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A TEXT-BOOK

HUMAN PHYSIOLOGY

INCLUDING A SECTION ON

PHYSIOLOGIC APPARATUS

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

PROFESSOR OP PHYSIOLOGY AND MEDICAL JURISPRUDENCE IN THE JEFFERSON MEDICAL COL-

LEGE; FORMERLY PROFESSOR OF PHYSIOLOGY IN THE PENNSYLVANIA COLLEGE OF
DENTAL SURGERY; FORMERLY LECTURER ON PHYSIOLOGY AND HYGIENE
IN THE DREXEL INSTITUTE OF ART, SCIENCE, AND INDUSTRY

FIFTH EDITION. REVISED AND ENLARGED
WITH 1 COLORED PLATE AND 359 ILLUSTRATIONS

PHILADELPHIA

P. BLAKISTON'S SON & CO.

1012 WALNUT STREET

-^sU^rC^^^^ >

THE. MAPLE .PRESS. YORK. PA

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LOYAL FRIEND COURTEOUS GENTLEMAN

GENEROUS PUBLISHER
THE PRESENT EDITION OF THIS WORK

IS
AFFECTIONATELY DEDICATED

PREFACE TO FIFTH EDITION

In the preparation of the fifth edition of the Text-book of Physiology
the attempt has been made once again to increase its value to those for
whom it is primarily intended, viz., students of medicine and practitioners
who desire to keep in touch with the progress of physiology. With this
view the text has been subjected to careful revision and in many sections
entirely rewritten. Some of the anatomic diagrams have been eliminated
and physiologic diagrams inserted. The suggestions and criticisms con-
tained in reviews and book notices, in letters from teachers and practi-
tioners have been considered and in many instances adopted, with benefit
to the text, for all of which the writer expresses his appreciation.

Among the subjects which have been added, enlarged or revised
may be mentioned the metabolism of the food principles; carbohydrate
metabolism; the electric currents of the heart and their graphic registra-
tion, the electro-cardiogram; animal heat; internal secretion; the localiza-
tion of functions in the cerebral cortex; the autonomic nerve system, etc.
These changes have necessitated the elimination of less important mate-
rial, but, nevertheless, some thirty-seven additional pages have been
added to the general body of the text.

To those teachers and students who have recommended and used this
work and to whom I am indebted for generous praise, kind criticisms, and
helpful suggestions, I wish to express my sincere thanks and trust that in
its improved form it will continue to meet their approval.

Once again I desire to express my appreciation of the unwearied and
invaluable assistance of Mr. I. A. Hagy in preparing the manuscript for
the press. '

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

A. P. B.

PREFACE TO FIRST EDITION.

The object in view in the preparation of this volume was the selection and
presentation of the more important facts of physiology, in a form which it is
believed will be helpful to students and to practitioners of medicine. Inas-
much 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 selec-
tion 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 investiga-
tion, 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.

PAGE

CHAPTER I.
INTRODUCTION i

CHAPTER II.
CHEMIC COMPOSITION OF THE HUMAN BODY 6

CHAPTER III.
PHYSIOLOGY OF THE CELL 24

CHAPTER IV.
HISTOLOGY OF THE EPITHELIAL AND CONNECTIVE TISSUES 31

CHAPTER V.
THE PHYSIOLOGY OF MOVEMENT . . ' 39

CHAPTER VI.
THE PHYSIOLOGY OF THE SKELETON 47

CHAPTER VII.
GENERAL PHYSIOLOGY OF MUSCLE-TISSUE S1

CHAPTER VIII.
THE GENERAL PHYSIOLOGY OF NERVE-TISSUE 9°

CHAPTER IX.
FOODS ....... "6

CHAPTER X.
DIGESTION • • X3^

CHAPTER XL
ABSORPTION 2IC

CHAPTER XII.
THE BLOOD 23°

CHAPTER XIII.
THE CIRCULATION OF THE BLOOD 4

CHAPTER XIV.
THE CIRCULATION OF THE BLOOD (Continued) « 32

xii CONTENTS.

PAGE
CHAPTER XV.

RESPIRATION 3?7

CHAPTER XVI.
ANIMAL HEAT .*.,... 43§

CHAPTER XVII.
EXCRETION .< 453

CHAPTER XVIII.
EXTERNAL SECRETIONS 47^

CHAPTER XIX.
INTERNAL SECRETION 49°

CHAPTER XX

METABOLISM . . , 5°9

CHAPTER XXI.
THE CENTRAL AND PERIPHERAL ORGANS OF THE NERVE SYSTEM 523

CHAPTER XXII.
THE MEDULLA OBLONGATA; THE ISTHMUS OF THE ENCEPHALON; THE BASAL GANGLIA . . 551

CHAPTER XXIII.
THE CEREBRUM .* 570

CHAPTER XXIV.
THE CEREBELLUM . 603

CHAPTER XXV.
THE CRANIAL NERVES 610

CHAPTER XXVI.
THE AUTONOMIC NERVE SYSTEM 640

CHAPTER XXVII.
PHONATION; ARTICULATE SPEECH 658

CHAPTER XXVIII.
THE SENSES OF TOUCH, TASTE AND SMELL 669

CHAPTER XXIX.
THE SENSE OF SIGHT 6»

CHAPTER XXX.
THE SENSE OF HEARING

CHAPTER XXXI.
REPRODUCTION 726

APPENDIX.

PHYSIOLOGIC APPARATUS . .

741

INDEX .

767

TEXT-BOOK OF PHYSIOLOGY

CHAPTER I
INTRODUCTION

An animal organism in the living condition exhibits a series of phe-
nomena which relate to growth, movement, mentality, and reproduction.
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. Through-
out 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 perpetuated.

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, physi-
ology 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 deter-
mine them.

This may naturally be divided into:

1. Individual physiology, the object of which is a study of the vital phenomena

or functions exhibited by the organs of any individual animal.

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 unfolding 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 and tissues 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 anatomv, it is essential that

2 .• ;TEXT-BOOK OF PHYSIOLOGY

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 composed
of a number of well-defined structures, such as heart, 'lungs, stomach, brain,
eye, etc., to which the term organ was originally applied, 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 individual — e.g., bones, muscles, nerves, skin,
teeth, glands, blood-vessels, etc. Indeed, any complexly organized struc-
ture capable of performing a given function may be described as an organ.
A description 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 :

1. Individual anatomy, the object of which is the investigation of the construc-
tion, form, and arrangement of the organs of any individual 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 microscopic 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 inter-
nal structure of the organs, the physical properties 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.

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.— The axial portion of all mammals, to which class
man zoologically belongs, as well as of all birds, reptiles, amphibians and
ish, is characterized by the presence of a bony, segmented axis
which extends in a longitudinal direction from before backward and which

known as the vertebral column or backbone. In virtue of the existence

this column all the classes of animals just mentioned form one great
division of the animal kingdom, the Vertebrata.

Each segment, or vertebra, of this axis consists of

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

INTRODUCTION 3

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 vertebrae are
variously modified and expanded, and, with the addition of new elements,
form the skull; at the posterior extremity they rapidly diminish in size,
and terminate in man in a short, tail-like process. In many animals, how-
ever, the vertebral column extends for a considerable distance beyond the
trunk into the tail. The vertebral column may be regarded as the founda-
tion 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.

The Dorsal Cavity.— 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 vertebrae and partly by the bones
of the skull. If a longitudinal section can 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 cavity nerves pass out which connect the
brain and spinal cord with all the structures of the body.

The Ventral Cavity. — The ventral cavity is confined mainly to the trunk
of the body. Its walls are formed by muscles and skin, strengthened in
most animals by bony arches, the ribs. Within the ventral cavity is con-
tained 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 con-
tinuation 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 peritoneal membranes, which, in addition,
are reflected over the surfaces of the organs contained within them.

The Surfaces of the Body. — The external surface of the body is covered
by the skin. This is composed of an inner portion, the derma, and an
outer portion, the epidermis. The former consists of connective-tissue
fibers, blood-vessels, nerves, etc.; the latter of layers of scales or cells. Em-
bedded within the skin are numbers of glands, which exude, in the different
classes of animals, sweat, oily matter, etc. Projecting from the surface of

4 TEXT-BOOK OF PHYSIOLOGY

the skin are hairs, bristles, feathers, claws. Beneath the skin are found
muscles, bones, blood-vessels, nerves, etc.

The internal surf ace— the surface of the alimentary tract and associated
cavities — is covered by mucous membrane.

The Appendicular Portion.— 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 loco-
motion 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 anatomists into
systems — e.g., the bones, collectively, constitute the bony or osseous system;
the muscles, the nerves, the skin, constitute, respectively, 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 necessary to the life of the individual,
everywhere it is found that two or more organs, though performing totally
distinct functions, are cooperating for the accomplishment of some larger
or compound function in which their individual functions are blended — e.g.,
the mouth, stomach, and intestines, with the glands connected with them,
constitute the digestive apparatus, the object or function of which is the com-
plete 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 func-
tions 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 physio-
logic apparatus— e.g., digestion, absorption of food, elaboration of blood,
circulation of blood, respiration, production of heat, secretion, and excretion
—are classified as nutritive functions, and have for their final object the
preservation of the individual and therefore the species.

The nerves and muscles constitute the nervo-muscle apparatus the
function of which is the production of motion. The eye, the ear, the nose,

INTRODUCTION 5

the tongue, and the skin, with their related structures, constitute, respec-
tively, the visual, auditory, olfactory, gustatory, and tactile apparatus, the
function of which, as a whole, is the reception of impressions and the trans-
mission of nerve impulses to the brain, where they give rise to visual, audi-
tory, olfactory, gustatory, and tactile sensations and volitional impulses.

The brain, in association with the sense organs, forms an apparatus
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 functions exhibited by the ap-
paratus just mentioned — viz., motion, sensation, language, mental and
moral manifestations — are classified as functions of relation, as they serve to
bring the individual into conscious 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 repro-
ductive apparatus characteristic of the two sexes. Their cooperation results
in the union of the germ-cell and sperm element and the consequent develop-
ment 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 corresponding parts
of different animals. Hence it is that in the study of human anatomy a
knowledge of the form, construction, and arrangement of the organs in
different types of animal life is essential to its correct interpretation; it
follows also that in the investigation and comprehension of the complex
problems of human physiology a knowledge of the functions of the organs
as they manifest themselves in the different types of animal life is indispen-
sable. 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 determina-
tion of the successive changes in composition of the living body is attended
with many difficulties. The living material, the bioplasm, is not only
complex and unstable in composition, 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 —

1. 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 sub-
stances 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 material 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, how-
ever, they must be obtained in the form under which they exist in the living
condition. The organic compounds consist of representatives of the carbo-
hydrate, fat, 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 per cent.; hydrogen, 9.10; nitrogen, 2.5; carbon, 13.50;
phosphorus, 1.15; calcium, 1.30; sulphur, 0.147; sodium, o.io; potassium,
0.026; chlorin, 0.085; fluorin, iron, silicon, magnesium, iodine, in small and
variable amounts.

CHEMIC COMPOSITION OF THE HUMAN BODY 7

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 converted 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 : i.
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., (i) amyloses,
including starch, dextrin, glycogen, and cellulose; (2) dextroses, 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, poly-
saccharids — multiples of six.

Though but few of the members of the carbohydrate group are con-
stituents 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 carbohydrates
will be stated in this connection.

Chemic Composition. — A chemic analysis of the carbohydrates shows
that they consist of carbon hydrogen and oxygen though the percentage
of these elements varies somewhat in different members of the group.
The average percentage composition of several carbohydrates is as follows:

C. H. o.

Starch 44-44 6.17 49-39

Dextrose 40.00 6.66 53-34-

Saccharose 42.10 6.44 51.46

i. 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 7o°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 dis-
appearing on heating, but reappearing on cooling.

Boiling starch with dilute sulphuric acid (25 per cent.) converts il
dextrose. In the presence of vegetable diastase or animal ferments, starch
is converted into maltose and dextrose, two forms of sugar.

8 TEXT-BOOK OF PHYSIOLOGY

Dextrin is a substance formed as an intermediate product in the trans-
formation of starch into sugar (maltose). There are at least two principal
varieties — erythrodextrin, which strikes a red color with iodin, and achroodex-
trin, 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 animal ferments erythrodextrin is converted into maltose.

Glycogen is a constituent of the animal liver, and, to a slight extent,
of muscles, 0.5 to 0.9 per cent., 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. Cellulose is
very stable toward dilute acids and alkalies. It is this property which is
made use of in the technical preparation of cellulose, in order to free it from
the other substances present in the plant material. When cellulose is
treated with strong sulphuric acid, and after disintegration diluted with
water and then boiled, it is completely hydrolyzed to dextrose.

2. DEXTROSES, C8H13O,.

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 water 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, it will be found that dextrose turns the plane
of polarized light to the right. It is therefore termed dextro-rotatory and
has received its name from this fact.

It has for a long time been known that when sugar, cupric hydroxid, and
an alkali— e.g., sodium or potassium hydroxid— are present in solution, the
sugar will abstract from the cupric hydroxid a portion of its oxygen, thus re-
ducing it to a lower stage of oxidation giving rise to cuprous oxid. Sugar has
a similar action on both silver and bismuth salts. 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

f sodium and potassium tartrate. It is made by dissolving cupric sulphate

34.64 grams, potassium hydroxid 125 grams, sodium and potassium tartrate

173 grams, m distilled water sufficient to make one liter.

The reaction is expressed by the following equation:'

CHEMIC COMPOSITION OF THE HUMAN BODY 9

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 cubic centimeter of
this solution, diluted with 4 c.c. of water, 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 sKowsltself 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 i c.c. of water the ^ of a milli-
gram of sugar. (Dextrose.)

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 suspected
solution is cautiously added from a buret until the blue color entirely dis-
appears. The strength of this solution is such that 10 c.c. is decolorized
by 50 milligrams of sugar (dextrose). Thus if 0.8 c.c. of the suspected solu-
tion, e.g., urine, decolorizes 10 c.c. of Fehling's solution, then it contains 50
milligrams of sugar, from which the percentage of sugar in the urine can be
determined.

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 ceremsia. The change
with dextrose is expressed in the following equation:

CeH12O6 -

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 solu-
tion 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, how-
ever, varies with the temperature and concentration of the solution. For this
reason it is turned levulo-rotatory and has received its name from this fact.

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.

Galactose is obtained by boiling milk-sugar (lactose) with dilute sul-
phuric 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, C12H22On.

Saccharose, or cane-sugar, is widely distributed throughout the vege-
table world, but is especially abundant in sugar-cane, sorghum cane, sugar-

io TEXT-BOOK OF PHYSIOLOGY

beet, Indian corn, etc. It crystallizes in large monoclinic prisms. It is
soluble in water and in dilute alcohol. Saccharose has no reducing power on
cupric hydroxid, and hence its presence cannot 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 fol-
lowing equation :

C12H220U + H20 =CaH1206 +C6H1206

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 inversion, as pre-
viously stated, after which it is readily fermented, yielding alcohol and
carbon dioxid.

Lactose is the form of sugar found exclusively in the milk of the mam-
malia, 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 :

C12H22On + H20 = 4C3H603
Lactose + Water =• Lactic Acid.

2C3H6O3 •= C4H8O2 + 2CO2 + 2H2

Lactic Acid — Buytric Acid + Carbon + Free

Dioxid Hydrogen.

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

2CaH1006 + H20 - C12H22on

Starch. Water. Maltose.

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 decomposition,
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-gluco-

CHEMIC COMPOSITION OF THE HUMAN BODY n

sazone 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 solution add 2 gm. of phenyl-hydrazin and 2 gm.
of sodium acetate, and boil for an hour. On cooling, the osazone crystal-
lizes 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.
The percentage composition of fat (stearin) is as follows: Carbon, 76.86;
hydrogen, 12.36; oxygen, 10.78. The fat found in animals is a mixture, in
varying proportions in different animals, 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:

C3H5(OH)3 + 3H.C18H3602 - C3HS(C18H3502)3 + 3H2O.
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 radical, tritenyl,
f TT

3 Stearin, C3H5(C18H35O2)3, is the chief constituent of the more solid
fats. It is solid at ordinary temperatures, melting at 55°C., then solidify-
ing again as the temperature rises, until at 7i°C. it melts permanently. It
crystallizes in square tables.

Palmitin, C3H5(C16H31O2)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, C3H5(C18H33O2)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 temperature
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:

3H5183323 32 = 353--1.834.a
Olein! Water. Glycerin. Oleic Acid.

The fat acids thus obtained are characterized by certain chemic fea-
tures, as follows:

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

12 TEXT-BOOK OF PHYSIOLOGY

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 hydroxid 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 + 3C18H3402 = 3KC18H3302 + 3H2O
Potassium Hydroxid. 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 potassium 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 centimeters of a solution of sodium
carbonate (0.25 per cent.) a small quantity 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.

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 characteristic 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 I5 -6 23-9 °-4 (Wiirtz).

Serum-albumin 53 .o 6.8 16.0 22.29 1.77 (Hammers ten).

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

Myosin . 52.82 7.11 16.77 21.90 i .27 (Chittenden and Cummins).

CHEMIC COMPOSITION OF THE HUMAN BODY 13

The molecular composition of the proteins is not definitely known
and the formulae which have been suggested are therefore only approxi-
mative. Leow assigns to albumin the formula C72H112N18O22S, while
Schutzenberger raises the numbers to C240H392N65O75S3, either of which
shows that the protein 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 investigation has shown
to be the case.

When any one of the typical proteins, found in animal or vegetable
tissues, is hydrolyzed by acids, alkalies and animal ferments under appro-
priate conditions, it can be resolved through a series of descending stages
into relatively simple nitrogen-holding bodies termed ammo-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-isovalerianic 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 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 reac-
tions. Such bodies are termed, according to their complexity, peptids and
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 diffusibility. 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 purification from crystalloid diffusible com-
pounds by the process of dialysis.

2. Solubility.— Some of the proteins 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 Droteins. owing to their complexity and instability, are prone

14 TEXT-BOOK OF PHYSIOLOGY

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 25°C. to 4o°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
follow the ingestion of foods in various stages of putrefaction are to
be attributed to these compounds. The terminal products are repre-
sented by hydrogen sulphid, ammonia, carbon dioxid, fats, phosphates,
nitrates, etc.

Classification. — The animal proteins by virtue of their structural
composition, their physical and chemic properties, permit of a provisional
arrangement into three groups as follows: Simple proteins, conjugate
proteins and protein derivatives.

SIMPLE PROTEINS

The simple proteins are so called because of the fact that when they
are hydrolyzed they yield only a-amino- and diamino-acids. The members
of this group are as follows:

PROTAMINS.

These proteins are derived for the most part from the heads of the sper-
matozoa of fish. They take their names from the species of fish from
which they are obtained, e.g., salmin (salmon), sturin (sturgeon), scom-
brin (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 constitute 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 com-
posed 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 complex
than the typical proteins; for on hydrolysis they not only yield the hexone
bases but in addition a large number of mono-amino-acids. They are, there-
fore, 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. They are precipitated from

CHEMIC COMPOSITION OF THE HUMAN BODY 15

neutral solution by alkaloid reagents. 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 solu-
tions, and in saturated solutions of sodium chlorid and magnesium 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 sub-
stance, soluble in water and in 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°.
(6) 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 54°C., and in
having a lower rotatory power, —35.5°.

(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 am-
monium 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
73°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 association
with serum-globulin and serum-albumin. It is also present in
lymph-tissue fluids and in pathologic transudates. It can be ob-
tained from blood-plasma which has been previously ^ 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) Paramyosinogen or Myosin.— This protein is a constituent of the
muscle-plasma from which it can be precipitated by a temperature

of 47°C.

(d) Myosinogen or Myogen.— 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

16 TEXT-BOOK OF PHYSIOLOGY

chemic change and contributes to the formation of an insoluble
protein known as myogen fibrin. 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 CO2 through a watery extract of the crystalline lens.

SCLERO-PROTEINS (ALBUMINOIDS).

The sclero-proteins constitute a group of substances similar to the pro-
teins in many respects, though differing from them in others. When ob-
tained 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 ligaments, or de-
calcified 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 protein known as chrondro-
mucoid.

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

PHOSPHO-PROTEINS.

The two members of this group are distinguished by yielding on decom-
position 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 calcium in a form known as calcium-
caseinogenate. It is precipitated by acetic acid and by magnesium
sulphate. It is coagulated by rennin, though the nature of the process
is not very clear. It was formerly taught that under the action of
rennin, an enzyme of the gastric mucous membrane, caseinogen was
separated into a solid portion, casein or tyrein, and a soluble portion.
The cleavage action of rennin thus indicated has not been verified by
subsequent investigations. It is more in accordance with the facts to
assume that the process is a double one and that the action of rennin is
to change the caseinogen to a soluble form, termed paracasein, after
which the lime salts present react with the paracasein in such a
manner as to cause it to assume the solid condition. 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, the casein will not be curdled by rennin, but it will neverthe-

CHEMIC COMPOSITION OF THE HUMAN BODY 17

less, undergo a change, for if the solution be treated as above and then
boiled to destroy the rennin, it will curdle upon the addition of calcium
salts.

(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 conjugated proteins are compounds in which the protein molecule is
combined with some other molecule or molecules, the chemic nature of
which varies considerably in the different members of the group, e.g.,
coloring matter, carbohydrates and nuclein. The chemic character of
the non- protein substance furnishes the basis for the following classification:

CHROMO-PROTEINS.

(a) Hemoglobin. — Hemoglobin is the coloring matter of the red cor-
puscles, of which it constitutes about 30 per cent, of the total weight.
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 oxy-
hemoglobin, and in the venous blood as deoxy- or reduced-hemoglobin.
When hydrolyzed by acids or alkalies, hemoglobin undergoes a
cleavage into a protein, globin, and a coloring matter, hemochromogen,
containing iron, which is easily oxidized to hematin.
(6) 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 protein 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 inter-
cellular substance of the connective tissues. It is readily precipitated
by acetic acid. When heated with dilute acids, mucin undergoes a
cleavage into a simpler protein and a carbohydrate termed mucose,
which is capable of reducing Fehling's solution.

(b) Mucoids.— The mucoids resemble the mucins though differing 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, tendon, bone, and in other situations. Chondromucoid
differs from other connective-tissue mucoids in the large amount of
chondroitin-sulphuric acid obtained upon decomposition. When
decomposed through the action of acid, the chondroitin-sulphuric
acid yields sulphuric acid and a nitrogenous body — chondroitin, which
in turn yields acetic acid and a new nitrogenous substance — chpn-
drosin, which has a more strongly reducing action on Fehling's solution

1 8 TEXT-BOOK OF PHYSIOLOGY

than dextrose. They differ slightly one from the other in proper-
ties and chemic composition. They yield on decomposition a
carbohydrate.

NUCLEO-PROTEINS.

The nucleo-proteins are obtained from the nuclei and cell-substance
of tissue-cells. Chemically they are characterized by the presence of
phosphorus in relatively large amounts. When hydrolyzed, 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, carbohy-
drates— mostly pentoses, pyrimidine bases — thymine, cytosine, and
uracyl, and the so-called purin bases, xanthin, hypoxanthin, adenin,
and guanin. All nucleins which yield the purin bases are termed
true nucleins.

DERIVATIVES OF PROTEIN

The protein derivatives include a variety of substances which arise
through a process of hydrolysis of simple proteins under the action of enzymes
and in the presence of acids and alkalies. The number of derivatives
obtained between the first cleavage of the protein molecule and its final
cleavage to amino-acids is large and will be presented at length in the para-
graph relating to protein digestion. The chief derivatives are as follows:

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 neutralization with sodium
hydroxid (o.i per cent, solution). After the precipitate is washed,
it is found to be insoluble in distilled water and in neutral saline solu-
tions. In acid solutions it is not coagulated by heat.

(6) Alkali-albumin.- — This is formed when a native albumin is treated
with a dilute alkali — e.g., o.i 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, PEPTONES AND POLYPEPTIDS.

During the progress of the digestive process, as it takes place in the stom-
ach and intestines, there is produced by the action of the gastric and pan-
creatic juices, out of the proteins of the food, a series of new proteins,
known as proteoses, peptones and polypeptids. 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 ioo°C. or to the prolonged action of alcohol. They are insoluble in
water, in dilute acids, and in neutral saline solutions.

CHEMIC COMPOSITION OF THE HUMAN BODY 19

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

(a) Fibrin. — Fibrin is derived from one of the blood proteins — viz.,
fibrinogen. It is not present under normal circumstances in the
circulating blood, but makes its appearance after the blood is with-
drawn 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
accumulates. When freed from blood by washing under water, it is
seen to consist of bundles of white elastic fibers or threads. It is in-
soluble in water, in alcohol, and ether. In dilute acids it swells, be-
comes 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 possesses the property of
decomposing hydrogen dioxid, H2O2 — i.e., liberating oxygen, which
accumulates in the form of bubbles on the fibrin. On incinera-
tion fibrin yields an ash which contains calcium phosphate and mag-
nesium phosphate.

Two views are held as to the origin of fibrin: first that it is the result
of the action of a special enzyme, termed thrombin on fibrinogen,
though the nature of the action is not very clear; second that it is
the result of a definite combination, physio-chemic in character, of
fibrinogen with thrombin which, however, is not regarded as an enzyme,
inasmuch as it is not destroyed by boiling, but a definite compound
partaking of the nature of an organic colloid. The amount of fibrin
formed from fibrinogen will be proportional to the amount of thrombin
present.

(b) Myosin fibrin and myogen fibrin are two insoluble proteins developed
out of the two chief proteins of muscle-plasma. Their develop-
ment after death is believed to be the cause of the stiffening of the
muscles. It is not definitely known whether this is the result of the
action of a special enzyme 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 gastric 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 protein 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 phenylalanin 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 coagulated
protein turns a purple-red color. This color is attributed to the
presence of tyrosin.

3. The blue-violet reaction. To the solution is added an excess of sodium

hydroxide, at least an equal volume, and then drop by drop, a very
dilute solution of copper sulphate. A blue-violet color is produced,
which deepens somewhat on heating, but no further change ensues.

20 TEXT-BOOK OF PHYSIOLOGY

This is also known as Piotrowski's test: As this same color is de-
veloped with the substance biuret, it is also known as the biuret
reaction. Biuret is formed by heating urea to i8o°C 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 change of state by virtue of which
they become solid. To this process the term ferment coagulation 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, 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 in-
dispensable to life. It is present in all the tissues and fluids without excep-
tion, 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 weighing 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. Possessing the capability of holding in solution a large number of
inorganic as well as some organic compounds, and being at the same time
diffusible, 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 (6J pounds) daily.
CALCIUM COMPOUNDS.

Calcium phosphate, Ca3(PO4)2, has a very extensive distribution
throughout the body. It exists largely in the bones, teeth, and to a slight ex-
tent in cartilage, blood, and other tissues. Milk contains 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 regarded as necessary to their structure.
It enters into chemic union with the organic matter, as shown by the fact
that it cannot be separated from it except by chemic means, such as immer-

CHEMIC COMPOSITION OF THE HUMAN BODY 21

sion in 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 formation 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, CaCO3, is present in practically the same situa-
tions in the body as the phosphate, and plays essentially the same rdle. It
is, however, found in the crystalline form, aggregated in small masses in the
internal ear, forming the otoliths, or ear stones. 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. 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 favorably the general nutritive process,
though the manner in which it acts is not very clear. During. its existence
in the body it undergoes chemic transformations or decompositions, yielding
its chlorin to form the potassium chlorid of the blood-corpuscles and muscles
and to form the hydrochloric acid of the gastric juice.

Sodium phosphate, Na2HPO4, 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, Na2CO3, 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, Na2SO4, is present in many of the tissues and fluids,
especially in the urine. Though introduced in the food, it is also, m 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, however, in the
tissue elements, especially in the muscle tissue, nerve tissue, and^red cor-
puscles. The plasma with which these structures are bathed contains but a

22 TEXT-BOOK OF PHYSIOLOGY

very small amount of this salt, but, as previously stated, a relatively large
amount 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 so-
dium chlorid.

Potassium phosphate, K2HPO4, 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, K2CO3, is generally found with the preceding
salt.

MAGNESIUM COMPOUNDS.

Magnesium phosphate, Mg3(PO4)2, is found in all tissues, in associa-
tion with calcium phosphate, though in much smaller quantity.

Magnesium carbonate, MgCO3, occurs only in traces in the blood.

IRON COMPOUNDS.

Iron is a constituent of the coloring-matter of the blood. Traces, how-
ever, are also found in lymph, bile, gastric juice, and in the pigment of the
eyes, skin and hair. The amount of iron contained in a body weighing 70
kilograms is about 2.2 gm. It exists under various forms — e.g., ferric oxid,
and in combination with organic compounds.

lodin is found in a number of organs — thyroid gland, lungs, ovaries,
liver, hypophysis, small intestines as well as in the blood and bile. The
iodine content of the thyroid gland is eight to ten times that of all the other
organs. The active ingredient of the thyroid colloid is the iodin-containing
substance called thyroidin. It is contained in that portion of the protein
which is soluble in physiological salt solution and precipitated upon half
saturation with ammonium sulphate. This portion is apparently a globulin
— called thyreoglobulin, and contains an easily separated carbohydrate group.
The thyroid glands of new-born children are iodin free.

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 characteristic pro-
portions, 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 greatest atomic
cohesion. O is noted for the number and intensity of its combinations.

As the properties of the compounds formed by the union of elements
must be the resultants of the properties of the elements themselves, 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 correspond-
ing 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.

CHEMIC COMPOSITION OF THE HUMAN BODY 23

PRINCIPLES OF DISSIMILATION

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

1. A number of organic acids, such as acetic, lactic, oxalic, butyric, pro-
pionic, 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 nitrogen-holding bodies, such as urea, uric acid, xanthin,
hippuric acid, creatin, creatinin, etc.

While some few of these compounds may possibly be regarded as neces-
sary 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 represent stages in their reduction
to simpler forms previous to being eliminated from the body.

CHAPTER III
PHYSIOLOGY OF THE CELL

A histologic analysis of the organs and tissues of the animal body shows
that they can be resolved 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, the first offspring of the
fertilized ovum. Ascending the line of embryonic development, it will be
found that every organized body originates in a single celt — 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 con-
sist mainly of a gelatinous substance forming the body of the cell, termed
cytoplasm or bioplasm, in which is embedded a smaller spheric body, the
nucleus. Within the nucleus there is frequently seen a still smaller body,
the nucleolus. 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
limit, ranging from 7.7^ (^Vcr of an incn> the diameter of a red blood-
corpuscle), to 13 5// CrJ-Q- of an inch, the diameter of the large cells in the
gray matter of the spinal cord). (See Fig. i.)

The cytoplasm consists of a soft, semifluid, gelatinous material, varying
somewhat in appearance in different tissues. Though frequently homogene-
ous, it often exhibits a finely granular appearance under medium powers
of the microscope. Young cells consist almost entirely of clear cytoplasm.
Mature cells contain, according to the tissue in which they are found,
material of an entirely different character — e.g., small globu'es of fat,
granules of glycogen, mucigen, pigments, digestive ferments, etc. Under
high powers of the microscope the cytoplasm is found to be pervaded by
a network of fibers, termed spongioplasm, in the meshes of which is con-
tained 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 cells. The arrangement of the
fibers forming the spongioplasm also varies, the fibers having sometimes a
radial direction, 'n others a concentric disposition, but most frequently being
distributed evenly in all directions. In many cells the outer portion of
the cell protoplasm 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 permeable

PHYSIOLOGY OF THE CELL 25

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 cytoplasm near
the center of the cell. In the resting condition of the cell it consists of a
distinct membrane, composed of amphipyrenin, inclosing the nuclear con-
tents. . The latter consists of a homogeneous 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

Nuclear membrane. x

Linin.

Nuclear fluid (matrix). -../

- Qejj memj-,rane>

Nucleolus. - --.

Chromatin-cords ,.
(nuclear network).

Nodal enlargements ^~'*'
of the chromatin.

Spongioplasm.
Hyaloplasm.

Foreign inclosures.

FIG. i. — DIAGRAM OF A CELL. Microsomes and spongioplasm are only partly drawn. — (Stohr.

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
nudeoli. At the pole of the nucleus, either within or just without in the
cytoplasm, is a small body, the centrosome, or pole corpuscle.

Chemic Composition of the Cell.— The composition of living 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 (char-
acterized by their large percentage of phosphorus), globulins, lipoids, such
as lecithin (a phosphorized fat) and cholesterin (a monatomic alcohol) and
possibly fat and carbohydrates. Inorganic salts, especially the potassium,

26 TEXT-BOOK OF PHYSIOLOGY

sodium, and calcium chlorids and phosphates, are almost invariable and
essential constituents.

MANIFESTATIONS OF CELL LIFE

Growth, the Maintenance of Nutrition, and Reproduction. — All

cells exhibit three fundamental properties of life — viz., growth, the mainte-
nance of their nutrition, and reproduction. 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 may be defined as the sum of the processes concerned in the
maintenance of the physiologic condition of the cell and includes both growth
and repair. So long as this is accomplished, 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
compounds which, apparently of no use in the cell, are eliminated from it.
It is, therefore, said that the living material undergoes molecular disintegra-
tion as a result of functional activity. To these changes the term katabolism
is also applied. Other of these changes are constructive, assimilative or
integrative in character, whereby a part at least of the food material furnished
by the blood-plasma is transformed, through a series of ascending chemic
stages into living material, and whereby it is repaired and its former physio-
logic condition restored. It is, therefore, said that the living material under-
goes molecular integration as a preparation for functional activity. To these
changes the term anabolism is also applied. During the course of its physio-
logic 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.

Living material has also a temperature varying in degree in different
species of animals as well as in different parts of the same animal. Here as
elsewhere the temperature is due to heat liberated from organic compounds
through disruption and subsequent oxidation to simpler compounds.
Though some of the heat liberated may come from the tissue molecules, the
larger part by far comes from the food molecules — sugar, fat, and protein,
constituents of the fluids circulating in the tissue spaces. These foods carry
into the body potential energy, ultimately derived from the sun. When they

PHYSIOLOGY OF THE CELL 27

are disrupted and oxidized the potential energy is transformed into kinetic
energy which manifests itself for the most part as heat. To the sum total
of all the chemic changes occurring in tissues and foods the term metabolism
is given.

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, or their cleavage products, the amino-
acids. Thus Voit contended 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 metabo-
lism 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 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 contended,
as previously stated, that the proteins of the tissue molecules, which he
distinguished as tissue proteins, do not metabolize to any appreciable extent
and confined practically all protein metabolism to the food proteins, now
distinguished as amino-acids, circulating in the tissue spaces and which he
characterized as circulating proteins. Even in starvation the tissue pro-
teins, as such, do not metabolize until they have been disintegrated in con-
sequence of chemic changes and transformed into circulating proteins.

Pfluger, however, asserted that the circulating proteins cannot be metabo-
lized as such 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 protein molecules which is followed in turn by con-
struction out of amino-acids during the subsequent rest, but that the greater
part of the protein metabolism takes place outside the cell, though in
contact with it.

Though the cell is, therefore, the seat of two opposing processes, assimila-
tion and dissimilation, it retains under normal conditions an average physio-
logic state, and so long as this is the case it is in a condition 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.

TEXT-BOOK OF PHYSIOLOGY

To the metabolism that takes place in the tissue protein the term endog-
enous protein metabolism is given while to that which takes place in the
tissue fluids the term exogenous protein metabolism is given.

Reproduction. — Cells reproduce themselves in the higher animals 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. 2) 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 por-
tions, which form the nuclei. Following the division of the nuclei, the
protoplasm divides. The process may be divided into three phases:

Close Skein
(viewed^ from

the side).
Polar field.

Loose Skein (viewed
from above — *. e., from

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

Spindle.

Mother Star (viewed Daughter Star Beginning Completed

from above). Division of the Protoplasm.

FIG. 2. — 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.)

i. 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 which 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
ends interdigitate on the opposite side of the nucleus — the antipole.
The polar field corresponds to the area occupied by the centrosome. This
arrangement is known as the close skein; but as the process goes on, the
chromosomes become thicker, shorter and less contorted, producing a

PHYSIOLOGY OF THE CELL 29

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 positions 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
correspond 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 mon-
aster. While moving toward the equator of the nucleus, and often
earlier, each chromosome 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 perhaps 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 destination,
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 chromo-
somes 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.

Physiologic Properties of Bioplasm.— All living bioplasm possesses
properties which 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 funda-
mental 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

30 TEXT-BOOK OF PHYSIOLOGY

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 movements,
is exhibited by many forms of cell bioplasm. In addition to the molecular
movements which take place in certain cells, other forms of movement ar
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 spermatozoa, the projection of pseudopodia,
etc. These movements, arising without any recognizable cause, are fre-
quently spoken of as spontaneous. Strictly speaking, however, all proto-
plasmic movement is the resultant of natural causes, the true nature c
which is beyond the reach of present methods of investigation.

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
'.vith blood-vessels and nerves. The form of the epithelial cell varies in
different situations, and may be flattened, cuboid, spheroid 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
;emented together by an intercellular substance.

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 as
/sociated body cavities) are both formed, in all situations, by the homogeneous
basement membrane, covered with one or more layers of cells. The glands

FIG. 3. — EPITHELIAL CELLS OF RABBIT, ISOLATED. X 560. i. Squamous cells (mucous
membrane of mouth) . 2. Columnar cells (corneal epithelium) . 3. Columnar cells, with cuticular
border, s (intestinal epithelium). 4. Ciliated cells; h, cilia (bronchial epithelium). — (Stohr.)

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 a sclero-protein (keratin), a
small quantity of water, and inorganic salts. In other situations, especially
on the mucous membranes, 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, pressure,
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 condi-
tions prevail, the epithelium is extremely soft. Epithelial tissues also possess

TEXT-BOOK OF PHYSIOLOGY

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 con-
ductors of heat, they assist in preventing too rapid radiation of heat from
the body, and cooperate with other mechanisms in maintaining the normal
temperature. 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-
f| vessels, the lymph-vessels, the peri-

g toneal, pleural, pericardial, and

other closed cavities are usually

3— ~

FIG. 4. — STRATIFIED SQUAMOUS
EPITHELIUM (LARYNX OF MAN).
X240. i. Columnar cells. 2. Prickle-
cells. 3. Squamous cells. — (Stohr.)

FIG. 5. — STRATIFIED CILIATED
EPITHELIUM. X 560. From the res-
piratory nasal mucous membrane
of man. i. Oval cells. 2. Spindle-
shaped cells. 3. Columnar cells. — •
(Stohr.}

termed endothelial cells. The cells in these situations 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 under-

lying structures which collectively form the true skin, thus protecting
them from the injurious influences of moisture^ air, dust, microorgan-
isms, etc., which would otherwise impair their vitality. Wherever con-
tinuous 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 membranes

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 r61e as absorbing agents in man and the higher animals. The

THE CONNECTIVE TISSUES 33

epithelium of the mucous membrane of the alimentary canal, particu-
larly that of the small intestine, is especially adapted, from its situa-
tion, consistency, and properties, to play the chief r61e in the absorp-
tion 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 connected
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 frame-
work 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 structure,
passing imperceptibly into one another, and perform practically the same
functions.

Areolar Tissue. — This variety is found widely distributed throughout
the body. It serves to unite the skin and mucous membrane to the struc-
tures 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, 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 micro-
scopically, the bundles can be shown to consist of extremely delicate, color-
less, 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 out-
line, a tendency to branch and unite with adjoining fibers, and to 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 corpus-
cles, connective-tissue corpuscles, plasma cells, and granule cells.

Adipose Tissue. — This tissue, which exists very generally throughout
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

TEXT-BOOK OF PHYSIOLOGY

size and shape, surrounded and penetrated by the fibers of connective
tissue. (See Fig. 6.) Microscopic examination shows that these masses
consist of small vesicles or cells, round, elliptical or polyhedral in shape,
depending somewhat on pressure. (See Fig. 7.) 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 synthesis of a fat-
acid and glycerin on the part of the cell material or of an enzyme contained
therein. As fat granules accumulate, at the expense of the cell proto-
plasm, they gradually coalesce, until there remains but a thin stratum
of the protoplasm, which forms the wall of the vesicle. Adipose tissue

super-
posed
layers.

FIG. 6. — ADIPOSE TISSUE. — (Stdhr.)

FIG. 7. — FAT -CELLS FROM THE
AXILLA OF MAN. i. The equator
of the cell in focus. 2. The ob-

somewhat elevated. 3, 4.

changed by pressure.

jective
Forms

Traces of protoplasm in the vicinity
of the flat nucleus k. — (Stohr.)

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 oxida-
tion 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 exami-
nation are seen to consist of delicate, wavy, transparent, homogeneous
fibers, which pursue an independent course, neither branching nor uniting
with adjoining fibers. (See Fig. 8.) A small amount of ground substance
serves to hold them together. Fibrous tissue is tough and inextensible, and

THE CONNECTIVE TISSUES

in consequence is admirably adapted to fulfil various mechanical functions
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 nuchae, 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 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, of their length before breaking. (See Fig. 9.)

FIG. 8. — CONNECTIVE-TISSUE
BUNDLES OF VARIOUS THICK-
NESSES OF THE INTERMUSCULAR
CONNECTIVE TISSUE OF MAN.
X

FIG. 9. — ELASTIC FIBERS OF THE
SUBCUTANEOUS AREOLAR TISSUE OF
A RABBIT.— (After Schdfer.)

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 substance. According to the amount
and texture of the ground substance, three principal varieties may be
distinguished:

i . Hyaline cartilage, in which the cells, relatively few in number, are embedded
in an abundant quantity of ground substance (Fig. 10, A.) The body of
the cells is in many instances distinctly marked off from the surround-
ing substance by concentric lines of 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 sub-
stance 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.

36 TEXT-BOOK OF PHYSIOLOGY

2. 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. io,c.) 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 vertebrae,
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 bones, in giving a certain degree of mobility to the joints, and in
diminishing the effects of shock and pressure imparted to the bones.

ABC

FIG. 10. — THE THREE TYPES OF CARTILAGE: A, HYALINE; B, ELASTIC; C, FIBROUS. — (Rad-
asch). a, b, Outer and inner layers of perichondrium; c, young cartilage cells; d, older cartilage
cells; e, f, capsule; g, lacuna.

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. IO,B.) As these fibers are elastic, they
impart to the cartilage a very considerable degree of elasticity. Yellow
fibro-cartilage is well adapted, therefore, for entering into the formation of
the external ear, epiglottis, Eustachian tube, etc. — structures which
require for their functional 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 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 dissolved 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 micro-
scopically, reveals a number of small, round, or oval openings, which repre-
sent 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 Haversian canals in the living state contain blood-
vessels and lymphatics. (See Fig. n.)

THE CONNECTIVE TISSUES

Around each Haversian canal is a series of concentric laminae, composed
of white fibers. Between every two laminae are found small cavities (lacunae) ,
from which radiate in all directions small canals (canaliculi), which com-
municate freely with one another. The Haversian canals, with their associ-
ated lacunae and canaliculi, form a system of intercommunicating passages,
which circulate lymph destined for the nourishment of bone. Each lacuna
contains the bone corpuscle, which bears a close resemblance to the usual
branched connective-tissue corpuscle, and whose function appears to be the
maintenance of the nutrition of the bone.

Periosteum.

Outer ground lamellae.

Haversian canals.

Haversian lamellae.

Interstitial lamellae.

Inner ground lamellae.

(*Y

7,1

Marrow.

FIG. ii. — FROM A CROSS- SECTION OF A METACARP OF MAN. X 50. The Haversian canals
contain a little marrow (fat-cells). Resorption line at h. — (Stohr).

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 concerned in the formation of bone, it is spoken of
as the osteogenetic 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 is spongy, and appears
to be made up of thin, bony plates, which intersect one another in all direc-
tions, and is found in greatest abundance 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.
In the cancellated tissue, near the extremities of the long bones, this fatty
deposition does not take place to the same extent, and the marrow appears
red. The cells of the red marrow are believed to give birth indirectly to
the red blood-corpuscles.

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

38 TEXT-BOOK OF PHYSIOLOGY

depends mainly on the quantity of water which enters into 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 r61es. Wher-
ever the various forms of connective tissue are found, their chemic composi-
tion and structure are in relation to their functions. If traction be the pre-
ponderating 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 can-
cellated. 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 trac-
tion, 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 oppos-
ing temporarily acting forces.

The Skeleton. — The connective tissues in their entirety constitute 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

Of the four phenomena presented by an animal, that which more im-
mediately interests the physiologist is movement, for the reason that it is
not only the animal's 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 of the body may for convenience be divided into two
groups, viz., external and internal.

The External or Skeleto-muscle Movements. — The external move-
ments are exhibited mainly by the head and extremities and may be either
special as when the animal changes the relation of one part of the body to
another, or general, as when it changes its position relatively to the en-
vironment as in the various acts of locomotion. The external movements
are the result of the cooperation of the bones of the skeleton and the muscles
which are attached to them. The skeleton imparts, nevertheless, a certain
degree of rigidity and fixity to the body; were it not for this the body would
be but a shapeless mass and incapable of performing any of its characteristic
external movements.

The joints of the skeleton permit various parts of the body to move
and to change their relation to each other. Without the presence of joints
such as are found in the vertebral column and in the limbs, external move-
ments would be impossible. The muscles impart movements to various
parts of the body. This they do by suddenly shortening and widening
whereby their extremities are approximated. The majority of the muscles
of the body are attached to the bones in such a manner that when their form
is altered, they change not only the relation of the bones with reference
to each other, but perhaps also the individual's relation to surrounding ob-
jects. The muscles thus become the active organs in both motion and
locomotion, in contradistinction to the bones which may be regarded as the
passive organs in the performance of the corresponding movements.

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

In the animal as in the physical machine, work is accomplished by the
intermediation of levers. In the animal machine, the levers are found in
the bones of the skeleton and more particularly in the long bones of the
extremities, the fulcra of which, the points around which they move, lie in
the joints.

40 TEXT-BOOK OF PHYSIOLOGY

That a lever may be effective as an instrument for the accomplishment of
work it must not only be capable of moving around its fulcrum, 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 environ-
ment, e.g., gravity, cohesion, friction, 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 muscles are therefore to
be regarded as the seat of those active energies that impart movement to
the levers.

The Internal or Vasculo- and Viscero-muscle Movements and
Gland Activities. — The internal movements are exhibited by the viscera,
the vascular apparatus and by glands, and, though less obvious, are no less
characteristic.

The viscera undergo variations in size from moment to moment as a
result of the contraction and relaxation of the non-striated muscle fibers,
composing in part their walls and thus regulate the reception and discharge
of their contents.

The vascular apparatus, and its adjunct, the lymph-vessel apparatus, is
engaged in the distribution of blood and nutritive material throughout the
body. The cavities of the heart are alternately increased and diminished
in size by the alternate relaxation and contraction of the muscle-fibers
composing the walls of the heart. During the relaxation the cavities fill
with blood and during the contraction the blood is driven through the vessels
in opposition to the friction presented by their walls; while the vessels them-
selves and especially the arteries, by virtue of the presence of elastic fibers
and the non-striated muscle-fibers in their walls, increase and decrease in
caliber from moment to moment and thus regulate the amount of blood
flowing through them in accordance with the physiologic needs of the organ
to which they are distributed.

The glands and more especially their epithelial investments are the seat
of certain molecular movements the result of which is the production and
discharge of a secretion destined to play a more or less important part in the
maintenance of the activities of the body.

In the performance of their functions these organs also meet resistances,
e.g., cohesion, friction, elasticity, etc., and when they are applied to the
overcoming of these resistances or forces, as they are in the performance of
their functions, it can also be said that they too are doing work. The co-
operation of external and internal organs is necessary, however, not only
for the maintenance of the life of the animal but also for the accomplishment
of external work.

Tissue Stimuli. — The various tissues of the body, mentioned in fore-
going paragraphs, though irritable do not possess spontaneity of action,
but require for the manifestation of their characteristic forms of activity
the application of a stimulus.

Thus the skeletal muscles and glands though capable of being excited
to activity by various artificial stimuli, require for the exhibition of their
normal activity the arrival of the physiologic stimulus, the nerve impulse,
developed in and transmitted to them by the nerve-tissue.

The visceral and vascular muscles though apparently capable of being

THE PHYSIOLOGY OF MOVEMENT

c. s.c.

-s.

FIG. 12. — DIAGRAM SHOWING THE RELATON OF SKELETAL, MUSCLE AND NERVE
TISSUES. (G. Bachman.) /.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.nt. 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; e}M» 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; a/.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.sx.
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 outv/ard from the spinal cord to the glands, blood-vessels, and the muscle
walls r>f tVif* vicrpra Tiavp fnr tVip catf> nf sirrmliritv been omitted from the diaerram.

42 TEXT-BOOK OF PHYSIOLOGY

excited to activity by agencies other than the nerve impulse are nevertheless
augmented or inhibited in their activity from moment to moment by nerve
impulses.

It is evident therefore that the activities of the organs and tissues which
are engaged in promoting the work of the body are excited to action and
controlled by the nerve-tissue, a fact which presupposes an anatomic con-
nection between them.

For an understanding of the mode of excitation of the motor organs and
the manner in which they cooperate in the performance of any given move-
ment, a brief preliminary account of the general arrangement and mode of
action of the nerve-tissue will be found helpful.

The General Relation of the Nerve-tissue to Peripheral Organs. —
The nerve-tissue is arranged partly in masses contained within the cavities

FIG. 13. — DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION OF REFLEX
ACTIONS. — (G. Bachntan.) r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor
cells in the anterior horn of the gray matter of the spinal cord, sp.c.; ef.n. efferent nerves distributed
to responsive organs, e.g., directly to skeletal muscles, sk.m., and indirectly through the inter-
mediation of sympathetic ganglia, sym.g., to blood-vessels, b.v., and to glands, g. The nerves
distributed to the walls of the viscera are not represented; v.r. and d.r., ventral and dorsal roots
and spinal nerves.

of the head and spinal column (the encephalon or brain and spinal cord),
forming the central organs of the nerve system, and partly in the form of
cords or nerves (the cranial and spinal nerves), forming the peripheral organs
of the nerve system. The latter connect the former not only with muscles,
glands, blood-vessels, and viscera, but with the skin, mucous membranes,
etc., as well.

(The relation of the nerve-tissue to the skeletal muscles, to glands, to
blood-vessels, and viscera are shown in Figs. 12, 13.)

The Spinal Cord. — The spinal cord is more especially the seat of origin
of the nerve energy that immediately excites and controls the activity of
the motor organs, and a knowledge of its structure, of its relations to these
organs, and the manner in which it is, in turn, excited to activity is necessary
to an understanding of the problem of movement.

THE PHYSIOLOGY OF MOVEMENT 43

The spinal cord is narrow and cylindric in shape and occupies the spinal
canal from the level of the first vertebra as far down as the second or third
lumbar vertebra. It presents both on its ventral and dorsal surfaces a
deep longitudinal fissure which partly divide the cord into halves, a right
and a left.

The Spinal Nerves. — To each side of the spinal cord there are attached
thirty-one nerves, which as they pass out through foramina in the walls of
the spinal column are termed spinal nerves. Each spinal nerve is con-
nected with the spinal cord by two roots, termed from their relation to the
ventral and dorsal surfaces, the ventral and dorsal roots. A short distance
from the spinal cord these two roots unite to form the spinal nerve proper,
after which the fibers composing it pass outward to be distributed to various
peripheral organs.

A transverse section of the spinal cord shows that each half is composed
externally of white matter, and internally of gray matter. The gray matter
in each half is arranged in the form somewhat of a crescent united in the
median line by a transverse band or commissure, the whole 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 (Fig. 13).

In the ventral horns of the gray matter are located large nerve-cells
which gives origin to nerve-fibers; these fibers early in embryologic develop-
ment become connected with structures for which they are by heredity
destined. With the growth of the embryo and the development of the
limbs there is a corresponding growth and development of the nerve-fibers
until they attain the size characteristic of the adult. The fibers collectively
constitute the ventral roots. Histologic investigation has shown that some
of the ventral roots contain two groups of nerve-fibers one of large and one
of small size. The fibers of large size pass forward and become directly
connected with the skeletal muscle. The fibers of small size pass forward
but for a short distance, after which they leave the ventral root and enter a
group of nerve-cells (the so-called sympathetic ganglion) around the den-
drites of which their terminal branches are disposed; from these cells new
non-medullated fibers arise which pass backward into the spinal nerve and
in association with the fibers of large size pass to vascular muscleand gland
epithelium more especially of the skin and mucous membrane. In some
situations the fibers of small size pass through or alongside of these ganglia
to other ganglia more or less distant in which they terminate in a similar
manner; from the cells of these ganglia new non-medullated nerve-fibers
arise which pass directly to visceral and vascular muscles and perhaps to gland
epithelium. The two portions of these nerves of small size are known re-
spectively, as pre- and post-ganglionic.

The nerve-fibers of small size found in the ventral roots of the spinal
nerves and in some of the encephalic nerves and distributed by way of
the ganglia to visceral and vascular muscles and to gland epithelium con-
stitute a system of nerves termed the autonomic system (see Chapter XXVI]

The dorsal root fibers originate from cells situated outside of, but
derived from the spinal cord. The cells in this situation at first develop two
processes from opposite ends, which at a later period shift their position,
unite and form a single process after which, a division into two branches

44 TEXT-BOOK OF PHYSIOLOGY

takes place, one of which passes toward and into the spinal cord and becomes
related in part with the nerve-cells in the ventral horn of the gray matter,
the other of which passes to the periphery and becomes connected with
the skin and mucous membrane covering the surfaces of the body (Fig. 13).
The surfaces of the body thus become associated anatomically and hence
physiologically with the motor organs.

With the growth and development of the brain and especially of the
cerebrum, nerve-cells make their appearance in the outer or cortical por-
tion. From the cells of certain specialized regions nerve-fibers pass down-
ward as far as the medulla oblongata, where for the most part, they cross
over to the opposite side and then descend the spinal cord to give off branches
or fibers at different levels which become related histologically with the
nerve-cells in the ventral horns of the gray matter. The cerebrum is thus
brought into relation through the intermediation of the spinal cord, with
skeletal, vascular and visceral muscles and gland epithelium (Fig. 12).

Coincident with the development of the nerve-fibers descending from the
cerebrum to the spinal cord, nerve-cells are developing in the more central
regions of the gray matter of the spinal cord from which nerve-fibers arise
that cross to the opposite side of the cord and then ascend directly or in-
directly to the cerebrum where their terminal branches come into physiologic
relationship with groups of specialized nerve-cells in different regions of the
cortex. Of the fibers of the dorsal roots some, as previously stated, pass
forward to the gray cells in the ventral horn; others, however, become
associated with the more centrally located nerve -cells above alluded to.
The surfaces of the body are thus brought into relationship through the
intermediation of the spinal cord with the cortex of the cerebrum.

The statements, made in the foregoing paragraphs in reference to the
spinal nerves, hold true for the twelve cranial nerves which correspond
physiologically at least with the ventral and dorsal roots of the spinal nerves.
Their relation to the cortex of the cerebrum is similar.

The nerve-fibers connecting the cerebrum with the motor organs con-
stitute the outgoing or efferent side of the nerve system. The nerve-fibers
connecting the surfaces of the body with the cerebrum constitute the in-
going or afferent portion of the nerve system.

The Efferent Spinal Nerve-cells. — It has been experimentally demon-
strated that each nerve-celt in the ventral horn not only generates but under
given conditions discharges a form of energy termed a nerve impulse, which
is transmitted by the nerve-fiber arising from it and by way of the ventral
roots of the spinal nerves directly to skeletal muscles and indirectly through
the sympathetic ganglia and their branches to glands, blood-vessels and
walls of viscera. (See Fig. 13.)

The arrival of the nerve impulse at once calls forth the form of activity
characteristic of the structure stimulated. Thus the muscle, for example,
passes from the passive to the active sjtate, 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, becomes 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 transformation of potential into kinetic energy is the

THE PHYSIOLOGY OF MOVEMENT 45

normal or the physiologic stimulus. The glands in response to the nerve
impulse pour out a secretion, the blood-vessels and viscera change their
caliber; all these tissues responding to the nerve impulse in a characteristic
manner are said to be irritable.

The nerve-cells in the ventral horns of the gray matter of the spinal cord
are therefore the sources of the energy requisite for the physiologic excitation
of the motor organs. If they are destroyed either experimentally or by
pathologic processes, the energy is no longer discharged and the motor organs
become incapable of performing their functions in a physiologic manner.

The nerve-cells, though extremely irritable, do not possess spontaneity
of action, but require for their excitation the arrival and stimulating action
of other nerve impulses. These may come (i) from the periphery through
afferent nerve-fibers by way of the dorsal roots of the spinal nerves; and
(2) from motor nerve-cells in the cortex of the cerebral portion of the brain,
through descending axons or nerve-fibers.

In the first instance the resulting movements taking place in response
to a peripheral or surface stimulation and independently of volitional or
emotional activity are termed reflex movements; in the second instance the
resulting movements taking place in response to volitional or emotional
activities are termed volitional or emotional movements.

In the case of reflex movements, the nerve impulses are primarily devel-
oped 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 efferent or motor nerve-cells which are in turn excited
to activity as a result of which, motor organs are aroused to action.

In the case of both the volitional skeletal-muscle movements and the
affective or emotional viscero-muscle and vasculo-muscle movements as
well as the activities of glands, the nerve impulses which cause the move-
ments are discharged from certain motor or efferent nerve-cells in the gray
matter of the cortex of the cerebrum and transmitted 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 movements due to cerebral or psychic activity are, however, the
immediate or the more or less 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, eye, ear, nose, tongue, and which have been developed by the impact of
objects in the external world.

The only organ that can be properly said to be excited to action by a
volitional act is the skeletal muscle; the glands, blood-vessels, and viscera
and the autonomic nerves which control them are apparently only influenced
in their activity by emotional states. Why this difference should exist it
is difficult to state.

The nerve-cells and their related nerve-fibers, responding by^the develop-
ment and conduction of nerve impulses are also said to be irritable. The
transformation of energy, however, manifests itself mainly as electricity and
molecular motion. The animal body in its entirety may therefore be regarded
as a machine for the transformation of potential energy into kinetic
energy, viz., heat and electricity, movements of muscles and bony levers,

46 TEXT-BOOK OF PHYSIOLOGY

secretion, sensation and other forms of nerve activity. When muscles and
bones are applied to the overcoming of opposing forces, mechanic work is
accomplished. In the following chapters some of the problems connected
with the activities of the primary mechanisms, the skeletal, muscle and
nerve tissues will be first considered and subsequently some of the problems
connected with the activities of the secondary mechanisms.

CHAPTER VI
THE PHYSIOLOGY OF THE SKELETON

The skeleton in its entirety determines the plan of organization of the
animal body and imparts to it its characteristic features. In its entirety it
serves for the attachment of muscles, the support of viscera and by reason of
the relation of the bones one to another, permits of a great variety of move-
ments. The skeleton may be divided into an axial and an appendicular
portion.

The Axial Portion. — The axial portion consists of the bones of the head,
of the vetebral column and the ribs. The vertebral column is the foundation
element and the center around which the appendicular portions are de-
veloped and arranged with a certain degree of conformity. It is composed
of a series of superimposed bones, the 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 vertebrae are united one to another by tough elastic discs
of fibre-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 vertebral column elastic and flexible, and enable it to
resist and diminish the force of shocks communicated to it. In all the static
and dynamic states of the body it plays a most essential r61e. 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 mobility.

The Appendicular Portion.— The appendicular portion consists
of the bones of the arms and legs, the scapular and pelvic arches^ By
reason of its character and anatomic arrangement, the animal body is en-
dowed with extreme mobility, enabling the animal to execute a great variety
of rapid and extensive movements which, however, vary in degree in different
animals in accordance with their organization and the nature of their
environment.

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 (i) 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.^ In this
connection, however, onlv those physical and physiologic peculiarities of the

48 TEXT-BOOK OF PHYSIOLOGY

skeleton, especially in its relation to 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 facilitates the move-
ments 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 nbro-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 membrane is composed
largely of connective tissue, the inner surface of which is lined by endo-
thelial cells, which secrete a clear, colorless, 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 inextensible but pliant,
ligaments assist in keeping the bones in apposition, and prevent dis-
placement 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 (i) diarthroses; (2)

amphiarthroses; (3) synarthroses.

A. 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 sur-
face 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:

i. Uniaxial Joints. — In this 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. Joints of this character — e.g., the elbow,
knee, ankle, the phalangeal joints of the fingers and toes — are, therefore,
termed ginglymi, or hinge-joints. Owing to the obliquity of their
articulating 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 modification of a simple hinge known as a

THE PHYSIOLOGY OF THE SKELETON 49

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. More-
over, 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 position 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 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 unequally

curved, though intersecting each other. When the surfaces 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 permitted 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, extension, adduction,
abduction, and circumduction.

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.
B. 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

50 TEXT-BOOK OF PHYSIOLOGY

united and held in position largely by the intervention of a firm, elastic
disc of fibro-cartilage. Each joint is also strengthened by ligaments.
C. 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.
Levers. — In the animal machine, as in physical machines generally,
work is accomplished by the intermediation of levers. The bones col-
lectively constitute a system of levers the fulcra of which lie in the joints.
The long bones more especially, are the levers which are employed by the
muscles to overcome the opposing forces or resistances. 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 organiza-
tion 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 i 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 osseous tissue and an
increased porosity, and, therefore, at both periods less capable of function-
ating as effectively as in the middle period of life. The animal body presents
many illustrations of the three orders of levers, their advantages and dis-
advantages from the mechanical point of view.

The nature of the opposing forces, however, is of such a character that
the animal and especially man, is but to a slight degree capable of over-
coming them with the natural anatomic and physiologic levers. With the
invention of tools (physical levers) of all kinds and their utilization by
man the effectiveness of the anatomic levers in the performance of work
has been enormously increased. Through their cooperation the progress
of man in the arts of civilization has been made possible.

The axial portion of the skeleton possesses largely, joints of the amphi-
arthrodial character which endow the vertebral column with certain forms
of movement which are necessary to the performance of many body activities.
While the range of movement between any two vertebrae is slight, the sum
total of movement of the entire series of vertebrae is considerable. In
different 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
vertebrae. Lateral flexion takes place in all portions of the column, but is
particularly marked in the cervical 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.

The appendicular portion possesses largely, joints of the diarthrodial
character which permit of free movement. The character of the
movement depends mainly on the shape and adjustment of the bones at
their points of union.

CHAPTER VII
GENERAL PHYSIOLOGY OF MUSCLE-TISSUE

The 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 attached to 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 contradistinc-
tion to the bones and joints, which are but passive agents in the performance
of the corresponding 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 nerve system. Muscles are divided into:

1. Skeletal muscles, comprising those muscles which are attached to the
various bones of the skeleton.

2. Visceral muscles, comprising those muscles which are found in and
which compose a portion of the walls of the hollow viscera.

As the skeletal muscles are capable of being excited to activity by nerve
impulses descending from the cerebrum as a result of volition they are
frequently termed voluntary muscles. By reason^ of their appearance as
seen under the microscope they are termed also striped or striated muscles.
As the visceral muscles are not capable of being excited to action by volition
they are frequently termed involuntary muscles. By reason of their ap-
pearance as seen under the microscope they are termed also non-striated
or smooth muscles.

Though for the most part the skeletal muscles are red in color, there are
certain muscles in man and other animals which are pale in color and in
many muscles, pale fibers are extensively distributed among the red fibers.

THE SKELETAL OR VOLUNTARY MUSCLE

All skeletal muscles consist of a central fleshy portion, the body or belly,
provided at either extremity with a tendon in the form of a cord or mem-
brane. The body is the active, contractile region, the source of the move-

TEXT-BOOK OF FHTSKHJOGTT

merit; tfu» tendon is the inactive region, thg passive transmitter of th^

.A. skeletal tttti^scie IS a KXEfMp*fT <Mys>n fnm<5i'*dliiiy of a i F^>ITM *m 4^*1 Ot

connective trasne, supporting niim Icvflms, blood-vessels, nerves, and
lymphatics. The general body of the musde is covered by a dense layer
of connective tissue, the epimysium, which blends with and partly forms
die 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 bv a special gh^ntfr, the
perim ysium. are prismatic in shape and on cross-section present an irregular
outline. The muscle-fibers composing ffa* fasciculi are separated one from
another and supported by a very delicate connective tissue, the gndo"iyj»'M|i>.
Hie connective tissue thus surrounding and penetrating the musde binds

the fibers into a distinct organ and
affords support to all remaining struc-
tures (Fig. 14).

Histology of the Skeletal Mus-
cle-fiber. — The muscle-fiber is the
ultimate anatomic unit of the mnvU*
system. The fibers for the most part
are arranged 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 micro-
millimeters. There are exceptional
fibers, however, which have a much
greater length. As the fibers have
but a limited length in the vast major-
ity of muscles, each end, more or less
pointed or beveled, is united to adjoin-
ing fibers by cement. In this way the
length of the muscles is built up.

When examined with the micro-
scope, the muscle-fiber is seen to be
cylindric or prismatic in shape and
to consist of a thin transparent mem-
brane, the sarcolemma, in which is
contained the true musde substance or sarcous substance. The sarco-
lemma is elastic and adapts itself to all changes of form the sarcous sub-
stance undergoes. Beneath the sarcolemma there are several nuclei
surrounded by granular material; a musde-fiber may therefore be re-
garded as a large multinucleated celL Each fiber also presents a series
of transverse bands alternately dim and bright which give to it a striated
appearance. If the 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 by Krause was regarded as the optic expression of a
membrane attached laterally to the sarcolemma. According to Rollet,
it is composed of a series of granules so closely applied as to give rise to the
appearance of a continuous line (Fig. 15).

Fre. 14.— Fzoif A CwJSS-SBcnox or THE
ADDUCTOR MUSCLE or A RABBIT. P. Peri-
mv^uin, containing two blood-vessels, at g;
m, musde-fibrrs; many are shrunken and be-
tween them toe endomrsnnn, p, can be seen ;
at x foe section of nmsde-fiber has fallen out.
X 60.— {Slokr.)

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE

The muscle-fiber also presents a longitudinal striation which indicates
that it is composed of finer elements placed side by side, termed fibrillae.
The fibrillae extend throughout the entire length of the fiber, though they
are not of uniform thickness. That portion of the fibrilla correspond-
ing in position to the dim band is thick, prismatic, or rod-like in shape, and
termed a sarcostyle; that portion corresponding in position to the bright
band is extremely thin and narrow and presents at its middle a slight en-
largement or nodule. The fibrillae are embedded in a clear transparent fluid
which, from its supposed nutritive character, is termed sarcoplasm, or
interfibrillar substance. The diminution in caliber of the fibrillae at different
levels would permit of the accumulation and storage of a larger amount of
this 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
by polarized light, the dim band ap-
pears bright and the bright band appears
Him against a dark background, indicat-
ing that the former is doubly refracting
or anisotropic, the latter singly refracting
or isotropic.

FIG. id— A. 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;
/, *, /, respectively the trans-
verse, the intermediate, and
lateral discs, n. Muscle
nuclei — (Piersol.)

FIG. 15. — MUSCLE-FIBER
OF A RABBIT, a. Dark
Iraitit b. Light band, c. I n-

termediaie line, n. Nucleus.
— (Landois and Stirling)

TEXT-BOOK OF PHYSIOLOGY

The Blood-supply. — Muscles in the physiologic condition require for
the maintenance of their activity a large amount of nutritive material. This
is obtained directly 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 reference to the muscle-fiber is
very characteristic. The arterial vessels, after entering the muscle, are
supported by the peri-mysium; in this situation they give off short transverse
branches, which immediately break up into a capillary network of rectangu-
lar shape within which the muscle-fibers are contained.

The muscle-fiber, in intimate relation with the capillary, is bathed with
lymph derived from it. Its contractile substance, how-
ever, 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 relation
with the living muscle material. The waste products
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 eliminating
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 middle point. Many
of the fibers pass directly to the muscle-fibers with which
they are connected; others are distributed 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 contain as many fibers as the muscle. In
I7;TDlA" such cases the nerve-fibers divide near their termination

I I I I -1.1.

Illlllllllll

FIG.

GRAM OF MUSCLE , ,.

STRIATIONS.— (Modi- until the number of branches equals the number of muscle-

jicil from^ "Stokr's fibers. The individual muscle-fiber is penetrated near

fibrillse* consists'1 hof its center b7 tne nerve where it terminates; the ends

alternate dark bands, being practically free from nerve influence. The stimulus

d.b., and light bands, that comes to the muscle-fiber acts primarily upon its

by K™use'sCTmtm- center, the effect of which then travels in both directions

brane, m.; a similar to the ends. The manner in which the nerve-fibers termi-

membrane crosses na^e jn muscie win be more fullv described in connection

the dim band accord- - ,,

ing to Heidenhain. with the histology of the nerve-tissue.

CHEMIC COMPOSITION OF MUSCLE

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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 55

Water 73.5

Proteins, including those of sarcolemma, connective tissue,

pigments 18 .02

Gelatin , i ,99

Fat 2 .27

Extractives 0.22

Inorganic salts 3 .12 (Halliburton.)

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 subsequently contracts and squeezes out an acid
muscle-serum. The coagulated protein partakes of the nature of fibrin 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 physio-
logic, it is regarded as a derivative of pre-existing proteins. An analysis of
muscle-plasma has shown the presence of at least two proteins which are
distinguished by their varying solubilities in different salt solutions, and by
the varying temperatures at which they coagulate. One of these proteins
coagulates at about 47°C. and because of its chemic relations has been
termed myosin or paramyosinogen; the other coagulates at about 56°C.
and for similar reasons has been termed myogen or myosinogen. The
latter is three or four times more abundant than the former. If the tempera-
ture of the cooled plasma be permitted to rise, both myosin and myogen
undergo a change of state termed coagulation. The substances resulting
are known as myosin fibrin and myogen fibrin. It is not known whether
these changes are due to the action of an enzyme or not. A similar change
in myosin and myogen occurs after death, giving rise to the condition known
as death stiffening or rigor mortis. The coagulation of these proteins in this
instance is probably caused by the presence and accumulation of metabolic
products. From the muscle-serum, according to Halliburton, may also be
obtained at 68°C. a globulin body termed myoglobulin and a small quantity
of myo-albumin. Among the proteins may be mentioned hemoglobin, which
gives the color to the muscles. SpectrcTscopic investigation reveals the
presence of a special pigment, myohematin, which is supposed to have a
respiratory function, inasmuch as its spectral 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 and 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
considerably in accordance with different states of the muscle. In a state of
tension it is hard and resistant; in the absence of tension it is soft and fluctu-
ating: to the sense of touch. Tension alone gives rise to hardness.

TEXT-BOOK OF PHYSIOLOGY

Cohesion. — The cohesion of a muscle is largely dependent on the quan-
tity of connective tissue it contains. A band of fresh human muscle i
square centimeter in cross-section has been found 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 ex-
tended beyond its normal length by 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. Muscles, therefore, possessing extensibility
and retractility are said to be elastic. 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 extensi-
bility of muscle does not follow the law of

elasticity as determined for inorganic bodies; FlG 'l8._ExTENSION CURVE
fc.£. , directly proportional to the weight and to the MUSCLE.— (Gad.)

length of the body extended; but that while in-
creasing in length with each successive weight, the increase is always in a
diminishing ratio. Thus, for example, as shown in Fig. 18: The exten-
sion 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 in-
crease in passing from 25 to 30 grams is only i millimeter. The exten-
sibility 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 behavior of 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 muscle asserts itself with gradu-

ux n i- K\J\J o iVj.uat;ivJL. If X. /YD- 11 • • i«l • 11'

scissa before; *', after exten- all7 increasing energy until its normal length is
sion.— (Landois and Stirling.) nearly, if not entirely, regained (Fig. 19). It is

usually stated that the elasticity of muscle is in-
complete, but 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 prob-
able that in the living body muscles possess perfect elasticity which enables
them completely to return to their normal length after extension. The
extension and retraction or elastic recoil of muscle depends on the main-
tenance of physiologic conditions. If the nutrition is impaired by fatigue,

FIG. 19. — CURVE OF ELAS-
TICITY PRODUCED BY CONTINU-
OUS EXTENSION AND RECOIL
OF A FROG'S MUSCLE, o x. Ab-

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 57

deficient blood-supply, or any pathologic condition, the elasticity is at once
impaired.

Tonicity. — Muscle tonus may be defined as a state of tension of a
muscle due to a slight but continuous contraction of its individual fibers in
consequence of which it tends to become shorter, and would do so, were it
not restrained by its attachments. As a result of this tension its efficiency
as a quickly responsive motor organ is increased. Though the skeletal
musculature of the body as a whole is in a state of tonus, individual muscles
vary in the degree of their tonus from time to time in consequence of varia-
tions in the causes that give rise to it. That such a tonus or tension exists
is apparently shown by the fact that when a muscle in a living animal is
divided the two portions will retract and separate a certain distance. This
would indicate that the muscle even in a state of relative rest is in a slight
degree of contraction.

This condition of tonus is attributed to the continuous arrival of nerve
impulses through efferent nerves discharged by nerve-cells in the spinal cord.
The tonus was therefore at one time attributed to an automatic activity of
the spinal cord. Brondgeest, however, showed that this was not the case,
but that the activity of the spinal cord and hence the tonus of the muscles is
partly reflex in origin inasmuch as it largely disappears on division of the
posterior or dorsal roots of the spinal nerves. The afferent impulses excit-
ing the cord reflexly may come from the skin in which they are developed
by the impressions made by external stimuli, or from the tendons and
muscles themselves in which they are developed by the slight degree of
extension and variations in extension to which these are subjected from
moment to moment. That this latter is a considerable factor in the pro-
duction of the tonus is shown by the effects which follow division of the
afferent nerves coming from any given muscle group; with the division of
the nerves the muscles relax and lose their usual tone. It is also probable
that the activity of the cord is partly the result of impulses descending the
cord in consequence of cerebral and sense organ activities.

Muscle tonus or elastic tension plays an important r61e in muscle con-
traction; 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 considerably 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 posi-
tion of a passive limb is the resultant also of the elastic tension of antago-
nistic groups of muscles. Again as a result of the slight but continuous
stimulation from the spinal cord the metabolic changes in the muscle material
are maintained at a certain level, with a corresponding production of heat.
A function of the tonicity would thus be the production of heat, other functions
which the tone subserves being more or less 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 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 condi-

58 TEXT-BOOK OF PHYSIOLOGY

tion. 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 connections 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 suspends 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 although containing 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 nutritive activity and irritability are maintained by a
due supply of oxygen, and of nutritive material, the removal of waste prod-
ucts, and a normal .temperature. The muscles of the cold-blooded animals,
for example 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.

Conductivity. — All muscle protoplasm possesses conductivity. 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 ends of the fiber;
the advance of the excitation process is immediately succeeded by the con-
traction process, the change of form which constitutes the contraction.
With the disappearance of the former, the latter also disappears and the
muscle resumes its previous passive condition. There is no evidence, how-
ever, 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 cannot be con-
ducted 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 current, the excitation and the contraction processes will
be conducted with extreme rapidity to the opposite end of the muscle. The

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 59

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 con-
traction 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 duration of the wave at any given point has
been shown to be, in the frog's muscle, about one-tenth of a second and its
length three-tenths of a meter.

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 im-
pulses" transmitted by the nerves from the 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.

1. 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 continuous but irregular pulsations
are produced. . .

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

or continuous pulsations if the strength of the solution 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-sub-
stance. Among these may be mentioned solutions of potassium and
sodium salts, weak solutions of the mineral and organic acids, amm
nium vapor, distilled water, glycerin, and sugar.

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

wire, causes the muscle to contract rapidly.

4 Electric Stimuli.— The most efficient stimulus and the one least injurious
to the tissue is the electric current. Either the constant or the
current may be used.1 . ,

The Constant Current.— It the ends of the wires in connection with an
electric cell be provided with non-polarizable electrodes and the latter place
on opposite ends of a freshly prepared sartorius muscle of a frog wh
been previously curarized, it will be found on closing or making the circui
that the muscle will exhibit a short, quick pulsation. During the acti
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 , state of continuous contraction. With the opening or
breaking of the current the muscle at once relaxes, or perhaps again co.
and then relaxes. The extent of the contraction depends mainly on ^t
strength of the current, being greater with strong, less with weak c

i Since the study of the physiologic properties of both ^cle-tissue and nerve t.sue

60 TEXT-BOOK OF PHYSIOLOGY

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 regularly greater than that occurring on the break or opening
of the circuit. Moreover, it has been shown in many ways that the con-
traction 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 cathode — and propagates itself to the opposite extrem-
ity; 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 irritability
and contractility of one extremity of a muscle with parallel fibers such as the

FIG. 20. — 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 elec-
trodes, anode and kathode, hi contact with a muscle, the injured end of which is more deeply
shaded. The arrows indicate the direction of the current.

sartorius. On applying non-polarizable electrodes to the muscle as in Fig.
20, 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. 20, 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 coil of the inductorium be con-
nected with an electric cell and the secondary coil be connected with a
muscle, it will be found that the current induced in the secondary circuit,
both on the make and break of the primary, will also cause the muscle to
pulsate sharply and rapidly if the two coils 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 peculiari-
ties 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 contractions 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.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 61

PHENOMENA FOLLOWING A MUSCLE STIMULATION
PHYSICAL PHENOMENA

Physiologic investigation has made it apparent that when a nerve impulse
-caches a muscle, it occasions a disruption of certain complex energy-
tiolding compounds and their subsequent oxidation to simpler compounds.
Coincidently with the chemic changes there is a transformation of the
potential energy of the molecules into kinetic energy which manifests
itself under three forms, heat, electricity and mechanic motion, or a change
of shape of the muscle. These phenomena vary in extent in accordance with
the intensity of the impulse as well as the frequency of its repetition.
Though the chemic changes are the first effects of the action of the nerve

FIG. 2i.— SHOWING THE CHANGES IN A MUSCLE AND MUSCLE-FIBER DURING

CONTRACTION.

impulse and the ones on which other phenomena depend, it will ^be found
convenient to consider the most evident effect, the physical change m t]

shape of the muscle, first. ii™,,;™ tho

Change of Shape.-The most obvious change m a muscle followmg the
arrival of a nerve impulse is that relating to its form. The muscle no only
becomes shorter, but at the same time thicker. The extent to wh ch t may
shorten when unopposed may amount to 30 per cent, or more of Us or^mal
length. The increase in thickness practically compensates for the di
tion in length, for there is no observable diminution ^ volume. ^ Th, chw ge
in form of the entire muscle results from a corresponding change of iorm
its individual fibers as determined by microscopic examination, each of
which becomes shorter and thicker. The successive changes in t
muscle and the individual fibers are represented in _Fig. ^ 21

When the contraction begins both the dim an,d,bn^sa^n^ Jn ^
length, but at the same time increase m breadth This continues unm
the contraction reaches its maximum. The diminution m
bright band is greater proportionally than the diminution m th length .ot the
dim band, a fact which gave rise to the supposition on the part of Engemann
that there is at the time of the contraction a passage of .fluid ^ mate^ ^
the bright into the dim band or from the sarcoplas m into *? s«cost^edSs
When fhe relaxation begins, a reverse change in the dim and bright bands
sets in and continues until they regain their former shape and volume. Com
cidently there is a passage of fluid material from the sarcostyle

TEXT-BOOK OF PHYSIOLOGY

sarcoplasm. As the contraction wave reaches its maximum the optic proper-
ties 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 compari-
son. 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 disappear, giving to the fiber an appearance

FIG. 22. — EXTENSION CURVES: B Bf, of the resting; b B'. of the contracting muscle.

of homogeneity. There is, 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.

Change of 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 increased and the elas-
ticity decreased is dependent on the amount of the resisting force. These
facts, as determined experimentally, are represented in Fig. 22. Let A B and
A b represent the length of the normal unweighted muscle, in 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 common 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 shortening.
It will be observed from these facts that while the muscle is extended in
both the passive and active states by corresponding 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 short-
ening of the muscle, the force of its contraction manifests itself physically
simply 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-
mined by the amount of the supported weights. When the weight is slight

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 63

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.

GRAPHIC REPRESENTATION OF THE CHANGE OF SHAPE

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 experimentally is the resultant
of the fusion of a greater or less number of separate and individual contrac-
tions or pulsations. To this enduring contraction the term tetanus has been
given. With the aid of appropriate 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
produce practically a normal physiologic tetanus. As in the experimental
study of the phenomena of muscle contraction it frequently becomes neces-
sary 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, such as the frog— in which, owing to the relatively slow
rate of the nutritive activities, the irritability and contractility endure for
a relatively long period of time, even though deprived of blood— are particu-
larly 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 chlond,
such a muscle will contract for a long period^of time on the application of
any form of stimulus, but especially the electric.

Method of Recording a Muscle Contraction.— Inasmuch as the
changes in the form of a muscle during a single contraction take plac
with extreme rapidity, their succession, peculiarities, and time relations
cannot be determined with any degree of accuracy by the unaided eye.
This difficulty can largely be overcome by the employment of the graphic
method, the principle of which consists in recording the movements by
means of a pen on some appropriate moving and receiving surface,
accomplish this object the muscle is attached at one extremity by a d*mp
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 point is applied
a smooth surface, such as glazed paper on a cylinder or ^ plate, cover
with lampblack. If the surface is stationary, the contraction is recor.
vertical line; if it is put in movement at a uniform rate by clockwork he
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 o
the contraction can be obtained by placing beneath the lever a writing pom
attached to an electro-magnet thrown into action by a tuning-fork yibrat ng
in hundredths of a second. In order to determine the rapidity ^™5
the contraction follows the stimulation, it is essential that the moment ot
the latter be also recorded. This is accomplished by an automatic
opening or closing of which develops the stimulus which excites the

TEXT-BOOK OF PHYSIOLOGY

A combination of these different appliances constitutes a myograph and the
curve of contraction a my o gram. (See Fig. 23.)

It is necessary for the purpose of placing the excised muscle under me-
chanic conditions similar to those found in the body and for the registration of
its movements under varying conditions to give the lever mass. This is accom-
plished by attaching weights to it beneath the muscle.

FIG. 23. — MYOGRAPH. K. Recording cylinder. M. Moist chamber. L.
Recording lever. W. Weight. I. Induction coil.

The Isotonic Myogram. — With the object of obtaining a curve of
the successive changes in the length of a muscle during a single contraction
and at the same time avoiding changes in tension and therefore an accelera-
tion of the lever, the weight attached to the lever should be applied close to its
axis, in accordance with the isotonic method. The curve of contraction thus
obtained is known as an isotonic myogram.1

With the muscle arranged as previously described and stimulated
directly with a single induced electric current, the contraction will be re-
corded in the form of a curve similar to that represented in Fig. 24, in which
the horizontal line represents the abscissa of time; a, the moment of stimula-
tion; and b c d, the degree of shortening and the subsequent relaxation at each
successive moment. The undulating line shows the time relations, the
distance from crest to crest representing hundredths of a second. The
curve may be divided into three portions:

1 The weighting of the lever or the loading of the muscle is accomplished in several ways: (i)
The weight is attached to the lever just beneath or in the immediate neighborhood of the point of
attachment of the muscle. This is known as the " loaded method" and has the effect of extending
the muscle beyond its normal length previous to the moment of its stimulation and contraction.
(2) The weight is attached to the lever at the same point as in the foregoing method, but by
means of a support beneath the lever, the weight is prevented from extending the muscle previous
to the moment of its stimulation and contraction. This is known as the "after-loaded" method.
In either case 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 addi-
tional movements which vitiate the true character of the curve. (3) The weight is attached to a
small pulley on the axis of the lever and therefore at some distance from the point of attachment
of the muscle. The advantage of this method lies in the fact that the initial tension of the muscle
induced by the load remains practically constant throughout the contraction period and hence
acceleration of the movement of the lever is prevented. This is known as the "isotonic method."

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 65

i. 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 o.oi second, but with the employment
of more accurate apparatus it has been reduced to 0.002 5 to 0.004 second.
During this period it is supposed that certain chemic changes are
taking place preparatory to the 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.

FIG. 24.— THE ISOTONIC MYOGRAM.

, 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 relaxation 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 a ter-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, an

of the period of relaxation 0.05 second. A single pulsation o te isolated

muscle of the frog therefore occupies, from the moment of stimulation ^to

termination, the tenth of a second. Muscles of many other ammds have

a contraction period the duration of which varies considerably from this.

Thus in man the time of a single contraction is one-twentieth of a seco id,

SSne in* Ss one three-hundredth of a second and in the turtle one second.

fied by both external and internal conditions, among which may b

proportional, within limits, to the strength of the latter Thu, using
as a stimulus the single induced current, it has been found that

66 TEXT-BOOK OF PHYSIOLOGY

strength of the current is progressively increased, the height of the con-
traction will correspondingly increase until a certain maximum height is
attained (Fig. 25, A, a b); 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. 25, B, d e). 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.

FIG. 25. — TEACING SHOWING THE EFFECTS OF A GRADUAL INCREASE IN THE STRENGTH
OF THE STIMULUS ON THE HEIGHT OF THE CONTRACTION, a. Minimal contraction; a b. pro-
gressive increase in the height; b c. first maximum (a number of contractions have been omitted
for economy of space) ; d e. second maximum.

It has also been shown that the rate at which the muscle is stimulated
with a given stimulus of uniform strength will influence the char-
/ acter of the contraction process. I|_t_he intervals between the successive
I stimulations be such as permit the muscle to recover from the effects of
i -fcr^cohtraction, it may contract as many as a thousand times witKout
j showing any particular variation from the normal form; but if the
/ intervals_are shorter than that just stated it is found that from the
i beginniiig~6f the stimulation each succeeding contraction slightly exceeds
\ in height the preceding contraction, until a certain maximum is reached
and maintained, indicating that for some reason the irritability and the
i energy of the contraction have been increased. This gradual increase
I in the height oj: the contraction has been termed the staircase effect, or
\the treppe. In the beginning of the period of stimulation there is some-
times observed a decrease in the height of the contraction following
several stimulations before the staircase effect develops, indicating
a temporary decrease in the irritability. These staircase 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 sub-
stances in the muscle arising as a result of. its katabolism, such as
carbon dioxid, mono-potassium phosphate, and paralactic acid. These
compounds, when present in small amounts or in larger amounts for a

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 67

short time, augment 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.
Variations in the Temperature.— The temperature at which all phases of
the contraction process, as represented by the myogram, attain their
physiologic maximum value is about 3o°C. If the temperature
of the muscle falls to 2o°C. there is a corresponding decline in activity,

FIG. 26. — SINGLE CONTRACTIONS OF THE GASTROCNEMIUS MUSCLE AT DIFFERENT TEMPERA-
TURES. Time tracing 200 per second. — (Brodie.)

as shown by an increase of the latent period, a decrease in the height
of curve — i.e., in the shortening of the muscle — 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 physiologic condition. At a temperature of 4o°C. to 5o°C.
the muscle suddenly contracts and passes into the condition of heat
rigor or rigor caloris. The protein constituents of the muscle are
coagulated and the irritability destroyed. (Fig. 26.)

FIG. 27.— CONTRACTIONS OF A GASTROCNEMIUS MUSCLE
WITH DIFFERENT LOADS. — (Brodie.)

3. Variations in 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 exami-
nation of Fig. 27, 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 duration of both the periods
of rising and falling energy.

4. Rapidly Repeated Stimulation.—Prolonged or excessive activity of our own

muscles is accompanied bv a feeling of stiffness or soreness and lassi-

68 TEXT-BOOK OF PHYSIOLOGY

tude. There is at the same time a diminution in the speed 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-yielding 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 dimmish 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 condition
of fatigue with its attendant phenomena is shown by stimulating
through its nerve an excised frog muscle with induced electric currents

FIG. 28. — FATIGUE CURVES. EVERY TWENTIETH CONTRACTION RECORDED.

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. 28.) If the stimu-
lation is continued the contractions gradually decline as the muscle
becomes exhausted. When a muscle will no longer respond to stimu-
lation 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 lie wholly 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 ex-
hibition of normal contractions. A diminution of blood supply or
an accumulation of waste products sooner or later impairs the irritability
and diminishes the vigor and extent of the contraction. Various
drugs — e.g., veratrin, barium, etc. — introduced 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.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE

The Isometric Myogram.— With the object of obtaining a curve of
the increase and decrease in the tension of a muscle during a single con-
traction, 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 practically
though not absolutely- sufficient to prevent shortening. To this method
the term isometric has been given, and the curve so obtained is an isometric
myogram or a tonogram. The recording portion of. the lever is prolonged
some distance so that the very slight
upward movement at its axis, close to
which the muscle is attached, 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 pul-
ley supported by the muscle clamp.

The curve of the variation in ten-
sion obtained by the isometric method
is shown in Fig. 29, b, in which the two
curves are contrasted. The form of the
curve indicates that the muscle attains its maximum of tension more
rapidly than its maximum 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 lengthens.

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 con-
traction of the muscle which is caused by the action of a constant or galvanic
current presents features which are somewhat 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

FlG- 29-— a. DIAGRAM OF ISOTONIC; b,
MDSCIE

FIG. 30. — MYOGRAM DUE TO THE ACTION OF A GALVANIC CURRENT, APPLIED DIRECTLY TO A
MUSCLE, WHEN THE CIRCUIT WAS CLOSED (c) AND WHEN IT WAS OPENED (0).

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 57.) In
Fig. 30 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 contracted
state known as galvanotonus; on opening the circuit at o the muscle again con-

TEXT-BOOK OF PHYSIOLOGY

tracted, after which it gradually relaxed and returned to its original con-
dition. The record shows also that during the actual passage of the cur-
rent the muscle substance was being stimulated by it.

The Work Accomplished by a Muscle during the Time of a Single

Contraction.-^By work is meant the overcoming of opposing . forces/

I In the physiologic activities of the body the muscles at each contraction not

Wly overcome the resistances of antagonistic muscles, the weight of the

limbs, the friction of joints, etc., but in addition overcome varjous external

resistances connected with the environment — e.g., gravity, cohesion, friction,

lelasticity, etc. The muscles may therefore be regarded as machines for

the accomplishment of work. Experimentally the work done by an iso-

lated muscle may be calculated if the height of the contraction is first obtained

and then multiplied by the weight
raised. The influence of the
weight on the height of the con-
traction is shown in Fig. 31, in
which only the height of the con-
traction or the degree of shorten-
ing and hence the lift of the weight
is represented. 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;
FIG. 31 .-TRACING SHOWING THE GRADUAL but as weights differing by a com-
DIMINUTION IN THE HEIGHT OF THE CONTRAC- mon increment are added, the
TION AS THE WEIGHT WAS INCREASED BY A COM- height of the contraction dimin-
MON INCREMENT OF 10 GRAMS FROM o TO 180 . , ° .. . , . • t^ f*

untl1 Wlth a glven weight it

GRAMS. MAGNIFICATION OF THE LEVER, 4.

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 gram-millimeters; 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.

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

Weight,
o grams

50 grams
100 grams
150 grams
200 grams
250 grams

Height.
14 mm.

9 mm.

7 mm.

5 mm.

2 mm.

o mm.

Work

o gram-

450 gram

700 gram-

750 gram

400 gram-

o gram

Done,
millimeters,
millimeters,
millimeters,
millimeters,
millimeters,
millimeters.

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 maxi-
mum 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.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 71

Absolute Muscle Force.— 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 'a muscle can raise and hold at its natural length after it has
been extended by this weight. In Fig. 22 this weight would be on the ab-
scissa B b' where it is cut by the extension curve of the active contracted
muscle. The absolute force is also measured by the weight which is just
sufficient to prevent the muscle from shortening when stimulated. 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 dependent 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 muscle-tissue, 1.058) by the average length of the
fibers. Assuming that the muscle weighs 609 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 114 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 the units of area — viz., one square centimeter. Rosenthal esti-
mates 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 to 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 preceding 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 char-
acter of the contraction for a long time except a slight increase, in the early
period, of the irritability as shown by the increased height of the curve
or shortening of the muscle. If, however, a second stimulus be applied
to a muscle during the period of relaxation, a second contraction immediately
follows which is added to or superposed on the first; the effect produced will
be greater than that produced by either stimulus separately, (See Fig. 32.)

A third stimulus applied during the relaxation of the second contraction
produces a third contraction which adds itself to the second, and so on.
The increment of increase in the extent of the successive contractions gradu-
ally diminishes, however, until the muscle reaches a maximum of contrac-
tion. The superposition of the second contraction on the first, the third on
the second, and so on, is termed summation of 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 would have been. (Fig. 32.)
The effects following both maximal and submaximal stimuli indicate that
the muscle cannot attain its maximum of shortening except through a

72 TEXT-BOOK OF PHYSIOLOGY

summation of several stimuli. If a second maximal stimulus enters 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 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 submini-
jmal stimuli, each of which is alone in-
sufficient to produce a contraction of
the muscle, be applied in rapid succes-
sion, a contraction frequently results.
This is termed the summation of submini-
mal stimuli.

Tetanus. — Tetanus may be defined
as a more or less continuous contraction
of a muscle which arises when the time
intervals between the stimuli are shorter
than the time of the contraction proc-
ess. Tetanus will be incomplete or
complete according to the number of
stimuli that reach the muscle in a sec-
ond of time. When a muscle is stimu-
lated 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 corre-
sponding number of contractions of
about equal extent. 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 relax completely before the
FIG. 32.— TRACING SHOWING THE EF- arrival of the succeeding stimulus, and
FECTS OF Two SUCCESSIVE STIMULI, a. a' hence remains in a more or less con-
WITH GRADUALLY DIMINISHING INTER- trarfpfi ~fatp Hnrino- whiVh it AYhiHitc *
VAL ON A MUSCLE CONTRACTION. To be I] Sta*e> a mS wmcn it exhibits a

read from below upward. 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. 33.

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 stimu-
lus, the fourth, the fifth, and so on. Owing largely to this summation
of the contractions there is a gradual rise in the height of the contraction
curve. This condition of the muscle, viz., continued contraction, com-
bined with diminished power of relaxation, is termed \.contracture. The
tracing also shows that as the stimulus continues, the base line, that con-
necting the lowest points of the contractions, gradually rises and takes the form

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 73

of a curve which increases in height as the stimulus continues. 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

FIG. 33. — CURVES SHOWING THE ANALYSIS OF TETANUS OF A FROG'S MUSCLE (GASTROC-
NEMIUS). 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.}

stimulation. 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 contraction-
remainder.

If the stimulation be still further increased in frequency, the individual
contractions become fused together and the curve described by the lever
becomes a continuous line. (See Fig. 33.) Notwithstanding the fact
that the individual contractions are no longer visIbTe, it can be shown by

FIG. 24. — DEVELOPMENT OF FATIGUE AND CONTRACTION. Muscle stimulated once a second by

a strong induced current.

other methods that the muscle is undergoing a series of slight alternate con-
tractions and relaxations or vibrations at least. After a varying length of
time the muscle becomes fatigued, relaxes, and returns to its natural^ con-
dition even though the stimulation be continued. The number of stimuli
per second necessary to 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

74 TEXT-BOOK OF PHYSIOLOGY

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. 34. It is evidently a combination of contracture and
fatigue. It will be observed that at the beginning of the stimulation there
is a staircase effect, a-b, combined with diminished 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.

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

2. Experimental.

3. Pharmacologic.

4. Pathologic

1. Physiologic Tetanus, i. 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 contractions are
states of tetanus, more or less complete; for the shortest possible volitional
contraction, however quickly it takes place, has always 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 impulses per second in consequence of the arrival
of nerve impulses coming 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
volitional 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 -fo
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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 75

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 instances 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 surface.

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 experimental
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 character 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 irritability 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 produce enormously increased.

4. Pathologic Tetanus, i. 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, characterized 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 having a selective action on these structures.
The clonic exacerbations are evoked from time to time by various forms of
peripheral stimulation.

2. Reflex.— A tetanus of individual muscles more or less continuous 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 ulcerative 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 char-
acter and reflex in origin.

76 TEXT-BOOK OF PHYSIOLOGY

The Muscle Sound. — If a stethoscope or a myophone with telephone
connections be placed on a muscle while in a condition of volitional tetanus
and at the same time kept in a certain degree of tension, there will be devel-
oped 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 receiving body such as a pillow, and the
masseter muscles made to contract 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 stimula-
tion. 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 relative rest are active and complex,
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.

If the muscle be stimulated through its related nerve 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. At the same time the muscle-
tissue changes from a neutral to an acid reaction, from the development of
sarcolactic acid and possibly phosphoric acid. The degree of the acidity
depends partly 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
the muscle 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 condi-
tion of extreme rigidity or contraction known as death stiffening 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 disappears, their
cohesion diminishes, and their irritability ceases. The time of the appear-
ance 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 irritability is impaired from any

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 77

cause, such as chronic disease or defective 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 extremities; finally in the trunk and lower ex-
tremities. It disappears in practically the same order. Chemic changes
of a marked character accompany 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 myosin and myogen within
the sarcolemma with the formation of two insoluble proteins, myosin fibrin
and myogen fibrin. . In the early stages of the coagulation restitution is
possible by the circulation 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 and myogen fibrins soluble, and possibly
to the action of various microorganisms which give rise to putrefactive

changes.

Source of the Muscle Energy.— The nature of the materials which are
the immediate sources of the muscle energy has been the subject of much
discussion. 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 protein ^materials.
The marked production of carbon, dioxid and sarcolactic acid points to the
decomposition of some unstable compound, of a carbohydrate character,
rich in carbon and oxygen. As the result of many investigations it has come
to be believed that glycogen is the compound which furnishes the energy
under physiologic conditions, 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
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 absence of
carbohydrate food, the muscle will utilize fat and protein, for experiment
has shown that the available glycogen is entirely consumed by the second <
third day. The mechanism by which the energy is liberated, whether
by direct oxidation or decomposition is uncertain. The general teen
experimental investigation points to the disruption of some carbohydrate,
perhaps glucose, derived from the stored glycogen and the oxidation o
intermediate products to carbon dioxid and water. The oxidizable com-
pound appears to be lactic acid. For if the muscle be made to co:
in an atmosphere deficient in oxygen, the amount of lactic acid produced
is relatively large and the amount of carbon dioxid relatively i
the surrounding atmosphere be rich in oxygen, the reverse, conditions
obtain. Under physiological conditions, when the muscle i supplied
with blood containing its customary percentage of oxygen i
that the products set free by the disruption of the sugar molecule are raj
oxidized to C02 and H2O, with the liberation of their Contained energy.
But the fact that muscle will contract in an atmosphere free <
that no free oxygen can be obtained from muscle, would support

78 TEXT-BOOK OF PHYSIOLOGY

that the .mechanism is one of decomposition. Hermann suggests that
the energy of a contraction is liberated by the splitting and subsequent
re-formation of a complex body belonging neither to the carbohydrates
nor fats, but to the proteins — 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 di-
oxid, sarcolactic acid, and a protein residue resembling myosin. On the
cessation of the contraction the muscle-cell recombines the protein 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 CO2 in large amount,
develops sarcolactic acid and myosin. There is thus a close analogy be-
tween 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 oxidation processes.

THERMIC PHENOMENA

The potential energy liberated in a muscle on the arrival and subsequent
action of a nerve impulse, manifests itself partly as heat and partly as
mechanic motion or a change of shape of the muscle. Though heat pro-
duction is taking place even during the passive condition, it is largely in-
creased by muscle activity. The amount of heat produced will vary however
with a variety of conditions, as strength of stimulus, tension, work done, etc.

Stimulus. — It has been experimentally determined that the skeletal
muscle of the frog, the gastrocnemius, shows after a single contraction a rise
in temperature of from o.ooi°C. to o.oo5°C. and after tetanization an
increase of from o.i4°C. to o.i8°C. It has also been shown that an increase
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 condi-
tions being the same, is the amount of heat liberated. If the muscle is
securely fastened at both extremities so that shortening is practically im-
possible during the stimulation, the maximum of heat production is reached.
In the tetanic state the great increase in temperature is due to the ten-
sion of antagonistic and strongly contracted muscles. In both instances,
mechanic motion being prevented, the liberated energy is transformed into
heat.

4 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 con-
servation of energy, the heat produced, stated in calories, plus the energy
required in the raising of the weight, expressed in kilogrammeters of work,
must equal the, potential energy transformed.
y A muscle during a tetanic contraction of short duration accomplishes

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 79

more work than during a single contraction, the weight in each case being
the same. In the former condition the height of contraction through sum-
mation, 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 tetanus, no further work is done. Internal
work is done, however, i.e., the continuous liberation of energy, as shown by
an increase in the temperature.

When a weight which is lifted by a muscle during a single contraction is
allowed to act on the muscle during the relaxation, no external work is
accomplished. All the energy set free manifests itself as heat. Internal
ork 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 intermediations of the bones of
the skeleton 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 employment 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 develop-
ment weighing 72 kilos can perform in eight hours has been variously esti-
mated. It will naturally vary according to the character of the occupation.
If the work done be calculated from the number of kilograms raised one
meter, the average laboring-man performs about 300,000 kilogrammeters of
work.

ELECTRIC PHENOMENA

Electric Currents from Injured Muscles. — The energy liberated as
the result of the action of a nerve impulse 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 deflection of the galvanometer needle. The
existence of this current is most conveniently demonstrated with Dingle
muscles the fibers of which are parallel— e.g., the sartorius, or the semimem-
branosus of the frog. If the tendinous ends of either of these muscles be
removed by a section 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 pro-
vided with non-polarizable electrodes, an electric current is at once de-
veloped. In all instances the current, as shown by the deflection of the
needle, originates at the transverse surface, passes through the muscle to

TEXT-BOOK OF PHYSIOLOGY

the longitudinal surface, thence through the galvanometer to the transverse
surface. The longitudinal surface is, therefore, electropositive, the trans-
verse surface electronegative. The two points exhibiting 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 longi-
tudinal 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 dis-
tances 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 cur-
rent is developed when two points on the longi-
tudinal surface equally distant from the equa-
tor, or two points on the transverse surface
equally distant from the central axis, are con-
nected. Such points are said to be isoelectric.
These facts are shown in Fig. 3 5 . The natural
ends of the muscle, enclosed by sarcolemma
and tendon, do not exhibit, if carefully pre-
served from injury, the negativity characteris-
tic 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 longitudinal surface with the
axis of the transverse surface have an electro-
motive force in the frog muscle of from 0.037
to 0.075 of a Daniell cell.

The electric currents in the muscle are
intimately associated with the chemic changes
underlying its nutrition, and hence their in-

FIG. 35. — 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.}

tensity rises and falls with all the conditions which maintain or impair
muscle nutrition and irritability. The currents observed in the injured
muscle during the inactive state have been termed currents of 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 com-
posed 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 electropositive. These currents Hermann
terms "demarcation currents."

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 81

Negative Variation of the Muscle Current.— If a muscle exhibiting a
current of injury be excited to activity by tetanizing induced currents
applied to the opposite end of the muscle, it will be observed that as the
contraction wave passes over the muscle there is a movement of the gal-
vanometer needle toward the zero point, indicating a diminution of the
potential on the longitudinal surface. To this diminution 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 posi-
tion. 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 temporary disinte-
gration of the muscle substance
(Fig. 36). With their disappear-
ance and the subsequent restora-
tion of the nutrition of the mus-
cle, the former electric condition
returns.

The primary deflection of the
galvanometer needle is due to the
demarcation current which arises FIG. 36.— THE NEGATIVE VARIATION OF THE

QC Q recruit r»f tViP HifFpTPnrA in DEMARCATION CURRENT. A. The contraction
af a /e r.enCG I11 wave, which as it passes beneath the electrode at B

electric potential produced by the causes a diminution of potential.

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 perfectly
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 contrac-
tion wave.

Action Currents.— When two isoelectric points on the longitudinal
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. 37, the muscle- tissue at that point becomes
electronegative toward B and a current at once passes through the galvan-
ometer from B to A, as shown by the deflection of the needle toward A. As
the contraction wave passes beneath B it in turn becomes electronegative,
and a temporary condition of 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 electropositive toward B, when a current will pass through the
galvanometer in the opposite direction from A to B, as shown by the move-
ment of the needle toward B, Fig. 38. 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 ir
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

TEXT-BOOK OF PHYSIOLOGY

observed, which, however, endures so long as the tetanic contraction is
maintained. To this current the term decremential 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.

FIG. 37. — THE CONDITION LEADING TO THE DEVELOPMENT or THE FIRST ACTION

CURRENT.

The presence of action currents in the muscle of the living body during a
single contraction was demonstrated by Hermann in the muscles of the
forearm. The arrangement of the experiment was, briefly, as follows:
The forearm was surrounded by two twine electrodes saturated with zinc

FIG. 38. — THE CONDITION LEADING TO THE DEVELOPMENT OF THE SECOND ACTION

CURRENT.

solution, one being placed at the physiologic middle — the nervous equator—-
the other at the wrist. Both electrodes were then connected with the galvan-
ometer. 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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 83

phase — atterminal — 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
currents 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 muscle 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 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
enarthrodial or ball-and-socket joints. Muscles which impart an angular
movement to the extremities to and from the median line of the body are
termed adductors and abductors respectively.

In addition to the actions of individual groups of muscles in producing
special movements, in some regions of the body, several groups of muscles
are coordinated for the accomplishment of certain
definite functions; e.g., the functions of respiration,

mastication, etc. The coordination of axial and * £

appendicutar muscles enables the individual to as-
sume certain postures, such as standing, sitting, and F •
lying; to engage in various acts of locomotion, as A W
walking, running, dancing, swimming. • I

Levers. — The function or special mode of action w"~ p A*'

of individual muscles can be understood only when FlG 39 _THE THREE OR-
the bones with which they are connected are regarded DERS OF LEVERS.
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 overcoming resistance.

In mechanics levers of three kinds or orders are recognized according
to the relative positions of the fulcrum or axis of motion, the applied
power, and the weight to be moved. (See Fig. 39.)

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 :

84 TEXT-BOOK OF PHYSIOLOGY

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

ment, 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 muscles attached to the tuberosity
of the ischium. The opposite movement is equally one of the first
order, but the relative positions 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 mentioned:

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 depressor muscles.

2. The raising of the body on the toes, in which movement the fulcrum 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 example 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 applied 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 generally obtains, viz., the
overcoming of a small resistance by the application of a large force acting
through a short distance. As a result there is a gain in the extent and rapid-
ity 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 :
i. 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 mus-
cular effort is, however, very slight, as the center of gravity of the head
lies but a short distance in front of the articulation.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 85

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 support 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 consequence 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 back-
ward 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 gravity 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 main-
tained 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 balancing 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 iri turn the active leg when
the foot touches the ground. Each leg is therefore alternately in an active
and in a 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 OR INVOLUNTARY 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 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,

TEXT-BOOK OF PHYSIOLOGY

elongated, and fusiform in shape. As a rule, they are extremely small,
measuring only from 40 to 250 micromillimeters in length and from 4 to 8
micromillimeters 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 enclosed 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 fibrillae surrounded by sarcoplasm (Fig. 40). The fibers are united
longitudinally and transversely by a cement material. The muscle is
increased in thickness by the superposition of successive layers. At vary-

FIG. 40. — 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.)

ing intervals the fibers are grouped into bundles or fasciculi by septa of
connective tissue (Fig. 41). Blood-vessels ramify in the connective tissue
and furnish the necessary nutritive material.

The visceral muscle receives stimuli from the spinal cord, not directly,
however, as in the case of 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 physiologically associated with the muscle-fiber.

Physiologic Properties. — The visceral muscles which have been sub-
jected to experiment are mainly those of the stomach, intestine, bladder,

ureter, and iris. From the results of
the experiments which have been pub-
lished, it is evident that all visceral
muscles possess elasticity, tonicity, irri-
tability, and conductivity.

The elasticity of the bladder mus-
cle 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 increment, 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.

The tonicity of visceral muscles is as pronounced in many situations as
is the tonicity of skeletal 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

Connective-tissue—
septum.

Nucleus.

Smooth muscle-fiber
in transverse section.

FIG. 41. — SECTION OF THE CIRCULAR
LAYER OF THE MUSCULAR COAT OF THE
HUMAN INTESTINE. — (Stohr.)

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 87

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 doubtless 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 direction at right angles to the
long axis of the intestine, partially obliterating its lumen. From this point
the conduction process indicated by the contraction wave passes in opposite
directions for some distance along the canal. As to whether this is accom-
plished 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 vigorous manner. The
contraction wave beginning at various points is propagated in both direc-
tions, running along the intestinal wall for a variable distance. A succession
of similar waves may be observed for some minutes. To the alternate
contraction and relaxation of the muscle-fibers, which are circularly ar-
ranged, 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 movements. Inas-
much as the cause is not apparent, these contractions are termed spontane-
ous or automatic.

Graphic Record of the Contraction.— For experimental purposes
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 prepared, the muscle may exhibit for several
hours a series of pulsations, 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 suc-
cession 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 constant current. A curve of such a contraction is shown in Fig. 42.
The contraction takes place more rapidly than the relaxation; the two
phases occupying five and thirty-five seconds respectively. The latent

TEXT-BOOK OF PHYSIOLOGY

FIG. 42. — 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.)

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 resp'onds to variations in [temperature, to strength of stim-
ulus, to the load, in a manner similar to, if not iden-
tical 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 passage through the
viscus or organ of the material contained within it.
The food in the stomach and intestines is subjected
to a churning process by the muscles, in consequence
of which the digestive fluids are more thoroughly in-
corporated and their characteristic action increased.
At the same time the food is carried through the
canal, the absorption of the nutritive material pro-
moted, 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 relaxation 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 re-
spective muscles. The mode of action of the individ-
ual muscles will be described in successive chapters.

Ciliary Movement.— The free surface of the epithelium covering the
mucous membrane in certain regions of the body is characterized by the
presence of delicate filamentous processes termed cilia. (See Fig. 43.)
Ciliated epithelium is found in man and mammals generally, in the nose,
Eustachian tube, larynx, with the exception 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 epithe-
lium is also found in all classes of animals, and espe-
cially 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 prolongation 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 and
rapid vibratile movement, so much so that the individual cilium cannot
be distinguished. In time, however, their vitality declines and the rapid-
ity of movement diminishes. When the movement of the individual
cilium falls to about eight or ten per second, its character can be readily

FIG. 43.— CILIATED EPI-
THELIUM.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 89

determined. It will then be seen that the movement is, as a rule, alter-
nately a backward and a forward one, the cilium lowering and then rais-
ing itself, the latter taking place more quickly and energetically than the
former. As the cilium raises itself it becomes somewhat flexed in a direc-
tion corresponding to that of the general movement. 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 4O°C., especially if they are
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 associated with
the function of the passages in which they are found. As 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. — The nerve- tissue, which unites and coordinates
the various organs and tissues of the body and brings the individual into
relationship with the external world, is conventionally arranged in two
systems, termed the encephalospinal or cerebrospinal and the sympathetic.

The encephalospinal system consists of:

1. 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:

1. A chain of ganglia situated on each side of the spinal column and extend-

ing 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 elaborate system of
intercommunicating nerves, many of which are connected with the
cerebrospinal system. (See Chapter XXVI.)

HISTOLOGY OF NERVE-TISSUE

The Neuron. — The nerve-tissue has been resolved by the investigations
of modern histologists into single morphologic units, to which the term
neurons 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 histologic -
ally 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 nerve 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 Fig. 44, A, the neuron may be said to
consist of: (i) 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 everywhere recognizable, they, exhibit a variety of
secondary features in different situations in accordance with peculiarities of
function.

The Nerve-cell. — The nerve-cell, or body of the neuron, presents a
variety of shapes and sizes in different portions of the nerve system.
Originally ovoid in shape, it has acquired, in course of development, pecul-
iarities of form which are described as pyramidal, stellate, pear-shaped,
spindle-shaped, etc. The size of the cell varies considerably, the smallest

GENERAL PHYSIOLOGY OF NERVE-TISSUE 91

having a diameter of not more thanio to 12 micro-millimeters, the largest
not more than 150 micro-millimeters. Each cell consists of granular,
striated cytoplasm, containing a distinct vesicular nucleus and a well-de-
fined nucleolus. A characteristic feature of the cytoplasm is the presence
of granules first described by Nissl, which stain deeply with methylene blue
and other dyes. For this reason these granules are spoken of as chromo-
phile granules. The remainder of the cytoplasm is penetrated in various
directions with nerve fibrils which are continuous with similar fibrils run-
ning through the axonic process as well as the dendrites. The physio-
logic significance of Nissl's granules is unknown. The nerve fibrils are
probably connected with the transmission of nerve impulses. A cell mem-
brane has not been observed. From the surface of the adult cell portions

A B-

FIG 44.-A. EFFERENT NEURON; B, AFFERFNT NEURON. FOUND IN BOTH SPINAL AND CRANIAL

NERVES.

of the cytoplasm are projected in various directions, which portions,
rapidly dividing and subdividing, form a series of branches, termed den-
drites or dendrLs. In some situations the ultimate branches of the den-
drites present short oclateral presses, known as lateral buds or gtmgte,
which impart to the branches a feathery appearance his character
istic is common to the cells of the cortex of the cerebrum and of the
cerebellum. The ultimate branches of the dendntes though formmg an
intricate feltwork, never anastomose with one another nor te witn
dendrites of adjoining cells. According to the number °' ax°f 1,™
are classified as monaxonic, diaxonic, polyaxomc. Most ot the
the nerve system of the higher vertebrates are moronic,
of the posterior or dorsal roots of the spinal am
they are diaxonic. In this situation the axons,
poles of the cell, either remain separate and pursue oW^-~ - _--- -
unite to form a common stem, which subsequent^ divides into two branche ,
which then pursue opposite directions. (See Fig. 44, B. (The nerve cell
maintains its own nutrition, and presides over that of t

92 TEXT-BOOK OF PHYSIOLOGY

axon as well. If the latter be separated in any part of its course from the
cell, it speedily degenerates and dies.

The Axon. — The axon, or nerve process, arises from a cone-shaped pro-
jection from the surface of the cell, and is the first outgrowth from its cyto-
plasm. 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 hyalin appearance. In structure,
the axon appears to consist of fine fibrillae embedded in a clear, semi-fluid
material, the neuroplasm. 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 felt-
work in the neighborhood of the cell. In the latter instance the axon
continues for an indefinite distance as an individual structure. In its
course, however, especially in the brain and spinal cord, it gives off a number
of collateral branches, which possess all its histologic features. The long
axons serve to bring the body of the cell into direct relation with peripheral
organs, or with more or less remote portions of the nerve system, thus con-
stituting association or commissural fibers. Physiologic investigations have
established the fact that the axon is the conducting agent of the nerve
impulses.

The Myelin. — At a short distance from the cell the more or less elon-
gated axon becomes invested with nucleated oblong cells, which subse-
quently become modified and constitute the medullary or my elm sheath.
When fresh the myelin is clear and semi-fluid; when treated with various
reagents it becomes opaque and imparts a white appearance to nerves.
The function of the myelin is unknown. All axons that possess a myelin
investment are known as myelinated nerve-fibers.

The Neurilemma. — The myelin in many situations is enclosed by a
thin transparent elastic membrane known as the neurilemma. In the
spinal cord and brain, the nerve fibers are for the most part wanting in
this membrane.

At intervals of about seventy-five times its diameter, the medullated 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 the axis-
cylinder and the surrounding plasma. Beneath the neurilemma in each
internodal segment there is a large nucleus surrounded by a small amount of
granular protoplasm.

The End Tufts. — The end-tufts 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 myelin 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE 93

94 TEXT-BOOK OF PHYSIOLOGY

termination, however, nerve-fibers retain their individuality, and never b<
come 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 throughout 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 space, in which is contained lymph destined for the
nutrition of the fiber. Near their ultimate terminations the nerve-fibers

FIG. 45. — TRANSVERSE SECTION OF A NERVE (MEDIAN), ep. Epineurium.
pe. Perineurium. ed. Endoneurium.— (Landois and Stirling.}

themselves undergo division, so that a single fiber may give origin to a num-
ber of branches, each of which contains a portion of the parent axis-cylinder
and myelin.

Sympathetic Ganglia. — A sympathetic ganglion consists essentially of
a connective-tissue capsule with an interior framework. The meshes of
this framework contain nerve-cells possessing dendrites and branching
axons. The majority of the axons are devoid of myelin and are therefore
known as non-myelinated nerve-fibers. Owing to the absence of the myelin
they present a rather pale or grayish appearance. 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 certain fine medullated nerve-fibers which leave the
spinal cord by way of the ventral roots of the spinal nerves. These nerve-
fibers are designated pre-ganglionic fibers, while those emerging from the cells
are designated post-ganglionic fibers.

Blood-supply. — Nerves being parts of living cells require for the main-
tenance of their nutrition a certain amount of blood. This is furnished by
the blood-vessels ramifying in and supported by the connective-tissue frame-
work. 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 globulins, a
nucleo-protein and neurokeratin) , certain lipoids, e.g., (a) cholesterin (a
monotomic alcohol free from both nitrogen and phosphorus), (b) several

GENERAL PHYSIOLOGY OF NERVE-TISSUE 95

cerebrosides or galactosides (nitrogen-holding bodies, free from phos-
phorous, compounds of a glucoside character, as shown by their yielding on
[hydrolysis the reducing carbohydrate galactose), (c) phosphatids (com-
pounds containing both nitrogen and phosphorus, e.g., lecithin, kephalin,
sphingo-myelin) , 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, practically 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 system is more active and extensive. In this situation any with-
drawal 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 G
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 intervertebral 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 backward 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 communication
between the brain and spinal cord, on the one hand, and the skeletal muscles,
glands, blood-vessels, visceral muscles, skin, mucous membrane, 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 organs, and so
accelerate or retard, augment or inhibit 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 determined that the anterior or. ventral

TEXT-BOOK OF PHYSIOLOGY

FIG. 46. — MOTOR NERVE-ENDINGS
or INTERCOSTAL MUSCLE-FIBERS OF
A RABBIT. X 150.— (St'ohr.)

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
sheaths. The axon or axis-cylinder then divides into a number of branches
which become directly and intimately associated with tissue-cells. The

particular mode of termination varies in

BHBSBnBBaHHBBMMBHi 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 rela-
tively large mass of sarcoplasm and nuclei,
the whole forming the so-called "motor
plate." Each muscle-fiber possesses one
such plate or end-organ in mammalia,
several in the frog. (Fig. 46.)

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 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. 47).

The afferent nerves as they ap-
proach their ultimate terminations un-
dergo similar changes. The end-tufts
become associated, in some situations,
with specialized end-organs which are extremely complex.

In the skin and mucous membranes the mode of termination varies con-
siderably. The following 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 hands and

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

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

FIG. 47. — 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 Moral.)

GENERAL PHYSIOLOGY OF NERVE-TISSUE

(A consideration of these end-organs will be found in the chapters de-
voted 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 neuromusde end-
organs. These spindles vary in length from i 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 contained 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
ventral horns of the gray matter of the spinal cord. These cells are the
modified descendants of independent, oval, pear-shaped cells — the neuro-
blasts — which migrate from the medullary tube. As they approach the
surface of the cord their axons are 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. (Fig. 48.)

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
nerve system and only subsequently
become connected with it. (See Fig.
48.) At the time of the closure of
the medullary tube a band or ridge of
epithelial tissue develops near the dor-
sal surface, which, becoming seg-
mented, moves outward and forms
the rudimentary spinal ganglia. The
cells in this situation develop two

axons, one from each end of the cell, ROOTS.— (Edinger, after His.)
which pass in opposite directions, one

toward the spinal cord, the other toward the periphery. In the adult
condition 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 sym-
pathetic nerve system), with visceral muscles, blood-vessels, and glands.

The afferent fibers are to be regarded as outgrowths from the cells of the

TEXT-BOOK OF PHYSIOLOGY

dorsal nerve ganglia, and serve to bring the skin, mucous membrane, and
certain visceral structures into relation with specialized centers in the central
nerve system.

Nerve Degeneration. — If any one of the cranial or spinal nerves be di-
vided 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 degenera-
tion is applied. The degenerative process begins simultaneously through-
out the entire course of the nerve, and consists in a disintegration and reduc-
tion of the myelin and axis-cylinder into nuclei, drops of myelin, and fat,
which in time disappear through absorption, leaving the neurilemma intact.
Coincident with these structural changes there is a progressive alteration and
diminution in the excitability of the nerve. From these facts it has been
assumed that the nerve-cells exert over the entire course of the nerve-fibers
a nutritive or a trophic influence. 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.

It has for a long time been stated that the portion of the divided nerve in
connection with the brain or spinal cord retains its normal condition with the
exception of a few millimeters at its peripheral end. This statement has
been disproved by recent observations which show that the cell body and
its attached axon also in the course of time undergo an atrophy which may
become permanent; also that the chromatin material of the cell, Nissl's gran-
ules, undergo dissolution and no longer stain with the usual dyes, a condition
which may last for two weeks or more. In some instances the granules
may be restored. These degenerative changes in the nerve cell are probably
the result of the cessation of its customary function.

lBlJfI3D 3fft In sbfeJUO qobvob ,89VX9fl

A CD

3in

FIG. 49. — DEGENERATION OF SPINAL NERVES AND NERVE-ROOTS AFTER SECTION. A,
Section of nerve-trunk beyond the ganglion. B, Section of ventral root. C, Section of dorsal.
D. Excision of ganglion, a. Ventral root. p. Dorsal root. g. Ganglion. — (Dalton.)

. btOD lunicffi 6ffa bi

The relation of the nerve-cells to the nerve-fibers, in reference to their
nutrition, is demonstrated by the results which follow section of Jthe ventral
and dorsal roots of the spinal nerves. If the ventral root alone be divided
the immediate degenerative process is confined to the peripheral portion, the
central portion remaining for a variable period normal. If the dorsal root
be divided on the peripheral side of the ganglion, degeneration takes place
only in the peripheral portion of the nerve. (See Fig. 49.) 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE 99

nerve ganglia, respectively, or, in other words, in the cells of which they are
an integral part. The structural changes which nerves undergo after separa-
tion 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.

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 infrequently happens that the func-
tions are restored because of a reunion of the separated ends and a regenera-
tion 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 neuri-
lemma. 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 transformed into myelin as in the
original fiber. The axis-cylinder continues to grow and extend itself
forward until it reaches its ultimate termination.

CLASSIFICATION OF NERVES .

The Efferent Nerves. — The efferent nerves may be classified, in accord-
ance with their distribution and the characteristic forms 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 directly to skeletal-muscles and excite them to activity.

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

way of ganglia and influence in one direction or another the degree of
their activity. Those which cause the formation and discharge of the
secretion peculiar to the gland, are known as secreto-motor, while those
which decrease or inhibit the secretion are known as secreto-inhibitor
nerves.

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

ioo TEXT-BOOK OF PHYSIOLOGY

inhibit or retard its rate and diminish its force. For this reason they
are termed cardiac nerves, one set of which is known as cardio-accelera-
tor and cardio-augmentor, the other as cardio-inhibitor nerves.

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

the muscle walls of the viscera and change in one direction or another
the degree of their contraction. Those which increase or augment the
contraction are known as viscero-augmentor, while those which decrease
or inhibit the contraction, are known as viscero-inhibitor nerves.

5. Hair bulb or pilo-motor nerves, those which transmit nerve impulses 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. 13.)

The Afferent Nerves. — The afferent nerves may also be classified, in ac-
cordance with their distribution and the character of the sensations or other
modes of nerve activity to which they give rise, into several groups, as follows:

1. Tegumentary nerves, comprising those distributed to skin, mucous mem-

branes and sense organs and which transmit nerve impulses from the
periphery to the nerve centers. They may be divided into reflex and
sensorifacient nerves.

A. 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 consequence in-
creased 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 consequence,
decreased activity of the peripheral organs. It is quite prob-
able that one and the same nerve may subserve both sensation
and reflex action, owing to the collateral branches which are
given off from the afferent roots as they ascend the posterior
column of the cord.

B. 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 sensations of well-being or dis-
comfort, hunger, thirst, fatigue, sex, want of air, etc.

2. Muscle nerves, comprising those distributed to muscles and tendons and

which transmit nerve impulses from muscles and tendons to the brain
where they give rise to the so-called muscle sensations, e.g., the direction

GENERAL PHYSIOLOGY OF NERVE-TiSSl E

101

and the duration of a movement, the resistance offered and the posture
of the body or of its individual parts.

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 composition. 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 stimulated. 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 histo-
logic 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 mus-
cles, without impairing the irritability 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-cells, acts upon the nerve-fibers in connection with
them. In the case of sensor or afferent nerves the stimuli act upon the pecul-
iar 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.

102 »{\5>t>!O'1 :TEXT,£OOK OF PHYSIOLOGY

Nerves respond to stimulation according to their habitual function;
thus, stimulation of a sensor nerve, if sufficiently strong, results in the sensa-
tion of pain; of the optic nerve, in the sensation of light; of a motor nerve,
in contraction of the muscle to which it is distributed; of a secretor nerve,
in the activity of the related gland, etc. It is, therefore, evident that pecul-
iarity 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 peculiarities of its central and peripheral end-organs.

Nerve stimuli may be divided into —

1. 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:

1. Mechanic: Sharp taps, sudden pressure, cutting, etc.

2. Thermic: Sudden application of heated object.

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

tion 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 gusta-

tory 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 chemic processes attended by
the liberation of energy, which propagates 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 dura-
tion. (See page 106.) Under strictly physiologic conditions the nerve
impulse passes in one direction only; in efferent nerves from the center to the

GENERAL PHYSIOLOGY OF NERVE-TISSUE 103

periphery, in afferent nerves from the periphery to the center. Experimen-
tally, however, it can be demonstrated that when a nerve impulse is aroused
in the course of a nerve by an adequate 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 direc-
tion, 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 pro-
duction of waste products, the liberation of heat, and in time by the phenom-
ena 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 differences of
physiologic actions, there are no characters, either histologic or chemic,
which serve to distinguish them from one another. Moreover, as the nerve
impulse is conducted through a nerve-fiber equally well in both directions,
as determined by experiments, it is probable that it does not differ in char-
acter 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 physiologically 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, secretion;
to a blood-vessel, variation in caliber; to special areas in the brain, sensa-
tions of light, sound, pain, etc.

Electric Excitation of Nerves.— For the purpose 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. 50.) When
kept moist, this preparation is extremely sensitive to either the galvanic
or the induced current.

IO4

TEXT-BOOK OF PHYSIOLOGY

Though the development and conduction of a nerve impulse may be
demonstrated by the deflection of the galvanometer needle or the move-
ment 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 impulse. The
preparation should be enclosed in a moist chamber and the nerve con-
nected with the inductorium through the intervention
of non-polarizable electrodes. The muscle may be at-
tached 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 elec-
tric 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 grad-
ually increasing the frequency of the stimulus. The
character of the contraction caused by indirect stimu-
lation— i.e., through the nerve — does not differ in any essential respect from
that due to direct stimulation.

FIG. 50. — NERVE*
MUSCLE PREPARA-
TION OF A FROG. F.
Femur. S. Sciatic
nerve. I. T e n d o
Achillis. — (Landois
and Stirling.}

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 surfaces constitutes the equator. From such
a cylinder strong currents are obtained when the natural longitudinal sur-
face 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 surface equidistant from the equator are united, no
current is obtainable. When asymmetric points on the longitudinal sur-
face 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 Ion-

GENERAL PHYSIOLOGY OF NERVE-TISSUE

105

gitudinal surface are electrically positive to the transverse surface and that
the point of greatest positive tension is situated near the equator (Fig. 51).

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., depehd 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. !•• j

Negative Variation of the Nerve Cur-
rent.— During the passage of the nerve impulse
the resting nerve current, or the demarcation
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 diminution of the difference
in potential between the positive longitudinal
and negative transverse sections. This nega-
tive variation of the demarcation current is
observed equally well from either the central
or peripheral end of the nerve. If the two
ends of the nerve are connected with galvanom-
eters and the nerve stimulated in the middle,
the demarcation currents simultaneously un-
dergo a negative variation. This may be
taken as a proof that the excitation process
propagates itself equally well in both direc-
tions. The negative variation is intimately
connected with changes in the molecular con-
dition of the nerve and is not due to any ex-
traneous 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 longi-
tudinal surfaces were then placed in connection with the electrodes of the
galvanometer wires and the current permitted to influence the needle.
The animal was then subjected to the action of strychnin. Upon the ap-
pearance 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 experi-
ment 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 cannot be demonstrated with the physio-

FIG. 51, — 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.)

io6 TEXT-BOOK OF PHYSIOLOGY

logic 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
transverse 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 summarize 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 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 dim-
cult to determine the presence of diphasic action currents during the pass-
age of an excitatory impulse through the nerve-fiber. The so-called negative
variation of the resting nerve current — the demarcation 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 demarca-
tion 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 negative 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 demarcation 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 under-
going 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 is 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 state-
ments to the nerve impulse, it may be said that it is a molecular disturb-
ance, traveling at the rate of about 28 meters a second, is wave-like in

GENERAL PHYSIOLOGY OF NERVE -TISSUE

107

character, the wave being 18 millimeters in length and occupying from
o 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 negative with reference
to all the rest of the nerve. This, however, is not the case, the absence of
the two opposing phases of the action current being explained on the suppo-
sition 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 galvan-
ometer needle During stimulation of the nerve, when two eventless or
isoelectric 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 ex
plained on similar grounds. It is true that an apparent ac ion current
is sometimes seen when the stimulating current is very powerful or the seat
of stimulation too near the diverting electrodes. This, however, must be
attributed to an electrotonic state of the nerve. ronctant

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

i. The development of a nerve impulse at the moment the jcurror t ent ers
and at the moment the current leaves the nerve, *.,., ****£™ Je
circuit is made and at the moment it is broken. The development
nerve impulse is made evident by the contraction of the ^scle 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.

^AGALVANOMETER

ANELECTROTONIC
CURRENTS

KATELECTROTONIC
CURRENTS

,. T* .i— °*, on e.cb _,ide of th, :

S*i

— ^ " the

nerve according to the direction of the polarizing curr

io8 ,

TEXT-BOOK OF PHYSIOLOGY

To this changed condition of the electromotive forces in a nerve the
term electrotonus was given (du-Bois Reymond). The currents them-
selves are known as electro tonic currents; from their relation to the anode
and kathode, they are termed anelectrotonic and katelectrotonic currents.
The condition of the nerve around the poles both in the intra-polar and
extra-polar regions is known as anelectrotonus and katelectrotonus.

The electrotonic currents vary considerably in strength and extent,
according to the intensity of the polarizing current, increasing steadily
with the intensity of the latter up to the point at which the polarizing cur-
rent begins to destroy the physical and chemic integrity of the nerve.
The electrotonic currents are strongest in the immediate 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 phys-
ical state of the nerve; for if it be ligated or injured beyond the polarized
portion, the electrotonic currents are abolished. The electrotonic currents
have no necessary 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 polari-
zation 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 stimulation — that
is, muscle contraction, sensation, and inhibition — are increased or decreased

N J

FIG. 53. — SCHEME OF THE ELECTROTONIC EXCITABILITY. — (Landais and Stirling.}

according to the strength and direction of the current. To this condition
the term electrotonus was also given (Pfliiger). This word has thus been
employed to express two distinct series of effects exhibited by a nerve through
a portion of which a constant galvanic current is passing. It appears desir-
able, 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 conductivity.
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 neighborhood of the anode or
positive pole and increased in the neighborhood o'f the kathode or negative
pole. These alterations in the excitability are most marked in the imme-
diate vicinity of the electrodes, though they extend for some distance into

GENERAL PHYSIOLOGY OF NERVE-TISSUE 109

both the extra-polar and intra-polar regions, though with gradually dimin-
ishing 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. 53). The extent to which the excitability is modi-
fied as well as the position of the neutral point will depend largely on the
strength of the polarizing or galvanic current.

<\ REGION OF

j INCREASED EXCITABILITY

ANODE KATHODE

SECONDARY COIL

FIG. 54. — DIAGRAM SHOWING THE REGION OF INCREASED EXCITABILITY CAUSED BY THE
PASSAGE OF A GALVANIC CURRENT THROUGH A PORTION OF A NERVE, STIMULATION OF
WHICH GIVES RISE TO INCREASED CONTRACTION.

The electrotonic alterations of excitability and conductivity can be
experimentally demonstrated on the muscle-nerve preparation in the fol-
lowing manner:

i. With a descending current of medium strength. Previous to the clo-
sure of the polarizing current, the nerve is stimulated first in the extra-
polar anodic region and the extra-polar kathodic region with an induc-

REGION OF/*
DECREASED EXCITABILITY £

FIG. 55.— DIAGRAM SHOWING THE REGION OF DECREASED EXCITABILITY CAUSED BY THE
PASSAGE OF A GALVANIC CURRENT THROUGH A PORTION OF A NERVE, STIMULATION or
WHICH GIVES RISE TO DECREASED CONTRACIION.

tion shock of medium intensity and the height of the contraction re-
corded. On repeating the stimulation after closure of the polarizing
current the contraction resulting 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. 540

TEXT-BOOK OF PHYSIOLOGY

2. With an ascending current of the same strength. After preliminary

testing of the excitability and the subsequent closure of 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 neighborhood 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. 55.)

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 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 regularity.
The order in which they occur under these varying conditions of experi-
mentation has been determined and tabulated as follows by Pfluger, and is
termed the law of contraction:

Current intensity

Ascending current

0 t c/ J H
8 AT ID - Descending current

_fcrfc

SMake Break

jviaKe

Weak

Contraction.
Contraction.
Rest.

Rest.

Contraction.
Contraction.

Contraction.
Contraction.
Contraction.

Rest.
Contraction.
Rest or weak con-
traction.

Medium

Strong

The results as above tabulated are sometimes complicated on the open-
ing of the circuit by a series of irregular pulsations of the muscle, an ap-
parent 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 ob-
served 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE in

'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 anelectrotonus,
both conditions being attended by a rise of excitability—not, however, by
the opposite changes. It is further assumed that the appearance of kat-
electrotonus is more effective as a stimulus than the disappearance of anelec-
trotonus. 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 descending, 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 contraction both on closing and
opening the circuit. The appearance of katelectrotonus and the disap-
pearance of anelectrotonus are both sufficiently powerful to generate an im-
pulse without, however, seriously 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 anelectrotonic 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 blocking of the kathodal stimulus by the
decrease of the excitability and conductivity at the anode. With the open-
ing 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 fc

With Ascending Current. With Descending Current.

Weak i. K. C. C.1 £2?S A or

Medium 2. K. C. C. • A. O. C/ K. C. C. A. O. C

Strong 3. — A. O. C.

Polar Stimulation of Human Nerves.-The preceding statements as
to changes in the excitability caused by the passage of a constant curr
as well as to the law of contraction, are based entirely on experiments ma
with the isolated nerve of the frog. It is probable, however, that the sun
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 gal-
vanic 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 and
region and a decrease in the excitability in the extra-polar katelecl
region, as shown by stimulating the nerve in the extra-polar regions v
fee induced current-results which are in apparent contradiction to

'K. C. C., kathodal closing contraction. ' A. O. C., anodal opening contracts.

H2 TEXT-BOOK OF PHYSIOLOGY

obtained with the isolated nerve. This want of accordance in the results
of the two classes of experiments 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 physio-
logic kathode with the induced current gives rise, therefore, to the phenome-
non of increased excitability 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 con-
tractions which follow the closing and opening of the constant curren,
have been thoroughly studied by Waller and de Watteville. These observers
employed a method similar to that of Erb, conjoining in one circuit th
testing and polarizing currents. By the graphic method they recorded
first the contraction produced by an induction shock alone; and, secondly

FIG. 56. — ANODE OF BATTERY. FIG. 57. — CATHODE OF BATTERY.

Polar region of nerve is anodic. Peri- Polar region of nerve is cathodic. Peri-
polar region of nerve is cathodic. polar region of nerve is anodic. — (Waller.)

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 electrodes,
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. 56 and 57). The peripolar regions also
experience similar alterations of excitability, though less in degree, accord-
ing 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 experimenting with
the frog's nerves, it is incorrect to speak of either ascending or descending
currents. Waller,1 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

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

GENERAL PHYSIOLOGY OF NERVE-TISSUE 113

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.
56 and 57). If this electrode be the anode of a current, the latter enters
the nerve by a series 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 series 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 — 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 collective kathode),
and admitting as proved that make excitation is kathodic, break excitation
anodic, we may, with a sufficiently strong current, expect to obtain a con-
traction 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, we obtain an anodic make contraction and an anodic break con-
traction. 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 strongei
than the anodic make contraction; the anodic break contraction is strongei
than the kathodic break contraction. Or, otherwise regarded, if, mst
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 :
of contraction for man reads as follows:

"Weak current K. C. C.

Medium current. . . . . . .K. C. C. A. C. C. A. O. 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 therapeutic
and diagnostic purposes. In accordance with the statements above ^quoted
one electrode should be applied to the part to be investigated, the
to some indifferent region. The electrode conveying the curr to o
from this part should be of a size sufficient to localize the current and

ii4 TEXT-BOOK OF PHYSIOLOGY

increase its density. It was discovered by Duchenne that there are certain
points all over the body stimulation of which is more quickly followed by
muscle contraction than others. It was subsequently discovered by Remak
that these points coincide with the entrance of the nerve into the muscle.
It is to these motor points that the one electrode should be applied.

Reactions of Degeneration. — In consequence of the degeneration
and changes in irritability which occur in nerves when separated 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 forms of electricity are of importance in the diagnosis
and therapeutics of the precedent lesions. The principal difference of
behayior 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 of degeneration. The synchronous diminished excitability of the
nerves is the same for either current. The term " partial reaction of degen-
eration" is used when there is a normal reaction of the nerves, with the de-
generative 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 acceleration in
the activity of various organs, are the result of stimulations 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 involuntary actions. A reflex action of skeletal muscles, glands,
or non-striated muscles of blood-vessels or of viscera, therefore, may be de-
fined as an action which takes place independent of volition and in response
to peripheral stimulation. As many of the processes to be described in
succeeding chapters are of this character, requiring for their performance
the cooperation of several organs and tissues associated through the inter-
mediation 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. 13, page 42, the necessary structures are as
follows:

1. A, receptive surface, skin, mucous membrane, sense-organs, 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 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE

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 subserve but a simple movement. The majority of
reflex activities, however, are extremely complex, and involve the coopera-
tion and coordination of a number of nerve centers situated at different
levels of the spinal cord on the same and opposite side, and of responsive
organs frequently situated at distances more or less remote from one another.
This implies that a number of neurons are
associated in function. The transference of
nerve impulses coming from a localized area
of a sentient surface to emissive cells situated
at different levels is accomplished by the inter-
calation 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 different levels of
the cord. (Fig. 58.)

For the excitation of a reflex action it is
essential that the stimulus applied to the re-
ceptive surface be of an intensity sufficient to
develop in the terminals of the afferent nerve a
series of nerve impulses, which, traveling in-
ward, will be distributed to and received by
the dendrites of the emissive or motor cell.
With the reception of these impulses there is
apparently a disturbance of the equilibrium
of its molecules, a liberation of energy, and, in
consequence, a transmission outward of im-
pulses 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., are illustrations of reflex activity.

FIG. 58. — DIAGRAM SHOWING
THE RELATION OF THE THIRD
NEURON a, TO THE AFFERENT
NEURON b, AND TO THE EFFERENT
NEURONS c, c, c. — (After Kottiker.)

CHAPTER IX
FOODS

The functional activity of every organ and tissue of the body is accom-
panied by a more or less active disintegration of the living material, the bio-
plasm, 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 reduced through a series of descending chemic
stages to a small number of simpler compounds which, being of no further
apparent value to the organism, are eliminated by the various eliminating
or excretory organs, the lungs, skin, kidneys, and liver. Among these
excreted compounds derived from tissue and from food metabolism the
most important are urea, uric acid, and carbon dioxid. Many other com-
pounds, organic as well as inorganic, are also eliminated from the body in
the various excretions, though they are present in but small amounts. Coin-
cident 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 performance
of their functions, it is essential that they be supplied with nutritive mate-
rials simila^ to those which enter into their own composition: viz., proteins,
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 capillary blood-vessels. The blood is therefore to be
regarded as a reservoir of nutritive material in a condition to be absorbed
and transformed into utilizable and living material. Inasmuch as the
materials which are lost to the body daily, through processes of disintegra-
tion and oxidation, are supplied by the blood, it is evident that this fluid
would diminish rapidly in volume, with a corresponding decline in func-
tional activity, were it not replenished by the introduction into the body of
new material in the food. This is brought about by the sensations of hunger
and thirst which periodically arise.

The Sensations of Hunger and Thirst. — For some' time it has been sup-
posed that hunger is a general sensation arising in consequence of nutritional
changes and referred to the epigastrium. The recent experiments of Can-
non, Carlson and others indicate that this is not the case, but that it arises
in consequence of a contraction of the musculature of the stomach after its
contents have been discharged into the intestine. By reason of the dietetic
habits these hunger contractions occur three or four times a day and when
once established occur on an average of about once a minute and last for
about half a minute. The contractions apparently stimulate nerve endings
as a result of which nerve impulses ascend to the cerebrum and evoke the

116

FOODS Ii;

sensation which is then referred to the stomach region. With the introduc-
tion of food these contractions disappear and the sensation subsides. The
characteristic peristaltic contractions occurring during digestion do' not give
rise to the hunger sensation. The sensation of thirst arises when there is a
deficiency of water in the body and is mainly referred to the mouth and fauces.
With the introduction of water into the stomach and intestine and its
absorption into the blood and tissues the sensation speedily disappears.
The nerve mechanism by which these various results are brought about is
unknown.

The foods which are consumed daily in response to sensations of hunger
and thirst are complex in composition and contain, though in varying
amounts, proteins, fats, carbohydrates, water, and inorganic salts, which,
in contradistinction to foods, are termed food principles or, as they main-
tain the nutrition, nutritive principles. These compounds also contain the
potential energy necessary to maintain the energy equilibrium of the body
which becomes manifest as heat and mechanic motion in the transforma-
tions of the material used in the nutritive processes.'

It has been stated in a previous chapter that the animal body may be
regarded as a machine capable of performing each day a certain amount of
work by the expenditure of a definite amount of energy. In the performance
of its work, whether it be the raising of weights against gravity, or the over-
coming 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, 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 corre-
sponding amount of food principles. If this condition is realized, the body
neither gains nor loses in weight, but remains in a condition of nutritive
equilibrium. The determination of the extent of the metabolism is made
from an analysis of the daily excretions. If therefore these are collected
and analyzed, it will become possible to determine from their chief constitu-
ents the extent and character of the tissue and food metabolized, and hence
to. calculate the relative quantities of the different food materials necessary
to replace the materials metabolized. Thus of the constituents the urea and
other nitrogen-holding compounds contained in the urine and feces represent
the proteins metabolized; the carbon dioxid and water represent the fat and
carbohydrates metabolized. Therefore it becomes possible to determine
approximately at least, from the amounts of the urea and carbon dioxid
eliminated, the different amounts of the food principles required to restore
the nutritive equilibrium under any given condition. As the activity of the
nutritive changes varies in accordance with age, weight, climatic conditions,
work done, etc., and as the excreted products vary in the same ratio, it is
obvious that the required amounts of food will vary in accordance with
these varying conditions, if nutritive equilibrium is to be maintained.

TEXT-BOOK OF PHYSIOLOGY

A Metabolism Experiment. — An experiment designed to determine the
character and the extent of the chemic transformations which tissue and food
materials undergo in their transit through the body is termed a metabolism
experiment. It consists primarily in placing an animal under conditions that
permit of the collection of all the excretions for purposes of analysis, the
object of which is to deduce from the amounts of urea and other nitrogen-
holding compounds, and of carbon dioxid excreted, or better, the total nitro-
gen and carbon they contain (i) the amount of tissue materials metabolized,
during a fasting period of longer or shorter duration and hence from the
nitrogen and carbon to calculate approximately, at least, the amounts of the
food principles and their ratio one to another that must be returned to the
body if nutritive equilibrium is to be restored; or (2) to deduce from the
total nitrogen and carbon excreted, the amounts of protein, fat, and carbo-
hydrates metabolized, that were contained in the customary foods during an

ffzSOf SodaLime H^SO*

FIG. 60. — A RESPIRATION CALORIMETER.

experimental period. If the outcome from the body balances the income
the weight of the animal remains stationary from which it is assumed that
the amounts consumed constitute a normal diet. Experiments having these
objects in view are made possible by enclosing the animal or man in a suit-
able chamber (Fig. 60) in which there is some provision for collecting the
urine and feces, and to which, in addition, there is attached an apparatus
for the absorption of water and carbon dioxid, both of which are caused to
pass from the chamber through the absorption apparatus under the action
of an aspirating pump. Simultaneously fresh air is introduced into the
chamber after passing through another absorption apparatus by which it
is freed from water and carbon dioxid. As the apparatus is traversed con-
stantly by a column of air of normal composition and the waste products
removed as rapidly as discharged, the experiment can be continued from
six to twenty hours or more without detriment to the subject of the ex-
periment. Previous to the beginning of the experiment the animal and
the absorption apparatus are carefully weighed. At the end of the experi-
ment the urine and feces are collected, weighed and analyzed for their chief

FOODS

119

constituent and their amounts determined. The absorption apparatus is
again weighed. The increase in weight represents the amounts of water
and carbon dioxid eliminated and absorbed. The animal is again weighed
and the loss in weight noted.

If at the same time it is desired to collect the heat dissipated from the
body it is necessary to surround the animal chamber with a water jacket, the
rise in temperature of which, expressed in calories, indicates the amount of
heat dissipated and collected. Since an apparatus of this character de-
termines the extent of the respiratory exchange as well as the amount of
heat dissipated, it is termed a Respiration Calorimeter.

For long-continued experiments on animals of large size and on man,
one of the larger and more accurately constructed forms of apparatus, such
as Benedict's (see Chapter on Animal Heat) must be employed.

In an experiment to determine the extent of the metabolism during a
fasting period there was collected on the first and second days 12.17 and
12.84 grams of nitrogen and 188.5 and I79-4 grams of carbon respectively.
From these amounts it was calculated that 76.1 and 80.3 grams of protein
and 206.1 and 191.6 grams of fat respectively were metabolized. From these
figures it is evident that at least equal amounts of protein and fat must be
consumed. As a matter of fact, however, these amounts would be insufficient
to maintain the energy equilibrium of the body. It has been estimated that
at least from 10 to 14 per cent, more food must be added.

In an experiment, made by Vierordt on the customary diet the following
results, somewhat rearranged were obtained. On the right, under the term
outcome, are arranged the amounts of the substances eliminated; on the left,
under the term income, the amounts of the food and tissue principles which
were calculated to have been metabolized. If the body is to retain its usual
weight it is evident that equivalent amounts of these food principles must be
introduced into the body.

COMPARISON OF THE INCOME AND OUTCOME

Income

Grams

Outcome

Grams

Protein

120

90
330
32
2818
744

Water

3114.00
26.00
33-8o
6.00
44.00
910.00

Fat

Urinary Solids
Feces

f Salts . .
Urea...
( Extractive

Carbohydrates ....

Salts . . ....

Water

Oxygen

Carbon dioxid .

4134

4134.00

In the foregoing experiment the total nitrogen contained in the urine
which was 15.8 grams is expressed in the table in terms of urea. The chemic
composition of urea, COH4N2 taken in connection with the amount stated,
indicates that it is the chief end-product of protein metabolism; and as i
gram of nitrogen corresponds to 2.14 grams of urea and either to 6.25 grams
of protein it is apparent that the urinary nitrogen corresponds to 98.80^ grams
of protein metabolized. The feces, however, contained 3.3 grams of nitrogen
corresponding to 20.62 grams of protein, making a total approximately of
120 grams of protein. From this it is apparent that an equal amount of

120

TEXT-BOOK OF PHYSIOLOGY

protein would have to be introduced into the body in order to restore protein
or nitrogen equilibrium.

The carbon dioxid excreted was 910 grams, equal to 248.8 grams of
carbon.1 The chemic composition of carbon dioxid, together with the
amount stated, indicates that it is the chief end-product of the metabolism
of fat or carbohydrate, or both, and hence from the amount eliminated it is
possible to determine in terms of fat or carbohydrate the amounts that
would have to be introduced into the body to restore carbon or fat and carbo-
hydrate equilibrium. As i gram of C corresponds to 3.66 grams of CO2
and either to 1.31 grams of fat it is apparent that the carbon, 184 grams,
or carbon dioxid, 676 grams, eliminated corresponds to 242 grams of fat
metabolized; again as i gram of C or 3.66 grams of CO2 corresponds to
2.25 grams of starch, it is apparent that the carbon or carbon dioxid elimi-
nated corresponds to 416 grams of starch. From these figures it is evident
that an equal amount of fat or starch would have to be introduced into the
body to restore the carbon or the fat or carbohydrate equilibrium, an amount
which in either case would be larger than could be readily disposed of by the
digestive apparatus and the assimilative capacities. Since the carbon dioxid
comes from both fat and starch or sugar, it is difficult to determine the per-
centage that comes from the metabolism of the fat and the percentage that
comes from the carbohydrate. From observation of the dietetic habits in
different countries and of the results of metabolism experiments, it has been
deemed advisable to apportion the fat to the carbohydrates in the ratio of i
to 3.5 to i to 7. In the foregoing table Vierordt regarded 90 grams of fat
and 330 grams of starch sufficient to restore the losses sustained.

The carbon dioxid excreted also indicates the amount of oxygen utilized
in the oxidation of the carbon and the surplus hydrogen of the fats and hence
the amount of oxygen that must have been absorbed from the air in the lungs.
The amount of oxygen stated in the table is, however, mainly an inference
and determined by deducting the loss in weight of the subject of the experi-
ment from the combined weight of the CO2 and the water eliminated. The
salts are balanced to a greater or less degree, day by day, by the salts intro-
duced in the foods. The excess of water discharged, 296 grams, beyond
that taken into the body arises from the union of oxygen with the surplus
hydrogen of the fats.

The following balance table, as given by Ranke, shows the relation
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

C. •

Outcome

Grams

IOO

T g £

?•} Q

Urea

71 < }

Fat

IOO

*3 •;>

JO -^

70-O

JB*»5 I

o c |

14.4

6.16

Carbohydrates . . .

250

93-°

Feces

'co,

i .1

10.84
208 oo

iS-5

225.0

iS.S

225.00

xlt must be remembered however that of the CO 2 eliminated a portion arises from the oxidation
of the carbon-holding residue of the protein, an amount in this experiment of approximately 64
grams, which would yield 234 grams of CO2. The remainder of the CO2, 676 grams, arose from
the oxidation of the fat and carbohydrate ingested.

FOODS

121

From the foregoing considerations it is essential that the constituents of
a normal diet should be present in certain amounts and should bear a certain
ratio one to another. Many attempts have been made to construct a suit-
able diet for a man weighing 70 kilos while doing light or moderate work.
The following are accepted estimates:

Ranke. Voit. Moleschott. Atwater. Hultgren.

Grains. Grams. Grams. Grams. Grains.

Protein 100 118 130 125 134

Fat 100 56 84 125 79

Starch 250 500 550 400 522

From the foregoing estimates it is assumed that for the maintenance
of nitrogen equilibrium an amount of protein, 100 grams or more, or about
1.5 to 1.7 grams for each kilogram of body weight must be consumed each
day; and that if the amount falls below this minimum the tissues will be
called upon to yield up a portion of their protein and thereby undergo
deterioration with a consequent loss of their efficiency.

It has, however, been established that nitrogen equilibrium can be main-
tained without apparent detriment to the body or its activities, for a variable
period of time, extending over months and years, on a diet much poorer in
its protein content than in any of the foregoing diets. Chittenden has demon-
strated by a long series of carefully conducted experiments on human
beings, that the protein intake can be reduced to 60 grams or 0.85 grams
for each kilogram of body weight without any impairment in the working
capacity of the tissues or of the individual. Even this amount is in actual
excess of the tissue needs as the protein metabolism according to Chittenden's
experiments probably does not amount to more than 0.75 gram for each
kilogram of body weight.

The daily observations of some twenty-four individuals who were placed
on a diet in which the protein content was low for a period varying from
five to eighteen months revealed the fact that they not only maintained the
nitrogen equilibrium, but that they gained in weight and strength as shown
by their capacity to meet successfully various endurance tests. These
experiments would therefore indicate that the consumption of 100 or more
grams of protein each day is unnecessary and that any amount beyond that
actually needed for tissue repair, approximately 60 grams or even less for
an individual weighing 70 kilograms is undesirable, for, as will be stated in
subsequent pages, all protein when metabolized yields a series of nitrogen-
holding bodies which must be subsequently eliminated by the kidneys and
perhaps the intestinal glands as well. This necessitates on the part of the
kidneys, the chief eliminating organs, the expenditure of a certain amount
of energy. The wear and tear of these organs will be proportional to the
amount of urea and other materials which they are called upon to excrete
and if the kidneys fail to excrete them, some may become deposited in the
tissues and give rise to certain nutritional disorders. Any unnecessary con-
sumption of proteins should therefore be avoided.

It must be remembered, however, as protein yields energy when me-
tabolized, that the heat value of. the excluded protein must be balanced by
an increase in the amount of either starch or fat or both, an increase that
will yield on oxidation an equivalent amount of heat.

An advantage to the body which a high protein diet (100 to 125 grams)

122 TEXT-BOOK OF PHYSIOLOGY

has over a low protein diet (60 to 80 grams) lies in the fact that the former
has a greater stimulating effect on the general metabolic process than the
latter as shown by an increase in the heat produced and dissiqated from the
body. This action of protein is to be attributed, however, to the action of
some of the amino-acids set free during digestion and subsepuently found
circulating in the tissue fluids. In this situation they stimulate, by reason
of their chemic features, the activity of the tissue cells, whereby their power
to metabolize carbohydrates or fats is considerably increased. The experi-
ments of Lusk have shown that when amino-acids, e.g., glycocoll, alanin, etc.,
are administered singly or combined to an animal, there is an increase in
heat production far beyond that which would result from oxidation of the
carbonaceous radical (glucose) which arises in the metabolism of the amino-
acids themselves; from this fact the inference is drawn that the increase in
metabolism, expressed in terms of heat, which follows the ingestion of meat
is to be attributed to the mass action of the absorbed amino-acids on the
tissue cells. For this reason proteins are said to have a specific dynamic
action on metabolism presumably beneficial.

CLASSIFICATION OF FOOD PRINCIPLES

Though the food principles are grouped as proteins, fats, carbohydrates,
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.

Gliadin and glutinin 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 1 x f {

Levulose or fruit-sugar j

Lactose or milk-sugar Milk.

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

Maltose Malt and malted foods.

gte k f Cereals, tuberous roots, and leguminous

\ plants.
Glycogen Liver, muscles.

4. INORGANIC
Water.

Sodium and potassium chlorid.

Sodium, potassium, and calcium phosphates

and carbonates.
Iron.

In nearly all animal and vegetable foods.

5. VEGETABLE ACIDS

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

6. ACCESSORY FOODS

Coffee, Tea, Cocoa, Condiments, Spices, Alcohol.

FOODS I23

THE DISPOSITION OF THE FOOD PRINCIPLES

The Proteins.— The protein principles of the food while in the ali-
mentary canal undergo a series of disintegrative changes by virtue of which
they are reduced in part to simple nitrogen-holding bodies, monoamino-
and diamino-acids and ammonia, and in part to their immediate antecedents
peptids and polypeptids, after which they are absorbed from the intestinal
contents. It has for some time been assumed that in the act of absorption,
the amino-acids and their immediate antecedents, are combined and trans-
formed into the form of protein characteristic of blood and tissue fluids,
viz., plasma albumin. This compound was looked upon as the immediate
source of the protein necessary for tissue growth and repair. The manner in
which the plasma* albumin was transformed into the protein character-
istic of each tissue has never been satisfactorily determined. Recently
evidence has been adduced which makes it probable that the amino-acids
undergo no change in the act of absorption but enter the blood as such and
are carried direct to the tissues. On reaching any given tissue the cells absorb
and synthesize, perhaps under the influence of an enzyme, such amino-acids
as they may need for their growth and repair. The surplus amino-acids,
i.e., those not utilized in the synthesis of tissue protein, may be synthesized
to plasma-albumin, or stored unchanged or be deaminized, i.e., separated
perhaps by the action of an enzyme, into the amino-group, NH2, and some
carbonaceous radical. The amino-group is then combined with hydrogen,
and subsequently with carbon dioxid, to form ammonium carbonate which
is then transformed into urea, a transformation that takes place to some
extent in the muscles (Folin); the carbonaceous remainder may be trans-
formed into fat or sugar, which is subsequently oxidized thus contributing
to the production of heat. In the process of tissue metabolism the protein
molecule undergoes disintegration and gives rise to amino-acids, the different
elements of which may undergo changes similar to those just stated. The
ammonia absorbed from the intestine is changed to ammonium carbonate
carried direct to the liver and transformed into urea.

The Fats. — The fat principles while in the alimentary canal also undergo
a series of changes whereby they are reduced by enzymic action to soap and
glycerin, under which forms they are absorbed. During the act of absorp-
tion the soap and glycerin are synthesized to human fat. The fine particles
thus formed in the intestinal wall are carried by the lymph vessels to the
thoracic duct, and thence into the blood stream, from which they rapidly
disappear. Though it is possible that a portion of the fat enters directly
into the formation of the living material in general, it is generally 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 synthe-
sized fat is 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 conditions, is mainly, however, a product of the transfomation
of carbo-hydrates.

The question has again been raised as to whether the fine granules of
emulsified fat can be absorbed without undergoing this preliminary cleavage
into fat acids and glycerin. The evidence adduced in support of this view
is conflicting and for its settlement further experimental work is necessary.

124 TEXT-BOOK OF PHYSIOLOGY

The Carbohydrates. — Carbohydrate principles are reduced during di-
gestion to simple forms of sugar, chiefly dextrose and levulose. Under
these forms they are absorbed into the blood. These compounds are then
carried to the liver and to the muscles where they are dehydrated and stored
under the form of starch, termed animal starch or glycogen. Subsequently
glycogen is transformed by hydra tion to sugar, after which it is oxidized to
carbon dioxid and water. The intermediate stages through which sugar
passes before it is reduced to carbon dioxid and water are only imperfectly
known. Though a large part of the carbohydrate material is at once oxi-
dized, it is now well established that another portion contributes to the forma-
tion of, if it is not directly converted into, fat. As the carbohydrates form a
large portion of the food, they contribute materially to the liberation of energy.

The Inorganic Principles. — The inorganic principles, though appar-
ently not playing 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 apparatus 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
rdle 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 deprivation of salt for a long period of
time, it 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 normal
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 maintaining

FOODS 125

the solidity of the bones and teeth, replacing the amount metabolized
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 de-
prived of it, the skeleton undergoes 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, however,
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 formation 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 10 to 90 milligrams daily, 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 retained in
the body and utilized again in the formation of new hemoglobin. ^ 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. According 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 contains only 3.2 milligrams per 100 grams of
body-weight.

Vegetable acids increase the secretions of the alimentary canal, anc
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 depnved
of these acids, the individual becomes scorbutic.

The Accessory Foods.— The accessory foods— coffee, tea, and cocoa-
when taken in moderation have a stimulating influence on the nervous sys-
tem, as shown by the removal of both mental and physical fatigue, by an
increased capacity for sustained mental work, and 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 pres-
sure, and hastens the general blood-flow. It has no influence either in ti
way of increasing or decreasing protein 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
asmuch as tannin also coagulates peptones, the excessive use of
beverage is apt to derange the digestive organs and the general pr<

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

The active principles in coffee, tea, and cocoa, and to which their effe<
are to be attributed, are caffein, them, and theobromin respectively,
alkaloids are chemically closely related one to the other and to the compo

126 TEXT-BOOK OF PHYSIOLOGY

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 cutane-
ous blood-vessels, a sensation of warmth, and an excitation of the brain.
In large quantities it acts as a paralyzant, 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. Whether* for this reason it can be regarded
as a food — that is, whether it can be substituted in part at least for fat or
carbohydrate material without impairing the protein metabolism — is at
present a subject of experimentation and discussion. According to some
investigators, alcohol does not retard protein metabolism, for when it is
introduced into the body in amounts equivalent to the carbohydrates with-
drawn 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 protein 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 con-
ditions, 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 infectious diseases and in cases of depres-
sion of the vital powers it is most useful as a restorative agent.

THE ENERGY OR HEAT VALUE OF THE 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 con-
structive 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 relatively 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
involved in the oxidation of the food principles.

It becomes, therefore, a matter of interest to determine the heat loss
from the body in twenty-four hours for the purpose of subsequently deter-
mining if the energy contained in the foods, expressed in terms of heat, is
present in amounts sufficient to compensate for the loss. The total quan-
tity of heat liberated in the body and dissipated from it in twenty-four hours

FOODS 127

is determined by placing the subject in a respiration chamber provided with
appliances containing water, by means of which the heat can be absorbed
and measured. (See chapter on Animal Heat.) The unit of heat measure-
ment is the Calorie, which is denned as the amount of heat necessary to raise
the temperature of one kilogram of water i°C. If therefore the volume
of the water employed in the experiment expressed in kilograms be multi-
plied by the number of degrees of temperature through which it has been
raised, the number of Calories will be known. The average number of
Calories dissipated by a human being in various ways, e.g., radiation, vapori-
zation of water from lungs and skin, warming of foods, air, etc., has been
estimated at from 2500 to 3000 each day. The question then to be deter-
mined is, whether any given diet scale contains this amount of energy and
whether it can be liberated as heat on oxidation in the body.

The amount of heat or energy which any given food principle will yield
can be determined by burning a definite amount (e.g., i 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., i 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 con-
sists essentially 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 calorimeters
and different food principles of the same group vary, though within certain
limits: e.g., i gram of casein yields 5.867 kilogram Calories; i gram of lean
beef, 5.656 Calories; i gram of fat yields 9.353, 9.423, 9.686 Calories; i gram
of carbohydrate, 4.182, 4.479, etc-> Calories. These numbers represent
the physical calorimetric 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 exception of the protein, which
is 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,
i .gram of urea will yield 2.523 kilogram Calories. As the urea which results
from the oxidation of i 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 kilogram
Calories produced when the food is burned to carbon dioxid, water, and
urea in the body :

i gram of protein yields. . . 4-124 Calories.

i gram of fat yields 9-353 Calories.

i gram of carbohydrate yields : : 4-*» Calories.

128 TEXT-BOOK OF PHYSIOLOGY

The total number of kilogram Calories or kilogram degrees of heat
yielded by any of the previously given diet scales can be readily determined
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 grams of fat yields 841 .77 Calories.

330 grams of starch yields 1358 .28 Calories.

2694.93 Calories

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 combined 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 maintaining 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 condition are
as follows: hunger, intense thirst, gastric and intestinal uneasiness and
pain, diminished pulse-rate and respiration, muscular weakness and emacia-
tion, a lessening in the amount of urine and its constituents, diminished
expiration of carbon dioxid, an exhalation of a fetid odor from the body,
vertigo, stupor, delirium, at times convulsions, a sudden fall in body tem-
perature, and finally death. The duration of life after complete depriva-
tion of food varies from eight to thirteen days or more, though this period
can be prolonged if the animal be supplied with water, this being more
essential under the circumstances than the organic materials which can be
supplied by the organism itself. The duration of the starvation period
will naturally vary in accordance with the previous condition of the animal
and the amount of reserve food, especially fat, the body contains.

The extent and the character of the metabolism that the body undergoes
in starvation can be determined from an examination of the excretions.
Thus the excretion of nitrogen declines very rapidly during the first few days
— a fact which has been attributed to the consumption of the surplus protein
food. The amount of nitrogen eliminated and hence the protein metabo-
lized depend, mainly on the amount of protein consumed daily before the
starvation period was inaugurated. At the end of four or five days when
the surplus protein has been metabolized and the tissues begin to metabo-
lize their own protein, the excretion remains fairly constant, from about 13
to 10 grams daily, until toward the close of the starvation period, when the
amount eliminated falls very rapidly. In several instances of prolonged
fasting by human beings the nitrogen elimination fell as low as 4 to 3.5
grams daily, indicating a protein metabolism only of from 25 to 18.75 grams.
As proteins contain about 16 per cent, of nitrogen, i part of nitrogen equals

FOODS

129

6.25 parts of protein. Hence, for every i 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, after the first five days therefore, indicates fairly accurately the extent
of the metabolism of the tissue protein.

It has been observed also that there is a steady diminution in the excre-
tion of carbon dioxid. As fat contains about 76 per cent, of carbon, i part
of carbon equals 3.66 parts of carbon dioxid or 1.31 parts of fat. Hence,
for every i gram of carbon or 3.66 grams carbon dioxid excreted it may be
assumed that 1.31 grams of fat have been metabolized. The daily excre-
tion of carbon, therefore, indicates the extent of fat metabolism. The car-
bohydrates are here left out of consideration, as they constitute only about
i per cent, of the body-weight. It must be borne in mind, however, that
in the metabolism of protein a certain quantity of fat or sugar is produced,
which also undergoes oxidation. The amount of the carbon or the CO2 that
the protein would give rise to, as previously determined, must therefore be
subtracted from that eliminated by the lungs, etc., in order to determine the
amount of body-fat metabolized.

In a fasting experiment, voluntarily undergone by a human being, and
lasting ten days, the metabolism of fat as shown by the CO2 excreted
amounted to 136.7 grams during the first four days and to 132 grams on
the tenth day. In another similar experiment lasting five days, the fat
metabolized amounted to 206 grams on the first day and 181 grams on the
fifth day. As the starvation period lengthens the amount of fat metabo-
lized gradually declines.

The following table shows the excretion of nitrogen and carbon and
the calculated amounts of protein and fat metabolized from an experiment
made by Ranke on himself during a fast of twenty hours, beginning twenty-
four hours after the last meal :

Disintegration of Tissue
(Calculated)

Expenditure
(Determined)

Nitrogen

Carbon

TJrea 17 firm

Nitrogen

Carbon

7-8
o.o

26.5

157-5

7.2
o.o

3-4
180.6

Uric acid, 0.2 gm /

Fat, 199.6 gm

Carbon dioxid

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 consequence of the final con-
sumption of all available foods, when death ensues. The immediate cause
of death is not, however, apparent.

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.

130

TEXT-BOOK OF PHYSIOLOGY

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 grams in weight,
are given in the following table, based on data furnished by Voit:

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 losing only 31 per cent, of their weight, yet furnish
429 grams of presumably protein material, for nutritive purposes. The
heart and nervous system experience but slight loss.

Organ

Percentage
Loss of weight

Actual Loss
of Tissue

Adipose tissue

Grams
267

Spleen . .

Liver . «

Testes

Muscles. . .... ....

•21

429

Blood..

Kidneys

Skin and hair

Lungs

•2

Intestines

Pancreas

Bones

Heart

•2

Nervous system ... . .

•2

Mixed Diet. — The chemic composition of the tissues, taken in con-
nection 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 investigation has also conclusively
established this fact. Moreover, the amounts of nitrogen and carbon elimi-
nated daily, and the ratio existing between them, indicate the amounts
of protein, 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 either
fat or 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 nitrogen equi-
librium, 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 cir-
cumstances, a human being weighing 70 kilograms would require more than
2000 grams of lean beef — an amount which, from the nature of the digestive
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

FOODS I3i

they are eliminated by the kidneys, accumulate 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 proteins, as shown by
the continuous excretion of urea, though the amount is less than during
starvation. An excess of fat retards the metabolism of proteins. The same
holds true for the carbohydrates.

Thus, in any well-arranged dietary there should be a combination
of proteins, fats, and carbohydrates in amounts sufficient to maintain nutritive
equilibrium; in other words, to repair the loss of tissue and to furnish the
requisite amount of energy 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 substances
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.

The following tables show the percentage composition of the edible
portions of foods as well as the amount of heat liberated per pound when
oxidized in the body, according to Atwater and Bryant.

Composition of Animal Foods. — The following table shows the average
percentage composition of various kinds of meats, cow's milk, and eggs:

Kind of Food w
Materials

Unavail-
ter able
Nutrients

Proteins

Fat

Carbo-
hydrates

Ash

Fuel Value
Per Lb.
453.6 Grams

P
ce

Beef:
Loin lean 6'

er Per
at. cent.

7 .O 1.2

*-7 i-9

5.0 1.0

3.4 1.6

3-7 , i-3
j.o 0.9

2.8 i 1.7

3.2 2.4

2.0 2.2
3.9 2.1

3-7 1.6
5-5 1-9
3-4 i-3
5.4 i.i
7.0 0.5

J.2 1.2

Per
cent.

19.1
17.0
20.7
18.9

19.7
9-7

17.9
iS-5

16.1
14.8
18.7
20.5
18.1
18.0

,1:1

Per
cent.

12 .1
26.2

.11

7-3
S-o

17.1
3i-4

28.6
27-5
iS-5

21.8

6.7

4-9
3-8
11.4

Per

cent.

Per

cent.

i .0
0.9
i.i

I.O

0.8

I.O

0.8
0.6

0.8
0.6
0.8
0.8
0.9
0.8

°-s

0.6

Calories

900
1470
735
"75

710
410

1095
1660

1555
1480
1040

853
650

570
310

755

Loin, fat 5^
Round lean 7<

Round fat 6<

Veal:
Cutlets (round) | y<

J-lver 7,
Mutton:
Leg 1 6

Loin ^<

Pork:

Ham . <

Fowl: 6,
Turkpv c

j. uxjs.cy 5

Halibut 7

Milk ;.. 8

5-o

^&8b> uuiicu /%

132 TEXT-BOOK OF PHYSIOLOGY ,

Meats. — It will be observed from these analyses that the meats contain
from 18 to 20 per cent, of protein material. The proteins are two in number
and are known as paramyosinogen or myosin and myosinogen or myogen,
both of which are in a semi-fluid condition. The latter is four or five times
as abundant as the former. After death these substances undergo coagu-
lation and give rise to two solid substances known as myosin-fibrin and
myogen-fibrin. After being subjected to the cooking process, meats contain
the albuminoid body gelatin, a product of the transformation of the proteins
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 large
percentage of fat represented in the foregoing table is due to the presence
in the food, as eaten, of adipose tissue which is an addition, not a constituent
of meat.

The carbohydrates vary from 0.5 to i per cent., and are represented
by glycogen. The principal inorganic salts are potassium phosphate and
sodium chlorid.

The composition of meat will vary in composition in nutritive value and
in energy- lib era ting capacity in accordance with the region of the body
from which the specimen is taken.

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 embryonic forms of tenia or
tapeworm, and Trichina spiralis, as well as bacterial growths, which fre-
quently infest the bodies of animals, are destroyed and made harmless.

Milk is the natural food of the young of all mammals, and is usually
regarded as typical on account of the ratio existing among its nutritive
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 1 0^00 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 membrane. The chief protein con-
stituent 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 caseinogen is precipitated as such and may be collected by
appropriate chemic methods. When taken into the stomach, caseinogen is
coagulated; that is, it is changed into casein or tyrein. This change is
brought about by the presence in the gastric juice of a special ferment
termed 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 quantity
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.

FOODS

133

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 soiir 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. Sodium and potassium chlorids are also present.

Eggs are ^also to be regarded as complete natural foods, inasmuch
as they contain all the necessary food principles. The analysis given in
the foregoing table represents the composition of the entire egg. The
white of the egg contains 12 per cent, of protein and 2 per cent, of fat.
The yolk, however, contains 15 per cent, of protein and 30 per cent, of fat.

Composition of Cereal Foods. — The average composition of the
principal cereals is shown in the following table:

Kind of Food w .
Material Water

Unavail-
able
Nutrients

Proteins

Fat

Carbo-
hydrates

Ash

Fuel Value
Per Pound
453.6 Grams

Per

cent.
Entire wheat flour.. . . n .4
Rye flour 12 9

Per
cent.

4-5
•* 6

Per
cent.
10.7
e •?

Per
cent.

i-7
o 8

Per
cent.
70.9

76 Q

Per

cent.
0.8

Calories
1645

Rice 12.3
Barley pearled ' 1 1 ?

3-7

o -o
6-5
6 6

o-3

/u.y
76.9

u-5
o-3
r> 8

1610

Buckwheat flour 13.6
Corn meal . 12 5

3-5
40

5-2

i.i

75-9

77 C

o'l

1630
1600

Oat meal 78

<; 6

•o

T-2 A

16 -J
6< 2

Whole wheat bread. . . 38.4
White bread 35 3

3-2

*J '4

7-5

0.8

I 2

w:> ••«
49.1

ro •}

1.4

1.0

o 8

*7V5

1125
HOC

Graham crackers 5.4

4.8

7-7

8-5

J* -O
72.$

i.i

1900

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 somewhat 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 cellulose and which contain the granules of starch, fat, small
quantities of protein, 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 portion, if not all,
of the gluten cells, so that such flour contains less nitrogenized 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 prepa-
ration of whole wheat flour.

Bread is an artificially prepared food made either of wheat or rye.
Owing to the fact that the proteins of the other cereals do not possess the
same adhesive properties when kneaded with water, they cannot be used

134

TEXT-BOOK OF PHYSIOLOGY

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 kept in a temperature of about
ioo°F. In the presence of heat and moisture the natural ferment of the
flour — diastase — converts a portion of the starch into sugar, which in turn
is split up into carbon dioxid and alcohol by the yeast plant. The sugar that
is added undergoes a similar change and hastens the process. 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 largely driven off; yeast cells
and other organisms are destroyed; the starch, particularly that on the
surface, is dextrinized. The principal salts contained in wheat flour are
potassium and magnesium phosphate.

Composition of Vegetable Foods. — The average composition of
some of the principal vegetables is shown in the following table:

Kind of Food
Material

Water

Unavail-
able
Nutrients

Proteins

Fat

Carbo-
hydrates

Ash

Fuel Value
Per Pound
453.6 Grams

Beans, lima, dried
Beans, lima, green —
Beans, white, dried. . .
Beans, string, cooked1
Peas dried

Per
cent.
10.4
68.5

12.6

95-3

9.C

Per
cent.
6.7
2.7

7-5
°-5
7 -6

Per
cent.

12.8

5-3

ISi
0.6

17 .3

Per
cent.
1.4
0.6
1.6

I.O

0.9

Per
cent.
65.6

21 .6

59-9

'•9
62.5

Per

cent.
3-i
i-3

2.6

0.7

2 .2

Calories

1565
525
1530
90
ie;o8

Peas, green, cooked1. .
Potatoes, boiled,
cooked. :
Potatoes sweet

73-8
75-5

t?I Q

2.5
J-7

5-1
1.9

2 2

3-i

O.I
I O

14.4

20.0
4.O 7

I.I
0.8

O 7

490
4i5

885

Beets cooked1.

88 6

I 2

I 7

O I

***»o

\j. i
I 2

170

Ca.bba.sfe

01 >?

O 7

I 2

O 3

e e

o 8

u.o

I4.O

Tomatoes

04. 3

O 4.

O.7

O 4.

1 8

O 4.

14U
TOO

Turnips . .

V^-J
80.6

0 8

I .O

O.2

7 8

o 6

17 ^

Effsr-ulant. .

02 .0

0.6

O.Q

O.7

\j.\j

O 4.

*/5
I2O

Spinach fresh

02 .3

1 .0

1.6

O.7

i 6

IOO

Asparagus, cooked. . .

y -J

91 .6

1.0

i-7

3-o

2.1

0.6

195

The vegetable foods, as a class, vary considerably in nutritive value and
digestibility, the latter depending on the amount of cellulose they contain.
A section of a vegetable shows not only the presence of an external cellulose
envelope, but also an inner framework which penetrates its substance in all
directions. The nutritive principles 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 protein, legumin, and
starch, and hence are especially valuable as nutritive foods. The presence

1 With butter etc., added.

FOODS

of the cellulose envelope, especially in ripe beans and peas, combined with
rather a dense texture, renders them somewhat difficult of digestion. Pota-
toes, though largely employed as food, are extremely 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 to the maintenance
of the normal nutrition. The want of green vegetables 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 nutritive 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, tartaric), 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 consumed. It has
been found by experiment that animal proteins are more easily and com-
pletely digested and absorbed than vegetable proteins; that cellulose is not
only highly indigestible, but by its presence in large quantities retards the
digestive process and impairs the activity of the entire digestive mechanism,
though in moderate quantity it undoubtedly aids digestion indirectly by
mechanically promoting peristalsis. The following table shows the relative
digestibility and availibility of the two classes of foods:

Weight of Food

Vegetable

Animal

Digested

Undigested

Digested

Undigested

Of 100
Of loo
Of 100

parts of protein
parts of fat
parts of carbohydrate

84.00
90.00
97.00

16. oo
10.00
3.00

97
•95
98

CHAPTER X
DIGESTION

Digestion is a process partly physical, partly chemic, by which the
nutritive principles of the foods are prepared for absorption. The reason
for these changes lies in the fact that the foods as consumed are hetero-
geneous 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, from which the nutritive prin-
ciples must be freed before they can be utilized; and in the further fact, that
even when consumed in the free state, the food principles are seldom in a
condition to be absorbed into the blood and subsequently 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. — The digestive apparatus comprises the
entire alimentary or food canal and its various appendages: the lips, the
teeth, the tongue,, the salivary glands, the gastric and intestinal glands, the
pancreas, and the liver (Fig. 60).

The alimentary canal is a musculo-membranous tube about eleven meters
in length, and extends from the mouth to the anus. It may be subdivided
into several distinct portions, as mouth, pharynx, esophagus, stomach,
small and large intestines. The mouth is provided (i) 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 connected with it by means of ducts.
Posteriorly the mouth opens into the pharynx or throat, a somewhat py-
ramidal-shaped structure about twelve centimeters in length, which in turn is
followed by the esophagus or gullet, a tube about twenty-two centimeters in
length. As the esophagus passes through the diaphragm it expands into the
stomach, a curved pyriform organ, which serves as a reservoir for the recep-
tion and retention of the food for a varying length of time. The small in-
testine is that portion of the alimentary canal extending from the end of the
stomach to the beginning of the large intestine in the right iliac fossa; owing
to its length, about eight meters, it presents a very convoluted appearance in
the abdominal cavity. Embedded in its walls are the intestinal glands
which open on its surface and secrete the intestinal fluid. In the upper por-
tion of the small intestine, within twelve centimeters of the stomach, there
is an orifice, the outlet of a small pouch, the Ampulla of Vater, into which

136

DIGESTION

137

open the terminations of the ducts of the liver and pancreas, organs which
secrete the bile and pancreatic juice respectively. The large intestine is
from one and three quarters to two meters in length and extends from the
end of the small intestine to the anus. Its walls contain a large number of
glands.

The general process of digestion is largely accomplished by the chemic
action of the digestive fluids secreted by glands, some of which are imbedded

Large,
Intestine.

Vermiform ,

FIG. 61.— DIAGRAM OF THE ALIMENTARY CANAL.— (Modified jrom Landots.)

in the walls of the canal while others are situated outside of it and com-
municate with it only by means of ducts. These fluids are the saliva tl
gastric, intestinal, and pancreatic juices, and the bile,
place throughout a large portion of the food canal, the process may be sut
divided into several stages: viz, prehension, mouth digestion, deglutu
gastric digestion, and intestinal digestion.

As a result of the action of these fluids the nutritive principles are pre
pared for absorption into the blood; the non-nutritive principles, along

138 TEXT-BOOK OF PHYSIOLOGY

with certain waste products, pass into the large intestine to be finally ex-
truded from the body.

FERMENTS; ENZYMES

In a preceding chapter it was stated that under favorable conditions
the carbohydrates, fats, and proteins undergo reduction to simpler com-
pounds as a result of the action of agents such as the yeast plant and various
forms of bacteria. To this process of reduction the term fermentation, and
to the agent which causes the fermentation the term ferment, or enzyme has
been given. As these compounds undergo reduction to simpler substances
somewhat different in character in the alimentary canal during the period of
digestion as a result of the action of ferments, it will be conducive to clearness
of ideas regarding the nature of the digestive process if the nature and prop-
erties of ferments in general are briefly considered at this time.

A ferment or an enzyme may be denned as an agent that induces a change
of state, or a change in composition of an organic compound, for the most
part hydrolytic, without itself being utilized in the process or appearing in
the end-results of the process.

Ferments have been divided for a long time into two classes, viz., organ-
ized and unorganized. Among the organized ferments may be mentioned
the yeast plant (Saccharomycetes) and various forms of bacteria; among
the unorganized ferments may be mentioned the diastase that transforms
the starch of barley, wheat, or other cereals into sugar, as well as ptyalin,
pepsin, steapsin or lipase, and other ferments contained in the digestive
fluids that transform or reduce the food principles to simpler compounds.

It will be recalled that if the yeast plant is added to a sugar solution
containing in addition some protein and various inorganic salts such as
phosphates and the solution kept at a favorable temperature the yeast cells
soon begin to grow and multiply. Coincidently the sugar is reduced for
the most part to carbon dioxid and alcohol. The carbon dioxid bubbling
through the solution as steam bubbles through water that is boiling, gave rise
to the expression fermentation (from fervere, to boil), and as this was
attributed to the life activities of the yeast plant it was called a ferment.

Again, if dead protein matter is exposed to air and moisture at a suitable
temperature it will be invaded by various species of bacteria, which in a short
time will begin to grow and multiply. Coincidently the protein molecules are
reduced to simpler compounds, such as hydrogen sulphid, ammonia,
carbon dioxid and a number of other compounds, the nature of which will
vary with the character of the protein. As this reduction is accompanied by
the bubbling of gases through the surrounding liquid, it too has received the
name of fermentation, and as the reduction is attributed to the life activities
of the bacteria they too have been called ferments. In both instances the
ferment is a unicellular plant possessing a distinct organization. For this
reason they have been termed organized ferments.

When grains of barley or other cereals containing starch are exposed to
moisture and a suitable temperature, the starch is gradually changed to
sugar, a transformation attributed to the action of a ferment. When the
starches, fats, proteins, and compound sugars are introduced into the
alimentary canal they are also' reduced to simpler compounds, a reduction
attributed to the action of a series of distinct and specific ferments. In

DIGESTION I39

addition to the changes that the food principles undergo in the alimentary
canal, the corresponding principles as well as many other compounds undergo
similar changes in the body tissues as the result of the action of ferments,
changes that underlie and condition many if not all the phenomena of the
nutritive process. In none of these instances, however, has the ferment been
satisfactorily isolated or its chemic or physical features determined. For
this reason these ferments have been termed unorganized ferments.
Investigations have demonstrated, however, that they are products of the
metabolism of the cells of plant and animal tissues.

In recent years the distinction between organized and unorganized
ferments has become untenable owing to the fact that chemists have succeeded
in extracting from yeast cells as well as from bacterial cells, enzymes or
ferments that produce in sugar and protein the same reduction effects under
the same conditions as in the case of yeast cells and bacteria themselves.
It is therefore probable that these organized cells act not directly by virtue of
their own activities, but indirectly, by virtue of an unorganized ferment
which they secrete and discharge into the surrounding medium. All enzymes
that produce their effects after being discharged from cells are termed
extra-cellular enzymes, while' those that produce their effects in the interior
of cells are termed intra-cellular enzymes.

The Nature of Enzymes. — An enzyme is in all probability organic in
character, though neither its chemic nature nor composition has been de-
termined. Some of them exhibit protein, others carbohydrate reactions, but
by reason of the difficulty in isolating enzymes and of freeing them absolutely
from all traces of protein and carbohydrates it is not possible to state
positively whether the reactions observed are due to the enzyme or its
associated organic matter. The purer the preparation, however, the less of
any chemic reaction is exhibited.

From what is known of their action, of the effects produced and of the condi-
tions under which they act, ferments have a resemblance to various inorganic sub-
stances or agents that produce changes of composition and decomposition
apparently by their presence alone^Mjpr, as far as the evidence goes, they neither
enter into the end-products of the inaction nor are they destroyed. A chemic
change thus produced is termed catalysis and the agent causing it is termed a
catalyzer or catalyst. The substance on which the catalyst acts is termed the
substrate. In most, if not in all instances a catalyst acts not as an initiator, but
as an accelerator of a change that would spontaneously take place with extreme
slowness and in some instances with results so slight as to be inappreciable.
Oxygen and hydrogen, for example, spontaneously combine, there are reasons for
believing, at room temperatures though at such a slow rate that the formation of
water cannot be detected, but if a small quantity of finely divided platinum be
added the combination takes place almost immediately; carburetted hydrogen and
oxygen combine when they pass over platinum with the formation of carbon
dioxid and water; saccharose and water in the presence of hydrochloric acid will
combine and be reduced to equal quantities of levulose and dextrose; dilute
peroxid of hydrogen will slowly decompose spontaneously and yield up oxygen,
but if finely divided platinum or silver be added the decomposition is greatly ac-
celerated. In all these instances, to which many more might be added, the
cata yst, simply by its presence accelerates a change spontaneously taking place
without itself appearing in the end-products of the reaction.

It has been experimentally demonstrated that the finer the catalyst is divided

140 TEXT-BOOK OF PHYSIOLOGY

or the greater the surface it presents the more energetically it acts. Thus, if platinum,
silver, or gold be changed to the colloidal state,1 a state in which the particles of ultra-
microscopic size are held in solution or perhaps suspension they become extremely
active catalyzers even in exceedingly small quantities.

From the foregoing facts and from many others it may be assumed
that the unorganized enzymes exist in the colloidal state.

The Rate and Completeness of Enzymic Action. — The rate and
completeness of enzymic action are influenced by a variety of conditions,
among the more important of which are temperature and the rapidity of
the removal of the products of their action.

Temperature. — All enzymes are sensitive to changes in temperature.
At o°C. they appear to be incapable of inducing changes in organic matter.
As the temperature is raised their reaction properties develop and increase
in velocity, until a temperature of 4O°C. to 5o°C. is reached, when they are
at their maximum. For this reason this degree of temperature is spoken of as
the optimum temperature. Beyond 5o°C. the velocity of their action begins
to decrease and at 6o°C. it comes to an end for the majority of enzymes.
At ioo°C. all reaction ceases for the reason that the enzymes are destroyed,
especially if they are moist.

The Removal of the Products of Enzymic Action. — The completeness
of enzymic action will depend on the rapidity with which the products of
enzyme activity are removed. This is illustrated in the following: If a
substrate such as fat and the enzyme lipase be mixed with water in a dialyzing
test-tube, the fat will combine with water, after which the fat will undergo
a cleavage into a fat acid and glycerin. If the products of the reaction are
removed practically as rapidly as they are formed the reaction will continue
until all the fat is so transformed. Under such circumstances the action
will be complete. If, however, the reaction takes place in a receptacle the
character of which prevents the removal of the fat acid and glycerin, the
reaction will in time come to an end, leaving apparently a percentage of
fat unchanged. The explanation at one time given for the cessation of the
reaction was that the accumulation of the products interfered with the
further action of the enzyme. It is, however, now generally admitted that
under the circumstances the ferment, shortly after the appearance of the
cleavage products, initiates a reverse action, i.e., recombines the fat acid and
glycerin with the re- formation of the fat until a condition of chemic
equilibrium is established between the opposing tendencies. The dis-
covery that many ferments are thus capable of secondarily reversing their
primary action has assisted in the interpretation of a number of obscure
physiologic processes. It must not be overlooked that in this instance the
enzyme does not initiate the reverse action, but merely hastens what would
take place by reason of a want of chemic equilibrium between the substances
present.2

1 The colloidal state may be developed by passing an electric current through electrodes of
these metals placed in distilled water. With the passage of the current particles of the metals
are discharged from one of the electrodes into the water in the form of a cloud.

2 Reversibility of a chemic reaction may be denned as a recombination of the products of the
reaction of the original compound until a condition of equilibrium is established between the
analytic and the synthetic tendencies. A classic illustration of the two phases of a chemic reaction
is the following: If chemically equivalent amounts of acetic acid and ethyl alcohol are mixed at a
definite temperature a reaction occurs which eventuates in the formation of ethyl acetate and

DIGESTION 141

The Specific Action of Enzymes. — The number of enzymes in the
digestive fluids and in the various tissues of the body has given rise to the
idea, which has been confirmed by experiment, that an enzyme exerts its
action on but one substrate, in other words, that its action is specific. Thus
an enzyme that would transform starch into sugar would not be capable of
causing a cleavage of fat into a fat acid and glycerin; an enzyme that would
cause a cleavage of saccharose would not be capable of causing a cleavage of
lactose. So with all other enzymes. Each seems to be specially adapted to
catalyze but one substrate under given conditions. Various other features
of enzymes, their mode of action, their origin from preexisting substances,
the methods by which they are made active, the conditions under which
they are most active, etc., will be mentioned in connection with a considera-
tion of the fluids and tissues in which they are present.

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 accomplished
fey the teeth and the movements of the lower jaw under the influence of
muscle contractions. Complete mechanic disintegration of the food is
important for 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 lite two sets
of teeth make their appearance. The first set constitute the temporary,
deciduous, or milk teeth; the second set constitute the permanent 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 smalle
than the permanent teeth, have the same general conformation Lhey
are divided into four incisors, two cuspids or canines, and

CTheWpermanent teeth, thirty-two in number, sixteen in each jaw, are
divided into four incisors, two cuspids or canines, four bicuspic
molars, and six molars for each jaw.

Each tooth may be said to consist of three portions: (i)

water. Irthis instanceaftera these

nets of the reaction begin to recombine with the formation of the °*

analytic and synthetic are increased in velocity. The enzyme, however, d<
merely hastens a reaction already taking place.

142

TEXT-BOOK OF PHYSIOLOGY

— O

or that portion which projects above the gums; (2) the root or fang, that
portion embedded in the alveolar socket; (3) the constricted portion 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 alveolar 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 represented in Fig. 62. In the center of the

dentine there is a cavity the general shape
of which varies in different teeth, and
which is occupied during the living condi-
tion by the tooth pulp.

Microscopic examination of the tooth
reveals the presence of irregular 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 extending 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 by a
thin membrane known as the cuticle or
membrane of Nasmyth.

The dentine is somewhat less dense than

FIG. 62.— VERTICAL SECTION OF the enamel. It is composed of connective-

tissue fibers embedded in a ground-sub-
C. Ce- stance, both of which have undergone
V. calcification in the course of development.
The dentine is penetrated 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
outward 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 portion and constitutes what is known as
the dentinal sheath or Neumann's sheath.

The cementum resembles bone because 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 for 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.

TOOTH IN JA

Dentine. P. M

brane. P.C. Pulp cavity.

ment. B. Bone of lower jaw.

Vein. a. Artery. N. Nerve. — (Stir-

ling.}

DIGESTION 143

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 terminations. Collectively these
processes are known as the dentine 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 fnto the
lacunae of the cementum.

The peridental membrane is composed of connective-tissue fibers abun-
dantly supplied with blood-vessels and nerves.

Movements of the Lower Jaw.— The lower jaw is capable of a down-
ward and upward, an antero-posterior, and a lateral movement, all depend-
ent on the peculiar construction of the joint.

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 liga-
ments. 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 pro-
vided 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 for-
ward carrying with it the interarticular fibro-cartilage, the upper concave
surface of which is applied to the convex surface of the eminentia articularis.
In the upward movement of the jaw both the condyles and the cartilages pass
backward and resume their normal position. The movements of depres-
sion and elevation are made possible by the slightly oblique direction of the
condyle. In the carnivorous animals, whose food requires considerable
cutting, these movements are especially well developed. In these animals
the condyles are transversely arranged and at right angles to the long axis
of the jaw. In the antero-posterior movement the jaw moves in a hori-
zontal direction and the condyles and the articular cartilages glide forward
and backward in the glenoid fossae. In the rodent animals the long axis
of the condyle runs in the antero-posterior direction, which allows c
considerable gliding movement. When the jaw performs > a lateral movement
the condyle Ld cartilage of one side may remain ^J^^Jff^
while the opposite condyle and cartilage glide forward m the gleno d fo sa,
directing the symphysis of the jaw to the opposite side of the medan line.
The lateral movements are well exhibited by the jrtfrm «*»*£
which they are quite extensive, and made possible *£j™***™
condyle and the large extent of articulating surface In man the s
of the joint is such as to admit of all these possibilities, and the lower jaw
acquires in consequence great freedom of movement. upmpntc: Of

The Functions of the Muscles of Mastication.-The movements of
the lower jaw are caused by the action of numerous muscles, which having
fixed points of origin, are attached to various points on its surface. The
muscles concerned in the movements of mastication are presente
following table:

144

TEXT-BOOK OF PHYSIOLOGY

Anterior belly of digastric

Mylohyoid

Geniohyoid

Temporal

Internal portion of masseter

Internal pterygoids

External pterygoids

External portion of masseter

Anterior fibers of temporal

Posterior fibers of temporal

Internal portion of masseter

Digastric, mylohyoid, and geniohyoid

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 normal
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, and the geniohyoid muscles, agree in having a similarity of
origin — the hyoid bone — and a common area of insertion, the anterior
portion of the lower jaw. Their anatomic relation is. such that their
combined action will depress the lower jaw and open the mouth.

The action of the elevator muscles becomes apparent when their points
of origin and insertion are considered. 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. After the
mouth has been opened, the simultaneous contraction of these muscles will
elevate the jaw and closes the mouth with considerable force. The power
of these muscles, which is very great, depends on the shortness and
thickness of the muscle-bundles.

The action of the rotator muscles, the external and internal pterygoids,
those which give rise to the lateral movements of the jaw, depends in like
manner on their origin and insertion. The first arises from the outer
surface of the external pterygoid plate and the great wing of the sphenoid
bone; the second arises mainly from the inner surface of the external ptery-
goid plate; 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 Mechanism1 of Mastication.— Mastication is a complex
act and involves the cooperation of a number of muscles, afferent and efferent
nerves, and a central mechanism by which they are excited to, and coordi-
nated in their activity. The central mechanism is located in the medulla
oblongata in the gray matter beneath the floor of the fourth ventricle.

During the intervals of mouth digestion the mouth is closed by the con-

1 By this term is meant a combination of nerves, afferent and efferent, and nerve centers
which when excited to action coordinates the actions of the organs with which it is associated.

DIGESTION *45

traction of the elevator muscles of the lower jaw. When the occasion arises

or the introduction of food, the mouth is opened by the depressor muscles;

ter the food is introduced into the mouth it is again closed and that

combination of muscle contractions initiated which when continued results

the nerve mechanism for
mastication are shown in the following table:

Efferent Nerves.
™t o< *

the trigeminal nerve. 2 HypOgiossal.

2. Glosso-pharyngeal. 3 Facial or portio dura.

The Efferent Nerves.-The efferent nerves that transmit nerve im-
pulses to the various muscles of mastication are the small root of the

* after emerging from .the cavity

'ttfssS&SG

the food develops in afferent nerves are rea ved by me That

thus falling into the category °f Je^^cSe?fc indicated by the
the masticatory movements are ^^^^^ volitional effort that
fact that they will be '? has been directed to some

of the tongue and mouth. stimulation of which

The Afferent Nerves.-The afferent : nerv , branches of the

146

TEXT-BOOK OF PHYSIOLOGY

in the peripheral terminations of these nerves, by reason of its physical and
chemic properties, nerve impulses which are then transmitted to the central
mechanism. If these nerves are divided, mastication is seriously impaired.
When divided and their central ends stimulated with induced electric
currents, the muscle will reflexly be thrown into contraction.

INSALIVATION

Insalivation is the incorporation of the saliva with the food, and takes
place for the most part during mastication. The saliva ordinarily present

in the mouth is a complex fluid composed
of the secretions of the parotid, submaxil-
lary, and sublingual glands and the muci-
parous follicles of the mouth, which col-
lectively constitute the salivary apparatus.

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 (Stenson's), which, after
crossing the masseter muscle to its anterior
border, turns inward, pierces the bucci-
nator muscle and opens on the surface of
the cheek opposite the second upper molar
tooth.

Acini- The submaxillary gland is situated

FIG. 63.— SCHEME OF THE HUMAN below the jaw in the anterior part of the
SUBMAXTLLARY GLAND.— (SwAr.) 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 mucous 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 number of ducts (the ducts of Walther)
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. 63.) The wall of the acinus is formed by a
reticulated basement membrane, 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 polygonal epithelial cells. The cells do not entirely fill up the cavity
of the acinus, but leave an intercellular space, the lumen, which constitutes

DIGESTION 147

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 interlobular 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 micro-
scopic appearance of their secreting cells, the salivary glands have been
divided by Heidenhain into two classes: viz., serous or albuminous, and
mucous glands. To the first class belong the parotid, a portion of the sub-
maxillary, and a portion of the glands of the tongue. To the second class
belong a portion of the submaxillary gland, the sublingual, 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 albu-
minous and mucous cells.

FIG. 64. — PAROTID GLAND AT REST. 1,1,
Acini; 2, duct; 3,3, albuminous cells filled
with fine granules; 4,4, nuclei almost con-
cealed. (Semi-diagrammatic.)

FIG. 65. — SUBMAXILLARY ^ GLAND AT
REST. 1,1, Acini; 2, duct; 3, 3, mucous cells
containing mucin; 4,4, nuclei, flattened and
dispersed toward the base of the cells; 5,5,
crescents of Giannuzzi. — {After Vialleton.)

In the serous glands the cells are more or less spheric in shape, nucleated,
and almost completely filled with dark granular material (Fig. 64). In the
mucous glands the cells are large, clear in appearance, and loaded with a
highly refracting material resembling mucin (Fig. 65). Between the base-
ment membrane and the clear cells are to be found in the acini of the sub-
maxillary 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.

148 TEXT-BOOK OF PHYSIOLOGY

Nerve-supply. — Histologic investigation has demonstrated that the
cells and blood-vessels of the salivary glands are supplied with 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 production of the secretions (secretor nerves), while others
influence the caliber of the blood-vessels causing either constriction or dilata-
tion (vaso-constrictor and vaso-dilatator nerves). (Fig. 68.) The secretor
fibers penetrate the basement membrane enclosing the gland acinus and
finally terminate between and on the surface of the secretor 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 fine
medullated nerve-fibers coming directly from the medulla oblongata and the
spinal cord. As they enter the ganglia, their terminal branches arborize
around and closely invest the cells of the ganglia and come into intimate
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-ganglionic 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
Stenson'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 protein matter, a trace of a sulpho-
cyanogen compound, and inorganic salts. The secretion 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 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, slightly
viscid, alkaline in reaction, and has a specific gravity of 1.002. It consists
of water, protein matter (mucin) , and inorganic salts.

The Sublingual Saliva. — The sublingual saliva is clear, extremely
viscid, and strongly alkaline in reaction. It consists of water, protein
matter (chiefly mucin) , and inorganic salts.

The small racemose glands embedded in the mucous membrane on
the inner surface of the cheeks and lips, on the hard and soft palate, and on
the tongue and pharynx, secrete a fluid which is grayish in color, and
extremely viscid and ropy. It contains a large amount of mucin.

Mixed Saliva. — The saliva of the mouth is a complex fluid composed
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 i.ooo to 1.006. The reaction is usually distinctly

DIGESTION I49

alkaline. It may, however, be neutral or even acid in reaction if there is
any fermentation of food particles in the mouth or in certain disorders of the
alimentary canal. When examined with the microscope, the saliva is seen
to contain epithelial cells, salivary corpuscles resembling leukocytes, particles
of food, and various microorganisms, especially Leptothrix buccalis.

The chemic composition of the saliva is shown in the following table:
COMPOSITION OF HUMAN SALIVA.

Water 995.16

Epithelium i .62

Soluble organic matter j [34

Potassium sulphocyanid o !o6

Inorganic salts . . x .g2

1000.00 1000.04

(Jacubowitsch.) (Hammerbacher.)

Water constitutes the main ingredient, amounting to 99.5 per cent.
The soluble organic matter is protein in character and is a mixture of
mucin, globulin, and serum-albumin. The potassium sulphocyanid is
mainly derived from the parotid gland. Its presence can be demon-
strated 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, and 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 consider-
able variation, depending upon habit, the nature of the food, etc. The
experiments of Professor Dalton and the results obtained by him are emi-
nently 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 wheat bread absorbed
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 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, suppos-
ing that it continues at the rate of 36 grams per hour, we have a total amount
°f 5T3+792 grams, or 1305 grams (19,780 grains), or about 2.8 pounds.

150

TEXT-BOOK OF PHYSIOLOGY

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. 66). During the
period of rest and just previous to secretor activity, the epithelial cells are
enlarged and swollen, and 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 almost to 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

FIG. 66. — PAROTID GLAND AFTER PRO-
LONGED ACTIVITY. 1,1, Acini; 2, duct; 3,3,
albuminous cells almost free of granules; 4,
nuclei clear and well denned. (Semi-diagram-
matic.)

FlG. 67. — SUB MAXILLARY GLAND AFTER

PROLONGED ACTIVITY. 1,1, Acini; 2, duct;
3,3, mucous cells free of mucin and filled
with fine granules; 4,4, nuclei rounded and
returned to the center of the cell; 5,5, cells of
Giannuzzi, large and distinct. (After
Vialleton.}

inferred that during rest the protoplasm of the cells gives rise to granular
material, and that during and after secretor activity there is an absorption
of new material from the lymph and a reconstruction 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. 67). 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, coalesce, and form a uniform mass. The chemic relations
of this substance indicate that it is the precursor of mucin — namely, mucigen.
During secretor 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 indicates 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 plavs an important r61e in the general digestive process. Experi-

DIGESTION I5I

ment has demonstrated that it has a two-fold action; viz., physical and
chemical.

Physically, saliva softens and moistens the food, unites its particles 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-
sists of a cellulose envelope or stroma in the meshes of which is contained the
true starch material, 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 con-
verted 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 transparent. On testing the solution from time to time
with iodin the characteristic blue reaction will be found to disappear, gradu-
ally, the color passing from blue to violet, to red, to yellow. If now the
solution be boiled with a solution of cupric hydroxid (Fehling's solution) a
copious red or yellow precipitate of cuprous oxid is formed, which indicates
the presence of sugar. The polariscope shows that this sugar is dextro-
rotatory. During the conversion of the starch 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 investi-
gators that this form can also in time be transformed into sugar. It is
possible 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:

Starch-Soluble Starch

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:

3.105 1-H1111 010.

Starch + Water = Maltose -f Dextnn.

The amylolytic1, amyloclastic, or starch-changing action of saliva depends

1 The term amylolytic has been objected to on the ground that it does not correctly express
the fact, but is analogous with electrolytic and means a transformation by means of starch.
Armstrong has suggested the use of the term amyloclastic as well as proteoclastic and lipoclastic
for the terms now generally employed.

152 TEXT-BOOK OF PHYSIOLOGY

on the presence of an unorganized ferment or enzyme known as ptyalin or
amylase. 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 protein. It is a prod-
uct in all probability of the katabolic activity of the secretor 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 moderately alkaline. Its
activity is arrested 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 ques-
tion 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 95° to io6°F.
the ptyalin acts most energetically, while its activity is entirely arrested by
reducing the temperature to the freezing-point or raising it to the boiling
point.

The Mode of Secretion of Saliva. — The secretion of saliva is a com-
plex act and involves the cooperation of gland-cells, blood-vessels, efferent
and afferent nerves contained in different cranial nerves, and a central
mechanism by which they are excited to and coordinated in activity. The
central mechanism is located in the medulla oblongata in the gray matter
beneath the floor of the fourth ventricle and subject to cerebral and surface
stimulation.

During the intervals' of mouth digestion 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, out of which they construct their characteristic con-
stituents. The blood-vessels possess that degree of dilatation necessary for
nutritive purposes.

With the development of the sensation of hunger, the sight and the odor of
agreeable foods develop visual and olfactory sensations, which give rise to
psychic states more or less agreeable. With their development the entire
mechanism is excited to activity and a more or less abundant discharge of
saliva into the mouth frequently takes place. This is, therefore, known as a
psychic secretion. The fluid thus secreted has given rise to the expression
"watering of the mouth."

With the introduction of food into the mouth and the stimulation of
afferent nerves (nerves of general sensibility and nerves of taste) and the
onset of mastication, the blood-vessels suddenly dilate, the blood-supply is
increased, and the gland-cells begin to discharge water, inorganic salts, and

DIGESTION

153

their organic constituents into the lumen of the acinus, materials that col-
lectively constitute the saliva characteristic of any one of the glands. This
continues until mastication ceases, when all the structures return to their
former condition of relative inactivity.

The Nerve Mechanism of Insalivation.— The nerves and nerve-cen-
ters that constitute the nerve mechanism for the secretion of saliva, as de-
termined by experimental investigations are shown in the following table:

Afferent Nerves.

i. Lingual and buccal branches of
the trigeminal nerve.

Nerve-centers.
Medulla oblongata.

Taste fibers
tympani.

in the chorda

3. Taste fibers in the glosso-
pharyngeal.

Efferent Nerves.

The chorda tympani and its post-
ganglionic continuations for the sub-
maxillary and sublingual glands;
the glosso-pharyngeal nerve and its
post-ganglionic continuations con-
tained in the auriculo-temporal
branch of the trigeminal nerve, for
the parotid gland.

The sympathetic nerve, both pre- and
post-ganglionic fibers, for all the
glands.

The Efferent Nerves.— The efferent nerve-fibers, as stated in the fore-
going paragraph, that transmit nerve impulses to the : submaxillary sub-
lingual, and parotid glands, as well as to their associated blood-vessels, belong
to the autonomic system, and are contained respectively in the chorda tym-
pani and its post-ganglionic continuations, in the glosso-pharyngeal and its
post-ganglionic continuations contained in the auriculo-temporal branch
of the fifth nerve, and in the post-gangliomc branches of the sympa
thetic nerve derived from the superior cervical ganglion. Jhat these nerves
transmit the nerve impulses to the salivary apparatus is shown by t]
that follow their division and stimulation.

rteCfe^r^^^

the facial (though it consists in part of autonomic nerve-fibers) the Mrunk of
which it leaves in the aqueduct of Fallopius. It then W^g?^
cavitv emerges through the Glaserian fissure, and joins the lingual bra
of Te'inTerior maxillary division of the fifth nerve. After passing forward
as far af the sublbgual gland, nearly all of the fibers leave the lingual nerve
S^ft^SB to'become connected by terminal Mf
ganglion cells in relation with the submaxillary and sublingual

on the secretion and flow of saliva from the submaxillary

^SS^ and stimulation of the
are shown in the following way: A cannula is inserted

154

TEXT-BOOK OF PHYSIOLOGY

That local ganglia are involved is shown by the effects which follow the
injection of nicotin into the circulation. After a sufficient dose — 10 milli-
grams for the cat — stimulation of the chorda has no effect. Stimulation of
the nerve-plexus beyond the ganglion, the post-ganglionic fibers, however, is
at once followed by vascular dilatation and secretion, a fact that would
indicate that the ganglia are not only stimulated by the chorda tympani but
that the conductivity of the nerve endings of the chorda around the ganglia
is impaired.

S&Mr*

Otic GantjUo.
Parotid Gland,

Jacoiseiis Nerue

SiihMaxllla.ry 6la

C&orela, 7ympani$erv&

. Cervical Ganglion.

Sympathetic Nerves
FIG. 68. — SCHEME OF THE NERVES INVOLVED IN THE SECRETION OF SALIVA.

It might be inferred that the increase in the flow of saliva is due to filtra-
tion, 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 due 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 mechanism is unimpaired,
for stimulation of the nerve, as in the previous instance, gives rise to a dilata-
tion of the vessels and an increased blood-supply. There is thus abundant
proof that the chorda tympani contains two sets of fibers — one regulating
the blood-supply to the gland, the other stimulating the secretor cells.

The efferent fibers, vaso-motor and secretor, which constitute in part

DIGESTION 155

he chorda tympani nerve have their origin in cells, the nucleus salivatorius,
ocated beneath the floor of the fourth ventricle, from which they emerge in
the nerve of Wrisberg or pars intermedia, and enter the trunk of the facial
nerve at the bottom of the internal auditory canal after which they pursue
;he course stated above.

The Glosso-pharyngeal Nerve. — The autonomic nerve-fibers that conduct
nerve impulses outward from the medulla to the parotid gland are believed
to pass through the glosso-pharyngeal 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 trunk of this latter nerve contains therefore post-ganglionic
Sbers that bear the same relation to the parotid gland and blood-vessels
that the post-ganglionic fibers from the submaxillary ganglion bear to the
submaxillary gland and blood-vessels.

The influence of the efferent fibers in the trunk of the glosso-pharyngeal
on the parotid gland is similar to the influence of the chorda tympani on the
submaxillary gland; for if the glosso-pharyngeal nerve or its post-ganglionic
continuations in the auriculo-temporal nerve be stimulated in any part of its
course with induced electric currents there follows a dilatation of the blood-
vessels and an abundant discharge of a thin saliva rich in water and salts
but poor in the amount of organic matter. Division of the glosso-pharyngeal
nerve, extirpation of the otic ganglion or division of the auriculo-temporal
nerve is followed by a loss of reflex secretion. Stimulation of the branch
connecting the glosso-pharyngeal with the otic ganglion ( Jacobson' s nerve)
gives rise to 'the secretion as shown by Heidenhain. Division of the nerve
is also followed by a loss of reflex secretion.

The Sympathetic Nerves.— The autonomic nerve-fibers which influence
the salivary secretion emerge from the spinal cord mainly through the second,
third, and fourth thoracic nerves. After passing into the sympathetic chain
they ascend to the superior cervical ganglion, with the cells of which they
become connected through the intermediation of fine terminal branches.
From this ganglion non-medullated nerve-fibers—sympathetic nerves proper
—follow the branches of the external carotid artery to the different glands.
There is no evidence that these fibers have any connection, anatomic
physiologic, with local ganglia at or near the submaxillary, sublmgual, or
parotid glands. If the sympathetic nerve in the neck, especially in the dog,
be divided and the peripheral end stimulated with induced electric currents,
there is at once a contraction of the smaller blood-vessels of the submaxillary
and sublingual glands and a diminution of the blood-supply, a result showing
the presence of vaso-constrictor fibers. Nevertheless both the submaxillary
and sublingual glands pour out a saliva which is different from that pou
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
the transformation of starch into sugar.

Stimulation of the sympathetic fibers passing to the parotid glan
followed by contraction of the vessels and an abolition of the secretion; but
at the same time there is an increased activity of the secretor cells, for sub-
sequent stimulation of the auriculo-temporal nerve not only caus
increase in the amount of water and inorganic salts, but an increase also in

156 TEXT-BOOK OF PHYSIOLOGY

the amount of organic matter far beyond that produced when the auriculo-
temporal alone has been stimulated. Histologic examination shows that
the small ducts of the gland are filled with thick organic matter after stimula-
tion of the cervical portion of the sympathetic chain.

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, chorda tympani and glosso-pharyngeal, 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 differences 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.

The Central Mechanism. — The central mechanism that excites the
glands and blood-vessels to activity through efferent nerves originating in its
cells maybe aroused to action (i) by nerve impulses descending from the cere-
brum in consequence of psychic states induced by the sight or the odor of
foods especially after long abstinence; (2) by nerve impulses transmitted
through afferent nerves from the mouth, developed by the contact of the food
with the peripheral terminations of the gustatory or general sensor nerves.

That psychic states, ideas and feelings aroused by the sight, odor, and
contemplation of food can give rise to a stimulation of the cells of the central
mechanism in the manner just stated is shown by the flow of saliva which is
familiarly known as watering of the mouth. This fact has been experimen-
tally demonstrated by Pavlov on dogs. 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 foods there was
at once a free discharge of saliva, the quantity and quality of which varied
with the character of the foods.

That the central mechanism can be excited to activity by nerve impulses
reflected from the periphery can be demonstrated by the introduction of food
into the mouth, as well as by stimulation of the branches of the afferent
nerves distributed to the mouth which constitute the afferent part of this
mechanism.

The Afferent Nerves. — The afferent nerves that transmit nerve impulses
from the mouth to the central mechanism, are the taste fibers in the chorda
tympani, the taste and sensor fibers of the glosso-pharyngeal, and the sensor
fibers of the lingual and buccal branches of the trigeminal nerve. This is
shown by the fact that if they are transversely divided there is a cessation of
the discharge of saliva when the peripheral nerve endings in the mouth are
stimulated by the presence of food. With these nerves intact the introduc-
tion of food into the mouth will invariably be followed by a flow of saliva.
Pavlov has apparently demonstrated that this general fact must be supple-
mented by the further fact, that there is a special adaptation between the
character of food and the different glands. Thus, solid dry foods, cause a
large flow of a thin saliva from the parotid glands, but a slight flow from the
submaxillary; moist foods and especially meat causes a large flow from the

DIGESTION 157

submaxillary gland, but a slight flow from the parotid. It is also probable
that the glands respond by discharging a secretion of special quality in
accordance with the properties of the different foods.

Stimulation of the afferent nerves with induced electric currents also gives
rise to a discharge of saliva. This can be demonstrated by exposing the
glands and the afferent nerves and subjecting them to experiment. Under
such circumstances, if a cannula be placed in the duct of the submaxillary
jgland, and the lingual nerve stimulated by induced electric currents of
moderate strength, a copious flow of saliva at once takes place. If now the
Iglosso-pharyngeal nerve or the central end of the divided chorda tympani
;nerve be stimulated in a similar manner, the effect on the secretion will be
ithe same. Division of these nerves in an animal, in such a way as to 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 pneumogastric, when the mucous membrane
of the stomach is stimulated; the sciatic, when after division, its central end
is stimulated.

R6sum6 of the Factors Involved in the Secretion of Saliva. — From
the foregoing statements it is apparent that the secretion of saliva is a complex
act involving the cooperation of several different factors. As the mechanism
for the elaboration of this secretion is typical of that for many secretions it
will be of advantage to summarize these factors and their specific functions.
These are as follows:

1. Epithelial cells, the physiologic actions of which are the production

of the specific characteristic constituents of the saliva, e.g., mucin,
albumin, the enzyme ptyalin, as well as the absorption and discharge of
water and inorganic salts.

2. Lymph, which contains the nutritive material necessary for the growth,

repair, and metabolic activities of the secreting cells.

3. Capillary blood-vessels, which permit the passage of those constituents of

the blood that collectively constitute lymph.

4. Vaso-motor nerves, some of which at the beginning of secretor activity

dilate the blood-vessels and thus increase the blood-supply and the
production of lymph (vaso-dilatator nerves) ; others of which at the
end of secretor activity perhaps actively contract the blood-vessels
and thus decrease the blood-supply to the previous condition (vaso-
constrictor nerves).

5. Secretor nerves, which stimulate the epithelial cells to increased activity

causing them to discharge their specific metabolic constituents along

with water and inorganic salt in characteristic proportions from the

orifices of the gland ducts (secreto-motor nerves).

The central mechanism is excited to coordinate activity, primarily, by
nerve impulses descending from the cerebrum as a result of psychic states
developed by the sight and odor of food, and secondarily, by nerve impulses,
transmitted by the nerves of gustation and general sensibility and developed
by the contact of food on their peripheral terminations during the act
of mastication.

Modifications of the Nerve Mechanism of Insalivation due to the
Physiologic Action of Drugs.— The functions of different portions of

158 TEXT-BOOK OF PHYSIOLOGY

the nerve mechanism of insalivation may be made apparent by an analysis of
the effects that follow the administration of physiologic or slightly toxic
doses of the alkaloids of various drugs. The effects can be shown to be due
to a depression or stimulation of the normal activity of one or more portions
of the mechanism. As a result the secretion may be decreased or increased
in volume. The following examples will illustrate the action of alkaloids
in general.

Nicotin. — When nicotin in sufficiently large doses is given to an animal
hypodermatically, the secretion of saliva after a variable period of time ceases
and the mouth becomes dry. If the chorda tympani nerve, i.e., the pre-
ganglionic portion, be then stimulated with induced electric currents the
usual phenomenon, viz., a free flow of saliva, fails to occur. If, however,
the nerve branches emerging from the submaxillary ganglion, i.e., the post-
ganglionic portion, be stimulated with electric currents, the saliva will be
discharged as usual. The inference is that the conductivity of the peripheral
terminations of the preganglionic chorda fibers is depressed so that the nerve
impulses discharged by the central mechanism fail to reach, and therefore to
stimulate, the submaxillary ganglion cells. The inference as to the seat of
action of nicotin is supported by the fact that painting the surface of the
superior cervical sympathetic ganglion with nicotin will impair the conductiv-
ity of the terminal branches of the pre-ganglionic fibers emerging from the
cord so that stimulation of these fibers fails to produce beyond the ganglion
the usual secretor effects. It is probable that nicotin has a similar action
on the peripheral terminations of Jacobson's nerve which arborize around
the nerve cells of the otic ganglion.

Atropin. — Atropin in doses of i milligram also causes a complete cessa-
tion in the flow of saliva and consequently an extreme dryness of the mouth.
After the occurrence of this condition neither stimulation of the pre-ganglionic
chorda tympani fibers nor of the post ganglionic fibers, will cause the glands to
secrete. But as stimulation of the sympathetic nerve in the cervical region
will excite a secretion the inference is that the atropin exerts a depressing
effect on the conductivity of the nerve endings in contact with the gland cells
thus interfering with the transmission of nerve impulses, rather than on the
gland cells themselves. The same holds true for the nerve terminations in the
post-ganglionic fibers distributed to the parotid gland. The action of atropin
is not limited, however, to the nerve terminations in connection with salivary
glands but extends to the nerve terminations in connection with many other
glands in the alimentary canal and skin. Even though the dose of atropin
be large, 10 to 15 milligrams for a dog, its action is confined to the terminal
nerve-fibers in connection with the gland cells, for when the chorda tympani
is stimulated the blood-vessels around the gland dilate as usual, a fact which
indicates that the submaxillary ganglion gives off fibers of a vaso-dilatator as
well as a secretor character. Unless the dose of atropin be largely increased,
e.g., 100 milligrams, it fails to depress the conductivity of the terminals oi
the sympathetic nerve-fibers.

Pilocarpin. — Pilocarpin in small doses, from 2 to 5 milligrams, hypo-
dermatically causes in the cat a free flow of saliva which may amount to half
a liter or more in the course of several hours. In human beings its effect on
the flow of saliva is equally marked. Ringer reports that in two patients,
after taking a medicinal dose, the amount of saliva discharged was 622 c.c.

DIGESTION i59

and 764 c.c. respectively. Since division of the nerves both pre- and post-
ganglionic, does not diminish or abolish the secretion, the inference is that
the pilocarpin exerts a stimulating action on the nerve endings in connection
with the gland cells. This inference is strengthened by the fact that the
pilocarpin effect is antagonized and the secretion checked by a suitable dose
of atropin, the seat of the action of which is known. The two alkaloids thus
appear to be in physiologic antagonism in their action on these nerve termi-
nations. The action of pilocarpin is not limited to the salivary glands, but
extends to glands found in the alimentary canal, respiratory passages, and
skin.

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 nerve system. The
deglutitory canal consists of the mouth, pharynx, and esophagus, each of
which presents certain anatomic features on which its physiologic action
depends.

The Mouth. — The cavity of the mouth is bounded in front and on the
sides by the alveolar arches and the teeth; the roof is formed by the hard
palate and the floor by the tongue. Anteriorily, the mouth communicates
with the exterior by a transverse opening the buccal orifice, and posteriorly
with the pharynx by an opening, the isthmus of the fauces. The cavity of
the mouth is lined by mucous membrane in which are imbedded numerous
mucous glands. The interior of the mouth is separated in part from the
.interior of the pharynx by an incomplete septum formed above by the soft
palate, laterally by the anterior and posterior palatal arches and below by
the tongue.

The Pharynx.— The pharynx is an oval-shaped cavity extending from
the base of the skull to the lower border of the cricoid cartilage, a distance
of about 12 centimeters. 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 con-
tractions. 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 communicate with the nasal chambers, the mouth, and the
ilarynx. 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 sepa-
rated from the mouth by the velum pendulum palati, a muscular structure
attached above to the hard palate; its lower edge or border^ is directed
idownward 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 respectively by the palato-glossei and palato-pharyngei
muscles. The superior laryngeal aperture 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

l6o TEXT-BOOK OF PHYSIOLOGY

cartilage, the epiglottis, placed just behind the tongue and so arranged that

The^iuscle coa? consists of an external layer of longitudinal fibers arranged
J thrrbands and of an internal layer composed of fibers arranged circularly
he upper part and obliquely in the lower part of fce esophagus. In the
upper tod 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-

J1 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 cardia muscle. By its action it prevents a
return under normal conditions of food into the esophagus.

The deglutitory 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 momentarily sus-
pended. The tip of the tongue is placed against the posterior surfaces of the
teeth. The tongue, by reason of its intrinsic musculature, then arches from
before backward against the roof of the mouth and by the contraction of the
hyoglossus muscle 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 the
pharynx and esophagus into the stomach, have been attributed until quite
recently entirely to peristaltic movements of their musculature.1 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, which, in
consequence of a rapid peristaltic movement running through the 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
musculature, 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 esophagus, is transferred
into the stomach, but that it is not the case with liquids has been demon-
strated 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 transfer-

1 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

and semisohd contents. It is characterized by a contraction of the muscle-fibers behind

ict and an inhibition or relaxation of the muscle-fibers in front of it. (Bayliss and Starling.)

DIGESTION l6l

ence 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 connectedexternally with a water manome
ter, 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 deglutition
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 liquid foods are
rapidly thrown down toward the end of the esophagus, peristalsis playing
no part in the process. The proof, however, of these statements was fur-
nished by Meltzer. This observer introduced into the pharynx and esophagus
rubber tubes, the ends of which were provided with thin-walled rubber bal-
loons which could be distended with air. The outer ends of the tubes were
connected with Marey's recording tambours. Any compression of the bal-
loon 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

FIG. 69. — TRACING OF THE ACT or DEGLUTITION, i. A indicates the compression of the
elastic bag caused by the bolus projected by the contraction of the mylohyoid muscles. B. Con-
traction 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 corresponding to A. D. Com-
pression by the residues of the bolus carried on by the contraction of the pharynx, B. E. Contrac-
tion of the esophagus. — (Landois and Stirling.}

applied against the surface of a revolving cylinder, it became possible, with
the addition of a chronograph, 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 how deep
the position of the esophageal balloon, it was compressed almost simultane-
ously 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 instant. 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. 69.) The pharyngeal curve, i,
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 exerted by the
contraction of the pharyngeal muscles. The interval of time between these

I6a TEXT-BOOK OF PHYSIOLOGY

rsssr? sssar^tfat. £5 sffistfsjs

the compression exerted by the bolus. The interval oi : time : betw een the
beginning of the first and second curves was not more than o.i second,
regardless of the depth to which the esophageal balloon was plunged. At a
[afer period a second rise of the lever was recorded; the time of its appear-
ance, height, duration, etc., were found to increase with the depth of the

1 These facts demonstrate that deglutition. consists of two phases: (i) a
rapid rise of pressure in the pharynx, as a result of which liquid or semi-
liquid foods are 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 cardia at the end of the
esophagus.

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.

The time required for a mouthful of liquid food to pass to the lower end
of the esophagus is approximately about o.i second. If the cardiac orifice
is normally closed, a period of about 6 or 7 seconds may elapse before the on-
coming peristaltic wave reaches the end of the esophagus and forces the
fluid into the stomach. If, however, a series of deglutitory acts follow one
another in quick succession there is an inhibition of the cardiac sphincter and
the peristaltic wave until after the last swallow. The time required for the
food to pass down the esophagus and into the stomach may vary in different
animals and in different human beings.

An examination of the action of the esophagus during deglutition, 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
consequence of the rapid and energetic contraction of the mylohyoid muscles.
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.— Because of the rapid
rise of pressure in the deglutitory canal during the act of swallowing it is
essential that the openings into the nasal and laryngeal cavities be closed to
prevent the entrance of food into them, which would otherwise take place.
Under normal circumstances this is done so effectually that it is seldom that
any portion of the food, liquid or solid, ever enters the nasal chambers
or the cavity of the larynx. The mechanism by which these openings are
closed is as follows:

At the moment the food passes into the pharynx the posterior nasal open-
ings are closed against the entrance of the food by a septum formed by the

DIGESTION

163

pendulous veil of the palate and the posterior half arches. The palate is
drawn upward and backward by the levator palati muscles, until it meets
the posterior wall of the pharynx, which at this moment advances. At the
same time it is made tense, by the action of the tensor palati muscles. (Fig
70) . This septum is completed by the advance toward the middle line of the
posterior half arches caused by the contraction of the muscles, the palato.
pharyngei, which compose them. When these structures are impaired in
their functional activity, as in diphtheritic paralysis and ulcerations, there
is not infrequently a regurgitation of food, especially liquids, into the
nose.

The larynx is equally protected against the entrance of food during
deglutition Bunder ^normal circumstances. That this accident occasionally
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 reasons: just preceding and dur-
ing the act of deglutition there is a complete suspension of the act of inspira-
tion, 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 movement
of the epiglottis, the result of the con-
traction of the aryteno-epiglottidean
muscle.

The action here attributed to the
epiglottis has been denied by Stuart
and McCormick. These observers
had the opportunity 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 dis-
tance. In no instance was it ever ob-
served to fold backward like a lid.

Because of the possibility that this
position of the epiglottis was due to
pathologic causes, Kanthack and An-
derson 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 observations on themselves, these observers reaffirm the gener-
ally 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 epiglottis
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 en-
trance of food, in the rabbit at least, by the closure of the glottis itself. This

Trachea*

FIG. 70. — DIAGRAM SHOWING THE MAN-
NER or CLOSURE or THE POSTERIOR NARES
AND LARYNX DURING DEGLUTITION. — (Lan-
dois and Stirling.}

i64 TEXT-BOOK OF PHYSIOLOGY

experimenter noticed, while observing the interior 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 mylohyoid 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 thyroid carti-
lage. 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 phenomenon. Under
such circumstances the closure of the glottis must have been due to the con-
traction of its intrinsic muscles and in consequence of a reflex action through
the inferior laryngeal nerves.

The Nerve Mechanism of Deglutition. — Deglutition is almost exclu-
sively a reflex act throughout its entire extent, and requires for its inaugura-
tion merely a stimulus to some portion of the mucous membrane in the
anterior part of the deglutitory canal. The first stage is primarily voluntary,
but from inattention to the process may become secondarily reflex. The
origin and course of the afferent nerves, stimulation of which excite reflexly
the movements of the pharynx and esophagus, however, are practically un-
known. In the rabbit deglutition 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 degluti-
tory movements, it is obvious that the terminals of this nerve cannot be the
source of the natural afferent impulses. Stimulation of the glosso-pharyn-
geal 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 alae cinereae. The efferent nerves comprise branches of the
facial, hypoglossal, motor filaments of the third division of the fifth nerve,
motor filaments of the glosso-pharyngeal and vagus nerves 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 coming from a single peripheral area, a series of nerve impulses
successively to successive 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 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 mechan-

DIGESTION

165

ism is temporarily suspended during deep anesthesia while the second per-
sists. (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
with sufficient vigor to force food through the cardiac orifice into the stom-
ach. The muscle coat of this portion of the esophagus is composed of non-
striated muscle-fibers, is supplied with a myenteric nerve plexus and resem-
bles lower portions of the alimentary canal. It is capable of developing a
peristalsis merely 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 composition.
The disintegration of the food inaugurated by mastication and insalivation

wric canal \
Sulcws inter/wcli'iis
Gastro-daodcnal wtisbiction Vestibule

FIG. 71. — ANATOMIC FEATURES OF THE STOMACH.

is still further carried on in the stomach by the solvent action of the acid fluid
there present, until the entire mass is reduced to a liquid or semi-liquid
condition.

The Stomach.— The stomach is the dilated and highly specialized por-
tion of the alimentary canal intervening between the esophagus and small
intestine. 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 abdomir
cavity, extending from the left hypochondrium to the right of the epigastrium.
The dimensions and capacity of the stomach undergo considerable perio
variation according to the extent to which it is distended by food. ^ I
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-po

i66 TEXT-BOOK OF PHYSIOLOGY

diameter from n to 12 centimeters. The capacity of the stomach varies
from 1500 to 1700 c.c. In the empty condition its walls are contracted and
partly in contact, and the entire organ is drawn up into the upper part of
the abdominal cavity. The stomach being a contractile and distensible organ
adapts itself to varying amounts of food and hence continually varies in
size. The opening through which the food passes into the stomach is known
as the esophago-gastric or the cardiac orifice. The opening through which
it passes into the intestine is known as pylorus, the pyloric, or gastro-duodenal
orifice, indicated on the outer surface by a slight constriction known as the
gastro-duodenal constriction. 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. Along
the lesser curvature there is a slight indentation due to a change in direction
of the right end of the stomach, known as the incisura angularis. If this
indentation is connected by a line with a point or angle almost opposite on
the greater curvature the stomach wilt be divided into a pyloric portion to the
right and a cardiac portion to the left. If a line be drawn transversely across
the stomach from the esophago-gastric or cardiac orifice to the greater curva-
ture, the cardiac portion will be subdivided into the fundus, to the extreme
left, and the cardiac portion proper or the body included between the fundus
and the pyloric portion.

If a vertical section of the stomach is made, it will be observed that the
walls are extremely thin to the left but gradually increase in thickness in
passing toward the pyloric orifice. The cavity of the pyloric portion, is,
when the stomach is empty, subdivided into a vestibule and a pyloric canal
which latter is said to begin at a point opposite the sulcus intermedius, and
to extend to the pyloric orifice. The pyloric canal is narrow and cylindric
and measures from 2.5 to 3.5 centimeters. The vestibule and pyloric canal
when distended with food form a tubular region which has been termed the
antrum or the gastric canal.

The Walls of the Stomach.— The walls of the stomach are formed by
four distinct coats united 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.
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 scattered, but toward the pyloric orifice they
are more numerous and form a tolerably thick layer which becomes con-
tinuous with the fibers of the small intestine. The circular fibers form a
complete 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 surrounding the
cardia the circular muscle-fibers form a true sphincter which is known as
the sphincter cardia. At the junction of the pyloric and cardiac regions, x-x,
Fig. 71, the circular fibers form a well-defined bundle — the transverse band —
which was supposed to contract periodically during digestion and thus par-
tially divide the stomach into two portions, the portion to the right of which

DIGESTION

167

was termed the antrum, and hence this transverse band has been termed the
sphincter antri pylorici. This function, however, has been questioned.

In the pyloric region the circular fibers are more closely arranged, form-
ing thick well-defined rings termed the antral muscles. At the pyloric open-
ing the circular fibers are crowded together and form a distinct muscle band
known as the sphincter pylori which projects for some distance into the inte-
rior of the canal. It has been stated by Rudinger that the inner fibers of
the longitudinal coat become connected with this circular band and consti-
tute a distinct muscle, the dilatator pylori. The oblique fibers are most dis-
tinct 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 consists 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 muscle tissue, the muscularis
mucosa, which supports the mucous membrane.

The internal or mucous coat is loosely attached to the muscle coat. In
the empty and contracted state of the stomach it is thrown into longitudinal
folds, or rugae, 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. In passing from the cardiac
toward and into the pyloric region the mucous membrane becomes thicker
and forms the inner wall of the vestibule and pyloric canal. It finally be-
comes continuous with the mucous membrane of the intestine. At the pyloric
orifice there is a circular involution of the mucous membrane 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 innumerable 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 membrane. A vertical section of the
gastric walls shows not only the position and the appearance of the
glands, but the relation of the various tissues which enter into the formation
of these walls. An examination of the mucous membrane in different
regions of the stomach reveals the presence of two distinct types of glands,
which from their situation are termed cardiac, and pyloric, which differ
not only in histologic structure, but also in function. Both types extend
through the entire thickness of the mucosa.

The cardiac glands are formed by an involution of the basement mem-
brane 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. 72).
The latter portion is wavy or tortuous and frequently subdivided into as
many as four distinct and separate tubules. The duct is lined by columnar
epithelial cells similar to that covering the surface of the mucosa. The
lumen of the gland is bordered by epithelial cells, cuboid in shape and con-
sisting of a granular protoplasm containing a distinct sphencal nucleus.
These cells are generally spoken of as the chief or central cells. In addi-

i68

TEXT-BOOK OF PHYSIOLOGY

tion to the chief cells, the cardiac glands contain 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 parietal
cell appears to be surrounded and penetrated by a system of passages which
open into the lumen of the gland by means of a delicate cleft or canaliculus
(Fig. 73). 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 mem-
brane and lined by epithelial cells (Fig. 74). The ducts are much longer
than the ducts of the cardiac glands. At its ex-
tremity each duct becomes branched, giving rise to a
number, from 2 to 10, of short tubes, each of which
has a large lumen and communicates with the duct by
a narrow short neck. The ducts are lined through-
out by columnar epithelium. According to Mall, the
total number of openings on the surface of the mucous
membrane of the dog's stomach is somewhat over
1,000,000, and the total number of blind tubes opposite
the muscularis mucosae exceeds 16,500,000. Accord.-
ing to Sappey, the surface of the mucous membrane
of the human stomach presents over 5,000,000 orifices
of gastric glands.

FIG. 72. — CARDIAC GLAND.
m Mouth of the duct; n, neck;
/. fundus; c, central cells;
p, parietal cells. (Landois
and Stirling.)

Lumen.

Secretor
capillaries.

FIG. 73. — SECTION OF CARDIAC GLAND
Oi 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. (Stdhr.)

Blood-vessels and Nerves. — The blood-vessels of the stomach after
entering the mucosa break up into a number of branches which are dis-
tributed to the muscle 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 mucosae.

The nerve elements are partly intrinsic and partly extrinsic. The in-

DIGESTION

169

trinsic nerve elements consist of two nerve plexuses, one which lies between
the muscle coats, another which lies between the muscle and the mucous
coats. They have been named after their discoverers, Auerbach?s and
Meissner's plexuses respectively. Each plexus consists of microscopic
ganglia and nerve processes which interlace with one another to form a
very complicated network. The nerve processes are distributed mainly to
muscle-fibers. As these plexuses are associated with a large part of the
alimentary canal they together have been termed the myenteric plexus.
The extrinsic nerves, which associate the central nerve system with the
gastric structures are the vagus and the great splanchnic. The vagus nerve,
through its efferent nerve-fibers is in physiologic relation with muscle-fibers
and gland-cells, through the intermediation of gang-
lionic cells. The great splanchnic through its efferent
nerve-fibers is in physiologic relation with the muscle-
fibers and blood-vessels through the intermediation
of the post-ganglionic fibers derived from the cells of
the semilunar ganglion.

Gastric Fistulae. — The general process of diges-
tion, 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, of necessary surgical or of experi-
mental 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 acci-
dent was about two and a half inches in circum-
ference 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
1831 at intervals made numerous experiments on
the nature of gastric digestion. As the result of an

admirable series of investigations it was established moutlofduct;J neck.
that the digestion of the food is largely a chemic (Landois.)
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 ac-
cording to the nature of the food, exercise, mental states, etc., and that the
process is aided by continuous movements of the muscle 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 proce:
when the food was introduced through the opening. Similar fistulae have
been established in both carnivorous and herbivorous animals with a view

TEXT-BOOK OF PHYSIOLOGY

of studying the process as it takes place in them. The results obtained in
these instances in many respects corroborate those obtained by Dr. Beau-
mont, though many new facts, unobserved by him, have been brought to
light.

Much additional information as to the mode of secretion and the char-
acteristics of the gastric juice has been obtained, since the introduction of two
new procedures by Pavlov. 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 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 ab-
domen but is separated from the rest of the stomach by a thin septum formed
of two layers of mucous membrane. (Fig. 75.) 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 con-
cerned, as the large stomach does. The phe-
nomena 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 gastric juice. As the swallowed
™ OPETAHE NHA™££ f°°d is discharged from the lower end of the
STOMACH TO THE MINIATURE divided esophagus the appetite continues, and

£S£?,££?S££ife £en" the aTal wi" eat. f?r sev<?al hours-

VISED BY PAVLOV, v. The nat- -^ tne second procedure it is possible to col-
urai stomach, s. The miniature lect gastric juice from the miniature stomach

^^CO^^^TA and .t0 study the effects on its quantity and
the abdominal walls. quality produced by psychic states, mastica-

tion, different articles of food, and by the

process of digestion itself as it goes on in the large stomach. In both in-
stances 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 con-
stant acid reaction, a slightly saline and acid taste, and a specific gravity
varying from 1.002 to 1.005. The juice obtained from the dog's stomach
possesses essentially the same characteristics, though its acidity as well as its
specific gravity are slightly greater. When kept from atmospheric influences,
it resists putrefactive 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. It may however be said to consist of water,

DIGESTION I71

organic matter, hydrochloric acid and various inorganic salts. The quan-
titative composition of the juice varies somewhat in different animals.

The organic matter present in gastric juice is a mixture of mucin and a
protein, products of the metabolic activity of the epithelial cells on the sur-
face of the mucous membrane and of the chief or central cells of the gastric
glands respectively. Associated with the protein material are two possibly
three ferment or enzyme bodies, termed pepsin, rennin and lipase. As is
the case with other enzymes, their true chemic nature is practically un-
known.

Pepsin.— 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 ante-
cedent 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 cardiac glands, it is also produced, though in less
amount, by the cells of the pyloric glands. Pepsin is the chief proteolytic
or proteoclastic agent of the gastric juice and exerts its influence most ener-
getically in the presence of hydrochloric acid and at a temperature of about
4O°C. Other acids — e.g., phosphoric, nitric, lactic, etc. — are also capable
of exciting it to activity, though with less intensity.

Rennin. — Rennin or pexin is present in the gastric juice not only of man
and all of 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 a transformation of soluble case-
inogen into a solid flaky body, casein.

Lipase. — Lipase, an enzyme found in pancreatic juice, has also been
shown to be present in gastric juice, the specific function of which appears
to be the digestion or hydrolysis of finely emulsified fat such as is found in
milk.

Hydrochloric Acid.— 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 its 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 most recent investigations show that the initial acidity of
the freshly secreted human gastric juice is between 0.32 and 0.48 per cent.
HC1. This initial acidity is reduced by combination with food, admixture
with saliva and gastric mucus, and by regurgitation of alkaline duodenal
contents, to 0.15 or 0.2 per cent. HC1, the optimum acidity for the proteolytic
activity of pepsin. As observed clinically, following various test meals,
the acidity of the gastric contents is seen to rise to a maximum as diges-
tion progresses, after which it falls to the optimum point of about 0.2 per
cent. HC1.

The immediate origin of the hydrochloric acid is difficult of explanation.
That it is derived, however, primarily from the chlorids of the food and
secondarily from the chlorids of the blood-plasma has been established by
direct experiment. If all the chlorids be removed from the food and all
the chlorids be withdrawn from the animal tissues by the administration of
various diuretics— e.g., potassium nitrate— there will be a total disappearance

172 TEXT-BOOK OF PHYSIOLOGY

of hydrochloric acid from the stomach. On the addition of sodium or potas-
sium 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, nothing
definite is known. Various theories of a chemic and physical character
have been offered, all of which are more or less unsatisfactory. As no hydro-
chloric 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 ante-
cedent zymogen compounds, pepsinogen and pexinogen (Warren) ; it imparts
activity to these ferments; it prevents and even arrests fermentative and
putrefactive changes in the food by destroying microorganisms; it softens
connective tissue, it dissolves and acidifies the proteins, thus making possible
the subsequent action of pepsin.

The inorganic salts of the gastric juice are probably only incidental and
play no part in the digestive process.

Mode of Secretion. — The observations of Dr. Beaumont and the experi-
ments 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 intro-
duction 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 alkaline or neutral reaction. The introduction,
however, of small portions of food or irritation with a glass rod causes a
change in the appearance of the mucous membrane. At the points of
irritation the membrane becomes red and vascular and in a few minutes
small 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 ^ Pavlov, who asserts that it is only an alkaline mucous the function of
which is protective in character. According to this investigator, mechanic
stimulation ^is incapable of exciting the secretion. The results of modern
methods of investigation make it apparent that the production and discharge
of the secretion is the result of the action of two different stimuli, a primary
and a secondary.

The primary stimulus to gastric secretion, according to Pavlov, 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 appear-
ance at the end of six minutes and during the time of the experiment, which

DIGESTION I73

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. Agree-
able emotions 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.

As a result of the psychic states induced by the sight and odor of food and
of the taste of food during mastication, nerve impulses not only descend from
the brain but are also transmitted from the mouth through afferent nerves,
to some central mechanism; and that from this mechanism, nerve impulses
must in turn be discharged to be transmitted through efferent nerve-fibers
which are distributed to the epithelium of the gastric glands. Experimental
investigations render it probable that the central mechanism is located in the
medulla oblongata and that the efferent path for the secretor fibers 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 uni-
form. The investigations of Pavlov seem to be the most reliable. He
found that after division of the nerve, secretion was arrested, and that stimu-
lation of the peripheral ends with induced electric currents at the rate of one
or two per second, caused after a latent period of several minutes' duration
a flow of gastric juice. Coincidently with the development of the psychic
secretion there is a dilatation of the gastric blood-vessels and an increase in
the supply of blood to the gastric glands. Whether this is due to the action
of vaso-dilatator fibers or to an inhibition of the action of vaso-constrictor
fibers is uncertain.

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 pro-
duction of the juice on a relatively large scale. That some factor is active
in keeping up the secretion in the stomach, is apparent from the in-
crease in the quantity and the change in the quality of the juice secreted by
the miniature stomach.

The secondary stimulus to the gastric secretion is in all probability
chemic in character and developed in the stomach or in its walls during
digestive 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 results of which lead to the inference that there is developed
in the mucous membrane of the pyloric region, by the action of certain
articles of food, e.g., dextrin, meat broths, soups, etc., or by the first products
of digestive activity, a chemic agent, which is absorbed by the blood and ]
carried to the glands throughout the stomach and which, on reaching t
glands, stimulates their cells in a specific manner. For this reason it has

174 TEXT-BOOK OF PHYSIOLOGY

been called the gastric hormone1 or the gastric secretin. Whatever the agen
or the mechanism 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 a secretion of 25 per cent, more pepsin than milk. Meat extract
and bouillon have a very stimulating effect on the quantity 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 gastric glands underg
changes in histologic structure which are believed to be connected with tv
production of the enzymes, pepsin and rennin, and the acid. In the resti%
period the protoplasm of the chief or central cells of the cardiac glands be-"
comes crowded with large and well-defined granules, which during the period
of secretory activity largely disappear, so much so that only the lumina
border of the cell is occupied by them, the outer border being clear and hya-
line in appearance. The parietal cells during rest are large and finely
granular, but after secretion they are smaller in size though still granular.

The cells of the pyloric glands, though containing granules, do not show
any marked difference between the resting and active conditions. According
to some observers they contain pepsinogen; according to others, mucin.
The epithelial cells lining the ducts of the pylorus and cardiac glands, if not
identical with the epithelial cells on the surface of the mucous membrane,
pass by transitional forms into them. Among these cells are found many
goblet cells which secrete a portion of the mucin 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 stimu-
lation through the^nerve-supply of the cells, and an output of a fluid to which
the name gastric juice is given.

The Physiologic Action of Gastric Juice.— In the study of the physi-
ology of gastric digestion as it takes place under normal conditions 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 nutri-
tive 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 on proteins, gradually disinte-
grates 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 protein. A most
convenient form, however, is fibrin obtained from blood by whipping and
thoroughly freed from corpuscles by washing under a stream of water. The

1 A hormone (from fynaw, "I excite") may be defined as an agent of known or unknown
composition which is secreted by some one organ, is discharged into and carried by the blood stream
to some correlated organ near or remote, on the functional activities of which it exerts an excitator
or stimulating influence. (See Chapter on Internal Secretion.)

DIGESTION I75

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 and the composition of foods (see pages 15 and 119).
For purposes of experimentation artificial gastric juice may be employed!
This is as effective as the normal secretion and in no essential respect differs
irom 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 iO4°F. (4O°C.) for an hour or two, they will be dissolved
and will entirely disappear, giving rise to a slightly opalescent mixture. In

je early stages of the process the fibrin becomes swollen and transparent
^ partly dissolved. If at this time the solution be carefully neutralized,

1 dissolved portion can be regained in the form of acid fibrin — a fact which
indicates that the first effect of the gastric juice is the acidification of the
protein. This having been accomplished, the pepsin becomes operative,

nd in a varying length of time transforms the acid-protein into a new form

. protdnjjtermed. peptone which differs from all other forms of protein
in being soluble in both acids and alkalies and non-coagulable by heat.
In the transformation of acid-protein into peptone it is possible to isolate
by the addition of magnesium sulphate and ammonium sulphate inter-
mediate bodies to which the term proteases 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. This supposed change produced by gastric juice
is represented by the following scheme:

Protein— Acid-protein— Proteose— Proteose— Peptone
(Primary) (Secondary)

From the fact that when peptones are subjected to the prolonged action
of pancreatic juice there arise compounds such as leucin, tyrosin, aspartic
acid, arginin, etc., it was believed that two kinds of peptones were formed
out of a simple protein one of which succumbed 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-peptone. It is generally admitted now, however, that there
is but one kind of peptone formed from any given protein, which under the
influence of pancreatic, and intestinal juice as well, is reduced by hydrolysis,
through successive stages to amino-acids or perhaps only to the antecedent
stage, in which two or more amino-acids yet remain united forming sub-
stances known as peptids.

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 un-
known, but there are reasons for thinking that it is a process ofj?ydra-
tion, attended by cleavage, with increasing solubility of the resulting
products. .

Characters of Peptones.— The peptones resulting from the digestic
different proteins, though resembling each other in many respects, yet possess

i76 TEXT-BOOK OF PHYSIOLOGY

different chemic characteristics, as shown by their reaction to various chemic
reagents. Though having some resemblance to the proteins from which
they are derived, they are to be distinguished from them by the following
general characteristics :

1. 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 proteins from which they are derived.

From the foregoing facts it may be inferred that in the digestion of pro-
teins there is a progressive diminution in the size of the molecules through a
series of hydrolytic changes. The molecules of the proteins, which from
various causes are coagulated, are transformed into smaller molecules which
are non-coagulable, soluble, and diffusible.

On liquid fat and hydrated starch gastric juice has no appreciable action.
It has apparently been demonstrated, however, that when fat in the emulsi-
fied state, the state in which it exists in milk, is introduced into the stomach
it undergoes a cleavage into fat acids and glycerin, in a manner similar to
that which fat undergoes in the intestine under the action of pancreatic
juice, as will be stated in a future paragraph. This presupposes the
existence of a ferment to which the name lipase has been given. Though
the action of saliva on starch is interfered with and even checked by a small
percentage of hydrochloric acid it is certain from the results of recent experi-
ments, that starch digestion continues for from twenty minutes to a half
hour or longer, for the reason that the acid as fast as it is secreted combines
with the proteins and is thus rendered inoperative and for the reason also
that the food is largely retained in the extreme fundic 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.

Notwithstanding the fact that dilute solutions of hydrochloric acid (0.3
per cent.) will promptly invert cane-sugar to 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.

Action of Gastric Juice on Foods.— The action of gastric juice on
proteins affords a key to its action 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 protein materials, such as meat, eggs, milk, etc.
Meat is disintegrated first by the conversion of the proteins 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 manner attacked and converted into peptones. The true muscle
or sarcous substance, consisting largely of myosin, undergoes a corresponding
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

DIGESTION 1 77

digested than when raw or firmly coagulated by prolonged boiling. In either
condition, however, the supporting tissue is dissolved and peptonized, after
which the native albumin undergoes the same change. The yolk of the egg
consists largely of fat held in suspension by a protein substance, vitellin,
which is also capable of transformation into peptone.

Adipose tissue is similarly reduced. The protein of the connective tissue
and of the fat vesicles is dissolved and peptonized and the fat-drops set
free.

Milk undergoes a peculiar change in composition before its chief protein
constituent, caseinogen, can be transformed into peptone. The caseinogen
in the presence of calcium salts is always in the soluble state. When acted
on by the gastric juice, the caseinogen undergoes a chemic change by reason
of which it combines with calcium salts and is then transformed into a solid
compound casein. This change is due to the presence and activity of the
enzyme, rennin. The necessity for this change in the process of digestion,
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. After its production, 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 they are boiled and dis-
integrated by the teeth, the gastric juice is enabled to penetrate the frame-
work and dissolve and peptonize the various protein constituents. As a
general rule, the vegetable proteins are more difficult of digestion than the
animal proteins.

Duration of Gastric Digestion. — The length of time the food 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 corrobo-
rated 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 under-
going digestion was about three and a half hours, the duration of the proc-
ess, however, 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 intes-
tines, this continuing for two to three hours until the stomach is completely
emptied. The relative digestibility of the different 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:

The time required for the stomach to discharge any given article o
food has been shown by Cannon to depend partly on its chemic composition
and partly on its capacity for absorbing hydrochloric acid. From an
examination of the stomach and duodenum of the cat by means of .
rays and the fluoroscopic screen, after the administration of equal quantities,

I78 TEXT-BOOK OF PHYSIOLOGY

25 c.c., of pure protein, fat, and carbohydrate, mixed with 5 grams of bismuth,
it became possible to determine the rate at which they left the stomach from
the length of the food masses in the duodenum and small intestine as indi-
cated by the shadows on the screen, at intervals of half an hour or longer.
The duration of the observations extended over a period of seven hours.

TABLE SHOWING THE DIGESTIBILITY OF VARIOUS ARTICLES OF FOOD

Hours. Minutes. Hours. Minutes.

Eggs, whipped i 20 Soup, barley, boiled i 30

Eggs, soft boiled 3 ; - * -, Soup, bean, boiled 3

Eggs, hard boiled 3 30 Soup, chicken, boiled 3

Oysters, raw 2 55 Soup, mutton, boiled 3 30

Oysters, stewed 3 3° Sausage 3 20

Lamb, broiled 2 30 Green corn, boiled 3 45

Veal, broiled 4 . . Beans, boiled 2 30

Pork, roasted 5 15 Potatoes, roasted 2 30

Beefsteak, broiled 3 . . Potatoes, boiled 3 30

Turkey, roasted 2 25 Cabbage, boiled 4 30

Chicken, boiled 4 . . Turnips, boiled 3 30

Chicken, fricasseed 2 45 Beets, boiled 3 45

Duck, roasted 4 . . Parsnips, boiled 2 30

When a pure protein, e.g., boiled beef free from fat, boiled haddock, or
the white meat of fowls is administered, foods which not only excite the
flow of gastric juice but readily absorb hydrochloric acid, the pylorus remains
closed for some time, scarcely any protein leaving the stomach during the first
half hour. Shortly after this when free hydrochloric acid makes its appear-
ance, the signal for the relaxation of the sphincter, the pylorus opens from
time to time and the passage of the protein into the duodenum begins and
gradually increases in rapidity until the maximum speed is attained, about
two hours after ingestion; from this time on, the speed of discharge gradually
diminishes until the end of the observation period.

When fat, e.g., beef, mutton, or pork fat, is administered, they remain
in the stomach for some time and when they begin to leave, the rate of dis-
charge is so slow that they are digested and absorbed almost as fast as
discharged and hence seldom accumulate in the small intestine. These
compounds delay the secretion of gastric juice and therefore free hydrochloric
acid, the presence of which appears to be necessary for the relaxation of the
pyloric sphincter.

When carbohydrates, e.g., starch paste, boiled rice, boiled mashed pota-
toes are administered, their discharge begins shortly after their entrance into
the stomach; they pass out rapidly, the velocity of discharge reaching its
maximum at the end of two hours, after which the speed declines to the end
of the observation period. The reason for the early and rapid discharge is
to be found in the fact that while the carbohydrates excite the secretion of
gastric juice they do not absorb the hydrochloric acid to any appreciable
extent. A combination of equal quantities of protein and carbohydrate
varies the rate of discharge of each separately. Thus under these circum-
stances the carbohydrates are not discharged so rapidly nor are the proteins
detained so long as usual; a combination of fat with either protein or carbo-
hydrate delays the time of discharge of both. From these facts it may be
inferred that the time any given food remains in the stomach will depend on its
chemic composition or the relative amounts of its contained protein, fat,
and carbohydrate principles.

DIGESTION

179

usually described as peristaltic in charae

also serve to

FIG. 75. — SHADOW SKETCHES
OF THE OUTLINES OF THE
STOMACH OF A CAT IMMEDI-
ATELY AFTER A MEAL (n.o),
AND AT VARIOUS INTERVALS
AFTERWARD (AT 12.0, AT 2.0,
3.30, 4.30).— (W. B. Camion.)

the

The movements of the human stomach as
described by Beaumont, as well as the movements
of the dog's stomach as stated by different ob-
servers are not in agreement in all respects, and
ai!e' moreover> °Pen to question for the reason
that they were not observed under strictly physio-
logic conditions. The more recent investigations
of Cannon have thrown new light on this sub-
ject. 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 subnitrate of bismuth!

Left

Post

FIG. 77. — 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, of
Physiology, Cannon.)

This substance, 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 change in shape that the stomach undergoes at
different periods of the digestive act. The results of these investigations
will be referred to in following paragraphs.

As a result of many methods of investigation it has become apparent that
the activities of different portions of the stomach, whereby food is admitted
into it, retained there, triturated and mixed with the gastric juice and finally
discharged into the duodenum, are due (i) to causes resident in the stomach
walls and the stomach contents and (2) to nerve impulses descending from
the central nerve system through the vagi and splanchnic nerves.

At the end of a digestive period the walls of the stomach contract and
almost obliterate its cavity. The sphincter cardiac and sphincter pylori

!8o TEXT-BOOK OF PHYSIOLOGY

are also contracted and the orifices they surround are more or less tightly
closed.

The Movements of the Sphincter Car dice. — The sphincter cardice 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 not only to the pass-
age of food but even to the introduction of a sound into the stomach. (Can-
non.) That the normal contraction is under the influence of the central
nerve system is shown by the effects which follow division and stimulation
of the peripheral end of the vagus. If it is stimulated with weak induced
currents, the contraction of the sphincter is somewhat inhibited and the
orifice enlarged; if it is stimulated with strong currents the contraction is
markedly increased and the orifice diminished. Apparently there are in the
vagus two sets of efferent nerve-fibers, one of which inhibits while the other
augments 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 also shown that at the beginning of each act of degluti-
tion, there is an inhibition of the sphincter muscle, and if the acts follow each
other in quick succession, the inhibition and relaxation are increased.
(Meltzer.) With the passage of food into the stomach the tonic contrac-
tion again supervenes. These effects also follow stimulation of the glosso-
pharyngeal nerve. Whether the sphincter inhibition is the result of an
inhibition of the center which maintains the tonus, or a stimulation of an
inhibitor center, is uncertain.

Though the tonus of the sphincter cardise is capable of being inhibited
and augmented by the central nerve system, the most frequent cause under
physiological conditions for an augmentation of the tonus, is the presence of
hydrochloric acid on the gastric side of the cardia. In the early stages of
digestion when the percentage of the acid is below the normal, the orifice
frequently opens and food is regurgitated for a short distance into the esopha-
gus. The ascent of the food far into the esophagus is prevented by a con-
traction and peristalsis of its muscle walls, by which the food is returned to
the stomach. As the free acid accumulates the sphincter tightly closes the
orifice. As this takes place after division of the nerves passing to this region
of the esophagus the inference is that the contraction of the sphincter is
brought about through the intermediation of a local reflex nerve mechanism
in the walls of the esophagus.

It has recently been reported by Cannon that a similar inhibition or
relaxation of the musculature of the cardiac end of the stomach is occasioned
by each act of deglutition and that it continues and increases if the acts
follow each other in quick succession. As the bolus descends the esophagus
and before it reaches its termination there is a relaxation of the musculature of
the cardiac end, a fall of intragastric pressure, an enlargement of the stomach
capacity and hence a readier receptivity of the bolus. That this inhibition
is caused by impulses descending the vagus is shown by the effects which
follow a moderate stimulation of the vagus nerve and by the fact that it
does not take place if the vagus nerves are divided. To this inhibition and

DIGESTION l8l

enlargement of the cardiac end of the stomach the term receptive relaxa-
tion has been given.

The investigations of Cannon previously referred to, show that not only
the sphincter cadiae, but other portions of the stomach walls exhibit differ-
ent forms of activity which for convenience of description are separately
described by him as follows :

1. The Movements of the Pyloric Portwn.—Thz movements of this region
of the stomach were first studied and sketched by means of the Rontgen
rays by Cannon. Some of the changes in shape due to these movements
are shown in Fig. 76. Within five minutes after a cat has finished a meal
of bread there is visible near the duodenal end of the antrum or vestibule
a slight annular contraction which moves peristaltically to the pyloric orifice;
this is followed by several waves recurring at regular intervals. Two or
three minutes after the first movement is seen, very slight constrictions ap-
pear 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 or vestibule 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 pyloric region has
lengthened, a wave takes about thirty-six seconds to move from the middle of
the stomach to the pyloric orifice. 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. 77.

2. The Movements of the Sphincter Pylori. — During the first ten or fifteen
minutes after the introduction of food the pyloric orifice is more or less
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 frequency with which
the pyloric orifice opens depends apparently on the degree to which the food
is softened. When the food is hard, the orifice is closed more tightly and
remains closed a longer period than when it is soft.

The physiologic cause for the relaxation or inhibition of the sphincter
pylori appears to be the presence of free acid at the pylorus; its contraction,
the presence of free acid in the duodenum. With the neutralization of the
acid in the duodenum, its influence on the sphincter muscle is weakened, after
which the muscle 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 proteins, which postpone the appearance 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 central nerve system, and through the intermediation of a local reflex
nerve mechanism, the myenteric plexus, in the walls of the pylorus.

182 TEXT-BOOK OF PHYSIOLOGY

This explanation of the opening of the pylorus is believed by its advocates
to supersede a former view, yet widely held, however, that the pylorus re-
mains open until closed reflexly from the duodenum by the passage into it
of acid, coarse food particles or other irritants from the stomach. When
these irritants are neutralized or removed, the pylorus relaxes, to be closed
again by another duodenal reflex. This intermittent opening and closing
continues until the stomach empties. Recent investigations apparently
suggest this interpretation of the mechanism to be the one that obtains for
the human pylorus. In these experiments the stomach was found to empty
more quickly when its contents were of a weak alkaline reaction than
when acid was present in the gastric contents.

If the pylorus is more or less open during the early stages of digestion it
is easier to understand how the intestinal juices can be regurgitated into the
stomach, as appears to be experimentally proven by the investigations of
Boldyreff, Spencer and his co-workers with the result of neutralizing the
superfluous acidity of the gastric juice. The initial high acidity of this fluid,
viz. : 0.32 to 0.48 causes, on its passage into the duodenum, a secretion of
pancreatic juice and bile and the development of an anti-peristaltic wave or
rhythmic pulsations which drives these alkaline fluids into the stomach until
the acidity is brought down to that point where the juice is no longer
irritating to the duodenal mucous membrane.

3. The Movements of the Cardiac Portion. — As digestion proceeds, the pre-
vestibular or cardiac portion 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 pyloric region to succeed
that which has been prepared and ejected into the duodenum. As the pre-
vestibular 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. 76. The fundus acts as a reservoir for the food and
forces out its contents a little at a time as the vestibular mechanism is ready
to receive them. Since peristaltic movements are absent from the fundus
portion, the food is not mixed with gastric juice, and therefore salivary diges-
tion can continue for a considerable period. There is no evidence of a circu-
lation of food in the stomach as sometimes described. On the contrary, the
movement through the elongated tube is in general a progressive though an
oscillating one. As the constriction waves rapidly pass over the food it is
advanced toward the pyloric orifice, but as 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 pyloric orifice
opens, 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 accumu-
lating which goes to show Jhat in human beings the walls of the stomach
exhibit constriction waves which are similar in all respects to those above
described. Evidence of this character has been furnished by experiments
conducted in the author's laboratory by Spencer and Meyer.

DIGESTION lg

Fig 78 is a record of the gastric contractions in man as recorded by the
lever of a Marey tambour in connection with a thin rubber sausage-shaped
balloon. The balloon was swallowed in the collapsed state and then in
flated under a pressure of about eight millimeters of mercury During the
experiment which lasted from one to an hour and a half, the subject was
placed in the recumbent position and frequently fell asleep The abdomen
was auscultated for sounds of peristalsis in the intestine. The passage of
food from the stomach into the duodenum was easily heard and coincided
with a fall of intragastric pressure as recorded in the tracing

The conditions necessary for the development of the gastric peristalsis
are (i) a condition of tonicity of the musculature, i.e., a slight degree of con-
traction whereby the muscle is shortened; (2) intragastric pressure. When
these two conditions are mutually adapted the musculature acquires a cer-
tain degree of tension whereupon the peristalsis arises. An excess or de-

FIG. 78. — GASTRIC CONTRACTIONS DURING DIGESTION. (Two hours after eating.)
The upper tracing shows respiratory excursions, the variation in the bases of which records the
changes in intragastric pressure. The heavier markings at the bases of the respiratory tracings
are heart-beats. Time marked in minutes. Analysis of the gastric contractions divides them into
long tonus waves of about 60 to 90 seconds duration, superposed upon which are shorter peristaltic
waves about 20 seconds in duration.

The lower tracing shows gastric digestive contractions with heart-beats, taken while holding the
breath at the end of an expiration. The intervals represent periods of respiration during which the
pen was removed from the drum. Were the intervals connected, a regular sequence of gastric tonus
waves with peristaltic waves superposed would appear.

ficiency of internal pressure as well as a loss of tonicity prevents peristalsis.
The peristalsis has no necessary fixed point of origin but arises at that
portion of the stomach in which the two factors previously mentioned bear
a certain relation one to the other. From their origin the peristaltic waves
pass toward the pylorus as a result of increased internal pressure. The
necessary degree of the preliminary tonus is imparted to the musculature
by nerve impulses descending the vagi. If these nerves are cut, the tonus is
impaired and peristalsis fails to develop. After a variable period the neuro-
muscular mechanism develops a tonus like that given to it by the vagi, after
which' the usual peristalsis returns. When once the peristalsis is well devel-
oped in digestion, division of the vagi has no effect. (Cannon.)

The Nerve Mechanism of the Stomach.— In preceding paragraphs
it was stated that during the period of gastric digestion the food is retained

!84 TEXT-BOOK OF PHYSIOLOGY

in the stomach because of the closure of the cardia (the esophago-gastric
orifice) and of the pylorus (the gastro-duodenal orifice) both orifices being
tightly closed by the tonic contraction of sphincter muscles; that both sphinc-
ters 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 fundus
muscles are tonically contracted and steadily pressing the food into and
through the cardiac region to the vestibule; that the pyloric or vestibular
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
physiologic 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 be-
tween the layers of muscle-fibers, it is not believed that they are the immedi-
ate sources of the stimulus to the contraction, though they may act as a coordi-
nating mechanism.

Though the activities of the pyloric muscles and of the pyloric sphincter,
if not the entire gastric musculature, are due in large measure to conditions
referred to in foregoing paragraphs, nevertheless the activities of all portions
of the musculature are susceptible to modifications by the central nerve sys-
tem either in the way of augmentation or inhibition and in response to intra-
gastric stimulation. The nerves more especially concerned in the main-
tenance and regulation of the gastric tonus and 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.

The Vagi. — 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 augmentation
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 contraction 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 augmentation occurs. May states that this was always
the case in his experiments. A similar inhibition may be brought about
reflexly by stimulation 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, apparently contain both inhibitor and augmentor nerve-
fibers for the gastric musculature.

The Splanchnics. — If the splanchnic nerves are divided and the peripheral
end stimulated with induced electric currents there follows an inhibition of
the peristalsis and a loss of tone. Morat, however, has observed a primary

DIGESTION

185

opposite effect. The splanchnic nerves therefore apparently contain both
inhibitor and augmentor fibers for the gastric musculature though the inhibi-
tor fibers largely predominate. From these facts it would appear that the
gastric muscles receive both inhibitor and augmentor fibers from two differ-
ent sources.

INTESTINAL DIGESTION

The physical and chemic changes which the food principles undergo in
the small intestine, and which collectively constitute intestinal digestion,
are complex and probably more important 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 proteins, proteoses, peptones, 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 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
constituents of the chyme. As soon as this has taken place, gastric diges-
tion 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 intes-
tinal digestion.

THE SMALL INTESTINE

The Small Intestine. — This portion of the alimentary canal is a convo-
luted 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 Walls of the Small Intestine. — The walls of the intestine consist
of four coats: viz., serous, muscle, submucous, and mucous.

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 de-
veloped 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 numerous, 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-cohc sphincter. It is
usually in a state of tonic contraction and regulates the passage of materials from

i86 TEXT-BOOK OF PHYSIOLOGY

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 muscle
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 valvulae 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 structures 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 gastric veins, forms the portal
vein. The nerve elements in the intestinal wall consist of two plexuses, one
(Auerbach's) lying between the muscle coats, the other (Meissner's) lying in the
submucous coat. To this nerve net, composed of nerve cells and nerve processes,
found in connection with the muscle coats of the stomach, of the small and of the
large intestine as well, the term myenteric plexus has been given. The lymphatics,
which originate in the mucous and 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 (Brunner's) and the intestinal
(Lieberkuhn'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 functions 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.

The surface of the mucous membrane presents throughout its entire extent
fine filiform or conical processes termed villi. The structure and function of the
villi will be considered in connection with the absorption of food materials.

The pancreas and liver are developed, during embryonic life, from the walls of
the intestine and are anatomically and physiologically associated with it.

PANCREAS

The Pancreas.— -This gland is long, narrow and flattened and is situated
deep in the abdominal cavity, lying just behind the stomach. It measures

DIGESTION

187

from fifteen to twenty centimeters in length, six in breadth, and two and a
half 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 of the
duodenum; the tail is directed to the left side and extends as far as the spleen

Pancreatic ducts. Common bile-duct-

Tail.

FIG. 79.— PANCREAS AND DUODENUM REMOVED FROM THE BODY AND SEEN FROM
BEHIND. THE GLAND is CUT TO SHOW THE DUCTS.— (Landois and Stirling.)

(Fig. 79). 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 in-

crea ses in size until it measures
about two or three millimeters
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
FIG. 80. FIG. 81. plexus.

ONE SACCULE OF THE PANCREAS OF THE RABBIT Histologic Structure.— In

IN DIFFERENT STATES OF ACTIVITY. Fig. 80. — 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 rilled with

fine granules. Fig. 81 . — 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. — (Kuhne and Lea.)

its structure the pancreas re-
sembles the salivary glands. It
consists of a connective-tissue
framework which divides the
gland tissue into lobules. Each
lobule is composed 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 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 their cen-
tral and peripheral ends (Fig. 80). 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.

i88

TEXT-BOOK OF PHYSIOLOGY

During the intervals of digestion the granular layer is very deep and oc-
cupies almost the entire cell; after active digestion the granular layer is
very narrow, while the clear zone is largely increased in depth (Fig. 81).
The blood-vessels of the pancreas are arranged around the acini in a
manner similar to that observed in the salivary glands. The ultimate ter-
minations of the nerves in the epithelium are probably by means of the
usual end-tufts.

The Islands ofLangerhans. — Throughout the body of the pancreas 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 colum-
nar bodies, seen in cross-section in Fig. 83, have been named, after their
discoverer, the islands of Langerhans.

Embryologic investigations have shown that these cells are outgrowths
from the primitive acini, to which they remain attached for some time by

FIG. 82. — SECTION OF HUMAN
PANCREAS, INCLUDING SEVERAL ACINI
AND Two DUCTS. THE CELLS PRE-
SENT A CENTRAL GRANULAR AND A
PERIPHERAL CLEAR ZONE. — (Piersol.}

FIG. 83. — SECTION OF HUMAN PAN-
CREAS SHOWING, a, a, ISLAND OF
LANGERHANS, AND b, THE USUAL ACINI. —
(Pier sol.)

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 carbohydrate
metabolism it has been suggested that the islands of Langerhans have a func-
tion 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 pancreas will be referred to in a subsequent
chapter.

Pancreatic Juice. — The pancreatic juice may be obtained by intro-
ducing 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

DIGESTION

189

the distal end of the cannula, when it can be collected. According to
Bernard, normal juice can be obtained only during the first twenty-four
hours of the experiment. The juice obtained from a temporary 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 completely 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 determined
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.

Human pancreatic juice obtained from a fistula of the duct was found to
be clear and limpid, resembling water, alkaline in reaction and with a sp. gr.
of 1.007. The total solids of two specimens amounted to about 1.270 and
1.244, grams in 100 grams of the juice. The amount of juice collected
varied from 420 c.c. to 884 c.c. daily.

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 considerably during this
period. Shortly after the 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 blood-supply is very much increased, from a dilatation of the blood-
vessels.

The secretion and discharge of the pancreatic juice is associated with
the introduction of food into the mouth and stomach and its early passage into
the duodenum and is brought about by the action of a primary and a second-
ary stimulus.

The primary stimulus is a psychic state according to Pavlov induced by
the sight, odor and taste of food and which leads to the discharge of nerve
impulses from nerve-cells in the medulla oblongata and their transmission by
efferent nerves in the trunk of the vagus nerve, to the cells of the acini. It is
probable that the impressions made by the food on the terminal filaments of
the afferent fibers in the vagus nerve develop nerve impulses which, when
transmitted to the medulla, occasion the discharge of nerve impulses that not
only excite the secretion but increase the blood-supply as well. The vaso-
motor nerve impulses reach the blood-vessel supplied to the gland, by
way of the great splanchnic nerve and the post-ganglionic fibers from the
semilunar ganglion. That the vagus nerve contains secretor fibers for the
pancreas has been established by Pavlov. This investigator states indeed
that the vagus nerve contains two classes of fibers for the pancreas, secreto-
motor and secreto-inhibitor, as well as vaso-dilatator fibers for the blood-
vessels, and therefore the effects of stimulation are often contradictory and
confused, but if the nerve be divided and time given for the degeneration of
the secreto-inhibitor and vaso-dilatator nerves, usually a period of four or
five days, then stimulation of the peripheral end of the nerve with induce*
electric currents is followed after a latent period of two to three minutes by

190 TEXT-BOOK OF PHYSIOLOGY

a discharge of the juice. Stimulation of the splanchnic nerve under similar
conditions also gives rise to a secretion.

Inasmuch as various agents, such as mineral and organic acids, placed
on the duodenal mucous membrane excite the flow, it is quite possible that
the passage of the acid contents of the stomach through the duodenum
acts as a powerful stimulus to this nerve mechanism. But as the secretion
and discharge of the juice is excited by the same conditions after the division
of all related nerves, other explanations have been sought for and found
in a secondary stimulus discovered by Bayliss and Starling.

The secondary stimulus is chemic in character and developed in the
glands of the mucous membrane of the duodenum by the action of the acids
of the chyme, that is, of the digested foods, coming through the pylorus.

These investigators made the discovery that if an extract of the gland
portion of the duodenal mucous membrane, made with hydrochloric acid 0.4
per cent, is injected into the blood it evokes a profuse discharge of pancreatic
juice. As hydrochloric acid alone will not produce this effect they assumed
that the extract contained an agent that excited or aroused the pancreas to
secretor activity and to which therefore they gave the name secretin. This
agent resists the temperature that usually destroys enzymes and therefore
is not regarded as a member of this class of agents. Since hydrochloric acid
appears to be necessary to the development of secretin, the further assumption
has been made that it is a derivative of a preexisting compound to which the
name prosecretin is given. The secretin thus developed is absorbed into the
blood and carried eventually to the pancreas and brought into relation with
the cells on which it exerts its stimulating action. To an agent of this class
Starling has given the name hormone.

Histologic Changes in the Cells during Secretor 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 interv als 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 relatively 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
Kuhne 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
disappear gradually as the secretion was poured out, while the outer zone
increased in width until almost the entire cell became clear and homogeneous.
(See Fig. 81.) After secretion ceased the granules again made their appear-
ance, the result, in all probability, of metabolic activity.

Physiologic Action of Pancreatic Juice.— Experimental investi-
gations have demonstrated the fact that pancreatic juice is the most complex
in its physiologic action of all the digestive fluids. By virtue of its contained
enzymes, pancreatic juice acts:

i. 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

DIGESTION i9I

transformed into maltose, passing through the intermediate stage of dextrin.
The process is in all respects similar to that observed in the digestion of
starch by saliva. Pancreatic juice, however, 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.

2. On protein. When protein compounds are subjected to the action of
artificial pancreatic juice, they are transformed into peptones which do not
differ in essential respects from those formed by the action of 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 and become translucent,
as it does when treated with hydrochloric acid and pepsin. On the con-
trary, it becomes corroded on the surface, fragile, and hi a short time under-
goes solution. The first product is a compound termed alkali-protein.
After solution has taken place, various chemic changes are initiated which
eventuate in the production of peptone. The intermediate stages in this process
have not been satisfactorily determined. At no time during artificial
pancreatic digestion is there any evidence of the presence of the primary
proteoses. The secondary proteoses, however, are usually present.

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. When proteins are artificially digested for a long
time there appears a number of compounds such as leucin, tyrosin, aspartic
acid, arginin, etc., which are representatives of a group of simple chemical
substances known as amino-acids. It would appear that the pancreatic
juice had the power under such circumstances of reducing the proteoses or
the peptones to their ultimate constituents. Whether this is due to the action
of trypsin or to the action of another enzyme erepsin is not very clear.

The'action of trypsin on proteins in an alkaline medium may be illustrated
by the following scheme:

Protein

Alkali-protein

Proteoses secondary

Peptone

Leucin Tyrosin Aspartic acid Arginin Ammonia

The view that the final stage in the digestion of proteins is the formation
of peptones, which in due time are absorbed and synthesized into blood
albumin, has been generally abandoned for there is an ever increasing
evidence that the final stage is the formation of the nitrogen-holding com-
pounds above mentioned; in other words, that the cleavage of the proteii
is far more complete than has heretofore been assumed. Indeed :
believed that they are reduced, if not to their ultimate constituents, the
amino- and diamino-acids, at least to one or more of the different polypepti

IQ2 TEXT-BOOK OF PHYSIOLOGY

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 compounds,
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, ammonia, etc., which in turn are
absorbed and transported by the blood direct to the tissues. The discovery
by Vernon of erepsin in pancreatic juice lends further support to this view.

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.
(38°C.), it will at the end of an hour or two be partially decomposed into
glycerin and the particular fat 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:

C3H5(C18H3302)3 + 3H20 = 3C18H3402 + C3H5(OH)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 fat 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 + C18H3i02 - 2NaC18H8302 + H2CO3

Coincident with the formation of the soap, the remaining portion of the
neutral oil will undergo division into globules of microscopic size, which are
held in suspension in the soap solution, forming what has been termed an
emulsion, which is white and creamy in appearance. The cause of this
minute subdivision of the fat and the necessity for it is unknown. It may
be assumed that by virtue of the subdivision a greater surface is exposed to
the action of the pancreatic enzyme and the digestion of the fat thereby
facilitated. 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 spontane-
ously. 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 is large, practically all of the fat undergoing this decomposition, with
the formation of soap and glycerin prior to their absorption.

According to Pavlov 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, protein, 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 positively known that this is the case
only with trypsin. This enzyme is a derivative of the zymogen, trypsinogen,
the production 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

DIGESTION I93

the former is accomplished, according to Pavlov, by a special activating
ferment secreted by the epithelium of the small intestine and termed entero-
kinase.1

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.

Intestinal Juice.— This fluid is a product of the activities of the cells
lining the follicles or glands of Lieberkiihn. Owing to its admixture with
other secretions and to the profound disturbance of the digestive function
caused by the establishment of intestinal fistulas, this fluid has rarely been
obtained 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 physi-
ologists, 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 i.oio. Chemic analysis
reveals the presence of proteins, mucin, and sodium carbonate.

The intestinal juice obtained by Tubbey and Manning from a small
portion of the human intestine (ileum) was opalescent, occasionally 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 proteins and mucins.

Physiologic Action of the Intestinal Juice. — The part played
by the intestinal juice in the digestive process is yet a subject of discussion,
as the results obtained by different observers are in some respects con-
tradictory, due to the fact that animals, as well as human beings, have been
the subjects of experimentation. By reason of its contained enzymes the
intestinal juice acts:

i. On Proteases and Peptones.— Even though the proteins have been reduced
by the action of the gastric and pancreatic juices to the stage of proteoses
and peptones they are not yet in a condition to be absorbed. The
further stage in their digestion appears to be their reduction, as stated"
in a foregoing paragraph, to amino-acids or their immediate anteced-
ents. This change has been attributed to the action of the intestinal
juice or a contained enzyme to which the name erepsin was given by its
discoverer Cohnheim.

1 An activator may be defined as an agent, which secreted by some one organ, is capably of
converting an inactive agent, secreted by some correlated organ, near or remote, into an active
agent by some chemical interaction. Thus the antecedents of many enzymes, either before or
after their discharge by gland cells are inactive and require to be modified in some unknown way
before they become functionally active. The term activator is usually applied to inorganic
agents, the term kinase to organic agents.
13

i94 TEXT-BOOK OF PHYSIOLOGY

2. On Compound Sugars. — Saccharose, maltose and lactose, the three

compound sugars, are believed by most observers to be not only non-
absorbable, but also non-assimilable and therefore are required to
undergo some digestive change before they can be absorbed and assimi-
. lated. An extract of the intestinal mucous membrane or the intestinal
juice of the dog added to a solution of saccharose will cause it to com-
bine chemically with water after which a cleavage into dextrose and
levulose will take place, which together constitute invert sugar. The
enzyme to which this action is attributed has been termed invertase or
saccharase. Maltose undergoes a similar change. After its combina-
tion with water it undergoes a cleavage into two molecules of dextrose.
Lactose appears to be unaffected by the pure juice. As it is non-assimi-
lable it has been supposed to undergo conversion into dextrose and
galactose while passing through the epithelial cells of the intestinal mu-
cosa. In either case the transformation is brought about by two fer-
ments known respectively as maltase and lactase.

3. On Trypsinogen. — This zymogen when first discharged from the pan-

creatic duct is inactive and incapable of effecting the necessary digestive
changes in the proteins. Shortly after its entrance into the intestine, it
becomes quite active and efficient, a change attributed to an agent
entero-kinase secreted by the mucosa in the upper part of the intestine.

THE LIVER

The Liver. — This highly vascular conglomerate gland is 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 various functions will
be deferred to a subsequent chapter. In this connection only the bile, its
physical properties, chemic composition, and physiologic action in relation
to the digestive process, will be considered. This fluid is a product of the
secretor activity of the liver cells. As it is poured into the intestine in man
and most mammals at a point corresponding to the orifice of the pancreatic
duct, and most abundantly at the time the food is passing through the duo-
denum, 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 unite finally to form 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 extremity of which expands into a
pear-shaped reservoir, the gall-bladder in which the bile is temporarily
stored. The duct formed by the union of the hepatic and cystic ducts, the
common bile-duct, 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 into a small receptacle, the ampulla
of Vater. The ampulla in turn opens on a small papilla into the intestine.
The walls of the biliary passages are composed of a mucous membrane
internally, a fibrous and muscular coat externally. The termination of the
common bile-duct is provided with a distinct band of circularly disposed

DIGESTION

muscle-fibers, which when in action completely close the orifice and prevent
the discharge of bile. It may therefore be regarded as a true sphincter
muscle, the structure and function of which were first pointed out by Oddi
Small racemose glands are embedded in the mucous membrane of the main

Physical Properties.— The bile obtained directly from the liver through
a cannula inserted into 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 the viscidity depending on the length of time it
remains in this reservoir. The specific gravity of human bile varies within
normal limits from i.oio to 1.020. The reaction is invariably alkaline 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-protein.

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 car-
nivorous animals it is orange or brown. In man it is green or a golden
yellow. The colors are due to the presence of pigments. Microscopic
examination fails to show the presence of structural elements.

Chemic Composition. — Human bile obtained from an accidental biliary
fistula was shown by Jacobson to contain the following ingredients, viz.:

COMPOSITION OF HUMAN BILE

Water 911.**

Sodium glycocholate p.^4

Sodium taurocholate a trace

Cholesterin 0.54

Free fat O.IO

Sodium palmitate and stearate i .36

Lecithin ; o .04

Organic matter, and pigments bifirubin and biliverdin 2 .26

Sodium chlorid 5.45

Potassium chlorid 0.28

Sodium phosphate i .33

Calcium phosphate 0.37

Sodium carbonate 0.93

In this analysis the solid ingredients constitute 22.6 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 ingredients
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 influence of hydrolizing agents, such as
dilute acids and alkalies, both acids will undergo cleavage into their re-

196 TEXT-BOOK OF PHYSIOLOGY

spective components — e.g., glycocoll and cholic acid, taurine and cholic acid.
Glycocoll and taurine are crystallizable nitrogenized compounds known
chemically as amido-acetic and amido-isethionic acids respectively.

From the results of the hydrolysis of the bile acids it may be inferred that
their formation in the liver cell is the result of an opposite process, viz. : a
synthesis of glycocoll and taurin with cholic acid. Glycocoll is an amino-
acid and there is much evidence that taurin is a derivative of cystin, another
amino-acid, both found in the small intestine. The origin of cholic acid is,
however, not so clear. The absorption of the foregoing compounds from
the intestine and their transmission to the liver cells by the portal vein fur-
nishes them with the necessary materials for the formation of the bile acids.
During the period the bile remains in the biliary passages, the biliary salts
hold cholesterin in solution.

There is also good evidence for the view, that after their discharge into
the intestine, the bile salts are absorbed, with the exception of a portion de-
stroyed by bacteria, and carried by the portal vein to the liver and again
excreted. By this circulation from liver to intestine and from intestine to
the liver, the work of the liver cells in the synthesis or secretion of bile acids,
is supposed to be reduced to a minimum. It is also probable that a portion
of the acids enters the general circulation and influences favorably the gen-
eral nutrition. It is stated by some investigators that the activities of the
liver cells are decidedly increased by the circulation of the bile salts and that
they are to be regarded as the natural stimuli to the secretion.

The presence of the bile salts can be demon-
strated by the employment of Pettenkofer's test
or reaction. It was shown by this investigator
that if to a solution of bile salts a small quantity
of a 10 per cent, solution of cane-sugar be added
and subsequently a small quantity of strong sul-
phuric acid, a brilliant red color appears which
soon passes into a rich purple. To secure the
best results in the performance of this test care
Stirling.} should be exercised to keep the temperature below

7o°C.; the characteristic colors appear to be due

to the action of the sulphuric acid on the cane-sugar by which a sub-
stance, furfurol, is produced, which in turn reacts with the cholalic acid.
This test can be applied to bile directly; thus if to bile in a test-tube cane-
sugar be added and the mixture thoroughly shaken, a portion of the bile
becomes quite frothy. If now sulphuric acid be carefully added, the red
and purple colors present themselves at once in the white froth — an indica-
tion that the bile salts are distributed through it.

Cholesterin. — Cholesterin can be obtained directly from bile or better
from white gall-stones by the employment of chemic methods described in
works on chemistry. When obtained in the pure state it undergoes crystal-
lization when it presents itself in the form of flat rectangular crystals which
are soluble in ether and hot alcohol. In the bile it is held in solution by
the bile salts. Chemic analysis shows that it is a monatomic unsaturated
alcohol with the formula CarH^OH. Cholesterin, though a constant con-
stituent of bile, is not confined to this fluid as it has been shown to be a
normal constituent of all animal and vegetable cells, though it is particularly

DIGESTION IQ7

abundant in the myelin of nerve-fibers. Though cholesterin has for a long
time been regarded merely as one of the products of the katabolism of living
material, it has come to be believed that it is necessary to the vitality of tissue
cells and especially to the blood cells. Entering into the composition of
the surface layer of cells, it prevents the entrance of certain toxins which
would have a destructive influence on their structure or composition. In
the metabolism of cells it is set free after which it passes into the blood to be
secreted by the liver. In the bile it frequently undergoes crystallization and
forms one of the forms of gall-stones. In the intestine it is converted into
stercorin and discharged 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 regarded as a derivative of
hematin, one of the cleavage products of hemoglobin, the coloring-matter
of the blood. In the liver the hematin 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 intes-
tine the bile pigments are finally reduced to hydrobilirubin or an allied sub-
stance, stercobilin, which becomes one of the constituents of the feces. A
portion of the latter is absorbed into the blood and ultimately discharged
into the urine where it is known as urobilin. The two substances are re-
garded as identical. 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 porce-
lain 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 gener-
ally as a white crystalline powder, though very frequently as a white waxy
mass which is soluble in ether and alcohol. Its chemic formula is C44H90-
NPO9. 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. Lecithin has been regarded as one of the decomposition
products of nerve- tissue, removed from the blood by the liver and thus be-
coming one of the constituents of the bile, in which it is held in solution by
the bile salts. Lecithin can be readily decomposed by various agents yield-
ing glycophosphoric acid, a fat acid and cholin.

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 determined 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 condi
tions of secretion therein are different from those in any other secretor
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

i98 TEXT-BOOK OF PHYSIOLOGY

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 considerable
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 discharged 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 retained until required for digestive purposes.
When acidulated food passes over the surface of the duodenum, there is an
increase in the secretion or at least the discharge of bile, and as this takes
place after the nerves distributed to the liver are divided, the assumption is
that an agent, possibly secretin, is developed in the duodenal mucous mem-
brane, which, absorbed into the blood, is ultimately distributed to' the liver
cells and by which they are excited to activity. At the same time there is
excited, through reflex 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 Influence of the Nerve System. — The storage and the discharge
of bile, brought about by the alternate contraction and relaxation of the
muscle walls of the gall-bladder and of the sphincter are regulated by the
nerve system. As the result of his experiments Doyon concludes, that dur-
ing the intervals of intestinal 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-badder, 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 con traction* 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 amylolytic or amyloclastic power of the pancreatic
juice is almost doubled and the same is true for its proteoclastic power,
while its lipoclastic or fat-splitting power is tripled.

The bile salts also dissolve insoluble soaps which may be formed during
digestion and thus favors the digestion of fat. If it be excluded from the
intestine 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

DIGESTION IQ9

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 fat acids and glycerin, with the formation of soaps, as
seems probable, the action of the bile becomes more apparent from the fact,
already stated, that it dissolves and holds in solution the soaps so formed
which would be necessary to their absorption by the epithelial cells. This
action has been attributed to the presence of the bile salts. As an aid to
digestion the bile has been regarded as important, for the reason that its
entrance into the intestine 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 body, itself
undergoing putrefactive changes very rapidly, it has been believed 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 is prevented from entering
the intestine there is an increase in the formation of gases and other products
which impart to the feces certain characteristics which are indicative of
putrefaction. As to the manner in which bile retards this process nothing
definite can be stated. It has been supposed to be a stimulant to the
peristaltic movements 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 evidence 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 influences 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 operation the abdomen was tumid and there was some rum-
bling 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 offensive. 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 operation. 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 about a pound and a half of beef-heart daily, but always refused fat.
There was an absence at all times of jaundice, fetor of the breath, and fall-
ing 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 intestine. The results of this and similar cases go to show that that por-
tion 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 becomes

200 TEXT-BOOK OF PHYSIOLOGY

fetid and there is a falling of the hair, showing some profound disturbance
of the general nutritive process.

The Movements of the Small Intestine. — During the period of intes-
tinal digestion, the walls of the intestine exhibit a series of movements which
triturate the food, mix it with the intestinal secretions, gradually transfer it
from the upper to the lower portions and promote the absorption of the pre-
pared food materials. The movements of the small intestine 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 contents and thereby the movements of the
intestinal walls visible on the fluorescent screen. There investigations re-
vealed the presence of two forms of activity, one of which is more or less
stationary and due to rhythmic contraction of circular muscle-fibers, the other
progressive, 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 contents into
small segments and for this reason was termed by Cannon rhythmic seg-
mentation; the latter activity is the well-known peristaltic wave.

Rhythmic Segmentation. — When the abdominal cavity is investigated by
the method above mentioned, it is observed that after the food has passed
into the intestine and formed a more or less consistent mass of variable
length, bands of circular muscle-fibers, situated at regular distances one
from another, begin to contract and divide a mass of food into segments,
after which they at once relax to be 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. 85). The lower
half of each segment then unites with the upper half of the segment of food
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 contractions make their appearance it would
seem that the mere presence of a segment of food 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 food forward to any
marked extent. In the cat the segmentation may proceed at the rate of
thirty divisions a minute.

Peristalsis. — After the food has been prepared by the process described
in the foregoing paragraph, 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 the mass of food and a relaxation
of the fibers in advance of it. The result is a movement forward of the food,
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 peristaltic wave
is usually extremely slow except in the duodenal region where it is quite rapid.

The first of these two movements to make its appearance is the peristaltic
which arises at the upper portion of the duodenum from which it sweeps
downward with considerable rapidity, carrying with it the food which has
been discharged from the stomach and which has accumulated in this region.
From the appearance, as shown by Rontgen ray examination, which the

DIGESTION

201

extreme upper portion of the duodenum presents, due to its distension by
food, it has been designated, the cap or pilleus ventriculi.

After the peristaltic wave has advanced the food a variable distance, it
disappears and the food comes to rest. By this procedure the incoming food
from the stomach is readily accommodated in the duodenal portion of the
intestine. With the disappearance of the peristaltic wave, rhythmic seg-
mentation again arises in the portion of the intestine corresponding to the
new situation of the segment of food. This in turn is succeeded by another
peristaltic wave which advances the food to a more distant region of the
intestine. This continues until at the end of gastric digestion a more or less
continuous column of food occupies the lumen of the small intestine from the
stomach to the ileo-cecal valve.

In addition to this characteristic physiologic movement it has also been
observed by different experimenters that the intestine manifests under special
circumstances two other forms of moving waves, waves moving downward
as well as upward from their point of
origin, but without being preceded by
an inhibition or relaxation. These waves
are therefore not regarded as true peris-
taltic waves. To avoid confusion, the
term diastalsis has recently been em-
ployed (Cannon) to designate the true
peristaltic movement, viz.: progressive
contraction preceded by inhibition, and
the terms katastalsis and anastalsis to
designate the descending and ascending

Contractions respectively, that OCCUr
without a forerunning inhibition.

Rush Peristalsis.-^^ conditions

that are perhaps not Strictly physiologic, ones come in between them and divide

a rapid and far-reaching peristalsis is

developed which may paSS Over trie in-
testine, from the duodenum to the CCCUm C.

without stopping in the course of 15

seconds, in the rabbit, and which has Mechanism.")

been designated rush peristalsis. It is . .

characterized by a wave of constriction preceded by a completely inhibit*

long section of intestine. The contents of the intestine are earned along

with extreme rapidity and vigor. The contractions may be increased by

purgative salts, ergot, barium chlorid, etc., the inhibition may be increased

by calcium chlorid, magnesium chlorid, etc. A combination of ergot and

calcium chlorid develops in the rabbit a pronounced rush peristalsis (Meltzer).

This movement appears to be under the control of the central nerve system

as it fails to develop after division of the vagus nerves.

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 accompanied 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

A, surface of a portion of the intestine,
showing six constrictions which divide the

to make the new segments shown in
Reetition of this rocess results in the

202 TEXT-BOOK OF PHYSIOLOGY

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 investigations to decide.

The Nerve Mechanism of the Small Intestine. — The causes of these two
forms of intestinal activity, rhythmic segmentation and peristalsis, have been
the subject of much investigation. 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 intestines and in close relation to the muscle cells, the so-called
myenteric plexus 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 contractions are myogenic or neurogenic.

As the rhythmic contractions continue though the peristaltic are abol-
ished by the introduction of nicotin into the blood, an agent which tempo-
rarily paralyses peripheral nerve-cells, it was concluded by Bayliss and
Starling that the rhythmic contractions are myogenic and that the peristaltic
contractions are reflex in character, the coordination being carried out by
the local nerve mechanisms and initiated by stimulation of the intestine.
This observation has been corroborated by Cannon who has shown that if
the myenteric plexus is divided by incisions extending around the intestine,
at intervals of 1.5 to 2 cm. for a distance of 45 cm., incisions reaching to the
submucous coat, the peristaltic movement is totally abolished though
rhythmic segmentation develops as usual and continues for long periods.

As the segmentation activity continues after interruption of the myenteric
plexus, the inference is justifiable that it is purely myogenic and is the re-
sponse to a stimulus within the intestine such as distension by food, when
the intestinal wall possesses the requisite degree of tonicity.

Though the orderly and coordinated contractions and relaxations of the
muscle coat, which constitutes a peristaltic movement, are mediated by the
myenteric plexus, and therefore termed a myenteric reflex, nevertheless the
contraction may be augmented or inhibited by the central nerve system
through the vagus and splanchnic nerves respectively.

Stimulation of the vagus is followed by an augmentation of the contrac-
tion, though not infrequently there is a primary inhibition of short duration.
Stimulation of the splanchnic is followed by a relaxation or inhibition of the
contraction, though according to some observers there is at times an opposite
effect.

The extent to which the contraction is initiated or augmented by stimula-
tion of the vagus depends upon the extent to which the contraction at the
moment is inhibited by the splanchnic. Thus after the splanchnics are
divided, stimulation of the vagus causes a much more pronounced contrac-
tion than would otherwise be the case; a fact that indicates that the
splanchnic nerve-center is in a state of tonus or tonic activity and therefore
exerting a constant inhibitor effect on the muscle-fibers. Stimulation of the
peripheral end of the divided splanchnic causes an arrest or inhibition of
a preexisting contraction.

The nerve-centers regulating the contraction and relaxation of the
muscle walls of the intestine are doubtless excited to activity by nerve impulses
transmitted through afferent nerves, probably the vagus, from the mucous
surface of the small intestine. These centers are also influenced by nerve
impulses descending from the cerebrum, though the route they take is not

DIGESTION 203

clearly defined. It is well known that mental states markedly influence the
contraction in one direction or another.

It has also been experimentally determined that the introduction of
various acids and gases into the intestinal canal is followed by an increase in
the contraction. It is probable therefore that the gases, acids, and perhaps
other compounds as well, developed by bacterial action also act as excitants
to muscle activity.

The Large Intestine.— The large intestine is that portion of the ali-
mentary canal situated between the termination of the ileum and the anus.
It varies in length from one and a quarter to one and a half meters, in
diameter from three and a half to seven centimeters. It is divided into
the cecum, the colon (subdivided into an ascending, transverse, and descend-
ing 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 ten centimeters in length — the vermiform 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 the activity of a sphincter
muscle, the ileo-colic, the contraction of which is regulated by the nerve
system, by way of the splanchnic nerves.

The colon ascends to the under surface of the liver, where it bends at a
right angle, forming the hepatic flexure, crosses the abdominal cavity to
the spleen, forming the splenic flexure, descends to the left iliac fossa, and
bends again. 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 1 8 centimeters in length. Within three centimeters of its termina-
tion at the anus it presents a constriction formed by a circular band of smooth
muscle-fibers known as the internal sphincter. The margin of the anus is
also surrounded by bands of striated 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 surround it completely. As the longitudinal bands are shorter than
the intestine itself, its surface becomes sacculated, each sac being partially
separated from adjoining sacs by narrow constrictions. The circular fibers
are arranged in the form of a thin layer over the entire intestine. Betwet
the sacculi, however, they are more closely arranged. The sacculi have beer
termed haustra from their supposed function, that of absorbing or drawm
water from the intestinal contents thus imparting to them a certain degree
of consistency. In the rectum the circular fibers are well developed, and at

204 TEXT-BOOK OF PHYSIOLOGY

a point two or three centimeters above the anus they form, as stated above,
the internal sphincter.

The mucous membrane of the large intestine possesses neither villi nor
valvulae conniventes. It contains a large number of tubules consisting 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.

The Movements 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, protein, starches, sugars,
and fats, undergo chemic changes and are transformed into amino-acids and
peptids, dextrose, soap and glycerin, fat acids, under which forms they are
absorbed. After the more or less complete digestion 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, vegetable seeds, cellulose, etc., the quantity and variety of
which depend on the nature of the food. These substances, passing into
the large intestine along with the excrementitious matter of the bile, become
incorporated with the mucous secretions and assist in the formation of the
feces.

Under the influence of a peristaltic movement similar to that wit-
nessed in the small intestine, all this excrementitious matter, deprived 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 or anastaltic 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-peristaltic waves may force some of the liquefied contents through the
iieo-colic opening into the small intestine because of the relaxation of the ilio-
colic sphincter muscle. It is questionable if this ascending movement is a
true peristalsis inasmuch as the advancing contraction is apparently not pre-
ceded by an area of inhibition or relaxation. It resembles rather the corre-
sponding movement manifested by the small intestine to which the term
anastalsis has been given, and is propagated along the muscle coat independ-
ently of the myenteric plexus.

In addition to this anastaltic wave, haustral contractions have been ob-
served which resemble the segmentation contractions of the small intestines
and which promote, it is believed, the absorption or drawing of water from
the intestinal contents.

The Function of the Large Intestine.— The large intestine, by reason
of its anatomic relation to the small intestine, serves, as a receptacle for the
temporary storage of the indigestible residue of the food together with certain
excretions of the intestinal glands both of which have descended from the
small intestine. Inasmuch as the contents of the large intestine, in that
portion known as the ascending colon are quite liquid, while the contents

DIGESTION 205

in the portion known as the descending colon are more or less solid, it is
apparent that an absorption of liquid must take place during the transit
from the cecum to the rectum. This is made possible by the retardation of
the intestinal contents caused by the antiperistaltic or the anastaltic wave,
together with the haustral contractions. Between the two modes of activity
different portions of the intestinal contents are exposed to the mucosa and
by which the liquids are in large measure absorbed. In the distal portion
of the large intestine, the characteristic movement is peristaltic by which the
more solid contents are carried to the sigmoid flexure where they accumulate
until the desire to evacuate the mass is experienced. By the contraction of
the pelvic portion of the descending colon the mass is forced into the rectum
from which it is discharged from the body. The function of the large
intestine is therefore to receive, to reduce to a proper consistency, to tempor-
arily store and subsequently discharge its contents, consisting of the indi-
gestible residue of the food, together with excretions of intestinal glands
which have descended from the small intestine and which constitute in part
the feces.

The Nerve Mechanism of the Large Intestine.— The nerve mechan-
ism of the large intestine includes both motor and inhibitor nerves. The
motor nerves comprise both pre- and post-ganglionic fibers; the former
have their origin in the spinal cord, from which they emerge in the third and
fourth sacral nerves and pass by way of the pelvic nerve to the pelvic
ganglia around the cells of which their fibers arborize; the latter (post-
ganglionic) fibers emerge from the cells of these ganglia and are distributed
to circular and longitudinal muscle-fibers of the intestinal wall.

The inhibitor fibers also comprise both pre- and post-ganglionic fibers;
the former have their origin in the lumbar region of the spinal cord, from
which they emerge in the second to the fifth lumbar nerves; they then pass into
and through the sympathetic chain and the inferior splanchnic nerves to the
inferior mesenteric ganglion around the cells of which they arborize; the
post-ganglionic fibers pass directly to the muscle-fibers of the intestinal wall.
Stimulation of the pelvic nerve with induced electric currents causes con-
traction of the muscle-fibers; stimulation of the hypogastric nerves causes
an inhibition of the contraction.

Intestinal Fermentation. — Owing to the favorable conditions in both
the small and large intestine for fermentative and putrefactive processes—
e.g., heat, moisture, oxygen, and the presence of various microorganisms-
some of 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.^ Dex-
trose and maltose are partially reduced to lactic acid; this to butyric acid,
carbon dioxid, and hydrogen. Fats are reduced to glycerol and fat acids, the
glycerol, according to the organisms present, yields succinic acid, carbon
dioxid, and hydrogen. Some of the protein derivatives — the amino-acids—
may be reduced to still simpler compounds under the action of bacteria.
Lysin, through the loss of a molecule of CO2, gives rise to cadaverin; orni-
thin to putrescin; tryptophan yields indol, skatol; tyrosin yields phenol and
probably cresol.

Indol.— This compound is of especial interest as it is the antecedent of
indican, found in the urine. Though it is in part discharged in the feces, it

2o6 TEXT-BOOK OF PHYSIOLOGY

is in part absorbed into trie portal blood and carried direct to the liver where
it is oxidized to indoxyl and combined or conjugated with potassium sulphate
forming the salt potassium indoxyl sulphate or indican, after which it enters
the blood, is carried to and eliminated by the kidneys. The presence of this
salt in the urine can be demonstrated by adding hydrochloric acid with a
small quantity of potassium chlorate; after this is done the indican combines
with water and undergoes a cleavage into indoxyl and potassium sulphate;
the former then combines with oxygen and gives rise to indigo blue. The
extent to which the indican is present is taken as a measure of the extent of
intestinal putrefaction.

Skatol. — This compound is also a derivative of the protein molecule, the
result of bacterial decomposition. It passes in part into the feces and gives
to them the characteristic odor. It is also in part absorbed into the portal
blood and carried direct to the liver where it is oxidized to skatoxyl, after
which it combines with potassium sulphate to form potassium skatoxyl sul-
phate after which it .enters the blood and is carried to the kidneys to be elimi-
nated in the urine.

Phenol and Cresol. — These compounds, also derivatives of the protein
molecule, are absorbed into the portal blood and undergo a similar conjuga-
tion. They too are finally eliminated by the kidneys.

The Feces. — The feces is a term applied to the mass of material ejected
from the rectum through the anus. They are characterized by consistency,
color and odor. The origin and the nature of this material have both a
physiologic and a clinic interest.

The consistency varies from day to day from liquid to solid, depending
partly on the character of the food, the rapidity with which it is transported
through the intestine and hence the extent to which absorption of water in
the large intestine takes place. On a meat diet the consistency is firm; on a
vegetable diet it is apt to be soft. The amount discharged from day to day
on a mixed diet varies from 120 to 170 grams containing from 30 to 42 grams
of dry matter. On a meat diet alone the quantity diminishes; on a vege-
table diet, especially if the articles of food are rich in cellulose, the quantity
will increase considerably beyond the customary amount.

The color on a mixed diet varies from a light yellow to black. The
usual brown color is due to the pigment urobilin or stercobilin, a derivative
of the pigments of the bile. On a meat diet the color deepens until it becomes
quite black due to the presence of sulphid of iron, the result of the union
of sulphuretted hydrogen with the iron derived from hematin contained in
the meat. On a vegetable diet the color lightens and may become slightly
yellow. If the contents of the intestine are carried forward too rapidly, the
time may be insufficient for a complete reduction of the bile pigments, hence
they appear in the feces imparting to them a green color. If there is an
obstruction to the discharge of bile into the intestine the feces may become
yellow or clay-colored.

The odor is characteristic and due to the presence of skatol and allied
bodies produced by the putrefaction of proteins by bacterial action. Sul-
phuretted hydrogen also contributes to the odor.

The chemic composition of the feces is complex. They consist of water,
mucin, an indigestible residue of food, decomposition products, excretions
from the intestinal glands, and inorganic salts. The residue of the food

DIGESTION 2o;

usually consists of the denser portions of the connective tissue of meats and
the cellulose of vegetables and cereals. When the latter are eaten in large
amounts the cellulose residue is increased and by its mechanic stimulation
increases the peristalsis and hastens the transfer of the feces through the
intestine. The decomposition products are derived from protein, fat, and
carbohydrate food by bacterial action and include skatol, indol, fat acids,
soaps, xanthin, ammonia, sulphuretted hydrogen, etc. The excretion from
the intestine itself contributes a considerable portion to the fecal mass. The
inorganic salts include phosphates of calcium and magnesium together with
various sodium and potassium compounds.

Defecation. — Defecation is the final act of the digestive process and
consists in the expulsion of the indigestible residue of the food and its asso-
ciated compounds 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 sphincter muscles and a contrac-
tion of the longitudinal and circular muscle-fibers, in consequence of which
the feces are expelled. These complex muscle actions are also aided by the
voluntary contractions of the diaphragm and abdominal muscles.

Nerve Mechanism of Defecation. — The act of defecation is primarily
reflex though somewhat influenced by voluntary efforts. The reflex charac-
ter of the act is especially noticeable in young children in whom by reason of
the imperfect development of the brain there is a lack of volitional control.
During the intervals of defecation the anal orifice is tightly closed by the
tonic contraction of the internal non-striated sphincter and the external
striated sphincter muscles, thus preventing the escape of gases or semi-liquid
material. The tonic contraction of both muscles is maintained by the
activity of nerve-centers located in the lumbar region of the spinal cord.
The circular and longitudinal fibers of the rectum proper are at the same
time in a relaxed or inhibited condition, the result of an inhibition, or a want
of stimulation, of their governing nerve-center or centers in the lumbar region
of the spinal cord. When the desire to evacuate the bowel is experienced,
impressions are being made by the feces on the afferent nerve endings in the
mucous membrane of the sigmoid flexure and of the rectum. The nerve
impulses thus developed are transmitted to the defecation or rectal nerve-
centers in the spinal cord and to the cerebrum and influence in one direction
or another their activities.

In the young child the arrival of the transmitted impulses in the spinal
cord is immediately followed by an inhibition of the sphincter centers and a
stimulation of the rectal muscle centers, as a consequence of which, the
sphincter muscles relax and the expulsive muscles contract thus discharging
the feces.

In the adult if the- act of defecation is to be permitted the same mechan-
ism is brought into action. In their expulsive efforts, these latter muscles
are assisted by the contraction of the diaphragm, abdominal, and other
muscles in response to volitional efforts. After the expulsion of the feces,

208 TEXT-BOOK OF PHYSIOLOGY

there is a return to the former condition, namely, a relaxation or inhibition
of the rectal muscles and a contraction of the sphincter muscles. If the act of
defecation is to be suppressed, the controlling influence of the nerve-center
on the contraction of the external sphincter may by an act of volition be
strengthened and the action of the reflex mechanism for a while antagonized.
The nerve mechanism therefore involves both efferent and afferent nerves
as well as nerve centers in the lumbo-sacral region of the spinal cord.

Efferent Nerves. — The efferent nerve-fibers for the external sphincter mus-
cle have their origin in the spinal cord from which they pass by way of the
third and fourth sacral nerves, the pelvic nerve and the inferior hemorrhoidal
nerve directly to the muscle.

The efferent nerve-fibers, for the longitudinally and circularly arranged
muscle-fibers of the rectum, including the specialized portion, the internal
sphincter, have their origin in nerve-cells in the lumbo-sacral region of the
spinal cord and pass to their destination by two paths. The fibers in the
first path leave the spinal cord by way of the second to the fifth lumbar
nerves, then pass into and through the sympathetic chain, through the inferior
splanchnics to the inferior mesenteric ganglion around the cells of which
their terminal branches arborize; from the cells of this ganglion new fibers
emerge which pass through the hyppgastric nerves to the muscles. The
fibers of the second path leave the spinal cord by way of the second to the
fourth sacral nerves, then pass into the pelvic or erigens nerve to small
ganglia along the sides of the rectum around the cells of which their
terminal branches arborize; from the cells of these ganglia new nerve-
fibers emerge which pass directly to the muscles. In both paths the nerves
coming from the cord are pre-ganglionic, those coming from the ganglia,
post-ganglionic.

The central mechanism that excites and coordinates the activities of the
rectal muscles is situated in the lumbo-sacral region of the spinal cord and
is designated the recto-anal center.

The Afferent Nerves. — The afferent nerve-fibers that excite the central
mechanism to activity are contained in the dorsal roots of the spinal nerves.
Although the anatomic relations of the various nerves composing this mechan-
ism are fairly well known, their physiologic actions are n6t clearly defined.
The results of experimental methods of investigation are neither uniform nor
in accord. The want of accord lies partly in anatomic peculiarities of the
animal the subject of the investigation, and partly perhaps in the character
of the stimulus employed.

Stimulation of the pelvic nerve causes, in the dog at least, a peristalic
contraction of the circular fibers of the rectum. Stimulation of the hypo-
gastric nerve causes an inhibition or relaxation of the circular fibers of the
rectum and of the internal sphincter as well. Inasmuch as these two groups
of fibers have opposite functions it may be assumed that the nerve centers
controlling them, both motor and inhibitor, are double centers and that they
can be made to act separately and alternately.

Recalling the events that take place, it may be assumed that peripheral
stimulation of the afferent nerves develops nerve impulses which, when
transmitted to the cord cause i, a stimulation of the motor center, a dis-
charge of nerve impulses through the pelvic nerve to the rectal muscles
calling forth a contraction; 2, a stimulation of the inhibitor center, a dis-

DIGESTION 209

charge of nerve impulses through the hypogastric nerve to the internal
sphincter and perhaps the external sphincter as well, calling forth their
relaxation or inhibition. With the discharge of the feces the former condi-
tion is re-established. A stimulus, the nature of which is not fully known,
causes a stimulation of the inhibitor center for the rectal muscles and a
stimulation of the motor center for the sphincters, the nerve impulses reaching
the muscles through the hypogastric and pelvic nerves respectively.

CHAPTER XI
ABSORPTION

Absorption is a process by which nutritive material from the tissue
spaces, from the serous cavities, from the interior of the lungs and from the
mucous surfaces of the body, and waste materials from the tissues are trans-
ferred into the blood.

The absorption of nutritive materials from the tissue spaces and from the
serous cavities may be regarded as an act of resorption or a return to the
blood of nutritive material which has passed across the walls of the capil-
lary blood-vessels in excess of that needed for purposes of nutrition, and
which if not returned would lead to an accumulation and the development of
edematous conditions; the absorption of oxygen from the lungs is essential
to the maintenance of nutritive activity, to the oxidation of foods and the
liberation of their energy ; the absorption of new nutritive materials from the
mucous surfaces of the entire alimentary canal, but more especially from
that of the small intestine, materials that have been produced out of the
foods by the digestive process, is essential to the maintenance of the
quantity and quality of the blood.

The absorption of the products of metabolism, of carbon dioxid, urea and
other nitrogen-holding compounds from the tissues into the blood is essential
to the continuance of their activities as well as a necessary preliminary to
their elimination from the body.

The anatomic mechanisms involved in the absorptive process are, pri-
marily, the tissue or lymph- spaces, the blood- and lymph-capillaries; second-
arily, the blood-vessels (veins,) and the lymph-vessels.

Tissue or Lymph-spaces. — Everywhere throughout the body, in the
connective-tissue system and in the interstices 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. 86). 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 surrounding blood-
vessels, all of which contain a greater or less quantity 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

210

ABSORPTION

211

surfaces of these cavities, however, are covered with a layer of endothelial
cells with sinuous margins. At intervals between these cells are to be found
small free openings which have received the name of stomata.

The Blood-capillaries. — The blood-capillaries not only permit of a
transudation of the liquid nutritive material from the blood across their
delicate walls, but are also engaged, if not in the resorption of a portion of
this transudate, at least in the absorption of waste products resulting from
tissue metabolism.

The Blood-vessels. — The blood-vessels — the venules — that arise from
the capillary-vessels gradually converge and unite to form veins which by
their union ever increase in size until they terminate in the inferior and
superior vena cava, both of which at their terminations communicate with
the right side of the heart. The blood containing the waste products ab-
sorbed from the tissues, is con-
ducted by these veins to the
right side of the heart, thence
by the pulmonic artery into the
lungs, where the carbon dioxid
is eliminated; the blood carrying
the remainder of the waste prod-
ucts is then conducted by the
pulmonic veins to the left side
of the heart by which it is dis-
charged into the arteries and dis-
tributed in part to the kidneys,
liver, skin and intestine; by these
organs the remaining waste prod-
ucts are excreted from the
blood.

The Lymph-capillaries.—
The lymph-capillaries in which
the lymph-vessels proper take
their origin are arranged in the
form of plexuses of quite ir-
regular shape. In most situa-
tions they are intimately inter-
woven with the blood-vessels, blood-vessels.— (Landois and Stirling.-)

from which they can be readily

distinguished by their larger caliber and irregular expansions. The wall of
the lymph-capillary 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. It was formerly believed
that the communication of the lymph-capillary with the tissue space was
a direct one, the lymph flowing from the latter into^the former through an
open passage-way. Recent investigation would indicate that this histologic
arrangement does not exist but that on the contrary the lymph-capillaries
are closed vessels and that the tissue space and the interior of the lymph-
capillary are separated one from the other by a thin partition of endothelial
cells. As the shape, size, etc., of both lymph-spaces and capillaries are

FIG. 86.— ORIGIN OF LYMPH-VESSELS FROM THE
CENTRAL TENDON OF THE DIAPHRAGM STAINED
WITH NITRATE OF SILVER, s. 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

212 TEXT-BOOK OF PHYSIOLOGY

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, how-
ever, may be regarded as similar: viz.: the reception and collection of the
excess of lymph which has transuded through the walls of the blood-vessels
and its transmission onward into the regular lymph-vessels.

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. From their origin they gradually converge toward the trunk
of the body, and finally empty into the thoracic duct. In their course they
anastomose very freely with adjoining vessels. The diameter of a lymph-
vessel varies from i to 2 mm. After the lymph-vessels have emerged from
the lymph-capillaries they acquire three distinct coats, each of which pos-
sesses definite histologic features.

The internal coat is composed of a delicate lamina of longitudinally dis-
posed elastic fibers covered with a layer of flattened nucleated endothelial
cells with wavy outlines.

The middle coat consists of white fibrous tissue arranged longitudinally
and of non-striated muscle and elastic fibers arranged transversely.

The external coat consists of practically the same structures, though the
muscle-fibers are longitudinally disposed.

The lymph-vessels are provided with valves which are so numerous and
located at such short intervals as to give the vessels a beaded appearance.
These valves are arranged in pairs and consist of two semi-lunar folds with
their concavities directed toward the larger vessels. They are formed by a
reduplication of the lining membrane, which is strengthened by fibrous tissue
derived from the middle coat.

Lymph-nodes, or glands. — In their course toward the thoracic duct the
lymph-vessels pass through a number of small pisiform bodies termed lymph-
nodes or 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 medullary portion. Each gland is covered exter-
nally by a dense membrane 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 its outer zone 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. ^ 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 re-

ABSORPTION 2I3

sistance to the passage of the lymph. The gland substance 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 sepa-
rated from the lymph sinus by a dense layer of a reticulum, which, how-
ever, 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 investing
membrane and then open directly into the lymph sinus. The vessels which
leave the gland are also in communication with the sinus. After the lym-
phatics 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 pri-
mary current, there is a secondary current flowing from the capillary blood-
vessels outward and into the sinus. The lymph flowing through the sinus
carries with it large numbers of lymph-corpuscles. 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 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 abdom-
inal 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 milli-
meters 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 junction of the internal jugular and subdavian
veins on the left side. The thoracic duct wall has the same general layers
as the wall of the lymph- vessel: viz., an internal or endothelial; a middle
elastic and muscular; an external or fibrous. It is also provided with
numerous valves.

The lymph-vessels of the right side of the head, of the right arm, and
a portion of the right side of the trunk terminate in the right thoracic duct,
which is about 25 to 30 mm. in length and which empties 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 diagram-
matically shown in Fig. 87.

LYMPH

Lymph is the clear fluid found within the tissue spaces and within the
lymph-vessels. Inasmuch as there are reasons for the view 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

214

TEXT-BOOK OF PHYSIOLOGY

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 thoracic duct during the intervals
of digestion or from one of the large trunks of the leg is a clear, colorless or
slightly opalescent fluid having an alkaline reaction and a specific gravity
of i. 020 to 1.040. Examined microscopically it is seen to hold in suspen-

FIG. 87. — DIAGRAM SHOWING THE COURSE or THE MAIN TRUNKS OF THE ABSORBENT SYSTEM.
The lymph-vessel of lower extremities (D) meet the lacteals of intestines (LAC) at the recep-
taculum 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 of Physiology.")

sion 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 8000 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 quan-
tity 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 granular appearance. When withdrawn

ABSORPTION 2IS

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 vanable 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 composition, 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 change not only its normal composition, but also the
relative amounts of its normal constituents.

Chemic analysis has shown that the lymph from the thoracic duct con-
tains from 3.4 to 4.1 per cent, of proteins (serum-albumin, fibrinogen),
0.046 to 0.13 per cent, of substances soluble in. ether (probably fat), o.i
percent, 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 magnesium salts. Fibrinogen is
seldom present beyond o.i per cent., which will account for the feeble and
slow coagulation. Lymph contains both free oxygen and carbon dioxid.
Of the former, however, there is but a small percentage; of the latter, about
45 yols. per cent., partially in the free state and partially combined with
sodium. Urea is also present in very small amounts. This analysis indi-
cates that lymph resembles blood-plasma in the character of its constituents,
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 reservoir 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 across 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 suffi-
cient to account for all the phenomena. For a consideration of the phenom-
ena 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 gener-
ally; less so, however, to colloid bodies, such as the proteins 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 con-
stituents 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

2I6 TEXT-BOOK OF PHYSIOLOGY

at least to those determining the passage of substances through animal
membranes as determined experimentally. The force giving rise to filtra-
tion 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 conditions will, of course, diminish the intracapillary pressure
and lymph production. Hemorrhages which lower the general blood-pres-
sure may so lower the capillary pressure as not only to stop the flow of
lymph to the tissues, but may give rise to a filtration 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 proteins, however, is somewhat less.
These facts are in accordance with what is known regarding the diffusibility
of both crystalloids and colloids through animal membranes.

According to Heidenhain, the production of lymph is not so much
due to intracapillary pressure as it is to the specialized activities 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 the injection of
peptones, albumin, the extract of the muscles of the leech, crab, mussel,
etc., is followed by an increase in the amount of lymph discharged from
the thoracic duct; the lymph also possesses a high degree of concentration,
being rich not only in inorganic but also in organic constituents. 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. These substances Heidenhain regarded as true lymphagogues.

It has also 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 the increased percentage
of salts and a higher osmotic pressure. The increased volume of blood
raises the intra-capillary blood pressure which in turn gives rise to a greater
flow of lymph.

The more recent experiments of Starling indicate that in addition to the
difference of pressure between the blood in the capillaries and the lymph in
the tissue spaces, a new factor must be considered and that is, the permea-
bility of the capillary wall. This he finds to vary considerably in different
parts of the vascular apparatus, being greatest in the capillaries of the

ABSORPTION 2I?

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 degree of permeability of the capillary
wall will account for the production of lymph in all regions of the body.

Another factor that has been invoked to account for the passage of the
constituents of lymph across the capillary wall, is an increased concentra-
tion of the intercellular lymph, the result of an accumulation of metabolic
products, and hence an increase in the osmotic pressure, which would lead
to an increase in the passage of the constituents of the blood into the lymph.
The activity of a tissue would thus indirectly lead to the formation of
lymph.

The Functions of Intercellular Lymph.— The origin and composition
of lymph, its situation and relation to the tissue cells indicate that its func-
tion is to provide the tissue cells with those nutritive materials necessary to
their growth, repair, and functional activities, and to receive from the tissue
cells the waste products of their metabolism 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 always 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
its own physiologic properties.

Coincidently with the loss of nutritive material, the lymph receives the
products of the metabolism 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 destructive 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 con-
tinually, 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 interference 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

2l8

TEXT-BOOK OF PHYSIOLOGY

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 investigations indicate, then additional factors, in explana-
tion of lymph absorption, 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 are

absorbed by the capillary blood-vessels. As
to whether the relatively feebly diffusible
colloids are so resorbed is as yet a matter of
investigation.

ABSORPTION OF FOODS

The most important of the absorbing sur-
faces, especially in its relation to the absorp-
tion of new material, is the mucous mem-
brane of the alimentary canal, and more par-
ticularly that portion lining the small intestine,
provided as it is with specialized absorbing
structures — the villi. Though certain sub-
stances 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 demon-
strated, however, that this takes place, if at
all, but to a slight extent. If, however, solu-
tions of inorganic salts, sugars, and peptones
possessing a concentration of at least 5 per
epi- cent. — a degree of concentration seldom re-
alized under normal conditions — are intro-

Basement membrane, d. Plate- duced into the stomach, their absorption will
cor6 connective"tissue elements of be effected, the rate of absorption following in
sorbcnt*' radicar^OT^cteaf— a genera^ waY tne increase, within limits, in
(Piersol) concentration. Water is practically not ab-

sorbed from the stomach. The absorption

of the products of digestion — i.e., dextrose, levulose, peptones, amino-acids,
soaps, glycerin, fat acids, salts, along with water, in which for the most
part Jthey ^are held in solution — is therefore limited very largely to the
small intestine, 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 i 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 mem-
brane (see Fig. 88) supporting tall columnar epithelial cells. Each cell is

ABSORPTION

219

composed of granular bioplasm containing a distinct nucleus. At its free
extremity a narrow border of the cell presents a striated appearance, as if it
were composed 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 arteries, 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 endothelial cells with sinuous margins. This capillary
probably begins by a blind extremity 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-fibers,
arranged longitudinally, 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 sub-
mucous coat of the intestine, which are
the ultimate branches of the intestinal
artery, and serve the purpose of de-
livering nutritive material to the capil-
lary plexus. While passing through
the latter a portion of the blood-
plasma transudes through the capil-
lary walls into the spaces of the retic-
ulated tissue, constituting lymph.
At the same time products of tissue
metabolism pass through the capillary
walls into the blood. The blood then
passes into the venules, which, leav-
ing the villus at its base, unite with
the veins of the submucous coat to
form the intestinal veins. These fi-
nally unite with the gastric and splenic
veins to form the portal vein, which enters the liver at the transverse fissure
(Fig. 89). The excess of lymph within the villus passes into the club-
shaped lymph-capillary, to be finally carried by the lymph-vessels 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 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 interior of the villi, from which they are subsequently removed by the
blood-vessels and lymph-vessels. As to the mechanism by which the epi-
thelial cells accomplish this result, nothing definite can be asserted. In-

FIG. 89. — DIAGRAM OF THE PORTAL
VEIN (pv) ARISING IN THE ALIMENTARY
TRACT AND SPLEEN (5), AND CARRYING THE
BLOOD FROM THESE ORGANS TO THE LIVER.
— (Yeo's "Text-book of Physiology.")

220 TEXT-BOOK OF PHYSIOLOGY

asmuch as the absorption of food does not take place in accordance with
the laws of osmosis as at present understood, it has been suggested that
the cells possess a "selective action" dependent on their organization and
physiologic activity, an activity which is to a great extent conditioned and
limited by the degree of diffusibility of the substances to be absorbed.

Absorption of Water and Inorganic Salts. — Water and inorganic salts
after their absorption from the intestine and transference into the lymph-
spaces of the villi pass through the walls of the capillary blood-vessels and
are carried, by the blood of the portal vein, into and through the liver into
the blood of the general circulation. Unless water be present in excessive
amounts, there is no appreciable absorption of water by the lymph-vessels.

Absorption of Sugar. — As previously stated, all the carbohydrates, with
the exception possibly of lactose, are transformed by the digestive 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 epithelial 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 to 0.25 per cent.; while after the injection of sugar into the intestine
the percentage may rise as high as 0.4 per cent. As chemic analysis of
lymph obtained from the thoracic duct shows no increase in the percentage
of sugar beyond that normally present (o.i per cent.), it is assumed that
sugar is not removed from the villi by the lymph- vessels.

On reaching the liver a portion of the sugar 12 to 20 per cent, passes from
the blood stream across the walls of the capillaries into surrounding lymph
spaces and comes into direct relation with the liver cells. Then through the
agency of an enzyme, the sugar is dehydrated, converted into starch and
stored for a variable length of time in the liver cells in the form of hyaline
masses which can be readily seen with the aid of the microscope. Under
this form the carbohydrate material is retained until the necessity arises
for its return to the blood, and this happens, when the percentage of sugar in
the blood falls below the normal, viz.: o.io to 0.15 per cent. Under such
circumstances the necessary amount of the liver starch is hydrated, con-
verted into sugar, and passed into the blood in quantities sufficient to restore
the normal percentage. The apparent necessity for this temporary storage
of sugar in the liver is to prevent its too rapid entrance into the arterial
blood and hence a rise in the percentage far beyond that which is normal.
Should this occur a condition known as hyperglycemia would result and
as a -consequence an elimination of the excess by the kidneys giving rise to
the condition known as glycosuria.

Absorption 0} the Products 0} Protein Digestion. — For the reason
that the proteins are for the most part transformed through hydration and
cleavage by the action of the gastric and pancreatic enzymes into peptones
and for the further reason that the peptones are diffusible bodies, it was
formerly believed that they represented the final stages in the digestion of
the proteins, and as such were absorbed out of the intestinal contents by
the action of the epithelium covering the villi. Inasmuch as chemic analysis
failed to detect more than a trace of either peptone or native protein in the

ABSORPTION

221

portal blood or in the lymph of the thoracic duct, it was assumed that the
epithelium after absorbing, also synthesized them into some form of coagu-
lable protein (plasma-albumin) which is readily assimilable by the blood.
The plasma-albumin thus supposed to be formed by the epithelial cells was
believed to pass into the interior of the villus, thence into the blood by
which it was carried to the liver. After passing through this organ it
entered the blood of the general circulation and became a part of the com-
mon store of protein, out of which each tissue cell constructed the particular
protein characteristic of it. That such a conversion of peptones to plasma-
albumin would be necessary in case of their absorption 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 coagulability of the blood,
coma, and death.

This general view as to the absorption of peptones is no longer enter-
tained. There are reasons, however, for believing that the change which
proteins undergo in digestion, is more far-reaching and complete, and that
the peptones in turn are disintegrated and reduced to even simpler bodies
such as polypeptids, peptids and amino-acids (see page 191). The extent
to which this disintegration proceeds will doubtless depend, to some extent,
on the quantity and variety of proteins consumed.

Since recent investigations make it highly probable that the final stage
in the digestion of protein is the formation of amino-acids it has come to
be believed that the problem of absorption is to be transferred to these
fragmentary bodies rather than to the peptones. Assuming this to be true,
the question at once arises as to what change, if any, they undergo during
their passage through the intestinal epithelium. The difficulty of detect-
ing the presence of amino-acids in the blood led to the assumption that after
their absorption they were synthesized, just as the peptone molecules were
supposed to be, and a protein molecule constructed similar to, if not identical,
with plasma-albumin.

The recent investigation of Abel, carried out with a new. form of
diffusion apparatus, renders it highly probable that amino-acids are
present in the circulating blood in readily determinable quantities. This
has led to the belief that the amino-acids, in part, at least, are absorbed as
such, pass through the intestinal epithelium, the portal blood and the liver
unchanged, and enter the blood of the general circulation to be carried
by it directly to the tissues where they are at once utilized by the tissue
cells or stored for future use. This view renders it much easier to Bunder-
stand, how out of the different proteins of the foods, varying widely in their
composition, the specific proteins of the tissues are constructed. It is only
necessary to assume that the tissue cells select from the variety of amino-
acids presented to them, only those which are necessary to the formation
of the protein by which they are characterized. The ^ plasma-albumin,
whatever its origin, might then be regarded as a protein surplus to be
called on if the protein ingested should be insunicient. It may be, however,
that future investigations will ascribe other functions to it.

Under whatever form the protein material is absorbed there is every reasc
to believe that it is carried by the portal circulation to the liver, through

222 TEXT-BOOK OF PHYSIOLOGY

which it passes to enter the blood of the general circulation. Ligation of
the thoracic duct does not interfere to any appreciable extent with protein
absorption nor with the normal production and elimination of urea, nor with
the weight of the animal.

Many facts in the physiologic chemistry of the body raise the question
as to what percentage of the amino-acids produced in the intestine daily is
utilized for tissue repair and growth. If the protein requirements of Chit-
tenden, viz.: 58 to 60 grams only, are necessary for repair and growth,
then approximately one-half the amino-acids produced from the protein
usually consumed must be disposed of in some other manner. The manner
of disposal of these unused (that is, unused for tissue repair and growth)
fragments of protein disintegration is doubtless varied; some of the amino-
acids, after absorption by the epithelial cells of the villi and mucosa, are
deprived of NH2 (the amino-acid nitrogen) or deaminized; the NH2 is then
converted into ammonia, combined with carbon dioxid to form ammonium
carbonate, carried to the liver, and changed into urea. That this is very
probably the case is rendered likely from the presence of a large quantity of
ammonia in the mucous membrane of the intestine and in the blood of the
portal vein, in which after a meal rich in protein it may be four times as
great as in the arterial blood. The remainder of the amino-acid molecule is
changed into some carbonaceous radical and finally into sugar or fat and
subsequently utilized by the organism for heat production. The dynamic
portion of the amino-acid is this deaminized remainder. Other of the surplus
amino-acids are acted on by intestinal bacteria, and converted into
simpler compounds, after which they are eliminated in the feces or absorbed
and carried to the liver where they undergo other changes and eventually
appear in the urine.

The ammonia set free during digestion is absorbed, carried to the liver
and transformed to urea.

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 objections. This view of fat absorption has largely been based on
the observation that during digestion fatty granules can be seen in all por-
tions 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 ^ brought about by the pancreatic enzyme, with the appear-
ance 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

ABSORPTION 223

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-vessels 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 micro-
scopic size. With the passage of the chyle into the thoracic duct it also
assumes the same milky appearance. 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 quanti-
tative composition as lymph, the difference being mainly in the large excess
of fat in the former. Indeed, chyle may be regarded as lymph with the
addition of fat.

With the absorption of the last of the fat-granules the contents of the
lymph-vessels of the mesentery— the lacteals — begin to lose their white
appearance near the intestine and as the fat-granules pass toward the
thoracic duct, the lymph-vessels gradually become transparent and disappear
from view. A similar change in the appearance of the thoracic duct ensues,
as the last portions of the fat ascend the duct to be discharged into the blood-
stream.

Routes for the Absorbed Food. — Physiologic experiments have dem-
onstrated 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.

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 circulation by
these two routes, as follows: (See Fig. 90.)

The water, inorganic salts, proteins or ammo-acids, and sugar after
entering the blood-vessels of the villus are carried by the blood of the in-
testinal 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 fat-granules, synthesized in the epithelial cells, after entering the
lymph-radicle of the villus are carried by the lymph-stream of the intestinal
lymph-vessels and emptied into the receptaculum chyli from which they
ascend into the thoracic duct, 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 dif-
ference 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 secretor activity of the capillary walls. As
soon as the pressure rises above that in the thoracic duct a forward move-
ment of lymph takes place. Other things being equal, the rate of move-
ment will be proportional 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

224

TEXT-BOOK OF PHYSIOLOGY

FIG. 90. — DIAGRAM SHOWING THE ROUTES BY WHICH THE ABSORBED FOODS REACH
THE BLOOD OF THE GENERAL CIRCULATION (G. Bachmari). 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 receptaculiim chyli, the lower expanded part of the thoracic
duct; th. d., the thoracic duct discharging lymph and chyle into the blood at the junction
of the internal jugular and subclavian veins; sup. v. c., the superior vena cava.

ABSORPTION 225

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 pres-
sure. When the muscle-fibers relax, regurgitation 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 for the absorption of new material. The rhythmic
contractions of the intestinal 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 contractions
of their contained muscle-fibers.

Inasmuch as the lymph-vessels lie in situations in which they are sub-
ject to compression by muscles during contraction, it is probable 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 observa-
tions have demonstrated the truth of this supposition. Alternate contraction
and relaxation of 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 intrathoracic pressure (that is, the positive pressure
exerted by the air in the lungs on the intrathoracic viscera, e.g., heart, veins,
thoracic duct, etc., which is less by about 6 millimeters of mercury than
the pressure in the lungs) decreases. The decrease is proportional to the
extent of the inspiration. With this decrease of pressure, the thoracic duct
expands and its internal pressure falls. As the intra-abdominal portion
of the thoracic duct and its tributaries are subjected to a higher pressure,
practically that of the atmosphere the lymph in these vessels is forced, by
reason of the difference in pressure between these two regions, into the
intrathoracic portion of the duct. During expiration, the rise of the in-
trathoracic pressure to its former value leads to a compression of the thoracic
duct and causes the lymph to be discharged rapidly into the blood-stream.
A regurgitation of the lymph is prevented by the closure of the numerous
valves throughout the course of the duct.

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 processes, 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 interpenetratipn 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,

226 TEXT-BOOK OF PHYSIOLOGY

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 in 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 denned as the passage of the molecules of water
through 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 through 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 through the membrane though usually
included under the term osmosis is more properly termed dialysis.

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 through the membrane until the water on both sides contains
the same number of molecules, or, in other words, until it is homogeneous in com-
position. 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 con-
taining 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 substances which by virtue of the small size of their molecules
pass through membranes, crystalloids, and all those which by virtue of the large size
of their molecules do not pass through 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 solution an
inorganic salt, the molecules of the salt-free water will osmose through 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 denned as the pressure exerted
by the molecules of the substance in solution against the outer layer of the mole-
cules of the solvent. If the solvent is enclosed by an elastic membrane it is
expanded and in consequence there is an osmosis of a surrounding solvent towards
and through it. The reason for this pressure lies in the fact that, when the mole-
cules of a substance are separated a certain distance, as they are when in solution,
they repel 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.: i. Physical. 2. The determination of the
freezing point. 3. By calculation.

ABSORPTION

227

i. 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. 91, which consists of an earthenware vessel (a), into the
upper open end of which a tall vertical glass tube has been hermetically sealed.
The pores of the earthenware vessel have been filled by a membrane made by
precipitating ferrocyanid 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 dissolved substance, is termed a semi-
permeable membrane, and its use is absolutely necessi-
tated when it is desired to obtain the actual pressure
exerted by any given substance in solution. An ap-
paratus of this character is termed an osmometer.

When, therefore, the osmometer containing a solution
of cane-sugar is placed in the vessel (b) containing water,
the following phenomena occur, viz.: an ascent of the
cane-sugar solution in the vertical glass tube, and a de-
scent of the level of the water in the vessel b. These
phenomena continue until the level of the fluid in the
glass tube reaches a certain height, when it becomes sta-
tionary, 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 earthen-
ware vessel, except at the open extremity of the vertical
glass tube. Inasmuch as the rigid walls of the osmometer
prevent any outward displacement of the molecules of the
water, the force of the impact of the sugar molecule is
directed against the molecules 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
rebound with the same velocity, and hence time is per-
mitted 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 permeable 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

Solution

*>/
Cane Sugar

—a

the glass tube continues to rise and the level of the fluid in pIG, 91.— AN OSMOMETER.
the vessel, b, continues to fall until the force of gravity

prevents any further upward 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 solution. A i per cent, solution of cane-sugar
at a temperature of from i3°C. to i6°C., as determined by this method, exerts
an osmotic -ps^ure of about 535 mm. Hg.; a 2 per cent, solution exerts an
osmotic pressureSpproximately twice this amount.

Experiments made with this and similar osmometers show—

1. That the osmotic pressure of any substance in solution is proportional to the

concentration, providing the temperature is constant.

2. That when the concentration is constant the osmotic pressure ns< s with, a:

id nrrmnrtinnal to. the temDCrature.

228 TEXT-BOOK OF PHYSIOLOGY

3. That when different substances are present in the same solvent the osmotic

pressure is equal to the sum of the individual or partial pressures.

4. That whatever the nature of the substance in solution it will exert the same

osmotic pressure, providing always the same number of molecules are
present; hence the molecular weights in grams per liter of different sub-
stances 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 other-
wise 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 arithmetic 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 obtain-
ing 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 dissolved 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., i.87°C. The osmotic pressure therefore of such a solution, as
determined 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 thermome-
ter the lowering of the freezing point in degrees centigrade, which is usually expres-
sed by the symbol A. Then the osmotic pressure is equal to A divided by 1.87°
C., and multiplied by 22.38 atmospheres, or 17,008 mm. of Hg. Thus if the
freezing point of any solution was found to be o.83°C. lower than water, its
osmotic pressure would be 0.83-:- 1.87X2 2. 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 o.56°C. Experimentally it has been determined
that the freezing point of water is lowered to the same level, when it contains so-
dium 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 preserving,
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 pres-
sure 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 hypo-isotonic. 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 hyperisotonic.

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 known that i gram of hydro-
gen 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, one gram-molecule is com-

ABSORPTION 22Q

pressed to a volume of i 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 i liter of water contains the same number of molecules
as a gram-molecule of hydrogen compressed to one liter, 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 i per
cent, solution of cane-sugar. The procedure is as follows: A gram-molecule of
cane-sugar (ClfHnOn) contains 342 grams; a i per cent, solution contains 10
grams to the liter; hence its osmotic pressure is 10-7-342X22.38 atmospheres or
0.65 atmosphere which is equal to 494 mm. of Hg.

Filtration.— Filtration may be denned as the passage of water, and of all sub-
stances dissolved in it, through a membrane as a result of a difference of hydro-
static pressure on the two sides. The difference between the two pressures con-
stitutes the force of nitration, and hence the greater the difference, 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 through the membrane are proportional
to the filtration force, and hence the filtrate will have a concentration similar to,
if not identical with, that of the original solution. The passage of colloids in solu-
tion 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 concentra-
tion 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 through which the constituents of the fluids, inorganic and
organic, are continually passing. Thus prepared foods in the intestine pass through
the intestinal wall into blood- and lymph-vessels; the constituents of the blood
pass through 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) through the sarcolemma in to the interior of the muscle fiber; (c) through
the limiting surface of, and into the interior of all other tissue cells. The waste
products, the result of tissue and food metabolism, pass from the interior of cells
through their limiting membranes or surfaces into the tissue spaces; thence through
the wall of the capillary vessel into the blood and finally through the wall of the
capillary vessel and the epithelium of the lung, the kidney, the liver, etc., to take
part in the formation of the excretions. These and other processes are believed
to be accomplished by the factors, diffusion, osmosis, and filtration.

The statements that have been made in foregoing paragraphs in reference 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
through these membranes, it is possible that the facts which have been obtained
experimentally are not strictly paralleled in the living body, and hence not strictly
applicable to the elucidation 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 through animal membranes in the living
condition.

CHAPTER XII
THE BLOOD

The blood may be defined as the nutritive fluid of the body since it con-
tains all those materials that are necessary to the maintenance of the nutri-
tion. The presence and proper circulation of the blood in the living or-
ganism are essential for the maintenance of tissue irritability and for the
manifestation of the activities of all physiologic mechanisms. The escape
of the blood from the vessels, especially in the higher animals, is followed by
cessation of the physiologic activities of all the tissues within a short period.
The irritability, however, persists for a variable length of time though it too
gradually declines and finally disappears. 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 constituents, including oxygen, are
given up to the tissues, by which they are utilized for growth, repair, and
the liberation of heat. At the same time the tissues yield up to the blood
a series of decomposition or katabolic products, 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 intestinal canal;
of oxygen, absorbed from the respiratory surface of the lungs; of katabolic
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 katabolic products, it neverthe-
less presents certain average physical, morphologic, and chemic properties
which distinguish it as an individual tissue.

The Physical Constitution 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 particles termed
corpuscles floating in it, of which there are two varieties, the red or the
erythrocytes and the white or the leukocytes. By appropriate methods it
can be shown that a third corpuscle, colorless in appearance and smaller
in size than the ordinary white corpuscle, is present in the blood-stream

THE BLOOD 23I

and known as the blood-platelet 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 cor-
puscles 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 result 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 distinguished
— the arterial, the color of which is a bright scarlet-red, 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
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
venous blood passes into and through the pulmonic capillaries the hemo-
globin absorbs a certain volume of oxygen after which it changes in color
and on emerging from the lungs imparts to the blood its characteristic
scarlet-red color. By reason of the union of the hemoglobin with the
oxygen it is generally termed while in the arteries, oxyhemoglobin. As
the arterial blood passes into and through the systemic capillaries, the oxy-
hemoglobin yields up a portion of its oxygen to the tissues after which it
again changes in color and on emerging from the tissues imparts to the
blood its characteristic bluish-red color. By reason of the loss of a portion
of its oxygen, the hemoglobin is generally termed while in the veins, deoxy-
or reduced hemoglobin.

2. Opacity.— Owing to the fact that the corpuscles have a refracting
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, at once becomes transparent.

3. Odor.— When freshly drawn the blood possesses a peculiar charac-
teristic odor which has been attributed to the presence of a volatile fat acid
in combination with an alkaline base. The intensity 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 1.051 and 1.059, averaging in man 1.056 and in woman 1.053. Normally,

232

TEXT-BOOK OF PHYSIOLOGY

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 ingestion of dry food, and by the elimination
of large quantities of water by the lungs, skin, and kidneys.

Inasmuch as the specific gravity of the blood varies from the normal in
one direction or the other in certain pathologic states, it is deemed desirable
for clinical purposes to determine the extent of this variation. Among the
methods suggested for this purpose that of Hammerschlag is the one most
generally resorted to. It is based on the principle, that a fluid in which a
drop of blood neither rises nor falls must have the same specific gravity as
the blood itself. As the specific gravity of the blood varies in different
pathologic states it is essential that the fluid employed is of such a character
that its specific gravity can be quickly varied in one direction or the other.
To meet this indication a fluid, a mixture of chloroform (specific gravity
1.526) and benzol (specific gravity 0.889) ^s nrst prepared in such propor-
tions that the mixture has a specific gravity of about i .040. With a pipette
a drop of blood is then placed in the mixture. If the drop rises the specific
gravity of the mixture is greater than that of the blood. Benzol is then
gradually added until the drop remains stationary. At this moment the
specific gravity of the mixture is the same as that of the blood. If the
drop falls the specific gravity of the mixture is less than that of the blood.
Chloroform is then gradually added until the drop remains stationary.
At this moment the specific gravity of the mixture is the same as that of
the blood. In either case the specific gravity of the mixture is determined
with a suitable hydrometer and the figure observed attributed to the blood.

5. Reaction. — The reaction of the blood has usually been stated as
alkaline from its effect on litmus paper. Thus, if blood is permitted to
remain for a few seconds on slightly reddened glazed litmus paper and then
washed off, a distinct blue color presents itself against a red or violet back-
ground. The alkalinity thus indicated has been attributed to disodium
phosphate, Na2HPO4, and sodium carbonate, Na2CO3. The degree of the
alkalinity is measured by the amount of a standard acid necessary to be
added before the indicator used shows an acid reaction. 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 unavoidable error in these estimates is about
30 milligrams. The alkalinity from this point of view varies but within
narrow limits in physiologic conditions. It is increased in the early stages
of digestion and decreased in the later stages as well as after prolonged
exercise.

In accordance with the ideas of physical chemistry the acidity of a fluid
is dependent on the presence_of hydrogen ions, H+, and the alkalinity on the
presence of hydroxyl ions, (OH). The reaction of any fluid, containing a
number of chemic compounds in solution, will be dependent therefore on the
relative proportions of hydrogen ions and hydroxyl ions that make their
appearance.

If the hydrogen ions are in excess the fluid is acid, if the hydroxyl ions
are in excess the fluid is alkaline in reaction. Tested by the methods of
physical chemistry blood and lymph are found to possess these opposite

THE BLOOD 233

ions in equal degree and therefore are neither acid nor alkaline but neutral
in reaction.

6. Temperature. — The temperature varies in degree in different parts
of the body. If the temperature variations correspond with the variations
in some other mammals, the highest temperature is in the hepatic veins
where, as in the dog, it registers about 4o.6°C., and the lowest temperature
is near the surface as in the mouth and axilla, where it is approximately
38.9°C.

7. Viscosity. — Viscosity may be denned as the resistance to the move-
ment of the molecules of a fluid homogeneous body among themselves. In
accordance with the degree of this resistance, which may also be spoken of
as internal friction, will the fluid at a given temperature be mobile or viscous.
Viscosity varies partly with the nature of the fluid and partly on its tempera-
ture. Thus at the same temperature water, syrup, and pitch possess different
degrees of viscosity. A rise in temperature of i°C. diminishes the viscosity
about 2 per cent. In all discussions relating to the viscosity of fluids, that
of distilled water is taken as a standard and regarded as unity.

Blood as a fluid is regarded by physiologists as possessing viscosity,
though the definition in the foregoing paragraph is not strictly applicable,
as it is not a homogeneous but a heterogeneous fluid consisting of plasma
the molecules of which show an inner friction and of corpuscles, which also
show friction. Blood having a complex composition as compared with
water has naturally a greater degree of viscosity or internal friction. Experi-
mental investigations render it certain that the observed viscosity is depend-
ent on the corpuscular elements to a greater extent than on the composition
of the plasma. About two-thirds of the viscosity is due to the corpuscles.

The viscosity of blood as compared with water may be determined by
permitting the two fluids to flow through capillary tubes of corresponding
caliber under a steadily acting pressure and then determining the volume
that flows through each in a given time, or by determining the distance to
which each fluid flows in a unit of time 'in these capillary tubes and then
in each instance, comparing the results one with the other. Normal human
blood is thus found to possess a viscosity 4.5 times that of distilled water
at body temperature. Dog's blood has- a viscosity 6 times that of water.
If the temperature of blood is raised the viscosity diminishes. Recalling
the statement that the viscosity is closely connected with the presence of
red corpuscles it would be expected that either an increase or decrease
in their number would change the viscosity in one direction or another.
In a case of polycythemia in which the red corpuscle count was 11,000,000
per cubic millimeter the viscosity was between 3 and 4 times the normal.
In certain other pathologic states of the blood characterized by a diminu-
tion in the number of red corpuscles the viscosity diminished one-half
or more. The ingestion of meat raises the viscosity, while the ingestion of
fats and carbohydrates diminishes it.

The determination of the viscosity for clinical purposes is accomplished
by the use of special forms of apparatus termed viscosimeters. These for
the most part consist of capillary tubes through which distilled water and
blood are caused to flow under the influence of a constant positive or nega-
tive pressure. The distance the water flows in a unit of time, compared
with that of blood, represents the degree of viscosity. Among these ap-

234

TEXT-BOOK OF PHYSIOLOGY

paratus those of Hess and Determann are generally employed, descriptions
of which will be found in works on diagnosis.

8. Coagulability. — Within a few minutes after the blood is withdrawn
from the vessels of a living animal it begins to lose its fluidity, becomes some-
what 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 coagula-
tion, 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
cpagulum 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 have 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. 92).

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 varying degrees of thick-

FIG. 92. — DIAGEAM 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.)

ness. Within a few hours the blood-mass detaches itself from the sides of
the vessel in consequence of the retraction 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 dot, 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 constitu-
ents of the plasma except the antecedents of the fibrin. The stages of coagu-
lation are shown in Fig. 92.

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, pro-
ducing the so-called buffy 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

THE BLOOD 235

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
is subjected to room-temperature, 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 corpuscles
is therefore not essential to the process of coagulation.

Rapidity of Coagulation. — The rapidity with which the blood coagu-
lates varies in different classes of animals under the same conditions: 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 changing 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 (i volume of a 25 per cent, solution to 3 volumes of
blood) ; of sodium sulphate (i volume of a saturated solution to 7 volumes of
blood). (3) The addition of potassium oxalate (i volume of a i per cent,
solution to 3 volumes of blood). (4) The injection into the circulating
blood of commercial peptone. (5) The mouth secretion of the leech.

Coagulation is hastened by the following agents, viz.: (i) a temperature
gradually increasing from 38°C. to 5o°C. (2) The addition 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; on 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 extensibility and retractility, and are therefore elastic.
The chemic features of fibrin have already been considered (see page 19).
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.

CHEMIC 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 specin<
gravity of 1.026 to 1.029. It is composed largely of water holding in solution
proteins, sugar, fat, inorganic salts, urea, cholestenn, lecithin, etc. In
composition it is quite complex, containing as it does not only the nutritive

236 TEXT-BOOK OF PHYSIOLOGY

materials derived from the digestion of the food, but also the substances
resulting from the disintegration of the tissues consequent on their functional
activity.

Serum. — The serum is the clear, transparent, slightly yellow fluid ex-
pressed 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 formation of fibrin.

The average composition of plasma is shown in the following table*

CHEMIC COMPOSITION OF PLASMA

Water 90.00

f Plasma-albumin 4-5°

Proteins ... \ Paraglobulin 3 .40

[ Fibrinogen o .30

Fatty matters 0.25

Sugar o . 10

Extractives o .60

Inorganic salts o .85

Plasma-albumin. — Of the protein constituents of the blood, plasma- or
serum-albumin is the most abundant, existing to the extent of from 4 to 5
per cent. It is readily obtained from plasma or serum by saturating either
of these fluids with magnesium sulphate, when all the proteins except
serum-albumin are precipitated. After their removal the remaining fluid^
is subjected to a temperature of from 70° to 75°C., when the serum-albumin
is precipitated in a coagulable form, after which it can be removed and its
chemic features determined. Recent investigations indicate that plasma-
albumin consists of, at least, more than one substance, since it can be separated
into a crystallizable and non-crystallizable or an amorphous portion both
of which however respond to the usual tests for albumin.

The origin and function of plasma-albumin are both obscure. Until
quite recently it has been regarded, owing to its similarity to egg-albumin,
and to its presence in the blood, as holding an important position as a
nutritive agent. It was assumed that it originated in a synthesis of the
products of protein digestion — peptones and amino-acids — in the epithelial
cells covering the villi and by them discharged into the blood of the portal,
and finally into the blood of the general circulation. On reaching the capil-
laries it was supposed to cross the endothelial wall, to become a constituent
of the lymph and then to be utilized by the tissue cells for growth and repair.
As was stated in the chapter on absorption, this view has been made untenable
by reason of the apparent fact that the products of protein digestion— the
amino-acids — are absorbed and enter the blood as such and carried direct
to the tissue cells by which they are directly synthesized to the protein
by which they are characterized. Plasma-albumin must therefore have,
if this view prevails, some other origin and function. Future experimenta-
tion may disclose both.

Paraglobulin. — This protein, 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

THE BLOOD 237

per dent, in the blood of man. As to the physiologic importance or ante-
cedents of paraglobulin 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 protein can be obtained from plasma, lymph,
pericardial, and peritoneal fluids, as well as from hydrocele fluid. 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 0.22 to 0.33 part per
hundred. Fibrinogen may be obtained from plasma which has been pre-
vented from coagulating, by the addition of magnesium sulphate in certain
quantities or by the addition of a saturated solution of sodium chlorid. In
a few minutes a flaky precipitate occurs. By repeated washing and pre-
cipitation with sodium-chlorid solutions of varying strength, the fibrinogen
may be obtained in a pure state. The history of fibrinogen is unknown,
though there is some experimental evidence for the belief that it is produced
in the liver though out of what has not been determined. Beyond the fact
that it contributes to the occasional formation of fibrin there is no positive
knowledge either as to its origin, its nutritive value, or its final disposition in
the blood under normal conditions.

Fat. — The plasma, and the serum as well, contains a very small quantity
of fat in the form of microscopic globules. Though the percentage is nor-
mally not above 0.25, yet just after a meal rich in fat 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. As
oxidation of fat in the blood has not been established the probabilities
are that the microscopic granules pass across the walls of the capillaries
and become constituents of the lymph. Soaps or alkaline salts of the
fat acids, though formed during the digestion of fats, are not present in
the blood. Lecithin and cholesterin are present in very small quantities.

Sugar.— The plasma also contains a small quantity of sugar in the form
of dextrose and is to be regarded as a normal constituent. ^ The percentage
of the sugar varies from o.io to 0.20 per cent. To this condition of the blood
in which the sugar is present in but normal amounts the term glyctmia has
been given. If the percentage falls below the normal amount, a. condition
of hypoglycemia is established; if on the contrary the percentage is increased
beyond the normal a condition of hyperglycemia is established, whereupon
the excess will be for the most part eliminated by the kidneys giving rise
to a condition known as glycosuria.

If the statement be accepted that the amount of blood in a body weigh-
ing 70 kilos is 3864 grams (i/ipth of the body weight) the amount of sugar
in the entire volume of blood is at most about 7.72 grams, an amount which
does not materially change under physiologic conditions.

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 ex-
cretory organs.

238

TEXT-BOOK OF PHYSIOLOGY

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 0.56 part per hundred. Calcium phosphate is
present in small quantity, 0.2 part per 100. This salt is not present to
the same extent in serum for the reason that it became a constituent of
fibrin at the time of coagulation. In other respects serum differs but
slightly from plasma in the proportions of its saline constituents.

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 field of
the microscope will be seen to be crowded with red corpuscles 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
network in the meshes of which lie
individual red and white corpuscles.
(See Fig. 93.) 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 extent prevented by defibrin-
ating the blood, it has been supposed
to be dependent on the formation of
some adhesive substance connected
with the formation of fibrin.

Color. — When viewed by trans-
mitted light, a single corpuscle is
slightly yellow or greenish in color;
but when a number are grouped to-
gether, the color deepens and the cor-
puscles appear red. In either case

FIG. 93. — CORPUSCLES FROM HUMAN
SUBJECT. A few colorless corpuscles are
seen among the colored discs, many of
which are arranged in rouleaux. — (Funke.)

the color is due to the presence in the corpuscle of a specific coloring-matter,
hemoglobin.

Shape.— The red corpuscle is a circular, flattened disc with rounded
Each surface is perfectly smooth and presents a shallow depression
in its center, so that it is also biconcave. A longitudinal section of a cor-
puscle would present, when viewed edgewise, an outline similar to that of
Fig. 94. 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 cen-
tral portion a dark spot If the objective be brought nearer and the center
accurately focused, the reverse appearance obtains; the central portion

THE BLOOD 239

becomes bright and the peripheral portion dark. The cause of this differ-
ence in optical appearance is the unequal 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 con-
ditions is 0.0075 mm-> its greatest thick-
ness is 0.0019 mm- Though this may be FIG. 94.— IDEAL TRANSVERSE
assumed as the average diameter, there SECTION OF A HUMAN RED CORPUS-
is a small percentage of distinctly smaller Siamet^^!^ ThidcneTs!"0 "' *'
and a small percentage of distinctly larger

corpuscles. The following table shows the results of measurements made
by different observers:

Normal Limits. Average Diameter.

Welcker diameter 0.0045-0.0095 mm 0.0070 mm.

Hayem diameter o .0060-0 .0088 mm o .0075 mm.

Gram diameter 0.0067-0.0093 mm 0.0078 mm.

Melassez o >OO76 mm.

0-00747 (jfa inch)

Structure. — The red corpuscle of man as well as of all other mammals
possesses neither a nucleus nor a limiting membrane, but appears to consist
of a homogeneous substance more or less semisolid in consistence. Under
the influence of certain reagents the corpuscle separates into two distinct
portions: viz., a colorless protoplasmic stroma and a coloring-matter which
diffuses into the surrounding liquid. The presence of the former can be
demonstrated by the addition of iodin, which imparts to it a faint yellow color.
The stroma is elastic, and determines not only the shape of the corpuscle but
gives to it the properties of extensibility and retractility.

The foregoing is the classic and generally accepted view as to the shape,
size, and structure of the red corpuscle. Nevertheless recent investigations
render it probable that the statements were based on observations of the
corpuscles under artificial rather than natural conditions, 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 cir-
culating blood, but bell- shaped, similar to that shown in Fig. 95. It was
not until 1902, after the publication of Weidenreich's investigations, that
this vie wj began to receive more attention than had hitherto been accorded
it. Weidenreich preserved in a moist chamber a hanging drop of human
blood, and on examination found that the red corpuscles were bell-shaped
though the depth of the bell cavity varied considerably. An examination
of the capillary circulation in the omentum of the rabbit revealed the fact
that the corpuscles in their natural medium were also bell-shaped. The
circular biconcave shape they ordinarily present when a drop of blood is
examined microscopically he attributes to cooling, evaporation and con-
centration 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 : —

24o L TEXT-BOOK OF PHYSIOLOGY

Greatest diameter 7 microns o .007 mm.

Diameter of cavity 3 microns o .003 mm.

Height of bell 4 microns o .004 mm.

Height of cavity 2.5 microns o .0025 mm.

Thickness of wall at apex 2 microns o .002 mm.

Thickness of wall at base 1.5 microns o .0015 mm.

The foregoing observations have been confirmed by many subsequent
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 discs 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. 95. Radasch found on examina-
tion of fetal tissues such as the spleen, kidney, liver, placenta, etc., that the
great majority of the corpuscles in all situations presented the bell shape
rather than the circular 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 surrounded by a
structureless, colorless membrane enclosing a colored but not nucleated
semi-fluid mass, which consists chemically of protein material, lecithin,

cholesterin, 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 the corpuscles are
so numerous and the spaces between them
so small that it seems almost impossible to
determine their number. This, however,

FIG. 95-RED CORPUSCLES has been accomplished for a cubic milli-
SKETCHED WHILE CIRCULATING IN meter of blood by various observers em-
THE VESSELS OF THE OMENTUM OF A ploying different methods with compara-

Htetogtf' (F' T' LCWiS in Stdkr'S tivel? uniform results. The average normal

number of corpuscles in one cubic milli-
meter 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 physio-
logic 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 diminution in the quantity of water in the blood — e.g., co-
pious 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 produce corresponding
changes in the total number of red corpuscles in the body. In women
lactation, menstruation, and the act of parturition 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 corresponding 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:

THE BLOOD

241

Place

Height Above Sea-level

Red Cells

Author

Christiania

o meter
148 meters
314 meters
414 meters
425 meters
700 meters
1800 meters
4392 meters

4,974,ooo

5,748,000
5,900,000
7,000,000
8,000,000

Laache.
Schaper.
Reinert.

Egger.
Viault.
(Koppe.)

Gottingen

Tubingen

Zurich

Auerbach

Reibaldsgriin

Arosa

The Cordilleras

This increase in the number of corpuscles takes place, according to
Viault's observations, within two or three weeks, and is apparently not con-
nected with either diet or mode of life, but rather with diminished atmos-
pheric, 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

FIG. 96. — HEMOCYTOMETER. a, Surface; b, section view; c, squares on the surf ace of ZJ magnified.
M, G,S, mouth piece, rubber tube and pipette.

account of the great number of corpuscles in i cubic millimeter of blood, it
becomes necessary for purposes of enumeration that the blood be diluted a
definite number of times and that the diluted mixture be placed in a counting
chamber possessing a definite capacity. By means of the pipette or mtlang-
eur of Potain and the counting chamber of Thoma both these objects are
attained.

The pipette consists of a capillary tube (Fig. 96) provided with an enlarge-
ment containing a freely movable small glass ball, E. One' end of the tube, S,
' is pointed, while to the other end is attached a rubber tube, G, for the purpose
of facilitating the introduction of the blood and the diluting fluid. The capillary
tube, which is accurately calibrated, carries marks, 0.5, i, 101, which sigmfy that
if the tube be filled with blood up to the mark i and the diluting fluid
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 0.5 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

242 TEXT-BOOK OF PHYSIOLOGY

ear or finger and sucked into the tube by introducing the end, M, of 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,1 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. 96 shows both a surface view, a, and a section view, b, of the counting
chamber. This consists of an oblong glass plate, o, on which are cemented two
small pieces of glass, one of which, WD, has in the center a circular opening
in which is placed the other, B, 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 o.i mm.
lower than that of the cover-glass, r. On the surface of the glass stage a series
of small squares is engraved, C, each one of which has a side length of ^ mm.
and an area of ^fa square mm. To facilitate counting, a group of 16 such
squares is surrounded by a thick line. Fig. 97. This group is separated from

adjoining groups, also enclosed by thick lines, by
an intermediate fine line, 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 -^Xo.i, or f^ cubic millimeter.

Before placing the diluted blood on the count-
ing 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
upon 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 dilu-
tion (100 or 200 as the case may be) and this divided by the cubic contents
of each small square (^oVo) 5 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
contain 500 corpuscles

FIG. 97.— MICROSCOPIC AP-
PEARANCE OF THE SMALL
SQUARES AND THE DISTRIBU-
TION OF THE CORPUSCLES.

500X200X4000.
80

:5,ooo,ooo 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 Preservation of the Red Corpuscles in the Plasma. — Within the
blood-vessels the physical conditions and chemic composition of the plasma
are such that both the form, and the composition of the corpuscle or the rela-
tion of the hemoglobin to the stroma, are maintained in the normal or physio-

1 Various solutions have been devised for diluting blood, any one of which may be employed, e.g. :
Hay em's Fluid: Toisson's Fluid:

Hydrarg. bichlor o . 5 gm.

Sodii sulphat 5 .o gm.

Sodii chlorid 2.0 gm.

Aquae destillat 200 .o gm.

Aquae destillat 160.00 parts.

Glycerinae 30 .00 parts

Sodii sulphat 8.00 parts.

Sodii chlorid i .00 part.

Methyl- violet 0.025 part.

Gower's Fluid:

Sodii sulphat gr. 104

Acid, acetic 5 j

Aquas dest q. s. ad §iv.

THE BLOOD 243

logic condition. The plasma is preservative of the 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 yield-
ing of water to the 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 purposes of
enumeration, it must be diluted, and unless special precautions are
observed the condition of equal osmotic pressure will be disturbed by the
diluting agent and the corpuscles will lose their characteristic form and
structure from either an absorption or loss of water.

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 inorganic constituents of the corpuscles (particularly potassium phos-
phate) 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 hypo-isotonic to it.

If on the contrary, water containing inorganic salts (particularly sodium
chlorid) is added 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 is essential
therefore in diluting the plasma with water, that the latter contains inorganic
salts in such amounts that the resulting mixture (plasma and water) possesses
an osmotic pressure equal to that of the original plasma or to that of the cor-
puscle. 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 Gowers alluded to on a preceding page. Because of the
fact that sodium chlorid is the chief inorganic constituent of the plasma it is
common in laboratory work to dilute the plasma of mammalian blood and
of frog's blood with solutions of sodium chlorid of 0.9 per cent, and 0.6 per
cent, respectively, which though not absolutely are sufficiently isotonic for
the purpose desired.

The Effects of Reagents. — 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. Wit
the escape and diffusion of the hemoglobin the blood becomes transparent
and changes to a dark red color to which the term "laky" has been given.
The mechanism by which the hemoglobin becomes dissociated and
charged from the corpuscle by these agents is unknown. The disinteg-
ration of the corpuscle and the diffusion of the hemoglobin into, and

244 TEXT-BOOK OF PHYSIOLOGY

solution by the surrounding medium, is termed hemolysis and the agents
by which it is produced are termed hemolytic agents.

CHEMIC COMPOSITION OF RED CORPUSCLES

When analyzed chemically the red corpuscles are found to consist of
water 65 per cent, and solid matter 35 per cent. The solids, moreover, have

been found to consist of a pigment hemo-
globin 33, protein 0.9, cholesterin and
lecithin 0.46, and inorganic salts (chiefly
potassium phosphate and chlorid and
sodium chlorid) 1.4 per cent, respectively.
Of the total solids the hemoglobin con-
stitutes about 94 per cent.

Hemoglobin. — In the normal condi-
tion of the corpuscle the hemoglobin is in
an amorphous condition and is com-
bined in some unknown way with the
stroma.

When- hemoglobin is decomposed in
the absence of oxygen it undergoes a
cleavage into a protein, globin, and an
iron-holding pigment, hemochromogen,
which constitutes about 4 per cent, of
the molecule. If a solution of hemo-
chromogen be exposed to air it absorbs
oxygen and is converted into hematin.
This latter compound can also be de-
rived directly from hemoglobin by the
action of acids and alkalies. It is to the
presence of hemochromogen in combina-
tion with the protein globin that the
hemoglobin is indebted for its power of
absorbing and carrying oxygen.
If blood which has been rendered laky, by water or any other of the
known agencies, be allowed to evaporate slowly, the dissolved hemoglobin
undergoes crystallization. The rapidity with which the crystals form varies
in the blood of different animals under similar conditions. 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. 98). 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. Notwith-
standing these slight differences, 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 defibrinated blood, especially that of the dog, rat,

FIG. 98. — CRYSTALLIZED H E M o
GLOBIN. a, b. Crystals from venous
Wood of man. c. Ffom blood of cat.
d. Of guinea-pig, e. Of marmot. /.
Of squirrel. — (Gautier.)

THE BLOOD

245

guinea-pig, and horse, with acetic or ethylic ether and then add a solution
i 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 m a practically pure form, and when subjected
to a temperature of ioo°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.

Dog

Horse

Dog

Guinea-pig

53-91
22.62
6.62
15.98
0-54
o-33

Jaquet.

5i-i5

'i.ti

17.94
°-39
o-33

Zinoffsky.

53.85
21.84

7-32
16.17

o-39
0-43

54-12
20.68
7.36
16.78
0.58
0.48

Hoppe-Seyler.

The percentage composition of hemoglobin is thus seen to vary slightly
in different animals, suggesting that there may be different kinds of hemo-
globin. The molecular composition is not known. On the assumption
that each molecule contains one atom of iron, Preyer suggested the following
empirical formula: C600H980N154O179S3Fe, with a molecular weight of
*3>332; Jaquet has suggested a different formula: C758H1203N195O218S3Fe,
with a molecular weight of 16,669. It is verv 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 one generally
employed. Chemic analysis has shown that hemoglobin contains 0.33 per
cent, and blood 0.046 per cent, of iron; with these two factors the quantity
of hemoglobin can be determined by the following formula: #=I00^°46 =
14 per cent. The total quantity of hemoglobin in the blood, assuming
the latter to be about 3684 grams (one-nineteenth of the body- weight, 70
kilos) will therefore amount to 515 grams; e.g., x= 368**14 =515. The
total amount of iron in the blood is obtained by the following formula:

vz.

,==I>7o grams.

Clinic Methods for the Determination of the Percentage of Hemo-
globin. — Under normal physiologic conditions the percentage of hemo-
globin undergoes but slight variation. In pathologic states there is fre-
quently a great diminution in the amount, especially in chlorosis, splenic
leukemia, and pernicious anemia, diseases in which it diminishes to a con-
siderable per cent, in many instances. For clinic purposes it becomes a
matter of importance to have some method by which the diminution of
hemoglobin can be determined. In the various methods employed the
normal amount of hemoglobin is considered as 100 per cent, and the normal

246 TEXT-BOOK OF PHYSIOLOGY

number of red corpuscles, 5,000,000 per cubic millimeter, is also considered
as 100 per cent. Under such conditions the corpuscles have a normal color
known as the color index. This is expressed by a fraction of which the
percentage of hemoglobin is the numerator and the percentage of corpuscles
the denominator. The normal color index is therefore i or unity. In
some pathologic states the hemoglobin alone diminishes, the number of the
corpuscles remaining the same; in this instance the color index is less than
unity, e.g., if the hemoglobin be reduced to 80 per cent, as determined by the
method to be described, then the color index will be T^°F = o.8 which indicates
that each corpuscle retains but eight-tenths of the normal amount of hemo-
globin, or stated in the reverse way, each corpuscle has lost two-tenths of the
normal amount of its hemoglobin. In other pathologic states there is both
a diminution in the percentage of the hemoglobin and in the percentage of
the corpuscles and the diminution may be equal or unequal in degree. If
the diminution' be equal the color index is unity; if it be unequal the color
index is less or greater than unity; e.g., if the percentage of hemoglobin be
but 60 and the percentage of red corpuscles, as determined by the method
of counting be but 80 (4,000,000 per cubic millimeter) then the color index
is 6-0 = 0.7 5 which indicates that each corpuscle retains but three-fourths
of the normal amount of hemoglobin; if on the contrary the percentage of
hemoglobin be but 60, and the percentage of red corpuscles be but 50 then
the color index is 1.2 which indicates that each corpuscle contains a larger
percentage of hemoglobin than normally. This condition is sometimes
observed in pernicious anemia.

For the determination of these variations in the hemoglobin for clinical
purposes two chromometric 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
present the same intensity of color, their richness in coloring-matter is the
same. There are two methods by which this can be done: (i) 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 wa ter and then
finding an identical color which represents a previously determined quantity
of hemoglobin (v. Fleischl).

Gowers' hemoglobinometer consists of two glass tubes of exactly the
same size and similar to those shown in Fig. 99. One, A, 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 i per cent, solution. The other tube, B, is ascendingly graduated
with 120 divisions, each one of which corresponds to 20 c.mm. With a
graduated pipette, D, 20 cubic millimeters of blood are accurately measured
and dropped into the bottom of the tube B, in 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 stand-
ard. ^ 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

THE BLOOD

247

standard tint at 60 divisions, the blood contained but 60 per cent, of the
normal amount of hemoglobin.

FIG. 99. — HALDANE'S MODIFICATION or COWERS' APPARATUS.

Haldane's Modification of Gowers' Method. — Haldane's hemoglobin-
ometer, Fig. 99, is a modification of that of Gowers. The tube A contains

FIG. loo.— VON FLEISCHL'S HEMOMETER. 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. Pinion to move K. S. Mirror of plaster-of-Pans.

also a i per cent, solution of blood having the normal percentage of hemo-
globin saturated with carbon monoxid. With the graduated capillary
pipette, D, 20 cubic millimeters of blood are then obtained from a slight

248 TEXT-BOOK OF PHYSIOLOGY

wound in the finger or elsewhere and then dropped into the tube B, in which
a small quantity of distilled water from E was previously placed to prevent
coagulation. The cap of G is then attached to a gas burner, through which
flows either pure CO or a gas containing CO and the rubber tube inserted
into D to the level of the water. After the gas has been flowing for a few
seconds the rubber tube is withdrawn, and while the glass tube is yet full of
the gas, it is closed with the thumb and gently shaken so as to convert all the
hemoglobin into carbon-monoxid hemoglobin. This is then diluted very
gradually as in the employment of the Gowers apparatus until the tint of
the solution in B corresponds to that in A. The level at which this is
observed indicates the percentage of hemoglobin in the blood used. The
error in this method scarcely exceeds i per cent.

Von Fleischl's hemometer consists of a metallic cell divided into two
compartments, a and a', by a vertical partition (Fig. 100). 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 similar pigment, the color of which passes
from a deep red at one end to clear glass at the other (Fig. 101). 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 illum-
ination of the blood and glass wedge is
FIG. IOI.-TINTED GLASS WEDGE OF THE accomplished by lamp-light reflected
VON FLEISCHL HEMOMETER. 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 the normal amount of hemoglobin.

Absorption Spectra.— Both oxyhemoglobin and reduced hemoglobin,
like other soluble pigments, have an absorbing influence 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 decomposes 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. 102. It consists of a
tube, B, which has at one end a slit that can be narrowed or widened by
means of a screw. The light, having passed through it, falls on an achro-
matic convex lens (called the collimator) at the opposite end of the tube
which renders the divergent rays of light parallel. These parallel rays sub-
sequently 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. 103. If a colored
medium be held in front of the slit so that the light has to pass through it first,
certain dark bands will appear in the spectrum, owing to the absorption of
certain rays.

THE BLOOD

249

Dilute solutions of arterial blood show absorption bands between the
Fraunhofer lines, D and E, in the green and yellow portion of the spectrum.

FIG. 102. — 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.)

(See Fig. 103.) The band nearest D, frequently designated as alpha, is dark
in the center and sharply denned. The band which lies toward E, desig-
nated as beta, is broader and less sharply defined.

Yellow.

Green.

Cyan Blue.

E b

100 no

Oxy

hemoglobin
0.8 %.

Oxy-

hemoglobin
0.28 %.

. •

Carbon
Monoxid
Hemoglobin

Reduced
Hemoglobin

FIG. 103.— SPECTRA OF HEMOGLOBIN AND SOME OF ITS COMPOUNDS.
— (Landois and Stirling.)

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

250

TEXT-BOOK OF PHYSIOLOGY

widths and intensities, it becomes necessary, in order to obtain the character-
istic bands, to employ only dilute solutions.

The absorption spectra, as seen with different strengths of solution one
centimeter thick, are shown graphically in Fig. 104. It will be observed
that solutions varying in strength from o.i per cent, to 0.6 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 slightly 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. Solutions less than o.oi 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. 103), frequently designated as gamma, broader

aBC D Eb f G h

FIG. 104. — GRAPHIC REPRESENTA-
TION or 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. — (Rollet.)

aBC D Eb F G h

FIG. 105. — GRAPHIC REPRESENTATION
OF THE ABSORPTION OF LIGHT IN A SPEC-
TRUM BY SOLUTIONS OF REDUCED HEMO-
GLOBIN OF DIFFERENT STRENGTHS. The
shading indicates the amount of absorp-
tion of the spectrum, and the numbers at
the side the strength of the solution. —
(Rollet.^)

and less marked between the lines D and E, but extending slightly beyond D.
Fig. 105 shows in the same graphic manner the increasing breadth of the
absorption band with increasing strengths of solution, as well as the simul-
taneous absorption of light at both the red and violet ends of the spectrum.
Oxyhemoglobin; Reduced 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 onee
combines with oxygen and is converted into oxyhemoglobin, which imparts
to the blood or solution a bright red or scarlet color. If the blood or solu-
tion be now deprived of oxygen, the oxyhemoglobin is converted into reduced
hemoglobin, which imparts to the blood or solution a dark bluish-red or pur-
ple color.

The quantity of oxygen absorbed by i gram of hemoglobin is estimated

THE BLOOD 25I

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, however, is not due entirely to a fall of
pressure but partly to the dissociation force of heat, which increases in power
as the pressure falls. 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 hemo-
globin 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 hemoglobin 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 portion 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 capillaries about 5 per cent,
only gives up its oxygen and becomes reduced, so that both kinds are present
in venous blood. In asphyxiated blood only reduced hemoglobin is present.
It is this capability 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 constituent of
coal-gas and more largely of 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 again to
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 forming a stable com-
pound with hemoglobin and thus interfering with its respiratory function
that carbon monoxid owes its poisonous properties. Examined spectro-
scopically, 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. 103.) A useful test for CO blood
is the addition of caustic soda, which produces a cinnabar red precipitate.

Carbo-hemoglobin.— When hemoglobin is exposed to an atmosphere
of oxygen and carbon dioxid, it will absorb the latter as well as the former,
though the union of the oxygen under such circumstances is not as strong
as when oxygen alone is present. To the compound formed by the union of

252 TEXT-BOOK OF PHYSIOLOGY

the carbon dioxid and the hemoglobin the name carbo-hemoglobin has been
given. The union, however, is not very stable as the carbon dioxid can be
readily dissociated. As the carbon dioxid absorption will take place,
even though the hemoglobin is practically saturated with oxygen, the as-
sumption has been made that the oxygen combines with hemochromogen,
and the carbon dioxid with the protein constituent — globin. If this be the
case the hemoglobin and hence the red corpuscle, can be regarded as con-
cerned in the absorption and transmission of carbon dioxid from the tissue
to the lungs.

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 alkalies decom-
poses it and develops one or more protein bodies to which the general term
globulin has been given, and .an iron-holding pigment 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 acids or alkalies, two combinations of hematin can be
obtained (acid and alkaline), each of which has special properties, giving
rise to different absorption bands.

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
extra vasa ted into the tissues. In its chemic composition 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.

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 condi-

THE BLOOD 253

tions, 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 situations 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
derivatives 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 con-
cerned in the further reduction and elimination of the hematin, is uncertain.
The genetic relationship 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 eliminated.
It appears also from chemic considerations that the hemoglobin will undergo
cleavage into a globulin body and hematin, which by the loss of its iron is
readily converted into the bile-pigment, bilirubin. The amount of this
latter pigment may therefore be taken as an index of the extent of corpuscular
destruction.

There is some evidence for the view that the iron set free by the reduc-
tion of the hematin, eventually reaches the blood-making organs and is
utilized by the developing corpuscles in the formation of the necessary
hemoglobin.

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 num-
ber 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 histologists that
in adult life the red corpuscles are derived from embryonic forms, the so-
called erythroblasts, cells of a large size with distinctly reticulated nuclei,
which are found chiefly in the red marrow of the long bones.1 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 appear-
ance most probably by a transformation of pre-existing marrow cells which
cross the capillary wall from without. 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 content. In the course of their development the nucleus
becomes smaller and denser, when the cells are known as normoblasts.
Subsequently the nucleus is extruded, carrying with it a portion of the peri-
nuclear cytoplasm, after which the remainder of the corpuscle assumes

1 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.
ii and 12; also paper by Prof. W. H. Howell, Journal of Morphology, vol. iv, 1892.

254

TEXT-BOOK OF PHYSIOLOGY

the shape and size of the adult corpuscle and is carried out into the general
circulation. After severe hemorrhage the formative processes in the marrow
may become so active that erythroblasts and normoblasts make their appear-
ance in the blood-stream before the extrusion of the nucleus has taken
place.

The Corpuscles of Other Vertebrated Animals. — In all mammals,
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 discs. In the animals excepted
the corpuscles are oval. The size, however, varies in different animals from
0.0092 mm. (-yV^ mcn) m tne elephant to 0.0023 mm. (T ^-^ mcn) m tne
musk-deer, while in most animals the average lies between 0.0084 mm. and
0.0050 mm. Inasmuch as the question may arise as to whether the corpus-
cles 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 is a matter of medicolegal as well as of
physiologic interest. Though the differences in size
are slight, yet it is possible for skilled microscopists,
when examining fresh blood, to make a diagnosis
between the corpuscles of man and those of the
domesticated animals, with the exception, perhaps,
of the guinea-pig. The diagnosis of the corpuscles
FIG. 106. FIG. 107. °f dried blood which have been altered by the ac-
AMPHIBIAN COLORED tion of various external agents, even though capa-
BLOOD-CORPUSCLES. ^ Fig. ble of a certain degree of restoration, is most dif-
ficult, and should not be attempted in criminal
cases without large experience in microscopy, in
measurements and methods of preparation of all

kinds of blood-corpuscles, and a proper conception 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) :

Stirling.')

Gulliver

Wormley

/-> 01. -J*. French M edi-
u- b(?hmidt colegal Soc. Formad
Mallmm Welcker

Inch

Mm.

Inch

Mm.

Inch

Mm.

-Inch

Mm.

Inch I Mm.

Man

.3200
•3S38
•3532
.3607
.4267
.4230
.4600
.4400
•S3°o
.6366

0.0079
0.0071
0.0071
0.0070
0.0060
0.0060
0.0057
0.0058
0.0048
0.0040

.3250 0.0078
.3223 0.0079
.3561 0.0071
.3653 0.0070
.4219 0.0060
.4268,0.0059
.4243 0.0059;
.4372 0.0058
i .4912 0.0031
i .6189 0.0041

.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
o. 0045
0.0040

• 3257
•32I3T
.3485
•3653
•4545
.4098
• 4545
• 3922
.5076
.5525

0.0078
0.0079
0.0073
0.0069
0.0056
o .0062
0.0056
0.0065:
0.0059
0.0046

.3200 0.0079
.3400 0.0075
.3580 0.0071
.3662 0.0069
.4200 0.0060
.4250 0.0060
.4310 0.0059

Guinea-pig

Dog...... ..

Rabbit...

ox. ;

Pig

Horse. . . .

Cat

Sheep
Goat

.5000 0.0051
.6100 0.0042

* Masson.

f Woodward.

In birds, reptiles, and amphibians the corpuscles are larger than in
mammals, are oval in shape, and nucleated. (See Figs. 106 and 107.)
As the scale of animal life is descended the corpuscles increase in size, until
in Proteus and Amphiuma the long diameter attains an average length of
o.o58mm. and 0.077 mm. respectively. In fish the corpuscles are smaller,

THE BLOOD 25S

oval, and nucleated, with the exception of the lamprey eels, in which they
are circular, biconcave, and nucleated, though the nucleus is generally con-
cealed 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!

The Function of the Red Corpuscles.— The red corpuscles, by virtue
of the capacity of their contained hemoglobin for oxygen absorption, 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, anol the
energy liberated through oxidation is correspondingly large. 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 substance is increased
in proportion to the extent to which it is subdivided.

The superficial area of a single human red corpuscle has been estimated
at 0.000128 sq. mm. If the number of corpuscles in i cubic millimeter of
blood averages 5,000,000, the superficial area would amount to 640 square
millimeters; and if the amount of blood in the body of a man weighing 70
kilos is taken as one-nineteenth of this weight — that is, 3864 grams (3659 c.c.)
—the total area of the corpuscular surface will amount to 2341 square meters.

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. 93). A careful examination of the blood by the employ-
ment of appropriate methods has revealed the presence of several varieties of
white corpuscles, to which reference will be made in a subsequent paragraph.
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 0.004 to 0.013 mm.,
though the average is about o.on mm. or about -j-sVfr 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 large. By various reagents it has been demonstrated that the
granules are fatty, protein, and carbohydrate (glycogen) in character. In
the fresh cells the existence of a nucleus is difficult of detection, though i
presence can be demonstrated by the addition of acetic acid, which renders
the perinuclear cytoplasm more transparent and makes the nucleus con-
spicuous and sharply defined. From its structure it is apparent that the
white corpuscle belongs to the group of undifferentiated tissues and resembles

256

TEXT-BOOK OF PHYSIOLOGY

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 90 per cent, is water and the remainder solid matter
consisting mainly of proteins, 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 corpuscles 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

FIG. 108.— AMCEBOID MOVEMENTS OF A WHITE CORPUSCLE FROM THE FROG. The form
changes occurred within ten minutes. The black particles are Chinese ink which had been
injected twenty-four hours before into the dorsal lymph sac.—(Rauber-Kopsch.)

variations in physiologic conditions. In the blood of the splenic artery there
is but i white to 2260 red, while in the splenic vein there is i white to every
60 red; or about thirty-eight times as many as in the artery. In the portal
vein there is i white to 740 red, while in the hepatic vein there is i 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 physiologic conditions. The
ingestion of food rich in protein material raises the count from 30 to 40 per
cent., as Compared with the count before the meal. 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 primiparae, the
number increases to 16,000 to 18,000. Many pathologic conditions of the
body also influence the count very considerably.

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

THE BLOOD

257

cubic millimeter, after which it rose to 1530, where it practically remained
for the succeeding three weeks of the fasting period.

When the number of leukocytes present in the peripheral blood exceeds
the normal, i.e., 10,000 per cubic millimeter the condition is termed leuko-
cytosis; when the number falls below the normal the condition is termed
leukopenia. Both conditions, however, may be only temporary and therefore
physiologic, or they may be permanent, associated with certain diseased states
of the body and therefore pathologic. It is therefore permissible to speak
of a physiologic and a pathologic leukocytosis and
leukopenia.

The method for counting the white corpuscles
is similar to that used in counting the red corpus-
cles. 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
that used for the red, and a much greater number
of squares in the counting chamber should be
counted, so as to diminish the percentage of error.

Physiologic Properties. — The white corpuscles
and especially the leukocytes possess the character-
istic property of exhibiting movements similar to
those exhibited by the amoeba, and are therefore
termed amoeboid. These movements consist in al-
ternate protrusions and retractions of portions of the
cell body, as a result of which they present from mo-
ment to moment, a great variety of forms. (See
Fig. 108.) The protruded process, the pseudopod,
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 reference to its environment. By
virtue of these amoeboid movements the corpuscle
can appropriate small particles of pigment, such
as indigo or carmine, and after a short time elimi-
nate them from various parts of the surface. It
is also capable of thrusting a process into and through the wall Jof the
capillary vessel, after which the remainder of the corpuscle follows (Fig.
109). 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 4O°C., beyond which they cease,
owing to the coagulation of the cell-substance. A low temperature also
arrests the movements. Induced electric currents also cause contraction
and death of the cell. Moisture and oxygen are necessary to their activity.
17

FIG. 109. — SMALL VES-
SEL SHOWING VARIOUS
STAGES IN THE DIAPEDESIS
OF LEUKOCYTES.— (G. Bach-
man.)

258 TEXT-BOOK OF PHYSIOLOGY

From their similarity to lower organisms the white corpuscles may be re-
garded 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 methy-
lene-blue stains, reveals the presence of five distinct varieties of leukocytes
and transitional forms which may be classified as follows:

1. Small lymphocytes, so called from their resemblance to the corpuscles

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. They vary in size from
0.004 to 0.007 mm.

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 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. Transitional forms, usually pre-
sent from i to 2 per cent, are very much like the large lymphocyte in
appearance and size, with the exception, however, that they possess a
cresentic or indented nucleus and have a somewhat greater affinity for
basic dyes. They are usually counted with the large lymphocytes.
Both varieties of lymphocytes are characterized by a cytoplasm
which is devoid of granules. Rarely, basophilic granules may be
present.

3. Polymorphonuclear neutrophiles. The nucleus of this cell is irregular

and assumes a great variety of shapes in different cells, a feature which
has suggested the name given to the cell. The perinuclear cystopksm
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. They
vary in size from 0.007 to o.oio of a mm.

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 shape and many cells possess several apparently dis-
tinct nuclei. The cytoplasm is ill-defined but its presence is easily
revealed through the large, intensely 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.

In abnormal states of the blood other forms of leukocytes are fre-
quently present, e.g., myelocytes, leukoblasts, myeloplaxes, etc., the
significance of which is not always apparent.

Origin of the White Corpuscles.— Of the various theories advanced 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 lympho-

(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 i 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 meth-
ylene-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, ii. Polynuclear Neutrophiles.

These cells are characterized by a polymorphous or polynuclear nucleus, surrounded by
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, polymorphous.

12, 13. Eosinophiles.

The nuclei are not unlike those of the polynuclear neutrophile, except that they are some-
what 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. Eosinophile 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 represented 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 5 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 cannot 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 n.
28, 29. Eosinophiles.

Compare with 12 and 13.

30. Eosinophilic Myelocyte.

Compare with 14.

31. Basophile. (Finely granular.)

This cell is characterized by the presence of exceedingly fine ^-granules, 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» 33, 34, 35, 36. Mast Cells.

The granules take a modified basic color, as shown by their royal-purple tint in this illus-
tration. Note their unusually large size and ovoid shape in 35, 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 chromatin of the mast cell is so deli-
cate and so feebly stained that it is barely visible. These cells were found in the blood of a
case of splenomedullary leukemia.

PLATE I.

.•3*?'2£"*ff'~

s — x

ooQ

THE LEUKOCYTES.
(2-16, Triacid Stain; 17-36, Eosin and Methylene-blue.)

(E. F. FABER, jec.)
(From DaCosta's " Clinical Hematology.")

THE BLOOD 259

cytes which take their origin entirely from the lymph-adenoid tissues of the
body, e.g., the lymph-glands, solitary and agminated follicles of the intes-
tines, etc. As the lymph flows through these structures the lymph-corpus-
cles, as the future lymphocytes 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 polymorphonuclear,
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 transi-
tional 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 unconvincing.

After an unknown period of life the leukocytes undergo dissolution and
disappear.

Functions. — The functions of the white corpuscles are but imperfectly
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 insoluble 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 particle or bacterium and digesting it. On account of this swallow-
ing action these cells were termed by Metchnikoff phagocytes and the process
phagocytosis. In their capacity as phagocytes it has been assumed that
they are made more or less efficient by the presence in the plasma of an
agent or agents which in some unknown manner render the bacteria less
resisting and thus make them more susceptible to the attacks of the leuko-
cytes. These agents which are supposed to be secreted by the tissue cells
are termed opsonins, from their supposed function, that of preparing the
bacteria for leukocytic digestion. 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 sug-
gested that they are also engaged in the absorption of fat from the lym-
phoid tissue of the intestine. In their dissolution they contribute to the
blood-plasma certain protein materials which assist under favorable
circumstances in the coagulation of the blood.

HISTOLOGY OF THE BLOOD -PLATELETS

The blood-platelets or plaques are small histologic elements circulating
in the blood-plasma. They were discovered and described in 1845 b7 Arnold.
Hayem, later applied to them the term hematoblasts, on the supposition
that they 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-

26o TEXT-BOOK OF PHYSIOLOGY

pig and bat), they are now regarded as normal constituents of the blood and
designated sometimes as the third corpuscle. When blood is freshly drawn
from the body, the plaques rapidly undergo disintegration and disappear;
but by treating the blood with osmic acid, the form and structure of the
plaque may be retained. They may also be preserved by preparing and
staining the tissues with Wright's blood stain.

The blood-platelet may be denned as a colorless, grayish- white, homo-
geneous or finely granular protoplasmic disc, varying in diameter 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, though the central portion is granular and
the peripheral portion clear. The ratio of the plaques to the red corpuscles
is i to 1 8 or 20, and the total number per cubic millimeter has been estimated
to be 250,000 to 300,000 or more.

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 blood-plaques can be seen with high powers of the microscope 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 examined microscopically with suitable powers will show large
numbers of plaques within the blood-vessels.

As to the origin of the blood-platelets there has been much difference of
opinion. Many theories have been proposed, none of which have been
accepted. As a result of long-continued observations Wright has recently
published results which make it probable that they are fragments or detached
portions of the cytoplasm of giant cells, megakaryocytes, found in the
marrow of the bones. The cytoplasm is prolonged into pseudopod-like
processes which become detached, and as they are in close relation to
the blood channels they are soon taken up and carried into the blood
of the general circulation when they are known as blood-platelets or
plaques.

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 coagula-
tion of the blood. Whenever they are diminished in number, as in purpura
and hemophilia, coagulation takes place very slowly.

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

THE BLOOD 26X

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 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 be-
comes apparent. By this method it has been shown that the ratio of blood
to body- weight in a human adult is i : 13; in an infant, i : 19; in a dog, i : 13;
in a cat, i 121.

The more recent investigations of Haldane and Smith and of Plesch
with the employment of a different method make it probable that the ratio
is approximately i '.19. Thus a man weighing 70 kilos would have 3684
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, J; liver, J; muscles, i;
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:

(Water 200 200

f Hemoglobin no

Solids 128 \ Other organic matter 10

[Salts 2

f Water 604 604

Fibrin 7

Plasma 672

L Solids 68

Albumin 5a

Fat i

Other organic matter 3

Potassium and sodium salts 4

Calcium and magnesium salts i

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 approxi-
mating the truth, are more or less incomplete and in some respects contra-
dictory. Since the coagulation is coincident with the appearance of ^th
fibrin, the antecedents of this substance, the physical and chemic conditions

262 TEXT-BOOK OF PHYSIOLOGY

which condition its development, and the succession of chemic changes
involved must be determined, before any consistent theory can be established.

Extra-vascular Coagulation. — At present it is generally believed
that the immediate factors concerned in extra-vascular coagulation are
fibrinogen, a calcium salt, and an agent thrombin. As to the manner in
which these three bodies react one with another there is a diversity of opinion.

As an outcome of a long series of experiments that have been performed
to determine the nature and the succession of the chemic phenomena
underlying the coagulation of the blood, the following facts seem to be well
established, viz: the immediate cause of the coagulation is the appearance
of fibrin, a derivative of an antecedent substance always present in the blood
termed fibrinogen; the cause of the conversion of the soluble fibrinogen into
the insoluble fibrin is the presence and activity, under the circumstances, of
an agent termed thrombin, the chemic nature of which is a subject of discus-
sion. By some chemists it is regarded as a ferment which causes a mo-
lecular rearrangement of the fibrinogen; by others it is regarded as a definite
organic colloidal body which unites in some physico-chemic manner with
the fibrinogen to form fibrin.

The crux of the problem is the source and the conditions necessary for
the production of the thrombin. It is generally conceded that thrombin is a
derivative of an antecedent substance prothrombin or thrombogen, a substance
always present in the blood plasma, a product of the decomposition of blood-
platelets and leukocytes. With prothrombin there is physiologically associ-
ated a calcium salt, the presence of which is absolutely essential for coagula-
tion or the conversion of prothrombin into thrombin as was conclusively
shown by Arthus and Pages: For if it is precipitated by the addition of
oxalate of potassium, coagulation will not take place. At all times then,
there are present in the blood, prothrombin, a calcium salt and fibrinogen.
Given the two former factors, the question arises, why do they not react to
form thrombin in the circulating blood, and why do they so react in shed
blood?

The answer of Morawitz is, that prothrombin requires an activating
agent, a kinase which is wanting in circulating blood but is present in shed
blood. It is supposed to develop in the disintegration of the cell elements of
the blood, leukocytes and blood-platelets, and perhaps from the cell elements
of the injured tissues as the blood flows over them. Shortly after its appear-
ance the kinase, with the aid of the calcium salt converts the prothrombin into
thrombin, after which it unites with fibrinogen to form fibrin. For this
reason the kinase has been termed thrombo-kinase.

The answer of Howell to the foregoing question is somewhat different and
based on a long series of experiments recently published. From the results
of these experiments the answer given is that prothrombin is prevented from
reacting with the calcium salt to form thrombin in the circulating blood, by
reason of the presence and possible union with prothrombin of an agent
termed anti-thrombin. So long as this relation is not disturbed the blood
remains fluid. When blood is shed there is supposed to develop from the cell
elements of the blood, the leukocytes and blood-platelets, and perhaps from
the cell elements of the injured tissues as well, a plastin, the specific action of
which is to combine with the anti-thrombin and thus set free the prothrombin.
This having been accomplished the calcium salt activates the prothrombin,

THE BLOOD 263

and converts it into thrombin, after which it combines with the fibrinogen.
For this reason the plastic agent has been termed thrombo-plastin. Experi-
ments indicate that this agent belongs to the group of bodies known as
phosphatids and similar in its properties to one member of this group, viz.,
kephalin.

Intra-vascular Coagulation. — So long as the relations of the blood
and the vascular apparatus remain physiologic, no coagulation occurs in the
vessels. The reasons assigned for this are: (i) the absence of thrombo-
kinase in sufficient amounts; or (2) the presence of anti- thrombin. On either
assumption the reaction between pro thrombin and calcium with the forma-
tion of thrombin does not take place. If the vessels are injured as they are
when ligated or torn or in any way impaired, coagulation promptly takes
place with the subsequent occlusion of the vessel. As to whether the injured
tissues or the blood-cells now generate an agent, thrombo-kinase, which
activates the prothrombin and calcium, or whether they generate an
agent thrombo-plastin, which neutralizes an anti-thrombin, is a subject of
discussion.

Under pathologic conditions of the circulatory apparatus, especially of
the internal lining, intra-vascular coagulation frequently arises, though the
process cannot 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 thrombus 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 the vessels, at the orifices of torn blood-
vessels, consist largely of blood-plaques. A thrombus so formed may con-
tain 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 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 physiologic 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 conditions 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 vary-
ing with their activity, with a certain velocity and under a given pressure.

The apparatus by which these results are attained is termed the circula-
tory 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. 1 10.)

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 mechan-
ism is of primary importance.

THE PHYSIOLOGIC ANATOMY OF THE HEART

The heart is a conic or pyramid-shaped hollow muscle 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 fibro-serous sac, the pericardium,
attached to the great vessels at its base.

The heart is a hollow, double muscle organ, consisting of a right and a
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

264

THE CIRCULATION OF THE BLOOD 267

the upper and central portion of the septum, there is, however, a small region
which is thin owing to the absence of muscle-tissue and composed of endo-
cardium only. This region is known as the pars membranacea septi.

The Cardio-pulmonic Vessels.— Though the two sides of the heart
are separated from each other by the auriculo-ventricular septum, they are
anatomically and physiologically connected by the intermediation of the
pulmonic system of vessels: viz., the pulmonic artery, capillaries, and
veins (Fig. no).

The pulmonic artery arises from the conus arteriosus of the right ven-
tricle. After a short upward course it divides into a right and a left branch,
which enter the corresponding lungs. The vessel at once divides and sub-
divides into a number of branches, which, after following the bronchial tubes
to their termination, give origin to capillaries that surround the air-cells of
the pulmonic 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-pulmonic air, and the
exchange of gases — the excretion of carbon dioxid and the absorption of
oxygen — for which the cardio-pulmonic vessels exist, is readily accom-
plished.

The pulmonic veins which return the arterialized 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 pulmonic
veins — two from each lung — enter the posterior wall of the left auricle.

The Course of the Blood through the Heart. — There is thus estab-
lished 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-pulmonic
vessels, and the left side of the heart.

The venous blood flowing toward the heart is emptied by the superior
and inferior vena cava into the right auricle, from which it passes through
the auriculo-ventricular opening into the right ventricle (Fig. 1 10) ; thence
into and through the pulmonic artery and its branches to the pulmonic
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 oxygen — and
changed in color from bluish-red to scarlet-red. The arterialized blood,
flowing toward the heart, is emptied by the pulmonic veins into the jeft
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 giving up of a portion of its oxygen to the tissues and the absorp-
tion 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 into the left side from the pulmonic
veins in equal quantities and velocities.

Though there is but one set of capillaries, as a rule, between arteries and
veins, there is an exception in the case of the arteries and veins of some of the
abdominal viscera. Thus the veins emerging from the capillaries of the

268 TEXT-BOOK OF PHYSIOLOGY

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 inferior vena cava just below the diaphragm.
From the foregoing facts physiologists frequently divide the general
circulation into:
i. The pulmonic 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.

FIG. 113.— RIGHT CAVITIES OF THE FIG. 114.— RIGHT CAVITIES OF THE HEART.
HEART. — Auricuio-venlricular valve open, Auriculo-ventricular valve closed, semilunar
semilunar valves closed. — (Dalton.) valves open. — (Dallon.)

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
relaxation of the muscle walls of the heart. That the movement may be a
progressive one, that there shall be no regurgitation during either the con-
traction or the relaxation, it is essential that some of the orifices of the
heart be closed during each of these periods. This is accomplished by the
heart valves.

The right auriculo-ventricular opening is surrounded and strengthened
by a ring of fibrous tissue to which is attached a membrane partially sub-
divided into three portions or cusps, which during the period of relaxation
are directed into the ventricle (Fig. 113); during the period of contraction
ley are raised and placed in complete apposition, when they act as a valve

THE CIRCULATION OF THE BLOOD

269

preventing a backward flow into the auricle (Fig. 114). In the former
position the valve is open; in the latter, shut. For these reasons this struc-
ture is known as the tricuspid valve. This valve is formed of fibrous tissue
derived from the fibrous ring, and some muscle-fibers, and 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 its being floated into the auricle.

The orifice of the pulmonic artery is also surrounded by a ring of
fibrous tissue to which are attached three semilunar or pocket-shaped mem-
branes, 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
Aurantii). 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 nod-
ule, 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 ap-
position and prevent a return of
the blood into the ventricle (Fig.
113); during the contraction they
are directed into the artery (Fig.
114). In the former position they
are shut; in the latter, they are open-

The left auriculo-ventricular opening is provided with a similar thou
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 respects to those of the tricuspid valve. The
orifice of the aorta is also surrounded by a ring of fibrous tissue to which
are attached three semilunar or pocket-shaped valves (Fig 112) which in
their arrangement, connections, and mode of action are similar in a 1 respects
to those at the orifice of the pulmonic artery. The anatomic relations of
the cardiac orifices one to the other and the appearance presented by t
valves when closed are represented in Fig. 115-

The Heart Muscle-fibers and Their Arrangement.- The muscle-
fibers of the heart represent in their structure a type between the ordinary
striated muscle and the smooth muscle. A longitudinal section of
heart-muscle shows a reticulated arrangement of the fibers the outcome of
a similar reticulated condition of the mesodermic material in which hey
develop The mesodermic reticulum containing numerous nuclei

FIG. 1 1 5.— VALVES or THE HEART, i. Right
auriculo-ventricular orifice, closed by the tri-
cuspid valve. 2. Fibrous ring. 3. Left auric-
ulo-ventricular orifice, closed by the mitral
valve. 4. Fibrous ring. 5. Aortic orifice and
valves. 6. Pulmonic orifice and valves. 7,8,9.
Muscular fibers. — (Bonamy and Beau.)

270

TEXT-BOOK OF PHYSIOLOGY

termed a syncytium. As the heart develops the muscle-fibers make their
appearance in the protoplasm and assume an arrangement which corre-
sponds to that of the trabeculae composing the reticulum (Fig. 116). In
the adult heart the intermediary spaces are reduced to narrow clefts in
consequence of the multiplication of the muscle-fibers. The clefts are
occupied with connective tissue, blood-vessels, lymphatics, etc. The indi-
vidual fiber consists of alternate dim
lateral union an(j \{gfa bands similar to the corre-

sponding bands of the ordinary skeletal
muscles, though it is devoid of a sar-
colemma. Among the fibers large oval
nuclei are distributed. At varying inter-
"vals the fibers are interrupted by inter-
calated discs. When the heart muscle
is treated with caustic potash the trabec-
ulae separate at the level of these discs,
forming what has hitherto been termed
the muscle cell or fiber.

The arrangement of the muscle-
fibers is quite complicated and in ac-
cordance with the functions of the in-
dividual portions of the heart. In the
auricles the fibers are arranged in two
sets: an outer transverse set, which
pass from auricle to auricle, and an
inner longitudinal 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 the

— _ ._ „ LONGITUDINAL SEC- auricles are practically independent of
TION OF A, PAPILLARY MUSCLE OF THE HUMAN each other. Circularly arranged fibers
HEART, x 360. — (Stokr.) are present near the terminations of

the venae cavae and pulmonic veins.

In the ventricles the muscle-fibers 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 vertical manner
after which they ascend to terminate in the wall of the septum, the columnae
carneae and musculi papillares. The superficial fibers of the posterior sur-
face 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 termi-
nate in the structures of the left ventricle, while those from the left ventricle
terminate in the structures of the right ventricle. Longitudinal fibers are
also found on the inner surface. The transverse fibers are very abundant

Nucleus of Nucleus of Intercalated

a muscle a connective disc.

THE CIRCULATION OF THE BLOOD 271

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
insure a complete and simultaneous discharge of blood from both auricles
as well as from both ventricles (Fig. 117).

FIG. 117.— ARRANGEMENT OF VENTRICULAR MUSCLE-FIBERS. — (After MacCallum.) I and
II, Superficial fibers of the left ventricle and conus arteriosus; III, deep layers of the left ven-
tricle; LAV, mitral orifice; RAV, tricuspid orifice; PA, pulmonic artery.— (From Hirschf elder.)

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 (and 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 anatomically and physiologically.

In 1893 Wilhelm His, Jr., discovered the existence of a band or bundle
of muscle-fibers which apparently took its origin from the posterior part of
the right side of the auricular septum, 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-bundle of His." In 1904 Retzer and
Braunig, working independently, 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

2?2 TEXT-BOOK OF PHYSIOLOGY

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 investiga-
tions 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 histologic 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. From their origin the fibers converge to form a dis-
tinct bundle which then passes forward on the right side of the auricular
septum between the lower edge of the fossa ovalis and the auriculo-ventricu-
lar septum; just above the insertion of the median cusp of the tricuspid
valve the bundle presents a very complicated network of muscle-fibers which
has been designated as a knot or the auriculo-ventricular node or the node
of Tawara; from the anterior portion of the node a bundle of fibers turns
downward and penetrates the auriculo-ventricular septum, beyond which it
passes below 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 endocar-
dium, 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 ven-
tricular 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 direc-
tions over the entire 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 sup-
posed function the " conduction sy stem " are histologically different from the
cardiac fibers, in so far as they are poorer in sarcoplasm and similar in their
appearance to embryonic muscle-fibers. In the auricular portion of the
bundle the fibers exhibit a more or less reticular arrangement; in the ven-
tricular portion, the fibers are more regularly arranged, are richer in sar-
coplasm and present a number of fibrillse near their periphery. In asso-
ciation with the muscle-fibers composing the auriculo-ventricular bundle
there is a special collection of nerve-cells and nerve-fibers. Their function
is unknown.

The ultimate termination of the system, beneath the endocardium, con-
stitutes 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 dis-
tributed in the human heart.

The Keith-Flack Node or the Sino-Auricular Node.— This is a
small body, discovered by the investigators whose names it bears, situated
in the sulcus terminalis "just below the fork formed by the junction of the
upper surface of the auricular appendix with the superior vena cava." It

THE CIRCULATION OF THE BLOOD 273

appears to be a remnant of primitive muscle tissue at what was formerly the
junction of the sinus venosus and the auricle. In its structure it resembles
the auriculo-ventricular (Tawara's) node, in that it consists of peculiar
muscle-fibers, nerve-cells, and nerve-fibers enclosed by connective tissue. It
is also provided with an abundant blood-supply. In the human heart', the
muscle-fibers of this remnant are striated, possess well marked and elongated
nuclei and are plexiform in arrangement. From the node the muscle-fibers
extend downward along the sulcus terminalis for about two centimeters.
The thickness of the bundle is about two millimeters. Superiorly the node
appears to be connected with or continuous with fibers in the superior vena
cava; inferiorly it is connected with the true auricular fibers. The dissection
of this node shows that the terminal branches of the vagi and sympathetic
nerves are in histologic relation with the nerve-cells. The situation, struc-
ture and relations of this neuro-muscle node appear to justify the assump-
tion that it is directly concerned in the initiation of the heart-beat.

THE MECHANICS OF THE HEART

Methods of Observation.— The movements of the heart, as well as
many phenomena connected with the flow of blood through its cavities, have
been determined by observation of, and experimentation on, the exposed heart
of a mammal — e.g., dog, cat, rabbit — supplemented and corrected by experi-
ments on the heart in its normal relations. Valuable information as to the
heart-beat and the influences which modify it has been obtained from experi-
ments made on the isolated heart of the turtle, frog, and allied animals.

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 of the heart
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. ^

Phenomena Observed. — From many observations and experiments it
has been determined that the heart at each beat presents two distinct move-
ments 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 — pulmonic artery and aorta; the
other is the movement of relaxation, or the diastole, followed by a pause
during which the cavities again fill up with blood from the venae cavae and
pulmonic veins.

The contraction of any part of the heart is termed the systole; the relaxa-
tion, 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 auricu-
lar 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.

It has also been ascertained that the contraction of the auricles and
ventricles as well as their subsequent relaxations, though occurring with

2?4 TEXT-BOOK OF PHYSIOLOGY

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, viz., the vena cava, 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.

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 possibly 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.

Changes in Position and Form. — It is also apparent under the condition
of the foregoing observation that the heart during each pulsation undergoes
changes of both position and form. In the diastolic condition, during which
the heart is in repose, the apex is directed obliquely downward and to the
left; the body of the heart is enlarged and its walls relaxed. As the
systole begins and reaches its maximum, the apex is tilted upward, the
entire heart is rotated on its axis from left to right and forced forward by
the expansion and elongation of the pulmonic artery and aorta. As the
diastole begins and rapidly passes to its completion a reverse series of
movements is presented, viz. : an ascent of the heart due to the recoil and
shortening of the pulmonic artery and aorta, a rotation of the heart on
its axis from right to left, and a fall of the apex. With the completion of
this latter event, the heart for a brief period is in repose.

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 diaphragm, all
of which cooperate in keeping the heart, and more particularly the right
ventricle, in close contact with the chest wall and limiting its movements.
By means of needles inserted into the apex of the heart, through the chest
walls, it has been shown by their slight movement that the apex is
practically a fixed point.

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 approxi-
mates that of a circle. In passing from the diastolic to the systolic condition
the transverse diameter diminishes while the antero-posterior diameter
increases, and the whole heart becomes somewhat more conic in shape.
It is questionable if the vertical diameter perceptibly 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 impulse is principally observed in the
space between the fifth and sixth ribs about an inch internal to a line drawn

THE CIRCULATION OF THE BLOOD 2?5

vertically from the middle of the clavicle. The cardiac impulse is synchron-
ous with the cardiac systole.

The cardiac impulse may be recorded with an appropriate apparatus
known as a cardiograph; the record obtained with it is known as a cardiogram
A cardiograph consists 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 cardiographic
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. 118 will be obtained, in which the slight elevation a represents the
contraction of the auricle which, completing
the filling of the ventricle, 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 relaxa-
tion of the ventricle; while e-f represents the
time of the diastole, during which the heart
cavities are enlarging with the incoming of a FIG. n8.— A CARDIOGRAM.
new volume of blood in consequence of which —(After Faction.)

the heart is pressing against the chest walls. The systolic plateau is charac-
terized by one or more elevations and depressions, the true cause of which is
unknown.

The Cardiac Cycle. — The term cardiac cycle is employed to express the
sequence of events from the beginning of one auricular systole and the
beginning of the auricular systole which immediately follows it. An examina-
tion of the heart shows that each pulsation may be divided into three phases,
viz. :

1. The auricular systole.

2. The ventricular systole.

3. The pause or period of repose during which both auricles and ven-

at rest.

The Graphic Record of the Cardiac Cycle. — For the purpose of obtaining accurate in-
formation as to the sequence of events, their time relations, as well as of the pressure within the
heart cavities during each phase of its activity, it is necessary to obtain graphic records of the
entire cardiac cycle.

This was first successfully accomplished by Chauveau and Marey, by means of sounds or
tambours (Fig. 119) introduced through the jugular vein into the cavities of the right heart. Each
tambour consists of a metallic frame covered by a thin rubber membrane. By means of flexible
tubes, a. v., the interior of each tambour can be placed in communication with the interior
of a second tambour provided with a recording lever. Pressure applied to the cardiac tambour
will be followed by a movement of the enclosed air toward the recording tambour indicated
by an outward movement of its membrane and a rise of the lever; removal of the pressure will
be followed by a movement of the enclosed air toward the cardiac tambour indicated by an
inward movement of the membrane and a fall of the lever.

When the tambours are introduced into, and carefully adjusted to the interior of the right
heart, the auricular and ventricular contractions will exert pressure on their enclosed tambours
as indicated by the rise of the levers of the recording tambours, which continues so long as
the pressure lasts. With the relaxation of the auricular and ventricular walls the pressure is

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TEXT-BOOK OF PHYSIOLOGY

removed and the levers fall to their former position. When

a. V a\\ nv the levers are applied to the surface of a recording cylinder

a record of auricular and ventricular contractions is obtained
such as that shown in Fig. 120.

A similar record would be obtained if the tambours were
placed in the cavities of the left side of the heart.

In the graphic record, Fig. 120 obtained
by the foregoing method, it is apparent that
during the period of repose there is a gradual
ascent of the tips of the recording levers, the
result of a gradual increase of pressure due to
the accumulation of blood within the heart
cavities. When this reaches a certain level the
auricular contraction occurs rather suddenly,
followed by an equally sudden relaxation, after
which the auricular walls remain at rest for a
relatively long period, though the pressure with-
in the auricle undergoes variations both in
m a the way of increase and decrease as shown by

small undulations on the curve.

With the close of the auricular systole, the
ventricular systole occurs quickly and energet-
ically and endures for some time, after which
the ventricular walls quickly relax and remain
at rest until the close of the next auricular con-
traction. The summit of the ventricular trac-
ing generally spoken of as the plateau presents
a series of elevations and depressions as stated

metal in a foregoing paragraph.

A comparison of the two traces shows that
. : f . . . . , 1,11

between the close of the auricular and the be-
ginning of the ventricular systole there is a slight
pause known as the inter systolic pause (Chauveau) . The tracings also show
that between the close of the ventricular contraction and the beginning of

FIG. 119.— CARDIAC SOUNDS,
Tambours to be inserted into

membrane surrounding
frame-work; a, v, ends of tubes in
connection with tambours. —

(Marey.)

FIG. 120. — A GRAPHIC RECORD OF (i) THE INTRA-AURICULAR PRESSURE; AND (2) THE INTRA-
VENTRICULAR PRESSURE OF THE HORSE. — (Chauveau and Marey.)

the succeeding auricular contraction there is a period during which the
whole heart is at rest and during which the cavities are filling with blood.

THE CIRCULATION OF THE BLOOD 277

For the purpose of obtaining the time of all these events, the recording
surface was divided into equal spaces by vertically drawn lines. The rate
of movement of the surface was such that each division corresponded to
one-tenth of a second. The record thus indicates that the auricular con-
traction lasted approximately 0.2 second, the ventricular contraction 0.4
second, and the pause 0.4 to 0.6 second.

From similar experiments made on other animals, e.g., the dog, similar
results have been obtained; but by reason of the employment of more
sensitive and more quickly responsive tambours, the curve of the auricular
contraction exhibits variations not recorded by the forms of tambour used
in earlier experiments. Reference to these variations will be alluded to in
subsequent paragraphs. The results obtained by recent observers now
generally accepted are in accord with the results obtained by Chauveau and
Marey by means of their cardiac tambours as shown in Fig. 120.

The Movement of the Blood Dur-
ing the Cycle.— From the characteris-
tic features of the foregoing record it is
apparent that with the relaxation of the
auricular walls, blood at once flows
from the venae cavae and the pulmonic
veins into the auricular cavities and
continues so to do throughout the entire
auricular diastole. With the relaxation
of the ventricular walls, however, the
blood that has accumulated in the
auricles up to this time, or its equiva-
lent coming from the venae cavae and pulmonic veins, now flows into the
ventricles until they are nearly filled. Before they are filled, however, the
auricular diastole comes to an end, the auricular walls again contract and
force some of their contained blood into the ventricles and thus rapidly com-
plete the filling. The ventricular systole immediately follows, during which the
blood is driven into the pulmonic artery and aorta. This having been ac-
complished, the ventricles relax, and the blood that has been accumulating
in the auricles begins to flow into the ventricles, after which the same series
of events follows as in the previous cycle.

The Curp of the Systole and Diastole of the Heart. — In the study
of the volume* changes of the heart by means of a specially devised cardi-
ometer, Henderson was enabled to record the contraction and relaxation
movements of the heart, obtaining thereby a curve that resembled the curve
of the contraction and relaxation of a skeletal muscle. If this curve is re-
versed and superposed on a curve of the intra-ventricular pressure, their
relation one to an other becomes apparent, Fig. 121.

The chief characteristic of the systolic portion of the curve, viz.: hs
ascent, though less than that of the pressure curve, shows that with
te opening of the semilunar valves, b, a large portion of the volume of the
blood in the ventricles, 90 per cent, according to Henderson, is quickly and
in a uniform manner discharged, after which the outflow slows and finally
ceases as indicated by the rounded apex of the curve. The chief character-
istic of the diastolic portion of the curve, viz. : its steep descent, though less
than that of the pressure curve, shows that with the opening of the auriculo-

FIG. 121. — THE VOLUME CURVE OF
THE HEART CONTRASTED WITH THE CURVE
OF THE INTRAVENTRICULAR PRESSURE.

278

TEXT-BOOK OF PHYSIOLOGY

ventricular valves, d, a large portion of the volume of the blood in the auricles
flows quickly into the ventricles during the early part of the diastole, nearly,
if not entirely refilling them, a procedure occupying a period of time ap-
proximately that required for the systolic discharge. In some experiments
the portion of the curve, recording the increase of volume does not reach the
abscissa but runs off parallel with it. In other experiments it gradually
approximates the abscissa or shows a slight fall at the time of the succeeding
auricular contraction. According to Henderson blood has ceased to flow
from the auricles into the ventricles during the foregoing period and the
contraction of the auricles adds but little to the volume of the blood already
in the ventricles. Other investigators attach more importance to this
event.

The portion of the volume curve between the apex and the beginning
of the succeeding ventricular systole, embracing the entire diastolic period,
has been subdivided into two portions, viz. : (a) from the apex to the be-
ginning of the horizontal portion, corresponding to the period of relaxation
and refilling — the diastole proper — and (6) the period of rest or the diastasis.
This latter period is of rather short duration in the normal rate of heart
beat. It lengthens if the rate decreases, and shortens if the rate increases.

The Action of the Valves During the Cycle. — As previously stated,
the forward movement of the blood is permitted and regurgitation prevented
by the alternate action of the semilunar and the auriculo-ventricular 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 ventricu-
lar 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 accom-
plished 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 lunulae,
come into complete apposition. By this means the orifices of the pulmonic
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 pulnf6nic
veins. Whether the ventricles exert a suction power at the moment of
their relaxation is an undecided question. A steady stream of blood ii4
the auricles and ventricles continues throughout the entire period of rest until
both cavities are filled. The tricuspid and bicuspid valves which hang down
into the ventricular cavities are now floated up by currents 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.

THE CIRCULATION OF THE BLOOD 279

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 accurately closed. This is readily
accomplished by reason of the position of the valves, which have been
floated^up and placed almost in apposition 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 into the auricles is prevented by their attachment to
the chorda tendinece, and the latter are kept from moving bodily upward
during the ventricular contraction by the compensatory downward pull of
the papillary muscles. 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 con-
traction, there comes a moment when the intra-ventricular pressure exceeds
the pressure in the aorta and in the pulmonic artery. As soon as this
occurs the semilunar valves of both vessels are thrown open and the blood
discharged.1 Both contraction and outflow continue until the ventricles
are practically empty, after which ventricular relaxation sets in. With
the discharge of the blood the pressure in both the pulmonic artery and aorta
rises, passing from a minimal to a maximal value. Coincidently the pres-
sure in the ventricles rises and even exceeds that in the pulmonic artery and
aorta and so continues until near the close of the systole when the two
opposing pressures are approximately equal. With the onset of the
ventricular relaxation the intra-ventricular pressure suddenly falls, and so
soon as it falls below the positive pressure of the blood in the sinuses of
Valsalva the semilunar valves are again closed, the column of blood is
supported, and regurgitation is prevented. In the meantime and while
the ventricles are contracting, blood is again flowing into, and accumulat-
ing in the auricles and thereby distending them preparatory to the next
systole. With the accumulation of blood in the auricles and ventricles
the cardiac cycle is completed.

The approximate changes in the shape of the heart, the variations in the
size of its cavities and in the size of the blood-vessels arising from them,
and the relative position of the valves during systole and diastole are shown
in Fig. 123.

Heart-sounds. — Two sounds accompany each pulsation of the heart,
both of which may be heard by applying the ear or the stethoscope 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 reproduced by pronouncing the syllables
lubb-dup, lubbdup. 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. 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

xThe discharge of the blood by the contraction of the ventricular walls is probably aided by
the simultaneous downward displacement of the more central portion of the auriculo-ventncular
septum, due to the contraction of the papillary muscles.

28o TEXT-BOOK OF PHYSIOLOGY

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 o.i 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
muscle walls of the ventricles, the simultaneous 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 charac-
terized 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 subse-
quent vibration of the aortic and pulmonic valves which occur 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 by hooks introduced
into the aorta and pulmonary artery. It is also possible that the vibration
of the column of blood produces an additional tone which adds itself to that
produced by the valves.

The foregoing events are shown in their order in the following diagram.

FIG. 122.— DIAGRAM SHOWING THE EVENT IN THE CARDIAC CYCLE.

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 pulmonic and systemic vessels is accomplished by the driving
power of the right and left ventricles respectively, aided, however, by minor
extra-cardiac forces. They may be regarded therefore SLS force-pumps.

If the heart consisted of ventricles only, the flow of blood from the vena?
cavae and pulmonic 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

THE CIRCULATION OF THE BLOOD 28l

filling of the ventricles, and hence their effective working as force-pumps is
more readily secured. The auricles may therefore be regarded as L/-
pumps. For this reason it is probable, notwithstanding the contraction of
the circular •muscle-fibers at the terminations of the venous system, that the
flow of blood into the auricles is never entirely arrested, but merely retarded
Regurgitation m these vessels does not occur for the reason that the re-
tardation develops a side pressure during the auricular contraction which
is equal to if it does not exceed the pressure in the auricle

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 synchron-
ously. That they do so can be shown by attaching levers to their walls, and
thus recording their activities. The synchronism is so perfect that until

S.a.-D.v.

D.a.-S.v.

FIG. 123. — 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. and C.i. Superior and inferior cavae; A.d. (atrium dextrum) right auricle;
A,s. (atrium sinistrum) left auricle; V.d. (ventriculus dexter) right ventricle; V.s. (ventriculus
sinister) left ventricle; P. pulmonic artery; A. aorta; P.P. papillary muscles. — (Landois.)

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 mechanism seems to lie, the synchronism of the former is
not interfered with; that the apical halves of the ventricles will beat syn-
chronously 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 neighboring
parts of the ventricle. It is therefore probable that the synchronism 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 i.

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TEXT-BOOK OF PHYSIOLOGY

ma* valve

mm valve

Intra- ventricular Pressure. — i. Positive Pressure. — It has been stated
that during the pause of the heart when its cavities are filling with blood
the semilunar valves are kept closed by the pressure of the blood in the
pulmonic artery and aorta, a pressure due to the resistance, as will be ex-
plained 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 is apparent therefore
that there must be, during the time of the systole, an intraventricular posi-
tive pressure sufficiently high to overcome and even exceed the pressure
ordinarily present in the aorta and pulmonic artery. It becomes, there-
fore, a matter of interest 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 respectively, and connecting its free
to manometer extremity with a mercurial manometer.
By the interposition of a double valve such
as represented in Fig. 124, it becomes possi-
ble, according to the direction in which 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. By the employment of a device of
this character Goltz found in the left ven-
tricle of the dog a maximal pressure of
114 to 135 mm. of mercury; in the right
ventricle, a maximal pressure of 35 to 62
mm.

The maximal pressure in the ventricles
during the systole, though always higher
than that in the arteries, is neither a fixed
nor an invariable pressure, as it rises and
falls with the latter from moment to mo-
ment. Within limits the cardiac power,
and therefore the intra-ventricular pressure, is capable of considerable in-
crease. The function of the heart is to drive the blood through the vessels
with a given velocity. This is possible only by first overcoming the resis-
tance to the flow offered by the vessels, as indicated by the arterial pres-
sure. As this is a variable factor, rising and falling very considerably at
times, the heart must meet and exceed each rise, within limits if the circu-
lation is to be maintained. This it does by calling on the reserve power
with which it is endowed. The power put forth by the heart is propor-
tional to the work it has to perform. If the arterial pressure continues
higher than the average for any length of time, the heart meets the condition
by an hypertrophy of its walls, but in so doing it encroaches on the reserve
power proportionally and when the latter has become exhausted the heart
may, on some sudden rise of pressure in the aorta, be unequal to the dis-
charge of blood from its cavities and hence remain in a state of permanent
diastole.

2. Negative Pressure. — It has also been demonstrated by the employ-
ment of the minimal valve that there is brief moment in the cycle when the

'to heart

FIG. 124. — 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.')

THE CIRCULATION OF THE BLOOD 283

intra-ventricular pressure is less than the pressure of the atmosphere, be-
coming indeed negative to it. This moment must be that between the be-
ginning of the relaxation of the ventricles and the opening of the auriculo-
ventricular valves. The extent of the negativity, its duration and frequency
have never been satisfactorily determined. Goltz, however, found in the
left ventricle of the dog a minimal pressure of — 23 to — 50 mm. of mercury.
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. A probable cause is an expansion of the base of
the ventricles due to the enlargement of the aorta and pulmonic 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.

The Intra-ventricular Pressure Curve of the Dog. — It was stated in
a previous paragraph that the contraction of the auricles and ventricles of
animals other than the horse have been graphically recorded. This is
especially true of the heart of the dog. A graphic record of the intra-
ventricular pressure, its course, its variations, and time relations is necessary
for the interpretation of the heart mechanisms. With such a record may be

FIG. 125. — V. CURVE OF THE PRESSURE IN THE VENTRICLE OF THE DOG. A. CURVE or
THE PRESSURE IN THE AORTA. The curves were taken simultaneously. 5, Tuning-fork vibrations
each corresponding to i/ioo of a second, a-b, line of atmospheric pressure. The ordinates
0-5 correspond in the two records, o. Closure of the auriculo-ventricular valve; i, opening
of the semilunar valves; 2, point of maximum pressure; 3, beginning of the ventricular relaxa-
tion; 4, closure of the semilunar valves; 5, opening of the auriclo-ventricular valve. (Hiirthle.)

compared the records of the pressures in the venae cavae and auricles 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 devised
manometers or tonometers or tonographs, 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 Hurthle. This consists of a small metallic
tambour 5 or 6 millimeters in diameter, covered by a thin rubber membrane.
A small button resting on the membrane plays against an elastic steel spring,
by the tension of which the pressure of the blood is counterbalanced. The
movements of the membrane 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

284 TEXT-BOOK OF PHYSIOLOGY

cannula and tambour are filled with an alkaline solution to prevent coagula-
tion of the blood, and then made 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. 125. To obtain the absolute value of this curve in millimeters
of mercury it is necessary to graduate the instrument previously. 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 throughout their entire course. A characteristic feature of this
curve is the 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 com-
bination of air and liquid, this portion of the curve is represented by a single
peak, which is taken as an indication that the maximal pressure once reached
is not maintained, but immediately begins to fall to its original level, not-
withstanding 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 approximating walls of the ventricle. There are
reasons for believing, however, that the former curve is the more correct repre-
sentation of the course of the intra-ventricular pressure. Bayliss and Star-
ling photographed on a moving surface the oscillations of a fluid, a solu-
tion 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 Hiirthle's membrane ma-
nometer.

The Relation of the Intra-ventricular Pressure Curve to the Intra-
cardiac Mechanisms. — By itself the curve of the intra-ventricular 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 auriculo-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. 125), and by comparing these with the
curve of the successive differences of pressure in these two cavities as deter-
mined by the "differential manometer,' ' it becomes possible to mark on the
ventricular pressure curve the points at which the foregoing events take
place. As the outcome of many observations and determinations, the
following statements may be made: As a point of departure for a considera-
tion of the relation of the intra-ventricular pressure to the time of action

THE CIRCULATION OF THE BLOOD 285

of the valves, the close of the ventricular systole may be convenientlv
selected.

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 difference 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 (see Fig. 125), 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 semilunar 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, the
"dicrotic" wave. This notch occurs at 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 descending portion.
As yet, however, the pressure is higher in the ventricle than in the auricle,
and continues so until near the line of atmospheric 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 a result of the auricular systole. Immediately following this
event the ventricular systole begins and as soon as the pressure in the ven-
tricle exceeds that in the auricle the mitral valve closes. This is marked
on the curve where the ordinate cuts it, at o. With the closure of the mitral
valve the blood becomes imprisoned within a closed cavity, closed at one
orifice by the mitral valve and at the other orifice by the semilunar valves.
As the blood is incompressible the intra-ventricular pressure under the force
of the ventricular contraction rapidly rises and continues so to do until the
pressure in the ventricle exceeds that in the aorta, at which moment the semi-
lunar valves are suddenly opened and the blood discharged. A comparison
of the aortic curve shows that for a short time during the ventricular systole
the pressure is falling, but at one point the curve turns at a sharp angle and
rapidly rises. This is an indication that the semilunar valves are suddenly
thrown open and the blood begins to pass into the aorta. This event occurs at
a moment marked on the ventricular curve by the ordinate i. 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 i and 4, the semilunar valves remain open and the blood
passes into the aorta.

In accordance with the foregoing: the ventricular systole may be sub-
divided into two periods:

286 TEXT-BOOK OF PHYSIOLOGY

1. The period of rising tension, from the beginning of the systole and the

closure of the auriculo-ventricular valves to the opening of the semi-
lunar valves, the pre-sphygmic period, occupying from 0.02 to 0.04
second.

2. The period of ejection, the sphygmic-period, 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, the post-sphygmic period,

from the end of the systole and the closure of the semilunar valves to
the opening of the auriculo-ventricular valves, occupying about 0.05
second.

2. The period of filling, from the opening of the auriculo-ventricular valves

to the beginning of the succeeding auricular systole.
The Time Relations of the Successive Periods of the Ventricular
Activity of the Human Heart. — The duration of each of the periods of
ventricular activity as stated in the foregoing paragraph holds true only for
the animal the subject of the experiment. The time relations of each period
vary somewhat with the animal as well as with the rate of the heart during
the time of the experiment. In human beings the same holds true. As the
outcome of different methods of investigation the average duration of each
period has been approximately estimated as follows :

Period. Rate 70. Rate So.

1. Presphygmic o .055 \ Systole o .051 1 Systole

2. Sphygmic 0.268/0.323 0.254/0.305

3. Post-sphygmic 0.050 1 Diastole 0.050 1 Diastole

4. Pause 0.490/0.540 0.395/0.445

Total Duration 0.863 0.750

The Intra-auricular Pressure. — During the auricular systole the
pressure within the auricle undergoes variations as shown by direct examina-
tion by means of a cannula inserted into the auricular cavity and connected
externally with a recording tambour, or by indirect examination by means
of an exploratory tambour placed over the right jugular vein in close relation
to the clavicle. The pressure variations in the jugular vein which are thus
recorded by means of a tambour provided with a writing lever are believed
to be caused by, closely follow and reproduce the pressure variations in the
auricle.

Among the most important of the direct examinations of the auricular
pressure are those of Porter, carried out by the insertion of a large cannula in
the auricular appendix, or in a pulmonary vein close to the auricle and con-
nected by its free extremity with a Hiirthle tambour. The curve of pressure
thus obtained, shown in Fig. 126,* is characterized by three positive and three
negative waves. Among the more important of the indirect determinations
of the auricular pressure variations are those of Bachmann, carried out with
highly sensitive recording tambours. The curve of pressure variations in
the jugular vein thus obtained, by Bachmann, Fig. 127, is placed in juxta-
position for purposes of comparison.

The first positive wave, a, is caused by the systole of the auricle and
amounts to about 9 millimeters of mercury. The first negative wave is due
to the relaxation of the auricle.

THE CIRCULATION OF THE BLOOD 287

The second positive wave,2 s, is not of auricular origin, but is due to
the systole of the ventricle in its early stage corresponding to the period
between^the closure of the auriculo- ventricular valve, and the opening of
the semilunar valves, the period of rising tension, and amounts to about
5 mm. of Hg. It is probably due to the bulging of the auriculo-ventricular
valve into the auricular cavity, by the still higher ventricular pressure, thus
diminishing its size and raising the pressure.

\J gf

Off

FIG. 126. — CURVE OF PRESSURE VARIATIONS FIG. 127. — CURVE OF PRESSURE VARIATIONS IN
IN THE AURICLES. (Enlarged.)— (Porter.} THE JUGULAR VEIN. (Enlarged.)— (Bachmann.)

The second negative wave, of, begins with the opening of the semi-
lunar valves, determined by comparison with a simultaneously recorded
curve of intra-ventricular pressure, and is due in part to the relaxation of
the auricular walls, but more especially to a descent of the more central
portions of the auriculo-ventricular septum, into the ventricular cavity,
due to the contraction of the papillary muscles during the ventricular
systole. The hollow cone thus formed enlarges the auricular cavity,
withdraws some of its contained blood, and hence lowers the pressure,
thus contributing materially to the filling of the auricle. This negative
pressure amounts to about — 10 mm. of Hg.

The third positive wave, v, occurs toward the end of the ventricular
systole and is probably caused by an inflow of blood from the veins as
well as by a return of the auriculo-ventricular septum to its normal position,
the result of a relaxation of the papillary muscles at a time when the intra-
ventricular pressure is still higher than the intra-auricular pressure. It
amounts to about 5 mm. of Hg.

The third negative wave, vf, appears very shortly after the relaxation of
the ventricle and though there is at this moment a rapid fall of intra-
ventricular pressure, on opening of the auriculo-ventricular valves and a
descent of blood into the ventricle, the fall of auricular pressure seldom
amounts to more than 0.5 mm. of Hg.

A B C F i The original tracing obtained by Porter is shown in the ac-

^^x^xUy' /-<_ companying Fig. 128. The letters designating the waves have

\X ': the following significance. A, systolic rise; AB, first diastolic

: '>, j fall; BC, first diastolic rise; CD, second diastolic. fall; E, second

D L ° diastolic rise; F, third diastolic fall; G, pause. In Fig. 126

the tracing has been enlarged and the waves relettered and named

FIG. 128.— CURVE OF -n accorda^ce ^h the terminology in vogue in the literature of
PRESSURE VARIATIONS IN clinical medicine.
THE AURICLE.— (Porter.}

2 The corresponding wave on the curve of the pressure variations in the jugular vein is
believed by Mackenzie to be due to the impact of the expanding carotid artery, and hence
the carotid, c, wave; inasmuch as it occurs in point of time with the beginning oi
lar systole, it is also called the systolic, s, wave.

288 TEXT-BOOK OF PHYSIOLOGY

A Graphic Record of the Auricular and Ventricular Contractions
of the Human Heart. — From the similarity of the anatomic arrangement
of the human heart to that of mammals in general it is permissible to assume
that a graphic record of the auricular and ventricular contractions of the

human heart would resemble in its general
features that of the hearts of mammals hereto-
fore experimented on, and that the same series
of events present themselves in the human heart
during each cycle, though by reason of the
difference in the rate of the beat, the duration
of each event in the cycle is somewhat different.
The nearest approach to obtaining a graphic
record of the auricular and ventricular con-
tractions of the human heart by the direct ap-
plication of exploratory tambours was made by
FIG. 129.— TRACINGS OF THE Francois Franck on a woman whose heart was
CoS^oNs^LoMTwo^ congenially displaced into the abdominal cavity.
WITH ECTOPIA OF THE HEART, a, An investigation revealed the fact that this
Auricular; v, ventricular.— (Fran- woman had a large opening in the anterior
(ois-Franck.) portion of the diaphragm through which the

ventricle had passed and formed a large protrusion in the epigastric
region. Through thin and relaxed abdominal walls the ventricular pul-
sations could be distinctly felt as well as the pulsation of what ap-
peared to be the inferior portion of the right auricle. A fibrous ring
around the edge of the opening in the diaphragm supported the heart
at the auriculo-ventricular groove. On the application of exploratory
tambours in connection with recording tambours one to the right
ventricle, the other to the right auricle, the record shown in Fig. 129 was
obtained of which the upper line represents the contraction of the auricle
and the lower line the contraction of the ventricle. A comparison of the
record with that obtained from the horse, Fig. 120, p. 276, shows that the
relation of the auricular to the ventricular systole is the same in the former
as in the latter and that in their general features the two records correspond,
from which it may be inferred that in the human heart the events occurring
during the cycle are practically identical with those occurring in the hearts
of other mammals. The small size of the auricular curve and the absence" of
undulations are probably due to the fact that the tambour was placed on
only a portion of the auricle.

A Schematic Representation of the Events of a Cardiac Cycle in
Man. — From graphic studies of the cardiac impulse, of the pressure changes
in the auricle and ventricle as indicated by pressure changes in the jugular
vein and carotid artery respectively it has become possible to construct a
diagram of the cardiac cycle of the human heart, to designate on the ventricu-
lar curve the time of the opening and closing of the valves, as well as the
time relations of the entire series of events. A scheme of this character is
shown in Fig. 130, based on that constructed by Fredericq.

Though the numerical values given for the duration of the auricular and
ventricular systole and diastole, viz.: auricular systole o.io second; auricular
diastole 0.70 second; ventricular systole 0.33 second; ventricular diastole and
pause 0.47 second, it must be borne in mind that they are true only for the

THE CIRCULATION OF THE BLOOD

289

heart beating approximately 75 times per minute. If the number of beats
increases, not only does the entire cycle diminish in duration, but its dif-
ferent subdivisions, auricular systole, ventricular systole, and diastole also

l/RICVL A R ,D/A STOL

'osurc ofScmi-luna?
valves

... Opening of
' ?lri

valve.

Closure or
ricttlo -ventricular
valve.

HNTRICVLAR SYSTOW VtNTRICUTAR DIA

Second^

0 J .2 .3 .4 .S .6 .7 .8 .1 .2 .3

FIG. 130.— A SCHEMATIC REPRESENTATION OF THE EVENTS or A CARDIAC CYCLE.

diminish in duration, though in unequal degrees. Thus it has been de-
termined that with each increase of 10 beats above 70, the ventricular systole
shortens by about 0.02 second and the ventricular diastole by about o.io
second. The opposite holds true if the num-
ber of beats decreases below 70 per minute.
The Relation of the Cardiogram to the
Events of the Cardiac Cycle.— A compari-
son of a typical cardiogram, such as is seen
in Figs. 118 and 131, with the curve of intra-
ventricular pressure, shows that they corre-
spond in essential features. The slight eleva-
tion (a) on the cardiogram represents the
contraction of the auricle, which completing
the filling of the ventricle causes it to press

c 11 I. IO1CJ U>t Cj VCUU.n-ui.aJ. viiaowix., \s,^st

more Vigorously against the Chest Wall j 0-C ciosing and opening of the auriculo-

represents the contraction of the ventricles, ventricular valves ;O',C', opening and

i . , i jj i j closing of the semilunar valves; C, Cr,

at which moment the apex is suddenly and period6of rising tension; O'£>, period of

forcibly driven agaillSt the chest wall; C-d ventricular discharge ;CC', time of the

represents the systolic plateau, the time dur- ^Jsen^se e°£v^£ £
ing which the ventricle is discharging blood

into the aorta; d-e represents the relaxation of the ventricle, white e-f rep-
resents the time of the diastole during which the heart cavities are enlarg-
ing with the incoming of a new volume of blood, in consequence of which
the heart is pressing against the chest walls. The systolic plateau is charac-
terized by one or more elevations and depressions, the true cause of which
is unknown.

CO CO

FIG. 131. — CARDIOGRAM.

Au-

29o TEXT-BOOK OF PHYSIOLOGY

From the correspondence of the curve of cardiac pressure against the
chest wall with the curve of intra-ventricular pressure it becomes possible
to indicate with approximate accuracy the time of the opening and closing
of the auriculo-ventricular valves and the semilunar valves and hence
the time of occurrence of the heart sounds and other features of the
cardiac cycle. Such a construction is shown in Fig. 131.

The Pulse Volume. — The pulse volume or the systolic output or the
amount of blood discharged by the ventricle at each systole has long been a
subject of investigation, but by . reason of the inherent difficulties of the
problem the results that have been obtained have varied within wide limits,
viz.: from 180 c.c. to 50 c.c. The methods that have been employed for
the determination of this volume are complicated and need not be detailed
here. Suffice it to say that the results of the more recent experiments
would indicate that the volume varies from 80 c.c. to 100 c.c. If the pulse
volume be assumed to weigh 100 grams and the total volume of blood in a
man weighing 70 kilograms to weigh 3864 grams then the pulse volume will
be about one-thirty-eighth of the total weight of blood, or about 0.00142 of
the body weight. In 38 heart beats therefore the entire amount of blood
will have passed through the heart. The systolic output is conditioned by
the factors which increase or decrease the length of the diastole and hence
the filling of the ventricle.

The Frequency of the Heart-beat.— The frequency of the heart-beat
varies with a variety of conditions: e.g., age, sex, posture, exercise, etc.

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 males 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 beat will vary as follows—
from 66 to 71 to 8 1 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 fall in blood-pressure is attended by an increase in the
rate.

The Electric Currents of the Heart.— The wave of contraction of the
heart muscle, like the wave of contraction of the skeletal, muscle (see page
81) is accompanied or perhaps preceded by the development of electric
currents the existence of which can be demonstrated by connecting the base
and apex, by means of non-polarizable electrodes, with a suitable galvan-
ometer. When the^base and apex of the frog's heart are thus connected, it
will be found that with each contraction, the galvanometer needle will deviate
from its position of rest and in a direction that indicates that there is not
only a change in their electric potential but also that the base becomes

THE CIRCULATION OF THE BLOOD 2gi

primarily electronegative to the apex which remains electropositive
With the development of the negativity a current passes down the heart
to the ^ apex, out through the galvanometer circuit and back to the point
of origin. This current has been termed the action current.

If the heart is connected with a more quickly responsive apparatus, e.g.,
a capillary electrometer (see appendix) it will be found that the mercury
will pulsate twice, but in opposite directions, with each contraction, indi-
cating that two action currents are developed, the first indicating that the
base is primarily negative to the apex which is electropositive, and the
second that the apex is secondarily negative to the base which has become
again electropositive: in other words the changes in electric potential are
diphasic. With the capillary electrometer Waller demonstrated that the

FIG. 132. — THE ELECTRONEGATIVE AND ELECTROPOSITIVE AREAS OF THE BODY SURFACE.
A and B respectively represents apex and base of the ventricular mass. Then, if at any moment
a difference of potential should arise between A and B, a current, c c c, will be established along
and around the axis C C; the line 0 O will represent the plane of zero potential or equator;
the lines a a a, b b b will represent equipotential curves around A and B. A difference of
potential between A and B will be manifested if the two leading-off electrodes are applied on
opposite sides of the equator, O O; no such difference will be manifested if both electrodes
are on the same side of the equator. The equator O 0 will divide the body into two asym-
metrical parts, (i) a portion b b b including the head and right upper extremity, (2) a portion
aaa including the three other extremities. — (Waller, Human Physiology.)

human heart also exhibits similar changes in its electro-potential with each
beat giving rise to electric currents which find their way through the sur-
rounding tissues to the surface of the body from which they can be led
off to the electrometer and then back to the surface of the body and finally
to the base of the heart. He was enabled to determine and locate the points
which are electronegative and electropositive. The results of his experi-
ments are embodied in the accompanying diagram Fig. 132, which, with
the legend is self-explanatory.

If any two points, a and b, on opposite sides of the equator O, O, are
connected with the electrometer the mercury will pulsate twice with each
heart-beat indicating the presence of two action currents which travel suc-
cessively in opposite directions. With suitable apparatus the oscillations,
of the mercury can be photographically recorded. By reason of the rapid

292

TEXT-BOOK OF PHYSIOLOGY

action of the heart and the inertia of the mercury, the exact changes of the
electric potential recorded are difficult of interpretation.

By means of the highly sensitive and quickly responsive string-galvan-
ometer devised by Einthoven (see appendix) not only can the variations of the
electric potential of the heart be detected, but the deviations of the string
caused by the passage of the most delicate current, can be magnified and a
shadow projected on a sensitive surface and photographed. The record
thus obtained is termed1

The Electrocardiogram. — An electrocardiogram obtained by photo-
graphing the oscillations of the galvanometer string during the cardiac cycle,
when different portions of the body surface, a and b, are connected with the
terminals of the string, is shown in Fig. 133, the different features of which
are designated by the letters P, Q, R, S, T. This curve, which represents
the relative strength, duration, succession and direction of the currents during
the cycle, it is believed furnishes a correct picture of the strength, duration,

succession and direction of the physio-
logic processes — the excitation and con-
traction processes — as they arise in the
heart muscle during its activity.

The interpretations of the electrocar-
diogram and the significance of its various
features are not, in some respects, in as
complete accord as seems desirable. In
general, it may be said that P corres-
ponds to the auricular systole and that
Q, R, S, T correspond to the ventricular
systole. R and T are always present in
physiological conditions at least, while Q
and S, may, either one or the other or
both, be wanting.

Einthoven is of the opinion that the general form of the electrocardio-
gram indicates the path and propagation of the stimulus or the excitation
process through the heart, as well as the order of contraction of different
portions of the ventricular walls, both of which are in harmony with the
origin, course and ultimate distribution, of the auriculo-ventricular con-
duction system as determined by Tawara (see page 272).

The record presents first a horizontal portion — the image of the gal-
vanometer string — which indicates that the string is at rest by reason of
the fact that all portions of the heart, during the diastolic pause are isoelectric
or of equal electropotential.

P is the result of the contraction of the auricle, indicating a condition
of negativity toward the ventricle which is now electropositive in conse-
quence of which, the electric currents pass down the heart, thence to the
body surface, thence through the galvanometer circuit and back to the heart,
causing the string as they pass through it, to move outward. With the
cessation of the auricular contraction the string returns to its position of
rest. The more or less horizontal line between P and Q indicates that both
auricles and ventricles are now at rest and in the same state of electric
potential. The passage of the excitation process through the conduction
system, which is now taking place, from its beginning to its terminations,

FIG. 133. — SCHEME OF THE ELECTRO-
CARDIOGRAM.— (Einthoven.)

THE CIRCULATION OF THE BLOOD 293

is, in and of itself, too feeble to cause a deviation of the thread under the
conditions of tension necessary for the registration of the other features of
the electrocardiogram.

The system of points or waves, negative and positive, Q, R, S in the first
part of the ventricular portion of the electrocardiogram permits of the infer-
ence that the stimulus arrives and the contraction arises, at once or almost
at once in different places of the ventricular masses. The first point Q, indi-
cates that the stimulus arrives first at a point lying near to the apex and
calls forth a contraction of moderate degree establishing a certain degree
of electronegativity toward the base which is now electropositive. The
direction of the point, below the horizontal portion, is due to the direction
of the current through the heart and galvanometer, which is of course the
reverse of the current that originates in the auricle and which gives rise to P.
The point R, which immediately follows, indicates that the stimulus has
arrived at the base of the ventricle calling forth a more pronounced contrac-
tion and a greater degree of electronegativity than that present at the apex.
The point S indicates that soon there-
after, the stimulus reaches regions
lying nearer to the apex of the ven-
tricle calling forth a contraction and
establishing an electronegativity which
soon gains the upper hand. The
horizontal portion of the electrocar-
diogram, between the system of points
Q, R, S, on the one side, and the point
T, on the other side, represents a con-
traction state in which the entire
muscle mass of the two ventricles
participates in the same measure and
hence all portions are in a condition
of equal negativity for which reason
the string remains stationary. It is during this period that the ventricles
are engaged in driving the blood out of their cavities into the aorta and
pulmonic artery.

T, the last peak of the ventricular portion of the electrocardiogram, is
directed upward and hence reveals the fact that the base of the ventricle is,
at this moment, in a condition of greater electronegativity than are por-
tions of the heart lying nearer the apex. The origin of the T peak has been
a subject of much discussion but the investigations of Gotch have made it
apparent that it is due to the contraction of muscle-fibers which have not
before been brought into action, that is, fibers near the root of the aorta,
in that portion of the ventricle which is the homologue of the bulbus arteriosus
in the lower animals. The contraction of this region completes the discharge
of the blood from the ventricular cavity.

The electrocardiogram has been further analyzed and elaborated by
Kraus and Nicolai. The results of their investigations are embodied in
Fig. 134. A somewhat different terminology for the different features of the
general record has been introduced. Thus in this scheme the letters
A, Ja, J, Jp, and F, replace the letters P, Q, R, S, T, of the Einthoven scheme.

FIG. 134.— SCHEME AND INTERPRETATION OF
THE ELECTROCARDIOGRAM. — (Nicolai.)

294 TEXT-BOOK OF PHYSIOLOGY

A, expresses auricular activity, Jl and F express respectively the initial and
final activity of the ventricle. The letters Ap indicate a negative wave
following auricular activity; h, represents the time during which the excita-
tion process is passing through the auriculo-ventricular bundle (bundle of
His) from its origin to its terminations. J designates a wave which initiates
the ventricular contraction and is the result of the contraction of the system
of papillary muscles. It is preceded and followed by negative waves Ja
and Jp. The horizontal portion of the record, t, represents the contraction
of the middle layers of the muscle fibers of the ventricle. F represents the
contraction of the muscle fibers in the region of the conus arteriosus; and is
likewise preceded and followed by negative waves Fa, and Fp. P represents
the pause in the heart activities.

The activities of the different parts of the heart indicated by the lettering
may be summarized as follows: (Ott.)

Phase of Electrocardiogram. Approximately.

A. Activity of i presystole. Period of filling of the heart (ventricle).

/. " " papillary system — intersystole. Period of distention of the heart.
t. u middle layer of ]

ventricular muscle \ systole. Period of ventricular discharge.
F. " " conus arteriosus J
P. Rest of the whole heart — pause of heart. Period of filling of the heart.

In pathologic conditions of the heart muscle there are many deviations
from the normal electrocardiogram which are more or less characteristic
and of diagnostic value and importance.

THE BLOOD-SUPPLY TO THE HEART

The nutrition of the heart, its irritability and contractility, the force
and frequency of the beat, are dependent on and maintained by the intro-
duction of arterialized blood into and the removal of waste products from
its tissue.

In frogs and allied animals the heart muscle is nourished by the blood
flowing through its cavities. During the diastole the blood, under the
influence of the slight pressure developed, passes from the interior
of the heart into a system of irregular passage-ways or channels, which
penetrate the heart-wall in all directions and thus comes into direct contact
with the heart-cells. With the beginning of the systole the blood is forced
out of these channels into the interior of the ventricle, bringing with it the
products of tissue metabolism.

In mammals the entire inner surface of the heart, as shown by
the investigations of Pratt, also presents a series of openings, the foramina
of Thebesius, which lead into a similar series of passage-ways penetrating
in various directions the heart-walls, and there are reasons for believing
that the heart of the mammal may be to some extent nourished in a manner
similar to the manner by which the frog heart is nourished. Thus, if a
glass tube be inserted and fastened into the aortic opening of the excised
heart of a cat and the interior of the ventricle filled with warm defibrinated
blood of the same animal, under a pressure of about 75 mm., the heart will
recommence and continue to beat for a period varying from one to several

1 / has the significance here of /.

THE CIRCULATION OF THE BLOOD 295

hours, thus showing that the mammalian heart may to some extent so
receive nutritive material. By reason of the fact that the metabolism of the
heart of the mammal is so much more active than that of the heart of the
frog, this method is far from being sufficient for nutritive purposes and hence
a more perfect and active blood-supply is necessitated for furnishing
nutritive material and the removal of the waste products. These results are
accomplished by the coronary arteries, on the one hand, and the coronary
veins, on the other.

The Coronary Vessels.— The coronary 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 directions, they
form a circle around the base of the ventricles and ultimately anastomose.
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 the visceral pericardium,
surrounded by connective tissue and fat. From their relation to the outer
surface of the heart they may be designated extra-mural vessels. From these
vessels small branches are given off \frhich penetrate the walls of the heart,
in which they divide into many branches and because of their relation to
the heart-muscle they may be designated intra-mural vessels. These vessels
are spoken of as terminal arteries in the sense that "the resistance in the
anastomosing branches is greater than the blood pressure in the- arteries
leading to those branches" (Pratt). From these small arteries arise a
rich network of capillary blood-vessels which closely invest the individual
muscle cells, and which permit here a rapid and extensive interchange of
nutritive and waste materials. The coronary veins arise from the union
of the small veins which emerge from the capillary areas. The veins 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 walls, as generally stated, but
in the walls of the ventricular cavities as well. They communicate through
a capillary plexus with both arteries and veins, and by special large passages
with the veins alone.

The Filling of the Coronary Arteries.— 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. Thus it was asserted and maintained by Briicke that
this event must occur during the diastole, because of the supposed fact that
the semilunar valves during the systole are so closely pressed against the
walls of the aorta and over the openings of the coronaty arteries as to prevent
the entrance of blood into them; but with the diastole and the return of the
valves to their former position the blood under the recoil of the walls of the

296

TEXT-BOOK OF PHYSIOLOGY

aorta would flow freely into them as well as into and through the intra-
mural arteries, capillaries and veins. According to Brucke the partial empty-
ing of the coronary arteries and their intra-mural branches, and the consequent
fall of pressure within them toward the end of the diastole facilitates the
systole of the ventricles, while the filling of the vessels and the consequent
rise of pressure promotes the diastole. This anatomic mechanism and its
associated functional activity constituted according to Brucke an apparatus
by which the activity of the heart could be self -regulated. This view as to
the time of the filling of the extra-mural coronary arteries has, however, been
disproved.

At the present time it is generally believed as the result of many forms
of experimentation that the extra-mural coronary arteries are filled during
the time of the systole. For it has been shown that the semilunar valves do
not close the openings of the coronary arteries by reason of the presence of
blood behind them under a high pressure; that a division of one of the
branches of these arteries is followed by a spurt of blood synchronous with
the systole. Moreover, if a kymographic trace of the pressure within the
coronary artery be compared with the trace of the pressure within the
carotid artery, it will be found that there is a complete agreement between
them as the pressures in the two vessels rise and fall simultaneously and
as a corollary are filled during the systole. 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 temporary retarda-
tion of the flow of the blood during the systole in the coronary (the extra-
mural) vessels, followed by a return of the velocity during the period of
diastolic repose.

During the diastole the blood flows freely from the extra-mural vessels
into the intra-mural arteries and capillaries. It is at this time too that the
heart-muscle receives from the capillary blood-vessels its nutritive material
and returns to the blood the products of its metabolism. During the systole
the intra-mural capillaries and veins are compressed and the blood driven
into the extra-mural veins. The greater the force and frequency of the beat,
the larger the volume of blood passing through the coronary system.

Vaso-motor Fibers for the Coronary Arteries. — The presence in the
vagus and sympathetic nerves, of vaso-motor fibers for the coronary arteries
has been a subject of much investigation and discussion. By reason of the
fact that stimulation of these nerves modifies the rate and the force of the
heart-beat, and these in turn modify the flow of blood through the vessels, it
is difficult to state whether the observed effects are the result of changes in
the caliber of the arteries or to a change in the character of the heart-beat.
Moreover owing to the anatomic relation which the arteries bear to the heart
muscle, the rapidity of the flow through them must vary with each contrac-
tion and relaxation and thereby the difficulty of interpretation is increased.
The results of direct experimental investigations of Porter, however, lead to
the conclusion that the existence of vaso-motor (constrictor) fibers for the
coronary arteries is highly probable.

These investigations have been corroborated by the investigations of
Barbour and Prince who have found that when isolated monkey hearts are
perfused with autogenous hirudin blood diluted with Locke's solution con-
taining minute doses of epinephrin, the coronary flow is decreased. From

THE CIRCULATION OF THE BLOOD 297

previous experiments on rings of the coronary artery of the human heart
the conclusion is drawn, that in man and the monkey, epinephrin constricts
the coronary arteries even though it dilates them in the dog, cat, rabbit,
and other animals. From these result the further conclusion is drawn that
in man and monkeys the coronary arteries are supplied with vaso-constrictor
nerves of true sympathetic (thoracico-lumbar) origin.

The Effects of Ligation of the Coronary Arteries.— 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, pro-
vided 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 declines
the heart-muscle frequently exhibits a series of independent contractions of
individual fibers and cells known as fibrillary contraction. All the results
which follow ligation are to be attributed in the light of experiment 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 i per cent, solution of potassium chlorid until the heart comes to rest and
then perfusing it with Ringer's solution.

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 or
the turtle will continue to beat under appropriate conditions for hours after
separation of all anatomic connections and removal -irojn 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 sameway. Nevertheless
there are good reasons for believing that though the heart has ceased to
respond to its customary stimulus, the irritability yet endures though
perhaps in lessened degree, by reason of the absence of blood, in the mam-
malian heart, in the coronary system of vessels. For if, after the heart has
ceased to beat for some time, warm defibrinated and oxygenated blood
or Locke's modification of Ringer's solution be passed through the coronary
vessels the beat will reappear and continue at its usual rate for some hours.
(See paragraph relating to the action of inorganic salts on the mammalian
heart, page 307.)

The reason for the longer continuance of the beat of the excised hea:
of the cold-blooded animal beyond that of the warm-blooded animal

298

TEXT-BOOK OF PHYSIOLOGY

lies probably in the difference in the rate of their respective metabolisms.
There is reason to believe that each cell 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 sepa-
rated 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 the contraction ceases. 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 maintenance of the
irritability. So long as this persists at a sufficiently high level the heart-
muscle will contract in response to the appropriate stimulus.

THE PHYSIOLOGIC PROPERTIES OF THE HEART-MUSCLE

The physiologic properties of the heart-muscle on which its efficiency as

a pumping organ depends, viz., irritability, conductivity, rhythmicity,

tonicity, automaticity, have been largely determined by a study of the heart

of the frog. As some of the facts to be stated in subsequent paragraphs have

reference to this heart, it will be found conducive to clearness if its anatomic

structure and physiologic action be understood. For this reason a brief

account of the frog heart will be found in the appendix.

i. 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 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
'iition of diastole. This can be done with the frog's heart, by
ing the t^tt^ at the sino-auricular junction, a procedure which
rents the f^pge of the contraction wave, which originates in the
sinus, over the auricles and ventricles (a fact that will be more fully
alluded to in a subsequent paragraph). With the heart thus prepared
and while still in situ, the apex may be connected with a recording
lever and its evoked contractions registered on a recording surface. In
this condition it will respond by a contraction to any form of an ade-
quate stimulus, such as the induced electric current.

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

THE CIRCULATION OF THE BLOOD 299

by a contraction to any adequate stimulus, physiologic or artificial.

a. The Blood-supply.— 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 (page 294).

b. The Influence oj Temperature. — For the manifestation of the irrita-
bility and contractility it is essential that the heart-muscle be kept at a suf-
ficiently high temperature in order that the physiologic or a given arti-
ficial stimulus may evoke a maximal contraction. This is accomplished
by immersing the suspended heart in a bath of Ringer's solution the tem-
perature of which can be readily decreased or increased by appropriate
means. The optimum temperature for the frog heart is about 25°C.
As the temperature is lowered .both rate and force decrease until at
about from 4°C. to o°C. both cease. Beyond 35°C. it also ceases to
contract, because of a coagulation of the muscle substance. The
mammalian heart attains its maximum activity at a temperature of
39°C. to 4i°C. .It ceases to beat at about 47°C. on the one hand and
at about i7°C. on the other hand.

Conductivity. — Conductivity of living material may be defined as the
ability to transmit through itself a condition of activity due to the
action of a stimulus. In muscle material the condition of activity is
characterized by a molecular process known as the excitation process,
followed almost immediately by a change of shape known as the con-
traction wave.

In skeletal muscle conductivity is developed to a high degree. Thus
if a stimulus, e.g., an induced electric current, be sent transversely
through one end of a muscle an excitation process is developed, followed
by a contraction wave, both of which are conducted through the muscle
without interruption to the other end with a speed, in the frog muscle,
of about 10 meters per second. In the cardiac muscle the physiologic
stimulus acts at or near the terminations of the venae cavae, from which
point an excitation process and a subsequent contraction wave are
conducted over the auricles, thence to the ventricles from base to apex
with extreme rapidity. It is evident therefore that the heart-muscle
also possesses conductivity to a high degree. It is now generally believed
that the propagation of both processes is accomplished by muscle-tissue
alone, independently of the nerve system. The conductivity, however,
is not equally well developed in every part of the heart.

In the frog heart this is especially true of the tissue at both the sino-
auricular and the auriculo-ventricular junctions. At these points the
contraction wave is delayed for an appreciable period (a condition at-
tributed to the embryonic character of the muscle-tissue), so that what
would otherwise be a single wave becomes divided into three smaller
waves, so that it becomes possible to observe and distinguish the con-
traction of the different chambers of the heart. In the frog s heart
the excitation process and the contraction wave begin in the sinus veno-
sus, from which they are conducted to the auricles, thence to the ven-
tricles. The successive contractions of the walls of the subdivisions o
the heart can be readily recorded with suitable apparatus. In Fig. 135,

300

TEXT-BOOK OF PHYSIOLOGY

which is a graphic record of the heart-beat, the two elevations of the
lever on the up-stroke, a and b, represent the contraction of the sinus
and the auricle respectively, followed by the vigorous and long con-
tinued contraction of the ventricle, while the two depressions, c and d,
indicate the delay in the transmission of the contraction wave at the
two junctions. There is here an anatomic obstacle to the conduction
of the contraction wave. The block between the sinus and the auricle

may be artificially increased to such an ex-
tent as to prevent absolutely the passage of
the contraction wave by ligation of the tis-
sue between them, a procedure introduced
by Stannius and now known as the first
Stannius ligature. /Under such circum-
stances the auricles and ventricle remain at
rest while the sinus continues to beat at its
usual rate. The obstacle between the auri-
cles and ventricle may be increased by the
same method also introduced by Stannius,
and now known as the second Stannius
ligature, or better by means of a suitable
and adjustable clamp. By carefully regulating the pressure of the clamp
it is possible to so block the wave that three or four auricular contrac-
tions may occur before the excitation process forces the block and ex-
cites a ventricular contraction. (Fig. 136.) 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 ex-
erts its action in the sinus venosus and that the auricular and ven-
tricular beats are in turn dependent on it.

I/ I/ cl/ V

FIG. 135. — RECORD or THE
CONTRACTION OF THE FROG'S
HEART.

FIG. 136. — RECORD or THE AURICULAR AND VENTRICULAR CONTRACTIONS BEFORE

AND AFTER THE CLOSURE OF THE CLAMP AT a.

In the mammalian heart the seat of the stimulus and the point oi
origin of the excitation process and the subsequent contraction wave
have been a subject of much investigation and discussion. For some
time it has been believed that these processes originate at the termina-
tions of, or between the terminations of the venae cavae in a region
corresponding to the sinus venosus in the frog heart1, from which they

JIn the mammalian heart the sinus venosus as a distinct chamber has been obliterated,
but it is represented by the following remnants: (i) The termination of the superior vena

THE CIRCULATION OF THE BLOOD 301

pass over the auricles, thence to the ventricles. On the basis of this belief
it has been assumed that there is a specialized area in which the stimulus
arises and which determines the rate and rhythm of the entire heart.
At present it is believed that this area is identical with the region occu-
pied by the sino-auricular node, the lower portion of thesulcus tenninalis.
With the view of determining the truth of this assumption Flack per-
formed a number of experiments on the hearts of dogs, cats, and rabbits,
some of the results of which, abstracted from his paper, are as follows:
The application of cold either through metallic tubes or by means of an
ethyl chlorid spray, the remainder of the heart being protected, caused
slowing of both auricles and ventricles. Weak electric stimulation
caused marked inhibition of both auricles and ventricles; slightly
stronger stimulation caused a mixed effect of inhibition and acceleration,
the latter usually predominating; still stronger stimulation gave rise to
marked acceleration of the whole heart rhythm or an altered rhythm of
auricles and ventricles. When electric stimuli were applied to other
regions of the superior vena cava or sulcus no effects were noticeable.

Mechanic stimulation as pinching the node with forceps called forth
similar results. Destruction of the node, however, had no effect on
the rhythm. The application of a weak solution of atropin abolishes
the customary effects of both vagus and sympathetic nerve stimulation.
From the foregoing facts it may be assumed that the usual seat of origin
of the stimulus to the cardiac contraction is the sino-auricular node, but
as the heart continues to contract after the node is destroyed, it is
evident that some other portion or portions of the auricular wall are
also capable of developing under the circumstances an adequate stimulus.

A further proof that the sino-auricular node is the initiator of the car-
diac contraction is found in its change of electric potential. It has long
been established that when any portion of living material enters into a
state of activity it becomes electro-negative to all other portions which
are at the same instant electro-positive. Lewis with special electrodes
in connection with a string galvanometer found in a series of determina-
tions that with the beginning of a cardiac contraction, the sino-auricu-
lar node was the point of initial electro-negativity, a fact that is in accord
with the general truth that the region of greatest activity exhibits the
greatest degree of negativity. The sino-auricular node may therefore
be regarded as the primary seat of the stimulus or excitation process
and the initiator of the beat.

From the sino-auricular node the excitation process is conducted to
the auricles and ventricles in quick succession, though between the end
of the auricular contraction and the beginning of the ventricular con-
traction there is also a perceptible interval similar to that observed in the
frog heart. For a long time it was assumed that the excitation process
and the contraction wave passed directly from auricles to ventricles
across the auriculo-ventricular junction as in the frog and that the interval

^*a (the right duct of Cuvier); (2) the coronary sinus (the left duct of Cuvier); (3), a
stratum submerged beneath auricular tissue at the taenia terminalis; (4) the remnants of
the venous valves, i.e., the Thebesian and Eustachian valves (Flack). In addition there
is a remnant of primitive tissue at the sino-auricular junction, that is,- where the superior
vena cava joins the tsnia terminalis of the right auricle, and known as the sino-auncular

joins
node.

3o2 TEXT-BOOK OF PHYSIOLOGY

between the auricular and ventricular contractions was due to an interfer-
ence with the passage of the contraction wave across the 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 excitation process passes, has been
found, as stated on page 271, in the system of muscle-fibers, described
in part by His, Retzer and Braunig, and Tawara and in part by Keith
and Flack and known as the conduction system of the heart, page 272. This
system it is believed constitutes the anatomic and physiologic path
across which the excitation process passes from auricles to ventricles.
The excitation process originating in the sino-auricular node passes
first to the auricular walls, exciting them to contraction and then into
and through the auriculo-ventricular bundle to the ventricular walls,
exciting them to contraction. The supposition that this was the case

has been 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 independently
of each other. These and other experiments
of a similar character have demonstrated be-
yond question that the auriculo-ventricular
bundle with its widespread ramifications is
the true conducting system between auricles
and ventricles. In this system the sino-auric-
ular node is regarded as the primary domi-

nating "puace maker" of the rate and rhythm

ing the auriculo-ventricular 01 the heart. Inasmuch, however, as the
bundle (AVB). SM, Septum heart will continue to beat, after the destruc-

membranaceum; MV, mitral r ,1 • i , ., .

valve.— (Hirschf elder.) tlon of the sino-auricular node it is evident

that it is not the only region that can initiate

the contraction. Whether the contraction under such circumstances
is due to an excitation arising in some other portion of the auricular wall
or in the subsidiary auriculo-ventricular node is a subject of discussion.
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. This time, estimated
from the beginning of the auricular systole to the beginning of the
ventricular systole amounts to from o.i to 0.2 second. The interval
between these two events, determined from the time between the oc-
currence of the a and the c or 5 waves on the jugular pulse tracing is
known as the a-c interval, or the As-Vs 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 ven-
tricular contraction, the inter-systolic period, by compression of a por-
tion of the tissues between auricles and ventricles including presum-

THE CIRCULATION OF THE BLOOD 303

ably the central part of the conducting system, the muscle bundle of
This has been accomplished in the dog by Erlanger by means
of a specially devised hook clamp (Fig. 137), which consists of an L-
shaped hook of steel wire the arm of which can be made to approach
a brass block by means of a bolt and screw. The L-shaped hook is
inserted into the right wall of the aorta, then passed downward and
backward into the left ventricle, then pushed through the ventricular
septum into the right ventricle. In this position it lies under the
auriculo-ventricular bundle. Compression is now brought about by
approximating the hook to the brass block by means of the nut.
When the compression 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 auri-
cles, the ventricular beat returns, at first slowly but with a gradually
increasing frequency until a definite but a comparatively slow rate is
attained. The rhythm thus developed is termed the ideo -ventricular
rhythm.

In experiments on the dog heart performed by Erlanger the following
results were obtained when the auriculo-ventricular bundle was com-
pletely crushed.

Aur. rate per minute. Yen. rate per minute. Ratio of Aur. to Yen.

Max. 216 Max. 69.8 3.39

Min. 117.8 Min. 34.8 3-38

Ave. 166.9 Ave. 52.3 3-J9

The reason assigned for the cessation of the ventricular contraction
is the non-arrival of the excitation process at the ventricular end of the
conducting system, because of the blocking or compression. 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 contract 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 totally different
rhythms.

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 compression gradually
applied, the first perceptible effect is a lengthening of the normal pause,
the inter-systolic, between the auricular and the ventricular 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 ven-
tricular 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 is capable of developing a ventricular beat, establishing what has

304 TEXT-BOOK OF PHYSIOLOGY

been termed the 2 .: i, 3 : i, 4 : i, rhythms respectively; 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 ventricu-
lar rhythm becomes manifest. Usually when the rhythm is of the 3 : i
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.

These experimental facts have afforded an explanation of the altered
rhythm between auricles and ventricles often found in that pathologic
condition known as Adams-Stokes 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.

Aur. rate per minute. Ven. rate per minute. Ratio of Aur. to Ven.

79-6 22-4 3-55

84-6 31.0 2.73

In a few cases of death from this disease a post-mortem examination
showed -a lesion of the auriculo- ventricular bundle.

3. Rhythmicity. — Rhythmicity may be defined as the ability to act in
regularly recurring cycles or the property of anything so acting. As
the heart-beat recurs in regular cycles or at regular intervals, it may
therefore be said that the heart-muscle is characterized by rhythmicity.
The beat of the heart as well as each phase of the beat occupies a
regular measure of time and is therefore rhythmic in character. Experi-
mental procedures, however, show that the rhythmic power or at least
the frequency of the rhythm varies in each of its subdivisions when they
are separated one from the other. Thus if the tissue between the sinus
and auricle in the frog or turtle heart be divided, the auriculo-ventricular
portion at once ceases to beat, while the sinus continues to beat as usual.
In a short time, however, the auricles and ventricles begin again to beat,
but with a slower rhythm. Division of the tissue between auricles and
ventricles is again followed by rest. In a short time the auricles begin
to beat, while the ventricle remains quiescent. If the ventricle now be
stimulated in a rhythmic manner it may resume rhythmic activity.
These facts are taken as an indication that the rhythmic power is
greatest in the sinus, less in the auricles, and least in the ventricles.
In the warm-blooded animal, e.g., dog, cat, rabbit, there is also a
difference in the rhythmicity of the auricles and ventricles. This is
shown by the effects which follow division of the auriculo-ventricular
bundle, or sudden and complete compression of that portion of the
auriculo-ventricular tissue containing it. 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

THE CIRCULATION OF THE BLOOD 305

develops a rhythm of its own, though it never attains that of the auricle.
From these facts it is probable that in each division of the heart a stimu-
lus similar to that acting in the sinus is developed when the heart
chambers are separated one from the other.

4. Tonicity. — Tonicity may be denned as a condition of muscle material

characterized by a slight degree of contraction which varies in extent,
however, from time to time under physiologic conditions. Whatever
the cause of the tonicity may be in any given form of muscle, the
slight degree of contraction which characterizes it not only resists undue
extension but permits of a quicker response to the action of a stimulus
and a more effective performance of work. The heart-muscle, like
the skeletal muscle, maintains continuously a certain degree of contrac-
tion, which not only prevents undue expansion of the heart during the
period of diastole, but increases its efficiency as a pumping organ at the
beginning and during the systole; The results obtained from subjecting
the heart muscle to the action of a calcium salt renders it probable that the
tonicity is in large measure the result of or is associated with the action of
such a salt (see page 306). The tonicity may, however, be increased
or decreased by the action of various external 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 tonicity, or the
contractile power, that complete relaxation is prevented, until finally
the heart comes to a standstill in the condition of systole. The passage
of dilute solutions of lactic acid, muscarine, etc., through the heart will,
on the contrary, so decrease the tonicity or the contractile power that
the normal contraction is not attained. The relaxation therefore
gradually increases until the heart finally comes to a standstill in the
condition of extreme diastole. In the first instance the tonicity is said
to be increased; in the second instance, decreased.

5. Automaticity.— Automaticity may be defined as the power of maintain-

ing activity by a self-acting cause or the power of acting independent
of external causes. 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

as to its origin and nature:

i. That it originates in the nerve-cells found in vanous parts of the heart
muscle- that it is a nerve impulse rhythmically and automatically
discharged by these cells and transmitted by their axons to the

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 consl
organic and inorganic, of the muscle-cells and those of the lymph
which they are surrounded. •

3o6 TEXT-BOOK OF PHYSIOLOGY

According to the first view the stimulus is neurogenic, according to the
second view myogenic.

The presence of nerve-cells; their relation to the muscle-cells; the pro-
nounced 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 never-
theless 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
wails 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 defibrin-
ated blood. Unless it be assumed that the heart-muscle contracts auto-
matically, 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 probability the outcome of a reaction between the chemic 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 approximating those in
which they exist in the blood.

The Action of Inorganic Salts. — i. On the Frog and Terrapin Heart. —
The inorganic salts which are most directly concerned in exciting and sustain-
ing the heart-beat are sodium chlorid, calcium phosphate or chlorid, and potas-
sium chlorid. A combination of these salts in the proportions in which they
exist in the blood was first suggested 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 c.c., 2 c.c. of a i 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 cavae 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 following: 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

THE CIRCULATION OF THE BLOOD 307

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 con-
tractions. 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 strips.
In a 0.7 per cent, sodium chlorid solution the strip beats rhythmically and
energetically, but only for a short period and with gradually diminishing force,
and a loss of tonicity until it entirely ceases to beat. A reason assigned for
this is the removal of other salts necessary to the excitation of the contrac-
tion. 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 period begin to contract rapidly and energetically and
for a longer time than when in sodium chlorid solution alone. The con-
tractions not infrequently 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 inorganic
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.; NaHCO3 0.02 per cent., dextrose o.i 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
contraction, for several hours at a time, on four consecutive days by perfusing
it with this solution saturated with oxygen and at a temperature of 35°(
No special precautions were observed other than keeping it cool (io°C.) and
moist during the intervals of experimentation. The duration of the irrita-
bility 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 i
was again revived by perfusing it with Ringer's solution. Altogether
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 inter-
vening 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 stimulation 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 doubl
pneumonia. It was made to beat rhythmically at a rate varying from 70 to 80
per minute when the solution had a temperature of 39°C., and at a raft

3o8 TEXT-BOOK OF PHYSIOLOGY

08 to 102 per minute when it had a temperature of 4i°C., though at this tem-
perature the beat became arrhythmic. All these instances demonstrate the
extreme persistence 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.

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. Potassium exerts a depressor or an inhibitor influence on the
irritability and contractility of the heart-muscle. Calcium exerts an accel-
erator and an augmentor influence on the irritability and contractility of the
heart.

A Theory 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 initiat-
ing the contraction and one apparantly 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 the 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 withdrawn.

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 combined 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 under-
goes a dissociation giving rise to a chain of events that culminate in a con-
traction. The initial step, therefore, is a dissociation of a complex unstable
molecule followed by an oxidation of the dissociated products. That an
active dissociation of some character takes place is evident from the consump-
tion 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 refrac-
tory to a stimulus during the systole, the probabilities are that all of the
unstable portion of the energy-yielding compound is dissociated with each
contraction. With the relaxation there is a renewal of the unstable
combination of calcium with the organic molecules, which increases
in amount until the maximum is again attained when another dis-
sociation occurs followed by another contraction. The rhythmicity of

THE CIRCULATION OF THE BLOOD 3o9

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 Engelmann, Langendorff and others
the dissociation is not spontaneous but is the result of the action of a specific
stimulus, an "inner stimulus," 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 stimu-
lus, whether chemic, electric or enzymic, nothing definite can be stated at
present.

The Response of the Heart to the Action of an Artificial 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 venosus 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 contracts. Contrary to what is observed
in skeletal muscles, the heart contraction, if it occurs 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 Engelmann is as follows: By reason of the fact that the
heart contracts at its maximal value to the action of any strength of stimulus,
under given conditions, there is always ensured a more or less complete
emptying of the ventricular contents and a uniform discharge of blood into
the arteries, which would not be the case if the e'xtent of the contraction
varied with the strength of the stimulus; and there are reasons for believing
that the normal stimulus 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 facts. 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 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 irrita-
bility. This is accomplished during the diastole. By virtue of this fact the
heart cannot act otherwise than in a periodic 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

3io

TEXT-BOOK OF PHYSIOLOGY

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 or non-responsive 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

will be developed, which is known as
the extra systole (Fig. 138).

The Extra Systole.— The extent
of this extra systole will be propor-
tional to the time at which the stimu-
lus is thrown into the ventricle as it
passes from the beginning to the end of
its relaxation. Whatever the extent of
the extra systole, its height is no
greater than that of the first systole.
For this reason it is believed a tetanic
contraction cannot be developed. If
the stimulus be thrown into the heart
just as the relaxation is completed, the
extra systole attains the same height as
the preceding systole. In passing from
the beginning to the end of the relaxa-
tion and into the diastolic or resting
period, it has been found that the
extra systole can be evoked by a stimu-
lus which is steadily decreased in in-
tensity. 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 (Fig. 139). For this
reason weak stimuli are more effective
in the later than in the earlier period
of the relaxation and the diastole.

After the development and disap-
pearance of the extra systole a consider-
able pause in the heart's action occurs
to which the term compensatory pause
has been given on the assumption that
it was necessary on the part of the
heart to compensate for the disturbance
of the rhythm by remaining at rest until the time of the next beat arid thus
restore the rhythm. This was thought to be a special property of the heart-
muscle. This view, however, is no longer entertained. For if an isolated
ventricle of a frog heart be employed and made to contract rhythmically by
an artificial stimulus, or if a spontaneously beating portion of the dog's
heart be employed for experimentation instead of the whole heart, the re-
sults of the same methods of stimulation are different. Though an extra

FIG. 138. — MYOGRAMS OF THE FROG'S
VENTRICLE SHOWING THE EFFECTS OF AN
INDUCED ELECTRIC CURRENT SENT INTO
THE VENTRICLE AT DIFFERENT TIMES OF
THE CYCLE. — (Marey.)

oo' indicates the beginning of the con-
traction in each series. The break in the
line e, indicates the time the stimulus is
sent in. In i, 2, 3, the stimulus falls into
the ventricle in the non-responsive period
or the refractory period, i.e., during the sys-
tole when the irritability has practically
disappeared. From 4 to 8 the stimulus
falls into the ventricle in the responsive
period or the period of returning and in-
creasing irritability, i.e., at the end of the
systole and during the diastole. The width
of the shaded lines is a measure of the
latent period, i.e., the period between the
time of stimulation and the beginning of
the resulting contraction which diminishes
as the diastole progresses.

THE CIRCULATION OF THE BLOOD 311

systole is called forth as usual, there is no compensatory pause; indeed, if
anything the pause is shorter than the regular pause. The theory that a
compensatory pause is necessitated for the restoration of the normal rhythm
is therefore not tenable.

The explanation assigned and generally accepted at present for the
production of a compensatory pause is as follows: In a spontaneously
beating heart the ventricular systole is evoked by the arrival of an excitation
process coming from the auricles. When the extra systole is induced by an
artificial stimulus, the next succeeding 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 second succeeding excitation, when it
responds and takes up the regular rhythm.

This fact is of great interest clinically for it frequently happens that
extra systoles of the ventricle arise in the human heart in conditions of the
circulation characterized by a high blood-pressure and especially when there

FIG no —DIAGRAM SHOWING THE VARIATIONS OF IRRITABILITY DURING THE SYSTOLE AND THE
DIASTOLE.— (Modified from Waller.)

is coincidently an impairment in the irritability and contractility of the heart-
muscle. Extra systoles, however, may have their origin in the auni

walls as well. , .,

If a series of successive stimuli be thrown into the heart-muscle tn
effect will vary in accordance with their time intervals. Should this be
than about three seconds there will be a gradual increase in the height fc
some half dozen contractions, a result to which the term staircase
"treppe" has been given. This increase in the height of the Contraction
is attributed to an increase in the irritability and contractility of the muscle
the result of the primary stimulating action of fatigue pro(

THE NERVE MECHANISM OF THE HEART

By this term is meant a combination of nerves and ne™ centers which
cooperate to increase or decrease either the rate or force of the heart s cor,
traction and hence the proper distribution of the blood, in accordance wil
SS&S* of differed organs of the body. That the *«™%
influenced by nerve centers in response to nerve impulses

312

TEXT-BOOK OF PHYSIOLOGY

them in consequence of cerebral (psychic) or physiologic activities in different
parts of the body is a matter of personal experience; that the heart is abnor-
mally influenced by the same nerve-centers in response to nerve impulses
transmitted to them in consequence of pathologic and traumatic processes
occurring in different regions of the body,, and that both heart and nerves

Emotional Centers

Hchilarating (BLUC)
Depressing (*£(>}

Ca/tfio -Inhibitor Center.

Ganglion Stellatum

Cantio -Accelerator Center

Affer,t,t\£xcitutor (BLUE.)
Atterent (inhibitor (BLUE)
ffinnt {inhibitor]* >ED)

Sympathetic M roes'

Accelerator &Augmentor

Intra-CardiacNerue Cells

FIG. 140.— DIAGRAM OF THE NERVE MECHANISM OF THE HEART.— (jG. Bachman.)

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. 140.

It was stated in a previous paragraph, page 297, that the contraction of
the heart-muscle is independent of its connection with the central organs of

THE CIRCULATION OF THE BLOOD 313

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 and chemic in character,
the result of a reaction between the chemic constituents, organic and in-
organic, of the muscle-cells and those in the lymph by which they are sur-
rounded. It has also been further shown that even in the living animal-
the heart will continue to beat and fulfil its functions after 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 exer-
tion than formerly, exhibited no striking disturbance of its functions.
Nevertheless groups of nerve-cells are 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 ganglion of Bidder; a third group
is found in the auricular septum, known as the septal ganglion, or the gan-
glion of v. Bezold or of Ludwig. 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 cavae, 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 pneumo-
gastric. Experimental investigation has established the fact that the sympa-
thetic is the motor nerve to the heart, the nerve which accelerates the rate
and augments the force of the normal beat; while the vagus is the inhibitor
nerve, the nerve which inhibits or controls the rate and the force of the beat
in accordance with the necessities of blood distribution. For this reason
these two nerves will be considered in the order stated. The course <
the fibers composing these nerves, from their origin to their termination, and
the relation they bear to one another and to neighboring structures, vary
somewhat in different animals. ,

The Origin and Distribution of the Sympathetic Nerves in Mammals.
-The sympathetic nerve-fibers which influence the action of the heart a
connected on the one hand with the heart-muscle itself and on the o
hand with nerve-fibers coming from the central nerve system.
are non-medullated and post-ganglionic, the latter medullated an

fibers have their origin in ^f

very probably from nerve-cells in the gray matter beneath the floor of
fourth ventricle. From this origin they descend the spinal cord as far as the
level of the second, third, and at times the fourth thoracic nerves. At 1

3I4 TEXT-BOOK OF PHYSIOLOGY

level they emerge from the cord in company with the nerve-fibers composing
the anterior roots of the second third, and fourth thoracic nerves. After
a short course, they enter the white rami communicantes, then the sympa-
thetic chain and pass upward to the ganglion Stella turn (the first thoracic),
and by way of the annulus of Vieussens (in the dog) to the inferior cervical
ganglion as well, around the nerve-cells of both of which their terminal
branches arborize.1 From the nerve-cells of both the stellate and inferior
cervical ganglia, the sympathetic nerves proper arise, which after emerging
from the ganglia pass toward the heart and become associated with the fibers
of the vagus and assist in the formation of the cardiac plexuses. On reach-
ing 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 tlie 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 scrip torius. 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. 140.)

The Origin and Distribution of the Sympathetic and Vagus Nerves
in the Frog. — In the frog and allied animals the relation of these two sets of
nerve-fibers, viz., the efferent sympathetic fibers and the efferent vagus

B1In man the annulus of Vieussens connects the inferior cervical with the middle cervical
ganglion.

THE CIRCULATION OF THE BLOOD 315

fibers, is somewhat different; and because of the fact that these nerves in this
animal are largely employed for determining experimentally 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-medullated fibers tarise* From this origin they
ascend, passing successively through the second sympathetic ganglion, the
annulus of Vieussens, the first sympathetic 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 vago-sympathetic 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 com-
mon 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. The mode of termination of both the vagus and sympa-
thetic 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 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 demon-
strate their respective functions first to divide the nerves, above their umo
at the vagus ganglion, and then stimulate their peripheral ends,
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 te,
or an augmentation in the force of the heart-beat or both, at the same time.

3l6 TEXT-BOOK OF PHYSIOLOGY

The effects of such a stimulation with induced currents of moderate intensity
are graphically shown inFig. 141 . The upper tracing shows that the heart was
first accelerated, the beats increasing from 15 per minute before stimulation,
to 30 per minute during stimulation. On the cessation of the stimulation,
the heart slowly returned to its former rate. Coincidently with this accelera-
tion 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 con-
traction which before stimulation was 9 mm., but during stimulation 12 mm.
In addition to the foregoing changes in the heart-beat there is an altera-
tion in the sequence of the beat. The natural delay in the conduction of the
excitation process from the auricles to the ventricle is increased, in conse-
quence of which the auricle completely relaxes before the ventricular con-
traction begins. Moreover, the auricular contraction again occurs before

FIG. 141. — TRACINGS SHOWING THE EFFECTS ON THE HEART-BEAT OF THE FROG FROM STIMU-
LATION 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 lowest tracing shows a similar series of
effects, the differences being only of degree. — (Brodie.)

the ventricle has completely relaxed. After the effect of the stimulatfon
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 sympa-
thetic 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
cwdio-accelerator or augmentor center. Since stimulation of the nerve in
any part of its course, which in all probability exaggerates its normal function,
is followed by an acceleration or an augmentation, the sympathetic is said

THE CIRCULATION OF THE BLOOD

317

to have an accelerator or an augmentor influence on the heart-beat; with
the cessation of the stimulation, and very frequently before, the heart returns
to its normal condition.

The Physiologic Action of the Vagus Nerve in the Frog.— Stimulation
of the intra-cranial roots of the vagus with very weak induced electric cur-
rents is followed by a gradual diminution in the rate and a diminution 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. 142.) 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

FIG. 142. — TRACING SHOWING THE EFFECT ON THE HEART-BEAT OF THE TOAD OF LONG
STIMULATION OF THE INTRA-CRANIAL ROOTS OF THE VAGUS WITH MODERATELY STRONG ELECTRIC
CURRENTS. — (Gaskell.)

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 cardio-
inhibitor center. Since stimulation of the nerve, either at its center, in its
course, or at its periphery, 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 influence on the beat of the heart.

During the continuance of the inhibition, the heart-muscle is relaxed,

FIG. I43--TRACING SHOWING THE DlMINUTION IN ™E RATE OF THE HEART-BEAT

FOLLOWING WEAK TETANIZATION OF THE VAGUS TRUNK.

its cavities dilated and filled with blood. The dilatation usually exceeds
that observed prior to the vagus stimulation, from which it is mferre
that some fibers of the vagus at least diminish the tomcity of the hear!

U 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 t
rate and force increase, until they attain or exceed that observed prior
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 durati<
the inhibitor effect varies with the duration of the stimulation. Thus during

3l8 TEXT-BOOK OF PHYSIOLOGY

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 (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 antagonistic action
of the inhibitor and accelerator nerve impulses. Thus stimulation of the
peripheral end of the divided trunk of the vagus in 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 some-
what stronger currents is followed by a diminution in the rate of the beat
because of the greater effect on the inhibitor nerve-fibers (Fig. 143). Stimu-
lation with strong tetanizing currents is followed by complete inhibition

(Fig. 144)- ¥

The foregoing facts lead to the inference that the cardio-accelerator
and the cardio-inhibitor centers have as their function the discharge of nerve
impulses which are conducted by their related nerves, the efferent sympathetic

FIG. 144. — TRACING SHOWING COMPLETE INHIBITION FOLLOWING STRONG TETAN-
IZATION OF THE VAGUS TRUNK.

and vagal fibers, to the heart, and which, in a manner, as yet unexplained
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 subse-
quent 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 inhib-
itor 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 sino-
auricular and auriculo-ventricular junctions, and an increase in the con-
ductivity 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 conducted 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 CIRCULATION OF THE BLOOD 319

The Physiologic Actions of the Sympathetic Nerves in Mammals.—

In the mammal the actions of the sympathetic nerves closely resemble their
action hi the frog. Thus stimulation of the nerves in any part of their
course, either through the rami communicantes, or after their emergence
from either the stellate or the inferior cervical ganglion 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.

FIG. 145.— ACCELERATION OP THE HEART-BEAT FOLLOWING STIMULATION OF THE CARDIAC
BRANCHES WHICH COME FROM THE ANNULUS OF VIEUSSENS.

The effect of the accelerator impulses is apparently a change in the inner
mechanism of the heart-muscle itself and not 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 (post-
ganglionic) 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 long latent period
intervenes between the moment of stimulation and the appearance of the
acceleration as shown in Fig. 145. A further peculiarity is that the accelera-
tion sometimes continues after the stimulus is
withdrawn, and sometimes ceases before it is
withdrawn.

The force of the heart-beat may be in-
creased without there being any increase in
the rate (Fig. 146). The auricular contraction
is probably increased whereby the ventricle is
more completely filled. TRACTION (CURVE OF PRESSURE IN

Though an increase in both the rate and RIGHT VENTRICLE) FROM STIMU-

- ,. & , «i, i -T_ LATION OF AUGMENTOR FIBERS.

force frequently occur simultaneously, mere is There is little or no change in
no necessary relation or connection between frequency. — (Franck, reduced.)
the two as they can and do occur independ-
ently of each other. For this reason it is generally assumed that the sym-
pathetic nerves contain two groups of fibers, viz., accelerators and augmen-
tors, the functions of which are respectively to 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 stimu-
late 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. . .

Division of the sympathetic nerves is at once followed by a diminutK
in the rate, the degree of which will depend to some extent on the rate

320 TEXT-BOOK OF PHYSIOLOGY

at which the heart was beating prior to the division. The results therefore
that follow stimulation and division of these nerves indicate that they are
transmitting nerve impulses from the centers from which they arise to the
heart upon which they exert a stimulating influence on the rate and force
of the beat.

The Physiologic Action of the Vagus Nerve in Mammals. — In the
mammal the same or similar effects in varying degree result from stimu-
lation of the vagus as in the frog. These results can be readily shown in
the dog or rabbit in the following way. The thorax of the animal is opened
and artificial respiration maintained. Under these circumstances the heart
will continue to beat in a practically normal manner for a long time. The
vagus nerve is then exposed on one side and divided, and its peripheral end
stimulated with induced electric currents of moderate strength; whereupon
the heart will 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 con-
traction. During the period of the inhibition the heart presents an
appearance similar to that presented by the heart of the cold-blooded
animal, viz. : completely relaxed walls and the cavities filled and distended
with blood. 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 contractions registered on a moving recording surface. The
effects on both auricles and ventricles which follow vagus stimulation will
then become more apparent. Fig. 147 is a tracing thus obtained. The
animal employed for the experiment was a rabbit.

Division of one vagus is followed in some mammals, e.g., dog by a marked
increase in the rate of beat and if both vagi are divided the increase may
amount to from 50 to 75 per cent. The results of stimulation and division
of the vagus nerves indicate that they are continuously transmitting nerve
impulses from the centers from which they arise, to the heart-muscle, on
the activity of which they exert a restraining or inhibitor influence.

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 frequently want-
ing. In this latter animal a complete standstill, 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 stimulus employed is quite strong. In all these
animals, however, after a very short time the nerve impulses lose their
inhibitor influence on 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 for
this loss of vagus control or escape of the heart from the vagus control has
as yet been offered.

THE CIRCULATION OF THE BLOOD 321

Seat of Action of the Vagus Impulses.— In the foregoing experiment of
which Fig. 147 is a graphic record, 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 impulses exert their influence mainly,
if not exclusively, on the auricle, and especially on the sino-auricular node,

FIG. I47.-RESULT OF THE STIMULATION OF THE PERIPHERAL END OF THE

DIVIDED LEFT VAGUS IN THE RABBIT.— (Brodte.)

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

the muscle bundle of His, there is for a time a complete cessation of ven

322 TEXT-BOOK OF PHYSIOLOGY

tricular 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 ven-
tricles will continue to beat at their usual rate. These and similar facts
lead to the conclusion that vagal inhibitor action is limited to the auricles.

From 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 Ho well,
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 sympa-
thetic 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. Again,
that an increase (within limits) 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 sensitiveness 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 foregoing effects of the sympathetic and vagus nerves on the heart
muscle, viz.: changes in its irritability, conductivity, rapidity, and the
energy of the beat, have been termed by Engelmann bathmo tropic, dromo-
tropic, chronotropic, and inotropic. Any one of these effects, e.g., the chrono-
tropic, may be modified in a positive direction by the sympathetic, or in a
negative direction by the vagus.

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

THE CIRCULATION OF THE BLOOD 323

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 possible inhibitor
influences, division of the sympathetic 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-accelera-
tor 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 cannot be stated
positively that this is the result of a reflex stimulation of the accelerator center.
Though earlier investigators believed this to be the correct interpretation,
the more recent experiments of Hunt apparently disprove it; for this investi-
gator has shown that if the vagus nerves are divided it is impossible to pro-
duce reflex acceleration of the heart. 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 possible.

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-in-
hibitor center.1 It is situated in the medulla oblongata or more exactly in
the gray matter beneath the floor of the fourth ventricle near the tip of the
calamus scriptorius. It is in all probability 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 j
feeble. In human beings the tone of the inhibitor apparatus is po

i Inasmuch as the nerve system is bilaterally arranged there are of course two nerve-centers
here as elsewhere, one on each side of the median line Aough they are associated in action b,
either commissural fibers or by a decussation of some of their efferen

324 TEXT-BOOK OF PHYSIOLOGY

developed in early childhood, as shown by the fact that the administration
of atropin, which removes temporarily inhibitor control, is not followed by 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 advanced 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 ac-
cordance 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 by nerve impulses transmitted to it through various
afferent or sensor nerves. Though both factors play an important part in
the maintenance of its activity, it is difficult to state which is the more
important of the two. That nerve impulses transmitted to the center
through afferent nerves lead now to an inhibition, now to an acceleration
of the heart has been abundantly established by the stimulation of afferent
nerves m almost any region of the body. Thus stimulation of the dorsal
roots of the spinal nerves, the trunks of the cranial sensor nerves the
splanchnic nerves, the pulmonary branches of the vagus, etc., gives rise to a
more or less pronounced inhibition of the heart. On the other hand
stimulation of afferent nerves sometimes leads to the opposite result, namelv
acceleration of the heart. As a rule, stimulation of the peripheral termina-
tions of these nerves is more effective than stimulation of their trunks
From facts^such as these, it has come to be believed that nerve impulses
developed m many regions of the body are being constantly transmitted
to this center, which not only stimulate it to activity but modify its activity
m one direction or another from moment to moment.

The first explanation, that acceleration of the heart, the result of a
peripherally acting stimulus, is due to a stimulation of the cardio-accelerator
>nter by the arrival of nerve impulses coming through afferent nerves,
having been made questionable and improbable by the results of Hunt's
experiments, the alternative explanation must be that while inhibition of the
heart is due to an excitation of the normal activity of the cardio-inhibitor
center, the acceleration of the heart is due to an inhibition of the normal
activity of the cardio-inhibitor center, and hence there follows 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
Tipherally excites or augments its activity, the other of which when stimu-
lated inhibits or depresses its activity, thus giving rise to a freer nlav ^ ^
cardio-accelerator center.

* °-f ^ arePresent in an7 one afferent

n™ . i tr!gemmus * is believed the excitator fibers
Followed h 6 VKV 6 T !°Vhat PeriPheral stimulation of this nerve is
e^s n™ 2??*i °f ?° ***** * *' sdatk' * is believed the inhib*or
divTded ne^ fa i ' VhC ^"n n that stimulati°n of the central end of the
livided nerve is followed generally by acceleration of the heart

THE CIRCULATION OF THE BLOOD 325

It is probable from the effects which follow gastro-intestinal disorders,
that the vagus nerve contains both classes of fibers as represented 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 thecardio-accelerator
center may be enabled to increase either the rate or the force or both, of the
heart movements. Palpitation of the heart from gastric irritation might
thus be explained.

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 activity 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 Influence of Psychic States. — It is a familiar personal experience
that emotional states, according to their suddenness or intensity may in-
crease or decrease the activity of the heart; thus depressing emotions may
diminish the activity almost to the point of complete inhibition, while joyous
emotions on the contrary may increase both rate and force to the point at
which the action becomes tumultuous and even irregular. The effects hi
either case are due to nerve impulses descending from the cerebrum to the
cardiac center in the medulla.

The nerve impulses discharged by cerebral cells during the occurrence
of both the depressing and joyous emotions exert their influence in all prob-
ability, on the cardio-inhibitor center alone. In the case of depressing emo-
tions, the nerve impulses excite or increase the normal degree of activity of
this center in consequence of which its inhibitor influence on the heart is
increased; in the case of joyous emotions, the nerve impulses inhibit the
normal degree of activity of the center in consequence of which its influence
on the heart is decreased. This permits of a freer play of the cardio-accel-
erator center and as a result there is an increased activity of the heart.

The Causes of the Variations in the Heart-beat.— It has been stated
elsewhere in the text (page 290), that the rate of the heart-beat is influenced
by age, muscle activity, the position of the body, meals, variations in blood
pressure, etc. The manner in which these changes are brought about is
not, however, always apparent. In addition to variations that are strictly
physiological in character 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 11
another or the same individual an inhibition which may be so pronounced
as to almost lead to a complete, though temporary standstill of the heart in
diastole The records of clinical medicine contain many instances wh
show that ocular, dental, gastric, intestinal, uterine and other organic : d«
orders, as well as various operative procedures in different regions o
body cause now an acceleration, now an inhibition of the heart whic
be so marked and pronounced as to give rise to serious apprehensu

The Depressor Nerve.— The vagus trunk also contains afferei

326

TEXT-BOOK OF PHYSIOLOGY

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 consideration of the physiologic
action of this nerve will be found in the section devoted to the nerve mechan-
isms concerned in the maintenance of the blood-pressure.

Modifications of the Nerve Mechanism of the Heart that Follow
Slightly Toxic Doses of Drugs. — The functions of different parts of the
nerve mechanism of the heart may be demonstrated by an analysis of the
effects that 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-muscle may at the same time be stimu-
lated or depressed in its action either in the same or in the opposite
direction to that of the nerve mechanism. 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 in-
crease in frequency is not very marked. In all
animals thus far experimented on after the ad-
ministration of atropin, neither stimulation of
the trunk of the vagus nor stimulation of the
intracardiac ganglia will arrest or even retard
the heart-beat. IThe inference, therefore, is
that the alkaloid exerts its action upon the gan-
glion cells and their terminal branches, impair-
ing their chemic integrity and abolishing their
normal function, that of conducting nerve im-
pulses from the vagus nerve proper to the heart-

^FR;A48'"~DlAGRAM VHOWING musde- Fig. 148. In consequence of this, the
THE RELATION OF THE VAGUS TO • n c •, ,..,.,. . ~.

THE HEART-MUSCLE CELL. influence or 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. Stimulation 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 accomplished 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.—Mter the administration of nicotin in sufficient amounts
the heart-beat is primarily decreased in frequency even to the point of stand-

Seat of action
of Mcotin

Sympathetic

Vagus nerve.

Seatofactim
of Atropin.

THE CIRCULATION OF THE BLOOD 32;

still in diastole for a few seconds, and secondarily increased both in fre-
quency 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-gangli-
omc, as well. After the secondary increase 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
peripheral terminations of the pre-ganglionic fibers of the vagus as they
arborize around the intra-cardiac ganglia, depressing them and suspending
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 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 (Cushny).

Pilocarpin and Muscarin— These alkaloids, whether administered in-
ternally 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 dias-
tole. For the reason that the internal administration or the local applica-
tion 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 consequence 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 ^corresponding decrease in the duration of the dias-
tole, 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 arises only when the vagus nerve is intact it is
very probably due to a stimulation of th e cardio-inhibitor center and a con-
sequent increase of its functional activity. Though either effect may be
produced in the frog the predominant effect is the increase in the contrac-
tion 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 stimulation 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 Vascular Apparatus in its entirety consists of a closed system of
vessels which not only contain the blood, but under the driving power of the
heart, transmit it to and from all regions of the body. It is usually divided
into a systemic and a pulmonic portion.

The Systemic Vascular Apparatus. — This portion of the general vas-
cular apparatus includes all the vessels extending from the left ventricle to
the right auricle: viz., 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 aduentitia.

The internal coat consists of a structureless elastic basement membrane,
the inner surface of which is covered by a layer of elongated spindle-shaped
endothelial cells. The middle coat consists of several layers of circularly
arranged, 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, exceeding largely in amount the muscle-tissue. It is also
more closely and compactly arranged. The external coat is well developed
in the large arteries (Fig. 149).

The presence in their walls of both elastic and contractile elements,
endows the arteries with the two properties of elasticity and contractility. .

Elasticity. — The elasticity is best developed in the large arteries, though
it is also present in arteries of relatively small size. By virtue of the elas-
ticity, the arteries are capable of being distended and elongated and when
the distending force is removed of recoiling or retracting and returning to
their former condition. Thus the capacity of the aorta and carotid artery
of the rabbit can be increased four times and six times respectively by raising
the intra-arterial pressure from o to 200 mm. of mercury. The elasticity
permits of a wide variation in the amount of blood the arterial system can
hold between its minimum and maximum distension. The arteries thus

328

THE CIRCULATION OF THE BLOOD

329

adapt themselves to the variations in the volume of blood discharged from
the ventricle at a single beat or in a unit of time. The elasticity also con-
verts the intermittent movement of the blood imparted to it by the heart
as it is ejected from the ventricle, into a remittent movement in the arteries
and finally into the continuous and equable movement observed in the capil-
laries. This is accomplished in the following manner: With each contrac-
tion 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 contrac-
tion, owing to the small caliber and friction of the arterioles. 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 reaction of the arterial walls from
without. With the cessation of the contrac-
tion the elastic walls recoil and propel the
blood toward the capillaries. The intermit-
tent action of the heart is thus succeeded by
the continuous reaction of the arterial wall.

As the blood advances toward the periph-
ery of the arterial system and larger amounts
pass into the capillaries, both the distention
and the elastic recoil diminish, and by the time
the blood reaches the capillaries its intermit-
tency of movement has been so far obliterated
by the elastic recoil that as it enters the capil-
laries the movement becomes equable and con-
tinuous. In this manner the arteries modify
and change the character of the blood flow
and in part adapt it for the conditions of the
blood flow in the capillary vessels.

In youth the arterial walls are highly dis-
tensible 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 I49._CoATS OF A SMALL

characteristics the flow of a fluid through a j^wx. a. En dot h'e Hum. b.
rigid tube under the intermittent action of a internal elastic lamina, c. Cir-
pump; that is, the intermittent movement im-
parted by the heart is not so completely con- dois and Stirling.)
verted into a continuous movement, and hence
the blood flows through the capillaries during the systole with greater velocity,
and during the diastole with less velocity, than is the case when the vesse
is normally elastic. For these and other reasons the tissues are not so well
nourished and hence their nutrition and functional activities decline.

Contractility.— The contractility permits of a variation in the amoi
of blood passing into a given capillary area in a unit of time. Normally ead
artery has a certain average caliber due to a given contraction of the 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
crease in the amount of blood beyond that supplied during functional mac-

330

TEXT-BOOK OF PHYSIOLOGY

tivity 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 region, therefore regulate the supply of blood to the tissues in
accordance with their functional needs.

Moreover, as will be stated in subsequent paragraphs the degree of
contraction of the arteriole muscle influences very markedly the degree of
friction which the 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 enlarged 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 this 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 functions of the arteries
therefore are as follows: (i) to
distribute the blood to all regions
of the body; (2) to accommodate
varying volumes of blood; (3) to
convert the intermittent flow of
blood as it leaves the heart into
a remittent and finally into a con-
tinuous and equable flow as it
passes into the capillaries; (4) to
regulate the volume of blood de-
livered to an organ in accordance
with its functional activities, and
(5) to present that degree of re-
sistance necessary to the main-
tenance of the normal blood pressure, through the activities of the muscle
fibers in the walls of the arterioles.

The Structure, Properties, and Functions of the Capillaries.— The
capillaries are small vessels continuous with the arteries on the one hand
and with the veins on the other hand. 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
ecrease 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
: mm. The wall of the capillary (Fig. 150) is composed of a single layer of
ideated endothehal cells with serrated edges united by a cementing mate-

FIG. 150. — CAPILLARIES. THE OUTLINES or
THE NUCLEATED ENDOTHELIAL CELLS WITH THE
CEMENT BLACKENED BY THE ACTION OF SILVER
NITRATE. — (Landois and Stirling.)

THE CIRCULATION OF THE BLOOD 33i

rial. Though extremely short, the capillaries divide and subdivide a number
of times, forming meshes or networks, the closeness and general arrange-
ment of which vary in different localities.

As the endothelial cells are living structures and characterized by irrita-
bility, contractility and tonicity, it may be assumed that the capillary wall as
a whole is characterized by the same properties. Upon the possession of
these properties the functions of the capillary depend.

The function 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 capil-
lary 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 measurement,
it readily permits, under certain conditions, of the necessary exchange of
materials between the blood and the tissues. The forces which are con-
cerned in the passage of materials across the capillary wall are embraced
under the terms diffusion, osmosis, and filtration. As a result of the inter-
change of materials the tissues are provided with nutritive materials and re-
lieved of the presence of the products of metabolism. As the blood loses
oxygen and gains carbon dioxid, it changes in color from a scarlet red to a
bluish red. In consequence of the exchange of materials, the blood under-
goes a change in composition, the extent and character of which varies in
accordance with the activities and character of the organ traversed by it.

In order that the nutritive materials may pass through the capillary
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 pressure and with a definite velocity.
These conditions are made possible by the cooperation of the physiologic
properties and physiologic actions of the heart and vascular apparatus, the
nature of which will be explained in subsequent pages.

The Structure, Properties, and Functions of the Veins.— The veins
arise from the distal side of the capillary vessels. As they emerge they are
quite small and designated venules. By their convergence and union the
the veins gradually increase in size in passing from the periphery toward
the heart. Their walls at the same time correspondingly increase in thick-
ness. 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 venae
cavae empty into the right auricle.

A typical vein consists of the same three coats as the artery: viz., the
tunica intima, the tunica meditkantii the tunica adventitia. The media,
however, does not possess as much of either the elastic or muscle tissue as
the artery, but a larger am&unt 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 furthering the movement
of the blood toward thtHieart, especially after a temporary obstruction.

Certain veins, especially the large veins of the extremities, are character-
ized by the presence of valves throughout their course. These are arranged
in pairs and formed by a reduplication of the internal coat, strengthened

332 TEXT-BOOK OF PHYSIOLOGY

by fibrous tissue. They are always directed toward the heart and in close
relation to the walls of the veins, so long as the blood is flowing forward.
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 accom-
modate 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. The veins of many viscera, of the bones, of
the cranial and vertebral cavities are devoid of valves. The function of
the veins is therefore to collect the blood from the capillary areas and trans-
mit it to the heart.

THE FLOW OF THE BLOOD THROUGH THE VESSELS
HYDRODYNAMIC CONSIDERATIONS

The blood flows through the arteries, capillaries and veins in accordance
with definite laws. During its transit certain phenomena are presented by
each of these three divisions of the vascular apparatus. These are mainly
velocity and pressure and in the arteries alone an alternate expansion and
recoil of the arterial wall with each heart beat, termed the pulse. 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 con-
sidered :

1. The flow of a liquid through a horizontal tube with rigid walls and of
uniform or variable diameter under a steadily acting pressure.

2. The flow of a liquid through a series of branching and again uniting
tubes with rigid walls under a steadily acting pressure.

3. The flow of a liquid through a tube with elastic walls under an inter-
mittently acting pressure.

THE FLOW OF A LIQUID THROUGH A HORIZONTAL TUBE WITH RIGID
WALLS UNDER A STEADILY ACTING PRESSURE

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. 151,
which consists of a horizontal tube into which is inserted at equal distances a series
of vertical tubes , and which is connected with a reservoir or pressure vessel, P. If
the reservoir be filled with a liquid, water for example, the latter under certain con-
ditions will exert a downward pressure and act as a propelling 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 show its downward pressure,
and in consequence it will be driven into and through the horizontal tube and
discharged from its free extremity with a definite velocity. At the same time the
fluid will rise in each vertical tube to a height directly proportional to the distance
of each tube from the free extremity. The velocity with which the fluid is dis-
charged can be determined by measuring the quantity, q, discharged in a unit of time,

(i second) and dividing it by the area of the tube, ;zT2; viz., v= A. Inasmuch as

THE CIRCULATION OF THE BLOOD

333

the tube is 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 the adhesion between the walls of
the tube and the water 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 adhesion 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 retarded in
its flow. Between the outer retarded layer and the axis of the stream, there is an
infinite » number of layers of molecules, the cohesion of which one for the other is
more and more overcome by the pressure in the vessel, P. The force of adhesion
between wall and fluid, together with the force of cohesion between the molecules
of the fluid give rise to the resistance of the fluid to the flow.

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. That such a pressure exists is shown

FIG. 151.— A PRESSURE VESSEL, P, WITH A HORIZONTAL OUTFLOW TUBE, O-n, INTO WHICH
VERTICAL TUBES OR MANOMETERS ARE INSERTED.

by the rise of the fluid in each of the vertical tubes, and the height to which it rises
in each tube is a measure of the pressure at its base. In the tube /, the fluid rises
to but a slight extent for the reason that the resistance yet to be overcome .is slight
in amount It is, however, a measure of the resistance or friction between the base
of the tube and the orifice of outflow. In the tube e the fluid rises twice as high
as in / because of the additional friction between the bases of the tubes e and /.
What is true of these two points is equally true of the points at the base of the tubes
dt c, b, a. Lines drawn to the pressure vessel from the top of the fluid in each tube
and parallel to the horizontal tube will show how much of the pressure force is
utilized in overcoming the friction in each section of the horizontal tube,
amount of the lateral pressure at any given point is theref ore indicated and measured
by the height to which the water rises in the tubes. For this reason these tube
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 *e reservoir
the outlet Therefore the pressure at any given point is proportional to the r
ance yet to be overcome and conversely the resistance to be overcome is indicate
by the amount of the pressure. (In the conduct of an experiment the propelling

334 TEXT-BOOK OF PHYSIOLOGY

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 utilized in overcoming the resistance
is approximately determined by drawing a line from the outlet to the reservoir,
uniting the upper levels of the water in the vertical tubes. The height of the fluid
at the point at which the line intersects the reservoir, y, is a measure, therefore,
not only of the resistance but also an indication of the relative amount of the pres-
sure used in overcoming it and is therefore known as the pressure height and
indicated by the letter, h.

The amount of the pressure consumed in imparting the observed velocity 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 veloc-
ity by twice the accelerating force of gravity, 980 centimeters, as expressed in the

v2
formula, — ; the quotient is the height and is known as the velocity height. Con-

versely if the moving fluid were discharged into empty space through an opening
in the tube at n, it would ascend an equal distance. If now this height is repre-
sented 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} or h' 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 mainly into two portions; one, the larger
by far,/*, is utilized in overcoming the resistance to the flow of the water; the other,
the smaller, h' 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 substituted a
tube the middle third of which is enlarged, the conditions will be similar to
the previous case until the fluid flows into the enlarged portion, when the velocity
will diminish, being inversely proportional to the area of the cross-section. The
resistance will be also diminished and therefore less of the pressure force or
driving power will be consumed than in the first section of the tube, and as a result,
the lateral pressure will fall less rapidly than in the first section. 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 similar to 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 resistance being increased at the
same time, there will be a rapid consumption of the pressure force and a steep fall
of lateral pressure. On flowing into the third section, the velocity again diminishes
and the pressure falls though more slowly to the end of the tube.

THE FLOW OF A LIQUID THROUGH A SERIES OF BRANCHING AND
AGAIN UNITING TUBES WITH RIGID WALLS UNDER A STEADILY

ACTING PRESSURE

In a system of this character, such as represented in Fig. 152, there must follow
as a result of the repeated branchings, a progressive increase in the total sectional
area of the collective tubes coincident with a progressive decrease in the sectional

THE CIRCULATION OF THE BLOOD

335

area o£ individual tubes in the section B c, while in the section c D, there must
follow a progressive decrease in the total sectional area of the collective tubes coin-
cident with a progressive increase in the sectional area of individual tubes, conse-
quently there will be a combination of the two conditions alluded to in the two
preceding paragraphs, namely, an enlargement of the stream bed coincident with
a diminution in size of the individual tubes composing it, in the middle section.
Moreover, for the purpose here intended it may be assumed that the tubes com-
posing the middle section c are microscopic in size and that their total sectional
area bears to the sectional area of tube A the ratio of 600 to i .

If the system is connected with a pressure vessel, as in the preceding instance,
and the stopcock is suddenly opened, the column of water will exert a downward

Fir i c 2 —PRESSURE VESSEL WITH A SERIES OF PROGRESSIVELY BRANCHING AND AGAIN UNITING

TUBES.

a, oc, a series of stopcocks by which the peripheral resistance can be increased or decreased.

pressure, and in consequence the water will be driven into and through the system
with a definite velocity and pressure.

The velocity of the fluid will gradually decrease from B to c in a ratio inversely
proportional to the total area of each cross-section until at c, it will attain its mini-
mal value; the velocity will again increase from c to D in a ratio inversely pro-
portional to the total area of each cross-section until at E, when it will attain the
value it had in A if the entrance and exit tubes have the same area.

The lateral pressure will gradually fall from the beginning to the end of the sys-
tem, though the fall must, be more rapid in B-C than in A-B as will be clear from

factors-viz, the widening of the stream bed which
resistance, and the narrowing of the individual tub es ^«^e,
the resistance-exert an opposing influence on the pressu re; hen,
pressure will be proportional to the ratio between these two factors. As

336

TEXT-BOOK OF PHYSIOLOGY

increase in the resistance due to the progressive decrease in the size of the individual
tubes preponderates considerably over the decrease in the resistance due to the
widening of the stream bed, there must be an increase in resistance in the area
B-C and therefore a more rapid fall of pressure than inA-B. This fall, however,
will not be as steep as it might be for the reason that the decrease in the velocity
is attended by a decrease in the resistance and hence a lessened consumption of
the propelling power. In the section C-D the two factors, viz. : the narrowing of
the stream bed which increases the resistance, and the enlarging of the individual
tubes which decreases the resistance, exert an opposing influence on the pressure,
hence the fall of pressure will be proportional to the ratio between these two factors.
As the decrease in the resistance due to the progressive enlargement of the individual
tubes preponderates considerably over the increase in the resistance due to the
narrowing of the stream bed, there should theoretically be a rapid fall of pressure
from c to E. This rapid fall, however, will be to some extent prevented for the
reason that the increase in velocity due to the narrowing of the stream bed in-
creases the resistance to a high value and hence the pressure falls less rapidly than
it otherwise would.

The pressure throughout the system is the result of (i) the downward pressure
of the water in the reservoir, and (2) the resistance to its flow, due to the cohesion
and friction of its molecules and its adhesion to the sides of the tubes, and its
extent in any one section will be proportional to the resistance yet to be overcome.
It will naturally be higher in the section A-B than in the section D-E, though the
difference in the level of the pressure between these two points will not be as great
as might theoretically be supposed from the small size of the tubes in c for the
decrease in velocity counterbalances in part the resistance which they offer.

The general curve of the fall of pressure in this system is indicted by the
curved line extending from the pressure vessel to the outlet of the horizontal tube.

The value of the pressures in these two sections and their relation to each other
could be varied either temporarily or permanently[by the insertion of a series of stop-
cocks a, x, along the course of the tubes between B and c in the neighborhood of
their ultimate branchings by which an additional resistance could be superposed
on the system from A to the 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 pressure be made to
act intermittently as it can be by alternately opening and closing the stopcock, at A
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. Coincidently there is an alternate temporary
increase and decrease of the lateral pressure.

THE FLOW OF A LIQUID THROUGH A TUBE WITH ELASTIC WALLS
UNDER AN INTERMITTENTLY ACTING PRESSURE

^When a tube with elastic walls is connected with a pressure vessel, the con-
ditions which are established on opening the stopcock and the consequent 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

THE CIRCULATION OF THE BLOOD 337

distention of the tube. This latter effect continues until the elastic recoil of the
walls of the tube exactly counterbalances the pressure of the water from within.
When this condition is established 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 alternately
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 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 becomes 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 are of such a nature that the resistance is very high and the
elasticity slight, the outflow will be intermittent. But if they are made to change
gradually, and this is especially the case with the resistance, from a slight to a
greater value, the outflow gradually changes from an intermittent to a remittent
and finally to a continuous 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 pressure, gives rise to
a distention of the tube, and acquires a certain velocity of outflow. In consequence
of the distention, a portion of the fluid accumulates. 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 be great and the elasticity
slight, this is impossible and the outflow will be entirely intermittent. ^ But if they
are made to increase in value, the proportionate amount of the fluid which accumu-
lates 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 duration, and hence the outflow
will change to a remittent and finally to a continuous outflow.

Coincident with the action and cessation of action of the primary pressure
there is a corresponding increase and decrease of the lateral pressure and when the
intermittent 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 witl
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, giv
rise to a wave on the surface of the tube which is propagated with more or les
rapidity-though with decreasing amplitude-from the beginning to ti
the tube and causing in each section a corresponding expansion and recoil, an
which is known as the expansion wave.

presented by the circulatory apparatus at the time of

THE APPLICATION OF THE FOREGOING FACTS TO THE
VASCULAR APPARATUS

The systemic vascular apparatus may be o
which have symmetrically divided and subdivided and
united and reunited in a corresponding manner. The arteries, a

33*

TEXT-BOOK OF PHYSIOLOGY

capillaries, venules, and veins may therefore be schematically arranged
(Fig. 153) in a manner identical with the schematic arrangement of tubes
represented on page 335. 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 developed during the contraction, to
the downward pressure of the water when the stopcock at A is opened.

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 capillaries the sectional area of
individual arteries, in consequence of repeated branching, diminishes,
though their total sectional area increases and in direct proportion to their

Veins

FIG. 153. — SCHEMATIC ARRANGEMENT OF THE VASCULAR APPARATUS.
a, x, the arteriole muscle by which the peripheral resistance can be increased or decreased.

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 i is to 600 or 800. This statement is based on
the estimated values of the sectional area of the aorta, 615 mm^ the velocity
of the blood in the aorta 300 mm. per second, the average velocity in the
capillaries 0.5 mm. per second. If these estimates are accepted as approxi-
mately correct, then the sectional area of the capillary system or bed is
obtained by the following formula:

x = — or 369,000 mm., a ratio of the aortic sectional area to the

THE CIRCULATION OF THE BLOOD 339

capillary sectional area of i to 600. 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 proportion to their distance
from the capillaries.

The stream-bed in the aorta is relatively narrow, but widens gradually as
it approaches the capillaries, where it attains its maximal width; it again
narrows gradually as it passes into the veins, until in the venae cavse it be-
comes 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 systenf to the

FIG. 154. — 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.)

terminations of the venae cavae, which result from the repeated branching
and subsequent uniting, as well as its relative width in the arteries, capil-
laries, and veins, are shown graphically in Fig. 154.

The Intra-ventricular Pressure.— When the heart contracts and the
intra-ventricular pressure rises above the pressure in the aorta, the aortic
valves are suddenly forced open and the blood is driven into and through the
arteries, capillaries, and veins to the right side of the heart and in accordance
with foregoing considerations with a definite velocity and against a certain
resistance, which in turn gives rise to a side pressure.

The Velocity.— The velocity of the blood in the systemic vascular ap-
paratus will gradually decrease from the aorta to the middle of the capillary
system in a ratio inversely proportional to the total area of any given cross-
section of the stream-bed, until in the capillaries it will attain its minimal
value, which is especially small because the resistance to the flow of blood in
the capillaries increases inversely as the square of their diameters, while in

340 TEXT-BOOK OF PHYSIOLOGY

the larger blood-vessels the increase is inversely proportional to the simple
diameter; the velocity will again increase from the middle of the capillary
system to the ends of the venae cavae in a ratio again proportional to the
total area of each cross-section of the stream-bed until in the venae cavae it
will attain its maximal value, though it will not attain its initial value in these
vessels because their combined sectional area is greater than that of the aorta.

The Lateral Pressure. — The lateral pressure will also gradually fall
from the beginning of the aorta to the ends of the venae cavae, though the
fall will be most rapid at the periphery of the arteries. In the arterial system
the fall of pressure will be proportional to the ratio between the increase in
resistance due to the narrowing of individual vessels, and the decrease in
resistance due to the widening of the stream-bed; as the former preponder-
ates over the latter there must be an increase in resistance from the aorta
to the capillaries and hence a sharper fall of pressure toward the termination
of the arterioles, which is very steep for reasons to be stated later. In the
venous system the fall of pressure will continue and its rate will be propor-
tional to the ratio between the increase in resistance due to the narrowing of
the stream-bed, and the decrease of resistance due to the enlarging of indi-
vidual vessels; as the latter preponderates over the former there should be
a rapid fall of pressure from the capillary system to the ends of the venae
cavae. This, however, is to some extent prevented for the reason that the
increase in velocity due to the narrowing of the stream-bed increases the
resistance to a relatively high value and hence the pressure falls less rapidly
than it otherwise would.

The Resistance. — The resistance to the flow of blood through the sys-
temic vascular apparatus is due to the cohesion and friction of its molecules
as well as that of the corpuscles and the adhesion of the blood to the sides of
the vessels. The resistance is also increased by the small diameter of the
capillary blood-vessels.

The high pressure characteristic of the arterial system contrasted
with the low pressure characteristic of the venous system determined by
experiment, cannot be accounted for alone by the resistance offered by the
small diameter of the vessels of the capillary system. This in itself would be
insufficient to maintain the observed differences in pressure in the different
sections of the vascular apparatus necessary for physiologic purposes. To
meet this necessity there has been developed at the periphery of the arterial
system, in the arteriole wall, a special muscle, a, x, Fig. 152 which by con-
tracting can add a physiologic resistance to what might be termed the phys-
ical resistance of the system. According to the degree of its contraction
will the resistance to the flow of blood from the arteries to the veins at the
periphery of the arterial system be increased and the arterial pressure be
raised and the venous pressure be lowered. According to the degree of its
relaxation will the resistance to the flow of blood from the arteries into the
veins be decreased at the periphery of the arterial system and the arterial
pressure be lowered and the venous pressure raised. By this means the
extent and the relation of the pressure in the two main sections of the
systemic vascular apparatus can be temporarily or permanently changed in
one direction or the other. The effect of the diminution in the caliber of
the arteriole due to the contraction of the muscle is spoken of as the
peripheral resistance.

THE CIRCULATION OF THE BLOOD 341

That the high pressure in the arteries is largely due to this physiologic factor
is shown by the rapid and pronounced fall of pressure that occurs when this
muscle suddenly relaxes as it does when the spinal cord is transversely
divided in the cervical region, thus cutting off from the arteriole muscle
those nerve influences that largely determine its contraction. Under
such circumstances the pressure in the dog may fall from approximately
140 mm. to 40 mm. of mercury or even less. Stimulation of the distal
extremity of the spinal cord will be followed by the temporary contraction of
the arteriole muscle and a rise of pressure to its former value.

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 mole-
cules. 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 percentage 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 be only
approximative.

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 aver-

(35)2
age velocity of the blood is 35 cm., the velocity pressure is equal to ---- or

0.62 centimeters of blood or 0.046 centimeters 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 i to 326.

The Pulse Wave.— Inasmuch as the heart's action is intermittent and
the walls of the arteries are elastic, there is a temporary increase and decrease
of the pressure with each beat of the heart, attended by a corresponding
expansion and recoil of the arteries, giving rise to a wave on the surface^of
the arteries which is propagated with more or less rapidity— though with
decreasing amplitude— from the beginning of the aorta to the capillaries,
and causing in each successive section a corresponding expansion and recoil,
and which is known as the expansion wave or pulse wave.

The general statements regarding the phenomena attending the flow of
blood through the systemic vascular apparatus contained in the foregoing
paragraphs will be further elaborated in the following pages. For special
reasons it is convenient to consider the pressure first.

BLOOD-PRESSURE

Blood-pressure may be defined as the pressure exerted radially or later-
ally by the moving blood-stream against the sides of the vessels. T
sure is the result of (i) the driving power of the heart, and (2) of t
ance offered to the forward movement of the blood-a resistance due to ti
cohesion and friction of the molecules of the blood of the blood corpuscles
and the adhesion of the blood to the sides of the blood-vessels. That there
such a pressure within the arteries, capillaries, and veins, different in amount

342 TEXT-BOOK OF PHYSIOLOGY

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 corresponding size.
When an artery is divided, the blood spurts from the opening for a consider-
able 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
and to a height 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 pressure. 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 difference in the height of the pressure in the arterial system as
contrasted with the venous system is due to the progressive diminution of
the resistance from the beginning of the aorta to the ends of the venae cavae,
together with the small diameter of the capillaries, increased to a variable
extent, by the tonic contraction of the arteriole muscle.

The same facts may be demonstrated in another and more striking way.
A dog or cat is anesthetized and securely fastened in an appropriate 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 di-
ameter. 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 correspond in a general way to the
respiratory movements. This experiment was originally performed on the
horse, by the Rev. Stephen Hales (1732).

In the venous tube the blood also rises to a certain height, after which it
remains quite stationary, as the effect of the cardiac contraction 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 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

THE CIRCULATION OF THE BLOOD

343

thing, to divide the vessel and insert the horizontal portion of a T-shaped
tube into the central and distal ends through which the blood can con-
tinue 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., i 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
i sq. mm. is the weight of 2000 mm. of blood. The weight of 2000 c.mm.
of blood is equal to 2.1 grams.

The Arterial Blood-pressure. — For accurate and long-continued ob-
servation 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

RECORDING BLOOD-PRESSURE.

artery (Fig

For the purpose of retarding coagulation of the blood and
revenmg escape of a large volume of blood from the vessels the
system is filled with a solution of carbonate of soda of sp. gr. 060 558
grams per 1000 C.C., or a 25 per cent, solution of magn ^um sulphate
sp gr 1060 and under a pressure approximately equal to that in the ves

of the Wood within, .nd the

of air

that

placed between the two limbs.

344 TEXT-BOOK OF PHYSIOLOGY

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 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. 155 and 156, 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

FIG. 156. — A PORTION OF A BLOOD-PRESSURE TRACING OBTAINED FROM THE CAROTID ARTERY
OP THE RABBIT WITH A MERCURIAL MANOMETER. The small oscillations are due to the heart-beat;
the large oscillations are due to the respiratory movements.

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 levels 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., (i) a more or less constant element 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
represented with certain limitations 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 pause; the
systolic pressure, by the distance between the base-line and the apices of

THE CIRCULATION OF THE BLOOD

345

max valve

mm valve

the curves following the cardiac systole. The relation of these two com-
ponents 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 in extent.

There are good reasons for believing, however, that this record does not
represent either the true diastolic or the true systolic pressure but that the
limits between the two are far more widely apart than here represented.
For, owing to its inertia, the mercury is not capable of following the rapid
variations of the pressure throughout their extent, that occur with each
heart-beat. The employment of one of the various forms of the quickly
responsive spring manometers such as are used in determining the rapid
variations of intra-cardiac pressure will show a much greater difference
between the diastolic and systolic pressures, often amounting to as much as
40 millimeters.

For the purpose of obtaining the maximum systolic and the minimum
diastolic pressures, it is best, however, to insert between the cannula and the
manometer a maximum and a minimum , to manometer

valve similar in principle to that shown
in Fig. 157. By permitting the blood to
exert its pressure first through the maxi-
mum valve and then permitting the mer-
curial column to exert its pressure through
the minimum valve in the reverse direction
for a certain length of time, the maximum
systolic and the minimum diastolic pres-
sures will be recorded. By this method
Dawson found an average maximum sys-
tolic pressure in the carotid artery of the
dog of 162, and a minimum diastolic pres-
sure of 103 mm. of mercury, a difference of
59 mm. Hg. The difference between these
two pressures is known as the pulse pres-

r . - ' , . f SU LllCIL lltt* 10.LIA-1 J-1414.J ~~ —~~

Sure. (A diagram showing the relation 0 maximum, minimum, or ordinary

these different pressures one to another will manometer according to the tap which

Q N is left open.— (Starling.)

be found on page 348.)

In a series of experiments it will be found that the blood-pressure in the
arteries, recorded with the mercurial manometer, though rising and falling a
certain number of millimeters, 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. Though the tracing fails to record accurately the diastolic
and systolic pressures it approximates a certain average or mean of the pres-
sure thus recorded, 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, the mean pressure is- the arithmetic mean
the systolic and diastolic pressures. This is, however, not strictly correct,
for if the pressure is recorded by means of a spring manometer or a sphygmo-
Sraph applied over the artery of man, a record much different in appearance
fndPsimilar to that shown in Fig. 158 will be obtained In such a record
it will be observed that the passage from the lowest diastolic pressure (a) to

'to heart

FIG. 157-— v. FRANK'S VALVE.
This is placed in the course of the

346

TEXT-BOOK OF PHYSIOLOGY

the highest systolic pressure (b) takes place quickly, occupying about o.i
second; and that the change from the highest systolic pressure to the succeed-
ing lowest diastolic pressure takes place slowly, in about 0.7 second, and that
the line of descent is interrupted by a secondary rise and fall of pressure be-
fore the former diastolic level is reached. 1 If a horizontal line is drawn across
the record parallel to a line uniting the lowest diastolic levels, and to a line
uniting the highest systolic levels and in the position of the arithmetic mean,
the triangular record of the pressure changes will be divided into two portions
of which the upper has a smaller area than the lower portion from which
it is apparent that the pressure is low for a longer period [that it is high
and hence the mean pressure cannot be the arithmetic mean between the
diastolic and systolic pressures. The mean pressure, however, can for a
given period at least be experimentally determined. 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 ab-
solute pressure. The same result can be obtained
by employing the compensatory manometer of
FIG. 158.— Spnygmogram or Marey which presents a constriction of this char-
acter. 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 expressed numerically by the statement that
it is equal in millimeters of mercury to the diastolic pressure plus one-third
of the pulse pressure. 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 represented 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 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 artery of different animals are the following: In the horse, from
122 lo 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

1 It must be remembered, however, that the cardiac systole endures though with diminishing
energy, even though the systolic pressure in the artery is falling, for 0.32 second, when a rapid
dilatation sets in attended by the closure of the semilunar valves, an event indicated in the tracing
by the notch preceding the second rise of pressure at c.

THE CIRCULATION OF THE BLOOD

347

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 resist-
ance suddenly and rapidly increases. Thus Volkman found simultaneously
in the carotid artery and in the metatarsal artery of the sheep a mean pressure
of 165 and 146 mm. Hg. respectively and this for the reason that the resist-
ance 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 constant
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 experi-
ments made on dogs. The figures represent in millimeters of mercury certain
average end pressures in the arteries named.

Artery

Systolic

Mean

Diastolic

Pulse pressure

. •

Brachio-cephalic. .......

163

121

103

Right carotid

160

118

Left carotid

160

123

101

Left subclavian . . .

168

123

I05

Left brachial . . •

160

118

Left renal . .....

165

123

i°3

Deep femoral

152

118

IO2

Thvroid

140

118

The Capillary Pressure.— The small size of the capillaries precludes
an investigation of their pressure by manometric methods. It may be state
however, to be approximately equal to the pressure required to obht
their lumina and to whiten the skin. The apparatus of v. Knes is based on
this theory. A small glass plate, from 2.5 to 5 sq. mm. is fastenec to the
under surface of a support of suitable size carrying a small scale pan
glass plate is placed on the skin near the root of a finger-nail and the seal
pan gradually weighted until the vessels are ob iterated, as shown by the
blanching of the skin. From results obtained with this apparatus v Knes
estimated the pressure in the capillaries of the hand at 37 mm. Hg. and

Pressure.-In passing from the capillaries to ^the heart
the pressure continues to fall. The increasing size of the : veins , penmte
again of manometric observations in different regions. In .he crural ym
the pressure has been found to be equal to 14 mm. Hg an
vein 9 mm. of Hg. In the jugular and subclavian and other ^es

348

TEXT-BOOK OF PHYSIOLOGY

the heart it is zero or even negative; that is, less than atmospheric pressure
to the extent of from i 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. 159.

16O

120

Line of
DIASTOLIC--
PRESSURE

Line of
SYSTOLIC PRESSURE

Line of
MEAN PRESSURE

PULSE PRESSURE

The differe?ice between

D/ASTOL/C

and
SYSTOLIC PRESSURE

FIG. 159. — 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 DlASTOLIC, THE MEAN, AND THE SYSTOLIC PRESSURES IN THE ARTERIAL SYSTEM. Based OH

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 6n the left give the pressure (approximately) in millimeters of mercury, h. Pressure
in heart, a. Arteriole region showing sudden fall of pressure, c. The fall of pressure in the
capllaries. v. The negative pressure in the large veins.

RESUMfi 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
re'sume" may be made: i. 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 contrac-
tion, 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 and its adhesion to the walls of the vessels. 4. The
resistance is inversely proportional to the diameter of the vessel and is there-
fore least in the large arteries and veins and greatest 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 venae cavae where it may be no greater than that of the atmos-
phere 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 apparatus
represents the resistance yet to be overcome. 8. The high pressure in the
arterial system and its marked fall at the periphery is more especially the
result of the very great resistance at this point, known as the peripheral

THE CIRCULATION OF THE BLOOD 349

resistance, the result of a rapid diminution in the diameter of the arterioles
and the capillary vessels, together with the tonic contraction of the arteriole
muscle. 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 the pressure to that of the
atmosphere. Even under normal circumstances this condition is approxi-
mated during the diastole. The recoil of the arterial wall by which the for-
ward 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 contained 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 ac-
commodated. This at once reestablishes the pressure at its former level.

During the contraction of the heart the kinetic energy is transformed
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 transformed into kinetic energy,
represented by the moving blood. The artery thus continues the work
of the heart during its period of inactivity. 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 previously 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 modii
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 i
pressure during and after the systole would be far more extensive than they
are, and would approximate the variations observed in tubes with i nj
walls. The elasticity, moreover, assists in the equalization c
stream, converting the intermittent and remittent flow characteristic of the
large arteries into the continuous equable stream characteristic : of the capil
laries. It also permits of wide variations in the amount of blood the art
can contain between their minimum and maximum distention.

35o TEXT-BOOK OF PHYSIOLOGY

VARIATIONS IN THE BLOOD -PRESSURE

A. In the Arterial Pressure. — It is apparent from the preceding state-
ments that the arterial blood-pressure as a whole may be increased above the
normal, by:

1. 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 it is further apparent that if the pressure is higher than the normal it
may be lowered to the normal by a decrease in either one or both of these
•factors.

It is also apparent that the arterial blood-pressure as a whole may be
decreased below the normal by:

1. A decrease in the rate and force of the heart's contraction.

2. K deer ease in the peripheral resistance.

3. A decrease in both the force of the heart and the peripheral resistance;
and it is again further apparent, that if the pressure is lower than the normal
it may be raised to the normal by an increase in either one or both of these
factors.

1. 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 accumu-
lation 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 be-
tween the normal rate and the volume discharged from the ventricle, and
that when the rate is increased, the volume discharged diminishes and hence
the pressure remains normal or even falls below the normal. Nevertheless,
if the inhibition of the heart be lessened, or entirely removed, as by division
of one or both vagi, there is generally a rise in the blood-pressure coincident
with the increased cardiac rate.

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.

2 . 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. The outflow of blood will at
the same time be diminished.

Arteriole contraction and a consequent rise of blood-pressure may be

THE CIRCULATION OF THE BLOOD 35I

experimentally brought about by an increase in the activity of the general
vaso-motor center due to the arrival of nerve impulses transmitted to it through
certain afferent nerves as when after division of the sciatic nerve in a curarized
animal the central end is stimulated with induced electric currents (Fig. 160),
or by injecting into the blood, adrenalin, which it is generally believed acts
on the muscle fibers of the vessel walls. Fig. 161 Physiologic causes may act

FIG. 1 60. — 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-MOTQR 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.

in the same way and reflexly cause arteriole contraction. A rise of
pressure from this cause much beyond the normal is to a large extent
prevented under physiologic conditions 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 depressor nerve, and a consequent reflex stimulation of the cardio-
inhibitor center, and not to a direct action on the heart-muscle, inasmuch
as the effect is not observed after division of the vagi.

FIG. 1 6 1.— TRACING SHOWING THE RISE OF BLOOD PRESSURE IN A CAT after the intra-
venous injection of minute doses of adrenalin injection at x. The abs
millimeters lower.

3. When both the force of the heart and the peripheral resistance are sim-
ultaneously increased there is a rapid increase in pressure, the former fact*
tends to increase, the latter factor, to decrease, the velocity of the outflow.
According as the one or the other preponderates, will there be
crease or decrease in velocity. If they balance each other, ther

352

TEXT-BOOK OF PHYSIOLOGY

no change. A rise of pressure from a combination of these factors is rather
a pathologic than a physiologic condition and is observed in certain diseases

of the vascular apparatus.

The converse of these statements also
holds true.

i. If when the general arterial pres-
sure is in a condition of equilibrium the
heart ejects into the arteries in a given
period of time a lessened quantity of
blood, either as a result of a decrease in
the rate or force, there will soon be a
diminution of the arterial distention and
a consequent fall in pressure. The
velocity at the same time diminishes.
This continues until the outflow no longer
exceeds the inflow. Equilibrium will
again be established, but the pressure
will be at a lower level.

The decrease in the rate and force of

the heart may be brought about suddenly

, j 11 t_ j- ,•

or more or less gradually by direct stimu-

lation of the inhibitor fibers in the1 trunk

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^arres^m the rate of the yagus nerve with Strong Or With
- & &

- . , . ,. , /T^.

stimulating the vagus nerve from "on" weak electric- currents respectively (Fig.

to " off.

162) ; or, by a stimulation of the cardio-
inhibitor center by nerve impulses trans-
mitted to it through certain afferent
nerves, e.g., the afferent fibers in the
vagus,. when stimulated with induced electric currents (Fig. 163).

2. If the peripheral resistance is diminished by a dilatation of the arteri-

with, the cessation of .the stknu-

ofTe LtUVi!
turned. (The abscissa should be 20 mm.
l°wer-)

i me ~ StfCt

Abscissa

FIG. 163. — TRACING SHOWING REFLEX INHIBITION OF THE HEART AND FALL OF BLOOD-PRESS-
URE IN THE CAT, following stimulation of the central end of the vagus nerve.

oles, the heart's contractions remaining the same, the outflow of blood at
once increases and the existing pressure soon diminishes.

As a rule a diminution in peripheral resistance is attended by an increase

THE CIRCULATION OF THE BLOOD 353

in the rate or force of the heart, and this is especially the case if the pressure
has been above the normal.

The decrease in the peripheral resistance, i.e., in the arteriole contraction
and its effect on the blood-pressure may be brought by a depression or in-
hibition of the activity of the general vaso-motor center due to the arrival
of nerve impulses transmitted to it by certain afferent nerves, e.g., the de-
pressor nerve (see page 386), when, after division, its central end is stimu-
lated; or by the inhalation of amyl nitrite, which it is generally believed,
acts locally on the muscle walls of the arterioles and causes their relaxation
(Fig. 164).

3. When both the force of the heart and .the peripheral resistance are sim-
ultaneously diminished, there will be a rapid fall in pressure. The former
factor tends to decrease, the latter factor to increase the velocity of 'outflow.

FIG 164— TRACING SHOWING THE FALL OF BLOOD-PRESSURE IN THE CAT, following the
inhalation 01 amyl nitrite. Inhalation begun at X.

According as the one or the other preponderates i 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 rather than a physiologic condition and
observed in states of profound depression due to serious injuries.

Local Variations in the Arterial Blood-supply .-The ^variations i
pressure and velocity from variations either in the activity of the heart or in
^peripheral resistance recorded in preceding ^^ffi^hS
the arterial system in its entirety; but it is f^^j^^TlK?

^^1^^^^^^^^^

354 TEXT-BOOK OF PHYSIOLOGY

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 accordance with its
functional needs. All variations in peripheral resistance 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 arteri-
oles. If the latter dilate in any given area, the capillary pressure increases
because of the propagation 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 contrary,
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 venous
pressure in a given area from local or temporarily acting causes — e.g. , aspira-
tion 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 Pressures. —
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 pres-
sure in the arteries and veins remain unchanged. If, however, either
factor changes in one direction or another, there will arise a change in the
relative degree of pressure in the different 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

THE CIRCULATION OF THE BLOOD 355

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.— Inas-
much as the blood-pressure undergoes considerable variation in both physio-
logic 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 pres-
sures may be determined at least approximately without resorting to any-
surgical procedure.

By reason of the fact that both the systolic and diastolic pressures are'
regarded as important factors in clinical conditions, and their determination
of value for diagnostic and therapeutic purposes in diseases of the circulatory
apparatus, it is desirable to have clear ideas of what is meant by these terms.
If the changes of pressure are registered by a sphygmograph, a curve re-
sembling that shown in Fig. 173, page 367, will be recorded, which shows
more or less accurately the qualitative, if not the quantitative variations, of
the pressure in the arteries during a cardiac cycle.

Systolic pressure may be denned as the highest pressure developed in
the artery during the systole of the heart and occurs in the first half of the
systole and is therefore of very short duration; after this the pressure begins
to fall, but endures until the close of the systole, indicated in this curve by the
notch preceding the elevation c.

Diastolic pressure may be defined as the lowest pressure in the artery
during the diastole of the heart and occurs at the end of the diastolic period
just before the time of the succeeding systole. The diastolic pressure
gradually falls from the time of closure of the semilunar valves to the end
of the diastolic period.

The instruments by which these pressures are determined are called
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 a hydrostatic
pressure to an artery, e.g., the temporal, radial, etc., until the lumen is com-
pletely obliterated as indicated by the disappearance of the pulse beyond the
point of compression, and at the same time the registration of the pressure
applied, by means of a mercurial or spring manometer. The pressure just
sufficient to obliterate the pulse or to allow it to reappear after obliteration,
is taken as the systolic pressure.

The principle involved in the second group is based on a suggestion o
Marey, 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
equalize the diastolic pressure within. Inasmuch as these pulsation
be transmitted to, taken up and reproduced by a mercurial column ir
tion with the pressure appliances, it becomes possible, when the maximum
oscillation of the mercurial column is attained, to read off the diast
pressure.

356

TEXT-BOOK OF PHYSIOLOGY

The truth of this suggestion was subsequently demonstrated by Howell
and Brush. These experimenters connected the right carotid artery of a
dog with a mercurial manometer, interposing along the course of the con-
necting tube a maximum and a minimum valve. The left carotid artery
was surrounded by a plethysmograph which was connected, 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 pur-
pose 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 sphyg-
momanometer surrounding the left carotid artery, when it was exhibiting
its maximum 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.

Many forms of sphygmomanometers adapted for clinic purposes, with
which both the systolic and the diastolic pressures can be obtained, have
been devised. With all forms however, the pressure is applied to the arm
by a rubber armlet which is at least 8 centimeters wide. This is the widest
armlet that can be adjusted to the average-sized arm and presents distinct
advantages over narrower armlets. This armlet is prevented from expand-
ing outward by a cuff composed of some unyielding material held in position
by straps which completely encircle the cuff. The rubber armlet is con-
nected by stiff-walled rubber tubes with a mercurial manometer on the one
hand and with a rubber bulb or air piston on the other hand. In using the
apparatus the pressure is raised by forcing air into the closed system — dis-
tending the rubber armlet and with the same degree of force — changing the
levels of the mercury in the manometer, until the pulse is no longer felt at
the wrist. By special devices the pressure is then carefully lowered until
the mercurial column begins to fall. At a given level it exhibits a consider-
able oscillation which may be mistaken for the actual systolic pressure but
which is probably due to the impact of the blood against the upper edge of
the rubber portion of the cuff. If the column of mercury be still further
lowered so that the pressure indicated is a trifle lower than the systolic pres-
sure the blood will be forced through the compressed artery and give rise
to a pulse wave, which may be felt at the wrist. The highest excursion of
the mercurial column noted by the eye at the moment the pulse reappears
is regarded as the systolic pressure.

The pressure is then lowered 5 millimeters at a time and the oscillations of
the mercurial column noted. As the pressure is thus slowly lowered there
will come a moment when the oscillations will attain a maximum value and
beyond which the oscillations again diminish. The lowest level of the
mercury column at the time of the sudden termination of the greatest
oscillation is taken as the diastolic pressure.

Erlanger's sphygmomanometer is a most valuable instrument for obtain-
ing both systolic and diastolic pressure. It possesses an advantage in that
it is provided, in addition to the mercurial manometer, with a tambour and
lever by which changes in pressure can also be recorded on a revolving
cylinder (Fig. 165). A complete description of this apparatus, the manner

THE CIRCULATION OF THE BLOOD

with i

357

eve™^^^^

It is difficult, therefore, to dateline atti^™

FIG. 165. — ERLANGER'S SPHYGMOMANOMETER.

sure indicated by the mercurial column just falls below the systolic pressure
and allows the blood to pass through. A new criterion for this determination
has been furnished by Erlanger. With a given speed of the drum, the up and
down strokes of the lever practically coincide. But if the speed of the drum
be slightly increased "so that each wave subtends about i Jto 2 mm. of smoked
paper (this speed is attained merely by removing the governor), the change

3S8 TEXT-BOOK OF PHYSIOLOGY

in form of the successive waves manifests itself usually as a more or less
abrupt separation of the ascending and descending strokes of the pulse record
(Fig. 1 66). The phenomenon may vary somewhat with the form of the
pulse wave and may even be obscured by fling, but there has been no great
difficulty in recognizing it in every case. It is often very clear when the
tracing shows no abrupt increase in amplitude whatsoever. It is just as
accurate an index to the systolic pressure as the ' sensory criterion' and that
of v. Recklinghausen. The change in form occurs because, at the moment
the pressure on the artery falls below systolic, blood succeeds in making its
way beneath the cuff. This must be squeezed out before the lever can re-
turn to the base line, whereas at higher pressures the lever is raised only
through the hydraulic ram action of the pulse wave upon the upper edge of
the cuff."

The conclusions of Erlanger regarding the results of his investigations
with this apparatus may be partially summed up in the following statements,

and as they hold true for other forms of
apparatus which determine both systolic
and diastolic pressures, they are here ap-
pended: "The pressure that is deter-
mined 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

FIG. 166.— TRACING SHOWING THE minimum lateral pressure of the artery

compressed and, therefore, as the mini-
NOTED.— (Erlanger.) mum lateral pressure is the same in all of

the larger arteries, the pulse pressure,

determined when the pressures in the brachial artery are observed, tends
to approximate the lateral pulse pressure in the aorta."

Any positive statement as to the numerical values of the different pres-
sures is somewhat difficult to make inasmuch as they will vary within physio-
logical limits in accordance with the position of the body, exercise, charac-
ter 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 corre-
sponding 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 65 to no mm.
Hg.; the pulse pressure ranges from 25 to 40 mm. Hg.

The Auscultatory Method of Determining the Blood-pressure. —
In 1905 a new method was introduced and described by Korotkow for the
determination of both the systolic and diastolic pressures, which in the
experience of clinicians is more accurate and satisfactory in both physiologic
and pathologic conditions than any of the other clinical methods. (See
papers by Goodman and Howell in the Univ. of Pa. Medical Bulletin, Nov.,
1910 and the American Journal of the Medical Sciences, September,
1911). It consists in the interpretation of certain sounds heard with the

THE CIRCULATION OF THE BLOOD 359

stethoscope, in the artery under observation when it is gradually released from a
pressure that has obliterated its lumen in a given region.

In the employment of this method the brachial artery is selected and
compressed in the usual manner with a wide cuff in connection with a
graduated mercurial manometer.

After the pulse has been obliterated the stethoscope is placed over the
artery below the cuff, care being taken to prevent undue pressure. On
releasing the pressure in the cuff very gradually as in the employment of
other methods a series of sounds corresponding with each arterial pulsation
is heard as the pressure falls from the systolic to the diastolic level. The series
of events are spoken of as phases of which five are recognized.

The first phase is characterized by a loud clear-cut snapping sound:
the second phase is characterized by a series of murmurs; the third phase
by a succession of loud clear snapping sounds which resemble very closely
those of the first phase but are less loud; the fourth phase is inaugurated by
a sudden decrease in the intensity of the murmurs of the third phase giving
rise to what is described as a dull tone that rapidly becomes weaker and soon
fades away; the fifth phase is one of silence.

These phases which are sharply defined and easily distinguishable are
believed to be associated with vibrations of the arterial walls. The first
sound is generally believed to be due to the sudden distention of the artery,
by the inrush of blood beneath the cuff, and indicates the systolic pressure
which can be at once observed by the height of the mercury in the manome-
ter. This sound lasts until the pressure falls about 14 millimeters. The
second sound, a succession of murmurs, is believed to be caused by whirl-
pool eddies in the blood stream as it is propelled from the partially con-
stricted artery into the non-constricted region below the cuff. These murmurs
last until the pressure falls about 20 millimeters. The third sound is at-
tributed to the vibration of the arterial wall but as the lumen of the artery
is so much greater than that of the compressed portion the rapidity of the
current is less and hence the sound is neither so sharp nor pronounced.
It lasts until the pressure falls about 6 millimeters. The transition from the
second to the third sound involves a fall of about 5 millimeters. The disap-
pearance of the sounds is coincident with the return of the artery to its nor-
mal size and hence a cessation of the vibration. It therefore indicates the
diastolic pressure, which can at once be observed by the height of the mer-
cury in the manometer.

The systolic pressure obtained by this method corresponds to the 1
sound that is heard over the brachial artery and is about 130 millimeters;
the diastolic pressure corresponds with the cessation of all sounds and is
about 8s millimeters.1 The pulse pressure is therefore 45 millimeters.

In pathologic states of the vascular apparatus the duration and intensity
of the sounds undergo considerable modification. In some diseases they
are quite characteristic and hence have both a diagnostic and 1
value.

i The statement that the fifth phase is the index of minimum pressure ^ ^fapute.
and Lutz using the Erlanger ^^^^^^^^^^

s^yg— ^

phase should be taken as index of minimum pressure.

36o TEXT-BOOK OF PHYSIOLOGY

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, 80 c.c., it is evident that the
blood must be flowing through the vascular apparatus with a certain velocity,
for during the minute the entire volume of blood, 3684 grams, must have
passed one and a half times through the heart. Direct observation of the
escape of blood from the central end of a divided artery, and from the pe-
ripheral 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 dif-
ferent parts of the vascular apparatus. In the arteries, moreover, the flow
is not quite uniform, but experiences an alternate acceleration and retarda-
tion with each heart-beat. In the capillaries and veins the flow is continu-
ous 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 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 80 c.c., and the number of heart-beats per minute is 72, the
total volume discharged per minute would be 5760 c.c., or 96 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 156
mm. per second. This being the case the velocity in the aortic arch at
least would be considerably less than in the carotid artery as will be stated
later, a fact which may however be explained on the assumption that owing
to the curvature of the aorta and the extensibility of its walls the lateral
pressure becomes very great; as a result the sectional area is increased and
the velocity diminished. With the cessation of the heart's activity, the elastic
recoil gives an impetus to the blood and increases its velocity.

The Mean Velocity in the Arteries. — The mean velocity of the blood
in the larger and more superficially lying arteries has been determined by
Volkmann with the hemodromometer, by Ludwig and Dogiel with the
Stromuhr, and by other investigators with different forms of apparatus.

THE CIRCULATION OF THE BLOOD 361

Since neither the blood nor any particle placed in it can be seen through the
walls of the artery, it occurred to Volkmann to intercalate 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. 167) 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. Previous to the inter- ;;

calation of the tube it is filled with serum or nor-
mal saline solution. With the turning of the cocks
as B the blood enters 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 capacity of the tube the velocity is
determined.

The Stromuhr or rheometer of Ludwig (Fig. 168)
is constructed on the same principle, but instead of
the glass tube having the same diameter it is

FIG. 167.— VOLKMANN' s HEMODROMOME
TER. C, C. Arterial cannulas.

FIG. 1 6 8— LUDWIG AND
DOGIEL'S RHEOMETER. X . V.
Axis of rotation. A, B. Glass
bulbs. h,k. Cannulas inserted
in the divided artery. f,ti,
rotates on g, /. c, d. Tubes.

362

TEDT-BOOK OF PHYSIOLOGY

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 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 differ-
ent phases of the cardiac cycle have been determined by Chauveau and

Lortet with the hematachometer
(Fig. 169). This consists of a me-
tallic tube carrying a graduated
disc. At one point the tube is
perforated but covered with a rub-
ber band through which passes an
index. When the tube is inserted
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 extent
of the excursion indicates the veloc-
ity. The apparatus is first gradu-
ated with currents of water of
known velocity. With this instru-
ment Chauveau found that in the
horse the velocity during the systole
was 520 mm. per second, at the

artery. C. Lateral tube connected with aTmanome- beginning of the diastole 22O mm.
ter. b. Index moving in a caoutchouc mem- per second, and during the pause
brane, a. G. Handle. , 6

150 mm. per second.

The Velocity in the Capillaries. — The rate of flow in the capillary
vessels cannot be experimentally determined. It has been estimated 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 determined 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 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 i.

HEMODROMOGRAPH OF CHATJ-

THE CIRCULATION OF THE BLOOD

363

The Velocity in the Veins.— In the venous system the velocity increases
in proportion as the sectional area decreases. In the jugular 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 com-
bined venae cavse is about twice that of the aorta; hence the relation of the
sectional area of the capillary system to the sectional area of the venae cavse
is about 300 to i.

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. 170.

The Relations of Blood-pressure and Velocity.— Though the pres-
sure of the blood bears a definite relation to the velocity it must be kept hi
mind that it is rather the difference in pressure between the beginning and

Arteries. Capillaries.
FIG. 170. , Blood-pressure. , Velocity.

Veins.
), Sectional area.

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 variations in pressure
and velocity, both in the same and opposite directions, which are theoretically
possible from a change in the force of the heart, or in the peripheral resistan
or both, are shown in the following table arranged by Waller. The pli
sign indicates increase, the minus sign, decrease, in effect.

The statements herein embodied have been established by Marey with an
artificial schema of the circulatory apparatus, and by Chauveau and Lortt
by experiments on animals with the hemodromograph, a specially c
apparatus for this purpose.

Though all the relations between pressure and velocity in the tabl
possible, those which are most physiological are probably 5 and 6, fc
instances there is a minimum alteration in pressure, but a maximum altera-

364

TEXT-BOOK OF PHYSIOLOGY

tion 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 pres-
sure 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.

No.

Heart

Arterioles

Blood-pressure

Blood flow

f Force constant

Resistance increased. . .

3
4

\ Force constant

f Force increased ....
\ Force diminished . . .

( Force increased

Resistance diminished .

Resistance constant. . . .
Resistance constant. . . .

Resistance diminished. .

i ;

6
8

\ Force diminished. . . .

( Force increased
\ Force diminished . . .

Resistance increased. . . .

Resistance increased . . .
Resistance diminished.

i i

n? -+

THE PULSE

The Arterial Pulse. — The pulse may be denned as a periodic expansion
and recoil of the walls of the arterial system. The expansion is caused by
the discharge from the heart into the arteries of a volume of blood, approxi-
mately 80 c.c., 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 diastole.

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
both in a transverse and longitudinal direction. 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, the expansion increases in
extent; at the same time adjoining segments 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.

The mode of development as well as the propagation of the expansion
and recoil movement of the arterial wall, which together constitute the pulse,
are illustrated in Fig. 171 in which A B represent the artery subdivided into
six equal parts indicated by the letters a to g. In accordance with this
subdivision of the artery the systole of the heart may be also divided into six
parts, during the first three of which the heart increases in power, and during
the last three of which it decreases in power, gradually falling to zero. The

THE CIRCULATION OF THE BLOOD

365

effect on the arterial wall of the discharge of blood from the ventricle is
illustrated in the figure. During the first one-sixth of the systole a certain
volume of blood is forced into the artery, which at this moment is already
full of blood. Of this volume a portion moves forward while another
portion moves sideways as the arterial wall begins to expand under the
pressure of the heart. At the end of the first one-sixth of the systole the
condition of the arterial wall may be represented by the lines ib. During
the second one-sixth the artery expands still more as the volume of blood
increases under the increasing force of the heart, so that at the end of the
second period the expansion of the arterial wall is not only greater at the
point a but in addition has extended over a greater length of the artery so
that the condition of the artery may be represented by the lines 20. Dur-
ing the third sixth the same process continues; the incoming volume of blood
still further expands the artery at a, as well as successive portions further on
as far as d, so that at the height of the systolic power the condition of the
artery may be represented by the lines 3 d.

FIG. 171.— DIAGRAM SHOWING THE DEVELOPMENT OF A PULSE WAVE. (Rollet.)

366 TEXT-BOOK OF PHYSIOLOGY

mic change in pressure at any given point of ,the arterial system; and in
amount, is the difference between the diastolic and the systolic pressures, at
the corresponding points. The volume of blood ejected from the ventricle
is frequently termed the pulse volume.

The Velocity of Propagation of the Pulse-wave. — The propagation
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 ob-
server 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 ex-
tremities somewhat different results, varying from 6.5 to n 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 condition unfavorable
to rapid propagation. For this reason a low arterial pressure will occasion
a delay in the appearance of the pulse- wave in any portion of the body; a
high arterial pressure 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 pulse-wave which thus spreads itself over the entire arterial system
with each systole of the heart can be perceived in certain localities 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, and in the arteries
of the retina under certain conditions, with the ophthalmoscope.

The Radial Pulse. — 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 expansion takes place quickly, the re-
coil relatively slowly; that the waves succeed one another with a certain fre-
quency, corresponding to the heart-beat; that the pulsations are rhythmic in
character, etc.

Inasmuch as the individuality of the pulse-wave varies at different periods
of life and under different physiologic and pathologic conditions, various
terms more or less expressive, have been suggested for its varying qualities.
Thus the pulse is said to be frequent or infrequent according as it exceeds or
falls short of a certain average number — 72 per minute; strong or weak
according to the energy with which the vessel expands ; quick or slow, accord-
ing to the suddenness with which the expansion 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 volume of blood ejected into the aorta, or, in other words,
the degree of fullness of the arterial system.

The three qualities which are of most value to the clinician are rate,
strength or force, and volume.

THE CIRCULATION OF THE BLOOD

367

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. (See page 290.) All
conditions which modify the rate of the heart will modify at the same time
the rate of the pulse.

For the purpose of accurately studying and analyzing the pulse-wave and
its characteristic features, it is necessary to obtain graphic records of the
alternate expansion and recoil of the artery under normal and abnormal
conditions. This is accomplished by means of—

The Sphygmograph. — The sphygmograph is, therefore, an apparatus
designed to take up, reproduce, and record the alternate expansion and recoil
of an artery caused by the temporary increase and decrease of pressure fol-
lowing 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

\1L

FIG 172 -VON FREY'S SPHYGMOGRAPH. GS. Metal framework. P. Button attached to spring.
[P. Vertical rod. U. Clock-work which turns the recording cylinder. VI. T

devised by Marey, Dudgeon, v. Frey, and many others. The instrument

of v. Frey is shown in Fig. 172. This consists, first, of a metal frameworl

GS by which the apparatus is fastened to the arm and support given

lever, recording surface, etc. The essential part is the spring carryin

ton P, which is placed over the artery, usually the radial, before it crosses tl

wrist-joint. A vertical rod F 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. The artery . .

usually selected for obtaining a sphygmogram is O^HYGMOGBAM

the radial. This artery lies quite superficially, '

FlG. i73._THE PULSE-CURVE
o, THE

368 TEXT-BOOK OF PHYSIOLOGY

shown in Fig. 173 is obtained. 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 sphygmogram or pulse-curve may be divided into two portions:
viz., a line of ascent from a to b, the anacrotic, and a line of descent from b
to d, the catacrotic (Fig. 170). In normal tracings the former is almost
vertical and caused by the sudden expansion of the artery immediately
following the ventricular contraction; the latter is in general oblique, occupies
a longer period of time, due to the slow recoil of the arterial walls, 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 gener-
ally sharp and pointed.

The vertical direction of the line of ascent is taken as an indication that
the arterial walls expand readily, that tne 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 conditions 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 developed
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 periphery; by others, that it is peripheral in origin,
beginning near the 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 ven-
tricular systole ceases and the aortic valves close. From this fact it is evi-
dent 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 increased by low arterial
pressure and highly elastic arteries. Both features are diminished by the
reverse conditions. The apex is sometimes rounded and even flat, indica-
tive of a great diminution in arterial elasticity. The sphygmogram not
infrequently varies considerably from the normal type in different pathologic
conditions of the circulatory apparatus. A consideration of these varia-
tions does not fall within the scope of this work.

The Carotid Pulse. — The carotid pulse can be readily recorded by
applying over the carotid artery, anterior to the sternocleidomastoid muscle,
on a level with the thyroid cartilage, a funnel-shaped tambour in con-
nection with a suitable recording tambour and lever. The sphygmogram
thus obtained resembles in all essential respects that obtained from the
radial artery. It is often of advantage in the investigation of certain

THE CIRCULATION OF THE BLOOD 369

problems of the heart, both physiologic and pathologic, to record the
carotid pulse and the cardiac impulse simultaneously.

The Venous Pulse. — By this term is meant a pulsation of the large veins
in the neighborhood of the heart but more especially in the jugular veins.
It is caused by variations of pressure transmitted backward into the veins
during and after the systole of the auricle. Though the venous pulsation is
not very marked in physiologic conditions it frequently becomes pronounced
in certain pathologic conditions of the heart.

The pressure variations in the jugular vein can be recorded by applying
over the vein a properly constructed tambour, a glass funnel or a Mackenzie
metal tambour connected with a suitable recording tambour. A graphic
record of a normal venous pulse thus obtained, shown in Fig. 174, is rather
complicated, consisting of three positive and three negative waves which
are related to variations of pressure in the right auricle, the result of the
successive contractions of the auricular and ventricular walls and the action
of intra-ventricular structures.

FIG I74.-SIMULTANEOUS TRACINGS OF THE JUGULAR PULSE, THE CAROTID PULSE 'AND THE

APEX ^.-(Bachmann.} At the bottom of the tracing the time is given in the fiftieths of a
fprnnrf The vertical lines o I, a, 3, etc., mark synchronous points on the curves. At T
auricular ^"h! fso-called' ^ave'caused by the systole ofthe ventricle; t, the stagnation wave

following the opening of the auriculo-ventricular valves.

As the venous pulse is a very evident symptom in some pathologic condi-
tions of the heart, and as its proper interpretation assists m th< : diagnos
these conditions, it has become of much significance m moderr chmca medi
cine For purposes of interpretation it is desirable to obtain si
graphic recPord^ not only of the venous pulse, but of the carotid or radial
pulse, and of the cardiac impulse as well. In the accompanying
these three records are represented.

The generally accepted interpretation of these waves is as follows.

»4

370 TEXT-BOOK OF PHYSIOLOGY

The first positive wave, a, is due to an expansion of the vein, the result
of a sudden rise of pressure. As it occurs before the ventricular systole, it
is pre-systolic in time and caused by the contraction of the auricle, the
effect of which is to cause a temporary retardation of the blood-stream
flowing toward the auricle and hence a backward wave of pressure.

The first negative wave is due to a recoil of the veins following a diminu-
tion of the pressure as the blood again moves forward in consequence of the
relaxation of the auricular walls.

The second positive wave, c or s, is also caused by a wave of positive pres-
sure in the vein, reflected from the auricle, though it is not of auricular origin.
As it begins with the ventricular contraction and develops during the closed
period, the protosystolic period, i.e., between the closure of the tricuspid
valve and the opening of the semilunar valves (see page 287), it is believed
to be due to the bulging of the auriculo-ventricular valve into the auricular
cavity, by the still higher intra-ventricular pressure thus diminishing its size
and raising its pressure.

The second negative wave, Af, is due to a marked fall of pressure, a col-
lapse of the walls of the vein and a rapid flow of blood to the auricular cavity.
These phenomena begin with the opening of the semilunar valves and are
due in part to the relaxation of the auricular walls, but more especially to a
descent of the more central portions of the auriculo-ventricular valve or
septum, into the ventricular cavity in consequence of the contraction of the
papillary muscles. The hollow cone thus formed, enlarges the auricular
cavity, withdraws some of its contained blood, and hence lowers the pressure,
which leads to the inflow of blood from the veins and hastens the auricular
filling.

The third positive wave, v, is caused by a third wave of pressure reflected
from the auricle. • It occurs toward the end of the ventricular systole and
is probably due to a slight retardation of the blood flow in consequence of
the return of the auriculo-ventricular septum to its normal position, the
result of a relaxation of the papillary muscles, when the intra-ventricular
pressure is still higher than the intra -auricular pressure.

The third negative wave, Vf, is caused by a third fall of pressure in the
vein and appears very shortly after the beginning of the ventricular relaxa-
tion, and the closure of the semilunar valves. It develops during the common
pause of auricles and ventricles. The fall of the venous pressure follows
the passage of the blood from the auricle into the ventricle. It continues
during the ventricular filling but disappears on the return of the auricular
contraction.

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 arterie,
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

THE CIRCULATION OF THE BLOOD

371

recording apparatus, e.g., a tambour with a lever or a piston recorder with
float and writing point. The space between the organ and vessel is filled
with normal saline, air, or oil. Such an apparatus is known as a plethysmo-
graph. Fig. 175. 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 essentially 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
of showing the action of vaso-motor nerves in changing the contraction of the
arterioles and hence the outflow of blood. Thus when an organ is enclosed

pIG< 175.— A PLETHYSMOGRAPH FOR THE ARM.

in a plethysmograph and the arterial contraction increased by either a direct
or reflex stimulation of the vaso-motor center there will be a rise in the
pressure, a diminution in the outflow of blood and a decrease in the
volume of the organ under observation; and on the contrary, if the artenole
contraction is diminished by a direct or reflex inhibition of the vaso-moto
center there will be a fall of pressure, an increased outflow of blood and ,
increase in the volume of the organ. From this it is learned that the func
tional activity of an organ which is attended and conditioned by an increased
blood-supply is always associated with an increase in volume. On plethys-
mographk records large undulations are frequently observed which are
regarded as of respiratory origin.

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

372

TEXT-BOOK OF PHYSIOLOGY

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. 176), 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 omen-
turn 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 adjustment to the

stage of the microscope. The abdo-
men 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 in-
testine should be so placed that it will
lie between the observer and the body
of the frog. The mesentery thus ex-
posed must be kept moist with nor-
mal saline solution.

When examined with low powers
of the microscope, arteries, veins, and
capillaries will be found occupying the
field of vision. Their general arrange-
ment, their size and connections, can
be readily determined. After a few
preliminary adjustments a region will

FIG. i76.-THE VESSELS OF THE FROG'S be found in which the blood is flowing
WEB. a. Trunk of vein, and (6, b) its in opposite directions. The vessel ap-
tributaries passing across the capillary net- parently carrying blood away from the

are pigment ce}h" '

y; the vessel

ently carrying blood toward the ob-
server 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-
tency, an alternate acceleration and retardation, corresponding to each
heart-beat; the capillary and venous streams are uniform and continuous.
The relative velocities in the three sets of vessels are indicated by the move-
ment of the red corpuscles. In the arteries they pass before the eye so rapidly
that they cannot 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 per-
ipheral portion, at the same time adhering to the sides of the vessel. Be-
tween the axial portion of the stream occupied by the red corpuscles and the
wall of the vessel there is a clear still layer of plasma, the result of an adhe-
sion of the plasma to the wall. It is this feature which gives rise to the

THE CIRCULATION OF THE BLOOD

373

friction between successive layers of the blood-stream, the resistance of the
blood flow, and the development of the 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 determined
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 di-
minishes the inflow, many of the smaller vessels
entirely disappear from view. The contraction and
relaxation of the arterioles will therefore deter-
mine the quantity of blood flowing into and
through the capillary system.

Migration of the White Corpuscles. — A
phenomenon frequently 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 condition of stasis is then
established. During the development of this condi-
tion the white corpuscles accumulate in large num-
bers 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 vessel-wall now
closes. The successive steps in this process are
shown in Fig. 177. As this migration occurs
mainly after the circulation 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

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
cav« that it had in the aorta, for the reason that the sectional area of the
verue'cavje is considerably larger than that of the aorta. The pressure also
is very low in the larger veins because the friction still to be overcc
relatively very slight.

FIG. 177. — DIAGRAM TO
SHOW VARIOUS STAGES E*
THE DIAPEDESIS OR MI-
GRATION OF WHITE COR-
PUSCLES.

374 TEXT-BOOK OF PHYSIOLOGY

The capacity of the venous system is considerably greater than that of
the arterial system, as there are usually two and even three veins accom-
panying each artery. This, taken in connection with its greater disten-
sibility, 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 of blood contained
in the two systems are regulated by the degree of contraction of the arteriole
muscles and this in turn by the vaso-motor nerves. The movement of the
blood through the veins is accomplished by the cooperation of several
forces, reference to which will be made in a following paragraph.

The Pulmonic Vascular Apparatus. — The pulmonic vascular appara-
tus consists of a closed system of vessels extending from the right ventricle
to the left auricle, and includes the pulmonic artery, capillaries, and pul-
monic veins. In its anatomic structure and physiologic properties it closely
resembles, with, the systemic apparatus.

The stream-bed widens from the beginning of the pulmonic artery to
the middle of the capillary system; it again narrows from this point to the
terminations of the pulmonic 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. The pressure in the pul-
monie artery of the dog has been shown by Beutner to be about one-third
that in the aorta; by Bradford and Dean to be one-fifth. Wiggers has
recently shown that in normally breathing dogs with arterial pressures ran-
ging from no to 112 mm. of mercury, the maximal or systolic pressure in the
pulmonic artery averaged 36 mm., and the minimal or diastolic averaged
5 mm. The reason for the low pressure may be found in the large size and
rich development of the pulmonic capillaries and the imperfect development
of an arteriole muscle at the periphery of the pulmonic artery, the result of
which is a diminution in the friction. Inasmuch as the friction is relatively
low, the work of the right heart is less than that of the left heart and hence
its walls are not so well developed. The pulmonic pressure being low the
intraventricular pressure of the right heart is relatively low as compared
with that of the left heart. The velocity of the blood-stream in each of the
three divisions of the system cannot 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
ventricle. Assuming the latter to be thirty seconds, the former would be
seven and one-half seconds.

The presence of vaso-motor nerves in the walls of the arterioles of the
pulmonic artery has not been definitely determined. Adrenalin which con-
stricts blood vessels supplied with vaso-motor nerves is stated by some
investigators to have this effect, but by others to be without it. If vaso-motor
nerves are present their action is relatively slight.

The capillary vessels are spread out in a very elaborate manner just
beneath the inner surface of the pulmonic 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

THE CIRCULATION OF THE BLOOD 375

through this system hourly and exposed to the respiratory surface is about
430 liters. The reason for the existence of the pulmonary circulation is
the renewal of the oxygen 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 variation 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 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 trans-
ferred, now take up and continue the work of the heart, and by recoil-
ing 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 the contrac-
tion and relaxation of the muscles there will be exerted an intermittent
pressure on the veins. With each contraction, the blood on the proxi-
mal side will at once be driven forward with increased velocity, Awhile
that on the distal side will be retarded, 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 arid force the blood forward.

3. Thoracic Aspiration.— The inspiratory movement aids the flow o

blood through the vense cavse and their tributaries. With each inspira-
tion the pressure within the thorax but outside the lungs undergoes a
diminution more or less pronounced in accordance with the extent of
the movement. As a result, the blood in the large veins outside of the
thorax, being subjected to a pressure greater than that^m the thorax,
flows more rapidly toward the heart. With each expiration t
verse 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 large!) ' P^ented
the valves, for each retardation is immediately checked by their clc
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,

376

TEXT-BOOK OF PHYSIOLOGY

if the arm be allowed to hang passively by the side of the body, the
veins, as seen 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 trie left ventricle per-
forms at each contraction when it discharges its contained volume of blood
into the aorta is :

1. To overcome the total resistance of the systemic vascular apparatus.

2. To impart velocity to the blood.

The resistance may be expressed in terms of aortic pressure. The
pressure in the aorta has not been absolutely determined, though for many
reasons it may be assumed to be about 150 mm. Hg., or its equivalent, a
column of blood 1.93 meters in height. If the volume, of blood which the
heart discharges is assumed to be 83 grams, the work done may be calculated
by multiplying the weight by the height to which it is raised: viz.,
0.083 X 1.93 = 0.16019 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 83
grams is estimated by squaring the velocity and dividing by the accelerating
force of gravity (2°'*28i) and multiplying the quotient by 0.083. The
value of the fraction given above represents the distance a body would have
to fall to acquire this velocity: viz., 0.0127 meter. The work done is there-
fore 0.083X0.0127, or 0.01054 kilogrammeter.

The entire work of the left ventricle is the sum of these two amounts,
or 0.17073 kilogrammeter. Assuming that the heart beats 72 times per
minute, the work done daily would be 0.17073X72X60X24, or 17701.3
kilogrammeters. The right ventricle approximately performs 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
23,600 kilogrammeters.

THE NERVE MECHANISM OF THE VASCULAR APPARATUS

By this expression is meant a combination of nerves and nerve-centers
by which the rate and force of the heart contractions and the contraction
of the arteriole muscles are maintained. It includes the cardiac nerves
(the cardio-accelerator and the cardio-inhibitor) and the vascular or vaso-
motor nerves (the vaso-augmentor or constrictor and the vaso-inhibitor or
dilatator nerves). The function of this mechanism is to maintain the
high blood-pressure characteristic of the arterial system, and to regulate
from moment to moment, the quantity of blood flowing into and out of organs
in accordance with their functional activities. The cardiac nerves have
been considered on pages 312-313.

Arterial Tonus. — The arteries, especially those in the peripheral region
of the arterial system, possess a well-defined layer of non-striated muscle-
fibers arranged in a circular direction or at right angles to the long axis of
the vessel. In the physiologic condition these arterioles are distended
beyond the natural condition by the side pressure of the blood flowing
through them, at the same time the muscle-fibers are in a state of tonic con-

THE CIRCULATION OF THE BLOOD 377

traction, thus giving to the arteries a certain average caliber which permits
a definite volume of blood to flow through them in a given unit of time. To
this condition of the arterial wall the term tonus is applied.

The cause of this tonic contraction is not definitely known. It has
been attributed to the action of local nerve-ganglia, to the pressure of blood
from within, to the influence of organic substances in the blood, the prod-
ucts of gland activity: e.g., adrenalin or epinephrin.

This tonic contraction of the vascular muscle is subject to increase or de-
crease, to augmentation or inhibition, in accordance with the action of various
agents. An augmentation of the contraction will result in a decrease of
the caliber and a reduction in the outflow of blood. An inhibition 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
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 intermediation of nerve-
fibers, termed vase-motor nerves. Of these there are two kinds, one which
increases or augments the contraction, the vase-constrictors or vaso-aug-
mentors; and another which decreases or inhibits the contraction, the vaso-
dilatators or vaso-inhibitors.

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
that compose the ventral 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 ventral roots as the white rami communicantes and enter
for the most part the vertebral or lateral sympathetic ganglia. From the
results of many observations and experiments it is probable that the great
majority of the vaso-constrictor 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 (i) the body walls; (2) the fore limbs; (3)
the head, neck and face; (4) the hind limbs; and (5) the abdominal viscera,
i. The vaso-constrictors for the blood-vessels of the walls of the trunk of
the body emerge from the spinal cord in the ventral roots of the spinal
nerves from the second thoracic to the third lumbar nerves inclusive.
After a short course they leave the ventral roots and pass by way of the
white rami into the corresponding thoracic and lumbar ganglia around
the nerve-cells of which they arborize. From these ganglia new nerve-
fibers arise which pass back by way of the gray rami, into the thoracic
and lumbar nerves and in association with them pass directly t
blood-vessels of the skin. (Fig. 178 A.}

2 The vaso-constrictors for the blood-vessels of the fore limbs emerge from
the spinal cord in the ventral roots of the fourth to the tenth thoracic
nerves inclusive. After a short course they leave the ventral roots pass
into the white rami, thence into the sympathetic chain after wh
take an upward direction and terminate around the cells of the ga
stellatum From this ganglion the new nerve-fibers enter by way of
the gray rami, the trunks of the cervical nerves which unite to form t]

378

TEXT-BOOK OF PHYSIOLOGY

brachial plexus and by this route pass to the blood-vessels of the skin
and possibly of the muscles of the fore limb.

The vaso-constrictors for the blood-vessels of the head, face and neck
emerge from the spinal cord in the ventral roots ol the first four thoracic
nerves. After a short course they leave the ventral roots by way of the
white rami and pass into the sympathetic chain after which they pass
upward through the ganglia and cord to the superior cervical ganglion,
around the cells of which these fibers arborize. From this ganglion
new fibers arise which follow the carotid artery and its branches to their
superficial terminations at least (Fig. 178$).

"vaso-constrictor center

Jjulbar vaso-constrictor center Blood vessels of head. & face

. / * *^----"" "~ S

Spinal vaso-constrictor center
A

Bloodvessels of abdominal viscera

Spinal vaso-constnctor center

C B

FIG. 178. — Diagrams showing the course of the pre- and post-ganglionic vaso-motor fibers
for the blood-vessels; of A, the walls of the trunk of the body; of B, the head and face; and of
C, the abdominal viscera.

4. The vaso-constrictors for the blood-vessels of the hind limbs emerge from

the spinal cord in the ventral roots of the tenth thoracic to the second or
third lumbar nerves inclusive. They then pass by way of the white
rami into the corresponding ganglia around the nerve-cells of which
some of the fibers arborize. Others, however, descend the sympathetic
chain to terminate in successive ganglia as far as the third sacral ganglion.
From all these ganglia new nerve-fibers arise which pass backward into
the corresponding nerves which enter into the formation of the lumbar
and sacral plexuses and by this route reach the blood-vessels of the skin
of the lower portions of the trunk and hind limbs.

5. The vaso-constrictors for the blood-vessels of the viscera of the abdominal

cavity emerge from the spinal cord in the ventral roots from the fifth
thoracic downward. After leaving these roots by way of the white
rami they pass into and across the ganglia composing the thoracic por-

THE CIRCULATION OF THE BLOOD 379

tion of the sympathetic chain and by their union subsequently assist in
the formation of the splanchnic nerves the terminal fibers of which
arborize around the cells of the collateral ganglia, viz. : the semilunar,
superior mesenteric renal, etc. From these various ganglia an elaborate
network of non-medullated fibers passes to the blood-vessels of the
stomach, intestines, and other viscera. The great splanchnic nerve is
one of the most important vaso-constrictor trunks of the body, on ac-
count of the large vascular area it controls (Fig. 178 C).
The existence, course, distribution, and functions of the vaso-constrictor
nerves have been determined by a variety of methods, physiologic and ana-
tomic. Stimulation of the nerve trunks under appropriate conditions
gives rise to a contraction, division to a dilatation 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 ultra- vascular injection
or the local application 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 stimulation of
pre-ganglionic fibers is without effect, though stimulation of the post-
ganglionic fibers is followed by the usual contraction.

The following facts 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 and a decided fall in
blood-pressure; stimulation of the peripheral end by their contraction and a
marked rise in blood-pressure. Division of the cervical cord of the sym-
pathetic 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-dilatator nerves have been
found in some of the cranial nerves and in association with vaso-constrictor
fibers in most of the nerve trunks of the body. Thus they have been
found in the nerve of Wrisberg, the glosso-pharyngeal, in the nerve trunks of
the upper and lower limbs, of the body walls, of the abdominal viscera (the
splanchnic nerve) and of the pelvic viscera.

The vaso-dilatator nerve-fibers that are found in some of the cranial
nerves have their origin in nerve-cells in the gray matter of the medulla
oblongata. From these cells they pass out in the trunk of the pars intermedia
or nerve of Wrisberg and in the trunk of the glosso-pharyngeal nerve.
Those fibers which are contained in the nerve of Wrisberg, enter, after a
short course, the trunk of the facial nerve and through its branches the
great petrosal and the chorda tympani, are ultimately distributed as pre-
ganglionic fibers to the spheno-palatine and submaxillary ganglia re-
spectively. From the spheno-palatine ganglion cells post-ganglionic fibers
are distributed to the blood-vessels of the mucous membrane of the nasal
chambers posteriorly, and to adjacent regions. From the subma
ganglion post-ganglionic fibers pass to the blood-vessels of the
and sublingual glands.

The vaso-dilatator fibers that are contained in the glosso-pharyngeal
nerve pass through the tympanic plexus by way of Jacobson s nerve to tl
otic ganglion, around the cells of which their end branches arborize, from
the cells of this ganglion post-ganglionic fibers pass to the walls of the
blood-vessels of the parotid gland and of the cheek and gums.

38o TEXT-BOOK OF PHYSIOLOGY

The vaso-dilatator nerve-fibers that are found in some of the sacral
nerves have their origin in nerve-cells in the gray matter of the lumbar
or sacral region of the spinal cord. From these cells they pass into the ventral
roots of the second and third sacral nerves to be ultimately distributed by
way of the pelvic nerve to sympathetic ganglia in the pelvic region around
the cells of which their terminal branches arborize. From these ganglia
post-ganglionic fibers emerge which pass to the blood-vessels of the organs of
generation and adjacent structures.

Antidromic Vaso-dilatator Nerve-fibers. — Vaso-dilatator nerve-fibers
are associated with the vaso-constrictor fibers and are present in the trunks of
the spinal nerves and are distributed to the blood-vessels of the skin of the
limbs and trunk. Though it has been generally believed that these vaso-
dilatator fibers have their origin in nerve-cells in the ventral horns of the
gray matter, that they pass outward through the ventral roots of the thoracic
and lumbar nerves, that they belong to the efferent system of nerves, yet
these facts have never been positively determined. While this may be the
correct interpretation doubt has been thrown upon it by the investigations
of Bayliss. From the results of a long series of experiments this investigator
concludes that vaso-dilatator nerves for the regions of the body just men-
tioned, do not leave the spinal cord in the ventral roots; that the vaso-dilata-
tion observed on stimulation of the mixed spinal nerve is due to the presence
of nerve-fibers that do not differ from the ordinary afferent or sensor, posterior
or dorsal root fibers; that these nerve -fibers, moreover, have their origin in the
nerve-cells of the ganglia of the dorsal roots. From the fact that they trans-
mit nerve impulses to blood-vessels in a direction contrary to that of other
afferent nerverfibers, the term antidromic has been given to them. The
centers from which they arise are capable apparently of being aroused to
activity by impulses transmitted to them from other regions of the body.

These statements are based on the following facts: Stimulation of the
peripheral ends of the divided dorsal roots of the upper thoracic and lumbo-
sacral nerves gives rise to vascular dilatation in the upper and lower limbs;
separation from the cord is not followed by their degeneration, hence they
are not efferent nerves; extirpation of the ganglia of the dorsal roots is,
however, followed by their degeneration, hence their trophic centers are
in these ganglia. Whether the blood-vessels of the abdominal viscera which
apparently receive vaso-dilatator nerve impulses are supplied by nerves
having the foregoing origin and action, is a subject for further investigation.

The course, distribution, and functions of the vaso-dilatator nerves have
been determined by the same methods as those employed in the investigation
of the vaso-constrictor nerves. Thus division and stimulation of the pe-
ripheral branches of the nerve of Wrisberg, e.g., the great petrosal and the
chorda tympani, is followed by an active dilatation of the blood-vessels of
the nasal chambers and palate, and of the blood-vessels surrounding the sub-
maxillary and sublingual gland. The inflow of blood is so great that the
submaxillary gland becomes bright red in color. Its tissues being unable
to consume all the oxygen, the blood emerges in the veins almost arterial
in color. Stimulation of Jacobson's nerve has the same effect on the blood-
vessels of the parotid gland. Stimulation of the branches of the sacral
nerves which collectively constitute the nervus erigens is followed by a dilata-
tion of the blood-vessels of the sexual organs.

THE CIRCULATION OF THE BLOOD 381

Slow stimulation of the peripheral ends of various spinal nerves is followed
by dilatation of the blood-vessels in the areas to which they are distributed.

From these and many other facts of a similar character it is probable
that the blood-vessels of some of the organs at least 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.

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 especially in the sacral
region.

Physiologic Properties of Vaso -motor Nerves.— The vaso-constrictor
and the vaso-dilatator nerve-fibers differ somewhat in their physiologic
properties, as shown by the results of experiment. Thus, when a mixed
nerve, i.e., one containing both classes of fibers — e.g., the sciatic — is stimu-
lated with frequently repeated induced currents, the constrictor effect is

FIG. 179.— 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 secor
for fifteen seconds. To be read from right to left. (Boroditch and Warren.)

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 i
Fig. 179, A and B.

In the experiment of which these tracings are the result the leg of a cat
enclosed in a plethysmograph and the variations in volume due to diktat*
or contraction of the vessels, following stimulation of the sciatic nerve
recorded by means of tambour and lever on a slowly revolving cylinder.
A the fall of the curve indicates a diminution of volume, from contract!
blood-vessels following a rate of stimulation of the sciatic nerve of i
second for fifteen seconds. In B the rise of the curve indicates an n
in volume from dilatation of the vessels following a rate of stimulation of i

3g2 TEXT-BOOK OF PHYSIOLOGY

per second for fifteen seconds. With different rates of stimulation some-
what 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 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 -constrictor Centers. — The nerve-cells throughout the spinal
cord from which the vaso-constrictor nerves take their origin may be re-
garded as nerve-centers which through their related nerve-fibers cause a
varying degree of contraction of the arteriole muscle. In how far these
centers are independent in their activity it is difficult to state. From the
results of experiments that have been made with a view of isolating these
centers, such as division of the cord at different levels, it is fairly well proven
that they respond to nerve impulses transmitted to them from different regions
of the body, as shown by the contraction of blood-vessels. This is especially
true of lower animals such as the frog and it may possibly be true of
mammals. Though it is probable that the spinal vaso-constrictor cells
possess a certain degree of tonicity, nevertheless they are subordinate in
their activity to, and dominated by, a group of nerve-cells in the upper part
of the floor of the fourth ventricle, and termed for this reason the medullary
bulbar vaso-constrictor center.

Though the blood-pressure falls to a very low level after the separa-
tion of the medulla from the spinal cord, the animal, if properly cared for,
may survive the operation and live for a considerable time. Under these
circumstances the arteries gradually recover their former degree of con-
traction. This is accepted as evidence that the nerve-cells in the spinal
cord have acquired an independent activity, or developed an activity that
had hitherto been dormant. After this, these nerve-centers can be excited
to activity by nerve impulses transmitted from the periphery.

The Medullary or Bulbar Vaso-constrictor Center. — The existence of
such a dominating center has been determined experimentally: thus if a
definite region of the medulla oblongata is punctured or in anyway destroyed
there is an 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 longitudi-
nally for a distance of four or five millimeters, terminating at a point four
millimeters above the tip of the calamus scriptorius. Because of the effects
that follow the destruction of this area the anatomic existence of a general
vaso-constrictor center has been assumed.

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 curarized and artificial respiration maintained, will
give rise to a marked contraction of the blood-vessels and a rise of blood-
pressure up to and far beyond the normal value.

If the experimental lesion is limited to the area mentioned in the foregoing
paragraph, the vascular dilatation also passes away after a time, the blood-

THE CIRCULATION OF THE BLOOD 383

vessels regain their normal tone, and the pressure again rises. These and
the foregoing facts indicate that there is in the gray matter beneath the floor
of the fourth ventricle a restricted area composed of nerve-cells, which main-
tains 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 throughout 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.

The Tonic Activity of the General Vaso-constrictor Center.—
Since the blood-vessels maintain a more or less constant tone in the physio-
logic condition, it is assumed that the vaso-motor center is in a state of
continuous or tonic activity or tonus, and as a result continuously discharging
nerve impulses through vaso-constrictor nerves to the blood-vessels. The
causes of this activity or tonicity have been difficult to formulate. It may be
accepted, however, from the results of experimentation that the activity of
the center is maintained (i) by the chemic character of the blood and
lymph by which it is surrounded and (2) by a continuous inflow of nerve im-
pulses transmitted from all regions of the body, or to both. The follow-
ing facts will show that both factors are probably involved.

The group of nerve cells which collectively constitute the vaso-motor
center has been shown by the experiments of Porter to consist of two groups
or centers, viz., a vaso-tonic center which maintains the vascular tonus, and a
vaso-reflex center which permit of various vaso-motor reflexes. After the
administration of curara the depressor and sciatic reflex changes in blood-
pressure are more than double, while the arterial tonus remains substantially
unchanged. Alcohol on the contrary diminishes or removes reflex activity,
but leaves the tonus unchanged.

Central Stimulation of the Vaso-tonic Center. — The general vaso-
motor (constrictor) center at least is markedly influenced b} the quantity and
quality of blood and lymph circulating around and through it. If the blood-
supply to the medulla and associated structures be diminished by compression
of the carotid arteries, the activity of the center is at once increased, as shown
by a rise of pressure, the result of increased vascular contraction. Restora-
tion of the blood-suply is followed by a return of the center to its normal
degree of activity. An increase of the percentage of carbon dioxid in the
blood or a decrease in the percentage of oxygen increases the activity^ the
center. In asphyxia, a condition in which there is a deficiency in the elimina-
tion of carbon dioxid and an insufficiency in the absorption of oxygen the
center becomes extremely excitable as shown by the rapid rise of the arterial
pressure. By reason of these and other facts it is assumed that the presence
and the normal pressure of CO2 in and around the vaso-motor center is
the efficient stimulus to its activtiy. In compression of the carotid arteries
as well as in asphyxia, the CO2 is not removed from the center sufficiently
rapidly and hence its pressure increases and unduly excites the center as
shown by the rapid rise of the blood-pressure. The subsidiary centers
the spinal cord are influenced by corresponding conditions.

Reflex Stimulation of the Vaso-reflex Center.— The results of expen
mentation render it certain that the vaso-reflex center may be augmented

384 TEXT-BOOK OF PHYSIOLOGY

or inhibited in its activity, and hence followed by increased or decreased
vascular contraction, by nerve impulses transmitted to it from the brain, or
from the periphery through afferent nerves. The effects may be local and
confined to the area in which the stimulus is acting or they may be general.
The following experiments may be cited in illustration:

Psychic states of an affective or emotional character influence the ac-
tivity of this center, 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 disappearance
of the emotional state, the blood-vessels return to their former degree of
contraction.

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 sub max-
illary gland. Stimulation of certain branches of the vagus nerve is followed
by a passive dilatation of blood-vessels and a marked fall of pressure.

The preceding statements as to the effects on the degree of vascular con-
traction, and hence on the blood-pressure which follow stimulation of differ-
ent afferent nerves, has lead 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-constrictor center. The former class is generally
termed pressor or excitator, the latter depressor or inhibitor fibers.

Theories of the Mode of Action of the Vaso-motor Centers. — A
satisfactory explanation of the manner in which peripheral stimuli influence
the activity of the vaso-motor centers and thus bring about the opposite
results mentioned in the foregoing paragraphs is difficult to present. Several
interpretations have been offered, viz.:

1. The peripheral stimuli according to their qualities act on either the ter-

minals of the pressor or the depressor fibers; according as they do, will
the vaso-constrictor center be augmented or inhibited in its activity
and followed by either an increase or decrease in the degree of the
previous vascular contraction.

2. The peripheral stimuli act simultaneously on the terminals of both

pressor and depressor fibers, and hence the vaso-constrictor center is
acted on simultaneously by two antagonistic influences. The resulting
effect on the blood-vessels, viz., increased or decreased contraction will
be the resultant of the action of these two influences on the vaso-con-
strictor center. If the stimuli act preponderantly on the pressor fibers,
the vaso-constrictor center will still be excited; if they act preponder-
antly on the depressor fibers the center will be inhibited, though in either
case not to the same degree as in the first instance.

3. The peripheral stimuli may act on the terminals of the pressor fibers alone.
The nerve impulses thus developed are transmitted to both centers but

THE CIRCULATION OF THE BLOOD

385

their effect will be to excite the activity of the vaso-constrictor centers
and to inhibit the activity of the vaso-inhibitor centers; or the peripheral
stimuli may act on the terminals of the depressor fibers alone. The
nerve impulses thus developed are likewise transmitted to both centers
but their effect will be to inhibit the activity of the vaso-constrictor centers
and to excite the activity of the vaso-inhibitor centers. In this view it is
assumed that both centers are
continuously active and in a
state of tonus, and that for
the necessary vascular reac-
tion one center must be
stimulated, and the other be
inhibited. As to which cen-
ter will be stimulated or in-
hibited, will probably depend
on the character of the stim-
ulus. Hence, the general
vascular tonus as well as its
variations from time to time,
in whole or in part, is the
resultant of the simultaneous
activity and variations in
activity of these two mutually
antagonistic but cooperative
centers.

Local Special Vaso-dilata-
tor Centers. — Thevaso-dilatator
centers in the medulla oblongata
and in the sacral region of the
spinal cord, which give origin to
nerve-fibers that regulate^ the
blood-supply to the salivary
glands on the one hand and to
the generative organs on the
other hand as previously stated
(page 379) are ordinarily in a
condition of relative inactivity
and hence the blood-vessels as-
sume a caliber just sufficient to
supply the materials ^ necessary
to maintain the nutritive activi-
ties of the organs concerned. ^,, ,

activity, either by nerve impulses descending from the cerebrum in conse-
quence of psychic states of an affective or emotional character, or by nerve
Impulses transmitted to them through afferent nerves, the blood-vessels of
theP salivary glands and of the organs of generation, at once actively dilate
and the amount of blood they transmit so great that on emerging from the
capillaries it still retains its arterial hue. The same phenomena arise ,
the efferent pre-ganglionic fibers, e.g., the chorda tympam and the nerve c
Jacobson, and the nervus erigens or pelvic nerve are stimulated
25

FIG 180. — DIAGRAM SHOWING THE ORIGIN
AND RELATION OF THE DEPRESSOR NERVE IN THE
R \BBIT. Depr. n., depressor nerve; vag. n., vagus
nerve; sup. 1. n., superior laryngeal nerve ; inf.c.g.,
inferior cervical ganglion; sym. n., sympathetic nerve;
car a., carotid artery; dig.m., digastric mus<
hvp n., hypoglossal nerve; sup. c. g., superior cer-
vical ganglion; inf. 1. n., inferior laryngeal nerve.

386 TEXT-BOOK OF PHYSIOLOGY

duced electric currents. The organs of generation in consequence of the
increased blood flow exhibit changes in volume and rigidity similar to those
exhibited previous to the act of coition.

In the particular instances in which stimulation of the peripheral termina-
tions of afferent nerves, associated with the nervus erigens 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 or
augment the activity of the special vaso-dilatator centers and for this reason
should be termed "reflex vaso-dilatator nerves" (Hunt).

The Depressor Nerve. — A striking illustration of the depressor or inhibitor
action of afferent nerves upon the vaso-constrictor (reflex) center is furnished
by the result of stimulation of a branch of the vagus, the so-called "depressor
nerve." In the rabbit, Fig. 180, there is a small nerve formed by the union
of a branch from the trunk of the vagus with a branch from the superior

FIG. 181. — 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 min-
utes. The line of zero pressure (0,0) should be 30 mm. lower than here shown. — (Bayliss.)

laryngeal. The peripheral distribution of this nerve is over the wall of the
ventricle and perhaps to some extent to the structures of the aorta near its
origin. A similar anatomic 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
on either 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. 181). The fall in
pressure, however, is due mainly to the dilatation of the arterioles, the result
of an inhibition of the general vaso-reflex center, and partly to the diminu-
tion in the rate and force of the heart, the result of a simultaneous stimula-
tion of the inhibitor center. This latter factor is of less importance than the
former for the fall of pressure occurs equally well after division of all the
cardiac nerves. For this reason the nerve was termed the depressor nerve
of the vaso-motor center.

On exposure of the abdominal cavity, it is observed during stimulation of
the depressor that there is a notable dilatation of the intestinal vessels.

THE CIRCULATION OF THE BLOOD 387

From this fact it was assumed that the action of the depressor 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 pressure, stimulation of the depressor nerve wili
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.

The Traube-Hering Waves.— Under certain experimental conditions
the arterial blood-pressure tracing exhibits, hi addition to the usual respira-
tory variations, certain longer rhythmic variations more or less wave-like in
character, which are known as Traube-Hering waves. They can be devel-
oped on a blood-pressure tracing by injecting magnesium sulphate or mor-
phine into the circulation, by tying the cerebral arteries, etc. These waves
indicate a periodic contraction and dilatation of the blood-vessels, the result
of a periodic stimulation of the vaso-motor center.

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 genera]
process of nutrition. Without a constant supply of oxygen and an equally
constant removal of the carbon dioxid, those chemic changes which under-
lie and condition all life phenomena could not be maintained.

The general process of respiration may be considered under the following
headings, viz.:*

1. The anatomy and general arrangement of the respiratory apparatus.

2. The mechanic movements of the thorax by which an interchange ol

atmospheric and intra-pulmonary air is accomplished.

3. The chemistry of respiration; the changes in composition undergone by

the air, blood, and tissues.

4. The nerve mechanism by which the respiratory movements are main-

tained and coordinated.

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. — 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 passages.
The mouth, though frequently serving as an entrance for air, is not primarily
a respiratory passage. Both the nasal chambers and the mouth com-
municate posteriorly with the pharynx, in which the respiratory and the
deglutitory passages cross each other, the former leading directly into the
larynx.

The Larynx. — The larynx is a complicated mechanism serving the
widely different though related functions of respiration and phonation.
It consists of a framework of cartilages, articulating one with another,
united by ligaments and capable of being moved, one on the other, by the
action of muscles; it is covered externally with fibrous tissue and lined with
mucous membrane. The superior laryngeal aperature is triangular in
shape, the base being directed upward and slightly forward, the apex
downward and backward. The inclination of the laryngeal aperature 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,

RESPIRATION

389

>ounded by the vocal bands, is also triangular in shape, though in this case
he base is directed backward, and the apex forward. (See chapter on Voice
jid Speech.)

The introduction of the vocal bands narrows at this level the air-passage
md to some extent interferes with the free entrance of air. According to
he investigations of Semon, the area of the air-passage above and below
he phonatory apparatus is about 200 sq. mm.; while the area bounded

the vocal apparatus is but 155 sq. mm. during quiet respiration.

FIG. 182.— Human larynx, trachea, bronchi, and lungs; shoeing the ramification of the bronchi,
and the division of the lungs into lobules.

390

TEXT-BOOK OF PHYSIOLOGY

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 convex and corresponds to the
general conformation of the thorax. The inner surface is concave and accom-
modates the contents of the mediastinal space. The under surface of the
lung is concave and rests on the diaphragm. The posterior border is con-
vex; the anterior border is thin. At about the middle of the inner surface
of the lung the blood-vessels which connect the heart with the interior of
the lung enter and leave in company with the branches of the bronchi,
bronchial arteries, veins, nerves, and lymphatics.

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 num-
ber of smaller branches, the bronchial tubes, which
penetrate the lung in all directions. With this re-
peated subdivision the tubes become narrower,
their walls thinner, their structure simpler. In
passing from the larger to the smaller tubes 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 diminu-
tion 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 bronchial muscley
between the mucous and submucous tissues, which
completely surrounds the tube and becomes especi-
ally well developed in those tubes devoid of cartilage.
The fibrous and mucous coats at the same time diminish in thickness.

Bronchial Innervation. — 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 that they are innervated by efferent fibers of the vagus nerve (broncho-
constrictors and possibly broncho-dilatators) inasmuch 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.
The cavity of the alveolus is termed the infundibulum. From the inner sur-
face of the alveolus and of the passageway leading into it, there project thin
partitions which subdivide the outer portion of the general cavity or infundib-
ulum into small spaces, the so-called air-sacs or air-cells (Fig. 183). The

FIG. 183. — SINGLE LOB-
ULE OF HUMAN LUNG. a.
Alveolar passage, b. Cav-
ity of lobule or infundibu-
lum. c. Pulmonary sacs.
— (Dalton.)

RESPIRATION 3gi

wall of the alveolus is extremely thin and consists of fibro-elastic tissue sup-
porting a very elaborate capillary network of blood-vessels. The bronchial
system as far as the alveolar passages is lined by ciliated epithelium. The
air-sacs are lined by flat epithelial plates of irregular shape, termed the re-
spiratory epithelium. The alveoli are united one to another by fibro-elastic
tissue.

The bronchial arteries which supply nutritive material to the pulmonary
structures arise from the aorta as a rule, though sometimes from an inter-
costal artery. Each lung receives two arteries which accompany the bronchi
as far as the distal ends of the alveolar passages. From the capillary net-
work formed out of the terminals of these arteries, 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 pulmonic veins. The blood
in the pulmonic veins, though largely arterialized, nevertheless contains
some venous blood derived from the veins arising from the capillary network
of the bronchial arterioles.

The nerves distributed to the muscle-fibers of the bronchial arteries, and
of the bronchial tubes and to the mucous membrane, are derived from the
vagus and the sympathetic and enter the substance of the lung at and around
its root.

In consequence of the presence of the elastic tissue, the lungs are disten-
sible and elastic. After removal from the body the elastic tissue at once
recoils, forcing out a portion of the con-
tained air. The condition of the lung
is now one of collapse. Under pressure,
however, the lung can be readily dis-
tended or inflated. These properties
endure for a long period after death,
if not indefinitely, if the lungs are prop-
erly 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 Pulmonic Blood-vessels. —
The pulmonic artery which conducts the
venous blood from the heart to the lungs
divides beneath the arch of the aorta into
a right and a left branch. Each branch *o
with its subdivisions enters the lung at

the hilum in company with the larger divisions of the bronchi
the lung the arteries divide and subdivide ma manner com
to that of the bronchial tubes, which they follow to their ultimat
minations. As the pulmonic lobules are approached; small artei
branch plunges into the wall of the lobule i x£w«S iTr

^SSwJsaKstMSsK

it lies nearer the inner than the

g4._ THE RELATION OP THE

392 TEXT-BOOK OF PHYSIOLOGY

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 pulmonic veins, out of the lungs and into the left auricle of the
heart. ' The main function of the pulmonic apparatus and the pulmonic
division of the circulatory apparatus is to afford a ready means for the
exhalation of the carbon dioxid and the absorption of oxygen. In conse-
quence 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 com-

FIG. 185. — 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. 9. Division to lower lobe.
10. Left branch of pulmonic artery, n. Right branch. 12. Left auricle of heart. 13. Left
superior pulmonic vein. 14. Left inferior pulmonic vein. 15. Right superior pulmonic vein
16. Right inferior pulmonic vein. 17. Inferior vena cava. 18. Left ventrice of heart. 19.
Right ventricle. — (Sappey.)

pressed from before backward. Its apex is directed upward, its base down-
ward. The walls of the thorax are composed, first, of a bony framework
or skeleton and, second, of muscles and fascia.

The bony framework is formed posteriorly by the thoracic vertebrae
and the posterior extremities 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 downward projection of
the sternum.

The ribs, which form a large part of the thoracic walls, constitute a series

RESPIRATION

393

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 themselves 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 costo-vertebral and costo-chondral and the chondro-sternal articula-
tions are diarthrodial in character and endow the thoracic walls with a con-
siderable 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 the
two adjoining vertebrae; the second, by the tubercle of the rib and the trans-
verse process. The costo-chondral 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

Ext. Intercostal

Int. Intercostal

Inttrcartilaginei

Levafores Costarum /
Extlntercostal

FIG. 186. — SHOWING THE SITUATION, THE
POINTS OF ATTACHMENT, AND DIRECTION OF
THE INTERCOSTAL MUSCLES, i. The inter-
costales externi. 2. The intercostales in-
terni. 3. The intercartilaginei.

MJntercostal

FIG. 187. — VIEW FROM BEHIND OF FOUR
DORSAL VERTEBRA AND THREE ATTACHED

RlBS, SHOWING THE ATTACHMENT OF THE

ELEVATOR MUSCLES OF THE RIBS AND THE
INTERCOSTALS. — (After Allen Thomson.)

as follows: the diaphram, the intercostales externi and interm, the levatores
costarum, the triangularis sterni, and the infra-costales.

The diaphragm is the musculo-membranous sheet which closes the in-
ferior opening of the thorax and completely separates its cavity from ths
the abdomen It consists of two muscles which arise from the bodies of
the first three or four lumbar vertebrae and neighboring fascia fr
border of the six lower ribs, and from the ensiform cartilage. ^
extensive origin the muscle-fibers pass centrally to be msertedmto a com
mon tendon As the direction of the fibers is from below .^^ and n-
ward, the diaphragm is somewhat dome-shaped. Its inferior border i
a short distance in contact with the sides of the -ax.

The intercostales externi, eleven in numberu «nf eaC^,Sld';JrCr?Pyo
between the ribs to which they are attached from the tubercle

186 and 187). Their fibers, which are arranged

394 TEXT-BOOK OF PHYSIOLOGY

in parallel bundles, are directed from above downward and from behind
forward. The point of attachment, therefore, of any given bundle of fibers
to the rib above, lies nearer the vertebral column, nearer the fulcrum, than
the point of attachment below.

The intercostales interni, eleven in number, occupy the spaces between,
and are attached to the ribs from the tubercle to the anterior extremity of
the cartilages. Their fibers, which are also arranged in parallel bundles,
are directed from above downward and backward (Figs. 186 and 187). The
portions of the internal intercostals between the cartilages are frequently
termed intercartilaginei.

The levatores costarum are twelve in number 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 downward and outward in a diverging manner to be inserted into the
ribs between the tubercle and the angle. Their action, as their name im-
plies, 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 inter-
costal 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 absolutely air-tight.

The Pleurae. — Each lung is surrounded by a closed invaginated serous
sac, the pleura, of which the inner portion is reflected over 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 dia-
phragm, and the viscera of the mediastinum. Under normal conditions
these two layers of the pleura, the pulmonic and parietal or thoracic 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 movement of the lungs.

THE MECHANIC MOVEMENTS OF THE THORAX

As the blood flows through the pulmonic capillaries it yields carbon dioxid
to, and receives oxygen from, the air in the pulmonic alveoli. As a re-
sult, the intra-pulmonic air changes in composition which interferes to a
greater or less extent with the further exchange of gases. That this ex-
change may continue, it is of primary importance that the air within the
alveoli be renewed 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 capacity of the lungs. During the former
there is an inflow of atmospheric air (inspiration), during the latter an out-
flow of intra-pulmonic air (expiration). The continuous recurrence of these
two movements brings about that degree of pulmonic ventilation necessary
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

RESPIRATION 395

short period of rest — viz. : between the end of an expiration and the beginning
of an inspiration — and a relatively long period of activity, including 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.1

THE STATIC CONDITION

Relation of the Thoracic Organs.— Intra-pulmonic Pressure : 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. 188). This condition is
maintained by the pressure of the air within the lungs, the mtra-pulmomc
pressure, which with the respiratory passages open, is that of the atmosphere,
760 mm. Hg. This relation persists so long as the thorax remains an
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. A pleural cavity, therefore, does not exist
physiological conditions, it b POteptSri^nJ^ The pressure of air within
LfrWsuTSn^^ at the moment the air is admitted,

the elastic tissue at once recoils and forces a large part of the air out of th
lung This is a proof that in the normal condition, the lungs, distended 1
atmospheric pressure from within, are in a state of elastic tension and ever
endeavoring to pull the pulmonic layer of the pleura away fror, , the^ pane^a

lhe

> It is a matter of discussion as to whe*er °T"^

of the thoracic walls at the end of expiration AJ^££ for ^ purposes here intended

men

o e ora or

if there is no absolute cessation, the ; movemen^ ^^ ^^ recogmzed that the forces both
a pause may be admitted. With this admiss , on* *, , h .R opposlte directog

elastic and muscular, which are ^ways acting on^ the thora ^ ^ ^ are practlcally

walk are stationary*

396 TEXT-BOOK 'OF PHYSIOLOGY

6 mm. This was taken by Bonders as a measure of the force with which
the lungs endeavor to recoil. The intra-thoracic pressure would be, there-
fore, atmospheric 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 ex-
tremities of the catheter and leads to a collapse of the lungs may be subse-
quently aspirated, when the lung returns to 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 differ-
ence 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

A S V.....

FIG. 1 88. — 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.)

tissue strives to recoil, and the extent to which it opposes the atmospheric
pressure. This subtracted from the atmospheric 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 experiences 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. 188.)

The cause of the negativity of the intra-thoracic pressure is connected
with the change in the relation of the lungs to the thorax attending the first
inspiration. Previous to birth the walls of the alveoli and bronchioles are
collapsed and in apposition. The larger bronchial tubes in all probability

RESPIRATION 397

contain fluid. The lungs therefore are devoid of air (atelectatic), and
haying 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 inspiration, 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 would assume unless distended, 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, the thorax and its contained organs undergo a
series of movements in consequence of which the relations among them
characteristic of the static condition, are temporarily changed. To these
movements the term respiratory has been given, as a result of which, the
ventilation of the lungs is accomplished.

The Respiratory Movements.— The respiratory movements consist of
an alternate increase and decrease in the capacity of the thorax, accompanied
by corresponding changes in the capacity of 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 thoracic capacity, the air passively flows out, of the lungs.
In both movements the lungs play an entirely passive part, their movements
being determined by the pressure of air within them and by the outward
movement of 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.

In inspiration the thorax is enlarged in all its diameters: viz., vertical,
transverse, and antero-posterior. In expiration the thorax is diminished in
all its diameters as it returns to its former condition.

Inspiratory Muscles.— The muscles which from their origin, direction,
and insertion contribute to the enlargement or expansion of 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 extraordinary inspirations. Among the muscles active
in ordinary inspirations may be mentioned the diaphragm, the intercostales
externi, the inter cartilaginei, the levatores costarum, the scaleni, and the ser-

398

TEXT-BOOK OF PHYSIOLOGY

ratus posticus superior. Among the muscles active in extraordinary inspira-
tions 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 convexity of the diaphragm diminished, but that portion in
contact with the thorax is drawn away, thus making a large free space
triangular in shape, termed the complementary pleural space, into which
the lateral and posterior portions of the lungs at once descend. The attach-
ment of the central tendon of the diaphragm to the pericardium prevents

any marked descent of this portion
except in forcible inspiratory efforts.
The vertical diameters are thus en-
larged, 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.
Coincidently with the descent of the diaphragm and the partial removal
of the pressure on the under surface of the lung, the intra-pulmonic air ex-
pands. As it expands it distends the lungs in the vertical direction, causing
them to follow the diaphragm and to occupy the so-called complementary
pleural space. With the expansion of the intra-pulmonic air there is a fall
in its pressure below the atmospheric pressure, to be followed immediately
by an inflow of air until atmospheric pressure is again established. This
occurs at the end of the inspiration.

The antero-posterior and transverse diameters are increased by the eleva-
tion 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-chondral and chondro-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 glid-
ing 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 some-
what 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

FIG. 189. — DIAGRAM SHOWING THE POSITION
AND SHAPE or THE DIAPHRAGM AT REST a AND
DURING INSPIRATION a' AND b. — (Boruttau.)

RESPIRATION

399

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
which still further increases the transverse diameters. Coincidently with
the increase in the transverse and antero-posterior diameters of the thorax,
and the partial removal of the pressure on the lateral surfaces of the lungs
there is also an additional expansion of the intra-pulmonic air. As it expands
it distends the lungs, causing them to occupy the available space thus estab-
lished. With the expansion of the intra-pulmonic air there is a still further
fall of pressure and an additional inrush of air. Between the descent of the
diaphragm and the elevation of the ribs and the advance of the sternum the
volume of air necessary for the ordinary respiratory needs is introduced into
the lungs.

This elevation and outward rotation of the ribs is the resultant of the coop
eration of the following muscles, viz. : the intercostales externi, the intercar-
tilaginei, the levatores costarum, the scaleni and the serratus posticus superior.

The action of the external intercostal muscles, as well as the action of
the intercartilaginei muscles, has been a subject of much discussion. Some

FIG IQO.— DIAGRAMS ILLUSTRATING THE ACTION OF THE EXTERNAL INTERCOSTAL AND IN-

TERCARTILAGINEI MUSCLES.

investigators have maintained that they are elevators of the ribs, and there-
fore inspiratory; others that they are depressors of the nbs, and therefc
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. Ii

rfSSdle of fiber, to the cutilage above lies nearer the

study of the schematic model first presented by Hamberger

400 TEXT-BOOK OF PHYSIOLOGY

this model v'-v' is a vertical support carrying two freely movable parallel
bars r r', united at their opposite ends with two other freely movable
and parallel bars cc, ccf carried by a second vertical support, s, representing
respectively the vertebral column, two adjoining ribs, two adjacent cartilages,
and the sternum. Diagram A shows the position of the different parts at
the end of expiration and B their position at the end of inspiration. The
parallel bars are joined to each other by elastic bands ei and ic having the
direction of and representing the external intercostal and intercartilaginei
muscles, respectively. The bars are depressed to sufficiently elongate and
tense the elastic bands and thus imitate the condition of the muscles in so
far as tension is concerned prior to their contraction. On releasing the
bars the elastic bands at once recoil and the bars representing ribs and
cartilages are raised. Although the elastic forces, acting in opposite direc-
tions as indicated by the arrows are equal, the bars are yet raised for the
reason that in accordance with the parallelogram of forces, the components
acting upward on the long arms of the levers preponderate over the com-
ponents acting downward on the short arms of the levers. This, taken in con-
nection with the fact that the distances between the adjoining bars are fixed,
leads not only to an elevation of the bars, but to a widening of the angle
between them and an advance of the second vertical support. The actions
of these bands thus disclose and illustrate the actions of both the external
intercostal and intercartilaginei muscles. It must therefore be concluded
that these muscles are the elevators of the ribs and cartilages and hence,
inspiratory in function. Of late the correctness of Hamberger's view has
been confirmed by experiments on living animals.

The levatores costarum, as is evident from their points of origin and in-
sertion, elevate the ribs posteriorly.

The scalenus muscles, anticus, medius, and posticus, arise from the trans-
verse 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
vertebrae. 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.

Forced Inspiration.; — In forced or extraordinary inspirations, whereby
the capacity of the thorax is still further increased, the foregoing muscles
are reinforced by the sterno-cleido-mastoideus, the trapezius, and the pectorales
minor and major. Their functions will become apparent from a considera-
tion 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 in-
spiratory 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

RESPIRATION 401

diminished by the recoil of 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. Coincident with the return of the
thoracic walls to their former condition there is a recoil of the elastic tissue
of the lungs, in consequence of which there is a compression of the intra-
pulmonic air. With its compression there is a rise of pressure above atmos-
pheric and at once there is an outflow of intra-pulmonic air until atmospheric
pressure is again established at the end of expiration.

It is somewhat uncertain if a normal expiratory movement necessitates
active muscle contraction. If, however, there is any impairment of the
elasticity of the lungs or ribs, or any interference with the free exit of the
intra-pulmonic air, it is highly probable that the elastic forces are assisted
by the internal intercostal and triangularis sterni muscles. It has been in-
sisted upon also that while the recoil of the elastic tissues is effective in the
early stages of an expiration, it is ineffective in the later stages. Hence there
arises a necessity for muscle assistance.

The action of the internal intercostals is less clearly understood than that
of the external intercostals. If, however, the direction of these muscles as
indicated in Fig. 190, diagram A, by the dotted line ii, ii, be considered it
would seem that their action would be the opposite of that of the external
intercostals— that is, it would be to depress the ribs. Employing the Ham-
berger model to elucidate the functions of these muscles it is apparent
that if the elastic band, ii, ii, recoils, the elastic force of the two halves, though
equal, will act in opposite directions, but as the component acting on the
long arm of the level preponderates over that acting on the short arm of
the lever, the ribs will be depressed. The action of the band is supposed
to disclose and illustrate the action of the muscle. If this is the case the
internal intercostal muscles must therefore be expiratory in function. ^

The triangularis sterni muscle, judging from its anatomic relations, in all
probability assists in expiration by depressing the cartilages to which H
attached and as a further result depressing the anterior extremities c

the ribs.

Forced Expiration.— After the elastic forces have ceased to act am
normal expiratory movement has been brought to a close, the thorax can be,
to a considerable extent, still further diminished in all its diameters by t
contraction, through volitional effort, of abdominal and thoracic muse
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 cessati^o! musdc
activity the elastic forces of the now-compressed thoracic walls, aided by
return of the upwardly displaced abdominal organs, at once :
thoracic walls to the position they had attained at the end of P"* «
tion. Of the muscles active in forced expiration in addition to the
costales interni and the triangularis sterni, the following may be menUo

.: the abdominales, the serratus posticus inferior, and

vz.

" njoint action of these muscles is to diminish the convexity of the
abdominal walls and to exert a pressure on the aM0^)^ns. These,
taking the line of least resistance, are forced upward against

402 TEXT-BOOK OF PHYSIOLOGY

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 ster-
num and the lower ribs and diminish the antero-posterior and transverse
diameters.

Movements of the Lungs. — As the thorax is enlarging in all its diame-
ters 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, inserting itself into the space developed between the thorax
and diaphragm as the latter contracts and is drawn away from the former.
In consequence 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 entirely passive and deter-
mined by the movements of the thorax.

The Changes in the Relation of the Thoracic Organs, and in the In-
tra-pulmonic and Intra- thoracic Pressures. — In the dynamic condition, as
previously stated, the relations of the thoracic organs undergo a change as
well as the intra-pulmonic and in tra- thoracic pressures. Thus during
inspiration the diaphragm descends, the ribs ascend and outwardly rotate
and the sternum advances, the result of which is an enlargement in the
diameters of the thorax. Coincidently 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.

During expiration the diaphragm ascends in consequence of the return
of the displaced abdominal viscera, the ribs descend and inwardly rotate
and the sternum recedes from the recoil of the elastic tissues, the result of
which is a diminution in the diameters of the thorax. Coincidently with the
diminution of the thorax there goes a decrease in the size and capacity of the
lungs in consequence of the recoil of their elastic tissue whereby the air in
the lungs is compressed.

The intra-pulmonic pressure in consequence of the alternate expansion
and compression of the intra-pulmonic air also undergoes a considerable
variation.

During inspiration the intra-pulmonic air expands. With the expansion
its pressure falls; but though it is now less than atmospheric pressure it is
yet much greater than the opposing force of the lung tissue. As a result of
the fall of intra-pulmonic pressure, there is a rapid inflow of air which con-
tinues until atmospheric pressure is restored; that is, at the end of the
inspiration.

During expiration the intra-pulmonic air becomes compressed. With
the compression its pressure rises above that of the atmosphere and in conse-

RESPIRATION 403

quence there is a rapid outflow of air, whiclTcontinues until atmospheric
pressure is again restored; that is, at the end of the expiration. (Fig. 192 ?A .)

The cause for the fall of intra-pulmonic pressure during inspiration and
the rise during expiration is to be found in the resistance offered by the air-
passages to the movement of the air, throughout their entire extent, and
especially at the level of the vocal bands. The greater the resistance, from
whatever cause, physiologic or pathologic, the greater the variations of
the pressure. If the inspiratory and expiratory movements take place slowly
the intra-pulmonic pressure may scarcely vary in either direction.

In quiet inspiration the fall of pressure, as indicated by a manometer
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. Thus it was found by Bonders
that with one nostril closed and a mercurial 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-
pulmonic pressure are graphically represented in the upper half of Fig. 191.

763

Inspiration -* /^

760 m?n.

Expiration,

7S8

A. INTRA-PULMONIC PRESSURES.

760mm,

Expiration

B. INTRA-THORACIC PRESSURE.

FIG. loi.— REPRESENTING THE CHANGES, i, IN THE INTRA-PULMONIC, AND 2, IN THE INTRA-THO-
RACIC PRESSURES DURING INSPIRATION AND EXPIRATION.

The intra-thoracic pressure also varies during both inspiration and expira-
tion. As the thorax enlarges and the intra-pulmonic pressure falls, the
recoil of the elastic tissue increases, with the result of still further dimin-
ishing the intra-thoracic pressure, until its maximum is reached near
the end of the inspiration. The fall of intra-thoracic pressure at
the end of a quiet inspiration reaches to about 9 mm. Hg.
forcible inspiratory efforts this fall in intra-thoracic pressure may
amount to 30 or 40 mm. of Hg. As the thorax again diminishes and
intra-pulmonic pressure rises above the atmospheric pressure, t
of the elastic tissue is again opposed, with the result of mcrea *mg tb
intra-thoracic pressure, until the former condition of pressure
been regained at the end of the expiration. Neither the fall nor
the subsequent rise of the intra-thoracic pressure takes place , howei
in a steadily progressive manner for the following reasons . a tracm;
were made of the variations in the circumference of the thorai
respiratory movement it would resemble in its mam features
Fig 191, and variations in any linear dimension of the lung wo
course in the same proportion. This amount of elongation of elast

4o4 TEXT-BOOK OF PHYSIOLOGY

in any direction would likewise be proportional to the force of elastic recoil.
Therefore the intra-thoracic pressure would vary from a uniform decrease
and increase just as the curve of Fig. 191 varies from uniform straight lines.
The changes in intra-thoracic pressure are graphically represented in Fig.
191, B.

The intra-thoracic pressure and its variations influence favorably the
flow of lymph through the thoracic duct (see page 225), as well as the flow
of blood from the extra-thoracic veins into the intra-thoracic veins, the right
side of the heart, and the cardio-pulmonary vessels. (See paragraphs at the
end of this chapter.)

The succession of events in the thorax at the time of a respiratory act
may be summarized as follows:

During Inspiration.

1. Enlargement of the thoracic diameters by muscle action.

2. Increase in the negativity of the intra-thoracic pressure.

3. Expansion of intra-pulmonic (alveolar) air.

4. Expansion of the lungs.

5. Lowering of the intra-pulmonic air pressure below the atmospheric

air pressure.

6. Inflow of atmospheric air, in consequence of its higher pressure, until

the intra-pulmonic air pressure rises to that of the atmosphere.
During Expiration.

1. Diminution of the thoracic diameters by the action of elastic forces.

2. Decrease in the negativity of the intra-thoracic pressure.

3. Recoil of the lungs.

4. Compression of the intra-pulmonic (alveolar) air.

5. Rise of intra-pulmonic air pressure above the atmospheric air

pressure.

6. Outflow of intra-pulmonic air, in consequence of its higher pres-

sure, until the intra-pulmonic 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 dimin-
ished 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 movement 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 required 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 considerable
oscillations, outward and inward, corresponding to the movements 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

RESPIRATION 4O$

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 regarded as strictly physiologic. The respiratory
-pace in quiet breathing is an isosceles triangle, with a length of 20 mm. and
, width at the base of 15.5 mm. with an area of 155 mm.

Respiratory Types.— Observation of the respiratory movements in the
two sexes shows that while the enlargement of the thoracic cavity is accom-
plished 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 diaphragmatic or abdominal type and in the
other the thoracic or costal type of respiration. The cause of this greater
mobility and activity of the thorax in the female has been a subject of much
discussion. It has been attributed, on the one hand, to the necessity for a
physiologic adjustment between respiration and child-bearing, and therefore
a specific sex peculiarity; on the other hand, it has been attributed to per-
sistent 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 thora-
cic muscles. It has been assumed that if inspiration were confined 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 ex-
changes be interfered with, but fetal life might be unfavorably influenced,
if not endangered. Modern investigations have not confirmed this assump-
tion, but, on the contrary, have corroborated 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 tem-
porary, for with its termination there is a return to the diaphragmatic type
of breathing.

Number of Respirations per Minute.— The number of respirations
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 years 15.0

i5-2oyears 20 30-50 years 17.0

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, however, the respiratory
movements often diminish in number as much as 30 per cent., at the san
time diminishing in depth.

Rhythm.— Each respiratory act takes place normally m a regular
methodic manner, each event occurring in a definite sequence and occupy-
ing the same relative period of time. This rhythm however, is not mfa
quently temporarily disturbed by emotions, volitional acts, muscle acti
phonation, changes in the composition of the blood, etc.; with the removal
of these disturbing factors, the respiratory mechanism soon ret
normal condition.

4o6

TEXT-BOOK OF PHYSIOLOGY

A graphic representation of the excursions of the thoracic walls, rhythmic
or otherwise, is obtained by fastening to the thorax an apparatus, a stethome-
ter or a pneumograph, which by means of a tambour takes up and trans-
mits the movement to a second tambour provided with a recording lever.
A simple form of pneumograph, suggested by Fitz (Fig. 192), 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

FIG. 192. — PNEUMOGRAPH. —

placed in communication either with 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 conditions obtain and the lever rises. If the lever be ap-
plied to the recording surface of a moving cylinder, a curve of the thoracic
movement, a pneumatogram, is obtained (Fig.
193), from which it is apparent that inspira-
tion takes place more abruptly and occupies
a shorter period of time than expiration; that
expiration immediately 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 toS.

<NSP.

FIG. 193.

A PNEUMATOGRAM. — (After
Marey.)

FIG. 194. — A SPIROMETER.
(Boruttau.)

Volumes of Air Breathed. — The volumes of air which enter and leave
the lungs with each inspiration and expiration naturally vary with the ex-
tent of the movement, though four volumes at least, may be determined:
(i) that of an ordinary inspiration; (2) that of an ordinary expiration; (3)
that of a forced inspiration; (4) that of a forced expiration.

The apparatus employed for the determination of these different volumes

RESPIRATION 407

is the spirometer, a modification of the gasometer. The form introduced by
Jonathan Hutchinson, of which Fig. 194 is a modification, consists of two
metallic cylinders, one containing water, the other containing air, the latter
being inserted into the former. The air cylinder is balanced by a weight so
accurately that it remains stationary in any position. A tube, penetrating
the base of the water cylinder, is continued upward through and above the
level of the water. The air-space above is thus placed in free communica-
tion with the atmospheric 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 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 the side support, graduated in cubic inches or centimeters, in-
dicates the volume of air inspired or expired.

With an apparatus of this character Hutchinson, from a long series of
observations, defined and determined the above-mentioned four volumes as
follows :

1. 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 (330
to 500 c.c.).

2. The complemented 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 (1800 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 expiration, and which amounts to
about ico cubic inches (1650 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 maintaining the
conditions essential to the free movement of blood through the capil-
laries and to the exchanges of gases between the blood and alveolar air.
As this volume of air cannot be displaced by volitional effort, but
resides permanently in the alveoli and bronchial tubes though constantly
undergoing renewal, it was termed —

4. The residual volume, the amount of which is difficult of determination,

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 expan-
sion, 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 inspira-
tion, this therefore excludes the residual volume. The vital capacity
was supposed to be an indication of an individual's respiratory power, not
only in physiologic but also in pathologic conditions. Though averaging
about 230 cubic inches (3770 c.c.) for an individual 5 feet 7 inches in height,
the vital capacity varies with a number of conditions, the most important

408

TEXT-BOOK OF PHYSIOLOGY

which is stature. It is found that between 5 and 6 feet the capacity in-
creases 8 inches (130 c.c.) for each inch increase in height.

The Total Volume of Air Breathed Daily. — For the solution of certain
problems connected with ventilation it is necessary to determine the total
volume of air taken into the lungs in the course of 24 hours. This can
be determined approximately if the two factors, the average volume of air
taken into the lungs at each inspiration, and the average number of res-

FIG. 195. — GAD'S PNEUMATOGRAPH.

pirations per minute be known. If it be accepted that the inspired volume
varies from 328 to 492 c.c. and that the respiratory frequency averages 18 per
minute, then the total volume breathed would amount to from 8500 to
12,752 liters.

The volume changes of the thorax indicated by the volumes of air en-
tering and leaving the lungs can be not only determined but graphically
represented by means of an apparatus similar in principle to the spirometer,
devised by Gad and known as the pneumato graph or aeroplethysmo graph

4OOO
350O
3OOO
Z5OO
WOO
1SOO
WOO
SOO
ft

A. \

L \

ZTSfifS/9f\
Vol. \

Vital

MT V\^/V

w 97

Capacity.

r/ •

V )

FIG. 196. — DIAGRAM REPRESENTING THE VOLUME CHANGES OF THE THORAX AND LUNGS. —

(Modi-fled, jrom Boruttau.)

(Fig. 195). 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
depression 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

RESPIRATION 409

air. Inspiration causes the lever to descend, expiration to ascend. Previous
graduation of the apparatus is necessary to determine the volumes breathed.
A graphic record of the volume changes is shown in Fig. 196.

Respiratory Sounds.— On applying the ear over the trachea and bronchi
there is heard during both inspiration and expiration a well-defined sound,
which is loud, harsh, and blowing in character, and which from its situation is
known as the bronchial sound. It is especially well heard between the
scapulae above the level of the fourth thoracic 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 production is uncertain. On applying the ear to almost any portion of
the chest-wall, but especially to the infrascapular area, there is heard during
both inspiration and expiration a delicate, sighing, rustling sound, which
from its supposed seat of origin, the air- vesicles or air-cells, is known as the
vesicular sound. This sound is supposed to be due to the sudden expansion
of the air-cells during inspiration a^H ^ thp fri^irm r>f ^A ajr

THE CHEMISTRY OF RESPIRATION

The general metabolic process as it takes place in the tissues involves
the assimilation of oxygen 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 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 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 exchanges between the blood and the tissues constitute internal
respiration, while the exchanges between the blood, the intra-pulmonic and
the atmospheric air, the result of the mechanic movements of the thorax,
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 physical
and chemic changes which are related to the exchange of gases between the
air in the lungs and the blood, on the one hand, and the exchange of gases
between the blood and tissues, on the other hand.

In consequence of the many and complex chemic changes which att
these gaseous exchanges, there arise changes in composition of:

1. 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 constitute the subject-matt
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.

4io TEXT-BOOK OF PHYSIOLOGY

Chemic analysis has shown that this change involves a loss of oxygen, a gain
in carbon dioxid, watery vapor, and organic matter. For the correct under-
standing 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 deductions can be made.

The results which have been obtained are represented in the following
table:

Inspired Air. Expired Air.

{Oxygen 20.80.
Carbon dioxid traces.
Nitrogen 79.20.
Watery vapor variable.

100

Oxygen.. 16.02.

Carbon dioxid . . 4.38.

Nitrogen 79.60.

Watery vapor., saturated.

Organic matter . a trace.

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 analy-
sis 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 containing barium or calcium hydrate solution. 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 atmospheric air has been maintained for some time. From the percent-
age 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 8,500 or 12,752 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 :: 8,500 : #=406 liters or 580 grams1
Or

100 : 4.78 :: 12,752 : # = 609 liters or 870 grams.

By the same method the total carbon dioxid exhaled is found to be either
372 liters or 735 grams, or 558 liters or 1103 grams; volumes in both instances
which agree very well with volumes obtained by other methods.

From the fact that only 558 liters of carbon dioxid are exhaled as compared
with 609 liters of oxygen absorbed, it is evident that not all of the oxygen
unites with carbon to form carbon dioxid and that the remainder of the
oxygen must unite with some other element. As there is usually an excess
of water eliminated over that introduced into the body, it is highly probable
that the oxygen combines with free hydrogen to form water. The relative
amounts of the oxygen so utilized are not fixed but variable, and depend on
the quality and quantity of the foods, exercise, etc. The ratio of the

1 i liter of oxygen weighs 1.4298 grams; i liter of carbon dioxid weighs 1.977 grams.

RESPIRATION 4II

volume of the carbon dioxid exhaled to the volume of oxygen absorbed is
known as the respiratory quotient, and is usually represented by the symbol

-Q^. 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 absorption
from the large intestine, in which decomposition of nitrogen-holding com-
pounds is taking place. It is generally believed that free nitrogen plays no
part in any phenomenon of combination or decomposition 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 tempera-
ture the percentage of water increases; with a fall, it decreases. By breath-
ing into a vessel, containing pumice stone saturated with sulphuric acid, the
vapor may be collected. The difference observed between the weight before
and after breathing is an indication of the amount by weight of water exhaled
during the time of breathing. It has been calculated that the amount of
water exhaled daily varies between 300 and 500 grams. Though invisible
at ordinary temperatures, it becomes visible at low temperature as soon as
it emerges from the respiratory tract. The loss of heat is followed by a con-
densation 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 2o°C. rises in temperature to
37°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 hi volume about one-ninth
of 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 statement of the
composition of the expired air, derived in part from the upper air-passages,
trachea, and bronchi, does not necessarily represent the composition of the
alveolar air. "It is very probable that the percentage ^ of carbon dioxid
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 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 ^f
volumes and consequently of the "partial pressures" of these two gases in

4I2 TEXT-BOOK OF PHYSIOLOGY

the trachea and alveoli that the degree of ventilation necessary for the ex-
change of gases between lungs and air is maintained. Though the respira-
tory movements doubtless create currents in the air-passages which carry,
oh the one hanct, a portion oTth~e inspired air drreetly-Hrte-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 approximately 122 mm. Hg., diffusion downward
will take place. Equilibrium, however, is never established, as the oxygen
is continually disappearing 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. Equilib-
rium 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 through the pulmonic artery into the lungs is
dark bluish-red, while that which flows from the lungs into the pulmonic
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 through the arteries into the tissues is scarlet red,
while that which flows from the tissues into the veins is bluish-red in color.
The blood while flowing through the tissue capillaries is changed from the
arterial to the venous condition. Since arterial blood when deprived 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 from the blood into the lungs
indicates that the carbon dioxid is as constantly passing from the tissues
across the capillary walls into the blood.

These considerations are confirmed by the results of analyses which have
been made of both arterial and venous 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
100 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 arterial and venous blood shows that each consists of oxygen, carbon
dioxid, and nitrogen, though in different amounts. Average composi-
tions of the gases extracted from dog's blood obtained from the carotid
artery and right ventricle respectively are given in the following table:

RESPIRATION 4I3

Arterial blood ^n' • : ; ; : • • ! 20 v°]a- Venous blood f Oxygen ........... I2 v°ls.

The changes produced in the blood by respiration, both external and in-
ternal, become apparent from a comparison of these analyses. The arterial
blood while passing through the capillaries of the tissues loses eight volumes
per cent, of oxygen and gains five volumes per cent, of carbon dioxid.
The venous blood while passing through the capillaries of the lungs gains
oxygen and loses carbon dioxid in corresponding amounts. These amounts
will vary somewhat in the analyses of the blood of different animals and
under .different physiologic conditions. 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 solubili-
ties 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 tempera-
ture, the pressure remaining constant, the absorptive power of water for each
of these gases diminishes. Thus at i5°C., the volumes 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 tempera-
ture of o°C., the volume of oxygen absorbed by one volume of water is
o 0489 c c.- at 1520 mm. of mercury, the same volume is absorbed but
weight is doubled. If the pressure falls below 760 mm. of mercury the
same volume is absorbed but its weight is diminished. (Law of
Because of the foregoing facts, it is necessary in all gaseous dete
tions to reduce for purposes of comparison the obtained volumes tc
temperature (o°C.) and pressure (760 mm. of mercury).

4i4 TEXT-BOOK OF PHYSIOLOGY

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, though the volume retained in the
liquid remains constant. The reason for this fact is, that under the condi-
tions, 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 temperature
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 pressure,"
and depends on the percentage volume of the gas present. If atmospheric
air contains at standard pressure and temperature 79.15 volumes percent,
of nitrogen, its partial pressure will be ^^ 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 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 milli-
meters 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 solution. If a liquid has a greater density than water, as
from the presence of inorganic salts, the absorptive power under standard
conditions 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 complete removal
of the atmospheric pressure. From this it might be inferred 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 follow the law of pressure as stated in foregoing para-
graphs. It is therefore necessary to test this supposed condition of the gases
in the blood by subjecting 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.

RESPIRATION 4!5

Oxygen.— If blood is subjected to a succession of pressures progressively
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 159
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
the latter, i.e., 70 mm. 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. Coin-
cidently 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, scarcely amounting to more than 0.36 c.c. per 100 c.c.
of blood. The agent therefore which holds the oxygen in combination is
the red corpuscle, or more exactly, the hemoglobin, which constitutes about
32 per cent, of its volume. This is proved by 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 absorp-
tion the hemoglobin changes in color from bluish-red to scarlet-red and
changes from hemoglobin to oxyhemoglobin. The reverse method, that of
subjecting 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 hemo-
globin combines with 1.34 c.c. of oxygen, and as the percentage 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. Thus the
hemoglobin in 100 c.c. .of blood would hold in combination 18.76 c.c. of
oxygen. This, together with the one c.c. held in solution in plasma, would
equal the volume obtained in the vacuum of the air-pump.

The union of the oxygen with the hemoglobin is therefore largely chemic
in character, dependent however on pressure. About one per cent, is
physically absorbed by or dissolved in the plasma; the remainder is chem-
ically combined with the hemoglobin.

The association or combination of oxygen is favored by a pressure
least 30 to 50 mm. Hg. and upward; the dissociation, by diminution c
pressure. In the conversion of hemoglobin into oxyhemoglobin two
tagonistic forces are at work, heat and chemic affinity,
tends to prevent, the latter to favor, the union. Chemic affinity increase
with the influence of mass, that is, in proportion to the number of i
unit of volume, with the density, and with the partial pressure of the oxyg<
Diminution of pressure reduces the mass influence and permits the
bring about dissociation (Bunge). The following table by Hufner shows

4i6 TEXT-BOOK OF PHYSIOLOGY

the relative proportion of hemoglobin and oxy hemoglobin in blood contain-
ing 14 per cent, hemoglobin and exposed to air at gradually diminishing
pressures :

Atmospheric Pressure Partial Pressure of Oxygen Hemoglobin Oxyhemoglobin

in mm. Hg. in mm. Hg. Percentage. Percentage.

760 159.3 I-49 98.51

524.8 no 2.14 97.86

357-8 75 3.11 96.89

238.5 50 4.60 95-40

119-3 25 8.79 91.21

47.7 10 19-36 80.64

23-8 5 32-51 67.49

O.O O.O IOO.OO 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 removed and the ex-
periment made with either plasma or serum. Even at zero pressure the fluid
contains carbon dioxid, as shown by its liberation on the addition of some
weak acid, as tartaric or phosphoric, an indication that it exists in a state oi
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 corpuscles has been attributed to hemoglobin, anc
especially when in the state of oxyhemoglobin. It is for this reason tha
blood yields all its carbon dioxid to the vacuum of the gas-pump.

The limit of pressure at which the plasma ceases to absorb carbor
dioxid physically and begins to combine it chemically 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 remainder being in a conditior
of both loose and stable combination.

An analysis of the serum, and presumably of the plasma, shows th<
presence of sodium salts, with which the carbon dioxid could enter into com
bination, viz.: sodium carbonate and dibasic sodium phosphate. Th<
sodium is thus partly divided between carbonic acid and phosphoric acid
The amount of the sodium which falls to carbon dioxid will depend on thi
mass influence of the latter; that is, its partial pressure.

At its origin in the tissues the carbon dioxid acquires a considerabl
tension, and its mass influence is correspondingly large. On entering th«
blood it combines with sodium carbonate, with the formation of sodiun
bicarbonate, as shown in the following equation:

2 + H2O=-2NaHCO3.

At the same time, having a greater mass influence than the phosphoric acid
it will withdraw from the dibasic sodium phosphate one-half of its sodium
with the formation of sodium bicarbonate and monobasic sodium phosphate
as shown in the following equation:

Na2HP04H-C02+H20=NaHC03 + NaH2P04.

In addition to the amount of CO2 physically absorbed by the plasma an
chemically combined with sodium, a third portion is more or less loosel
combined with the protein constituent, globin, of the red corpuscle. A
this is the case the corpuscles must be regarded as carriers of CO2 from th
tissues to the lungs as well as carriers of oxygen. It has also been suggeste

RESPIRATION 4I?

that the presence of the CO2 in the corpuscle, assists in the dissociation of
the oxygen as the blood passes through the capillaries. 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 com-
bination of the carbon dioxid with the basic salts depends on its partial
pressure; 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 volumes varying in
amount and in accordance with their partial pressures until equilibrium is
established. If the pressure of any one gas in the atmosphere is greater than
the pressure of the same gas in the liquid, it is absorbed; if the pressure is
less the gas is discharged. Knowing the pressure of the gases in percent-
ages of an atmosphere, at the beginning 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 aerotonometer s 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 presents a large surface
to the action of the contained gases. In the aerotonometer 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.
states that the oxygen tension in the pulmonic 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-dioxid tension in the tissues varies from
45 to 68 mm. Hg., or from 6 to 9 per cent, of an atmosphere; while m the
venous blood it varies from 30 to 41 mm. Hg., or from 3.8 to 5.4 per cent,
and in the pulmonic alveoli it is about 21 mm. or 2.8 per cent.
27

4I8 TEXT-BOOK OF PHYSIOLOGY

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
cannot be demonstrated by the usual methods of gas analysis. Only in the
saliva and in the blood of the placental umbilical vein can 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 decomposition or cleavage in consequence of
cell activity.

Nevertheless many facts from the fields of comparative physiology and
physiologic che'mistry 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 characteristic 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 pigments 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 colorless compound, which, however, on exposure recom-
bines with oxygen and regains the original color.

Though free oxygen cannot 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 improb
able 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 CO2 even for
several hours.

RESPIRATION

419

Naturally the absorption of oxygen and the discharge of carbon dioxid
and the changes of composition which are incident to nutrition 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 corre-
sponding degrees of respiratory change. 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.

Atmospheric Air.

Ox-iss mm.flg ', or 20.85 per cent ,

COz - 0.3 m. ™ H&, or 0. 04 joer cen t,

of an atmosphere.

Ox-Tension

38 mm If?,

S per cent.

C0x-7ension-

4J.8mm H#,

&5 per cent.

J%^

— —

Alveolus

Arterial
Blood.

Venous
*\Blood.

OzTension O.
C&fenston i

6 to 9 per cent.

Ox-Tension.

2O6m,nt. ffff,

J4percent

COz -Tension

38 m, nt fig,

Jper cent

Tissues.

FIG. I97.-DIAGRAM SHOWING THE RELATIVE TENSION OF OXYGEN AND CARBON DlOX
THE LUNGS, IN THE BLOOD, AND IN THE TISSUES.

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 1

oxygen. Both tissues discharged carbon dioxid in amounts proper

the oxygen absorbed. The same respiratory changes may be more satis-

factorily" demonstrated by passing bl ood, througl: .the ^ °M^ted

organs and the tissues of recently living animals. The analys

before and after perfusion shows a loss of oxygen and a gam i

nsion of the Gases in the Tissues. -As the presence of Jree oxygen
cannot be demonstrated, its tension there must be regarded as m/.

42o TEXT-BOOK OF PHYSIOLOGY

tension of the carbon dioxid is quite high, though difficult of exact deter-
mination. 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 rela-
tions to each other, are shown in Fig. 197, expressed in mm. Hg. and
percentages of an atmosphere.

The Mechanism of the Gaseous Exchange. — In these pressure differ-
ences 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 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 capil-
laries, the blood enters a region in which the oxygen tension in the surround-
ing 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 dissociated oxygen also passes through the capillary
wall into the surrounding lymph and so to the tissue cells where it is stored
and utilized. On passing into the 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 condition is regained.

The sojourn of the blood in the capillaries being short, the oxyhemo-
globin can part with but a portion of its oxygen, sufficient, however, to sat-
isfy 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 pulmonic 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 dissociation 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 exchanges of gases,
it is possible that the alveolar or respiratory epithelium may also play an
essential r61e. It is believed by some investigators 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 investigators, 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 dif-
ferent 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

RESPIRATION 42I

to considerable criticism and not generally accepted, some other force than

iffusion would have to be found to explain the facts. It would then remain

to be determined in how far the alveolar epithelium could be regarded as

an active agent in both absorption and excretion in opposition to pressure.

THE TOTAL RESPIRATORY EXCHANGE

The total quantities of oxygen absorbed and carbon dioxid discharged
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 different 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 by experiments with spirometric
apparatus, and from the percentage loss of oxygen and gain of carbon dioxid
shown by an analysis of the respired air, it can be calculated at least ap-
proximately what the total amounts of oxygen absorbed and carbon dioxid
exhaled must be. If it be assumed that the minimum daily volume of air
breathed is 8500 liters and the maximum volume 12,752 liters, and the
percentage loss of oxygen is 4.78, then the total volume of oxygen absorbed
is 406 liters (580 grams) or 609 liters (870 grams). By the same method the
total carbon dioxid exhaled daily is found to be either 372 liters (735 grams)
or 558 liters (1103 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, Pettenkofer and Voit's — it is only
possible, however, to determine the amount of carbon dioxid and water exhaled
and from them to calculate 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 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 H2SO4, by which the water I
collected; then into long tubes containing barium hydroxid, by which the
carbon dioxid is absorbed; thence into a small meter, by which its amount
registered. From the amount of water and carbon dioxid thus obtainec
the amounts of both in the total air breathed are calculated. The water and
carbon dioxid previously present in the air are simultaneously determined
a corresponding absorption apparatus and deducted from the amoun
tained from the respired air. As the apparatus is traversed constantly by a
column of air of normal composition and the waste products removed as rapidly
as discharged, the experiment can be continued for periods varying ]
six to twenty-four hours without detriment to the subject of the experiment.

422 TEXT-BOOK OF PHYSIOLOGY

With those forms adapted only for animals — Regnault's and Reiset's, or
Jolyet and Regnard's — it is possible to determine simultaneously the ab-
sorption of oxygen and the discharge of carbon dioxid. As the apparatus
employed is completely closed, the carbon dioxid must be remoyed as soon
as discharged and the oxygen renewed as soon as absorbed. The former is
accomplished by the aspiratory action of moving bulbs containing an alkali,
the latter by a steadily acting pressure on a reservoir of oxygen. This
apparatus consists essentially of a glass 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 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.

Of the results obtained by these and other methods a few are given in
the following table:

Oxygen Absorbed. Observer. Carbon Dioxid Discharged.

746 grams. Vierordt. 876 grams.

700 grams. Pettenkofer and Voit. 800 grams.

663 grams. Speck. 770 grams.

The amounts of oxygen absorbed in Pettenkofer and Voit's experiments
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 carbon dioxid 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 surplus hydrogen of the fat with the forma-
tion 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 kilo-
gram) 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 by the carbon
dioxid discharged. But as the carbon dioxid is more easily estimated than
the oxygen, the former 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 func-

RESPIRATION 423

tional 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 living tissues in twenty-four
hours, as given m the following table:

QUANTITY OF O2 AND CO2 ABSORBED AND EXHALED Dl
HOURS, IN CUBIC CENTIMETERS

By 100 Grams of: Absorbed.
Muscle ,.« a „ „

JRING TWENTY-FOUR

Carbon Dioxid
Exhaled.
56.8 c.c.
42.8c.c.
15.6 c.c.
15. 4 c.c.
27.5 c.c.

R T f r-

Brain

Kidneys

Spleen

37 -° c-c-

Testicles

27-3 c-c-

Pounded bones. .

17 .2 r r

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 condi-
tioned^ 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 condition of body repose to one of
marked activity there ought to be an increase in the amount of oxygen
absorbed and CO2 discharged. Pettenkofer and Voit found that a man in
repose who absorbed daily 807.8 grams of oxygen and discharged 930
grams CO2, absorbed during work 1006 grams of oxygen and discharged 1137
grams of CO2. Edward Smith, who estimated only the CO2, found that a
man in repose who discharged carbon dioxid at the rate of 161.6 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 absorp-
tion of oxygen as much as 17 per cent, and the discharge of CO 2 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 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 3-2°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 corresponding amounts for the same period <
time were for oxygen 13.9 grams and for carbon dioxid 13.12 grams. Lavoi-
sier and Sequin, having reference only to the oxygen, found that a man

424 TEXT-BOOK OF PHYSIOLOGY

at a temperature of i5°C. consumed 38.31 grams of oxygen, while at a tem-
perature 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 decreased activity,
coincident with a rise in temperature. The leaver ^temperatures act as a
stimulus to the peripheral terminations of the nerve systeffiy^bdagiAg about
reflexly increased activity of the body at large. , The muscles especially are
not onfy 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 atmos-
phere 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 34°C. there was discharged 325 c.c. per kilo per hour.
Intermediate temperatures were attended by corresponding increases in
the amounts of CO 2 discharged. The reason for this difference in the two
classes of animals is probably to be found in the cold-blooded animals, in
the want, 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.

THE NERVE MECHANISM OF RESPIRATION

The nerve mechanism by which the respiratory muscles are excited to
and coordinated in activity is extremely complex and involves the action of
both afferent and efferent nerves and their related nerve-centers in the
central nerve system. For the free introduction of air into the lungs it is
essential that the nasal and laryngeal passages and the cavity of the thorax
be simultaneously enlarged. The muscles by which these results are ac-
complished have already been mentioned and described. Their simultane-
ous and coordinate contraction implies the coordinate activity of nerve-
centers and their related motor nerves; thus the action of the nasal and
laryngeal muscles (the dilatator naris and the posterior crico-arytenoid)
involves the activity of the facial and inferior laryngeal nerves respectively,
the centers of origin of which lie in the gray matter beneath the floor of
the fourth ventricle; the diaphragm and intercostal muscles involve re-
spectively 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,

RESPIRATION 425

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 inspiratory
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 essential not only 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 accomplished by the recoil of the
elastic tissue of the lungs and thorax, the return of the displaced abdominal
organs aided by atmospheric pressure, 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 rhythmically 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, at the
level of the sensory end-nucleus of the vagus nerve, a region the sudden de-
struction of which on one side is followed by a cessation of respiratory move-
ments on the corresponding side, though they continue 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
"noeud vital" (vital point) 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

* Though its existence has not been anatomically determined beyond
question, it is permissible to speak of the central mechanism as a
located in the medulla oblongata.

The Cause of the Rhythmic Activity of the Inspiratory Center-
It has long been a subject of discussion as to whether the periodic activity

426 TEXT-BOOK OF PHYSIOLOGY

of the inspiratory center is automatic or autochthonic (Gad) in character,
expressive of the idea that the rhythmic discharge of nerve impulses is due
to some stimulating agent generated in the nerve-cells of the center itself, the
activity of which is conditioned by the gaseous condition of the blood;
or whether it is reflex in character, that is, due to the action of nerve im-
pulses transmitted from different regions of the body through afferent nerves.
The solution of this problem has apparently been settled by experiments
the object of which was the division of all afferent nerve-paths that might
have central connections with the center. The results of experiments of
this character are somewhat as follows: When the vagus nerves are divided
the respiratory movements at once diminish in number per minute but at
the same time increase in depth and amplitude. The number of respiratory
movements under these circumstances varies in different animals from four
to eight per minute, a rate which continues practically constant so long as the
animal lives, which may be a period varying from a few days to several
weeks. The relative duration of the respiratory phases also undergoes a
change, inspiration becoming longer than expiration and at the same time
becoming more or less spasmodic in character.

Inasmuch as it is a familiar observation that the normal rate of the respira-
tory movement is frequently disturbed by nerve impulses transmitted to the
center, through afferent nerves other than the vagi, as well as from higher
centers in the brain, section of the vagi has been supplemented by a trans-
verse section of the spinal cord at the level of the first dorsal nerve, by section
of the dorsal roots of the cervical nerves and by a transverse section of the
region of the brain just posterior to the corpora quadrigemina, a series of pro-
cedures which practically isolates the center from all transmitted impulses.
Nevertheless, the inspiratory center still continues to discharge nerve im-
pulses to the respiratory muscles at a rate not differing much from that
witnessed after section of the vagi. At most the diminution in the rate
will not be more than two or three more per minute. The results of these
experimental procedures would seem to indicate that the fundamental rate
of discharge of nerve impulses is approximately from four to six per minute.
This conclusion has been strengthened by the results of experiments designed
to suspend the activity for some minutes by the withdrawal of the blood
by temporarily occluding the blood-vessels passing to the head. With the
resuscitation of the center, after the release of the blood-stream and at a
time when there are reasons for believing that the afferent paths are still
incapable of conduction, the initial rate of discharge was practically constant,
about four per minute in the cat. The same result was observed in some
instances in cats when in addition to producing anemia the vagi as well
as the region posterior to the corpora quadrigemina were divided (Stewart).

It may therefore be assumed that the respiratory center possesses an in-
dependent automatic rhythm which is, however, much slower than that
characteristic of it when all afferent paths leading to it are intact.

Accepting the statement that the fundamental rhythm of the inspiratory
center is automatic — that is, due to a stimulus generated within itself —
the question at once arises as to the nature of the stimulating agent. By
some investigators it has been assumed that the stimulus is connected with
the content or pressure of oxygen, by others with the content or pressure of
carbon dioxid, and that the variations in the respiratory rhythm are depend-

RESPIRATION 42?

ent on variations in the pressure of one or the other of these two gases.
As a result of a long series of experiments made on animals and human
beings, with the respiratory nerve mechanism intact, it is now the generally
accepted opinion that the more efficient cause for the respiratory rhythm
isjmjnCTea^e in the Pressure of carbon dioxid in the blood and hence in the
center itself rather than a decrease in the pressure of the' oxygen. Whether
the~pressure of the carbon dioxid be the efficient cause or not of the funda-
mental respiratory rhythm, there is abundant evidence that the activity or the
irritability of the center is modified to an extraordinary extent by variations
in the pressure of the carbon dioxid when the nerve system is intact. Proofs
in support of this statement will be given in a subsequent paragraph.

Reflex Stimulation of the Inspiratory Center.— Whather the inspira-
tory center is automatic in character or not, it may be influenced directly
by nerve impulses descending from the brain in consequence of volitional
acts or emotional states, and indirectly by nerve impulses brought to it
from the general periphery through various afferent nerves, in consequence of
agencies acting on their peripheral terminations: e.g., cold applied to the skin,
irritating gases to the nasal and bronchial mucous membrane, distention and
collapse of the pulmonary alveoli.

The Relation of the Vagus Nerves to the Inspiratory Center.—
The vagus nerves, of all the afferent nerves, appear to be the most influential
in maintaining the normal rhythmic discharge of nerve impulses from the
inspiratory center, as shown by the effects that follow their separation from
the center. (Fig. 198.) 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. At the same
time the inspirations become deeper and somewhat spasmodic in character.
The duration of the inspiratory movement is also increased beyond that
of the expiratory movement. If now the central end of one of the divided
vagi be stimulated with weak induced electric currents, the respiratory
movements are again 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 seem to indicate that the
vagus nerve contains nerve-fibers which, under physiologic conditions, trans-
mit nerve impulses which inhibit the inspiratory discharge and lead to an
expiratory movement sooner than would otherwise be the case, and thus
maintain the normal rate and extent of the inspiratory discharge.
stimulus to the development of these nerve impulses is generally believed
to be the distension of the air cells at the beginning of the inspiratory
movement.

If however the central end of the divided vagus be stimulated with
induced electric currents of moderate intensity, the opposite effect is pro-
duced, viz.: an increase in the extent of the inspiratory movement ana
decrease in the extent of the expiratory movement until the inspiratory
muscles pass into the tetanic state and the chest walls come to rest
condition of a forced inspiration. This would indicate that the vagus nen
also contains nerve-fibers which, with the proper degree of stimula
capable of so exciting or augmenting the activity of the inspiratory ce
and therefore the extent of the inspiratory movement, as to lead i
dition of tetanus of the inspiratory muscles. If, on the other hand, the

428

TEXT-BOOK OF PHYSIOLOGY

central end of the divided superior laryngeal nerve be stimulated with
induced electric currents, an effect the opposite of the foregoing is produced,
viz.: a decrease in the extent of the inspiratory and an increase in the extent
of the expiratory movement until the inspiratory muscles pass into the
state of relaxation and the chest walls come to rest in the condition of com-
plete expiration. This would indicate that the superior laryngeal nerve
contains nerve-fibers which are capable, when sufficiently stimulated of

/n e ct. ob.

FIG. 198. — DIAGRAM SHOWING THE RELATION OF THE PULMONIC FIBERS OF THE VAGUS TO
THE INSPIRATORY CENTER AND THE CONNECTIONS OF THE LATTER WITH THE PHRENIC AND
INTERCOSTAL NERVE CENTERS AND THEIR RELATED MUSCLES. — (G. Bachmari). med. ob. Medulla
oblongata. sp. c. Spinal cord. p. v. r. Pulmonic vagus nerve, excitator and inhibitor, in sp. c.
Inspiratory center, phr. c. Phrenic nerve-centers, phr. n. Phrenic nerve, int. n. c. Intercostal
nerve-centers, int. c. n. Intercostal nerves, ext. int. c. m. External intercostal muscles.

inhibiting the activity of the inspiratory center, and therefore the extent of
the inspiratory movement, as to lead to the condition of complete relaxation
of the inspiratory muscles with a consequent expiratory standstill.

The same results, viz.: a complete relaxation of the inspiratory muscles
leading to an expiratory standstill, not infrequently follows strong stimulation
of the central ends of divided vagi, and always after the administration
of large doses of chloral.

RESPIRATION

429

The results of these experiments would seem to indicate that the vagus
nerve contains two classes of nerve-fibers, one of which, when stimulated
with weak induced electric current, inhibits and regulates the discharge
of:,nerve energy from the inspiratory center, and thereby the extent and
frequency of the inspiratory movement; the other of which when stimulated
excites or augments the discharge of nerve energy from the inspirator^
center and thereby leads to an increase in the depth or amplitude of the
inspiratory movement. According as the one or the other of these two
classes of fibers are excessively stimulated, will the inspiratory center be
inhibited or augmented in its activity to such an extent that the chest walls

will come to rest in the first
instance in the state of expira-
tory standstill, in the second
instance in the state of inspira-
tory standstill.

The stimulus adequate to
the excitation of the pulmonic
terminations of the vagus
FIG. 199. — POSITIVE VENTILATION (Head), tfnder nerve-fibers in the physiologic
the influence of positive ventilation, the inspiratory
contractions of the diaphragm become less and less
till they disapoear completely.

Diaphragm.
Seconds.

condition was formerly be-
lieved 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 pulmonic 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
simultaneously recorded (Fig. 200), a result similar in all respects to that

Negative
ventilation.

Diaphragm.

Seconds.

FIG. 200.— NEGATIVE VENTILATION. (Head). At a negative ventilation was commenced.
Fhe expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it
inally enters into continued contraction.

broduced by stimulation of the superior larnygeal nerve. On the other hand,
if the lungs are collapsed by the artificial withdrawal of air (negative ventila-
tion) there will be produced an augmentation of the inspiratory and an
Inhibition of the expiratory movements until the inspiratory muscles are in
a, condition of tetanic contraction as indicated by the contraction of the
iiaphragm (Fig. 201) and by the state of the thorax which is that charac-
;eristic of extreme inspiration, a result similar in all respects to that produced
by moderate stimulation of the central end of the divided vagus.

43o TEXT-BOOK OF PHYSIOLOGY

Theories of the Mode of Action of the Respiratory-nerve Mechanism.

— A satisfactory explanation of the action of the respiratory-nerve 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 expira-
tory center is involved with the activity of the inspiratory center either
simultaneously or successively.

The following theories have each found adherents.

1. 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 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 discharge of the inspiratory
center and thus permit of an expiratory movement. TheJnhibitoj^fibers
aresuprjp^edjo_be stimulated

WKhTherecoil of tKe Tungs f
inherent excitation of the 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 expanding lung. This explanation is in accordance with the results
which follow stimulation of the superior laryngeal nerve or the trunk of
the vagus with induced electric currents of slight intensity.

2. If it is assumed, on the contrary, that the inspiratory center is not in a
state of constant excitation leading to a frequent periodic 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 con-
sideration the presence of nerve-fibers in the vagus which increase or aug-
ment 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 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. 198); and just as the inhibitor fibers
are stimulated by the expansion of the lungs, so the excitator fibers are stimu-
lated in turn by the recoil of the lungs. The nerve impulses developed by
the recoil of the walls of the alveoli ascend the vagus nerve to the inspiratory
center, excite it to activity, and thus call forth a new inspiration sooner
than it would otherwise take place. The nerve impulses developed by
the expansion of the walls of the alveoli in turn ascend the vagus nerve to the
inspiratory center and inhibit its activity and thus lead to an expiratory
movement, sooner than it would otherwise take place. According to this
view the respiratory mechanism is self-regulative and maintained by the
alternate expansion and recoil of the lungs.

3. 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 al
the same time as shown by the effects which follow; the inhibitor fibers

RESPIRATION 43i

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 expansion 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 expira-
tion, 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 investigator 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 cooper-
ate 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, de-
veloped during the expansion of the lung, inhibit the inspiratory while aug-
menting the expiratory center, and that impulses developed during the re-
coil of the lungs inhibit the expiratory while stimulating the inspiratory
center.

The Effect of a Change in the Pressure of the Blood Gases on the
Activity of the Inspiratory Center. — It has long been known that the in-
spiratory center is very sensitive to a change in the composition of the blood in
so far as its gaseous constituents are concerned. So long as the composition
remains normal the center retains its normal irritability and rhythm. As
stated in a previous paragraph it has been a subject of discussion as to whether
the center is more responsive to an increase in the pressure of the carbon
iioxid or to a decrease in the pressure of the oxygen. As the outcome of
i long series of experiments it is now the generally accepted opinion that an
ncrease in the percentage and pressure of the carbon dioxid in the blood
ind hence in the center itself is. more efficient in raising the irritability of the
:enter than a decrease in the percentage and pressure of the oxygen. Thus
f an animal is caused to inhale air containing but 2 per cent, of CO2 more
.han normal the respiratory movements will be increased in frequency and
depth, while a 'corresponding diminution in the percentage of oxygen will
DeTwIthout effect.

It has been shown by Haldane and Priestley that when an individual was
breathing normal air and the rate of the respiratory movement, 14 per minute,
•he average depth was 63 7 c.c. and the total ventilation was 8.918 liters per
minute. On raising the percentage of the CO2 in the inspired air from
D.CH percent, to 0.79 per cent, the average depth increased to 739 c.c. and
:he total ventilation to 10.346 liters per minute, the rate remaining the same
When the percentage of the CO2 was raised to 2 per cent, the average depth
ncreased to 864 c.c., the rate to 15, and the total ventilation to 12.960 liters
Der minute; and when the CO2 in the inspired air was raised to 6 per cent
:he average depth was increased to 2104 c.c., the rate to 27 per minute, and
lie total ventilation to 56.808 liters. The results of these experiments
ndicate that an increase in the percentage of the CO in the inspired air
eads to an increase in the percentage and pressure of the GO, in it
erial blood and hence in the inspiratory center, as a result of which the

432 TEXT-BOOK OF PHYSIOLOGY

center becomes more irritable and discharges its energy more frequently
and to a greater degree as shown by the increase in the rate and the depth
of the inspiratory movement.

The same observers have also shown that when an individual is caused
to inhale air the percentage of the oxygen of which had been reduced from
20 to 13 and therefore to about 8 per cent, in the alveolar air instead of about
15 per cent, no particular change in either the frequency or the depth of
the inspiratory movements was noticed, but when the percentage of the
oxygen was lowered below this amount the inspiratory center became more
irritable as shown by an increase in the rate and depth of the inspiratory
movement. As a rule the oxygen percentage in the alveolar air must be
reduced fully one-half and thereby the percentage and pressure of the oxygen
in the arterial blood fully one-third before the respiratory center is stimulated
to increased activity. A reason assigned for this result is the presence in
the blood of some non-oxidized metabolic product, probably lactic acid, that
is acting as the stimulus. All recent experimental work confirms the view
that the specific stimulus to the inspiratory center is the normal pressure of
the CO2 in the blood and so responsive is it to this agent that an increase
in even 0.2 per cent, in the alveolar air is sufficient to almost double the
respiratory ventilation.

The Establishment of Respiration after Birth. — Previous to birth
the exchange of the gases which constitutes the respiratory activity in the
mammalian fetus, takes place in the placenta. The venous blood is carried
by the umbilical artery to this organ in which the blood of the fetus comes
into close relation with the blood of the mother, the two fluids being separated
only by an extremely thin partition. The venous blood while passing
through the placenta yields up its carbon dioxid to and receives oxygen
from the maternal blood; after this exchange of gases the now-arterialized
blood is returned to the fetus by the umbilical veins.

Immediately after birth and the detachment of the placenta, this method
of gaseous exchange is abolished and if the life of the child is to be main-
tained the respiratory movement must be established. The cause of the
first inspiration, therefore, must be associated with an increase in the per-
centage of carbon dioxid or a decrease in the percentage of oxygen in the
blood. In accordance with statements in foregoing paragraphs the former
condition is more likely to be the efficient cause. The rapid accumulation
of carbon dioxid with its increasing pressure in the inspiratory center so
raises its irritability as to lead to a discharge of nerve impulses which are
conducted to the inspiratory muscles and cause their contraction. With
the first inspiration thus established the nerve mechanism described, pages
424, 426 comes into play.

Inasmuch as cold water applied to the skin of the adult profoundly ex-
cites at times the inspiratory center it has been assumed that an additional
factor leading to an excitation of the inspiratory center is the rapid cooling oi
the surface of the child by the evaporation of the amniotic fluid from the
surface of the skin. The nerve impulses thus developed are transmitted
through cutaneous nerves to the inspiratory center. This assumption is
somewhat strengthened by the fact that in delayed inspiration the stimula-
tion of the skin by the application of cold water frequently leads to a sudder
inspiratory movement.

RESPIRATION 433

MODIFICATIONS OF THE RESPIRATORY RHYTHM

The rhythmic 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 respiratory
apparatus itself.

Eupriea. — 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 from the inspiratory center
to the inspiratory muscles, of sufficient energy and frequency for the main-
tenance 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 probably in depth is termed
hyperpnea.

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 inspiratory center, in consequence of which its ir-
ritability 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 absorbing power of hemoglobin, and thus by diminish-
ing the quantity of oxygen absorbed lead to more frequent respirations.
To the hyperpnea induced by heat the term thermo-polypnea is frequently

Muscle activity, especially if it is violent and indulged in by those unac-
customed 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 attrib-
uted to the production and discharge into the blood of metabolic products
which act as stimuli to the respiratory center and thus increase its activity.
Of these metabolic products CO2 is undoubtedly one of the most efficient,
as stated in foregoing paragraphs. Emotional states ^^^~
respiratory activity. With their disappearance the normal condition returns.
Apnea.-Apnea may be defined as a temporary cessation of the respira-
tory movements. It may be developed by rapid and fePinsP^10^.^uue
o volitional efforts, by rapid mechanic inflation of the lungs, and by stimu-
ati7on Tvarious afferent nerves. If one volitionally breathes rapidly and
deeply for a period varying from two to ten minutes, it will be found on

lungs of an animal be rapidly
trachea, a similar condition is

434 TEXT-BOOK OF PHYSIOLOGY

lished by volitional efforts or by mechanic inflation, the respiratory move-
ments gradually return. At first they are feeble but soon increase in
amplitude and frequency until the normal is reached. At one time the
apnea that results from rapid ventilation of the lungs, whether volitional or
mechanical, was attributed, on the assumption that a deficiency of oxygen
in the arterial blood is the physiologic stimulus to the activity of the inspira-
tory center, to an excess of oxygen in the blood, the result of the forced
ventilation, complete saturation of the plasma and the hemoglobin, in
consequence of which the inspiratory center remained inactive. The
apneic state is at present attributed, on the assumption that carbon dioxid
in the arterial blood is the physiologic stimulus to the inspiratory center, to a
diminution in the percentage of the carbon dioxid in the alveolar air (to 4 per
cent, or less), in the blood, and therefore in the center, the result of the forced
ventilation. The increased ventilation eliminates the carbon dioxid to such
an extent that the percentage and pressure in the blood is insufficient to
arouse the center to activity. To the condition of the blood that results
from this rapid ventilation, viz.: a diminished percentage of CO2, the term
acapnia has been given. An apnea which is thus developed is termed apnea
chemica or apnea vera. As previously stated, stimulation of certain afferent
nerves, especially the vagus, will induce a similar cessation of the respiratory
movements. Thus if the central end of the divided vagus be stimulated
with induced electric currents of marked intensity, the thorax will come to
rest in the state characteristic of deep expiration from inhibition of the in-
spiratory center. Inasmuch as stimulation of the vagus causes an apnea
resembling that caused by rapid inflation of the lungs, it has been suggested
that in the development of apnea the inspiratory center is inhibited in its
activity simultaneously with the elimination of the CO 2, from the mechanic
stimulation of the pulmonic terminations of the vagus. An apnea caused
by stimulation of the vagus is termed apnea vagi or apnea inhibitoria.

In the apnea that results from voluntary or mechanic inflation of the
lungs it is difficult to state in how far the condition is due to a diminution
in the pressure of the CO2 and in how far to a stimulation of the vagus.
But inasmuch as apnea can be established, though not of such long duration,
after division of the vagus nerves, the probabilities are that the diminished
percenta^e_of_the_CO2 is the main cause.

^0ys^nea^— -Excessive and laborious respiratory movements constitute
a condition known as dyspnea. Movements of this character indicate that
the bloo4 contains a greater percentage of CO^^j^n^rmal^or a diminished
fin ' etffier case the'irntability of the inspiratory center

is abnormally- heightened. Of the two conditions, the former is by far the
more common. While-iris true that a deficiency of oxygen in the arterial
blood gives rise to an increase in the rate and depth of the inspiratory move-
ments, this does not arise until the deficiency of the oxygen falls to about
one-third of the normal. On the other hand, an increase of even 0.2 per
cent, of CO 2 in the alveolar air will almost double the inspiratory activity.
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 ventila-

RESPIRATION 435

tion. A deficiency in the amount or the quality of the hemoglobin is usually
attended with more or less dyspnea.

Asphyxia.— If the state of the blood observed in dyspnea be exaggerated
—that is, if the increase in the percentage of carbon dioxid become more
marked — the respiratory movements become more laborious. A con-
tinuance 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: (i) 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 asphyxi-
ated condition is as follows: Increased rate and depth of the respiratory
movements, passing rapidly from hyperpnea to dyspnea, with an active con-
traction of all the muscles concerned in respiration, ordinary and extraor-
dinary; 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 ex-
piration, followed by general convulsions; collapse, characterized by un-
consciousness, 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-inhibi-
tory and vaso-motor centers from the accumulation of the carbon dioxid.
With the exhaustion of the nerve-centers, there is a general relaxation of the
skeletal muscles, the cardiac muscle, a fall of the blood-pressure, and dilata-
tion of the pupils.

The Cheyne-Stokes Respiration.— A modification of the respiratory
movements characterized by periods of rest alternating with periods of
activity was described in 1818 and in 1854 by the two writers whose names
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 I
twenty to thirty respiratory movements.

Each period of rest of the respiratory mechanism is closed by the appear-
ance of a slight shallow respiratory movement, which is immediately fc
lowed by a second, slightly deeper, and this in turn by a third, a four
fifth, and so on, each becoming deeper than the preceding until a certain
maximum is reached, after which, each succeeding movement gradua
diminishes in depth until finally the movement becomes imperceptible ai
new period of rest supervenes. A graphic representation of the C
Stokes type of respiration is shown in Fig. 201 This type of respiration «
frequently an accompaniment of certain pathologic conditions, eg., uremic
states, cerebral hemorrhage, heart diseases, arteriosclerosis, etc, though no

436 TEXT-BOOK OF PHYSIOLOGY

satisfactory explanation of it has yet been presented. A similar though far
less marked periodicity in the respiratory movements is frequently observed
during 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.

FIG. 201. — TRACING SHOWING THE CHEYNE-STOKES FORM OF RESPIRATION. — (Hill.}

THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF

BLOOD THROUGH THE INTR A -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 a similar way
cause the blood of the extra-thoracic veins to flow into, through, and
out of the intra- thoracic vessels. From the tendency of the pulmonic
elastic tissue to recoil, the blood-vessels in the thorax at the end of an expira-
tion sustain a positive pressure, the intra-thoracic pressure (see page 305),
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 under atmospheric pressure will flow
steadily toward the intra-thoracic veins, the venae cavae, and the right
side of the heart, i.e., from a point of high to a point of low pressure. During
inspiration there is a decrease in the intra-thoracic pressure, the decrease
being proportional to the extent of the inspiration. With this decrease
of pressure, the intra-thoracic veins expand and their internal pressure falls.
As the systemic or extra-thoracic veins are subjected to atmospheric pressure,
the blood in these vessels is forced, by reason of the difference of pressure
between these two regions, to flow more rapidly and freely into the intra-
thoracic veins and right side of the heart. The right heart being more
generously 'filled with blood will discharge a larger volume with each con-
traction into the pulmonic artery.

Coincident with these effects a similar effect is produced in the arterioles
and capillaries of the pulmonic 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 the intra-pulmonic pressure; and at the
end of an inspiration it will be considerably less. With the inspiration there-
fore there will occur a dilatation of these vessels, and hence a larger flow of
blood through them and into the pulmonic veins. The left heart, being

RESPIRATION 437

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 undulations 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 fesults~1i^rea3fl}ra?^^ by the cGBKence 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
expiration; for a certain period after the beginning of an inspiration the
pressure continues to fall, and for a certain period after the beginning 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 begin-
ning of the inspiration, when the vessels dilate, the blood-flow momentarily
slows; the left heart continuing to discharge small volumes into the aorta,
the pressure continues 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
I 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
1 volume of blood transmitted by them becomes less, the output of the left heart

declines, and the pressure falls.

3. On the Flow of Lymph. — The fall of the intrathoracic pressure
during an inspiratory movement has an accelerating effect on the flow of
lymph from the abdominal portion of the thoracic duct into the thoracic
portion. The rise of the pressure during an expiratory movement causes a
more abundant discharge of lymph from the end of the duct, into the venous
blood. The advantages of this mechanism have been alluded to on page 225.

CHAPTER XVI
ANIMAL HEAT

The animal body possesses a temperature that is perceptible to the sense
of touch and determinable by a thermometer. This temperature is the
result of the liberation of heat which attends the chemic changes taking
place in the tissues and organs of the living body and which underlie all
manifestations of life. In consequence of this each animal 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 main-
tain within limits a constant temperature which is independent of and gen-
erally above that of the surrounding atmosphere. As the temperature of
these animals is high and perceptible to the sense of touch, they were origi-
nally designated "warm-blooded" animals. As this temperature is con-
stant notwithstanding the great variations in external temperature during
the summer and winter seasons, they are more appropriately termed con-
stant-temperatured or homoio-thermous 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 4O°C. The
causes of this variation are doubtless connected with peculiarities of organi-
zation. In birds the rectal temperature is usually higher, varying from
4o.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 suffi-
ciently active heat-regulating mechanism, the temperature 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 8.9°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 elevation of the temperature restores their vitality. In
summer-time, on the contrary, the body-temperature may attain to 38°C.
Similar variations 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 temperature is
inconstant, varying with the temperature of the surrounding medium, they
are more appropriately termed "variable-temperatured" or poikilo-ther-
mous animals.

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

ANIMAL HEAT 439

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. Afr a result of many observa-
tions it has been found that the temperature of the rectum is, on
the average, 37.2°C.; that the mouth, 36.8°C.; that of the axilla, 36.9°C.
Owing to radiation and conduction, the surface temperature 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 2O°C.
the skin of the pectoral region has a temperature of 34.7°; that of the
cheek, 34.4°; 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 muscles, the brain, alimentary canal,
etc., it is evident that these organs have a higher temperature than the
rectum.

perature is practically uniform. — (After Bernard.)

As the chemic changes underlying physiologic activity vary in intensity
and extent in different regions of the body, there would be marked vana
tions in their temperature were it not that the blood having a
capacity for heat-absorption, distributes the heat almost ™*<™g**
portions of the body, so that at a short distance beneath the surface

44o TEXT-BOOK OF PHYSIOLOGY

Variations in the Mean Temperature. — The mean temperature of
the human body for twenty-four hours, which for the mouth and the rectum
may be accepted at 36.8°C. and 37.2°C. respectively, is subject to variations
from a variety of circumstances, such as age, periods of the day, food,
exercise, etc.

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 tem-
perature 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 o.5°C. to i.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 processess, 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 tempera-
ture but slightly — not more than o.5°C. Insufficiency of food lowers
the temperature; total withdrawal of food, as in starvation, 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-regulating mechanism. Alcoholic drinks
lower the temperature about o.5°C. In large toxic doses in persons un-
accustomed 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,
dilatation 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 i.5°C. as a result of increased activity in chemic changes in the
muscles themselves. A rise beyond this point is prevented by the increased
activity of the circulatory apparatus, the removal 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 temperature of the air,
the temperature of the body remains almost constant. The same is true for
the seasonal variations in the temperature of the temperate regions.

The Residual Heat of the Body. — As a preliminary to a consideration
of heat-production and heat-dissipation, it is of interest to determine the
actual quantity of heat expressed in Calories, that resides in the body at all
times. This can be approximately determined from the chemic composi-
tion and the temperature. A chemic analysis of the body shows that it con-
sists of water 0.6, and of tissue 0.4. If the weight be assumed to be 70
kilograms then 42 kilograms consist of water, and as the temperature is
37°C, the 42 kilos of water will contain 42 X 37 or 1554 kilogram calories;

ANIMAL HEAT 441

the remaining 28 kilograms consist of tissues, the specific heat of which is
but 0.8 that of water, hence the 28 kilograms of tissue will contain 28 X 0.8
Calories, the equivalent of 22.4 kilograms of water. Since the temperature
of the body is 37°C. the additional number of Calories will be 22.4 X*37
or 828, making a total of 2382 Calories an amount of heat absolutely necessary
to maintain the body-temperature at the physiological level. Notwith-
standing the constant liberation of large amounts of heat each day, it is
dissipated more or less rapidly in accordance with variations in temperature,
character of clothing and a variety of other conditions, and so accurately
is this done, that at the end of the twenty-four hours the body possesses
its customary quantity of heat and its physiologic temperature.

HEAT-PRODUCTION. THERMOGENESIS

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 secretion, each
exhibition of nerve-force, is accompanied by the evolution of heat. The
chemic changes are for the most part of the nature of oxidations, the union
of oxygen with the elements, carbon and hydrogen, 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: (i) By deter-
mining experimentally the heat values of different food principles by direct
oxidation; (2) by collecting and measuring with a suitable apparatus, a cal-
orimeter, the heat evolved by the oxidation of the food within, and dissipate

fr°™' ^Direct Oxidation.— The amount of heat which any given food prin-
ciple will yield can be determined by burning a definite amount-**., i
Jam-to carbon dioxid and water and ascertaining the extent to which
the heat thus liberated will raise the temperature of a given amount c
e z i kilogram The amount of heat may be expressed in gram or
'

am drees or calories; a gram calorie or kilogram ^**g£
of heat required to raise the temperature of a gram or a kilogram

grams) of

The aaratus employed for this purpose B termed

442 TEXT-BOOK OF PHYSIOLOGY

physical values, and indicate the quantity of heat such quantities of foods
give rise to when completely oxidized to carbonic acid and water. In the
animal 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 practically liberate as much heat as they do when oxidized outside
the body. The proteins, 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 protein.
According to Rubner, i gram of urea will yield 2.523 kilogram calories.
As about one-third of a gram of urea results from the oxidation of i gram
of protein, the amount of heat to be deducted from the heat value of the
protein is J of 2.523, or 0.841 Calories. It has also been shown by the same
investigator that some of the ingested protein is found in the feces, the heat
value of which must also be determined and deducted. This having been
done, the physiologic heat value of protein thus becomes 4.124 Calories.

The following estimates give approximately the number of kilogram
calories which should be liberated within the body when the protein is burned
to the stage of urea, and the fat and carbohydrate to the stage of carbon dioxid
and water:

i gram of protein 4.124 Calories

i gram of fat 9-353 Calories

i gram of carbohydrate 4.116 Calories

The total number of kilogram calories yielded by the various diet scales
can be readily determined by multiplying the quantities of the food prin-
ciples consumed by the foregoing factors.

2. Calorimetric Measurements. — It has been determined from a long
series of experiments that the animal body dissipates a variable amount of
heat from day to day, an amount that can be collected and measured by
placing the animal in a suitable apparatus — a calorimeter. A calorimeter
is therefore an apparatus designed for the direct determination of the quan-
tity of heat dissipated from the body in any given time. The amount
obtained by this method expressed in Calories is taken as a measure of the
heat liberated by the oxidation of the food, providing of course, the tempera-
ture of the animal remains unchanged. The substance employed for col-
lecting and measuring the heat is usually water. 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 defi-
nite amount' of water (Fig. 203). The animal is placed in the inner box,
which is also provided with tubes for the entrance of fresh and the exit
of expired or vitiated air. The heat radiated is absorbed by the water and
its temperature raised. To prevent loss by radiation and to render it in-
dependent of changes in the surrounding temperature the calorimeter is
surrounded by a poorly conducting material, such as wool. The tempera-
ture 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 experi-
ment, then the heat absorbed by the water represents the amount produced
by the animal. If, on the contrary, the temperature of the animal rises
or falls, the number of calories so retained or lost must be added to or sub-
tracted from the amount absorbed by the calorimeter. In the determination

ANIMAL HEAT

443

)f the absolute amount of heat retained or lost by the animal above or
Delow the initial temperature, as well as that absorbed by the materials of
;he apparatus in these various instances, the water equivalent of the tissues of
;he animal and the materials of the calorimeter must be obtained, and then
idded to or subtracted from, as the case may be, the amount of water in the
:alorimeter , and the amount thus obtained multiplied by its rise in temperature.

When means are taken to collect for purposes of analysis, the urine and
;he feces and also the expired air, the apparatus is designated a respira-
ion calorimeter. The nitrogen in the urine and feces and the carbon dioxid
n the expired air permit of the determination of the amounts of protein
and fat metabolized. The ventilation of the chamber is accomplished by
some form of aspirating
apparatus.

In properly conducted
experiments in which the
sources of error are reduced
to a minimum there is a
very close correspondence
between the total physio-
logic heat value of the food
and the amount collected
by the calorimeter. Thus,
in an experiment detailed
by Rubner, a dog was
given during twelve days
228.06 grams of protein and
340.4 grams of fat, the
physical heat value of which
was estimated at 4429 Cal-

FIG. 203.— WATER CALORIMETER or 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

im - for the purpose of distributing the absorbed heat uni-

• , Ti/forpc formly. To prevent the escape of heat with the expired

The urine and feces *P gj tube g, is wound many times in the water-space

beneath the animal cage.

ones.

during this period were col-

determined, which amounted to 305 Calories. The heat which theoretic-
ally therefore, should have been produced was 4"4 Calories.
SerLnt the calorimeter actually absorbed 395 ! Calorie, ; a j difference
between the theoretic and experimental results of ^Calo

to hold true Inasmuch as the calorimeter of Benedict differs in some re-

444

TEXT-BOOK OF PHYSIOLOGY

These calorimeters consist of two parts, one for the collection of the
heat liberated by the subject of the experiment, and the other for the col-
lection of the carbon dioxid and water exhaled and for the measurement
of the oxygen consumed. Means are also provided for the collection of the
urine and feces when necessary.

The first part of the calorimeter consists, Fig. 204, of a chamber of sufficient
size with metallic walls, copper and zinc so arranged as to prevent either
absorption or radiation of heat, in other words that they will be heat-
proof. At one side there is a door of sufficient size to admit the subject
as well as a window to admit light. The heat dissipated by the subject of
the investigation is taken up by a current of water of even but low tem-
perature and flowing constantly under a steadily acting pressure through a
copper pipe of 6 mm. internal and 10 mm. external diameter. The tem-
perature of the water is determined before it enters and after it leaves the

O+ Soda Lime HXSO+
FIG. 204. — DIAGRAM TO SHOW THE PRINCIPLE OF THE AT WATER-BENEDICT CALORIMETER

chamber. The amount of water passing through the tube in a given time,
is collected and weighed. The difference in temperature, multiplied by
the weight of the water gives the amount of heat dissipated by the individual.
To facilitate the absorption of the heat a large number of copper discs 5
centimeters in diameter are soldered to the water pipe at a distance of 5
millimeters from one another. As the absorption pipe is some 5 meters
in length, the area presented for absorption approximates about 4.7 square
meters. The heat thus absorbed is soon conducted to the water followed by a
rise in its temperature. Inasmuch as many experiments extend over a
period of several days or more, special arrangements have been devised for
the introduction of food which do not interfere with the accurate working
of the calorimeter. The door of the food aperture is provided with glass
for the transmission of light into the chamber. In some calorimeters a
bed is provided, in others a chair as well as devices which enable the subject
to perform a measurable quantity of mechanical work.

ANIMAL HEAT

445

The second part of the calorimeter consists of a closed system of tubes
tnd absorption vessels through which the air is kept moving under the action
)f a blower and thus kept in a respirable condition. As the air leaves the
:hambeMt passes through two absorption vessels by which the water and
:arbon dioxid are successively absorbed and collected by sulphuric acid and
>oda^lime respectively. The air then passes through an additional sul-
phuric-acid vessel which absorbs any water carried from the soda-lime vessel
Dy the air and so back into the chamber. By weighing these absorbing
vessels before and after the experiment the amount of water and carbon
iioxid are readily determined. The oxygen of the air of the chamber that
s utilized for respiration purposes is restored by the admission of oxygen
:rom a cylinder in necessary amounts by special automatic devices. The
imount of oxygen entering the chamber is determined by weighing the
:ylinder before and after the experiment. The loss in weight shows the
oxygen consumed. Of course, the air of the chamber must be analyzed
o correct certain errors that may piossibly arise.

With the calorimeters described in the foregoing paragraphs it is pos-
sible not only to determine directly, but also indirectly, from the amounts
protein, fat, and carbohydrate metabolized (calculated from the C,H,O,
ind N eliminated and O absorbed) the heat produced and dissipated each
iay under a great variety of conditions e.g., when the subject is fasting or
iving on the customary diet; when resting or doing a fair days work; when
in health or in disease, etc.

i. During Fasting. — In an experiment extending over a fasting period
of seven days' duration, recorded at length by Benedict, the heat was
determined directly, and also indirectly, from the materials metabolized.
Some of the results of this experiment are shown in the following tables:

METABOLISM OF S. A. B. DURING A SEVEN-DAY FAST

Day

Calories

Grams

R.Q.

Urine

Directly
Deter-
mined

Calcu-
lated
from
Metab.

Per
Kg-

Per Sq.
M.

Pro-
tein

Fat

Glyco-
gen

Ratio

N:S.

Ratio
N:P2Oi

1765
1768
1797

1775
1649

1553
1568

1796
1790
I78S
1734
1636

1547
1546

29.7
29.9
30.8
30.8
29.0

27-5
28.0

941
946
969
966

985
856
869

73-4
74-7
78.1
69.8
65.2
64.4
60.8

126.4
147-5
i53-o
144.7
144-7
129.8

132-5

64.9
23.1

5-4
25.2

8.2

21.7

18.7

-78

•75
•74
•75
•74
•75
•74

19.6
18.6

I7-38
i6.n
16.26
16.27
16.28

8-55
5-55
6.34
4-83
5-23
5-iQ
4-S;

•2.

4
6

Many similar experiments have been made by Benedict and by o her* witi
different forms of apparatus. As an average result it may be state
in the fasting condition and doing light work there is an average , heat-.
tion of 3t or 32 calories for each kilogram of body-weigh , thus making a

the food

—ary diet, and '
condition, the amount of heat produced and liberated

nee,

446 TEXT-BOOK OF PHYSIOLOGY

creased. Under these circumstances there will be an increase of from 10 to
14 per cent, thus raising the heat-production and dissipation per kilogram of
body- weight from approximately 32 Calories to 35.5 or 36.5 or a total of
2485 to 2555 Calories. The heat liberated by the metabolism during inan-
ition is not sufficient to maintain the body in heat equilibrium and there-
fore, an increase in the food supply of from 10 to 14 per cent, beyond that
metabolized is necessitated. A reason assigned for this necessity is as
follows: During digestion and in the earlier stages of assimilation, the foods
undergo a reduction and cleavage, in consequence of which a portion of their
contained energy is liberated as free heat which however cannot be utilized by
tissue cells in the performance of their various activities. This heat is
probably directly dissipated in an environment of 33°C. The energy that
cells require for the manifestations of their physiologic activities must
apparently be derived only from the metabolism of food materials within
themselves. Therefore, food materials in amount just sufficient to replace
the amount metabolized in inanition, would furnish an amount of heat
which would be insufficient to maintain the body in heat equilibrium.
Though this holds true for a man weighing 70 kilograms it is not strictly
true for men who weigh less or have a different skin area. It has been
pointed out by Rubner that the number of Calories dissipated per kilogram
of body- weight varies with the weight and the skin area : that the metabolism
is proportional to the surface area or, in other words, whatever the weight,
there will be for a given surface area, a certain heat loss leading in turn to a
given metabolism of food materials. In the following table the weights of
different men, their skin areas, total heat-production as well as Calories per
kilogram while doing light work are presented. (Rubner.)

Weight Area in Calories of Calories

in Kg. Sq. M. Metabolism. per Kg.

80 2.283 2864 35-8

70 2.088 2631 37.7

60 1.885 2368 39.5

50 1.670 2102 42.O

40 1.438 1810 45.2

In the foregoing table it will be observed that in the man weighing 80
kilograms, the ratio of skin area to kilogram is 0.0285, while in the man
weighing 40 kilograms the ratio is 0.0359. The heat liberated by this slight
increase in skin area amounts to 9.4 calories per kilogram weight.

3. During Work. — The foregoing estimates as to the amounts of heat
produced have reference mainly 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 other-
wise 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 aver-
age 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

ANIMAL HEAT 447

(when engaged in active muscle movements produced 271.2 calories and
bsorbed 119.84 grams of oxygen per hour. The increase in heat-produc-
tion per hour during activity was thus almost doubled, though the sum total
produced 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
here is a greatly diminished heat-production, not more than 40 calories per
hour being produced. The preceding data may be tabulated as follows
Him) :

Day of Rest. Day of Work.

Heat units (Calories) pro- } Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs.
duced / 2470.4 320 1235.2 2169.6 320

2790.4 3724-8

Increased muscle activity is associated with increased metabolism and
k more rapid dissipation of heat, which must be compensated for by a cor-
responding heat-production if heat equilibrium is to be maintained. This
necessitates a larger amount of food ingested. From Calorimetric inves-
tigations, as well as from a calculation based on the amounts of the excreted
products, various diet scales have been arranged by different investigators,
which it is believed will furnish the extra amount of energy. A few are
here appended.

DIETARIES FOR A MAN OF 70 KILOS, DURING HARD WORK
Voit. Rubner. Playfair. Hultgren.

Protein 145 165 155 134 grams

Fat 100 70 70 79 grams

Carbohydrate 500 565 567 522 grams

3574 3362 36l9 3436 Calories

The Mechanism of Heat-production.— Heat is liberated to a greater
or less degree in all tissues in the cells of which food materials^ are under-
going disruption and oxidation or metabolism. The tissues in which metabo-
lism and therefore heat liberation takes place most energetically, e.g., muscles
and glands are more especially to be regarded as thermogenic tissues. ^

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.

It will be recalled that all muscles possess tonicity by which is meant a
slight degree of contraction, the result of the continuous arrival of nerve
impulses through efferent nerves discharged from motor nerve-cells in the
spinal cord this discharge being maintained largely by nerve impulses com-
ing through afferent nerves from the muscles themselves, the joints, tendons,
and skin. As a result of this slight but constant stimulation of the spinal
cord, the metabolic changes in muscle material are maintained at a certain
level, with a corresponding liberation of heat. The chief result of the tonicity
would thus be the production of heat. Any physiologic condition that
leads to a greater discharge of nerve impulses from the spinal cord and
hence increased muscle activity, must be attended by increased heat-produc-
tion. Therefore work and exercise of all kinds which involve a more rapid
contraction of the skeletal muscles is attended with increased heat-produc-
tion. By reason of their mass and more or less continuous activity the muscles
are justly regarded as the chief thermogenic organs.

448 TEXT-BOOK OF PHYSIOLOGY

The consumption of foods that have a higher potential heat value also
contribute to the amount of heat produced. Foods have different physio-
logic heat values. If the food consumed contains much potential energy and
quantity consumed be larger than the average daily requirements, there will
be an increase in heat-production. (See page 442.)

The chief external factor that increases metabolism in these and other
organs and tissues is a low external temperature. A fall of the external
temperature, such as is experienced in the fall and winter seasons, causes,
through cutaneous afferent nerve stimulation, a stimulation of the spinal
motor centers, and to a larger discharge of nerve impulses to muscles, glands,
and other tissues. The. increased metabolism thus developed leads to the
consumption of increased amount of food and oxygen. As a result there is
an increased heat-production. If at the same time volitional activities of
muscles be evoked, as is not infrequently the case, there will be a still further
increase in metabolism and heat liberation. When all these conditions,
increased muscle activity, increased amount of food with high potential
energy, and a low external temperature coexist, heat-production attains its
maximum, amounting to as much as 4726 Calories daily (Hultgren).

HEAT-DISSIPATION. THERMOLYSIS

•</-..

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 main-
tenance of the body-temperature. Should this heat be retained, the tem-
perature of the body would be raised at the end of twenty-four hours, an
additional 18° or 2o°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 37°C. it is essential that the heat liberated
be dissipated 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 dis-
sipation of the heat is accomplished in several ways: (i) In warming the
food and drink to the temperature of the body. (2) In warming 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 con-
duction from the skin. The quantities of heat lost to the body by these
different processes it is difficult for obvious reasons to accurately determine,
and the estimates usually given must be regarded only as approximative.

The number of Calories which are theoretically liberated by the various
diet scales can be readily determined by multiplying the quantities of food
principles consumed by the usual factors (see page 442). Thus the total
number of Calories furnished by the various diet scales would be as follows:
Voit's, 3387; Vierordt's, 2695; Ranke's, 2335; Moleschott's, 2984; Atwater's,
3331; Hultgren's, 3436. As the body- weight may not increase and as the
temperature in physiological conditions does not rise, the assumption is
that the food is oxidized in the body to urea, carbon dioxid and water with
the liberation of the foregoing amounts of heat and their equally rapid
dissipation.

Assuming 2500 Calories to be an average of heat liberated during a day
of repose, the losses, in the ways stated in the foregoing paragraph, may be
tabulated as follows:

ANIMAL HEAT 449

1. In Warming Food and Drink. — The average temperature of food and

drink is about i2°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 3X0.8
X25°C. = 6o 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 i2°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 at-
tains the temperature of the body will therefore amount to 15 Xo. 26 X 25°
= 97.5 Calories = 3. 8 per cent. The expired air removes from the
body a corresponding amount.

3. In the Evaporation of 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.

4. In the Evaporation of Water from the Skin.— The quantity of water evapor-

ated from the skin may be estimated at 660 grams, causing a loss of
heat by this channel of 660X0.582 = 384.1 Calories = i5.3 per cent,
c 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, 25oo-77744 = i725-6 Calories = 69 per cent,
would represent the average amount lost by radiation and conduction.
The Mechanism of Heat-dissipation.— As stated in foregoing para-
graphs heat-dissipation is accomplished mainly by radiation and conduction
from the skin, 70 per cent., and by evaporation of water from the skin and
lungs 25 per cent. The heat dissipated in warming food, drink and air
inspired, or what amounts to the same thing in raising the excretions, urinary,
fecal and respiratory to the body-temperature, may be here neglected.
The relative amounts dissipated by these two routes, radiation and evapora-
tion of water, will depend largely on the external temperature in so far as
it is not interfered with by clothing and artificial temperatures. ^

The mechanism by which the dissipation is accomplished consists of
the cutaneous and respiratory blood-vessels and the sweat-glands together
with the heart and respiratory apparatus, which collectively therefore may
be regarded as thermolytic organs, all of which are made to cooperate by
the intermediation of the nerve system, especially the vaso-motor and
secretory portions of it. With a given external temperature such as charac-
terizes the spring months, there is a certain ratio between the percentage of
heat lost bymdiation and by water evaporation. As the temperature rises
as it does during the summer months, the cutaneous vessels dikte as a
result of a reflex inhibition of the vaso-motor center due to the stimula mg
action of the heat on the cutaneous nerve endings, which brings to the surface
a larger volume of blood, a condition favorable to increased radiation This,
however, is to some extent prevented by reason of a diminution of he differ,
ence in temperature between that of the atmosphere and that of the body.
The sTat-glands at the same time are stimulated to increased activ-

45o TEXT-BOOK OF PHYSIOLOGY

Ity, and in consequence of the additional volumes of blood brought to
the skin a larger amount of sweat is secreted, which speedily undergoes
evaporation. As each gram of water for its evaporation requires 0.582
of a calorie, it is evident that increased secretion of sweat favors heat-dissipa-
tion. 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 evapora-
tion of water from the lungs and thus occasion a greater loss of heat.

When the external temperature falls as, it does in the autumn and winter
months, there is a decrease in the physiologic 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 due to increased activity of the general
vaso-motor center. The blood, being prevented from coming to the surface,
is retained in the deeper portion of the body, and in consequence the con-
ditions for radiation are diminished. Nevertheless by reason of the in-
crease in the difference of temperature between that of the air and that of the
body, radiation takes place to a greater extent than during the summer
months. These variations in the cutaneous circulation in response to varia-
tions in the external temperature 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 tem-
perature 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 quali-
ties, and hence are adapted for warm weather. Radiation from the skin is
somewhat interfered with by subcutaneous fat, the extent of the interference
being dependent on its amount.

Inasmuch as the mean temperature of the body remains practically
constant, notwithstanding seasonal variations, it is apparent that heat-dis-
sipation must be exactly balanced by heat-production. 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 muscle, vascular, and
secretor elements, coordinated by the nerve system.

The Influence of the Nerve System. — Even though it is admitted that
the heat liberated is the result of the metabolizing power of muscles
and glands, and other tissues as well, and even though it is further admitted
that the heat liberated is increased when the tissues (muscles) are excited to
action under the stimulating influence of the efferent nerve system, never-
theless the question has been raised whether in addition, there are not special
nerves and nerve-centers, the specific function of which is to stimulate

ANIMAL HEAT 451

metabolism at all times and thus provide for a continuous production of heat,
the amount of which from day to day, varying with the extent of the losses.
In this case the heat liberated by muscles at the time of their contraction
would be incidental and not to be regarded as a necessary part of the fun-
damental heat-producing mechanism.

This view is apparently supported by the increase in heat-production
that follows injuries or lesions of certain portions of the nerve system. Thus
injury to the caudate nucleus or to the tuber cinereum, in an animal is promptly
followed by a rise in the body-temperature of from four to about seven de-
grees; while lesions of the cortex of the animal in the neighborhood of the
cruciate sulcus and at the junction of the supra-sylvian and post-sylvian
fissures is promptly followed by a fall in the body-temperature. These and
many other facts have led to the formation of a theory as to the manner in
which heat-production is excited and regulated by the nerve system in ac-
cordance with variations in external temperature. In this theory it is
assumed that, throughout the anterior horns of the gray matter of the spinal
cord there are nerve-cells that give origin to nerve-fibers that pass out in the
ventral roots of the spinal nerves, to be distributed in company with them
to muscles at least, and excite heat-production even though the muscles are
not in active contraction. These centers and nerves are termed, respectively,
thermogenic centers and ihermogenic nerves. The thermogenic centers it is
further assumed, though continuously active, are not influenced directly by
nerve impulses transmitted to them from the skin in consequence of changes
in the external temperature nor by changes in the temperature of the blood,
but are influenced by nerve impulses descending the nerve system from
higher levels.

Experimental investigations have apparently demonstrated that an injury
or stimulation of the caudate nucleus or of the tuber cinereum is very
promptly followed by an increase in heat-production, which leads to the
inference that there are in these regions groups of nerve-cells which when
stimulated increase heat-production. To this group of cells the term thermo-
accelerator has been given.

Experimental investigations have also apparently demonstrated the fact
that injuries or stimulation of the cerebral cortex in the neighborhood of the
cruciate sulcus and at the junction of the supra-sylvian and post-sylvian
fissures (dog) is promptly followed by a decrease in heat-production which
leads to the inference that there are in these regins groups of nerve-cells stimu-
lation of which decreases heat-production. To this group of cells the term
thermo-inhibitor has been given. Both these centers are in direct connec-
tion by nerve-fibers with the thermogenic centers in the spinal cord and
through which they alternately accelerate or inhibit their activity.

The thermo-accelerator and thermo-inhibitor centers it is assumed are
both connected with the cutaneous surface through the intermediation of
afferent nerves and, therefore, in a position to be influenced by changes ^in
the external temperature. Evidence is also at hand that they may be in-
fluenced by changes in the temperature of the blood passing to and around
them. A fall of the external temperature develops nerve impulses which
transmitted to the thermo-accelerator center excite it to action and thus
increase heat-production; a rise in external temperature, on the other hand,
has an opposite effect by stimulation of the inhibitor center.

452 TEXT-BOOK OF PHYSIOLOGY

The regulation and maintenance of the mean temperature or thermotaxis
is, therefore, the result of an adjustment between heat-production and heat-
dissipation, both of which are variable factors, and is accomplished by a
complex, self-regulating mechanism consisting of muscle, vascular and secretor
elements coordinated by the nerve system. Given this mechanism and the
variations in the external temperature the metabolizing power of the living
material is caused to vary in one direction or another as the external tem-
perature rises and falls.

CHAPTER XVII
EXCRETION

Excretion may be defined as the process by which the end-products of
tissue metabolism are removed from the body, the nature of the process, how-
ever, differing in no essential particulars from that underlying the process
of secretion as stated in preceding chapters. 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 ingredi-
ents 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, with the exception of those given
off in the lungs, complex fluids in which are to be found in varying propor-
tions the chief end-products of metabolism.

THE URINE

The urine is a fluid formed by the activities of the kidneys and character-
ized by well-marked physical properties and a complex chemical composi-
tion. After its formation in the kidneys it .passes through the ureters into
the bladder, where it is temporarily retained before being discharged

the body.

Physical Properties.— 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 dimrmshe
from the presence of mucus, calcium and magnesium phosphates, am

e color, which varies within physiologic limits from a pale yellow to a
reddish-brown, is due to the presence of the coloring-matters, urochro me,
uroWin, and uroerythrin, all of which are derivatives of the bile pigments ab
sorbed from the liver or the alimentary canal.

The odor of the urine is characteristic and due to the presence c
organic compounds. . ,

The reaclwn of the urine is generally acid to litmus. The reaction has
been for a long time attributed to the presence of sodium dihydrpgen ph<
phate, though it is more probably due to the presence of various acid radicals
The intensity of the acidity will depend on the extent to which any particular
acid is dissociated with the liberation of the hydrogen ions as it is the 1
factor that imparts acidity to the fluid. 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, especially if the food

454 TEXT-BOOK OF PHYSIOLOGY

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 excreted in the urine. The phosphoric acid
which enters into combination 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 frequently passed from
a variety of causes.

Chemic Composition. — The chemic composition of the urine is very
complex and is determined partly by the metabolism of the constituents of
the tissues and partly by the quantity and the quality of the food consumed
and metabolized. Hence the composition will vary from day to day in
accordance with the character of the food. An average composition is
presented in the following table:

THE CHEMIC COMPOSITION OF URINE

Water 1500.00 c.c.

Total solids 72 .00 grams.

Urea 33 . 18 grams.

Uric acid (urates) 0.55 grams.

Hippuric acid (hippurates) 0.40 grams.

Kreatinin, xanthin, hypoxanthin, guanin, ammonium \ arum*

salts, pigment, etc J

Inorganic salts; sodium and potassium sulphates, phos-
phates, and chlorids; magnesium and calcium phos-

phates.

27.00 grams.

Organic salts: lactates, acetates, formates in small

amounts

Sugar a trace

Gases, nitrogen, and carbonic acid.

The Total Solids. — It is frequently a matter of clinic interest to deter-
mine the total amount of the solid constituents excreted in twenty-four hours.
This may be attained approximately by multiplying the last two figures of
the specific gravity by the coefficient 2.33 of Haeser or Christison. The
coefficient of Jones, 2.6, is believed by some observers to give more accurate
results for conditions existing in this country. 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 daily.

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 urinarv solids.

EXCRETION 455

Urea. — 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
CON2H4. When subjected to prolonged boiling, it combines with water,
giving rise to ammonium 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 + 2H2O = (NH4)2CO3.

The amount of urea excreted each day varies from 30 to 40 grams, the
average being about 34 grams and therefore represents an amount of
protein metabolized equivalent to from 90 to 120 grams or an average
of about 100 grams. The remaining nitrogen-holding compounds in the
urine represent as shown by their nitrogen content a protein metabolism
of about 1 2 grams. As to how much of the urea or of the total nitrogen is
derived from the metabolism of tissue protein and how much from the
metabolism of the food protein that is not elaborated into tissue protein,
is difficult to state. It has been observed however in human beings in the
fasting condition that for a period of 10 days, there is a daily excretion
of about 21 grams of urea equivalent to about 63 grams of protein metabo-
lized. If it be accepted that approximately 63 grams of tissue protein are
metabolized each day then of the 34 grams of urea excreted, 13 grams must
come from about 40 grams of metabolized food protein. That the urea that
comes from the tissue protein is a rather constant factor and that the urea
that comes from the food protein is a variable factor is shown by the fact that
the amount of urea excreted rises and falls proportionately to the protein con-
sumed. As to the particular tissues that are undergoing protein metabolism
there is much obscurity. Contrary to what might be expected there is ap-
parently but little protein metabolism in muscle tissue for there is no parallel
ism between urea production and muscle work. Even after severe labor
extending over a period of some hours there is no noticeable increase in the
urea excreted.

Seat of Formation and Antecedents of Urea. — It has been stated in a
foregoing paragraph that the excretory organs are engaged in the process of
eliminating from the blood, rather than in elaborating, the end-products of
metabolism. Therefore the supposition is that the kidneys are not the seat
of urea formation but only the means by which it is eliminated from the
blood. This supposition is rendered highly probable from the following
facts: the blood of the renal artery contains from one-third to one-half
more urea than the blood of the renal vein; ligation of the renal arteries
or removal of the kidneys leads to an accumulation of urea in the blood to an
extent four times the normal amount in 24 hours; perfusion of the excised
kidney, which still retains its physiologic activity, with blood containing
known antecedents of urea is unattended with urea formation. These and
other facts of a similar character confirm the view that the kidney does not
manufacture but simply excretes urea brought to it by the blood. Since
urea is always present in the blood to an extent of from 0.04 per cent, to
0.06 per cent., i.e., from 4 to 6 grams per 10,000 grams, and that it is being ex-
creted at the rate of about 1.5 grams per hour, it is evident that it is being as
constantly formed in some one or more organs, and discharged into the
blood.

456 TEXT-BOOK OF PHYSIOLOGY

The experimental evidence now at hand indicates the liver as the chief
organ engaged in this process. The following facts support this view, viz.:
destructive diseases of the liver, e.g., acute yellow atrophy, interstitial hepa-
titis, and suppuration, largely diminish the production of urea but increase the
amount of the ammonium salts in the urine; the establishment of an Eck
fistula (the union of the portal vein with the ascending vena cava whereby
the livfcr is almost entirely excluded from receiving compounds absorbed
from the intestine) is followed by a decrease in the production of urea and
an increase in the ammonium content of the urine; the perfusion of the
liver of a recently killed animal with a given amount of blood containing
ammonium salts will be followed after the lapse of several hours by an amount
of urea in the blood two or three times the normal quantity. These and
other facts indicate that the chief seat of urea formation is to be found in
the liver cells.

The antecedents of urea, out of which the hepatic cells construct urea
have, for chemic reasons as well as from the foregoing experimental results,
been shown to be the salts of ammonia the carbonate, carbamate, and
lactate. The increase in the ammonia of the urine simultaneously with
the decrease in the urea renders it extremely probable that these salts are
antecedents of urea and that the transformation takes place in the liver
cells. The chemic change that takes place is simply the abstraction of two
molecules of water as shown in the following formula:

(NH4) 2C03 - 2H20 = CON2H4.

The source of the ammonia is probably in part the intestine, as this
compound is one of the products of the hydrolysis and cleavage of the proteins
during digestion. That this is the case is apparent from the fact that the
blood of the portal vein always contains more ammonia that the blood of
any other region of the vascular apparatus. The advantage to the body
that results from the conversion of ammonia to urea is that it prevents an
ammonia intoxication with its attendant evils that would otherwise arise.

It will be remembered that the amino-acids, as tyrosin, leucin, glutamic,
and aspartic acids, diamino-acids and bases, as lysin, arginin, histidin which
are also products of the hydrolysis of proteins during digestion are capable
of being absorbed as such by the epithelial cells of the villi and mucous mem-
brane. After absorption they are transmitted by the blood to the tissues of
the body generally by which they are directly utilized in the formation of
tissue protein or perhaps stored for future use, for there is evidence for the
belief that tissues have a capacity for the storage of amino-acids, which in
the case of muscles amounts to from 70 to 80 milligrams per 100 grams.
After saturation of the tissues, so to speak, with amino-acids, there yet re-
mains a certain percentage in the blood which undergoes a cleavage into an
NH2 portion and an organic portion; the former is then converted to am-
monia and subsequently to urea by the liver cells, the latter, the organic
portion, contributes to the production of fat or sugar, which in due time is
oxidized and thus contributes to the store of body-heat.

From the foregoing facts it is evident that given the presence of ammonia
salts in the blood of the portal vein the appearance of urea in the urine is
readily accounted for. There is also evidence for the belief that in the
muscles and perhaps other tissues as well, amino-acids, those resulting from

EXCRETION 457

the metabolism of tissue protein, as well as those stored, undergo a similar
chemic change with the formation of urea. This being the case, the muscles
must be regarded as a seat of urea formation.

Uric Acid. — Uric acid is one of the constant ingredients of the urine.
It is a crystalline nitrogen-holding body closely resembling urea, its formula
being C5H4N4O3. The total quantity excreted daily varies from 0.2 to i
gram. It is doubtful if uric acid exists in a free state in the urine, the indi-
cations being that it is combined with sodium and potassium in the form of a
quadriurate. The urates when in excess are frequently deposited 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 protein metabolism. 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 protein and
nucleic acid are formed, from the latter of which uric acid is derived. Nu-
cleic acid when decomposed yields bases, such as guanin, adenin, etc., which
under the action of the enzymes guanase andadenase are combined with water,
deaminized, and converted into xanthin and hypoxanthin. Because of the
fact that these bodies can also be obtained from a synthesized 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
experimentally to derive one from the other. When hypoxanthin, however,
is given internally it is oxidized and converted into uric acid. It is ex-
tremely 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 therefore partly endogenous and
partly exogenous in origin.

There is some evidence that not all the uric acid produced in the body is
excreted as such, but, that a portion, perhaps one-half, is changed to urea.

Adenin, Guanin, Xanthin, Hypoxanthin. — These compounds are also
found in urine in small but variable amounts. They are nitrogenized com-
pounds derived mainly from the metabolism of the nuclein bodies.

Kreatinin. — This is a crystalline nitrogenous compound closely resem-
bling kreatin, one of the constituents of muscle-tissue. The amount excreted
daily is about i gram. The origin of kreatinin is not very clear. It is
probable, however, that if kreatin is capable of transformation into kreatinin
a certain portion is derived from the kreatin contained in the meat con-
sumed as food. But as kreatinin is steadily excreted though in less amounts
on a diet from which meat is excluded it is certain that this portion at least
must have some other source containing nitrogen, and the inference is that
it is one of the end-products of the protein metabolism that is taking
places in tissues generally and more particularly in muscle-tissue.

Hippuric Acid. — This acid in combination with sodium and potassium
is very generally present in urine, though in small amounts. It ;s 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

458 TEXT-BOOK OF PHYSIOLOGY

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 precursors, or related bodies. Various
compounds of this class are found in vegetable foods, a fact which may ac-
count for the increase in the excretion of hippuric acid on a vegetable diet.

Indol, Skatol, Phenol, Cresol. — These compounds, products of the
putrefactive changes in the derivatives of protein are present in variable
amounts, associated with potassium sulphate (see page 205). These com-
pounds are known as the ethereal sulphates. The extent to which they are
present is taken as a measure of the extent of intestinal putrefaction; their
presence can be determined by various tests. Of these compounds the one
generally tested for is potassium indoxyl sulphate or indican. If hydro-
chloric acid and a small quantity of sodium hypochlorite be added to sus-
pected urine, together with a few cubic centimeters of chloroform, the indican
if present will be separated by the acid into indoxyl and potassium sulphate.
The former compound will then be oxidized by the oxygen set free from the
sodium hypochlorite and form indigo blue. The chloroform will absorb the
indigo blue as fast as formed and when the reaction is completed will accumu-
late at the bottom of the test-tube. The depth of the color is indicative of
the quantity present and of the extent of the intestinal putrefaction.

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 magnesium phosphates,
known as the earthy phosphates, are present to the extent of i gram. Though
insoluble in water, they are held in solution in the urine by its acid constitu-
ents. 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 phosphoric 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 proteins
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. Each kidney resembles a bean in shape, is
from 10 to 12 centimeters in length, 2 centimeters in breath, and weighs from
144 to 170 grams. These organs are situated in the lumbar region, one on
each side of the vertebral column behind the peritoneum, and extend from
the eleventh rib to the crest of the ilium. The anterior surface is convex,
the posterior surface concave. The latter presents a deep notch — thehilum.
The kidney is surrounded by a thin smooth membrane composed of white
fibrous and yellow elastic tissue; though it is attached to the surface of the
kidney by minute processes of connective 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

EXCRETION

459

duct (Fig. 205). This cavity is mainly occupied by the upper part of the
1 renal duct, the ureter, the interior of which is termed the pelvis. The ureter
divides into several portions which terminate in small caps or calyces which
receive the apices of the pyramids. The parenchyma of the kidney consists
of two portions, viz. :

1. An internal or medullary portion, consisting of a series of pyramids or

cones, some twelve or fifteen in number, which present a distinctly
striated appearance. r»

2. An external or cortical por-

tion, half an inch in
thickness and distinctly
friable in character.

A histologic analysis of
the kidney shows that it is
composed of a connective-
tissue framework supporting
secreting tubules, blood-ves-
sels, lymphatics and nerves,
all of which are directly con-
nected with the removal of
the urinary constituents from
the blood.

The Tubules of the Kid-
ney. — From its structure it
is apparent that the kidney
is a compound tubular gland,
the orifices of exit being di-
rected toward the pelvis. If
the apex of each pyramid be
examined with a lens, it will
present a number of small
orifices which may be re-
garded as the beginnings
of the uriniferous tubules.
From this point the tubules

pass Olltward in a Straight but n Ureter. C. Renal calyx, i. Cortex, i'. Medullary

Somewhat diverging manner ra'ys. i"/ Labyrinth, or cortex proper. 2. Medulla.

tnwarH tV.P rnrtpv aivinP" off 2'. Papillary portion of medulla, or medulla proper.

toward tne COrteX, giving On ^i^ jy~ ^ ^ medulla. 3, 3- Transverse sec-

at aCUte angles a number Of tion through the axes of the tubules of the border layer.
branches (Fig. 206). From . Fat of the renal sinus 5 ,5-

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 be-
comes enlarged and twisted, and, after pursuing an extremely convoluted

FIG. 205. — LONGITUDINAL SECTION THROUGH THE

c°ursmg meduUa rayS'~

course

, turns backward into the medullary portion for some distance, form-
ing 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
comes convoluted, and finally terminates in an ovoid mvaginated en large-
ment known as Muller's or Bowman's capsule, which receives a small
of blood-vessels— the glomerulus. Each tubule consists of a basement mem-

460

TEXT-BOOK OF PHYSIOLOGY

brane lined throughout its entire extent by epithelial cells. The epithelium
as well as the tubule varies in shape and size in different parts of its course.
In the capsule the epithelium is flattened, lining not only the inner surface

Lobule.

Connecting
piece.

Intercalated
piece.

Convoluted
tubule.

Renal corpuscle.- ^ll _

Intercalated
piece.

Thick

.Thin

division of the
loop of Henle,

Collecting ----- -

tubule.

Papillary duct. -

SkTunica albuginea.

Stellate vein.

Interlobular

artery.

Interlobular

vein.

Arcif orrn artery

Arcitorm vein.

•Interlobar artery.
Interlobar vein.

Papilla.

FIG. 206. — SCHEME OF THE COURSE OF THE URINIFEROUS TUBULES AND THE RENAL

VESSELS. (Stohr.)

of the capsule but reflected over the blood-vessels as well. This is known
as the glomerular epithelium. In the convoluted portions of the tubules the
epithelium is cuboid, granular, and somewhat striated; in Henle's loop it is
more or less flattened.

EXCRETION

461

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 reunite to
form an efferent vessel the caliber of which is less than that of the afferent
artery. In consequence of this, there is a greater resistance 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 physiologic point of view. As stated above,
the glomerulus is received into and surrounded by the terminal expansion
or capsule of the tubule. This capsule, formed by an invagination of the
terminal portion of the tubule, consists of two walls, an outer one consisting
of an extremely thin basement membrane, covered by flattened epithelial
cells, and an inner one consisting apparently only of flattened epithelium
which is reflected over and closely invests the glomerular blood-vessels
(Fig. 207). 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 secretor 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 efferent vessel
of the glomerulus soon again divides
and subdivides to form an elaborate
capillary plexus which surrounds 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 the interlobular veins
as ] well as the veins at the base
of the pyramids. The renal veins
formed by the union of these latter FIG 207>_ScHEME OF THE RENAL o* MAL-

Veins emerges from the kidney at the PIGHIAN CORPUSCLE, i. Interlobular artery.

hilum and finally empties into the 2. Afferent vessel. 3. Efferent vessel. 4. Outer

. . . J wall. <;. Inner wall. 6. Glomerulus. 7. JNecfc

vena cava inferior. of tubuie.— (Stohr.)

The Nerves of the Kidney.— .

The nerves distributed to, and associated with the functional activities of
the kidney consist of both pre- and post-ganglionic fibers. The latter have
their origin in the cells of small ganglia situated close to the semilunar
ganglion. From their origin they pass through the renal plexus and follow
the course of the blood-vessels to their termination. The former, the pre-
ganglionic nerves, have their origin in the lower portion of the thoracic region

462 TEXT-BOOK OF PHYSIOLOGY

of the spinal cord and pass forward in the small splanchnic nerves. Experi-
ment has shown that these nerve-fibers have both vaso-constrictor and
vaso-dilatator functions. The presence of specific secretor fibers for the
epithelium has not been satisfactorily demonstrated.

The Renal Duct. — The renal duct, the ureter, is a musculo-membranous
tube about 5 mm. in diameter when distended, 30 cm. in length, and extends
from the hilum to the base of the bladder into which it empties. 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 physiologic mechanism concerned in the secretion of the urine, re-
duced to its simplest terms, consists of (i) the afferent vessel, the glomeru-
lus, the efferent vessel, the capillaries and the veins; (2) the glomerular cap-
sule and the epithelium of the tubule throughout its entire extent. Fig. 208.

The secretion of urine by this mechanism is a complex process and
susceptible of several interpretations. It was originally inferred by Bowman,
an inference based on the histologic structure of the blood-vessels and tubules
and their relation one to the other, that there is here present (i) an ap-
paratus for filtration, the capsule with its enclosed glomerulus, and (2) an
apparatus for secretion, the tubule with its epithelium, and therefore 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 filtration, under pressure, through the walls of
the glomerular capillaries, while the organic constituents are removed by the
epithelium lining the tubules; that the water, descending the tubules from the
capsules, plays the part of a solvent for the crystallizable compounds secreted
by the epithelium, in consequence of which they are the more readily carried
by the moving fluid into the pelvis of the kidney.

Ludwig, influenced largely by the facts of blood-pressure, advanced the
view a few years later that the factors concerned in the secretion of urine
are purely physical; that in consequence of the high pressure in the vessels of
the glomeruli, due to the high pressure in the renal artery on the one hand,
and to the resistance offered by the smaller efferent vessel on the other hand,
all the urinary constituents are filtered off in a state of extreme dilution.
In order to account for the higher percentage of the organic constituents in
the urine than in the blood, it was assumed that as the dilute urine passed
through the tubules the water and possibly other substances as well, that were
necessary for the nutrition of the body, were partly reabsorbed, passing by
diffusion into the lymph and blood until the urine acquired its normal
characteristics and degree of concentration. 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

EXCRETION

463

;:ause and effect, and that the formation of urine was accomplished entirely
py purely physical forces.

The progress of physiologic investigation, however, has thrown some
lloubt on the validity of this physical interpretation.

Heidenhain brought forward a series of facts which supported the view,
.hat all the constituents of the urine are eliminated from the blood by a pro-
:ess of secretion on the part of the epithelial cells both of the glomerulus and
he tubules; that these cells have the power of selection in removing from the
}lood normal diffusible constituents when in excess, and abnormal con-
stituents, i.e., urea, uric acid, etc., when present in any amount, recognizing
it the same time that the secretor activity of these cells is modified in one
direction or another by the blood-pressure and the variations which it under-

JV M

FIG. 208.-THE MECHANISM FOR THE SECRETION OF THE URINE. I A, interlobular artery ;
av, afferent vessel; cv, efferent vessel; c, capillary vessels; v, vein; IV, interlobular vein, ct,
!j contorted tubule.

goes from moment to moment. In other words that the secretion of urine is
ft physiologic or vital process, rather than a physical or mechanical process.
I As evidence that the cells of the tubules possess a selective power, he pre-
jsented the following experiment: The spinal cord of an anima is divided u
(the neck for the purpose of lowering the blood-pressure in the kidney below
:the pressure at which the urine is secreted. Five to twenty c.c. of a satur-
ated solution of indigo-carmine are injected into the blood-vessels; after
: intervals varying from ten minutes to one hour the animal is 1
blood-vessles washed out with alcohol for the purpose of preciprta ing 1
indigo-carmine in situ. Section of the kidney shows a uniform blue stai,
of the cortex alone (Fig. 209). Microscopic examination revea the fee
that the blue stain is due to the deposition of the pigmen in the ^lumen a
in the lumen border of the cells of the convoluted tubules (Fig. 210) and

464

TEXT-BOOK OF PHYSIOLOGY

the ascending limb of Henle's loop; while the epithelium of Bowman's cap-
sule as well as the glomerular epithelium present no evidence of pigmentation.
The physiologic action of the cells of the convoluted tubules in elimination
of indigo-carmine, is supposed to indicate their action in the elimination of
urea and other nitrogen-holding compounds. The absence of the pigment,
from the glomerular epithelium lends support to the view, that its function
is the elimination of water and highly diffusible inorganic salts.

Another experiment which apparently shows the action of the epithelium
is the following: A solution of uric acid in piperazin is injected into thej
blood of a rabbit. At .the end of from twenty to sixty minutes the animal is
killed. The kidney is sectioned and examined microscopically, whereupon
it is found that crystals of uric acid are present in the epithelial cells of the con-
torted tubules, especially toward their inner border, while in the medullary
tubules the crystals are confined to the lumen. These facts lead to the
inference that under these conditions at least, the uric acid passes from the

FIG. 209. — KIDNEY OF A RAB-
BIT. Cortex alone stained with
the rindigo-carmine at the end of
one hour. — (Heidenhain.)

FIG. 210. — MICROSCOPIC APPEARANCE OF
THE LUMEN OF THE CONVOLUTED TUBULES
CONTAINING THE INDIGO-CARMINE. — (Hei-
denhain.}

blood by way of the epithelium into the interior of the tubule to be finally
discharged into the pelvis of the kidney.

Nussbaum attempted to establish the secretory power of the epithelium
in another way. In the frog the kidney receives blood from 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 determined. After
so doing the now 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 excreted 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 experiments
that the secreting structures of the kidney consist of two distinct systems,
the glomerular and the tubular; the former secreting water, salts, sugar,

EXCRETION 465

peptone, etc., the latter urea, uric acid, etc. These and similar facts in-
dicate that the renal epithelium possesses a secretor 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.

The Present View as to the Mode of Formation of the Urine.—
The still further progress of physiologic investigation has apparently re-
established the view, subject to some modifications, that the gtomerulus is
after all but a passive apparatus permitting the filtration of water and diffus-
ible salts, as Bowman and Ludwig suggested, and has confirmed the view of
Heidenhain that the epithelium is a secretory apparatus, and in addition
has established the further fact that the epithelium is an absorptive apparatus
as well. Without presenting the details of a long series of experiments it
suffices to say that the results of these investigations make it reasonably
certain that the glomerulus permits of the passage of a filtrate, not of the
character and composition asserted by Ludwig, but having the character-
istics and composition of the blood-plasma, i.e., having the chemic composi-
tion and the same degree of concentration as this fluid, less its protein con-
tent; that as this fluid passes through the tubules it receives the organic con-
stituents which are secreted by the epithelium and in addition a certain
volume of water as well; also that it loses water and inorganic salts which
are returned to the blood not by diffusion as Ludwig believed but by an
act of absorption on the part of the epithelial cells, when these substances
are needed for nutritive purposes.

The probability that the renal epithelium possesses absorptive functions
is indicated by a greater or less constriction of the tubules in those animals
in which the retention of water at least, is desirable. Thus in fish where
the supply of water is abundant and no need for its retention in the body
exists, the tubules are short, wide and free from any narrowing which would
retard the downward flow of the fluid; in frogs which live partly on land and
I which lose water readily by evaporation from the skin, the retention of water
is necessary, hence in these animals the tubules are long and much con-
stricted; in tortoises where there is no evaporation from the skin, the tubes
are short, wide and devoid of constrictions. In mammals the constricted
portion, the loop of Henle, is long and narrow, and very probably serves t
retard the flow of the liquid thus permitting of the absorption of water am
other constituents. ,. .

Summarizing the foregoing it may be stated that the formation i of urine
is accomplished by the cooperation of the glomerulus and the epithelium o

^^IgUmeruius permits the passage of a nitrate similar if not identical
with the blood plasma— minus the protein.

The epithelium, especially that lining the contorted portion of the tubule,
(i) secretes water, various salts previously in combination with protein, eg
phosphates, urea, uric acid, various other nitrogen-holding compounds
organic matters; (2) absorbs such constituents of. the urinary fluid as water
and inorganic salts which may be necessary to the nutrition of the body^
final result of the entire process is the maintenance of the normal composition
of the blood and body fluids.

466

TEXT-BOOK OF PHYSIOLOGY

The Influence of Blood-pressure. — Whether the elimination of thfe
urinary constituents is entirely secretor (physiologic) in character or not there
can be no doubt 4;hat the whole process is largely determined by the pressure
and velocity of the blood in the glomerular capillaries, or, to state it more ao
curately, on the difference of pressure between the blood in the capillaries and
the urine in the capsules. As a rule, this latter pressure is at a minimum. li
the urine should accumulate in the ureter and tubules either from ligation 01
mechanical obstruction until its pressure approximated that of the blood, the
secretion should be diminished if not abolished. It is difficult to determine
the average pressure or velocity of the blood in the glomerular capillaries,
though they both must be greater than in capillaries 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.

Glomerular Pressure. — The pressure of the blood in the glomeruli is
the resultant of the pressure in the renal artery and the resistance to the
outflow of blood through the efferent vessel and the capillaries beyond.

The pressure of blood in the renal artery may be augmented ani
the velocity of the blood stream increased:
i. By an increase in blood-pressure generally.

2. By an increase in the blood-pressure
of the renal artery alone.

The first condition may be caused
by an increase in either the force oa
frequency of the heart's action or by
a contraction of the arterioles of the
vascular areas in any or all parts of the
body, excepting, of course, the renal vas-
cular area. Should this condition arise,
the blood would be forced into the renal
artery in larger volumes and in conse-
quence its pressure would be increased
The second condition is brought about bj
a dilatation of the renal artery alone and
possibly by a simultaneous contraction
of the efferent vessels of the glomeruli,
The pressure of the blood in the
renal artery and therefore in the glomer-
uli may be diminished and the velocit}
decreased :

1. By a decrease in the blood-pressure
generally.

2. By a decrease in the blood-pressure
of the renal artery alone.

The first condition may be caused by a decrease in either the force 01
frequency of the heart's action or by a dilatation of the arterioles of large
vascular areas in any or all parts of the body. Should this condition arise;
the volume of blood delivered to the kidney in the unit of time would be
diminished and hence its pressure would fall. The dilatation of the cutane-
ous vessels in summer, the result of the high temperature leads to a di-

FIG. 211. — 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 Morat.)

EXCRETION

467

inished blood-supply to the kidney and a diminution in the amount of urine
ecreted. The second condition is brought about by contraction of the
•enal artery alone and possibly by a simultaneous dilatation of the efferent
'essels of the glomeruli. Moreover the pressure in the vessels of the glomer-
.li may be varied according to the degree of contraction or relaxation of the
uscle coat of the afferent and efferent vessels. See Fig. 211 and the ac-
:ompanying explanation.

Coincident with the rise and fall of pressure in the glomerular capillaries
:here is a rise and fall in the rate of urinary flow. Thus it has been found that
,n increase in the aortic pressure from 127 to 142 mm. of mercury, from
igation of the carotid, femoral, and vertebral arteries, increased the rate of
.rinary 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
ivision of the spinal cord is followed by a total abolition of the urinary flow,
'hese facts serve to indicate the dependence of the secretion on blood-
oressure.

The period of functional activity of the kidney is accompanied by an
increase in the volume of blood flowing through it as is evident from an in-

FIG. 212.— SCHEME OF A RENAL ONCOMETER OR PLETHYSMOGRAPH. K, kidney; RT, receiving
tambour or capsule; PB, pressure bottle; PR, recording piston.

spection of the organ. At this time it is enlarged, swollen, and red in color.
The blood in the renal vein is bright red in color and contains more oxygen
and less carbon dioxid than venous blood generally. During the intervals
of activity the kidney is supplied with a less amount of blood and hence it
diminishes in size, becomes pale in color and the blood of the renal vein
becomes dark and venous in character. These variations in the volume of
the kidney have also been experimentally determined and registered by
means of the oncometer and oncograph devised by Roy.

The Renal Oncometer.— The oncometer consists of a metallic capsule,
(Fig. 212) composed of halves which open and close by means of a hinge.
The capsule encloses two thin membranous and distensible sacs. These are
connected with a piston recorder by means of a tube. The kidney, with-
drawn from the body, is placed within the oncometer. Through an opening
in the side pass the artery, vein, and ureter. A thin light oil is then poured
through a side tube from a pressure bottle until the membranous sacs
are completely filled and surround the kidney on all sides. When the

468 TEXT-BOOK OF PHYSIOLOGY

tube from the pressure bottle is closed, the conditions are such that all varia-
tions in the volume of the kidney are taken up and reproduced by the level
attached to the piston recorder. A curve of the variations in the volume
of the kidney is shown in Fig. 213. An examination of this curve shows thai
the volume-changes exhibit not only the respiratory but also the cardiac
undulations.

The Influence of the Nerve System. — The influence of the nerve system
in regulating the blood-supply to the kidney is evident from the results oi
experimentation. If the nerves which accompany 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 induced electric currents the artery contracts, the kidnej
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-dilata-
tor nerves. The vaso-constrictor nerves 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 the centers of origin
of these nerves gives rise to contraction or to a dilatation of the artery, a

FIG. 213. — CURVE OF THE VARIATIONS IN THE VOLUME OF THE KIDNEY.

diminution or a swelling of the kidney, and a decrease or an increase in
secretion, independent of any variation in general blood-pressure according
to the nature of the cause acting.

The route of the vaso-constrictor nerves is, in the dog at least, through
the lesser splanchnics, the terminal branches of which arborize around the cells
of the renal ganglia; from these ganglia new fibers arise which pass through
the renal plexus into the kidney to be distributed to the muscle coat of the
renal artery branches. 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 proba-
bility situated in the medulla oblongata in close proximity to the general
vaso-motor centers, though subordinate centers are doubtless 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 to local vaso-motor centers, thence through the
splanchnics to the renal ganglia, thence through the renal plexus to the
blood-vessels.

EXCRETION 469

The Influence of Variations in the Composition of the Blood. — As it

b the function of the kidneys to excrete water, inorganic salts, and various
bid-products of metabolism from the blood and thus maintain a general
sverage composition, it is highly probable that as soon as they accumulate
beyond a certain percentage they themselves act as stimuli to renal activity,
Either by acting directly on the renal epithelium or by increasing the glomeru-
iar pressure. There is evidence at least that urea acts in the former manner
'- ,nd that an excess of water in the blood, from copious drinking or from a sud-
Ln checking of the skin from a fall of temperature, acts in the latter manner,
the introduction into the blood of inorganic salts, such as potassium nitrate,
odium 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
jnanner'in which these agents and other members of their class, the so-called
aline diuretics, increase renal activity is yet a subject of discussion. On the
bne hand, it is stated that they promote an absorption of water from the
issues to such an extent that a condition of hydremic plethora is produced,
vhich in itself increases not only the general blood-pressure but the local
•enal pressure as well, and that it is this factor which is the cause of the
ncreased flow of urine. On the other hand, it is asserted that though the
,alts increase the local pressure and the volume of the kidney, they never-
heless act specifically on the renal epithelium, and therefore may be regarded
is secreto-motor agents. An increase in the percentage of sugar or urea in
he blood has a similar influence on the kidney.

The Storage and Discharge of Urine.— Urination.— The urinary con-
stituents, as soon as they are eliminated from the blood, pass into and through
-he urinif erous tubules and by them are discharged into the pelvis of the
tidney. They then enter the ureter by which they are conducted to the
aladder. 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

the ureter.

The Bladder.— The bladder is a reservoir for the temporary reception of
he urine prior to its expulsion from the body. When distended it is ovoid
n 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 collectively encircle the en-
tire organ. As these fibers by their contraction expel the urine from the
bladder they are known collectively as the detrusor unna muscle. At 1
exit of the bladder the circular fibers are somewhat increased in number
laiving rise to the appearance of a distinct muscle band which has been terme
the sphincter vesica muscle. The presence of this muscle has, however, been
denied and the retention of the urine has been attributed to mechanic condi
tions at the neck of the bladder. The urethra just beyond the bladder is pro
vided with a distinct circular muscle composed of striated fibers, the sph^ter
urethra muscle. At the close of an act of urination or micturition the bladd
fcSall, contracted, and its cavity is almost obliterated but as urine is con-
Itinually descending the ureter and entering the bladder at its .base the
detrusor muscle gradually relaxes or becomes sufficiently inhibited from
momen to moment to receive it. The escape of unne into the urethra is

47o TEXT-BOOK OF PHYSIOLOGY

prevented either by mechanic conditions or by the contraction of the sphincter
muscle at the vesic orifice. When the accumulating urine reaches a certain
volume, it gives rise to an intra- vesic pressure. When this pressure rises to
about 80 cm. of water the delrusor urin& acquires a certain degree of tension
or tonus. This is followed by rhythmic contractions of the detrusor urinse
which increase in extent and vigor as the urine continues to accumulate until
finally a general contraction develops, the force of which overcomes the
constricting influences at the bladder orifice and the fluid is discharged.
This action of the detrusor muscle is generally reinforced by the contraction
of the abdominal muscles. The latter portions of the urine are ejected
through the urethra by the rhythmic action of the accelerator urinae muscles.

The Nerve Mechanism of Urination. — The expulsion of urine is pri-
marily a reflex act though subject to a variable amount of volitional control.
The reflex character of the act is especially noticeable in young children in
whom, by reason of the imperfect development of the brain, there is a lack
of volitional control. During the intervals of urination the orifice of the
bladder is closed by the tonic contraction of the sphincter vesicae and sphinc-
ter urethrae muscles, thus preventing the immediate exit of urine after its
descent into the bladder. The tonic contraction of both muscles is main-
tained by the activity of nerve-centers in the lumbar region of the spinal
cord. The detrusor muscle is at the same time in a more or less relaxed
condition, the result of an inhibition of its governing center in the spinal cord.

When the accumulating urine reaches a certain volume it causes, as
previously stated, an intra- vesic pressure, an increased tonus of the detrusor
muscle, followed by slight rhythmic contractions of its fibers.

When the desire to urinate is experienced impressions are being made on
the afferent nerve endings in the mucous membrane of the bladder. The
nerve impulses thus developed are transmitted to the urination center in the
spinal cord and to the cerberum and influence in one direction or another
their activities. In a young child the arrival of the transmitted impulses in
the spinal cord is immediately followed by an inhibition of the sphincter
centers and a stimulation of the detrusor center, as a result of which the
sphincter muscles relax and the detrusor muscle contracts, thus expelling
the urine. In the adult, if the act of urination is to be permitted the same
mechanism is brought into action. In its expulsive efforts the detrusor mus-
cle is assisted by the contraction of the abdominal muscles and possibly the
diaphragm in response to volitional efforts. After the discharge of urine
there is a return to the former condition, namely, a contraction of the sphincter
muscles and a gradually inhibition of the detrusor muscle. If the act of
urination is to be suppressed volitional impulses descend the cord and cause
an increased contraction of the sphincter urethrae muscle, whereby the action
of the reflex mechanism is for a while opposed.

The nerve mechanism therefore involves both efferent and afferent nerves
as well as nerve-centers in the lumbo-sacral region of the spinal cord.

Efferent Nerves. — The efferent nerve-fibers for the sphincter urethrae
muscle have their origin in the spinal cord from which they pass by way of
the third and fourth sacral nerves, the pelvic nerve and the inferior hemor-
rhoidal nerve directly to the muscle.

The efferent nerve-fibers, for the detrusor muscle, including the specialized
portion, the internal sphincter, have their origin in nerve-cells in the lumbo-

EXCRETION 471

jacral region of the spinal cord and pass to their destination by two paths,
f he fibers in the first path leave the spinal cord by way of the second to the
ifth lumbar nerves, then pass into and through the sympathetic chain,
hrough the inferior splanchnics to the inferior mesenteric ganglion around
he cells of "which their terminal branches arborize; from the cells of this
janglion new fibers emerge which pass through the hypogastric nerves to the
nuscles. The fibers of the second path leave the spinal cord by way of the
iecond to the fourth sacral nerves, then pass into the pelvic or erigens nerve
o small ganglia (pelvic ganglia) along the sides of the bladder around the
:ells of which their terminal branches arborize; from the cells of these ganglia
^iew nerve-fibers emerge which pass directly to the muscles. In both paths
:he nerves coming from the cord are pre-ganglionic, those coming from the
ganglia, post-ganglionic.

The central mechanism that excites and coordinates the activities of the
vesical muscles is situated in the lumbo-sacral region of the spinal cord and
!.s designated the vesical or urination center.

Afferent Nerves — The afferent nerve-fibers that excite the central mech-
inism to activity, are contained in both the nerve-paths described in fore-
going paragraphs and enter the spinal cord in the dorsal roots of the lumbar
ind spinal nerves.

Though the origin, course and distribution of the nerves composing this
mechanism are fairly well known, their mode of action is somewhat obscure
and the results of experimentation not always in accord. According to v.
Zeissl stimulation of the peripheral ends of the divided hypogastric nerves
causes mainly a contraction of the sphincter muscles and a relaxation of the
detrusor muscle, while a stimulation of the peripheral ends of the divided
sacral nerves causes a vigorous contraction of the detrusor muscle and a re-
laxation of the sphincter muscles. The lumbar centers would therefore
cause a reception and a retention of the urine, and the sacral centers would
cause its expulsion.

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 to 1.006. 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 determi-
nation, for the reason that the rate of secretion varies greatly with variations in
temperature, food, drink, season of the year, etc.

Chemic analysis of the sweat shows that it contains but from 0.5 t<
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 o.i per
cent The amount, however, may be very much increased in uremic
conditions, the result of acute or chronic disease of the kidneys The inor-
ganic constituents consist mainly of sodium chlond and alkaline and

472 TEXT-BOOK OF PHYSIOLOGY

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 important as an excrementi-
tious 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 1.17 to 1.35 square meters in
man and from i.i to 1.17 square meters in woman. It varies in thickness
in different localities of the body from J to y^ 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 pap-
illary layer. The reticulated layer consists of white fibrous and 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 con-
sists 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. 225) .

The epidermis is an extra-vascular structure consisting entirely of epi-
thelial cells. It may also be subdivided into two layers — the Malpighian or
pigmentary layer, and the corneous or horny layer. The former is closely ap-
plied to the papillary layer of the true skin and is composed of large nucle-
ated cells, the lowest layer of which, the "prickle cells," contains the pig-
ment granules which give to the skin its varying hues in different individu-
als 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 over the cutaneous surface, though
they are more thickly disposed in some situations than others. They
probably average 400 to the square centimeter; 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 regulated to a large extent by

EXCRETION 473

'the nerve system. Here, as in other secreting glands, the fluid is derived
from materials in the lymph-spaces, furnished 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., vase-motor nerves, which regulate the blood-supply, and
secretor nerves, which stimulate the gland-cells to activity.

The Sweat Nerves. — The sweat nerves which excite the activities of the
epithelium of the sweat-glands have their origin in nerve-cells located in the
anterior and lateral gray matter of the spinal cord. The sweat nerves, like
ithe vaso-motor nerves, do not pass directly to the gland-cells, but indirectly
I by way of the ganglia of the sympathetic chain. In these ganglia the sweat
I nerves terminate, their end-branches aborizing around the nerve-cells. From
I the cells of these ganglia new nerve-fibers arise which then pass without
! interruption to their final destination. The former are termed pre-gangli-
! onic, the latter post-ganglionic. From their origin ^and distribution it is
apparent that the sweat nerves are constituent portions of the autonomic

nerve system.

From experiments made on animals and from observation of clinic con-
ditions in human beings, it may be stated in a general way, that the pre-
ganglionic fibers which emerge from the spinal cord in the ventral roots of
spinal nerves between the second thoracic and third lumbar nerves may be
1 divided into four groups, viz.: (i) Those distributed to the sweat-glands of
the skin of the trunk of the body; (2) those for the sweat-glands of the fore
limbs- (3) those for the sweat-glands of the head, neck and face; (4) those
for the sweat-glands of the hind limbs. These nerves pursue the same route
as the vaso-motor nerves with which they are associated in function (see

The exact course for the sweat nerves has been experimentally deter-
mined only for the cat and dog. In these animals, however sweat-glands
are found only in the balls of the feet. According to Langley s observations
the sweat nerves for the forefeet leave the spinal cord ^in the thoracic
nerves from the fourth to the tenth inclusive. After passing into the sym-
pathetic chain they ascend to the stellate ganglion, around the cells of which
their end-branches arborize. From this ganglion non-mediated fibers pass
in the gray rami communicantes to the nerves composing -the bracmal plexus
and then to the feet. The sweat nerves for the hind feet leave the cord
mainly in the first and second lumbar and terminate in sympathetic ganglia
from which the post-ganglionic nerves pass into the nerve-trunks me luded
between the sixth lumbar and the second sacral nerves, which enter into
the formation of the sacral plexus and through which ^^ <**»£*;

The existence of a dominating sweat center in the medulla oblongata i
probable, though its location has never been definitely determined.

474

TEXT-BOOK OF PHYSIOLOGY

ten to fifteen minutes after ligation of the blood-vessels of the limb or even
after its amputation, when the corresponding nerve is stimulated.

As the sweat-glands are always in a state of more or less activity it is
assumed that the sweat centers possess a certain degree of tonicity, though
the cause for such tonicity has not been made apparent. It is quite possible
that the presence of carbon dioxid in the blood, as well as the temperature,
may be important factors, since an increase in the venosity of the blood or
a rise in temperature is usually followed by an increase in the amount of
sweat excreted. The centers may also be excited to increased activity by
both central and peripheral causes, e.g., psychic states of an emotional
character and a rise in the external temperature. 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 cells themselves. As in
the case of the salivary glands atropin sus-
pends the activity of the terminal branches of
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 cuta-
neous surface.

They consist of a root and a shaft. The
shaft is oval in shape and about 60 micro-
millimeters in diameter; it consists of fibrous
tissue, covered externally by a layer of im-
FIG. 214.— LARGE SEBACEOUS bricated cells, and internally by cells contain-
GLAND. r. Hair in its follicle. 23, m~ granular and pigment material.

4, c. Lobules of the gland. 6. Ex- ° & . . r ° . . , ,, , . ,,

cretory duct traversed by the hair. The root of the hair is embedded in the
—(Sappey.} hair-follicle, formed by a tubular depression

of the skin, extending nearly through to the

subcutaneous 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 pro-
jection of amorphous matter, corresponding 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, consti-
tuting 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 excited 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, pro-
teids, fat, cholesterin, and inorganic salts.

EXCRETION 475

The sebaceous glands are simple and compound racemose glands
ppening by a common excretory duct on the surface of the epidermis or
t into the shaft of a hair-follicle (Fig. 2 14) . These glands are extremely numer-
ous 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
i epithelium.

The sebum is not produced by an act of true secretion, but is 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 lubri-
cate the hair and skin and prevent them from becoming dry and harsh.

The surface of the fetus is generally covered with a thick layer of seba-
ceous 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 XVIII
EXTERNAL SECRETIONS

Secretion is a term applied to a process by which complex fluids are
formed from the constituents of the lymph which 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 filtra-
tion. These separated materials may be utilized in several ways:

1. 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 and specific materials of widely different application.
The fluids and specific materials thus formed are utilized for the most
part to meet some special need of the body. All such fluids and mate-
rials are termed secretions, and the organs by which they are formed are
termed secretor organs. Secretions whether simple or complex may in
a general way be divided into two groups, viz. : external and internal.
External Secretions. — An external secretion may be defined as a more
or less complex fluid formed by the secretor activities of epithelial cells of
glands, which is discharged through well-defined ducts on the surfaces of
the body, the skin or mucous membrane. The glands by which they are
formed or secreted are known as glands of external secretion.

Internal Secretions. — Internal secretions may be defined as more or
less complex materials or agents formed by the activities of epithelial cells of
organs, and which are discharged into, and distributed by the blood to organs
and tissues near and remote, the activities of which they influence in varying
ways and degrees. The glands by which they are formed or secreted are
known as glands of internal secretion.

Organs of External Secretions. — All organs belonging to this group
consist 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 resem-
blance one to another, their physiologic function varies in different situations,
in accordance probably with their ultimate chemic structure, a fact which
determines the difference in the character of the secretions.

These organs may consist of a single layer of cells or a group of cells, and
may be subdivided into—

1. 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 com-

EXTERNAL SECRETIONS 477

posed 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 physiologic 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, semi-transparent, alkaline fluid, containing
leukocytes and epithelial cells. It is composed chemically of water, mineral
salts and an albuminoid 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, and 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 membranes enclose
what are practically large lymph-sacs or spaces, and the fluid they contain
resembles lymph in all respects and is practically 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 considerable 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 eye,
the fluid of the internal ear, the cerebrospinal fluid, etc., will be considered
in connection with the organs with which they are associated.

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 communication 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 —

1. 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 seba-
ceous 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 pass-
age of the plasma across the capillary wall have already been considered

478 TEXT-BOOK OF PHYSIOLOGY

in connection with the production of lymph. They include the physical
processes, diffusion, osmosis, and filtration combined with a secretor activity
of the cells of the capillary wall. The question as to which of these pro-
cesses is the more active is yet a subject of investigation.

As the chemic composition and the chemic features of the organic con-
stituents of all secretions have been demonstrated to be the outcome of
metabolic processes going on within the epithelial cells, it must be assumed
at least that these differences are correlated with differences in the histo-
logic features and chemic composition of the epithelium. The discharge of
the secretion is, as a rule, intermittent; that is, there are 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 diges-
tive enzymes, granules of glycogen, globules of fat, sugar, and proteins,
as in the case of the mammary gland. In how far all these compounds are
the result of 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 vascular and the blood emerging by the
veins frequently retains its customary 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.

The Influence of the Nerve System. — The activity of every gland is con-
trolled by nerve-centers situated in the central nerve system. These centers
may be excited to activity either by impressions made on the peripheral
terminations of afferent nerves 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 receptive surface (skin,
mucous membrane, or sense-organ) , an afferent nerve, an emissive cell from
which emerges an efferent nerve to be distributed to a responsive organ, the
gland epithelium, though the secretion may in some instances be initiated
by a psychic state.

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 156) and will again be in subsequent chapters.

The structure of the glands of external secretion, the composition and

EXTERNAL SECRETIONS

479

physiologic actions of their secretions have in large part been considered
in the foregoing chapter on Digestion. There remains, however, to be con-
sidered the mammary glands, the liver and the sebaceous glands.

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
presents at its convexity a small conical
eminence termed the mammilla or nipple,
surrounded by a circular area of pig-
mented skin, the areola. The gland
proper is covered by a layer of adipose

FIG. 215- — MAMMARY GLAND. i.
Lactiferous ducts. 2. Lobuli of the
mammary gland.

FIG. 216 — ACINI OF THE MAMMARY
GLAND or A SHEEP DURING LACTATION.
a. Membrana propria. b. Secretory
epithelium.

tissue anteriorly and is attached posteriorly to the pectoral muscles by a net-
work of fibrous tissue.

During utero-gestation the mammary glands become larger, firmer,
and more lobulated; the areola darkens and the blood-vessels, especially
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 consists
of an aggregation of some 15 or 20 irregular pyramidal lobes, each of which
is surrounded by a framework of fibrous tissue. This tissue affords support
for blood-vessels, lymph-vessels, and nerves. Each lobe is provided with a
single excretory duct, the lactiferous duct, which as it approaches the areo a
expands into a fusiform ampulla or reservoir. At the base of the nipple
the ampulla contract to form some 15 or 20 narrow ducts, which, ascending
the nipple, open by constricted orifices 0.5 mm. in diameter on its apex

Flg6n 'tracing the lactiferous duct into a lobe, it is found to divide and
subdivide into a number of branches, which pass into smaller masses-tne
lobules. The lobule in turn is composed of a large number of tubular acini
or alveoli, the final terminations of the lobular ducts. Each acinus consists

48o

TEXT-BOOK OF PHYSIOLOGY

of a basement membrane lined by a single layer of low cuboidal epithelial
cells (Fig. 216). Externally the acinus is surrounded by blood-vessels,
nerves, and lymphatics.

MILK

Physical Properties. — 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, holding in
suspension an enormous number of small, highly refractive oil-globules,
which measure on the average about ToVo" °f an mc^ in diameter. It has
been asserted by some observers that each globule is surrounded by a thin
protein envelope which enables it to maintain the discrete form. This, how-
ever, is at present disbelieved.

The quantity of milk secreted daily by the human female averages about
1200 c.c.

Chemic Composition. — The chemic analysis of milk shows that it con-
sists of all the different classes of nutritive principles, which are necessary
to the growth and development of the body. The only exception appears
to be an insufficient amount of iron (3 or 4 milligrams per 1000 grams of
milk) for the formation of the coloring matter of the blood, the hemoglobin.
This is provided, however, by the liver in which the iron accumulates during
intra-uterine life. According to Bunge the liver of a rabbit newly born con-
tains as much as 18.2 milligrams per 100 grams of body-weight, while at
the end of twenty-four days it contains only 3.2 milligrams per 100 grams of
body-weight. The composition, however, varies not only in different classes
of mammals but in the same animal at different times and under different
physiologic conditions. Average compositions of milk of certain animals are
shown in the following table:

THE COMPOSITION OF MILK

Constituents

Human

Cow

Goat

Mare

Ass

Water

87.80

87 .00

86.01

90.00

oo.oo

Caseinogen \

I . ZO

3 .20

3 .60

i. 80

2 .IO

Lactalbumm /
Fat

3 • ^o

v8o

4.00

0.30
i .30

1.30

Lactose..

7 OO

500

A AC

e CQ

6 30

Inorganic Salts

o .20

o . c,o

0.86

' O.3O

0.30

Caseinogen is the chief protein constituent of milk. Associated with it,
however, are two other proteins, 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 entan-
gled may be collected by appropriate chemic methods. On the addition of
rennet, an alcoholic extract of the mucous membrane of the calf's stomach,
which contains the enzyme rennin or pexin, the caseinogen undergoes a
conversion into an insoluble protein, casein or tyrein. 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

EXTERNAL SECRETIONS

481

the milk be completely decalcified by the addition of potassium oxalate.
After coagulation, the more or less solid mass of milk separates into a liquid
portion, the serum, and a solid portion, the coagulum. The former, gener-
ally termed whey, consists of water, salts, lactalbumin, sugar; the latter, the
curd, consists 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 coagulates the caseinogen in a manner similar to,- if not iden-
tical with, this process, which appears to be essential to the normal digestion
of the milk.

Fat is present in the condition of a very fine emulsion and 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 34°C. When milk is allowed to stand for
some time, the fat-globules rise to the surface and form a thick layer known
as cream. Churning the milk or cream causes the fat-globules to run to-
gether 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: C^H^O^.
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 precipi-
tated as a more or less consistent mass.

Inorganic salts are always present and are chiefly those of potassium,
sodium, calcium, and magnesium phosphates and chlorids. The following
table of Bunge gives the quantitative amounts of these constituents in both
human and cow's milk:

In 1000 Parts

Potas-
sium

Sodium

Calcium

Magne-
sium

Iron
Oxid

Phos-
phoric
Acid

Chlorin

Human milk

o 78

O.2<?

0.33

0.06

o .0036

0.47

0.43

Cow's milk

1.76

I .11

I. 50

O.2I

0.0030

1.97

1.69

Iron is also present in small amounts possibly from 3 to 5 milligrams per
1000 c.c. Citric acid to the extent of 0.05 per cent, is also present.

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 follow-
ing 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;
constriction 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

482 TEXT-BOOK OF PHYSIOLOGY

which are discharged separately into the lumen, while others remain for a
time associated with the detached portion of the cell (Fig. 217). From
these histologic changes it is inferred that the caseinogen and fat are products
of the metabolism of the cell protoplasm and not derived directly from
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 secretor activity rather

than of diffusion and filtration. This is ren-
dered probable from the fact that the propor-
tions 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 passage of
the water and salts into the lumen of the acinus
the proteids undergo disintegration and solution
and the liquid assumes the characteristics of
milk.

The discharge of milk is occasioned by the
FIG. 217.— SECTION OF THE suction efforts on the part of the child, aided by
MAMMARY GLAND OF A CAT atmospheric pressure and the contractions of
TAT'ON ETcav"yEoraiveAoU the non-striated muscle-fibers of the lactiferous

filled with granules and glob- ducts.

of fat. i, 2, > Epithe- xhe Developmental Changes and the Se-

cretion of Milk.-Previous to menstruation the
glands are quite rudimentary, but with the estab-
lishing of this function they increase in size as a result of the growth of con-
nective tissue and the deposition of fat. The glandular tissue is, however,
more or less inactive, but with impregnation it too undergoes a rapid devel-
opment. The alveoli increase in number and size and toward the end of
the period of pregnancy begin to secrete the forerunner of milk, the fluid
colostrum. From the foregoing it is apparent that there is a close relation-
ship between the activities of the ovaries and the uterus and the activities
of the mammary glands though the mechanism by which the two groups of
activities are mediated is not very clear. Starling and Lane-Claypon have
thrown some light on the mechanism by the demonstration that the injection
of water extracts of developing feti into the body of a non-pregnant mam-
mal—a rabbit — for a period of from 15 to 17 days was followed by the devel-
opment of the mammary glands, similar if not identical with the develop-
ment that occurs during an actual pregnancy. From this fact the inference
is drawn that during intra-uterine life, the fetus as it develops secretes an
agent which, entering the maternal blood, is carried to the mammary glands
and by it stimulated to growth. The agent thus produced by the fetus
would belong to the class of agents known as hormones. Since those kata-
bolic changes in the cells of the glands which eventuate in the formation of
milk do not arise during the intra-uterine life of the fetus, but which soon
arise after birth, the further assumption is made that this hormone has an
inhibitor influence on the gland-cells which prevents the formation of milk;
but with its withdrawal after birth secretion at once begins.

Influence of the Nerve System. — Judging from analogy, it is
probable that the secretion of milk is regulated by impulses emanating

EXTERNAL SECRETIONS 483

(from the nerve system, though the exact nerve-channels for the trans-
mission of such impulses have not been determined experimentally.
The results of experiments made on the nerve distribution to the
mammary gland of the goat are unsatisfactory and contradictory and
do not shed much light on the subject. Even after division of all nerves
to this gland in this animal, secretion continues in an apparently normal
manner. Nevertheless, it is well known that emotional states on the part of
the mother modify the quantity as well as quality of milk, indicating a con-
nection between the gland-cells and the central organs of the nerve system.
Nerve terminals have been discovered in and around the epithelial cells— a
fact which supports this view.

Colostrum.— Within a day or two after parturition the alveoli become
filled with a fluid which in some respects resembles milk and which has been
termed colostrum. This is a watery fluid containing disintegrated epithelial
cells and fat-globules, as well as a colostrum corpuscles, which are probably
leukocytes containing fine fat-globules. Colostrum is distinguished from
milk in being richer in sugar and inorganic salts. It also differs from milk
in undergoing coagulation by heat which is supposed to be due to the pres-
ence of a globulin. Its coagulation point is about 72°C. 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 ligaments 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 duo-
denal 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
transverse 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 num-
ber of small masses, rounded, ovoid, or polygonal in shape, called lobules,
measuring about one millimeter in diameter and separated from one another
by a narrow space in which are to be found blood-vessels, lymphatics, and
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. 218). 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. 219). Each cell
possesses one and at times two nuclei. There is no evidence for the exist-
ence of a distinct cell-wall. The cell protoplasm frequently contains glob-
ules of fat, granules of a protein nature, granules of glycogen, pigment ma-
terial, etc. The appearance presented by the cell will vary considerably, ac-
cording to the time it is observed. Thus there may be a complete absence of
these constituents, when the cell may present a series of vacuoles separated
by bands of protoplasm. The cells are the secreting structures of the liver,

484 TEXT-BOOK OF PHYSIOLOGY

and hence are in close relation to capillary blood-vessels, lymph spaces,
nerves, and irregular channels or passageways. 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 transverse
fissure. After penetrating its substance they divide and subdivide into
smaller and smaller branches', which ulimately occupy the space between the
lobules, completely surrounding and lim-
iting them. From their situation they are
termed interlobular veins and arteries.

The interlobular veins give off small cap-
illary vessels which penetrate the lobule at LB^&foBt&SS&ai , central vein.
all points of its surface. These capillaries,

Interlobular vein. Hepatic duct.

FIG. 219. — SCHEME or A HEPATIC
LOBULE, REPRESENTED IN TRANSVERSE
SECTION BELOW AND, BY PARTIAL LEV-
FIG. 218. — SECTION OF LIVER OF PIG, SHOWING ELING, IN LONGITUDINAL SECTION
V^ERY DIAGRAMMATICALLY THE LOBULES, a. Interlobu- ABOVE. In the left half the blood-
lar connective tissue, b, c. Branches of portal vein vessels are drawn; in the right half
and of hepatic artery, d. Bile-ducts, e. Intralobular only the cords of hepatic cells. X 20.
vein.— (Pier sol.) —(Stohr.)

though frequently anastomosing, form a radial meshwork which converges
toward the center of the lobule. In the meshes of this plexus are 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 intralobu-
lar 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. 220). 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

EXTERNAL SECRETIONS

485

walls, receive from the cells some of the products of their secretor 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 beginnings of the hepatic or bile-ducts
lying in the interlobular spaces. The interlobular bile-ducts possess a dis-
tinct wall lined by flattened epithelium. 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.

FIG. 2 20.— 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.)

The Nerve Supply.— 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 vagi and of the splanch-
nics as far as the semilunar ganglion, around the cells of which the terminal
fibers arborize. * From the cells of this ganglion post-ganghonic fibers arise
which pass by way of the hepatic plexus along the course of the hepatic
artery and portal vein. Many of the nerves which enter the liver are vaso-
motor in function; as to whether others are secretor in character is yet a sub-
ject of investigation. It has been asserted that nerve filaments have I
demonstrated running between the cells and even penetrating their substance.
This fact would indicate that the metabolic processes of the liver ar<
the control of the central nerve system.

Functions of the Liver.— The anatomic and histologic peculiarities
the liver would indicate that it has a variety of relations to the general pr<
cesses of the body. Experimental investigation has brought some of thes
relations to light. Though its physiologic actions are not yet wholly u
stood, it may be said that it is engaged in :

1. The elaboration and excretion of bile.

2. The production of starch (glycogen) and sugar (glucose).

486 TEXT-BOOK OF PHYSIOLOGY

3. The formation of urea.

4. The conjugation of products of protein putrefaction.

The Elaboration of Bile. — The physical properties and chemical com-
position of the bile have already been considered (page 195). The character-
istic salts of the bile, sodium glycocholate 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 capillaries. 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 composition indicates that they are derivatives of the proteins,
though the intermediate stages in their production are unknown. The
origin of the cholalic acid with which they are combined is equally obscure.
The bile salts as they are found in the bile are produced however 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 hemoglobin.
It is supposed that the liver cells bring about this change by combining water
with hematin, with the abstraction of iron. The product thus formed is bili-
rubin, which is excreted, while the iron is for the most part retained in the
liver cells.

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 in the
same way as in all other glands.

When once formed, the liver cells discharge these various compounds
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 increas-
ing pressure which arises from the secretion and accumulation of bile, this
fluid flows from the smaller into the larger bile-ducts, and finally is emptied
either directly into the intestine 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 digestion- 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 experi-
ment. Thus, division of the splanchnic nerves is followed by an increased
discharge of bile, apparently due to the dilatation of the portal vessels; stimu-
lation 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 discharge of bile leads to its accumulation,

EXTERNAL SECRETIONS 487

ja rise of pressure beyond that of the capillary blood-vessels, and a reabsorp-
ition by the lymph-vessels of the bile constituents. After their discharge into
jthe blood from the thoracic duct these constituents are deposited in part
i in various tissues, giving rise to the phenomena of jaundice, and in part are
eliminated in the urine.

The Production of Starch (Glycogen) and Sugar (Glycose or Glucose) .
—In 1857 Bernard discovered the fact that the liver normally during life
produces a substance, analogous in its chemic composition to starch and
known as liver starch or animal starch. 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 proteins are removed by nitration. The nitrate thus obtained
is opalescent and resembles a solution of starch. The starch may be
precipitated from this solution with alcohol. It may subsequently be ob-
tained free by drying, when it presents itself as a white amorphous powder,
soluble in hot or cold water. Chemic analysis shows that it consists of
C6H10O5, 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 pres-
ence of sugar.

For the reason that this starch is capable of being transformed into or of
generating glycose or glucose it received the name of glycogen; and inasmuch
as the liver continually produces glycogen it may be said to have a starch-
forming or an amylogenetic or a glycogenic function.

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 starch but a
relatively large amount of sugar. The inference drawn is that after death
the starch is transformed by some agent, possibly a ferment, into sugar
(glucose). From this fact as well as from the results of different lines of
investigation, it is the generally received opinion that the same change is
constantly taking place in the living condition and therefore the liver is said
to have a sugar-forming or a glycogenetic function. The liver cells are thus
characterized by two processes amylogenesis zndglyco genesis. To the trans-
formation of glycogen into sugar the term glycogenolysis has recently been
applied.

The presence of glycogen in the liver cells can be shown microscopically
in the form of discrete hyaline and refractive masses, which show a blue or
violet color with iodin. 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. By
experimental methods it has been shown that the production of glycogen i<
dependent very largely on the consumption of carbohydrates, the greatd
the amount of sugar and starch in the food, the greater being the production
of glycogen. Nevertheless it is also certain that glycogen can be derived
from proteins; for if the carbohydrates are excluded from the food and the

488 TEXT-BOOK OF PHYSIOLOGY

animal fed on a pure protein diet plus fat, glycogen will continue to be
formed in the liver though in far less amounts.

The facts connected with the formation of glycogen, as well as with its
transformation as at present generally accepted, may be stated as follows:
The dextrose or glucose into which the carbohydrates are mainly converted by
the action of the digestive fluids is absorbed into the blood of the portal vein
and carried direct to the liver, where a certain portion of it diffuses across
the capillary walls into the surrounding lymph-spaces; by the action of the
cells or by a special enzyme it is then 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 glucose or dextrose by the addition
of a molecule of water, after which it is returned to the blood, by which it
is transported to the systemic capillaries, where it disappears again, diffusing
across the walls of the capillaries into the surrounding lymph-spaces to play
a part in the general nutritive process. Though the final disposition of the
sugar is uncertain it is highly probable that after its delivery to the muscles,
for example, it may be directly oxidized or temporarily stored as glycogen
or possibly be used in the formation of living material. Ultimately, how-
ever, through oxidation (glycolysis) it yields heat and contributes to the
production of muscle energy. Should there be a failure on the part of the
liver cells to store up its usual percentage of the absorbed sugar, 10 to 20
per cent., by reason of impaired nutrition, disturbance of the portal circu-
lation, or a larger excess of sugar in the blood of the portal vein, it would
pass through the liver into the blood of the general circulation and increase
the percentage amount of sugar above the normal (o.i to 0.2 per cent.)
establishing the condition of hyperglycemia. This would soon be followed
by its elimination from the blood by the kidneys and its appearance in
the urine, giving rise to a glycosuria.

In opposition to this view, Dr. Pavy, after years of accurate experimentation,
states that the blood on the cardiac side of the liver never under normal circum-
stances 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 re-
converted into sugar, and denies that the liver produces sugar, to be discharged
into the blood; the function of the liver is merely to arrest the passage of sugar,
and so to shield the general circulation from an excess; 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 glucosid
constitution of protein material in general, accounts for the presence of glycogen
in; muscles and other tissues on the assumption that during the cleavage of the
protein 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
proteins necessary to the construction of the tissues.

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 ammonium salts, e.g., the car-

EXTERNAL SECRETIONS 489

bonate, carbamate, and lactate, which are constantly present in the blood.
These salts, which result from protein metabolism, may be absorbed from
the tissues or from the intestines, 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 cava whereby the liver is largely excluded from
acting on products absorbed from the intestines) there is a marked diminu-
tion in the production of urea while the ammonia content of the urine
largely increases. One large source for the ammonium which is trans-
formed into urea by the liver cells, is that split off from the protein molecule
! during digestion by the action of the gastric and pancreatic juices. The
ammonia is then absorbed and combined with carbon dioxid with formation
of ammonium carbonate. When this compound is transported to the liver
by the portal blood, the cells convert it into urea in a manner shown in the
following formula:

(NH4) 2C03 - 2H20 = CON2H4.

Destructive diseases of the liver — e.g., acute yellow atrophy, suppuration,
cirrhosis— largely diminish the production of urea, but increase the quanti-
ties of the ammonium salts in the urine. The same is true when the liver
cells are destroyed during acute phosphorus poisoning.

The Conjugation of Products of Protein Putrefaction.— One of the
important functions of the liver is the conversion of toxic compounds, the
products of the putrefaction of proteins, into non-toxic compounds. These
compounds are formed in the intestine, are absorbed and carried by the blood
of the portal vein to the liver. In their passage through the capillaries of
the liver they are conjugated for the most part with potassium sulphate by
the action of the liver cells and thus deprived of their toxicity. Among the
substances thus conjugated are indol, skatol, phenol, and cresol. After
absorption indol and skatol are oxidized to indoxyl and skatoxyl and then
combined with potassium sulphate giving rise to potassium mdoxyl sul-
phate and potassium skatoxyl sulphate. Phenol and cresol are apparently
directly combined with potassium sulphate. All of these compounds then
pass into the blood of the general circulation and finally are eliminated by
the kidneys. Potassium indoxyl sulphate or indican is the source of the
indigo-forming substance found in the urine. Other compounds ^like-
wise reduced in toxicity by the liver cells though the methods by which tl
is accomplished vary with the nature of the compound. The liver t
presents a chemic defense against the entrance of more or less toxic agents
into the blood of the general circulation.

CHAPTER XIX
INTERNAL SECRETION

An internal secretion may be defined as a more or less complex material
or agent, produced by the secretor activities of epithelial cells of organs and
tissues, and which are discharged into the blood and distributed to organs
more or less remote, the activities of which they influence in varying ways
and degrees. Some increase, some inhibit physiologic processes while others
stimulate growth and in different ways modify metabolism. The internal
secretion in many, if not all instances belongs to a class of agents known as
hormones, agents which according to Starling, are of known or unknown
composition, characterized by a relatively simple chemic or molecular com-
position, an easy diffusibility across the walls of the capillary blood-vessels,
a ready susceptibility to oxidation and a rapid elimination, as a result of
which, their action does not continue indefinitely.1

The internal secretions and the organs by which they are produced,
constitute a secondary coordinating mechanism, which supplement the pri-
mary coordinating nerve mechanisms of the body; the cooperation of both,
however, being necessary for the maintenance of the activities of individual
organs as well as of the activities of the body as a whole.

Though the term internal secretion has been applied to various sub-
stances such as urea and carbon dioxid, which arise in consequence of tissue
metabolism and which, after being discharged into the blood, influence in
many ^ ways physiologic processes, yet the term is more applicable to the
secretions of organs possessing epithelial cells in anatomic relation to blood-
vessels. The glands by which these specific materials or hormones are
secreted are known as

Glands of Internal Secretion or Endocrinous Glands.— The glands
consist mainly of epithelial cells in close relation to the walls of capillary
blood-vessels and lymphatics, and in some instances, if not all, under the
control of the central nerve system. By reason of the absence of ducts and
their relation to blood-vessels they have also been termed ductless glands
and vascular glands and inasmuch as the secretion is discharged internally
(into the blood) they have been designated endocrinous glands.

The glands which fall into this category are the thyroid, the parathyroids,
the hypophysis cerebri or pituitary, the adrenals, the pancreas, the ovaries,
and testicles.

THE THYROID

The thyroid gland or body consists of two lobes situated on the lateral
aspect of the upper part of the trachea (Fig. 221). Each lobe is pyriform

1 By reason of the fact that some of the active principles of the internal secretions instead
of exciting, relax or inhibit activity, the term hormone for the class is inapplicable according to

schafer, and he therefore suggests for the latter class, the term 'chalone' from \a\&w, to relax
PJz.m s*ack. Because they act like certain drugs he also proposes the term 'acoid' from

akos, a remedy, to denote this quality, and to denote that they arise within the body, the prefix
aut can be employed. We thus get the complete expression autocoid substance to denote any

irug-like principle which is produced in or can be separated from the internally secreting organs
and we may divide these autocoid substances into hormones or excitatory agents, and chalones
or inhibitory agents.

490

INTERNAL SECRETION 491

an 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 o.i mm. in diameter. Each sac is composed of a thin homogen-
.ous membrane lined by cuboid epithelium. 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
capillary blood-vessels and lymphat-
ics. The individual sacs are united
and supported by connective tissue,
which forms, in addition, a cover-
ing for the entire gland. Non-
medullated nerve fibers derived from
the middle and inferior cervical gan-
glia pass into the gland and are dis-
tributed to the walls of the smaller
blood-vessels and to the epithelium ^
lining the vesicles (Berkley).

The role played by the thyroid
gland and its secretion, especially in

mammals in the regulation of the ^^

functional activities of various ^ aai>_VlEW or THYROID BoDY. M

Organs and tissues Of the body has Thyroid isthmus. 2. Median portion of crico-
been determined from the examina- thyroid membrane. 3. Crico-thyroid muscle.
nion Of the effects that follow its 4- Lateral lobe of thyroid I

arrested development in the early

years of childhood, its degeneration in adults, its surgical removal in human
beings when this is necessitated by pathologic conditions, its extirpation in
animals, and the ingestion of extracts of the gland or the>gland itself.

The Effects of Arrested Development.— A congenital absence of the
thyroid or an arrested development in early childhood is followed by a de
fective physical and mental development characterized by a group of pn
nomena termed cretinism. The most characteristic of these Phenomena are
a small and irregular-shaped body; a swollen face; puff y eyelids; a broad
nose, a large tongue inclined to hang out of the mouth; a swollen abdomen,
short thick legs; mental dullness and stupidity; and even more or ^^
The thyroid, as shown on post-mortem examination presents an atroph
appearance and consists mainly of hard fibrous tissue.

The Effects that Follow Degeneration in the Adult. -The de
erative processes that occur in the thyroid in the adult give r- to a zrouD
of phenomena the most striking of which is a swollen condil
the result of the hyperplasia of the subcutaneous connective tissue,
embryonic type and rich in mucinoid material, to ™lc^.tie,tff^r^ecomes
is given. Partly in consequence of this change in the skin, the face become
broader, swollen and flattened, giving rise to a oss ofj^r^sion. The iea-
turesinsomeinstancesbecomecoarseandirregular. With the progress of the

492 TEXT-BOOK OF PHYSIOLOGY

disease the mind becomes dull and clouded, the memory becomes defective,
dementia supervenes and finally the condition of idiocy may be established.

The Effects of Surgical Removal in Man and Animals. — The com-
plete removal of the thyroid gland by surgical procedures necessitated by
grave pathologic conditions has been followed in human beings by a serious
morbid condition to which the terms cachexia strumapriva or thyroprima
have been applied. Within a relatively short time the patients complained
of muscle fatigue, a sense of heaviness in the limbs and more or less pain.
With the progress of the disordered condition there developed a swelling of
the face, hands and feet similar to that observed in myxedema. The mental
processes became sluggish, the speech difficult and movements slow. A
mental condition, approximating idiocy was finally developed before death
supervened. With the development of knowledge, regarding the function
of the thyroid it was found possible to remove a large portion of the diseased
gland without the development of unfortunate results, providing a small
portion, sufficiently large, however, to maintain the necessary amount oi
internal secretion was left behind.

The extirpation of the thyroid in animals is usually fatal though the
effects differ according to the animal operated on. Herbivorous animals
as a rule survive the removal of the gland far better than carnivorous animals,
though no sufficient reason for this difference has been presented. Dogs
usually die in a few weeks, death being preceded by tremors, and convul-
sions. Monkeys, according to Horsley's experiments and observations, gen-
erally die within a few weeks. 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; tremors 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 termin-
ating in idiocy; fall 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 from the foregoing facts, that the presence 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, render-
ing them innocuous and thus preserving the body from a species of auto-
intoxication, is gradually yielding to the more probable view that the epi-
thelium 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 syrnptoms which
follow thyroidectomy.

The Effects of Feeding the Thyroid Gland.— On the supposition that
the thyroid gland secreted and discharged into the blood an agent necessary
to the maintenance of the normal metabolism, it was thought probable that
the internal administration of the gland or its extracts might provide the
body with the necessary amount of the secretion. Acting on the suggestion,
fresh thyroids of various animals, principally from sheep, have been admin-

INTERNAL SECRETION 493

tetered to myxedematous patients with the result of relieving them of their
'• unfortunate symptoms and of restoring them to a normal physiologic condi-
'.ion. Equally, if not more surprising, are the results which have followed
r.he internal administration of thyroids to cretins. The long-continued
:reatment changed the entire metabolism resulting in an increase in growth
and weight and an awakening of the child's mentality.

Hyperthyroidism. — If the gland extracts are administered in sufficient
quantity and for a sufficient length of time when the body is in a normal con-
dition the internal secretion influences very markedly the general metabolism
is shown by an increased oxidation of fat and protein and a decline hi body
'weight. If the dosage be large toxic symptoms may arise, e.g., vertigo, in-
leased cardiac action, flushing, tremors, glycosuria, and in monkeys, exoph-
thalmos and a widening of the palpebral fissure. From these facts the
inference has been drawn from the clinical side that the symptoms com-
prised under the term exophthalmic goiter, viz.: rapid action of the heart, pul-
sation of the large arteries at the base of the neck, protrusion of the eyeballs
and fine tremors of the hands are due to an enlargement of the gland and a
hypersecretion of the thyroid cells, a condition spoken of as hyperthyroidism.
This inference has apparently been confirmed by the disappearance of the
symptoms after the removal of a large portion of the gland, care being taken
to leave a small portion sufficiently large, however, to produce the necessary
amount of the internal secretion.

The Thyroid Secretion.— The chemic features of the material sec
and obtained from the structures of the thyroid indicate that it is a complex
protein containing iodin, which, under the influence of various reagents,
undergoes cleavage, giving rise to a non-protein residue, which carries with
it the iodin and phosphorus. The amount of iodin in the thyroid varies
from o ss to i milligram for each gram of tissue. To this compound the
term iodo-thwin has been given. The administration of this compound
produces effects similar to those which follow the therapeutic administration
of the fresh thyroid itself, viz.: a diminution of all myxedematous symptoms.
1 In normal states of the body, iodo-thyrin influences very actively the general
metabolism. It gives rise to a decomposition of fats and proteins and to a
decline in body-weight. In large doses it may produce toxic symptoms, e.g.,
increased cardiac action, vertigo, and glycosuria.

The Functions of the Thyroid Gland.-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 wrncri
follow when they alone are removed. From their situation and
tionship to the thyroid gland it is generally accepted that in the earl
periments, especially those made on cats and dogs and some other carnivor-
ous animals, both sets of glands were removed and hence some of the symp-
toms which developed after the removal of the thyroids were due to the loss
of function not of the thyroid but of the parathyroids.

This is especially true of the fibrillar contractions, tremors and spasms^
These it is now more generally believed arise only in consequence of 1
simultaneous removal of the parathyroids. nrnH1irp an

The function or the physiologic action of the thyroid is to produce an
internal secretion which ato its entrance into the blood promotes favorably

494 TEXT-BOOK OF PHYSIOLOGY

the metabolism of the neuro-muscular systems at least. The myxedema and
the failure of the mental powers are attributed to the loss or degeneration of
the gland and hence its internal secretion, and cretinism to the arrest of its
development.

THE PARATHYROIDS

The parathyroids are small bodies, usually four in number, two on each
side. They are divided into superior and inferior. The superior are situ-
ated internally and on the posterior surface in close relation to, and frequently
imbedded in, the substance of the thyroid; the inferior are situated externally,
sometimes in contact with, and at other times removed a variable distance
from the thyroid (Fig. 222). Microscopically the parathyroids consist oi
thick cords of epithelial cells separated by septa of fine connective tissue
and surrounded by capillary blood-vessels.

Effects of Parathyroid Removal.— The surgical removal of the para-
thyroids 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 symptoms which are em-
braced under the general term "tetany." These symptoms are fibrillary
contractions of muscles, tremors, spasmodic contractures and paralyses oi

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
movements which frequently become dysp-
neic 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.) The administration of calcium
causes the tetany to disappear and relieves
the animal of these various symptoms, from
which it is inferred that the parathyroids
influence or regulate calcium metabolism.

These results for the most part occur
only when all the parathyroids are removed
It is asserted that even if one gland is re-
tained the animal does not die. The above
described symptoms may manifest them-
selves, however, but they are slight in
degree. Under these circumstances there
is a diminished tolerance for sugar as shown
by the appearance of sugar in the urine
FIG. 222.— The position of the para- when the normal amount is ingested,
thyroid glands.— (Zuckerkandi.) Vincent and Jolly have recently pub

lished 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

INTERNAL SECRETION 495

(is more than 51 per cent, of animals survived the operation for a prolonged
>eriod and of these 68 per cent, showed no specific symptoms of any kind.
?rom the contradictory observations it is evident that the subject needs
urther investigation.

THE PITUITARY

The pituitary is a small body lodged in the sella turcica of the sphenoid
j^ne. It measures 14 mm. from side to side, 8 mm. from before backward,
md 6 mm. from above down, and consists of an anterior lobe somewhat
pink in color, and a posterior lobe yellowish-gray in color. The anterior
lobe is much the larger and partly embraces the posterior lobe (Fig. 223).
The anterior lobe is developed from an invagination of the ectoderm of the
buccal cavity and consists of gland tissue surrounded by a thin envelope of
connective tissue. It becomes separated from the mouth by the fusion of the
sphenoid cartilages. The posterior lobe is an outgrowth from the mid-brain
and is connected with the infundibulum of the third ventricle by a short
stalk. In the early stages of its development it presents a central cavity
which is, however, soon obliterated by the growth of special tissues. It
(persists in the cat. It has been suggested that the term
hypophysis cerebri be reserved for the anterior lobe and
the term infundibular body for the posterior lobe. This
distinction appears to be desirable inasmuch as in their
origin, structure and functions they are separate and
distinct bodies.

Histology of the Pituitary Body.— If a mesial

sagittal section be made through the pituitary it will ^_

present an appearance which in a general way is the FK 223 _SAGITTAL

same in many animals though the details vary some- SECTION OF THE PITU-

what in each animal. In the monkey the arrangement F^^J^1^;

of the anatomic parts (Fig. 224) is similar to the ar- mGpARTOFTfeiRDVEN-

rangement in man. It will be observed that the posterior TRICLE. a. Anterior

'lobe is solid and that there is no open connection with ^ita^Jgfg3

the cavity of the third ventricle; that it is invested, over of the infundibulum, b.

a large part of its surface, by a thin layer of epithelium, posterior lobe comiected

: The anterior lobe, which lies in front of it is separated g^^^^sSi

! by a cleft which is the remnant of the cavity of the tion of optic chiasm.-

'buccal pouch. Though the appearance of the ante- (Sdwalbefrom Qu*m.)

rior lobe, and the epithelial investment of the posterior

lobe is somewhat different, the latter is but a differentiation of the former, a

procedure that takes place in fetal life. The eF^W;^^^tJs

usually spoken of as the pars intermedia and regarded histo log ca ly and

physiologically as a part of the posterior lobe. Superiorly the anterior I

and the pars intermedia are united, though a portion of he at ter pas es

upward and embraces, if it does not entirely surround the _infund ,bi Jar

stalk- inferiorlv and posteriorly the two bodies also unite. The posterior

S^ investment m the mi J -toe.

The extent to which the epithelium invests the posterior lobe vanes in dii

f erent animals, In the cat and dog it is almost complete.

496

TEXT-BOOK OF PHYSIOLOGY

bk d

arranged in columns between which pass large thin-walled blood-vessels
In view of the physiologic importance of this lobe it is believed that the
granules of the cell represent an internal secretion, which passes into the
blood-stream and is thus distributed to various regions of the body.

The pars intermedia consists of several layers of finely granular epithelial
cells which develop a colloid material that subsequently passes into the pos-
terior lobe where it becomes hyaline in character. The epithelial investment
is separated from the posterior lobe by a layer of blood-vessels though
columns of cells penetrate it.

The posterior lobe consists of neuroglia cells and fibers. True nerve-
cells are apparently wanting. Throughout the lobe there are numerous small
hyaline bodies which are apparently streaming upward to the ventricular
cavity. In view of the physiologic importance of this infundibular body or
pars nervosa, these hyaline masses are believed to represent an internal secre-
tion which passes upward through loose tissue channels toward the infundi-
bulum to be discharged into the fluid of the third ventricle. If the stalk be
divided there is an accumulation of these bodies in the posterior lobe.
Both parts of the pituitary are well supplied with blood though from different
sources.

Effects of Total Removal. — The effects which were observed by the
earlier investigators to follow total removal of the hypophysis were not
always in accord by reason of the difference in the operative methods

pursued, injuries to the brain,
infections, imperfect removals
as shown by post-mortem ex-
amination, etc. Some inves-
tigators claimed that after total
removal animals lived for long
periods and that therefore the
gland was not essential to life;
others claimed, however, its
total removal was followed very
shortly by death preceded by a
series of characteristic symp-
toms and that therefore it was
absolutely essential to life.
The introduction of a new

method of procedure for the re-

FIG. 224.— MESIAL SAGGITAL SECTION OF THE PIT- , f f, hvnnnhv«;i<; hv

UITARY BODY OF THE MONKEY, a, Optic chiasm; b, moval ol tfte . nypopliysis

process of the pars intermedia; c, third ventricle; d, PaulesCO and its employment

anterior lobe proper;/, posterior lobe or pars nervosa; by Gushing and his CO-WOrkers

i, epithelium investment of the posterior lobe; £,epithe- ,J i j 4. u i,- u

lium of the pars intermedia passing over the neighbor- has led to results which are for

ing brain mass; e, cleft.— (After Herring.} the most part in general agree-

ment. This method involves

an approach to the gland through the temporal bone instead of through the
buccal cavity as was formerly the case. The temporal muscles are first
dissected away from the skull on both sides and reflected downward.
Large openings are made in the bone and dura of both sides. The temporal
lobe on one side is lifted up with a spoon-shaped spatula sufficiently large
to expose the hypophysis, hanging from the infundibulum. Owing to the

INTERNAL SECRETION 497

bpposite opening in the skull, the opposite half of the cerebrum is displaced
knd protruded so that injury to the brain from compression is prevented.
OThe gland can then be picked up with forceps and removed. Paulesco
Ireported that the total removal of the gland in 24 dogs resulted in death in
&4 hours. In seven other animals the fatal result was postponed for variable
periods. One animal survived for five months and another for a year
Without exhibiting any very characteristic symptoms. As a post-mortem
examination showed that the gland was only partially removed it was
assumed that the remaining portion had been sufficient to maintain life.
Removal of the anterior lobe alone was followed by death as certainly as when
the entire gland was removed. Removal of the posterior lobe alone was not
followed by noticeable effects. From these facts Paulesco asserted that the
hypophysis is an organ indispensable to life as its removal rapidly eventuates in
death, and that of its different parts the anterior lobe is the more important.
Crowe, Gushing and Homans have more recently reported a series of 100
operations for the removal of the hypophysis, the results of which are corrobo-
rative in many respects of the results of Paulesco. It was found that the
duration of life in adult dogs was from two to three days and in young dogs
about eleven days. In a few cases the animals survived for several weeks
but a post-mortem examination showed that small viable portions of the
gland had escaped removal. Among the many symptoms that followed to-
tal hypophysectomy according to these experimenters the more striking after
24 hours were a lowering of body- temperature, unsteadiness of gait and stiff-
ness of movement, a fall of blood-pressure, feeble and slow respiration,
muscle twitchings, lethargy, coma, and death. In old animals there was
occasional glycosuria; in young animals polyuria. • Total removal of the
anterior lobe alone in this series of experiments was also as fatal as removal
of the entire gland.

The Effects of Partial Removal of the Anterior Lobe.— When only a
portion of the anterior lobe was removed the animal survived for a much
longer period than when the removal was Complete. The duration of
apparently depended on the amount and the cellular activity of the parts left
behind As a result of the partial removal only there developed a series of
phenomena to which the term cachexia hypophyseopnva has been given.
These phenomena varied somewhat in accordance with the age of
animal Adult animals became adipose and degenerated sexually young
animals likewise became adipose but they remained undersized and fail
to develop sexual characteristics and hence sexual infantilism persisted.
The organs of reproduction in both sexes remained rudimentary,
temperature was subnormal and nutritive disorders of the skin .developed.
These various symptoms were attributed at the time to the partial removal
of the anterior lobe alone and hence a deficiency of secretion but he results
of a series of experiments, subsequently published by Gushing led to
belief that some of these phenomena were due to injury or impairment of
normal function of the posterior lobe at, or subsequent to, the , time o ft
operation. Just which of these phenomena were to be Attributed
diminished secretion of the anterior lobe and which to a ^diminished secrehon
of the posterior lobe was left for future experiments to determine
important the function of the anteror lobe may be in relation to .the ph
nomena just mentioned, it is apparent that it must have some additional func-

498 TEXT-BOOK OF PHYSIOLOGY

tion of even greater significance or else its removal would not be attended
with fatal results.

The Effects of Pathologic Conditions. — In recent years the idea has
gradually developed that certain pathologic states of the body are associated in
some way with pathologic states of the pituitary body. Thus the condition
of giantism which begins in youth and the condition of acromegaly which
appears in adult life are believed to be the result of a hypersecretion of
the anterior lobe, which in turn may be due to a hyperplasia of the gland
elements excited by a variety of causes. In both giantism and acromegaly
there is an increased activity in the nutritive process leading to an over-
growth of osseous tissue and the overlying structures. In the former con-
dition the overgrowth is general; in the latter it is confined to the face and
the extremities, hands and feet. To this phase of pituitary activity the term
hyperpituitarism has been given.

The opposite condition, infantilism and adiposity, have also been shown
to be associated with pathologic changes in the pituitary. In these cases
not only is the individual of small size but the genital organs are undeveloped.
In addition there may be a subnormal temperature, loss of hair, etc. These
phenomena may be due to a diminished or defective secretion partly of the
anterior lobe and partly of the posterior lobe. To this condition the term
hypopituitarism has been given.

Goetsch has reported recently that the feeding of an extract of the anterior
lobe to young rats "has a stimulating effect upon the growth of the animal
and upon its sexual development and activity." Infantilism and a want oi
sexual development may therefore be due to a deficiency of the anterior-lobe
secretion early in life.

The Effects of Removal of the Posterior Lobe.— Gushing in a series
of experiments (1911) has demonstrated that the posterior lobe with its epi-
thelial investment exerts, contrary to general opinion, a profound influence
on metabolism and more especially on the metabolism of the carbohydrates,
either alone or in conjunction with other glands having internal secretions.
These experiments also led to the belief that some of the phenomena detailed
in the foregoing paragraph, especially the deposition of fat, the subnormal
temperature and perhaps the imperfect development of the sexual organs are
due rather to a deficiency or absence of the secretion of the posterior lobe
than to a deficiency or absence of the secretion of the anterior lobe.

It has apparently been demonstrated by Gushing that the hyaline bodies
found in the posterior lobe represent an internal secretion; that they are
discharged into the cavity of the third ventricle where they undergo solution
in the cerebrospinal fluid, by means of which the dissolved material
enters the blood-stream, since compression of the infundibular stalk gives
rise to an accumulation of the hyaline material. The presence in the cerebro-
spinal fluid of an agent that produces the same physiologic effects when
intravenously injected, as injections of extracts of the posterior lobe do, has
also been established.

In the various operative procedures incident to the removal of the entire
hypophysis or of the anterior lobe a transient glycosuria is frequently ob-
served, a phenomenon attributed to the discharge under the circumstances of
an excessively large amount of the reserve hyaline substance or of the
posterior lobe secretion into the cerebrospinal fluid in the third ventricle.

INTERNAL SECRETION 499

his secretion diminishes temporarily the normal toleration or assimilation
i sugar and in some unknown way leads to a hyperglycemia and glycosuria.1
"f the customary amount of sugar is ingested a portion of it will be eliminated

n the urine.

If the posterior lobe with its epithelial investment is totally removec
)r if the infundibular stalk is compressed by a clip so as to prevent the dis-
-harge of the secretion into the ventricle the animal becomes very tolerant of
jugar and is enabled to assimilate larger quantities than formerly without
:he development of alimentary glycosuria. As a probable result of the in-
Teased carbohydrate assimilation, a condition of nutrition is established,
Characterized by a general deposition of fat suggesting a conversion of the
sugar into fat. There is probably at the same time an imperfect oxidation
of the carbohydrates as indicated by the lowered temperature.

That the condition of generalized adiposity is probably due to deficient
posterior lobe secretion is shown by the fact that the increased tolerance for
sugar can be lowered very promptly by the coincident intravenous or sub-
cutaneous injection of extracts of the posterior lobe.

From the foregoing facts it may be assumed that the secretion ot
posterior lobe in some unknown way influences the metabolism of sugar.
From the facts at hand it may be assumed that a hypersecretion from any
cause whatever, leads to a diminished tolerance for or assimilation of sugar,
as shown by the hyperglycemia and glycosuria, though the manner in which
the hyperglycemia is developed, whether by a more rapid conversion <
elycogen to sugar or by an inefficient storage of sugar as glycogen is unknown.
A hyposecretion from any cause leads to an increased tolerance for or
assimilation of sugar which eventually contributes to the formation and
deposition of fat. In the complexus of symptoms that accompany patho-
logic changes in the hypophysis either in the anterior or posterior lobe i
difficult to indicate those which are to be attributed to increased or '****"*
secretion of either the anterior or posterior lobe by reason of their close
juxtaposition and their possible simultaneous involvement; again it is ate
uncertain as to whether the secretions produce their effects alone or through
the cooperation of the secretions of other organs having more or less influ-
ence in the metabolism of the carbohydrates.

The Effects of Injections of Extracts.-The extracts of the anterior
lobe when intravenously injected appear to be without "W^J^
on any of the physiologic mechanisms. Injections of the extracts of
posterior lobe, however, give rise very promptly as shown by Howel o an
increase in the blood-pressure which appears to be due to an men
*5£ of the arterLle muscle rather than to a •
motor centers, as the contraction takes place even after
spinal cord and medulla oblongata. The action of
of the extract eral as

normal tolerance for cane-sugar in te

urine

indkafng iaf te assimilation limit has been exceeded

BOO TEXT-BOOK OF PHYSIOLOGY

the cardio-inhibitor center as the retardation is partly prevented at least
when the vagus is divided or its function suspended by atropin. Even aftei
this has been done, however, a slowing of the heart may still be induced a
fact which suggests that the extract acts directly on the heart-muscle as well
Schafer and his co-workers have also demonstrated that pituitary extracts
cause dilatation of the renal vessels and stimulate specifically the renal cellj
to activity, thus causing a marked diuresis. The extract also stimulates thi
non-striated muscles of the intestines, bladder and uterus, giving rise in each in*
stance to vigorous contractions; it also causes a marked discharge of milk
from the mammary gland during lactation due to the contraction of the walls
of the milk ducts; the dilatator muscle of the iris is also stimulated, causing
dilatation of the pupil. A pharmaceutical preparation of the pituitary
pituitnn", in a similar manner raises the blood-pressure for a considerable
period, stimulates intestinal peristalsis and excites contractions of the uterus
during and after labor.

The Functions of the Pituitary or Hypophysis.— The functions of the
pituitary body are related to the activities of the anterior and posterior lobes.
The anterior lobe, through its internal secretion stimulates the growth of the
skeleton and associated tissues as apparently shown by the fact that an excess
of secretion in early life leads to giantism and in adult life to acromegalia
while a deficiency of secretion leads to defective growth and the establish-
ment of infantilism. The posterior lobe through its internal secretion assists
m the regulation of carbohydrate metabolism as shown by the fact that an
excess of secretion lowers the assimilation capacity and thus develops gly-
cosuna, while a deficiency of the secretion raises the assimilation capacity
and leads to the production and accumulation of fat.

ADRENAL GLANDS

The adrenal glands or suprarenal capsules are two flattened bodies, some-
what 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 6 to 8 mm in
thickness. The weight of each is about 4 gm. Accessory glands are some-
times found m the surrounding connective tissue along the abdominal svm-
pathetic and in the neighborhood of the genital organs.

In some animals such as the dog, cat and rabbit, these glands have no
anatomic connection with the kidneys, but are situated at varying dis-
tances from them.

Histology.— The gland is covered externally by a fibrous tissue from
which septa pass into the more central portions thus forming a framework
for the support of blood-vessels and cells.

A section of the gland shows just beneath the capsule an outer portion
termed the cortex and an inner portion termed the medulla (Fig. 225) The
cortex consists mainly of cuboid cells arranged in cylindric columns.' The
°Une!IlauyerS °f °ells are arranged in ^regular masses forming what has been
called the zona glomerulosa. The medulla consists of uniting and interlacing
cords of polyhedral cells, the cytoplasm of which contains granular matter
and a distinct nucleus. When treated with chromic acid or chromium salts
the cytoplasm stains a dull brown or yellow color. For this reason they are
termed chromaffin cells. Similar cells are found in sympathetic ganglia

INTERNAL. SECRETION 501

The gland is abundantly supplied with blood-vessels and nerves. The
rteries are branches of the aorta, the phrenic, and renal arteries. After
enetrating the gland they divide into smaller branches and capillaries which
Itimately come into close relation with the cells of both the cortex and
aedulla. The veins emerge from the gland at the hilum and empty on
tie right side into the vena cava and on the left side into the renal vein.
The nerves passing to the gland are derived for the most part from the
utonomic system. The pre-ganglionic fibers pass from the cord by way
f the splanchnics to the semilunar ganglion. The post-ganglionic pass
rom the semilunar ganglia through its branches direct to the gland. Ac-
ording to Bergmann nerves come from the phrenic and vagus also.

Embryologic Development. — Embryologic
avestigations have shown that the mature adre-
al gland consists of two distinct tissues derived
rom two different portions of the embryo. The
ortex is derived from that portion of the meso-
erm from which is evolved the precursor of the
idney, the Wolffian body; the medulla is de-
ived from the embryonic sympathetic ganglia
nd consists primarily of nerve-cells, which,
owever, in the course of development become
rofoundly modified. Comparative anatomic
tudies have shown the relation of these two
omponents of the adrenal body. In the elasmo-
>ranch fishes, the shark, ray, etc., the cortex is
epresented early by an internal body somewhat
od-shaped and elongated and situated toward
he posterior portion of the kidney. In the
)ony fishes this organ becomes paired. The
nedulla is represented by a series of paired
bodies extending along the vertebral column
ind in close relation to the sympathetic ganglia
and contain a number of chromaffin cells.

These two elements unite to form the com- FlG> 225>_SECTION OF HUMAN
xnmd adrenal. In the mammals the larger por- SUPRARENAL BODY, a, fibrous
ion of this chromaffin material fuses and be- capsule; 6, zona glomerulosa; c,
:omes enveloped on each side by the internal ^±g^^5^±SS
uody with the formation of the existing adrenal n'ei; gt ganglion-cells. (Piersol).
Dody. The unincorporated portions of the chro-
maffin bodies remain as masses of varying size, found in connection with
the sympathetic nerve system. The most important of these bodies is the
, ' abdominal chromaffin body" extending along the aorta from the level^of
•the adrenals to the bifurcation. It is readily exposed in the dog by stain-
ing with a solution of bichromate of potassium. Accessory adrenal bodies
may consist of either cortical or chromaffin material alone or of both.
The Effects of Disease and Removal of the Adrenal Glands.-
!is a profound disturbance of the nutrition first described by Addison and
subsequently termed by Trousseau, Addison's disease, which is characte
ized'by extreme muscular weakness and an incapacity for sustained muscle
activity a bronze-like discoloration of the skin and mucous membranes, dis

502 TEXT-BOOK OF. PHYSIOLOGY

turbance of the digestive functions, indicated by indigestion, vomiting and
diarrhea; a feeble action of the heart; a small feeble pulse; a low blood-pres-
sure; a subnormal temperature and a feeble respiration. This condition,
which is thus largely characterized by a loss of tone in the skeletal as well aj
the vascular musculature, terminates fatally from a paralysis of the respira-
tory muscles. Post-mortem examination reveals in all cases a more or less
extensive disease of one or both adrenals. A very common lesion is a tuber-
cular degeneration. These symptoms were attributed by Addison to a loss
of function of the glands.

The removal of these bodies from various animals by surgical procedure;
is invariably and in a short time followed by death, preceded by some of th<
symptoms characteristic of Addison' s disease. Thus, shortly after their re
moval the animal becomes tranquil and apathetic; the respiration soon be
comes feeble and difficult; prostration supervenes and the animal appears a;
though paralyzed, but the irritability of the skeletal muscles and nerves ii
normal; the heart becomes slow, feeble and irregular; the blood-pressure fall
promptly 20 to 30 mm. of mercury, after which it steadily falls to a low level
the appetite fails, the temperature declines and death occurs in from twelvi
to forty-eight hours. In some instances a pigmentation of the skin simila
to that seen in Addison's disease has been observed. From the fact tha
animals so promptly die after extirpation of these bodies, and the further fac
that the blood of some animals is toxic to the subjects of recent extirpation
but not to normal animals, the conclusion was drawn that the function of th
adrenal bodies is to remove from the blood some toxic product of musd
metabolism. Its accumulation after extirpation was supposed to causi
death through autointoxication. This view is, however, not generall;
accepted.

The Effects of the Injection of Gland Extracts.— On the suppositio]
that the adrenals might secrete and pour into the blood a specific materia
that favorably influences general metabolism, Schafer and Oliver injecte(
hypodermatically glycerin and water extracts of the medulla into the bodie
of various animals and observed at once an increased rate of the heart
beats and of the respiratory movements. The effects however were onl
transitory. When these extracts were injected into the veins directly, ther
followed in a short time a cessation of the auricular contraction though th
ventricular contraction continued vigorously but with a slower rhythm
The blood-pressure at the same time was markedly increased. If the vag
were cut previous to the injection or if the inhibitor influence of the vag
was removed by an injection of atropin the reverse effects were produced
viz., an increase in the rapidity and vigor of both the auricular and ventricula
contraction accompanied by a still more marked rise of blood-pressure. Thi
latter effect is the result partly of the increased action of the heart but ver
largely the result of a vigorous contraction of the muscle-fibers in the walls of th
arterioles. This is attributed to a direct stimulation of the arterioles and no
to a stimulation of the vaso-constrictor center. The contraction of th
arterioles is quite general as shown by plethysmographic studies of the limbs
the spleen, kidney, etc. The arterioles of the lungs and brain do not con
tract under its influence to the same extent as do the arterioles in othe
regions of the body, possibly for the reason that the arteriole muscles ii
these organs are not so abundantly supplied with vaso-motor nerves as the

INTERNAL SECRETION 503

.are in other regions of the body. Applied locally to the mucous membranes,

[adrenalin extract produces contraction of the blood-vessels as shown by the

[pallor which follows. 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.

In the foregoing instances the extract apparently produces its effects by

an augmentation of the normal tonus of the arteriole muscle. The effects

follow the injection of an extract of the medulla only. An extract of the

cortex appears to be without influence.

The injection of small doses of the active principle of the gland into the
peritoneal cavity or into the blood is also followed by glycosuria in the
course of an hour which may last for several hours.

The Internal Secretion.— It is apparent from the results of these experi-
;ments that the adrenal bodies are engaged in elaborating and pouring into
; the blood a specific material which on the one hand stimulates to increased
, activity the muscle-fibers of the heart and arteries, thus assisting in main-
taining the normal blood-pressure, and on the other hand maintaining the
tonicity of the skeletal muscles. An alkaloidal substance was isolated by
Abel from extracts of this gland, to which the term epinephrin was given. A
crystallizable substance was isolated first by Takamine and later by Aldrich,
to which the term adrenalin was given. Both substances are apparently
equally efficacious in causing contraction of the blood-vessels and in raising
the blood-pressure. Epinephrin or adrenalin represents the active principle
of the gland and is regarded as a product of the secretor activity of the
cells composing the medulla. As the effects following the intravenous injec-
tion of adrenalin are of short duration, the supposition is that it is speedily
oxidized. It is regarded as the typical hormone.

The action of adrenal extract however is not limited to the non-striated
muscle-fibers of the arterioles but extends itself to the non-striated fibers
found in the the walls of the viscera, e.g., stomach and intestines, gall-
bladder, urinary bladder, uterus, etc. The administration of this secretion
is followed however, in these regions, by an inhibition of the tonus and
subsequent relaxation of the visceral walls, with the exception of the uterus.
In the case of the isolated virgin uterus of many mammals, adrenal augments
the tone and the vigor of the spontaneous contractions. In addition to the
foregoing phenomena, dilatation of the pupil, increased flow of saliva and
glycosuria have been observed.

From an examination of the effects that follow the intravenous injection
of adrenalin or adrenal extracts, it is apparent that they closely resemble the
effects that follow stimulation of the sympathetic nerve in a large part if not
in all parts, of its distribution.

It has been a subject of discussion as to whether adrenalin acts on the
muscle-fiber directly or upon the endings of the sympathetic nerves with which
they are functionally associated. By reason of the fact that non-striated
muscles that have no connections with the sympathetic nerve system, are
not influenced by adrenalin; and the further fact that non-striated
muscles that have been deprived of their nerve connections through degenera-
tive changes following division of the nerves, are influenced by adrenalin,
have led to the assumption that it acts neither on muscle nor nerve, but on
some material which intervenes between the nerve endings and the muscle

5°4

TEXT-BOOK OF PHYSIOLOGY

but which is intimately related to the muscle. To this material Langley
has applied the term "receptive substance." In all instances the adrenalin
stimulates the receptive substances as a result of which the normal effect
produced by the sympathetic nerve impulse is intensified.

The Influence of the Nerve System. — The secretory activity of the
adrenals is regulated by the nerve system. Thus Dreyer found that the blood
of the adrenal vein after stimulation of the splanchnics was capab le of causing
to a much greater extent the usual physiologic effects when injected into an
animal than blood of the adrenal vein before stimulation and this independ-
ent of the vascular changes that were simultaneously provoked. It has also
been shown by Ascher that a high blood-pressure can be maintained by
prolonged stimulation of the splanchnics. Cannon has reported that major
emotional disturbances such as fright lead to an increase in the secretion
of the adrenals as shown by the fact that the blood taken from the vena cava
above the level of the adrenal veins will promptly produce an inhibition
of a contracting intestinal strip, while blood taken from the animal previous
to the fright, had no such effect. After ligation of the veins or the removal
of the adrenals there was a failure of this effect upon excitement.

Emotional excitement in cats at least is also attended with hypergly-
cemia and glycosuria which is probably due to an increase of the adrenal
secretion in the blood inasmuch as .a similar effect follows the injection of the
extract into the blood. The hyperglycemia and glycosuria caused either
by the intravenous injection of the extract or by an increased activity of the
adrenals following emotional excitement, fear or rage, is difficult of explana-
tion. It may be the result of a direct action or an indirect action through
secretor nerves on the liver cells, in consequence of which the stored glycogen
is rapidly transformed into sugar and discharged into the blood.

An advantage that would accrue to the animal from the accumulation
of sugar in the blood under these circumstances, would be a quickly avail-
able source of energy-yielding material, for the continued muscle activity
that would attend either flight or defense.

The Function of the Adrenal Gland. — The function of the adrenal
gland, at least of the medullary portion, is to furnish an internal secretion
which serves apparently to stimulate the receptive substance at the myo-
neural junction and thus cooperates .with the sympathetic system to maintain
that degree of frequency and force of the heart-beat and the contraction of
the arteriole muscle necessary to maintain the normal blood-pressure; to
inhibit as occasion requires, the tonus of muscle walls of various viscera;
to cause a mobilization of sugar in the blood when this is necessary, and
to increase in some unexplained way the tonus and activity of the skeletal
musculature.

The Chromaphil Tissue. — The chromaphil tissue lying along the aorta
and in other situations as well — that portion of the original embryonic
chromaphil tissue that was not incorporated in the medulla of the adrenal
gland — has been shown to possess physiologic actions similar to, if not
identical with extracts of the adrenal medulla itself. Thus it causes not only
a marked rise of blood-pressure but, according to the recent investigations of
Fulk and Macleod, in many mammals it inhibits the spontaneous contrac-
tions of isolated intestinal muscle and augments the tone and contractions of
the isolated virgin uterus.

INTERNAL SECRETION 505

The Pancreas.— The pancreas though engaged in the production of an

xternal secretion is yet, by reason of the specialized group of cells, the

slands of Langerhans, to be regarded as an organ of an internal secretion

s well. These islands it is generally believed are engaged in the secretion

f an agent which after entering the blood is carried to the muscles where it

ctivates or assists a glycolytic enzyme in promoting the oxidation of sugar;

r it may inhibit normally the stimulating action of adrenalin on the liver

ells and thus prevent an excessive output of sugar and the development of

yperglycemia (see page 519). If the entire pancreas is extirpated and the

Unimal survive the operation, a glycosuria is soon established, followed by a

Series of symptoms resembling those observed in diabetes mellitus as it occurs

Jin man, viz.: thirst, polyuria, loss of energy, decline in body-weight, etc.,

followed by death in a few weeks. Pathologic processes that involve a large

portion of the pancreas likewise give rise to a similar series of phenomena,

as ligation of the pancreatic duct, a procedure that leads to a destruction of

all portions of the pancreas except the islands of Langerhans and without

developing glycosuria has led to the inference that these islands are the

agents engaged in the production of the internal secretion.

The Testicles and Ovaries. — The testicles and ovaries are regarded at
the present time as glands for the production of an internal secretion, as well
as for the production of the characteristic reproductive elements. The
testicles possess in the epithelial cells lining the peripheral portions of the
seminiferous tubules an apparatus for the development of spermatozoa, and
in the cells in the connective tissue between the tubules — the so-called inter-
stitial cells — an apparatus for the production of the internal secretion which
is discharged into the blood, and favorably influences the development of the
secondary sexual characters, as well as the body as a whole. This general
view is based on facts such as the following:

The removal of the testicles early in life and before the age of puberty
leads to imperfect development of the vesiculae seminales and the prostate
gland; in addition to these defects, there is a failure of development of the
various and distinctly sexual characters peculiar to man and other animals as
well. Sexual desire is wanting and the body frequently remains in the in-
fantile state. In some instances, however, there is an increase in the bones of
the skeleton and hence the animal increases in size. It has been suggested
that the internal secretion controls the growth of the bones by antagonizing
the action of some other agent, as the secretion of the anterior lobe of the
pituitary, which promotes the growth of the skeleton. That these results are
not due to the loss of the structures producing the sperm elements, is shown
by the fact that ligation of the vas deferens only, while destroying these
structures, does not interfere with development of the sexual characters.
Transplantation of the testicles, in cocks and in certain of the smaller mam-
mals that have been castrated, has led to the development of secondary
sexual characters which in no apparent way differed from those of control
animals. In old age the testicles undergo degenerative changes characterized
by a decline in physical vigor, inactivity and a cessation of sexual activity.
From the foregoing facts it is apparent that these organs are engaged in the
production of an internal secretion which influences favorably the growth,
development, and general vigor of the entire body.

Extracts of the testicles have been prepared and injected into the body

5o6 TEXT-BOOK OF PHYSIOLOGY

in advanced years, with a view of reestablishing the former condition of
body vigor. Experiments of this character were made originally by Brown-
Sequard on himself and reported that his general health, his muscular power
and mental activity were much improved. These results have not received
general confirmation.

The ovaries are also regarded as glands for the production of an internal
secretion, as well as for the production of characteristic reproductive ele4
ments. The ovary possesses in the epithelium of the Graafian follicles ail
apparatus for the development of the ova, and in the cells of the intervening
stroma — the so-called interstitial cells — an apparatus for the production o|
an internal secretion which is poured into the blood and influences not onljii
the development of sexual organs but influences favorably the development
of the secondary sexual characters as well as the body as a whole.

The removal of the ovaries of human beings early in life is an operation
that is not often performed and hence it is difficult to state the results that
might arise. Their removal in certain animals leads to an atrophy of the!
uterus, and in addition, to a failure of development of secondary sexual
characters. Menstruation does not occur and the body does not reach matu-
rity. The removal of the ovaries in adult life results in a cessation of men-
struation, and the appearance of a variety of disorders of a body and mental
character. Similar phenomena are frequently observed at the menopause,
when the ovaries undergo degenerative changes. The administration oi
extracts of the ovaries — oophorin tablets — is claimed to relieve some of the
symptoms following the removal of the ovaries or occurring during the
menopause. The transplantation of an ovary into the wall of the uterus 01
into the broad ligament after ovariotomy in women has, even after the lapse
of two years, reestablished menstruation and awakened sexual desire. From
facts such as the foregoing it has come to be believed that ovaries also produce
an internal secretion which has a marked influence on the body and
mental states of women and mammals generally.

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 lefl
hypochondrium between the stomach and the diaphragm. In this situation
it is held in position by a fold of the peritoneum which passes from the uppei
border to the diaphragm.

Structure. — A section of the spleen shows that it consists of connective
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 trabeculae 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 trabeculse in some animals contain numerous non-striated
muscle-fibers. In man they are relatively few in number. The blood-ves-
sels which enter the spleen are supported by the connective-tissue septa
As they pass toward the center of the organ they divide very rapidly anc
soon diminish in size. In their course small branches are given off, whict
penetrate the inter-trabecular tissue and become encased with spheric 01

INTERNAL SECRETION 5°7

oylindric masses of adenoid tissue known as Malpighian corpuscles. These
corpuscles are composed largely of leukocytes. In some animals the leuko-
cytes, instead of being arranged in masses, are distributed along the walls
of the artery as a continuous layer. Within the corpuscles the arteries pass
into capillaries; whether the artery passes directly to the splenic pulp or indi-
rectly by way of the corpuscles, its ultimate branches terminate 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 dis-
puted question as to whether the spaces are lined by epithelium, thus form-
ing a continuous blood channel, or whether they are wanting in this histologic

element.

The Splenic Pulp.— The spaces of the connective-tissue framework
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 experiments
which have been made to determine the functions of the spleen, it cannot 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 inter-
fering with the normal metabolism would indicate that its function is not very
important. The chief changes observed after such a procedure are an en-
largement 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 contnt
utes 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 lack-
ing positive confirmation. . . ,

Volume Variations of the Spleen.— It was shown some years since by
Roy, with the aid of the plethysmograph, that the spleen undergoes rhyth-
mic variations in volume from moment to moment. In the cat and m t.
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 recording apparatus
being filled with oil, each variation in volume was attended by a to-and-fro
displacement and a corresponding movement of the recordi ng lever The
special form of plethysmograph used for this purpose is known as t
cometer or bulk measurer, and the recording apparatus as the oncograph
The cause of these variations in volume Roy attributed to a rhythmic
contractility of the non-striated muscle-fibers in the capsule and .trabecute
and not to changes in the arterial blood-pressure, as the curve of the pressure
taken simultaneously remained practically uniform. The effect of the

5o8 TEXT-BOOK OF PHYSIOLOGY

rhythmic contractions of the splenic muscle-tissue is to force the blood
through the organ, a condition 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 simultaneously with the contraction of the
arterioles generally. On the contrary, the splenic volume increase is coinci-
dent 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
condition.

Influence of the Nerve System. — The nerves which supply the vascu-
lar 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
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 possesses some
contractile mechanism independent to some extent of the nerve system.

CHAPTER XX
METABOLISM

TEXT-BOOK OF PHYSIOLOGY

The term metabolism may, therefore, be applied to the chemic change
which materials undergo, under the influence of living cells. It comprises
(i) the elaboration of substances of relatively low composition into sub-
stances of higher and more complex composition — anabolism; and (2)
their subsequent reduction to substances of lower and less complex com-
position, as well as the reduction of the relatively less complex food materials
— katabolism.

For many reasons it is desirable to know, as far as this is possible the
extent and character of the tissue changes, as well as the successive chemic
changes which the food materials undergo in their transit through the
body from day to day.

In order to obtain this information it is necessary to place the subject of
the experiment under conditions which permit of the collection of the ex-
cretions for purposes of analysis, and then to deduce from the nitrogen and
carbon they contain, the amounts of the tissue materials metabolized in the
absence of food; or the amounts of the food materials metabolized when the
ordinary amounts of food are consumed. Supplementary facts from the
field of physiologic chemistry throw much light on many of the intermediate
chemic stages.

The methods by which a metabolism experiment is conducted have been
detailed on pages 118-120.

In various sections of the preceding pages some of the facts pertaining
to the metabolism of the body have been presented and considered, and
it will suffice in this connection to summarize some of the facts alread}
alluded to and to add others not heretofore mentioned.

Before considering the metabolism of the individual organic food ma-
terials as it unfolds itself in the animal body under physiologic conditions,
it is of advantage to consider the metabolism of living material as it mani-
fests itself when the animal or the human being is deprived of food for a
variable length of time or during the starvation process.

THE METABOLISM OF LIVING MATERIAL DURING STARVATION

When an animal is subjected to the conditions of a metabolism experi-
ment and deprived of all food except water and oxygen, it is a relatively simple
matter to collect the excretions, to analyze them for nitrogen and carbon and
then to calculate from the amounts of each the amounts of protein and fal
metabolized from day to day. The carbohydrates may be left out of con-
sideration in a prolonged experiment. The following experiment is
illustrative. The man fasted five days and performed light work in the
respiration chamber.

METABOLISM OF J. A. IN STARVATION

N elimination

C elimination

D f f

Urine

Feces

Total

Urine

Feces

Respiration

Total

12 .04

0.13

12.17

8.0

1. 1

188.5

197.6

12.72

0-13

12.84

8-3

1. 1

179.4

188.8

13.48

0.13

13.61

9.9

1. 1

172.2

183.2

13.56

0.13

13.69

10.3

I.I

169.4

180.8

11-34

0.13

11.47

9-3

1. 1

165.8

176.2

METABOLISM

From the foregoing figures the following table of the general metabolism
*s made:

Day of fasting

Protein

Fat

Calories
from protein

Calories
from fat

Calories,
total

76.1

206. 1

303-5

1916.9

2220.4

80.3

191.6

320.5

1781.9

2102.4

85.1

181.2

339-4

1684.7

2024.1

85.6

177.6

341-4

1651.9

1992.3

71.7

181.2

286.1

1684.7

1970.8

— From Lusk's "Nutrition."

From the foregoing table it will be readily observed that with each sus-
:eeding day there was metabolized as the result of activity and for purposes
>f heat-production a certain quantity of protein and fat which on the
ifth day amounted to 71.7 grams and iSi.21 grams, respectively, yielding
'.ogether 1970.8 Calories.

In prolonged starvation the metabolism of protein and fat continues
;hough in diminishing amounts until both reach the minimum just^ previous
;o death, which for the protein is about 18 or 20 grams. The minimum for
:he fat at this period depends on the amount of fat in the body prior to the
starvation period.

During the course of the starvation there is a corresponding decline in
the body weight. Coincidently certain disorders of nutrition arise. ^ (For
further particulars regarding starvation metabolism, the loss^of weight of
different tissues and the post-mortem appearance the reader is referred to
pages 128 and 129.)

THE METABOLISM OF THE BODY ON A MIXED DIET

As an illustration of the result of a metabolism experiment on a mixed
diet the following experiment of Pettenkofer and Voit may be cited,
subject was a man, weighing 70 kilograms and at rest.

On a mixed diet the materials under outcome were collected; i >m
their amounts it was calculated that the body had received the amounts
the food principles under income.

COMPARISON OF THE INCOME AND OUTCOME^

Income

Outcome

Nin
grams

Cin
grams

Nin
grams

C in

gicims

HzOin

grams

137 grams albumin
117 grams fat . . .

| 19-5

3I5-5

Urine . •

17-4

2.1

12.7

14.5
248.6

275.8

1279

83
828

2190

Feces

352 grams carbohydrate . .
2016 grams water

19-5

It will be observed that the body was in nitrogenous equilibrium, hat
it stored up 39.7 grams of carbon equivalent to 52 grams of fat, a
it eliminated 174 more grams of water than were consul

> The fat is calculated from the carbon remaining after deducting ; th< > carbon denved
the protein which is equal to 3.28 grams for each gram of nitrogen ehr

512 TEXT-BOOK OF PHYSIOLOGY

THE METABOLISM OF THE PROTEINS

It will be recalled that the protein constituents of the food introduce
into the alimentary canal are reduced by the action of the enzymes of th
gastric and pancreatic juices to relatively simple compounds, viz. : ammoni;
amino- and diamino-acids which are for the most part at once absorbed int
the blood. A portion of the ammonia and a portion of some of the aminc
acids pass into the large intestine where they give rise under the destructiv
action of bacteria to substances of a simpler character such as indol, skato
and phenol. These and other putrefactive substances are in part discharge
from the body in the feces and in part absorbed into the blood and ultimate!
eliminated in combination with potassium sulphate by the kidney (see page
458 and 488). The ammonia absorbed is converted during its passag
through the liver into urea.

The absorbed amino-acids, after passing through the liver and gainin
entrance into the general circulation, are distributed to the tissues whei
they are in part utilized for the reconstruction of the tissue molecules ths
have undergone metabolism in consequence of their functional activity, an
in part separated by the action of tissue enzymes into the amine elemer
NH2, and a carbonaceous radical. The percentage of the amino-acids use
for the former purpose varies at different periods of life and under varyin
circumstances. In youth when tissues are developing and growing and a
thesame time are undergoing an active metabolism, a larger percentage :
utilized for repair and growth than in adult life when the body-growth i
least has ceased, though metabolism continues more or less actively.

The amine element, NH2, and the carbonaceous radical arising froi
the cleavage of the amino-acids not utilized for tissue growth and repair ai
disposed of as follows. The NH2 is combined with hydrogen and then wit
CO2 to form ammonium carbonate which, perhaps in the tissues, certain!
in the liver is transformed into urea; the carbonaceous radical is oxidize
to CO2 and water with the liberation of heat. The heat-producing powe
of protein — 4 Calories per gram — is almost entirely due to this oxidatior
Since scarcely more than half the protein consumed is utilized for tissu
repair the question has been raised as to whether a smaller amount tha
that usually consumed viz.: 100 to 120 grams, would not be equally satie
factory. For statements regarding the advantages of a low- and higt
protein diet see pages 121 and 122.

The tissue protein, sooner or later, also undergoes katabolism in consc
quence of functional activity, and the natural supposition would be that
too is reduced to its constituent amino-acids which are then disposed c
by the methods already alluded to. Nevertheless there are reasons for thin!
ing that this is not wholly the case. Folin has presented a series of fact
that lead to the supposition that this portion of the protein is represente
in part by creatinin. Thus it has been shown that while the urea excrete
rises and falls with the protein consumed, the amount of creatinin excrete
remains more or less stationary; again, in individuals of pronounced muse!
development, after unusual muscle activity and in diseases involving
destruction of muscle-tissue there is an increase in creatinin eliminatior
In the opposite conditions the amount excreted is low. For these and othe
reasons creatinin is regarded as a specific final end-product and an indicate
of the metabolism of muscle protein.

METABOLISM 5!3

The Metabolism of the Nucleo-prolems.—The tissue cells of animal and
many vegetable foods contain nuclei into the composition of which nucleo-
proteins enter. In the metabolism of the nucleo-proteins it is assumed on
the basis of their reaction with hydrolyzing agents that they undergo a
cleavage into a protein and nuclein. The nuclein subsequently gives rise
to nucleic acid which, under the influence of an enzyme, is separated into
two bases, guanin and adenin ; these bases under the action of two enzymes,
guanase and adenase, are combined with water, deaminized and transformed
into xanthin and hypoxanthin. The oxidation of these latter compounds
gives rise to uric acid. (See page 13 and 457.)

THE METABOLISM OF THE FAT

It will be recalled that during the digestive process the neutral fats are
gradually reduced by the action of the enzyme lipase of the pancreatic juice
to corresponding fat acids ^ucLglycerin; that subsequently the fat acids com-
bine with alkalis to form soaps, after which both soaps and glycerin are
absorbed. After absorption and during their passage through the epithelial
cells covering the villi, the soap and glycerin are synthesized into fat globules
and deposited in the cell material. Subsequently the fat granules enter
the lymph- vessels of the mesentery and by which they are discharged into
the thoracic duct and finally into the blood at the junction of the -subclavian
and internal jugular veins. After a meal rich in fat, the amount of fat in
the blood may be so great as to impart to it a distinctly pink color. In the
course of several hours the fat disappears. As to the change it undergoes
before it passes across the capillary wall there is much obscurity. It is
stated by Hanriot, that a lipase in blood again hydrolizes the fat to fat acids
and glycerin and in consequence of their solubility, pass out into the lymph-
spaces. By ultra-microscopic methods of illumination the fat may be ob-
served as "blood dust," in vigorous Brownian movement (Bayliss). It is
possible that under this form they pass into the lymph-spaces. As to the
final disposition of the fat the general belief is, that when it is consumed
in normal amounts and under physiologic conditions it promptly undergoes
oxidation in the tissue cells yielding carbon dioxid and water with the libera-
tion of heat.

The intermediate stages through which fat passes have not been wholly
determined. Nevertheless it is a well-founded supposition that the first
stage is a cleavage of the neutral fats to fat acids — stearic, palmitic and
oleic, and glycerin.

These fat acids are the higher members of the fat acids and are character-
ized by a high molecular weight. Before they can undergo complete oxida-
tion they must be reduced to lower acids of the fat acid series, acids of a low
molecular weight.

This is accomplished presumably by successive cleavages and oxidations.
Thus stearic acid which consists of 18 carbon groups is successively reduced
by oxidation of the carbon atom in the beta position until the four carbon
stage is reached when the resulting acid is known as butyric acid, e.g.,
CH3.CH2.CH2.COOH. Upon oxidation of the beta carbon atom there is
formed /3-oxybutyric acid, viz.: CH3.CHOH.CH2.COOH.

The succeeding oxidation results in the removal of the two hydrogen
atoms attached to the beta carbon atom and results in the formation of

5i4 TEXT-BOOK OF PHYSIOLOGY

aceto-acetic acid or diacetic acid, viz.: CH3.CO.CH2.COOH, which by the
loss of a molecule of CO2 yields acetone, viz.: CH3.CO.CH3.

Acetone by a further oxidation is converted into carbon-dioxid and
water. Thus by these successive oxidations the stored-up energy of the
fat is liberated as heat.

If fat is consumed in too large a quantity the last stage may not be
who lly completed and some of the acetone produced may be eliminated in
the urine. Analysis of the urine generally shows the presence of acetone
to the extent of o.oi to 0.03 grams daily.

In diabetes these three compounds — /3-oxybutyric acid, aceto-acetic acid
and acetone — make their appearance in the urine. Since all oxidations are
preceded by a cleavage the inference is that in this disease there is an
absence of the enzyme necessary to effect the cleavage of the oxybutyric
acid and thus permit of further oxidation. The accumulation of this
acid in the body establishes the condition known as acidosis and in turn to
coma and death.

The Origin of the Body Fat. — i. From Protein.— It is a familiar
observation that on the customary diet the animal body frequently accu-
mulates a large amount of fat. The question, therefore, has arisen as to its
origin, and many experiments have been made to determine it. If an animal
is placed oft a protein diet, fat is very seldom deposited. If the protein is
consumed in normal amounts, and the fats and carbohydrates in excess of
the normal amounts, there is generally an accumulation and storage of fat.
It was for a long time taught by v. Voit that the carbonaceous radical set
free in the metabolism of protein was retained in the body and deposited as
fat. This origin of fat has largely been disproven. The experimental
work of subsequent investigators has shown that v. Voit's results were due
to a faulty analysis of the food consumed ; that if certain errors are eliminated,
all the carbon arising from protein metabolism can be detected in the usual
excretions. The prevailing opinion is that fat does not arise from protein.

The appearance of fat granules in the cells of tissues undergoing certain
pathologic changes was formerly regarded as evidence that the protein con-
stituents of these cells in their metabolism give rise to fat. This view is also
no longer entertained. The facts observed lead to the inference that the
fat thus appearing is either transported from the connective-tissue cells or is
deposited directly from the blood-stream; but owing to the destructive changes
in the cell it is no longer able to metabolize it. The fat observed in muscle -
and gland cells is to be regarded as an evidence of deposition rather than of
degeneration.

2. From Fat. — If now body fat is not a derivative of protein, the only two
other sources are the fats and the carbohydrates of the food. That the fat
of the food can be deposited is now a general belief, contrary to the former
belief, that the fat of the food was at once oxidized, thus sparing from oxida-
tion the fat arising from the protein. This belief is based on the results of
experiments which however are not strictly physiologic in character. Thus
two dogs were starved for a definite period. One was then given a large
amount of linseed-oil and the other large amounts of mutton fat. At the
end of several weeks both had accumulated fat. A post-mortem examina-
tion of the fat of the first dog showed that it was liquid at o°C. while the
fat of the second dog was solid at 50 °C. In another experiment a dog

METABOLISM 5*5

w allowed to starve until its weight had been reduced 40 per cent. It was
inen given large amounts of fat with small amounts of meat. At the end
f)f five days, the animal was killed and found to contain 1353 grams of fat.
Of this amount only 131 could have come from the protein. The inference
' herefore is that the food fat is capable of being deposited as such. The
I )resence in the animal's fat of foreign fat acids such as erucic acid has
\ )een detected when the animal has been fed on colza or rape-seed oil.

Though these facts hold true it is highly probable that under physiologic
S:onditions all the varieties of fats consumed as foods are digested and sub-
sequently synthesized by the epithelial cells of the villi into the form of fat
:haracteristic of the animal, and if not immediately needed for oxidation
Purposes is transported to the connective tissues and deposited. Regardless
)f the nature of the fat in the food, the fat of the animal is peculiar to it and
possesses physical and chemic properties which serve in large measure to
distinguish it.

3. From Carbohydrates.— That carbohydrates are capable of being
-ransformed into fat when consumed in amounts beyond that necessary
IOT heat-production is a generally well-recognized fact, though the successive
steps by which this is brought about have never been disclosed. It has not
been possible to effect this transformation by any known chemic procedure.
Animals fed on a diet containing the customary amounts of protein and fat
but containing a somewhat larger amount of carbohydrates than usual soon
begin to lay on fat. The many experiments on the fattening of animals have
placed this beyond question. This article of food must be looked on as the
chief source of the body fat.

THE METABOLISM OF THE CARBOHYDRATES

It was stated in a previous chapter that all the starch and sugar that are
consumed daily are converted by the action of the salivary, pancreatic and

•intestinal enzymes, for the most part into glucose; that after absorption the

'glucose is transported by the blood of the portal vein to the liver; that while
passing through the liver capillaries, a portion passes across the capillary wall
into the surrounding lymph and into the liver cells in which it is dehydrated
by enzymic action and converted into starch (glycogen); that subsequently

|this starch is again hydrated by the same or a different enzyme and con-
verted into sugar, glucose or glycose, after which it passes into the blood to
take the place of the sugar which has left the blood^ and which has
oxidized in the tissues. By this process, glycogenolysis the normal percent-
age of the sugar in the blood is maintained. The metabolism of the carbo-

: hydrates in the body includes a brief statement of the formation of glycogen
and sugar in other tissues than the liver and more especial y in the muscles.
Muscle Glycogen.-Glycogen is also found in muscles and to some
extent in the placenta, and embryonic tissues generally. Chemic analysi
has shown that muscles contain from 0.5 per cent, to i per • cen . and as
these organs amount to about 40 per cent. (28 kgm.) of weight of the body,
70 kgm., they generally contain from 140 to 280 grams of glycogen Inas-
Luch as chemic analysis has failed to demonstrate the presence of glycogen
in the blood, the inference is that it arises in the muscle-cell in a manner
similar to that observed in the liver-cell, viz.: by a transformation, through

5i6 TEXT-BOOK OF PHYSIOLOGY

de-hydra tion, of the sugar of the blood. By reason of this fact it may be said
that the muscle also possesses a starch-forming or a glycogenic or an amylo-
genetic function. If it is a fact that of the sugar absorbed only from 12 to
20 per cent, is temporarily arrested by the liver, the remainder passing on
into the blood of the general circulation, it is readily conceivable that the
storage of the sugar under the form of glycogen by the muscle-cells is neces-
sary not only for the activity of the muscle itself, but as a means of preventing
an abnormal percentage of sugar in the circulating blood. It is generally
admitted that though the glycogen is the source of the energy expended by
the muscle, it cannot be disrupted and oxidized as such, but that it must
first be transformed into sugar (glucose); and for this purpose the assump-
tion is made that a special enzyme is present and active. The muscle is
therefore said to possess or exhibit a sugar-forming or a glycogenetic func-
tion. The muscle-cells are thus like the liver cells characterized by the two
processes amylogenesis and glycogenesis. During the periods of prolonged
activity of the muscles the percentage of glycogen rapidly diminishes, a fact
that leads to the inference that it is the source in large part of the energy
expended by the muscle. During the period of rest the percentage oi
glycogen rapidly increases until the normal is regained.

The Influence of the Nerve System.— The physiologic mechanism by
which glycogen is changed to glucose (glycogenolysis) in amounts just suffi-
cient to supply the needs of the tissues without perceptibly increasing the
amount of sugar in the blood is obscure and but imperfectly known. Thai
the nerve system is in some way concerned in the regulation of glycogenolysis
is apparent from observation of the effects of injuries, major emotional states
as well as of the results of experimental procedures, but whether the regula-
tion is direct, i.e., through its action on the liver-cells, or indirect, i.e., through
its action on the glands of internal secretion, e.g., adrenals and hypophysis,
is a subject of investigation and discussion. 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 lasts for from five tc
six hours in the rabbit and from two to three or even seven in the dog. Foi
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 (hyperglycemia). 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 con-
version of glycogen into sugar (glycogenolysis). This supposition receives
support from the fact that the degree of the hyperglycemia, and the subse-
quent glycosuria, will depend on the amount of glycogen previously in the
liver. If the animal has been well fed on carbohydrates, the resulting gly-
cosuria will be pronounced; if, on the contrary, it has been allowed to fast
for several days, the glycosuria will be slight.

Assuming that the nerve-cells, which constitute the diabetic area, influ-
ence the conversion of glycogen into sugar, the question arises as to whether
the puncture destroys the nerve-cells, or whether it stimulates them in some
way to increased activity. The results of experiment lead to the latter sup-
position. Thus stimulation of sensor nerves in different regions of the body,
injuries to the central nerve system, major emotional states, etc., which have

METABOLISM 51?

5i8 TEXT-BOOK OF PHYSIOLOGY

nerve impulses become more efficient. These facts do not decide the ques
tion, however, as to the character of the nerves involved, that is, whether the}
are purely vaso-motor or secretory in character. The evidence for the exist
ence of glycogenolytic nerves is not decisive.

Glycosuria. — The mechanism by which the sugar is stored in the livei
as glycogen, and subsequently transformed into glucose, and discharged int<
the blood in amounts just sufficient to meet the needs of the tissues withou
giving rise to hyperglycemia and glycosuria, as well as the mechanism b]
which it is oxidized is apparently very delicate and easily disturbed as indi
cated by the ease with which a glycosuria more or less pronounced and o
longer or shorter duration can be established. The causes of glycosuria ar<
many and their mode of action often obscure.

Alimentary Glycosuria. — When carbohydrate food, sugar more especiall]
is consumed in amounts beyond what constitutes the assimilation or storagi
limit, the excess soon appears in the urine. This limit varies somewhat ii
different animals. The tissues collectively of a normal human being havi
an assimilation capacity of approximately 150 grams of glucose. Shoulc
an amount of sugar be consumed much beyond this at one time, the excesi
will be eliminated in the urine. Inasmuch as carbohydrate material con
sumed each day is represented mainly by starch, and as digestion and ab
sorption proceed slowly and metabolism continually, a glycosuria due to {
large amount of aliment is not of frequent occurrence, under physiologii
conditions. The assimilation limit may be lowered or raised beyond thi
normal in accordance with variations in the activity of the hypophysis
(See chapter on Internal Secretion.)

Adrenal Glycosuria. — As stated in a previous paragraph the subcutaneou;
injection of adrenalin chlorid or of the watery extract of the medulla of thi
gland is very soon followed by a glycosuria and hyperglycemia as well. Thi
degree of the glycosuria will depend on the extent to which glycogen hai
been stored in the liver. It will also be recalled that stimulation of the lef
splanchnic nerve is followed by glycosuria by reason of a greater dischargi
of adrenalin into the blood. The increased transformation of glycogen t<
sugar has been attributed to an exaltation of the irritability of the nerve end
ings of the hepatic plexus or to a direct action on the liver cells themselves
As to which view is more probable, future experiments may decide. Ii
either case the adrenalin excites an increased transformation of glycogen t(
sugar. Any factor therefore which would more or less permantly increase
adrenal activity would cause a more or less permanent glycosuria. (See
page 504.)

Pancreatic Glycosuria. — It has been known for some years that if the
pancreas be extirpated and the animal survive the operation, glycosuria is
promptly established, followed by a series of symptoms which gradually, in-
creasing in severity, and lead to the death of the animal in from two to foui
weeks. In addition to the presence of sugar, acetone, aceto-acetic, /3-oxybu-
tyric acid, and an excess of urea have been found in the urine. The
quantity of sugar excreted and the gravity of the attendant symptoms maj
be much diminished, if not entirely prevented by allowing a portion of the
gland to remain even though its capacity for the production of pancreatic
juice is entirely abolished. Transplantation of various portions of the pan-
creas into the subcutaneous tissue, in the walls of the abdomen, will alsc

METABOLISM 5*9

view

520 TEXT-BOOK OF PHYSIOLOGY

It is assumed by the supporters of the second view that the production
of sugar in the liver cells is regulated by two opposing factors or hormones,
that secreted by the adrenal glands and that secreted by the pancreas, the
former exciting, the latter inhibiting the process. In the absence of the
pancreatic hormone the adrenal hormone unduly excites the production of
sugar. The following facts support this view: After extirpation of the adre-
nals or ligation of the adrenal veins, by which the secretion is prevented from
entering the venous blood, the removal of the pancreas is not followed as
usual by glycosuria. The simultaneous injection of an extract of pancreas
and an injection of that amount of adrenalin ordinarily necessary to produce
glycosuria prevents its development.

Pharmacologic Glycosuria. — It is well known that various pharmacologic
agents when introduced into the body frequently give rise to glycosuria,
though the manner in which they do so is not always clear. Thus, the ad-
ministration of ether, curare, the uranium salts, phlorhizin, etc., is generally
followed by the elimination of sugar in varying amounts. Of these agents
the one generally used for experimental purposes is:

Phlorhizin. — Phlorhizin is a glucosid obtained from the root bark of the
cherry, plum and apple tree. It can readily be separated into glucose and
phloretin on the latter of which its action depends. If, therefore, either
phlorhizin or phloretin be injected subcutaneously or administered by the
mouth, sugar very promptly will appear in the urine in amounts varying,
with the dosage, from 5 to 15 per cent. This glycosuria is, however, tem-
porary, but may be made more or less continuous by repeated injections at
least three or four times daily. Coincidently there is a diminution in the
percentage of sugar in the blood (hypoglycemia), the opposite condition to
that observed in the glycosurias heretofore mentioned. Inasmuch as this
deficiency calls for a larger discharge of sugar from the liver this organ and
others as well, soon become free from glycogen especially if the animal be
deprived of carbohydrate food. Experimental investigations lead to the con-
clusion that the seat of action of the phlorhizin is in the kidney itself (whether
in the glomerular or in the renal epithelium is uncertain) , in consequence of
which the sugar of the blood is permitted to pass along with other constitu-
ents into the tubules and hence into the urine. As to the change which the
kidney structures undergo under the action of phlorhizin not nruch is known.
It has been assumed that ordinarily the sugar is in combination with colloid
material in the blood and by reason of this combination cannot pass across
the wall of the glomerular blood-vessels. Any uncombined sugar, as is the
case in hyperglycemia, will diffuse readily in the urine. Under the influence
of phlorhizin the kidney structure is presumed to acquire the power of break-
ing up this combination setting the sugar free, whereupon it at once diffuses
into the urine. When phlorhizin is administered to an animal living on a
meat and fat diet alone, after all sugar has been discharged from the body,
sugar still appears in the urine indicating that it must have some other origin
than the glycogen of the liver or other tissues. The ratio of the dextrose to
the nitrogen in the urine — 2.8 to i (Minkowski) or 3.65 to i (Lusk), a ratio
which corresponds to that found in the urine of diabetic patients living on a
meat-fat diet, leads to the conclusion that the sugar arises from the metabo-
lism of protein in a mariner already alluded to.

Pituitary Glycosuria. — It was discovered by Gushing during his experi-

METABOLISM 521

mental and surgical procedures incidental to the removal of the pituitary
in whole or in part, that glycosuria frequently develops which, however,
gradually passes away. Subsequently it was shown that mechanical or
electrical stimulation of the intact posterior lobe gave rise to a similar glycosu-
ria provided of course there was a sufficiently large amount of glycogen in
the liver. This effect has been attributed to an increased discharge of the
internal secretion of the posterior lobe into the cerebro-spinal fluid and finally
into the blood, which in some way lowers the assimilation limit of the ani-
mal and hence leads to an elimination of sugar (see chapter on Internal
Secretion).

Parathyroid Glycosuria. — When the thyroids and at least three of the
parathyroid bodies are removed in animals, there is a disturbance of carbo-
hydrate metabolism, a diminished tolerance for sugar, as shown by the
appearance of glycosuria. A similar condition is established when three
parathyroids and but one thyroid are removed. It would thus appear that
the parathyroids rather than the thyroids have an influence, though un-
defined in the regulation of carbohydrate metabolism.

The problem of the storage of sugar in the body, its release and subse-
quent oxidation in accordance with metabolic needs is of great interest by
reason of the fact that it is intimately related with that grave condition of
persistent glycosuria seen in man and to which has been given the term:

Diabetes. — This term has been applied to a syndrome characterized by
persistent hyperglycemia and glycosuria accompanied by thirst, wasting of
the tissues and imperfect oxidation of the fats. The sugar discharged into
the urine varies in amounts but continues even in the absence of all carbo-
hydrate food. The condition is usually permanent, enduring for months or
years but eventually terminating in the death of the individual.

The cause of the disease in each instance is not always clear nor the man-
ner in which the storage, release or oxidation of the sugar is disturbed.

A characteristic feature of the disease is the continuous elimination of
sugar even in the absence of carbohydrate food. Under these circumstances
there is evidence that the sugar is derived from the protein consumed or
from the protein of the tissues or from both. It is well known that in the
metabolism of protein it undergoes a cleavage into an NH2 element and an
organic acid radicle. It is further known that the NH2 element is combined
successively with hydrogen and carbon dioxid to form ammonium carbonate
which is transformed in the liver into urea; that the organic acid radical is
converted into sugar. It has also been determined that the dextrose yielded
in the metabolism of protein bears to the nitrogen yield, the ratio of 3.65
to i, indicating that for every 3.65 grams of sugar and every gram of nitrogen
6.25 grams of protein have been metabolized. In diabetes, in the absence
of carbohydrate from the diet, this ratio of dextrose to nitrogen in the urine,
expressed by the symbol D : N, has been found to exist from which the de-
duction is made that the origin of the sugar is to be sought for in the metab
lism of the proteins. . - - ,

The necessity for heat-production leads to a larger consumption of fat
and this in turn impairs or overtaxes the capacity of the body tissues for fat
oxidation and hence there arises imperfectly oxidized cleavage products such
as /3-oxybutyric acid, aceto-acetic acid and acetone which are finally e
nated in the urine thus establishing the condition of acidosis.

522 TEXT-BOOK OF PHYSIOLOGY

It is, therefore, apparent that the phenomena of diabetes in man closely
resemble the phenomena that arise in animals when the pancreas is removed;
hence there is a general belief that destructive disease of the pancreas is the
most frequent cause of diabetes.

From the foregoing facts it is clear that a glycosuria more or less pro-i
nounced may be due to the following causes:

1. An imperfect abstraction of sugar from the portal blood and its storage

as glycogen by the liver cells.

The imperfect abstraction and storage of sugar may be due to an impair-
ment in the functional activities of the liver cells or to the ingestion and
absorption of excessive quantities of sugar from the intestine. In the
latter instances the resulting glycosuria is said to be of alimentary origin.

2. A too rapid conversion of glycogen to sugar on the part of the liver cells

(glycogenesis or glycogenolysis).

The increased conversion of glycogen to glucose may be due to impair-
ment of the nerve-centers regulating the normal process or to stimula-
tion of the liver cells by one or more hormones discharged into the blood
in unusual quantities by some organ of internal secretion, e.g., adrenalin.
See page 504.

3. An incomplete oxidation of sugar (glycolysis) in the cells of muscle and

perhaps other tissues as well, in consequence of which it accumulates
in the blood beyond the normal amount, thus establishing the condition
of hyperglycemia.

The imperfect oxidation of sugar in the muscle- tissue is probably the
result of an absence of the necessary glycolytic enzymes. The more
rapid metabolism of the protein constituents of the tissues whereby
their glucose radicals are liberated in large amount maybe necessitated by
the inability to oxidize the sugar normally brought to them by the blood.
The final disposition of sugar in the body is an oxidation to carbon
dioxid and water. The chief if not the only intermediate stage is lactic
acid. It appears from chemical relations that the molecule of sugar is trans-
formed into two molecules of lactic acid and these in turn into carbon dioxid
and water. In its final oxidation the contained energy is liberated as heat.

CHAPTER XXI

THE CENTRAL AND PERIPHERAL ORGANS OF THE NERVE

SYSTEM

The nerve system has been resolved by histologic investigation into single
morphologic units termed neurons. Though they have a common origin
they have assumed different forms, in different regions of the system in the
course of their development. Nevertheless it is apparent from an ex-
amination of their structure that they have many features in common.
The neurons in their totality constitute the neuron or nerve tissue.
Arranged in both a serial and a collateral manner into a regular and con-
nected whole, they form the neuron or nerve system. The neurons, more-
over, are grouped into more or less complexly organized masses termed organs
which in accordance with their location may be divided into (i) central organs
and (2) peripheral organs.

The Central Organs.— 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, occu-
pying 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 foss* of the cranium; (3) the isthmus of
the encephalon, the more or less pyramidal-shaped portion connecting t
cerebrum and cerebellum with each other and both with (4) the medulU

narrow and cylindric in shape. It occupies the spinal
canal as far as the second or third lumbar vertebra

The central organs of the nerve system are bilaterally symmetric, c
sisting of distinct halves united in the median line The cerebrum ,
divided by a deep fissure, running antero-postenorly, into two ovoid mass
termed clebral hemispheres; the cerebellum is also partially *^vtted^
hemispheres; the isthmus likewise, presents in the *^ *JeJJ^
division into halves; the medulla oblongata and spinal cord are s iivided
by an anterior or ventral and a posterior or dorsal fissure into

^ThePeripheral Organs.-The peripheral organs of the nerve system
in c relation with the central organs are the ence-

phalic and the spinal nerves. , lpr):qn i:ne

The encephalic nerves, twelve in number on each side of the median oe

5t^^^

are usually termed cranial nerves

524 TEXT-Bf)OK OF PHYSIOLOGY

The spinal nerves, thirty-one in number on each side, are in anatomic
relation 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 of the nerve system.

The spinal nerves consist of two groups of nerve-
"fibers, a ventral and a dorsal group. Though
closely intermingled in the common trunk of the
spinal nerve they are distinctly separated near the
spinal cord. Owing to their connection with the
ventral and dorsal surfaces of the spinal cord they
have been termed respectively the ventral and dor-
sal roots. Peripherally the ventral root fibers are
distributed to skeletal muscles, glands, walls of
blood-vessels and walls of various viscera: the dor-
sal root fibers are distributed to skin, mucous mem-
branes, muscles, joints, etc.

The central organs of the nerve system are sup-
ported and protected by three membranes named,
in their order from without inward, the dura mater,
the arachnoid, and the pia mater.

The Encephalo-spinal Fluid. — The general
subarachnoid space, as well as certain cavities within
the encephalon, contain a clear transparent fluid,
termed the encephalo-spinal fluid. This fluid has
an alkaline reaction and a specific gravity of 1.007
or 1.008. It is composed of water, proteins (pro-
teoses and serum globulin), and pyrocatechin
C6H4(OH)2, 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.

The Functions of the Nerve System. — It will
be recalled that the entire nerve system can be re-
solved into simple morphologic units, termed neu-
rones, each of which possesses certain histologic fea-
tures and physiologic properties; that their relation
one to the other both in a serial and collateral man-
ner gives rise to the general architecture of the nerve
system. Recalling the functions of neurons in their
individual and collective capacities the functions of the nerve system may be
formulated.

The functions of the nerve system are twofold: (i) It unites and asso-
ciates the organs and tissues of the body in such a manner that they are
enabled to cooperate for the accomplishment of a definite object. (2) It

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.y
ms. The upper and lower
limits of the spinal cord.
The remaining letters in-
dicate the region and num-
ber of the spinal nerves. — •
(Quain, after Bourgery.}

THE SPINAL CORD 525

serves to arouse in the individual a consciousness of the existence of an
external world, by virtue of the impressions which it makes on his sense
organs, and consequently to enable him to adjust himself to his environment.

By virtue of the anatomic and physiologic association, a stimulus, 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
I coordination of action is accomplished mainly by the spinal cord and the
[medulla oblongata. Alljactions which take place in response to a periph-
eral stimulus and independently of volition are termed reflex actions. The
reflex activities connected with digestion, the circulation of the blood,^with
respiration, excretion, etc., are illustrations of the coordinating capabilities
of the nerve-centers located in these portions of the central nerve system.

By virtue of the physiologic activities of the encephalon and more
particularly of the cerebrum and of the relation existing between it and the
sense organs of the body, consciousness of the existence of an external
world and the individual's relation to it is developed. Experimental and
clinic investigations show that of the parts of the encephalon, the cerebrum
is the chief, though not perhaps the sole organ of the mind and that its
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 organs of the nerve system
and the remaining structures of the body.

The nerve-trunks constituting this part of the nerve system may I
divided into two groups, as follows:

1 The first group comprises nerves in connection with the special sense

organs, e.g., skin, eye, ear, nose, tongue, as well as nerves in connection
with the general or organic sense-organs, e.g., mucous membranes,
viscera, etc., which are connected primarily with nerve-cells in the spinal
cord and medulla oblongata, and secondarily with nerve^cells in 1(
ized areas of the cerebral cortex. . .

2 The second group comprises nerves which terminate mainly in the

muscle apparatus and which constitute the continuation of nei
which have their origin in nerve-cells of localized areas of the c

ThTfirst group of nerves, the afferent, especially those connected with
the special sense-organs, are excited to activity ^PfJ^^f^
peripheral terminations by agencies m the external world The nerve
impulses thus generated are transmitted m part only as far
cord and medulla oblongata while the remainder ascend ;™e'c^ '"
localized areas of the cerebral cortex where they evoke sensations.

»-

in their related n.rve-ce Is .te 1 '«°">>" v™

c •*— ffi

become physical expressions of mental states, and if direct

526 TEXT-BOOK OF PHYSIOLOGY

manner to the overcoming of the resistance offered by the external world
they become capable of modifying it in accordance with the mental states.
The efferent nerves thus become a means of communication 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, after a brief pre-
sentation of the relation of the spinal nerves to the spinal cord, to consider
them in the order of their complexity, beginning with the spinal cord and
ending with the cerebrum.

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 gray matter. From their origin 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
^taaJLganglia. 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 ascending branches. The former
probably constitute the comma tract, the terminal branches of which sur-
round 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.

Classification of Spinal -nerve Fibers. — The ventral root axons have
been shown by histologic and physiologic methods of investigation to be dis-
tributed to skeletal muscles, glands, blood-vessels, and viscera. The dorsal
root axons have been shown by the same methods to be distributed to skin,
mucous membranes, and muscles. Hence the ventral and dorsal root fibers
may be classified in accordance with their distribution, and the character-
istic modes of activity to which they give rise into several groups as shown
in the tabulation on pages 99, 100.

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

THE SPINAL CORD 527

•oots contain all the efferent fibers, the posterior or dorsal roots all the afferent
iibers. The proofs in support of this view are as follows:
Stimulation of the ventral root fibers produces:

1. Tetanic contraction of skeletal muscles.

2. Discharge of secretions from glands.

3. Increase in the degree of the contraction, the tonus, of the muscle

walls of the peripheral arteries.

4. Variations in the degree of the contraction, the tonus, of the muscle

walls of certain viscera either in the way of augmentation or in-
hibition.1
Division oj the ventral root fibers is followed by:

1. Relaxation of skeletal muscles and loss of movement.

2. Cessation in the discharge of secretions from glands.

3. Temporary dilatation and loss of the tonus of blood-vessels.

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.
'eripheral stimulation of the dorsal root fibers produces:

1. Reflex excitation of spinal centers, in consequence of which there is an

increased activity of skeletal muscles, glands, blood-vessels, and
visceral walls.

2. Reflex inhibition of spinal nerve-centers, in consequence of which

there may be a decrease in the activities of skeletal muscles, glands,

blood-vessels, and viscera.

7. Sensations of touch, temperature, pressure, and pain.
4 Sensations of the duration and direction of muscle movements, Of

resistance offered and of the position of the body or of its individ

parts (muscle sensations).
Division of the dorsal root fibers is followed by:

1 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.

2 Loss of sensation in all parts to which they are distributed.

The ventral roots are therefore efferent in function, transmitting nerve
impulses from the spinal cord to the peripheral organs which exci

dorsal roots are afferent in function, transmitting nerve impulses
o (a] the

- •sff i tiaras

organs.

528 TEXT-BOOK OF PHYSIOLOGY

The efferent nerves taken collectively are also distributed to skeletal
muscles, glands, blood-vessels and viscera.

The afferent nerves taken collectively are distributed to the skin, mucous
membrane and to specialized sense organs — eye, ear, nose, tongue and skin.
Some of the afferent nerves contain efferent fibers which are distributed to
glands, blood-vessels and viscera.

The origin and distribution of both efferent and afferent encephalic nerves
and the phenomena that follow their stimulation and divisions, and the func-
tions attributed to them will be fully considered in a subsequent chapter, j

THE SPINAL CORD

The narrow elongated portion of the central nerve system contained
within the spinal canal is named, from its situation and appearance, the
spinal cord. 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 lengthTfFom 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, ihefilum terminate. It is divided
by the anterior and posterior longitudinal fissures into halves, and is there-
fore bilaterally symmetric. A transverse section of the cord shows that
it is composed of both white and gray matter, the former covering the surface,
the latter- occupying the center.

The 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 physiologically 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, glands, blood-vessels,
skin, etc.

If the exact distribution of the nerves of any segment were known,
its function could be readily stated. By virtue of this segmentation it
becomes possible for each segment to act independently of or in cooperation
with other segments near or remote, with which they are associated by the
intrinsic or associative cells and their axons; and by the same cooperative
action the spinal cord itself is enabled to act as a unit.

The Structure of the Gray Matter. — The gray matter is arranged in
the form of two crescents, united in the median line by a transverse band 01
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 portion termed the intermediate gray sub-
stance. 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. 227.)

The anterior horn is short and broad and entirely surrounded by white
matter. The posterior horn is narrow and elongated and extends quite

THE SPINAL CORD

529

ip to the surface of the cord, where it is capped by gelatinous matter, the
^ubstantia gelatinosa. In the lower cervical and thoracic regions a portion
:>f the intermediate gray substance projects outward and forms the so-called
ateral horn. The gray matter fundamentally consists of a framework of
jfine neuroglia supporting blood-vessels, lym-
phatics, medullated and non-medullated nerves,
and groups of nerve-cells.

The Nerve-cells. — The nerve-cells of the
cord are very numerous and they present a va-
riety of shapes and sizes in different regions.
They are usually arranged in groups which ex-
tend for some distance up and down the gray
matter, 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 regions, 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 9 7 ). In the course of their mi-
gration they developed dendrites which form
an intricate felt-work throughout the anterior
horn. One of the processes, the axon, ap-
proached the surface of the cord, penetrated it,
grew outward, became covered with myelin and
neurilemma, and developed into an anterior
root fiber. These nerve-cells, with their den-
drites, axons, and terminal branches, J )rm of the sixtl? cervical nerve. B.

efferent neurons of the first order. The inti- At the mid-dorsal region, c.

th lumbar

FIG. 227. — SECTIONS THROUGH

mate histologic and physiologic relationship _At
existing between the nerve-cell and the axon is part Of the conus meduiiaris. i.
revealed by the degenerative changes which ^^-ts^^An^nor^
arise in the latter when separated from the £ssure ^ Central canal _( Uor.
former. The cell apparently determines the rzy "Anatomy," after Schwdbe.)
nutrition of the axon and may be regarded as

trophic in function. Some of the cells of the 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 reenter the gray matter at different levels. They are probably asso-
ciative in function. Other cells send their axons into that portion of the
white matter on the same and opposite sides known as Gower s
lateral tract (Fig. 228).

53°

TEXT-BOOK OF PHYSIOLOGY

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 extends from the
seventh or eighth cervical nerves downward to the 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 the 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 distribu-
tion of their axons are shown in Fig. 228.

Donai

l/entral

FIG. 228. — 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 chiefly 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: i, Entering fibers of the posterior root, ending, from within
outward, as follows: Clarke's column, posterior horn of opposite side, anterior horn same side (re-
flex 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.

Classification of Nerve-cells. — The cells of the gray matter may be
divided into three main groups: viz., intrinsic or associative, receptive or
afferent, and emissive or efferent.

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 down-
ward. At various levels they reenter the gray matter and arborize around
other intrinsic cells.

The receptive cells are largely sentient or afferent 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

THE SPINAL CORD

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.

The emissive cells are efferent or motor in function, inasmuch as the
excitation arising in them is transmitted outwardly through their axons to
muscles, glands, blood-vessels and viscera, imparting to them motion,
either molar or molecular. As the efferent fibers in the ventral roots of
the spinal nerves are classified (see page 99) in accordance with their physio-
logic action into motor, secretor, vaso-motor, 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 Structure of the White Matter. — A transverse section of the cord
shows that the white matter completely covers the gray matter except where

FIG 220.— TRANSECTION OF THE CERVICAL SPINAL CORD SHOWING ITS CHIEF SUBDIVISK

(After Mills.) \^

the posterior horns reach the surface. Anteriorly the white matter of each
lateral half is connected by a narrow strip or bridge of white matl
anterior commissure. Microscopic examination shows that the white
is composed of vertically disposed 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
of the cord is anatomically divided into an anterior, a lateral, and a post,
column by the anterior and posterior roots of the spinal nerves

532

TEXT-BOOK OF PHYSIOLOGY

and functions. Some of the more important tracts are shown in Fig. 229.
They may be divided, however, into efferent, afferent, and associative fibers

1. The anterior column, comprising that portion between the anterioi
longitudinal fissure and the anterior roots, has been subdivided into:

(a) The direct pyramidal tract, or column of Tiirck. This tract 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 com-
missure to enter the gray matter of the anterior horn. These fibers are the
continuations of fibers which take their origin in cells which 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
descending degeneration.

(b) The antero-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 com-
missural 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 they 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:

(a) The antero-lateral tract of Gowers. This tract is somewhat crescentic
in shape and situated on the lateral aspect of the cord external to the antero-
lateral root zone. It extends throughout the entire length of the cord.
When divided it undergoes ascending degeneration, which would indicate
that the axons originate in nerve-cells in the gray matter.. This tract is
therefore probably afferent in function.

The majority of the fibers composing this tract on reaching the pons
turn backward, pass through the superior medullary velum to terminate in
the dorsal vermis of the cerebellum.

(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 after transverse section
and are in all probability associative fibers which come from nerve- cells in
the gray matter to reenter at lower and higher levels. It is also believed by
some investigators that the anterior portion contains efferent and the pos-
terior portion afferent fibers; for this reason it is frequently termed the
mixed lateral tract.

(c) The crossed pyramidal tract. This tract occupies the posterior por-
tion of the lateral column, though its exact position varies somewhat in
different regions of the cord. In the cervical and thoracic regions it is
covered by a layer of fibers. In the lumbar region, however, it comes to the
surface. From above downward this tract gradually diminishes in size,
for the reason that its fibers and their collaterals enter the gray matter at
successive levels. The terminal branches of these fibers are in close physio-

THE SPINAL CORD 533

logic 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 below 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 degener-
ates 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 subdivided into:

(a) The postero-external 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 reenter
the gray matter at different levels. Another portion of this tract is made up
of nerve-fibers derived from the dorsal roots of the spinal nerves, which
cross this column toward the median line in an oblique or horizontal direc-
tion. 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 probably as-
sociative 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) Lissauer's 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 ascending and de-
scending branches, which finally terminate around cells in the posterior
horn.

In addition to the tracts described in foregoing paragraphs a number
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:

i The antero-lateral tract of Marchi and Lowenthal, situated at the
anterior and inner angle of the anterior column, which degenerates down-

534

TEXT-BOOK OF PHYSIOLOGY

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 down-
ward.

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 lumbosacral region. They
arise from nerve-cells in the gray matter. They undergo descending
degeneration when divided, but not after division pf the dorsal roots.

THE FUNCTIONS OF THE SPINAL CORD

Anatomic investigation has demonstrated that the segments of the spinal
cord are associated through their related spinal nerves with the organs and
tissues of definite areas^ of the body. Physiologic investigation has also
demonstrated that the segments by reason of the presence of nerve-cells
and nerve-fibers may be regarded as composed of:

1. Nerve-centers, each of which has certain special functions, and

2. Conduction paths by which these centers are brought into relation not

only with one another, but with the cerebrum and its subordinate or
underlying parts, e.g., the medulla oblongata, pons varolii and
cerebellum.

A. THE SPINAL CORD SEGMENTS AS LOCAL NERVE -CENTERS.

The efferent cells of the spinal segments are the immediate sources of
the nerve energy that excites activity in skeletal muscles, glands, vascular,
and to some extent visceral muscles.

The discharge of their energy may be caused:

1. By variations in the composition of the blood or lymph by which they

are surrounded or as the outcome of a reaction between the chemic
constituents of the lymph on the one hand and the chemic constituents
of the nerve-cell on the other hand. The excitation of the cell thus
occasioned is termed automatic or autochthonic excitation.

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 cells in

the cortex of the cerebrum- or subordinate regions.

The excitation in the former instance is said to be reflex or peripheral
in origin; in the latter instance direct or cerebral in origin. In the direct
or cerebral excitations the skeletal muscle movements are due to psychic
states of a volitional, or an affective or emotional character; the gland dis-
charges and vascular and visceral muscle movements to affective or emotional
phases of cerebral activity only.

Automatic Excitation. — By this expression is meant a discharge of
energy from the spinal nerve-cells occasioned by (a) a change in the chemic
composition of the blood or lymph by which they are surrounded or prob-
ably a reaction between the constituents of the lymph and the constituents
of the nerve-cell or (b) the development within the cell of a stimulus, the
so-called "inner stimulus," the outcome of metabolic activity.

As no effect arises without a sufficient cause the term automatic has been

THE SPINAL CORD 535

objected to and the term autochthonic has been suggested, as more nearly ex-
pressing the facts stated. A center so acting could not be regarded as prima-
rily a center for reflex activity, however much it might be influenced second-
arily by afferent nerve impulses. If the cell excitation is continuous though
variable from time to time, it is said to possess tonus and the organ or tissue
thus excited is also said to possess tonus or to be in a state of tonic activity.
If the cell discharge is intermittent in character it imparts to certain muscles,
e.g., the respiratory muscles, a rhythmic activity. It must, however, be kept
in mind that the tonus of nerve-centers as well as of peripheral organs can also
be developed and maintained by the inflow of nerve impulses transmitted
from the periphery. The reason for the belief that the cord and its upper
prolongation, the medulla oblongata, are endowed with autochthonic activity
is based on the fact that certain peripheral organs are in a state of continu-
ous activity and apparently uninfluenced to any marked extent except tem-
porarily by nerve impulses transmitted to the cord through afferent nerves.
As illustrations of such continuous activity may be mentioned: (a) the
contraction of the abductor muscle of the larynx (the posterior crico-arytenoid)
whereby the vocal membranes are separated and the- glottis kept open under
all circumstances except during the emission of a vocal sound; (b) the con-
traction of the dilatator muscle of the iris; (c) the contraction of the anal
and vesic sphincters; (d) the periodic contraction of the respiratory muscles
(see page 425); (e) the acceleration of the heart-beat (page 423).

Though automatic activity of the spinal cord is yet upheld by some
physiologists, the fact must be recognized that with increasing knowledge
of reflex activities many phenomena previously regarded as automatic have
been found to be dependent on peripheral stimulation and therefore reflex
in origin. Whether this will eventually be found true for all instances of so-
called automatic or autochthonic activity will depend on the results of future
investigations. Among the phenomena removed from the sphere of auto-
matic, to the sphere of reflex activity may be mentioned muscle tonus, vascular
tonus and, trophic tonus.

Muscle Tonus.— All the skeletal muscles of the body are at all times^in
a state of slight but continuous contraction, termed tonus, by virtue of which
their efficiency as quickly responsive organs is increased. That such a
slight contraction is present even in a state of rest is shown by the fact that
if a muscle be divided in the living animal the two portions will contract
and separate to a certain distance. The condition of the muscle 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-varyin
position of the limbs and trunk (see page 57).

Vascular Tonus.— The arteriole muscles throughout the vascular appara-
tus are also constantly in a state of slight but continuous contraction which
assists in the maintenance of an average arterial pressure and i
continuous discharge of nerve energy from the general or dominating vaso-
tonic (constrictor) center in the medulla oblongata. This center it will

536

TEXT-BOOK OF PHYSIOLOGY

recalled has been shown by Porter to consist of two portions, a vaso-tonic
and a vase-reflex. The former is in a state of continuous tonus of activity ;
the latter is capable of being influenced in its activity not only by variations
in the composition of the blood but by nerve impulses transmitted to it
from all regions of the body (see page 383).

Trophic Tonus. — The normal metabolism of muscle, gland, and con-
nective tissue which underlies the assimilation of food, the production and
storage of energy-holding compounds, and the production of new compounds,
is dependent, in the higher animals at least, on the connection of these tissues
with the central nerve system; for if the efferent nerves be divided, not only
will they themselves 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 inac-
tivity, but rather to a loss of nerve influence. It would appear from facts of

,sp.c.

FIG. 230. — DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION OF REFLEX
ACTIONS, G. Bachman. r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor cells in
the anterior horn of the gray matter of the spinal cord, sp.c.; ef.n. efferent nerves distributed to
responsive organs, e.g., directly to skeletal muscles, sk.m., and indirectly through the interme-
diation of sympathetic ganglia, sym.g., to blood-vessels, b.v., and to glands, g. The nerves
distributed to viscera are not represented.

this character that the normal metabolism is dependent for its continuance
on nerve influences. There is no evidence, however, as to the existence of
special trophic nerves, separate from those which impart to glands and mus-
cles their customary activities. The trophic centers and the motor centers
are identical, though the two modes of their activity are separate and distinct.
The activity of the so-called trophic centers which was at one time believed
to be automatic is now regarded as due to reflex influences.

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 investigation will disclose the
existence and pathway of these afferent fibers. \; -;^j]

Reflex Excitation.— It has already been stated that the nerve-cells in'the
spinal cord are capable of receiving and transforming afferent nerve impulses,

THE SPINAL CORD

537

the result of peripheral stimulation, into efferent nerve impulses, which are
reflected outward to skeletal muscles, exciting contraction; to glands, pro-
voking secretion; to blood-vessels, changing then* caliber; and to organs, in-
hibiting or augmenting their activity. All such actions taking place through
the spinal cord and medulla oblongata independently of sensation or volition
are termed reflex actions. The mechanism involved hi every reflex action
consists of at least the following structures (Fig. 230) :

1. A receptive surface; e.g., skin, mucous membrane, sense organ, etc.

2. An afferent fiber and cell.

3. An emissive cell, from which arises —

4. An efferent nerve, distributed to —

5. A responsive organ, as 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 surface,
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 re-
ceived by, the dendrites of the emissive cell in the anterior horn of the gray
matter. With the reception of these impulses there will be a disturbance_in
the equilibrium of the molecules of the cells,
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 ex-
tremely 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 mus-
cles situated at distances more or less remote
from one another. The transference of nerve
impulses coming from a localized area of a
receptive surface, to emissive cells situated at
different levels is accomplished by the ^ inter-
mediation 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 colla-
teral branches with the dendrites of the efferent

neurons situated at different levels. (Fig. 231.) ^^ ^ ^ ^ ^ __
A histologic and physiologic mechanism NEURONS c, c, c.— (After Koiiiker.)
this character readily explains how a localized .

stimulation can give rise to reflex actions extremely complex in character.

The reflex contractions of skeletal muscles are best studied after division

the central nerve system at the upper limit of the spinal cord Aft.

Spinal centers can act independently of, and uninfluenced
^ or volitional efforts on the part of the animal.
Me to provoke reflex contractions under such circumstances
aXals, they are, as a rule, incomplete and of short duration,

FIG. 231. — DIAGRAM SHOWING
THE RELATION OF THE THIRD
NEURON a, TO THE AFFERENT
b, AND TO THE EFFERENT

538 TEXT-BOOK OF PHYSIOLOGY

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 to the study of reflex actions.

The separation of the spinal cord from the brain is readily effected by
destroying the medulla oblongata. This can be done by inserting a pin
through the skin and the occipito-atlantal membrane covering the space be-
tween the occipital bone and the atlas, until it strikes the bodies of the
vertebrae below. If the pin is properly directed it passes through the med-
ulla. Care should be taken to avoid injury to the blood-vessels on either
side. The brain itself should then be destroyed, so as to remove all con-
sciousness, by inserting the pin into the brain 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 the frog 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 execute spontaneous
movements. Any stimulus applied to the skin, however, provided it is of
sufficient intensity, will be followed by a more or less pronounced move-
ment. Mechanic, chemic, or 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 movements, 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 movements 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 coordinated and apparently pur-
posive and defensive in character, can be produced. The coordinated and
purposive character of the movements exhibited by a brainless frog led
Pfliiger to the assumption that the spinal cord in this as well as in other cold-
blooded animals is possessed of sensorial functions, and endowed with
rudimentary consciousness. 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 experi-
ment 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 favor-

THE SPINAL CORD 539

able, 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 decapi-
tated that reflex muscle contractions could be elicited by stimulating the skin
after the lapse of an hour after execution. " While the right arm was lying
extended by 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 n centimeters in
extent, without making any pressure on the subjacent muscles. We im-
mediately 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 Unilaterality. — 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 pro-

pagated 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 to the medulla oblongata; motor reaction then becomes
general, and it is propagated up and down the cord, so that all the mus-
cles of the body are thrown into action, the medulla oblongata acting
as a focus whence radiate all reflex impulses.

Reflex Irritability.— The general irritability or quickness of response of
the mechanism involved in reflex action can be approximately 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 Turck 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, solution of sulphuric acid. The time is determined by means
of a metronome beating one hundred times a minute. Stimulation of the
skin can also be effected by the induced electric current, as suggested
Gaskell A single shock is, however, ineffective. The currents must

540

TEXT-BOOK OF PHYSIOLOGY

follow each other with a rapidity sufficient to give rise to a summation of
effects in the nerve-centers which will then be followed by a muscle response.
It is highly probably that the chemic stimulation gives rise to a similar sum-
mation of effects.

The period of time thus obtained is distributed over the entire mechan-
ism. The true reflex time, however — i.e., the time occupied in the passage
of the nerve impulses through the spinal mechanism — is shorter and is obtained
by subtracting from the whole period the time occupied by the passage of the
impulses through the afferent and efferent nerves as well as the latent period

of muscle contraction. 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 total reflex time, that is, the time
elapsing between the application of the
stimulus and the response of an organ
may be shortened by influences which
heighten the irritability of any one or
more portions of the reflex arc: e.g.

olf.n.

Separation of the Brain from the Cord.
— This is at once followed by an in-
crease in reflex irritability, and is
taken as evidence that the brain is
normally exerting an inhibitor in-
fluence over the reflex centers of the
cord. The same increase is ob-
served upon hemisection of the cord,
though the increase is limited to the
same side.

The Toxic Action of Drugs. — Many
drugs increase the irritability of the
spinal cord, though the most efficient
is strychnin. This drug, even in
small doses, increases 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 in-
coordinated 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 impulses 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 direc-
tions. Absolute repose of the animal and the exclusion of all external
stimuli greatly diminish the tendency to the occurrence of spasms.
Degeneration of the Pyramidal Tracts. — In primary lateral sclerosis, a
pathologic condition characterized primarily by a degeneration of the
terminal filaments of the pyramidal tract fibers, the reflex activity of

-tned.ob.

FIG. 232. — 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; med. ob. medulla oblon-
gata; IV. v. fourth ventricle.

THE SPINAL CORD 541

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 stimu-
lated. The explanation offered is practically the same as hi division
of the cord: viz., withdrawal of the inhibitor and controlling influence
of the brain.

The total reflex time may be lengthened by influences which lower the
irritability of any one or more portions of the reflex arches.

1. Stimulation of Certain Regions of the Brain. — It was discovered by Setche-

now that when the frog brain is divided just anterior to the optic lobes
(Fig. 232) and the reflex time subsequently determined according to the
method of Tiirck, the time can be considerably lengthened by stimula-
tion of the optic lobes. This is readily 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 in-
fluence over centers in the spinal cord through descending nerve-fibers.
This 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 accom-
plished through the intermediation of this center localized by Setchenow.

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 reac-
tion completely inhibited.

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 Drugs— e.g., chloroform, chloral— which are believed

to exert a depressing action on the nerve-cells themselves.
Special Reflex Actions.— 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 conditions 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 reflexes.

2. The tendon or deep reflexes.

3. The organ reflexes.

The skin reflexes, characterized by contraction of underlying muscles,
are induced by stimulation of the afferent nerve-endings of the skin— eg.,
by pricking, pinching, scratching, etc. The following are the principal skin

reflexes: . ,

Plantar reflex, consisting of contraction of the muscles of the foot, induced
by stimulation of the sole of the foot; it takes place through the seg-
ments of the cord which give rise to the second and third sacral nerves.
Gluteal reflex, consisting of contraction of the glutei muscles when the :
over the buttock is stimulated; it takes place through the segments

giving origin to the fourth and fifth lumbar nerves.
Cremasteric reflex, consisting of a contraction of the cremaster muscle

542 TEXT-BOOK OF PHYSIOLOGY

and a retraction of the testicle toward the abdominal ring when the skin
on the inner side of the thigh is stimulated; it takes place through the
segments which give 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; it takes
place through the spinal segments which give origin to the nerves from
the eighth to the twelfth thoracic.

5. Epigastric reflex, consisting of a slight muscular contraction in the neigh-

borhood of the epigastrium when the skin between the fourth and
sixth ribs is stimulated; it takes place through the segments of the cord
which give origin to the nerves from the fourth to the seventh thoracic
inclusive.

6. Scapular reflex consisting of a contraction of the scapular muscles when

the skin between the scapulae is stimulated; it takes place through the
segments of the cord which give rise to the nerves from the fifth cervical
to the third thoracic inclusive.

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," are characterized by a movement of cer-
tain parts of the body due to the contraction of certain muscles and are
elicited by a sharp tap on their tendons. The fundamental condition for
the production of the tendon reflex is a certain degree of tonus 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, the muscle spindles, of the afferent nerves. When the mus-
cle is passively extended, as it must be 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 tendon reflexes are of much value in the diagnosis of certain lesions of
the spinal cord.

The following are the principal forms of the tendon reflexes :

1. The Patellar tendon reflex or knee-jerk. This phenomenon is characterized

by a quick extension of the leg from the knee downward, due to the
contraction of the extensor 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 degen-
eration of the cord; it is absent in locomotor ataxia and in atrophic
lesions of the anterior gray cornua.

2. The tendo-Achillis reflex or ankle-jerk. This phenomenon is characterized

by a flexion of the foot due to a contraction of the gastrocnemius muscle
when the tendo-Achillis is struck. To elicit the contraction, the leg
should be extended and the dorsum of the foot be pressed toward the

THE SPINAL CORD 543

leg so as to give to the gastrocnemius a slight degree of extension. If
the tendon be now sharply struck a quick flexion of the foot is produced.
. Ankle clonus. — This phenomenon consists of a series of rhythmic 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 extend the foot at the ankle suddenly and energetically,
thus putting the tendo-Achillis and the gastrocnemius muscle on 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. — This phenomenon is characterized by a flexion of the
foot, then of. the leg and perhaps of the thigh when the great toe is
strongly and suddenly flexed. It is present in those diseases of the
spinal cord in which there is a pronounced patellar reflex.
, The Wrist and Elbow Reflex. — These phenomena are characterized by an
extension movement of the hand and arm when the tendons of the ex-
tensor muscles are sharply tapped. These reflexes are especially
marked in primary lateral sclerosis of the cord in the upper portion.
The organ reflexes, e.g., the activities of the genito-urinary organs, the
tomach, intestines, gall-bladder, etc., which are induced by peripheral
timulation have been considered in connection with the physiologic action
f these organs. The genito-urinary center is located in the lumbar region
>f the spinal cord. In diseased conditions of this region the genito-urinary
eflexes are sometimes increased, at other times decreased or even abolished.
Direct or Cerebral Excitation. — The activity of the emissive cells of
he spinal cord segments, due to the arrival of nerve impulses descending
he spinal cord from the cerebrum, in consequence of psychic states of a
volitional or of an affective or emotional character, will be considered in a
ubsequent paragraph entitled " encephalo-spinal conduction."

3. THE SPINAL CORD SEGMENTS AS CONDUCTORS.

The white matter of the spinal cord consists of nerve-fibers the special
unctions of which are

1. To conduct nerve impulses from one segment of the cord to another.

2. To conduct nerve impulses coming 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 con-
sists 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
spinal segments it is essential that they should be united by commissura
or associative fibers. This is, in fact, accomplished by the axons of the mtr:
sic cells of the gray matter, which constitute such a large part of the ant<
lateral and posterior root zones. In consequence of this association,
the cord becomes capable of complex coordinated and purposive ref

ome of the nerve-fibers arising in the intrinsic cells beco™
with the cerebellum through the direct cerebellar tract and the antero-lateral

544 TEXT-BOOK OF PHYSIOLOGY

cerebellar tract. By this means the cord and cerebellum are associated ir
functional activity.

Spino-encephalic or Sensor Conduction. — The spino-encephalic 01
sensor pathway extends from different levels of the spinal cord to the cortex
of the cerebrum, and, consists of two or more consecutively arranged groups
of neurons. The first group of these neurons extends from the spinal cord
to the thalamus and is known as the spino-thalamic system; the second group
of these neurons extends from the thalamus to the cortex of the cerebrum and
is known as the thalamo-cortical system.

The spino-thalamic system is connected with the cutaneous and mucous
surfaces, and with the tendons, joints and muscles by means of the dorsa'
root fibers of the spinal nerves. Histologic and clinical investigations have
shown that the central terminations of the nerve-fibers corning from the skin
become associated with afferent cells on the same side of the gray matter
at the same or at somewhat different levels; that the axons of these cells
cross the median plane and then pass upward through the cord, medulla
pons and crura cerebri to the thalamus. Similar methods of invest] gati or
have shown that the nerve-fibers coming from the tendons, joints, and mus-
cles ascend the white matter of the spinal cord of the same side, as far as the
upper limits of the posterior column, where their terminal branches become
associated with the cells of the cuneate and gracile nuclei. From the cells
of these nuclei, axons arise which cross the middle plane of the medulla tc
join the fibers ascending the spinal cord. The former fibers are known as
the internal arcuate fibers and assist in the formation of the lemniscus o]
fillet (Fig. 233). The afferent or sensor pathway thus decussates in part,
at different levels of the spinal cord, and in part at the level of the gracik
and cuneate nuclei. The former is often termed the lower, the latter the
upper sensor decussation or crossway.

The afferent pathway on passing toward the thalamus receives additional
fibers at the level of the medulla and pons from the cells of the opposite
side with which the terminations of the afferent cranial nerves, the trigem-
inal, glossopharyngeal, and vagus, are associated.

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. On reaching the cord the cutaneous nerve im-
pulses are received by afferent or receptive nerve-cells in the gray matter,
and transmitted by their axons across the median plane of the spinal cord
and then upward to and through the medulla, the posterior part of the pons,
the posterior part of the crura cerebri, and for the most part to the ventral
portion of the thalamus opticus.

On reaching the thalamus the nerve impulses are received by nerve-cells,
and then transmitted by their axons which pass by way of the internal cap-
sule to the cells of the cortex of the cerebrum. It is probable however that
some fibers from the cord and medulla pass direct to the cortex. When
thus transmitted ^through the cord to the cerebral hemispheres directly or
indirectly, they are received by specialized nerve-cells in the cortex and trans-
lated into conscious sensations. The nerve impulses developed in tendons
and muscles are received by localized groups of nerve-cells, the nucleus
gracilis and nucleus cuneatus, situated at the top of the posterior column,

THE SPINAL CORD 545

which are reached by the upward coursing of some of the dorsal root fibers
through the columns of Goll and Burdach. They are then transmitted by
the internal arcuate fibers and the fillet either directly or indirectly to the
cortex of the cerebrum and translated into conscious sensations. The sensa-
tions thus arising may be divided intox special and general sensations. Of
the former may be mentioned the sensations of temperature, pain, touch,
pressure, passive position and movements of parts due to the activity of
skeletal muscles; of the latter may be mentioned hunger, thirst, fatigue,
well-being, etc.

Though all the impulses that give rise to *these varied sensations are
contained within the fibers of the afferent peripheral nerves, they are on
reaching the cord distributed by the intraspinal mechanisms to different
tracts of nerve-fibers, each of which transmits to localized areas of the cerebral
cortex, the somesthetic areas, a special group of impulses which give rise to
sensations of various kinds such as those just mentioned.

The current views regarding the physiologic activities of the afferent
portion of the peripheral nerve system and its relation to the production of
different forms of sensibility have been enlarged by the results of the investi-
gations that have been made by Head. Thus, he has shown that the afferent
nerves consist of three systems, each of which when excited to activity evokes
in consciousness a different and distinct group of sensations as follows :

1. One system of nerves which when stimulated evoke sensations through

which is gained the power of cutaneous localization, of the discrimina-
tion of two points of a compass, of the finer grades of temperature, and
of light touch. To this form of cutaneous sensibility the term epicritic
has been applied.

2. A second system of nerves which when vigorously stimulated, as by the

prick of a pin or by the application of a hot or cold object, evoke sensa-
tions of pain or heat and cold. To this form of cutaneous sensibility
the term protopathic has been applied. This form of sensibility is
unaccompanied by a definite appreciation of the locality stimulated for
the reason that the stimulus causes a widespread or radiating sensation
which at times is referred to parts far removed from the part stimulated.

3. A third system of nerves which when stimulated evoke sensations of pres-

sure, of the passive position and the movements of parts of the body, and
sensations of pain as well, if the stimulus (pressure) be severe, or if the
underlying structures are injured, e.g., the rupture of a joint. The
nerves subserving this form of sensibility are contained in the trunks
of the motor (muscle) nerves and are distributed to muscles, tendons
and joints. To this form of sensibility the term deep has been applied.
The pathways through the spinal cord that conduct these afferent im-
pulses 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.

As the outcome of many investigations it may be said that a^transverse
section of one lateral half of the cord in the monkey, or a lesion involv-
ing the one lateral half in man, as a rule abolishes many if not all forms of
cutaneous sensibility on the opposite side below the injury. This would seem

546

TEXT-BOOK OF PHYSIOLOGY

to prove that the nerve impulses cross the median line of the cord immedi-
ately or very shortly after entering and then ascend the corresponding half
of the cord on their way to the thalamus. At the same time, muscle sen-

FIG. 233. — DIAGRAM OF THE SENSOR PATHWAYS IN THE SPINAL CORD ENLARGED ABOVE BY
FIBERS OF THE SENSOR CRANIAL NERVES AND NERVES OF SPECIAL SENSE.

sibility is abolished on the same below the injury. This would seem to
prove that the fibers of the posterior roots that enter and cross the
column of Burdach and ascend in the column of Goll to terminate around the
cells of the gracile and cuneate nuclei are derived mainly from the muscles.

THE SPINAL CORD 547

;t is, however, believed by some investigators that those fibers which sub-
. erve the sense of touch do not decussate at once, but ascend in the column
k>f Goll as far as the medulla oblongata, where they, in common with the
ibers coming from the muscles, arborize around the nerve-cells hi the gracile
ind cuneate nuclei.

The pathways for the impulses that give rise to the different sensation

lave been variously located by different observers, e.g., in the gray matter,

n the limiting layer, and hi the antero-lateral tract of Gowers; the pathway

or the impulses that give rise to temperature sensations has been located in

,he gray matter; the pathway for tactile impressions has been located in the

posterior columns, though this is not beyond dispute. The pathway for

)ain sensations has been located in Gowers' tract.

The results on both sides of the body, which follow a transverse lesion,
Experimental or traumatic, of one-half of the spinal cord in the thoracic
•egion for example, are shown in the subjoined table, a grouping of results
vhich is known as the Brown-Se"quard "Symptom Complex." It is under-
stood, of course, that these results are observed below the level of the lesion.

THE BROWN-SEQUARD SYMPTOM COMPLEX

Side Opposite of Lesion. Side of Lesion,

c. Temperature sensibilities abolished, i. Temperature sensibilities re-
tained.

2. Painful sensibilities abolished. 2. Painful sensibility retained.

3. Pressure (painful) sensibilities abol- 3. Pressure (painful) ; sensibilities
ished. retained.

\. Passive position of limb and direc- 4- Passive position of limb and di-
tibn of movement, retained. rection of movement, abolished.

5. Light pressure or light touch, may or 5. Light pressure or light touch,
may not be abolished. retained.

5. Tactile discrimination, retained. 6. Tactile discrimination, abol-
ished.

7. Cutaneous localization, abolished. 7- Cutaneous localization, retained.

k Voluntary motion, retained. 8. Voluntary motion, abolished.

From a study of this table it is apparent: (i) that some forms of sensor
impulses (those of pain and temperature sensibility) cross soon after their
Entrance and pass up the opposite side of the cord; (2) that other forms of
sensor impulses (those of the sense of passive position of a limb and
direction of movement and tactile discrimination (Head)) do not cross, but
pass up on the same side as the entering dorsal nerve roots; (3) hat tacti
sensibility may or may not be abolished on the side opposite the lesion; am
i,(4) that the sense of cutaneous localization may be dissociated from the sense
of passive position, and remain intact when the latter is absent Head)

PEncephalo-sp nal or Motor Conduction.-The encephalo-spinal or
motor pathway extends from the cerebral cortex to ffff*j"^**

encephalic motor nerves and the ventral spinal nerves all of which ar«
tributed to skeletal muscles.

548 TEXT-BOOK OF PHYSIOLOGY

The reason for the development of this pathway is apparent from the
following considerations:

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.

Embryologic and histologic examination of the spinal cord and medulla
show that so far as their mechanisms for independent physiologic activities
are concerned both are fully developed. Similar investigations of the cere-
bral hemispheres and of the nerve-fibers which bring their nerve-cells intc
relation with the spinal segments show that the cells of the cortex are not onl)
immature, but that their descending axons are incompletely invested witli
myelin. With the growth of the child, psychic life unfolds and volitional
control of muscles is acquired. Coincidently the cells of the cerebral cortes
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 hemispheres 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 surface of the crura cerebri, behind the pons to the medulla,
of which they constitute the anterior pyramids (Fig. 234). At this poinl
the pyramidal tract1 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, constituting the direct
pyramidal tract or column 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 cerebral hemisphere eventually are brought into relation with
the cells of the gray matter of the opposite side of the cord.

1 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 pyramidal tract. The base of the pyramid in-
cludes ^ the convolutions of the cortex in front of the Rolandic fissure. The summit of the
pyramid is truncated and covers the pyramidal region of the internal capsule.

THE SPINAL CORD

549

That the pyramidal tracts are the conductors of volitional impulses
iroughout the length of the cord to its various segments has been made

FIG. 234.-DiAGRAMOF.THE PYRAMIDAL TRACT OR MOTOR PATH III.

TV Pathetic nerve V Motor division of the tngemmal nerve. VI. The

evident by the results of section, electric stimulation, and disease. Division
of the anterior and lateral columns of one side of the cord in any part of it
extent is invariably followed by a loss of motion or paralysis of the muscl(

550 TEXT-BOOK OF PHYSIOLOGY

below the section, while electric stimulation of the peripheral end of the
isolated crossed pyramidal tract is followed by marked characteristic move-
ments of the muscles. Similar results follow division of the pyramidal trac^
in any part of its course from the cerebral cortex downward. Electrid
stimulation of the cortical cells which give origin to the pyramidal tract ig
also followed by contraction of the muscles of the opposite side, while theii
destruction is attended by paralysis of the same muscles. As the nutrition oi
the fibers is governed by the cells, it follows that when the axon is separated
from its cell-body it degenerates. It has been found that a lesion of the
pyramidal tract in any part of its course is followed by descending degenera-
tion, which is taken in evidence that it conducts nerve impulses from above
downward. Thus experimental investigation and pathologic 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 pyramidal tracts are also the conductors of the nerve impulses dis-
charged by the cells of the cerebrum during the occurrence of the affective
or emotional states, that excite to activity the lower nerve centers; those that
give origin to the autonomic nerve fibers which excite to activity the epithelium
of glands, the non-striated muscles in the walls of the blood-vessels and viscera.

CHAPTER XXII

THE ANATOMIC RELATIONS OF THE MEDULLA OBLONGATA;

THE ISTHMUS OF THE ENCEPHALON; THE CORPORA

QUADRIGEMINA; THE BASAL GANGLIA

THE MEDULLA OBLONGATA

The medulla oblongata is that portion of the central nerve system im-
mediately 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. 235 and
236). 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 regular-
ity. 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 developed and
present a collection of cells near their bases which can be traced upward and
downward for some distance. At the level of the decussation of the py-
ramidal tracts the head of the anterior horn becomes 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 extensive network of nerve-fibers known as the
reticular formation. In its meshes are found collections of nerve-cells of
varying size. Toward 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., hypoglossal, efferent fibers of the vagus, and
glossopharyngeal.

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 continuous
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 pyramidal tract), which decussate at
the anterior portion of the medulla. The united fibers can be traced up-
ward to the pons, where they disappear from view.

The lateral column is composed of fibers continuous with those of the lateral

552

TEXT-BOOK OF PHYSIOLOGY

column of the cord. As the fibers pass upward, however, 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

FIG. 235. — 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. 19. Fifth nerve. 20. Motor
oculi externus. 21. Facial nerve. 22.
Auditory nerve. 23. Nerve of Wris-
b e r g. 24. Glosso-pharyngeal nerve.
25. Pneumogastric. 26, 26. Spinal
accessory. 27. Sublingual nerve. 28,
29, 30. Cervical nerves. — (Sappey.}

FIG. 236. — POSTERIOR OR DORSAL VIEW
OF THE MEDULLA OBLONGATA, ISTHMUS,
AND BASAL GANGLIA, i. Corpora quad-
rigemina. 2. Corpus quadrigeminum an-
terius (pregeminum). 3. Corpus quadri-
geminum posterius (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, n, 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. Antero-inferior wall of the fourth ventri-
cle. 20. Acoustic nerve. 21. Spinal cord.
22. The postero-median column. 23. The
posterior pyramids. — (Sappey.)

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.

ISTHMUS OF THE ENCEPHALON 553

The posterior column is composed largely of fibers continuous with those
f the posterior column of the cord. The subdivision of this column into a
>ostero-external (Burdach) and a postero-internal (Goll) is more marked
n the medulla than in the cord. The former is here known as ihefuniculus
uneatus, the latter as the funiculus gracilis. These two strands of fibers
,re apparently continued into the restiform body. Owing to the divergence
if the restiform bodies a V-shaped space is formed, the floor of which is
overed with epithelium resting on the ependyma. At the upper extremity
>f the funiculus cuneatus and funiculus gracilis, two collections of gray
natter are found, known respectively as the nucleus cuneatus and nucleus
rracilis. Around the cells of these nuclei many of the fibers of the posterior
:olumn end in brush-like expansions (see Fig. 233).

The Fillet or Lemniscus. — From the ventral surface of the cuneate and
Cradle nuclei axons emerge which pass forward and upward through the
*ray matter and decussate with corresponding fibers coming from the op-
posite nuclei. They then assume a position just posterior to the pyramids
md between the olivary bodies. These fibers thus form a new tract, termed
he fillet or lemniscus. As this tract ascends toward the cerebrum it receives
idditional axons from the sensor end-nuclei of all the afferent cranial
lerves of the opposite side with the exception of the auditory. From the
md-nuclei of the auditory nerve, new axons ascend as a distinct tract situated
icar the lateral aspect of the pons. From their position, these two separate
racts have been termed the mesial and lateral fillets respectively.

THE ISTHMUS OF THE ENCEPHALON

The isthmus of the encephalon comprises that portion of the central
uerve system connecting the cerebrum above, the cerebellum behind, and the
medulla below. Its ventral surface presents below an enlargement, convex
from side to side, the pons Varolii. On each side the fibers of which the
pons consists converge to form a compact 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 diverge some-
what from below upward, enter the base of the cerebrum and are known as
crura cerebri. Embracing the crura above are two large bands of wh]
matter, the optic tracts (Fig. 235).

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. 236). At the extreme upper part of this surface
there are four small grayish eminences, the corpora quadngemma.
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 T.
space is an expansion of the central cavity of the cord, the result of
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 ventricle. The floor of the fourth
ventricle is covered with a layer of epithelium resting on the ependyma con-
tinuous with that lining the central canal of the cord. Beneath this

f en's varoHi comprises in a general way that portion of the central

554 TEXT-BOOK OF PHYSIOLOGY

nerve system situated between the medulla 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 ventricle. 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, arranged
longitudinally and transversely (Fig. 237). The longitudinal fibers are
largely continuations of the pyramidal tracts, or the fibers composing in part
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 transverse
fibers. The transverse fibers are divided into
a superficial and a deep set. Among these
fibers are groups of nerve-cells which collec-
tively are known as the nucleus pontis. Some
of the transverse fibers, especially the super-
ficial ones, are commissural in character — i.e.,

FIG.237.-TRANSECTIONOPTHE *<* con?«* corresponding parts of the gray
PONS THROUGH ITS MIDDLE FOR- matter of the lateral halves of the cerebellum;
TION, SHOWING THE RELATION others coming from the gray matter of the
a rTsHCoNMPoVsEDTEASS/°; D™S cerebellum cross the median line and terminate
longitudinal fasciculus. L.c. and around the cells of the nucleus pontis; others
c. LOCUS ceruleus. L.f. Lateral agajn 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: (i) The fillet; (2) the formatio
reticularis; (3) the medial longitudinal bundle; (4) groups of efferent and
afferent nerve-cells.

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 acoustic
tract.

The formatio reticularis is a continuation of that of the medulla.
The medial longitudinal bundle is a band of nerve-fibers, triangular in
shape, placed on either side of the median line just beneath the floor of the
fourth ventricle, and the aqueduct of Sylvius. It consists of both afferent
(ascending) and efferent (descending) fibers. The afferent fibers are the
axons of sensor end-nuclei located in the upper segments of the spinal cord
as well as of axons of sensor end-nuclei of cranial nerves. As they pass
upward some of the fibers, as well as collateral branches, arborize around

ISTHMUS OF THE ENCEPHALON

555

the nuclei of origin of the various motor cranial nerves of the same and
opposite sides. The ascending fibers thus associate anatomically sensor
end-nuclei of both spinal and cranial nerves with the nuclei of the motor
cranial nerves. The efferent fibers are the axons of nerve-cells located in
the corpora quadrigemina and in a special nucleus in the floor of the third
ventricle. From this origin the fibers soon cross the median line, pursue a
downward course and come into close relation with the ascending fibers.
Iri their descent fibers pass successively to the nuclei of origin of the motor
cranial nerves and to nuclei in the upper segments of the spinal cord. The
efferent fibers thus associate anatomically the nuclei from which they arise
with the nuclei just alluded to. • *- •-•

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-later-
ally, decussate in the median line,
and form the lateral fillet of the oppo-
site side. Some few axons go to the
lateral fillet of the same side.

The groups of efferent nerve-cells
lying just beneath the floor of the
fourth ventricle give origin to axons
composing the motor portion of the
fifth, the sixth, the seventh cranial
nerves. The groups of afferent cells
are the sensor end-nuclei of the fifth
and eighth cranial nerves from which

new axons paSS as a part Ot the

mesial and lateral fillets toward the

ereorum. ^

The crura cerebri comprise that

portion of the Central nerve System
, i i i

situated between the pons below and
the cerebrum above. They are com-
posed 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. 238). 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 bundl<
of fibers derived from the frontal, and from the temporal and occipital por-
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 tegmenlum,
are continuous with those which pass upward from the medulla and pons,
e g the fillet, both mesial and lateral, the formatio reticularis, the media
longitudinal bundle, and, in addition, the fibers of the superior peduncles of
the cerebellum. Above, the fibers terminate largely in collections of gray
matter at the base of the cerebrum.

The aqueduct of Sylvius is a short narrow canal which connects the cavity

FIG. 238.— SCHEME OF TRANSVERSE SEC-

p.i.b. Posterior longitudinal bundle. F-

Fillet or lemniscus. RN. Red nucleus. SN.

Substantia nigra. III. Third nerve. Py.
Pyramidal tracts. Fc. Fronto-cerebellar;

ana TOC, temporo-ocdpitai fibers of the

crusta. CC. Caudato-cerebe liar fibers in
part of crusta. — (After V\/ ermcke and,

556 TEXT-BOOK OF PHYSIOLOGY

of the fourth with the cavity of the third ventricle. It is lined by the epen-'
dyma 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 superior pair are the larger and are
known as the superior quadrigeminal bodies, the superior colliouli or the pre-
gemina; the inferior pair are the smaller and are known as the inferior quad-
rigeminal bodies, the inferior colliculi, or the post-gemina.

External and somewhat inferior to the corpora quadrigemina are two
small collections of gray matter the more external of which has been termed
the external geniculate body or the pregeniculum, the more internal of which
has been termed the internal geniculate body or the post-geniculum.

Though these bodies are closely associated anatomically, they differ in
origin, in their relations, and in their functions.

On either side the fibers composing the optic tract pass to and through
the geniculate bodies in which some of the fibers- terminate, while others
pass onward to the superior and inferior quadrigeminal bodies and there
terminate. The bands of white matter associating the superior or external
and the inferior or internal geniculate bodies, with the corresponding quad-
rigeminal bodies are known as the superior and inferior brachia respectively.
The internal geniculate body gives origin to and receives fibers from the
mesial portion of the optic tract which is in reality not a portion of the optic
tract proper, but a commisural band (Gudden) which associates the body
from which it arises with that of the opposite side. The point of decussa-
tion is in the posterior part of the optic chiasm.

The external geniculate body is a terminal station for a portion of the
fine visual fibers coming from the retina. From the cells of this body
new axons arise which course forward and upward, enter the internal
capsule and pass by way of the optic radiation to the cortex of the occipital
region of the cerebrum.

The corpora quadrigemina show on microscopic examination that they
are composed of nerve-cells and nerve-fibers, both of which are so intricately
arranged that it is difficult to trace their relation one to another and to ad-
joining structures. Some of the cells of the superior quadrigeminal body give
origin to axons which pass downward and forward and terminate in brush-
like expansions around the nuclei of origin of the oculo-motor, trochlear, and
abducent nuclei ; other cells are surrounded by the terminal branches of some
of the fibers of the optic tract, though it is not probable that they are true visual
fibers. Still other cells receive the terminal branches of axons. the cells of
origin of which are located in the occipital cortex of the cerebrum and which
reach the superior quadrigeminal body by way of the optic radiation and
internal capsule.

The cells of the post-geminum give origin to axons which pass upward,

BASAL GANGLIA 557

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, termi-
nate in brush-like expansions around these same cells. There is thus es-
tablished a connected pathway between the cochlea and the temporo-sphe-
noidal cortex. The cells of the temporal cortex, however, send axons in the
reverse direction by way of the auditory tract to the cells of the post-geminum.
There is thus established a double communication between the occipital
and temporal region of the cerebral cortex, and the pre-geminal and post-
geminal bodies respectively.

THE BASAL GANGLIA : THE CORPORA STRIATA AND OPTIC

THALAMI

The basal ganglia are collections of ganglionic matter, situated at the
base of the cerebrum along the course of the nerve-fibers that pass to and

FIG. 239.-CoRpoRA STRIATA, OPTIC THALAMI, CORPORA QUADRIGEMINA, CEREBELLUM AND
ASSOCIATED STRUCTURES, i, Corpora quadrigemina; 2 valve of Vieussens; ; 3 P^Pedunc
4 upper part of medi-peduncle; 5, upper part of crus; 6, lateral fillet; 7, band of I
bracWum; 9, frenulum; xo, gray matter of valve of Vieussens; 11, ^^S!^3}^S
missure; 13, 14, center of cerebellum; 15, post-commissure; 1 6 peduncles of i gland 17,

pineal gland- 18, 10, posterior and anterior tubercles of the thalamus; 20, tema semicircular* , 21,
vessels of the corpus striatum; 22, fornicolumn; 23, corpus stnatum; 24, septum lucidum.-
(Sappey.)

from its cortical expansion. Among these ganglia the more important are
the corpora striata and the optic thalami. They are made visible upon
removal of the cerebrum. The general relations of these ganglia an

Tghe2corpus striatum, the more anterior of the two, is an ovoid col-

558

TEXT-BOOK OF PHYSIOLOGY

AMYGDAIA

lection of gray and white matter and receives its name from the fact that it

presents on cross-section a striated appearance. The larger portion of this

body is embedded in the cerebral white matter, while the smaller portion

projects into the anterior part of the lateral ventricle. A dissection of this

nucleus shows that it is subdivided by a band of white matter into two

smaller nuclei, viz. : the caudate and the lenticular nuclei.

i. The caudate nucleus is a pyriform body which corresponds with the

intra-ventricular portion of the corpus striatum. It consists of a head,

an arching body and a tail. The head, which is thick and large, projects

into the anterior cornu of the ventri-
cle; the body arches across the ven-
tricle from before backward and

/M^^^ **"*%. from within outward, while the tail

is directed downward and forward
to become associated with the collec-

m t-1-'" m tion of gray matter situated beneath

the lenticular nucleus and known as
^/ the amygdaline nucleus. Anteriorly,

^^^ the caudate nucleus is united with

the lenticular nucleus by a narrow
bridge of gray matter, partially sub-
divided by small bands or strands of
nerve-fibers passing through it.
2. The lenticular nucleus is an irregularly
triangular pyramidal- shaped body
and corresponds with the extra-ven-
tricular portion of the corpus stria-
tum, the portion embedded in the
cerebral white matter. The apical
extremity of the nucleus is directed
toward the median line while its
_^ convex base is directed toward and

runs almost parallel with the gray
FIG 24o.-Two VIEWS OF A MODEL matter of the Island of ReiL The

OF THE STRIATUM. A, Lateral aspect ; B, , , , ,. r

mesial aspect.— (Spitzka) general appearance and relation of

these nuclei, are shown in Figs. 240

and 241. A horizontal section of the lenticular nucleus shows that it is
divided by two lamina of white matter into three portions. The two in-
ner, from their pale yellow color, form the globus pallidus, the outer,
somewhat darker in color, is termed the putamen. External to the len-
ticular nucleus is a thin stratum of gray matter, arranged more or less
vertically, and placed between the outer surface of the lenticular nucleus
and the cortex of the Island of Reil, and known as the claustrum.
The optic thalamus is an oblong mass of gray matter situated between
the sensor, afferent pathway and the cortex of the cerebrum. The anterior
and posterior extremities of each thalamus present enlargements known
respectively as the anterior tubercle and the posterior tubercle or pulmnar.
The mesial surface of the thalamus forms the lateral wall of the third ventri-
cle 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

--AMYGDALA

BASAL GANGLIA

559

out penetrated by white matter, which subdivides its contained gray cells
':nto four more or less distinct masses termed nuclei, viz.: an anterior, a
lateral, occupying the external part of the thalamus, a ventral, close to the
entire ventral surface, and a posterior, situated beneath the pulvinar. Be

FIG 241.— HORIZONTAL SECTION THROUGH THE CEREBRUM SHOWING THE NATURAL RELATIONS

OF THE VARIOUS STRUCTURES.

'neath 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 substantia nigra. f

Though the thalamus has extensive connections with many po

56o TEXT-BOOK OF PHYSIOLOGY

the central nerve system, the most important are with the cortex, the tegj
mentum, and the optic tracts.

From the cells of these various nuclei axons emerge which pass into thl
internal capsule, and through the corona radiata to various portions of th^
cortex. Those which come from the pulvinar and pass to the occipital lobd
constitute a part of the optic radiation; those from the lateral and ventral
nuclei ultimately reach the parietal lobe; those from the anterior nucleu^
pass to the hippocampal and uncinate convolutions. In a similar manne^
various 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 exacj
distribution of various strands of fibers is a subject of much discussion.
Most of the fibers of the mesial fillet end in tufts around the cells of the
ventral and lateral nuclei; other fibers pass directly to the cortex.

The optic tract sends fibers directly into the pulvinar,. the external
geniculate body, and the superior corpus quadrigeminum, around the cells
of which they terminate in brush-like expansions.

The Internal Capsule. — The lenticular nucleus is enclosed on all sides
by ascending and descending nerve-fibers. From the manner in which
they surround and enclose the nucleus they have collectively been called the
lenticular capsule. If a horizontal section of the cerebrum be made at a
certain level so as to cut across the capsule and the enclosed nucleus an
appearance similar to that shown in Fig. 241 will be presented. That portion
of the capsule that lies between the caudate nucleus and the optic thalamus
internally and the lenticular nucleus externally is known as the internal
portion of the lenticular capsule or in its abbreviated form as the internal
capsule, while that portion between the external convex border of the len-
ticular nucleus and the claustrum is known as the external portion of the
lenticular capsule or in its abbreviated form as the external capsule. At a
given level the internal capsule may be said to consist of two segments or
limbs, an anterior, situated between the caudate nucleus and the anterioi
extremity of the lenticular nucleus, and a posterior, situated between the
optic thalamus and the posterior extremity of the lenticular nucleus. The
two segments unite at an obtuse angle, termed the knee, which is directed
toward the median line. The appearance which is presented at different
levels varies however considerably.

SUMMARY OF THE STRUCTURE OF THE MEDULLA, ISTHMUS, AND

BASAL GANGLIA

Structure of the Central Gray Matter. — Though the general arrange-
ment of the central gray matter has been incidentally alluded to in the fore-
going 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

MEDULLA AND BASAL GANGLIA 561

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
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 denned axon. As a rule, the cells are arranged in groups, or clusters,
or nests, partially surrounded and enclosed by supporting tissue, and situ-
ated beneath the floor of the fourth ventricle and the floor of the aqueduct of
(Sylvius.

Classification of Nerve-cells.— The cells of the gray matter may be
divided into three main groups; viz., intrinsic or associative, receptive or
afferent and emissive or efferent.

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 down-
ward. At various levels they re-enter the gray matter and arborize around
other intrinsic cells.

The receptive cells are largely sentient or afferent in function, inasmuch as
they receive the nerve impulses transmitted to them by afferent cranial
nerves. As the afferent nerve fibers are classified, in accordance with the sensa-
tions to which they give rise, as sensor, thermal, tactile, gustatory, auditory,
etc., so these 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.

The emissive cells are efferent or motor in function, inasmuch as
excitation arising in them is transmitted outwardly through their axons to
muscles, glands, blood-vessels, and viscera, imparting to them motion,
either molar or molecular. As the efferent fibers are classified (see page 99)
in accordance with their physiologic action into motor, vaso-motor, secretor,
viscero-motor, so the nerve-cells of which the nerves are integral parts may
be classified physiologically as motor, vaso-motor, secretor, and viscero-
motor. Collections or groups of such cells are termed centers.

Structure of the White Matter.— The white matter is composed 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 (i) a ventral or pedal portion and (2) a dorsal or
tegmental portion. The fibers constituting the ventral or pedal portion may
te convenience be said to extend from the cerebral cortex through the crus
cerebri to the pons, medulla, and spinal cord. They may be divided r
three distinct tracts: e.g., the pyramidal tract, the fronto-cerebellar tract,
and the occipito-temporo-cerebellar tract (Fig. 242).

The pyramidal or motor tract descends from the cortex of the cerebrum
mainly from the gyrus anterior to the central fissure, passes through
posterior one-third of the anterior segment and the anterior two-thirds of the
posS segment of the internal capsule, the middle two-fifths of the crusta,

562 TEXT-BOOK OF PHYSIOLOGY

behind the transverse fibers of the pons, to become the anterior pyramid 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 gyri
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

FIG. 242. — SCHEMA OF THE PROJECTION FIBERS OF THE CEREBRUM AND OF THE PEDUNCLES OP
THE CEREBELLUM; LATERAL VIEW OF THE INTERNAL CAPSULE. A, Tract from the frontal gyri
to the pons nuclei, and so to the cerebellum (frontal cerebro-cortico-pontal tract); B, the motor
(pyramidal) tract; C, the sensory (body sense) tract; Dt the visual tract; E, the auditory tract; F,
the fibers of the superior peduncle of the cerebellum; G, fibers of the middle peduncle uniting with
A in the pons; H, fibers of the inferior peduncle of the cerebellum; /, fibers between the auditory
nucleus and the inferior quadrigeminal body; K, motor (pyramidal) decussation in the bulb; Vtt
fourth ventricle. The numerals refer to the cranial nerves. — (Modified from Starr.)

temporal lobes, passes to the inner side of the lenticular nucleus, and con-
tinues 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.
In this tract, not shown in the figure, are to be found the afferent fibers
constituting the visual tract, D and the auditory tract E.

The fibers constituting the dorsal or tegmental portion of the longitudinal
system may be said for convenience to extend from the posterior portion of
the medulla and pons to the optic thalamus and cerebrum. They may be
subdivided into several tracts, the more important of which are the fillet and
the dorsal longitudinal bundle.

FUNCTIONS OF THE MEDULLA OBLONGATA 563

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 the spinal and various sensor cranial nerves, occupies
a region in the ventral and mesial portion 'of the tegmentum throughout its
entire extent. Superiorly this mesial fillet terminates for the most part
around nerve cells in the nuclei of the thalamus. From these nuclei new
fibers arise which pass for the most part to the cortex of the post-central
and parietal gyri. The fibers coming from the sensor end-nucleus of the
auditory nerve (the lateral fillet) lie on the lateral aspect of the pons and
cms. Superiorly they terminate in the post-geminum (the inferior quadri-
geminal body) and in the internal geniculate body. From these nuclei the
fibers composing the auditory tract pass to the super-temporal convolution.

The dorsal longitudinal bundle, an upward extension of the fibers com-
posing a portion of the ground bundle of the spinal cord,' is located on
either 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. Supe-
riorly some of the fibers become related to cells in the thalamus and sub-
thaiamic region. This bundle of fibers appears to be mainly commissural in
character.

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 cortex 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 op-
posite side. Beyond this point they pass to the cerebellar cortex. From
their anatomic relations it is probable that these transverse fibers are com-
missural in character, bringing 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, CORPORA
QUADRIGEMINA, AND BASAL GANGLIA

Microscopic examination of the white and gray matter of these various
parts of the central nerve system shows that they are composed 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 mon
complicated and involved. The functions of these closely related struct
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 collectively r
regarded as consisting:

i Of nerve centers each of which has a special function, and
2. Of conducting paths by which these centers are brought into relation

564 TEXT-BOOK OF PHYSIOLOGY

not only with one another but with the cerebrum, the cerebellum and
the spinal cord.

A. The Medulla and Isthmus, as Local Nerve-centers. — The emis-
sive or efferent cells situated in the gray matter beneath the floor of the
fourth ventricle and beneath the aqueduct of Sylvius and arranged in more
or less well-defined groups, are the sources of. the nerve energy that excites
activity in the skeletal muscles, glands, vascular and to some extent visceral
muscles of the head, neck, thorax and abdomen.

The efferent fibers emerging from these cells constitute in part, the trunks
of the efferent encephalic or cranial nerves, the origin, course, distribution
and functions of which will be considered in a subsequent chapter.
The discharge of their energy may be caused:

1. By variations in the composition of the blood or lymph by which they

2. By the arrival of nerve impulses, coming through afferent cranial and

spinal nerves from the general periphery, skin, mucous membrane,
etc.

3. By the arrival of nerve impulses descending from cells in the cortex of

the cerebrum or subordinate regions.

The excitation in the former instance is said to be reflex or peripheral
in origin; in the latter instance direct or cerebral in origin. In the direct or
cerebral excitations the skeletal muscle movements are due to psychic states
of a volitional or an affective or emotional character; the gland discharges and
vascular and visceral muscle movements to emotional phases of cerebral
activity only.

The activities of these centers are thus called forth by autochthonic, reflex
or peripheral, and direct or cerebral excitations. The forms of activity excited
in peripheral organs resemble those excited by the spinal nerve-centers.
See page 451.

Special Nerve-centers. — 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 r61e in
this respect. As the accomplishment of these functions requires the co-
operation 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 co-
ordinating mechanisms consisting of nerve-cells and nerve-fibers which are
associated in various ways for the accomplishment of definite functions.
To such a Coordinating mechanism the term "center" has been given:
e.g., respiratory, cardiac, deglutitory, etc.1

Experimentation has shown that the medulla and pons contain a
number of such centers, the more important of which are as follows:
i. The cardiac centers, which exert (i) an accelerator action over the heart's
pulsations through nerve-fibers emerging from the spinal cord in the

1 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 be accurately
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 MEDULLA AND ISTHMUS 565

roots of the first and second dorsal nerves and reaching 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 322 and 323.)

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 382.)

3. A respiratory center, which coordinates the muscles concerned in the

production of the respiratory movements. (See page 425.)

4. A mastication center, which excites to activity and coordinates the muscles

of mastication. (See page 144.)

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 164.)

6. An articulation center, which coordinates the muscles necessary to the

production of articulate speech.

^ A diabetic center stimulation of which gives rise to glycosuria.
8. A salivary center, stimulation of which excites the discharge of saliva.

B. The Medulla and Isthmus as Conductors. — The anterior pyra-
mids of the medulla and their continuations through the more ventral por-
tions of the pons, being portions of the general pyramidal tract, serve to
conduct volitional efferent nerve impulses from higher portions of the brain
to the spinal cord. Division 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, 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 im-
pairment 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 consists ventrally of fibers which are largely derived
from the pyramidal tracts and are continuous with the longitudinal fibers
of the ventral portion of the pons and medulla; and dor sally of fibers continu-
ous with those coming through the lower portions of the tegmentum. Hence
they are conductors 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 Corpora Quadrigemina. — From the anatomic relation of the
superior quadrigeminal body (the pre-geminum) to the optic tract, the
inference can be drawn that it is in some way essential to the performance
of various reflex ocular movements and perhaps to the variations in size of
the pupil. Experimental investigations and pathologic changes support the

inference.

Irritation of the pre-geminum in monkeys on one side is followed by
diminution of the pupils first on the opposite side and then almost immedi-
ately on the same side. The eyes at the same time are also widely opened and
the eyeballs turned upward and to the opposite side. If the irritation be

566 TEXT-BOOK OF PHYSIOLOGY

continued, motor reactions are exhibited in various parts of the body.
Destruction of the pre-geminum in both monkeys and rabbits is followed by
blindness, dilatation and immobility of the pupils, with marked disturbance
of equilibrium and locomotion (Ferrier).

From the anatomic relation of the inferior quadrigeminal body (the
post-geminum) 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 post-geminum gives rise to cries and various forms
of vocalization. Pathologic states of this body are also attended by impair-
ment of hearing and disorders of the equilibrium.

From the foregoing facts it is probable that the corpora quadrigemina are
associated with station and locomotion. Ferrier assumes that in these
bodies " sensory impressions, retinal and others, are coordinated with adapt-
ive 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
activities. 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.

It is stated by some experimenters that localized injuries, both experi-
mental 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
intermediaries between the peripheral sense-organs and the cortex.

The results of experimental stimulation and destruction of the thalami
are extremely contradictory and fail to throw much light on their functions.
Ferrier states that destruction of the posterior part of one thalamus pro-
duced blindness in the opposite eye and impairment of the sense of touch
and pain in the opposite side of the body. In a patient under the care of
Hughlings 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 diminution of the sense of touch on the left side of the body. Post-
mortem examination showed a patch of softening in the posterior part of
the right thalamus, the remainder of the organ being normal.

It is probable that in the thalamus visual, tactile, and labyrinthine im-
pressions are received, coordinated, and reflected outward, with the result
of producing various adaptive motor reactions connected with station and
equilibrium. The thalamus is believed by some investigators to act also as
an intermediary between emotional states and their expression in the muscles

FUNCTIONS OF THE INTERNAL CAPSULE

567

f the face, this power being lost in certain pathologic conditions. The
|<power of regulating the temperature of the body has been also assigned to
;he thalamus, as destruction of its anterior extremity is usually followed by
i 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, a pathway
for the transmission of nerve impulses from the cerebral cortex to the pons,
medulla, and spinal cord, which give rise to contraction 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

(Elb),s 0*H (DIG.), ABDO^N (Abd) H* KNEE (Kn.), D.orrs o* Fool
(Dig.). S. Sensor tract. O. T. Optic tract. A.T. Auditory tract.

reason that it transmits 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 interesting parts of the central nerve system As shown m Fig
243, it consists of two segments or limbs united at an obtuse ange, the knee
or elbow, which is directed toward the median line. The motor tea.
confined to the posterior one-third of the anterior ^g^.^^*6,^™
two-thirds of the posterior segment. The sensor tract is confined to the
posterior one-third of the posterior segment, the extreme end of whi
contains the optic and auditory tracts. contains

The region of the anterior segment in front of the motor tract
the fibersTthefronto-cerebellar tract, the function of which is unknown,
region contains fibers which descend from the cerebral cortex

568

TEXT-BOOK OF PHYSIOLOGY

to nerve-centers situated in the gray matter beneath the aqueduct of Sylvius,
in the gray matter beneath the floor of the fourth ventricle, 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. 243. The relation of the internal capsule to the caudate
nucleus and the optic thalamus internally, and to the lenticular nucleus ex-
ternally, is also shown in a vertical section of the cerebrum made in front of
the gray commissure (Fig. 244).

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.

FIG. 244.— A TRANSVERSE SECTION OF ONE-HALF OF THE CEREBRUM. Arteries supplying
the lenticular nucleusLAT, the internal capsule 7C, and the caudate nucleus CA7. i. The middle
cerebral. 2, 2. Lenticular arteries. 3. The lenticulo-striate artery passing through the ex-
ternal capsule.

This condition, to a greater or less extent, frequently arises in consequence
of a rupture of the blood-vessels which supply the caudate nucleus, the len-
ticular nucleus and the internal capsule. These arteries, branches of the
middle cerebral, penetrate the brain in the anterior and posterior perforated
spaces (Fig. 244). By reason of a diseased condition of their walls.
especially of the lenticulo-striate artery, the vessel ruptures and the blood
is extravasated into the surrounding tissues.

If the extravasation occurs in the anterior two-thirds of the posterior
limb of the internal capsule, there will be a destruction of the efferent or
motor fibers, and a separation of the cortical motor area from the medullary
and spinal motor nuclei and, therefore, a loss of volitional control of the mus-
cles of the face, arm and leg of the opposite side. The extent of this paraly-
sis will depend, of course, on the extent of the extravasation. The muscles

FUNCTIONS OF THE INTERNAL CAPSULE 569

the larynx, thorax and abdomen, those associated with symmetrical move-
nents escape paralysis for the reason that in all such movements each side of
the brain controls the muscles of both sides of the body. If the extravasation
of blood also destroys the fibers of the posterior, one-third of the internal
capsule hi the region between the optic thalamus and the lenticular nucleus,
there will be a destruction of the afferent or sensor fibers and a separation of
the cutaneous areas from the cortical sensor areas and, therefore, a loss of
sensation on the opposite side of the body. If the extravasation is confined
to the external limits of the posterior one-third of the capsule, there would be
a loss of sensation without impairment of motion, a rare condition however.
In these hemorrhages the optic and auditory tracts are seldom involved.
Coincident with the rupture of these vessels there is a loss of consciousness
as a rule, and the patient falls as if struck and hence this condition received
the name of apoplexy. With the recovery of consciousness the paralysis of
motion becomes apparent and is usually permanent. Not infrequently, how-
ever, partial recovery of motion and sensation takes place after the blood
coagulates and the serum is absorbed, thus relieving pressure on adjoining
nerve-tracts which were not otherwise injured.

CHAPTER XXIII
THE CEREBRUM

The cerebrum is the largest portion of the encephalon, constituting
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 by 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 by a broad white band of
nerve-fibers, the corpus callosum. Each hemisphere or hemi-cerebrum is
convex on its outer aspect, and corresponds in a general way with each side
of the cavity of the skull; the inner or mesial surface is flat and forms the
lateral boundary of the longitudinal fissure.

The surface of each hemi-cerebrum presents a series of alternate indenta-
tions and elevations, known respectively as fissures or sulci, and convolutions
or gyri. 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 physiologic processes, clinical phenomena,
and surgical procedures. The general arrangement of the primary fissures
and convolutions is represented in Figs. 245 and 246.

Fissures. —

1. The Sylman fissure y 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 emi-
nence 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 develop-
ment 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 Rolandic or central fissure, equally important, is found on the superior

and lateral aspects of the cerebrum. It runs from a point on the con-
vexity 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 is such as to form with the longitudinal fissure an angle
of about 70 degrees.

3. The intra-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 CEREBRUM

The parieto-occipital fissure, situated on the mesial surface of the hemi-
spheres, 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. 246.)

SUPfKCfNTRAL f.

'1C. 245.-FlSSURES AND GYRI ON THE LATERAL SURFACE OF THE LEFT HEMI-CEREBRUM--

F. Fissure. G. Gyrus. R. Ramus.— (Spitzka.)

F,G. ,46-F.SSUBES AND G^ OF THE_M|SUL S^ACE OF THE LEFT HEM,-,

The calcarine Assure begins on the posterior extremity of the mesial surface
'• of tt ocdpitaHobe gFrom this point it passes downward and forward
to unite with the parieto-occipital fissure.

572 TEXT-BOOK OF PHYSIOLOGY

6. The par a- central fissure begins at the supero-mesial border of the hemis-

phere. It then passes downward and forward for a variable distance
and then turns upward enveloping a lobule known as the para-
central lobule.

7. The super-callosal 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 are
found the pre-central, the super -frontal, medi-frontal and sub-frontal fissures;
in the temporal lobe the super-temporal and medi-temporal fissures.

Convolutions. — The convolutions or gyri are the portions of the cere-
bral surface comprised between the fissures. The arrangement of the sur-
face 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.

1. The anterior or pre-central convolution or gyrus is situated just in front oi

the Rolandic or central fissure, with which it corresponds in direction.
It is continuous above with the super-frontal and below with the sub-
frontal convolution.

2. The super -frontal convolution or gyrus is bounded internally by the longi-

tudinal 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 medi-frontal convolution or gyrus 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 or gyrus winds around the pre-sylvian brand

of the fissure of Sylvius in the anterior and inferior portion of the fronta
lobe. It is continuous posteriorly with the lower end of the pre-centra
convolution.

The Parietal Lobe. — The parietal lobe presents three well-markec
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.

1. The posterior or post-central convolution or gyrus is situated just behind tht

Rolandic or central fissure, with which it corresponds in direction
Above, it is continuous with the super-parietal convolution; below, witl
the marginal and the pre-central convolutions.

2. The super-parietal convolution or gyrus is bounded internally by the longi

tudinal fissure and externally by the intra-parietal fissure. From th<
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 or gyrus is connected anteriorly with th

THE CEREBRUM 573

post-central convolution. Passing backward, it winds around the
superior extremity of the fissure of Sylvius, in which situation 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-temporaj fissure, while the other runs downward and back-
ward, 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
in its external surface three well-marked convolutions: viz., the super-, the
iedi-, and the sub-temporal, separated by the super- and medi-temporal
ssures. These three convolutions are in a general way parallel with each
ther, and pursue a direction from before backward and upward. Ante-
Lorly, they are fused together, but posteriorly their connections are some-
hat different. The super-temporal is continuous behind and above with
ae supra-marginal convolution, and behind and below with the angular
onvolution or gyrus. The medi-temporal blends with the preceding am
Vith the middle occipital. The sub-temporal is continuous with the

erior occipital. A

The Occipital Lobe.— The occipital lobe is triangular in shape and
orms the posterior apex of the hemisphere. Its base on the externa
urface is formed by an imaginary line drawn from the paneto-occiptal
issure to the pre-occipital notch on the inferior and lateral border. 1 he
eternal surface presents three convolutions— the superior, middle, am

574

TEXT-BOOK OF PHYSIOLOGY

4. The quadrate lobule, or precuneus, a square-shaped convolution, is situated

between the posterior termination of the para-central fissure and th$
parieto-occipital fissure. It blends with the callosal convolution, on thl
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 mesial surface of the occipital Iob0
between the parieto-occipital and calca-
rine fissures.

The Insula or Island of Reil.— This
anatomic structure consists of a triangular-j
shaped cluster of six small convolutions situa-
ted 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 tem-
poral lobes.

Structure of the Gray Matter of the
Cortex. — The gray matter, the cortex of the
cerebrum, varies from two to four millime-
ters in thickness. When examined with a lens
of low power, it presents a laminated appear-
ance, due to differences in color and arrange-
ment of its constituent elements. With
higher magnification the cortex is seen to con-
sist of neuroglia cells, nerve-cells with special-
ized dendrites and axons, medullated and
non-medullated nerve-fibers, blood-vessels,
connective tissue, etc., all of which are arranged
and interblended in a most intricate man-
ner. Notwithstanding the complexity of its
structure, modern histologic methods have
enabled Cajal to divide it into four fairly dis-
tinct layers or zones, from without inward, as
follows (Fig. 247):

i . The Molecular Layer. — The most superficial
portion of this layer consists mainly ol
neuroglia or glia cells, the processes oi
which interlace in all directions, forming
a distinct sheath just beneath the pia,
The deeper portions of this layer con-
tain a specialized type of nerve-cells (Cajal cells), of which then
are several varieties. These cells give off nerve-fibers which pur-
sue a horizontal direction for a variable distance, but in theii
course give off collateral branches which ascend to the outer surface
of the layer. Among these structures are to be found, also, den-
dritic processes of cells situated in the subjacent layer. The termina
filaments of medullated nerve-fibers coming from nerve-cells in lowe]
regions of the encephalo-spinal axis are also present, but for the mos
part they pursue a tangential direction.

FIG. 247.— SECTION OP 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. —
(PiersoL)

THE CEREBRUM 575

The Layer of Small Pyramidal Cells. — This layer consists mainly of
nerve-cells, the majority of which are pyramidal 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 process of the pyramidal cells is broad at the base,
but narrows rapidly as it passes upward. It frequently divides into sev-
eral branches, each of which develops club-shaped processes or gem-
mules, which give to it a feathery appearance. 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 medu-
lated 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 preceding layer, with
the exception that the apical process is larger, better developed, and
branches more freely. All the dendrites are extensively provided with
gemmules. The axon is well developed, sharply denned, and smooth.
After giving off collateral branches, the axon descends into the cere-
brum and becomes a medullated nerve-fiber.

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, soon branches, and becomes gemmulated. The axon

is well defined and it soon descends into the white matter.

The Number of Cortical Cells.— Attempts have been made by various

dstologists to estimate the total number of functional nerve-cells in the cere-

>ral cortex of man. Though the estimates are widely different, the lowest

>resents numbers which are beyond comprehension. Thus, Meynert s

stimate is 612 millions; Donaldson's estimate for the entire brain is 12,000

nillions; and Thompson's 9283 millions.

Structure of the White Matter.— The white matter of the cerebrum
:onsists of medullated nerve-fibers which, though intricately arranged, may
^e divided into three systems: viz., the commissural, the association, an

)roiection. .,

[ 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 corresponding areas of the temporo-sph
noidal lobes cross in the anterior commissure. All the commissura.
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.
The association system.-The fibers which compose this system un
neighboring as well as distant parts of the same hemisphere, and may
therefore be divided into long and short fibers They associate the in-
excitable or association areas with the excitable or projection areas.

The projection system.-The fibers composing this 9^.?^»
areas of the cortex of the cerebrum with the basal ganglia the , pons,
medulla oblongata, and spinal cord. They may be divided mto . (i)
afferent fibers which have their origin in the lower nerve-centers at dii

576 TEXT-BOOK OF PHYSIOLOGY

ferent levels and thence pass to the cortex; and (2) efferent fibers whicl
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-efferent or cortico-fugal.
The afferent fibers, the so-called sensor tract, which transmit nerve im-
pulses coming from the general periphery and the sense-organs, pass througl
the tegmen'tum as the mesial and lateral fillets, and thence to the cortex
directly by way of the internal capsule, or indirectly through the intermedia
tion of the thalamic and subthalamic nuclei. (See Fig. 242, page 562.) Th<
distribution of these fibers to. the various areas of the cortex will be stated ii
subsequent paragraphs.

The efferent fibers of the so-called motor tract which transmit motor o]
volitional nerve impulses from the cortex to the pons, medulla, and spina
cord, emerge from the layer of pyramidal cells of the gray matter of the an
terior or the pre-central convolution, the para-central lobule, and immedi
ately adjacent areas. From this origin the axons descend through th(
white matter of the corona radiata, converging toward the internal cap
sule, into and through which they pass, occupying the anterior two-third
of the posterior limb or segment. Beyond the capsule they continue t<
descend, occupying the middle three-fifths of the pes or crusta of the crui
cerebri, the ventral portion of the pons, and eventually the anterio:
pyramid of the medulla oblongata. At this point the tract divides int<
two portions, viz. :

1. A large portion, containing from ninety-one to ninety-seven per cent, o

the fibers, which decussates at the lower border of the medulla anc
passes down the lateral column of the cord, constituting the crossei
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 sidi
of the anterior column of the same side, constituting the direct pyramida
tract or column of Tiirck.

After passing through the internal capsule, and as it descends througl
the crus, pons, and medulla, the cortico-efferent tract gives off a number o
fibers which cross the median line and arborize around the nerve-cells o
the gray matter beneath the aqueduct of Sylvius (the nuclei of origin of th<
third and fourth cranial nerves), and around the nerve-cells in the gra]
matter beneath the floor of the fourth ventricle (the nuclei of origin of th<
remainder of the motor cranial nerves). The remaining fibers go to forn
the crossed and direct pyramidal tracts and arborize around the cells in th<
anterior horn of the gray matter of the opposite side of the cord at successiv<
levels. By this means the cortex is brought into anatomic and physiologic
relation with the general musculature of the body through the various crania
and spinal motor nerves. (See Fig. 234, page 549.)

The fronto-cerebellar and the occipito-temporo-cerebellar tracts are als<
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 t<
and through the internal capsule, occupying the anterior one-third of th<
anterior segment. It then descends along the inner side of the crus cerebr
to the pons, where its fibers arborize around the cells of the nucleus pontis
Through the intermediation of these cells this tract is brought into relatioi

THE CEREBRUM 577

th the cerebellum of the same but chiefly of the opposite side. The
cipito-temporal tract, originating in the cells of the cortex of both the
cipital and temporal lobes, passes downward and inward toward the
iticular nucleus, beneath which it passes to enter the outer one-fifth of the
ista. It then enters the pons, and through the nucleus pontis also comes
to relation with the cerebellum of both sides. (See Fig. 242, page 562.)

THE FUNCTIONS OF THE CEREBRUM

The functions of the cerebrum comprehend, in man at least, all that
rtains to sensation, cognition, feeling, and volition. All subjective experi-
ces, which in their totality constitute mind, are dependent on and asso-
ited with the anatomic integrity and the physiologic activity of the cere-
urn and its related sense-organs, the eye, ear, nose, tongue, etc.

From an examination of the anatomic development of the brain in different
isses of animals, in different men and races of men, and from a study of the
thologic lesions and the results of experimental lesions of the brain, evi-
nce has been obtained which reveals in a striking manner the intimate
nnection of the cerebrum and all phases of mental activity.
Comparative anatomic investigations show that there is a general connec-
tion 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 enor-
mous increase in size over that of the highest animals, the anthropoid
apes. The most cultivated races of men have the greatest cranial
capacity, that of the educated European or American being approxi-
mately 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 incompatible 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 brain
weight of 96 distinguished men has been -found to be 51.9 ounces
(1473 grams) (Spitzka).

Pathologic lesions 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 hem-
orrhage destroys consciousness. Physical and chemic alterations of
the gray matter of the cerebrum have been shown to coexist with insan-
ity, loss of memory, loss of articulate speech, etc. Congenital defects of
organization are accompanied by a deficiency in mental capacity and
the higher 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 fe small, imperfectly developed,
and wanting in proper chemic composition.

Experimental lesions of the brain in lower animals are attended by res
similar to those observed in disease or after injury in man.

578 TEXT-BOOK OF PHYSIOLOGY

of the cerebrum in the pigeon completely abolishes intelligence anc
destroys the capability of performing volitional movements. Th(
pigeon remains in a state of profound stupor, though retaining the cap
ability of executing reflex or instinctive movements. It can temporarily,
be aroused by loud noises, light placed before the eyes, pinching of th<
toes, etc., but it soon relapses into a condition of quietude. Coinciden
with the destruction of the cerebrum there occurs a loss of memory
reason, and judgment, and the animal fails to associate the impression
with any previous train of ideas. The higher the animal in the scale o
development, the more striking is the loss of mentality after remova
of the cerebrum.

4. Experimental interference with the blood-supply to the cerebrum is followec
by a diminished or complete cessation of its activities. There is per
haps no organ of the body that is so directly dependent upon its blood
supply for the continuance of its activities as the cerebrum. Thi
supply of blood is furnished by four large blood-vessels: viz., the twc
carotid and the two vertebral arteries. These vessels, after entering
the cavity of the skull, give off branches which unite to form the "circl<
of Willis." From this circle, large branches are given off which ente:
the cerebrum and distribute blood to all its parts. After passing througl
-v the capillaries the blood, greatly altered in chemic composition, ii
returned by large veins. The large volume of blood that is present ii
the brain and the marked changes in composition that it undergoes whil<
passing through the brain indicate a very active and complex metab
olism in this organ. By means of the anatomic arrangement of th<
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 vessel;
are occluded by pathologic deposits or surgical procedures, brail
activity continues, though perhaps diminished in degree. Occlusior
of all' four vessels, however, is at once followed by a complete abolitior
of all forms of cerebral activity. An experiment performed by Brown
S6quard 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 evidences of life had entirely ceased
Four tubes connected with a reservoir of warm defibrinated blooc
were then connected with the four arteries of the head. By means of i
pumping apparatus imitating the action of the heart the blood wai
driven into and through the brain. After a few minutes cerebral activ
ity returned, as shown by contractions of the muscles of the face anc
eyes. The character of the contractions were such as to convey tht
idea that they were directed by the will. These vital manifestations
continued for a period of fifteen minutes, when on the cessation of tht
artificial circulation they disappeared, and the head exhibited once
more the usual phenomena observed in dying: viz., contraction anc
then dilatation of the pupils and convulsive movements of the muscles
of the face. ,

The Localization of Functions in the Cerebrum. — By the term
localization of functions, is meant the assignment of definite physiologit
functions to definite anatomic areas of the cerebral cortex. From experi
ments made on the brains of animals, by the observation and association o

THE CEREBRUM 579

580 TEXT-BOOK OF PHYSIOLOGY

her 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 discharge of nerve
impulses excite contraction of special groups of muscles and which, from
their coordinate and purposive character, are conventionally termed voli-
tional. 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 ex-
perimentation: 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 of a given area is attended by phenomena which indi-
cate that the animal is experiencing 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. If the stimulation or excitation
of any given area is followed by contraction, and its destruction by paralysis
of muscles, it is. assumed that the area is motor in function — is an area of
motion.

The animals generally employed for experiments of this character are
dogs and monkeys, though other animals have frequently been employed by
different investigators. Of all animals, the monkey is the most frequently
selected, as the configuration of the brain in its general outlines more closely
resembles that 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 clinical symp-
toms which arise during the development of pathologic processes, and the
phenomena which occur during surgical procedures for the removal of
growths and pathologic cortical areas, justify the conclusion that the chart
of the sensor and motor areas of the monkey brain may be transferred to the
human brain without introducing any serious errors.

The Sensor and Motor Areas of the Monkey Brain.— From experi-
ments made on the brains of monkeys, Ferrier, Schafer, Horsley, and many
others have mapped out, though not with a high degree of definiteness and
certainty, a number of areas, stimulation of which gives rise to sensation,
while their destruction is followed by a loss of sensation on the opposite
side of the body. Collectively these areas are known as sensor areas and
are as follows:

1. A tactile area or an area for tactile sensibility.

2. An olfactory and a gustatory area or areas for olfactory and gustatory
^ sensibility.

3. An auditory area or an area for auditory sensibility.

4. A visual area or an area for visual sensibility.

These same investigators have mapped out a number of other areas, stimu-
lation of which gives rise to contraction of muscles on the opposite side of
the body producing movements which are so coordinate and purposive in

THE CEREBRUM

character that they may be regarded as identical with those produced by
volitional effort, while destruction of these areas is followed by a loss of
motion or paralysis of these muscles. Collectively these areas are known as
motor areas and are as follows :
i. A head and eye area.

A face area.

An arm area.

A trunk area.

A leg area.

The location of these areas will be apparent from an examination of Figs.
248 and 249.

FIG 248 —DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE LATERAL SURFACE OF
THE MONKEY BRAIN. — (After Horsley and Schdfer.)

Motor Reactions Following Electric Stimulation of Sensor Areas.-

Electric stimulation of the sensor areas is attended by certain motor reac-
tions which vary in accordance with the area stimulated Thus, wh<
electrodes are applied to different portions of the occipital lobe the eyeballs
are coniugately turned upward, downward, or laterally and to the opposite
side; when placed on the upper portion of the superior temporal convolu-
tion 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 convolu-
tion there is movement of torsion of the nostril and lips of the same side_
was then assumed that the movements were reflex m character inasmuch
as they resembled the movements caused by strong impressions mad,
sense organs and due to the development of sensations by the electric cu
ren s and not an evidence that the area in question is a motor area in the
sense that this term is applied to the area along the Rolandic fissure especially

s their destruction is not followed by paralysis of any of the corresponding
muscl s The vlw that these reactions are reflex in character has been

Tstantiated I by the anatomic fact that in the immediate neighborhood of
each sense area though not overlapping it, motor cells are present from
whth fferen nerve-fibers pass in close relation to the afferent fibers toward
Tnd to lower nerve-centers and presumably to the centers of origin of related

S82

TEXT-BOOK OF PHYSIOLOGY

cranial nerves. This interpretation is supported by the experiments of Serial er,
which showed that the contraction of the eye muscles which followed stimula-
tion 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 can-
not be to and through the frontal lobe, but probably through some lower
center. The same facts hold true for the reactions of the ear muscles follow-
ing stimulation of the temporal lobe.

|| For these reasons it came to be believed that each sense area is associated
with a motor area, though the two are not identical but separate in theii
distribution. The associated motor areas assist in the formation of a
mechanism by which reflex movements are executed when sense organs
are stimulated.

The view that the cortex of the cerebrum can be divided into separate and
independent though physiologically related motor and sensor areas has,.how-

FIG. 249. — DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE MESIAL SURFACE OF
THE MONKEY BRAIN. — (After Horsley and Schafer.}

ever, been questioned in recent years, and a somewhat different interpreta-
tion 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 separa-
tion, it is thought the term sensori-motor should be employed as more descrip-
tive 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 become
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 myelini-

THE CEREBRUM

,ation are three tracts numbered by Flechsig i, 2 and 3,. which arise largely
torn the median nucleus of the thalamus and the medial lemniscus and pass
o the anterior and posterior convolutions, to the para-central lobule and
oot of the superior frontal convolution, and to the foot of the third frontal
Convolution respectively. It is these fibers which convey nerve impulses to
he cortex and furnish information regarding changes taking place in the
>ody itself and thus lead to the performance of muscle movements. This
,rea is therefore primarily a sensor area, an area for body-feelings, cutaneous,
actile, muscle, and visceral, and secondarily a motor area. The afferent
ibers to this region become myelinated during the ninth month of intra-
iterine life, the efferent fibers from it become myelinated during the third
nonth of extra-uterine life.

By the same method of reasoning the gustatory, olfactory, auditory, and
visual sense areas are to be regarded as sensori-motor in character, for embryo-
ogic investigations show that subsequently to the myelimzation of the
ifferent tracts connecting the sense-organs with the cortex, efferent nerve-
racts arise from or near to the same centers and undergo myelimzation.
)ther words, these areas are primarily sensor and secondarily motor, and
therefore should be termed sensori-motor. In Flechsig's own terminology
each corticopetal or afferent tract is accompanied by a corticofugal c

^The view, viz. : the coincidence of sensor and motor areas has had general
acceptance for the reason that it seemed more in accordance with tl
than the earlier view. Nevertheless there were many facts both of a physio-
logic and pathologic character which were difficult to harmonize with it, am
in recent years the accumulation of facts and the weight **b"
toward the view that the areas are anatomically separate and

the Chimpanzee Brain.-In a series of experi-

ments mde by Sherrington and Grunbaum on the brain ^JgQ
it was discovered that the so-called motor area was not so widely distribu e
as in the monkeys generally, but was confined almost exclusively to the
KnWutLnuTt C front of the fissure of Rolando, as it was impossible to
oE any movement on direct stimulation of the convolution J
All points on the surface of the pre-central ^ofeti^S%
. formine- the wall of the Rolandic fissure itself, were found to be
! Krfductive of movement when stimulated^ ^S^

areas

584 TEXT-BOOK OF PHYSIOLOGY

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 following diagrams (Figs. 250 and 251), 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 indicate, as
suggested by Mills, that they are not sharply delimited, but that they inter-
fuse or interdigitate with surrounding regions.

In the following paragraphs these areas are considered in the order oj
their development and physiologic activities.

The Sensor Areas. — The sensor areas occupy regions corresponding in
a general way with those of the monkey brain.

i\The Cutaneous Area. — The area of cutaneous or tactile sensibility has been
assigned to the post-central convolution on the lateral aspect, and to a
portion of the super-frontal convolution and the lower half of the para-

CONCRLTE CONCEPT

FIG. 250. — THE AREAS AND CENTERS OF THE LATERAL ASPECT OF THE HUMAN HEMICEREBRUM.—

(C. K. Mills.)

central lobule on the mesial aspect of the hemicerebrum. It has been
stated by Flechsig, who bases his statements on the results of embryo-
logic investigations as to the course, time of myelinization and termina-
tion of certain afferent tracts, that the cutaneous area must also be
assigned though perhaps in less degree to the pre-central convolution
as well.

Clinic observations and post-mortem findings, together with the results
of more recent experimental investigations make it extremely probable that
the cutaneous area is confined entirely to the regions posterior to the central
fissure or fissure of Rolando. Dr. Charles K. Mills, whose skill in interpret-
ing the phenomena of the diseases of the brain is well known, states in this
connection, that "innumerable cases have been reported of lesions of the
motor (the pre-central convolution) without the slightest impairment of sen-
sibility." In many instances portions of the motor cortex of the human
brain have been excised by necessary surgical procedures. In these cases

THE CEREBRUM 585

also, careful studies have failed to show any impairment of sensibility. On
the other hand, destruction of the post-central convolution in monkeys by
the electro-cautery and in man by disease has invariably led to a loss of sen-
sibility, hemianesthesia, on the opposite side of the body without at the
same time causing any loss of motion. The location and extent of the an-
esthesia corresponds, of course, with the location and extent of the lesion of
the cortex.

, Gushing has recently recorded the results of stimulation with the electrical
current of the post-central convolution in two conscious patients. The local-
ized stimulus gives rise to definite sensations which were likened in one
case to a sensation of numbness, and in the other to a definite tactual im-
pression. From the foregoing facts it may be stated in a general way that
the post-central convolution is the region for tactile (light touch) and asso-
ciated forms of sensation such as tactile discrimination and localization and

FIG 2 m —THE AREAS AND CENTERS OF THE MESIAL ASPECT OF THE HUMAN HEMICEREBRUM.-

(C. K. Mills.)

perhaps of the finer shades of temperature, in other words of epi-critic

Tn^Musde Sense Area.-The area of muscle sensibility has been assigned
to the region posterior to but adjoining the post-central convolution am
includes the anterior part of the super-parietal and sub-paneta j
volutions and perhaps the supra-marginal convolution on he latera
aspect and a portion of the callosal convolution on the mesial aspe

£ sSS^ich are evoked in response to the action of nerve
impulses coming from tendons, muscles, etc are those o passive po^ ion
and the direction and duration of movements of parts of the body.
^J™£*™dp™t-m°«™ findings indicate that lesions of this area are
ol ^owed by a loss of the muscle sense. Owing to the close juxtaposi ion
of the areas of cutaneous and muscle sensibility it is unusual to find a loss

586 TEXT-BOOK OF PHYSIOLOGY

of muscle sense without impairment of the tactile sense, not infrequently
they coexist. Surgical removal of a small angioma posterior to the post-
central convolution and about the junction of the super- and sub-parietal
convolutions was followed by a loss of the muscle sense in the opposite hand
and forearm without any disturbance of other sensations (Starr and McCosh).
In addition to sensations of passive position and direction of movements,
the sensations of temperature and deep pressure are also associated with
the physiologic activities of this region of the parietal lobe. There is much
obscurity as to the location, however, of the area in which sensations of pain
are evoked. These two areas together constitute the area of the body-feelings
or the someaesthetic area.

Subdivisions of the Cutaneous and Muscle Sense Areas. — Clinic observa-
tions and post-mortem findings also warrant the deduction that the general
areas of cutaneous and muscle sense areas are physiologically subdivided
as is the general motor area (see page 590) into areas for the skin and
muscles of the face, arm, trunk, and leg which occupy respectively areas
that adjoin the corresponding subdivisions of the motor area representing
these parts of the body (Mills). On the mesial surface of the cerebrum there
are sensor areas for a portion of the leg, anus, genitalia and viscera.

The afferent pathway through which the nerve impulses, developed in
the sense-organs of the skin, tendons and muscles, pass to the cortical areas
and evoke the characteristic sensations has been described on pages 544
and 545.

3. The Stereo gnostic Area. — The area of stereognostic perception. Stereog-
. nosis is the recognition of an object when placed in the hands, through

its form, density, temperature, etc. The area associated with stereog-
nostic perception has been assigned to a portion of the super-parietal
convolution and to the precuneus.

This perception depends on the integrity and cooperation of the tactile,
the pressure, the temperature and muscle senses as well as the power of dis-
criminating points in contact with the skin. A lesion of this area impairs or
destroys the power of recognition of objects and establishes the condition of
aster eognos^s. This judgment, however, would also be impaired or abolished
if either the tactile or muscle sense were impaired or abolished, since both are
necessary factors in the series of events that lead to the formation of the
judgment. The existence of such a center has been made highly probable
by clinical cases in which astereognosis existed without impairment of either
the cutaneous or muscle sensibility. Inasmuch as focal lesions of the parietal
cortex give rise to localized impairment of stereognostic perception as well as
impairment of the cutaneous and muscle senses, it is stated that the area is
also capable of subdivision into smaller areas for the face, arm, trunk and
leg (Mills).

4. The Gustatory Area. — The area for gustatory sensibility has been assigned

to the sub-collateral convolution on the mesial aspect of the temporo-
sphenoidal lobe.

Disease processes involving this area give rise frequently to subjective
sensations of taste. Electric stimulation of this area in mammals causes
movements of the lips, tongue, etc., which are usually associated with sen-
sations of taste.

The afferent pathway by which nerve impulses, developed by action of

THE CEREBRUM 587

>rganic matter in solution on the terminal portions of the gustatory nerves,
ire transmitted to the cortical area is considered in connection with the
ense of taste.
The Olfactory Area. — The area for olfactory sensibility has been assigned

to the anterior portion of the hippo-campal convolution (the uncinate

region) and the anterior portion of the callosal convolution or gyrus

fornicatus.

Disease processes in this region give rise frequently to subjective sensa-
tions of odors which as a rule are of an unpleasant character. Destruction
of this area is followed by a loss of odor sensations. Electric stimulation
of this area in mammals was found by Ferrier to be attended with a peculiar
torsion of the nostril and lips on the same side similar to the reaction brought
about by the application of an odorous substance more or less disagreeable
to the nostril.

The afferent pathway, by which nerve impulses developed in the ter-
minal portion of the olfactory nerve by the contact of odorous particles are
transmitted to the cortical areas, is considered in connection with the
olfactory nerve.
6. The Auditory Area.— The area of auditory sensibility has been assigned

to portions of the temporal lobe and may be divided into primary and

secondary areas.

The primary area is located in the posterior portion of the super-

temporal convolution, and perhaps the posterior portion of the msula.

The secondary areas are located one below and in advance, and the other

below and some what behind the primary area, both extending into the

medi-temporal convolution.

Unilateral destruction of the primary area is followed, however, only
by a partial loss of hearing in the opposite ear, owing to partial decussation
of the auditory nerve, which, however, may be recovered from, after a time,
owing probably to a compensatory activity of the insular convolution.
Bilateral destruction of this region is followed by complete deafness.
primary area is connected on the one hand with the basal auditory center
(the internal geniculate body) by the auditory radiation, and on the other

„«
<±s *

ih," UghteTide, The am. holds true for the [.reept.or, of senM.ons of

**%Sa£* 5.'*™,% which ne,,e impute, developed in the terminal
pjons Sfhe ,Pudito,,7n«™ by the imp.c, of atmo.phenc vibrat™, .«,
Iransmitted to the auditory are., is considered in connecu

588 TEXT-BOOK OF PHYSIOLOGY

temporal convolution as well, there is an area in which, it is believed by
some clinicians, words are associated with concrete ideas of objects recog-
nized by one or more of the senses. To this area the term naming area
has been given. It is connected by association fibers with the areas for>
word hearing and word seeing.

7. The Visual Area. — The area for visual sensibility has been assigned to
portions of the occipital and parietal lobes and may be divided into
primary and secondary areas.

The primary area is located in a triangular-shaped area on the mesial
surface of the occipital lobe, which includes the gray matter between
the parieto-occipital and the calcarine fissures (the cuneus), and in
the gray matter of the first occipital convolution on the lateral aspect
of the occipital lobe. The secondary areas are located partly on the,
lateral aspect of the occipital lobe and partly in the supra-marginal
and angular convolutions of the parietal lobe.

Focal lesions of the primary area on one side are followed by lateral
homonym ous hemianopsia, which, however, does not involve, as a rule,
the fovea or macula.1 It is, therefore, the area of homonym ous half- retinal
representation. The location of the area for macular or central vision is
uncertain. Henschen locates it in the anterior part of the area near the ex-
tremity of the calcarine fissure, and asserts that in each area both maculae
are represented. From experiments made on monkeys Schafer locates it in
the same region.

The primary area is connected, on the one hand, with the basal visual
centers (the external geniculate body and the thalamus) by the optic radiation
and, on the other hand, with the secondary areas by association fibers.

The secondary areas on the lateral aspect of the occipital lobe are rather
extensive, reaching down as far as the third and fourth occipital convolu-
tions. Clinical evidence indicates that the cortex of this entire area is
associated with the registration or memorization of the visual sensations
and perceptions of objects, though it may be subdivided into smaller areas
for the registration of the visual sensations of different groups of objects
such as geometric and architectonic forms, of persons, places and natural
objects. Diseased processes in this region of the brain may result in the
condition known as object blindness. The area on the lateral aspect of the
parietal lobe (the supramarginal and angular convolutions) are associated
with the memorization of the visual sensations and perceptions of words,

1 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 con-
sequence 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 hemianopsia has been applied. As a result of this want of functional activity of these reti-
nal 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 homonymous hemianopsia, indicative of the
fact that objects in the field of vision to the left of the binocular fixation point are invisible.

THE CEREBRUM 589

tters, numbers, and perhaps objects. If the visual word area is destroyed
";y disease, word blindness is established, and the patient is unable to under-
'and written or printed language because of his inability to revive memory
nages of words. Letter and number blindness may or may not be present
ccording to the extent of the lesion.

All the special sense areas may, therefore, be said to consist of two smaller
>reas, a sensor and a psychic, e.g., a cutaneous sensor and a cutaneous psychic,
visual sensor and a visual psychic, etc.; the former is for the development
,f crude sensation; the latter, more complex in character, is for the correla-
x>n and formation of sensations whereby judgments or definite conceptions
.re formed (Campbell).

The Sensori-motor Areas. — It will be observed on examination of dia-
rams 250 and 251, that each sense area has closely associated with it a
ense motor area, each of which is to be regarded, from one point of view at
iiast, as a constituent part of the motor area of the cortex.

On a previous page it was stated that electric stimulation of sensor areas
s attended with certain motor reactions in the muscle groups associated with
he activities of the sense-organs, reactions which resemble those caused by
trong impressions made on the sense-organs by external agents; that some
nvestigators, therefore, believed that the areas in question were motor and
,-ensor in function and that their anatomic substrata were so intermingled
md so difficult of separation that the term sensori-motor should be employed
is more descriptive and more in accordance with the facts; but that the
progress of experimental work, together with the accumulation of facts from
he clinical and pathologic fields, incline to the view that the groupings of the
motor and sensor cells are anatomically and physiologically separate and
distinct though related functionally. Corresponding areas are now believed
to be present in the human brain. For this reason each of the special sense
areas with the exception of the cutaneous, is represented in Fig. 250, as asso-
ciated with a motor area, viz.: a gustatory, an olfactory, an auditory and
a visual motor area. The cutaneous sensor area is, however, also associated
with a cutaneous motor area in the posterior portion of the pre-central con
volution These associated motor and sensor areas are here represented
not as overlapping, but contiguous, though perhaps interdigitating with
each other. Their location and relations are apparent from an examina-
tion of the diagrams. From the results of experimental investigations it is
.generally believed the sense motor areas are more ^^\^f*
sensori-motor reflex actions rather than to voluntary actions and that they
constitute the efferent element of a reflex arc, since destruction of the area
interferes with the former rather than the latter Hpmnn,trated that

It will be recalled that the investigations of Flechsig demonstrate
from these areas efferent or motor nerves pass through the sense tracts or
radTations in the reverse way, to the mid-brain, where they become related

SZ^crfSS^tS motor cranial nerves which excite to action those

, n

590

TEXT-BOOK OF PHYSIOLOGY

ance with the part stimulated. All these movements are similar to those
which follow gustatory, olfactory, auditory, and visual sensations, evoked
by unexpected stimulation of the peripheral sense-organs themselves.

The Motor Areas. — The motor areas especially those associated with
the execution of volitional movements have been assigned to the pre-central
convolution, the contiguous portions of the base of the medi- and sub-frontal
convolutions and the para-central lobule.

The exclusion of the post-central convolution from the motor area, tq
which it belongs in the monkey brain, 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 abso-
lutely inexcitable to electric stimulation. It is quite probable that with the

FIG. 252. — SCHEME OF THE MOTOR AREA OF THE HUMAN BRAIN AND ITS SUBDIVISIONS. — (After

Mills.)

growth of the brain in size and complexity, the motor area has come to occupy
a position somewhat farther forward in the human brain than in the monkey
brain.

This general area is capable of subdivision into areas of variable size, e.g.,
head and eyes, face, arm, trunk, and leg areas which are related through the
efferent nerve-fibers composing in part the pyramidal tract to groups of
muscles, e.g., head and eye muscles,' face, arm, trunk and leg muscles, on
the opposite side of the body and the activities of which they initiate and
control. (See Figs. 250 and 252.)

These large subdivisions of the cortical motor area are in turn capable
of a further subdivision into smaller areas which are in relation with one or
more muscles of the larger groups. These smaller areas indicated in the
diagram by compressed italics contain groups of nerve-cells which excite to
action, through their efferent axons and their medullary and spinal connec-

THE CEREBRUM 591

dons, the muscles which impart movements to the regions corresponding to

:he words in compressed italics.

The main motor areas are as follows:

li. The Head and Eye Area. — This area has been assigned to the contiguous
portions of the medi- and sub-frontal convolutions just anterior to the
pre-central convolution. It is subdivided into smaller areas which initiate
and govern the movements of the head and eyeballs. Stimulation of
this area, in the chimpanzee at least, produces turning of the head to
the opposite side with conjugate deviation of the eyes to that side.

2. The Face Area. — This area has been assigned to the lower portion of the

pre-central convolution and extends from below upward to about the
level of the genu of the central fissure. This rather large area may be
subdivided into (a) an upper portion including about one-third of the
whole and (b) a lower portion including the remaining two-thirds. In
both the upper and lower portions, there are groups of nerve-cells which
excite to action the muscles imparting movements to (a) the angle of
the mouth, the eyelids and jaws and (b) the movements of the vocal
bands or cords, the opening and closing of the mouth, the protrusion
and retraction of the tongue. All of these movements have their areas
of representation in the face area.

3. The Arm Area.— This area has been assigned to the pre-central convolu-

tion just above and contiguous to the face area which it exceeds some-
what in extent. It is the largest of all the subdivisions of the general
area. It may be divided into at least five smaller areas, the cells of
which excite to action the muscles imparting movements to the thumb,
the fingers, the wrist, the elbow and the shoulder.

4. The Trunk Area.— This area has been assigned by^ Sherrington and

Greenbaum to the pre-central convolution just superior to the arm area
and is rather limited in extent. Horsley located a portion of this area
on the mesial and lateral edges of the hemisphere in front of the leg
area. The nerve-cells of this area when electrically stimulated excite
to action the muscles which impart movements to the spinal column,
such as arching rotation, etc.

< The Leg Area.— This area has been assigned to the extreme upper portion
of the pre-central convolution and to the adjoining mesial surface, the
upper portion of the para-central lobule. The area on the lateral aspect
of the cerebrum may be subdivided into at least four smaller areas con-
taining groups of nerve-cells which excite to action the muscles impart
ing movements to the toes, ankle, knee and hip.

Evidence from the clinical side has demonstrated the fact that a
localized irritative lesion of any one of these areas gives rise to convuls
movements of the muscles of the opposite side of the body, simil
character to those resulting from electric simulation of the correspo
ing 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
Se purpose of localizing obscure irritative lesions prior to surgical pro-
cedures on the brain gives rise to similar convulsive movements.

ne motor speechlnd the motor writing areas will be considered
the following paragraphs.

592 TEXT-BOOK OF PHYSIOLOGY

Language. — Language may be defined as the expression of ideas by a
succession of motor acts and may be divided into (i) spoken language (ar-
ticulate speech), and (2) written language. The expression of ideas byj
words (speech) and by verbal signs (writing) depends primarily on the power
of reviving images or memories of objects, words, letters, numbers, etc., seen
and heard, and secondarily on the power of reviving the images or memories
of the muscle movements which were previously employed in an effort to1
imitate or reproduce the sounds of objects, words, letters, etc. (speech) or
the visual impressions by verbal signs (writing).

For both spoken and written language two phases of activity are to be
recognized, viz. : a receptive or sensor and an emissive or motor,
i. The Receptive or Sensor Phase.— Before language can be acquired by
a developing child, impressions of external objects must be made on the
visual, auditory and other sense-organs as well, which in consequence
lead to the development in the corresponding cortical areas of sensations
and ultimately of conceptions regarding the nature of the objects im-
pressing themselves on the peripheral sense-organs.

Thus, through the visual apparatus in its entirety, definite concrete
conceptions regarding the shape, size, color, etc., of an object or word,
letters and numbers, etc., are obtained. For future use the sensations
must not only be perceived but registered or memorized. The memor-
ization of the sensations produced by luminous objects, is supposed to
be associated with the activities of the cortex on the lateral aspect of the
occipital lobe ; the sensations produced by words, letters, and numbers
with the activities of the cortex of the supra-marginal and angular con-
volutions of the parietal lobe. This is made possible by means of
association fibers which unite the primary visual area in and around
the cuneus, with the secondary visual areas. That the visual memories
" are associated with the previously mentioned convolutions of the parietal
and occipital lobes is apparent from the fact, that if they are destroyed
by disease there is a loss of these memories even though the individual
sees the objects as formerly; but though seeing them they cannot be
remembered or recalled. To this condition the terms word and object
blindness have been given.

Coincidently with the development of visual sensations and their memori-
zation, the child receives through the auditory apparatus in its entirety,
definite concrete conceptions regarding the intensity, pitch and timbre or
quality of sounds produced by atmospheric vibrations, due to vibrating
bodies; sounds which are subsequently associated with the vocal and articu-
lating apparatus of human beings when pronouncing words, letters and
numbers, or with objects such as bells, musical instruments, etc. For
future use the sensations thus aroused must not only be perceived but like-
wise registered or memorized. The memorization of the sensations pro-
duced by atmospheric vibrations is supposed to be associated with the
activities of the temporal lobe and more particularly of the posterior two-
thirds of the super-temporal and medi-temporal convolutions. This is made
possible by means of association fibers which unite the primary auditory area
with the secondary auditory areas. That the auditory memories are associ-
ated with the previously mentioned convolutions is apparent from the fact
that if they are destroyed by disease, there is a loss of these memories

THE CEREBRUM 593

jven though the individual hears the sounds as formerly, but though hear-
jig them, they cannot be remembered or recalled. To this condition the
!;erms word deafness and object deafness have been given.

From the concrete conceptions formed of individual objects of a similar
:haracter or of a class, there are developed abstract conceptions, by the
uniting into a single idea the elements which are common to all the objects
bf the class. This involves an analysis and a synthesis, a recognition of
the points of similarity and dissimilarity. The development of abstract
conceptions is believed to be associated with the activities of the cortex of
one or the other of the association areas (see page 596).

For the emission of the sounds of words which constitute spoken
language it is essential that the sensations produced by the muscle move-
ments shall not only be perceived but registered or memorized. Without
this the capacity to express words would not be possible. This registration
is believed to be associated with the lower portion of the area for muscle
sensibility.

2. The Emissive or Motor Phase. — In an attempt to express ideas by
spoken or written language the muscles determining the action of the
larynx, jaws and' teeth, tongue, lips, hands, etc., are excited to action
by nerve impulses descending through nerves, the nuclei of origin of
which lie in the gray matter beneath the floor of the medulla oblongata
and in the gray matter of the spinal cord. The nuclei of these nerves
are in turn excited to action by nerve impulses descending by way of
the internal capsule from the cortical areas for the face and arm
respectively.

The Motor Speech Area. — By this term is meant an area of the cortex,
the function of which is to arrange language for outward expression; for the
use of the executive centers concerned with speech, viz.: the laryngeal, lingual
and facial centers located at the foot of the pre-central convolution. This
area, i.e., the motor speech area, has been assigned to the posterior part of
the subfrontal convolution (Broca's convolution) on the left side in those who
are right-handed and on the right side in those who are congenially left-
handed, and in the anterior part of the insular or perhaps the pre-insular
convolutions. Unipolar faradic stimulation of this area fails to call forth
any motor response; its destruction by disease, however, is followed by a
more or less extensive loss of the faculty of articulate speech or the faculty
of expressing ideas with words, a condition usually spoken of as motor
aphasia or aphemia. This area and the area at the foot of the pre-central
convolution are united by association fibers.

The Motor Writing Area.— By this term is meant an area of the coi
the function of which is to arrange language for outward projection; for 1
use of the executive centers concerned with writing, viz., the arm
located in the middle portion of the pre-cental convolution. This area, t.^.,
the motor writing area, has been assigned to the posterior half or third oi
medi-frontal convolution. Unipolar faradic stimulation of this area fail
call forth any motor response; its destruction by disease, however, is followec
by an inability to express ideas by writing, a condition usually spc
of as agraphia. This area and the general arm center in the pr<
convolution are united by association fibers.
38

594

TEXT-BOOK OF PHYSIOLOGY

ith om

The Language Zone. — These different areas are connected with
another by association fibers, and, taken collectively, constitute the languagi
zone, which in people who are congenitally right-handed is located on th<
left side of the brain only. Their situation and relations are shown in Fig
253. In this figure the dotted lines coming from the eye (v) and ear (a
represent the visual and auditory tracts through which nerve impulses pas
to the visual centers (V) and the auditory (A) respectively. Similar lint

coming from the muscles involved L
speech and writing might also be reprc
sented to indicate the paths of th
nerve impulses to the muscle sens
areas not shown in the diagram. Th
single continuous lines on the surface
of the cortex represent nerve-fiber
which associate the visual and audi
tory centers with the speech and writ
ing centers. The double lines assc
ciate the visual and the auditory area
with the frontal association area on th
one hand and the association area wit;
the motor speech and the motor writ
ing area respectively. The dotted line
coming from the speech and writin
centers represent the tracts comin
from the areas in the pre-central con
volutions, and through which nerve im
pulses pass to the muscle of the larym
tongue, mouth, and lips, and to th
muscles of the hand. The anatomi
and physiologic association of the vari
ous areas is essential to the registratio]

Xf J^IS3t ^TC^ °^ tne impressions made on the ea

^^CC---^^^ /( and eve and for the expression of th
^ *i<^u-wA^ ideas evolved from them by word

(speech) and signs (writing). Thei
collective action is essential to the ac
quisition of language. Destruction o
any part of this cerebral mechanisn
is attended by an impairment of or ;
total loss in either the power of ob
taining auditory images of words hear<

FIG. 253. — DIAGRAM SHOWING THE RE-
LATION OF THE CENTERS or LANGUAGE AND
THEIR PRINCIPAL ASSOCIATIONS. A. Audi-
tory center. V. Visual center. M. Motor
speech center. E. Motor writing center.
O O. Intellectual center.— (A /ter Gr asset.)

and visual images of words seen, or th

power of expressing ideas by speech and writing. To this pathologic con
dition the term aphasia has been given.

Aphasias are of many degrees and kinds, though they may be include(
in the two general divisions, motor and sensor.

Motor aphasia may be either aphemic or agraphic. In aphemic aphasi;
the patient is unable to express or communicate his thoughts by spokei
words, owing to an inability to arrange words for outward expression am
hence to execute those movements of the mouth, tongue, etc., necessary fo

THE CEREBRUM 595

speech without there being any paralysis of these muscles. The lesion is

usually in the third frontal convolution and most frequently associated with

tight hemiplegia. In agraphic aphasia the patient is unable to communicate

, his ideas by writing through an inability to arrange verbal signs for outward

expression and hence to execute the movements of the hand and arm neces-

sary for writing. In this form of aphasia the lesion is in the writing area,

ijin the posterior half or third of the medi-frontal convolution. These two

f Jorms of motor aphasia are not infrequently associated.

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 neighbor-

hood of the supra-marginal convolution. In auditory aphasia or amnesia

.the patient cannot understand articulate or vocal speech, though capable of

hearing and understanding other sounds, through an inability to distinguish

the associations 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.

Many other forms of aphasia have been observed and described by
clinicians which are fully considered in works relating to diseases of the

nerve system.

The statements .regarding the mechanism of speech, the functions

signed to the motor speech area (Broca's convolution) and the motor writing

area though very generally accepted, have been questioned in recent years

1 by Marie who on the basis of clinico-pathologic facts has presented a some-

what different view which has found many adherents.

Marie's Theory of Aphasia.— Marie states that there is but one aphasia
and but one speech center, which he locates somewhere in the left temporo-
parietal lobe, and which he designates as a specialized intellectual center
speech. Motor aphasia in the accepted sense, he states is a combination of
word blindness and word deafness (both, however being defects of the speech
center) and defective articulation (anarthria) the result of a lesion of the
motor tracts necessary to the excitation of the muscles for articulation. Th
ksion causing the anarthria is in the lenticular zone, in close relation to the
enticular nucleus. This zone, in a horizontal section of the brain is limited

uceus. ,

b£a vertical plane level with the anterior sulcus of the ins,
by a similar plane level with the posterior sulcus of the insula;
tL^lrll ventricle and externally by the surface of the insula
This zone i< anatomically associated with the supra-marginal and angular
Jon^o utos and with tJposterior portions of the super and ^-touponj

many others are at variance with it.

596

TEXT-BOOK OF PHYSIOLOGY

Bilateral Representation.— Though highly specialized movements
such as those performed by the arms and hands, legs and feet, have thei:
areas of representation on one side of the cerebrum only, and that oppositi
to the side of the movement, less highly specialized movements, such as thi
masticatory, phonatory, respiratory and various trunk movements, whicl
require for their performance the cooperation of muscles on both sides o

Motor and tactile area.

Parietal association area

Island of Reil.

Occipito-temporal
association area.

Auditory area.
Motor and tactile area.

Parietal association area
(Precuneus).

Visual area
(cuneus) .

Frontal

association

area.

Occipito-temporal
association area.

Olfactory lobe.
Olfactory tract.

Olfactory area.

Gyrus hippocampus.

FIG. 254.— DIAGRAMS TO SHOW THE POSITION AND THE RELATION OF THE ASSOCIATION AND
PROJECTION AREAS. THE PROJECTION AREAS ARE DOTTED.— (After Flechsig.)

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 unilat-
eral; 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

THE CEREBRUM 597

:otal cerebral cortex. There yet remain large regions to which it has been
mpossible to assign specific functions based on physiologic experiments.
Three or four such regions separated by the sensor and motor centers are to
pe recognized on the lateral and mesial aspects of the hemisphere. In Fig.
£54 the location, extent, and names of these regions are represented. The
.ibers which are found in these regions belong almost exclusively to the
Association system, and become medullated at a later period than do the fi-
bers of the projection system; moreover, from the method of their medulliza-
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
:onvolutions. From histologic and pathologic evidence these regions were
termed by Flechsig association centers or areas, implying the idea that
through the intervention of their cell mechanisms the sense areas are in-
directly associated anatomically and physiologically, 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 associating 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 case 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 num-
ber 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 charac-
teristics 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

598 TEXT-BOOK OF PHYSIOLOGY

and the bone. These sinuses, from the very nature of the tissues whid
enter into their formation, have rigid walls and will therefore withstand an}
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.

The various sinuses have opening into them, the veins which return the
blood from the cortex and subjacent white matter, and from the inner struc-
tures of the brain. Neither sinuses nor veins have valves and most of th<
veins which empty into the superior longitudinal sinus have their mouth?
directed forward, hence the blood discharged from these veins must flo^
against the current in the sinus. The venous blood leaves the craniun
mainly by way of the internal jugular veins which are direct continuations ol
the lateral sinuses.

The Intra-cranial Lymph -spaces. — In order to understand the phe*
nomena attending the circulation of blood through the cranium it is necessar)
to take into consideration an important fact, viz. : that the brain and spina!
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 mate]
is'filled with a liquid, the so-called cerebrospinal fluid, which being interposec
between the brain and the skull on the one hand and the spinal cord and the
vertebrae 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 communicatior
with that in the subarachnoid space through the foramen of Magendie and tb<
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 tc
from 60 to 80 c.c.

The Mechanism of the Intra-cranial Circulation. — As previousl)
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 condition of the arteries during the
cardiac diastole, the relations of these structures are somewhat as follows : the
cerebrospinal fluid occupies all the available lymph-space, but under a pres-
sure approximately equal to that in the large veins and hence not materiall)
above that of the atmosphere; the pressure in the arteries, capillaries and
veins presents the Usual values in these different regions of the vasculai
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 accomplished 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 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

THE CEREBRUM 599

ts activities, even to the point of unconsciousness. If this is not to occur
he maximum expansion of the arteries, and hence the brain, must be checked
ind controlled. This is accomplished in the following way: As the brain
i/pproaches that degree of expansion permitted by the displacement of the
erebrospinal fluid, it begins to exert a compression of the pial veins. This
.ompression by narrowing the lumen of the veins diminishes their capacity
ind hence increases the pressure of their contained blood until it is equiva-
ent to the pressure exerted by the brain against the veins. At this moment
he pressures in the arterioles, capillaries and veins approximate each other
n 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
ncrease 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
n the cerebral veins is regarded therefore as the factor which, by limiting
ibrain expansion, checks the rise of the intra-cranial pressure beyond physio-
llogic limits. With the diastole of the heart and the recoil of the artenes,
the former relation of the blood, brain, cerebrospinal fluid and cranial walls
is regained. Because 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 expiratory rise of venous pres-
sure than by the inspiratory rise of general arterial pressure,
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 mtra-
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. H
to be 40 to 50 mm. of Hg. in the convulsions of strychnin poisoning and a
little less than zero in a patient standing erect.

The Regulation of the Volume of Blood Entering the Brain. -
generally believed that the cerebral vessels are not F^ed wi^^^otor
5 T^™ sttPmnt to Drove their existence either by physiologic or

be depende't on changes affecting the arterial and venous pressures

tt *JS?^ in increasing or decreasing the blood-supply
to therresfd'sTthe power of the ^7*""^^^^

600 TEXT-BOOK OF PHYSIOLOGY

bringing about a rise or a fall in the general arterial pressure, regulates thi
blood-supply to the brain, and controls its amount in accordance with it:
needs.

Brain Activity. — Brain activity is characterized by an active conscious
ness, the development of sensations, ideas, feelings, and the exercise o
volitional power (which manifests in muscle movement) and is the result of ;
physiologic condition of the body at large. For the manifestation of brail
activity it is essential that the irritability of the brain cells and more especialh
of those composing in large measure the cerebral cortex be maintained at I
normal physiologic level, so that they may respond in the manner peculia:
to them to the action of nerve impulses transmitted through afferent nerve:
from all regions of the body. Here as elsewhere throughout the body, th<
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, witl
a given^ velocity and under a definite pressure. So long as these condition;
are maintained in the strictly physiologic condition, so long will the brair
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 externa
world, viz.: the eyes, ears, nose, tongue, and skin. The maintenance o:
these structures in a strictly physiologic condition is also one of the essentia
conditions for brain activity.

Judging from the changes in the character and composition of the blooc
which occur during its passage through the brain capillaries, there is coin-
cidently with brain activity an active metabolism, which eventuates, at the
end of a variable number of hours, in the decline of the irritability, a reduc-
tion of functional activity, and the establishment of the condition of fatigue.
The irritability of the sense organs, especially of the eyes and ears, in al]
probability declines in a similar manner. These structures pass into the
condition of fatigue and become less responsive to external stimuli. The
results ^of all these conditions is a less active stimulation of the brain cells,
which in connection 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 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 mucous membranes from diminished secre-
tion. The skin appears to be less sensitive to mechanic pressure and other
forms of stimulation.

THE CEREBRUM 601

In addition to the foregoing phenomena, experimental investigations
ave shown that there is a shunting of a portion of the blood-stream from
le brain to other regions of the body, especially to the skin and perhaps to
le abdominal viscera as well, whereby it becomes incapable of functionating
ihysiologically. The fact that the brain receives a lessened quantity of
iJood during sleep has been shown by trephining the skull and inserting in
fee orifice a glass plate through which the circulatory conditions of the brain
fan be observed. In the waking condition the blood-vessels on the surface
• f the brain are prominent, and turgid with blood and the whole organ
'ompletely fills the cranial cavity, indicating that the blood-vessels in the
iterior of the brain are in a similar condition. With the onset of sleep the
•arger blood-vessels begin to diminish in size, the smaller vessels disappear
•rom view, the brain tissues become pale and the volume of the brain shrinks.
)uring the continuance of deep sleep, this anemic condition persists. As
•he period of sleep approaches its termination, the smaller blood-vessels
.gain fill with blood, the surface of the brain flushes, and in a very short
line 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,
las been shown by inserting an arm or leg in a plethysmograph by which
neans 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
irm 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
naximum which was from one to two hours after the beginning of sleep.
<\fter this period the volume remains practically the same for several hours,
diminishing as the intensity of sleep diminishes and the waking state is
ipproached. 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
Df 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 withdrawal 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 depth of sleep varies from hour to hour.
Attempts have been made to estimate the intensity by measunng 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 maxi-
mum between the first and second hours, after which it rapidly decreases un-
til 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 du
the latter period when the brain is reviving that dreams arise, the ele
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 satisfactory,
all the facts which have been presented it would appear that one cause
decline in the irritability of the nerve-cells of the brain and associated sense

602 TEXT-BOOK OF PHYSIOLOGY

organs, and the development of fatigue conditions, the result of prolongs
activity.

A second cause is the withdrawal of a large portion of trie blood from th<
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 resul
of the withdrawal of the blood there has been much difference of opinion
Howell has offered a plausible explanation for the withdrawal of the blooi
from the brain to the cutaneous vessels, based on the activity of the vase
motor center. He assumes that for a variable number of hours, correspond
ing to the usifcal waking state, this center possesses a certain average tonus
due in all probability to reflex influences, by virtue of which it maintains ;
certain average contraction of the cutaneous vessels. But at the end c
this period it too becomes fatigued, declines in irritability, becomes les
responsive to reflex influences, and hence loses its control over the vesseh
As a result they dilate and thus reduce the amount of blood flowing to the brai
to a level insufficient to maintain its activity, after which sleep supervenes
During sleep the irritability and tonus of the center are restored, when ii
control of the blood-vessels is regained. Unless the brain in its functiona
activities differs from all other organs of the body, it may be inferred tha
cessation of activity or repose is the result partly of fatigue and partly of ;
diminution of the blood-supply.

CHAPTER XXIV
THE CEREBELLUM

The cerebellum is situated in the inferior fossae of the occipital bone
eneath the posterior lobes of the cerebrum, from which it is separated by the
ntorium cerebelli, a semilunar fold of the dura mater. It is partially
ivided into hemispheres by a longitudinal fissure, more apparent on the
iferior surface, though united by a central lobe, the vermiform process,
ach hemisphere is connected with the cerebrum, the pons, medulla, and
Dinal cord by three bundles of nerve-fibers known respectively as the superior,
liddle and inferior peduncles. The surface of the cerebellum presents a
eries of lobes and fissures of which the former have received more or less
anciful names. A section of the cerebellum shows that it is composed
f gray matter externally and white matter internally. The general
ppearance presented on section is shown in Fig. 255.

Structure of the Gray Matter .-The gray matter consists mainly of
lerve-cells of varying size and shape, which are arranged in two layers:
iz an outer or molecular and an inner or granular.

The molecular layer consists of stellate and multipolar cells of small

ize from which dendrites and axons pass horizontally and vertically

anular kyer consists, as its name implies, of granular-shaped cells and

rTe stellate cells These cells are characterized by the possession of den-

^ Sd axoS! the course and relation of which have not been clearly

determined^ molecular layer presents a series of large cells

r JnallvTscribed by Purkinje and known by his name From the outer
nl nfL rell body one or more dendrities emerge which soon divide and

604

TEXT-BOOK OF PHYSIOLOGY

with those coming from the opposite side, while others pursue a straigh
direction, terminating on the same side. Through the intervention of fiber
which arise in the red nucleus and ascend to the cerebral cortex, the hemi
sphere is thus connected with both sides of the cerebellum, though chiefl-
with the opposite side.

Efferent fibers also leave the cerebellum by the middle peduncle and pas
directly to the nucleus pontis, around the cells of which their terminal
arborize. Efferent fibers also descend the inferior peduncles and constituti
the tract known as the Lowenthal and Marchi tract, situated 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 th<

FIG. 256. — SECTION or CEREBELLAI
CORTEX. A. Outer or moleculai
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.)

FlG- 255.— VIEW OF CEREBELLUM IN SECTION AND
or FOURTH VENTRICLE, WITH THE NEIGHBORING PARTS.
— (From Sappey.) i. Median groove fourth ventricle,
ending below in the calamus scriptorius, with the longi-
tudinal eminences formed by the fasciculi 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 quadrigemina.— (After Hirschfeld
and LeveiUe.)

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 in-
ferior 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 formei
coming from the gracile and cuneate nuclei of the opposite side, the lattei
from the same side, which also pass to the superior vermis; (3) the acoustico-
cerebellar tract, composed of fibers which are the axons of the sensory
end-nuclei (Deiters) of the vestibular portion of the auditory nerve. It is
probable that all these fibers decussate prior to their final termination.

THE CEREBELLUM 605

The cerebellum through this system of efferent and afferent fibers is
Bought into relation with many different regions of the cerebrum, pons,
[edulla, and spinal cord. Each half of the cerebellum is connected with the
[regoing structures of the same side, and of the opposite side.

THE FUNCTIONS OF THE CEREBELLUM.

! From the observations of the results of experimental lesions, from the
Jialysis of clinico-pathologic facts, and from the comparative anatomic
bvelopment in different animals, the deduction has been drawn that the
trebellum coordinates and harmonizes the action of those muscles the
i:tivities of which are necessary to the maintenance of body equilibrium
oth during station and progression.

By equilibrium of the body is understood a condition which may be main-
lined for a variable length of time without displacement, and is possible only
i) long as a vertical line passing through the center of gravity falls within the
[ase of support. The support offered by the earth to the feet neutralizes and
[imnteracts the force of gravity. In standing, when the body is in the erect or
hilitary position, the arms by the side, the center of gravity lies between the
Ficrum and the last lumbar vertebra, and the line of gravity falls between the
[;et and within the base of support. The entire skeleton for the time being
I rendered .fixed and rigid at all its joints by the combined action of the
mscles connected with it. That this position may be maintained all the
ifferent groups of antagonistic but cooperative muscles must be accurately
Dordinated in their actions. Any failure in this respect is at once attended
y a disturbance of the equilibrium and displacement.

In progression, walking, running, dancing, etc., the body is translated
rom point to point by the alternate action of the legs. Whether the direction
f the translation be rectilinear or curvilinear, as the legs change their position
:om moment to moment, the center of gravity also changes, and at once the
quilibrium is menaced. If it is to be maintained and displacement prevente
Here must be a prompt readjustment in the relation of all parts of the
lody so that the line of gravity falls again within the base of support.
nore complicated the moments of progression, or the 'narrower the base ot
upport, the greater is the danger to the equilibrium, and hence the necessity
br rapid and compensatory changes in coordinated muscle activity. All
movements of this character, in man at least, are primarily volitional and
lequire for their performance the constant exercise of the attention. With
requent repetition they gradually come to be performed independently c
oSciousness and fall info the category of secondary or acquired reflexes.

Though coordinating power is exhibited by the spinal cord, med
aid basal ganglia, it is only in the cerebellum that this power attains it
highest development and differentiation. To it is assigned the power
Defecting and grouping muscles, not in any restricted P^.^^f^
,he body, and coordinating their actions in such a manner as to pr(

iqUThe Results of Experimental Lesions.-If the cerebellum in its totality
Coordinates and harmonizes the action of the muscles on the

6o6

TEXT-BOOK OF PHYSIOLOGY

the cerebellum are attended by such results. The phenomena observed a:
many and complex. They differ in extent and character in different anima
and in accordance with the extent and location of the lesion, though the no
ofjncoordination runs through them all.

FIG. 257.— ATTITUDE ASSUMED AFTER DESTRUCTION OF THE LEFT HALF OF THE CEREBELLUM,
(Moral and Doyon, after Thomas.)

Removal of one lateral half of the cerebellum in the dog is followed by a
inability to maintain the equilibrium necessary to the erect position. 0
attempting to stand, the animal at once falls toward the side of the lesion, tt
muscles of which at the same time contract and give to the body a distinct!
curved condition (Fig. 257). The anterior limbs are extended to theopposil
side. On making efforts to regain the standing position, the animal ma

roll over around the long axis of ft
body. Conjugate deviation of th
eyes is frequently observed as well a
nystagmus.

After a few days the symptoms pa]
tially subside and the animal acquire
the power of sitting on the abdome
when the anterior limbs are widel
extended (Fig. 258). As the daysg
by the improvement continues, and th
animal recovers the power of walking
though each step is attended wit
tremor and oscillations of the bod)
Any change in the center of gravit
such as results^when one leg is lifted may result in a fall toward the side c
the lesion, owing to an inability to promptly bring about the necessar
compensatory muscle actions. With time the animal continues to improv
in its power of adjustment, though it never completely recovers it. Move
ments of progression are apt to be characterized by stiffness and accom
panied by tremor suggestive of volitional efforts.

Total removal of the cerebellum is followed by a different train of symp
toms. The extensor muscles apparently preponderate in their action, fo
the limbs are extended and abducted, the head and neck are retracted, an<
opisthotonos is established. In time these effects also partially subside
though all attempts at walking are permanently accompanied by tremor am
oscillations. The characteristic effect which follows section of the peduncle
is again incoordination, manifesting itself in deviation of the head, eyes, in
ability to walk, tremor on exertion, etc. The effects vary, however, accordinj
to the peduncle divided. Section of the middle peduncle gives rise to th.

FIG.- 2$%. — ATTITUDE IN REPOSE AFTER
THE .COMPLETE REMOVAL OF THE CERE-
BELLUM BUT DURING THE PERIOD OF RES-
TORATION OF FUNCTION. — (Morat and Doyon,
after Thomas.)

THE CEREBELLUM 607

iost pronounced effects. The head and the anterior part of the body are
% once drawn toward the pelvis on the side of the section. A voluntary
itfort on the part of the animal causes it to lose all control of its muscles and
t.e body is rotated around its longitudinal axis from 40 to 60 times a
binute before it comes to rest. According as the lesion is made from be-
Jnd or before, the rotation is from or to the side of the section. In time
Hiese symptoms subside, though the animal never completely recovers.

The partial recovery of the power of coordination, observed after removal
\: a portion or the whole of the cerebellum, indicates that the centers in the
ford, medulla, pons, and cerebrum
I'idowed with corresponding
Lough less developed power, de-
plop compensatory activity and
pquire to some extent the capa-
bilities of the cerebellum itself

fig- 259).

: Clinico-pathologic facts partly
brroborate the results of physio-
hgic investigations. In various Fm> 259._PROGRESSION AFTER DESTRUCTION

l)rms of uncomplicated cerebellar 0F THE VERMIS.— (Moral and Doyon, ajter Thomas.)

fisease, vertigo, tremor on making

Voluntary efforts, difficulty in maintaining the erect position, unsteadiness

ii walking, opisthotonos, pleurothotonos, are among the symptoms generally

ibserved.

Comparative anatomic investigations reveal a remarkable correspondence
letween the development of the cerebellum and the complexity of the move-
tents exhibited by animals. In those animals whose movements are corn-
flex and require for their performance the cooperation of many groups of
nuscles the cerebellum attains a much greater development in reference to
he rest of the brain than in animals whose movements are relatively simple
h character. This relative increase in the development of the cerebellum
3 found in many animals, as the kangaroo, the shark, the swallow, and the
iredaceous birds generally.

The Coordinating Mechanism.— Though it is not known how 1
erebellum selects and coordinates groups of muscles for the performance
,f any complex movement, it is known that its activity is largely reflex in
rigin and excited by impulses which come to it from peripheral organs,
his as in other forms of reflex activity the mechanism involves (i) afferc
lerves, e.g., cutaneous, muscle, optic, and vestibular, and their related end-
,rgans, tactile corpuscles, muscle spindles, retina, and semicircular cam
til indirectly connected with (2) the cerebellar centers; (3 1 efferent nerves
ndirectly connected with (4) the general musculature of the body
;tation and progression are directly dependent on the development and
ransmission of ffferent impulses from the previously ^"^p^M
'sense-organs to the cerebellum. Tactile, muscle, visual, and jabynntlune
impressions and sensations not only cooperate in the deve opment and or-
?anization of the motor adjustments necessary to the maintenance ot

t^ but even after their organization

ar ^ necessary to the excitation of cerebellar activity The manner in
^th "Soothe development of this capability on the part of the cere-

£8 TEXT-BOOK OF PHYSIOLOGY

Aellum is conjectural. Their ever-present influence is shown by the effect
7which follow their removal, as the following facts indicate.

The prevention of the development of tactile impulses by freezing q
anesthetizing the soles of the feet, and the blocking of normally develops
impulses through destruction of afferent pathways in diseases of the spina
cord lead at once to make impairment in the coordinating power. Th
removal of the skin from the hind legs of the frog, previously deprived of it
cerebrum, destroys its coordinating power, which it would otherwise posses
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 infoni
us of the activity and the degree of activity of our muscles, the location o
limbs, ^ the amount of effort necessary to produce a given movement, etc,
also gives rise to much incoordination. A blocking of both tactile am
muscle impulses frequently exists in degeneration or sclerosis of the posterio
columns of the spinal cord. The coordinating power is so much impairs
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 removet
by closure of the lids. Walking becomes extremely difficult; the gait i
irregular and jerky, and equilibrium is maintained only by keeping th<
eyes fixed on the ground in front and by artificially increasing the basis o
support by the use of canes.

An interference with the development of the customary visual impres
sions which in a measure maintain the sense of relation of the individua
to surrounding objects also gives rise to equilibratory disturbances, j?
rapid change in the relation of the individual to surrounding objects or th«
reverse; a change in the direction of one optic axis from the use of a prisn
or from paralysis of an eye muscle; the destruction of an eye— these an<
similar conditions frequently give rise to such marked disturbances of th<
equilibratory power that displacement is difficult to prevent.

An interference with the development of the so-called labyrinthine im
pressions by destruction of the semicircular canals gives rise to the mos
remarkable disturbances in this respect. Section of one horizontal canal
in the pigeon is followed by oscillations of the head in a horizontal plant
around a vertical axis. Bilateral section so increases these oscillations tha
the pigeon is unable to maintain equilibrium and forced to fall and turn con
tinuously around the vertical axis. Bilateral section of the posterior vertica
canals gives rise to oscillations around a horizontal axis which frequently be
come so exaggerated as to eventuate in the turning of backward somersaults
head over heels. Similar phenomena follow division of the superior vertica
canals.

Bilateral destruction of both sets of canals is attended by extraordinary
disturbances in the equilibrium. From the moment of the operation th<
animal, the pigeon, loses all control of its motor mechanisms. It can neithe;
maintain a fixed attitude nor execute orderly movements of progression; it;
activity, continuous and uncontrollable, is characterized by spinning arounc
a vertical axis, turning somersaults, dashing itself against surrounding object!
until life is endangered. If the animal be protected from injury, these dis

1 The physiologic anatomy of the semicircular canals is described in the chapter devotee
to the ear, to which the reader is referred

THE CEREBELLUM 609

irbances gradually subside, and in the course of a few months the equilibra-
)ry power is so far regained that standing and walking at least become
ossible. In this condition, however, the coordinating power is directly de-
endent on visual impulses, for with the closure of the eyes all the previous
lotor disturbances at once recur. These and similar facts indicate that the
gmicircular canals are the peripheral sense-organs from which come the
erve impulses most essential to the excitation of the cerebellar coordina-
te centers in their control of equilibrium and of progression.

The cerebellum may therefore be regarded as the essential, most highly
ifferentiated portion of the coordinating mechanism concerned in the main-
enance of equilibrium, during both station and progression. The manner

which the cerebellum accomplishes this result is unknown, though it is
ertain, from the foregoing facts, that its special mode of activity is dependent
to the excitatory action of nerve impulses transmitted from a variety of
Peripheral sense-organs.

CHAPTER XXV
THE ENCEPHALIC OR CRANIAL NERVES

The nerve-trunks which serve as channels of communication between th
encephalon and the structures of the head, the face, and in part the organ
of the thorax and abdomen, pass through foramina in the walls of the cranium
and for this reason are termed cranial nerves.

According to the classification now generally adopted, there are twelvi
cranial nerves on either side of the median line, which, enumerated iron
before backward, are as follows (Fig. 260) :

The First or Olfactory. The Seventh or Facial.

The Second or Optic. The Eighth or Acoustic.

The Third or Oculo-motor. The Ninth or Glosso-pharyngeal.

™6 l??l or Trochlear. The Tenth or Pneumogastric or Vagus.

The Fifth or Trigemmal. The Eleventh or Spinal Accessory.

The Sixth or Abducent. The Twelfth or Hypoglossal.

The cranial nerves may be classified physiologically in accordance with thei]
functional manifestations into three groups, viz. :

i. The Nerves of Special Sense: e.g., Olfactory, Optic, Acoustic, Gustatory (Glosso-phar
yngeal and Chorda tympani).

2' and pS^M °f Ge-neral Sensibmtv: e-8; Large root of the Trigeminal, Glosso-pharyngeal

3. The Nerves of Motion: *.#., Oculo-motor, Trochlear, the small root of the Trigeminal
Abducent, Facial, Spinal Accessory, and Hypoglossal.

Though this classification in the main holds true, it must be borne in mine
that modern investigations have demonstrated that the glosso-pharyngea
and pneumogastric nerves contain even at their junction with the medulla
oblongata a number of efferent or motor fibers, and to this extent are mixec
nerves.

The Origins of the Cranial Nerves.— In accordance with moderr
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 jugulai
ganglia. From the cells of these ganglia dendrites pass peripherally tc
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
ceH there emerges a short axon process which soon divides into a central and a
peripheral branch. The former passes toward and into the gray matter
located beneath the floor of the fourth ventricle, where its end-tufts arborize

610

THE ENCEPHALIC OR CRANIAL NERVES

611

bout nerve-cells. The latter (the peripheral branch) passes toward the
eneral periphery to be distributed to skin and mucous membranes.

The nerves of motion have their origin in the nerve-cells in the gray matter
•eneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle.
?he axons emerging from these cells course peripherally to be distributed

0 skeletal muscles. In some of the motor nerves, and in some sensor
lerves as well, there are to be found efferent fibers of smaller size which
'lave a similar origin and which become related through the intervention
>f sympathetic ganglia (peripheral neurons) with visceral and vascular
nuscles 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 of special sense for the
inost part terminate in primary basal
ganglia, around the cells of which their
:entral end-tufts arborize. From these
:ells axons arise which pass upward and
directly or indirectly come into physio-
ogic relation with sensor nerve-cells in
;he cerebral cortex.

The nerves of general sensibility ter-
minate in the gray matter beneath the
floor of the fourth ventricle, around the
nerve-cells of which their end-tufts arbor-
ize. 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 cere-
brum, where they also come into physio-
logic relation with sensor nerve-cells.

The axons in both of these classes of
Inerves thus originate in the cells of the
central nerve system and continue up-
Iward to the cerebrum, the primary affer-
ent path.

1 The motor nerves which have their
origin in the cells of the gray matter

FIG. 260. — SUPERFICIAL ORIGIN OF
THE CRANIAL NERVES FROM THE BASE
OF THE ENCEPHALON. i. Olfactory. 2.
Optic. 3. Motor oculi. 4. Trochlear.
5. Trigeminal. 6. Abducent 7. Facial.
f. Nerve of Wrisberg. 8. Acoustic. 9.
Glosso-pharyngeal. 10. Pneumogastric.
IT. Spinal accessory. 12. Hypoglossal. —
(Moral and Doyon.)

beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle
are in physiologic relation with nerve-cells in the 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 I
ginning in the cerebral cortex is thus continued by the motor nerves

geThe1threePgroups of nerves, those of special sense, of general sensibility,
, and the motor nerves, are neurons of the first order; the nerve^cell
1 fibers which constitute the cerebral connections are neurons of the <

order. It is probable that the sensor cells in the cerebral cortex are neuro

of a third order.

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TEXT-BOOK OF PHYSIOLOGY

THE FIRST NERVE. THE OLFACTORY

The first cranial nerve, the olfactory, is situated in the upper third of the
nasal fossa, in the r egio olfactoria. It consists of from 20 to 30 branches, the
fibers of which are non-medullated.

Origin. — The olfactory nerve is composed of centrally coursing 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 be-
come related by their end-tufts with
structures in the gray matter of
the olfactory bulb (Fig. 261).

Cortical Connections. — The
olfactory bulb and olfactory tract,
formerly called the olfactory nerve,
are portions of the cerebrum (the
olfactory lobe) which arise em-
bryologically by a protrusion oi
the walls of the cerebral cavity,
The bulb is oval-shaped and con-
sists of both gray and white matter,
It rests on the cribriform plate oi
the ethmoid bone and is embraced
by the olfactory nerves. As seen
on sagittal section, there is just be-
neath the surface a layer of large
pyramidal and spindle-shaped
cells (termed also mitral cells),
each provided with an apical and
two lateral dendrites. The apical dendrite passes toward the surface and
ends in a brush- or basket-like expansion which interlaces with the end-
tufts of the olfactory nerves, forming what are known as the olfactory glom-
erules. 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 back-
ward 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 lateral root-fibers pass outward into the fossa oi
Sylvius and come into relation with nerve-cells in the inferior extremity oi

FIG. 261. — THE RELATION OF THE OLFACTORY
NERVES TO THE OLFACTORY 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. — (Moral and- Doyon.}

THE ENCEPHALIC OR CRANIAL NERVES

613

the gyrus hippocampus and the gyrus uncinatus. 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. 262).

Histologic and embryologic methods of research have shown that some
of the fibers in the olfactory tract are centrifugal in function. They originate
in the olfactory cortical areas, pass toward the periphery 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 with the
bulb of the opposite side, and
carries impulses from the cor-
tex to the bulb. The two op-
posite cerebral olfactory 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 olfac-
tory end-organ to the action
of many substances is remark-
able, responding, for example, to the y TUTTO o"o °f a gram of oil of roses and to
the 2y6iro tro °f 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. Destructive
lesions of the hippocampal and uncinate gyri are followed by similar results.

ca.

T.o.

FIG. 262. — 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. P.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.) — AJter Quain.)

THE SECOND NERVE. 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 bas« of the cerebrum.

Origin. — The axons which constitute the optic nerve have their origin
in the ganglion cells in the anterior part of the retina. Through their
dendrites these cells are brought into relation posteriorly with successive

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TEXT-BOOK OF PHYSIOLOGY

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. 263) :

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 cho-
rioid 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.1

The Decussation of the Optic Nerves.— The extent
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 temporal two-
thirds of the retina of the right eye. In a similar man-
ner the fibers 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 the retina of the left
eye (Fig. 264). 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. Tran-
section of the optic nerve shows that it is composed of an enormous number
of non-medullated nerve-fibers, estimated by Salzer at from 450,000 to
800,000, enclosed in a sheath of the dura mater.

The visual fibers comprising the optic nerve may be physiologically
divided into two clases, .(a) those coming from the peripheral portion of the
retina, and (b) those coming from that central area 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 ex-
amination of a cross-section of the optic nerve shows the presence of a wedge-
shaped tract occupying the center of the nerve which is regarded as

1 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 is composed
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, there being no anatomic
or physiologic distinction between the optic nerve so called and the optic tract. Both are out-
growths from the brain and hence possess properties which differentiate them from other cranial
nerves.

FIG. 263. — RETI-
NAL CELLS, s', 2'.
Visual cells with
their peripheral ter-
minations, s. Rods.
z. Cones, b. Bi-
polar cells, g. Gan-
glion cells from which
arise the axons of the
optic nerve.

VISUAL IJ£LD

VISUAL HEW

THE ENCEPHALIC OR CRANIAL NERVES

composed of the macular fibers. At the chiasm this bundle of fibers un-
dergoes a partial decussation 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) . With the excep-
tion of the fibers passing to the ante-
rior quadrigeminal body, 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). These fibers are
not a part of the optic nerve proper,
but commissural fibers associating
the internal geniculate bodies of the
two sides. (Gudderfs Commissure.}

Cortical Connections. — After
entering the pulvinar and the lateral
or external geniculate body the visual
fibers terminate in end-tufts which LESION^OF'THE'IRIGHT OP£C~TR^CT OR^THE
arborize around nerve-cells. From RIGHT CUNEUS. THE SHADED LINES IN THE
these cells new axons arise which VISUAL FIELDS INDICATE THE DARKENED AREA.
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 mesial 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 superior quadrigeminal body.

Function. — The function of the optic nerve and tract 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 physio-

616 TEXT-BOOK OF PHYSIOLOGY

logic 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 sensation.

Owing to the partial decussation of the optic nerve-fibers at the chiasma,
the impressions made on the temporal side of the retina are received by the
cuneus of the same side, while the impressions made on the nasal side are re-
ceived by the cuneus of the opposite side. Each eye is, therefore, associated
with the cuneus of each cerebral hemisphere.

Pupillary Fibers. — The optic nerve also contains nerve-fibers some-
what larger in caliber than the usual visual fibers, which are sup-
posed to form the afferent path for those nerve impulses which excite
reflexly a contraction of the sphincter pupilla muscle, thus varying 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 find-
ings. 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 pupil-
lary 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. 268, page 620). 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 followed 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 obscuration or total obliteration of nearly one-half of the
visual field,1 to which the term hemianopsia is given. If, for example, the
right optic tract 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 hemianopsia (Fig.
266). 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. 2

1 The visual field comprises that portion of the external world from which, with the eyes
stationary, rays of light pass to the retinae and is the area included between the extremes of
the visual lines entering the pupil. The center of the visual field is the area the rays of light from
which are focalized on the fovea centralis. The visual field is somewhat irregular in outline by
reason of the position of the eyeball in the orbit cavity, and the consequent interference with
the entrance of light by the bridge of the nose, the cheek bones, and the eyebrows. The hori-
zontal diameter of the visual field for the right eye is about 150°, of which 90° pertain to the
temporal and 60° to the nasal portion. The vertical diameter is about 115°, of which 45° pertain
to the superior and 70° to the inferior portion. By reason of the position of the eyes in the orbit
cavity the two visual fields, viz.: that of the right and of the left eye, overlap to a variable extent in
their nasal divisions.

2 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.

THE ENCEPHALIC OR CRANIAL NERVES

617

The existence of a 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 apparent on reference to Fig. 265. 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 invisible 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.

THE THIRD NERVE. THE
OCULO-MOTOR

The third cranial nerve, the oculo-
motor, consists of some 15,000 pe-
ripherally coursing nerve-fibers which
serve to bring the nerve-cells from
which they arise into relation with a
large portion of the general muscula-
ture of the eye.

Origin. — The axons composing
the third nerve arise from a series 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. 266).

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 investigation of the results of pathologic processes
have not only shown that this is the case, but also succeeded in locating the
position of the nucleus for any given muscle. Though there is some differ-
ence 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 follow-
ing order:

1. The sphincter pupillae.

2. The tensor chorioideae (the accommodation nucleus).

FIG. 265. — DIAGRAM TO SHOW THE EXIST-
ENCE or HEMIANOPSIA. The lesion is sup-
posed to be in the right optic tract.

6i8

TEXT-BOOK OF PHYSIOLOGY

3. The convergence nucleus, a common nucleus for the conjoint action of

the two internal recti muscles.

4. The superior rectus.

5. The inferior rectus.

6. The levator palpebrae.

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 inter-
nal 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.

Distribution. — After their origin the
axons converge to form a common trunk,
which emerges from the base of the enceph-
alon, on the inner side of the crus 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 palpebra muscles; the lattei
is distributed to the internal and inferior
recti and inferior oblique muscle (Fig. 267),
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 constituting 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 oi
the pupil. The ciliary nerves are not por-
tions of the third nerve proper, but periph-
eral sympathetic neurons. As the ciliary
ganglion receives filaments from the caver-
nous plexus of the sympathetic and fila-
ments 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 encepha-
lon is followed by contraction of the muscles to which it is distributed
with the following results, viz. :
i. Diminution in the size of the pupil.

FIG. 266. — 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 &), 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.)

THE ENCEPHALIC OR CRANIAL NERVES

619

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 compression
from a pathologic cause is followed by a relaxation of the muscles, with the
following effects, viz.:

Dilatation of the pupil, the iris
responding neither to light nor
to efforts of accommodation.

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 torsion toward the mesial
line due to the unopposed
action of external- rectus and
the superior oblique muscles.

5. Double vision or diplopia. The

image of the eye of the para- Sensory
i . i . . , , ' **•«•*•

lyzed 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 im-
pulses from the nuclei of origin to all the muscles of the eye except the ex-
ternal 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 trans-
mission of nerve impulses by the constituent fibers of the nerve from their
related nuclei. This is made evident by the effects which follow stimula-
tion and division of the nerve or lesions of the nuclei themselves.

The central nuclei can be excited to activity (i) by nerve impulses de-
scending the motor tract, from the cerebral cortex, (2) by nerve impulses
coming through various afferent nerves. This holds true more especially
for the sphincter pupillae 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 inten-
sity 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 con-
traction of the radiating fibers of the iris, which collectively constitute the
dilatator pupillce muscle. With the entrance of light into the eye, the pupil

FIG. 267. — INTRA-ORBITAL PORTION OF THE
THIRD NERVE, i. Optic nerve. 2. Third
nerve. 3. Superior branch. 4. Injerior 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. 1 1 .
fibers. 12. Sympathetic fibers. 13.
Ciliary nerves. — (Sappey.)

620

TEXT-BOOK OF PHYSIOLOGY

diminishes in size, in consequence of the contraction of the sphincter pupilla
caused by a stimulation of the peripheral ends of the pupillary fibers of the
retina, the degree of contraction 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 pupil-
lary fibers in the optic
nerve; an emissive center,
the sphincter nucleus of
the motor oculi center; effer-
ent nerves, including fibers
in the trunk of the motor
oculi and in the ciliary
nerves; and a responsive
organ, the muscle. (See
Fig. 268). That this is the
mechanism involved in this
reflex, is shown by the fact
that when any portion
of it is destroyed, the
reflex contractions of the
sphincter are impaired or
abolished.

As stated in a preceding
paragraph the central ter-
mination of the afferent
pupillary fibers concerned
in this reflex is not posi-
tively known. No one has
as yet succeeded in tracing
these fibers directly to the
sphincter nucleus. It has
been shown, however, that
as the optic tract ap-
proaches its termination
the visual and the pupil-
lary fibers separate and it
FIG. 268. — DIAGRAM DESIGNED TO SHOW THE MEGHAN- has been assumed that the
ISM OF THE IRIS REFLEX The central termination of the j ^ ^to anatomic

pupillary fibers is hypothetical. . J

relation with some inter-
calated 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 evidence for the view that this intercalated system has its
origin in the anterior corpora quadrigemina as shown in the accompanying
diagram.

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 contrac-
tion 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

Sup. Cervical Ganglion,
fibers^

•JF^Thoracic Newt

Transection of Spinal Cord

THE ENCEPHALIC OR CRANIAL NERVES 621

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 reflex contractions of
the sphincter muscles are consensual, that is, bilateral reflex actions because
of the decussation of the pupillary fibers at the chiasm. Contraction 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 Jmpillae muscle, but partly to the contraction of
the dilatator pupillae muscle, which -is kept normally in a state of tonic con-
traction 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. 268), 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 the 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 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 617 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 en-
tirely 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 toward 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. 268 in which the course of the pupillary fibers
is shown and especially if it be accepted that these fibers at their central

622 TEXT-BOOK OF PHYSIOLOGY

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 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 toward 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 pupillary response manifests itself. The course
of the pupillary fibers in this instance will also become apparent from an
examination of Fig. 268. 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.

THE FOURTH NERVE. THE TROCHLEAR

The fourth cranial nerve, the trochlear, consists of peripherally coursing
axons which serve to bring the cells from which they arise into relation with
the superior oblique muscle. t

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. After emerging from the nucleus the nerve-fibers pass down-
ward for a short distance, then curve dorsally 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 trochlear nerve is in his-
tologic and physiologic connection with the motor area of the cerebral cor-
tex. Nerve-cells in this region give off axons which enter the pyramidal
tract and descend through the internal capsule and the cms 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 be-
low the posterior quadrigeminal body, crosses the superior cerebellar pe-
duncle, and winds around the cms cerebri to the anterior border pf the pons
Varolii. It then enters the orbit cavity through the sphenoid fissure and
finally terminates in the superior oblique muscle. In its course the nerve
receives filaments from the cavernous plexus of the sympathetic and the
ophthalmic division of the trigeminal.

Properties. — Stimulation of. the nerve- trunk is followed by spasmodic
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 action of the inferioi
oblique muscle, the anterior pole of the eyeball is turned upward and in-
ward 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 inclined toward that of the normal eye.

THE ENCEPHALIC OR CRANIAL NERVES

623

Function. — The function of the trochlear nerve is to transmit nerve
impulses to the superior oblique muscle and to excite it to contraction.

THE FIFTH NERVE. THE TRIGEMDSTAL

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 Affer-
ent 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

n Jf^. ., N FIG. 269. — SCHEME OF ORIGIN AND CONSTITUTION OF THE

penpnerally (rig. ^ 209). TRIGEMINAL NERVE, i. Centrally coursing fibers. 2, 3,
The Centrally directed 4- Peripherally coursing fibers of the cells of the ganglion of
branches rollertivplv fnrm Gasser- R> N- Nuclei of origin of the efferent fibers. 6.

lively lorm Motor root Central terminations of the large root
the so-called large or sen-
sor root; the peripherally directed branches collectively constitute the three
main divisions of the nerve: viz., the ophthalmic, the superior maxillary,
and the inferior maxillary. Branches of the carotid plexus of the sym-
pathetic enter the nerve in the neighborhood 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 sub-
stantia gelatinosa.

2. The Peripheral Branches. — The peripheral axons emerge from the
peripheral end of the ganglion of Gasser in three distinct and separate
branches, each of which is distributed to a different 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
forehead and the nose, the lachrymal gland and caruncle, and the
mucous membrane of the nose.

624 TEXT-BOOK OF PHYSIOLOGY

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.

3. The inferior maxillary branch passes through the foramen ovale, after

which it subdivides into three branches — the auriculo- temporal, 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 alveolar process of the lower jaw and
the integument of the lower part of the face.

The afferent axons thus serve to bring into relation the skin, mucous
membranes of the head and face, and other sentient structures, 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 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 pathologic 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. Exposure 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
are distributed. The skin and mucous membranes, the eye, nose, or teeth
may be experimentally injured without any evidences of pain on the part oi
the animal. Various reflexes, e.g., those of mastication, insalivation, degluti-
tion, the afferent paths of which are formed in part by the fifth nerve, are
often seriously impaired. At the same time the lachrymal secretion dimin-
ishes and the pupil contracts. The same results are observed in human
beings in whom the nerve has been divided for relief from severe 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 infre-
quently 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; and uteeration sets in, which may lead to com-
plete destruction of the eyeball. The mucous membrane of the nose be-

THE ENCEPHALIC OR CRANIAL NERVES 625

comes swollen, vascular, and liable to hemorrhage on the slightest irritation.
The degenerative changes may lead to a complete loss 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 condition and by the
changed vascular supply from division of the vasomotor 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 quadngem-
ina, gives origin to axons which descend and join the axons from the chief
motor nucleus (Fig. 269).

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 th<
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 antenor portion
of the digastric. A few axons are also distributed to the tensor tympam 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 mu

°f SticarConnections.--^ 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 internal «Pg^*J^"
cerebri, after which they cross to the opposite side. Their end-tufts arbor-
ize around the cells of nuclei in the medulla oblongata. ^

Properties.-5/^/a/^ of the small root gives rise to convulsive move-
ments oJ the muscles of mastication. Division of the "^^^
a Daralvsis of these muscles. Contraction or paralysis of the tensor tympam
^d "S pl£ti muscles would also be ^^^fffffSSi

Functions.— The function of the afferent fibers of the tit re is

the transmission of nerve impulses from its P«^"«t±S3KS3
the medulla oblongata; (b) through its afferent cortical tracts to the cereb ral
cortex where they evoke sensations. The nerve therefore endows all

^e1 flSon St^-^S^snussio, of nerve impulses
centrally with the nuclei of origin of the efferent ^^^™JJg

"Terijhlfstoulation of different areas in the distribution of the
afferent fibers eT, conjunctiva, nasal and oral mucous membranes teeth,
efc causes a Vafie'ty of reflex activities in the muscles associated with the

626 TEXT-BOOK OF PHYSIOLOGY

eyes, face, the respiratory and cardiac mechanisms, which indicate that the
afferent fibers are centrally in relation with a number of motor nerve-
centers.

THE SIXTH NERVE. THE ABDUCENT

The sixth cranial nerve, the abducent, consists of peripherally coursing
axons which serve to bring the nerve-cells from which they arise into rela-
tion 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 prob-
able 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 med-
ulla oblongata and the pons Varolii just external 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. In its course the nerve
receives 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 cms cerebri, after which
they cross to the opposite side, where their end-tufts arborize around the
cells of the nucleus already described.

Properties. — Stimulation of the nerve is followed by spasmodic con-
traction of the external rectus muscle and external deviation of the eyeball.
Division of the nerve is followed by paralysis or relaxation 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 impulses to
the external rectus muscle and excite it to contraction.

THE SEVENTH NERVE. 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 supercilli, orbicularis
palpebrarum, levator labii superioris alaeque nasi, zygomatici, the pyramidalis
nasi, compressor nasi, depressor alae nasi, levator anguli oris, buccinator,
orbicularis oris, depressor anguli oris, depressor labii inferioris, levator
menti, posterior belly of the digastric, stylohyoid, and platysma myoides.
These muscles by their individual and cooperative contraction express ideas
and feelings and are therefore termed muscles of expression.

Origin. — The nerve-fibers or axons composing the seventh nerve arise
for the most part from a nucleus of large multipolar nerve-cells situated

THE ENCEPHALIC OR CRANIAL NERVES 627

? about five millimeters beneath the upper half of the floor of the fourth ven-
tricle 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 ependyma of the ven-
tricle; they then turn on themselves, forming an arch that encloses the nu-
' cleus of the sixth nerve; they then course downward 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 ttie 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 corrugator
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 capsule, and the crus 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 bone above and be-
tween the cochlea and vestibule, the nerve makes a sharp bend, forming the
genu facialis, turns backward and enters the aqueduct of Fallopius, the gen-
eral 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 temporo-facial and the cervico-facial, the ultimate branches
of which are distributed as previously stated to the superficial muscles of the
head and face.

Properties. — Electric stimulation of the trunk of the nerve after its emer-
gence 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. — situated
along the course of the nerve or at its nuclear origin, or in the cortical or
sub-cortical regions, frequently give rise to spasmodic movements of the
facial muscles on one side, 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 of the corresponding side. 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 paraly-
sis, are as follows: A relaxed and immobile condition of the side of the

628 TEXT-BOOK OF PHYSIOLOGY.

face corresponding to the lesion; separation of the eyelids from paralysis of
the orbicularis palpebrarum and the unopposed contraction of the levator
palpebrse muscle; abolition of the ability to wink; 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;
articulation is impaired from paralysis of the orbicularis oris muscle, the
labial sounds especially being imperfectly produced.

Functions. — The function of the facial nerve is the transmission of
nerve impulses from the nerve-cells in which it arises to the superficial
muscles of the face, those more especially concerned in the expression of the
feelings. By reason of the association of the cortical facial area and the
nucleus of origin of the facial nerve the latter becomes the medium of
communication between the cortical area <md the facial muscles and serves
for the transmission to the muscles of those nerve impulses developed by
and associated with psychic states. The muscles thus excited to action
individually and collectively express in a general way the character of
the psychic state. For this reason the facial nerve is termed the nerve of
expression.

Branches of the Facial Nerve ; Their Origin, Properties and Func-
tions.— The branches usually described in works on anatomy as coming
from the facial nerve are as a matter of fact, not branches of the facial nerve
proper (with one exception, viz. : the tympanic) but are branches of autonomic
nerves (vaso-motor and secretor) which after leaving the central nerve sys-
tem enter the sheath of the facial. After a short distance, however, they
leave it to be distributed around sympathetic ganglia. In one of these
branches (the chorda tympani) afferent nerve-fibers are present which minis-
ter to the sense of taste. These fibers very naturally cannot be regarded as
integral parts of the facial proper.

Between the facial and the acoustic nerve there is a small nerve known
as the pars intermedia, the nervus iritermedius or the nerve of Wrisberg. The
true nature of this nerve has long been a subject of investigation. The
results of histologic investigation and physiologic experimentation would
indicate that it is composed of both efferent and afferent fibers. The efferent
fibers which constitute in part the nerve of Wrisberg have their origin in a
group of cells situated beneath the floor of the fourth ventricle near the
median line between the nucleus of the facial and the nucleus of the motor
root of the trigeminal nerve and known as the nucleus salivatorius. The
afferent fibers arise from nerve-cells composing in large part the ganglionic
enlargement found on the genu of the facial nerve at the point where it turns
backward to enter the aqueduct of Fallopius. The cells of this geniculate
ganglion, originally bipolar present single axons which soon divide into
centrally and peripherally coursing branches. The centrally coursing
branches constitute in part the nerve of Wrisberg, which entering and pass-
ing through the pons terminates directly or indirectly around the sensor
end-nucleus of the glosso-pharyngeal nerve. The peripherally coursing
branches enter the sheath of the facial nerve and accompany it as far
as a point about 5 millimeters above the stylo-mastoid foramen.

From its mode of origin, the nerve of Wrisberg cannot be regarded as

THE ENCEPHALIC OR CRANIAL NERVES 629

an integral part of the facial nerve proper, but must be considered as an
independent nerve composed of both afferent and efferent fibers.

At the beginning and in the course of the aqueduct of Fallopius the
facial trunk gives off the following branches: the large superficial petrosal, the
small superficial petrosal, the stapedius and the chorda tympani (Fig. 270).
i. The large superficial petrosal nerve is given off near the geniculate gan-
glion. It then passes forward into the spheno-maxillary fossa and be-
comes associated with the spheno-palatine or Meckel's ganglion. In its
course it receives a filament known as the deep petrosal, from the car-
otid plexus of the sympathetic. The nerve-trunk formed by the union of
these two nerves is known as the
Vidian nerve and terminates as
stated above. The character and
function of the large petrosal
nerve have been a subject of
much discussion. As the out-
come of modern methods of in-
' vestigation it may be concluded
that it is composed mainly, if
not entirely, of fine medullated
nerve-fibers which are the con-
tinuations of corresponding fi-
bers in the nerve of Wrisberg
and that their destination is
the spheno-palatine ganglion, FIG. 270.— CHORDA TYMPANI NERVE, i, 2,
around the nerve-Cells of 3> 4- Facial nerve passing through the aqueductus
whirh their term in a 1 hranrhp*; Fallopii. 5. Ganglioform enlargement. 6.

5 Great petrosal nerve. 7. Spheno-palatine gan-
arbonze. glion. 8. Small' petrosal nerve. 9. Chorda tym-

Stimulation of the large petrosal, Pa™- I0> IJ> I2> J3- Various branches of the

.,1 . j j ! , . facial. 14. 14, 15. Glosso-pharyngeal nerve. —

with induced electric currents, (Hirsckfeld.)
gives rise to a dilatation of the

blood-vessels of, and a secretion from the mucous membrane, of the
nose, soft palate, upper part of the pharynx, roof of the mouth, gums,
and upper lip — the regions of distribution of the post-ganglionic fibers
of cells of the spheno-palatine ganglion (see page 647). The nerve
therefore contains both vaso-dilatator and secretor fibers which belong
to the autonomic system of nerves. As after the administration of
nicotine stimulation of this nerve is without effect, and as stimula-
tion of the spheno-palatine ganglion gives rise to the usual vaso-dila-
tator and secretor effects it may be inferred that the ganglion is the
way station between the pre-ganglionic fibers and the blood-vessels and
glands. The deep petrosal, which joins the large petrosal is in all
probability a vaso-constrictor nerve coming from the superior cervical
ganglion of the sympathetic. There is no evidence that the large pe-
trosal contains any fibers from the facial proper for the innervation of:
any striated muscle of the palate.

2. The small superficial petrosal nerve is given off from the facial at a
point somewhat external to the large petrosal nerve. In its course
it is joined by a small filament derived from Jacobson's branch of the
glosso-pharyngeal. Together they pass into the otic ganglion, where

630 TEXT-BOOK OF PHYSIOLOGY

the fibers arborize around the nerve-cells composing it. Experiments
are lacking as to the function of the small petrosal. The small size of
its nerve-fibers and their termination would lead to the conjecture
that they are probably vaso-dilatator and secretor. Stimulation of Jacob-
son's nerve gives rise to a dilatation of the blood-vessels of, and secretion
from, the mucous membrane of the cheek, lips, and gums and of the
parotid and orbit glands, the regions of distribution of the post-ganglionic
fibers of the otic ganglion. This nerve therefore contains both vaso-
dilatator and secretor fibers (see pages 155, 634.).

3. The tympanic nerve or stapedius nerve is distributed directly to the

stapedius muscle, and as this muscle is of the striated or skeletal
variety it is innervated by the facial proper.

4. The chorda tympani nerve is given off from the facial at a point about

5 millimeters above the stylo-mastoid foramen. It then passes upward
and forward and enters the tympanum through the iter chords 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 nerve. Some of its fibers can
be traced to the mucous membrane of the dorsum of the tongue, others
to the submaxillary and sublingual ganglia with which they become
associated.

The^ determination of the origin, course, and functions of the chorda
tympani nerve has given rise to many investigations and discussions, and it
cannot be said that the results thus far attained are as satisfactory as might
be desired.

If the chorda tympani nerve is divided there follows a contraction of
the blood-vessels in the neighborhood of and a diminution in the secretion
from the submaxillary and sublingual glands. Stimulation of the peripheral
end of the divided nerve gives rise to a dilatation of the blood-vessels and an
increased production and discharge of saliva from these glands (see page
153). From these results it is certain that the chorda tympani contains both
vaso-dilatator and secretor 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 stimulated. 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 vaso-dilatator and
secretor fibers in the chorda tympani are the continuations of the efferent
fibers found in the pars intermedia and that they too have their origin in the
nucleus salivatorius.

^ If the nerve be divided as it crosses the tympanic cavity or before it unites
with the lingual branch of the fifth nerve, there follows a loss of taste in the
anterior two-thirds of the tongue on the corresponding side, though the sensi-
bility remains unimpaired. For this and other reasons, the chorda tympani
has long been regarded as the nerve of taste for this region. The nerve-fibers
subserving the sense of taste are believed to be the peripherally coursing
fibers which have their origin in the nerve-cells of the geniculate ganglion
and which descending in the aqueduct of Fallopius are continued as the
chorda tympani. The nerve impulses developed in the peripheral termina-
tions of this nerve by the action of organic matter in solution are transmitted

THE ENCEPHALIC OR CRANIAL NERVES 631

through the chorda tympani, along the facial nerve as far as the geniculate
ganglion. 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 evi-
dence seems to favor the view that the gustatory fibers have their origin in the
geniculate 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.

THE EIGHTH NERVE. THE ACOUSTIC

The eighth cranial nerve, the acoustic, 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 equilibratory.

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. 271). From this origin they
pass centrally into the central canal of the modiolus, at the base of which
they emerge in well-defined bundles and enter the internal auditory meatus.
Dendritic processes from these cells pass peripherally to terminate on the
ciliated epithelial cells of the organ of Corti.

The axons comprising the vestibular 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
portion. Dendritic processes from these cells pass peripherally into the
internal ear, where they terminate in the epithelial cells situated 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 tfie lateral acous-
tic nucleus, around the cells of which its end-tufts arborize. The vestib-
ular 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-internal or chief auditory nucleus and
the dorso-external or Deiters' nucleus. Some of the fibers of the vestibu-
lar branch descend through the pons and medulla as far as, the cuneat(

Cortical Connections.— From the nerve-cells of the ventral and lateral
acoustic nuclei axons arise, some of which cross the median plane to enter

632

TEXT-BOOK OF PHYSIOLOGY

the lateral lemniscus for fillet of the opposite side, while others enter the
lemniscus of the same side. The lemniscus then passes upward to the in-
ternal geniculate body around the nerve-cells of which its fibers terminate.
From the geniculate body the acoustic radiation arises which then passes
upward and outward through the posterior limits of the internal capsule
to terminate finally around the cells of the super-temporal convolution.
From the nuclei around which the vestibular branch terminates, axons

arise which in all probability pursue a
somewhat similar course and terminate in
the temporal lobe.

Properties. — Stimulation of the coch-
1 ear nerve is unattended by either motor
or sensor phenomena. Division of the
nerve is followed by a loss of the sense
of hearing. Irritative pathologic lesions
give rise to sensations of sound of vary-
ing character and intensity. Degenera-
tion of the nerve or destruction by tumors,
etc., will also be followed by a loss of
the sense of hearing.

Experimental lesions of the semicir-
cular canals involving a destruction 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 by disturbances of equi-
librium, e.g., vertigo, or a sense of sway-
ing, 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 transmitted by the auditory tract
to the acoustic area in the cerebral cortex
where they evoke sensations of sound and
its different qualities, intensity, 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 ampullae of the semicircular
canals caused by inertia of the endolymph during changes in the position of
the head and body. The impulses carried by the vestibular nerve give rise

FIG.

OF THE. ACOUSTIC 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.
8. Vestibular nerve. 9. Dorso-external
nucleus (D eiters). 10. Dorso-internal
nucleus. — (After Moral and Doyon.}

THE ENCEPHALIC OR CRANIAL NERVES 633

reflexly to certain adaptive and protective movements by which the equi-
librium of the body -in both dynamic and static conditions is maintained.

THE NINTH NERVE. THE GLOSSO-PHARYNGEAL

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 by 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 cer-
tain 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 peripher-
ally. 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^iuclei. 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 stylo-pharyngeal 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.

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 mus-
culature of the fauces and pharynx. These nerve-cells are located in the
lateral portion of iheformatio reticularis at some distance below the floor of
the fourth ventricle. They constitute the upper portion of a collection of
the cells known as the nucleus ambiguus.

Distribution. — From this origin the efferent fibers pass dorsally to near
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-pharyngeus, and to the middle constrictor
muscle of the pharynx. In addition to the foregoing efferent fibers the
glossopharyngeal 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 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 connected with
the general motor area of the cortex through fibers descending in the

634 TEXT-BOOK OF PHYSIOLOGY

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.— Stimulation of the glosso-pharyngeal trunk with induced el-
ectric currents calls forth evidence of pain and contraction of the stylo-pharyn-
geus 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, develops^ nerve
impulses which transmitted to the cortex evoke sensations 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. These nerve-fibers are thus
members of the autonomic system of nerves.

Functions. — The afferent fibers of the glosso-pharyngeal transmit
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 afferent fibers are therefore divisable into nerves of
general sensibility and nerves of special sense. 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. The fibers excit-
ing secretion have their origin in the nucleus salivatorius •, from which the
efferent autonomic fibers in the chorda tympani nerve arise.

THE 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, in-
dependent of those derived in its course from adjoining 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 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 periph-
erally directed branches collectively form a portion of the common trunk
of the nerve.

Distribution. — The axon of the centrally directed branches after entering
the medulla pass toward its dorsal aspect, where they bifurcate, give collat-

THE ENCEPHALIC OR CRANIAL NERVES 635

erals, and terminate in fine end-tufts in the immediate neighborhood of two
groups of nerve-cells, the vagal sensor end^nudei.

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 in-
testine, and also to the heart. The afferent fibers thus serve to bring into
anatomic and physiologic relation the mucous membrane of these organs
and certain sensor enoVnuclei 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 muscles of the lower two-thirds of the
esophagus; to the muscle-fibers of the stomach and perhaps the intestines;
to the walls of the gall-bladder and to the sphincter of the common bile-duct;
and to the non-striated muscle-fibers of the bronchial tubes, and to the heart.
Among the efferent fibers are some which are distributed to the gastric
glands and to the pancreas. From this distribution it is apparent that the
efferent fibers in the vagus are largely if not entirely members of the auto-
nomic system of nerves.

The efferent fibers 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 as well as the heart and
gastric glands.

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

Vagal Branches. — As the vagus passes down the neck it gives off the
following main branches:

1. The pharyngeal nerves, which, after entering into the formation of 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; according to B eevor and Horsley the nerves
for these muscles are branches of the spinal accessory.

2. The esophageal nerves, which after entering into the formation of the

esophageal plexus, are distributed to the mucous membrane, and to the
muscles of the lower two-thirds of the esophagus.

3. The superior laryngeal nerve which, entering the larynx through the

thyro-hyoid membrane, is distributed to the mucous membrane 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
whirh in thp rabbit unite to form a single nerve, the so-called depressor

636 TEXT-BOOK OF PHYSIOLOGY

nerve. (See page 325.) It is distributed to the surface of the ventricle
and perhaps to structures at the root of the aorta. 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 con-
strictor of the pharynx. These motor fibers are derived from the spinal
accessory.

5. The cardiac nerves which, after entering into the formation of the cardiac

plexus, are distributed to the heart.

6. The pulmonic 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 membrane and

muscle walls of the stomach, intestines, and gall-bladder. 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 esophagus, stomach, and possibly
of the intestine and of the pulmonary apparatus, and at the same time causes
an inhibition of the heart. Division of the nerve is followed by a loss of
sensibility in the mucous membrane of the alimentary tract and of the pul-
monary apparatus, together with a loss of motility of the structures above
mentioned, and a loss of the inhibition .of the heart.

Stimulation of the trunk of the nerve in different parts of its course pro-
duces a variety of results dependent to some extent on the presence of anas-
tomosing 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 esopha-
gus. Division of these nerves is followed by a loss of sensibility in the parts
to which they are distributed, a partial paralysis of the muscle coat and an
impairment of deglutition.

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 contraction of the muscles
of deglutition, and of the muscles concerned in the act of coughing; inhibi-
tion of the inspiratory movement and arrest of respiration in the condition
of expiratory standstill, with perhaps a tetanic contraction of the expiratory
muscles, and contraction of the laryngeal muscles with closure of the glottis.

Peripheral 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 mem-
brane, paralysis of the crico-thyroid muscle with a consequent lowering of

THE ENCEPHALIC OR CRANIAL NERVES 637

the pitch, and a diminution in the clearness of the voice. In consequence
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 depres-
sor nerve is without effect; stimulation of the central end retards and even
arrests the heart's pulsations and lowers the general blood-pressure. These
two effects, though associated, are nevertheless independent of each other. If
the vagus nerves be divided on both sides between the origin of the depres-
sor 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-inhibitor 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 consequence of a depres-
sion of that portion of the general vaso-motor center which maintains through
the splanchnic nerves a tonic contraction of their walls. It has been satis-
factorily 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). Evi-
dently, 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 contraction of the abductor muscles (the
posterior crico-arytenoids) ; for phonatory purposes the glottis is closed and
the vocal membranes approximated by the contraction of the adductor mus-
cles. It has been shown that these opposed groups of muscles have inde-
pendent nerve-supplies; that two sets of fibers 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.
With an increase in the strength of the stimulus, however, the reverse obtains:
namely, adduction which increases until the glottis is completely 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 laryngeal 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 stimulation 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. Accord-
ing to the recent investigations of Schaternikoff and Friedenthal, they come
direct in the vagus, from a nucleus near the vagal motor nucleus in the med-
ulla, the spinal accessory sending no branches to the heart. In the frog

638 TEXT-BOOK OF PHYSIOLOGY

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 Pulmonic Nerves.— The pulmonic nerves, given off from the trunk
after its entrance into the thorax, do not lend themselves readily to ex-
perimentation. Division of both vagi in the neck above the point of exit
of the pulmonic 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 divided vagus with weak induced
electric currents increases the frequency, but decreases the amplitude, of the
respiratory movements. This would indicate that in the physiologic state
these nerve-fibers conduct afferent nerve impulses that inhibit the inspiratory
discharge and lead to an expiratory movement sooner than would otherwise
be the case. If the stimulation be increased in intensity the inspiratory
movement gradually so exceeds the expiratory that the inspiratory muscles
pass into the condition of tetanus and the chest walls come to rest in the
condition of forced inspiration.

Stimulation of the vagus with strong induced electric currents not infre-
quently inhibits the inspiratory movement and increases the expiratory until
there is a complete cessation of movement in the condition of expiratory
standstill. The effect thus produced is similar to, if not identical with, that
produced by stimulation of the superior laryngeal nerve.

Faradization of the trunks of the pulmonic branches or stimulation of
their peripheral terminations in the mucous membrane of the bronchial
tubes or alveoli by the inhalation of chemic vapors causes arrest of respira-
tory movements, a fall of blood-pressure, and a reflex inhibition of the heart
(Brodie).

The 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 sen-
sibility of the mucous membrane of the stomach, impairs motility, and inter-
feres 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 circulation, 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 contraction
of the muscle of the lower two-thirds of 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 thejnaintenance of certain organic

THE ENCEPHALIC OR CRANIAL NERVES

639

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 con-
sidered in foregoing pages.

THE ELEVENTH NERVE. THE SPINAL ACCESSORY

The eleventh cranial nerve, the spinal accessory, consists of peripherally

coursing fibers which bring the nerve-cells

from which they arise into relation with

separate but functionally related muscles,

such as those entering into the formation

of the larynx. It consists of two por-
tions, the medullary or bulbar and the

spinal.

Origin. — The axons comprising the

medullary portion arise from a group of

nerve-cells in the extreme lower part of

the nucleus ambiguus, known as the nidus

laryngei. 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 por-
tion 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 cer-
vical 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 mag-
num, 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. 272.)

Distribution. — After emerging from

the cranial cavity the nerve soon sepa^

rates into two branches:

i. An internal or anastomotic branch,
consisting chiefly of filaments com-
ing from the medulla oblongata. It
soons enters the trunk of the vagus,
from which fibers pass through the
pharyngeal plexus to the superior

and inferior constrictor muscles of the pharynx, to thepalato-pharyngeus,
to the levator palati and azygos uvulae muscles (Beevor and Horsley) ; to

Glosso-pharyngeal nerve. 3, 3. Pneumo-

gastric. 4,4,4- Trunk of the spinal acces-

sory. S. Sublingual nerve. 6. Superior

cervicai ganglion. 7, 7. Anastomosis of

the first two cervical nerves. 8. Carotid

14, 15. Branches of the facial. 16. Otic
ganglion. 17. Auricular branch of the
pneumogastric. 18. Anastomosing branch
from the spinal accessory to the pneu-
mogastric. 19. Anastomosis of the first

with the Sec0nd pair of cervical nerves. 21.
Pharyngeal plexus. 22. Superior laryn-

64o TEXT-BOOK OF PHYSIOLOGY

the muscles of the larynx through the inferior laryngeal nerve, and to the
heart according to some authorities.
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 it is distributed.
Destruction of the medullary root is followed by impairment of deglutition
from a paralysis of the muscles of the pharynx and palate 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 ac-
celeration 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 the cervical nerves. In consequence of division of the external
branch 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 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 regu-
late the tension of the vocal bands during phonation and the muscles which
control the respiratory movements associated with sustained or prolonged
muscle efforts. The fibers may also convey nerve impulses which exert
an inhibitor influence on the heart.

THE TWELFTH NERVE. 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 collec-
tion of nerve-cells situated beneath the floor of the fourth ventricle. This
nucleus is elongated and extends from the medullary striae downward as fax
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

THE ENCEPHALIC OR CRANIAL NERVES 641

derived from the cervical plexus pass to muscles which elevate and depress
the hyoid bone.

Cortical Connections. — The hypoglossal nerve nuclei are connected
with nerve-cells in the lower third of the general motor area around the in-
ferior termination of the fissure of Rolando by axons which descend in the
pyramidal tract.

Properties. — Faradization of the nerve gives rise to convulsive move-
ments of the muscles to which it is distributed. Division of the nerve is
followed by a loss of motion and an interference with deglutition, mastication,
and articulation, especially in the pronunciation of the consonantal sounds.
In hemiplegia, complicated with paralysis 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 unopposed action of the muscle of the opposite side.

Function. — The hypoglossal nerve transmits nerve impulses from its
origin to the intrinsic and extrinsic muscles of the tongue, exciting them to
activity. The coordinate activity of these muscles favorably assists mastica-
tion, articulation, and deglutition.

CHAPTER XXVI
THE AUTONOMIC NERVE SYSTEM

Introductory Statements. — The motor tissues of the body may be
divided into two groups, the one, comprising the striated skeletal muscles,
the other, comprising the striated cardiac, and the non- striated vascular and
visceral muscles, and the epithelium of glands. The movements to which
they give rise are, in the first instance, those exhibited mainly by the head,
upper and lower extremities; and in the second instance, those exhibited
by the heart and blood-vessels; by the walls of the viscera, e.g., stomach, in-
testines, bile and urinary bladders, etc., and those exhibited by glands (the
secretion and discharge of fluids), e.g., salivary, gastric, perspiratory, mam-
mary, etc., as well as the specific agents, e.g., adrenalin, pituitrin, etc., dis-
charged into the blood. The activities which these two groups of motor
tissues exhibit are excited mainly by two ultimate and separate factors.

The Skeletal Muscles. — The skeletal muscles in the different regions of
the body are excited to action by nerve impulses, which come to them directly
from special groups of emissive or motor cells situated in the gray matter
beneath the aqueduct of Sylvius, beneath the floor of the fourth ventricle,
and in the ventral cornua of the gray matter of the spinal cord. The nerve
impulses, discharged by these cells, are transmitted directly through but one
axon to the muscle. The nerve-cells in these different regions, though
irritable, do not possess spontaneity of action, but require for the manifesta-
tion of their activity the arrival and the stimulating influence of nerve im-
pulses. These may come (i) from peripheral regions of the body through
afferent spinal nerves in consequence of the action of external agents, in
which case the resulting movement is termed a reflex movement; or (2) from
the cerebrum through descending nerve-fibers in consequence of an act of
volition, in which case the resulting movement is termed a volitional or
voluntary movement. In the performance of daily work the skeletal muscle
activities, though frequently caused by reflected nerve impulses are, in the
vast majority of instances,, caused by nerve impulses of cerebral (volitional)
origin. From moment to moment these muscles are excited to action, guided
and controlled for the most part by this dominating and in some respect
arbitrary factor. By the constant exercise of the will, these muscles are
compelled to act constantly, and for this reason it is said that they do not
possess independent activity, except in a minor degree.

The Striated Cardiac, the Non-striated Vascular and Visceral
Muscles and the Epithelium of Glands. — The striated cardiac, the vas-
cular and visceral muscles, and the epithelium of glands in all regions of the
body, if not primarily excited to action, are at least controlled and regulated
in their activities, by nerve impulses that come to them indirectly from special
groups of emissive or motor nerve-cells situated in (i) a small region at the
extreme upper part of the gray matter beneath the aqueduct of Sylvius; (2)
in special regions in the gray matter beneath the floor of the fourth ventricle;

642

THE AUTONOMIC NERVE SYSTEM 643

(3) in the ventral horns of the gray matter of the spinal cord between and
including the levels of origin of the second thoracic to the second or third
lumbar nerves; and (4) in the gray matter of the lower portion of the cord
between and including the levels of origin of the second to the fourth sacral
nerves. The nerve impulses discharged by these cells are transmitted indi-
rectly, not through one, but through two successively arranged neurons. The
first, a fine white medullated fiber, emerges from the medulla or spinal cord,
in association with the large motor root fibers passing to the skeletal muscles,
and after a variable distance leaves these fibers to arborize around and to
become physiologically related to a sympathetic ganglion; the second, a fine,
dark non-medullated fiber, emerges from one of the cells of the ganglion,
which, after pursuing a longer or shorter course, branches and becomes his-
tologically and physiologically related to non-striated muscle-fibers and
epithelium. The first neuron is termed pre-ganglionic, the second post-
Jandionic, or in accordance with long-established usage, sympathetic.

The nerve-cells in the regions which give origin to the pre-ganglionic
neurons though irritable, do not possess spontaneity of action, but require
for the manifestation of their activities, the arrival and stimulating influence
of nerve impulses. These may likewise come (i) from peripheral regions
of the body through afferent nerves in consequence of the action of external
asents— in which case the resulting movement is termed a reflex movement;
or (2) from the cerebrum through descending nerve-fibers in consequence
of affective or emotional psychic states, in which case the resulting movement
or modification of movement, is termed an affective or an emotional move-
ment In the performance of the functions of vascular and visceral organs
and glands, the non-striated muscles and epithelium are in the vast majority
of instances caused by reflected nerve impulses, though frequently modified
by nerve impulses of cerebral (affective or emotional) origin.1 .

From the fact that the nerve-centers of the pre-ganglionic nerve-fibers ir
the cranio-bulbar and spinal-cord regions are removed from and not subject
to volitional control; and from the further fact that they are in the vast
maiority of instanced stimulated to activity by nerve impulses transmitted
^peripheral regions (though modified from time to time by the ever vary-
ing phases of affective or emotional psychic activity) this systerr
regarded as in a sense independent, i.e., of volitional control,
or autonomous in its activity, and, therefore, has been des'
nomic nerve system (Langley). The tissues to which it is
akn been designated the autonomic tissues.

In a cons deration of this subject it must be borne in ^md that there are

grees, by peripheral and cerebral activities.

i It fa onty necessary to recall the ^
vessels and sweat-glands of the necl ; d £ace 1 = oserva ™ ^ o£ Cannon of the effects

c"teo • and intesunal move-

merits and the secretion of the adrenal glands.

644 TEXT-BOOK OF PHYSIOLOGY

THE PHYSIOLOGIC ANATOMY OF THE AUTONOMIC NERVE SYSTEM

In a consideration of the essential facts of the physiologic anatomy of
this system, it will be found convenient to consider first the sympathetic
ganglia, and the distribution of their post-ganglionic fibers.

The Sympathetic Ganglia. — The sympathetic ganglia may for con-
venience 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 is regarded by some anatomists as belonging to the
pre-vertebral series. The thoracic portion consists of ten or twelve 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 hypogas-
tric, or pelvic.

The peripheral ganglia are in more or less close relation with tissues and
organs in different regions of the body. Among the members of this group
may be mentioned the ciliary or ophthalmic, the spheno-palatine, the otic
and the submaxillary ganglia; the ganglia in the walls of the heart, in the
walls of the respiratory organs, in the walls of the stomach, intestines and
at the base of the bladder (the pelvic ganglia).

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 nucle-
ated capsule by which each cell is surrounded, branch and re-branch and
interlace to form a pericapsular plexus. Each cell gives origin to an axon,
which as it leaves the cell becomes invested with a sheath continuous with
the capsule surrounding the cell-body. It is, however, wanting in a medul-
lary sheath, and hence the nerve presents a gray color. Such a structure,
in its entirety, is known as a sympathetic neuron. The axonic processes as
they emerge from the cells become united and in many instances form dis-
tinct nerve strands, but subsequently as they approach their distribution
divide and subdivide, forming smaller and even smaller bundles, which pass
in different directions to regions varying in position according to the situa-
tion of the ganglion from which they come. These post-ganglionic fibers
are for descriptive purposes divided into two groups, viz. : (i) the gray rami
communicantes , and (2) the rami viscerates.

THE ANATOMIC RELATIONS OF THE SYMPATHETIC GANGLIA
TO PERIPHERAL STRUCTURES

The Vertebral Ganglia. — i. The grey rami communicantes. The fibers
composing these branches, as the name implies, communicate with the

THE AUTONOMIC NERVE SYSTEM 645

trunks of the spinal nerves. Each ganglion of the vertebral chain gives
origin to one or more of these post-ganglionic branches which pass backward
and outward and enter the sheath of the corresponding spinal nerve. In
the cervical region, however, where the ganglia do not correspond in number
with the cervical spinal nerves, the ganglia give off two or more gray rami.
Thus in man the superior cervical ganglion sends branches to the first four
cervical nerves. The middle and inferior ganglia send a branch to the fifth
and sixth and the seventh and eighth cervical nerves respectively. The first
thoracic ganglion sends several branches into the trunks of the nerves that
enter into the formation of the brachial plexus. The ganglia in the thoracic,
lumbar and sacral regions, send at least one gray ramus, in some instances
two, into the sheath of the corresponding thoracic, lumbar and sacral nerves.

The gray rami which thus enter the sheath of the spinal nerve trunks, pass
in company with their efferent motor fibers, to the periphery, to be finally
distributed to structures in the skin, viz.: non-striated muscles of blood-
vessels, non-striated muscles of hair follicles, and epithelium of sweat-glands.
The blood-vessels and sweat-glands of the skin of the neck receive their
ganglionic nerve-supply from the superior and middle cervical ganglia; those
of the skin of the arm, from the inferior cervical and first thoracic ganglia;
those for the skin of the trunk, from the thoracic ganglia; those for the skin
of the hip and leg, from the lumbar and upper sacral ganglia; those for the
skin of the external genital organs, from the lower sacral ganglia.

2. The rami viscerales. — The fibers composing these branches were sup-
posed, as the name implies, to pass directly to viscera, though it is apparent
from the course they pursue that they communicate, if not with spinal, at
least with cranial nerves.

The superior cervical ganglion gives off from its cephalic extremity two
visceral branches, which subsequently divide and subdivide forming the
carotid and cavernous plexuses; from these plexuses slender branches follow
the course of the more superficial arteries at least, to their terminations, while
others pass into the trunks of the trigeminal, abducent, and the superior and
deep petrosal branches of the facial nerve, to be distributed to blood-vessels
and glands of special regions of the head and face. Still other branches
pass down the neck and in their course become associated with correspond-
ing branches from the middle and inferior cervical ganglia. Interlacing in
an intricate manner they assist in forming the cardiac plexuses.

The middle cervical ganglion gives off visceral branches, which in asso-
ciation with branches from the inferior cervical ganglion, pass to the thyroid
gland to be distributed to the walls of the blood-vessels and to the gland cells
as well. Stohr, Berkley and others have shown that fine non-medullated
nerve filaments terminate on and between the epithelial cells lining the folli-
cles, though the supply is not very abundant.

The inferior cervical ganglion gives off visceral branches which pass down-
ward and forward and are ultimately distributed to the heart-muscle. With
these fibers there are usually associated, according to the animal considered,
visceral fibers which come from the first thoracic or stellate ganglion. These
fibers are known as the sympathetic cardiac fibers and have for the physiolo-
gist and clinician great interest as they are associated with the activities of
the heart. (See pages 311 and 319.)

The thoracic, lumbar and sacral ganglia also give off visceral branches

6^6 TEXT-BOOK OF PHYSIOLOGY

which pass for the most part to neighboring structures, though from the
lower lumbar and sacral ganglia branches pass to viscera in the lower ab-
dominal and pelvic regions.

In accordance with the law of distribution and relations of the fibers of
the sympathetic ganglia to peripheral organs, it can be assumed that to what-
ever organ the visceral branches are distributed they too ultimately terminate
in the non-striated muscle-cells of the walls of the blood-vessels and the walls
of hollow viscera, and in some situations in the epithelium of glands as well.

The Pre-vertebral Ganglia. — The pre-vertebral ganglia are located in
the abdominal cavity.

The semilunar, the renal, and the superior mesenteric are situated in the
neighborhood of the celiac axis and on a level with the adrenal bodies. These
ganglia give off an enormous number of visceral branches which interlace in
a very intricate manner forming what is known as the solar plexus. Sub-
divisions of this plexus, taking their names from the regions to which they
are distributed, are known as the gastric, renal, adrenal, splenic, hepatic and
superior mesenteric. The terminals of the fibers composing these plexuses
are distributed to the non-striated muscle-fibers of blood-vessels of the stom-
ach, kidney, adrenal body, liver, and small intestine; to the muscle- walls of
the stomach, the small intestine, the walls of the gall-bladder, as well as the
sphincter muscles surrounding the pyloric and the ileo-colic orifices.

The inferior mesenteric ganglion situated close to the origin of the inferior
mesenteric artery gives off visceral fibers, which also interlace with other
fibers to form the hypogastric plexus, from which fibers pass to the muscle-
walls of the colon, bladder, uterus, vagina and to the blood-vessels of the
pelvic viscera.

The Peripheral Ganglia. — The peripheral ganglia, as previously stated,
are in more or less close relation with tissues and organs in different regions
of the body. Among the members of this group may be mentioned the
ciliary or ophthalmic, the sphenopalatine, the otic and the submaxillary
ganglia; the ganglia in the walls of the heart, in the walls of the respiratory
organs, in the walls of the stomach, and intestines and the ganglia at the
base of the bladder (the pelvic ganglia).

The ganglia situated in the head are usually described in connection with
and as constituent parts of the cranial nerve system. They, however, bear
the same relation to the cranial that the vertebral and pre-vertebral ganglia
bear to the spinal nerves. They consist of ganglion cells from which post-
ganglionic fibers pass to glands, blood-vessels, and viscera. Motor and sensory
nerves pass through one or more ganglia, though they have no anatomic
connection with them. In their structure, distribution and functions they
closely resemble the collateral ganglia.

1. 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 con-
nective-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.

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.

THE AUTONOMIC NERVE SYSTEM 647

3. The otic ganglion is situated just below the foramen ovale and internal

to the third division of the fifth nerve. The cells give origin to post-
ganglionic fibers which 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.
Motor fibers from the small or motor root of the fifth nerve pass
through this ganglion to the tensor tympani muscle.

4. The submaxillary ganglion is situated close to the corresponding gland.

The post-ganglionic fibers to the blood-vessels and gland-cells.

5. The cardiac ganglia are situated in different regions in the walls of the

heart; their visceral branches are distributed directly to the heart muscle-
cells.

6. The pelvic ganglia are situated close to the base of the bladder; their

visceral branches are distributed to the blood-vessels of the generative
organs and to the muscle-walls of the bladder, rectum, etc.
From the distribution of the branches emerging from all the different
groups of ganglia there is every reason to believe that they are in some way
associated with vaso-augmentor and vaso-inhibitor, viscero-augmentor and
viscera -inhibitor, secreto-motor and secreto-inhibitor ', and pilo-motor phenom-
ena.

THE ANATOMIC RELATION OF THE CENTRAL NERVE SYSTEM TO THE

SYMPATHETIC GANGLIA

The central nerve system is associated anatomically and physiologically
with the sympathetic ganglia through the intermediation of fine medullated
nerve-fibers, the pre-ganglionic, which have their origin in nerve-cells situated
in four different regions, viz.:

T. The Mid-brain Region. — The pre-ganglionic nerve-fibers that leave the
brain in this region arise from groups of nerve-cells situated high up in
the gray matter beneath the aqueduct of Sylvius just where it widens
to form the cavity of the third ventricle. From this origin they enter
the trunk of the oculo-motor nerve and in association with it enter the
orbit cavity. In this situation these pre-ganglionic fibers leave the
oculo-motor nerve and enter the ciliary or ophthalmic ganglion around
the nerve-cells of which their terminal branches arborize. The gray
post-ganglionic fibers arising in the gray cells of this ganglion enter the
eyeball and are ultimately distributed to the sphincter muscle of the
iris and to the ciliary muscle.

2. The Bulbar Region. — The pre-ganglionic fibers that leave the brain in
this region arise from nerve-cells situated in the gray matter beneath the
floor of the fourth ventricle a little above and below the calamus scrip-
torius. These fibers leave this region by three routes, viz.: in the
trunks of (i) the pars intermedia or nerve of Wrisberg, (2) the glosso-
pharyngeal and (3) the vagus.

The pre-ganglionic fibers that leave by the nerve of Wrisberg enter
the facial nerve and subsequently pass by way of the great superficial
petrosal nerve to the spheno-palatine ganglion, and by way of the chorda
tympani nerve to the submaxillary ganglion, around the nerve-cells of
which their terminal branches arborize. The gray post-ganglionic fibers
which arise in the cells of these ganglia are distributed to the blood-

648 TEXT-BOOK OF PHYSIOLOGY

vessels and glands of the nose and mouth and to the blood-vessels and
epithelium of the submaxillary and sublingual glands respectively.

The pre-ganglionic fibers that leave by way of the glosso-pharyngeal
nerve pass into the tympanic branch or nerve of Jacobson and ulti-
mately arborize around the cells of the otic ganglion. The gray post-
ganglionic fibers which arise in the cells of this ganglion pass by way of
the auriculo-temporal branch of the trigeminal nerve to the blood-vessels
and epithelium of the parotid gland.

The pre-ganglionic fibers that leave in the trunk of the vagus nerve
are ultimately distributed to the ganglia of the heart, stomach, small
intestine, etc., around the nerve-cells of which their terminal branches
arborize. The gray post-ganglionic fibers which arise in these ganglia
pass to the heart-fibers, to the non-striated muscle -fibers in the walls of
the stomach, intestines, etc. These fibers contained in the facial, glosso-
pharyngeal and vagus nerves, together with their ganglionic continua-
tions, have collectively been termed the bulbar autonomic system.
Together with the fibers in the oculo-motor nerve they have been termed
the cranio-bulbar autonomic system.

3. The Mid-spinal Cord Region. — The pre-ganglionic nerve-fibers that
leave the spinal cord in this region arise from groups of nerve-cells situ-
ated in the gray matter between the levels of origin of the second thoracic
and the second and third, perhaps the fourth, lumbar nerves. From this
origin the fine pre-ganglionic fibers emerge from the cord in the ventral
roots of the thoracic and upper lumbar nerves and hence naturally fall
into two groups, viz.: the thoracic and the lumbar. Both groups of
nerves accompany the ventral motor roots of the spinal nerves to about
the point where each nerve divides into an anterior and a posterior
branch; they then leave and enter the vertebral or sympathetic chain
of ganglia. The branches of communication are known as the white
rami communicantes. The nerve-fibers composing these communicating
branches all terminate around the nerve-cells of the ganglia at the same
and at somewhat different levels and in different regions.

The thoracic pre-ganglionic fibers in accordance with their distribu-
tion may be divided into five groups, viz. :

(a) Those fibers which emerge from the spinal cord in each of the tho-
racic nerves and which terminate around the cells of the ganglia at
the same level.

(b) Those fibers which emerge from the spinal cord in the thoracic
nerves from the fourth to the tenth and which after entering the
vertebral chain turn upward and finally terminate around the cells
of the first thoracic or stellate ganglion.

(c) Those fibers which emerge from the cord in the second and third
thoracic nerves and which after entering the vertebral chain turn
upward and terminate around the cells of the inferior cervical gan-
glion. '.«

(d) Those fibers which emerge from the cord in the second, third and
perhaps the fourth thoracic nerves and which, after entering the
vertebral chain turn upward and pass through the various ganglia
and cord and finally terminate around the cells of the superior cer-
vical ganglion.

THE AUTONOMIC NERVE SYSTEM 649

(e) Those fibers which emerge from the spinal cord in the thoracic
nerves from the fifth to the tenth and which after entering the verte-
bral chain pass across it and turn downward and forward uniting
at different levels to form the greater and lesser splanchnic nerves,
the terminal branches of which arborize around the cells of the semi-
lunar, the renal, and the superior mesenteric ganglia.

^ The lumbar ^ pre-ganglionic fibers in. accordance with their distribu-
tion may be divided into three groups, viz. :

(a) Those fibers which, after pursuing a similar course to the preceding
terminate around the cells of the ganglia at the same level.

(b) Those fibers which after entering, cross the vertebral chain and
then pass forward as the inferior splanchnic nerves to terminate
around the cells of the inferior mesenteric ganglion.

(c) Those fibers which after entering, descend the vertebral chain to
terminate around the cells of the successive ganglia as far as the
third sacral. It is probable that some fibers from the lower thoracic
nerves also descend the lumbar chain.

By reason of this anatomic distribution, the thoracico-lumbar pre-
ganglionic fibers become related to all the vertebral and the pre- vertebral
ganglia. From these various ganglia, as stated on page 645 post-gan-
glionic fibers arise which either as gray rami communicantes enter the
spinal nerves or as rami viscerales pass direct to their termination. In
either case the nerve-fibers ultimately become histologically and physio-
logically related to non-striated vascular and visceral muscle-fibers and
to epithelium of glands. The specific distributions of the post-gan-
glionic fibers are stated in the paragraphs relating to the ganglia them-
selves. A connection is thus established between the cells of the spinal
cord and the motor tissues by means of the pre- and post-ganglionic
nerve-fibers. These nerves include, therefore, all the vaso-motor (con-
strictor) nerves, the secreto-motor nerves for the sweat glands, the thy-
roid and the adrenal glands, and a large part of the viscero-motor (in-
hibitor) nerves.

The thoracico-lumbar pre-ganglionic nerves together with their post-
ganglionic continuations together constitute the thoracico-lumbar auto-
nomic nerve system.

4. The Sacral Spinal-cord Region. — The pre-ganglionic nerve-fibers that
leave the spinal cord in this region arise from groups of nerve-cells situa-
ted in the gray matter between and including the levels of origin of the
second, the third and the fourth (?) sacral nerves. From this origin the
pre-ganglionic fibers emerge from the cord in association with the large
motor fibers composing the ventral roots of these sacral nerves and pass
with them to the interior of the pelvis. Here they leave the sacral
nerves and enter the pudendal or pelvic nerve (the nervus erigens) and
finally terminate around the cells of the pelvic ganglia. From these
ganglia post-ganglionic fibers arise which pass onward to be distributed
to the non-striated muscle-fibers of pelvic viscera and the blood-vessels
of the external generative organs. These fibers contained in the sacral
nerves together with their post-ganglionic continuation have collectively
been termed the sacral autonomic system. It may be regarded as a

650 TEXT-BOOK OF PHYSIOLOGY

special nerve system for the anal end of the gut and structures develop-
mentally connected with it. » . .

From the distribution of the pre-ganglionic fibers and their histologic
connection with the sympathetic ganglia and from the distribution and
histologic connection of the branching fibers from the ganglia there is
every reason to believe that the autonomic nerve system includes all the
vaso-motor, some of the vaso-inhibitor, the viscero-motor and the vis-
cero-inhibitor, the secreto-motor and the secreto-inhibitor nerves.
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 centrally located; the latter 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 interconnect-
ing cords, as a rule, transmit nerve impulses from the more centrally to the
more peripherally located ganglia, and are, therefore, termed rami efferentes.
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 or interconnecting cords, there are some which
possess special interest for the physiologist, viz. :

1. The cervical which connects the thoracic ganglia with the superior cervical

ganglion. It is composed mainly of pre-ganglionic medullated fibers
derived originally from the thoracic spinal cord.

2. The great splanchnic nerve formed mainly by the union of pre-ganglionic

fibers which cross the vertebral chain in the region of the fifth to the
tenth thoracic ganglia. It connects the cells of the spinal cord with the
cells of the semilunar ganglion.

3. The small splanchnic nerve formed by the union of pre-ganglionic medul-

lated fibers that cross the vertebral chain in the region of the ninth and
tenth thoracic ganglia. It connects the cells in the spinal cord with
the renal and perhaps the superior mesenteric ganglion.

4. The inferior splanchnic nerves formed by the union of pre-ganglionic

medullated fibers that cross the upper lumbar chain in the region of the
second and third lumbar ganglia. They connect the cells in the spinal
cord with the inferior mesenteric ganglion.

Afferent Sympathetic Fibers. — With the foregoing groups of pre-gan-
glionic autonomic fibers in the thoracic and lumbar regions there is associated
a certain number of afferent fibers which impart a certain degree of sensibility
to the organs of the thoracic and abdominal regions. Though these organs in
the physiologic condition do not appear to be endowed with much sensibility,
nevertheless, in pathologic conditions they become exceedingly sensitive and
give rise to extremely painful sensations. The afferent nerves which mediate
these sensations have their origin in nerve-cells located in the ganglia of the
dorsal roots of the spinal nerves; from this origin they pass outward by way
of the white rami into nerve trunks which pass to the organs in the thoracic
and abdominal cavities, e.g., the cardiac plexus and the splanchnic nerves
and the sacral nerves to the pelvic organs. The presence of afferent nerves
in the splanchnics can be shown by stimulating the central end or even the
white rami with electric currents, a procedure that gives rise to pain as well
as a reflex rise of blood-pressure. The number of afferent nerve-fibers in

THE AUTONOMIC NERVE SYSTEM 651

any of the these nerve trunks is quite small in comparison with the efferent
fibers. In pathologic conditions of the organs the sensations to which they
give rise are frequently referred to areas of the cutaneous surface overlying
the lesion.

THE FUNCTIONS OF THE AUTONOMIC NERVE SYSTEM

The view according to which the sympathetic ganglia are to be regarded
as independent organs endowed with functions of their own and in nowise
directly dependent for their activities on the central nerve system is at the
present very largely 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 interchanges between it and
the blood and lymph by which it is surrounded, maintaining its own nutri-
tion and exerting possibly a favorable influence over the nutrition of periph-
eral tissues to which its post-ganglionic branches are distributed.

From the number and fineness of the ultimate terminations of the
branches given off from the ganglion cells as well as their extensive distribu-
tion in all regions of the body, it has been suggested that they are distribu-
tors of the nerve impulses discharged in consequence of the stimulating influ-
ence of nerve impulses coming from the spinal cord, through the pre-gan-
glionic fibers.

From the distribution of the post-ganglionic fibers, viz.: to the walls of
the blood-vessels, to the walls of the viscera, and to the epithelium of glands
and in some animals to the muscles of the hair follicles, it may be inferred
that the ganglia are associated with vaso-motor, viscero-motor, secreto-motor
and pilo-motor phenomena; and since the pre-ganglionic fibers are always
associated with the ganglia, anatomically and physiologically, it can be said
that they too are associated with the same series of phenomena.

The functions of the autonomic nerve system, as determined from its
anatomic distribution and the results of experimental investigations, are to
augment or to inhibit the tonus of the blood-vessels including the heart, the
tonus of visceral walls and the activity of the epithelium of glands, and are,
therefore, the sum total of the functions of the vaso-motor, viscero-motor
and secreto-motor nerves, that is, the nerves which collectively constitute
this system. 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 strength of the stimulus and the portion of the system subjected to
experiment, as will be apparent from the following statements.

The Functions of the Mid-brain Autonomic Nerves.— The functions
of the efferent pre-ganglionic nerve-fibers contained in the trunk of the oculo-
motor nerve, together with their post-ganglionic continuations (the ciliary
ganglion and its branches), are (*) To augment the tonus or degree of con-
traction of the sphincter pupilla or the sphincter iridis in accordance with in-
creasing intensities of light, and (b) to augment the tonus or degree of con-
traction of the ciliary muscle accompanying efforts of accommodation. ^ The
result of the contraction of the sphincter pupillse is a diminution in the diame-
ter of the pupil and hence a diminution in the amount of light entering the
eye so that the perception of the image may be sharp and distinct. The
result of the contraction of the ciliary muscle is a relaxation of the suspensory

652 TEXT-BOOK OF PHYSIOLOGY

ligament of the lens whereupon the latter becomes more convex on its ant(
rior surface, and hence adjusted for near vision. These and similar facts
are shown by stimulation and division of these nerves either pre- or post-
ganglionic with induced electric currents.

The Functions of the Bulbar Autonomic Nerves. — The efferent pre-
ganglionic nerve -fibers contained primarily in the nerve of Wrisberg or pars
intermedia, and secondarily in the facial nerve may be divided into two
groups, viz.: (a) those passing by way of the great petrosal nerve to the spheno-
palatine ganglion, and (b) those passing by way of the chorda-tympani nerve
to the submaxillary and sublingual ganglia. The functions of the great
petrosal fibers together with their post-ganglionic continuations (the spheno-
palatine ganglion and its branches) are (a) To inhibit the contraction of the
blood-vessels of the mucous membrane of the nose, soft palate, upper part of
the pharynx, the roof of the mouth and upper lip, and (b) to augment the
secretion of the mucous glands of the corresponding region. The functions
of the chorda-tympani fibers and their ganglionic continuations (the sub-
maxillary and sublingual ganglia and their branches) are (a) To inhibit the
tonus of the blood-vessels of the submaxillary and sublingual glands and
(b) to augment the activities of their epithelium and thus increase their
secretions.

The function of the efferent pre-ganglionic fibers contained in the trunk
of the glosso-pharyngeal nerve together with -their post-ganglionic continua-
tions (the otic ganglion and its branches) are (a) To inhibit the contraction
of the blood-vessels of the parotid gland and (b) to augment the activities of
its epithelium and thus increase its secretion.

The efferent pre-ganglionic nerve-fibers contained in the trunk of the
vagus nerve together with their post-ganglionic continuations are as follows:
(a) To inhibit the tonus and contractile power of the heart-muscle; (b) to
inhibit and sometimes augment the contraction of the esophagus and of the
sphincter car dice] (c) to inhibit the tonus of the cardiac end of the stomach for
the purpose of receiving incoming amounts of food; (d) to augment the tonus
and the contractile power of the gastric musculature and the pyloric sphinc-
ter; (e) to augment and sometimes inhibit the tonus of the bronchial muscula-
ture; (/) to inhibit the sphincter muscle of the common bile duct and to aug-
ment the tonus and contractile power of the visceral muscles in the walls of
the gall-bladder; (g) to augment the tonus and contractile power of the vis-
ceral muscles of the walls of the small intestine, and (h) to augment and some-
times to inhibit the activities of the epithelium of the gastric and pancreatic
glands and thus increase their secretions. These various phenomena result
when these various efferent nerves are divided and their peripheral extremi-
ties are appropriately stimulated.

The Functions of the Thoracic and Lumbar Autonomic Nerves.—
The efferent pre-ganglionic nerve-fibers that leave the spinal cord in the
ventral roots of the thoracic and upper lumbar nerves together with their
post-ganglionic continuations, have been shown by experiment to be vaso-
motor (constrictor), secreto-motor, viscero-motor and in some animals pilo-
motor in functions. The origin, course and distribution of these nerve -fibers
have been stated on pages 377 and 473. The specific functions of these
fibers will depend on the regions of the body to which they are distributed
(See page 648.)

THE AUTONOMIC NERVE SYSTEM 653

The functions of the thoracic fibers that terminate in the ganglia of the
same levels are (a) to augment the tonus and the degree of the contraction
of the blood-vessels of the skin of the arm and trunk of the body; and (b) to
augment the activities of the epithelium of the sweat-glands in the correspond-
ing regions. Stimulation of either the pre- or post-ganglionic fibers produces
the foregoing phenomena.

The functions of the fibers that ascend the sympathetic chain to terminal
in the first thoracic, the inferior cervical ganglia are (a) to accelerate the rate
and the force of the contraction of the heart-muscle, and (b) perhaps augment
the activities of the epithelium of the thyroid gland. The pre-ganglionic-
fibers having these important functions emerge from the spinal cord for the
most part in the ventral roots of the second and third thoracic nerves. After
leaving these nerves by way of the rami communicantes they enter the thoracic
chain and pass upward to terminate in and around- the cells of the inferior
cervical ganglion in man and in the first thoracic ganglion in certain other
mammals. The post-ganglionic visceral fibers— the cardiac nerves—that
arise in the cells of these ganglia pass downward and forward and reach the
heart by way of the cardiac plexus. Stimulation of these nerves causes the
same effects in the heart as stimulation of the pre-ganglionic fibers in any
Dart of their course. The center of origin of these accelerator and aug-
mentor cardiac fibers has not been definitely located but there are reasons
for thinking it is in the medulla oblongata. .

The functions of the fibers which in their upward course form the cervica
cord and finally terminate around the nerve cells of the superior ,

gan(?)° \Taugment the tonus and increase the- contraction of the blood-
vessels of the skin and mucous membrane of different portions of he head,
ce and neck- (6) to augment the contraction of the blood-vessels of the sub-
maxmary ^ the'sublingua" and the parotid glands; and (c) to augment the activ-
tty of the epithelium of the sweat glands of the head, face and neck and
Dossiblv of the salivary glands as well, (e) To augment the tonus and the
con Sons of the dilatator pupill* muscle, thus antagonizing and overcom-
ing the ^contraction of sphincter pupill* muscle and establishing a dilatation

°f irfSeeoine effects may be demonstrated by exposing and dividing
" T°h:nf uncttnfo?1 thXeTs that cross the sympathetic chain and unite

6$4 TEXT-BOOK OF PHYSIOLOGY

spleen; and (b) to inhibit the tonus and diminish the degree of contraction
of the muscle walls of the stomach and intestine; (c) to inhibit the tonus
during digestion, of the sphincter muscle of the common bile duct; and (d)
to augment the tonus and degree of contraction of the muscle-walls of the
gall-bladder; (e) to augment the tonus of the ileo-colic sphincter muscle; (/)
to augment the secretion of the adrenal bodies.

These effects are readily observed if the great splanchnic nerve is first
divided and then its peripheral end stimulated with induced electric currents.
With the division of the nerve the blood-vessels of the splanchnic area, as
well as of neighboring organs, dilate, attended by a sudden fall of the general
blood-pressure; with the beginning of the stimulation the blood-vessels of
the corresponding areas at once contract, coincidently with which there is a
rapid rise in the general blood-pressure.1 The division of the splanchnics
is followed by a rise in the tonus of the muscle-walls of the intestine (due to
the augmentor activity of the now unopposed vagi nerves); stimulation of
the peripheral end of the splanchnics is followed by a marked inhibition of
the tonus.

The functions of the lumbar fibers that terminate in the ganglia of the
same levels are: (a) To augment the tonus and the degree of contraction of
the blood-vessels of the skin of the trunk to which they are distributed; and
(b) to augment^ the activities of the sweat-glands of the corresponding regions.
^The functions of the nerve-fibers that cross the sympathetic chain and
unite to form the inferior splanchnic nerves, together with their ganglionic
continuations (the inferior mesenteric ganglion and the hypogastric plexus),
are: (a) To augment the tonus and the contractile power of the blood-vessels
of the pelvic viscera; (b) to inhibit the tonus and diminish the contractile
power of the muscle- walls of the large intestine; (c) to augment the tonus of
the sphincter muscle of the urinary bladder, and to inhibit the tonus of the
muscle- walls of the bladder during the intervals of urination; (d) to augment
the tonus and contractile power of the uterus.

These various phenomena arise when either the pre-ganglionic or the
post-ganglionic fibers in the hypogastric plexus are stimulated with induced
electric currents.

^ The functions of the fibers which descend the sympathetic chain to ter-
minate around the cells of the lumbar and sacral ganglia are (a) to augment
the tonus and the degree of contraction of the blood-vessels of the skin of the
hip and leg; and (b) to augment the activities of the epithelium of the sweat-
glands of the corresponding parts. By reason of the distribution of the
visceral branches of the sacral ganglia a similar contraction of the blood-
vessels is observed in portions of the pelvic viscera and the external genitalia
The Functions of the Sacral Autonomic Nerves.— The functions of the
pre-ganglionic nerve-fibers that leave the spinal cord by way of the ventral
roots of the second, third and perhaps the fourth sacral nerves, together with
their post-ganglionic continuations (the pelvic ganglia and their branches)
are: (a) To inhibit the tonus and cause a dilatation of the blood-vessels of
the generative organs; (b) to inhibit the tonus of the sphincter muscle of the

e ^f11™ is followe<l by a slight primary inhibition and dila-
t t , would lead to the inference that vaso-dilatator fibers are also

is £?hee ±1' 7?^ d° T belo?g Pr°Perly to the autonomic system as their center of
> in the ganglia of the dorsal roots of the spinal nerves. (See pages 380 )

THE AUTONOMIC NERVE SYSTEM 655

bladder; (c) to augment the contraction of the detrusor muscle during urina-
tion; (d) to augment the tonus and contractile power of the muscle- walls of
the colon and rectum, and the rectal sphincters.

Summary.— The autonomic system consists of:

The autonomic tissues, viz. : the non-striated, visceral and vascular muscle

tissue, cardiac striated muscle tissue and epithelial tissue.
The autonomic nerves.

The autonomic nerves consist of two successively arranged neurons.
The first has its origin in cells of the central nerve system; the second has
its origin in cells of the sympathetic ganglia. The former is termed pre-
ganglionic, the latter post-ganglionic. The pre-ganglionic neuron arborizes
peripherally around the cells of the sympathetic ganglia; the post-ganglionic
terminates by fine branchings around the cells of the tissues.

The pre-ganglionic neurons have their origin in four separate regions ot
the central nerve system, viz.: (i) The mid-brain region; (2) the medulla
oblongata; (3) the mid-spinal cord region and (4) the sacral^ region.

The autonomic system of nerves as evident from their distribution com-

e viscero-augmentor and viscera-inhibitor nerves for the non-striated
muscles of the eyeball (iris and ciliary muscle) of the lower portion of
the esophagus, of the trachea and bronchial tubes, of the stomach, of
the small and large intestine, the gall-bladder and the pelvic organs.
The vase-dilatator nerves for the non-striated muscles of the blood-vessels
of the salivary glands, the mucous and serous glands of the nose, mouth
and pharynx, and of the genital organs.

7 The vaso-constrictor nerves for the non-striated muscles of the blood-ves-
sels of the skin of the head, face, neck, trunk, arms, and legs, alimentary
canal, liver, spleen, kidney, adrenals and genital organs
A. The cardio-augmentor and cardio-inhibitor nerves for the heart
t The Tecreto-augmentor and seer eto -inhibitor n^s teto*f^*»*
of the salivary glands, of the mucous and serous glands of the mouth,
nose and pharynx, of the trachea and bronchial tubes of the gastric
Ss of ?he thyroid, of the pancreas, of the adrenals, of the liver, and
the sweatglands'of the skin of the head, face neck, trunk, arms, legs
and of the glands in connection with the genital organs.
The centers of origin of the autonomic nerves possess a certain degree of
tonus mamtaTned in all probability by changes of a chemic character be-
tween ent of ?he nerve cells and the constituents of the blood by

tween

which fl^ed. In consequence of this tonus, they maintain a

Certain degree of activity in the tissues to which the autonomic fibers are dis-
trihuted o'ven this tonus the autonomic centers may be excited tc in-
creased actS (i) By nerve impulses transmitted by afferent nerves from
creased activity wj r impuises descending from the

-tent of the resulting activity of the
autonomic tissues.

656 ' TEXT-BOOK OF PHYSIOLOGY

NOTE. — Langley to whom we are so largely indebted for the present state of our
knowledge of the anatomy and physiologic relations, not only of different portions
of this system, but of the system in its entirety, introduced the term " autonomic,"
on account of the insufficiency of the older nomenclature, e.g., "sympathetic,"
"ganglionic," "vegetative," "organic," "visceral," etc., to fully express its ana-
tomic relations and physiologic action. By this term it is implied that this system is
independent in action; that is, independent of volitional control, that its activity
is determined by nerve impulses coming from the periphery, though subject to
modifications by cerebral states of an affective or emotional character. Thus he
states (Journal of Physiology, vol. 23) " I propose to substitute the word auto-
nomic " * * *; "the autonomic nervous system means the nervous system of the
glands and of the involuntary muscle." "I propose the term autonomic nervous
system for the sympathetic system and the allied nervous systems of the cranial
and sarcal nerves and for the local nervous system of the gut " * * *. "I conclude
that there is no fundamental difference between the pre -ganglionic fibers of the
body, whether they belong to the cranial, the sympathetic or the sacral autonomic
systems." "The word autonomic nervous system consists of the sympathetic sys-
tem, of the cranial autonomic system, and the enteric system, the plexuses of
Auerbach and Meissner."

Gottlieb and Meyer, partly on the basis of the difference in the physiologic
action of certain drugs on different portions of this system, and partly on the ap-
parent antagonism in the action of the nerves which arise from the bulbar and
sacral region of the spinal cord, to those which arise from the thoracic and upper
lumbar regions, proposed a modification of the terminology. Thus they applied
to the entire system the term "vegetative," but at the same time retained the term
autonomic for the fine medullated pre-ganglionic fibers in the cranio-bulbar and in
the sacral nerves, i.e., those nerves not in physiologic relation with the chain of
sympathetic ganglia, though related to peripheral ganglia. The pre-ganglionic
cranio-bulbar and sacral nerve-fibers together with their post-ganglionic continua-
tions innervate the anterior and posterior ends of the digestive tract and associated
structures respectively, while the pre-ganglionic thoracico-lumbar nerve-fibers and
their ganglionic continuations innervate the sweat-glands and blood-vessels of the
head, face, extremities and the body-walls and the blood-vessels and muscle- walls
of the abdominal viscera. Associated with these latter nerves are the efferent fibers
of the vagus which innervate the heart, the blood-vessels and glands of the viscera
of the thorax and part of the abdomen as well.

By reason of the double innervation of various organs of the body and the op-
posite effects produced by stimulation on the one hand of the autonomic cranio-
bulbar and sacral nerve-fibers and on the other hand of the pre- and post-ganglionic
fibers of the thoracico-lumbar nerves, Gottlieb and Meyer concluded, that there is
a fundamental difference in the functions of the cranio-bulbar and sacral regions
and the thoracico-lumbar regions of the spinal cord. This antagonism is shown
by the effects observed in the iris and in the action of the heart by alternate stimula-
tion of the cranio-bulbar and thoracic fibers which are distributed to these struc-
tures. It should be borne in mind, however, that even though pre-ganglionic
fibers leave the spinal cord by way of the thoracic nerves, the nerve-cells from which
they arise may lie in other and even distant regions. Thus it is well known that
the vaso-motor center which through thoracic nerves constricts the blood-vessels
of the salivary glands, the mucous glands of the mouth and nose lies high up the
bulbar region, not far from the vaso-dilatator center, which through bulbar fibers
(in the chorda tympani and glosso-pharyngeal) supplies and dilates the same blood-
vessels of the corresponding structures. It is also believed that the dilatator pupillae
or iridis center which, through nerve-fibers that leave the spinal cord through the
second thoracic nerve and then pass upward in the sympathetic chain to the supe-
rior cervical ganglion and thence to the dilatator fibers of the iris, is also located

THE AUTONOMIC NERVE SYSTEM 657

Lar the group of nerve-cells which gives origin to the nerve-fibers for the sphincter
luscle of the iris. It is also believed that the accelerator nerves for the heart
Ihich emerge from the spinal cord in the second and third thoracic nerves come
lorn a center lying in the bulbar region not far from the center from which the
ihibitor fibers in the vagus come.

Nevertheless there are other instances in which this antagonism does exist, e.g.,
I the effects of the efferent fibers of the vagus and the splanchnic nerves on the
Jtuscle-wall of the intestine, the former augmenting, the latter inhibiting the con-
•J action and so in other regions. But this does not necessitate the employment of
jw terms. It suffices to say that there is a difference in the action of different
arts of the general autonomic system. It is just as probable, however, that the
difference in the effects observed following stimulation of the two classes of nerves
•epends on the character of their peripheral terminations rather than on a differ-
nce in the character of the central cells.

Huber, in a recent review of the morphology of the sympathetic system, says
,1 regard to the foregoing division, "such a division of the autonomic nerve system
oes not seem justifiable when viewed in the light of a morphologic study of the
anglia with their constituent neurons, the terminations of the neuraxis of the
.eurones in the various tissues, and the connections of these neurones with the
erebrospinal axis by means of the pre-ganglionic fibers; morphologically con-
idered, the entire autonomic system is- a unit and will be treated as such. The
ninor structural differences, more apparent than real, observed in the neurons of
:ertain of the cranial autonomic ganglia and in the entire system, do not warrant,
t would seem to me, a regional subdivision of the autonomic system, whencon-
idered from the viewpoint of structure."

CHAPTER XXVII
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 fore part 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 interpolation 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 framework, 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 horizontal 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 11.5 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.

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

PHONATION; ARTICULATE SPEECH

659

runs from before backward, and is known as the false vocal band or cord.
Between the true and false bands there is a cavity or space prolonged up-
ward 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, cor-
responding to the lumen of the trachea.

(Fig. 273.)

The Laryngeal Cartilages, Articula-
tions, and Ligaments.— The cartilages which
compose the framework of the larynx are nine
in number, three of which are single: viz., the
cricoid, the thyroid, and the epiglottis, while
six occur in pairs: viz., the arytenoids, the
cornicula laryngis, and the cuneiform. (Figs.
274 and 275.)

The cricoid cartilage is the foundation
cartilage, and affords support to the remain-
ing cartilages and the structures 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 at-
tached 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 downward, out-
ward, and forward.

The thyroid, the largest of the laryngeal

cartilages, is composed of two flat quadrilat- J^la^M/"?'. Section of the pos-
eral plates, united anteriorly at an angle of terior portion of the cricoid carti-
aboutP9o degrees. Each plate is directed ^^^*S£
backward and outward and terminates m a ^ superior border of the cricoid
free border, which is prolonged upward and cartilage, jo. Section of ***£
downward for some distance, terminating in ^.^^ Se'carit'y of'the lax;
two processes, the superior and inferior cornua. ^ I3- Arytenoid gland. 14,
The upper border to the thyroid is deeply
notched in front. The inferior border over-

FIG. 273. — LONGITUDINAL SEC-
TION OF THE HUMAN LARYNX,
SHOWING THE VOCAL BANDS, i.
larynx. 2. Supe-
3. Inferior vocal
cord. 4. Arytenoid cartilage. 5.

j?> 2Q Trachea.-(Sa#>o>.)

66o

TEXT BOOK OF PHYSIOLOGY

ment 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 point 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 backward and almost at right angles to the long axis of
the cricoid facet.

The cornicula 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., ant-eater — extends between these two cartilages.

FIG. 274. FIG. 275.

FIG. 274. — LARYNGEAL CARTILAGES AND LIGAMENTS, ANTERIOR SURFACE, i. Hyoid bone.
2 1 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 ligaments, u. Trachea. — (Sappey.)

FIG. 275. — LARYNGEAL CARTILAGES AND LIGAMENTS, POSTERIOR SURFACE, i, i. Thyroid
cartilage. 2. Cricoid cartilage. 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. Epiglot-
tis. 8. Ligament uniting it to the reentering angle of the thyroid cartilage. — (Sappey.)

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 articu-
lating sufaces of the cricoid and arytenoid cartilages. This joint is provided

PHONATION; ARTICULATE SPEECH 661

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:
(i) 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 backward
with outward rotation of the vocal process. In either case the process
describes an arc of a circle. (3) A sliding movement toward 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 contiguous 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 be-
comes thinner, and is finally attached anteriorly to the thyroid near the
median line, and posteriorly to the vocal process of the arytenoid, thus con-
stituting 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 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.

The Vocal Bands. — 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 immediate 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 cartil-
ages. 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 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. 276 and 277).

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,

662

TEXT-BOOK OF PHYSIOLOGY

from which it takes its origin. The fibers of the muscle pass upward and
outward 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
outward, and as the vocal band is carried with it the glottis is widened
a condition necessary to the free entrance of air into the lungs (Fig. 278)'
Since the contraction of the crico-arytenoid has this result, it is generally
spoken of as the abductor or respiratory muscle.

mh K the side of the

From thls point its fibers are directed upward and backward

to be inserted into the external process of the arytendd Itfactfon is to

f°rWard ^ ™d' *» approximating
The thyro-arytenoid muscle arises from the inferior two-thirds of the

is*5H^s^-*BgtSaa£-

PHONATION; ARTICULATE SPEECH

663

of, the vocal band. The action of the thyro-arytenoid muscle in conjunction
with the lateral crico-arytenoid is to rotate the arytenoid cartilage around the
vertical axis and to draw the vocal process forward and inward, thus carry-
ing 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. 279).

The arytenoid muscle consists (i) of transversely arranged fibers which
arise from and are inserted into the outer surface of the opposite 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 processes, the rima respirator la ^ and so direct the
expiratory blast of air toward and through the rima vocalis.

FIG. 278. — GLOTTIS WIDELY OPENED
FROM SIMULTANEOUS CONTRACTION or
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. 279. — 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.)

The collective actions of the three foregoing muscles is to close or con-
strict the glottis, and for this reason they are spoken of as the adductor ,

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 anterior part of the cricoid car-
tilage toward the thyroid, which remains stationary and to swing the
auadrate Plate of the cricoid and the arytenoid cartilages downward and
backward5 TMs movement has the result of tensing the voca bands
The cricoid is at the same time drawn backward by the action of the more

^rves'of £aerdynx!-The nerves which innervate the muscles of the
larvnx Ind enlow the mucous membrane with sensibility are derived from
th7va^s mmk The superior laryngeal is for the most part sensor and
HkHbuted to the mucous membrane, though it contains motor fibers for
fhe rTcotyro £ rTscle. The inferior laryngeal is purely motor and is
distributed lo all the muscles with the exception of the cnco-thyroid.

664

TEXT-BOOK OF PHYSIOLOGY

THE MECHANISM OF PHONATION

Phonation, the production of vocal sounds in the larynx, is the result oi
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 approxi-
mately closed and the vocal bands be made more or less tense.

The closure of the glottis — the approximation of the vocal processes 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-thyroic
and the thyro-arytenoid muscles, the former by the backward displacement
of the cricoid and arytenoid cartilages, the latter by converting the natural
concave edge of the vocal band to a straight line. The lengthening and tens-
ing of the vocal bands by the crico-thyroid muscle is regarded by some inves-
igators as a coarse means, the approximation of the free edges by the thyro-
arytenoid, as a finer means, of adjustment for the producton 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

FIG. 280.- — 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. 281. — 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.}

emitted. The appearance presented by the glottis just previous to the emis-
sion of a note of medium pitch, as determined by laryngologic examination, is
shown in Fig. 280. When the foregoing conditions in the glottis are realized
the air stored or collected in the lungs is forced by the contraction ol
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 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 tc
which the air in the lungs and trachea is subjected was determined by Latoui
to vary from 160 mm. of water for sounds of moderate, to 940 mm. of watei
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 other music instruments, all vocal sounds are characterized
by intensity, pitch and quality, tone or color.

PHONATION; ARTICULATE SPEECH

665

The intensity, or loudness of a sound depends on the extent or amplitude
of the to-and-fro vibration or the extent of the excursion of the vex
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 moi
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.

The pitch of the voice depends on the num-
ber 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 sub-
jected. In the emission of sounds of highest WITH THE LARYNGOSCOPE DUR.
pitch the tension of the vocal bands and the ING THE EMISSION OF HIGH-
narrowing of the glottis attain their maximum. j^J*u*** x, **•.
In the emission of sounds of lowest pitc B re- ^ ^ 6 Pharynx> 7. Arv-
verse conditions obtain. In passing from the tenoid cartilages. 8. Opening
lowest to the highest pitched sounds in the range betj^aa^^l «£
of the voice peculiar to any one individual, io^ Cartiiage Of Santorini. u.
there is a progressive increase in both the ten- cuneiform cartilage. 12. Su-

, IS a pAU£^ . r rior vocal cords. 13. In-

cords.— (Le Bon.)

80 and 240

In the

of notes due to vibrations lying between

and

more closely approximated and the vocal aperture reduced to a
the timbre or tone-color, depends on

tions (overtones). 1 ne t .m 01 uic . Th quaiity of the sound

the blending of a number ^oi the mouth

b changes in

to the voice a

somewhat different quality
The Varieties of Voice

are

music scale) comprisillg
regic , laryngeal sounds

^ Cerent individuals of the

666 TEXT-BOOK OF PHYSIOLOGY

same sex. It is customary to classify voices, especially 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', do,, or from 85 to 256 vibrations per second; those of the baritone
from A, Ia1? to P, fa3, or from 106 to 341 vibrations per second; those of the
tenor from c , do2, to a', las, or from 128 to 427 vibrations per second- those
of the contralto from e, mia, to c", do4, or from 160 to 512 vibrations per
second; those of the mezzo-soprano from g, so!2, to e", mi4, or from 102 to
640 vibrations per second; those of the soprano from b, si,, to g" sol 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 exceeded this range, but they are exceptional.

Speech is the expression of ideas by means of articulate sounds. These
sounds may be divided into vowel and consonant sounds.

The vowel sounds, a, e, i, o, u, are laryngeal tones modified by the
superposition and reinforcement of certain overtones developed in the mouth
. and pharynx by changes in their shapes. The number of vibrations under-
lying the production of each vowel sound is a matter of dispute.

Consonant sounds are produced by the more or less complete interruption
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, /, n, r, sh, zh.

5. Posterior linguo-palatals, k, g, h, y.

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 qmet respiration the vocal bands are widely separated and almost
stationary, moving but slightly during either inspiration or expiration At
this time, according to the investigations of Semon, the area of the glottis is
approximately 160 sq. mm., somewhat less than the area of either the
supraglottic or mfraglottic regions, which is about 200 sq. mm. This con-

in Of the glottis is maintained by the steady continuous contraction of the
posterior cnco-arytenoid muscles, the abductors of the vocal bands

tor phonatory purposes it is essential that the respiratory function be
temporarily suspended and the vocal bands closely approximated. This is

>mplished by the contraction of the remaining muscles of the larynx

PHONATION; ARTICULATE SPEECH 667

rith the exception of the crico- thyroid, which are collectively known as the
dductors of the vocal bands. During phonation the adductor muscles over-
ome the activity of the abductors. With the cessation of phonation the
.bductors immediately restore the vocal bands to their former respiratory

position.

ILiUii.

The activities of these two antagonistic groups of muscles are under the
control of the central nerve system. The only pathway for the excitator
lerve impulses is through the fibers of the inferior or recurrent laryngeal
lerve. The relation of these nerve-fibers both centrally and peripherally, as
veil as their physiologic action, has been the subject of much experimenta-
ion. The results have not always been in accord, owing to the choice of
inimal, the use of anesthetics, strength of stimulus, etc,

As the outcome of many investigations it is believed that each muscle
rroup is innervated by its own bundle of nerve-fibers, both of which are con-
'ained in the inferior laryngeal, though coming from two separate centers in
the medulla oblongata. Russell succeeded in separating the fibers for the
ibductors from the fibers for the adductors in the inferior laryngeal, and in
tracing them to their terminations. So completely was this done that
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. Stimulation
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.
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

'center was located by the same investigators in the medulla
near the ala cinerea and the upper border of ^.^™™*™ptonus Stimu-
lation of this area was invariably followed by bilateral adduction of the vocal

movements has also been located

the lower portion of the pre-central convolution, near the anterior border.
kti^ o'f this area gives rise to marked adduction of both vocal bands,
indicating that the representation is therefore bilateral.

Faradic stimulation of the inferior laryngeal nerve during slight etne
gives rise to closure of the glottis; the same stimulation
anesthetization gives rise to opening or ilatation of

of the muscle ,nd increased tension oi the voc.l b.ncta. It „

668 TEXT-BOOK OF PHYSIOLOGY

believed that these fibers are derived originally from the efferent fibers of tl
glosso-pharyngeal nerve. The remaining fibers of the superior larynge
endow the upper portion of the larynx with extreme sensibility which to
certain extent protects the air-passages against the entrance of forei^
bodies. Irritation of the terminal filaments of this nerve by particles
food, solid or liquid, gives rise to marked reflex spasm of the adduct<
muscles and closure of the glottis, followed by a strong expiratory blast of a
from the lungs by which the offending particles are removed. Division «
this nerve on both sides is followed by a paralysis of the crico-thyroid muscle
a lowering of the tension of the vocal bands, and a loss of sensibility of tl
laryngeal mucous membrane.

CHAPTER XXVIII
THE SENSES OF TOUCH, TASTE AND SMELL

Introductory.— It is one of the functions of the nerve system to bring
he individual into conscious relation with the external world. This is ac-
-omplished in part through the intermediation of afferent nerves, connected
Iripherally with highly specialized terminal organs, and centrally wit.
,necialized areas in the cerebral cortex.

Excitation of the terminal organs by material changes in the environment
develops nerve impulses which, transmitted to the cortical areas, evoke
Usatfons. These sensations, differing in character from those vague ill-
defined sensations-^., fatigue, well-being, discomfort, etc -caused .by
changes occurring within the body, are termed special sensations-
temperature; taste; smell; light and its varying
, hue, and tint; sound and its varying qualities, intensity,

special sensations are

optic, and auditory. Each mechanism responds MX

stimulus and to no other. Thus, the stimulus for the

£kvr>itci tir»n nf thp different mecnamsms.

CALlLcttlUil Ui tiAv/ vj. L j_i. ^ j. ^

TVi f t involved in the production of the sensations inciuue ^i; a

" " " of the

peripheral org.n-.ith . ''' ni.lll co.te* in

0, ,he
the ^'''" « e h.tevet their character-

object in comparison with former expenences.

J 660

670 TEXT-BOOK OF PHYSIOLOGY

THE SENSE OF TOUCH

^ The physiologic mechanism involved in the sense of touch includes tL
skin and the mucous membrane lining the mouth, the afferent nerves, thei
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. T<
the skin, therefore, is ascribed a touch sense and a temperature sense
Of the touch sensations two kinds may be distinguished: viz., pressure
sensations and place sensations. With the contact of an external bod]
there arises the perception not only of the pressure, but also the perceptioi
of the place or locality of the contact. In accordance with this, it i<
customary to^attribute to the skin a pressure sense and a location sense.

The specific physiologic stimuli to the terminal organs in the skin an^
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 deli-
cacy in different regions, though its structure is everywhere essentially the
same. As the physiologic anatomy of the skin has elsewhere been detailed
(page 472), 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 ac
inner layer of rather loose connective tissue and an outer layer of condensed
connective tissue. The latter, known as the epidermis, consists of an innei
layer of pigment cells and a thick outer layer of epithelial cells. The
derma is characterized by the presence of elevations (papilke) which are
everywhere 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 inter-
mediates 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 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 termi-
nation of afferent nerves adapted for the impress of a specific stimulus.
The peripheral organs of afferent nerves found in the skin and oral mucous
membrane 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.

3. 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 consisting of a connective-tissue capsule containing a
number of tactile discs with which the nerve-fibrils are connected. If

THE SENSE OF TOUCH 671

the afferent nerve is traced to the capsule, it is found to lose both its
neurilemma and its 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.
Hair Wreaths.— Just, below the openings of the sebaceous glands 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 regarded 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 lamel-
lated 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 connected with the bulb.

Other forms of peripheral organs are found in special regions of the skin
as well as in different animals.

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, tor 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. I hey a
also especially abundant in the immediate neighborhood of the hair-follicles.
The peripheral end-organ associated with the touch spots in the neigh
borhood 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
Meissner corpuscle, for in the palmar surface of the distal phalanx of the
index-finger, where the touch sense is quite acute, about 20 corpuscles a
present in each square millimeter of surface. The specific stimulus neces-
sary 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 external body is attended by a
certain amount of pressure, which, however, must attain a certain degree
before the sensation can be evoked. This is known as the thres ihold ya ue
or the degree of liminal intensity. Since the sensations are the result of
pressure, they are termed pressure sensations, and their intensity may 1
pxnressed in terms of pressure.

The sensitivity of the skin as determined by the pressure sense ? vanes m
different regions 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
mUHme'ters for he skin of the forehead is 0.002 gram; for the flexor aspect
oTthT forearm, 0.003 gram; and for the hips, thigh, and abdomen 0.005
gram- for the palmar surface of the finger, 0.019 gram; for the heel, i gram.

672 TEXT-BOOK OF PHYSIOLOGY

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 of the sensation. Not all changes in the stimulus,
however, are attended by a change in the sensation. It has been deter-
mined 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 sensa-
tion, to an appreciable extent, will occur only when the objective stimulus
changes in a definite ratio. This ratio, however, will vary not only in dif-
ferent regions of the skin, in different individuals, but with the sense-organ
investigated.

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 sensation related in some way to the part stimulated. 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
sensor 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, there-
fore, 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 distinguishing the sensa-
tions 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 sensor circle. Within this circle the two sensations become
fused into one sensation. The discriminative sensibility of different regions
as determined by compass points is shown in the following table; the
numbers represent the distances at which two sensations are recognized:

mm.

Tip of tongue I . I

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 SENSE OF TOUCH

673

The discriminative sensibility of any portion of the body is a function of
> mobility. This is shown by the fact that it increases rapidly from the
Loulders to the fingers and from the hips to the toes.

The Temperature Sense.— The sensations of heat and cold which are

broerienced from time to time are caused by changes in the temperature of

''he skin produced in a variety of ways. As these sensations are specifically

ifferent from those of touch, as well as different from each other, it is highly

•robable that for each sensation there are special nerve-endings distributed

•hroughout the skin. Investigations have shown that all over the skin there

re 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. 1 ne

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

irJSli

numerous than the heat spots in almost all regions of the body. (See Fig.

4-3

674 TEXT-BOOK OF PHYSIOLOGY

external temperature caused by natural or artificial means, which diminishej
the radiation, the temperature of the skin will at once rise, the end-organ*
will be stimulated, and a sensation of warmth developed. If, on the othe:
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, and £
sensation of cold developed. Experiment also teaches that the intensity 01
a warm or cold sensation will depend on the existing temperature of the skin
and not upon the absolute temperature of the object. Thus, water at 2o°C,
will evoke a sensation of heat or cold respectively 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 musculature oi
the body or even of its individual parts, there arises in consciousness 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 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 are central in origin and partake of the nature of a

discharge of nerve impulses from the nerve-cells through the motoi
nerves to the muscles, the entire process being accompanied by sensation
This is known as the innervation theory.

2. That the processes are peripheral in origin, initiated by stimulation o:

specialized end-organs in the muscles and tendons which are connectec
through the intermediation of afferent nerves with nerve-cells in the
cerebral cortex.

Pr.e.

FIG. 284. — A NEURO-MUSCLE SPINDLE OF A CAT. (Ruffim.) c. Capsule, pr. e. Primary
ending, s. e. Secondary ending, pi. e. Plate ending. (All these are probably sensor in function.]
— (Starling1 s "Physiology."}

The physiologic mechanism subserving the muscle sense, according to the
second theory, now held by many physiologists, thus involves peripheral end-
organs, afferent nerves, their cortical connections 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 body 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 abundant. From their shape
they are known as muscle spindles. They vary in length from 2 to 12 mm.

THE SENSE OF TOUCH 675

and in breadth from 0.15 to 0.4 mm. Each spindle (Fig. 284) consists of a
connective- tissue capsule containing 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. The sensor fiber loses its external investments as it approaches the
(capsule. The naked axis-cylinder then penetrates the capsule, and after
i-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
i 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 histo-
i logic methods Sherrington has traced afferent fibers from the muscle spindles
directly into the spinal nerve ganglia. The contractions of muscles from
electric stimulation as well as the contractions known as muscle cramp,
due to unknown agents, give rise to sensations of pain, a fact which in-
dicates the presence in muscles of afferent or sensor nerves.

Cortical Area.— Pathologic findings have shown that an impairment
or a loss of the muscle sense is associated with destructive lesions of perhaps
the super- and sub-parietal convolutions (Figs. 249 and 252). In a case re-
ported 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 t
surfaces of external objects implies the cooperation of the skin am
muscles. The sensations which are evoked are combinations of contact
and muscle sensations. The union of these sensations forms the basis of
the perception of hardness, softness, smoothness, and roughness of

THE SENSE OF TASTE

The physiologic mechanism involved in the sense of taste includes the
tongue the gustatory nerves, their cortical connections and nerve-cells in the
graf mattergof the sub-collateral convolution. The peripheral excitation of
ffi.S£S£ gives rise to nerve impulses which transmitted to the brain
evoke the sensations of taste. The specific physiologic stimulus is matter,

OT^<^!^^^ of the chorda tympani (page

LTiddTK glosso-pharyngeal (page 634) after entering the medulla
obSngata terminate around certain sensor end nuclei, the exact location of

676

TEXT-BOOK OF PHYSIOLOGY

which is uncertain. From these nuclei axons arise which in all probability
cross the median plane and ascend to the sub-collateral convolution. The
exact course of this gustatory tract is, however, obscure.

The Tongue. — The tongue consists of both intrinsic and extrinsic mus-
cles, in virtue of which it is susceptible of a change both in shape and in
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.

1. The filiform papilla, the most numerous, cover the anterior two-thirds of

the tongue; they are conical or filiform in shape and covered with
horny epithelium which is often prolonged into filamentous tufts.

2. The jungiform 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 V. They
consist of a central projection surrounded by a wall or circumvallation
from which they take their name.

The Peripheral End-organs. The Tastebuds. — 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. 285). 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
wall of the bud is composed of elongated curved epithe-
lium. The interior contains narrow spindle-shaped
neuro-epithelial cells provided at their outer extremity with
stiff hair-like filaments which project into the taste-pore.
The neuro-epithelial cells are in physiologic relation
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 with the nerves of taste is rendered prob-
able from the fact of their degeneration after division of
the nerves.

The Taste Area. — The taste area, though confined
for the most part to the tongue, extends in different in-
dividuals 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 consequence of
impressions made by different substances on the peripheral apparatus of
this area are in so many instances combinations of taste, touch, temperature,

FIG. 285.— TASTE-
BUD FROM CIRCUM-
VALLATE PAPILLA OF
A CHILD. The oval
structure is limited to
the epithelium (e)
lining the furrow,
encroaching slightly
upon the adjacent
connective tissue (/);
o, taste-pore through
which the taste-cells
communicate with
the mucors surface.
— (After Piersol.}

THE SENSE OF SMELL 677

ind smell that they are extremely difficult of classification. Nevertheless
Ax primary tastes can be recognized: bitter, sweet, acid or sour, salt or saline,
ilkaline and metallic. Though the contact of any bitter, sweet, acid, salt,
jtc., substance with any part of the tongue will, if the substance be present
n sufficient quantity or concentration, develop a corresponding sensation,
iome regions of the tongue are more sensitive and responsive than others.
[Thus, the posterior portion is more sensitive to bitter substances than the
.nterior; the reverse is true for sweet substances and perhaps for acids and

.lines.

The intensity of the resulting sensation in any given instance will depend
m the degree of concentration of the substance, while its massiveness will
lepend on the area affected.

THE SENSE OF SMELL

The physiologic mechanism involved in the sense of smell includes the
nasal fosss, the olfactory nerves, the olfactory tracts, and nerve-cells in those
I areas of the cortex known as the uncinate convolution and anterior part
the gyrus fornicatus. Peripheral stimulation 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 vaporous 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 turbmat*
bones. Each fossa opens anteriorly and posteriorly by the anterio
posterior nares, the latter communicating 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. 1
former, the mucous membrane over the septum and superior turbmat
bone is somewhat thicker than elsewhere and covered with a neuro-epithel:

Tne i-enpnerai End-organ.-This consists of a basement membrane
supporting two kinds of cells, the olfactory and the sustentacular. T
olfactory cells are bipolar nerve-cells, the center of which contains a large
spheric nucleus. The peripheral pole is cylindric or conic ir [shape and
provided at its extremity with several bur-like Processes ^central
pole becomes the axon process and passes directly to the olfact ory buHx

The sustentacular cells are epithelial m character and, as their nam
imnlies suDDort or sustain the olfactory cells.

Fo^hePappreciation of odorous particles the air must be drawn through
the nasal foss* with a certain degree of velocity. If the particles are widely
doused in ttelir they must be drawn not only more quickly but more
*$& nto contact wi'th the olfactory hairs, as in the act of sniffing the
result of short energetic inspirations. To many substances the oHf
apples is exttemfly sensitive. Thus, it has been shown that a parUcle
of mercaptan the actual weight of which was calculated
nf a milligram eives rise to a distinct sensation.

TheStory Sensations. -The sensations which arise m consequence
of the excitation of the olfactory apparatus are very numerous and their

678 TEXT-BOOK OF PHYSIOLOGY

classification is extremely difficult. For this reason it is customary to divide
them into two groups: viz., agreeable and disagreeable, 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 parl
in the acquisition of knowledge. 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 XXIX
THE SENSE OF SIGHT

The physiologic mechanism involved in the sense of sight includes the
jyeball, the optic nerve, the optic tracts, the thalamo-occipital tract or the
Dptic radiation, and nerve-cells in the cuneus and adjacent gray matter.
jPeripheral stimulation of this mechanism develops nerve impulses ^ which
transmitted 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, distance, and direc-
tion 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 turned in
any direction by the contraction of the muscles attached to it. ^

The ball is spheroid in shape, measuring about 24 millimeters in its
antero-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.

, ,

»Tcor».a.-Tte n 1. the thick op,,™ membrane
of th, ball. It is compo,«do layers o

tL optic axl By virtue of its firmness and density the sclera gives form to
meridian 11 mm. The curvature is, therefore, sharper in the latt

68o TEXT-BOOK OF PHYSOLOGY

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 trans-
parent fibrils of connective tissue continuous with those found in the sclera.
Lymph-spaces are present throughout the cornea, in which are to be found
lymph-corpuscles. The anterior surface of the cornea is covered with several
layers of nucleated epithelium supported by a structureless membrane, the
anterior elastic lamina. The posterior surface also is covered by a layer of
epithelium supported by a similar membrane, the posterior elastic lamina or
the membrane of Descemet, which at its periphery becomes continuous

Visual axis Optic axis

^^jSt?"\

Suspensory li^ ^^\\ ~^<\ f..-fris

/\ /Aqueou\s\ hmnor \.><

'',;.A\/ - - \ CN. v V

Equatorial \ diameter

Optic

FIG. 286. — THE RIGHT EYE VIEWED IN HORIZONTAL IN SECTION. — (After Toldt.)

with the iris. At the junction of the cornea and sclera there is a circular
groove, known as the canal of Schlemm.

The posterior elastic lamina, near the margin of the cornea, breaks up
into fibers to form a network structure, the intervals between the fibers of
which are known as the spaces of Fontana. These spaces are in communi-
cation with the canal of Schlemm.

The Chorioid, Iris, Ciliary Muscle, and Ciliary Processes. The
Chorioid. — 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 composed
largely of blood-vessels, arteries, capillaries, and veins, supported by con-
nective tissue. Externally it is loosely connected to the sclera; internally it
is. lined by a layer of hexagonal cells containing black .pigment which,

THE SENSE OF SIGHT 681

iinough usually described as a part of the chorioid, are now known to belong,
embryologically and physiologically, 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 con-
i rained 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 tem-
perature to the retina in contact with it.

The Iris.— 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 circum-
ferential 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 supporting 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 cor-
nea The posterior surface is formed by a thin structureless membrane sup-
porting a layer of pigment cells continuous with those. lining the chorioid.
The color which the iris presents in different individuals depends on the rela-

on the ciliary border of the same. h. Ligamentum pectmatum.-(4/to

tive amount of pigment in the connective-tissue corpuscles. In blue eyes the

Thl ^sctnltsrof the non-striated variety and arranged in two sets,

of the iris Contraction of this band of fibers diminishes, relaxation increases,
the size of the pupil. This muscle is known as the sphmcter pup** <
sphincter ™- more or less continuous layer in the posterior

682

TEXT-BOOK OF PHYSIOLOGY

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 an increase 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 653).

The Ciliary Muscle. — The ciliary muscle is a gray circular band about two
millimeters in width, consisting of non-striated muscle-fibers. The majority
of 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 in-
serted into the chorioid coat opposite the ciliary processes. The inner por-
tion of the muscle is interrupted by bundles of fibers which pursue a circular
direction (Fig. 287). They collectively constitute the annular or ring
muscle of Muller. 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 cho-
rioid coat, and for this reason
it is frequently termed the ten-

External limiting membrane. SOr choriOldeCB.

The Retina.— The retina
is the internal coat of the eye,
extending forward almost to
the ciliary processes, where it
terminates in an indented bor-
der, known as the ora serrata.
In the living condition it is
clear, transparent and pink
in color. After death it be-
comes opaque. The retina is
abundantly supplied with
blood-vessels, derived from
the arteria centralis retina, a
branch of the ophthalmic,

FIG. 288.-VERTicAL SECTION or HUMAN RETINA. which pierces the optic nerve

near the sclera, runs forward
in its center, to the retina, in
The veins arising from the

Pigment-layer (not shown) .

2. Layer of rods and cones.

4. Outer nuclear layer.

Outer molecular layer.
Inner nuclear layer.

7. Inner molecular layer.

8. Layer of ganglion cells.

9. Layer of nerve-fibers.

288. — VERTICAL SECTION OF HUMAN RETINA.
—(Schaper.)

which its terminal branches are distributed
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 about 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 lutea. 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
pptic nerve.

The retina is remarkably complex in structure, presenting an appearance,
when viewed microscopically, something like that represented in Fig. 288,
indicating that it is composed of different cellular elements arranged in
layers. These have been named, from behind forward, as follows:

THE SENSE OF SIGHT

683

1. The layer of pigment cells.

2. The layer of rods and cones, or Jacobson's layer.

3. The external limiting membrane.

4. The outer nuclear or granular layer.

5. The outer molecular or reticular layer.

6. The inner nuclear or granular layer.

7. The inner molecular or reticular layer.

8. The layer of ganglion cells,
o. 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
fibers of Muller. 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 neu
glia tissue and the manner in which they unite to form the above-mentioned
layers are schematically shown in Fig. 289.

The pigment-layer is composed of
hexagonal cells. Though formerly
described as forming a part (the inner
layer) of the chorioid, these cells belong
embryologically to the retina. From
their retinal surf ace delicate pigmented
processes extend into and between
the rods and cones. On exposure to
light these procesess elongate and
push themselves between the rods.
In the dark they retract and withdraw
into the cell-body.

The visual cells which form the
layer of rods and cones are of two varie-
ties, the rod shaped and the cone

shaped.

The rod-shaped visual cell con-

£& £$ £S?«3££ rsj&.&T4sx

Of Jacobson's membrane and a fane ^inner granuies). F. Inner molecular layer.

fiber containing a nucleus, which,

FIG. 289.— CROSS-SECTION OF THE RETINA
FROM A MAMMAL. A. Layer of rods and

^GangUon ceU,
n« .f ^

Lower ramifica.

containing

after piercing the external limiting f ^ ^^ ^

membrane, passes into the outer mo- tion of a bipolar rod. «• I

lecular iayer! where it terminate „ , a ^ ^— Lgl

spheric enlargement. 1 he outer poi ^ z Bipolar contact of rods and cones t.

tinn of the rod is clear and homo- Muller's supporting fibers. S. Centrifugal

geneous, though containing a pigment nerve-fiber,-^ — , <W
known as visual purple or rhodopsm;

684

TEXT-BOOK OF PHYSIOLOGY

ing 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 fourteen rods to one cone. In the macula the rods are
entirely absent, cones alone being present.

The layer of visual cells together with the neuroglia constitutes the first
of the three layers of the retina proper. The external limiting membrane is
formed by the bending of the ends of neuroglia cells.

The bipolar cells consist of a central portion, found in the inner nuclear
layer, from which are given off two processes which pass in opposite direc-
tions, one toward the visual cells, the other toward the ganglion cells. The

FIG. 290. — 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, i.
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.")

former terminate in tufts which arborize around the tufts and spheric en-
largements of the visual cells, and assist in the formation of the outer molec-
ular 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 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 processes 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 molecular layer.

THE SENSE OF SIGHT 685

From the relation of the ganglion cells, in which the optic nerve-fibers
nake their origin, to the visual cells and the bipolar cells, the former may
Ibe regarded as the terminal visual organ, the intermediary between the
lether vibrations and the ganglion cell. The visual cells are directed toward
the chorioid, away from the entering light, dipping 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 reduced 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. 290).

It is estimated that the optic nerve contains about 500,000 nerve-nt rs,
and that for each fiber there are about 7 cones, 100 rods, and 7 pigment
cells In accordance with this estimate there would be about 3,500,000
cones, 50,000,000 rods, and 3>5°°>000 pigment cells. The distance be-
tween the centers of two adjacent cones in the fovea is 4 micromillimeters.
The Refracting Media.— The refracting media enclosed by the fore-
going membranes are the aqueous humor, the lens and the vitreous humor.
The Aqueous Humor.— The aqueous humor is small in amount in compari-
son with the 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 it into an anterior and a 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
this origin it passes through the pupil into the anterior chamber,
to keep the cornea tense and smooth. The ocular tension depends partly
on the presence of this fluid in the eyeball. There is every reason for believ-
ing that there is a constant stream of fluid from the blood-vessels into the
eye and from the eye through the spaces of Fontana at the base of the iris
into the canal of Schlemm, and so into the blood. Any interference with the
exit of this fluid rapidly increases the intra-ocular tension. Inasmuch as
the aqueous humor has the same refracting power as the cornea the two
be regarded as a single body. .

' The Lens —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^
mm., the diameter about 9 mm. It consists of a transparent capsule contain
ing elongated hexagonal fibers which, having their ori.m near the anterior

the anterior surface of the lens near the equator. The space between he
two kyers of "he ligament is the canal of Petit. The anterior surface of the
Sament presents a series of plications conforming to correspondmg &<*

largest of the refracting

686 TEXT-BOOK OF PHYSIOLOGY

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 con-
cavity, in which the crystalline lens is lodged. The vitreous humor consists
of water (97 per cent.), organic matter and salts, enclosed in a transparent
membrane, the tunica hyaloidea. The mass of the vitreous humor is pene-
trated by a species of connective tissue.

These bodies, the cornea and aqueous humor, the lens and the vitreous
humor, together form a refracting system by which parallel rays of light,
which enter the eye when it is in repose, are converged and brought to a focus
on the retina.

The relations of the parts entering into the structure of the eye are shown
in Fig. 286.

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 percipient elements oi
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, that stimulate 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 re-
spects as far as space relations go, does the image on the retina differ from
the appearance of the object, aside from the fact that the object has usually
three, the image only two, dimensions — viz., in size and position. What-
ever the distance, 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 side of the image.

The Dioptric or Refracting Apparatus. — The formation of an image is
made possible by the introduction of a complex refracting apparatus consist-
ing 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, homo-
centric rays — arriving at the eye must traverse successively the different re-
fracting media. In their passage from one to the other, they undergo at the
surfaces changes in direction before they are finally converged to a focal point.
In order to follow mathematically the rays in all their deviations through the
media, to determine their focal points and to construct an image, a knowl-
edge of the form of the refracting surfaces, the refractive indices of the dif-
ferent media, and the distance of the surfaces from one another must
be known.

The following constants are now accepted: The radius of curvature
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,
T-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 prac-

THE SENSE OF SIGHT 687

tically parallel, and as the index of refraction of the aqueous humor is the
same as that of the cornea, they may be regarded 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

Parallel rays of light entering the eye pass from 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

j the lens is congenially absent or

i has been removed by surgical pro- ^

; cedures. Perfect vision, however, ^ 2 _RErRACTION OF HOMOCENTRIC
requires that the convergence ot RAYS AND TBE FORMATION OF AN IMAGE.

sESSSSittSM: trass

sufficient to focalize them on the

in every complicated system oi ^ re*a g^ .^ ^ ^.^

spheric surfaces there may be assu ^ ^ and

which the system may ^Sm. either by calculation

properties be k™^J^path of the refracted ray, and the position

if those of the object in the first

medium be known. placed as Gauss showed, by a single

um e . placed as Gauss showe, y a snge

ZSSSJSSfJSA * +- «-*- • *•

-f t tV,p rornea is not parallel to the anterior surface,

• Strictly speaking, the postenor •»»<•«<*** ^£ Ln Aat of the aqueous humor, viz :
the index of refraction of .the ~™^ ".^rt^ due to the higher index is almost exactly
* « .

and e n . .rt ue o

,377. But as the increasn * ^«\^e h{ghl curvature of tte postenor cornea.

accurate results-

688

TEXT-BOOK OF PHYSIOLOGY

common axis — e.g., two focal points, two principle points, two nodal points, ]
two focal planes, two principal planes, and two nodal planes.

Properties of the Cardinal Points. — The first focal point, Fv in Fig. j
292, has the property that every ray which before refraction passes through1
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, -H\; that is,
rays in the first medium which go through the first principal point pass after

FIG. 292. — DIAGRAM SHOWING THE POSITION AND RELATION OF THE CARDINAL POINTS.

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 which in
the first medium is -directed to the first nodal point passes after refraction
through the second nodal point, and the direction of the rays before and
after refraction are paralled to each other. In Fig. 292 let A B represent
the axis. The distance of the first focal point, Fv from the first principal

FIG. 293. — DIAGRAM TO FIND THE IMAGE IN LAST MEDIUM OF A LUMINOUS POINT IN

THE FIRST.

plane, Hv is the anterior focal distance. The distance of the second focal
point, F2, from the second principal plane, H2, is the posterior focal distance.
The distance of the first nodal point, Nv from the first focal point, Fv is equal
to the posterior focal distance H2 F2. The distance of the second nodal
point, N2, from the second focal point, F2, is equal to the anterior focal
distance, Hl FL. It is evident, therefore, that the distance of the corre-
sponding 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 proportional to the

THE SENSE OF SIGHT

689

•efractive 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 determined, and the

bourse of a refracted ray in the last medium be constructed if its direction in

'the first be given according to the following rules:

fi. To find the image in the last medium of a luminous point in the first : Let
A (Fig. 293) be this given point. Draw A B parallel to the axis until it
meets the second principal plane in £; then B F2 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 second nodal point parallel to A C.
This will be the refracted ray in the last medium. Where the two re-
fracted rays, BF2 and D £, intersect, the image of A will be Ar*

' 2 To find the refracted ray in the last medium of a given ray in the first
medium: Let A B (Fig. 294) be the given ray. Continue this ray until

™ or A GIVEt

it meets the first principal plane in C. Draw C D

Now assume any point, such as E, in the given ray, and find its

E, by the Rule i. Then D E, becomes the course of the refract

The'Schematic Eye.-Accepting the system of cardinal points, Listing,
Bonders, and v. Helmholtz have constructed "schematic" eyes to be sub-
ctitntpH for the refracting system of the natural eye.

For thfs purpose U is necessary to make use of the various estunates of
^he indices of refraction of the different media, of the radii of curvatures
3 the different refracting surfaces, and of the distances separating them to
deduce an Ivlge eye ^a basis for calculation. The most widely accepted
attempts thatofv. Helmholtz. The data he assumed are as follows: The
re Se index of air-i; of the cornea and aqueous humor, 1.3365; of he

„ the point A is infinite,,

A, owhere all paraUe, to the n

meet.

69o

TEXT-BOOK OF PHYSIOLOGY

mm. before the anterior surface of the cornea; the second focal point i
situated 15.619 mm. behind the posterior surface of the lens; the firs
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 foca
distance of this schematic eye, the distance between Fl and H19 there
fore amounts to 15.498 mm., and the posterior focal distance, H2 to F2, t<
20.713 mm.

When the eye, however, is accommodated for near vision, the relation
of the cardinal points are changed and will be as follows, if the point accom
modated for lies 152 mm. from the cornea: Anterior focal distance, i3-99<
mm.; posterior focal distance, 18.689 mm-5 distance from cornea of the firs

FIG. 295.— DIAGRAM SHOWING THE POSITION OF THE CARDINAL POINTS IN THE "SCHEMATIC
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.

and second principal points, 1.858 and 2.257 mm- respectively; distance of
the posterior focus, 20.955 mm- fr°m cornea. Given this schematic eye in the
accommodated state, the course of the rays and the determination 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 disre-
garded, 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

THE SENSE OF SIGHT

aodal point, which latter becomes the center of curvature of the single re-
dacting surface. The dimensions of this "reduced" eye are as follows (see
:?ijr 206) From the anterior surface of the cornea, corresponding to the
brincipal plane H, to the nodal point N, 5.215 mm., from the anterior focal
point F , to the principal plane H, i.e., the anterior focal distance/ , 15.49
tarn • from the principal plane H to the posterior focal point Fv i.e., the
boste'rior 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
la radius of curvature, r, of 5.125 mm. In such an eye luminous rays emanat-
ing from the anterior focal point are parallel to the axis after refraction in
the interior of the eye. Also rays parallel to the axis before refraction unit.

at 'fly mean7off this reduced eye the construction of the refracted ray, the
various calculations as to the size of the image, the size of diffusion c es,

hJ*^*?£^ an object. From A a pencil of rays falls
on the single refracting surface. One of the rays, the nodal ray, falling on
Ae surface perpendicularly, passes unrefracted through the single nodal point,

TT FIG 207 —THE FORMATION OF AN IMAGE IN THE

FIG. 296.— THE REDUCED EYE. REDUCED EYE.

N to the posterior focal plane. The remaining rays, partially r.
n the figure falling on this surface under varying degrees of

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 ^^ ^
A B- a b=fN:N g, or a fc—^^X' and therefore /— p+r'

692

TEXT-BOOK OF PHYSIOLOGY

FIG. 298. — 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) ft. — (From Stewart.}

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 distance 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 diameter
of the object by 15.5 mm. and divide
by the distance of the object from
the eye.

The Visual Angle.— The visual
angle is defined as the angle formed
by the intersection 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 in-
verted or reversed image of the object on the retina. The size of the
visual angle increases with the nearness and decreases with the remote-
ness of the object; the retinal image correspondingly increases and de-
creases in size. These facts will become apparent from an examination of
Fig. 298. As the size of the retinal image diminishes when the visual angle
diminishes either as a result of the removal 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 separately. When this limit is reached the size of the angle ex-
pressed 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 01
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 oi
such an object or interval, to the nodal point, subtend an angle of 60 seconds.1
Beyond this the two points are indistinguishable. In other words the
emmetropic eye possesses the power of distinguishing the correspondingly

1 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. 316, in which A B represents
the size of the object 1.454 mm.;
N, the nodal point; CAT, the
line which bisects the object,
represents the distance of the
object from the nodal point;
fl, the visual angle subtended
and whose value it is desired
to know, and b one-half of the
angle a. By trigonometry the
size of the angle a can be de-
termined in the following way: FIG. 209.— FIGURE SHOWING THE METHOD OF OBTAINING
one-half the size of the object THE VISUAL ANGLE EXPRESSED IN DEGREES OR FRACTION
A B, is divided by its distance OF A DEGREE OF AN ARC.
from the nodal point; the quo-
tient is the tangent of half the angle. Thus 0.727 -5- 5000 =-0.0001454. By reference to tables ol
natural tangents, 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.

THE SENSE OF SIGHT 693

small interval between the two images on the retina of the two objective
points. The size of the image or the interval between the two retinal points,
determined from the foregoing factors by the formulae on page 692 is 0.004
mm., which would correspond 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 cones on the retina, correspond-
ing 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

FIG. 300.— STANDARD LETTERS, FOR TESTING THE ACUITY OF VISION.

(for the same visual angle) it is evident that the object would have to be one-
fifth of i 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. by the distance. The standard letters
are so constructed that each is inscribed within a square, the sides of which
at a specific distance, 5 meters, subtend an angle of 5 minutes and which is
again subdivided into 25 small squares each side of which subtends an angle
of i minute. These partial little squares correspond to the details of the
letter while the whole letter of course, embraces an angle of 5 minutes both
as to height and to breadth (Fig. 300). The letter that could be distinctly
seen at a distance of 5 meters, would have, therefore, a vertical and a hori-
zontal dimension of 5 times i.454 mm. or 7.27 mm. (Fig. 300 A), and
meters corresponding dimension of 14-54 mm., etc. (Fig. 300^.)

If with the accommodation suspended, the emmetropic eye could
clearly distinguish at a distance of 5 meters a letter 7.27 mm. m size which
would, therefore, subtend an angle of 5 minutes, then the acuity of the
vision would be normal and could be expressed as follows: V =^or V -T*
If on the contrary at this distance the smallest letter that could be clearly
seen is one that would subtend an angle of 5 minutes at a distance of 10 meters
then the visual acuity would be only one-half the normal, and could be
expressed as follows: V = A or V - J, etc. The acuity of vision is expressed,

694

TEXT-BOOK OF PHYSIOLOGY

FIG. 301. — THE REFRACTION or PARALLEL AND
DIVERGENT RAYS IN THE EMMETROPIC EYE IN THE
PASSIVE AND IN THE ACTIVE OR ACCOMMODATED
CONDITION.

therefore, by a fraction the numerator of which is the distance at which the;
test is made and whose denominator is the distance at which the smallest!
letters distinguished by the patient subtend an angle of 5 minutes, or inj

other words the distance at
which the patient reads di-j
vided by the distance at which]
he ought to read the smallest!
letters seen by him on the!
chart.

Accommodation. — Ac-
commodation may be denned
as the power which the eye,
possesses of adjusting itself to
vision at different distances;
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 cannot focus
rays coming from a distant
and a near object at the same time. Thus, if an object is held before
one eye at a distance of 22 centimeters, for example, and the vision
is directed to a distant object it is evident that the near object is indistinctly
seen; but if the vision is then directed to the near object, it in turn becomes
clear and distinct, while the distant object becomes blurred and indistinct.
It is evident, therefore, that rays of light coming from a distant and a near
object cannot be simultaneously, but only alternately, 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, parallel rays of light (Fig. 301, 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 intercepted by the retina before they
are focused, and give rise to the formation of diffusion-circles and indis-
tinctness of vision. The reverse is also true. When the eye is adjusted
for the refraction and focusing of divergent rays (Fig. 301, P) parallel rays
will be brought to a focus before reaching the retina, and, again diverging,
will form diffusion-circles. It is evident, therefore, that it is impossible
to focus simultaneously both parallel and divergent rays, and to see dis-
tinctly 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 :

istance of Luminous Point.

5 m.
e m.
2 m.
5 m.
- m'.
i room.
o'.75om.
0375111.
o i88m.
0.094 m.
0.088 m.

THE SENSE OF SIGHT

Distance of the Focal

Point behind the Posterior

Surface of the Retina.

o.o mm.
0.005 mm.
0.012 mm.

695

Diameter of the Diffusion-circle.

0.050 mm.
o. 100 mm.
0.20 mm.
mm.

mm'
mm.

mm.
mm.

0.40

i. 60
3-20
3-42

o.o

o.oon

0.0027

0.0050

0.0112

o.o222

0.0443

0.0825

o.i6i6

0.3122

0.5768

0.6484

mm.
mm.
mm.
mm.
mm.
mm.
mm.
mm.
mm.
mm.
mm.
mm.

From the foregoing table it is evident that between infinity and 65 meters,
he diffusion-circles are so slight that no perceptible accommodative effort
s required to eliminate them. From 65 meters to 6 meters the diffusion-
ircles gradually become larger, though they are yet so faint as to require
or their correction an accommodative effort which is scarcely measurable.
rom 6 meters up to 6 centimeters, however, a progressive increase in ace
modative power is demanded for distinct vision.

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
of adjustment, however, for near vision the eye passes into an active £ te,
the result of a muscle effort, the energy of which is proportional to the near-
ness of the object toward which the eye is directed.

Mechanism of Accommodation.-Inasmuch as neither the corneal
curvature nor the shape of the eyeball undergoes any change during accom-
modation, 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 and
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 tnree
images in the eye, one at the surface *****^
two at the pupillary margin (Fig. 302). Of the two latter,
one is auite large and situated apparently in front ot the
small, and inverted. The midcUe

of the lens> c. In_
verted image, from

tnira, wmcii is i«miu, »»*• .

image is_reflected 6^JJ^j£S±MS

rtw

ccording to Helmholtz, during accommodation the entire antenor sur-

696 TEXT-BOOK OF PHYSIOLOGY

face of the lens becomes more convex, while at the same time it slightly ad-
vances, possibly as much as 0.4 mm. in extreme efforts. This change is
represented in Fig. 303. According to Tscherning, the increase in con-
vexity of the anterior surface is confined to the central portion, the re-
mainder of the surface becoming somewhat flattened. There is, moreover, j
no evidence that there is any advance of the surface or any increase in the!
thickness of the lens. A series of new and ingenious experiments lend!
support to Tscherning' s view, though of late Hess has brought forward defi-
nite experimental evidence in favor of the view of Helmholtz. The radius
of curvature in either case approximates 6 mm. in extreme efforts of ac-
commodation. The increase in convexity naturally increases the refracting
power.

Whichever view is accepted, the nearer the object — that is, the greater the
degree of divergence of the light rays — the more pronounced must be the
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. Coincidently with the lens change, the pupillary margin advances
and the pupil itself becomes smaller. By this means an indistinctness of the

FIG. 303 . — THE LEFT HALF REPRESENTS THE EYE IN A STATE OF REST. THE RIGHT
HALF IN STATE OF ACCOMMODATION.

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
aberration.

The Function of the Ciliary Muscle.— Though it is generally admit-
ted that the increase in the convexity of the lens is caused by the contrac-
tion 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 ligament. 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 for-
ward and becomes more convex. In consequence of this latter fact the
refracting power is proportionally increased. In extreme efforts of accom-
modation it is believed by some observers that the- circularly arranged
fibers, the so-called annular muscle, contract anil exert a pressure on the
periphery of the lens and thus aid other mechanisms in relaxing the liga-
ment 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

THE SENSE OF SIGHT 697

very much hypertrophied, thus reinforcing the meridional fibers. In
myopia, on -the contrary, where the accommodative effort is at^a mini-
mum, the entire muscles 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 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 wit.
Tscherning'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 required for thei:
focalization. So long as the luminous point remains between infinity and
6s meters, the eye, directed toward it, remains 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
at ^distance of^ meters or beyond.1 If the luminous point ^aduaUy
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
Sctelslffl its near point or puncture proximum This near point in a
healthy boy of twelve years will lie at a point situated 7 cm. from the eye
while t^e same point lies 20 cm. distant in a man of forty years Of ol > ects
which lie nearer than the punctum proximum the eye **«&

images. The distance between the punctum remotum and the
mum is termed the range of accommodation.

: l™ the refracting power of which is such as to enable it to produce the

lSfeasi

b Tme,,ur, of the force «,««<* to

ut. o fTo'cm ™d b me,,ur, of the force «,««<* to «ch a la.

aftsa««s»5 ' - - - - - - -

698

TEXT-BOOK OF PHYSIOLOGY

point to a focus on the retina. A lens of this character is said to have a
refracting power of 5 dioptrics.1

The refracting media of the human eye in repose have collectively a
refracting power of about 64 dioptrics, the reciprocal of its anterior focal
distance. The refracting power of the corneal surface alone is equivalent to
42 dioptrics. The crystalline lens by reason of its relations and situation
in the optic media has a refracting power of about 20 dioptrics.

The capability of the lens to increase its refraction during accommodative
efforts beyond the 20 dioptrics varies considerably at different periods of life.
From youth to old age, the elasticity of the lens steadily declines, and the
range of accommodation diminishes from the recession of the near point.

The following table shows the decrease in the accommodative power with
increasing years, the shortening of the range of accommodation and the
distance to which the near point has receded from the eye.

Age, Years

Range of Accom-
modation, Dioptrics

Near point,
cm.

Age, Years

Range of Accom-
modation, Dioptrics

Near point,
cm.

14.0

7.0

3-5

28.0

10. 0

IO.O

5°
60

2-5
i .0

40.0
IOO.O

7.0

14.0

Zero

4-5

22 .O

Convergence of the Eyes during Accommodation. — When the eyes
are at complete rest and directed to some far distant object the visual lines
are parallel and the optic axes are directed outward at an angle of about
5 degrees, which is known as the angle alpha. If, however, it is desired to
see distinctly with both eyes, any object within the range of accommodation,
the eyeballs must be converged toward the median line, the object being to
enable the rays of light emanating from a point to fall directly into the foveae
so that the two retinal images shall give rise to but a single impression and
thus prevent double vision or diplopia which would otherwise result. Con-
vergence of the eyeballs, therefore, increases as the visual axes are directed
toward objects which are placed progressively nearer the eyeballs. This is
accomplished by the conjoint and harmonious action of the internal recti
muscles balanced by the action of their physiologic antagonists — the external
recti muscles. Convergence of the eyeballs is always accompanied by in-
crease in the accommodation, the two being necessary for distinct binocular
vision.

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

1 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 i 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 re-
fractive power of 2, 5, 10 dioptrics respectively, obtained by dividing 100 cm. by 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, and using a
corresponding negative figure. Thus a lens which diverges parallel rays in such a way as to make
them appear to radiate from a point 20 centimeters behind itself is said to have a refractive power of
minus 5 dioptrics.

THE SENSE OF SIGHT 699

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 respectively contract and relax with increasing and decreasing in-
tensities of the light. The size of the pupil, therefore, through which the

L 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 pupillae 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 sphincter contraction is greater when
the light falls directly upon the fovea. Contraction of this muscle is 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 con-
tractions and relaxations of. the sphincter pupilla and dilatator ^puptila
muscles. The contraction of the sphincter is entirely reflex and involves
those structures necessary to the performance of any reflex act, viz.: a recep-
tive 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. I he
contraction of the dilatator pupilte muscle is determined by the activity
of a continuously active nerve-center in the medulla oblongata which trans-
mits its nerve impulses through the spinal cord, along the first and second
dorsal nerves to the superior cervical ganglion, and thence to the ms by
way of the fifth nerve. (See Fig. 268, page 620.) These two muse
appear to bear an antagonistic relation to each other, or section of the motor
oculi is followed by relaxation of the sphincter muscle and dilatation of the
Jupil. Stimulation of the sympathetic is followed by a more >'?™
dilatation. The size of the pupil is the resultant of a balancing o
two forces.

OPTIC DEFECTS

yopia —Presbyopia may be defined as a condition of the normal

ic^

impossible at ordinary distances. As age advances the lens , gradu-
atoSd* and hence its power to increase in convexity and thick-
ness to the s^e extent as in earlier life, in response to efforts of accommo-
da on The^efractive power is, thereby, lessened and the eye is no longer
ab e to see distinctly at the normal reading distances, viz. : 22 to 28 cm.

rom a luminous point at the normal reading distances are

and hence the diffusion circles mcrease

;oo

TEXT-BOOK OF PHYSIOLOGY

in size. The near point, the point from which divergent rays can be focal-
ized, therefore advances toward the far point, or recedes from the individual.
The range of accommodation is, thereby, diminished. At forty years the near
point is about 22 cm., indicating an increase in refracting power on the part
of the lens of 4.5 dioptrics; at forty-five years the near point has receded to
28 cm. This indicates that the lens at this period has only a refracting
power of approximately 3.5 dioptrics, showing therefore a loss in the five
years of i dioptry; at fifty years the near point recedes to 40 cm., and at sixty
to 100 cm., indicating a loss in refracting power, ptf the part of the lens of
2 and 3.5 dioptrics respectively. Convex lenses placed before the eyes hav-
ing a refracting power of i, 2, and 3.5 dioptrics 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 4.5
dioptrics would therefore be required by such a man to return the near point
to its normal position, 22 cm. from the eye.

Myopia. — Myopia may be defined as a condition of the eye characterized
by .an increase in the antero-posterior diameter or by a hypernormal refrac-
ting power of the lens. The former is the usual condition. In either case
parallel rays of light which enter the eye are brought to a focus in front of

FIG. 304. — MYOPIA. Parallel rays
focus at F, cross and form diffusion-
circles; divergent rays from A focus
on the retina.

FIG. 305. — CORRECTION OF MYOPIA BY
A CONCAVE LENS.

the retina, after which they diverge and give rise to diffusion circles and indis-
tinctness of vision. Divergent rays, however, which enter the eye are focal-
ized as usual on the retina even in its new position. The distant point, the
punctum remotum is always at a finite 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. 304).

The increase in the length of the antero-posterior diameter may range
from a fraction of a millimeter up to 3.8 mm. With an increase of 0.16 mm.
the far point is but 200 cm. distant; and with an increase of 3.8 mm. it is but
10 cm. distant. Inasmuch as only divergent rays can be focalized by the
myopic eye normal vision can be established by placing before the eyes a bi-
concave lens with a diverging power in the first instance of 0.5 dioptry and
the second of 10 dioptrics (Fig. 305).

Hypermetropia. — Hypermetropia may be defined as 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 con-
dition. In either case parallel rays of light which enter the eye are, therefore,
not brought to a focus when the accommodation is suspended. Falling on
the retina previous to focalization, they give rise to diffusion-circles and indis-
tinctness of the image. As no object can be seen distinctly no matter how

THE SENSE OF SIGHT 701

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 eye, they would, on emerging, take a divergent direction, as shown
in Fig. 306, 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 the eye, it is termed negative. Since rays coming from
the retina take a divergent direction on emerging from the eye, it is evident

FIG. 306. — THE HYPERMETROPIC EYE. 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.

that only converging rays can be focalized by a passive hypermetropic eye.
The hypermetropic person attempts, and partially succeeds, in focalizing the
rays by increasing the convexity of the lens through an increased accommoda-
tive effort which often gives rise to accommodation fatigue and headache.
As there are no convergent rays in nature, it is necessary for distinct vision
that all rays, parallel and divergent, shall be given a convergent direction
before entering the eye. The decrease in the length of the antero-posterior
diameter may range from a fraction of a millimeter up to 2.78 mm. Normal
vision may be established by placing before the hypermetropic eye convex

FlG 30? _ HYPERMETROPIA. PAR- FIG. 308.— CORRECTION OF HYPER-

ALLEL RAYS FOCUSED BEHIND THE METROPIA BY A CONVEX LENS.

RETINA.

lenses with a converging power, in the first instance, of 0.5 dioptry and, in the
second instance, of 10 dioptrics (Figs. 307 and 308).

Astigmatism.— Astigmatism may be defined as a condition of the eye
characterized by an inequality of curvature of its refracting surfaces in con-
sequence 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.

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

702 TEXT-BOOK OF PHYSIOLOGY

distances is so slight that the error in the formation of the image is scarcely
noticeable. A transverse section of a cone of light coming from the cornea is
practically a circle. If, however, the vertical curvature exceeds the normal
to any marked extent, the rays passing in the vertical plane will be more
sharply refracted and brought to a focus much sooner than the rays passing
through the horizontal plane. 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. 309, which represents the appearance pre-
sented on cross-section both before and after focalization of each set of rays.
Though the vertical plane has usually the sharper curvature, it not infre-
quently happens as illustrated in this figure, 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.

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 demonstrated in the human
eye. That this condition is present to but a slight extent in the normal

FIG. 309. — REFRACTION BY AN ASTIGMATIC SURFACE. — (Hansell and Sweet.)

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 or 5 dioptrics.

Chromatic Aberration. — When a beam of 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 refrac-
tion 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 eye is so slight
that the mind fails to take cognizance of it. That the eye is incapable simul-
taneously of focalizing rays of widely different refrangiblity, 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

THE SENSE OF SIGHT

703

and clearly denned, the periphery blue and ill-defined. The reason
for this is dear. 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. With 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 o

the refracting surfaces. In first-class optic instruments the lenses are

i centered— that is, their optic centers are situated on the same axis. In

I 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

Temporal Jige^

eve A line passing through the center of the cornea and .the center

of the

^s about 5°. In hypermetropia it is greater, amountmg to 7 or 8 ,

704 TEXT-BOOK OF PHYSIOLOGY

giving to the eye an appearance of divergence. In myopia it is mucbi
smaller — 2° — or in extreme cases may be abolished, the line of visioni
corresponding with the optic axis or even passing beyond it. The
angle gamma is of value in determining 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 transforming 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—
are transformed into nerve impulses, but little is known. It is probable
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 differences in the intensity of light, while the cones, in

FIG. 311.— DIAGRAM FOR OBSERVING THE SITUATION OF THE BLIND SPOT —

(Helmholtz.)

addition, are impressed by qualitative differences in color. The nerve-
fibers themselves 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 Bonders, 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 be demonstrated by the familiar experiment
of Mariotte, which consists in placing before the eye two objects having the
relation to each other shown in Fig. 311. 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

THE SENSE OF SIGHT

705

Lais anatomic arrangement, a portion of the light coming through the pupil
till be intercepted by the vessels and a shadow projected on the layer of
bds and cones. Ordinarily, these shadows are not perceived, for the reason
;hat the shaded parts are more sensitive, so that the small amount of light
tiassing through the vessels produces as strong an impression on this part
Is does the full amount of light on the unshaded parts of the retina, and
Lrhaps because the mind has learned to disregard them. But if light be
fnade to enter the eye obliquely, the position of the shadows will be changed,
jvhen at once they become apparent. This can be shown in the following
>vay: If in a darkened room a lighted candle be held several inches to the
idde'and to the front of the eye, and then moved up and down, there will
oe perceived, apparently in the field of vision, an arborescent figure corres-
ponding 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.-^ retina is not equally excitable in .M parts
of its extent The maximum degree of sensibility is found m the macula
tatea and especially in its central portion, the fovea. In this region the
"almost entirely disappear, the layers of rods and cones

rx'tr *£%^&tttt*-*f * *"ked

g^l£S3is3£s^

7o6 TEXT-BOOK OF PHYSIOLOGY

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, although that it plays some important r61e 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 mem-
brane. These conditions are represented in Fig. 312.

The Eye a Living Camera. — In its construction, in the arrangement
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 simi-
larity between them; e.g., the sclera and chorioid may be compared to the
walls of the camera ; the combined refracting media to the component glasses
of the 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 of the amount of light to be admitted, and
for the partial exclusion of those marginal rays which
give rise to spheric aberration; the ciliary muscle to the
adjusting screw, by means of which the image is brought
to a focus on the sensitive plate, notwithstanding the
FIG. 313.— RETINA OF varying distances of the object from the lens. The
o^wiN^r^H PreseKnce of the visual purple in .the rods of the retina
METERS DISTANT a. capable of being altered by light makes the compan-
Yellow spot, b, b. son still more striking.

nSv^fibefs.^l^o'1 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, 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. 313). That portion
of the image corresponding to the window lights will be quite bleached in
appearance from the action of the light on the pigment, while that corre-
sponding to window bars will have the usual color of the retina. Dur-
ing life the regeneration of the visual purple must take place with extreme
rapidity if a similar change takes place with the formation of each image.
The visual purple 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

THE SENSE OF SIGHT 7°7

Lensation. If the direction of either visual axis be changed by pressure on
the eyeball, tKere 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, on two corresponding points; while 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 emanating 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 correspond physiologically are

shown in Fig. 314- In this figure the — u^ ^_L_^

retinal area is divided into quadrants FKJ _COBMSPONDING AREAS OF THE
by vertical and horizontal lines of sepa- RETINA.

ration, 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 the eye, fall on corresponding areas the sensation of but one
object arises. If, however, they fall on non-corresponding areas, two sen-
sations arise. Normal binocular vision enlarges very considerably the area
of the visual field, permits of a better estimation of the size and distance
of objects, enables the mind to form more readily a perception of depth,
and increases the intensity of sensations.

The Horopter.— When the eyes are in a so-called secondary position
—that is in a position in which the visual axes are converged 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
eve, pass through the nodal point, and fall on corresponding parts of the
two retinae and give rise to but single images. All such points he, 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. 315, in which the objects I, II, I.I project
their rays into both eyes and upon 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 will
proiect its rays on parts of the retinae which do not correspond, and hence
dve rise to double images. This is evident from examination of Fig. 316.
While the eyes are directed to figure 2, of which there is but a single image,
the objects7 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

2alAtAall times, therefore, double images are formed on the retinae the
existence of which is scarcely noticed unless the attention is directed to hem.
ThisTs due to the fact that many of the images fall on the peripheral, less
Jensitve parts of the retinae. At the same time, from a want of accommo-

7o8 TEXT-BOOK OF PHYSIOLOGY

dation and the formation of diffusion-circles, they are indistinct. For these
reasons they are readily neglected.

In the primary position of the eyes — that is, a position in which the
visual axes are parallel — the horopter is a plane at infinity. In the tertiary
positions .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 rotated 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. 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 passes
forward to the upper and inner angle of the orbit cavity, at which point its

FIG. 31 s. — HOROPTER FOR THE FIG. 316. — SCHEME OF IDENTICAL AND

SECONDARY POSITION, WITH CON- NON-IDENTICAL POINTS OF THE RETINA. —

VERGENCE OF THE VISUAL AXES. (Landois.}

— (Landois.)

tendon passes through a cartilaginous pulley, after which it is reflected back-
ward to be inserted into the superior surface 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 line of
regard. Around this antero-posterior axis the eye may be regarded as per-
forming its circular rotation or torsion. At right angles to this line, and
joining the centers of rotation of both eyes, is the horizontal or transverse

THE SENSE OF SIGHT

709

axis, around which the movements of elevation (up to 34 degrees) and de-
pression (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— respectively. 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, form-
ing 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 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 movements of the eye at least three muscles are necessary (see
following table):

Inward .

Rectus internus.

Inward and

Rectus internus.

Outward . .

Rectus externus.

downward

Rectus inferior.

Upward

Downward
Inward and

Rectus superior.
Obliquus inferior.
Rectus inferior.
Obliquus superior.
Rectus internus.

Outward and
upward

Outward and

Obliquus superior.
Rectus externus.
Rectus superior.
Obliquus inferior.
Rectus externus.
Rectus inferior.

upward

Ivcctus superior.
Obliquus 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 and tertiary.
Both eyes always move simultaneously, which is called the associated move-
ment oj the eyes. There are three forms of associated movements: (i) move-
ment of both eyes in the same direction; (2) movements of convergence by
which the visual lines are converged 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 (i) and (3) takes place for
certain positions of the object looked at.

Color-perception.— A beam of sunlight passed through a glass pns
is decomposed into a series of colors— red, orange, yellow, green, blue, and

fio

TEXT-BOOK OF PHYSIOLOGY

violet — the so-called spectral colors, so well exemplified in the rainbow.
The spectral colors are termed simple colors, because they cannot be any
further decomposed by a prism. Objectively, the spectral colors consist of
very rapid transverse electro-magnetic 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 colors, either
at the same time or in rapid succession. The following table shows the
results of such experiments as performed by v. Helmholtz (Dk. =dark;
Wh. -whitish):

Violet

Indigo

Cyan-blue

Bluish-green

Green

Greenish-
yellow

Yellow

Red.
Orange.
Yellow

Purple.
Dk.-rose.
Wh -rose

Dk.-rose.
Wh.-rose.
White

Wh.-rose.
White.
Wh -green

White.
Wh.-yellow.
Wh -yellow.

Wh.-yellow.
Yellow.
Gr -yellow

Gold-yellow.
Yellow.

Orange.

Gr.-yellow.
Green.
Bluish-green.

White.
White-blue.
Water-blue.

Wh.-green.
Water-blue.
Water-blue.

Wh.-green.
Bl.-green.

Green.

Cyan- blue

Indigo

These are the mixed colors. But it is to be observed that only two new color-
sensations can be produced, white and purple, the remaining 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^ODtained, however, by adding color pig-
ments. 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 we must distinguish three primary qualities: (i)
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 depending 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 differ-
ently, as is shown by the following table giving the limits up to which the
colors are recognized:

White. Blue. Red. Green.

80°

55°

Externally 90°

Internally 60°

Superiorly 45°

Inferiorly 70°

40"
60°

65°
50°

35o

45°

5°o

40°
30°
35°

THE SENSE OF SIGHT 711

Theories of Color-perception. — The theory of v. Helmholtz, originated
by Thomas Young (1807), assumes in its latest form the existence in the
luman retina of three different kinds of end-organs, 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 sub-
stance, which is affected mainly by the red part of the spectrum; the second
rroup 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-sensations
arise. If a light mainly excites the red- or green- or blue-sensitive substance
of a retinal area, we term it red, green, or blue, respectively. BuHf two of
these photo-chemical substances are stimulated simultaneously, quite differ-
ent sensations arise. Thus simultaneous 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.
Simultaneous stimulation of all three substances of a certain area produces
the sensation of white. According to this theory, complementary colors
are any two which together excite all 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 underlying
assumption that the process of restitution in a nerve-element is capable of
exciting a sensation. This theory asserts that there are three visual sub-
stances 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 assimila-
tion in this substance produces the sensation of black. Similarly, red light
produces dissimilation or decomposition in the red-green substance, and
this, again, the sensation of red. Green light, however, favors the process
of 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 dissimilation and assump-
tion in the same visual substance antagonize each other, and consequently
produce no color-sensation by means of this substance but only
sensation of white, by reason of decomposition by both colors, in i
white-black substance. Thus, 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 produce only the sensat on
ofS as both yellow and blue decompose the white-black material
Co^rbUndness is explained by the assumption of the absence of either the
red-green or the yellow-blue visual substance in the retina.

7I2 TEXT-BOOK OF PHYSIOLOGY

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 support-
ing 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 by a semilunar plate of dense
fibrous ^ tissue, the tarsus. The cutaneous edge of the lid is bordered with
short stiff hairs. At the inner extremity each eyelid presents a small opening,
the punctum 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. Their ducts open on the free border of the lid.
These glands secrete an oleaginous material 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 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 distributed 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 XXX
THE SENSE OF HEARING

The physiologic mechanism involved in the sense of hearing includes the

ear, the acoustic nerve, the acoustic tract (the lateral fillet or lemniscus), the
acoustic radiation, and nerve-cells in the cortex o the temporal lobe.

Peripheral stimulation of this mechanism develops nerve impulse which,
transmitted to the cerebral cortex, evoke the sensation of sound and
ins Qualities— intensity, pitch, and timbre.

The specific physiologic stimulus to the terminal organ, the organ of
Corgis the impact of atmospheric pulsations of varying energy and rapuhty.

THE PHYSIOLOGIC ANATOMY OF THE EAR

The ear the organ of hearing, is lodged within *e petrous portion of

(FiTh3eExternal Ear.-The external ear consists of the ***** or auricle

and the external auditory canal. cartilage which

The Pinna.— The pinna is composed of a thin layer ol caruiag

5TL±r a^itAelow the Liter, a

d"SS£S£^

glands. They secrete cerumen or ear-

IS^ii^sSS'Si-SSlBSE

external auditory canal and the ^^^L long in its antero-posterior
though wider above than J^l°w- ^ portion is known as the attic. The
and vertical diameters. _ ^nU^seterioriy with the mastoid cells, anteriorly

with the" ph^ynx ^S^J^^ the tympanic cavity and

1 ne tLUSi acni'd A^^^nir^f^T QQtVip Eustacnian tuue. IL

7'4

TEXT-BOOK OF PHYSIOLOGY

membrane covered with ciliated epithelium. Near the middle of its course
the tube is contracted, though expanded at either extremity (Fig ,17)
measures about 40 mm. in length. Its general direction from fhe pharyn-
de rees * °UtWard> backward' and upward at an angle of about 45

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 cavities

The interior of the cavity is crossed from side to side by a chain of

hTnx a mUC°US membrane continuous with that lining the

The Membrani Tympani.-Tte 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

ctdMonWr TK 6 fCtal Tdid°n Can be easi'y removed- but in the adult
idition cannot be removed, owing to its consolidation with the surrounding

This membrane consists primarily of a layer of fibrous tissue which
uoCv r eXtfrna"y ^ a thm layer of skin continuous with that lining the
iitory canal and internally by a thin mucous membrane. The tympanic
membrane is pkced obliquely at the bottom of the auditory canal, Liming
five de±f andbehmd downward and forward at an angle of about forty5
five degrees. The external surface of this membrane presents a funnel-
shaped depression, the sides of which are slightly convex

mfwif T"'~^Tning.aCr°SS the ^P*™ cavity and forming an
gular line of joined levers is a chain of bones, which articulate onelith

THE SENSE O"F HEARING

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.

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 tympam.

[The incus or anvil bone presents a concave articular surface which receives

| the head of the malleus. The stapes, or stirrup bone, articulates externally

1 with the long process of the incus, and internally, by its oval base, with me

! edges of an oval opening, the foramen ovale. The entire chain is partially

supported by a ligament attached to the shrot process of the incus am

to the walls of the tympanic cavity.

The Tensor Tympani Muscle.— This is a delicate muscle, about 15 mm.
in length, situated in a narrow groove just above the Eustachian tube (Tig.

320). It arises from the car-
s' tilaginous portion of the Eusta-
chian tube and the adjacent
portion of the sphenoid bone.
From thisTorigin it passes nearly
horizontally backward to the
tympanic cavity; just opposite

FlG - g —TYMPANIC MEMBRANE AND THE AUDI-
TOR OSSICLES (LEFT) SEEN FROM WITHIN, »«.,
FROM THE TYMPANIC CAVITY. M. Manubnum
or handle of the malleus. T. Insertion of the
"ensor tvmpani. h. Head. IF. Long process of
the malleusf a. Incus, with the short (K) and t
ong ( process. 5. Plate of the stapes A*,
JS^s the common axis of rotation of the auditory
ossicles. S'. The pinion-wheel arrangement be-
tween the malleus and incus.— (Landois.)

TIG. 319.— 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.)

inserted into the handle of ^

its point of articulation with the incus .g located

The Internal Ear.— > ronsists 0{ an osseous and a

taS within the former.

into vestibule> se r

the petrous portion of

'e Vllt-The vestibule is a small, triangular-shaped cavity between

7i6

TEXT-BOOK OF PHYSIOLOGY

the semicircular canals and the cochlea. It is separated from the cavity oi'
the middle ear by an osseous partition which presents near its center an oval;
opening, the foramen ovale. (See Fig. 322.) In the living condition 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.

The Semicircular Canals. — The semicircular
canals are three in number, and named from their
position, the superior vertical, the posterior verti-
cal and the horizontal. These canals are at
right angles one to the other and open by five
orifices into the vestibule, one of the orifices, how-
ever, being common to two of the canals. Each
canal near the vestibular orifice is enlarged to
almost twice the size of the rest of the canal form-
ing what has been termed the ampulla. The
superior canal is placed transversely to the long
axis of petrous portion of the temporal bone; the
posterior canal is placed almost parallel to the posterior surface of the petrous
portion of the temporal bone. The lateral or horizontal canal is placed
horizontally to the vertical canals. Their relation to one another on opposite
sides of the median plane of the body is shown in Fig. 321. From the ana-
tomic relations of these canals, it is apparent that the horizontal or lateral
canals lie in one and the same plane; that the two superior vertical canals,
as well as the two posterior vertical canals, lie in planes which are at right
angles to each other, and that the superior vertical canal of the one side,

FIG. 320. — M, THE TENSOR
TYMPANI MUSCLE — THE Eus-
TACHIAN TUBE (LEFT). —
(Landois.)

FIG. 321. — DIAGRAMS OF SEMICIRCULAR CANALS TO SHOW THEIR POSITION IN THREE PLANES
AT RIGHT ANGLES TO ONE ANOTHER. It will be seen that the two horizontal canals lie in the
same plane (H), and that the superior vertical of one side (SV) is in the same plane as the
posterior vertical (PV) of the other side. — (from Ewald.)

lies in a plane parallel to the plane in which the posterior vertical canal of
the opposite side lies.

The Cochlea. — 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 coch-
lea is partially subdivided into two cavities by a thin spiral plate of bone
which projects from the inner wall, known as the lamina ossea spiralis. In
the natural condition this partition is completed by a connective-tissue mem-
brane, so that the two passages are completely separated from each other.

THE SENSE OF HEARING

717

fhe upper passage or scala is in free communication with the vestibule,
nd is known as the scala vestibuli; the lower passage or scala in the deadcon-
ition communicates with the tympanum by means of a round opening, the
iramen rotundum, and is, therefore, known as the scala tympani. In the liv-

41 5' 6

FIG. 322. — BONY 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.

FIG. 323. — i. Utricle. 2.
Saccule. 3. Vestibular end of
cochlea. 4. Canalis reuniens.
5. Membranous cochlea. 6.
Membranous semicircular

canals.

;ing condition this opening is completely closed by a membrane, a second
membrana tympani. Both the scalae 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, diverg-
ing from the central canal, pass through
the osseous lamina spiralis and transmit
fibers of the auditory nerve. The interior
f the bony labyrinth is lined by periosteum
1 jvered 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 may be subdivided into vestibule, semi-
circular canals, and cochlea (Fig. 323).

The Vestibular Portion.— 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 endolymphaticus.

The Semicircular Canals.— The semicir-
cular canals, three in number, 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 ar
semicircular canals are composed of connective tissue lined by epithelium
At the points of entrance of the auditory nerve, the macula acusttca, in all

FIG. 324.— SECTION OF WALL OF
UTRICLE or THE INTERNAL EAR,
through macular region, from
rabbit, showing otoliths (o), em-
bedded within granular substance
(g). h. Ciliated cells with proc-
esses. (/>), extending between
sustentacular elements (s). m.
Basement membrane, n. Nerve-
fibers within fibrous tissue (D
passing toward hair-cells and
becoming non-medullated at base-
ment-membrane.— (After Piersol.)

7i8

TEXT-BOOK OF PHYSIOLOGY

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 project into the cavity. In the saccule anc
utricle the hair-like processes are covered by a layer of small crystals oJ
calcium carbonate held together by a gelatinous material. The crystals are
known as otoliths (Fig. 324).

The Vestibular Nerve.— The fibers of the vestibular nerve, arising from
the cells of the ganglion of Scarpa in the internal auditory meatus, send theii
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 maculae acusticae they subdivide into
delicate fibnllse, which ultimately become historically and physiologically
related to the neuroepithelium. From the relation of the nerve-fibers to
the epithelium, the latter must be regarded as the highly specialized terminal
organ of the vestibular portion of the auditory nerve.

The Cochlea.— The cochlea is a closed membranous tube situated between
the osseous lamina spiralis and the outer bony wall. A transverse section of
the entire cochlea shows the relation of the osseous and membranous portions
(*ig- 325)- The cochlear tube is triangular in shape. The base is attached
to the bony wall, the apex to the edge of osseous 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 extraordinarily modi-
fied and specialized along the surface
of the basilar membrane, to constitute
what is known as

The Organ of Corti.— In Fig. 325
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 con-
sists secondarily of a series of colum-
nar 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 ex-
ternal to the outer and inner hair cells are supporting or sustentacular in
character.

The organ of Corti extends the entire length of the cochlea. The num-
ber of rods which, standing side by side, form the inner limb of the arch is
imated at 5600; the number which form the outer limb is estimated at
3050. ^ 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,

FIG.

325. — A TRANSVERSE SECTION OF
TURN OF THE COCHLEA.

THE SENSE OF HEARING 719

elongated, and project outward, forming a reticulated membrane through
;he meshes of which the hair-like processes of the cells project.

From the connective-tissue thickening on the upper surface of the os-
seous lamina spiralis there extends outward over the organ of Corti a thin
membrane, the membrana tectoria. The common cavity between the walls
of the osseous and membranous labyrinth in the vestibule, the semi-
circular 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 processes of the epithelial cells covering the
maculae acusticae and the rods of Corti are consequently bathed by endo-
lymph. 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 endolymph through the endolymphatic duct.

The Cochlear Nerve. — 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 termination of these fibers in the hair cells is
unknown.

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 HEARING

The general function of the ear is the reception of atmospheric vibrations
and the transmission of the effects they produce 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, transmitted by the chain of bones to the
labyrinth to pass successively through the perilymph, the membranous walls,
and the endolymph, to the hair cells. The nerve impulses generated by these
vibrations are then transmitted by the cochlear (acoustic) nerve to the acous-
tic centers of the cerebrum, where the sensations of sound are evoked. In
this general process each one of the individual structures composing the ear
in its entirety, has a special function. In order to fully appreciate these
functions the characteristic features of atmospheric vibrations, viz. : intensity
pitch and quality must be kept in mind.

Atmospheric Vibrations.— The vibrations of the atmosphere, the ob-
jective causes of the sensations of sound, are imparted to the atmosphere 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, r
suiting in a succession of alternate condensations and rarefactions which are
propagated in all directions. The impact of a rhythmic succession of such
condensations on the ear gives rise to musical sounds; the impact of an
arrhythmic or irregular succession gives rise to noises.

720 TEXT-BOOK OF PHYSIOLOGY

If a writing point attached to a tuning-fork in vibration be placed in con-
tact with a traveling recording surface, each vibration will be recorded in the
form of a wave. For this reason atmospheric vibrations are generally
spoken of as sound-waves. A line drawn horizontally 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 sensations of sounds which physiologically result from the stimula-
tion of the auditory apparatus are characterized by loudness, pitch and qual-
ity or timbre and are the result of the intensity or vigor, frequency, and form
of the atmospheric vibrations.

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 vibrat-
ing 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 the G clef, of the music scale, corresponds 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 n octaves. Vibrations less than 14 per
second cannot 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 important part in the
collection of sound-waves. In man the pinna plays but a subordinate part
in this process. Nevertheless an individual with defective hearing may have
the perception of sound increased by placing the pinna at an angle of oo
degrees to the side of the head or by placing the hand behind it. The external
auditory canal transmits the sonorous vibrations to the tympanic membrane.
From the pbliquity of this canal it has been supposed that the vibrations,

r 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

THE SENSE OF HEARING

re transmitted to it. This it does by vibrating in unison with them. The
ibrations which the membrane exhibits correspond in amplitude, in fre-
lency and in form to those of the atmosphere. That this membrane
rtuallv reproduces all vibrations within the range of audibility has been
xperimentally demonstrated. The membrane, not being fixed as far as it
ension is concerned, does not possess a fixed fundamental note like a station-
"ry fixed membrane, and is, therefore, just as well adapted for the reception of
tee set of vibrations as another. This is made possible by variations in Us
ension in accordance with the pitch or frequency of the atmospheric vibra-
ions In the absence of vibration the membrane is in a condition of :

xat'ion- with the advent of sound-waves possessing a gradual increase of
£ as in the ascent of the music scale, the tension of the membrane m-
- eases until its maximum is reached at the upper limit of the range of
audibility. By this change" in tension certain tones become perceptible and
iistinct while others become imperceptible and indistinct

The Function of the Tensor Tympani Muscle. -The function of this

..« ...responding ,« .n mc,,«.

bones is to transmit the effects 01 me atmo P fa d becomes evident

labyrinth. The manner t uch h hjgjj one another and to the

and to the fluid of the labyrinth on

the other. surface of the tympanic membrane

When P'"^.^0^^1^^ handle of the malleus, the
it is at once pushed "f^J^mrf around an axis corresponding to its
head at the same :t,me rot ^emoves inward a small ledge of bone

and hence pushes mwa -

722 TEXT-BOOK OF PHYSIOLOGY

the long process of the incus. Since this process is united at almost '•
right angle to the stapes bone, the latter is forced toward and into th
foramen ovale, thus producing a pressure on the perilymph. With th
cessation of the pressure the elastic forces of the membrane and of the lij
ments return the handle of the malleus to its former position; by the unlock
ing of the malleo-mcudal joint the entire chain also returns to its former posi
tion without exerting undue traction on the basal attachment of the stapes

As the long process of the incus is shorter than the handle of the mallei
and as the movement between them takes place around an axis from be for'
backward, it follows that the excursion of the incus and stapes will be les
than that of the malleus, while the force will be greater. Hence as th
vibrations are transferred from the tympanic membrane of large area to th,
base of the stapes of small area (*> to 1.5), they lose in amplitude but in
crease is force. Their pressure on the perilymph is therefore 13.3 time
greater than on the membrana tympani. In addition to its function as i
transmitter of vibrations, the chain of bones serves as a point of attachmen
for muscles which regulate the tension of the tympanic membrane ant
the pressure on the labyrinth.

The Function of the Stapedius Muscle.-The function of thestapediu.
muscle is a subject of much discussion. According to Henle, its functior
is so to adjust the stapes bone that it will be prevented from exerting an undu<
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 stape<
inward, make it a fixed point, and place the anterior part in such a positior
that it will vibrate freely and accurately.

The Function of the Eustachian Tube.— In order that the tympanic
membrane may vibrate freely it is essential that the air pressure on both

fl? w -<.eqUf \ at t " tlmCS- This is made P°ssible by the Eustachian
tube. Were it not for this passageway, with each inward swing of the mem-
brane the air m the tympanic cavity would be condensed and its pressure
raised, in consequence of which the movement of the membrane would be
:d; 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 mem-
brane to a given vibration could not be attained and the resulting sound
would be muffled and indistinct. But as with each vibration of the
membrane the air can pass into and out of the tympanum through this
partially closed tube, inequalities of pressure are prevented and a free
vibration permitted.

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

ressV Eustachian tube into the tympanum, thus increasing the

2. By closing the^mouth and nose and then making an effort of deglutition.

AS is act is attended by an opening of the pharyngeal end of the
man tube, the air in the tympanum is partly withdrawn and the
pressure lowered. In each instance hearing is impaired. After
either experiment the normal condition is restored by swallowing with
the nasal passages open.

THE SENSE OF HEARING 723

The Functions of the Internal Ear.— From the anatomic arrange-
icnt of the structures of the internal ear it is evident that if the vibrations
I the stapes bone are to reach the peripheral organs— the hair cells— of
,oth the vestibular and cochlear nerves, they must traverse successively
he perilymph, the membranous walls, and the endolymph. As the penlymph
3 incompressible, the inward movement of the stapes would be prevented
vere it not for the elastic character of the membrane closing the foramen
•otundum The pressure wave occasioned by each inward movement of
stapes is transmitted through the scala vestibuli, the helicotrema, and
;cala tympani, to this membrane, which by virtue of its elasticity is pressed
nto the tympanic cavity. With the outward movement of the stapes,

^ThTFunctionTof thVcoclilea.-The cochlea is the portion of the in-
ternal ear which is concerned in the perception of tones. The arrangement
oHhe histologic elements of the organ of Corti indicates that they m some
way respond To the vibrations of varying frequency and *rm and through
Ae 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 distm-
S thtpLh and the quality or timbre of a tone most physiologists prefer
ETof Helmholtz, who regarded the transverse fibers of *e basilar mem-
•Trane as the elements immediately concerned, and compared them both in

724 TEXT-BOOK OF PHYSIOLOGY

cochlea to 0.495 mm- at tne apex, and, according to Retzius, are about 24,000
in number. As the human ear usually cannot distinguish more than 11,000
tones, it is evident that there is a sufficient anatomic basis for this theory.

The Functions of the Semicircular Canals.— From the pronounced
disturbances of equilibrium and progression which follow injury or destruc-
tion of one or more of the semicircular canals or of the vestibular branch of
the acoustic nerve (phenomena which have been alluded to in connection
with a consideration of the functions of the cerebellum (see page 608), it is
apparent that they are among the peripheral sense-organs, the physiologic
action of which is the development of nerve impulses, which when trans-
mitted to the brain assist the equilibratory mechanism to maintain the equi-
librium of the body, both in the standing position and in the various modes
of progression. The character of the stimulus, however, and the manner in
which it acts on the specialized portion of the sense-organs (the hair cells) is
not entirely clear.

In any explanation it must be recalled (i) that the membranous canals
are in direct connection with the utriculus, and that both are filled with
endolymph; and (2) that these canals act in pairs at least, in the varying
positions or movements of the head, e.g., the two horizontal or lateral canals
act together when the head is rotated around a vertical axis; the two superior
and the two posterior verticals act together when the head is rotated around
a horizontal axis; the superior and posterior vertical of one side act together
with the corresponding canals of the opposite side when the head is rotated
around an antero-posterior or sagittal axis; the superior vertical of one side
acts with the posterior vertical of the opposite side when the head is rotated
around an oblique axis.

Goltz was the first to present the theory that the stimulus is to be found
in the pressure of the endolymph. He stated that in any given position oi
the head, the endolymph would gravitate to the most dependent portion oi
the canals and thus press on the nerve endings in one or more of the ampullae,
and thus develop nerve impulses which, when transmitted to the brain,
would evoke more or less conscious sensations. These sensations would
indicate the position of the head, and lead to the necessary effort to control
the movement, to maintain equilibrium and to restore the head to the norma]
position. If this mechanism is injured this knowledge and this control is
interfered with and a want of equilibrium is developed.

Later Breuer propounded the view that the stimulus was to be found in
a movement of the endolymph in one or more canals when the head was
rotated, a movement which varied in rapidity in proportion to the rate oi
head rotation. Mach subsequently presented the view that the stimulus was tc
be found in a variation of pressure, positive or negative in one or more oi
the ampullae rather than in an actual movement of the fluid. Crum Brown
made the further suggestion that in addition to the movement or pressure
of the endolymph there was a movement of the perilymph as well which
involved also the walls of the canal. This general theory is usually desig-
nated as the dynamic in contradistinction to the theory of Goltz which is
designated as the static. In the dynamic theory, the movement of the fluid
in the canal or canals, the plane of which corresponds to the plane in which
the head is rotated, though in the opposite direction, is over the hair cells at
the ampullae and thus acts as a stimulus. Thus, if the head is rotated to the

THE SENSE OF HEARING 725-

hght around a -vertical axis and in a horizontal plane the endolymph in the
tfght half of each horizontal or lateral canal will flow toward the ampulla,
ind in the left half away from the ampulla. If the head is rotated to the
left the reverse movements would arise.

If the head is rotated backward around a horizontal axis the endolymph
will flow in both posterior vertical canals in the opposite direction, i.e., away
from the ampullae and if the head is rotated around the same axis forward,
the reverse movement takes place. If the head is rotated around an oblique
axis, the endolymph will flow in the superior vertical canal of one side away
from the ampulla and in the posterior vertical canal of the other side toward
the ampulla. Thus whatever the plane of rotation of the head may be,
there is always a flow of endolymph in the opposite direction, which
movement, or variation in pressure, becomes the effective stimulus. The nerve
impulses developed by this stimulation when transmitted to the brain evoke
more or less conscious sensations, which inform us as to the direction and the
extent of movement of the head and calls forth the action of the mechanism
by which the movement is regulated and controlled and the equilibrium of
the body maintained.

CHAPTER XXXI

REPRODUCTION

Reproduction is the process by which a new individual is initiated and
developed and the species to which it belongs is preserved. Reproduc-
tion 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 reproduc-
tive organs characteristic of
the two sexes.

Embryology is a depart-
ment 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 OR-
GANS OF THE FEMALE

The reproductive organs of
the female comprise the ova-
ries, Fallopian tubes, uterus,
and vagina.

The Ovaries.— The ova-
ries 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 to-
ward its penpheral portions. It is estimated that each human ovary con-
tains from 20,000 to 40,000 follicles. The follicles vary considerably in size;

726

FIG. 726.— SECTION OF CORTEX OF CAT'S OVARY,
EXHIBITING LARGE GRAAFIAN FOLLICLES, a. Pe-
ripheral zone of condensed stroma. b. Groups of im-
maiuic tuaictes. c. Theca of follicle, d. Membrana
granulosa. e. Discus proligerus. /. Zona pellucida.
g. Vitcllus. .h. Germinal vesicle, i. Germinal spot.
k. Cavity of liquor folliculi.— (After PiersoL)

REPRODUCTION

727

'hile many are .visible to the unaided eye, others require for their detection
igh powers of the microscope. Although the follicles are present in the

'ary at the time of birth, it is not until the period of puberty that they as-

.me functional activity.

From this time on to the catamenial period there is a constant growth
.nd development of these follicles. Each follicle consists of an external
.ivestment of fibrous tissue and blood-vessels, and an internal investment of
:ells, the membrana granulosa. At the lower portion of this membrane there
an accumulation of cells, the proligerous .disc (Fig. 326). The cavity of
he follicle contains a slightly yellowish, alkaline, albuminous fluid, a
ransudate in all probability from the blood-vessels. The Graafian follicle
s of especial interest, for it is in this structure, and more especially in the
.roligerous disc, that the true germ-cell or ovum is developed.

FIG. 327. — 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 cannot be
seen. — (Stohr.)

FIG. 328. — FRONTAL SEC-
TION OF THE UTERUS, i. 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.)

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
iSIrt*, and a nucleolus or gennmal spot The cytoplasm present
foward its central portion a quantity of granular material, partly fatty in
character, the deutoplasm or vitellus. The penpheral portion of the cyto-
Dlasm is surrounded by a clear thick membrane, the zona pelluctda, external
£ which is aTayer of radially placed columnar epithelium, the corona radtoto

Thueus 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 affinity for certain staining
mSerials and hence are known as chroma**, in the meshes of which lies a
material that stains less deeply and known as achromatin.

728 TEXT-BOOK OF PHYSIOLOGY

The Fallopian Tubes. — The Fallopian tubes are about 12 centimeters]
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
fimbrise, one of which is attached to the ovary. The tube consists of three J
coats — an external or fibro-serous; a middle or muscle, the fibers of which!
are arranged longitudinally and circularly; and an internal or mucous, which!
is folded longitudinally. The surface of the mucous coat is covered with a|
layer of ciliated epithelial cells, the direction of motion of which is toward!
the uterus.

The Uterus. — The uterus is pyriform in shape and divided into a body]
and neck. It measures, before the first pregnancy, about 7 cm. in length,!
5 cm. in breadth and 2 J cm. in thickness. A frontal section of the uterus
shows a central cavity which in the body is triangular in shape, in the neck
oval or fusiform (Fig. 328). 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 complicated manner. The interior of the uterus is lined
by mucous membrane covered with cylindric ciliated epithelial cells, the
motion of which is toward the external os. Tubular glands are found in
large numbers in the mucous membrane lining 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 rectum and bladder. It extends
from the surface of the body to the brim of the pelvis, and embraces 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 augmen-
tation 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 fully understood, is received by the fimbriated
extremity 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 of, 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 chfomatin
material. The extruded portions are known as the polar bodies. The non-
extruded portion of the chromatin material is known as the female pronu-
cleus or germ nucleus. The chromosomes are reduced to one-half the somatic
number. The succession of changes which the nucleus undergoes is termed

REPRODUCTION

729

maturation. .As the nucleus is regarded as the part of the ovum which
transmits parental characteristics it is assumed that the extrusion of a por-
tion 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 ex-
tends over four or five days and the amount of blood discharged varies
from 180 c.c. to 200 c.c. Menstruation is 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 condition of the blood-vessels. Subsequently to these changes
the epithelial surface, as well as the more superficial portions of the connec-
tive 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 membrane 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 glutin-
ous substance, now become convoluted 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 impregnated.

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 un-
impregnated and pregnant condition is expressed in the following table by
Dal ton:

CORPUS LUTEUM OF MENSTRUATION.

At the end of three
weeks.
One month

Three-quarters of an incli in d
wall pale.
Smaller; convoluted wall bright

Two months

yellow; clot still reddish.
Reduced to the condition of an
insignificant cicatrix.

Absent

CORPUS LUTEUM OF PREGNANCY.

Larger; convoluted wall bright yellow;
clot still reddish.

Seven-eighths of an inch in diameter; con-
voluted wall bright yellow; clot perfectly
decolorized.

Seven-eighths of an inch in diameter; clot
pale and fibrinous; convoluted wall dull
yellow.

Still as large as at the end of second
month; clot fibrinous; convoluted wall
paler.

Half an inch in diameter; central clot con-
verted into a radiating cicatrix; external
wall tolerably thick and convoluted, but
without any bright yellow color.

730

TEXT-BOOK OF PHYSIOLOGY

THE REPRODUCTIVE ORGANS OF THE MALE

The reproductive organs of the male comprise the testicles, vasa deferen-
tia, vesiculae 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. 329) 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 in-
ward in a diverging manner. The spaces between the septa are occupied
by the true gland substance, the seminiferous tubules.

FIG. 329. — DIAGRAM OF A VER-
TX.CAL SECTION THROUGH A TES-
TICLE, i. Mediastinum testis. 2,
2. Trabeculae. 3. One of the
lobules. 4, 4. Vasa recta. 5.
Globus major of the epididymis.
6. Globus minor. 7. Vas def-
erens. — (Holden.}

PIG. 330. — VAS DEFERENS, VESICUL.E
SEMINALES, 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, 1. Prostate. — (Liegeois.)

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 mediasti-
num testis, the convolutions disappear, and after uniting with one another
terminate in from twenty to thirty straight tubes, of small diameter,
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 con-
voluted. 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 consists of a basement membrane lined by granular
nucleated epithelium.

REPRODUCTION 73*

The vas dcferens, the excretory duct of the testicle, is about 60 cm. in
length and from 2 to 3 mm. in diameter, and extends upward from^th
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
unites with the duct of the vesicula seminalis to form the ejaculatory duct.

The vesiculse seminales are two lobulated pyriform bodies, about 40
mm in length, situated on the under surface of the bladder. Each vesicuk
seminalis consists of an external fibrous coat, a middle, muscular coat, and
internal mucous coat. The mucous coat contains a number of smi
tubular albumin-producing glands which secrete a characteristic fluid.

The ejaculatory duct, formed by the union of the vas deferens and th
duct of the vesicula seminalis, opens into the prostatic portion of 1

(FlThe°prostate gland is a musculo-glandular mass surrounding the
posterior extremity of the urethra. It contains a large number of tubu
more or less branched and convoluted, and lined by columnar epithelium,
Thev secrete a fluid which is poured into the urethra at the time of the ejacu-
lation of semen and impart motility to the spermatozoa or spermia
' The penis consists of three parts: the corpus spong^osum below through
which passes the urethra, and the two corpora cavernosa, one on ei

- %^r^-'Kr4rreir^rrjf;

dorsum of the penis and unite to form a large vein by which the blood is r

732

TEXT-BOOK OF PHYSIOLOGY

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. 331). The head contains a nucleus
of chromatin material. The total length of a spermatozoon varies from
50 to 80 micro-millimeters. 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 con-
siderable rapidity. The power of locomotion depends on
the possession of the tail which, by lashing the sur-
rounding 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 phy-
siologic 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 — spermatids — which subse-
quently develop into adult spermatozoa. In this process
the primary nuclear chromatin undergoes a division, so
that each spermatozoon receives but a fractional amount
representing one-half the number of somatic chromo-
somes. 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 quan-
tity 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 introduction of the sper-
matozoa 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 vi-
tality for some days. The migration is effected by the
propelling power of the filamentous tail and by the action of the cilia of the
uterus and tubes.

From observations made on the behavior of the spermatozoa toward
the ovum in lower animals, and on the manner in 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 appar-
ently 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.
Notwithstanding their enormous numbers it is generally believed that but a
single spermatozoon effects an entrance into the ovum. With the accom-
plishment of this, however, the spermatozoon loses its mobility, after which
the tail disappears.

FIG. 331. —HUMAN
SPERMATOZO&N. i.
Front view, 2, side
view, of the head.
k. Head. m. mid-
dle piece. /. Tail.
e. Terminal fila-
ment.— (After Ret-
zius.}

REPRODUCTION 733

The germ nucleus proceeds to the middle of the ovum where it is followed
by head and middle-piece of the spermium; the middle-piece forms a central
spindle while the germ nucleus and head of the spermium each resolves
itself into one-half the number of chromatic loops of a somatic cell. In this
condition the fertilized" ovum represents a parent cell, that possesses the
physiologic activities and characters of both ancestral cells. From this
parent cell the offspring develops through successive division, multiplication
and differentiation of the resulting cells. The chromatic material of the
germ nucleus and head of the spermium represent the transmitters of in-
herited characters.

The Fixation of the Ovum.— The ovum, after fertilization in the
oviduct, continues to divide and pass slowly to the uterus (8 to 10 days)
where it is retained until the end of gestation. A menstrual mucosa haying
developed the ovum lodges on a smooth thick area and gradually sinks
beneath the surface. During the passage down the oviduct the zona pellu-
cida has become attenuated and has been finally replaced by a thick layer
of ameboid and phagocytic cells called the trophoderm. Upon lodgment
of the ovum these cells destroy the underlying mucosa and produce a cavity
into which the ovum sinks. As the ovum increases in size the mucosa
gradually covers it; that portion of the mucosa toward the uterine cavity is
called the decidua capsularis (d. reflexa), that beneath the ovum the decidua
basilaris (placental d.), while the remainder constitutes the decidua panetahs
(d. vera). As development proceeds the decidua basilaris becomes greater,
ultimately developing into the placenta.

Segmentation of the Ovum.— Immediately after fertilization the ovun
divides and redivides, within the diminishing zona pellucida, forming an
irregular mass called the morula. The peripheral cells form a layer, the
trophoderm, beneath the attenuated zona pellucida ultimately replacing
that structure. The remaining cells of the morula differentiate into three
masses— ectodermal, entodermal and mesodermal; the central cells of these
masses liquefy and disappear forming thus the ectodermal or ammotic
cavity limited by the ectoderm; the entodermal cavity, limited by the ento-
dernv and the mesodermal or celomic cavity, limited by the extra-embryonic
mesoderm. Meanwhile cells in various parts of the thickened trophoderni
have disappeared leaving this layer in the form of delicate trophode
villi the future chorionic and placental villi.

The Embryonic Shield.— The floor of the amniotic cavity, consisting
of ectoderm and entoderm, constitutes the embryonic shield or disc. As
the shield increases in size a median longitudinal thickening is seen
occupying the caudal half of the area. This is the primitive streak, a tem-
porary7 structure that is soon overshadowed by changes in the areas jus in
Front of it. Here is formed a median, longitudinal, grooved ridge of ecto-
derm, that develops rapidly in length. This is the neural groove and/d*,
The dorsal lips of the groove approach each other in the mid-line and fuse,
seSiS from the original ectoderm which closes over the ectodermal
tube This ectodermal tube is the neural tube from which the nerve system

1S dlTthe ^mediate vicinity of the head end of the primitive streak is seen
a darkened area, Hensen's node that represents the beginning imagination
of the Ectoderm m the formation of the embryonic mesoderm and notochord

734

TEXT-BOOK OF PHYSIOLOGY

to be considered later. That portion of the embryonic shield that gives
rise to the embryo itself becomes distinctly outlined laterally and in the head
and tail regions of the neural groove. Just external to this area, the embry-
onic area proper, is a transparent area, the area pellucida, beyond which is the
area opaca in which the first blood-vessels appear.

Mesoderm and Notochord. — So far in the embryonic area only ecto-
derm and entoderm exist. Hensen's node at the head end of the .primitive
streak represents an invagination (gastrulation) of ectoderm between ecto-
derm and entoderm. This invagination elongates headward in the embry-
onic area constituting a tube of ectodermal cells, the chordal canal. Later
the ventral wall of the canal and the adjacent entoderm disappear so that
the chordal ectoderm temporarily forms the dorsal median boundary of the
entodermal cavity. By this process a communication is established between
the entodermal cavity and neural groove, called the neur enteric canal. The
chordal ectoderm separates from the entoderm and then forms a solid cord
of cells, the notochord, between entoderm and neural groove, the neurenteric
canal, however, persisting for some time. In the meantime other ecto-
dermal cells in the region of the chordal invagination spread between ecto-
derm and entoderm and form the anlage of the mesoderm. These cells by
rapid proliferation soon separate ectoderm and entoderm and join the
extra-embryonic mesoderm. The separation of ectoderm and entoderm is
complete except in the regions of the bucco-pharyngeal and cloacal membranes.
Upon each side of the neural groove the mesoderm becomes transversely
grooved on its ectodermal surface, forming a number of successive block-
like masses called primitive somites or segments. Of these there are thirty-
eight of the trunk and possibly four
for the head region. Each segment
consists of three parts — the sclero-
tome, the myotome and the dermatome.
Lateral to the somite is a thickened
mass of mesoderm, the intermediate
cell-mass, that laterally splits into two
layers, the outer accompanies the ecto-
derm forming the somatopleure which
gives rise to the body-wall, the inner
joins the entoderm forming the
splanchnopleure from which the gut-
tract, vitelline duct and yolk-sac are
derived.

Fetal Membranes. — As the primi-
tive streak and neural groove are form-
ing, the extra-embryonic mesoderm
that lies beneath the trophoderm invades the trophodermal villi forming thus
the chorion with its villi. Gradually the mesoderm of the roof 6f the am-
niotic cavity splits into two layers, the upper constituting chorionic meso-
derm while the under one attached to the ectoderm of the amniotic forms
with the latter the amnion. In the chick and some mammals, the amnion
is derived from the somatopleure in the folding off of the body. In amniotes
the amniotic cavity is at first small but rapidly increases in size. It con-
tains a clear, transparent liquid, the amniotic fluid, which amounts to about

FIG. 332. — HUMAN EMBRYO AND ITS EN-
VELOPES AT THE END OF THE THIRD MONTH.

— (Dalton.)

REPRODUCTION

735

one liter at term; it serves to protect the fetus during gestation and at
parturition it dilates the os cervicis, and flushes the birth canal. This liquid
is derived mainly from the blood as it contains albumin, sugar, fat and in-
organic salts. Traces of urea indicate that some of its constituents are de-
rived from the embryo itself.

The caudal end of the embryonic area is left connected with the chorion
by a heavy band of mesoderm termed the belly-stalk, to which the caudal
part of the amnion is attached. The entoderm is invaginated into the
belly-stalk for a short distance constituting the allantois of higher forms.
In oviparous forms the allantois grows out between the closing somatopleuric
folds that form the body-wall and constitutes a free sac upon which vessels
(allantoic arteries and veins) develop from the embryo. This sac then
spreads beneath the white shell membrane forming the organ of nutrition
and respiration of these forms during the last half of their incubation periods.
In mammals the extra-embryonic portion of the allantois is of little
importance (Fig. 332).

Placenta Formation. — The chorionic villi increase rapidly in size and
number and usually surround the whole fetal sac, giving it a peculiar shaggy

Chorionic villi.

Intervillous spaces.
Floating villus.

Attached villi.
Vein.

Spiral artery.
Gland.

Vein.

Muscularis. \

FIG. 3*3- — DIAGRAM OF HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. — (Schaper.1)

appearance. Blood-vessels now proceed from the embryo along the belly-
stalk (not the allantois in higher forms as formerly stated) . These, the
umbilical arteries and veins, pass to the chorionic villi and send branches to
those of the placental area; these vascularized villi constitute the chorion
frondosum, while the avascular villi form the chorion leve. The villi of the
latter disappear during the second month, leaving the chorionic membrane
smooth. The villi of the chorion frondosum now penetrate the uterine glands
of the decidua basilaris, which by this time have been denuded of epithelium,
and have gained connection with the blood-vessels of themucosa; in this
manner these uterine glands have become converted into blood sinuses. The
chorionic villi either attach themselves to the tunica propria of the mucosa

736 TEXT-BOOK OF PHYSIOLOGY

( fixed mill] or remain free (floating villi) . At the edge of the placental area
very few villi develop, leaving a circular channel called the marginal sinus.
This attachment of villi becomes marked from the third month on and
this is considered the beginning of placentation. From this time on to term
there is merely an increase in number of villi and vessels with thus a cor-
responding increase in the size of the placenta. (Fig. 333.)

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 1 8 to 24 cm. in diameter
and weighs from 400 to 600 grams. It is most frequently attached to the
upper and back part of the uterine wall. Though exceedingly complex
in structure it consists essentially of two portions, a fetal and a maternal.

The fetal portion consists primarily of those villi on the chorion in relation
with the decidua basilaris. These structures gradually increase in size and
number, and receive the ultimate branches of the umbilical arteries. The
maternal portion consists primarily of the decidua basilaris. As gestation
advances the placental villi rapidly increase in size and number, and re-
ceive the branches of the umbilical arteries. At the same time the decidua
basilaris becomes hypertrophied 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 basilaris and its contained blood-vessels undergo certain histo-
logic changes which result in the formation of large cavities, sinuses, or
lakes, into which the blood of the uterine vessels is emptied. As the pla-
centa 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 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 structures, the placenta is to be regarded as
both an absorptive and a respiratory organ. So long as these exchanges are
permitted to take place in a normal manner the nutrition of the embryo is
secured.

The Nutritive Supply of the Embryo. — The growth and development
of the embryo from the period of fertilization to the period of birth require
a continuous and ever increasing supply of food materials. This is derived
from several sources and requires for its utilization, the development, in
different classes of animals, of specialized forms of the circulatory apparatus,
the relative importance of which varies in accordance with the source of
the food supply. These are known as the vitelline, the allantoic, and the
placental circulations. All these forms are present at successive stages in
the development of the human embryo but only the last is of major
importance.

As the ovum passes down the oviduct it imbibes its nutritive material
from the mucosa. When it lodges itself in the uterus it probably receives

REPRODUCTION

737

Iditional material in the same way. The period during which it does so
, however, very limited.

The Vitelline Circulation. — The vitelline circulation, which in oviparous

ttimals, e.g., the chick, is of primary importance because of the large

mount of food stored in the vitellus or yolk, is in mammals of relatively

ight importance because of the limited supply of food in the vitellus. It

nevertheless present in early stages.

The Allantoic Circulation which in oviparous animals is also of primary
nportance in the latter half of the incubation period both as an absorption

eta

superi'or ven°cavx i Hepatic vein. ^.Venaporte. ,. #«. Pulmonary vem.
vejn< — (From Kollmann.}

and respiratory apparatus is also present in mammals to a slight extent,
but it is merely a transition stage in the development of placenta! circulation
The Placental Circulation.-The development of the fetal or placental
circulatory apparatus by which the fetus obtains its food supply and nece:
sar} oxygen and frees itself from carbon dioxid has been alluded to ma
foregoing paragraph relating to the formation of the placenta. After the
blood-vessels of the embryo, the umbilical arteries and vein have come into
and physiologic relations with the uterine blood-vessels, the
nSrials and the oxygen are derived entirely from the maternal

738 TEXT-BOOK OF PHYSIOLOGY

blood-stream which at the same time receives carbon dioxid and perhaps
other waste products from the fetal blood-stream. The placenta thl
serves as a digestion and respiratory organ. The blood having undergo^
these changes now leaves the placenta and returns to the fetus by the UT&
bilical vein. This blood is relatively rich in nutritive material and off
scarlet red color by reason of the presence of an increased amount of oxygeJ
As it passes into the abdominal cavity a portion, about one-half, of the bk><3
is directed by the ductus venosus into the inferior vena cava, while the ri
mainder is emptied into the portal vein, by which it is distributed to tM
liver and from which it emerges by the hepatic veins and is poured into thl
inferior vena cava. The blood in the vena cava is thus a mixture of venou]
blood from the lower extremities and liver, and oxygenated blood from thl
placenta. After its discharge into the right auricle the blood is directed bjj
a fold of the lining membrane, the Eustachian valve, through an opening id
the interauricular septum, the foramen ovale, into the left auricle. It theiS
flows through the auriculo-ventricular opening into the left ventricle, thend
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 superioi
vena cava into the right auricle, but as it passes in front of the Eustachiafl
valve, it flows directly into the right ventricle and then into the pulmonarj
artery. On account of the unexpanded 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 fai
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
liver and upper half of the body. As the blood passes through the aorta,
a portion is directed from the main current by the hypogastric and umbilical
arteries to the placenta, where it loses carbon 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 ttye 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 contractions which are somewhat peristaltic in char-
acter; these contractions, 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
vagina. ^ After a short period of rest the uterine contractions return and
rapidly increase in vigor and duration. As a result of the pressure thus

REPRODUCTION 739

exerted from a-11 sides on the body of the child, the head gradually descends
still further into the vagina and finally emerges through the vulva to be
iollowed in a short time by 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, with 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
which the vessels are compressed and permanently 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 addition it is very probable that the stimulation of the inspiratory
center is also occasioned by the arrival of nerve impulses from the skin,
developed by the cooling of the skin due to the vaporization of the amniotic
fluid.

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 contracts, and sends a
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 mem-
branous fold grows upward and backward from the edge of the foramen
ovale on the left side; the ductus arteriosus also contracts. With the first in-
spiration and the expansion of the lungs, the blood which enters the pul-
monary artery 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 Eu-
stachian 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 um-
bilical vein and ductus venosus, at the end of four or five days, have also
become almost impervious from the contraction of their walls. The
proximal ends of the hypogastric arteries remain open and carry blood to the
walls of the bladder. The distal ends of the arteries are converted into im-
pervious cords.

Lactation.— As pregnancy advances the mammary glands increase
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 resem-
bles 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 coag-

740

TEXT-BOOK OF PHYSIOLOGY

ulates on boiling, and certain inorganic salts which have a laxative effect]
on the new-born child. Normal lactation aad 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 blastoder-
mic membranes gradually increases in complexity and variety, as the individ-
ual 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 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 eliminated by the placental structures;
but as metabolism increases in complexity the liver and kidney assume ex-
cretory 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, in-
dicating the establishment of both hepatic and renal activity. Contractions
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 subserve general
metabolism begin to functionate and in the course of a week are fully estab-
lished. 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 propor-
tional to the food consumed; the digestive glands are elaborating their re-
spective enzymes, digestion proceeding as in the adult. The hepatic secre-
tion is active and the lower bowel is emptied of its contents; the coordinate
activites of the nerve-, muscle-, and gland-mechanisms are entirely reflex in
character. Psychic activities are in abeyance by reason of the incomplete
development 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 appli
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 dec naty,
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 •»JW|£gWj
acquaintance with those appliances by means of whic.

**£&$$& aTapparL composed of different elements, which
by virtue of chemic actions taking place among them, generate : and co
electricity In its simplest form an electnc cell consists of two m<
zinc and copper, or carbon, or platinum, etc., immersed m an exciting fluid,

^ThelTekmenHs 'hfoKc^on chemically by the sulphuric add
* With the immersion of these elements m a solution of H SO a

formula: Zn + H2SO,-ZnSO. + H,

The zinc sulphate passes into the solution, while the hydrogen accu-

742

TEXT-BOOK OF PHYSIOLOGY

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
material, its length, and the area of its cross-section. 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, therefore, not depend entirely on the electro-motive force, but
rather on the ratio between the electro-motive force and the resistance.

335.—AN
ELECTRIC CELL.

FIG. 336. — Two SIMPLE ELECTRIC CELLS JOINED
IN SERIES. C. Copper. Z. Zinc.

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 mercury 106.3 cm- l°ng
and i sq. mm. 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 the ohm, one coul-
omb of electricity in one second of time yielding a current strength of one
ampere.

The relation may be expressed in the following formula, Ohm's law:

Electro-motive force (E. M. F.) Volts

C (current strength) = — — ==-- or Ampers = =..--

Resistance (R) Ohms

In practical work it is often necessary to increase the strength of the
current. This is done by uniting two or more cells in series, i.e., uniting the
copper of one cell to the zinc of a second, and so on (Fig. 336). If the
resistance remains the same the total voltage and current are those 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:

i. 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.

PHYSIOLOGIC APPARATUS

743

2. The accumulation of hydrogen bubbles on the surface of the copper
linders the passage of the electricity. In a short time they develop a current
n the opposite direction, which also tends to weaken the original current.
Tiis action is termed polarization of the elements.

Cells of this character are not suited for physiologic work, in which

onstancy in the strength of the current is absolutely necessary. To over-

ome these disadvantages, cells have been devised which are less violent i

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

physiologic purposes is — .

The Daniell cell.— This consists of a porous cup containing a satural
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
H2SO4 (1:15). In this solution is immersed a roll of sheet zinc (Fig. 337).
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 liber-
ates 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. Polariza-
tion 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

FIG. 337. — DANIELL CELL.

EnterUresistance of the cell must be taken ^J^g*^ o£
The Drv Cell.— The commercial dry cell is a convenient source u

When

744

TEXT-BOOK OF PHYSIOLOGY

the plates are united by a conjunctive wire the current within the cell flowi
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 (thi
negative pole).

Leads.— By means of insulated wires attached to the poles of a cell, tW
electricity may be conducted from the cell and used for exciting or stimulate
ing purpose. As the wires thus become practically prolongations of th<
plates their ends become the corresponding poles. In experimental worl
the ends of the wires are provided with special devices, termed—

Non-polarizable Electrodes.— The necessity for the employment o;
such electrodes arises from the fact that when the ends of the wires from i
cell are placed in direct contact with the tissues chemic changes are producec
in a short time, which lead to their polarization. As a result, a curreni

opposite in direction to that of the cell i<
developed, which tends to weaken or neu-
tralize it. This polarization current vitiates
the result of many experiments made witl
highly irritable tissue such as nerve-tissue,
Whether for stimulating purposes or for the
purpose of detecting the existence of electric
currents in living tissues, it is essential thai
the electrodes used shall be non-polarizable,
The earliest electrodes of this character were
made by du Bois-Reymond and were based
on the fact discovered by Regnault that a
strip of chemically pure zinc or amalga-
mated zinc (Matteucci) immersed in a satu-
rated solution of zinc sulphate would not
polarize. One form made by du Bois-Rey-
mond is shown in Fig. 338. It consists of
a flattened glass tube attached to a universal
FIG. 338.— NON-POLARIZABLE ELEC- joint and supported by an insulated brass
Vo°nDFiS*- *• DuB<^R^ond's. s. stand. The lower end of the tube is

Von -bleischPs. 3. d' Arson val's. i j -Ai i i. ,-,, . ,

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 amal-
gamated 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
distance.

^Any one of these three electrodes is suitable for physiologic experimen-
tation, 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 electrodes) of the wires of a cell

PHYSIOLOGIC APPARATUS

745

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 neces-
sary 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. 339). The surface of the vulcanite
plate carries two rectangular blocks of brass, each
of which has two holes 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 in-
terposing 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.

2. 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 con-

tinued 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 apparatus. MOND FRICTION KEY.

The amount of the current which is returried

to the cell through the short circuit will be proportional to the resistance

of the longer circuit. As the latter is usually great in comparison with

the former, practically all the current is short-circuited When the

bridge is lowered, therefore, the current is short-circuited; when n

raised, the current flows into the longer circuit through the tissue

ThPePMercury 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. 340). At opposite points there are binding
nosts one of which is provided with a rigid fixed copper rod passing into
?he mercury; the othe? is provided with a movable bent rod which may
be m™de to dip into or be withdrawn from the mercury by the ,

FIG. 339.— Du BOIS-REY-

e 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—

746

TEXT-BOOK OF PHYSIOLOGY

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. 341) 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 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 re-
spectively. 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,

FIG. 340. — A MERCURY KEY.

FIG. 341. — RHEOCORD.

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 proportional to the
difference of potential between A and S. If S is close to A, the difference of
potential is slight. If S is removed from A toward 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 direction of the current,
in other experiments to deflect it, without changing the position of the elec-
trodes. Both these results may be accomplished by the use of —

PohPs commutator. This is a round block of wood with six cups,
each of which is in connection with a binding post (Fig. 342). In each ofxthe
two cups marked i and 2, + and — , is inserted one end of a copper wire
bent at right angles. The other ends of the wires are supported and in-
sulated by a hard-rubber handle. To the top of each wire is soldered a
semicircular copper wire. This arrangement permits of a rocking move-

PHYSIOLOGIC APPARATUS

747

ment whereby the opposite ends of the semicircular wires can be made to dip
Sto cups 3 and 4, and into cups 5 and 6 alternately. . Two wires crowed m
the middle of the block serve to connect opposite pairs of cups. Wher
use, the cups are filled with clean mercury. The method of using the

« Current R^erser.-Tht positive and negative poles of thfe electric
cell are connected by wires with binding posts i and 2 respectively.
A key is interposed in the circuit. Wires are then earned 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 sem cir
cular wires dip into cups 3 and 4. The direction of the current will be
onlhe closureof the circuit from r to 3, then from 3 along a wire to and
throughthetissueandbackto4,andthencetothecell. « the P™1™ °J
the rocker be now reversed so that the opposite ends of the semicircular

FIG. 342.— POHL'S

A. Arranged as
rent deflector.

reverse,; B, as a c,

-' sssssrss? »— * «-

748

TEXT-BOOK OF PHYSIOLOGY

tinuous flow of the current through the primary circuit there is no evidence
of a current in the secondary circuit. The induced current is but of momen-
tary duration. The current flowing through the primary circuit is termed
the inducing, the current flowing through the secondary circuit the inducec
current.

The induced^ current is opposite in direction to that of the inducing cur-
rent when the circuit is made or closed; it is 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, othei
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, increas-
ing as the coils are ap-
proximated, decreasing
as they are separated.

Approximation or
separation of the coils
while the current is flow-
ing through the primary
circuit develops an in-
duced current, which
disappears, however, the
moment the movement
of the coil ceases. A
sudden increase or de-
crease in the strength of the inducing current also develops an induced
current.

When the coils are approximated W the primary current increased in
strength, the induced current is opposite in direction to that of the inducing
current; 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 insulted copper wire, termed primary and
secondary (Fig. 343).

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, which,
as soon as the circuit is made, become magnetized, with the effect of
increasing the action of the inducing current.

FIG. 343- — INDUCTORIUM OF DU BOIS-REYMOND. K/, Pri-
mary, R" secondary spiral. B. Board on which R" moves.

i. Scale. H . 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 Neef's hammer is not required.

PHYSIOLOGIC APPARATUS 749

The secondary coil, R", consists of a much greater number of turns of
finer copper wire, the ratio being about 40 to i, also wound around a spool,
aving a tunnel sufficiently large to enable it to slide over the primary. By
fee means the strength of the induced current is increased. As a result
f the construction of the inductorium, the low electro-motive force of t
ell is transformed into the high electro-motive force characteristic of tt
iduce d current. As the number of turns of wire in the secondary coil is to
he number in the primary, so are the electro-motive forces in the sec.

^hetclndS colSsllong a track, B, which permits it to be moved
oward or a°way from the primary6 The distance between the two coils can
e measured and the strength of the induced current again reproduced
the? things being equal, by means of a centimeter-millimeter scale paste,

>n Tte1nds°of\e wire of the secondary coil are fastened to two binding
>osts to which conducting wires provided with hand electrodes can be

attaTkhedinductorium may be used for obtaining either a single current or a

serieA:f^^^ «* ^ ** r rr otive tforce>

. penetrative power, and short duration, the single induced current is a

^r^p-- .

The explanation offered ft, this g~:^Eg^£%S££*$&

S&rSSKttSSftSS^ *•»- in *• ndghbonng C01k a ^

750 TEXT-BOOK OF PHYSIOLOGY

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 electricity is, however, the same in both cases.

If the secondary be pushed further along the slideway until it largeli
covers the primary coil, a position will be reached when the make-induc.ed
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 thl
make-induced current becomes more powerful and efficient than the breaks
induced current, as shown by the greater contraction of the muscle. Thid
result is explained by the fact that the make-extra current is now able oj
itself to induce a current in the secondary coil, on account of its proximity a
which, added to that induced by the battery current, produces a current]
greater than that induced on the break of the circuit.1

Rapidly Repeated Induced Currents. — As the single-induced current is oi
extremely short duration, it is inefficient as a stimulus in the conduct oi
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. 344)
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 uppei
surface there is a small plate of platinum which is in contact with an adjust-
able, platinum- tipped screw, S', carried by a plate of brass in connection witt
binding post S".

For the purpose of interrupting tfye primary circuit frequently in a unil
of time, and thus developing induced currents in quick succession, the
apparatus is arranged in the following way: The positive and negative
poles of the electric cell are connected by wires with binding post P' and P"
a key being interposed in the circuit. If the screw S' is in contact witt
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', tc
the coils surrounding the vertical bars B', then to P", and so back tc
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 demagnetized
and the hammer is restored to its original position by the elasticity of the
spring. The circuit is thus reestablished, 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 wil]
vary with the length of the spring.

1 "On certain peculiarities of the inductorium," Prof. Colin C. Stewart, "Univ. Pa. Medica
Bulletin," Feb., 1904.

PHYSIOLOGIC APPARATUS 75*

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 contact
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 procedure as
tetanization or Faradization. These cur-
rents also increase in strength as the
secondary approaches the primary.

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 con-

sists (Fig. 362) in connecting with a wire binding
P' and S", and in providing binding post P" wi

FIG. 344. — HELMHOLTZ'S MODIFI-
CATION or 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.

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 arrange-
ment is practically a short-circuiting key by which a
portion of the current is returned to the cell without
ever entering the primary coil. The same arrange-
ment, though differently lettered, is shown in Fig. 363.
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-circuiting key
is opened, the full volume of the primary current flows
through the primary coil. When the short-circuiting key is closed, most of the current fails tc
enter the coil taking the easier path through the key. Some of the current, however, always flows
through the coil and is never diverted. The cycle of changes in the electric condition of the
primary coil is thus altered for two reasons:

"First we no longer have an alternation between a full primary current and none at all— rath
an alternation between a full primary current and a weaker one. The difference in the phases .
thus lessened, the extent of the change on making and breaking is lessened and corresper
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
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
?he disappearing primary current in the same direction, produces the same effect as if the primary
current Self were to disappear slowly. As a result the disappearance of the primary current loses
iS7orme?emciency as an inducer of secondary currents, and the break-induction current is reduced

t0"Thts £3M%$jSf& -ke- 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
Sebreak-exira current has an easier path through the short-circuiting key, 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 moveme
can be translated in one direction, they may be recorded in dii

attaching the moving structure— e.g., heart, muscle, etc.— to a

752

TEXT-BOOK OF PHYSIOLOGY

FIG. 345.— A RECEIVING TAMBOUR.

delicate lever the free extremity of which is provided with a writing
point.

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 tambour. 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 con-
dition. If the mem-
brane of the first tam-
bour 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 disc. The membrane of the first
tambour is frequently provided with a button, which is placed over the
moving structure. The "inward movement 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 at-
tended by a fall of the
lever. The first tam-
bour is termed the re-
ceiving, the second the
recording tambour
(Figs. 345, 346).

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 communication with a
recording apparatus — e.g., (i) a piston provided with a lever (page 467) ;
or (2) a tambour and lever (page 357); or (3) a mercurial manometer
carrying a float and pen (page 343). The space between the part
investigated and the vessel is filled with fluid. The variations in volume
of the organ cause a displacement of the fluid and give rise to a to-and-
fro movement which is taken up and reproduced by the recording
apparatus.

'vhe writing point may be (i) some form of pen carrying ink which
rev. ; the movement on a white paper surface, or (2) a piece of
metaV glass, or paper which records the movement on smoked paper
or glass.

FIG. 346. — A RECORDING TAMBOUR. — (Marey.)

or.ic APK.

The Recording Surface. — The surface which receives and records the.
mo/ements 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 framework. 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, how-
ever, 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 move-
ment 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 pf 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. 347)-

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.

' The Extent of a Movement.— As
the lever not only takes up and re-
produces 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,
magnification of the lever is readily determined by dividing the dis

etween 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 dividn
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 c
curve the duration of the entire movement, or of any one portion o
be determined by means of a time marking or chronographic appar;
sisting of (i) a small signal magnet provided with a movable armature t
which is attached a writing style; (2) an automatic interrupter; and (3) •

eleThe signal Magnet.-The magnet (Fig. 348) is actuated by the electric
current made and brokenat regularand known intervals by an automati

FIG. 347.— KYMOGRAPH. (Boruttau's,
zold, Leipzig.)

754

TLXT-BOOK OF PHYSIOLOGY

FIG. 348. — SIGNAL MAGNET.

acting interrupter placed in the circuit. With each make ai'id break of the cirl
cuit the armature and style move alternately downward and upward. ThJ
excursion of the style can be readily recorded on a traveling surface. The
character and number of the interruptions per second VviVi determine
the character of the tracing. If they occur in a rhythmic manner, the tracing '

will be sinusoidal or wave-like
in form. If the time of inter-]
ruption is of short duration as]
compared with the time of
closure of the circuit, the trac-^
ing 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. 349. This consists of a metallic frame
carrying a coil of wire in the center of which there is a core of 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 cir-
cuit 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 mer-
cury, when the circuit is again re-
stored. The reed may be so con-
structed that it will be raised and
lowered 50, 100, or 200 times a
second. The armature of the
signal magnet undergoes a corre-
sponding number of elevations
and depressions. If the reed vi-
brates 100 times in a second, the
distance from crest to crest of the
wave tracing will represent T-J-g- of FlG-
a second. Interrupters of various
kinds have been devised which make and break the circuit from i 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 secure this, a
moist chamber is employed (Fig. 350). 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 holding the femur of a nerve-muscle prepa-

; ration, as well as a horizontal rod for supporting at least three pairs of

non-polarizable electrodes. A groove around the outer edge of the platform

349-

-PAGE'S VIBRATING REED.
modification.)

(Reichert's

PHYSIOLOGIC APPARATUS

755

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, modified at one
end to carry the delicate axis of a light stiff recording lever.
lever is pointed, to enable it to write on a smoked glass 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, ^tne
after-loading screw, also enables the lever to be placed ^OTi^talpoation.
The portion of the lever near the axis is provided with a double hook, tl
lower portion of which serves for the attachment of the weight by which tl

no as in the registration of a muscle contraction under

varying conditions, it is necessary to give the lever mass by attaching weights

FIG. 350.— MOIST CHAMBER.

paper, . piece of tm.el

th, ,«„„!„„

the

756

TEXT-BOOK OF PHYSIOLOGY

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 i per cent, solution of potassium chlorid to each 100 c.c.
The Galvanometer and Capillary Electrometer. — In the detection
and investigation of the electric currents of muscles, nerves, and other
tissues, the physiologist is limited to the galvanometer and capillary elec-
trometer. 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 direction of the
current. The essential requisite of any galvanometer used for physiologic

purposes is that it will re-
spond 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 directive influence of
the earth's magnetism is
eliminated, and the multi-
plication of the number of
turns of the wire in the coils
which surround the needle.
The tangent galvanom-
eter, or boussole, as con-
structed by Wiedemann,

FIG. 351-WlEDEMANN's BOUSSOLE. ^ ^ ^ ^ fre(?uently

employed in physiologic investigations (Fig. 351). It consists primarily
of a thick copper cylinder, through which a tunnel has been bored.
Within this tunnel is suspended 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 alumin-
ium rod, surmounted by a circular plane mirror. Above the mirror rises a
glass tube, which carries on top, on an ebonite support, a little windlass, cap-
able of being centered by three small screws. On the windlass is wound a
single filament of silk, which passes down the tube and is attached to the
mirror. The magnet can, by this contrivance, 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

PHYSIOLOGIC APPARATUS 757

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 oscillations. 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 pointing 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 imparted to the end of the accessory magnet in the direc-
tion 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 astro-
nomic 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 number 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 :

the same purposes.

The Capillary Electrometer.— Not withstanding the extreme ,ensi-
tiveness of the modern galvanometer, it has been found desirable, in the in-
vestigation 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 by Lippmann of the cap
lary electrometer. The principle of this apparatus 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 L by hy-
drogen taking place at its surface. If a capillary glass tube be filled wilt mer-
curv and its lower end inserted into a solution of sulphuric acid, and the former
connected with the positive and the latter with the negative electroc
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 coi
quence of the diminution of its capillary constant or the tension of its surface
contact with the acid. As a reverse movement follows a cessation of the c
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,
such as these Lippmann constructed the capillary electrometer a con-
venient modification of which devised by M. v. Frey, «f ^^*£
This consists of a glass tube, A, forty millimeters in length, three milliinet
in diameter, the lower end of which is drawn out to a fine capillary point
The tuTe is filled with mercury and its capillary point immersed in a 10 per
cent solution of sulphuric acid. The vessel containing the acid is filled to
the exten of several millimeters with mercury also. The mercury in the

758

TEXT-BOOK OF PHYSIOLOGY

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. 353).
The special advantage of the electrometer is, that it will respond instantly
to any variation in the electro-motive force and indicated a difference of

FIG. 352. — VON FREY'S CAPILLARY ELECTROMETER.

FIG. 353. — CAPILLARY
ELECTROMETER. R.
Mercury in tube; capil-
lary tube. s. Sulphuric
acid. q. Hg. B. Ob-
server. M. Microscope.

potential, according to Lippmann's observation, as slight as the nrrsir °^ a
Daniell. These rapid oscillations can be recorded 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 con-
nection with it.

The String Galvanometer. — The string galvanometer (Fig. 354) as de-
vised by Einthoven is a far more sensitive apparatus than either the usual
galvanometer or the capillary electrometer, and responds to feeble electric
currents more quickly than either. It is based on the principle that a con-
ductor placed near a stationary magnet will tend to change its position when
a current is passed through it, just as a magnet, e.g., a delicate magnetic
needle, placed near a conductor will change its position when a current is
passing through the conductor. For the purpose of developing a high degree

PHYSIOLOGIC APPARATUS

759

of sensitiveness, two strong magnets are employed, across the field of which
runs a very fine conductor usually made by drawing out molten quartz, by
means of a bow and arrow, to a thread much thinner than the smallest possi-
ble metal wire, and then silvering the filament to enable it to conduct elec-
tricity. When the filament is securely fastened at its extremities, it will be
found that it will deviate to the right or the left according to the direction of
the current which enters it.

By reason of the small size of the filament employed, it must be strongly
illuminated, and the shadow of the thread must be magnified by an optical
device and then projected across a narrow slit on a sensitive surface behind
it. As the shadow of the thread moves from side to side at the same time
that the sensitive surface moves in a direction at right angles to it, a graphic

, 1HE CAMBRIDGE ELECTRO-CARDIOGRAPH OR STRING GALVANOMETER.

,354

record will be obtained in which the abscissa will indicate time and the ordi-
nates the displacement of the thread or the strength of the current passing
through it. A typical normal record or electro-cardiogram (Fig. 133, page
202) shows three upward directed waves separated by two smaller down wai
directed waves and these are commonly designated, following Emthoven,
by the letters P, Q, R, S, T in order. In the conduct of an expenmen
patient's extremities are connected with some suitable low-resistance ele
trades Under such circumstances a part of the heart's action currents may
be led off to the filament of the galvanometer. If the circuit is comp lefc
from the right arm and the left arm, the arrangement is called lead ]
the right and left leg lead II; if from the left arm and the left leg, lead III
The apparatus is customarily arranged so that the current produced by

76o TEXT-BOOK OF PHYSIOLOGY

electro-negative auricles and electro-positive ventricles gives an upward loop
to the tracing, one centimeter of height being made to correspond to an elec-
tro-motive force of one millivolt on suitable adjustment of the quartz filament.
Various forms of string galvanometers, based on the principle stated in!
a previous paragraph, have been devised. The apparatus shown in Fig. 354
is known as the' Cambridge Electro-cardiograph.

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 experimental 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 togethef, as
when in their usual physiologic relation. For this latter purpose the gas-
trocnemius muscle and sciatic nerve are employed, constituting the so-called
"nerve-muscle preparation."

For these, and many other reasons, the student should familiarize him-
self 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 visceFa 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 (Fig. 355, 356). 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 semi-membranosus (sm) and
the rectus internus minor or gracilis (ri"). Between these two groups,
note the biceps femoris (b). Above the thigh observe the gluteus (gl), the
ileo-coccygeus (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 (ri7), and minor (ri"), the adductor
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).

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

PHYSIOLOGIC APPARATUS

761

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 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
urostyle with the forceps and separate it from the last vertebra. With

the scissors cut off the vertebral column above the seventh vertebra. Place

he legs on the dorsal surface and then divide the seventh eighth and mn^

vertebra lengthwise. With the forceps lift up one lateral hal of the vertebra

aid free the nerve as far as the knee by dividing connective tissue and nerve

Theompleted preparation consists of the gastrocnemius muscle the sci-
atfc nervt with half of the seventh, eighth, and ninth vertebra and the lower

half of the femur.

762 TEXT-BOOK OF PHYSIOLOGY

THE ANATOMY OF THE FROG HEART AND THE VASCULAR

APPARATUS

The heart of the frog can be readily exposed after the animal has been
made insensible by destruction of the brain. The sternum is divided longi-
tudinally and each half drawn outward by gentle traction of the anterior
extremities. The pericardium is then divided and turned aside.

When viewed from the ventral surface, Fig. 375, the heart shows two
auricles, a right and left, a single ventricle and a more or less conical vessel,
the conus arteriosus, which arises from the right side of the base of the ven-
tricle. When viewed from the dorsal surface, Fig. 376, it presents a tri-
angular-shaped vessel, the sinus venosus, formed by the union of the
terminations of the two superior and inferior venae cavae. A dissection of
the heart shows that the cavity of the sinus venosus communicates with

FIG. 357. — VENTRAL SURFACE OF THE FROG HEART. (After Howes.) ra. Right auricle
la. Left auricle, v. Ventricle, ca. Conus arteriosus. p'. Pulmo-cutaneous trunk. $'. Sys-
temic aortic trunk, c' . Carotid trunk, ac. Left anterior caval vein.

the cavity of the right auricle by means of a transversely oval foramen, in
the posterior wall of the auricle. This opening is provided with two
valves, a ventral and dorsal, the free edges of which are directed toward
the cavity of the auricle. Two pulmonary veins, a right and left, penetrate
the dorsal wall of the left auricle.

A longitudinal section, Fig. 377, of the heart shows that the auricles,
though separated by a septum, communicate below by a common orifice
with the cavity of the single ventricle. This orifice, the auriculo-ventricu-
lar, is provided with two valves the free edges of which are directed toward
the cavity of the ventricle.

PHYSIOLOGIC APPARATUS 7^3

The conus.arteriosus is separated from the ventricle by three semilunar
valves The interior of the conus is traversed by a longitudinally disposed
membranous valve attached to its dorsal surface; the ventral edge is, however,
free The upper extremity of the conus passes into the bulbus aorta, from
which it is separated by a semilunar valve and the free extremity of the
longitudinal valve. From the bulbus aortae arise two arge branches, a
right and a left, each of which is subdivided by two longitudinal partitions
fnto three vessels, the carotid trunk, the aortic arch, and the pulmo-cutaneous
trunk (See Fig 377.) The carotid and aortic trunks communicate
Separately with The cavity of the bulbus, while the pulmo-cutaneous trunk
communicates with the conus arteriosus by a single orifice, just ^bdow the
free end of the longitudinal valve. After pursuing a short course these

carotid trunks respectively

organs of the body, ine iwo , at > pulmo-cutaneous

- *e lung and a cutaneous

branch which is distributed to the skin. therefore, as follows:

The course of the blood through the heart cavils, tt« ^.^
The venous blood poured by the ^ "^ rf ht auricie. While the
through the .sino:auricular foramen ^° the "^ auride is being filled
right auricle is being filled frojt ' *^OUK*> <<when the auricles contract,
by blood coming through the pulmonary veins corresponding

which they do simultaneously ^^ each pa^ es its blooa ^ ^^^ ^

S -us arteriosus springs from

7^4 TEXT-BOOK OF PHYSIOLOGY

the right side of the ventricle it will at first receive only venous blood which
on ^the contraction of the conus might pass either into the bulbus aortd
or into the aperture of the pulmo-cutaneous trunks. But the carotid and
systemic trunks are connected with a much more extensive capillary system*
than the pulmo-cutaneous and the pressure in them is proportionally great ]
so that it is easier for the blood to enter the pulmo-cutaneous trunks than to
force aside the valves between the conus and the bulbus. A fraction of a
second is, however, enough to get up the pressure in the pulmonary and
cutaneous arteries, and in the meantime the pressure in the arteries of the
head, trunk, etc., is constantly diminishing, owing to the continual flow of
blood toward the capillaries. Very soon, therefore, the blood forces the

FIG. 359-— FROG HEART WITH VENTRAL SURFACE DISSECTED AWAY TO SHOW ITS STRUC-
TURE (After Parker and Has^ell.} ra. Right auricle, la. Left auricle. ,a. Septum between
aur: .ies. sao. Smo-auricular opening, ca. Conus arteriosus. sv. Semilunar valves av
Aunculo-ventncular valves. Iv. Longitudinal valve, sv' . Semilunar valve. *' s' c' Pulmo-
cutaneous aortic and carotid trunks respectively, cc. Columns carneee.

valves aside and makes its way into the bulbus aorta?. Here again the
course taken is that of least resistance; owing to the presence of the carotid
gland, the passage of blood into the carotid trunks is less free than into
the wide elastic systemic trunks. These will, therefore, receive the next
portion of blood which, the venous blood having been mostly driven to the
lungs, will be a mixture of venous and arterial. Finally as the pressure
rises in the systemic trunks the last portion of blood from the ventricle
which coming from the left side is arterial, will pass into the carotids and so
supply the head" (Parker and Haswell).

The muscle-fibers composing the walls of the heart from the sinus venosus

• the conus arteriosus are continuous, though at the sino-auricular, the

iculo-ventncular, and the ventriculo-conic junctions the continuity is to

PHYSIOLOGIC APPARATUS 7^5

ome extent interrupted by bands of circularly disposed fibrous tissue, serving
or the support of the valves, which momentarily interfere- with the ready
Passage of the contraction wave from one division of the heart to another.
Ihe frog heart receives its nutritive material from the blood flowing through
ts cavities. During the diastole the blood, under the influence of the slight
pressure developed, passes from the interior of the heart into a system of
rregular passageways or channels which penetrate the heart-wall in all
directions, and thus comes into direct contact with the heart-eel s. With
-he 'beginning of theSystole the blood is forced out of these channels into the
nterior of the ventricle, bringing with it the products of tissue metabolism.
1-he Heart-beat.-iJf the heart while beating is lifted up by a ligature
attached to thev apex it will be observed that the contraction begins in tb
walls of the sinus venosus, then passes to the auricles, thence to the ventnc
and finally to the conus; from this it may be inferred that the physiologic
stimulus acts primarily in the walls of the sinus from which its effect, viz
the excitation process, is conducted from one cavity to another in quick
succession.

[

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UNIVERSITY OF CALIFORNIA LIBRARY

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