of the auricular septum, and from which point it
passed forward just above the auriculo-ventricular septum to a point
near the aortic opening, where it divided into two portions, a right and
a left, of which the latter apparently ended in the basis of the aortic
leaflet of the mitral valve. This bundle has been termed "the
muscle band of His." In 1904 Retzer and Braunig, working inde-
pendently, corroborated the existence of this bundle and described its
anatomic course more completely. The investigations of Braunig
led to the conclusion that this bundle of muscle-fibers which was
constantly present in all animals examined, including man, began
on the right side of the auricular wall below the fossa ovalis from
which point it passed forward, and anteriorly penetrated the auriculo-
ventricular septum to become connected with the musculature of the
ventricular septum just below the pars membranacea septi. Though
both these observers state that the bundle divides into a right and left
limb as it enters the ventricular septum, the ultimate distribution and
termination of these limbs was not clearly determined. Retzer
estimated that this bundle was 18 mm. long, 2.5 mm. broad, and 1.5
mm. thick. By these investigators this bundle was termed the
" auriculo-ventricular bundle."

In 1906 Tawara published the results of an extended series of
investigations made on the embryonic and adult hearts of many
mammals including man, which resulted in a further increase of
knowledge concerning the development, anatomic course and histo-
logic features of this bundle, and established beyond doubt that it is
the pathway along which the contraction process is conducted from
the auricles to the ventricles.

A brief summary of Tawara's account of this bundle is as follows:
It arises near the opening of the coronary sinus where it is connected
with the true auricular fibers. It then passes forward on the right
side of the auricular septum between the lower edge of the fossa ovalis
and the auriculo-ventricular septum; just above the insertion of the
median cusp of the tricuspid valve the bundle presents a very com-
plicated network of muscle-fibers which has been designated as a knot;
from the anterior portion of the knot, a bundle of fibers turns down-
ward and penetrates the auriculo-ventricular septum, beyond which
it passes through the pars membranacea septi to the upper limit of the
muscle portion of the ventricular septum. It then divides into two
limbs or branches which descend on either side of the septum under
the endocardium, the right limb lying somewhat deeper than the left.
Each of these limbs is enclosed by a layer of connective tissue which
isolates it from the musculature of the ventricular septum as far
as the lower third of the ventricular cavities. In this region they
divide into a number of bundles some of which enter the papillary
muscles, while others forming tendon-like strands branch freely
beneath the endocardium and spread in all directions over the entire

282 TEXT-BOOK OF PHYSIOLOGY.

inner surface of the ventricle and enter into histologic connection with
the true cardiac muscle-fibers.

The fibers composing this system, and termed by Tawara from its
supposed function the "conduction system" are histologicly different
from the cardiac fibers, insofar as they are poorer in sarcoplasm and
similar in their appearance to embryonic muscle-fibers. In the auric-
ular portion of the bundle the fibers exhibit a more or less reticular
arrangement; in the ventricular portion, the fibers are more regularly
arranged, are richer in sarcoplasm and present a number of fibrilla
near their periphery.

The ultimate termination of the system, beneath the endocardium,
constitutes the so-called Purkinje fiber layer. In the sheep, calf and
in other animals these fibers are abundant and readily recognized;
though they are not so well developed, they are nevertheless present
and extensively distributed in the human heart.

More recently the connection, in the human heart, between the
distal extremities of the two limbs of the muscle bundle and the sub-
endocardial layer of Purkinje fibers has been questioned by Fahr, who
has been unable to verify Tawara' s statements in this respect. His
findings led him to the opinion that the terminations of the limbs
merge directly without any division into the musculature of the ven-
tricle.

THE MECHANICS OF THE HEART.

The immediate cause of the movement of the blood through the
vessels is the contraction and relaxation of the muscle-walls of the
heart, and more particularly of the walls of the ventricles, each of
which plays alternately the part of a force-pump, and to a slight
extent of a suction-pump. The motive power is furnished by the
heart itself, by ' the transformation of potential energy, stored up
during the period of rest, into kinetic energy — i. e., heat and mechanic
motion.

The contraction of any part of the heart is termed the systole;
the relaxation, the diastole. As each side of the heart has two cavities
the walls of which contract and relax in succession, it is customary to
speak of an auricular systole and diastole, and a ventricular systole
and diastole. As the two sides of the heart are in the same anatomic
relation to each other, they contract and relax in the same periods
of time.

The movements of the heart, as well as many phenomena con-
nected with the flow of blood through its cavities, have been deter-
mined by observation of, and experiment on, the exposed heart of a
mammal — e. g., dog, cat, rabbit — supplemented and corrected by
experiments on the heart in its normal relations. Valuable informa-
tion as to the heart-beat and the influences which modify it has been
obtained from experiments made on the isolated heart of the turtle,
frog, and allied animals.

THE CIRCULATION OF THE BLOOD. 283

If the thorax of a dog, completely anesthetized, is opened and
artificial respiration established, the heart will be observed in active
movement inside the pericardium. If this sac is divided and turned
aside, the heart will be fully exposed to view. At the normal rate
of movement characteristic of the dog it will be almost impossible
to determine either the succession of events or their duration. But
by observing the heart under different conditions at different rates of
movement and with instrumental aids physiologists have succeeded
not only in analyzing the movements, but in describing their sequence
and in estimating their time duration.

Thus it has been determined that the heart presents two distinct
movements which alternate with each other in quick succession. One
is the movement of contraction, or the systole, by which the blood
contained within its -cavities is ejected into the arteries — pulmonary
artery and aorta; the other is the movement of relaxation, or the dias-
tole, followed by a pause during which the cavities again fill up with
the blood from the venae cava? and pulmonary veins.

Sequence of Events. — It has been ascertained that the contrac-
tion of the auricles and ventricles as well as their subsequent relaxa-
tions, though occurring with extreme rapidity, do not take place
simultaneously but successively; that the contraction process passes
over the heart in the form of a wave; that it begins, indeed, at
the terminations of the great veins, then passes to and over the
auricles, thence to and over the ventricles from base to apex with great
rapidity, but occupying in these different regions unequal periods of
time; that the relaxation immediately succeeds the contraction, in
the same order, and that at the close of the ventricular relaxation
there is a period during which the whole heart is in repose, passively
filling with blood.

Changes in Position and Form. — In passing from the diastolic
to the completed systolic condition the exposed heart undergoes changes
both of position and form as the contraction rises to its maximum.
This having been attained, the heart undergoes reverse changes until
the original diastolic condition is regained. Thus at the time of the
venticular systole the apex is tilted upward, the entire heart is twisted
on its axis from left to right and forced downward by the expansion
and elongation of the pulmonary artery and aorta. At the time of the
diastole, the reverse movements take place.

It is probable, however, that these movements are not permitted to
the same extent in the unopened chest, for the following reasons:
the heart is enclosed in the pericardium, is supported posteriorly by
the expanded lungs, and both posteriorly and inferiorly by the dia-
phragm, all of which cooperate in keeping the heart, and more particu-
larly the right ventricle, in close contact with the chest-wall and limit-
ing its movements. By means of needles inserted into the apex of the
heart, through the chest-walls, it has been shown by their slight move-
ment that the apex is practically a fixed point.

284 TEXT-BOOK OF PHYSIOLOGY.

In the diastolic condition the shape of the heart near the base is
elliptic on cross-section, the long diameter extending from side to side.
In the completed systolic condition the shape of the same cross-section
is that of a circle. In passing from the diastolic to the systolic con-
dition the transverse diameter diminishes while the antero-posterior
diameter increases, while the whole heart becomes somewhat more
conic in shape. It is questionable if the vertical diameter percept-
ibly shortens. During the systole the heart hardens, increases in
convexity, and is more forcibly pressed against the chest-wall. As
this takes place suddenly, it gives rise to a marked vibration of the chest-
wall, known as the cardiac impulse. This is principally observed in
the space between the fourth and fifth ribs, between the left edge of
the sternum and a line drawn vertically through the nipple. The car-
diac impulse is synchronous with the cardiac systole.

The cardiac impulse may be recorded with an appropriate appa-
ratus known as a cardiograph; the record obtained with it is known

as a cardiogram. A cardiograph con-
sists of a tambour covered with a thin
rubber membrane provided with a
button. The tambour is supported
by a metallic frame which permits of
an easy and accurate adjustment of
the button over the seat of the cardiac
impulse. A rubber tube connects the
Fig. 132.— a Cardiogram. cardiography tambour with a second

tambour provided with a recording
lever and thus transmits all variations in the pressure of the air in the
former to the latter.

When all adjustments are carefully made a tracing similar to that
shown in Fig. 132 will be obtained in which the slight elevation a
represents the contraction of the auricles which, completing the filling
of the ventricles, causes the apex of the heart to press more vigorously
against the chest wall; b-c represents the contraction of the ventricles
at which moment the apex is suddenly and forcibly driven against the
chest wall; c-d represents the systolic plateau, the time during which
the ventricle is discharging blood into the aorta; d-e represents the
relaxation of the ventricle while e-j represents the time of the diastole
during which the heart cavities are enlarging with the incoming of a
new volume of blood and in consequence of which the heart is press-
ing against the chest walls. The systolic plateau is characterized
by one or more elevations and depressions, the true cause of which is
unknown.

If the heart sounds are listened to at the same time the record
is being taken it becomes possible to indicate by means of an electro-
magnet signal the time during the beat that they occur. While it is
generally admitted that the first sound is heard at b, there is some
difference of opinion as to the time the second sound is heard. Thus

THE CIRCULATION OF THE BLOOD.

285

while Landois places it as occurring at d, Edgren places it at e, while
Marey places it at a point midway between d and e.

The Cardiac Cycle. — The entire period of the heart's pulsation
may be divided into three phases, viz. :

1. The auricular contraction.

2. The ventricular contraction.

3. The pause or period of repose, during which both auricles and

ventricles are at rest.
These three phases collectively constitute a cardiac cycle or a
cardiac revolution. The duration of a cycle, as well as the duration
of each of its three phases, varies in different animals in accordance
with the number of cycles which recur in a unit of time. In human
beings in adult life there are about 72 cycles to the minute; the average
duration therefore is 0.83 second.
From this it follows that the time
occupied by any one of the three
phases must be extremely short and
difficult of determination. From
observations made on human beings
and from experiments on animals
the following estimates have been
made and accepted as approxi-
mately correct:

1. The auricular systole, 0.16 soun

second.

2. The ventricular systole, 0.32

second.

3 . The period of rest for both auric-

les and ventricles, 0.32 second.
The relations of these three phases to one another may be illus-
trated by the accompanying diagram (Fig. 133), in which the space 1-2 is
the duration of a cardiac cycle divided into eight equal spaces, each
of which represents one-tenth of a second. The line A represents the
auricular, the line V the ventricular phase. The rise in the line A
represents the contraction; the fall and subsequent continuation, the
relaxation and pause. The rise in the line V and its continuation
represent the contraction; the fall and subsequent continuation, the
relaxation and the pause. From this it is apparent that the auric-
ular contraction or systole has a brief duration, 0.16 second, while
the relaxation or diastole has a long duration, 0.64 second; that the
ventricular contraction immediately following the auricular has a
duration of 0.32 second, while the relaxation and diastole have a
duration of 0.48 second; that the pause of the entire heart, that is, the
period between the termination of the ventricular systole and the be-
ginning of the next auricular systole, is only 0.32 second.

The frequency of the heart-beat varies with a variety of con
ditions: e. g., age, sex, posture, exercise, etc.

Fig. 133. — The Phases of the Heart's
Pulsation.

286 TEXT-BOOK OF PHYSIOLOGY.

Age. — The most important normal condition which modifies the
activity of the heart is age. Thus:

Before birth, the number of beats a minute averages 140

During the first year it diminishes to 128

During the third year it diminishes to 95

From the eighth to the fourteenth year it averages 84

In adult life it averages 72

Sex. — The heart-beat is more rapid in females than in males.
Thus while the average beat in males is 72, in females it is usually
8 or 10 beats more.

Posture. — Independent of muscle efforts the rate of the beat is
influenced by posture. It has been found that when the body is
changed from the lying to the sitting and to. the standing position,
the heart will vary as follows — from 66 to 71 to 81 on the average.

Exercise and digestion also temporarily increase the number of beats.

A rise in blood-pressure from any cause whatever is usually attended
by a decrease, while a jail in blood-pressure is attended by an increase
in the rate.

The Action of the Valves. — As previously stated, the forward
movement of the blood is permitted and regurgitation prevented by
the alternate action of the auriculo-ventricular and the semilunar
valves. As a point of departure for a consideration of the action of
the valves and their relation to the systole and diastole of the heart,
the close of the ventricular systole may be conveniently selected.

At this moment, if the blood is not to be returned to the ventricles,
the semilunar valves must be instantly and completely closed. This
is accomplished in the following manner: During the outflow of blood
from the ventricles the valves are pushed outward toward the walls
of the vessels, though not coming into contact with them, for behind
them are the pouches of Valsalva, containing blood, continuous with
and under the same pressure as that in the vessels themselves. With
the cessation of the outflow and the beginning of the relaxation the
pressure of the blood behind the valves suddenly forces them inward
until their free edges, including the lunulas, come into complete appo-
sition. By this means the orifices of the pulmonary artery and aorta
are securely closed and a return flow prevented. Reversal of the
valves is prevented by their mode of attachment to the fibrous rings
of the orifices.

During the ventricular systole the relaxed auricles have been
filling with blood. With the ventricular relaxation this volume, or
its equivalent, flows readily into the empty and easily distensible
ventricles, its place being taken by an additional volume of blood
flowing from the venae cavae and pulmonary veins. Whether the
ventricles exert a suction power at the moment of their relaxation is
an undecided question. A steady stream of blood into the auricles
and ventricles continues throughout the entire period of rest until
both cavities are filled. The tricuspid and bicuspid valves which

THE CIRCULATION OF THE BLOOD.

287

hang down into the ventricular cavities are now floated up by cur-
rents of blood welling up behind them until they are nearly closed.
The auricles now contract, forcing their contained volumes, or at
least the larger portions of them, into the ventricles, which become
fully distended.

With the cessation of the auricular systole the ventricular systole
begins. If the blood is not to be returned to the auricles at this
moment, the tricuspid and mitral valves must be suddenly and com-
pletely closed. This is readily accomplished by reason of the position

S.a.-D.v

D.a.-S.v

Fig. 134. — Diagrammatic Representation of the Auricular Systole, S.a.,
with the Ventricular Diastole, D.v., and of the Auricular Diastole, D.a., with
the Ventricular Systole, S.v. C.s. andC.i. Superior and inferior cavae; A.d. (atrium dex-
trum) right auricle; A.s. (atrium sinistrum) left auricle; V.d. (ventriculus dexter) right ven-
tricle; Y.s. (ventriculus sinister) left ventricle; P. pulmonary artery; A. aorta; P.P. papillary
muscles. — (Landois.)

of the valves, which have been floated up and placed almost in apposi-
tion by the blood itself. With the beginning of the ventricular pressure
the blood is forced upward against the valves until their free edges
are brought together and the orifices closed. Reversal of these
valves is prevented by the contraction and shortening of the papil-
lary muscles, which in consequence exert a traction on their under
surfaces. The blood now confined in the ventricle between the
closed auriculo-ventricular and semilunar valves is subjected to
pressure from all sides. As the pressure rises proportionately to the
vigor of the contraction, there comes a moment when the intra-vcn-
tricular pressure exceeds that in the aorta and pulmonary artery.

288 TEXT-BOOK OF PHYSIOLOGY.

Immediately the semilunar valves of both vessels are thrown open and
the blood discharged. Both contraction and outflow continue until
the ventricles are practically empty, after which ventricular relaxa-
tion sets in, attended by a rapid fall of pressure. Under the influence
of the positive pressure of the blood in the sinuses of Valsalva the
semilunar valves are again closed, the column of blood supported,
and regurgitation is prevented. With the accumulation of blood in
the auricles the cardiac cycle is completed.

The changes in the shape of the heart, the variations in the
size of its cavities and in that of the blood-vessels arising from them,
the relative position of the valves during systole and diastole are shown
in Fig. 134.

Relative Functions of Auricles and Ventricles. — Though
both auricles and ventricles are essential to the continuous movement
of blood, they possess unequal values in this respect. The passage
of the blood through the pulmonary and systemic vessels is accom-
plished by the driving power of the right and left ventricles respec-
tively, aided, however, by minor extra-cardiac forces. They may
be regarded therefore as force-pumps.

If the heart consisted of ventricles only, the flow of blood from
the vense cavse and pulmonary veins would be temporarily arrested
during their systole and their subsequent refilling delayed. This is
obviated, however, by the addition of the auricles; for during the
ventricular systole the blood continues to flow into the auricles, in
which it is temporarily stored until the ventricular relaxation sets in.
With this event the accumulated blood passes into the ventricles, which
are thus practically filled before the auricular systole occurs by which
the filling is completed. By this means there is no delay in the filling
of the ventricles, and hence their effective working as force-pumps
is more readily secured. The auricles may therefore be regarded
as feed-pumps. For this reason it is probable, notwithstanding the
contraction of the circular muscle-fibers at the terminations of the
venous system, the flow of blood into the auricles is never entirely
arrested. Regurgitation in these vessels does not occur for the reason
that the pressure in the auricles is not higher than, if as high as, in
the great veins.

Synchronism of the Two Sides of the Heart. — If the balance
of the circulation is to be maintained, the two sides of the heart must
act synchronously. That they do so can be shown by attaching
levers to their walls, and thus recording their activities. The syn-
chronism is so perfect that until recently it was generally believed to
be dependent on nerve connections; but Porter has shown that if the
ventricles are cut away from the auricles, in which the nerve mechan-
ism seemed to lie, the synchronism of the former is not interfered
with; that the apical halves of the ventricles will beat synchronously
if perfused with blood through an artery; that a very small bridge of
muscle-tissue will carry the wave of excitation from one part to neigh-

THE CIRCULATION OF THE BLOOD.

to manometer

mm valve

boring parts of the ventricle. It is therefore probable that the syn-
chronism is accomplished through muscle connections only. The
left ventricle, in keeping with the greater work it has to do, has a
greater development than the right, and therefore contracts more
energetically. The ratio between the energy of the left and right
sides is approximately 3 to 1.

Intra-cardiac Pressure. — It has been stated that during the
pause of the heart when its cavities are rilling with blood the semilunar
valves are kept closed by the pressure of the blood in the pulmonary
artery and aorta, a pressure clue to the resistance, as will be explained
later, offered to the flow of the blood mainly by the smaller arteries
and capillaries; that they are opened only when the pressure of the
blood within the ventricle exceeds that
in the arteries. It becomes, therefore,
a matter of importance to determine
the extent of this pressure as well as
its variations during the course of a
cardiac cycle. This can be done by
inserting a long catheter into either the
right or left ventricle, through the
jugular vein or the carotid artery re-
spectively, and connecting its free ex-
tremity with a mercurial manometer.
By the interposition of a double valve
such as represented in Fig. 135, it
becomes possible, according to the
direction the blood is permitted to
flow, to obtain either the maximal or
the minimal pressure that occurs in
the heart during a series of cycles.
Thus Goltz found in the left ventricle
of the dog a maximal pressure of 114
to 135 mm.; in the right ventricle,

Minimal pressures of — 2$ to — 52 mm. for the left ventricle have also
been obtained.

The maximal pressure in the ventricles during the systole, though
always higher than that in the arteries, is not a fixed or an invariable
pressure, as it rises and falls with the latter from moment to moment.
Within limits the cardiac power, and therefore the intra-cardiac
pressure, is capable of considerable increase. The function of the
heart is to drive the blood through the vessels with a given velocity.
This is only possible by first overcoming the resistance to the flow
offered by the vessels, as indicated by the arterial pressure. As this
is a variable factor, rising and falling very considerably at times, the
heart must meet and exceed each rise, if the circulation is to be main-
tained. The power put forth by the heart is proportional to the
work it has to perform. If the arterial pressure continues higher

TQ

to heart

Fig. 135. — v. Frank's Valve.
This is placed in the course of the
tube between heart and manometer,
so that the latter may be used as a
maximum, minimum, or ordinary
manometer according to the tap which
is left open. — (Starling.)

a pressure of 35 to 62 mm.

290

TEXT-BOOK OF PHYSIOLOGY.

than the average for any length of time, the heart meets the condition
by an hypertrophy of its walls.

The Intra-ventricular Pressure Curve. — An accurate interpre-
tation of the play of the heart mechanism necessitates the obtaining of
a graphic record of the course of the intra-ventricular pressure, its varia-
tions and time relations. With such a record may be compared the
records of the pressures in the venae cava?, on the one hand, and in the
aorta, on the other hand, and their relations one to another accurately
defined.

The intra-ventricular pressure has been obtained by specially de-
vised manometers or tonometers or tonography, as they are variously
termed, the construction of which is such as to enable them to respond
instantly to the very rapid variations of the pressure which occur
during the brief cardiac cycle. One of the best is that of Hiirthle.

This consists of a small metallic tam-
bour 5 or 6 millimeters in diameter,
covered by a thin rubber membrane.
A small button resting on the mem-
brane plays against an elastic steel
spring, by the tension of which the
pressure of the blood is counterbal-
anced. The movements of the mem-
brane are taken up, magnified, and
recorded by a suitable lever. A long
cannula is inserted into the right
ventricle through the jugular vein or
into the left ventricle through the
carotid artery. Both cannula and
tambour are filled with an alkaline
solution to prevent coagulation of the
blood, and then joined air-tight.
The pressure of the blood in the ventricle is thus transmitted by a
liquid column to the tambour and to its attached lever. With such
a manometer a curve is registered similar to that shown in Fig. 136.
To obtain the absolute value of this curve in millimeters of mercury
it is necessary to previously graduate the instrument. An examination
of the curve shows that previous to the ventricular contraction there
is a very slight rise of pressure above that of the atmosphere, repre-
sented by the line a -- b. This may be due to the inflow of blood
from the auricle during the diastole. At o the pressure suddenly
rises, passes quickly to its maximum value, (2), which is maintained
with slight variations for some time, and then suddenly (3) begins
to fall, and rapidly reaches the line of atmospheric pressure, or even
passes below it, becoming negative in fact for a short period. The
curve may also be taken as a record of the ventricular contraction,
for there are reasons to believe that the two closely coincide through-
out their entire course. A characteristic feature of this curve is the

Fig. 136. — V. Curve of the pres-
sure in the ventricle of the dog. —
(Hiirthle.) A. Curve of the pressure
in the aorta. The curves were taken
simultaneously, s. Tuning-fork vi-
brations, 100 per second.

THE CIRCULATION OF THE BLOOD. 291

more or less horizontal portion comprised between the points 2 and 3,
marked by several elevations and depressions, which has been termed
the systolic plateau.

With other forms of elastic manometers, especially those in which
the transmission of the intra-ventricular pressure is effected by air or by
a combination of air and liquid, this portion of the curve is represented
by a single peak, which is taken as an indication that the maximum
pressure once reached is not maintained, but immediately begins to
fall to its original level, notwithstanding the continued contraction of
the ventricle. Those who adhere to this view attribute the plateau
to the closure of the orifice of the catheter by the contracting and ap-
proximating walls of the ventricle. There are reasons for believing,
however, that the former curve is the more correct representation of the
course of the intra-ventricular pressure. Bayliss and Starling photo-
graphed on a moving surface the oscillations of a fluid, a solution of
sodium sulphate, in a capillary glass tube one end of which was closed,
the other end placed in connection with an intra-cardiac catheter, the
oscillations representing the variations in pressure. The photogram
thus obtained resembles the curve obtained by Hurthle's membrane
manometer.

The Relation of the Intra-ventricular Pressure Curve to the
Intra-cardiac Mechanisms. — By itself the curve of the intra-ventric-
ular pressure affords no indication as to events occurring within the
heart: i. e., as to the times during the systole, of the closure of the
auricolo-ventricular valves and the opening of the semilunar valves,
or the times during the diastole, of the closure of the semilunar valves
and the opening of the auriculo-ventricular valves.

By registering the curve of pressure in the aorta simultaneously
with the pressure in the left ventricle (Fig. 136), and by comparing
these with the curve of the successive differences of pressure in these
two cavities as determined by the "differential manometer," it be-
comes possible to mark on the ventricular pressure curve the points
at which the foregoing events take place.

During the systolic plateau the blood is passing from the ventricle
into the aorta. Independent of the slight elevations and depressions
there is an absolute fall of pressure between the beginning and the
end of the plateau. There is also a corresponding fall in the aortic-
pressure, corresponding to these two points. The curve of the dif-
ference of pressure shows, however, that the ventricular pressure is
slightly higher than the aortic. This fall in both ventricular and
aortic pressures is due to the escape of blood from the arterial into
and through the capillary system. At 3, however, whether completely
emptied or not, the ventricle suddenly relaxes, and its pressure soon
falls below that in the aorta. As soon as this takes place the semi-
lunar valves must close, if regurgitation into the ventricular cavity
is to be prevented. A comparison of the aortic pressure curve shows
a slight notch, the "dicrotic notch," just preceding a slight elevation,

292 TEXT-BOOK OF PHYSIOLOGY.

the "dicrotic" wave. This notch is taken as the moment when the
semilunar valves close. The corresponding point on the ventricular
pressure curve has been placed just where the ordinate 4 cuts the de-
scending portion. As yet, however, the pressure is higher in the ven-
tricle than in the auricle, and so continues until near the line of atmos-
pheric pressure. At this point the pressure in the auricle, due to the
accumulation of blood during the ventricular systole, now forces open
the mitral valve and the blood flows into the ventricle. The opening
of the mitral valve occurs about the point where the ordinate 5 cuts the
curve.

The ventricular pressure curve affords but slight, if any, indication
of the auricular systole. It apparently does not give rise to any
noticeable increase in the ventricular pressure. The slight rise in
the pressure curve, which just precedes the abrupt rise due to the
ventricular systole, may be taken as an indication of an increasing
pressure due to the inflow of blood from the auricle. As soon as the
pressure in the ventricle exceeds that in the auricle the mitral valve
closes. This is marked on the curve where the ordinate cuts it, at o.
Coincident with this, the ventricular systole begins, and as the blood is
contained within a closed cavity the pressure abruptly rises. A
comparison of the aortic curve shows that for a short time during
the ventricular systole, the pressure is falling, but at one point it turns at
a sharp angle and rapidly rises. This is an indication that the semi-
lunar valves are suddenly thrown open and the blood begins to pass
into the aorta. This event occurs at a moment marked on the ventric-
ular curve by the ordinate 1. Beyond this point the pressure continues
to rise, for the aortic pressure must not only be exceeded, but a certain
velocity must be imparted to the blood. Between the ordinates 1
and 4, the semilunar valves remain open and the blood passes into
the aorta.

In accordance with the foregoing
The ventricular systole may be subdivided into two periods:

1. The period of rising tension, from the beginning of the systole to

the opening of the semilunar valves, occupying from 0.02 to
0.04 second.

2. The period of ejection, from the opening of the semilunar valves

to the end of the systole, occupying about 0.2 second.
The ventricular diastole may also be divided into two periods:

1. The period of falling tension or relaxation, from the end of the

systole to the time of lowest pressure in the ventricle, occupying
about 0.05 second.

2. The period of filling, from the opening of the mitral valve to the

beginning of the systole.
Negative Pressure. — As shown by the ventricular pressure curve
there is a moment when the pressure falls below atmospheric pres-
sure, becoming negative to it. The extent to which this takes place,
its duration and frequency, have never been satisfactorily determined.

THE CIRCULATION OF THE BLOOD. 293

The cause of the negative pressure, its influence on the opening of
the auriculo-ventricular valves, and on the entrance of blood into the
ventricles are equally unknown. The most probable cause is an
expansion of the base of the ventricles due to the enlargement of the
aorta and pulmonary artery. That it is not due to the expansion
of the thorax is evident from the fact that it occurs when the thorax
is open and the heart exposed.

Heart-sounds. — Two sounds accompany each pulsation of the
heart, both of which may be heard by applying the ear or the stetho-
scope to the chest-walls, especially over the region of the heart. One
of these sounds is low in pitch, dull and prolonged; the other is high
in pitch, clear and short. These sounds can be approximately repro-
duced by pronouncing the syllables lubb-dupp, lubb-dupp. The
long dull sound occurs with the systole, the first phase of a new cardiac
cycle, and is therefore termed the -first sound; the short clear sound
occurs at the beginning of the diastole, with the second phase of the
cardiac cycle, and is therefore termed the second sound. The first
sound is the systolic, the second the diastolic, sound. With the ear
it can readily be determined that there is a brief pause between the
first and second sounds, and a longer pause between the second and the
first sounds. The duration of the first sound is almost equal to the
duration of the systole — viz., 0.3 second; the duration of the second
sound is not more than 0.1 second. The systolic sound is heard
most distinctly over the body of the heart; the diastolic sound is heard
most distinctly in the neighborhood of the third rib to the right of the
sternum.

The causes of the heart-sounds have enlisted the attention of
clinicians and physiologists for years, and many factors have been
assigned for their production. At present it is generally believed
that the first sound is the product of at least two, possibly three, factors :
viz., the contraction of the muscular walls of the ventricles, the simul-
taneous closure and subsequent vibration of the tricuspid and mitral
valves, and the sudden increase of pressure of the apex of the heart
against the chest-wall.

That the contraction of the ventricular muscle gives rise to a sound
is certain from the fact that it is perceptible in an excised heart when
the cavities are free from blood and when the valves are prevented
from closing. The explanation of this sound is extremely difficult, as
the contraction, though prolonged, is not of the nature of a tetanus and
therefore not characterized by rapid variations of tension. The apex
element may be eliminated by placing the individual in the recumbent
position.

The second sound is the product of the simultaneous closure and
subsequent vibration of the aortic and pulmonary valves which occurs
at the beginning of the ventricular diastole as the blood surges back
against the closed valves. This has been definitely proved by the fact
that the sound disappears when the valves are destroyed or held back

294 TEXT-BOOK OF PHYSIOLOGY.

by hooks introduced into the aorta and pulmonary artery. It is also
possible that the vibration of the column of blood produces an addi-
tional tone which adds itself to that produced by the valves.

The relation of the sounds to the systole and diastole of the heart
is represented in Figs. 133 and 137.

The Blood-supply to the Heart. — The nutrition of the heart,
its contractility, the force and frequency of the beat, are dependent
on and maintained by the introduction of arterialized blood into and
the removal of waste products from its tissue. This is accomplished
by the coronary arteries, on the one hand, and the coronary veins, on
the other. The arteries, two in number, the right and left, arise from
the aorta in the pouches of Valsalva just above the right and left
. semilunar valves. Turning in opposite direc-
tions, they ultimately anastomose, forming a
circle around the base of the ventricles. From
both the right and left artery branches are
given off which run over the walls of both
auricles and ventricles, the most important of
which in man are the anterior and posterior
inter-ventricular. These main vessels lie in
grooves on the surface of the heart beneath
Fig. 137. — Scheme of fae visceral pericardium, surrounded by con-
fnne^Sd' S Jhat active tissue and fat. Small branches pene-
events occur in the heart, trate the heart-muscle in which they divide
and the outer, the relation mt0 capillaries. From the capillary areas

of the sounds and snences n • • i • i • 1 1 1

to these events. small veins arise which, passing backward,

converge to form the coronary veins. These
follow the course of the arteries and finally terminate in the coronary
sinus, located in the auriculo-ventricular groove on the posterior
surface of the heart. This sinus opens into the right auricle between
the opening of the inferior vena cava and the auriculo-ventricular
opening. Its orifice is guarded by a valve, which is usually single,
though sometimes double.

While by far the larger portion of the blood is returned by the
coronary veins, it is also certain that some of it is returned by small
veins which open into little pits or depressions on the inner surface of
the heart-walls, known as the foramina Thebesii. It has, however, been
shown by Pratt that these foramina are present not only in the auricular
wall, as generally stated, but in the walls of all the cavities. These
foramina communicate through a capillary plexus with both arteries
and veins, and by special large passages with the veins alone.

The period of time in the cardiac cycle during which the coronary
(the extra-mural) arteries are filled with blood, whether during the
systole or the diastole, has been a subject of much discussion. At
present, however, as the result of many experiments it is generally
believed that they are filled at the time of the systole. A comparison
of the tracings of the pulse-wave taken simultaneously in the carotid

THE CIRCULATION OF THE BLOOD. 295

and coronary arteries shows that the pressure rises and falls simul-
taneously in both vessels; that there is a complete agreement between
the two tracings, and as a corollary both vessels are filled during the
systole. But because of the pressure which the heart-muscle must
exert upon the smaller arteries and veins within its own substance
during systole, it is probable that there is a freer circulation in the
coronary (the extra-mural) vessels, during the period of diastolic
repose.

During the systole the intra-mural vessels are compressed and the
blood driven out of the capillaries into the veins; during the diastole,
these vessels again dilate and permit the blood to re-enter freely from
the extra-mural arteries. The greater the force and frequency of
the beat, the greater the volume of blood passing through the coronary
system.

As stated in a foregoing paragraph the nutrition of the heart-
muscle, its irritability and contractility, depend on the blood-supply
derived from the coronary vessels. This is shown by the effects which
follow its withdrawal. Ligation of both coronary arteries in the dog is
followed by a diminution in the force and frequency of the heart-beat,
and in a few minutes by complete cessation. Ligation of even a single
branch of a coronary artery of the dog heart provided it supply a
sufficiently large territory — e. g., the arteria circumflexa — is sufficient
to cause arrest in at least 80 per cent, of animals (Porter). With the
ligation of this vessel there occurs a gradual diminution in the force and
frequency of the systole. As the power of coordinate contraction ceases
the heart-muscle frequently exhibits a series of independent contraction
of individual fibers and cells known as fibrillary contraction. All the
results which follow ligation are to be attributed in the light of experi-
ment to the sudden anemia which is thus established. The removal
of the ligature and the return of the blood will restore the nutrition and
re-establish coordinate contractions. The excised heart of the mammal
which has passed into the condition of fibrillary contraction may be
again made to beat rhythmically and vigorously, by first cooling it
with normal saline, and then perfusing it with warm defibrinated blood
through the coronary vessels under a suitable pressure. The same
result can be brought about by first perfusing it with a r per cent,
solution of potassium chlorid until the heart comes to rest and then
perfusing it with Ringer's solution.

In frogs and allied animals the heart is nourished by blood flow-
ing, during the diastole, from the interior of the heart into a system
of irregular channels which penetrate the walls in all directions.
With the systole the blood is returned to the cavities. The excised
heart of the mammal — e. g., the cat — may be partially nourished in
a similar manner through the foramina Thebesii. If the warm
defibrinated blood of the same animal be introduced into the ventricle
under a pressure of about 75 mm. of blood, the heart will recommence
and continue to beat for a period varying from one to several hours.

296 TEXT-BOOK OF PHYSIOLOGY.

The Beat of the Excised Heart.— The beat of the heart, its
frequency and regularity, its continuance from the early stages of
fetal development till death, has long been an interesting subject
for physiologic investigation. Though related to the functional
activities of the body at large, the activity of the heart is in a sense
independent of them, for it will continue for a variable length of time
after they have ceased. The heart of the frog and the turtle will
continue to beat under appropriate conditions for hours after separa-
tion of all its anatomic connections and removal from the body.
The heart of the dog or cat will, however, beat but for a few minutes.
The human heart would in all probability act in the same way. Never-
theless there are good reasons for believing that though the spontaneous
beat has ceased, the irritability yet endures though perhaps in lessened
degree. For if, after the heart has ceased to beat for some time, warm
defibrinated and oxygenated blood be passed through the coronary
vessels the beat will reappear and continue at its usual rate for some
hours.

The reason for the longer continuance of the beat of the excised
heart of the cold-blooded animal beyond that of the warm-blooded
animal lies probably in the difference in the rate of their respective
metabolisms. There is reason to believe that each cejl of the heart-
muscle, in common with other tissue-cells, during life stores up and
holds in reserve a larger quantity of nutritive material than is necessary
for its immediate needs. When separated from the general blood-
supply, the cells begin to utilize this reserved material. With its
consumption the irritability declines and after a variable period of time
disappears. As the metabolism is far more rapid in the warm-blooded
than in the cold-blooded animal, it is probable that the reserved
nutritive material is utilized more quickly in the former than in the
latter other conditions being equal. So long as it lasts in either class,
the irritability and contractility persist.

Whatever the immediate or exciting cause of the heart contraction
may be, the fundamental condition for its manifestation is the main-
tenance of the irritability. So long as this persists at a sufficiently
high level the heart-muscle will contract in response to the appropriate
stimulus.

PROPERTIES OF THE HEART-MUSCLE.

The heart-muscle is characterized by the following properties, viz.,
irritability, conductivity, rhythmicity, tonicity, and automaticity.
r. Irritability. — The heart-muscle in common with other muscles
possesses irritability by virtue of which it responds by a change
of form to the action of a stimulus. Whatever the stimulus,
here, as elsewhere, there is a conversion of potential into kinetic
energy — heat, electricity and mechanic motion. The normal
physiologic stimulus has not been positively determined. In

THE CIRCULATION OF THE BLOOD. 297

common with other forms of muscle-tissue, the heart-muscle
may be made to contract by artificial stimuli — e. g., mechanic,
thermic, chemic, and electric.

For the demonstration of this fact it is necessary to eliminate the
action of the physiologic stimulus and to bring the heart to rest
in the condition of diastole. This can be done with the frog
heart, by ligating the tissues between the sinu-auricular junction,
a procedure which prevents the passage of the contraction wave
which originates in the sinus, over the auricles and ventricles
(a fact that will be more fully alluded to later). With the heart
thus prepared and while still in situ, the apex may be connected
with a recording lever and its provoked contractions registered
on a recording surface. In this condition it will respond by a
contraction to any form of an adequate stimulus and especially
to the induced electric current. Experiment has shown that the
irritability is most marked in the neighborhood of the venae cava?
terminations. It is least marked in the ventricles.

In its irritability, contractility, and manner of response to
stimuli, the heart of the mammal corresponds in all essential
respects to the heart of the frog or turtle. •

The irritability of the heart-muscle depends primarily on the
blood-supply and secondarily on the maintenance of a normal
temperature, and so long as both conditions are maintained the
muscle will respond by a contraction to any adequate stimulus,
physiologic or artificial.

a. The Blood -sup ply. — The supply of blood to the mammalian
heart is derived from the coronary arteries which, though filled
during the systole, deliver the blood to the intra-mural arterioles
and capillaries during the diastole. The facts relating to the
blood-supply have been presented fully in a foregoing paragraph.

b. The Influence of Temperature. — For the manifestation of the
irritability and contractility it is essential that the heart-muscle
be kept at a sufficiently high temperature in order that the physio-
logic or a given artificial stimulus may provoke a maximal con-
traction. This is accomplished by immersing the suspended
heart in a bath of Ringer's solution the temperature of which
can be readily decreased or increased by appropriate means.
The optimum temperature for the frog heart is about 2^° C.
As the temperature is lowered both rate and force decrease until
at about from 40 C. to o° C.both cease. Beyond 3 5 ° C . it also
ceases to contract, because of a coagulation of the muscle sub-
stance. The mammalian heart attains its maximum activity
at a temperature of 370 C. It ceases to beat at about 470 C.
on the one hand and at about 170 C. on the other hand.

2. Conductivity. — The heart-muscle possesses conductivity. The
excitation process and the subsequent contraction wave, both of
which take their rise under physiologic conditions near the venae

298 TEXT-BOOK OF PHYSIOLOGY.

cavae terminations, are conducted over the auricles, thence to the
ventricles from base to apex. It is now generally believed that
the propagation of both processes is accomplished by muscle-
tissue alone, independently of the nerve system. The conduc-
tivity, however, is not equally well developed in every part of the
heart. This is especially true of the tissue at both the sinu-
auricular and the auriculo-ventricular junctions. At these points
in the frog heart the contraction wave is delayed for an appreciable
period, a condition attributed to the embryonic character of the
muscle-tissue. In the frog's heart the excitation process begins
in the sinus venosus, from which it passes to the auricles, thence
to the ventricles. In Fig. 138, which is a graphic record of the

heart-beat, the two elevations of the
level on the up-stroke, a and b, repre-
sent the contraction of the sinus
and the auricle respectively, while
the two depressions, c and d, indi-
cate the delay in the transmission
of the contraction wave at the two
junctions. There is here an ana-
tomic obstacle to the conduction of
the contraction wave. This may
Fig. 138.— Record of the ^ artificially increased by com-

CONTRACTION OF THE FROG'S j \

Heart. pressing the heart between the au-

ricles and ventricles with a clamp.
By carefully regulating the pressure it is possible to so block the
wave that three or four auricular contractions may occur before
the excitation process forces the block and excites a ventricular
contraction. (Fig. 139.) If the block is complete, rather than
partial, the ventricle will come to rest and so remain. From
the foregoing facts it is evident that the physiologic stimulus
exerts its action in the sinus venosus and that the auricular and
ventricular beats are in turn dependent on it.

In the mammalian heart the contraction wave arising at the
terminations of, or at a point between the terminations of, the
venae cavae, is likewise conducted over the auricles and thence to
the ventricles; between the end of the auricular and the beginning
of the ventricular contraction there is also a perceptible interval
similar to that observed in the frog heart. For a long time this
was attributed to an interference with the passage of the contrac-
tion wave across the auriculo-ventricular junction because of the
extreme scarcity of the muscle-fibers in this region or to their
embryonic character. In recent years, however, this view has
been abandoned because the real bond of union between the
auricular and ventricular tissues, across which the contraction
wave passes, has been found, as stated on page 280, in the system
of muscle-fibers, described in part by His, Retzer and Braunig

THE CIRCULATION OF THE BLOOD. 299

and completed by Tawara and termed by him the conduction
system of the heart. This system it is now believed constitutes
the anatomic and physiologic path across which the contraction
wave passes from auricles to ventricles. The supposition that
this was the case was demonstrated by Hering and others who
succeeded in dividing the muscle-bundle in the excised hearts
of rabbits and dogs, kept actively beating by perfusion with
Ringers' solution. On division of the bundle both auricles and
ventricles continued to beat though with different rates and in-
dependently of each other. These and experiments of a similar
character have demonstrated beyond question that the auriculo-
ventricular bundle with its widespread ramifications is the true

Fig. 139. — Record of the Auricular and Ventricular Contractions before
and after the closure of the clamp at a.

conducting system between auricles and ventricles. The cause
assigned by Tawara, for the interval between the auricular and
ventricular contraction is not so much the embryonic character
of the fibers of the system, as it is the length of the system as a
whole, which he estimates at from 4 to 6 centimeters. Because
of this fact, the excitation process requires time to pass from the
beginning to the ends of the system and to all parts of the ventricles,
which time is represented by the inter-systolic period or interval.
With the mammalian heart as with the frog heart it is possible
to increase the length of the interval between the auricular and
the ventricular contraction, the inter-systolic period, by com-
pression of the tissues between auricles and ventricles including
presumably the central part of the conducting system, the muscle
band of His. This has been accomplished in the dog by Erlanger
by means of a specially devised hook clam]). When the com-
pression is brought about suddenly and completely the ventricles
at once cease beating though the auricles continue to beat with
their customary rate and regularity. After a variable period of
time, varying from a few seconds to 70 seconds, during which the
ventricles are relaxed and gradually filling with blood from the

■. rate per minute.

V

216
117.8
162. 1

3-7
2.4
3-°5

TEXT-BOOK OF PHYSIOLOGY.

auricles, the ventricular beat returns, at first slowly but with a
gradually increasing frequency until a definite but a comparatively
slow rate is attained.

In experiments on the dog heart performed by Erlanger the
following results were obtained when the auriculo-ventricular
bundle was completely crushed.

Ven. rate per minute.

Max. 69.8
Min. 34.8
Ave. 54.1

The reason assigned for the cessation of the ventricular contrac-
tion is the non-arrival of the excitation process at the ventricular
end of the conducting system, because of the blocking or com-
pression. Under physiologic conditions the ventricular beat is
directly dependent on the arrival of the excitation process from
the auricles and if it fails to arrive the ventricle does not con-
tract for some seconds. The return of the beat during complete
blocking is attributed to the development of a hitherto dormant
inherent rhythmicity. When this is established both auricles and
ventricles continue to beat though with a totally different rhythm.

The effects which follow gradual compression of the muscle-
bundle are somewhat different from those which follow sudden
compression. If the clamp is accurately adjusted and the com-
pression gradually applied, the first perceptible effect is a length-
ening of the normal pause between the auricular and the ventric-
ular contraction. With an increase in the compression there
will come a moment when one of the auricular contraction waves
fails to reach the ventricle, or if it does, it is so enfeebled that
it is incapable of exciting the ventricle, which in consequence
fails to contract. This dropping out of a ventricular contraction
may occur once in every 10, 9, 8, 7, 6, etc., auricular beats, in
accordance with the degree of compression. With a further
tightening of the clamp, the blocking of the excitation process
may be still further increased so that only every second, third or
fourth auricular beat will be followed by a ventricular beat, es-
tablishing what has been termed the 2:1, 3:1, 4:1, rhythms re-
spectively; and finally when the blocking is complete no excitation
process can reach the ventricle.

Owing to the capability of the mammalian ventricle to develop
an independent rhythm when not stimulated by the auricles for
a few seconds or more, it is not always possible to state at what
particular moment in the successive stages of compression the
independent ventricular rhythm becomes manifest. Usually
when the rhythm is of the 3 : 1 type, i. e., when the third auricular
contraction fails to reach the ventricle, it will begin to beat of
itself. Under such circumstances the auricles and ventricles
become dissociated even though the block is not quite complete.

THE CIRCULATION OF THE BLOOD. 301

These experimental facts have afforded an explanation of the
altered rhythm between auricles and ventricles in that pathologic
condition known as Stokes-Adams disease. In this disease the
rhythm may be any one of the rhythms stated in the foregoing
paragraph. In two instances the following ratio of the ventricle
to the auricle was observed by Erlanger.

Ven. rate per minute. Aur. rate per minute. ^

22.4 79.6 3.55

31.0 84.6 2.73

In a few cases of death from this disease a postmortem ex-
amination showed a pathologic lesion of the auriculo-ventricular
bundle.

3. Rhythmicity. — The beat of the heart is a uniform movement,

occurring at regular intervals. Each phase of each beat occupies a
regular measure of time. The beat is therefore rhythmic in char-
acter. The heart-muscle as a whole varies in rhythmic power in
its different parts. It is best developed in the frog and tortoise,
in the sinus venosus, less so in the auricles, least in the ventricles.
This may be shown by division of the tissue between sinus and
auricles in situ. At once the auriculo-ventricular portion ceases
to beat, while the sinus continues contracting as usual. In a
short time the auricles and ventricles begin to beat, though less
rapidly than formerly. Separation, of the auricle from the ven-
tricle is again followed by rest. In due time the auricle begins
to beat, while the ventricle remains quiescent. If the ventricle
be now stimulated in a rhythmic manner, it may resume rhyth-
mic activity. These facts are taken as an indication that the
rhythmic power is developed in unequal degree in the three
divisions of the heart.

In the warm-blooded animal, e. g., dog, cat, rabbit, there is
also a difference in the rhythmicity between the auricles and
ventricles. This is shown by the effects which follow division of
the auriculo-ventricular bundle, or sudden and complete com-
pression of the heart at the auriculo-ventricular groove. In
either case the ventricle for a short time remains at rest, though
the auricles continue to beat at their usual rate. After a variable
number of seconds the ventricle develops a rhythm of its own,
though it never attains that of the auricle.

4. Tonicity. — The heart-muscle, like the vascular muscle, main-

tains continuously a certain degree of contraction, termed tone,
upon which the efficiency of the heart as a pumping organ is
largely dependent. In the physiologic condition the ventricular
muscle neither contracts nor relaxes to its fullest possible extent,
but maintains an intermediate position between the two extremes
and for this reason is said to possess tonicity. This tone may,
however, be increased or decreased by the action of various ex-

3o2 TEXT-BOOK OF PHYSIOLOGY.

ternal agents. Thus the passage of dilute solutions of various drugs
— e. g., alkalies, digitalis — through the cavities of the excised heart
will so increase the tone, or the contractile power, that complete
relaxation is prevented, until finally the heart comes to a stand-
still in the condition of systole. The passage of dilute solutions
of lactic acid, muscarine, etc., through the heart will, on the con-
trary, so decrease the tone or the contractile power that the nor-
mal contraction is not attained. The relaxation therefore gradu-
ally increases until the heart finally comes to a standstill in the
condition of diastole. In the first instance the tonicity is said
to be increased; in the second instance, decreased.

5. Automaticity. — Inasmuch as the heart continues to contract in
a perfectly rhythmic manner after removal from the body and
apparently without the aid of an external stimulus, it is said that
the heart-muscle is automatic or spontaneous in action. Strictly
speaking, however, this is not the case, for the reason that all
movement, that of the heart included, is the resultant of the action
of natural causes though their true nature may be beyond the
reach of present methods of investigation,
The Nature of the Stimulus.— As the heart continues to beat

after removal from the body, it is evident that the stimulus does not

originate in the central nerve system but in the heart itself. Two

views have been held as to its origin and nature:

1. That it originates in the nerve-cells found in various parts of the

heart-muscle; that it is a nerve impulse rhythmically and auto-
matically discharged by these cells and transmitted by their
axons to the heart-muscle cells.

2. That it originates in the muscle-cells themselves; that it is chemic in

character and due to a reaction between the chemic constituents,
organic and inorganic, of the muscle-cells and those in the lymph
by which they are surrounded.

According to the first view the stimulus is neurogenic, according
to the second view myogenic, in origin.

The presence of nerve-cells; their relation to the muscle-cells; the
pronounced rhythmic activity of the sinus and auricles in which the
nerve-cells are abundant; the feeble activity of the apex, in which they
are wanting — these and other facts lend support to the view that the
stimulus originates in the nerve-cells. To them have been attributed
the power of automatic activity.

The absence of nerve-cells in portions of the heart-muscle, which
nevertheless exhibit rhythmic contractions for quite a long period
of time; the rhythmic beat of the embryonic heart before the migration
of nerve-cells to its walls shows, that the stimulus does not necessarily
originate in nerve-cells. Moreover, Porter has conclusively shown
that the apex of the dog's heart, which is generally believed to be
totally devoid of nerve-cells, can be made to beat for hours by feeding
it through its nutrient artery with warm defibrinated blood. Unless

THE CIRCULATION OF THE BLOOD. 303

it be assumed that the heart-muscle contracts automatically, without a
cause, it is a fair assumption that the exciting cause of the contraction
arises within the muscle-cells themselves, and that it is in all proba-
bility the outcome of a reaction between the chemic constituents
and more especially the inorganic constituents of the blood or lymph
on the one hand, and the chemic constituents of the muscle-cells on
the other. The discovery that some of the inorganic salts of the blood
have a specific physiologic action on the heart-muscle was made in
1882 by Ringer. Since then, many attempts have been made to
isolate these constituents, to determine not only their individual, but
also their collective action, when combined in proportions approxi-
mating those in which they exist in the blood.

The Action of Inorganic Salts. — 1. On the Frog and Terrapin
Heart. — The inorganic salts which are most directly concerned in ex
citing and sustaining the heart-beat are sodium chlorid, calcium phos-
phate or chlorid, and potassium chlorid. A combination of these
salts in the proportions in which they exist in the blood was first sug-
gested by Ringer and is made by saturating a 0.65 per cent, solution
of sodium chlorid with calcium phosphate, and then adding to each
100 ex., 2 c.c. of a 1 per cent, solution of potassium chlorid. A frog's
heart immersed in this solution will continue to beat for some hours.
A combination of the chlorids of sodium, calcium, and potassium
in amounts which will vary for different animals is equally efficient
in maintaining the heart-beat.

The collective as well as the individual actions of these salts have
been strikingly brought out by the experiments of Profs. Howell and
Greene, from whose published results the following statements are
derived. Instead of employing the entire heart, they used for various
reasons strips from the terminations of the venae cava? and from the
ventricle of the terrapin heart. The proportion of the inorganic salts
most favorable for the contraction of the vena cava strips is the follow-
ing: viz., sodium chlorid, 0.7 per cent.; calcium chlorid, 0.026 per
cent.; potassium chlorid, 0.03 per cent. When vena cava strips are
immersed in this solution, they begin in a short time to exhibit rhythmic
contractions which may continue for several days. In the same
strength of solution the ventricular strips remain inactive but if the
percentage of the calcium chlorid be raised from 0.026 per cent, to
0.04, or 0.05 per cent., spontaneous contractions soon develop and
continue for several days or more. In the foregoing solution when the
calcium chlorid is present only to the extent of 0.026 per cent., though
the ventricular strip does not contract, it is kept in good condition for
contraction, for even after many hours the raising of the percentage
of calcium chlorid to 0.04 or 0.05 per cent, will call forth after a brief
latent period, rapid and energetic contractions. From this fact it is
inferred that the vena? cava? region is more sensitive to the combined
action of the salts than is the ventricle.

The action of the individual salts is also best shown with ventricular

304 TEXT-BOOK OF PHYSIOLOGY.

strips. In a 0.7 per cent, sodium chlorid solution the strip beats
rhythmically and energetically, but for a short period and with gradually
diminishing force, until it entirely ceases to beat. A reason assigned
for this is the removal of other salts necessary to the excitation of the
contraction. In a calcium chlorid solution — 0.9 per cent. — i. e.,
isotonic with the sodium chlorid — the heart strip is thrown into strong
tone, but does not rhythmically contract. If, however, the strip is
placed in normal saline, and calcium chlorid added in amounts equal
to that present in the blood, it will after a very short latent period
begin to contract rapidly and energetically and for a longer time than
when in sodium chlorid solution alone. The contractions not infre-
quently occur before relaxation is completed, so that the strip passes
into the condition of contracture.

In potassium chlorid solutions isotonic — 0.9 per cent. — with
sodium chlorid solution the heart strip also fails to contract. This
is the case also when the potassium is added to the sodium chlorid
in amount practically equal to that found in the blood.

2. On the Mammalian Heart. — The collective action of the in-
organic salts on the isolated heart of all members of this class of animals,
which have been made the subject of experimentation, is as marked, if
not more so, than it is on the heart of the frog or terrapin especially
when the coronary blood-vessels are perfused with Ringer's solution
or the modification of it suggested by Locke, as follows: NaCl 0.90
per cent.; CaCl2 0.024 Per cent.; KC1 0.042 per cent.; NaHC03 0.02
per cent., dextrose 0.1 per cent. The reviving and sustaining power
of this solution is extraordinary. Locke and Rosenheim were able to
revive the isolated heart of a rabbit and to excite it to active contrac-
tion, for several hours at a time, on four consecutive days by perfusing
it with this solution saturated with oxygen and at a temperature of
350 C. No special precautions were observed other than keeping it
cool (io° C.) and moist during the intervals of experimentation. The
duration of the irritability and contractility extended over a period
of 95 hours. Kuliabko revived the heart of a rabbit for an hour
nearly three days after removal from the body of the animal. It was
then placed on ice, and after four days it was again revived by per-
fusing it with Ringer's solution. Altogether this heart retained its
irritability for seven days. Hering revived the heart of a monkey on
three different occasions, the first, 4^ hours, the second, 28 hours, and
the third 54 hours after the death of the animal. In the intervening
periods the heart was also kept on ice. In this animal it was even
possible to increase and decrease the activity of the heart by stimula-
tion of the nerves which normally control the rate of the beat. Kuliabko
was also able to revive the isolated heart of a child 20 hours after death
from a double pneumonia. It was made to beat rhythmically at a
rate varying from 70 to 80 per minute when the solution had a tem-
perature of 390 C, and at a rate of 98 to 102 per minute when it had a
temperature of 41 ° C, though at this temperature the beat became

THE CIRCULATION OF THE BLOOD. 305

arrhythmic. All these instances demonstrate the extreme longevity
of the irritability of the heart-muscle under appropriate conditions.

The action of individual salts has been shown experimentally on
the hearts of rabbits, cats, dogs, monkeys, by Gross, Howell and others.
Thus it has been found that when an isolated heart is rhythmically
beating in response to the perfusion of Ringer's or Locke's solution,
the addition of potassium chlorid in small amounts is followed by a
decrease in the rate and force of the contraction, and in larger amounts
by a complete cessation of the contraction and a standstill in diastole.
On the withdrawal of the potassium, the former frequency and vigor
are regained. Potassium exerts a depressor or an inhibitor influence
on the irritability and contractility of the heart- muscle.

Under the same conditions, the addition of calcium chlorid in
sufficient amounts is followed by an increase in the rate and in the
vigor of the contractions; on its withdrawal both rate and force return
to the previous condition. Calcium exerts an accelerator and an aug-
mentor influence on the irritability and contractility of the heart.

The Cause of the Heart-beat. — From the foregoing facts it seems
probable that the heart-beat is connected with and dependent on
the presence and interaction of the inorganic salts present in the
lymph, though as to the manner in which they interact to initiate the
beat, there is some obscurity. A very plausible theory as to the part
played by the inorganic salts in initiating the contraction and one in
accordance with the facts has been presented by Howell as follows:

The heart-muscle, it is assumed, contains a stable organic energy-
yielding compound of which potassium is one of its constituents and
on which its stability depends. This compound must be present in
relatively large amounts as the heart will continue to contract and
expend energy for many hours after the blood-supply has been with-
drawn.

During the diastole a reaction takes place between this compound
and the calcium or the calcium and the sodium salts, whereby a portion
of the organic compound is freed from potassium and is then com-
bined with calcium, or with calcium and sodium. In consequence,
this portion of the organic compound in combination with the calcium
acquires and gradually increases in instability, reaching its maximum
at the end of the diastole, when it undergoes a dissociation giving rise
to a chain of events that culminate in a contraction. The initial
step, therefore, is a dissociation of a complex unstable molecule followed
by an oxidation of the dissociated products. That an active dissocia-
tion of some character takes place is evident from the consumption
of oxygen, the production of carbon dioxid, the liberation of heat,
electricity and mechanic motion.

Inasmuch as the contraction is always maximal and as the heart
is refractory to a stimulus during the systole, the probabilities are
that all of the energy-yielding unstable compound is dissociated with
each contraction. With the relaxation there is a renewal of the unstable

306 TEXT-BOOK OF PHYSIOLOGY.

molecules along the lines stated above, which increase in number
and instability until the maximum is again attained when another
dissociation occurs followed by another contraction. The rhyth-
micitv of the heart's action, the appearance of a refractory condition
during the systole and its gradual disappearance during the diastole,
as well as other phenomena, are readily explained by the foregoing
hypothesis.

The cause of the dissociation of the energy-yielding material is,
however, a subject of discussion. According to Howell it is not
necessary to assume the presence of any cause other than the extreme
instability of the organic compound in question. According to
Engelman, Langendorff and others, the dissociation is not spontaneous
but is the result of the action of a specific stimulus, an "inner stimu-
lus," arising within the muscle elements themselves through metabolic
processes; and so long as these processes are chemically and physically
conditioned by blood or tissue fluids containing the inorganic salts,
so long will this stimulus be produced. As to the nature of this stimulus,
whether chemic, electric or enzymic, nothing definite can be stated at
present.

The Response of the Heart to the Action of a Stimulus.—
The heart of the frog as well as of some other animals may be brought
to a standstill by the ligation of the tissues between the sinus veno-
sus and the auricle, a procedure first introduced by Stannius and
now known as the first Stannius ligature. Under such circumstances
the heart may be made to contract by stimulating it with the single
induced current. With each passage of the current the heart con-
tracts. Contrary to what is observed in other muscles, the heart-
muscle, if it contracts at all, at once reaches its maximal value. Any
increase in the strength of the stimulus above the threshold value has
no greater effect on the extent or force of the contraction than the
minimal stimulus. A conclusion which may be drawn from this
fact, according to Engelman, is as follows: By reason of the
fact that the heart contracts at its maximum value to the action of
any strength of stimulus, under given conditions, there is always en-
sured a complete emptying of the ventricular contents and a uni-
form discharge of blood into the arteries, which would not be the
case if the extent of the contraction varied with the strength of the
stimulus; and there are reasons for believing that the normal stimu-
lus for the contraction varies within wide limits above the threshold
value both in normal and abnormal conditions of the heart. The
changes in the extent or force of the contraction are the result, not of
changes in the intensity of the stimulus, but of changes in the heart-
muscle, caused by variations in mechanical resistances.

The periodicity of the heart's action or its rhythm may also be
elucidated by the foregoing fact. There are reasons for believing
that at the time of the contraction practically all of the available
energy-yielding material is completely utilized, after which the heart

THE CIRCULATION OF THE BLOOD. ' 307

relaxes and remains at rest in the diastolic condition for a given period;
and before a second excitation wave can be developed and pass from
the sinus over the heart there must be a re-accumulation of energy-
yielding material, and a restoration of the irritability. This is accom-
plished during the diastole. By virtue of this fact the heart can not
act otherwise than in a periodic or rhythmic manner.

Inasmuch as there is a conversion of all of the potential energy
into kinetic energy during the systole, there is of necessity, a lowering of
the irritability, and to so great an extent is this the case that the heart
will not respond to the action of a second stimulus either physiologic
or artificial during the systolic period. This non-responsiveness of
the heart may be shown by throwing into it a second stimulus at any
moment during the systole. Whatever the moment or whatever the
strength of the stimulus may be the extent of the contraction remains
the same. During the systolic period the heart is said, therefore, to
be refractory to a second stimulus. If,
however, a second stimulus of average
strength be thrown into the ventricle at
any moment during the relaxation, a
second contraction or extra systole will be
developed. The extent of this contraction coJ^^' Tnd^h/com-
will be proportional to the time at which pensatory Pause. The break
the stimulus is thrown into the ventricle as in the horizontal line indicates

, ,, , ,, , r the moment the electric cur-

it passes from the beginning to the end of rent passes through the heart.

its relaxation. Whatever the extent of the

extra contraction it superposes itself on the first, though its height is no
greater than the first. For this reason it is believed a tetanic con-
traction can not be developed. If the stimulus be thrown into the
heart just as the relaxation is completed, the extra contraction attains
the same height as the preceding contraction. In passing from the
beginning to the end of the relaxation and into the diastolic or resting
period, it has been found that the extra contraction can be provoked
by a stimulus which is steadily decreased in intensity. It is evident
from this fact that the restoration of the energy-yielding material and
the return of the irritability gradually increases from the beginning of
the relaxation to the end of the diastole. For this reason weak stimuli
are more effective in the later than in the earlier period of the relaxa-
tion and the diastole.

After the development and disappearance of the extra contraction
a considerable pause in the heart's action occurs to which the term
compensatory pause has been given (Fig. 140), on the assumption that
it was necessary on the part of the heart to compensate for the dis-
turbance of the rhythm by remaining at rest until the time of the next
beat and thus restore the rhythm. This was thought to be a special
property of the heart-muscle. This view, however, is no longer enter-
tained. For if an isolated ventricle of a frog heart be employed and
made to contract rhythmically by an artificial stimulus, or if a sponta-

308 TEXT-BOOK OF PHYSIOLOGY.

neously beating portion of the dog's heart be employed for experi-
mentation instead of the whole heart, the results of the same methods
of stimulation are different. Though an extra contraction is called
forth as usual, there is no compensatory pause; indeed, if anything the
pause is shorter than the regular pause. The theory of a necessary
compensation is unnecessary.

The explanation assigned and generally accepted at present for
the production of a compensatory pause is as follows: In a sponta-
neously beating heart the ventricular contraction is provoked by the
arrival of an excitation process coming from the auricles. When the
extra contraction is induced by an artificial stimulus, the next succeed-
ing excitation from the auricle falls into the refractory period and
hence the ventricle is not stimulated. It, therefore, simply waits
for the arrival of the third succeeding excitation, when it responds and
takes up the regular rhythm.

If a series of successive stimuli be thrown into the heart-muscle
the effect will vary in accordance with their time intervals. Should
this be less than about three seconds there will be a gradual increase in
the height for some half dozen contractions, a result to which the term
"staircase" or "treppe" has been given. This increase in the height
of the contraction is attributed to an increase in the irritability and con-
tractility of the muscle the result of the primary stimulating action of
fatigue products.

THE NERVE MECHANISM OF THE HEART.

By this term is meant a combination of nerves and nerve-centers
which cooperate to increase or decrease either the rate or force — or
both — of the heart's contraction in accordance with the needs of the
system. That the heart is normally influenced by the central organs
of the nerve system in response to the action of nerve impulses re-
flected to them from many organs of the body is a matter of personal
experience; that it is abnormally influenced by the same or other organs
in response to nerve impulses reflected to them in consequence of
pathologic and traumatic processes, occurring in different regions of
the body, and that both heart and nerves are modified in different
ways by the action of drugs introduced into the body, are matters of
daily clinical experience.

The nerves comprising this mechanism and the relation they bear
one to another are represented in Fig. 141.

It was stated in a previous paragraph, page 296, that the con-
traction of the heart-muscle is independent of its connection with
the central organs of the nerve system, and that it will continue to
contract in a rhythmic manner for a variable length of time even
after its removal from the body of the animal, the length of time
varying with the animal and the conditions to which it is subjected;
that the stimulus is myogenic in origin and chemic in character, the

THE CIRCULATION OF THE BLOOD.

3°9

result -of '-.a^ reaction between the chemic constituents, organic and
inorganic, of the muscle-cells and those in the lymph by which they are
surrounded. It has also been further shown that even in the living
animal the .heart will continue to beat and fulfil its functions after

Emotional Centers

Exhilarating (blue)
Depressing f#Eo)

Cardio -Inhibitor Center.

Ganglion Stella turn

Intra-CardiacNerue Cells\

Cardio -Accelerator Center

VagasNerve

Jef „AExcitutor (blue)
fffennt [Inhibitor (red)

Sympath etic Nerves

Accelerator <£ Aug men tor

Fig. 141. — Diagram of the Nerve Mechanism of the Heart. — (G. Bachman.)

division of all nerves in connection with it. A dog thus experimented
on lived for eleven months, and beyond the fact of becoming fatigued
more readily upon exertion than formerly, exhibited no striking dis-
turbance of his functions. Nevertheless groups of nerve-cells are

3io TEXT-BOOK OF PHYSIOLOGY.

present in certain portions of the heart in all classes of vertebrate
animals, which bear an anatomic and physiologic relation to the heart-
cells on the one hand, and to the nerves connecting them with the
central organs of the nerve system on the other hand.

Intra-cardiac Nerve-cells. — In the frog heart a group of nerve-
cells is found in the sinus at its junction with the auricle, known as
the crescent or ganglion of Remak; a second group is found at the
base of the ventricle on its anterior aspect, and known as the gan-
glion of Bidder; a third group is found in the auricular septum, known
as the septal ganglion, or v. Bezold's or Ludwig's. The majority of
the cells are situated on the surface of the heart just beneath the
pericardium. From the cell-body fine non-medullated fibers pass into
the substance of the heart, to become histologically and physiologically
related with the muscle-fiber.

In the dog heart and in the mammalian heart generally, though
nerve-cells are present, they are not arranged in such definite groups,
but are more widely distributed in the terminations of the venae cava?,
pulmonary veins, the walls of the auricles, and in the neighborhood
of the base of the ventricles.

Extra-cardiac Nerves. — The extra-cardiac nerves which connect
the heart with the central nerve system and through which the activities
of the heart are influenced are two: viz., the sympathetic and the vagus
or pneumogastric. Experimental investigation has established the
fact that the sympathetic is the motor nerve to the heart, the nerve
which accelerates the rate and augments the force of the natural beat;
while the vagus is the inhibitor nerve, the nerve which inhibits or con-
trols the rate and the force of the beat in accordance with the necessi-
ties of blood distribution. For this reason these two nerves will be
considered in the following order. The course of the fibers compos-
ing these nerves, from their origin to their termination, and the relation
they bear to one another and to neighboring structures, vary some-
what in different animals.

The Origin and Distribution of the Sympathetic Nerves in
Mammals. — The sympathetic nerve-fibers which influence the action
of the heart, are connected on the one hand with the heart-muscle
itself and on the other hand with nerve-fibers coming from the central
nerve system. The former are non-medullated and post-ganglionic,
the latter medullated and pre-ganglionic.

The pre-ganglionic fibers have their origin in the medulla oblongata
and very probably from nerve-cells in the gray matter beneath the
floor of the fourth ventricle. From this origin they descend the spinal
cord as far as the level of the second and third thoracic nerves. At
this level they emerge from the cord in company with the nerve-fibers
composing the anterior roots of the second and third thoracic nerves.
After a short course, they enter the white rami communicantes, enter
the sympathetic chain and pass upward to the ganglion stellatum,
and by way of the annulus of Vieussens to the inferior cervical ganglion

THE CIRCULATION OF THE BLOOD. 311

as well around the nerve-cells of which their terminal branches arborize.
From the nerve-cells of both the stellate and inferior cervical ganglia,
the sympathetic nerves proper arise which after emerging from the
ganglia pass towards the heart and become associated with the fibers
of the vagus and assist in the formation of the cardiac plexuses. On
reaching the heart they may terminate directly in the muscle-cell
or indirectly through the intermediation of intra-cardiac nerve-cells.
The former mode of termination is the more probable. Experiment
has shown that both the pre- and post-ganglionic fibers are efferent
in function.

The Origin and Distribution of the Vagus Nerve in Mammals.
— The vagus nerve-fibers which influence the heart are connected on
the one hand with the heart through the intermediation of the intra-
cardiac cells, and on the other hand with the central nerve system.
Histologic investigation has shown that the vagus nerve-trunk of man
and mammals generally, contains medullated fibers of large and small
size. Experiment has shown that the large fibers are afferent, the
small fibers efferent in function.

The large afferent fibers arise in the ganglia situated on the trunk
of the nerve. From their contained nerve-cells a short axon process
proceeds which soon divides into a central and a peripheral branch.
The central branch passes toward and into the gray matter beneath
the floor of the fourth ventricle where its end-tufts arborize around
nerve-cells; the peripheral branch passes toward the general periphery
to be distributed to the mucous membrane of the lungs, stomach,
intestine, etc. The small efferent fibers are the peripherally coursing
axons of nerve-cells situated in the gray matter beneath the floor of
the fourth ventricle at the tip of the calamus scriptorius. The exact
course of these fibers from their origin into the trunk of the vagus
is not positively known. According to some investigators, they leave
the medulla by way of the spinal accessory nerve and enter the trunk
of the vagus through its internal or anastomotic branch; according to
recent investigations made by Schaternikoff and Friedenthal, they
leave the medulla along the path by which the afferent fibers enter
and never become associated with the spinal accessory nerve at its
origin.

In the neighborhood of the inferior or recurrent laryngeal nerves,
branches containing efferent fibers are given off, which pass to the
heart by way of the cardiac plexus. The terminal branches of these
fibers are not distributed directly to the heart-muscle, but to the intra-
cardiac nerve-cells, around the bodies of which they end in basket-
like formations. The fibers in the vagus are pre-ganglionic; those
of the nerve-cells post-ganglionic. (See Fig. 142).

In the frog and allied animals the relation of these two sets of
nerve-fibers, viz., the efferent sympathetic fibers and the efferent
vagus fibers, is somewhat different; and because of the fact that these
nerves in this animal are largely employed for determining experi-

312

TEXT-BOOK OF PHYSIOLOGY.

thetic Neuron

mentally their respective actions on the heart, this relation should be
clearly understood.

The sympathetic nerve-fibers in this animal are also in connection
with the heart on the one hand and with nerve-fibers coming from
the central nerve system on the other hand. The pre-ganglionic
fibers take their origin very probably in nerve-cells in the medulla
oblongata. From this origin they descend and emerge from the
spinal cord in the anterior roots of the third spinal nerve, then pass
through the white rami communicantes to the third sympathetic
ganglion around the nerve-cells of which their terminal fibers arborize.
From the nerve-cells of this ganglion, the sympathetic nerves proper,
the post-ganglionic, non-meclullatecl fibers arise. From this origin

they ascend, passing successively through
the second sympathetic ganglion, the
annulus of Vieussens, the first sympa-
thetic ganglion, to the ganglion on the
trunk of the vagus, at which point they
enter the sheath of the vagus fibers and
in company with them pass to the heart.
For this reason the common trunk is
generally spoken of as the vagosympa-
thetic nerve.

The vagus nerve is connected with
the medulla oblongata by a series of from
six to eight roots. A short distance from
the medulla, the nerve trunk passes
through a large opening in the cranium
beyond which it presents an enlargement, termed the vagus ganglion.
The peripheral end of this ganglion gives off two trunks, one the
glossopharyngeal, the other the vagus proper.

The vagus nerve proper in the frog also consists of both afferent
and efferent fibers which have practically the same origin, distribution
and termination as the corresponding fibers in the mammal.

After the union of the sympathetic fibers with the vagus fibers, the
common trunk passes forward to the angle of the jaw, winds around
the pharynx just beneath the border of the petro-hyoid muscle and
in close relation with the carotid artery. As the nerve approaches the
heart it divides into two branches, the pulmonary and the cardiac.
At the sinus venosus some of the fibers become related, histologically
and physiologically, with the ganglion cells, while others plunge into
the heart, course along the auricular septum on the left side and finally
terminate at or near the ganglion cells of the base of the ventricle.
'flic mode of termination of both the vagus and sympathetic fibers
is similar to that observed in the mammals.

The Physiologic Actions of the Sympathetic Nerves in the
Frog. The information now possessed regarding the influence
which the central nerve system exerts on the heart through these

Fig. 142. — Diagram showing
the Relation of the Vagus to
the Heart Muscle-cell.

THE CIRCULATION OF THE BLOOD. 313

nerves, has been derived largely from experiments made on the nerves
of the frog, toad, and turtle. Inasmuch as the sympathetic and vagus
nerves in the frog and related animals are bound up in a common
sheath, it is necessary in order to demonstrate their respective functions
to first divide the nerves, before their union at the vagus ganglion, and

Fig. 143. — Tracings Showing the Effects on the Heart-beat of the Frog from
Stimulation of the Sympathetic Nerves Prior to Their Union with the Vagus
Nerve. The upper tracing shows an increase in the rate which before stimulation was 15
per minute and during stimulation 30 per minute. Before stimulation the height of the
ventricular beat was 9 mm. and during the stimulation it was 12 mm. The lower tracing
shows a similar series of effects, the differences being only of degree. — (Brodie.)

then stimulate their peripheral ends. The heart should be exposed
and attached to a recording lever so that its movements may be taken
up and recorded on a moving recording surface.

Stimulation of the sympathetic fibers with induced electric currents,
prior to their union with the vagus, is followed by an increase in the
rate or an augmentation in the force of the heart-beat or both at the
same time. The effects of such a stimulation with induced currents
of moderate intensity are graphically shown in Fig. 143. The upper
tracing shows that the heart was first accelerated, the beats increasing
from 15 per minute before stimulation, to 30 per minute during stim-
ulation. On the cessation of the stimulation, the heart slowly returned
to its former rate. Coincidently with this acceleration of the rate there
was an augmentation of the force of the ventricular contraction as
shown by an increase in the height of the ventricular contraction which
before stimulation was 9 mm., but during stimulation 12 mm.

In addition to the foregoing changes in the heart-beat there is an
alteration in the sequence of the beat. The natural delay in the con-
duction of the excitation process from the auricles to the ventricle is

3i4 TEXT-BOOK OF PHYSIOLOGY.

increased, in consequence of which the auricle completely relaxes before
the ventricular contraction begins. Moreover, the auricular contrac-
tion again occurs before the ventricle has completely relaxed. After
the effect of the stimulation passes away, the acceleration diminishes,
the augmentation declines and a reverse change in the sequence occurs.
The lower tracing shows a similar series of effects. If the stimulus
be applied to the pre-ganglionic sympathetic nerves, an acceleration or
augmentation of the heart follows, similar in all respects to that which
follows stimulation of the post-ganglionic or sympathetic fibers proper;
and the inference may be drawn that if the stimulus could be applied
directly to the nerve-cells in the medulla oblongata from which the fibers
take their origin, the same acceleration or augmentation would follow;
for this reason this collection of nerve-cells is known as the cardio-
accelerator or augmentor center. Since stimulation of the nerve in any
part of its course, which in all probability exaggerates its normal func-
tion, is followed by an acceleration or an augmentation, the sympa-
thetic is said to have an accelerator or an augmentor influence on
the heart-beat; with the cessation of the stimulation, and very fre-
quently before, the heart returns to its normal condition.

The Physiologic Action of the Vagus Nerve in the Frog. — Stimu-
lation of the intra-cranial roots of the vagus with very weak induced elec-
tric currents is followed by a gradual diminution in the rate and a dim-

Fig. 144. — Tracing showing the Effect on the Heart-beat ot the Toad of
long Stimulation of the Intra-cranial Roots of the Vagus with Moderately
strong Electric Currents. — (Caskell.)

inution in the force of the heart-beat. If the induced currents are
moderate in strength, the heart will at once come to a standstill in
diastole. (Fig. 144.) If the stimulus be applied to the trunk or the
peripheral portion of the vagus, for example close to the sinu-auricular
junction, an inhibition occurs similar in all respects to that which
follows stimulation of the intra-cranial roots, and judging from what
is known regarding the action of nerve cells, the inference may be
drawn that if the stimulus could be applied directly to the group of
nerve-cells from which the efferent fibers arise, the same inhibition
would follow; for this reason this collection of nerve-cells is known
as the car dio -inhibitor center. Since stimulation of the nerve, either,
at its center, in its course, or at its periphery, and which in all probability
exaggerates its normal function, is followed by a period of rest or
inactivity, the vagus is said to have a retarding or an inhibitor in-
fluence on the beat of the heart.

J Airing the continuance of the inhibition, the heart-muscle is

THE CIRCULATION OF THE BLOOD.

3i5

relaxed, its cavities dilated and filled with blood. The dilatation
usually exceeds that observed prior to the vagus stimulation.

After cessation of the stimulation, the heart resumes its activity.
At first the beat usually is slow and feeble, but with each succeeding
beat both rate and force increase, until they attain or exceed that
observed prior to the stimulation. In some cases, however, the heart
begins to beat with as much and even more vigor than it did prior to
the stimulation. The duration of the inhibitor effect varies with the
duration of the stimulation. Thus during and after a stimulation of
thirty-eight seconds the heart of the toad remained at rest for 292
seconds (Gaskell) ; the heart of a snake for from one-half to one hour

Fig. 14 :

-Tracing showing the Diminution in the Rate of the Heart-beat
following Weak Tetanization of the Vagus Trunk.

(Meyer) ; the heart of a turtle for four and a half hours (Mills) . The
period of inhibition will depend on the strength of the electric current
employed, the nerve stimulated, the season of the year, etc.

The effects on the heart-beat which will follow stimulation of the
vago-sympathetic in its course vary, however, because of the antagon-
istic action of the inhibitor and accelerator nerve impulses. Thus
stimulation of the peripheral end of the divided trunk of the vagus in

Fig. 146.-

-Tracing showing Complete Inhibition following Strong Tetan-
ization of the Vagus Trunk.

the frog or the toad with weak tetanizing induced electric currents
is followed by an increase in the rate of the heart-beat because of the
stimulation of the accelerator fibers which apparently respond before
the inhibitor fibers; stimulation with somewhat stronger currents is
followed by a diminution in the rate of the beat because of the greater
effect on the inhibitor nerve-fibers (Fig. 145). Stimulation with
strong tetanizing currents is followed by complete inhibition (Fig. 146).
The foreo;oin<r facts lead to the inference that the cardio-accelerator

3i6 TEXT-BOOK OF PHYSIOLOGY.

and the cardio-inhibitor centers have as their function the discharge
of nerve impulses which are conducted by their related nerves, the
efferent sympathetic and vagal fibers, to the heart, and which, in an as
vet unexplained manner, accelerate or augment or inhibit, the action
of the heart. The relation which these two centers bear one to the
other and the manner in which they are influenced in their activities
both directly and reflexly and thus regulate the action of the heart from
moment to moment will be considered in a subsequent paragraph.

Changes in the Conductivity of the Heart. — In addition to
the changes in the rate and force of the heart caused by stimulation
of the inhibitor and the augmentor nerves, it is stated by Gaskell
that there is also during the inhibition a decrease in the conductivity
of the heart at both the sinu-auricular and auriculo- ventricular junc-
tions, and an increase in the conductivity during acceleration of the
beat. The decrease in conductivity may be so pronounced that only
every second or third contraction of the auricle will be followed by a
contraction of the ventricle. In other instances both auricles and
ventricles remain at rest while the sinus maintains its usual rate.
The increase in conductivity is shown by first artificially blocking
the contraction wave at the auriculo-ventricular junction with the
clamp, until only every second or third auricular contraction is con-
ducted to the ventricle, and then stimulating the sympathetic. At
once the auricular contraction forces the block, and passes to the
ventricle, calling forth a normal contraction.

The Physiologic Actions of the Sympathetic Nerves in Mam-
mals.— In the mammal, stimulation of the sympathetic nerves in any
part of their course, either through the rami communicantes, the ven-
tral portion of the annulus of Vieussens, or after their emergence from
the stellate or inferior cervical ganglia is followed by effects similar to
those observed in the frog: viz., an acceleration or augmentation, or
both, of the heart-beat. The percentage increase in the acceleration
varies in different animals. In some instances the increase varies
from 58 per cent, to 100 per cent. (Hunt). If the heart is beating
slowly before stimulation, the acceleration is more marked than if
it is beating rapidly.

The effect of the accelerator impulses is apparently a change in
the inner mechanism of the heart-muscle itself and not to a change in
the peripheral portion of the inhibitor apparatus. This is indicated
by the fact that acceleration occurs after the full physiologic action of
atropin which acts upon, and impairs the conductivity of, the intra-
cardiac nerve-cell terminals.

A peculiarity of the sympathetic nerve is that it does not
respond to stimulation as rapidly as do many nerves, so that a rather
Jong latent period intervenes between the moment of stimulation and
the appearance of the acceleration as shown in Fig. 147. A further pe-
culiarity is that the acceleration sometimes continues after the stim-
ulus is withdrawn, and sometimes ceases before it is withdrawn.

THE CIRCULATION OF THE BLOOD.

3*7

Though an increase in both the rate and force frequently occur
simultaneously, there is no necessary relation or connection between the
two as they can and do occur independently of each other. For this
reason it is generally assumed that the sympathetic nerves contain
two groups of fibers, viz., accelerators and augmentors, the functions
of which are to respectively accelerate the rate and augment the force
of the heart-beat. From the fact that both auricles and ventricles
exhibit these changes it is assumed that the nerve impulses stimulate
both chambers. This is rendered probable also from the experiments
of Erlanger, who found that after complete heart block, stimulation
of the sympathetic caused independent acceleration of both auricles
and ventricles.

VW*AMNY\MAi\N\n«v«*«ivi>flj« /^'v',«^J^J\l^^f\J^J^f\/^^Vl^

Fig.

-Acceleration of the Heart following Stimulation of the Cardiac
Branches which come from the Annulus of Vieussens.

The Physiologic Action of the Vagus Nerve in Mam-
mals.— In the mammal the same or similar effects result from
stimulation of the vagus as in the frog. If the thorax of the
dog is opened and artificial respiration maintained the heart will
continue to beat in a practically normal manner for a long time.
Under such conditions if the vagus nerve on one side be divided
and its peripheral end stimulated with induced electric currents of
moderate strength the heart will be seen to come to a standstill almost
immediately in the condition of diastole, and may be so kept for a
variable period, from fifteen to thirty seconds or more, during which
its walls are relaxed and its cavities filled with blood. On cessation of
the stimulation the contractions return and in a very short time the
former rate and force of the beat are regained. If the electric currents
are of feeble strength, the heart will come to rest gradually through a
gradual diminution in the rate and force of the contraction. During
the period of the inhibition the heart presents an appearance similar to
that presented by the heart of the cold-blooded animal. When the
heart of an animal is thus exposed, the auricle and the ventricle of
one side may be attached by threads to writing levers and their contrac-
tions registered on a moving recording surface. The effects on both
auricles and ventricles which follow vagus stimulation will then become
more apparent. Fig. 148 is a tracing thus obtained. The animal
employed was a rabbit.

The inhibitor effect of the vagus varies in degree and duration
in different animals. In the dog the effect of vagus stimulation is
usually pronounced, lasting from 15 to 30 seconds; in the rabbit it is
perhaps equally well pronounced but somewhat less in duration; in
the cat it is almost wanting. In this latter animal a complete standstill,

3i«

TEXT-BOOK OF PHYSIOLOGY.

even for a few seconds, is very rarely seen; usually there is produced
merely a slight diminution in the rate of the beat even though the stim-
ulus employed is quite strong. In all these animals, however, after
a very short time the nerve impulses lose their inhibitor influence on

Fig. 14S.

-Result of the Stimulation of the Peripheral End of the
Divided Left Vagus in the Rabbit. — (Brodie.)

the heart-muscle, and notwithstanding continued stimulation of the
vagus, the heart returns to its former rate and vigor. This result is
in marked contrast to that observed during stimulation of the vagus
in the cold-blooded animals, in which the heart may be kept at rest
for relatively very long periods of time. No satisfactory explanation

THE CIRCULATION OF THE BLOOD. 319

for this loss of vagus control or escape of the heart from the vagus
control has as yet been offered.

Seat of Action of the Vagus Impulses.— In a foregoing experi-
ment of which Fig. 148 is a graphic result, stimulation of the left vagus
with a fairly strong current was followed by a diminution in both the
rate and force of the contraction of both auricles and ventricles, though
the effect was most marked in the auricles. From this and similar
facts it has come to be the general belief that the inhibitor nerve im-
pulses exert their influence mainly, if not exclusively, on the auricle,
and that the cessation of ventricular action is a secondary effect due
to the non-arrival across the conducting apparatus of the normal
excitation process from the auricle. This is the case undoubtedly in
the cold-blooded animals, and the experiments of Erlanger on the heart
of the dog indicate that the same holds true for the mammals. This
investigator has found that when the auriculo-ventricular tissues are
suddenly clamped, including presumably the muscle band of His, there
is for a time a complete cessation of ventricular activity, but after a
variable period of time, fifty seconds or more, the ventricle develops
an independent rhythm which gradually increases in frequency, but
seldom , if ever, attains that of the auricles. Under such circumstances
tetanic stimulation of the auriculo-ventricular tissues by means of the
clamp now transformed into stimulating electrodes, failed to bring
about a stoppage of the ventricles. Moreover, if during the time the
clamp is applied and after the ventricle has developed a rhythm of its
own, the vagus is stimulated, the auricles will cease to beat as usual,
but the ventricles will continue to beat at their usual rate. These and
similar facts lead to the conclusion that vagal inhibitor action is lim-
ited to the auricles.

From the foregoing facts it is apparent that the accelerator and
augmentor effects of the sympathetic nerve impulses, and the inhibitor
effects of the vagus nerve impulses, closely resemble on the one hand
the accelerator and augmentor effects of increasing amounts of diffusible
calcium salts, and on the other hand the inhibitor effects of increasing
amounts of diffusible potassium salts in the blood or other circulating
fluid; and so closely do these two sets of phenomena resemble each
other, that they are by some observers regarded as identical.

Some additional facts in this connection have been presented by
Howell, viz., that an increase (within limits) and a decrease in the
percentage of diffusible calcium salts in a circulating fluid passing
through the cavities of the mammalian (cat) heart, increases on the
one hand, and decreases on the other hand, the sensitiveness of the
heart to sympathetic acceleration and augmentation. From this the
inference is deduced that the acceleration and augmentation of the
heart-beat which follow stimulation of the sympathetic nerves are
due to the presence in the heart tissue of a certain percentage of
diffusible calcium salts, which have been freed from combination with
organic matter by the action of the sympathetic nerve impulses.

32o TEXT-BOOK OF PHYSIOLOGY.

Again, that an increase (within limit) and a gradual decrease in the
percentage of diffusible potassium salts in a circulating fluid passing
through the cavities of the frog and the cat heart, increases on the one
hand and decreases and finally abolishes on the other hand the sensi-
tiveness of the heart to vagus inhibition. From this the inference is
deduced that the inhibition of the heart-beat which follows stimulation
of the vagus nerve is due to the presence in the heart tissue of a certain
percentage of diffusible potassium salts, which have been freed from
combination with organic matter by the action of the vagus nerve
impulses.

The Cardio-accelerator Center. — The collection of nerve-cells
from which the pre-ganglionic fibers of the sympathetic system arise
is known as the cardio-accelerator or augmentor center. The exact
location of this center in the central nerve system has not been as
yet accurately determined. It is" probably located in the medulla
oblongata.

From experiments which have been made on the sympathetic
nerve apparatus in its entirety, it is believed that the function of this
center is the discharge of nerve impulses which, conducted to the heart
by the pre-ganglionic and post-ganglionic sympathetic fibers, cause an
acceleration in the rate or an augmentation in the force, or both, of
the heart-beat. It is also generally believed since the publication
of Hunt's investigations that this center is in a state of tonic activity.
This is shown by the fact that after the division of the vagus nerves
and the removal of all inhibitor influences, division of the sympa-
thetic nerves or extirpation of the stellate or inferior cervical ganglion,
is yet followed by a decrease in the rate of the heart-beat. After
division of the sympathetic nerves and the removal of accelerator
influences it is also easier to bring about inhibition through vagus
stimulation.

The Factors which Determine the Activity of the Cardio-ac-
celerator Center. — The question has been raised as to whether the
tonic activity of this center is maintained by central or peripheral
stimuli, i. e., whether it is maintained by causes within itself, the
result of an interaction between the constituents of the cell substance
and those of the surrounding lymph, or whether it is maintained by
nerve impulses reflected to it through various afferent or sensor nerves.
Inasmuch as there is no way of determining whether the causes are
central, except by dividing all afferent nerves, it is impossible to state
how much influence is to be attributed to this factor. On the contrary,
though it is readily demonstrable that stimulation of many afferent
nerves will cause an acceleration of the heart it can not be stated
positively that this is the result of a reflex stimulation of the accelerator
renter. Though earlier investigators believed this to be the correct
interpretation, the more recent experiments of Hunt apparently dis-
prove it; for this investigator has shown that if the vagus nerves are
divided it is impossible to produce reflex acceleration of the heart.

THE CIRCULATION OF THE BLOOD. 321

His conclusion, confirming that of others, is that cardiac acceleration
is the result of an inhibition of the cardio-inhibitor center. A freer play
to the tonic activity of the accelerator center would thus be made pos-
sible.

The Cardio-inhibitor Center. — The collection of nerve-cells
from which the small efferent fibers of the vagus nerve arise is known
as the cardio-inhibitor center. It is situated in the medulla oblongata
and more especially in the gray matter beneath the floor of the fourth
ventricle near the tip of the calamus scriptorius. It is in all prob-
ability, a part of the nucleus ambiguus.

From the experiments which have been made on the vagus inhibitor
apparatus in its entirety it is believed that the function of this center
is the discharge of nerve impulses, which conducted to the heart by
the vagus fibers cause an inhibition of its beat of greater or less extent.
In the dog, and probably in many other mammals, this center exerts
a more or less constant inhibitor or restraining influence on the heart's
activity. This is indicated by the fact that the rate of the beat is
very much increased by simultaneous division of both vagi. The
degree of the inhibition which this center exerts varies greatly, however,
in different animals. In the cat and in the rabbit the inhibitor control
is normally so slight that there is but a relatively slight increase in the
rate of the beat after division of the vagi. The tone of the vagus in
these animals is, therefore, said to be slight or feeble. In human
beings the tone of the inhibitor apparatus is poorly developed in earlv
childhood, as shown by the fact that the administration of atropim
which removes temporarily inhibitor control is not followed bv an
increase in the rate of the beat. It develops steadily and reaches
a maximum at from the twenty-fifth to the thirtieth year. In ad-
vanced years the tone again declines. For these and other reasons it
is believed that this center is in a state of tonic activity in many if
not all mammals, discharging nerve impulses which exert a regulative
influence on the cardiac mechanism in accordance with its needs and
especially in reference to the variable resistances offered to the flow of
blood which the heart must overcome.

The Factors which Determine the Activity of the Cardio-
inhibitor Center. — The question has also been raised as to whether the
tonic activity of this center is maintained by central or peripheral
stimuli, i. e., whether it is maintained by causes within itself the result
of an interaction between the constituents of the cell substance and
those of the surrounding lymph, or whether it is maintained bv nerve
impulses reflected to it through various afferent or sensor nerves.
Though both factors play an important part in the maintenance of its
activity, the trend of evidence points to the conclusion that the reflected
impulses are by far the more important of the two. This latter sup-
position is supported by the results of direct experimentation upon
sensor nerves in almost any region of the body. Thus stimulation
of the dorsal roots of the spinal nerves, the trunks of the cranial sensor

322 TEXT-BOOK OF PHYSIOLOGY.

nerves, the splanchnic nerves, the pulmonary branches of the vagus,
etc., gives rise to a more or less pronounced inhibition of the heart.
As a rule, stimulation of the peripheral terminations of these nerves is
more effective than stimulation of their trunks, hence an explanation
is at hand for the cardiac inhibition which results from sudden disten-
tion of the stomach and intestines, or operative procedures in the nose,
mouth, and larynx.

Reflex inhibition of the heart, even to. the stage of absolute and
permanent standstill, eventuating in the death of the individual is
a not infrequent result of peripherally acting causes of a pathologic
or operative character. From the results of experimental procedures
the inference is drawn that normally, nerve impulses, developed by
the action of physiologic causes, are reflected continuously from many
peripheral regions of the body, which falling into this center gently
stimulate and maintain it in a condition of necessary tonicity or activ-
ity.

The Causes of the Variations in the Heart-beat. — It has been
stated elsewhere in the text (page 286), that the rate of the heart-beat
is influenced by age, muscle activity, the position of the body, meals,
variations in blood pressure, etc. In addition to these factors there is
abundant evidence that other factors, e.g., the action of peripheral
stimuli of a physiologic or pathologic character in various regions of
the body, can and do cause reflexly at one time or in one individual
an acceleration of a marked character, and at another time or in
another or the same individual an inhibition which may be so pro-
nounced as to lead to a complete standstill in diastole. The records
of clinical medicine contain many instances which show that gastric,
intestinal, uterine and other organic disorders as well as various opera-
tive procedures in different regions of the body cause now an accelera-
tion, now an inhibition of the heart.

The first explanation, that acceleration of the heart, the result of a
peripherally acting stimulus, is due to a stimulation of the cardio-
accelerator center by the arrival of nerve impulses coming through
afferent nerves, having been made questionable and improbable by the
results of Hunt's experiments, the alternate explanation must be, that
the acceleration is due to an inhibition of the normal activity of the
cardio-inhibitor center, and that inhibition is due to an excitation of
the normal activity of the cardio-inhibitor center, and hence there fol-
lows the corollary that afferent nerves contain two sets of nerve-fibers
which are in physiologic relation with the cardio-inhibitor center, one
of which when stimulated peripherally inhibits its activity, the other
of which when stimulated excites or augments its activity.

The extent to which both sets of fibers are present in any one
afferent nerve is unknown. In the trigeminus it is believed the ex-
citator fibers preponderate for the reason that peripheral stimulation
of this nerve is followed by inhibition of the heart; in the sciatic, it is
believed the inhibitor nerves preponderate, for the reason that stimu-

THE CIRCULATION OF THE BLOOD. 323

lation of the central end of the divided nerve is followed generally by
acceleration of the heart.

It is probable from the effects which follow gastro-intestinal dis-
orders, that the vagus nerve contains both classes of fibers as repre-
sented in Fig. 141, inasmuch as stimuli of a pathologic character in
one individual may reflexly excite or increase the activity of the cardio-
inhibitor center, to be followed by an inhibition of the heart; and in
another individual, may reflexly inhibit the activity of the same center
and to such an extent that the cardio-accelerator center may be enabled
to increase either the rate or the force or both, of the heart move-
ments. Palpitation of the heart from gastric irritation might thus be
explained.

The Influence of Psychic States.— The cardio-inhibitor and
the cardio-accelerator centers may be increased in activity also by
nerve impulses descending from the cerebrum, the result of emo-
tional states; thus depressing emotions according to their intensity
may so increase the activity of the cardio-inhibitor center, that the
heart's action may not only be retarded but even completely inhibited;
joyous emotions, on the contrary, may so increase the activity of the
cardio-accelerator center or what is more probable inhibit the activity
of the cardio-inhibitor center that the heart's action will be increased
in both its rate and force.

From the results of stimulation of the sympathetic (accelerator) and
vagus (inhibitor) nerves under a great variety of conditions it has been
established that their respective centers are mutually antagonistic;
that the activity of the accelerator center at one moment limits the ac-
tivity of the inhibitor and at another moment is limited in turn by it;
that the rate of the heart-beat at each moment is the resultant of the
relative degree of activity of the two centers.

The Depressor Nerve.— The vagus trunk also contains afferent
fibers stimulation of which not only brings about a reflex inhibition
of the heart, but also a dilatation of the peripheral arteries and a fall
of blood-pressure through a depressive influence on the vaso-motor
centers. To this nerve the term depressor has been given. A con-
sideration of the physiologic action of this nerve will be found in the
section devoted to the nerve mechanisms concerned in the mainte-
nance of the blood-pressure.

Modifications of the Nerve Mechanism of the Heart due
to the Physiologic Action of Drugs.— The functions of different
parts of the nerve mechanism of the heart may be demonstrated by an
analysis of the effects which follow the administration of slightly toxic
doses of the alkaloids of various drugs. The effects can be shown to be
due to a stimulation or to a depression of the normal activity of one or
more portions of the mechanism. The alkaloid may exert its specific
action on the central portions in the medulla, or on the peripheral
portions in the heart, or on both simultaneously. The heart- mus-
cle may at the same time be stimulated or depressed in its action

324 TEXT-BOOK OF PHYSIOLOGY.

either in the same or in the opposite direction to that of the nerve mech-
anism. As a result the heart-beat may be increased or decreased both
in rate and force.

The following examples will illustrate the action of alkaloids in
general.

Atropin. — After the administration of atropin in sufficient amounts
the heart-beat increases in frequency in all animals in which the cardio-
inhibitor centers exert a steady inhibitor influence over the heart.
This is especially true in man and the dog. In animals in which the
inhibitor control is slight, as the rabbit and frog, the increase in fre-
quency is not very marked. In all animals thus far experimented
on after the administration of atropin, neither stimulation of the
trunk of the vagus nor stimulation of the intra-cardiac ganglia will
arrest or even retard the heart-beat. The inference, therefore, is that
the alkaloid exerts its action upon the ganglion cells and their ter-
minal branches, impairing their chemic integrity and abolishing their
normal function, that of conducting nerve impulses from the vagus
nerve proper to the heart-muscle. In consequence of this, the influ-
ence of the cardio-inhibitor center is cut off and the cardio-accelerator
being unopposed in its activity, the rate of the beat is increased.
After a variable period the heart returns to its normal rate. Stimu-
lation of the vagus is again followed by the usual inhibition. As
atropin is partly oxidized, and partly excreted, it is assumed that the
nerve terminals have been restored by nutritive forces to their normal
condition and their conductivity regained. This having been accom-
plished the vagus nerve impulses can again reach the heart-muscle and
the cardio-inhibitor center is therefore enabled to re-establish inhibitor
control and antagonize the activity of the cardio-accelerator center.

Nicotin. — After the administration of nicotin in sufficient amounts
the heart-beat is primarily decreased in frequency even to the point of
standstill in diastole for a few seconds, and secondarily increased both
in frequency and force beyond the normal. If the vagus nerves be
first divided this primary decrease is not so marked and the inference
is that the alkaloid primarily stimulates the cardio-inhibitor center and
increases its normal function and perhaps the terminal branches of the
vagus fibers, the pre-ganglionic, as well. After the secondary in-
crease in the rate is established stimulation of the vagus trunk fails to
inhibit the heart, though stimulation of the intra-cardiac ganglia is at
once followed by the usual inhibitor phenomenon, arrest of the heart in
diastole. For this reason it is believed that nicotin acts on the periph-
eral terminations of the pre-ganglionic fibers of the vagus as they
arborize around the intra-cardiac ganglia, depressing them and sus-
pending their normal function, that of conducting nerve impulses from
the vagus to the ganglion cells. Since stimulation of the pre-ganglionic
fibers of the accelerator apparatus fails to accelerate the rate of the
heart-beat, though stimulation of the post-ganglionic fibers has the
usual accelerating effect, the inference is that nicotin acts upon and

THE CIRCULATION OF THE BLOOD. 325

suspends the conductivity of their terminal branches in the ganglia.
The acceleration of the heart must therefore be attributed either to a
stimulation of the post-ganglionic fibers or of the cardiac muscle
itself (Cushney).

Pilocarpin and Muscarin. — These alkaloids, whether adminis-
tered internally or applied locally to the heart, diminish the frequency
and the force of the beat to such an extent that it very shortly comes
to rest in diastole. For the reason that the internal administration or
the local application of atropin in proper doses, which has a depressive
action on the intra-cardiac cell terminations, removes the inhibition
and restores the normal rhythm, the inference is drawn that both these
alkaloids either increase the irritability of the nerve-cells or heighten
the conductivity of their terminal fibers. The return of the heart-beat
is attributed to a decline in irritability to the normal level in conse-
quence of the antagonistic action of the atropin.

Digitalin.— -The administration of digitalin gives rise to effects,
the character and extent of which vary in different animals. In the frog,
as a rule, the only effect produced is a gradual increase in the duration
and force of the ventricular systole, with a corresponding decrease in
the duration of the diastole, until the heart comes to rest in the systolic
state. As this effect is observed after division of the vagus trunk
and also after the suspension of the activity of the intra-cardiac cell-
fibers by atropin, it is evidently due to a direct stimulation of the heart-
muscle. In some instances, however, the opposite effect is produced,
viz., a gradual increase in the length of the diastole, a decrease in the
duration of the systole, until the heart comes to rest in the diastolic
state. As this effect only arises when the vagus nerve is intact it is
very probably due to a stimulation of the cardio-inhibitor center and
a consequent increase of its functional activity. Though either effect
may be produced in the frog the predominant effect is the increase in
the contraction of the heart-muscle rather than an inhibition of the beat.

In mammals both effects are observed, viz., a diminution in the
rate of the beat, a lengthening of the diastole and an increase in the
vigor of the systole, which are evidently due to a simultaneous stim-
ulation of the cardio-inhibitor center and of the cardiac muscle.
Digitalin thus expends itself on two opposing mechanisms; as to
which gains the ascendency will depend on the dosage and the character
of the animal.

CHAPTER XIV.
THE CIRCULATION OF THE BLOOD (Continued).

THE VASCULAR APPARATUS: ITS STRUCTURE AND FUNCTIONS.

The systemic vascular apparatus consists of a closed system of
vessels extending from the left ventricle to the right auricle, and in-
cludes the arteries, capillaries, and veins. Though serving as a whole
to transmit blood from the one side of the heart to the other, each one
of these three divisions has separate but related functions, which are
dependent partly on differences in structure and physiologic properties,
and partly on their relation to the heart and its physiologic activities.

The Structure, Properties and Functions of the Arteries. —
The arteries serve to transmit the blood ejected from the heart to the
capillaries; that this may be accomplished they divide and subdivide
and ultimately penetrate each and every area of the body. Their
repeated division is attended by a diminution in size, a decrease in the
thickness and a change in the structure of their walls.

A typical artery consists of three coats: an internal, the tunica
intima; a middle, the tunica media; an external, the tunica adventitia.

The internal coat consists of a structureless elastic basement mem-
brane, on the inner surface of which rests a layer of elongated spindle-
shaped endothelial cells. The middle coat consists of several layers of
circularly disposed, non-striated muscle-fibers, between which are
networks of elastic fibers. The external coat consists of bundles of
connective tissue of the white fibrous and yellow elastic varieties.
Between the external and middle coats there is an additional elastic
membrane. In the small arteries there is but a single layer of muscle-
fibers. In the large arteries the elastic tissue is very abundant, ex-
ceeding largely in amount the muscle-tissue. It is also more closely
and compactly arranged. The external coat is well developed in the
large arteries (Figs. 149 and 150).

In virtue of the presence in their walls of both elastic and con-
tractile elements, the arteries possess the two properties of elasticity
and contractility.

The elasticity is especially well developed in the large arteries, which
are capable, therefore, of both distention and elongation, and, when the
distending force is withdrawn, of returning to their previous condition.
The elasticity permits of a wide variation in the amount of blood the
arterial system can hold between its minimum and maximum disten-
tion. Thus the capacity of the aorta and carotid artery of the rabbit
can be increased four times and six times respectively by raising the in-

326

THE CIRCULATION OF THE BLOOD.

327

b

tra-arterial pressure from o to 200 mm. of mercury. The elasticity
also converts the intermittent movement of the blood imparted to it
by the heart as it is ejected from the ventricle, into a remittent move-
ment in the arteries and finally into the continuous and equable
movement observed in the capillaries. This is accomplished in the
following manner: With each contraction of the left ventricle more
blood is ejected into the aorta than the arteries can discharge
into the capillaries and veins during the time of the contraction. The
portion not so discharged exerts a lateral
pressure against the walls of the arteries
which at once dilate until a condition of
equilibrium is established between the
pressure from within and the elastic re-
action of the arterial walls from without.
With the cessation of the contraction the
elastic walls recoil and propel the blood
toward the capillaries. The intermittent
action of the heart is thus succeeded by
the continuously reacting arterial wall.

As the blood advances toward the
periphery of the arterial system and larger
amounts pass into the capillaries, both
the distention and the elastic recoil di-
minish, and by the time the blood reaches
the capillaries its intermittency of move-
ment has been so far obliterated by the
elastic recoil, that as it enters the capil-
laries the movement becomes equable
and continuous. The elasticity thus

equalizing the
throughout the

'«

■d

%

H

S

Fig. 149. — Coats or a Small
Artery, a. Endothelium, b.
Internal elastic lamina, c. Cir-
cular muscular fibers of the middle
coat. d. The outer coat. — (Lan-
dois and Stirling.)

serves the purpose of
movement of the blood
arterial system.

In youth the arterial walls are highly
distensible and elastic; in advanced years
they are frequently relatively rigid and
inelastic; and in consequence the flow of blood toward and into the
capillaries approximates in its characteristics the flow of a fluid through
a rigid tube under the intermittent action of a pump; that is, the inter-
mittent movement imparted by the heart is not so completely converted
into a continuous movement, and hence the blood flows through the
capillaries during the systole with greater velocity, and during the dias-
tole with less velocity, than is the case when the vessel is normally
elastic. For these and other reasons the tissues are not so well nour-
ished and hence their nutrition and functional activities decline.

The contractility permits of a variation in the amount of blood
passing into a given capillary area in a unit of time. Normally each
artery has a certain average caliber due to a given contraction of the

328 TEXT-BOOK OF PHYSIOLOGY.

muscle coat. Beyond this average condition the artery can pass in one
direction or the other by either a relaxation or increased contraction
of the muscle coat. During the functional activity of any organ or
tissue there is need for an increase in the amount of blood beyond
that supplied during functional inactivity or rest. This is accomplished
by a relaxation of the muscle-fibers. With the cessation of activity
the muscle-fibers again contract and reduce the amount of blood to
that required for nutritive purposes only. An increased contraction
of the muscle-fibers beyond the average, diminishes the outflow of
blood, and if sufficiently great may give rise to anemia and pallor.
The contractile elements at the periphery of the arterial system, in the

so-called arteriole re-
gion, therefore regu-
late the supply of
blood to the tissues in
accordance with their
| functional needs.

||b Moreover, as will

be stated in subse-
quent paragraphs the
-Trai sverse Section of Part of the de of contraction

Wall of the Posterior Tibial Artery (Man). °

—{Schajer.) a. Endothelium lining the vessel, ap- 01 the arteriole mus-
pearing thicker than natural from the contraction of cle influences verv
the outer coats, b. The elastic layer of the intima. rnnrl-prll-w tVip Hpotpp
c. Middle coat composed of muscle-fibers and elastic ]r . . y me utbice

tissue, d. Outer coat consisting chiefly of white of friction which the
fibrous tissue.— (From Yeo's "Physiology.") blood has to overcome

in passing from the
arteries into the capillaries. If the muscle contracts vigorously the
caliber of the arteriole is diminished and the friction increases; if the
muscle relaxes, the caliber of the arteriole is augmented and the friction
decreases. By virtue of its tonic activity, the arteriole muscle at the
periphery of the arterial system offers considerable resistance to the
outflow of the blood and which is therefore spoken of generally as the
peripheral resistance, though there is included under this term the
resistance, offered by the small caliber of the capillary blood-vessel
as well. This latter factor is constant, the former variable.

The Structure, Properties and Functions of the Capillaries. —
The capillaries are small vessels that connect the arteries with the veins.
Though different in structure from a small artery or vein, there is no
sharp boundary between them, as their structures pass imperceptibly
one into the other. A true capillary, however, is of uniform size in
any given tissue and does not undergo any noticeable decrease in size
from repeated branchings. The diameter varies in different tissues
from 0.0045 mm- to 0.0075 mm., just sufficiently large to permit the
easy passage of a single red corpuscle. The length varies from 0.5 mm.
to 1 mm. The wall of the capillary (Fig. 151) is composed of a single
layer of nucleated endothelial cells with serrated edges united by a

THE CIRCULATION OF THE BLOOD.

329

cement material. Though extremely short, the capillaries divide and
subdivide a number of times, forming meshes or networks, the close-
ness and general arrangement of which very in different localities.

As the endothelial cells are living structures and characterized by
irritability, contractility and tonicity, it may be assumed that the cap-
illary wall as a whole is characterized by the same properties. Upon
the possession of these properties, the functions of the capillary depend.

The junction of the capillary wall is to permit of a passage of the
nutritive materials of the blood into the surrounding tissue spaces
and of waste products from
the tissue spaces into the
blood. The structure of
the capillary wall is well
adapted for this purpose.
Composed as it is of but a
single layer of endothelial
cells, the thickness of which
defies accurate measure-
ment, it readily permits,
under certain conditions,
of the necessary exchange
of materials between the
blood and the tissues. The
forces which are concerned
in the passage of materials
across the capillary wall are
embraced under the terms
diffusion, osmosis, and filtra-
tion. As a result of the in-
terchange of materials the tissues are provided with nourishment and
relieved of the presence of waste products. The blood at the same time
changes to a variable extent in chemic composition; because of the loss
of oxygen and the gain of carbon dioxid it also changes in color from
red to bluish-red.

In order that the nutritive materials may pass through the cap-
illary wall in amounts sufficient to maintain the necessary supply of
lymph in the lymph or tissue spaces, it is essential that the blood shall
flow into and out of the capillary vessels constantly and equably, in
volumes varying with the activities of the tissues, under a given pres-
sure and with a definite velocity. These conditions are made possible
by the cooperation of the physical properties and physiologic functions
of the heart and vascular apparatus, the nature of which will be ex-
plained in subsequent pages.

The Structure, Properties and Functions of the Veins.— The
veins serve to collect the blood from the capillary areas and return
it to the right side of the heart. As they emerge from the capillary
areas the veins, which in these regions are termed venules, are quite

Fig. 151. — Capillaries. The Outlines of
the Nucleated Endothelial Cells with the
Cement Blackened by the Action of Silver
Nitrate. — (Landois and Stirling.)

330 TEXT-BOOK OF PHYSIOLOGY.

small. By their convergence and union the veins gradually increase
in size in passing from the periphery toward the heart. Their walls
at the same time correspondingly increase in thickness. The veins
from the lower extremities, the trunk, and abdominal organs finally
terminate in the inferior vena cava. The veins from the head and
upper extremities terminate in the superior vena cava. Both vense
cavse empty into the right auricle.

A typical vein consists of the same three coats as the artery: viz.,
the tunica intima, the tunica media, and the tunica adventitia. The
media, however, does not possess as much of either the
elastic or muscle tissue as the artery, but a larger
amount of the fibrous tissue. Hence they readily
collapse when empty. In virtue of their structure the
veins also possess both elasticity and contractility ,
though in a far less degree than the arteries. These
properties come into play and are of value in further-
ing the movement of the blood toward the heart, es-
pecially after a temporary obstruction. •

Veins are distinguished by the presence of valves
Valves ° of a throughout their course. These are arranged in pairs
Vein. v. Semi- and formed by a reduplication of the internal coat,
lunar valve, i. strengthened by fibrous tissue. They are always
valve.6— \r>avid- directed toward the heart and in close relation to the
son.) walls of the veins, so long as the blood is flowing for-

ward (Fig. 152). An obstruction to the flow causes
the valves to turn backward until they meet in the middle line, when
they act as a barrier to regurgitation. Under these circumstances the
elastic tissue permits the veins to distend and accommodate the
blood. With the removal of the obstruction the recoil of the elastic
tissue, and perhaps the contraction of the muscle-tissue, forces the
blood quickly onward.

HYDRODYNAMIC CONSIDERATIONS.

The blood flows through the arteries, capillaries and veins in ac-
cordance with definite laws. During its transit certain phenomena
are presented by each of these three divisions of the vascular appara-
tus. Since these phenomena, as well as the laws which govern them
are similar to, though more complex than the phenomena presented
by relatively simple tubes with rigid or elastic walls, while liquids are
flowing through them under a steadily acting or an intermittently
acting pressure, it will be conducive to clearness of conception of the
mechanics of the vascular apparatus, if there be considered :

1. The flow of a liquid through a horizontal tube with rigid
walls and of uniform or variable diameter under a steadily acting
pressure, and

2. The flow of a liquid through a tube with elastic walls under
an intermittently acting pressure.

THE CIRCULATION OF THE BLOOD.

33i

THE FLOW OF A LIQUID THROUGH A HORIZONTAL TUBE
WITH RIGID WALLS.

The phenomena and the laws which govern them, that attend the flow
of a liquid through a rigid tube of uniform diameter under a steadily acting
pressure may be readily observed in an apparatus similar to that represented
in Fig. 153, which consists of a reservoir or pressure vessel, P, provided with a
horizontal tube into which is inserted at equal distances a series of vertical
tubes. If the reservoir be filled with a liquid, water for example, the latter
under certain conditions will exert a downward pressure and act as a pro-
pelling or driving power, the degree of which will depend on the height of
the column and may be represented by H. If the stopcock at O be
opened the column of water, which has heretofore been exerting an equal
pressure in all directions, will now exert a downward pressure only, and

Fig. 153. — A Pressure Vessel, P, with an outflow horizontal tube, O-n, into
which vertical tubes or manometers are inserted.

in consequence it will be driven into and through the horizontal tube and
discharged from its free extremity with a definite velocity. The velocity
can be determined by measuring the quantity, q, discharged in a given

time, t, (1 sec.) and dividing it by the area of the tube, jt»«; e.g., v= — -

Moreover in a tube of uniform diameter the velocity through each cross-
section will be the same.

As the water flows through the horizontal tube it meets with resistance,
namely, the cohesion and friction of its molecules, and which must be
overcome if the flow is to continue. Because of the fact that water will
moisten most surfaces with which it comes in contact there will be an adhe-
sion between the walls of the tube and the outer layer of the column of
water, in consequence of which it will become more or less stationary.
Between the outer stationary layer and the axis of the stream, there is an
infinite number of layers of molecules, the cohesion of which one for the
other steadily diminishes in passing from the periphery to the center of the
stream. The extent of this cohesion will increase as the tube is lengthened.

332 TEXT-BOOK OF PHYSIOLOGY.

and decrease as it is widened. In tubes of large diameter the axial cohe-
sion is slight and hence the friction of the molecules, the resistance to the
flow, is readily overcome; in the tubes of small diameter the axial cohesion
is relatively great and the resistance less easily overcome.

As a result of the resistance the forward movement of the water under
the pressure in P is somewhat retarded, and as a consequence it will exert
a lateral or radial pressure against the walls of the tube, as shown by the
rise of the water in the vertical tubes. The amount of the lateral pressure
at any given point is indicated and measured by the height to which the
water rises. For this reason these tubes are termed pressure tubes or
piezometers.

Since the resistance in a tube of uniform diameter is proportional to its
length the lateral pressure will gradually but progressively decrease from
the reservoir to the outlet. Therefore the pressure at any given point is
proportional to the resistance yet to be overcome and conversely the resist-
ance to be overcome is indicated by the height of the pressure. (In the
conduct of an experiment the propelling power should be kept constant by
permitting fluid to flow into the reservoir as rapidly as it flows out of the
horizontal tube.)

The power, or force which overcomes the resistance in the horizontal
tube and imparts velocity to the fluid is the downward pressure of the
water in the reservoir, represented by H. The amount of this power util-
ized in overcoming the resistance is approximately indicated by the height
of the fluid, y, at which point the line uniting the upper limits of the water
in the vertical tubes intersects it. The height of the fluid at this point is a
measure, therefore, not only of the resistance but also an indication of the
relative amount of the pressure used in overcoming it and is therefore
known as the pressure height.

The amount of the pressure consumed in imparting the observed ve-
locity is determined by ascertaining the height from which a particle must
fall in empty space to acquire this velocity. This is obtained by dividing
the square of the velocity by twice the accelerating force of gravity as ex-
pressed in the formula, ~; the quotient is the height and is known as the
velocity height. Conversely if the moving fluid were discharged into empty
space through an opening in the tube at n, it would ascend an equal dis-
tance. If now this height is represented by F, and a line be drawn from
it, parallel to the line of pressure until it meets the reservoir at x, it will be
seen what percentage, x y, of the primary propelling power is consumed in
imparting the observed velocity.

Of the total pressure a small portion is left over which is utilized in
forcing into, and overcoming the resistance offered by, the orifice of the
horizontal tube. The initial pressure in P therefore divides itself into two
portions; one, the larger by far, is utilized in overcoming the resistance to
the flow of the water; the other, the smaller, in imparting velocity.

Thus the two phenomena presented by the flow of a liquid through a
tube with rigid walls and of uniform diameter are velocity and pressure, of
which the former is the same for each cross-section, and the latter at any
point directly proportional to the resistance to be overcome.

If, instead of a horizontal tube of uniform diameter, there be substi-
tuted a tube, the middle third of which is enlarged, the conditions will be
the same as in the previous case until the fluid flows into the enlarged por-
tion, when the velocity will diminish and will become inversely proportional

THE CIRCULATION OF THE BLOOD.

333

to the area of the cross-section. The resistance will be also diminished
and as a result the pressure rises or at least less of the initial pressure is
utilized in this than in the first section of the tube. When the liquid flows
into the narrow or third section, the primary velocity returns. Though the
resistance again increases the amount to be overcome is small, and hence
there is a rapid and steady fall of pressure.

On the contrary, if a tube be substituted, the middle third of which is
narrowed, the conditions will be the same as in the previous cases until the
liquid flows into the narrowed section, when at once the velocity increases
and becomes inversely proportional to the area of the cross-section; the re-
sistance being increased at the same time, there will be a rapid consump-
tion and a steep fall of pressure. On flowing into the third section, the
velocity again diminishes and the pressure falls though less slowly to the
end of the tube.

THE FLOW OF A LIQUID THROUGH A SERIES OF BRANCHING
AND AGAIN RE-UNITING TUBES WITH RIGID WALLS.

In a system of this character, such as represented in Fig. 154, there may
be assumed as a result of the repeated branchings, a progressive increase
in the total sectional area of the collective tubes coincident with a progres-
sive decrease in the sectional area of individual tubes in the section B c,

Fig. 154. — A Series of Branching and again Re-uniting Tubes.

while in the section c d, there is a progressive decrease in the total sectional
area of the collective tubes coincident with a progressive increase in the
sectional area of individual tubes.

Assuming the system to be connected with a pressure vessel, as in the
preceding instance, but which has been omitted from the figure, and the
stop cock to be suddenly opened, the column of water will t now exert a
downward pressure, and in consequence the water will be driven into and
through the system with a definite velocity and pressure.

The velocity of a fluid through such a system should theoretically
be decreased from b to c in a ratio inversely proportional to the total area
of each cross-section. This, however, will not be the case because of the
presence of the angles formed by the repeated branchings which, as de-
termined experimentally, simultaneously increase the velocity. The extent
to which the initial velocity will be changed will therefore be proportional

334 TEXT-BOOK OF PHYSIOLOGY.

to the ratio between these two factors. If they balance each other there
will be no change; according as the one or the other preponderates, there
will be a corresponding decrease or increase in velocity.

In flowing from c to d the velocity will be changed in the opposite di-
rection and for the reverse reasons and in E it will have regained the value
it had in A, if the entrance and exit tubes are of the same diameter.

The lateral pressure should theoretically be increased from B to c be-
cause of the increase in the total sectional area and the consequent diminu-
tion of friction, but because of the increase in resistance due to the decrease
in sectional area of individual tubes as well as by the angles formed by the
branching of the tubes it will be simultaneously decreased. The extent to
which the pressure will be changed will also be proportional to the ratio
between these two factors. If they balance each other, there will be no
change; according as one or the other preponderates will there be an in-
crease or decrease in pressure. In passing from c to D the pressure will be
changed in the same direction but for the reverse reasons.

In a general way it may be said that in a system in which the succes-
sive branchings are accompanied by a progressive decrease in diameter, the
influence of an increase in the total sectional area on velocity will prepon-
derate over that due to the production of angles, and hence the initial
velocity will be decreased from B to c, and increased from c to D, for the
reverse reasons; and if the tubes in the center of the system are capillary
in character, the resistance offered by them and hence the consumption of
the propelling power will preponderate over the decrease in resistance due
to the widening of the stream bed, and hence as the liquid flows from b to c
there will be a fall in pressure, and in flowing from c to D there will be an
additional fall of pressure, owing to the narrowing of the stream bed and
the increase in resistance, which is however slight in amount.

The pressure throughout this system is the result of the resistance to
the flow of the water and the extent of the pressure in any one section
will be proportional to the resistance yet to be overcome. It will
naturally be high in the section a-b and low in the section d-e. The
value of these two pressures in these sections and their relation to each
other may be varied temporarily or permanently by the introduction along
the course of the tubes between B and c of a series of stopcocks. If the
lumen of each stopcock has a certain average value, so as to permit of a
certain outflow of water, the pressure will have a certain value in both
a-b and d-e. But if the lumen of each stopcock is decreased, there will
be an increase in the resistance and hence a rise of pressure in a-b and
a fall of pressure in d-e. If, on the contrary, the lumen of each stopcock
is increased, there will be a decrease in the resistance and hence a fall of
pressure in a-b and a rise of pressure in d-e. The stopcocks may be
spoken of as a variable peripheral resistance.

In the foregoing exposition it has been assumed that in all instances the
pressure in the pressure vessel was steadily acting. If, however, the pres-
sure be made to act intermittently as it can be by alternately opening and
closing the stopcock, both the velocity and the pressure will be alternately
increased and decreased. The outflow of the fluid during the moment the
pressure is acting will be rapid, and during the moment the pressure is not
acting the outflow will cease. It becomes therefore intermittent. Coinci-
dently there is an alternate temporary increase and decrease of the lateral
pressure.

THE CIRCULATION OF THE BLOOD. 335

THE FLOW OF A FLUID THROUGH A TUBE WITH ELASTIC

WALLS.

When a tube with elastic walls is connected with a pressure vessel, the
conditions which are established on opening the stopcock and the conse-
quent flow of water, will soon approximate those observed in a tube with
rigid walls. As the water moves forward, it encounters friction, exerts a
lateral pressure and causes a distention of the tube. This latter effect con-
tinues until the elastic recoil of the walls of the tube exactly counterbal-
ances the pressure of the water from within. When this condition is es-
tablished the tube becomes practically a tube with rigid walls, and hence
so long as the primary pressure is uniform, the velocity and lateral pressure
will obey the laws which hold true for rigid tubes.

If, however, the primary pressure be intermittently applied or alter-
nately increased or decreased, and the water forced into the tube, previously
filled with water but under no particular pressure, it will be forced out of
the peripheral end of the tube more or less rapidly during the period of the
increase of pressure and less rapidly during the period of the decrease of
pressure or it may cease entirely. The extent to which the outflow be-
comes merely remittent, or entirely intermittent, will depend on the amount
of resistance, whether this be due to length of tube or a narrowed outlet,
and the degree of elasticity.

When these factors have a negative value the outflow will be intermit-
tent. But if they are made to gradually change, and this is especially the
case with the resistance, from a negative to a positive value, the outflow
gradually changes from an intermittent to a remittent and finally to a con-
tinuous outflow and for the following reasons:

With a given resistance and elasticity, the fluid which is driven into the
tube by the action of the primary pressure exerts more or less lateral pres-
sure, gives rise to a distention of the tube and acquires a certain velocitv
of outflow. In consequence of the distention, a portion of the fluid accumu-
lates. With the cessation in the action of the primary pressure, the elastic
walls recoil and force the accumulated fluid forward and so maintain more
or less effectively the same velocity of outflow until there is a return of the
pressure. If the resistance and elasticity have a negative value this is im-
possible and the outflow will be entirely intermittent. But if they are
made to increase in value, the proportionate amount of the fluid which ac-
cumulates during the action of the primary pressure will also increase in
amount and hence there will be an increase in the distention of the tube.
The elastic recoil will therefore be greater in amount and longer in dura-
tion, and hence the outflow will change to a remittent and finallv to a con-
tinuous outflow.

Coincident with the action and cessation of action of the primary pres-
sure there is a corresponding increase and decrease of the lateral pressure
and when the intermittency in their action is sufficiently rapid, the excess
of fluid entering the tube over that discharged becomes sufficiently great to
maintain a certain average or mean pressure, which, however, undergoes
an alternate increase and decrease with each variation in the primary
pressure.

The temporary increase and decrease of the pressure and the consequent
expansion and recoil of the tube in the neighborhood of the pressure vessel,
gives rise to a wave on the surface of the fluid which is propagated with

336

TEXT-BOOK OF PHYSIOLOGY.

more or less rapidity — though with decreasing size from the beginning to
the end of the tube and causing in each section a corresponding expansion
and recoil, and known as the expansion wave.

THE APPLICATION OF THE FOREGOING FACTS TO THE
VASCULAR APPARATUS.

The systemic vascular apparatus may be conceived of as a system
of tubes which have symmetrically divided and subdivided and after-
wards again united and reunited in a corresponding manner. The
arteries, arterioles, capillaries, venules and veins may therefore be sche-
matically arranged (Fig. 155) in a manner identical with the schematic
arrangement of tubes represented on page 333. The heart, with which
they are in connection, when filled with blood may be compared with
the' reservoir filled with water, and the intra-ventricular pressure devel-

CflPILLARIES

ARTERIES

VEINS

Fig. 155. — Schematic Arrangement of the Vascular Apparatus.

oped during the contraction, to the downward pressure of the water
when the stopcock at O is opened (See Fig. 153).

The Stream-bed. — The stream-bed, the path along which the
blood flows, varies widely in its total sectional area in different parts
of its course, being least in the aorta and venae cavae, and greatest in
the capillaries. In passing from the base of the aorta toward the capil-
laries the sectional area of individual arteries, in consequence of re-
peated branching, diminishes, though their total sectional area in-
creases and in direct proportion to their distance from the heart.
In the capillary system the sectional area of an individual capillary
attains its minimal value, though the total sectional area attains its
maximal value. Comparing one with the other, it has been estimated
that the total sectional area of the aortic bed is to the total sectional
area of the capillary bed as 1 is to 600 or 800. In passing from the
capillary into the venous system the sectional area of individual veins
increases, though the total sectional area decreases and in direct pro-
portion to their distance from the capillaries.

The stream-bed in the aorta is relatively narrow, but widens grad-

THE CIRCULATION OF THE BLOOD.

337

ually as it approaches the capillaries, where it attains its maximum
width; it again narrows gradually as it passes into the veins, until in
the venae cavae it becomes almost as narrow as in the aorta. As the
combined sectional areas of the venae cavae are greater than the sectional
area of the aorta, the stream-bed of the former never becomes as narrow
as that of the latter.

The gradual increase in the width of the stream-bed from the
beginning of the aorta to the middle of the capillary system, and the
gradual decrease in the width of the stream-bed from the middle of
the capillary system to the terminations of the venae cavae, which

Fig. 156. — Diagram Designed to Give an Idea of the Aggregate Sectional
Area of the Different Parts of the Vascular System. A. Aorta. C. Capillaries.
V. Veins. The transverse measurement of the shaded part may be taken as the width of
the various kinds of vessels, supposing them fused together. — (Yeo.)

results from the repeated branching and subsequent reuniting, as
well as its relative width in the arteries, capillaries and veins, is shown
graphically in Fig. 156.

When the heart contracts and when the intra- ventricular pressure
rises above the pressure in the aorta, the aortic valves are forced
open and the blood is driven into and through the arteries, capillaries and
veins and empties into the right side of the heart with a definite velocity
and pressure. Because of the fact that the stream-bed progressively
increases in width from the aorta to the middle of the capillar}- system
the velocity through each cross-section should theoretically be inversely
proportional to its total sectional area. This, however, strictly speak-
ing, is not the case, owing to the accelerating influence of the angles
formed by the repeated branchings. But for the reason that the in-
fluence of the former factor so largely preponderates over the influence

338 TEXT-BOOK OF PHYSIOLOGY.

of the latter factor, it may be accepted that the initial velocity of the
blood in the aorta gradually decreases inversely as the sectional area
increases, until it attains its minimal value in the capillary system.
The actual velocity is the resultant of these two opposing forces.
For the reverse reason, viz., the narrowing of the stream-bed and the
retarding influence of the angles, the velocity of the blood gradually
increases from the middle of the capillary system to the end of the vense
cavse though it will never attain its initial velocity in these vessels
because their combined sectional areas are greater than that of the
aorta. The accelerating effect of the narrowing of the stream-bed
preponderates over the retarding effects of the angles and the velocity
will be the resultant again of these two opposing forces. The same
facts of course hold true for the pulmonic vascular apparatus.

In its passage through the vessels the blood meets with resistance,
viz., the cohesion and friction of its molecules which must be over-
come if the flow or the movement of the blood is to continue. The
statements made in previous paragraphs regarding the causes and the
results of friction in tubes with either rigid or elastic walls hold true
for the most part also for the blood-vessels. As a result of friction
the forward movement of the blood is somewhat retarded and in
consequence will exert a lateral or radial pressure against the walls of
the vessels, and from the facts previously stated, this pressure should
diminish in a more or less progressive manner from the origin of the
aorta to the ends of the venae cavse. That this is the case can be
demonstrated for the arteries and veins at least by the insertion of pres-
sure tubes and into which the blood can ascend. The height of the
pressure at any given point in the system will be proportional to the
resistance yet to be overcome.

Inasmuch as the stream-bed is not uniform in diameter the fall
in pressure will not be uniform. As it enlarges in its middle portion
the resistance lessens and theoretically there should be a rise of pressure
in passing towards the capillary system, but as the repeated branching
of the vessels is attended by an enormous diminution in the diameter of
individual vessels, the resistance is proportionately increased and for
this reason there should be a great consumption of the propelling power
and a marked and rapid fall of pressure at the periphery of the arterial
and throughout the capillary systems. That this is the case, though
not to the extent it might be, is because the effect of the increase in
resistance so largely preponderates over the effect of the increase in
the width of the stream-bed. The actual fall of pressure is therefore
the resultant of these two opposing factors. For the reverse reasons,
however, viz., the narrowing of the stream-bed and the increase in
the sectional area of individual tubes, there will be a further consump-
tion of the propelling power and a further fall of pressure to the ends
of the venae cavae. The effect of the increase in resistance of the rapidly
narrowing stream-bed preponderates over the effect of the enlarge-
ment of diameter of individual vessels. The fall of pressure from

THE CIRCULATION OF THE BLOOD. 339

the middle of the capillary system to the ends of the venae cavae will
be again the resultant of these two factors.

The pressure throughout the vascular apparatus is, of course, the
result of the resistance offered to the flow of the blood and therefore
it will be high in the aorta and its main branches and low in the large
veins. The amount and the relation of these two pressures in these
two sections of the vascular apparatus can be temporarily or permanently
changed in one direction or another by an increase or decrease in the
resistance offered to the flow of blood from the arteries through the
capillaries into the veins. This variation in the resistance is brought
about by an increase or a decrease in the degree of the contraction of
the arteriole muscles. Thus, if the muscle contraction increases, the
resistance is increased and the pressure in the arteries rises; if, on the
contrary, the muscle contraction decreases, the resistance diminishes
and the pressure falls in the arteries and rises in the veins. The
contraction of the arteriole muscle is spoken of as the peripheral
resistance.

The Distribution of the Intra-ventricular Pressure. — The
pressure developed during the ventricular contraction is thus expended
in imparting velocity to the blood and overcoming the cohesion and
friction of its molecules. The percentage of the pressure utilized in
overcoming the resistance could be approximately determined from
the pressure in the aorta if this were accurately known; the percent-
age of the pressure utilized in imparting velocity could be determined
with the formula ^, if the actual velocity of the blood in the aorta
could be experimentally determined. On account of the difficulty in
obtaining this latter factor at least, the results must only be approxi-
mative.

An idea of the ratio between the velocity pressure and the resistance
pressure, however, may be obtained from the distribution of the aortic
pressure in the dog in reference to the carotid artery. Thus, if it be
assumed that the average velocity of the blood is 35 cm., the velocitv
pressure is equal to -^ or 0.62 centimeters of blood or 0.046 centi-
meters of mercury, and if the average aortic pressure is 150 mm. of
mercury, the ratio of the velocity pressure to the resistance pressure is
as 1 to 326.

The phenomena which for the most part characterize the flow of
blood through the blood-vessels are velocity and pressure, combined
with an alternate expansion and recoil of the arterial vessels due to the
intermittent character of the heart-beat. For special reasons it is
convenient to consider the pressure first.

BLOOD-PRESSURE.

From theoretic considerations alone it may be inferred that the
blood, as it flows through the vascular apparatus, exerts a pressure
against the walls of the vessels, and that this pressure is greatest at the

34o TEXT-BOOK OF PHYSIOLOGY.

beginning of the aorta, and least at the ends of the vense cavae. The
fact that the blood flows from the aorta to the vense cavae indicates
that there is a higher pressure in the former than in the latter. The
same holds true for the pulmonary artery and veins. So long as this
is the case, the blood must flow from the point of high to the point of
low pressure.

To this pressure the term blood-pressure is given, and may be
defined as the pressure exerted radially or laterally by the moving
blood-stream against the sides of the vessels. That there is such a
pressure within the arteries, capillaries, and veins, different in amount
in each of these three divisions of the vascular apparatus, is evident
from the results which follow division of an artery or a vein of corre-
sponding size. When an artery is divided, the blood spurts from the
opening for a considerable distance and with a certain velocity. The
reason for this lies in the fact that the vessel has been distended by the
pressure from within and its walls thrown into a condition of elastic
tension, so that at the moment there is an outlet, the vessel suddenly
recoils and forces the blood out with a velocity proportional to the
distention. When a vein is divided, the blood as a rule merely wells
out of the opening with but slight momentum, and for the reason
that the vessel has been but slightly, if at all distended by the pres-
sure. These results indicate that the blood in the arteries stands
under a pressure considerably higher than that of the atmosphere,
while that in the veins stands under a pressure perhaps but slightly
above that of the atmosphere. Especially true is this of the larger
veins.

The same facts may be demonstrated in another and more striking
way. A dog or cat is anesthetized and securely fastened in an appro-
priate holder. The carotid artery on the right side and the jugular
vein on the left side are freely exposed and clamped. Into the artery
there is inserted on the distal side of the clamp and in the direction of
the heart a cannula to which is connected a tall glass tube, 200 cm.
high and of about 4 mm. internal diameter. Into the vein there is
passed on the proximal side of the clamp and in the direction of the
capillaries a second cannula, to which is connected a similar tube,
though of less height. If the two clamps are removed at the same
time, the blood will mount in both tubes simultaneously. In the
arterial tube the blood will ascend by leaps corresponding to the heart-
beats until a certain height is reached, when the column becomes
relatively stationary, being kept in equilibrium by the blood-pressure
within the vessel and the atmospheric pressure without. Though
stationary in a general sense, nevertheless, the blood-column oscillates,
rising and falling with each contraction and relaxation of the heart.
Not infrequently larger excursions of the column are seen which corre-
spond in a general way to the respiratory movements. This experi-
ment was originally performed on the horse, by the Rev. Stephen
Hales ('1732).

THE CIRCULATION OF THE BLOOD.

34i

In the venous tube the blood also rises to a certain height, after
which it remains quite stationary, as the effect of the cardiac con-
traction is not propagated under normal conditions beyond the arterial
system. The height to which it rises is but slight as compared with
that in the arterial tube. The pressure in both vessels is thus recorded
in millimeters of blood. Strictly speaking the pressure thus obtained
does not represent the lateral pressure in the carotid artery but in the
vessel from which it arises. The central end of the carotid is, under
the circumstances, but a continuation of the cannula and the pressure
thus obtained is the lateral pressure of either the innominate artery

B. P TRACING

^v-V

P^P\

W

Fig. 157. — Diagram to show the Relation of the Mercurial Manometer to
the Artery, on One Hand, and to the Recording Cylinder, on the Other Hand,
when Arranged for Recording Blood-pressure.

or the aorta as the case may be. In order to obtain the lateral pressure
in the carotid or any other artery it is only necessary to take the end
pressure of any one of its branches or what amounts to the same thing,
to divide the vessel and insert the horizontal portion of a T-shaped
tube into the central and distal ends and through which the blood can
continue to flow, and to connect the vertical portion with a vertical
pressure tube or with a mercurial manometer. The absolute pressure
on any given unit of vessel surface — e. g., 1 sq. mm. — is obtained
by multiplying the height of the column, expressed in millimeters,
by the unit of surface, and then determining the weight of this mass
of blood. Thus if the height of the column of blood in the carotid
artery tube is 2000 mm., then the pressure on r sq. mm. is 2000 mm.
of^blood. The weight of 2000 c.mm. of blood is equal to 2.1 grams.

342

TEXT-BOOK OF PHYSIOLOGY.

The Arterial Blood-pressure. — For accurate and long-continued
observation the arterial blood-pressure is more conveniently studied
by means of a U-shaped tube (a manometer) partially filled with
mercury. One limb of the manometer is connected by means of a
tube and a cannula with an artery (Fig. 157). For the purpose of
retarding coagulation of the blood and for preventing the escape of a
large volume of blood from the vessels, the system is filled with a solu-
tion of carbonate of soda of sp. gr. 1060, 5.58 grams per 1000 c.c,
and under a pressure approximately equal to that in the vessel of the

r6o

/v

f*5

f4o

r35

~3°

f25

~TZ6

■ 1

p5

fio Time Record in Seconds

rsi 1 1 | 1 1 | 1 | 1 1 l 1 1 III

Line of Atmospkeric Pressure

Fig. 158. — A Portion of a Blood-pressure Tracing Obtained from the Carotid
Artery of the Rabbit with a Mercurial Manometer. The small oscillations are
due to the heart-beat, the large oscillations are due to the respiratory movements.

animal as determined in previous experiments. When commun-
ication is established between the vessel and the cannula, the mercurial
column adjusts itself to the pressure in the artery and at once exhibits
the same cardiac oscillations and respiratory undulations as did the
column of blood in the previous experiment.

The height of the mercurial column kept in equilibrium by the
pressure of the blood within, and the pressure of air without the vessel
is that between the lower level of the mercury in the proximal, and
the higher level in the distal limb of the manometer, both of which
can be read off on a scale placed between the two limbs.

The height of the mercury as well as its oscillations in the distal
limb may be recorded by placing on the top of the mercury a light

THE CIRCULATION OF THE BLOOD.

343

max valve

to manometer

mm valve

float, the upper end of which carries a writing point. When the
latter is placed in contact with the moving blackened surface of a
recording cylinder or kymograph, the height and the oscillations are
recorded in the form of a tracing similar to that shown in Figs. 157
and 158, in which the smaller oscillations represent the changes in
pressure due to the systole and diastole of the heart and the larger
oscillations to variations in the average pressure due to the respiratory
movements. The height of the mercurial column kept in equilibrium
at any particular moment is determined by measuring the distance
between a base-line or abscissa, which represents the position of the
mercury at atmospheric pressure, and any given point on the trace
above, and multiplying it by 2, for the reason that the mercury sinks
in the proximal limb as high as it rises
in the distal limb of the manometer
and hence the column of mercury sup-
ported is that observed between the
upper and lower level of the mercury
in the distal and proximal limbs of
the manometer.

The blood-pressure as revealed by
the tracing may be resolved into two
components: viz., (1) a constant ele-
ment and represented by the pressure
in the arteries during the period of the
cardiac diastole, which is termed the
diastolic or minimum pressure; and
(2) a variable element and represented
by that additional pressure occurring
at;the time of the cardiac systole, which
is termed the systolic or maximum
pressure. The diastolic pressure is
represented by the distance between

the base-line and the points of the curve corresponding to the diastolic
rest; the systolic pressure, by the distance between the base-line and
the apices of the curves following the cardiac systole. The relation
of these two components varies in different animals and in the same
animal at different times. If the diastolic pressure is low, the systolic
increase may be considerable; if the former is high, the latter may be
slight. The relation, however, of these components is not so accurately
shown by the mercurial manometer, owing to the inertia of the mercury,
as by one of the various forms of quickly responsive spring man-
ometers used in determining the rapid variations of intra-cardiac
pressure. These instruments show a much larger rise of pressure
during the systole, often amounting to as much as one-third or one-
fourth of the diastolic pressure.

For the purpose of obtaining the maximum systolic and the mini-
mum diastolic pressures, it is best, however, to insert between the

to heart

Fig. 159. — v. Frank's Valve.
This is placed in the course of the
tube between heart and manometer,
so that the latter may be used as a
maximum, minimum, or ordinary
manometer according to the tap which
is left open. — (Starling.)

344 TEXT-BOOK OF PHYSIOLOGY.

cannula and the manometer a maximum and a minimum valve
similar in principle to that shown in Fig. 159. By permitting the blood
to exert its pressure first through the maximum valve and then per-
mitting the mercurial column to exert its pressure through the mini-
mum valve in the reverse direction for a certain length of time, or by
permitting each to exert its pressure alternately with each heart-beat,
the maximum systolic and the minimum diastolic pressures will be re-
corded. By this method Dawson found an average maximum pres-
sure in the carotid artery of the dog of 162, and a minimum pressure
of 103 mm. of mercury, a difference of 59 mm. Hg. The difference
between these two pressures is known as the pulse pressure. (A dia-
gram showing the relation of these different pressures one to another
will be found on page 346.)

In a series of experiments it will be found that the blood-pressure
in the arteries, though rising and falling a certain number of milli-
meters, yet retains a fairly constant general average, the result of an
adjustment between the number of heart-beats per minute and the
amount of the resistance offered to the escape of blood into the capil-
laries and veins. Between the two extremes there is, of course, a
certain average or mean pressure which represents the power driving
the blood through the vessels. It is frequently stated that in a tracing
in which the respiratory undulations are absent, that the mean
pressure is the arithmetic mean of the systolic and diastolic pressures.
This is, however, not strictly correct, as can be demonstrated experi-
mentally. Thus, if at some one point between the artery and the
manometer, the lumen of the connecting tube be largely obliterated
by a constriction, the variations in the pressure following the systole and
diastole of the heart will be largely, if not entirely excluded, and the
mercury, instead of rising rapidly in the manometer and fluctuating
with each heart-beat, will rise slowly to a certain level and then remain
at rest. The number of millimeters of mercury thus supported
represents the mean or absolute pressure. The same result can be
obtained by employing the compensatory manometer of Marey which
presents a constriction of this character. From many experiments
made by Dawson it has been learned that the mean pressure lies
nearer to" the diastolic than to the systolic pressure and may be ex-
pressed numerically by the statement that it is equal in millimeters
of mercury to the diastolic pressure plus one-third of the pulse pres-
sure. In a tracing in which the respiratory undulations are present
the mean pressure can be calculated. The method by which this is
done, however, is rather complicated and need not be detailed here.
In a general way the mean pressure in such a tracing may be repre-
sented by a line drawn horizontally across the tracing midway between
the apex and trough of the undulation.

Estimates of the Mean Arterial Pressure. — Because of the
difficulty in obtaining the pressure in small arteries, the experimental
determinations have for the most part been confined to large arteries

THE CIRCULATION OF THE BLOOD.

345

such as the carotid, brachial and femoral, and hence the results which
have been obtained have reference to the lateral pressure in the aorta
or in the large vessels which immediately arise from it. The pressure
obtained in the usual way at the central end of a divided carotid is
generally known as. the "end pressure" and represents the mean
lateral pressure in the aorta or in the innominate artery. Among the
results thus obtained in different experiments from the carotid arterv
of different animals are the following: In the horse, from 122 to 214
mm. Hg.; in the dog, from 140 to 160 mm.; in the cat, 150 mm.; in the
rabbit, from 90 to 100 mm.; in the sheep, 170 mm.; in the calf, from 133
to 165 mm. In two observations made on human beings previous to
the amputation of a limb, the pressure was found in the brachial
artery of one patient to vary from no mm. to 120 mm. Hg., and in the
anterior tibial artery of the other patient from no mm. to 160 mm. Hg.

The investigations made in different parts of the arterial system
indicate that the mean pressure is remarkably constant and uniform
and does not show any noticeable falling off until near the arteriole
region where the resistance suddenly and rapidly increases. Thus
Volkman found simultaneously in the carotid artery and in the meta-
tarsal artery of the sheep a mean pressure of 165 and 146 mm. Hg.
respectively and this for the reason that the resistance throughout
the arterial system does not markedly increase, until the arteriole
region is reached. The careful investigations of Dawson show that
in the large blood-vessels of the dog the diastolic pressure is as con-
stant as the mean pressure though it undergoes slight variations in
different regions; but that the systolic pressure, as shown by taking
the end pressure in the thyroid and similar sized arteries in different
parts of the arterial tree, undergoes a considerable falling off, though
it, too, remains high in large arteries.

The numerical expressions of these various pressures in different
parts of the arterial system are shown in the following table abstracted
from the more extensive tables of Dawson. The results were obtained
from experiments made on dogs. The figures represent in milli-
meters of mercury certain average end pressures in the arteries named.

Artery-

Systolic.

Mean.

Diastolic.

Pulse Pressure.

Brachio-cephalic

Right carotid. . . .

163
160

121

ttS

i°3
no

IOi

105
no
103
102
97

60

Left carotid

160 123
168 123
160 11S
165 123
152 118

4

59
63

Left subclavian

Left brachial

Left renal

b->

Deep femoral

Thyroid

5°
43

.

The Capillary Pressure. — The small size of the capillaries pre-
cludes an investigation of their pressure by manometric methods.
It may be stated, however, to be approximately equal to the pressure

346

TEXT-BOOK OF PHYSIOLOGY.

required to obliterate their lumen and to whiten the skin. The
apparatus of v. Kries is based on this theory. A small glass plate,
from 2.5 to 5 sq. mm., is fastened to the under surface of a support
of suitable size carrying a small scale pan. The glass plate is placed
on the skin near the root of a finger-nail and the scale pari gradually
weighted until the vessels are obliterated, as shown by the blanching
of the skin. From results obtained with this apparatus v. Kries

Fig. 160. — A Diagram Designed to Show the Amount and the Relation of
the Blood-pressure in the three Divisions of the Vascular Apparatus, as well
as the Relation of the Diastolic. The mean and the systolic pressures in the arterial
system. Based on experiments made on dogs. H. Heart. A. Arteries. C. Capillaries.
V. Large veins. O, O, being the zero line (= atmospheric pressure), the pressure is
indicated by the height of the curve The numbers on the left give the pressure (approxi-
mately) in millimeters of mercury, h. Pressure in heart, a. Arteriole region showing
sudden fall of pressure, c. The fall of pressure in the capillaries, v. The negative pres-
sure in the large veins.

estimated the pressure in the capillaries of the hand at 37 mm. Hg.
and in the ear at 20 mm.

The Venous Pressure. — In passing from the capillaries to the
heart the pressure continues to fall. The increasing size of the veins
permits again of manometric observations in different regions. In the
crural vein the pressure has been found to be equal to 14 mm. Hg., and
in the brachial vein 9 mm. of Hg. In the jugular and subclavian and
other vessels near the heart it is zero or even negative; that is, less than
atmospheric pressure to the extent of from 1 to 10 mm. of mercury.

The amount and relation of the different pressures in the three
divisions of the systemic vascular apparatus are approximately shown
in Fig. 160.

THE CIRCULATION OF THE BLOOD. 347

RESUME OF THE FACTS OF THE BLOOD-PRESSURE AND OF
THE FACTORS WHICH CAUSE IT.

From a consideration of the foregoing facts and statements the
following resume may be made: 1. The blood during its flow
exerts a pressure against the sides of the blood-vessels. 2 . This pressure
is the resultant on the one hand of the intra-ventricular pressure
developed at the time of the contraction, and on the other hand of the
resistance to the forward movement of the blood. 3. The resistance
is to be sought for in the cohesion and friction of the molecules of the
blood. 4. The resistance is proportional to the diameter of the
vessel and is therefore least in the large arteries and veins and great-
est in the arterioles and capillaries. 5. The pressure is highest in the
aorta where it may amount in man to 150 mm. of mercury above that
of the atmosphere, and lowest at the ends of the vena? cavae where it
may be no greater than that of the atmosphere or may be even 10
mm. Hg. below it. 6. The pressure falls from the beginning to the
end of the vascular apparatus, though not progressively, for throughout
the large vessels of the arterial system it continues relatively high. 7.
The high pressure in the aorta is due to the total resistance of the
vascular apparatus and the pressure at any given point of the appa-
ratus represents the resistance yet to be overcome. 8. The high
pressure in the arterial system and its marked fall at its periphery is
more especially the result of the very great resistance at this point,
known as the peripheral resistance, the result of a rapid diminution
in the diameter of the arterioles and the capillary vessels, modified
by the tonic contraction of the arteriole muscles. 9. The pressure
in the arterial system undergoes considerable variation both above
and below the mean pressure during the systole and diastole of the
heart.

The Heart. — The primary factor in the production of the pressure
is the pumping action of the heart. Should there be any cessation
in its activity, the elastic walls of the arteries would recoil and force
the blood into the veins. There would be coincidently a fall of pres-
sure equal to that of the atmosphere. Even under normal circum-
stances this condition is approximated during the diastole. The
recoil of the arterial wall by which the forward movement of the blood
is maintained is attended by a fall in pressure. But before this reaches
any considerable extent, the heart again contracts and forces its con-
tained volume of blood into the arteries.

That this may be accomplished it is essential that the cardiac
energy be sufficient not only to drive a portion of the blood through
the capillaries into the veins, but to oppose the recoiling arteries, and
to distend them to their previous extent, so that the incoming volume
of blood may be accommodated. This at once reestablishes the
pressure at its former level.

During the contraction of the heart the kinetic enerew is trans-

348 TEXT-BOOK OF PHYSIOLOGY.

formed into potential energy, represented by the tense distended walls
of the arteries. With the relaxation of the heart and the closure of
the semilunar valves the potential energy of the arteries is again trans-
formed into kinetic energy, represented by the moving blood. The
artery thus continues the work of the heart during its period of in-
activity. The rapidity with which the cardiac contractions succeed
each other prevents the pressure from sinking below a certain average
level.

The Resistance.- — The secondary factor is the resistance to the
flow of blood through the vessels, the nature of which has been pre-
viously stated. So long as. the resistance, and especially that variable
element of it at the periphery of the arterial system, maintains a
certain average value, so long will the pressure in each division of the
vascular apparatus maintain an average or a physiologic value.
Should the resistance at the periphery of the arterial system vary in
either direction, the result of an increase or a decrease in the degree
of the contraction of the arteriole muscle, there will arise a change in
the relative degree of pressure in each of the three divisions of the
vascular apparatus.

The Elasticity of the Vessel Walls. — A tertiary factor is the
elasticity of the arterial wall. While it can hardly be said that the
elasticity is a cause of the pressure, there can be attributed to it the
capability of modifying and assisting in the maintenance of the
pressure at a more or less constant level; for were it not for this property
of the vessel wall the variations in pressure during and after the systole
would be far more extensive than they are, and would approximate
the variations observed in tubes with rigid walls. The elasticity,
moreover, assists in the equalization of the blood-stream, converting
the intermittent and remittent flow characteristic of the large arteries
into the continuous equable stream characteristic of the capillaries.
It also permits of wide variations in the amount of blood the arteries
can contain between their minimum and maximum distention.

VARIATIONS IN THE BLOOD-PRESSURE.

A. In the Arterial Pressure. — It is evident from the preceding
statements that the arterial blood-pressure as a whole may be increased
by:

i. An increase in the rate or force of the heart's contraction.

2. An increase in the peripheral resistance.

3. An increase in both the force of the heart and the peripheral
resistance.

And that it may be decreased by:

1. A decrease in the rate and force of the heart's, contraction.

2. A decrease in the peripheral resistance.

3. A decrease in both the force of the heart and the peripheral
resistance.

THE CIRCULATION OF THE BLOOD. 349

If when the arterial pressure is in a condition of equilibrium the
heart ejects into the arteries in a given period of time an increased
quantity of blood as a result of an increased rate of contraction, there
will be an accumulation of blood temporarily in the arteries and a rise
of pressure (the peripheral resistance remaining the same), for the
reason that the pressure is only sufficient to force into the capillaries
a given volume, in the same period of time. As the pressure rises the
velocity and the outflow will be increased until equilibrium is restored
though at a somewhat higher level. A rise of pressure from an
increase in the rate of the beat alone has been questioned, for it has
apparently been demonstrated that there is a definite relation between
the normal rate and the volume discharged from the ventricle, and
that when the rate is increased, the volume discharged diminishes

Fig. 161. — A Tracing showing an Increase in the Blood-pressure in the
Carotid Artery of a Rabbit due to an Increase in the Peripheral Resistance
from a Contraction of the Arterioles Caused by Reflex Stimulation of the
Vaso-motor Center. The nerve stimulated was the sciatic. Stimulation began at s.
The rate of the heart-beat is unchanged. With the cessation of the stimulation the blood-
pressure falls for the reverse reasons.

and hence the pressure remains normal or even falls below the normal"
An increase in the pressure is readily brought about by an increase
in the force or power of the contraction, the frequency remaining the
same. An increase in the volume of blood ejected at each contraction
will necessarily lead to an accumulation. With the accumulation
there goes an increased distention of the artery and a corresponding
increase of pressure. In a short time, therefore, the increased pressure
will force out of the arteries at a higher rate of speed this excess of
blood until the outflow again equals the inflow. This restores the
equilibrium but establishes the mean pressure at a higher level.

If the peripheral resistance is increased by a contraction of the
muscle walls of the arterioles, the frequency and force of the heart
remaining the same, there will also be an accumulation of blood in the
arteries, an increased distention and consequent rise of pressure (Fig.
161). The outflow of blood will at the same time be diminished. A rise
of pressure from this cause much beyond the normal is to a large extent
prevented by a simultaneous decrease in the rate and force of the
heart-beat. This is due to a stimulation of the peripheral ends of the

35°

TEXT-BOOK OF PHYSIOLOGY.

depressor nerve, and a consequent reflex stimulation of the cardio-
inhibitor center, and not to a direct action on the heart -muscle, inas-
much as the effect is not observed after division of the vagi. When
both the force of the heart and the peripheral resistance are simul-
taneously increased there is a rapid increase in pressure ; the former
factor tends to increase, the latter factor, to decrease, the velocity of
the outflow. According as the one or the other preponderates, will
there be an increase or decrease in velocity. If they balance each

other, there will be no change.
A rise of pressure from a combi-
nation of these factors is rather
a pathologic than a physiologic
condition and is observed in cer-
tain diseases of the vascular ap-
paratus.

The converse of these state-
ments also holds true. If when
the general arterial pressure is
in a condition of equilibrium
the heart ejects into the arteries
in a given period of time a less-
ened quantity of blood, either as
a result of a decrease in the rate
or force, there will soon be a
diminution of the arterial disten-
tion and a consequent fall in
pressure (Fig. 162). The velocity
at the same time diminishes.
This continues until the outflow
no longer exceeds the inflow.
Equilibrium will again be estab-
lished, but the pressure will be at
a lower level.

If the peripheral resistance
is diminished by a dilatation of
the arterioles, the heart's con-
tractions remaining the same, the existing pressure soon diminishes.
The outflow of blood at once increases in velocity.

As a rule a diminution in peripheral resistance is attended by an
increase in the rate or force of the heart, and this is especially the case
if the pressure has been above the normal.

When both the force of the heart and the peripheral resistance are
simultaneously diminished, there will be a rapid fall in pressure. The
former factor tends to decrease, the latter factor to increase the velocity
of outflow. According as the one or the other preponderates will
there be a decrease or an increase in velocity. If they balance each
other there will be no change. This condition is also a pathologic

Abscissa

Fig. 162. — A Tracing of the Blood-
pressure in the Carotid Artery of a
Rabbit, showing a sudden decrease in
the pressure due to an arrest in the rate
and force of the heart -beat the result of
stimulating the vagus nerve from " on "
to " off." With the cessation of the stimu-
lation the pressure began to rise as the
rate and the force of the heart-beat re-
turned. (The abscissa should be 20 mm.
lower.)

THE CIRCULATION OF THE BLOOD. 351

rather than a physiologic condition and observed in states of profound
depression due to serious injuries.

Local Variations in the Arterial Blood-supply. — The varia-
tions in pressure and velocity from variations either in the activity of
the heart or in the peripheral resistance recorded in preceding para-
graphs, have reference to the arterial system in its entirety; but it is
evident from many facts that similar variations take place in special re-
gions or organs of the body. Thus, it is a well-known fact that for the
exhibition of the functional activity of every organ there must be an in-
crease in the volume of blood supplied to it in each unit of time. This
is accomplished by an active dilatation of the arterioles of the artery
of supply, and unless the area or organ supplied is large, as the
splanchnic area for example, there will be no necessary diminution
in either the general blood-pressure or the average velocity. With the
cessation of functional activity, there is no longer any need for so
large a blood-supply and hence the arterioles contract, diminish the
outflow, and raise the pressure. If, on the other hand, the area to be
supplied be large, as the splanchnic area, the dilatation of the
intestinal arteries will be attended by such a large inflow of blood that
not only will there be a fall of pressure in these vessels, but a fall of
pressure in other arteries as well, combined with a diminution in
velocity through them. With the contraction of the intestinal arteries
the reverse conditions at once arise. By constant variations in the
peripheral resistance of individual arteries in each and every region
of the body, and in association with variations in the rate or force of the
heart, the blood is shunted now into this, now into that organ in ac-
cordance with its functional needs. All variations in peripheral re-
sistance are largely brought about reflexly by the vaso-motor nerves,
the origin, distribution and mode of action of which will be considered
in subsequent paragraphs.

B. In Capillary Pressure. — The pressure in the capillaries,
though for the most part possessing a permanent value, is subject
to variations in accordance with variations in the pressure in either
the arterial or venous systems or both. The marked difference in the
pressure in the large arteries and the capillaries is partly due to the
resistance offered by the narrow arterioles. If the latter dilate in any
given area, the capillary pressure increases because of the propaga-
tion into them of the arterial pressure. The reverse condition would
decrease the pressure. On the other hand, any interference with
the outflow from any given area, due to venous compression, would
likewise increase the pressure; any factor which would, on the con-
trary, favor the outflow would decrease the pressure. Independent
of any change in the arteriole resistance, it is evident that a rise in
arterial pressure alone would increase the capillary pressure. If
both arterial and venous pressures rise, the capillary pressure increases;
if both fall, it decreases.

C. In Venous Pressure. — Independent of any change in the

352 TEXT-BOOK OF PHYSIOLOGY.

venous pressure in a given area from local or temporarily acting
causes — e. g., aspiration of the thorax or heart, muscle contractions,
change of position, etc. — the general venous pressure will be increased
by a decrease in the value of those factors which produce the difference
of pressure between the arteries and veins. An increase in the value
of these factors would necessarily decrease the pressure.

Variations in the Relation of the Arterial and Venous Pres-
sures.^— So long as the heart maintains a given rate and force and the
resistance at the periphery of the arterial system (due to the contraction
of the arteriole muscle) a given value, will the usual physiologic
difference between the pressure in the arteries and veins remain un-
changed. If, however, either factor changes in one direction or another,
there will arise a change in the relative degree of pressure in the dif-
ferent divisions of the vascular apparatus. Thus if the heart force
increases and a larger volume of blood is discharged into the arteries
in a unit of time, the amount of blood in the venous system diminishes,
and the result is a rise of the arterial and a fall of the venous pressures.
If, on the contrary, the heart force decreases or the mitral valve permits
of a regurgitation, a smaller volume of blood is ejected into the arteries
in a unit of time, the amount of blood in the venous system increases,
and the result is a fall of the arterial and a rise of the venous pressure.

Again if the arteriole muscle relaxes and a larger volume of blood
flows from the arteries into the veins in a unit of time, the result will
be a fall of arterial and a rise of venous pressure. If, on the contrary,
the arterial muscle contracts and a smaller volume of blood flows into
the veins, the reverse change of pressure obtains.

The Determination of the Arterial Blood-pressure in Man.—
Inasmuch as the blood-pressure undergoes considerable variation in
both physiologic and pathologic conditions as well as in response to
the action of drugs, it seemed desirable to possess some means by which
an accurate knowledge of the pressure under a variety of conditions
could be obtained both for diagnostic and therapeutic purposes.
The foregoing method of obtaining the blood-pressure not being of
general application to human beings for obvious reasons, special
instruments have been devised by which the pressures may be deter-
mined at least approximately without resorting to any surgical proce-
dure. These instruments are turned sphygmomanometers. Some
of the many forms of this instrument are adapted for obtaining the
systolic pressure only, while others are adapted for obtaining either
the systolic or the diastolic pressure, or both.

The principle involved in the first group is the application of an
elastic pressure to an artery, e. g., the temporal, radial, etc., until the
lumen is completely obliterated as indicated by the disappearance of
the pulse beyond the point of compression, and at the same time reg-
ister the pressure applied, by means of a mercurial or spring man-
ometer. The pressure just sufficient to obliterate the pulse or to allow
it to reappear after obliteration, is taken as the systolic pressure.

THE CIRCULATION OF THE BLOOD.

353

The principle involved in the second group is based on a suggestion
of Marev, that the maximum pulsation of the artery or the maximum
distention and recoil following a heart-beat would be most likely to
take place when an elastic pressure applied to the outside of an artery
is just sufficient to equalize the diastolic pressure within. Inasmuch
as these pulsations can be transmitted to, taken up and reproduced by
a mercurial column in connection with the pressure appliances, it
becomes possible, when the maximum oscillation of the mercurial,
column is attained, to read off the diastolic pressure.

With either form of apparatus it became necessary to devise a
suitable elastic sac or tube enclosed bv non-elastic or rigid walls and

Fig. i 6v — The Sphygmomanometer of Mosso.

which could be made to encircle a finger or an arm, and which could
in turn be connected with a pressure apparatus, and with a manometer
by which any given pressure could be registered.

One of the best known of the sphygmomanometers in that of Mosso
represented in Fig. 163. It consists essentially of rubber capsules,
contained within metallic tubes and into which two fingers of each
hand can be inserted. This system is connected, on the one hand,
with a pressure apparatus, and, on the other, with a manometer pro-
vided with a scale. A float and writing-pen record the movements
of the mercurial column on a moving blackened surface. In using
this apparatus the pressure is adjusted to the point at which the mer-
curial column exhibits the greatest oscillations.

Mosso's interpretation of the results obtained with this apparatus

was that when the greatest oscillations of the mercurial column were

taking place, the external pressure was just equal to the mean arterial

pressure, the latter being the mean between the maximum pressure

23

354 TEXT-BOOK OF PHYSIOLOGY.

during the systole and the minimum pressure during the diastole of
the heart. It was only necessary, therefore, to take the readings cor-
responding to the excursions of the mercurial column and to determine
from them the mean arterial pressure.

It has been experimentally demonstrated, however, by Howell and
Brush that this interpretation, either for this or any similar form of
apparatus, is not correct, but that the maximum oscillations take place
when the pressure applied to the exterior of the artery is just equal
to the pressure within the artery at the end of the cardiac diastole;
or in other words, the pressure in the manometer from which the
greatest oscillation takes place indicates diastolic pressure. These
experimenters connected the right carotid artery of a dog with a mercur-
ial manometer, interposing along the course of the connecting tube
a maximum and a minimum valve. The left carotid artery was sur-
rounded by a plethysmograph which was connected, on the one hand,
with both a mercurial and a spring manometer, the former for the
purpose of indicating the pressure necessary to obtain the greatest
oscillation, the latter for the purpose of magnifying and recording the
pulsation. When the observations were simultaneously made it
was found that the diastolic pressure in the right carotid measured by
the minimum manometer was almost exactly equal to the pressure
measured by the manometer in connection with the sphygmomanometer
surrounding the left carotid artery, when it was exhibiting its maxi-
mum excursions. The difference in the results of the two sides
scarcely exceeded more than one or two millimeters of mercury.
It was, therefore, established that the greatest oscillations record
diastolic pressure. It was also shown by the same investigators
that Mosso's apparatus is not adapted for obtaining systolic pressure.
Among the many forms of sphygmomanometers adapted for clinic
purposes and with which both systolic and diastolic pressures may be
obtained is that devised by Stanton* (Fig. 164). The pressure is ap-
plied to the arm by the rubber armlet h, which is 3 1 inches wide.
This is the widest armlet that can be adjusted to the average-sized
arm and presents distinct advantages over the narrow armlet hitherto
employed. This armlet is prevented from expanding outward by a
cuff, f, of double thick canvas with inserted strips of tin, which is
held in place by two straps which completely encircle the cuff. On
the rigidity of this depends to a large extent the transmission of
pulsation. The rubber armlet is. connected by glass with a stiff- walled
rubber tube, g, which in turn connects with the manometer. The
manometer is perhaps the most important part of the apparatus. It
is constructed entirely of metal except for the glass tube containing
the mercury column. The chamber c communicates by means of a
metal tube with the glass column d, which is connected by a screw-
thread at 3, the caliber of c being approximately 100 times that of d.

*The following description of this apparatus is abstracted from the Univ. of Pa.
Medical Bulletin, Feb., 1903.

THE CIRCULATION OF THE BLOOD.

355

The cap of the chamber, which screws on, is provided with a metal T
which is connected at 2 with the rubber armlet and at 1 with the bulb,
used as an air-pump. At a is a stopcock shutting the rubber bulb
completely from the rest of the apparatus while at b is a screw-valve
which allows the air to escape from the closed system. When de-
sired, the manometer can be made portable (without removing the
mercury) by screwing the caps 1 and 2 into either end of the T at
1 and 2. The manometer is then tilted away from the glass column
d until all the mercury has run into the chamber, the glass is then

Fig. 164. — Stanton's Sphygmomanometer.

unscrewed and cap 3 screwed in. Before removing cap 3 the man-
ometer must always be tilted, else the mercury will be lost.

The rubber bulb is similar to those found on atomizers.

In using this apparatus the pressure is raised by the air-bulb
forcing air into the closed system — distending the rubber armlet and
with the same degree of force displacing the mercury in c, driving
it up the glass column d. When the pulse is no longer felt, the bulb
still being compressed, the arm of valve a is turned until it is at right
angles with the thumb and finger. The valve b is now slowly unscrewed
until the mercury column begins to fall. With the eye on the scale
the point at which the pulse reappears is mentally noted as the systolic
pressure. Often considerably before the reappearance of the pulse

356

TEXT-BOOK OF PHYSIOLOGY.

to palpation, a pulsation is seen in the mercury column. As the
column slowly falls this increases up to its greatest oscillation and then
diminishes. The lowest point of the greatest pulsation is noted as
the diastolic pressure.

Fig. 165. — Erlanger's Sphygmomanometer.

Erlanger's sphygmomanometer is, also, a most valuable instrument
for obtaining both systolic and diastolic pressure. It possesses, an
advantage in that it is provided, in addition to the mercurial man-
ometer, with a tambour and lever by which the changes in pressure can

THE CIRCULATION OF THE BLOOD. 357

also be recorded on a revolving cylinder (Fig. 165). A complete de-
scription of this apparatus, the manner of using it and the results that
can be obtained with it will be found in the Johns Hopkins Hospital
Reports, Vol. XII.

Any statement as to the numerical values of the different pressures
is somewhat difficult to make inasmuch as they will vary within phys-
iological limits in accordance with the position of the body, exercise,
character of psychic states, digestion, temperature and other conditions.
For comparative investigations it is necessary, therefore, to place the
subject of the investigation in one and the same position, to apply the
cuff to the corresponding arm, to use always a uniform width of cuff
and to select the same time of day with reference to meals, etc.

It may be stated, however, that in adult life the systolic pressure
in the brachial artery ranges from no to 135 millimeters of Hg. in
men and about 10 mm. less in women; the diastolic pressure ranges
from 6s to no mm. Hg. ; the pulse pressure ranges from 25 to 40 mm.

The conclusions of Erlanger regarding the results of his investi-
gations with this apparatus may be partially summed up in the follow-
ing statements, and as they hold true for other forms of apparatus
which determine both systolic and diastolic pressures, they are here
appended: "The pressure that is determined by occluding an artery
is probably the maximum end pressure of the artery occluded. The
pressure determined by the method of maximal oscillations is the
minimum lateral pressure of the artery compressed, and, therefore,
as the minimum lateral pressure is the same in all of the larger ar-
teries, the pulse pressure, determined when the pressures in the brachial
artery are observed, tends to approximate the lateral pulse pressure
in the aorta."

THE VELOCITY OF THE BLOOD.

From the number of heart-beats per minute, 72, and the amount
of blood discharged from the left ventricle at each beat, 180 c.c, it is
evident that the blood must be flowing through the vascular appa-
ratus with a certain velocity, for during the minute the entire volume
of blood, 5769 grams, must have passed twice through the heart.
Direct observation of the escape of blood from the central end of a
divided artery, and from the peripheral end of a divided vein, as well
as of the flow through the capillaries as seen with the microscope,
shows that the velocity of the flow varies in different parts of the vas-
cular apparatus. In the arteries, moreover, the flow is not quite
uniform, but experiences alternate acceleration and retardation with
each heart-beat. In the capillaries and veins the flow is continuous
and uniform, as the conditions of the arterial walls are such as to
completely overcome the intermittency.

If the systemic vascular apparatus be conceived of as a system

358 TEXT-BOOK OF PHYSIOLOGY.

of tubes which have symmetrically divided and subdivided, and have
again united and reunited in a corresponding manner, it is clear that
the total sectional area will steadily increase from the beginning to
the middle of the system, and then as steadily decrease from the middle
to the end of the system. In such a system the same volume of blood
must pass through any given section in a unit of time if the balance
of the circulation is to be maintained. As the velocity of a fluid is
inversely as the sectional area of the tubes through which it flows, it
follows that the initial mean velocity of the blood in the aorta will
steadily decrease as it flows into the steadily enlarging stream- bed
until it reaches a minimal value in the middle of the capillary system;
and that it will again steadily increase as it flows into the narrowing
stream-bed until it reaches the heart. The initial mean velocity of
the blood in the aorta will not be attained in the venae cavae, for the
reason that the total sectional area of the latter is somewhat greater
than that of the former. The same facts hold true for the pulmonic
vascular system.

The Mean Velocity in the Aorta. — From the well-known fact
that the velocity with which a fluid is flowing through a tube may be
determined by dividing its sectional area into the quantity discharged
in a unit of time, attempts have been made to determine the mean
velocity of the blood at the beginning of the aorta. If it be assumed
that the volume discharged at each contraction is 180 c.c, as stated by
Vierordt, and the number of heart-beats per minute at 72, the total
volume discharged per minute would be 12,960 c.c, or 215 c.c. per
second. The sectional area of the aorta at its origin is 6.15 sq. cm.
On the principle above stated, these two factors would show a velocity
of 350 mm. per second. An objection to this estimate is that the amount
of blood discharged — i. e., the contraction volume— is much larger
than recent investigations warrant. Different observers have estimated
that in man the contraction volume is considerably less, probably not
more than 80 c.c.

The Mean Velocity in the Arteries. — The mean velocity of the
blood in the larger and more superficially lying arteries has been de-
termined by Volkmann with the hemodromometer, by Ludwig and
Dogiel with the stromuhr, and by other investigators with different
forms of apparatus.

Since neither the blood nor any particle placed in it can be seen
through the walls of the artery, it occurred to Volkmann to inter-
calate along the course of a vessel a U-shaped glass tube about one
meter in length with a lumen the diameter of that of the selected
vessel, into and through which the blood could be made to flow.
The mechanic construction of the apparatus is such (Fig. 166) that
the blood can be made to flow directly into the distal portion of the
artery across the base or indirectly by way of the glass tube. Pre-
vious to the intercalation of the tube it is filled with serum or normal
saline solution. With the turning of the cocks at B the blood enters

THE CIRCULATION OF THE BLOOD.

359

the glass tube and drives the serum ahead of it into the arterial system.
From the difference in time between the moment the blood enters and
the moment it leaves the tube and from the length of the tube the veloc-
ity is determined.

The stromuhr or rheometer of )(

Ludwig (Fig. 167) is constructed on
the same principle, but instead of the
glass tube having the same diameter
it is considerably enlarged on its two
sides. The bulbs are fastened to a
metallic disk which rotates around an
axis in the metallic base which carries
the tubes to be inserted into the arteries.

Fig. 166. — Volkmann's Hemodromom-
eter. C, C. Arterial cannulas.

Fig. 167. — Ludwig and
Dogiel's Rheometer. X, Y.
Axis of rotation. A, B. Glass
bulbs, h, k. Cannulas inserted
in the divided artery, e, eIt
rotates on g, f. c, d. Tubes.

With this device it is possible to place either bulb in connection with the
proximal end of the artery. Previous to the experiment the proximal
bulb is filled with oil, the distal bulb with serum or normal saline.
On removing the clips on the artery the blood flows into the proximal

360

TEXT-BOOK OF PHYSIOLOGY

bulb and drives the oil into the distal bulb. As soon as the former is
filled with blood the bulbs are reversed and the same relative condi-
tions are attained. This is repeated a number of times. Knowing
the capacity of the bulbs, and the number of times they are filled in a
given period, the total quantity of blood discharged is obtained. This
divided by the sectional area of the artery gives the velocity. The
following values have thus been obtained : For the carotid of the dog,
205 to 357 mm. per second; for the carotid of the horse, 306 mm.;
for the metatarsal artery of the horse, 56 mm. (Volkmann). For the
carotid of rabbits, 94 to 226 mm.; for the carotid of the dog, 349 to 733
mm. (Dogiel).

The variations in the velocity of the blood in the arteries during

the different phases of the
cardiac cycle have been de-
termined by Chauveau and
Lortet with the hematach-
ometer (Fig. 168). This con-
sists of a metallic tube carry-
ing a graduated disk. At one
point the tube is perforated
but covered with a rubber
band through which passes an
index. When the tube is in-
serted into the divided ends of
an artery, the current of blood
strikes the short arm of the
index and gives to the outer
long arm a movement in the
opposite direction. The ex-
tent of the excursion indicates
the velocity. The apparatus
is first graduated with currents
of water of known velocity.
With this instrument Chau-
veau found that in the horse the velocity during the systole was 520
mm. per second, at the beginning of the diastole 220 mm. per second,
and during the pause 150 mm. per second.

The Velocity in the Capillaries. — The rate of flow in the capil-
lary vessels can not be experimentally determined. It has been esti-
mated by Vierordt at 0.5 mm. per second in his own retinal capillaries;
by Weber at 0.8 mm. In frogs the velocity can be fairly well deter-
mined by observing the time required for a corpuscle to pass over one
or more divisions of an ocular micrometer. Weber calculated in this
way that the velocity is 0.5 mm. per second.

As the velocity varies inversely with the sectional area, it becomes
possible to approximately determine the relation of the sectional area
of the capillary system to that of the aorta from the above-mentioned

Fig. 168. — The Hematachometek of Chau-
veau and Lortet. A, B. Tube inserted in
artery. C. Lateral tube connected with a manom-
eter, b. Index moving in a caoutchouc mem-
brane, a. G. Handle.

THE CIRCULATION OF THE BLOOD.

?6i

velocities. If it be assumed that the velocity in the aorta averages
300 mm. and in the capillaries 0.5 mm. per second, then the sectional
area of the capillaries is to that of the aorta as 600 to 1.

The Velocity in the Veins. — In the venous system the velocity
increases in proportion as the sectional area decreases. In the jugu-
lar vein Volkmann found the velocity 225 mm. per second, which
was about one-half that in the aorta of the same animal. The reason
for the slow rate of movement in the jugular vein is to be found in the
fact that the sectional area of the combined venae cavas is about twice
that of the aorta; hence the relation of the sectional area of the cap-
illary system to the sectional area of the venae cavae is about 300 to 1.

Arteries. Capillaries. Veins.

Fig. 169. , Blood-pressure. , Velocity. — o — o — o — o, Sectional area.

The blood-pressure, the velocity of the blood, the sectional area
of the vascular apparatus, and their relation one to the other are
shown in Fig. 169.

The Relations of Blood-pressure and Velocity. — Though the
pressure of the blood bears a definite relation to the velocity it must be
kept in mind that it is rather the difference in pressure between the
beginning and the termination of the arterial system, rather than the
mean pressure that influences the velocity. Thus, with a given force
of the heart and a given peripheral resistance, the velocity will have
a given value, and so long as these factors remain constant will the
velocity remain constant, even though the mean pressure should fall,
as from a hemorrhage, or should rise, as from an injection of some
indifferent fluid.

If, however, the primary factors, viz., the cardiac force or the
peripheral resistance, change their values in either the same or opposite
directions, there will be a change at once in the velocity. The varia-
tions in pressure and velocity, both in the same and opposite directions,
which are theoretically possible from a change in the force of the heart.

362

TEXT-BOOK OF PHYSIOLOGY.

or in the peripheral resistance, or both, are shown in the following
table arranged by Waller. The plus sign indicates increase, the
minus sign, decrease, in effect.

No.

Heart

Arterioles

Blood-pressure

Blood-flow

i

2

/ Force constant ....
\ Force constant ....

Resistance increased
Resistance diminished

+

+

3
4

/ Force increased
\ Force diminished .

Resistance constant
Resistance constant

+

+

5
6

/ Force increased ....
\ Force diminished . .

Resistance diminished
Resistance increased

+ -

- +

+ +

7
8

f Force increased ....
\ Force diminished . .

Resistance increased
Resistance diminished

+ +

+ -
- +

The statements herein embodied have been established by Marey
with an artificial schema of the circulatory apparatus, and by Chau-
veau and Lortet by experiments on animals with the hemodromo-
graph, a specially devised apparatus for this purpose.

Though all the relations between pressure and velocity in the table
are possible, those which are most physiological are probably 5 and
6, for in both instances there is a minimum alteration in pressure, but
a maximum alteration in blood flow or velocity. The first instance
is the condition most favorable for the functional activity of organs,
for the reason that the volume of blood which the organ receives in a
unit of time is increased without any change in pressure; and it is an
established fact that within physiological limits it is the volume of
blood which an organ receives rather than the pressure under which it
is received, that determines its activity. In the second instance, on
the cessation of activity the velocity is decreased and the normal
condition restored without any appreciable change in pressure.

THE PULSE.

The pulse may be defined as a periodic expansion and recoil of
the arterial system. The expansion is caused by the discharge from
the heart into the arteries of a volume of blood during the systole; the
recoil is due to the elastic reaction of the arterial walls on the blood,
driving it forward, into, and through the capillaries, during the dias-
tole.

At the close of the cardiac diastole the arterial system is full of
blood and considerably distended. During the occurrence of the
succeeding systole, a definite volume of blood is again discharged into
the aorta. The incoming volume of blood is now accommodated by the
discharge of a portion of the general blood volume into the capillaries
and by the expansion of the arteries. The expansion naturally begins
at the root of the aorta and at the beginning of the systole. As the
blood continues to be discharged from the heart, adjoining segments

THE CIRCULATION OF THE BLOOD. 363

of the aorta and its branches expand in quick succession, and by the
time the systole is completed the expansion has traveled over the entire
arterial system as far as the capillaries. With the cessation of the
systole and perhaps even before, the recoil of the arterial walls at once
occurs, beginning at the root of the aorta and rapidly passing over the
arteries to the capillaries.

This expansion movement which thus passes from the beginning
to the end of the arterial system in the form of a wave is known as the
pulse-wave or pulse. Preceding and causing the expansion and recoil
of the arterial system there is an alternate increase and decrease of the
general blood-pressure, as shown by the small curves on a blood-pressure
tracing, and for this reason the pressure which causes the expansion
and recoil is termed the pulse pressure. It is defined as the rhythmic
change in pressure at any given point of the arterial system; and in
amount, is the difference between the diastolic and the systolic pres-
sures, at the corresponding points. The volume of blood ejected
from the ventricle is frequently termed the pulse volume.

The pulse-wave which thus spreads itself over the entire arterial
system with each systole of the heart can be perceived in certain locali-
ties by the eye, by the sense of touch, and investigated with various
forms of apparatus or instrumental means. The pulse-wave, or at
least the elevation of the soft tissues overlying it, can be seen in the
radial artery, where it passes across the wrist-joint, in the carotid
artery, in the temporal artery, in the arteries of the retina under certain
conditions, with the ophthalmoscope. If the ends of the fingers are
firmly placed over the radial artery, not only the increase and decrease
of pressure, but also many of the peculiarities of the pulse-wave, may
be perceived. Without much difficulty it may be perceived that the ex-
pansion takes place quickly, the recoil relatively slowly; that the waves
succeed one another with a certain frequency, corresponding to the
heart-beat; that the pulsations are rhythmic in character, etc. Inas-
much as the individuality of the pulse-wave varies at different periods
of life and under different physiologic and pathologic conditions, vari-
ous terms more or less expressive, have been suggested for its varying
peculiarities. Thus the pulse is said to be frequent or infrequent accord-
ing as it exceeds or falls short of a certain average number — 72 per
minute; quick or slow, according to the suddenness with which the ex-
pansion takes place or strikes the fingers; hard or soft, tense or easily
compressible, according to the resistance which the vessel offers to its
compression by the fingers; large, full, or small, according to the vol-,
ume of blood ejected into the aorta, or, in other words, the degree of
fullness of the arterial system.

Frequency of the Pulse. — As the pulse or the arterial expansion
and recoil is the direct result of the heart's action, its frequency must,
under physiologic conditions, coincide with that of the heart. All
conditions which modify the rate of the heart will modify at the same
time the rate of the pulse.

364

TEXT-BOOK OF PHYSIOLOGY.

The Velocity of Propagation of the Pulse-wave.— The propag-
ation of the pulse-wave from its origin at the root of the aorta to any
given point of the arterial system occupies an appreciable period of
time. The difference in time between the systole and the appearance
of the pulse-wave at the dorsal artery of the foot can be appreciated
by the sense of touch. The absolute time occupied by the wave in
reaching this point was determined by Czermak to be 0.193 second.
The rate at which the wave is propagated over the vessels of the lower
extremity has been estimated by the same observer at 11. 16 meters
per second, and for the upper extremities at but 6.7 meters per second.
Other experimenters have obtained for the lower extremities some-

Fig. 170. — Von Frey's Sphygmograph. G. S. Metal framework. P. Button at
tached to spring. F. Vertical rod. U. Clock-work which turns the recording cylinder.
VI. Time markei.

what different results, varying from 6.5 to 11 meters per second.
Weber's original estimate was from 7.92 to 9.24 meters per second.
The slower rate of movement in the vessels of the upper extremities
has been attributed to a greater distensibility of their walls, a condi-
tion unfavorable to rapid propagation. For this reason a low arterial
tension will occasion a delay in the appearance of the pulse-wave in
any portion of the body; a high arterial tension will of course have the
opposite effect. The difference in the speed of the pulse-wave and
the blood-current shows that they are not identical and must not be
confounded with each other.

The Sphygmograph. — The sphygmograph is an apparatus de-
signed to take up, reproduce, and record the alternate expansion and
recoil of an artery caused by the temporary increase and decrease of
pressure following each heart-beat. The tracing or record obtained
with it is termed the pulse-curve or the sphygmogram. Different forms
of this apparatus have been devised by Marey, Dudgeon, v. Frey, and

THE CIRCULATION OF THE BLOOD. 365

many others. The instrument of v. Frey is shown in Fig. 170. This
consists first of a metal framework by which the apparatus is fastened
to the arm and support given to the lever, recording surface, etc. The
essential part is the spring carrying a button which is placed over the
artery, usually the radial, before it crosses the wrist-joint. A vertical
rod transmits the movement of the spring to the recording lever; the
movements of the latter are recorded on a small cylinder inclined
slightly so that the upstroke may be vertical. A small electro-magnet
serves to record the time relations of the changes in the blood-pressure.
An average tracing taken from the radial artery is shown in Fig. 171.
This, however, is not a tracing of the pulse-wave, but rather a record
of the changes in pressure, their succession and time relations, which
follow each beat of the heart. The
artery usually selected for obtaining a
sphygmogram is the radial. This artery
lies quite superficially, covered only by
connective tissue and skin and supported
by the flat surface of the radial bone,
conditions most favorable to technical
investigation. FlG i?i._The pulse-curve or

The sphygmogram or pulse-curve Sphygmogram.

may be divided into two portions: viz.,

a line of ascent from a to b, and a line of descent from b to d (Fig.
171). In normal tracings the former is almost vertical and caused by
the sudden expansion of the artery immediately following the ventric-
ular contraction; the latter is in general oblique, due to the recoil of
the arterial walls, occupies a longer period of time, and is marked by
several elevations and depressions, both of which indicate that the
restoration to equilibrium is neither immediate nor uncomplicated.
One of these elevations is quite constant and known as the dicrotic wave,
c; the depression or notch just preceding it is known as the dicrotic
notch. Pre- and post-dicrotic waves are not infrequently present. The
summit is generally sharp and pointed.

The vertical direction of the line of ascent is taken as an indica-
tion that the arterial walls expand readily, that the blood is discharged
quickly, and that the ventricular action is not impeded. An oblique
direction of the line of ascent is an indication that the reverse condi-
tions obtain. The height varies inversely as the arterial pressure,
other things being equal; being high with a low pressure, and low with
a high pressure.

The dicrotic elevation shows that a second expansion wave is de-
veloped which interrupts temporarily the recoil of the arterial walls.
The origin of this second expansion has been the subject of much
investigation, and at present it may be said that the question is not
fully decided. It is asserted by some investigators that it is central
in origin, beginning at the base of the aorta and passing to the per-
iphery; by others, that it is peripheral in origin, beginning near the

:66

TEXT-BOOK OF PHYSIOLOGY.

capillary region and reflected to the heart. The former view is the
one more generally accepted. According to it, the expansion is the
result of the sudden closure of the aortic valves, and a backward surge
of the blood column against them. The sudden arrest of the blood and
its accumulations again expands the aorta.

The dicrotic notch is therefore taken as the moment at which the
ventricular systole ceases -and the aortic valves close. From this fact
it is evident that immediately after the first expansion the pressure
begins to fall, even though the ventricular systole continues, owing
to the discharge of blood from the arterial into the capillary and venous
systems. The height of the dicrotic wave or the depth of the dicrotic
notch is favored by low arterial tension and highly elastic arteries.
Both features are diminished by the reverse conditions. The apex is
sometimes rounded and even flat, indicative of a great diminution in
arterial elasticity. The sphygmogram not infrequently varies con-

AJ

Fig. 172. — Mosso's Plethysmography G. Glass vessel for holding a limb. F.
Flask for varying the water-pressure in G. T. Recording apparatus. — (Landois and
Stirling.)

siderably from the normal type in different pathologic conditions of
the circulatory apparatus. A consideration of these variations does
not fall within the scope of this work.

The Volume Pulse. — If an individual artery expands with each
systole and recoils with each diastole of the heart, the same is true of
all arteries, and as a result the volume of any organ or part of the body
must undergo similar changes. To such alternate changes in volume
the term volume pulse is given. The extent to which an organ will
increase in volume will depend to some extent on its elasticity. The
reason for the increase in volume is the resistance offered to the flow
of blood into and through the capillaries; the decrease in volume to
the overcoming of the resistance through the arterial recoil.

The variations in volume may be recorded by enclosing the organ
in a rigid glass or metal vessel, which at one point is in communication
with a recording apparatus, e. g., a tambour with a lever or mer-
curial manometer with float and pen. The space between the organ
and vessel is filled with normal saline, air, or oil. Such an apparatus
is known as a plelhy sinograph. A well-known form of plethysmograph

THE CIRCULATION OF THE BLOOD.

367

is that of Mosso (Fig. 172). Many forms of this apparatus have been
devised in accordance with the character of the organ — spleen, kidney,
etc. — to be investigated, though the principle underlying them is es-
sentially the same. In addition to changes in volume due to the heart's
action, most organs undergo additional changes in volume from vaso-
motor and respiratory causes.

Indeed the plethysmographic is the most generally employed method
to show the action of vaso-motor nerves in changing the resistance
of the arterioles and hence the
outflow of blood. Thus when
an organ is enclosed in a plethys-
mograph and the arterial pres-
sure raised by either a direct
stimulation of vaso-motor nerves
or a reflex stimulation of the
vaso-constrictor center, there is
always a decrease in the volume
of the organ under observation;
and on the contrary, when the
vaso-dilatator nerves or centers
are stimulated and the vessels
dilated, there will be a fall of
pressure, an increased outflow of
blood and an increase in the
volume of the organ. From this
it is learned that the functional
activity of an organ which is
attended and conditioned by an
increased blood-supply is always
associated with an increase in
volume. On plethysmographic
records large undulations are frequently observed which are regarded
as of respiratory origin.

Fig. 173. — The Vessels of the Frog's
Web. a. Trunk of vein, and (b, b) its
tributaries passing across the capillary net-
work. The darkispots are pigment cells. —
(Yeo's "Physiology.")

THE CAPILLARY CIRCULATION.

In certain regions of the body of many animals it is possible, on
account of the delicacy and transparency of the tissues, to observe
not only the flow of blood through the smaller arteries, capillaries,
and veins, but many of the phenomena connected with it, to which
reference has already been made. The structures usually selected
for the observation of these phenomena are the interdigital membranes
(Fig. 173), the tongue, the lung, the bladder, and the mesentery of the
frog. Though any one of these structures will afford an admirable
view of the blood-flow, the mesentery for many reasons is the most
satisfactory. For a comparison of the phenomena observed in the cold-
blooded animals with those in the warm-blooded animals the omentum

368 TEXT-BOOK OF PHYSIOLOGY.

of the guinea-pig may be employed. If the frog is the subject of ex-
periment, it should be slightly curarized and the brain destroyed by
pithing. The animal is then placed on a small board capable of ad-
justment to the stage of the microscope. The abdomen is then opened
along the side and a loop of intestine withdrawn and placed around
a cork ring which surrounds an opening in the side of the frog board.
The loop of the intestine should be so placed that it will lie between
the observer and the body of the frog. The mesentery thus exposed
must be kept moist with normal saline solution.

When examined with low powers of the microscope, arteries, veins,
and capillaries will be found occupying the field of vision. Their
general arrangement, their size and connections, can be readily deter-
mined. After a few preliminary adjustments a region will be found
in which the blood is flowing in opposite directions. The vessel
apparently carrying blood away from the observer is an artery; the
vessel apparently carrying blood toward the observer is a vein; the
smallest vessels are capillaries. The blood in the artery is of a brighter
color than the blood in the vein; the blood in the capillaries is almost
colorless. The arterial blood-stream not infrequently shows remit-
tancy, an alternate acceleration and retardation, corresponding to each
heart-beat; the capillary and venous streams are uniform and con-
tinuous. The relative velocities in the three sets of vessels are indicated
by the movement of the red corpuscles. In the arteries they pass
before the eye so rapidly that they can not be distinguished; in the
capillaries they pass so slowly that both form and structure may be
determined; in the veins, though again moving rapidly, they can often
be distinguished.

The relative positions of the red and white corpuscles in the
blood-stream are also apparent; the former occupy the central, the
latter the peripheral portion, at the same time adhering to the sides
of the vessel. Between the axial portion of the stream occupied by
the red curpuscles and the wall of the vessel there is a clear still layer
of plasma, the result of an adhesion of the plasma to the wall. It is
this feature which gives rise to the friction between successive layers
of the blood-stream, the resistance to the blood-flow, and the devel-
opment of blood-pressure. The relative breadth of the still layer
and amount of friction are greater in small than in large vessels.

The volume of blood passing into any given capillary area is de-
termined by the degree of contraction of the arterioles. Thus on the
application of warm saline solution, which relaxes the arterioles, there
is a large increase in the inflow of blood; vessels previously invisible
suddenly come into view as the blood with its corpuscles passes into them.
On the application of cold water, which contracts the arterioles and
diminishes the inflow, many of the smaller vessels entirely disappear
from view. The alternate contraction and relaxation of the arterioles
will therefore determine the quantity of blood flowing into and through
the capillary system.

THE CIRCULATION OF THE BLOOD.

369

Migration of the White Corpuscles. — A phenomenon fre-
quently observed in the capillary vessels of the mesentery or of the
bladder of the frog is the passage of the white corpuscles through the
walls into the surrounding lymph-spaces. To this process the term
migration or diapedesis is given. After the tissues have been exposed
to the air for some time or subjected to an irritant, the vessels dilate
and become distended with blood. In a short time the blood-stream
slows, and finally comes to rest. The condi-
tion of stasis is then established. During the
development of this condition the white cor-
puscles accumulate in large numbers along the
inner surface of the vessels and soon begin to
pass through the vessel-walls. This they do
by protruding a portion of their substance and
inserting it into and through the vessel-wall.
This once accomplished, the remainder of the
cell in due time follows until it has entirely
passed out into the tissue-space. The opening
in the cell-wall now closes. The successive
steps in this process are shown in Fig. 174.
As this migration occurs mainly after the cir-
culation has ceased or when the tissues present
the phenomena of approaching inflammation,
it is difficult to state in how far it is strictly a
physiologic process.

The Venous Circulation. — The blood,
having passed through the capillary vessels, is
gathered up by the veins and conveyed to the
right side of the heart. As the veins converge
and unite to form larger and larger trunks the
sectional area gradually diminishes, and hence
the velocity of the blood-flow increases, though
it never attains the velocity, even in the venae
cavae, that it had in the aorta, for the reason
that the sectional area of the venae cavae is
considerably larger than that of the aorta.
The pressure also is very low in the larger
veins because the friction still to be overcome is relatively very slight.

The capacity of the venous system is considerably greater than
that of the arterial system, as there are usually two and even three
veins accompanying each artery. This, taken in connection with its
greater distensibility, makes of the venous system a reservoir in which
blood can be stored. On this reservoir the arterial system can call
for that amount of blood necessary for the maintenance of its normal
volume and pressure, and into it any excess can be discharged. The
relative amounts contained in the two systems are regulated by the
nervous svstem. The movement of the blood through the veins is

Fig. 174. — Diagram to
show Various Stages in
the diapedesis or mi-
GRATION oe White Cor-
puscles.

37o TEXT-BOOK OF PHYSIOLOGY.

accomplished by the cooperation of several forces, reference to which
will be made in a following paragraph.

THE PULMONIC VASCULAR APPARATUS.

The pulmonic vascular apparatus consists of a closed system
of vessels extending from the right ventricle to the left auricle, and
includes the pulmonary artery, capillaries, and pulmonary veins. In
its anatomic structure and physiologic properties it closely resembles,
if it is not identical with, the systemic apparatus.

The stream-bed widens from the beginning of the pulmonary
artery to the middle of the capillary system; it again narrows from
this point to the terminations of the pulmonary veins.

The movement of the blood from the beginning to the end of the
system is due to a difference of pressure between these two points,
the result of the friction between the blood and the vascular walls.
From the difference in the extent of the pulmonic and systemic systems
it is evident that, other things being equal, the friction .is less, and
therefore also the pressure is less in the former than in the latter. This
view is supported by the difference in the thickness of the walls of
the right and left sides of the heart. The pressure in the pulmonary
artery of the dog was shown by Beutner to be about one-third that
in the aorta; by Bradford and Dean to be one-fifth. The velocity
of the blood-stream in each of the three divisions of the system can
not well be determined. The time occupied by a particle of blood
in passing from the right to the left ventricle has been estimated at
one-fourth the time required to pass from the left to the right ven-
tricle. Assuming the latter to be thirty seconds, the former would be
seven and one-half seconds.

The capillary vessels are spread out in a very elaborate manner
just beneath the inner surface of the pulmonary air-cells, and form,
by their close relation to it, a mechanism for the excretion of carbon
dioxid and the absorption of oxygen. The extent of the capillary
surface is very great. It has been estimated at 200 square meters.
The amount of blood flowing through this system hourly and exposed
to the respiratory surface is about 800 liters. The reason for the ex-
istence of the pulmonary circulation is the renewal of the oxygen
volume in the blood and the elimination of the carbon dioxid; for the
accomplishment of both objects ample provision is here made. The
flow of blood through the cardio-pulmonary vessels is subject to varia-
tion during both inspiration and expiration in consequence of their
relation to the respiratory apparatus. The mechanism by which these
variations are produced will be considered in the chapter devoted to
Respiration.

FORCES CONCERNED IN THE CIRCULATION OF THE BLOOD.

1. The Contraction of the Heart. — The primary forces which keep
the blood flowing from the beginning of the aorta to the right

THE CIRCULATION OF THE BLOOD. 371

side of the heart and from the beginning of the pulmonary artery
to the left side are the contractions of the left and right ventricles
respectively. This is evident from the fact that each ventricle
at each contraction not only overcomes the pressure in the aorta
and pulmonary artery, the sum of all resistances, but imparts a
given velocity to the blood. Since the pressure continuously
falls from the beginning to the end of each system, it follows that
the blood must flow from the point of high to the point of low
pressure. During the interval of the heart's activity the walls of
the arteries, to which the heart's energy was largely transferred,
now take up and continue the work of the heart, and by recoiling
drive the blood forward and into the venous system. Though
the heart's energy is probably sufficient to drive the blood into
the opposite side of the heart, it is supplemented by other
forces — e. g.:

2. Muscle Contraction. — As a result of the relation which the veins

bear to the muscles in all parts of the body it is clear that with
each contraction and relaxation of the muscles there will be exerted
an intermittent pressure on the veins. With each contraction,
the blood on the proximal side will at once be driven forward
with increased velocity, while that on the distal side will be re-
tarded, will accumulate and distend the veins, owing to the
closure of the valves; with the relaxation of the muscle the elastic
and contractile tissues in the walls of the veins will come into
play and force the blood forward.

3. Thoracic Aspiration. — The inspiratory movement aids the flow

of blood through the venae cavae and their tributaries. With
each inspiration the pressure within the thorax but outside the
lungs undergoes a diminution more or less pronounced in ac-
cordance with the extent of the movement. As a result, the
blood in the large veins outside of the thorax, being now subjected
to a pressure greater than that in the thorax, flows more rapidly
toward the heart. With each expiration the reverse obtains.

4. Action of the Valves. — It is quite probable that gravity opposes

to some extent the flow of blood through the veins below the level
of the heart. This opposition to the upward flow is largely pre-
vented by the valves, for each retardation is immediately checked
by their closure and support given to the column of blood. The
influence of gravity is shown when the relation of the arm to
the heart is changed. Thus, if the arm be allowed to hang pas-
sively by the side of the body, the veins, especially on the back of
the hand, will become distended with blood. If now the arm be
raised, the blood will flow rapidly toward the heart, as shown by
the rapid emptying of the veins.
Work Done by the Heart. — The work which the left ventricle

performs at each contraction when it discharges its contained volume

of blood into the aorta is:

372 TEXT-BOOK OF PHYSIOLOGY.

i. To overcome the total resistance of the systemic vascular appa-
ratus expressed in terms of aortic pressure; and —
2. To impart velocity to the blood.

The pressure in the aorta is not absolutely determined, though
for many reasons it may be assumed to be about 150 mm. Hg.,orits
equivalent, a column of blood 1.93 meters in height. If the volume
of blood which the heart discharges is assumed to be 188 grams, the
work done may be calculated by multiplying the weight by the height :
viz., 0.188 X 1.93 = 0.3628 kilogrammeter.

The velocity of the blood in the aorta has been approximately
estimated at 0.5 meter per second. The work done in imparting this
velocity to 188 grams is estimated by squaring the velocity and dividing
by the accelerating force of gravity (oX598l) and multiplying the
quotient by 0.188. The quotient of the first two values represents
the distance a body would have to fall to acquire this velocity: viz.,
0.0127 meter. The work done is therefore 0.188 X 0.0127, or 0.0023
kilogrammeter.

The entire work of the left ventricle is the sum of these two amounts,
or 0.3651 kilogrammeter. Assuming that the heart beats 72 times per
minute, the work done daily would be 0.604 X 72 X 60 X 24, or
37,857.6 kilogrammeters. The right ventricle approximately per-
forms about one-third of this amount of work in overcoming the
resistance offered by the pulmonary system and in imparting velocity
to its contained volume of blood. The work of the entire heart would
therefore be for the twenty-four hours about 50,476 kilogrammeters.

THE NERVE MECHANISM OF THE VASCULAR APPARATUS.

The middle coat of the arteries, and especially of those in the
peripheral region of the arterial system, consists of well-defined
layers of non-striated muscle-fibers arranged in a circular direction or
at right angles to the long axis of the vessel. In the physiologic con-
dition these fibers are in a state of continuous contraction, more or
less pronounced, and give to the arteries a certain average caliber
which permits a definite volume of blood to flow through them in a
given unit of time.

The cause of this tonic contraction is not definitely known. It
has been attributed to the action of local nerve-ganglia, to the pres-
sure of blood from within, to the influence of organic substances in the
blood, the products of gland activity: e. g., adrenalin or cpinephrin.

This tonic contraction of the vascular muscle is subject to increase
or decrease in accordance with the action of various agents. In-
creased contraction will result in a decrease of the caliber and a
reduction in the outflow of blood. Decrease of the contraction or
relaxation will result in an increase both of the caliber and outflow
of blood. The small arteries thus determine the volume of blood

THE CIRCULATION OF THE BLOOD. 373

passing to any given area or organ in accordance with its functional
activities.

The Vaso-motor Nerves. — The activities of the vascular muscle
are regulated by the central nerve system through the intermedia-
tion of nerve-fibers, termed vaso-motor nerves. Of these there are
two kinds, one which increases or augments the contraction, the
vaso-constrictors or vaso-augmentors; another which decreases or
inhibits the contraction, the vaso-dilatators or vaso-inhibitors.

The vaso-motor nerves of both classes, unlike the ordinary motor
nerves, do not pass directly to the muscle-fiber, but indirectly by way
of the ganglia of the sympathetic nerve system. In these- ganglia
the vaso-motor nerves, which come from the central nerve system,
terminate, breaking up into tufts, which arborize around the nerve-
cells. From the cells new nerve-fibers arise which then pass without
interruption to their final destination.

The nerve-fibers which emerge from the central nerve system
are extremely fine in caliber and medullated; those which emerge
from the sympathetic ganglia are equally fine, but non-medullated.
The former are termed pre-ganglionic, the latter post- ganglionic
fibers. The ganglion in which the pre-ganglionic fibers end is not
necessarily found in the pre- vertebral or lateral chain; it may be
found in the collateral or even in the peripheral group of ganglia.

The Vaso-constrictor Nerves.— The vaso-constrictor nerves take
their origin from nerve-cells located in the anterior horns and lateral
gray matter of the spinal cord. They emerge from the cord in company
with the fibers which compose the anterior roots of the spinal nerves
from the second thoracic to the second or third lumbar nerves inclusive.
A short distance from the cord they leave the anterior roots as the
white rami communicantes and enter the pre-vertebral or lateral
sympathetic ganglia. From the results of many observations and
experiments it is probable that the great majority of the vaso-con-
strictor nerves terminate in these ganglia; that is to say, it is here that
the pre-ganglionic fibers arborize around the contained nerve-cells.
From the nerve-cells new fibers arise, the post-ganglionic, which pass
to the blood-vessels of the head, to the upper and lower extremities,
and to the thoracic and abdominal viscera.

The vaso-constrictors for the head emerge from the spinal cord
in the first four thoracic nerves, thence pass successively into and
through the ganglion stellatum (the first thoracic), the annulus of
Vieussens, the inferior cervical ganglion, the sympathetic cord to
the superior cervical ganglion, around the cells of which they arborize.
From this ganglion the new fibers follow the carotid artery and its
branches to their terminations.

The vaso-constrictors for the fore-limbs emerge from the cord in
the roots of the fourth to the tenth thoracic nerves inclusive. Through
the white rami they pass into the sympathetic chain, after which they
take an upward direction and terminate around the cells of the gan-

374 TEXT-BOOK OF PHYSIOLOGY.

glion stellatum. From this ganglion the new fibers enter, by way of
the gray rami communicantes, the trunks of the cervical nerves which
unite to form the brachial plexus and by this route pass to the blood-
vessels.

The vaso-constrictors for the hind-limbs emerge from the cord
in the roots of the eleventh dorsal to the second or third lumbar nerves
inclusive. They then pass through the white rami to the lower
lumbar and upper sacral ganglia. Thence by way of the gray rami
they pass into the nerve-trunks which unite to form the sacral nerves
and by this route pass to the blood-vessels.

The vaso-constrictors for the viscera of the abdominal cavity pass
by way of the splanchnic nerves directly into the collateral ganglia,
the semilunar, the superior mesenteric, the inferior mesenteric, and
the sacral. From these ganglia an elaborate network of non-medul-
lated fibers passes to the blood-vessels of the stomach, intestines, and
other viscera. The great splanchnic nerve is one of the most im-
portant vaso-constrictor trunks of the body, on account of the large
vascular area it controls.

The existence, course, distribution, and functions of the vaso-
constrictor nerves have been determined by a variety of methods,
physiologic and anatomic. Stimulation of the nerve-trunks under
appropriate conditions gives rise to a contraction, division to a dilata-
tion of the blood-vessels. The physiologic continuity of the pre-
ganglionic fibers with the nerve-cells of the sympathetic ganglia
has been shown by the intra-vascular injection or the local applica-
tion of nicotin. This agent, as shown by Langley, has a selective
action on the arborizations of the pre-ganglionic fibers, and when
given in sufficient doses suspends their conductivity; hence stimu-
lation of the pre-ganglionic fibers is without effect, though stimulation
of the post-ganglionic fibers is followed by the usual contraction.

The following will serve as illustrations of the functions of vaso-
constrictor nerves. Division of the great splanchnic is followed by
a marked dilatation of the blood-vessels of the intestinal tract; stimu-
lation of the peripheral end by their contraction. Division of the
cervical cord of the sympathetic is followed by dilatation of the blood-
vessels of the side of the head; stimulation of the peripheral end by
their contraction.

The Vaso-dilatator Nerves. — The vaso-dilalalor nerves have
their origin for the most part in nerve-cells situated in the region of the
spinal cord included between the origins of the second dorsal to the
second lumbar nerves inclusive, though they are not confined to this
region. Some vaso-dilatator fibers have their origin in the medulla
oblongata, others in the sacral region of the spinal cord.

The general course of the dilatator fibers for the intestinal tract
is the same as that of the vaso-constrictors, though instead of becom-
ing related to the nerve-cells in the pre-vertebral ganglia, they pass
by way of the splanchnics to the collateral ganglia, the semilunar,

THE CIRCULATION OF THE BLOOD. . 375

the superior and inferior mesenteric, and perhaps to peripheral ganglia
in or near the blood-vessels themselves.

The vaso-dilatators for the limbs are found in the common nerve-
trunks associated with the usual motor and sensor fibers, though the
exact route by which they pass from the spinal cord to the peripheral
nerves has not in all cases been determined. Their cell stations have
not been definitely located. The vaso-dilatator nerves for the blood-
vessels of the submaxillary gland arise in the medulla, pass outward
in the trunk of the facial nerve, and reach the gland by way of the
chorda tympani branch. Their cell station is in the ganglion near
the hilum of the gland. The vaso-dilatator nerves for the blood-
vessels of the corpora cavernosa of the penis, the nervi erigentes,
have their origin in the sacral region of the spinal cord; and emerge
in the roots of the second and third sacral nerves. Their cell station
is in the ganglion near the organ.

The existence, course, distribution, and functions of the vaso-
dilatator fibers have been determined by the same methods employed
in the investigation of the vaso-constrictors. Thus division and
stimulation of the peripheral end of the chorda tympani nerve are
at once followed by an active dilatation of the blood-vessels of the
submaxillary gland. The inflow of blood is so great that the gland
becomes bright red in color. Its tissues being unable to appropriate
all the oxygen, the blood emerges in the veins almost arterial in char-
acter. Stimulation of the peripheral ends of the divided nervi erigentes
is followed by similar effects in the blood-vessels of the corpora caver-
nosa. Slow stimulation, once per second, of the peripheral end of a
divided sciatic nerve is followed by dilatation of the blood-vessels of
the leg.

From these and many other facts of a similar character it is highly
probable that the blood-vessels of each organ are under the control
of two antagonistic classes of nerve-fibers, one augmenting the degree
of their contraction, the vaso-constrictors, the other diminishing it
through inhibition, the vaso-inhibitors. Through the cooperative
antagonism of these two classes of nerves the caliber of the blood-
vessels and thereby the volume of the blood is accurately adapted to the
needs of each organ both during rest and during activity. It is also
to the alternate activity of these nerves that the variations occurring
from time to time in the volume of organs are to be attributed.

Physiologic Properties. — The vaso-constrictors and the vaso-
dilatators differ somewhat in their physiologic properties, as shown by
the results of experiment. Thus, when a mixed nerve, i. e., one con-
taining both classes of fibers — e. g., the sciatic — is stimulated with
frequently repeated induced currents, the constrictor effect is the more
pronounced, the dilatator effect being wanting or prevented; when
stimulated with slowly repeated induced currents, the dilatator effect
is the more pronounced. These different effects are strikingly shown
in Fig. 175, A and B.

376

TEXT-BOOK OF PHYSIOLOGY.

In the experiment of which these tracings are the result the leg
of a cat was enclosed in a plethysmograph and the variations in volume
due to dilatation or contraction of the vessels, following stimulation
of the sciatic nerve, were recorded by means of a tambour and lever
on a slowly revolving cylinder. In A the fall of the curve indicates
a diminution of volume, from contraction of blood-vessels following
a rate of stimulation of the sciatic nerve of 16 per second for fifteen
seconds. In B the rise of the curve indicates an increase in volume
from dilatation of the vessels following a rate of stimulation of i per
second for fifteen seconds (Bowditch and Warren). With different
rates of stimulation somewhat different results are obtained.

After division of a mixed nerve the vaso-constrictors degenerate
and lose their influence over the blood-vessel in from four to five

^

1

f^i^

Fig. 175. — Plethysmograms of the Hind-leg of the Cat following Stimulation
of the Sciatic Nerve. In A the rate of stimulation was sixteen per second, in B one
per second for fifteen seconds.

days, the vaso-dilatators in from seven to ten days, as shown by the
response to electrical stimulation.

When a nerve is cooled, the vaso-constrictors lose their irritability
before the vaso-dilatators.

Vaso-motor Centers. — The nerve-cells throughout the spinal
cord from which the vaso-motor nerves take their origin may be
regarded as nerve-centers which through their related nerve-fibers
exert from time to time either a constrictor or a dilatator influence
over the blood-vessels. Though both the vaso-constrictor and vaso-
dilatator centers are in a state of continuous activity, the former
decidedly preponderates, as shown by the maintenance of a tonic
contraction of the blood-vessels. The activity of both centers may
be increased or decreased, augmented or inhibited, by nerve impulses
reflected to them from the periphery through afferent nerves or through
nerve-fibers descending the cord from higher levels of the nerve-
system. Experiment has shown that when a definite region of the
medulla oblongata is punctured or in anywise destroyed there is an

THE CIRCULATION OF THE BLOOD. 377

immediate dilatation of the blood-vessels throughout the body and a
fall of blood-pressure below one-half or one-third of the normal value.
This region has a width of one and a half millimeters and extends
longitudinallv for a distance of four or five millimeters, terminating
at a point four millimeters above the tip of the calamus scriptorius.

A transection of the medulla above the upper limit of this area is
without effect on the blood-pressure. A similar section below it.
however, is at once followed by vascular dilatation, a loss of vascular
tone, and a general fall of blood-pressure. Subsequent stimulation
of the peripheral end of the divided medulla, the animal being curar-
ized and artificial respiration maintained, will give rise to a marked
contraction of the blood-vessels and a rise of blood-pressure up to
and Tar beyond the normal value.

If the experimental lesion is limited to the area mentioned in the
foregoing paragraph, the vascular dilatation passes away after a
time, the blood-vessels regain their normal tone, and the pressure
again rises. These facts indicate that there is in the gray matter
beneath the floor of the fourth ventricle, a restricted area composed of
nerve-cells, which maintains through efferent nerve-fibers the tonus
of the blood-vessels by virtue of its dominating influence over the vaso-
motor centers in the cord, and which is therefore to be regarded as the
general vaso-motor {constrictor) center. The vaso-motor centers through-
out the cord are to be regarded as subsidiary centers. The nerve-
fibers which transmit the regulative nerve impulses from the general to
the subsidiary centers are to be found in the lateral columns of the
spinal cord.

Since the blood-vessels maintain a more or less constant tone, it is
assumed that the vaso-motor center is in a state of continuous activity.
In how far, however, this activity is the result of chemic changes
between the cells and the surrounding lymph and blood, or the result
of constantly arriving nerve impulses reflected from the periphery
or from higher regions of the nerve system, is not readily deter-
minable. Both factors are probably involved.

A general vaso-dilatator center has never been located and there are
many reasons for thinking that such a center has no anatomic existence.
There are, however, special or local vaso-dilatator centers in the
medulla oblongata and in various regions of the spinal cord and
especially in the lumbar region.

Direct Stimulation of the Vaso-motor Centers. — The general
vaso-motor (constrictor) center at least is markedly influenced by
the quantity and quality of blood and lymph circulating around and
through it. If the blood-supply to the medulla and associated struc-
tures be diminished by compression of the carotid arteries, the activity
of the center is at once increased, as shown by increased vascular
contraction and a rise of pressure. Restoration of the blood-supply
is followed by a return of the center to its normal degree of activity.
Increased blood-supply, as in cerebral hyperemia, is attended by a

378 TEXT-BOOK OF PHYSIOLOGY.

fall in blood-pressure indicating a decrease in the activity of the center.
A diminution in the percentage of oxygen or an increase in the per-
centage of C02 in the blood will increase the activity of the center.
In asphyxia especially, the center is extremely excitable, as shown by
a rise of the arterial tension. The subsidiary centers in the spinal
cord are influenced by corresponding conditions.

Reflex Stimulation of the Vaso-motor Centers. — The results
of experiment make it certain that the degree of vascular contraction
maintained by the vaso-motor centers can be increased or decreased
by nerve impulses reflected to the cord and medulla from the periphery
or from the brain. The effect may be general, or local and confined
to the area from which the impulses arise. The following experi-
ments may be cited as illustrations:

Stimulation of the central end of a divided posterior root of a
spinal nerve gives rise to increased vascular contraction, as shown
by the rise of blood-pressure. Stimulation of the central end of the
divided sciatic will give rise to opposite results, according to the strength
of the stimulus, weak stimuli producing dilatation, strong stimuli
producing contraction of the vessels. Stimulation of the central end
of the divided vagus gives rise to dilatation of the vessels of the lips,
cheeks, and nasal and palatal mucous membranes. Stimulation of
the tongue is followed by dilatation of the vessels of the submaxillary
gland. Stimulation of certain branches of the vagus nerve is followed
by a passive dilatation of blood-vessels and a marked fall of pressure.
Stimulation of the peripheral terminations of the afferent nerves of
the glans penis will give rise to an active dilatation of the blood-vessels
of the corpora cavernosa, etc.

A satisfactory explanation of these different results is, however,
wanting. By some investigators it is believed that the usual variations
in the arteriole contraction are the outcome of corresponding varia-
tions in the activity of the general vaso-constrictor center the result
of nerve impulses coming through afferent nerves.

The preceding statements as to the effects on the degree of vascular
contraction, and hence on the blood-pressure which follow stimulation
of different afferent nerves, has led to the assumption that there are in
most afferent nerves two classes of nerve-fibers, though perhaps in
varying proportions, one of which when in activity augments, the
other of which when in activity inhibits the activity of the vaso-con-
strictor center. The former class is generally termed pressor, the
latter depressor fibers.

It is possible, therefore, that under physiologic conditions, physio-
logic stimuli act on the peripheral terminations of either the one or the
other; according as they do will the center be augmented or inhibited
in its activity, and attended by either an increase or a decrease in the
degree of the previous vascular contraction.

Again it may be assumed, from the results of experimentation on
afferent nerves, that the physiologic stimuli may act simultaneously

THE CIRCULATION OF THE BLOOD.

379

on the peripheral terminations of both classes of fibers and that the
vaso-constrictor center is acted on by the two antagonistic influences.
In this assumption the resultant effect on the blood-vessels, viz.,
increased or decreased contraction, will be the resultant of their action
on the vaso-constrictor center. If the stimuli act preponderantly
on the depressor fibers the
center will be depressed
and the vessels will dilate;
if they act preponderantly
on the pressor fibers the
center will be stimulated
and the vessels will con-
tract.

Inasmuch as the vas-
cular dilatation is often
greater than the dilatation
which follows division of
the vaso-motor fibers them-
selves, it has been assumed
by some that the general
vascular tonus, as well as
its variations from time to
time, is the resultant of the
simultaneous activity and
variations in activity of
both vaso-constrictor and
vaso-dilatator centers; that
in the afferent nerves there
are two sets of fibers, one
of which when stimulated
augments the activity of the
vaso-constrictor center and
inhibits the activity of the
vaso-dilatator center; the
other of which auguments
the activity of the vaso-
dilatator center and in-
hibits the activity of the
vaso-constrictor center.
The result, either contrac-
tion or dilatation, which

follows stimulation of their peripheral terminations will depend on the
character of the physiologic stimulus.

In those particular instances in which stimulation of the peripheral
terminations of afferent nerves, e. g., the nervi erigentes and chorda
tympani, is followed by active dilatation of the blood-vessels, it has been
assumed that there are afferent nerve-fibers which directly stimulate

sym.n
depr.n
vag.n

car. a

Fig. 176. — Diagram showing the Origin
and Relation of the Depressor Nerve in the
Rabbit. Depr. n., depressor nerve; vag. n., vagus
nerve; sup. 1. n., superior laryngeal nerve; inf. e.g.,
inferior cervical ganglion; sym. n., sympathetic nerve ;
car. a., carotid artery; dig. m., digastric muscle;
hyp. n., hypoglossal nerve; sup. c. g., superior cer-
vical ganglion; inf. 1. n., inferior laryngeal nerve.

380 TEXT-BOOK OF PHYSIOLOGY.

or augment the activity of a special vaso-dilatator center and for this
reason should be termed "reflex vaso-dilatator nerves" (Hunt).

The Influence of Emotional States. — The vaso-motor centers
are capable of being influenced in their activities by emotional states,
doubtless as a result of the arrival of nerve impulses from the cortex
of the cerebrum. Thus it is well known that fear causes a contraction
of the blood-vessels of the head and face and that shame causes a
dilatation of the same vessels. With the cessation or the disappear-
ance of the emotional state, the blood-vessels return to their former
degree of contraction.

The Depressor Nerve. — A striking illustration of the depressor or
inhibitor action of afferent nerves upon the vaso-constrictor center is
furnished by the result of stimulation of a branch of the vagus, the so-
called "depressor nerve." In the rabbit, Fig. 176, there is a small
nerve formed bv the union of a branch from the trunk of the vagus

FIG j77, — Fall of Blood-pressure from Excitation of the Depressor Nerve.
The cylinder was stopped in the middle of the curve and the excitation maintained for
seventeen minutes. The line of zero pressure (0,0) should be 30 mm. lower than here
shown. — (Bayliss.)

with a branch from the superior laryngeal. The peripheral distribu-
tion of this nerve is over the wall of the ventricle and perhaps to some
extent to the structures of the arota near its origin. A similar ana-
tomic arrangement is met with in the horse, pig, and hedge hog. In
some other animals, as the dog, it is bound up in the vago-sympathetic.
In man it is also present, though shortly after its origin it enters the
trunk of the vagus. Division of this nerve is without effect either on
the heart or the vessels. Stimulation of the peripheral end has neither
an accelerator nor an inhibitor action on the heart. Stimulation of
the central end is followed by a fall in blood-pressure, frequently
to a level below one-half the normal value; at the same time there is a
diminution, brought about reflexly, in the rate of the heart-beat (Fig.
[77). The fall in pressure, however, is not due to this cause, for it
occurs equally well after division of all the cardiac nerves. For this

THE CIRCULATION OF THE BLOOD. 381

reason the nerve was termed the depressor nerve of the vaso-motor
center.

On exposure of the abdominal cavity, it is observed during stimula-
tion of the depressor that there is a notable dilatation of the intestinal
vessels. From this fact it was assumed that the action of the depres-
sor nerve was to lower the general pressure through reflex dilatation
of these vessels. It has been shown by Porter and Beyer that if the
splanchnics are divided and the peripheral end stimulated so as to
maintain the tonus of the intestinal vessels, and hence the general pres-
sure, stimulation of the depressor nerve will nevertheless be followed
by a fall of the blood-pressure almost as great as when the splanchnics
are intact. From this it is evident that the depressor nerve is related
to centers which influence the vascular apparatus in its entirety.
It has been supposed that through it the heart can protect itself from
injurious results of an excessive rise of arterial pressure.

Thus, when the intra-cardiac pressure or the intra-aortic pressure
rises beyond a normal amount from increased resistance, the peripheral
terminations of this nerve are stimulated with the result that the vaso-
motor center is inhibited and the arterioles relaxed. Through this
means the pressure falls and the work of the heart is lessened.

CHAPTER XV.
RESPIRATION.

Respiration is a process by which oxygen is introduced into, and
carbon dioxid removed from, the body. The assimilation of the
former and the evolution of the latter take place in the tissues as a
part of the general process of nutrition. Without a constant supply
of oxygen and an equally constant removal of the carbon dioxid,
those chemic changes which underlie and condition all life phenom-
ena could not be maintained.

The general process of respiration may be considered under the
following headings, viz. :

i. The anatomy and general arrangement of the respiratory appa-
ratus.

2. The mechanic movements of the thorax by which an interchange

of atmospheric and intra-pulmonary air is accomplished.

3. The chemistry of respiration, the changes in composition under-

gone by the air, blood, and tissues.

4. The nerve mechanism by which the respiratory movements are

maintained.

THE RESPIRATORY APPARATUS.

The respiratory apparatus consists essentially of:

1. The lungs and the air-passages leading into them: viz., the nasal

chambers, mouth, pharynx, larynx, and trachea.

2. The thorax and its associated structures.

The nasal chambers are the natural entrances for the inspired
air. Their complicated structure slightly retards the movement of
the air, in consequence of which its temperature and moisture are
adjusted to the physiologic conditions of the lower respiratory pas-
sages. The mouth, though frequently serving as an entrance for air,
is not primarily a respiratory passage. Both the nasal chambers
and the mouth communicate posteriorly with the pharynx, in which
the respiratory and the deglutitory passages cross each other, the
former leading directly into the larynx.

The larynx is a complicated mechanism serving the widely dif-
ferent though related functions of respiration and phonation. It
consists of a framework of cartilages, articulating one with another,
united by ligaments and moved by muscles; it is covered externally
with fibrous tissue and lined with mucous membrane. The superior
opening of the larynx, the glottis, is triangular in shape, the base '

382

RESPIRATION.

383

being directed upward and forward, the apex downward and back-
ward. The inclination of the glottic opening is almost vertical.

The cavity of the larynx is partially subdivided by the interposition
of the vocal bands into a superior and an inferior portion. The
opening, bounded by the vocal bands, is also triangular in shape,

Fig. 178. — Trachea axd Bronchial Tubes, i, 2. Larynx. 3, 3. Trachea. 4.
Bifurcation of trachea. 5. Right bronchus. 6. Left bronchus. 7. Bronchial division
to upper lobe of right lung. 8. Division to middle lobe. 9. Division to lower lobe.
10. Division to upper lobe of left lung. n. Division to lower lobe. 12, 12, 12, 12.
Ultimate ramifications of bronchi. 13, 13, 13, 13. Lungs, represented in contour
14, 14. Summit of lungs. 15, 15. Base of lungs. — (Sappey.)

though in this case the base is directed backward, the apex forward.
(See chapter on Voice and Speech.)

The introduction of the vocal bands narrows at this level the air-
passage and to some extent interferes with the free entrance of air.
According to the investigations of Semon, the area of the air-passage
above and below the phonatory apparatus is about 200 sq. mm.;
while the area bounded by the vocal apparatus is but 155 sq. mm.
during quiet respiration.

584

TEXT-BOOK OF PHYSIOLOGY.

The trachea is a tube, some 12 centimeters in length, from one-
half to three-fourths of a centimeter in breadth, extending from the
lower border of the larynx to a point opposite the fifth dorsal verte-
bra. It consists of an external fibrous and an internal mucous mem-
brane, between which is a series of superposed C-shaped arches or
rings of elastic cartilage, some 18 or 20 in number. Between the
fibrous and mucous coats posteriorly, and occupying the space be-
tween and attached to the free ends of the cartilages, there is a layer
of transversely arranged non-striated muscle-fibers, known as the
tracheal muscle. The alternate contraction and relaxation of this
muscle would by varying the distance between the ends of the cartil-
ages, either diminish or increase the caliber of the trachea. The

surface of the mucous membrane

is covered by a layer of stratified
columnar ciliated epithelium (Fig.
179). In the submucous tissue
there are a number of glands the
ducts of which open on the free
surface.

Opposite the fifth dorsal verte-
bra the trachea divides into a
right and a left bronchus. Each
bronchus again subdivides into two
or three branches, which penetrate
the corresponding lung.

The lungs, in the physiologic
condition, occupy the greater part
of the cavity of the thorax. They are separated from each other by
the contents of the mediastinal space: viz., the heart, the large
blood-vessels, the esophagus, etc. Each lung is somewhat pyramidal
in shape with the apex directed upward. The outer surface is con-
vex and corresponds to the general conformation of the thorax. The
inner surface is concave and accommodates the contents of the me-
diastinal space. At about the middle of the inner surface there
enter the lung, the bronchi, and blood-vessels. The under surface
of the lung is concave and rests on the diaphragm. The posterior
border is convex; the anterior border is thin.

A histologic analysis of the lung shows it to consist of the branches
of the bronchi, their subdivisions and ultimate terminations, blood-
vessels, lymphatics and nerves, imbedded in a stroma of fibrous and
elastic tissue. The anatomic relations which these structures bear
one to another is as follows: —

Within the substance of the lung the bronchi divide and subdivide,
giving origin to a large number of smaller branches, the bronchial
tubes, which penetrate the lung in all directions. With this repeated
subdivision the tubes become narrower, their walls thinner, their
structure simpler. In passing from the larger to the smaller tubes

Fig. 179. — Transverse Section of
the Trachea of a Kitten. — (Stirling.)

RESPIRATION.

385

the cartilaginous arches become shorter and thinner, and finally are
represented by small angular and irregularly disposed plates. In the
smallest tubes the cartilage entirely disappears. With the diminution
of the caliber of the tube and a decrease in the thickness of its walls,
there appears a layer of non-striated muscle-fibers, the so-called bron-
chial muscle, between the mucous and submucous tissues, which com-
pletely surrounds the tube and becomes especially well developed in
those tubes devoid of cartilage. The fibrous and mucous coats at the
same time diminish in thickness.

The bronchial muscles are presumably in a state of tonic contraction
and impart to the bronchial tubes a certain average caliber best
adapted for respiratory purposes. Experimental investigations indicate

Bronchiole

Infundibulum

Air-cell.

Fig. 180. — Scheme of a Bronchiole Ter-
minating in Alveolar Passages, those Leading
into Infundibula beset with Air-cells.—
(Landois and Stirling.)

Fig. 181. — Single Lob-
ule of Human Lung. a.
Alveolar passage, b. Cav-
ity of lobule or infundib-
ulum. c. Pulmonarv sacs.
— (Dalton.)

that they are innervated by efferent fibers of the vagus nerve, inas-
much as stimulation of this nerve is usually followed by a contraction
of the muscles and a narrowing of the lumen of the bronchial system.
These muscles may also be thrown into increased activity by the
inhalation of irritating gases and into a tetanus by pathologic causes
as seen in the various forms of asthma.

When the bronchial tube has been reduced to the diameter of about
one millimeter, it is known as a bronchiole or a terminal bronchus.
From the sides of the terminal bronchus and from its final termination
there is given off a series of short branches which soon expand to form
lobules or alveoli (Fig. 180). The cavity of the alveolus is termed
the infundibulum. From the inner surface of the alveolus and of the
passageway leading into it, there project thin partitions which sub-
25

386

TEXT-BOOK OF PHYSIOLOGY.

Fig. 182. — Section of Silvered Lung of Kitten,
including Portions of Infundibulum and Air-
sac, a. Small polyhedral epithelial cells covering
the wall of the infundibulum. b. Fibro-elastic
framework, c. Large flattened epithelial plates lining
air-sac, among which lie small groups of small cells
(d).— (Pier soil)

divide the outer portion of the general cavity or infundibulum into
small spaces, the so-called air-sacs or air-cells (Fig. 181). The wall
of the alveolus is extremely thin and consists of fibro-elastic tissue,

supporting a very elab-
orate capillary network
of blood-vessels. The
bronchial system as far
as the aveolar passages
is lined by ciliated epi-
thelium. The air-sacs
are lined by flat epithe-
lial plates of irregular
shape, termed the respi-
ratory epithelium (Fig.
182). The alveoli are
united one to another by
fibro-elastic tissue.

The bronchial arter-
ies which supply nutri-
tive material to the pul-
monary structures arise
from the aorta as a rule,
though sometimes from
an intercostal artery. Each lung receives two arteries which ac-
company the bronchi as far as the distal ends of the alveolar passages.
From the capillary network formed out of the terminals of these ar-
teries, two systems of veins arise,
one of which returns the blood from
the larger tubes and empties it into
the azygos vein; the other of which
returns the blood from the smaller
tubes and the alveolar passages,
and empties it into the pulmonary
veins.' The blood in the pulmonary
veins, though largely arterialized,
nevertheless contains some venous
blood derived from the veins aris-
ing from the capillary network of
the bronchial arterioles.

The nerves distributed to the
muscle-fibers of the bronchial arter-
ies, and of the bronchial tubes and
to the mucous membrane, are de-
rived from the vagus and the sym-
pathetic and enter the substance of the lung at and around its root.

In consequence of the presence of the elastic tissue, the lungs
are distensible and elastic. After removal from the body the elastic

Fig. 183. — The Relation of the
Pulmonary Artery, PA, and the
Pulmonary Vein, PV, to the Lobules,
AA. B. The Bronchiole.

RESPIRATION.

387

tissue at once recoils, forcing out a portion of the contained air. The
condition of the lung is now one of collapse. Under pressure, how-
ever, the lung can be readily distended or inflated. These proper-
ties endure for a long period after death, if not indefinitely, if the lungs
are properly preserved. The capacity of the lungs can be made to
vary within rather wide limits in virtue of the presence of the elastic
tissue.

The Pulmonary Blood-vessels. — The pulmonary artery which
conducts the venous blood from the heart to the lungs divides beneath

Fig 184. — Bronchi and Lungs, Posterior View, i, i. Summit of lungs. 2, 2.
Base of lungs. 3. Trachea. 4. Right bronchus. 5. Division to upper lobe of lung.
6. Division to lower lobe. 7. Left bronchus. 8. Division to upper lobe. 9. Division
to lower lobe. 10. Left branch of pulmonary artery. 11. Right branch. 12. Left
auricle of heart. 13. Left superior pulmonary vein. 14. Left inferior pulmonary vein.
15. Right superior pulmonary vein. 16. Right inferior pulmonary vein. 17. Inferior
vena cava. 18. Left ventricle of heart. 19. Right ventricle. — (Sappey.)

the arch of the aorta into a right and a left branch. Each branch with
its subdivisions enters the lung at the hilum in company with the larger
divisions of the bronchi. Within the lung the arteries divide and sub-
divide in a manner corresponding to that of the bronchial tubes,
which they follow to their ultimate terminations. As the pulmonary
lobules are approached, a small arterial branch plunges into the wall
of the lubule (Fig. 183), in which it forms an elaborate capillary net-
work which surrounds and embraces the air-sacs on all sides. As
this network is to subserve the respiratory exchange of gases it lies
nearer the inner than the outer surface of the lobule and in close rela-

288

TEXT-BOOK OF PHYSIOLOGY.

tion to the respiratory epithelium. The air and blood are thus brought
into intimate relationship, being separated only by the respiratory
epithelium and the wall of the capillary vessel. The blood emerging
from the capillary vessels is conducted by a corresponding converging
system of vessels, the pulmonary veins, out of the lungs and into the
left auricle of the heart. The main function of the pulmonary appara-
tus and the pulmonary division of the circulatory apparatus is to afford
a ready means for the exhalation of the carbon dioxid and the absorption

of oxygen. In consequence of this
exchange of gases the blood changes
in color from dark bluish-red to
scarlet red. The relations of the
heart and its vessels to the lungs
and bronchial tubes are shown in
Fig. 184.

The Thorax.— The thorax, in
which the respiratory organs and
their associated structures are
lodged, is conic in shape, though
somewhat compressed from before
backward. Its apex is directed up-
ward, its base downward. The
walls of the thorax are composed,
first, of a bony framework or skel-
eton and, second, of muscles and
fascia. The bony framework is
formed posteriorly by the thoracic
vertebrae and the posterior extremi-
ties of the ribs, laterally by the
ribs, and anteriorly by the costal
cartilages and the sternum. The
superior opening, through which
pass the trachea, esophagus, and
blood-vessels, is oval in outline and
measures from side to side about 12.5 cm., and from before backward
about 6.25 cm. The inferior opening is of large size, but irregular in
its boundaries from the upward inclination of the ribs and the down-
ward projection of the sternum.

The ribs which form a large part of the thoracic walls constitute a
series of bony arches attached posteriorly to the vertebrae and anteriorly
to the sternum through the intermediation of their cartilages. The
last two form an exception. The ribs are somewhat twisted upon them-
selves and pursue an oblique direction from above downward and
forward. As a result the anterior extremity lies at a lower level than the
posterior. The costal cartilages are directed upward and forward, with
the exception of the upper three, which are almost horizontal. The
general arrangement and appearance of the thorax are shown inFig. 185.

Fig. 185. — Thorax, Anterior View
1. Manubrium sterni. 2. Gladiolus. 3
Ensiform cartilage of xiphoid appendix
4. Circumference of apex of thorax. 5
Circumference of base. 6. First rib
7. Second rib. 8, 8. Third, fourth,
fifth, sixth, and seventh ribs. 9. Eighth,
ninth, and tenth ribs. 10. Eleventh and
twelfth ribs. 11, n. Costal cartilages.

RESPIRATION.

389

The costo-vertebral and costo-chondral and the chondro-sternal
articulations are diarthrodial in character and endow the thoracic
walls with a considerable degree of mobility. The costo-vertebral
joints are two in number, the first being formed by the beveled head
of the rib and the bodies of two adjoining vertebrae; the second, by
the tubercle of the rib and the transverse process. The costo-chon-
dral and the chondro-sternal articulations, as their names imply,
are formed by the ribs, cartilages, and sternum.

The muscles which complete the formation of the thoracic walls
are as follows: the
diaphragm, the in-
tercostales externi
and interni, the
levatores costarum,
t h e triangularis
sterni, and the in-
fra-costales.

The diaphragm
is the musculo-
membranous sheet
which closes the
inferior opening of
the thorax and
completely sepa-
rates its cavity
from that of the ab-
domen. It con-
sists of two mus-
cles which arise
from the bodies of
the first three or
four lumbar verte-
brae and neighbor-
ing fascia, from the
border of the six

lower ribs, and from the ensiform cartilage (Fig. 186). From this ex-
tensive origin the muscle-fibers pass centrally to be inserted into a
common tendon. As the direction of the fibers is from below up-
ward and inward, the diaphragm is somewhat dome-shaped. Its
inferior border is for a short distance in contact with the sides of the
thorax.

The intercostales externi, eleven in number on each side, occupy
the spaces between the ribs to which they are attached from the
tubercle to the anterior extremity (Figs. 187 and 188). Their fibers,
which are arranged in parallel bundles, are directed from above
downward and from behind forward. The point of attachment,
therefore, of anv given bundle of fibers to the rib above, lies nearer

Fig. 186. — Diaphragm, Inferior Aspect, i. Anterior
and middle leaflet of central tendon. 2. Right leaflet. 3.
Left leaflet. 4. Right cms. 5. Left cms. 6, 6. Intervals
for phrenic nerves. 7. Muscular fibers, from which the
ligamenta arcuata originate. 8. Muscular fibers that arise
from the inner surface of the six lower ribs. 9. Fibers
that arise from ensiform cartilage. 10. Opening for inferior
vena cava. 11. Opening for esophagus. 12. Aortic open-
ing. 13, 13. Upper portion of transversalis abdominis,
turned upward and outward. 14. Anterior leaflet of trans-
versalis aponeurosis. 15, 15. Quadratus lumborum. 16,
16. Psoas magnus. 17. Third lumbar vertebra.

39°

TEXT-BOOK OF PHYSIOLOGY.

the vertebral column, nearer the fulcrum, than the point of attach-
ment below.

The intercostales inter ni, eleven in number, occupy the spaces
between, and are attached to, the ribs from the tubercle to the anterior

extremity of the cartil-

ages,

The

lr

fibers,

which are also arranged
in parallel bundles, are
directed from above
downward and back-
ward (Figs. 188 and 189).
The levatores costa-
rum are twelve in num-
ber on either side. They
arise from the tips of the
transverse processes of
the last cervical and the
thoracic vertebrae with
the exception of the last.
From the point of origin
the fibers pass down-
ward and outward in a
111 % s diverging manner to be

inserted into the ribs
between the tubercle and
the angle. Their action,
as their name implies, is
to elevate the posterior
portion of the ribs.

The triangularis
sterni arises from the side
of the posterior surface
of the lower third of the
sternum and is inserted
by fleshy slips into the
cartilages of the ribs
from the second to the
sixth.

From the fact that
the inferior opening of
the thorax as well as the intercostal spaces are completely closed by
the foregoing muscles, and from the further fact that the superior is
closed by fascia except at those points through which pass the
trachea, blood-vessels and esophagus, the cavity of the thorax is abso-
lutely air-tight.

The Pleurae. — Each lung is surrounded by a closed invaginatcd
serous sac, the pleura, of which the inner portion is reflected over

T
Fig. 187. — Showing the Situation, the Points
of Attachment, and Direction of the Intercostal
Muscles, i. The intercostales exlerni. 2. The inter-
costales interni. 3. The intercartilaginei. — (Deaver.)

RESPIRATION.

39i

and is closely adherent to the surface of the entire lung as far as its
root; the outer portion is reflected over the inner wall of the thorax,
the superior surface of the diaphragm, and the viscera of the mediasti-
num. Under normal conditions these two layers of the pleura, the
visceral and parietal, are in contact, or at most separated only by a
thin capillary layer of lymph. The presence of this fluid prevents
appreciable friction as the two surfaces play against each other in
consequence of the movements of the lungs.

THE MECHANIC MOVEMENTS OF THE THORAX.

The blood receives oxygen from, and yields carbon dioxid to, the
alveoli of the lungs, as it flows through the pulmonary capillaries.
That this exchange of gases may continue, it is of primary import-
ance that the air within the alveoli be removed as rapidly as it is
vitiated. This is accomplished by an
alternate increase and decrease in the
capacity of the thorax, accompanied
by corresponding changes in the ca-
pacity of the lungs. During the former
there is an inflow of atmospheric air
(inspiration), during the latter an out-
flow of intra-pulmonary air (expira-
tion). The continuous recurrence of
these two movements brings about that
degree of pulmonary ventilation neces-
sary to the normal exchange of gases
between the blood and the air. The
two movements together constitute a
respiratory act or cycle.

In the course of the respiratory
cycles the thorax presents alternately
a short period of rest — viz., between
the end of an expiration and the be-
ginning of an inspiration — and a
relatively long period of activity, in-
cluding both inspiration and expiration. The former may be regarded
as the static, the latter as the dynamic condition of the thorax. In the
static condition, the thorax and its contained and associated organs
sustain a definite relation one to another; in the dynamic conditions
these relations undergo a change the extent of which is proportional to
the extent of the movements.*

* It is a matter of dispute as to whether or not there is an absolute cessation of move-
ment of the thoracic walls at the end of expiration. A graphic record of the movement
shows that if there is no absolute cessation, the movement is so slight that, for the pur-
poses here intended, a pause may be admitted. With this admission it is, however, recog-
nized that the forces, both elastic and muscular, which are always acting on the thoracic
walls, though in opposite directions, have not ceased to act, but have become so nearly
equal that for a brief period they are practically in a condition of equilibrium, during
which the thoracic walls are stationary.

Fig. 188. — View from behind of
Four Dorsal V e r t e b r .e and
Three Attached Ribs, showing
the Attachment of the Elevator
Muscles of the Ribs and the
Intercostals. 1. Long and short
elevators. 2. External intercostal. 3
Internal intercostal.-— (Allen Thomson )

392 TEXT-BOOK OF PHYSIOLOGY.

THE STATIC CONDITION.

Relation of the Thoracic Organs. — Intra-pulmonary Pres-
sure : Intra-thoracic Pressure. — In the static condition of the
thorax the lungs, by virtue of their distensibility, completely fill all
parts of the thorax not occupied by the heart and great blood-vessels
(Fig. 189). This condition is maintained by the pressure of the air
within the lungs, the intra-pulmonary pressure, which with the respira-
tory passages open, is that of the atmosphere, 760 mm. Ffg. This
relation persists so long as the thorax remains air-tight. If the skin
and muscles covering an intercostal space be removed the lung can
be seen in close contact with the parietal layer of the pleura gliding by
with each inspiration and expiration. If, however, an opening be
now made in the pleura sufficient to admit air, the lung immediately
collapses and a pleural cavity is established. The pressure of air within
and without the lung counterbalancing, at the moment the air is
admitted, the elastic tissue at once recoils and forces a large part of
the air out of the lung. This is a proof that in the normal condition,
the lungs, distended by atmospheric pressure from within, are in a
state of elastic tension and ever endeavoring to pull the visceral layer
of the pleura away from the parietal layer. That they do not succeed
in doing so is due to the fact that the atmospheric pressure from with-
out is prevented from acting on the lung by the firm unyielding walls
of the thorax.

Intra-thoracic Pressure.- — As a result of the elastic tension of
the lungs a fractional part of the intra-pulmonary pressure, 760 mm.
Hg., is counterbalanced or opposed, so that the heart and great vessels
and other intra-thoracic viscera are subjected to a pressure somewhat
less than that of the atmosphere; the amount of this pressure will be
that of the atmosphere less that exerted by the elastic tissue of the
lung in the opposite direction, expressed in terms of millimeters of
mercury. In the thorax but outside the lungs, there then prevails
a pressure, negative to the pressure inside the lungs and which is known
as the intra-thoracic pressure.

The amount of this intra-thoracic pressure can be approximately
determined in several ways. Thus, if shortly after death a mer-
curial manometer be inserted air-tight into the trachea of a human
being and the thorax opened, the lungs will recoil and compress their
contained air. The mercurial manometer will at once show an
excess of pressure in the trachea of about 6 mm. This was taken
by Donders as a measure of the force with which the lungs endeavor
to recoil. The intra-thoracic pressure would be, therefore, atmos-
pheric pressure, 760 mm., less 6 mm., or 754 mm. Hg. Another
method is to insert a rubber catheter through a small opening in an
intercostal space into the thoracic cavity. The air which enters
through the open extremities of the catheter and leads to a collapse
of the lungs may be subsequently aspirated, when the lung returns to

RESPIRATION.

393

its normal position. The catheter is then placed in connection with
a water manometer. On establishing a communication between
them, by the turning of a stopcock, the water will rise in the proximal
and fall in the distal limb of the manometer, indicating a pressure
in the thorax negative to that in the lung. The difference in the
level of the water in the two limbs of the manometer, expressed in
millimeters of mercury, would also represent the force with which
the elastic tissue strives to recoil, and the extent to which it opposes
the atmospheric pressure. This subtracted from the atmospheric

Fig. 189. — Section of Thorax with the Lungs, Heart, and Principal Vessels.
5. Catheter introduced into the pleural space and connected with a manometer. — (After
Moral and Doyen.)

pressure would give the intra-thoracic pressure. In the living dog
this latter is less than the former, to the extent of from 3.5 to 5.5 mm.
For the same reason the superior surface of the diaphragm also ex-
periences a pressure less than that of the atmosphere. Owing to the
soft and yielding character of the abdominal walls the atmospheric
pressure is transmitted through the abdominal organs to the inferior
surface of the diaphragm. The pressure being greater from below
than above, the diaphragm is forced upward until it assumes the dome-
like appearance it usually presents. (These relations are shown in
Fig. 189.)

The cause of the negativity of the intra-thoracic pressure is con-
nected with the change in the relation of the lungs to the thorax at-
tending the first inspiration. Previous to birth the walls of the alveoli

394 TEXT-BOOK OF PHYSIOLOGY.

and bronchioles are collapsed and in apposition. The larger bron-
chial tubes in all probability contain fluid. The lungs therefore are
devoid of air (atelectatic), and, having a specific gravity greater than
water, readily sink when placed in this fluid. The capacity of the
thorax does not exceed the volume of the lungs. With the first inspira-
tion, however, the thoracic walls take a new position. The air at once
rushes into the lungs and distends them. But as the capacity of the
thorax even at the end of the expiration is now greater than the volume
which the lungs could assume without considerable distention, there
at once arises the elastic recoil in the opposite direction, the condition
which gives rise to the negativity of the pressure in the thoracic cavity.
It is also probable that as the child develops, the thorax grows more
rapidly than the lungs, giving rise to a condition which would increase
and accentuate the elastic tension and thus increase the negativity
of the intra-thoracic pressure.

THE DYNAMIC CONDITION.

In the dynamic condition as previously stated these relations
and pressures change. Thus the diaphragm descends, the ribs
ascend, the sternum advances and the lungs expand. The intra-
pulmonary pressure varies during both inspiration and expiration.
With the enlargement of the thorax through muscle activity, there
goes a corresponding increase in the size and capacity of the lungs,
in consequence of the expansion and pressure of the air in the pulmonary
alveoli. With the expansion of the air its pressure falls; but though it
is now less than atmospheric, it is yet much greater than the opposing
force of the lung tissue. As a result of the fall of intra-pulmonary pres-
sure there is a rapid inflow of air, which continues until atmospheric
pressure is restored; that is, at the end of the inspiration. With the
diminution of the thorax, through the recoil of the elastic tissue of the
thoracic and abdominal walls, there goes a corresponding decrease of
lung capacity, in consequence of the recoil of the elastic tissue of the
lungs. As a result, the air in the lungs becomes compressed, its pres-
sure rises above that of the atmosphere, and a rapid outflow of air
takes place, which continues until atmospheric pressure is again re-
stored; that is, at the end of the expiration.

The cause for the fall of intra-pulmonary pressure during in-
spiration and the rise during expiration is to be found in the resist-
■ ance offered by the air-passages to the movement of the air, through-
out their entire extent, and especially at the level of the vocal bands.
The greater the resistance, from whatever cause, physiologic or patho-
logic, the greater the variations of the pressure.

In quiet inspiration the fall of pressure, as indicated by a man-
ometer inserted into one nostril, seldom amounts to more than 1.5
mm. of Hg., the rise in expiration, 2.5 to 3 mm. of Hg. In forcible
inspiratory and expiratory efforts these limits may be largely exceeded.

RESPIRATION.

395

Insp.

Intra-pulmonary pressure.
B ExP-

Thus it was found by Donders that with one nostril closed and a mer-
curial manometer inserted into the other the pressure by voluntary
efforts could be made to fall 57 mm. during inspiration and to rise
87 mm. during expiration. The changes in intra-pulmonary pressure
are graphically represented in the upper half of Fig. 190.

The infra-thoracic pressure also varies during both inspiration and
expiration. As the intra-pulmonary pressure falls, the recoil of the
elastic tissue increases, with the result of diminishing the intra-thoracic
pressure, though not in a steadily progressive manner. The fall of
intra-thoracic pressure at the end of a quiet inspiration amounts to
about 9 mm. Hg. In forcible inspiratory efforts this fall in intra-
thoracic pressure may amount to 30 or 40 mm. of Hg. As the intra-
pulmonary pressure rises above the atmospheric pressure during
expiration, the recoil of the elastic tissue is again opposed, with the
result of increasing the
intra-thoracic pressure,
though not in a steadily
progressive manner.
The changes in intra-
thoracic pressure are
graphically represented
in the lower half of Fig.
190.

Respiratory Move-
ments.— As previously
stated, the ventilation
of the lungs is accom-
plished by an alternate
increase and decrease in

the capacity of the thorax, accompanied by corresponding changes
in the lungs, the two movements being known as inspiration- and
expiration respectively. During the increase in the thoracic capacity,
the air passively flows into the lungs; during the decrease in the thora-
cic capacity, the air passively flows out of the lungs. In both move-
ments the lungs play an entirely passive part, their movements being
determined by the pressure of air within them and by the thoracic
walls, with which they are in close contact.

1. Inspiration is an active process, the result of muscle activity.

2. Expiration is a passive process, the result mainly of the recoil of

the elastic tissue of the walls of the thorax and abdomen and of
the elastic tissue of the lungs.
Ih inspiration the thorax is enlarged in all its diameters: viz.,
vertical, transverse, and antero-posterior. In expiration these diam-
eters are in turn decreased as the thorax returns to its previous con-
dition.

Inspiratory Muscles. — The muscles which from their origin.
direction, and insertion contribute to the enlargement or expansion of

760 mm

C 760 mm

Intra-thoracic pressure.

Fig. 190. — Representing the Changes, i, in
the Intra-pulmonary, and 2, in the Intra-tho-
racic Pressures during Inspiration and Ex-
piration.

396

TEXT-BOOK OF PHYSIOLOGY

the thorax are quite numerous, and include those muscles which
enter into the formation of the thoracic walls (intrinsic muscles),
as well as certain muscles which, having their origin elsewhere, are
attached to the thoracic walls at different points (extrinsic muscles),
though the extent to which they are called into activity depends
on the necessity for either tranquil or energetic inspirations. The
gradations between a minimum and a maximum inspiration are
very slight, and it is difficult to state at what particular instant any
given muscle begins to act. It is customary, however, to divide
the muscles into two groups: (i) Those active in the average or
ordinary inspirations, and (2) those active in maximum or extra-
ordinary inspirations. Among the muscles active in ordinary inspira-
tions may be mentioned the diaphragm, the inter co stales externi,

the intercartilagenei, the levator es costarum,
the scaleni, and the serratus posticus superior.
Among the muscles active in extraordinary
inspirations may be mentioned, in addition to
the foregoing, the sterno-cleido-mastoideus , the
trapezius, and the pectorales minor and major.
The vertical diameter is increased by the
contraction and descent of the diaphragm,
and more especially of its lateral muscular
portions. At the end of an expiration the
diaphragm is relaxed, and the lower portion
closely applied to the walls of the thorax. At
the beginning of an inspiration the muscle-
fibers contract, shorten, and approximate a
straight line, whereby not only is the con-
vexity of the diaphragm diminished, but that
portion in contact with the thorax is drawn
away, thus making a large free space into
which the lateral and posterior portions of the
The attachment of the central tendon of the
diaphragm to the pericardium prevents any marked descent of this
portion except in forcible inspiratory efforts (Fig. 191). The vertical
diameters are thus enlarged, though unequally in different regions of
the thorax.

As the diaphragm descends it displaces the abdominal viscera,
forcing them downward against the abdominal walls, which advance
and become more convex. In forcible inspiration the diaphragm,
acting from the central tendon as the more fixed point, would draw
the lower portion of the thorax inward were this not prevented by the
outward pressure of the displaced viscera.

The anlero- posterior and transverse diameters arc increased by
the elevation and outward rotation of the ribs and an advance of the
sternum, both movements made possible by the construction and
arrangement of the costo-vertebral and costo-chrondral and chrondro-

Fig. 19T. — Diagram

SHOWING INTERVAL BE-
TWEEN the Position of
the Diaphragm in Ex-
piration (e, e) and In-
spiration (i, i). The In-
crease in Capacity is
Shown by the White
Areas. — (Yeo.)

lungs at once descend.

RESPIRATION.

397

sternal articulations. The construction of these articulations is such
as to permit at the first a slight elevation and depression of the head
of the rib, and at the second a gliding of the tubercle on the transverse
process. The axis around which the rib rotates practically coincides
with the axis of the rib neck, which in the upper part of the thorax is
almost horizontal, in the lower part somewhat sagittal in direction.
Hence when the ribs are elevated the upper part of the thorax increases
in its antero-posterior, the lower part in its transverse diameters.
At the same time, the lower portion of the sternum is pushed forward
and upward by the elevation of the anterior extremity of the ribs and
the widening of the angle of the costo-chondral articulation. With
the elevation of the ribs there goes an eversion or outward rotation

Fig. 192. — Diagram illustrating the Action of A, the External Intercostal
and B, the Internal Intercostal Muscles. V, V. Vertical support. R, R'.
Parallel bars. S. Vertical strip, representing respectively the vertebral column, two ribs,
and sternum.

which gives an additional increase to the transverse diameters. This
elevation and outward rotation of the ribs is the resultant of the coop-
eration of the following muscles, viz.: the intercostales externi, the
intercartilagenei, the levatores costarum, the scaleni and the serratus
posticus superior.

The action of the external intercostal muscles has been a subject
of much discussion. Some investigators have maintained that they
are elevators of the ribs, and therefore inspiratory; others that they are
depressors of the ribs, and therefore expiratory in function. At the
present time the general consensus of opinion is that the former view
is the one most in accordance with the facts. The situation of the
muscles and the shortness of their fibers render it extremely diffi-
cult to obtain myographic tracings of their action. This is supposed,
however, to be disclosed by the play of the apparatus suggested orig-
inally by Bernouilli, which consists, as shown in Fig. 192, of a vertical
support carrying two freely movable parallel bars united at their outer
extremities by a short vertical strip, representing respectively the
vertebral column, two adjoining ribs, and a piece of the sternum.

398 TEXT-BOOK OF PHYSIOLOGY.

The parallel bars are joined to each other by a short elastic band
having the direction of and representing the external intercostal muscles.
If the bars are depressed, the elastic band is elongated and made
tense. On releasing the bars the band at once recoils and elevates
them. Although the elastic force is the same in both directions, the
bars are yet elevated for the reason that in accordance with the parallel-
ogram of forces the component acting upward on the long arm of
the lever preponderates over the component acting downward on the
short arm of the lever. The action of the band is supposed to disclose
and illustrate the action of the muscle.

The inter car tilaginei, those portions of the intercostales interni
which occupy the space between the costal cartilages from the sternum
to their outer extremity, bear the same relation to the cartilages in
reference to the sternum that the external intercostals bear to the ribs
in reference to the vertebral column; that is, the point of attachment
to the cartilage above, lies nearer the sternum, nearer the fulcrum,
than the point of attachment below. Hence the same action is at-
tributed to them as to the external intercostals: viz., elevation of the
cartilages and the anterior extremities of the ribs.

The levatores costarum, as is evident from their points of origin
and insertion, elevate the ribs posteriorly.

The scaleni muscles, anticus, medius, and posticus, arise from the
transverse processes of the cervical vertebrae, and after pursuing a
downward and forward direction are inserted into the sternal end of
the first and second ribs. The action of the first two, at least, is to
elevate the first rib and thus establish a fixed point from which the
intercostal muscles can act. The posticus has doubtless a similar
action on the second rib.

The serratus posticus superior, a quadrilateral sheet of muscle-
fibers, arises mainly from the spines of the last cervical and first and
second thoracic vertebras. The anterior extremity is serrated and
attached to the outer surfaces of the second, third, fourth, and fifth
ribs beyond the angle. The action of the muscle is the elevation of
the ribs to which it is attached.

In forcible or extraordinary inspirations, whereby the capacity of
the thorax is still further increased, the foregoing muscles are rein-
forced by the sterno-cleido-mastoideus , the trapezius, and the pectorales
minor and major. Their functions will become apparent from a
consideration of their origins and insertions.

Expiratory Forces and Muscles. — Expiration, as previously
stated, is a passive process brought about by the recoil of the elastic
tissues of the thoracic and abdominal walls, and of the lungs, all of
which have been stretched and made tense during inspiration. With
the cessation of the inspiratory effort the elastic forces, assisted by the
weight of the ribs, sternum, and soft tissues, return the thorax to its
former condition. The result is a diminution of all the diameters of
the thorax. The vertical diameter is diminished by the recoil of

RESPIRATION. 399

the tense abdominal walls, the replacement of the abdominal organs
and the consequent ascent of the diaphragm to its former position.
The transverse and antero- posterior diameters are diminished by the
descent of the ribs, sternum, and lungs. It is somewhat uncertain
if a normal expiratory movement necessitates active muscle contrac-
tion. If, however, there is any impairment of the elasticity of the
lungs or ribs, or any interference with the free exit of the intra-pulmo-
nary air, it is highly probable that the elastic forces are assisted by the
internal intercostal and triangularis sterni muscles.

The action of the internal intercostals is less clearly understood
than that of the externals, and for the same reasons. If, however,
Bernouilli's model discloses the action of the latter, it equally well
discloses the action of the former. Thus, if the parallel bars be joined
by an elastic band having the direction of and representing the inter-
nal intercostals, and then forcibly elevated, the band is elongated and
made tense. On releasing the bars, the elastic band at once recoils
and depresses them. Here, again, though the elastic force is the same
in both directions, the bars are depressed, for the reason that the
component acting downward on the long arm of the lever prepon-
derates over that acting upward on the short arm of the lever. The
action of the band is supposed to disclose and illustrate the action of
the muscle.

The triangularis sterni muscle, judging from its anatomic re-
lations, in all probability assists in expiration by depressing the car-
tilages to which it is attached and as a further result the anterior
extremities of the ribs.

After the elastic forces have ceased to act and the normal expira-
tory movement has been brought to a close, the thorax can be, to a
considerable extent, still further diminished in all its diameters by
the contraction, through volitional effort, of abdominal and thoracic
muscles. To this decrease in the capacity of the thorax, as a result
of which a much larger volume of air is expelled from the lungs than
during passive expiration, the term forced expiration has been given.
With the cessation of muscle activity the elastic forces of the now-
compressed thoracic walls, aided by the return of upward displaced
abdominal organs, at once restore the thoracic walls to the position
they had attained at the end of passive expiration. Of the muscles
active in forced expiration in addition to the intercostales interni and
the triangularis sterni, the following may be mentioned, viz.: the
abdominales, the serratus posticus inferior, and the quadratus lum-
borum.

The externus abdominis arises by a series of muscle slips from the
outer surface of the lower eight ribs. After pursuing an oblique
direction downward and forward, the slips blend to form a single
muscle, which is inserted mainly into the outer lip of the anterior
half of the crest of the ilium and into the central abdominal tendon
or aponeurosis.

4oo TEXT-BOOK OF PHYSIOLOGY.

The internus abdominis arises mainly from the anterior two-thirds
of the inner crest of the ilium and the lumbar fascia. Its fibers pass
upward and forward to be inserted into the cartilages of the last three
ribs and into the central abdominal tendon.

The rectus abdominis arises from the crest of the pubes and is
inserted above into the cartilages of the fifth, sixth, and seventh ribs,
and occasionally into the ensiform cartilage.

The transversalis arises from the cartilages of the last six ribs,
the lumbar fascia, and the anterior half of the crest of the ilium.
After passing transversely across the abdomen, the fibers are inserted
mainly into the linea alba.

The conjoint action of these muscles is to diminish the convexity
of the abdominal walls and to exert a pressure on the abdominal
organs. These, taking the line of least resistance, are forced upward
against the inferior surface of the diaphragm, which in consequence
becomes more strongly curved and ascends higher into the thorax.
The vertical diameter of the thorax is thus diminished. Acting from
the pelvis as a fixed point, these muscles will also draw downward
and inward the lower end of the sternum and the lower ribs and
diminish the antero-posterior and transverse diameters.

The serratus posticus inferior arises from the spines of the last
two thoracic and first two lumbar vertebras. The fibers pass upward
to be inserted into the lower border of the last four or five ribs beyond
the angle. Their action is to depress the ribs and assist in expira-
tion.

The quadratus lumborum has a similar action on the last rib.

Movements of the Lungs. — As the thorax is enlarging in all its
diameters during inspiration, through muscle activity, the lungs are
correspondingly enlarging in all. their diameters, by virtue of their
distensibility, through the pressure of the air within them. The lungs
must therefore move downward, outward and forward. That this
is the case is made evident both by an examination of the lungs through
an intercostal space after removal of the skin and intercostal muscles
and by the methods of percussion. The inferior border of each lung
descends from the lower border of the sixth to the eleventh rib, insert-
ing itself into the space developed between the thorax and diaphragm
as the latter contracts and is drawn away from the former. In con-
sequence of the lateral expansion the anterior border of each lung
advances toward the middle line until the heart is almost covered.
With the beginning and continuance of expiration the lungs exhibit
a reverse movement which continues until they reach their original
position. At all times, however, the movements of the lungs are en-
tirely passive and determined by the movements of the thorax.

The succession of events in the thorax at the time of a respiratory
act may be summarized as follows:

During Inspiration.
i. Enlargement of the thoracic diameters by muscle action.

RESPIRATION. 401

2. Expansion of intra-pulmonary (alveolar) air.

3. Expansion of the lungs.

4. Lowering of the intra-pulmonary air pressure below the atmos-

pheric air pressure.

5. Increase in the negativity of the intra-thoracic pressure.

6. Inflow of atmospheric air, in consequence of its higher press-

ure, until the intra-pulmonary air pressure rises to that of
the atmosphere.
During Expiration.

1. Diminution of the thoracic diameters by the action of elastic

forces.

2. Recoil of the lungs.

3. Compression of the intra-pulmonary (alveolar) air.

4. Rise of intra-pulmonary air pressure above the atmospheric

air pressure.

5. Decrease in the negativity of the intra-thoracic pressure.

6. Outflow of intra-pulmonary air, in consequence of its higher

pressure, until the intra-pulmonary air pressure falls to that
of the atmosphere.

Respiratory Movements of the Upper Air-passages. — The
resistance to the entrance of air into and through the respiratory tract
is much diminished by respiratory movements of the nares and larynx
which are associated and occur synchronously with the movement
of the thorax.

The nares at each inspiration are dilated by the outward move-
ment of their alae or wings, the result of muscle activity. At each
expiration they are diminished by the return of their cartilages through
the play of elastic forces. The larynx, as shown by observation with
the laryngoscope, exhibits corresponding movements of the vocal
membranes. Their introduction at this level naturally narrows the
tract, and would interfere with both the entrance and the exit of air
were they not kept widely asunder during the time they are not re-
quired for purposes of phonation. This is accomplished by the tonic
contraction of the posterior crico-arytenoid muscles, which are entirely
respiratory in function.

It is not infrequently stated that these membranes exhibit consider-
able oscillations, outward and inward, corresponding to the periods
of inspiration and expiration. The statements of the majority of
laryngologists do not favor this view. During tranquil breathing
the membranes are widely separated and almost stationary, seldom
moving in either direction more than a few millimeters. In labored
respirations these movements are naturally increased in extent. The
irregular movements of the membranes occasioned by the unskilful use
of the laryngoscope, especially with nervous patients, are not to be re-
garded as strictly physiologic. The respiratory space in quiet breath-
ing is an isoceles triangle, with a length of 20 mm. and a width at the
base of 15.5 mm. with an area of 155 mm.
26

4o2 TEXT-BOOK OF PHYSIOLOGY.

Respiratory Types. — Observation of the respiratory movements
in the two sexes shows that while the enlargement of the thoracic
cavity is accomplished both by the descent of the diaphragm (as shown
by the protrusion of the abdomen) and the elevation of the thoracic
walls, the former movement preponderates in the male, the latter in
the female, giving rise to what has been termed in the one case the dia-
phragmatic or abdominal and in the other the thoracic or costal type of
respiration. The cause of this greater mobility and actvity of the tho-
rax in the female has been a subject of much discussion. It has been
attributed, on the one hand, to the necessity for a physiologic adjust-
ment between respiration and child-bearing, and therefore a specific
sex peculiarity; on the other hand, it has been attributed to persistent
constriction of the waist, in consequence of which the full play of the
diaphragm is prevented and the burden of inspiration is thrown on the
thoracic muscles. It has been assumed that if inspiration were con-
fined in women to the diaphragm, there would arise in the latter stages
of gestation such an increase in intra-abdominal pressure that not only
would respiratory exchanges be interfered with, but fetal life might be
unfavorably influenced, if not endangered. Modern investigations
have not confirmed this assumption, but, on the contrary, have corrob-
orated the view that the preponderance of thoracic movement is due
to the influences of dress restrictions, for with their removal the so-called
costal type of breathing entirely disappears. While gestation may
lead to a greater activity of the thorax, this is but temporary, for with
its termination there is a return to the diaphragmatic type of breathing.

Number of Respirations per Minute. — The number of respira-
tions which occur in a unit of time varies with a variety of conditions,
the most important of which is age. The results of the observations of
Quetelet on this point, which are generally accepted, are as follows:

Age. Respirations per Minute. Age. Respirations per Minute

o- i year, 44 20-25 years, 18.7

5 years, 26 25-30 " 16.0

15-20 " 20 30-5° " I^-°

From these observations it may be assumed that the average number
of respirations in the adult is eighteen per minute, though varying
from moment to moment from sixteen to twenty. During sleep, how-
ever, the respiratory movements often diminish in number as much
as 30 per cent., at the same time diminishing in depth.

Rhythm. — Each respiratory act takes place normally in a regular
methodic manner, each event occurring in a definite sequence and
occupying the same relative period of time. This rhythm, however,
is not infrequently temporarily disturbed by emotions, volitional acts,
muscle activity, phonation, changes in the composition of the blood,
etc.; with the removal of these disturbing factors, the respiratory
mechanism soon returns to its normal condition.

A graphic representation of the excursions of the thoracic walls,
rhythmic or otherwise, is obtained by fastening to the thorax an appara-

RESPIRATION.

40;

Fig. 193. — Pneumograph. — (Fits.)

tus, a stethometer or a pneumograph, which by means of a tambour
takes up and transmits the movement to a second tambour provided
with a recording lever. A simple form of pneumograph, suggested
by Fitz (Fig. 193), consists of a coil of wire two and a half centimeters
in diameter and about 40 centimeters in length, enclosed by thin rubber
tubing, one end of which is closed, the other placed in communication
with either a tambour and lever or with a piston recorder. By means
of an inelastic cord or chain the apparatus is securely fastened to the
chest. With each inspiration the spring is elongated, the air within
the system is rarefied,
and as a result the lever
falls; with each expira-
tion the reverse condi-
tions obtain and the
lever rises. If the lever
be applied to the record-
ing surface of a moving
cylinder, a curve of the
thoracic movement, a pneumatogram, is obtained (Fig. 194), from
which it is apparent that inspiration takes place more abruptly and
occupies a shorter period of time than expiration; that expiration im-
mediately follows inspiration, but that there is a slight pause between
the end of the expiration and the beginning of the inspiration. The
time relations of the two movements can be obtained by a magnet-
signal actuated by an electric current interrupted once a second. The
ratio of inspiration to expiration has been represented as 5 to 6, or 6 to 8.
Volumes of Air Breathed. — The volumes of air which enter
and leave the lungs with each inspiration and expiration naturally

vary with the extent of the move-
ment, though four at least may be
determined: (1) that of an ordinary
inspiration; (2) that of an ordinary
expiration; (3) that of a forced in-
spiration; (4) that of a forced ex-
piration.

The apparatus employed for
the determination of these different
volumes is the spirometer, a modification of the gasometer. The
form introduced by Jonathan Hutchinson (Fig. 195) consists of two
metallic cylinders, one (a) containing water, the other (b) containing
air, the latter being inserted into the former. The air cylinder is
balanced by weights so accurately that it remains stationary in any
position. A tube, penetrating the base of the water cylinder, is con-
tinued upward through and above the level of the water. The air-
space above is thus placed in free communication with the external air.
A stopcock at the outer end of this tube prevents the escape of the air
when this is not desirable. To the free end of the tube a rubber tube

Fig. 194. — A Pneumatogram.
Marey.)

{After

404

TEXT-BOOK OF PHYSIOLOGY.

provided with a suitable mouthpiece is attached, through which air
can be breathed into or out of the air cylinder. With each inspiration
the air cylinder descends; with each expiration it ascends. A scale,
on one of the side supports, graduated in cubic inches or centimeters,
indicates the volume of air inspired or expired.

With this apparatus Hutchinson, from a long series of observa-
tions, defined and determined the above-mentioned four volumes
as follows:

i. The tidal volume, that which flows into and out of the lungs with

each inspiration and expiration, which
varies from 20 to 30 cubic inches (312
to 468 c.c).

2. The complemental volume, that which
flows into the lungs, in addition to the
tidal volume, as a result of a forcible
inspiration, and which amounts to about
no cubic inches (1748 c.c).

3. The reserve volume, that which flows out
of the lungs, in addition to the tidal
volume, as a result of a forcible expira-
tion, and which amounts to about 100
cubic inches (1562 c.c).

After the expulsion of the reserve
volume there yet remains in the lungs an
unknown volume of air which serves the
mechanic function of distending the air-
cells and alveolar passages, thus main-
taining the conditions essential to the
free movement of blood through the
capillaries and to the exchanges of gases
between the blood and alveolar air. As
this air can not be displaced by voli-
tional effort, but resides permanently in the alveoli and bron-
chial tubes though constantly undergoing renewal, it was termed —
The residual volume, the amount of which is difficult of determina-
tion, but has been estimated by different observers at 914 c.c,
1562 c.c, 1980 c.c.
The Vital Capacity of the Lungs. — From foregoing statements
it is clear that the thorax and lungs are capable of a maximum degree
of expansion, at which moment the lungs contain their maximum
volume of air. This volume, whatever it may be, represents the entire
capacity of the lungs in the physiologic condition, and includes the
tidal, the complemental, the reserve, and the residual volumes. Mr.
Hutchinson, however, defined the vital or respiratory capacity of the
lungs as the amount of air which can be expelled by the most forcible
expiration after the most forcible inspiration, and which therefore ex-
cludes the residual volume. The vital capacity was supposed to be

Fig. 195. — Spirometer.
(Hutchinson.)

4-

RESPIRATION.

405

Fig. 196. — Pneumatograph. — (Gad.)

an indication of an individual's respiratory power, not only in phys-
iologic but also in pathologic conditions. Though averaging about
230 cubic inches (3593 c.c.) for an individual 5 feet 7 inches in height,
the vital capacity varies with a number of conditions, the most im-
portant of which is stature. It is found that between 5 and 6 feet the
capacity increases 8 inches (125 c.c) for each inch increase in height.

The volume changes of the
thorax indicated by the volumes
of air entering and leaving the
lungs can be not only determined
but graphically represented by
means of an apparatus similar in
principle to the spirometer, de-
vised by Gad and known as the
pneumatograph or aero pie thy sino-
graph (Fig. 196). This consists
of a quadrangular box with double walls, the space between which
is filled with water. The center of the box is an air chamber. A thin-
walled mica box sinks into the water. Posteriorly it is attached to
and rotates around an axis, which permits of an elevation or depres-
sion of the anterior portion. It is also carefully counterpoised. A
light lever attached to the mica box records its movements. The
interior of the box communicates by a tube with a large reservoir into

which the individual breathes, the
object being to prevent a too rapid
vitiation of the air. Inspiration
causes the lever to descend, expira-
tion to ascend. Previous gradua-
tion of the apparatus is necessary
to determine the volumes breathed.
A graphic record of the volume
changes is shown in Fig. 197.

Respiratory Sounds. — On ap-
plying the ear over the trachea and
bronchi there is heard during both
inspiration and expiration a well-
defined sound, loud, harsh, and
blowing in character, which from
its situation is known as the bron-
chial sound. It is especially well
heard between the scapula? above
the fourth dorsal vertebra. This sound is produced in the larynx,
for with its separation from the trachea the sound disappears. The
cause of the sound is to be found in the narrowing of the air-passage
at the level of the vocal membranes, though the mechanism of its pro-
duction is uncertain. On applying the ear to almost anv portion of
the chest-wall, but especially to the infrascapular area, there is heard

•-W

ao c.in

TIDALVOL

Fig. 197. — Representing the Vol-
ume Changes of the Thorax and
Lungs (Diagrammatic).

4o6 TEXT-BOOK OF PHYSIOLOGY.

during both inspiration and expiration a delicate, sighing, rustling
sound, which from its supposed seat of origin, the air-vesicles or -cells,
is known as the vesicular sound. This sound is supposed to be due
to the sudden expansion of the air-cells during inspiration and to the
friction of the air in the alveolar passages.

THE CHEMISTRY OF RESPIRATION.

The general nutritive process as it takes place in the tissues involves
the assimilation of oxvgen and the evolution of carbon dioxid. The
former is the first, the latter the last, of a series of chemic changes
the continuance of which is essential to the maintenance of all life
phenomena. A constant supply of oxygen and an equally constant
removal of carbon dioxid are necessary conditions for tissue activity.
The respiratory movements constitute the means by which the oxygen
of the air is brought into, and the carbon dioxid expelled from, the
lungs into the surrounding air. The blood is the medium by which
the oxygen is transported from the lungs to the tissues and the carbon
dioxid from the tissues to the lungs.

The exchanges between blood and tissues constitute internal res-
piration, in contradistinction to the thoracic movements by which
the air is brought into relation with the blood, and which constitute
external respiration. The transfer of the oxygen by the blood from
the interior of the lungs to the tissues, and of the carbon dioxid from
the tissues to the interior of the lungs, is the outcome of a series of
chemic changes which are related to the exchange of gases between
the air in the lungs and the blood, on the one hand, and between the
blood and tissues, on the other.

In consequence of the many and complex chemic changes which
attend these gaseous exchanges, there arise changes in composition of:
i. The air breathed.

2. The blood, both arterial and venous.

3. The tissue elements and the lymph by which they are surrounded.
The investigation of the nature of these changes, the mechanism

of their production, and their quantitative relations constitutes the
subject-matter of the chemistry of respiration.

CHANGES IN THE COMPOSITION OF THE AIR.

Experience teaches that the air during its sojourn in the lungs
undergoes such a change in composition that it is rendered unfit for
further breathing. Chemic analysis has shown that this change in-
volves a loss of oxygen, a gain in carbon dioxid, watery vapor and
organic matter. For the correct understanding of the phenomena
of respiration it is essential, that not only the character but the extent
of these changes be known. This necessitates an analysis of both the
inspired and expired airs, from a comparison of which certain deduc-
tions can be made

RESPIRATION. 407

The results which have been obtained are represented in the
following table:

Inspired Air. Expired Air.
f Oxygen, 20.80. f Oxygen, 16.02.

100 J Carbon dioxid, traces. | Carbon dioxid, . . . 4.38.

vols. I Nitrogen, 79.20. , < Nitrogen, 79.60.

I Watery vapor, variable. ' | Watery vapor, . . . .saturated.

[ Organic matter.

These analyses indicate that under ordinary conditions the air
loses oxygen to the extent of 4.78 per cent, and gains carbon dioxid
to the extent of 4.38 per cent.; that it gains in nitrogen to the extent
of 0.4 per cent, and in watery vapor from its initial amount to the
point of saturation, as well as in organic matter. It is to these changes
in their totality that those disturbances of physiologic activity are
to be attributed which arise when expired air is re-breathed for any
length of time without having undergone renovation.

Special forms of apparatus have been devised for the collection
and analysis of gases. Their construction as well as the methods
of analysis involved are complicated and need not be described in
this connection. The presence of the carbon dioxid, however, may
be readily shown by breathing through a glass tube into a vessel con-
taining barium or calcium hydrate. The turbidity which immediately
follows is due to the formation of barium or calcium carbonate, which
can be due only to the presence of carbon dioxid. That this turbidity
is not due to the carbon dioxid normally present in the air is shown by
the fact that the solution remains clear until the passage of the atmos-
pheric air has been maintained for some time. From the percentage
loss of oxygen and gain in carbon dioxid, the total oxygen absorbed and
carbon dioxid exhaled may be approximately calculated. Thus, if
the volume of air breathed daily be accepted at either 10,800 or 12,-
240 liters, and the percentage loss of oxygen be 4.78, the total oxygen
absorbed may be obtained by the rule of simple proportion, e. g.:

100 : 4. 78 :: 10,800 : # = 516 liters
Or

100 : 4.78 :: 12,240 : # = 585 liters.

By the same method the total carbon dioxid exhaled is found to be
either 473 or 526 liters; volumes in both instances which agree very
well with volumes obtained by other methods.

From the fact that when one volume of oxygen combines with
carbon it gives rise to but one volume of carbon dioxid, it is evident
that of the oxygen absorbed the greater portion by far is utilized in
the oxidation of the carbon, while the smaller portion is utilized in
the oxidation of other substances, but especially hydrogen, as shown
by the increase in water eliminated beyond that consumed. These
amounts, however, are not fixed but variable, and depend on the
quality and quantity of the foods, exercise, etc. The ratio of the volume
of the carbon dioxid exhaled to the volume of oxygen absorbed is

4o8 TEXT-BOOK OF PHYSIOLOGY.

known as the respiratory quotient, and is usually represented by the
svmbol -§~ Thus in the foregoing analysis the respiratory quotient is
0.916.

The gain in nitrogen is a variable factor, ranging from zero to 0.9
per cent. This gain is probably of accidental occurrence, due to ab-
sorption from the large intestine, in which decomposition of nitrogen-
holding compounds is taking place. It is generally believed that free
nitrogen plays no part in any phenomenon of combination or decom-
position within the body.

The gain in watery vapor will depend on the amount previously
present in the air. This is conditioned by the temperature. With
a rise in temperature the percentage of water increases; with a fall,
it decreases. By breathing into a vessel containing pumice stone satu-
rated with sulphuric acid, the vapor may be collected. The difference
observed between the weight before and after breathing is an indica-
tion of the amount by weight of water exhaled during the time of
breathing. It has been calculated that the amount of water exhaled
daily approximates 500 grams. Though invisible at ordinary temper-
atures, it becomes visible at low temperature as soon as it emerges from
the respiratory tract. The loss of heat is followed by a condensation
of the vapor, which appears at once as a cloudy precipitate.

The gain in organic matter is also variable. The amount present
is not sufficient to permit of a thorough chemic analysis, but there are
reasons for believing that it belongs to the proteid group of bodies.
If it accumulates in the air, especially at high temperatures, it readily
undergoes decomposition, with the production of offensive odors.
Traces of free ammonia have also been found in the expired air.
In addition to these chemic changes, the air experiences physical
changes; e. g., a rise in temperature and an increase in volume. The
rise in temperature can be shown by breathing through a suitable
mouthpiece into a glass tube containing a thermometer. By this
means it has been shown that inspired air at 20 ° C. rises in temperature
to 370 C; at 6.3 ° to 29.8 ° C. The increase in the temperature will
depend upon that of the air inspired and the time it remains in the lungs.
If retained a sufficient length of time it will always become that of the
body. As a result of the heat absorption the expired air increases in
volume about one-ninth over that of the inspired air. When corrected
for temperature and pressure and freed from aqueous vapor, the volume
of the expired air is less than that of the inspired air by about one-two
hundred and fiftieth.

The Composition of the Alveolar Air. — The foregoing state-
ment of the composition of the expired air, derived in part from the
upper air-passages, trachea, and bronchi, does not necessarily repre-
sent Ihe composition of the alveolar air. It is very probable that the
percentage of carbon dioxid is greater, the percentage of oxygen less,
in the latter than in the former. This is made evident by collecting
in several portions the expired air as it escapes from the respiratory

RESPIRATION. 409

tract and subjecting it to analysis. The last portion always contains
a larger amount of carbon dioxid and a smaller amount of oxygen
than the first portion. The determination of the composition of the
alveolar air is extremely difficult. It has been estimated to contain
from 5 to 6 per cent, of carbon dioxid and from 14 to 18 per cent, of
oxygen.

Pulmonary Ventilation. — It is owing largely to this inequality
of volumes and consequently of the "partial pressures" of these two
gases in the trachea and alveoli that the degree of ventilation necessarv
for the exchange of gases between lungs and air is maintained. Though
the respiratory movements doubtless create currents in the air-passages
which carry, on the one hand, a portion of the inspired air directly
into the alveoli, and, on the other hand, carry a portion of the alveolar
air directly out of the body, other portions find their way into and out
of the alveoli in accordance with the laws of diffusion. If the pressure
of the oxygen in the trachea is 158 mm. Hg. and in the alveoli approxi-
mately 122 mm. Hg., diffusion downward will take place. Equilibrium,
however, is never established, as the oxygen is continually disappear-
ing by passing into the blood. On the contrary, if the carbon dioxid
pressure in the alveoli is approximately 28 to 40 mm. Hg., and in the
trachea 0.3 mm. Hg., diffusion will take place upward. Equilibrium
will never be established, however, as the carbon dioxid is constantly
coming out of the blood. Pulmonary ventilation may also be aided
by those alternate changes in volume of the heart, great vessels, and
lungs occurring as the result of the heart-beat and producing the so-
called cardio-pneumatic movements.

CHANGES IN THE COMPOSITION OF THE BLOOD.

The blood which flows into the lungs through the pulmonary
artery is dark bluish-red, that which flows from the lungs into the
pulmonary veins is scarlet red, in color. The blood is changed, while
flowing through the lung capillaries, from the venous to the arterial
condition. As the air in the lungs gains carbon dioxid and loses oxygen,
it is fair to assume that what the air gains the blood loses, and what the
air loses the blood gains. In other words, the blood, while passing
through the lungs, is changed from venous to arterial by the loss of
carbon dioxid and the gain of oxygen. The change in color of venous
blood from dark bluish to scarlet red is strikingly shown by shaking
it in a test-tube with oxygen or atmospheric air.

The blood which flows into the tissues through the arteries is red,
that which flows from the tissues into the veins is bluish, in color.
The blood while flowing through the tissue capillaries is changed from
the arterial to the venous condition. Since arterial blood when de-
prived of oxygen becomes bluish-red, the indication is that the change
in color is associated with, if not entirely due to, the escape of oxygen
into the tissues. The constant elimination of carbon dioxid froni the

4io TEXT-BOOK OF PHYSIOLOGY.

blood into the lungs indicates that the carbon dioxid is as constantly
passing from the tissues through the capillary walls into the blood.

These considerations are confirmed by the results of analyses which
have been made of both venous and arterial blood. The presence
of gas in the blood is demonstrated by subjecting it under appropriate
conditions to the vacuum of the mercurial air-pump, into which it at
once escapes. From ioo volumes, an average of 60 volumes of gas at
standard pressure, 760 mm. Hg. and temperature o° C, can thus be
obtained.

Gases of the Blood. — An analysis of the volumes of gas removed
from both venous and arterial blood shows that each consists of oxygen,
carbon dioxid, and nitrogen, though in different amounts. An aver-
age composition of the gases extracted from dog's blood obtained from
the right ventricle and carotid artery is given in the following table:

Venous blood J ^jf V ' :\ 9'12 ™ls- Arterial blood \ °Xy,ge "' W ' '.V 2° v°ls-
t < Carbon dioxid, 4S , { Carbon dioxid, 40

IOO VOlS. -vr-, ^J i, IOO VOls. ,T.. ' ^ ,,

[ JNitrogen, .... 1- 2 [ Nitrogen, . . . . 1- 2

The changes produced in the blood by respiration, both external
and internal, become apparent from a comparison of these analyses.
The venous blood while passing through the lungs gains from eight
to eleven volumes per cent, of oxygen and loses five volumes per cent,
of carbon dioxid. The arterial blood while passing through the tissues
loses oxygen and gains carbon dioxid in corresponding amounts. The
volume of nitrogen is not appreciably changed.

The Relation of the Gases in the Blood. — The mechanism
by which the gases become associated with the blood at the moment
of their entrance into it, and again become dissociated just prior to
their exit from it, as well as their relation to the blood while in transit,
will be more readily understood after reference to a few elementary
facts relative to the absorption of gases by liquids in general and the
conditions of temperature and pressure by which it is influenced.

It is well known that liquids will absorb or dissolve at any constant
pressure unequal volumes of different gases in accordance with their
solubilities and with variations in temperature. Water, for example,
will absorb, in accordance with the foregoing conditions, oxygen, carbon
dioxid and nitrogen, as well as many other gases. The volume of
any gas thus absorbed is known as the coefficient- of absorption, and may
be defined as the number of cubic centimeters of the gas which one
cubic centimeter of water will absorb when the gas, in contact with
the water, stands under a pressure of one atmosphere or 760 mm. of
mercury and at a temperature of o° C. The volume absorbed, '
however, varies inversely as the temperature. Thus at o° C. the
volume of oxygen absorbed by one volume of water is 0.0489 c.c. ;
of carbon dioxid 1.713 c.c; of nitrogen 0.0234 c.c. With a rise of tem-
perature, the pressure remaining constant, the absorptive power of
water for each of these gases diminishes. Thus at 150 C, the volumes

RESPIRATION. 411

of oxygen, carbon dioxid and nitrogen absorbed are 0.0310 c.c., 1.0025
c.c. and 0.0168 c.c. respectively. Though the volume of the gas
absorbed diminishes as the temperature rises, it is independent of
pressure, for no matter to what extent the pressure may vary the
volume absorbed is always the same. (Law of Henry.)

If the weight of the gas absorbed be considered rather than the
volume (that is the product of the volume and the density or the
number of molecules in the volume), then the temperature remaining
constant, the weight of the volume absorbed increases and decreases
proportionately as the pressure rises and falls. Thus at a pressure of
760 mm. of mercury and at a temperature of o° C, the volume of
oxygen absorbed by one volume of water is 0.0489 c.c; at 1520 mm.
of mercury, the same volume is absorbed but its weight is doubled.
If the pressure falls below 760 mm. of mercury the same volume is ab-
sorbed but its weight is diminished. (Law of Dalton.) Because of
the foregoing facts, it is necessary in all gaseous determinations to
reduce for purposes of comparison the obtained volumes to standard
temperature (p° C.) and. pressure (760 mm. of mercury).

When the liquid is once saturated with a gas at a constant pressure
and temperature, there is coincidently with the entrance of the gas
into the liquid, an equivalent exit of the gas from it, tnough the volume
retained in the liquid remains constant. The reason for this fact is,
that under the conditions, the volume of the gas dissolved by the
liquid though small in amount exerts a pressure in the opposite direction
equivalent to the pressure acting upon the liquid. If one cubic
centimeter of water absorbs 0.0489 c.c. of oxygen at 760 mm. and
o° C, this volume will exert a pressure opposite in direction of 760 mm.
of mercury. For this reason the entrance and exit of the gas are
equal and opposite.

If water be exposed to atmospheric air consisting of oxygen, carbon
dioxid, and nitrogen in the ordinary proportions, at any given tem-
perature and pressure, the water will absorb unequal volumes of each of
the three gases. The pressure under which each gas is absorbed is a
part only, however, of the total atmospheric pressure at the time. The
pressure exerted by any one of these gases is known as its " partial pres-
sure," and depends on the percentage volume of the gas present. If
atmospheric air contains at standard pressure and temperature 79.15
volumes per cent of nitrogen, its partial pressure will be 2f^ of 760,
or 601.54 mm. Hg.; if the air contains 0.04 volume per cent, of carbon
dioxid and 20.85 volumes per cent, of oxygen, the partial pressure of
each will be 0.30 mm. Hg. and 158.46 mm. Hg. respectively. The
absorption of each gas is independent of all the rest, and is the same
for nitrogen, for example, as if it alone were present at a pressure of
601.54 mm. Hg. -

Again, if water holding in solution a certain volume of a gas — ■
carbon dioxid, for example — be exposed to an atmosphere containing
but 0.04 volume per cent, of carbon dioxid, and having therefore a

4i2 TEXT-BOOK OF PHYSIOLOGY.

pressure of but 0.3 mm. Hg., the gas will at once begin to leave the
water, and continue to do so until the pressure of the carbon dioxid
in the atmosphere balances the pressure of the gas in the water, at which
moment the escape of the gas ceases. The pressure of a gas in a liquid
is equal to that pressure in millimeters of mercury of the same gas in
the atmosphere which is required to keep it in solution. What is
true for the carbon dioxid is true for any other gas that may be in solu-
tion. If a liquid has a greater density than water as from the pres-
ence of inorganic salts, the absorptive power under standard condi-
tions of temperature and pressure becomes less. It is for this reason
that blood-plasma contains less oxygen, nitrogen and carbon dioxid
than water.

It will be recalled that the blood yields up its gases when subjected
to the vacuum of the mercurial pump; that is, to a diminution or com-
plete removal of the atmospheric pressure. From this it might be in-
ferred that the gases are merely held in solution by pressure, and at
once escape the moment they are exposed to a space in which there
is a very slight or a total absence of pressure. In other words that the
absorption of gases by the blood and their escape from it follows the law
of pressure as stated in foregoing paragraphs. It is therefore neces-
sary to test this supposed condition of the gases in the blood by sub-
jecting the latter to gradually diminishing pressures, with a view of
determining in how far the discharge of the gases follows the law of
falling pressures. For convenience the conditions of each gas will be
considered separately.

Oxygen. — If blood is subjected to a succession of pressures pro-
gressively less than the standard, it is found that though oxygen is
evolved, its evolution is not in accordance with the law of partial
pressures; that is, in proportion to the diminution of pressure. Within
wide limits — e. g., from 760 to 332 mm. atmospheric pressure, to which
correspond oxygen pressures of 160 and 70 mm. respectively — there
is but a slight increase in the amount of oxygen evolved; and it is not
until the pressure of the oxygen falls below this that it begins to be
liberated in large amounts. From this on, the oxygen continues to be
liberated with decreasing pressures, until the zero point is reached, when
all gaseous discharge ceases. Coincidently the blood changes in color
from a bright red to a deep bluish-red. It is evident from the results
of this procedure that the condition of the oxygen in the blood is but
to a slight extent one of physical absorption. The indications are that
the union is of the nature of a chemic combination.

If the red corpuscles are removed from the blood and the plasma
alone treated in the manner above described, it will be found that the
oxygen liberated now follows the law of partial pressure. The amount
so liberated, however, is small — about one per cent, of the total oxygen
of the blood. The agent therefore which holds the oxygen in com-
bination is the red corpuscle, and more especially the hemoglobin,
which constitutes about 30 per cent, of its volume. This is proved by

RESPIRATION. 413

the fact that a solution of gas-free hemoglobin of a strength equivalent
to that of the blood (14 per cent.), exposed to oxygen under a gradually
increasing pressure from zero up to 50 to 70 mm. pressure, will absorb
large quantities of oxygen; beyond this point the amount absorbed
is again small in comparison. At 70 mm. pressure the hemoglobin
is almost saturated. Coincidently with this absorption the hemo-
globin changes in color from dark blue to bright red; changes from
hemoglobin to oxyhemoglobin. The reverse method, that of subject-
ing oxyhemoglobin to gradually diminishing pressures, yields opposite
results, that is the oxygen becomes dissociated and the force by which
this is accomplished is known as the force of dissociation. As one
gram of hemoglobin combines with 1.59 c.c. of oxygen, and as the per-
centage of hemoglobin is 13.50 to 14, it is evident that there is sufficient
hemoglobin to combine with practically all the oxygen usually present
in the blood.

The union of the oxygen with the hemoglobin is therefore largely
chemic in character, dependent however on pressure. About one-half
of one per cent, is physically absorbed by or dissolved in the plasma;
the remainder is chemically combined with the hemoglobin.

The association or combination of oxygen is favored by a pressure
of at least from 30 to 50 mm. Hg. and upward; the dissociation, by
diminution of pressure. In the conversion of hemoglobin into oxy-
hemoglobin two antagonistic forces are at work, heat and chemic
affinity. The former endeavors to prevent, the latter to favor, the
union. Chemic affinity increases with the influence of mass, that is,
in proportion to the number of atoms in a unit of volume, with the
density and with the partial pressure of the oxygen. Diminution of
pressure reduces the mass influence and permits the heat to bring
about dissociation (Bunge). The following table by Hiifner shows
the relative proportion of hemoglobin and oxyhemoglobin in blood
containing 14 per cent, hemoglobin and exposed to air at gradually
diminishing pressures:

pheric Pressure

Partial Pressure of

Hemoglobin-

Oxyhemoglobin

IN MM. Hg.

Oxygen in mm.

Hg.

Percentage.

Percentage.

760

159-3

1.49

98.51

524.8

no

2.14

97.86

357-3

75

3-11

96.89

238-5

5°

4.60

95-40

"9-3

25

S.79

91. 2L

47-7

10

19.36

S0.64

23.8

5

32-5x

67.49

0.0

0.0

100.00

O.OO

Carbon Dioxid. — The blood yields up its contained carbon dioxid
to the vacuum of the gas-pump as completely as it does its oxygen.
The same is not the case, however, if the red corpuscles are first re-
moved and the experiment made with either plasma or serum. Even
at zero pressure the fluid contains carbon dioxid, as shown by its libera-
tion on the addition of some weak acid, as tartaric or phosphoric, an

4i4 TEXT-BOOK OF PHYSIOLOGY.

indication that it exists in a state of firm combination. The same
result follows the addition of the red blood-corpuscles, which act in a
manner similar to the acids just mentioned. This property of the cor-
puscles has been attributed to hemoglobin, and especially when in
the state of oxyhemoglobin. It is for this reason that blood yields all
its carbon dioxid to the vacuum of the gas-pump.

The limit of pressure at which the plasma ceases to physically
absorb carbon dioxid and 'begins to chemically combine it is not very
clearly defined. It has been estimated that of the entire amount,
38 to 45 volumes, only about 2.5 volumes are so absorbed, the remain-
der being in a condition of both loose and stable combination.

An analysis of the serum, and presumably of the plasma, shows
the presence of sodium salts, with which the carbon dioxid could
enter into combination, viz.: sodium carbonate and dibasic sodium
phosphate. The sodium is thus partly divided between carbonic acid
and phosphoric acid. The amount of the sodium which falls to carbon
dioxid will depend on the mass influence of the latter; that is, its partial
pressure.

At its origin in the tissues the carbon dioxid acquires a consider-
able tension, and its mass influence is correspondingly large. On
entering the blood it combines with sodium carbonate, with the forma-
tion of sodium bicarbonate, as shown in the following equation:

Na2C03 + CCV+ H20 = 2NaHC03.

At the same time, having a greater mass influence than the phos-
phoric acid, it will withdraw from the dibasic sodium phosphate one-
half of its sodium, with the formation of sodium bicarbonate and mono-
basic sodium phosphate, as shown in the following equation:

Na2HP04 + C02 + H20 = NaHC03 + NaH2P04.

With the diffusion of the carbon dioxid from the blood into the alveoli
its tension in the venous blood falls, its mass influence diminishes,
while that of the phosphoric acid relatively increases. As a result,
the sodium is withdrawn from the sodium bicarbonate, an additional
liberation of carbon dioxid takes place and dibasic sodium phosphate
is re-formed. The association or combination of the carbon dioxid
with the basic salts depends on its partial pressure; its dissociation in
the lungs, on a diminution of pressure.

Nitrogen. — This gas exists in both arterial and venous blood in a
state of solution. There is no evidence that it enters into combination
with any other element.

Tension of the Gases in the Blood. — It will be recalled that a
Liquid holding in solution one or more gases will on exposure to an
atmosphere composed of the same gases either give up or absorb vol-
umes varying in amount and in accordance with their partial pressures
until equilibrium is established. Jf the pressure of any one gas in the
atmosphere is greater than the pressure of the same gas in the liquid,

RESPIRATION. 415

it is absorbed; if the pressure is less the gas is discharged. Knowing
the pressure of the gases in percentages of an atmosphere, at the be-
ginning and the end of an experiment, the original tension or pressure
of the gases in the liquid can be easily calculated. On this principle
various forms of apparatus known as aerotonometers have been devised
by which the tension of the gases in the blood can be determined.

These appliances consist essentially of a glass tube containing
oxygen, carbon dioxid and nitrogen in known amounts and tensions.
The blood from an animal is then allowed to flow directly from an artery
or vein into the tube. As it flows down its sides in a thin layer it pre-
sents a large surface to the action of the contained gases. In the aero-
tonometer of Fredericq the blood made non-coagulable by the injection
of peptone is returned from the opposite extremity of the tube to the
animal. This enables the experiment to be continued for an hour or
more. A knowledge of the tensions of the blood gases is of interest,
as it affords a clue to the mechanism by which the interchange takes
place between the lungs and the blood, on the one hand, and the blood
and tissues, on the other. The results, however, of different observers
are not sufficiently in accord to permit of positive deductions.

In the well-known experiments of Strassburger, the tension of the
oxygen in the arterial blood of the dog was found to be 29.64 mm. Hg.,
or 3.9 per cent, of an atmosphere, and in the venous blood 22.04 mm.
Hg., or 2.9 per cent. The tension of the carbon dioxid in the venous
blood was found to be 41.14 mm. Hg., or 5.4 per cent. of an atmosphere,
and in the arterial blood 21.8 mm. Hg., or 2.8 per cent. Very different
results have been obtained by Fredericq with the aerotonometer
devised by him and by the employment of a method different from that of
Strassburger. Thus he states that the oxygen tension in the pulmonary
alveoli is 136 mm. Hg., or 18 per cent, of an atmosphere while in the
arterial blood it is 106 mm. Hg., or 14 per cent.; while the carbon-di-
oxid tension in the tissues varies from 38 to 68 mm. Hg., or from 5 to 9
per cent. of an atmosphere; while in the venous blood it varies from 30 to
41 mm. Hg., or from ^.S to 5.4 per cent, and in the pulmonary alveoli
it is about 21 mm. or 2.8 per cent.

CHANGES IN THE COMPOSITION OF THE TISSUES AND LYMPH.

From previous statements the inferences can be drawn that the
oxygen leaves the blood as the latter flows through the capillaries;
that it passes through the capillary wall into the surrounding lymph
and so to the tissue-cells; that it oxidizes food materials in the tissue-
cells whereby the potential energy of the former is liberated as kinetic
energy; that the carbon dioxid so evolved passes into the lymph and
through the wall of the capillary into the blood.

While this is doubtless the case, the presence of free oxygen in the
tissues can not be demonstrated by the usual methods of gas analysis.
Only in the saliva and in the blood of the placental umbilical vein can

4i6 TEXT-BOOK OF PHYSIOLOGY.

it be shown that oxygen has directly passed through the capillary wall.
For this reason it has been claimed by a few investigators that oxygen
does not leave the blood, but that the field of its activity as an oxidizing
agent is limited to the blood-current, where it meets with and oxidizes
easily reducible substances entering from the tissues. On this view
the potential energy of the food would be liberated by mere decom-
position or cleavage in consequence of cell activity.

Nevertheless many facts from the fields of comparative physi-
ology and physiologic chemistry combine to support the view that
oxygen is absolutely necessary to the maintenance of the life of all
tissue-cells. Though they will continue to manifest their character-
istic activities — e. g., contraction on the part of a muscle, secretion by a
gland, the conduction of a nerve impulse by the nerve, etc. — for a
variable length of time after oxygen is prevented from gaining access
to them, nevertheless they will in due time die.

The necessity for oxygen on the part of the tissues and the avidity
with which they absorb it, is shown by their power of reducing pig-
ments such as alizarine blue. If this pigment be injected into the
blood-vessels of an animal and the animal killed in about ten minutes,
it will be found that while the blood exhibits a deep blue color the
tissues present their usual colors. But after exposure to the air or to
free oxygen the latter also acquire the characteristic blue color. The
explanation offered for this fact is that the tissues in their need for
oxygen absolutely extract it from the pigment, reducing it to a color-
less compound, which, however, on exposure recombines with oxygen
and regains the original color.

Though free oxygen can not be shown to be present in the tissues,
there are many reasons for believing that it is continually passing into
them by way of the lymph-stream. Its rapid disappearance would
indicate that it is immediately utilized for the production of carbon
dioxid (which is improbable on other grounds), or that the tissues
possess a capacity for oxygen storage, of placing it in reserve under
some combination or other, by which it can be securely retained
until required for oxidation purposes. This is rendered probable
from the fact that the carbon dioxid evolved at any given moment is
not necessarily dependent on the oxygen just absorbed, for if oxygen
be withheld from a nutritive fluid which is being artificially circulated
through a recently isolated organ, carbon dioxid will continue to be
discharged for some time. A muscle, or even a living animal — e. g.,
a frog — placed in an atmosphere of pure nitrogen will remain active
and evolve C02 for even several hours.

Naturally the absorption of oxygen and the discharge of carbon
dioxid and the changes of composition which arc incident to nutri-
tion will be most marked in those tissues characterized by the greatest
degree of physiologic activity. Muscle-tissue exhibits these changes
to a greater degree than bone. Tissues with intermediate degrees
of activity should exhibit corresponding degrees of respiratory change.

RESPIRATION.

4i7

Experiment confirms this view. Thus, 100 grams each of muscle,
spleen, and broken bone from a recently living animal exposed to the
air for twenty-four hours absorbed respectively 50.8 c.c, 27.3 c.c,
and 17.2 c.c. of oxygen, while each discharged during the same period
56.8 c.c, 15.4 c.c, and 8.1 c.c. of carbon dioxid respectively. In
another series of experiments by a different observer 100 grams of
muscle absorbed in three hours 23 c.c. of oxygen, and 100 grams of
bone 5 c.c. of oxygen. Both tissues discharged carbon dioxid in
amounts proportional to the oxygen absorbed. The same respiratory
changes may be more
satisfactorily demon-
strated by passing
blood through the tis-
sues of isolated organs
and the tissues of re-
cently living animals.
The analysis of the
blood before and after
perfusion shows a loss
of oxygen and a gain
in carbon dioxid.

Tension of the
Gases in the Tis-

ATMOSPHERIC AIR.
O— I SB MM MS,OR 20.85 PC
CO 0.3 MM HG OR 0.04- PC

O-TEMSION

22.04- MM HO OR

CO TENSION
•♦I.O* MM HG OR
5.4 P.C.

ALVEOLUS

VtNOUS

BLOOD

ARTERIAL
BLOOD

O -TENSIOI
O.OO M M H

CO-TENSIO>

O — TENSION

CO —TENSION

Fig. 198. — Diagram showing the Relative Tension
of Oxygen and Carbon Dioxid ln the Lungs, ln the
Blood, and est the Tissues.

sues. — As the pres-
ence of free oxygen
can not be demon-
strated, its tension
there must be re-
garded as nil. The
tension of the carbon
dioxid is quite high,
though difficult of
exact determination.
It has been estimated
at from 45 to 68 mm.
Hg., or from 6 to 9 per cent, of an atmosphere.

The variations of tension or pressure of these two gases in the
lungs, in different parts of the vascular apparatus, and in the tissues,
and their relations to each other, are shown in Figure 198, expressed
in mm. Hg. and percentages of an atmosphere. The figures inserted
are those of Strassburger and are of less value as far as the oxygen
tension is concerned than those of Fredericq.

The Mechanism of the Gaseous Exchange.— In these pressure
differences sufficient cause is found for the exchange of the gases.
The oxygen pressure in the alveoli being in excess of that in the blood,
the gas passes through the thin alveolo-capillary wall into the plasma.
As the oxygen pressure in the plasma rises and approximates that in

4i8 TEXT-BOOK OF PHYSIOLOGY.

the alveoli, a portion of the oxygen combines with the hemoglobin
until the latter is almost saturated. The corpuscle is then carried
through the arterial system surrounded by oxygen under a definite
pressure which is sufficient to keep the absorbed oxygen in union
with the hemoglobin. On passing into the systemic capillaries, the
blood enters a region in which the oxygen tension in the surrounding
tissues is nil. At once the oxygen dissolved in the plasma passes
through the capillary wall into the surrounding tissue-spaces. The
pressure removed from the corpuscle, a dissociation of the oxygen
and of the hemoglobin takes place, after which the oxygen also passes
through the capillary wall into the surrounding lymph and so io the
tissue-cells where it is stored and utilized. On passing into tnc
venous system the dissociation of the oxygen and the hemoglobin is
checked by the rise of oxygen pressure in the plasma. On reaching
the lungs the oxygen again passes into the blood until the former con-
dition is regained.

The sojourn of the blood in the capillaries being short, the
oxyhemoglobin can part with but a portion of its oxygen, sufficient,
however, to satisfy the needs of the tissues.

The carbon dioxid pressure in the tissues being in excess of that
in the blood, it passes through the capillary wall into the blood, where
it exists in the free and combined states. On passing into the pul-
monic capillaries the blood enters a region in which the carbon dioxid
in the alveoli is less than in the blood. At once a diffusion and dis-
sociation of the carbon dioxid takes place through the alveolo-capillary
wall until equilibrium is established. This, however, is of very
short duration, for the carbon dioxid so eliminated is rapidly removed
from the lungs by the respiratory movements.

While diffusion, in response to physical and chemic conditions,
thus plays a large part in, and is sufficient to account for, the ex-
changes of gases, it is possible that the alveolar or respiratory epithe-
lium may also play an essential role. It is believed by some in-
vestigators that it is active in both the absorption of oxygen and the
excretion of carbon dioxid. This view has been suggested as a means
of interpreting the results of the experiments of more recent investi-
gators, made with a view of determining the tension of the blood
gases. It was found by Bohr that the tension of the oxygen in arterial
blood was often as high as 101 to 144 mm. Hg., and in many instances
higher than the tension of the oxygen in the trachea, while the carbon
dioxid tension in the trachea was higher than in the blood. Haldane
and Smith by a different method found an oxygen tension in the
arterial blood of 200 mm. Hg. If these results should prove to be
correct, though they are at present subject to considerable criticism
and not generally accepted, some other force than diffusion would
have to be found to explain the facts. It would then remain to deter-
mine in how far the alveolar epithelium could be regarded as an active
agent in both absorption and excretion in opposition to pressure.

RESPIRATION. 419

THE TOTAL, RESPIRATORY EXCHANGE.

The total quantities of oxygen absorbed and carbon dioxid dis-
charged by a human being in twenty-four hours are measures of the
intensity of the respiratory process, and an indication of the extent and
character of the chemic changes attending all life phenomena. Their
determination and their relation to each other are matters of interest
and importance. The quantities which have been obtained by differ-
ent observers are the outcome of calculations based on certain groups
of data and of experiments made with special forms of apparatus.

Thus from the total air breathed daily, estimated from the amounts
obtained during a longer or shorter period, of experiments with spiro-
metric apparatus, and from the percentage loss of oxygen and gain of
carbon dioxid shown by an analysis of the respired air, it can be cal-
culated at least approximately what the total amounts of oxygen ab-
sorbed and carbon dioxid exhaled must be. If it be assumed that
the minimum daily volume of air breathed is 10,800 liters and the
maximum volume 12,240 liters, and the percentage loss of oxygen is
4.78, then the total volume of oxygen absorbed is 516 liters (735.17
grams) or 585 liters (836.42 grams). By the same method the total
carbon dioxid exhaled daily is found to be either 473 liters (931.8 grams)
or 526 liters (1036 grams). The direct experiments which have been
made with specially devised forms of apparatus, both on human beings
and animals, have yielded similar results. With those forms which
are adapted for both human beings and animals — Scharling's, Petten-
kofer and Voit's — it is only possible, however, to determine the amount
of oxygen absorbed. This is done by deducting the loss in weight
by the man or animal during the experiment from the combined
weights of the carbon dioxid and water discharged. The difference
represents the oxygen absorbed.

The Pettenkofer-Voit apparatus (Fig. 199) consists essentially
of a chamber large enough to admit a man and capable of being
made air-tight with the exception of an inlet for air for breathing
purposes. The respired air is drawn through a tube and measured
by a large meter turned by a water or gas motor. By means of a side
tube a fractional quantity of the main column of air is diverted to an
absorption apparatus by a small pump. This air first passes into
a vessel containing H,S04, by which the water is collected; then
into long tubes containing barium hydroxid, by which the carbon
dioxid is absorbed; thence into a small meter, by which its amount is
registered. From the amount of water and carbon dioxid thus ob-
tained the amounts of both in the total air breathed are calculated.
The water and carbon dioxid previously present in the air are simulta-
neously determined by a corresponding absorption apparatus and de-
ducted from the amounts obtained from the respired air. As the
apparatus is traversed constantly by a column of air of normal com-
position and the waste products removed as rapidly as discharged,

420

TEXT-BOOK OF PHYSIOLOGY.

the experiment can be continued for periods varying from six to

twenty-four hours without detriment to the subject of the experiment.

AYith those forms adapted only for animals — Regnault's and

1 C -C ^

r/l _ *J O

Reiset's, or Jolyet and Regnard's — it is possible to determine simul-
taneously the absorption of oxygen and the discharge of carbon
dioxid. As the apparatus employed is completely closed, the carbon
dioxid must be removed as soon as discharged and the oxygen re-

RESPIRATION.

421

newed as soon as absorbed. The former is accomplished by the as-
piratory action of moving bulbs containing an alkali, the latter by a
steadily acting pressure on a reservoir of oxygen. This apparatus
(Fig. 200) consists essentially of a bell-jar in which the animal is
placed. This is brought into connection by tubes, on the one hand,
with the oxygen reservoir, and, on the other hand, with the aspiratory

Fig. 200. — Regnault's and Reiset's Respiration Apparatus. A. Bell-jar for the
reception of the animal, surrounded by a compartment, B, containing water. N, N, N.
Reservoirs of oxygen communicating, on the one hand, with the animal chamber, and,
on the other hand, with pressure bottles, P, by which the oxygen is driven into the animal
chamber. G, G. Aspiratory bulbs containing sodium hydroxid in solution for the absorp-
tion of the carbon dioxid. The bulbs are given an alternate up-and-down movement
by a falling weight or electric motor.

bulbs, kept in motion by some form of motor. The construction of
each of these forms of apparatus is so complex, the conduct of an
experiment and the final determination of the results so complicated,
that a detailed description would be out of place in a work of this
character.*

Among the results obtained by these and other methods a few are
given in the following table:

* Both forms of apparatus are in use in the Physiological Laboratory of the Jefferson
Medical College and are fully described by Prof. H. C. Chapman in his text-book on
Physiology, to which the reader is directed for further information.

422 TEXT-BOOK OF PHYSIOLOGY.

Oxygen Absorbed.

Observer.

Carbon Dioxid Discharged,

746 grams.

700 "

Yierordt.
Pettenkofer and Voit.

876 grams.
Soo "

663 "

Speck.

770 "

The amounts of oxygen absorbed in Pettenkofer and Voit's experi-
ments varied from 594 to 1072 grams; of carbon dioxid exhaled, from
686 to 1285 grams.

In all these results it is evident on examination that the volume
of oxygen absorbed is always greater than the volume of carbon
dioxid exhaled, or, what amounts to the same thing, the weight of the
oxygen absorbed is always greater than the weight of the oxygen
entering into the formation of the carbon dioxid exhaled. The reason
for this difference between the amounts of oxygen in the inspired air
and in the C02 exhaled is found in the fact that on a mixed diet — one
containing fat — a portion of the oxygen is utilized in the oxidation
of the hydrogen of the fat with the formation of water. Under such
a diet the respiratory quotient is always less than unity, usually 0.907.
On a purely carbohydrate diet — one in which there is no surplus
hydrogen — all the oxygen will combine with carbon and be returned
as carbon dioxid, and hence the respiratory quotient will be unity.
The respiratory quotient therefore indicates the extent to which the
oxygen absorbed is utilized in oxidizing carbon, on the one hand, and
hydrogen, on the other.

Since the total oxygen absorbed and carbon dioxid discharged
will vary considerably with the size of the animal, it is customary,
for purposes of comparison, to reduce all total results to the unit of
body- weight (one kilogram) and to the unit of time (one hour).

Respiratory Activity. — The activity or the intensity of the
respiratory process may be measured either by the oxygen absorbed
or the carbon dioxid discharged. But as the carbon dioxid is more
easily estimated than the oxygen, it is usually taken as the index of
the activity, though there are reasons for believing that it would be
more accurately indicated or represented by the oxygen.

Whatever factor may be accepted as the measure, it is certain that
the respiratory activity varies in different tissues in accordance with
their functional activities, being least in bones and greatest in muscles.
This is shown by the relative amounts of oxygen absorbed and carbon
dioxid discharged by equal amounts of each of these and other tissues
in twenty-four hours, as shown in the following table:

QUANTITY OF 0 AND CG2 ABSORBED AND EXHALED DURING
TWENTY-FOUR HOIKS, IN CUBIC CENTIMETERS.

By 100 Grams of: Oxygen Absorbed. Carbonic Acid Exhaled.

Muscle, 50.8 c.c. 56.8 c.c.

Brain, 45.8 " 42.8 "

Kidneys, 37.0 " 15.6 "

Spleen, 27.3 " 15.4 "

Ti tit les, 18.3 " 27.5 "

Pounded bones 17.2 " 8.1 "

RESPIRATION. 423

The total respiratory change therefore of the body as a whole is the
resultant of the respiratory changes of its individual organs and
tissues, and is conditioned by all influences which retard or hasten
their activity. Among these influences the more important are the
following:

Muscle Activity. — As the muscles constitute a large part of the
body, about 40 per cent., and as muscle-tissue absorbs and discharges
relatively large quantities of oxygen and carbon dioxid, it is readily
apparent that an increase in their activity would be followed or attended
by an increase in the respiratory exchange. In passing from a con-
dition of body repose to one of marked activity there ought to be an
increase in the amount of oxygen absorbed and C02 discharged.
Pettenkofer and Voit found that a man in repose who absorbed daily
807.8 grams of oxygen and discharged 930 grams C02 absorbed during
work 1006 grams of oxygen and discharged 1137 grams of C02.
Edward Smith, who estimated only the C02, found that a man in
repose who discharged carbon dioxid at the rate of 16 1.6 c.c. per
minute increased the amount while walking at the rate of two and
three miles an hour to 569 c.c. and 851 c.c. respectively. Similar
results have been obtained by other investigators.

Digestive Activity. — The activity of the alimentary canal,
involving contraction of its muscle coat through its entire length
as well as secretion of its related glands called forth by the ingestion
of food, materially influences the absorption of oxygen and discharge
of carbon dioxid, independent of the increase due to the oxidation
of food materials after absorption. It was found that in a fasting
man a dose of sodium sulphate increased the absorption of oxygen as
much as 17 per cent, and the discharge of C02 24 per cent. (Lowy).
It is difficult to determine how much of the increase after a meal is
therefore due to food oxidation and how much to functional activity
of the canal itself. The consumption of nitrogenized meals, however,
has a greater effect than non-nitrogenized meals.

Temperature. — A rise in temperature of the surrounding air
has as an effect a diminution in the amounts of oxygen consumed and
carbon dioxid discharged. A fall in temperature has the opposite
effect. Thus a cat at a temperature of — 3.20 C. consumed during a
period of six hours 21.39 grams of oxygen and discharged 22 grams
of carbon dioxid, while at a temperature of 29.6 ° C. the correspond-
ing amounts for the same period of time were for oxygen 13.9 grams
and for carbon dioxid 13.12 grams. Lavoisier and Sequin, having
reference only to the oxygen, found that a man at a temperature of
15 ° C. consumed 38.31 grams of oxygen, while at a temperature of
32. 8° C. the corresponding amount was but 35 grams. Similar
results have been obtained by other observers with different animals.
The explanation of these facts is to be found in the increased activity
of all physiologic mechanisms coincident with a fall, and in the de-
creased activity, coincident with a rise in temperature. The lower

424 TEXT-BOOK OF PHYSIOLOGY.

temperatures act as a stimulus to the peripheral terminations of the
nerve system, bringing about reflexly increased activity of the body
at large. The muscles especially are not only reflexly but volitionally
excited to greater activity. This leads naturally to an increase in
the consumption of oxygen and in the production of carbon dioxid
and in the evolution of heat.

In cold-blooded animals the respiratory exchange is influenced in
a manner the reverse of that observed in warm-blooded animals.
With a rise of external temperature and a corresponding rise of body-
temperature the discharge of carbon dioxid steadily increases. Thus
a frog in an atmosphere at o° C. with a body-temperature of i° C.
discharged per kilogram per hour 4.31 c.c. of carbon dioxid; in an
atmosphere of 35 ° C. with a body-temperature of 340 C. there was
discharged 325 c.c. per kilo per hour. Intermediate temperatures
were attended by corresponding increases in the amounts of C02
discharged. The reason for this difference in the two classes of
animals is probably to be found in the want, in the cold-blooded
animals, of a self-adjusting heat-regulating mechanism.

Age. — In early youth, as a result partly of the more pronounced
activity of the nutritive energies and partly of a cutaneous surface
relatively greater, as. compared with the mass of the body, than in
adult life, the absorption of oxygen and the discharge of carbon dioxid
are greater both absolutely and relatively. Thus, in a boy of nine
and a half years with a weight of 22 kilograms it was found that in
twenty-four hours there was a discharge of carbon dioxid amounting
to 488 grams, or 0.92 gram per kilo per hour, and in man with a
weight of 65.5 kilograms there was a discharge of 804.72 grams, or 0.51
gram per kilo per hour.

MODIFICATIONS OF THE RESPIRATORY RHYTHM.

The character of the respiratory movements is materially modified
by a change in the quantitative and qualitative composition of the
air and blood as well as by changes of a pathologic nature of the res-
piratory apparatus itself.

Eupnea. — So long as the air retains its normal composition and
the respiratory mechanism its structural integrity, so long do the
respiratory movements exhibit a normal rhythm and frequency.
To the condition of easy tranquil breathing the term eupnea is given.
In this condition the percentages of oxygen and carbon dioxid in the
blood are such as to favor at least the rhythmic discharge of nerve
impulses to the respiratory muscles, of sufficient energy and frequency
for the maintenance of normal respiration.

Hyperpnea. — The normal rate of the respiratory movements is
increased by a rise in body-temperature, by active exercise, and by
emotional states. Whatever the cause, the increase in rate and prob-
ably in depth is termed hyperpnea.

RESPIRATION. 425

Febrile states characterized by a rise in the temperature of the
blood increase considerably the respiratory activity. This is due in
all probability to a warming of the respiratory center, in consequence
of which its excitability is heightened; for surrounding the carotid
arteries with warm tubes and heating the blood on its way to the
medulla has the same effect. It is also possible, however, that the
high temperature of febrile conditions may interfere with the absorb-
ing power of hemoglobin, and thus by diminishing the quantity of
oxygen absorbed lead to more frequent respirations. To the hy-
perpnea induced by heat the term thermo- polypnea is frequently
given.

Muscle activity, especially if it is violent and indulged in by those
unaccustomed to exercise, is generally followed by increased rate
and depth of breathing, and not infrequently it is attended with
such extreme difficulty that the condition approximates that of dyspnea.
This condition is attributed to the production and discharge into the
blood of unknown waste products which act as irritants to the respira-
tory center and thus increase its activity. As they apparently can not
be isolated and their chemic nature determined, it is presumable that
they are speedily oxidized or reduced in the blood. Experiment
has shown that the increase of carbon dioxid does not account for the
increased rate of breathing. Emotional states temporarily increase
respiratory activity. With their disappearance the normal condition
returns.

Apnea. — If the respiratory movements are voluntarily increased
in frequency and depth for a short time it will be found on cessation
that for a variable length of time the respiratory mechanism remains
in a condition of complete rest or inaction. To this complete cessation
of activity the term apnea is given. The same phenomenon is wit
nessed in animals when the lungs are rapidly inflated with air by
means of bellows. At one time this was attributed to an excess of
oxygen in the blood (the result of the forced ventilation of the lungs),
complete saturation of the plasma and hemoglobin, in consequence of
which the respiratory center remained inactive. This has been dis-
proved, however, by modern chemic analyses of the blood. The
condition is now attributed:

1. To increased ventilation of the lungs and an increased percentage

of oxygen in the alveoli, as a result of which the normal percentage
of oxygen in the blood can be maintained for a longer period than
usual.

2. To a stimulation of the peripheral terminations of the pneumo-

gastric nerve whereby the discharge of nerve impulses from the
respiratory center is temporarily inhibited. Division of the
pneumogastric nerve prevents the development of the apneic
condition.
Dyspnea. — Excessive and laborious respiratory movements con-
stitute a condition known as dyspnea. Movements of this character

426 TEXT-BOOK OF PHYSIOLOGY.

indicate that the blood is deficient in oxygen or overcharged with
carbon dioxid. In either case the excitability of the respiratory center
is abnormally heightened. These conditions of the blood may be
caused: (i) By all those pathologic conditions of the respiratory
apparatus which limit the free entrance of oxygen into and the free
exit of carbon dioxid from the blood; (2) by those alterations in the
composition of the air and subsequently in the blood which arise when
the individual is confined in a space of moderate size with imperfect
ventilation. The want of oxygen in the blood gives rise to more
forcible inspirations; the presence of C02 in excess, to more forcible
expirations — showing that the former condition affects the inspiratory
portion of the respiratory center, the latter condition the expiratory
portion. A deficiency in the amount or quality of the hemoglobin
is usually attended with dyspnea.

Asphyxia. — If the state of the blood observed in dyspnea be ex-
aggerated— that is, if the decrease in the percentage of oxygen and
the increase in the percentage of carbon dioxid become more marked
— the respiratory movements become more laborious. A continuance
of this changed composition of the blood eventuates in death. Before
this occurs the individual exhibits a succession of phenomena, to
the totality of which the term asphyxia is given.

Asphyxia may be caused: (1) By a sudden interference with the
entrance of oxygen into and the exit of carbon dioxid from the blood,
as in drowning, occlusion of the trachea from any cause, double
pneumothorax, etc. (2) By confinement in a small space the air
of which speedily undergoes a loss of oxygen and an accumulation of
carbon dioxid. In the first instance death may occur in a few minutes;
in the second instance it may be postponed several hours or more,
the time varying with the size of the space.

The succession of phenomena presented by an individual in the
asphyxiated condition is as follows: Increased rate and depth of the
respiratory movements, passing rapidly from hyperpnea to dyspnea,
with an active contraction of all the muscles concerned in respira-
tion, ordinary and extraordinary; a blue cyanosed condition of the
face from the rapid accumulation of carbon dioxid and disappearance
of the oxygen of the blood; a diminution in the depth of inspiration
and an increase in the force and extent of expiration, followed by
general convulsions; collapse, characterized by unconsciousness, loss
of the reflexes, relaxation of the muscles, a weak action of the heart,
a disappearance of the pulse and death. As shown by observation
of the circulatory apparatus in artificially induced asphyxia, there is
primarily an increase in the activity of the heart, soon followed by
retardation; a rise of blood-pressure in the early stages and a fall to
zero after collapse has set in. The retardation and final cessation of
the heart, as well as the rise of the blood-pressure, are to be attributed
to stimulation of the cardio-inhibitory and vaso-motor centers from
the accumulation of the carbon dioxid. With the exhaustion of the

RESPIRATION.

427

nerve-centers, there is a general relaxation of the muscles and a fall
of the pressure.

The Cheyne-Stokes Respiration. — A modification of the respir-
atory movements characterized by periods of rest alternating with
periods of activity -was described in 1818 and in 1854 by the two
writers whose name it bears. The periods of rest vary in duration
from twenty to thirty seconds; the periods of activity from thirty to
sixty seconds and may include from twenty to thirty respiratory
movements.

Each period of rest of the respiratory mechanism is closed by the
appearance of a slight shallow respiratory movement, which is imme-
diately followed by a second, slightly deeper, and this in turn by a

Fig. 201. — Tracing Showing the Cheyne-Stokes Form of Respiration. — (Hill.)

third, a fourth, a fifth, and so on, each becoming deeper than the
preceding until a certain maximum is reached, after which, each
succeeding movement gradually diminishes in depth until finally the
movement becomes imperceptible and a new period of rest super-
venes. A graphic representation of the Cheyne-Stokes type of respira-
tion is shown in Fig. 201. This type of respiration is frequently an
accompaniment of certain pathologic conditions, e. g., uremic states,
cerebral hemorrhage, heart diseases, arteriosclerosis, etc., though no
satisfactory explanation of it has yet been presented. A similar
though far less marked periodicity in the respiratory movements is
frequently observed during normal sleep, especially in children. A
periodicity can also be developed by dividing transversely the medulla
oblongata just above the calamus scriptorius which either injures the
respiratory center or removes from it some cerebral influence.

THE NERVE MECHANISM OF RESPIRATION.

The nerve mechanism by which the respiratory muscles are ex-
cited to action is extremely complex and involves the action of both
afferent and efferent nerves and their related nerve-centers in the cen-
tral nerve svstem. For the free introduction of air into the lunsrs it is

428 TEXT-BOOK OF PHYSIOLOGY.

essential that the nasal and laryngeal passages and the cavity of the
thorax be simultaneously enlarged. The muscles by which these
results are accomplished have already been mentioned and described.
Their simultaneous and coordinate contraction implies the coordinate
activity of motor nerves and their centers; thus, the nasal and laryn-
geal muscles (the dilatator naris and the posterior crico-arytenoid)
involve the activity of the facial and inferior laryngeal nerves re-
spectively, the centers of origin of which lie in the gray matter beneath
the floor of the fourth ventricle; the diaphragm and intercostal muscles
involve respectively the activity of the phrenic and intercostal nerves,
the centers of origin of which lie in the anterior horn of the gray matter
of the spinal cord at a level, for the phrenic, of the fourth, fifth, and
sixth cervical nerves, and for the intercostals at the level of the upper
thoracic nerves. Division of any one of these nerves is followed by
paralysis of its related muscle.

Inspiratory Center. — The coordinate contraction of the inspira-
tory muscles implies a practically simultaneous discharge of nerve
impulses from each of the foregoing nerve-centers, accurately graduated
in intensity in accordance with inspiratory needs. This has been
supposed to necessitate the existence in the central nerve system of a
single group of nerve-cells from which nerve impulses are rhythmically
discharged and conducted to the previously mentioned nerve-centers
in the medulla oblongata and spinal cord, by which they are in turn
excited to activity. To this group of cells the term "inspiratory
center" has been given.

For the free exit of air from the lungs it is not only essential that
the air-passages be open, but that the air in the lungs be compressed
until its pressure rises above that of the atmosphere. This is accom-
plished by the recoil of the elastic tissue of the lungs and thorax, the
return of the displaced abdominal organs aided by atmospheric pres-
sure, and the contraction of the expiratory muscles. In how far
muscle action is necessary for expiratory purposes will depend on the
resistance offered to the outflow of air and on the degree of efficiency
of the elastic forces.

Expiratory Center. — The simultaneous and coordinate activity
of the expiratory muscles in impeded expirations also involves the
action of motor nerves and nerve-centers. The simultaneous and
coordinate discharge of nerve impulses, also graduated in intensity
for expiratory needs, apparently implies the existence in the central
nerve system of a single center from which nerve impulses are rhyth-
mically discharged which excite and coordinate the lower nerve-centers.
To this group of cells the term "expiratory center" has been given.
The two centers taken together constitute the so-called "respiratory
center."

The anatomic existence, however, of a definite group of cells which
initiates the respiratory movements has not as yet been demonstrated.
Nevertheless there is in the dorsal portion of the medulla oblongata,

RESPIRATION. 429

at the level of the sensory end-nucleus of the vagus nerve, a region the
sudden destruction of which on one side is followed by a cessation of
respiratory movements on the corresponding side, though they con-
tinue on the opposite side, a fact which indicates that the area, though
acting as a unit, is bilateral. The bilateral character of the area is
also shown by the continuance of the respiratory movements on both
sides after longitudinal division of the medulla. Destruction of the
entire region is followed by a complete cessation of respiratory activity
and death of the animal. For this reason the term "nceud vital"
was applied to it. In this area the respiratory center was located.
It has, however, been shown by Gad that if this area be gradually
destroyed by cauterization the respiratory movements do not cease,
but continue until the cauterization has reached a point far forward
in the formatio reticularis, in which the respiratory center was assumed
to lie.

Though its existence has not been anatomically determined beyond
question, it is permissible to speak of the central mechanism as a
"center" located in the medulla oblongata.

The activity of the inspiratory center has long been described as
automatic or autochthonic (Gad) in character, expressive of the idea
that the rhythmic discharge of nerve impulses is conditioned by the
composition of the blood or lymph by which it is surrounded, though
susceptible to the action of nerve impulses reflected to it through
afferent nerves. Thus so long as the blood retains its normal com-
position the inspiratory movements are normal. If, however, the blood
becomes richer in oxygen and relatively poorer in carbon dioxid, the rate
of discharge of nerve impulses and hence the inspiratory movements, di-
minish until the condition of apnea results. If, on the contrary, the
blood becomes poorer in oxygen and richer in carbon dioxid, the reverse
condition obtains: viz., an increased rate of discharge of nerve impulses,
increased frequency of inspiration, hyperpnea, and dyspnea. This
view of the automaticity of the inspiratory center is supported by the
fact that the center continues more or less active after division of the
medulla oblongata just posterior to the corpora quadrigemina, of the
spinal cord at the level of the seventh cervical nerve, of the vagus
nerves and the posterior roots of the cervical nerves, channels through
which the majority of nerve impulses reach the center. Whether the
center would continue active after division of all remaining afferent
nerves, e. g., trigeminal and glossopharyngeal, it is impossible to state,
since from the nature of the case such an experiment would be most
difficult to perform.

The first inspiration after birth is supposed to be due to the direct
stimulation of the respiratory center by the increase in the carbon
dioxid present in the blood, though it may be aided by the cooling
of the skin due to vaporization of the amniotic fluid.

Reflex Stimulation of the Inspiratory Center. — Whether
the inspiratory center is automatic in character or not, it may be

430 TEXT-BOOK OF PHYSIOLOGY.

influenced directly by nerve impulses descending from the brain in
consequence of volitional acts or emotional states, and indirectly
by nerve impulses reflected to it from the general periphery through
various afferent nerves, in consequence of agencies acting on their
peripheral termination: e. g., cold applied to the skin, irritaing gases
to the nasal and bronchial mucous membrane, distention and collapse
of the pulmonary alveoli.

Of all afferent nerves, the vagus appears to be the most influential in
maintaining the rhythmic discharge of nerve impulses from the inspira-
tory center. (Fig. 202). Thus, if while the animal is breathing regularly
and quietly both vagi are cut, the respiratory movements become much
slower, falling perhaps to one-third their original number per minute.
If the central end of the divided vagus be stimulated with weak faradic
currents, the respiratory movements are increased in frequency and
their depth diminished until the normal rate is restored. With the
cessation of the stimulation the former condition at once returns.
This would indicate that in the physiologic state afferent impulses
are ascending the vagus fibers which influence the rate of discharge
from the inspiratory center. If, however, the stimulation is increased
in strength, the inspiratory movement gradually so exceeds the expi-
ratory that the muscles pass into the tetanic state and the chest-walls
come to rest in the condition of forced inspiration. The vagus appa-
rently contains fibers which augment the inspiratory movement.
If, on the other hand, the central end of the divided superior laryngeal
nerve be stimulated with faradic currents, the opposite effect is pro-
duced: viz., an excess of the expiratory over the inspiratory move-
ment until the chest-walls come to rest in the condition of passive
expiration. The superior laryngeal nerve apparently contains fibers
which inhibit the inspiratory movement.

The same result, an expiratory standstill, not infrequently follows
strong stimulation of the divided vagus, and always after the admin-
istration of large doses of chloral.

The vagus apparently contains two classes of nerve-fibers, one of
which when stimulated increases the extent of the inspiratory move-
ment until the thorax comes to a standstill in the state of forced in-
spiration; the other of which when stimulated increases the extent of
the expiratory movement at the expense of the inspiratory until the
thorax comes to a standstill in a state of deep expiration.

The stimulus adequate to the excitation of the nerve-fibers in the
physiologic condition was formerly believed to be the chemic action
of carbon dioxid; it is now believed to be a mechanic action, the result
of the alternate distention and collapse of the walls of the pulmonary
alveoli. Thus, it has been shown by Head that if the lungs are actively
inflated (positive ventilation) there will be produced an inhibition
of the inspiratory and an augmentation of the expiratory movement
until the inspiratory, muscles are completely relaxed as indicated by
the relaxation of the diaphragm, the movements of which are simulta-

RESPIRATION.

43i

neously recorded (Fig. 203), a result similar in all respects to that
produced by stimulation of the superior laryngeal nerve. On the
other hand, if the lungs are collapsed by the artificial withdrawal
of air (negative ventilation) there will be produced an augmentation

med. ob.

Fig. 202. — Diagram Showing the Relation of the Pulmonary Fibers of the
Vagus to the Inspiratory Center and the Connections of thf Latter with the
Phrenic and Intercostal Nerve Centers and their Related Muscles. {G. Bach-
man.) med. ob. Medulla oblongata, sp.c. Spinal cord, p.v.r. Pulmonary vagus nerve, ex -
citator and inhibitor, in sp.c. Inspiratory center, phr.c. Phrenic nerve centers, phr.n.
Phrenic nerve int.n.c. Intercostal nerve centers, int.cn. Intercostal nerves, ext.int.c.m.
External intercostal muscles.

of the inspiratory and an inhibition of the expiratory movements until
the inspiratory muscles are in a condition of tetanic contraction as in-
dicated by the contraction of the diaphragm (Fig. 204) and by the
state of the thorax which is that characteristic of extreme inspiration.

43 2

TEXT-BOOK OF PHYSIOLOGY.

Positive
ventilation.

a result similar in all respects to that produced by moderate stimu-
lation of the central end of the divided vagus.

A satisfactory explanation of the action of the respiratory mechanism
is very difficult to present. Theories vary in accordance with the
estimate of an investigator as to the degree of automaticity of the
inspiratory center, of the effects of vagus stimulation and as to the

extent to which the ex-
piratory center is in-
volved with the activity
of the inspiratory center
either simultaneously or
successively.

If it is assumed that
the inspiratory center is
automatic and in a state
of continuous excitation
the result of the action
of carbon dioxid in the
blood circulating around
it, then it is only necessary to assume the existence, in the trunk of
the vagus of but one set of nerve-fibers, viz., inhibitor fibers, the
central terminations of which arborize around the inspiratory center
and the function of which is to check or inhibit the action of the
inspiratory center and thus permit of an expiratory movement. The
inhibitor fibers are supposed to be stimulated peripherally by the
expansion of the lungs. With the recoil of the lungs the inhibitor
effect gradually dies away, while the inherent excitation of ,the

Fig. 203. — Positive Ventilation (Head). Under
the influence of positive ventilation, the inspiratory
contractions of the diaphragm become less and less
till they disappear completely.

Seconds.

Fig. 204. — Negative Ventilation (Head). At a negative ventilation was com-
menced. The expiratory relaxation of the diaphragm is seen to become more and more
incomplete, until it finally enters into continued contraction.

inspiratory center again returns, to be followed by another discharge
of nerve impulses and a new inspiratory movement, which will in turn
be again inhibited as the inhibitor fibers are stimulated by the ex-
panding lung. This explanation is in accordance with the results

RESPIRATION. 433

which follow stimulation of the superior laryngeal nerve or the trunk
of the vagus with strong induced electric currents.

If it is assumed, on the contrary, that the inspiratory center is not
in a state of constant excitation leading to a continuous discharge
of nerve impulses, but requires the arrival of a stimulus to call forth
its normal activity, then this theory does not suffice, inasmuch as it
leaves out of consideration the presence of nerve-fibers in the vagus
which increase or augment the activity of the inspiratory center; and
that such fibers are present is apparently indicated by the effects of
stimulation of the central end of the vagus nerve with weak and
moderately strong induced electric currents and from the experiments
of Hering and Breuer, and later of Head. These observers assume,
therefore, that in addition to the inhibitor fibers there are also present
in the vagus, excitator fibers, the central terminations of which are in
relation with the inspiratory center also (Fig. 202; ; and just as the inhib-
itor fibers are stimulated by the expansion of the lungs so the excitator
fibers are stimulated in turn by the recoil of the lungs. The nerve im-
pulses thus developed ascend to the inspiratory center, excite it, and call
forth a new inspiration sooner than it would otherwise take place. Ac-
cording to this view the respiratory mechanism is self-regulative and
maintained by the alternate expansion and recoil of the lungs.

Many experimenters, however, find difficulty in accepting the
view that the recoil of the lungs should stimulate nerve endings and
hence this theory has not received general acceptance.

Another explanation which is satisfactory in many respects has been
presented by Meltzer. This investigator asserts also the existence in
the trunk of the vagus the two classes of nerve-fibers, the inhibitor
and the excitator; but that for some reason they do not respond to
stimulation at the same time as shown by the effects which follow; the
inhibitor fibers respond first and the excitator fibers somewhat later.
Therefore when they are stimulated simultaneously the primary effect
is an inhibition of the inspiratory center followed by an expiratory
movement. The secondary effect is a stimulation of the inspiratory
center followed by a new inspiratory movement. In this view ex-
pansion of the lungs stimulates both the inhibitor and the excitator
fibers, but during the expansion and for a short time after, the effect
of the inhibitor stimulation, viz., cessation of inspiration and the
advent of expiration, alone manifests itself. With the cessation of
expiration, the inhibitor stimulation dies away and the late effect or
the long after-effect of the excitator stimulation, viz., a new inspiration,
manifests itself. This author assumes the surface of the lung to be
the peripheral organ of the respiratory, reflexes.

When it is assumed that both inspiratory and expiratory centers
cooperate in a respiratory movement, as they do in labored respiration
either simultaneously or successively, the difficulties of the problem are
manifestly much greater. In this case it may be supposed that afferent
impulses, developed during the expansion of the lung, inhibit the
2S

434 TEXT-BOOK OF PHYSIOLOGY.

inspiratory while augmenting the expiratory center, and that impulses
developed during the recoil of the lungs inhibit the expiratory while
stimulating the inspiratory center.

THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW

OF BLOOD THROUGH THE INTRA-THORACIC VESSELS AND

ON THE ARTERIAL PRESSURE.

i. On the Intra-thoracic Vessels. — The forces which cause the
air to flow into and out of the lungs will at the same time and in the
same way cause the blood of the systemic vessels to flow into, through,
and out of the intra-thoracic vessels. From the tendency of the pul-
monary elastic tissue to recoil, the blood-vessels in the thorax at the
end of an expiration sustain a positive pressure, about six millimeters
of mercury less, than that in the lungs, or, in other words, a pressure
negative to that of the atmosphere by six millimeters. As a result
the blood in the systemic vessels standing under atmospheric pressure
will flow steadily toward the intra-thoracic veins, the vense cavse,
and the right side of the heart; i. e., from a point of high to a point of
low pressure. Since during inspiration, with the increasing tendency
to pulmonary recoil, the positive pressure on the veins and heart may
diminish by thirty millimeters of mercury, the blood will flow in in-
creased volume from the systemic to the intra-thoracic vessels. The
right heart, being more generally filled with blood, will discharge a
larger volume with each contraction into the pulmonary artery.

Coincident with these effects a similar effect is produced in the
arterioles and capillaries of the pulmonary alveoli. Situated between
the two elastic layers of the alveolar wall, embedded in a meshwork
of connective tissue, the pressure to which they are subjected at the
end of an expiration will also be a few millimeters less than that of
the intra-pulmonary air; and at the end of an inspiration it will be
considerably less. With- the inspiration there will occur a dilatation
of the vessels, a larger flow of blood through them and into the pul-
monary veins. The left heart, being in consequence more generously
filled with blood, will discharge a larger volume into the aorta at each
contraction. During expiration the flow of blood through the intra-
thoracic vessels will be diminished for the reverse reasons.

2. On the Arterial Pressure. — An examination of a tracing of
the arterial pressure will show that it is characterized by small un-
dulations due to the cardiac beat and large undulations due to the
respiratory movements, inspiration being accompanied by a rise,
expiration by a fall of pressure. These results are readily accounted
for by the difference in the volume of blood discharged by the left
heart into the aorta during the time of the two movements. If a
tracing of the respiratory movements and of the blood-pressure be
taken simultaneously, it will be found that the rise of pressure does not
exactly correspond with inspiration, nor the fall of pressure with

RESPIRATION. 435

expiration; for a certain period after the beginning of an inspiration
the pressure continues to fall, and for a certain period after the begin-
ning of an expiration the pressure continues to rise. During the
remainder of the period, however, inspiration is attended by a rise,
expiration by a fall of pressure. The explanation of these results
lies in the fact that at the beginning of the inspiration, when the
vessels dilate, the blood-flow momentarily slows; the left heart con-
tinuing to discharge small volumes into the aorta, the pressure con-
tinues to fall. So soon as the left heart begins to be better filled, the
pressure at once begins to rise. At the end of an expiration the flow
of blood into the left heart continues and the pressure rises, but with
the return of the intra-thoracic pressure the vessels diminish in caliber,
the volume of blood transmitted by them becomes less, the output
of the left heart declines, and the pressure falls.

The Traube-Hering Waves. — Under eertain experimental con-
ditions the arterial blood-pressure tracing exhibits, in addition to the
usual respiratory variations, certain longer rhythmic variations more
or less wave-like in character, which are known as Traube-Hering
waves. They can be developed on a blood-pressure tracing by in-
jecting magnesium sulphate or morphine into the circulation, by tying
the cerebral arteries, -etc. These waves indicate a periodic contrac-
tion and dilatation of the blood-vessels the result of a stimulation of
the vaso-motor centers.

CHAPTER XVI.

ANIMAL HEAT.

The chemic changes which take place in the tissues and organs
of the living body and which underlie all manifestations of life are
attended by the evolution of heat. In consequence of this each ani-
mal acquires a certain body-temperature.

In man, as well as in other mammals and in birds, the chemic
changes are extremely active and the evolution of heat very great.
Through some special heat-regulating mechanism, by which heat-
production and heat-dissipation are kept in equilibrium, these animals
have acquired and maintain within limits a constant temperature which
is independent of and generally above that of the surrounding atmos-
phere. As the temperature of these animals is high and perceptible
to the sense of touch, they were originally designated "warm-blooded"
animals. As this temperature is constant notwithstanding the great
variations in external temperature during the summer and winter
seasons, they are more appropriately termed constant-temperatured
or homoiothermous animals. The intensity of the body-temperature
determined by the insertion of a thermometer in the rectum varies in
different classes of mammals from 37.2 ° C. to 400 C. The causes of
this variation are doubtless connected with peculiarities of organiza-
tion. In birds the rectal temperature is usually higher, varying from
40.9 ° C. in the pigeon to 44 ° C. in the titmouse and the swift.

In reptiles, amphibians, and fish chemic changes, as a rule, are
not very active and heat-production relatively slight. As they are
devoid of a sufficiently active heat-regulating mechanism, the tempera-
ture of the body is largely dependent on that of the medium in which
they live, though it is always one or more degrees above it. In winter
the body-temperature of frogs, for example, may decline as low as 8.90
C, the temperature of the surrounding medium being 6.7 ° C. When
subjected to temperatures below zero, the temperature of the body
may fall below the freezing-point also, when the lymph and fluids of
the body become ice. Though apparently dead, the gradual eleva-
tion of the temperature restores their vitality. In summer-time, on the
contrary, the body-temperature may attain to 38 ° C. Similar varia-
tions have been observed in other animals. As the temperature of
these animals is low and perceptibly below that of our own bodies,
they were originally termed "cold-blooded" animals; as their temper-
ature is inconstant, varying with the temperature of the surrounding
medium, they are more appropriately termed "variable temperatured"
or poikilo-thermous animals.

436

ANIMAL HEAT. 437

THE TEMPERATURE OF THE HUMAN BODY.

The determination of the temperature of the human body under
the changing conditions of life is a matter of the greatest physiologic
and clinical interest. The temperature of the superficial portions of
the body may be obtained by the introduction of a thermometer into
the mouth, the rectum, the vagina, or the axilla. As a result of many
observations it has been found that the temperature of the rectum is,
on the average, 37.2 ° C; that of the mouth, 36.8 ° C; that of the
axilla, 36.9 ° C. Owing to radiation and conduction the surface tem-
perature is lower than that of either the mouth or rectum, and varies
to a slight extent in different regions of the body: e. g., at a room-
temperature of 20 ° C. the skin of the pectoral region has a temperature
of 34.70; that of the cheek, 34.40; that of the calf, 33. 6°; that of the tip
of the ear, only 28.8 °, etc.

In the interior of the body, especially in organs in which oxidation
takes place rapidly, and which at the same time are protected by
their anatomic surroundings from rapid radiation, the temperature
is higher than that observed in the rectum. From an investigation
of the temperature of the blood as it emerges from the liver, the mus-
cles, the brain, alimentary canal, etc., it is evident that these organs
have a higher temperature than the rectum.

As the chemic changes underlying physiologic activity vary in
intensity and extent in different regions of the body, there would be
marked variations in their temperature were it not that the blood,
having a large capacity for heat-absorption, distributes the heat al-
most uniformly to all portions of the body, so that at a short distance
beneath the surface the temperature does not vary but within a few
degrees.

In the dog the temperature of the blood in the aorta and in its
principal branches is approximately 38.3 ° C. In passing through
the systemic capillaries the temperature falls from radiation and con-
duction to surface temperature, to again rise as the venous blood ap-
proaches the deeper regions of the body. In the neighborhood of
the renal veins and in the superior vena cava the temperature is again
that of the aorta. In the portal vein the temperature rises to 40. 2 ° C;
in the hepatic vein, to 40. 6° C. In the right ventricle, owing to the ad-
mixture of blood from different localities having different temperatures,
the temperature falls to 38.2 ° to 40.40. In passing through the pul-
monary capillaries the temperature of the blood again falls, so that in
the left ventricle it will register from 38 ° C. to 40. 2 ° C. There is thus
usually a difference between the two sides of the heart of about 0.20 C.

Variations in the Mean Temperature. — The mean tempera-
ture of the human body for twenty-four hours, which for the mouth
and the rectum may be accepted at 36.7 ° C. and 37.20 C. respectively,
is subject to variations from a variety of circumstances, such as age,
periods of the day, food, exercise, etc.

438 TEXT-BOOK OF PHYSIOLOGY.

Age. — At birth the temperature of the infant is slightly higher
than that of the mother, registering in the rectum about 37.5 ° C. In
a few hours it rapidly declines to about 36.5 °, to be followed in the
course of twenty-four hours by a rise to the normal or slightly beyond.
During childhood the temperature gradually approximates that of
the adult. In old age the temperature rises, as a rule, and attains a
maximum at eighty years of 37.4 ° C.

Periods of the Day. — The observations of Jiirgensen show that
there is a diurnal variation in the mean temperature of from 0.50 C.
to 1.5 ° C, the maximum occurring late in the afternoon, from 5 to 7
o'clock, the minimum early in the morning, from 4 to 7 o'clock. This
diurnal variation in the mean temperature is related to corresponding
variations in many other physiologic processes, and its causes are to
be found in the ordinary habits of life as regards the time of meals,
periods of exercise, sleep, etc.

Food and Drink. — The ingestion of a hearty meal increases the
temperature but slightly — not more than 0.5 ° C. Insufficiency of
food lowers the temperature; total withdrawal of food, as in starva-
tion, is followed by a steady though slight decline, until just preceding
the death of the animal, when it falls abruptly to from 6° to 8° C.
Cold drinks lower, hot drinks raise the temperature. Food and drinks,
however, only temporarily change the mean temperature, and after a
short period equilibrium is restored through the activity of the heat-regu-
lating mechanism. Alcoholic drinks lower the temperature about
0.5 ° C. In large toxic doses in persons unaccustomed to their use
the temperature may be lowered several degrees. This is attributed
not to a diminution in heat-production, but rather to an increase in
heat-dissipation (Reichert) from increased action of the heart, dilata-
tion of the blood-vessels of the skin, and increased activity of the sweat-
glands. "

Exercise. — The temperature may be raised by active muscular
exercise from i° to 1.50 C. as a result of increased activity in chemic
changes in the muscles themselves. A rise beyond this point is pre-
vented by the increased activity of the circulatory apparatus, the re-
moval of the heat to the surface, and its rapid radiation.

External Temperature. — The external temperature influences but
slightly the mean temperature of the human body. In the tropic as
well as in the arctic regions, notwithstanding the change in the tem-
perature of the air, that of the body remains almost constant. The
same is true for the seasonal variations in the temperature of the tem-
perate regions.

THE SOURCE AND TOTAL QUANTITY OF HEAT FRODUCED.

The Source of Heat. — The immediate source of the body-heat
is to be found in the chemic changes which take place in all the tissues
and organs of the body. Each contraction of a muscle, each act of

ANIMAL HEAT. 439

secretion, each exhibition of nerve-force, is accompanied by the evo-
lution of heat. The chemic changes are for the most part of the nature
of oxidations, the union of oxygen with the elements, carbon and hy-
drogen, of the food principles either before or after they have become
constituents of the tissues. The ultimate source of the body-heat is
the latent or potential energy in the food principles, which was absorbed
from the sun's energy and stored up during the growth of the vegetable
world. In the metabolism of the animal body the food principles are
again reduced through oxidation, directly or indirectly, to relatively
simple bodies, such as urea, carbon dioxid, and water, with a liberation
of a large portion of their contained energy which manifests itself as
heat and mechanic motion.

The Total Quantity. — The total quantity of heat liberated in
the body daily may be approximately determined in at least two ways:
(1) By determining experimentally the heat values of different food
principles by direct oxidation; (2) by collecting and measuring with
a suitable apparatus, a calorimeter, the heat evolved by the oxidation
of the food within, and dissipated from, the body daily.

1. Direct Oxidation. — The amount of heat which any given food
principle will yield can be determined by burning a definite amount—
e. g., 1 gram — to carbon dioxid and water and ascertaining the extent
to which the heat thus liberated will raise the temperature of a given
amount of water, e. g., 1 kilogram. The amount of heat may be
expressed in gram or kilogram degrees or calories; a gram calorie
or kilogram calorie being the amount of heat required to raise the
temperature of a gram or a kilogram (1000 grams) of water i° C.
The apparatus employed for this purpose is termed a calorimeter,
which consists essentially of a closed chamber, in which the oxidation
takes place, surrounded by a water-jacket. The rise in temperature
of the water indicates the amount of heat produced.

The results obtained by investigators employing different calor-
imeters and different food principles of the same class vary, though
within narrow limits: e. g., 1 gram casein yields 5.867 kilogram calor-
ies; 1 gram of lean beef, 5,656; 1 gram of fat, 9.353, 9.423, 9.686 calor-
ies; 1 gram of starch or sugar, 4. 116, 4.182, 4.479, etc-> calories. These
results are, however physical values, and indicate the quantitv of
heat such quantities of foods give rise to when completely oxidized
to carbonic acid and water. In the human body the carbohydrates
and the fats, with the exception of the small portion which escapes
digestion, are reduced to carbon dioxid and water, and hence prac-
tically liberate as much heat as they do when oxidized outside the body.
The proteids, however, are only reduced to the stage of urea. As this
compound is capable of further reduction in the calorimeter to carbon
dioxid and water, with the liberation of heat, the quantity of heat it
contains must therefore be deducted from the physical heat value of the
proteid. According to Rubner, 1 gram of urea will yield 2.523 kilogram
calories. As about one-third of a gram of urea results from the oxi-

44o TEXT-BOOK OF PHYSIOLOGY.

dation of i gram of proteid, the amount of heat to be deducted from
the heat value of the proteid is -J of 2.523, or 0.841 calories. It has
also been shown by the same investigator that some of the ingested
proteid is found in the feces, the heat value of which must also be deter-
mined and deducted. This having been done, the physiologic heat
value becomes 4.124 calories.

The following estimates give approximately the number of kilo-
gram calories which should be liberated within the body when the
proteid is burned to the stage of urea, and the fat and carbohydrate
to the stage of carbon dioxid and water :

1 gram of proteid 4.124 calories

1 " fat 9.353 "

1 " carbohydrate 4. 116 "

The total number of kilogram calories yielded by the various diet
scales can be readily determined by multiplying the quantities of the
food principles consumed by the foregoing factors. The diet scale
of Yierordt, for example, yields the following:

120 grams of proteid 494. 88 calories

90 " fat 841.77 "

330 " starch 135S.28

Total, 2694.93 "

The total calories obtained from other diet scales would be as follows :
Ranke's, 2335; Voit's, 3387; Moleschott's, 2984; Atwater's, 3331;
Hultgren's, 3436. These numbers indicate theoretically the total
heat-production in the body daily.

2. Calorimetric Measurements.— -By this method the heat dissipated
from the body of an animal is directly collected and measured, and
the amount so obtained is taken as a measure of the heat evolved by
the oxidation of the food. A calorimeter is therefore an apparatus
for the direct estimation of the quantity of heat dissipated from the
body in a given time. The substance employed for collecting and
measuring the heat is either water or air. The calorimeters in general
use consist essentially of two metallic boxes placed one within the other,
though separated by a space sufficiently large to hold a definite amount
of water (Fig. 205). The animal is placed in the inner box, which
is also provided with tubes for the entrance of fresh and the exit of
expired air. The heat radiated is absorbed by the water and its tem-
perature raised. To prevent loss by radiation and to render it inde-
pendent of changes in the surrounding temperature the calorimeter is
surrounded by a poorly conducting material, such as wool. The
temperature of the animal is taken at the beginning and the end of the
experiment. If the temperature of the animal remains the same at
the end of the experiment, then the heat absorbed by the water repre-
sents the amount produced by the animal. If, on the contrary, the
temperature of the animal rises or falls, the number of calories so re-

ANIMAL HEAT.

441

tained or lost must be added to or subtracted from the amount ab-
sorbed by the calorimeter. In the determination of the absolute
amount of heat retained or lost by the animal above or below the initial
temperature, as well as that absorbed by the materials of the appara-
tus in these various instances, the water equivalent of the tissues of the
animal and the materials of the calorimeter must be obtained, and then
added to or subtracted from, as the case may be, the amount of water
in the calorimeter, and the amount thus obtained multiplied by its
rise in temperature. In properly conducted experiments in which
the sources of error are reduced to a minimum there is a very close
correspondence between the total physiologic heat value of the food
and the amount col-
lected by the calor-
imeter. Thus, in an
experiment detailed
by Rubner, a dog was
given during twelve
days 228.06 grams of
proteid and 340.4
grams of fat the
physical heat value of
which was estimated
at 4419 calories. The
urine and feces during
this period were col-
lected and their heat
value determined,
which amounted to
305 calories. The
heat which theoretic-
ally should have been
produced was 41 19
calories. During the

experiment the calorimeter actually absorbed 3958 calories, a differ-
ence between the theoretic and experimental results of 156 calories;
thus of the total energy liberated 96 per cent, appeared as heat.

Calorimetric experiments on man corresponding to those made
by Rubner on dogs have not been successful, owing purely to tech-
nical difficulties. Various attempts have been made, however, to
determine the daily heat-dissipation. Liebermeister immersed a
man in a bath with a temperature lower than that of the man's body.
From the rise in temperature of the water it was calculated that the
man produced daily 3525 calories. Leyden placed the leg alone of
a man in a calorimeter. In one hour 6 calories were absorbed. As-
suming that the total superficial area of the body was fifteen times that
of the leg, he calculated, taking into consideration various sources o'f
error, that the entire body would produce daily 2376 calories. Ott,

Fig. 205. — Water Calorimeter of Dulong. D
and D'. Tubes for the entrance and exit of air. T and T'.
Thermometers for ascertaining the temperature of the
water. S. A mechanic contrivance for stirring the water
for the purpose of distributing the absorbed heat uni-
formly. To prevent the escape of heat with the expired
air, the tube D' is wound many times in the water-space
beneath the animal cage.

442 TEXT-BOOK OF PHYSIOLOGY.

employing a water calorimeter, found that the body of a man produced
103 calories during an afternoon, or at the rate of 2472 calories daily.
These and similar experiments, while not free from many objections,
furnish results which indicate that the heat dissipated from the body
approximates the physiologic heat values of the foods.

HEAT-DISSIPATION AND REGULATION OF THE TEMPERATURE.

Heat-dissipation. — From the preceding statements it is evident
that the body is continually liberating heat in amounts daily far in
excess of that necessary for the maintenance of the body-temperature.
Should this heat be retained, the temperature of the body would be
raised at the end of twenty-four hours an additional 18 ° or 200 C. —
a temperature far in excess of that compatible with the maintenance
of physiologic processes. That the body may be kept at the mean
temperature of 3 7. 8° C. it is essential that the heat liberated be dissi-
pated as fast as it is produced, or to state the problem in another way,
the heat dissipated by the body must be replaced by an equal amount
liberated, if equilibrium of temperature is to be maintained. The
dissipation of the heat is accomplished in several ways: (1) In warm-
ing the food and drink to the temperature of the body. (2) In warm-
ing the inspired air to the same temperature. (3) In the evaporation
of water from the lungs. (4) In evaporating water from the skin.
(5) In radiation and conduction from the skin. The quantities of
heat lost to the body by these different processes it is difficult for ob-
vious reasons to accurately determine, and the estimates usually given
must be regarded only as approximative.

Assuming 2500 calories to be an average of heat liberated during
a day of repose, the losses, in the ways stated above, may be given as
follows :

1. In Warming Food and Drink. — The average temperature of food

and drink is about 12 ° C; the amount of both together is about
3 kilograms; the specific heat of food about 0.8 that of water.
The absorption of body-heat therefore by the food amounts
approximately to 3X0.8X250 C. = 60 calories = 2.8 per cent. With
the removal of the end-products of the foods and drink from the
body an equal amount of heat is carried out.

2. In Warming the Inspired Air. — The average temperature of the

air is 120 C.; the amount of inspired air, about 15 kilograms; the
specific heat of air, 0.26. The absorption of body-heat by the air
until it attains the temperature of the body will therefore amount
to 15 X 0.26 X 25°= 97.5 calories = 3.8 per cent. The expired air
removes from the body a corresponding amount.

3. In the Evaporation 0} Water from the Lungs. — The quantity of

water evaporated from the lungs may be estimated at 400 grams;
as each gram requires for its evaporation 0.582 calorie, the
quantity of heat lost by this channel would be 400X0.582 =
232.8 calories = 9.4 per cent.

ANIMAL HEAT. 443

4. /;/ the Evaporation of Water from the Skin. — The quantity of water

evaporated from the skin may be estimated at 660 grams, caus-
ing a loss of heat by this channel of 660 X 0.582 = 384.1 calories =
15.3 per cent.

5. In Radiation and Conduction from the Skin. — The amount of

heat lost by this process can be indirectly determined only by
subtracting the total amount lost by the above-mentioned channels
from the total amount produced. Thus, 2500 — 774.4= 1725.6
calories = 69 per cent, would represent the average amount lost by
radiation and conduction.
Regulation of the Mean Temperature. — In order that the
mean temperature of the body may remain practically constant, the
heat dissipated must be exactly balanced by the heat liberated. Should
there be any want of correspondence between the two processes, there
would arise either an increase or a decrease in the mean temperature.
As both heat-production and heat-dissipation are variable factors,
dependent on a variety of internal and external conditions, their
adjustment is accomplished by a complex self-regulating mechanism
involving muscular, vascular, and secretory elements, coordinated by
the nerve system. Heat-production varies in intensity and amount,
in accordance with a number of conditions, but principally with
variations in physiologic activity, the quantity and quality of the
food, and changes in the external temperature. All physiologic and
especially muscle activity is attended by chemic changes and the
evolution of heat. The greater the activity, the larger is the volume
of heat. Foods have different physiologic heat values. If the food
consumed contains much potential energy and the quantity con-
sumed be larger than the average daily requirements, there will be
an increase in heat-production. A lowering of the external tem-
perature, as in the winter season, leads to increased heat-production
through stimulation of the nerve-centers. When all these conditions,
increased muscular activity, increased amount of food of high poten-
tial energy, and a low external temperature coexist, heat-production
attains its maximum, amounting to as much as 4726 calories daily
(Hultgren).

Heat-dissipation varies in rapidity in accordance with variations
of a number of factors, but principally with variations in the external
temperature and the activity of the perspiratory apparatus. The
heat is dissipated mainly by the skin, 69 per cent., in consequence of
radiation and conduction and by the evaporation of the sweat. The
loss by this channel as well as from the lungs is dependent for the
most part on a difference of temperature of the surrounding air and
of the body. If the surrounding temperature is high, there is an
increase in the activity of both the circulatory and respiratory mechan-
isms, brought about by the central nervous system. In addition to
an increased action of the heart, the blood-vessels of the skin dilate,
and deliver to the surface a larger volume of blood in a given time,

444 TEXT-BOOK OF PHYSIOLOGY.

thus increasing the conditions favorable to radiation. The sweat-
glands at the same time are stimulated to increased activity, and
in consequence of the additional volumes of blood brought to the skin
a larger amount of sweat is secreted, which speedily undergoes evap-
oration. As each gram of water for its evaporation requires 0.582 of
a calorie, it is evident that increased secretion of sweat favors heat-
dissipation. The nerve-centers influencing the activity of the sweat-
glands may be stimulated not only reflexly, but directly by an excess
of heat in the blood. If, however, the atmosphere itself possesses a
high percentage of moisture, evaporation from the body is much
diminished and the value of sweating as a means of lowering the
body-temperature is much impaired. Evaporation is hastened by
air in motion. Hastened respiratory movements and the dilatation
of blood-vessels of the respiratory surface also increase the evaporation
of water from the lungs and thus occasion a greater loss of heat.

If the external temperature falls there is a decrease in the physio-
logic activity of the skin from a contraction of the blood-vessels, a
diminution of the blood-supply, and a cessation in the secretion of
sweat. The blood, being prevented from coming to the surface, is
retained in the deeper portion of the body, and in consequence the
conditions for radiation are diminished. These variations in the
cutaneous circulation in response to variations in the external tem-
perature are brought about by the vaso- motor nerve mechanism; and
as they take place with extreme promptness heat-dissipation and heat-
production are quickly adjusted and the mean temperature maintained.

Radiation from the skin is modified to some extent by clothing.
An excess of clothing diminishes, a diminution of clothing increases
radiation. The quality of clothing is also an important factor. Wool
is a poor conductor of heat but a good absorber and retainer of moisture,
and hence is adapted for cold weather. Linen and cotton possess the
opposite qualities, and hence are adapted for warm weather. Radia-
tion from the skin is somewhat interfered with by subcutaneous fat,
the extent of the interference being dependent on its amount.

The foregoing estimates as to the amounts of heat produced have
reference only to the body in repose. When the body passes into a
state of muscle activity, there is at once a notable increase in heat-
production in consequence of the increase in the activity of the chemic
changes which underlie body activity, as shown by the increase in the
consumption of oxygen and the production of carbon dioxid. Not
all of the potential energy set free, however, appears as heat; for,
if the muscles are engaged in doing work a part of the energy which
would otherwise manifest itself as heat is converted into mechanic
motion. From the work done during a period of eight hours it has
been estimated that about 500 calories are so transformed or utilized.
Hirn calculated from an average of five experiments that a man
weighing 67 kilos in repose produced 154-4 calories per hour and
absorbed 30.7 grams of oxygen per hour; but when engaged in active

ANIMAL HEAT. 445

muscle movements produced 271.2 calories and absorbed 119.84
grams of oxygen per hour. The increase in heat-production per hour
during activity was thus almost doubled, though the sum total pro-
duced daily in which there was a working period of eight or ten hours
was only about one-third more than during a day of repose. During
sleep there is a greatly diminished heat-production, not more than
40 calories per hour being produced. The preceding data may be
tabulated as follows (Martin) :

Day of Rest. Day of Work.

Heat units (calor- \ Rest 16 hrs. Sleep S hrs. Rest S hrs. Work 8 hrs. Sleep 8 hrs.
ies) produced .. / 2470.4 320 I235-2 2169.6 320

2790.4 3724-S

CHAPTER XVII.
SECRETION.

Secretion. — Secretion is a term applied to a process by which a
portion of the constituents of the blood are separated from the blood-
stream, by the activities of the endothelial cells of the capillary wall, as
the blood flows through the capillary blood-vessels. In this process the
endothelial cell is aided by the physical forces, diffusion, osmosis and fil-
tration. The materials thus separated are collectively termed lymph.
This separated or secreted material may be utilized in several ways:
i. For the repair of the tissues, for growth, for the liberation of energy.
2. For the elaboration or production by specialized organs of a variety
of complex fluids of widely different application. The fluids
thus formed are utilized for the most part to meet some special
need of the body. All such fluids are termed secretions.

All secretions are products of the activities of epithelial cells
covering a flat, or lining a more or less complexly involuted,
membrane which in each instance may be termed a secretor organ.
As the fluids are poured out on the surface of the body, they have
been termed external secretions: e. g., mucus, saliva, gastric juice,
milk, sebaceous matter, etc. Within recent years it has been demon-
strated that the epithelium of certain organs and particularly of those
which do not possess a duct, such as the thyroid, thymus, adrenals,
hypophysis, etc., also produces certain specific constituents which are
reabsorbed into the blood, and which in some unknown but yet favor-
able way influence the general nutrition. To such products of these
organs the term internal secretions has been given.

The blood, in addition to its nutritive constituents, contains a
number of principles, derived from the tissues, which are to be re-
garded as waste products, the outcome of the katabolic activity of
the tissues and of no further use to the body. If retained, they would
seriously if not fatally interfere with the normal physiologic activities
of the different tissues. They are therefore removed by specialized
organs after their separation from the blood-stream. The waste
products in solution thus removed are not capable of being utilized for
any special purpose, and arc therefore termed excretions: e. g., urine,
perspiration, etc. Excretion, however, is performed by the activities
of epithelial cells aided by the physical forces of diffusion, osmosis
and nitration; and though a distinction is made between the two
classes of fluids, no sharp line can be drawn between the cell processes
which take place in secretor and excretor organs.

446

SECRETION. 447

All secretor organs may be divided into —
i. Epithelial.

2. Reticular and vascular, the latter term indicating merely their re-
lation to blood-vessels.

The Epithelial Secretor Organs. — The epithelial secretor or-
gan consists primarily of a thin delicate homogeneous membrane, one
side of which is covered with a layer of epithelial cells and the other
side of which is closely invested by a network of capillary blood-
vessels, lymph- vessels and nerves. Though the epithelial cells have a
general histologic resemblance one to another, their physiologic function
varies in different situations, in accordance probably with their ulti-
mate chemic structure, a fact which determines the difference in the
character of the secretions.

The epithelial secretor organs may consist of a single layer of cells
or a group of cells, and may be subdivided into —
i. Secreting membranes.
2. Secreting glands.

The secreting membranes are the mucous membranes lining
the gastro-intestinal, the pulmonary, and the genito-urinary tracts,
and the serous membranes lining closed cavities, such as the pleural,
pericardial, peritoneal, and synovial membranes.

The mucous membranes are soft and velvety in character and
are composed of a condensed connective tissue forming a basement
membrane beneath which is a layer of blood-vessels and muscle-fibers,
and on which is a layer of epithelium, the histologic as well as physio-
logic characters of which vary in different situations. The mucus
secreted by the various epithelial forms will very naturally possess a
somewhat different composition, according to the locality in which it
is formed. In a general way it may be said that mucus is a pale,
semitransparent, alkaline fluid, containing leukocytes and epithelial
cells. It is composed chemically of water, mineral salts and an al-
buminoid body, mucin, to the presence of which it owes its viscidity.
Much of the mucus is secreted by the goblet cells on the surface of the
mucous membranes. The principal varieties of mucus are the nasal,
bronchial, vaginal, urinary, gastro-intestinal.

The serous membranes are composed of thin membrane formed
by a condensation of connective tissue and covered by a single layer
of large, flat, nucleated cells with irregular margins. These mem-
branes enclose what are practically large lymph sacs or spaces, and
the fluid they contain resembles lymph in all respects and is prac-
tically identical with it. It serves to diminish friction when the viscera
they enclose move over one another. The most important of the serous
membranes are the pleural, pericardial, and peritoneal.

The synovial membranes in and around joints resemble serous
membranes. The cells covering them, however, secrete a clear,
colorless fluid resembling lymph, but differing from it in containing
a mucin-like substance, a nucleo-albumin, which imparts to it con-

44S TEXT-BOOK OF PHYSIOLOGY.

siderable viscidity. This synovial fluid serves to diminish friction
between the opposing surfaces of the bones as they glide over one
another during movement.

Other secretions, such as the aqueous and vitreous humors of the
eve, the fluid of the internal ear, the cerebrospinal fluid, etc., will be
considered in connection with the organs with which they are associated,
as have been the digestive secretions.

The secreting glands are formed of the same histologic elements
as the secreting membranes. They are formed by an involution of
the mucous membrane or skin the epithelium of which is variously
modified structurally and functionally in the various situations in
which they are formed. Like the membranes themselves, the glands
are invested by capillary blood-vessels and supplied with lymph- vessels
and nerves, of which the latter are in direct connection with the blood-
vessels and epithelial cells. The interior of each gland is in com-
munication with the free surface by one or more passageways known
as ducts.

These glands may be classified according as the involution is
cylindrical or dilated as —

i. Tubular. The tubular glands may be simple — e. g., sweat-
glands, intestinal glands, fundus glands of the stomach; or compound
— e. g., kidney, testicle, salivary, and lachrymal glands.

2. Alveolar. The alveolar glands may also be simple — e. g.,
the sebaceous glands, the ovarian follicles, meibomian glands; or
compound, as the mammary glands and salivary glands.

For the production of a secretion it is necessary that the plasma
of the blood, the common material, be delivered to the lymph-spaces
with which the epithelial cells are in close relation. The processes
involved in the passage of the plasma across the capillary wall have
already been considered in connection with the production of lymph.
They include the physical processes, diffusion, filtration, and osmosis,
combined with a secretory activity of the cells of the capillary wall.
The question as to which of these processes is the more active is yet
a subject of investigation.

As the chemic composition and the chemic features of the organic
constituents of all secretions have been demonstrated to be the out-
come of metabolic processes going on within the epithelial cells, it
must be assumed at least that these differences are correlated with
differences in the histologic features and molecular structure of the
epithelium. The discharge of the secretion is, as a rule, intermittent;
that is, there are periods of activity on the part of the gland followed
by periods of inactivity or rest. In rest more especially the epithelial
cells, after the assimilation of lymph, accumulate within themselves
such characteristic products as globules of mucin, granules which
apparently are the antecedents of the digestive enzymes, granules
of glycogen, globules of fat, sugar, and proteid, as in the case of the
mammary gland. In how far all these compounds are the result of

SECRETION. 449

secretor activity or of a cell degeneration and disintegration it is
impossible to state in the light of present knowledge. During the
period of gland rest the blood-supply to the gland is merely sufficient
for nutritive purposes. When the occasion arises for gland activity,
the blood-vessels, under the influence of the vaso- motor nerves, dilate
and deliver to the gland an amount of blood far beyond that required
for nutritive purposes. As a result the gland becomes red and vas-
cular and the blood emerging by the veins frequently retains its cus-
tomary arterial color. The increased blood-supply favors a rapid
transudation of water and salts into the lymph-spaces from which they
are speedily absorbed and transmitted by the epithelial cells into the
interior of the gland lumen. Coincident with the passage of water
through the cell, the organic constituents are extruded from the end of
the cell bordering the lumen to become dissolved, or in the case of fat
to be suspended, in the water. The secretion thus formed accumulates
and with .the rise of pressure which inevitably follows it at once passes
into the ducts to be discharged on the surface of the mucous membrane
or skin, as the case may be.

Influence of the Nerve System. — The activity of every gland
is controlled by nerve-centers situated in the central nerve system.
These centers may be excited to activity either by impressions made
on the peripheral terminations or by emotional states, or, possibly, by
changes in the composition of the blood itself. As a rule, all normal
secretion is a reflex act involving the usual mechanism: viz., a sentient
surface (skin, mucous membrane, or sense-organ), an afferent nerve, an
emissive cell from which emerges an efferent nerve to be distributed
to a responisve organ, the gland epithelium.

For the production of the secretion by the epithelial cell it is believed
by some experimenters that two physiologically distinct, efferent
nerve-fibers are involved — one stimulating the production of the organic
constituents {trophic nerves), the other stimulating the secretion of
water and inorganic salts {secretor nerves). The evidence for the
influence of the nerve system on secretion and the mode of connection
of the nerve-fibers with the gland-cells have been alluded to (page 163)
and will again be in subsequent chapters.

The reticular and vascular glands, though not possessing any
common histologic features, are grouped together merely for con-
venience, and will be considered in a separate chapter in connection
with the problems of internal secretion.

MAMMARY GLANDS.

The mammary glands, which secrete the milk, are two more or
less hemispheric organs situated in the human female on the anterior
surface of the thorax. Though rudimentary in childhood, they
gradually increase in size as puberty approaches. The gland pre-
sents at its convexity a small conical eminence termed the mammilla
29

45°

TEXT-BOOK OF PHYSIOLOGY.

or nipple, surrounded by a circular area of pigmented skin, the areola.
The gland proper is covered by a layer of adipose tissue anteriorly
and is attached posteriorly to the pectoral muscles by a network of
fibrous tissue.

During utero-gestation the mammary glands become larger,
firmer, and more lobulated; the areola darkens and the blood-vessels,
especiallv the veins, become more prominent. At the period of
lactation the gland is the seat of active histologic and physiologic
changes correlated with the production of milk. At the close of
lactation these activities cease, the glands diminish in size, undergo

involution, and gradually return
to their former non-secreting
condition.

Structure of the Mammary

Gland. — Each mammary gland

p|| consists of an aggregation of

||| some 15 or 20 irregular pyram-

llfj'j idal lobes, each of which is

(-a

m

i^ii'i

' 1

m

Fig. 206. — Mammary Gland. i.
Lactiferous ducts. 2. Lobuli of the
mammary gland.

Fig. 207. — Acini of the Mammary
Gland of a Sheep During Lactation.
a. Membrana propria. b. Secretory
epithelium.

surrounded by a framework of fibrous tissue. • This tissue affords
support for blood-vessels, lymphatics, and nerves. Each lobe is pro-
vided with a single excretory duct, the lactiferous duct, which as it
approaches the areola expands into a fusiform ampulla or reservoir.
At the base of the nipple the ampullae contract to form some 16 or 18
narrow ducts, which, ascending the nipple, open by constricted orifices
0.5 mm. in diameter on its apex (Fig. 206).

On tracing the lactiferous duct into a lobe, it is found to divide
and subdivide into a number of branches, which pass into smaller
masses — the lobules. The lobule in turn is composed of a large
number of tubular acini or alveoli, the final terminations of the lobu-
lar ducts. Each acinus consists of a basement membrane lined by a
single layer of low cuboidal epithelial cells (Fig. 207). Externally
the acinus is surrounded by blood-vessels, nerves, and lymphatics.

SECRETION. 451

MILK.

Milk as obtained during active lactation is an opaque bluish-
white fluid, almost inodorous, with a sweet taste, an alkaline reaction,
and a specific gravity of from 1.025 to 1.040. Examined micro-
scopically, it is seen to consist of a clear fluid, the milk plasma, hold-
ing in suspension an enormous number of small, highly refractive
oil-globules, which measure on the average about T u \ 0 u of an inch
in diameter. It has been asserted by some observers that each globule
is surrounded by a thin proteid envelope which enables it to maintain
the discrete form. This, however, is at present disbelieved.

The quantity of milk secreted daily by the human female averages
about 1200 c.c.

Chemic analysis has shown that the milk of all the mammalia
consists of all the different classes of nutritive principles, though .in
different proportions, which are necessary to the growth and devel-
opment of the body. The only exception appears to be an insuffi-
cient amount of iron for the formation of the coloring-matter of the
blood, the hemoglobin.

Caseinogen is the chief proteid constituent of milk. Associated
with it, however, are two other proteids, lactalbumin and lactoglobulin,
both of which are present in but small quantity. When milk is treated
with acetic acid, sodium chlorid, or magnesium sulphate to saturation,
the caseinogen is precipitated as such, and after the removal of the
fat with which it is entangled may be collected by appropriate chemic
methods. On the addition of rennet prepared from the mucous
membrane of the calf's stomach, which contains the enzyme rennin
or pexin, the caseinogen undergoes cleavage into an insoluble proteid,
casein or tyrein, and a small quantity of a new soluble proteid. To
this process the term coagulation has been given. The presence of
calcium phosphate appears to be essential to this process, inasmuch
as it does not take place if the milk be completely decalcified by the
addition of potassium oxalate. x\fter coagulation, the more or less
solid mass of milk separates into a liquid portion, the serum, and a
solid portion,- the coagulum. The former, generally termed whey,
consists of water, salts, lactalbumin, sugar; the latter, the curd, con-
sists of the casein and entangled fat. Boiling the milk retards and
even prevents the coagulation by rennet, owing to the precipitation
of the calcium phosphate. When milk is taken into the stomach, it
is probable that the rennin coagules the caseinogen in a manner similar
to, if not identical with, this process, which appears to be essential to
the normal digestion of the milk.

The fat of milk is more or less solid at ordinary temperatures.
It is a compound of olein, palmitin, and stearin with small quantities
of butyrin and caproin. The melting-point of butter varies between
31 ° and 340 C. When milk is allowed to stand for some time, the
fat-globules rise to the surface and form a thick lavcr known as cream.

452

TEXT-BOOK OF PHYSIOLOGY.

Churning the milk or cream causes the fat-globules to run together
and form a coherent mass termed butter.

Lactose is the particular form of sugar characteristic of milk.
It belongs to the saccharose group and has the following composition:
CI2H22OXI. Though incapable of undergoing fermentation by the
action of the yeast plant, it is readily reduced by the Bacillus acidi
lactici to lactic acid and carbon dioxid, the former of which imparts
to milk an acid reaction and a sour taste. With the accumulation of
the lactic acid the caseinogen is precipitated as a more or less consistent
mass.

The inorganic salts of milk are chiefly potassium, sodium,
calcium, and magnesium phosphates and chlorids. Iron is also
present in small amount. The following table of Bunge gives the
quantitative amounts of these constituents in both human and cow's
milk:

In iooo Parts.

Potas-
sium.

Sodium.

Calcium.

Magne-
sium.

Iron
Oxid.

Phos-
phoric
Acid.

Chlorin.

0.78
1.76

0.25
1. 11

°-33
1-59

0.06
0.21

0.0036
0.0030

0.47
1.97

°-43
1.69

Cow's milk,

Mechanism of Milk Secretion. — During the time of lactation
the mammary gland exhibits periods of secretory activity which
alternate with periods of repose. Coincidently with these periods
certain histologic changes take place in the secreting epithelium
At the close of a period of active secretion and after the discharge of
the milk each acinus presents the following features: The epithelial
cells are short, cubical, nucleated, and border a relatively wide lumen,
in which is found a variable quantity of milk. After the gland has
rested for some time active metabolism again begins. The cells grow
and elongate; the nucleus divides into two or three new nuclei; con-
striction takes place and the inner portion is detached and discharged
into the lumen of the acinus. During the time these changes are
taking place oil-globules make their appearance in the cell protoplasm,
some of which are discharged separately into the lumen, while others
remain for a time associated with the detached portion of the cell
(Fig. 208). From these histologic changes it is inferred that the case-
inogen and fat are products of the metabolism of the cell protoplasm
and not derived directly through the lymph from the blood. The
lactose apparently has a similar origin, as appears from the fact that
it is not found either in the blood or any other tissue, and that it is
formed independently of carbohydrate food. The water, and especially
the inorganic salts, are the result of secretory activity rather than of
rl illusion and filtration. This is rendered probable from the fact that
the proportions of the inorganic salts of milk are more closely allied
to those of the tissues of the new-born child than to blood. With the

SECRETION. 453

passage of the water and salts into the lumen of the acinus the proteids
undergo disintegration and solution and the liquid' assumes the charac-
teristics of milk.

The discharge of milk is occasioned by the suction efforts on the
part of the child, aided by atmospheric pressure and the contractions
of the non-striated muscle-fibers of the lactiferous ducts.

Influence of the Nerve System. — Judging from analogy, it is
probable that the secretion of milk is regulated by influences emanating
from the nerve system, though the exact nerve-channels for the trans-
mission of such influences have not been
determined experimentally. Various at-
tempts have been made to isolate and
study these nerves, but the results are in-
conclusive. It is well known that emo-
tional states on the part of the mother
modify the quantity as well as quality of
milk, indicating a connection between the
gland-cells and the central organs of the
nerve system. Nerve terminals have been
discovered in and around the epithelial Fig. 208.— Section of the

cells — a fact which supports this view. Mammary Gland of a Cat

Colostrum.— Within a day or two after IN THE Early Stages of Lac-

J _.. tation. A. Cavitv of alveoli

parturition the alveoli become filled with a filled with granule's and glob-
fluid which in some respects resembles ules of fat- *> 2> 3- Epithe-
milk and which has been termed colostrum. l£Ziti™-(YeoTS °f ^
This is a watery fluid containing disinte-
grated epithelial cells, fat-globules, as well as colostrum corpuscles,
which are probably emigrated leukocytes. Colostrum is distinguished
from milk in being richer in sugar and inorganic salts. It is said to
possess constituents which act as a laxative to the young child.

THE LIVER.

The liver is a large gland situated in the upper and right side
of the abdominal cavity, where it is held in position largely by liga-
ments formed by reduplications of the peritoneal investment. In
the adult it weighs, freed of blood, from 1300 to 1700 grams. The
liver is connected with the duodenal portion of the intestine by the
hepatic duct. It receives blood both from the hepatic artery and from
the portal vein, and in this respect differs from all other glands in
the body. The epithelial structures of the liver are inclosed by a
firm fibrous membrane, known as Glisson's capsule. At the trans-
verse fissure it invests and follows the blood-vessels, which there
enter, in all their ramifications through the gland.

Structure of the Liver. — The liver is composed of an enormous
number of small masses, rounded, ovoid, or polygonal in shape, called
lobules, measuring about one millimeter in diameter and separated

454

TEXT-BOOK OF PHYSIOLOGY.

from one another by a narrow space in which are to be found blood-
vessels, lymphatics, hepatic ducts, supported by connective tissue.
In the pig this space and its contained elements is quite distinct,
sharply marking out the border of the lobule (Fig. 209). This is not
so apparent in man. Each lobule is made up of irregular or polygonal
shaped cells measuring about 30 to 40 micromillimeters in diameter.
These cells are arranged in a radial manner from the center to the
circumference of the lobule (Fig. 210). Each cell possesses one and
at times two nuclei. There is no evidence for the existence of a distinct
cell-wall. The cell protoplasm frequently contains globules of fat,
granules of a proteid nature, granules of glycogen, pigment material,

etc. The appearance
presented by the cell will
vary considerably, ac-
cording to the time it is
observed. Thus there
may be a complete ab-
sence of these constitu-
ents, when the cell may
present a series of vacu-
oles separated by bands
of protoplasm. The
cells are the secreting
structures of the liver,
and hence are in close
relation with capillary
blood-vessels, lymphatic
spaces, nerves, and ir-
regular channels or pas-
sageways. The latter
running between the epithelial cells may be compared to the lumen of
other secreting glands.

Blood-vessels and Their Distribution.— The blood-vessels
which are in relation with the liver are:

1. The portal vein.

2. The hepatic artery.

3. The hepatic vein.

The portal vein and the hepatic artery enter the liver at the trans-
verse fissure. After penetrating its substance they divide and sub-
divide into smaller and smaller branches, which ultimately occupy
the space between the lobules, completely surrounding and limiting
them. From their situation they are termed interlobular veins and
arteries.

The interlobular veins give off small capillary vessels which pene-
trate the lobule at all points of its surface. These capillaries, though
frequently anastomosing, form a radial meshwork which converges
toward the center of the lobule. In the meshes of this plexus are

i:^i

Fig. 209. — Section of Liver of Pig, showing
very Diagrammatically the Lobules, a. Interlobu-
lar connective tissue, b, c. Branches of portal vein
and of hepatic artery, d. Bile- ducts, e. Intralobular
vein. — (Pier sol.)

SECRETION.

455

■ Trabecule of
hepatic cells.

Central vein.

found, arranged in a corresponding radial manner, the liver cells.
The interlobular arteries are distributed to the walls of the portal
vein, to the connective tissue, and finally terminate in the portal vein
capillaries. The intralobular capillaries thus receive and transmit
blood which is an admixture of both arterial and venous blood. In
the center of each lobule there is a large vein, formed by the union of
the intralobular capillaries, known as the intralobular vein, which
collects all the blood of the lobule and transmits it through the lobule
to an underlying or sublobular vein (Fig. 211). These latter vessels,

uniting and reuniting, ultimately
form the hepatic vein, which empties
the blood into the inferior vena cava.
Bile Capillaries and Hepatic
Ducts. — The bile capillaries are
narrow channels which penetrate
the lobule in all directions and are
generally found running along the
sides of the cells. These channels,
which are devoid of walls, receive
from the cells some of the products
of their secretory activity, and hence
are comparable to the lumen of the
alveoli of other secreting glands.
At the periphery of the lobules the
bile capillaries communicate with
larger channels which are the begin-
nings of the hepatic or bile-ducts
lying in the interlobular spaces. The
interlobular bile-ducts possess a dis-
tinct wall lined by flattened epithe-
lium. There is, however, no distinct
line of demarcation between the cells
of the interlobular ducts and the
secreting cells of the liver proper, as
the two blend insensibly, the one into the other. As the hepatic ducts
increase in size they gradually acquire the structure characteristic of
the main hepatic duct: viz., a mucous, a muscle, and a fibrous coat.

Nerves. — Experimental investigations have demonstrated that
the liver is supplied with nerves derived from the central nerve system.
The route of these nerves is probably by way of the splanchnics
and the vagi. Many of the nerves which enter the liver are vaso-motor
in function; as to whether others are secretory in character is yet a
subject of investigation. It has been asserted that nerve terminals
have been demonstrated running between the cells and even pene-
trating their substance. This fact would indicate that the metabolic
processes of the liver are under the control of the central nerve system.
Functions of the Liver. — The anatomic and histologic pecul-

Interlobular vein. Hepatic duct.

Fig. 210. —Scheme of a Hepatic
Lobule, represented in Transverse
Section below and, by Partial Lev-
eling, in Longitudinal Section
Above. In the left half the blood-
vessels are drawn; in the right half
only the cords of hepatic cells. X 2°-
—(Stohr.)

456 TEXT-BOOK OF PHYSIOLOGY.

iarities of the liver would indicate that it has a variety of relations to
the general processes of the body. Experimental investigation has
brought some of these relations to light. Though its physiologic
actions are not yet wholly understood, it may be said that it —
i. Secretes bile.

2. Produces and stores glycogen.

3. Assists in the formation of urea.

Secretion of Bile. — The physical properties and chemic com-
position of the bile have already been considered (page 202). The
characteristic salts of the bile, sodium glycochlate and taurocholate,
do not pre-exist in the blood, and therefore must be formed by the liver
cells out of materials derived from the blood of the intralobular capil-

Fxg. 211. — Transverse Section of a Single Hepatic Lobule, i. Intralobular
vein, cut across. 2, 2, 2, 2. Afferent branches of the intralobular vein. 3, 3, 3, 3, 3, 3,
3, 3, 3. Interlobular branches of the portal vein, with its capillary branches, forming the
lobular plexus, extending to the radicles of the intralobular vein. — (Sappey.)

larics. The antecedents of the bile salts, glycocoll and taurin, are
crystallizable nitrogenized compounds, and known chemically as
amido-acetic and amido-ethylsulphonic acids. Their chemic composi-
tion indicates that they are derivatives of the proteids or the albu-
minoids, though the intermediate stages in their production are un-
known. The origin of the cholalic acid with which they are com-
bined is equally obscure. The bile salts as they are found in the bile
are produced in the liver cells by metabolic activity.

The primary coloring-matter of the bile, bilirubin, has been shown
to be a derivative of hematin, a product of the disintegration of hemo-
globin. It is supposed that the liver cells bring about this change
by combining water with hematin, with the abstraction of iron. The
prod ml thus formed is bilirubin, which is excreted, while the iron is
for 1 he mosl pari retained.

SECRETION. 457

Cholesterin is a waste product derived largely from the nerve-
tissue. It is brought to the liver and simply excreted by the cells.
The remaining constituents of the bile, water and inorganic salts, are
secreted here as in all other glands.

When once formed, the liver cells discharge these various com-
pounds into the channels by which they are surrounded; they then
pass into the open mouths of the bile-ducts at the periphery of the
lobules. Under the increasing pressure which arises from the secre-
tion and accumulation of bile, this fluid flows from the smaller into the
larger bile-ducts, and finally is emptied either directly into the in-
testine or into the gall-bladder, where it is stored until required for
digestive purposes. The secretion of bile, as observed by means of
a biliary fistula, is continuous and not intermittent, though the rate of
flow is subject to considerable variation.

The liver cells, as far as the secretion of bile is concerned, appear
to be independent of the nerve system. Their activity, however,
is stimulated by the increased blood-supply which arises during di-
gestion in consequence of the dilatation of the intestinal vessels,
since it is at this period that the rate of discharge is the greatest.
The same results have been shown by experiment. Thus, division
of the splanchnic nerves is followed by an increased discharge of
bile, apparently due to the dilatation of the portal vessels; stimula-
tion of their peripheral ends is followed by a decreased discharge of
bile in consequence of the contraction of the portal vessels. The
bile salts appear to be the most efficient stimulants to the activity of
the liver cells, for their administration and absorption is followed by
an increase not only in the amount of water, but of the inorganic
salts and other solid constituents as well.

The flow of bile from the bile capillaries to the main hepatic
duct, though primarily dependent on differences of pressure, is aided
by the contraction of the muscular walls of the bile-ducts and the
inspiratory movements of the diaphragm. Any obstacle to the dis-
charge of bile leads to its accumulation, a rise of pressure beyond
that of the capillary blood-vessels, and a reabsorption by the lymph-
vessels of the bile constituents. After their discharge into the blood
from the thoracic duct these constituents are deposited in part in
various tissues, giving rise to the phenomena of jaundice, and in part
are eliminated in the urine.

The Production of Glycogen. — In 1857 Bernard discovered the
fact that the liver normally during life produces a sugar-forming
substance, analogous in its chemic composition to starch, to which he
gave the name glycogen. This substance can be obtained by the
following method: Small pieces of the liver of an animal recently
killed, preferably after a meal rich in carbohydrates, are placed in
acidulated boiling water for a few minutes; then rubbed up in a
mortar with sand, again boiled, after which the proteids are removed
by filtration. The filtrate thus obtained is opalescent and resembles

458 TEXT-BOOK OF PHYSIOLOGY.

a solution of starch. The glycogen may be precipitated from this
solution with alcohol as a white amorphous powder, soluble in water.
Chemic analysis shows that it consists of C6HioO-, or a multiple of it.

When either the original solution obtained by boiling or a solution
of this amorphous powder is treated with iodin, it strikes a port-wine
color. When digested with saliva, pancreatic juice, or boiled with
dilute acids, the solution becomes clear, and testing with Fehling's
solution reveals the presence of sugar.

If the liver be allowed to remain in the body of an animal for a
period of twenty-four hours before the decoction is made as above
described, it will be found that the solution contains only a small
amount of glycogen but a relatively large amount of sugar. The
inference drawn is that after death the glycogen is transformed by
some agent, possibly a ferment, into sugar (dextrose).

The presence of glycogen in the liver cells can be shown micro-
scopically in the form of discrete hyaline and refractive granules.
As they are soluble in water they can be readily dissolved out from
the cells, leaving small vacuoles separated from one another by strands
of cell substance. The amount of glycogen in a well-fed animal varies
from 1.5 to 4 per cent, of the total weight of the liver. The production
of glycogen is dependent very largely on the consumption of car-
bohydrates, the greater the amount of sugar and starch in the food,
the greater being the production of glycogen. On a pure proteid diet
it is still produced, though in small amounts.

Glycogen is also found in muscles, placenta and embryonic tissues
generally. Muscles contain from 0.5 per cent, to 1 per cent, and as
they amount to a.bout 40 per cent, of the weight of the body, 70 kilo-
grams, they contain from 140 to 280 grams of glycogen. During
periods of prolonged activity of the muscles the percentage of glycogen
rapidly diminishes, a fact 'which leads to the inference that it is the
source in large part of the energy expended by the muscle. During
rest the percentage of glycogen again rapidly increases until the
normal percentage is regained. The source of the muscle glycogen
is undoubtedly the liver in which it is temporarily stored.

The facts connected with the formation of glycogen, as well as its
disposition as at present generally accepted, may be stated as follows:
The dextrose into which the carbohydrates arc converted by the
action of the digestive fluids is absorbed into the blood of the portal
vein and carried direct into the liver, where by the action of the cells
it is abstracted, dehydrated, and temporarily deposited under the
form of the non-diffusible body glycogen. At a subsequent period and
in proportion to the needs of the system the liver cells, through the
agency of a ferment, transform the glycogen into dextrose, return it to
the circulation, by which it is transported to the systemic capillaries,
where it disappears. The blood of the hepatic vein therefore contains
more sugar than the blood of any other part of the body, and the
blood of l he a ri cries more than the blood of the other veins. Should

SECRETION. 459

there be a failure on the part of the liver cells to abstract the sugar,
it would pass through the liver into the general circulation, from which
it would be eliminated by the kidneys. The final fate of the sugar
is uncertain. It is, however, probable that after its delivery to the
muscles, for example, it may be directly oxidized, or stored as glycogen
or possibly utilized in the formation of living material. Ultimately,
however, through oxidation it yields heat, and contributes to the pro-
duction of muscle energy.

In opposition to this view, Dr. Pavy, after years of accurate ex-
perimentation, states that the blood on the cardiac side of the liver
never under normal circumstances contains a larger percentage of
sugar than is to be found in any part of the circulation, except in the
portal vein. He states that glycogen is never reconverted into sugar,
and denies that the liver produces sugar, to be discharged into the
blood; that the function of the liver is merely to arrest the passage of
sugar, and so to shield the general circulation from an excess; that the
sugar which arises in the liver after death is a post-mortem product
and not an illustration of what takes place during life. Dr. Pavy,
having apparently demonstrated the glucoside constitution of proteid
material in general, accounts for the presence of glycogen in muscles
and other tissues on the assumption that during the cleavage of the
proteid molecule the carbohydrate element is set free and temporarily
stored as glycogen. He thus accounts for the production of sugar
in the body, even in the absence of all sugar and starch from the food.
Pavy believes that the glycogen produced in the liver is utilized in the
formation of fat and the synthesis of complex proteids necessary to
the construction of the tissues.

The Influence of the Nerve System. — The results of various ex-
perimental investigations indicate that the production of sugar from
the glycogen in the liver is influenced by the activities of the nerve sys-
tem. It was discovered by Bernard that puncture of the floor of the
fourth ventricle, at a point between the acoustic and vagus nerves, near
the middle line, is followed within an hour or two by the appearance of
sugar in the urine, which lasted for from five to six hours in the rab-
bit and from two to three or even seven in the dog. For this reason
Bernard gave to this area the name of "diabetic area."

Coincident with the appearance of sugar in the urine (glycosuria)
there is an increase in the percentage of sugar in the blood (hyper-
glycemia). The liver at the same time contains a higher percentage
of sugar than normally. Apparently the initial step in this series of
phenomena is an increased conversion of glycogen into sugar. This
supposition receives support from the fact that the degree of the hyper-
glycemia, and the subsequent glycosuria, will depend on the amount of
glycogen previously in the liver. If the animal has been well fed on
carbohydrates, the resulting glycosuria will be pronounced; if, on the
contrary, it has been allowed to fast for several days, the glycosuria
will be slight.

460 TEXT-BOOK OF PHYSIOLOGY.

Assuming that the nerve-cells which constitute the diabetic area
influence the conversion of glycogen into sugar, the question arises as
to whether the puncture destroys the nerve-cells, or whether it stimulates
them to increased activity. The results of experiment lead to the
latter supposition. Thus if the vagus nerve is divided in the neck and
its central end stimulated there is developed a glycosuria. Stimulation
of other sensor nerves has a similar effect. As stimulation of the vagus
has the same effect as the puncture, the inference is that the center is
normally excited to physiologic activity by impulses reflected from
some surface or organ in the peripheral distribution of this nerve.

If the nerve-cells in the diabetic area regulate the production of
sugar in the liver, the further question arises as to the pathway through
which the nerve impulses emanating from them reach the liver, whether
by way of the vagi or by way of the spinal cord and splanchnic nerves.
That it is not by way of the vagi is shown by the fact that the glyco-
suria established by the puncture does not disappear when they are
divided; that it is by way of the spinal cord, as far at least as the
first dorsal nerve, and subsequently the splanchnic nerves, is indicated
by the fact that a cross- section of the spinal cord above this level, destruc-
tion of the upper three dorsal roots as well as division of the splanchnic
nerves prevents the development of the glycosuria which follows punc-
ture of the medulla. Though stimulation of the upper dorsal (pre-
ganglionic) nerve-fibers gives rise to glycosuria, yet, contrary to
expectation, stimulation of the splanchnic (post-ganglionic) nerve-
fibers does not have the same effect. This may be due, however, to
changes in the relation of the capillary blood-vessels to the liver cells
or to the character of the stimulus employed.

A further question arises as to whether the nerve impulses which
pass from the diabetic center to the liver are vaso-motor in character
and exerting their effect on the blood-vessels, or whether they are secretor
in character and exerting their effect on the liver cell. Bernard was
of the opinion that they are vaso-motor in character and that the dia-
betic area was a part of the general vaso-motor center. More recent
investigators are of the opinion that they are secretor in character, for
the reason that whether the blood-pressure rises from a stimulation
of the central end of the divided vagus, or falls from a stimulation
of the depressor nerve, in each instance there follows a glycosuria.

If the production of sugar in the liver is a reflex act as Bernard
supposed, taking place through a mechanism consisting of an afferent
pathway, the vagus nerve, and an efferent pathway consisting of the
spinal cord and splanchnic nerves, the question arises as to the seat
of action of the stimulus. This Bernard located in the lungs, for the
reason that though division of the vagus in the neck checks the produc-
tion of the sugar, division below the origin of the pulmonary branches
had no such effect.

Diabetes.- Diabetes is a chronic disease characterized by the
appearance of sugar in the urine in variable amounts. This patho-

SECRETION. 461

logic condition has usually been associated with derangements of the
glycogen function of the liver, though doubtless derangements of
other organic functions will produce the same condition. At the
present time it is believed that the excretion of sugar by the kidneys
depends on two causes: (1) An ineffectual abstraction and storage
of sugar due to some impairment in the activity of the liver cells; (2)
a rapid cleavage of the proteid constituents of the tissues, in consequence
of some profound alteration in the nutritive process, whereby their
glucose radicals are liberated in unusual amounts. The physiologic
mechanism by which the normal metabolism of the carbohydrates
is regulated is unknown. That it is complex in character is shown
by the phenomena which follow not only puncture of the medulla,
but also removal of the pancreas and the administration of various
toxic agents.

Removal of the pancreas from the body of a dog or other animal
is at once followed by a rise in the percentage of sugar in the blood
and its elimination by the kidneys. In a short time acetone, aceto-
acetic and oxybutyric acids make their appearance, attended by the
usual symptoms characteristic of glycosuria in man. The quantity
of sugar excreted and the gravity of the attendant symptoms may be
much diminished by allowing a portion of the gland to remain in situ.
even though its capacity for the production of pancreatic juice is en-
tirely abolished. Transplantation of the pancreas to the subcutaneous
tissue or to the abdominal cavity will practically prevent the glycosuria.
The explanations which have been offered as to the manner in which
the pancreatic tissue prevents and its absence gives rise to the ex-
cretion of sugar are purely hypothetical. It has been claimed by some
investigators that the pancreas secretes a specific material, which enters
the blood and promotes oxidation of the sugar. In the absence of this
material the sugar accumulates, and is finally eliminated by the kidneys.
Since the discovery of the islands of Langerhans it has been suggested
by some investigators that the production of the material which regu-
lates carbohydrate metabolism should be attributed to them rather than
to the pancreas as a whole. The sugar excreted doubtless in part
comes from the glycogen of the liver, as this disappears in a short time.
But as sugar continues to be excreted, even though all carbohvdrates
be withdrawn from the food, the conclusion is justifiable that it arises
in consequence of increased proteid metabolism. This supposition is
strengthened by the fact that the quantity of urea excreted rises and
falls with the quantity of sugar excreted.

Phloridzin, a glucoside obtained from the root bark of the cherry
and plum tree, gives rise to the appearance of sugar in the urine, in
amounts beyond that which might come from the glucose normally
present in the blood or from the glycogen of the liver. As there is a
concomitant increase in the amount of urea excreted, the supposition
is that phloridzin increases proteid metabolism.

Curara, in doses sufficient to paralyze the muscles, also gives rise

462 TEXT-BOOK OF PHYSIOLOGY.

to the appearance of sugar in the urine. This is not due, however,
to an increased production on the part of the liver, but rather to a
want of consumption on the part of the muscles, due to their inac-
tivity. The accumulation of the sugar in the blood which takes place
for this reason leads very promptly to its removal by the kidneys.

The Formation of Urea. — It is now generally believed that the
liver is the most active of all the organs which may be engaged in the
production of urea. This belief is based on numerous physiologic
and pathologic data. The compounds out of which the hepatic cells
construct urea have been for chemic reasons asserted to be the am-
monium salts, e. g., the carbonate, lactate, carbamate, which are
constantlv present in the blood. These salts, which result from
proteid metabolism, may be absorbed from the tissues or from the in-
testines carried to the liver, and there synthesized to urea. This
supposition is supported by an experiment as follows: The liver of
an animal recently living is removed from the body and its vessels
perfused continuously with blood (the urea content of which is known)
containing the ammonium salts. An analysis of this blood shows,
after a time, a diminution of these salts, and a large increase in the
amount of the urea. After the establishment of an Eck fistula
(the union of the portal vein with the ascending vena cave whereby
the liver is largely excluded from acting on products absorbed from
the intestines) there is a marked diminution in the production of urea
while the ammonia content of the urine largely increases. The leucin
and tvrosin which result from the prolonged action of pancreatic juice
on hemi-peptone are also capable of being converted to urea by the
hepatic cells, and in all probability are so disposed of. Destructive
diseases of the liver — e. g., acute yellow atrophy, suppuration, cirrhosis
—largely diminish the production of urea, but increase the quantities
of the ammonium salts in the urine. The same is true when the liver
cells are destroyed during acute phosphorus-poisoning.

VASCULAR OR DUCTLESS GLANDS.

INTERNAL SECRETIONS.

The metabolism of the body generally, as well as that of individual
organs, has been shown to be related not only to the physiologic ac-
tivity of such organs as the liver and pancreas, but also to the activity
of the so-called vascular or ductless glands. The influence of the
pancreas in regulating the production of glycogen by the liver, and
the influence of the liver in the maintenance of the general metabo-
lism through the production of glycogen and the formation of urea,
are now established facts. That the vascular or duct less glands to
an equal extent, though perhaps in a different way, assist in the main-
tenance of physiologic processes, appears certain from the results
of animal experimentation. The explanation given for the influence

SECRETION.

463

of these glands is that they produce specific substances, which are
poured into the blood or lymph and carried direct to the tissues, to
the activities of which they appear to be essential; for without these
substances the nutrition of the tissues declines and in a short time
a fatal termination ensues.

Inasmuch as these partly unknown substances are formed by cell
activity and are poured into the interstices of the tissues, they have
been termed "internal secretions." Though the term internal secre-
tions is applicable to all substances which arise in consequence of
tissue metabolism, and which, after being poured into the blood,
influence in varying degrees
and ways physiologic proc-
esses, yet the term in this
connection will be applied
only to the secretions of the
thyroid gland, hypophysis
cerebri, and adrenal bodies.

Thyroid Gland.— The
thyroid gland or body con-
sists of two lobes situated on
the lateral aspect of the upper
part of the trachea (Fig. 212).
Each lobe is pyriform in
shape, the base being directed
downward and on a level with
the fifth or sixth tracheal ring.
The lobe is about 50 mm. in
length, 20 mm. in breadth,
and 25 mm. in thickness. As

a rule, the lobes are united by a narrow band or isthmus of the same
tissue. The gland is reddish in color, and abundantly supplied with
blood-vessels and lymphatics.

Microscopic examination shows that the thyroid consists of an
enormous number of closed sacs or vesicles, variable in size, the largest
not measuring more than 0.1 mm. in diameter (Fig. 213). Each sac is
composed of a thin homogeneous membrane lined by cuboid epithe-
lium. The interior of the sac in adult life contains a transparent,
viscid, fluid containing albumin and termed "colloid" substance.
Externally, the sacs are surrounded by a plexus of capillarv blood-
vessels and lymphatics. The individual sacs are united and sup-
ported by connective tissue, which forms, in addition, a covering for
the entire gland.

Function of the Thyroid. — The knowledge at present possessed
as to the function of the thyroid gland, especially in mammals, is the
outcome of a study of the effects Avhich follow its arrested development
in the child, its degeneration in the adult, and its extirpation in the
human being as well as in animals. The results, however, which fol-

Fig. 212. — View of Thyroid Body. i.
Thyr°id isthmus. 2. Median portion of crico-
thyroid membrane. 3. Crico-thyroid muscle.
4. Lateral lobe of thyroid body. — {After Morris.)

464

TEXT-BOOK OF PHYSIOLOGY.

low its extirpation are not always uniform in all animals, though suf-
ficient reasons for the lack of uniformity can not always be assigned.
Cretinism, a condition characterized by a want of physical and
mental development, is associated with, if not directly dependent on,
a congenital absence of the thyroid, or its arrested development dur-
ing the early years of childhood.

Myxedema, a condition of the skin in which there is a hyperplasia
of the connective tissue, of an embryonic type, rich in mucin, is gener-
allv regarded as one of the effects of degenerative processes in the

thyroid. Partly in
consequence of this
change in the skin the
face becomes broader,
swollen, and flattened,
giving rise to a loss of
expression. At the
same time the mind
becomes dull, clouded,
even approximat-
ing the idiotic type.
This supposed infil-
tration of the skin
with mucin was
termed myxedema by
Ord, who at the same
time associated it with
a change in the struc-
ture of the thyroid as
a result of which it
became functionally
useless.

Extirpation of the
thyroid, for relief from symptoms due to grave pathologic changes,
has been followed in human beings by symptoms similar to those of
myxedema. To this condition the terms operative myxedema and
cachexia strumipriva have been applied.

After the publication of the history of the myxedema which fol-
lowed surgical removal of the thyroid, Schiff, in 1887, repeated his
earlier experiments on dogs, and found again that removal of the thyroid
was speedily followed by tremors, convulsions, and death. Similar
experiments were made by Horsley on monkeys, with results which
resembled those characteristic of myxedema. Among the symptoms
which developed within a few days after the removal of the gland may
be mentioned loss of appetite; fibrillar contractions of muscles; trem-
ors and spasms; mucinoid degeneration of the skin, giving rise to
puffiness of the eyelids and face and to a swollen condition of the
abdomen; hebetude of mind, frequently terminating in idiocy; fall

Fig 213. — A Lobule from a Thin Section of the
Thyroid Gland of an Adult Man. i. Colloid sub-
stance. 2. Epithelium. 3. Tangential section of a tubule,
the epithelium viewed from the surface. 4. Tubule in
transverse section. 5. Connective tissue. — (Stohr.)

SECRETION. 465

of blood-pressure; dyspnea; albuminuria; atrophy of the tissues,
followed by death of the animal in the course of from five to eight
weeks. The complexus of symptoms observed in monkeys was
divided by Horsley into three stages: viz., the neurotic, the mucinoid,
and the atrophic.

It is evident that the pressure of the thyroid is essential to the
normal activity of the tissues generally. As to the manner in which
it exerts its favorable influence, there is some difference of opinion.
The view that the gland removes from the blood certain toxic bodies,
rendering them innocuous and thus preserving the body from a species
of auto-intoxication, is gradually yielding to the more probable view
that the epithelium is engaged in the secretion of a specific material,
which finds its way into the blood or lymph and in some unknown
way influences favorably tissue metabolism. This view of the function
of the thyroid is supported by the fact that successful grafting of a
portion of the thyroid beneath the skin or in the abdominal cavity will
prevent the usual symptoms which follow thyroidectomy. The same
result is obtained by the intravenous injection of thyroid juice or by
the administration of the raw gland. It was shown by Murray that
myxedematous patients could be benefited, and even cured, by feeding
them with fresh thyroids or with the dry extract.

The chemic features of the material secreted and obtained from
the structures of the thyroid indicate that it is a complex proteid con-
taining iodin, which, under the influence of various reagents, under-
goes cleavage, giving rise to a non-proteid residue, which carries with
it the iodin and phosphorus. The amount of iodin in the thyroid
varies from 0.33 to 1 milligram for each gram of tissue. To this com-
pound the term thyroiodhi has been given. The administration of this
compound produces effects similar to those which folloAv the therapeu-
tic administration of the fresh thyroid itself; viz., a diminution of all
myxedematous symptoms. In normal states of the body, thyroiodin
influences very actively the general metabolism. It gives rise to a de-
composition of fats and proteids and to a decline in body-weight.
In large doses it may produce toxic symptoms, e. g., increased cardiac
action, vertigo, and glycosuria.

The conclusions as to the functions of the thyroid gland which
have been drawn from the results that have followed its removal from
animals by surgical procedures, have been made questionable, since
the discovery of the parathyroid glands and a study of the phenomena
which follow when they alone are removed. From their situation and
close relationship to the thyroid gland it is generally accepted, that in
the earlier experiments, especially those made on cats and dogs, and
some other carnivorous animals, both sets of glands were removed
and hence the symptoms which developed after the removal of the
thyroids were due to the loss of function not only of the thyroid but
of the parathyroids as well.

The Parathyroids. — The parathyroids are small bodies, usually
3°

466 TEXT-BOOK OF PHYSIOLOGY.

four in number, two on each side. They are divided into superior
and inferior. The superior are situated internally and on the poste-
rior surface in close relation to, and frequently imbedded in, the sub-
stance of the thyroid; the inferior are situated externally, sometimes
in contact with, and at other times removed a variable distance from
the thyroid. Microscopically the parathyroids consist of thick cords
of epithelial cells separated by septa of fine connective tissue and sur-
rounded by capillary blood-vessels. Chemic analysis shows that they
also contain iodin in combination with some organic compound.

Effects of Parathyroid Removal. — The surgical removal of the
parathyroids is followed in the course of from two to five days by the
death of the animal preceded in most instances by a series of symp-
toms which are embraced under the general term "tetany." These
symptoms are fibrillary contractions of muscles, tremors, spasmodic
contractions and paralyses of groups of muscles and not infrequently
convulsive seizures and coma. During the convulsion there is an
acceleration of the heart-beat, and increase in the respiratory move-
ments which frequently become dyspneic in character. There is also
a loss of appetite, nausea, mucous vomiting, and diarrhea. Death
may occur during a convulsion or from coma. (Morat and Doyon.)

These results for the most part occur only when all the parathy-
roids are removed. It is asserted that even if one gland is retained
the animal does not die. The above described symptoms may mani-
fest themselves, however, but they are slight in degree.

Vincent and Jolly have recently published the results of a series
of experiments which seem to negative to some extent the preceding
statements. These experimenters state that while it is true, that, as a
rule, the removal of both thyroids and parathyroids in the carnivora
is a fatal operation, there are nevertheless many exceptions; and in
the mammalia generally, e.g., cats, dogs, foxes, guinea-pigs, rats and
monkeys, the exception becomes the rule as more than 51 per cent, of
animals survived the operation for a prolonged period and of these 68
per cent, showed no specific symptoms of any kind. From the con-
tradictory observations it is evident that the subject needs further in-
vestigation.

The Pituitary Body. — This is a small body lodged in the sella
turcica of the sphenoid bone. It consists of an anterior lobe, somewhat
red in color, and a posterior lobe, yellowish-gray in color. (Fig. 214.)
The former is much the larger and partly embraces the latter. The
anterior lobe is developed from an invagination of the epiblast of the
mouth cavity, and consists of distinct gland tissue. The posterior
lobe is an outgrowth from the brainj and is connected with the in-
fundibulum by a short stalk. It has been suggested that the term
infundibular body be reserved for the posterior lobe, and the term
hypophysis cerebri for the anterior lobe. This distinction appears
to be desirable, inasmuch as in their origin and structure they are
separate and distinct bodies.

SECRETION.

467

Removal of the hypophysis cerebri, or the pituitary body, is always
followed by a fatal result, preceded by symptoms not unlike those
which follow removal of the thyroid: viz., anorexia, tremors, spasms,
etc. Degeneration of the pituitary body has been found in connection
with a hypertrophic condition of the bones of the face and extremities
to which the term acromegalia has been given.

Intravenous injection of an extract of the pituitary increases
the force of the heart-beat without any change in its frequency, and
causes a rise of blood-pressure from a stimula-
tion of the arterioles (Schafer and Oliver). The
material secreted by the pituitary has not been
isolated, hence its chemic features are unknown.
After its formation it probably passes through a
system of ducts into the cerebrospinal fluid, after
which it influences the metabolism of the nerve
and osseous tissues as well as the force of the
heart-muscle.

An extract of the anterior lobe itself exerts no
appreciable effect on the blood-pressure or on
the rate of the heart-beat, nor does it influence
the circulatory and respiratory organs. An ex-
tract of the infundibular body intravenously
injected, however, gives rise to increased blood-
pressure and to a slowing of the heart-beat
(Howell).

Adrenal Bodies, or Suprarenal Capsules.
— These are two flattened bodies, somewhat
crescentic or triangular in shape, situated each
upon the upper extremity of the corresponding
kidney, and held in place by connective tissue.
They measure about 40 mm. in height, 30 mm.
in breadth, and from 6 to 8 mm. in thickness.
The weight of each is about 4 gm.

Function of the Adrenal Bodies. — It was observed by Addison
that a profound disturbance of the nutrition, characterized by a bronze-
like discoloration of the skin and of the mucous membranes of the
mouth, extreme muscular weakness, and profound anemia, was asso-
ciated with, if not dependent on, pathologic conditions of the suprarenal
glands. In the progress of the disease the asthenia gradually increases,
the heart becomes weak, the pulse small, soft, and feeble, indicating a
general loss of tone of the muscular and vascular apparatus. Death
ensues from paralysis of the respiratory muscles. The essential nature
of the lesion which gives rise to these symptoms has not been deter-
mined.

Removal of these bodies from various animals is invariably and
in a short time followed by death, preceded by some of the symptoms
characteristic of Addison's disease. Their development, however,

Fig. 214. — Sagittal
Section* of the Pitu-
itary Body and In-
fundibulum with ad-
JOINING Part of Third
Ventricle, a. Ante-
rior lobe. a'. A projec-
tion from it toward the
front of the infundibu-
lum. b. Posterior lobe
connected by a stalk
with the infundibulum,
i. I.e. Lamina cinerea.
0. Right optic nerve.
ch. Sectionofoptic
chiasm, r.o. Recess of
ventricle above the
chiasma. cm. Corpus
mammillare.-(Sc/',"<:,(7rtf,
from Qua in.)

468 TEXT-BOOK OF PHYSIOLOGY.

is more acute. From the fact that animals so promptly die after ex-
tirpation of these bodies, and the further fact that the blood of such
animals is toxic to the subjects of recent extirpation, but not to normal
animals, the conclusion was drawn that the function of the adrenal
bodies is to remove from the blood some toxic product of muscle
metabolism. Its accumulation after extirpation gives rise to death
through auto-intoxication.

On the supposition that the adrenals might secrete and pour into
the blood a specific material which favorably influences general
metabolism, Schafer and Oliver injected hypodermi'cally glycerin
and water extracts, and observed at once an increased activity of the
heart-beats and of the respiratory movements. The effects, however,
were only transitory. When these extracts are injected into the veins
directly, there follows in a short time a cessation of the auricular
contraction of the heart, though the ventricular contraction continues
with an independent rhythm. If the vagi are cut previous to the
injection or if the inhibition is removed by atropin, the rapidity and
vigor of both auricles and ventricles are increased. Whether the
inhibitory influence is removed or not, there is a marked increase in
the blood-pressure, though it is greater in the former instance. This
is attributed to a direct stimulation and contraction of the muscle-
fibers of the arterioles themselves, and not to vaso-motor influences,
as it occurs also after division of the cord and destruction of the bulb.
The contraction of the arterioles is quite general, as shown by plethys-
mographic studies of the limbs, spleen, kidney, etc. Applied locally
to the mucous membranes, adrenal extract produces contraction of
the blood-vessels and pallor. The skeletal muscles are affected by
the extract very much as they are by veratrin. The duration of a
single contraction is very much prolonged, especially in the phase of
relaxation or of decreasing energy.

It is evident from these experiments that the adrenal bodies are
engaged in elaborating and pouring into the blood a specific material
which stimulates to increased activity the muscle-fibers of the heart
and arteries, and thus assist in maintaining the normal blood-pres-
sure as well as the tonicity of the skeletal muscles. An alkaloid al
substance was isolated by Abel from extracts of this gland, to which
the term epinephrin was given. A crystallizable substance was iso-
lated first by Takamine and later by Aldrich, to which the term adren-
alin was given. Both substances are apparently equally efficacious
in causing contraction of the blood-vessels and in raising the blood-
pressure. The question as to which of these two substances represents
the active principle of the gland is as yet a subject of discussion.

The Spleen. — The spleen is a soft bluish-red organ, oval in shape,
from twelve to fifteen centimeters long by eight broad and four thick.
It is situated in the left hypochondrium between the stomach and the
diaphragm. In this situation it is held in position by a fold of the perit-
oneum which passes from the upper border to the diaphragm.

SECRETION.

469

Structure. — A section of the spleen shows that it consists of con-
nective tissue, blood-vessels, lymph-corpuscles, and lymphoid tissue.
The surface of the spleen is covered by a capsule composed of dense
fibrous tissue, from the inner surface of which septa or trabecular
pass inward toward the center of the organ. In their course they give
off a series of processes which unite freely, forming a spongy connective-
tissue framework. The capsule and the main trabecular in some
animals contain numerous non-striated muscle-fibers. In man they
are relatively few in number. The blood-vessels which enter the spleen
are supported by the connective-tissue septa. As they pass toward the
center of the organ they divide very rapidly and soon diminish in size.
In their course small branches are given off, which penetrate the inter-
trabecular tissue and be-
come encased with spheric
or cylindric masses of
adenoid tissue known as
Malpighian corpuscles.
These corpuscles are com-
posed largely of leukocytes.
In some animals the leuko-
cytes, instead of being ar-
ranged in masses, are dis-
tributed along the walls of
the artery as a continuous
layer. Within the corpus-
cles the arteries pass into
capillaries; whether the
artery passes directly to the
splenic pulp or indirectly
by way of the corpuscles,
its ultimate branches ter-
minate in capillaries which

open into the spaces of the splenic pulp. From these spaces a net-
work of venules gathers the blood and transmits it to the veins. It is
a disputed question as to whether the spaces are lined by epithelium,
thus forming a continuous blood channel, or whether they are wanting
in this histologic element.

The Splenic Pulp. — The spaces of the connective-tissue frame-
work are filled with a dark red semifluid mass known as the splenic
pulp. When microscopically examined, the pulp presents a fine
loose network of adenoid tissue, large numbers of leukocytes or lymph-
corpuscles, red corpuscles in various stages of disintegration, and
pigment granules. Chemic analysis reveals the presence of a number
of nitrogen-holding bodies, e. g., leucin, tyrosin, xanthin, uric acid;
organic acids, e. g., acetic, lactic, succinic acids; pigments containing
iron, and inorganic salts.

The Functions of the Spleen. — Notwithstanding all the ex-

Fig. 215. — Malpighian Corpuscle of a Cat's
Spleen Injected, a. Artery, b. Meshes of the
pulp injected, c. The artery of the corpuscle rami-
fying in the lymphatic tissue composing it.

47°

TEXT-BOOK OF PHYSIOLOGY.

periments which have been made to determine the functions of the
spleen, it can not be said that any very definite results have been
obtained. The fact that the spleen can be removed from the body
of an animal without appreciably interfering with the normal metabo-
lism would indicate that its function is not very important. The
chief changes observed after such a procedure are an enlargement
of the lymphatic glands and an increase in the activity of the red
marrow of the bones. The presence of large numbers of leukocytes
in the splenic pulp and in the blood of the splenic vein suggested
the idea that the spleen is engaged in the production of leukocytes,
and to this extent contributes to the formation of blood. The presence

of disintegrated red blood-
corpuscles has suggested
the view that the spleen
exerts a destructive action
on functionally useless red
corpuscles. These and
other theories as to splenic
functions have been offered
by different observers, but
all are lacking positive con-
firmation.

Volume Variations of
the Spleen. — It was shown
some years since by Roy,
with the aid of the plethys-
mograph, that the spleen
undergoes rhythmic varia-
tions in volume from mo-
ment to moment. In the
cat and in the dog the diminution in the volume (the systole) and the
increase in volume (the diastole) together occupied about one minute.
This fact was determined by withdrawing the spleen through
an opening in the abdominal wall and enclosing it in a box with
rigid walls, the interior of which was connected with a piston record-
ing apparatus. The system being filled with oil, each variation
in volume was attended by a to-and-fro displacement and a cor-
responding movement of the recording lever. The special form of
plethysmograph used for this purpose is known as the oncometer or
bulk measurer, and the recording apparatus as the oncograph (Fig.
216 and Fig. 222;.

The cause of these variations in volume Roy attributed to a rhyth-
mic contractility of the non-striated muscle-fibers in the capsule and
trabecular, and not to changes in the arterial blood-pressure, as the
curve of the pressure taken simultaneously remained practically
uniform. The effect of the rhythmic contractions of the splenic
muscle tissue is to force the blood through the organ, a condition

Fig. 216. — Spleen Oncometer Laid Open.

SECRETION. 471

necessitated perhaps by the pressure relations within, though what
function is thereby fulfilled is not apparent.

It was subsequently shown by Schafer and Moore that the splenic
volume is extremely responsive to all fluctuations of the arterial blood-
pressure; that though the spleen may passively expand and recoil in
response to the rise and fall of the blood-pressure, nevertheless the
reverse conditions may obtain: viz., that the splenic volume may
diminish as the pressure rises, if the splenic arterioles contract simul-
taneously with the contraction of the arterioles generally. On the
contrary, the splenic volume may increase coincident with a dilatation
of the splenic and systemic arterioles. In addition to the rhythmic
variations, the spleen steadily increases in volume for a period of five
hours after digestion, and then gradually returns to its former con-
dition.

Influence of the Nerve System. — The nerves which supply the
vascular and visceral muscles in the spleen are derived directly
from the semilunar ganglion (post-ganglionic fibers) and pass to it
in company with the splenic artery. The nerve-cells from which
they arise are in physiologic relation with nerve-fibers (pre-ganglionic
fibers) which emerge from the spinal cord in the anterior roots of
the third thoracic to the first lumbar nerves inclusive, though they
are found most abundantly in the sixth, seventh, and eighth thoracic
nerves. Their center of origin is in the medulla oblongata.

Stimulation of the nerves in any part of their course gives rise
to a diminution in splenic volume; division of the nerves is followed
by an increase in the volume. In asphyxia the spleen is small and
contracted, a condition attributed to a stimulation of the centers in
the medulla by the venosity of the blood.

The musculature of the spleen may also be excited to contraction
by reflex influences, as shown by the fact that stimulation of the central
end of a sensory nerve is attended by a diminution of volume.

Inasmuch as the excised spleen will continue to exhibit variations
in volume when perfused with blood, it would appear that it possess
some mechanism independent to some extent of the nerve system.

CHAPTER XVIII.

EXCRETION.

As stated in the preceding chapter, the term excretion is limited to
the process by which the end-products of tissue metabolism are re-
moved from the body, the nature of the process, however, differing
in no essential particulars from that underlying the process of secre-
tion. The histologic structures involved and the forces at work
being of the same general character, it is impossible to draw any
sharp line of distinction between them. As a general fact it may
be stated that in their composition all the characteristic ingredients
of the excretions are incapable either of entering into the formation
of tissue or of undergoing oxidation for the purpose of heat-production.
As the retention of these end-products in the body would exert a
deleterious influence on normal metabolism, their prompt removal
becomes essential to the maintenance of physiologic activity. The
principal excretions of the body — urine, perspiration, and bile — are
complex fluids in which, with the exception of those given off in the
lungs, are to be found in varying proportions the chief end-products
of metabolism.

THE URINE.

Normal urine has a pale yellow or amber color, an aromatic
odor, an acid reaction, and a specific gravity of 1.020. As a rule, it
is perfectly transparent, though its transparency may be diminished
from the presence of mucus, calcium and magnesium phosphates, and
mixed urates.

The color, which varies within physiologic limits from a pale
yellow to a reddish-brown, is due to the presence of the coloring-
matters urobilin, urochrome, and uroerylhrin, all of which are de-
rivatives from the bile pigments absorbed from the liver or the alimen-
tary canal.

The reaction of the urine is acid, owing to the presence of the acid
phosphates of sodium and calcium. The degree of acidity, however,
varies at different periods of the day. Urine passed in the morning
is strongly acid, while that passed during and after digestion, espe-
cially if the food be largely vegetable in character and rich in alkaline
salts, is either neutral or alkaline in reaction. The diminished acidity
after meals is attributed to the formation of hydrochloric acid by the
gastric glands and the consequent liberation of bases which are ex-
creted in the urine. The phosphoric acid which enters into com-

472

EXCRETION. 473

bination with sodium and potassium bases is a product of tissue
metabolism.

The specific gravity is about 1.020, though it varies from 1.015
to 1.025. It will diminish, other things being equal, with increased
consumption of water and diminished activity of the skin; it will be
increased of course by the opposite conditions.

The quantity of urine excreted in twenty-four hours varies from
1200 to 1700 c.c. Amounts both above and below these are fre-
quently passed from a variety of causes.

The odor of the urine is characteristic and due to the presence of
aromatic compounds.

COMPOSITION OF URINE.

Water, 1500.00 c.c.

Total solids, 72.00 grams.

Urea, 33-iS "

, Uric acid (urates) , 0.55 "

Hippuric acid (hippurates), .' 0.40 "

Kreatinin, xanthin, hypoxanthin, guanin, ammo- \ «

nium salts, pigment, etc.
Inorganic salts: sodium and potassium sulphates, ]
phosphates, and chlorids; magnesium and cal- |
cium phosphates, \ 27.00 "

Organic salts: lactates, acetates, formates in small |
amounts,

Sugar, a trace

Gases, nitrogen, and carbonic acid.

The estimation of total urinary solids in any given sample of
urine is frequently a matter of clinical interest. This may approxi-
mately be attained by multiplying the last two figures of the specific
gravity by the coefficient of Haeser or Christison, 2.33. The result
expresses the total solids in 1000 parts: e. g., urine with a specific
gravity of 1.020 would contain 20 X 2.33, or 46.60 grams of solid
matter per 1000 c.c. If the amount passed in twenty-four hours be
1500 c.c, the total solids would amount to 69.9 grams.

The Water of the Urine. — The amount of urinary water and its
ratio to the solid constituents will vary with the amount consumed
and the activity of the skin and lungs. In summer the foods, liquid
and solid, remaining the same, the quantity of water in the urine is
diminished in consequence of increased activity of skin and lungs
and the ratio of water to solids decreased. In winter the reverse
conditions obtain. The food remaining the same, the consumption
of large quantities of water hastens at least the removal of end-products
from the tissues and thus increases the urinary solids.

Urea is the most abundant of the organic constituents of the
urine and is present to the extent of from 2 to 3 per cent. It is a
colorless neutral substance, crystallizing under varying conditions
in long silky needles or in rhombic prisms. It is soluble in water
and alcohol. It is composed of CON,H4. When subjected to pro-
longed boiling, it combines with water, "iving rise to ammonium

474 TEXT-BOOK OF PHYSIOLOGY.

carbonate. The presence of Micrococcus urea in urine will also
convert the urea, by combining it with two molecules of water, into
ammonium carbonate, CON2H4 + 2H20 = (NH4)2C03.

The average amount of urea excreted daily varies from 30 to 34
grams. As urea is now known to be the principal end-product of
proteid metabolism within the body, it is evident that the quantity
produced and eliminated in the twenty-four hours will depend on the
quantity of proteid food consumed and on the extent to which the
proteid constituents of the tissues are metabolized. In the condition of
nutritive equilibrium, when the proteid ingested is 100 grams and the
urea egested 31.5 grams, it is difficult to state the percentage of urea
which is derived from the metabolism of the proteid food (circulating
proteid) and that derived from the metabolism of the proteids of the
tissues (organ proteid). In this condition, however, it is found that
if the proteid consumed is varied within limits above or below the
standard amount of 100 grams, the quantity of urea excreted rises and
falls in practically the same ratio, indicating apparently that the
production of urea is directly dependent on the proteid supply. On
the contrary, it has been observed in human beings in the fasting
condition that for a period of ten days there is a daily excretion of
about 21 grams of urea, equivalent to about 70 grams of proteid.
Again, contrary to former views, the metabolism of proteid and the
production of urea are practically independent of muscular work.
Even after severe labor extending over a period of some hours there
is no noticeable increase in the urea eliminated.

Seat of Urea Formation. — It is quite certain in the light of present
knowledge that urea is partly formed in the liver by the action of
the cells out of cleavage products of proteid metabolism. The par-
ticular compounds out of which the cells synthetize urea are the
ammonium salts, especially the carbamate and carbonate. The
experimental reasons for this view have already been stated on page
462.

Uric acid is one of the constant ingredients of the urine. It is a
crystalline nitrogen-holding body closely resembling urea, its formula
being CSH4N403. The total quantity excreted daily varies from 0.2
to 1 gram. It is doubtful if uric acid exists in a free state in the
urine, the indications being that it is combined with sodium and
potassium in the form of a quadriurate. The urates are frequently
deposited when in excess from the urine as a brick-red sediment,
the color being due to their combination with the coloring-matter
uroerythrin. When pure, uric acid crystallizes in the rhombic form,
though it assumes a variety of forms. Uric acid was long regarded
as a product of general proteid metabolism and for chemic reasons
an antecedent of urea. This view has been abandoned. At present
it is believed that it is a cleavage product of nuclein, a constituent
of all cell nuclei. In the metabolism of nuclein a proteid and nucleic
acid are formed, from the latter of which uric acid is derived. Nu-

EXCRETION. 475

cleic acid when decomposed yields a series of bases, such as xanthin,
hypoxanthin, adenin, guanin, etc. Because of the fact that these
bodies can also be obtained from a synthetized body termed purin
they are known collectively as the purin bases. Though there is a
close relationship between uric acid and the purin bases, it has been
impossible to experimentally derive one from the other. When hy-
poxanthin, however, is given internally it is oxidized and converted
into uric acid. It is extremely probable, therefore, that uric acid is
an oxidation product of one or more of the purin bases.

It is probable, however, that not all of the uric acid eliminated is
derived from the nuclein of tissue-cells and their decomposition
products, the purin bases. Some of it is undoubtedly derived from
the nucleins contained in foods. The uric acid eliminated is there-
fore partly endogenous and partly exogenous in origin.

Xanthin, hypoxanthin, guanin, etc., are also found in urine
in small but variable amounts. They are nitrogenized compounds
derived mainly from the metabolism of the nuclein bodies.

Kreatinin is a crystalline nitrogenous compound closely resem-
bling kreatin, one of the constituents of muscular tissue. The amount
excreted daily is about i gram. Though kreatinin may arise in conse-
quence of proteid metabolism, it is probable that it is largely derived
from a transformation of the kreatin contained in the meat consumed
as food.

Hippuric acid in combination with sodium and potassium is
very generally present in urine, though in small amounts. It is more
abundant in the urine of the herbivora than the carnivora. In man
the amount excreted daily is about 0.7 gram, though the amount may
be raised by a diet of asparagus, plums, cranberries, etc., and by the
administration of benzoic and cinnamic acids. There is evidence that
hippuric acid is formed in the kidney from benzoic acid, its pre-
cursors, or related bodies. Various compounds of this class are found
in vegetable foods, a fact which may account for the increase in the
excretion of hippuric acid on a vegetable diet.

Leucin, tyrosin, phenol, cystin, indoxyl, skatoxyl, are found
in small amounts even under normal conditions. They arise from
putrefactive change in the intestine.

Inorganic Salts. — Sodium and potassium phosphates, known
as the alkaline phosphates, are found in both blood and urine. The
total quantity excreted daily is about 4 grams. Calcium and mag-
nesium phosphates, known as the earthy phosphates, are present to
the extent of 1 gram. Though insoluble in water, they are held in
solution in the urine by its acid constituents. If the urine be rendered
alkaline, they are at once precipitated. Sodium and potassium
sulphates are also present to the extent of about 2 grams. The phos-
phoric and sulphuric acids which are combined with these bases enter
the body for the most part in the foods, though there is evidence that
they also arise by oxidation in consequence of the metabolism of proteids

476

TEXT-BOOK OF PHYSIOLOGY.

which contain phosphorus and sulphur. Sodium chlorid is the most
abundant of the inorganic salts. It is derived mainly from the food.
The amount excreted is about 15 grams in twenty-four hours.

THE KIDNEYS.

The kidneys are the organs engaged in the excretion of the urinary
constituents from the blood. They resemble a bean in shape, are

from 10 to 12 centi-
meters in length, 2 in
breadth, and weigh
from 144 to 170
grams. They are situ-
ated in the lumbar
region, one on each
side of the vertebral
column behind the
peritoneum, and ex-
tend from the eleventh
rib to the crest of the
ilium. The anterior
surface is convex, the
posterior surface con-
cave. The latter pre-
sents a deep notch —
the hilum. The kid-
ney is surrounded by
a thin smooth mem-
brane composed of
white fibrous and
yellow elastic tissue;
though it is attached
to the surface of the
kidney by minute
processes of connec-
tive tissue, it can very
readily be torn away.
The substance of the
kidney is dense but
friable.

Upon making a
longitudinal section
of the kidney it will
be observed that the
hilum extends into the interior of the organ and expands to form a
cavity known as the sinus, in which are found the blood-vessels, nerves,
and duct (Fig. 217). This cavity is mainly occupied by the upper
part of the renal duct, the ureter, the interior of which is termed the

Fig. 217. -—Longitudinal Section through the
Kidney, the Pelvis of the Kidney, and a Number
of Renal Calyces. A. Branch of the renal artery.
U. Ureter. C. Renal calyx, i. Cortex, i'. Medullary
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 sec-
tion through the axes of the tubules of the border layer.
4. Fat of the renal sinus. 5, 5. Arterial branches.
*. Transversely coursing medulla rays. — {Tyson, after
Henle.)

EXCRETION. 477

pelvis. The ureter divides into several portions which terminate in
small caps of calyces which receive the apices of the pyramids. The
parenchyma of the kidney consists of two portions: viz. —
i. An internal or medullary portion, consisting of a series of pyramids
or cones, some twelve or fifteen in number, which present a dis-
tinctly striated appearance.
2. An external or cortical portion, half an inch in thickness and dis-
tinctly friable in character.
The Histology of the Kidney. — The kidney is composed of a
connective-tissue framework supporting secreting tubules, blood-
vessels, lymphatics, and nerves, all of which are directly connected
with the removal of the urinary constituents from the blood. The
kidney is structurally a compound tubular gland. If the apex of
each pyramid be examined with a lens, it will present a number of
small orifices which may be regarded as the beginnings of the urinifer-
ous tubules. From this point the tubules pass outward in a straight
but somewhat diverging manner toward the cortex, giving off at
acute angles a number of branches (Fig. 218). From the apex to the
base of the pyramids they are known as the tubules of Bellini. In
the cortical portion of the kidney the tubule becomes enlarged and
twisted, and, after pursuing an extremely convoluted course, turns
backward into the medullary portion for some distance, forming the
ascending limb of Henle's loop; it then turns upon itself, forming the
descending limb of the loop, reenters the cortex, again expands and
becomes convoluted, and finally terminates in an ovoid enlargement
known as Midler's or Bowman's capsule, in which is contained a
small tuft of blood-vessels — the glomerulus. Each tubule consists of
a basement membrane lined throughout its entire extent by epithelial
cells. The epithelium as well as the tubule vary in shape and size in
different parts of its course. In the capsule the epithelium is flattened,
lining not only the inner surface of the capsule but reflected over
the blood-vessels as well. This is known as the glomerular epi-
thelium. In the convoluted portions of the tubules the epithelium
is cuboidal, granular, and somewhat striated; in Henle's loop it is
more or less flattened.

The Blood-vessels of the Kidney. — The renal artery enters the
kidney at the hilum behind the ureter; it soon divides into several
large branches which penetrate the substance of the kidney between
the pyramids and pass outward into the cortex. At the base of the
pyramids branches of the arteries form an anastomosing plexus.
From this plexus vessels are given off, some of which follow the straight
tubules toward the apex of the pyramids, vasa recta, while others
enter the cortex and pass to its surface (Fig. 218). In the course of
the latter small branches are given off, each of which soon divides and
subdivides to form a ball of capillary vessels known as the glomer-
ulus. These capillaries, however, do not anastomose, but soon re-
unite to form an efferent vessel the caliber of which is less than that

478

TEXT-BOOK OF PHYSIOLOGY.

of the afferent artery. In consequence of this, there is a greater re-
sistance to the outflow of blood than to the inflow, and therefore a
higher blood-pressure in the glomerulus than in capillaries generally.
The relation of the glomerulus to the tubule is important from a

Lobule.

Lobule.

Tunica albuginea.

Renal corpuscle. --___

Intarcalated
piece.

Thick.. _•___

Thin - ■'-]

division of thi
loop of Henk

Collecting
tubule.

Papillary rluct

Papilla.

Interlobular
artery.
— Interlobular
vein.

Arciform artery

Arcitorm vein.

-•Interlobar artery.
..Interlobar vein.

I p,. 218. Scheme of the Course of the Uriniferous Tubules and the Renal

Vessels.

EXCRETION.

479

physiologic point of view. As stated above, the glomerulus is re-
ceived into and surrounded by the terminal expansion or capsule of
the tubule. This capsule, formed by an indentation of the terminal
portion of the tubule, consists of two walls, an outer one consisting
of an extremely thin basement membrane, covered by flattened epi-
thelial cells, and an inner one consisting apparently only of flattened
epithelium which is reflected over and closely invests the glomerular
blood-vessels (Fig. 219). The blood is thus separated from the interior
of the capsule by the epithelial wall of the capillary and the epithelium
of the reflected wall of the
capsule. During the periods
of secretory activity the blood-
vessels of the glomerulus are
filled with blood to such an
extent that the sac cavity is
almost obliterated. After its
exit from the capsule the effer-
ent vessel of the glomerulus
soon again divides and sub-
divides to form an elaborate
capillary plexus which sur-
rounds and closely invests the
convoluted tubules. From
this plexus as well as from the
plexus which surrounds the
straight tubules veins arise
which pass toward and empty
into veins at the base of the
pyramids. The renal vein
formed by the union of these
latter veins emerges from the
kidney at the hilum and finally empties into the vena cava inferior.

The nerves of the kidney are derived from the renal plexus and
follow the course of the blood-vessels to their termination.

The Renal Duct. — The excretory duct of the kidnev, the ureter,
is a musculo-membranous tube about 5 mm. in diameter when dis-
tended, 30 cm. in length, and extends from the hilum to the base of
the bladder. The upper extremity is expanded and within the renal
sinus becomes irregularly branched, giving rise to a number of short
tubes, called calyces, each of which embraces the apex of a Malpighian
pyramid. The interior of the expanded portion of the ureter is
known as the pelvis. The wall of the ureter consists of a mucous
membrane, a muscle coat, and an external fibrous investment.

MECHANISM OF URINE SECRETION.

The secretion of urine is a complex process and susceptible of
several interpretations. It was originally inferred by Bowman that,

Fig. 219. — Scheme of the Renal or Mal-
pighian Corpuscle, i. Interlobular artery.
2. Afferent vessel. 3. Efferent vessel. 4. Outer
wall. 5. Inner wall. 6. Glomerulus. 7. Neck
of tubule. — (Stohr.)

4So TEXT-BOOK OF PHYSIOLOGY.

as the kidney presents anatomically an apparatus for nitration, the
capsule with its enclosed glomerulus, and an apparatus for secretion,
the epithelium of the urinary tubules, the elimination of the urinary
constituents from the blood is accomplished by the two processes of
filtration and secretion; that the water and highly diffusible inorganic
salts simply pass by diffusion, under pressure, though the walls of
the glomerular capillaries, while the organic constituents are removed
by the epithelium lining the tubules.

Influenced largely by the facts of blood-pressure Ludwig advanced
the view that the factors concerned in the secretion of urine were
purely physical; that in consequence of the high pressure in the vessels
of the glomeruli, due to the resistance offered by the smaller efferent
vessel, all the urinary constituents were filtered off in a state of extreme
dilution. In order to account for the higher percentage of the organic
constituents in the urine, it was assumed that as the dilute urine passed
through the tubules the water was partly reabsorbed, passing by
diffusion into the lymph and blood until the urine acquired its normal
characteristics. In support of this view, a large number of facts
relating to the influence of an increase and a decrease of pressure in
the blood-vessels of the glomeruli, the velocity of the blood-stream,
etc., in determining the rate of urinary flow were adduced, all of which
apparently indicated that the former stood to the latter in the relation
of cause and effect, and that the formation of urine was accomplished
entirely by physical forces.

The progress of physiologic investigation, however, has thrown
some doubt on the validity of this physical interpretation, and has
rather served to support the view of Bowman that the organic con-
stituents at least are removed from the blood by a process of selection
on the part of the epithelium of the convoluted part of the urinary
tubules; in other words, that the secretion of urine is physiologic
rather than physical. Heidenhain has brought forward a series of
facts which support this view. As evidence that the cells possess a
selective power, he presents the following experiment: The spinal
cord of an animal is divided in the neck for the purpose of lowering the
blood-pressure in the kidney below the pressure at which the urine is
secreted; a solution of incligo-carmine is injected into the blood-
vessels; after the lapse of ten minutes the animal is killed, the blood-
vessels washed out with alcohol for the purpose of precipitating the
indigo-carmine in situ. Section of the kidney shows a uniform blue
stain of the cortex alone. Microscopic examination reveals the fact
that the blue stain is due to the deposition of the pigment in the lumen
and in the lumen border of the cells of the convoluted tubules and
the ascending limb of Henle's loop; while the epithelium of Bow-
man's capsule as well as the glomerular epithelium present no evi-
dence of pigmentation.

Nussbaum attempted to establish the secretory power of the epi-
thelium in another way. in the frog the kidney receives blood from

EXCRETION. 481

two sources: the glomeruli receive their blood from the renal artery,
the tubules from the capillaries formed by the anastomosis of branches
of the efferent vessel of the glomerulus and the branches of the renal
portal vein. Nussbaum believed that by ligating the renal artery
all glomerular activity could be abolished and the part played by the
epithelium could be established. After so doing the flow of urine was
at once checked; the injection of urea at once reestablished it. This
fact was taken as a proof that the tubular epithelium not only ex-
creted urea, but water and perhaps other constituents as well. It
was also found that sugar, peptones, carmine, etc., which are always
eliminated from the blood under normal conditions, are not removed
after ligation of the renal artery. It was concluded from these ex-
periments that the secreting structures of the kidney consist of two
distinct systems, the glomerular and the tubular; the former secreting
water, salts, sugar, peptone, etc. ; the latter urea, uric acid, etc. These
and similar facts indicate that the renal epithelium possesses a secretory
rather than an absorptive function. Heidenhain and those who
agree with him assert that even the water and inorganic salts which
pass through the glomerular epithelium do so in consequence of cell
selection and cell activity; that the entire process is one of secretion,
though conditioned by blood-pressure, blood velocity, etc.

Influence of Blood-pressure. — Whether the elimination of the
urinary constituents is entirely secretory (physiologic) in character
or not there can be no doubt that the whole process is largely deter-
mined by the pressure and velocity of the blood in the glomerular
capillaries, or, to state it more accurately, on the difference of pres-
sure between the blood in the capillaries and the urine in the capsules.
As a rule, this latter pressure is at a minimum. If the urine should
accumulate in the ureter and tubules either from ligation or mechan-
ical obstruction until its pressure approximates that of the blood, the
secretion would be diminished if not abolished. It is difficult to
determine the average pressure or velocity of the blood in the glomers
ular capillaries, though they both must be greater than in capillarie-
in other parts of the body, from the fact that the efferent vessel is
narrower than the afferent, and therefore offers great resistance to the
outflow of blood, a condition most favorable to the production of a
high pressure in the glomerulus.

The pressure of the blood in the glomeruli may be raised and the
velocity increased :

1. By an increase in blood-pressure generallv.

2. By an increase in the pressure of the renal artery alone.

The first condition may be brought about by an increase in either
the force or frequency of the heart's action or by a contraction of the
arterioles of vascular areas in any or all parts of the body, excepting,
of course, the renal vascular area. The second condition is brought
about by a dilatation of the renal artery alone and possibly by a con-
traction of the efferent vessels of the crlomeruli.

TEXT-BOOK OF PHYSIOLOGY.

The pressure of the blood in the glomeruli may be diminished
and the velocity decreased:

i. Bv a decrease in the blood-pressure generally.
2. By a decrease in the pressure of the renal artery alone.

The first condition is brought about by a decrease in either the
force or frequency of the heart's action or by a dilatation of the arteri-
oles of large vascular areas in any or all parts of the body. The
second condition is brought about by contraction of the renal artery
alone and possibly by a dilatation of the efferent vessels of the glom-
eruli. The effect of the contraction and relaxation of either the

afferent or efferent vessels on the
pressure within the glomerulus
is shown in Figure 220.

Coincident with the rise and
fall of pressure in the glomerular
capillaries there is a rise and fall
in the rate of urinary flow. Thus
it has been found that an in-
crease in the aortic pressure from
127 to 142 mm. of mercury, by
ligation of the carotid, femoral,
and vertebral arteries, increased
the rate of urinary flow from
8.7 grams in thirty minutes to
21.2 grams. On the contrary, a
decrease in aortic pressure below
40 mm. of mercury caused by
division of the spinal cord is fol-
lowed by a total abolition of the
urinary flow. These facts serve
to indicate the dependence of
the secretion on blood-pressure.
That there is an increase in

Bowm
Caps

r ^=^

Fig. 220. — To Illustrate the Effect
of Active Changes in the Vasa Affer-
entia and efferentia on the pressure
in the Glomerular Capillaries. A. Re-
nal arteries. G. Glomerular capillaries.
C. Tubular capillaries. V. Vein. The short
thick lines represent the vasa afferentia and
efferentia. The continuous heavy line repre-
sents the mean average pressure. If the vas
afferens dilates and the vas efferens contracts
separately or conjointly, the pressure will rise,
as indicated by the upper dotted line. If the
vas afferens contracts and the vas efferens
dilates separately or conjointly, the pressure
will fall, as indicated by the lower dotted
line. — {After Moral.)

the volume of the blood flowing through the kidney during its functional
activity is apparent from inspection. It is enlarged, swollen, and red
in color. The blood in the renal vein is bright red in color and con-
tains more oxygen and less carbon dioxid than venous blood generally.
During the intervals of activity the kidney diminishes in size, is pale
in color and the blood of the renal vein dark and venous in character.
These variations in the volume of the kidney have also been experi-
mentally determined and registered by means of the oncometer and
. oncograph devised by Roy.

The oncometer consists of a metallic box (Fig. 221) composed
of halves which open and close by means of a hinge. It is connected
with a recording apparatus, the oncograph (Fig. 222), through the
tube T. The kidney, withdrawn from the body, is placed within the
oncometer. Through an opening in the side pass the artery, vein,

EXCRETION.

483

and ureter. Between the kidney and the wall of the capsule there is
placed a thin membrane. Oil is then poured through the side tube
I until the space between the capsule and the kidney, as well as the
tube leading to the chamber of the oncograph, are completely filled.
When the tube I is closed, the conditions are such that all variations
in the volume of the kidney are taken up and reproduced by the
recording lever attached to the piston of the oncograph. A curve

- -,Fig. 221. — Oncometer. K. Kidney; the thick line is the metallic capsule. h. Hinge.
I. .Tube for filling apparatus. T. Tube to connect with T/; a, v, u. Artery, vein,
ureter. — (Stirling, after Roy.)

Fig. 222. — Oxcograph. C. Chamber filled with oil, communicating by T, with
T. p. Piston. /. Writing-lever. — (Stirling, after Roy.)

of the variations in the volume of the kidney is shown in Figure 223
taken simultaneously with the curve of the blood-pressure. An
examination of this curve shows that the volume-changes coincide
with changes in the blood-pressure, exhibiting not only the respiratorv
but also the cardiac undulations.

Influence of the Nerve System. — The influence of the nerve

B.P.

Fig. 223. — B. P. Blood-pressure curve. K. Curve of the volume of the kidnev.
T. Time curve; intervals indicate a quarter of a minute. A. Abscissa. — (Stirling, after
Roy.)

system in regulating the blood-supply to the kidney is evident from
the results of experimentation. If the nerves which accompanv
the renal artery into the kidney are divided, the artery at once dilates,
the kidney enlarges, and a copious flow of urine takes place. If the
peripheral ends of these nerves be stimulated with the induced electric

4S4 TEXT-BOOK OF PHYSIOLOGY.

current, the artery contracts, the kidney diminishes in size, and the
flow of urine ceases. In addition to these vaso-constrictor nerves,
there is evidence that the kidney also receives vaso-dilator nerves
which emerge from the spinal cord and are found in the anterior roots
of the eleventh, twelfth, and thirteenth dorsal nerves, in the dog.
Direct and reflex stimulation of these nerves gives rise to a dilatation
of the artery, a swelling of the kidney, and an increase in secretion,
independent of any variation in general blood pressure.

The route of the vaso-constrictor nerves is, in the dog at least,
through the splanchnics. Section of these nerves is followed by a
dilatation of the renal vessels and an increase in the flow of urine.
Stimulation of the peripheral ends is followed by a constriction of the
vessels and a cessation of the flow of urine.

The vaso-motor center for the blood-vessels of the kidney is in all
probability situated in the medulla oblongata in close proximity to
the general vaso-motor centers, though subordinate centers are doubt-
less present in the spinal cord. It was found by Bernard that puncture
of the medulla was occasionally followed by a profuse secretion of
urine without the presence of sugar. The route of the vaso-motor
impulses which influence the renal blood-supply is down the cord
through the splanchnics and through the renal plexus.

Influence of Variations in the Composition of the Blood. —
As it is the function of the kidneys to excrete water, inorganic salts,
and various end-products from the blood and thus maintain a gen-
eral average composition, it is highly probable that as soon as they
accumulate beyond a certain percentage they themselves act as stimu-
lants to renal activity, either by acting directly on the renal epithelium
or by increasing the glomerular pressure. There is evidence at least
that urea acts in the former manner. An excess of water in the blood,
that from copious drinking or from a sudden checking of the skin from
a fall of temperature, will act in the latter way. The introduction into
the blood of inorganic salts, such as potassium nitrate, sodium acetate,
etc., will in a short time lead to increased activity of the .kidneys, as
shown by an increase in the quantity of urine excreted. The manner
in which these agents and other members of their class, the so-called
saline diuretics, increase renal activity is yet a subject of discussion.
On the one hand, it is stated that they promote an absorption of water
from the tissues to such an extent that a condition of hydremic plethora
is produced, which in itself increases not only the general blood-
pressure but the local renal pressure as well, and that it is this factor
which is the cause of the increased flow of urine. On the other hand,
it is asserted that though the salts increase the local pressure and the
volume of the kidney, they nevertheless act specifically on the renal
epithelium, and therefore may be regarded as secreto-motor agents.
An increase in the percentage of sugar or urea in the blood has a similar
influence on the kidney.

The Storage and Discharge of Urine.— Urination. — The urin-

EXCRETION. 485

ary constituents, as soon as they are eliminated from the blood,
pass into and through the uriniferous tubules and by them are dis-
charged into the pelvis of the kidney. They then enter the ureter by
which they are conducted to the bladder. The immediate cause of
this movement is undoubtedly a difference of pressure between the
terminal portions of the tubules and the terminal portion of the ureter,
aided by the peristaltic contraction of the muscle wall of the ureter.

The bladder is a reservoir for the temporary reception of the urine
prior to its expulsion from the body. When distended it is ovoid in
shape and is capable of holding from 600 to 800 cu. cm. The bladder
is composed of four coats: viz., serous, muscle, areolar, and mucous.
The muscle coat consists of external longitudinal and internal circular
and oblique layers of fibers of the non-striated variety which collec-
tively encircle the entire organ. As these fibers by their contraction
expel the urine from the bladder, they are known collectively as the
detrusor urincz muscle. At the exit of the bladder the circular fibers
are somewhat increased in number, giving rise to the appearance of a
distinct muscle which has been termed the sphincter vesica, muscle.
The presence of this muscle has, however, been denied and the reten-
tion of the urine has been attributed to mechanic conditions at the
neck of the bladder. The urethra just beyond the bladder is provided
with a distinct circular muscle composed of striated fibers, the sphincter
urethra muscle. When the urine passes into the bladder it is retained
there and prevented from escaping by the contraction of this latter
muscle. Under normal conditions the urine accumulates to a con-
siderable extent before the intra-vesic pressure gives rise to a charac-
teristic sensation and the desire for urination.

The Nerve Mechanism of Urination. — The muscle mechan-
isms which retain as well as expel the urine are under the control of
the nerve system. The sphincter urethrse muscle, which by the orifice
of the bladder is closed, is kept in a state of tonic contraction by nerve
impulses coming from the spinal cord through the anterior roots of
the third and fourth sacral nerves. The detrusor urinas muscle is
excited to contraction by impulses coming likewise through the sacral
nerves and through the upper lumbar nerves from the cord. The
centers of origin for these two sets of motor nerves are located in
the cord in the neighborhood of the fifth lumbar vertebra. The
expulsion of the urine is largely a reflex act, though under the con-
trol of the will. When the desire to urinate is experienced, nerve
impulses are coming through sensory nerves from the mucous mem-
brane of the bladder which are reflected to the centers governing
the sphincter urethras and detrusor urinas muscles and to the brain.
The effect of the reflected impulses is to inhibit the sphincter center
and to stimulate the detrusor center. If the act of urination is to be
permitted, volitional impulses descend through the spinal cord which
have the effect of still further inhibiting the sphincter center and
stimulating the detrusor center, the result being a relaxation of the

4S6 TEXT-BOOK OF PHYSIOLOGY.

sphincter muscle and a contraction of the detrusor muscle and the
expulsion of the urine. If the act of urination is to be suppressed,
volitional impulses inhibit the detrusor center and stimulate the
sphincter.

PERSPIRATION; SEBUM.

The perspiration or sweat, the chief secretion of the skin, is a
clear colorless fluid, slightly acid in reaction and saline to the taste.
Its specific gravity varies from 1.003 t0 i-°o6. Unless collected from
the soles of the feet and the palms of the hand, it is apt to be mixed
with epithelial cells and sebum. The total quantity of perspiration
secreted daily has been variously estimated at from 700 to 1000 grams;
the exact amount, however, is difficult of determination, for the reason
that the rate of secretion varies readily with variations in tempera-
ture, food, drink, season of the year, etc.

Chemic analysis of the sweat shows that it contains but from 0.5
to 2.5 per cent, of solid constituents, the variation in the percentage
depending on the quantity of water secreted. The solids consist
of traces of urea, neutral fats, lactic and sudoric acids in combination
with alkaline bases, and inorganic salts (Fovel). Other observers,
however, have not been able to detect the presence of either lactic
or sudoric acid. Urea is a constant ingredient, though its percentage
is extremely small, possibly not more than 0.1 per cent. The amount,
however, may be very much increased in uremic conditions, the
result of acute or chronic disease of the kidneys. The inorganic
constituents consist mainly of sodium chlorid and alkaline and earthy
phosphates. Carbonic acid is also present in the free state as well as
in combination with alkaline bases.

The very small quantity of the solid constituents in the sweat,
taken in connection with the fact that it is excreted most abundantly
when the external temperature is high, indicates that it is not so im-
portant as an excrementitious fluid as it is as a means for the regulation
of the temperature of the body.

The sweat is a product of the secretory activity of specialized
glands, the sweat-glands, embedded in the skin, to the histologic
structures of which they bear a special relation.

THE SKIN.

The skin is a complexly organized structure investing the entire
external surface of the body. Its total area varies from 16 to 20 feet
in man and from 12 to 16 feet in woman. It varies in thickness in
different localities of the body from -J- to yl(r of an inch. The skin
consists of two principal layers: viz., a deep layer, the derma or corium,
and a superficial layer, the epidermis.

The derma or corium may be subdivided into a reticulated and a
papillary layer. The reticulated layer consists of white fibrous and

EXCRETION.

'4S7

yellow elastic tissue, non-striated muscle-fibers, woven together in
every direction and forming an areolar network, in the meshes of
which are deposited masses of fat and a structureless amorphous
matter; the papillary layer consists mainly of club-shaped elevations
or projections of the amorphous matter constituting the papillae.
The reticulated layer serves to connect the skin with the underlying
structures and to afford support for the blood-vessels, nerves, and
lymphatics which are distributed to the papillae (Fig. 224).

The epidermis
is an extra-vascular
structure consist-
ing entirely of epi-
thelial cells. It
may also be sub-
divided into two
layers — the Mal-
pighian or pig-
mentary layer, and
the corneous or
horny layer. The
former is closely
applied to the pa-
pillary layer of the
true skin and is
composed of large
nucleated cells, the
lowest layer of
which, the "prickle
cells," contains the
pigment granules
which give to the
skin its varying
hues in different
individuals and in
different races of
men; the corneous
layer is composed
of flattened cells which from their exposure to the atmosphere, etc.,
are hard and horny in texture.

The Sweat-glands. — These glands are tubular in shape, the inner
extremity of each being coiled upon itself a number of times, forming
a little ball situated in the derma or the subcutaneous connective
tissue. From this coil the duct passes up in a straight direction to the
epidermis, where it makes a few spiral turns, after which it opens
obliquely on the surface. The gland consists of a basement membrane
lined with epithelial cells. It is supplied abundantly with blood-
vessels and nerves. The sweat-glands are extremely numerous all

j^W ^

Fig. 224. — Section Perpendicularly Through the
Healthy Skin. a. Epidermis or scarfskin. b. Rcte mu-
cosum, or rete malpighii. c. Papillary layer, d. Derma,
corium, or true skin. e. Panniculus adiposus, or fatty tis-
sue. /, g, h. Sweat-gland and duct, i, k. Hair, with its
follicle and papilla. /. Sebaceous gland.

488 TEXT-BOOK OF PHYSIOLOGY.

over the cutaneous surface, though they are more thickly disposed
in some situations than others. They probably average 2500 to the
square inch; the total number has been estimated at from 2,000,000
to 2,500,000.

The Influence of the Nerve System on the Production of
Sweat. — The secretion of sweat, though a product of the activity
of epithelial cells and dependent on a variety of conditions, is reg-
ulated to a large extent by the nerve system. Here as in other secreting
glands the fluid is derived from materials in the lymph-spaces, fur-
nished by the blood. Generally the two conditions, increased blood-
flow and increased glandular action, coexist. At times, however,
a profuse clammy perspiration is secreted with diminished blood-flow.
Two sets of nerves are evidently concerned in this process: viz.,
vaso-motor nerves, which regulate the blood-supply, and secret or
nerves, which stimulate the gland cells to activity.

The nerve-centers which control the sweat-glands are situated in
the spinal cord, though the number of such centers and their exact
location for the different regions of the body have not yet been satis-
factorily determined. In a general way it may be stated that the
centers for the head and face lie in the upper cervical portion of the
cord; for the upper extremities, in the lower cervical portion; for the
lower extremities, in the lower dorsal and upper lumbar portion.
The secretor nerves which emerge from these centers reach the glands
of the face and head through the cervical sympathetic; of the arms
and legs, through the brachial plexus and the sciatic nerves. It is
probable that there is also a general dominating sweat center located
in the medulla oblongata.

That the sweat-glands are stimulated to activity by nerve impulses
is shown by the fact that stimulation of the peripheral end of the
divided cervical sympathetic, of the brachial plexus, or of the sciatic
nerve is followed in a few seconds by a profuse secretion. Though
under physiologic conditions there is a simultaneous dilatation of the
blood-vessels and an increased supply of blood, this is merely a con-
dition and not a cause of the secretion; for the secretion can be excited
and the flow maintained for a period of from ten to fifteen minutes
after ligation of the blood-vessels of the limb or even after its ampu-
tation, when the corresponding nerve is stimulated.

The sweat-glands may be excited to activity by their related nerve-
centers, either by central, reflex, or peripheral influences. Among
the first may be mentioned mental emotions, venosity of the blood,
increased temperature of the blood, hot drinks, violent muscular
exercise, etc. Among the second may be mentioned powerful stim-
ulation of various afferent or sensor nerves, heightened external
temperature, etc. Among the last may be mentioned various drugs.
Pilocarpin injected into the blood causes a profuse secretion even
when the nerves have been divided. Its action is supposed to be
exerted on the terminal branches of the nerves and possibly on the

EXCRETION.

489

cells themselves. As in the case of the salivary glands atropin sus-
pends the activity of the terminal branches of the secretor nerves.

Hairs. — Hairs are found in almost all portions of the body, and
can be divided into — •

1. Long, soft hairs, on the head.

2. Short, stiff hairs, along the edges of the eyelids and nostrils.

3. Soft, downy hairs on the general cutaneous surface.

They consist of a root and a shaft. The shaft is oval in shape
and about -^j^ of an inch in diameter; it consists of fibrous tissue,
covered externally by a layer of imbricated cells, and internally by
cells containing granular and pigment material.

The root of the hair is embedded in the hair-follicle, formed by
a tubular depression of the skin, ex-
tending nearly through to the sub-
cutaneous tissue; its walls are formed
by the layers of the corium, covered
by epidermic cells. At the bottom of
the follicle there is a papillary projec-
tion of amorphous matter, corres-
ponding to a papilla of the true skin,
containing blood-vessels and nerves,
upon which the hair-root rests. The
investments of the hair-roots are
formed of epithelial cells, constituting
the internal and external root-sheaths.

The lower portion of the hair-
follicle is connected with the upper
surface of the derma by bundles of
non-striated muscle-fibers which are
termed arrectores pilorum muscles.
Their inclination and insertion are
such that their contraction is followed
by erection of the hair-follicle and hair-shaft. These muscles are ex-
cited to action by nerves termed pilo-motor nerves.

THE SEBUM.

The sebum or sebaceous matter is a peculiar oily material
produced by specialized glands in the skin. It consists of water,
epithelium, proteids, fat, cholesterin, and inorganic salts.

The sebaceous glands are simple and compound racemose
glands opening by a common excretory duct on the surface of the
epidermis or into the shaft of a hair-follicle (Fig. 225),. These glands
are extremely numerous and found in all portions of the body, with
the exception of the palms of the hands and soles of the feet, and most
abundantly in the face. They are formed by a delicate structureless
membrane lined by polyhedral epithelium.

The sebum is not produced by an act of true secretion, but is

Fig. 225. — Large Sebaceous
Gland, i. Hair in its follicle. 2, 3,
4, 5. Lobules of the gland. 6. Ex-
cretory duct traversed by the hair.
— (Sa'ppey.)

4QO TEXT-BOOK OF PHYSIOLOGY.

formed by a proliferation and degeneration of the gland epithelium.
"When first poured on the surface, the sebum is oily and semiliquid
in character, but soon hardens and acquires a cheese-like consistence.
It serves to lubricate the hair and skin and prevent them from be-
coming dry and harsh.

The surface of the fetus is generally covered with a thick layer of
sebaceous matter, the vernix caseosa, which possibly keeps the skin
in a normal condition by protecting it from the effects of the long-
continued action of the amniotic fluid in which the fetus is suspended.

CHAPTER XIX.

THE CENTRAL ORGANS OF THE NERVE SYSTEM AND
THEIR NERVES.

The central organs of the nerve system are the encephalon
and the spinal cord lodged within the cavity of the cranium and the
cavity of the spinal or vertebral column respectively. The general
shape of these two portions of the nerve system corresponds with
that of the cavities in which they are contained. The encephalon is
broad and ovoid, the spinal cord is narrow and elongated.

The encephalon is subdivided by deep fissures into four distinct,
though closely related portions: viz., (i) the cerebrum, the large ovoid
mass, occupying the entire upper part of the cranial cavity; (2) the
cerebellum, the wedge-shaped portion placed beneath the posterior
part of the cerebrum and lodged within the cerebellar fossae of the
cranium; (3) the isthmus of the encephalon, the more or less pyramidal-
shaped portion connecting the cerebrum and cerebellum with each
other and both with (4) the medulla oblongata. (Fig. 226.)

The spinal cord is narrow and cylindric in shape. It occupies
the spinal canal as far as the second or third lumbar vertebra. The
central nerve system is bilaterally symmetric, consisting of distinct
halves united in the median line. The cerebrum is subdivided by a
deep fissure, running antero-posteriorly, into two ovoid masses termed
cerebral hemispheres; the cerebellum is also partially subdivided into
hemispheres; the isthmus likewise presents in the median line a partial
division into halves; the medulla oblongata and spinal cord are sub-
divided by an anterior or ventral and a posterior or dorsal fissure into
halves, a right and a left.

The peripheral organs of the nerve system in anatomic and
physiologic relation with the central organs are the encephalic and the
spinal nerves. The encephalic nerves, twelve in number on each
side of the median line, are in relation with the base of the encephalon,
and because of the fact that they pass through foramina in the walls of
the cranium they are usually termed cranial nerves.

The spinal nerves, thirty-one in number on each side, are in re-
lation with the spinal cord, and because of the fact that they pass through
foramina in the walls of the spinal column they are termed spinal nerves.
As both cranial and spinal nerves are ultimately distributed to the
structures of the body — i. e., the general periphery — they collectively
constitute the peripheral organs 0} the nerve system.

The central organs of the nerve system are supported and protected
by three membranes named, in their order from without inward, the
dura mater, the arachnoid, and the pia mater.

4Qi

49:

TEXT-BOOK OF PHYSIOLOGY.

The dura mater is a tough membrane composed of fibrous tissue.
It consists of two layers, the outer of which lines the cranial cavity and
forms an internal periosteum; the inner layer is closely attached to
the outer except at certain regions where it
separates and forms supporting structures,
such as the falx cerebri, falx cerebelli, ten-
torium cerebelli, etc.; at the margin of the
foramen magnum the outer layer becomes con-
tinuous with the periosteal tissue, while the
inner layer invests the cord down to its ulti-
mate termination. (Fig. 227.)

The arachnoid is a delicate serous mem-
brane. The external surface is smooth and
well defined and separated from the dura by
a narrow space, the subdural space. The
inner surface sends inward fine connective-
tissue processes which interlace in every direc-
tion, constituting the subarachnoid tissue.
This tissue is abundant in the cranium, much
less so in the spinal canal. The spaces be-
tween the connective tissue, taken collectively,
constitute the general subarachnoid space.
Around the spinal cord this space is well de-
fined, and at the base of the encephalon ex-
pands to form large cavities known as the
cisterna magna, cisterna pontis, etc.

The pia mater is a delicate membrane
composed of areolar tissue. It closely invests
the encephalon and spinal cord, dipping into
the various fissures. It is exceedingly vascular
and sends small blood-vessels for some distance
into the brain and spinal cord.

The Encephalo-spinal Fluid. — The gen-
eral subarachnoid space, as well as certain
cavities within the encephalon, contain a clear
transparent fluid, termed the encephalo-spinal.
This fluid has an alkaline reaction and a
specific gravity of 1.007 or 1-008. It is com-
posed of water, proteids (proteoses and serum-
globulin), and a compound pyrocatechin,
capable of reducing copper salts, though not
exhibiting any other of the properties of sugar.
In many respects this fluid resembles lymph.
The subarachnoid space and the general encephalic cavities, termed
ventricles, communicate with one another by an opening in the pia
mater (the foramen of Magendie) as it passes over the lower part of
the fourth ventricle.

Fig. 226. — The Central
Organs of the Nerve
System, f. t. o. Frontal,
temporal, and occipital
lobes of the cerebrum, c.
Cerebellum, p. Pons. mo.
Medulla oblongata. ms.,
ms. The upper and lower
limits of the spinal cord.
The remaining letters in-
dicate the region and num-
ber (if the spinal nerves. —
(Quain, after Bonrgery.)

THE ENCEPHALO-SPINAL MEMBRANES.

493

the

It was stated in Chapter VIII that the entire nerve or neuron system
can be resolved into a single morphologic unit, the neuron: the histo-
logic features and the physiologic properties of the neuron were there
also described; the anatomic relation of the neurons constituting the
peripheral organs of the nerve system, to the neurons constituting the
central organs of the nerve system were also stated and illustrated in
part diagrammatically, page 125. From the statements made regarding
the functions of the different neurons in their individual and collective
capacity the functions of the nerve system will become apparent.

The Functions of the Nerve System. — The functions of
nerve system are twofold: (1) It unites
and coordinates the organs and tissues
of the body in such a manner that they
are enabled to cooperate for the accom-
plishment of a definite object. (2) It
serves to arouse in the individual a con-
sciousness of the existence of an external
world, by virtue of the impressions which
it makes on his sense organs, and conse-
quently to enable him to adjust himself
to his environment.

By virtue of the coordination, a stim-
ulus, if of sufficient intensity, applied to
one organ or tissue will call forth activity
in one or more organs near or remote

from the part stimulated. This coordi-
nation is accomplished mainly by the
spinal cord and the medulla oblongata.
All actions which take place in response
to a peripheral stimulus and independ-
ently of volition are termed reflex ac-
tions. The reflex activities connected
with digestion, the circulation of the
blood, with respiration, excretion, etc.,
are illustrations of the coordinating capa-
bilities of the nerve-centers located in
these portions of the central nerve system.

Consciousness of the existence of the external world and of the re-
lation existing between it and the individual is associated with the
physiologic activities of the encephalon, and more particularly of the
cerebral hemispheres. This portion of the nerve system is the chief,
though perhaps not the sole, organ of the mind, and its main functions
are for the most part mental.

The function of a part at least of the peripheral nerve system is to
afford a means of communication between the central nerve system
and the remaining structures of the bodv. The nerve-trunks consti-

Fig. 227. — The Membranes of
the Spinal Cord. i. Dura
mater. 2. Arachnoid. 3. Poste-
rior root of spinal nerve. 4. An-
terior root of spinal nerve. 5.
Ligamentum dentatum. 6. Linea
splendens. — (Morris, ajter Ellis.)

tuting this part may be divided into two groups, as follows:

494 TEXT-BOOK OF PHYSIOLOGY.

i. The first group comprises nerves in connection with the special
sense-organs, e. g., eye, ear, nose, tongue, skin, as well as nerves
in connection with the general or organic sense-organs, e. g., mu-
cous membranes, viscera, etc., which transmit nerve impulses
to certain localized areas in the cerebral cortex, where they are
translated into conscious sensations. These sensations, both
special and general, by their grouping and combinations are the
primary elements of intelligence.
2. The second group comprises those nerves which terminate in the
muscle apparatus and which transmit nerve impulses, by way of
the medulla and spinal cord, from localized areas in the cerebral
cortex to the muscles of the face, trunk and extremities, which are
in consequence excited to activity. The muscle movements thus
become physical expressions of mental states, and if directed in a
definite manner to the overcoming of the resistances offered by
the external world become capable of modifying it in accordance
with the mental states.

The first group of nerves, the afferent, especially those connected
with the special sense-organs, are excited to activity by impressions
made on their peripheral terminations by agencies in the external
world, and thus become a means of communication between the
physical and the mental worlds.

The second group of nerves, the efferent, are excited to activity
bv those molecular disturbances in their related nerve-cells which
accompany volitional efforts, and thus they become a means of com-
munication between the mental and the physical worlds.

The central nerve system is thus composed of a number of separate
though closely related parts, to each of which a separate function has
been assigned. In the study of the structure and function of these
separate parts it will be found convenient, and conducive to clearness,
to consider them in the order of their complexity, beginning with the
spinal cord and ending with the cerebrum.

THE SPINAL CORD.

The spinal cord is the narrow elongated portion of the central
nerve system contained within the spinal canal. It is cylindric in
shape though presenting an enlargement in both the lower cervical
and lower lumbar regions corresponding to the origins of the nerves
distributed to the upper and lower extremities. The cord varies in
length from 40 to 45 cm., measures 12 mm. in diameter, weighs 42 gms.,
and extends from the atlas to the second lumbar vertebra, beyond
which it is continued as a narrow thread, the filum terminate. (Fig.
228.) It is divided by the anterior and posterior longitudinal fissures
into halves, and is therefore bilaterally symmetric. A transverse sec-
tion of the cord shows that it is composed of both white and gray mat-
ter, the former covering the surface, the latter occupying the center.

THE SPIXAL CORD.

495

Structure of the Gray Matter. — The gray matter is arranged
in the form of two crescents, united in the median line by a trans-
verse band or commissure forming a figure resembling the letter H.
Though varying in shape in different regions of the cord, the gray
matter in all situations presents on either side an anterior or ventral
and a posterior or dorsal horn. Between the two horns there is a

Superior or Cervical Segment
of Spinal Cord.

Middle or Dorsal Portion
of Cord.

Inferior Portion of Cord and
Cauda Equina.

Fig. 228.- — Superior, Middle, and Inferior Portions of Spinal Cord. i. Floor
of ^fourth ventricle. 2. Superior cerebellar peduncle. 3. Middle cerebellar peduncle.
4-»Inferior cerebellar peduncle. 5. Enlargement at upper extremity of postero-median
column. 6. Glosso-pharyngeal nerve. 7. Vagus. S. Spinal accessory. 9, 9, 9, 9.
Ligamentum denticulatum. 10, 10, 10, 10. Posterior roots of spinal nerves. 11, 11, n.
11. Postero-lateral fissure. 12, 12, 12, 12. Ganglia of posterior roots. 13, 13. Anterior
roots. 14. Division of united roots into anterior and posterior nerves. 15. Terminal
extremity of cord. 16, 16. Filum terminale. 17, 17. Cauda equina. I, VIII. Cervical
nerves. I, XII. Dorsal nerves. I, V. Lumbar nerves. I, V. Sacral nerves. — (Sappcy.)

portion termed the intermediate gray substance. The commissure
presents in its center a narrow canal which extends throughout the
entire length of the cord. This canal is lined by cylindric epithelium
and surrounded by gelatinous material. (Fig. 229.)

The anterior horn is short and broad and entirely surrounded
by white matter. The posterior horn is narrow and elongated and
extends quite up to the surface of the cord, where it is capped by gelat-

496

TEXT-BOOK OF PHYSIOLOGY.

inous matter, the substantia gelatinosa. In the lower cervical and
thoracic regions a portion of the intermediate gray substance pro-
jects outward and forms the so-called lateral horn. The gray matter
fundamentally consists of a framework of tine neuroglia supporting

blood-vessels, lymphatics, medullated
and non-medullated nerves, and groups
of nerve-cells.

The Nerve-cells. — The nerve-cells
of the cord are very numerous and pre-
sent a variety of shapes and sizes in
different regions. They are usually
arranged in groups which extend for
some distance up and down the cord,
forming columns more or less continuous.
In the anterior horn two well-marked
groups are found, one situated at the
anterior and inner angle, known as the
antero-median group, the other situated
at the posterior and lateral angle and
known as the postero-lateral group. In
the lower cervical and upper thoracic re-
gions, in the region of the lateral horn,
another group of cells is found, known as
the intermediate group. In the central
portion of the horn there is also a central
group.

The cells of the anterior horns are of
large size, nucleated and multipolar.
They are the modified descendants of
pear-shaped cells, the neuroblasts, which
migrated from the medullary tube (see
page 105). In the course of their migra-
tion they developed dendrites which form
an intricate felt-work throughout the
anterior horn. One of the processes, the
axon, approached the surface of the c6rd,
penetrated it, grew outward, became
covered with myelin and neurilemma,
and developed into an anterior root-
fiber. These nerve-cells, with their
dendrites, axons, and terminal branches,
form efferent neurons of the first order.
The intimate histologic and physiologic
relationship existing between the nerve-cell and the axon is revealed by
the degenerative changes which arise in the latter when separated from
the former. The cell apparently determines the nutrition of the axon
and may be regarded as trophic in function. wSome of the cells of the

C

D

Fig. 229. — Sections
Different Regions
Spinal Cord. A. At
of the sixth cervical nerve.
At the mid-dorsal region.

THROUGH
OF THE

the level
B.
C.
At the center of the lumbar
enlargement. D. At the upper
pari of the conus medullaris. 1.
Posterior roots. 2. Anterior roots.
3. Posterior fissure. 4. Anterior
6 in-. 5. Central canal. — (Mor-
ris' "Anatomy," ajter Schwalbe.)

THE SPIXAL CORD.

497

anterior horn send their axons into the immediately surrounding white
matter of the same side, after which they divide into two branches,
one passing up, the other down, the cord, to re-enter the gray matter at
different levels. They are probably associative in function. Other
cells send their axons into that portion of the white matter on the same
and opposite sides known as Gower's antero-lateral tract. (Fig. 230.)
In the posterior horn nerve-cells are also present, though they
are not so numerous as in the anterior horn. At the base of the horn
and on its inner side there is a well-marked group of cells which ex-
tends from the seventh or eighth cervical nerves downward to the

Oorsal

l/entral

Fig. 230. — Scheme of the Structure of the Cord. — (Howell after Lenhossek.)
On the right the nerve cells; on the left the entering nerve fibers. Right side: i, Motor
cells, anterior horn, giving rise to the fibers of the anterior root; 2, tract cells whose axons
pass into the white matter of the anterior and lateral columns; 3, commissural cells whose
axons pass chief!}' through the anterior commissure to reach the anterior columns of the
other side; 4, Golgi cells (second type), whose axons do not leave the gray matter; 5, tract
cells whose axons pass into the white matter of the posterior column. Left side: 1, Enter-
ing fibers of the posterior root, ending, from within outward, as follows: Clarke's column,
posterior horn of opposite side, anterior horn same side (reflex arc), lateral horn of same
side, posterior horn of same side; 2, collaterals from fibers in the anterior and lateral
columns; 3, collaterals of descending pyramidal fibers ending around motor cells in anterior
horn.

second or third lumbar nerves, being most prominent in the thoracic
region. This column is known as Clarke's vesicular column. From
the nerve-cells constituting this column axons pass obliquely outward
into that portion of the white matter known as the direct cerebellar tract.
Other nerve-cells send their axons into the white matter in the
posterior portion of the cord bordering the posterior median fissure.
Some of the nerve-cells, their situation and the distribution of their
axons are shown in Fig. 230.

Classification of Nerve-cells. — The cells of the gray matter may
be divided into three main groups: viz., intrinsic, efferent, and atferent.

498 TEXT-BOOK OF PHYSIOLOGY.

The intrinsic cells are associative in function. The axons to which
these cells give origin pass more or less horizontally into the white
matter, where they divide into two branches, one of which passes
upward, the other downward. At various levels they re-enter the
gray matter and arborize around other intrinsic cells.

The efferent cells, independently of their trophic influence, are
also motor in function, inasmuch as the excitation arising in them
is transmitted outwardly through their axons to muscles, blood-vessels,
glands and viscera, imparting to them motion, either molar or molec-
ular. As the efferent fibers in the ventral roots of the spinal nerves
are classified (see page 107) in accordance with their physiologic action
into motor, vaso-motor, secretor, viscero-motor and pilo-motor nerves,
so the nerve-cells of which the nerves are integral parts may be classified
physiologically as motor, vaso-motor, secretor, viscero-motor and pilo-
motor. Collections or groups of such cells are termed "centers."

The afferent cells are largely sentient or receptive in function,
inasmuch as the excitations brought to the spinal cord by the afferent
nerves in the dorsal roots from the general periphery are received
by them and transmitted by and through their axons to the cortex of the
cerebrum, where they are translated into conscious sensations. As
the nerve-fibers in the dorsal roots of the spinal nerves are classified,
in accordance with the sensations to which they give rise, as sensor,
thermal, tactile, etc., so these nerve-cells may be similarly classified
according as they transmit their excitations to those specialized areas
in the cerebral cortex in which these different sensations arise.

Structure of the White Matter. — A transverse section of the
cord shows that the white matter completely covers the gray matter
except where the posterior horns reach the surface. Anteriorly the
white matter of each lateral half is connected by a narrow strip or
bridge of white matter, the anterior commissure. Microscopic ex-
amination shows that the white matter is composed of vertically dis-
posed medullated nerve-fibers which are devoid of a neurilemma.
These fibers are supported partly by a framework of connective
tissue, and partly by neuroglia. The white matter of each side of
the cord is anatomically divided into an anterior, a lateral, and a pos-
terior column by the anterior and posterior roots of the spinal nerves.
Classification of the Nerve-fibers. — From a study of the em-
bryologic development of the white matter and of the degenerative
changes which follow its pathologic and experimental destruction, it
has been differentiated into a number of specialized tracts which have
different origins, destinations, and functions. Some of the more im-
portant tracts are shown in Fig. 231. They may be divided, however,
into efferent, afferent, and associative fibers.

1. The anterior column, comprising that portion between the
anterior longitudinal fissure and the anterior roots, has been sub-
divided into:

(a) The direct pyramidal tract, or column of Tiirck. This tract

THE SPINAL CORD.

499

borders the longitudinal fissure and extends from the upper extremity
of the cord as far down as the mid-thoracic region. From above
downward this tract diminishes in size, for the reason that its fibers
or their collaterals cross at successive levels to the opposite side of the
cord by way of the anterior commissure to enter the gray matter of
the anterior horn. These fibers are the continuations of fibers which
take their origin in cells wdiich are located in the cortex of the cerebral
hemisphere of the same side. The terminal filaments of these fibers
or axons are in physiologic relation either directly or indirectly through
intercalated neuron cells with the dendrites of the cornual cells.
When divided in any part of their course, these fibers undergo de-
scending degeneration. They are therefore efferent neurons and of the
second order.

Fig. 231. — Transection of the Cervical Spinal Cord Showing Its Chief Sub-
divisions.— (After Mills.)

(b) The ant ero -lateral ground bundle or root zone. This tract lies
external to the pyramidal tract, surrounds the anterior horn of the
gray matter and extends throughout the length of the cord. It is
composed of short commissural or associative fibers which come from
nerve-cells in the gray matter from the same and opposite sides of the
cord. After entering the white matter the}7 divide into two branches,
pursue opposite directions, then re-enter the gray matter at higher
and lower levels and come into relation with other nerve-cells.

2. The lateral column, comprising that portion between the
ventral and dorsal roots, has been divided into :

500 TEXT-BOOK OF PHYSIOLOGY.

(a) The antero-lateral tract of Gowers. This tract is somewhat
crescentic in shape and situated on the lateral aspect of the cord ex-
ternal to the antero-lateral root zone. It extends throughout the entire
length of the cord. When divided it undergoes ascending degenera-
tion, which would indicate that the axons originate in nerve-cells
in the gray matter. This tract is therefore afferent in function.

(b) The lateral limiting tract. This tract, which is quite narrow, lies
close to the external border of the gray matter. It is composed of
fibers which do not degenerate to any considerable extent and are in
all probability associative fibers which come from nerve-cells in the
gray matter to re-enter at lower and higher levels. It is also believed
by some investigators that the anterior portion contains efferent and
the posterior portion afferent fibers; for this reason it is frequently
termed the mixed lateral tract.

(c) The crossed pyramidal tract. This tract occupies the posterior
portion of the lateral column, though its exact position varies some-
what in different regions of the cord. In the cervical and thoracic
regions it is covered by a layer of fibers. In the lumbar region, how-
ever, it comes to the surface. From above downward this tract grad-
ually diminishes in size, for the reason that its fibers and their col-
laterals enter the gray matter at successive levels. The terminal
branches of these fibers are in close physiologic relation either directly
or indirectly through intercalated neuron cells with the dendrites of
the cornual cells. These fibers are the continuations of fibers which
take their origin in cells which are located in the cortex of the cerebral
hemispheres of the opposite side. When divided in any part of their
course, they undergo descending degeneration. They are therefore
efferent neurons and of the second order.

(d) The direct cerebellar tract, or column of Flechsig. This tract
is situated on the surface of the lateral column external to the crossed
pyramidal tract. It slightly increases in size from belowr upward.
It is composed of fibers the cells of which are found on the inner side
and base of the posterior horn (Clark's vesicular column). From
this origin the fibers pass obliquely outward to the surface and then
directly upward to terminate, as its name implies, in the cerebellum.
Decussation of these fibers takes place in the superior vermiform lobe
of the cerebellum. When divided this tract degenerates upward. It
is therefore in all probability an afferent tract and of the second order.

3. The posterior column, comprising that portion between the
dorsal roots and the posterior longitudinal fissure, has been sub-
divided into:

(a) The poster o-extemal tract of Burdach. This tract lies just
within the posterior horns. A portion of this tract is composed
of ground fibers which, though vertically disposed, have but a short
course. They take their origin in cells in the gray matter, and after
entering this tract divide into ascending and descending branches,
which with their collaterals re-enter the gray matter at different

THE SPINAL CORD. 501

levels. Another portion of this tract is made up of nerve-fibers de-
rived from the dorsal roots of the spinal nerves, which cross this col-
umn toward the median line in an oblique or horizontal direction.
The fibers of the upper portion of this tract terminate around the
nucleus cuneatus at the medulla oblongata. When divided, these
fibers degenerate for but a short distance. The ground fibers are prob-
ably associative in function.

(b) The postero-internal tract, or column of Goll. This tract is
separated from the former by a septum of connective tissue which is
most marked above the eleventh thoracic segment. The fibers which
compose this tract are long and derived for the most part from the
dorsal roots of the spinal nerves of the same side. This is shown by
the fact that division of these roots central to the ganglion is followed
by ascending degeneration of the column of Goll as far as the nucleus
gracilis in the medulla oblongata. Fibers derived from cells in the
gray matter are also contained in this column. This tract is largely
afferent in function.

(c) Lissauefs tract. This tract embraces the tip of the posterior
horn and is composed principally of fibers from the dorsal roots of the
spinal nerves. After entering the tract the fibers divide, into ascend-
ing and descending branches, which finally terminate around cells
in the posterior horn.

In addition to the tracts described in foregoing paragraphs a num-
ber of small narrow tracts have been discovered in different regions
of the spinal cord the functional significance of which, however, has
not been determined. Of these may be mentioned:

1. The antero-lateral tract of Marchi and Lowenthal, situated at
the anterior and inner angle of the anterior column, which degenerates
downward after removal of one-half of the cerebellum.

2. The comma tract a narrow bundle of fibers situated in the
anterior portion of the column of Burdach. When it is divided it
degenerates downward.

3. The septo-marginal tract, an oval shaped tract situated along
the margin of the posterior longitudinal fissure.

4. The cornu-commissural tract found along the border of the
anterior portion of the posterior column as far forward as the posterior
commissure. Both of these tracts are best developed in the lumbo-
sacral region. They arise from nerve-cells in the gray matter. They
undergo descending degeneration when divided, but not after division
of the dorsal roots.

The Relation of the Spinal Nerves to the Spinal Cord. — The

spinal nerves present near the spinal cord two divisions which from
their connection with the anterior or ventral and the posterior or
dorsal surfaces are known as the ventral and dorsal roots.

The ventral roots are the axons of various groups of nerve-cells
situated in the anterior horns of the grav matter. From their origin

5o2 TEXT-BOOK OF PHYSIOLOGY.

these axons pass almost horizontally forward through the anterior
column in three distinct bundles. After emerging from the cord
they curve downward and backward to join the dorsal roots. The
dorsal roots are the centrally directed axons of nerve-cells in the
spinal ganglia. After entering the cord they divide into two main
groups, a lateral and a mesial. A portion of the lateral group enters
the posterior horn directly through the caput cornu; the other portion
turns upward and runs through Lissauer's tract and ultimately enters
the posterior horn. The mesial group passes into the postero-external
column (Burdach), where the fibers divide into descending and as-
cending branches. The former constitute the comma tract, the
terminal branches of which surround cells in the gray matter; the
latter (ascending) cross the column obliquely and enter the postero-
internal column (Goll), in which they pass upward to terminate around
the cells of the nucleus gracilis of the same side. As these root fibers
pass up and down the cord, collateral branches are given off which
enter the gray matter at successive levels and come into physiologic
relation with the cells of Clark's vesicular column on the same and
opposite sides and with the cells of the anterior horn.

The peripherally directed axons of the nerve-cells in the spinal
nerve ganglia become associated with the axons of the ventral roots
and together they pass as a spinal nerve to peripheral organs.

The ventral root axons are distributed to skeletal muscles, blood-
vessels, glands and viscera. The dorsal root axons are distributed to
skin, mucous membranes and muscles. The classification of the
nerve-fibers in the ventral and dorsal roots in accordance with the
functions they subserve will be found on pages 107, 108.

Though both the efferent and afferent fibers of the spinal nerves
are directly connected with nerve-cells in the spinal cord, they are
also indirectly connected by efferent and afferent nerve-tracts with
the cerebral cortex.

Experimentally, it has been determined that the anterior or ventral
roots contain all the efferent fibers, the posterior or dorsal roots all the
afferent fibers. The proofs in support of this view are as follows:
Stimulation of the ventral roots produces:

1. Tetanic contraction of skeletal muscles.

2. Variations in the degree of the contraction, the tonus, of the
muscle walls of the peripheral arteries either in the way of aug-
mentation or inhibition.

3. Discharge of secretions from glands.

4. Variations in the degree of the contraction, the tonus, of the
muscle walls of certain viscera either in the way of augmentation
or inhibition.

Division of the ventral roots is followed by:

1. Relaxation of skeletal muscles and loss of movement.

2. Temporary dilatation and loss of the tonus of blood-vessels.

3. Cessation in the discharge of secretions from glands.

Diagram Indicating the Course or the Motor and Sensory Fibers of the
Spinal Cord and Medulla. — (Gordinier.)

a, a. Motor cells of the cerebral cotrex. b, b. Arborizations of the fibers of the sensory
tract in the cerebral cortex, c. Nucleus of the column of Burdach, showing terminal
arborizations of the long sensory fibers of the cord. d. Nucleus of the column of Goll,
showing terminal arborizations of the long sensory fibers of the cord. e. Section of the
medulla, showing sensory decussation. /. Section of medulla, showing motor or pyramidal
decussation, g, g. Motorial end plates, h. Section through the cervical region of the cord,
showing termination in the anterior horn of the motor fibers of the direct pyramidal tract
after they have crossed in the anterior commissure; also fiber of crossed pyramidal tract
ending about anterior horn cell of same side, i, i. Posterior spinal ganglia, j, k. Sensory
fibers of short course. /. Sensory fibers of long course, terminating in medulla, in, m,
m. Sensory end organs, n. Section through lumbar cord.

PLATE II.

THE SPINAL CORD. 503

4. Temporary impairment of the normal activities of the visceral
muscles from loss of central nerve control; the degree of impair-
ment depending on the nature of the viscus involved.
Peripheral stimulation of the dorsal roots produces:

1. Sensations of touch, temperature, pressure and pain.

2. Reflex excitation of spinal nerve centers in consequence of which
there is an increased activity of skeletal muscles, blood-vessels,
glands and visceral walls.

3. Reflex inhibition of spinal nerve centers in consequence of which
there may be a decrease in the activities of skeletal muscles,
blood-vessels, glands and viscera.

4. Sensations of the duration and direction of muscle movements,
of the resistance offered and of the position of the body or of its
individual parts^ (muscle sensations) .

Division of the dorsal roots is followed by:

1. Loss of sensation in all parts to which they are distributed.

2. Loss of the power of exciting or inhibiting reflexly the activities
of spinal nerve centers and in consequence a loss of the power of
exciting or inhibiting the activities of peripheral organs.

The ventral roots are therefore efferent in function transmitting
nerve impulses from the spinal cord to the peripheral organs which
excite them to activity.

The dorsal roots are afferent in function transmitting nerve impulses
from the general periphery to (a) the spinal cord where they excite
its contained nerve-centers to activity, and (b) to the cerebrum where
they excite its centers to activity with the development of sensations.

Segmentation of the Spinal Cord. — For the elucidation of many
problems connected with the physiologic actions of the spinal cord,
as well as of the symptoms which follow its pathologic impairment,
it will be found helpful to consider the cord as consisting physiologicly
of a series of- segments placed one above the other, the number of
segments corresponding to the number of spinal nerves. Each spinal
segment would therefore comprise that portion of the cord to which
is attached a pair of spinal nerves. The nerve-cells in each segment
are in histologic and physiologic relation with definite areas of the
body, embracing muscles, blood-vessels, glands, skin, etc.

If the exact distribution of the nerves of any segment were
then known, its function could be readily stated. By virtue of this
segmentation it becomes possible for each segment to act inde-
pendently, or in cooperation with other segments near or remote,
with which they are associated by the intrinsic or associative cells
and their axons; and the spinal cord itself is enabled to act as a unit.

FUNCTIONS OF THE SPINAL CORD.

Physiologic investigation has demonstrated that the spinal cord,
by virtue of the presence of nerve-cells and nerve-fibers, may be re-
garded as composed of:

5o4 TEXT-BOOK OF PHYSIOLOGY.

i. Independent nerve-centers, each of which has a special function;

and —
2. Conducting paths by which these centers are brought into relation

with one another and with the cerebrum and its subordinate or

underlying parts, e. g., medulla and pons Varolii.
The Spinal Cord Segments as Independent Centers. — The
efferent cells of the spinal segments are the immediate sources of
the nerve energy which excites activity in muscles,, blood-vessels, glands.
The discharge of their energy may be caused:
i. By variations in the composition of the blood or lymph by which

they are surrounded. The activity of the cell thus occasioned

is termed automatic or autochthonic (Gad).

2. By the arrival of nerve impulses coming through afferent nerves

from the general periphery, skin, mucous membrane, etc.

3. By the arrival of nerve impulses descending the spinal cord from

the cerebrum or subordinate structures. The peripheral ac-
tivity in the former instance is said to be reflex or peripheral in
origin; in the latter instance, direct or cerebral in origin. In this
latter instance, also, the muscle movements are due to volitional,
the vascular variations and glandular discharges to emotional,
forms of cerebral activity.
Each segment of the spinal cord may be regarded, therefore,
because of its contained nerve-cells:

1. As a center for automatic activity.

2. As a center for the reception of nerve impulses arising either at the

periphery or in the cerebrum, and for their subsequent trans-
mission through efferent nerves to various peripheral organs.
Automatism. — The growth, the nutrition and multiplication of
the cells of various tissues, and their continuous and rhythmic activity,
have been attributed to an automatic action of the spinal nerve-cells.
By this expression is meant a discharge of energy from the cells oc-
casioned by a change in their environment, i. e., in the chemic com-
position of the blood or lymph by which they are surrounded, and in-
dependent of any excitation coming through afferent nerves. If the
cell activity is continuous, though variable in degree from time to
time, it gives rise to what is termed tonus, e. g., trophic tonus, vas-
cular, muscle tonus, etc. If the cell activity is intermittent, it imparts
to muscles a certain rhythmic activity, e. g., the respiratory movements.
As no effect arises without a sufficient cause, the term automatic
has been objected to and the term autochthonic has been suggested
(Gad), expressive of the idea that the energy originates in the nerve-
cell as a result of a reaction between the cell and its ever-changing
environment. A center so acting could not be regarded as primarily
a center for reflex action, however much it might be influenced or
conditioned secondarily by afferent impulses. Though automatic
activity of the spinal cord centers is advocated by some physiologists,
the fact must be recognized that with increasing knowledge of reflex

THE SPINAL CORD. 505

activities some of the phenomena hitherto regarded as automatic
have been found to be reflex in origin. Whether this will eventually
be found true for all forms of so-called automatic or autochthonic
activity remains to be seen.

Trophic Tonus. — The normal metabolism of muscle, gland, and
connective tissue which underlies the assimilation of food, the storing
of energy, and the production of new compounds, is dependent, in the
higher animals at least, on the connection of these tissues with the cen-
tral nerve system; for if the efferent nerves be divided, not only will
they undergo degeneration in their peripheral portions, but the muscles,
glands, and connective tissues to which they are distributed will also
undergo similar changes. This is to be attributed not merely to in-
activity, but rather to a loss of nerve influence, inasmuch as inactivity
leads merely to atrophy and not to degeneration. It would appear
from facts of this character that the normal metabolism is dependent
for its continuance on nerve influences. There is no evidence, how-
ever, as to the existence of special trophic nerves, separate from those
which impart to glands and muscles their customary activities. The
trophic centers and the motor centers are identical, though the two
modes of their activity are separate and distinct.

Vascular Tonus. — The state of moderate contraction of the
arterioles throughout the body, in consequence of which the average
arterial pressure is maintained, has been attributed to constant activity
of the vaso-motor centers, this activity being conditioned by variations
in the composition of blood, either an increase in the quantity of
carbon dioxid or a decrease in the quantity of oxygen. The vaso-
motor centers are regarded as primarily automatic, though capable
of being influenced secondarily by reflected excitations from the
periphery or direct excitations from the cerebrum.

Muscle Tonus. — It is well known that if a muscle be divided in
the living animal the two portions will contract and separate them-
selves to a certain distance. This indicates that the muscle when in
a state of rest is in a slight degree cf contraction. This condition of
the muscle, to which the term muscle tonus is given, was formerly
attributed to an automatic and continuous discharge of energy from
the nerve-cells. Brondgeest, however, showed that this tonus is
entirely reflex in origin and immediately disappears on division of
the posterior roots of the spinal nerves, which would not be the case
if the cells in the cord were acting automatically. The afferent nerves
in this reflex arise in the muscle or its tendons, and the stimulus is
the slight degree of extension to which the muscle is subjected in virtue
of its attachments and the ever-varying position of the limbs and trunk.

The tonic contraction of the visceral muscles — e. g., the pyloric,
the vesical, the anal sphincters — though regarded as automatic by
some, is probably reflex in origin, dependent on the arrival of afferent
impulses from the periphery. It is probable that future investiga-
tion will disclose the existence and pathway of these afferent fibers.

506

TEXT-BOOK OF PHYSIOLOGY.

Reflex Actions. — It has already been stated that the nerve-cells
in the spinal cord are capable of receiving and transforming afferent
nerve impulses, the result of peripheral stimulation into efferent nerve
impulses, which are transmitted outward to muscles, exciting contrac-
tion; to glands, provoking secretion; to blood-vessels, changing their
caliber; and to organs, inhibiting or accelerating their activity. All
such actions taking place through the spinal cord and medulla oblon-
gata independently of sensation or volition are termed reflex actions.
The mechanism involved in every reflex action consists of at least the
following structures (Fig. 232):

sp.c

Fig. 232. — Diagram Showing the Structures Involved in the Production of
Reflex Actions, G. Bachman. r.s. Receptive surface; af.n. afferent nerve; [ex.
emissive or motor cells in the anterior horn of the gray matter of the spinal cord, sp.c;
ej.n. efferent nerves distributed to responsive organs, e. g., directly to skeletal muscles,
sk.m., and indirectly through the intermediation of sympathetic ganglia, sym. g., to blood-
vessels, b.v., and to glands, g. The nerves distributed to viscera are not represented.

i. A receptive surface; e. g., skin, mucous membrane, sense
organ, etc.

2. An efferent fiber and cell.

3. An emissive cell, from which arises —

4. An efferent nerve, distributed to a responsive organ, as—

5. Muscle, gland, blood-vessel, etc.

In this connection the reflex contractions of skeletal muscles only
will be considered.

If a stimulus of sufficient intensity be applied to the receptive sur-
face, there will be developed in the terminals of the afferent nerve a
series of nerve impulses which will be transmitted by the afferent nerve
to, and received by, the dendrites of the emissive cell in the anterior
horn of the gray matter. With the reception of these impulses there

THE SPINAL CORD.

5°7

will be a disturbance in the equilibrium of the molecules of the cell,
a liberation of energy and a transmission of nerve impulses outward
through the efferent nerve to the muscle.

A reflex mechanism or arc of this simplicity would subserve but a
simple movement. The majority of the reflexes, however, are extremely
complex and involve the cooperation and coordination of a number
of centers at different levels of the spinal cord and medulla, on the same
and opposite sides, and of muscles situated at distances more or less
remote from one another. The transference of nerve impulses com-
ing from a localized area of a receptive surface, to emissive cells situated
at different levels is accomplished by the intermediation of a third
neuron situated in the gray matter which
is in connection, on the one hand with
the central terminals of the afferent
nerve, and, on the other hand through
collateral branches with the dendrites of
the efferent neurons situated at different
levels. (Fig. 233.) A histologic and
physiologic mechanism of this character
readily explains how a localized stimula-
tion can give rise to reflex actions ex-
tremely complex in character.

The reflex contractions of skeletal
muscles are best studied after division of
the central nerve system at the upper
limit of the spinal cord. xAiter this pro-
cedure the spinal centers can act inde-
pendently of, and uninfluenced by either
sensation or volitional efforts on the part
of the animal. Though.it is possible to
provoke reflex contractions under such
circumstances in warm-blooded animals,
they are, as a rule, incomplete and of
short duration, owing to disturbances of
the circulation and respiration and the
consequent loss of tissue irritability. In frogs and in cold-blooded
animals generally, the spinal cord retains its irritability for a long
period of time after removal of the brain, and therefore is well adapted
for the study of reflex actions.

The separation of the spinal cord from the brain is readilv effected
by destroying the medulla oblongata. This can be done by inserting
a pin through the skin, the occipito-atlantal membrane covering the
space between the occipital bone and the atlas, until it strikes the bodies
of the vertebras below. If the pin is properly directed it passes through
the medulla. Care should be taken to avoid injury to the blood-
vessels on either side. The brain itself should then be destroved, so
as to remove all consciousness, by inserting the pin into the brain

Fig. 233. — Diagram Showing
the Relation of the Third
Neuron a, to the Afferent
Neuron b, and to the Efferent
Neurons c, c, c. — (After KdUiker.)

5oS TEXT-BOOK OF PHYSIOLOGY.

cavity through the foramen magnum, and giving it a few rotatory
movements.

A frog so prepared, and placed on the table and allowed to remain
at rest for a few moments until the shock of the operation passes away,
will draw the limbs close to the body and assume a position not unlike
that of a normal frog. If then the posterior limbs be extended, they
will immediately be drawn close to the side of the trunk in the usual
flexed position. If the toes are pinched with forceps, the foot will
execute a series of movements as if it were trying to free itself from
the source of irritation.

If the frog be suspended, the limbs, through the force of gravity,
will be gradually extended and hang down freely. In this, as in the
sitting position, the animal will remain perfectly quiet and will not
exhibit spontaneous movements. Any stimulus applied to the skin,
however, provided it is of sufficient intensity, will be followed by a
more or less pronounced movement. Mechanic, chemic and electric
stimuli applied to any part of the skin will call forth the characteristic
reflex movements. Chemic stimuli such as weak solutions of sulphuric
or acetic acid placed on the toes will be followed by feeble flexion of
the corresponding leg, to be succeeded in a short time by extension.
Stronger solutions will produce more extensive and vigorous move-
ments, the foot at the same time being rubbed against the thigh,
apparently for the purpose of freeing it from the irritant. Similar
phenomena follow the application of the acid to the fingers or the
trunk. As a rule, the extent and complexity of the movement is within
limits proportional to the strength of the stimulus. By limiting the
sphere of action of the stimulus to definite but different areas of the
skin a great variety of movements, more or less complex and coordin-
ated and apparently purposive and defensive in character, can be
produced. The coordinated and purposive, character of the move-
ments exhibited by a brainless frog led Pfluger to the assumption that
the spinal cord in this as well as in other cold-blooded animals is pos-
sessed of sensorial functions, and endowed with rudimentary con-
sciousness. This view, however, is not generally accepted, the
movement being attributed to specialized mechanisms in the cord,
partially inherited, which permit of one and the same movement with
mechanic regularity and precision, so long as the conditions of the
experiment remain the same.

In warm-blooded animals similar results may be obtained for a
short time after division of the cord, especially if artificial respiration
is maintained and the circulation of the blood continued. The cord
will then retain its irritability for some time. If the conditions of
experimentation were favorable, it is highly probable that the human
spinal cord would execute similar movements. Thus it was observed
by Robin in a man who had been decapitated that reflex muscle con-
tractions could be elicited by stimulating the skin after the lapse of an
hour after execution. "While the right arm was lying extended by

THE SPINAL CORD. 509

the side, with the hand about 25 centimeters distant from the upper
part of the thigh, I scratched with the point of a scalpel the skin of the
chest at the areola of the nipple, for a space of 10 or 11 centimeters
in extent, without making any pressure on the subjacent muscles.
We immediately saw a rapid and successive contraction of the great
pectoral muscle, the biceps, probably the brachialis anticus, and
lastly the muscles covering the internal condyle. The result was a
movement by which the whole arm was made to approach the trunk;
with rotation inward and half-flexion of the forearm upon the arm;
a true defensive movement, which brought the hand toward the
chest as far as the pit of the stomach. Neither the thumb, which
was partially bent toward the palm of the hand, nor the fingers, which
were half bent over the thumb, presented any movements. The arm
being replaced in its former position, we saw it again execute a similar
movement on scratching the skin, in the same manner as before, a
little below the clavicle. This experiment succeeded four times, but
each time the movement was less extensive; and at last scratching the
skin over the chest produced only contractions in the great pectoral
muscle which hardly stirred the limb" (Dalton).
Laws of Reflex Action (Pfliiger).

1. Law of Unilateral it y. — If a feeble irritation be applied to one or

more sensory nerves, movement takes place usually on one side
only, and that the same side as the irritation.

2. Law of Symmetry. — If the irritation becomes sufficiently intense,

motor reaction is manifested, in addition, in corresponding
muscles of the opposite side of the body.

3. Law of Intensity. — Reflex movements are usually more intense

on the side of irritation; at times the movements of the opposite
side equal them in intensity; but they are usually less pronounced.

4. Law of Radiation. — If the excitation still continues to increase, it

is propagated upward, and motor reaction takes place through
centrifugal nerves coming from segments of the cord higher up.

5. Law of Generalization. — When the irritation becomes very intense,

it is propagated in the medulla oblongata; motor reaction then
becomes general, and it is propagated up and down the cord,
so that all the muscles of the body are thrown into action, the
medulla oblongata acting as a focus whence radiate all reflex
movements.
Special Reflex Movements. — Among the reflexes connected
with the more superficial portions of the body there are some which
are so frequently either increased or diminished in pathologic con-
ditions of the spinal cord that their study affords valuable indications
as to the seat and character of the lesions. They may be divided into:

1. The skin or superficial, and

2. The tendon or deep reflexes.

3. The organ reflexes.

The skin reflexes, characterized bv contraction of underlyine; mus-

5io TEXT-BOOK OF PHYSIOLOGY.

cles, are induced by stimulation of the skin — e. g., pricking, pinching,
scratching, etc. The following are the principal skin reflexes:
i. Plantar reflexe, consisting of contraction of the muscles of the foot,
induced by stimulation of the sole of the foot; it involves the
integrity of the reflex arc through the lower end of the cord.

2. Gluteal reflex, consisting of contraction of the glutei muscles

when the skin over the buttock is stimulated; it takes place
through the segments giving origin to the fourth and fifth lumbar
nerves.

3. Cremasteric reflex, consisting of a contraction of the cremaster

muscle and a retraction of the testicle toward the abdominal
ring when the skin on the inner side of the thigh is stimulated;
it depends upon the integrity of the segments giving origin to
the first and second lumbar nerves.

4. Abdominal reflex, consisting of a contraction of the abdominal

muscles when the skin upon the side of the abdomen is gently
scratched; its production requires the integrity of the spinal
segments from the eighth to the twelfth dorsal nerves.

5. Epigastric reflex, consisting of a slight muscular contraction in

the neighborhood of the epigastrium when the skin between the
fourth and sixth ribs is stimulated; it requires the integrity of
the cord between the fourth and seventh dorsal nerves.

6. Scapular reflex consisting of a contraction of the scapular muscles

when the skin between the scapulae is stimulated; it depends
upon the integrity of the cord between the fifth cervical and third
dorsal nerves.

The skin or superficial reflexes, though variable, are generally
present in health. They are increased or exaggerated when the gray
matter of, the cord is abnormally excited, as in tetanus, strychnin-
poisoning, and disease of the lateral columns.

The so-called "tendon reflexes,'1'1 characterized by the contraction
of a muscle, are also of much value in the diagnosis of lesions of the
cord and are elicited by a sharp tap on a given tendon. The term,
tendon reflex, is, however, somewhat inaccurate. The fundamental
condition for the production of the tendon reflex is the normal tone of
the muscle, which is a true reflex, maintained by afferent nerve impulses
developed in the muscle itself in consequence of its extension and hence
compression of the end-organs of the afferent nerves, the muscle
spindles. When the muscle is passively extended, as it is when the
reflex is to be elicited, there is an exaltation of the tonus and an increase
in the irritability. To this condition of the muscle due to passive
tension, the term myotatic irritability has been given. If the muscle
extension be now suddenly increased, as it is when the tendon is sharply
tapped, the increased compression of the muscle spindles will develop
additional afferent impulses which after transmission to the spinal
cord will give rise to contraction of the corresponding muscle.

The following are the principal forms of the tendon reflexes:

THE SPINAL CORD. 511

1. Patellar reflex or knee-jerk, consisting of a contraction of the ex-

tensor muscles of the thigh when the ligamentum patellae is
struck between the patella and tibia. This reflex is best ob-
served when the legs are freely hanging over the edge of a table.
The patella reflex is generally present in health, being absent
in only 2 per cent.; it is greatly exaggerated in lateral sclerosis,
in descending degeneration of the cord; it is absent in locomotor
ataxia and in atrophic lesions of the anterior gray corn.ua.

2. Ankle-jerk or Ankle Reflex.— If the extensor muscles of the leg be

placed on the stretch and the tendo Achillis be sharply struck, a
quick extension of the foot will take place.

3. Ankle Clonus.— This consists of a series of rhythmic reflex con-

tractions of the gastrocnemius muscle, varying in frequency from
six to ten per second. To elicit this reflex, pressure is made upon
the sole of the foot so as to suddenly and energetically flex the
foot at the ankle, thus putting the tendo Achillis and the gas-
trocnemius muscle upon the stretch. The rhythmic movements
thus produced continue so long as the tension within limits
is maintained. Ankle clonus is never present in health, but is
very marked in lateral sclerosis of the cord.
The toe reflex, peroneal reflex, and wrist reflex are also present in
sclerosis of the lateral columns and in the late rigidity of hemiplegia.
The organ reflexes, e. g., the activities of the genito-urinary organs,
the stomach, intestines, gall-bladder, etc., and which are induced by
peripheral stimulation have been considered in connection with the
physiologic action of these organs. The genito-urinary center is
located in the lumbar region of the spinal cord. In diseased conditions
of this region the genito-urinary reflexes are sometimes increased, at
other times decreased or even abolished.

Reflex Irritability. — The general irritability or quickness of
response of the mechanism involved in reflex action can be approxi-
mately determined by observation of the length of time that elapses
between the application of a minimal stimulus and the appearance of
the muscle response. The method of Tiirck is sufficiently accurate
for general purposes. This consists in suspending a frog, after
removal of the brain, and immersing the foot in a 0.2 per cent, solu-
tion of sulphuric acid. The time is determined by means of a metro-
nome beating one hundred times a minute. Stimulation of the skin
can also be effected by the induced electric current, as suggested by
Gaskell. A single shock is, however, ineffective. When the shocks
follow each other with sufficient rapidity, they give rise to a summa-
tion of effects in the nerve-centers which will soon be followed by a
muscle response. It is highly probable that the chemic stimulation
gives rise to a similar summation of effects.

The period of time thus obtained is distributed over the entire
mechanism. The true reflex time, however — i. e., the time occupied
in the passage of the nerve impulses across the spinal mechanism —

5i2 TEXT-BOOK OF PHYSIOLOGY.

is shorter and is obtained by subtracting from the whole period
the time occupied by the passage of the impulses through the affer-
ent and efferent nerves as well as the latent period of muscle con-
traction. This corrected period, the true reflex time, has been found
to be twelve times longer than the time occupied by the passage of the
nerve impulse through the nerves, including the latent period of the
muscle.

The reflex irritability is increased by:
i. Separation of the Brain from the Cord. — This is at once followed
by an increase in reflex irritability, and is taken as evidence that
the brain normally exerts an inhibitor influence over the reflex
centers of the cord. The same increase is observed upon hemi-
section of the cord, though the increase is limited to the same
side.

2. The Toxic Action of Drugs. — Strychnin even in small doses in-

creases the irritability to such an extent that a minimal stimulus
is sufficient to call forth spasmodic contractions of all the
skeletal muscles. Under its influence the usual coordinated
reflexes disappear and are succeeded by incoordinated reflexes.
The explanation of this fact is believed to be a diminution in the
resistance offered by the cord to the passage of the afferent im-
pulses rather than to a direct stimulation of the efferent cells.
So much is this resistance decreased that the nerve impulses,
instead of being confined to their accustomed paths, are radiated
in all directions. Absolute repose of the animal and the exclu-
sion of all external stimuli greatly diminish the tendency to
the occurrence of spasms.

3. Degeneration of the Pyramidal Tracts. — In primary lateral scle-

rosis, a pathologic condition characterized primarily by a degen-
eration of the terminal filaments of the pyramidal tract fibers,
the reflex activity of the cord becomes exalted. As the disease
progresses the irritability increases to such an extent that violent
spasmodic contractions of the arms and legs arise when the skin
or tendons are mechanically stimulated. The explanation
offered is practically the same as in division of the cord: viz.,
withdrawal of the inhibitor and controlling influence of the
brain.

The reflex excitability may be decreased by:
1. Stimulation of Certain Regions of the Brain. — It was discovered
by Setchenow that when the frog brain is divided just anterior
to the optic lobes and the reflex time subsequently determined
according to the method of Tiirck, that the time can be consider-
ably lengthened by stimulation of the optic lobes. This is read-
ily accomplished by placing small crystals of sodium chlorid on
the optic lobes. It was concluded from this fact that these lobes
contain centers which exert an inhibitor influence over centers
in the spinal cord through descending nerve-libers. This

THE SPINAL CORD.

5i3

conclusion is strengthened by the fact that division of the
brain just behind the optic lobes causes a temporary inhibition
of the reflexes in consequence of a mechanical irritation of these
fibers. It is quite probable that the volitional inhibition of
certain reflexes is accomplished through the intermediation of
this center localized by Setchenow. (Fig. 234.)

2. Stimulation of Sensor Nerves. — If during the application of a

stimulus sufficient to call forth a characteristic reaction in a
definite period of time, a sensor
nerve in a distant region of the
body be simultaneously stimu-
lated, it will be found that the
reflex time will be lengthened
or the reaction completely in-
hibited.

3. Lesions of the spinal cord; e. g.,

atrophy of the multipolar cells
of the anterior horns of the
gray matter; degeneration of
the terminals of the dorsal root
fibers.

4. The toxic action of various drugs

— e. g., chloroform, chloral —
which are believed to exert a
depressing action on the nerve-
cells themselves.
The Spinal Cord as a Con-
ductor.— The white matter of the
spinal cord consists of nerve-fibers
the specific function of which is

1. To conduct nerve impulses from

one segment of the cord to
another.

2. To conduct nerve impulses com-

ing to the cord through afferent
nerves, directly or indirectly to
various areas of the encephalon .

3. To conduct nerve impulses from the encephalon to the spinal cord

segments.
Intersegmental or Associative Conduction. — The spinal cord
consists of a series of physiologic segments each of which has specific
functions and is associated through its related spinal nerve with a
definite segment of the body. For the harmonious cooperation and
coordination of all the spinal segments it is essential that they should
be united by commissural or associative fibers. This is. in fact,
accomplished by the axons of the intrinsic cells of the gray ma iter,
which constitute such a large part of the antero-lateral and posterior
3.3

7/icd. ob.

Fig. 234. — Diagram of the Brain of
the Frog. olj. n. olfactory nerves; olj.l.
olfactory lobes ; c.h. cerebral hemispheres ;
op. thl. optic thalamus; op. I. optic lobes;
c. cerebellum; mod. of. medulla oblon-
gata; IV. v. fourth ventricle.

5i4 TEXT-BOOK OF PHYSIOLOGY.

root zones. In consequence of this association, the cord becomes ca-
pable of complex coordinated and purposive reflex actions.

Spino-encephalic or Sensor Conduction. — The nerve impulses
that arise in consequence of impressions made on the terminals of
the nerves in the cutaneous and mucous surfaces, in the viscera and
in the muscles, are transmitted through the dorsal roots of the spinal
nerves to the cord. When transmitted through the cord to the cere-
bral hemispheres directly or indirectly, they are received by specialized
nerve-cells in the cortex and translated into conscious sensations.
The sensations thus arising may be divided into special and general
sensations. Of the former may be mentioned pain, touch, pressure,
temperature; of the latter may be mentioned hunger, thirst, fatigue,
well-being, etc.

The pathways through the spinal cord that conduct these afferent
impulses to the brain are ill defined and imperfectly known, and only
for a few sensations can it be said that their pathways have been
determined. The reason for this obscurity lies partly in the difficulties
of experimentation, partly in the difficulties of interpretation. Clinical
observations are for special reasons more or less untrustworthy.

Section of one lateral half of the cord, or a lesion involving the
one lateral half, as a rule abolishes all forms of cutaneous sensibility
on the opposite side below the injury. This would seem to prove that
the nerve impulses cross the median line of the cord immediately or
very shortly after entering. At the same time, muscle sensibility is
abolished on the corresponding side below the injury. This would
seem to prove that the fibers of the posterior roots which enter and
cross the column of Burdach and ascend in the column of Goll are
derived mainly from the muscles. It is, however, believed by some
investigators that those fibers which subserve the sense of touch do
not decussate at once, but ascend in the column of Goll as far as the
medulla oblongata, where they, in common with the fibers coming
from the muscles, arborize around the nerve-cells in the gracile and
cuneate nuclei. The afferent path is then continued by new nerve-
fibers which emerge from these cells, and which, after crossing the
median plane and decussating with the fibers coming from the oppo-
site side, join the afferent path from the spinal cord. These fibers
are known as the internal arcuate fibers and assist in the formation
of the lemniscus or fillet. (Fig. 235.) The sensor pathway decus-
sates in part at different levels of the spinal cord and in part at the
level of the gracile and cuneate nuclei. The former is often termed
the lower, the latter the upper sensor decussation.

The pathways for the impulses that give rise to the different sen-
sation have been variously located by different observers, e. g., in the
gray matter, in the limiting layer, and in the antcro-lateral tract of
Govvers; the pathway for the impulses that give rise to temperature
sensations has been located in the gray matter; the pathway for
tactile impressions has been located in the posterior columns, though

THE SPINAL CORD.

5i5

this is not beyond dispute. The pathway for pain sensations has
been located in Gowers' tract.

Encephalo-spinal or Motor Conduction. —At birth the child is
capable of performing all the functions of organic life, such as sucking,
swallowing, breathing, etc. It is, however, deficient in psychic activity
and in volitional control of its muscles. Its movements are therefore
largely, if not entirely, reflex in character.

Fig. 235. — Diagram of the Sensor Pathways in the Spinal Cord Augmented
above by Fibers of the Sensor Cranial Nerves and Nerves of Special Sense.
V. The trifacial nerve. VIII. The vestibular branch of the acoustic nerve. IX. The
glosso-pharyngeal nerve. X. The pneumogastric nerve. — (Van Gehuchten.)

Embryologic and histologic examination of the spinal cord and
medulla show that so far as their mechanisms for independent phys-
iologic activities are concerned both are fully developed. Similar
investigations of the cerebral hemispheres and of the nerve-fibers
which bring their nerve-cells into relation with the spinal segments
show that the cells of the cortex are not only immature, but that their
descending axons are incompletely invested with myelin. With the

5i6 TEXT-BOOK OF PHYSIOLOGY.

growth of the child, psychic life unfolds and volitional control of mus-
cles is acquired. Coincidently the cells of the cerebral cortex grow
and develop and the fibers become covered with myelin.

The nerve-fibers which have their origin in the cells of the cerebral
cortex, and which terminate in tufts around the cells in the anterior
horns of the gray matter of the spinal segments, are to be regarded as
long commissural tracts uniting and associating these two portions
of the central nerve system.

Experimental investigations and observations of pathologic lesions
accord with the view that physiologically these fibers are efferent
pathways for the transmission of motor or volitional impulses from
the cortex to the spinal segments. The nerve-cells in which the
motor impulses originate are located for the most part, as will be fully
stated later, in the central portion of the cortex of the cerebral hemi-
spheres in the neighborhood of the central or Rolandic fissure. The
axons of these cells from each hemisphere descend through the corona
radiata to and through the internal capsule, along the inferior sur-
face of the crura cerebri, behind the pons to the medulla, of which
they constitute the anterior pyramids. (Fig. 236.) At this point the
pyramidal tract* of each side divides into two portions, viz. :

1. A large portion, containing from 80 to 90 per cent, of the fibers,

which decussates at the lower border of the medulla and passes
downward in the posterior part of the lateral column of the
opposite side, constituting the crossed pyramidal tract; as it descends
it gradually diminishes in size as its fibers or their collaterals
enter the gray matter of each successive segment.

2. A small portion, containing from 20 to 10 per cent, of the fibers,

which does not decussate at the medulla but passes downward on
the inner side of the anterior column of the same side, consti-
tuting the direct pyramidal tract or column of Tiirck. This
tract can be traced down, as a rule, only as far as the mid-dorsal
region. As it descends it becomes smaller as its fibers cross the
anterior commissure to enter the gray matter of the opposite
side. Thus all the fibers of the pyramidal tract from each cere-
bral hemisphere eventually are brought into relation with the
cells of the gray matter of the opposite side of the cord.
That the pyramidal tracts are the conductors of volitional impulses
throughout the length of the cord to its various segments has been
made evident by the results of section, electric stimulation, and disease.
iJivision of the anterior and lateral columns of one side of the cord in
any part of its extent is invariably followed by a loss of motion or paral-

*From the fact that the region included between the origin of these fibers and
the internal capsule presents somewhat the form of a pyramid with four sides,
Charcot designated it the pyramidal region and the fibers composing it the pyram-
idal tract. The base of the pyramid includes the cortex of the convolutions
around the Rolandic fissure. The summit of the pyramid is truncated and covers
the pyramidal region of the internal capsule.

THE SPINAL CORD.

5i7

ysis of the muscles below the section, while electric stimulation of the
peripheral end of the isolated crossed pyramidal tract is followed by
marked "characteristic movements of the muscles. Similar results
follow division of the pyramidal tract in any part of its course from

x

'<

%

m

Fig. 236. — Diagram of the Pyramidal Tract or Motor Path. III. Common
oculo-motor nerve. IV. Pathetic nerve. V. Motor division of the trigeminal nerve.
VI. The abducens nerve. VII. Facial nerve. IX. and X. Motor divisions of the glosso-
pharyngeal and pneumogastric nerves. XL Spinal accessory nerve. XII. Hypoglossal
nerve. — ( Van Geh iichten.)

the cerebral cortex downward. Electric stimulation of the cortical
cells which give origin to the pyramidal tract is also followed by con-
traction of the muscles of the opposite side, while their destruction is
attended by paralysis of the same muscles. As the nutrition of the

5i8 TEXT-BOOK OF PHYSIOLOGY.

fibers is governed by the cells, it follows that when the axon is separated
from its cell it degenerates. It has been found that a lesion of the py-
ramidal tract in any part of its course is followed by descending degen-
eration, which is taken in evidence that it conducts nerve impulses
from above downward. Thus experimental investigation and path-
ologic observation are in accord in the view that physiologically these
nerve-fibers are the pathways for the transmission of motor or volitional
impulses from the encephalon to the spinal cord.

The relation of the motor and sensor pathways to each other in
the spinal cord and brain are shown in Plate II. The afferent fibers
which decussate at various levels through the spinal cord are not repre-
sented.

CHAPTER XX.

THE MEDULLA OBLONGATA; THE ISTHMUS OF THE EN-
CEPHALON ; THE BASAL GANGLIA.

THE MEDULLA OBLONGATA.

The medulla oblongata is that portion of the central nerve system
immediately superior to and continuous with the spinal cord. It has
the shape of a truncated cone, the base of which is directed upward,
the truncated apex downward. It is 38 mm. in length, 18 mm. in
breadth, and 12 mm. in thickness. By the continuation upward of
the anterior and posterior median fissures, the medulla is divided into
symmetric halves (Figs. 237 and 238). Like the cord, of which it is a
continuation, it is composed of white matter externally and gray matter
internally.

Structure of the Gray Matter. — The gray matter of the medulla
is continuous with that of the cord, though owing to the shifting of
position of the different tracts of the white matter it is arranged with
much less regularity. The appearance which the gray matter presents
on transverse section varies also at different levels.

At the level of the first cervical nerve the posterior horns are narrow,
elongated, and directed outward. The lateral horns are well devel-
oped and present a collection of cells near their bases which can be
traced forward and backward for some distance. At the level of the
decussation of the pyramidal tracts the head of the anterior horn be-
comes detached from the rest of the gray matter and is pushed backward
toward the posterior horn ; the bases of the anterior horns become spread
out to form a layer of gray matter near the dorsal aspect of the medulla.
Transverse sections of the medulla at all levels show a more or less ex-
tensive network of nerve-fibers known as the reticular formation. In
its meshes are found collections of nerve-cells of varying size. To-
ward the dorsal aspect of the medulla special groups of cells are found
from which axons arise to become the fibers of various efferent cranial
nerves, e. g., the hypoglossal, the efferent fibers of the vagus, and glosso-
pharyngeal.

Structure of the White Matter. — The white matter is composed
of nerve-fibers supported by connective tissue and neuroglia. It is
subdivided on either side by grooves into three main columns: viz.,
an anterior column or pyramid, a lateral column, and a posterior column.

The anterior column or pyramid is composed partly of fibers con-
tinuous with those of the anterior column of the spinal cord (the direct
pyramidal tract), and partly of fibers continuous with those of the
lateral column of the cord of the opposite side (the crossed pyram-

5i9

520

TEXT-BOOK OF PHYSIOLOGY.

Fig. 237. — Anterior or Ventral
View of the Medulla Oblongata
and Isthmus, i. Infundibulum. 2.
Tuber cinereum. 3. Corpora albi-
cantia. 4. Cerebral peduncle. 5.
Tuber annulare. 6. Origin of the
middle peduncle of the cerebellum.
7. Anterior pyramids of the medulla
oblongata. 8. Decussation of the an-
terior pyramids. 9. Olivary bodies.
10. Restiform bodies, n. Arciform
fibers. 12. Upper extremity of the
spinal cord. 13. Ligamentum dentic-
ulatum. 14, 14. Dura mater of
the cord. 15. Optic tracts. 16.
Chiasm of the optic nerves. 17.
Motor oculi communis. 18. Patheti-
cus. jo. Fifth nerve. 20. Motor
oculi externus. 21. Facial nerve. 22.
Auditory nerve. 23. Nerve of Wris-
berg. 24. Glosso-pharyngeal nerve
25. Pneumogastric. 26, 26. Spinal
accessory. 27. Sublingual nerve. 28,
29, 30. Cervical nerves. — (Sappey.)

Fig. 238. — Posterior or Dorsal View
of the Medulla Oblongata, Isthmus,
and Basal Ganglia, i. Corpora quad-
rigemina. 2. Corpus quadrigeminum an-
terior (pregeminum). 3. Corpus quadri-
geminum posterior (post-geminum). 4.
Tract of fibers (brachium) passing to the
corpus geniculatum externum. 5. Tract of
fibers (brachium) passing to 6, the corpus
geniculatum internum. 7. Posterior com-
missure. 8. Pineal gland. 9. Superior cere-
bellar peduncle. 10, 11, 12. The valve of
Vieussens. 13. The pathetic nerve. 14.
Lateral groove of the isthmus. 15. Triangu-
lar bundle of the isthmus. 16. Superior
cerebellar peduncle. 17. Middle cerebellar
peduncle. 18. Inferior cerebellar peduncle.
19. Anteroinferior wall of the fourth ventri-
cle. 20. Acoustic nerve. 21. Spinal cord.
22. The postero-median column. 23. The
posterior pyramids. — {Sappey.)

ISTHMUS OF THE ENCEPHALON. 521

idal tract), which decussate at the anterior portion of the medulla.
The united fibers can be traced upward to the pons, where they dis-
appear from view.

The lateral column is composed of fibers continuous with those
of the lateral column of the cord. As the fibers pass upward, how-
ever, they diverge in several directions. The fibers of the crossed
pyramidal tract cross the median line, as previously stated, to enter
into the formation of the anterior column; the fibers of the direct
cerebellar tract gradually curve backward, and in so doing unite with
other fibers to form the restiform body, after which they enter the
cerebellum by way of the inferior peduncle. Situated between the
anterior pyramid and the restiform body is a small oval mass, the
olivary body, composed of both white and gray matter.

The posterior column is composed largely of fibers continuous with
those of the posterior column of the cord. The subdivision of this
column into a postero-external (Burdach) and a postero-internal
(Goll) is more marked in the medulla than in the cord. The former
is here known as the funiculus cuneatus, the latter as the funiculus
gracilis. These two strands of fibers are apparently continued into
the restiform body. Owing to the divergence of the restiform bodies
a V-shaped space is formed, the floor of which is covered with epithe-
lium resting on the ependyma. At the upper extremity of the funicu-
us cuneatus and funiculus gracilis, two collections of gray matter are
found, known respectively as the nucleus cuneatus and nucleus gracilis.
Around the cells of these nuclei many of the fibers of the posterior
column end in brush-like expansions.

The Fillet or Lemniscus. — From the ventral surface of the cu-
neate and gracile nuclei axons emerge which pass forward and upward
through the gray matter and decussate with corresponding fibers com-
ing from the opposite nuclei. They then assume a position just pos-
terior to the pyramids and between the olivary bodies. These fibers
thus form a new distinct tract, termed the fillet or lemniscus. As this
tract ascends toward the brain it receives additional axons from the
sensory end-nuclei of all the afferent cranial nerves of the opposite
side with the exception of the auditory. From the end-nuclei of the
auditory nerve new axons ascend as a distinct tract situated near the
lateral aspect of the pons. From their position these two separate
tracts have been termed the mesial and lateral fillets respectively.

Before proceeding to a consideration of the functions of the medulla
oblongata it will be found conducive to clearness to sketch the salient
anatomic features of the parts anterior to it and their relations one to
another.

THE ISTHMUS OF THE ENCEPHALON.

The isthmus of the encephalon comprises that portion of the
central nerve system connecting the cerebrum above, the cerebellum
behind, and the medulla below. Its ventral surface presents below

52-

TEXT-BOOK OF PHYSIOLOGY.

an enlargement, convex from side to side, the pons Varolii. On
each side the fibers of which the pons consists converge to form a com-
pact bundle, the middle peduncle, which enters the corresponding half
of the cerebellum. Above the pons, this surface presents two large
columns of white matter which, diverging somewhat from below up-
ward, enter the base of the cerebrum and are known as the crura cerebri.
Embracing the crura above are two large bands of white matter, the
optic tracts (Fig. 237).

The dorsal surface presents below two diverging columns of white
matter, the inferior peduncles; above, two converging columns, the
superior peduncles of the cerebellum (Fig. 238). At the extreme
upper part of this surface there are four small grayish eminences,
the corpora quadrigemina. From the disposition of the white matter
on the dorsal surface of the isthmus and medulla, there is formed a

lozenge-shaped space, the fourth ventricle.
The space is merely an expansion of the
central cavity of the cord, the result of
the changed relations of the white and
gray matter in this region of the central
nerve system. Above, this ventricle
communicates by a narrow canal, the
aqueduct of Sylvius, with the third ven-
tricle. The floor of the fourth ventricle
is covered with a layer of epithelium
resting on the ependyma continuous with
that lining the central canal of the cord.
Beneath this is a layer of gray matter.

The pons Varolii comprises in a
general way that portion of the central
nerve system situated between the me-
dulla oblongata and the crura cerebri.
The ventral surface is convex from side to side; the lateral surface,
owing to the convergence of the fibers of which it is composed, is
contracted to form the middle peduncle of the cerebellum; the posterior
surface is flat and forms the upper half of the floor of the fourth ven-
tricle. The pons consists of white fibers and gray matter supported
by connective tissue and neuroglia. Transverse sections of the pons
show that it is divided into an anterior or ventral, and a posterior or
dorsal portion, the latter being usually termed the tegmentum.

The ventral portion consists for the most part of white fibers, ar-
ranged longitudinally and transversely (Fig. 239). The longitudinal
fibers are largely continuations of the pyramidal tracts, or the fibers
composing the anterior pyramid of the medulla. In the lower part of
the pons these fibers are compactly arranged, but at higher levels they
are separated into a number of bundles by the interlacing of the trans-
verse fibers. The transverse fibers are divided into a superficial and
a deep set. Among these fibers are groups of nerve-cells which collec-

Fig. 239. — Transection of the
Pons through its Middle Por-
tion, Showing the Relation
of the Nerve Tracts of Which
it is Composed. D. 1. f. Dorsal
longitudinal fasciculus. L.c. and
c. Locus ceruleus. L.f. Lateral
fillet.

ISTHMUS OF THE ENCEPHALON. 523

tively are known as the nucleus ponds. Some of the transverse fibers,
especially the superficial ones, are commissural in character — i. e.,
they connect corresponding parts of the gray matter of the lateral
halves of the cerebellum; others coming from the gray matter of the
cerebellum cross the median line and terminate around the cells of
the nucleus pontis; others again are connected with the gray cells of
the same side. Through the intermediation of the nucleus pontis
and certain of the longitudinal fibers of the pons, the cerebellum is
brought into relation with the cerebrum.

The dorsal or tegmental portion consists of: (1) The fillet; (2) the
formatio reticularis; (3) the posterior longitudinal bundle; (4) the
substantia ferruginosa; (5) groups of nerve-cells from which arise
various cranial nerves— e. g., the fifth, sixth, seventh, and eighth.

The fillet or lemniscus in this region is divided into a mesial and a
lateral portion. The fibers of the mesial portion are partly the axons
of the nerve-cells of the gracile and cuneate nuclei of the opposite side
of the medulla, and partly of the axons of the sensor nerve-cells of the
afferent cranial nerves with the exception of the auditory. The fibers
of the lateral portion are mainly the axons of the cells in the floor of the
fourth ventricle around which the auditory nerve-fibers end. They
are therefore a continuation of the auditory tract.

The formatio reticularis is a continuation of that of the medulla.

The posterior longitudinal bundle is triangular in shape and situated
behind the formatio reticularis and close to the median line. The
fibers composing it are largely derived from the ground fibers of the
antero-lateral column of the spinal cord.

The superior olive is a cylindric mass of gray matter situated in
the pons in the anterior part of the formatio reticularis. It consists
of nerve-cells the axons of which pass dorso-laterally, decussate in
the median line, and form the lateral fillet of the opposite side. Some
few axons go to the lateral fillet of the same side.

The substantia ferruginosa is composed mainly of pigmented cells.

The groups of nerve-cells lying just beneath the floor of the fourth
ventricle give origin to axons of the motor portion of the fifth, the sixth,
the seventh cranial nerves. Some of the groups are the sensor end-
nuclei of the fifth and eighth cranial nerves.

The crura cerebri comprise that portion of the central nerve sys-
tem situated between the pons below and the cerebrum above. They
are composed of strands of nerve-fibers which are divided, as shown
on cross-section, into a ventral and a dorsal portion by a crescentic
shaped layer of gray matter, the substantia nigra (Fig. 240). Of the
fibers which compose the ventral portion of each crus, the crusta or pes,
the larger part is continuous below, through the longitudinal fibers of
the pons, with the pyramid of the medulla and the pyramidal tract;
above they assist in the formation of the internal capsule. On the
inner and on the outer side of each crusta there is a bundle of fibers
derived from the frontal, and from the temporal and occipital por-

5^4

TEXT-BOOK OF PHYSIOLOGY.

Fig. 240.- — Scheme of Transverse Sec-
tion of the Cerebral Peduncles. CQ
Corpora quadrigemina. Aq. Aqueduct
p. Lb. Posterior longitudinal bundle. F
Fillet or lemniscus. RX. Red nucleus. SN
Substantia nigra. III. Third nerve. Py
Pyramidal tracts. Fc. Fronto-cerebellar;
and TOC, temporo-occipital fibers of the
crusta. CC. Caudate-cerebellar fibers in
upper part of crusta. — {After Wernicke and
Gowers.)

tions of the cerebrum respectively. These fibers are connected directly
with the nuclei pontis and indirectly with the cerebellum of the same
and opposite sides. The fibers which compose the dorsal portion, the
tegmentum, are continuous with those which pass upward from the

medulla and pons, e. g., the fillet,
both mesial and lateral, the
formatio reticularis, the posterior
longitudinal bundle, and, in ad-
dition, the fibers of the superior
peduncles of the cerebellum.
Above, the fibers terminate
largely in collections of gray
matter at the base of the cere-
brum.

The aqueduct of Sylvius is a
short narrow canal which con-
nects the cavity of the fourth
with the cavity of the third
ventricle. It is lined by the
ependyma and surrounded by a
layer of gray matter continuous
with that forming the floor of
the fourth ventricle.. In that
portion of the gray matter lying
beneath or ventral to the aqueduct there are groups of nerve-cells
which give origin to axons which unite to form the third and fourth
cranial nerves.

THE CORPORA QUADRIGEMINA.

The corpora quadrigemina are four small grayish eminences
situated beneath the posterior border of the corpus callosum and behind
the third ventricle. They rest upon the lamina quadrigemina, which
forms the roof of the aqueduct of Sylvius. The anterior pair are termed
the nates, or the pregemina, the posterior pair the testes, or the post-
gem ina.

From the external surface of each body there pass outward bundles
of fibers termed brachia. The fibers which compose the brachium
of the pregeminum pass outward and enter a small collection of gray
matter, the corpus geniculalum externum, and the optic tract. The
fibers which compose the brachium of the postgeminum are divided
into two bundles, one of which enters a second small collection of gray
matter, the corpus geniculatum internum, while the other passes forward
beneath this body to enter the internal capsule, beyond which it passes
to the cortex of the temporal region of the cerebrum.

Though these bodies are closely associated anatomically, they
differ in origin, in their relations and in their functions.

Microscopic examination of sections of the quadrigeminal bodies

BASAL GANGLIA. 525

shows that they are composed of nerve-ceils and nerve-fibers, both of
which are so intricately arranged that it is difficult to trace their rela-
tion one to another and to adjoining structures. Some of the cells of
the pregeminum give off axons which course outward and forward,
enter the internal capsule, and pass through the optic radiation to the
cortex of the occipital region of the cerebrum. Many fibers of the
optic tract, axons of the cells of the retina, end in brush-like expansions
around these same cells. There is thus formed a connected pathway
between the retina and the occipital cortex.

The cells of the occipital cortex, however, send axon fibers in the
reverse direction through the optic radiation to terminate around the
cells of the pregeminum, while axons of pregeminal cells pass for-
ward to the retina and to the cells of origin of the third nerve.

The cells of the postgeminum give origin to axons which pass up-
ward, forward, and outward, enter the internal capsule, and pass by
way of the auditory tract to the cortex of the temporo-sphenoidal
region of the cerebrum. Many of the fibers of the lateral fillet, a
portion of the auditory tract, terminate in brush-like expansions
around these same cells. There is thus established a connected path-
way between the cochlea and the temporo-sphenoidal cortex. The
cells of the temporal cortex, however, send axons in the reverse direc-
tion by way of the auditory tract to the cells of the postgeminum.
There is thus established a double communication between the occipital
and temporal region of the cerebral cortex, and the pregeminal and
postgeminal bodies respectively.

THE BASAL GANGLIA; THE CORPORA STRIATA AND
OPTIC THALAMI.

The basal ganglia surmount the crura cerebri, but are only made
visible by removal of the cerebrum (Fig. 241).

The corpora striata are two large ovoid collections of gray and
white matter situated at the base of the cerebrum. The larger portion
of each body is embedded in the cerebral white matter, while the
smaller portion projects into the anterior part of the lateral ventricle.
A transection of the corpus striatum shows that it is divided by a band of
white matter into two portions, viz. :

1. The caudate nucleus, the intra- ventricular portion, convex in shape

with its base directed forward, its apex or tail directed backward
and downward.

2. The lenticular nude us, the extra-ventricular portion, somewhat bicon-

vex in shape and embedded largely in the white matter. Each len-
ticular nucleus is subdivided by two lamina of white matter into
three portions. The two inner, from their pale yellow color,
form the globus pallidas, the outer, somewhat darker in color, is
the put a men.
The Internal Capsule. — The band of white matter separating the
caudate from the lenticular nucleus has been termed the internal cap-

526 TEXT-BOOK OF PHYSIOLOGY.

sule from the manner in which it embraces the inner surface of the
lenticular nucleus. It consists of nerve-fibers which associate histo-
logically and physiologically all portions of the cerebral cortex with the
optic thalamus, pons, medulla, spinal cord, and cerebellum. The
relation of the capsule to the nuclei through which it passes is readily

Fig. 241. — Dissection of Brain, from above, Exposing the Lateral Fourth and
Fifth Ventricles with the Surrounding Parts, h. — a. Anterior part, or genu of
corpus callosum. b. Corpus striatum, b'. The corpus striatum of left side, dissected so
as to expose its gray substance, c. Points by a line to the taenia semicircularis. d. Optic
thalamus, e. Anterior pillars of fornix divided; below they are seen descending in front
of the third ventricle, and between them is seen part of the anterior commissure; in front
of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum
lucidum. /. Soft or middle commissure; g is placed in the posterior part of the third
ventricle; immediately behind the latter are the posterior commissure (just visible) and the
pineal gland, the two crura of which extend forward along the inner and upper margins
of the optic thalami. h. and i. The corpora quadrigemina. k. Superior crus of cere-
bellum. Close to k is the valve of Vieussens, which has been divided so as to expose the
fourth ventricle. /. Hippocampus major and corpus fimbriatum, or taenia hippocampi.
m. Hippocampus minor, n. Eminentia collateralis. 0. Fourth ventricle, p. Posterior
surface of medulla oblongata, r. Section of cerebellum. 5. Upper part of left hemisphere
of cerebellum exposed by the removal of part of the posterior cerebral lobe. — {Hirschjeld
and Leveille.)

shown on cross-section (Fig. 242). The appearance which it presents,
however, varies considerably at different levels. At a given level it
may be said to consist of two segments or limbs, an anterior, situated
between the caudate nucleus and the anterior extremity of the lenticu-
lar nucleus, and a posterior, situated between the optic thalamus and
the posterior extremity of the lenticular nucleus. The two segments

BASAL GANGLIA.

527

unite at an obtuse angle, termed the knee, which is directed toward the
median line.

The optic thalami are two oblong masses of gray matter situated
upon the crura cerebri and behind the corpora striata. The anterior and
posterior extremities of each thalamus present enlargements known
respectively as the anterior tubercle and the posterior tubercle or pul-
vinar. The mesial surface of the thalamus forms the lateral wall of t he-
third ventricle and is covered by epithelium resting on a thin layer of
ependyma.

A transection of the thalamus shows
that it is not only covered externally but
penetrated by white matter, which sub-
divides its contained gray cells into four
more or less distinct masses termed
nuclei, viz., an anterior, a lateral, occu-
pying the external part of the thalamus,
a ventral, close to the entire ventral sur-
face, and a posterior, situated beneath
the pulvinar. Beneath and somewhat
internal to each optic thalamus there is
a region, the subthalamic, consisting of
an intricate network of nerve-fibers and
several nuclei of gray matter, e. g., the
red or tegmental nucleus, the subthalamic
nucleus, or Luys' body, and the sub-
stantia nigra.

Though the thalamus has extensive
connections with many portions of the
central nerve system, the most important
are with the cortex, the tegmentum, and
the optic tracts.

From the cells of these various nuclei
axons emerge which pass into the in-
ternal capsule, and through the corona
radiata to all portions of the cortex.
Those which come from the pulvinar and pass to the occipital lobe
constitute a part of the optic radiation; those from the lateral and
ventral nuclei ultimately reach the parietal lobe; those from the
anterior nucleus pass to the hippocampal and uncinate convolutions.
In a similar manner all portions of the cortex are brought into relation
with the thalamus, axons from the cortical cells passing downward to
terminate in tufts around the thalamic nuclei.

The tegmentum is intimately related to the thalamus, though the
exact distribution of various strands of fibers is a subject of much dis-
cussion. Most of the libers of the mesial fillet end in tufts around the
cells of the ventral and lateral nuclei; other fibers pass directly to the
cortex.

Fig. 242. — Horizontal Sec-
tion of the Internal Cap-
sule showing its Relations
to the Caudate Nucleus,
Optic Thalamus, and the
Lenticular Nucleus, i.
Caudate nucleus. 2. Anterior
segment 0/ the internal capsule.
3. External capsule. 4. Len-
ticular nucleus. 5. Claustrum.
6. Posterior segment of internal
capsule. 7. Optic thalamus. —
(Modified from Landois.)

528 TEXT-BOOK OF PHYSIOLOGY.

The optic tract sends fibers directly into the pulvinar, around the
cells of which they terminate in brush-like expansions.

SUMMARY OF THE STRUCTURE OF THE MEDULLA, ISTHMUS,
AND BASAL GANGLIA.

Structure of the Central Gray Matter. — Though the general
arrangement of the central gray matter has been incidentally alluded
to in the foregoing presentation of the anatomic features of the medulla
and isthmus, it will be convenient to summarize its arrangement and
structure at this point.

The gray matter of the cord, of the dorsal aspect of the medulla
and pons, of the region surrounding the aqueduct of Sylvius, and of the
lining of the third ventricle, constitute practically a continuous system,
though presenting modifications in various parts of its extent. In
the transition region of the spinal cord and medulla the gray matter
of the former becomes much changed in shape owing to the shifting
of position of the various tracts of white matter, until in the medulla
and pons it is spread out in the form of a thin layer near their dorsal
surfaces, where, together with the ependyma, it forms the floor of
the fourth ventricle.

In the region of the aqueduct of Sylvius the gray matter again
converges and ultimately surrounds the canal, to again expand at its
anterior extremity, to form the lining of the third ventricle.

The Nerve-cells. — The nerve-cells in these different regions do
not differ morphologically from those in the gray matter of the spinal
cord. The corpus, or body of the cell, presents a number of dendrites
as well as the sharply defined axon. As a rule, the cells are arranged
in groups, or clusters, or nests, partially surrounded and enclosed by
supporting tissue, and situated beneath the floor of the fourth ventricle
and the floor of the aqueduct of Sylvius. From some of the cell groups
axons pass ventrally through the white matter to merge on the ventral
and lateral surfaces of the medulla, pons, and crura, where they are
known as efferent or motor cranial nerves. From other groups of cells,
axons cross the. median line, and after joining the mesial fillet ascend
toward the cerebrum. Around these latter cells the terminal filaments
of the afferent or sensor cranial nerves arborize. The collection of
cells found in the central gray matter may be divided into two groups —
efferent and afferent.

The efferent cells, like those of the cord independent of a trophic
influence, are motor in function, inasmuch as the excitation arising
in them is transmitted outward through their related axons to mus-
i les, glands, viscera or blood-vessels, imparting to them motion, either
molar or molecular.

The afferent cells are largely sentient or receptive in function,
inasmuch as the excitations brought to them by the afferent cranial
nerves from skin and mucous membranes and from sense-organs,

MEDULLA AND BASAL GANGLIA.

529

such as the tongue and ear, are received by them and transmitted
through their ascending axons to the cortex of the cerebrum, where
thev are translated into conscious sensations.

Structure of the White Matter. — The white matter is com-
posed of medullated nerve-fibers, and though arranged in a very
complex manner may be divided into longitudinal and transverse
fibers.

The longitudinal -fibers which compose the main portion of the
isthmus may be subdivided into (1) a ventral or pedal portion and (2)

Fig. 243. — Diagrammatic Arrangement of the Projection Tracts Connecting
the Cerebral Cortex with the Lower Nerve-centers. A. Fronto-cerebellar
tract. B. The pyramidal or motor tract. C. Sensory tract. D. Visual tract from
optic thalamus (6.T.) to the occipital lobe. E. Central auditory tract. F. Superior
cerebellar peduncle. G. Middle cerebellar peduncle. H. Inferior cerebellar peduncle.
C.N. Caudate nucleus. C.Q. Corpora quadrigemina. Vt. Fourth ventricle. The
numerals refer to cranial nerves. J. Eighth nerve nucleus. — {After Starr.)

a dorsal or tegmental portion. The fibers constituting the ventral
or pedal portion may for convenience be said to extend from the cere-
bral cortex to the pons, medulla, and spinal cord. They may be di-
vided into three distinct tracts: e. g., the pyramidal tract, the fronto-
cerebellar tract, and the occipito-temporo-cerebellar tract (Fig. 243).
The pyramidal tract descends from the cortex of the cerebrum
bordering the fissure of Rolando, passes through the posterior one-
third of the anterior segment and the anterior two-thirds of the pos-
terior segment of the internal capsule, the middle two-fifths of the
crusta, behind the transverse fibers of the pons, to become the anterior
34

530 TEXT-BOOK OF PHYSIOLOGY.

pyramids of the medulla, beyond which it divides into the direct and
crossed pyramidal tracts of the cord. In its course some of the fibers
and their collaterals arborize around efferent cells from the anterior
extremity of the aqueduct of Sylvius to the termination of the spinal
cord.

The fronto-cerebellar tract descends from the cortex of the frontal
portion of the anterior lobe, passes through the anterior portion of the
anterior segment of the internal capsule, the inner fifth of the crusta
to the pons, where its fibers terminate or arborize around'the nucleus
pontis of the same and opposite sides.

The occipito-temporo-cerebellar tract descends from the occipital
and temporal lobes, passes to the inner side of the lenticular nucleus,
and continues downward on the outer side of the crusta, occupying
about one-fifth of its bulk, to the pons, where its fibers also arborize
around the nucleus pontis of the same and opposite sides. By means
of fibers in the middle peduncle these descending fibers are brought
into relation with the cerebellum.

The fibers constituting the dorsal or tegmental portion of the longit-
udinal system may be said for convenience to extend from the pos-
terior portion of the medulla and pons to the optic thalamus and cere-
brum. They may be subdivided into several tracts: viz., the fillet,
the posterior longitudinal bundle, Gowers' tract, etc.

The fillet or lemniscus, consisting of fibers having their origin
partly from the cells of the cuneate and gracile nuclei and partly from
the cells of the sensor end-nuclei of various sensor cranial nerves,
occupies a region in the ventral and mesial portion of the tegmentum
throughout its entire extent. Superiorly this mesial fillet divides
into two portions, one of which passes to the thalamus and pregem-
inum (anterior corpus quadrigeminum), the other to the cortex of the
parietal and limbic lobes. The fibers coming from the sensor end-
nucleus of the auditory nerve (the lateral fillet) lie on the lateral aspect
of the pons and crus. Superiorly they terminate in the postgeminum
(the posterior corpus quadrigeminum).

The posterior longitudinal bundle, an upward extension of the fibers
composing a portion of the ground bundle of the spinal cord, is located
on cither side of the median line just beneath the floor of the fourth
ventricle and the aqueduct of Sylvius. As it passes upward collateral
branches are given off, some of which arborize around the cell nuclei
of the third, fourth, and sixth cranial nerves of the same side, while
others cross the median line and arborize around the corresponding
cell nuclei of the opposite side. Superiorly some of the fibers become
related to cells in the thalamus and subthalamic region. This bundle
of fibers appears to be mainly commissural in character.

Gowers' tract, the antero-lateral tract of the spinal cord, occupies
a position in the lateral region of the formatio reticularis both in the
medulla and pons. Continuing upward, it enters the mesial fillet,
and in company with it passes through the posterior division of the

FUNCTIONS OF THE MEDULLA OBLONGATA. 531

internal capsule and finally terminates around cells in the cortex of
the parietal lobe.

The transverse -fibers of the isthmus are found in the pons. The
fibers of the ventral as well as those of the more dorsal regions have
their origin in nerve-cells in the cprtex of the cerebellum. From their
origin they pass through the cerebellar white matter, and through
the middle peduncle as far as the median line, where they decussate
with fibers coming from the opposite side. Beyond this point they
pass to the cerebellar cortex. From their anatomic relations it is prob-
able that these transverse fibers are commissural in character, bring-
ing into relation opposite but corresponding regions of the cerebellar
cortex. In addition to the commissural fibers other transverse fibers
associate the cerebellar cortex with the gray matter in the pons on
both the same and opposite sides. In this way the cerebellum is
brought into relation with longitudinal fibers coming from and going
to the cerebrum.

FUNCTIONS OF THE MEDULLA OBLONGATA, ISTHMUS, AND
BASAL GANGLIA.

Microscopic examination of the white and gray matter of these
various parts of the central nerve system shows that they are com-
posed of nerve-cells and nerve-fibers which morphologically do not
differ in essential respects from those found in the spinal cord, though
their arrangement is far more complicated and involved. The func-
tions of these closely related structures are in consequence equally
complex and involved and but imperfectly known.

In a general way it may be said that by virtue of the presence
of nerve-cells and definite tracts of nerve-fibers these structures col-
lectively may be regarded as consisting:

1. Of centers for reflex actions; and —

2. Of conducting paths by which the various parts are brought into

relation one with another and with the spinal cord, the cerebel-
lum, and the cerebrum.

The Medulla Oblongata and Pons. — The gray matter situated
in these structures — i. e., just beneath the floor of the fourth ventricle
— contains nerve-cells arranged in more or less well-defined groups
which may be divided into efferent and afferent.

The efferent cells are the immediate sources of nerve impulses which
are transmitted through efferent axons to various peripheral organs-
muscles, glands, viscera and blood-vessels. Their activity may be ex-
cited by the same influences which excite the efferent cells of the spinal
cord: e. g., variations in the composition of the blood or lymph; the ar-
rival of nerve impulses coming through afferent pathways in the spinal
cord and through afferent cranial nerves; the arrival of nerve impulses
coming through efferent pathways from the cerebrum. The peripheral
activity resulting from their excitation may therefore be automatic or
autochthonic, peripheral (reflex) or cerebral (volitional) in origin.

532 TEXT-BOOK OF PHYSIOLOGY.

The afferent cells are sentient or receptive in function, inasmuch
as they receive nerve energies coming through lower afferent pathways
and transmit them through their related axons to the cortex of the
cerebrum, where they are translated into conscious sensations.

The efferent cells give origin tq nerve-fibers which pass ventrally
and become the efferent or motor cranial nerves.

The afferent cells give origin to fibers which pass to the cerebral
cortex. Around both groups of cells, the afferent or sensor cranial
nerves terminate in tuft-like expansions. In a subsequent section the
origin, course, and distribution of the various cranial nerves will be
considered. But as the function of the nerve is but to transmit energy
from the cell of which it constitutes a part, the function ascribed to it
may without impropriety be transferred to the cell itself.

Since it is by means of nerve-cells and their associated fibers that
many important functions of organic life are initiated and maintained,
it would naturally be expected from its extensive nerve connections
that this region of the nerve system plays an extensive role in this
respect. As the accomplishment of these functions requires the
cooperation and coordination of a number of separate but related
structures, it is evident that there must exist in the medulla and pons
a number of coordinating mechanisms consisting of nerve-cells and
nerve-fibers which are associated in various ways for the accomplish-
ment of definite functions. To such a coordinating mechanism the
term "center" has been given: e. g., respiratory, cardiac, deglutitory,
etc.*

The Medulla Oblongata and Pons as Centers for Reflex Activ-
ities.— Experimentation has shown that the medulla and pons contain
a number of such centers, the more important of which are as follows:
i. Cardiac centers, which exert (i) an accelerator action over the
heart's pulsations through nerve-fibers emerging from the spinal
cord in the roots of the first and second dorsal nerves and reach-
ing the heart through the sympathetic nerve; (2) an inhibitor
or retarding action on the rate of the heart beat through efferent
fibers in the trunk of the pneumogastric or vagus nerve. (See pages

3*2> 3I3-)

2. A vaso-motor center, which regulates the caliber of the blood-

vessels throughout the body in accordance with the needs of the
organs and tissues for blood, through nerve-fibers passing by
way of the spinal nerves to the walls of the blood-vessels. (See
page 376.)

3. A respiratory center, which coordinates the muscles concerned in

the production of the respiratory movements. (See page 428.)

* By the term center as here employed is meant a collection of nerve-cells and nerve-
fibers occupying an area of greater or less extent, though its exact anatomic limits may
not Ik- ac( urately defined. That an area may merit the term center, it is necessary that
its stimulation should increase, its destruction should abolish or impair, functional
activity.

FUNCTIONS OF THE CRURA CEREBRI. 533

4. A mastication center, which excites to activity and coordinates the

muscles of mastication. (See page 150.)

5. A deglutition center, which excites and coordinates the muscles

concerned in the transference of the food from the mouth to
the stomach. (See page 170.)

6. An articulation center, which coordinates the muscles necessary to

the production of articulate speech.

In addition, the gray matter contains centers which influence the
secretion of saliva, provoke vomiting, coordinate the muscles of the
face concerned in expression, and control the secretion of the perspira-
tion.

As Conducting Pathways. — The anterior pyramids of the medulla
and their continuations through the more ventral portions of the pons,
being portions of the general pyramidal tract, serve to conduct voli-
tional efferent nerve impulses from higher portions of the brain to the
spinal cord. Division of either of these pathways is at once followed
by a loss of volitional control of the muscles below the section.

The dorsal or tegmental portion, containing the fillet and Gowers'
tract, serves to transmit afferent nerve impulses from the spinal cord
to higher portions of the brain. Transverse division of one-half
of the dorsal portion of the pons is followed by complete anesthesia
of the opposite half of the body without any impairment of motion.

The restiform bodies constitute a pathway between the spinal cord
and the cerebellum. The transverse fibers of the pons associate
opposite but corresponding portions of the cerebellar hemispheres.

The Crura Cerebri. — The crura cerebri consist ventrally of fibers
which are largely derived from the pyramidal tracts and are con-
tinuous with the longitudinal fibers of the ventral portion of the pons
and medulla; and dorsally of fibers continuous with those coming
through the lower portions of the tegmentum. Hence they are con-
ductors of motor impulses in the former and of sensor impulses in the
latter region. It is not definitely known as to whether reflex actions
take place through the gray matter, the locus niger, or not.

The gray matter beneath the aqueduct of Sylvius contains nerve-
cell groups which are centers for reflex actions in connection with
ocular movements: e. g., closure of the lids, contraction of the sphinc-
ter pupillae, convergence of the eyes, etc.

The Corpora Quadrigemina.— From the anatomic relation of
the anterior quadrigeminal body (the pregeminum) to the optic tract,
on the one hand, and to the optic radiation, on the other, the inference
can be drawn that it is in some way essential to the performance of
the visual process. Experimental investigations and pathologic
changes support the inference.

Irritation of the pregeminum in monkeys on one side is followed
by dilatation of the pupils first on the opposite side and then almost
immediately on the same side. The eyes at the same time are also
widely opened and the eyeballs turned upward and to the opposite

TEXT-BOOK OF PHYSIOLOGY.

side. If the irritation be continued, motor reactions are exhibited
in various parts of the body. Destruction of the pregeminum in both
monkeys and rabbits is followed by blindness, dilatation and immo-
bility of the pupils, with marked disturbance of equilibrium and
locomotion (Ferrier).

From the anatomic relation of the posterior quadrigeminal body
(the postgeminum) to the lateral fillet, the basal tract for hearing,
the inference may be drawn that it is in some way connected with the
auditory process.

Stimulation of the postgeminum gives rise to cries and various forms

of vocalization. Pathologic
states of this body are also
attended by impairment of
hearing and disorders of
the equilibrium.

From the foregoing facts
it is probable that the cor-
pora quadrigemina are as-
sociated with station and
locomotion. Ferrier as-
sumes that in these bodies
"sensory impressions, ret-
inal and others, are co-
ordinated with adaptive
motor reactions such as are
involved in equilibration
and locomotion."

The Corpora Striata.
— The relation of these
bodies to the pyramidal
motor tract would indicate
that they are in some way
connected with motor ac-
tivities. Their function,
however, is obscure. While
stimulation of one corpus
produces convulsion of the muscles of the opposite side of the body,
and destruction gives rise to paralysis of the corresponding muscles, it
is difficult, owing to the intimate association of the white and the gray
matter, to state to which the phenomena are to be attributed. The
evidence at hand points to the conclusion that if a lesion is limited to
the gray matter the paralysis which might result would be but
temporary and of short duration. The pathologic evidence is of a
similar character. Gowers is of the opinion, that if the lesion is small
and at a sufficient distance from the white fibers of the capsule, there
may even be no initial hemiplegia; neither motor nor sensory paralysis
will arise if the lesion is confined to the gray matter.

Fig. 244. — Horizontal Section of the Inter-
nal Capsule Showing the Position and Rela-
tion oe the Motor Tracts for the Eye, Head
(Hd.), Tongue (Tg.), Mouth (Mth.), Shoulder
(Shi.), Elbow (Elb.), Digits of Hand (Dig.),
Abdomen (Abd.), Hip, Knee (Kn.), Digits of
Foot (Dig.). S. Sensor tract. O. T. Optic tract.
A. T. Auditory tract. 1. Caudate nucleus. 2.
Anterior segment of internal capsule. 3. Ex-
ternal capsule. 4. Island of Reil. 5. Lenticular
nucleus. 6. Claustrum. 7. Posterior segment of
internal capsule. — (Modified from Landois.)

FUNCTION'S OF THE INTERNAL CAPSULE.

It is stated by some experimenters that localized injuries, both
experimental and pathologic, are followed by a persistent rise of
temperature, varying from i° to 2.6° C.

The Optic Thalami. — From the anatomic relation of the optic
thalami to the general and special sense nerve-tracts, on the one hand,
and to the cerebral cortex, on the other hand, it is assumed that they
are connected with the production of sensations both general and
special, and act as intermediates between the peripheral sense-organs
and the cortex.

The results of experimental stimulation and destruction of the
thalami are extremely con-
tradictory and fail to throw
much light on their func-
tions. Ferrier states that
destruction of the posterior
part of one thalamus pro-
duced blindness in the op-
posite eye and impairment
of the sense of touch and
pain in the opposite side of
the body. In a patient
under the care of Hugh-
lings- Jackson there was
blindness in the right half
of each eye, loss of hearing
in the left ear, impairment
of taste on the left side of
the tongue, and a diminu-
tion of the sense of touch
on the left side of the body.
Postmortem examination
showed a patch of soften-
ing in the posterior part of
the right thalamus, the re-
mainder of the organ being
normal.

Fig. 245. — Vertical Sectiox Through the-
Right Cerebral Hemisphere in Front of the
Gray Commissure. 1. Caudate nucleus. 2. Cor-
pus callosum. 3. Pillars of the fornix. 4. Internal
capsule. 5. Optic thalamus. 6. Gray commissure.
7. External capsule. S. Claustrum (1, 2, 3, the
three divisions of the lenticular nucleus). — (Landois.)

It is probable that in the thalamus visual, tactile, and labyrinthine
impressions are received, coordinated, and reflected outward, with
the result of producing various adaptive motor reactions connected
with station and ecpuilibrium. It is also believed by some investigators
to act as an intermediate between emotional states and their expres-
sion in the muscles of the face, this power being lost in certain patho-
logic conditions. The power of regulating the temperature of the
body has also been assigned to the thalamus, as destruction of its
anterior extremity is usually followed by a rise in temperature.

The Internal Capsule. — The internal capsule has been shown by
the results both of experiment and of pathologic processes to be, first,

536 TEXT-BOOK OF PHYSIOLOGY.

a pathway for the transmission of nerve impulses from the cerebral
cortex to the pons, medulla, and spinal cord, which give rise to con-
traction of the muscles of the opposite side of the body; and, second,
a pathway for the transmission of nerve impulses coming from skin,
mucous membrane, muscles, and special sense-organs to the cortex,
where they give rise to sensations general and special. It is therefore
the common motor and sensor pathway. For the reason that it trans-
mits both motor and sensor impulses, and for the further reason that
it is frequently the seat of pathologic lesions which are followed by
either a loss of motion or sensation or both, the internal capsule is one
of the most important parts of the central nerve system. As shown
in Fig. 244, it consists of two segments or limbs united at an obtuse
angle, the knee or elbow, which is directed toward the median line.
The motor tract is confined to the posterior one-third of the anterior
segment and the anterior two-thirds of the posterior segment. The
sensor tract is confined to the posterior one-third of the posterior seg-
ment, the extreme end of which also contains the optic and auditory
tracts.

The region of the anterior segment in front of the motor tract
contains the fibers of the fronto-cerebellar tract, the function of which
is unknown.

The motor region contains fibers which descend from the cerebral
cortex to nerve-centers situated in the gray matter beneath the aque-
duct of Sylvius, in the gray matter beneath the floor of the fourth ven-
tricle, and in the anterior horns of the gray matter of the spinal cord,
and which in turn are connected by the cranial and spinal nerves with
the muscles of the eye, head, face, trunk, and limbs. The positions
occupied by these different tracts are shown in Fig. 244.

The relation of the internal capsule to the caudate nucleus and
the optic thalamus internally, and to the lenticular nucleus externally,
is also shown in a vertical section of the cerebrum made in front of the
gray commissure (Fig. 245). From the fact that the internal capsule
contains efferent or motor tracts, and afferent or sensor tracts, it is
evident that a destructive lesion of the motor tract would be followed
by a loss of motion; and of the sensor tract, by a loss of sensation on
the opposite side of the body.

CHAPTER XXI.
THE CEREBRUM.

The cerebrum is the largest portion of the encephalon, constitut-
ing about 85 per cent, of its total weight. In shape it is ovate, convex
on its outer surface, narrow in front and broad behind. It is divided
bv a deep longitudinal cleft or fissure into halves, known as the cerebral
hemispheres. The hemispheres are completely separated anteriorly
and posteriorly by this fissure, but in their middle portions are united
bv a broad white band, the corpus callosum. Each hemisphere or
hemi-cerebrum is convex on its outer aspect, and corresponds in a
general way with the cavity of the skull; the inner or mesal surface is
flat and forms the lateral boundary of the longitudinal fissure.

The surface of each hemi-cerebrum presents a series of alternate
indentations and elevations, known respectively as fissures or sulci,
and convolutions or gyres. A knowledge of the situation and extent
of the principal fissures and convolutions, as well as of their relation
one to another, is essential to a clear understanding of many phys-
iologic processes, clinical phenomena, and surgical procedures, The
general arrangement of the primary fissures and convolutions is rep-
resented in Figs. 246 and 247.

Fissures. —

1. The fissure of Sylvius, one of the most important of the primary

fissures, is found on the side of the cerebrum. It begins at the
base and extends upward, outward, and backward to a point
corresponding to the eminence of the parietal bone, where it
usually terminates in a more or less vertically directed branch,
the epi- sylvian branch. Anteriorly a short branch is given off
which passes upward and forward into the frontal lobe and
known as the pre-sylvian; a horizontal branch is known as the
sub-sylvian. The Sylvian fissure is the first to appear in the de-
velopment of the fetal brain, becoming visible at the third month.
In the adult it is deep and well marked and divides the hemi-
cerebrum into a frontal and a temporo-sphenoidal lobe.

2. The fissure of Rolando, or central fissure, equally important, is

found on the superior and lateral aspects of the cerebrum. It
runs from a point on the convexity of the hemisphere near the
median line transversely outward and downward toward the
fissure of Sylvius, but as a rule does not pass into it. It divides
the frontal from the parietal lobe. The inclination of the central
fissure h such as to form with the longitudinal fissure an angle
of about 67 degrees.

537

53*

TEXT-BOOK OF PHYSIOLOGY.

The infra-parietal fissure arises a short distance behind the central
fissure. It then runs upward, backward, and downward to
terminate near the posterior extremity of the parietal lobe. It
divides the parietal lobe into a superior and an inferior portion.

The parieto-occipital fissure, situated on the mesal surface of the
hemisphere, divides the latter into a parietal and an occipital
lobe. It begins as a deep notch on the surface of the hemisphere,
and is then continued downward and forward until it enters the
calcarine fissure. (Fig. 247.)

Fig. 246. — Diagram Showing Fissures and Convolutions on the Lateral
Aspect of the Left Hemi-Cerebrum. F. Frontal. P. Parietal. T. Temporal and
O. Occipital lobes. S. Fissure of Sylvius. EPS. Epi-sylvian, P R S. Pre-sylvian, S B S.
Sub-sylvian fissures. C. Central fissure or Fissure of Rolando. P R C. Pre-central fis-
sure. SPFR. Super-frontal fissure. MEFR. Medi-frontal fissure. SBFR. Sub-
frontal fissure. PC. PC. Post-central fissure. PTL. Parietal fissure. PAROC.
Par-occipital, EXOCC. Ex-occipital fissures. SPTMP. Super-temporal fissure. MTMP
Medi-temporal fissure.

5. The calcarine fissure begins on the posterior extremity of the mesal

surface of the occipital lobe. From this point it passes downward
and forward to unite with the parieto-occipital fissure.

6. The para-central fissure begins at the supero-mesal border of the

hemisphere. It then passes downward and forward for a varia-
ble distance and then turns upward enveloping a lobule known
as the para-central lobule.

7. The su per -c alio sal fissure extends from a point just anterior to the

para-central lobule downward and forward below the rostrum of

the corpus callosum.
Secondary fissures of more or less importance are present in the
different lobes, subdividing the surface into convolutions: e. g., in the
frontal lobe arc found the pre-central, the super- frontal, medi-jrontal
and sub-jrontal fissures; in the temporal lobe the super-temporal and
medi- temporal fissures.

THE CEREBRUM.

5^9

Convolutions. — The convolutions or gyres are the portions of
the cerebral surface comprised between the fissures. The arrange-
ment of the surface is such that only the more superficial portions are
visible. The depth of the convolution, the portion bordering the
fissure, is concealed from view. Each lobe presents a series of such
convolutions which differ considerably in their relative physiologic
importance.

The Frontal Lobe. — The frontal lobe presents on its convex

surface four convolutions : viz., the anterior or pre-central convolution,

and the super-, medi, and sub-frontal convolutions.

i. The anterior or pre-central convolution is situated just in front of

the Rolandic or central fissure, with which it corresponds in

Fig. 247. — Diagram Showing Fissures and Convolutions on the Mesal As-
pect of the Left Hemi-cerebrum. C. Upper extremity of the central fissure. PARC.
Para-central fissure. S P C L. Super-callosal fissure. C L. Callosal fissure. O C. Oc-
cipital fissure. CLC. Calcarine fissure. CLT. Collateral fissure.

direction. It is continuous above with the super-frontal and
below with the sub-frontal convolution.

2. The super-frontal convolution is bounded internally by the

longitudinal fissure and externally by the super-frontal fissure.
From the upper end of the pre-central convolution, with which
it is continuous, it runs forward and downward to the anterior
extremity of the frontal lobe, where it turns backward and rests
on the orbital plate of the frontal bone.

3. The medA- frontal convolution is situated on the side of the lobe,

between the super-frontal fissure above and the medi-frontal
fissure below. Its general direction is downward and forward.

4. The sub-frontal convolution winds around the pre-sylvian branch

of the fissure of Sylvius in the anterior and inferior portion of

54o TEXT-BOOK OF PHYSIOLOGY.

the frontal lobe. It is continuous posteriorly with the lower end

of the pre-central convolution.
The Parietal Lobe. — The parietal lobe presents three well-
marked convolutions: viz., the posterior or post-central convolution,
and the super- and sub-parietal. The latter is again subdivided
into the marginal and the angular convolution.

i. The posterior or post-central convolution is situated just behind
the Rolandic or central fissure, with which it corresponds in
direction. Above, it is continuous with the super parietal
convolution; below, with the marginal and the pre-central con-
volutions.

2. The super-parietal convolution is bounded internally by the longit-

udinal fissure and externally by the intra-parietal fissure. From
the upper end of the post-central convolution, with which it is
connected, it runs downward and backward as far as the parieto-
occipital fissure.

3. The sub-parietal convolution is connected anteriorly with the

post-central convolution. Passing backward, it winds around
the superior extremity of the fissure of Sylvius, in which situa-
tion it is known as the supra-marginal convolution. Beyond
this point it divides into two portions, one of which runs forward
into the temporal lobe above the super-temporal fissure, while
the other runs downward and backward, following the intra-
parietal fissure to its termination. At this point it makes a sharp
bend and runs forward into the temporal lobe just beneath the
super-temporal fissure. In the neighborhood of the bend it is
generally known as the angular convolution or gyrus.

The Temporo-sphenoidal Lobe. — The temporo-sphenoidal lobe
presents on its external surface three well-marked convolutions: viz.,
the super-, the medi-, and the sub-temporal, separated by the super- and
medi- temporal fissures. These three convolutions are in a general
way parallel with each other, and pursue a direction from before
backward and upward. Anteriorly, they are fused together, but
posteriorly their connections are somewhat different. The super-
temporal is continuous behind and above with the supra-marginal
convolution, and behind and below with the angular convolution or
gyre. The medi-temporal blends with the preceding and with the
middle occipital. The sub-temporal is continuous with the inferior
occipital.

The Occipital Lobe. — The occipital lobe is triangular in shape
and forms the posterior apex of the hemisphere. Its base on the
external surface is formed by an imaginary line drawn from the
parieto-occipital fissure to the pre-occipital notch on the inferior and
lateral border. The external surface presents three convolutions —
the superior, middle, and inferior occipital.

The inner or mesal surface of the hemisphere, formed in part

THE CEREBRUM. 541

by the frontal, the parietal, the occipital, and the temporal lobes, pre-
sents several convolutions of much physiologic interest, viz. :

1. The callosal convolution, situated between the super-callosal

fissure and the corpus callosum. From its origin anteriorly
at the base of the brain this convolution passes backward, grad-
ually increasing in width as it approaches the posterior extremity
of the corpus callosum. At this point it again narrows and
descends between the calcarine and hippocampal fissures to blend
with the hippocampal convolution.

2. The gyrus hippocampus, formed by the union of the posterior

extremity of the callosal convolution and the sub-calcar-
ine convolution is situated just below the dentate or hippo-
campal fissure. Anteriorly it becomes enlarged, and just
behind the apex of the temporal lobe turns backward and
inward to form a hook-shaped eminence, the uncinate gyrus
or uncus.

The limbic lobe is the name given to an area of the brain which
includes, among other structures, the callosal convolution, the gyrus
hippocampus, and the uncus. As forming a part of this general
lobe may be mentioned the dentate fascia, the strise and peduncle
of the corpus callosum, the septum lucidum, the fornix, and the
infracallosal gyrus.

3. The collateral convolution is bounded by the collateral fissure

above, and its inferior border extends from the occipital lobe to
the anterior pole of the temporal lobe.

4. The quadrate lobule, or precuneus, a square-shaped convolution,

is situated between the posterior termination of the para-central
fissure and the parieto-occipital fissure. It blends with the callosal
convolution, on the one hand, and with the parietal lobule on the
other.

5. The cuneus, a triangular or wedge-shaped convolution or lobule,

is situated on the mesal surface of the occipital lobe between the
parieto-occipital and calcarine fissures.

The Insula or Island of Reil. — This anatomic structure con-
sists of a triangular shaped cluster of six small convolutions situated
at the bifurcation of the Sylvian fissure and concealed from view
by the convolutions bordering it, spoken of collectively as the oper-
culum. These convolutions are connected with the frontal, the
parietal, and the temporal lobes.

Structure of the Gray Matter of the Cortex. — The gray matter,
the cortex of the cerebrum, varies from two to four millimeters in
thickness. When examined with a lens of low power, it presents a
laminated appearance, due to differences in color and arrangement
of its constituent elements. With higher magnification the cortex
is seen to consist of neuroglia cells, nerve-cells with specialized den-
drites and axons, medullated and non-medullated nerve-fibers, blood-
vessels, connective tissue, etc. — all of which are arranged and inter-

54-

TEXT-BOOK OF PHYSIOLOGY.

blended in a most intricate manner. Notwithstanding the complexity
of its structure, modern histologic methods have enabled Cajal to
divide it into four fairly distinct layers or zones, from without inward,
as follows (Fig. 248) :

1. The Molecular Layer. — The most superficial portion of this layer

consists mainly of neuroglia or glia cells,
the processes of which interlace in all
directions, forming a distinct sheath
just beneath the pia. The deeper por-
tions of this layer contain a specialized
type of nerve-cell (Cajal cells), of which
there are several varieties. These cells
give off nerve-fibers which pursue a
horizontal direction for a variable dis-
tance, but in their course give off
collateral branches which ascend to the
outer surface of the layer. Among
these structures are to be found, also,
dendritic processes of cells situated in
the subjacent layer. The terminal
filaments of medullated nerve-fibers
coming from nerve-cells in lower re-
gions of the encephalo-spinal axis are
also present, but for the most part they
pursue a tangential direction.
The Layer of Small Pyramidal Cells. —
This layer consists mainly of nerve-
cells, the majority of which are pyra-
midal in shape and of small size.
Other cells, however, are present, which
present a variety of shapes, for which
reason the layer was at one time termed
the ambiguous layer. The apical proc-
ess of the pyramidal cells is broad at
the base, but narrows rapidly as it
passes upward. It frequently divides
into several branches, each of which
develops club-shaped processes or gem-
mules, which give to it a feathery ap-
pearance. Dendrites are also given
off from the sides and base of the cell-
body. From the base a single axon
descends which ultimately becomes the axis-cylinder of a medul-
la u-d nerve.
The Layer of Large Pyramidal Cells. — The nerve-cells of this
layer, as the name implies, are also pyramidal in shape, but of
large size. Each cell presents the same features as the cells of the

Fig. 248. — Section of the
Cerebral Cortex (Motor Area)
of Child, Stained by Golgi's
Silver Method. A. Layer of
neuroglia cells. B. Layer of small
pyramidal ganglion cells. C.
Layer of large pyramidal cells. D.
Layer of irregular smaller cells. —
(Pier sol.)

THE CEREBRUM. 543

preceding layer, with the exception that the apical process is
larger, better developed, and branches more freely. All the den-
drites are extensively provided with gemmules. The axon is
well developed, sharply defined, and smooth. After giving off
collateral branches, the axon descends into the cerebrum and
becomes a medullated nerve-fiber.
4. The Layer of Polymorphous Cells. — In this layer the nerve-cells
present a variety of forms: e. g., spindle, polygonal, pyramidal,
etc. The spindle form is the most common. From either end
of the spindle a large dendrite emerges which soon branches and
becomes gemmulated. The axon is well defined and it soon de-
scends into the white matter.
The Number of Cortical Cells. — Attempts have been made by
various histologists to estimate the total number of functional nerve-
cells in the cerebral cortex of man. Though the estimates are widely
different, the lowest presents numbers which are beyond compre-
hension. Thus, Meynert's estimate is 612 millions; Donaldson's
1200 millions; while Thompson's is 9200 millions.

Structure of the White Matter. — The white matter of the
cerebrum consists of medullated nerve-fibers which, though intri-
cately arranged, may be divided into three systems: viz., the com-
missural, the association, and the projection.

1. The commissural system. The fibers which compose this system

unite corresponding areas of the cortex of each hemisphere.
The fibers from the frontal, parietal, and occipital lobes cross
in the median line and form a band of transversely arranged
fibers, the corpus callosum. The fibers which unite the corre-
sponding areas of the temporo-sphenoidal lobes cross in the an-
terior commissure. All the commissural fibers are the axons of
nerve-cells, in the cortex, the terminals of which are to be found
in the cortex of the opposite side.

2. The association system. The fibers which compose this system

unite neighboring as well as distant parts of the same hemisphere,
and may therefore be divided into long and short fibers. They
associate the inexcitable or association areas with the excitable
or projection areas.

3. The projection system. The fibers composing this system unite

certain areas of the cortex of the cerebrum with the basal gan-
glia, the pons, medulla oblongata, and spinal cord. They may
be divided into: (1) afferent fibers which have their origin in
the lower nerve-centers at different levels and thence pass to
the cortex; and (2) efferent fibers which have their origin in the
cortex and thence pass to the lower nerve-centers, terminating
at different levels. The former are also termed the cortico-
afferent or cortico- petal; the latter, cortico-efjercnt or cortico-fugal.
The afferent fibers, the so-called sensor tract, which transmit
nerve impulses coming from the general periphery and the sense-

544 TEXT-BOOK OF PHYSIOLOGY.

organs, -pass through the tegmentum as the mesial and lateral fillets,
and thence to the cortex directly by way of the internal capsule, or
indirectly through the intermediation of the thalamic and subthalamic
nuclei. The distribution of these fibers to the various areas of the
cortex will be found in following paragraphs.

The efferent fibers of the so-called motor tract which transmit
motor or volitional nerve impulses from the cortex to the pons, medulla,
and spinal cord, emerge from the layer of pyramidal cells of the gray
matter of the anterior or the pre-central convolution, the paracentral
lobule and immediately adjacent areas. From this origin the axons
descend through the white matter of the corona radiata, converging
toward the internal capsule, into and through which they pass, occupy-
ing the anterior two-thirds of the posterior limb or segment. Be-
yond the capsule they continue to descend, occupying the middle
three-fifths of the pes or crusta of the crus cerebri, the ventral portion
of the pons, and eventually the anterior pyramid of the medulla oblon-
gata. At this point the tract divides into two portions, viz.:
i. A large portion, containing from ninety-one to ninety-seven per
cent, of the fibers, which decussates at the lower border of the
medulla and passes down the lateral column of the cord, con-
stituting the crossed pyramidal tract.
2. A small portion, containing from three to nine per cent, of the
fibers, which does not decussate at the medulla, but passes
down the inner side of the anterior column of the same side,
constituting the direct pyramidal tract or column of Tiirck.
After passing through the internal capsule, and as it descends
through the crus, pons, and medulla, the cortico-efferent tract gives
off a number of fibers which cross the median line and arborize around
the nerve-cells in the gray matter beneath the aqueduct of Sylvius
(the nuclei of origin of the third and fourth cranial nerves), and around
the nerve-cells in the gray matter beneath the floor of the fourth ven-
tricle (the nuclei of origin of the remainder of the motor cranial nerves).
The remaining fibers go to form the crossed and direct pyramidal tracts
and arborize around the cells in the anterior horn of the gray matter
of the opposite side of the cord at successive levels. By this means
the cortex is brought into anatomic and physiologic relation with the
general musculature of the body through the various cranial and
spinal motor nerves. (See Fig. 236, page 517.)

The fronlo-cerebellar and the occipito-lemporo-cerebellar tracts
are also efferent tracts and parts of the projection system. The
fronto-cerebellar, originating in the nerve-cells of the cortex of the
frontal lobe, passes down to and through the internal capsule, occupy-
ing the anterior one-third of the anterior segment. It then descends
;ilon,^ the inner side of the crus cerebri to the pons, where its fibers
arborize around the cells of the nucleus pontis. Through the inter-
im flialion of these cells this tract is brought into relation with the
cerebellum of the same but chiefly of the opposite side. The occipito-

THE CEREBRUM. 545

temporal tract, originating in the cells of the cortex of both the occip-
ital and temporal lobes, passes downward and inward toward the
lenticular nucleus, beneath which it passes to enter the outer one-fifth
of the crusta. It then enters the pons, and through the nucleus pontis
also comes into relation with the cerebellum of both sides. (See Fig.

243> Page 529-)

THE FUNCTIONS OF THE CEREBRUM.

The functions of the cerebrum comprehend, in man at least,
all that pertains to sensation, cognition, feeling, and volition. All
subjective experiences, which in their totality constitute mind, are
dependent on and associated with the anatomic integrity and the
physiologic activity of the cerebrum and its related sense-organs, the
eye, ear, nose, tongue, etc.

From an examination of the anatomic development of the brain
in different classes of animals, in different men and races of men,
and from a study of the pathologic lesions and the results of experi-
mental lesions of the brain, evidence has been obtained which reveals
in a striking manner the intimate connection of the cerebrum and all
phases of mental activity.

1. Comparative anatomic investigations show that there is a general

connection between the size of the brain, its texture, the depth
and number of convolutions, and the exhibition of mental power.
Throughout the entire animal series an increase in intelligence
goes hand in hand with an increase in the development of the
brain. In man there is an enormous increase in size over that
of the highest animals, the anthropoid apes. The most culti-
vated races of men have the greatest cranial capacity, that of the
educated European or American being approximately 92.1 cubic
inches (1835 c.c); while that of the Australian is but 81.7 cubic
inches (1628 c.c). ' Men distinguished for great mental power
usually have large and well-developed brains; e. g., that of Cuvier
weighed 64.4 ounces (1830 grams); that of Abercrombie, 63
ounces (1786 grams). A large intelligence, however, is not incom-
patible with a much smaller brain weight; thus, the brain of
Helmholtz weighed but 50.8 ounces (1440 grams) ; that of Leidy,
49.9 ounces (1415 grams); that of Liebig, 47.7 ounces (1352
grams). The average arithmetic brain weight of 96 distinguished
men was found to be 51.9 ounces (1473 grams) (Spitzka).

2. Pathologic IcsioJis and mechanic injuries which disorganize the

cerebrum are at once followed by a disturbance or an entire
suspension of mental activity. - Concussion of the brain or
sudden compression from a hemorrhage destroys consciousness.
Physical and chemic alterations of the gray matter of the cere-
brum have been shown to coexist with insanity, loss of memory,
of articulate speech, etc. Congenital defects of organization are
accompanied by a deficiency in mental capacity and the higher
35

546 TEXT-BOOK OF PHYSIOLOGY.

instincts. Under such circumstances no great advance in brain
development is possible and the intelligence remains at a low
level. In congenital idiocy the brain is small, imperfectly devel-
oped, and wanting in proper chemic composition.

3. Experimental lesions of the brain in lower animals are attended

by results similar to those observed in disease or after injury
in man. Removal of the cerebrum in the pigeon completely
abolishes intelligence and destroys the capability of performing
volitional movements. The pigeon remains in a state of pro-
found stupor, though retaining the capability of executing reflex
or instinctive movements. It can temporarily be aroused by loud
noises, light placed before the eyes, pinching of the toes, etc.,
but it soon relapses into a condition of quietude. Coincident
with the destruction of the cerebrum there occurs a loss of mem-
ory, reason, and judgment, and the animal fails to associate the
impressions with any previous train of ideas. The higher the
animal in the scale of development, the more striking is the loss
of mentality after removal of the cerebrum.

4. Experimental interference with the blood-supply to the cerebrum

is followed by a diminished or complete cessation of its activities.
There is perhaps no organ of the body that is so directly depend-
ent upon its blood-supply for the continuance of its activities
as the cerebrum. The supply of blood is furnished by four large
blood-vessels: viz., the two carotid and the two vertebral arteries.
These vessels, after entering the cavity of the skull, give off
branches which unite to form the "circle of Willis." From this
circle, large branches are given off which enter the cerebrum
and distribute blood to all its parts. After passing through the
capillaries the blood, greatly altered in chemic composition, is
returned by large veins. The large volume of blood that is
present in the brain and the marked changes in composition
that it undergoes while passing through the brain indicate a
very active and complex metabolism in this organ. By means
of the anatomic arrangement of the blood-vessels at the base
of the brain, the blood-supply is equalized. It also explains
why, when one, or even two, of the four large vessels are oc-
cluded by pathologic deposits or surgical procedures, brain
activity continues, though perhaps diminished in degree. Occlu-
sion of all four vessels, however, is at once followed by a complete
abolition of all forms of cerebral activity. An experiment per-
' formed by Brown-Sequard illustrates the dependence of cerebral
activity on the blood-supply. A dog was beheaded at the junction
of the neck and chest. After a period of ten minutes all evi-
dences of life had entirely ceased. Four tubes connected with a
reservoir of warm defibrinated blood were then connected with
the four arteries of the head. By means of a pumping apparatus
imitating the action of the heart the blood was driven into and

THE CEREBRUM. 547

through the brain. After a few minutes cerebral activity returned,
as shown by contraction of the muscles of the face and eyes. The
character of the contractions were such as to convey the idea
that they were directed by the will. These vital manifestations
continued for a period of fifteen minutes, when on the cessation of
the artificial circulation they disappeared, and the head exhibited
once again the usual phenomena observed in dying: viz., con-
traction and then dilatation of the pupils and convulsive move-
ments of the muscles of the face.
Localization of Functions in the Cerebrum. — By the term
localization of functions is meant the assignment of definite phys-
iologic functions to definite anatomic areas of the cerebral cortex.
From experiments made on the brains of animals, by the observation
and association of clinical symptoms with pathologic lesions of the
central nerve system, and from observation of the developmental stages
of the embryonic brain, it has been established in recent years:

1. That the general and special sense-organs of the body are associ-

ated through afferent nerve-tracts with definite though perhaps not
sharply delimited areas of the cerebral cortex; and —

2. That certain areas of the cortex are associated through efferent

nerve-tracts with special groups of skeletal or voluntary muscles.

Experimental excitation of a cortical area associated with a sense-
organ is undoubtedly attended by the production of a sensation at
least similar to that produced by peripheral excitation of the sense-
organ itself; destruction of the area is followed by an abolition of all
the sensations associated with the sense-organ. For these reasons
such areas are termed sensor.

Excitation of a cortical area associated with a group of skeletal
muscles is attended by their contraction; destruction of the area is
followed by their relaxation or paralysis. For these reasons such areas
are termed motor.

Since the sense-organs are remote from the brain and the impres-
sions made upon them by the objective world can be utilized by the
mind, only after they have been reproduced in the cortical areas, it
may be said that each sense-organ has its special area in the cortex
by which it is represented, or, in other words, each sense-organ has
a cortical area of representation.

Since the muscles are remote from the brain and since they contract
in response to the discharge of nerve impulses from the cells of the
cortical motor areas, it may be said that the activities of the motor areas
are represented by the contractions of the muscles; in other words, that
the cortical motor areas have areas of representation in the general
skeletal musculature. It is usually stated, however, in the reverse
way: viz., that the muscle movements have areas of representation
in the cortex.

,The cortex of the cerebrum may therefore be compared to a mosaic
made up, partially at least, of sensor and motor areas which respectively

548 TEXT-BOOK OF PHYSIOLOGY.

represent sense-organs and motor organs, and which hear a definite
anatomic and physiologic relation one to the other. Their cooperation
is essential to the normal performance of all forms of cerebral activity.

A knowledge of the situation of these areas, the order of their devel-
opment, the effects that arise from their stimulation or follow their
destruction, are matters of the highest importance in the study of cere-
bral activity and indispensable to the physician in the localization of
lesions which manifest themselves in perversions or abolition of sensa-
tions and in convulsive seizures or paralyses.

The Sensor Areas. — The sensor areas which should theoretically
be present in the cortex are primarily those which receive and translate
into conscious sensations nerve impulses, developed by changes going
on in the body itself; and secondarily those which receive and translate
into conscious sensations the nerve impulses developed in the special
sense-organs by the impact of the external or objective world. In the
former areas, are received the nerve impulses that come from the skin,
mucous membranes, muscles, viscera, etc., and give rise to cutaneous,
muscle, and visceral sensations. In the latter areas are received the
nerve impulses that come from the sense-organs and give rise to tactile,
gustatory, olfactory, auditory, and visual sensations. A number of such
sense areas may be predicated: e. g., areas of cutaneous and muscle
sensibility, of gustatory, olfactory, auditory, and visual sensibility.

The Motor Areas. — The motor areas which should theoretically
be present in the cortex are those which in consequence of the dis-
charge of nerve impulses excite contraction of special groups of mus-
cles and which, from their coordinate and purposive character, are
conventionally termed volitional. Five such general motor areas
may be predicated: e. g., one for the muscles of the head and eyes,
one for the muscles of the face and associated organs, and others for
the muscles of the arm, leg, and trunk. They are usually designated
as head and eye, face, arm, leg, and trunk motor areas.

The existence and anatomic location of these areas in the cortex
of animals have been determined by the employment of two methods
of experimentation: viz., stimulation and destruction or extirpation;
the first by means of the rapidly repeated induced electric currents,
the second by the electric cautery and the knife. If the stimulation
or excitation of any given area is followed by contraction and its de-
struction by paralysis of muscles, it is assumed that the area is motor
in function — is a center of motion. If the stimulation of a given area
is attended by phenomena which indicate that the animal is experienc-
ing sensation, and its destruction by a loss of this capability or the loss
of a special sense, it is assumed that the area is sensor in function-
is an area of special sense. The animals generally employed for ex-
periments of tin's character are dogs and monkeys, though other animals
have frequently been employed by different investigators. Of all
animals, the monkey is the mosl frequently selected, as the configura-
tion of the brain in its general outlines more closely resembles that

THE CEREBRUM.

549

of man than does the brain of any other animal. The results therefore
which are obtained, there is every reason to believe, are the results,
in their general outlines, that would follow stimulation of the human
brain if this were possible under the same conditions. Indeed, the
clinic symptoms which arise during the development of pathologic
processes, and the phenomena which occur during surgic procedures
for the removal of growths and pathologic cortical areas, justify the
conclusion that the chart of the motor and sensor areas of the monkev
brain may be transferred to the human brain without introducing any
serious errors.

The Sensor Areas of the Monkey Brain. — From experiments
made on the brains of monkeys, Ferrier, Schafer, Horsley, and many
others have mapped out, though not with a high degree of defmite-

Fig. 249. — Diagram of the Motor and Sensor Areas on the Lateral Surface
OF the Monkey Brain. — {After Horsley and Schafer.)

ness and certainty, the sensor areas, stimulation of which gives rise to
sensation, destruction to loss of sensation. A diagrammatic repre-
sensation of these areas is shown in Fig. 249 and Fig. 250.

The tactile area or area of tactile perception has not been accurately
or definitely located. Ferrier assigned it to the hippocampal region.
Schafer and Horsley assigned it to the limbic lobe, and especially to
that portion known as the gyrus fornicatus, as destruction of this con-
volution was followed by hemianesthesia of the opposite side of the
body which was more or less marked and persistent. These observers
conclude that the limbic lobe "is largely if not exclusively concerned
in the appreciation of sensation, painful and tactile." Other ex-
perimenters question this conclusion and locate the area near to. if not
within, the Rolandic area. The difference of opinion regarding the

55o TEXT-BOOK OF PHYSIOLOGY.

location and probable limitation of the area of tactile sensibility renders
necessary additional and more conclusive experiments.

The olfactory and gustatory areas or areas of olfactory and gusta-
tory perception have been located in the uncinate gyrus and the adjacent
region, though their exact limits have not been determined by the ex-
periments thus far performed.

The auditory area or area of auditory perception was located by
Ferrier in the upper two-thirds of the superior temporo-sphenoidal
convolution. Bilateral cauterization of this region gave rise to com-
plete deafness, which endured to the time of the animal's death, more
than a vear later. Unilateral destruction of this region gave rise to
deafness in the opposite ear only. The results of experiments made

Fig. 250. — Diagram of the Motor and Sensor Areas on the Mesial Surface
of the Monkey Brain. — (After Horsley and Schajer.)

subsequently by other observers would indicate that the auditory area
is somewhat more extended than that designated by Ferrier, as ap-
parently animals recovered their hearing, to some extent at least, after
complete recovery from the operation. The limit or extension of the
area is, however, uncertain.

The visual area or area of visual perception has been located in
the occipital lobe, though in this, as in the previous instances, its exact
limits have not been positively determined. Experimenters also are
not in accord as to the relative functions of its different parts. Ferrier
located this area in the occipital lobe and that adjacent portion of the
parietal lobe on the outer surface known as the angular gyrus. He
found that extirpation of the angular gyrus alone was followed by a
temporary blindness of the opposite eye, which was, however, not

THE CEREBRUM. 551

hemiopic in character.* He also found that destruction of the occip-
ital lobe together with the angular gyrus gave rise to a more or less
enduring hemianopsia, in addition to the transient blindness of the
opposite eye. From these and similar facts he concluded that the an-
gular gyrus is the area of representation for the macular or central
.region of the retina, and the occipital lobe for the corresponding halves
of the peripheral portions of the retina.

It was, however, found by Munk, Schaf er, and others that the angular
gyrus was not concerned in any way with vision; that extirpation of the
occipital lobe alone, especially if the line of division be carried a little
further forward on the mesial and inferior surfaces, was followed
by homonymous hemiopia (loss of retinal function on the same side),
and therefore homonymous hemianopsia. Additional experiments
lead to the conclusion that the area for macular vision is near the an-
terior extremity of the calcarine fissure, while the area for peripheral
vision is in the posterior portion of the mesial surface and for a variable
distance on the outer surface. Moreover, there is reason to believe that
the area for macular vision is in relation with homonymous halves of
the two maculae luteae. The supposed error, the assignment of macu-
lar vision to the angular gyrus, has been attributed to destruction of
the fibers of the optic radiation, which in their course to the occipital
lobe pass close to this gyrus.

The Motor Areas of the Monkey Brain.— From experiments
made on the brains of monkeys Ferrier mapped out a number of areas
stimulation of which give rise to muscle contractions on the opposite
side of the body which are so purposive and coordinate in character
that they may be regarded as identical with those produced volitionally.
Destruction of these areas is followed by paralysis. The results of
Ferrier' s earlier work are represented in Fig. 251, the descriptive text
to which renders them intelligible. In a general way it may be said
that the upper third of the anterior and posterior central convolutions
presides over the movements of the leg of the opposite side of the body;

* In a consideration of this subject certain facts connected with visual perception, both
in physiologic and pathologic conditions, must be kept in mind. Thus, visual sensation
may arise from stimulation of either the central portion, the macula, or the peripheral
portion of the retina or both. In the first instance the vision is termed central or macular;
in the second instance, peripheral or retinal. Macular vision is clear, sharp, and distinct;
retinal vision progressively indistinct from the center toward the periphery. Division of
one optic tract is followed, in consequence of the partial decussation of the optic nerve-
fibers at the chiasma, by a loss of function in the outer two-thirds of the retina of the
same side, both in the central (macular) as well as in its peripheral portions, and the
inner one-third of the retina of the opposite side. To this condition the term hemiopia
has been applied. As a result of this want of functional activity of these retinal portions
on the side of the lesion, rays of light emanating from objects situated in the opposite
side of the field of vision will not be perceived when both eyes are directed to the fixation
point. To this " blindness " in the opposite half of the field of vision the name hemianopsia
is given. In the lesion under consideration (division of one optic tract) the hemianopsia
is bilateral, and as it affects the corresponding portions associated in normal vision it is of
the homonymous variety. Division of the right optic tract is followed by left lateral homon-
ymous hemianopsia, indicative of the fact that objects in the held of vision to the Left of
the binocular fixation point are invisible.

55^

TEXT-BOOK OF PHYSIOLOGY.

the middle third over the movements of the arm; the inferior third over
the movements of the face and tongue. Collectively these areas are
known as the motor area or motor zone; and as it is ranged along the
Rolandic fissure, it is sometimes termed the Rolandic area.

The experiments of Horsley and Schafer added additional facts
and enabled them to construct a new diagrammatic representation of

the motor area and more ac-
curately define the special
areas upon the lateral and
mesial aspects of the brain
of the monkey. The bound-
aries of the general and
special areas, as determined
by these observers, will be
readily apparent from an
examination of Fig. 249.
Their experiments have en-
abled them also to subdivide
the general into special areas
as follows:

1. The head area or area for
visual direction into
areas excitation of which
causes "opening of the
eyes, dilatation of the
pupils and turning the
head to the opposite
side with conjugate de-
viation of the eyes to
that side."

2. The leg area maybe sub-
divided into (a) an area
both on the lateral and
mesial surfaces which
presides over the move-
ments of the hip and
thigh; (b) an area in the
posterior part which
presides over the move-

ments of the legs and
toes; (c) an area in the paracentral lobule for the movements of
the hallux or great toe.

3. The trunk area, situated largely on the mesial surface, may be sub-

divided into an anterior and a posterior area, which respectively
preside over the movements of the spinal column as arching and
rotation, and the movements of the pelvis and tail.

4. The arm area maybe subdivided as follows: (a) an area superiorly

Fig. 251. — Left Hemisphere of Monkey,
Showing Details of Motor Areas Indicated
by the Movements Following Stimulation of:
1. Superior parietal lobule; exciting advance of the
hind limb. 2. Top of ascending frontal and
parietal convolutions; flexion and outward rotation
of thigh; flexion of toes. 3. On ascending frontal
convolution near semilunar sulcus; movements
of hind limb, tail and extremity of trunk. 4. On
adjacent margins of ascending frontal and parietal
convolution; adduction and extension of arm,
pronation of hand. 5. Top of ascending frontal
near superior frontal convolution; forward extension
of arm. a, b, c, d. On ascending parietal; move-
ments of various muscles of the forearm. 6.
Ascending frontal convolution; flexion of forearm
and supination of hand which is brought toward
mouth. 7. Retraction and elevation of corner
of mouth. 8. Elevation of nose and lip. 9 and 10.
Opening mouth and motions of tongue, n. Re-
traction of angle of mouth. 12. Middle and
superior frontal convolutions; movements of head
and eyelids. 13 and 13'. Anterior and posterior
Jim I is of angular gyrus; movements of eyeballs.
14. Superior .temporo-sphenoidal convolution, ear
pricked and head moved. 15. Movement of lip
and nostril. — (Ferrier.)

THE CEREBRUM. 553

which controls the movements of the shoulder; (b) an area pos-
teriorly and below this, which controls the movements of the elbow;
(c) an area anteriorly and below the preceding, governing the
movements of the wrist and fingers; (d) an area posteriorly and
below governing the movements of the thumb.
5. The face area may be divided into an upper part, comprising
about one-third, and a lower part, comprising the remaining two-
thirds. In the upper part are areas governing the movements
of the opposite angle of the mouth and of the lower face. In the
lower part anteriorly there is an area governing the movements
of the vocal membranes or bands (the laryngeal area) ; posteriorly
areas governing the opening and closing of the mouth, the protru-
sion and retraction of the tongue.
Electric stimulation of the sensor areas is attended by certain
motor reactions which vary in accordance with the area stimulated.
Thus, when the electrodes are applied to different portions of the
occipital lobe the eyeballs are conjugately turned upward, downward,
or laterally and to the opposite side; when placed on the upper por-
tion of the superior temporal convolution, the ear is pricked up or
retracted, the head is turned to the opposite side and the pupils are
dilated; when placed on the hippocampal convolution, there is move-
ment of torsion of the nostril and lips of the same side.

Ferrier assumed that these movements were the result of the origin-
ation of subjective sensations and not an evidencet that the area in
question is a motor area, in the sense that this term is applied to the
areas of the Rolandic region, especially as their destruction is not
followed by paralysis of any of the corresponding muscles. This
interpretation is supported by the experiments of Schafer, which showed
that the contraction of the eye-muscles which followed stimulation of
the occipital lobe took place between 0.2 and 0.3 second later than
when the frontal lobe was stimulated; and that as the motor reaction
takes place after extirpation of the frontal region, the route of the
efferent impulse cannot be to and through the frontal lobe, but prob-
ably through some lower center. The same facts hold true for the
reactions of the ear-muscles following stimulation of the temporal
lobe.

The view that the cortex of the cerebrum can be divided into
separate and independent though physiologically related motor and
sensor areas has been questioned in recent years, and a somewhat
different interpretation given to the facts. It is believed by many
physiologists and neurologists that the so-called motor and sensor areas
are so closely related that it is almost impossible to distinguish one
from the other either anatomically or physiologically. Thus the Rolandic
region is believed to be both motor and sensor in function, the former,
however, being more predominant in the pre-central, the latter in the
post-central, convolution. As these two functions are so intimately
blended and their anatomic substrata so difficult of separation, it is

554 TEXT-BOOK OF PHYSIOLOGY.

thought the term sensori-motor should be employed as more descriptive
and more in accordance with the facts to the entire Rolandic region.

This view has been strengthened by the results of the embryologic
investigation of Flechsig, which show that different nerve-tracts be-
come medullated or receive their myelin investment at successively
later periods and that the tracts which first become myelinated and
are hence first functionally active, belong to the afferent system.
Among the first to undergo myelinization are three tracts numbered
by Flechsig i, 2 and 3, which arise largely from the median nucleus
of the thalamus and the medial lemniscus and pass to the anterior
and posterior convolutions, to the para-central lobule and foot of
the superior frontal convolution, and to the foot of the third frontal
convolution respectively. It is these fibers which convey nerve im-
pulses to the cortex and furnish information regarding changes taking
place in the body itself and thus lead to the performance of muscle
movements. This area is therefore primarily a sensor area, an area
for body-feelings, cutaneous, tactile, muscle, and visceral, and second-
arily a motor area. The afferent fibers to this region become myel-
inated during the ninth month of intra-uterine life, the efferent fibers
from it become mvelinated during the third month of extra-uterine
life.

By the same method of reasoning the gustatory, olfactory, audi-
tory, and visual sense areas are to be regarded as sensori-motor in
character, for embryologic investigations show that subsequently
to the myelinization of the afferent tracts connecting the sense-organs
with the cortex, efferent nerve-tracts arise from or near to the same
centers and undergo myelinization. In other words, these areas are
primarily sensor and secondarily motor, and therefore should be termed
sensori-motor. In Flechsig's own terminology each cortico-petal or
afferent tract is accompanied by a cortico-fugal or efferent tract.

In this view sensations, or the mental processes the outcome of
sensations, are the immediate cause of the movements of the muscles
connected with both the sense-organs and skeletal structures. Though
this interpretation — viz., the coincidence of sensor and motor areas — ap-
pears more in accordance with the facts than the earlier view, it must be
admitted that there are many facts both of a physiologic and pathologic
character which it is difficult to harmonize with it.

The Motor Area of the Chimpanzee Brain. — In a series of
experiments made by Sherrington and Grunbaum on the brain of the
chimpanzee it was discovered that the so-called motor area was not
so widely distributed as in the monkeys generally, but was confined
almost exclusively to the convolution just in front of the fissure of
Rolando, as it was impossible to obtain any movement on direct stim-
ulation of the convolution just behind it. All points on the surface of
the pre-central convolution, including the portion forming the wall
of the Rolandic fissure itself, were found to be excitable and productive
of movement when stimulated. The sequence of representation from

THE CEREBRUM.

555

below upward is similar to that observed in the monkey. One pecul-
iarity, however, was the location of the area for conjugate deviation
of the eyeballs to the opposite side. This is situated far forward in the
middle and inferior frontal convolutions, and separated from the- areas
in the pre-central convolution by a region apparently inexcitable.
These facts are of great interest and value in the assignment of the motor
areas in the cortex of the human brain, as in its development and con-
figuration the chimpanzee brain more closely resembles the human
brain than does the monkey's.

The Localization of Sensor and Motor Areas in the Human
Brain. — The observation of clinical symptoms and their interpreta-
tion by postmortem findings, the phenomena observed during surgical

CONCRETE CONCEPT

Fig. 252. — The Areas and Centers of the Lateral Aspect of the Human Hemi-
cerebrum. — (C. K. Mills.)

procedures, and the results of embryologic investigations, point to the
conclusion that corresponding areas both for sensations and move-
ments exist in the cerebral cortex of the human brain, though it is
probable that their locations do not in all respects coincide with
those characteristic of the monkey or even the ape brain. In the fol-
lowing diagrams (Figs. 252 and 253), the sensor and motor areas are
at least provisionally located, in accordance with recent observations.
They are represented as limited or bounded by a serrated line to in-
dicate, as suggested by Mills, that they are not sharply defined, but
that they interfuse or interdigitate with surrounding regions.

The Sensor Areas. — The sensor areas occupy regions correspond-
ing in a general way with those of the monkev brain.

556

TEXT-BOOK OF PHYSIOLOGY,

The cutaneous and muscle sense areas have been assigned to the
post-central, a portion of the super and sub-parietal convolutions
on the lateral aspect, and to portions of the frontal convolution and
of the callosal convolution on the mesial aspect. It is also probable
that the tactile (cutaneous) area may be assigned, though in less
degree, to the pre-central convolution, the general motor area. This
is in accordance with the embryologic investigations of Flechsig, who
concludes that the entire Rolandic region is to be regarded as sensor
as well as motor in function, and names it the area of body feelings,
or the somesthetic area.

The clinic and postmortem evidence as to the extent of the area
of tactile sensibility and its coincidence with the motor area is sorae-

Fig. 253. — The Areas and Centers of the Mesial Aspect of the Human Hemi-
cerebrum. — (C. K. Mills.)

what contradictory, and in some respects apparently in opposition
to the view of Flechsig. Thus, Dr. C. K. Mills, whose skill in inter-
preting the phenomena of disease is well known, states in this con
nection in his work on nervous diseases that "innumerable cases have
been reported of lesions of the motor cortex without the slightest im-
pairment of sensibility." In several cases of excision of the human
cortex in the Rolandic region by surgical operations careful studies
of the patients failed to show any impairment of sensation. Other
competent observers, however, have reported a number of cases in
lii< h anesthesia more or less pronounced and persistent has accom-
panied lesions of the motor area. The explanation of these contra-
dictory observations is not apparent.

The olfactory area has been assigned to the uncinate convolution,

THE CEREBRUM. 557

the anterior part of the callosal convolution, and the posterior part of the
base of the frontal lobe. Lesions in this region are frequently accom-
panied by subjective olfactory sensations.

The gustatory area has been assigned to the collateral convolu-
tion.

The auditory area has been assigned to the posterior portion of the
super temporal convolution and to the retro-insular convolutions,
the island of Reil. Unilateral destruction of this region is followed
by only a partial loss of hearing in the opposite ear (owing to the par-
tial decussation of the cochlear nerve), which, however, may be re-
covered from after a time, owing probably to a compensatory activity
of the insular convolutions. Bilateral disease of this region is followed
by complete deafness. Within this area there is a smaller region,
disease of which is accompanied by word-deafness only, the patient
being unable to distinguish the tone intervals between words and syl-
lables and therefore hearing only confused noises. Object hearing
has also a separate area of representation.

The visual area has been assigned to a triangular shaped area on
the mesal surface of the occipital lobe, which includes the gray matter
above and below the calcarine fissure (the cuneus and upper part of
the lingual lobe), and to the gray matter of the first occipital convolution
on the lateral aspect of the occipital lobe. Focal lesions of this area
on one side are followed by lateral homonymous hemianopsia, which,
however, does not involve, as a rule, the fovea or macula. It is, there-
fore, the area of homonymous half-retinal representation. The loca-
tion of the area for macular or central vision is uncertain. Henschen
locates it in the anterior part of the area near the extremity of the cal-
carine fissure, and asserts that in each area both maculse are represented.
From experiments made on monkeys Schafer locates it in the same
region. Beyond the limits of this visual area and on the lateral aspect
of the parietal lobe there is a region (the supra-marginal convolution
and angular gyrus) in which impressions of words and letters seen have
their representation. Destruction of this area by diseases is followed
by word- and perhaps letter-blindness, the patient being unable to
recognize words and letters seen because of failure to revive the mem-
ory images of words and letters. The areas for visual sensations and
optic memory pictures are therefore separate, a fact which has led to
a division of the visual area into a lower and a higher area.

It was stated in a previous paragraph that electric stimulation of
the sensor areas of the monkey brain is attended by certain motor
reactions which vary with the area stimulated. Corresponding areas
are believed to be present in the human brain and that their stimula-
tion would be followed by similar motor reactions. Their location is
shown in Figs. 252 and 253, and named visual, auditory, olfactory,
and gustatory motor.

The stereo gnostic area or area of stereo gnostic perception, by which
objects are recognized through their form independent of vision and

558

TEXT-BOOK OF PHYSIOLOGY.

by the sense of touch alone, has been located in the super parietal
convolution and the precuneus (Mills). The existence of such an
area is rendered probable by the fact that cases have been recorded
in which there was a loss of this power (astereognosis) uaccompanied
by either sensor or motor disturbances. Postmortem investigations
showed that in these cases there was a destruction of the superior
parietal convolution.

Equilibratory, intonation, and orientation areas have been pro-
visionally located in the sphenotemporal lobe.

The Motor Area. — The general motor area (Fig. 254) is represented
as occupying the pre-central convolution, the base of the super-frontal
convolution, both on its lateral and mesial aspects, and the paracentral

Fig. 254. — Scheme of the Motor Area of the Human Beain and its Subdivisions.
—(Ajter Mills.)

lobule. The exclusion of the post-central convolution from the motor
area is in accordance with the embryologic researches of Flechsig, which
indicate that the efferent fibers which compose the pyramidal tract come
from the region anterior to the central fissure, and with the experiments
of Sherrington and Griinbaum on the brain of the chimpanzee, which
demonstrate that the post-central convolution is absolutely inexcitable
to electric stimulation. It is quite probable that with the growth of the
brain in size and complexity, the motor area has come to occupy a posi-
tion somewhat farther forward in the human brain than in the monkey
brain.

This general area is also capable of subdivision into^areas of vari-
able size, in which the movements of the face and associated structures,

THE CEREBRUM. 559

the head and eyes, the arm, trunk, and leg, are represented. (Fig.
254.) The sequence of their representation from below upward is
similar to that observed in the monkey and chimpanzee. In each of
these five main areas there are yet smaller areas in which the move-
ments of localized regions of the body are in part represented and
which are indicated in diagram (Fig. 254) by corresponding words.
The words in the areas marked, eyes and head, face, arm, trunk, and
leg, indicate the location of nerve-cells which through the discharge
of nerve impulses excite to contraction the muscles, which impart to
the regions indicated by these words, their characteristic movements.
A localized irritative lesion of any one of these areas gives rise to
convulsive movements of the muscles of the opposite side of the body,
similar in character to those resulting from electric stimulation of the
corresponding areas of the monkey and ape brains. Destruction of
these areas from the growth of tumors, softening, etc., is followed by
paralysis of the muscles. Electric stimulation of these areas of the
human brain for the purpose of localizing obscure irritative lesions
prior to surgical procedures on the brain gives rise to the same con-
vulsive movements.

Language. — The succession of motor acts by which ideas are
expressed, is known as language, which may be divided into (1) artic-
ulate or spoken, and (2) written.

The expression of ideas both by words and signs depends primarily
on the power of reviving the images or memories of words and letters
heard and seen; and secondarily on the power of reviving the images
or memories of the muscle movements which were previously em-
ployed in an effort to imitate or reproduce the words (speech) or the
verbal signs (writing).

Clinico-pathologic investigations have shown that words and
letters heard and seen have areas of representation in the cortex, the
former in the general auditory area, the latter in the supra-marginal
convolution and angular gyrus (Fig. 252). Destruction of these
areas is followed by word-deafness and word-blindness respectively.
The same methods of investigation have shown that the muscle move-
ments employed to reproduce the words and the verbal signs also have
areas of representation in the cortex; the former in the sub-frontal
convolution (Fig. 252), and probably in the adjacent region, the island
of Reil, on the left side in the great majority of people; the latter in front
of the arm region of the general motor area. Destruction of these
areas is followed in the first instance by a loss of the power of executing
the movements of the muscles employed in speech, and in the second
instance, of those employed in writing.

These different areas are connected with one another by associa-
tion fibers, and, taken collectively, constitute the language zone Their
situation and relations are shown in Fig. 255. In this figure the
dotted lines coming from the ear (a) and the eye (v) represent
the auditory and visual tracts through which nerve impulses pass

560

TEXT-BOOK OF PHYSIOLOGY.

to the auditory (A) and the visual centers (V) respectively. Similar
lines coming from the muscles involved in speech and writing might
also be represented to indicate the paths of the nerve impulses to the
motor speech (M) and the motor writing center (E). The continuous
lines on the surface of the cortex represent nerve-fibers which associate

the auditory and visual centers
with the speech and writing cen-
ters and with higher psychic
centers (O O) as well. The
dotted lines coming from the
speech and writing centers repre-
sent the tracts through which
nerve impulses pass to the muscle
of the larynx, tongue, mouth, and
lips, and to the muscles of the
hand. The anatomic and physi-
ologic association of the various
areas is essential to the registra-
tion of the impressions made on
the ear and eye and for the ex-
pression of the ideas evolved
from them by words (speech)
and signs (writing). Their col-
lective action is essential to the
acquisition of language. De-
struction of any part of this
cerebral mechanism is attended
by an impairment or a total loss
either in the power of obtaining
auditory images of words heard
and visual images of words seen,
or in the power of expressing
ideas by speech and writing. To
this pathologic condition the
term aphasia has been given.

Aphasia. — It was discovered
by Bouillaud that a destructive
lesion of the third frontal convo-
lution on the left side was accom-

Fig. 255. — Diagram Showing the Re-
lation of the Centers op Language and
their Principal Associations. A. Audi-
tory center. V. Visual center. M. Motor
speech center. E. Motor writing center.
O O. Intellectual center. — (After Grasset.)

panied by a partial or complete loss of the faculty of articulate speech,
the power to express ideas with words. To this condition the term
aphasia was given. Though of limited application ctymologically,
the word is now em ployed in a wider sense to signify "partial or com-
plete loss of the power of expression or comprehension of the con-
ventional signs of language," words cither spoken or written, due to
ie ions of different portions of the cortex, and especially on the left
side.

.THE CEREBRUM. 561

Aphasias are of many degrees and kinds, though they may be in-
cluded in the two general divisions, motor and sensor.

Motor aphasia may be either ataxic or agraphic. In ataxic aphasia
the patient is unable to express or communicate his thoughts by spoken
words, owing to an inability to execute those movements of the mouth,
tongue, etc., necessary for speech without there being any paralysis
of these muscles. The lesion is usually in the third frontal convolution
and most frequently associated with right hemiplegia. In agraphic
aphasia the patient is unable to communicate his ideas by writing through
an inability to execute the necessary movements, though retaining his
mental processes. In this form of aphasia the lesion is in the writing
area. These two forms of motor aphasia are not infrequently asso-
ciated.

Sensor aphasia or amnesia may be either visual or auditory. In
visual aphasia or amnesia the patient is unable to recognize a letter
or word, printed or written (though capable of seeing other objects),
a condition known as letter- or word-blindness. It is usually associated
with lesions in the neighborhood of the supra-marginal convolution.
In auditory aphasia or amnesia the patient cannot understand articu-
late or vocal speech, though capable of hearing and understanding other
sounds, through an inability to distinguish tone intervals of words and
letters — a condition known as word-deafness. It is associated with
lesions of the auditory area.

Paraphasia is an inability to recall the proper words to associate
with ideas and necessary to their expression.

Concept aphasia is the inability to recall the names of objects.
It is associated with lesions of the cortex of the mid-temporal or third
temporal convolution (Mills). This area is known as the concept or
naming area.

Bilateral Representation. — Though highly specialized move-
ments, such as those performed by the arms and hands, legs and feet,
have their areas of representation on one side of the cerebrum only,
and that, opposite to the side of the movement, less highly specialized
movements, such as the masticatory, phonatory, respiratory and
various trunkal movements, which require for their performance the
cooperation of muscles on both sides of the body, have their areas of
representation on both sides of the cerebrum; the area of either side
exciting to action the muscles on both sides of the body. In the case
of specialized movements the representation is unilateral; in the case
of the more general movements the representation is bilateral.

Association Centers. — The sensor and motor areas to which
specific functions have been assigned do not constitute more than
one-third of the total cerebral cortex. There yet remain large regions
to which it has not been possible to assign specific functions based on
physiologic experiments. Three or four such regions separated by
the sensor and motor centers are to be recognized on the lateral and
mesial aspects of the hemisphere. In Fig. 256 the location, extent,
36

,-6:

TEXT-BOOK OF PHYSIOLOGY.

and names of these regions are represented. The fibers which are
found in these regions belong almost exclusively to the association
system, and become medullated at a later period than do the fibers of
the projection system; moreover, from the method of their medulliza-

Motor and tactile area.

Parietal association area

-*-^S§

w?mm*

\ 1 V J :&■?:}

Frontal

association

area.

Occipito-temporal
association area.

Auditory area.

Motor and tactile area.

Parietal association area
(Precuneus).

Frontal

association

area.

Occipito-temp
association area

Olfactory lobe.
Olfactory tract.

Olfactory area.

Gyrus hippocampus.

Fig. 256. — Diagrams to show the Position and the Relation of the Association
and Projection Areas. The Projection Areas are Dotted. — (After Flechsig.)

tion it would appear that many of these fibers grow out directly from
the sensor centers into these regions and become related to the nerve-
cells of their convolutions, while others grow out from adjacent as
well as distant convolutions. From histologic and pathologic evidence
these regions were termed by Flechsig association centers or areas, im-
plying the idea that through the intervention of their cell mechanisms

THE CEREBRUM. 563

the sense areas are indirectly associated anatomically and physiolog-
ically, and together constitute a mechanism by which sensations are
associated and elaborated into concrete forms of knowledge or related
to definite forms of movement.

It has been assumed by Flechsig that the frontal association center,
from its connections with the sensor and motor areas of the Rolandic
region, the olfactory, and perhaps other regions, is engaged in asso-
ciating and registering body sensations and volitional acts, and that the
knowledge thus gained has reference largely to the personality of the
individual; that the parieto-occipital association area, from its relation
to the visual, auditory, and tactile sense areas, is engaged in associating
and registering visual, auditory, and tactile sensations, and that the
knowledge thus gained has reference mainly to the external world.
These assumptions in a general way are supported by the phenomena
of disease. In certain lesions of the frontal lobe the symptoms indicate
a loss or change of ideas regarding personality rather than of the
objective world, while the reverse is true in disease of the parieto-
occipital lobe.

The Intra-cranial Circulation. — The circulation within the
cranium presents certain peculiarities which distinguish it from that
in other parts of the body. These peculiarities reside in part in the
anatomic arrangement of the blood-vessels, in the probable absence of
vaso-motor nerves to the blood-vessels, and in greater part in the fact
that the brain and its blood-vessels are contained in a box with rigid,
unyielding and closed walls.

The Blood-supply. — As stated in a previous paragraph the
arteries supplying the brain with blood are four in number viz. : The
two internal carotids and the two vertebrals. These four arteries
anastomose very freely at the base of the brain, the anastomosis
constituting the circle of Willis. From this circle there arise the
anterior, middle and posterior cerebral arteries which are distributed
to the cortex and the underlying white matter. The basal ganglia,
the capsule and adjacent white matter are supplied by a number of
branches which arise from the circle of Willis or from the three cerebral
arteries immediately after their origin. From the distribution of
these two sets of vessels they have been named the cortical and the
central ganglionic respectively.

The venous blood is returned by a system of vessels which present
characteristics of physiologic interest. These vessels consist of large
sinuses formed by folds of the dura mater or, as at the base of the cranium,
by the dura mater and the bone. These sinuses, from the very nature
of the tissues which enter into their formation, have rigid walls and will
therefore withstand any pressure to which they may be subjected under
physiologic conditions. The same obtains at their points of exit from
the cranium where a free outflow is in consequence always assured.

564 TEXT-BOOK OF PHYSIOLOGY.

The various sinuses have opening into them, the veins which
return the blood from the cortex and subjacent white matter, and
from the inner structures of the brain. Neither sinuses nor veins
have valves and most of the veins which empty into the superior
longitudinal sinus have their mouths directed forward, hence the blood
discharged from these veins must flow against the current in the sinus.
The venous blood leaves the cranium mainly by way of the internal
jugular veins which are direct continuations of the lateral sinuses.

The Intra-cranial Lymph Spaces. — In order to understand the
phenomena attending the circulation of blood through the cranium
it is necessary to take into consideration an important fact, viz. : that
the brain and spinal cord are surrounded on all sides by a relatively
large and continuous lymph space. This space which is found
between the arachnoid and the pia mater is filled with a liquid, the
so-called cerebrospinal fluid, which being interposed between the
brain and the skull on the one hand and the spinal cord and the vertebras
on the other hand, acts as a water cushion protecting these delicate
organs from the injury which might result from sudden jars. The
ventricles of the brain are also filled with cerebrospinal fluid which is
in communication with that in the subarachnoid space through the
foramen of Magendie and the foramina of Key and Retzius. The
cerebrospinal fluid may also penetrate into the perineural lymph
spaces surrounding the cranial and spinal nerves. The quantity of
the cerebrospinal fluid is relatively small, amounting to from 60 to
80 c.c.

The Mechanism of the Intra-cranial Circulation. — As pre-
viously stated, by virtue of the physical relations existing between the
blood, the brain, the cerebrospinal fluid and rigid walls of the cranium,
the flow of the blood through the brain and cranial cavity, is attended
by certain phenomena which are peculiar to this region and present
in no other situation.

Taking as a point of departure the conditions during the diastole
of the arteries,, the relations of these structures are somewhat as
follows: the cerebrospinal fluid occupies all the available lymph
space, but under a pressure approximately equal to that in the large
veins and hence not materially above that of the atmosphere; the
pressure in the arteries, capillaries and veins presents the usual values
in these different regions of the vascular apparatus; the brain presents
a volume which may be termed diastolic.

With the occurrence of the succeeding cardiac systole, the cerebral
vessels, receiving an additional volume of blood, expand and occasion a
corresponding increase in the volume of the brain, which is accom-
plished by a partial displacement of the cerebrospinal fluid into
extra-cranial lymph spaces. Because of the fact that the displacement
of the cerebrospinal fluid is insufficient to permit of the complete
expansion of the brain, there is developed in the intra-cranial lymph
spaces a counter pressure (the so-called intra-cranial pressure) which

THE CEREBRUM. 565

would keep pace with and finally equalize the rising pressure in the
arteries. In consequence of this, the brain tissue, it is believed, would
be subjected to a pressure sufficiently great to interfere with its activi-
ties, even to the point of unconsciousness. If this is not to occur the
maximum expansion of the arteries, and hence the brain, must be
checked and controlled. This is accomplished in the following way:
As the brain approaches that degree of expansion permitted bv the
displacement of the cerebrospinal fluid, it begins to exert a com-
pression of the pial veins. This compression by narrowing the
lumen of the veins diminishes their capacity and hence increases the
pressure of their contained blood until it is equivalent to the pressure
exerted by the brain against the veins. At this moment the pressures
in the arterioles, capillaries and veins approximate each other in value.

From these factors it will be seen that the circulation through the
brain approximates a circulation through a system of rigid tubes.
The result is an increase in the velocity of the outflow and a diminution
of the blood-pressure. As an additional result the pulse wave of the
arterial system is transmitted to the blood of the large veins and sinuses
which therefore exhibit normally pulsations synchronous with those
of the arteries. The rise of the pressure in the cerebral veins is
regarded therefore as the factor which, by limiting brain expansion,
checks the rise of the intra-cranial pressure beyond physiologic limits.
With the diastole of the heart and arteries, the former relation of the
blood, brain, cerebrospinal fluid and cranial walls is regained. Be-
cause of this change of relation with each heart-beat, the brain pulsates
synchronously with the arteries.

The brain differs from other organs, also, in that normally its
volume is more influenced in a positive direction by the expiratorv
rise of venous pressure than by the inspiratory rise of general arterial
pressure. Thus the rise of pressure in the thoracic veins which occurs
with each expiratory act, causes a damming back of the venous blood
in the sinuses and pial veins, resulting in a further increase in the
volume of the brain and in the intra-cranial pressure. The reverse
takes place in inspiration.

It has been ascertained experimentally that the intra-cranial
pressure may vary considerably and consciousness still be preserved.
Hill found it to be 40 to 50 mm. of Hg. in the convulsions of strychnia
poisoning and a little less than zero in a patient standing erect.

The Regulation of the Volume of Blood Entering the Brain.—
li is generally believed that the cerebral vessels are not provided with
vaso-motor nerves. Every attempt to prove their existence either by
physiologic or histologic methods has thus far failed of convincing
proof. In the absence of vaso-motor nerves, the regulation of the
circulation in the brain must necessarily be dependent on changes
affecting the arterial and venous pressures in other regions of the
body.

The most effective factor in increasing or decreasing the blood-

566 TEXT-BOOK OF PHYSIOLOGY.

supply to the brain resides in the power of the vaso-motor center to
cause a contraction or dilatation of the cutaneous and splanchnic vessels.
Thus if the vaso-motor center declines in its tonus from any cause what-
ever, there is a relaxation of the blood-vessels in one or both of these
regions, an increase in the volume of the blood flowing into them, and
in consequence, a decrease in the volume of the blood flowing through
the brain. If on the contrary the vaso-motor center is increased in
its tonus, the reverse conditions prevail in the cutaneous and splanch-
nic vessels and the quantity of blood flowing into the brain is increased.
Thus in an indirect way the vaso-motor center, by bringing about a
rise or a fall in the general arterial pressure, regulates the blood-supply
to the brain, and controls its amount in accordance with its needs.

Brain Activity and Brain Repose or Sleep. — Brain activity is
characterized by an active consciousness, the development of sensations,
ideas, feelings, and the exercise of volitional power (which manifests
in muscle movement) and is the result of a physiologic condition of
the body at large. For the manifestation of brain activity it is essential,
that the irritability of the brain cells and more especially of those com-
posing in large measure the cerebral cortex be maintained at a normal
physiologic level, so that they may respond in the manner peculiar
to them, to the action of nerve impulses reflected through afferent
nerves from all regions of the body. Here as elsewhere throughout
the body, the irritability depends on, and is maintained by, the presence
of blood flowing into and out of the brain in varying quantity from
moment to moment, with a given velocity and under a definite pressure.
So long as these conditions are maintained in the strictly physiologic
condition, so long will the brain respond to stimuli by the development
of sensations. The avenues through which nerve impulses pass to the
cortical cells are those beginning in the special and general sense
organs of the body in contact with the external world, viz. : the eyes,
ears, nose, tongue, and skin. The maintenance of these structures
in a strictly physiologic condition is also one of the essential conditions
for brain activity.

Judging from the changes in the character and composition of the
blood which occur during its passage through the brain capillaries,
there is coincidently with brain activity an active metabolism, which
eventuates, at the end of a variable number of hours, in the decline
of the irritability, a reduction of functional activity, and the establish-
ment of the condition of fatigue. The irritability of the sense organs,
especially of the eyes and ears, in all probability declines in a similar
manner. These structures pass into the condition of fatigue and
become less responsive to external stimuli. The result of all these
conditions is a less active stimulation of the brain cells, which in con-
nection with other factors predisposes to

Brain Repose or Sleep. — Brain repose or sleep is characterized
by a greater or less degree of unconsciousness, the non-development
of sensations, ideas, feelings and volitional acts, and is the result of a

THE CEREBRUM. 567

diminution in the physiologic activities of the body at large and more
especially of the brain, sense organs and spinal cord. Coincident with
the cessation of brain activity and the onset of sleep, there is a
diminution in the rate and force of the heart beat, and in the frequency
and depth of the respiratory movements, and a relaxation of the skeletal
muscles, especially those employed in voluntary movements.

The sense organs are in part protected from the action of external
stimuli. The eyeball is so turned, that its anterior pole is directed
far upward under the eyelid, while the pupil is markedly diminished
in size, and in consequence the entrance of light largely prevented.
The ear is protected against the reception of sounds of ordinary
pitch by an increased tension of the tympanic membrane. The
nose and mouth are less responsive to various stimuli because of the
dryness of their mucou's membranes from diminished secretion. The
skin appears to be less sensitive to mechanic pressure and other forms
of stimulation.

In addition to the foregoing phenomena, experimental investigations
have shown, that there is a shunting of a portion of the blood stream
from the brain to other regions of the body, especially to the skin and
perhaps to the abdominal viscera as well, whereby it becomes incapable
of functionating physiologicly. The fact that the brain receives a
lessened quantity of blood during sleep has been shown by trephining
the skull and inserting in the orifice a glass plate through which the
circulatorv conditions of the brain can be observed. In the wakinsr

- O

condition the blood-vessels on the surface of the brain are prominent,
and turgid with blood and the whole organ completely fills the cranial
cavity, indicating that the blood-vessels in the interior of the brain are
in a similar condition. With the onset of sleep the larger blood-vessels
begin to diminish in size, the smaller vessels disappear from view,
the brain tissues become pale and the volume of the brain shrinks.
During the continuance of deep sleep, this anemic condition persists.
As the period of sleep approaches its termination, the smaller blood-
vessels again fill with blood, the surface of the brain flushes, and in a
very short time the former circulatory conditions return, the volume
of the brain increases and the waking state is reestablished.

The fact that the skin receives an increased volume of blood during
sleep, has been shown by inserting an arm or leg in a plethysmograph
by which means a record of any change in volume can be obtained.
Howell thus succeeded in obtaining graphic records in the variations
of the volume of the arm during sleep. These records disclosed
the fact that with the onset of sleep the volume of the arm gradually
increased in size until it attained a maximum which was from one to
two hours after the beginning of sleep. After this period the volume
remains practically the same for several hours, diminishing ■ as the
intensity of sleep diminishes, and the waking state is approached.
Just previous to the return of consciousness there is a rapid diminution
in the volume of the arm. If it be accepted that the enlargement of

568 TEXT-BOOK OF PHYSIOLOGY.

the cutaneous vessels is followed by a diminution in size of the cerebral
vessels, it follows that the former condition stands to the latter in the
relation of cause and effect, whereby a portion of the blood is diverted
from the brain to the skin. It also naturally follows that the with-
drawal of the blood from the brain to the skin and possibly other
regions as well, is the fundamental condition for brain repose.

The Intensity of Sleep. — Observations of individuals during
sleep show, that the intensity or the deepth of sleep varies from hour
to hour. Attempts have been made to estimate the intensity by
measuring the intensity, or the loudness of a sound caused in several
ways, that is necessary to awaken the sleeper. Accepting this criterion
it may be stated from the results of many experiments, that sleep
increases in intensity or depth and reaches its maximum between the
first and second hours, after which it rapidly decreases until the end
of the third hour, when consciousness is so nearly restored, that
but a very slight stimulus is required to awaken the sleeper. It is
during the latter period when the brain is reviving, that dreams arise
the element of which are formed of previous sensations.

The Causes of Sleep. — Different theories have been proposed to
account for the causes of sleep, none of which have been wholly satis-
factorv. From all the facts which have been presented it would appear
that one cause is a decline in the irritability of the nerve-cells of the
brain and associated sense organs, and the development of fatigue
conditions, the result of prolonged activity.

A second cause is the withdrawal of a large portion of the blood
from the brain on the presence of which, here as elsewhere, normal
activity depends. As to whether the diminished activity of the brain
is the cause of, or the result of the withdrawal of the blood there has
been much difference of opinion. Howell has offered a plausible
explanation for the withdrawal of the blood from the brain to t he-
cut aneous vessels, based on the activity of the vaso-motor center.
He assumes that for a variable number of hours, corresponding to
the usual waking state, this center possesses a certain average tonus,
due in all probability to reflex influences, by virtue of which it main-
tains a certain average contraction of the cutaneous vessels. But
at the end of this period it, too, becomes fatigued, declines in irritability,
becomes less responsive to reflex influences, and hence loses its control
over the vessels. As a result they dilate and thus reduce the amount
•of blood flowing to the brain to a level insufficient to maintain its
activity, after which sleep supervenes. During sleep the irritability
and tonus of the center are restored when its control of the blood-
vessels is regained. Unless the brain in its functional activities differs
from all other organs of the body, it may be inferred that cessation of
activity or repose is the result partly of fatigue and partly of a diminu-
i ion of the blood-supply.

CHAPTER XXIT.
THE CEREBELLUM.

The cerebellum is situated in the inferior fossae of the occipital
bone, beneath the posterior lobes of the cerebrum, from which it
is separated by the tentorium cerebelli, a semilunar fold of the dura
mater. It is partially divided into hemispheres by a longitudinal
fissure, more apparent on the inferior surface, though united by a
central lobe, the vermiform process. Each hemisphere is connected
with the cerebrum, the pons, medulla and spinal cord by three bundles
of nerve-fibers known respectively as the superior, middle, and inferior
peduncles. The surface of the cerebellum presents a series of lobes and
fissures of which the former have received more or less fanciful names.
A section of the cerebellum shows that it is composed of gray matter
externally and white matter internally. The general appearance
presented on section is shown in Fig. 257.

Structure of the Gray Matter. — The gray matter consists
mainlv of nerve-cells of varying size and shape, which are arranged in
two layers: viz., an outer or molecular and an inner or granular.

The molecular layer consists of stellate and multipolar cells of
small size, from which dendrites and axons pass horizontally and
vertically. The granular layer consists, as its name implies, of
granular shaped cells and large stellate cells. These cells are character-
ized by the possession of dendrites and axons, the course and relation
of which have not been clearly determined.

The inner border of the molecular layer presents a series of
large cells originally described by Purkinje and known by his name.
From the outer end of the cell-body one or more dendrites emerge which
soon divide and subdivide into a number of branches which pass
toward the cerebellar surface. The general arrangement of these
dendrites gives to the entire cell a tree-like appearance (Fig. 258).
From the inner end of the cell an axon emerges which passes centrally
into the white matter.

Structure of the White Matter. — The white matter consists of
nerve-fibers which are arranged in association and projection systems.

The Association System. — The libers which compose this system
are of variable lengths and unite adjacent as well as distant regions
of the cerebellar cortex. They doubtless associate them both anatom-
ically and physiologically.

The Projection System. — The fibers composing this system con-
nect the cerebellar cortex with certain structures in the cerebrum,
pons, medulla, and spinal cord. They may be divided into efferent
and afferent systems.

569

57°

TEXT-BOOK OF PHYSIOLOGY.

The efferent fibers have their origin in the cells of Purkinje and
the dentate nucleus. Some of these fibers emerge from the cere-
bellum in the superior peduncles through which they pass toward -and
beneath the corpora quadrigemina to terminate around the cells of
the red nucleus. As they approach this nucleus some of the fibers
cross the median line and decussate with those coming from the op-
posite side, while others pursue a straight direction, terminating on the

same side. Through the
intervention of fibers
which arise from the
red nucleus and ascend
to the. cerebral cortex,
the cortex is thus con-
nected with both sides
of the cerebellum, though
chiefly with the opposite
side.

Efferent fibers also
leave the cerebellum by
the middle peduncle and
pass directly to the nu-
cleus ponds, around the
cells of which their term-
inals arborize. Efferent
fibers also descend the
inferior peduncles and
constitute the tract
known as the Lowenthal
and Marchi tract, situ-
ated in the antero-lateral
region of the spinal cord
in its upper part.

The afferent fibers
come from a variety of
sources. Those found
in the superior peduncles come from the red nucleus; those in the
middle peduncles from the nucleus pontis of the opposite side, having
crossed or decussated at the raphe near the anterior surface of the
pons; those contained in the inferior peduncles are the most abundant
and important, and are represented by (i) the direct cerebellar tract,
which terminates in the superior vermis after decussation; (2) the
anterior and posterior arcuate fibers, the former coming from the
gracile and cuneate nuclei of the opposite side, the latter from the
same side, which also pass to the superior vermis; (3) the acustico-
cerebellar tract, composed of fibers the axons of the sensory end
nuclei (Deiters) of the vestibular portion of the auditory nerve. It
is probable that all these libers decussate prior to their final termination.

Fig. 257. — View of Cerebellum in Section, and
of Fourth Ventricle, with the Neighboring
Parts. — {From Sappey.) 1. Median groove fourth
ventricle, ending below in the calamus scriptorius,
with the longitudinal eminences formed by the fas-
ciculi teretes, one on each side. 2. The same groove,
at the place where the white streaks of the auditory
nerve emerge from it to cross the floor of the ventricle.
3. Inferior peduncle of the cerebellum, formed by the
restiform body. 4. Posterior pyramid; above this is
the calamus scriptorius. 5, 5. Superior peduncle of
cerebellum, or processus e cerebello ad testes. 6, 6.
Fillet to the side of the crura cerebri. 7, 7. Lateral
grooves of the crura cerebri. 8. Corpora quadri-
gemina.— {After Hirschfeld and Leveille.)

THE CEREBELLUM.

57i

The cerebellum through this system of efferent and afferent fibers
is brought into relation with many different regions of the cerebrum,
pons, medulla, and spinal cord. Each half of the cerebellum is con-
nected with the foregoing structures of the same side, but more espe-
cially of the opposite side.

THE FUNCTIONS OF THE CEREBELLUM.

From the observations of the results of experimental lesions, from
analysis of clinico-pathologic facts, and from its comparative anatomic
development in different animals, the deduction has been drawn that
the cerebellum coordinates and har-
monizes the action of those muscles
the activities of which are necessary to
the maintenance of body equilibrium
both during station and progression.

By equilibrium of the body is un-
derstood a condition which may be
maintained for a variable length of
time without displacement, and if
possible only so long as a line passing
through the center of gravity falls
within the base of support. The sup-
port offered by the earth to the feet
neutralizes and counteracts the force
of gravity. In station, when the body
is in the erect or military position,
the arms by the side, the center of
gravity lies between the sacrum and
the last lumbar vertebra, and the line
of gravity falls between the feet and
within the base of support. The en-
tire skeleton for the time being is rendered fixed and rigid at all its
joints by the combined action of the muscles connected with it. That
this position may be maintained all the different groups of antagonistic
but cooperative muscles must be accurately coordinated in their ac-
tions. Any failure in this respect is at once attended by a disturbance
of the equilibrium and displacement.

In progression, walking, running, dancing, etc., the body is trans-
lated from point to point by the alternate action of the legs. Whether
the direction of the translation be linear or curvilinear, as the legs
change their position from moment to moment, the center of gravity
also changes, and at once the equilibrium is menaced. If it is to be
maintained and displacement prevented there must be a prompt
readjustment in the relation of all parts of the body so that the line
of gravity falls again within the base of support. The more com-
plicated the movements of progression, or the narrower the base of
support, the greater is the clanger to the equilibrium, and hence the

Fig. 258. — Section of Cerebellar
Cortex. A. Outer or molecular
layer. B. Inner or granular layer.
C. White matter, a. Cell of Purk-
inje. b. Small cells of inner layer
c. Dendrites of these cells, d. A
similar cell lying in the white matter.
— (Stirling.)

572 TEXT-BOOK OF PHYSIOLOGY.

necessity for rapid and compensatory changes in coordinated muscle
activity. All movements of this character, in man at least, are pri-
marilv volitional and require for their performance the constant exercise
of the attention. With frequent repetition they gradually come to
be performed independently of consciousness and fall into the cate-
gorv of secondary or acquired reflexes.

Though coordinating power is exhibited by the spinal cord, medulla,
and basal ganglia, it is only in the cerebellum that this power attains
its highest development and differentiation. To it is assigned the
power of selecting and grouping muscles, not in any restricted part,
but in all parts of the body, and coordinating their actions in such a
manner as to preserve the equilibrium.

The Results of Experimental Lesions. — If the cerebellum in
its totality, coordinates and harmonizes the action of the muscles
on the opposite sides of the body, any derangement of its structure or
its connections with the cord, medulla, pons, or basal ganglia should
at once be followed by incoordination of muscles and a want of har-

x '4a. ■'

< ^

Fig. 259. — Attitude Assumed After Destruction of the Left Half of the
Cerebellum. — (Morat and Doyon, after Thomas.)

mony in their action. Experimental lesions of the cerebellum are
attended by such results. The phenomena observed are many and
complex. They differ in extent and character in different animals
and in accordance with the extent and location of the lesion, though
the note of incoordination runs through them all.

Removal of one lateral half of the cerebellum in the dog is followed
by an inability to maintain the equilibrium necessary to the erect posi-
tion. On attempting to stand, the animal at once falls toward the side of
the lesion, the muscles of which at the same time contract and give
to the body a distinctly curved condition (Fig. 259). The anterior
limbs are extended to the opposite side. On making efforts to re-
gain the standing position, the animal may roll over around the long
axis of its body. Conjugate deviation of the eyes is frequently ob-
1 r ■ 1 :d as well as nystagmus.

After a few days the symptoms partially subside and the animal
acquires the power of sitting on the abdomen when the anterior limbs
are widely extended (Fig. 260). As the days go. by the improve-
ment continues, and the animal recovers the power of walking, though

THE CEREBELLUM.

573

Fig. 260. — Attitude in Repose after
the Complete Removal of the Cere-
bellum BUT DURING THE PERIOD OF RES-
TORATION of Function. — {Moral and Doyon,
after Thomas.)

each step is attended with tremor and oscillations of the body. Any
change in the center of gravity such as results when one leg is lifted
may result in a fall toward the side of the lesion, owing to an inability
to promptly bring about the necessary compensatory muscle actions.
With time the animal continues to improve in its power of adjustment,
though it never completely recovers it. Movements of progression
are apt to be characterized by
stiffness and accompanied by
tremor suggestive of volitional
efforts.

Total removal of the cerebel-
lum is followed by a different
train of symptoms. The extensor
muscles apparently preponderate
in their action, for the limbs are
extended and abducted, the head
and neck are retracted, and
opisthotonos is established. In
time these effects also partially
subside, though all attempts at
walking are permanently accom-
panied by tremor and oscillations. The characteristic effect which
follows section of the peduncles is again incoordination, manifesting
itself in deviation of the head, eyes, inability to walk, tremor on
exertion, etc. The effects vary, however, according to the peduncle
divided. Section of the middle peduncle gives rise to the most pro-
nounced effects. The head and the anterior part of the body are at
once drawn toward the pelvis on the side of the section. A voluntary

effort on the part of the

animal causes it to lose all
control of its muscles and
the body is rotated in the
direction of its longitudinal
axis from 40 to 60 times a
minute before it comes to
rest. According as the
lesion is made from behind
or before, the rotation is
from or to the side of the
section. In time these
symptoms subside, though the animal never completely recovers.

The partial recovery of the power of coordination, observed after
removal of a portion or the whole of the cerebellum, indicates that
the centers in the cord, medulla, pons, and cerebrum endowed with
corresponding though less developed power, develop compensatory
activity and acquire to some extent the capabilities of the cerebellum
itself (Fig. 261).

Fig. 261. — Progression after Destruction
of the Vermis. — {Moral and Doyon, after Thomas.)

574 TEXT-BOOK OF PHYSIOLOGY.

Clinico- pathologic facts partly corroborate the results of phys-
iologic investigations. In various forms of uncomplicated cerebellar
disease, vertigo, tremor on making voluntary efforts, difficulty in main-
taining the erect position, unsteadiness in walking, opisthotonos,
pleurothotonos, are among the symptoms generally observed.

Comparative anatomic investigations reveal a remarkable correspond-
ence between the development of the cerebellum and the complexity
of the movements exhibited by animals. In those animals whose
movements are complex and require for their performance the coopera-
tion of many groups of muscles the cerebellum attains a much greater
development in reference to the rest of the brain than in animals whose
movements are relatively simple in character. This relative increase
in the development of the cerebellum is found in many animals, such
as the kangaroo, the shark, the swallow, and the predaceous birds
generally.

The Coordinating Mechanism. —Though it is not known how
the cerebellum selects and coordinates groups of muscles for the per-
formance of any complex movement, it is known that its activity is
largely reflex in origin and excited by impulses reflected to it from
peripheral organs. In this as in other forms of reflex activity the
mechanism involves (i) afferent nerves, e. g., cutaneous, muscle, optic,
and vestibular, and their related end-organs, tactile corpuscles, muscle
spindles, retina, and semicircular canals, all indirectly connected with
(2) the cerebellar centers; (3) efferent nerves indirectly connected
with (4) the general musculature of the body. Both station and pro-
gression are directly dependent on the development and transmission
of afferent impulses from the previously mentioned peripheral sense-
organs to the cerebellum. Tactile, muscle, visual, and labyrinthine
impressions and sensations not only cooperate in the development
and organization of the motor adjustments necessary to the main-
tenance of the equilibrium and locomotive coordination, but even after
their organization they are necessary to the excitation of cerebellar
activity. The manner in which they lead to the development of this
capability on the part of the cerebellum is conjectural. Their ever-
present influence is shown by the effects which follow their removal,
as the following facts indicate.

The prevention of the development of tactile impulses by freezing
or anesthetizing the soles of the feet, and the blocking of normally de-
veloped impulses through destruction of afferent pathways in diseases
of the spinal cord lead at once to marked impairment in the coordinat-
ing power. The removal of the skin from the hind legs of the frog,
previously deprived of its cerebrum, destroys its coordinating power,
which it would otherwise possess in a high degree.

The blocking in consequence of destructive lesions of the spinal
cord, of the impulses, which come from the muscles, tendons, etc.,
and which inform us of the activity and the degree of activity of our
muscles, the location of the limbs, the amount of effort necessary

THE CEREBELLUM. 575

to produce a given movement, etc., also gives rise to much incoor-
dination. A blocking of both tactile and muscle impulses frequently
exists in degeneration or sclerosis of the posterior columns of the
spinal cord. The coordinating power is so much impaired in this
disease that the patient is unable to maintain, without strained effort,
the erect position and especially if the directive power of the eyes be
removed by closure of the lids. Walking becomes extremely difficult;
the gait is irregular and jerky, and equilibrium is maintained only
by keeping the eyes fixed on the ground in front and by artificially
increasing the basis of support by the use of canes.

An interference with the development of the customary visual
impressions which in a measure maintain the sense of relation of the
individual to surrounding objects also gives rise to equilibratory dis-
turbances. A rapid change in the relation of the individual to sur-
rounding objects or the reverse; a change in the direction of one
optic axis from the use of a prism or from paralysis of an eye muscle;
the destruction of an eye; — these and similar conditions frequently
give rise to such marked disturbances of the equilibratory power that
displacement is difficult to prevent.

An interference with the development of the so-called labyrin-
thine impressions by destruction of the semicircular canals gives rise
to the most remarkable disturbances in this respect. Section of one
horizontal canal* in the pigeon is followed by oscillations of the head
in a horizontal plane around a vertical axis. Bilateral section so
increases these oscillations that the pigeon is unable to maintain
equilibrium and forced to fall and turn continuously around the
vertical axis. Bilateral section of the posterior vertical canals gives
rise to oscillations around a horizontal axis which frequently become
so exaggerated as to eventuate in the turning of backward somersaults,
head over heels. Similar phenomena follow division of the superior
vertical canals.

Bilateral destruction of both sets of canals is attended by extra-
ordinary disturbances in the equilibrium. From the moment of the
operation the animal, the pigeon, loses all control of its motor mechan-
isms. It can neither maintain a fixed attitude nor execute orderly
movements of progression; its activity, continuous and uncontrollable,
is characterized by spinning around a vertical axis, turning somer-
saults, dashing itself against surrounding objects until life is en-
dangered. If the animal be protected from injury, these disturbances
gradually subside, and in the course of a few months the equilibratory
power is so far regained that standing and walking at least become
possible. In this condition, however, the coordinating power is
directly dependent on visual impulses, for with the closure of the
eyes all the previous motor disturbances at once recur. These and
similar facts indicate that the semicircular canals are the peripheral

* The physiologic anatomy of the semicircular canals is described in the chapter
devoted to the ear, to which the reader is referred.

576 TEXT-BOOK OF PHYSIOLOGY.

sense-organs from which come the nerve impulses most essential to
the excitation of the cerebellar coordinative centers in their control of
equilibrium and of progression.

The cerebellum may therefore be regarded as the essential, most
highly differentiated portion of the coordinating mechanism con-
cerned in the maintenance of equilibrium, during both station and
progression. The manner in which the cerebellum accomplishes
this result is unknown, though it is certain, from the foregoing facts,
that its special mode of activity is dependent on the excitatory action
of nerve impulses reflected from a variety of peripheral sense-organs.

CHAPTER XXIII.
THE CRANIAL NERVES.

The nerve-trunks which serve as channels of communication
between the encephalon and the structures of the head, the face,
and in part the organs of the thorax and abdomen, pass through for-
amina in the walls of the cranium, and for this reason are termed
cranial nerves.

According to the classification now generally adopted, there are
twelve cranial nerves on either side of the median line, which, enu-
merated from before backward, are as follows (Fig. 262) :

First or Olfactory. Seventh or Facial.

Second or Optic' Eighth or Auditory.

Third or Oculo-motor. Ninth or Glosso-pharyngeal.

Fourth or Patheticus. Tenth or Pneumogastric or Vagus.

Fifth or Trigeminal. Eleventh or Spinal Accessory.

Sixth or Abducens. Twelfth or Hypoglossal.

The cranial nerves may be classified physiologically in accordance
with their functional manifestations into three groups, viz. :

1. Nerves of Special Sense: e.g., Olfactory, Optic, Auditory, Gustatory (Glosso-

pharyngeal).

2. Nerves of General Sensibility: e.g., Large root of the Trigeminal, Glosso-

pharyngeal, and Pneumogastric.

3. Nerves of Motion : e.g., Oculo-motor, Patheticus, the small root of the Trigeminal,

Abducens, Facial, Spinal Accessory, and Hypoglossal.

Though this classification in the main holds true, it must be borne in
mind that modern investigations have demonstrated that the glosso-
pharyngeal and pneumogastric nerves contain even at their junction
with the medulla oblongata a number of efferent or motor fibers,
and to this extent are mixed nerves.

The Origins of the Cranial Nerves. — In accordance with modern
views as to the origins of nerves in general, it may be stated that—

The nerves of special sense have their origin respectively in the
neuro-epithelial cells in the mucous membrane of the olfactory region
of the nose, in the ganglion cells of the retina, in the cells of the spiral
ganglion of the cochlea and the ganglion of Scarpa, and in the cells
of the petrous and jugular ganglia. From the cells of these ganglia
dendrites pass peripherally to become associated with specialized
end-organs, while axons pass centrally in well-defined bundles to
become related by means of their end-tufts with primary basal ganglia.

The nerves of general sensibility have their origin in the ganglia
on their trunks, and in this respect resemble the spinal nerves. From
the ganglion cell there emerges a short axon process which soon
divides into a central and a peripheral branch. The former passes

37 577

78

TEXT-BOOK OF PHYSIOLOGY.

toward and into the gray matter located beneath the floor of the fourth
ventricle, where its end-tufts arborize about nerve-cells. The latter
(the peripheral branch) passes toward the general periphery to be
distributed to skin and mucous membranes (Fig. 263).

The nerves of motion have their origin in the nerve-cells in the gray
matter beneath the aqueduct of Sylvius and beneath the flo6r of the
fourth ventricle (Fig. 264). The axons emerging from these cells
course peripherally to be distributed to skeletal muscles. In some of

the motor nerves, and in some sen-
sory nerves as well, there are to be
found efferent fibers of smaller size
which have a similar origin and
which become related through the
intervention of sympathetic ganglia
(peripheral neurons) with visceral
muscles and glands. These nerves
have been termed autonomic nerves.
The Cortical Connections of
the Cranial Nerves. — Each of
these three groups of cranial nerves
has special connections with the
cerebral cortex.

The nerves 0] special sense for
the most part terminate in primary
basal ganglia, around the cells of
which their central end-tufts arbor-
ize. From these cells axons arise
which pass upward and directly or
indirectly come into physiologic re-
lation with sensor nerve-cells in the
cerebral cortex.

The nerves of general sensibility
terminate in the gray matter be-
neath the floor of the fourth ventri-
cle, around the nerve-cells of which
their end-tufts arborize. These
groups of nerve-cells are known as sensor end-nuclei. Though once
regarded as the centers of origin of the sensor nerves, they are now
regarded as the centers of origin of axons which pass upward to the
cortex of the cerebrum, where they also come into physiologic relation
with sensor nerve-cells.

The axons in both of these classes of nerves thus originate in the
cells of the central nerve system and continue upward to the cere-
brum, the primary afferent path.

The motor nerves which have their origin in the cells of the gray
matter beneath the aqueduct of Sylvius and beneath the floor of the
fourth ventricle are in physiologic relation with nerve-cells in the

Fig. 262. — Superficial Origin of
the Cranial Nerves from the Base
of the Encephalon. i. Olfactory. 2
Optic. 3. Motor oculi. 4. Patheticus.
5. Trigeminal. 6. Abducens. 7. Facial.
7'. Nerve of Wrisberg. 8. Auditory. 9.
Glosso-pharyngeal. 10. Pneumogastric.
11. Spinal accessory. 12. Hypoglossal. —
' Morat and Doyon.)

THE CRANIAL NERVES.

579

motor region of the cortex through descending axons contained in the
pyramidal tract, the end-tufts of which arborize around the nerve-
cells. The efferent path beginning in the cerebral cortex is thus
continued by the motor nerves to the general periphery.

The three groups of nerves, those of special sense, of general
sensibility, and the motor nerves, are neurons of the first order; the

Fig. 263. — Ganglia of Origin of the
Sensor Cranial Nerves, i. Trigem-
inal (ganglion of Gasser). 2. Nerve of
Wrisberg. 3. Auditory (ganglion of
Scarpa). 4. Glosso-pharyngeal (gang-
lion of Andersch). 5. Pneumogastric
(ganglion plexiformis). — {After Moral and
Doyon.)

Pig. 264. — Nuclei of Origin of the
Motor Cranial Nerves, i. Motor
oculi. 2. Patheticus. 3. Motor root of
the trigeminal. 4. Abducens. 5. Fa-
cial. 6. Mixed nucleus for efferent fibers
of the glosso-pharyngeal vagus and spinal
accessory. 7. Ffypoglossus. S. Spinal
accessory, q. Spinal nerves. — {After
Moral and Doyon.)

nerve-cells and fibers which constitute the cerebral connections are
neurons of the second order. It is probable that the sensor cells in
the cerebral cortex are neurons of a third order.

FIRST PAIR. THE OLFACTORY.

The first cranial nerve, the olfactory, is situated in the upper
third of the nasal fossa, in the regio olfactoria. It consists of from 20
to 30 branches, the fibers of which are non-medullated.

Origin. — The olfactory nerve is composed of centrallv coursing

58o

TEXT-BOOK OF PHYSIOLOGY.

axons which have their origin in the central ends of bipolar, rod-
shaped, or spindle-shaped nerve-cells interspersed among the epithelial
cells covering the mucous membrane in the regio olfactoria; the
peripheral ends of these cells give off a number of dendrites which are
spread out to form a delicate feltwork over the surface of the mucous
membrane. From their origin the axons gradually converge to form
bundles which ascend to the cribriform plate of the ethmoid bone,
through the foramina of which they pass to become related by their

end-tufts with structures in
the gray matter of the olfac-
tory bulb (Fig. 265).
6 Cortical Connections.

—The olfactory bulb and
. olfactory tract, formerly
called the olfactory nerve,
are portions of the cere-
brum (the olfactory lobe)
which arise embryologically
by a protrusion of the walls
of the cerebral cavity. The
bulb is oval-shaped and
consists of both gray and
white matter. It rests on
the cribriform plate of the
ethmoid bone and is em-
braced by the olfactory
nerves. As seen on sagittal
section, there is just be-
neath the surface a layer of
large pyramidal and spin-
dle-shaped cells (termed
also mitral cells), each pro-
vided with an apical and
The apical dendrite passes toward the surface
or basket-like expansion which interlaces with

Fig. 265. — The Relation oe the Olfactory
Nerves to the Oleactory Tract, i. Ol-
factory nerve-cell. 2. Axon process. 3. Epi-
thelial cells. 4. Glomerulus. 5. Mitral cells.
6. Centrally coursing axons of the olfactory
tract. — (Morat and Doyon.)

two lateral dendrites.

and ends in a brush

the end-tufts of the olfactory nerves, forming what are known as the

olfactory glomerules. The lateral dendrites end free.

The axons of the pyramidal cells pass toward the center of the
bulb and bend at right angles, after which they pursue a horizontal
direction toward and into the olfactory tract. This tract is about five
centimeters in length, prismatic in shape on cross-section and divisible
into a ventral and a dorsal portion. It emerges from the posterior
extremity of the bulb, passes backward to the posterior part of the
anterior lobe, where it divides into three roots: viz., a lateral or external,
a mesial or internal, a middle or dorsal. The fibers of the lateral and
mesial roots are derived almost exclusively from the ventral portion
of the tract, the fibers of which come from the mitral cells in the bulb.

THE CRANIAL NERVES.

58i

The lateral root-fibers pass outward into the fossa of Sylvius and come
into relation with nerve-cells in the inferior extremity of the gyrus
hippocampus and the gyrus uncinates. The mesial fibers pass inward
and come into relation with nerve-cells in the pre-callosal part at least
of the gyrus, fornicatus. The fibers thus far considered are undoubt-
edly true olfactory fibers, pursuing a centripetal direction, carrying
nerve impulses from the olfactory cells to the cerebrum (Fig. 266).

Histologic and embryologic methods of research have shown that
some of the fibers in the
olfactory tract are cen-
trifugal in direction.
They originate in the
olfactory cortical areas,
pass toward the peri-
phery as far as the an-
terior commissure, where
they cross to become the
dorsal root, enter the
olfactory tract, and
finally terminate in the
bulb. This tract serves
to connect the cortex
v ith the bulb of the op-
posite side, and carries
impulses from the cortex
to the bulb. The two
opposite cerebral olfac-
tory areas are also united
by commissural fibers
which decussate at the
anterior commissure.

Function. — The
function of the olfactory
system in its entirety, is
the transmission of nerve
impulses from its origin
in the olfactory region of

the nose to the cerebral cortex, where they evoke sensations of odor.
The stimulus to its excitation is the impact and chemic action of
gaseous or volatile organic matter on the dendrites of the olfactory cells.
The sensitiveness of the olfactory end-organ to the action of many suit-
stances is remarkable, responding, for example, to the T2000 ¥o~ °f a
gram of oil of roses and to the 276^00Tr of a gram of mercaptan.

Division or destruction of the olfactory path at any point is followed
by an abolition of the sense of smell on the corresponding side. De-
structive lesions of the hippocampal and uncinate gyri are followed
by similar results.

Fig. 266. — Olfactory Lobe of the Human Brain.
— Bu. Olfactory bulb. T. Tract. Tr.o. Trigone. R.
Rostrum of corpus callosum. p.' Peduncle of corpus
callosum, passing into G. s., gyrus subcallosus (diagonal
tract, Broca). Br. Broca's area. F.p. Fissura prima.
F.s. Fissura serotina. C.a. Position of anterior com-
missure. L.t. Lamina terminalis. Ch. Optic chiasma.
T.o. Optic tract, p. olf. Posterior olfactory lobule (or
anterior perforated space), m.r. Mesial root. l.r.
Lateral root of tract.— (His.) — (After Qiiain.)

<82

TEXT-BOOK OF PHYSIOLOGY.

SECOND PAIR. THE OPTIC.

The second cranial nerve, the optic, consists of centrally coursing
axons of neurons, which connect the essential part of the organ of
vision, the retina, with sensory end-nuclei or ganglia situated at the
base of the cerebrum.

Origin. — The axons which constitute the optic nerve have their
origin in the ganglionic cells in the anterior part of the retina. Through
their dendrites these cells are brought into relation posteriorly with
successive layers of cells which collectively constitute the retina.
Though the retina is said to consist of ten or eleven layers, it may be
reduced practically to three, viz. (Fig. 267):

1. The layer of visual cells.

2. The layer of bipolar cells.

3. The layer of ganglionic cells.
The visual cells present peripherally modified

dendrites, known as the rods and cones; centrally
they give off an axon which after a short course
terminates in an end-tuft. The* bipolar cells also
possess dendrites and an axon; the former interlace
with the end-tufts of the visual cell axon, the latter
with the dendrites of the ganglion cell. The retina
may be regarded therefore as the peripheral end-
organ in which the optic nerve originates. From
their origin the axons turn backward, at the same
time converging to form a distinct bundle which
passes through the chorioid coat and sclera. After
emerging from the eyeball the nerve-bundle (the
optic nerve) passes backward as far as the sella
turcica, traversing in its course the orbit cavity and
the optic foramen. At the sella turcica there is a
union and partial decussation in man and other
mammals of the two nerves, forming the optic
chiasm*

Decussation of the Optic Nerves. — The ex-
tent to which the fibers from each eye decussate at the chiasm is a
subject of dispute but the results of various methods of research would
seem to indicate, that the fibers from the nasal third of the retina of
the left eye cross in the chiasm, to unite with the fibers from the tem-
poral two-thirds of the retina of the right eye. In a similar manner
the libers from the nasal third of the retina of the right eye cross in
the chiasm, and unite with the fibers from the temporal two-thirds of

* Though the foregoing is the usual method of stating the origin and course of the optic
nerve, nevertheless morphologically the true optic nerve lies wholly within the retina and
compo ed of the visual cells there found. The remainder of the visual system from
and including the ganglion cells of the retina to the optic basal ganglia, is the optic tract,
being no anatomi* or physiologic distinction between the optic nerve so called and
the optic tract. Both are outgrowths from the brain and hence possess properties which
differentiate them from other cranial nerves.

Fig. 267. — Reti-
nal Cells, s', z'.
Visual cells with
their peripheral ter-
minations. 5. Rods.
2. Cones, b. Bi-
polar cells, g. Gan-
glion cells from which
arise the axons of the
optic nerve.

THE CRANIAL NERVES.

583

the retina of the left eye (Fig. 268). Posterior to the chiasm the crossed
and uncrossed fibers form the so-called optic tracts, which after winding
around the crura cerebri enter the optic basal ganglia. Transection of
the optic nerve shows that it is composed of an enormous number of
non-medullated nerve-fibers, estimated by Salzer at from 450,00c to
800,000, enclosed in a sheath of the dura mater.

The visual fibers comprising the optic nerve may be physiologicly
divided into two classes, (a) those coming from the peripheral portion of
the retina, and (b) those com-
ing from that central area VISUAL HOD VISUAL HELD
known as the macula lutea.
The retinal fibers are by far
the more abundant, and make
up the major portion of the
nerve; the macular fibers are
less abundant. An examina-
tion of a cross-section of the
optic nerve shows the presence
of a wedge-shaped tract occu-
pying the center of the nerve
and which is regarded as com-
posed of the macular fibers
At the chiasm this bundle of
fibers undergoes a partial de-
cussation similar to that of the
fibers coming from the more
peripheral portions of the
retina. In the left optic tract,
therefore, fibers from at least
four different regions are to be
found: viz., the two-thirds of
the temporal side of the left
retina, the temporal half of
the left macula, the nasal third
of the right retina, and the
nasal half of the right macula.
Corresponding fibers are to be
found in the right optic tract.

As the optic tract passes around the crus cerebri it divides into a lateral
or outer, and a mesial or inner bundle, which then terminate in the
optic basal ganglia. The fibers of the lateral bundle are traceable
into the lateral or external geniculate body (the pre-geniculum) , the
pulvinar of the optic thalamus, and the anterior quadrigeminal body
(the pre-geminum). These are in all probability the true visual
fibers. The fibers of the mesial bundle are traceable into the internal
geniculate body (the post-geniculum) and the posterior quadrigeminal
body (the post-geminum).

Fig. 268. — Diagram Illustrating Left
Homonymous Lateral Hemianopsla from
a Lesion of the Right Optic Tract or
the Right Cuneus. The Shaded Lixes
in the Visual Fields Indicate the
Darkened Area.

584 TEXT-BOOK OF PHYSIOLOGY.

Cortical Connections. — After entering the basal ganglia the
visual fibers terminate in end-tufts which arborize around nerve-cells.
From these cells new axons arise which ascend through the posterior
part of the internal capsule, at the same time curving backward to form
the optic radiation of Gratiolet, and terminate finally around nerve-
cells in the gray matter of the cuneus and in the gray matter bordering
the calcarine fissure, both situated on the mesal aspect of the occipital
lobe.

Centrifugal Fibers of the Optic Nerve. — All the fibers previously
alluded to have been afferent or centripetal in direction; but the optic
nerve also contains efferent or centrifugal fibers which come from
nerve-cells in the basal ganglia and ramify around special cells, the
amacrine cells, in the retina. Their function is unknown. It has
been suggested that they regulate the vascular supply to the retina.
Centrifugally coursing fibers also connect the visual areas of the cortex
with the basal ganglia.

Function. — The function of the visual apparatus in its entirety
is the transmission of nerve impulses from the retina to the cerebral
cortex where they evoke the sensations of light and its different
qualities — colors. The specific physiologic stimulus to the retinal
visual cells is the impact of the undulations of the ether. In general
it may be said that, at least for the same color, the intensity of the
objective undulation or vibration, determines the intensity of the sen-
sation.

Pupillary Fibers. — The optic nerve also contains nerve-fibers
somewhat larger in caliber than the usual visual fibers, which are
supposed to form the afferent path for those nerve impulses which
excite reflexly a contraction of the sphincter pupillce muscle, thus vary-
ing the size of the pupil. These fibers, termed pupillary fibers, come
from all portions of the retina but most abundantly from the posterior
pole in and around the macula. The existence of these fibers is.
confirmed by pathologic findings. In a manner similar to that of the
visual fibers they, too, undergo a decussation in the optic chiasm, so
that in the optic tract there are pupillary fibers which come from the
temporal side of the eye of the corresponding side, and fibers which
come from the nasal side of the eye of the opposite side (Fig. 272).
The central termination of these fibers is not positively known.

Hemiopia and Hemianopsia. — Division of the optic nerve
between the eyeball and the optic chiasm is followed by complete
blindness in the eye of the corresponding side. Owing to the partial
decussation of the fibers in the chiasm, division of an optic tract is fol-
lowed by a loss of sight in the outer two-thirds of the eye of the same side
and in the inner third of the eye of the opposite side. To this loss of
visual power in the retina the term hemiopia is given. In consequence of
this loss of visual power in the retina there is a corresponding obscura-
tion or total obliteration of nearly one-half of the visual field, to which
the term hemianopsia is given. Jf, for example, the right optic tract

THE CRANIAL NERVES.

58;

is divided there will be hemiopia in the outer two-thirds of the right
eye and the inner third of the left eye, with left lateral hemianopsia,
and as the portions of the retina which are affected are associated in
vision the loss of the visual fields is spoken of as homonymous hemi-
anopsia (Fig. 268). A destructive lesion of the cerebral visual area,
the cuneus and the adjacent gray matter on the right side, is also
followed by left lateral hemianopsia.*

The existence of an homonymous hemianopsia becomes evident
when the individual is directed to focus the vision on an object placed,
directly in front and with its
center in the median plane of
the body, when if the lesion be
on the right side, the left half
of the object will be invisible.
The reason for this will be ap-
parent on reference to Fig. 269.
All the light rays emanating
from the left half of the object
fall on the retina on the side of
the injury, and hence there will
be no sensation. If, however,
the object be moved to the right
without change in the position
of the head, the entire object
will be visible, as all the rays
fall on the normal side. If, on
the contrary, the object be
moved to the left, it will be in-
visible for the opposite reason.

Hemianopsia may be the
result of either destruction of
the optic tract or of the cortical

visual area. The seat of lesion in any given case is indicated by a
peculiarity of the iris reflex pointed out by Wernicke, which will be
referred to in connection with the consideration of the oculo-motor
nerve.

THIRD PAIR. THE OCULO-MOTOR.

The third cranial nerve, the oculo-motor, consists of some 15,000
peripherally coursing nerve-fibers which serve to bring the nerve-cells
from which they arise into relation with a large portion of the general
musculature of the eye.

Origin. — The axons composing the third nerve arise from a series

* It should be borne in mind that in both instances the retina itself is unaffected. The
impact of light generates, as usual, nerve impulses which proceed as far backward as the
point of division or destruction. In consequence those portions of the cerebral cortex
stimulation of which evokes the sensation of light remain unaffected and the individual
does not become aware through sensation, of the presence of a luminous body in the left
side of the visual field.

Fig. 269. — Diagram to Show the Exist-
ence of Hemianopsia. The lesion is sup-
posed to be in the right optic tract.

;S6

TEXT-BOOK OF PHYSIOLOGY.

of seven or eight groups of nerve-cells, located in the gray matter
beneath the floor of the aqueduct of Sylvius. From each of these
groups or nuclei, bundles of axons emerge, which after a short course
unite to form the common trunk. The large majority of the fibers in

the nerve come directly from the nuclei
of the same side; the remainder come
from a group of cells on the opposite
side of the median line. There is
thus a partial decussation of its fibers
(Fig. 270).

The different groups of cells, the
nuclei of origin, are arranged in a serial
manner. The anatomic arrangement
of these nuclei would indicate that
each nucleus is related to an individual
member of the eye-group of muscles.
Clinical observation and the investiga-
tion of the results of pathologic pro-
cesses have not only shown that this
is the case, but have succeeded in
locating the position of the nucleus
for any given muscle. Though there
is some difference of opinion in regard
to the exact location of one or two of
the nuclei, the tabulation subjoined is
approximately correct.

Enumerating them from before
backward, the nuclei occur in the
following order:

The sphincter pupilke.

Fig. 270. — Diagrammatic View of
the Situation and Relation of
the Nuclei of Origin of the
oculo-motor and patheticus
(Trochlearis) Nerves. The oculo-
motor nuclei consist of an anterior
nucleus, the Edinger-Westphal nucleus
(a and b), and a posterior nucleus;
the posterior nucleus has a dorsal, a
ventral, and a mesial portion; the de-
cussation of fibers from the dorsal
portion of the posterior nucleus is also
shown. The decussation of the fibers
of the fourth nerve is also represented.
— (Edinger.)

2. The tensor chorioideas (the accom-
modation nucleus).

3. The convergence nucleus, a com-
mon nucleus for the conjoint ac-
tion of the two internal recti
muscles.

4. The superior rectus.

5. The inferior rectus.

6. The levator palpebral.

7. The inferior oblique.

Cortical Connections. — The oculo-motor nuclei are in histologic
and physiologic relation with the motor area of the cerebrum. Nerve-
cells in the cortex give off axons which, entering the pyramidal tract,
descend through the internal capsule, and the crus cerebri, from which
they cross to the opposite side. The end-tufts arborize around the
nuclei of the oculo-motor nerve with the exception of the nucleus for
the iris sphincter.

THE CRANIAL NERVES.

537

Distribution. — After their origin the axons converge to form a
common trunk, which emerges from the base of the encephalon, on
the inner side of the cms cerebri, in front of the pons Varolii. The
nerve then passes forward through the sphenoid fissure into the orbit
cavity, where it divides into a superior and an inferior branch. The
former is distributed to the superior rectus and the levator palpebral
muscles; the latter is distributed to the internal and inferior recti and
inferior oblique muscles (Fig. 271).

From the inferior branch a short bundle of fibers passes to the
ciliary or ophthalmic ganglion, where they terminate, arborizing around
the ganglion cells. These fibers are smaller in size than those con-
stituting the bulk of the nerve and belong to the system known as
the autonomic. These cells
give origin to new axons, the
ciliary nerves, which enter the
eyeball, pass forward between
the sclera and chorioid coat,
and terminate in the ciliary
muscle and the sphincter of the
pupil. The ciliary nerves are
not portions of the third nerve
proper, but peripheral sym-
pathetic neurons. As the
ciliary ganglion receives fila-
ments from the cavernous
plexus of the sympathetic and
filaments which become a part
of the trigeminal nerve, it is
probable that the ciliary
nerves contain not only motor,
but vaso-motor and sensor
fibers as well.

Properties. — Stimulation of the nerve near its exit from the en-
cephalon is followed by contraction of the muscles to which it is dis-
tributed with, the following results, viz.:

1. Diminution in the size of the pupil.

2. Accommodation of the eye for near vision.

3. Elevation of the upper eyelid.

4. Internal deviation and rotation upward and inward of the anterior

pole of the eye, combined with a small amount of torsion toward

the mesial line, due to preponderating action of the internal rectus

and inferior oblique muscles.
Division of the nerve either experimentally or as a result of com-
pression from a pathologic cause is followed by a relaxation of the
muscles, with the following effects, viz.:
1. Dilatation of the pupil, the iris responding neither to light nor to

efforts of accommodation.

Fig. 271. — Intra-orbital Portion of the
Third Nerve, i. Optic nerve. 2. Third
nerve. 3. Superior branch. 4. Inferior branch.
5. Abducens. 6. Trifacial. 7. Ophthalmic
branch divided. 8. Nasal branch. 9. Ciliary
ganglion. 10. Motor branch to this ganglion
from the inferior branch of the third nerve. 11.
Sensory fibers. 12. Sympathetic fibers. 13.
Ciliary nerves. — (Sappey.)

588 TEXT-BOOK OF PHYSIOLOGY.

2. Loss of the accommodative power.

3. Falling of the upper eyelid (ptosis).

4. External deviation and rotation downward and outward of the

anterior pole of the eyeball combined with a small amount of tor-
sion toward the mesial line due to the unopposed action of exter-
nal rectus and the superior oblique muscles.

5. Double vision or diplopia. The image of the eye of the paralyzed

side is projected to the opposite side of the true image and to the
upper part of the visual field. Owing to the slight mesial torsion
the false image is inclined away from the true image.

6. Immobility and slight protrusion of the eyeball.

Function. — The function of the third nerve is to transmit nerve
impulses from the nuclei of origin to all the muscles of the eye except
the external rectus and superior oblique and excite them to activity.
The majority of the ocular movements, the power of accommodation,
the variations in the size of the pupil in accordance with variations
in the intensity of the light, the power of convergence of the visual
axes, are all excited by the transmission of nerve impulses by the con-
stituent fibers of the nerve from their related nuclei. This is made
evident by the effects which follow stimulation and division of the
nerve or lesions of the nuclei themselves.

The central nuclei can be excited to activity (1) by nerve impulses
descending the motor tract, from the cerebral cortex, (2) by nerve
impulses coming through various afferent nerves. This holds true
more especially for the sphincter pupillse nucleus.

The Iris Reflex or the Pupillary Reflex. — These are terms
applied to the variations in the size of the pupil that follow variations
in the intensity of the light. In the absence of light the pupil widely
dilates, due largely to the relaxation of the sphincter pupillce muscle
and partly to a contraction of the radiating fibers of the iris which
collectively constitute the dilatator pupilla muscle. With the entrance
of light into the eye, the pupil diminishes in size, in consequence of the
contraction of the sphincter pupillce caused by a stimulation of the
peripheral ends of .the pupillary fibers of the retina, the degree of con-
traction depending within limits on the intensity of the light.

The action of the sphincter pupillae muscle is therefore a reflex
action and involves the usual mechanism, viz. : A receptive surface,
the retina; afferent nerves, the pupillary fibers in the optic nerve;
an emissive center, the sphincter nucleus of the motor oculi center;
efferent nerves, including fibers in the trunk of the motor oculi and
in the ciliary nerves; and a responsive organ, the muscle. (See Fig.
272.) That this is the mechanism involved in this reflex, is shown
by the fact that when any portion of it is destroyed, the reflex con-
tractions of the sphincter are impaired or abolished.

As slated in a preceding paragraph the central termination of the
afferenl pupillary fibers concerned in this reflex is not positively known.
No one has as yet succeeded in tracing these fibers directly to the

THE CRANIAL NERVES.

589

sphincter nucleus. Experimental and pathologic data apparently
disprove the probability of their terminating in the anterior corpora
quadrigemina. It has been shown, however, that as the optic tract
approaches its termination the visual and the pupillary fibers separate
and it has been assumed that the latter come into anatomic relation
with some intercal-
ated system which in
turn is connected with
the sphincter nucleus.
As to the situation,
origin and course of
this system nothing
positively is known.
There is some evi-
dence for the view
that these two sys-
tems are associated by
commissural fibers.

The contraction of
the sphincter and a
diminution in the size
of the pupil, may be
direct, as when the
light which enters one
eye causes a reflex
contraction of the
sphincter of one and
the same side; or it
may be indirect or
consensual, as when
the light, which enters
one eye only, causes
a contraction of the
sphincter not only in
the eye of the same,
but in the eye of the
opposite side also. It
is, however, highly
probable that all re-
flex contractions of

Pupillary fibers
in Optic Ti

J)irecL.

Crossed,

'ht-Corp.Quad-
Fostga/ig/ioriic fibers.

\Sup.Ceroical Ganglion.
Pnganylivnic fibers.

■■/-Tiwracic A (//(.

Trail seitio/i of Spinal Co/ri

Fig. 272. — Diagram Designed to Show the Mechanism
of the Iris Reflex. The central termination of the
pupillary fibers is hypothetical.

the sphincter muscles are consensual, that is, bilateral reflex actions
because of the decussation of the pupillary fibers at the chiasm. Con-
traction of both pupils also occurs as an associated movement in the
convergence of the eyes during accommodation.

The dilatation of the pupil is, however, not due exclusively to the
relaxation of the sphincter pupillas muscle, but partly to the contraction
of the dilatator pupillas muscle, which is kept normally in a state of

5QO TEXT-BOOK OF PHYSIOLOGY.

tonic contraction by impulses emanating from a nerve-center in the
medulla oblongata.

The axons which arise in this center pass down the cord, emerge
through the first thoracic nerve, and then ascend to the superior
cervical ganglion (see Fig. 272), in which their terminal branches
arborize around its nerve-cells. From these cells new axons of the
sympathetic system arise which pass successively to the ophthalmic
division of the fifth nerve, the nasal nerve, the long ciliary nerve and
he iris.

Experimental research renders it highly probable that the dilatator
center is in a state of continuous activity and the dilatator muscle in
a state of tonic contraction. Whatever the normal stimulus may be,
the center is increased in activity by dyspneic blood, by severe muscle
exercise, by emotional excitement, and by stimulation of various
sensor nerves. That the efferent pathway just alluded to transmits
the impulses to the iris is shown by the fact that division in any part
of the course is followed by narrowing, stimulation by active dilatation
of the pupil.

The variations in the size of the pupil, though largely a reflex
act under the control of the oculo-motor nerve, are nevertheless partly
due to the active cooperation of the dilatator nerves and their related
muscle. The size of the pupil necessary from moment to moment
for the admission of just that amount of light essential to the formation
and perception of a distinct image is the result of two nicely adjusted
and delicately balanced forces.

Wernicke's Hemianopic Pupillary Reaction. — It was stated
on page 585 that a modification of the pupillary reaction is observed in
some cases of hemianopsia, which indicates approximately the seat of
the lesion. This reaction, or inaction as it is sometimes called, is
present when the lesion is along the course of the optic tract between
the chiasma and the anterior quadrigeminal body. In a case of left
lateral hemianopsia, the lesion being in the right optic tract, the method
of testing for the reaction is as follows : The eye of the left side is first
carefully shielded from the light. A fine ray of light is then projected
into the right eye in such a manner that it falls entirely on the non-
sensitive (the temporal) side of the retina. There will be an absence
of the usual pupillary response, or rather the pupil remains inactive;
but if the light is gradually directed towards the sensitive (the nasal)
side of the retina, there will come a moment, as the central line is crossed
and the light falls on the sensitive side, when the usual pupillary response
manifests itself, viz. : a contraction of the sphincter pupillae and a
diminution in the size of the pupil. The explanation of these facts
will become apparent from an examination of Fig. 272 in which the
course of the pupillary fibers is shown and especially if it be accepted
that these fibers at their central terminations decussate or are in relation
either directly or indirectly with the sphincter centers.

The eye of the right side is then in turn shielded from the light

THE CRANIAL NERVES.

591

and the same method of examination is carried out. In this case,
however, the light is projected first on the nasal, which is the non-
sensitive side of the retina; there will again be no response in the pupil.
But if the light is gradually directed towards the sensitive (the temporal)
side, there will come a moment, as the central line is crossed and the
light falls on the sensitive portion of the retina, when the usual pupillarv
response manifests itself. The course of the pupillary fibers in this
instance will also become apparent from an examination of Fig. 272.
It is evident, however, that in either case a bilateral pupillary reaction
will follow stimulation of the sensitive side of either eye because of the
central decussation of the pupillary fibers.

FOURTH PAIR. THE PATHETICUS.

The fourth cranial nerve, the patheticus, consists of peripherally
coursing axons which serve to bring the cells from which they arise
into relation with the superior
oblique muscle.

Origin. — The axons of this
nerve arise from a group of cells
located beneath the aqueduct of
Sylvius just posterior to the last
nucleus of the third nerve. xAiter
emerging from the nucleus the
nerve-fibers pass downward for
a short distance, then curve dor-
sally around the aqueduct of
Sylvius, and enter the valve of
Vieussens, where they completely
decussate with the nerve-fibers of
the opposite side.

Cortical Connections. —
The nucleus of the pathetic
nerve is in histologic and physi-
ologic connection with the motor
area of the cerebral cortex.
Nerve-cells in this region give off
axons which enter the pyramidal
tract and descend through the
internal capsule and the crus
cerebri, after which they cross to

the opposite side. Their end-tufts arborize around the cells of the
nuclei already described.

Distribution. — After its decussation the nerve-trunk emerges
just below the posterior quadrigeminal body, crosses the superior
cerebellar peduncle, and winds around the crus cerebri to the anterior
border of the pons Varolii. It then enters the orbit cavity through
the sphenoid fissure and finally terminates in the superior oblique

Fig. 273. — Distribution of the
Patheticus. I. Olfactory nerve. II. Op-
tic nerves. III. Motor oculi communis. IV.
Patheticus, by the side of V the ophthalmic
branch of the fifth, and passing to the supe-
rior oblique muscle. VI. Motor oculi ex-
ternus. i. Ganglion of Gasser. 2, 3, 4,
5, 6, 7, 8, 9, 10. Ophthalmic division of
the fifth nerve, with its branches. — (Hirscli-
jeld.)

592

TEXT-BOOK OF PHYSIOLOGY.

muscle. In its course the nerve receives filaments from the cavernous
plexus of the sympathetic and the ophthalmic division of the trigeminal
(Fig. 273).

Properties. — Stimulation of the nerve-trunk is followed by spas-
modic contraction of the superior oblique muscle, the anterior pole
of the eyeball being turned downward and outward, combined with
slight torsion away from the middle line.

Division of the nerve is followed by a relaxation or paralysis of

the muscle. In
consequence of the
now unopposed ac-
tion of the inferior
oblique muscle, the
anterior pole of the
eyeball is turned
upward and inward
with slight torsion
toward the middle
line. The diplopia
consequent upon
this paralysis is
homonymous, the
images appearing
one above the
other. The image
of the paralyzed
eye is below that of
the normal eye and
its upper end in-
clined toward that
of the normal eye.
Function. — The function of this nerve is to transmit nerve im-
pulses to the superior oblique muscle and to excite it to contraction.

3 4

Fig. 274. — Scheme oe Origin and Constitution of the
Trigeminal Nerve, i. Centrally coursing fibers. 2, 3,
4. Peripherally coursing fibers of the cells of the ganglion of
Gasser. R, N. Nuclei of origin of the efferent fibers. 6.
Motor root. Central terminations of the large root.

FIFTH PAIR. THE TRIGEMINAL.

The fifth cranial nerve, the trigeminal, consists of both afferent and
efferent axons which for the most part are separate and distinct.
The afferent axons constitute by far the major portion, the efferent
fibers the minor portion, of the nerve.

Origin of the Afferent Axons. — The afferent axons have their
origin in the monaxonic cells in the ganglion of Gasser, which rests
on the apex of the" petrous portion of the temporal bone. The cells
of this ganglion give origin to a short process which soon divides into
two branches, one of which passes centrally, the other peripherally
(Fig. 274). The centrally directed branches collectively form the so-
called large or sensor root; the peripherally directed branches collec-

THE CRANIAL NERVES.

593

tively constitute the three main divisions of the nerve: viz., the oph-
thalmic, the superior maxillary, and the inferior maxillary. Branches
of the carotid plexus of the sympathetic enter the nerve in the neighbor-
hood of the ganglion of Gasser and accompany some of its branches
to their terminations.

Distribution. — i. The Central Branches. — The axons of the
large root pass backward into the pons Varolii on its lateral aspect.
After entering the pons each axon divides into two branches, one
of which passes upward a short distance, the other passes downward,
descending as far as the second cervical segment. Both branches
give off a number of collaterals, some of which terminate in fine end-
tufts around nerve-cells in the substantia gelatinosa.

2. The Peripheral Branches.
— The . peripheral axons emerge
from the peripheral end of the
ganglion of Gasser in three dis-
tinct and separate branches, each
of which is distributed to a differ-
ent region of the face and head.
i. The ophthalmic branch passes

forward and subdivides into

three large branches, the

frontal, the lachrymal, and

the nasal. The ultimate

termination of the branches

of these nerves is as follows :

viz., the conjunctiva and

skin of the upper eyelid, the

cornea, the skin of the fore-
head and the nose, the

lachrymal gland and car-
uncle, and the mucous

membrane of the nose (Fig.

275)-

2. The superior maxillary branch passes forward through the foramen

rotundum, crosses the spheno-maxillary fossa, enters the infra-
orbital canal, and emerges at the infra-orbital foramen. In its
course it gives off a number of branches which are distributed
as follows: viz., to the integument and conjunctiva of the lower
lid, the nose, cheek, and upper lip, the palate, the teeth of the
upper jaw, and the alveolar processes (Fig. 276).

3. The inferior maxillary branch passes through the foramen ovale,

after which it subdivides into three branches — the auriculo-tem-
poral, the lingual, and the inferior dental. The ultimate branches
are distributed as follows: viz., the external auditory meatus, the
side of the head, the mucous membrane of the mouth, the anterior
portion of the tongue, the arches of the palate, the teeth and
38

F16. 275. — Ophthalmic Branch of the
Fifth, i. Ganglion of Gasser. 2. Oph-
thalmic division of the fifth. 3. Lachrymal
branch. 4. Frontal branch. 5. External
frontal. 6. Internal frontal. 7. Supra-
trochlear. 8. Nasal branch. 9. External
nasal. 10. Internal nasal. — (Hirschjeld.)

594

TEXT-BOOK OF PHYSIOLOGY

alveolar process of the lower jaw and the integument of the lower

part of the face (Fig. 277),
The afferent axons thus serve to bring into relation the skin,
mucous membranes of the head and face,, and other sentient struc-
tures, with certain sensor end-nuclei in the pons, medulla oblongata,
and adjoining structures.

Cortical Connections. — The afferent portion of the trigeminal
nerve is brought into physiologic relation with the sensor portion of
the cerebral cortex by means of nerve-fibers, which have their origin in
the cells around which the terminal branches of the centrally coursing
fibers arborize. The cells situated in the substantia gelatinosa give

Fig. 276. — 1. Superior maxillary nerve. 2, 3, 4, 5. Dental nerves. 6. Spheno-
palatine ganglion. 7. \ idian nerve. 8. Large superficial petrosal. 9. Carotid branch
of large petrosal. 10. Oculo-motor. 11. Superior cervical ganglion. 12. Carotid
branches of this ganglion. 13. Facial,. 14. Glosso-pharyngeal. 15. Jacobson's nerve,
and 16, 17, iS, 19, branches to the sympathetic, fenestra rotunda, Eustachian tube. 20.
Deep external petrosal. 21. Deep internal petrosal. — (H'irschjeld.)

off axons, which after a short course cross the median line, enter the
fillet and then ascend in the general sensor tract to the cortex where
they in turn arborize around sensor nerve-cells.

Properties. — Irritative pathlogic lesions, e. g., pressure by tumors,
aneurysms, neuritis, degenerative changes in the ganglion cells, or
lesions which in any way gradually impair the physical or chemic
integrity of the nerve-fibers, give rise to a variety of painful sensations
referable to the seat of the lesion or to one or more regions in the
peripheral distribution of the nerve. Many of the various forms of
trigeminal neuralgia are caused by lesions of this character. Ex-
posure of the dental nerves from caries of the teeth, the presence of
minute foreign bodies in the conjunctiva, operative procedures in the
nasal chambers, all testify to the extreme sensibility of the nerve.
Division of the large root within the cranium is followed at once by
complete abolition of all sensibility in the head and face to which its
branches arc distributed. The skin and mucous membranes, the eye,

THE CRANIAL NERVES.

595

nose, or teeth may be experimentally injured without any evidences
of pain on the part of the animal. Various reflexes, e. g., those of
mastication, insalivation, deglutition, the afferent paths of which are
formed in part by the fifth nerve, are often seriously impaired. At the
same time the lachrymal secretion diminishes and the pupil contracts.
The same results are observed in human beings in whom the nerve

v .

V

JWL*

Fig. 277. — Inferior Maxillary Branch of the Trigeminal Nerve, i. Branch
to the masseter muscle. 2. Filament of this branch to the temporal muscle. 3. Buccal
branch. 4. Branches anastomosing with the facial nerve. 5. Filament from the buccal
branch to the temporal muscle. 6. Branches to the external pterygoid muscle. 7.
Middle deep temporal branch. S. Auriculotemporal nerve. 9. Temporal branches.
10. Auricular branches. 11. Anastomosis with the facial nerve. 12. Lingual branch.
13. Branch of the small root to the mylo-hyoid muscle. 14. Inferior dental nerve, with
its branches (15, 15). 16. Mental branch. 17. Anastomosis of this branch with the
facial nerve. — {Hirschfeld.)

has been divided for relief from neuralgia. Anesthesia or a loss of
sensibility may also be caused by pathologic lesions of the nerve-trunks
or of the sensor end-nuclei.

Division of the large root at or near the ganglion of Gasser has
not infrequently been followed by an alteration in the nutrition of the
eye and nose. In the course of twenty-four hours the eye becomes
vascular and inflamed; the cornea becomes opaque; ulceration sets
in which may lead to complete destruction of the eyeball. The

5g6 TEXT-BOOK OF PHYSIOLOGY.

mucous membrane of the nose becomes swollen, vascular, and liable
to hemorrhage on the slightest irritation. The degenerative changes
may lead to a complete loss of the sense of smell. These results were
formerly attributed to a loss of trophic influence which it was believed
the nerve exercised over these structures. Modern experimentation
and various surgical procedures have demonstrated that the nutritive
disorders are septic in origin, made possible by the anesthetic condi-
tion and by the changed vascular supply from division of the vaso-
motor fibers which join the nerve at or near the ganglion.

Origin of the Efferent Axons. — The efferent axons arise for the
most part from nerve-cells located in the gray matter beneath the upper
half of the floor of the fourth ventricle. A group of cells known as the
superior or accessory nucleus, situated posterior to the corpora quad-
rigemina, give origin to axons which descend and join the axons from
the chief motor nucleus (Fig. 274).

Distribution. — From their origin the fibers pass forward through
the pons and emerge on its lateral aspect, forming the so-called small
root of the fifth nerve. This then passes forward beneath the' ganglion
of Gasser, leaves the cavity of the skull through the foramen ovale,
and joins the inferior maxillary division already described. Its
axons are ultimately distributed to the muscles of mastication: viz.,
the masseter, the temporal, the external and internal pterygoids, the
mylohyoid, and the anterior portion of the digastric. A few axons
are also distributed to the tensor tympani and tensor palati muscles.
The efferent or peripherally coursing axons thus serve to bring the
nerve-cells from which they arise into relation with the muscles of
mastication.

Cortical Connections. — The nuclei of origin of the small root
are in histologic and physiologic relation with the lower third of the
motor area of the cerebral cortex. Nerve-cells in this region give
off axons which enter the pyramidal tract, descend through the in-
ternal capsule and the crus cerebri, after which they cross to the
opposite side. Their end-tufts arborize around the cells of nuclei in
the medulla oblongata.

Properties. — Stimulation of the small root gives rise to convulsive
movements of the muscles of mastication. Division of the nerve is
followed by a paralysis of these muscles. Contraction or paralysis of
the tensor tympani and tensor palati muscles would also be observed
under the same conditions. .

Functions. — The function of the afferent fibers of the fifth nerve
is the transmission of nerve impulses from its peripheral distribution
to (a) the medulla oblongata; (b) through its afferent cortical tracts
to the cerebral cortex where they evoke sensations. The nerve there-
fore endows all the parts to which it is distributed with sensibility.

The function of the efferent fibers is the transmission of nerve im-
pulses from the cells from which they take their origin, to the muscles of
mastication which are excited to activity by them. The afferent nerves

THE CRANIAL NERVES.

597

are in relation centrally with' the nuclei of origin of the efferent nerves,
hence the latter can be excited not only voluntarily but refiexly as in
the usual acts of mastication. The afferent fibers from the mouth
doubtless assist in the reflex secretion of saliva.

Peripheral stimulation of different areas in the distribution of the
afferent fibers, e. g., conjunctiva, nasal and oral mucous membranes,
teeth, etc., causes a variety of reflex activities in the muscles associated
with the eyes, face, the respiratory and cardiac mechanisms, which
indicate that the afferent fibers are centrally in relation with a number
of motor nerve centers.

SIXTH PAIR. THE ABDUCENS.

The sixth cranial nerve, the abducens, consists of peripherally
coursing axons which serve to bring the nerve-cells from which they
arise into relation with the external rectus muscle.

Origin. — The axons arise from
a group of cells located in the gray
matter beneath the upper half of
the floor of the fourth ventricle. It
is quite probable that a few fibers
in each nerve-trunk come from the
nucleus on the opposite side of the
middle line.

Distribution. — The nerve-
fibers pass forward from their
origin through the gray and white
matter and emerge through the
groove between the medulla oblon-
gata and the pons Varolii just ex-
ternal to the anterior pyramid. The
nerve then passes through the
sphenoid fissure into the orbit
cavity, where it is distributed to
the external rectus muscle (Fig.
278). In its course the nerve re-
ceives filaments from the carotid
plexus of the sympathetic.

Cortical Connections. — The
nucleus of the sixth nerve is in histologic and physiologic connection
with the motor area of the cerebral cortex. From nerve-cells in this
region axons are given off which enter the pyramidal tract, descend
through the internal capsule and crus cerebri, after which they cross
to the opposite side, Avhere their end-tufts arborize around the cells
of the nucleus already described.

Properties. — Stimulation of the nerve is followed by spasmodic
contraction of the external rectus muscle and external deviation of
the eyeball. Division of the nerve is followed by paralysis or relaxation

Fig. 2 78. — Distribution of the
Motor Oculi Externus or Abducexs.
1. Trunk of the motor oculi communis,
■with its branches (2, 3, 4, 5, 6, 7). 8.
Motor oculi externus, passing to the ex-
ternal rectus muscle. 9. Filaments of
the motor oculi externus anastomosing
with the sympathetic. 10. Ciliary nerves.
—iHirsckfeld.)

59S TEXT-BOOK OF PHYSIOLOGY.

of the muscle. As a result of the unopposed action of the internal
rectus the anterior pole of the eyeball is turned toward the middle
line (internal strabismus). In consequence of this deviation there
is homonymous diplopia. The images are on the same level and
parallel. The image of the paralyzed eye lies external to that of the
normal eye.

Function. — The function of this nerve is to transmit nerve im-
pulses to the external rectus muscle and excite it to contraction.

SEVENTH PAIR. THE FACIAL.

The seventh cranial nerve, the facial, consists of peripherally
coursing nerve-fibers, which serve to bring the nerve-cells from which
they arise into relation with most of the superficial muscles of the head
and face.

The muscles supplied by this nerve, as stated by the general
anatomists, are as follows: The occipito-frontalis, corrugator super-
cilii, orbicularis palpebrarum, levator labii superioris, alaeque nasi,
zvgomatici, the pyramidalis nasi, the compressor nasi, the depressor
alae nasi, levator anguli oris, buccinator, orbicularis oris, depressor
anguli oris, depressor labii inferioris, the levator menti, the posterior
belly of the digastric, the stylo-hyoid, and the platysma myoides.

Origin. — The nerve-fibers or axons composing the seventh nerve
arise for the most part from a nucleus of large multipolar nerve-cells
situated about five millimeters beneath the upper half of the floor
of the fourth ventricle toward the middle line.

From this nucleus, which is about four millimeters long, axons
emerge which at first pass inward and backward as far as the epen-
dyma of the ventricle; they then turn on themselves, forming an arch
that encloses the nucleus of the sixth nerve; they then course down-
ward and outward, emerging from the pons at its lower border between
the olivary and restiform bodies. As the axons approach the floor
of the ventricle collateral branches are given off which, crossing the
median line, arborize around the nerve-cells of the opposite facial
nucleus.

Clinic observations and histologic investigations, however, render
it probable that the fibers distributed to the occipito-frontalis, the cor-
rugator supercilii, and the upper half of the orbicularis palpebrarum,
are derived from the oculo-motor nucleus, and, descending the posterior
longitudinal bundle, enter the trunk of the facial as it turns to pass
forward through the pons. It is also probable, for similar reasons,
that the fibers distributed to the orbicularis oris are derived from the
hypoglossal nucleus.

Cortical Connections. — The nucleus of the facial nerve is in
histologic and physiologic connection with the facial region of the
general motor area of the cerebral cortex. From the cells of this
region axons descend through the pyramidal tract, the internal cap-

THE CRANIAL NERVES.

599

sule, and the cms cerebri, beyond which they cross to the opposite
side and arborize around the cells of the nucleus already described.

Distribution. — From its superficial origin the trunk of the nerve
passes into the internal auditory meatus beside the auditory nerve.
After passing forward and outward for a short distance through the

Fig. 279. — Superficial Branches of the Facial and the Fifth. — 1. Trunk of the
facial. 2. Posterior auricular nerve. 3. Branch which it receives from the cervical
plexus. 4. Occipital branch. 5, 6. Branches to the muscles of the ear. 7. Digastric
branches. 8. Branch to the stylo-hyoid muscle. 9. Superior terminal branch. 10.
Temporal branches. 11. Frontal branches. 12. Branches to the orbicularis palpe-
brarum. 13. Nasal or suborbital branches. 14. Buccal branches. 15. Inferior ter-
minal branch. 16. Mental branches. 17. Cervical branches. 18. Superficial temporal
nerve (branch of the fifth). 19, 20. Frontal nerves (branches of the fifth). 21, 22, 23,
24, 2%, 26, 27. Branches of the fifth. 28, 29, 30, 31, 32. Branches of the cervical nerves.
~(H ir sch j eld.)

bone above and between the cochlea and vestibule, the nerve makes
a sharp bend, forming the genu facialis, turns backward and enters
the aqueduct of Fallopius, the general course of which it follows as
far as the stylo-mastoid foramen. After emerging from this foramen
the nerve passes downward and forward as far as the parotid gland,
within which it terminates by dividing into two main branches, the

6oo

TEXT-BOOK OF PHYSIOLOGY.

temporo-facial and the cervicofacial, the ultimate branches of which
are distributed as previously stated to the superficial muscles of the
head and face (Fig. 279).

The Pars Intermedia or Nerve of Wrisberg.— The facial nerve
at the genu, the point where it turns backward to enter the aqueduct
of Fallopius, presents a slight enlargement, grayish in color, and in
which nerve-cells are contained. This enlargement is known as the
geniculate ganglion. The cells of this ganglion, originally bipolar, pre-
sent single axons which soon divide into centrally and peripherally cours-
ing branches. The former collectively constitute the pars intermedia

or nerve of Wrisberg, which,
entering and passing through
the pons, terminates around
the sensor end-nucleus of the
glosso-pharyngeal nerve; the
latter, the peripherally directed
branches, enter the sheath of
the facial and accompany it
as far as a point about five
millimeters above the stylo-
mastoid foramen.

From its mode of origin
the nerve of Wrisberg can not
be regarded as an integral
part of the facial nerve proper,
but is to be regarded as an
independent sensor nerve. As
to the true function and rela-
tion of this nerve there is
much conflict of opinion.
Branches of the Facial.
—In the aqueduct of Fallopius the facial gives off the following
branches : the greater and lesser petrosals, the stapedius, and the chorda
tympani (Fig. 280).

1. The greater petrosal nerve is given off near the geniculate ganglion.

It then passes forward into the spheno-maxillary fossa and ter-
minates in the spheno-palatine ganglion by an arborization of
its fibers around the ganglion cells.

2. The lesser petrosal nerve is given off at a point somewhat external

to the preceding. It leaves the skull by a small foramen and
terminates in the otic ganglion by an arborization of its fibers
around the ganglion cells.

3. The stapedius branch leaves the aqueduct of Fallopius somewhat

further down by a small foramen, enters the pyramid of the
middle ear, and is finally distributed to the stapedius muscle.

4. The chorda tympani is given off from the facial at a point about

live millimeters above the stvlo-mastoid foramen. Il then passes

Fig. 280. — Chorda Tympani Nerve, i, 2
3, 4. Facial nerve passing through the aqueduc-
tus Fallopii. 5. Ganglioform enlargement. 6.
Great petrosal nerve. 7. Sphenopalatine gan-
glion. 8. Small petrosal nerve. 9. Chorda tym-
pani. 10, 11, 12, 13. Various branches of the
facial. 14, 14, 15. Glosso-pharyngeal nerve. —
(Hirsch/eld.)

THE CRANIAL NERVES. 601

upw'ard and forward and enters the tympanum through the iter

chordae posterius, crosses the tympanic membrane between the

malleus and incus, leaves the tympanum by the iter chordae

anterius or canal of Huguier, and finally joins the lingual branch

of the fifth. Some of its fibers can be traced to the dorsum of

the tongue, others to the submaxillary and sublingual ganglia,

where they terminate in tufts around the ganglion cells.

Properties. — Electric stimulation of the trunk of the nerve after

its emergence from the stylo-mastoid foramen produces convulsive

movements in all the muscles to which its branches are distributed.

The same results follow stimulation of the intra-cranial portion of the

nerve in an animal recently killed.

Irritative pathologic lesions — e. g., tumors, aneurysms, etc. — sit-
uated along the course of the nerve or at its nuclear origin, fre-
quently give rise to spasmodic movements of the facial muscles which
may be tonic or clonic in character.

Division of the facial nerve after its emergence from the stylo-
mastoid foramen is followed by a complete relaxation or paralysis
of the superficial facial muscles. The same result follows compres-
sion of the nerve-trunk in any part of its intra-cranial course.

The phenomena presented by an individual suffering from division
or compression of the facial nerve, and which collectively constitute
facial paralysis, are as follows: A relaxed and immobile condition of
the side of the face corresponding to the lesion; separation of the eye-
lids from paralysis of the orbicularis palpebrarum and the unopposed
contraction of the levator palpebrse muscles; abolition of the act of
winking; drooping of the angle of the mouth; an escape of saliva
from the mouth; contraction of the muscles and distortion of the
opposite side of the face; on attempting to laugh or talk the distortion
of the face is increased; during mastication the food accumulates
between the teeth and cheek, from paralysis of the buccinator; articu-
lation is impaired from paralysis of the orbicularis oris muscle, the
labial sounds especially being imperfectly produced.

Properties of the Branches Given off in the Aqueduct of
Fallopius. — The great petrosal nerve is in all probability composed
of efferent fibers (vaso-dilatator and secretor) which leave the pons
by way of the nerve of Wrisberg, or pars intermedia, to be distributed
around the cells of the spheno-palatine ganglion; for stimulation
either of this nerve or of the ganglion is followed by the same results:
viz., dilatation of the blood-vessels of, and secretion from, the mucous
membrane of the nose, soft palate, upper part of the pharynx, roof
of mouth, gums, and upper lip.

The small petrosal nerve is also composed of efferent fibers; shortly
after leaving the facial it is joined by a small nerve, derived from
Jacobson's branch of the glosso-pharyngeal, which is also efferent
in function; for stimulation of Jacobson's nerve as well as stimulation
of the otic eanglion is followed bv the same result: viz., dilatation

6o2 TEXT-BOOK OF PHYSIOLOGY.

of the blood-vessels of, and secretion from, the mucous membrane of
the cheek, lower lip, and gums, and of the parotid and the orbit glands.

The stapedius nerve, distributed directly to the stapedius muscle,
is motor in function.

The Chorda Tympani. — Stimulation of the chorda tympani nerve
in the tympanic cavity produces dilatation of the blood-vessels of,
and an increased production and discharge of saliva from, the sub-
maxillary and sublingual glands.

Division of this nerve is followed by a contraction of the blood-
vessels and a diminution of the secretion. From these results it is
certain that the chorda tympani contains both vaso-dilatator and secre-
tor fibers. Nicotin applied to the submaxillary and sublingual ganglia
abolishes the effects of stimulation of the chorda tympani. It does
not prevent the same effects when the ganglia themselves are stimu-
lated. It is clear, therefore, that the vaso-dilatator and secretor
fibers arborize around the cells of the ganglia and are not distributed
directly to the gland structures. It is highly probable that the efferent
fibers in the chorda tympani emerge from the pons by way of the
pars intermedia, or nerve of Wrisberg.

Division of the chorda in the tympanum is also followed by a loss
of taste in the anterior two-thirds of the tongue. For this and other
reasons the chorda tympani has long been regarded as the nerve of
taste for this region. The specific physiologic stimulus to the chorda
tympani nerve is organic matter in solution acting on the peripheral
terminations of the nerve in the mucous membrane of the tongue.
The exact pathway for these afferent or gustatory fibers beyond the
geniculate ganglion has long been a subject of much discussion.
According to some observers these fibers enter the great petrosal
nerve, pass forward as far as the spheno-palatine ganglion, then into
the superior maxillary division of the trigeminal, and so to the brain.
According to others, these fibers pass into the pars intermedia, into
the pons, where they terminate around the sensor end-nucleus of the
glosso-pharyngeal. The evidence for and against either of these two
views is most conflicting and insufficient to justify positive statements
one way or the other. To the writer the weight of evidence seems to
favor the view that the gustatory fibers have their origin in the genicu-
late ganglion; that they pass centrally through the pars intermedia;
that they are similar in function to the glosso-pharyngeal; and that
they are indeed but aberrant branches of this nerve.

Functions. — The function of the facial nerve is the transmission
of nerve impulses from the nerve-cells from which it arises to the
muscles of the face. As these muscles express ideas and emotions
the nerve has been termed the nerve of expression. Because of the
presence of efferent fibers which leave the main trunk by way of the
chorda tympani nerve, it regulates the caliber of the blood-vessels of the
submaxillary and sublingual glands and excites the glands to activity.
It also influences hearing by its action on the stapedius muscle.

THE CRANIAL NERVES.

60 •

EIGHTH PAIR. THE AUDITORY.

The eighth cranial nerve, the auditory, consists of the centrally
coursing axons of neurons which connect the essential organ of hearing
with sensor end-nuclei in the pons Varolii. This nerve consists of
two portions: viz., a cochlear or auditory and a vestibular or equilibra-
tory.

Origin.— The axons comprising the cochlear portion have their
origin in the bipolar nerve-cells of the spiral ganglion located in the
spiral canal near the base of the

osseous lamina spiralis (Fig. 281). ..-''

From this origin they pass cen-
trally into the central canal of the
modiolus, at the base of which
they emerge in well-defined bun-
dles and enter the internal audi-
tory meatus. Dendritic processes
from these cells pass peripherally
to terminate on the ciliated epi-
thelial cells of the organ of Corti.

The axons comprising the vesti-
bular portion have their origin in
the bipolar nerve-cells of the
ganglion of Scarpa located in the
internal auditory meatus. From
this origin they pass centrally in
connection with the cochlear por-
tion. Dendritic processes from
these cells pass peripherally into
the internal ear, where they
terminate on epithelial cells situ-
ated on the inner surface of the
utricle and saccule and in the
ampullae of the semicircular canals.

The common trunk of the
auditory nerve, consisting of both
cochlear and vestibular divisions
after emerging from the internal
auditory meatus, passes backward,
inward, and downward as far as
the lateral aspect of the pons where
the two divisions again separate.

The cochlear nerve, the external root, passes to the outer side of
the restiform body and enters the ventral acoustic nucleus and the
lateral acoustic nucleus, around the cells of which its end-tufts arborize.
The vestibular nerve, the internal root, passes on the inner side of
the restiform body to the dorsal portion of the pons, where, after
bifurcating, the end-tufts of the axons arborize around the dorso-

Fig. 281. — Origin* and Termination
of the .-Auditory Nerve, i. Cochlea.
2. Spiral ganglion (Corti). 3. Cochlear
nerve. 4. Ventral acoustic nucleus. 5.
Lateral acoustic nucleus. 6. Semi-
circular canals. 7. Ganglion of Scarpa.
S. Vestibular nerve. 9. Dorso-external
nucleus (D eiters). 10. Dorso-internal
nucleus. — (After Moral and Doyon.)

6o4 TEXT-BOOK OF PHYSIOLOGY.

internal or chief auditory nucleus and the dorso-external or Deiters'
nucleus. Some of the fibers of the vestibular branch descend through
the pons and medulla as far as the cuneate nucleus.

Cortical Connections. — The cochlear nerve is ultimately con-
nected with the cerebral acoustic area, in the temporal lobe of the
opposite side through the intermediation of the auditory tract. This
tract is complex and involved. In a general way it may be said to
consist in part of fibers which come direct from the cochlear branch.
After passing through the ventral nucleus and the trapezoid body
they cross the median line, enter the lemniscus or fillet, and finally
terminate in the pre- and post-geminal bodies. In their course they
give off collateral branches to these various nuclei through which
they pass. Other fibers taking their origin from cells in these various
nuclei proceed to the cortex where they terminate.

Properties. — Stimulation of the cochlear nerve is unattended by
either motor or sensor phenomena. Division of the nerve is fol-
lowed by a loss of the sense of hearing. Irritative pathologic lesions
give rise to sensations of sound of varying character and intensity.
Degeneration of the nerve or destruction by tumors, etc., will also
be followed by a loss of the sense of hearing.

Experimental lesions of the semicircular canals involving a de-
struction of the physiologic relations of the vestibular nerve are followed
by a loss of the coordinating and equilibratory power. Disordered
movements, such as rotation to the right or left, somersaults back-
ward and forward, follow destruction of these canals. Pathologic
lesions in the peripheral distribution of the nerve are attended in man
bv disturbances of equilibrium, e. g., vertigo, a sense of swaying,
pitching, and staggering.

Functions. — The function of the cochlear nerve is to convey
nerve impulses from its origin to the pons, from which they are trans-
mitted by the auditory tract to the acoustic area in the cerebral cortex
where they evoke sensations of sound and its different qualities, inten-
sity, pitch and timbre. The specific physiologic stimulus to the
development of these impulses is the impact of atmospheric undulations
on the tympanic membrane, received and transmitted by the chain
of bones to the structures of the internal ear — the organ of Corti—
with which the peripheral terminations of the nerve are connected.

The function of the vestibular nerve is the transmission of nerve
impulses to the pons, whence they are transmitted to the cortex of
both the cerebrum and cerebellum and to other centers. The specific
physiologic stimulus is supposed to be a variation in pressure in the
ampulla of the semicircular canals caused by movements of the en-
dolymph induced by changes in the position of the head and body.
The impulses carried by the vestibular nerve give rise renexly to certain
adaptive and protective movements by which the equilibrium of the
body in both dynamic and static conditions is maintained.

THE CRANIAL NERVES. 605

NINTH NERVE. THE GLOSSOPHARYNGEAL.

The ninth cranial nerve, the glosso-pharyngeal, consists, as shown
by both histologic and experimental methods of research, of both
afferent and efferent nerve-fibers, of which the former, however, are
bv far the more abundant. Near its exit from the cavity of the skull
the nerve presents two ganglionic enlargements known as the petrosal
and jugular ganglia.

Origin of the Afferent Fibers. — The afferent fibers serve to
bring certain end-nuclei in the medulla oblongata into anatomic and
physiologic relation with portions of the mucous membrane of the
tongue, pharynx, and middle ear. The afferent fibers are axons of
the monaxonic cells of the petrosal and jugular ganglia. The single
axon from each of these cells soon divides into two branches, one
of which passes centrally, the other peripherally. The centrally
directed branches collectively form the so-called roots, four or five in
number, which enter the medulla between the olivary and restiform
bodies. The peripherally directed branches collectively form the
two main divisions, from the distribution of which, to the tongue and
pharynx, the nerve takes its name.

Distribution. — The axons of the centrally directed branches
after entering the medulla pass toward its dorsal aspect, where they
bifurcate, give off collateral branches, and terminate in fine end-tufts
in the immediate neighborhood of two groups of nerve-cells, the
sensor end-nuclei. The axons of the peripherally directed branches,
after emerging from the base of the skull through the jugular foramen,
pass forward and inward under cover of the stylorpharyngeal muscle;
winding around this muscle they divide into terminal branches which
are distributed to the mucous membrane of the posterior one-third
of the tongue, pharynx, soft palate, uvula, and tonsils (Fig. 283).

Origin of the Efferent Fibers.— The efferent fibers serve to bring
the nerve-cells from which they arise into connection with a portion
of the musculature of the fauces and pharynx. These nerve-cells
are located in the lateral portion of the formatio reticularis at some
distance below the floor of the fourth ventricle. They constitute
the upper portion of a collection of cells known as the nucleus am-
bi vuus.

Distribution. — From this origin the efferent fibers pass dorsally
to near the sensor end-nuclei, then turn outward and forward and
finally emerge from the medulla in intimate association with the
afferent fibers. They are ultimately distributed to the stylo-pharyn-
geus, the palato-glossui and to the middle constrictor muscles of the
pharynx. In addition to the foregoing efferent fibers the glosso-
pharyngeal nerve contains at its emergence from the medulla both
vaso-motor and secretor fibers.

Jacobson's Nerve. — This is a small branch which leaves the glosso-
pharyngeal at the petrous ganglion. After passing through a small
canal in the base of the skull it enters the tympanic cavity, within

606 TEXT-BOOK OF PHYSIOLOGY.

which it gives off branches to the great and lesser petrosal nerves, to
the mucous membrane of the foramen ovale, the foramen rotundum,
and to the Eustachian tube.

Cortical Connections. — The motor nucleus is doubtless con-
nected with the general motor area of the cortex through fibers de-
scending in the pyramidal tract. The exact location of the cortical
area for the pharynx is not well determined, but is most likely to be
found in the lower part of the general motor area near the termination
of the Rolandic fissure. The exact cortical connections of the afferent
tract are unknown, but are most likely to be found in the general
sensor area.

Properties. — Electric stimulation of the glosso-pharyngeal trunk
calls forth evidence of pain and contraction of the stylo-pharyngeus
and middle constrictor muscles. Peripheral stimulation of the termi-
nals of the nerve fibers in the mucous membrane of the posterior third
of the tongue with different kinds of organic matter in solution, de-
velops nerve impulses which transmitted to the cortex evoke sensa-
tions of taste. Division of the nerve abolishes sensibility in the
mucous membrane to which it is distributed, impairs the sense of
taste in the posterior third of the tongue, and gives rise to paralysis
of the above-mentioned muscles.

Stimulation of Jacobson's nerve is followed by dilatation of the
blood-vessels of, and secretion from, the mucous membrane of the
lower lip, cheek, and gums, and from the parotid gland. Division
of the nerve is followed by the opposite results. The course of the
fibers which give rise to these results is by way of the lesser petrosal
to the otic ganglion, around the cells of which the fibers arborize.
From the cells of this ganglion non-medullated fibers pass to the
blood-vessels and gland cells.

Functions. — The afferent fibers of the glosso-pharyngeal trans-
mit nerve impulses from the parts to which they are distributed to the
cerebral cortex, where they evoke sensations of pain and sensations
of taste; they also assist in all probability in the performance of certain
reflexes connected with deglutition. The efferent fibers transmit
impulses to muscles, exciting them to activity, and to the otic ganglion,
which in turn dilates blood-vessels and excites secretion.

TENTH NERVE. THE PNEUMOGASTRIC OR VAGUS.

The tenth cranial nerve, the pneumogastric or vagus, consists,
as shown by histologic methods of research, of both afferent and
efferent fibers, independent of those derived in its course from adjoin-
ing motor or efferent nerves. Near the exit of the nerve from the
cavity of the cranium it presents two ganglionic enlargements known
respectively as the ganglion of the root (the jugular) and the ganglion
of the trunk (the plexiform).

Origin of the Afferent Fibers. — The afferent fibers take their

THE CRAXIAL NERVES. 607

origin in the monaxonic cells of the ganglia on the root and trunk.
The single axon from each of these cells soon divides into two branches,
one of which passes centrally, the other peripherally. The centrally
directed branches collectively form the so-called roots, ten to fifteen
in number, which enter the medulla between the restiform body and
the lateral column. The peripherally directed branches collectively
form a portion of the common trunk of the nerve.

Distribution. — The axons of the centrally directed branches
after entering the medulla pass toward its dorsal aspect, where they
bifurcate, give off collaterals, and terminate in fine end-tufts in the
immediate neighborhood of two groups of nerve-cells, the vagal
sensor end-nuclei.

The axons of the peripherally directed branches unite to form a
portion of the common trunk, which, as it descends the neck and
enters the thorax and abdomen, gives off a number of branches which
are ultimately distributed to the mucous membrane of the esophagus,
larynx, lungs, stomach, and intestine, and also to the heart. The
afferent fibers thus serve to bring into anatomic and physiologic relation
the mucous membrane of these organs with certain sensor end-nuclei
in the medulla oblongata.

Origin of the Efferent Fibers. — The efferent fibers take their
origin from nerve-cells located in the lateral portion of the formatio
reticularis at some distance below the floor of the fourth ventricle.
These cells constitute the lower portion of the nucleus ambiguus.

Distribution. — From their origin the efferent axons pass dorsally
to near the sensor end-nuclei, then turn outward and forward, and
finally emerge from the medulla in close association with the afferent
branches. They are ultimately distributed to the levator palati,
azygos uvulae, and palato-pharyngeus muscles; to the superior and
inferior constrictor muscles of the pharynx, and to the muscles of the
esophagus; to the muscle-fibers of the stomach and perhaps the in-
testines; and to the non-striated muscle-fibers of the bronchial tubes.
Among the efferent fibers are some which are distributed to the gastric
glands and to the pancreas.

According to Beevor and Horsley, in the monkey the motor fibers
for the levator palati come from the spinal accessory nerve.

The efferent fibers thus serve to bring the nerve-cells from which
they arise into anatomic and physiologic connection with a portion of
the musculature of the alimentary canal and its diverticulum, the
lung.

Communicating Branches. — At or near the ganglia the vagus
receives communicating branches from the eleventh nerve, the spinal
accessory, the facial, the hypoglossal, and the anterior branches of
the two upper cervical nerves. Owing to this manifold origin of the
efferent fibers in the trunk and peripheral branches of the vagus,
it is, in some instances, difficult, if not impossible, to determine to
which of these nerves a given muscle contraction is to be referred.

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TEXT-BOOK OF PHYSIOLOGY

Vagal Branches. — As the vagus passes down the neck it gives
off the following main branches (Fig. 282) :
1. The pharyngeal nerves which, after entering into the formation of

Fig. 282. — Distribution of the Pneumogastric. — 1. Trunk of the left pneumo-
gastric. 2. Ganglion of the trunk. 3. Anastomosis with the spinal- accessory. 4.
Anastomosis with the sublingual. 5. Pharyngeal branch (the auricular branch is not
shown in the figure). 6. Superior laryngeal branch. 7. External laryngeal nerve. 8.
Laryngeal plexus. 9, 9. Inferior laryngeal branch. 10. Cervical cardiac branch. 11.
Thoracic cardiac branch. 12,13. Pulmonary branches. 14. Lingual branch of the fifth.
15. Lower portion of the sublingual. 16. Glosso-pharyngeal. 17. Spinal accessory.
18, 19, 20. Spinal nerves. 21. Phrenic nerve. 22, 23. Spinal nerves. 24, 25, 26, 27,
28, 29, 30. Sympathetic ganglia. — (Hirschjeld.)

the pharyngeal plexus, are distributed to the mucous membrane
and to the muscles of the pharynx; e. g., superior and inferior
constrictors, the levator palati, and the azygos uvulae.
2. The esophageal nerves, which after entering into the formation of

THE CRANIAL NERVES. 609

the esophageal plexus, are distributed to the mucous membrane,
and to the muscles of the esophagus.

3. The superior laryngeal nerve which, entering the larynx through

the thyro-hyoid membrane, is distributed to the mucous mem-
brane lining the interior of the larynx and to the crico-thyroid
muscle. From the superior laryngeal and the main trunk small
branches are given off which in the rabbit unite to form a single
nerve, the so-called depressor nerve. (See page 380.) It is dis-
tributed to the heart-muscle. Though this anatomic arrangement
is not found in man, there are many reasons for believing that
analogous fibers are present in the vagus trunk of man and other
animals.

4. The inferior laryngeal nerve which is distributed ultimately to all

the muscles of the larynx (except the crico-thyroid) and to the
inferior constrictor of the pharynx.

5. The cardiac nerves which, after entering into the formation of. the

cardiac plexus, are distributed to the heart.

6. The pulmonary nerves distributed to the mucous membrane of the

bronchial tubes and their ultimate terminations, the lobules and
air-cells, as well as to their non-striated muscle-fibers.

7. The gastric and intestinal nerves, distributed to the mucous mem-

brane and muscular walls of the stomach and intestines. Other
fibers in all probability pass to the liver, spleen, kidney, and
suprarenal bodies.

Properties of the Pneumogastric or Vagus Nerve and its
Various Branches. — Faradization of the vagus nerve close to the
medulla oblongata gives rise to sensations of pain and to contraction
of the musculature of a portion of the alimentary tract: viz., the
palate, pharynx, esophagus, stomach, and possibly of the intestine
and of the pulmonary apparatus. Division of the nerve is followed
by a loss of sensibility in the mucous membrane of the alimentary
tract and of the pulmonary apparatus, together with a loss of motility
of the structures above mentioned.

Stimulation of the trunk of the nerve in different parts of its course
produces a variety of results dependent to some extent on the presence
of anastomosing branches from adjoining nerves.

The Pharyngeal Nerves. — Faradization of the pharyngeal nerves
consisting of both afferent and efferent fibers, gives rise to sensations
of pain, contraction of the pharyngeal muscles, and perhaps to vomiting.
Division of these nerves is followed by a loss of sensibility in the parts
to which they are distributed and by paralysis of the muscles with a
consequent impairment of deglutition.

The Esophageal Nerves. — Faradization of the esophageal nerves
gives rise to sensations of pain and to contractions of the muscle coat
of the esophagus. Division of these nerves is followed by a loss of sen-
sibility in the parts to which they are distributed, a partial paralysis
of the muscle coat and an impairment of deglutition (see page 170).

39

610 TEXT-BOOK OF PHYSIOLOGY.

The Superior Laryngeal Nerve. — Faradization of the superior
laryngeal nerve gives rise to sensations of pain, and to contraction of
the crico-thyroid muscle. Through reflected impulses it causes con-
traction of the muscles of deglutition, and of the muscles concerned
in the act of coughing; inhibition of the inspiratory movement and
arrest of respiration in the condition of expiratory standstill, with
perhaps a tetanic contraction of the expiratory muscles, and con-
traction of the laryngeal muscles with closure of the glottis. Periph-
eral stimulation of this nerve — e. g., the contact of foreign particles-
gives rise to a similar series of phenomena. Division of these nerves
is followed by a loss of sensibility in the laryngeal mucous membrane,
paralysis of the crico-thyroid muscle with a consequent lowering of
the pitch, and a diminution in the clearness of the voice. In conse-
quence of the loss of the sensibility there is an inability to perceive
the entrance of foreign bodies into the larynx.

The Depressor Nerve. — Stimulation of the peripheral end of the
depressor nerve is without effect; stimulation of the central end re-
tards and even arrests the heart's pulsations and lowers the general
blood-pressure. These two effects, though associated, are neverthe-
less independent of each other. If the vagus nerves be divided on
both sides between the origin of the depressor and the origin of the
cardiac nerves, and the former stimulated, there will be a fall of
pressure without retardation of the heart. The effect on the heart
is attributed to a stimulation of the cardio-inhibitory mechanism in
the medulla oblongata.

The fall of general blood-pressure was formerly attributed to a
sudden dilatation of the splanchnic blood-vessels alone, in conse-
quence of a depression of that portion of the general vaso-motor center
which maintains through the splanchnic nerves a tonic contraction of
their walls. It has been satisfactorily demonstrated that this is not
the sole cause; for after division of the splanchnic nerves, stimulation
of the depressor causes a still further fall of from 30 to 40 per cent.
in the general pressure (Porter and Beyer). Evidently, not any one,
but all portions of the vaso-motor center are subject to the effects of
depressor stimulation.

The Inferior Laryngeal Nerves.- — Faradization of the inferior
laryngeal nerves produces effects which vary in accordance with the
strength of the stimulus, with different animals, and with the same
animal at different periods of life. In the adult dog and in man, the
glottis is kept widely open for respiratory purposes by the tonic con-
traction of the abductor muscles (the crico-arytenoids) ; for phonatory
purposes the glottis is closed and the vocal membranes approximated
by the contraction of the adductor muscles. It has been shown that
these opposed groups of muscles have independent nerve-supplies;
thai two sets of libers in the common trunk can be separated and
stimulated independently of each other. Feeble stimulation of the
common trunk produces a still further abduction of the vocal cords.

THE CRANIAL NERVES. 611

With an increase in the strength of the stimulus, however, the reverse
obtains: namely, adduction which increases until the glottis is com-
pletely closed. Division of the nerves is followed by paralysis of both
the phonatory and respiratory muscles, the abductors and adductors,
with the result of seriously impairing both phonation and respiration
and not infrequently causing death. The fibers of the inferior laryn-
geal nerve are derived from the eleventh nerve, the spinal accessory.
The Cardiac Nerves. — Faradization of the trunk of the vagus or
of the peripheral end of the divided nerve gives rise to a diminution
in the frequency and force of the heart's contractions; and if the stim-
ulation be sufficiently powerful, completely arrests it in the phase of
diastole. To these results the term inhibition is applied. Division
of the vagi or of the cardiac branches is followed by an increase in
the number of contractions from loss of inhibitor influences. The
inhibitor fibers of the vagus are generally believed to be derived from
the spinal accessory, though this has been questioned. According to
the recent investigations of Schaternikoff . and Friedenthal, they come
direct in the vagus, from a nucleus near the vagal motor nucleus in
the medulla, the spinal accessory sending no branches to the heart.
In the frog and other batrachia the vagus contains also accelerator
or augmentor fibers derived from the sympathetic; hence stimulation,
especially if feeble, may increase the heart's action.

The Pulmonary Nerves.— The pulmonary nerves, given off from
the trunk after its entrance into the thorax, do not lend themselves
readily to experimentation. Division of both vagi in the neck above
the point of exit of the pulmonary branches is followed by a decrease
in the frequency of the respiratory acts, with an increase in their depth.
At the same time there is a loss of sensibility of the mucous membrane
of the trachea and lungs and a paralysis of non-striated muscle-fibers.
Stimulation of the central end of the vagus increases the frequency,
but decreases the amplitude, of the respiratory movements. If the
stimulation be increased in intensity the respiratory movements in-
crease in frequency until the inspiratory muscles pass into the con-
dition of tetanus.

Feeble stimulation of the vagus not infrequently inhibits the in-
spiratory movement and increases the expiratory until there is a
complete cessation of movement in the condition of expiratory stand-
still. The effect thus produced is similar to, if not identical with,
that produced by stimulation of the superior laryngeal nerve. This
would seem to indicate the presence in the vagus trunk of two sets of
afferent fibers coming from the lungs through the pulmonary branches,
one of which inhibits inspiration, the other inhibits expiration.

Faradization of the trunks of the pulmonary branches or stimula-
tion of their peripheral terminations in the mucous membrane of
the bronchial tubes or alveoli by the inhalation of chemic vapors
causes arrest of respiratory movements, a fall of blood-pressure, and
a re Ilex inhibition of the heart (Brodie).

612 TEXT-BOOK OF PHYSIOLOGY.

Gastric Nerves. — Stimulation of the peripheral end of a divided
vagus nerve causes a distinct contraction of the right half of the
stomach and secretion from the gastric glands. Division of the nerve
abolishes the sensibility of the mucous membrane of the stomach,
impairs motility, and interferes with the secretion of the gastric juice.

Similar experimentation on the trunk of the vagus has shown that
the nerve excites contraction of the upper part of the small intestine
and of the gall-bladder, the secretion of the pancreas, the renal cir-
culation, the secretion of urine, etc.

Functions. — The afferent fibers transmit nerve impulses from the
area of their distribution to the medulla and thence through cortical
connections to the sensor cerebral areas, where they evoke sensations.
They therefore endow all parts to which they are distributed with
sensibility.

The efferent fibers transmit impulses outward which excite contrac-
tion of the muscles of the pharynx, the esophagus, the stomach, the small
intestine, and the gall-bladder, and the muscles of the bronchial tubes;
excite secretion from the glands of the stomach, pancreas, and kidney
and exert an inhibitor influence on the activity of the heart. The
efferent fibers belong to the autonomic system of nerves and are not
connected with the ganglia of the vagus, but with local peripheral
ganglia.

The afferent fibers also assist in the maintenance of certain organic
reflex actions which are highly essential to the life of the individual,
e.g., respiration, the heart beat, blood pressure, etc., all of which
have been considered in foregoing pages.

ELEVENTH PAIR. THE SPINAL ACCESSORY.

The eleventh cranial nerve, the spinal accessory, consists of per-
ipherally coursing fibers which bring the nerve-cells from which they
arise into relation with separate but functionally related muscles.
It consists of two portions, the medullary or bulbar and the spinal.

Origin. — The axons comprising the medullary portion arise from
a group of nerve-cells in the lower part of the nucleus ambiguus.
From this origin the axons pass forward and outward to emerge from
the medulla just below and in series with the roots of the vagus nerve.

The axons comprising the spinal portion have their origin in
nerve-cells in the lateral margin of the anterior horn of the gray matter
in the cervical portion of the cord as far down as the fifth cervical
vertebra. From this origin the fibers pass to the surface of the cord
to emerge between the ventral and dorsal roots in from six to eight
filaments, after which they unite from below upward to form a distinct
nerve. This enters the cranial cavity through the foramen magnum,
where it joins with the medullary portion to form the common trunk,
which then passes forward to emerge from the cranium through the
jugular foramen. (Fig. 283.)

THE CRANIAL NERVES.

613

Distribution. — After emerging from the cranial cavity the nerve
soon separates into two branches:

1. iVn internal or anastomotic branch, consisting chiefly of filaments

coming from the medulla
oblongata. It soon enters
the trunk of the vagus, from
which fibers pass to the
muscles of the pharynx, to
the muscles of the larynx
through the inferior laryn-
geal nerve, and to the heart
according to most author-
ities.

2. An external branch, consisting

chiefly of the accessory

fibers from the spinal cord.
It is distributed to the sterno-

cleido-mastoid and trapezius

muscles.
Cortical Connections. —
The nucleus of origin of the
medullary branch at least is in
relation with nerve-cells in the
lower third of the general cerebral
motor area, the axons of which
descend in the pyramidal tract.

Properties. — Faradization
of the medullary portion of the
nerve near its origin gives rise to
contraction of the muscles to
which they are distributed. De-
struction of the medullary root
is followed by impairment of
deglutition and a loss of the
power of producing vocal sounds
on account of paralysis of the
constrictor muscles of the larynx.
According to some authorities,
there is also an acceleration of
the heart's action from a loss of
inhibitor influences.

Stimulation of the external
branch gives rise to contraction
of the sterno-cleido-mastoid and
trapezius muscles, though division of the branch does not give rise to
complete paralysis, as they are supplied with motor fibers also from
lhe cervical nerves. In consequence of division of the external branch

Fig. 2S3. — Spinal Accessory Xerve.
1. Trunk of the facial nerve. 2. 2.
Glossopharyngeal nerve. 3, 3 Pneumo-
gastric. 4, 4, 4. Trunk of the spinal acces-
sory. 5. Sublingual nerve. 6. Superior
cervical ganglion. 7, 7. Anastomosis of
the first two cervical nerves. 8. Carotid
branch of the sympathetic. 9, 10, 11,
12, 13. Branches of the glosso-pharyngeal.
14, 15. Branches of the facial. 16. Otic
ganglion. 17. Auricular branch of the
pneumogastric. iS. Anastomosing branch
from the spinal accessory to the pneu-
mogastric. 10. Anastomosis of the first
pair of cervical nerves with the sublingual.
20. Anastomosis of the spinal accessory
with the second pair of cervical nerves. 2 1 .
Pharyngeal plexus. 22. Superior laryn-
geal nerve. 23. External laryngeal nerve.
24. Middle cervical ganglion. — {Hirsch-
jeld.)

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TEXT-BOOK OF PHYSIOLOGY.

animals experience extreme shortness of breath during exercise, from a
want of coordination of the muscles of the fore-limbs and the muscles
of respiration.

Functions. — The function of the fibers of the spinal accessory
nerve is the transmission of nerve impulses from the cells from which

Fig. 284. — Distribution of the Hypoglossal Nerve. — 1. Root of the fifth nerve.
2. Ganglion of Gasser. 3, 4, 5, 6, 7, 9, 10, 12. Branches and anastomoses of the fifth
nerve. 11. Submaxillary ganglion. 13. Anterior belly of the digastric muscle. 14.
Section of the mylo-hyoid muscle. 15. Glosso-pharyngeal Nerve. 16. Ganglion of
Andersch. 17, 18. Branches of the glosso-pharyngeal nerve. 19, 19. Pneumogastric.
20, 21. Ganglia of the pneumogastric. 22, 22. Superior laryngeal branch of the pneu-
mogastric. 23. Spinal accessory nerve. 24. Sublingual nerve. 25. Descendens noni.
6. Thyro-hyoid branch. 27. Terminal branches. 28. Two branches one to the genio-
hyo-glossus and the other to the genio-hyoid muscle. — (Sappey.)

they take their origin to the muscles to which they are distributed.
They therefore excite to action some of the muscles of deglutition; the
muscles which regulate the tension of the vocal bands during phonal ion
and the muscles which control the respiratory movements associated
with sustained or prolonged muscle efforts. The fibers also convey
nerve impulses which exert an inhibitor inllucncc on the heart.

THE CRANIAL NERVES. 615

TWELFTH PAIR. THE HYPOGLOSSAL.

The twelfth cranial nerve, the hypoglossal, consists of peripherally
coursing nerve-fibers which serve to connect the nerve-cells from
which they arise with the musculature of the tongue.

Origin. — The axons composing the hypoglossal nerve arise from
a collection of nerve-cells situated beneath the floor of the fourth
ventricle. This nucleus is elongated and extends from the medullary
striae downward as far as the lower border of the olivary body. It- is
located ventro-laterally to the spinal canal. After leaving the cells
of the nucleus the axons pass forward and outward toward the surface
of the medulla, from which they emerge in ten or twelve small bundles
or filaments in the groove between the olivary body and the anterior
pyramid. Beyond this point they unite to form a common trunk.

Distribution. — The common trunk thus formed passes out of
the cranial cavity through the anterior condyloid foramen. In its
course it receives filaments from the first and second cervical nerves,
the sympathetic and vagus. It is finally distributed to the intrinsic
muscles of the tongue and to the genio-hyo-glossus, hyo-glossus, and
stylo-hyoid muscles. Branches derived from the cervical plexus
pass to muscles which elevate and depress the hyoid bone. (Fig. 284.)

Cortical Connections. — The hypoglossal nerve nuclei are con-
nected with nerve-cells in the lower third of the general motor area
around the inferior termination of the fissure of Rolando by axons
which descend in the pyramidal tract.

Properties. — Faradization of the nerve gives rise to convulsive
movements of the muscles to which it is distributed. Division of the
nerve is followed by a loss of motion and an interference with deglu-
tition, mastication, and articulation, especially in the pronunciation
of the consonantal sounds. In hemiplegia, complicated with paral-
ysis of the tongue from injury to the hypoglossal tract, the opposite
side of the tongue is involved in the paralysis. On protrusion of
the tongue the tip is deviated to the paralyzed side, due to the un-
opposed action of the muscle of the opposite side.

Function. — The hypoglossal nerve transmits nerve impulses
from its center of origin to the intrinsic and extrinsic muscles of the
tongue, endowing them with motility. The coordinate activity of
these muscles favorably assists mastication, articulation, and deg-
lutition.

CHAPTER XXIV.

THE SYMPATHETIC NERVE SYSTEM.

• The sympathetic nerve system consists of a number of ganglia
united one to another by intervening cords of nerve-fibers. These
ganglia may for convenience of description be divided into three
groups: viz., the vertebral or lateral, the pre- vertebral or collateral,
and the peripheral or terminal.

The vertebral ganglia are arranged in the form of chains, one on
each side of the vertebral column. The number of ganglia in the
chain varies in animals of different and in animals of the same species.
In man the number varies from 20 to 22. Each chain may be divided
into a cervical, a thoracic, a lumbar, a sacral, and a coccygeal portion.
The cervical portion is usually described as consisting of three ganglia
— a superior, a middle, and an inferior. This statement is open to
question, however, as the middle one is frequently absent and the
inferior one is regarded by some anatomists as belonging to the pre-
vertebral series. The thoracic portion consists of ten or eleven ganglia,
the lumbar and sacral portions of four each and the coccygeal portion
of one, the so-called ganglion impar.

The pre-vertebral ganglia are also united in the form of a chain
situated in the abdominal cavity. The ganglia constituting this
chain are known as the semilunar, the renal, the superior and inferior
mesenteric, and hypogastric.

The peripheral ganglia are in more or less close relation with the
tissues and organs in different parts of the body. As members of
this group may be mentioned the ciliary or ophthalmic, the spheno-
palatine, the otic, the submaxillary and the sublingual ganglia; the
ganglia in walls of the heart, the respiratory organs, the intestines,
the bladder, etc.

The general arrangement of the sympathetic ganglia, their inter-
connecting cords and branches, is shown in Figs. 285 and 286.

Structure of the Ganglia. — Each ganglion consists of a capsule
or stroma of connective tissue in which are contained large numbers
of nerve-cells, nerve-fibers, medullated and non-medullated, and
blood-vessels. The nerve-cells give origin to two or more dendrites,
which, perforating a nucleated capsule by which each cell is sur-
rounded, branch and rebranch and interlace to form a pcricapsular
plexus. Each cell gives origin also to an axon, which as it leaves
the cell becomes invested with a sheath continuous with the capsule
surrounding the cell-body. Jt is, however, wanting in a medullary
sheath, and hence the nerve presents a gray color. Such a structure,
in its entirety, is known as a sympathetic neuron.

616

THE SYMPATHETIC NERVE SYSTEM.

617

Fig. 2S5. — Cervical and Thoracic Portiox of the Sympathetic, i, i, i. Right
pneumogastric. 2. Glossopharyngeal. 3. Spinal accessory. 4. Divided trunk, of the
sublingual. 5, 5, 5. Chain of ganglia of the sympathetic. 6. Superior cervical ganglion.
7. Branches from this ganglion to the carotid. S. Nerve of Jacobson. 9. Two filaments
from the facial, one to the spheno-palatine and the other to the otic ganglion. 10. M< it< •;
oculi externus. 11. Ophthalmic ganglion, receiving a motor filament from the motor
oculi communis and a sensory filament from the nasal branch of the fifth. 12. Spheno-
palatine ganglion. 13. Otic ganglion. 14. Lingual branch of the fifth nerve. 15. Sub-

6i8 TEXT-BOOK OF PHYSIOLOGY.

Structure of the Interconnecting Cords. — The interconnecting
cords are composed of non-medullated and medullated nerve-fibers.
The former are the axons of cells found in the ganglia more centrallv
located; the latter, as will be stated later, are derived from the spinal
nerves, from the fibers of which, however, they differ in character,
being much smaller and finer. The fibers of the interconnecting
cords, as a rule, transmit nerve impulses from the more centrally to
the more peripherally located ganglia, and are therefore termed rami
ejferentes. In the vertebral chain some of the cords transmit nerve
impulses upward, others downward, others again forward, to the pre-
vertebral and peripheral ganglia.

Among the rami efferentes, interconnecting cords, there are some
which possess special interest for the physiologist, viz.:
r. The cervical, which connects the thoracic ganglia with the superior
cervical ganglion. It is composed mainly of medullated nerve-
fibers which are derived originally from the spinal nerves.

2. The' great splanchnic nerve, formed by the union of branches from

the fifth to the tenth thoracic ganglia. It connects these ganglia
with the semilunar ganglion.

3. The small splanchnic nerve, formed by the union of branches from

the ninth and tenth thoracic ganglia. It connects these ganglia

with the solar and renal plexuses.
Distribution of the Sympathetic Fibers. — It has been demon-
strated by histologic and physiologic methods of investigation that
the sympathetic non-medullated fibers which have their origin in
the cells of the sympathetic ganglia, vertebral, pre-vertebral, and
peripheral, are distributed ultimately and directly to but two struc-
tures: viz., non-striated muscle and secretor epithelium. Moreover,
there is no evidence to warrant the assumption that these structures
ever receive nerve impulses directly from the spinal or cranial nerves.
All nerve impulses which influence their activities, either in the way
of augmentation or inhibition, emanate directly though not originally
from the sympathetic ganglion cells. Since non-striated muscles
are found in the walls of blood-vessels, in the walls of hollow viscera,

maxillary ganglion. 16, 17. Superior laryngeal nerve. 18. External laryngeal nerve.
19, 20. Recurrent laryngeal nerve. 21, 22, 23. Anterior branches of the upper four
cervical nerves, sending filaments to the superior cervical sympathetic ganglion. 24. An-
terior branches of the fifth and sixth cervical nerves, sending filaments to the middle
cervical ganglion. 25, 26. Anterior branches of the seventh and eighth cervical and the
first dorsal nerves, sending filaments to the inferior cervical ganglion. 27. Middle cervical
ganglion. 28. Cord connecting the two ganglia. 29. Inferior cervical ganglion. 30,31.
Filaments connecting this with the middle ganglion. 32. Superior cardiac nerve, t,^.
Middle cardiac nerve. 34. Inferior cardiac nerve. 35, 35. Cardiac plexus. 36.
fjanglioii of the cardiac plexus. 37. Nerve following the right coronary artery. 38,38.
Intercostal nerves, with their two filaments of communication with the thoracic ganglia.
39,40,41. Great splanchnic nerve. 42. Lesser splanchnic nerve. 43,43. Solar plexus.
II- I.'-l'i pneumogastric. 45. Right pneumogastric. 46. Lower end of the phrenic
nerve. 17. Si ■< tion of the right bronchus. 48. Arch of the aorta. 49. Right auricle.
50. Right ventricle. 51,52. Pulmonary artery. 53. Right half of the stomach. 54. Sec-
tion of the diaphragm. (Sappey.)

THE SYMPATHETIC NERVE SYSTEM.

619

and around hair-follicles, and since secretor epithelium is found in
all glands, there is every reason to believe that the ganglia in some

Fig. 286. — Lumbar and Sacral Portions of the Sympathetic, i. Section of the
diaphragm. 2. Lower end of the esophagus. 3. Left half of the stomach. 4. Small
intestine. 5. Sigmoid flexure of the colon. 6. Rectum. 7. Bladder. S. Prostate. 9.
Lower end of the left pneumogastric. 10. Lower end of the right pneumogastric. n.
Solar plexus. 12. Lower end of the great splanchnic nerve. 13. Lower end of the lesser
splanchnic nerve. 14, 14. Last two thoracic ganglia. 15, 15. The four lumbar ganglia.
16, 16, 17, 17. Branches from the lumbar ganglia. iS. Superior mesenteric plexus. 19,
21,22,23. Aortic lumbar plexus. 20. Inferior mesenteric plexus. 24,24. Sacral portion
of the sympathetic. 25, 25, 26, 26, 27, 27. Hypogastric plexus. 28, 29, 30. Tenth,
eleventh, and twelfth dorsal nerves. 31, 32, ^^, 34, 35, 36, 37, ^S, 39. Lumbar and sacral
nerves. — (Sa ppey.)

62o TEXT-BOOK OF PHYSIOLOGY.

way are associated with vaso-augmentor and vaso-inhibitor, viscero-
motor and viscero-inhibitor, pilo-motor and secretor phenomena.

The Anatomic Relations of the Sympathetic and Cerebro-
spinal Systems. — The sympathetic ganglia are connected with the
spinal nerves by two branches, one white, the other gray in color,
and known respectively as the white and gray rami communicantes.
These two rami differ somewhat in their topographic distribution.
The white rami are found passing only from those spinal nerves
included between the first thoracic and second or third lumbar and
their corresponding ganglia. The gray rami, on the contrary, are
found passing from the ganglia to each of the spinal nerves. In the
cervical region, where the ganglia do not correspond in number with
the cervical nerves, each ganglion gives off two or more gray rami.
In man the superior cervical ganglion sends gray rami . to the first
four cervical nerves; the middle and inferior ganglia apparently send
gray rami to the fifth and sixth, the seventh and eighth nerves re-
spectively.

The white rami are composed of fine medullated nerve-fibers
which arise from nerve-cells situated in the lateral portion of the gray
matter in the thoracic and lumbar regions of the spinal cord. From
this origin they pass forward into the ventral roots of the spinal nerves,
in which they are contained until the spinal nerve formed by the union
of the ventral and dorsal roots divides into its anterior and posterior
divisions. At this point the fine medullated nerve-fibers leave the
common trunk and pass forward into the corresponding vertebral
ganglion, around the cell-bodies of which some of the fibers at once
arborize. Other fibers, however, pass through this ganglion and
ascend or descend the cord for a variable distance, and arborize around
the cells of more or less distant ganglia; others again pass forward
into the pre-vertebral and even the peripheral ganglia before they
finally terminate. The nerve-cells in the spinal cord are thus brought
into relation with the ganglia of all three chains, though for each cell
there is but one ganglion terminal, one cell station, between the spinal
cord and the tissues. Though innervated by the spinal cord, these
structures receive their nerve impulses, as previously stated, not
directly but indirectly through the ganglion cells. The medullated
nerve-fibers coming from the spinal cord are known as pre- ganglionic
-fibers; the non-medullated fibers, passing from the ganglia, as post-
ganglionic fibers.

The gray rami are composed of non-mcdullated nerve-fibers,
axons of the cells in the vertebral or lateral ganglia. After their
emergence from the ganglia they take a backward direction and enter
the spinal nerve-trunks, in company with which they pass to the
periphery, to be finally distributed to structures in the skin: viz., non-
striated muscles of blood-vessels, non-striated muscles of the hair-
follicles and epithelium of glands. They may therefore be regarded
as having vaso-motor, pilo-motor, and secretor functions.

THE SYMPATHETIC NERVE SYSTEM. 621

Afferent Sympathetic Fibers.— With the foregoing groups of
efferent fibers, the sympathetic nerves, in the thoracic and lumbar
regions more especially, contain a number of afferent fibers which
when stimulated give rise to sensations of pain or to reflex phenomena.
The routes by which these afferent fibers reach the spinal cord lead,
on the one hand, into and through the gray rami to the ganglia on
the posterior roots, where they have their cells of origin; and, on the
other hand, into and through the white rami. The number of afferent
fibers in any trunk in comparison with the efferent is quite small.

FUNCTIONS OF THE SYMPATHETIC SYSTEM.

The view according to which the sympathetic system is to be
regarded as an independent apparatus endowed with functions of its
own and in nowise directly dependent for its activities on the spinal
cord, is at the present time discarded. Peripheral structures cease
to exhibit their characteristic functions after division of the spinal
nerves in connection with their related ganglia. This does not exclude
the possibility of the sympathetic cell-body, in virtue of the inter-
changes between it and the blood and lymph by which it is surrounded,
maintaining its own nutrition and exerting a favorable influence over
the nutrition of the peripheral tissues to which its efferent branches
are distributed.

The nerve-tissue in its entirety may be regarded as a single system
which may be functionally divided into a nerve system of animal and
a nerve system of vegetative life, according as the nerve energies
originating in and emanating from the central nervous system are
transmitted directly to the skeletal muscles or indirectly, through
the intervention of a sympathetic neuron, to visceral muscles and
glands. In the former system but one neuron, the spino-periph-
eric, connects the spinal cord proper with the muscle; in the latter
system there are two, the spino-ganglionic and the ganglio-peripheric.

From the distribution of the post-ganglionic fibers it may be in-
ferred that the activities of the vascular and visceral muscles, either
in the way of augmentation or inhibition, the activities of the muscles
of the hair-follicles, and of the epithelium of glands, are called forth
by the ganglia in consequence of the arrival of nerve impulses coming
from the spinal cord through the pre-ganglionic fibers. Experimental
observations show this to be true. The extent to which these different
modes of activity manifest themselves in one or more regions of the
body will depend to some extent on the portion of the sympathetic
system subjected to experimental procedures.

The Functions of the Cervical Portion. — If the sympathetic
cord central to the superior cervical ganglion be stimulated with the
induced electric current, among the resulting phenomena there will be
observed dilatation of the pupil, retraction of the nictitating mem-
brane in animals possessing it, contraction of the blood-vessels of

622 TEXT-BOOK OF PHYSIOLOGY.

the skin and mucous membrane in different parts of the head and
face, contraction of the blood-vessels of the salivary glands, increase
of secretion from the submaxillary gland, the perspiratory and mucous
glands, erection of hairs in different localities of the head and neck,
and in the dog dilatation of the blood-vessels of the lips, gums, and
hard palate. If the cervical cord be divided, opposite effects will be
observed: viz., contraction of the pupil, dilatation and passive conges-
tion of the blood-vessels, a rise in temperature, and a loss of the power
of erecting hairs. Stimulation of the peripheral end causes a dis-
appearance of the latter and a reappearance of the former phenomena.
These facts indicate that the cervical portion is efferent in function.
The fibers composing it are medullated nerve-fibers derived from the
thoracic or dorsal nerves from the first to the fourth. From the
several sources the fibers pass via the white rami into the vertebral
chain, and thence without interruption to the superior cervical ganglion,
in and around the cells of which their end-tufts arborize in their char-
acteristic manner.

That the superior cervical ganglion is the cell station between the
spinal cord and the peripheral organs is shown by the fact discovered
and applied by Langley that the intravenous injection of nicotin or
the local application of it to the ganglion itself, impairs the conductivity
of the terminals of pre-ganglionic fibers, after which their stimulation
has no effect on the ganglion cells, though the latter retain their
activity, as shown on direct stimulation. Of the nerve-centers in the
spinal cord which through pre-ganglionic fibers influence peripheral
structures, some appear to be in a state of constant activity: e. g.,
the vaso-constrictor centers and the pupillo-dilatator centers. In how
far this action is automatic or autochthonic, or reflex, is uncertain.

The Functions of the Thoracic Portion. — The phenomena
which follow stimulation of this portion of the sympathetic system
resemble in a general way those observed in the head when the cervical
portion is stimulated. The situation of the resulting phenomena
will vary in accordance with the part the subject of the experiment.
For an understanding of the results of experiment the origin and
distribution of the following nerve-branches must be kept in view:

(a) The cardiac nerves which take their origin in part in cells in the first

thoracic or stellate ganglion, and in part in cells in the inferior
cervical ganglion. From this origin they pass do wn ward and for-
ward and reach the heart by way of the cardiac plexus. Stimu-
lation of these nerves gives rise to an increased frequency and an
augmentation in the force of the heart-beat. The pre-ganglionic
libers by which these cells are excited to activity emerge from the
cord by the first and second thoracic nerves.

(b) The splanchnic nerves the roots of which emerge from the fourth
to the tenth or eleventh thoracic ganglia. The fibers composing
these nerves arc for the most pari pre-ganglionic and derived
from the corresponding spinal nerves. The cell stations of the

THE SYMPATHETIC NERVE SYSTEM. 623

splanchnic fibers are in the semilunar, superior mesenteric, and
renal ganglia. From these ganglia non-medullated post-gan-
glionic fibers pass peripherally to the walls of the intestines, the
blood-vessels of the intestines, liver, kidneys, spleen, etc. Stim-
ulation of the great splanchnic produces inhibition of the in-
testinal movements, a marked primary contraction of the intes-
tinal movements, a marked primary contraction of the intestinal
blood-vessels and other viscera, followed by dilatation, coincidently
with which there is a primary rise succeeded by a fall of blood-
pressure throughout the body. Division of the nerve is followed
by dilatation of the intestinal vessels and a fall of blood-pressure.
Stimulation of the central end of the divided nerve excites the
activity of the general vaso-motor center, as shown by the rise of
the general blood-pressure. Stimulation of the smaller splanchnics
gives rise to a slight primary contraction of the blood-vessels, soon
followed by a marked dilatation. These facts indicate that the
splanchnic nerves contain visceral nerves which inhibit intestinal
movements, vaso-motor fibers both augmentor and inhibitor.
The presence of secretor nerves for the intestinal glands is dis-
puted.
(c) The cutaneous nerves for the trunk leave the lateral ganglia by
the gray rami, enter the thoracic spinal nerves, and pass in com-
pany with them to their terminations, to be ultimately distrib-
uted to the walls of the blood-vessels, the arrectores pilorum
muscles, and the sweat-glands. The pre-ganglionic fibers come
from the spinal nerves by the white rami. Their functions are
vaso-motor, pilo-motor, and secretor. The cutaneous nerves
for the fore-limbs have their origin from cells in the stellate gan-
glion (first dorsal). After a short upward course they enter
the trunks of the nerves composing the brachial plexus. The
pre-ganglionic fibers come from the white rami of the fourth to the
ninth thoracic nerves. After entering the lateral chain they take
an upward direction and arborize around the cells of the stellate
ganglion. The cutaneous nerves for the liind-limbs are derived
from the lower lumbar and the upper sacral ganglia. They also
enter the spinal nerves by the gray rami and pass to the blood-
vessels and glands of the skin. The pre-ganglionic fibers come
from the twelfth thoracic to the third lumbar nerves. In both
the brachial and sciatic nerves vaso-motor fibers (constrictors
and dilatators) and secretor nerves are present, as shown by
experimental methods (see page 374).
The Functions of the Lumbo-sacral Portion. — From the
ganglia of the lumbar and sacral regions gray rami enter the lumbar
and sacral nerves and accompany them to their distribution. In
the lumbar region the vertebral chain contains a number of. pre-
ganglionic fibers which have descended from the thoracic region as
well as fibers which have come into the chain bv the white rami from

624 TEXT-BOOK OF PHYSIOLOGY.

the lumbar nerves themselves. Many of these fibers pass to the
inferior mesenteric ganglion, in which they find their cell station.
Fibers from the sacral cord pass into the hypogastric plexus. The
course and distribution of the individual nerves is complicated and
involved. In a general way it may be said that these two regions of
the lateral chain send viscero-motor and viscero-inhibitor, vaso-con-
strictor and dilator nerves to the pelvic viscera and to the external
organs of generation. Their function therefore is to regulate the
activities of the viscera as well as the blood-supply in accordance
with functional needs.

The Functions of the Cephalic Ganglia. — The ganglia situated
in the head are usually described in connection with and as con-
stituent parts of the cranial nerve system. They, however,- bear the
same relation to the cranial nerves that the ganglia of the trunk bear
to the spinal nerves. They consist of ganglion cells from which post-
ganglionic fibers pass to glands, blood-vessels, and non-striated
muscles, and to which pre-ganglionic fibers pass from the cranial
nerves. Motor and sensor nerves pass through one or more ganglia,
though they have no anatomic connection with them. In their struc-
ture, distribution, and functions they closely resemble the collateral
ganglia of the abdominal sympathetic:

i. The ciliary or ophthalmic ganglion is situated in the orbital cavity
posterior to the eyeball. It is small in size, gray in color, and
consists of a connective-tissue stroma containing nerve-cells.
From these cells post-ganglionic fibers emerge which, after a
short course forward, penetrate the eyeball and terminate in the
circular fibers of the iris and the ciliary muscle. Pre-ganglionic
fibers of small size, and similar in their anatomic features to
the fibers of the white rami of the spinal nerves, leave the motor
oculi by a short root from the inferior division and arborize
around the ganglionic cells. Stimulation of the pre-ganglionic
fibers gives rise to contraction of the circular fibers of the iris,
with a diminution in the size of the pupil, and contraction of the
ciliary muscle with accommodation of the eye for near vision.
Division of these fibers is followed by the opposite results.
Post-ganglionic fibers from the superior cervical ganglion which
come through the cavernous plexus pass through the ciliary
ganglion to the blood-vessels of the iris and retina which are
vaso-constrictor in function. Sensor fibers from the peripheral
division of the fifth nerve pass to the cornea and endow it with
sensibility.
2. The spheno- palatine ganglion is situated in the spheno-maxillary
fossa. Its nerve-cells send non-medullated post-ganglionic fibers
to the blood-vessels and glands of the mucous membrane of the
nasal and oral regions. Stimulation of the ganglion gives rise
to dilatation of the blood-vessels and increase of secretion in this
• ntirc region. The pre-ganglionic fibers are derived from the

THE SYMPATHETIC NERVE SYSTEM. 625

seventh or facial nerve by way of the great petrosal. Sensor
fibers from the superior maxillary division of the fifth nerve pass
through the ganglion to the same regions.

3. The otic ganglion is situated just below the foramen ovale and

internal to the third division of the fifth nerve. The postgang-
lionic fibers pass to the parotid gland by way of the auriculo-
temporal division of the fifth nerve, and to the blood-vessels of the
mucous membrane of the lower lip, cheek, and gums. The pre-
ganglionic fibers are derived from the efferent fibers in the glosso-
pharyngeal or ninth nerve, by way of Jacobson's nerve and the
small petrosal. Stimulation of these nerves in any part of their
course gives rise to vascular dilatation and increase of secretion in
the region of their distribution. Motor fibers from the small
or motor root of the fifth nerve pass through this ganglion to
the tensor tympani muscle.

4. The submaxillary and sublingual ganglia are situated close to the

corresponding glands. Their post-ganglionic fibers pass to the
blood-vessels and gland-cells. The pre-ganglionic fibers are
derived from the seventh or facial nerve through the chorda
tympani branch. Stimulation of the chorda or of the ganglia
themselves gives rise to marked dilatation of the blood-vessels
and an increased flow of saliva. It therefore contains vaso-
dilatator and secretor fibers for these glands. Yaso-constrictor
and a few secretor nerves, it will be recalled, come to these glands
from the superior cervical ganglion.
Peripheral Ganglia. — Among the peripheral ganglia may be
mentioned those in the heart and those in the intestinal walls. The
pre-ganglionic fibers are contained in the trunk of the vagus nerve.
Stimulation of the peripheral end of the divided vagus gives rise to
inhibition of the heart, contraction of the walls of the stomach and
intestines, secretion from the gastric and perhaps the pancreatic gland.

40

CHAPTER XXV.
PHONATION; ARTICULATE SPEECH.

Phonation, the emission of vocal sounds, is accomplished by the
vibration of two elastic membranes which cross the lumen of the
larynx antero-posteriorly and which are thrown into vibration by a
blast of air from the lungs.

Articulate speech is a modification of the vocal sounds or the voice
produced by the teeth and the muscles of the lips and tongue and is
employed for the expression of ideas.

The larynx, the organ of the voice, is situated in the forepart of
the neck, occupying the space between the hyoid bone and the upper
extremity of the trachea. In this situation it communicates with the
cavity of the pharynx above and the cavity of the trachea below.
From its anatomic relations and its internal structure — the interpola-
tion of the elastic membranes — the larynx subserves the two widely
different yet related functions, respiration and phonation.

THE ANATOMY OF THE LARYNX.

The larynx consists primarily of a series of cartilages united one
with another in such a manner as to form a more or less rigid frame-
work, yet possessing at its different joints, a certain amount of motion;
and secondarily, of muscles and nerves which conjointly impart to
the cartilages the degree of movement necessary to the performance
of the laryngeal functions. It is covered externally by fibrous tissue
and lined throughout by mucous membrane continuous with that
lining, the pharynx and trachea.

The larynx presents a superior or pharyngeal and an inferior or
tracheal opening. The pharyngeal opening is triangular in shape,
the base being directed forward, the apex backward. The plane of
this opening in the living subject is almost vertical. The tracheal
opening is circular in shape and corresponds in size with the upper
ring of the trachea. Viewed from above, the general cavity of the
larynx is seen to be partially subdivided by two membranous bands —
the vocal bands or cords — which run from before backward in a hori-
zontal plane. The space between the bands, the glottis, varies in
size and shape from moment to moment in accordance with respira-
tory and phonatory necessities. The average width of the glottis, at
its widest part, during quiet respiration is about 13.5 mm. in men
and n. 5 mm. in women. With the advent of phonation the vocal
membranes are at once approximated, and to such an extent that the
glottic opening is reduced to a mere slit. It is then spoken of as the
rima glottidis, or chink of the glottis.

626

PHONATION; ARTICULATE SPEECH.

627

The space above the vocal bands, the supra-glottic or supra-rimal
space, is triangular in shape and extends from the pharyngeal opening
to the plane of the vocal bands. The mucous membrane lining the
walls of this space, presents on either side, just above the vocal bands,
a crescentic fold which runs from be-
fore backward, and is known as the
false vocal band or cord. Between
the true and false bands there is a
cavity or space prolonged upward and
outward for some distance, forming
what is known as the ventricle of the
larynx. The space below the vocal
bands, the infra-glottic or infra-rimal
space, is narrow above and elongated
from before backward, but wide and
circular below, corresponding to the
lumen of the trachea. (Fig. 287.)

The Laryngeal Cartilages, Artic-
ulations, and Ligaments. — The
cartilages which compose the frame-
work of the larynx are nine in num-
ber, three of which are single: viz.,
the cricoid, the thyroid, and the epi-
glottis, while six occur in pairs: viz.,
the arytenoids, the cornicula laryngis,
and the cuneiform. (Figs. 288 and
289.)

The cricoid cartilage is the founda-
tion cartilage, and affords support to
the remaining cartilages and the struc-
tures attached to them. In shape it
resembles a signet-ring, the broad
quadrate portion of which is directed
backward, while the narrow circular
portion is directed forward. It rests
upon the upper ring of the trachea, to
which it is firmly attached by fibrous
tissue. The posterior upper border of
the quadrate portion presents on either
side an oval convex facet for articula-
tion with the arytenoid cartilage. The
long axis of this facet is directed down-
ward, outward, and forward.

The thyroid, the largest of the laryngeal cartilages, is composed
of two flat quadrilateral plates, united anteriorly, at an angle of about
90 degrees. Each plate is directed backward and outward and ter-
minates in a free border, which is prolonged upward and downward

Fig. 2S7. — Longitudinal Section
of the Human Larynx, Showing
the Vocal Bands, i. Ventricle of
the larynx. 2. Superior vocal cord
3. Inferior vocal cord. 4. Arytenoid
cartilage. 5. Section of the arytenoid
muscle. 6, 6. Inferior portion of the
cavity of the larynx. 7. Section of
the posterior portion of the cricoid
cartilage. S. Section of the anterior
portion of the cricoid cartilage. 9.
Superior border of the cricoid car-
tilage. 10. Section of the thyroid
cartilage. 11, 11. Superior portion
of the cavity of the larynx. 12, 13.
Arytenoid gland. 14, 16. Epiglottis.
15, 17. Adipose tissue. iS. Section
of the hyoid bone. 19, 19, 20.
Trachea. — {Sap fey.)

628

TEXT-BOOK OF PHYSIOLOGY.

for some distance, terminating in two processes, the superior and
inferior cornua. The upper border of the thyroid is deeply notched
in front. The inferior border overlaps laterally the cricoid.

The epiglottis is a leaf-shaped piece of cartilage attached to the
thyroid at the median notch. It is firmly united by membranes and
ligaments to the thyroid and arytenoid cartilages and to the base of
the tongue.

The arytenoid cartilages are two in number and symmetric in

Fig. ,288. — Laryngeal Cartilages
and Ligaments, Anterior Surface.
1. Hyoid bone. 2, 2, 3, 3. Greater and
lesser cornua. 4. Thyroid cartilage. 5.
Thyro-hyoid membrane. 6. Thyro-hyoid
ligaments. 7. Cartilaginous nodule. 8.
Cricoid cartilage. 9. The crico-thyroid
membrane. 10. The crico-thyroid liga-
ments. 11. Trachea. — (Sappey.)

Fig. 289. — Laryngeal Cartilages
and Ligaments, Posterior Surface.
1, 1. Thyroid cartilage. 2. Cricoid car-
tilage. 3, 3. Arytenoid cartilages. 4, 4.
Crico-arytenoid articulations. 5, 5. Crico-
thyroid articulations. 6. Union of the
cricoid cartilage and of the trachea. 7.
Epiglottis. 8. Ligament uniting it to
the reentering angle of the thyroid car-
tilage.— {Sappey.)

shape. Each cartilage is a triangular pyramid, the apex of which is
rei urved, and directed backward and inward. The base presents three
angles — an anterior, an external, and an internal. The anterior angle
is long and pointed and projects forward in a horizontal plane. It
serves for the attachment of the vocal membranes and is therefore
termed the vocal process. The external angle is short, rounded, and
prominent, and serves for the attachment of muscles. The internal
angle affords a poinl of insertion for a ligament. The inferior surface
of the arytenoid is concave for articulation with the convex surface
of the cricoid facet. Its long axis, however, is directed from before

PHONATION; ARTICULATE SPEECH. 629

backward and almost at right angles to the long axis of the cricoid
facet.

The cornicida laryngis and the cuneiform cartilages are small
nodules of yellow elastic cartilage embedded in a fold of membrane
which unites the arytenoid and the epiglottis. They are fragments
of a ring of cartilage which in some animals — e. g., anteater — extends
between these two cartilages.

The crico-thyroid articulation is formed by the opposition of the
tip of the inferior cornu of the thyroid cartilage and an articular facet
on the side of the cricoid. The joint is provided with a synovial
membrane and enclosed by a capsular ligament. The movements
permitted at this joint take place around a horizontal axis and consist
of an upward and downward movement of both the thyroid and
cricoid, combined with a sliding movement of the latter upward and
backward.

The crico-arytenoid articulation is formed by the apposition of
the articulating surfaces of the cricoid and arytenoid cartilages. This
joint is provided with a synovial membrane and enclosed by a loose
capsular ligament which would permit of an extensive sliding of the
arytenoid cartilage downward and outward were it not prevented
by the posterior crico-arytenoid ligament, which is attached, on the
one hand, to the cricoid, and, on the other, to the inner angle of the
arytenoid. The movements permitted at this joint are : (1) Rotation
of the arytenoid around a vertical axis which lies close to its inner
surface. (2) A sliding motion inward and forward with inward
rotation of the vocal process, or a sliding motion outward and back-
ward with outward rotation of the vocal process. In either case the
process describes an arc of a circle. (3) A sliding movement towards
the median line in consequence of which the inner surfaces of the
arytenoids are brought almost in contact.

The crico-thyroid membrane is composed mainly of elastic tissue.
It may be divided into a mesial and two lateral portions. The mesial
portion is well developed, triangular in shape, and unites the con-
tiguous borders of the cricoid and thyroid cartilages. The lateral
portion is attached below to the superior border of the cricoid. From
this attachment it passes upward and inward under cover of the
thyroid. As it ascends it elongates and becomes thinner, and is
finally attached anteriorly to the thyroid near the median line, and
posteriorly to the vocal process of the arytenoid, thus constituting
the inferior thyro-arytenoid ligament. It is covered internally by
mucous membrane and externally by the internal thyro-arytenoid
muscle. The free edge of this ligament forms the basis of the true
vocal band. A superior thyro-arytenoid ligament forms the basis
of the false vocal band.

The thyro-hyoid membrane, composed of elastic tissue, unites the
superior border of the thyroid to the hyoid bone.

The mucous membrane lining the larynx is thin and pale. As

6.^o

TEXT-BOOK OF PHYSIOLOGY.

it passes downward it is reflected over the superior thyro-arytenoid
ligament, and assists in the formation of the false vocal band; it then
passes into and lines the ventricle, after which it is reflected inward
over the superior border of the thyro-arytenoid muscle and ligament,
and assists in the formation of the true vocal band; it then returns
upon itself and passes downward over the lateral portion of the crico-
thyroid membrane into the trachea.

'■;■'.■*$ 'Mm

~MJ

Fig. 290. — Posterior View of the
Muscles of the Larynx, i. Posterior
crico-arytenoid muscle. 2, 3, 4. Differ-
ent fasciculi of the arytenoid muscle.
5. Aryteno-epiglottidean muscle. — (Sap-
pey.)

The thin, free, reduplicated
edge of the mucous membrane
constitutes the true vocal band.
The surface of the mucous mem-
brane is covered by ciliated
epithelium except in the im-
mediate neighborhood of the
vocal bands.

The vocal bands are attached anteriorly to the thyroid cartilage
near the receding angle and posteriorly to the vocal processes of the
arytenoid cartilages. They vary in length in the male from 20 to 25
mm. and in the female from 15 to 20 mm.

The Muscles of the Larynx.— The muscles which have a direct
action on the cartilages of the larynx and determine the position of the
vocal bands both for respiratory and phonatory purposes, and which

Fig. 291. — Lateral View of the
Muscles of the Larynx, i. Body of
the hyoid bone. 2. Vertical section of
the thyroid cartilage. 3. Plorizontal sec-
tion of the thyroid cartilage turned down-
ward to show the deep attachment of
the crico-thyroid muscle. 4. Facet of
articulation of the small cornu of the
thyroid cartilage with the cricoid cartilage.
5. Facet on the cricoid cartilage. 6.
Superior attachment of the crico-thyroid
muscle. 7. Posterior crico-arytenoid
muscle. 8, 10. Arytenoid muscle. 9.
Thyro-arytenoid muscle, n. Aryteno-
epiglottidean muscle. 12. Middle thyro-
hyoid ligament. 13. Lateral thyrohyoid
ligament. — (Sappey.)

PHONATION; ARTICULATE SPEECH.

631

regulate their tension as well, are nine in number and take their names
from their points of origin and insertion: viz., two posterior crico-
arytenoids, two lateral crico-arytenoids, two thyro-arytenoids, one
arytenoid, and two crico-thyroids (Figs. 290 and 291).

The posterior crico-arytenoid muscle lies on the posterior surface
of the quadrate plate of the cricoid cartilage, on either side of the
median line, from which it takes its origin. The fibers of the muscle
pass upward and outwrard and in their course converge to be inserted
into the external angle of the arytenoid cartilage. The superior and
more horizontally directed fibers rotate the arytenoid around its
vertical axis; the inferior and obliquely directed fibers draw the cartilage
downward and inward. As a result of the action of the muscle in its
entirety, the vocal process is turned upward and outwrard, and as the
vocal band is carried with it the glottis is widened, a condition nec-

ttt$£&£

Fig. 292. — Glottis Widely Opened
from Simultaneous Contraction oe
Both Crico-arytenoid Muscles, b.
Epiglottis, rs. False vocal band. ri.
True vocal band. ar. Arytenoid car-
tilages, a. Space between the arytenoids.
c. Cuneiform cartilages, ir. Interarytenoid
fold. rap. Aryepiglottic fold. cr. Car-
tilage rings. — {Mandl.)

Fig. 293. — Position of the Vocal
Bands Due to the Simultaneous
Contraction of Both Lateral Crico-
arytenoid Muscles and Both Thyro-
arytenoid Muscles, b. Epiglottis, rs.
False vocal band. ri. True vocal band.
or. Space between the arytenoid cartil-
ages, the glottis respiratoria. ar. Ary-
tenoid cartilages, c. Cuneiform carti-
lages, rap. Aryepiglottic fold. ir. In-
terarytenoid fold. — {Mandl.)

essary to the free entrance of air into the lungs (Fig 292). Since
the contraction of the crico-arytenoid has this result, it is genera 11 v
spoken of as the abductor or respiratory muscle.

The lateral crico-arytenoid muscle arises from the side of the cricoid
cartilage. From this point its fibers are directed upward and back-
ward to be inserted into the external process of the arytenoid. Its
action is to draw the arytenoid cartilage forward and inward, thus
approximating and relaxing the vocal band.

The thyro-arytenoid muscle arises from the inferior two-thirds
of the inner surface of the thyroid cartilage just external to the median
line. From this origin the fibers pass backward and outward, to be
inserted into the anterior surface and external angle of the arytenoid
cartilage. The inner portion of the muscle lies close to and supports,
if it does not constitute a part of, the vocal band. The action of the
thyro-arytenoid muscle in conjunction with the lateral crico-arvtenoid
is to rotate the arytenoid cartilage around the vertical axis and to

632 TEXT-BOOK OF PHYSIOLOGY.

draw the vocal process forward and inward, thus carrying the vocal
cord toward the median line. When the muscles of the two sides
simultaneously contract, the vocal bands are closely approximated
and the space between them, the rima vocalis, reduced to a mere slit,
one of the conditions essential to phonation (Fig. 293).

The arytenoid muscle consists (1) of transversely arranged fibers
which arise from and are inserted into the outer surface of the oppo-
site arytenoid cartilages, and (2) of obliquely directed fibers which
arise from the outer angle of one arytenoid to be inserted into the
apex of the other. In their course they decussate in the median line.
The action of this muscle is to approximate the arytenoid cartilages
and thus obliterate that portion of the glottis between the vocal proc-
esses, the rima respiratoria, and so direct the expiratory blast of air
toward and through the rima vocalis.

The collective actions of the three foregoing muscles is to close or
constrict the glottis, and for this reason they are spoken of as the
adductor or pJwnatory muscles.

The crico-thyroid muscle arises from the side and front of the
cricoid cartilage and is inserted above into the lower border of the
thyroid cartilage. The action of this muscle is to draw up the an-
terior part of the cricoid cartilage toward the thyroid, which remains
stationary, and to swing the quadrate plate of the cricoid and the
arytenoid cartilages downward and backward. This movement
has the result of tensing the vocal bands. The cricoid is at the same
time drawn backward by the action of the more longitudinally dis-
posed fibers.

Nerves of the Larynx. — The nerves which innervate the muscles
of the larynx and endow the mucous membrane with sensibility are
derived from the vagus trunk. The superior laryngeal is for the
most part sensor and distributed to the mucous membrane, though
it contains motor fibers for the crico-thyroid muscle. The inferior
laryngeal is purely motor and is distributed to all the muscles with
the exception of the crico-thyroid.

THE MECHANISM OF PHONATION.

Phonation, the production of vocal sounds in the larynx, is the
result of the vibration of the vocal bands caused by an expiratory
blast of air from the lungs. That a sound may arise it is essential
that the glottis be approximately closed and the vocal bands be made
more or less tense.

The closure of the glottis — the approximation of the vocal proc-
- and the vocal bands — is accomplished, it will be recalled, by
the contraction of the lateral crico-arytenoid, the arytenoid, and the
thyro-arytenoid muscles. The increase in tension is accomplished
by the contraction of the crico-thyroid and the thyro-arytenoid muscles,
the former by the backward displacement of the cricoid and arytenoid

PHOXATIOX; ARTICULATE SPEECH.

633

cartilages, the latter by converting the natural concave edge of the
vocal band to a straight line. The lengthening and tensing of the
vocal bands by the crico-thyroid muscle is regarded by some investi-
gators as a coarse means, the approximation of the free edges by the
thyro-arytenoid, as a finer means, of adjustment for the production
of slight changes in the pitch of sounds. The extent to which the
glottis is closed and the membranes tensed will depend, however,
on the pitch of the sound to be emitted. The appearance presented
bv the glottis just previous to the emission of a note of medium pitch,
as determined by laryngologic examination, is shown in Fig. 294.
When the foregoing conditions in the glottis are realized, the air stored
or collected in the lungs is forced by the contraction of the expiratory
muscles, through the narrow space between the bands. As a result
of the resistance offered by this narrow outlet and the force of the
expiratory muscles the air within the lungs and trachea is subjected
to pressure, and as soon as the pressure attains a certain level the vocal

Fig. 294. — Position of the Vocal
Bands Previous to the Emission of a
Sound, b. Epiglottis, rs. False vocal
band. ri. True vocal band. ar. Ary-
tenoid cartilages. — (Mandl.)

Fig. 295. — Position of the Vocal
Bands in the Production of Notes
of Low Pitch. /. Epiglottis, or. Glottis.
ns. False vocal cord. ni. True vocal cord.
ar. Arytenoid cartilages. — (Mandl.)

bands are thrown into vibrations, which in turn impart to the column
of air in the upper air-passages a corresponding series of vibrations by
which the laryngeal vibrations are reinforced. The degree of pressure
to which the air in the lungs and trachea is subjected was determined
by Latour to vary from 160 mm. of water for sounds of moderate,
to 940 mm. of water for sounds of highest intensity. With the escape
of the air or the separation of the vocal bands the vibration ceases
and the sound dies away.

The Characteristics of Vocal Sounds. — In common with the
sounds produced by all other music instruments, all vocal sounds
are characterized by intensity, pitch and quality, tone or color.

The intensity or loudness of a sound depends on the extent or am-
plitude of the up-and-down vibration or the extent of the excursion
of the vocal band on either side of the position of equilibrium or rest ;
and this in turn depends on the force with which the blast of air strikes
the band. The more forceful the blast of air, the larger, other things
being equal, will be the primary vibrations of the bands, and hence
the secondary vibrations of the air in the upper air-passages.

634

TEXT-BOOK OF PHYSIOLOGY.

fejL

The pitch of the voice depends on the number of vibrations in a
unit of time, a second. This will be conditioned by the length of the
bands in vibration or the length and width of the aperture through
which the air passes and the degree of tension to which the bands are
subjected. In the emission of sounds of highest pitch the tension of
the vocal bands and the narrowing of the glottis attain their maxi-
mum. In the emission of sounds of lowest pitch the reverse conditions
obtain. In passing from the lowest to the highest pitched sounds in
the range of the voice peculiar to any one individual, there is a pro-
gressive increase in both the tension of the vocal bands and the narrow-
ing of the glottic aperture. In the production of low-pitched notes

of men, those due to vibrations lying be-
tween 80 and 240 per second, the tension
is regulated by the crico-thyroid muscle;
the aperture of the glottis during this time
being elliptic in shape and relatively wide
(Fig. 295). In the production of notes due
to vibrations lying between 240 and 512
vibrations per second, the anterior fibers of
the cricothyroid muscle relax and the thyro-
arytenoid muscle comes into play; by its
action the vocal bands are more closely
approximated and the vocal aperture re-
duced to a linear slit. In the high-pitched
notes emitted by soprano singers the vocal
bands are so closely applied to each other
that only a very small portion in front,
bounding a small oval aperture, is capable
of vibrating (Fig. 296). The difference in
the pitch of the voice in men and women is
due largely to the greater size and develop-
ment of the vocal bands in the former than in the latter.

The quality of the voice, the timbre or color, depends on the form
combined with the intensity and pitch of the vibration. As with
sounds produced by music instruments, the primary or fundamental
vibration of the vocal band is complicated by the superposition of
secondary or partial vibrations (overtones). The form of the vibration
will therefore be a resultant of the blending of a number o'f different
vibrations. The quality of the sound produced in the larynx is,
however, modified by the resonance of the mouth and nasal cavities;
1 ertain of the overtones being reinforced by changes in the shape of the
mouth cavity more especially, thus giving to the voice a somewhat
different quality.

The Varieties of Voice. — The region of the music scale, com-
prising all vibrations between 32 and 2048 per second, with which
laryngeal sounds arc in accord will vary in the two sexes and in different
individuals of the same sex. It is customary to classify voices, es-

Fig. 296. — Glottis Seen
with the Laryngoscope dur-
ing the Emission of High-
pitched Sounds, i, 2. Base
of the tongue. 3, 4. Epiglot-
tis. 5, 6. Pharynx. 7. Ary-
tenoid cartilages. 8. Opening
between the true vocal cords.

9. Aryteno-epiglottidean folds.

10. Cartilage of Santorini. 11.
Cuneiform cartilage. 12. Su-
perior vocal cords. 13. In-
ferior vocal cords. — (Le Bon.)

PHONATIOX; ARTICULATE SPEECH. 635

pecially those of singers, into bass, baritone, tenor, contralto, mezzo-
soprano, and soprano, in accordance with the regions of the music
scale with which they correspond. Thus the succession of notes
characteristic of the bass voice vary in pitch from F, fa', to C, do3,
or from 87 to 256 vibrations per second; those of the baritone from
A, la, to F', fa3, or from 106 to 341 vibrations per second; those of
the tenor from C, do2, to a', la3, or from 128 to 435 vibrations per
second; those of the contralto from e, mi2, to C" , do4, or from 160 to
512 vibrations per second; those of the mezzo-soprano from g, sol,,
to e", mi4, or from 192 to 640 vibrations per second; those of the
soprano from b, si2, to g", sol4, or from 240 to 768 vibrations per second.
The range of the voice is thus seen to embrace from one and three-
quarters to two octaves. Some few individual singers have far ex-
ceeded this range, but they are exceptional.

Speech is the expression of ideas by means of articulate sound?.
These sounds may be divided into vowel and consonant sounds.

The vowel sounds, a, e, i, 0, u, are laryngeal sounds modified by
the superposition and reinforcement of certain overtones developed
in the mouth and pharynx by changes in their shapes. The number
of vibrations underlying the production of each vowel sound is a
matter of dispute. According to Konig, the sound of a is the result
of 940 vibrations; of e, 1880 vibrations; of i, 3760 vibrations; of o,
470 vibrations; of ou, 235 vibrations.

Consonant sounds are produced by the more or less complete in-
terruption of the vowel sounds during their passage through the organs
of speech. These may be divided into:

1. Labials, p, b, m.

2. Labio-dentals, /, v. .

3. Linguo-dentals, s, z.

4. Anterior linguo-palatals, t, d, 1, n.

5. Posterior linguo-palatals, k, g, h, y, r.

The names of these different groups of consonants indicate the
region of the mouth in which they. are produced and the means by
which the air blast is interrupted.

THE NERVE MECHANISM OF THE LARYNX.

The nerve mechanism by which the musculature of the larynx
is excited to action and coordinated so as to subserve both respiration
and phonation involves the fibers contained in the superior and inferior
laryngeal nerves (both branches of the vagus) and their related nerve-
centers in the central nerve system.

For respiratory purposes it is essential that the lumen of the glottis
shall be sufficiently large to permit the entrance and exit of air without
hindrance. Laryngoscopic examination of the larynx in the human
being shows that during quiet respiration the vocal bands are widely
separated and almost stationary, moving but slightly during either

636 TEXT-BOOK OF PHYSIOLOGY.

inspiration or expiration. At this time, according to the investigations
of Semon, the area of the glottis is approximately 160 sq. mm., some-
what less than the area of either the supraglottic or infraglottic regions,
which is about 200 sq. mm. This condition of the glottis is maintained
bv the steady continuous contraction of the posterior crico-arytenoid
muscles, the abductors of the vocal bands.

For phonatory purposes it is essential that the respiratory function
be temporarily suspended and the vocal bands closely approximated.
This is accomplished by the contraction of the remaining muscles of
the larynx, with the exception of the crico-thyroid, which are collectively
known as the adductors of the vocal bands. During phonation the
adductor muscles overcome the activity of the abductors. With the
cessation of phonation the abductors immediately restore the vocal
bands to their former respiratory position.

The activities of these two antagonistic groups of muscles are under
the control of the central nerve system. The only pathway for the
excitatory nerve impulses is through the fibers of the inferior or re-
current laryngeal nerve. The relation of these nerve-fibers both
centrally and peripherally, as well as their physiologic action, has been
the subject of much experimentation. The results have not always
been in accord, owing to the choice of animal, the use of anesthetics,
strength of stimulus, etc.

As the outcome of many investigations it is believed that each
muscle group is innervated by its own bundle of nerve-fibers, both
of which are contained in the inferior laryngeal, though coming from
two separate centers in the medulla oblongata. Russell succeeded
in separating the fibers for the abductors from the fibers for the ad-
ductors in the inferior laryngeal, and in tracing them to their termi-
nations. So completely was this done that it became possible to
produce at will, through stimulation, either abduction or adduction,
without contraction of the muscle of opposite function.

The laryngeal respiratory center was located by Semon and Horsley,
in the cat, in the upper part of the floor of the fourth ventricle. Stim-
ulation of this area during etherization was followed by abduction
of the vocal bands. The efferent fibers of this center are believed by
some investigators to leave the central nerve system in the spinal
accessory nerve, by others in 'the lower roots of the vagus.

From the continuous activity of the abductor muscle, and the
stationary position of the vocal bands, it is probable that the medullary
center is in a state of continuous activity or tonus, the result probably
of reflex influences.

A cortical representation for laryngeal respiratory movements
has been determined by Semon and Horsley in different classes of
animals. In the cat especially, stimulation of the border of the
olfactory sulcus gives rise to complete abduction of the vocal bands
on both sides. The representation is therefore bilateral.

The phonatory center was located by the same investigators in

PHOXATION; ARTICULATE SPEECH. 637

the medulla near the ala cinerea and the upper border of the calamus
scriptorius. Stimulation of this area was invariably followed by
bilateral adduction of the vocal bands and closure of the glottis.

A cortical representation for phonatory movements also was
located in the lower portion of the precentral convolution, near the
anterior border. Stimulation of this area gives rise to marked ad-
duction of both vocal bands, indicating that the representation is
also bilateral.

Faradic stimulation of the inferior laryngeal nerve during slight
ether anesthetization gives rise to closure of the glottis; the same
stimulation, however, during deeper anesthetization gives rise to
opening or dilatation of the glottis, a fact indicating that either the
adductor muscles or their nerve terminals are depressed by the action
of the ether before the muscles and nerves of opposite function. The
superior laryngeal nerves contain motor fibers for the crico-thyroid
muscles. Stimulation of the nerve gives rise to contraction of the
muscle and increased tension of the vocal bands. It is believed that
these fibers are derived originally from the efferent fibers of the glosso-
pharyngeal nerve. The remaining fibers of the superior laryngeal
endow the upper portion of the larynx wTith extreme sensibility which
to a certain extent protects the air-passages against the entrance of
foreign bodies. Irritation of the terminal filaments of this nerve by
particles of food, solid or liquid, gives rise to marked reflex spasm
of the adductor muscles and closure of the glottis, followed by a strong
expiration blast of air from the lungs by which the offending particles
are removed. Division of this nerve on both sides is followed by a
paralysis of the crico-thyroid muscles, a lowering of the tension of the
vocal bands, and a loss of sensibility of the laryngeal mucous mem-
brane.

CHAPTER XXVI.
THE SPECIAL SENSES.

It is one of the functions of the nerve system to bring the individual
into conscious relation with the external world. This is accomplished
in part through the intermediation of afferent nerves, connected per-
ipherally, with highly specialized terminal organs, and centrally, with
specialized areas in the cerebral cortex.

Excitation of the terminal organs by material changes in the en-
vironment develops nerve impulses which, transmitted to the cortical
areas, evoke sensations. These sensations, differing in character
from those vague ill-defined sensations — e. g., fatigue, well-being,
discomfort, etc. — caused by material changes occurring within the
body, are termed special sensations — e. g., touch; pressure; pain;
temperature; taste; smell; light and its varying qualities, intensity,
hue, and tint; sound and its varying qualities, intensity, pitch, and
timbre.

The terminal organs which receive the impress of the external
world are the skin, tongue, nose, eye, and ear, and collectively con-
stitute the special sense-organs. The physiologic mechanisms which
underlie and develop these special sensations are known respectively
as the tactile, gustatory, olfactory, optic, and auditory. Each mechan-
ism responds to but a single form of stimulus and to no other. Thus,
the stimulus for the skin is mechanic pressure; for the tongue, soluble
organic and inorganic matter; for the nose, volatile or gaseous matter;
for the eye, ether vibrations; for the ear, atmospheric undulations.
These stimuli alone are adequate to the physiologic excitation of the
different mechanisms.

The factors involved in the production of the sensations include
(i) a special physical stimulus; (2) a specialized terminal organ; (3)
an afferent nerve pathway, and (4) a specialized receptive sensor cell
in the cerebral cortex.

Though the resulting sensations in each instance differ widely in
their characteristics, it is difficult to present a satisfactory explanation
for these differences. If it be assumed that the nerve impulses which
ascend the different nerves of special sense are alike in quality, then
it must be admitted that the character of the sensation is the expression
of a specialization and organization of the cortical area. If, on the
other hand, specialization of the cortex is denied, then there must be
admitted a specialization of the peripheral organ — with a resulting
difference in quality or rapidity of the nerve impulses which would
impress or excite the non-specialized cortex in such a way as to call
forth the characteristic sensation, it is possible, however, that neither

638

THE SENSE OF TOUCH. 639

supposition is wholly correct, and that the character of the sensation
depends on the construction and adaptation of the entire sense appara-
tus to the character of the stimulus.

Whatever the conditions for their origin and whatever their char-
acteristics, sensations in themselves do not constitute knowledge;
they are but elementary states of consciousness, raw materials out
of which the mind elaborates conceptions and forms judgments as
to the character of any given object in comparison with former ex-
periences.

THE SENSE OF TOUCH.

The physiologic mechanism involved in the sense of touch in-
cludes the skin and the mucous membrane lining the mouth, the
afferent nerves, their cortical connections, and nerve-cells in the cortex
of the parietal lobe.

Peripheral excitation of this mechanism develops nerve impulses
which, transmitted to the cortex, evoke sensations of touch and
temperature. To the skin, therefore, is ascribed a touch sense and
a temperature sense. Of the touch sensations two kinds may be dis-
tinguished: viz., pressure sensations and place sensations. With the
contact of an external body there arises the perception not only of the
pressure, but also the perception of the place or locality of the contact.
In accordance with this, it is customary to attribute to the skin a pres-
sure sense and a location sense.

The specific physiologic stimuli to the terminal organs in the skin
and oral mucous membrane are mechanic pressure and thermic
vibrations.

The Skin. — The skin, which constitutes the basis for the sense
of touch, covers and closely invests the entire body. It varies in
thickness and delicacy in different regions, though its structure is
everywhere essentially the same. As the physiologic anatomy of the
skin has elsewhere been detailed (page 486), it is only necessary to
state here that it is divided into a deep and a superficial layer. The
former, known as the derma, consists of an inner layer of rather loose
connective tissue and an outer layer of condensed connective tissue.
The latter, known as the epidermis, consists of an inner layer of pig-
ment cells and a thick outer layer of epithelial cells. The derma is
characterized by the presence of elevations (papilla?) which are every-
where extremely abundant. Throughout the derma ramify blood-
vessels and nerves.

The Peripheral or Terminal Organs. — Between the contact
surface and the afferent nerves specialized structures are found which
serve as intermediates between the stimulus, on the one hand, and the
afferent nerves, on the other hand. By virtue of their structure they
are far more irritable than the nerve-fibers and hence respond more
quickly to the physiologic stimulus than the nerve-fiber itself. To

640

TEXT-BOOK OF PHYSIOLOGY.

these specialized organs, found not only in the skin but in other sense-
organs as well, the term peripheral or terminal organ is given. It is
these structures that are primarily excited to activity by the physiologic
stimulus, and that in turn arouse the nerve to activity. Peripheral
organs are to be regarded as special modes of termination of afferent
nerves and adapted for the impress of a specific stimulus. The periph-
eral organs of afferent nerves found in the skin and oral mucous mem-
brane present a variety of forms, some of which are as follows:

1. Free Endings. — These are pointed or club-shaped processes, the

ultimate terminations of afferent nerve-fibrils, found in and
among epidermic cells.

2. Tactile Cells. — These are oval nucleated bodies found in the

deeper layers of the epidermis. They rest upon or are embraced
by a crescentic shaped body, the tactile meniscus, which in turn is

directly connected with the nerve-
fibril and probably a modification
of it (Fig. 297).
The Corpuscles of Meissner and
Wagner. — In the papillae of the
derma, especially in the palm of
the hand and in the finger-tips,
are found elliptical bodies con-
sisting of a connective-tissue cap-
sule containing a number of
tactile discs with which the
nerve-fibrils are connected. If
the afferent nerve is traced to the
capsule, it is found to lose both
its neurilemma and medulla, after
which the naked fibril penetrates the capsule, breaks up into a
number of branches, and after pursuing a more or less spiral
course becomes connected with the tactile discs (Fig. 298).
Hair Wreaths. — Just below the openings of the sebaceous gland
the hair-follicles are surrounded by naked axis-cylinder fibrils
in the form of a wreath, which in all probability terminate in
the cells of the external root-sheath. These, too, are to be re-
garded as part of the touch apparatus.

Corpuscles of Vater or Pacini. — These are oval-shaped structures
found along the nerves distributed to the palms of the hands and
the soles of the feet, on the nerves distributed to the external
genital organs, to joints and other structures. They consist of
a thick capsule of lamellated connective tissue in the interior
of which is a bulb resembling granular protoplasm. The axis-
cylinder of the nerve-fiber enters the capsule and becomes con-
nected with the bulb (Fig. 299).

Other forms of peripheral organs are found in special regions of
the skin as well as in different animals.

Fig. 297. — Tactile Cells from Snout
OF Pig. a. Tactile cell. m. Tactile disc.
n. Xerve-fiber. — (Stirling.)

5-

THE SENSE OF TOUCH.

641

Touch Sense. — The area, stimulation of which evokes sensations
of touch, is coextensive with the skin and that limited portion of the
mucous membrane lining the mouth. Careful stimulation of the
skin by means of a fine stiff bristle has revealed the fact, however, that
the touch area is not continuous, but discrete, presenting itself under
the form of small areas or spots, separated by relatively large areas
insensitive to the same agent. Stimulation of these spots always calls
forth a sensation of touch. For this reason they are known as "touch
spots." The number of such spots in any given area of skin varies
considerably. Thus, in the skin of the calf fifteen such spots have
been counted in a square centimeter. In the
palm of the hand from forty to fifty have been
counted in an area of the same extent. They
are also especially abundant in the immediate
neighborhood of the hair-follicles.

The peripheral end-organ associated with
the touch spots in the neighborhood of a hair-
follicle is in all probability the wreath of nerve-
fibrils surrounding the follicle. In regions
devoid of hairs the end-organ is the Meissner
corpuscle, for in the palmar surface of the last
phalanx of the index-finger, where the touch
sense is quite acute, about 20 corpuscles are
present in each square millimeter of surface, f
The specific stimulus necessary to evoke the
sensation of touch is a deformation of the skin;
and the greater this is within physiologic limits,
the more pronounced is the sensation.

Pressure Sense. — The contact of an ex-
ternal body is attended by a certain amount
of pressure, which, however, must attain a cer-
tain degree before the sensation can be evoked.
This is known as the threshold value, or the
degree of liminal intensity. Since the sensa-
tions are the result of pressure, they are termed
pressure sensations, and their intensity may be
expressed in terms of pressure.

The sensitivity of the skin as determined
by the pressure sense varies in different re-
gions of the body and in accordance with the

size of the area pressed. Thus, the liminal intensity of a stimulus
for an area of nine square millimeters for the skin of the forehead is
0.002 gram; for the flexor aspect of the forearm, 0.003 gram; and for
the hips, thigh, and abdomen, 0.005 gram5 f°r the palmar surface of
the finger, 0.019 gram; for the heel, 1 gram. The delicacy of the sense
of touch is measured by the slight increase or decrease in the intensity
of the stimulus that will produce an appreciable change in the intensity
41

Fig. 298. — Touch-cor-
puscle of Meissxer axd
Wagner, b. Papilla of
cutis. d . Nerve-fiber of
touch-corpuscle, e, /.
Nerve-fiber in touch-cor-
puscles, g. Cells of Mal-
pighian layer. — (From
Stirling.)

642

TEXT-BOOK OF PHYSIOLOGY.

of the sensation. Not all changes in the stimulus, however, are
attended by a change in the sensation. It has been determined that
the latter will change only when the former changes in a definite
ratio, which for the volar surface of the third phalanx of the index-
finger is as 29 is to 30. Thus, other things being equal, a sensation
caused by a given weight will only change with moderate stimulation
when one-thirtieth of the weight is either added or subtracted. The
ratio of change, however, varies in different regions of the body: thus,
for the back of the hand the ratio varies from one-tenth to one-twentieth;
for the tongue, one-thirtieth to one-fortieth. The difference of stimulus
necessary to evoke a sensation is known as the threshold difference.
It seems to be a law not only for the skin, but for other senses as well,
that a change in the intensity of a sensation,
to an appreciable extent, will occur only when
the objective stimulus changes in a definite
ratio. This ratio, however, will vary not only
in different regions of the skin, in different
individuals, but with the sense-organ investi-
gated.

Place Sense. — The sensation evoked by
stimulation of the skin is always, under normal
conditions, referred to the place stimulated.
This holds true not only for two or more points
near or widely separated on the same side, but
also for corresponding points on opposite sides
of the body, even when the stimuli have the
same intensity and are simultaneously applied.
The cause for this localizing power is to be
found in a difference in the quality of the sen-
sation related in some way to the part stimu-
lated. Each cutaneous area is supposed to
give to the tactile sensation a quality or local
sign by virtue of which the mind is enabled to
localize the point of contact.
Each cutaneous area which has a local sign of its own is known
as a sensory circle, for the reason that the mind does not refer the
sensation to a point, but to an area more or less circular in outline.
The skin may therefore be regarded as composed of myriads of such
circles varying in size in different regions of the body.

The delicacy of the localizing power in any part of the skin is
determined by testing the power which the part possesses, of dis-
tinguishing the sensations produced by the contact of the- points of a
pair of compasses placed close together. The distance to which the
points must be separated in order to evoke two separate recognizable
sensations is a measure of the diameter of the sensory circle. Within
this circle the two sensations become fused into one sensation. The
discriminative sensibility of different regions as determined by com-

Fig. 299. — Pacinian
Corpuscles, c. Capsules.
d. Endothelial lining sepa-
rating the latter, n. Nerve.
/. Funicular sheath of nerve.
m. Central mass. n'. Ter-
minal fiber; and a. Where
it splits up into finer fibrils.
— (Stirling.)

THE SENSE OF TOUCH. 643

pass points is shown in the following table; the numbers represent
the distances at which two sensations are recognized:

MM.

Tip of tongue, 1.1

Palmar surface of third phalanx of index finger, 2.2

Red surface of lips, 4.5

Palmar surface of first phalanx of finger, 5.5

Tip of nose, 6.8

Palm of hand, 8.9

Lower part of forehead, 22.6

Dorsum of hand, 31.6

Dorsum of foot, 40.6

Middle of the back, 67.7

The discriminative sensibility of any portion of the body is a
function of its mobility. This is shown by the fact that it increases
rapidly from the shoulders to the fingers and from the hips to the toes.

The Temperature Sense. — The sensations of heat and cold which
are experienced from time to time are caused by changes in the tem-
perature of the skin produced in a variety of ways. As these sensations
are specifically different from those of touch, as well as different from
each other, it is highly probable that for each sensation there are
special nerve-endings distributed throughout the skin. Investigations
have shown that all over the skin there are innumerable spots of
varying size which if stimulated evoke sensations of heat or cold.
Such points are termed heat and cold spots. Each responds to but
one kind of stimulus. A warm object applied to a heat spot will
evoke a sensation of warmth. It will have no effect on the cold spot.
The reverse is also true. Between the cold and heat spots there are
areas that are neutral, insensitive to either heat or cold. The cold
spots are more numerous than the heat spots in almost all regions of
the body. (See Fig. 300.)

The sensitivity of the skin to temperature changes is very acute,
as shown by the fact that even 0.050 C. is readily appreciable. This
holds true, however, only when the temperature of the object lies
between 270 and 330 C. This capability varies in different regions
of the skin, and depends on the number of heat and cold spots present,
the thickness of the epidermis, the thermal conductivity of the object
touching it, and the extent to which it is habitually exposed or protected.

The physiologic stimulus to the thermic end-organs is the passage
of heat through the skin from the interior of the body to the sur-
rounding air. If the radiation is continuous and uniform, the end-
organs soon adapt themselves to the temperature of the surrounding
air and the sensation of heat, under physiologic conditions, is not
evoked. If there is a sudden rise in the external temperature caused
by natural or artificial means, which diminishes the radiation, the
temperature of the skin will at once rise, the end-organs will be stim-
ulated, and a sensation of warmth developed. If, on the other hand,
there is a sudden fall in temperature and an increased radiation,
the temperature of the skin will fall, the end-organs will be stimulated,

644

TEXT-BOOK OF PHYSIOLOGY.

and a sensation of cold developed. Experiment also teaches that the
intensity of a warm or cold sensation will depend on the existing tem-
perature of the skin, and not upon the absolute temperature of the
object. Thus, water as 200 C. will evoke a sensation of heat or cold
according as the skin has previously been cooled below or warmed
above this temperature.

The Muscle Sense. — As a result of the activities of the muscula-
ture of the body or even of its individual parts, there arises in con-
sciousness a series of sensations, which are termed muscle sensations.
These sensations give rise to the perception —
1. Of the direction and duration of both passive (due to external

K3

aifc illllllll It"
*:llff

Fig. 300. — Cold and Hot Spots from the Anterior Surface of the Forearm.
a. Cold spots, b. Hot spots. The dark parts are the most sensitive, the hatched the
medium, the dotted the fe^'v and the vacant spaces the non-sensitive. — (Landois and
Stirling.)

causes) and active movements (due to internal, volitional efforts)
which take place without hindrance;

2. Of the resistance offered to movements by external bodies; and —

3. Of the posture of the body or of its individual parts.

As to the seat of the physiologic processes which precede and
underlie the development of the sensations two views, at least, may
be advanced, viz.:

1. That the processes arc central in origin and partake of the nature

of a discharge of nerve impulses from the nerve-cells through
the motor nerves to the muscles, the entire process being accom-
panied by sensation. This is known as the innervation theory.

2. That the processes are peripheral in origin, initiated by stimulation

of specialized end-organs in the muscles and tendons which are

THE SENSE OF TOUCH.

645

connected through the intermediation of afferent nerves with
nerve-cells in the cerebral cortex.

The physiologic mechanism subserving the muscle sense, accord-
ing to the second theory, now held by many physiologists, thus in-
volves peripheral end-organs, afferent nerves, their cortical connec-
tions and nerve-cells in the cerebral cortex at or near the junction
of the superior and inferior parietal convolutions.

The End-organs. — These are small fusiform structures found in
and among the muscle bundles of all the muscles of the bodv with
the exception of the diaphragm and eye muscles. In the muscles of
the arm and in the small muscles of the hand they are especially abund-
ant. From their shape they are known as muscle spindles. Thev
vary in length from 2 to 12 mm. and in breadth from 0.15 to 0.4 mm.
Each spindle (Fig. 301) consists of a connective-tissue capsule con-
taining from two to ten longitudinally arranged striated muscle fibers
of fine diameter. In the middle or equatorial region of these intra-fusal
fibers there is frequently found a quantity of non-striated protoplasmic
matter. The spindle is supplied with both sensor and motor nerves.

Fig. 301. — A Neuro-muscle Spixdle of a Cat. (Ruffini.) c. Capsule, pr. e.
Primary ending. 5. e. Secondary ending. i>l. e. Plate ending. (All these are probably
sensor in function.) — (Starling's "Physiology.")

The sensor fiber loses its external investments as it approaches the
capsule. The naked axis-cylinder then penetrates the capsule, and
after dividing several times terminates in a ribbon-like or spiral manner
around the intra-fusal muscle fiber. This ending was described by
and is known as Ruffini's "annulo-spiral ribbon." The motor nerve
also penetrates the capsule and terminates in the polar extremities
of the intra-fusal fiber. Sensor end-organs supposed to be connected
with the muscle sense are also found in the tendons of muscles.

Afferent Nerves. — That muscles are abundantly supplied with
afferent nerves has been proved by different methods of investigation.
With histologic methods Sherrington has traced afferent fibers from
the muscle spindles directly into the spinal nerve ganglia. The con-
tractions of muscles from electric stimulation as well as the con-
tractions known as muscle cramp, due to unknown agents, give rise
to sensations of pain, a fact which indicates the presence in muscles
of afferent or sensor nerves.

Cortical Area. — Pathologic findings have shown that an im-
pairment or a loss of the muscle sense is associated with destructive

646

TEXT-BOOK OF PHYSIOLOGY.

lesions of perhaps the super and sub-parietal convolutions (Figs.
252, 253). In a case reported by Starr the removal of a small tumor
in the pia mater situated over the junction of the superior and inferior
parietal lobules was followed by a loss of the muscle sense and marked
ataxia in the right hand for a period of six weeks, after which recovery
took place. These symptoms were attributed to injury of the cortex
from unavoidable surgical procedures.

The muscle sensations, as stated in foregoing paragraphs, form the
basis of the perception not only of the direction and the duration of a
body movement and the resistance experienced, but also of the position
and the tension of the muscle groups. The latter fact more especially
makes it possible for the mind to direct the muscles and to graduate
the energy necessary to the accomplishment of a definite purpose.

Active Touch. — Active touch or the application of the fingers
to the surfaces of external objects implies the cooperation of the skin
and the muscles. The sensations which are evoked are combina-
tions of contact and muscle sensations. The union of these sensa-
tions forms the basis of the perception of hardness, softness, smooth-
ness, and roughness of bodies.

THE SENSE OF TASTE.

The physiologic mechanism involved in the sense of taste includes
the tongue, the gustatory nerves (the chorda tympani and the glosso-
pharyngeal), their cortical connections and
nerve-cells in the gray matter of the fourth
temporal convolutions. The peripheral ex-
citation of this apparatus gives rise to nerve
impulses which transmitted to the brain evoke
the sensations of taste. The specific physio-
logic stimulus is matter, organic and inorganic,
in a state of solution.

The Tongue. — The tongue consists of
both intrinsic and extrinsic muscles, in virtue
of which it is susceptible of both a change in
shape and position. The movements of the
tongue, though not essential to taste, are made
use of in the finer discrimination of tastes.

The tongue is covered over by mucous
membrane continuous with that lining the oral
cavity. The dorsum of the tongue presents a
series of papillae richly supplied with blood-
vessels and nerves. Of these there are three
varieties, the filiform, the fungiform, and the
circumvallate (Fig. 302).

1. The filiform papilla, the most numerous, cover the anterior two-
thirds of the tongue; they are conical or filiform in shape and

Fig. ,302. — The Tongue.

1. Papilla; circumvallatac.

2. Papillae fungiformes.

THE SENSE OF TASTE.

647

covered with horny epithelium which is often prolonged into
filamentous tufts.

2. The fungiform papilla, found chiefly at the tip and sides of the

tongue, are less numerous but larger than the preceding and of a
deep red color.

3. The circumvallate papilla:, from eight to ten in number, are

situated at the base of the tongue arranged in the form of the
letter A They consist of a central projection surrounded by
a wall or circumvallation from which they take their name.

The Peripheral End-organs. The Taste-buds. — Embedded
in the epithelium covering the mucous membrane not only of the
tongue but of the palate and posterior surface of
the epiglottis are small ovoid bodies which from
their relation to the gustatory nerves are regarded
as their peripheral end-organs and known as taste-
buds or taste-beakers. Each bud is ovoid in shape
(Fig. 303). Its base rests on the tunica propria; its
apex comes up to the epithelium, where it presents
a narrow funnel-shaped opening, the taste-pore.
The wrall of the bud is composed of elongated curved
epithelium. The interior contains narrow spindle-
shaped neuro-epithelial cells provided at their outer
extremity wTith stiff hair-like filaments wdiich pro-
ject into the taste-pore.

The neuro-epithelial cells are in physiologic re-
lation with the nerves of taste. The terminal
branches, after entering the bud at its base, develop
fine tufts which come into contact with the cells.
That the taste-buds are connected writh the nerves
of taste is rendered probable from the fact of their
degeneration after division of the nerves.

The Taste Area. — -The taste area, though con-
fined for the most part to the tongue, extends itself
in different individuals to the mucous membrane of
the hard palate, to the anterior surface of the soft palate, to the uvula,
the anterior and posterior half arches, the tonsils, the posterior wall
of the pharynx, and the epiglottis.

The Taste Sensations. — The sensations which arise in conse-
quence of impressions made by different substances on the peripheral
apparatus of this area are in so many instances combinations of taste,
touch, temperature, and smell that they are extremely difficult of
classification. Nevertheless four primary tastes can be recognized:
viz., bitter, sweet, acid or sour, salt or saline. Though the contact
of any bitter, sweet, acid, or salt substance with any part of the tongue
will, if the substance be present in sufficient quantity or concentration,
develop a corresponding sensation, some regions of the tongue are
more sensitive and responsive than others. Thus, the posterior

Fig. 303. — Taste-
bud from Circum-
yallate p.apilla of
A Child. The oval
structure is limited to
the epithelium (e)
lining the furrow,
encroaching slightly
upon the adjacent
connective tissue (/);
0, taste-pore through
which the taste-cells
communicate with
the mucous surface.
— (Ajter Piersol.)

648 TEXT-BOOK OF PHYSIOLOGY.

portion is more sensitive to bitter substances than the anterior; the
reverse is true for sweet substances and perhaps for acids and salines.
The intensity of the resulting sensation in any given instance
will depend on the degree of concentration of the substance, while
its massiveness will depend on the area affected.

THE SENSE OF SMELL.

The physiologic mechanism involved in the sense of smell in-
cludes the nasal fossae, the olfactory nerves, the olfactory tracts^ and
nerve-cells in those areas of the cortex known as the uncinate con-
volution and anterior part of the gyrus fornicatus. Peripheral stimu-
lation of this mechanism develops nerve impulses which, transmitted
to the cortex, evoke the sensations of odor. The specific physiologic
stimulus is matter in the gaseous or volatile state.

The Nasal Fossae. — The nasal fossa? are irregularly shaped
cavities separated by a vertical septum formed by the perpendicular
plate of the ethmoid bone, the vomer, and the triangular cartilage.
The outer wall presents three recesses separated by the projection
inward of the turbinated bones. Each fossa opens anteriorly and
posteriorly by the anterior and posterior nares, the latter communicat-
ing with the pharynx. Both fossae are lined throughout by mucous
membrane. The upper part of the fossa is known as the olfactory,
the lower portion as the respiratory region. In the former, the mucous
membrane over the septum and superior turbinated bone is somewhat
thicker than elsewhere and covered with a neuro-epithelium which
constitutes —

The Peripheral End-organ. — This consists of a basement mem-
brane supporting two kinds of cells, the olfactory and the sustentacular.
The olfactory cells are bipolar nerve-cells, the center of which contains
a large spheric nucleus. The peripheral pole is cylindric or conic
in shape and provided at its extremity with several hair-like processes.
The central pole becomes the axon process and passes directly to the
olfactory bulb.

The sustentacular cells are epithelial in character and, as their
name implies, support or sustain the olfactory cells.

For the appreciation of odorous particles the air must be drawn
through the nasal fossae with a certain degree of velocity. If the
particles are widely diffused in the air, they must be drawn not
only more quickly but more forcibly into contact with the olfactory
hairs, as in the act of sniffing, the result of short energetic inspira-
tions. To many substances the olfactory apparatus is extremely
sensitive. Thus, it has been shown that a particle of mercaptan
the actual weight of which was calculated to be Too'irFfrinTo' °f a milli-
gram gave rise to a distinct sensation.

The Olfactory Sensations. — The sensations which arise in conse-
quence of ilif excitation of the olfactory apparatus are very numerous

THE SENSE OF SMELL. 649

and their classification is extremely difficult. For this reason it is
customary to divide them into two groups: viz., agreeable and dis-
agreeable, in accordance with the feelings they excite in the individual.
As the olfactory sensations give rise to feelings rather than ideas,
this sense plays in man a subordinate part in the acquisition of knowl-
edge. In lower animals this sense is employed for the purpose of
discovering and securing food, for detecting enemies and friends,
and for sexual purposes. In land animals the entire olfactory appa-
ratus is well developed and the sense keen; in some aquatic animals, as
the dolphin, whale, and seal, the apparatus is poorly developed and
the sense dull.

CHAPTER XXVII.
THE SENSE OF SIGHT.

The physiologic mechanism involved in the sense of sight includes
the eyeball, the optic nerve, the optic tracts, their cortical connections,
and nerve-cells in the cuneus and adjacent gray matter. Peripheral
stimulation of this mechanism develops nerve impulses which trans-
mitted to the cortex evoke (i) the sensation of light and its different
qualities — colors; (2) the perception of light and color under the form
of pictures of external objects; and (3) in connection with the ocular
muscles, the production of muscle sensations by which the size, dis-
tance, and direction of objects may be judged.

The specific physiologic stimulus to the terminal end-organ,
the retina, is the impact of ether vibrations. In general, it may be
said that, at least for the same color, the intensity of the objective
vibration determines the intensity of the sensation.

THE PHYSIOLOGIC ANATOMY OF THE EYEBALL.

The eyeball is situated at the fore part of the orbit cavity, and
in such a position as to permit of an extensive range of vision. It is
loosely held in position by a fibrous membrane, the capsule of Tenon,
which is attached, on the one hand, to the eyeball itself, and, on the
other, to the walls of the orbit cavity. Thus suspended, the eyeball
is susceptible of being moved in any direction by the contraction of
the muscles attached to it.

The ball is spheroid in shape, measuring about 24 millimeters
in its anterio-posterior diameter and a little less in its transverse and
vertical diameters. When viewed in profile, it is seen to consist of
the segments of two spheres, of which the posterior is the larger,
occupying five-sixths, and the anterior is the smaller, occupying one-
sixth of the ball. It is composed of several concentrically arranged
membranes enclosing various refracting media essential to vision.

The membranes, enumerating them from without inward, are
as follows: the sclera and cornea, the chorioid and iris, and the retina.
The refracting media are the aqueous humor, the crystalline lens,
an el the vitreous humor.

The Sclera and Cornea. — The sclera is the thick opaque mem-
brane covering the posterior five-sixths of the ball. It is composed
of layers of connective tissue which are arranged transversely and
longitudinally. It is pierced posteriorly by the optic nerve about

650

THE SENSE OF SIGHT.

651

15 13 1G

3 or 4 millimeters internal to the optic axis. By virtue of its firmness
and density the sclera gives form to the eyeball, protects delicate
structures enclosed by it, and serves for the attachment of the muscles
by which the ball is moved (Figs. 304, 309.) The cornea is the
transparent membrane forming the anterior one-sixth of the ball. It
is nearly circular in shape, measuring in its horizontal meridian 12 mm.,
in its vertical meridian 11 mm. The curvature is therefore sharper
in the latter than in the former. The radius of curvature of the
anterior surface at that central portion ordinarily used in vision is
7.829 mm.; that of the posterior surface about 6 mm.

The substance of the cornea is made up of thin layers of delicate
transparent fibrils of connective tissue continuous with those found in
the sclera. Lymph-
spaces are present
throughout the cor-
nea, in which are to
be found lymph-cor-
puscles. The anterior
surface of the cornea
is covered with several
layers of nucleated
epithelium supported
by a structureless
membrane, the an-
terior elastic lamina.
The posterior surface
also is covered by a
layer of epithelium
supported by a similar
membrane, the mem-
brane of Descemet,
which at its periphery
becomes continuous
with the iris. At the
junction of the cornea
and sclera there is a
circular groove,
known as the canal
of Schlemm.

The Chorioid, Iris, Ciliary Muscle, and Ciliary Processes.—
The chorioid is the dark brown membrane which extends forward
nearly to the cornea, where it terminates in a series of folds, the ciliary
processes. Posteriorly, it is pierced by the optic nerve. It is com-
posed largely of blood-vessels, arteries, capillaries, and veins, sup-
ported by connective tissue. Externally it is loosely connected to the
sclera; internally it is lined by a layer of hexagonal cells containing
black pigment which, though usually described as a part of the chorioid.

Fig. 304. — Chorioid Coat of the Eye. i. Optic
nerve. 2, 2, 2, 2, 3, 3, 3, 4. Sclerotic coat divided and
turned back to show the chorioid. 5, 5, 5, 5. The
cornea, divided into four portions and turned back.
6, 6. Canal of Schlemm. 7. External surface of the
chorioid, traversed by the ciliary nerves and one of the
long ciliary arteries. 8. Central vessel into which
open the vasa vorticosa. 9, 9, 10, 10. Chorioid zone.
11, 11. Ciliary nerves. 12. Long ciliary artery. 13,
13, 13, 13. Anterior ciliary arteries. 14. Iris. 15, 15.
Vascular circle of the iris. 16. Pupil. — {Sappey.)

652 TEXT-BOOK OF PHYSIOLOGY.

is now known to belong, embryologicly and physiologicly, to the
retina. Lying within the outer layer of arteries and veins there is a
thick layer of small arterioles and capillaries, known as the chorio-
capillaris. The chorioid with its contained blood-vessels bears an
important relation to the nutrition and function of the eye. It provides
a free supply of lymph and presents a uniform temperature to the
retina in contact with it.

The iris is the circular, variously colored membrane in the anterior
part of the eye just behind the cornea. It presents a little to the
nasal side of the center a circular opening, the pupil. The outer or
circumferential border is united by connective tissue to the cornea,
sclera, and ciliary muscle; the inner border forms the boundary of
the pupil. The iris consists of a framework of connective tissue sup-
porting blood-vessels, muscle-fibers, and pigmented connective-tissue
cells. The anterior surface is covered by a layer of cells continuous
with those covering the posterior surface of the cornea. The pos-
terior surface is formed by a thin structureless membrane supporting
a layer of pigment cells continuous with those lining the chorioid.
The color which the iris presents in different individuals depends on
the relative amount of pigment in the connective-tissue corpuscles.
In blue eyes the pigment is wanting. In gray, brown, and black
eyes the pigment is present in progressively increasing amounts. The
blood-vessels are connected with those of the chorioid coat.

The muscle-fibers are of the non-striated variety and arranged in
two sets, one circularly, the other radially, disposed.

The circular fibers are found close to the pupil near the posterior
surface of the iris. Contraction of this band of fibers diminishes,
relaxation increases, the size of the pupil. This muscle is known
as the sphincter pupillce or sphincter iridis.

The radial fibers form a more or less continuous layer in the
posterior part of the iris, extending from the margin of the pupil,
where they blend with the circular fibers, to the outer border. Con-
traction of the fibers enlarges the size of the pupil. The muscle is
known as the dilatator pupillce.

The nerves exciting the sphincter pupillce to action are the ciliary
nerves, axons of nerve-cells located in the ciliary, or ophthalmic gang-
lion. Stimulation of these fibers gives rise to contraction of the
sphincter and diminution in the size of the pupil. The nerves excit-
ing the dilatator pupillce to action are axons of nerve-cells located in
the superior cervical ganglion. They reach the iris by way of the
cervical sympathetic, the ophthalmic division of the fifth, and the long
ciliary nerve. Stimulation of these nerves is followed by contraction
of the dilatator and enlargement in the size of the pupil. Both the
ciliary and superior cervical ganglia are in relation with pre-ganglionic
fibers coming from the central nerve system., (See page 589.)

The ciliary muscle is a gray circular band about two millimeters
in width, consisting of non-striated muscle-fibers. The majority of

THE SENSE OF SIGHT.

653

these fibers pursue a radial or meridional direction. Taking their
origin from the junction of the sclera, cornea, and iris, they pass
backward to be inserted into the chorioid coat opposite the ciliary
processes. The inner portion of the muscle is interrupted by bundles
of fibers which pursue a circular direction. (Fig. 305.) They collec-
tively constitute the annular or ring muscle of Miiller. The ciliary
muscle in common with the circular fibers of the iris receives its nerve-
supply direct from the nerve-cells in the ciliary ganglion. Contraction
of the ciliary muscle tenses the chorioid coat, and for this reason it
is frequently termed the tensor chorioidece.

The Retina. — The retina is the internal coat of the eye, extending
forward almost to the ciliary processes, where it terminates in an
indented border, known as the ora serrata. In the living condition
it is clear, transparent, and pink in color. After death it becomes

Fig. 305. — Section through the Ciliary Region of the Human Eye. a. Radi-
ating bundles of the ciliary muscle, b. Deeper bundles, c. Circular network, d.
Annular muscle of Miiller. e. Tendon of ciliary muscle. /. Muscle-fibers on posterior
side of the iris. g. Muscles on the ciliary border of the same. h. Ligamentum pectin-
atum. — {After Iwanoff.)

opaque. The retina is abundantly supplied with blood-vessels, de-
rived from the arteria centralis retina, a branch of the ophthalmic,
which pierces the optic nerve near the sclera, runs forward in its
center, to the retina, in which its terminal branches are distributed.
The veins arising from the capillary plexus leave the retina by the
same route.

In the posterior portion of the retina, at a point corresponding
with the axis of vision, there is a small oval area bout 2 mm. in its
transverse and about 0.8 mm. in its vertical diameter. From the fact
that it presents a yellow appearance, it is known as the macula lu/ea.
This area presents in its center a depression with sloping sides, known
as the fovea centralis. About 3.5 mm. to the nasal side of the macula
is the point of entrance of the optic nerve.

The retina is remarkably complex in structure, presenting an
appearance when viewed microscopically, something like that repre-
sented in Fig. 306, indicating that it is composed of different cellular

654

TEXT-BOOK OF PHYSIOLOGY.

Uril A(

elements arranged in layers. These have been named, from behind
forward, as folloAvs:

i. The layer of pigment cells.
2. The layer of rods and cones, or Jacobson's layer.
The external limiting membrane.
The outer nuclear or granular layer.
The outer molecular or reticular layer.
The inner nuclear or granular layer.
The inner molecular or reticular layer.
The layer of ganglion cells.
9. The layer of nerve-fibers.
Modern histologic methods of research have made it possible to
reduce the retina, exclusive of the pigment cells, to three successive
layers of nerve-cells, supported by a highly developed neuroglia,
forming what has been termed the fibers of Miiller. These nerve-
cells are as follows:

1. The visual cells.

2. The bipolar cells.

3. The ganglion cells.

The relation of these nerve-cells one to another and to the supporting
neuroglia tissue and the manner in which they unite to form the

above-mentioned layers
are schematicly shown
in Fig. 307.

The pigment layer is

3. External limiting membrane, composed of hexagonal

cells. Though formerly
described as forming a
part (the inner layer) of
the chorioid, these cells
belong embryologicly to
the retina. From their
retinal surface delicate
pigmented processes ex-
tend into and between
the rods and cones. On
exposure to light these
processes elongate and
push themselves between
the rods. In the dark
they retract and with-
draw into the cell-body.
The visual cells which

form the layer of rods and cones are of two varieties, the rod-shaped

and the cone-shaped.

The rod-shaped visual cells consist of a straight elongated cylinder

extending through the entire thickness of Jacobson's membrane and

1
If

Fig. 306.-

Pigment-layer (not shown).

Layer of rods and cones.

Outer nuclear layer.

5. Outer molecular layer.

6. Inner nuclear layer.

7. Inner molecular layer.

8. Layer of ganglion cells.

9. Layer of nerve-fibers.

-Vertical Section of Human Retina.
— (Schaper.)

THE SENSE OF SIGHT.

655

a fine fiber containing a nucleus, which, after piercing the external
limiting membrane, passes into the outer molecular layer, where it
terminates in a spheric enlargement. The outer portion of the rod
is clear and homogeneous, though containing a pigment known as
visual purple or rhodopsin; the inner portion of the rod is slightly
granular.

The cone-shaped visual cells also consist of two portions, a conic
portion situated in Jacobson's membrane between the rods, and a
fine fiber, containing a nucleus, which, after piercing the external
limiting membrane, passes into
the outer molecular layer, where
it terminates in a fine tuft. The
inner portion of the cone is
thicker than the rod and rests on
the limiting membrane; the outer
portion tapers to a fine point and
is known as the cone-style. The
cones, as a rule, are shorter than
the rods. The proportion of
rods to cones varies in different
parts of the retina, though there
are on the average about four-
teen rods to one cone. In the
macula the rods are almost en-

The layer of visual cells to-
gether with the neuroglia consti-
tute the first three layers of the
retina proper. The external
limiting membrane is formed by
the blending of the ends of neu-
roglia cells.

The bipolar cells consist of a
central portion, found in the
inner nuclear layer, from which
are given off twro processes which
pass in opposite directions, one
toward the visual cells, the other toward the ganglion cells. The
former terminate in tufts which arborize around the tufts and spheric
enlargements of the visual cells, and assist in the formation of the
outer molecular layer; the latter terminate in similar tufts in the inner
molecular layer.

The ganglion cells are arranged in a single layer, as a rule. They
are large and nucleated. From the inner side of each cell there is
given off a single axon which passes toward the center of the retina
(forming the nerve-fiber layer), where it enters and assists in forming

Fig. 307. — Cross-section of the Retixa
from a Mammal. A. Layer of rods and
cones. B. Visual cells (outer granules).
C. Outer molecular layer. E. Bipolar cells
(inner granules). F. Inner molecular layer.
G. Ganglion cells. H. Layer of nerve-
fibers, a. Rods. b. Cones. e. Bipolar
rod. f. Bipolar cone. r. Lower ramifica-
tion of a bipolar rod. f. Lower ramification
of a bipolar cone, g, h, i, j, k. Ganglion
cells in various stages, branching from F.
x, z. Bipolar contact of rods and cones, t.
Muller's supporting fibers. S. Centrifugal
nerve-fibers. — {After Ramon y Cajal.)

6;6

TEXT-BOOK OF PHYSIOLOGY.

the optic nerve. From the outer side of the ganglion cell dendrites
pass into and assist in forming the inner molecular layer. These
dendrites come into physiologic relation with those of the inner proc-
esses of the bipolar cells.

Horizontally disposed nerve-cells are also present in the outer
molecular layer in relation with the visual cells. Spongioblasts or
amacrine cells are also present at the border of and in the inner molec-
ular layer.

From the relation of the ganglion cells, from which the optic nerve-
fibers take their origin, to the visual cells and the bipolar cells, the
latter may be regarded as the terminal visual organ, the intermediate

imtiiiuiiii

Fig. 308. — Horizontal Section through the Macula and Fovea of a Man Sixty
Years Old. The section is not through the exact center of the fovea, for there are only
cone visual cells and no remnants of the confluence of the inner granule and ganglion cell
layers are present. 1. Cones. 2. External limiting membrane. 3. Outer nuclear
layer. 4. Henle's fiber layer. 5. Outer molecular or reticular layer. 6. Inner nuclear
layer. 7. Inner molecular or reticular layer. 8. Layer of ganglion cells. 9. Nerve-
fiber layer. — {After Schaper, Stohr's "Histology")

between the ether vibrations and the ganglion cell. The visual cells
are directed toward the chorioid, away from the entering light, dip-
ping into the pigment cells. They, with the pigment layer, are the
elements by which the ether vibrations are transformed into nerve
energy.

In the fovea most of the retinal elements are wanting or are re-
duced in thickness. The cones alone are present. The cone-fibers
with their nuclei are directed 'obliquely upward and outward along
the slope of the fovea, to end in tufts which come into physiologic
relation with the dendrites of the ganglion cells which at the top of
the fovea are generally increased in number (Fig. 308).

It is estimated that the optic nerve contains about 500,000 nerve-
fibers, and that for each fiber there are about 7 cones, noo rods, and

THE SENSE OF SIGHT.

657

7 pigment cells. In accordance with this estimate there would be
about 3,500,000 cones, 50,000,000 rods, and 3,500,000 pigment cells.
The distance between the centers of two adjacent cones in the fovea
is 4 micromillimeters.

The vitreous humor is the largest of the refracting media and
occupies by far the largest portion of the interior of the eyeball. From
its position it gives support to the retina. Anteriorly it presents a
concavity, in which the crystalline lens is lodged. The vitreous
humor consists of water (97 per cent.), organic matter and salts, en-
closed in a transparent membrane, the tunica hyaloidea. The mass of
the vitreous humor is penetrated by a species of connective tissue.

Fig. 309. — Horizontal Section of the Eyeball, i. Sclera. 2. Cornea. 3.
Chorioid. 4. Iris. 5. Ciliary muscle. 6. Retina. 7. Lens. 8. Suspensory ligament.
9. Canal of Schlemm. 10. Canal of Petit, n. Optic nerve. — (Deaver.)

The aqueous humor is small in amount in comparison with tte
vitreous and is found in the space bounded by the cornea, the ciliary
body, the suspensory ligament, and the lens. The projection of
the iris into this space partially divides into an anterior and posterior
portion or chamber. The aqueous humor is a clear, watery, alkaline
fluid derived from or secreted by the capillary blood-vessels of the
ciliary body. From this origin it passes through the pupil into the
anterior chamber. It serves to keep the cornea tense and smooth.
The ocular tension partly depends on the presence of this fluid in
t he eyeball. There is every reason for believing that there is a constant
stream of fluid from the blood-vessels into the eye and from the ey<
through the spaces of Fontana at the base of the iris into the canal of

658 TEXT-BOOK OF PHYSIOLOGY.

Schlemm, and so into the blood. Any interference with the exit of
this fluid rapidly increases the ocular tension.

The lens is the transparent biconvex body situated just behind
the iris, in the concavity of the vitreous. The thickness of the lens
is 3.6 mm. j the diameter about 9 mm. It consists of a transparent
capsule containing elongated hexagonal fibers which, having their
origin near the anterior central portion of the lens, pass out toward
the margin, where they bend around to terminate in a triradiate
figure on the opposite side. Chemicly the lens consists of water,
a globulin body (crystallin) , and salts.

The Suspensory Ligament. — The lens is held in position by the
suspensory ligament, formed in part by the hyaloid membrane and
in part by fibers derived from the ciliary processes. The former be-
comes attached to the posterior surface, the latter to the anterior
surface of the lens near the equator. The space between the two
layers of the ligament is the canal of Petit. The anterior surface of
the ligament presents a series of plications conforming to correspond-
ing plications on the surface of the ciliary processes.

The relations of all the parts entering into the structure of the eye
are shown in Fig. 309.

THE PHYSIOLOGY OF VISION.

The Retinal Image. — The general function of the eye is the
formation of images of external objects on the free ends of the per-
cipient elements of the retina, the rods and cones. The existence of
an image on the retina can be readily seen in the excised eye of an
albino rabbit, when placed between a lighted candle and the eye of
an observer. Its presence in the human eye can be demonstrated
with the ophthalmoscope. It is this image, composed of focal points
of luminous rays, which stimulates the rods and cones, which is the
basis of our sight-perceptions, and out of which the mind constructs
space relations of external objects. In only two essential respects
does the image on the retina differ from the object, aside from the
fact that the object has usually three, the image only two, dimensions —
viz., in size and relative arrangement of its parts. Whatever the dis-
tance, the image is generally smaller than the object; it is also reversed,
the upper part of the object becoming the lower part of the image,
and the right side of the object the left of the image.

The Dioptric Apparatus. — The formation of an image is made
possible by the introduction of a complex refracting apparatus con-
sisting of the cornea, aqueous humor, lens, and vitreous humor.
Without these agencies the ether vibrations would give rise only to a
sensation of diffused luminosity. Rays of light emanating from
any one point — that is, homocentric rays — arriving at the eye must
traverse successively the different refracting media. In their passage
from one to the other, they undergo at the surfaces changes in direc-
tor] before they are finally converged to a focal point. In order to

THE SENSE OF SIGHT. 659

mathematically follow the rays in all their deviations through the
media, to determine their focal points and to construct an image, a
knowledge of the form of the refracting surfaces, the refractive indices
of the different media, and the distances of the surfaces from one
another must be known.

The following constants are now accepted: The radius of curva-
ture of that portion of each refracting surface used for distinct vision is
for the cornea 7.829 mm., for the anterior and posterior surfaces of
the lens 10 and 6 mm., respectively. The indices of refraction of the
different media are as follows: cornea and aqueous humor, 1.3365;
lens, 1. 4371; vitreous body, 1.3365. The distance from the vertex of
the cornea to the lens is 3.6 mm.; the thickness of the lens, 3.6 mm.;
the distance from the posterior surface of the lens to the retina, 15 mm.
As the two surfaces of the cornea are practically parallel, and as the
index of refraction of the aqueous humor is the same as that of the
cornea, they may be re-
garded as but one medium.
The refracting surfaces may
therefore be reduced to the
anterior surface of the
cornea, the anterior surface
of the lens, and the posterior
surface of the lens.*

Parallel ravs of light
, • ,1 " p Fig. 310. — Refraction of Homocentric

entering tne eye pass irom rays and the Formation of an Image.
air, with an index of re-
fraction of 1.00025, into the cornea, with an index of refraction
of 1.3365. In passing from the rarer into the denser medium they
undergo refraction in accordance with the laws of optics and are
rendered somewhat convergent. The extent of this first refraction
and convergence is sufficiently great to bring parallel rays, if con-
tinued, to a focus about 10 mm. behind the retina. This would be
the condition in aphakia whether the lens is congenitally absent or
has been removed by surgical procedures. Perfect vision, however,
requires that the convergence of the light must be great enough to
bring the focal point, the image, on the retina. This is accomplished
by the introduction of an additional refracting body, the lens. On
entering the lens the rays are for the same reason — i. e., the passage
from a rarer into a denser medium — again refracted and converged,
and if continued would come to a focus about 6.5 mm. behind the
retina. On passing from the lens into the vitreous — i. e., from a
denser into a rarer medium — the rays are once more converged and to
an extent sufficient to focalize them on the retina (Fig. 310).

* Strictly speaking, the posterior surface of the cornea is not parallel to the anterior
surface, and the index of refraction of the cornea is a trifle greater than that of the
aqueous humor, viz., 1.377. But as the increase in the corneal refraction due to a higher
index is almost exactly counteracted by a decrease in refraction, due to the higher curva-
ture of the posterior corneal surface, the usual assumptions furnish quite accurate result?.

660 TEXT-BOOK OF PHYSIOLOGY.

While it is thus possible to geometricly follow the rays through
these media by means of the above-mentioned factors, the procedure
is attended with many difficulties. Moreover, as the relations all
change when rays enter the eye from objects situated progressively
nearer the eye, a separate calculation is necessitated for each distance
for the determination of the size of the image.

A method by which these difficulties are much reduced was sug-
gested by Gauss and developed by Listing. It was demonstrated by
Gauss that in every complicated system of refracting media separated
by centered spheric surfaces there may be assumed certain ideal
or cardinal points, to which the system may be reduced, and which,
if their relative position and properties be known, permit of the de-
termination, either by calculation or geometric construction, of the
path of the refracted ray, and the position and size of the image in
the last medium, of the object in the first.

Every dioptric system can be replaced, as Gauss showed, by a
single system composed of six cardinal points and six planes per-
pendicular to the common axis — e. g., two focal points, two principal

jr,

prV

j>

Fig. 311. — Diagram showing the Position and Relation of the Cardinal Points.

points, two nodal points, two focal planes, two principal planes, and
two nodal planes.

Properties of the Cardinal Points. — The first focal point, F„ in
Fig. 311, has the property that every ray which before refraction passes
through it, after refraction is parallel to the axis.

The second focal point, F2, has the property that every ray which
before refraction is parallel to the axis, passes after refraction
through it.

The second principal point, H2, is the image of the first, FLt', that
is, rays in the first medium which go through the first principal point
pass after the last refraction through the second. Planes at right
angles to the axis at these points are principal planes. The second
principal plane is the image of the first. Every point in the first
principal plane has its image after refraction at a corresponding point
in the second principal plane at the same distance from the axis and
on the same side.

The second nodal point, N2, is the image of the first, NT: a ray
whi( h in the first medium is directed to the first nodal point passes
after refraction through the second nodal point, and the directions of
the rays before and after refraction are parallel to each other. In

THE SENSE OF SIGHT.

66 1

Fig. 311 let A B represent the axis. The distance of the first focal
point, FJ} from the first principal plane, Hx, is the anterior focal
distance. The distance of the posterior focal point, F2, from the
second principal plane, H2, is the posterior focal distance. The dis-
tance of the first nodal point, N1} from the first focal point is equal
to the second focal distance. The distance of the second nodal
point, N2, from the posterior focal point is equal to the anterior focal
distance. It is evident, therefore, that the distance of the correspond-

Fig. 312. — Diagram to Find the Image in Last Medium of a Luminous Point in

the First.

ing principal and nodal points from each other is equal to the differ-
ences between the two focal distances. Also the distance of the two
principal points from each other is equal to the distance of the two
nodal points from each other. Finally, the focal distances are pro-
portional to the refractive indices of the first and last media. Planes
passing through the focal points vertically to the axis are known as
focal planes.

From these properties of the cardinal points the position of an
image in the last medium of a luminous point in the first may be

Yic. 313. — Diagram to Find the Refracted Ray in the Last Medium of a Given
Ray in the First Medium.

determined, and the course of a refracted ray in the last medium be
constructed if its direction in the first be given according to the fol-
lowing rules:

1. To find the image in the last medium of a luminous point in the
first: Let A (Fig. 312) be this given point. Draw A B parallel to
the axis until it meets the second principal plane in B; then BF2
will be this ray after refraction. Draw a second ray from A to
the first nodal point; then draw another ray, D E, from the

662

TEXT-BOOK OF PHYSIOLOGY.

second nodal point parallel to .4 C. This will be the refracted
ray in the last medium. Where the two refracted rays, BF2 and
D E, intersect, the image of A will be Az*

To find the refracted ray in the last medium of a given ray in the
first medium: Let A B (Fig. 313) be the given ray. Continue
this ray until it meets the first principal plane in C. Draw C D
parallel to. the axis. Now assume any point, such as E, in the
given ray, and find its image Ex by the Rule 1. Then D ET
becomes the course of the refracted ray.

The Schematic Eye. — Accepting the system of cardinal points,

Fig. 314. — Diagram showing the Position of the Cardinal Points in the "Sche-
matic Eye." The continuous lines in the upper half of the figure show their position
in the passive emmetropic eye. The dotted lines indicate the change in their position
in an eye accommodated for the object A at the distance a from the cornea, or 152 mm.
The lower half of the figure shows the formation of a distinct image on the retina of an eye
accommodated for the object A at the distance a from the cornea.

Listing, Donders, and v. Helmholtz have constructed "schematic"
eyes to be substituted for the refracting system of the natural eye.

For this purpose it is necessary to make use of the various esti-
mates of the indices of refraction of the different media, of the radii
of curvatures of the different refracting surfaces, and of the distances
separating them, to deduce an average eye as a basis for calculation.
The most widely accepted attempt is that of v. Helmholtz. The data
he assumed are as follows: The refractive index of air = 1; of the
cornea and aqueous humor, 1.3365; of the lens, 1.4371; of the vitreous

*If the point A is infinitely far from the eye, all the rays striking the eye will be paral-
lel to each other. The nodal ray must therefore be drawn, and the point where this
nodal ray meets the second focal plane will be the image of A =A^ where all rays parallel
to the nodal ray will meet.

THE SENSE OF SIGHT. 663

humor, 1.3365; the radius of curvature of the cornea, 7.829 mm.;
of the anterior surface of the lens, 10 mm.; of the posterior surface,
6 mm. ; the distance from the apex of the cornea to the anterior surface
of the lens, 3.6 mm.; thickness of lens, 3.6 mm. From the above-
mentioned data v. Helmholtz calculated the position of the cardinal
points for the eye as follows (see Fig. 314): The first focal point is
situated 13.745 mm. before the anterior surface of the cornea; the
posterior focal point is situated 15.619 mm. behind the posterior
surface of the lens; the first principal point, 1.753 mm- behind the
cornea; the second principal point, 2.106 mm. behind the cornea;
the first and second nodal points, 6.968 and 7.321 mm. behind the
apex of the cornea, respectively. The anterior focal distance of this
schematic eye, the distance between FT and HT, therefore amounts
to 15.498 mm., and the posterior focal distance, H2 to F2, to 20.713
mm.

When the eye, however, is accommodated for near vision, the
relations of the cardinal points are changed and will be as follows,

if the point accommodated for, lies 152

mm. from the cornea: Anterior focal » A ^\

distance, is-QQO mm.; posterior focal dis- IfCsk \

> o yy ' r isms mm \^II±"" I

tance, 18.689 rnm. ; distance from cornea F' y- V " 207^m ¥•

of the first and second principal points, \ J

1.858 and 2.257 mm. respectively; dis- ^**=B=a^^

tance of the posterior focus, 20.955 mm- Fig. 315.— The Reduced Eye.
from cornea. Given this schematic eye

in the accommodated state, the course of the rays and the determina-
tion of the position of an image in the last medium of a luminous
point in the first can easily be determined by the rules already given.

The Reduced Eye.— As suggested by Listing, this schematic, eye
may be yet further simplified or reduced to a single refracting surface
bounded anteriorly by air and posteriorly only by aqueous or vitreous
humor. Without introducing any noticeable error in the determination
of the size of the retinal image, the anterior principal and the anterior
nodal points may be disregarded, owing to the minuteness of the
distances (0.39 mm.) separating the two systems of points. There
is thus obtained one principal point and one nodal point, which latter
becomes the center of curvature of the single refracting surface. The
dimensions of this "reduced" eye are as follows (see Fig. 315). From
the anterior surface of the cornea, corresponding to the principal
plane H, to the nodal point N, 5.215 mm., from the anterior focal
point FI} to the principal plane H, i. e., the anterior focal distance /',
15.498 mm.; from the principal plane H to the posterior focal point
F2, i.e., the posterior focal distance /", 20.713 mm.; the index of
refraction is 1.3365. There is thus substituted for the natural eye a
single refracting surface with a radius of curvature, r, of 5.215 mm.
In such an eye luminous rays emanating from the anterior focal
point are parallel to the axis after refraction in the interior of the eve.

664 TEXT-BOOK OF PHYSIOLOGY.

Also rays parallel to the axis before refraction unite at the posterior
focal point.

By means of this reduced eye the construction of the refracted ray,
the various calculations as to the size of the image, the size of diffusion
circles, etc., are greatly facilitated: e. g.,

In Fig. 316 let A B represent an object. From A homocentric
rays fall on the single refracting surface. One of the rays, the nodal

A_ ray, falling on the surface

/r ^v perpendicularly, passes

^^\^~~--~^^^ ^_^\p unrefracted through the

- - - ■"-'^^d(g^ single nodal point, N, to

-- —" — "~" ""\ ^jr trie Posterior focal plane.

-=-""" ^w—^^^ The remaining rays, par-

tially represented in the
Fig. 316.-THE Formation of an Image in the figure fallinfir on this sur-
Reduced Eye. . ° . ° . .

face under varying de-
grees of incidence, undergo corresponding degrees of refraction, by
which they form a converging cone of rays which unite at a point
situated on the nodal ray. These two points, A, a, are known as
conjugate foci. The same holds true for homocentric rays emanating
from B or any other point of the object.

The Size of the Retinal Image. — The size of the retinal image,
I (in Fig. 316 a b), may now be easily calculated, when the size of the
object, O (in Fig. 316 A B), and its distance, D, from the refracting
surface with radius of curvature, r, are known, by the following
formula:

O: I = D + r:f"— r.

For, as the triangles A N B and a N b are similar, we have

A B: ab = / N: N g, or a b = A B ' *N 8.

Independent of the foregoing method, the size of the retinal image
may be calculated if it is remembered that the eye, like any optic
system, has a point of such a quality that a ray of light which before
entering the eye was directed toward it, after refraction continues as
if it came from this point. In other words, there is in the eye a point
which allows a ray of light to pass unrefracted. This point, termed
the nodal point of the eye, determines the size of the image; for if a
line be drawn from both the upper and lower ends of an object through
this nodal point, it is clear that the images of the respective points
must lie on these two rays where they intersect the retina. The dis-
tance of this nodal point from the retina is 15.498 mm. It is clear,
therefore, that the size of the object is to the size of the image, as the
distance of the object from the nodal point is to the distance of the
nodal point from the retina; or, in other words, to find the size of the
retinal image: multiply the size of the object by 15.5 mm. and divide
by the distance of the object from the eye.

The visual angle is defined as the angle formed by the intersection

THE SENSE OF SIGHT.

665

of two lines drawn from the extremities of an object to the nodal point
of the eye. Beyond the nodal point, however, the lines again diverge
and form an inverted or reversed image of the object on the retina.
The size of the visual angle increases with the nearness and decreases
with the remoteness of the object; the retinal image correspondingly

increases and decreases in size.
A

These facts will become ap-
parent from an examination

6'

of Fig. 317. As the size of

Fig. 317.

-Drawing Designed to show
how the Visual Angle and Size of Retinal
Image Varies with the Distance of an
Object of Given Size. For the distant position
of A-B the visual angle is a; for the near
position (dotted lines) /3. (From Stewart.)

the retinal image diminishes
as the visual angle diminishes
either as a result of the re-
moval of a given object from
the eye, or of a diminution of
the size of the object, there
comes a limit in the size of the
visual angle, beyond which it
is impossible to see the two end points (A and B) of the object
separatelv. When this limit is reached the size of the angle expressed
in degrees of the circle, may be determined if the distance between the
two points and their distance from the eye be known. Thus it has been
experimentally determined that at a distance of 5 meters, the smallest
object or the smallest interval between two points which permits the
eye to distinguish them as such, is about 1.454 mm. Lines drawn from
the extremities of such an object or interval, to the nodal point, subtend
an angle of 60 seconds.* Beyond this the two points are indistinguish-
able. In other words the emmetropic eye possesses the power of distin-
guishing the correspondingly small interval between the two images on
the retina of the two objective points. The size of the image or the inter-
val between the two retinal points, determined from the foregoing factors
by the formulas on page 664 is 0.004 mm., and which would correspond

*The size of the visual angle, under which an object of this size and situated at a
distance of 5 meters is distinctly seen, can be determined from the following Fig. 318,
in which A B, represent
the size of the object
1.454 mm.: Ar, thenodal
point; CN, the line
which bisects the object,
represent the distance of
the object from the nodal
point; a, the visual angle
subtended and whose
value it is desired to
know, and b one-half of
the angle (7. By trigo-
nometry the size of the
angle </ can be deter-
mined in the following way: one-half the size of the object A B, is divided by its distance
from the nodal point; the quotient is the tangent of half the angle. Thus 0.727 -r- 5000 =
0.0001454. By reference to logarithmic tables it will be found that the angle or fraction
of the circle corresponding to this tangent is 30 seconds, and that therefore the whole angle
is 60 seconds.

Fig. 318. — Figure showing the Method of Obtaining
the Visual Angle Enpressed in Degrees or Fraction
of a Degree of the Circle.

666 TEXT-BOOK OF PHYSIOLOGY.

to a visual angle of 60 seconds. If the retinal distance is less than this
the two sensations fuse into one. The reason assigned for this is,
that the distance between the centers of two adjoining cones in the
macula is 0.004 mm. With a visual angle not less than 60 seconds,
the two foci fall on separate cones. With a smaller visual angle the
two foci fall on, and excite but a single cone and hence there arises
the sensation of but a single point. The acuteness of vision, therefore,
of the emmetropic eye depends on its power of distinguishing the
smallest retinal image or the smallest interval between two points on
the retina corresponding to a visual angle of 60 seconds.

In ophthalmic practice it is customary in testing the acuteness of
vision to employ test letters of specific sizes for specific distances. The
letters are so proportioned that when they are placed at the specified
distances, the extremities of the letters subtend an angle of 5 minutes.
The letters have been constructed on the following basis: Since to
an angle of 60 seconds there corresponds an object of 1.454 mm. at
the distance of 5 meters as shown before and as the object decreases
in proportion to the distance (for the same visual angle) it is evident

E T

E L

B.
Fig. 319. — Standard Test Letters.

that the object would have to be one-fifth of 1.454 mm. or 0.2908 mm.
in order to subtend an angle of 60 seconds at one meter. From this
the size for any other distance in meters is found simply by multiplying
0.2908 mm. with the distance.

The standard letters are so constructed that each meridian is
composed of five squares, each of which at a specified distance, sub-
tends an angle of 1 minute, and hence the entire letter an angle of 5
minutes. The letter that could be distinctly seen at a distance of 5
meters, would have, therefore, a vertical and a horizontal meridian
of 5 times 1.454 mm. or 7.27 mm. (Fig. 319 A), and at 10 meters
corresponding meridians of 14.54 mm., etc. (Fig. 319 B.)

If with the accommodation suspended, the emmetropic eye could
clearly distinguish a letter 7.27 mm. in size at a distance of 5 meters
and which would, therefore, subtend an angle of 5 minutes, the acuity
of 1 he vision would be normal and could be expressed as follows:
V=| or V = i. If on the contrary at this distance the smallest
letter that could be clearly seen is one that subtends an angle of 5
minutes at a distance of 10 meters then the visual acuity would be only
one half l he normal and could be expressed as follows V=-yV or V=|,
etc. The acuity of vision is expressed, therefore, by a fraction the
enumerator of which is the distance at which the test is made, and

THE SENSE OF SIGHT.

667

whose denominator is the distance at which the smallest letters dis-
tinguished by the patient, subtend an angle of 5 minutes, but which
in the instance cited above subtends an angle of 10 minutes; or in
other words the distance at which the patient reads divided bv the
distance at which he ought to read the smallest letters seen by him were
his visual acuity normal.

Accommodation. — Accommodation may be defined as the power
which the eye possesses of adjusting itself to vision at different dis-
tances; or in other words, the power of focusing rays of light on the
retina, which come from different distances at different times. That
such a power is a necessity is apparent from the fact that it can not
focus rays coming from a distant and a near object at the same time.
Thus, if an object is held before the eye at a distance of 22 centimeters,
for example, and the
vision is directed to a
distant object it is evi-
dent that the near ob-
ject is indistinctly seen;
but if the vision is then
directed to the near ob-
ject, it in turn becomes
clear and distinct, while
the distant object be-
comes blurred and indis-
tinct. It is evident,
therefore, that rays of
light coming from a dis-
tant and a near object
can not be simultane-
ously, but only alter-
nately, focused on the
retina. The observer at
the same time becomes

conscious, as the vision is directed from the distant to the near object,
of a change in the eye itself, a change that involves time and effort.
The reasons for these facts will become apparent from a consideration
of the following facts:

In a normal or emmetropic eye, homocentric parallel rays of light
(Fig. 320, a, b) after passing through the optic media are converged
and brought to a focus on the retina, /. Rays, however, which come
from a luminous point situated near the eye, P, and are therefore
divergent, passing through the optic media at the same time, are inter-
cepted by the retina before they are focused, and give rise to the for-
mation of difjusion-circles and indistinctness of vision. The reverse
is also true. When the eye is adjusted for the refraction and focusing
of divergent rays (Fig. 320, P) parallel rays will be brought to a focus
before reaching the retina, and, again diverging, will form ditlusion-

Fig. 320. — The Refraction of Parallel and
Divergent Rays in the Emmetropic Eye in the
Passive and in the Active or Accommodated
Condition.

668

TEXT-BOOK OF PHYSIOLOGY.

Distance of the Focal
oint behind the Posterior
Surface of the Retina.

Diameter of the Diffusion-circle

o.o mm.

0.0 mm.

0.005 "

O.OOII "

0.012 "

0.0027 "

0.025 "

0. 0*056 "

0.050 "

O.OII2 "

O.IOO "

0.0222 "

0.20 "
0.40 "
o.So "

0.0443 "
0.0825 "
0.1616 "

1.60 "

0.3122 "

3.20 "
3-42 "

0.5768 "
0.6484 "

circles. It is evident, therefore, that it is impossible to simultaneously
focus both parallel and divergent rays, and to see distinctly at the same
time, two objects which are situated at different distances. The eye
must be alternately adjusted first to one object and then to another.
To this adjustment the term accommodation has been given.

The following table of Listing shows the size of the diffusion-
circles formed of objects situated at different distances when the
accommodative power is suspended in an emmetropic eye:

Distance of Luminous Point.

6

3
1.500

0.750

0-375
0.188
0.094
0.088

The normal eye when adjusted for distant vision is in a passive
condition, and hence vision of distant objects is unattended with
fatigue. In the act of adjustment, however, for near vision the eye
passes into an active state, the result of a muscle effort, the energy of
which is proportional to the nearness of the object toward which the
eye is directed.

From the foregoing table it is evident that between infinity and
65 meters, the diffusion-circles are so slight that no perceptible ac-
commodative effort is required to eliminate them. From 65 meters
to 6 meters the diffusion-circles gradually become larger, though they
are yet so faint as to require for their correction an accommodative
effort which is scarcely measurable. From 6 meters up to 6 centi-
meters, however, a progressive increase in accommodative power is
demanded for distinct vision.

Mechanism of Accommodation. — Inasmuch as neither the
corneal curvature nor the shape of the eyeball undergoes any change
during accommodation, the necessary change, whatever it may be,
is to be sought for in the lens. As to the character of the changes
in this body, two views are held, based largely on the fact and its
interpretation, that images of a luminous point reflected from the
anterior surface of the cornea, the anterior and posterior surfaces
of the lens, change their relative positions during accommodation.

Thus, if in a darkened room a lighted candle be placed in front
of and to the side of an individual whose eye is directed to a distant
object, an observer placed in the same relative position as the candle
will observe three images in the eye, one at the surface of the cornea,
two at the pupillary margin (Fig. 321;. Of the two latter, one is

THE SENSE OF SIGHT. 669

quite large and situated apparently in front of the third, which is faint,
small, and inverted. The middle image is reflected from the convex
surface of the lens, the last from the concave surface. These images
of reflection are known as catoptric images. If now the individual
be directed to fix the gaze on a near object, the second image changes
its position, advances toward the corneal image and at the same time
becomes smaller, a change which, in accordance with the laws of
optics, could only be due to an increase in the convexity of the anterior
surface of the lens. A slight displacement of the third image some-
times observed indicates a possible increase in the convexity of the
posterior surface lens.

According to Helmholtz, during accommodation the entire anterior
surface of the lens becomes more convex, while at the same time it
slightly advances, possibly as much as 0.4 mm. in
extreme efforts. This change is represented in
Fig. 322. According to Tscherning, the increase
in convexity of the anterior surface is confined to
the central portion, the remainder of the surface
becoming somewhat flattened. There is, moreover,
no evidence that there is any advance of the surface a b c

or any increase in the thickness of the lens. A FlG ,2i— Catop-
series of new and ingenious experiments lend sup- tric Images in the
port to Tscherning's view. The radius of curvature ?YE- ?■ J-Pn.gbt

i . , ° . , „ image of reflection,

in either case approximates 6 mm. in extreme efforts fror£ tne cornea, b.
of accommodation. The increase in convexity Upright image from
naturally increases the refracting power. the anterior surface

TTT1 - . . .°f of the lens. c. In-

Whichever view is accepted, the nearer the ob- verted image, from
ject— that is, the greater the degree of divergence the posterior surface
of the light rays — the more pronounced must be the holtzA
increase in convexity in order that they may be
sufficiently converged and focalized on the retinal surface. Changes in
the convexity of the lens, either of increase or decrease, are attended
by changes in the distinctness of images. Coincident with the lens
change, the pupillary margin advances and the pupil itself becomes
smaller. By this means an indistinctness of the image is prevented
by cutting off the rays which would give rise, owing to the angle at
which they fall on the surface, to diffusion-circles, from spheric aber-
ration.

The Function of the Ciliary Muscle.— Though it is generally
admitted that the increase in the convexity of the lens is caused by
the contraction of the ciliary muscle, the exact manner in which this
is accomplished is not clearly understood. According to Helmholtz,
when the eye is in repose and directed to a distant object the lens is
somewhat flattened from a traction exerted by the suspensory liga-
ment. When the eye is directed to a near object, the ciliary muscle
contracts, thereby relaxing the ligament, as a result of which the lens,
by virtue of an inherent elasticity, bulges forward and becomes more

6jo

TEXT-BOOK OF PHYSIOLOGY.

convex. In consequence of this latter fact the refracting power is
proportionally increased. In extreme efforts of accommodation it is
believed by some observers that the circularly arranged fibers, the
so-called annular muscle, contract and exert a pressure on the periphery
of the lens and thus aid other mechanisms in relaxing the ligament
and in increasing the convexity. This view appears to be supported
by the fact that in hypermetropia, where a constant effort is required
to obtain a distinct image of even distant objects, the annular muscle
becomes very much hypertrophied, thus reinforcing the meridional
fibers. In myopia, on the contrary, where the accommodative effort
is at a minimum, the entire muscle possesses less than its average size
and development.

According to Tscherning, a different explanation of the action of
the ciliary muscle must be given. Thus, when it contracts, the antero-
internal angle, that portion in close relation with the suspensory
ligament, recedes and exerts on the ligament a pressure which in turn
exerts a traction on the peripheral portions of the anterior surface of

UptfAeZtum.
■_fion>mansJfem&r£tn>6
Gyrrtea proper
-Itejeemef 'Jfeminxne
-EndptheZiusrv
SpOzcterJridis
CbuiaZ <5c?tfemSri£L
%*&&aryJfajcZe
■ivV •fcZera, ■

IProc&sraj CUiarit

Fig. 322. — The Left Half Represents the Eye in a State of Rest. The Right
Half in State of Accommodation.

the lens, which produces the deformation observed. At the same
time the postero-external portion of the muscle exerts traction on the
chorioid, thus sustaining the vitreous and indirectly the lens.

The reason for the flattening of the periphery of the lens from
zonular compression and the sharpening of the central convexity is
to be found in the fact that the convexity of the more solid central
portion, the nucleus, is greater than that of the lens itself. Hence it
is easily understood why a zonular traction would give rise to peripheral
flattening.

There is, however, one point which seems difficult to harmonize
with Tschcrning's view; that is, the fact that during accommodation
the lens appears to be slightly tremulous, thus showing relaxation,
and not increased tension, of the suspensory ligament.

Range of Accommodation.— It has been stated that rays of
light coming from a luminous point situated at any distance beyond
65 meters are so nearly parallel that no accommodative effort is re-
quired for their focalization. So long as the luminous point remains
between infinity and 65 meters, the eye, directed toward it, remains

THE SENSE OF SIGHT. 671

completely relaxed. The point at which the object can be distinctly
seen without accommodation is termed the far point or the punctum
remotum. This for the normal eye is at a distance of 65 meters or
beyond.* If the luminous point gradually approaches the eye from
a point 65 meters distant, the accommodative power comes into play
and gradually increases until it attains its maximum. The nearest
point up to which the eye is able to form distinct images of objects
is called its near point or punctum proximum. This near point
in a healthy boy of twelve years will lie at 2§ inches from the eye,
while the same point lies only at 8 inches or 20 cm. in a man of forty
years. Of objects which lie nearer than the punctum proximum the
eye cannot form distinct images. The distance between the punctum
remotum and the punctum proximum is termed the range of accommo-
dation.

Force of Accommodation. — The increase in curvature of the
lens necessary to focalize rays wdien the eye is directed from the far
to the near point necessitates the expenditure of energy on the part
of the ciliary muscle. The energy expended in the act of accommo-
dation may be measured by a lens, the refracting power of which is
such as to enable it to produce the same result — that is, to give the
diverging rays coming from the near point, e. g., 20 cm., a parallel
direction. A lens, therefore, which has for a near point a focal dis-
tance of 20 cm. would be a measure of the force expended, for such a
lens placed in front of the crystalline lens, when in a state of repose,
would, with the assistance of the latter, bring diverging rays coming
from the near point to a focus on the retina. A lens of this character
is said to have a refracting power of 5 dioptries.

Since lenses of the same curvature made from different materials
have different refracting powers, it becomes necessary to have, for
purposes of comparison, some unit of measurement. The unit now
accepted is the refracting power of a glass lens which is sufficient
to focalize parallel rays at a distance of 100 cm. or 1 meter. This
amount of refracting power is termed a dioptry. Lenses which
would focalize parallel rays at a distance of 50, 20, or 10 cm. are
said to have a refractive power of 2, 5, or 10 dioptries, respectively,
obtained by dividing into 100 cm. the focal distance. The refracting
power of a biconcave lens is determined by prolonging backward in
the direction the parallel rays have come, the rays which have been
rendered divergent by the lens.

The refracting media of the human eye in repose have collectively
a refracting power of about 64 dioptries, the reciprocal of its focal
length. The refracting power of the corneal surface alone is equiva-
lent to 42 dioptries. The crystalline lens could in the schematic eye
be replaced by a lens of about 13 dioptries in front of the eye, as is
done after the extraction of a cataract. But owing to its position in a

*In practical ophthalmic work a point six meters distant is taken as the far point for
the reason that the rays at this distance are practically parallel.

672 TEXT-BOOK OF PHYSIOLOGY.

medium denser than air, it has been calculated that its refracting
power is about 20 dioptries.

The capability of the lens to increase its refraction during accom-
modative efforts beyond the 20 dioptries varies considerably at differ-
ent periods of life. At ten years the increase is 14 dioptries, as the
near point is 7 cm.; at thirty years the increase is but 7 dioptries, as
the near point is 14 cm.; at sixty the increase is but 1 dioptry and
the near point 100 cm.; at seventy it is zero. From youth to old
age, the elasticity of the lens steadily declines, and the range of accom-
modation diminishes from the recession of the near point.

Convergence of the Eyes during Accommodation. — In binocu-
lar vision of near objects the eyes are turned inward and the optic
axis of each — a line passing through the center of the cornea and
the center of the eye — turned toward the median line during accom-
modation. So long as the eyes are directed toward the far point, 65
meters or beyond, the optic axes are parallel. When the eyes are
directed to any point within 65 meters the optic axes are converged,
the convergence increasing steadily as the near point is approached.
In this way the fovea of each eye is directed to the same point and
single vision made possible. Were this not the case, double vision
would result.

Functions of the Iris.— For purposes of distinct vision it is essen-
tial that the quantity of light entering the interior of the eye shall
be so adjusted that the formation and subsequent perception of the
image shall be sharp and distinct. This is accomplished by the .iris,
the circular fibers of which alternately contract and relax with in-
creasing and decreasing intensities of the light. The size of the pupil,
therefore, through which the light passes, will vary from moment
to moment and in accordance with variation in the light intensity.
The quantity of light necessary to distinct vision is thus regulated.

In the total absence of light the sphincter pupillse muscle is relaxed
and the pupil widely dilated. With the appearance of light and an
increase in its intensity the muscle again contracts and the pupil
progressively narrows. With a given intensity in the light, the sphinc-
ter contraction is greater when the light falls directly into the fovea.
Contraction of this muscle also occurs as an associated movement in
the convergence of the eyes during accommodation and in consensus
with the other eye.

In addition to this function of the iris, it constitutes, by virtue of
the sphincter muscle contraction, an important corrective apparatus.
Being non-transparent, it serves as a diaphragm intercepting those
rays which would otherwise pass through the peripheral portions of
the lens and by spheric aberration give rise to indistinctness of the
image. The movements of the iris by which the size of the pupil is
determined are caused by the contractions and relaxations of the
sphincter piipiUai and dilatator pupillcR muscles. The contraction of
the sphincter is entirely reflex and involves those structures necessary

THE SENSE OF SIGHT. 673

to the performance of any reflex act, viz.: a receptive surface, the
retina; afferent nerves, the pupillary fibers of the optic nerve; a central
emissive center situated in the gray matter beneath the aqueduct of
Sylvius; and efferent nerves, the motor oculi and the ciliary nerves.
The stimulus requisite to the excitation of this mechanism is the impact
of light waves or ether vibrations on the rods and cones. According
to the intensity of these vibrations will be the resulting contraction of
the muscle. The contraction of the dilatator pupillae muscle is deter-
mined by the activity of a continuously active nerve-center in the
medulla oblongata which transmits its nerve impulses through the
spinal cord, along the* first and second dorsal nerves to the superior
cervical ganglion, and thence to the iris by way of the fifth nerve.
(See Fig. 272, page 589.) These two muscles appear to bear an an-
tagonistic relation to each other, for section of the motor oculi is fol-
lowed by relaxation of the sphincter muscle and dilatation of the pupil.
Stimulation of the sympathetic is followed by a more pronounced
dilatation. The size of the pupil is the resultant of a balancing of these
two forces.

OPTIC DEFECTS.

Presbyopia. — This is a condition of the eye characterized by a
defective or diminished accommodative power. As age advances the
lens loses its elasticity and the power to increase its refraction, and
vision at the normal reading distance becomes impossible. The near
point, therefore, advances toward the far point, or recedes from the indi-
vidual. The range of accommodation is also diminished. At fortv
years the near point is about 22 cm.; at forty-five years it has receded
to 28 cm. This would indicate that the lens in these five years has
lost 1 dioptry of refracting power; at fifty years the near point recedes
to 43 cm., and at sixty to 200 cm., indicating a loss in refracting power
on the part of the lens of 2 and 4 dioptries respectively. Convex
lenses placed before the eyes having a refracting power of 1, 2, and 4
dioptries would in the three instances return the near point to its
normal position. At the age of seventy the lens is incapable of any
increase during an accommodative effort. A lens of 4 dioptries
would therefore be required by such a man, for near vision at 10 inches.

Myopia. — This is a condition of the eye characterized by an in-
crease in the antero-posterior diameter or a hypernormal refracting
power of the lens. The former is the usual condition. Parallel rays
of light brought to a focus in front of the retina again diverge, giving
rise to diffusion-circles and indistinctness of the image. Divergent
rays alone are capable of being focalized on the retina in its new
position. The punctum remotum is always at a definite distance,
but approaches the eye as the myopia increases. The near point
is usually much nearer the eye than 20 cm. For this reason the
condition is termed near sight. (Fig. 323.)

The increase in the length of the antero-posterior diameter may
43

674

TEXT-BOOK OF PHYSIOLOGY.

range from a fraction of a millimeter up to 10 mm. With an increase
of 0.16 mm. the far point is but 200 cm. distant; and with an increase
of 3.2 mm. it is but 10 cm. distant. Inasmuch as only divergent
rays can be focalized by the myopic eye normal vision can be restored
by the use of a biconcave lens with a diverging power in the first
instance of 0.5 dioptry and the second of 10 dioptries. (Fig. 324.)

Hypermetropia. — This is a condition of the eye characterized by
decrease of the normal antero-posterior diameter or by a subnormal
refracting power of the lens. The former is the usual condition.

Fig. 323. — Myopia. Parallel rays
focus at F, cross and form diffusion-
circles; divergent rays from A focus
on the retina. — (Hansell and Sweet.)

Fig. 324. — Correction of Myopia
by a Concave Lens. — (Hansell and
Sweet.)

Parallel rays of light do not, therefore, come to a focus when the
accommodation is suspended. Falling on the retina previous to
focalization, they give rise to diffusion-circles and indistinctness of the
image. As no object can be seen distinctly no matter how remote,
there is no positive far point. The near point is abnormally distant—
sometimes as far as 200 cm. For this reason the condition is termed
far sight. A hypermetropic eye without accommodative effort can
focalize only converging rays on the retina. If rays of light were to
come from the retina of such an eve, they would, on emerging, take

■/...:.":.-"-"-■--■»- k

I [G. 325. — The Hypermetropic Fye. Parallel rays (A, B) can be focused only at a
point behind the eye, as at /; rays of light coming from the retina take, on emerging from
the eye, a divergent direction, C, D. K. The negative punctum remotum.

a divergent direction, as shown in Fig. 325, dotted line C and D.
If these same rays were to be prolonged backward, they would meet
at the point K, which is the punctum remotum; and as it is behind
tin- eye, it is termed negative. Since rays coming from the retina
take a divergent direction on emerging from the eye, it is evident
that only converging rays can be focalized by a passive hyperme-
tropic eye. As there are no convergent rays in nature, it is necessary
for distinct vision that all rays, parallel and divergent, shall be given

THE SENSE OF SIGHT.

675

a convergent direction before entering the eye. This is done by
placing before the eye convex. lenses the converging power of which
is proportional to the degree of hypermetropia (Figs. 326, 327).

Astigmatism. — This is a condition of the eye characterized by
an inequality of curvature of its refracting surfaces in consequence of
which not all of a homocentric bundle of rays are brought to the
same focus. The inequality may be either in the cornea or lens, or
both, though usually in the cornea.

Fig. 326. — Hypermetropia. Par-
allel Rays Focused behind the
Retixa. — {Hansel! and Sweet.)

Fig. 327. — Correction of Hyper-
metropia by a Convex Lens.
— (Hanscll and Sweet.)

In the normal cornea the radius of curvature in the vertical meridian
is a trifle shorter, 7.6 mm., than that of the horizontal, 7.8 mm., and
hence its focal distance is slightly shorter. The difference, however,
in the focal distances is so slight that the error in the formation of the
image is scarcely noticeable. A transection of a cone of light coming
from the cornea is practically a circle. If, however, the vertical cur-
vature exceeds the normal to any marked extent, the rays passing
through this meridian will be more sharply refracted and brought
to a focus much sooner than the rays passing through the horizontal

Fig. 328. — Refraction by an Astigmatic Surface. — (Hanscll and Sweet.)

meridian. The result will be that the cone of light will be no longer
circular, but more or less elliptic. The variations of the shape of
this cone are shown in Fig. 328, which represents the appearances
presented on cross-section both before and after focalization of each
set of rays. Though the vertical meridian has usually the sharper
curvature, it not infrequently happens that the reverse is true. For
the reason that the rays from one point do not all come to the same
focus or point, the condition is termed astigmatism.

676 TEXT-BOOK OF PHYSIOLOGY.

Spheric Aberration. — When the rays of light which emanate
from a point fall upon a spheric lens, they do not after passing through
it reunite at one point because of the fact that the more peripheral
rays have a shorter focus than the central rays. To this condition
the term spheric aberration is given. Spheric aberration can be dem-
onstrated in the human eye. That this condition is present to but
a slight extent in the normal eye is due to the presence of the iris,
which intercepts those rays which would otherwise pass through the
marginal portions of the refracting media. In widely dilated eyes
the spheric aberration of the peripheral parts may amount to as much
as 4.5 dioptries.

Chromatic Aberration. — When a beam of white light is made
to pass through a prism, it is decomposed into the primary colors
owing to a difference in the refrangibility of the rays. In passing
through the refracting media of the eye the different rays composing
white light also undergo unequal refraction and those rays which
give rise to one color are brought to a focus at a point somewhat
different from those which give rise to other colors. If the eye is
accommodated for one set of rays, it is not for another, and the result
is a fringe of colors around the image. This defect in the normal
eve is so slight that the mind fails to take cognizance of it. That the
eye is incapable of simultaneously focalizing rays of widely different
refrangibility, as those which give rise to .the blue and red colors,
is shown by the following experiment: The eye being directed to a
luminous point, a plate of cobalt-glass is placed between the light and
the observer close to the eye. This substance has the property of
intercepting all rays but the red and the blue and hence these alone
will be seen. The center of the image produced will be red and clearly
defined, the periphery blue and ill defined. The reason for this is
clear. The eye more readily accommodates itself for the red rays,
and hence their focal point is distinct. The blue rays, having a
higher degree of refrangibility, come to a focus, cross and diverge,
and give rise to diffusion-circles. If a biconcave glass be placed before
the cobalt, the blue rays can be focalized on the retina, while the red
will fall on the retina without focalization. The image will now be
blue and distinct in the center, the periphery red and ill defined.
Wil h the removal of the minus glass the reverse condition again obtains.

Imperfect Centering. — From a purely physical point of view, the
eye is not a perfect optic instrument. In addition to the defects
noticed in the foregoing paragraphs, there is yet another, viz.: an
imperfect centering of the refracting surfaces. In first-class optic
instruments the lenses are centered — that is, their exact centers are
.situated on the same axis. In viewing an object through such a
system the visual line corresponds with the axis of the lens system.
This is not the case with the refracting system of the eye. A line
pas-ring through the center of the cornea and the center of the eye,
Hi- optic axis (O A in Fig. 329), does not pass exactly through the

THE SENSE OF SIGHT.

677

2.

center of the lens and does not fall into the point for most distinct
vision, the fovea. This has led to the recognition of other lines the
relations of which must be kept in mind in all optic discussions, viz. :
1. The visual axis or visual line (V L), the line connecting the point
viewed, the nodal point and the fovea centralis.
The line 0} fixation or line of regard (V C), the line connecting the
point viewed with the center of rotation, the latter being situated
6 mm. behind the nodal point of the eye and 9 mm. before the
retina. The relation of these lines and certain angles connected
with them are shown in Fig. 329. The angle included between
the line D D (the major axis of the corneal ellipse) and the visual
line is the angle alpha, amounting on the average to 50. The
angle included between the optic axis and the line of fixation or

temporal- Sic£e

Fig. 329. — Diagram showing the Corneal Axis D D, the Optic Axis O A,
the Visual Axis V L, and the Line of Fixation V C; also the Three Adgels, a, /i, y,

regard is the angle gamma, while the angle between the optic
axis and the line of vision is the angle beta. In emmetropia the
angle alpha is about 50. In hypermetropia it is greater, amount-
ing to 70 or 8°, giving to the eye an appearance of divergence.
In myopia it is much smaller — 2° — or in extreme cases may be
abolished, the line of vision corresponding with the optic axis
or even passing beyond it. The angle gamma is of value in de
termining the actual deviation of the eye in squint.
Functions of the Retina. —Of all the layers of the retina, the
rods and cones appear to be the most essential to vision. It is only
this layer that is capable of receiving the light stimulus and of trans-
forming it into some specific form of energy, which in turn arouses
in the fibers of the optic nerve the characteristic nerve impulses.
A ray of light entering the eye passes entirely through the various
layers of the retina, and is arrested only upon reaching the pigmentary
epithelium in which the rods and cones are embedded. As to the
manner in which the objective stimuli — light and color, so called —

67S TEXT-BOOK OF PHYSIOLOGY.

are transformed into nerve impulses, but little is known. It is prob-
able that the ether vibrations are transformed into heat, which excites
the rods and cones. These, acting as highly specialized end-organs
of the optic nerve, start the impulses on their way to the brain, where
the seeing process takes place. As to the relative function of the
rods and cones, it has been suggested, from the study of the facts
of comparative anatomy, that the rods are impressed only by differ-
ences in the intensity of light, while the cones, in addition, are im-
pressed by qualitative differences in color. The nerve-fibers them-
selves are insensible to the impact of the ether vibrations, and require
for their excitation some intermediate form of energy. That this is
the case was shown by Donders, who reflected a beam of light on
the optic nerve at its entrance without the individual experiencing any
sensation of light. This region, occupied only by the optic-nerve
fibers and devoid of any special retinal elements, is therefore an
insensitive or blind spot. The diameter of this spot is about 1.5 mm.,
and occupies in the field of vision a space of about 6°. It is situated
about 3.5 mm. to the nasal side of the visual axis. Its existence can

Fig. 330. — Diagram for Observing the Situation of the Blind Spot. —

(Helmholtz.)

be demonstrated by the familiar experiment of Mariotte, which con-
sists in placing before the eye two objects having the relation to each
other as in Fig. 330. With the left eye closed and the right eye directed
to the cross, both objects may be visible. But by moving the figure
away from or toward the eye, there will be found a distance, about
30 cm., when the circle will be invisible. This occurs when the image
falls on the optic nerve at its entrance. The experiment of Purkinje
as described in the following paragraph demonstrates also the fact
that the sensitive portion of the retina is to be found only in the layer
of rods and cones.

It is well known that the blood-vessels of the retina are situated
in its innermost layers a short distance behind the optic-nerve fibers.
Owing to this anatomic arrangement, a portion of the light coming
through the pupil will be intercepted by the vessels and a shadow
projected on the layer of rods and cones. Ordinarily, these shadows
are not perceived, for the reason that the shaded parts arc more sen-
sitive, so that the small amount of light passing through the vessels
produces as strong an impression on this part as does the full amount
of light on the unshaded parts of the retina, and perhaps because the
mind has learned to disregard them. But if light be made to enter

THE SENSE OF SIGHT. 679

the eye obliquely, the position of the shadows will be changed, when at
once they become apparent. This can be shown in the following way:
If in a darkened room a lighted candle be held several inches to the
side and to the front of the eye, and then moved up and down, there
will be perceived, apparently in the field of vision, an arborescent
figure corresponding to the retinal blood-vessels. This is due to the
falling of the shadows on unusual portions of the layer of rods and
cones.

Excitability of the Retina. — The retina is not equally excitable
in all parts of its extent. The maximum degree of sensibility is found
in the macula lutea, and especially in its central portion, the fovea.
In this region the layers of the retina almost entirely disappear, the
layer of rods and cones alone remaining, and in the fovea only the
latter are present. That this area is the point of most distinct vision
is shown by the observation that when the eye is directed to any given
point of light, its image always falls in the fovea. Any pathologic
change in the fovea is attended by marked indistinctness of vision.
The sensibility of the retina gradually but irregularly diminishes from
the macula toward the periphery. This diminution in sensibility
holds true for monochromatic as well as white light.

As stated above, the nature of the molecular processes which take
place in the retinal tissue, caused on one hand by the light vibrations,
and on the other hand developing nerve impulses, is entirely un-
known. The discovery of the visual purple in the outer segment of
the rods gave promise of some explanation of the process, especially
when it was shown to undergo changes when exposed to the action
of light. But as the pigment is wanting in the cones, and especially
in the fovea, it cannot be considered essential to distinct vision, al-
though that it plays some important role in the visual process is highly
probable. It was observed by Van Genderen Stort, that when an
animal is kept in darkness some time before death, the cones are long
and filiform; but if the animal has been exposed to light, they are
short and swollen. It was discovered by Boll that if an animal
is kept in darkness an hour or two before death the pigment is
massed at the ends of the rods and cones, but after exposure to light
it becomes displaced and extends over and between the rods almost
to the external limiting membrane. These conditions are represented
in Fig. 331.

The Eye a Living Camera.— In its construction, in the arrange-
ment of its various parts, and in their mode of action the eye may be
compared to a camera obscura. Though the comparison may not be
absolutely exact, yet in a general way it is true that there are many
striking points of similarity between them; e. g., the sclera and chorioid
may be compared to the walls of the camera; the combined refracting
media to the single lens, the action of which results in the focusing
of the light rays; the retina to the sensitive plate receiving the image'
formed at the focal point; the iris to the diaphragm for the regulation

68o

TEXT-BOOK OF PHYSIOLOGY.

of the amount of light to be admitted, and for the partial exclusion
of those marginal rays which give rise to spheric aberration; the ciliarv
muscle to the adjusting screw, by means of which the image is brought
to a focus on the sensitive plate, notwithstanding the varying distances
of the object from the lens. The presence of the visual purple in the
rods of the retina capable of being altered by light makes the com-
parison still more striking.

Kiihne even succeeded in obtaining a fixed image or an optogram
of an external object in a manner similar to that by which an image
is fixed on the sensitive plate of a camera. An animal is kept in
the dark for about ten minutes in order to permit the retinal pigment
to be completely regenerated. The animal, with the eyes covered,

Fig. 331. — Section of the Retina of a Frog. A. In darkness. B. In light. —
(After Van Genderen Stort, from Tscherning's "Physiologic Optics.")

is then brought into a room with a single window. While the head
is steadily directed to the window, the eye is exposed for several
minutes. The eyes are again covered, the animal killed, and the
eyes removed by the light of a sodium flame. The retina is then
placed in a 4 per cent, solution of alum. In a short time the image
of the window, the optogram, will be fixed (Fig. 332). That portion
of the retina corresponding to the image is quite bleached in appear-
ance from the action of the light on the pigment. During life the
regeneration of the visual purple must take place with extreme rapidity.
It is believed to be derived from a pigment secreted by the layer of
pigment cells.

Binocular Vision. -Though two images are formed, one on each
retina, when the eyes are directed to a given object, there results but
one sensation. If the direction of either visual axis be changed by

THE SENSE OF SIGHT.

68 1

Fig. 332. — Retina of
a Rabbit. Optogram
of a Window Four
Meters Distant a.
Yellow spot. b, b.
White streak of nerve -
fibers. — (Kiihne.)

pressure on the eyeball, there arise two sensations, and the object
appears to be doubled. The reason assigned for this, in the first
instance, is that the two images fall into the foveae, two corresponding
points; wrhile in the second instance they fall on non-corresponding
points. It would appear, therefore, that for the purpose of seeing
an object singly when the eyes are directed toward it, the rays eman-
ating from it must fall on corresponding parts of the retina. As all
portions of the retina are sensitive to light, though in varying degrees,
it is not essential that the images always fall in
the fovese. The parts of the retinae which cor-
respond physiologicly are shown in Fig. 333.
In this figure the retinal area is divided into
quadrants by vertical and horizontal lines of
separation, as they are termed. If one retina is
placed in front of or over the other, it will be
found that the quadrants bearing similar letters
cover each other. So long as the rays of light,
entering lhe eye, fall on corresponding areas the
sensation of but one object arises. If, however,
they fall on non-corresponding areas, two sensa-
tions arise. Normal binocular vision enlarges

very considerably the area of the visual field, permits of a better estima-
tion of the size and distance of objects, enables the mind to form more
readily a perception of depth, increases the intensity of sensations and
makes sensation more uniform by off-setting retinal rivalry.

The Horopter. — When the eyes are in the so-called secondary
position — that is, in a position in which the visual axes are con-
verged and directed to a point in front of and in the middle plane of

the body — it will be found on
examination that rays of light
from a number of other objects
enter the eye, pass through the
nodal point, and fall on corres-
ponding parts of the two retinas
and give rise to but single im-
ages. All such points lie, for the
horizontal line of separation, on
a line termed the horopter. The form of this line is that of a circle
which passes through the fixation point and the two nodal points.
Any object on the horopter will give rise to but a single image. This
is shown in Fig. 334, in which the objects I, II, III project their rays
into both eyes which fall on corresponding areas.

In addition to the horopter for the horizontal line of separation,

there is also an horopter for the vertical line of separation. At a

distance of two meters the vertical horopter is a plane. Within this

distance it is concave to the face; beyond this distance it is convex.

An object which lies either in front of or behind the fixation point

Fig. ->>i2). — Corresponding Areas of the

Retina.

682 TEXT-BOOK OF PHYSIOLOGY.

will project its rays on parts of the retinae which do not correspond,
and hence give rise to double images. This is evident from examina-
tion of Fig. 335. While the eyes are directed to figure 2, of which
there is but a single image, the objects B and A give rise to double
images, for reasons already given. If the eyes are now directed to
B, double images will be formed of 2 and A.

At all times, therefore, double images are formed on the retinae
the existence of which is scarcely noticed unless the attention is directed
to them. This is due to the fact that many of the images fall on the
peripheral, less sensitive parts of the retinae. At the same time, from
a want of accommodation and the formation of diffusion-circles, they
are indistinct. For these reasons they are readily neglected.

B

Fig. 334. — Horopter for the
Secondary Position, with Con-
vergence of the Visual Axes.
— (Landois.)

Fig. 335.- — Scheme of Identical and
Non-identical Points of the Retina. —
(Landois.)

In the primary position of the eyes — that is, a position in which
the visual axes are parallel — the horopter is a plane in infinity. In
the tertiary position the horopter is a curve of complex form.

Movements of the Eyeball. — The almost spheric eyeball lies
in the correspondingly shaped cavity of the orbit, like a ball placed
in a socket, and is capable of being moved to a considerable extent
by the six muscles which are attached to it. These muscles are
the superior and inferior recti, the external and internal recti, and
the superior and inferior obliqui (Fig. 336). The four recti muscles
arise from the apex of the orbit cavity, from which point they pass
forward to be inserted into the sclera about 7 to 8 mm. from the
corneal border. The superior oblique muscle having a similar origin

THE SENSE OF SIGHT.

passes forward to the upper and inner angle of the orbit cavity, at
which point its tendon passes through a cartilaginous pulley, after
which it is reflected backward to be inserted into the superior sur-
face of the sclera about 16 mm. behind the corneal border. The
inferior oblique muscle arises from the inner and inferior angle of the
orbit cavity. It then passes outward, upward, and backward, to be
inserted into the upper, posterior and temporal portion of the sclera
about 4 or 5 mm. from the optic nerve entrance.

The movements of each eye are referred to three fixed lines or
axes, which have their origin at the point of rotation of the eyeball,
this point lying about 1.7 mm. behind the center of the globe. If the
eye looks straight forward in the horizontal plane (the head being
erect), the line joining the center of rotation with the object looked at
is the line of fixation or . 7

line of regard. Around i^t:

this antero-posterior axis
the eye may be regarded
as performing its circular
rotation or torsion. At
right angles to this line,
and joining the center of
rotation of both eyes, is
the horizontal or trans-
verse axis, around which
the movements of eleva-
tion (up to 34 degrees)
and depression (down to
57 degrees) take place.
At right angles to both
of these lines there is the
vertical axis, around
which the movements of
adduction (toward the
nose up to 45 degrees)
and abduction (toward the temple up to 42 degrees) occur. The
six muscles may be divided into three pairs, each of which has a
common axis around which it tends to move the eyeball. But
only the common axis of the internal and external recti coincides
with one of three axes before mentioned — namely, with the vertical
axis — thus moving the ball only inwardly or outwardly — respec-
tively. The other two pairs, however, have their own axes of
action, and their movements of the ball must be, therefore, analyzed
with regard to all the three axes, each of these four muscles producing
rotation, elevation, and depression, and abduction or adduction. The
superior and inferior recti muscles, forming one pair, move the eye
around a horizontal axis which intersects the median plane of the body
in front of the eyes at an angle of 63 degrees, and the superior and

Fig. 336. — Muscles or the Eye. Tendon or
Ligament of Zinn. i. Tendon of Zinn. 2. Ex-
ternal rectus divided. 3. Internal rectus. 4.
Inferior rectus. 5. Superior rectus. 6. Superior
oblique. 7. Pulley for superior oblique. 8. In-
ferior oblique. 9. Levator palpebrte superioris.
10, 10. Its anterior expansion, n. Optic ner,-e. —
(Sappey.)

684

TEXT-BOOK OF PHYSIOLOGY.

inferior oblique muscles forming the third pair rotate the globe around
a horizontal axis which cuts the median plane of the body behind the
eyes at an angle of 39 degrees. Thus it is that each muscle moves
the eye as follows, the movement for practical purposes being referred
to the cornea: The rectus externus draws the cornea simply to the
temporal side, the rectus internus simply to the nose; the superior
rectus displaces the cornea upward, slightly inward, and turns the
upper part toward the nose (medial torsion) ; the inferior rectus moves
the cornea downward, slightly inward, and twists the upper part away
from the nose (lateral torsion) ; the superior oblique displaces the cornea
downward, slightly outward, and produces medial torsion; while
the inferior oblique moves the cornea upward, slightly outward, and
produces lateral torsion. These facts show that for certain move-
ments of the eye at least three muscles are necessary (see following
table) :

Inward, Rectus internus.

Outward, Rectus externus.

jj . , / Rectus superior.

* ' \ Obliquus inferior.

J Rectus inferior.
\ Obliquus superior.
f Rectus internus.

upward, \ Rectus superior.

( Obliquus inferior.

Downward, .
Inward and

Inward and [ Rectus internus.

downward, \ Rectus inferior.

[ Obliquus superior.
Outward and | Rectus externus.

upward, \ Rectus superior.

[ Obliquus inferior.
Outward and \ Rectus externus.

downward, . . . . \ Rectus inferior.

[ Obliquus superior.

If both eyes have their line of vision in the horizontal plane parallel
with each other and with the median plane of the body, they are
said to be in the primary position. All other positions are called
^secondary. Both eyes always move simultaneously, which is called
the associated movement of the eyes. There are three forms of asso-
ciated movements: (1) movement of both eyes in the same direction;
(2) movements of convergence by which the visual lines are con-
verged on a point in the middle line of the body; (3) movements of
divergence, by which the eyes are brought back from convergence to
parallelism, or even to divergence, as in certain stereoscopic exercises.
A combination of (1) and (2) or of (1) and (3) takes place for certain
positions of the object looked at.

Color-perception. — A beam of sunlight passed through a glass
prism is decomposed into a series of colors — red, orange, yellow,
green, blue, and violet — the so-called spectral colors, so well exem-
plified in the rainbow. The spectral colors are termed simple colors,
because they can not be any further decomposed by a prism. Ob
jectively, the spectral colors consist of very rapid transverse vibrations
of the ether, from about 400 millions of millions per second for red
to about 760 millions of millions for violet, but subjectively they are
sensations caused by the impact of the ether-waves on the percipient
layer of the retina.

It is possible to mix or blend these spectral color-sensations in
the eye by stimulating the same area of the retina by different spectral

THE SENSE OF SIGHT.

685

colors, cither at the same time or in rapid succession. The following
table shows the results of such experiments as performed by v. Helm-
holtz (Dk. = dark; Wh. = whitish):

Red.

Orange.

Yellow.

Gr. -yellow.

Green.

Bluish-green.

Cyan-Blue.

Violet.

Purple.

Dk.-rose.

Wh.-rose.

White.

White-blue.

Water-blue.

Indigo.

Indigo.

Dk.-rose.

Wh.-rose.

White.

Wh. -green.

Water-blue.

Water-blue.

Cyan-
blue.

Wh.-rose.
White.
Wh. -green.
WTh. -green.
Bl. -green.

Bluish-
green.

Green.

Greenish-
yellow.

Yel-
low.

White. Wh. -yellow. Gold-yellow. Orange.

Wh. -yellow. Yellow. Yellow.

Wh. -yellow. Gr.-yellow.

Green.

These are the mixed colors. But it is to be observed that only two
new color-sensations can be produced, white and purple, the remain-
ing mixed colors already finding their equivalent in the spectrum.
White and purple, therefore, are color-sensations which have no
objective equivalent in a simple number of ether- vibrations like the
spectral colors.

Two spectral colors which by their mixture produce the sensation
of white are called complementary colors. Such are red and green-
blue, golden yellow and blue, green and violet. The mixture of all
the spectral colors produces white again. This is the result of
adding two or more color-sensations. Different results are obtained,
however, by adding color pigments. Yellow and blue, for example,
produce in the eye white, but on the painter's palette green. The
colors of nature are usually mixtures of simple colors, as can be shown
by spectroscopic analysis or by a synthesis of spectral colors.

In all color-sensations wre must distinguish three primary qualities :
(1) hue; (2) purity or tint; (3) brightness or luminosity. The first
quality gives the main name to the color — e. g., red or blue — this de-
pending on the spectral color or the mixture of two spectral colors
with which it can be matched. The second quality, the tint, depends
on the admixture of white with the ground color; and the third quality,
brightness, depends on the objective intensity of the light and the
subjective sensitiveness of the retina. Color-perception thus far refers
only to the most sensitive part of the retina. At the more peripheral
parts of the retina the colors are seen somewhat differently, as is
shown by the following table giving the limits up to which the colors
are recognized:

White. Blue. Red. Green.

Externally oo° So0 6s° 500

Internally 6o° S5° So0 400

Superiorly 4s0 4°° 3^° 30°

Inferiority 700 6o° 450 35°

Theories of Color-perception. — The theory of v. Helmholtz,

originated by Thomas Young (1807), assumes in its latest form the
existence in the human retina of three different kinds of end-organs.

686 TEXT-BOOK OF PHYSIOLOGY.

each of which is loaded with its own photo-chemical substance capable
of being decomposed by a certain color, and thus exciting the fiber
of the optic nerve.

In the first group these end-organs are loaded with a red-sensitive
substance, which is affected mainly by the red part of the spectrum;
the second group has its end-organs provided with a green-sensitive
substance, which is mainly excited by the green color; while the third
group is provided with a blue-sensitive substance, this latter being
mainly affected and decomposed by the blue-violet portion of the
spectrum. All these three different end-organs are present in every
part of the most sensitive area of the retina, and are connected by
separate nerve-fibers with special parts of the brain, in the cells of
which each calls up its separate sensation of red or green or blue.

Out of these three primary color-sensations all other color-sensa-
tions arise. If a light mainly excites the red- or green- or blue -sensi-
tive substance of a retinal area, we term it red, green, or blue, re-
spectively But if two of these photo-chemical substances are stimu-
lated simultaneously, quite different sensations arise. Thus simul-
taneous stimulation of the red and green substances gives rise to the
sensation of yellow, that of red and blue to the sensation of purple,
and that of blue and green to the sensation of blue-green. Simul-
taneous stimulation of all three substances of a certain area produces
the sensation of white. According to this theory, complementary
colors are all those which together excite all the three substances.
Color-blindness is explained by this theory, on the assumption that
two of the photo-chemical substances have become similar or equal
in composition to each other.

The theory of Hering, brought forward in 1874, has the under-
lying assumption that the process of restitution in a nerve-element
is capable of exciting a sensation. This theory asserts that there are
three visual substances in the retina — a white-black, a red-green, and
a yellow-blue visual substance. A destructive process in the white-
black substance, such as is induced not only by white light, but also
by any other simple or mixed color, produces the sensation of white,
while the process of restitution or assimilation in this substance pro-
duces the sensation of black. Similarly, red light produces clis-
p,Ssimilation or decomposition in the red-green substance, and this,
again, the sensation of red. Green light, however, favors the process
<>\ restitution or assimilation in the red-green substances, and thus
gives rise to the sensation of green. In the same way the sensation
of yellow has its cause in the decomposition of yellow-blue substance
induced by yellow light, while the sensation of blue is produced by
an assimilative process in the same substance. Simultaneous processes
of disassimilation and assimilation in the same visual substance an-
tagonize each other, and consequently produce no color-sensation
by means of this substance, but only the sensation of white, by reason
of decomposition, by both colors, in the white-black substance. Thus,

THE SENSE OF SIGHT.

687

yellow and blue, impinging on the same retinal area, have no effect
on the yellow-blue substance, because they are antagonistic in their
action on this substance, but only produce the sensation of white, as
both yellow and blue decompose the white-black material. Color-
blindness is explained by the assumption of the absence of either the
red-green or the yellow-blue visual substance in the retina.

Accessory Structures. — The eyeball is protected anteriorly by
the eyelids and their associated structures, the Meibomian glands,
the lachrymal glands, and tears.

The eyelids consist of a central framework of connective tissue
supporting muscle tissue (the orbicularis palpebrarum muscle) and
glands, and covered externally by skin and internally by a modified
skin, the conjunctiva. The free border of each lid is strengthened

Ai IfHSte

&

Fig. 337. — The Lacrimal and Meibomian Glands, and Adjacent 'Organs of the
Eye. i, 1. Inner wall of orbit. 2, 2. Inner portion of orbicularis palpebrarum. 3, 3.
Attachment to circumference of base of orbit. 4. Orifice for transmission of nasal artery.
5. Muscle of Horner (tensor tarsi). 6, 6. Meibomian glands. 7. 7. Orbital portion of
lacrimal gland. 8,9,10. Palpebral portion, n, 11. Mouths of excretory ducts. 12,13.
Lacrimal puncta. — (Sappey.)

by a semilunar plate of dense fibrous tissue, the tarsus. The cuta-
neous edge of the lid is bordered with short stiff hairs. At the inner
extremity each eyelid presents a small opening, the pu net um lacrimale,
the beginning of the lachrymal duct. The two ducts after uniting
open into the nasal duct.

The Meibomian glands are modified sebaceous glands imbedded
in the posterior portion of the lids (Fig. 337). Their ducts open on
the free border of the lid. These glands secrete an oleaginous ma-
terial resembling sebaceous matter which accumulates along the
margin of the lid and prevents the tears from flowing down the cheek.

The lachrymal gland is situated at the upper and outer part of the
orbit cavity. It consists of a series of compound tubules lined by

6S8 TEXT-BOOK OF PHYSIOLOGY.

epithelium. The secretion (the tears) is conducted from the gland
to the outer part of the conjunctiva by seven or eight ducts. The
lachrymal secretion consists of water and inorganic salts. It is dis-
tributed over the corneal surface during the act of winking, thus
keeping it moist and free from foreign particles. It eventually passes
into the lachrymal ducts and then into the nose. The lachrymal glands
receive secretory fibers by way of the fifth nerve and the cervical
sympathetic. The secretion can be excited reflexly from stimulation
of sensor nerves as well as by emotional states.

CHAPTER XXVIII.
THE SENSE OF HEARING.

The physiologic mechanism involved in the sense of hearing in-
cludes the ear, the auditory nerve, its cortical connections, and nerve-
cells in the cortex of the temporal lobe.

Peripheral excitation of this mechanism develops nerve impulses
which, transmitted to the cortex, evoke the sensation of sound and
its varying qualities— intensity, pitch, and timbre.

The specific physiologic stimulus; to the terminal organ, the organ
of Corti, is the impact of atmospheric undulations of varying energy
and rapidity.

THE PHYSIOLOGIC ANATOMY OF THE EAR.

The ear, the organ of hearing, is lodged within the petrous portion
of the temporal bone. It may, for convenience of description, be
divided into three portions: viz., the external, the middle, and the
internal portions (Fig. 338).

The external ear consists of the pinna or auricle and the external
auditory canal. The pinna is composed of a thin layer of cartilage
which presents a series of elevations and depressions. It is attached
by fibrous tissue to the outer edge of the auditory canal and covered
by a layer of skin continuous with that covering adjacent structures.
The general shape of the pinna is concave. Its anterior surface pre-
sents, a little below the center, a deep depression — the concha.

The external auditory canal extends from the concha inward for
a distance of from 25 to 30 mm. It is directed at first upward, for-
ward, inward, and then downward to its termination. It is composed
partly of bone and partly of cartilage and lined by a reflection of
the skin covering the pinna. At the external portion of the canal
the skin contains a number of tubular glands, the ceruminous glands,
which resemble in their conformation the perspiratory glands. They
secrete cerumen or ear-wax.

The middle ear, or tympanum, is an irregularly shaped cavity
hollowed out of the temporal bone and situated between the external
auditory canal and the internal ear. It is narrow from side to side,
though wider above than below. It is relatively long in its antero-
posterior and vertical diameters. The upper portion is known as the
attic. The middle ear is in communication posteriorly with the
mastoid cells, anteriorly with the pharynx through the Eustachian
tube.

The Eustachian Tube. — The passageway between the tympanic

44 689

690

TEXT-BOOK OF PHYSIOLOGY

cavity and the nasopharynx is known from its discoverer as the
Eustachian tube. It is composed internally of bone, externally of
cartilage, and is lined by mucous membrane covered with ciliated
epithelium. Near the middle of its course the tube is contracted,
though expanded at either extremity (Fig. 338). It measures about
40 mm. in length. Its general direction from the pharyngeal orifice
is outward, backward, and upward at an a.ngle of about 45 degrees.
The middle ear cavity is separated from the external ear by a
membrane — the membrana tympani — and from the internal ear by
an osseo-membranous partition which forms a common wall for both

Fig. 338. — The Ear. i. Pinna, or auricle. 2. Concha. 3. External auditory canal.
4. Membrana tympani. 5. Incus. 6. Malleus. 7. Manubrium mallei. S. Tensor tym-
pani. 9. Tympanic cavity, to. Eustachian tube. 11. Superior semicircular canal.
12. Posterior semicircular canal. 13. External semicircular canal. 14. Cochlea. 15.
Internal auditory canal. 16. Facial nerve. 17. Large petrosal nerve. 18. Vestibular
branch of auditory nerve. 19. Cochlear branch. — (Sappey.)

cavities. The interior of the cavity is crossed from side to side by a
chain of bones and lined by a mucous membrane continuous with
that lining the pharynx.

The membrana tympani is a thin, translucent, nearly circular
membrane, measuring about 10 mm. in diameter, placed at the inner
termination of the external auditory canal. It is inclosed in a ring
of bone which in the fetal condition can be easily removed, but in
the adult condition can not be removed, owing to its consolidation
with the surrounding bone. This membrane consists primarily of
a layer of fibrous tissue which is covered externally by a thin layer
oi skin continuous with thai lining the auditor}' canal, and internally

THE SENSE OF HEARING.

691

by a thin mucous membrane. The tympanic membrane is placed
obliquely at the bottom of the auditory canal, inclining from above
and behind downward and forward at an angle of about forty-five
degrees. The external surface of this membrane presents a funnel-
shaped depression, the sides of which are slightly convex.

The Ear-bones. — Running across the tympanic cavity and form-
ing an irregular line of joined levers is a chain of bones, which articu-
late one with another at their extremities. These bones are known
as the malleus, incus, and stapes. The form and arrangement of these
bones are shown in Figs. 339, 340.

The malleus, or hammer bone, consists of a head, neck, and handle,
of which the latter is attached to the inner surface of the membrana
tympani. The incus or
anvil bone presents a con-
cave articular surface which
receives the head of the
malleus. The stapes, or
stirrup-bone, articulates ex-
ternally with the long pro-
cess of the incus, and in-
ternally, by its oval base,
with the edges of an oval
opening, the foramen ovale.
The entire chain is partially
supported by a ligament
attached to the short pro-
cess of the incus and to the
walls of the tympanic cavity.

The Tensor Tympani
Muscle. — This is a delicate
muscle, about 15 mm. in
length, situated in a nar-
row groove just above the
Eustachian tube (Fig. 341).
It arises from the cartila-

Fig. 339. — Tympanic Membrane and the Audi-
tory Ossicles (Left) seex from within, i. c
from the Tympanic Cavity. M. Manubrium
or handle of the malleus. T. Insertion of the
tensor tympani. h. Head. IF. Long process of
the malleus, a. Tncus. with the short (K) and the
long (I) process. 5. Plate of the stapes. Ax,
Ax, is the common axis of rotation of the auditory
ossicles. Sz. The pinion-wheej arrangement be-
tween the malleus and iro;s. — (Landois.)

ginous portion of the Eusta-
chian tube and the adjacent portion of the sphenoid bone. From
this origin it passes nearly horizontally backward to the tympanic
cavity; just opposite to the foramen ovale, its tendon bends at a right
angle over the processus cochleariformis and then passes outward
across the tympanic cavity to be inserted into the handle of the malleus
near the neck.

The stapedius muscle emerges from the cavity of a pyramid of
bone which projects from the posterior wall of the tympanum. Its ten-
don passes forward to be inserted into the neck of the stapes bone
near its point of articulation with the incus.

The internal ear, or labyrinth, is located within the petrous

6a:

TEXT-BOOK OF PHYSIOLOGY.

Fig. 340. — Audi-
tory Ossicles, i
Head of malleus. 2
Processus brevis. 3
Processus gracilis
4. Manubrium. 5
Long process of in-
cus. 6. Articulation
between incus and
stapes. 7. Stapes.
— (Sappey.)

portion of the temporal bone. It consists of an osseous and a mem-
branous portion, the latter contained within the former.

The osseous labyrinth is subdivided into vestibule, semicircular
canals, and cochlea.

The vestibule is a small, triangular-shaped cavity
between the semicircular canals and the cochlea. It
is separated from the cavity of the middle ear by an
osseous partition which presents near its center an
oval opening, the foramen ovale. In the living con-
dition this opening is closed by the base of the
stapes bone, which is held in position by an annular
ligament. The inner wall presents a number of
openings for the passage of nerve-fibers (Fig. 342).
The semicircular canals are three in number, a
superior vertical, an inferior vertical, and a hori-
zontal, each of which opens by two orifices into the
cavity of the vestibule, with the exception of the two
vertical, which unite at one extremity and then open
by a single orifice. Each canal near its vestibular
orifice is enlarged to almost twice the size of the
rest of the canal, forming what is known as the ampulla.

The cochlea, the anterior portion of the labyrinth, is a gradually
tapering canal, about 35 mm. in length, wound spirally two and a
half times around a central bony axis, the modiolus. The cavity of
the cochlea is partially subdivided into
two cavities by a thin spiral plate of bone
which projects from the inner wall,
known as the lamina osseous spiralis. In
the natural condition this partition is
completed by a connective-tissue mem-
brane, so that the two passages are com-
pletely separated from each other. The
upper passage or scala is in free com-
munication with the vestibule, and is
known as the scala vestibuli; the lower
passage or scala in the dead condition
communicates with the tympanum by
means of a round opening, the foramen
rotundum, and is therefore known as the
scala tympani. In the living condition

this opening is completely closed by a membrane, a second membrana
tympani. Both the scalse vestibuli and tympani communicate at the
apex of the cochlea by a small opening, the helicotrema. The modiolus,
the central bony axis, is perforated from base to apex by a canal for
the passage of the auditory nerve-fibers; lateral canals, diverging from
the central canal, pass through the osseous lamina spiralis and transmit
fibers of the auditory nerve. The interior of the bony labyrinth is

Fig. 341. — M, The Tensor
Tympani Muscle — the Eus-
tachian Tube (Left). —
(Landois.)

THE SENSE OF HEARING.

6Q3

Fig. 342. — Boxy Cochlea, i.
Ampulla of superior semicircular
canal. 2. Horizontal canal. 3.
Junction of superior and posterior
semicircular canals. 4. The pos-
terior semicircular canal. 5. Fora-
men rotundum. 6. Foramen ovale.
7. Cochlea.

lined by periosteum covered by epithelium and in communication
with lymph-spaces at the base of the skull by means of the aqueduct
of the vestibule.

The membranous labyrinth, lying within the osseous labyrinth,
corresponds with it in form, though it is smaller in size. It mav be
subdivided into vestibule, semicircular canals, and cochlea (Fig. 343)

The vestibular portion consists of two
small sacs, the utricle and the saccule,
which communicate .with each other by
means of the two branches of a duct
passing through the aqueduct of the
vestibule — the ductus endolympkaticus.

The semicircular canals communicate
with the utricle in the same manner as
the bony canals communicate with the
vestibule. The saccule communicates
with the membranous cochlea by a short
canal, the canalis reuniens. The walls
of the utricle, saccule, and semicircular
canals are composed of connective tissue
lined by epithelium. At the points of
entrance of the auditory nerve, ihemaculce

acusticce, in all three structures, the epithelium undergoes a marked
change in appearance and structure. It becomes columnar in shape
and provided with stiff hair-like processes or threads, which projecl
into the cavity. In the saccule and utricle the hair-like processes are
covered by a layer of small crystals of calcium carbonate held together
by a gelatinous material. The crystals are known as otoliths (Fig. 344).
The fibers of the vestibular nerve, arising from the cells of the
ganglion of Scarpa in the internal auditory
meatus, send their peripherally directed
branches through the foramina in the inner
wall of the vestibule, through the walls of
the utricle and semicircular canals near the
ampulla. As the fibers approach the macula?
acusticse they subdivide into delicate fibrillar,
which ultimately become histologically and
physiologically related to the neuro-epithe-
lium. From the relation of the nerve-fibers
to the epithelium, the latter must be re-
garded as the highly specialized terminal.
organ of the vestibular portion of the auditory nerve.

The cochlea is a closed membranous tube situated between the
osseous lamina spiralis and the outer bony wall. A transection of the
entire cochlea shows the relation of the osseous and membranous
portions (Fig. 345). The cochlear tube is triangular in shape. The
base is attached to the bony wall, the apex to the edge of osseous

Fig. 343. — 1. Utricle. 2.
Saccule. 3. Vestibular end of
cochlea. 4. Canalis reuniens.
5. Membranous cochlea. 6.
Membranous semicircular

canal. — (Potters "Anatomy.")

694

TEXT-BOOK OF PHYSIOLOGY.

Fig. 344. — Section of Wall of
Utricle of the Internal Ear,
through macular region, from
rabbit, shewing otoliths (0), em-
bedded within granular substance
(g). h. Ciliated cells with proc-
esses, (p), extending between
sustentacular elements (s). m.
Basement membrane. 11. Nerve-
fibers within fibrous tissue (I)
passing toward hair-cells and
becoming non-medullated at base-
ment-membrane.— (After Piersol.)

lamina spiralis. One side of the tube forms in part the membrane of

Reissner, the other side forms in part the basilar membrane. The

sides of the cochlea toward the scala vestibuli and scala tympani are

covered with epithelium. The triangular cavity of the cochlear tube

is known as the scala media. The inner
surface of the cochlear tube is lined by
epithelium, which becomes extraordi-
narily modified and specialized along the
surface of the basilar membrane, to con-
stitute what is known as —

The Organ of Corti.— In Fig. 345
this organ is represented as it appears
on cross-section of the cochlea. It con-
sists primarily of an arch composed of
two modified epithelial cells known as
the rods or pillars of Corti, which rest
below on the basilar membrane, but meet
and interlock above; it consists second-
arily of a series of columnar epithelial
cells provided with hair-like processes
which rest upon and are supported by
the rods both on the inner and outer
aspects of the arch. The space beneath
the arch is known as the tunnel. The

inner hair cells are not nearly so numerous as the outer hair cells

The epithelial cells external to the outer and inner hair cells are sup

porting or sustentacular in character.

The organ of Corti extends the entire length of the cochlea. The

number of rods which, standing

side by side, form the inner limb

of the arch is estimated at 5600;

the number which form the outer

limb is estimated at 3850. The

outer rods are broader than the

inner and at some places articu-
late with two or three inner rods.

The upper edges of the rods are

flattened, elongated, and project

outward, forming a reticulated

membrane through the meshes

.of which the hair^ike processes

of the cells project.

From the connective-tissue

thickening on the upper surface

of the osseous lamina spiralis there extends outward over the organ of

Corti a thin membrane, the membrana lectoria. The common cavity

between the walls of the osseous and membranous labyrinth in the

Fig. 345. — A Transverse Section of a
Turn of the Cochlea.

THE SENSE OF HEARING. 695

vestibule, the semicircular canals, in the scala vestibuli and scala
tympani of the cochlea, is filled with a clear fluid — the perilymph; the
common cavity within the walls of the entire membranous labyrinth
is also filled with a similar fluid — the endolymph. The hair-like proc-
esses of the epithelial cells covering the maculae acusticae and the
rods of Corti are consequently bathed by endolymph. Both fluids
are in relation with the subarachnoid lymph-spaces at the base of the
brain, the perilymph through the aqueduct of the vestibule, the endo-
lymph through the endolymphatic duct.

The fibers of the cochlear nerve, arising from the ganglion cells
of the spiral ganglion situated in the osseous lamina spiralis near
the modiolus, send their peripheral branches to the saccule and to
the organ of Corti. As they approach this structure they lose their
medullary sheath and become naked axis-cylinders. The fibers then
divide into two parts, of which one passes to the inner hair cells; the
other passes between the inner rods and crosses the tunnel between
the outer rods to the outer hair cells. The exact method of termina-
tion of these fibers in the hair cells is unknown, but doubtless it is
both histologic and physiologic.

From the relation of the nerve-fibers to the organ of Corti the
latter must be regarded as the highly specialized terminal organ of
the cochlear division of the auditory nerve.

THE PHYSIOLOGY OF AUDITION.

The general function of the ear is the reception and transmission
of atmospheric vibrations from the concha to the percipient elements
— the hair cells — of the organ of Corti. The vibratory excitation of
these end-organs thus caused, is the basis of auditory perceptions.
The atmospheric vibrations are collected by the pinna and concha,
conveyed by the auditory canal to the tympanic membrane, trans-
mitted by the chain of bones to the labyrinth to pass successively
through the perilymph, the membranous walls, the endolymph, to
the hair cells. The nerve impulses generated by these vibrations are
then transmitted by the cochlear nerve to the auditory centers of the
cerebrum, where the sensations of sound are evoked. In order to
appreciate the function of the individual structures concerned in this
general function there must be kept in mind a few of the character-
istics of atmospheric vibrations.

Atmospheric Vibrations. — The vibrations of the atmosphere
which are the objective causes of the sensations of sound are com-
municated to it by the vibrations of elastic bodies such as tuning-
forks, rods, strings, membranes, etc. These produce in the air a
to-and-fro movement of its particles, resulting in a succession of
alternate condensations and rarefactions which are propagated in
all directions. The impact of a rhythmic succession of such con-
densations on the ear gives rise to musical sounds; the impact of an
arrhvthmic or irregular succession gives rise to noises.

696 TEXT-BOOK OF PHYSIOLOGY.

If a writing point attached to a tuning-fork in vibration be placed
in contact with a traveling recording surface, each vibration will be
recorded in the form of a wave. For this reason atmospheric vibra-
tions are generally spoken of as sound-waves. A line drawn hori-
zontally through such a curve indicates the position of rest of the fork;
the extent of the curve on each side of this line indicates the excursion
of the fork or the amplitude of its movement.

The sounds which physiologically result from the impact and
transmission of the effects of sound-waves, possess intensity, pitch, and
quality or tone.

The intensity or loudness of a sound depends on the amplitude
of the vibration which causes it. The greater the amplitude or
swing of the vibrating body, the greater is the energy with which it
strikes the ear.

The pitch of a sound depends on the number of vibrations which
strike the ear in a unit of time— a second. The greater the number,
the higher the pitch. Thus while the pitch of the sound caused by
the note C, on the first leger line below, of the music scale, corre-
sponds to 256 vibrations, the pitch of the sound caused by the note C
an octave above, corresponds to 512 vibrations. The lowest rate of
vibration which can produce a distinct sound varies in different
individuals from 14 to 18; the highest rate varies from 35,000 to
40,000 per second. Between these two extremes lies the range of
audibility, which embraces about 11 octaves. Vibrations less than
14 per second can not be perceived as a continuous sound; vibrations
beyond 40,000 also fail to be so perceived. In the ascent of the
music scale from the lowest to the highest regions there is a gradual
increase in the vibration frequency.

The quality of a sound depends on the form of the vibration. It
is this feature which gives rise to those differences in sensations which
permit one to distinguish one instrument from another when both
are emitting the same note. The form of the sound-wave in any
given instance is the resultant of a combination of a fundamental
vibration and certain secondary vibrations of subdivisions of the
vibrating body. These secondary vibrations give rise to what is
known as overtones. By their union with and modification of the
fundamental vibration there is produced a special form of vibration
which gives rise not to a simple but a composite sensation. It is
for this reason that the same note of the piano, the violin, and the
human voice varies in quality.

The Function of the Pinna and External Auditory Canal.—
In those animals possessing movable ears the pinna plays an im-
portant part in the collection of sound-waves. In man it is doubtful
if ii plays a part at all necessary for hearing. Nevertheless an indi-
vidual with defective hearing may have the perception of sound
increased by placing the pinna at an angle of 90 degrees to the side
of the head or by placing the hand behind it. The external auditory

THE SENSE OF HEARING. 697

canal transmits the sonorous vibrations to the tympanic membrane
From the obliquity of this canal it has been supposed that the vibra-
tions, after passing the concha, undergo a series of reflections on their
way to the tympanic membrane, which, owing to its inclination, would
be struck by them in a much more effective manner.

The Function of the Tympanic Membrane. — The function of
the tympanic membrane is the reception of the atmospheric vibrations
which are transmitted to it. This it does by vibrating in unison with
them. The vibrations which the membrane exhibits correspond in
amplitude, in frequency, and in form to those of the atmosphere.
That this membrane actually reproduces all vibrations within the
range of audibility has been experimentally demonstrated. The
membrane not being fixed, as far as its tension is concerned, does
not possess a fixed fundamental note, like a stationary fixed mem-
brane, and is therefore just as well adapted for the reception of one
set of vibrations as another. This is made possible by variations in
its tension in accordance with the pitch of frequency of the atmos-
pheric vibrations. In the absence of vibration the membrane is in
a condition of relaxation; with the advent of sound-waves possessing
a gradual increase of pitch, as in the ascent of the music scale, the.
tension of the membrane increases until its maximum is reached at
the upper limit of the range of audibility. By this change in tension
certain tones become perceptible and distinct, while others become
imperceptible and indistinct.

The Function of the Tensor Tympani Muscle. — The function
of this muscle is, as its name indicates, to change and to fix the tension
of the tympanic membrane, so that it can most readily vibrate in unison
with vibrations of varying degrees of rapidity. The tendon of this
muscle playing around the processus cochleariformis is attached
almost at a right angle to the handle of the malleus. Hence as the
muscle contracts it exerts its traction from the process and draws the
handle of the malleus inward, thus increasing the convexity of the
tympanic membrane and at the same time its tension. With the
relaxation of the muscle the handle of the malleus passes outward,
and the convexity and tension diminish.

In the ascent of the music scale, each note corresponding to an
increase in vibration frequency, requires for its perception an increase
in tension and an increase in the force of the contraction of the tensor
muscle. In the descent of the music scale the reverse conditions
obtain. The contraction of the muscle is of the nature of a single
twitch, and of just sufficient force and duration to tense the membrane
for a given rate of vibration.

The contraction of the muscle is excited rerlexly. The afferent
path is through fibers of the trigeminal nerve distributed to the tym-
panic membrane; the efferent path is through fibers in the small root
of the trigeminal. The stimulus is sudden pressure on the tympanic
membrane. The more frequently and forcibly the stimulus is applied.

698 TEXT-BOOK OF PHYSIOLOGY.

the greater is the muscle response. The tensor tympani muscle may
therefore be regarded as an accommodative apparatus by which the
tympanic membrane is adjusted for the reception of vibrations of
varying degress of frequency.

The Function of the Chain of Bones. — The function of the
chain of bones is to transmit the effects of the atmospheric vibrations
to the fluid of the labyrinth. The manner in which this is accom-
plished becomes evident from the relation which the bones of this
chain bear to one another and to the tympanic membrane on the
one hand and to the fluid of the labyrinth on the other.

When pressure is made on the outer surface of the tympanic mem-
brane it is at once pushed inward, carrying with it the handle of
the malleus, the head at the same time rotating outward around an
axis corresponding to its ligamentous attachments. As the handle
moves inward a small ledge of bone just below the malleo-incudal
joint locks with, and hence pushes inward, the long process of the
incus. Since this process is united at almost a right angle to the
stapes bone, the latter is forced toward and into the foramen ovale,
thus producing a pressure on the perilymph. With the cessation of
the pressure the elastic forces of the membrane and of the ligaments
return the handle of the malleus to its former position; by the un-
locking of the malleo-incudal joint the entire chain also returns to
its former position without exerting undue traction on the basal attach-
ment of the stapes.

As the long process of the incus is shorter than the handle of the
malleus, and as the movement between them takes place around an
axis from before backward, it follows that the excursion of the incus
and stapes will be less than that of the malleus, while the force will
be greater. Hence as the vibrations are transferred from the tym-
panic membrane of large area to the base of the stapes of small area
(20 to 1.5), they lose in amplitude but increase in force. Their pres-
sure on the perilymph is therefore thirty times greater than on the
membrana tympani. In addition to its function as a transmitter of
vibrations, the chain of bones serves as a point of attachment for
muscles which regulate the tension of the tympanic membrane and
the pressure on the labyrinth.

The Function of the Stapedius Muscle. — The function of the
stapedius muscle is a subject of much discussion. According to
Henle, its function is to so adjust the stapes bone that it will be pre-
vented from exerting an undue pressure on the perilymph during the
inward excursions of the incus process. According to Toynbee,
its function is to press the posterior part of the stapes inward, make-
it a fixed point, and place the anterior part in such a position that
it will vibrate freely and accurately.

The Function of the Eustachian Tube.— In order that the tym-
panic membrane may vibrate freely it is essential that the air pressure
on both sides shall be equal at all times. This is made possible by

THE SENSE OF HEARING. 699

the Eustachian tube. Were it not for this passageway, with each
inward swing of the membrane the air in the tympanic cavity would
be condensed and its pressure raised, in consequence of which the
movement of the membrane would be retarded; with each outward
swing, the air would be rarefied and its pressure lowered below that
of the atmosphere, and in consequence the movement outward would
be retarded; the maximum response, therefore, of the membrane to
a given vibration could not be attained and the resulting sound would
be muffled and indistinct. But as with each vibration of the mem-
brane the air can pass into and out of the tympanum through this
tube, inequalities of pressure are prevented and a free vibration per-
mitted.

The impairment in the acuteness of hearing which is caused by
either a rise or fall of pressure in the middle ear can be shown —

1. By closing the moutH and nose and then forcing air from the

lungs through the Eustachian tube into the tympanum, thus in-
creasing the pressure.

2. By closing the mouth and nose and then making an effort of

deglutition. As this act is attended by an opening of the phar-
yngeal end of the Eustachian tube, the air in the tympanum is
partly withdrawn and the pressure lowered. In each instance
hearing is impaired. After either experiment the normal con-
dition is restored by swallowing with the nasal passages open.
The Functions of the Internal Ear. — From the anatomic
arrangement of the structures of the internal ear it is evident that if
the vibrations of the stapes bone are to reach the peripheral organs —
the hair cells — of both the vestibular and cochlear nerves, they must
traverse successively the perilymph, the membranous walls, and the
endolymph. As the perilymph is incompressible, the inward move-
ment of the stapes would be prevented were it not for the elastic
character of the membrane closing the foramen rotundum. The
pressure wave occasioned by each inward movement of the stapes
is transmitted through the scala vestibuli, the helicotrema, the scala
tympani, to this membrane, which by virtue of its elasticity is pressed
into the tympanic cavity. With the outward movement of the stapes,
equilibrium is at once restored.

The Functions of the Cochlea. — The cochlea is the portion
of the internal ear which is concerned in the perception of tones.
The arrangement of the histologic elements of the organ of Corti
indicates that they in some way respond to the vibrations of varying
frequency and form, and through the development of nerve impulses,
evoke the sensations of pitch and quality. The manner in which
this is accomplished is largely a matter of speculation. While many
theories have been offered in explanation of the power to distinguish
the pitch and the quality or timbre of a tone, most physiologists prefer
that of Helmholtz, who regarded the transverse fibers of the basilar
membrane as the elements immediately concerned, and compared

7oo TEXT-BOOK OF PHYSIOLOGY.

them, both in their arrangement and power of sympathetic vibration,
with the strings of a piano. He said: "If we could so connect every
string of a piano with a nerve-fiber that the nerve-fiber would be
excited as often as the string vibrated, then, as is actually the case
in the ear, every musical note which affected the instrument would
excite a series of sensations exactly corresponding to the pendulum-
like vibrations into which the original movements of the air can be
resolved; and thus the existence of each individual overtone would
be exactly perceived, as is actually the case with the ear. The per-
ception of tones of different pitch would, under these circumstances,
depend upon different nerve-fibers, and hence would occur quite
independently of each other. Microscopic investigation shows that
there are somewhat similar structures in the ear. The free ends of
all the nerve-fibers are connected with small elastic particles which
we must assume are set into sympathetic vibration by sound-waves."
(Stirling.)

The mechanism might be regarded, therefore, somewhat as
follows: The sound-waves received by the membrana tympani and
transmitted by the chain of bones to the fenestra ovalis produce
variable pressures in the fluids of the internal ear; these pressures
varv in intensity, in number, and in quality, and correspond with
the intensity, pitch, and quality of the tones. If, therefore, a com-
pound wave of pressure be communicated by the base of the stapes,
it will be resolved into its constituents by the different transverse
fibers of the basilar membrane, each picking out its peculiar portion of
the wave and thus stimulating corresponding nerve filaments. Thus
different nerve impulses are transmitted to the brain, where they are
fused in such a manner as to give rise to a sensation of a particular
quality, but still so imperfectly fused that each constituent, by a
strong effort of attention, may be still recognized. The transverse
fibers of the basilar membrane vary in length from 0.04155 mm. at
the base of the cochlea to 0.495 mm- at the aPex> and, according to
Retzius, are about 24,000 in number. As the human ear usually
cannot distinguish more than 11,064 tones, it is evident that there
is a sufficient anatomic basis for this theory.

The functions of the semicircular canals have already been
stated in connection with the chapter relating to the functions of the
cerebellum.

CHAPTER XXIX.
REPRODUCTION.

Reproduction is the process by which a new individual is initiated
and developed and the species to which it belongs is preserved. Re-
production is the result of the union and subsequent development
of germ- and sperm-cells. These cells are produced and their union
accomplished by the cooperation of the reproductive organs charac-
teristic of the two sexes.

Embryology is a department of anatomic science which has for
its object the investigation of the successive stages that the new being
passes through during its gradual development prior to birth.

THE REPRODUCTIVE ORGANS OF THE FEMALE.

The reproductive organs of the female comprise the ovaries,
Fallopian tubes, uterus, and vagina (Fig. 346).

The Ovaries. — The ovaries are two small, flattened bodies,
measuring about 40 mm. in length and 20 in breadth. They are
situated in the cavity of the pelvis, one on either side, and embedded
in a fold of the peritoneum, known as the broad ligament. A section
of the ovary shows that it consists externally of a thin, firm, connective-
tissue membrane and internally of a fine connective-tissue stroma,
supporting blood-vessels, non-striated muscle-fibers and nerves, and
containing in its meshes a very large number of spheric sacs named
after their discoverer, de Graaf , the Graafian sacs or follicles. These
follicles are very numerous and are present in all portions of the
ovary, though they are most abundant toward its peripheral portions.
It is estimated that the human ovary contains from 20,000 to 40,000
follicles. The follicles vary considerably in size; while many are visible
to the unaided eye, others require for their detection high powers of
the microscope. Although the follicles are present in the ovary at
the time of birth, it is not until the period of puberty that they assume
functional activity.

From this time on to the catamenial period there is a constant
growth and development of these follicles. Each follicle consists of
an external investment of fibrous tissue and blood-vessels, and an
internal investment of cells, the membra na granulosa. At the lower
portion of this membrane there is an accumulation of cells, the pro-
ligerous disc (Fig. 347). The cavity of the follicle contains a slightly
yellowish, alkaline, albuminous fluid, a transudate in all probability
from the blood-vessels. The Graafian follicle is of especial interest,

701

702

TEXT-BOOK OF PHYSIOLOGY.

for it is in this structure, and more especially in the proligerous disc,
that the true germ-cell or ovum is developed.

The ovum is a spheric body measuring about 0.3 mm. in diameter.
It consists of a mass of living, protoplasmic material, cytoplasm, a
nucleus or germinal vesicle, and a nucleolus or germinal spot. The
cvtoplasm presents toward its central portion a quantity of granular
material, partly fatty in character, the deutoplasm or vitellus. The
peripheral portion of the cytoplasm is surrounded by a delicate radially
striated border, the zona pellucida or radiata (Fig. 348).

The nucleus consists of a nuclear membrane enclosing contents.
The latter consist of an amorphous material in which is embedded
a network, some of the threads of which have a strong affinitv for cer-

Fig. 346. — Uterus, Fallopian Tubes axd Ovaries; Posterior View. 1,1. Ovaries.
2, 2. Fallopian tubes. 3, 3. Fimbriated extremity of the left Fallopian tube seen from
its concavity. 4. Opening of the left tube. 5. Fimbriated extremity of the right tube,
posterior view. 6, 6. Fimbriae which attach the extremity of each tube to the ovary.
7, 7. Ligaments of the ovary. 8, 8, 9, 9. Broad ligament. 10. Uterus. 11. Cervix
uteri. 12. Os externum. 13, 13. Vagina.

tain staining materials, and hence are known as chromatin, while others
stain less deeply and are known as achromatin.

The Fallopian Tubes. — The Fallopian tubes are about 12 centi-
meters in length and extend from the upper angles of the uterus to
the ovaries. Each tube is somewhat trumpet-shaped, the narrow
portion being close to the uterus, the wide portion close to the ovary.
The outer extremity of the tube is expanded and subdivided, and
presents a series of processes termed fimbriae, one of which is attached
to the ovary. The tube consists of three coats — an external or serous;
a middle or muscular, the fibers of which are arranged longitudinally
and transversely; and an internal or mucous. The surface of the
mucous coat is covered with a layer of ciliated epithelial cells, the
motion of which is toward the uterus.

The Uterus. — The uterus is pyriform in shape and divided into
a bodv and neck. It measures, before the first pregnancy, about

PLATE III.

1 IIAGRAM OF FCETAL CIRCULATION.

W Preyer del

REPRODUCTION.

703

US

-d

^7

7 cm. in length, 5 cm. in breadth and 2\ cm. in thickness. A frontal
section of the uterus shows a central cavity which in the body is tri-
angular in shape, in the neck oval or fusiform (Fig. 349). At the
upper angles of the uterus the cavity is continuous with the cavity of
each Fallopian tube. At the junction of the body and the neck, the
cavity presents a constriction, the internal os. The constriction at
the end of the neck is known as the external os. The walls of the
uterus are extremely thick and composed of non-striated muscle-
fibers arranged in a very j,
complicated manner.
The interior of the uterus
is lined by mucous mem-
brane covered with cylin-
dric ciliated epithelial
cells, the motion of which
is toward the external os.
Tubular glands are found
in large numbers in the
mucous membrane lin-
ing the cavity, while
racemose glands are
found in the mucous
membrane lining the
neck. Owing to the
flattening of the uterus
from before backward
the walls are almost in
contact and the cavity
almost obliterated.

The Vagina. — The
vagina is a musculo-
membranous canal, from
12 to 18 cm. in length,
situated between the rec-
tum and bladder. It
extends from the surface
of the body to the brim
of the pelvis, and em-
braces at its upper extremity the neck of the uterus.

Ovulation. — After the establishment of puberty a Graafian follicle
develops and ripens or matures periodically, usually every twenty-
eight days. During the time of maturation the follicle increases in
size, from an augmentation of its fluid contents, and approaches the
surface of the ovary, where it forms a projection varying from 6 to
12 mm. in size. When maturation is complete the vesicle ruptures,
and the ovum and liquid contents are discharged. The ovum, by a
mechanism not fullv understood, is received bv the fimbriated ex-

W8k

w*

^■ss^zSssM^s^y^Sis

Or

Fig. 347. — Section of Cortex of Cat's Ovary,
Exhibiting Large Graafian" Follicles, a. Per-
ipheral zone of condensed stroma, b. Groups of im-
mature follicles, c. Theca of follicle, d. Membrana
granulosa, e. Discus proligerus. /. Zona pellucid a.
g. Yitellus. /;. Germinal vesicle, i. Germinal spot.
k. Cavity of liquor folliculi. — (After Piersol.)

■04

TEXT-BOOK OF PHYSIOLOGY.

tremity of the Fallopian tube and enters its cavity. The ovum is
then transferred through the tube by the peristaltic contraction^of
its muscle-fibers and by the action of the cilia of its lining epithelium.
The time occupied in the transference of the ovum from the ovary
to the interior of the uterus has been estimated to be from four to
ten days.

Either at the time, or very shortly after, its discharge from the
follicle, the ovum, and more especially the nucleus, undergoes a series
of histologic changes which eventuates in an extrusion of a portion
of the chromatin material. The extruded portions are known as the
polar bodies. The non-extruded portion of the chromatin material

vl

: \

Fig. 348. — Ovum of a Cow. i. Zona
pellucida. 2. Cytoplasm, vitellus. 3. Nu-
cleus, germinal vesicle. 4. Nucleolus, germ-
inal spot. 5. Corona radiata. The radial
striation of the zona pellucida can not be
seen. — (Stohr.)

Fig. 349. — Frontal Sec-
tion of the Uterus. 1. Cav-
ity of the body. 2, 3. Lateral
walls. 4,4. Cornua. 5. Os
internum. 6. Cavity of the
cervix. 7. Arbor vitae of the
cervix. 8. Os externum. 9.
Vagina. — (Sappey.)

is known as the female pronucleus. The succession of changes which
the nucleus undergoes is termed maturation. As the nucleus is
regarded as the part of the ovum which transmits parental character-
istics it is assumed that the extrusion of a portion of the nuclear
material is a means by which an excess of inherited substance is
prevented.

Menstruation. — Menstruation is a periodic discharge of blood
and mucus from the surface of the mucous membrane of the uterus,
and occurs about every twenty-eight days. The duration of the
menstrual period extends over four or five clays and the amount of
blood discharged varies from 180 c.c. to 200 c.c. Menstruation is

REPRODUCTION.

705

usually an accompaniment of ovulation, though the latter process may
take place independently of the former. It is characterized by both
local and systemic changes. The local changes are most marked
in the uterus, the mucous membrane of which increases in thickness
from a proliferation of the connective tissue and a hyperemic condi-
tion of the blood-vessels. Subsequently to these changes the epithe-
lial surface, as well as the more superficial portions of the connective
tissue, undergo degeneration and exfoliation, after which the finer
blood-vessels rupture and permit of an escape of blood into the uterine
cavity. At the end of the menstrual period regenerative changes set
in which continue until the normal condition of the mucous mem-
brane is reestablished.

The Corpus Luteum. — With the rupture of the Graafian follicle
there is an effusion of blood into the follicular cavity which soon
coagulates, loses its color and assumes the characteristics of fibrin.
The walls of the follicle, which have become thickened from the
deposition of a reddish-yellow glutinous substance, now become con-
voluted and undergo a still further hypertrophy, until they encroach
upon and almost obliterate the follicular cavity. In a few weeks the
mass loses its red color and becomes decidedly yellow, when it is
known as the corpus luteum. With the continuance of reparative
changes this body gradually disappears until at the end of two months
nothing remains but a small cicatrix on the surface of the ovary.
Such are the changes in the follicle if the ovum has not been impreg-
nated.

The corpus luteum, after impregnation has taken place, undergoes
a much slower development, becomes larger, and continues during
the entire period of gestation. The difference between the corpus
luteum of the unimpregnated and pregnant condition is expressed in
the following table by 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 condition
of an insignificant cicatrix.

Absent or unnoticeable.

Absent.

Absent.

Larger; convoluted wall bright yel-
low; clot still reddish.

Seven-eighths of an inch in di-
ameter; convoluted wall bright yellow;
clot perfectly decolorized.

Seven-eighths of an inch in diame-
ter; clot pale and fibrinous; convo-
luted wall dull yellow.

Still as large as at the end of second
month; clot fibrinous; convoluted wall
paler.

Half an inch in diameter; central
clot converted into a radiating cicatrix:
externa] wall tolerably thick and con-
voluted, but without any bright yellow
color.

jo6

TEXT-BOOK OF PHYSIOLOGY.

THE REPRODUCTIVE ORGANS OF THE MALE.

The reproductive organs of the male comprise the testicles, vasa
deferentia, vesicular seminales, and penis.

The Testicles. — The testicles are oblong glands, about 40 mm. in
length, 30 mm. in breadth and 20 mm. in thickness, and contained
within the cavity of the scrotum. A section of the testicle (Fig. 350)
reveals the presence externally of a dense fibrous membrane, the tunica
albuginea, and internally a connective-tissue framework consisting
mainly of septa, which enter the organ on its posterior aspect at the
mediastinum testis, passing inward in a diverging manner. The
spaces between the septa are occupied by the true gland substance,
the seminiferous tubules.

The seminiferous tubules are very numerous, the estimate as to

their number varying from 800 to 1000.
When unraveled they measure from 30
to 40 cm. in length and 0.3 mm. in
diameter. At their peripheral extremities
the tubules are very much convoluted,
but as they pass toward the mediastinum
testis, the convolutions disappear, and
after uniting with one another terminate
in from twenty to thirty straight tubes,
the vasa recta, which pass through the
mediastinum and form the rete testis.
At the upper part of the mediastinum
the tubules unite to form from nine to
thirty small ducts, the vasa efferentia,
which soon become very much convo-
luted. After a short course they unite
to form a single tortuous tube, about 7
meters in length and 0.4 mm. in diameter,
which descends behind the testicle to its
lower border. This tube is known as
the epididymis. The seminal tubule con-
sists of a basement membrane lined by granular nucleated epithelium.
The vas deferens, the excretory duct of the testicle, is about 60 cm.
in length and from 2 to 3 mm. in diameter, and extends upward from
the epididymis to the inguinal canal, through which it passes into the
abdominal cavity and then to the under surface of the base of the
bladder, where it unites with the duct of the vesicula seminalis to form
the ejaculatory duct

The vesiculae seminales are two lobulated pyriform bodies, about
40 mm. in length, situated on the under surface of the bladder. Each
vesicula seminalis consists of an external fibrous coat, a middle, mus-
cular coat, and an external mucous coat. The mucous coat contains
a) number of small tubular albumin-producing glands which secrete
a characteristic fluid.

Fig. 350. — Diagram of a Ver-
tical Section throvgh a Tes-
ticle. 1. Mediastinum testis. 2,
2. Trabecular. 3. One of the
lobules. 4, 4. Vasa recta. 5.
Globus major of the epididymis.
6. Globus minor. 7. Vas def-
erens.— {H olden.)

REPRODUCTION.

The ejaculatory duct, formed by the union of the vas deferens and
the duct of the vesicula seminalis, opens into the prostatic portion
of the urethra (Fig. 351).

The prostate gland is a musculo-glandular mass situated at the
posterior extremity of the urethra. It contains a large number of
tubules, more or less branched and convoluted, and lined by columnar
epithelium. They secrete a fluid which is poured into the urethra
at the time of the ejaculation of semen.

The penis consists of three parts: the corpus spongiosum below,
through which passes the urethra, and the two corpora cavernosa,
one on either side and above.
The corpus spongiosum termi-
nates anteriorly in a conic-shaped
structure, the glans penis; the
corpora cavernosa consist ex-
ternally of a fibrous investment
and internally of erectile tissue.
These bodies are abundantly
supplied with blood, which after
entering their substance by the
arteries, passes into sinuses or
reservoirs, from which it is car-
ried away by veins. These ves-
sels pass to the dorsum of the
penis and unite to form a large
vein by which the blood is re-
turned to the general circulation.
By virtue of the erectile tissue
in the corpora cavernosa the
penis becomes erect and rigid
when the blood supply is in-
creased. This takes place in
response to peripheral stimula-
tion or emotional states, or both
combined. When these conditions are established nerve impulses
pass outward through nerves, the nervi erigentes, which have their
origin in the lumbar region of the spinal cord, and bring about an
active dilatation of the arteries and a relaxation of the non-striated
muscle-fibers in the corpora cavernosa. With these events there is a
rapid influx of blood and a distention and an erection of the organ.
This condition is furthered and maintained by a partial compression
of the dorsal vein by the fibrous capsule.

Semen. — The semen is a complex fluid composed of the secretions
of the testicles, the vesiculae seminales, the prostatic tubules, and
urethral glands. It is grayish-white in color, mucilaginous in con-
sistence, characteristic in odor, and somewhat heavier than water.
In response to appropriate stimulation the muscle-fibers in the walls

Fig. 351. — Vas Deferens, Vesicul.e
Seiiinales, and Ejaculatory Ducts.
a. Vas deferens, b. Seminal vesicle.
c. Ejaculatory duct. d. Termination of
the ejaculatory duct. e. Opening of the
prostatic utricle. /, g. Veru montanum.
h, I. Prostate. — (Liegeois.)

7o8

TEXT-BOOK OF PHYSIOLOGY

of the vasa deferentia, vesiculae seminales, and prostatic tubules
contract and discharge their contents into the urethra, from which
they are forcibly ejected by the rhythmic contraction of the ejaculatory
muscles, the ischio- and bulbo caver no si. The amount of semen dis-
charged at each ejaculation varies from i to 5 c.c.

Spermatozoa. — The spermatozoa are peculiar morphologic ele-
ments which arise within the seminiferous tubules as a result of com-
plex histologic changes in the lining epithelium.
An adult spermatozoon consists of a conoid slightly
flattened head, from the posterior part of which
there projects a short straight rod, provided with a
long filamentous tail or cilium and an end-piece
(Fig. 352). The head contains a nucleus of chro-
matin material. The total length of a spermato-
zoon varies from 50 to 80 micromillimeters. The
characteristic physiologic feature of spermatozoa is
incessant locomotion when in a suitable medium.
So long as they are confined to the vas deferens
they are quiescent, but with their advent into the
vesicula seminalis and dissemination in its contained
fluid, they become extremely active and move around
with considerable rapidity. The power of locomo-
tion depends on the possession of the tail which, by
lashing the surrounding fluid now in this and now
in that direction, propels the head from place to
place. The vitality of spermatozoa is such as to
enable them to retain their physiologic activities in
the uterus for more than eight days.

The development of spermatozoa from testicular
cells as observed in lower animals indicates that
each cell gives rise to four embryonic forms — sper-
matids— which subsequently develop into adult
spermatozoa. In this process the primary nuclear
chromatin undergoes a division, so that each sper-
matozoon receives but a fractional amount. The changes by which
this condition is brought about are comparable to the changes exhibited
by the ovum, and have for their result a reduction in the quantity of
hereditary substance to be transmitted.

Fecundation. — Fecundation is the union of the spermatozoon
(the sperm-cell) with the ovum (the germ-cell) and takes place in the
great majority of instances in the Fallopian tube. After the intro-
duction of the spermatozoa into the vagina during the act of copulation,
they soon begin to pass upward, into and through, the uterine cavity
and out into the Fallopian tube, where they accumulate in large
numbers and retain their vitality for some days. The migration is
effe< led by the propelling power of the filamentous tail.

From observations made on the behavior of the spermatozoa

Fig. 352. — Human
Spermatozoon. i.
Front view, 2, side
view, of the head.
k. Head. m. mid-
dle piece. /. Tail.
e. Terminal fila-
ment. — (After Rel-
zins.)

REPRODUCTION.

709

toward the ovum in lower animals, and on the manner by which their
union is effected, the inference may be drawn that a similar procedure-
takes place in mammals. In lower animals the spermatozoa on
approaching an ovum take on increased activity, swimming around
it in all directions and apparently seeking a point of entrance. In
fish and molluscs the zona pellucida presents a distinct opening, the
micropyle, through which the spermatozoon passes. Inasmuch as
the mammalian ovum is devoid of such an opening, the mechanism of
entrance of the spermatozoon is not clearly understood. Notwith-
standing their enormous numbers it is generally believed that but a
single spermatozoon effects an entrance into the ovum. With the
accomplishment of this, however, the spermatozoon loses its vitality,
after which the body and tail disappear. The head, which in this
instance also is the transmitter of the inherited material, advances to

Fig. 353. — Impregnated Uterus,
with Folds of Decidua Growing up
Around the Egg. The narrow open-
ing, where the folds approach each
other, is seen over the most prominent
portion of the egg. — (Dalton.)

Fig. 354. — Impregnated Uterus;
showing the connection between the
villosities of the chorion and the decidual
membranes. — (Dalton.)

meet and unite with the nucleus of the ovum. A series of histologic
changes now arise, which eventuate in the production of a new cell, a
parent cell, possessing all the features of cell structure and the physio-
logic activities and characteristics of both ancestral cells. From this
parent cell the new being develops through successive division, multi-
plication, and differentiation of cells.

The Fixation of the Ovum. — If the ovum is to develop into a
new being it is essential that it be retained within the cavity of the
uterus. This is accomplished by the development of specialized
structures on the surface of the uterine mucosa and on the surface
of the ovum. With the fertilization of the ovum, the mucous mem-
brane of the uterus takes on an increased growth; it becomes hyper-
trophied and vascular, and develops small elevations known as villi.
Inasmuch as this membrane is detached and discharged at the birth
of the fetus, it is known as the decidua vera. With the fertilization

710

TEXT-BOOK OF PHYSIOLOGY.

of the ovum, the zona pellucida or radiata also develops villosities,
and as it passes from the Fallopian tube into the uterus the villi inter-
digitate, and its further progress is retarded. (Figs. 353 and 354.)
In a short time a portion of the decidua vera grows up on all sides
and encloses the ovum. Its retention is thus secured. That portion
of the decidua which grows around the ovum is termed the decidua
reflexa; while the portion to which the ovum attaches itself is termed
the decidua serotina, and is of interest for the reason that it becomes
the seat of development of the placenta, the organ by which the fetus
is nourished. As development advances the decidua reflexa also
increases in size and extent, and about the end of the fourth month
comes into contact with the decidua vera, with which it ultimately
fuses.

DEVELOPMENT OF FETAL ACCESSORY STRUCTURES.

Segmentation of the Ovum. — Shortly after the formation of the
parent cell, segmentation of the nucleus and cytoplasm takes place
in accordance with karyokinetic methods. The two new cells thus
formed undergo a similar division into four, the four into eight, the

eight into .sixteen,
and so on until the
space within the
zona pellucida is
completely filled
with a large number
of small cells, each
possessing the char-
acteristic cell struc-
tures. The peri-
pheral cells then ar-
range themselves in
the form of a mem-
brane, and as they
are, at the same
time, subjected to
mutual pressure
they assume a poly-
hedral shape, and
give to the mem-
brane a mosaic ap-
pearance (Fig. 355). The central cells then undergo liquefaction. At
some point on the inner surface of the membrane, cells accumulate
which by their division and multiplication form a second membrane.
The two' together are known as the external and internal blastodermic
membranes.

Germinal Area. — At about this period there is an accumulation
of cells at a certain spot in the substance of the blastodermic mem-

Fig. 355. — Primitive Trace of the Embryo, a.
Primitive trace, b. Area pellucida. c. Area opaca. d.
Blastodermic cells, e. Villi beginning to appear on the
surface of the zona pellucida. — (Lugois.)

REPRODUCTION.

711

branes which marks the position of the future embryo. This spot,
at first circular, soon becomes elongated. A slight indentation now
develops into what is known as the primitive trace. Around this area
there is a clear space, the area pellucida, which is in turn surrounded
by a darker region, the area opaca. The primitive trace soon dis-
appears and the area pellucida becomes guitar-shaped. A second
groove, the medullary groove, is now formed, which develops from
before backward and becomes the neural medullary canal.

Blastodermic Membranes. — The embyro, at this period, con-
sists of three layers — viz., the external and the internal blastodermic
membranes and a middle membrane formed by a genesis of cells
from their internal surfaces. These

layers are known from without inward as ^

the epiblast, mesoblast, and hypoblast.

The epiblast gives rise to the central
nerve system, the epidermis and its ap-
pendages, and the primitive kidneys.

The mesoblast gives rise to the dermis,
muscles, bones, nerves, blood-vessels,
sympathetic nerve system, connective
tissue, the urinary and reproductive ap-
paratus, and the walls of the alimentary
canal.

The hypoblast gives rise to the epi-
thelial lining of the alimentary canal and
its glandular appendages, the liver and
pancreas, and the epithelium of the
respiratory tract.

Dorsal Laminae. — As development
advances, the true medullary groove
deepens, and there arise two longitudinal
elevations of the epiblast — the dorsal
lamina, one on either side of the groove
— which grow up, arch over, and unite so
as to form a closed tube, the primitive
central nerve system.

The Chorda Dorsalis. — Just beneath the neural canal there arises
a group of hypoblastic cells which arrange themselves in the form of a
cylindric rod, which marks out the position of the future bony axis
of the body. This rod is known as the chorda dorsalis or notochord.

Primitive Vertebrae. — On either side of the neural canal the
cells of the mesoblast undergo, a longitudinal thickening, which
develops and extends around the neural canal and the chorda dorsalis,
and forms the arches and the bodies of vertebrae. They become
divided transversely into segments.

The mesoblast now separates into two layers: the external, joining
with the epiblast, forms the somatopleural the internal, joining with

- -pp

Fig. 356. — Diagram Repre-
senting the Relation of Pri
mary Structures in a Develop-
ing Chicken; Vertical Trans-
verse Section. The medullar}'
groove and chorda dorsalis are seen
in section; the alimentary canal
pinched off from the yolk-sac is
completely closed, a. Amnion, a,
c. Amniotic cavity filled with
amniotic fluid, pp. Space be-
tween amnion and chorion con-
tinuous with the pleuro-peritoneal
cavity inside the body. vt. Vitel-
line membrane, or zona pellucida.
ys. Yolk-sac, or umbilical vesicle.
— (Foster and Balfour.)

712

TEXT-BOOK OF PHYSIOLOGY

the hypoblast, forms the s plane Jaw pleura; the space between them
constitutes the pleuro- peritoneal cavity (Fig. 356).

Visceral Laminae. — The walls of the pleuro-peritoneal cavity
are formed by a downward prolongation of the somatopleura (the
visceral lamina), which, as they extend around in front, pinch off
a portion of the yolk-sac (formed by the splanchnopleura), which
becomes the primitive alimentary canal; the lower portion, remaining
outside of the body cavity, forms the umbilical vesicle.

The Fetal or Embryonic Membranes. — With the appearance
of the visceral laminae two membranes develop in succession, both
of which play an important part in the subsequent life of the embryo.
These are known as the amnion and the allantois.

The amnion is formed by folds of the epiblast and the external
layer of the mesoblast rising up in front, behind, and at the sides.
These folds gradually extend over the back of the embryo to a certain

Fig. 357. — Diagram of Fecundated
Egg. a. Umbilical vesicle, b. Amniotic
cavity, c. Allantois. — (Dalton.)

Fig. 358. — 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 lavers of the amniotic folds. —
(Dalton.)

point where they meet, coalesce, and enclose a cavity known as the
amniotic cavity. The membranous partition between the folds is
absorbed, after which the outer layer recedes and becomes blended
with the primitive enveloping membrane of the ovum and thus assists
in the formation of the chorion — the external covering of the embryo.

The cavity enclosed by the amnion is at first quite small, but soon
enlarges from the accumulation of a clear, transparent fluid, the
amniotic fluid. It gradually increases in amount up to the latter
period of gestation, when its volume reaches about one liter. This
fluid is derived mainly from the blood, as it contains albumin, sugar,
fatty matter, and inorganic salts. Traces of urea indicate that some
of its constituents arc derived from the embryo itself.

The allantois is primarily a pouch or diverticulum which develops
from the posterior portion of the alimentary canal. As il develops
it enlarges, and in its growth inserts itself between the two layers of
the amnion, coming into contact more especially with the external layer.

REPRODUCTION.

7i3

It finally completely surrounds the embryo, after which its edges
become fused together (Figs. 357 and 358).

The allantois is of especial interest and importance, as it is the
means by which the blood of the embryo is brought into relation
with the blood of the mother. As it develops, two arteries, the hypo-
gastrics, one from each internal iliac, pass out of the abdominal cavity
within the walls of the allantois, and follow it in its course around
the embryo. The ultimate branches of these arteries penetrate the
villous processes which develop on the surface of the chorion and
which take part in the formation of the placenta. A single large
vein emerges from the placenta and returns the blood to the embryo.
In its course it winds around the arteries in a spiral manner a number
of times. These vessels — the umbilical arteries and vein — are en-
closed by the walls of the allantois and amnion, and together constitute
the umbilical cord which at the
end of gestation is about 60 cm.
in length. (Figs. 359, 360).

The Chorion. — The chorion,
the external investment of the
embryo, is formed by the fusion
of the primitive egg membrane
— the zona pellucida — the ex-
ternal layer of the amnion, and
the allantois. Very early in de-
velopment its external surface
becomes covered with homogen-
eous, granular, club-shaped proc-
esses, which by continued bud-
ding and growth, give to the
membrane a shaggy appearance.

At about the end of the second month these processes begin to atrophy
and disappear from the surface of the chorion, with the exception of
that portion which is in contact with the decidua serotina. At this
point the processes or villi continue to grow and develop, and insert
themselves more deeply into the mucous membrane. Corresponding
processes from the mucous membrane insert themselves between
the villi of the chorion, which by their growth and fusion secure, among
other things, the retention of the embryo.

The Nutrition of the Embryo. — Coincident ly with the develop-
ment of the amnion, allantois, and chorion, there arises within the
body of the embryo the early forms of many, if not all, of the future
viscera. The nutritive material required for their growth is partly
contained within the umbilical vesicle lying without the body cavity.
That this material may be utilized, blood-vessels emerge from the
body and ramify within the walls of the vesicle. The capillaries to
which these vessels give rise come into close relation with and absorb
the food material, after which it is carried bv veins to the heart, bv

Fig. 359. — Human Embryo and its En-
velopes at the End of the Third Month.

7M

TEXT-BOOK OF PHYSIOLOGY.

which it is distributed to all parts of the embryo. These vessels
are collectively known as the omphalo-mesenteric arteries and veins.
This primitive or vitelline circulation is of short duration in mam-
mals, as the nutritive material in the vesicle is small in amount and
is soon exhausted. In birds, however, it is of primary importance.
The main supply of nutritive material, however, is derived from
the mother by means of a highly developed and specialized organ —
The Placenta. — Of all the embryonic structures the placenta
is the most important. It is formed by the end of the third- month,
after which it gradually increases in size up to the end of the eighth
month, by which time it is fully developed. It then measures from
18 to 24 cm. in diameter and weighs from 400 to 600 grams. It is

most frequently situated at
the upper and back part of
the uterine cavity. Though
exceedingly complex in
structure it consists essen-
tially of two portions, a
fetal and a maternal.

The fetal portion con-
sists primarily of those villi
on the chorion in relation
with the decidua serotina.
These structures gradually
increase in size and num-
ber, and receive the ulti-
mate branches of the um-
bilical arteries. The ma-
ternal portion consists
primarily of the decidua
serotina. As gestation ad-
vances the chorionic villi
rapidly increase in size and
number, and receive the
branches of the umbilical
arteries. At the same time the decidua serotina becomes hyper-
trophied and vascular. With the continued growth and development
of these two structures they gradually fuse together and finally become
inseparable. In accordance with the needs of the embryo, the decidua
serotina and its contained blood-vessels undergo certain histologic
changes which result in the formation of large cavities, sinuses, or
lakes, into which the blood of the uterine vessels is emptied. Coin-
< irlcntly the villi of the chorion grow and give off numerous branches,
which project themselves in all directions into the blood of uterine
sinuses (Figs. 361, 362). As the placenta develops, the structures
separating the blood of the mother from that of the child gradually
become modified until they are represented by a thin cellular or

Fig. 360. — Human Embryo, with Amnion and
Allantois, in the Third Week. There are as
yet no limbs; the embryo and its appendages are
surrounded by the tufted chorion. — (Haeckel.)

REPRODUCTION.

7^3

homogeneous membrane. The conditions now are such as to permit
of a free exchange of material between the mother and child. Whether
by osmosis or by an act of secretion, the nutritive materials of the
maternal blood pass through the intervening membrane into the
fetal blood on the one hand, while waste products pass in the reverse
direction into the maternal blood on the other hand. Inasmuch
as oxygen is absorbed and carbon dioxid exhaled by the same struc-
tures, the placenta is to be regarded as both a digestive and a respira-
tory organ. So long as these exchanges are permitted to take place
in a normal manner the nutrition of the embrvo is secured.

Fig. 361. — Diagram showing the Relations of the Fetal Membranes. Ant.
Amnion. Ch. Chorion. M. Muscle wall of uterus. R. Decidua reflexa. 5. Serotina.
V. Decidua vera. Y. Yolk stalk. — (McMurrich.)

The Fetal Circulation. — The composition of the blood as well
as the course it pursues through the heart and vascular apparatus
presents peculiarities which have arisen in consequence of the neces-
sity of obtaining nutritive material through the placenta and the
almost impervious condition of the pulmonary capillaries. On re-
turning from the placenta, the blood in the umbilical vein is relatively
rich in nutritive material and scarlet red in color from the presence
of oxygen. As it passes into the abdominal cavity a portion of the
blood is directed by the duct us venosus into the vena cava, while
another portion is emptied into the branches of the portal vein, by
which it is distributed to the liver and from which it emerges bv the

■i6

TEXT-BOOK OF PHYSIOLOGY.

hepatic veins and poured into the vena cava. The blood in the vena
cava is thus a mixture of venous blood from the lower extremities
and liver, and oxygenated blood from the placenta. After its dis-
charge into the right auricle the blood is directed by a fold of the
lining membrane, the Eustachian valve, through an opening in the
interauricular septum, the foramen ovale, into the left auricle. It then
flows through the auriculo-ventricular opening into the left ventricle,
thence into the aorta, and by its branches is distributed to all parts
of the body.

The blood from the head and upper extremities is emptied by
the superior vena cava into the right auricle, but as it passes in front
of the Eustachian valve, it flows directly into the right ventricle and
then into the pulmonary artery. On account of the unexpanded

Amnion.
Chorion.

■ Chorionic villi.

HI

Intervillous spaces.
^^.S^l}^1,' Floal

,- Attached villi.
J

Vein.

Fig. 362. — Diagram of Human Placenta at the Close of Pregnancy. — (Schaper.)

Muscularis

condition of the lungs and the almost impervious condition of the
pulmonary capillaries, but a small portion of the blood passes through
them, while the larger portion by far passes into the aorta directly
through a duct, the ductus arteriosus, which enters at a point below
the origin of the left carotid and subclavian arteries. A comparison
of the blood distributed to the head and upper extremities, with that
distributed to the lower extremities, will show a larger percentage
of nutritive material and oxygen in the former than in the latter,
a fact which has been offered as an explanation of the more rapid
growth of the upper half of the body. As the blood passes through
the aorta, a portion is directed from the main current by the hypogas-
tric and umbilical arteries to the placenta, where it loses carbon

REPRODUCTION.

m

dioxid and gains oxygen, and changes in color from a bluish red to a
scarlet red.

Parturition. — At the end of gestation — approximately 280 days
from the time of conception — a series of changes occur in the uterine
structures which lead to an expulsion of the child, the placenta, and
decidua vera. To this process in its entirety the term parturition
is given. At this time, from causes not clearly defined, the uterine
walls begin to exhibit throughout their extent a series of slight con-
tractions which are somewhat peristaltic in character; these con-
tractions, which gradually increase in frequency and vigor, bring
about a dilatation of the internal os and a descent of the membranes
into the cervical canal. The pressure exerted by these membranes
during the time of the contraction materially assists in the relaxation
of the circular fibers and a dilatation of the external os. When the
dilatation has so far advanced that the diameter of the external os
attains a measure of 7 or 8 cm., the tension of the membranes becomes
sufficiently great to lead to their rupture and to a partial escape of
the amniotic fluid. With this event, the presenting part of the child,
usually the head, descends into the cervical canal. After a short
period of rest the uterine contractions return and rapidly increase in
vigor and duration. As a result of the pressure thus exerted from all
sides on the body of the child, the head gradually descends into the
vagina and finally emerges through the vulva to be followed in a short
time by the expulsion of the trunk and limbs, and a discharge of the
remaining amniotic fluid. With the expulsion of the child the uterine
contractions cease for a period of ten or fifteen minutes, when they
again recur, wTith the result of detaching the placenta and expelling
it into the vagina. It is then removed by the cooperative action of the
abdominal and perineal muscles. The hemorrhage which would
naturally occur with the detachment of the placenta and the laceration
of the maternal vessels is prevented by the firm continuous contraction
of the uterine walls, by wrhich the vessels are compressed and perma-
nently closed.

The Establishment of Inspiration and the Adult Circulation.
— After the birth of the child and the detachment of the placenta,
there speedily occurs a decrease in the quantity of oxygen and an
increase in the quantity of carbon dioxid in the blood, a condition
which causes a discharge of nerve energy from the inspiratory center,
a contraction of the inspiratory muscles, an expansion of the thorax,
and an inflow of air into the lungs.

In the later months of intrauterine life the vascular apparatus
undergoes certain anatomic changes which favor the transition from
the placental to the adult circulation. Thus the ductus venosus con-
tracts, and shunts a larger volume of blood into and through the
liver; the Eustachian valve diminishes in size and at the time of birth
has almost disappeared; a membranous fold grows upward and
backward from the edge of the foramen ovale on the left side; the

7i8 TEXT-BOOK OF PHYSIOLOGY. '

ductus arteriosus also contracts. With the first inspiration and the
expansion of the lungs, the blood which enters the pulmonary arterv
passes through the pulmonary capillaries in large volume and is
returned by the pulmonary veins to the left auricle. The entrance
of the blood into this cavity presses the membranous fold against
the margins of the foramen ovale and thus prevents the further flow
of blood from the right auricle. The blood entering the right auricle
by the inferior vena cava now flows into the right ventricle, which
is favored by the small size of the Eustachian valve. The foramen
ovale is permanently closed at the end of a week or ten days; the
ductus arteriosus at the end of four days. The umbilical vein and
ductus venosus, at the end of four or five days, have also become
almost impervious from the contraction of their walls. The hypo-
gastric arteries remain open and carry blood to the walls of the bladder.

Lactation. — As pregnancy advances the mammary glands in-
crease in size, partly from a deposition of fat and connective tissue
and partly from a multiplication of the secreting acini. The lining
epithelial cells at the same time increase in size, and toward the end
of pregnancy begin to exhibit functional activity. At the time of
birth, or within a day or so after birth, the acini are filled with a fluid
which in its qualitative composition resembles milk and is known as
colostrum. It is distinguished from milk more especially in the fact
that it contains in large quantity a proteid which coagulates on boiling,
and certain inorganic salts which have a laxative effect on the new-
born child. Normal lactation and the phenomena which accompany
it are fully established by the end of the second or third day.

The composition of milk and the mechanism of its production
have been stated in the chapter on Secretion.

Physiologic Activities of the Embryo. — During intrauterine
life the evolution of structure is accompanied by an evolution of
function. The relatively simple and uniform metabolism of the
undifferentiated blastodermic membranes gradually increases in
complexity and variety, as the individual tissues and organs make
their appearance and assume even a slight degree of functional activity.
As to the periods at which different organs begin to functionate, but
little is positively known.

The primitive heart, in all probability, begins to pulsate very
early, as in an embryo from fifteen to eighteen days old and measuring
but 2.2 mm. in length, Coste found the amnion, the allantois, the
omphalo-mesenteric vessels, and the two primitive aortae developed.
In the earlier weeks, all products of metabolism are doubtless elimi-
nated by the placental structures; but as metabolism increases in
complexity the liver and kidney assume excretory activity. Thus,
at the end of the third month the intestine contains a dark, greenish,
viscid material — meconium — composed of bile pigments, bile salts,
and desquamated epithelium; the amniotic fluid, as well as the fluid
within the bladder, contains urea at the end of the sixth month,

REPRODUCTION. 719

indicating the establishment of both hepatic and renal activity. Con-
tractions of the skeletal muscles of the limbs begin about the fifth
month, from which it may be inferred that the mechanism for muscle
activity, viz., muscles, efferent nerves, and spinal centers, has become
anatomically developed and associated, and capable of coordinate
activity. These contractions are, in all probability, automatic or
autochthonic in character due to stimuli arising within the spinal
centers. The remaining organs remain more or less inactive.

After birth, with the first inspiration and the introduction of food
into the alimentary canal, the physiologic mechanisms which sub-
serve general metabolism begin to functionate and in the course of a
week are fully established. At this time the cardiac pulsation averages
about 135 a minute; the respiratory movements vary from 30 to 35
a minute, and are diaphragmatic in type; the urine, which was at first
scanty, is now abundant and proportional to the food consumed;
the digestive glands are elaborating their respective enzymes, digestion
proceeding as in the adult. The hepatic secretion is active and the
lower bowel is emptied of its contents; the coordinate activities of the
nerve-, muscle-, and gland-mechanisms are entirely reflex in character.
Psychic activities are in abeyance by reason of the incomplete develop-
ment of the cerebral mechanisms.

APPENDIX.

PHYSIOLOGIC APPARATUS.

The study of the physical and physiologic properties of muscles and
nerves necessitates the employment of some stimulus which, when
applied to either tissue, will call forth a contraction of the muscle, or
the development of a nerve impulse in the nerve. The most convenient
stimulus is electricity, for the reason that, with appropriate apparatus,
its intensity and duration can be graduated with the utmost nicety.
Moreover, it does not destroy the tissues, as do many chemic, physical,
and mechanic stimuli.

It is therefore necessary that the student should have a practical
acquaintance with those appliances by means of which
electricity is generated, applied and controlled.

The electric cell is an apparatus composed of
different elements, which, by virtue of chemic actions
taking place among them, generate and conduct elec-
tricity. In its simplest form an electric cell consists
of two metals — zinc and copper, or carbon, or platinum,
etc., immersed in an exciting fluid, usually dilute sul-
phuric acid (Fig. 363).

The zinc element is the one acted on chemically
by the sulphuric acid, and at the expense of which the
electricity is maintained. It is known as the gener-
ating element. The copper is the collecting and con-
ducting element.

With the immersion of these elements in a solution of H2S04 a
chemic action at once takes place between the zinc and the acid, with
the formation of zinc sulphate and the liberation of hydrogen, as
expressed in the following formula:

Zn + H2S04 = ZnS04 + H2.

The zinc sulphate passes into the solution, while the hydrogen accu-
mulates on the surface of the copper element.

As all chemic action is accompanied by the development of elec-
tricity, it can be shown by appropriate means that this is the case at
the surface of the zinc. Such a combination is the means of establish-
ing a difference of potential between two points; the point of highesl
potential being the surface of the zinc or the positive clement, the
point of lowest potential being the copper or the negative element.

46 72 T

Fig.
Electric Cell.

TEXT-BOOK OF PHYSIOLOGY.

So long as the elements remain unconnected there is no movement
of electricity, no current.

If the ends of the elements projecting beyond the fluid are connected
by a copper wire, a pathway or circuit is established, and a movement of
the electricity takes place. As electricity flows from the point of high to
the point of low potential, it follows that inside the cell the current flows
from the zinc to the copper, and outside the cell from the copper to the
zinc. Such a current is termed a continuous, a galvanic or a voltaic
current. Inasmuch as there is a progressive fall in potential between
the highest and lowest points, it follows that any two points in the
circuit will exhibit a similar difference, of potential. For this reason
the projecting end of the copper element is at a higher potential than
the projecting end of the zinc element. The end of the copper is,
therefore, termed the positive, + pole or anode, the end of the zinc the
negative, — pole or kathode.

Electric Units. — Owing to the difference of the electric potential
in the cell, the electricity leaves the cell under a certain degree of
pressure, termed the "electro-motive force." As it passes through the
circuit it meets with resistance, the amount of which will depend on

the nature of the circuit

»-}

£^\

ShGJ -*

H2SO

Fig. 364.
ix Series.

—Two Simple Electric Cells Joined
C. Copper. Z. Zinc.

material, its length, and
the area of its cross-sec-
tion. In accordance
with the resistance will
depend the quantity of
electricity that a given
electro-motive force will
press through in a unit
of time. The strength
of the current will there-
fore not depend entirely
on the ratio between the

on the electro-motive force, but, rather,
electro-motive force and the resistance.

For the measurement of electric quantities, a system of units has
been devised. The unit of electro-motive force is the volt; the unit of
resistance is the ohm, i. e., the resistance offered by a column of mer-
cury 106.3 cm- l°n§ and T scl- mm- m section at o° C; the unit of
quantity is the coulomb; the unit of time is one second. One volt
is the electro-motive force which, when steadily applied, will press
through a resistance of one -ohm, one coulomb of electricity in one
second of time yielding a current strength of one ampere.

This relation may be expressed in the following formula, Ohm's
law:

C (current strength) =

[■.!■ i tro mol iv, fon c (F M. 1 . )

-or Ampere

Voll
( >hm

l-'.i -1 lam i- i R )

In practical work it is often necessary to increase the strength of the
current. This is done bv uniting two or more cells in series, *'. e.,

PHYSIOLOGIC APPARATUS.

723

uniting the copper of one cell to the zinc of a second, and so on (Fig.
364). If the resistance remains the same the total voltage is that of
one cell multiplied by the number of cells united.

The cell as above described cannot maintain a current of constant
strength for any length of time, for the following reasons:

1. The sulphuric acid solution, in consequence of its chemic action,
soon becomes nothing more than a saturated solution of zinc sulphate,
after which its chemic activity ceases. The current, therefore, soon
diminishes in strength.

2. The accumulation of hydrogen bubbles on the surface of the
copper hinders the passage of the electricity. In a short time they
develop a current in the opposite direction, which also tends to weaken
the original current. This action is termed polarization of the elements.

Cells of this character are
not suited for physiologic
work, in which constancv in
the strength of the current
is absolutely necessary. To
overcome these disadvantages,
cells have been devised which
are less violent in action,
which prevent polarization,
and which maintain a current
of constant strength for a long
period of time. One of the
most generally used for physi-
ologic purposes is—

The Daniell cell. This
consists of a porous cup con-
taining a saturated solution
of CuS04, copper sulphate,
in which is immersed a copper

plate or rod. This combination is placed in a glass vessel containing
a solution of H2S04 (1:15). In this solution is immersed a roll of
sheet zinc (Fig. 365). Each of the plates is provided with a binding
screw. When the cell is in action the sulphuric acid attacks the
zinc, forming zinc sulphate, and liberates hydrogen; the cup being
porous, the hydrogen passes into the copper sulphate solution, where
it combines with the sulphuric acid radicle, and liberates metallic
copper. Polarization of the copper is thus prevented. The metallic
copper is deposited on the copper plate, which is thus kept bright.
The copper sulphate solution is kept at the point of saturation by
packing around the copper cylinder a quantity of the crystals of the
salt. The sulphuric acid passes back into the porous cup, to take the
place of that used. This cell is remarkably constant for these reasons,
and well adapted for physiologic as well as other purposes where a
current of uniform strength is necessary.

Fig. 365. — Daxiell Cell.

724

TEXT-BOOK OF PHYSIOLOGY.

The projecting ends of the copper and zinc plates are termed respec-
tively the positive pole or anode, and the negative pole or kathode.
The electro-motive force of a Daniell cell is practically i volt; but when
the two poles are connected by a wire of i ohm resistance, the current
strength will be less than i ampere, possibly only 0.7, owing to the
resistance offered to the flow of electricity by the fluids between the
zinc and the copper. In all measurements, the internal resistance
of the cell must be taken into consideration.

The Dry Cell. — The commercial dry cell is a convenient source of
electricity for general laboratory work. It consists of a cup of zinc,
the inner surface of which is covered over with a thick layer of a paste
of plaster-of-Paris, saturated with ammonium chlorid. In the center

of the cup there is a rod of carbon.
Surrounding this rod and occupying
the space between it and the plaster-
of-Paris paste, is a mixture of man-
ganese dioxid and charcoal. The
upper surface of the cell is sealed to
prevent evaporation. The electricity
is generated at the surface of the zinc
cup by the chemic action of the chlorin
which arises from the dissociation of
the ammonium chlorid. When the
plates are united by a conjunctive wire
the current within the cell flows from
the zinc (the positive element) to the
carbon (the negative element), and
without the cell from the carbon (the
positive pole) to the zinc (the nega-
tive pole).

Leads. — By means of insulated

wires attached to the poles of a cell,

the electricity may be conducted from

the cell and used for exciting or

stimulating purpose. As the wires

thus become practically prolongations of the plates their ends become

the corresponding poles. In experimental work the ends of the wires

are provided with special devices, termed —

Non-polarizable electrodes. The necessity for the employmen
of such electrodes arises from the fact that when the ends of the wires
from a cell are placed in direct contact with the tissues chemic changes
arc produced in a short time, which lead to their polarization. As a
result, a current opposite in direction to that of the cell is developed,
which tends to weaken or neutralize it. This polarization current
vitiates the result of many experiments made with highly irritable
tissue such as nerve-tissue. Whether for stimulating purposes or for
I he purpose of detecting the existence of electric currents in living

Fig. 366. — Non-polarizable Elec
trodes. 1. Du Bois-Reymond's. 2
Yon Fleischl's. 3. d'Arsonval's.

PHYSIOLOGIC APPARATUS.

725

tissues, it is essential that the electrodes used shall be non-polarizable.
The earliest electrodes of this character were made by du Bois-Rey-
mond and were based on the fact discovered by Regnault that a strip
of chemically pure zinc or amalgamated zinc (Matteucci) immersed
in a saturated solution of zinc sulphate would not polarize. One
form made by du Bois-Reymond is shown in Fig. 366. It consists
of a flattened glass tube attached to a universal joint and supported
by an insulated brass stand. The lower end of the tube is closed with
kaolin or China clay made into a paste with a 0.6 per cent, solution of
sodium chlorid. It can be moulded into any desired shape. The
interior of the tube is partially filled with a
saturated solution of sulphate of zinc in
which is immersed the strip of amalga-
mated zinc. To the upper end of the zinc
the conducting wire is attached.

The v. Fleischl brush electrode is similar
to the preceding except that the end of the
tube is closed by the brush of a camel's-hair
pencil.

The d'Arsonval electrode consists of a
glass tube containing a silver rod coated
with fused silver chlorid. The interior of
the tube is filled with normal salt solution
0.6 per cent, and the end closed with a
thread or plug of asbestos which is made
to project beyond the tube for a short dis-
tance.

Any one of these three electrodes is
suitable for physiologic experimentation, as
their free ends neither corrode the tissues
nor develop electric currents.

Keys. — Muscle and nerve-tissues are
conductors of electricity. When, therefore,
the terminals (the non-polarizable elec-
trodes) of the wires of a cell are placed in
contact with either a muscle or a nerve a
circuit is made through which a current of electricity flows; when one
or both are removed, the circuit is broken and the current ceases. In
practical work it is often necessary to keep the electrodes in contact
with the tissues for a variable length of time. The circuit, however,
may be alternately made and broken at will by interposing along the
return wire a mechanic contrivance known as a key, of which there
are many forms.

The du Bois-Reymond Friction Key. — This consists of a plate
of vulcanite attached to a screw clamp by which it can be fastened to
the edge of a table (Fig. 367). The surface of the vulcanite plate
carries two rectangular blocks of brass, each of which has two holes

Fin. 367. — Du Rois-Rey-
mond Friction Key.

726

TEXT-BOOK OF PHYSIOLOGY.

2.

drilled through it, for the insertion of wires, which are held in position
by small screws. A movable bridge of brass, provided with an ebonite
handle, serves to make connection between the blocks. There are
two ways of interposing this key in the circuit.

1. As a Simple Key. — For this purpose one of the wires, usually the
negative, is carried from the cell to one block and then continued
from the second block. When the bridge is down, the circuit is
made and the current passes; when it is up, the circuit is broken.
As a Short-circuiting Key. — When used for this purpose, the wires
of the cell are carried to the inner holes of each block and then
continued from the outer holes to the tissues or to some form of
apparatus which it is desired to actuate. When the key is closed
i. e., when the bridge is down, the current on reaching the key,
will divide, one portion passing across the bridge and so back to

the cell, the other portion
passing to the tissue or ap-
paratus. The amount of
the current which is returned
to the cell through the short
circuit will be proportional
to the resistance of the
longer circuit. As the latter
is usually great in compar-
ison with the former; prac-
tically all the current is
short-circuited. When the
bridge is lowered, therefore,
the current is short-circuited ;
when it is raised, the current flows into the longer circuit through
the tissue or apparatus.
The Mercury Key. — In this form the connection is established by
means of mercury. It consists of a circular block in the center of which
there is a cup containing mercury (Fig. 368). At opposite points
there are binding posts, one of which is provided with a rigid fixed
copper rod passing into the mercury; the other is provided with a
movable bent rod which may be made to dip into or be withdrawn
from the mercury by the ebonite handle.

The effect of a constant or galvanic current on a muscle or nerve
will depend to some extent on its strength. This may be accurately
regulated by means of an apparatus known as —

The Rheocord. With this apparatus an electric current may be
divided, one portion continuing through a conductor back to the
battery, the other portion being sent off through the nerve. The
strengths of these two currents are inversely proportional to the
resistances of their circuits. A simple form of rheocord (Fig. 369)
consists of a long wire arranged for convenience in parallel lines on a
small wooden base and connected at its two ends with binding posts

Fig. 368. — A Mercury Key.

PHYSIOLOGIC APPARATUS.

727

A and B. The resistance of this wire, 1.6 ohms, can be increased by
the introduction of small resistance coils, between D and B, varying
from 5 to 20 ohms.

The two binding posts A and B are connected with the positive and
negative poles of an electric cell respectively. A simple key is placed in
the circuit.

From A, a wire passes to one of the electrodes on which the muscle
or nerve rests. A second wire passes from the second electrode to a
clamp S, by way of the binding post C, which can be fastened to the
long wire at any given point. The current, on reaching A, will divide
into two branches, one of which will pass along the wire A, B, and
thence back to the cell; the other will pass through the nerve and back
to S and thence also to the
cell. The amount of current
passing through the nerve
circuit will be inversely pro-
portional to the resistance of
the nerve and directly propor-
tional to the difference of
potential between A and S.
If S is close to A, the differ-
ence of potential is slight.
If S is removed from A to-
ward B, the difference of
potential is increased and the
current sent through the nerve
circuit is increased.

In many experiments it is
necessary to reverse the direc-
tion of the current, in other
experiments to deflect it, with-
out changing the position of
the electrodes. Both these
results may be accomplished by the use of —

Pohl's commutator. This is a round block of wood with six
cups, each of which is in connection with a binding post (Fig. 370). In
each of the two cups marked 1 and 2, + and — , is inserted one end
of a copper wire bent at right angles. The other ends of the wires
are supported and insulated by a hard-rubber handle. To the top
of each wire is soldered a semicircular copper wire. This arrange-
ment permits of a rocking movement, whereby the opposite ends of
the semicircular wires can be made to dip into cups 3 and 4, and into
cups 5 and 6 alternately. Two wires crossed in the middle of the
block serve to connect opposite pairs of cups. When in use, the
cups are filled with clean mercury. The method of using the com-
mutator is as follows:
1. As a Current Reverser. — The positive and negative poles of the

Fig. 369. — Rheocord.

72S TEXT-BOOK OF PHYSIOLOGY.

electric cell are connected by wires with binding posts i and 2
respectively. A key is interposed in the circuit. Wires are then
carried from binding posts 3 and 4 to the electrodes in connection
with the muscle or nerve. The rocker of the commutator is so
turned that the ends of the semicircular wires dip into cups 3 and 4.
The direction of the current will be on the closure of the circuit
from 1 to 3, then from 3 along a wire to and through the tissue
and back to 4, and thence to the cell. If the position of the
rocker be now reversed so that the opposite ends of the semi-
circular wires dip into cups 5 and 6, the direction of the current
through the tissue will be reversed. The positive current, after
entering binding post 1, will flow to 5; then along one of the cross
wires to 4; then along a wire to and through the tissue and back
to 3, along the opposite cross wire to 6. thence to 2 and so back
to the cell.

Fig. 370. — Pohl's Commutator.

A. Arranged as a current reverser; B, as a cur-
rent deflector.

2. As a Current Deflector. — When it is desirable to deflect the current

to two pairs of electrodes differently situated, wires are carried

from binding posts 3 and 4 to one pair, and from 5 and 6 to the

other pair. The cross wires are then removed. According to

the position of the rocker the current will be deflected to one or

the other.

The Inductorium. — This is an apparatus designed for the purpose

of obtaining single or rapidly succeeding electric currents by induction.

Its construction is based on facts discovered by Faraday, some of

which are the following:

If two circuits, a primary and a secondary, are placed parallel to
each other, the former connected with a galvanic cell, the latter with
a galvanometer, it is found that, at the moment the primary circuit is
made, and at the moment it is broken, a current is induced in the
secondary circuit, as shown by a momentary deflection of the galvano-
meter needle. During the continuous How of the current through the
primary circuit there is no evidence of a current in the secondary

PHYSIOLOGIC APPARATUS.

729

circuit. The induced current is but of momentary duration. The
current flowing through the primary circuit is termed the inducing,
the current flowing through the secondary circuit the induced current.

The induced current is opposite in direction to that of the inducing
current when the circuit is made or closed; it is in the same direction,
however, when the circuit is broken or opened.

If the circuits are arranged in the form of coils, it is found that,
other things being equal, the strength of the induced currents will be
proportional to the number of turns in the coils.

If the coils are placed at varying distances from each other, the
strength of the induced current varies, increasing as the coils are
approximated, decreasing as they are separated.

Approximation or separation of the coils while the current is flowing
through the primary circuit develops an induced current, which dis-
appears, however,
the moment the
movement of the
coil ceases. A
sudden increase or
decrease in the
strength of the in-
ducing current also
develops an in-
duced current.

When the coils
are approximated
or the primary cur-
rent increased in
strength, the in-
duced current is
opposite in direc-
tion to that of the
inducing cur-rent ;
with the reverse conditions, the induced current has the same direction.

The induced currents have been termed, in honor of their discoverer,
Faradic currents.

The du Bois-Reymond inductorium, based on the foregoing
facts, consists essentially of two coils of insulated copper wire, termed
primary and secondary (Fig. 371).

The primary coil, R', consists of thick copper wire wound around
a wooden spool attached to a vertical support. The beginning of this
coil is at the binding post S", its termination either at binding post
P" or S'". In the course of this primary wire or circuit, there are
placed two vertical bars of soft iron, B', connected at their bases to
form a horseshoe magnet, around the ends of which the wire is coiled.
The object of this device will be explained later.

Inside the primary coil there is placed a bundle of soft iron wires,

Fiq. 371- — Ixductoritjji of du Bois-Reyiioxd. R', Pri-
mary, R". secondary spiral. B. Board on which R" moves.

1. Scale. -\ . Wires from battery. P', P". Pillars. H.

Neef's hammer. B'. Electro-magnet. S'. Binding screw
touching the steel spring (H). S" and S'". Binding screws to
which to attach wires where Xeef's hammer is not required.

73o TEXT-BOOK OF PHYSIOLOGY.

which, as soon as the circuit is made, become magnetized, with the
effect of increasing the action of the inducing current.

The secondary coil, R", consists of a much greater number of turns
of a finer copper wire, the ratio being about 40 to 1, also wound around
a spool, having a tunnel sufficiently large to enable it to slide over the
primary. By these means the strength of the induced current is
increased. As a result of the construction of the inductorium, the low
electro-motive force of the cell is transformed into the high electro-
motive force characteristic of the induced current. As the number
of turns of wire in the secondary coil is to the number in the primary,
so are the electro-motive forces in the secondary coil to those in the
primary coil.

The secondary coil slides along a track, B, which permits it to be
moved toward or away from the primary. The distance between the
two coils can be measured and the strength of the induced current
again reproduced, other things being equal, by means of a centimeter-
millimeter scale pasted on the edge of B.

The ends of the wire of the secondary coil are fastened to two
binding posts to which conducting wires provided with hand electrodes
can be attached.

The inductorium may be used for obtaining either a single current
or a series of rapidly repeated induced currents.

The Single Induced Current. — On account of its high electro-motive
force, its penetrative power, and short duration, the single induced
current is a most convenient and suitable form of stimulus for many
purposes. In order to obtain such a current, the positive wire of the
cell is carried to binding post S", and the negative wire either to S'"
or P". A key is placed in the primary circuit. The course of the
current will then be on the closure of the circuit from the cell to S",
thence around R' to S'", and so back to the cell; or if the negative
wire is connected with P", the course of the current on leaving R'
will be through the coils surrounding the two vertical bars B', thence
to P", and so back to the cell. If the secondary coil be placed close
to the primary and the wires of the secondary brought into contact
with a muscle, it will be found that with both the make and the break
of the primary circuit a current is induced in the secondary, as shown
by a short quick pulsation of the muscle; but during the time of closure
of the circuit, the induced current is wanting, as shown by the quiescent
condition of the muscle. It will be apparent, however, from the
energy of the contraction that the break induced current is a more
efficient stimulus than the make induced current. That this is the
case is made evident by removing the secondary to the end of the slide-
way and then gradually bringing it toward the primary half a centi-
meter at a time, making and breaking the circuit after each move-
ment until a pulsation of the muscle occurs. It will be found to
occur first on the break of the circuit. As the secondary approaches
the primary a position will be reached when a pulsation occurs on

PHYSIOLOGIC APPARATUS. 731

the make as well as on the break of the circuit, though it will be less
pronounced.

The explanation offered for this difference in the strength of the two induced currents
is as follows: With the make of the circuit and the passage of the battery current through
the primary coil there is induced in the neighboring and parallel turns of the wire an extra
current opposite in direction to the primary current. This extra or self-induced current
antagonizes and prevents the current from attaining its maximum development as quickly
as it otherwise would, and therefore its efficiency as an inducer of a current in the secondary
is diminished. On the break of the circuit the primary current disappears quickly, and
as there is nothing to retard its disappearance its efficiency as an inducer of a current in
the secondary coil is not diminished. It is not infrequently stated that the disappearance
of the primary current induces in the neighboring coils a break extra current corresponding
in direction which assists in the development of the induced current. This is not the case,
however, as no break extra current is developed in the inductorium as ordinarily used
when actuated by a battery current of moderate strength.

As it is not so much the intensity of the current as it is rapid variations in intensity
•that produce effects, it is readily apparent why the induced current developed at the
break of the primary is more effective as a stimulus than the induced current developed
at the make of the primary circuit. The quantity of the electricity is, however, the same
in both cases.

If the secondary be pushed further along the slideway until it
largely covers the primary coil, a position will be reached when the
make induced current equals in its efficiency as a stimulus the break
induced current; and if the secondary be yet further advanced, a
position is reached when the make induced current becomes more
powerful and efficient than the break induced current, as shown by
the greater contraction of the muscle. This result is explained by
the fact that the make extra current is now able of itself to induce a
current in the secondary coil, on account of its proximity, which, added
to that induced by the battery current, produces a current, greater
than that induced on the break of the circuit.*

Rapidly Repeated Induced Currents. — As the single induced current
is of extremely short duration, it is inefficient as a stimulus in the con-
duct of many experiments. It is necessary, therefore, to develop it
with a frequency that is sufficient to give rise to a summation of effects.
The duration of the stimulation may be thus considerably prolonged.
This is accomplished by introducing in the primary circuit close to the
primary coil an automatic interrupter, usually Neef's modification of
Wagner's hammer (Fig. 371). This consists of a vertical post, P',
to the top of which is fastened a metallic spring carrying at its opposite
end a steel or iron hammer, H, which hangs over, but does not touch,
the two vertical bars of soft iron around which the wire of the primary
coil is wound. About the middle of the spring on its upper surface
there is a small plate of platinum which is in contact with an adjustable,
platinum-tipped screw, S', carried by a plate of brass in connection
with binding post S".

For the purpose of interrupting the primary circuit frequently in a
unit of time, and thus developing induced currents in quick succession,
the apparatus is arranged in the following way: The positive and

* " On certain peculiarities of the inductorium," Prof. Colin C. Stewart, " Univ. Pa.
Medical Bulletin," Feb., 1904.

732 TEXT-BOOK OF PHYSIOLOGY.

negative poles of the electric cell are connected by wires with binding
posts P' and P", a key being interposed in the circuit. If the screw
S' is in contact with the spring, the current on the closure of the circuit
will enter P', pass along the spring to S', thence into and through the
primary coil R', to the coils surrounding the vertical bars B', then to
P", and so back to the cell.

As the current passes around the vertical bars, they are magnetized.
The magnetization draws down the hammer, and, in so doing, breaks

the circuit at the tip of the screw, S'.
The vertical bars are at once demag-
netized, and the hammer is restored
to its original position by the elasticity
of the spring. The circuit is thus
re-established, the current flows
through the coils, the bars are again
magnetized, the hammer is drawn
down, to be followed by a second break
of the circuit.

The number of times the circuit is
thus made and broken per second will
vary with the length of the spring.

As each interruption of the primary
circuit develops an induced current,
it follows that the latter must succeed
each other with a frequency corres-
ponding with the frequency of the
former. If while the primary circuit
is thus being interrupted the wires of
the secondary coil be placed in con-
tact with a muscle, the induced current will give rise to contractions
which will succeed each other so rapidly that they fuse together,
producing a spasm or tetanus of the muscle. For this reason these
currents are frequently spoken of as tetanizing currents, and the pro-
cedure as tetanization or Faradization. These currents also increase
in strength as the secondary approaches the primary.

Fig. 372. — Helmholtz's Modifi-
cation of Neef's Hammer. As
long as c is not in contact with d,
g h remains magnetic; thus c is at-
tracted to d and a secondary circuit,
a, b, c, d, e, is formed; c then springs
back again, and thus the process
goes on. A new wire is introduced
to connect a with /. K. Battery.

Helmholtz's Modification of the Inductorium. — With a view of equalizing the
strengths of the induced currents, Helmholtz suggested a device the adoption of which
accomplishes this to a certain extent. It consists (Fig. 372) in connecting with a wire
binding posts P' and S", and in providing binding post P" with an adjustable screw which
can be raised until the spring comes in contact with it, when the hammer is drawn down
by the electromagnet B'. This latter arrangement is practically a short-circuiting key by
which a portion of the current is returned to the cell without ever entering the primary coil.
'I b.i nne arrangement, though differently lettered, is shown in Fig. 371 By the use of
the entire device the changes in the primary coil are made not by making and breaking
the primary current, but by alternately long- and short-circuiting the current. "When the
short -< ircuiting key is opened, the full volume of the primary current flows through the pri-
mal 1 oil. When the short-circuiting key is closed, most of the current fails to enter the
coil, taking tin- easier path through the key. Some of the current, however, always flows
through (he coil and is never diverted. The cycle of changes in the electric condition of
the primary coil is thus altered for two reasons:

PHYSIOLOGIC APPARATUS.

733

"First, we no longer have an alternation between a full primary current and Done at al
— rather an alternation between a full primary current and a weaker one. The difference
in the phases is thus lessened, the extent of the change on making and breaking is lessened ,
and correspondingly the efficiency of the make and break currents induced in the secondary
coil is slightly decreased.

"Second,'on making the primary current, as in the ordinary coil, the sudden appearance
of the primary current is antagonized by the opposing make extra current, with the result
that the make induced current is still further reduced; while on breaking the current the
break extra current can now flow through the primary coil across the short-circuiting key.
This current, trailing behind the disappearing primary current in the same direction, pro-
duces the same effect as if the primary current itself were to disappear slowly. As a result
the disappearance of the primary current loses its former efficiency as an inducer of second-
ary currents, and the break induction current is reduced to about the efficiency of the make.
' "This so-called 'equalizing' of the make and break induced currents is never perfect,
if for no other reason, because the make extra current must take the long circuit through
the battery, while the break extra current has an easier path through the short-circuiting
kev, and is thus greater than the make extra current." (C. C. Stewart."!

THE GRAPHIC METHOD.

The term graphic is applied to a method by which curves or tracings
are obtained which represent the extent, duration, and time relations of
the movements accompanying physiologic processes. If these move-
ments can be trans-
lated in one direc-
tion, they may be
recorded in different
ways :

i. By attaching the
moving struc-
ture — e. g . ,
heart, muscle,
etc. — to a deli-
cate lever the
free extremity
of which is pro-
vided with a

Fig.

-A Receiving Tambour.

writing point.
2. By transmitting the movement through a column of air enclosed
in a rubber tube the two ends of which are attached to a metallic
capsule, covered by a rubber membrane, termed a drum or tam-
bour. When the membrane of the first tambour is pressed or
driven inward, the air is forced through the rubber tube into the
second tambour and its membrane is pushed outward. As
soon as the primary pressure is removed, the membranes return
to their former condition. If the membrane of the first tambour
is drawn outward, the air in the system is rarefied and the mem-
brane of the second tambour is pressed inward. For the purpose
of registering the movement transmitted by the column of air,
the second tambour is provided with a light lever supported by a
vertical bearing resting on a small metallic disk. The mem-
brane of the first tambour is frequently provided with a button,

734

TEXT-BOOK OF PHYSIOLOGY.

which is placed over the moving structure. The inward move-
ment of the membrane of the first tambour produces an outward
movement of the membrane of the second tambour, indicated,
though magnified, by the rise of the free end of the lever. The
reverse movement of the membrane is attended by a fall of the
lever. The first tambour is termed the receiving, the second the
recording tambour (Figs. 373, 374).
3. By enclosing an organ — e. g., kidney, spleen, arm, finger, etc. —
in a rigid glass or metal vessel which at one point is in communic-
ation with a recording apparatus — e. g., (1) a piston provided
with a lever (page 483); or (2) a tambour and lever (page 336);
or (3) a mercurial manometer carrying a float and pen (page 341).
The space between the part investigated and the vessel is filled
with fluid. The variations in volume of the organ cause a dis-
placement of the fluid and give rise to a to-and-fro movement
which is taken up and reproduced by the recording apparatus.
The writing point may be (1) some form of pen carrying ink which
records the movement on a white paper surface, or (2) a piece of

metal, glass, or paper
wdiich records the
movement on smoked
paper or glass.

The Recording
Surface. — The sur-
face which receives
and records the move-
ments of a pen or
lever is usually that of
a cylinder which is covered with glazed paper and coated with a thin
layer of soot, obtained by passing the cylinder through the flame of
a gas burner. The axis of the cylinder is supported by a metal frame-
work. If the writing point of the lever be placed against the cylinder
and a movement be imparted to it, a portion of the soot is rubbed off,
leaving a white line behind. If the cylinder be stationary, the rise and
fall of the lever are recorded as a vertical line. Such a record shows
only the extent of a movement. If the cylinder is traveling, however,
at a uniform rate, the rise and fall of the lever are recorded in the form
of 'a curve the width of the two arms of which will depend partly on
the rapidity of the movement of the lever and partly on the rate of
movement of the cylinder. The cylinder movement is initiated and
maintained by clock-work or by the transmission of power by belting
to a system of pulleys in connection with its axis. As the tracing is
wave-like in form, the cylinder is frequently spoken of as a kymograph
or wave recorder (Fig. 375).

From the record thus obtained it is possible to determine not only
the extent but also the duration, the form, and the rate of recurrence
of any given movement.

Fig. 374. — A Recording Tambour. — (Marey.)

PHYSIOLOGIC APPARATUS.

735

The Extent of a Movement. — As the lever not only takes up and
reproduces a movement, but at the same time magnifies it, it is essential
that the degree of magnification be known, in order to determine the
actual extent of the movement. The magnification of the lever is
readily determined by dividing the distance between the axis of the
lever and its writing point by the distance between the axis and the
point of attachment of the structure, and then dividing the height of
the tracing by this quotient. The final quotient represents the extent
of the movement.

The Time Relations of a Movement. — When recorded in the form
of a curve, the duration of the entire movement, or of any one portion
of it, can be determined by
means of a time marking or
chronographic apparatus, con-
sisting of (i) a small signal
magnet provided with a mov-
able armature, to which is
attached a writing style; (2)
an automatic interrupter; and
(3) an electric cell.

The Signal Magnet.—
The magnet (Fig. 376) is
actuated by the electric cur-
rent made and broken at
regular and known intervals
by an automatically-acting in-
terrupter placed in the circuit.
With each make and break of
the circuit the armature and
style move alternately down-
ward and upward. The ex-
cursion of the style can be
readily recorded on a travel-
ing surface. The character
and number of the interrup-
tions per second will deter-
mine the character of the
tracing. If they occur in a
rhythmic manner, the tracing
will be sinus oidal or wave-
like in form. If the time of interruption is of short duration as
compared with the time of closure of the circuit, the tracing will be a
horizontal line with short vertical elevations at regular intervals.

The Automatic Interrupter. — The circuit may be interrupted by
vibrating reeds, tuning-forks, metronomes, etc. A well-known form
of vibrating reed is shown in Fig. 377. This consists of a metallic
frame carrving a coil of wire in the center of which there is a core of

Fig. 375. — Kymograph.
zold, Leipzig.)

(Boruttau's, Pet-

736

TEXT-BOOK OF PHYSIOLOGY.

Fig. 376. — Signal Magnet.

soft iron. To the vertical part of the frame there is fastened the reed,
the distal end of which is bent to dip into an adjustable mercury cup.
When in circuit the current enters the coil, then flows into and through
the frame and the reed to the mercury, and thence back to the cell.
On the closure of the circuit and the magnetization of the iron core
the reed is withdrawn from the mercury, the circuit broken, and the
core demagnetized. The elasticity of the spring returns it to the
mercury, when the circuit is again restored. The reed may be so
constructed that it will be raised and lowered 50, 100, or 200 times a

second. The armature
of the signal magnet un-
dergoes a corresponding
number of elevations
and depressions. If the
reed vibrates 100 times
in a second, the distance
from crest to crest of the
wave tracing will represent y^ of a second. Interrupters of various
kinds have been devised which make and break the circuit from 1 to
250 times a second.

Moist Chamber. — In many experiments, it is necessary to keep the
nerve or muscle preparation in a uniformly moist atmosphere. To se-
cure this, a moist chamber is employed (Fig. 378). This consists of a
hard-rubber platform, supported by a piece of brass, which slides up
and down a vertical rod, and which can be clamped at any height. By
means of a short lever the
vertical rod can be turned,
carrying the platform from
side to side. The rod is
secured to a firm iron base.
Six double binding posts
for the attachment of wires
pass through the platform.
Near the side of the upper
surface of the platform
there rises a vertical rod,
carrying a clamp for hold-
ing the femur of a nerve-
muscle preparation, as well
as a horizontal rod for supporting three pairs of non-polarizablc
electrodes. A groove around the outer edge of the platform receives a
glass shade, which covers the whole. The air of the chamber is kept
moist by placing in it pieces of blotting-paper saturated with water.

From the under surface of the platform there descends a rod, which,
by means of a double binding screw, supports a horizontal rod, modi-
fied at one end to carry the delicate axis of a light stiff recording lever.
The end of this lever is pointed, to enable it to write on a smoked glass

377

-Page's Vibrating Reed.
modification.)

(Reichert's

PHYSIOLOGIC APPARATUS.

737

or paper. Beneath the axis is a strip of brass, carrying a screw, which
gives support to the lever until the instant the contraction of the
muscle begins. This screw, the after-loading screw, also enables
the lever to be placed in a horizontal position. The portion of the
lever near the axis is provided with a double hook, the lower portion
of which serves for the attachment of the weight by which the muscle
is counterpoised.

In some experiments, as in the registration of a muscle contraction
under varying conditions, it is necessary to give the lever mass by
attaching weights directly beneath the muscle. This, however,
introduces certain errors in the movements of the lever, which some-
what deform what would otherwise be the normal curve. If the
weight be attached, not opposite to the muscle attachment, but close

Moist Chamber.

to the axis of the lever, the undesirable acceleration of the lever move-
ment, during both contraction and relaxation, is largely prevented.
The lever may be a straw, a strip of celluloid or aluminium. It should
be as light as possible. The writing point may be made of stiff paper,
a piece of tinsel, glass or aluminium. It should have sufficient elas-
ticity to keep it in contact with the cylinder during the excursion of the
lever. The writing point should be placed as nearly parallel as possible
to the surface of the cylinder.

Normal Saline Solution. — To prevent drying and a loss of irrita-
bility the tissue under investigation should be kept moist with the nor-
mal saline solution (NaCl 0.6 per cent.). This solution very largely
47

TEXT-BOOK OF PHYSIOLOGY.

prevents either absorption or extraction of water from the tissues and
thus retards chemic changes in their composition.

Ringer's solution, largely used for the same purpose, is made by
saturating 0.65 per cent. NaCl solution with calcium phosphate and
then adding 2 c.c. of a 1 per cent, solution of potassium chlorid to
each 100 c.c.

The Galvanometer and Capillary Electrometer. — In the de-
tection and investigation of the electric currents of muscles, nerves,
and other tissues, the physiologist is limited to the galvanometer and
capillary electrometer. The principle of the galvanometer is based
on the fact that an electric current flowing through a wire parallel in
direction with a magnetic needle will tend to set the needle at right

angles to the direc-
tion of the current.
The essential requi-
site of any galvano-
meter used for physi-
ologic purposes is
that it will respond
quickly to the influ-
ence of extremely
weak currents. This
is realized by the use
of small light needles,
the adoption of the
astatic system, or
some similar device
by which the direc-
tive influence of the
earth's magnetism is
eliminated, and the
multiplication of the
number of turns of the wire in the coils which surround the needle.

The tangent galvanometer, or boussole, as constructed by Wiede-
mann, is the form most frequently employed in physiologic investi-
gations (Fig. 379). It consists primarily of a thick copper cylinder,
through which a tunnel has been bored. Within this tunnel is sus-
pended a magnetized ring, just large enough to swing clear of the sides
of the chamber. The object of making the magnet ring-shaped is
to increase its strength in proportion to its size, and to get rid of the
central inactive part. Connected with and passing upward from the
magnetized ring through the copper cylinder is an aluminium rod,
surmounted by a circular plane mirror. Above the mirror rises a
glass tube, which carries on top, on an ebonite support, a little wind-
lass, capable of being centered by three small screws. On the wind-
lass is wound a single filament of silk, which passes down the tube
and is attached to the mirror. The magnet can, by this contrivance,

Fig. 379. — Wiedemann's Boussole.

PHYSIOLOGIC APPARATUS. 739

be raised or lowered and centered in the copper chamber. Deflections
of the mirror from currents of air are prevented by inclosing it with a
brass cover provided with a glass window. The coils are placed on
each side of the copper chamber, and supported by a rod, on which
they slide. By this arrangement they can be approximated until
they meet and completely conceal the cylinder. By varying the
position of the coils the influence of the current upon the needle can be
increased or diminished. An advantage which this galvanometer
possesses is the damping of the oscillation of the needle, so that it
quickly comes to rest after deflection. This is accomplished by the
development of induction currents in the copper cylinder, the direction
of which is opposite to that of the movement of the needle. The
instrument, therefore, is aperiodic — that is to say, when the needle
is influenced by a current it moves comparatively slowly until the
maximum deflection is reached, when it comes to rest without oscilla-
tions. When the circuit is broken the needle swings slowly back to
zero, and again comes to rest without oscillations.

Inasmuch as the needle is not astatic, it is rendered so by the use
of an accessory magnet — the so-called Hauy's bar. This magnet,
supported by a rod directed perpendicular to the coils, is placed in the
magnetic meridian, horizontal to the needle, with its north pole point-
ing north. By sliding the magnet toward the needle the directive
influence of the earth's magnetism is gradually diminished, and when
it is reduced to a minimum the needle acquires its highest degree of
instability. By means of a pulley an angular movement can be im-
parted to the end of the accessory magnet in the direction of the
magnetic meridian, which serves to keep the needle on the zero of
the scale. The deflections of the needle are observed by means of an
astronomic telescope, above which is placed a scale divided into
centimeters and millimeters, and distant from the galvanometer about
six or eight feet. As the numbers on the scale are reversed, they will
be seen in the mirror in their natural position, and with the deflection
of the needle the numbers will appear as if drawn across the mirror.
The extent of the deflection is readily determined when the needle
comes to rest.

The reflecting galvanometer of Sir William Thompson is also
used for the same purposes.

The Capillary Electrometer. — Notwithstanding the extreme
sensitiveness of the modern galvanometer, it has been found desirable,
in the investigation of many physiologic processes, to possess some
means which will respond even more promptly to slight variations 'in
electro-motive force. This has been realized in the construction bv
Lippmann of the capillary electrometer. The principle of this appa-
ratus rests upon the fact that the capillary constant or the surface-
tension of mercury undergoes a change upon the passage of an electric
current, in consequence of a polarization by hydrogen taking place
at its surface. If a capillary glass tube be filled with mercury and

74o

TEXT-BOOK OF PHYSIOLOGY.

its lower end inserted into a solution of sulphuric acid, and the
former connected with the positive and the latter with the nega-
tive electrode, it will be observed, upon the passage of the current,
that a definite movement of the mercury takes place, in the direction
of the negative electrode, in consequence of the diminution of its
capillary constant or the tension of its surface in contact with the acid.
As a reverse movement follows a cessation of the current, a series of
oscillations will follow a rapid making and breaking of the current.
If the direction of the current is reversed, the capillary constant is
increased and the mercury ascends the tube toward the negative
pole. From facts such as these Lippmann constructed the capillary

electrometer, a con-
venient modification of
which devised by M. v.
Frey, is shown in Fig.
380. This consists of a
glass tube, A, forty
millimeters in length,
three millimeters in
diameter, the. lower end
of which is drawn out to
a fine capillary point.
The tube is filled with
mercury and its capillary
point immersed in a 10
per cent, solution of sul-
phuric acid. The vessel
containing the acid is
filled to the extent of
several millimeters with
mercury also. The mer-
cury in the tube is put
in connection with a
platinum wire (a), and
the acid in the vessel
with a second wire (b). When a constant current passes into the
apparatus in the direction from b to a the mercury is pushed up the
tube, and, upon the breaking of the current, it may or may not return
to the zero-point. For the purpose of measuring in millimeters of
mercury the pressure necessary to compensate this change in the
capillary constant produced by the electro-motive force of polarization,
the apparatus is provided with a pressure-vessel, H, and a manometer,
B. This electrometer can be applied to any microscope having a
reversible stage. The oscillations of the mercury can then be observed
with the microscope provided with an ocular micrometer (Fig. 381).
The special advantage of the electrometer is, that it will respond
instantly to any variation in the electro-motive force, and indicate a

Fig.- 380. — Von Frey's Capillary Electrometer.

PHYSIOLOGIC APPARATUS.

74i

difference of potential, according to Lippmann's observation, as
slight as the totts o" °f a Daniell. These rapid oscillations can be re-
corded by photographic methods.

In using either the galvanometer or the electrometer for detecting
the existence of electric currents or differences of potential in living
tissues, it is absolutely essential that non-polarizable electrodes be
employed in connection with it.

DISSECTION OF THE HIND-LEG OF THE FROG.

Much of our knowledge of the physiologic properties of muscles
and nerves has been derived from the study of the muscles and nerves
of the cold-blooded animals, especially of the frog, for the reason that
in these animals the tissues retain their vitality
under appropriate conditions for a considerable
period of time after death or removal from the
body. The muscles generally employed for ex-
perimental purposes are the gastrocnemius, the
sartorius, the semimembranosus, the gracilis, and
the hyoglossus. The nerve generally employed
is the sciatic. Both muscle and nerve may be
studied independently of each other, or they may
be studied together, as when in their usual physi-
ologic relation. For this latter purpose the gas-
trocnemius muscle and sciatic nerve are em-
ployed, constituting the so-called "nerve-muscle
preparation."

For these, and many other reasons, the stu-
dent should familiarize himself with the general
anatomy of the frog, and especially with the
anatomy of the posterior extremities.

Preparation of the Frog. — Destroy the frog
by plunging a pin through the skin and soft
tissues covering the space between the occipital
bone and the first vertebra until the point is
stopped by the vertebra. Turn the pin toward
the head and push it into the brain cavity;
move it from side to side and destroy the brain. Pass the pin into the
spinal canal and destroy the spinal cord. With a stout pair of scissors
cut off the body behind the fore-limbs. Remove the viscera and the
abdominal walls. Draw the hind-legs out of the skin. Place the
legs on a glass plate, back uppermost, and moisten them freely with
normal saline solution.

Observe on the outer side of the dorsal surface of the thigh the follow-
ing muscles (Figs. 382, 383). The triceps femoris (tr), made up of the
rectus anticus (ra), the vastus externus (ve), and the vastus internus
(vi), not seen from behind; on the inner side, the semimembranosus

Fig. 381. — Capillary
Electrometer. R.
Mercury in tube; capil-
lary tube. 5. Sulphuric
acid, q. Hg. B. Ob-
server. M. Microscope.

742

TEXT-BOOK OF PHYSIOLOGY.

(sm) and the rectus interims minor or gracilis (ri"). Between these
two groups, note the biceps femoris (b). Above the thigh observe
the gluteus (gl), the ileococcygeus (ci), and the pyriformis (p).

In the leg observe the gastrocnemius (g) with its tendon (the tendo
Achillis), the tibialis anticus (ta), and the peroneus (pe).

Turn the frog on its back and note the muscles on the ventral surface
of the thigh, the rectus internus major (ri'), and minor (ri"), the ad-
ductor magnus (ad"), the sartorius (s), the adductor longus (ad'),
and the vastus internus (vi). In the leg, in addition to those already
seen from behind, note the tibialis posticus (tp) and the extensor
cruris (ec).

ec

Fig. 382. — Leg Muscles of the Frog.
Ventral Surface. — (Ecker.)

Fig. 383. — Leg Muscles of the Frog.
Dorsal Surface. — (Ecker.)

Note in the abdominal cavity the three large spinal nerves, the
seventh, eighth, and ninth.

Dissection of the Sciatic Nerve. — The sciatic nerve is composed
of the seventh, eighth, and ninth spinal nerves. After its emergence
from the pelvic cavity, it passes down the thigh between the semi-
membranosus and the biceps muscles, in company with the femoral
blood-vessels. Below the knee it divides into the tibialis and peroneus
nerves; the former sending branches into the gastrocnemius. In its
course, the sciatic sends branches to the muscles of the entire leg.

Carefully separate the biceps and semimembranosus by tearing
the connective tissue uniting them. The sciatic nerve and femoral
blood-vessels come into view; with a bent glass rod gently separate the

PHYSIOLOGIC APPARATUS. 743

nerve from its surroundings from the knee to the thigh. Begin at the
knee. In order to expose the nerve at the pelvis, it will be necessary
to divide the pyriformis and the ileo-coccygeus muscles. Care must
here be exercised, so as not to injure the nerve which lies immediately
beneath. Lift up the uro-style with the forceps and separate it from
the last vertebra. With the scissors cut off the vertebral column above
the seventh vertebra. Place the legs on the dorsal surface and then
divide the seventh, eighth, and ninth vertebrae lengthwise. With the
forceps lift up one lateral half of the vertebras and free the nerve as far
as the knee by dividing connective tissue and nerve branches. Be
careful not to injure the nerve with scissors or forceps.

The Nerve-Muscle Preparation.— Divide the tendo Achillis
just below its nbro-cartilaginous thickening at the heel, and detach the
gastrocnemius up to the knee. Cut through the tibio-fibular bone
just below the knee-joint. Cut the femur transversely near its middle
and remove the muscles from the lower end, carefully avoiding injury
to the nerve. The completed preparation consists of the gastroc-
nemius muscle, the sciatic nerve, with half of the seventh, eighth,
and ninth vertebrae and the lower half of the femur.

INDEX.

Abducens nerve, 597
Aberration, chromatic, 676

spheric, 676
Absorption, 213

by epithelium of villi, 224
of foods, 223
of fat, 227
of proteins, 226
of sugar, 225
of water, 225
of lymph, 222
spectra of blood, 256
Accommodation of the eye, 667

convergence of eyes during, 672
force of, 671
mechanism of, 668
range, 670
Action currents of muscles, 86
of nerves, 115
reflex, 123

of medulla oblongata, ^32
of spinal cord, 506
Adrenal bodies, 467
Agraphia, 561
Albuminoids, 17
Albumins, 16
Alcohol, effects of, 134
Alimentary canal, 146
Allantois, 712
Animo-acids, 14
Amnion, 712
Amylopsin, 197
Amyloses, 8
Animal body, structure of, 3

heat, 437
Ankle clonus, 511

jerk, 511
Aphasia, 560
ataxic, 561
amnesic, 561
Apnea, 425
Arterial circulation, 326

pressure, 341
Arteries, structure and properties of, 326
Articulate speech, 635
Asphyxia, 426

Association centers of cerebrum, 561
Astigmatism, 675
Auditory area, 557
nerve, 603

Basal ganglia, 525
Bile, 201

composition of, 202

Bile, mode of secretion, 204
physiologic action, 205
pigments, 203
salts, 203
Bilirubin, 203
Biliverdin, 203
Bioplasm, 30

physiologic properties, 4
Blastodermic membranes, 711
Blind spot, 678
Blood, 236

changes in, during respiration, 409
circulation of, 271
coagulation of, 238
chemistry of, 267
extravascular, 268
intravascular, 269
constituents of, 236
corpuscles, 242, 2.61, 265
defibrinated, 240
general composition of, 267
physical properties of, 237
plates, 265
pressure, 339
arterial, 341
capillary, 345
causes of, 347

determination of, in man, 352
methods of estimation, 344, 352
variations in, 34S
venous, 346
quantity of, 266
serum, 240

velocity of, in arteries, 358
of, in capillaries, 360
of, in veins, 361
Burdach, column of, 466

Calcium salts of the body, 22
Calorimeter, 441
Capillary blood-vessels, 32 S
functions of, 329

circulation, 367

electrometer, 730
Capsule, internal. 525

functions of, 536
Carbohydrates, 8
Carbon monoxid hemoglobin, 269
Cardiac cycle, 285
Cardio-accelerator center, 320

factors which determine its activity, 320
Cardio-inhibitor center, 321

factors which determine its activity, 321
Cardio-pulmonarv vessels. 27,

745

746

INDEX.

Caseinogen, iS, 451
Caudate nucleus, 525
Cells, structure of, 26

chemic composition, 27

manifestatations of life by, 28

reproduction of, 31
Central organs of the nerve system, 491
Cerebellar tract, 405
Cerebellum, 569

functions of, 571

results of experimental lesions, 572
Cerebrum, 537

convolutions of, 539

fissures of, 537

functions of, 545

localization of function in, 547

motor area of the chimpanzee brain,

554

motor area of the human brain, 558
motor area of the monkey's brain, 551
sensor areas of the human brain, 555
sensor areas of the monkey's brain, 549
structure of the gray matter, 541
structure of the white matter, 543

Chemic composition of the body, 7

Chimpanzee brain, motor area of, 554

Cholesterin, 203

Chorda tympani nerve, 161

Chorion, 713

Chromo-proteins, 19

Chyle, 227

Ciliary movement, 94
muscle, 625

function of, 669

Circulation of blood, 271

forces concerned, 370

Clark's vesicular column, 455

Classification of food principles,":^©

Coagulated proteins, 20

Cochlea, 693

functions of, 699

Colostrum, 453

Commutator, 727

Complemental air, 404

Conjugated proteins, 18

Connective tissues, 35

physical and physiologic properties of,

4i
Corpora quadrigemina, 524

functions of, 533
striata, 525

functions of, 534
Corpus luteum, 705
Cranial nerves, 577
Crura cerebri, 523

functions of, 533

origins of, 577
Crystalline lens, 658

Daily ration of U. S. soldier, 144
Decidual membrane, 710
Defecation, 2 it

nerve mechanism of, 211
Deglutition, 163

nerve mechanism of, 170

Demarcation current, 85

Depressor nerve, 323, 378, 380

Dextrin, 9

Dextroses, 9

Diabetes, 460

Diapedesis of leucocytes, 369

Diaphragm, 389

Dietaries, 143

Diffusion, 230

Digestion, 145

Digestive apparatus, 145

Dilatator pupillae muscle, 652

Direct cerebellar tract, 500

pyramidal tract, 498
Ductless glands, 462
Ductus arteriosus, 718

venosus, 717
Dyspnea, 425

Electrodes, non-polarizable, 724
Electrotonic alterations in excitability of
nerves, 117

current, 116
Electrotonus, 116
Encephalo-spinal fluid, 492
Endocardium, 274
Enterokinose, 200
Epidermis, 487
Epididymis, 706
Epinephrin, 468

Epithelial tissues, functions of, ^^, 34
Equilibration, mechanism of, 574
Erepsin, 199

Erlanger's sphygmomanometer, 356
Erythrocytes, 242
Eupnea, 424

Eustachian tube, 689, 698
Excretion, 472

Expiratory forces and muscles, 398
Expired air, composition of, 407
Eye, cardinal points of, 660

dioptric apparatus of, 658

muscles of, 683

physiologic anatomy of, 650

reduced, 663

schematic, 662

Facial nerve, 598

paralysis of, 601
Fallopian tube, 702
Fat, 12

absorption of, 227

digestion of, 199

emulsification of, 13

saponification of, 13
Feces, 210
Fecundation, 708
Fchling's solution, 9
Fetal circulation, 715

membranes, 712

structures, 712
Fibrin, 20
Fibrinogen, 241
Fillet, 521
Filtration, 234

INDEX.

747

Follicle, Graafian, 701
Food, 127

animal, 139

cereal, 141

composition of, 139

disposition of, 130

heat value of, 134

principles, 130

quantities required daily, 12S

vegetable, 142
Forces aiding the. movement of lymph

and chyle, 228
Fovea, 653, 656

Galactose, n

Gall-bladder, 201

Galvanic current, effect of, on nerves, 116

Galvanometer, 738

Ganglia, cephalic, 624

Gaseous exchange in lungs, 407

in tissues, 415
Gases of blood, relation of, 410

tension of, 414

carbon dioxid, 413

oxygen, 412
Gastric digestion, 171
fistula?, 176
glands, 174
juice, 177

mode of secretion, 178

physiologic action of, 182
Globulins, 17

Glossopharyngeal nerve, 605
Glycogen, 10, 458

Glycogenic function of the liver, 457
Gluco-proteins, 19
Gmelin's test for bile pigments, 204
Goll, columns of, 501
Gowers' antero-lateral tract, 500
Graafian follicle, 701
Graphic method, 733
Green vegetables, 142

Hairs, 489

Hearing, sense of, 689

Heart, 271

action of sympathetic nerve on, 3 1 2 , 3 1 6

of vagus nerve on, 314, 317
auriculo-ventricular bundle, 280
beat, nature of the stimulus, 302

action of inorganic salts, 303

frequency of, 285

of the excised heart, 296
blood-supply, 294
causation of, 305
causes of the variations of, 322
course of blood through, 276
cycle of, 285
intracardiac nerve-cells, 310

pressure, 289
intraventricular pressure curve, 290
mechanics of, 282
modifications of beat due to the action

of drugs, 323
muscle-band of His, 280

Heart, muscle-fibers of, 279
negative pressure ofr 292
nerve, mechanism of, 308
orifices and valves, 277, 278
origin and distribution of the sympa-
thetic nerves to, 310
origin and distribution of the vagus

nerve to, 311
physiologic anatomy of, 271
relative function of auricles and ven-
tricles, 288
sounds, 293

synchronism of the two sides, 288
valves, action of, 286
work done by, 371
Heart-muscle, properties of, 296
automaticity, 302
conductivity, 297
irritability, 296
response to action of a stimulus,

306
rhythmicity, 301
tonicity, 301
Heat dissipation, 442
income, 438
relation to work, 444
rigor, 70
Helmholtz's theory of color perception, 685
Hematin, 260
Hemianopsia, 584
Hemoglobin, 252

absorption spectra, 256
chemic composition of, 253
compounds of, 258
quantity of, 254
Hemoglobinometer, Gowers', 255
Hemometer, v. Fleischl's, 256
Hering's theory of color perception, 686
Histons, 16
Horopter, 681
Hypermetropia, 674
Hyperpnea, 424
Hypoglossal nerve, 615

Incus, 691

Induced currents, 730, 731
Inductorium, 729
Infra-proteins, 20
Insalivation, 151

nerve mechanism of, 155
Inspiration, 395

movements of thorax, 391

muscles, 395
Insula, 541

Intercostal muscles, 389.
Internal capsule, 525

functions of, 536

secretion, 462
Intestinal digestion, 191

fermentation, 210

juice, 193

physiologic action of, 200

movements, 206

nerve mechanism of, 20S
Intracardiac presssure, 289

748

INDEX.

Intracranial circulation, 563

mechanism of, 564
Intrapulmonary pressure, 392
Intrathoracic pressure, 392
Intravascular coagulation, 269
Invertin, 200
Iris, 652

functions of, 672

nerve mechanism of, 588, 673
Iron of the body, 24, 254
Irritability of muscles, 59

of nerves, 109
Island of Langerhans, 195

of Reil, 541
Isometric myogram, 72
Isotonic myogram, 67
Isthmus of encephalan, 521
functions of, 527

Jacobsen's nerve, 605
Joints, 49

classification of, 49

Kidney, 476

histology of, 477
Knee-jerk, 475
Kymograph, 734, 735

Labyrinth of ear, 691
Lacrimal glands, 687
Lactation, 718
Lacteals, 227
Lactose, n
Language, 559
Large intestine, 208
Larynx, 626

nerve mechanism of, 635

structure of, 627
Lateral columns of the spinal cord, 499
Law of contraction, 119
Lecithin, 204
Lemniscus, 521
Lens, crystalline, 658
Lenticular nucleus, 526
Leukocytes, 261

chemic composition of, 262

classification of, 263

functions of, 265

number of, 262

origin of, 265

physiologic properties, 263
Levers, 88
Levulose, 10
Limbic lobe; 506
Liver, 201, 483

formation of urea in, 462

functions of, 455

influence of the nerve system on, 459

production of glycogen, 457

secretion of bile, 456
Localization of functions in cerebrum, 547
Lungs, structure of the, 384
Lymph, 218

absorption of, 222

1 omposition of, 219

Lymph, functions of, 221

movement of, 228

physical properties, 219

production of, 220
Lvmph capillaries, 214
Lymph-glands, 215
Lymph-vessels, 214
Lymphocytes, 219, 264

Macula lutea, 653
Malleus, 691
Maltose, n
Mammary gland, 449
Mastication, 147
muscles of, 149
nerve mechanism of, 150
Meats, composition of, 139
Medulla oblongata, 519

reflex activities of, 532
Meibomian glands, 687
Membrana tympani, 690

functions of, 697
Menstruation, 704
Metabolism on protein diet, 138

on fat and carbohydrate diet, 139
Methemoglobin, 260
Migration of leukocytes, 264, 369
Milk, 451

composition of, 139, 452
mechanism of secretion, 452
Moist chamber, 736
Mosso's plethysmograph, 366

spygmomanometer, 353
Motor area of chimpanzee brain, 554
of human brain, 558
of monkey brain, 549
oculi nerve, 585
Mouth digestion, 147
Movements of the eyeball, 6S2
of the intestines, 206
of the lower jaw, 149
of the lungs, 400
of the stomach, 186
Muscle action currents, 86
contraction, 65

chemic phenomena of, 80
electric phenomena of, 83
graphic record of, 66
modifying influences of, 68
physical phenomena of, 63
rigor mortis, 89
summation effect, 74
tetanus, 75

thermic phenomena of, 82
electric currents from, 85
electric currents, negative variation

of, 85
energy, source of, 81
fatigue, 70

groups, special action of, '87
sense, 644
sound, 80
spindle, 645
stimuli, 60
tissue, 53

INDEX.

749

Muscle tissue, chemic composition of, 56

elasticity, 58, 64

histology of, 54, 91

irritability, 59

physical properties of, 57

physiologic properties of, 60

tonicity, 59
Myopia, 673
Myosinogen, 17, 57
Myxedema, 427

Nerve, abducens, 597

auditory, 603

facial, 598

glossopharyngeal, 605

hypoglossal, 615

irritability, 109

motor oculi, 585

olfactory, 579

optic, 582

patheticus, 591

pneumogastric, 606 '

spinal accessory, 612

stimuli, no

tissue, histology of, 96

trigeminal, 592
Nerve impulse, no
Nerve-muscle preparation, 112, 743
Nerve system, functions of, 493
Nerve tissue, 96

histology of, 96
Nerves, chemic composition and metab-
olism of, 101

classification of, 107

degeneration of, 105

development of, 104

effects of galvanic current on, 116

electric currents of, 114

electric currents of, negative varia-
tion of, 113

electric excitation of, 112

electric phenomena of, 113
action currents, 115
diphasic action currents, 115

peripheral endings of, 103

physiologic properties of, 109

pilo-motor, 108

polar stimulation of, 119, 121

relation of, to central nerve system,
102

stimuli of, 109
Neuron, 96

Nicotin, actions of, 324
Nucleo-proteins, 19
Nucleus caudatus, 525

cuneatus, 501

gracilis, 501

lenticularis, 526
Nutrition of the embryo, 713

Oculo-motor nerve, 585
Ohm's law, 722
Olein, 13

Olfactory nerve, 579
Oncograph, 483

Oncometer, 483
Ophthalmic ganglion, 624
Optic constants, 659

thalamus, 527

functions of, 535
Optic nerve, 582
Optogram, 680
Organ of Corti, 694
Osazones, 12
Osmometer, 231
Osmosis, 230
Osmotic pressure, 231
Ossicles of ear, 691
Otic ganglion, 625
Ovary, 701
Ovulation, 703
Ovum, 702
Oxygen in blood, 412

in tissues, 415

quantity absorbed daily, 422
Oxyhemoglobin, 258

Pacinian corpuscle, 640
Palmitin, 13
Pancreas, 194
Pancreatic juice, 196

mode of secretion, 196

physiologic action of, 197
Parathyroids, 465
Partial pressure of gases, 411
Parturition, 717
Pathetic nerve, 591
Pepsin, 178
Peptones, 184
Perspiration, 486
Peripheral organs of the nerve system,

100, 491
Petrosal nerves, 600
Pettenkofer-Voit respiration apparatus,

420
Pexin, 178
Phagocytosis, 265
Phloridzin diabetes, 461
Phonation, 626

mechanism of, 632
Phospho-proteins, 18
Physiology of the cell, 26

of movement, 43
Pilo-motor nerves, 454
Pituitary body, 466
Placenta, 714

Plasma of blood, composition of 240
Pleura, 390
Pneumatograph, 405
Pneumogastric nerve, 606
Pneumograph, 403
Polar stimulation, 119

of human nerves, 121
Pons varolii, 522

functions of, 523
Portal vein, 227
Postures, 89
Presbyopia, 673
Prosecretin, 197
Protamins, 16

75°

INDEX.

Proteins, 14

cheraic composition, 14

color reactions, 21

physical properties, 15

precipitation tests, 21

structure of, 14
Ptyalin, 159
Pulmonary vascular apparatus, 370

ventilation, 409
Pulse, 362

frequency, 363

wave, velocity of, 364
Punctum proximuni, 671

remotum, 671
Pyramidal tracts of spinal cord, 498, 501

Reaction of degeneration, 124
Red corpuscles, 242

chemic composition of, 252
effects of reagents, 248
function of, 251
life history of, 251
number of, 246
of vertebrated animals, 249
Reduced hemoglobin, 258
Reflex action, 125, 506

laws of, 509
Refractory period of the heart, 307
Regnault's and Reisset's respiration ap-
paratus, 421
Relation of gases in the blood, 410
Rennin, 178
Reproduction, 701

Reproductive organs of the female, 701
Reproductive organs of the male, 706
Reserve air, 404 .
Residual air, 404
Respiration, 382

changes in composition of air during,

406
changes in composition of blood, 409
changes in tissues, 415
chemistry of, 406

expiratory forces and muscles, 398
frequency of, 402

mechanism of gaseous exchange, 417
nerve mechanism of, 427
Respiration, total respiratory exchange, 419

volumes of air breathed, 403
Respiratory apparatus, 382
movements, 395
muscles, 395

effects of, on arterial pressure,

434
effects of, on the flow of blood
through the thoracic vessel,

434
of upper air passages, 401
I-!' .-.ures, 392
quotient, 408, 422
rhythm, 402

modifii ation of, 429
Chcyne-Stokcs, 427
sounds, 405
i> pes, 402

Retina, 653

functions of, 677
Retinal image, 658

size of, 664
Rheocord, 726
Rigor mortis, 80
Rima glottidis, 626

respiratoria, 632

vocalis, 632
Routes of the absorbed food, 227

Saccharose, n
Saliva, 154

physiologic action of, 158
Salivary glands, 152

histologic changes in, during secre-
tion, 157

nerve mechanism of, 160
Sebaceous glands, 489
Sclero-proteins, 17
Sebum, 489
Secretin, 197
Secretion, 446

internal, 462
Semen, 707

Semicircular canals, 575
Sensor areas of human brain, 555

of monkey brain, 549
Serum, 240

Setchenow's center, 477
Sight, sense of, 650
Skeleton, physiology of, 48
Skin, 486

nerve endings in, 603
reflexes, 509
Sleep, 565
Smell, sense of, 648
Sodium glycocholate, 203
Sodium taurocholate, 203
Spectroscope, 115
Speech, 635
Spermatozoa, 708
Spheno-palatine ganglion, 624
Sphygmograph, 364
Sphygmomanometer, 355, 356
Spinal accessory nerve, 612
cord, 494

encephalo-spinal conduction, 515

functions of, 503

as a conductor, 513

as an independent center,

5°4
nerve-cells, classification of, 497
nerve fibres of, 498

classification of, 463, 498
reflex actions of, 506
reflex irritability of, 511
relation of spinal nerves to, 501
segmentation of, 503
spinal nerve roots, functions of,

502
spino-encephalic conduction, 514
structure of gray matter, 495
structure of white matter, 498
tracts of, 499

INDEX.

75i

Spirometer, 404
Splanchnic nerves, 622
Spleen, 468

functions of, 469
Stanton's sphygmomanometer, 355
Stapes, 691
Starch, 8

digestion of, 158
Starvation, 136
Stearin, 12

Stereognostic area, 557
Stomach, 171

movements of, 186
nerve mechanism of, 189
Suprarenal capsules, 467
Sweat-glands, 487

Sweat, influence of nerve system on pro-
duction of, 488
Sympathetic nerve system, 616

cephalic ganglia of, 624
functions of the cervical por-
tions, 621
functions of the lumbosacral

portions, 623
functions of the thoracic por-
tion, 622

Taste buds, 647

nerve of, 646

sense of, 646
Teeth, 147
Tegmentum, 488
Temperature of body, 437

regulation of, 443

sense, 643
Tendon reflexes, 510
Tension of gases in blood, 414

tissues, 417
Tensor tympani muscle, 691

functions of, 697
Testicles, 706
Tetanus, 75

experimental, 79

pathologic, 79

pharmacologic, 79

physiologic, 78
Thoracic duct, 217
Thorax, 388

dynamic condition of, 394

mechanic movements of, 391

static condition of, 392
Thyroid gland, 463

functions of, 463
Tidal air, 404
Tissue spaces, 213
Tongue, 646
Total carbon-dioxid exhaled, 422

oxygen absorbed, 422

respiratory exchange, 410
Touch, sense of, 639
Trachea, 384
Tracts of spinal cord, 465
Traube-Hering waves, 435
Trigeminal nerve, 592
Trypsin, 198

Ttirck, column of, 49S
Tympanum, 689

Umbilical cord, 713
Upper air-passages, respiratory move-
ments of, 401
Urea, 473

seat of formation, 462, 474
Uric acid, 474
Urine, 472

composition of, 473
mechanism of secretion, 479

influence of blood composition,

484
influence of nerve system, 483
relation of blood-pressure to, 481
Urination, 484

nerve mechanism of, 485
Uterus, 702

Vagus nerve, 606

influence on heart, 314, 317
Valves of heart, 2 78
Vasa deferentia, 706
Vascular apparatus, 326

glands, 462

hydrodynamic considerations, 330,

333

stream bed, 336

nerve mechanism of, 372
Vaso-motor center, 376

direct stimulation, 377

nerves, 373

reflex stimulation, 378
Veins, 313

structure and function, 329
Velocity of blood, 357, 359
Venous circulation, 369
Vertebral column, 51
Vesiculae seminales, 706
Villi, 223

functions of, 224
Visceral muscle, 90

functions of, 93

properties of, 91
Vision, 650

accommodation, 667

astigmatism, 675

binocular, 6S0

color perception, 6S4

functions of retina, 677

hypermetropia, 674

myopia, 673

presbyopia, 673
Visual angle, 664
Vital capacity of lungs, 404
Vocal bands, 630

sounds, 633
Voice and speech, 635
Volume pulse, 366

Walking, 90

Wallerian degeneration, 106
Water, amount of, in the body, 22
Watery vapor in breath, 40S

752 INDEX.

Wernicke's pupillary reaction, 590 Yellow spot, 653

White blood-corpuscles, 261

function of, 265 Zona pellucida, 710

migration of, 369 Zymogen, 178
origin of, 265 pepsinogen, 159, 173

varieties of, 263 ptyalogen, 159

Wrisberg, nerve of, 600 trypsinogen, 200

QP3+ 3 83