MEDICAL

Gift of

Panama-Pacific Intern1
imposition Company.

A TEXT-BOOK

OF

HUMAN PHYSIOLOGY

BRUBAKER

A TEXT-BOOK

OF

HUMAN PHYSIOLOGY

INCLUDING A SECTION ON

PHYSIOLOGIC APPARATUS

BY

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

PROFESSOR OF PHYSIOLOGY AND MEDICAL JURISPRUDENCE IN THE JEFFERSON MEDICAL COL-
LEGE; FORMERLY PROFESSOR OF PHYSIOLOGY IN THE PENNSYLVANIA COLLEGE OF

DENTAL SURGERY; LECTURER ON PHYSIOLOGY AND HYGIENE IN

THE DREXEL INSTITUTE OF ART, SCIENCE, AND INDUSTRY

FOURTH EDITION. REVISED AND ENLARGED
WITH 1 COLORED PLATE AND 377 ILLUSTRATIONS

PHILADELPHIA

P. BLAKISTON'S SON & CO.

1012 WALNUT STREET
1913

COPYRIGHT, 1912, BY P. BLAKISTON'S SON & Co.

THE- M A P L K . I» H K S S . YORK. I' A

'

TO

KENNETH M. BLAKISTON

LOYAL FRIEND COURTEOUS GENTLEMAN

GENEROUS PUBLISHER
THE PRESENT EDITION OF THIS WORK

IS
AFFECTIONATELY DEDICATED

85

PREFACE TO FOURTH EDITION.

The preparation of a fourth edition of this Text Book of Physiology has
furnished the opportunity for revision and the incorporation of new matter.
Through condensation and elimination it has become possible to insert addi-
tional matter equivalent to some fifty pages without increasing the size of
the volume. The new paragraphs which have been inserted in the various
chapters contain facts relating to the mechanic movements of the stomach
and intestines and the nerve mechanisms regulating them; the digestion and
absorption of the proteins; the viscosity, specific gravity and coagulability
of the blood; the physiologic mechanism of the heart and the properties of
the cardiac muscle; the venous pulse; the auscultatory method of determin-
ing blood pressure; the modifications of the respiratory rhythm; the physio-
logic action of the pituitary gland and the adrenals, etc. These additions,
it is believed, will enhance its value to the medical student and practitioner.

In the preparation of this, as of preceding editions the aim has been to
present the facts in a form that will familiarize students with the essential
problems of physiology.

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.

I am also indebted to Dr. Lucius Tuttle, demonstrator of physiology,
for the reading of the proof.

Vll

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 23

CHAPTER IV.

HISTOLOGY OF THE EPITHELIAL AND CONNECTIVE TISSUES 30

CHAPTER V.

THE PHYSIOLOGY OF MOVEMENT 38

CHAPTER VI.
THE PHYSIOLOGY OF THE SKELETON 44

CHAPTER VII.
GENERAL PHYSIOLOGY OF MUSCLE- TISSUE 48

CHAPTER VIII.
THE GENERAL PHYSIOLOGY OF NERVE-TISSUE 87

CHAPTER IX.
FOODS 115

CHAPTER X.
DIGESTION 133

CHAPTER XI.
ABSORPTION 205

CHAPTER XII.
THE BLOOD 227

CHAPTER XIII.
THE CIRCULATION OF THE BLOOD 261

CHAPTER XIV.

THE CIRCULATION OF THE BLOOD (Continued) , 319

xi

xii CONTENTS.

PAGE
CHAPTER XV.

RESPIRATION 377

CHAPTER XVI.
ANIMAL HEAT 429

CHAPTER XVII.
SECRETION 438

CHAPTER XVIII.
EXCRETION 470

CHAPTER XIX.
THE CENTRAL ORGANS OF THE NERVE SYSTEM AND THEIJI NERVES 498

CHAPTER XX.
THE MEDULLA OBLONGATA; THE ISTHMUS OF THE ENCEPHALON; THE BASAL GANGLIA . 519

CHAPTER XXI.
THE CEREBRUM 537

CHAPTER XXII.
THE CEREBELLUM 565

CHAPTER XXIII.
THE CRANIAL NERVES 572

CHAPTER XXIV.
THE AUTONOMIC OR SYMPATHETIC NERVE SYSTEM 608

CHAPTER XXV.
PHONATION; ARTICULATE SPEECH 620

CHAPTER XXVI.
THE SPECIAL SENSES 631

CHAPTER XXVII.
THE SENSE OF SIGHT 642

CHAPTER XXVIII.
THE SENSE OF HEARING 677

CHAPTER XXIX.
REPRODUCTION 688

APPENDIX.
PHYSIOLOGIC APPARATUS • . . . . 704

INDEX 727

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. Special 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 anatomy, 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. Special 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 of all mammals, to which class man zoologically
belongs, as well as of all birds, reptiles, amphibians, and osseous fish, is
characterized by the presence of a bony, segmented axis, which extends in a
longitudinal direction from before backward, and which is known as the
vertebral column or backbone. In virtue of the existence of 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 —

INTRODUCTION. 3

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

2. A bony arch arising from the dorsal aspect and surmounted by a spine-
like process.

At the anterior extremity of the body of the animal the 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 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 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 fo$ 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 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 contained a musculo-mem-
branous 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 extremity 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
alimentary canal and the various cavities connected with it are lined through-
out by a mucous membrane.

The 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

4 TEXT-BOOK OF PHYSIOLOGY.

latter of layers of scales or cells. Embedded within the skin are numbers of
glands, which exude, in the different classes of animals, sweat, oily matter,
etc. Projecting from the surface of the skin are hairs, bristles, feathers, claws.
Beneath the skin are found muscles, bones, blood-vessels, nerves, etc.

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

Anatomic Systems. — All the organs of the body which have certain
peculiarities of structure in common are classified by 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.

INTRODUCTION. 5

The nerves and muscles constitute the nervo-muscle apparatus, the
function of which is the production of motion. The eye, the ear, the nose,
the tongue, and the skin, with their related structures, 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, fatty, and protein groups of organic bodies; the inorganic compounds
consist of water, various acids, and inorganic salts.

The compounds or proximate principles thus obtained can be further
resolved by an ultimate analysis into a small number of chemic elements
which are identical with elements found in many other organic as well as
inorganic compounds. The different chemic elements which are thus
obtained, and the percentages in which they exist in the body, are as follows
—viz., oxygen, 72 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, 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 2:1.
The molecule of the carbohydrates above mentioned consists of either six
atoms of carbon or a multiple of six; in the latter case the quantity of
hydrogen and oxygen taken up by the carbon is increased, though the
ratio remains unchanged.

The carbohydrates may be divided into three groups — viz.: (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.

i. AMYLOSES, (C8H10O5)n.

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

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

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 — ery thro dextrin, which strikes a red color with iodin, and achroo dex-
trin j 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.

8 TEXT-BOOK OF PHYSIOLOGY.

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; dilute acids can
convert it into dextrose.

2. DEXTROSES, C6H12Oe.

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

When obtained from any source, it is soluble in 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 — are present in solution, the sugar
will abstract from the cupric hydroxid a portion of its oxygen, thus reducing
it to a lower stage of oxidation giving rise to cuprous oxid. Sugar has a
similar action on both silver and bismuth. On this property of sugar a
standard solution of cupric hydroxid was suggested by Fehling which may
be employed for both qualitative and quantitative tests for the presence of
sugar in solution.

Fehling's Test Solution. — This is a solution of cupric hydroxid made
alkaline by an excess of sodium or potassium hydroxid with the addition
of sodium and potassium tartrate. It is made by dissolving cupric sulphate
34.64 grams, potassium hydroxid 125 grams, sodium and potassium tartrate
173 grams, in distilled water sufficient to make one liter.
The reaction is expressed by the following equation :

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

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

CHEMIC COMPOSITION OF THE HUMAN BODY. 9

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:

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-
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:

io TEXT-BOOK OF PHYSIOLOGY.

C,2H22On + H20 = C6H12Ofl + C6H12Oa
Saccharose + Water — Levulose-f 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 baciHus it is decomposed into
lactic acid, and finally into butyric acid, as expressed in the following
equation:

C12H220U + H^o - 4C3H603

Lactose + Water = Lactic Acid.

2C3H603 = C4H802 + 2C02 + 2H2
Latic 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:

2C6H1005 + 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-
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

CHEMIC COMPOSITION OF THE HUMAN BODY. n

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(HO)3 + 3C18H3602 = C3H5(C18H35O2)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 radicle, tritenyl,
C3H5.

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 71° 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:

C3H5(C18H3302)3+3H20 = C3
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.

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 + 3
Potassium. Oleic Acid. Potassium Oleate. W;

ater.

12 TEXT-BOOK OF PHYSIOLOGY.

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 15.6 23.9 0.4 (Wurtz).

Serum-albumin 53.0 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).

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
Schiitzenberger 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.

CHEMIC COMPOSITION OF THE HUMAN BODY. " 13

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 amino-acids and
diamino-arids, 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 proteins, owing to their complexity and instability, are prone
to undergo disintegration and reduction to simpler compounds. This
decomposition or putrefaction occurs most readily when the conditions
most favorable to the growth of bacteria are present — viz., a temperature
varying from 25° C. to 40° 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

i4 TEXT-BOOK OF PHYSIOLOGY.

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 o£ the fact that when they
are hydrolyzed they yield only 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-
brom (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 heocone
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 certain number of 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. 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.

CHEMIC COMPOSITION OF THE HUMAN BODY. 15

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°.

(b) Egg-albumin. — Though not a constituent of the human body, egg-
albumin resembles the foregoing in many respects. When obtained
in the solid form from the white of the egg, it is a yellow mass without
taste or odor. Though similar to serum-albumin, it differs from
it in being precipitated by ether, in coagulating at 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
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 CO 2 through a watery extract of the crystalline lens.

16 TEXT-BOOK OF PHYSIOLOGY.

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.

(6) 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.

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

(d) Keratin is the substance found in all horny and epidermic tissues,
such as hairs, nails, scales, etc. It differs from most 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, coagulation does not take place.

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

CHEMIC COMPOSITION OF THE HUMAN BODY. 17

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 hydrolysed by acids or alkalies, hemoglobin undergoes a
cleavage into a protein, globin, and a pigment hematin.

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

GLUCO-PROTEINS.

(a) Mucin. — Mucin is the 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, and in other situations. They differ slightly one from the
other in properties and chemic composition. They yield on decom-
position 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 hydrolysed, they
separate into a protein and a nuclein. The nucleins derived from
cell nuclei can be still further separated into a simpler protein and
nucleic acid, which latter in turn yields phosphoric acid and the so-
called purin bases, xanthin, hypoxanthin, adenin, and guanin. All
nucleins which yield the purin bases are termed true nucleins.

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

i8 TEXT-BOOK OF PHYSIOLOGY.

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 ammo-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.

(b) 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 100° C. or to the prolonged action of alcohol. They are insoluble in
water, in dilute acids, and in neutral saline solutions.

In this same group may be included also those coagulated 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 pro-
duct is an alkali-albumin. Fibrin possesses the property of decom-
posing hydrogen dioxid, H2O2 — i.e., liberating oxygen, which accu-
mulates in the form of bubbles on the fibrin. On incineration fi-
brin yields an ash which contains calcium phosphate and magnesium
phosphate.

CHEMIC COMPOSITION OF THE HUMAN BODY. 19

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 (Howell).

(b) Myosin fibrin andMyogen 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
ty rosin.

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. A few drops of copper sulphate solution are

first added to the protein solution, and then an excess of sodium
hydroxid. A blue-violet color is produced, which deepens somewhat
on heating, but no further change ensues. This is also known as
Piotrowski's test: As this same color is developed with the substance
biuret, it is also known as the biuret reaction. Biuret is formed by
heating urea to 180° 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.j 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

20 TEXT-BOOK OF PHYSIOLOGY.

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-
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 r61e. 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

CHEMIC COMPOSITION OF THE HUMAN BODY. 21

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, in all
probability, formed in the body from the decomposition and oxidation of
the proteids.

POTASSIUM COMPOUNDS.

Potassium chlorid, KC1, is met with in association with sodium chlorid
in almost all situations in the body. It preponderates, 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
very small amount of this salt, but, as previously stated, a relatively large
quantity of sodium chlorid. Though introduced to some extent in the food,
it is very likely that it is also formed through the decomposition of the 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

22 TEXT-BOOK OF PHYSIOLOGY.

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.

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.

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 nitrogenized 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 cell — the ovum. As
the cell is the elementary unit of all tissues, the function of each tissue must
be referred to the function of the cell. Hence the cell may be defined as the
primary anatomic and physiologic unit of the organic world, to which every
exhibition of life, whether normal or abnormal, is to be referred.

Structure of Cells. — Though cells vary in shape and size and internal
structure in different portions of the body, a typical cell may be said to 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 7.7/1 (^-gVo" °f an inch, the diameter of a red blood-
corpuscle), to 135/4 GrJ-o- 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 globules 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, in 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

23

TEXT-BOOK OF PHYSIOLOGY.

completely incloses the cell substance. The cell membrane is permeable
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. %

Linin.

Nuclear fluid (matrix). - ./

Nucleolus.

Chromatin-cords ,,.
(nuclear network).

Nodal enlargements ^-''
of the chromatin.

• • Cell membrane.
Exoplasm.

Microsomes.
Centrosoma.

' ~ 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

PHYSIOLOGY OF THE CELL. 25

possibly fat and carbohydrates. Inorganic salts, especially the potassium,
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 denned 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,

26 TEXT-BOOK OF PHYSIOLOGY.

constituents of the fluids circulating in the tissue spaces. These foods carry
into the body potential energy, ultimately derived from the sun. When they
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. Thus Voit contended that the tissue
molecules are comparatively stable in composition and under ordinary con-
ditions of nutrition do not undergo any material change during either rest
or activity, and that metabolism is confined to the food materials occupying
spaces in and around the living cell. The cause which initiates this metab-
olism is unknown, but is supposed to reside in the cell, if it is not a property
of the cell itself. Because of the fact that but a very small amount, if any,
of sugar or fat enters into the composition of bioplasm it is generally admitted
that these foods are metabolized in the tissue spaces and in the manner just
alluded to. The problem, however,, is different in the case of the proteins.
Voit contended, as previously stated, that the proteins of the tissue molecules,
which he distinguished as tissue proteins, do not metabolize and confined all
protein metabolism to the food proteins circulating in the tissue spaces and
which he distinguished as circulating proteins. Even in starvation the tissue
proteins, as such, do not metabolize until they have been dissolved in con-
sequence of chemic changes and transformed into circulating protein.

Pfliiger, however, asserted that the circulating proteins cannot be metab-
olized but that they must first be built up into tissue proteins. The metab-
olism of protein is, therefore, confined, in this view, to the molecules of
the living material. It is possible, however, that both views are correct and
that in the physiologic condition the activity of the tissues is attended by a
partial destruction of the tissue molecules which is followed in turn by con-
struction 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.

PHYSIOLOGY OF THE CELL.

27

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 — i. 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.
Th 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
much looser arrangement, known as the loose skein. During the
formation of the loose skein, the centrosome divides into two portions,

28 TEXT-BOOK OF PHYSIOLOGY.

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
of bioplasm found in nerves, which serves to transmit, with extreme rapidity,

PHYSIOLOGY OF THE CELL. 29

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 are
exhibited, more or less constantly, by many cells in the animal body — e.g.,
the waving of cilia, the ameboid movements and migrations of white blood-
corpuscles, the activities of 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 of
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
with 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
cemented 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

30

THE CONNECTIVE TISSUES. 31

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-
vessels, the lymph-vessels, the peri-
toneal, pleural, pericardial, and
other closed cavities are usually

i

FIG. 4.— STRATIFIED SQUAMOUS FIG. 5.— STRATIFIED CILIATED

EPITHELIUM (LARYNX OF MAN). EPITHELIUM. X 56°- From the res-

X240. i. Columnar cells. 2. Prickle- piratory nasal mucous membrane

cells. 3. Squamous cells. — (Stohr.} 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

32 TEXT-BOOK OF PHYSIOLOGY.

feeble r61e as absorbing agents in man and the higher animals. The
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 struct-
ures 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

THE CONNECTIVE TISSUES.

33

kidneys, and in the bones, is practically but a modification of areolar tissue.
In these situations it presents itself in small masses or lobules of varying
size and shape, surrounded and penetrated by the fibers of connective
tissue. (See Fig. 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 retrograde change in
the protoplasmic material of the connective-tissue cells. When this proto-
plasm becomes rich in carbon and hydrogen, it is speedily converted into
fat, which makes its appearance in the form of minute drops in different

In

super-
posed
layers.

FIG. 6.— ADIPOSE TISSUE.— (Stokr.)

FIG. 7. — FAT-CELLS FROM THE
AXILLA OF MAN. i. The equator
of the cell in focus. 2. The ob-
jective somewhat elevated. 3, 4.
Forms changed by pressure, p.
Traces of protoplasm in the vicinity
of the flat nucleus k.—(Stohr.)

portions of the cell. As the drops accumulate, at the expense of the cell
protoplasm, they gradually coalesce, until there remains but a thin stratum
of the protoplasm, which forms the wall of the vesicle. Adipose tissue
may, therefore, be regarded as areolar tissue, in which, and at the expense
of some of its elements, fat is stored for the future needs of the organism.
A diminution of food, especially of fat and carbohydrates, is promptly
followed by an absorption of fat by the blood-vessels and by its transference
to the tissues, where it is either utilized for tissue construction or for 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

3

34

TEXT-BOOK OF PHYSIOLOGY.

found, can be resolved into elementary bundles, which on microscopic exam-
ination 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
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,

FIG. 8. — CONNECTIVE-TISSUE
BUNDLES OF VARIOUS THICK-
NESSES or THE INTERMUSCULAR
CONNECTIVE TISSUE OF MAN.
X 240.— (Stohr.)

FIG. 9. — ELASTIC FIBERS OF THE
SUBCUTANEOUS AREOLAR TISSUE OF
A RABBIT.— (After Schafer.}

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.)

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 embeded
in an abundant quantity of ground substance (Fig. IO,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

THE CONNECTIVE TISSUES.

35

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.

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

13

c —

ABC

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

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.
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.

TEXT-BOOK OF PHYSIOLOGY.

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.)

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.

Marrow.

FIG. ii. — FROM A CROSS-SECTION OF A METACARP OF MAN. X 50. The Haversian canals
contain a little marrow (fat-cells) . Respiration 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,

THE CONNECTIVE TISSUES. 37

which arise through the transformation of the cell protoplasm. In the
cancellated tissue, near the extremities of the long bones, this fatty transfor-
mation 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
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 roles. 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 movements 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 an other, or general as when it changes its position
relatively to the environment as in the various acts of locomotion. The
external movements are the result of the cooperation of the skeletal muscles
and the bones of the skeleton to which they are attached. The muscles
possess the power of suddenly shortening or contracting and by virtue of
their relation to the bones impart to them all the external movements char-
acteristic of the animal. The change of relation of the bones and hence of
the parts of the animal of which they form a part, are dependent on the
construction of the joints.

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.

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.

38

THE PHYSIOLOGY OF MOVEMENT. 39

The internal movements are exhibited by the viscera, the vascular appa-
ratus, and by glands, and, though less obvious, are no less characteristic.

The viscera, by virtue of the presence of non-striated muscle-fibers in
their walls, are capable of changing their caliber from moment to moment
either in the way of an increase or decrease and thus regulate and control
the passage of their contents through them.

The vascular apparatus, and its adjunct, the lymph-vessel apparatus, is
engaged in the distribution of blood and nutritive material throughout the
body. The heart drives the blood through the vessels in opposition to the
friction presented by their walls, while the vessels themselves and especially
the arteries, by virtue of 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.

When these various organs are applied to the overcoming of various
resistances or forces, as they are in the performance of their functions, it can
also be said that they too are doing work. The cooperation 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.

The various tissues of the body, mentioned in foregoing 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
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 the Motor Organs. —
The nerve tissue is arranged partly in masses contained within the cavities
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,

40

TEXT-BOOK OF PHYSIOLOGY.

c. s.c.

DC..

SC

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.m. a
skeletal muscle acting in an upward direction and the source of the active power to be ap-
plied to the lever; sp.c. transection of the spinal cord showing the relation of the white and
the gray matter: m.c. a motor cell in the anterior horn of the gray matter; ef.n. an effer-
ent nerve-fiber connecting the motor cell from which it arises with the skeletal muscle and
contained in the ventral roots of the spinal nerves; 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
ford on the other hand and contained in the dorsal roots of the spinal nerves; c.s.c.
coronal section of the cerebrum showing the relation of the gray to the white matter; v.c.
a volitional or motor cell; d.a. a descending axon or nerve-fiber connecting the volitional
cell from which it arises with the motor cell in the spinal cord; s.c. a sensor cell; a.a. an
ascending axon or nerve-fiber connecting a receptive cell from which it arises (not shown in
the diagram) with the sensor cell in the gray matter of the cerebrum. The nerve-fibers
which pass outward from the spinal cord to the glands, blood-vessels, and the muscle
walls of the viscera, have for the sake of simplicity been omitted from the diagram.

THE PHYSIOLOGY OF MOVEMENT. 41

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 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 excited to activity is necessary to an understanding of the prob-
lem of movement.

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

,sp.c.

FIG. 13. — 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 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, vr. and d.r., ventral and dorsal roots
and spinal nerves.

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. To each side of the cord there is attached thirty-one nerves,
which as they pass out through foramnia in the walls of the spinal column are
termed spinal nerves. Each spinal nerve is connected with the spinal cord
by two roots, termed from their relation to the ventral and dorsal surfaces,
the ventral and dorsal roots.

Experimental investigation has demonstrated that the ventral roots are
connected peripherally with the motor organs and transmit to them nerve
energy developed in the spinal cord; that the dorsal roots are connected
peripherally with the skin, mucous membranes, etc., and transmit nerve
energy developed in their terminations to the spinal cord. The ventral
and dorsal roots are therefore termed from their function, efferent and afferent
nerves respectively.

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

42 TEXT-BOOK OF PHYSIOLOGY.

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.

In the ventral horns of the gray matter are located large nerve-cells
which give origin to nerve- fibers; these fibers in their growth pass forward
through the cord and emerge as ventral roots; continuing to grow, these
fibers gradually reach and become connected with the motor organs to
which they are by heredity directed.

It has been experimentally demonstrated that each nerve-cell 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 ganglia of the sympathetic nerve system 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 state, that is, the muscle becomes shorter
and thicker, and the bone to which it is attached is moved. This is at
once followed by a return of the muscle to the passive state; that is, it
lengthens, 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
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.

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 are
apparently only influenced in their activity by emotional states.

In the case of reflex movements, the nerve impulses are primarily devel-

THE PHYSIOLOGY OF MOVEMENT. 43

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 nerve-cells which are in turn excited to activity.

In the case of volitional skeletal-muscle movements, the nerve impulses
which cause the movements 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. Fig. 12.

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

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

A consideration of the body movements involves an account of (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, only those physical and physiologic peculiarities of the
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:

i. 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

44

THE PHYSIOLOGY OF THE SKELETON. 45

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 fibro-cartilage are inserted
between the surfaces of the bones.

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

entirely inclosing the cavity of the joint. This 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.

i. 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
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

46 TEXT-BOOK OF PHYSIOLOGY.

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.

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

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

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

opposing surfaces of the bones are immovably united, and hence do not

THE PHYSIOLOGY OF THE SKELETON. 47

permit of any movement — e.g., the joints between the bones of the
skull.

The Vertebral Column. — In all static and dynamic states of the body
the vertebral column plays a most essential r61e. Situated in the middle of
the back of the trunk, it forms the foundation of the entire skeleton. It is
composed of a series of superimposed bones, termed vertebrae, which increase
in size from above downward as far as the brim of the pelvic cavity. Superi-
orly, 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 nbro-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.

The amphiarthrodial character of the intervertebral joints endows the
entire 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 movement, 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.

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 connected with the bones of the
skeleton in such a manner that, by an alteration in their form, they can
change not only the position of the bones with reference to one another, but
can also change the individual's relation to surrounding objects. They are
therefore, the active organs of both motion and locomotion, in 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. Voluntary muscles, comprising those the activity of which is called

forth by an act or effort of volition.

2. Involuntary muscles, comprising those the activity of which is entirely
independent of the volition.

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

THE VOLUNTARY OR SKELETAL MUSCLE.

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-
ment; the tendon is the inactive region, the passive transmitter of the move-
ment to the bones.

A skeletal muscle is a complex organ consisting of a framework of
connective tissue, supporting muscle-fibers, blood-vessels, nerves, and

48

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

49

P

lymphatics. The general body of the muscle is covered by a dense layer
of connective tissue, the epimysium, which blends with and partly forms
the tendon. From the under surface of this covering, septa of connective
tissue pass inward, dividing and grouping the fibers into larger and smaller
bundles, termed fasciculi. The fasciculi, invested by a special sheath, the
perimysium, are prismatic in shape and on cross-section present an irregular
outline. The muscle-fibers composing the fasciculi are separated one from
another and supported by a very delicate connective tissue, the endomysium.
The connective tissue thus surrounding and penetrating the muscle 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 trie-
ultimate anatomic unit of the muscle
system. The fibers for the most part
are arranged parallel one to another
and in a direction corresponding to
the longj 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 m ^muscle-fibers; many are shrunken and be-

cylindric or prismatic in shape and
to consist of a thin transparent mem-
brane, the sarcolemma, in which is
contained the true muscle 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 muscle-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).

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

FIG. 14. — FROM A CROSS-SECTION OF THE
ADDUCTOR MUSCLE or A RABBIT. P. Peri-
mysium, containing two blood-vessels, at g;

tween them the endomysium, p, can be seen;
at x the section of muscle-fiber has fallen out.
X 6o.—(Stohr.)

TEXT-BOOK OF PHYSIOLOGY.

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
internbrillar 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 polarised light, the dim band ap-
pears bright and the bright band appears
dim against a dark background, indicat-
ing that the former is doubly refracting
or anisotropic, the latter singly refracting
or isotropic.

FIG. 16 — 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. — (Pier sol.')

FIG. 15. — MUSCLE-FIBER
OF A RABBII. a. Dark
band. b. Light band. c. In-
termediate line. n. Nucleus.
— (Lai"lois and Stirling)

This interpretation of the structure of the muscle-fiber has been subjected
to criticism in recent years by Heidenhain. This observer regards the trans-
verse line in the bright band as did its discoverer Krause as a true membrane
which is attached laterally to the sides of the sarcolemma. The fibrillae he
also regards as continuous but of uniform thickness, and passing directly
through the transverse membrane by which they are supported and main-
tained in their normal relation. In this view the fibrilla consists of alternate
regions of a doubly refracting and a singly refracting material. The sarco-
plasm is, therefore, confined to the internbrillar spaces. Fig. 17.

The fiber of the pale muscle is similar histologically to the fiber of the
red muscle. It, however, does not contain so much granular protoplasm as
does the fiber of the red muscle and hence does not intercept the light to the
same extent. The greater the quantity of granular protoplasm the darker
the muscle.

The Blood-supply. — Muscles in the physiologic condition require for
the maintenance of their activity a large amount of nutritive material. This

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

cd

bl

Illllil

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
such cases the nerve-fibers divide near their termination GRAM
until the number of branches equals the number of muscle- ,7

-, r™ • j- -j i i r-i ' , j *W fro™ Stohr's

fibers. The individual muscle-fiber is penetrated near Histology."} The
its center by the nerve where it terminates; the ends fibrillae consists of
being practically free from nerve influence. The stimulus ^ndHght bands"'
that comes to the muscle-fiber acts primarily upon its /.'&.*' /.&. is crossed
center, the effect of which then travels in both directions by Krause's mem-
to the ends. The manner in which the nerve-fibers termi- ™&£j * ctSes
nate in muscle will be more fully described in connection the dim band accord-
with the histology of the nerve tissue. 'm% to Heidenhain.

Illllil

FIG. 17.— DIA-
OF MUSCLE

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 :

52 TEXT-BOOK OF PHYSIOLOGY.

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 myoalbumin. Among the proteins may be mentioned hemoglobin, which
gives the color to the muscles. Spectroscopic 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.

GENERAL PHYSIOLOGY OF MUSCLE TISSUE.

53

Cohesion. — The cohesion of a muscle is largely dependent on the quan-
tity of connective tissue it contains. A band of fresh human muscle one
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 '^.—EXTENSION CURVE OF
i.e. , 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 grains 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-
ally increasing energy until its normal length is
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-

FIG. 19. — CURVE OF ELAS-
TICITY PRODUCED BY CONTINU-
OUS EXTENSION AND RECOIL
OF a FROG'S MUSCLE, o x. Ab-
scissa before; x'} after exten-
sion.— (Landois and Stirling.}

54 TEXT-BOOK OF PHYSIOLOGY.

tenance of physiologic conditions. If the nutrition is impaired by fatigue,
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 tfyat 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 role 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

GENERAL PHYSIOLOGY OF MUSCLE TISSUE. 55

the withdrawal of the stimulus the muscle again undergoes a reverse change
of form, becoming longer and narrower, and returning to its original condi-
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, with which it is
closely associated, are maintained by a due supply of oxygen, and of nutritive
material, the removal of waste products, 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

56 TEXT-BOOK OF PHYSIOLOGY.

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
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, ammo-
nium vapor, distilled water, glycerin, and sugar.

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

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 induced

current may be used.1

The Constant Current. — If the ends of the wires in connection with an
electric cell be provided with non-polarizable electrodes and the latter placed
on opposite ends of a freshly prepared sartorius muscle of a frog which has
been previously curarized, it will be found on closing or making the circuit
that the muscle will exhibit a short, quick pulsation. During the actual
passage of the current, especially if it is weak, there may be no apparent

1 Since the study of the physiologic properties of both muscle-tissue and nerve-tissue
involves the employment of electricity as a stimulus, it is necessary for the student to
familiarize himself with certain forms of apparatus by which it is generated, controlled, and
applied. To avoid interrupting the continuity of the text this information is embodied
in an appendix. The facts therein contained should be mastered at this time by the student.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 57

change in the muscle. If the current is strong, the muscle may, on the
contrary, remain in a state of continuous contraction. With the opening or
breaking of the current the muscle at once relaxes, or perhaps again contracts
and then relaxes. The extent of the contraction depends mainly on the
strength of the current, being greater with strong, less with weak currents.
When the current is sufficiently strong to elicit both making and breaking
contractions, it is found that the contraction occurring on the make or closure
of the circuit is 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

v

A. B.

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, in 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

TEXT-BOOK OF PHYSIOLOGY.

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.

PHENOMENA FOLLOWING A MUSCLE STIMULATION.
PHYSICAL PHENOMENA.

Physiologic investigation has made it apparent that when a nerve impulse
reaches a muscle, it occasions a disruption of certain complex energy-
holding 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. 21. — 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 in the
shape of the muscle, first.

Change of Shape. — The most obvious change in a muscle following the
arrival of a nerve impulse is that relating to its form. The muscle not only
becomes shorter, but at the same time thicker. The extent to which it may
shorten when unopposed may amount to 30 per cent, or more of its original
length. The increase in thickness practically compensates for the diminu-
tion in length, for there is no observable diminution in volume. The change
in form of the entire muscle results from a corresponding change of form of
its individual fibers as determined by microscopic examination, each of
which becomes shorter and thicker. The successive changes in both the
muscle and the individual fibers are represented in Fig. 21.

When the contraction begins both the dim and bright bands diminish in
length, but at the same time increase in breadth. This continues until the
contraction reaches its maximum. The diminution in the length of the

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

59

bright band is greater proportionally than the diminution in the length of the
dim band, a fact which gave rise to the supposition on the part of Englemann
that there is at the time of the contraction a passage of fluid material from
the bright into the dim band or from the sarcoplasm into the sarcostyles.
When the relaxation begins, a reverse change in the dim and bright bands
sets in and continues until they regain their former shape and volume. Coin-
cidently there is a passage of fluid material from the sarcostyles to the
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

— o

FIG. 22. — EXTENSION CURVES: B B', 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

60 TEXT-BOOK OF PHYSIOLOGY.

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
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 muscle 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 perio d 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 chlorid,
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 place
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. To
accomplish this object the muscle is attached at one extremity by a clamp
to a firm support, and at the other extremity to a weighted lever, which is,
however, sufficiently light to enable it to take up, reproduce, and magnify
its movements. The end of the lever provided with a point is applied to
a smooth surface, such as glazed paper on a cylinder or plate, covered
with lampblack. If the surface is stationary, the contraction is recorded as a
vertical line; if it is put in movement at a uniform rate by clockwork, the

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

61

contraction is recorded in the form of a curve, the width of the arms of which
will depend on the rate of movement. The time relations of the phases of
the contraction can be obtained by placing beneath the lever a writing point
attached to an electro-magnet thrown into action by a tuning-fork vibrating
in hundredths of a second. In order to determine the rapidity with which
the contraction follows the stimulation, it is essential that the moment of
the latter be also recorded. This is accomplished by an automatic key, the
opening or closing of which develops the stimulus which excites the muscle.
A combination of these different appliances constitutes a myograph and the
curve of contraction a my o gram. (See Fig. 23.)

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

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.

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

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."

62 TEXT-BOOK OF PHYSIOLOGY.

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 bed, 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 :

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

FIG. 24. — THE ISOTONIC MYOGRAM.

taking place preparatory to the exhibition of the movement. The
duration of the latent period in influenced by a variety of conditions,
e.g., temperature, fatigue, strength of stimulus, etc.

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

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

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

The relaxation as shown by the character of the curve begins slowly,
then proceeds rapidly, and again slowly as the muscle attains its
original length. The termination of the 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 after-vibrations,
which are probably due to the inertia of the lever as well as to changes
in the muscle elasticity.

The duration of the period of shortening is about 0.04 second, and
of the period of relaxation 0.05 second. A single pulsation of the isolated
muscle of the frog therefore occupies, from the moment of stimulation to
termination, the tenth of a second. Muscles of many other animals have
a contraction period the duration of which varies considerably from this.
Thus, in man the time of a single contraction is one-twentieth of a second,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 63

in some insects one three-hundredth of a second, and in the turtle one second.
Pale muscles have a shorter period than the red.

Influences Modifying the Effect of the Stimulus. — The contraction
process in its entirely as well as in its individual parts is considerably modi-
fied by both external and internal conditions, among which may be mentioned
the following:

i. Character 0} the Stimulus. — As the contraction is the response of the
muscle to a stimulus, it may be inferred that the vigor of the former is
proportional, within limits, to the strength of the latter. Thus using
as a stimulus the single induced current, it has been found that if the
strength of the current is progressively increased, the height of the 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

A. B.

FIG. 25. — TRACING 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.

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.

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. If the intervals between the successive
stimulations be such as permit the muscle to recover from the effects of
the contraction, it may contract as many as a thousand times without
showing any particular variation from the normal form; but if the
intervals are shorter than that just stated it is found that from the
beginning of the stimulation each succeeding contraction slightly exceeds
in height the preceding contraction, until a certain maximum is reached
and maintained, indicating that for some reason the irritability and the
energy of the contraction have been increased. This gradual increase

64 TEXT-BOOK OF PHYSIOLOGY.

in the height of 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-

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

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
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.

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

2. 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 30° C. If the temperature
of the muscle falls to 20° C. there is a corresponding decline in activity,
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 40° C. to 50° C.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 65

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.)

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.

Rapidly-repeated Stimulation. — Prolonged or excessive activity of our own
muscles is accompanied by a feeling of stiffness or soreness and lassi-
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 diminish its contractile power and its capacity
for doing work. The more rapidly they are removed, the sooner is
a fatigued muscle restored to its normal condition. The 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
5

66 TEXT-BOOK OF PHYSIOLOGY.

impulse to the muscle. By this means it is protected from absolute
exhaustion.

5. 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.
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-

FIG. 29.— a. DIAGRAM OF ISOTONIC; b, sion obtained by the isometric method
Z< 4»Sg? MUSCLE CURVES-("S 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
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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

67

state known as galvanotonus; on opening the circuit at o the muscle again con-
tracted, after which it gradually relaxed and returned to its original
condition. 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.

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

In the physiologic activities of the body the muscles at each contraction not
only overcome the resistances of antagonistic muscles, the weight of the
limbs, the friction of joints, etc., but in addition overcome various external
resistances connected with the evironment — e.g., gravity, cohesion, friction,
elasticity, etc. The muscles may therefore be regarded as machines for
the accomplishment of work. Experimentally the work done by an iso-
lated muscle maybe 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;
but as weights differing by a com- ^ _TRACING SHOWING THE GRADUAL
mon increment are added, the DIMINUTION IN THE HEIGHT OF THE CONTRAC-
height of the contraction dimin- TION AS THE WEIGHT WAS INCREASED BY A COM-

ishes until with a given weight it MON INCREMENT OF I0 GRAMS FROM ° T0 l8°

GRAMS. MAGNIFICATION OF THE LEVER, 4-

IS ml.

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.

68 TEXT-BOOK OF PHYSIOLOGY.

Weight. Height. Work Done.

o grams 14 mm. o gram-millimeters.

50 grams 9 mm. 450 gram-millimeters.

100 grams 7 mm. 700 gram-millimeters.

150 grams 5 mm. 750 gram-millimeters.

200 grams 2 mm. 400 gram-millimeters.

250 grams o mm. o gram- 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.

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 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 144 sq. centi-
meters 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 grad-

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

69

ually 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— thaHs, 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 summa-

tion of several stimuli. If a second max-

imal 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 to-

gether, 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-
mal 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 FIG. 32.— TRACING SHOWING THE EF-
complete according to. the number of -- gSSfifSSSSStt.
stimuli that reach the muscle in a^sec- VAL ON A MUSCLE CONTRACTION. To be

Ond Of time. When a muscle is Stimu- read from below upward.

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 corresponding 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 com-
pletely before the arrival of the succeeding stimulus, and hence remains in
a more or less contracted state, during which it exhibits a series of alternate
partial contractions and relaxations. To this condition of muscle activity

70 TEXT-BOOK OF PHYSIOLOGY.

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-

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.)

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
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

FIG. 34. — DEVELOPMENT OF FATIGUE AND CONTRACTION. Muscle stimulated once a second by

a strong induced current.

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
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.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 71

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 visible, it can be shown by
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
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: —

™ . , . f Volitional.
. i. Physiologic {Reflex_

2. Experimental.

3. Pharmacologic.

•0^1 • f Bacterial.

4. Pathologic (Reflex

i. 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

72 TEXT-BOOK OF PHYSIOLOGY.

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 -^
of a second in duration.

2. Reflex. — A tetanus of muscle, physiologic in character, arises during
the performance of many muscle movements in consequence of peripherally
acting causes and may therefore be termed a reflex tetanus. The duration
of a tetanus thus induced, like the duration of a volitional tetanus, will vary
with the duration of the exciting cause. Reflex tetani are presented by the
muscles of the lower jaw during mastication, by the intercostal muscles
during breathing, by the muscles of the limbs during walking, etc. In these
and other 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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 73

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.

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 18 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

74 TEXT-BOOK OF PHYSIOLOGY.

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
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. — Notwithstanding many investigations,
the nature of the materials which are the immediate source of the muscle
energy is not known. The absence of any noticeable increase in the quan-
tity of urea or other nitrogen-holding compounds excreted renders it prob-
able 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. It has been suggested that glycogen furnishes
the energy, inasmuch as this substance, generally present in muscle, dis-
appears 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 pre-
vented from contracting accumulates glycogen. Bunge is of the opinion
that though the carbohydrates are the main, they are not the only sources of
muscle energy. If there is a deficiency or 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 or third day. The
mechanism by which the energy is liberated, whether by direct oxidation or
decomposition is uncertain. The general trend of experimental investigation

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 75

points to the disruption of some carbohydrate, perhaps sugar, derived from
the stored glycogen and the oxidation of the intermediate products to carbon
dioxid and water. The oxidizable compound appears to be lactic acid.
For if the muscle be made to contract in an atmosphere deficient in oxygen,
the amount of lactic acid produced is relatively large and the amount of
carbon dioxid relatively small. If the surrounding atmosphere be rich
in oxygen, the reverse conditions obtain. Under physiological conditions,
when the muscle is supplied with blood containing its customary percentage
of oxygen, it is probable that the products set free by the disruption of the
sugar molecule are rapidly oxidized to CO2 and H2O, with the liberation of
their contained energy. But the fact that muscle will contract in an atmos-
phere free of oxygen, that no free oxygen can be obtained from muscle,
would support the idea that the mechanism is one of decomposition. Her-
mann 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
dioxid, 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 0.001° C. to 0.005° C. an(^ after tetanization an
increase of from o. 14° C. to o. 18° 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,

76 TEXT-BOOK OF PHYSIOLOGY.

mechanic motion being prevented, the liberated energy is transformed into
heat.

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

A muscle during a tetanic contraction of short duration accomplishes
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
work is done, as shown by the fact that the muscle becomes fatigued.

Work Done Daily. — The muscle system in its entirety is to be regarded
as a machine for the transformation of potential into kinetic energy, and in
so doing accomplishes work. Through the 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

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

77

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 single
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 elec-
tric current is at once developed. In all in-
stances the current, as shown by the deflec-
tion of the needle, originates at the transverse

face, passes through the muscle to the
longitudinal surface, thence through the gal-
vanometer to the transverse surface. The
longitudinal surface is, therefore, electroposi-
tive, the transverse surface electronegative.
The two points exhibiting the greatest differ-
ence of potential, and hence the most power-
ful current, lie in the equator and in the cen-
ter of the transverse surface. Currents of
gradually diminishing intensity are obtained
when the electrode placed on the longitudinal
surface is removed toward either end. Feeble
currents are developed when two points situ-
ated at unequal distances, either on correspond-
ing or opposite sides of the equator, are con-
nected; in either case the current flows from
the point lying nearest the equator to the point
farthest from it. Similar currents are obtained
when two points on the cross-section situated
at unequal distances from the central axis
are connected, in which case the direction of
the currents will be from the point lying near-
est the periphery toward the center. On the
contrary, no current is developed when two points on the longitudinal surface
equally distant from the equator, or two points on the transverse surface
equally distant from the central axis, are connected. Such points are said to
be isoelectric. These facts are shown in Fig. 35. The natural ends of the
muscle, enclosed by sarcolemma and tendon, do not exhibit, if carefully
preserved from injury, the negativity characteristic of the artificial transverse
ends.

Similar electric conditions are exhibited by the muscles of man and other
mammals, by the muscles of birds, reptiles, amphibia, etc. The currents
developed by connecting the equator on the longitudinal surface with the

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.')

78 TEXT-BOOK OF PHYSIOLOGY.

axis of the transverse surface have an electromotive 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 intensity 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 composed of molecules each of which exhibits
the same difference of potential on its longitudinal and transverse surfaces
as the muscle prism itself. Hermann denies the existence of currents in
normal resting muscle and attributes them to injuries of the surface, due to
methods of preparation, in consequence of which the tissue dies and becomes
electronegative to the uninjured area, which remains electropositive. These
currents Hermann terms "demarcation currents."

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 galvanometer needle toward
the zero point, indicating a dimin-
ution of the potential on the longi-
tudinal surface. To this dimin-
ution in the strength of the cur-
rent the term negative variation
was given. On the withdrawal of
FIG. 36.— THE NEGATIVE VARIATION OF THE the stimulus the needle again re-
DEMARCATION CURRENT. A. The contraction turns in a short time to its former
wave, which as it passes beneath the electrode at B

causes a diminution of potential. position. The diminution of po-

tential on the longitudinal surface

of the muscle is now attributed to the passage of the excitation and contrac-
tion processes, to a temporary disintegration of the muscle substance (Fig.
36). With their disappearance and the subsequent restoration of the
nutrition of the muscle, the former electric condition returns.

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

Electric Currents from Non-injured Muscles.— Though 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.

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE.

79

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

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

CUREENT.

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

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

CURRENT.

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 .

8o TEXT-BOOK OF PHYSIOLOGY.

The term phasic is applied to these currents. The first current flows in the
muscle in the direction of progress of the contraction wave — first phase; the
second current flows in the reverse direction — second phase; the current is
therefore diphasic. When a muscle is tetanized, there is but a single current
observed, which, however, endures so long as the tetanic contraction is
maintained. To this current the term 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.

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
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
phase — atterminal — the wrist became positive and the current passed in the
muscle toward its termination; and in the second — ab terminal — 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.,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 81

the functions of respiration, mastication, etc. The coordination of axial
and appendicular muscles enables the individual to assume certain postures,
such as standing, sitting, and lying; to engage in various acts of locomotion,
as walking, running, dancing, swimming.

Levers. — The function or special mode of action of individual muscles
can be understood only when the bones with which they are connected are
regarded as levers whose fulcra or fixed points lie
in the joints where the movement takes place, and
the muscles as sources of power for imparting move-

ment to the levers with the object of overcoming
resistance. F «

w

In mechanics levers of three kinds or orders are A W
recognized according to the relative positions of the • J

fulcrum or axis of motion, the applied power, and vT" p AIO;

the weight to be moved. (See Fig. 39.) FlG> 39._THE THREE OR_

In levers of the first order the fulcrum, F, lies DERS OF LEVERS.
between the weight or resistance, W, and the power

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

1. The elevation of the trunk from the flexed position. The axis of 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
6

8o TEXT-BOOK OF PHYSIOLOGY.

The term phasic is applied to these currents. The first current flows in the
muscle in the direction of progress of the contraction wave — first phase; the
second current flows in the reverse direction — second phase; the current is
therefore diphasic. When a muscle is tetanized, there is but a single current
observed, which, however, endures so long as the tetanic contraction is
maintained. To this current the term 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.

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
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
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.,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 81

the functions of respiration, mastication, etc. The coordination of axial
and appendicular muscles enables the individual to assume certain postures,
such as standing, sitting, and lying; to engage in various acts of locomotion,
as walking, running, dancing, swimming.

Levers. — The function or special mode of action of individual muscles
can be understood only when the bones with which they are connected are
regarded as levers whose fulcra or fixed points lie
in the joints where the movement takes place, and If

the muscles as sources of power for imparting move- * A *{f)

ment to the levers with the object of overcoming ?

resistance. F • | . .

In mechanics levers of three kinds or orders are A W
recognized according to the relative positions of the •
fulcrum or axis of motion, the applied power, and w"~ p A

the weight to be moved. (See Fig. 39 ) FlG 39._THE THREE OR_

In levers of the first order the fulcrum, F, lies DERS OF LEVERS.
between the weight or resistance, W, and the power

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

1. The elevation of the trunk from the flexed position. The axis of 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
6

82 TEXT-BOOK OF PHYSIOLOGY.

of application, acts at a great mechanic disadvantage in many instances,
especially in levers of the third order.

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

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

1. By balancing the head on the apex of the vertebral column. This is

done by the action of the muscles on the back of the neck. The 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.

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,

GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 83

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 denned as a progression in a forward horizontal direction
due to the alternate action of both legs. In walking one leg becomes for the
time being the active or supporting leg, carrying the trunk and head; the
other the passive but progressing leg, to become in turn the active leg when
the foot touches the ground. Each leg is therefore alternately in an active
and 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 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,

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.}

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 connective-tissueJ

skeletal muscle. In some animals the

visceral fiber presents a longitudinal

striation suggesting the existence of Nucleus.

fibrillae surrounded by sarcoplasm (Fig. Smooth muscie-fibei

40). The fibers are united longitudi- intransver£

nally and transversely by a cement FIG. 41.— SECTION OF THE CIRCULAR

material. The muscle is increased in LAYER OF THE MUSCULAR COAT OF THE

thickness by the superposition of succes- HuMAN

sive layers. At varying 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

86 TEXT-BOOK OF PHYSIOLOGY.

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 especi-
ally 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
G 43THELruMrEI ceU 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
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 40° 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 encephalo spinal 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.

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, pecu-
liarities of form which are described as pyramidal, stellate, pear-shaped,
spindle-shaped, etc. The size of the cell varies considerably, the smallest

88

TEXT-BOOK OF PHYSIOLOGY.

having a diameter of not more than 10 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

AXON

A. B.

FIG. 44. — A. EFFERENT NEURON; B. AFFERENT NEURON.

directions with nerve fibrils which are continuous with similar fibrils run-
ning through the axonic process as well as the dendrites. The physiolo-
gic significance of Nissl's granules is unknown. The nerve fibrils are prob-
ably connected with the transmission of nerve impulses. A cell mem-
brane has not been observed. From the surface of the adult cell portions
of the cytoplasm are projected in various directions, which portions,
rapidly dividing and subdividing, form a series of branches, termed den-
drites or dendrons. In some situations the ultimate branches of the den-

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 89

drites present short oclateral presses, known as lateral buds, or gemmules,
which impart to the branches a feathery appearance. This character-
istic is common to the cells of the cortex of the cerebrum and of the
cerebellum. The ultimate branches of the dendrites, though forming an
intricate feltwork, never anastomose with one another nor unite with
dendrites of adjoining cells. According to the number of axons, nerve-cells
are classified as monaxonic, diaxonic, polyaxonic. Most of the cells of
the nerve system of the higher vertebrates are monaxonic. In the ganglia
of the posterior or dorsal roots of the spinal and cranial nerves, however,
they are diaxonic. In this situation the axons, emerging from opposite
poles of the cell, either remain separate and pursue opposite directions, or
unite to form a common stem, which subsequently divides into two branches,
which then pursue opposite directions. (See Fig. 44, B.) The nerve-cell
maintains its own nutrition, and presides over that of the dendrites and the
axon as well. If the latter be separated in any part of its course from the
cell, it speedily 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 myelin 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 filers.

The Neurliemma. — 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

90 TEXT-BOOK OF PHYSIOLOGY.

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
the brain and spinal cord the tufts are in more or less intimate relation
with the dendrites of adjacent neurons.

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

The neurons composing the spinal and cranial nerves are represented in
Fig. 44, which are connected peripherally by their terminal branches with
muscles on the one hand and with epithelium of skin, mucous membrane, etc.,
on the other hand. In the spinal cord the terminal branches of the afferent
neuron come into histologic and physiologic relation with the dendrites of a
second neuron, the axonic process of which in many instances ascends the
cord to different levels or even as far as the brain, where its terminal branches
come into relation with the dendrites of still another neuron, the axonic
process of which is in turn connected with neurons in the cortex of either
the cerebrum or cerebellum. The surfaces of the body are thus brought
into relation with the cerebral and cerebellar neurons. The neurons
arranged in this serial manner constitute the afferent side of the nerve
system.

In a similar way the efferent neurons of the spinal and cranial nerves are
brought into relation with the cortex of the cerebrum. Large pyramidal-
shaped neurocytes situated in specialized regions of the cortex of the cere-
brum send their axonic processes down through the brain and cord. As they
approach their destination the terminal branches become related histo-
logically and physiologically with the dendrites of the neurons composing
the cranial and spinal nerves. The cortex of the cerebrum is thus brought
into relation with the general musculature of the body. The neurons
arranged in this serial manner constitute the efferent side of the nerve system.

Neurons, moreover, are grouped into more or less complexly organized
masses, termed organs, which in accordance with their locations may be
divided for convenience into central and peripheral organs.

The Central Organs of the Nerve System. — The central organs con-
sist of the encephalon and spinal cord, contained within the cavities of the
head and spinal column respectively. They consist of neurons arranged

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 91

in a very complex manner. In a subsequent chapter the anatomic arrange-
ment of their constituent parts will be detailed.

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

The nerve-trunks connect the brain and cord with all the remaining
structures of the body. Each nerve is invested by a thick layer of lamel-
lated connective tissue, known as the epineurium. A transverse section of a
nerve shows (see Fig. 45), that it is made up of a number of small bundles of

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

fibers, each of which possesses a separate investment of connective tissue —
the perineurium. Within this membrane the nerve-fibers are supported by
a fine stroma — the endoneurium. After pursuing a longer or shorter course,
the nerve-trunk gives off branches, which interlace very freely with neigh-
boring branches, forming plexuses, the fibers of which are distributed to
associated organs and regions of the body. From their origin to their
termination, however, nerve-fibers retain their individuality, and never be-
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

92 TEXT-BOOK OF PHYSIOLOGY.

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 anterior roots of the spinal nerves. These
nerve-fibers are designated pre-ganglionic fibers, while those emerging
from the cells are designated post-ganglionic fibers. (See Sympathetic
System.)

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
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
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

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 93

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
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 different situations. These termina-
tions 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 smalt branches which appear to be embedded in a relatively 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.

94

TEXT-BOOK OF PHYSIOLOGY.

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 pene-

SenFory
nerve-fiber..

Muscle
fibers.

Motor plate.

Nerve-fiber
bundle.

FIG. 46. — MOTOR NERVE-ENDINGS OF INTERCOSTAL MUSCLE-FIBERS OF A RABBIT.

X 150.— (Stohr.)

trate the acinus wall and end on the gland-cell. The fibers present a varicose
appearance (Fig. 47).

The afferent nerves as they approach their ultimate terminations
undergo similar changes. The end-tufts become associated, in some situ-
ations, 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

B

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.}

in other situations.

5. End-bulbs of Krause in the conjunc-
tiva, clitoris, penis, etc.
(A consideration of these end-organs

will be found in the chapters devoted to the organs of which they form
a part.)

In the skeletal muscles afferent fibers become associated with small
spindle-shaped structures known as muscle-spindles or neuromuscle end-
organs. These spindles vary in length from 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

95

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 ^.

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
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 portion in connection with the brain or cord retains its

96

TEXT-BOOK OF PHYSIOLOGY.

normal condition with the exception of a few millimeters at its peripheral
end. The degenerative process begins simultaneously throughout the entire
course of the nerve, and consists in a disintegration and reduction of the
myelin and axis-cylinder into nuclei, drops of myelin, and fat, which in time
disappear through absorption, leaving the neurilemma intact. Coincident
with these structural changes there is a progressive alteration and diminution
in the excitability of the nerve. Inasmuch as the central portion of the nerve,
which retains its connection with the nerve-cell, remains histologically
normal, it has been assumed that the nerve-cells exert over the entire course
of the nerve-fibers a nutritive or a trophic 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.

The relation of the nerve-cells to the nerve-fibers, in reference to their
nutrition, is demonstrated by the results which follow section of the ventral
and dorsal roots of the spinal nerves. If the ventral root alone be divided
the degenerative process is confined to the peripheral portion, the central

a

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.}

portion remaining 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 nerve ganglia, respectively, or,
in other words, in the cells of which they are an integral part. The structural
changes which nerves undergo after separation from their centers are degen-
erative in character, and the process is usually spoken of, after its discoverer,
as the Wallerian degeneration.

When the nerve-cells from which the nerve-fibers arise, whether efferent
or afferent, undergo degeneration from any cause whatever, the nerve-fiber
becomes involved in the degenerative process and when it is completed the
structures to which they are distributed, especially the muscles, undergo an
atrophic or fatty degeneration, with a change or loss of their irritability.
This is, apparently, not to be attributed merely to inactivity, but rather to a
loss of nerve influences, inasmuch as inactivity merely leads to atrophy and
not to degeneration.

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 97

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 conincidently with the degenerative
process there occurs a regenerative process, consisting in a multiplication of
the nuclei lying just beneath the neurilemma and an accumulation around
them of a granular protoplasm which in due time completely fill the neuril-
emma. At this stage the fiber is known as a band-fiber. If now the physical
conditions are such as to permit of a reunion of the nerve, this takes place,
and under the nutritive influence of the cell the axis-cylinder grows into the
band-fiber and the protoplasm becomes 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 may be classified, in accordance with the character-
istic 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 cause the formation and discharge of the secretion
peculiar to the gland.

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

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

98 TEXT-BOOK OF PHYSIOLOGY.

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

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 99

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.

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

loo TEXT-BOOK OF PHYSIOLOGY.

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 104.) Under strictly physiologic conditions the nerve
impulse passes in one direction only; in efferent nerves from the center to the
periphery, in afferent nerves from the periphery to the center. 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 101

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 influ-
ence 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 pro-
duced 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, sensations 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 prepara-
tion (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 ex- Fl6- 5°-— NERVE,
tremely sensitive to either the galvanic or the induced TION OF A FROG ARp\

Current. P>mur. S. Sciatic

Though the development and conduction of a nerve "ecr^jis ^_ 7L^0fs
impulse may be demonstrated by the deflection of the and Stirling.)™
galvanometer needle or the movement of the mercury in
the capillary electrometer, it is more conveniently demonstrated by the con-
traction 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 en-
closed in a moist chamber and the nerve connected with the inductorium
through the intervention of non-polarizable electrodes. The muscle may
be attached to the muscle-lever and its contractions recorded.

102

TEXT-BOOK OF PHYSIOLOGY.

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

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

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 cur-
rents are obtained when the natural longitud-
inal surface and the transverse surface are
connected with the electrodes of the galvano-
meter circuit. The strength of the current
thus obtained will diminish or increase accord-
ing as the electrode on the longitudinal sur-
face 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
surface are connected, weak currents are ob-
tained, in which case the point lying nearer the equator becomes positive
to the point more distant, which becomes negative. From these facts it is
evident that all points on the longitudinal surface are electrically positive to
the transverse surface and that the point of greatest positive tension is sit-
uated near the equator (Fig. 51).

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.)

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 103

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

Negative Variation of the Nerve Current. — 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 varia-
tion, 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
negative 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 galvanometers and the nerve stimulated in the
middle, the demarcation currents simultaneously undergo a negative varia-
tion. This may be taken as a proof that the excitation process propagates
itself equally well in both directions. The negative variation is intimately
connected with changes in the molecular condition of the nerve and is not
due to any extraneous electric or other influence. And du Bois-Reymond
was also enabled to obtain a negative variation of the current in the nerves
of a living frog which were yet in connection with the spinal cord. In this
experiment the sciatic nerve was divided at the knee and freed from its
connections up to the spinal column; the transverse and longitudinal surfaces
were then placed in connection with the electrodes of the galvanometer wires
and the current permitted to influence the needle. The animal was then
subjected to the action of strychnin. Upon the appearance of the muscle
spasms the needle was observed to swing backward toward the zero point to
the extent of from i to 4 degrees, and upon the cessation of the spasms to
return to its previous position. In an experiment of this nature it is obvious
that the negative variation was the result of a physiologic stimulation of the
nerve arising within the spinal cord.

The question also here arises as to whether the negative variation is due
to a steady, continuous decrease of the natural current, or whether it is due
to successive and rapidly following variations in its intensity, similar to that
observed in muscles. Though this cannot be demonstrated with the physio-
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

104 TEXT-BOOK OF PHYSIOLOGY.

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 diffi-
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 statements
to the nerve impulse, it may be said that it is a molecular disturbance,
traveling at the rate of about 28 meters a second, is wave-like in character,
the wave being 18 millimeters in length, and occupying from 0.0006 to
0.0008 of a second in passing any given point.

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

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 105

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 currentless 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 action 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.

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:

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

2. The development of electric currents on each side of the positive pole
or anode, and the negative pole or kathode (see Fig. 52), which can be led

* J / ^X

• GALVANOMETER

\

ANELECTROTONIC KATELECTROTONIC

CURRENTS CURRENTS

FIG 52. — ELECTROTONIC CURRENTS.

off by means of wires into a galvanometer circuit from either the artificial
transverse and longitudinal surfaces, or from any two points on the longi-
tudinal surface as shown by the deflection of the galvanometer needle.
The direction of these electric currents in the nerve coincides with that of
the galvanic or "polarizing current." The " natural nerve currents," the
currents of injury or demarcation currents, as they are variously termed,
are at the same time increased and decreased at opposite extremities of the
nerve according to the direction of the polarizing current.

To this changed condition of the electromotive forces in a nerve the
term electrotonus was given (du Bois-Reymond). The currents them-
selves are known as electrotonic 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 current

io6

TEXT-BOOK OF PHYSIOLOGY.

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

n\ e

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

according to the strength and direction of the current. To this condition
the term electro tonus 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
electrotonus the expression, electrotonic alteration of excitability and con-
ductivity.

During the passage of the current the excitability of the intra-polar as
well as the extra-polar regions undergoes a change which, as shown on
examination, is found to be diminished in the neighborhood of the anode or
positive pole and increased in the neighborhood of 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
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.

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

107

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-

REGION OF
INCREASED EXCITABILITY

SECONDARY COIL

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

polar anodic region and the extra-polar kathodic region with an induc-
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

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 OF
WHICH GIVES RISE TO DECREASED CONTRACHON.

stimulation of the kathodic region will be decidedly increased. (See

Fig- 540

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

io8

TEXT-BOOK OF PHYSIOLOGY.

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 Pfliiger, and is
termed the law of contraction:

Current intensity

Ascending current

Descending current
Make Break

Make Break

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 Pfliiger.
All the phenomena of the law of contraction were explained by Pfliiger on
the assumption that the current stimulates the nerve only at the one electrode,
at the kathode on closing, and at the anode on opening; or, in other words, by
the appearance of katelectrotonus or by the disappearance of anelectrotonus,

GENERAL PHYSIOLOGY OF NERVE-TISSUE. 109

both conditions being attended by a rise of excitability — not, however, by
the opposite changes. It is further assumed that the appearance of kate-
lectrotonus 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 inferfere 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 follows:

With Ascending Current. With Descending Current.

Weak i. K. C. C.1 K. C. C.

Medium 2. K. C. C. A. O. C.2 K. C. C. A. O. C.

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

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

If the electrodes connected with the wires of a sufficiently strong 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 anelectrotonic
region and a decrease in the excitability in the extra-polar katelectrotonic
region, as shown by stimulating the nerve in the extra-polar regions with
the induced current— results which are in apparent contradiction to those
obtained with the isolated nerve. This want of accordance in the results
of the two classes of experiments arises from a failure to recognize the fact

*K. C. C., kathodal closing contraction. a A. O. C., anodal opening contraction.

no TEXT-BOOK OF PHYSIOLOGY.

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 phenom-
enon 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 current
have been thoroughly studied by Waller and de Watteville. These observers
employed a method similar to that of Erb, conjoining in one circuit the
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
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.

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

GENERAL PHYSIOLOGY OF NERVE-TISSUE. in

"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 stronger
than the anodic make contraction; the anodic break contraction is stronger
than the kathodic break contraction. Or, otherwise regarded, if, instead
of comparing the contractions obtained with a sufficiently strong current,
we observe the order of their appearance with currents gradually increased
from weak to strong, we shall find that the kathodic make contraction appears
first, that the kathodic break contraction appears last, and the formula
of contraction for man reads as follows :

'Weak current .K. C. C.

Medium current K. C. C. A. C. C. A. 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 other
to some indifferent region. The electrode conveying the current to or
from this part should be of a size sufficient to localize the current and to
increase its density. It was discovered by Duchenne that there are certain
points all over the body stimulation of which is more quickly followed by

112

TEXT-BOOK OF PHYSIOLOGY.

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. The
position of some of these points on the forearm is shown in Fig. 58.

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

M. biceps brachii.

M. brach. anticus.
N. medianus.

M. pronator teres.

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

M. flex, digitor. sublim.

M. flex. dig. subl. (dig. ind. et min.)
LM. flex. poll,
longus.

N. ulnaris.

M. flexor carpi ulnaris.

N. ulnaris.

FIG. 58. — MOTOR POINTS OF THE MEDIAN AND ULNAR NERVES, WITH THE MUSCLES
SUPPLIED BY THEM. — (Landois and Stirling.}

and therapeutics of the precedent lesions. The principal difference of
behavior is observed in the muscles, which exhibit diminished or abolished
excitability to the induced current, while at the same time manifesting
an increased excitability to the constant current; so much so is this the
case that a closing contraction is just as likely to occur at the positive as
at the negative pole. This peculiarity of the muscle response is termed
the reaction 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

GENERAL PHYSIOLOGY OF NERVE-TISSUE.

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-
nned 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 41, the necessary structures are as
follows:

1. A receptive surface, skin, mucous mem-

bane, sense-organs, etc.

2. An afferent nerve-fiber and cell.

3. An emissive cell, from which arises —

4. An efferent nerve, distributed to a respon-

sive organ, as

5. Skeletal muscle, gland, blood-vessel, etc.
Such a combination of structures consti-
tutes a reflex mechanism or arc, the nerve
portion of which, in the case of skeletal mus-
cles, 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 nerve-cells of a peri-
pheral sympathetic ganglion. The reflex arc
is then continued by the processes of the gang-
lion 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 intercalation of a third neuron situated in
the gray matter which is in connection, on the one hand, with the central
terminals of the afferent neuron, and, on the other hand, through its colla-
teral branches with the dendrites of the efferent neurons situated at different
levels of the cord. (Fig. 59.)

For the excitation of a reflex action it is essential that the stimulus applied
to the receptive surface be of an intensity sufficient to develop in the terminals
of the afferent nerve a series of nerve impulses, which, traveling inward, 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

8

FIG. 59. — DIAGRAM SHOWING
THE RELATION OF THE THIRD
NEURON a, TO THE AFFERENT
NEURON b, AND TO THE EFFERENT
NEURONS c, c, c.— (After Kolliker.)

ii4 TEXT-BOOK OF PHYSIOLOGY.

the equilibrium of its molecules, a liberation of energy, and, in consequence,
a transmission outward of impulses through the efferent nerve to muscle,
gland, or blood-vessel, separately or collectively, with the production of
muscle contraction, a secretion, vascular dilatation or contraction, etc.
The reflex actions take place, for the most part, through the spinal cord and
medulla oblongata, which, by virtue of their contained centers, coordinate
the various organs and tissues concerned in the performance of the organic
functions. The movements of mastication; the secretion of saliva; the
muscle, gland, and vascular phenomena of gastric and intestinal digestion;
the vascular and respiratory movements; the mechanism of micturition,
etc., are illustrations of reflex activity.

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 similar 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. With the diminution of the volume of the blood and
an insufficient supply to the tissues, there arise the sensations of hunger and
thirst, which lead to the consumption of food and the subsequent restoration
of the physiologic condition of the tissues. These two sensations are also
partially dependent on the empty condition of the stomach and the dryness
of the mucous membrane of the mouth and throat.

The foods which are consumed daily in response to sensations of hunger
and thirst are complex in composition and contain, though in vary-
ing amounts, proteins, fats, carbohydrates, water, and inorganic salts,
which, in contradistinction to foods, are termed food principles, or as they
maintain the nutrition, nutritive principles. These compounds also contain
the potential energy necessary to maintain the energy equilibrium of the

n6 TEXT-BOOK OF PHYSIOLOGY.

body which becomes manifest as heat and mechanic motion in the
transformations 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 overcoming of friction, cohesion, or elasticity, the machine
suffers disintegration and metabolizes a portion of the food materials and
loses a portion of its available energy. Unlike other machines, however,
it possesses the power, within limits, of self-renewal, 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 examination of the quantity and composition of the daily excretions.
If therefore these are collected and analyzed, it will become possible to de-
termine from their chief constituents the extent and character of the tissue
and food metabolized. Thus the urea and other nitrogen-holding com-
pounds contained in the urine represent the proteins metabolized; the carbon
dioxid and water represent the fat and carbohydrates metabolized. There-
fore it becomes possible to determine from the amounts of the urea and
carbon dioxid eliminated, the different amounts of the food principles re-
quired 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 equilibrium is to be maintained.

An experiment designed to collect the excretions for purposes of analysis
is termed a metabolism experiment; its object is to deduce from the amounts
of urea and other nitrogen-holding compounds, of carbon dioxid and water
discharged, the amount of the tissue and food metabolized, and hence from
them to calculate 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. This is accomplished by one of the many forms of respiration
appliances, which have been devised for animals and for man. The best
form of apparatus for determining the metabolism of man is that designed
by Benedict.

Many metabolism experiments have been performed by different inves-
tigators under a great variety of conditions. The results, though differing
in some respects, have nevertheless a general average value. The following
table shows the results of a series of experiments made by Vierordt. On
the right under the term outcome, are arranged the amounts of the sub-
stances eliminated; on the left, under the term income, the amounts of the

FOODS.

117

food principles which were calculated as necessary to replace the tissue and
food metabolized.

COMPARISON OF THE INCOME AND OUTCOME.

Income

Grams

Ounces

Outcome

Grams

Ounces

Protein

I2O

427

Water

2818

OO 3O

Fat

OO

317

Urea

A.O

yy -ow

I A.O

Carbohydrates

•J5Q

1 1 64.

Feces dry

38

I 34.

Salts

32

I 13

Salts'

o°

•JO

I 13

Water

2818

00 3O

Carbon dioxid

O2 2

32 37

Oxygen

7^6

26.66

Water formed in body

y*4

296

IO ^O

4146

146.13

4146

146.13

Other estimates as to the amounts of the organic food principles required
daily based on the amount of the excreted products are as follows:

Protein . . .

Ranke.
Grams.

IOO

Voit.
Grams.
118

Moleschott.
Grams.

I3O

Atwater.
Grams.

12 ^

Hultgren.
Grams.

134.

Fat.

IOO

s6

84

12 ^

7O

Starch..

. . : . . 2 *o

^00

t^O

4.OO

«2

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 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 grams 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 equilibrum 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

n8

TEXT-BOOK OF PHYSIOLOGY.

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, they become deposited in the tissues
and give rise to certain nutritional disorders. Any unnecessary consump-
tion of proteins should therefore be avoided.

It must be remembered, however, as protein yields energy when meta-
bolized, 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.

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

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

N.

C.

Protein

IOO

I e ?

C-7 O

Fat

IOO

JO tVP

7O O

Carbohydrates

2 CO

Q-7 O

15-5

225.0

Outcome

Grams

N.

C.

Urea

,1 e\

Uric acid . ...

o. c I

14.4

6.16

Feces '

i i

10 84

CO,..

208 oo

15-5

225.00

From the above it will be observed that the daily discharge for each
kilogram of body-weight is 0.22 gram of nitrogen and 3.21 grams of carbon;
the relation of the two being £=1.46. On a diet in which there is an
excess of either protein or carbohydrates this ratio necessarily changes.

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:

FOODS. 119

i. 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 oik In seeds, grains, nuts, fruits, and other

vegetable tissues.

3. CARBOHYDRATES.

Dextrose or grape-sugar 1 In fad

Levulose or fruit-sugar J

Lactose or milk-sugar Milk.

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

Maltose Malt and malted foods.

Starcn ( 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 Acros.

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

6. ACCESSORY FOODS.

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

Disposition of Food. — The protein principles of the food while in the
alimentary canal undergo a series of disintegrative changes by virtue of
which they are reduced in part to simple nitrogen-holding bodies, amino-
and diamino-acids and ammonia, and in part to their immediate antecedents
peptids and polypeptids. Under these forms the nitrogen-holding con-
stituents of the food are absorbed from the intestine. During the act of
absorption they are for the most part transformed into the forms of protein
characteristic of blood and more particularly that form known as plasma
or serum albumin. After being distributed by the blood-stream to the
tissues, they are brought into relation with the living cells. The disposition
made of the protein material by the bioplasm of the cell has not been definitely
determined. According to Voit, of the protein thus brought into con-
tact with the living tissues, only a small percentage is utilized and assim-
ilated for tissue repair. This he terms tissue or organ protein. The re-
maining large percentage circulating in the interstices of the tissues, though
not forming an integral part of them, is acted on directly by them, merely
by virtue of contact — split, oxidized, and reduced to simpler compounds.
This he terms circulating protein.

120 TEXT-BOOK OF PHYSIOLOGY.

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

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

The amino-acids that are not utilized in the synthesis of the necessary
blood proteins are absorbed by the intestinal epithelium and deprived of their
amidogenic nitrogen (NH2). The latter is then converted into ammonium
carbonate, which is then carried to the liver and converted into urea. The
remainder of the amino-acid is carried into the circulation, and is eventually
oxidized, thus giving rise to heat. It is also possible that some of the amino-
acids are carried to the tissues and there directly used in tissue formation.

The fat principles while in the alimentary canal also undergo a series of
changes whereby they are reduced to soap and glycerin, under which forms
they are absorbed. During the act of absorption 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, 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 ingested 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 also a product of the metabolic activity of connective-tissue cells and is a
derivative of both proteins and carbohydrates.

The carbohydrate principles are reduced during digestion 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 hydration 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 oxidized, it is now well estab-
lished that another portion contributes to the formation of, if it is not directly
converted into, fat. As the carbohydrates form a large portion of the food,
they contribute materially to the liberation of energy.

The inorganic principles, though apparently not playing as active

FOODS. 121

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
r61e 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
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
deprived of it, the skeleton pidergoes 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.

122 TEXT-BOOK OF PHYSIOLOGY.

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

Accessory foods — coffee, tea, and cocoa — when taken in moderation-
have a stimulating influence on the nervous system, as shown by the
removal of both mental and physical fatigue, by an increased capacity
for sustained mental work, and by the persistent wakefulness among those
unaccustomed to their use. Coffee more especially increases the frequency
and force of the heart-beat, raises the arterial pressure, and hastens the
general blood-flow. It has no influence either in the way of increasing or
decreasing 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. In-
as much as tannin also coagulates peptones, the excessive use of tea as a
beverage is apt to derange the digestive organs and the general process of
digestion.

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

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

Alcohol when taken in small quantities stimulates the digestive glands
to increased activity and thus promotes digestive power. Its absorption into
the blood is followed by increased action of the heart, dilatation of the 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

FOODS. 123

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
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 defined 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

i24 TEXT-BOOK OF PHYSIOLOGY.

raised, the number of Calories will be known. The average number of
Calories dissipated in various ways, e.g., radiation, vaporization 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 determined is, whether
any given diet scale contains this amount of heat and whether it can be
liberated 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 ' 4.116 Calories.

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:

FOODS. 125

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 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 urea declines very rapidly during the first few days —
a fact which has been attributed to the consumption of the surplus protein
food. After this period, when the tissues begin to metabolize their own
protein, the excretion remains fairly constant, about 13 grams, until
toward the close of the starvation period, when the amount eliminated
falls very rapidly. As proteins contain about 16 per cent, of nitrogen,
i part of nitrogen equals 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, though this is greatest in the last few days. As fat
contains about 76 per cent, of carbon, i part of carbon equals 1.31 parts of
fat. Hence, for every i gram of carbon or 3.66 grams carbon dioxid ex-
creted it may be assumed that 1.31 grams of fat have been metabolized.

126

TEXT-BOOK OF PHYSIOLOGY.

The daily excretion of carbon, therefore, indicates the extent of fat metab-
olism. The carbohydrates are here left out of consideration, as they con-
stitute 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 fat that the protein would give rise to, as previously deter-
mined, must therefore be subtracted from that eliminated by the lungs, etc.,
in order to determine the amount of body-fat metabolized. Observations
of human beings in the fasting condition show that for a period of ten days
there is a daily excretion of about 24 grams of urea, equivalent to about 72
grams of protein. This amount, however, may be reduced to from 40 to 50
per cent, if the individual has a surplus of body-fat. Human beings under
similar circumstances may lose during the first few days from 180 to 200
grams of fat.

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

Protein, 50 gm
Fat, 199.6 gm

Nitrogen

Carbon

Urea 17 s?m

Nitrogen

Carbon

7.8

0.0

26.5

157-5

7-2
o.o

3-4
180.6

184.0

Uric acid o 2 gm j

Carbon dioxid

7.8

184.0

7.«

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, in all probability, from
a cessation in the action of the heart.

Post-mortem Appearances. — It has been experimentally determined that
animals die when the body-weight has declined to about 40 per cent. Post-
mortem examination shows that the loss of material, though very generally
distributed throughout the body, is greatest in organs and tissues least
essential to life.

The results of an analysis of the organs and tissues of a cat after a thirteen-
day period of starvation, during which the animal lost 1017 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.

FOODS.

127

Organ

Percentage
Loss of weight

Actual Loss
of Tissue

Adipose tissue

07

Grams
267

Spleen

67

6

Liver „ ,

54

40

Testes

4O

I

^Muscles

•7T

420

Blood

27

27

Kidneys

11

7

Skin and hair

21

80

Lungs.

18

Intestines .

18

21

Pancreas

17

I

Bones ,

14

ec

Heart

o

Nervous system

7

i

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 equilibrium,
it was necessary to increase the proteins from two to three times the usual
amount. Thus, a dog weighing 30 to 35 kilograms required from 1500 to
1800 grams of flesh daily in order to get the requisite amount of carbon to
prevent consumption of its own adipose tissue. Under similar circumtances,
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
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.

128

TEXT-BOOK OF PHYSIOLOGY.

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
Materials

;
Water

Unavail-
able
Nutrients

Proteins

Fat

Carbo-
hydrates

Ash

Fuel Value
Per Lb.
453.6 Grams

Beef:
Loin, lean

Per

cent.

67 .0

Per
cent.

I .2

Per
cent.

IQ . I

Per

cent.

12. 1

Per

cent.

Per

cent.

i .0

Calories
ooo

Loin, fat

£.4.7

I .0

17 .O

26.2

o.o

1470

70 o

I O

2O .7

7 . "?

i .1

73s,

Round, fat

VM!«

60.4

1.6

18.9

18-5

i .0

"75

Cutlets (round)
Liver. . .

70.

7 3

7
o

I

o

•3

Q

19

•7

7

7-3

t: o

o
i

.8
o

710

4IO

Mutton:
Leg

62

8

I

7

17

Q

T7 T

o

g

IOQC

T »

Loin

CO

2

2

1C,

C

' 3i 4

o

6

1660

Pork:
Loin chops ...

C,2

o

2

2

16

I

28.6

o

8

ICCC

Ham

C3 .

Q

2

. I

14

8

27 . cr

o

6

1480

Fowl:
Turkey

if:

cc

7

I
I

.6

18
20

7

c

15-5

21 8

o

o

.8

s

1040

Mackerel

79

•3

18

I

6 7

o

Q

6 co

Halibut. .

7 r

I

18

o

4 0

o

s

Milk • ..

87.

o

O

. c

7

2

3-8

5 o

0

c

2,10

Eggs boiled

77

2

2

12

8

II .4

o

§

7CC

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-nbrin and
myogen-fibrin. After being subjected to the cooking process, meats contain

FOODS. 129

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, in 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-liberating 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.
When subjected to the churning process, fat-globules run together and
form a coherent mass — butter.

Lactose is the particular form of sugar found in milk. In the presence

of Bacillus acidi lactici, the lactose is decomposed into lactic acid and

carbon dioxid, the former of which not only imparts a sour taste to the milk,

but causes a precipitation of the caseinogen. The chief salt found in milk

9

130

TEXT-BOOK OF PHYSIOLOGY.

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 1 5 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
Material

Water

Unavail-
able
Nutrients

Proteins

Fat

Carbo-
hydrates

Ash

Fuel Value
Per Pound
453.6 Grams

Entire wheat flour. . . .
Rye flour
Rice
Barley, pearled
Buckwheat flour
Corn meal.

Per
cent.
11.4
12.9
12.3

"•5

13-6

12 5

Per
cent.

tl

3-7
4.0

3-5

4O

Per
cent.
10.7
5-3
6-5
6.6

5-2

7 ^

Per

cent.

r-7

0.8

o-3
i .0
i .1

I 7

Per
cent.
70.9
76.9
76.9
76.10
75-9

73 "%

Per
cent.
0.8
o-5
°-3
0.8
0.7
o 8

Calories

1645
1610
1610
1630
1600
162 ^

Oat meal.

7 8

<; 6

I"? 4.

66

6c 2

I 4.

I7QC

Whole wheat bread. . .
White bread

38-4

•1C -2

3-2
33

7-5

7 I

0.8

I 2

49.1

C2 3

I .O

o 8

1125
IICK

Graham crackers

f A

A 8

4

7 7

8 c

72 . C

i i

IQOO

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
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
100° 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

FOODS.

13 1

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
o <

Per
cent.
6.7
2.7
7-5
°-5
76

Per
cent.

12.8

5-3
15-8
0.6

17 "?

Per
cent.
1,4

0.6
1.6
i .0
o o

Per
cent.
65.6

21.6

59-9

/'9
62 <;

Per

cent.
3-i

:i

0.7

2 2

Calories

J565
525
*53°
90

I^O8

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

73-8
75-5

Cj Q

2-5

1.7

•2 .O

5-i
1.9

2 2

3-i

O.I
I Q

14.4
20. o

40 3

I.I

0.8

O 7

490
4i5

88 c

Beets cooked1...

88 6

I .2

I .7

O.I

72

I 2

«°o

I7O

Cabbage. . ...

QI . t;

0.7

I .2

O.3

t f

o 8

I4O

Tomatoes ... . .

04- 3

0.4

O.7

O 4

•3 S

O 4

IOO

Turnips
Egg-plant. .

89.6

02 .O

0.8
0.6

I .O
o.o

0.2
O-1

7-8

4.0

0.6

O 4

J75

I2O

Spinach, fresh

02 .3

i .0

1.6

O.3

7 .2

i 6

IOO

Asparagus, cooked. . .

y <3

91 .6

i .0

i.7

3-o

2.1

0.6

195

1 With butter etc., added.

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
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.

132

TEXT-BOOK OF PHYSIOLOGY.

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 of the two classes of foods:

Weight of Food

Vegetable

Animal

Digested

Undigested

Digested

Undigested

Of 100 parts of solids

75-5
46.6

90-3

24-5
53-4
9-7

89.9
81.2
96.9

10. 1

18.8
3-i

Of 100 parts of protein

Of 100 parts of fats or carbohydrates .

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 assimilated by the tissues.
When foods are consumed in their natural state or after they have been
subjected to the cooking process, they are subjected while in the food canal
to the solvent action of various fluids by which they are disintegrated and
reduced to the liquid condition. The nutritive principles freed from their
combinations are changed in chemic composition and transformed into
substances capable of absorption. To all the physical and chemic changes
which foods undergo in the food canal the term digestion has been given.

The digestive apparatus comprises the entire alimentary or food canal
and its various appendages: the lips, the teeth, the tongue, the salivary
glands, the gastric and intestinal glands, the pancreas, and the liver (Fig. 60).

The canal itself is a musculo-membranous tube about thirty-two feet
in length, and extends from the mouth to the anus. It may be subdivided
into several distinct portions, as mouth, pharynx, 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 five inches in length, which in turn is
followed by the esophagus or gullet, a tube about nine inches in length. As
the esophagus passes through the diaphragm it expands into the stomach,
a curved pyriform organ, which serves as a reservoir for the reception and
retention of the food for a varying length of time. The small intestine is
that portion of the alimentary canal extending from the end of the stomach
to the beginning of the large intestine in the right iliac fossa; owing to its
length, about twenty-two feet, 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 portion of
the small intestine, within five inches of the stomach, there is an orifice, the
outlet of a small pouch, the Ampulla of Vater, into which open the termina-
tions of the ducts of the liver and pancreas, organs which secrete the bile and

133

134

TEXT-BOOK OF PHYSIOLOGY.

pancreatic juice respectively. The large intestine is from five to six feet in
length and extends from the end of the small intestine to the anus. Its walls
contain a large 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
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, the

Salivary 6fan<f

Vermiform

FIG. 60. — DIAGRAM OF THE ALIMENTARY CANAL. — (Modified, from Landois.)

gastric, intestinal, and pancreatic juices, and the bile. Though taking
place throughout a large portion of the food canal, the process may be sub-
divided into several stages: viz., prehension, mouth digestion, deglutition,
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
with certain waste products, pass into the large intestine to be finally ex-
truded from the body.

DIGESTION. 135

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 is 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 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 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
addition to the changes that the food principles undergo in the alimentary
canal, the corresponding principles as well as many other compounds under-

i36 TEXT-BOOK OF PHYSIOLOGY.

go 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 produces changes of composition and decomposition
apparently by their presence alone, for, as far as the evidence goes, they neither
enter into the end-products of the reaction 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 hydiogen 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
catalyst, 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
or the greater the surface it presents the more energetically it acts. Thus, if platinum,

DIGESTION. 137

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 40° C. to 50° C. is reached, when they are
at their maximum. For this reason this degree of temperature is spoken of as
the optimum temperature. Beyond 50° C. the velocity of their action begins
to decrease and at 60° C. it comes to an end for the majority of enzymes.
At 100° 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
water. In this instance after a certain* percentage of these substances has thus united the prod-

TEXT-BOOK OF PHYSIOLOGY.

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
by 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 life 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 smaller
than the permanent teeth, have the same general conformation. They
are divided into four incisors, two cuspids or canines, and four molars for
each jaw.

The permanent teeth, thirty-two in number, sixteen in each jaw, are
divided into four incisors, two cuspids or canines, four bicuspids or pre-
molars, and six molars for each jaw.

Each tooth may be said to consist of three portions: (i) the crown,
or that portion which projects above the gums; (2) the root or fang, that

ucts of the reaction begin to recombine with the formation of the original compounds until the
opposing tendencies are in equilibrium, a state in which they remain so long as the conditions
remain unchanged. Again, if maltose and water be mixed, a reaction occurs which eventuates in
the formation of dextrose. In time the dextrose molecules combine to form maltose and water
until a condition of equilibrium is established. If the yeast enzyme be added the reactions both
analytic and synthetic are increased in velocity. The enzyme, however, does not initiate, but
merely hastens a reaction already taking place.

DIGESTION.

139

RKt

pa-

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. 61. 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
the enamel. It is composed of connective-
tissue fibers embedded in a ground-sub-
stance, both of which have undergone
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.

FIG. 61. — VERTICAL SECTION OF
TOOTH IN JAW. E. Enamel. D.
Dentine. P. M. Periodontal mem-
brane. P.C. Pulp cavity. C. Ce-
ment. B. Bone of lower jaw. V.
Vein. a. Artery. N. Nerve. — (Stir-
ling.)

I4o TEXT-BOOK OF PHYSIOLOGY.

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 into 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 of a
considerable gliding movement. When the jaw performs a lateral movement,
the condyle and cartilage of one side may remain in their natural position
while the opposite condyle and cartilage glide forward in the glenoid fossa,
directing the symphysis of the jaw to the opposite side of the median line.
The lateral movements are well exhibited by the herbivorous animals, in
which they are quite extensive, and made possible by the small size of the
condyle and the large extent of articulating surface. In man the structure
of the joint is such as to admit of all these possibilities, and the lower jaw
acquires in consequence great freedom of movement.

The Functions of the Muscles of Mastication. — The 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 presented in the
following table:

DIGESTION.

141

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.

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.

i42 TEXT-BOOK OF PHYSIOLOGY.

During the intervals of mouth digestion the mouth is closed by the con-
traction of the elevator muscles of the lower jaw. When the occasion arises
for the introduction of food, the mouth is opened by the depressor muscles;
after the food is introduced into the mouth it is again closed and that
combination of muscle contractions initiated which when continued results
in the mechanical division of the food.

The nerves and nerve centers constituting the nerve mechanism for
mastication are shown in the following table:

Afferent Nerves. Nerve-center. Efferent Nerves.

1. Lingual and buccal branches of Medulla oblongata. i. Small root of the trigeminal
the trigeminal nerve. nerve.

2. Glosso-pharyngeal. 2. Hypoglossal.

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 tri-
geminal, the hypoglossal, and the facial.

The small root of the trigeminal nerve after emerging from the cavity
of the cranium through the foramen ovale joins the inferior maxillary divi-
sion of the large sensor root. After a short course the efferent fibers sepa-
rate into two groups, an upper and a lower; the upper group is distributed to
the masseter, temporal, internal and external pterygoid muscles, the lower
group is distributed to the mylohyoid and anterior belly of the digastric
muscles. The hypoglossal nerve, after emerging from the cranium through
the anterior condyloid foramen, passes downward and forward to be dis-
tributed to the intrinsic and extrinsic muscles of the tongue. The facial
or portio dura after emerging from the stylo-mastoid foramen is distributed
to the superficial muscles of the face.

Stimulation of any one of these nerves with induced electric currents
gives rise to convulsive movements in the muscles to which it is distributed
while its division is followed by paralysis of the muscles.

The Central Mechanism. — The central mechanism that excites and
coordinates the action of the nerve- cells from which these nerves emerge, may
be excited to activity (i) by nerve impulses descending from the cerebrum as
a result of volitional efforts; and (2) by nerve impulses transmitted through
afferent nerves from the mouth. Though movements of mastication are pri-
marily volitional and may so continue, nevertheless when once initiated they
continue for an indefinite period, so long in fact as the nerve impulses which
the food develops in afferent nerves are received by the central mechanism,
thus falling into the category of secondary or acquired reflex acts. That
the masticatory movements are of this reflex character is indicated by the
fact that they will be maintained, even though the volitional effort that
called them forth has subsided and the attention has been directed to some
entirely different subject. It would appear that all that is necessary under
such circumstances is the stimulating action of the food upon the peripheral
terminations of the afferent nerves distributed to the mucous membrane
of the tongue and mouth.

The Afferent Nerves. — The afferent nerves, stimulation of which
excites the central mechanism, are the lingual and buccal branches of the
superior and inferior maxillary divisions of the trigeminal nerve, the lingual

DIGESTION.

143

Excretory
duct.

branches of the glosso-pharyngeal, and in all probability the gustatory fibers
of the chorda tympani. The introduction of food into the mouth develops
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 various secretions of the parotid,
submaxillary, and sublingual glands and
the muciparous follicles of the mouth, which
collectively constitute the salivary ap-
paratus.

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 buccin-
ator muscle and opens on the surface of
the cheek opposite the second upper molar
tooth.

The submaxillary gland is situated
below the jaw in the anterior part of the
submaxillary triangle. From the gland
there emerges a duct (Wharton's) which opens into the mouth by a minute
orifice on the surface of a papilla by the side of the tongue.

The sublingual gland is situated just beneath the 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. 62). The wall of the acinus is formed by a
reticulated basement membrane, surrounded externally by blood-vessels,

144

TEXT-BOOK OF PHYSIOLOGY.

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
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-

FIG. 63. — 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.)

FlG. T64. — 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.)

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.

In the serous glands the cells are more or less spheric in shape, nucleated,
and almost completely filled with dark granular material (Fig. 63). In the
mucous glands the cells are large, clear in appearance, and loaded with a
highly refracting material resembling mucin (Fig. 64). 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.

DIGESTION. 145

The glands embedded in the mucous membrane covering the tongue,
lips, cheek, palate, and pharynx are for the most part lined with epithelial
cells which contain a highly refracting matter similar to, if not identical with,
that found in the cells of the submaxillary and sublingual glands.

Nerve-supply. — Histologic investigation has demonstrated that the
cells and blood-vessels of the salivary glands are supplied with 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. 67.) 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 nerve-
trunks 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 inter-
mediation, 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

10

146 TEXT-BOOK OF PHYSIOLOGY.

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
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 . 16 994 . 20

Epithelium i . 62 2 . 20

Soluble organic matter i .34 i .40

Potassium sulphocyanid o .06 o .04

Inorganic salts i .82 2 . 20

1000.00 1000.04

(Jacubowitsch.) (Hammerbacher.)

Water constitutes the main ingredient, amounting to 99.5 per cent,
the soluble organic matter is 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

DIGESTION.

147

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 mastica-
tion be added to the amount secreted during the remaining twenty-two hours,
supposing that it continues at the rate of 36 grams per hour, we have a total
amount of 513+792 grams, or 1305 grams (19,780 grains), or about 2.8
pounds.

Histologic Changes in the Salivary Glands during Secretion.—
During and after secretion very remarkable changes take place in the cells
lining the acini, which are in some way connected with the production of the
essential constituents of the salivary fluids. In the case of the parotid gland,
which may be regarded as the type of a serous or albuminous gland, the
following changes have been observed by Langley (Fig. 65). During the

FIG. 65. — PAROTID GLAND APTER 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. 66. — SUBMAXILLARY 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.)

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
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. 66). 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

i48 TEXT-BOOK OF PHYSIOLOGY.

on the addition of carmine. When treated with water or dilute acids, the
globules swell up, coalesce, and form a uniform mass. The chemic relations
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 plays an important r61e in the general digestive process. Experi-
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:

DIGESTION. 149

/ Achroodextrin.

Starch-Soluble Starch- " Maltose.

This change consists in the assumption by the starch of a molecule of water,
and for this reason the process is termed hydrolysis. The nature of the
chemic change is shown in the following formula:

6105 3-I,2an 6

Starch + Water - Maltose + Dextrin.

The amylolytic1, amyloclastic, or starch-changing action of saliva depends
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 106° 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 Nerve Mechanism of the Secretion of Saliva. — The secretion of
saliva is a complex 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.

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-

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.

i5o TEXT-BOOK OF PHYSIOLOGY.

tents. The blood-vessels possess that degree of dilatation necessary for
nutritive purposes.

With the introduction of food into the mouth 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 their organic
constituents into the lumen of the acinus, materials that collectively consti-
tute 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 nerves and nerve centers that constitute the nerve mechanism for
the secretion of saliva, as determined by experimental investigations are
shown in the following table:

Afferent Nerves. Nerve-centers. Efferent Nerves.

1. Lingual and buccal branches of Medulla oblongata. The chorda tympani and its post-

the trigeminal nerve. ganglionic continuations for the sub-

maxillary and sublingual glands;
the glosso-pharyngeal nerve and its
postganglionic continuations con-
tained in the auriculo-temporal

2. Taste fibers in the chorda branch of the trigeminal nerve, for

tympani. the parotid gland.

3. Taste fibers in the glosso- The sympathetic nerve for all the

pharyngeal. 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, are
contained respectively in the chorda tympani and its postganglionic con-
tinuations, in the glosso-pharyngeal and its postganglionic continuations
contained in the auriculo-temporal branch of the fifth nerve, and in the
branches of the sympathetic nerve derived from the superior cervical ganglion.
That these nerves transmit the nerve impulses to the salivary apparatus is
shown by the effects that follow their division and stimulation.

The Chorda Tympani. — The chorda tympani nerve is apparently a
branch of the facial, the trunk of which it leaves in the aqueduct of Fallopius.
It then crosses the tympanic cavity, emerges through the Glaserian fissure,
and joins the lingual branch of the inferior maxillary division of the fifth
nerve. After passing forward as far as the sublingual gland, nearly all of
the fibers leave the lingual nerve by four or five strands to become connected
by terminal branches with nerve ganglion cells in relation with the sub-
maxillary and sublingual glands. (See Fig. 67.)

The effects on the secretion and flow of saliva from the submaxillary
gland which follow division and stimulation of the chorda tympani nerve
are shown in the following way: A cannula is inserted into Wharton's duct
and the rate of flow estimated; the nerve is then divided, after which the
flow ceases. The peripheral end of the nerve is then stimulated with
induced electric currents when a copious secretion of a thin saliva takes
place, accompanied by a marked dilatation of the blood-vessels of the gland.
The quantity of blood passing through the vessels is so great as to give to
the venous blood an arterial hue and to the small veins a distinct pulsation.
It would appear from these effects that the chorda contains two sets of fibers,

DIGESTION.

one of which inhibits the action of a local vaso-motor mechanism permitting
the blood-vessels to dilate (vaso-dilatator fibers), the other of which stimu-
lates the secretor cells to activity, through the intermediation of local ganglia.
That local ganglia are involved is shown by the effects which follow the
injection of nicotin into the circulation. After a sufficient dose — 10 milli-
grams for the cat — stimulation of the chorda has no effect. Stimulation of
the nerve-plexus beyond the ganglion, the postganglionic 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.

GlossoJ*/iarynaeal^\ \ / Vf

/), .> , "l^fe^

Utic vanrjl.i

Parotid Gland,
Jaco&sens ^lerut

Sul-Maxillary Ganqlion_ ~*

SiibMaxilla.ry Git

Chorda, Jympanillfertte,

Sup.Cervical Ganglion

erces

FIG. 67. — 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 dilatation

152 TEXT-BOOK OF PHYSIOLOGY.

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
the chorda tympani nerve have their origin in cells, the nucleus salivatorius,
located 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
the course stated above.

The Glosso-pharyngeal Nerve.— The 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 auriculotemporal
nerve. The trunk of this latter nerve contains therefore postganglionic
fibers that bear the same relation to the parotid gland and blood-vessels
that the postganglionic 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 sympathetic 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 point non-medullated nerve-fibers follow the branches of the
external carotid artery to the different glands. There is no evidence that
these fibers have any connection, anatomic or physiologic, with local ganglia
at or near the submaxillary, sublingual, or parotid glands. If the sympa-
thetic nerve in the neck, especially in the dog, be divided and the peripheral
end stimulated with induced electric currents, there is at once a contrac-
tion 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 poured out when the
chorda tympani is stimulated. The quantity is less, it is more viscid, richer
in organic matter, of a higher specific gravity, and more active in the trans-
formation of starch into sugar.

DIGESTION. 153

Stimulation of the sympathetic fibers passing to the parotid gland is
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 causes an
increase in the amount of water and inorganic salts, but an increase also in
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 sympathetic.

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 experiment-
ally demonstrated by Pawlow on dogs. This investigator cause 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 of
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-

154 TEXT-BOOK OF PHYSIOLOGY.

tion of food into the mouth will invariably be followed by a flow of saliva.
Pawlow 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
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
gland, and the lingual nerve stimulated by induced electric currents of
moderate strength, a copious flow of saliva at once takes place. If now the
glosso-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
the 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.

Resum6 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

DIGESTION. 155

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
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 preganglionic 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 preganglionic
chorda tympani fibers nor of the postganglionic 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
postganglionic 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

156 TEXT-BOOK OF PHYSIOLOGY.

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 of
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.
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 cavity of the mouth communicates posteriorly with the pharynx by
a narrow orifice, the isthmus of the fauces. This orifice is bounded above
by the soft palate, laterally by the anterior and posterior half arches, and
below by the tongue.

The pharynx is an oval-shaped cavity extending from the base of the
skull to the lower border of the cricoid cartilage, a distance of about 12 centi-
meters. (See Fig. 72.) Its walls are formed mainly by three pairs of
muscles — the superior, middle, and inferior constrictors — each consisting
of red, striated muscle-fibers, and hence capable of rapid and energetic
contractions. Superiorly the pharynx is attached to and supported by the
basilar process of the occipital bone; inferiorly it becomes continuous with
the esophagus. The anterior wall of the pharynx is imperfect and presents
openings which communicate with the nasal chambers, the mouth, and the
larynx. The lateral wall on either side presents the opening of the Eustachian
tube which leads directly into the cavity of the middle ear. The interior of
the pharynx is lined by mucous membrane. The pharynx is partially separ-
ated from the mouth by the velum pendulum palati, a muscular structure
attached above to the hard palate; its lower edge or border is directed
downward and backward and presents in the middle line a conical process,
the uvula. On either side the palate presents two curved arches, the anterior

DIGESTION.

157

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
cartilage, the epiglottis, placed just behind the tongue and so arranged that
it can easily be depressed and elevated.

3 'The esophagus, the continuation of the deglutitory canal, extends down-
ward from the lower border of the cricoid cartilage for a distance of from

FIG. 68.— VERTICAL SECTION OF THE NASAL FOSSA AND MOUTH, i. Left nares. 2. Lateral
cartilage of the nose. 3. Portion of the internal alar cartilage forming the skeleton of the lower
part. 4. Superior meatus. 5. Middle meatus. 6. Inferior meatus. 7. Sphenoidal sinuses.
8. External boundary of the posterior nares. 9. Internal elliptical opening of the Eustachian
tube. 10. Soft palate, n. Vestibule of the mouth. 12. Vault of palate. 13. Genioglossus
muscle. 14. Geniohyoid muscle. 15. Cut margin of the mylohyoid muscle. 16. Anterior
pillar of the palate (anterior half-arch), presenting a triangular figure with the base inferiorly,
covering partly the tonsil. 17. Posterior pillar (posterior half -arch) of the palate. 18. Tonsil.
19. Follicular (mucous) glands at the base of the tongue. 20. Cavity of the larynx. 2 1. Ventricle
of the larynx. 22. Epiglottitis. 23. Cut os hyoides. 24. Cut thyroid cartilage. 25. Thyrohyoid
membrane. 26. Section of posterior portion of the cricoid cartilage. 27. Section of the anterior
portion of the same cartilage. 28. Crico- thyroid membrane. — (Sappey.)

22 to 25 centimeters, to a point opposite the ninth thoracic vertebra, where
it expands into the stomach. Its walls are composed of an internal or
mucous and an external or muscle coat, united by areolar tissue. The
muscle coat consists of an external layer of longitudinal fibers arranged in
three bands and of an internal layer composed of fibers arranged circularly
in the upper part and obliquely in the lower part of the esophagus. In the
upper third the fibers are striated; in the middle third they are a mixture of
both striated and non-striated; in the lower third they are entirely non-
striated.

The muscle fibers surrounding the esophago-gastric orifice are arranged
in the form of and play the part of a sphincter muscle, and for this reason

158 TEXT-BOOK OF PHYSIOLOGY.

may be termed the sphincter cardies muscle. By its action it prevents a
return under normal conditions of food into the esophagus.

The deglutitive act may be for convenience divided into three stages, viz. :

1. The passage of the food from the mouth into the pharynx.

2. The passage of the food through the pharynx into the esophagus.

3. The passage of the food through the esophagus into the stomach.

In the first stage the bolus of food is placed on the superior surface of the
tongue. The mouth is then closed and respiration is 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 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 the exceptional rather than the usual method
has been demonstrated by Kronecker, Falk, and Meltzer.

In 1880 the first of these experimenters made the observation that the
sensation in the stomach following the swallowing of a mouthful of cold water
occurred too quickly to be explained by the prevalent belief that its transfer-
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 connected externally with a water mano-
meter, 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 food is rapidly thrown
down into the stomach, peristalsis playing no part in the process. The
proof, however, of these statements was furnished by Meltzer. This ob-
server introduced into the pharynx and esophagus rubber tubes, the ends of
which were provided with thin-walled rubber balloons which could be
distended with air. The outer ends of the tubes were connected with
Marey's recording tambours. Any compression of the balloon would be
followed by the passage of the air into the tambour and an elevation of the

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
solid and semisolid contents. It is characterized by a contraction of the muscle-fibers behind
the object and an inhibition or relaxation of the muscle-fibers in front of it. (Bayliss and Starling.)

DIGESTION. 159

lever. With one balloon in the pharynx and the other in the esophagus at
varying depths, and the recording levers of the tambours applied against
the surface of a revolving cylinder, it became possible, with the addition of a
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,

FIG. 69. — TEACING OF THE ACT OF DEGLUTITION, i. A indicates the compression of the
elastic bag caused by the bolus projected by the contraction of the mylohyoid muscles. B. 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.}

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
two crests amounts to not more than 0.3 second. In the esophageal curve,
3, the elevation, C, corresponds to the elevation, A, and is likewise due to
the compression exerted by the bolus. The interval of time between the
beginning of the first and second curves was not more than o.i second,
regardless of the depth to which the esophageal balloon was plunged. At a
later period a second rise of the lever was recorded; the time of its appear-
ance, height, duration, etc., were found to increase with the depth of the
balloon.

These facts demonstrate that deglutition consists of two phases: (i) a
rapid rise of pressure in the pharynx, as a result of which the bolus is sud-
denly shot down to the lower end of the esophagus; (2) a peristaltic contrac-
tion 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.

160 TEXT-BOOK OF PHYSIOLOGY.

The immediate cause of the sudden rise of pressure was shown by
Meltzer to be the contraction of the mylohyoid muscles. When the nerves
going to these muscles were divided in a dog, deglutition was practically
abolished. These muscles are probably assisted in their action by the
contraction of the hyoglossus muscles as well as the tongue itself.

It was also demonstrated in these experiments that the contraction of
the esophagus did not partake of the character of ordinary peristalsis. It
was found that the esophagus contracted in three distinct segments, corre-
sponding in all probability to the difference in the character of their muscular
fibers. The first segment, about six centimeters in length, was found to
begin to contract about 1.2 seconds after the beginning of the first curve and
acts for 2 seconds; the second segment, about twelve centimeters in length,
beginning to contract about 1.8 seconds or 3 seconds after the beginning of
the first section, and lasting for from 5 to 7 seconds; the third segment,
six centimeters in length, contracting from 6 to 7 seconds. The beginning
and the end of the contraction for each segment occurred simultaneously
throughout its entire extent. If, however, a series of deglutitory acts follow
each other in quick succession, there is an inhibition of the peristaltic con-
tractions until after the final swallow.

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
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.

DIGESTION.

161

B

The larynx is equally protected against the entrance of food during
deglutition under 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 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 ex-
periments 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
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

Trachea*

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

162 TEXT-BOOK OF PHYSIOLOGY.

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 own 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 of the deglu-
titory 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 unknown. 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 deglutitory movements, it is
obvious that the terminals of this nerve cannot be the source of the natural
afferent impulses. Stimulation of the glosso-pharyngeal 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-
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-

DIGESTION.

163

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
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 is the dilated and highly specialized portion of the alimen-
tary canal intervening between the esophagus and small intestine. When

OBLIQUE FIBERS OF
MUSCLE COAT

POSITION
FTHE
SPHINCTER
PYLORI

POSITION OF THE
SPHINCTER CARDIAE

ESOPHAGO-GASTRIC
ORIFICE;THE CARDIA

FUNDUS

DUODENUM

PYLORUS
ANTRUM

SPHINCTER
ANTRI PYLORICI

CIRCULAR FIBERS

OF
MUSCLE COAT

PREANTRAL OR CARDIAC REGION

FIG. 71. — ANATOMIC FEATURES OF THE STOMACH.

moderately distended with food, it is somewhat conical or pyriform in shape
and slightly curved on itself. It is situated obliquely and in some individuals
almost vertically in the upper part of the abdominal cavity, extending from
the left hypochondrium to the right of the epigastrium. The dimensions
and capacity of the stomach undergo considerable periodic variation accord-
ing to the extent to which it is distended by food. In the average condition
it measures in its long diameter from 25 to 35 centimeters, in its vertical
diameter at the cardia 15 centimeters, in its antero-posterior diameter from
ii 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 opening through which the food passes into the stomach is known as
the esophago-gastric orifice or the cardia. The opening through which' it
passes into the intestine is known as the pylorus, the pyloric or gastro-
duodenal orifice. Between these two orifices the stomach along its upper

164 TEXT-BOOK OF PHYSIOLOGY.

border presents a curve and along its lower border a much larger curve,
known as the lesser and greater curvatures respectively. The extreme left
end of the stomach is termed the fundus. Toward the pyloric orifice there
is a region of the stomach included between the pylorus and a line uniting a
small indentation on the lesser curvature with a point or angle almost oppo-
site on the greater curvature, known as the antrum. The region included
between the ill-defined limits of the fundus and the antrum is known as
the preantral or cardiac region.

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 juntion of the antrum with the preantral region
the circular fibers are arranged in a well-defined bundle termed the sphincter
antri pylorici. In the pyloric region the circular fibers are more closely
arranged, forming thick well-defined rings termed the antral muscles. At
the pyloric opening the circular fibers are again crowded together and
form a distinct muscle band — the sphincter pylori — which projects for some
distance into the interior of the stomach. It has been stated by Riidinger
that the inner fibers of the longitudinal coat become connected with this
circular band and constitute a distinct muscle, the dilatator pylori. The
oblique fibers are most distinct over the cardiac portion of the stomach,
but extend from left to right as far as the junction of the middle and last
thirds of the stomach. They are continuations of the circular fibers of the
esophagus.

The submucous coat 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 muscular tissue, the muscularis
mucoscB, which supports the mucous membrane.

The internal or mucous coat is loosely attached to the muscular coat. In
the empty and contracted state of the stomach it is thrown into longitudinal
folds, or 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. At the pylorus there is a circular
involution of the mucous membrane which is known as the pyloric valve.

DIGESTION.

165

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 preantral or 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 preantral or cardiac glands are formed by an
involution of the basement membrane of the mucosa
and lined by epithelial cells. Each gland may be said
to consist of a short duct, or neck, and a body, or fun-
dus (Fig. 72). The latter portion is wavy or tortuous
and frequently subdivided into as many as four dis-

Vo \

Lumen.

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

FIG. 73. — SECTION OF FUNDUS GLAND
OF MOUSE. Left upper half drawn after
an alcohol preparation, right upper half
after a Golgi preparation. The entire
lower portion is a diagrammatic combi-
nation of both preparations. (Stdhr.)

tinct and separate tubules. The duct is lined by columnar epithelial cells
similar to those covering the surface of the mucosa. The lumen of the
gland is bordered by epithelial cells, cuboid in shape, and consisting of a
granular protoplasm containing a distinct spherical nucleus. These cells
are generally spoken of as the chief or central cells. In addition to the
chief cells, the preantral or 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

i66

TEXT-BOOK OF PHYSIOLOGY.

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 fundic glands. At its extremity 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 throughout by columnar epi-
thelium. 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 muscu-
laris mucosae exceeds 16,500,000. According to
Sappey, the surface of the mucous membrane of the
human stomach presents over 5,000,000 orifices of
gastric glands.

Blood-vessels, Nerves, and Lymphatics. — The
blood-vessels of the stomach after entering the mucosa
break up into a number of branches which are 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 proc-
esses of smooth muscle-fibers passing up from the
muscularis mucosae.

The nerve-fibers distributed to the stomach are
derived from the vagus and the sympathetic branches
of the solar plexus. After piercing the serous coat
the fibers form or unite with a plexus of fibers situa-
STOMACH! te(^ between the circular and longitudinal layers of
m, Mouth of duct; n, neck, the muscle-coat. At the nodal points of this plexus
(Landois) large nerve-ganglion cells are to be found, the whole

forming the mechanism known as Auerbach's plexus. A similar plexus
of cells and fibers in more or less intimate anatomic connection with the
foregoing is found between the muscle and submucous coats, and is known
as Meissner's plexus. From this plexus fine nerve filaments are distrib-
uted to muscle-fibers, blood-vessels, and glands. In the latter structure ter-
minal arborizations have been detected in close contact with the secreting
cells themselves.

The lymphatics, which are quite numerous, originate in the meshes of the
mucosa. The larger trunks enter lymph-glands lying along the greater and
lesser curvatures of the stomach.

Gastric Fistulas. — The general process of digestion, as it takes place in
the stomach, has been studied in human beings and animals with a fistula in

DIGESTION.

167

the walls of the stomach and abdomen, the result either of accident or of
necessary surgical or experimental procedures.

The earliest observations on gastric digestion were made by Dr. Beau-
mont on Alexis St. Martin, who, as the result of a gunshot wound, was left
with a permanent fistulous opening into the fundus of the stomach. This
opening two years after the accident was about two and a half inches in
circumference and usually closed from within by a fold of mucous mem-
brane 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 investi-
gations it was established that the digestion of the food is largely a chemic
act, due to the presence of an acid fluid secreted by the mucous membrane;
that this fluid is secreted most abundantly after the introduction of food into
the stomach; that different articles of food
possess varying degrees of digestibility; that
the duration of digestion varies according to
the nature of the food, exercise, mental states,
etc., and that the process is aided by contin-
uous 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 process when
the food was introduced through the opening.
Similar fistulae have been established in both
carnivorous and herbivorous animals with a
view of studying the process as it takes place

them. The results obtained in these in-

m

FIG. 75. — DIAGRAM SHOWING
THE RELATION OF THE NATURAL
STOMACH TO THE MINIATURE
STOMACH OR POUCH MADE AC-
CORDING TO THE PROCEDURE DE-
VISED BY PAWLOW. V. The nat-
ural stomach. 5. The miniature
stomach, e, e. The septum formed
by the mucous membrane. A, A.
The abdominal walls.

stances in many respects corroborate those
obtained by Dr. Beaumont, though many new
facts, unobserved by him, have been brought
to light.

Much additional information as to the mode of secretion and the char-
acteristics of the gastric juice has been obtained, since the introduction of two
new procedures by Pawlow. The first consists in establishing a gastric
fistula and subsequently dividing the esophagus in the neck, and then so
adjusting the divided ends that they heal 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
oftwo 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 concerned, as the
large stomach does. The phenomena which are observed in it may be taken

i68 TEXT-BOOK OF PHYSIOLOGY.

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 food is discharged from the lower end of the
divided esophagus the appetite continues, and hence the animal will eat for
several hours. By the second procedure it is possible to collect gastric juice
from the miniature stomach and to study the effects on its quantity and
quality produced by psychic states, mastication, different articles of food,
and by the process of digestion itself as it goes on in the large stomach. In
both instances the juice is obtained free from admixture with saliva or food.

Gastric Juice. — The gastric juice obtained from the human stomach
free from mucus and other impurities is a clear, colorless fluid with a 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. The following analyses represent the com-
position of a sample obtained by Schmidt from the stomach of a woman who
had a fistula, but who was nevertheless in good health; also the composition
of the juice from a dog:

COMPOSITION OF GASTRIC JUICE.

Human. Dog.
(Schmidt.)

Water 994-04 973 .06

Organic matter 3.19 17 . 13

Hydrochloric acid o . 20 3 .34

Calcium chlorid o .06 0.26

Sodium chlorid i . 46 2 . 50

Potassium chlorid ° • 55 i .12

Calcium phosphate ] 1.73

Magnesium phosphate V 0.12 o .23

Ferric phosphate J o .08

Ammonium chlorid o .47

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 ferment
or enzyme bodies, termed pepsin and rennin. As is the case with other
enzymes, their true chemic nature is practically unknown.

Pepsin, though present in gastric juice, is not present as such in the chief
cells of the glands, but is derived from a zymogen, propepsin or pepsinogen,
when the latter is treated with hydrochloric acid. This antecedent com-
pound is related to the granules observed in and produced by the cell proto-
plasm during the period of rest. Though pepsin is largely produced by the
central cells of the preantral glands, it is also produced, though in less amount,

DIGESTION. 169

by the cells of the pyloric glands. Pepsin is the chief proteolytic or proteo-
clastic agent of the gastric juice and exerts its influence most energetically
in the presence of hydrochloric acid and at a temperature of about 40° C.
Other acids — e.g., phosphoric, nitric, lactic, etc. — are also capable of exciting
it to activity, though with less intensity.

Rennin or 'pexin is present in the gastric juice not only of man and all the
mammalia, but also of birds and even fish. In its origin from a zymogen
substance ; in its relation to an acid medium and an optimum temperature
it bears a close resemblance to pepsin. Its specific action is the coagulation
of milk, a condition due to a transformation of soluble caseinogen into a
solid flaky body, casein.

Hydrochloric acid is the agent which gives to the gastric juice its normal
acidity. Though the juice frequently contains lactic, acetic, and even phos-
phoric acids, it is generally believed that they are the result of fermentation
changes occurring in the food, the result of bacterial action. The percentage
of hydrochloric acid has been the subject of much discussion. The analysis
of human gastric juice made by Schmidt shows a percentage of 0.02, while
that of the dog is 0.34. It is probable, however, that the low percentage of
HC1 in human gastric juice was due to the admixture with saliva. At pres-
ent it is believed from analyses made for clinical purposes that the acid is
present to the extent of at least 0.2 per cent. This degree of acidity is not
constant during the entire process of digestion. In the earlier as well as in
the later stages it is much less.

The immediate origin of the hydrochloric acid is difficult of 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
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 ekher 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

i7o TEXT-BOOK OF PHYSIOLOGY.

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 Pawlow, 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 primary stimulus to gastric secretion, according to Pawlow, is a
psychic state induced, on the one hand, by the sight or the odor of food
especially if the animal is hungry and the food appetizing; and on the other
hand by the mastication of food which is agreeable to the animal. Thus
when a dog was tempted by the sight of food, the secretion made its appear-
ance at the end of six minutes and during the time of the experiment, which
lasted for an hour and a half, 80 cubic centimeters of the juice were obtained.
This is known as psychic or appetite juice. The character of a psychic
state, however, greatly influences the amount of the juice secreted. 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 Pawlow seem to be the most reliable. He

DIGESTION. 171

found that after division of the nerve, secretion was arrested, and that stim-
ulation 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 pyloric mucous membrane, 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, is carried to the glands
throughout the stomach and which, on reaching the glands, stimulates their
cells in a specific manner. For this reason it has been called the gastric
hormone or the gastric secretin. Whatever the agent 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 undergo
changes in histologic structure which are believed to be connected with the
production of the enzymes, pepsin and rennin, and the acid. In the resting
period the protoplasm of the chief or central cells of the preantral or cardiac
glands becomes crowrded with large and well-defined granules, which during
the period of secretory activity largely disappear, so much so that only the
luminal border of the cell is occupied by them, the outer border being clear
and hyaline in appearance. The parietal cells during rest are large and
finely granular, but after secretion they are smaller in size though still granu-
lar. (See Fig. 76, A and B.)

The cells of the pyloric glands, though containing granules, do not show

172

TEXT-BOOK OF PHYSIOLOGY.

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 fundus 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.

_...&

A ' B

FIG. 76. — SECRETIONS OF DEEP ENDS OF FUNDUS GLANDS OF THE CAT IN DIFFERENT SECRE-
TIVE PHASES. X 1000. — (Bensley). A . From a fasting stomach. The chief cells are filled with
large zymogen granules; nuclei near the outer ends of cells. Gentian- violet preparation.
b b b. Border cells. B. Six hours after an abundant meal of raw flesh. The chief cells exhibit
two zones, the inner occupied by large zymogen granules, the outer by a deeply staining, obscurely
fibrillar element, prozymogen; the nuclei lie at the junction of the two zones, b b b. Border
cells, pr. Prozymogen. c. Mucin-secreting cells, similar to those found hi the neck of the gland.
Gentian- violet preparation. — (Hemmeter after Bensley.}

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
chemic features of proteins, as well as the typical forms contained in the

DIGESTION. 173

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
from it. A glycerin extract of the mucous membrane acidulated with 0.2
per cent, hydrochloric acid is probably the best.

If the small pieces of fibrin be suspended in clear gastric juice and kept
at a temperature of 104° F. (40° C.) for an hour or two, they will be dissolved
and will entirely disappear, giving rise to a slightly opalescent mixture. In
the early stages of the process the fibrin becomes swollen and transparent
and partly dissolved. If at this time the solution be carefully neutralized,
the dissolved portion can be regained in the form of acid 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,
and in a varying length of time transforms the acid-protein into a new form
of protein, termed 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 (primary)
Proteose (secondary)
Peptone.

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, white 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 ammo-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-

174 TEXT-BOOK OF PHYSIOLOGY.

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

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

DIGESTION. 175

change. If the quantity of meat be not too large and the gastric juice be
secreted in proper amount, it is possible that all the meat will be digested in
the stomach. It is quite probable, however, that this is not the case and that
a portion of the semidigested meat passes into the intestine, where its final
solution is effected.

The white of egg, especially when slightly boiled, is much more readily
digested than when raw or firmly coagulated by prolonged boiling. In either
condition, however, the 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. Beamount 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 corrob-
orated 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 pro-
cess, 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

176 TEXT-BOOK OF PHYSIOLOGY.

mode of preparation and the time required for the digestion of different
foods are exhibited :

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 30 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

The time required for the stomach to discharge any given article of
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 Rontgen
rays and the fluoroscopic screen, after the administration of equal quantities,
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.

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 potatoes, 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 carbo-
hydrates excite the secretion of gastric juice they do not absorb the hydro-
chloric acid to any appreciable extent. A combination of equal quantities

DIGESTION.

177

of protein and carbohydrate varies the rate of discharge of each separately.
Thus under these circumstances the carbohydrates are not discharged so
rapidly nor are the proteins detained so long as usual; a combination of fat
with either protein or carbohydrate 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.

Movements of the Stomach.— During the period of gastric digestion
the muscle walls of the stomach become the seat of a series of movements,

peristaltic in character, which not only incorpo-
rate the gastric juice with the food, but also serve
to eject the liquefied portions of the food into
the small intestine.

The movements of the human stomach as
described by Beaumont, as well as the movements
of the dog's stomach as stated by different ob-
servers are not in agreement in all respects, and
are, moreover, open 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-
, Q , / Ject- By means of the Rontgen rays he has been

Right.

Left

FIG. 77. — 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. Cannon.}

Post

FIG. 78. — 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 FZ, 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.}

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. 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. Some of these changes are represented in Fig. 77. The anatomic
features of the cat stomach of interest in this connection are represented
in Fig. 78.

12

178 TEXT-BOOK OF PHYSIOLOGY.

These investigations show that different portions of the stomach walls
exhibit different forms of activity, which for convenience of description are
separately described by Cannon as follows:

i. The Movements of the Pyloric Part. — Within five minutes after
a cat has finished a meal of bread there is visible near the duodenal end
of the antrum a slight annular contraction which moves peristaltically to
the pylorus; this is followed by several waves recurring at regular intervals.
Two or three minutes after the first movement is seen, very slight constric-
tions appear near the middle of the stomach, and, pressing deeper into the
greater curvature, course slowly toward the pyloric end. As new regions
enter into constriction, the fibers just previously contracted become relaxed,
so that there is a true moving wave, with a trough between two crests. When
a wave swings round the bend in the pyloric part, the indentation made by
it deepens; and as digestion goes on the antrum elongates and the constrictions
running over it grow stronger, but, until the stomach is nearly empty, they
do not entirely divide the cavity. After the antrum has lengthened, a wave
takes about thirty-six seconds to move from the middle of the stomach to
the pylorus. At all periods of digestion the waves recur at intervals of almost
exactly ten seconds. It results from this rhythm that when one wave is
just beginning several others are already running in order before it. Be-
tween the rings of constriction the stomach is bulged out, as shown in the
various outlines in Fig. 77.

Movements of the Pyloric Sphincter. — During the first ten or fifteen
minutes after the first constriction of the antrum the pylorus is tightly
closed. After this period it opens at irregular intervals to permit the passage
of liquefied food which is ejected by peristaltic waves for a distance of two
or three centimeters into the duodenum. The frequency with which the
pylorus opens depends apparently on the degree to which the food is softened.
When the food is hard, the pylorus closes more tightly and remains closed a
longer period than when it is soft.

The 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 nerve system.

The Activity of the Cardiac Portion. — As digestion proceeds, the pre-
antral part of the stomach elongates and assumes the shape of a tube,
which becomes the seat also of peristaltic constriction waves. As a result,
some of the food is gradually forced into the antrum to succeed that which
has been prepared and ejected into the duodenum. As the pre-antral tube
is emptied of its contents the longitudinal and circular fibers of the fundus

DIGESTION. 179

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. 77. The fundus acts as a reservoir for the food and forces out its
contents a little at a time as the antral mechanism is ready to receive them.
Since peristaltic movements are absent from the cardiac portion the food is
not mixed with gastric juice, and therefore salivary digestion can continue
for a considerable period. There is no evidence of a circulation of food
in the stomach as sometimes described. On the contrary, the movement
through the pre-antral tube and antrum is in general a progressive though
an oscillating one. As the constriction waves rapidly pass over the food it is
advanced toward the pyloric opening, 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 pylorus relaxes,
when the advancing constriction wave expels it into the intestine. With its
expulsion room is afforded for an additional quantity of 'food, and hence
there is a general advance of the food mass toward the pylorus.

Though these observations were made on the cat, evidence is accumu-
lating which goes to show that in human beings the walls of the stomach
exhibit constriction waves which are similar in all respects to those above
described.

The Nerve Mechanism of the Stomach. — In preceding paragraphs
it was stated that during the period of gastric digestion the food is retained
in the stomach because of the closure of the cardia (the esophago-gastric
orifice) and of the pylorus (the gastro-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
pyloric or antral muscles are vigorously active throughout the digestive
period, triturating the food, mixing it with gastric juice, and finally driving
it through the temporarily open pylorus into the intestine.

These separate but related groups of muscle-fibers, by reason of their
endowments, and possibly by virtue of the presence of local nerve mechan-
isms, 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 vary-
ing from an hour to an hour and a half. Though nerve-cells and nerve-
fibers (Auerbach's plexus) are present in the walls of the stomach between
the layers of muscle-fibers, it is not believed that they are the immediate
sources of the stimulus to the contraction, though they may act as a coordinat-
ing mechanism. The stimulus in all probability develops in the muscle-
fiber itself and is therefore myogenic.

The sphincter cardia 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

i8o TEXT-BOOK OF PHYSIOLOGY.

nerves distributed to it. The contraction may be so pronounced as to offer
considerable resistance not only to the passage of food but even to the intro-
duction of a sound into the stomach. (Cannon.) That the normal con-
traction is under the influence of the central nerve system is shown by the
effects which follow stimulation of the peripheral end of the divided vagus.
If it is stimulated with weak induced currents, the contraction is somewhat
inhibited and the orifice enlarged; if it is stimulated with strong currents the
contraction is markedly increased. Apparently there are in the vagus two
sets of efferent nerve-fibers, one of which augments, while the other inhibits
the contraction, and corresponding to the nerves there must be in the medulla
oblongata two centers from which they arise, an augmentor and an
inhibitor.

Observation has shown that at the beginning of each act of deglutition,
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 the food into the stomach the tonic con-
traction 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.

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

The degree of activity of both the pyloric sphincter and the antral muscles
is modified also by the central nerve system either in the way of inhibi-
tion or augmentation and in response to gastric stimulation. The nerves
more especially concerned in the maintenance and regulation of the gastric
contractions are the vagi and the splanchnics. The afferent fibers through
which nerve impulses pass to the nerve centers are in all probability con-
tained in the trunk of the vagus nerve; the efferent fibers through which
nerve impulses from the centers reach the stomach, are contained partly in
the trunk of the vagus and partly in the trunk of the splanchnic nerve.

If the vagus nerves are divided in the neck, there is a loss of muscle tonus
though the contractions do not wholly disappear. Stimulation of the per-
ipheral 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 maybe 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

DIGESTION. 181

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, contain both inhibitor and augmentor nerve-fibers for the
gastric musculature, though the augmentor nerves largely preponderate.

Stimulation of the peripheral end of a divided splanchnic is followed by
an inhibition of the peristalsis and a loss of tone. Morat, however, has
observed a primary opposite effect. The splanchnic nerves, therefore,
also contain both inhibitor and augmentor fibers for the gastric musculature
though the inhibitor fibers largely preponderate. From these facts it would
appear that the gastric muscles receive both inhibitor and augmentor fibers
from two different sources.

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 fche musculature acquires a cer-
tain degree of tension whereupon the peristalsis arises. An excess or de-
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
lost and peristalsis fails to develop. When once the peristalsis is well
developed in digestion, division of the vagi has no effect. (Cannon.)

INTESTINAL DIGESTION.

The physical and chemic changes which the food principles undergo in
the small intestine, and which collectively constitute intestinal digestion,
are probably more important and complex than those taking place in the
stomach, for the food is, in this situation, subjected to the solvent action of
the pancreatic and intestinal juices, as well as to the action of the bile, each
of which exerts a transforming influence on one or more substances and
still further prepares them for absorption into the blood.

To rightly appreciate the physiologic actions of the digestive juices
poured into the intestine, the nature of the partially digested food as it
comes from the stomach must be kept in mind. This consists of water,
inorganic salts, acidified 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

182 TEXT-BOOK OF PHYSIOLOGY.

absorbable and assimilable compounds which collectively constitute intes-
tinal digestion.

THE SMALL INTESTINE.

The small intestine, in which this stage of digestion takes place, is a
convoluted tube, measuring about seven meters in length and 3.5 cm. in
diameter, and extending from the pyloric orifice of the stomach to the begin-
ning of the large intestine.

The intestine consists 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 developed in the upper than in the lower portions of the intestine.
The muscle coat consists of two layers of fibers: (i) an external or longitudinal,
and (2) an internal or circular layer. The longitudinal fibers are most
marked at that border of the intestine free from peritoneal attachment,
though they form a thin layer all over the intestine. The circular fibers are
much more 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-colic sphincter. It is usually
in a state of tonic contraction and regulates the passage of materials from
the small into the large intestine, and possibly also in the reverse direction
under special circumstances.

The submucous coat consists of areolar tissue and serves to unite the
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 penetating 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,

DIGESTION. 183

uniting with the splenic and gastric veins, forms the portal vein. The nerves
are derived from the lower part of the semi- lunar ganglia. The branches
follow the blood-vessels and become associated with two plexuses, one
(Auerbach's) lying between the muscle coats, the other (Meissner's) lying
in the 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 (Lieberkiihn's) glands.

The duodenal glands are situated beneath the mucous membrane and
open by a short wide duct on its free surface. They are racemose glands
lined by nucleated epithelium. The secretion of these glands is clear,
slightly viscid, and alkaline. Its chemic composition and 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 in-
testinal juice.

Intestinal Juice. — Owing to its admixture with other secretions and
to the profound disturbance of the digestive function caused by the establish-
ment of intestinal fistulae, 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 physiologists, by the employ-
ment 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.

i84

TEXT-BOOK OF PHYSIOLOGY.

PANCREAS.

The pancreas is a long flattened gland, situated deep in the abdominal
cavity, lying just behind the stomach. It measures from fifteen to twenty
centimeters in length, six in breadth, and two and a half in thickness. Itfis
usually divided into a head, body, and tail. The head is directed to the right

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.}

side and is embraced by the curved portion of the duodenum; the tail is di-
rected to the left side and extends as far as the spleen (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 ap-
proaches the head of the gland it gradually increases 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
plexus.

Histologic Structure. — In
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-

FIG. 80. FIG. 81.

ONE*SACCULE OF THE PANCREAS OF THE RABBIT
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 filled 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.}

DIGESTION.

185

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.
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 of Langerhans. — 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 or rods or columns,
separated from the acini and from one another by layers of connective

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, ay a, ISLAND OF
LANGERHANS, AND b, THE USUAL ACINI. —
(Piersol.}

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
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. ^ n

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

i86 TEXT-BOOK OF PHYSIOLOGY.

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
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 100° 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 stomach and its early passage into the duo-
denum and is brought about by the action of a primary and a secondary
stimulus.

The primary stimulus is 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 trans-
mitted 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 Pawlow. This investigator states 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-

DIGESTION. 187

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 induced electric currents
is followed after a latent period of two or three minutes by 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 intervals of digestion, it will be observed
that the inner zone is broad, highly granular, occupying nearly the entire cell,
while the outer zone is narrow and clear. If, however, the gland be examined
shortly after a period of active secretion, the reverse conditions will be
observed; that is, the inner zone will be narrow, containing 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.

z88 TEXT-BOOK OF PHYSIOLOGY.

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:

1. On starch. When normal pancreatic juice or a glycerin extract of
the gland is added to a solution of hydrated starch, the latter is speedily
transformed into maltose, passing through the 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 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 in 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 and certain nitrogenized bodies,
leucin, tyrosin, aspartic acid, etc. 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. It
will be recalled that when the peptone of peptic digestion is subjected to the
action of trypsin a portion of it is decomposed into leucin and tyrosin, while
another portion presumably is not so decomposed, for which reason the
latter was called anti- and the former, hemi-peptone. It is now believed that
anti-peptone is not a peptone at all, but a compound termed carnic acid,
which can be decomposed into simpler nitrogen-holding bodies such as
leucin, tyrosin, arginin, etc.

The action of trypsin on proteins in an alkaline medium may be illustrated
by the following scheme:

Protein

Alkali-protein

Secondary proteoses

Peptone

1

Leucin

Tyrosin

Aspartic acid

Arginin Amn

When the proteins which have escaped digestion in the stomach pass
into the small intestine and mingle with the pancreatic juice, they are

DIGESTION. 189

doubtless digested in the course of the intestinal canal, passing through the
stages just described. As leucin and tyrosin are found in the intestine during
digestion, it is probable that a portion of the peptone undergoes decompo-
sition into these bodies; but as to the extent to which this takes place or in
how far it is a necessary process under normal conditions is yet a subject of
investigation. It is certain that it takes place when there is an excess of
protein food or when for any reason digestion is prolonged or absorption is
delayed.

Though 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 accepted, there is an ever increasing
evidence that it is not wholly true, and that the final stage may be the forma-
tion of the nitrogen-holding compounds above mentioned; in other words,
that the cleavage of the proteins is far more complete than has heretofore
been assumed. Indeed it has been asserted that they are reduced, if not to
their ultimate constituents, the amino- and diamino-acids, at least to one or
more of the different polypeptid stages. Ever since the discovery by Cohn-
heim 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, etc., which
in turn are absorbed and synthesized to blood or tissue albumin. 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 100° 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 + C3H6(HO)3
Triolein. Water. Oleic Acid. Glycerin.

If to this acidified oil. there be added an alkali, e.g., potassium or sodium
carbonate, the latter will at once combine with the 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 + C18H3402 = 2NaC18H,302 + H2CO8

Coincident with the formation of the soap the remaining neutral oil under-
goes division into drops of microscopic size, which float in the soap solution,
forming what has been termed an emulsion, which is white and creamy
in appearance. The action of the pancreatic juice may then be said to
consist in the cleavage of the neutral fats into fatty acids and glycerin, after
which the formation of the soap and the division of the fat takes place spon-
taneously. 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

IQO TEXT-BOOK OF PHYSIOLOGY.

it is large, practically all of the fat undergoing this decomposition, with
the formation of soap and glycerin prior to their absorption.

According to Pawlow, the relative amounts of the pancreatic enzymes
produced, are conditioned by the character and amounts of the food principles
consumed. Thus, if chyme contains an excess of either starch, 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
the former is accomplished, according to Pawlow, by a special activating
ferment secreted by the epithelium of the small intestine and termed entero-
kinase.

The rapidity with which pancreatic juice in the presence of bile and
hydrochloric acid (under conditions such as are present in the duodenum)
can develop sufficient fatty acid to form an emulsion was determined by
Rachford to be two minutes. The activity of steapsin is thus shown to be
very great.

Physiologic Action of the Intestinal Juice. — The part played
by the intestinal juice in the digestive process is yet a subject of discussion,
as the results obtained by different observers are in some respects con-
tradictory, due to the fact that animals, including human beings, have
been the subjects of experimentation. Notwithstanding the actions of
saliva, gastric and pancreatic juice, there yet remain in the food saccharose,
maltose, and lactose, three forms of sugar which are believed by most to
observers to be non-assimilable and therefore require some change before
they can be absorbed and assimilated. . An extract of the intestinal mucous
membrane or the intestinal juice of a dog, added to a solution of saccharose,
will in a very short time convert it into dextrose and levulose, which together
constitute invert sugar. The enzyme by which this inversion is produced,
though nothing definite is known as to its nature, has been termed invertase
or saccharase. Tubbey and Manning state that the human intestinal juice
as obtained by them has the same action. In the case of intestinal fistulas
reported by Busch, which were supposed to be located in the upper third of
the intestine, it was found that when saccharose was introduced into the
lower opening, it was not inverted but appeared in the feces unchanged.

Maltose is also rapidly transformed into dextrose. Lactose appears
to be unaffected by the pure juice. As it is non-assimilable it has been
supposed to undergo conversion into dextrose and galactose while passing
through the epithelial cells of the intestinal mucosa. In either case the
transformation is brought about by two ferments known respectively as
maltase and lactase.

The intestinal juice also contains the two ferments enterokinase and
erepsin. The former activates the trypsinogen of the pancreatic juice and
converts it into trypsin; the latter acts on the peptones and reduces them to
peptids and amino-acids..

DIGESTION. 191

THE LIVER.

The liver is a highly vascular conglomerate gland situated in the right
hypochondriac region and connected with the intestine by a duct.

Inasmuch as the liver performs several functions related to both secretion
and excretion, a consideration of its structure and its various functions will
be deferred to a subsequent chapter. In this connection only the bile, and its
physical properties and chemic composition in relation to the digestive
process, will be considered.

The bile 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 corre-
sponding to the orifice of the pancreatic duct, and most abundantly at the
time the food is passing through the duodenum, it is usually regarded as a
digestive fluid possessing an influence favorable if not necessary to the com-
pletion 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 (Fig. 60). 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 termina-
tion of the common bile-duct is provided with a distinct band of circularly
disposed muscle-fibers, which when in action completely close the orifice
and prevent the discharge of bile. It may therefore be regarded as a true
sphincter muscle. Small racemose glands are embedded in the mucous
membrane of the main ducts.

Physical Properties and Chemic Composition of Bile. — 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 dis-
charged from the liver, but may become neutral in the gall-bladder. The
alkalinity depends on the presence of sodium carbonate and sodium phos-
phate. When fresh, it is inodorous; but it readily undergoes putrefactive
changes, and soon becomes offensive. Its taste is decidedly bitter. When
shaken with water, it becomes frothy — a condition which lasts for some
time and which is due to the presence of mucin. In ox bile the mucin is
replaced by a nucleo-proteid.

The color of bile obtained from the hepatic duct is variable, usually a
shade between a greenish-yellow and a brownish-red. In different animals

iQ2 TEXT-BOOK OF PHYSIOLOGY.

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.

Human bile obtained from an accidental biliary fistula was shown by
Jacobson to contain the following ingredients, viz. :

COMPOSITION OF HUMAN BILE.

Water 977 .40

Sodium glycocholate 9-94

Sodium taurocholate a trace

Cholesterin o . 54

Free fat o . 10

Sodium palmitate and stearate 1 .36

Lecithin o .04

Organic matter, and pigments bilirubin and biliverdin 2 .26

Sodium chlorid 5 .45

Potassium chlorid o . 28

Sodium phosphate i .33

Calcium phosphate o .37

Sodium carbonate o . 93

1000.00

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 hydrating agents, such as
dilute acids and alkalies, both acids will undergo cleavage into their re-
spective components — e.g., glycocoll and cholalic acid, taurine and cholalic
acid. Glycocoll and taurine are crystallizable nitrogenized compounds
known chemically as amido-acetic and amido-isethionic acids respectively.
The bile salts are produced in the liver by a true act of secretion, as they
are not found in any of the tissues and fluids of the body. After being dis-
charged into the intestine they undergo chemic changes, after which they
can no longer be recognized. In all probability they are resorbed into the
blood and play some ulterior part in the nutrition of the body.

The presence of the bile salts can be demonstrated 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 sulphuric 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 should be exercised
to keep the temperature below 70° C.; the characteristic colors appear to
be due to the action of the sulphuric acid on the cane-sugar by which a
substance, 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

DIGESTION. 193

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 is a constant ingredient of bile, though it is
not confined to this fluid, as its presence has been determined in the crystal-
line lens, blood-corpuscles, nerve-tissue, and various pathologic fluids. It
is an organic non-nitrogenized compound resem-
bling the fats in some particulars, but differing
from them in not being capable of saponification
with alkalies. It presents itself in the form of
thin transparent rectangular crystals, insoluble
in water but soluble in ether and boiling alcohol
(Fig. 84). It is held in solution in bile by the bile
salts. If they are deficient in amount, the choles- FIG. 84.— C HO LEST ERIN-
terin may pass out of solution, collect around some CRYSTALS. — (Landois and
foreign matter, and form a gall-stone. Choles-
terin is largely a product of the metabolism of nerve-tissue, from which it
is absorbed by the blood, carried to the liver, and excreted. In the intes-
tine it is converted into stercorin and discharged from the body in the feces.

Bilirubin, Biliverdin. — These two pigments impart to the bile its
red and green colors respectively. Bilirubin is present in the bile of human
beings and the carnivora, biliverdin in the bile of the herbivora. As the
former pigment readily undergoes oxidation in the gall-bladder, giving rise
to the latter pigment, almost any specimen of bile may present any shade
of color between red and green. Bilirubin is 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. An
oxidation of the bilirubin can be produced by nitroso-nitric acid. If this
agent is added to a thin layer of bile on a porcelain surface, a series of colors
will rapidly succeed one another, commencing with green and passing to
blue, orange, purple, and yellow. This is the basis of the well-known test
for bile pigments suggested by Gmelin.

Lecithin. — Lecithin is regarded, because of its physical properties and
chemic composition, as a complex fat. When pure it presents itself 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. It is readily decomposed, yielding with various reagents
glyco-phosphoric acid, a fat acid (stearic), and the alkaloid, cholin. Leci-
thin has been regarded as one of the decomposition products of nerve-tissue,
removed from the blood by the liver and thus becoming one of the constitu-
ents of the bile, in which it is held in solution by the bile salts.

The Mode of Secretion and Discharge of Bile. — The manner in which
the bile flows from the liver into the main hepatic ducts, the variations in the

194 TEXT-BOOK OF PHYSIOLOGY.

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
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 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 experi-
ments Doyon concludes, that during 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-bladder, thus establishing the conditions for the storage of bile; but
when intestinal digestion is inaugurated the splanchnic nerve carries nerve
impulses which inhibit the sphincter and augment the contraction of the
walls of the gall-bladder, thus establishing the condition for the discharge of
the bile.

The total quantity of bile secreted daily has been estimated to be from
500 to 800 grams.

Physiologic Action of Bile. — Notwithstanding our knowledge of the
complex composition of bile, the quantity discharged daily, and the time
and place of its discharge, its exact relation to the digestive process has not
been fully determined. No specific action can be attributed to it. It has
but a slight, if any, diastatic action on starch. It is without influence on

DIGESTION. 195

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
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

196 TEXT-BOOK OF PHYSIOLOGY.

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
fetid and there is a falling of the hair, showing some profound disturbance
of the general nutritive process.

The Movements of the Small Intestine. — 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 revealed 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 intes-
tinal contents into small segments and for this reason was termed by Can-
non rhythmic segmentation; 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

DIGESTION.

197

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.

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

FIG. 85. — THE DIVISIVE OR SEGIMENTING MOVEMENTS OF THE SMALL INTESTINE. A, surface
of a portion of the intestine, showing six constrictions which divide the contents into five
segments, as shown in B: a~s these constrictions pass away new ones come in between them and
divide each segment of the contents into two, the adjoining halves of neighboring segments
fusing to make the new segments shown in C. Repetition of this process results in the condition
shown in D. — (Modified, after Hough andSedgewick, " The Human Mechanism")

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
peristaltic waves. To avoid confusion, the term diastalsis has recently been
employed (Cannon) to designate the true peristaltic movement, viz.: pro-
gressive 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. — Under conditions that are perhaps not strictly
physiologic, a rapid and far-reaching peristalsis is developed which may pass
over the intestine, from the duodenum to the cecum without stopping in the
course of 15 seconds, in the rabbit, and which has been designated rush
peristalsis. It is characterized by a wave of constriction preceded by a

198 TEXT-BOOK OF PHYSIOLOGY.

completely inhibited long section of intestine. The contents of the intestine
are carried 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 combina-
tion 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
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 nevertheless the contraction may be augmented or inhibited
by the central nerve system through the vagus and splanchnic nerves.

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-

DIGESTION. 199

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
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.

The colon ascends to the under surface of the liver, where it bends at a
right angle, crosses the abdominal cavity to the spleen, bends again, and
descends to the left iliac fossa. At this point it turns upon itself to form the
sigmoid flexure. The rectum is a dilated pouch, situated within the true
pelvis. It measures from 15 to 18 centimeters in length. Within three
centimeters of its termination 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.

2oo TEXT-BOOK OF PHYSIOLOGY.

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. Between
the sacculi, however, they are more closely arranged. In the rectum they
are well developed, and at a point an inch above the anus they form, as
stated above, the internal sphincter.

The mucous membrane of the large intestine possesses neither villi nor
valvulae conniventes. It contains a large number of tubules consisting of a
basement membrane lined by columnar epithelium. They resemble the
follicles of Lieberkiihn. 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 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 ileo-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 preceded by an area
of inhibition or relaxation. It resembles rather the corresponding move-
ment manifested by the small intestine to which the term anastalsis has been
given, and is propagated along the muscle coat independently of the myen-
teric plexus.

DIGESTION. 201

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 postganglionic 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 postganglionic 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
postganglionic 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 the
intestine for fermentative and putrefactive processes — e.g., heat, moisture,
oxygen, and the presence of various microorganisms — the food, when con-
sumed in excessive quantity or when acted on by defective secretions, under-
goes a series of decomposition changes which are attended by the production
of gases and various chemic compounds. Dextrose and maltose are partially
reduced to lactic acid; this to butyric acid, carbon dioxid, and hydrogen.
Fats are reduced to glycerol and fat acids, the glycerol, according to the
organisms present, yields succinic acid, carbon dioxid, and hydrogen. The
proteins under the prolonged action of the erepsin of the intestinal juice are
reduced, with the production of leucin and tyrosin. These crystalline
compounds are in turn reduced to simpler forms. The former yields valer-
ianic acid, ammonia, and carbon dioxid; the latter, tyrosin, gives rise to indol,
which is the antecedent of indican, found in the urine. This compound is
discharged in part in the feces though it is in part absorbed into the portal
blood and carried direct to the liver where it is oxidized to indoxyl and com-
bined 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 under-
goes 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, another derivative of the protein molecule, the result of bacterial
decomposition, passes in part into the feces and gives to them the character-
istic odor. It is also in part absorbed and oxidized to skatoxyl, after which
it combines with potassium sulphate to form potassium skatoxyl sulphate.
It is eliminated in the urine 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,

202 TEXT-BOOK OF PHYSIOLOGY.

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
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
ncreases 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

DIGESTION. 203

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 nerves 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 an-
other their activities. If the act of defecation is to take place there is an
inhibition of the nerve-centers maintaining the tonus or contraction of the
two sphincter muscles and a stimulation of the nerve-centers exciting or
augmenting the contraction of the rectal muscles with the result of a dis-
charge of the fecal mass. 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,
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 efferent nerve-fibers for the external sphincter 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 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 hypogastric nerve 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

204 TEXT-BOOK OF PHYSIOLOGY.

coming from the cord are preganglionic, those coming from the ganglia,
postganglionic.

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 nerve fibers that excite the central mechanism to activity,
are contained in both the nerve paths described in foregoing paragraphs
and enter the spinal cord in the dorsal roots of the lumbar and spinal nerves.
Although the anatomic relations of the various nerves composing this mechan-
ism are fairly well known, their physiologic actions are not 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 peristaltic
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-
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 XL
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 through 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 the 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 lymph- and blood-capillaries; second-
arily, the lymph-vessels and the veins.

Tissue or Lymph-spaces ; Lymph-capillaries. — Everywhere through-
out 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,

205

206

TEXT-BOOK OF PHYSIOLOGY.

peritoneal, pericardial, etc., are also to be regarded as lymph-spaces. The
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 lymph-capillaries in which the lymph-vessels proper take their
origin are arranged in the form of plexuses of quite irregular shape. In
most situations they are intimately interwoven with the blood-vessels, from
which they can be readily distinguished by their larger caliber and irregular
expansions. The wall of the lymph-capillary is formed by a single layer of
endothelial cells with characteristic sinuous outlines. These capillaries anas-
tomose 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 histo-
logic arrangement does not exist
but that on the contrary the
lymph-capillaries are closed ves-
sels and that the tissue space and
the interior of the lymph-capil-
lary are separated one from the
other by a thin partition of endo-
thelial cells. As the shape, size,
etc., of both lymph- spaces and
capillaries are determined largely
by the nature of the tissue in
which they are found, it is not always possible to separate one from the
other. Their function, however, may be regarded as similar: viz., the
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.

The blood-capillaries not only permit of a transudation of the liquid
nutritive material from the blood through 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.

Lymph- vessels. — The lymph-vessels constitute a system of minute,
delicate, transparent vessels found in nearly all the organs and tissues of the
body, and take their origin from the lymph-capillaries and spaces above
described (Figs. 87 and 88.) 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

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
blood-vessels. — (Landois and Stirling.}

ABSORPTION. 207

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 possesses 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.

FIG. 87. — LYMPH- VESSELS AND LYMPH-NODES OF THE HEAD AND NECK.

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

208

TEXT-BOOK OF PHYSIOLOGY.

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 medul-
lary portion, they subdivide and form
bands or cords which interlace in every
direction and constitute a loose meshwork
the spaces of which communicate with one
another and with the alveoli (Fig. 89).
Within the meshes of this framework the
proper gland substance is contained. In
the cortical compartments it is moulded
into pear-shaped masses; in the medullary
meshwork it assumes the form of rounded
cords which are connected with one another.
In both regions, however, it is separated
from the septa by a space termed a lymph
sinus, through which the lymph flows as it
passes through the gland. The lymph
sinus is crossed by a network of retiform
connective tissue which offers considerable
resistance to the passage of the lymph.
The gland 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 separated from the
lymph sinus by a dense layer of a reti-
culum, which, however, does not prevent
lymph and even corpuscles from passing
through it into the lymph sinus.

The lymph-glands are abundantly sup-
plied 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 lymphatics enter the
gland they lose their external and middle coats, retaining only the internal
or endothelial coat, which lines the inner surface of the lymph sinus.

FIG. 88. — LYMPH-VESSELS OF THE
ARM. — (D caver.)

ABSORPTION. 209

The current of lymph, therefore, is from the afferent vessels through the
lymph sinus into the efferent vessels. In addition to this primary current,
there is a secondary current flowing from the capillary blood-vessels out-
ward and into the sinus, which 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

tr.

FIG. 89. — DIAGRAMMATIC SECTION OF A LYMPH-NODE, a. I., Afferent; e. I., efferent lymph-
vessel, C. Cortical substance. M. Recticular cords of the medulla. /. s. Lymph sinus, c
Capsule, with trabeculae, tr. — (Landois and Stirling.}

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 subclavian
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. 90.
14

210

TEXT-BOOK OF PHYSIOLOGY.

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
tissue spaces as intercellular lymph, and that found in the lymph-vessels as
intravascular lymph.

FIG. 90. — DIAGRAM SHOWING THE COURSE OF 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.")

The Physical Properties of Lymph. — Whether obtained from tissue
spaces or from lymph-vessels, the lymph presents practicallyjthe 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-
sion a large number of corpuscles similar to those seen in the lymph-glands

ABSORPTION. 211

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
from the vessels lymph undergoes a spontaneous coagulation, though
the coagulum is never as firm as that observed in the coagulation of the
blood. The cause of the coagulation is the appearance of fibrin. After
a variable length of time the coagulum separates into a liquid and a solid
portion, the serum and the clot.

The Chemic Composition of Lymph. — Although the lymph obtained
from the tissue spaces, from the lymph-vessels, as well as from the so-called
serous cavities has the same general chemic characteristics, 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
per cent, of sugar, and from 0.8 to 0.9 per cent, of inorganic salts, of which
sodium chlorid (0.55 per cent.) and sodium carbonate (0.24 per cent.)are
the most abundant (Munk). There are usually in most specimens small
quantities of potassium, calcium, and 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 vols. 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 through the capillary wall and is received into the tissue-spaces,
where it comes into intimate contact with the tissue-cells.

The forces concerned in the passage of the constituents of the blood-
plasma through 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-

212 TEXT-BOOK OF PHYSIOLOGY.

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
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 other investigators, 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 secre-
tion 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 sub-
stances into the blood apparently increases the activity of the endothelial
cells, as shown by an increased lymph production without any appreciable
increase of intracapillary pressure. Thus it has been shown that after the
injection into the blood of sugar, sodium chlorid, sodium sulphate, urea, etc.,
there is an increase in the flow of lymph from the thoracic duct. The lymph,
however, under these circumstances is richer in water than is normally the
case. As the blood at the same time increases its percentage of water, it is
assumed that the water is extracted from the tissues, by reason of an increased
percentage of salts in the tissue spaces due to increased activity of the endo-
thelial cells. A higher percentage of salts in the lymph than in the blood is
difficult to account for on the diffusion-filtration theory. The injection of
peptones, albumin, the extract of the muscles of the leech, crab, mussel, etc.,
is also followed by an increase in the amount of lymph discharged from the
thoracic duct; but in this instance the lymph possesses a higher degree of
concentration, being richer not only in inorganic but also in organic con-

ABSORPTION. 213

stituents. The cause of this increase in both the quantity and quality
of the lymph is believed to be an increased activity in the secreting power of
the endothelial cells.

The more recent experiments of Starling indicate that in addition to the
difference of pressure 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
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 which has been invoked to account for the passage of the
constituents of lymph through 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. It is possible that all these facts may be otherwise interpreted;
the subject is yet a matter of investigation.

The Functions of Intercellular Lymph. — The origin and composition
of lymph, its situation and relation to the tissue cells indicate that its 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.

214 TEXT-BOOK OF PHYSIOLOGY.

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
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 surfaces, especially in its relation
to the absorption of new material, is the mucous membrane of the alimentary
canal, and more particularly that portion lining the small intestine, provided
as it is with specialized absorbing structures — the villi. Though certain
substances can be absorbed from the mouth, it is not probable that any food
is so absorbed. From the changes which the food principles undergo in the
stomach it might naturally be inferred that their absorption would promptly
follow. Experimental researches have demonstrated, however, that this
takes place, if at all, but to a slight extent. If, however, solutions of inor-
ganic salts, sugars, and peptones possessing a concentration of at least 5 per
cent. — a degree of concentration seldom realized under normal conditions —
are introduced into the stomach, their absorption will be effected, the rate of
absorption following in a general way the increase, within limits, in concen-
tration. Water is practically not absorbed from the stomach. The absorp-
tion of the products of digestion — i.e.y dextrose, levulose, peptones, amino-
acids, soaps, glycerin, fat acids, salts, along with water, in which for the
most part they 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. 91) supporting tall columnar epithelial cells. Each cell is
composed of granular bioplasm containing a distinct nucleus. At its free

ABSORPTION.

215

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

<Zs

FIG. 91. — LONGITUDINAL SEC-
TION OF A VILLUS FROM IN-
TESTINE OF THE DOG, HIGHLY
MAGNIFIED, a. Columnar epi-
thelium containing goblet-cells (ft)
and migratory leukocytes (h). c.
Basement membrane, d. Plate-
like connective-tissue elements of
core, e, e. Blood-vessels. /. Ab-
sorbent radical or lacteal. —
(Piersol.)

FIG. 92. — SECTION OF INJECTED SMALL
INTESTINE OF CAT. a, b. Mucosa. g.
Villi. i. Their absorbent vessels, h. Sim-
ple follicles, c. Muscularis mucosae. ;'. Sub-
mucosa. g, e'. Circular and longitudinal
layers of muscle. /. Fibrous coat. All the
dark lines represent blood-vessels filled with
the injection mass. — (PiersoL)

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
toj;he 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 delivering nutritive material to the capillary
plexus (Fig. 92). While passing through the latter a portion of the blood-
plasma transudes through the capillary walls into the spaces of the reticu-

2l6

TEXT-BOOK OF PHYSIOLOGY.

lated tissue, constituting lymph. At the same time products of tissue metab-
olism pass through the capillary walls into the blood. The blood then passes
into the venules, which, leaving the villus at its base, unite with the veins of
the submucous coat to form the intestinal veins. These finally unite with
the gastric and splenic veins to form the portal vein, which enters the liver
at the transverse fissure (Fig. 93). 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 re-
moval from the villus of anything but
the excess of lymph and metabolic
products.

Function of the Villi.— The villi,
and especially the epithelial cells cover-
ing 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
epithelial cells accomplish this result,
nothing definite can be asserted. In-
asmuch as the absorption of food does
not take place in accordance with the
laws of osmosis as at present under-
stood, it has been suggested that the
cells possess a "selective action" de-
pendent 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

FIG. 93'. —DIAGRAM OF THE PORTAL

VEIN (pv) ARISING IN THE ALIMENTARY

TRACT AND SPLEEN (s), AND CARRYING THE
BLOOD FROM THESE ORGANS TO THE LIVER.
— (Yea's "Text-book of Physiology."}

ABSORPTION. 217

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 through 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 minute
granules 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., 0.05 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 of the Products o) Protein Digestion. — For the reason
that the proteins are for the most part transformed through hydra tion 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. Though the production of pep-
tones was believed to be a necessary process before the absorption of protein
material could be effected, yet it was apparently demonstrated by the results
of experimentation that unchanged native protein, e.g., egg- albumin and
partially digested proteins, e.g., proteoses, were also absorbed though in
far less amounts. It has also been demonstrated that native proteins can
be absorbed by the mucous membrane of the large intestine. Inasmuch
as chemic analysis has failed to detect more than a trace of either peptone or
native protein in the portal blood or in the lymph of the thoracic duct, it
must be assumed that the epithelium after absorbing must also synthesize
them into some form of coagulable protein (plasma-albumin) which is
readily assimilable by the blood. That such a reconversion is necessary
would appear from the fact that the introduction of peptones even in small
amounts into the blood is followed by their elimination unchanged in the
urine. When injected into the blood in large amounts, they act as toxic
agents, giving rise to a fall of blood-pressure, a diminished coagulability of
the blood, coma, and death.

After passing through the epithelium into the spaces of the villi the recon-
structed or synthesized plasma-protein molecules are removed by the blood-

218 TEXT-BOOK OF PHYSIOLOGY.

vessels and carried direct to the liver. Even though there is no appreciable
increase in the amount of protein in the portal blood during digestion, there
is every reason to think that this is the route by which it reaches the general
circulation. Ligation of the thoracic duct does not interfere to any appre-
ciable extent with protein absorption nor with the normal elimination of urea
nor with the weight of the animal.

The foregoing statements are based on the view that the final stage in
the digestion of proteins is the formation of peptones. There are reasons
however for believing that the change is more far-reaching and complete, and
that the peptones in turn are disintegrated and reduced to still less complex
bodies represented by polypeptids, peptids, and even amino-acids. (See
page 189.) The extent to which this disintegration proceeds will doubtless
depend on the quantity and variety of proteins consumed.

If the reduction of the protein molecule to this fragmentary condition is
the outcome of protein digestion, as recent investigations indicate, then the
problem of absorption is transferred to these fragmentary bodies rather than
to the peptone molecule. Inasmuch as the presence of the peptids and the
amino-acids in the blood of the portal vein has not been demonstrated
beyond question, the supposition is that after their absorption by the intes-
tinal epithelium, they are synthesized and a protein molecule constructed,
similar to, if not identical with, the plasma-albumin. This view renders
it much easier to understand how out of the different proteins, vary-
ing widely in their composition, the specific proteins of the blood are con-
structed. It is only necessary to assume that the epithelial cell selects from
the variety of fragments presented to it, only those which are necessary to
the formation of the plasma-albumin and the plasma-globulin, and to syn-
thesize them to these characteristic compounds.

The plasma-albumin thus becomes the common protein out of which
each tissue constructs the particular kind of protein characteristic of it,
thus bringing about repair and growth. Whether this is accomplished by
the simple incorporation of the protein molecule directly or whether it must
be first reduced to amino-acids before the tissues can construct their own
protein is unknown. That the plasma-albumin bears an intimate relation
to the nutritive activities of the tissues is apparent from the decline in the
general nutrition and a marked loss of body weight, when in consequence of
diseases of the kidney it escapes in the urine. An alternate assumption
however is conceivable, viz., that the amino-acids, though not readily demon-
strable in the blood of the portal vein, are nevertheless absorbed as such, pass
through' the liver, enter the blood of the general circulation, and are carried
direct to the tissue-cells in which they are directly synthesized into the form
of protein characteristic of them. The plasma-albumin might then be
regarded as a protein surplus to be called upon if the protein ingested
should be insufficient.

Many facts in the physiologic chemistry of the body raise the question
as to what percentage of the amino-acids is utilized for tissue repair and
growth and what percentage for heat production. If the protein require-
ments of Chittenden, viz., 58 to 60 grams only, are necessary for repair and
growth, then approximately one-half the amino-acids ordinarily produced
are used for heat production. The manner of disposal of these unused

ABSORPTION. 219

(that is unused for tissue repair and growth) fragments of protein
disintegration is doubtless varied; a large portion is undoubtedly ab-
sorbed by the epithelial cells of the villi and mucosa after which they
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 sugar or fat and subsequently utilized by the
organism for heat production. The dynamic portion of the amino-acid is
this deaminized remainder. Another portion is 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.

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
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
presents 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 ad-
dition of fat.

220 TEXT-BOOK OF PHYSIOLOGY.

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. 94).

The water, inorganic salts, proteins, and sugar after entering the blood-
vessels of the villus are carried by the blood of the intestinal veins directly
into the liver by the portal vein; after circulating through the capillaries of
the liver and being influenced by the liver cells, they are discharged by the
hepatic veins into the inferior or ascending vena cava.

The fats after entering the lymph-radicle of the villus are carried by the
lymph-stream of the intestinal lymph-vessels and emptied into the recep-
taculum 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
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

ABSORPTION.

221

FIG. 94.— DIAGRAM SHOWING THE ROUTES BY WHICH THE ABSORBED FOODS REACH
THE BLOOD OF THE GENERAL CIRCULATION (G. Bachman). 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, in}, v. c., the
inferior vena cava; int. I. v., the intestinal lymph vessels converging to discharge their
contents, chyle, into rec. c. the receptaculum chyli, the lower expanded part of the thoracic
duct; th. <2.,'the thoracic duct discharging lymph and chyle into the blood at the junction
of the internal jugular and subclavian veins; sit p. v. c., the superior vena cava.

222 TEXT-BOOK OF PHYSIOLOGY.

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 interpenetration of
the molecules takes place, which continues until a condition of homogeneity is
established.

Again, when a soluble substance, inorganic or organic, is placed in water,
the molecules of the substance will at once begin to separate themselves and
to diffuse throughout the water until the solution becomes homogeneous, and
notwithstanding the fact that the dissolved substance possesses weight, the
solution remains homogeneous. The force of gravity is overcome by the force
of diffusion.

The velocity with which the molecules of a substance will diffuse through
a solvent like water, varies considerably. The experiments of Graham show
that if the molecules of a given weight of hydrochloric acid diffuse completely
in a unit of time, the molecules 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

ABSORPTION. 223

will continue to pass through the membrane until the water on both sides contain
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.

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. 95, 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 necessitated when it is desired
to obtain the actual pressure exerted by any given substance in solution. An
apparatus of this character is termed an osmometer.

When, therefore, the osmometer containing a solution of cane-sugar is placed
in the vessel (£) containing water, the following phenomena occur, viz.: an ascent
of the cane-sugar solution in the vertical glass tube, and a descent of the level
of the water in the vessel b. These phenomena continue until the level of the
fluid in the glass tube reaches a certain height, when it becomes stationary, and
no further effect takes place.

In explanation of the foregoing phenomena it may be said that the molecules

224

TEXT-BOOK OF PHYSIOLOGY.

of the sugar strike or press against the outer layer of the molecules of the solvent,
which at all points are in contact with the rigid walls of the earthenware vessel,
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
the glass tube continues to rise and the level of the fluid in
the vessel, b, continues to fall until the force of gravity
prevents any further 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 ex-
pressed 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 13° C. to 16° C., as determined by this method,
exerts an osmotic pressure of about 535 mm. Hg.; a 2
per cent, solution exerts an osmotic pressure approxi-
mately twice this amount.

Experiments made with this and similar osmometers
show —

1. That the osmotic pressure of any substance in solu-

tion is proportional to the concentration, providing
the temperature is constant.

2. That when the concentration is constant the osmotic
pressure rises with, and is proportional to, the tempera-
ture.

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.
.. 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;
weights in grams per liter of different substances
exert the same osmotic pressure at the same temperature.
Because of the fact that when certain substances, e.g., many inorganic salts,
many acids and bases, are dissolved, some of their molecules undergo ionization,
i.e., separation into parts which are charged with electricity, and hence the two
together, molecules and ions, exert a greater osmotic pressure than would 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-

Solution

°f
Cane Sugar

hence the molecular

ABSORPTION. 225

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., 1.87° 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 thermom-
eter 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 0.83° C. lower than water, its
osmotic pressure would be 0.83-^-1.87X22.38 atmospheres or 9.847 atmospheres
= 7,483 mm. Hg. If any two solutions have the same freezing point they contain
the same number of molecules and hence have the same osmotic pressure. Blood
plasma has a freezing point of 0.56° 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-
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 (C12 H22 On) contains 342 grams; a i per cent, solution contains 10
grams to the liter; hence its osmotic pressure is io-=-342X 22.38 atmospheres or
0.65 atmosphere which is equal to 494 mm. of Hg.

Filtration. — Filtration may be defined as the passage of water, and of all 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-
15

226 TEXT-BOOK OF PHYSIOLOGY.

stitutes the force of filtration, 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; (6) through the sarcolemma into 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 is a highly complex nutritive fluid, the presence and proper
circulation of which in the living organism are essential to the maintenance
and activity of all physiologic 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 of time. The
immediate dependence of the functional activities of the tissues and organs
on the presence of the blood can be demonstrated by the following experi-
ment: If the nozzle of a syringe, adapted to the size of the animal, be intro-
duced through the jugular vein into the right side of the heart and the blood
be suddenly withdrawn, 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
functional activity. 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.

Constituents of Blood. — A microscopic examination of the blood as it
flows through the capillary vessels of the web of the frog or the mesentery of
the rabbit shows that it is not a homogeneous fluid, but that it consists of two
distinct portions: viz., (i) a clear, transparent, slightly yellow fluid, the
plasma or liquor sanguinis; (2) small 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 cor-
puscle, colorless in appearance and smaller in size than the ordinary white
corpuscle, is present in the blood-stream 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

227

228 TEXT-BOOK OF PHYSIOLOGY.

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 corpuscles 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 centri-
fuge 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, and the venous, the color
of which is a dark bluish-red or purple. The cause of the color as well as
the difference in color is the presence in the red corpuscles of a coloring-
matter, hemoglobin, in different degrees of combination with oxygen. The
intensity of the color in either kind of blood is dependent on the thickness of
the blood-stream, for in the finest capillaries, as seen under the microscope,
there is an almost total absence of color. As the arterial blood passes into
and through the systemic capillaries, the hemoglobin yields up a portion of
its oxygen to the tissues and changes in color, though the change is not
appreciable by the eye. On passing into the veins, however, the blood-
stream soon presents its characteristic dark bluish-red color, which deepens
as it approaches the lungs. On passing into and through the capillary
vessels of the lungs the hemoglobin absorbs a new volume of oxygen, changes
in color, and on emerging from the lungs the blood presents its characteristic
scarlet color.

2. Opacity. — Owing to the fact that the corpuscles have a 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 fatty 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,
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

THE BLOOD. 229

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) is nrst prepared in such propor-
tions that the mixture has a specific gravity of about 1.040. 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, Na2 HPO4, and sodium carbonate, Na2 CO3. 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
ions in equal degree and therefore are neither acid nor alkaline but neutral
in reaction.

6. Temperature. — The temperature varies from 36.78° C. (98.2° F.) in
the superior vena cava to 39.7° C. (103.4° F.) in the hepatic vein, the mean
being about 38° C. (100° F.).

23o TEXT-BOOK OF PHYSIOLOGY.

7. Viscosity. — Viscosity may be defined 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 and comparing 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
diminution 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 a capillary tube through which the blood is caused
to flow under the influence of a constant positive or negative pressure. The
distance the blood flows in a unit of time, compared with that of water,
represents the degree of viscosity. Among these apparatus 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

THE BLOOD.

231

coagulum becoming dislodged. If a portion of such a jelly-like mass be
examined microscopically, it will be found to be penetrated in all directions
by a felt- work of extremely fine delicate fibrils, which, having made their
appearance before the corpuscles 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. 96).

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-
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

FIG. 96. — DIAGRAM TO ILLUSTRATE THE PROCESS or COAGULATION, i. Fresh blood, plasma
and corpuscles. 2. Coagulating blood (birth of fibrin). 3. Coaglulated blood (clot and serum). —
(Waller.)

time the clear fluid increases in amount and accumulates along the sides
and bottom of the vessel. The shrinkage in the volume of the red coagulum
and the increase of the volume of the clear fluid which is expressed from its
meshes continue for a period varying from ten to fifteen hours, according
to certain external conditions. The blood has now become separated into
two distinct portions: viz., a solid contracted red mass, termed clot, and a
clear fluid, termed serum. The clot consists of the fibrin containing in its
meshes the red and white corpuscles; the serum consists of all the constitu-
ents of the plasma except the antecedents of the fibrin. The stages of coagu-
lation are shown in Fig. 96.

If the blood coagulates slowly the red corpuscles, owing to their greater
specific gravity, subside to the bottom of thebloodmass, 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 bu/y coat. In certain inflammatory conditions the
coagulating power of the blood is much diminished, and the corpuscles,
having time to subside, a well-developed buffy coat is formed which at one
time had much interest for pathologists. As the contraction of the fibrin
takes place more actively in the center, there being here less resistance than
at the sides of the coagulum, the upper surface usually becomes depressed
or cupped.

Coagulation of Plasma. — Clear plasma may be obtained by means of
the centrifuge from blood to which sufficient magnesium sulphate has been
added to prevent coagulation, or from horse's blood which has been allowed
to flow into a tall vessel surrounded by a cooling mixture so as to prevent
coagulation and thus permit the red corpuscles to subside. If such plasma

232 TEXT-BOOK OF PHYSIOLOGY.

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 commercialpeptone. (5) The mouth secretion of the leech.

Coagulation is hastened by the following agents, viz.: (i) a temperature
gradually increasing from 38° C. to 50° 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 18).
The remaining fluid, similar in its physical appearance to the blood, is
termed defibrinated blood. When such blood is allowed to remain at rest for
a few days, the remaining red corpuscles gradually sink to the bottom of the
fluid, above which will be found the clear serum.

COMPOSITION OF PLASMA.AND SERUM.

Plasma. — The plasma obtained by any of the methods previously
described is a clear, colorless, transparent, slightly viscid fluid, with a specific
gravity of 1.026 to 1.029. It is composed largely of water holding in solution
proteins, sugar, fatty matter, inorganic salts, urea, cholesterin, lecithin, etc.
In composition it is quite complex, containing as it does not only the nutritive
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:

THE BLOOD. 233

COMPOSITION OF PLASMA.

Water 90 .00

f Serum-albumin 4-5°

Proteins . . . \ Paraglobulin 3 .40

[ Fibrinogen o . 30

Fatty matters . 0.25

Sugar o . 10

Extractives o . 60

Inorganic salts 0-85

100.00

Serum-albumin. — Of the protein constituents of the blood, serum-
albumin is the most abundant, existing to the extent of from 4 to 5 per cent.
From its similarity to egg-albumin it is regarded as holding an important
position as a nutritive agent, for it is out of this common protein that in all
probability each individual tissue elaborates the special protein characteristic
of it, since during starvation the albumin steadily diminishes in amount. As
it passes through the walls of the capillary vessels it is found in the lymph,
pericardial fluid, and similar secretions in various parts of the body, as well
as in various pathologic transudates. It is also present in serum. While
circulating in the lymph-spaces the serum-albumin is utilized in replacing
the proteins which have undergone disintegration during tissue metabolism.
Its supply in the blood is maintained by the absorption of peptones or simpler
products of protein digestion, e.g., amino-acids, which are formed from the
proteins of the food and which during the time of absorption are changed in
some unknown way into serum-albumin. It is readily obtained from plasma
or serum by saturating either of these fluids with magnesium sulphate, when
all the 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.

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
per cent, 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 parts 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,

234 TEXT-BOOK OF PHYSIOLOGY.

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 as well as the serum 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 fatty matter the amount
may be so great as to give to the blood a milky or opalescent appearance.
Within a few hours, however, this excess of fat disappears from the blood,
though its immediate disposition is unknown. Soaps or alkaline salts of the
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. — Sugar is present in the blood in the form of dextrose, and is
now regarded as a normal constituent. The amount is about i part per
thousand, though it may be present to the extent of 15 parts per thousand.
Beyond this, the excess soon appears in the urine.

Extractives. — The blood contains a series of nitrogenized bodies, such
as urea, uric acid, creatin, creatinin, xanthin, etc., which result from the
katabolic changes in nerve- and muscle-tissues as well as from subsequent
chemic combinations and decompositions. Though constantly absorbed
from the tissues, they seldom accumulate beyond a small amount, since
they are constantly being eliminated from the blood by the various ex-
cretory organs.

Inorganic Salts. — The inorganic salts of the plasma are chiefly sodium
and potassium chlorids, sulphates, and phosphates, together with calcium
and magnesium phosphates. Of the salts, sodium chlorid is the most
abundant, amounting to 0.56 parts per hundred. Calcium phosphate is
present in small quantity — 2 parts per 1000. 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.
97.) 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 defibrinating the blood, it has been
supposed to be dependent on the formation of some adhesive substance con-
nected with the formation of fibrin.

THE BLOOD.

235

Color. — When viewed by transmitted light, a single corpuscle is slightly
yellow or greenish in color; but when a number are grouped together, the
color deepens and the corpuscles appear red. In either case the color is due
to the presence in the corpuscle of a specific coloring-matter, hemoglobin.

Shape. — The red corpuscle is a
circular, flattened disk with rounded
edges. Each surface is perfectly
smooth and presents a shallow de-
pression in its center, so that it is
also biconcave. A longitudinal sec-
tion of a corpuscle would present,
when viewed edgewise, an outline
similar to that of Fig. 98. This
difference in the thickness of the
peripheral and central portions of
the corpuscle gives rise to differences
in optical appearances when ex-
amined microscopically. At a certain
distance of the object-glass the cor-
puscle presents in its peripheral por-

FIG. 97. — CORPUSCLES FROM HUMAN
SUBJECT. A few colorless corpuscles are
seen among the colored discs, many of
which are arranged in rouleaux. — (Funke.)

FIG. 98. — IDEAL TRANSVERSE
SECTION OF A HUMAN RED CORPUS-
CLE. (Magnified 5000 times.) a, b.
Diameter, c, d. Thickness. -

tion a bright rim, and in its central
portion a dark spot If the objective
be brought nearer and the center
accurately focused, the reverse ap-
pearance obtains; the central portion becomes bright and the peripheral
portion dark. The cause of this difference in optical appearance is the
unequal distribution of the transmitted light in consequence of the shape

c 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
assumed as the average diameter, there
is a small percentage of distinctly smaller
and a small percentage of distinctly larger
corpuscles. The following table shows the results of measurement made
by different observers:

Normal Limits. Average Diameter.

Welcker diameter o .0045-0 .0095 mm o .0070 mm.

Hayem ; . . . .diameter 0.0060-0.0088 mm 0.0075 mm.

Gram diameter 0.0067-0.0093 mm 0.0078 mm.

Melassez o .0076 mm.

0.00747 (goVo 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.

236 TEXT-BOOK OF PHYSIOLOGY.

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 cor-
puscle is not circular and bi-
concave in shape, in the cir-
culating blood, but bell-
shaped, similar to that shown
in Fig. 99. It was not until
1902, after the publication of
Weidenreich's investigations
that this view began to receive
more attention than had
hitherto been accorded it.
c. d. Weidenreich preserved in a

FIG. 99.— THE SHAPE OF THE RED CORPUSCLE moist chamber a hanging drop
IN DIFFERENT MAMMALS. (Weidenreich.) a. Man. of human blood, and on CX-
b. Dog. c. Pig. d. Rabbit. . ,. r

animation 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 pre-
sent when a drop of blood is examined microscopically he attributes to cool-
ing, evaporation and concentration of the drawn blood. Experimentally
it was shown that when blood was added to 0.6 to 0.65 per cent, solution
of sodium chlorid all the corpuscles were bell shaped; but if the solution
was increased or decreased in strength, this form was at once changed.

The dimensions of the bell-shaped cell according to Weidenreich are as
follows: —

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 disks of the conventional
form. He also observed that the corpuscles in the capillary blood-vessels
of the omentum of the guinea-pig were bell-shaped and presenting an
appearance similar to that shown in Fig. 100. 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

THE BLOOD.

237

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 hemoglo-
bin. 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,
has been accomplished for a cubic milli-
meter of blood by various observers em-
ploying different methods with compara-
tively 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:

FIG. roc.— RED CORPUSCLES
SKETCHED WHILE CIRCULATING IN
THE VESSELS OF THE OMENTUM or A
GUINEA-PIG. (F. T. Lewis in Stohr's
Histology.}

Place

Height Above Sea-level

Red Cells

Author

Christiania

o meter

4,974,000

Laache.

Gottingen'

148 meters

5,225,000

Schaper.

Tubingen

314 meters

5,322,000

Reinert.

Zurich

414 meters

5,752,000

Steirlin.

Auerbach

425 meters

5,748,000

Koppe.

Reibaldsgriin

700 meters

5,900,000

„ rr

Koppe.

Arosa

1800 meters

7,000,000

Egger.

The Cordilleras

4392 meters

8,000,000

Viault.

(Koppe.)

This increase in the number of corpuscles takes place, according to
Viault's observations, within two or three weeks, and is apparently not con-

238 TEXT-BOOK OF PHYSIOLOGY.

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
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 melang-
eur of Potain and the counting chamber of Thoma both these objects are
attained.

The pipette consists of a capillary tube (Fig. 101) 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 signify that

FIG. 101. — HEMOCYTOMETER. a, Surface; b, section view; c, squares on the surface of B magnified-
M, G, S, mouth piece, rubber tube and pipette.

if the tube be filled with blood up to the mark i and the diluting fluid be
sucked into the tube up to the mark 101, the blood will be diluted 100 times.
If the blood be sucked up to the mark 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
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
1 Various solutions have been devised for diluting blood, any one of which may be employed, e.g.:
Hayem'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, o .025 part.

Gower's Fluid:

Sodii sulphat ................................................. gr.

Acid, acetic ................................................... 5 j

Aquae dest .............................................. q. s. ad 5i

104

THE BLOOD.

239

This group is separated from

sucked into the tube up to the mark lo'i . On shaking the pipette for a few minutes,
the admixture will take place, aided by the movements of the glass ball.

Fig. 101 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. 102.
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 ToVo 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. FIG. 102.— MICROSCOPIC AP-
The number of corpuscles in at least five series PEARANCE OF THE SMALL SQUARES
of sixteen small squares is then counted; this AND THE ^DISTRIBUTION OF THE
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) '•> tne product is then divided by the number of squares
counted (So in the instance given above) : e.g., five series of sixteen small squares
contain 500 corpuscles

§0^x^X4000 ^ 5)000j00o 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-
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

24o TEXT-BOOK OF PHYSIOLOGY.

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* hyper tonic, 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 G owers 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. With
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 dis-
charged from the corpuscle by these agents is unknown. The disintegra-
tion of the corpuscle and the diffusion of the hemoglobin into and its solu-
tion by the surrounding medium is termed hemolysis and the agents by
which it is produced are termed hemolytic agents.

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 disks. In the animals excepted
the corpuscles are oval. The size, however, varies in different animals from
0.0092 mm. (YTTF incn) m tne elephant to 0.0023 mm. (TYT2T mcn) in the
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-

THE BLOOD.

241

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 of dried blood which have been altered by the
action of various external agents, even though capable of a certain degree
of restoration, is most difficult, 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) :

Gulliver

Inch Mm

Wormley

Inch | Mm.

C. Schmidt
Mallinin

French Medi-
colegal Soc.
Welcker

Formad

Inch Mm. Inch Mm. Inch Mm.

Man

Guinea Pig

Dog

Rabbit

Ox

Pig

Horse

Cat

Sheep

Goat

,3200 o
• 3538.0

,3607 o.
.4267 o.
,4230 o.
,4600 o.
.4406 o.
.5300 o.
.6366 o.

0079

0071
0071
0070
0060
0060
0057
0058
0048
0040

.32500

.3223 o
.3561 o
.3653 o
.4219 o
.4268 o
.4243 o

.4372 O

.4912 o
.6189 o

,0078
0079
,0071
.0070
,0060
.0059
0059
,0058
,0031
0041

.3300
.3300*
.3636
.3968

• 4354
.4098
.4464

• 4545
.5649
.6369

0.0077

0.0077 I

o .0070

0.0064

0.0058

0.0062

0.0057

0.0056

0.0045

o . 0040

•3257 O

.3213-1-0

.3485 o

.3653 o

•4545 o

.4098 jo

•4545 b

.3922 o

.5076 jo

•5525 JO

.0078;

.0079!
.0073!
,0069
.0056
.0062
.0056]
.0065 .
.0059 i
.0046 i

3200,0

3400 O,

.35800
3662 o,
,4200 o ,
4250:0,
431°!°'

0079
0075
0071
0069
0060
0060
0059

500050.0051
6100 o. 0042

In birds, reptiles, and amphibians the corpuscles are larger than in
mammals, are oval in shape, and nucleated. (See Figs. 103 and 104.)
As the scale of animal life is descended the corpuscles increase in size, until
in Proteus and Amphiuma the long diameter at-
tains an average length of 0.058 mm. and 0.077 mm.
respectively. In fish the corpuscles are smaller,
oval, and nucleated, with the exception of the
lamprey eels, in which they are circular, biconcave,
and nucleated, though the nucleus is generally 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, not-
withstanding the similarity of shape, be readily dis-
tinguished from them.

The Function of the Red Corpuscles.— The
red corpuscles, by virtue of the capacity of their
contained hemoglobin for oxygen absorption, maybe
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 t he
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

FIG. 103. FIG. 104.
AMPHIBIAN COLORED
BLOOD-CORPUSCLES. Fig.
103, on the flat; Fig. 104,
on edge. — (Landois and
Stirling.)

* Masson.
16

f Woodward.

242 TEXT-BOOK OF PHYSIOLOGY.

respiration is active, the quantity of oxygen absorbed is large, and the energy
evolved through oxidation great. In those animals, on the contrary, in
which the corpuscles are large and relatively few in number, the reverse
conditions obtain. This is in accordance with the fact that the superficial
area of any given volume of 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.

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-
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.

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 menstral 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

1 For an admirable re'sume' 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.

THE BLOOD.

243

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. At
first they are large, homogeneous, colorless, perhaps slightly tinged with
hemoglobin and distinctly nucleated. They increase in number by kary-
okinesis 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 perinuclear cytoplasm, after which the
remainder of the corpuscle assumes 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 erythro-
blasts and normoblasts make their appearance in the blood-stream before
the extrusion of the nucleus has taken place.

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

FIG. 105. — CRYSTALLIZED H E M o
GLOBIN. a, b. Crystals from venous
blood of man. c. From blood of cat.
d. Of guinea-pig, e. Of. marmot. /.
Of squirrel. — (Gautier}.

244 TEXT-BOOK OF PHYSIOLOGY.

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. 105). 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, a-11 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,
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 in a practically pure form, and when subjected
to a temperature of 100° C. its water of crystallization is driven off, after
which it can be analyzed. In the subjoined table the results of several
analyses are given for 100 parts of hemoglobin.

Dog

Horse

Dog

Guinea-pig

C.

'
53-Qi

22 .62
6.62
15.98
0-54

o-33
Jaquet.

5I-I5
23-43
6.76
17.94
o-39
o-33

Zinoffsky.

53-85
21.84

7-32
16.17

o-39
o-43

54.12
20.68
7-36
16.78
0.58
0.48

o

H. . . .

N.. .

S

Fe

Hoppe-Seyler.

The elementary composition of hemoglobin is thus seen to vary slightly
in different animals, suggesting that there may be different kinds of hemo-
globin. The rational molecular formula is not known. On the assumption
that each molecule contains one atom of iron, Preyer suggested the following
empirical formula: C600H960Nl54O179S3Fe, with a molecular weight of
i37332; Jaquet has suggested a different formula: C758H1203N19?O218S3Fe,
with a molecular weight of 16,669. It is very evident from this that the
molecule is of enormous size and exceedingly complex.

Quantity of Hemoglobin. — The quantity of hemoglobin in blood as
determined by chemic, chromometric, and spectro-photometric methods
amounts to about 14 per cent, in man and 13 per cent, in woman. Of the
chemic methods, that based on the amount of iron is the one generally
employed. Chemic analysis has shown that hemoglobin contains 0.42 per
cent, and blood 0.056 per cent, of iron; with these two factors the quantity
of hemoglobin can be determined by the following formula: x= 0°*°'°S6 =
T3-33 Per cent. The total quantity of hemoglobin in the blood, assuming

THE BLOOD. 245

the latter to be about 3684 grams (one-nineteenth of the body-weight, 70
kilos) will therefore amount to 491 grams; e.g., # = 3684i*oI3'3 = 49i. The
total amount of iron in the blood is obtained by the following formula:
viz., * = 3684*°-°s6=2.o6 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
number of red corpuscles, 5,000,000 per cubic millimeter, is also considered
as 100 per cent, tinder 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 T8o°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 |-J- = o. 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 water and then
finding an identical color which represents a previously determined quantity
of hemoglobin (v. Fleischl).

246

TEXT-BOOK OF PHYSIOLOGY.

Cowers' hemoglobinometer consists of two glass tubes of exactly the
same size and similar to those shown in Fig. 106. Oney 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

FIG. 106. — HALDANE'S MODIFICATION OF GOWERS' APPARATUS.

more water must be added, it is in excess; if less, it is diminished. If, for
example, the 20 cubic millimeters of blood from an anemic patient gave the
standard tint at 60 divisions, the blood contained but 60 per cent, of the
normal amount of hemoglobin.

Haldane's Modification of Gowers* Method. — Haldane's hemoglobin-
ometer, Fig. 106, is a modification of that of Gowers. The tube A contains
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
wound in the ringer 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

THE BLOOD.

247

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.

, FIG. 107. — 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-Paris.

Von Fleischl's hemometer consists of a metallic cell divided into two
compartments, a and a', by a vertical partition (Fig. 107). 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. 108). 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
accomplished by lamp-light reflected
from the white reflecting surface
beneath. The depth of color of the
glass opposite 100 on the scale corresponds to that of normal blood. If,
therefore, the colors are identical at 75 divisions, the blood contains but 75
per cent, of 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

FIG. 108.— TINTED GLASS WEDGE or THE
VON FLEISCHL HEMOMETER.

248

TEXT-BOOK OF PHYSIOLOGY.

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

FIG. 109. — 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. — (Landosi and Stirling.}

Yellow.

Green.

Cyan Blue.

Oxy

hemoglobin
0.8 %.

FIG. 1 10. — SPECTRA OF HEMOGLOBIN AND SOME or ITS COMPOUNDS.
— (Landois and Stirling.')

spectroscope in common use is that shown in Fig. 109. 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

THE BLOOD.

249

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. no. 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.

Dilute solutions of arterial blood show absorption bands between the
Fraunhofer lines, D and E, in the green and yellow portion of the spectrum.
(See Fig. 1 10.) The band nearest D, frequently designated as alpha, is dark
in the center and sharply defined. The band which lies toward E, desig-
nated as beta, is broader and less sharply defined.

aBC D

FIG. in. — GRAPHIC REPRESENTA-
TION OF THE ABSORPTION OF LIGHT IN
A SPECTRUM BY SOLUTIONS OF OXY-

HEMOGLOBIN OF DIFFERENT STRENGTHS.

The shading indicates the amount of
absorption of the spectrum, and the
numbers at the side the strength of the
solution. — (Rollet.)

aBC D

G h

FIG. 112. — GRAPHIC REPRESENTATION
OF THE ABSORPTION OF LIGHT IN A SPEC-
TRUM BY SOLUTIONS OF REDUCED HEMO-
GLOBIN OF DIFFERENT STRENGTHS. The
thading indicates the amount of absorp-
sion of the spectrum, and che numbers at
the side rhe strength of the solution. —
(Rollet.)

As the amount of light absorbed varies with the concentration of the
solution as well as its thickness, and gives rise to absorption bands of different
widths and intensities, it becomes necessary, in order to obtain the 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. in. 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

250 TEXT-BOOK OF PHYSIOLOGY.

absorption band (see Fig. no), frequently designated as gamma, broader
and less marked between the lines D and E, but extending slightly beyond D.
Fig. 112 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.

Compounds of Hemoglobin. — The coloring-matter of the blood is
characterized by the property of combining with and of again yielding up
oxygen. The union is a chemic one, taking place under certain pressure
conditions. It therefore may exist in two states of oxidation, distinguished
by a difference in color and their absorption spectra. If hemoglobin either
in blood or in solution be shaken with air, it at once combines with oxygen
and is converted into oxyhemoglobin, which imparts to the blood or solution
a bright red or scarlet color. If the blood or solution be now deprived of
oxygen, the oxyhemoglobin is converted into reduced hemoglobin, which
imparts to the blood or solution a dark bluish or purple color.

The quantity of oxygen absorbed by i gram of hemoglobin is estimated
at 1.56 c.c. measured at o° C. and 760 mm. of mercury. The compound
formed by the union of oxygen and hemoglobin is a very feeble one; for
when the pressure is lowered the union becomes less stable, and as the zero
point is approached, as in the Torricellian vacuum, a rapid dissociation of
the oxygen takes place. This, 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

THE BLOOD. 251

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. ExarAined 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. no.) A useful test for CO blood
is the addition of caustic soda, which produces a cinnabar red precipitate.

Methemoglobin. — This is a pigment, closely related to oxyhemoglobin,
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 vasated 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,

252 TEXT-BOOK OF PHYSIOLOGY.

containing a proteid resembling cell-globulin, lecithin, cholesterin, and
inorganic salts, among which potassium phosphate is especially abundant.

HISTOLOGY OF THE WHITE CORPUSCLES OR LEUKOCYTES.

The presence of white corpuscles in the blood can be readily observed
under the same conditions as the red corpuscles are observed. Thus when
the mesentery of the frog or the guinea-pig is examined with the microscope
the white corpuscles are seen adhering to the walls of the blood-vessels; in
a drop of freshly drawn blood they are found in the spaces between red
corpuscles (Fig. 97.) 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 -grVo- mc^'

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 its
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
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
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

THE BLOOD.

253

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 86 1 per
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-

FIG. 113. — 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.)

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 corpuscles. 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 dimmish the percentage of error.

Physiologic Properties. — The white corpuscles and especially the
leukocytes possess the characteristic property of exhibiting movements
similar to those observed in the amoeba, and are therefore termed amoeboid.
These movements consist in alternate protrusions and retractions of portions
of the cell body, as a result of which they exhibit a great variety of forms.

254

TEXT-BOOK OF PHYSIOLOGY.

(See Fig. 113.) 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 \irtue 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 of the capillary vessel, after which the

remainder of the corpuscle follows (Fig. 114).
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 pas-
sage of the white blood-corpuscles through the
capillary wall the term diapedesis is given. The
movements of the white corpuscles are increased by
a rise in temperature up to 40° C., beyond which
they cease, owing to the coagulation of the cell-
substance. A low temperature also arrests the
movements. Induced electric currents also cause
contraction and death of the cell. Moisture and
oxygen are necessary to their activity. From their
similarity to lower organisms the white corpuscles
may be regarded as independent organisms living
in the animal fluids, just as the amoeba lives in its
natural liquid medium.

Varieties of Leukocytes. — A detailed study of
the blood with the aid of the triacid staining fluid
of Ehrlich or any of the various eosin and methy-
lene-blue stains, reveals the presence of five distinct
varieties of leukocytes and transitional forms which
may be classified as follows:

i. Small lymphocytes, so called from their resem-
blance 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

FIG. 114. — SMALL VES-
SEL SHOWING VARIOUS
STAGES IN THE DIAPEDESIS
OF LEUKOCYTES. (G. Bach'
man.)

(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 methylene-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 n the nuclear structure is
obviously separated into four parts; in 9 it is moderately, and in 10 markedly, poly-
morphous.

12, 13. Eosinophiles.

The nuclei are not unlike those of the polynuclear neutrophile, except that they are
somewhat less convoluted, and poorer in chromatin, staining less intensely. The
protoplasm is filled with coarse eosinophile granules, the characteristics of which
are clearly illustrated by 13, a "fractured" eosinophile.

14. Eosinophilic Myelocyte.
Compare with 15.

15, 1 6. Myelocytes. (Neutrophili-c.)

These cells are morphologically similar to 14, except that they contain neutrophile
instead of eosinophile granules. Note that the granules of the myelocyte are identical
with those of the polynuclear neutrophile. A dwarf form of myelocyte is repre-
sented by 1 6.

{Eosin and Methylene-bhie.)

17, 1 8. 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 can-
not be detected, so that they must be classed as large lymphocytes.

23. Transitional Form.

The distinction between this cell and 24 is not marked; the nucleus of the latter
simply being somewhat more basic and convoluted.

24, 25, 26, 27. Polynuclear Neutrophiles.

With this stain these cells show a feebly acid protoplasm, and lack granules. Note
that the more twisted the nucleus the deeper it is stained. Compare with 9, 10
and ii.
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 5-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
illustration. 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 chro-
matin of the mast cell is so delicate and so feebly stained that it is barely visible.
These cells were found in the blood of a case of splenomedullary leukemia.

PLATE I.

O

THE LEUKOCYTES.
(2-16, Triacid Stain; 17-36, Eosin and Methylene-blue.)

(E. F. FABER, jec.}
(From DaCoslas " Climcal H ematology .")

THE BLOOD. 255

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 cystoplasm
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-
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.

256 TEXT-BOOK OF PHYSIOLOGY.

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. The cells engaged in this process are the polymorphonuclear
leukocytes and the large and the small lymphocytes. He regards them as
the general scavengers of the body. It has been suggested that they are
also engaged in the absorption of fat from the lymphoid tissue of the intestine.
In their dissolution they 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 by 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-
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 defined as a colorless, grayish-white, homo-
geneous or finely granular protoplasmic disk, 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

THE BLOOD. 257

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
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 1:13; in an infant, 1:19; in a dog, 1:13;
in a cat, 1:21.

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 1:19. 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, J;
other organs, J.
17

258 TEXT-BOOK OF PHYSIOLOGY.

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:

Cells..

Plasma.

328 {

[ Solids

. 128 \

Hemoglobin

. 116

Other organic matter

10

f Water

\

60A

Salts

2

604

672
t Solids

.. 68'

Fibrin

7

Albumin

• 53

I

Fat

Other organic matter

Potassium and sodium salts
Calcium and magnesium salts. . . .

4
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 the
fibrin, the antecedents of this substance, the physical and chemic conditions
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 insoluole fibrin is the presence and activity, under the circumstances, of
an agen^ 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
tiie 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-

THE BLOOD. 259

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 union with prothrombin of an agent termed
anti-thrombin. So long as this combination 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,
and converts it into thrombin, after which it combines with the fibrinogen.
For this reason the plastic agent has been termed thrombo-plastin.

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; (2) the presence of an anti-thrombin. On either
assumption the reaction between prothrombin 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

26o TEXT-BOOK OF PHYSIOLOGY.

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. 115.)

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 nbro-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

261

262

TEXT-BOOK OF PHYSIOLOGY.

3

cavities, the right auricle and the right ventricle constitute the venous heart;
the left auricle and the left ventricle, the arterial heart.

The right auricle is quadrangular in shape and presents on its posterior

aspect two large openings, the terminations
of the two final trunks of the venous system,
the superior and inferior vena cava (Fig. 116).
Below the auricle communicates with the ven-
tricle by a large opening which, from its posi-
tion, is termed the auriculo-ventricular open-
ing. The walls of the auricle are extremely
thin, not measuring more than two millimeters
in thickness.

The right ventricle, as shown on cross-
section, is crescentic in shape owing to the
projection of the ventricular septum. It pre-
sents at its upper left angle a cone-shaped
prolongation, the conus arteriosus. From this
prolongation, and continuous with it, arises
the pulmonary artery. The wall of the ven-
tricle measures in the middle about four milli-
meters in thickness. The inner surfaces of
the ventricle show: (i) a complicated system
of muscle ridges and bands, the columns car-
nece (fleshy columns), and (2) a set of muscle
projections, the musculi papillares (papillary
muscles), which arise by a broad base from
the walls of the ventricle and project upward
toward the auriculo-ventricular opening.
From the apex of each papillary muscle there
are given off fine tendinous cords, the chorda
tendinea, which become attached above to the
under surface of the auriculo-ventricular valve.
The left auricle, similar in general shape to
the right, presents posteriorly four openings,
the terminations of the four final trunks of the
venous system of the lungs, the pulmonary
veins. Below is found the corresponding
auriculo-ventricular opening. The wall of the
auricle measures about 3 mm. in thickness.
The left ventricle (Fig. 117) is conic in shape
from above downward and oval or circular in
shape on cross-section. At its upper inner
angle it presents a circular orifice, the mar-
gins of which give attachment to the walls of
the aorta, the main arterial trunk of the sys-
temic circulation. The inner surfaces of the
ventricle show a similar though better developed system of columnae car-
neae, musculi papillares, chordae tendineae, etc. The wall of the left ven-
tricle measures about 11.5 mm. in thickness in the middle.

FIG. 115. — DIAGRAM OF THE
CIRCULATION, i. Heart. 2.
Lungs. 3. Head and upper ex-
tremities. 4. Spleen. 5. Intestines.
6. Kidney. 7. Lower extremities.
8. Liver.— (After Dalton.)

THE CIRCULATION OF THE BLOOD.

263

The Endocardium. — The cavities of both the right and left sides of the
heart are lined by a thin, firm connective-tissue membrane, closely adherent
to the muscle-tissue, termed the endocardium. It contains also elastic fibers
and smooth muscle-fibers. Its entire surface is covered with a layer of
polygonal endothelial cells. This membrane serves partially to resist undue

FIG. 116 FIG. 117.

FIG. 116. — INTERIOR OF RIGHT AURICLE AND VENTRICLE, EXPOSED BY THE REMOVAL OF
A PART OF THEIR WALLS, i, Superior vena cava; 2, inferior vena cava; 2', hepatic veins; 3, $',$".
inner wall of right auricle; 4, 4, cavity of right ventricle; 4', papillary muscle; 5, 5', 5", flaps of
tricuspid valve; 6, pulmonary artery in the wall of which a window has been cut; 7, on aorta
near the ductus arteriosus; 8, 9, aorta and its branches; 10, n, left auricle and ventricle. — (Allen
Thomson.}

FIG. 117. — LEFT AURICLE AND VENTRICLE, OPENED AND PART OF THEIR WALLS REMOVED
TO SHOW THEIR CAVITIES, i, Pulmonary vein cut short; i', cavity of left auricle; 3,3", thick
wall of left ventricle; 4, portion of the same with papillary muscle attached; 5, the other papillary
muscles; 5', wall of the ventricle; 6, 6', the segments of the mitral valve; 7, the figure in aorta is
placed over the semilunar valves; 7', aorta; 8, pulmonary artery; 10, branches of aorta. — (Allen
Thomson.)

distention of the heart during contraction and to prevent separation of the
muscle-fibers. The endocardium is continuous with the lining membrane
of the blood-vessels.

The inter- auricular septum is quite thin and composed of the two layers
of the endocardium, between which is a layer of muscle-fibers. It presents
at its lower portion an oval depression, the fossa ovalis.

The inter- ventricular septum is quite thick and well developed, and com-

264 TEXT-BOOK OF PHYSIOLOGY.

posed of the two layers of the endocardium enclosing the muscle-fibers. In
the upper and central portion of the septum, there is, however, a small region
which is thin owing to the absence of muscle-tissue and composed of endo-
cardium only. This region is known as the pars membranacea septi.

The Cardio-pulmonary Vessels. — Though the two sides of the heart
are separated from each other by the auriculo- ventricular septum, they are
anatomically and physiologically connected by the intermediation of the
pulmonary system of vessels: viz., the pulmonary artery, capillaries, and
veins (Fig. 115).

The pulmonary 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 pulmonary lobules.

The capillaries in this situation are extremely abundant and well developed.
They lie close to the inner surfaces of the air-cells. The blood is thus
brought into intimate relationship with the intra-pulmonary air, and the
exchange of gases, — the excretion of carbon dioxid and the absorption of
oxygen — for which the cardio-pulmonary vessels exist, is readily accom-
plished.

The pulmonary veins which return the blood to the heart are formed
by the convergence and union of the small veins which emerge from
the capillary system. The final trunks thus formed, the four pulmonary
veins — two from each lung — enter the posterior wall of the left auricle.

The Course of the Blood through the Heart. — There is thus estab-
lished a pathway between the venae cavse on the right side and the aorta on
the left side, by way of the right side of the heart, the cardio-pulmonary
vessels, and the left side of the heart.

The venous blood flowing toward the heart is emptied by the superior
and inferior venae cavae into the right auricle, from which it passes through
the auriculo-ventricular opening into the right ventricle (Fig. 115); thence
into and through the pulmonary artery and its branches to the pulmonary
capillaries, where it is arterialized by the exchange of gases — the giving up of
a portion of carbon dioxid to the lungs and the absorption of oxygen — and
changed in color from bluish-red to scarlet-red. The arterialized blood,
flowing toward the heart, is emptied by the pulmonary veins into the left
auricle, from which it passes through the auriculo-ventricular opening into
the left ventricle; thence into the aorta and its branches to the systemic
capillaries, where it is de-arterialized by a second but opposite exchange of
gases — the 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 from the pulmonary veins into the left
side in equal quantities and velocities.

Though there is but one set of capillaries, as a rule, between arteries and

THE CIRCULATION OF THE BLOOD.

265

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
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 pulmonary circulation, which includes the course of the blood from

the right side of the heart through the lungs to the left side of the

heart.

FIG. 118. — RIGHT CAVITIES OF THE FIG. 119.— RIGHT CAVITIES OF THE HF.ART.
HEART. — Auriculo-venlricular valve open, Auriculo-ventricular valve closed, semilunar
semi-lunar valves closed.— (.Da//0«.) valves open.— (Dalton.}

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-

266

TEXT-BOOK OF PHYSIOLOGY.

divided into three portions or cusps, which during the period of relaxation
are directed into the ventricle (Fig. 118); during the period of contraction
they are raised and placed in complete apposition, when they act as a valve
preventing a backward flow into the auricle (Fig. 119). 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 pulmonary 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 nbro-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.
1 1 8); during the contraction they
are directed into the artery (Fig.
119). In the former position they
are shut; in the latter, they are open.
The left auriculo-ventricular opening is provided with a similar though
better developed fibrous ring and membranous valve. It is, however,
subdivided into but two portions or cusps, and is therefore termed the
bicuspid valve, or, from its fancied resemblance to a bishop's mitre, the
mitral valve. The general arrangement, connections, and mode of action
of this valve are similar in all 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. 117), which in
their arrangement, connections, and mode of action are similar in all respects
to those at the orifice of the pulmonary artery. The anatomic relations of
the cardiac orifices one to the other and the appearance presented by the
valves when closed are represented in Fig. 120.

The Heart Muscle-fibers and Their Arrangement. — The muscle-
fibers of the heart represent in their structure a type between the ordinary

FIG. 120.— VALVES OF 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.)

THE CIRCULATION OF THE BLOOD.

267

lateral union

striated muscle and the smooth muscle. A longitudinal section of the
heart-muscle shows a reticulated arrangement of the fibers, the outcome of
a similar reticulated condition of the mesodermic material in which they
develop. The mesodermic reticulum containing numerous nuclei is
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. 121). 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
and light 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 disks. When the heart muscle
is treated with caustic potash the trabec-
ulae separate at the level of these disks,
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
auricles are practically independent of
ecah other. Circularly arranged fibers ™*R°TF A
are present near the terminations of
the venae cavae and pulmonary 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

Nucleus of Nucleus of Intercalated
a muscle a connective disc.

fiber. tissue cell.

FIG. 121. — FROM A LONGITUDINAL SEC-
HUMAN

268 TEXT-BOOK OF PHYSIOLOGY.

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
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

FIG. 122. — 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, pulmonary artery. — (From Hirschf elder.)

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. 122).

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

THE CIRCULATION OF THE BLOOD. 269

constantly present in all animals examined, including man, began on the
right side of the auricular wall below the fossa ovalis, from which point it
passed forward, and anteriorly penetrated the auriculo-ventricular septum
to become connected with the musculature of the ventricular septum just
below the pars membranacea septi. Though both these observers state that
the bundle divides into a right and left limb as it enters the ventricular
septum, the ultimate distribution and termination of these limbs was not
clearly determined. Retzer estimated that this bundle was 18 mm. long,
2.5 mm. broad, and 1.5 mm. thick. By these investigators this bundle was
termed the "auriculo-ventricular bundle."

In 1906 Tawara published the results of an extended series of 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-ventric-
ular 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 system " are historically 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 sarco-
plasm and present a number of fibrillae near their periphery. In associa-
tion 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

270 TEXT-BOOK OF PHYSIOLOGY.

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
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 — pulmonary 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 the blood from the venae cavae and
pulmonary veins.

THE CIRCULATION OF THE BLOOD. 271

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
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 caves, 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 pulmonary 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 pulmonary 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

272 TEXT-BOOK OF PHYSIOLOGY.

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
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. 123 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
FIG 123.— A CARDIOGRAM. cavities are enlarging with the incoming of a
(After Pachon.) 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.

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-
tricles are at rest.

For the purpose of obtaining accurate information 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. 124) introduced through the jugular vein into the cavities of the right heart. Each

THE CIRCULATION OF THE BLOOD.

273

a

tambour consists of a metallic frame covered by a thin rub-
ber membrane. By means of flexible tubes, a. v., the in- a •
terior of each tambour can be placed in communication
with the interior of a second tambour provided with a re-
cording 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 pres-
sure will be followed by a movement of the enclosed air
toward the cardiac tambour indicated by an inward move-
ment 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 re-
cording tambours, which continues so long as the pressure
lasts. With the relaxation of the auricular and ventricular
walls the pressure is removed and the levers fall to their
former position. When 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. 125.

A similar record would be obtained if the tambours were
placed in the cavities of the left side of the heart.

In this record the upper and lower lines rep-
resent respectively the contraction and relaxa-
tion of the auricle and ventricle as well as the
variations of pressure occurring within them.
A study of this record shows 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 accumu-
lation of blood within the heart cavities. When
this reaches a certain level the auricular con-
traction 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 within the auricle undergoes variations both in
the way of increase and decrease as shown by small undulations on the curve.

m

FIG. 124. — CARDIAC SOUNDS.
v, Tambours to be inserted into
the ventricle; a, tambour to be in-
serted into the auricle; m, rubber
membrane surrounding metal
frame- work; a, v, ends of tubes in
connection with tambours. —

FIG. 125. — TRACINGS OF (i) THE INTRA-AURICULAR PRESSURE; AND (2) THE INTRA-VENTRICULAE
PRESSURE OF THE HORSE. — (Chauveau and Marey.)

With the close of the auricular systole, the ventricular systole occurs
quickly and energetically and endures for some time, after which the ventricu-

18

274 TEXT-BOOK OF PHYSIOLOGY.

lar walls quickly relax and remain at rest until the close of the next auricular
contraction. The summit of the ventricular tracing generally spoken of as
the plateau presents a series of elevations and depressions as stated in a
foregoing paragraph.

A comparison of the two traces shows that between the close of the
auricular and the beginning 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 the
succeeding auricular contraction there is a period during which the whole
heart is at rest and the cavities filling with blood.

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. 125.

From the foregoing facts it is apparent that with the relaxation of the
auricular walls, blood at once flows from the venae cavae and the pulmonary
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 equivalent
coming from the venae cavae and pulmonary veins, now flows into the ventricles
until they are nearly filled. Before they are filled, however, the auricular dias-
tole comes to an end, the auricular walls again contract and force their con-
tained blood into the ventricles and thus rapidly complete the filling. The
ventricular systole immediately follows, during which the blood is driven into
the pulmonary artery and aorta. This having been accomplished, the ven-
tricles 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 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

THE CIRCULATION OF THE BLOOD. 275

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 pulmonary
artery and aorta are securely closed and a return flow prevented. Reversal
of the valves is prevented by their mode of attachment to the fibrous rings of
the orifices.

During the ventricular systole the relaxed auricles have been filling with
blood. With the ventricular relaxation this volume, or its equivalent, flows
readily into the empty and easily distensible ventricles, its place being taken
by an additional volume of blood flowing from the venae ca\ae and pulmonary
veins. Whether the ventricles exert a suction power at the moment of
their relaxation is an undecided question. A steady stream of blood into
the auricles and ventricles continues throughout the entire period of rest until
both cavities are filled. The tricuspid and bicuspid valves which 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.

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 completely 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
that in the aorta and pulmonary artery. As soon as this occurs the semi-
lunar valves of both vessels are thrown open and the blood discharged. The
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-ventricular septum, due to the contraction of the
papillary muscles. Both contraction and outflow continue until the ven-
tricles are practically empty, after which ventricular relaxation sets in,
attended by a rapid fall of pressure. Under the influence of the positive
pressure of the blood in the sinuses of Valsalva the semilunar valves are
again closed, the column of blood supported, and regurgitation is prevented.
In the meantime and while the ventricles are contracting, blood is again
flowing into, and accumulating in the auricles and thereby distending them
preparatory to the next systole. With the accumulation of blood in the
auricles the cardiac cycle is completed.

The approximate changes in the shape of the heart, the variations in the

276

TEXT-BOOK OF PHYSIOLOGY.

size of its cavities and in that of the blood-vessels arising from them, and
the relative position of the valves during systole and diastole are shown in
Fig. 126.

Relative Functions of Auricles and Ventricles. — Though both
auricles and ventricles are essential to the continuous movement of blood,
they possess unequal values in this respect. The passage of the blood
through the pulmonary and systemic vessels is accomplished by the driving
power of the right and left ventricles respectively, aided, however, by minor
extra-cardiac forces. They may be regarded therefore as force-pumps.

If the heart consisted of ventricles only, the flow of blood from the venae
cavae and pulmonary veins would be temporarily arrested during their systole

S.a.-D.v.

D.a.-S.v.

FIG. 126. — 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. pulmonary artery; A. aorta; P.P. papillary muscles. — (Landois.)

and their subsequent refilling delayed. This is obviated, however, by the
addition of the auricles; for during the ventricular systole the blood continues
to flow into the auricles, in which it is temporarily stored until the ventricular
relaxation sets in. With this event the accumulated blood passes into the
ventricles, which are thus practically filled before the auricular systole occurs
by which the filling is completed. By this means there is no delay in the
filling of the ventricles, and hence their effective working as force-pumps is
more readily secured. The auricles may therefore be regarded as feed-
pumps. For this reason it is probable, notwithstanding the contraction of
the circular muscle-fibers at the terminations of the venous system, that the
flow of blood into the auricles is never entirely arrested. Regurgitation in

THE CIRCULATION OF THE BLOOD.

277

max valve

to manometer

mi'n valve

these vessels does not occur for the reason that the pressure in the auricles
is not higher than, if as high as, in the great veins.

Synchronism of the Two Sides of the Heart. — If the balance of the
circulation is to be maintained, the two sides of the heart must act 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
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.

Intra-ventricular 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 pulmonary
artery and aorta, a pressure due to the resis-
tance, as will be explained later, offered to
the flow of the blood mainly by the smaller
arteries and capillaries; that they are opened
only when the pressure of the blood within
the ventricle exceeds that in the arteries. It
becomes, therefore, a matter of importance
to determine the extent of this pressure as
well as its variations during the course of a
cardiac cycle. This can be done by inserting a long catheter into either the
right or left ventricle, through the jugular vein or the carotid artery respec-
tively, and connecting its free extremity with a mercurial manometer. By
the interposition of a double valve such as represented in Fig. 135, it becomes
possible, 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. Thus Goltz found in the left ventricle of the dog a
maximal pressure of 114 to 135 mm.; in the right ventricle, a pressure of 35
to 62 mm. Minimal pressures of —23 to —52 mm. for the left ventricle have
also been obtained.

The maximal pressure in the ventricles during the systole, though always
higher than that in the arteries, is neither a fixed nor an invariable pressure,
as it rises and falls with the latter from moment to moment. Within limits
the cardiac power, and therefore the intra-ventricular pressure, is capable of
considerable increase. The function of the heart is to drive the blood
through the vessels with a given velocity. This is possible only by first over-

to heart

FIG. 127. — 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.)

278 TEXT-BOOK OF PHYSIOLOGY.

coming the resistance to the flow offered by the vessels, as indicated by the
arterial pressure. As this is a variable factor, rising and falling very con-
siderably at times, the heart must meet and exceed each rise, within limits if
the circulation 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
proportional 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 discharge
of blood from its cavities and hence become paralyzed.

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
compared the records of the pressures in the vense 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

FIG. 128. — V. CURVE OF THE PRESSURE IN THE VENTRICLE OF THE DOG. A. CURVE OF
THE PRESSURE IN THE AORTA. The curves were taken simultaneously, s, Tuning-fork vibrations
each corresponding to i/ioo of a second. The ordinates 0-5 correspond in the two records,
o, Closure of the auriculo- ventricular valve; i, opening of the semi-lunar valves; 2, point of
maximum pressure; 3, beginning of the ventricular relaxation; 4, closure of the semi-lunar valves;
5, opening of the auriclo-ventricular valve. (Hurthle.}

construction of which is such as to enable them to respond instantly to the
very rapid variations of the pressure which occur during the brief cardiac
cycle. One of the best is that of Hiirthle. This consists of a small metallic
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 CIRCULATION OF THE BLOOD. 279

the jugular vein or into the left ventricle through the carotid artery. Both
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. 128. 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 solution
of sodium sulphate, in a capillary glass tube one end of which was closed,
the other end placed in connection with an intra-cardiac catheter, the oscil-
lations representing the variations in pressure. The photogram thus
obtained resembles the curve obtained by Hlirthle's membrane manometer.

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. 128), 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-

28o TEXT-BOOK OF PHYSIOLOGY.

tion of the relation of the intra-ventricular pressure to the time of action
of the valves, the close of the ventricular systole may be conveniently
selected.

During the systolic plateau the blood is passing from the ventricle in to 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. 128), 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.

THE CIRCULATION OF THE BLOOD. 281

In accordance with the foregoing: the ventricular systole may be sub-
divided into two periods:

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.

Negative Pressure. — As shown by the ventricular pressure curve there
is a moment when the pressure falls below atmospheric pressure, becoming
negative to it. The extent to which this takes place, its duration and fre-
quency, have never been satisfactorily determined. 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 pulmonary artery. That it is not due to the
expansion of the thorax is evident from the fact that it occurs when the
thorax is open and the heart exposed.

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 amount of blood. In 38 heart beats
therefore the entire amount of blood will have passed through the heart.

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-

282 TEXT-BOOK OF PHYSIOLOGY.

nected by its free extremity with a Hiirthle tambour. The curve of pressure
thus obtained, shown in Fig. 129,* 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. 130, 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.

V

\J if

af

FIG. 129. — CURVE OF PRESSURE VARIATIONS FIG. 130. — CURVE OF PRESSURE VARIATIONS IN
IN THE AURICLE. (Enlarged). — (Porter.} THE JUGULAR VEIN. (Enlarged.) — (Bachmann.}

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.

The second negative wave, a/, 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

4f.f l The original tracing obtained by Porter is shown in the ac-

companying Fig. 131. The letters designating the waves have
*. tne following significance. A, systolic rise; AB, first diastolic

A \, J, fall; BC, first diastolic rise; CD, second diastolic fall; E, second

diastolic rise; F, third diastolic fall; G, pause. In Fig. 129
the tracing has been enlarged and the waves relettered and named
m * accordance with the terminology in vogue in the literature of

Chnical medlcme'

*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 calls it
the carotid, c, wave; inasmuch as it occurs in point of time with the beginning of the ventric-
ular systole, it is also called the systolic, s, wave.

THE CIRCULATION OF THE BLOOD. 283

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 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

Francois Frank on a woman whose heart was FIG. 132.— TRACINGS OF
congenitallydisplaced into the abdominal cavity.
An investigation revealed the fact that this WITH ECTOPIA OF THE HEART, a,
woman had a large opening in the anterior Auricular; v, ventricular.— (Fran-
portion of the diaphragm through which the f™

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. 132 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. 125, p. 273, 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

284

TEXT-BOOK OF PHYSIOLOGY.

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. 133, based on that constructed by Fredericq.

AURICULAR D/ A STOLE

'oswe ofSonlluna?

valves

nifio of
Semi-lunar valves.

Opening of

" valves

.Closure of
urlculo vmtricula
valve.

Closure of
u.ricalo -ventricular
valve.

\l\TPJCULAKSYSTOLE\ \\ttNJR I CUTAR DIA

Seconds

0 J .2 .3 .2 .S .6 .7 .8 .1 .2 .3

FIG. 133. — A SCHEMATIC REPRESENTATION OF THE EVENTS OF A CARDIAC CYCLE.

Though the foregoing numerical values are given for the duration of the
auricular and ventricular systoles and of the common pause, it must be
borne in mind that they are true only for the heart beating approximately
70 times per minute. If the number of beats increases, not only does the
entire cycle diminish in duration, but its different subdivisions, auricular
systole, ventricular systole, and diastole also diminish in duration, though
in unequal degrees. Thus it has been determined that with each increase
of 10 beats, the ventricular systole shortens by about 0.02 second and the
ventricular diastole by about o.io second. The opposite holds true if the
number of beats decreases below 70 per minute.

The Relation of the Cardiogram to the Events of the Cardiac Cycle.
— A comparison of a typical cardiogram, such as is seen in Figs. 123 and
134, with the curve of intra-ventricular pressure, shows that they correspond
in essential features. The slight elevation (a) on the cardiogram represents
the contraction of the auricle, which completing the filling of the ventricle
causes it to press more vigorously against the chest wall ; b-c represents the
contraction of the ventricles, at which moment the apex is suddenly and
forcibly driven against the chest wall; c-d represents the systolic plateau,
the time during which the ventricle is discharging blood into the aorta ; d-e
represents the relaxation of the ventricle, while e-f represents the time of the
diastole during which the heart cavities are enlarging with the incoming of

THE CIRCULATION OF THE BLOOD.

285

a

f

0' C'O

FIG. 134.— CARDIOGRAM,

o, Au-

a new volume of blood, in consequence of which the heart is pressing
against the chest walls. The systolic plateau is characterized by one or
more elevations and depressions, the true cause of which is unknown.

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 oc-
currence of the heart sounds and other fea-
tures of the cardiac cycle. Such a construc-
tion is shown in Fig. 134.

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. 1 nese closing and opening of the auriculo-

sounds can be approximately reproduced by ventricular valves; O', C', opening and
pronouncing the syllables lubb-d»p, lubb- ttSttSSESsR

dup. The long dull SOUnd OCCUrs With the ventricular discharge ;CC', time of the

systole, the first phase of a new cardiac cycle, occurrence of the first and second
and is therefore termed the first sound; the ^nds respectively^
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
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 pulmonary valves which occur at the

286 TEXT-BOOK OF PHYSIOLOGY.

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 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 Blood-supply to the Heart.— The nutrition of the heart, its
irritability and contractility, the force and frequency of the beat, are depend-
ent on and maintained by the introduction 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
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

THE CIRCULATION OF THE BLOOD. 287

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 arteries, two in number, the right and left, arise
from the aorta in the pouches of Valsalva just above the right and left semi-
lunar valves. Turning in opposite directions, they ultimately anastomose,
forming a circle around the base of the ventricles. From both the right and
left artery branches are given off which run over the walls of both auricles
and ventricles, the most important of which in man are the anterior and
posterior inter-ventricular. These main vessels lie in grooves on the surface
of the heart beneath 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 which penetrate the walls of the heart, in which they divide
into many branches before terminating in a capillary system. Because of
their relation to the heart-muscle they may be designated intra-mural vessels.
From the capillary areas small veins arise which, passing backward, con-
verge to form the coronary veins. These follow the course of the arteries
and finally terminate in the coronary sinus, located in the auriculo-ventricu-
lar 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 coronary 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 flows freely into them. It was
further assumed that the coronary arteries empty themselves into the capil-
laries during the time of the systole. According to Briicke the emptying of
the coronary arteries and the consequent fall of pressure within them pro-
moted the contraction of the ventricle, while the filling of the vessels and the
consequent rise of pressure facilitated the diastole. This anatomic mechan-
ism and its associated functional activity constituted according to Briicke an
apparatus by which the activity of the heart could be self-regulated. This
theory, however, has been disproved and is no longer entertained.

288 TEXT-BOOK OF PHYSIOLOGY.

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 pressure 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 inceased.
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 quite probable.

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

THE CIRCULATION OF THE BLOOD. 289

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 from the body. The
heart of the dog or cat will, however, beat but for a few minutes. The
human heart would in all probability act in the same way. Nevertheless
there are good reasons for believing that though the spontaneous beat has
ceased, the irritability yet endures though perhaps in lessened degree. For
if, after the heart has ceased to beat for some time, warm defibrinated and
oxygenated blood 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 299.)

The reason for the longer continuance of the beat of the excised heart
of the cold-blooded animal beyond that of the warm-blooded animal
lies probably in the difference in the rate of their respective metabolisms.
There is reason to believe that each 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
19

29o TEXT-BOOK OF PHYSIOLOGY.

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
condition of diastole. This can be done with the frog's heart, by
ligating the tissues at the sino-auricular junction, a procedure which
prevents the passage of the contraction wave which originates in the
sinus, over the auricles and ventricles (a fact that will be more fully
alluded to 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
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 286).

b. The Influence oj Temperature. — For the manifestation of the irrita-
bility and contractility it is essential that the heart-muscle be kept at asuf-
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
37° C. It ceases to beat at about 47° C. on the one hand and at about
17° C. on the other hand.

THE CIRCULATION OF THE BLOOD. 291

2. 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 de-
layed for an appreciable period (a condi-
tion attributed to the embryonic charac-
ter 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 dis-
tinguish the contraction of the different
chambers of the heart. In the frog s HEART.
heart the excitation process and the con-
traction wave begin in the sinus venosus, from which they are
conducted to the auricles, thence to the ventricles. The successive
contractions of the walls of the subdivisions of the heart can be
readily recorded with suitable apparatus. In Fig. 135, 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 continued 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 extent as to prevent absolutely the passage of the
contraction wave by ligation of the tissue, as first suggested by Stannius.
Under such circumstances the auricles and ventricle remain at rest
while the sinus continues to beat at its usual rate. The obstacle between
the auricles and ventricle may be increased by the same method or

292 TEXT-BOOK OF PHYSIOLOGY.

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 contractions may occur before the excitation
process forces the block and excites 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 exerts its action in the sinus venosus and that
the auricular and ventricular beats are in turn dependent on it.

In the mammalian heart the seat of the stimulus and the point of
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

FIG. 136. — RECORD OF THE AURICULAR AND VENTRICULAR CONTRACTIONS BEFORE
AND AFTER THE CLOSURE OF THE CLAMP AT a.

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 the sulcus terminalis.
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

1In 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
cava (the right duct of Cuvier); (2) the coronary sinus (the left duct of Cuvier); (3), a
stratum submerged beneath auricular tissue at the tsenia 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 taenia terminalis of the right auricle, and known as the sino-auricular
node.

THE CIRCULATION OF THE BLOOD. 293

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
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 268, 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. 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

294 TEXT-BOOK OF PHYSIOLOGY.

with different rates and independently of each other. These and
other experiments of a similar character have demonstrated beyond
question that the auriculo-ventricular bundle with its widespread
ramifications is the true conducting system between auricles and
ventricles. In this system the sino-auricular node is regarded as the
primary dominating "pace maker" of the rate and rhythm of the heart.
Inasmuch, however, as the heart will continue to beat, after the destruc-
tion 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 be-
tween these two events, determined from the
time between the occurrence of the a and c
or s 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
ventricular contraction, the inter-systolic period,
by compression of a portion of the tissues be-
tween auricles and ventricles including pre-
sumably the central part of the conducting sys-
tem, the muscle bundle of His. 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. Com-
pression 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 auricles, 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.

FIG. 137. — The Erlanger
heart-block clamp compress-
ing the auriculo-ventricular
bundle (AVB). SM, Septum
membranaceum; MV, mitral
valve. — (Hirschf elder.}

THE CIRCULATION OF THE BLOOD. 295

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. Ven. rate per minute. Ratio of Aur. to Ven.

Max. 216 Max. 69.8 3.09

Min. 117.8 Min. 34.8 3.38

Ave. 166.9 Ave. 52.3 3^9

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
been termed the 2 :i, 3 :i, 4:1, 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 ventric-
ular 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.

296 TEXT-BOOK OF PHYSIOLOGY.

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 denned 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
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. This tone 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

THE CIRCULATION OF THE BLOOD. 297

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 held

as to its origin and nature:

1. That it originates in the nerve-cells found in various parts of the heart-

muscle; that it is a nerve impulse rhythmically and automatically
discharged by these cells and transmitted by their axons to the heart-
muscle cells.

2. That it originates in the muscle-cells themselves; that it is chemic in

character and due to a reaction between the chemic constituents,
organic and inorganic, of the muscle-cells and those of the lymph by
which they are surrounded.

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
walls shows that the stimulus does not necessarily originate in nerve-cells.
Moreover, Porter has conclusively shown that the apex of the dog's heart,
which is generally believed to be totally devoid of nerve-cells, can be made
to beat for hours by feeding it through its nutrient artery with warm 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

298 TEXT-BOOK OF PHYSIOLOGY.

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
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,
until it entirely ceases to beat. A reason assigned for this is the removal of
other salts necessary to the excitation of the contraction. In a calcium
chlorid solution — 0.9 per cent. — i.e., iso tonic with the sodium chlorid — the
heart strip is thrown into strong tone, but does not rhythmically contract.
If, however, the strip is placed in normal saline, and calcium chlorid added
in amounts equal to that present in the blood, it will after a very short latent
period begin to contract rapidly and energetically and for a longer time
than when in sodium chlorid solution alone. The contractions not infre-
quently occur before relaxation is completed, so that the strip passes into the
condition of contracture.

THE CIRCULATION OF THE BLOOD. 299

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° C.
No special precautions were observed other than keeping it cool (10° 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 it
was again revived by perfusing it with Ringer's solution. Altogether this
heart retained its irritability for seven days. Hering revived the heart of a
monkey on three different occasions, the first, 4^ hours, the second, 28
hours, and the third 54 hours after the death of the animal. In the 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 double
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 rate of
98 to 102 per minute when it had a temperature of 41° 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. Potassium exerts a depressor or
an inhibitor influence on the irritability and contractility of the heart-
muscle.

Under the same conditions, the addition of calcium chlorid in sufficient
amounts is followed by an increase in the rate and in the vigor of the con-
tractions; on its withdrawal both rate and force return to the previous condi-
tion. Calcium exerts an accelerator and an augmentor influence on the
irritability and contractility of the heart.

300 TEXT-BOOK OF PHYSIOLOGY,

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 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 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

THE CIRCULATION OF THE BLOOD. 301

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 extent 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 fact. There are reasons for believing that at the time of
the contraction practically all of the available energy-yielding material is
completely utilized, after which the heart 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
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.

The Extra Systole. — The extent of this extra systole will be propor-
tional to the time at which the stimulus 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 (Fig.
139) . 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 relaxation and into the diastolic or
resting period, it has been found that the extra systole can be evoked by a
stimulus which is steadily decreased in intensity. It is evident from this

302

TEXT-BOOK OF PHYSIOLOGY.

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. 138). 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 disappearance of the extra systole a consider-
able pause in the heart's action occurs to which the term compensatory pause

FIG. 138. — DIAGRAM SHOWING THE VARIATIONS OF IRRITABILITY DURING THE SYSTOLE AND THE
DIASTOLE. — (Modified from Waller.)

has been given (Fig. 139), on the assumption that it was necessary on the
part of the heart to compensate for the disturbance of the rhythm by remain-
ing at rest until the time of the next beat and thus restore the rhythm. This
was thought to be a special property of the heart-muscle. This view, how-
ever, 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 experi-
mentation instead of the whole heart, the results of the same methods of
stimulation are different. Though an extra 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 necessi-
tated for the restoration of the normal rhythm is therefore not tenable.

The explanation assigned and generally ac-
cepted at present for the production of a com-
pensatory pause is as follows: In a spontaneously
beating heart the ventricular systole is evoked byfcthe
arrival of an excitation process coming from the
auricles. When the extra systole is induced by an

FIG. 139.— THE EXTRA
SYSTOLE AND THE COMPEN-
SATORY PAUSE. The break
in the horizontal line indicates artificial stimulus, the next succeeding excitation
the moment the electric cur- from the auricle falls into the refractory period and
sthroughthe heart' 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

THE CIRCULATION OF THE BLOOD. 303

is coincidently an impairment in the irritability and contractility of the heart-
muscle. Extra systoles, however, may have their origin in the auricular
walls as well.

If a series of successive stimuli be thrown into the heart-muscle the
effect will vary in accordance with their time intervals. Should this be less
than about three seconds there will be a gradual increase in the height for
some half dozen contractions, a result to which the term "staircase" or
"treppe" has been given. This increase in the height of the contraction
is attributed to an increase in the irritability and contractility of the muscle
the result of the primary stimulating action of fatigue products.

THE NERVE MECHANISM OF THE HEART.

By this term is meant a combination of nerves and nerve-centers which
cooperate to increase or decrease either the rate or force — or both — of the
heart's contraction in accordance with the needs of the system. That the
heart is normally influenced by the central organs of the nerve system in
response to the action of nerve impulses reflected to them from many organs
of the body is a matter of personal experience; that it is abnormally influenced
by the same or other organs in response to nerve impulses reflected to them
in consequence of pathologic and traumatic processes occurring in different
regions of the body, and that both heart and nerves are modified in different
ways by the action of drugs introduced into the body, are matters of daily
clinical experience.

The nerves comprising this mechanism and the relation they bear one to
another are represented in Fig. 140.

It was stated in a previous paragraph, page 289, that the contraction of
the heart-muscle is independent of its connection with the central organs of
the nerve system, and that it will continue to contract in a rhythmic manner
for a variable length of time even after its removal from the body of the
animal, the length of time varying with the animal and the conditions to
which it is subjected; that the stimulus is myogenic in origin and chemic in
character, the result of a reaction between the chemic constituents, organic
and inorganic, of the muscle-cells and those in the lymph by which they are
surrounded. It has also been further shown that even in the living animal
the heart will continue to beat and fulfil its functions after 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 v. Bezold's
or Ludwig's. The majority of the cells are situated on the surface of the

304

TEXT-BOOK OF PHYSIOLOGY.

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

Emotion a I Cen ters

Exhilarating (BLUE)
*eo)

Cantio -Inhibitor Center.

Ganglion Stellatum

Intra-Carctiactferoe Cells

Canfio -Accelerator Center

erve

tutor (BLUI.}
bitor (BLUE)

£ffenntInhibltor(RED)

Sympath etic Mroes '

Accelerator & Auymen tor

FIG. 140. — DIAGRAM OF THE NERVE MECHANISM OF THE HEART. — (G*. Bachnnm.}

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-

THE CIRCULATION OF THE BLOOD. 305

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 of
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, are
connected on the one hand with the heart-muscle itself and on the other
hand with nerve-fibers coming from the central nerve system. The former
are non-medullated and post-ganglionic, the latter medullated and pre-
ganglionic.

The pre-ganglionic fibers have their origin in the medulla oblongata and
very probably from nerve-cells in the gray matter beneath the floor of the
fourth ventricle. From this origin they descend the spinal cord as far as the
level of the second and third thoracic nerves. At this level they emerge from
the cord in company with the nerve-fibers composing the anterior roots of
the second and third thoracic nerves. After a short course, they enter the
white rami communicantes, enter the sympathetic chain and pass upward
to the ganglion stellatum, and by way of the annulus of Vieussens to the
inferior cervical ganglion as well, around the nerve-cells of which their
terminal branches arborize. From the nerve-cells of both the stellate and
inferior cervical ganglia, the sympathetic nerves proper arise, which after
emerging from the ganglia pass toward the heart and become associated
with the fibers of the vagus and assist in the formation of the cardiac plexuses.
On reaching the heart they may terminate directly in the muscle-cell or
indirectly through the intermediation of intra-cardiac nerve-cells. The
former mode of termination is the more probable. Experiment has shown
that both the pre- and post-ganglionic fibers are efferent in function.

The Origin and Distribution of the Vagus Nerve in Mammals.—
The vagus nerve-fibers which influence the heart are connected on the one
hand with the heart, through the intermediation of the intra-cardiac cells, and
on the other hand with the central nerve system. Histologic investigation has
shown that the vagus nerve-trunk of man and mammals generally, contains
medullated fibers of large and small size. Experiment has shown that the
large fibers are afferent, the small fibers efferent in function.

The large afferent fibers arise in the ganglia situated on the trunk of the
nerve. From their contained nerve-cells a short axon process proceeds
which soon divides into a central and a peripheral branch. The
central branch passes toward and into the gray matter beneath the floor of
the fourth ventricle, where its end-tufts arborize around nerve-cells; the
peripheral branch passes toward the general periphery to be distributed to
the mucous membrane of the lungs, stomach, intestine, etc. The small
efferent fibers are the peripherally coursing axons of nerve-cells situated in
the gray matter beneath the floor of the fourth ventricle at the tip of the
calamus scriptorius. The exact course of these fibers from their origin into

306 TEXT-BOOK OF PHYSIOLOGY.

the trunk of the vagus is not positively known. According to some investi-
gators, 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. 147.)

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
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 arise. 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

THE CIRCULATION OF THE BLOOD.

307

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 union
at the vagus ganglion, and then stimulate their peripheral ends. The heart
should be exposed and attached to a recording lever so that its movements
may be taken up and recorded on a moving recording surface.

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.)

Stimulation of the sympathetic fibers with induced electric currents,
prior to their union with the vagus, is followed by an increase in the rate,
or an augmentation in the force of the heart-beat or both, at the same time.
The effects of such a stimulation with induced currents of moderate intensity
are graphically shown in Fig. 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 acceler-
ation 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.

308 TEXT-BOOK OF PHYSIOLOGY.

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
the ventricle has completely relaxed. After the effect of the stimulation
passes away, the acceleration diminishes, the augmentation declines and a
reverse change in the sequence occurs. The lower tracing shows a similar
series of effects. If the stimulus be applied to the pre-ganglionic sympathetic
nerves, an acceleration or augmentation of the heart follows, similar in all
respects to that which follows stimulation of the post-ganglionic or 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
cardio-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
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 alt 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,
its cavities dilated and filled with blood. The dilatation usually exceeds
that observed prior to the vagus stimulation, from which it is inferred

THE CIRCULATION OF THE BLOOD. 309

that some fibers of the vagus at least diminish the tonicity of the heart-
muscle.

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 the
rate and force increase, until they attain or exceed that observed prior to the
stimulation. In some cases, however, the heart begins to beat with as much
and even more vigor than it did prior to the stimulation. The duration of
the inhibitor effect varies with the duration of the stimulation. Thus during
and after a stimulation of thirty-eight seconds the heart of the toad remained

FIG. 143. — TRACING SHOWING THE DIMINUTION IN THE RATE OF THE HEART-BEAT
FOLLOWING WEAK TETANIZATION OF THE VAGUS TRUNK.

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 wilt 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

FIG. 144. — TRACING SHOWING COMPLETE INHIBITION FOLLOWING STRONG TETAN-
IZATION OF THE VAGUS TRUNK.

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). Stim-
ulation with strong tetanizing currents is followed by complete inhibition
(Fig. 144).

The foregoing facts lead to the inference that the cardio-accelerator
and the cardie-inhibitor centers have as their function the discharge of nerve
impulses which are conducted by their related nerves, the efferent sympathetic

3io TEXT-BOOK OF PHYSIOLOGY.

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 Physiologic Actions of the Sympathetic Nerves in Mammals.—
In the mammal, stimulation of the sympathetic nerves in any part of their
course, either through the rami communicantes, the ventral portion of the
annulus of Vieussens, or after their emergence from the stellate or inferior

FIG. 145. — ACCELERATION OF THE HEART FOLLOWING STIMULATION OF THE CARDIAC
BRANCHES WHICH COME FROM THE ANNULUS OF VIEUSSENS.

cervical ganglia is followed by effects similar to those observed in the frog:
viz., an acceleration or augmentation, or both, of the heart-beat. The
percentage increase in the acceleration varies in different animals. In some
instances the increase varies from 58 per cent, to 100 per cent. (Hunt). If
the heart is beating slowly before stimulation, the acceleration is more marked
than if it is beating rapidly.

The effect of the accelerator impulses is apparently a change in the inner
mechanism of the heart-muscle itself and not a change in the peripheral
portion of the inhibitor apparatus. This is indicated by the fact that
acceleration occurs after the full physiologic action of atropin, which acts
upon, and impairs the conductivity of, the intra-cardiac nerve-cell terminals.

A peculiarity of the sympathetic nerve is that it does not respond to
stimulation as rapidly as do many nerves, so that a rather 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.

Though an increase in both the rate and force frequently occur simul-

THE CIRCULATION OF THE BLOOD. 311

taneously, there is no necessary relation or connection between the two as
they can and do occur independently of each other. For this reason it is
generally assumed that the sympathetic nerves contain two groups of fibers,
viz., accelerators and augmentors, the functions of which are 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 stimulate both chambers. This is rendered probable
also from the experiments of Erlanger, who found that after complete heart-
block, stimulation of the sympathetic caused independent acceleration of
both auricles and ventricles.

The Physiologic Action of the Vagus Nerve in Mammals. — In the
mammal the same or similar effects result from stimulation of the vagus as
in the frog. If the thorax of the dog is opened and artificial respiration
maintained the heart will continue to beat in a practically normal manner for
a long time. Under such conditions if the vagus nerve on one side be divided
and its peripheral end stimulated with induced electric currents of moderate
strength, the heart will be seen to come to a standstill almost immediately in
the condition of diastole, and may be so kept for a variable period, from
fifteen to thirty seconds or more, during which its walls are re axed and its
cavities filled with blood. On cessation of the stimulation the contractions
return and in a very short time the former rate and force of the beat are
regained. If the electric currents are of feeble strength, the heart will
come to rest gradually, through a gradual diminution in the rate and force
of the contraction. During the period of the inhibition the heart presents
an appearance similar to that presented by the heart of the cold-blooded
animal. When the heart of an animal is thus exposed, the auricle and the
ventricle of one side may be attached by threads to writing levers and their
contractions registered on a moving recording surface. The effects on both
auricles and ventricles which follow vagus stimulation will then become
more apparent. Fig. 146 is a tracing thus obtained. The animal employed
for the experiment was a rabbit.

The inhibitor effect of the vagus varies in degree and duration in
different animals. In the dog the effect of vagus stimulation is usually
pronounced, lasting from 15 to 30 seconds; in the rabbit it is perhaps equally
well pronounced but somewhat less in duration; in the cat it is almost 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.

Seat of Action of the Vagus Impulses. — In a foregoing experiment of
which Fig. 146 is a graphic result, stimulation of the left vagus with a fairly
strong current was followed by a diminution in both the rate and force of the

3i2 TEXT-BOOK OF PHYSIOLOGY.

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 that the cessation of ventricular action
is a secondary effect due to the non-arrival across the conducting apparatus

FIG. 146. — RESULT OF THE STIMULATION OF THE PERIPHERAL END OF THE
DIVIDED LEFT VAGUS IN THE RABBIT. — (Brodie.)

of the normal excitation process from the auricle. This is the case un-
doubtedly in the cold-blooded animals, and the experiments of Erlanger on
the heart of the dog indicate that the same holds true for the mammals.
This investigator has found that when the auriculo-ventricular tissues are
suddenly clamped, including presumably the muscle band of His, there is for a
time a complete cessation of ventricular activity, but after a variable period of
time, fifty seconds or more, the ventricle develops an independent rhythm
which gradually increases in frequency, but seldom, if ever, attains that of

THE CIRCULATION OF THE BLOOD. 313

the auricles. Under such circumstances tetanic stimulation of the auriculo-
ventricular tissues by means of the clamp now transformed into stimulating
electrodes, failed to bring about a stoppage of the ventricles. Moreover,
if during the time the clamp is applied and after the ventricle has developed
a rhythm of its own, the vagus is stimulated, the auricles will cease to beat
as usual, but the ventricles will continue to beat at their usual rate. These
and similar facts lead to the conclusion that vagal inhibitor action is
limited to the auricles.

From the foregoing facts it is apparent that the accelerator and augmentor
effects of the sympathetic nerve impulses, and the inhibitor effects of the
vagus nerve impulses, closely resemble on the one hand, the accelerator and
augmentor effects of increasing amounts of diffusible calcium salts, and on
the other hand, the inhibitor effects of increasing amounts of diffusible
potassium salts in the blood or other circulating fluid; and so closely do these
two sets of phenomena resemble each other, that they are by some observers
regarded as identical.

Some additional facts in this connection have been presented by Howell,
viz., that an increase (within limits) and a decrease in the percentage of
diffusible calcium salts in a circulating fluid passing through the cavities of
the mammalian (cat) heart, increases on the one hand, and decreases on
the other hand, the sensitiveness of the heart to sympathetic acceleration
and augmentation. From this the inference is deduced that the acceleration
and augmentation of the heart-beat which follow stimulation of the 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 bathmotropic , 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
discharge of nerve impulses which, conducted to the heart by the pre-

3i4 TEXT-BOOK OF PHYSIOLOGY.

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 Cardie-Inhibitor Center. — The collection of nerve-cells from
which the small efferent fibers of the vagus nerve arise is known as the
cardio-inhibitor center. It is situated in the medulla oblongata 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 slight or
feeble. In human beings the tone of the inhibitor apparatus is poorly
developed in early childhood, as shown by the fact that the administration

THE CIRCULATION OF THE BLOOD. 315

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 is1 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 reflected to it through various
afferent or sensor nerves. Though both factors play an important part in
the maintenance of its activity, the trend of evidence points to the conclusion
that the reflected impulses are by far the more important of the two. This
latter supposition is supported by the results of direct experimentation upon
sensor nerves in almost any region of the body. Thus stimulation of the
dorsal roots of the spinal nerves, the trunks of the cranial sensor nerves, the
splanchnic nerves, the pulmonary branches of the vagus, etc., gives rise to a
more or less pronounced inhibition of the heart. As a rule, stimulation of
the peripheral terminations of these nerves is more effective than stimulation
of their trunks, hence an explanation is at hand for the cardiac inhibition
which results from sudden distention of the stomach and intestines, or
operative procedures in the nose, mouth, and larynx.

Reflex inhibition of the heart, even to the stage of absolute and permanent
standstill, eventuating in the death of the individual is a not infrequent
result of peripherally acting causes of a pathologic or operative character.
From the results of experimental procedures the inference is drawn that
normally, nerve impulses, developed by the action of physiologic causes,
are reflected continuously from many peripheral regions of the body, and
falling into this center gently stimulate and maintain it in a condition of
necessary tonicity or activity.

The Causes of the Variations in the Heart-beat. — It has been stated
elsewhere in the text (page 286), that the rate of the heart-beat is influenced
by age, muscle activity, the position of the body, meals, variations in blood
pressure, etc. 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 in
another or the same individual an inhibition which may be so pronounced
as to lead to a complete standstill in diastole. The records of clinical medi-
cine contain many instances which show that gastric, intestinal, uterine and
other organic disorders as well as various operative procedures in different
regions of the body cause now an acceleration, now an inhibition of the
heart.

316 TEXT-BOOK OF PHYSIOLOGY.

The first explanation, that acceleration of the heart, the result of a
peripherally acting stimulus, is due to a stimulation of the cardio-accelerator
center by the arrival of nerve impulses coming through afferent nerves,
having been made questionable and improbable by the results of Hunt's
experiments, the alternative explanation must be that the acceleration is due
to an inhibition of the normal activity of the cardio-inhibitor center, and
that inhibition is due to an excitation of the normal activity of the cardio-
inhibitor center, and hence there 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 peripherally inhibits its
activity, the other of which when stimulated excites or augments its activity.

The extent to which both sets of fibers are present in any one afferent
nerve is unknown. In the trigeminus it is believed the excitator fibers
preponderate for the reason that peripheral stimulation of this nerve is
followed by inhibition of the heart; in the sciatic, it is believed the inhibitor
nerves preponderate, for the reason that stimulation of the central end of the
divided nerve is followed generally by acceleration of the heart.

It is probable from the effects which follow gastro-intestinal 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 the cardio-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.

The Influence of Psychic States. — The cardio-inhibitor and the cardio-
accelerator centers may be increased in activity also by nerve impulses
descending from the cerebrum, the result of emotional states; thus depressing
emotions according to their intensity may so increase the activity of the
cardio-inhibitor center that the heart's action may not only be retarded but
even completely inhibited; joyous emotions, on the contrary, may so increase
the activity of the cardio-accelerator center or what is more probable inhibit
the activity of the cardio-inhibitor center that the heart's action will be
increased in both its rate and force.

From the results of stimulation of the sympathetic (accelerator) and
vagus (inhibitor) nerves under a great variety of conditions it has been
established that their respective centers are mutually antagonistic; that the
activity of the accelerator center at one moment limits the 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 Depressor Nerve. — The vagus trunk also contains afferent fibers
stimulation of which not only brings about a reflex inhibition of the heart,
but also a dilatation of the peripheral arteries and a fall of blood-pressure
through a depressive influence on the vaso-motor centers. To this nerve
the term depressor has been given. A 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.

THE CIRCULATION OF THE BLOOD.

Seat of action
of

Modifications of the Nerve Mechanism of the Heart due to the
Physiologic Action of Drugs. — The functions of different parts of the
nerve mechanism of the heart may be demonstrated by an analysis of the
effects which follow the administration of slightly toxic doses of the alkaloids
of various drugs. The effects can be shown to be due to a stimulation or to
a depression of the normal activity of one or more portions of the
mechanism. The alkaloid may exert its specific action on the central
portions in the medulla, or on the peripheral portions in the heart, or on
both simultaneously. The heart-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. The 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-
muscle. Fig. 147. In consequence of this, the
influence of the cardio-inhibitor center is cut off
and the cardio-accelerator being unopposed in
its activity, the rate of the beat is increased. After a variable period
the heart returns to its normal rate. 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. — After the administration of nicotin in sufficient amounts
the heart-beat is primarily decreased in frequency even to the point of stand-
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-

Uqgws nerve .

Seatofaction
of Atropin.

FIG. 147. — DIAGRAM SHOWING
THE RELATION OF THE VAGUS TO
THE HEART MUSCLE-CELL.

3i8 TEXT-BOOK OF PHYSIOLOGY.

onic, 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 a 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 the 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 systemic vascular apparatus consists of a closed system of vessels
extending from the left ventricle to the right auricle, and includes the arteries,
capillaries, and veins. Though serving as a whole to transmit blood from
the one side of the heart to the other, each one of these three divisions has
separate but related functions, which are dependent partly on differences in
structure and physiologic properties, and partly on their relation to the heart
and its physiologic activities.

The Structure, Properties and Functions of the Arteries. — The
arteries serve to transmit the blood ejected from the heart to the capillaries;
that this may be accomplished they divide and subdivide and ultimately
penetrate each and every area of the body. Their repeated division is
attended by a diminution in size, a decrease in the thickness and a change in
the structure of their walls.

A typical artery consists of three coats: an internal, the tunica intima; a
middle, the tunica media; an external, the tunica adventitia.

The internal coat consists of a structureless elastic basement membrane,
on the inner surface of which rests a layer of elongated spindle-shaped en-
dothelial cells. The middle coat consists of several layers of circularly
arranged, non-striated muscle-fibres, 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. 148).

In virtue of the presence in their walls of both elastic and contrac-
tile elements, the arteries possess the two properties of elasticity and
contractility.

The elasticity is especially well developed in the large arteries, which are
capable, therefore, of both distention and elongation, and, when the distend-
ing force is withdrawn, of returning to their previous condition. The
elasticity permits of a wide variation in the amount of blood the arterial
system can hold between its minimum and maximum distention. 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 also converts 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 con-

3*9

320

TEXT-BOOK OF PHYSIOLOGY.

tinuous and equable movement observed in the capillaries. This is accom-
plished in the following manner: With each contraction of the left ventricle
more blood is ejected into the aorta than the arteries can discharge into the
capillaries and veins during the time of the contraction. The portion not so
discharged exerts a lateral pressure against the walls of the arteries which at
t once dilate until a condition of equilibrium is established between the pres-
sure from within and the elastic reaction of the arterial walls from without.

With the cessation of the contraction the elas-
tic walls recoil and propel the blood toward
the capillaries. The intermittent 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, ancj, 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. The elasticity thus serves the pur-
pose of equalizing the movement of the blood
throughout the arterial system.

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
characteristics the flow of a fluid through a
rigid tube under the intermittent action of a
pump; that is, the intermittent movement im-
parted by the heart is not so completely con-
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 vessel is normally elastic. For these and other
reasons the tissues are not so well nourished and hence their nutrition
and functional activities decline.

The contractility permits of a variation in the amount of blood passing
into a given capillary area in a unit of time. Normally each artery has a
certain average caliber due to a given contraction of the muscle coat. Be-
yond this average condition the artery can pass in one direction or the other
by either a relaxation or increased contraction of the muscle coat. During
the functional activity of any organ or tissue there is need for an increase in
the amount of blood beyond that supplied during functional inactivity or
rest. This is accomplished by a relaxation of the muscle-fibers. With
the cessation of activity the muscle-fibers again contract and reduce the
amount of blood to that required for nutritive purposes only. An increased
contraction of the muscle-fibers beyond the average, diminishes the outflow

FIG. 143. — COATS OF A SMALL
ARTERY, a. E n d o t h e 1 i u m. b.
Internal elastic lamina, c. Cir-
cular muscular fibers of the middle
coat. d. The outer coat. — (Lan-
dois and Stirling.)

THE CIRCULATION OF THE BLOOD.

321

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 augmented and the friction decreases. By virtue of its
tonic activity, the arteriole muscle at the periphery of the arterial system
offers considerable resistance to the outflow of the blood and this is there-
fore spoken of generally as the peripheral resistance, though there is included
under this term the resistance
offered by the small caliber of
the capillary blood-vessel as well.
This latter factor is constant,
the former variable.

The Structure, Properties,
and Functions of the Capilla-
ries.— The capillaries are small
vessels that connect the arteries
with the veins . Though different
in structure from a small artery
or vein, there is no sharp bound-
ary between them, as their struc-
tures pass imperceptibly one into
the other. A true capillary,
however, is of uniform size in
any given tissue and does not
undergo any noticeable decrease
in size from repeated branchings.
The diameter varies in different
tissues from 0.0045 mm- to °-°°75 mm., just sufficiently large to permit the
easy passage of a single red corpuscle. The length varies from 0.5 mm. to
i mm. The wall of the capillary (Fig. 149) is composed of a single layer of
nucleated endothelial cells with serrated edges united by a cement material.
Though extremely short, the capillaries divide and subdivide a number of
times, forming meshes or networks, the closeness and general arrangement
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,

FIG. 149. — CAPILLARIES. THE OUTLINES OP
THE NUCLEATED ENDOTHELIAL CELLS WITH THE
CEMENT BLACKENED BY THE ACTION OF SILVER
NITRATE. — (Landois and Stirling.)

322 TEXT-BOOK OF PHYSIOLOGY.

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 waste products. The blood at the same time changes
to a variable extent in chemic composition; because of the loss of oxygen and
the gain of carbon dioxid it also changes in color from red to bluish-red.

In order that the nutritive materials may pass through the 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 physical
properties and physiologic functions 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
serve to collect the blood from the capillary areas and return it to the right side
of the heart. As they emerge from the capillary areas the veins, which in
these regions are termed venules, are quite small. By their convergence
and union the veins gradually increase in size in passing from the periphery
toward the heart. Their walls at the same time correspondingly increase
in thickness. The veins from the lower extremities, the trunk, and abdom-
inal 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 media, and the tunica adventitia. The media,
however, does not possess as much of either the elastic or muscle tissue as
the artery, but a larger amount of the fibrous tissue. Hence they readily
collapse when empty. In virtue of their structure the veins also possess
both elasticity and contractility, though in a far less degree than the arteries.
These properties come into play and are of value in furthering the movement
of the blood toward the heart, especially after a temporary obstruction.

Veins are distinguished by the presence of valves throughout their course.
These are arranged in pairs and formed by a reduplication of the internal
coat, strengthened 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 accommodate the blood. With the removal of the
obstruction the recoil of the elastic tissue, and perhaps the contraction of the
muscle-tissue, forces the blood quickly onward.

HYDRODYNAMIC CONSIDERATIONS.

The blood flows through the arteries, capillaries and veins in accordance
with definite laws. During its transit certain phenomena are presented by

THE CIRCULATION OF THE BLOOD. 323

each of these three divisions of the vascular apparatus. Since these phenom-
ena, as well as the laws which govern them are similar to, though more com-
plex than the phenomena presented by relatively simple tubes with rigid or
elastic walls while liquids are flowing through them under a steadily acting
or an intermittently acting pressure, it will be conducive to clearness of
conception of the mechanics of the vascular apparatus, if there be
considered :

1 . The flow of a liquid through a horizontal tube with rigid walls and of
uniform or variable diameter under a steadily acting pressure.

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.

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. 150,
which consists of a reservoir or pressure vessel, P, provided with a horizontal tube
into which is inserted at equal distances a series of vertical tubes. If the reservoir
be filled with a liquid, water for example, the latter under certain conditions will
exert a downward pressure and act as a 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 exert-
ing ari equal pressure in all directions, will now exert a downward pressure only,
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, Tzr2; viz., v= — 2. Inasmuch as

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 stationary.
Between the outer stationary layer and the axis of the stream, there is an infinite
number of layers of molecules, the cohesion of which one for the other 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
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

324

TEXT-BOOK OF PHYSIOLOGY.

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
dy c, 6, 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. The
amount of the lateral pressure at any given point is therefore indicated and measured
by the height to which the water rises in the tubes. For this reason these tubes are
termed pressure tubes or piezometers.

Since the resistance in a tube of uniform diameter is proportional to its length
the lateral pressure will gradually but progressively decrease from the reservoir to
the outlet. Therefore the pressure at any given point is proportional to the resist-

FIG. 150. — A PRESSURE VESSEL, P, WITH A HORIZONTAL OUTFLOW TUBE, O-nt INTO WHICH
VERTICAL TUBES OR MANOMETERS ARE INSERTED.

ance yet to be overcome and conversely the resistance to be overcome is indicated
by the amount of the pressure. (In the conduct of an experiment the propelling
power should be kept constant by permitting fluid to flow into the reservoir as
rapidly as it flows out of the horizontal tube.)

The power or force which overcomes the resistance in the horizontal tube and
imparts velocity to the fluid is the downward pressure of the water in the reservoir,
represented by H. The amount of this power utilized in overcoming the resistance
is approximately indicated by the height of the fluid, yt at which point the line
uniting the upper limits of the water in the vertical tubes intersects it. The height
of the fluid at this point is a measure, therefore, not only of the resistance but also
an indication of the relative amount of the pressure used in overcoming it and is
therefore known as the pressure height.

The amount of the pressure consumed in imparting the observed 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-

v2
ity by twice the accelerating force of gravity as expressed in the formula, - ; the

quotient is the height and is known as the velocity height. Conversely if the mov-
ing fluid were discharged into empty space through an opening in the tube at n,

THE CIRCULATION OF THE BLOOD. 325

it would ascend an equal distance. If now this height is represented by F, and a
line be drawn from it, parallel to the line of pressure until it meets the reservoir at
x, it will be seen what percentage, x y, or h' of the primary propelling power is con-
sumed 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, h, 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.

In a system of this character, such as represented in Fig. 151, 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
area of 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 stop cock is suddenly opened, the column of water will exert a downward
pressure, and in consequence the water will be driven into and through the system
with a definite velocity and pressure.

The velocity of 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-

326

TEXT-BOOK OF PHYSIOLOGY.

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
the following considerations.

In the section B-C the two factors — viz., the widening of the stream bed which
decreases the resistance, and the narrowing of the individual tubes which increases
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
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 in A-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

D E

FIG. 151.-

-PRESSURE VESSEL WITH A SERIES OF PROGRESSIVELY BRANCHING AND REUNITING

TUBES.

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 CIRCULATION OF THE BLOOD. 327

The pressure throughout the system is the result of the resistance to the flow of
the water 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
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:

328 TEXT-BOOK OF PHYSIOLOGY.

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
intermittency in their action is sufficiently rapid, the excess of fluid entering the
tube over that discharged becomes sufficiently great to maintain a certain average
or mean pressure, which, however, undergoes an alternate increase and decrease
with each variation in the primary pressure.

The temporary increase and decrease of the pressure and the consequent
expansion and recoil of the tube in the neighborhood of the pressure vessel, give
rise to a wave on the surface of the tube which is propagated with more or less
rapidity — though with decreasing amplitude, from the beginning to the end of
the tube and causing in each section a corresponding expansion and recoil, and
known as the expansion wave.

THE APPLICATION OF THE FOREGOING FACTS TO THE
VASCULAR APPARATUS.

The systemic vascular apparatus may be conceived of as a system of tubes
which have symmetrically divided and subdivided and afterwards again
united and reunited in a corresponding manner. The arteries, arterioles,
capillaries, venules, and veins may therefore be schematically arranged
(Fig. 152) in a manner identical with the schematic arrangement of tubes
represented on page 325. The heart, with which they are in connection,
when filled with blood may be compared with the reservoir filled with water,
and the in tra- ventricular pressure developed during the contraction, to
the downward pressure of the water when the stopcock at is A 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
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. 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 CIRCULATION OF THE BLOOD.

329

The stream-bed in the aorta is relatively narrow, but widens gradually as
it approaches the capillaries, where it attains its maximum width; it again
narrows gradually as it passes into the veins, until in the venae cavae it 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 system to the

Veins

FIG. 152. — SCHEMATIC ARRANGEMENT OF THE VASCULAR APPARATUS.

terminations of the venae cavae, which result from the repeated branching
and subsequent reuniting, as well as its relative width in the arteries, capil-
laries, and veins, are shown graphically in Fig. 153.

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 with a definite velocity and pressure.

The 'velocity of the blood in the systemic vascular apparatus will gradu-
ally decrease in accordance with foregoing considerations 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 the larger blood-vessels the increase is
inversely proportional to the simple diameter; the velocity will again in-
crease from the middle of the capillary system to the ends of the venae cavae

330 TEXT-BOOK OF PHYSIOLOGY.

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 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 pe-
riphery 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 narrow-
ing,of individual vessels, and the decrease in resistance due to the widening
of the stream-bed ; as the former preponderates 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 proportional to the ratio between the increase in

FIG. 153. — 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.)

resistance due to the narrowing of the stream-bed, and the decrease of resist-
ance due to the enlarging of individual 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 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

nsufficient to maintain the observed differences in pressure in the different

THE CIRCULATION OF THE BLOOD. 331

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 phy-
sical 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.

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 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

2g

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-

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 phenomena which for the most part characterize the flow of blood
through the blood-vessels are velocity and pressure, combined with an
alternate expansion and recoil of the arterial vessels due to the intermittent
character of the heart-beat. For special reasons it is convenient to consider
the pressure first.

332 TEXT-BOOK OF PHYSIOLOGY.

BLOOD-PRESSURE.

From theoretic considerations alone it may be inferred that the blood,
as it flows through the vascular apparatus, exerts a pressure against the
walls of the vessels, and that this pressure is greatest at the beginning of the
aorta, and least at the ends of the venae cavae. The fact that the blood
flows from the aorta to the venae cavae indicates that there is a higher pres-
sure in the former than in the latter. The same holds true for the pulmonary
artery and veins. So long as these conditions are maintained, the blood
must flow from the point of high to the point of low pressure.

To this pressure the term blood-pressure is given, and may be defined
as the pressure exerted radially or laterally by the moving blood-stream
against the sides of the vessels. That there is such a pressure within the
arteries, capillaries, and veins, different in amount in each of these three
divisions of the vascular apparatus, is evident from the results which follow
division of an artery or a vein of corresponding size. When an artery is
divided, the blood spurts from the opening for a considerable distance and
with a certain velocity. The reason for this lies in the fact that the vessel
has been distended by the pressure from within and its walls thrown into a
condition of elastic tension, so that at the moment there is an outlet, the
vessel suddenly recoils and forces the blood out with a velocity 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 atmos-
phere. Especially true is this of the larger veins.

The same facts may be demonstrated in another and more striking way.
A dog or cat is anesthetized and securely fastened in an 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

THE CIRCULATION OF THE BLOOD.

333

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
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-

B.P.TRACING

ABSCISSA..-

FIG. 154. DIAGRAM TO SHOW THE RELATION or THE MERCURIAL MANOMETER TO THE ARTERY,
ON ONE HAND, AND TO THE RECORDING CYLINDER, ON THE OTHER HAND, WHEN ARRANGED FOR
RECORDING BLOOD-PRESSURE.

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 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
artery (Fig. 154). For the purpose of retarding coagulation of the blood and
for preventing the escape of a large volume of blood from the vessels, the
system is filled with a solution of carbonate of soda of sp. gr. 1060, 55.8
grams per 1000 c.c., or a 25 per cent, solution of magnesium sulphate of

334 TEXT-BOOK OF PHYSIOLOGY.

sp. gr. 1060, and under a pressure approximately equal to that in the vessel
of the animal as determined in previous experiments. When communication
is established between the vessel and the cannula, the mercurial column
adjusts itself to the pressure in the artery and at once exhibits the same
cardiac oscillations and respiratory undulations as did the column of blood
in the previous experiment.

The height of the mercurial column kept in equilibrium by the pressure
of the blood within, and the pressure of air without the vessel is that
between the lower level of the mercury in the proximal, and the higher level
in the distal limb of the manometer, both of which can be read off on a scale
placed between the two limbs.

The height of the mercury as well as its oscillations in the distal limb
may be recorded by placing on the top of the mercury a light float, the upper

FIG. 155. — A PORTION or A BLOOD-PRESSURE TRACING OBTAINED FROM THE CAROTID ARTERY
OF THE RABBIT WITH A MERCURIAL MANOMETER. The small oscillations are due to the heart-beat;
the large oscillations are due to the respiratory movements.

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. 154 and 155, in which the smaller oscillations represent
the changes in pressure due to the systole and diastole of the heart and the
larger oscillations to variations in the average pressure due to the respiratory
movements. The height of the mercurial column kept in equilibrium at any
particular moment is determined by measuring the distance between a base-
line or abscissa, which represents the position of the mercury at atmospheric
pressure, and any given point on the trace above, and multiplying it by 2,
for the reason that the mercury sinks in the proximal limb as high as it rises

THE CIRCULATION OF THE BLOOD.

335

max^valve

mm valve

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 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 , to manometer

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 repre-
sented. For, owing to the inertia of the
mercury, it 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 dia-
stolic and systolic pressures, often amount-
ing 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 valve similar in principle to that
shown in Fig. 156. By permitting the blood to exert its pressure first through
the maximum valve and then permitting the mercurial column to exert its
pressure through the minimum valve in the reverse direction for a certain
length of time, or by permitting each to exert its pressure with alternate
heart-beats, the maximum systolic and the minimum diastolic pressures
will be recorded. By this method Dawson found an average maximum
pressure in the carotid artery of the dog of 162, and a minimum pressure of
103 mm. of mercury, a difference of 59 mm. Hg. The difference between
these two pressures is known as the pulse pressure. (A diagram showing
the relation of these different pressures one to another will be found on
page 338).

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

to heart

FIG. 156. — 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.}

336 TEXT-BOOK OF PHYSIOLOGY.

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 of
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-
graph applied over the artery of man a record much different in appearance
and similar to that shown in Fig. 157 will be obtained. In such a record
it will be observed that the return of the pressure from the systolic to the
diastolic level not only occupies a longer time than the passage from the
diastolic to the systolic level, but that the line of descent is interrupted by a
secondary rise and fall of pressure before the original diastolic level is
reached. It is evident, therefore, that the pressure is low for a longer period
than 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 man-
ometer and fluctuating with each heart-beat, will rise slowly to a certain level
and then remain at rest. The number of millimeters of mercury thus
supported represents the mean or absolute pressure. The same result can be
obtained by employing the compensatory manometer of Marey which
presents a constriction of this character. From many experiments made by
Dawson it has been learned that the mean pressure lies nearer to the diastolic
than to the systolic pressure and may be expressed numerically by the state-
ment 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 undu-
lations are present the mean pressure can be calculated. The method by
which this is done, however, is rather complicated and need not be detailed
here. In a general way the mean pressure in such a tracing may be repre-
sented by a line drawn horizontally across the tracing midway between the
apex and trough of the undulation.

Estimates of the Mean Arterial Pressure. — Because of the difficulty
in obtaining the pressure in small arteries, the experimental determinations
have for the most part been confined to large arteries 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 to 214 mm. Hg.; in the dog, from 140 to 160 mm.; in the cat, 150 mm.;

THE CIRCULATION OF THE BLOOD.

337

in the rabbit, from 90 to 100 mm.; in the sheep, 170 mm.; in the calf, from
133 to 165 mm. In two observations made on human beings previous
to the amputation of a limb, the pressure was found in the brachial
artery of one patient to vary from no mm. to 120 mm. Hg., and in the
anterior tibial artery of the other patient from no mm. to 160 mm. Hg.

The investigations made in different parts of the arterial system indicate
that the mean pressure is remarkably constant and uniform and does not
show any noticeable falling off until near the arteriole region where the 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

12 I

TQ7

60

Right carotid. ...

1 60

118

iuo
no

en

Left carotid

1 60

123

IOI

y

Cn

Left subclavian

168

123

IO?

6*

Left brachial

1 60

118

no

"O

CQ

Left renal

i6<;

123

IO3

62

Deep femoral

1^2

118

1 02

CO

Thyroid

I4O

118

07

42

The Capillary Pressure. — The small size of the capillaries precludes
an investigation of their pressure by manometric methods. It may be stated,
however, to be approximately equal to the pressure required to obliterate
their lumina and to whiten the skin. The apparatus of v. Kries is based on
this theory. A small glass plate, from 2.5 to 5 sq. mm., is fastened to the
under surface of a support of suitable size carrying a small scale pan. The
glass plate is placed on the skin near the root of a finger-nail and the scale
pan gradually weighted until the vessels are obliterated, as shown by the
blanching of the skin. From results obtained with this apparatus v. Kries
estimated the pressure in the capillaries of the hand at 37 mm. Hg. and in the
ear at 20 mm.

The Venous Pressure. — In passing from the capillaries to the heart
the pressure continues to fall. The increasing size of the veins permits
again of manometric observations in different regions. In the crural vein

338

TEXT-BOOK OF PHYSIOLOGY.

the pressure has been found to be equal to 14 mm. Hg., and in the brachial
vein 9 mm. of Hg. In the jugular and subclavian and other vessels near
the heart it is zero or even negative; that is, less than atmospheric pressure
to the extent of from 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. 157.

L ine of
SYSTOLIC PRESS UP£

Lm<t of
ME AH PRESSUPE

Lin* of

D/ASTOLIC
Pressure

PULSE

T^e difference between
SYSTOLIC

O/ASTOL/C PPE3SVP£

FIG. 157.— -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, Bated OD

experiments made on dog*. H. Heart. A. Arteries. C. Capillaries. V. Large veins. O, O,
being the zero line ( — atmospheric pressure), the pressure is indicated by the height of the curve.
The numbers on the left give the pressure (approximately) in millimeters of mercury, h. Pressure
in heart, a. Arteriole region showing sudden fall of pressure, c. The fall of pressure in the
capillaries, v. The negative pressure in the large veins.

RESUME OF THE FACTS OF THE BLOOD-PRESSURE AND OF THK
FACTORS WHICH CAUSE IT.

I From a consideration of the foregoing facts and statements the folio
r&ume* may be made: i. The blood during its flow exerts a pres
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 f riaion of the
molecules of the blood, 4. The resistance is inversely proportional to the
diameter of the vessel and is therefore least in the large arteries ami vein ami
greatest in the arterioles and capillaries, 5, The pressure i highest in the
aorta where it may amount in man to 150 mm. of m< hove that of the

atmosphere, and lowest at the ends of the venae cavae where it may be no
greater than that of the atmosphere or may be even 10 mm. \\\'i. below it.
6. The pressure falls from the befnnnin^ to the eml of the vascular apparat
though not progressively, for throughout the large vessels of the Arterial
System it Continues relatively hiVh. 7. The hi;/h pressure in the aorta is Hue
to the total resistance of the vaseul-ir ;ipp;ir;itu nno1 the pressure al ;my

THE CIRCULATION OF THE BLOOD. 339

point of the apparatus represents the resistance yet to be overcome. 8. The
high pressure in the arterial system and its marked fall at its periphery is
more especially the result of the very great resistance at this point, known
as the peripheral resistance, the result of a rapid diminution in the diameter
of the arterioles and the capillary vessels, modified by the tonic contraction
of the arteriole muscles. 9. The pressure in the arterial system undergoes
considerable variation both above and below the mean pressure during the
e 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

.rv in either direction, the result of an increase or a decrease in the

^e 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

The Elasticity of the Vessel Walls.— A tertiary factor is the elasticity
of the arterial wall. While it can hardly be said that the elasticity is a cause
of the pressure, there can be attributed to it the capability of modifying
and assisting in the maintenance of the pressure at a more or less constant
were it not for this property of the vessel wall the variations in
pressure during and after the systole would be far more ex: :han they

are, and would approximate the variations observed in tubes with rigid
walls. The elasticity, moreover, assists in the equalization of the blood-
stream, convening the intermittent and remittent flow charac of the

340 TEXT-BOOK OF PHYSIOLOGY.

large arteries into the continuous equable stream characteristic of the capil-
laries. It also permits of wide variations in the amount of blood the arteries
can contain between their minimum and maximum distention.

VARIATIONS IN THE BLOOD -PRESSURE.

A. In the Arterial Pressure. — It is evident from the preceding 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 that it may be brought back to the normal by a decrease in either one
or both of these factors.

It is also evident that the arterial blood-pressure may be decreased below
the normal by:

1. A decrease in the rate and force of the heart's contraction.

2. A decrease in the peripheral resistance.

3. A decrease in both the force of the heart and the peripheral resistance
and that it may be raised to the normal by an increase in either one or both
of these factors.

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.

An increase in the pressure is readily brought about by an increase in the
force or power of the contraction, the frequency remaining the same. An
increase in the volume of blood ejected at each contraction will necessarily
lead to an accumulation. With the accumulation there goes an increased
distention of the artery and a corresponding increase of pressure. In a
short time, therefore, the increased pressure will force out of the arteries
at a higher rate of speed this excess of blood until the outflow again equals the
inflow. This restores the equilibrium but establishes the mean pressure at a
higher level.

If the peripheral resistance is increased by a contraction of the muscle
walls of the arterioles, the frequency and force of the heart remaining the
same, there will also be an accumulation of blood in the arteries, an increased
distention and consequent rise of pressure (Fig. 158). The outflow of
blood will at the same time be diminished. A rise of pressure from this cause
much beyond the normal is to a large extent prevented by a simultaneous

THE CIRCULATION OF THE BLOOD.

341

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 stimu-
lation of the cardio-inhibitor center, and not to a direct action on the heart-

FIG. 158. — A TRACING SHOWING AN INCREASE IN THE BLOOD-PRESSURE IN THE CAROTID
ARTERY OF A RABBIT DUE TO AN INCREASE IN THE PERIPHERAL RESISTANCE FROM A CONTRACTION
OF THE ARTERIOLES CAUSED BY REFLEX STIMULATION OF THE VASO-MOTOR CENTER. The nerve
stimulated was the sciatic. Stimulation began at s. The rate of the heart-beat is unchanged.
With the cessation of the stimulation the blood-pressure falls for the reverse reasons.

muscle, inasmuch as the effect is not observed after division of the vagi.
When both the force of the heart and the peripheral resistance are simul-
taneously increased there is a rapid increase in pressure; the former factor
tends to increase, the latter factor, to
decrease, the velocity of the outflow.
According as the one or the other pre-
ponderates, will there be an increase
or decrease in velocity. If they balance
each other, there will be no change. A
rise of pressure from a 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. If when the general arterial
pressure is in a condition of equilibrium
the heart ejects into the arteries in a given
period of time a 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 (Fig. 159).
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.

If the peripheral resistance is1 dimin-
ished by a dilatation of the arterioles, the
heart's contractions remaining the same,
the existing pressure soon diminishes,
increases.

FIG. 159. — A TRACING OF THE BLOOD-
PRESSURE IN THE CAROTID ARTERY OF A
RABBIT, showing a sudden decrease in
the pressure due to an arrest in the rate
and force of the heart-beat the result of
stimulating the vagus nerve from " on "
to "off." With the cessation of the stimu-
lation the pressure began to rise as the
rate and the force of the heart-beat re-
turned. (The abscissa should be 20 mm.
lower.)

The outflow of blood "at once

342 TEXT-BOOK OF PHYSIOLOGY.

As a rule a diminution in peripheral resistance is attended by an increase
in the rate or force of the heart, and this is especially the case if the pressure
has been above the normal.

When both the force of the heart and the peripheral resistance are simul-
taneously diminished, there will be a rapid fall in pressure. The former
factor tends to decrease, the latter factor to increase the velocity of outflow.
According as the one or the other preponderates will there be a decrease or
an increase in velocity. If they balance each other there will be no change.
This condition is also a pathologic 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 in
pressure and velocity from variations either in the activity of the heart or in
the peripheral resistance recorded in preceding paragraphs, have reference to
the arterial system in its entirety; but it is evident from many facts that
similar variations take place in special regions or organs of the body. Thus,
it is a well-known fact that for the exhibition of the functional activity of
every organ there must be an increase in the volume of blood supplied to it
in each unit of time. This is accomplished by an active dilatation of the
arterioles of the artery of supply, and unless the area or organ supplied is
large, as the splanchnic area for example, there will be no necessary diminu-
tion in either the general blood-pressure or the average velocity. With the
cessation of functional activity, there is no longer any need for so large a
blood-supply and hence the arterioles contract, diminish the outflow, and raise
the pressure. If, on the other hand, the area to be supplied be large, as the
splanchnic area, the dilatation of the intestinal arteries will be attended by
such a large inflow of blood that not only will there be a fall of pressure in
these vessels, but a fall of pressure in other arteries as well, combined with a
diminution in velocity through them. With the contraction of the intestinal
arteries the reverse conditions at once arise. By constant variations in the
peripheral resistance of individual arteries in each and every region of the
body, and in association with variations in the rate or force of the heart, the
blood is shunted now into this, now into that organ in 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.

THE CIRCULATION OF THE BLOOD. 343

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
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 physi-
ologic 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. These instruments are termed 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 sys-
tolic 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 of
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

344 TEXT-BOOK OF PHYSIOLOGY.

an elastic pressure applied to the outside of an artery is just sufficient to
equalize the diastolic pressure within. Inasmuch as these pulsations can
be transmitted to, taken up and reproduced by a mercurial column in connec-
tion with the pressure appliances, it becomes possible, when the maximum
oscillation of the mercurial column is attained, to read off the diastolic
pressure.

With either form of apparatus it becomes necessary to devise a suitable
elastic sac or tube enclosed by non-elastic or rigid walls and capable of
being made to encircle a finger or an arm, which can in turn be connected
with a pressure apparatus, and with a manometer by which any given
pressure can be registered.

One of the best known of the sphygmomanometers is that of Mosso
represented in Fig. 160. It consists essentially of rubber capsules, which are
contained within metallic tubes and into which two fingers of each hand can
be inserted. This system is connected, on the one hand, with a pressure
apparatus, and, on the other, with a manometer provided with a scale. A

FIG. 160. — THE SPHYGMOMANOMETER OF Mosso.

float and writing-pen record the movements of the mercurial column on a
moving blackened surface. In using this apparatus the pressure is adjusted
to the point at which the mercurial column exhibits the greatest oscillations.

Mosso's interpretation of the results obtained with the apparatus was
that when the greatest oscillations of the mercurial column were taking place,
the external pressure was just equal to the mean arterial pressure, the latter
being the mean between the maximum pressure during the systole and the
minimum pressure during the diastole of the heart. It was necessary, there-
fore, only to take the readings corresponding to the excursions of the mer-
curial column and to determine from them the mean arterial pressure.

It has been experimentally demonstrated, however, by Howell and Brush
that this interpretation, either for this or any similar form of apparatus, is
not correct, but that the maximum oscillations take place when the pressure
applied to the exterior of the artery is just equal to the pressure within the

THE CIRCULATION OF THE BLOOD.

345

artery at the end of the cardiac diastole; or in other words, the pressure in
the manometer from which the greatest oscillation takes place indicates
diastolic pressure. These experimenters connected the right carotid artery
of a dog with a mercurial manometer, interposing along the course of the
connecting 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 purpose of magnifying and recording the pulsation. When
the observations were simultaneously made it was found that the diastolic
pressure in the right carotid measured by the minimum manometer was

FIG. 161. — STANTON'S SPHYGMOMANOMETER.

almost exactly equal to the pressure measured by the manometer in connec-
tion with the sphygmomanometer 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. It was also shown by the same investigators that Mosso's appara-
tus is not adapted for obtaining systolic pressure. *

Among the many forms of sphygmomanometers adapted for clinic pur-
poses and with which both systolic and diastolic pressures may be obtained
is that devised by Stanton1 (Fig. 161). The pressure is applied to the arm by

1 The following description of this apparatus is abstracted from the Univ. of Pa. Medical
Bulletin, Feb., 1903.

346 TEXT-BOOK OF PHYSIOLOGY.

the rubber armlet H, which is 3! inches wide. This is the widest armlet
that can be adjusted to the average-sized arm and presents distinct advan-
tages over the narrow armlet hitherto employed. This armlet is prevented
from expanding outward by a cuff, F, of double thick canvas with inserted
strips of tin, which is held in place by two straps which completely encircle
the cuff. On the rigidity of this depends to a large extent the transmission
of pulsation. The rubber armlet is connected by glass with a stiff-walled
rubber tube, G, which in turn connects with the manometer. The man-
ometer is perhaps the most important part of the apparatus. It is constructed
entirely of metal except for the glass tube containing the mercury column.
The chamber c communicates by means of a metal tube with the glass
column D, which is connected by a screw-thread at 3, the caliber of c being
approximately 100 times that of D. The cap of the chamber, which screws
on, is provided with a metal T which is connected at 2 with the rubber armlet
and at i with the bulb, used as an air-pump. At A is a stopcock shutting the
rubber bulb completely from the rest of the apparatus while at B is a screw-
valve which allows the air to escape from the closed system. When desired,
the manometer can be made portable (without removing the mercury) by
screwing the caps i and 2 into either end of the T at i and 2. The man-
ometer is then tilted away from the glass column D until all the mercury has
run into the chamber, the glass is then unscrewed and cap 3 screwed in.
Before removing cap 3 the manometer must always be tilted, else the mercury
will be lost. The rubber bulb is similar to those found on atomizers, with
the addition of a distensible reservoir to obliterate the air pulse.

In using this apparatus the pressure is raised by the air-bulb forcing air
into the closed system — distending the rubber armlet and with the same
degree of force displacing the mercury in c, driving it up the glass column D.
When the pulse is no longer felt, the bulb still being compressed, the arm of
valve A is turned until it is at right angles with the thumb and finger. The
valve B is now slowly unscrewed until the mercury column begins to fall. At
a given level it exhibits a considerable 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 pressure 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 maximal value and
beyond which the oscillations again diminish. The lowest level of the
mercury column at the time of the greatest oscillation is taken as the diastolic
pressure.

Erlanger's sphygmomanometer is, also, a most valuable instrument for
obtaining both systolic and diastolic pressure. It possesses an advantage
in that it is provided, in addition to the mercurial manometer, with a tam-
bour and lever by which changes in pressure can also be recorded on a re-
volving cylinder (Fig. 162). A complete description of this apparatus,

THE CIRCULATION OF THE BLOOD.

347

the manner of using it and the results that can be obtained with it will be
found in the Johns Hopkins Hospital Reports, Vol. XII.

With this apparatus, the lever often exhibits a considerable oscillation
even when the pressure exerted on the arm exceeds the systolic pressure.

COPYRIGHT 1906, BY SCHNEIDER BROS., N.Y.

FIG. 162.— ERLANGER'S SPHYGMOMANOMETER.

It is difficult therefore to determine at times the moment at which the pres-
sure indicated by the mercurial column just falls below the systolic pressure
and allows the blood to pass through. A new criterion for this determina-
tion 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 ijto 2 mm. of smoked

348 TEXT-BOOK OF PHYSIOLOGY.

paper (this speed is attained merely by removing the governor), the change
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. 163). 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. 163.— TRACING SHOWING THE minimum lateral pressure of the artery
POINT, INDICATED BY THE ARROW, AT ronrnreccPrj o^rl therefore a<; the mini
WHICH THE SYSTOLIC PRESSURE is TO BE 1Q> l re> as

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 physi-
ological 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,

THE CIRCULATION OF THE BLOOD. 349

1911). It consists in the interpretation of certain sounds heard with the
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 manom-
eter. 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 first
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 85 millimeters. 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 therapeutic value.

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

350 TEXT-BOOK OF PHYSIOLOGY.

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 per-
ipheral 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 indif-
ferent parts of the vascular apparatus. In the arteries, moreover, the flow
is not quite uniform, but experiences alternate acceleration and retarda-
tion with each heart-beat. In the capillaries and veins the flow is contin-
uous 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 wilt 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.

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

THE CIRCULATION OF THE BLOOD.

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. 164) 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. 165)
is constructed on the same principle, but instead of
the glass tube having the same diameter it is

FIG. 164. — VOLKMANN'S HEMODROMOM-
ETER. C, C. Arterial cannulas.

FIG. 165. — LUDWIG AND
DOGIEL'S RHEOMETER. X, Y.
Axis of rotation. A, B. Glass
bulbs, h, k. Cannulas inserted
in the divided artery, e, elt
rotates on g, /. c, d. Tubes.

considerably enlarged on its two sides. The bulbs are fastened to a metallic
disk which rotates around an axis in the metallic base which carries the tubes
to be inserted into the arteries. With this device it is possible to place either
bulb in connection with the proximal end of the artery. Previous to the

352

TEXT-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. 1 66). This consists of a me-
tallic tube carrying a graduated
disk. 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 grad-
uated 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 a manom- beginning of the diastole 22O mm.

per second, and during the pause
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.

FIG. 1 66. — THE HEMATACHOMETER OF CHAU-
VEAU AND LORTET. A, B. Tube inserted in

eter. b. Index moving
brane, a. G. Handle.

in a caoutchouc mem-

THE CIRCULATION OF THE BLOOD.

353

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 cavae 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 cavae
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. 167.

Arteries. Capillaries.
FIG. 167. , Blood-pressure. , Velocity.

Veins.
>, Sectional area.

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 in
mind that it is rather the difference in pressure between the beginning and
the termination of the arterial system, rather than the mean pressure that
influences the velocity. Thus, with a given force of the heart and a given
peripheral resistance, the velocity will have a given value, and so long as
these factors remain constant will the velocity remain constant, even though
the mean pressure should fall, as from a hemorrhage, or should rise, as from
an injection of some indifferent fluid.

If, however, the primary factors, viz., the cardiac force or the peripheral
resistance, change their values in either the same or opposite directions,
there will be a change at once in the velocity. The 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 resistance
or both, are shown in the following table arranged by Waller. The plus
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 Lortet
by experiments on animals with the hemodromograph, a specially devised
apparatus for this purpose.
23

354

TEXT-BOOK OF PHYSIOLOGY.

No.

Heart

Arterioles Blood-pressure

Blood-flow

i

2

f Force constant
\ Force constant

Resistance increased . . .
Resistance diminished .

+

+

3
4

f Force increased ....
\ Force diminished . . .

Resistance constant
Resistance constant. . . .

1

+

I

f Force increased
\ Force diminished. . . .

Resistance diminished. .
Resistance increased. . . .

+ -
- +

+ +

8

( Force increased
\ Force diminished . . .

-
Resistance increased . . .
Resistance diminished.

+ +

+ -
- +

Though all the relations between pressure and velocity in the table are
possible, those which are most physiological are probably 5 and 6, for in both
instances there is a minimum alteration in pressure, but a maximum altera-
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.

THE PULSE.

The Arterial Pulse. — The pulse may be defined 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.
W ith 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
p assing over the arteries to the capillaries.

The mode of development as well as the propagation of the expansion
an d recoil movement of the arterial wall, which together constitute the pulse,
are illustrated in Fig. 168 in which A B represent the arter y subdivided into

THE CIRCULATION OF THE BLOOD.

355

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
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

FIG. 168. — DIAGRAM SHOWING THE DEVELOPMENT OF A PULSE WAVE. (Rollet.)

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 2 c. 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.

The force of the heart now begins to decline and from this moment on, the
elastic force of the artery preponderates and in consequence the arterial wall
begins to recoil at the point a. At the end of the fourth sixth of the systole,
therefore, the arterial wall at a, has recoiled to 2, while the expansion at a
has advanced to b 3 where the force of the heart and the elastic force of the
artery are equal. At this moment the condition of the artery may be repre-
sented by the lines 2, 63, e. During the two remaining sixths of the cardiac
systole, the same process continues until, through elastic recoil, the artery
has returned to its original condition at a, and the expansion has extended
as far as g, while the height of the expansion has advanced to ^3 where the
force of the systole and the force of the elastic recoil balance each other. At
the end of the systole the condition of the arterial wall may be represented
by the lines a, d$, g, which indicates that the expansion and recoil of the
artery, which together constitute the pulse, partake of the form of a wave the
length of which is represented by the line o, 0, and the height by the distance ^3.

This expansion and recoil which thus pass from the beginning to the
end of the arterial system assumes the form of a wave and therefore is known
as the pulse-wave or pulse. Preceding and causing the expansion and recoil
of the arterial system there is an alternate increase and decrease of the gen-

356 TEXT-BOOK OF PHYSIOLOGY.

eral blood-pressure, as shown by the small curves on a blood-pressure tracing,
and for this reason the pressure which causes the expansion and recoil is
termed the pulse pressure. It is denned as the rhythmic change in pressure
at any given point of the arterial system; and in amount, is the difference
between the diastolic and the systolic 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, 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 con-
ditions, various terms more or less expressive, have been suggested for its
varying peculiarities. 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; quick or slow, according 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 CIRCULATION OF THE BLOOD.

357

Frequency of the Pulse. — As the pulse or the arterial expansion and
recoil is the direct result of the heart's action, its frequency must, under
physiologic conditions, coincide with that of the heart. All conditions which
modify the rate of the heart will modify at the same time the rate of the pulse.

The Sphygmograph. — The sphygmograph is 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 following each

FIG. 169. — VON FREY'S SPHYGMOGRAPH. G. S. Metal framework. P. Button attached to spring.
F. Vertical rod. U. Clock-work which turns the recording cylinder. VI. Time marker.

heart-beat. The tracing or record obtained with it is termed the pulse-
curve or the sphygmogram. Different forms of this apparatus have been
devised by Marey, Dudgeon, v. Frey, and many others. The instrument
of v. Frey is shown in Fig. 169. This consists first of a metal framework
GS by which the apparatus is fastened to the arm and support given to the
lever, recording surface, etc. The essential part is the spring carrying a but-
ton P, which is placed over the artery, usually the radial , before it crosses the
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 OR SPHYGMOGRAM OF THE RADIAL
the radial. This artery lies quite superficially, ARTERY-
is covered only by connective tissue and skin and is supported by the flat
surface of the radial bone, conditions most favorable to technical investiga-
tion. An average tracing taken from the radial artery is shown in Fig. 170.
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:

FIG. 170. — THE PULSE-CURVE

358 TEXT-BOOK OF PHYSIOLOGY.

viz., a line of ascent from a to b, and a line of descent from b to d (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, due to the recoil of the arterial walls, occupies
a longer period of time, and is marked by several elevations and depressions,
both of which indicate that the restoration to equilibrium is neither immediate
nor uncomplicated. One of these elevations is quite constant and known as
the dicrotic wave, c; the depression or notch just preceding it is known as the
dicrotic notch. Pre- and post-dicrotic waves are not infrequently present.
The summit is generally sharp and pointed.

The vertical direction of the line of ascent is taken as an indication that
the arterial walls expand readily, that the blood is discharged quickly, and
that the ventricular action is not impeded. An oblique direction of the
line of ascent is an indication that the reverse 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
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.

THE CIRCULATION OF THE BLOOD. 359

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. 171, 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. 171. — SIMULTANEOUS TRACINGS OF THE JUGULAR PULSE, THE CAROTID PULSE, AND THE
APEX BEAT. — (Bachmann.} At the bottom of the tracing the time is given in the fiftieths of a
second. The vertical lines o, i, 2, 3, etc., mark synchronous points on the curves. A, The
auricular wave; s, the so-called c wave caused by the systole of the ventricle; v, the stagnation wave
caused by the filling of the auricle. It will be noticed that the c wave (marked 5 in the tracing)
occurs at the beginning of the ventricular systole as marked on the apex beat, and shortly before
the pulse in the carotid artery. The height of the v wave is reached just after the occurrence of
the dicrotic notch on the carotid wave, and coincides with the opening of the auriculoventricular
valves; Af, the negative wave caused by the effect of the ventricular systole; Vf, the negative wave
following the opening of the auriculoventricular valves.

As the venous pulse is a very evident symptom in some pathologic condi-
tions of the heart, and as its proper interpretation assists in the diagnosis of
these conditions, it has become of much significance in modern clinical medi-
cine. For purposes of interpretation it is desirable to obtain simultaneously
graphic records not only of the venous pulse, but of the carotid or radial
pulse, and of the cardiac impulse as well. In the accompanying figure 171
these three records are represented.

The generally accepted interpretation of these waves is as follows :
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

360 TEXT-BOOK OF PHYSIOLOGY.

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 282), 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 arteries,
and as a result the volume of any organ or part of the body must undergo
similar changes. To such alternate changes in volume the term volume
pulse is given. The extent to which an organ will increase in volume will
depend to some extent on its elasticity. The reason for the increase in
volume is the resistance offered to the flow of blood into and through the
capillaries; the decrease in volume to the overcoming of the resistance through
the arterial recoil.

The variations in volume may be recorded by enclosing the organ in a
rigid glass or metal vessel, which at one point is in communication with a
recording apparatus, e.g., a tambour with a lever or a piston recorder with
float and writing point. The space between the organ and vessel is filled

THE CIRCULATION OF THE BLOOD.

361

with normal saline, air, or oil. Such an apparatus is known as a plethysmo-
graph. Fig. 172. Many forms of this apparatus have been devised in
accordance with the character of the organ — spleen, kidney, etc. — to be
investigated, though the principle underlying them is 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

FIG. 172. — A PLETHYSMOGRAPH.

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 arteriole
contraction is diminished by a direct or reflex inhibition of the vaso-motor
center there will be a fall of pressure, an increased outflow of blood and an
increase in the volume of the organ. From this it is learned that the 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-
mographic 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
of blood through the smaller arteries, capillaries, and veins, but many of
the phenomena connected with it, to which reference has already been made.

362

TEXT-BOOK OF PHYSIOLOGY.

The structures usually selected for the observation of these phenomena are
the interdigital membranes (Fig. 173), the tongue, the lung, the bladder,
and the mesentery of the frog. Though any one of these structures will
afford an admirable view of the blood-flow, the mesentery for many reasons
is the most satisfactory. For a comparison of the phenomena observed in
the cold-blooded animals with those in the warm-blooded animals the omen-
tum 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
be found in which the blood is flowing
in opposite directions. The vessel ap-
parently carrying blood away from the
observer is an artery; the vessel appar-
ently carrying blood toward the obser-
ver 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
friction between successive layers of the blood-stream, the resistance of the

Fro. 173. — THE VESSELS OF THE FROG'S
WEB. a. Trunk of vein, and (b, V) its
tributaries passing across the capillary net-
work. The dark spots are pigment cells. —
( Yeo's ' ' Physiology. ' ')

THE CIRCULATION OF THE BLOOD.

363

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. 174. 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
process.

The Venous Circulation. — The blood, having passed through the
capillary vessels, is gathered up by the veins and conveyed to the right side
of the heart. As the veins converge and unite to form larger and larger
trunks the sectional area gradually diminishes, and hence the velocity of the
blood-flow increases, though it never attains the velocity, even in the venae
cavae, that it had in the aorta, for the reason that the sectional area of the
venae cavae is considerably larger than that of the aorta. The pressure also
is very low in the larger veins because the friction still to be overcome is
relatively very slight.

FIG. 174. — DIAGRAM TO
SHOW VARIOUS STAGES IN
THE DIAPEDESIS OR MI-
GRATION OF WHITE COR-
PUSCLES.

364 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 apparatus consists of a closed system of
vessels extending from the right ventricle to the left auricle, and includes
the pulmonary artery, capillaries, and pulmonary veins. In its anatomic
structure and physiologic properties it closely resembles, with, the systemic
apparatus.

The stream-bed widens from the beginning of the pulmonary artery to
the middle of the capillary system; it again narrows from this point to the
terminations of the pulmonary veins.

The movement of the blood from the beginning to the end of the system
is due to a difference of pressure between these two points, the result of the
friction between the blood and the vascular walls. The pressure in the pul-
monary 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 rang-
ing from no to 112 mm. of mercury, the maximal or systolic pressure in
the pulmonary 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 pulmonary capillaries and the
imperfect development of an arteriole muscle at the periphery of the-
pulmonary 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 pulmonary 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
can not well be determined. The time occupied by a particle of blood in
passing from the right to the left ventricle has been estimated at one-
fourth the time required to pass from the left to the right ventricle. As-
suming the latter to be thirty seconds, the former would be seven and
one-half seconds.

The capillary vessels are spread out in a very elaborate manner just
beneath the inner surface of the pulmonary air-cells, and form, by their
close relation to it, a mechanism for the excretion of carbon dioxid and the
absorption of oxygen. The extent of the capillary surface is very great.
It has been estimated at 200 square meters. The amount of blood flowing

THE CIRCULATION OF THE BLOOD. 365

through this system hourly and exposed to the respiratory surface is about
800 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 prox-
imal side will at once be driven forward with increased velocity, while
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 and force the blood forward.

3. Thoracic Aspiration. — The inspiratory movement aids the flow of

blood through the venae cavae and their tributaries. With each 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 in the thorax,
flows more rapidly toward the heart. With each expiration the re-
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 largely prevented by
the valves, for each retardation is immediately checked by their closure
and support given to the column of blood. The influence of gravity

366 TEXT-BOOK OF PHYSIOLOGY.

is shown when the relation of the arm to the heart is changed. Thus,
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 the 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.,
0.083X1.93 = 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 1083
grams is estimated by squaring the velocity and dividing by the accelerating
force of gravity (2^&I) 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 310-311.

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

THE CIRCULATION OF THE BLOOD. 367

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 continuous
contraction, more or less pronounced, and give to the arteries a certain
average caliber which permits a definite volume of blood to flow through
them in a given unit of time. 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
decrease, 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 vaso-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-motor nerves of both classes, unlike the ordinary motor nerves,
do not pass directly to the muscle-fiber, but indirectly by way of the ganglia
of the sympathetic nerve system. In these ganglia the vaso-motor nerves,
which come from the central nerve system, terminate, breaking up into
tufts, which arborize around the nerve-celts. From the cells of these gang-
lia new nerve-fibers arise which then pass without interruption to their
final destination.

The nerve-fibers which emerge from the central nerve system are ex-
tremely fine in caliber and medullated; those which emerge from the sym-
pathetic ganglia are equally fine, but non-medullated. The former are termed
pre-ganglionic or autonomic, the latter post-ganglionic or sympathetic fibers.
The ganglion in which the pre-ganglionic fibers end is not necessarily found in
the pre- vertebral or lateral chain; it may be found in the collateral or even in
the peripheral group of ganglia. (See sympathetic or autonomic nerve system.}

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 or third
lumbar nerves inclusive. A short distance from the cord they leave the ventral
root as the white rami communicantes and enter the pre-vertebral or lateral
sympathetic ganglia. From the results of many observations and experi-
ments 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 the

368 TEXT-BOOK OF PHYSIOLOGY.

skin of the head and face, to the blood-vessels of the skin of the upper and
lower extremities and trunk and to the blood-vessels of the thoracic and
abdominal viscera.

The vaso-constrictors for the head and face emerge from the spinal
cord in the first four thoracic nerves, thence pass successively by way of the
white rami communicantes into and through the ganglion stellatum (the
first thoracic), the annulus of Vieussens, the inferior cervical ganglion, the
sympathetic cord to the superior cervical ganglion, around the cells of which
they arborize. From this ganglion the new fibers follow the carotid artery
and its branches to their terminations.

The vaso-constrictors for the fore-limbs emerge from the cord in the roots
of the fourth to the tenth thoracic nerves inclusive. Through the white
rami they pass into the sympathetic chain, after which they take an upward
direction and terminate around the cells of the ganglion stellatum. From
this ganglion the new fibers enter, by way of the gray rami communicantes,
the trunks of the cervical nerves which unite to form the brachial plexus
and by this route pass to the blood-vessels of the skin and possibly of the
muscles of the fore-limb.

The vaso-constrictors for the hind-limbs emerge from the cord in the
roots of the eleventh dorsal to the second or third lumbar nerves inclusive.
They then pass through the white rami to the lower lumbar and upper
sacral ganglia. Thence by way of the gray rami they pass into the nerve-
trunks which unite to form the sacral nerves and by this route pass to the
blood-vessels of the skin and possibly the muscles of the hind limb.

The vaso-constrictors for the walls of the trunk of the body emerge
from the spinal cord between the second thoracic and third lumbar nerves.
They then pass by way of the white rami into the thoracic and lumbar
ganglia around the cells of which they arborize. From these ganglia new
fibers arise which pass by way of the gray rami into the thoracic and lum-
bar nerves and directly to the blood-vessels of the skin.

The vaso-constrictors for the viscera of the abdominal cavity after
emerging from the spinal cord pass by way of the branches that unite to
form the splanchnic nerves directly into the collateral ganglia, the semilunar,
the superior mesenteric, the inferior mesenteric, and the sacral. From
these ganglia an elaborate network of non-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 account of the large vascular area it controls.

The existence, course, distribution, and functions of the vaso-constrictor
nerves have been determined by a variety of methods, physiologic and anat-
omic. 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 intra-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 CIRCULATION OF THE BLOOD. 369

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 their
origin for the most part as generally believed in nerve-cells situated in
the region of the spinal cord included between the origins of the second
dorsal to the second lumbar nerves inclusive, though they are not confined
to this region. Some vaso-dilatator fibers have their origin in the medulla
oblongata, others in the sacral region of the spinal cord.

Vaso-dilatator fibers have been found in association with vaso-constrictor
fibers in most of the nerve trunks of the body though they are perhaps not
so abundant. Thus the results of experimentation indicate that they are
present in the cervical portion of the sympathetic in the nerve trunks of
the upper and lower limbs, in the nerve trunks of the body walls, and in the
splanchnics. The exact path, however, through which they pass from the
spinal cord to the peripheral nerve trunks has not in all cases been positively
determined. The cell stations also in which the pre-ganglionic fibers
terminate have not in all cases been located. There are reasons for the
opinion, however, that they follow somewhat the same paths as the vaso-
constrictor fibers.

The vaso-dilatator fibers that arise in the medulla oblongata 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 ulti-
mately distributed as pre-ganglionic fibers to the spheno-palatine and sub-
maxillary and sublingual ganglia respectively. 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 submaxillary and sublingual ganglia post-ganglionic fibers pass to
the blood-vessels of the submaxillary 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 the
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.

The vaso-dilatators that emerge from the sacral region of the spinal
cord, pass by way of the second and third sacral nerves as preganglionic
fibers to ganglia situated near the blood-vessels supplying in both sexes,
the organs of generation and adjacent structures. From the ganglia post-
ganglionic fibers are distributed to the walls of these blood-vessels.

The existence, course, distribution, and functions of the vaso-dilatator
fibers have been determined by the same methods employed as in the investi-
gation of the vaso-constrictors. Thus division and appropriate stimulation
of the peripheral end of the cervical sympathetic will be followed by a con-
24

370 TEXT-BOOK OF PHYSIOLOGY.

gestive dilatation of the blood-vessels of the upper and lower lips, gums,
cheeks, nasal mucous membrane, and corresponding cutaneous regions.
Stimulation of peripheral nerve trunks, providing the stimulation is not too
rapid will be followed by a dilatation of the blood-vessels and an increase in the
volume of the limb. Stimulation of the peripheral end of the divided
chorda tympani nerve is at once followed by an active dilatation of the
blood-vessels of the submaxillary gland. The inflow of blood is so great that
the gland becomes bright red in color. Its tissues being unable to appropriate
all the oxygen, the blood emerges in the veins almost arterial in character.
Stimulation of the peripheral ends of the divided nervi erigentes is followed
by similar effects in the blood-vessels of the corpora cavernosa. Slow stimu-
lation, once per second, of the peripheral end of a divided sciatic nerve is
followed by dilatation of the blood-vessels of the leg.

From these and many other facts of a similar character it is probable
that the blood-vessels of each organ are under the control of two antagonistic
classes of nerve-fibers, one augmenting the degree of their contraction,
the vaso-constrictors, the other diminishing it through inhibition, the
vaso-inhibitors. Through the cooperative antagonism of these two classes
of nerves the caliber of the blood-vessels and thereby the volume of the
blood is accurately adapted to the needs of each organ both during rest
and during activity. It is also to the alternate activity of these nerves that
the variations occurring from time to time in the volume of organs are to be
attributed.

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.

Antidromic Vaso-dilatator Nerve-fibers. Though it has been generally be-
lieved that the vaso-dilatator nerves for the blood vessels of the limbs and trunk
arise from nerve cells in the ventral horns of the grey matter; that they pass out-
ward 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 ex-
periments this investigator concludes that special vaso-dilatator nerves for the
regions of the body just mentioned, do not leave the spinal cord in the ventral
roots; that the vase-dilatation 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 transmit nerve impulses to blood vessels in a direction contrary to
that of other afferent nerve fibers, 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; extirpa-
tion 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 CIRCULATION OF THE BLOOD.

the abdorminal viscera which apparently receive vaso-dilatator nerve impulses
are supplied by nerves having the foregoing origin and action is a subject for
further investigation.

Physiologic Properties. — The vasoconstrictors and the vaso-dilatators
differ somewhat in their physiologic properties, as shown by the results
of experiment. Thus, when a mixed nerve, i.e., one containing both classes
of fibers — e.g., the sciatic — is stimulated with frequently repeated induced
currents, the constrictor effect is the more pronounced, the dilatator effect
being wanting or prevented; when stimulated with slowly repeated induced
currents, the dilatator effect is the more pronounced. These different effects
are strikingly shown in Fig. 175, A and B.

A B

FIG. 175. — PLETHYSMOGRAMS OF THE HIND- LEG OF THE CAT FOLLOWING STIMULATION OF
THE SCIATIC NERVE. In A the rate of stimulation was sixteen per second, in B one per second
for fifteen seconds.

In the experiment of which these tracings are the result the leg of a cat was
enclosed in a plethysmograph and the variations in volume due to dilatation
or contraction of the vessels, following stimulation of the sciatic nerve, were
recorded by means of tambour and lever on a slowly revolving cylinder. In
A the fall of the curve indicates a diminution of volume, from contraction of
blood-vessels following a rate of stimulation of the sciatic nerve of 16 per
second for fifteen seconds. In B the rise of the curve indicates an increase
in volume from dilatation of the vessels following a rate of stimulation of i
per second for fifteen seconds (Bowditch and Warren). With different rates
of stimulation somewhat different results are obtained.

After division of a mixed nerve the vaso-constrictors degenerate and lose
their influence over the blood-vessel in from four to five days, the vaso-
dilatators in from seven to ten days, as shown by the response to electrical
stimulation.

When a nerve is cooled, the vaso-constrictors lose their irritability before
the vaso-dilatators.

Vaso-motor Constrictor Centers. — The nerve-cells thoughout the
spinal cord from which the vaso-constrictor nerves take their origin may be
regarded 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

372 . TEXT-BOOK OF PHYSIOLOGY.

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 longitudin-
ally 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-
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 vase-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
impu ses 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, it is assumed
that the vaso-motor center is in a state of continuous tonic activity or
tonus, and as a result continuously discharging nerve impulses through vaso-

THE CIRCULATION OF THE BLOOD. 373

constrictor nerves to the blood-vessels. The causes of this activity or tonicity
have been difficult to formulate. In how far the activity of the center is
maintained by the chemic character of the blood and lymph by which it is
surrounded and in how far by a continuous inflow of nerve impulses
transmitted from all regions of the body is not readily determinable. The
following facts will show that both factors are probably involved.

Direct Stimulation of the Vaso-motor Centers. — The general vaso-
motor (constrictor) center at least is markedly influenced by the quantity and
quality of blood and lymph circulating around and through it. If the blood-
supply to the medulla and associated structures be diminished by compression
of the carotid arteries, the activity of the center is at once increased, as shown
by increased vascular contraction and a rise of pressure. Restoration of
the blood-supply is followed by a return of the center to its normal degree of
activity. Increased blood-supply, as in cerebral hyperemia, is attended by a
fall in blood-pressure indicating a decrease in the activity of the center. A
diminution in the percentage of oxygen or an increase in the percentage
of CO 2 in the blood will increase the activity of the center. In asphyxia
especially, the center is extremely excitable, as shown by a rise of the arterial
pressure. The subsidiary centers in the spinal cord are influenced by
corresponding conditions.

Reflex Stimulation of the Vaso-motor Centers. — The results of
experiment make it certain that the degree of vascular contraction maintained
by the vaso-constrictor centers can be increased or decreased by nerve im-
pulses reflected to the cord and medulla from the periphery or from
the brain. The effect may be general, or local and confined to the area
from which the impulses arise. The following experiments may be cited
as illustrations:

Stimulation of the central end of a divided posterior root of a spinal nerve
gives rise to increased vascular contraction, as shown by the rise of blood-
pressure. Stimulation of the central end of the divided sciatic will give rise
to opposite results, according to the strength of the stimulus, weak stimuli
producing dilatation, strong stimuli producing contraction of the vessels.
Stimulation of the central end of the divided vagus gives rise to dilatation of
the vessels of the lips, cheeks, and nasal and palatal mucous membranes.
Stimulation of the tongue is followed by dilatation of the vessels of the submax-
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.

A satisfactory explanation of these different results is, however, wanting.
By some investigators it is believed that the usual variations in the arteriole
contraction are the outcome of corresponding variations in the activity of
the general vaso-constrictor center, the result of nerve impulses coming
through afferent nerves.

The preceding statements as to the effects on the degree of vascular 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.

374

TEXT-BOOK OF PHYSIOLOGY.

It is possible, therefore, that under physiologic conditions, physiologic
stimuli act on the peripheral terminations of either the one or the other;
according as they do will the center be augmented or inhibited in its activity,
and followed by either an increase or a decrease in the degree of the previous
vascular contraction.

Again it may be assumed, from the results of experimentation on afferent
nerves, that the physiologic stimuli may act simultaneously on the periph-
eral terminations of both classes
of fibers and that the vaso-con-
stricter center is acted on by the
two antagonistic influences. In
this assumption the resultant
effect on the blood-vessels, viz.,
increased or decreased contrac-
tion, will be the resultant of their
action on .the vaso-constrictor
center. If the stimuli act pre-
ponderantly on the depressor
fibers the center will be depres-
sed and the vessels will dilate;
if they act preponderantly on
the pressor fibers the center will
be stimulated and the vessels
will contract.

Inasmuch as the vascular
dilatation is often greater than
the dilatation which follows divi-
sion of the vaso-constrictor fibers
themselves, it has been assumed
by some that the general vascular
tonus, as well as its variations
from time to time, is the result-
ant of the simultaneous activity
and variations in activity of both
vaso-constrictor and vaso-dilata-
tor centers; that in the afferent
nerves there are two sets of fibers,
one of which when stimulated
augments the activity of the vaso-
constrictor center and inhibits

FIG. 176. — DIAGRAM SHOWING THE ORIGIN
AND RELATION OF THE DEPRESSOR NERVE IN THE
RABBIT. Depr. n., depressor nerve; vag. n., vagus
nerve; sup. 1. n., superior laryngeal nerve ; inf. c. g.,
inferior cervical ganglion; sym. n., sympathetic nerve;
car. a., carotid artery; dig. m., digastric muscle;
hyp. n., hypoglossal nerve; sup. c. g., superior cer-
vical ganglion; inf. 1. n., inferior laryngeal nerve.

the activity of the vaso-dilatator
center; the other of which aug-
ments the activity of the vaso-dilatator center and inhibits the activity of
the vaso-constrictor center. The result, either contraction or dilatation,
which follows stimulation of their peripheral terminations will depend on
the character of the physiologic stimulus.

In those particular instances in which stimulation of the peripheral
terminations of afferent nerves, associated with the nervi erigentes and
chorda tympani, is followed by active dilatation of the blood-vessels, it has

THE CIRCULATION OF THE BLOOD. 375

been assumed that there are afferent nerve-fibers which directly stimulate
or augment the activity of a special vaso-dilatator center and for this reason
should be termed "reflex vaso-dilatator nerves" (Hunt).

The Influence of Emotional States. — The vaso-constrictor centers
are capable of being influenced in their activities by emotional states,
doubtless as a result of the arrival of nerve impulses from the cortex of the
cerebrum. Thus it is well known that fear causes a contraction of the blood-
vessels of the head and face and that shame causes a dilatation of the same
vessels. With the cessation or the disappearance of the emotional state,
the blood-vessels return to their former degree of contraction. The vaso-
dilatator centers in the medulla and in the sacral region of the spinal cord are
influenced in a similar manner by emotional states.

The Depressor Nerve. — A striking illustration of the depressor or in-
hibitor action of afferent nerves upon the vaso-constrictor center is furnished
by the result of stimulation of a branch of the vagus, the so-called "depressor
nerve." In the rabbit, Fig. 176, there is a small nerve formed by the union
of a branch from the trunk of the vagus with a branch from the superior

FIG. 177. — FALL OF BLOOD-PRESSURE FROM EXCITATION OF THE DEPRESSOR NERVE. The
cylnder 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. 177). The fall in
pressure, however, is not due to this cause, for it occurs equally well after
division of all the cardiac nerves. For this 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.

376 TEXT-BOOK OF PHYSIOLOGY.

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 will
nevertheless be followed by a fall of the blood-pressure almost as great as
when the splanchnics are intact. From this it is evident that the depressor
nerve is related to centers which influence the vascular apparatus in its
entirety. It has been supposed that through it the heart can protect itself
from injurious results of an excessive rise of arterial pressure.

Thus, when the intra-cardiac pressure or the intra-aortic pressure
rises beyond a normal amount from increased resistance, the peripheral
terminations of this nerve are stimulated with the result that the vaso-motor
center is inhibited and the arterioles relaxed. Through this means the
pressure falls and the work of the heart is lessened.

CHAPTER XV.

RESPIRATION.

Respiration is a process by which oxygen is introduced into, and carbon
dioxid removed from, the body. The assimilation of the former and the
evolution of the latter take place in the tissues as a part of the general
process of nutrition. Without a constant supply of oxygen and an equally
constant removal of the carbon dioxid, those chemic changes which 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 of

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 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 for the lower respiratory passages. The mouth,
though frequently serving as an entrance for air, is not primarily a respira-
tory passage. Both the nasal chambers and the mouth communicate
posteriorly with the pharynx, in which the respiratory and the deglutitory
passages cross each other, the former leading directly into the larynx.

The larynx is a complicated mechanism serving the widely different
though related functions of respiration and phonation. It consists of a
framework of cartilages, articulating one with another, united by ligaments
and moved by muscles; it is covered externally with fibrous tissue and lined
with mucous membrane. The superior opening of the larynx, the glottis,
is triangular in shape, the base being directed upward and forward, the
apex downward and backward. The inclination of the glottic opening is
almost vertical.

The cavity of the larynx is partially subdivided by the interposition of
the vocal bands into a superior and an inferior portion. The opening,
bounded by the vocal bands, is also triangular in shape, though in this case

377

378

TEXT-BOOK OF PHYSIOLOGY.

the base is directed backward, and the apex forward. (See chapter on Voice
and Speech.)

The introduction of the vocal bands narrows at this level the air-passage
and to some extent interferes with the free entrance of air. According to
the investigations of Semon, the area of the air-passage above and below
the phonatory apparatus is about 200 sq. mm.; while the area bounded
by the vocal apparatus is but 155 sq. mm. during quiet respiration.

FIG. 178. — TRACHEA AND BRONCHIAL TUBES, i, 2, Larynx. 3, 3. trachea. 4. Bifurcation
ot trachea. 5. Right bronchus. 6. Left bronchus. 7. Bronchial division to upper lobe of right
lung. 8. Division to middle lobe. 9. Division to lower lobe. 10. Division to upper lobe of left
lung. ii. Division to lower lobe. 12, 12, 12, 12. Ultimate ramifications of bronchi. 13, 13, 13,
13. Lungs, represented in contour. 14, 14. Summit of lungs. 15, 15. Base of lungs. — (Sappey.)

The trachea is a tube, some 12 centimeters in length, from two to
two and a half centimeters in breadth, extending from the lower border
of the larynx to a point opposite the fifth dorsal vertebra. It consists of
an external fibrous and an internal mucous membrane, between which is
a series of superposed C-shaped arches or rings of elastic cartilage, some
1 8 or 20 in number. Between the fibrous and mucous coats posteriorly,
and occupying the space between and attached to the free ends of the car-
tilages, there is a layer of transversely arranged non-striated muscle-fibers,
known as the tracheal muscle. The alternate contraction and relaxation of
this muscle would by varying the distance between the ends of the cartil-
ages, either diminish or increase the caliber of the trachea. The surface of

RESPIRATION. 379

the mucous membrane is covered by a layer of stratified columnar ciliated
epithelium (Fig. 179). In the submucous tissue there are a number of glands
the ducts of which open on the free surface.

Opposite the fifth dorsal vertebra the trachea divides into a right and a
left bronchus. Each bronchus again subdivides into two or three branches,
which penetrate the corresponding lung.

The lungs, in the physiologic condition, occupy the greater part of the
cavity o'f the thorax. They are separated from each other by the contents
of the mediastinal space: viz., the heart, the large blood-vessels, the esoph-
agus, 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 arter-
ies, 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

, . , . , ,, -, FIG. 179. — TRANSVERSE SECTION OF

relations which these structures bear one THE TRACHEA OF A KITTEN.— (Stirling.)
to another is as follows: —

Within the substance of the lung the bronchi divide and subdivide,
giving origin to a large number of smaller branches, the bronchial tubes,
which penetrate the lung in all directions. With this repeated subdivision
the tubes become narrower, their walls thinner, their structure simpler.
In passing from the larger to the smaller tubes the cartilaginous arches
become shorter and thinner, and finally are represented by small angular
and irregularly disposed plates. In the smallest tubes the cartilage entirely
disappears. With the diminution of the caliber of the tube and a decrease
in the thickness of its walls, there appears a layer of non-striated muscle-
fibers, the so-called bronchial muscle, between the mucous and submucous
tissues, which completely surrounds the tube and becomes especially
well developed in those tubes devoid of cartilage. The fibrous and mucous
coats at the same time diminish in thickness.

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

TEXT-BOOK OF PHYSIOLOGY.

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

Bronchiole..

Infundibulum

FIG. 180. — SCHEME OF A BRONCHIOLE TER-
MINATING IN ALVEOLAR PASSAGES, THOSE LEADING
INTO INFUNDIBULA BESET WITH AIR-CELLS. —
(Landois and Stirling.)

FIG. 181. — SINGLE LOB-
ULE OF HUMAN LUNG. a.
Alveolar passage, b. Cav-
ity of lobule or infundib-
uium. c. Pulmonary sacs.
— (Dalton.)

millimeter, it is known as a bronchiole or a terminal bronchus. From the
sides of the terminal bronchus and from its final termination there is given
off a series of short branches which soon expand to form lobules or alveoli

(Fig. 1 80). The cavity of the
alveolus is termed the infundib-
ulum. 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.
181). The wall of the alveolus
is extremely thin and consists of
nbro-elastic tissue, supporting a
very elaborate capillary network
FIG. 182.— SECTION OF SILVERED LUNG OF KITTEN, of blood- vessels. The bron-
INCLUDING PORTIONS OF INFUNDIBULUM AND AIR- , . i
SAC. a. Small polyhedral epithelial cells covering phial System as far as the alveo-

the wall of the infundibuium. b. Fibre-elastic lar passages is lined by ciliated

framework, c. Large flattened epithelial plates lining epithelium. The air-SaCS are
air-sac, among which lie small groups of small cells ..r . ,

(d).— (Pier sol* lined by flat epithelial plates of

irregular shape, termed the res-
piratory epithelium (Fig. 182). The alveoli are united one to another by
nbro-elastic tissue.

The bronchial arteries which supply nutritive material to the pulmonary

RESPIRATION. 381

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 pulmonary veins. The blood
in the pulmonary 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, p v. lii^V— — PA

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 Pulmonary Blood-vessels.—
The pulmonary artery which conducts the

venous blood from the heart to the lungs FIG.^.-THE RELATION _ „
divides beneath the arch of the aorta into PULMONARY ARTERY, PA, AND THE
a right and a left branch. Each branch PULMONARY VEIN, PV, TO THE LOBULES,
with its subdivisions enters the lung at AA' B- THE BRONCHIOLE-
the hilum in company with the larger divisions of the bronchi. 'Within
the lung the arteries divide and subdivide in a manner corresponding
to that of the bronchial tubes, which they follow to their ultimate ter-
minations. As the pulmonary lobules are approached, a small arterial
branch plunges into the wall of the lobule (Fig. 183), in which it forms
an elaborate capillary network which surrounds and embraces the air-
sacs on all sides. As this network is to subserve the respiratory exchange
of gases it lies nearer the inner than the outer surface of the lobule and in
close relation to the respiratory epithelium. The air and blood are thus
brought into intimate relationship, being separated only by the respiratory
epithelium and the wall of the capillary vessel. The blood emerging from
the capillary vessels is conducted by a corresponding converging system of
vessels, the pulmonary veins, out of the lungs and into the left auricle of the
heart. The main function of the pulmonary apparatus and the pulmonary
division of the circulatory apparatus is to afford a ready means for the
exhalation of the carbon dioxid and the absorption of oxygen. In conse-

OF THE

382

TEXT-BOOK OF PHYSIOLOGY.

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-
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

10

18

V5

FIG. 184. — BRONCHI AND LUNGS, POSTERIOR VIEW, i, i. Summit of lungs. 2, 2. Base of
lungs. 3. Trachea. 4. Right bronchus. 5. Division to upper lobe of lung. 9. Division to lower lobe.
10. Left branch of pulmonary artery, n. Right branch. 12. Left auricle of heart. 13. Left
superior pulmonary vein. 14. Left inferior pulmonary vein. 15. Right superior pulmonary vein.
16. Right inferior pulmonary vein. 17. Inferior vena cava. 18. Left ventricle of heart. 19.
Right ventricle. — (Sappey.)

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
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

RESPIRATION.

383

three, which are almost horizontal.
The general arrangement and appearance
of the thorax are shown in Fig. 185.

The costo-vertebral and costo-chon-
dral and the chondro-sternal articulations
are diarthrodial in character and endow
the thoracic walls with a considerable
degree of mobility. The costo-vertebral
joints are two in number, the first being
formed by the beveled head of the rib
and the bodies of the two adjoining ver-
tebrae; the second, by the tubercle of the
rib and the transverse 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 as
follows: the diaphragm, the intercostales
externi and interni, the levatores costa-
rum, the triangularis sterni, and the
infra-costales.

The diaphragm is the musculo-mem-
branous sheet which closes the inferior

13

FIG. 1 86. — DIAPHRAGM, INFERIOR ASPECT, i. Anterior
and middle leaflet of central tendon. 2. Right leaflet. 3.
Left leaflet. 4. Right crus. 5. Left crus. 6, 6. Intervals
for phrenic nerves. 7. Muscular fibers, from which the
ligamenta arcuata originate. 8. Muscular fibers that arise
from the1 inner surface of the six lower ribs. 9. Fibers
that arise from ensiform cartilage. 10. Opening for inferior
vena cava. n. Opening for esophagus. 12. Aortic open-
ing. 13, 13. Upper portion of transversalis abdominis,
turned upward and outward. 14. Anterior leaflet of trans-
versalis aponeurosis. 15, 15. Quadratus lumborum. 16,
1 6. Psoas magnus. 17. Third lumbar vertebra.

FIG. 185. — THORAX, ANTERIOR VIEW.
i. Manubrium sterni. 2. Gladiolus. 3.
Ensiform cartilage of xiphoid appendix.
4. Circumference of apex of thorax. 5.
Circumference of base. 6. First rib.
7. Second rib. 8, 8. Third, fourth,
fifth, sixth, and seventh ribs. 9. Eighth,
ninth, and tenth ribs. 10. Eleventh and
twelfth ribs, n, n. Costal cartilages.

opening of the thorax
and completely sepa-
rates its cavity from that
of the abdomen. It con-
sists of two muscles
which arise from the
bodies of the first three
or four lumbar verte-
brae and neighboring
fascia, from the border
of the six lower ribs, and
from the ensiform car-
tilage (Fig. 186). From
this extensive origin the
muscle-fibers pass cen-
trally to be inserted into
a common tendon. As
the direction of the fibers
is from below upward
and inward, the dia-
phragm is somewhat
dome shaped. Its infe-
rior border is for a short
distance in contact with
the sides of the thorax.

384

TEXT-BOOK OF PHYSIOLOGY.

The intercostales externi, eleven in number on each side, occupy the
spaces between the ribs to which they are attached from the tubercle to the
anterior extremity (Figs. 187 and 188). Their fibers, which are arranged
in parallel bundles, are directed from above downward and from behind

forward. The point of attach-
ment, 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 at-
tached to the ribs from the
tubercle to the anterior extrem-
ity of the cartilages. Their
fibers, which are also arranged
in parallel bundles, are direc-
ted from above downward and
backward (Figs. 187 and 188).
The portions of the internal
intercostals between the carti-
lages are frequently termed in-
tercartilaginei.

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 verte-
brae 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 implies, is to elevate the
posterior portion of the ribs.
The triangularis sterni arises from the side of the posterior surface of the
lower third of the sternum and is inserted by fleshy slips into the cartilages
of the ribs from the second to the sixth.

From the fact that the inferior opening of the thorax as well as the 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

FIG. 187. — SHOWING THE SITUATION, THE POINTS
OF ATTACHMENT, AND DIRECTION OF THE INTERCOSTAL
MUSCLES, i. The intercostales externi. 2. The inter-
costales interni. 3. The intercartilaginei. — (Deaver.)

RESPIRATION. 385

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 visceral and parietal, are in contact, or
at most separated only by a thin capillary layer of lymph. The presence of
this fluid prevents appreciable friction as
the two surfaces play against each other
in consequence of the movements of the
lungs.

THE MECHANIC MOVEMENTS OF THE
THORAX.

The blood receives oxygen from, and
yields carbon dioxid to, the alveoli of the
lungs, as it flows through the pulmonic
capillaries. That this exchange of gases
may continue, it is of primary importance
that the air within the alveoli be renewed FlG< jgs.— VIEW FROM BEHIND OF
as rapidly as it is vitiated. This is accom- FOUR DORSAL VERTEBRA AND
plished_by an alternate increase and de- ^n£^OTI^I£>™
crease in the capacity of the thorax, ac- MUSCLES OF THE RIBS AND THE
companied by corresponding changes in INTERCOSTALS. i. Long and short
the capacity of the lungs. During the for-
mer there is an inflow of atmospheric air
(inspiration), during the latter an out-flow of intra-pulmonic air (expira-
tion). 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 con-
stitute a respiratory act or cycle.

In the course of the respiratory cycles the thorax presents alternately a
short period of rest — viz., between the end of an expiration and the 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

1 It is a matter of discussion as to whether or not there is an absolute cessation of movement
of the thoracic walls at the end of expiration. A graphic record of the movement shows that
if there is no absolute cessation, the movement is so slight that, for the purposes here intended,
a pause may be admitted. With this admission it is however, recognized that the forces, both
elastic and muscular, which are always acting on the thoracic walls, though in opposite directions,
have not ceased to act, but have become so nearly equal that for a brief period they are practically
in a condition of equilibrium, during which the thoracic walls are stationary.

25

386

TEXT-BOOK OF PHYSIOLOGY.

occupied by the heart and great blood-vessels (Fig. 189). This condition is
maintained by the pressure of the air within the lungs, the intra-pulmonic
pressure, which with the respiratory passages open, is that of the atmosphere,
760 mm. Hg. This relation persists so long as the thorax remains air-
tight. If the skin and muscles covering an intercostal space be removed the
lung can be seen in close contact with the parietal layer of the pleura gliding
by with each inspiration and expiration. If, however, an opening be now
made in the pleura sufficient to admit air, the lung immediately collapses and
a pleural cavity is established. The pressure of air within and without the
lung counterbalancing, at the moment the air is admitted, the elastic tissue
at once recoils and forces a large part of the air out of the lung. This is a proof
that in the normal condition, the lungs, distended by atmospheric pressure
from within, are in a state of elastic tension and ever endeavoring to pull the

FIG. 189. — SECTION OF THORAX WITH THE LUNGS, HEART, AND PRINCIPAL VESSELS. 5.
Catheter introduced into the pleural space and connected with a manometer. — (After Moral and
Doyen.}

visceral layer of the pleura away from the parietal layer. That they do not
succeed in doing so is due to the fact that the atmospheric pressure from
without is prevented from acting on the lung by the firm unyielding walls of
the thorax.

Intra-thoracic Pressure. — As a result of the elastic tension of the lungs a
fractional part of the intra-pulmonary pressure, 760 mm. Hg., is counter-
balanced or opposed, so that the heart and great vessels and other intra- thoracic
viscera are subjected to a pressure somewhat less than that of the atmosphere;
the amount of this pressure will be that of the atmosphere less that exerted by
the elastic tissue of the lung in the opposite direction, expressed in terms of
millimeters of mercury. In the thorax but outside the lungs, there then
prevails a pressure, negative to the pressure inside the lungs and which is
known as the intra-thoracic pressure.

RESPIRATION. 387

The amount of this intra-thoracic pressure can be approximately deter-
mined in several ways. Thus, if shortly after death a mercurial manometer
be inserted air-tight into the trachea of a human being and the thorax opened,
the lungs will recoil and compress their contained air. The mercurial
manometer will at once show an excess of pressure in the trachea of about
6 mm. This was taken by 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
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. 189.)

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
contain fluid. The lungs therefore are devoid of air (atelectatic) , and,
having a specific gravity greater than water, readily sink when placed in this
fluid. The capacity of the thorax does not exceed the volume of the lungs.
With the first 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 of the static con-

388 TEXT-BOOK OF PHYSIOLOGY.

dition are 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 dificult 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 car tilaginei, the levatores costarum, the scaleni, and the ser-
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
(Fig. 190). The vertical diameters are thus enlarged, though unequally in
different regions of the thorax.

As the diaphragm descends it displaces the abdominal viscera, forcing

RESPIRATION.

389

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 tower part some-
what sagittal in direction. Hence
when the ribs are elevated the upper
part of the thorax increases in its an-
tero-posterior, the lower part in its
transverse diameters. At the same
time, the lower portion of the sternum FIG. 190.— DIAGRAM SHOWING THE POSITION

k -miQViprl fnrwarH anH iir>warH V»v AND SHAPE OF THE DIAPHRAGM AT REST a AND

.Dy DURING INSPIRATION a' AND b.—(Boruttau.}
the elevation of the anterior extremity

of the ribs and the widening of the angle of the costo-chondral articulation.
With the elevation of the ribs there goes an eversion or outward rotation
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.

390

TEXT-BOOK OF PHYSIOLOGY.

The action of the external intercostal muscles, as well as the action of
the inter car tilaginei muscles, has been a subject of much discussion. Some
investigators have maintained that they are elevators of the ribs, and there-
fore inspiratory; others that they are depressors of the ribs, and therefore
expiratory in function. At the present time the general consensus of opinion
is that the former view is the one most in accordance with the facts. In
the following explanation as to their action, their relation to the ribs and to
the cartilages, must be recalled to mind. The relation of the external inter-
costals is such that the point of attachment of any given bundle of fibers
to the rib above lies nearer the vertebral column, nearer the fulcrum,
than the point of attachment to the rib below. The relation of the inter-
cartilaginei to the cartilages is such that the point of attachment of any
given bundle of fibers to the cartilage above lies nearer the sternum, nearer
the fulcrum, than the point of attachment to the cartilage below. The
situation of the muscles and the shortness of their fibers render it extremely
difficult to obtain myographic tracings which would elucidate their action
in elevating the ribs and cartilages.

A clear conception of their action, however, may be arrived at by the
study of the schematic model first presented by Hamberger. Fig. 191. In

FIG. 191. — DIAGRAMS ILLUSTRATING THE ACTION OF THE EXTERNAL INTERCOSTAL AND IN-

TERCARTILAGINEI MUSCLES.

this model v-vr is a vertical support carrying two freely movable parallel
bars rr', united at their opposite ends with two other freely movable
and parallel bars cc, carried by a second vertical supports, 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 thus and 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 component

RESPIRATION. 391

acting upward on the long arm of the lever preponderates over the com-
ponent acting downward on the short arm of the lever. This taken in con-
nection with the fact that the distance between the adjoining bars is 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 action
of these bands thus disclose and illustrate the action of both the external
intercostal and intercartilagenei 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
vertebras. The anterior extremity is serrated and attached to the outer
surfaces of the second, third, fourth, and fifth ribs beyond the angle. The
action of the muscle is the elevation of the ribs to which it is attached.

In forcible or extraordinary inspirations, whereby the capacity of the
thorax is still further increased, the foregoing muscles are reinforced by the
sterno-cleido-mastoideus , the trapezius, and the pectorales minor and major.
Their functions will become apparent from a consideration of their origins
and insertions.

Expiratory Forces and Muscles. — Expiration, as previously stated, is
a passive process brought about by the recoil of the elastic tissues of the
thoracic and abdominal walls, and of the lungs, all of which have been
stretched and made tense during inspiration. With the cessation of the 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
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-

392 TEXT-BOOK OF PHYSIOLOGY.

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, we consider the direction of these
muscles as indicated in Fig. 191, diagram, A by the dotted line ii, ii, it would
seem that their action would be the opposite of that of the external inter-
costals— that is, it would be to depress the ribs. By the shortening of the
muscles, the two forces, indicated by the direction of the arrows, are equal
and opposite, but as the component acting on the long arm of the lever pre-
ponderates over that acting on the short arm of the lever, the ribs are
depressed. If this is the case these muscles must therefore be expiratory in
function. The action of the band is supposed to disclose and illustrate the
action of the muscle.

The triangularis sterni muscle, judging from its anatomic relations, in all
probability assists in expiration by depressing the cartilages to which it is
attached and as a further result depressing the anterior extremities of
the ribs.

Forced Expiration. — After the elastic forces have ceased to act and the
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 the
contraction, through volitional effort, of abdominal and thoracic muscles.
To this decrease in the capacity of the thorax, as a result of which a much
larger volume of air is expelled from the lungs than during passive expiration,
the term forced expiration has been given. With the cessation of muscle
activity the elastic forces of the now-compressed thoracic walls, aided by the
return of the upwardly-displaced abdominal organs, at once restore the
thoracic walls to the position they had attained at the end of passive expira-
tion. Of the muscles active in forced expiration in addition to the inter-
cos tales interni and the triangularis sterni, the following may be mentioned,
viz.: the abdominales, the serratus posticus inferior, and the quadratus
lumborum.

The conjoint action of these muscles is to diminish the convexity of the
abdominal walls and to exert a pressure on the abdominal organs. These,
taking the line of least resistance, are forced upward against the inferior
surface of the diaphragm, which in consequence becomes more strongly
curved and ascends higher into the thorax. The vertical diameter of the
thorax is thus diminished. Acting from the pelvis as a fixed point, these
muscles will also draw downward and inward the lower end of the 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 diam-
eters 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

RESPIRATION. 393

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 intra-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-
quence there is a rapid outflow of air, which continues 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 Donders

394 TEXT-BOOK OF PHYSIOLOGY.

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. 192.
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. In
forcible inspiratory efforts this fall in intra-thoracic pressure may
amount to 30 or 40 mm. of Hg. As the thorax again diminishes and the
intra-pulmonic pressure rises above the atmospheric pressure, the recoil
of the elastic tissue is again opposed, with the result of increasing the

760 m?n.

Inspirati<ut+ /^

z \?nm^^ . Expiration.
A. INTRA-PULMONIC PRESSURES.

760mm ^-^J0^^7 -*- Expiration

Xtt

B. INTRA-THORACIC PRESSURE.

FIG. 192. — REPRESENTING THE CHANGES, i, IN THE INTRA-PULMONIC, AND 2, IN THE INTRA-THO-
RACIC PRESSURES DURING INSPIRATION AND EXPIRATION.

intra-thoracic pressure, until the former condition of pressure has
been regained at the end of the expiration. Neither the fall nor
the subsequent rise of the intra-thoracic pressure takes place, however,
in a steadily progressive manner for the following reasons: If a tracing
were made of the variations in the circumference of the thorax during a
respiratory movement it would resemble in its main features the tracing in
Fig. 194, and variations in any linear dimension of the lung would be of
course in the same proportion. This amount of elongation of elastic tissue
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. 194 varies from uniform straight lines.
The changes in intra-thoracic pressure are graphically represented in
Fig. 192, B.

The intra-thoracic pressure and its variations influence favorably the
flow of lymph through the thoracic duct (see page 222), 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:

RESPIRATION. 395

During Inspiration.

1. Enlargement of the thoracic diameters by muscle action.

2. Increase in the negativity of the in tra- 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 periods of inspira-
tion and expiration. The statements of the majority of laryngologists do
not favor this view. During tranquil breathing the membranes are widely
separated and almost stationary, seldom moving in either direction more than
a few millimeters. In labored respirations these movements are naturally
increased in extent. The irregular movements of the membranes occasioned
by the unskilful use of the laryngoscope, especially with nervous patients,
are not to be regarded as strictly physiologic. The respiratory space in
quiet breathing is an isosceles triangle, with a length of 20 mm. and a 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

396 TEXT-BOOK OF PHYSIOLOGY.

preponderates in the male, the latter in the female, giving rise to what has been
termed in the one case the diaphragmatic or abdominal 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 discus-
sion. It has been attributed, on the one hand, to the necessity for a physi-
ologic adjustment between respiration and child-bearing, and therefore a
specific sex peculiarity; on the other hand, it has been attributed to persistent
constriction of the waist, in consequence of which the full play of the dia-
phragm is prevented and the burden of inspiration is thrown on the thoracic
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

15-20 years, 20 30-50 years, 17 .o

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 same
time diminishing in depth.

Rhythm. — Each respiratory act takes place normally in a regular
methodic manner, each event occurring in a definite sequence and occupy-

FIG. 1 93: — PNEUMOGRAPH. — (Fitz.)

ing the same relative period of time. This rhythm, however, is not infre-
quently temporarily disturbed by emotions, volitional acts, muscle activity,
phonation, changes in the composition of the blood, etc.; with the removal
of these disturbing factors, the respiratory mechanism soon returns to its
normal condition.

RESPIRATION.

397

A graphic representation of the excursions of the thoracic walls, rhythmic
or otherwise, is obtained by fastening to the thorax an apparatus, a stethom-
eter or a pneumo graph, 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. 193), consists of a
coil of wire two and a half centimeters in diameter and about 40 centimeters in
length, enclosed by thin rubber tubing, one end of which is closed, the other
placed in communication 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.
194), 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 to 8.

INSP.

FIG. 194. — A PNEUMATOGRAM. — (After
Marey.)

FIG. 195. — A SPIROMETER.
(Baruttau.)

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 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
is the spirometer, a modification of the gasometer. The form introduced by
Jonathan Hutchinson, of which Fig. 195 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 external air. A stopcock at the outer end of this tube prevents

398 TEXT-BOOK OF PHYSIOLOGY.

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, indicates the volume
of air inspired or expired.

With an apparatus of this character Hutchinson, from a long series of
observations, denned 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 & 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 100 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 volume4, 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, denned 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 of
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-

RESPIRATION.

399

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,

FIG. 196. — GAD'S, PNEUMATOGRAPH.

devised by Gad and known as the pneumato graph or aeroplethysmograph
(Fig. 196). This consists of a quadrangular box with double walls, the
space between which is filled with water. The center of the box is an air
chamber. A thin walled mica box sinks into the water. Posteriorly it is
attached to and rotates around an axis, which permits of an elevation or
depression of the anterior portion. It is also carefully counterpoised. A
light lever attached to the mica box records its movements. The interior

4OOO
3SOO
3OOO
2300
200O
1SOO
100O
SOO
ft

A \

A

/ \

rnr\

Vital

%$\l _ \/\ /y

>

W

Capacity.

rv

\/

V J

FIG. 197. — DIAGRAM REPRESENTING THE VOLUME CHANGES OF THE THORAX AND LUNGS. —

(Modified from Boruttau.)

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 ot the
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. 197.

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

400 TEXT-BOOK OF PHYSIOLOGY.

known as the bronchial sound. It is especially well heard between the
scapulae above 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 uncer-
tain. 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
and to the friction of the air in the alveolar passages.

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 blood and tissues constitute internal respiration,
in contradistinction to the thoracic movements by which the air is brought
into relation with the blood, and which constitute external respiration. The
transfer of the oxygen by the blood from the interior of the lungs to the tissues,
and of the carbon dioxid from the tissues to the interior of the lungs, is the
outcome of a series of 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 between the blood and tissues, on the other.

In consequence of the many and complex chemic changes which attend
these gaseous exchanges, there arise changes in composition of:

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-matter of
the chemistry of respiration.

CHANGES IN THE COMPOSITION OF THE AIR.

Experience teaches that the air during its sojourn in the lungs undergoes
such a change in composition that it is rendered unfit for further breathing.
Chemic analysis has shown that this change 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.

RESPIRATION. 401

The results which have been obtained are represented in the following
table:

Inspired Air. Expired Air.

f Oxygen, 20 .80.

ioo I Carbon dioxid, traces.

vols. j Nitrogen, 79 .20.

( Watery vapor, variable.

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.:

ioo : 4.78 :: 8,500 : # = 406 liters or 580 grams1
Or

ioo : 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, excercise, etc. The ratio of the
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

CO

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

1 i liter of oxygen weighs 1.4298 grams; i liter of carbon dioxid weighs 1.977 grams.
26

402 TEXT-BOOK OF PHYSIOLOGY.

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 20° 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 in 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 is
greater, the percentage of oxygen less, in the latter than in the former. This
is made evident by collecting in several portions the expired air as it escapes
from the respiratory tract and subjecting it to analysis. The last portion
always contains a larger amount of carbon dioxid and a smaller amount of
oxygen than the first portion. The determination of the composition of the
alveolar air is extremely difficult. It has been estimated to contain from
5 to 6 per cent, of carbon dioxid and from 14 to 18 per cent, of oxygen.

Pulmonary Ventilation. — It is owing largely to this inequality of
volumes and consequently of the "partial pressures" of these two gases in
the trachea and alveoli that the degree of ventilation 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,
on the one hand, a portion of the inspired air directly into the alveoli, and,

RESPIRATION. 403

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 into the lungs through the pulmonary artery is
dark bluish-red, that which flows from the lungs into the pulmonary veins
is scarlet red, in color. The blood is changed, while flowing through the
lung capillaries, from the venous to the arterial condition. As the air in the
lungs gains carbon dioxid and loses oxygen, it is fair to assume that what
the air gains the blood loses, and what the air loses the blood gains. In
other words, the blood, while passing through the lungs, is changed from
venous to arterial by the loss of carbon dioxid and the gain of oxygen. The
change in color of venous blood from dark bluish to scarlet red is strikingly
shown by shaking it in a test-tube with oxygen or atmospheric air.

The blood which flows into the tissues through the arteries is scarlet red,
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 through the
capillary walls into the blood.

These considerations are confirmed by the results of analyses which have
been made of both venous and arterial blood. The presence of gas in the
blood is demonstrated by subjecting it under appropriate conditions to the
vacuum of the mercurial air-pump, into which it at once escapes. From
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 venous and arterial blood shows that each consists of oxygen, carbon
dioxid, and nitrogen, though in different amounts. An average composi-
tion of the gases extracted from dog's blood obtained from the right ventricle
and carotid artery is given in the following table:

Venous blood \: ' V, ....... I2 vo^' Arterial blood f ^en'\: ' ' :: ' ' 2O vo}s-

loovols. Carbon dioxid, ...... 45 vob. IOO vols. Carbon dioxid,.. 40 vok.

Nitrogen, ........... 1-2 vols. [ Nitrogen, ....... 1-2 vols.

404 TEXT-BOOK OF PHYSIOLOGY.

The changes produced in the blood by respiration, both external and in-
ternal, become apparent from a comparison of these analyses. The venous
blood while passing through the lungs gains from eight to eleven volumes
per cent, of oxygen and loses five volumes per cent, of carbon dioxid. The
arterial blood while passing through the tissues loses oxygen and gains
carbon dioxid in corresponding amounts. The volume of nitrogen is not
appreciably changed.

The Relation of the Gases in the Blood. — The mechanism by which
the gases become associated with the blood at the moment of their entrance
into it, and again become dissociated just prior to their exit from it, as well
as their relation to the blood while in transit, will be more readily understood
after reference to a few elementary facts relative to the absorption of gases
by liquids in general and the conditions of temperature and pressure by
which it is influenced.

It is well known that liquids will absorb or dissolve at any constant
pressure unequal volumes of different gases in accordance with their 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 15° 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
0.0489 c.c.; at 1520 mm. of mercury, the same volume is absorbed but its
weight is doubled. If the pressure falls below 760 mm. of mercury the
same volume is absorbed but its weight is diminished. (Law of Dalton.)
Because of the foregoing facts, it is necessary in all gaseous determina-
tions to reduce for purposes of comparison the obtained volumes to standard
temperature (o° C.) and pressure (760 mm. of mercury).

When the liquid is once saturated with a gas at a constant pressure
and temperature, there is coincidently with the entrance of the gas into the
liquid, an equivalent exit of the gas from it, though the volume retained in the
liquid remains constant. The reason for this fact is, that under the condi-

RESPIRATION. 405

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.

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

4o6 TEXT-BOOK OF PHYSIOLOGY.

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 that it begins to be liberated in large amounts. From this on, the
oxygen continues to be liberated with decreasing pressures, until the zero point
is reached, when all gaseous discharge ceases. Coincidently the blood
changes in color from a bright red to a deep bluish-red. It is evident from
the results of this procedure that the condition of the oxygen in the blood
is but to a slight extent one of physical absorption. The indications are
that the union is of the nature of a chemic combination.

If the red corpuscles are removed from the blood and the plasma alone
treated in the manner above described, it will be found that the oxygen
liberated now follows the law of partial pressure. The amount so liberated,
however, is small — about one per cent, of the total oxygen of the blood.
The agent therefore which holds the oxygen in combination is the red corpus-
cle, 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 absorption the hemo-
globin changes in color from bluish-red to scarlet-red and changes from hemo-
globin to oxyhemoglobin. The reverse method, that of subjecting oxy-
hemoglobin to gradually diminishing pressures, yields opposite results, that
is, the oxygen becomes dissociated and the force by which this is accomplished
is known as the force of dissociation. As one gram of hemoglobin com-
bines 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 looc.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 chemi-
cally combined with the hemoglobin.

The association or combination of oxygen is favored by a pressure of at
least 30 to 50 mm. Hg. and upward; the dissociation, by diminution of
pressure. In the conversion of hemoglobin into oxyhemoglobin two an-
tagonistic forces are at work, heat and chemic affinity. The former
tends to prevent, the latter to favor, the union. Chemic affinity increases
with the influence of mass, that is, in proportion to the number of atoms in a
unit of volume, with the density, and with the partial pressure of the oxygen.
Diminution of pressure reduces the mass influence and permits the heat to
bring about dissociation (Bunge). The following table by Hiifner shows
the relative proportion of hemoglobin and oxyhemoglobin in blood contain-
ing 14 per cent, hemoglobin and exposed to air at gradually diminishing
pressures:

RESPIRATION. 407

Atmospheric Pressure Partial Pressure of Oxygen Hemoglobin Oxy hemoglobin

in mm. Hg. in mm. Hg. Percentage. Percentage.

760 159.3 J-49 98-51

524.8 no 2.14 97.86

357.8 75 3-n 96-89

238.5 50 4.60 95-40

119.3 25 8.79 91.21

47.7 10 I9-36 80.64

23.8 5 32-Si 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 of
firm combination. The same result follows the addition of the red blood-
corpuscles, which act in a manner similar to the acids just mentioned.
This property of the corpuscles has been attributed to hemoglobin, and
especially when in the state of oxy hemoglobin. It is for this reason that
blood yields all its carbon dioxid to the vacuum of the gas-pump.

The limit of pressure at which the plasma ceases to absorb carbon
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 condition
of both loose and stable combination.

An analysis of the serum, and presumably of the plasma, shows the
presence of sodium salts, with which the carbon dioxid could enter into com-
bination, viz.: sodium carbonate and dibasic sodium phosphate. The
sodium is thus partly divided between carbonic acid and phosphoric acid.
The amount of the sodium which falls to carbon dioxid will depend on the
mass influence of the latter; that is, its partial pressure.

At its origin in the tissues the carbon dioxid acquires a considerable
tension, and its mass influence is correspondingly large. On entering the
blood it combines with sodium carbonate, with the formation of sodium
bicarbonate, as shown in the following equation:

Na2C03 + C02 + H20-2NaHC03.

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:

Na2HP04 + CO2 + H20=NaHCO8+NaH2P04.

With the diffusion of the carbon dioxid from the blood into the alveoli its
tension in the venous blood falls, its mass influence diminishes, while that of
the phosphoric acid relatively increases. As a result, the sodium is withdrawn
from the sodium bicarbonate, an additional liberation of carbon dioxid takes
place and dibasic sodium phosphate is re-formed. The association or com-
bination of the carbon dioxid with the basic salts depends on its partial
pressure; dissociation in the lungs, on a diminution of pressure.

4o8 TEXT-BOOK OF PHYSIOLOGY.

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. Thus he
states that the oxygen tension in the pulmonary alveoli is 136 mm. Hg., or
18 per cent, of an atmosphere while in the arterial blood it is 106 mm. Hg.,
or 14 per cent.; while the carbon-dioxid tension in the tissues varies from
45 to 68 mm. Hg., or from 6 to 9 per cent, of an atmosphere; while in the
venous blood it varies from 30 to 41 mm. Hg., or from 3.8 to 5.4 per cent,
and in the pulmonary alveoli it is about 21 mm. or 2.8 per cent.

CHANGES IN THE COMPOSITION OF THE TISSUES AND LYMPH.

From previous statements the inferences can be drawn that the oxygen
leaves the blood as the latter flows through the capillaries; that it passes
through the capillary wall into the surrounding lymph and so to the tissue-
cells; that it oxidizes food materials in the tissue-cells whereby the potential
energy of the former is liberated as kinetic energy; that the carbon dioxid

RESPIRATION. 409

so evolved passes into the lymph and through the wall of the capillary into
the blood.

While this is doubtless the case, the presence of free oxygen in the tissues
can not be demonstrated by the usual methods of gas analysis. Only in the
saliva and in the blood of the placental umbilical vein can 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 chemistry combine to support the view that oxygen is absolutely
necessary to the maintenance of the life of all tissue-cells. Though they
will continue to manifest their 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.

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.

4io

TEXT-BOOK OF PHYSIOLOGY.

Thus, 100 grams each of muscle, spleen, and broken bone from a recently
living animal exposed to the air for twenty-four hours absorbed respectively
50.8 c.c., 27.3 c.c., and 17.2 c.c. of oxygen, while each discharged during the
same period 56.8 c.c., 15.4 c.c., and 8.1 c.c. of carbon dioxid respectively.
In another series of experiments by a different observer 100 grams of muscle
* absorbed in three hours 23 c.c. of oxygen, and 100 grams of bone 5 c.c. of
oxygen. Both tissues discharged carbon dioxid in amounts proportional to
the oxygen absorbed. The same respiratory changes may be more satis-
factorily demonstrated by passing blood through the tissues of isolated

Atmospheric Air.

Ox-iss mm,.ng , vr 2O.85 per cent ,
COz'0.3 ™ ™ Hy, or 0. 04 jeer cent,
of a/i atmosphere.

//S\Z»,k/r\\

Ox-Tension

38 mm ff&,

3 per cent.

5.5 per cent.

tfpercent.
\CfcTcnswn >

Alveolus

Arterial
Blood.

Venous
'load.

OxTension
Qjgension

6 && percent.

/ft?,

J4 percent

38

Jper eent.

Tissues.

FIG. 198. — DIAGRAM SHOWING THE RELATIVE TENSION OF OXYGEN AND CARBON DIOXID IN
THE LUNGS, IN THE BLOOD, AND IN THE TISSUES.

organs and the tissues of recently living animals. The analysis of the blood
before and after perfusion shows a loss of oxygen and a gain in carbon
dioxid.

Tension of the Gases in the Tissues.— As the presence of free oxygen
cannot be demonstrated, its tension there must be regarded as nil. The
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-

RESPIRATION. 411

tions to each other, are shown in Figure 198, 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 oxy hemoglo-
bin can part with but a portion of its oxygen, sufficient, however, to satisfy
the needs of the tissues.

The carbon dioxid pressure in the tissues being in excess of that in the
blood, it passes through the capillary wall into the blood, where it exists in the
free and combined states. On passing into the 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 role. 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
to considerable criticism and not generally accepted, some other force than
diffusion would have to be found to explain the facts. It would then remain
to be determined in how far the alveolar epithelium could be regarded as
an active agent in both absorption and excretion in opposition to pressure.

4i2 TEXT-BOOK OF PHYSIOLOGY.

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 is
collected; then into long tubes containing barium hydroxid, by which the
carbon dioxid is absorbed; thence into a small meter, by which its amount is
registered. From the amount of water and carbon dioxid thus obtained
the amounts of both in the total air breathed are calculated. The water and
carbon dioxid previously present in the air are simultaneously determined by
a corresponding absorption apparatus and deducted from the amounts ob-
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 from
six to twenty-four hours without detriment to the subject of the experiment.

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

RESPIRATION. 413

employed is completely closed, the carbon dioxid must be removed 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.1

Of the results obtaind 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 CO 2 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 formation of
water. Under such a diet the respiratory quotient is always less than unity,
usually 0.907. On a purely carbohydrate diet — one in which there is no
surplus hydrogen — all the oxygen will combine with carbon and be returned
as carbon dioxid, and hence the respiratory quotient will be unity. The
respiratory quotient therefore indicates the extent to which the oxygen
absorbed is utilized in oxidizing carbon, on the one hand, and hydrogen,
on the other.

Since the total oxygen absorbed and carbon dioxid discharged will vary
considerably with the size of the animal, it is customary, for purposes of
comparison, to reduce all total results to the unit of body- weight (one 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, it is usually taken as the index of the activity, though there are
reasons for believing that it would be more accurately indicated or repre-
sented 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-

1 Both forms of apparatus are in use in the Physiological Laboratory of the Jefferson Medical
College and are fully described by Prof. H. C. Chapman in his text-book on Physiology, to which
the reader is directed for further information.

4i4 TEXT-BOOK OF PHYSIOLOGY.

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 in the following table:

QUANTITY OF O2 AND CO2 ABSORBED AND EXHALED DURING TWENTY-FOUR
HOURS, IN CUBIC CENTIMETERS.

Oxygen Carbon Dioxid

By 100 Grams of: Absorbed. Exhaled.

Muscle, 5o.8c.c. 56.8c.c.

Brain, 45.8c.c. 42.8c.c.

Kidneys, 37.0 c.c. i5.6c.c.

Spleen, 27.3 c.c. I5.4c.c.

Testicles, 18 .3 c.c. 27 . 5 c.c.

Pounded bones 17.2 c.c. 8 . i c.c.

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 CO2 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 of

RESPIRATION. 415

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
at a temperature of 15° 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 lower temperatures act as a
stimulus to the peripheral terminations of the nerve system, bringing about
reflexly increased activity of the body at large. The muscles especially are
not only reflexly but volitionally excited to greater activity. This leads
naturally to an increase in the consumption of oxygen and in the production
of carbon dioxid and in the evolution of heat.

In cold-blooded animals the respiratory exchange is influenced in a
manner the reverse of that observed in warm-blooded animals. With a
rise of external temperature and a corresponding rise of body-temperature
the discharge of carbon dioxid steadily increases. Thus a frog in an atmo-
sphere 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 ^a discharged 325 c.c. per kilo per hour.
Intermediate temperatures were attended by corresponding increases in
the amounts of CO2 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
action is extremely complex and involves the action of both afferent and effer-
ent 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 accomplished have already been
mentioned and described. Their simultaneous 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 respectively the activity of the phrenic and
intercostal nerves, the centers of origin of which lie in the anterior horn of

416 TEXT-BOOK OF PHYSIOLOGY.

the gray matter of the spinal cord at a level, for the phrenic, of the fourth,
fifth, and sixth cervical nerves, and for the intercostals at the level of the
upper thoracic nerves. Division of any one of these nerves is followed by
paralysis of its related muscle.

Inspiratory Center. — The coordinate contraction of the 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
"nceud vital" was applied to it. In this area the respiratory center was
located. It has, however, been shown by Gad that if this area be gradually
destroyed by cauterization the respiratory movements do not cease, but
continue until the cauterization has reached a point far forward in the
formatio reticularis, in which the respiratory center was assumed to lie.

Though its existence has not been anatomically determined beyond
question, it is permissible to speak of the central mechanism as a "center"
located in the medulla oblongata.

RESPIRATION. 417

The Cause of the Rhythmic Activity of the Inspiratory Center.—

It has long been a subject of discussion as to whether the periodic activity
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 impulses
received 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 numbers 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
27

4i8 TEXT-BOOK OF PHYSIOLOGY.

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-
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
is an increase 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.

The first inspiration after birth is supposed to be due to the direct
stimulation of the respiratory center by the increase in the carbon dioxid
present in the blood, though it may be aided by the cooling of the skin due
to vaporization of the amniotic fluid.

Reflex Stimulation of the Inspiratory Center. — Whether the 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.

Of all afferent nerves, the vagus appears 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. 199.) 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 the divided vagus
be stimulated with weak faradic 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 indicate that in the physiologic state afferent
impulses are ascending the vagus fibers which influence the extent of dis-
charge from the inspiratory center, or, in other words, inhibit the inspiratory
discharge and lead to an expiratory movement sooner than would other-
wise be the case. If, however, the stimulation is increased in strength,
the inspiratory movement gradually so exceeds the expiratory that the mus-
cles pass into the tetanic state and the chest-walls come to rest in the con-
dition of forced inspiration. The vagus apparently contains fibers which
are capable of so exciting or augmenting the activity of the inspiratory center,
and therefore the extent of the inspiratory movement, as to lead to the con-
dition of tetanus of the inspiratory muscles. If, on the other hand, the

RESPIRATION.

419

central end of the divided superior laryngeal nerve be stimulated with
induced electric currents, the opposite effect is produced: viz., an excess of
the expiratory over the inspiratory movement until the chest-walls come to
rest in the condition of passive expiration. The superior laryngeal nerve
apparently contains fibers which gradually inhibit activity of the inspiratory
center and hence the inspiratory movement.

mect.ob.

FIG. 199. — DIAGRAM SHOWING THE RELATION OF THE PULMONARY 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. Pulmonary vagus nerve, excitator and inhibitor. iri*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.

The same result, an expiratory standstill, not infrequently follows strong
stimulation of the divided vagi, and always after the administration of
large doses of chloral.

The results of these experiments would seem to indicate that the vagus
nerve contains two classes of nerve-fibers, one of which, when stimulated,
inhibits and regulates the discharge of nerve energy from the inspiratory

42O

TEXT-BOOK OF PHYSIOLOGY.

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 inspiratory 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 stimu-
lated, 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 in-
stance 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 pulmon-
ary terminations of the vagus
nerve-fibers in the physiologic
condition was formerly be-

Diaphragm.
Seconds.

FIG. 200. — POSITIVE VENTILATION (Head). Under
the influence of positive ventilation, the inspiratory lieved to be the chemiC action
contractions of the diaphragm become less and less of Carbon dioxid: it IS now

believed to be a mechanic

till they disapoear completely.

action, the result of the alternate distention and collapse of the walls of the
pulmonary alveoli. Thus, it has been shown by Head that if the lungs
are actively inflated (positive ventilation) there will be produced an in-
hibition 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

Negative
ventilation.

Diaphragm.

Seconds.

FIG. 201. — NEGATIVE VENTILATION. (Head). At a negative ventilation was commenced.
The expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it
finally enters into continued contraction.

recorded (Fig. 200), a result similar in all respects to that produced by
stimulation of the superior laryngeal nerve. On the other hand, if the
lungs are collapsed by the artificial withdrawal of air (negative ventilation)
there will be produced an augmentation of the inspiratory and an inhibition
of the expiratory movements until the inspiratory muscles are in a condi-
tion of tetanic contraction as indicated by the contraction of the dia-
phragm (Fig. 201) and by the state of the thorax which is that characteristic
of extreme inspiration, a result similar in all respects to that produced by
moderate stimulation of the central end of the divided vagus.

A satisfactory explanation of the action of the respiratory mechanism

RESPIRATION. 421

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,

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 action of the inspiratory
center and thus permit of an expiratory movement. The inhibitor fibers
are supposed to be stimulated peripherally by the expansion of the lungs.
With the recoil of the lungs the inhibitor effect gradually dies away, while the
inherent excitation of the 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 feeble intensity.

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. 199); 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 thus developed
ascend to the inspiratory center, excite it, and call forth a new inspiration
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.

Many experimenters, however, find difficulty in accepting the view that
the recoil of the lungs stimulates nerve endings and hence this theory has
not received general acceptance.

Another explanation which is satisfactory in many respects has been
presented by Meltzer. This investigator asserts also the existence in the
trunk of the vagus the two classes of nerve-fibers, the inhibitor and the
excitator; but that for some reason they do not respond to stimulation at
the same time as shown by the effects which follow; the inhibitor fibers
respond first and the excitator fibers somewhat later. Therefore when
they are stimulated simultaneously the primary effect is an inhibition of
the inspiratory center followed by an expiratory movement. The secondary

422 TEXT-BOOK OF PHYSIOLOGY.

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 author assumes the surface of the lung to be the peripheral organ of the
respiratory reflexes.

When it is assumed that both inspiratory and expiratory centers 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 recoil
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
dioxid or to a decrease in the pressure of the oxygen. As the outcome of
a long series of experiments it is now the generally accepted opinion that an
increase in the percentage and pressure of the carbon dioxid in the blood
and hence in the center itself is more efficient in raising the irritability of the
center than a decrease in the percentage and pressure of the oxygen. Thus
if an animal is caused to inhale air containing but 2 per cent, of CO2 more
than normal the respiratory movements will be increased in frequency and
depth, while a corresponding diminution in the percentage of oxygen will
be without 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,
the average depth was 637 c.c. and the total ventilation was 8.918 liters per
minute. On raising the percentage of the CO2 in the inspired air from
0.04 percent, to 0.79 per cent, the average depth increased to 739 c.c. and
the 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
increased to 864 c.c., the rate to 15, and the total ventilation to 12.960 liters
per minute; and when the CO2 in the inspired air was raised to 6 per cent,
the average depth was increased to 2104 c.c., the rate to 27 per minute, and
the total ventilation to 56. 808 liters. The results of these experiments indicate
that an increase in the percentage of the CO2 in the inspired air leads to an
increase in the percentage and pressure of the CO2 in the arterial blood
and hence in the inspiratory center, as a result of which the 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.

RESPIRATION. 423

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.

MODIFICATIONS OF THE RESPIRATORY RHYTHM.

The character of the respiratory movements is materially modified by a
change in the quantitative and qualitative composition of the air and
blood as well as by changes of a pathologic nature of the respiratory appa-
ratus itself.

Eupnea. — So long as the air retains its normal composition and the
respiratory mechanism its structural integrity, so long do the respiratory
movements exhibit a normal rhythm and frequency. To the condition of
easy tranquil breathing the term eupnea is given. In this condition the
percentages of oxygen and carbon dioxid in the blood are such as to favor at
least the rhythmic discharge of nerve impulses to the respiratory mus-
cles, of sufficient energy and frequency for the maintenance of normal
respiration.

Hyperpnea. — The normal rate of the respiratory movements is increased
by a rise in body-temperature, by active exercise, and by emotional states.
Whatever the cause, the increase in rate and 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 respiratory center, in consequence of which its excitabil-
ity 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 inter-
fere with the absorbing power of hemoglobin, and thus by diminishing the
quantity of oxygen absorbed lead to more frequent respirations. To the
hyperpnea induced by heat the term thermo-polypnea is frequently given.

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-

424 TEXT-BOOK OF PHYSIOLOGY.

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 temporarily increase
respiratory activity. With their disappearance the normal condition returns.

Apnea. — Apnea may be denned as a temporary cessation of the respira-
tory movements. It may be developed by rapid and deep inspirations due
to volitional efforts, by rapid mechanic inflation of the lungs, and by stimu-
lation of various afferent nerves. If one volitionally breathes rapidly and
deeply for a period varying from two to ten minutes, it will be found on
cessation of the effort that a condition of apnea is established which may
last for from thirty seconds to several minutes. One experimenter succeeded
after forcible inspiration for two and a half minutes, in establishing in
himself an apnea that lasted for several minutes before there was the slightest
desire to breathe. Before the cessation of the apnea, the face became pale
and corpse- like, indicative of a marked condition of anoxhemia. If the
lungs of an animal be rapidly inflated through a cannula inserted in the
trachea, a similar condition is developed. Whether the apnea be estab-
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,
the thorax will come to rest in the state characteristic of deep expiration
from inhibition of the inspiratory 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 CO2,
from the mechanic stimulation of the pulmonary 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

RESPIRATION. 425

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
percentage of the CO2 is the main cause.

Dyspnea. — Excessive and laborious respiratory movements constitute
a condition known as dyspnea. Movements of this character indicate that
the blood contains a greater percentage of CO2 than normal or a diminished
percentage of oxygen. In either case the excitability of the inspiratory center
is abnormally heightened. Of the two conditions, the former is by far the
more common. While it is 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 respiratory activity.

A deficiency in the amount or the quality of the hemoglobin is usually
attended with more or less dyspnea. These conditions of the blood may be
caused: (i) By all those pathologic conditions of the respiratory apparatus
which limit the free entrance of oxygen into and the free exit of carbon
dioxid from the blood; (2) by those alterations in the composition of the air
and subsequently in the blood which arise when the individual is confined in a
space of moderate size with imperfect ventilation.

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-

426 TEXT-BOOK OF PHYSIOLOGY.

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 it
bears. The periods of rest vary in duration from twenty to thirty seconds;
the periods of activity from thirty to sixty seconds and may include from
twenty to thirty respiratory movements.

Each period of rest of the respiratory mechanism is closed by the appear-
ance of a slight shallow respiratory movement, which is immediately fol-
lowed by a second, slightly deeper, and this in turn by a third, a fourth, a
fifth, and so on, each becoming deeper than the preceding until a certain
maximum is reached, after which, each succeeding movement gradually

FIG. 202. — TRACING SHOWING THE CHEYNE-STOKES FORM OF RESPIRATION. — (Hill.)

diminishes in depth until finally the movement becomes imperceptible and a
new period of rest supervenes. A graphic representation of the Cheyne-
Stokes type of respiration is shown in Fig. 202. This type of respiration is
frequently an accompaniment of certain pathologic conditions, e.g., uremic
states, cerebral hemorrhage, heart diseases, arteriosclerosis, etc., though no
satisfactory explanation of it has yet been presented. A similar though far
less marked periodicity in the respiratory movements is frequently observed
during sleep, especially in children. A periodicity can also be developed
by dividing transversely the medulla oblongata just above the calamus
scriptorius, which either injures the respiratory center or removes from it
some cerebral influence.

THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF
BLOOD THROUGH THE INTRA-THORACIC VESSELS, AND ON
THE ARTERIAL PRESSURE.

i. On the Intra-thoracic Vessels. — The forces which cause the air
to flow into and out of the lungs will at the same time and in 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 pulmonary
elastic tissue to recoil, the blood-vessels in the thorax at the end of an expira-

RESPIRATION. 427

tion sustain a positive pressure, the intra-thoracic pressure (see page 386),
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 pulmonary artery.

Coincident with these effects a similar effect is produced in the arterioles
and capillaries of the pulmonary alveoli. Situated between the two elastic
layers of the alveolar wall, embedded in a meshwork of connective tissue,
the pressure to which they are subjected at the end of an expiration will
also be a few millimeters less than 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 pulmonary veins. The left heart, being
in consequence more generously filled with blood, will discharge a larger
volume into the aorta at each contraction. During expiration the flow of
blood through the intra-thoracic vessels will be diminished for the reverse
reasons.

2. On the Arterial Pressure. — An examination of a tracing of the arterial
pressure will show that it is characterized by small 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 results are readily accounted for by the difference in the volume of
blood discharged by the left heart into the aorta during the time of the two
movements. If a tracing of the respiratory movements and of the blood-
pressure be taken simultaneously, it will be found that the rise of pressure
does not exactly correspond with inspiration, nor the fall of pressure with
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
flow of blood into the left heart continues and the pressure rises, but with the
return of the intra-thoracic pressure the vessels diminish in caliber, the
volume of blood transmitted by them becomes less, the output of the left heart
declines, and the pressure falls.

428 TEXT-BOOK OF PHYSIOLOGY.

The Traube-Hering Waves. — Under certain experimental conditions
the arterial blood-pressure tracing exhibits, in 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 stimulation of the vaso-motor centers.

CHAPTER XVI.
ANIMAL HEAT.

The chemic changes which take place in the tissues and organs of the
living body and which underlie all manifestations of life are attended by the
evolution of heat. In consequence of this each 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-ihermous 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 40° C. The
causes of this variation are doubtless connected with peculiarities of organ-
ization. In birds the rectal temperature is usually higher, varying from
40.9° C. in the pigeon to 44° C. in the titmouse and the swift.

In reptiles, amphibians, and fish chemic changes as a rule are not very
active and heat-production relatively slight. As they are devoid of a 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-tempera tured" 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
clinical interest. The temperature of the superficial portions of the body

429

430 TEXT-BOOK OF PHYSIOLOGY.

may be obtained by the introduction of a thermometer into the mouth,
the rectum, the vagina, or the axilla. As 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 20° 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.

As the chemic changes underlying physiologic activity vary in intensity
and extent in different regions of the body, there would be marked varia-
tions in their temperature were it not that the blood, having a large ca-
pacity for heat-absorption, distributes the heat almost uniformly to all por-
tions of the body, so that at a short distance beneath the surface the tem-
perature varies but a few degrees.

In the dog the temperature of the- blood in the aorta and in its principal
branches is approximately 38.3° C. In passing through the systemic cap-
illaries the temperature falls from radiation and conduction to surface
temperature, to again rise as the venous blood approaches the deeper regions
of the body. In the neighborhood of the renal veins and in the superior
vena cava, the temperature is again that of the aorta. In the portal vein
the temperature rises to 40.2° C.; in the hepatic vein, to 40.6° C. In the
right ventricle, owing to the admixture of blood from different localities
having different temperatures, the temperature falls to 38.2° or 40.4°. In
passing through the pulmonary capillaries the temperature of the blood
again falls, so that in the left ventricle it will register from 38° C. to 40.2° C.
There is thus usually a difference between the two sides of the heart of about

0.2° C.

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, exer-
cise, 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 0.5° C. to 1.5° C., the

ANIMAL HEAT. 431

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 0.5° 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 0.5° 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 1.5° 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 SOURCE AND TOTAL QUANTITY OF HEAT PRODUCED.

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-

432 TEXT-BOOK OF PHYSIOLOGY.

orimeter, the heat evolved by the oxidation of the food within, and dissipated
from, the body daily.

i. Direct Oxidation. — The amount of heat which any given food prin-
ciple 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 kilo-
gram degrees or calories; a gram calorie or kilogram Calorie being the amount
of heat required to raise the temperature of a gram or a kilogram (1000
grams) of water i° C. The apparatus employed for this purpose is termed
a calorimeter, which consists essentially of a closed chamber, in which the
oxidation takes place, surrounded by a water-jacket. The rise in tempera-
ture of the water indicates the amount of heat produced.

The results obtained by investigators employing different calorimeters
and different food principles of the same class vary, though within narrow
limits: e.g., i gram casein yields 5.867 kilogram Calories; i gram of lean
beef, 5.656; i gram of fat, 9.353, 9.423, 9.686 Calories; i gram of starch or
sugar, 4.116, 4.182, 4.479, etc-j Calories. These results are, however,
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
human body the carbohydrates and the fats, with the exception of the small
portion which escapes digestion, are reduced to carbon dioxid and water,
and hence 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-fhird 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 becomes 4.124 Calories.

The following estimates give approximately the number of kilogram
Calories which should be liberated within the body when the proteid is burned
to the stage of urea, and the fat and carbohydrate to the stage of carbon dioxid
and water:

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. The diet scale of Vierordt, for
example, yields the following:

120 grams of proteid 494.88 Calories

90 grams of fat 841.77 Calories

330 grams of starch 1358.28 Calories

Total 2694.93 Calories

ANIMAL HEAT.

433

The total Calories obtained from other diet scales would be as follows:
Ranke's, 2335; Volt's, 3387; Moleschott's, 2984; Atwater's, 3331; Hultgren's,
3436. These numbers indicate theoretically the total heat-production in
the body daily.

2. Calorimetric Measurements. — By this method the heat dissipated
from the body of an animal is directly collected and measured, and the
amount so obtained is taken as a measure of the heat evolved by the oxidation
of the food. A calorimeter is therefore an apparatus for the direct estimation
of the quantity of heat dissipated from the body in given time. The sub-
stance employed for collecting and measuring the heat is either water or
air. The calorimeters in general use consist essentially of twojjmetallic
boxes placed one within the other, though separated by a space sufficiently
large to hold a definite 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 air. The heat radiated is absorbed by the water and
its temperature raised. To
prevent loss by radiation
and to render it independ-
ent of changes in the
surrounding temperature
the calorimeter is sur-
rounded by a poorly con-
ducting material, such as
wool. The temperature of
the animal is taken at the
beginning and the end of
the experiment. If the tem-
perature of the animal re-
mains the same at the end
of the experiment, then the
heat absorbed by the water
represents the amount pro-
duced by the animal. If,
on the contrary, the tem-
perature of the animal rises

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
for the purpose of distributing the absorbed heat uni-
formly. To prevent the escape of heat with the expired
air, the tube D' is wound many times in the water-space
beneath the animal cage.

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
of the absolute amount of heat retained or lost by the animal above or
below the initial temperature, as well as that absorbed by the materials of
the apparatus in these various instances, the water equivalent of the tissues
of the animal and the materials of the calorimeter must be obtained, and
then added to or subtracted from, as the case may be, the amount of water
in the calorimeter, and the amount thus obtained multiplied by its rise in
temperature. In properly conducted experiments in which the sources
of error are reduced to a minimum there is a very close correspondence
between the total physiologic heat value of the food and the amount col-
lected by the 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 4419 Calories. .^The
28

434 TEXT-BOOK OF PHYSIOLOGY.

urine and feces during this period were collected and their heat value
determined, which amounted to 305 Calories. The heat which theoretic-
ally should have been produced was 4119 Calories. During the experiment
the calorimeter actually absorbed 3958 Calories, a difference between the
theoretic and experimental results of 156 Calories; thus of the total energy
liberated 96 per cent, appeared as heat.

Calorimetric experiments on man corresponding to those made by
Rubner on dogs have not been successful, owing purely to technical diffi-
culties. Various attempts have been made, however, to determine the daily
heat-dissipation. Liebermeister immersed a man in a bath with a tem-
perature lower than that of the man's body. From the rise in temperature
of the water it was calculated that the man produced daily 3525 Calories.
Leyden placed the leg alone of a man in a calorimeter. In one hour 6
Calories were absorbed. Assuming that the total superficial area of the body
was fifteen times that of the leg, he calculated, taking into consideration
various sources of error, that the entire body would produce daily 2376
Calories. Ott, employing a water calorimeter, found that the body of a
man produced 103 Calories during an afternoon, or at the rate of 2472 Cal-
ories daily. These and similar experiments, while not free from many
objections, furnish results which indicate that the heat dissipated from the
body approximates the physiologic heat values of the foods.

HEAT -DISSIPATION AND REGULATION OF THE TEMPERATURE.

Heat-dissipation. — From the preceding statements it is evident that
the body is continually liberating heat in amounts daily far in excess of
that necessary for the maintenance of the body-temperature. Should
this heat be retained, the temperature of the body would be raised at
the end of twenty-four hours an additional 18° or 20° C. — a temperature
far in excess of that compatible with the maintenance of physiologic pro-
cesses. 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 dissipation 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 eva-
poration of water from the lungs. (4) In evaporating water from the skin.
(5) In radiation and conduction from the skin. The quantities of heat lost
to the body by these different processes it is difficult for obvious reasons to
accurately determine, and the estimates usually given must be regarded only
as approximative.

Assuming 2500 Calories to be an average of heat liberated during a day

of repose, the losses, in the ways stated above, may be given as follows:

i. In Warming Food and Drink. — The average temperature of food and

drink is about 12° C.; the amount of both together is about 3 kilograms;

the specific heat of food about 0.8 that of water. The absorption

of body-heat therefore by the food amounts approximately to 3X0.8

X25° C. = 6o Calories = 2. 8 per cent. With the removal of the end-

ANIMAL HEAT. 435

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 12° 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 15X0.26X25°
= 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.

5. In Radiation and Conduction from the Skin. — The amount of heat lost

by this process can be indirectly determined only by subtracting the
total amount lost by the above-mentioned channels from the total
amount produced. Thus, 2500—777.44= 1725.6 Calories = 69 per cent,
would represent the average amount lost by radiation and conduction.
Regulation of the Mean Temperature. — In order that the mean
temperature of the body may remain practically constant, the heat dissipated
must be exactly balanced by the heat liberated. Should there be any want
of correspondence between the two processes, there would arise either an
increase or a decrease in the mean temperature. As both heat-production
and heat-dissipation are variable factors, dependent on a variety of internal
and external conditions, their adjustment is accomplished by a complex self-
regulating mechanism involving muscle, vascular, and secretor elements,
coordinated by the nerve system.

Heat-Production. — 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 coming through afferent nerves from the muscles themselves,
the joints, tendons, and skin. As a result of this slight but constant stimula-
tion of the spinal cord, the metabolic changes in muscle material are main-
tained 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 production. Therefore work and exercise of all kinds which involve
a more rapid contraction of the skeletal muscles is attended with increased
heat production. The consumption of foods that have a higher potential
heat value also contribute to the amount of heat produced. Foods have
different physiologic heat values. If the food consumecl contains much

436 TEXT-BOOK OF PHYSIOLOGY.

potential energy and quantity consumed be larger than the average daily
requirements, there will be an increase in heat-production. A lowering
of the external temperature, as in the winter season, leads to increased heat-
production through stimulation of the nerve-centers. When all these con-
ditions, viz : 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 as4726 Calories daily (Hultgren).
In winter time, the lowering of the external temperature leads through
reflex stimulation of the spinal motor centers to a larger discharge of
nerve impulses to muscles leading to increased activity and increased heat-
production.

Heat-Dissipation. — Heat-dissipation varies in rapidity in accordance
with variations of a number of factors, but principally with variations in the
external temperature and the activity of the perspiratory apparatus. The
heat is dissipated mainly by the skin, about 85 per cent., in consequence of
radiation and conduction and by the evaporation of the sweat. The loss
by this channel as well as from the lungs is dependent for the most part on
a difference of temperature of the surrounding air and of the body. If the
surrounding temperature is high, there is an increase in the activity of both
the circulatory and respiratory mechanisms, brought about by the central
nerve system. In addition to an increased action of the heart, the
blood-vessels of the skin dilate, and deliver to the surface a larger volume
of blood in a given time, thus increasing the conditions favorable to
radiation. The sweat glands at the same time are stimulated to
increased activity by the central nerve system (the sweat centers in the
spinal cord) by the action of nerve impulses transmitted from the skin
developed by the action of the higher temperature on the nerve
endings; hence, 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.

If on the contrary, the external temperature falls 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.
The blood, being prevented from coming to the surface, is retained in the
deeper portion of the body, and in consequence the conditions for radiation
are diminished. These variations in the cutaneous circulation in reponse
to variations 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
temperature maintained.

ANIMAL HEAT. 437

Radiation from the skin is modified to some extent by clothing. An
excess of clothing decreases, 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.

The foregoing estimates as to the amounts of heat produced have refer-
ence only to the body in repose. When the body passes into a state of
muscle activity, there is at once a notable increase in heat-production in con-
sequence of the increase in the activity of the chemic changes which underlie
body activity, as shown by the increase in the consumption of oxygen and
the production of carbon dioxid. Not all of the potential energy set free,
however, appears as heat; for, if the muscles are engaged in doing work a
part of the energy, which would otherwise manifest itself as heat is converted
into mechanic motion. From the work done during a period of eight hours
it has been estimated that about 500 Calories are so transformed or utilized.
Hirn calculated from an average of five experiments that a man weighing
67 kilos in repose produced 154.4 Calories per hour and absorbed 30.7 grams
of oxygen per hour; but when engaged in active muscle movements pro-
duced 271.2 Calories and absorbed 119.84 grams of oxygen per hour. The
increase in heat-production per hour during activity was thus almost doubled,
though the sum total 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 there is a greatly diminished heat-production, not more
than 40 calories per hour being produced. The preceding data may be
tabulated as follows (Martin) :

Day of Rest. Day of Work.

Heat units (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

CHAPTER XVII.
SECRETION.

Secretion. — Secretion is a term applied to a process by which complex
fluids are formed from the constituents of the lymph which is separated
from the blood-stream by the activities of the endothelial cells of the capil-
lary 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 nitration. This separated or secreted material 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 of widely different application. The fluids thus formed
are utilized for the most part to meet some special need of the body.
All such fluids are termed secretions.

All secretions are products of the activities of epithelial cells covering
a flat, or lining a more or less complexly involuted, membrane which in
each instance may be termed a secretor organ. As the fluids for the most
part are poured out. on the surface of the body, they have been termed
external secretions: e.g., mucus, saliva, gastric juice, milk, sebaceous matter,
etc. Within recent years it has been demonstrated that the epithelium of
certain organs, for example, of those which do not possess a duct, such as
the thyroid, adrenals, hypophysis, etc., also produces certain specific con-
stituents which are however returned to the blood, and which in some un-
known but yet favorable way influence the general nutrition. To such
products of these organs the term internal secretions has been given.

The blood, in addition to its nutritive constituents, contains a number of
principles, derived from the tissues, which are to be regarded as waste pro-
ducts, the outcome of the katabolic activity of the tissues and apparently
of no further use to the body. If retained, they would seriously if not
fatally interfere with the normal physiologic activities of the different tissues.
They are therefore removed by specialized organs after their separation from
the blood-stream. The waste products in solution thus removed are not
capable of being utilized for any specific purpose, and are therefore termed
excretions: e.g., urine, perspiration, etc. Excretion, also, is performed
by the activities of epithelial cells aided by the physical forces of diffusion,
osmosis and filtration; and though a distinction is made between the two
classes of fluids, no sharp line can be drawn between the cell processes which
take place in secretor and excretor organs.
All secretor organs may be divided into —

1. Epithelial.

2. Reticular and vascular, the latter term indicating merely their relation

to blood-vessels.

438

SECRETION. 439

The Epithelial Secretor Organs. — The epithelial secretor organ
consists primarily of a thin delicate homogeneous membrane, one side of
which is covered with a layer of epithelial cells and the other side of which
is closely invested by a network of capillary blood-vessels, lymph-vessels,
and nerves. Though the epithelial cells have a general histologic 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.

The epithelial secretor 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-
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, as have been
the digestive secretions.

The secreting glands are formed of the same histologic elements as
the secreting membranes. They are formed by an involution of the mucous

440 TEXT-BOOK OF PHYSIOLOGY.

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
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 molecular structure 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 protein's,
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

SECRETION.

441

ducts to be discharged on the surface of the mucous membrane or skin, as the
case may be.

Influence of the Nerve System. — The activity of every gland is 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.

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 94) and will again be in subsequent chapters.

The reticular and vascular glands, though not possessing any common
histologic features, are grouped together merely for convenience, and will
be considered in a separate chapter in connection with the problems of
internal secretion.

MAMMARY GLANDS.

The mammary glands, which secrete the milk, are two more or less
hemispheric organs situated in the human female on the anterior surface of

the thorax. Though rudimentary in
childhood, they gradually increase in
size as puberty approaches. The gland
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. 204. — MAMMARY GLAND. i.
Lactiferous ducts. 2. Lobuli of the
mammary gland.

FIG. 205. — ACINI OF THE MAMMARY
GLAND OF 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.

442 TEXT-BOOK OF PHYSIOLOGY.

During utero-gestation the mammary glands become larger, firmer,
and more tabulated; 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 areola
expands into a fusiform ampulla or reservoir. At the base of the nipple
the ampullae 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
(Fig. 204).

On tracing the lactiferous duct into a lobe, it is found to divide and
subdivide into a number of branches, which pass into smaller masses — the
lobules. The lobule in turn is composed of a large number of tubular acini
or alveoli, the final terminations of the lobular ducts. Each acinus consists
of a basement membrane lined by a single layer of low cuboidal epithelial
cells (Fig. 205). Externally the acinus is surrounded by blood-vessels,
nerves, and lymphatics.

MILK.

Milk as obtained during active lactation is an opaque bluish- white fluid,
almost inodorous, with a sweet taste, an alkaline reaction, and a specific
gravity of from 1.025 to 1.040. Examined microscopically, 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 To"iroir of an mcn m diameter. It has been asserted by some observers
that each globule is surrounded by a thin proteid envelope which enables it
to maintain the discrete form. This, however, is at present disbelieved.

The quantity of milk secreted daily by the human female averages about
1200 c.c.

Chemic analysis has shown that the milk of all the mammalia consists
of all the different classes of nutritive principles, though in different propor-
tions, which are necessary to the growth and development of the body. The
only exception appears to be an insufficient amount of iron for the formation
of the coloring-matter of the blood, the hemoglobin.

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

SECRETION.

443

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
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 coagules the caseinogen in a manner similar to, if not identical
with, this process, which appears to be essential to the normal digestion of
the milk.

The fat of milk is more or less solid at ordinary temperatures. It is a
compound of olein, palmitin, and stearin with small quantities of butyrin
and caproin. The melting-point of butter varies between 31° and 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 together and form a coherent mass termed
butter.

Lactose is the particular form of sugar characteristic of milk. It belongs
to the saccharose group and has the following composition: 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.

The inorganic salts of milk are chiefly potassium, sodium, calcium,
and magnesium phosphates and chlorids. Iron is also present in small
amount. The following table of Bunge gives the quantitative amounts of
these constituents in both human and cow's milk:

In 1000 Parts

Potas-
sium.

Sodium.

Calcium

Magne-
sium.

Iron
Oxid.

Phos-
phoric
Acid.

Chlorin.

Human milk

0.78

O.2S

0.33

0.06

0.0036

0.47

0.47

Cow's milk

1.76

i .11

i . qo

O.2I

0.0030

i .07

i. 60

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

444 TEXT-BOOK OF PHYSIOLOGY.

into the lumen of the acinus. During the time these changes are taking
place oil-globules make their appearance in the cell protoplasm, some of
which are discharged separately into the lumen, while others remain fora
time associated with the detached portion of the cell (Fig. 206). 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 nitration. 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. 206.— SECTION OF THE suction efforts on the part of the child, aided by
MAMMARY GLAND OF A CAT atmospheric pressure and the contractions of
rATZEriavTHyEof°aiveAoCH *e non-striated muscle-fibers of the lactiferous

filled with granules and glob- ducts.

ules of fat. i, 2, 3. Epithe- Influence of the Nerve System. — Judging
milk" torn analogy, it is probable that the secretion of
milk is regulated by impulses emanating from the
nerve system, though the exact nerve-channels for the transmission of such
impulses have not been determined experimentally. Various attempts
have been made to isolate and study these nerves, but the results are incon-
clusive. It is well known that emotional states on the part of the mother
modify the quantity as well as quality of milk, indicating a connection
between the gland-cells and the central organs of the nerve system. Nerve
terminals have been discovered in and around the epithelial 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
emigrated leukocytes. Colostrum is distinguished from milk in being
richer in sugar and inorganic salts. It is said to possess constituents which
act as a laxative to the young child.

THE LIVER.

The liver is a large gland situated in the upper and right side of the
abdominal cavity, where it is held in position largely by 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

SECRETION.

445

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. 207). This is not so ap-
parent in man. Each lobule is made up S&tftii&ZP^ ^Trabecuiae of
of irregular or polygonal shaped cells
measuring about 30 to 40 micromilli-
meters in diameter. These cells are ar-
ranged in a radial manner from the center to

XTrabeculae of
hepatic cells.

Central vein.

Interlobular vein. Hepatic duct.

FIG. 208.— SCHEME or A HEPATIC
LOBULE, REPRESENTED IN TRANSVERSE
SECTION BELOW AND, BY PARTIAL LEV-
FIG. 207. — SECTION OF LIVER OF PIG, SHOWING ELING, IN LONGITUDINAL SECTION
VERY DIAGRAMMATIC ALLY 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.— (Piersol.) —(Stohr.)

the circumference of the lobule (Fig. 208). Each cell possesses ome and
at times two nuclei. There is no evidence for the existence of a distinct
cell-wall. The cell protoplasm frequently contains globules of fat, gran-
ules of a protein nature, granules of glycogen, pigment material, etc.
The appearance presented by the cell will vary considerably, according
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,
and hence are in close relation to capillary blood-vessels, lymphatic 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:

446

TEXT-BOOK OF PHYSIOLOGY.

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 limiting them. From their situation they
are termed interlobular veins and arteries.

The interlobular veins give off small capillary vessels which penetrate the
lobule at all points of its surface. These capillaries, though frequently
anastomosing, form a radial meshwork which converges toward the center
of the lobule. In the meshes of this plexus are found, arranged in a corres-
ponding radial manner, the liver cells. The interlobular arteries are distrib-
uted to the walls of the portal vein, to the connective tissue, and finally
terminate in the portal vein capillaries. The intralobular capillaries thus
receive and transmit blood which is an admixture of both arterial and venous

FIG. 209. — 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.)

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 underly-
ing or sublobular vein (Fig. 209) . These latter vessels, uniting and reuniting,
ultimately form the hepatic vein, which empties the blood into the inferior
vena cava.

Bile Capillaries and Hepatic Ducts. — The bile capillaries are narrow
channels which penetrate the lobule in all directions and are generally found
running along the sides of the cells. These channels, which are devoid of
walls, receive from the cells some of the products of their 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

SECRETION. 447

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.

Influence of the Nerve System. — 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 splanch-
nics and the vagi. Many of the nerves which enter the liver are vaso-motor
in function; as to whether others are secretor in character is yet a subject
of investigation. It has been asserted that nerve filaments have been demon-
strated running between the cells and even penetrating their substance.
This fact would indicate that the metabolic processes of the liver are under
the control of the central nerve system.

Functions of the Liver. — The anatomic and histologic peculiarities of
the liver would indicate that it has a variety of relations to the general pro-
cesses of the body. Experimental investigation has brought some of these
relations to light. Though its physiologic actions are not yet wholly under-
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).

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 191) . 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

448 TEXT-BOOK OF PHYSIOLOGY.

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,
a rise of pressure beyond that of the capillary blood-vessels, and a reabsorp-
tion by the lymph-vessels of the bile constituents. After their discharge into
the blood from the thoracic duct these constituents are deposited in part
in various tissues, giving rise to the phenomena of jaundice, and in part are
eliminated in the urine.

The Production of 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 filtration. The filtrate 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 glucose it received the name of glycogen; and inasmuch as the

SECRETION. 449

liver continually produces glycogen it is said to have a starch-forming or a
glycogenic or an amylogenic 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 glyco- genetic Junction.

The presence of glycogen in the liver cells can be shown microscopically
in the form of discrete hyaline and refractive masses. 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 produc-
tion of glycogen is dependent very largely on the consumption of carbohy-
drates, the greater the amount of sugar and starch in the food, the greater
being the production of glycogen. Nevertheless it is also certain that gly-
cogen can be derived from proteins; for if the carbohydrates are excluded
from the food and the animal fed on a pure protein diet, glycogen will con-
tinue to be formed in the liver though in far less amounts.

The facts connected with the formation of glycogen, as well as with its de-
struction as at present generally accepted, may be stated as follows: The
dextrose 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
directly to the liver, where a certain portion of it diffuses through the cap-
illary walls into the surrounding lymph spaces; by the action of the cells
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, trans-
form the glycogen into glucose or dextrose, return it to the blood, by which
it is transported to the systemic capillaries, where it disappears again, diffus-
ing through 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. Ulti-
mately, however, through oxidation 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.
29

450 TEXT-BOOK OF PHYSIOLOGY.

In opposition to this view, Dr. Pavy, after years of accurate experimenta-
tion, states that the blood on the cardiac side of the liver never under nor-
mal circumstances contains a larger percentage of sugar than is to be found in
any part of the circulation, except in the portal vein. He states that glycogen
is never reconverted into sugar, and denies that the liver produces sugar, to
be discharged into the blood; 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 mate-
rial 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 pro-
duced in the liver is utilized in the formation of fat and the synthesis of
complex proteins necessary to the construction of the tissues.

The Influence of the Nerve System.— The results of various experi-
mental investigations indicate that the production of sugar from the glycogen
in the liver is influenced by the activities of the nerve system. It was dis-
covered 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 to six hours in the rabbit and from two to three or even seven in
the dog. For this reason Bernard gave to this area the name of "diabetic
area."

Coincident with the appearance of sugar in the urine (glycosuria) there
is an increase in the percentage of sugar in the blood (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. This supposition receives support from the
fact that the degree of the hyperglycemia, and the subsequent glycosuria,
will depend on the amount of glycogen previously in the liver. If the animal
has been well fed on carbohydrates, the resulting glycosuria will be pro-
nounced; 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 influence
the conversion of glycogen into sugar, the question arises as to whether the
puncture destroys the nerve-cells, or whether it stimulates them to increased
activity. The results of experiment lead to the latter supposition. Thus
if the vagus nerve is divided in the neck and its central end stimulated there
is developed a glycosuria. Stimulation of other sensor nerves has a similar
effect. As stimulation of the vagus has the same effect as the puncture, the
inference is that the center is normally excited to physiologic activity by
impulses reflected from some surface or organ in the peripheral distribution
of this nerve.

If the nerve-cells in the diabetic area regulate the production of sugar in
the liver, the further question arises as to the pathway through which the
nerve impulses emanating from them reach the liver, whether by way of the

SECRETION. 451

vagi or by way of the spinal cord and splanchnic nerves. That it is not by
way of the vagi is shown by the fact that the glycosuria established by the
puncture does not disappear when they are divided; that it is by way of the
spinal cord, as far at least as the first dorsal nerve, and subsequently the
splanchnic nerves, is indicated by the fact that a cross-section of the spinal
cord above this level, destruction of the upper three dorsal roots as well as
division of the spanchnic nerves prevents the development of the glycosuria
which follows puncture of the medulla. Though stimulation of the upper
dorsal (pre-ganglionic) nerve-fibers gives rise to glycosuria, yet, contrary to
expectation, stimulation of the splanchnic (post-ganglionic) nerve-fibers
does not have the same effect. This may be due, however, to changes in
the relation of the capillary blood-vessels to the liver cells or to the character
of the stimulus employed.

A further question arises as to whether the nerve impulses which pass
from the diabetic center to the liver are vaso-motor in character, exerting
their effect on the blood-vessels, or whether they are secretor in character
and exerting their effect on the liver cells. Bernard was of the opinion that
they are vaso-motor in character and that the diabetic area was a part of the
general vaso-motor center. More recent investigators are of the opinion
that they are secretor in character, for the reason that whether the blood-
pressure rises from a stimulation of the central end of the divided vagus, or
falls from a stimulation of the depressor nerve, in each instance there follows
a glycosuria.

If the production of sugar in the liver is a reflex act as Bernard supposed,
taking place through a mechanism consisting of an afferent pathway, the
vagus nerve, and an efferent pathway consisting of the spinal cord and
splanchnic nerves, the question arises as to the seat of action of the stimulus.
This Bernard located in the lungs, for the reason that though division of the
vagus in the neck checks the production of the sugar, division below the
origin of the pulmonary branches had no such effect.

Muscle Glycogen. — Glycogen is also found in muscles and to some
extent in the placenta, and embryonic tissues generally. Chemic analysis
has shown that muscles contain from 0.5 per cent, to i per cent, 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-
much 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
hydration, of the sugar of the blood. By reason of this fact it may be said
that the muscle also possesses a glycogenic 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 necessary 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 assumption is made that a special enzyme is present

452 TEXT-BOOK OF PHYSIOLOGY.

and active. The muscle is therefore said to possess or exhibit a glyco-gen-
etic function. 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 of glycogen rapidly increases un-
until the normal is regained.

The metabolism of the carbohydrates or the manner in which they are
stored, transformed and finally oxidized is a subject about which there is
much obscurity and uncertainty. Some light is thrown on the problem by a
consideration of the pathologic state known as

Diabetes. — Diabetes is a chronic disease characterized by the appear-
ance of sugar in the urine in variable amounts. Under normal circumstances
all the sugar consumed as food undergoes oxidation to carbon dioxid and
water, none appearing in the urine except the merest trace. In some dis-
ordered states of nutrition this oxidation is imperfect or entirely lacking.
The sugar therefore accumulates in the blood after which it is eliminated in
the urine in large, though variable amounts, a condition which may endure
for months or years though eventually leading to the death of the individual.
The pathologic condition underlying this incomplete oxidation is imper-
fectly understood and has usually been associated with derangements of the
glycogenic function of the liver, though doubtless derangements of other
organic functions will produce the same condition. At the present time it
is believed that the persistent excretion of sugar by the kidneys depends on
several causes: (i) An ineffectual abstraction and storage of sugar due to
some impairment in the activity of the liver cells ; (2) an incomplete oxida-
tion in the muscles by reason of the absence of the necessary enzymes; (3) a
cleavage of the protein constituents of the tissues, in consequence of some
profound alteration in the nutritive process, whereby their glucose radicals
are liberated in unusual amounts.

The physiologic mechanism by which the normal metabolism of the carbo-
hydrates is regulated is obscure and but imperfectly known. That it is com-
plex in character is shown by the phenomena which follow not only puncture
of the medulla, and other injuries to and disturbances of the nerve system, but
also removal of the pancreas and the administration of various toxic agents.

Nerve influences, whether the result of injuries to the nerve system or of
various pathologic states occasionally apparently precede and cause the
diabetic state, though the manner in which they do so is practically unknown.
Some light is thrown on the process by a consideration of the facts
detailed in a previous paragraph relating to the influence of the nerve system
in the production and storage of glycogen in the liver. It is quite possible
that nerve impulses abnormally developed may lead to an incomplete
storage of glycogen by the liver-cells, the result of an imperfect nutrition or
a deranged circulation.

Removal of the pancreas from the body of a dog or other animal is in a
few days followed by a rise in the percentage of sugar in the blood and its
elimination by the kidneys. In a short time acetone, aceto-acetic and
/?-oxybutyric acids make their appearance, attended by the usual symptoms
characteristic of glycosuria in man. Death usually occurs at the end of
three or four weeks. The quantity of sugar excreted and the gravity of the

SECRETION. 453

attendant symptoms may be much diminished by allowing a portion of the
gland to remain in situ, even though its capacity for the production of pan-
creatic juice is entirely abolished. Transplantation of the pancreas to the
subcutaneous tissue or to the abdominal cavity will practically prevent the
glycosuria. The explanations which have been offered as to the manner in
which the pancreatic tissue prevents and its absence gives rise to the excre-
tion of sugar are purely hypothetical. It has been claimed by some in-
vestigators that the pancreas secretes a specific material, which after its en-
trance into the blood and its distribution to the tissues, particulary the muscles,
promotes oxidation of the sugar. In the absence of this material the
oxidizing power is lost and hence the sugar accumulates, and is finally
eliminated by the kidneys. Since the discovery of the islands of Langer-
hans it has been suggested by some investigators that the production of the
material which regulates carbohydrate metabolism should be attributed to
them rather than to the pancreas as a whole. The presence however of a
glycolytic enzyme in either the pancreas or the muscle has not been positively
demonstrated, but if sugar be subjected to the action of a mixture of pancre-
atic and muscle juices, it is quickly oxidized, from which it has been inferred
that the secretion of the pancreas activates the enzyme of the muscle. That
the pancreas is actively associated with carbohydrate metabolism is indica-
ted by the fact that in a considerable percentage of cases of diabetes lesions
of the pancreas, more or less extensive, have been found. The sugar excreted
doubtless in part comes from the glycogen of the liver, as this disappears in a
short time. But as sugar continues to be excreted, even though all carbohy-
drates be withdrawn from the food, the conclusion is justifiable that it
arises in consequence of increased protein metabolism. This supposition is
strengthened by the fact that the quantity of urea excreted rises and falls with
the quantity of sugar excreted.

Phlorizin, a glucoside obtained from the root bark of the cherry and
plum tree, gives rise to the appearance of sugar in the urine, in amounts
beyond that which might come from the glucose normally present in the
blood or from the glycogen of the liver. As there is a concomitant increase
in the amount of urea excreted, the supposition is that phloridzin increases
protein metabolism.

Curara, in doses sufficient to paralyze the muscles, also gives rise to the
appearance of sugar in the urine. This is not due, however, to an increased
production on the part of the liver, but rather to a want of consumption on
the part of the muscles, due to their inactivity. The accumulation of the
sugar in the blood which takes place for this reason leads very promptly to
its removal by the kidneys.

The Formation of Urea. — It is now generally believed that the liver
is the most active of all the organs which may be engaged in the production
of urea. This belief is based on numerous physiologic and pathologic
data. The compounds out of which the hepatic cells construct urea have
been for chemic reasons asserted to be the ammonium salts, e.g., the car-
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

454 TEXT-BOOK OF PHYSIOLOGY.

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 the amino-acid compounds in the in-
testine which are not needed for the reconstruction of the protein molecule.
These compounds are absorbed by the epithelial cells of the villi and
mucous membrane generally, deamidized or deprived of their amino-acid
nitrogen (NH2) which is at once converted into ammonia. The ammonia
in turn combines 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) 2CO3 - 2H2O = 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 indoxyl sulph-
ate 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 are like-
wise reduced in toxicity by the liver cells though the methods by which this
is accomplished vary with the nature of the compound. The liver thus
presents a chemic defense against the entrance of more or less toxic agents
into the blood of the general circulation.

VASCULAR OR DUCTLESS GLANDS.

INTERNAL SECRETIONS.

The metabolism of the body generally, as well as that of individual
organs, has been shown to be related not only to the physiologic activity
of such organs as the liver and pancreas, but also to the activity of the so-

SECRETION.

455

-4

called vascular or ductless glands. The influence of the pancreas in regulat-
ing the oxidation of sugar and the influence of the liver in the maintenance
of the general metabolism through the production of glycogen and the
formation of urea, are now established facts. That the vascular or ductless
glands to an equal extent, though perhaps in a different way, assist in the
maintenance of physiologic processes, appears certain from the results of
animal experimentation. The explanation given for the influence of these
glands is that they produce specific substances, which are poured into the
blood or lymph and carried direct
to the tissues, to the activities of
which they appear to be essential;
for without these substances the
nutrition of the tissues declines and
in a short time a fatal termination
ensues.

Inasmuch as these partly un-
known substances are formed by
cell activity and are poured into
the circulating blood, they have
been termed " internal secretions."
Though the term internal secre-
tions is applicable to all sub-
stances which arise in consequence
of tissue metabolism, and which,
after being poured into the blood,
influence in varying degrees and
ways physiologic processes, yet the term in this connection will be applied
only to the secretions of the thyroid and parathyroid glands, pituitary body
or hypophysis cerebri, and adrenal bodies.

Thyroid Gland. — The thyroid gland or body consists of two lobes situated
on the lateral aspect of the upper part of the trachea (Fig. 210). Each lobe
is pyriform in shape, the base being directed downward and on a level
with the fifth or sixth tracheal ring. The lobe is about 50 mm. in length,
20 mm. in breadth, and 25 mm. in thickness. As a rule, the lobes are united
by a narrow band or isthmus of the same tissue. The gland is reddish in
color, and abundantly supplied with blood-vessels and lymphatics.

Microscopic examination shows that the thyroid consists of an enormous
number of closed sacs or vesicles, variable in size, the largest not measuring
more than o.i mm. in diameter (Fig. 211). Each sac is composed of a thin
homogenous 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 lymphatics. The individual sacs are
united and supported by connective tissue, which forms, in addition, a cov-
ering for the entire gland.

Effects of Removal of the Thyroid. — The knowledge at present pos-
sessed as to the function of the thyroid gland, especially in mammals, is the
outcome of a study of the effects which follow its arrested development in
the child, its degeneration in the adult, and its extirpation in the human

FIG. 210. — VIEW OF THYROID BODY. i.
Thyroid isthmus. 2. Median portion of crico-
thyroid membrane. 3. Crico-thyroid muscle.
4. Lateral lobe of thyroid body. — (After Morris.)

456

TEXT-BOOK OF PHYSIOLOGY.

being as well as in animals. The results, however, which follow its extirpation
are not always uniform in all animals, though sufficient reasons for the
lack of uniformity cannot always be assigned.

Cretinism, a condition characterized by a want of physical and mental
development, is associated with, if not directly dependent on, a congenital
absence of the thyroid, or its arrested development during the early years
of childhood.

Myxedema, a condition of the skin in which there is a hyperplasia of
the connective tissue, of an embryonic type, rich in mucin, is generally

regarded as one of the
effects of degenerative pro-
cesses in the thyroid in the
adult. Partly in conse-
quence of this change in
the skin the face becomes
broader, swollen, and flat-
tened, giving rise to a loss
of expression. At the same
time the mind becomes
dull, clouded, even approxi-
mating the idiotic type.
This supposed infiltration
of the skin with mucin was
termed myxedema by Ord,
who at the same time asso-
ciated it with a change in
the structure of the thyroid
as a result of which it be-
came functionally useless.

Extirpation of the thy-
roid, for relief from symp-
toms due to grave patho-
logic changes, has been followed in human beings by symptoms similar to
those of myxedema. To this condition the terms operative myxedema and
cachexia, strumipriva have been applied.

After the publication of the history of the myxedema which followed
surgical removal of the thyroid, Schiff, in 1887, repeated his earlier experi-
ments on dogs, and found again that removal of the thyroid was speedily
followed by tremors, convulsions, and death. Similar experiments were
made by Horsley on monkeys, with results which resembled those character-
istic of myxedema. Among the symptoms which developed within a few
days after the removal of the gland may be mentioned loss of appetite;
fibrillar contractions of muscles; 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 terminating 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.

FIG 211. — A LOBULE FROM A THIN SECTION OF THE
THYROID GLAND OF AN ADULT MAN. i. Colloid sub-
stance. 2. Epithelium. 3. Tangential section of a tubule,
the epithelium viewed from the surface. 4. Tubule in
transverse section. 5. Connective tissue. — (Stohr.)

SECRETION. 457

It is evident 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, rendering them innocuous
and thus preserving the body from a species of auto-intoxication, is gradually
yielding to the more probable view that the epithelium is engaged in the
secretion of a specific material, which finds its way into the blood or lymph
and in some unknown way influences favorably tissue metabolism. This
view of the function of the thyroid is supported by the fact that successful
grafting of a portion of the thyroid beneath the skin or in the abdominal
cavity will prevent the usual symptoms which follow thyroidectomy. The
same result is obtained by the intravenous injection of thyroid juice or by
the administration of the raw gland. It was shown by Murray that myxe-
dematous patients could be benefited, and even cured, by feeding them with
fresh thyroids or with the dry extract.

The chemic features of the material secreted and obtained from the
structures of the thyroid indicate that it is a complex 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 0.33 to i
milligram for each gram of tissue. To this compound the term thyro-iodin
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. In normal
states of the body, thyro-iodin 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.

It has also been suggested from the clinical side that the symptoms
comprised under the term exophthalmic goiter, viz., enlargement (hyper-
trophy) of the thyroid, rapid action of the heart, pulsation of the large
arteries at the base of the neck, protrusion of the eye-balls and fine tremors
of the hands are due to a hypersecretion of the thyroid cells, a condition
spoken of as hyperthyroidism.

The conclusions as to the functions of the thyroid gland which have been
drawn from the results that have followed its removal from animals by
surgical procedures, have been made questionable, since the discovery of
the parathyroid glands and a study of the phenomena which follow when
they alone are removed. From their situation and close relationship to the
thyroid gland it is generally accepted, that in the earlier experiments, espe-
cially those made on cats and dogs, and some other carnivorous animals, both
sets of glands were removed and hence some of the symptoms which devel-
oped 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 the
simultaneous removal of the parathyroids. The myxedema and the failure
of the mental powers are attributed to the loss or degeneration of the thyroid,
and cretinism to the arrest of its development.

458

TEXT-BOOK OF PHYSIOLOGY.

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 situated 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. 212). Micro-
scopically the parathyroids consist of 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 contractions and paralyses of

groups of muscles and not infrequently
convulsive seizures and coma. During the
convulsion there is an acceleration of the
heart-beat, and increase in the respiratory
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.)

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.

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 experiment-
ers state that while it is true, that, as a
rule, the removal of both thyroids and para-
thyroids in the carnivora is a fatal opera-
tion, there are nevertheless many excep-
tions; and in the mammalia generally, e.g.,

cats, dogs, foxes, guinea-pigs, rats, and monkeys, the exception becomes the
rule as more than 51 per cent, of animals survived the operation for a pro-
longed period and of these 68 per cent, showed no specific symptoms of any
kind. From the contradictory observations it is evident that the subject
needs further investigation.

The Pituitary Body. — The pituitary is a small body lodged in the
sell a turcica of the sphenoid bone. It measures 14 mm. from side to
side, 8 mm. from befcre backward, and 6 mm. from above down, and con-
sists of an anterior lobe somewhat pink in color, and a posterior lobe yellow-
ish-gray in color. The anterior lobe is much the larger and partly embraces

FIG. 212. — The position of the para
thyroid glands. — (Zitckerkandl.)

SECRETION. 459

the posterior lobe. (Fig. 213) The anterior lobe is developed from an in-
vagination 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 func-
tions 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
same in many animals though the details vary some- SECTION "I'
what in each animal. In the monkey the arrangement ITARY BODY AND INFUN-

of the anatomic parts (Fig. 214) is similar to the ar- DIBULUM WITH ADJOIN-

T, \n ? , 7 , , . ING PART OF THIRD VEN-

rangement in man. It will be observed that the posterior TRICLE. a. Anterior

lobe is solid and that there is no open connection with lobe. a'. A projection
the cavity of the third ventricle; that it is invested, over oT^eS^fundlbulumT
a large part of its surface, by a thin layer of epithelium. Posterior lobe connected
The anterior lobe, which lies in front of it is separated by a stalk ™ih the in-
by a cleft which is the remnant of the cavity of the £f £^c ctasm.-
buccal pouch. Though the appearance of the ante- (Schwaibefrom Quain.)
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 epithelial investment is
usually spoken of as the pars intermedia, and regarded histologically and
physiologically as a part of the posterior lobe. Superiorly the anterior lobe
and the pars intermedia are united, though a portion of the latter passes
upward and embraces, if it does not entirely surround, the infundibular
stalk; inferiorly and posteriorly the two bodies also unite. The posterior
surface of the posterior lobe is free from epithelial investment in the mid-line.
The extent to which the epithelium invests the posterior lobe varies in dif-
ferent animals. In the cat and dog it is almost complete.

Microscopic examination of the anterior lobe shows the presence of
granular epithelial cells, the descendents of the original buccal epithelium,
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-

460

TEXT-BOOK OF PHYSIOLOGY.

bk d

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
parsnervosa, these hyaline masses are believed to represent an internal secretion
which passes upward through loose tissue channels toward, the infundibulum
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. 214.— MESIAL SAGGITAL SECTION OF THE PIT- movai f tup hvnonhvsi* hv

UITARY BODY OF THE MONKEY, a, Optic chiasm; 6, r inf . nyP°

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 in vestment of the posterior lobe; &,epi the- -, , -, , , . ,

Hum 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
opposite opening in the skull, the opposite half of the cerebrum is displaced
and protruded so that injury to the brain from compression is prevented.
The gland can then be picked up with forceps and removed. Paulesco
reported that the total removal of the gland in 24 dogs resulted in death in
24 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.

SECRETION. 461

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 corrob-
orative 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 life appar-
ently depended on the amount and the cellular activity of the parts left be-
hind. As a result of the partial removal only there developed a series of
phenomena to which the term cachexia hypophyseopriva has been given.
These phenomena varied somewhat in accordance with the age of the
animal. Adult animals became adipose and degenerated sexually, young
animals likewise became adipose but they remained undersized and failed
to develop sexual characteristics and hence sexual infantilism persisted.
The organs of reproduction in both sexes remained rudimentary. The
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 the results
of a series of experiments, subsequently published by Gushing, led to the
belief that some of these phenomena were due to injury or impairment of the
normal function of the posterior lobe at, or subsequent to, the time of the
operation. Just which of these phenomena are due to a diminished secre-
tion of the anterior lobe and which to a diminished secretion of the posterior
lobe future investigations only will determine.

The Effects of Pathologic Conditions. — In recent years the idea has gradu-
ally developed that certain pathologic states of the body are associated in
some way with pathologic states of the pituitary body. Thus the condition
of gigantism 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 gigantism 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

462 TEXT-BOOK OF PHYSIOLOGY.

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 are believed to be due to a diminished or defective secretion
partly of the anterior lobe and partly of the posterior lobe. To this condi-
tion the term hypopituitarism has been given.

The Effects of Removal of the Posterior Lobe. — Gushing in a series of ex-
periments (1911) has demonstrated that the posterior lobe with its epithelial
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,
as will be explained more fully in a following paragraph. These experi-
ments also led to the belief that some of the phenomena detailed in the
foregoing paragraph, especially the deposition of fat, the subnormal tem-
perature 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 cerebro-spinal fluid, by means of which the dissolved material
enters the blood-stream. The presence in the cerebro-spinal fluid of an
agent that produces the same physiologic effects when intravenously in-
jected, 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 third ventricle. This secretion in some
unknown way leads to a hyperglycemia and glycosuria. At the same time
the animal becomes unable to tolerate or assimilate the usual amount of
sugar experimentally ingested without increasing the glycosuria, which is
assumed therefore to be of alimentary origin.

If the posterior lobe with its epithelial investment is totally removed
or if the infundibular stalk is compressed by a clip so as to prevent the dis-
charge of the secretion into the ventricle the animal becomes very tolerant of
sugar and is enabled to assimilate larger quantities than formerly without
the development of alimentary glycosuria. As a probable result of the in-
creased 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

SECRETION. 463

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 of the
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 of
glycogen to sugar or by an inefficient storage of sugar as gtycogen 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 it is
difficult to indicate those which are to be attributed to increased or decreased
secretion of either the anterior or posterior lobe by reason of their close
juxtaposition and their possible simultaneous involvement; again it is also
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 any appreciable effect
on any of the physiologic mechanisms. Injections of the extracts of the
posterior lobe, however, give rise very promptly, as shown by Howell, to an
increase in the blood-pressure which appears to be due to an increased
contraction of the arteriole muscle rather than to a stimulation of the vaso-
motor centers, as the contraction takes place even after destruction of the
spinal cord and medulla oblongata. The action of the active constituent
of the extract appears to be very general as there is a simultaneous diminution,
as shown by plethysmographic investigations, in the volume of various
organs. On the heart the extract has an inhibitor action which takes place
concomitantly with the contraction of the arterioles and the rise of the
pressure as shown by Howell. This is attributed to a direct stimulation of
the cardio-inhibitor center as the retardation is partly prevented at least
when the vagus is divided or its function suspended by atropin. Even after
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 cells
to activity, thus causing a marked diuresis. The extract also stimulates the
non-striated muscles of the intestines, bladder, uterus, as well as the dilatator
muscle of the iris.

Adrenal Glands, or Suprarenal Capsules. — These are two flattened
bodies, somewhat crescentic or triangular in shape, situated each upon the
upper extremity of the corresponding kidney, and held in place by connective
tissue. They measure about 40 mm. in height, 30 mm. in breadth, and
from 6 to 8 mm. in thickness. The weight of each is about 4 gm. Acces-
sory glands are sometimes found in the surrounding connective tissue along
the abdominal sympathetic and in the neighborhood of the genital organs.

In some animals such as the dog, cat and rabbit, these glands have no

464

TEXT-BOOK OF PHYSIOLOGY.

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 shpws just beneath the capsule an outer portion
termed the cortex and an inner portion termed the medulla (Fig. 215). The
cortex consists mainly of cuboid cells arranged in cylindric columns. The
outer layers of cells are arranged in irregular 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 chro-
maffin cells. Similar cells are found in sym-
pathetic ganglia.

The gland is abundantly supplied with
blood-vessels and nerves. The arteries are
branches of the aorta, the phrenic, and renal
arteries. After penetrating the gland they
divide into smaller branches and capillaries
which ultimately come into close relation with
the cells of both the cortex and medulla.
The veins emerge from the gland at the hilum
and empty on the 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 autonomic system. The
pre-ganglionic fibers pass from the cord by way
of the splanchnics to the semilunar ganglion.
The post-ganglionic pass from the semilunar
FIG. 215.— SECTION OF HUMAN ganglia through its branches direct to the gland.
SUPRARENAL BODY, a, fibrous According to Bergmann nerves come from the

The Effects of Disease and Removal of

capsule; b, zona glomerulosa; c, .

zona fasciculata; rf, zona reticularis; phrenic and vagUS also.

e, medullary cords;/, venous chan-

id; ft ganglion-ceils. (Piersol). the Adrenal Glands. -It was observed by Addi-
son that a profound disturbance of the nutrition, characterized by a
bronze-like discoloration of the skin and of the mucous membranes
of the mouth, extreme muscular weakness, and profound anemia, were asso-
ciated with, if not dependent on, pathologic conditions of the suprarenal
glands. In the progress of the disease the asthenia gradually increases, the
heart becomes weak, the pulse small, soft, and feeble, indicating a general
loss of tone of the muscular and vascular apparatus. Death ensues from
paralysis of the respiratory muscles. The essential nature of the lesion
which gives rise to these symptoms has not in all instances been determined.
A very common lesion is a tuberculous degeneration. The symptoms were
attributed by Addison to a loss of the function of the glands.

Removal of these bodies from various animals is invariably and in a

SECRETION. 465

short time followed by death, preceded by some of the symptoms characteristic
of Addison's disease. Thus, shortly after their removal the animal becomes
tranquil and apathetic; the respiration soon becomes feeble and difficult;
prostration supervenes and the animal appears as though paralyzed, but the
irritability of the skeletal muscles and nerves is normal; the heart becomes
slow, feeble and irregular; the blood-pressure falls 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 twelve to forty-eight hours.
In some instances a pigmentation of the skin similar to that seen in Addison's
disease has been observed. From the fact that animals so promptly die after
extirpation of these bodies, and the further fact that the blood of such
animals is toxic to the subjects of recent extirpation, but not to normal
animals, the conclusion was drawn that the function of the adrenal bodies
is to remove from the blood some toxic product of muscle metabolism. Its
accumulation after extirpation was supposed to cause death through auto-
intoxication. This view is, however, not generally accepted.

The Effects of the Injection of Gland Extracts. — On the supposition
that the adrenals might secrete and pour into the blood a specific material
that favorably influences general metabolism, Schafer and Oliver injected
hypodermatically glycerin and water extracts of the medulla into the bodies
of various animals and observed at once an increased rate of the heart-
beats and of the respiratory movements. The effects however were only
transitory. When these extracts were injected into the veins directly, there
followed in a short time a cessation of the auricular contraction though the
ventricular contraction continued vigorously but with a slower rhythm.
The blood-pressure at the same time was markedly increased. If the vagi
were cut previous to the injection or if the inhibitor influence of the vagi
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 ventricular
contraction accompanied by a still more marked rise of blood-pressure. This
latter effect is the result partly of the increased action of the heart but very
largely the result of a vigorous contraction of the muscle-fibers in the walls of the
arterioles. This is attributed to a direct stimulation of the arterioles and not
to a stimulation of the vasoconstrictor center. The contraction of the
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 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.

It is apparent from these experiments that the adrenal bodies are engaged
in elaborating and pouring into the blood a specific material which stimu-
lates to increased activity the muscle-fibers of the heart and arteries, and
thus assists in maintaining the normal blood-pressure as well as the tonicity
of the skeletal muscles. An alkaloidal substance was isolated by Abel from
3°

466 TEXT-BOOK OF PHYSIOLOGY.

extracts of this gland, to which the term epinephrin was given. A crystalli-
zable 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. The question as to which of these two substances represents the
active principle of the gland is as yet a subject of discussion.

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 contraction and
a subsequent relaxation of the visceral walls. In these instances the extract
or the active principle produces its effects apparently by an inhibition of
the tonus of the visceral muscle.

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 nerve connections with the sympathetic 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
but which is intimately related to the muscle. To this material Langley
has applied the term "receptive substance."

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 capable 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 indepen-
dent 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 and 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 intraveneous 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

SECRETION. 467

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 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 situ-
ated in the left 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 upper 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 trabeculae 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 and
soon diminish in size. In their course small branches are given off, which
penetrate the inter-trabecular tissue and become encased with spheric or
cylindric 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; pig-
ments 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 can not be
said that any very definite results have been obtained. The fact that the
spleen can be removed from the body of an animal without appreciably 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 contrib-

468 TEXT-BOOK OF PHYSIOLOGY.

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 in the
dog the diminution in the volume (the systole) and the increase in volume
(the diastole) together occupied about one minute.

This fact was determined by withdrawing the spleen through an opening
in the abdominal wall and enclosing it in a box with rigid walls, the interior
of which was connected with a piston recording apparatus. The system
being filled with oil, each variation in volume was attended by a to-and-fro
displacement and a corresponding movement of the recording lever. The
special form of plethysmograph used for this purpose is known as the on-
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-fibres in the capsule and trabeculae,
and not to changes in the arterial blood-pressure, as the curve of the pressure
taken simultaneously remained practically uniform. The effect of the
rhythmic contractions of the splenic muscle tissue is to force the blood
through the organ, a condition 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 fibres) and pass to it in company with the splenic
artery. The nerve-cells from which they arise are in physiologic relation
with nerve-fibres (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

SECRETION. 469

reflex influences, as shown by the fact that stimulation of the central end of a
nerve Js 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 XVIII.
EXCRETION.

As stated in the preceding chapter, the term excretion is limited to the
process by which the end-products of tissue metabolism are removed from
the body, the nature of the process, however, differing in no essential particu-
lars from that underlying the process of secretion. 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.

Normal urine has a pale yellow or amber color, an aromatic odor, an
acid reaction, and a specific gravity of 1.020. As a rule, it is perfectly
transparent, though its transparency may be diminished from the presence
of mucus, calcium and magnesium phosphates, and mixed urates.

The color, which varies within physiologic limits from a pale yellow to
a reddish-brown, is due to the presence of the coloring-matters urobilin,
urochrome, and uroerythrin, all of which are derivatives of the bile pig-
ments absorbed from the liver or the alimentary canal.

The reaction of the urine is acid, owing to the presence of the acid phos-
phates of sodium and calcium. The degree of acidity, however, varies at
different periods of the day. Urine passed in the morning is strongly acid,
while that passed during and after digestion, especially if the food be largely
vegetable in character and rich in alkaline salts, is either neutral or alkaline in
reaction. The diminished acidity after meals is attributed to the formation
of hydrochloric acid by the gastric glands and the consequent liberation of
bases which are 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 i-Q25-
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

470

EXCRETION. 471

1700 c.c. Amounts both above and below these are frequently passed from
a variety of causes.

The odor of the urine is characteristic and due to the presence of aromatic
compounds.

COMPOSITION OF URINE.

Water 1500.00 c.c.

Total solids 72 .00 grams.

Urea 33 .18 grams.

Uric acid (urates) o . 55 grams.

Hippuric acid (hippurates) o .40 grams.

Kreatinin, xanthin, hypoxanthin, guanin, ammonium

salts, pigment, etc.
Inorganic salts; sodium and potassium sulphates, phos-
phates, and chlorids; magnesium and calcium phos-
phates.

ii .21 grams.

27.00 grams.

Organic salts: lactates, acetates, formates in small

amounts

Sugar a trace

Gases, nitrogen, and carbonic acid.

The estimation of total urinary solids in any given sample of urine is
frequently a matter of clinical interest. This may approximately be attained
by multiplying the last two figures of the specific gravity by the coefficient
of Haeser or Christison, 2.33. The result expresses the total solids in 1000
parts: e.g., urine with a specific gravity of 1.020 would contain 20X2.33, 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 urinary solids.

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 hi 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 12 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

472 TEXT-BOOK OF PHYSIOLOGY.

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.

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 dimmish 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 liver 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
oiher 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,

EXCRETION. 473

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.

The amino-acids, as tyrosin,leucin, glutamic, and aspartic acids, diamino-
acids and bases, as lysin, arginin, histidin which are also products of the hy-
drolysis of proteins during digestion are capable of being absorbed as such
by the epithelial cells of the villi and mucous membrane, in which they
undergo a cleavage into an NH2 portion and an organic portion; the former
is then converted to ammonia and subsequently to urea by the liver cells,
the latter the organic portion contributes to the production of fat or sugar,
which are in due time oxidized and thus contribute to the store of body heat.
A portion of the ammo-acids, such as is not used in the formation of tissue
protein is apparently disposed of in this way.

From the foregoing facts it is evident that given the presence of am-
monia salts and ammo-acids in the blood of the portal vein the appearance of
urea in the urine is readily accounted for. However it must be remembered
that though the intestine is a source of the ammonia and the amino-acids it is
probably not the only source for there is evidence that the proteins that
enter into the composition of all tissues and tissue fluids, are undergoing at
all times a hydrolysis under the influence of enzymes whereby products
are produced similar to if not identical with those produced in the intestine.
These after their discharge into the blood stream are carried to the liver
where they undergo the same change as though derived from the intestine.

The question arises however as to what percentage of the urea is derived
from the products of the metabolism of the tissue proteins (endogenous
urea) and what percentage is derived from the products of the metabolism
of the food proteins in the alimentary canal (exogenous urea). The answer
to this question is connected with the further question as to the amount of
protein necessary to keep the body in nitrogen equilibrium at its lowest
level compatible with health and efficiency. If this amount be from 30 to
50 grams as recent experiments would seem to show then the endogenous
urea would be approximately from 10 to 17 grams. Accordingly as this
lower level is raised will the amount of the endogenous urea be increased.

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-

474 TEXT-BOOK OF PHYSIOLOGY.

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 a series of bases, such as xanthin, hypo-
xanthin, adenin, guanin, etc. Because of the fact that these bodies can also
be obtained from a synthesized body termed purin they are known collect-
ively 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 extremely probable, therefore,
that uric acid is an oxidation product of one or more of the purin bases.

It is probable, however, that not all of the uric acid eliminated is derived
from the nuclein of tissue-cells and their decomposition products, the
purin bases. Some of it is undoubtedly derived from the nucleins contained
in foods. The uric acid eliminated is 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.

Xanthin, hypoxanthin, guanin, etc., are also found in urine in small
but variable amounts. They are nitrogenized compounds derived mainly
from the metabolism of the nuclein bodies.

Kreatinin is a crystalline nitrogenous compound closely resembling
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 as 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 in combination with sodium and potassium is very gener-
ally present in urine, though in small amounts. It is more abundant in the
urine of the herbivora than the carnivora. In man the amount excreted
daily is about 0.7 gram, though the amount may be raised by a diet of
asparagus, plums, cranberries, etc., and by the administration of benzoic and
cinnamic acids. There is evidence that hippuric acid is formed in the
kidney from benzoic acid, its precursors, or related bodies. Various com-
pounds of this class are found in vegetable foods, a fact which may account
for the increase in the excretion of hippuric acid on a vegetable diet.

Indol, skatol, phenol, cresol, products of the putrefactive changes in the
derivatives of protein are present in variable amounts, associated with
potassium sulphate (see page 454). These compounds are known as the

EXCRETION. 475

ethereal sulphates. The extent to which they are present is taken as a meas-
ure of the extent of intestinal putrefaction; their presence can be deter-
mined by various tests. Of these compounds the one generally tested for is
potassium indoxyl sulphate or indican. If [hydrochloric acid and a small
quantity of potassium chlorate be added to suspected urine, the indican if
present will be separated into indoxyl and potassium sulphate. The former
compound will then be oxidized and form indigo blue. The depth of the
color is indicative of the quantity present and 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 resembles a bean in shape, is from 10
to 12 centimeters in length, 2 in breath, and weigh from 144 to 170 grams.
They 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 — the hilum. 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
duct (Fig. 216). This cavity is mainly occupied by the upper part of the
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.

2. An external or cortical portion, half an inch in thickness and distinctly

friable in character.

476

TEXT-BOOK OF PHYSIOLOGY.

The Histology of the Kidney. — The kidney is composed of a connective-
tissue framework supporting secreting tubules, blood-vessels, lymphatics,
and nerves, all of which are directly connected with the removal of the urinary
constituents from the blood. The kidney is structurally a compound tubular
gland. If the apex of each pyramid be examined with a lens, it will present
a number of small orifices which may be regarded as the beginnings of the
uriniferous tubules. From this point the tubules pass outward in a straight

y, but somewhat diverging

I/1' manner toward the cortex,

giving off at acute angles a
number of branches (Fig.
217). From the apex to the
base of the pyramids they
are known as the tubules of
Bellini. In the cortical por-
tion of the kidney the tubule
becomes enlarged and twist-
ed, and, after pursuing an
extremely convoluted course,
turns backward into the
medullary portion for some
distance, forming the ascend-
ing limb of Henle's loop; it
then turns upon itself, form-
ing the descending limb of
the loop, reenters the cortex,
again expands and becomes
convoluted, and finally ter-
minates in an ovoid enlarge-
ment known as Muller's or
Bowman's capsule, in which
is contained a small tuft of
blood-vessels — the glomeru-
fas. Each tubule consists of
a basement membrane lined

thrOughout its entire extent
. , . , . ; p

Labyrinth, or cortex proper. 2. Medulla, by epithelial Cells. I lie epl-

thelium as Well as the tubule
. -L j • •

FIG. 216. — LONGITUDINAL SECTION THROUGH THE
KIDNEY, THE PELVIS OF THE KIDNEY, AND A NUMBER
OF RENAL CALYCES A. Branch of the renal artery.
U. Ureter. C. Renal calyx, i. Cortex, r. Medullary
rays. i". Labyrinth, or c

2'. Papillary portion of medulla, or medulla proper,
2". Border layer of the medulla. 3, 3. Transverse sec-
tion through the axes of the tubules of the border layer.
4. Fat of the renal sinus. 5, 5. Arterial branches, different parts of its course.
*. Transversely coursing medulla rays.— (Tyson, after jn faQ capsule the epithelium

is flattened, lining not only

the inner surface of the capsule but reflected over the blood-vessels as well.
This is known as the glomerular 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.

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

EXCRETION.

477

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 torwad th apexeof the pyramids,

Lobule.

Lobule.

.Thin

division of the
loop of Henle.

"N,. Tunica albuginea.

Stellate vein.

Interlobular

artery.

Interlobular

vein

, Arcif orm artery

Arcitorm vein.

Interlobar artery.

Interlobar vein.

Papillary duct. -

FIG. 217. — SCHEME OF THE COURSE OF THE URINIFEROUS TUBULES AND THE RENAL

VESSELS.

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-

478

TEXT-BOOK OF PHYSIOLOGY.

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 indentation of the
terminal portion of the tubule, consists of two walls, an outer one consisting
of an extremely thin basement membrane, covered by flattened epithelial
cells, and an inner one consisting apparently only of flattened epithelium
which is reflected over and closely invests the glomerular blood-vessels
(Fig. 218). 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 in vests the convoluted tubules.
From this plexus as well as from the
plexus which surrounds the straight
tubules veins arise which pass toward
and empty into veins at the base
of the pyramids. The renal vein
formed by the union of these latter
veins emerges from the kidney at the
hilum and finally empties into the
vena cava inferior.

The nerves of the kidney are de-
rived directly from small ganglia near

the semi-lunar ganglion. From this origin they pass through the renal plexus
and follow the course of the blood-vessels to their termination. Experi-
ment has shown that these nerves are both vaso-constrictors and vaso-
dilatators.

The Renal Duct. — The excretory duct of the kidney, 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.

FIG. 218. — SCHEME OF THE RENAL OR MAL-
PIGHIAN- CORPUSCLE, i. Interlobular artery.
2. Afferent vessel. 3. Efferent vessel. 4. Outer
wall. 5. Inner wall. 6. Glomerulus. 7. Neck
of tubule.— (Stohr.)

EXCRETION. 479

MECHANISM OF URINE SECRETION.

The secretion of urine is a complex process and susceptible of several
interpretations. It was originally inferred by Bowman that, as the kidney
presents anatomically an apparatus for filtration, the capsule with its enclosed
glomerulus, and an apparatus for secretion, the epithelium of the urinary
tubules, 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.

Influenced largely by the facts of blood-pressure Ludwig advanced the
view that the factors concerned in the secretion of urine were purely physical;
that in consequence of the high pressure in the vessels of the glomeruli,
due to the high pressure in the renal artery on the one hand and to the resis-
tance offered by the smaller efferent vessel on the other hand all the urinary
constituents were filtered off in a state of extreme dilution. In order to ac-
count for the higher percentage of the organic constituents in the urine, it
was assumed that as the dilute urine passed through the tubules the water
and possibly other substances as well 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
cause and effect, and that the formation of urine was accomplished ent rely
by physical forces.

The progress of physiologic investigation, however, has thrown some
doubt on the validity of this physical interpretation, and has rather served
to support the view of Bowman that the organic constituents at least are
removed from the blood by a process of selection on the part of the epithelium
of the convoluted urinary tubules; in other words, that the secretion
of urine is physiologic rather than physical. Heidenhain has brought
forward a series of facts which support this view. As evidence that the cells
possess a selective power, he presented the following experiment: The
spinal cord of an animal is divided in the neck for the purpose of lowering the
blood-pressure in the kidney below the pressure at which the urine is secreted.
Five to twenty c.c. of a saturated solution of indigo-carmine is injected into
the blood-vessels; after intervals varying from ten minutes to one hour the
animal is killed, the blood-vessels washed out with alcohol for the purpose of
precipitating the indigo-carmine in situ. Section of the kidney shows a uni-
form blue stain of the cortex alone. (Fig. 219.) Microscopic 'examination
reveals the fact that the blue stain is due to the deposition of the pigment in
the lumen and in the lumen border of the cells of the convoluted tubules
(Fig. 220) and the ascending limb of Henle's loop; while the epithelium of
Bowman's capsule 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

480

TEXT-BOOK OF PHYSIOLOGY.

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.

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 flow of urine was at once checked; the injection of urea at
once reestablished it. This fact was taken as a proof that the tubular
epithelium not only excreted urea, but water and perhaps other constituents

FIG. 219. — KIDNEY OF A RAB-
BIT. Cortex alone stained with
the indigo-carmine at the end of
one hour. — (Heidenhain.)

FIG. 220. — MICROSCOPIC APPEARANCE OF
THE LUMEN OF THE CONVOLUTED TUBULES
CONTAINING THE INDIGO-CARMINE. — (Hei-
denhain.')

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,
peptone, etc.; the latter urea, uric acid, etc. These and similar facts in-
dicate that the renal epithelium possesses a secretor rather than an absorptive
function. Heidenhain and those who agree with him assert that even the
water and inorganic salts which pass through the glomerular epithelium
do so in consequence of cell selection and cell activity; that the entire process
is one of secretion, though conditioned by blood-pressure, blood velocity, etc.
Influence of Blood-pressure. — Whether the elimination of the urinary
constituents is entirely secretor (physiologic) in character or not there can
be no doubt that the whole process is largely determined by the pressure and
velocity of the blood in the glomerular capillaries, or, to state it more accu-
rately, 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. If

EXCRETION.

481

the urine should accumulate in the ureter and tubules either from ligation or
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.

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 effer-
ent vessel and the capillaries beyond.

The pressure of blood in the renal artery may be augmented and
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, or
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
by a dilatation of the renal artery alone
and possibly by a contraction of the
efferent vessels of the glomeruli.

The pressure of the blood in the

renal artery and therefore in the glomer- sents the mean average pressure. If the vas
lili mav be HiminUripH anH the veloritv afferens dilates and the vas efferens con tractr

6 Velocity ratel or conjointly, the pressure will rise

decreased:

1. By a decrease in the blood-pressure
generally.

2. By a decrease in the blood-pressure line.— (After Morat.}
of the renal artery alone.

The first condition may be caused by a decrease in either the force or
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-
minished blood supply to the kidney and a diminution in the amount of urine
secreted. The second condition is brought about by contraction of the
renal artery alone and possibly by a dilatation of the efferent vessels of the
glomeruli. Moreover the pressure in the vessels of the glomeruli may be
varied according to the degree of contraction or relaxation of the muscle
31

Bow
Capsule

FIG. 221 .—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-
ean average pressure. If the vas
aff erens dilates and the vas efferens contracts

mjo

as" indicated by the up'per 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

482

TEXT-BOOK OF PHYSIOLOGY.

coat of the afferent and efferent vessels. See Fig. 221 and the accompanying
explanation.

Coincident with the rise and fall of pressure in the glomerular capillaries
there is a rise and fall in the rate of urinary flow. Thus it has been found that
an increase in the aortic pressure from 127 to 142 mm. of mercury, from
ligation of the carotid, femoral, and vertebral arteries, increased the rate of
urinary flow from 8.7 grams in thirty minutes to 21.2 grams. On the
contrary, a decrease in aortic pressure below 40 mm. of mercury caused by
division of the spinal cord is followed by a total abolition of the urinary flow.
These facts serve to indicate the dependence of the secretion on blood-
pressure.

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-
spection of the organ. At this time it is enlarged, swollen, and red in color.

FIG. 222. FIG.

FIG. 222. — ONCOMETER. K. Kidney; the thick line is the metallic capsule, h. Hinge. I.
Tube for filling apparatus. T. Tube to connect with T, a, v, u. Artery, vein, ureter. — (Stirling,
after Roy.)

FIG. 223. — ONCOGRAPH. C'. Chamber filled with oil, communicating by T, with T. p.
Piston. 1. Writing-lever. — (Stirling, after Roy.)

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 oncometer consists of a metallic box (Fig. 222) composed of halves
which open and close by means of a hinge. It is connected with a recording
apparatus, the oncograph (Fig. 223), through the tube T. The kidney,
withdrawn from the body, is placed within the oncometer. Through an
opening in the side pass the artery, vein, and ureter. Between the kidney
and the wall of the capsule there is placed a thin membrane. Oil is then
poured through the side tube I until the space between the capsule and the
kidney, as well as the tube leading to the chamber of the oncograph, are
completely filled. When the tube I is closed, the conditions are such that all

EXCRETION. 483

variations in the volume of the kidney are taken up and reproduced by the
recording lever attached to the piston of the oncograph. A curve of the
variations in the volume of the kidney is shown in Figure 223 taken simul-
taneously with the curve of the blood-pressure. An examination of this
curve shows that the volume-changes coincide with changes in the blood-
pressure, exhibiting not only the 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 of
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 kidney
diminishes in size, and the flow of urine ceases. In addition to these vaso-
constrictor nerves, there is evidence that the kidney also receives vaso-dilata-
tor nerves which emerge from the spinal cord and are found in the anterior

B.P.

ney.
intervals indicate a quarter of a minute. A. Abcissa. — (Stirling, after Roy.)

roots of the eleventh, twelfth, and thirteenth dorsal nerves, in the dog.
Direct and reflex stimulation of these nerves gives rise to a dilatation of the
artery, a swelling of the kidney, and an increase in secretion, independent
of any variation in general blood-pressure.

The route of the vaso-constrictor nerves is, in the dog at least, through
the 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.

Influence of Variations in the Composition of the Blood. — As it is
the function of the kidneys to excrete water, inorganic salts, and various

484 TEXT-BOOK OF PHYSIOLOGY.

end-products of metabolism from the blood and thus maintain a general
average composition, it is highly probable that as soon as they accumulate
beyond a certain precentage they themselves act as stimuli to renal activity,
either by acting directly on the renal epithelium or by increasing the glomer-
ular pressure. There is evidence at least that urea acts in the former manner
and that an excess of water in the blood, from copious drinking or from a sud-
den 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,
sodium acetate, etc., will in a short time lead to increased activity of the
kidneys, as shown by an increase in the quantity of urine excreted. The
manner in which these agents and other members of their class, the so-called
saline diuretics, increase renal activity is yet a subject of discussion. On the
one hand, it is stated that they promote an absorption of water from the
tissues to such an extent that a condition of hydremic plethora is produced,
which in itself increases not only the general blood-pressure but the local
renal pressure as well, and that it is this factor which is the cause of the
increased flow of urine. On the other hand, it is asserted that though the
salts increase the local pressure and the volume of the kidney, they never-
theless act specifically on the renal epithelium, and therefore may be regarded
as secreto-motor agents. An increase in the percentage of sugar or urea in
the blood has a similar influence on the kidney.

The Storage and Discharge of Urine. — Urination. — The urinary con-
stituents, as soon as they are eliminated from the blood, pass into and through
the uriniferous tubules and by them are discharged into the pelvis of the
kidney. They then enter the ureter by which they are conducted to the
bladder. The immediate cause of this movement is undoubtedly a difference
of pressure between the terminal portions of the tubules and the terminal
portion of the ureter, aided by the peristaltic contraction of the muscle wall
of the ureter.

The bladder is a reservoir for the temporary reception of the urine prior
to its expulsion from the body. When distended it is ovoid in shape and is
capable of holding from 600 to 800 cu. cm. The bladder is composed of
four coats: viz., serous, muscle, areolar, and mucous. The muscle coat
consists of external longitudinal and internal circular and oblique layers of
fibers of the non-striated variety which collectively encircle the entire organ.
As these fibers by their contraction expel the urine from the bladder, they
are known collectively as the detrusor urina muscle. At the exit of the bladder
the circular fibers are somewhat increased in number, giving rise to the
appearance of a distinct muscle band which has been termed the sphincter
vesicce muscle. The presence of this muscle has, however, been denied and
the retention of the urine has been attributed to mechanic conditions at the
neck of the bladder. The urethra just beyond the bladder is provided with
a distinct circular muscle composed of striated fibers, the sphincter urethra
muscle. At the close of an act of urination or micturition the bladder is
small, contracted, and its cavity is almost obliterated, but as urine is con-
tinually descending the ureter and entering the bladder at its base, the
detrusor muscle gradually relaxes or becomes sufficiently inhibited from
moment to moment to receive it. The escape of urine into the urethra is
prevented either by mechanic conditions or by the contraction of the sphincter

EXCRETION. 485

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 detrusor urincR 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 muscle mechanisms which
retain as well as expel the urine are under the control of the nerve system.
The nerves involved in the regulation of this mechanism reach the bladder
by two different routes, viz: (I) by way of the lumbar nerves and their con-
tinuations, the hypogastrics, and (II) by way of the sacral nerves and their
continuations in the pelvic plexus. The centers of origin of these special
nerve fibers are located in the spinal cord in the neighborhood of the third,
fourth and fifth lumbar segments.

The lumbar nerves from the third to the fifth give off branches (pre-
ganglionic) which pass forward to the inferior mesenteric ganglion, around
the cells of which their terminal branches arborize; from the cells of this
ganglion new fibers arise (post-ganglionic), which descend through the hypo-
gastric nerves to the muscle coat of the bladder.

The sacral nerves from the second to the fourth, give off branches which
emerge from the sacral foramina and then pass forward in the nervi erigentes
(pre-ganglionic) to small ganglia in the pelvic or vesical plexus around the
cells of which their terminal fibers arborize ; from these ganglia new fibers
arise (post-ganglionic) which are distributed also to the muscle coat of the
bladder. Afferent fibers pass from the mucous coat through the posterior
roots of the sacral nerves and reach the spinal cord centers from which
the efferent fibers for the muscle coat emanate.

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.

The expulsion of the urine is primarily a reflex act, though in the adult it
is subject to a variable amount of volitional control. When the accumulated
urine has reached a certain volume it causes, as previously stated, an
intra-vesic pressure, a muscle tonus, and slight rhythmic contractions of the
detrusor muscle. Coincidently nerve impulses are developed in the termin-
als of the afferent nerves in the mucous coat which are then transmitted to
the spinal cord centers and to the brain, where the characteristic sensation
and the desire to urinate arises. This desire is probably reinforced by

486 TEXT-BOOK OF PHYSIOLOGY.

another sensation due to the passage of a small quantity of urine into the
urethra. In a young child the arrival of the reflex impulses in the spinal
cord is immediately followed by an inhibition of the sphincter center and
a stimulation of the detrusor center, as a result of which the sphincter muscle
relaxes and the detrusor muscle contracts, thus expelling the urine. In
the adult if the act of urination is to be permitted volitional impulses
descend the cord which cause a contraction of the abdominal muscles which
through pressure on the bladder assist in the expulsion of the urine. If the
act of urination is to be suppressed volitional impulses descend the cord and
cause a contraction of the sphincter urethrae muscle and thus temporarily
prevent the discharge of the urine. After urination the entrance of urine
from the ureter brings about a reflex contraction of the sphincter muscle
by stimulation of the lumbar sphincter center and an inhibition of the
detrusor muscle by stimulation of the lumbar inhibitor center in con-
sequence of which the urine is received and retained until the pressure of the
accumulated urine again causes 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 to 2.5
per cent, of solid constituents, the variation in the percentage depending on
the quantity of water secreted. The solids consist of traces of urea, neutral
fats, lactic and sudoric acids in combination with alkaline bases, and inorganic
salts (Fovel). Other observers, however, have not been able to detect the
presence of either lactic or sudoric acid. Urea is a constant ingredient,
though its percentage is extremely small, possibly not more than 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 chlorid and alkaline and
earthy phosphates. Carbonic acid is also present in the free state as well
as in combination with alkaline bases.

The very small quantity of the solid constituents in the sweat, taken in
connection with the fact that it is excreted most abundantly when the external
temperature is high, indicates that it is not so 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

EXCRETION.

487

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 -^^ 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 pa-
pillary 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 underly-
ing structures and to afford
support for the blood-ves-
sels, nerves, and lymphat-
ics 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 individ- FIG. 225. — SECTION PERPENDICULARLY THROUGH THE
uals and in different races HEALTHY SKIN, a Epidermis or scarfskin. b. Rete mu-
. ,, , cosum, or rete malpighn. c. Papillary layer, a. Derma,

of men; the corneous layer corium> or true skin. e. Pannicuius aiposus, or fatty tis-

is Composed Of flattened sue. /, g, h. Sweat-gland and duct, i, k. Hair, with its

cells which from their ex- follicle and PaPilla' *• Sebaceous sland- ,
posure 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

488 TEXT-BOOK OF PHYSIOLOGY.

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
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., vaso-motor nerves, which regulate the blood-supply, and
secretor nerves, which stimulate the gland cells to activity.

The nerve-centers which control the sweat-glands are situated in the
spinal cord, though the number of such centers and their exact location for
the different regions of the body have not yet been satisfactorily determined.
From observation of clinic and pathologic conditions in human beings and
from experiments made on animals it may ^be stated in a general way
that the centers for the head and face lie in the upper thoracic region of the
cord; for the upper extremities, in the upper two-thirds of the thoracic
region; for the lower extremities, in the lower thoracic and upper lumbar
region. The secretor nerves which emerge from these centers are contained
in the ventral roots of the thoracic and upper lumbar nerves, which they leave
by way of the white rami communicantes as medullated (pre-ganglionic) fibers
to enter the sympathetic ganglia, around the cells of which they arborize.
From these ganglia non-medullated (post-ganglionic) fibers emerge, re-enter
the spinal nerves, with the exception of those for the head and face, and
then pass to the sweat glands in various regions of the body, following a
course similar to that pursued by the vaso-constrictor nerves for correspond-
ing regions. It is probable, though it has not been demonstrated, that
there is also in the medulla a general dominating sweat center.

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 fore-feet 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-medullated fibers pass
in the gray rami communicantes to the nerves composing the brachial 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 non-medullated nerves pass into the nerve-trunks included
between the sixth lumbar and the second sacral nerves, which enter into
the formation of the sacral plexus and through which they pass to the feet.

That the sweat-glands are stimulated to activity by nerve impulses is
shown by the fact that stimulation of the peripheral end of the divided
cervical sympathetic, of the brachial plexus, or of the sciatic nerve is followed
in a few seconds by a profuse secretion. Though under physiologic con-
ditions there is a simultaneous dilatation of the blood-vessels and an increased
supply of blood, this is merely a condition and not a cause of the secretion;

EXCRETION.

489

for the secretion can be excited and the flow maintained for a period of from
ten to fifteen minutes after ligation of the blood-vessels of the limb or even
after its amputation, when the corresponding nerve is stimulated.

The sweat-glands may be excited to activity by their related nerve-centers,
either by central, by reflex, or by local peripheral influences. Among the first
may be mentioned mental emotions, venosity of the blood, increased tempera-
ture of the blood, hot drinks, violent muscular exercise, etc. Among the
second may be mentioned powerful stimulation of various afferent or sensor
nerves, heightened external temperature, etc. Among the last may be
mentioned various drugs. Pilocarpin injected into the blood causes a profuse
secretion even when the nerves have been divided. Its action is supposed
to be exerted on the terminal branches of the nerves and possibly on the
cells themselves. As in the case of the
salivary glands atropin suspends the activity
of the terminal branches of the secretor
nerves.

Hairs. — Hairs are found in almost all
portions of the body, and can be divided
into—

1. Long, soft hairs, on the head.

2. Short, stiff hairs, along the edges of the
eyelids and nostrils.

3. Soft, downy hairs on the general 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-
bricated cells, and internally by cells contain-
ing granular and pigment material.

nni_ , <• , i i_ • • i_ j j j • ,1

Cheroot of the hair is embedded in the
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.

FIG. 226.— LARGE SEBACEOUS
GLAND, i. Hair in its follicle. 2,3,

4, <. Lobules of the eland. 6. Ex-

£etory duct traversed by the hair.

490 TEXT-BOOK OF PHYSIOLOGY.

The sebaceous glands are simple and compound racemose glands
opening by a common excretory duct on the surface of the epidermis or
into the shaft of a hair- follicle (Fig. 226) . 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
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 vernioc caseosa, which possibly keeps the skin in a normal
condition by protecting it from the effects of the long-continued action of
the amniotic fluid in which the fetus is suspended.

CHAPTER XIX.

THE CENTRAL ORGANS OF THE NERVE SYSTEM AND
THEIR NERVES.

The central organs of the nerve system are the encephalon and the
spinal cord, lodged within the cavity of the cranium and the cavity of the
spinal or vertebral column respectively. The general shape of these two
portions of the nerve system corresponds with that of the cavities in which
they are contained. The encephalon is broad and ovoid, the spinal cord
is narrow and elongated.

The encephalon is subdivided by deep fissures into four distinct, though
closely related portions: viz., (i) the cerebrum, the large ovoid mass, 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 fossae of the cranium; (3) the isthmus of
the encephalon, the more or less pyramidal-shaped portion connecting the
cerebrum and cerebellum with each other and both with (4) the medulla
oblongata. (Fig. 227.)

The spinal cord is narrow and cylindric in shape. It occupies the
spinal canal as far as the second or third lumbar vertebra. The central
nerve system is bilaterally symmetric, consisting of distinct halves united in
the median line. The cerebrum is subdivided by a deep fissure, running
antero-posteriorly, into two ovoid masses termed cerebral hemispheres; the
cerebellum is also partially subdivided into hemispheres; the isthmus like-
wise presents in the median line a partial division into halves; the medulla
oblongata and spinal cord are subdivided by an anterior or ventral and a
posterior or dorsal fissure into halves, a right and a left.

The peripheral organs of the nerve system in anatomic and phy-
siologic relation with the central organs are the encephalic and the spinal
nerves. The encephalic nerves, twelve in number on each side of the
median line, are in relation with the base of the encephalon, and because of
the fact that they pass through foramina in the walls of the cranium they are
usually termed cranial nerves.

The spinal nerves, thirty-one in number on each side, are in 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 periph-
eral organs of the nerve system.

The central organs of the nerve system are supported and protected
by three membranes named, in their order from without inward, the dura
mater, the arachnoid, and the pia mater.

The dura mater is a tough membrane composed of fibrous tissue. It
consists of two layers, the outer of which lines the cranial cavity and forms

49 1

492

TEXT-BOOK OF PHYSIOLOGY.

an internal periosteum; the inner layer is closely attached to the outer
except at certain regions where it separates and forms supporting structures,
such as the falx cerebri, falx cerebelli, tentorium cerebelli, etc. ; at the margin
of the foramen magnum the outer layer becomes continuous with the
periosteal tissue, while the inner layer invests the cord down to its ultimate
termination. (Fig. 228.)

The arachnoid is a delicate serous membrane.
The external surface is smooth and well denned
and separated from the dura by a narrow space,
the subdural space. The inner surface sends in-
ward fine connective-tissue processes which inter-
lace in every direction, constituting the subarach-
noid tissue. This tissue is abundant in the cra-
nium, much less so in the spinal canal. The
spaces between the connective tissue, taken collec-
tively, constitute the general subarachnoid space.
Around the spinal cord this space is well denned,
and at the base of the encephalon expands to
form large cavities known as the cisterna magna,
cisterna pontis, etc.

The pia mater is a delicate membrane com-
posed of areolar tissue. It closely invests the en-
cephalon and spinal cord, dipping into the various
fissures. It is exceedingly vascular and sends small
blood-vessels for some distance into the brain and
spinal cord.

The Encephalo-spinal Fluid. — The general
subarachnoid space, as well as certain cavities within
the encephalon, contain a clear transparent fluid,
termed the encephalo-spinat 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 serumglobulin) , 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.

It was stated in Chapter VIII that the entire
nerve or neuron system can be resolved into single
morphologic units, the neurons: the histologic fea-
tures and the physiologic properties of the neuron
were there also described; the anatomic relation of the neurons constitut-
ing the peripheral organs of the nerve system to the neurons constituting
the central organs of the nerve system, were also stated and illustrated in
part diagrammatically, page 49. From the statements made regarding the

FIG. 227. — THE CENTRAL
ORGANS OF THE NERVE
SYSTEM. F. T. o. Frontal,
temporal, and occipital
lobes of the cerebrum, c.
Cerebellum. P. Pons. mo.
Medulla oblongata. ms.,
ms. The upper and lower
limits of the spinal cord.
The remaining letters in-
dicate the region and num-
ber of the spinal nerves. —
(Quain, ajter Bourgery.)

THE ENCEPHALO-SPINAL MEMBRANES.

493

functions of the different neurons in their individual and collective capacity
the functions of the nerve system will become apparent.

The Functions of the Nerve System. — The functions of the nerve
system are twofold: (i) It unites and associates the organs and tissues of
the body in such a manner that they are enabled to cooperate for the accom-
plishment of a definite object. (2) It serves to arouse in the individual a
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 environ-
ment.

By virtue of the anatomic and physiologic
association, a stimulus, if of sufficient inten-
sity, applied to one organ or tissue will call
forth activity in one or more organs near or
remote from the part stimulated. This coor-
dination of action is accomplished mainly by
the spinal cord and the medulla oblongata.
All actions which take place in response to a
peripheral stimulus and independently of voli-
tion are termed reflex actions. The reflex
activities connected with digestion, the circu-
lation of the blood, with respiration, excretion,
etc., are illustrations of the coordinating capa-
bilities of the nerve-centers located in these
portions of the central nerve system.

Consciousness of the existence of the ex-
ternal world and of the relation existing be-
tween it and the individual is associated with
the physiologic activities of the encephalon,
and more particularly of the cerebral hemi- tenor root of spinal nerve. 5.

Spheres. This portion Of the nerve system is Ligamentum dentatum. 6. Linea

the chief, though perhaps not the sole, organ sPlendens"
of the mind, and 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 may be 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 local-
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 nerve paths
which have their origin in nerve-cells of localized areas of the cerebral
cortex.
The first group of nerves, the afferent, especially those connected with

FlG> 228.— THE MEMBRANES OF
THE SPINAL CORD. i. Dura

494 TEXT-BOOK OF PHYSIOLOGY.

the special sense-organs, are excited to activity by impressions made on their
peripheral terminations by agencies in the external world. The nerve
impulses thus generated are transmitted in part only as far as the spinal
cord and medulla oblongata while the remainder ascend to nerve-cells in
localized areas of the cerebral cortex where they evoke sensations. These
sensations by their grouping and combinations become the primary elements
of intelligence. The afferent nerves thus become a means of communication
between the physical and the mental worlds.

The second group of nerves, the efferent, are excited to activity by those
molecular disturbances in their related nerve-cells which accompany voli-
tional efforts. The nerve impulses thus developed and discharged from
localized areas in the cerebral cortex are transmitted by way of the med-
ulla and spinal cord to the muscles of the face, trunk and extremities
which are in consequence excited to activity. The muscle movements thus
become physical expressions of mental states, and if directed in a definite
manner to the overcoming of the 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, to consider them in
the order of their complexity, beginning with the spinal cord and ending with
the cerebrum.

THE SPINAL CORD.

The spinal cord is the narrow elongated portion of the central nerve
system contained within the spinal canal. It is cylindric in shape though
presenting an enlargement in both the lower cervical and lower lumbar
regions corresponding to the origins of the nerves distributed to the upper
and lower extremities. The cord varies in length from 40 to 45 cm., measures
12 mm. in diameter, weighs 42 gms., and extends from the atlas to the second
lumbar vertebra, beyond which it is continued as a narrow thread, the
{Hum terminate. (Fig. 229.) It is divided by the anterior and posterior
longitudinal fissures into halves, and is therefore 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.

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 or
commissure forming a figure resembling the letter H. Though varying
in shape in different regions of the cord, the gray matter in all situations
presents on either side an anterior or ventral and a posterior or dorsal horn.
Between the two horns there is a 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. 230.)

The anterior horn is short and broad and entirely surrounded by white

THE SPINAL CORD.

495

matter. The posterior horn is narrow and elongated and extends quite
up to the surface of the cord, where it is capped by gelatinous matter, the
substantia gelatinosa. In the lower cervical and thoracic regions a portion
of the intermediate gray substance projects outward and forms the so-called
lateral horn. The gray matter fundamentally consists of a framework of
fine neuroglia supporting blood-vessels, lymphatics, medullated and non-
medullated nerves, and groups of nerve-cells.

Superior or Cervical Segment Middle or Dorsal Portion Inferior Portion of Cord

of Spinal Cord. of Cord. and Cauda Equina.

FIG. 229. — SUPERIOR, MIDDLE, AND INFERIOR PORTIONS OF SPINAL CORD. i. Floor of
fourth ventricle. 2. Superior cerebellar peduncle. 3. Middle cerebellar peduncle. 4. Infe-
rior cerebellar peduncle. 5. Enlargement at upper extremity of postero-median column. 6.
Glosso-pharyngeal nerve. 7. Vagus. 8. Spinal accessory. 9, 9, 9, 9. Ligamentum denticula-
tum. 10, 10, 10, 10. Posterior roots of spinal nerves, n, u, u, n. Postero-lateral fissure.
12, 12, 12, 12. Ganglia of posterior roots. 13, 13. Anterior roots. 14. Division of united roots
into anterior and posterior nerves. 15. Terminal extremity of cord. 16, 16. Filum terminale.
17,17. Cauda equina. I, VIII. Cervical nerves. I, XII. Dorsal nerves. I, V. Lumbar nerves.
I, V. Sacral nerves. — (Sappey.)

The Nerve-cells. — The nerve cells of the cord are very numerous and
they present a variety of shapes and sizes in different regions. They are
usually arranged in groups which extend 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 at 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

496

TEXT-BOOK OF PHYSIOLOGY.

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 multi-
polar. They are the modified descendants of pear-shaped cells, the neuro-
blasts, which migrated from the medullary tube (see page 95). In the course
of their migration 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, form
efferent neurons of the first order. The inti-
mate histologic and physiologic relationship
existing between the nerve-cell and the axon is
revealed by the degenerative changes which
arise in the latter when separated from the
former. The cell apparently determines the
B 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 im-
mediately surrounding white matter of the same
side, after which they divide into two branches,
one passing up, the other down, the cord, to
re-enter the gray matter at different levels.
They are probably associative in function.
C Other cells send their, axons into that portion
of the white matter on the same and opposite
sides known as Gower's antero-lateral tract.
(Fig. 231.)

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
D group of cells which extends from the seventh
or eighth cervical nerves downward to the sec-
L or third lumbar nerves, being most promi-
OF THE nent in the thoracic region. This column is
SPINAL CORD. A. At the level known as Clarke's vesicular column. From the

of the sixth cervical nerve. B.

At the mid-dorsal region, c. nerve-cells constituting this column axons pass

At the center of the lumbar obliquely outward into the portion of the white

part'oMhe* comis ^meduiiaris.PPi! matter known as the direct cerebellar tract.

Posterior roots. 2. Anterior roots! Other nerve-cells send their axons into the white

3. Posterior fissure. 4. Anterior matter in the posterior portion of the cord

fissure, c. Central canal. — (Mor- , , . .-, • T r o

ris' "Aruitomy," after Schwaibe.) bordering the posterior median fissure. Some

of the nerve-cells, their situation and the dis-
tribution of their axons are shown in Fig. 231.

Classification of Nerve-cells. — The cells of the gray matter may be
divided into three main groups: viz., intrinsic, efferent, and afferent.

FIG. 230
DIFFERENT

SECTIONS THROUGH
REGIONS

THE SPINAL CORD.

497

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 efferent cells, independently of their trophic influence, are also motor
in function, inasmuch as the excitation arising in them is transmitted out-
wardly 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 97)
in accordance with their physiologic action into motor, secretor, vaso-motor,
viscero-motor and pilo-motor nerves, so the nerve-cells of which the nerves

Dorsal

FIG. 231. — SCHEME OF THE STRUCTURE OF THE CORD. — (Harwell 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.

are integral parts may be classified physiologically as motor, vaso-motor,
secretor, viscero-motor and pilo-motor. Collections or groups of such cells
are termed "centers."

The afferent cells are largely sentient or receptive in function, inasmuch
as the excitations brought to the spinal cord by the afferent nerves in the
dorsal roots from the general periphery are received by them and transmitted
by and through their axons to the cortex of the cerebrum, where they are
translated into conscious sensations. As the nerve- fibers in the dorsal roots
of the spinal nerves are classified, in accordance with the sensations to which
they give rise, as sensor, thermal, tactile, etc., so these nerve-cells may be
similarly classified according as they transmit their excitations to those
32

498

TEXT-BOOK OF PHYSIOLOGY.

specialized areas in the cerebral cortex in which these different sensations
arise.

Structure of the White Matter. — A transverse section of the cord
shows that the white matter completely covers the gray matter except where
the posterior horns reach the surface. Anteriorly the white matter of each
lateral half is connected by a narrow strip or bridge of white matter, the
anterior commissure. Microscopic examination shows that the white matter
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 side
of the cord is anatomically divided into an anterior, a lateral, and a posterior
column by the anterior and posterior roots of the spinal nerves.

FIG. 232. — TRANSECTION OF THE CERVICAL SPINAL CORD SHOWING ITS CHIEF SUBDIVISIONS. —

(After Mills.)

Classification of the Nerve-fibers. — From a study of the embryologic
development of the white matter and of the degenerative changes which follow
its pathologic and experimental destruction, it has been differentiated into
a number of specialized tracts which have different origins, destinations,
and functions. Some of the more important tracts are shown in Fig. 232.
They may be divided, however, into efferent, afferent, and associative fibers.

i. The anterior column, comprising that portion between the anterior
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 SPINAL CORD. 499

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 re-enter 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 physi-
ologic relation either directly or indirectly through intercalated neuron
cells with the dendrites of the cornual cells. These fibers are the continua-
tions 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

500 TEXT-BOOK OF PHYSIOLOGY.

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 re-enter
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:

1. The antero-lateral tract of Marchi and Lo wen thai, situated at the
anterior and inner angle of the anterior column, which degenerates down-
ward after removal of one-half of the cerebellum.

2. The comma tract, a narrow bundle of fibers situated in the anterior
portion of the column of Burdach. When it is divided it degenerates down-
ward.

3. The septo-mar ginal 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

THE SPINAL CORD. 501

arise from nerve-cells in the gray matter. They undergo descending
degeneration when divided, but not after division of the dorsal roots.

The Relation of the Spinal Nerves to the Spinal Cord.— The spinal
nerves present near the spinal cord two divisions which from their connection
with the anterior or ventral and the posterior or dorsal surfaces are known
as the ventral and dorsal roots.

The ventral roots are the axons of various groups of nerve-cells situated
in the anterior horns of the 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
spinal ganglia. After entering the cord they divide into two main groups,
a lateral and a mesial. A portion of the lateral group enters the posterior
horn directly through the caput cornu; the other portion turns upward and
runs through Lissauer's tract and ultimately enters the posterior horn. The
mesial group passes into the postero-external column (Burdach), where
the fibers divide into descending and 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.

The ventral root axons are distributed to skeletal muscles, blood-vessels,
glands and viscera. The dorsal root axons are distributed to skin, mucous
membranes, and muscles. The classification of the nerve-fibers in the
ventral and dorsal roots in accordance with the functions they subserve
will be found on pages 97, 98.

Though both the efferent and afferent fibers of the spinal nerves are
directly connected with nerve-cells in the spinal cord, they are also indirectly
connected by efferent and afferent nerve-tracts with the cerebral cortex.

Experimentally, it has been determined that the anterior or ventral
roots contain all the efferent fibers, the posterior or dorsal roots all the afferent
fibers. The proofs in support of this view are as follows:

Stimulation oj the 'ventral root fibers produces:

1. Tetanic contraction of skeletal muscles.

2. Discharge of secretions from glands.

3. Variations in the degree of the contraction, the tonus, of the muscle

walls of the peripheral arteries either in the way of augmentation
or inhibition.

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.

502 TEXT-BOOK OF PHYSIOLOGY.

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.

Peripheral 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, blood-vessels, glands, 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, blood-
vessels, glands, and viscera.

3. Sensations of touch, temperature, pressure, and pain.

4. Sensations of the duration and direction of muscle movements, of the

resistance offered and of the position of the body or of its individual
parts (muscle sensations).

Division oj 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 excite them to
activity.

The dorsal roots are afferent in function, transmitting nerve impulses
from the general periphery to (a) the spinal cord where they excite its con-
tained nerve-centers to activity or to a more or less complete cessation of
activity (inhibition), and (b) to the cerebrum where they excite its centers
to activity with the development of sensations.

Segmentation of the Spinal Cord. — For the elucidation of many
problems connected with the physiologic actions of the spinal cord, as well as
of the symptoms which follow its pathologic impairment, it will be found
helpful to consider the cord as consisting physiologically of a series of segments
placed one in advance of the other, the number of segments corresponding to
the number of spinal nerves. Each spin alsegment 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 SPINAL CORD. 503

THE FUNCTIONS OF THE SPINAL CORD.

Anatomic investigation has demonstrated that the spinal cord is com-
posed of a series of segments which 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 instances 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 volitional, the
gland discharges and vascular and visceral muscle movements to emo-
tional phases of cerebral activity.

Automatic Activity. — By this expression is meant a discharge of energy
from the spinal nerve-cells occasioned by (a) a change in the chemic composi-
tion of the blood or lymph by which they are surrounded or probably 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
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

504 TEXT-BOOK OF PHYSIOLOGY.

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 contin-
uous 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 417); (e) the acceleration of the heart-beat (page 313).

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.

Trophic Tonus. — The normal metabolism of muscle, gland, and
connective 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 per-,
ipheral portions, but the muscles, glands, and connective tissues to which
they are distributed will also undergo similar changes. This is to be attrib-
uted not merely to inactivity, but rather to a loss of nerve influence. It
would appear from facts of 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 muscles 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.

Vascular Tonus. — The arteriole muscles throughout the vascular
apparatus are also constantly in a state of slight but continuous contraction
which assists in the maintenance of an average arterial pressure and is due
to the continuous discharge of nerve energy from the general or dominating
vasomotor (constrictor) center in the medulla oblongata. The automaticity
of this center has also largely been discredited; but whether it is automatic
or not it is capable of being influenced in its activity not only by variations
in the composition of the blood but by nerve impulses reflected to it from all
regions of the body (see page 372.)

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

THE SPINAL CORD.

505

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- varying
position of the limbs and trunk, (see page 54.)

The tonic contraction of the visceral muscles — e.g., the pyloric, the
vesical, the anal sphincters — though regarded as automatic by some, is

,sp.c

FIG. 233. — DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION or 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. #., 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.

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.

Reflex Activity. — It has already been stated that the nerve-cells in the
spinal cord are capable of receiving and transforming afferent nerve impulses,
the result of peripheral stimulation, into efferent nerve impulses, which are
transmitted outward to skeletal muscles, exciting contraction; to glands,
provoking secretion; to blood-vessels, changing their caliber; and to organs,
inhibiting 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 in every reflex action
consists of at least the following structures (Fig. 233) :

TEXT-BOOK OF PHYSIOLOGY.

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. 234.)
A histologic and physiologic mechanism of
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
of the central nerve system at the upper limif of the spinal cord. After
this procedure the spinal centers can act independently of, and uninfluenced
by either sensation or volitional efforts on the part of the animal. Though
it is possible to provoke reflex contractions under such circumstances in
warm-blooded animals, they are, as a rule, incomplete and of short duration,
owing to disturbances of the circulation and respiration and the consequent
loss of tissue irritability. In frogs and in cold-blooded animals generally,
the spinal cord retains its irritability for a long period of time after removal
of the brain, and therefore is well adapted 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

FIG. 234. — DIAGRAM SHOWING
THE RELATION or THE THIRD
NEURON a, TO THE AFFERENT
NEURON b, AND TO THE EFFERENT
NEURONS c, c, c. — (After Kolliker.}

THE SPINAL CORD. 507

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 medulla.
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 conscious-
ness, by inserting the pin into the brain cavity through the foramen mag-
num, 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-
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 decap-
itated 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

5o8 TEXT-BOOK OF PHYSIOLOGY.

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 loor 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 oj 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 oj 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 oj 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 oj 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 oj 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.

Special Reflex Movements. — Among the reflexes connected with the
more superficial portions of the body there are some which are so frequently
either increased or diminished in pathologic 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 skin — e.g., pricking, pinching, scratching,
etc. The following are the principal skin reflexes:
i. Plantar reflex consisting of contraction of the muscles of the foot, induced

THE SPINAL CORD. 509

by stimulation of the sole of the foot; it involves the integrity of the
reflex arc through the second and third sacral nerves.

2. Gluteal reflex, consisting of contraction of the glutei muscles when the skin

over the buttock is stimulated; it takes place through the segments
giving origin to the fourth and fifth lumbar nerves.

3. Cremasteric reflex, consisting of a contraction of the cremaster muscle

and a retraction of the testicle toward the abdominal ring when the skin
on the inner side of the thigh is stimulated; it depends upon the integ-
rity of the segments giving origin to the first and second lumbar
nerves.

4. Abdominal reflex, consisting of a contraction of the abdominal muscles

when the skin upon the side of the abdomen is gently scratched; its
production requires the integrity of the spinal segments from the
eight to the twelfth thoracic nerves.

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 requires the integrity of the cord between
the fourth and seventh thoracic nerves.

6. Scapular reflex consisting of a contraction of the scapular muscles when

the skin between the scapulae is stimulated; it depends upon the in-
tegrity of the cord between the fifth cervical and third thoracic nerves.
The skin or superficial reflexes, though variable, are generally present in
health. They are increased or exaggerated when the gray matter of the
cord is abnormally excited, as in tetanus, strychnin-poisoning, and disease
of the lateral columns.

The so-called "tendon reflexes,'1'' are characterized by the contraction of a
muscle and are elicited by a sharp tap on its tendon. They are also of much
value in the diagnosis of lesions of the cord. 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 of the afferent nerves, the muscle spindles.
When the muscle is passively extended, as it is when the reflex is to be
elicited, there is an exaltation of the tonus and an increase in the irri-
tability. To this condition of the muscle due to passive tension, the term
myotatic irritability has been given. If the muscle extension be now sud-
denly increased, as it is when the tendon is sharply tapped, the increased
compression of the muscle spindles will develop additional afferent impulses
which after transmission to the spinal cord will give rise to contraction of the
corresponding muscle.

The following are the principal forms of the tendon reflexes :
i. The Patellar tendon reflex or knee-jerk. This phenomenon is characterized
by a contraction of the extensor muscles of the thigh imparting to the leg a
forward movement when the ligamentum patellae is struck between the
patella and tibia. This reflex is best observed when the legs are freely
hanging over the edge of a table. The patella reflex is generally present
in health, being absent in only 2 per cent. ; it is greatly exaggerated in
lateral sclerosis, in descending degeneration of the cord; it is absent in
locomotor ataxia and in atrophic lesions of the anterior gray cornua.

5io TEXT-BOOK OF PHYSIOLOGY.

2. The tendo achillis reflex or ankle- jerk. This is characterized by a corf-

traction of the gastrocnemius muscle and a flexion of the foot. To
elicit the contraction the leg should be extended and the dorsum of the
foot be pressed toward the leg so as to give to the gastrocnemius
a slight degree of extension. If the tendon be now sharply struck a quick
extension of the foot is produced.

3. Ankle clonus. — This consists of a series of rhythmic reflex contractions

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 dorsi-flex the foot at the ankle suddenly and energetically, thus
putting the tendo Achillis and the gastrocnemius muscle upon the
stretch. The rhythmic movements thus produced continue so long as
the tension within limits is maintained. Ankle clonus is never present
in health, but is very marked in lateral sclerosis of the cord.

The toe reflex, peroneal reflex, and wrist reflex are also present in sclerosis
of the lateral columns and in the late rigidity of hemiplegia.

The organ reflexes, e.g., the activities of the genito-urinary organs, the
stomach, intestines, gall-bladder, etc., which are induced by peripheral
stimulation have been considered in connection with the physiologic action
of these organs. The genito-urinary center is located in the lumbar region
of the spinal cord. In diseased conditions of this region the genito-urinary
reflexes are sometimes increased, at other times decreased or even abolished.

Reflex Irritability. — The general irritability or quickness of response of
the mechanism involved in reflex action can be 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 Ttirck 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 by
Gaskell. A single shock is, however, ineffective. The currents must
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 reflex irritability is increased by :

i. 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
normally exerts an inhibitor influence over the reflex centers of the cord.

THE SPINAL CORD.

olf.n.

The same increase is observed 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 stim-
ulus* is sufficient to call forth spasmodic contractions of all the skeletal
muscles. Under its influence the usual coordinated reflexes disappear
and are succeeded by incoordinated reflexes. The explanation of this
fact is believed to be a diminution in
the resistance offered by the cord to
the passage of the afferent 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 ra-
diated in all directions. Absolute
repose of the animal and the ex-
clusion of all external stimuli greatly
diminish the tendency to the occur-
rence of spasms.

Degeneration 0} 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 the
cord becomes exalted. As the dis-
ease progresses the irritability in-
creases to such an extent that vio-
lent spasmodic contractions of the
arms and legs arise when the skin
or tendons are mechanically stimu- FlG. 235.— DIAGRAM OF THE BRAIN OF

lated. The explanation offered is THE FROG. olf. n. olfactory nerves ;olf.l.

olfactory lobes ; c. h. cerebral hemispheres ;
op. thl. optic thalamus; op. I. optic lobes;
c. cerebellum; med. ob. medulla oblon-

practically the same as in division

of the cord: viz., withdrawal of the

inhibitor and controlling influence gata; IV- v- fourth ventricle.

of the brain.

The reflex excitability may be decreased by:

Stimulation of Certain Regions oj the Brain. — It was discovered by Setche-
now that when the frog brain is divided just anterior to the optic lobes
(Fig. 235) 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

512 TEXT-BOOK OF PHYSIOLOGY.

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 oj spinal cord; e.g., atrophy of the multipotar cells of the anterior

horns of the gray matter; degeneration of the terminals of the dorsal
root fibers.

4. The toxic action of various drugs — e.g., chloroform, chloral — which are

believed to exert a depressing action on the nerve-cells themselves.

B. THE SPINAL CORD SEGMENTS AS CONDUCTORS.

The white matter of the spinal cord consists of nerve-fibers the special
function of which is

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 the
spinal segments it is essential that they should be united by commissural
or associative fibers. This is, in fact, accomplished by the axons of the intrin-
sic cells of the gray matter, which constitute such a large part of the antero-
lateral and posterior root zones. In consequence of this association,
the cord becomes capable of complex coordinated and purposive reflex
actions.

Spino-encephalic or Sensor Conduction. — The nerve impulses that
arise in consequence of impressions made on the terminals of the nerves
in the cutaneous and mucous surfaces, in the viscera and in the muscles, are
transmitted through the dorsal roots of the spinal nerves to the cord. On
reaching the cord they are received by nerve-cells, the axons of which pass up-
ward to and through the medulla, the posterior part of the pons, the poste-
rior part of the crura cerebri, and for the most part to the ventral portion of the
thalamus opticus, forming what is known as the spino-thalamic system.
On reaching the thalamus they are received by nerve cells, the axons of
which pass by way of the internal capsule to the cells of the cortex of the
cerebrum forming what is known as the thalamo -cortical system. 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 translated into conscious sensations. The sensations
thus arising may be divided into special and general sensations. Of the

THE SPINAL CORD. 513

former may be mentioned the sensations of pain, touch, pressure, tem-
perature, 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, but especially to sensations of pain, temperature
(heat and cold), touch, passive position, and movement of parts, due to the
action of skeletal muscles.

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
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-
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.
It is, however, believed by some investigators that those fibers which sub-
serve the sense of touch do not decussate at once, but ascend in the column
of Goll as far as the medulla oblongata, where they, in common with the
fibers coming from the muscles, arborize around the nerve-cells in the gracile
and cuneate nuclei. The afferent path originating in the spinal cord in-
creases in size at successive levels as it passes upward, to and through the
medulla and successive structures, to the thalamus. The fibers that com-
pose the afferent path originating in the cells of the gracile and cuneate
nuclei, cross over the median plane and after decussating with the fibers
coming from the opposite side, join the afferent path from the spinal cord.
These fibers are known as the internal arcuate fibers and assist in the forma-
tion of the lemniscus or fillet. (Fig. 236.) The sensor pathway decussates
in part at different levels of the spinal cord and in part at the level of the gra-
cile and cuneate nuclei. The former is often termed the lower, the latter
the upper sensor decussation.

The afferent pathway on passing toward the thalamus receives addi-
tional fibers at the level of the medulla and pons from the cells with which
the terminations of the afferent cranial nerves, the trigeminal, glosso-
pharyngeal, and vagus, are associated.

The pathways for the Impulses that give rise to the different sensation

33

Su TEXT-BOOK OF PHYSIOLOGY.

have been variously located by different observers, e.g., in the gray matter,
in the limiting layer, and in the antero-lateral tract of Gowers; the pathway
for the impulses that give rise to temperature sensations has been located in

FIG. 236. — DIAGRAM OF THE SENSOR PATHWAYS IN THE SPINAL CORD ENLARGED ABOVE BY
FIBERS OF THE SENSOR CRANIAL NERVES AND NERVES OF SPECIAL SENSE.

the gray matter; the pathway for tactile impressions has been located in the
posterior columns, though this is not beyond dispute. The pathway for
pain sensations has been located in Gowers' tract.

THE SPINAL CORD. m 515

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 group 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 group 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 group 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.
As previously stated, the pathways in the spinal cord for the transmission
of afferent nerve impulses are imperfectly known, and for their determina-
tion in human beings reliance must be placed on observations of the results
of disease of the cord, supplemented by experiments made on the cord in
mammals such as monkeys and apes.

From the classification of the various forms of sensibility by Head, the
following table showing the effects or symptoms following a transverse lesion
of one-half of the cord in man has been constructed by Turner and Stewart.

SCHEME SHOWING THE BROWN-SEQUARD ' 'SYMPTOM-COMPLEX/'

BASED ON HEAD'S OBSERVATIONS.
Side of lesion. Side opposite lesion.

Motor paralysis. No paralysis.

Retention of tactile, light pressure, Tactile and light pressure sensibili-
painful, and thermal sensibilities. ties may or may not be impaired.

Painful and thermal sensibilities

abolished.

Painful pressure retained. Painful pressure abolished.

Impairment or abolition of tactile Retention of sense of position and
discrimination, and sense of posi- of tactile discrimination,
tion of limbs.

Retention of cutaneous localiza- Cutaneous localization depends up-
tion. on the state of tactile sensibility.

Si6 . TEXT-BOOK OF PHYSIOLOGY.

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 and of movement and
tactile discrimination, Head) do not cross, but pass up on the same side as
the entering posterior nerve roots; (3) that tactile sensibility may or may not
be abolished on the side opposite the lesion; and (4) that the sense of cuta-
neous localization may be dissociated from the sense of passive position, and
remain intact when the latter is absent (Head).

Encephalo-spinal or Motor Conduction. — At birth the child is cap-
able of performing all the functions of organic life, such as sucking, swallow-
ing, 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 into
relation with the spinal segments show that the cells of the cortex are not only
immature, but that their descending axons are incompletely invested with
myelin. With the growth of the child, psychic life unfolds and volitional
control of muscles is acquired. Coincidently the cells of the cerebral cortex
grow and develop and the fibers become covered with myelin.

The nerve-fibers which have their origin in the cells of the cerebral cortex,
and which terminate in tufts around the cells in the anterior horns of the
gray matter of the spinal segments, are to be regarded as long commissural
tracts uniting and associating these two portions of the central nerve system.

Experimental investigations and observations of pathologic lesions
accord with the view that physiologically these fibers are efferent pathways
for the transmission of motor or volitional impulses from the cortex to the
spinal segments. The nerve-cells in which the motor impulses originate
are located for the most part, as will be fully stated later, in the central
portion of the cortex of the cerebral 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. 237.) At this point
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

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 around the Rolandic fissure. The summit of the pyramid
is truncated and covers the pyramidal region of the internal capsule.

THE SPINAL CORD. 517

does not decussate at the medulla but passes downward on the inner
side of the anterior column of the same side, constituting the direct

FIG. 237. — DIAGRAM OF THE PYRAMIDAL TRACT OR MOTOR PATH. III. Common oculo-motor
nerve. IV. Pathetic nerve. V. Motor division of the trigeminal nerve. VI. The abducens
nerve. VII. Facial nerve. IX. and X. Motor divisions of the glosso-pharyngeal and pneumogas-
tric nerves. XI. Spinal accessory nerve. XII. HypoglossaJ nerve. — (Van Gehuchten.}

pyramidal tract or column Turck. This tract can be traced down, as
a rule, only as far as the mid-dorsal region. As it descends it becomes

5i8 TEXT-BOOK OF PHYSIOLOGY.

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.
That the pyramidal tracts are the conductors of volitional impulses
throughout the length of the cord to its various segments has been made
evident by the results of section, electric stimulation, and disease. Division
of the anterior and lateral columns of one side of the cord in any part of its
extent is invariably followed by a loss of motion or paralysis of the muscles
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 tract
in any part of its course from the cerebral cortex downward. Electric
stimulation of the cortical cells which give origin to the pyramidal tract is
also followed by contraction of the muscles of the opposite side, while their
destruction is attended by paralysis of the same muscles. As the nutrition of
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.

CHAPTER XX.

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. 238 and
239). 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.

520

TEXT-BOOK OF PHYSIOLOGY.

The lateral column is composed of fibers continuous with those of the lateral
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,

FIG. 233. — ANTERIOR OR VENTRAL
VIEW OF THE MEDULLA OBLONG AT A
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-
berg. 24. Glosso-pharyngeal nerve
25. Pneumogastric. 26, 26. Spinal
accessory. 27. Sublingual nerve. 28,
29, 30. Cervical nerves. — (Sappey.)

FIG. 239. — 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.)

as previously stated, to enter into the formation of the anterior column;
the fibers of the direct cerebellar tract gradually curve backward, and in so
doing unite with other fibers to form the restiform body, after which they
enter the cerebellum by way of the inferior peduncle. Situated between the

ISTHMUS OF THE ENCEPHALON. 521

anterior pyramid and the restiform body is a small oval mass, the olivary
body, composed of both white and gray matter.

The posterior column is composed largely of fibers continuous with those
of the posterior column of the cord. The subdivision of this column into a
postero-external (Burdach) and a postero-internal (Goll) is more marked
in the medulla than in the cord. The former is here known as ihefuniculus
cuneatus, the latter as the funiculus gracilis. These two strands of fibers
are apparently continued into the restiform body. Owing to the divergence
of the restiform bodies a V-shaped space is formed, the floor of which is
covered with epithelium resting on the ependyma. At the upper extremity
of the funiculus cuneatus and funiculus gracilis, two collections of gray
matter are found, known respectively as the nucleus cuneatus and nucleus
gracilis. Around the cells of these nuclei many of the fibers of the posterior
column end in brush-like expansions, (see Fig. 236).

The Fillet or Lemniscus. — From the ventral surface of the cuneate and
gracile nuclei axons emerge which pass forward and upward through the
gray matter and decussate with corresponding fibers coming from the op-
posite nuclei. They then assume a position just posterior to the pyramids
and between the olivary bodies. These fibers thus form a new tract, termed
the fillet or lemniscus. As this tract ascends toward the cerebrum it receives
additional axons from the sensor end-nuclei of all the afferent cranial
nerves of the opposite side with the exception of the auditory. From the
end-nuclei of the auditory nerve, new axons ascend as a distinct tract situated
near the lateral aspect of the pons. From their position, these two separate
tracts have been termed the mesial and lateral fillets respectively.

THE ISTHMUS OF THE ENCEPHALON.

The isthmus of the encephalon comprises that portion of the central
nerve system connecting the cerebrum above, the cerebellum behind, and the
medulla below. Its ventral surface presents below 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 white
matter, the optic tracts (Fig. 238).

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. 239). At the extreme upper part of this surface
there are four small grayish eminences, the corpora quadrigemina. From
the disposition of the white matter on the dorsal surface of the isthmus and
medulla, there is formed a lozenge-shaped space, the fourth ventricle. The
space is an expansion of the central cavity of the cord, the result of the
changed relations of the white and gray matter in this region of the central
nerve system. Above, this ventricle communicates by a narrow canal, the
aqueduct of Sylvius, with the third ventricle. The floor of the fourth
ventricle is covered with a layer of epithelium resting on the ependyma con-

522 TEXT-BOOK OF PHYSIOLOGY.

tinuous with that lining the central canal of the cord. Beneath this is a
layer of gray matter.

The pons varolii comprises in a general way that portion of the central
nerve system situated between the 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. 240). 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.,

Fto.*^-T«*HS«enoK0»iH. they conrneuct 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
°TF ,TsHCo*LEpoVsEDTRA£T/F DoS cerebellum cross the median line and terminate
longitudinal fasciculus. L.c. and around the cells of the nucleus pontis; others
^LOCUS cemleus. L.f. Lateral again are connected with the gray cells of the

same side. Through the intermediation of

the nucleus pontis and certain of the longitudinal fibers of the pons, the
cerebellum is brought into relation with the cerebrum.

The dorsal or tegmental portion consists of: (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

ISTHMUS OF THE ENCEPHALON.

523

(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
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.
In 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 FIG. 241.— SCHEME OF TRANSVERSE SEC-
and eighth cranial nerves from which ™NJDF_^HE^CEJRP^7EDAUNCLAES* ^3
new axons pass as a part of the
mesial and lateral fillets toward the
cerebrum.

The crura cerebri comprise that

Fillet or lemniscus. RN. Red nucleus. SX.
Substantia nigra. III. Third nerve. Py.
Pyramidal tracts. Fc. Fronto-cerebellar;
and TOC, temporo-occipital fibers of the

portion of the Central nerve System crusta. CC. Caudato-cerebellar^ fibers in

upper part of crusta. — {After Wernicke and
Gowers.}

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. 241). Of the fibers which compose the ventral
portion of each crus, the crusta or pes, the larger part is continuous below,
through the longitudinal fibers of the pons, with the pyramid of the medulla
and the pyramidal tract; above they assist in the formation of the internal
capsule. On the inner and on the outer side of each crusta there is a bundle
of fibers derived from the frontal, and from the temporal and occipital por-
tions of the cerebrum respectively. These fibers are connected directly
with the nuclei pontis and indirectly with the cerebellum of the same and
opposite sides. The fibers which compose the dorsal portion, the tegmentum,
are continuous with those which pass upward from the medulla and pons,

524 TEXT-BOOK OF PHYSIOLOGY.

e.g., the fillet, both mesial and lateral, the formatio reticularis, the medial
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
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 colliculi 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

BASAL GANGLIA. 525

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,
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 ot the cerebrum along the course of the nerve-fibers that pass to and
from its cortical expansion. Among these ganglia the more important are
the corpora striata and the optic ffialami. They are made visible upon
removal of the cerebrum. The general relations of these ganglia are shown
in Fig. 242.

The corpus striatum, the more anterior of the two, is an ovoid col-
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 ventricle; the body arches across the ven-
tricle from before backward and from within outward, while the tail is
directed downward and forward to become associated with the collection
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 subdivided
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-ventricular portion of the corpus striatum,
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 matter of the

526 TEXT-BOOK OF PHYSIOLOGY.

Island of Reil. The general appearance and relation of these nuclei,
are shown in Figs. 243 and 244. A horizontal section of the lenticular
nucleus shows that it is divided by two lamina of white matter into three
portions. The two inner, from their pale yellow color, form the globus
pallidus, the outer, somewhat darker in color, is termed the putamen.
External to the lenticular 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 daustrum.

FIG. 242. — CORPORA STRIATA, OPTIC THALAMI, CORPORA QUADRIGEMINA, CEREBELLUM AND
ASSOCIATED STRUCTURES, i, Corpora quadrigemina; 2, valve of Vieussens; 3, pre-peduncle
4, upper part of medi-peduncle; 5, upper part of crus; 6, lateral fillet; 7, band of Reil; 8, post-
brachium; 9, frenulum; 10, gray matter of valve of Vieussens; n, medi-commissure; 12, pre-com-
missure; 13, 14, center of cerebellum; 15, post-commissure; 16, peduncles of the pineal gland; 17
pineal gland; 18, 19, posterior and anterior tubercles of the thalamus; 20, teniasemicircularis; 21,
vessels of the corpus striatum; 22, fornicolumn; 23, corpus striatum; 24, septum lucidum. —
(Sappey.)

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 pulvinar
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
but penetrated by white matter, which subdivides its contained gray cells

BASAL GANGLIA.

527

into 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-
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.

Though the thalamus has extensive connections with many portions of
the central nerve system, the most important are with the cortex, the teg-
mentum, and the optic tracts.

From the cells of these various nuclei axons emerge which pass into the
internal capsule, and through the corona radiata to various portions of the
cortex. Those which come from the
pulvinar and pass to the occipital lobe
constitute a part of the optic radiation;
those from the lateral and ventral nuclei
ultimately reach the parietal lobe; those
from the anterior nucleus pass to the
hippocampal and uncinate convolutions.
In a similar manner 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 exact dis-
tribution 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 quadri-
geminum, around the cells of which they
terminate in brush-like expansions.

The Internal Capsule.— The lenti-
cular 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. 244 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

AMYGDALA

FIG. 243. — Two VIEWS OF A MODEL
OF THE STRIATUM. A, Lateral aspect; B,
mesial aspect. (Spitzka.)

528 TEXT-BOOK OF PHYSIOLOGY.

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 anterior
extremity of the lenticular nucleus, and a posterior, situated between the

FIG. 244. — HORIZONTAL SECTION THROUGH THE CEREBRUM SHOWING THE NATURAL RELATIONS

OF THE VARIOUS STRUCTURES.

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.

MEDULLA AND BASAL GANGLIA. 529

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,
until in the medulla and pons it is spread out in the form of a thin layer near
their dorsal surfaces, where, together with the ependyma, it forms the floor
of the fourth ventricle.

In the region of the aqueduct of Sylvius the gray matter again converges
and ultimately surrounds the canal, to again expand at its anterior extremity
to form the lining of the third ventricle.

The Nerve- cells. — The nerve-cells in these different regions do not
differ morphologically from those in the gray matter of the spinal cord. The
corpus, or body of the cell, presents a number of dendrites "as well as the
sharply defined axon. As a rule, the cells are arranged in groups, or clusters,
or nests, partially surrounded and enclosed by supporting tissue, and situ-
ated beneath the floor of the fourth ventricle and the floor of the aqueduct of
Sylvius. From some of the cell groups axons pass ventrally through the
white matter to merge on the ventral and lateral surfaces of the medulla,
pons, and crura, where they are known as efferent or motor cranial nerves.
From other groups of cells, axons cross the median line, and after joining
the mesial fillet ascend toward the cerebrum. Around these latter cells the
terminal filaments of the afferent or sensor cranial nerves arborize. The
collection of cells found in the central gray matter may be divided into two
groups — efferent and afferent.

The efferent cells are motor in function, inasmuch as the excitation
arising in them is transmitted outward through their related axons to, and
exciting movement in, skeletal muscles, glands, viscera or blood-vessels.

The afferent cells are largely sentient or receptive in function, inasmuch
as the excitations brought to them by the afferent cranial nerves from skin
and mucous membranes and from sense-organs, such as the tongue and ear,
are received by them and transmitted through their ascending axons to the
cortex of the cerebrum, where they are translated into conscious sensations.

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
for convenience be said to extend from the cerebral cortex through the crus
cerebri to the pons, medulla, and spinal cord. They may be divided into

34

530

TEXT-BOOK OF PHYSIOLOGY.

three distinct tracts: e.g., the pyramidal tract, the fronto-cerebellar tract,
and the occipito-temporo-cerebellar tract (Fig. 245).

The pyramidal or motor tract descends from the cortex of the cerebrum
mainly from the gyms anterior to the central fissure, passes through the
posterior one-third of the anterior segment and the anterior two-thirds of the
posterior segment of the internal capsule, the middle two-fifths of the crusta,
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.

FIG. 245. — SCHEMA OF THE PROJECTION FIBERS OF THE CEREBRUM AND OF THE PEDUNCLES OF
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; D, 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; Vt,
fourth ventricle. The numerals refer to the cranial nerves. — (Modified from Starr.)

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 poris,
where its fibers terminate or arborize around the nucleus pontis of the same
and opposite sides.

The occipito-temporo-cerebellar tract descends from the occipital and
temporal lobes, passes to the inner side of the lenticular nucleus, and 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
eseht descending fibers are brought into relation with the cerebellum.

FUNCTIONS OF THE MEDULLA OBLONGATA. 531

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.

The^//e/ orlemniscus, 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 more

532 TEXT-BOOK OF PHYSIOLOGY.

complicated and involved. The functions of these closely related structures
are in consequence equally complex and involved and but imperfectly known.
In a general way it may be said that by virtue of the presence of nerve-
cells and definite tracts of nerve-fibers these structures. collectively may be
regarded as consisting:

1. Of centers for reflex actions; and —

2. Of conducting paths by which the various parts are brought into relation

one with another and with the spinal cord, the cerebellum, and the
cerebrum.

The Medulla Oblongata and Pons. — The gray matter situated in these
structures — i.e., just beneath the floor of the fourth ventricle — contains
nerve-cells arranged in more or less well-defined groups which may be divided
into efferent and afferent.

The efferent cells are the immediate sources of nerve impulses which are
transmitted through efferent axons to various peripheral organs — skeletal
muscles, glands, viscera, and blood-vessels. Their activity may be excited
by the same influences as excite the efferent cells of the spinal cord: e.g.,
variations in the composition of the blood or lymph; the arrival of nerve
impulses coming through afferent pathways in the spinal cord and through
afferent cranial nerves; the arrival of nerve impulses coming through efferent
pathways from the cerebrum. The peripheral activity resulting from their
excitation may therefore be automatic or autochthonic, peripheral (reflex)
or cerebral (volitional) in origin.

The afferent cells are sentient or receptive in function, inasmuch as they
receive nerve impulses coming through lower afferent pathways and transmit
them through their related axons to the cortex of the cerebrum, where they
evoke sensations.

The efferent cells give origin to nerve-fibers which pass ventrally and be-
come the efferent or motor cranial nerves.

The afferent cells give origin to fibers which pass to the cerebral cortex.
Around both groups of cells, the afferent or sensor cranial nerves terminate
in tuft-like expansions. (In a subsequent section the origin, course, dis-
tribution, and functions of the various cranial nerves will be considered).
But as the function of the nerve is only to transmit energy from the cell of
which it constitutes a part, the function ascribed to the nerve may without
impropriety be transferred to the cell itself.

Since it is by means of nerve-cells and their associated fibers that many
important functions of organic life are initiated and maintained, it would
naturally be expected from its extensive nerve connections that this region
of the nerve system plays an extensive r61e in this respect. As the accomplish-
ment of these functions requires the cooperation and coordination of a number
of separate but related structures, it is evident that there must exist in the
medulla and pons a number of coordinating mechanisms consisting of nerve-
cells and nerve-fibers which are associated in various ways for the accomplish-
ment of definite functions. To such a coordinating mechanism the term
" center" has been given: e.g., respiratory, cardiac, deglutitory, etc. *

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 CRURA CEREBRI. 533

The Medulla Oblongata and Pons as Centers for Reflex Activities: —

Experimentation has shown that the medulla and pons contain a number
of such centers, the more important of which are as follows:

1. Cardiac centers, which exert (i) an accelerator action over the heart's

pulsations through nerve-fibers emerging from the spinal cord in the
roots of the first and second dorsal nerves and 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 313, 314.)

2. A vaso-motor center, which regulates the caliber of the blood-vessels

throughout the body in accordance with the needs of the organs and
tissues for blood, through nerve-fibers passing by way of the spinal
nerves to the walls of the blood-vessels. (See page 372.)

3. A respiratory center, which coordinates the muscles concerned in the

production of the respiratory movements. (See page 416.)

4. A mastication center, which excites to activity and coordinates the muscles

of mastication. (See page 142.)

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 162.)

6. An articulation center, which coordinates the muscles necessary to the

production of articulate speech.

7. A diabetic center stimulation of which gives rise to glycosuria.

In addition, the gray matter contains centers which influence the secretion
of saliva, provoke vomiting, coordinate the muscles of the face concerned in
expression, and control the secretion of the perspiration.

As Conducting Pathways. — The anterior pyramids of the medulla and
their continuations through the more ventral portions of the pons, being
portions of the general pyramidal tract, serve to conduct volitional efferent
nerve impulses from higher portions of the brain to the spinal cord. Divi-
sion 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. — 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 dorsally of fibers continuous with those coming through the lower por-
tions 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 gray matter beneath the aqueduct of Sylvius contains nerve-cell

534 TEXT-BOOK OF PHYSIOLOGY.

groups which are centers for reflex actions in connection with ocular move-
ments: e.g., closure of the lids, contraction of the sphincter pupillae, con-
vergence of the eyes, etc.

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
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 adap-
tive 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

FUNCTIONS OF THE INTERNAL CAPSULE.

535

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

FIG. 246. — HORIZONTAL SECTION OF THE INTERNAL CAPSULE SHOWING THE POSITION AND
RELATION OF THE MOTOR TRACTS FOR THE EYE, HEAD, TONGUE, MOUTH, SHOULDER (Shi.),
ELBOW (Elb.), DIGITS OF HAND (Die.), ABDOMEN (Abd.), HIP, KNEE (Kn.), DIGITS OF FOOT
(Dig.). S. Sensor tract. O. T. Optic tract. A. T. Auditory tract.

equilibrium. The thalamus is believed by some investigators to act also as
an intermediary between emotional states and their expression in the muscles
of the face, this power being lost in certain pathologic conditions. The
power of regulating the temperature of the body has been also assigned to
the thalamus, as destruction of its anterior extremity is usually followed by
a rise in temperature.

The Internal Capsule. — The internal capsule has been shown by the
results both of experiment and of pathologic processes to be, first, 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

536 TEXT-BOOK OF PHYSIOLOGY.

sense-organs to the cortex, where they give rise to sensations general and
special. It is therefore the common motor and sensor pathway. For the
reason that it 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 in Fig.
246, it consists of two segments or limbs united at an obtuse angle, the knee
or elbow, which is directed toward the median line. The motor tract is
confined to the posterior one-third of the anterior segment and the anterior
two-thirds of the posterior segment. The sensor tract is confined to the
posterior one-third of the posterior segment, the extreme end of which also
contains the optic and auditory tracts.

The region of the anterior segment in front of the motor tract contains
the fibers of the fronto-cerebellar tract, the function of which is unknown.

The motor region contains fibers which descend from the cerebral cortex
to nerve-centers situated in the gray matter beneath the 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. 246.

The relation of the internal capsule to the caudate nucleus and the optic
thalamus internally, and to the lenticular nucleus externally, is also shown
in a vertical section of the cerebrum made in front of the gray commissure
(Fig. 237). From the fact that the internal capsule contains efferent or
motor tracts, and afferent or sensor tracts, it is evident that a destructive
lesion of the motor tract would be followed by a loss of motion; and of the
sensor tract, by a loss of sensation on the opposite side of the body.

CHAPTER XXL
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 fiat 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. 247 and 248.

Fissures.

1. The Sylvian fissure, 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.

537

538

TEXT-BOOK OF PHYSIOLOGY.

4. 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. 248.)

SUPfftCfNTRAL F.

FIG. 247. — FISSURES AND GYRI ON THE LATERAL SURFACE OF THE LEFT HEMI-CEREBRUM.
F. Fissure. G. Gyrus. R. Ramus.

FIG. 248. — FISSURES AND GYRI OF THE MESIAL SURFACE OF THE LEFT HEMI-CEREBRUM

(Spitzka.)

5. The calcarine fissure begins on the posterior extremity of the mesial surface
of the occipital lobe. From this point it passes downward and forward
to unite with the parieto-occipital fissure.

THE CEREBRUM. 539

6. The para-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 of

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 branch

of the fissure of Sylvius in the anterior and inferior portion of the frontal
lobe. It is continuous posteriorly with the lower end of the pre-central
convolution.

The Parietal Lobe. — The parietal lobe presents three well-marked
convolutions: viz., the posterior or post-central convolution, and the super-
and sub-parietal. The latter is again subdivided into the marginal and the
angular convolution.

1. The posterior or post-central convolution or gyrus is situated just behind the

Rolandic or central fissure, with which it corresponds in direction.
Above, it is continuous with the super-parietal convolution; below, with
the marginal and the pre-central convolutions.

2. The super -parietal convolution or gyrus is bounded internally by the longi-

tudinal fissure and externally by the intra-parietal fissure. From the
upper end of the post-central convolution, with which it is connected,
it runs downward and backward as far as the parieto-occipital fissure.

540 TEXT-BOOK OF PHYSIOLOGY.

3. The sub-parietal convolution or gyrus is connected anteriorly with the
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-temporal 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
on its external surface three well-marked convolutions: viz., the super-, the
medi-, and the sub-temporal, separated by the super- and medi-temporal
fissures. These three convolutions are in a general way parallel with each
other, and pursue a direction from before backward and upward. Ante-
riorly, they are fused together, but posteriorly their connections are some-
what different. The supertemporal is continuous behind and above with
the supra-marginal convolution, and behind and below with the angular
convolution or gyrus. The medi-temporal blends with the preceding and
with the middle occipital. The sub- temporal is continuous with the in-
ferior occipital.

The Occipital Lobe. — The occipital lobe is triangular in shape and
forms the posterior apex of the hemisphere. Its base on the external
surface is formed by an imaginary line drawn from the parieto-occipital
fissure to the pre-occipital notch on the inferior and lateral border. The
external surface presents three convolutions — the superior, middle, and in-
ferior occipital.

The inner or mesial surface of the hemisphere, formed in part by the
frontal, the parietal, the occipital, and the temporal lobes, presents several
convolutions of much physiologic interest, viz. :

1. The callosal convolution, or gyrus, situated between the super-callosal

fissure and the corpus callosum. From its origin anteriorly at the base
of the brain this convolution passes backward, gradually increasing in
width as it approaches the posterior extremity of the corpus callosum.
At this point it again narrows and descends between the calcarine and
hippocampal fissures to blend with the hippocampal convolution.

2. The hippocampal gyrus, formed by the union of the posterior extremity

of the callosal convolution and the sub-calcarine convolution is situated
just below the dentate or hippo-campal fissure. Anteriorly it becomes
enlarged, and just behind the apex of the temporal lobe turns backward
and inward to form a hook-shaped eminence, the uncinate gyrus or
uncus.

The limbic lobe is the name given to an area of the brain which
includes, among other structures, the callosal convolution, the gyrus
hippocampus, and the uncus. As forming a part of this general
lobe may be mentioned the dentate fascia, the striae and peduncle of
the corpus callosum, the septum lucidum, the fornix, and the in-
fra-callosal gyrus.

3. The sub-collateral convolution or gyrus is bounded by the collateral fissure

THE CEREBRUM.

54i

above, and its inferior border extends from the occipital lobe to the
anterior pole of the temporal lobe.

4. The quadrate lobule, or precuneus, a square-shaped convolution, is situated

between the posterior termination of the para-central fissure and the
parieto-occipital fissure. It blends with the callosal convolution, on the
one hand, and with the parietal lobule on the other.

5. The cuneus, a triangular or wedge-shaped convolution or lobule, is situated

on the mesial surface of the occipital lobe
between the parieto-occipital and calca-
rine fissures.

The Insula or Island of Reil.— This
anatomic structure consists of a triangular-
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, con-
nective tissue, etc., all of which are arranged
and interblended in a most intricate man-
Notwithstanding the complexity of its

ner.

structure, modern histologic methods have
enabled Cajal to divide it into four fairly dis-
tinct layers or zones, from without inward, as
follows (Fig. 249):

i. The Molecular Layer. — The most superficial
portion of this layer consists mainly of
neuroglia or glia cells, the processes of
which interlace in all directions, forming
a distinct sheath just beneath the pia.

yrr

FIG. 249. — SECTION OF THE
CEREBRAI^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 deeper portions of this layer con-
tain a specialized type of nerve-cells (Cajal cells), of which there
are several varieties. These cells give off nerve-fibers which pur-
sue a horizontal direction for a variable distance, but in their
course give off collateral branches which ascend to the outer surface
of the layer. Among these structures are to be found, also, den-
dritic processes of cells situated in the subjacent layer. The terminal

542 TEXT-BOOK OF PHYSIOLOGY.

filaments of medullated nerve-fibers coming from nerve-cells in lower
regions of the encephalo-spinal axis are also present, but for the most
part they pursue a tangential direction.

2. 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 med-
ulated nerve.

3. 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 defined, and smooth.
After giving off collateral branches, the axon descends into the cere-
brum and becomes a medullated nerve-fiber.

4. The Layer of Polymorphous Cells. — In this layer the nerve-cells present a

variety of forms: e.g., spindle, polygonal, pyramidal, etc. The spindle
form is the most common. From either end of the spindle a large
dendrite emerges, 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
histologists to estimate the total number of functional nerve-cells in the cere-
bral cortex of man. Though the estimates are widely different, the lowest
presents numbers which are beyond comprehension. Thus, Meynert's
estimate is 612 millions; Donaldson's estimate for the entire brain is 12000
millions; and Thompson's 9283 millions.

Structure of the White Matter. — The white matter of the cerebrum
consists of medullated nerve-fibers which, though intricately arranged, may
be divided into three systems: viz., the commissural, the association, and the
projection.

1. The commissural system. The fibers which compose this system unite

corresponding areas of the cortex of each hemisphere. The fibers
from the frontal, parietal, and occipital lobes cross in the median line
and form a band of transversely arranged fibers, the corpus callosum.
The fibers which unite the corresponding areas of the temporo-sphe-
noidal lobes cross in the anterior commissure. All the commissural fib-
ers are the axons of nerve-cells in the cortex, the terminals of which are to
be found in the cortex of the opposite side.

2. The association system. The fibers which compose this system unite

neighboring as well as distant parts of the same hemisphere, and may
therefore be divided into long and short fibers. They associate the in-
excitable or association areas with the excitable or projection areas.

THE CEREBRUM. 543

3. The projection system. The fibers composing this system unite certain
areas of the cortex of the cerebrum with the basal ganglia, the pons,
medulla oblongata, and spinal cord. They may be divided into: (i)
afferent fibers which have their origin in the lower nerve-centers at dif-
ferent levels and thence pass to the cortex; and (2) efferent fibers which
have their origin in the cortex and thence pass to the lower nerve-centers,
terminating at different levels. The former are also termed the cortico-
afferent or corticopetal; the latter, corticoefferent or corticofugal.
The afferent fibers, the so-called sensor tract, which transmit nerve im-
pulses coming from the general periphery and the sense-organs, pass through
the tegmentum as the mesial and lateral fillets, and thence to the cortex
directly by way of the internal capsule, or indirectly through the intermedia-
tion of the thalamic and subthalamic nuclei. See Fig. 245, page 530. The
distribution of these fibers to the various areas of the cortex will be stated in
following paragraphs.

The efferent fibers of the so-called motor tract which transmit motor or
volitional nerve impulses from the cortex to the pons, medulla, and spinal
cord, emerge from the layer of pyramidal cells of the gray matter of the an-
terior or the pre-central convolution, the paracentral lobule, and immediately
adjacent areas. From this origin the axons descend through the white
matter of the corona radiata, converging toward the internal capsule, into
and through which they pass, occupying the anterior two-thirds of the poste-
rior limb or segment. Beyond the capsule they continue to descend, occupy-
ing the middle three-fifths of the pes or crusta of the crus cerebri, the ventral
portion of the pons, and eventually the anterior pyramid of the medulla
oblongata. At this point the tract divides into two portions, viz. :

1. A large portion, containing from ninety-one to ninety-seven per cent, of

the fibers, which decussates at the lower border of the medulla and
passes down the lateral column of the cord, constituting the crossed
pyramidal tract.

2. A small portion, containing from three to nine per cent, of the fibers,

which does not decussate at the medulla, but passes down the inner side
of the anterior column of the same side, constituting the direct pyramidal
tract or column of Tiirck.

After passing through the internal capsule, and as it descends through
the crus, pons, and medulla, the cortico-efferent tract gives off a number of
fibers which cross the median line and arborize around the nerve-cells of
the gray matter beneath the aqueduct of Sylvius (the nuclei of origin of the
third and fourth cranial nerves), and around the nerve-cells in the gray
matter beneath the floor of the fourth ventricle (the nuclei of origin of the
remainder of the motor cranial nerves). The remaining fibers go to form
the crossed and direct pyramidal tracts and arborize around the cells in the
anterior horn of the gray matter of the opposite side of the cord at successive
levels. By this means the cortex is brought into anatomic and physiologic
relation with the general musculature of the body through the various cranial
and spinal motor nerves. (See Fig. 237, page 517.)

The fronto-cerebellar and the occipito-temporo-cerebellar tracts are also
efferent tracts and parts of the projection system. The fronto-cerebellar,

544 TEXT-BOOK OF PHYSIOLOGY.

originating in the nerve-cells of the cortex of the frontal lobe, passes down to
and through the internal capsule, occupying the anterior one-third of the
anterior segment. It then descends along the inner side of the crus cerebri
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 relation
with the cerebellum of the same but chiefly of the opposite side. The
occipito-temporal tract, originating in the cells of the cortex of both the
occipital and temporal lobes, passes downward and inward toward the
lenticular nucleus, beneath which it passes to enter the outer one-fifth of the
crusta. It then enters the pons, and through the nucleus pontis also comes
into relation with the cerebellum of both sides. (See Fig. 245, page 530.)

THE FUNCTIONS OF THE CEREBRUM.

The functions of the cerebrum comprehend, in man at least, all that
pertains to sensation, cognition, feeling, and volition. All subjective experi-
ences, which in their totality constitute mind, are dependent on and asso-
ciated with the anatomic integrity and the physiologic activity of the cere-
brum and its related sense-organs, the eyej ear, nose, tongue, etc.

From an examination of the anatomic development of the brain in different
classes of animals, in different men and races of men, and from a study of the
pathologic lesions and the results of experimental lesions of the brain, evi-
dence has been obtained which reveals in a striking manner the intimate
connection of the cerebrum and all phases of mental activity.

1. Comparative anatomic investigations show that there is a general 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).

2. 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

THE CEREBRUM. 545

brain development is possible and the intelligence remains at a low
level. In congenital idiocy the brain is small, imperfectly developed,
and wanting in proper chemic composition.

3. Experimental lesions of the brain in lower animals are attended by results

similar to those observed in disease or after injury in man. Removal
of the cerebrum in the pigeon completely abolishes intelligence and
destroys the capability of performing volitional movements. The
pigeon remains in a state of 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 the
toes, etc., but it soon relapses into a condition of quietude. Coincident
with the destruction of the cerebrum there occurs a loss of memory,
reason, and judgment, and the animal fails to associate the impressions
with any previous train of ideas. The higher the animal in the scale of
development, the more striking is the loss of mentality after removal
of the cerebrum.

4. Experimental interference with the blood-supply to the cerebrum is followed

by a diminished or complete cessation of its activities. There is 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. The
supply of blood is furnished by four large blood-vessels: viz., the two
carotid and the two vertebral arteries. These vessels, after entering
the cavity of the skull, give off branches which unite to form the "circle
of Willis." From this circle, large branches are given off which enter
the cerebrum and distribute blood to all its parts. After passing through
the capillaries the blood, greatly altered in chemic composition, is
returned by large veins. The large volume of blood that is present
in the brain and the marked changes in composition that it undergoes
while passing through the brain indicate a very active and complex met-
abolism in this organ. By means of the anatomic arrangement of the
blood-vessels at the base of the brain, the blood-supply is equalized.
It also explains why, when one, or even two, of the four large vessels
are occluded by pathologic deposits or surgical procedures, brain
activity continues, though perhaps diminished in degree. Occlusion
of all four vessels, however, is at once followed by a complete abolition
of all forms of cerebral activity. An experiment performed by Brown-
Se*quard 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 blood
were then connected with the four arteries of the head. By means of a
pumping apparatus imitating the action of the heart the blood was
driven into and through the brain. After a few minutes cerebral activ-
ity returned, as shown by contractions of the muscles of the face" and
eyes. The character of the contractions were such as to convey the
idea that they were directed by the will. These vital manifestations
continued for a period of fifteen minutes, when on the cessation of the
artificial circulation they disappeared, and the head exhibited once
more the usual phenomena observed in dying: viz., contraction and
35

546 TEXT-BOOK OF PHYSIOLOGY.

then dilatation of the pupils and convulsive movements of the muscles

of the face.

Localization of Functions in the Cerebrum. — By the term localiza-
tion of functions is meant the assignment of definite physiologic functions to
definite anatomic areas of the cerebral cortex. From experiments made on
the brains of animals, by the observation and association of clinical symp-
toms with pathologic lesions of the central nerve system, and from obser-
vation of the developmental stages of the embryonic brain, it has been es-
tablished in recent years:

1. That the general and special sense-organs of the body are associated

through afferent nerve-tracts with definite though perhaps not sharply
delimited areas of the cerebral cortex; and—

2. That certain areas of the cortex are associated through efferent nerve-

tracts with special groups of skeletal or voluntary muscles.

Experimental excitation of a cortical area associated with a sense-organ is
undoubtedly attended by the production of a sensation at least similar to
that produced by peripheral excitation of the sense-organ itself; destruction
of the area is followed by an abolition of all the sensations associated with the
sense-organ. For these reasons such areas are termed sensor.

Experimental excitation of a cortical area associated with a group of skele-
tal muscles is attended by their contraction; destruction of the area is followed
by their relaxation or paralysis. For these reasons such areas are termed
motor.

Since the sense-organs are remote from the brain and the impressions
made upon them by the objective world can be utilized by the mind only
after they have been reproduced in the cortical areas, it may be said that
each sense-organ has its special area in the cortex by which it is represented,
or, in other words, each sense-organ has a cortical area of representation.

Since the muscles are remote from the brain and since they contract in
response to the discharge of nerve impulses from the cells of the cortical motor
areas, it may be said that the activities of the motor areas are represented
by the contractions of the muscles; in other words, that the cortical motor
areas have areas of representation in the general skeletal musculature. It
is usually stated, however, in the reverse way: viz., that the muscle move-
ments have areas of representation in the cortex.

The cortex of the cerebrum may therefore be compared to a mosaic
made up, partially at least, of sensor and motor areas which respectively
represent sense-organs and motor organs, and which bear a definite anatomic
and physiologic relation one to the other. Their cooperation is essential to
the normal performance of many forms of cerebral activity.

A knowledge of the situation of these areas, the order of their develop-
ment, the effects that arise from their stimulation or follow their destruction,
are matters of the highest importance in the study of cerebral activity and
indispensable to the physician in the localization of lesions which manifest
themselves in perversions or abolition of sensations and in convulsive seizures
or paralyses.

The Sensor Areas. — The sensor areas which should theoretically be
present in the cortex are primarily those which receive and translate into con-
scious sensations nerve impulses, developed by changes going on in the body

THE CEREBRUM. 547

itself; and secondarily those which receive and translate into conscious sensa-
tions the nerve impulses developed in the special sense-organs by the impact of
the external or objective world. In the former areas, are received the nerve
impulses that come from the mucous membranes, muscles, joints, viscera, etc.,
and give rise to muscle, and visceral sensations. In the latter areas are
received the nerve impulses that come from the sense-organs and give rise
to cutaneous, e.g., tactile, thermal, painful, gustatory, olfactory, auditory,
and visual sensations. A number of such sense areas may be predicated:
e.g., areas of cutaneous and muscle sensibility, of gustatory, olfactory, auditory,
and visual sensibility.

The Motor Areas. — The motor areas which should theoretically be
present in the cortex are those which in consequence of the 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 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 a center of
motion. If the stimulation of a given area is attended by phenomena
which indicate 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. The animals gener-
ally 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 configura-
tion 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 symptoms 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 Areas of the Monkey Brain. — From experiments made on
the brains of monkeys, Ferrier, Schafer, Horsley, and many others have
mapped out, though not with a high degree of definiteness and certainty,
the sensor areas, stimulation of which gives rise to sensation, destruction to
loss of sensation. A diagrammatic representation of these areas is shown
in Fig. 250 and Fig. 251.

The tactile area or area of tactile perception has not been accurately or

548

TEXT-BOOK OF PHYSIOLOGY.

definitely located. Ferrier assigned it to the hippocampal region. Schafer
and Horsely assigned it to the limbic lobe, and especially to that portion
known as the gyms fornicatus, or callosal gyms, as destruction of this convolu-
tion was followed by hemianesthesia of the opposite side of the body which was
more or less marked and persistent. These observers conclude that the limbic
lobe "is largely if not exclusively concerned in the appreciation of sensation,
painful and tactile." Other experimenters question this conclusion and
locate the area near to, if not within, the Rolandic area. The difference of
opinion regarding the location and probable limitation of the area of tactile
sensibility renders necessary additional and more conclusive experiments.
The olfactory and gustatory areas or areas of olfactory and gustatory

FIG. 250. — DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE LATERAL SURFACE OF THE
MONKEY BRAIN. — (After Horsley and Schafer.}

perception have been located in the uncinate gyrus or uncus and the adjacent
region, though their exact limits have not been determined by the experi-
ments thus far performed.

The auditory area or area of auditory perception was located by Ferrier
in the upper two-thirds of the superior temporo-sphenoidal convolution.
Bilateral cauterization of this region gave rise to complete deafness, which
endured to the time of the animal's death, more than a year later. Unilateral
destruction of this region gave rise to deafness in the opposite ear only. The
results of experiments made subsequently by other observers would indi-
cate that the auditory area is somewhat more extended than that designated
by Ferrier, as apparently animals regained their hearing, to some extent
at least, after complete recovery from the operation. The limit or extent of
the area is, however, uncertain.

The visual area or area of visual perception has been located in the occipital
lobe, though in this, as in the previous instances, its exact limits have not
been positively determined. Experimenters also are not in accord as to the
relative functions of its different parts. Ferrier located this area in the
occipital lobe and that adjacent portion of the parietal lobe on the outer

THE CEREBRUM.

549

surface known as the angular gyrus. He found that extirpation of the an-
gular gyrus alone was followed by a temporary blindness of the opposite eye,
which was, however, not hemianopsic in character.1 He also found that
destruction of the occipital lobe together with the angular gyrus gave rise
to a more or less enduring hemianopsia, in addition to the transient
blindness of the opposite eye. From these and similar facts he concluded
that the angular gyrus is the area of representation for the macular or central
region of the retina, and the occipital lobes for the corresponding halves
of the peripheral portions of the retina.

It was, however, found by Munk, Schafer, and others that the angular
gyrus was not concerned in any way with vision; that extirpation of the

FIG. 251. — DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE MESIAL SURFACE OF THE MON-
KEY BRAIN. — (After Horsley and Schafer.)

occipital lobe alone, especially if the line of division be carried a little further
forward on the mesial and inferior surfaces, was followed by homonymous
hemianopsia. Additional experiments lead to the conclusion that the
area for macular vision is near the anterior extremity of the calcarine fissure,

1 In a consideration of this subject certain facts connected with visual perception, both hi
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 ret-
inal 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.

550 TEXT-BOOK OF PHYSIOLOGY.

while the area for peripheral vision is in the posterior portion of the mesial
surface and for a variable distance on the outer surface. Moreover, there
is reason to believe that the area for macular vision is in relation with homony-
mous halves of the two maculae luteae. The supposed error, the assignment
of macular vision to the angular gyrus, has been attributed to destruction of
the fibers of the optic radiation, which in their course to the occipital lobe
pass close to this gyrus.

The Motor Areas of the Monkey Brain. — From experiments made on
the brains of monkeys Ferrier mapped out a number of areas stimulation of
which give rise to muscle contractions on the opposite side of the body which
are so purposive and coordinate in character that they may be regarded as
identical with those produced volitionally. Destruction of these areas is
followed by paralysis. Collectively these areas are known as the motor area
or motor zone; and as it is ranged along the Rolandic fissure, it is sometimes
termed the Rolandic area.

The experiments of Horsley and Schafer added additional facts and
enabled them to construct a new diagrammatic representation of the motor
area and more accurately define the special areas upon the lateral and mesial
aspects of the brain of the monkey. The boundaries of the general and
special areas, as determined by these observers, will be readily apparent
from an examination of Fig. 250. Their experiments have enabled them
also to subdivide the general into special areas as follows:

1. The head area or area for visual direction into areas excitation of which

causes " opening of the eyes, dilatation of the pupils and turning the
head to the opposite side with conjugate deviation of the eyes to that
side."

2. The leg area may be subdivided into (a) an area both on the lateral and

mesial surfaces which presides over the movements of the hip and
thigh; (b) an area in the posterior part which presides over the move-
ments of the legs and toes; (c) an area in the paracentral lobule for the
movements of the hallux or great toe.

3. The trunk area, situated largely on the mesial surface, may be subdivided

into an anterior and a posterior area, which respectively preside over
the movements of the spinal column as arching and rotation, and the
movements of the pelvis and tail.

4. The arm area may be subdivided as follows: (a) an area superiorly

which controls the movements of the shoulder; (b) an area posteriorly
and below this, which controls the movements of the elbow; (c) an
area anteriorly and below the preceding, governing the movements of
the wrist and fingers; (d) an area posteriorly and below governing the
movements of the thumb.

5. The face area may be divided into an upper part, comprising about one-

third, and a lower part, comprising the remaining two-thirds. In the
upper part are areas governing the movements of the opposite angle
of the mouth and of the lower face. In the lower part anteriorly there
is an area governing the movements of the vocal membranes or bands
(the laryngeal area); posteriorly areas governing the opening and
closing of the mouth, the protrusion and retraction of the tongue.

THE CEREBRUM. 551

Electric stimulation of the sensor areas is attended by certain motor
reactions which vary in accordance with the area stimulated. Thus, when
the electrodes are applied to different portions of the occipital lobe the eye-
balls are conjugately turned upward, downward, or laterally and to the
opposite side; when placed on the upper portion of the superior temporal
convolution, the ear is pricked up or retracted, the head is turned to the
opposite side and the pupils are dilated; when placed on the hippocampal
convolution, there is movement of torsion of the nostril and lips of the same
side.

Ferrier assumed that these movements were the result of the origination
of subjective sensations and not an evidence that the area in question is a
motor area, in the sense that this term is applied to the areas of the Rolandic
region, especially as their destruction is not followed by paralysis of any of
the corresponding muscles. This interpretation is supported by the ex-
periments of Schafer, which showed that the contraction of the eye-muscles
which followed stimulation of the occipital lobe took place between 0.2 and
0.3 second later than when the frontal lobe was stimulated; and that as the
motor reaction takes place after extirpation of the frontal region, the route of
the efferent impulse cannot be to and through the frontal lobe, but probably
through some lower center. The same facts hold true for the reactions of
the ear-muscles following stimulation of the temporal lobe.

The view that the cortex of the cerebrum can be divided into separate and
independent though physiologically related motor and sensor areas has
been questioned in recent years, and a somewhat different interpretation
given to the facts. It is believed by many physiologists and neurologists
that the so-called motor and sensor areas are so closely related that it is
almost impossible to distinguish one from the other either anatomically or
physiologically. Thus the Rolandic region is believed to be both motor and
sensor in function, the former, however, being more predominant in the per-
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-
zation are three tracts numbered by Flechsig i, 2 and 3, which arise largely
from the median nucleus of the thalamus and the medial lemniscus and pass
to the anterior and posterior convolutions, to the para-central lobule and
foot of the superior frontal convolution, and to the foot of the third frontal
convolution respectively. It is these fibers which convey nerve impulses to
the cortex and furnish information regarding changes taking place in the
body itself and thus lead to the performance of muscle movements. This
area is therefore primarily a sensor area, an area for body-feelings, cutaneous,
tactile, muscle, and visceral, and secondarily a motor area. The afferent
fibers to this region become myelinated during the ninth month of intra-

552

TEXT-BOOK OF PHYSIOLOGY.

uterine life, the efferent fibers from it become myelinated during the third
month 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-
logic investigations show that subsequently to the myelinization of the
afferent tracts connecting the sense-organs with the cortex, efferent nerve-
tracts arise from or near to the same centers and undergo myelinization. In
other words, these areas are primarily sensor and secondarily motor, and
therefore should be termed sensori-motor. In Flechsig's own terminology
each corticopetal or afferent tract is accompanied by a corticofugal or
efferent tract.

CONCRETE CONCEPT

FIG. 252. — THE AREAS AND CENTERS OF THE LATERAL ASPECT OF THE HUMAN HEMICEREBRUM. —

(C. K. Mills.)

In this view sensations, or the mental processes the outcome of sensations,
are the immediate cause of the movements of the muscles connected with both
the sense-organs and skeletal structures. Though this interpretation — viz.,
the coincidence of sensor and motor areas — appears more in accordance
with the facts than the earlier view, it must be admitted that there are many
facts both of a physiologic and pathologic character which it is difficult to
harmonize with it.

The Motor Area of the Chimpanzee Brain. — In a series of experi-
ments made by Sherrington and Griinbaum on the brain of the chimpanzee
it was discovered that the so-called motor area was not so widely distributed
as in the monkeys generally, but was confined almost exclusively to the
convolution just in front of the fissure of Rolando, as it was impossible to
obtain any movement on direct stimulation of the convolution just behind it.
All points on the surface of the pre-central convolution, including the portion
forming the wall of the Rolandic fissure itself, were found to be excitable
and productive of movement when stimulated. The sequence of representa-

THE CEREBRUM.

553

tion from below upward is similar to that observed in the monkey. One
peculiarity, however, was the location of the area for conjugate deviation of
the eyeballs to the opposite side. This is situated far forward in the middle
and inferior frontal convolutions, and separated from the areas in the pre-
central convolution by a region apparently inexcitable. These facts are of
great interest and value in the assignment of the motor areas in the cortex
of the human brain, as in its development and configuration the chimpanzee
brain more closely resembles the human brain than does the monkey's.
The Localization of Sensor and Motor Areas in the Human Brain.—
The observation of clinical symptoms and their interpretation by post-mortem
findings, the phenomena observed during surgical procedures, and the results

FIG. 253. — THE AREAS AND CENTERS OF THE MESIAL ASPECT OF THE HUMAN HEMICEREBRUM. —

(C. K. Mills.)

of embryologic investigations, point to the conclusion that corresponding
areas both for sensations and movements exist in the cerebral cortex of the
human brain, though it is probable that their locations do not in all respects
coincide with those characteristic of the monkey or even the ape brain. In
the following diagrams (Figs. 252 and 253), the sensor and motor areas are
at least provisionally located, in accordance with recent observations. They
are represented as limited or bounded by a serrated line to indicate, as
suggested by Mills, that they are not sharply delimited, but that they inter-
fuse or interdigitate with surrounding regions.

The Sensor Areas. — The sensor areas occupy regions corresponding in
a general way with those of the monkey brain.

The cutaneous and muscle sense areas have been assigned to the post-
central, a portion of the super- and sub-parietal convolutions on the lateral
aspect, and to portions of the frontal convolution and of the callosal convolu-
tion on the mesial aspect. It is also probable that the tactile (cutaneous)
area may be assigned, though in less degree, to the pre-central convolution,
the general motor area. This is in accordance with the embryologic investi-

554 TEXT-BOOK OF PHYSIOLOGY.

gations of Flechsig, who concludes that the entire Rolandic region is to be
regarded as sensor as well as motor in function, and names it the area of
body feelings, or the somesthetic area.

The clinic and post-mortem evidence as to the extent of the area of tactile
sensibility and its coincidence with the motor area is somewhat contradictory,
and in some respects apparently in opposition to the view of Flechsig. Thus,
Dr. C. K. Mills, whose skill in interpreting the phenomena of disease is well
known, states in this connection in his work on nervous diseases that "innu-
merable cases have been reported of lesions of the motor cortex without the
slightest impairment of sensibility." In several cases of excision of the
human cortex in the Rolandic region by surgical operations careful studies of
the patients failed to show any impairment of sensation. Other competent
observers, however, have reported a number of cases in which anesthesia
more or less pronounced and persistent has accompanied lesions of the motor
area. The explanation of these contradictory observations is not apparent.

The olfactory area has been assigned to the uncinate convolution, the
anterior part of the callosal convolution, and the posterior part of the base of
the frontal lobe. Lesions in this region are frequently accompanied by
subjective olfactory sensations.

The gustatory area has been assigned to the collateral convolution.

The auditory area has been assigned to the posterior portion of the super
temporal convolution and to the retro-insular convolutions, the island of
Reil. Unilateral destruction of this region is followed by only a partial
loss of hearing in the opposite ear (owing to the partial decussation of the
cochlear nerve), which, however, may be recovered from after a time, owing
probably to a compensatory activity of the insular convolutions. Bilateral
disease of this region is followed by complete deafness. Within this area
there is a smaller region, disease of which is accompanied by word-deafness
only, the patient being unable to distinguish the tone intervals between
words and syllables and therefore hearing only confused noises. Object-
hearing has also a separate area of representation.

The visual area has been assigned to a triangular shaped area on the
mesial surface of the occipital lobe, which includes the gray matter above
and below the calcarine fissure (the cuneus and upper part of the lingual
lobe), and to the gray matter of the first occipital convolution on the lateral
aspect of the occipital lobe. Focal lesions of this area on one side are followed
by lateral homonymous hemianopsia, which, however, does not involve, as a
rule, the fovea or macula. It is, therefore, the area of homonymous 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
extremity 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. Beyond the limits of this visual area and on the lateral
aspect of the parietal lobe there is a region (the supra-marginal convolution
and angular gyrus) in which impressions of words and letters seen have
their representation. Destruction of this area by diseases is followed by
word- and perhaps letter -blindness, the patient being unable to recognize
words and letters seen because of failure to revive the memory images of
words and letters. The areas for visual sensations and optic memory

THE CEREBRUM.

555

pictures are therefore separate, a fact which has led to a division of the
visual area into a lower and a higher area.

It was stated in a previous paragraph that electric stimulation of the
sensor areas of the monkey brain is attended by certain motor reactions
which vary with the area stimulated. Corresponding areas are believed to
be present in the human brain and that their stimulation would be followed
by similar motor reactions. Their location is shown in Figs. 252 and 253,
and named visual, auditory, olfactory, and gustatory motor.

The stereo. gnostic area or area of stereognostic perception, by which objects
are recognized through their form independent of vision and by the sense of
tpuch alone, has been located in the super-parietal convolution and the
precuneus (Mills). The existence of such an area is rendered probable by

FIG. 254. — SCHEME OF THE MOTOR AREA OF THE HUMAN BRAIN AND ITS SUBDIVISIONS. — (After

Mills.)

the fact that cases have been recorded in which there was a loss of this
power (astereognosis) unaccompanied by either sensor or motor distur-
bances. Post-mortem investigations showed that in these cases there was a
destruction of the superior parietal convolution.

Equilibratory, intonation, and orientation areas have been provisionally
located in the spheno-temporal lobe.

The Motor Areas. — The general motor area (Fig. 252) is represented as
occupying the pre-central convolution, the base of the super-frontal con-
volution, both on its lateral and mesial aspects, and the paracentral lobule.
The exclusion of the post-central convolution from the motor area is in ac-
cordance 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
Grunbaum on the brain of the chimpanzee, which demonstrate that the
post-central convolution is absolutely inexcitable to electric stimulation.

556 TEXT-BOOK OF PHYSIOLOGY.

It is quite probable that with the 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 also capable of subdivision into areas of variable size,
in which the movements of the face and associated structures, the head and
eyes, the arm, trunk, and leg, are represented. (Fig. 254.) The sequence
of their representation from below upward is similar to that observed in the
monkey and chimpanzee. In each of these five main areas there are yet
smaller areas in which the movements of localized regions of the body are
in part represented and which are indicated in diagram (Fig. 254) by corre-
sponding words. The words in the areas marked, eyes and head, face,
arm, trunk, and leg, indicate the location of nerve-cells which through the
discharge of nerve impulses excite to contraction the muscles which im-
part to the regions indicated by these words their characteristic movements.
A localized irritative lesion of any one of these areas gives rise to convulsive
movements of the muscles of the opposite side of the body, similar in char-
acter to those resulting from electric simulation of the corresponding areas
of the monkey and ape brains. Destruction of these areas from the growth
of tumors, softening, etc., is followed by paralysis of the muscles. Electric
stimulation of these areas of the human brain for the purpose of localizing
obscure irritative lesions prior to surgical procedures on the brain gives
rise to the same convulsive movements.

Language. — The succession of motor acts by which ideas are expressed,
is known as language, which may be divided into (i) articulate or spoken,
and (2) written.

The expression of ideas both by words and signs depends primarily
on the power of reviving the images or memories of words and letters heard
and seen; and secondarily of the power or reviving the images or memories
of the muscle movements which were previously employed in an effort to
imitate or reproduce the words (speech) or the verbal signs (writing).

Clinico-pathologic investigations have shown that words or letters heard
and seen have areas of representation in the cortex, in the general auditory
area, in the supra-marginal convolution and angular gyrus respectively (Fig.
252). Destruction of these areas is followed by word-deafness and word-
blindness. The same methods of investigation have shown that the muscle
movements employed to reproduce the words and the verbal signs also
have areas of representation in the cortex; the former in the sub-frontal
convolution (Fig. 252), and probably in the adjacent region, the island of
Reil, on the left side in the great majority of people; the latter in front of
the arm region of the general motor area. Destruction of these areas is
followed in the first instance by a loss of the power of executing the move-
ments of the muscles employed in speech, and in the second instance, of
those employed in writing.

These different areas are connected with one another by association
fibers, and, taken collectively, constitute the language zone. Their situ-
ation and relations are shown in Fig. 255. In this figure the dotted lines
coming from the ear (a) and the eye (v) represent the auditory and visual
tracts through which nerve impulses pass to the auditory (A) and the visual
centers (V) respectively. Similar lines coming from the muscles involved

THE CEREBRUM.

557

in speech and writing might also be represented to indicate the paths of the
nerve impulses to the motor speech (M) and the motor writing centers (E).
The continuous lines on the surface of the cortex represent nerve-fibers
which associate the auditory and visual centers with the speech and writing
centers and with higher psychic centers (O O) as well. The dotted lines
coming from the speech and writing centers represent the tracts through
which nerve impulses pass to the muscle of the larynx, tongue, mouth, and
lips, and to the muscles of the hand. The anatomic and physiologic
association of the various areas is
essential to the registration of the
impressions made on the ear and eye
and for the expression of the ideas
evolved from them by words (speech)
and signs (writing). Their collective
action is essential to the acquisition
of language. Destruction of any
part of this cerebral mechanism is at-
tended by an impairment of or a total
loss in either the power of obtaining
auditory images of words heard and
visual images of words seen, or the
power of expressing ideas by speech
and writing. To this pathologic con-
dition the term aphasia has been
given.

Aphasia. — It was discovered by
Bouillaud that a destructive lesion of
the third frontal convolution on the left
side was accompanied by a partial or
complete loss of the faculty of articu-
late speech, the power to express ideas
with words. To this condition the
term aphasia was given. Though of
limited application etymologically, the
word is now employed in a wider
sense to signify " partial or complete
loss of the power of expression or com-
prehension of the conventional signs
of language," words either spoken or
written, due to lesions of different por-
tions of the cortex, and especially on
the left side.

Aphasias are of many degrees and kinds, though they may be included
in the two general divisions, motor and sensor.

Motor aphasia may be either ataxic or agraphic. In atoxic aphasia the
patient is unable to express or communicate his thoughts by spoken words,
owing to an inability to execute those movements of the mouth, tongue, etc.,
necessary for speech without there being any paralysis of these muscles,
the lesion is usually in the third frontal convolution and most frequently

FIG. 255. — DIAGRAM SHOWING THE RE-
LATION OF THE CENTERS OF LANGUAGE AND
THEIR PRINCIPAL ASSOCIATIONS. A. Audi-
tory center. V. Visual center. M. Motor
speech center. E. Motor writing center.
O. Intellectual center. — (After Grasset.)

558 TEXT-BOOK OF PHYSIOLOGY.

associated with right hemiplegia. In agraphic aphasia the patient is unable
to communicate his ideas by writing through an inability to execute the
necessary movements, though retaining his mental processes. In this form of
aphasia the lesion is in the writing area. These two forms of motor aphasia
are not infrequently 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.

Bilateral Representation. — Though highly specialized movements,
such as those performed by the arms and hands, legs and feet, have their
areas of representation on one side of the cerebrum only, and that opposite
to the side of the movement, less highly specialized movements, such as the
masticatory, phonatory, respiratory and various trunk movements, which
require for their performance the cooperation of muscles on both sides of
the body, have their areas of representation on both sides of the cerebrum ;
the area of either side exciting to action the muscles on both sides of the
body. In the case of specialized movements the representation is unilateral;
in the case of the more general movements the representation is bilateral.

Association Centers. — The sensor and motor areas to which specific
functions have been assigned do not constitute more than one-third of the
total cerebral cortex. There yet remain large regions to which it has been
impossible to assign specific functions based on physiologic experiments.
Three or four such regions separated by the sensor and motor centers are to
be recognized on the lateral and mesial aspects of the hemisphere. In Fig.
256 the location, extent, and names of these regions are represented. The
fibers which are found in these regions belong almost exclusively to the
association system, and become medullated at a later period than do the
fibers of the projection system; moreover, from the method of their medulliza-
tion it would appear that many of these fibers grow out directly from the
sensor centers into these regions and become related to the nerve-cells of
their convolutions, while others grow out from adjacent as well as distant
convolutions. From histologic and pathologic evidence these regions were
termed by Flechsig association centers or areas, 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

THE CEREBRUM.

559

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

Motor and tactile area.

Parietal association area

and of Reil.

Occipito-temporal
association area.

Auditory area.

Motor and tactile area.

Parietal association area
(Precuneus).

Olfactory lobe.
Olfactory tract.

Occipito-temporal
association area.

\ Olfactory area.

Gyrus hippocampus.

FIG. 256.— DIAGRAMS TO SHOW THE POSITION AND THE RELATION OF THE ASSOCIATION AND
PROJECTION AREAS. THE PROJECTION AREAS ARE DOTTED. — (After Flechsig.)

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.

560 TEXT-BOOK OF PHYSIOLOGY.

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
and the bone. These sinuses, from the very nature of the tissues which
enter into their formation, have rigid walls and will therefore withstand any
pressure to which they may be subjected under physiologic conditions. The
same obtains at their points of exit from the cranium where a free outflow
is in consequence always assured.

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 the
veins which empty into the superior longitudinal sinus have their mouths
directed forward, hence the blood discharged from these veins must flow
against the current in the sinus. The venous blood leaves the cranium
mainly by way of the internal jugular veins which are direct continuations of
the lateral sinuses.

The Intra-cranial Lymph Spaces. — In order to understand the phe-
nomena attending the circulation of blood through the cranium it is necessary
to take into consideration an important fact, viz. : that the brain and spinal
cord are surrounded on all sides by a relatively large and continuous lymph
space. This space which is found between the arachnoid and the pia mater
is filled with a liquid, the so-called cerebrospinal fluid, which being interposed
between the brain and the skull on the one hand and the spinal cord and the
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 communication
with that in the subarachnoid space through the foramen of Magendie and the
foramina of Key and Retzius. The cerebrospinal fluid may also penetrate
into the perineural lymph spaces surrounding the cranial and spinal nerves.
The quantity of the cerebrospinal fluid is relatively small, amounting to
from 60 to 80 c.c.

THE CEREBRUM. 561

The Mechanism of the Intra-cranial Circulation. — As previously
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 materially
above that of the atmosphere; the pressure in the arteries, capillaries and
veins presents the usual values in these different regions of the vascular
apparatus; the brain presents a volume which may be termed diastolic.

With the occurrence of the succeeding cardiac systole, the cerebral
vessels, receiving an additional volume of blood, expand and occasion a
corresponding increase in the volume of the brain, which is 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
its activities, even to the point of unconsciousness. If this is not to occur
the maximum expansion of the arteries, and hence the brain, must be checked
and controlled. This is accomplished in the following way: As the brain
approaches that degree of expansion permitted by the displacement of the
cerebrospinal fluid, it begins to exert a compression of the pial veins. This
compression by narrowing the lumen of the veins diminishes their capacity
and hence increases the pressure of their contained blood until it is equiva-
lent to the pressure exerted by the brain against the veins. At this moment
the pressures in the arterioles, capillaries and veins approximate each other
in value.

From these factors it will be seen that the circulation through the brain
approximates a circulation through a system of rigid tubes. The result is an
increase in the velocity of the outflow and a diminution of the blood-pressure.
As an additional result the pulse wave of the arterial system is transmitted
to the blood of the large veins and sinuses which therefore exhibit normally
pulsations synchronous with those of the arteries. The rise of the pressure
in the cerebral veins is regarded therefore as the factor which, by limiting
brain expansion, checks the rise of the intra-cranial pressure beyond physio-
logic limits. With the diastole of the heart and the recoil of the arteries,
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. Thus the rise
of pressure in the thoracic veins which occurs with each expiratory act,
causes a damming back of the venous blood in the sinuses and pial veins,
36

562 TEXT-BOOK OF PHYSIOLOGY.

resulting in a further increase in the volume of the brain and in the intra-
cranial pressure. The reverse takes place in inspiration.

It has been ascertained experimentally that the intra-cranial pressure
may vary considerably and consciousness still be preserved. Hill found it
to be 40 to 50 mm. of Hg. in the convulsions of strychnin poisoning and a
little less than zero in a patient standing erect.

The Regulation of the Volume of Blood Entering the Brain. — It is
generally believed that the cerebral vessels are not provided with vaso-motor
nerves. Every attempt to prove their existence either by physiologic or
histologic methods has thus far failed of convincing proof. In the absence
of vaso-motor nerves, the regulation of the circulation in the brain must neces-
sarily be dependent on changes affecting the arterial and venous pressures in
other regions of the body.

The most effective factor in increasing or decreasing the blood-supply
to the brain resides in the power of the vaso-motor center to cause a contrac-
tion or dilatation of the cutaneous and splanchnic vessels. Thus if the
vaso-motor center declines in its tonus from any cause whatever, there is a
relaxation of the blood-vessels in one or both of these regions, an increase in
the volume of the blood flowing into them, and in consequence, a decrease
in the volume of the blood flowing through the brain. If on the contrary the
vaso-motor center is increased in its tonus, the reverse conditions prevail in
the cutaneous and splanchnic vessels and the quantity of blood flowing into
the brain is increased. Thus in an indirect way the vaso-motor center, by
bringing about a rise or a fall in the general arterial pressure, regulates the
blood-supply to the brain, and controls its amount in accordance with its
needs.

Brain Activity. — Brain activity is characterized by an active conscious-
ness, the development of sensations, ideas, feelings, and the exercise of
volitional power (which manifests in muscle movement) and is the result of a
physiologic condition of the body at large. For the manifestation of brain
activity it is essential that the irritability of the brain cells and more especially
of those composing in large measure the cerebral cortex be maintained at a
normal physiologic level, so that they may respond in the manner peculiar
to them to the action of nerve impulses transmitted through afferent nerves
from all regions of the body. Here as elsewhere throughout the body, the
irritability depends on, and is maintained by, the presence of blood flowing
into and out of the brain in varying quantity from moment to moment, with
a given velocity and under a definite pressure. So long as these conditions
are maintained in the strictly physiologic condition, so long will the brain
respond to stimuli by the development of sensations. The avenues through
which nerve impulses pass to the cortical cells are those beginning in the
special and general sense organs of the body in contact with the external
world, viz.: the eyes, ears, nose, tongue, and skin. The maintenance of
these structures in a strictly physiologic condition is also one of the essential
conditions for brain activity.

Judging from the changes in the character and composition of the blood
which occur during its passage through the brain capillaries, there is 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-

THE CEREBRUM. 563

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 all
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.

In addition to the foregoing phenomena, • experimental investigations
have shown that there is a shunting of a portion of the blood stream from
the brain to other regions of the body, especially to the skin and perhaps to
the abdominal viscera as well, whereby it becomes incapable of functionating
physiologically. The fact that the brain receives a lessened quantity of
blood during sleep has been shown by trephining the skull and inserting in
the orifice a glass plate through which the circulatory conditions of the brain
can be observed. In the waking condition the blood-vessels on the surface
of the brain are prominent, and turgid with blood and the whole organ
completely fills the cranial cavity, indicating that the blood-vessels in the
interior of the brain are in a similar condition. With the onset of sleep the
larger blood-vessels begin to diminish in size, the smaller vessels disappear
from view, the brain tissues become pale and the volume of the brain shrinks.
During the continuance of deep sleep, this anemic condition persists. As
the period of sleep approaches its termination, the smaller blood-vessels
again fill with blood, the surface of the brain flushes, and in a very short
time the former circulatory conditions return, the volume of the brain
increases and the waking state is reestablished.

The fact that the skin receives an increased volume of blood during sleep,
has been shown by inserting an arm or leg in a plethysmograph by which
means a record of any change in volume can be obtained. Howell thus
succeeded in obtaining graphic records in the variations of the volume of the
arm during sleep. These records disclosed the fact that with the onset of
sleep the volume of the arm gradually increased in size until it attained a
maximum which was from one to two hours after the beginning of sleep.

564 TEXT-BOOK OF PHYSIOLOGY.

After this period the volume remains practically the same for several hours,
diminishing as the intensity of sleep diminishes and the waking state is
approached. Just previous to the return of consciousness there is a rapid
diminution in the volume of the arm. If it be accepted that the enlargement
of 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 measuring 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 max-
imum 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 during
the latter period when the brain is reviving that dreams arise, the elements
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. From
all the facts which have been presented it would appear that one cause is a
decline in the irritability of the nerve-cells of the brain and associated sense
organs, and the development of fatigue conditions, the result of prolonged
activity.

A second cause is the withdrawal of a large portion of the blood from the
brain, on the presence of which, here as elsewhere, normal activity depends.
As to whether the diminished activity of the brain is the cause of, or the result
of the withdrawal of the blood there has been much difference of opinion.
Howell has offered a plausible explanation for the withdrawal of the blood
from the brain to the cutaneous vessels, based on the activity of the vaso-
motor center. He assumes that for a variable number of hours, correspond-
ing to the usual waking state, this center possesses a certain average tonus,
due in all probability to reflex influences, by virtue of which it maintains a
certain average contraction of the cutaneous vessels. But at the end of
this period it too becomes fatigued, declines in irritability, becomes less
responsive to reflex influences, and hence loses its control over the vessels.
As a result they dilate and thus reduce the amount of blood flowing to the brain
to a level insufficient to maintain its activity, after which sleep supervenes.
During sleep the irritability and tonus of the center are restored, when its
control of the blood-vessels is regained. Unless the brain in its functional
activities differs from all other organs of the body, it may be inferred that
cessation of activity or repose is the result partly of fatigue and partly of a
diminution of the blood-supply.

CHAPTER XXII.
THE CEREBELLUM.

The cerebellum is situated in the inferior fossae of the occipital bone,
beneath the posterior lobes of the cerebrum, from which it is separated by the
tentorium cerebelli, a semilunar fold of the dura mater. It is partially
divided into hemispheres by a longitudinal fissure, more apparent on the
inferior surface, though united by a central lobe, the vermiform process.
Each hemisphere is connected with the cerebrum, the pons, medulla, and
spinal cord by three bundles of nerve-fibers known respectively as the superior,
middle, and inferior peduncles. The surface of the cerebellum presents a
series of lobes and fissures of which the former have received more or less
fanciful names. A section of the cerebellum shows that it is composed
of gray matter externally and white matter internally. The general
appearance presented on section is shown in Fig. 257.

Structure of the Gray Matter. — The gray matter consists mainly of
nerve-cells of varying size and shape, which are arranged in two layers:
viz., an outer or molecular and an inner or granular.

The molecular layer consists of stellate and multipolar cells of small
size, from which dendrites and axons pass horizontally and vertically. The
granular layer consists, as its name implies, of granular-shaped cells and
large stellate cells. These cells are characterized by the possession of den-
drites and axons, the course and relation of which have not been clearly
determined.

The inner border of the molecular layer presents a series of large cells
originally described by Purkinje and known by his name. From the outer
end of the cell-body one or more dendrities emerge which soon divide and
subdivide into a number of branches which pass toward the cerebellar sur-
face. The general arrangement of these dendrities gives to the entire cell a
tree-like appearance (Fig. 258). From the inner end of the cell an axon
emerges which passes centrally into the white matter.

Structure of the White Matter. — The white matter consists of nerve-
fibers which are arranged in association and projection systems.

The Association System. — The fibers which compose this system are of
variable lengths and unite adjacent as well as distant regions of the cer-
bellar cortex. They doubtless associate them both anatomically and physio-
logically.

The Projection System. — The fibers composing this system connect the
cerebellar cortex with certain structures in the cerebrum, pons, medulla,
and spinal cord. They may be divided into efferent and afferent systems.

The efferent fibers have their origin in the cells of Purkinje and the
dentate nucleus. Some of these fibers emerge from the cerebellum in the
superior peduncles through which they pass toward and beneath the corpora

565

566

TEXT-BOOK OF PHYSIOLOGY.

quadrigemina to terminate around the cells of the red nucleus. As they
approach this nucleus some of the fibers cross the median line and decussate
with those coming from the opposite side, while others pursue a straight
direction, terminating on the same side. Through the intervention of fibers
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 chiefly
with the opposite side.

Efferent fibers also leave the cerebellum by the middle peduncle and pass
directly to the nucleus pontis, around the cells of which their terminals
arborize. Efferent fibers also descend the inferior peduncles and constitute

FIG. 258. — SECTION OF CEREBELLAR
CORTEX. A. Outer or molecular
layer. B. Inner or granular layer.
C. White matter, a. Cell of Purk-
inje. 6. Small cells of inner layer.
c. Dendrites of these cells, d. A
similar cell lying in the white matter.
— (Stirling.)

FIG. 257. — VIEW OF CEREBELLUM IN SECTION, AND
OF 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 Leveille.*)

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 the
superior peduncles come from the red nucleus; those in the middle peduncles
from the nucleus pontis of the opposite side, having crossed or decussated at
the raphe near the anterior surface of the pons; those contained in the 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 former
coming from the gracile and cuneate nuclei of the opposite side, the latter

THE CEREBELLUM. 567

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 through this system of efferent and afferent fibers is
brought into relation with many different regions of the cerebrum, pons,
medulla, and spinal cord. Each half of the cerebellum is connected with the
foregoing 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
analysis of clinico-pathologic facts, and from the comparative anatomic
development in different animals, the deduction has been drawn that the
cerebellum coordinates and harmonizes the action of those muscles the
activities of which are necessary to the maintenance of body equilibrium
both during station and progression.

By equilibrium of the body is understood a condition which may be main-
tained for a variable length of time without displacement, and is possible only
so long as a vertical line passing through the center of gravity falls within the
base of support. The support offered by the earth to the feet neutralizes and
counteracts the force of gravity. In standing, when the body is in the erect or
military position, the arms by the side, the center of gravity lies between the
sacrum and the last lumbar vertebra, and the line of gravity falls between the
feet and within the base of support. The entire skeleton for the time being
is rendered fixed and rigid at all its joints by the combined action of the
muscles connected with it. That this position may be maintained all the
different groups of antagonistic but cooperative muscles must be accurately
coordinated in their actions. Any failure in this respect is at once attended
by a disturbance of the equilibrium and displacement.

In progression, walking, running, dancing, etc., the body is translated
from point to point by the alternate action of the legs. Whether the direction
of the translation be rectilinear or curvilinear, as the legs change their position
from moment to moment, the center of gravity also changes, and at once the
equilibrium is menaced. If it is to be maintained and displacement prevented
there must be a prompt readjustment in the relation of all parts of the
body so that the line of gravity falls again within the base of support. The
more complicated the moments of progression, or the narrower the base of
support, the greater is the danger to the equilibrium, and hence the necessity
for rapid and compensatory changes in coordinated muscle activity. All
movements of this character, in man at least, are primarily volitional and
require for their performance the constant exercise of the attention. With
frequent repetition they gradually come to be performed independently of
consciousness and fall into the category of secondary or acquired reflexes.

Though coordinating power is exhibited by the spinal cord, medulla,
and basal ganglia, it is only in the cerebellum that this power attains its
highest development and differentiation. To it is assigned the power of
selecting and grouping muscles, not in any restricted part, but in all parts of
the body, and coordinating their actions in such a manner as to preserve the
equilibrium.

568

TEXT-BOOK OF PHYSIOLOGY.

The Results of Experimental Lesions.— If the cerebellum in its totality
coordinates and harmonizes the action of the muscles on the opposite sides of
the body, any derangement of its structure or its connections with the cord,
medulla, pons, or basal ganglia should at once be followed by incoordination
of muscles and a want of harmony in their action. Experimental lesions of
the cerebellum are attended by such results. The phenomena observed are
many and complex. They differ in extent and character in different animals
and in accordance with the extent and location of the lesion, though the note
of incoordination runs through them all.

FIG. 259. — ATTITUDE ASSUMED AFTER DESTRUCTION or 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 an
inability to maintain the equilibrium necessary to the erect position. On
attempting to stand, the animal at once falls toward the side of the lesion, the
muscles of which at the same time contract and give to the body a distinctly
curved condition (Fig. 259). The anterior limbs are extended to the opposite
side. On making efforts to regain the standing position, the animal may

roll over around the long axis of its
body. Conjugate deviation of the
eyes is frequently observed as well as
nystagmus.

After a few days the symptoms par-
tially subside and the animal acquires
the power of sitting on the abdomen
when the anterior limbs are widely
extended (Fig. 260). As the days go
by the improvement continues, and the
animal recovers the power of walking,
though each step is attended with
tremor and oscillations of the body.
Any change in the center of gravity
such as results when one leg is lifted may result in a fall toward the side of
the lesion, owing to an inability to promptly bring about the necessary
compensatory muscle actions. With time the animal continues to improve
in its power of adjustment, though it never completely recovers it. 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, for
the limbs are extended and abducted, the head and neck are retracted, and

FIG. 260. — ATTITUDE IN REPOSE AFTER
THE COMPLETE REMOVAL OF THE CERE-
BELLUM BUT DURING THE PERIOD OF RES-
TORATION OF FUNCTION. — (Moral and Doyon,
after Thomas.)

THE CEREBELLUM.

569

opisthotonos is established. In time these effects also partially subside,
though all attempts at walking are permanently accompanied by tremor and
oscillations. The characteristic effect which follows section of the peduncles
is again incoordination, manifesting itself in deviation of the head, eyes, in-
ability to walk, tremor on exertion, etc. The effects vary, however, according
to the peduncle divided. Section of the middle peduncle gives rise to the
most pronounced effects. The head and the anterior part of the body are
at once drawn toward the pelvis on the side of the section. A voluntary
effort on the part of the animal causes it to lose all control of its muscles and
the body is rotated around its longitudinal axis from 40 to 60 times a
minute before it comes to rest. According as the lesion is made from be-
hind or before, the rotation is from or to the side of the section. In time
these symptoms subside, though the animal never completely recovers.

The partial recovery of the power of coordination, observed after removal
of a portion or the whole of the cerebellum, indicates that the centers in the
cord, medulla, pons, and cerebrum
endowed with corresponding
though less developed power, de-
velop compensatory activity and
acquire to some extent the capa-
bilities of the cerebellum itself
(Fig. 261).

Clinico -pathologic facts partly
corroborate the results of physio-
logic investigations. In various

- £ -I . j "L 1 1 •*..•.*-*•• 4.W.L. JL JXV/VJXX.JJ>OiJXV^i-« ^LJL- X XL.XN. J_X iJO JL. JV \J \^ X 1\J J.H

torms Ot uncomplicated cerebellar 0F THE VERMIS.— (Morat and Doyon, after Thomas.)

disease, vertigo, tremor on making

voluntary efforts, difficulty in maintaining the erect position, unsteadiness

in walking, opisthotonos, pleurothotonos, are among the symptoms generally

observed.

Comparative anatomic investigations reveal a remarkable correspondence
between the development of the cerebellum and the complexity of the move-
ments exhibited by animals. In those animals whose movements are com-
plex and require for their performance the cooperation of many groups of
muscles the cerebellum attains a much greater development in reference to
the rest of the brain than in animals whose movements are relatively simple
in character. This relative increase in the development of the cerebellum
is found in many animals, as the kangaroo, the shark, the swallow, and the
predaceous birds generally.

The Coordinating Mechanism. — Though it is not known how the
cerebellum selects and coordinates groups of muscles for the performance
of any complex movement, it is known that its activity is largely reflex in
origin and excited by impulses which come to it from peripheral organs. In
this as in other forms of reflex activity the mechanism involves (i) afferent
nerves, e.g., cutaneous, muscle, optic, and vestibular, and their related end-
organs, tactile corpuscles, muscle spindles, retina, and semicircular canals,
all indirectly connected with (2) the cerebellar centers; (3) efferent nerves
indirectly connected with (4) the general musculature of the body. Both
station and progression are directly dependent on the development and

FIG. 261. — PROGRESSION AFTER DESTRUCTION

570 TEXT-BOOK OF PHYSIOLOGY.

transmission of afferent impulses from the previously mentioned peripheral
sense-organs to the cerebellum. Tactile, muscle, visual, and labyrinthine
impressions and sensations not only cooperate in the development and or-
ganization of the motor adjustments necessary to the maintenance of the
equilibrium and locomotive coordination, but even after their organization
they are necessary to the excitation of cerebellar activity. The manner in
which they lead to the development of this capability on the part of the cere-
bellum is conjectural. Their ever-present influence is shown by the effects
which follow their removal, as the following facts indicate.

The prevention of the development of tactile impulses by freezing or
anesthetizing the soles of the feet, and the blocking of normally developed
impulses through destruction of afferent pathways in diseases of the spinal
cord lead at once to make impairment in the coordinating power. The
removal of the skin from the hind legs of the frog, previously deprived of its
cerebrum, destroys its coordinating power, which it would otherwise possess
in a high degree.

The blocking, in consequence of destructive lesions of the spinal cord,
of the impulses, which come from the muscles, tendons, etc., and which inform
us of the activity and the degree of activity of our muscles, the location of
limbs, the amount of effort necessary to produce a given movement, etc.,
also gives rise to much incoordination. A blocking of both tactile and
muscle impulses frequently exists in degeneration or sclerosis of the posterior
columns of the spinal cord. The coordinating power is so much impaired
in this disease that the patient is unable to maintain, without strained effort,
the erect position and especially if the directive power of the eyes be removed
by closure of the lids. Walking becomes extremely difficult; the gait is
irregular and jerky, and equilibrium is maintained only by keeping the
eyes fixed on the ground in front and by artificially increasing the basis of
support by the use of canes.

An interference with the development of the customary visual impres-
sions which in a measure maintain the sense of relation of the individual
to surrounding objects also gives rise to equilibratory disturbances. A
rapid change in the relation of the individual to surrounding objects or the
reverse; a change in the direction of one optic axis from the use of a prism
or from paralysis of an eye muscle; the destruction of an eye; — these and
similar conditions frequently give rise to such marked disturbances of the
equilibratory power that displacement is difficult to prevent.

An interference with the development of the so-called labyrinthine im-
pressions by destruction of the semicircular canals gives rise to the most
remarkable disturbances in this respect. Section of one horizontal canal1
in the pigeon is followed by oscillations of the head in a horizontal plane
around a vertical axis. Bilateral section so increases these oscillations that
the pigeon is unable to maintain equilibrium and forced to fall and turn con-
tinuously around the vertical axis. Bilateral section of the posterior vertical
canals gives rise to oscillations around a horizontal axis which frequently be-
come so exaggerated as to eventuate in the turning of backward somersaults,

1 The physiologic anatomy of the semicircular canals is described in the chapter devoted
to the ear, to which the reader is referred.

THE CEREBELLUM. 571

head over heels. Similar phenomena follow division of the superior vertical
canals.

Bilateral destruction of both sets of canals is attended by extraordinary
disturbances in the equilibrium. From the moment of the operation the
animal, the pigeon, loses all control of its motor mechanisms. It can neither
maintain a fixed attitude nor execute orderly movements of progression; its
activity, continuous and uncontrollable, is characterized by spinning around
a vertical axis, turning somersaults, dashing itself against surrounding objects
until life is endangered. If the animal be protected from injury, these dis-
turbances gradually subside, and in the course of a few months the equilibra-
tory power is so far regained that standing and walking at least become
possible. In this condition, however, the coordinating power is directly de-
pendent on visual impulses, for with the closure of the eyes all the previous
motor disturbances at once recur. These and similar facts indicate that the
semicircular canals are the peripheral sense-organs from which come the
nerve impulses most essential to the excitation of the cerebellar coordina-
tive centers in their control of equilibrium and of progression.

The cerebellum may therefore be regarded as the essential, most highly
differentiated portion of the coordinating mechanism concerned in the main-
tenance of equilibrium, during both station and progression. The manner
in which the cerebellum accomplishes this result is unknown, though it is
certain, from the foregoing facts, that its special mode of activity is dependent
on the excitatory action of nerve impulses transmitted from a variety of
peripheral sense-organs.

CHAPTER XXIII.
THE ENCEPHALIC OR CRANIAL NERVES.

The nerve-trunks which serve as channels of communication between the
encephalon and the structures of the head, the face, and in part the organs
of the thorax and abdomen, pass through foramina in the walls of the cranium,
and for this reason are termed cranial nerves.

According to the classification now generally adopted, there are twelve
cranial nerves on either side of the median line, which, enumerated from
before backward, are as follows (Fig. 262) :

First or Olfactory. Seventh or Facial.

Second or Optic. Eighth or Acoustic.

Third or Oculo-motor. Ninth or Glosso-pharyngeal.

Fourth or Trochlear. Tenth or Pneumogastric or Vagus.

Fifth or Trigeminal. Eleventh or Spinal Accessory.

Sixth or Abducent. Twelfth or Hypoglossal.

The cranial nerves may be classified physiologically in accordance with their
functional manifestations into three groups, viz. :

1. Nerves of Special Sense: e.g., Olfactory, Optic, Acoustic, Gustatory (Glosso-pharyngeal)

2. Nerves of General Sensibility: e.g., Large root of the Trigeminal, Glosso-pharyngeal, and
Pneumogastric.

3. Nerves of Motion: e.g., Oculo-motor, 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 mind
that modern investigations have demonstrated that the glosso-pharyngeal
and pneumogastric nerves contain even at their junction with the medulla
oblongata a number of efferent or motor fibers, and to this extent are mixed
nerves.

The Origins of the Cranial Nerves. — In accordance with modern
views as to the origins of nerves in general, it may be stated that—

The nerves of special sense have their origin respectively in the neuro-
epithelial cells in the mucous membrane of the olfactory region of the nose,
in the ganglion cells of the retina, in the cells of the spiral ganglion of the
cochlea and the ganglion of Scarpa, and in the cells of the petrous and jugular
ganglia. From the cells of these ganglia dendrites pass peripherally to
become associated with specialized end-organs, while axons pass centrally
in well-defined bundles to become related by means of their end-tufts with
primary basal ganglia.

The nerves of general sensibility have their origin in the ganglia on their
trunks, and in this respect resemble the spinal nerves. From the ganglion
cell there emerges a short axon process which soon divides into a central and a
peripheral branch. The former passes toward and into the gray matter
located beneath the floor of the fourth ventricle, where its end-tufts arborize
about nerve-cells. The latter (the peripheral branch) passes toward the
general periphery to be distributed to skin and mucous membranes.

572

THE CRANIAL NERVES.

573

The nerves of motion have their origin in the nerve-cells in the gray matter
beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle.
The axons emerging from these cells course peripherally to be distributed
to skeletal muscles. In some of the motor nerves, and in some sensor
nerves as well, there are to be found efferent fibers of smaller size which
have a similar origin and which become related through the intervention
of sympathetic ganglia (peripheral neurons) with visceral muscles and
glands. These nerves have been termed autonomic nerves.

The Cortical Connections of the Cranial Nerves. — Each of these three
groups of cranial nerves has special connections with the cerebral cortex.

The nerves of special sense for the
most part terminate in primary basal
ganglia, around the cells of which their
central end- tufts arborize. From these
cells axons arise which pass upward and
directly or indirectly come into physio-
logic relation with sensor nerve-cells in
the 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
nerves thus originate in the cells of the
central nerve system and continue up-
ward to the cerebrum, the primary affer-
ent path.

The motor nerves which have their
origin ' in the cells of the gray matter
beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle
are in physiologic relation with nerve-cells in the 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 be-
ginning in the cerebral cortex is thus continued by the motor nerves to the
general periphery.

The three groups of nerves, those of special sense, of general sensibility,
and the motor nerves, are neurons of the first order; the nerve-cells and
fibers which constitute the cerebral connections are neurons of the second
order. It is probable that the sensor cells in the cerebral cortex are neurons
of a third order.

FIG. 262. — 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.
7'. Nerve of Wrisberg. 8. Acoustic. 9.
Glosso-pharyngeal. 10. Pneumogastric.
n. Spinal accessory. 12. Hypoglossal. —
(Moral and Doyon,)

574

TEXT-BOOK OF PHYSIOLOGY.

FIRST NERVE. THE OLFACTORY.

The first cranial nerve, the olfactory, is situated in the upper third of the
nasal fossa, in the regio olfactoria. It consists of from 20 to 30 branches, the
fibers of which are non-medullated.

Origin. — The olfactory nerve is composed of 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 f eltwork 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. 263).

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 of
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 of
the ethmoid bone and is embraced
by the olfactory nerves. As seen
FIG. 263 .—THE RELATION OF THE OLFACTORY On sagittal section, there is just be-

KSn™ Jce.1 02LTZEYproTeEsAsCI-3. "E^ neath *' surface a layer of large
theiial cells. 4. Glomerulus. 5. Mitral cells, pyramidal and spindle-shaped

6. Centrally coursing axons of the olfactory ceHs (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 of

... i

-- -3

THE CRANIAL NERVES.

575

Sylvius and come into relation with nerve-cells in the inferior extremity'of
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. 264).

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 TTimy or °f a gram of oil of roses and to
the 2T6~ro o"o °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.

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 base 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

CH.

T.o.

FIG. 264. — 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. ol}. Posterior olfactory lobule (or
anterior perforated space), m.r. Mesial root. l.r.
Lateral root of tract. — (His.) — After Quain.)

576

TEXT-BOOK OF PHYSIOLOGY.

dendrites these cells are brought into relation posteriorly with successive
layers of cells which collectively constitute the retina. Though the retina is
said to consist of ten or eleven layers, it may be reduced practically to three,
viz. (Fig. 265):

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

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. 266). 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-

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. 265. — RETI-
NAL CELLS, s', z'.
Visual cells with
their peripheral ter-
minations, s. Rods.
2. Cones, b. B i-
polar cells, g. Gan-
glion cells from which
arise the axons of the
optic nerve.

THE CRANIAL NERVES.

577

VISUAL ff£LD

VISUAL HEW

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
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. (Gudden's Commissure.)

Cortical Connections. — After
entering the pulvinar and the lateral
or external geniculate body the visual
fibers terminate in end-tufts which
arborize around nerve-cells. From
these cells new axons arise which
ascend through the posterior part of the internal capsule, at the same time
curving backward to form the optic radiation of Gratiolet, and terminate
finally around nerve-cells in the gray matter of the cuneus and in the gray
matter bordering the calcarine fissure, both situated on the 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.

37

Optic

FlG- 266.— DIAGRAM ILLUSTRATING LEFT
HOMONYMOUS LATERAL HEMIANOPSIA FROM A
LESION OF THE RIGHT OPTIC TRACT OR THE
RIGHT CUNEUS. THE SHADED LINES IN THE
VISUAL FIELDS INDKATE THE DARKENED A*EA.

578 TEXT-BOOK OF PHYSIOLOGY.

Function. — The function of the visual apparatus in its entirety is the
transmission of nerve impulses from the retina to the cerebral cortex where
they evoke the sensations of light and its different qualities — colors. The
specific physiologic stimulus to the retinal visual cells is the impact of the
undulations of the ether. In general it may be said that, at least for the
same color, the intensity of the objective undulation or vibration determines
the intensity of the sensation.

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 re-
flexly a contraction of the sphincter pupillce 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. 270). 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

The existence of a homonymous hemianopsia becomes evident when

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 eye-brows. 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.

3 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 CRANIAL NERVES.

579

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. 267. 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.

THIRD NERVE. THE OCULO-
MOTOR.

The third cranial nerve, the oculo-
motor, consists of some 15,000 per-
ipherally 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. 268).

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 difference
of opinion in regard to the exact location of one or two of the nuclei, the
tabulation subjoined is approximately correct.

Enumerating them from before backward, the nuclei occur in the follow-
ing order:

1. The sphincter pupillae.

2. The tensor chorioideae (the accommodation nucleus).

FIG. 267. — DIAGRAM TO SHOW THE EXIST-
ENCE OF HEMIANOPSIA. The lesion is sup-
posed to be in the right optic tract.

580

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 cms 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-
aton, 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 latter
is distributed to the internal and inferior
recti and inferior oblique muscle (Fig. 269).
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 of
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 enceph-
alon is followed by contraction of the muscles to which it is distributed
with the following results, viz. :

FIG. 268. — DIAGRAMMATIC VIEW OF
THE SITUATION AND RELATION OF
THE NUCLEI OF ORIGIN OF THE
OCULO-MOTOR AND PATHETICUS
(TROCHLEARIS) NERVES. The oculo-
motor nuclei consist of an anterior
nucleus, the Edinger-Westphal nucleus
(a and b), and a posterior nucleus;
the posterior nucleus has a dorsal, a
ventral, and a mesial portion; the de-
cussation of fibers from the dorsal
portion of the posterior nucleus is also
shown. The decussation of the fibers
of the fourth nerve is also represented.
— (Edinger.)

THE CRANIAL NERVES. 581

1. Diminution in the size of the pupil.

2. Accommodation of the eye for near vision.

3. Elevation of the upper eyelid.

4. Internal deviation and rotation upward and inward of the anterior pole

of the eye, combined with a small amount of torsion toward the mesial
line, due to preponderating action of the internal rectus and inferior
oblique muscles.
Division of the nerve either experimentally or as a result of compression

from a pathologic cause is followed by a relaxation of the muscles, with the

following effects, viz.:

1. Dilatation of the pupil, the iris

responding neither to light nor
to efforts of accommodation.

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 ^ ^r^pT^™ °A™

line due tO the Unopposed nerve. 3. Superior branch. 4. Inferior branch.

action Of external rectUS and 5- Abducens. 6. Trifacial. 7. Ophthalmic

trip Qnnprirvr nhlimip mimrlpc branch divided. 8. Nasal branch. 9. Ciliary

the Superior Oblique muscles. gangKonm I0. Motor branch to this ganglion

5. Double Vision Or diplopia. The from the inferior branch of the third nerve, n.

image Of the eye Of the para- Sensory fibers. 12. Sympathetic fibers. 13.
*\ . T J . Ciliary nerves. — (Sappey.)

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-

582

TEXT-BOOK OF PHYSIOLOGY.

traction of the radiating fibers of the iris, which collectively constitute the
dilatator pupilla muscle. With the entrance of light into the eye, the pupil
diminishes in size, in consequence of the contraction of the sphincter 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. 270). 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 sphinc-
ter are impaired or abol-
ished.

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. Experi-
mental and pathologic data
apparently disprove the
probability of their ter-
minating in the superior
corpora quadrigemina. It
has been shown, however,
that as the optic tract ap-
proaches its termination

llan/nben
in Optic Tract
Direct,.^
Crossed..

Transect ion of Spin ul Cord

FIG. 270. — DIAGRAM DESIGNED TO SHOW THE MECHANISM
OF THE IRIS REFLEX. The central termination of the
pupillary fibers is hypothetical.

the visual and the pupillary fibers separate and it has been assumed that
the latter come into anatomic relation with some intercalated system which
in turn is connected with the sphincter nucleus. As to the situation, origin
and course of this system nothing positively is known. There is some evi-
dence for the view that these two systems are associated by commissural
fibers.

THE CRANIAL NERVES. 583

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
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 mu'scles 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 pupillae 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. 270), 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 578 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 th e 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 rath er 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 t he central line is crossed and the
light falls on the sensitive side, whe n the usual pupillary response manifests

584

TEXT-BOOK OF PHYSIOLOGY.

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. 270 in which the course of the pupillary fibers
is shown and especially if it be accepted that these fibers at their central
terminations decussate or are in relation either directly or indirectly with the
sphincter centers.

The eye of the right side is then in turn shielded from the light 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. 270. 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 pupil-
lary fibers.

FOURTH NERVE. THE TROCHLEAR.

The fourth cranial nerve, the troch-
lear, consists of peripherally coursing
axons which serve to bring the cells from
which they arise into relation with the
superior oblique muscle.

Origin. — The axons of this nerve
arise from a group of cells located be-
neath the aqueduct of Sylvius just pos-
terior to the last nucleus of the third
nerve. After emerging from the nucleus
the nerve-fibers pass downward 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 crus cerebri, after
which they cross to the opposite side. Their end-tufts arborize around the
cells of the nuclei already described.

Distribution. — After its decussation the nerve-trunk emerges just be-
low the posterior quadrigeminal body, crosses the superior cerebellar pe-
duncle, and winds around the crus cerebri to the anterior border of the pons
Varolii. It then enters the orbit cavity through the sphenoid fissure and
finally terminates in the superior oblique muscle. In its course the nerve

FIG. 271. — DISTRIBUTION OF THE
PATHETICUS. I. Olfactory nerve. II.
Optic nerves. III. Motor oculi commu-
nis. IV. Trochlear, by the side of V the
ophthalmic branch of the fifth, and passing
to the superior oblique muscle. VI. Motor
oculi externus. i. Ganglion of Gasser.
2> 3> 4> S» 6, 7, 8, 9, 10. Ophthalmic divi-
sion of the fifth nerve, with its branches. —
(Hirschfeld.)

THE CRANIAL NERVES.

585

receives filaments from the cavernous plexus of the sympathetic and the
ophthalmic division of the trigeminal (Fig. 271).

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 inferior
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.

Function. — The function of the trochlear nerve is to transmit nerve
impulses to the superior oblique muscle and to excite it to contraction.

FIFTH NERVE. THE TRIGEMINAL.

The fifth cranial nerve, the trigeminal, consists of both afferent and
efferent axons which for the most part are separate and distinct. The
afferent axons constitute
by far the major portion,
the efferent fibers the
minor portion, of the
nerve.

Origin of the 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

FiG 272._ScHEME F ORIGIN AND CONSTITUTION OF THE

peripherally (Fig. 272). TRIGEMINAL NERVE, i. Centrally coursing fibers. 2, 3,
The Centrally directed 4- Peripherally coursing fibers of the cells of the ganglion of

branches collectively form

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

586

TEXT-BOOK OF PHYSIOLOGY.

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 (Fig. 273).
2. The superior maxillary branch
passes forward through the fora-
men rotundum, crosses the
spheno-maxillary fossa, enters
the infra-orbital canal, and emer-
ges at the infra-orbital foramen.
In its course it gives off a num-
ber of branches which are dis-
tributed as follows: viz., to the
integument and conjunctiva of
the lower lid, the nose, cheek,
and upper lip, the palate, the
teeth of the upper jaw, and the
alveolar processes (Fig. 274).
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 (Fig. 275).
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

FIG. 273. — OPHTHALMIC BRANCH OF THE
FIFTH, i. Ganglion of Gasser. 2. Oph-
thalmic division of the fifth. 3. Lachrymal
branch. 4. Frontal branch. 5. External
frontal. 6. Internal frontal. 7. Supra-
trochlear. 8. Nasal branch. 9. External
nasal. 10. Internal nasal. — (Hirschjeld.)

THE CRANIAL NERVES.

587

FIG. 274. — i. Superior maxillary nerve. 2, 3, 4, 5. Dental nerves. 6. Spheno-palatine gan-
glion. 7. Vidian nerve. 8. Large superficial petrosal. 9. Carotid branch of large petrosal.
10. Oculo-motor. u. Superior cervical ganglion. 12. Carotid branches of this ganglion. 13.
Facial. 14. Glosso-pharyngeal. 15. Jacobson's nerve, and 16, 17, 18, 19, branches to the
sympathetic, fenestra rotunda, Eustachian tube. 20. Deep external petrosal. 21. Deep internal
petrosal. — (Hirschfeld.)

FIG. 275. — INFERIOR MAXILLARY BRANCH OF THE TRIGEMINAL NERVE, i. Branch to the
masseter muscle. 2. Filament of this branch to the temporal muscle. 3. Buccal branch. 4.
Branches anastomosing with the facial nerve. 5. Filament from the buccal branch to the temporal
muscle. 6. Branches to the external pterygoid muscle. 7. Middle deep temporal branch. 8.
Auriculo-temporal nerve. 9. Temporal branches. 10. Auricular branches, n. Anastomosis
with the facial nerve. 12. Lingual branch. 13. Branch of the small root to the mylo-hyoid
muscle. 14. Inferior dental nerve, with its branches (15, 15). 16. Mental branch. 17. Anas-
tomosis of this branch with the facial nerve. — (Hirschfeld.)

588 TEXT-BOOK OF PHYSIOLOGY.

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 of
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 ulceration sets in, which may lead to com-
plete destruction of the eyeball. The mucous membrane of the nose be-
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 quadrigem-
ina, gives origin to axons which descend and join the axons from the chief
motor nucleus (Fig. 272).

Distribution. — From their origin the fibers pass forward through the
pons and emerge on its lateral aspect, forming the so-called small root of
the fifth nerve. This then passes forward beneath the ganglion of Gasser,
leaves the cavity of the skull through the foramen ovale, and joins the
inferior maxillary division already described. Its axons are ultimately
distributed to the muscles of mastication: viz., the masseter. the temporal,
the external and internal pterygoids, the mylohyoid, and the anterior portion
of the digastric. A few axons are also distributed to the tensor tympani and

THE CRANIAL NERVES. 589

tensor palati muscles. The efferent or peripherally coursing axons thus serve
to bring the nerve-cells from which they arise into relation with the muscles
of mastication.

Cortical Connections. — The nuclei of origin of the small root are in
histologic and physiologic relation with the lower third of the motor area
of the cerebral cortex. Nerve-cells in this region give off axons which enter
the pyramidal tract, descend through the internal capsule and the cms
cerebri, after which they cross to the opposite side. Their end-tufts arbor-
ize around the cells of nuclei in the medulla oblongata.

Properties. — Stimulation of the small root gives rise to convulsive move-
ments of the muscles of mastication. Division of the nerve is followed by
a paralysis of these muscles. Contraction or paralysis of the tensor tympani
and tensor palati muscles would also be observed under the same conditions.

Functions. — The function of the afferent fibers of the fifth nerve is
the transmission of nerve impulses from its peripheral distribution to (a)
the medulla oblongata; (b) through its afferent cortical tracts to the cerebral
cortex where they evoke sensations. The nerve therefore endows all the
parts to which is it distributed with sensibility.

The function of the efferent fibers is the transmission of nerve impulses
from the cells from which they take their origin, to the muscles of mastication,
which are excited to activity by them. The afferent nerves are in relation
centrally with the nuclei of origin of the efferent nerves, hence the latter
can be excited not only voluntarily but reflexly as in the usual acts of masti-
cation. The afferent fibers from the mouth doubtless assist in the reflex
secretion of saliva.

Peripheral stimulation of different areas in the distribution of the
afferent fibers, e.g., conjunctiva, nasal and oral mucous membranes, teeth,
etc., causes a variety of reflex activities in the muscles associated with the
eyes, face, the respiratory and cardiac mechanisms, which indicate that the
afferent fibers are centrally in relation with a number of motor nerve
centers.

SIXTH 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 (Fig. 276). 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

590

TEXT-BOOK OF PHYSIOLOGY.

tract, descend through the internal capsule and crus 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 mid-
dle line (internal strabismus). In con-
sequence 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.

SEVENTH NERVE. THE FACIAL.

FIG. 276. — DISTRIBUTION OF THE
MOTOR OCULI EXTERNUS OR ABDUCENS.
i. Trunk of the motor oculi communis,
with its branches (2, 3, 4, 5, 6, 7). 8.
Motor oculi externus, passing to the exter-
nal rectus muscle. 9. Filaments of the
motor oculi externus anastomosing with
the sympathetic. 10. Ciliary nerves. —
(Hirschfeld.)

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, cor-
rugator supercilii, 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, de-
pressor labii inferioris, levator menti, posterior belly of the digastric, stylo-
hyoid, and platysma myoides.

Origin. — The nerve-fibers or axons composing the seventh nerve arise
for the most part from a nucleus of large multipolar nerve-cells situated
about five millimeters beneath the upper half of the floor of the fourth 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 the median line, arborize around the nerve-cells
of the opposite facial nucleus.

Clinic observations and histologic investigations, however, render it
probable that the fibers distributed to the occipito-frontalis, the corrugator

THE CRANIAL NERVES. 591

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 cms cerebri, beyond which
they cross to the opposite side and arborize around the cells of the nucleus
already described.

Distribution. — From its superficial origin the trunk of the nerve passes
into the internal auditory meatus beside the auditory nerve. After passing
forward and outward for a short distance through the 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 (Fig. 277).

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-cramal 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, frequently give rise to
spasmodic movements of the facial muscles which may be tonic or clonic in
character.

Division of the facial nerve after its emergence from the stylo-mastoid
foramen is followed by a complete relaxation or paralysis of the superficial
facial muscles. The same result follows compression 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
face corresponding to the lesion; separation of the eyelids from paralysis of
the orbicularis palpebrarum and the unopposed contraction of the levator
palpebrae muscles; 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

592

TEXT-BOOK OF PHYSIOLOGY.

muscles of the face. These muscles by their individual and cooperative
contraction express ideas and feelings and are therefore termed muscles of
expression. 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 'and the facial muscles and serves
for the transmission to the muscles of those nerve impulses developed by

FIG. 277. — SUPERFICIAL BRANCHES or THE FACIAL AND THE FIFTH. — i. Trunk of the facial.
2. Posterior auricular nerve. 3. Branch which it receives from the cervical plexus. 4. Occipital
branch. 5, 6. Branches to the muscles of the ear. 7. Digastric branches. 8. Branch to the
stylo-hyoid muscle. 9. Superior terminal branch. 10. Temporal branches, n. Frontal
branches. 12. Branches to the orbicularis palpebrarum. 13. Nasal or suborbital branches.
14. Buccal branches. 15. Inferior terminal branch. 16. Mental branches. 17. Cervical
branches. 18. Superficial temporal nerve (branch of the fifth) . 19, 20. Frontal nerves (branches
of the fifth). 21, 22, 23, 24, 25, 26, 27. Branches of the fifth. 28, 29, 30, 31, 32. Branches of the
cervical nerves. — (Hirschfeld.)

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.— Between the facial and the acoustic nerve there is a small nerve known
as the pars intermedia, the nervus intermedius or the nerve of Wrisberg. The
true nature of this nerve has long been a subject of investigation. The

THE CRANIAL NERVES. 593

results of histologic investigation and physiologic experimentation would
indicate that it is composed of both afferent and efferent fibers. The afferent
fibers arise from nerve-cells composing in large part the ganglionic enlarge-
ment found on the genu of the facial nerve at the point where it turns back-
ward 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.

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 sali-
vatorius. From its mode of origin,
the nerve of Wrisberg cannot be re-
garded as 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 petro-

sal, the small superficial petrosal, FIG. 278.— CHORDA TYMPANI NERVE, i,
the Stapedius and the chorda tym- 3, 4- Facial nerve passing through the aqueductus
_Q • /-£• _Q\ Fallopii. 5. Ganglioform enlargement. 6.

pam, (*ig. 278). Great petrosal nerve. 7. Sphenopalatine gan-

I. The large superficial petrosal nerve glion. 8. Small petrosal nerve. 9. Chorda tym-

is given off near the geniculate Pa™- I0> «» I2> *£. Va-™™ branches of the

0 ,. ... , r facial. 14, 14, 1 5. Glosso-pharyngeal nerve. —

ganglion. It then passes for- (Hirschfeld.}
ward into the spheno-maxillary

fossa and becomes associated with the spheno-palatine or Meckel's gan-
glion. In its course it receives a filament known as the deep petrosal,
from the carotid 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 outcome
of modern methods of investigation it may be concluded that it is
composed mainly, if not entirely, of fine medullated nerve-fibers which are
the continuations of corresponding fibers in the nerve of Wrisberg and
that their destination is the spheno-palatine ganglion, around the
nerve-cells of which their terminal branches arborize.
Stimulation of the large petrosal, with induced electric currents, 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,
38

594 TEXT-BOOK OF PHYSIOLOGY.

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 Sympathetic). 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 stimulation of the spheno-palatine ganglion
gives rise to the usual vaso-dilatator 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 petrosal 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
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 check, 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 152, 598.)

3. The stapedius nerve or tympanic 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 chordae posterius,
crosses the tympanic membrane between the malleus and incus, leaves
the tympanum by the iter chordae anterius or canal of Huguier, and
finally joins the lingual branch of the fifth 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 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

THE CRANIAL NERVES. 595

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
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.

Division of the chorda tympani nerve is also followed by 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
150.) 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.

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. 279). 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

596

TEXT-BOOK OF PHYSIOLOGY.

portion. Dendritic processes from these cells pass peripherally into the
internal ear, where they terminate on 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,
I0 passes backward, inward, and downward

..''' as far as the lateral aspect of the pons

where the two divisions again separate.

The cochlear nerve, the external root,
passes to the outer side of the restiform
body and enters the ventral acoustic
nucleus and the lateral acoustic nucleus,
around the cells of which its end-tufts
arborize. The vestibular nerve, the in-
ternal 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 nu-
cleus and the dorso-external or Deiters'
nucleus. Some of the fibers of the ves-
tibular branch descend through the pons
and medulla as far as the cuneate
nucleus.

Cortical Connections. — The coch-
lear nerve is ultimately connected with
the cerebral acoustic area, in the tem-
poral lobe of the opposite side through
the intermediation of the auditory tract.
This tract is complex and involved.
^-n a genera^ waY & may ^e said to con-

FIG. 279. — ORIGIN AND TERMINATION
OF THE AUDITORY NERVE, i. Cochlea.

nerve. 4. Ventral acoustic nucleus. 5.
Lateral acoustic nucleus. 6. Semi-
circular canals. 7. Ganglion of Scarpa.
8. Vestibular nerve. 9. Dorso-external
nucleus (Deiters). 10. Dorso-internal
nucleus. — {After Moral and Doyon.}

2. Spiral ganglion (Cord). 3. Cochlear sist in part of fibers which come direct

from the cochlear branch. After pass-
ing through the ventral nucleus and the
trapezoid body they cross the median
line, enter the lemniscus or fillet, and
finally terminate in the pre- and post-
geminal bodies. In their course they give off collateral branches to these
various nuclei through which they pass. Other fibers taking their origin
from cells in these various nuclei proceed to the cortex where they terminate.
Properties. — Stimulation of the cochlear nerve is unattended by either
motor or sensor phenomena. Division of the nerve is followed by a loss
of the sense of hearing. Irritative pathologic lesions give rise to sensations
of sound of varying character and intensity. Degeneration of the nerve or
destruction by tumors, etc., will also be followed by a loss of the sense of
hearing.

Experimental lesions of the semicircular canals involving a destruction
of the physiologic relations of the vestibular nerve are followed by a loss of

THE CRANIAL NERVES. 597

the coordinating and equilibratory power. Disordered movements, such as
rotation to the right or left, somersaults backward and forward, follow
destruction of these canals. Pathologic lesions in the peripheral distribution
of the nerve are attended in man by disturbances of equilibrium, e.g., vertigo,
or a sense of swaying, pitching, and staggering.

Functions. — The function of the cochlear nerve is to convey nerve
impulses from its origin to the pons, from which they are 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
reflexly to certain adaptive and protective movements by which the equi-
librium of the body in both dynamic and static conditions is maintained.

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-nuclei. The axons
of the peripherally directed branches, after emerging from the base of the
skull through the jugular foramen, pass forward and inward under cover of
the stylo-pharyngeal muscle; winding around this muscle they divide into
terminal branches which are distributed to the mucous membrane of the

598 TEXT-BOOK OF PHYSIOLOGY.

posterior one-third of the tongue, pharynx, soft palate, uvula, and tonsils
(Fig. 281).

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.

Jacobsorfs 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
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. Divison 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

THE CRANIAL NERVES. 599

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 in all probability their origin in the nucleus salivatorius.

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-
erals, and terminate in fine end-tufts in the immediate neighborhood of two
groups of nerve-cells, the vagal sensor end-nuclei.

The axons of the peripherally directed branches unite to form a portion
of the common trunk, which, as it descends the neck and enters the thorax and
abdomen, gives off a number of branches which are ultimately distributed to
the mucous membrane of the esophagus, larynx, lungs, stomach, and 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 end-nuclei in the medulla oblongata.

Origin of the Efferent Fibers. — The efferent fibers take their origin
from nerve-cells located in the lateral portion of theformatio 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 autonomic 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

6oo

TEXT-BOOK OF PHYSIOLOGY.

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

FIG. 280. — DISTRIBUTION OF THE PNEUMOGASTRIC. — i. Trunk of the left pneumogastric.
2. Ganglion of the trunk. 3. Anastomosis with the spinal accessory. 4. Anastomosis with the
sublingual. 5. Pharyngeal branch (the auricular branch is not shown in the figure). 6. Superior
laryngeal branch. 7. External laryngeal nerve. 8. Laryngeal plexus. 9, 9. Inferior laryngeal
branch. 10. Cervical cardiac branch, u. Thoracic cardiac branch. 12, 13. Pulmonary
branches. 14. Lingual branch of the fifth. 15. Lower portion of the sublingual. 16. Glosso-
pharyngeal. 17. Spinal acessory. 18, 19, 20. Spinal nerves. 21. Phrenic nerve. 22, 23.
Spinal nerves. 24, 25, 26, 27, 28, 29, 30. Sympathetic ganglia. — (Hirschfeld.}

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 (Fig. 280) :

THE CRANIAL NERVES. 601

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
which in the rabbit unite to form a single nerve, the so-called depressor
nerve. (See page 375.) 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 utimately 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. Division of the nerve is
followed by a loss of sensibility in the mucous membrane of the alimentary
tract and of the pulmonary apparatus, together with a loss of motility of
the structures above mentioned.

Stimulation of the trunk of the nerve in different parts of its course 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 esoph-
agus. Division of these nerves is followed by a loss of sensibility in the parts

602 TEXT-BOOK OF PHYSIOLOGY.

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 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
crico-ary tenoids) ; for phonatory purposes the glottis is closed and the
vocal membranes approximated by the contraction of the adductor muscles.
It has been shown that these opposed groups of muscles have independent
nerve-supplies; that two sets of fibers in the common trunk can be separated
and stimulated independently of each other. Feeble stimulation of the com-
mon 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

THE CRANIAL NERVES. 603

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
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.

Feeble stimulation of the vagus not infrequently 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 pro-
duced is similar to, if not identical with, that produced by stimulation of the
superior laryngeal nerve. (See page 418.)

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).

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

604 TEXT-BOOK OF PHYSIOLOGY.

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 the maintenance of certain organic
reflex actions which are highly essential to the life of the individual, e.g.,
respiration, the heart-beat, blood-pressure, etc., all of which have been con-
sidered in foregoing pages.

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 portions, 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 portion have their origin in nerve-cells
in the lateral margin of the anterior horn of the gray matter in the cervical
portion of the cord as far down as the fifth cervical vertebra. From this
origin the fibers pass to the surface of the cord to emerge between the ventral
and dorsal roots in from six to eight filaments, after which they unite from
below upward to form a distinct nerve. This enters the cranial cavity
through the foramen magnum, where it joins with the medullary portion to
form the common trunk, which then passes forward to emerge from the
cranium through the jugular foramen. (Fig. 281.)

Distribution. — After emerging from the cranial cavity the nerve soon
separates into two branches:

i. An internal or anastomotic branch, consisting chiefly of filaments coming
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 the palato-pharyngeus,
to the levator palati and azygos uvulae muscles (Beevor and Horsley) ; to
the muscles of the larynx through the inferior laryngeal nerve, and to the
heart according to some authorities.

THE CRANIAL NERVES.

605

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 me-
dullary portion of the nerve near its
origin gives rise to contraction of the
muscles to which it is distributed. De-
struction 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 pro-
ducing vocal sounds on account of pa-
ralysis of the constrictor muscles of the
larynx. According to some authorities,
there is also an acceleration of the heart's
action from a loss of inhibitor influences.

Stimulation of the external branch
gives rise to contraction of the sterno-
cleido-mastoid and trapezius muscles,
though division of the branch does not
give rise to complete paralysis, as they
are supplied with motor fibers also from
the cervical nerves. In consequence of
division of the external branch animals
experience extreme shortness of breath

during exercise, from a Want Of COOrdma- Glosso-pharyngeal nerve. 3, 3 Pneumo-

tion Of the muscles Of the fore-limbs and gastric. 4,4,4. Trunk of the spinal acces-

tVi^ mncrlpQ nf rpcnirarirm sory- 5- Sublmgual nerve. 6. Superior

me muscles Ol respiration. ^ cervical ganglion. 7, 7. Anastomosis of

Functions. — The function Of the the first two cervical nerves. 8. Carotid

fibers of the spinal accessory nerve is branch of the sympathetic. 9, 10, n

. . x - ' i f 12,13. Branches of the glosso-pharvngeal.

the transmission of nerve impulses from I4> £ Branches of the facial 16. Otic

the Cells from which they take their ganglion. 17. Auricular branch of the

origin to the muscles to which they are FZTh?a^al \cce~tf"hge"
distributed. They therefore excite to mogastric. 19. Anastomosis of the first

action some Of the muscles Of deglutition; Pair of cervical nerves with the sublingual.

the muscles which regulate the tension ^^^^^^^7.

Of the VOCal bands during phonation and Pharyngeal plexus. 22. Superior laryn-

the muscles which control the respiratory geal""T/ 23- External laryngeal nerve.
, . , , . j 24. Middle cervical ganglion. — (Hirsch-

movements associated with sustained or jdd^

prolonged muscle efforts. The fibers

may also convey nerve impulses which exert an inhibitor influence on the

heart.

6o6

TEXT-BOOK OF PHYSIOLOGY.

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 far
as the lower border of the olivary body. It is located ventro-laterally to the

FIG. 282. — DISTRIBUTION OF THE HYPOGLOSSAL NERVE. — i. Root of the fifth nerve. 2.
Ganglion of Gasser. 3, 4, 5, 6, 7, 9, 10, 12. Branches and anastomoses of the fifth nerve, u.
Submaxillary ganglion. 13. Anterior belly of the digastric muscle. 14. Section of the mylo-
hyoid muscle. 15. GLOSSO-PHARYNGEAL NERVE. 16. Ganglion of Andersch. 17,18. Branches
of the glosso-pharyngeal nerve. 19, 19. Pneumogastric. 20,21. Ganglia of the pneumogastric.
22, 22. Superior laryngeal branch of the pneumogastric. 23. Spinal accessory nerve. 24.
Sublingual nerve. 25. Descendens noni. 26. Thyro-hyoid branch. 27. Terminal branches.
28. Two branches one to the genio-hyo-glossus and the other to the genio-hyoid muscle. —
(Sappey.)

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

THE CRANIAL NERVES. 607

filaments from the first and second cervical nerves, the sympathetic and
vagus. It is finally distributed to the intrinsic muscles of the tongue and to
the genio-hyo-glossus, hyo-glossus, and stylo-hyoid muscles. Branches
derived from the cervical plexus pass to muscles which elevate and depress
the hyoid bone. (Fig. 282.)

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 XXIV.
THE AUTONOMIC NERVE SYSTEM.

The autonomic nerve system consists of i, the sympathetic ganglia and
their branching nerve-fibers, and 2, fine medullated nerve-fibers contained
in the trunks of some of the cranial and some of the spinal nerves, which
serve to bring the nerve-cells in which they arise into relation with the sym-
pathetic ganglia. The fine-medullated nerve-fibers arising in cells in dif-
ferent parts of the central nerve system and passing outward in the trunks
of various nerves are termed from their relation to the sympathetic ganglia,
pre-ganglionic fibers; the non-medullated fibers arising in and emerging from
the sympathetic ganglia are termed post-ganglionic fibers.

This system of nerves is distributed almost exclusively to the epithelium
of secretor organs and to the non-striated muscle-fibers in the walls of the
blood-vessels, including the striated fibers of the heart, and the non-striated
muscle-fibers in the walls of the hollow viscera.

Inasmuch as this system of nerves is supposed to be in a measure inde-
pendent, self-regulative, or autonomous in its activity, but at the same time
under the control of a higher power it has received the name of the auto-
nomic nerve system. (Langley.)

It will be found convenient to consider first the sympathetic ganglia and
the distribution of their post-ganglionic fibers, and second, the origin, course
and distribution of the pre-ganglionic fibers. The sympathetic ganglia may
for convenience of description be divided into three groups: viz., the vertebral
or lateral, the pre-vertebral or collateral, and the peripheral or terminal.

The vertebral ganglia are arranged in the form of chains, one on each
side of the vertebral column. The number of ganglia in the chain varies in
animals of different and in animals of the same species. In man the number
varies from 20 to 22. Each chain may be divided into a cervical, a thoracic,
a lumbar, a sacral, and a coccygeal portion. The cervical portion is usually
described as consisting of three ganglia — a superior, a middle, and an inferior.
This statement is open to question, however, as the middle one is frequently
absent and the inferior one is regarded by some anatomists as belonging to
the pre-vertebral series. The thoracic portion consists of ten or 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
hypogastric, or selvic.

The periphearl ganglia are in more or less close relation with the tissues
and organs in different parts of the body. As members of this group
•may be mentioned the ciliary or ophthalmic, the spheno-palatine, the otic,
the submaxillary and the sublingual ganglia; the ganglia in walls of the heart,
the respiratory organs, the stomach and intestines, the bladder, etc.

608

THE SYMPATHETIC NERVE SYSTEM.

609

40

54

FIG. 283. — CERVICAL AND THORACIC PORTION OF THE SYMPATHETIC, i, i, i. Right pneumo,
gastric. 2. Glosso-pharyngeal. 3. Spinal accessory. 4. Divided trunk of the sublingual.
5> 5y 5- Chain of ganglia of the sympathetic. 6. Superior cervical ganglion. 7. Branches from
this ganglion to the carotid. 8. Nerve of Jacobson. 9. Two filaments from the facial, one to
the spheno-palatine and the other to the otic ganglion. 10. Motor oculi externus. u. Ophthal-
mic ganglion, receiving a motor filament from the motor oculi communis and a sensory filament
from the nasal branch of the fifth. 1 2 . Spheno-palatine ganglion. 13. Otic ganglion. 14. Lingual
branch of the fifth nerve. 15. Sub-maxillary ganglion. 16, 17. Superior laryngeal nerve. 18.
External laryngeal nerve. 19, 20. Recurrent laryngeal nerve. 21, 22, 23. Anterior branches
of the upper four cervical nerves, sending filaments to the superior cervical sympathetic gan-
glion. 24. Anterior branches of the fifth and sixth cervical nerves, sending filaments to the middle

39

6io TEXT-BOOK OF PHYSIOLOGY.

The general arrangement of the sympathetic ganglia, their inter-connect-
ing cords and branches, is shown in Figs. 285 and 286.

Structure of the Ganglia. — Each ganglion consists of a capsule or
stroma of connective tissue in which are contained large numbers of nerve-
cells, nerve-fibers, medullated and non-medullated, and blood-vessels.
The nerve-cells give origin to two or more dendrites, which, perforating a
nucleated capsule by which each cell is surrounded, branch and rebranch and
interlace to form a pericapsular plexus. Each cell gives origin also to an axon,
which as it leaves the cell becomes invested with a sheath continuous with
the capsule surrounding the cell-body. It is, however, wanting in a medullary
sheath, and hence the nerve presents a gray color. Such a structure, in its
entirety, is known as a sympathetic neuron.

The axonic processes as they emerge from the cells divide and sub-
divide forming ever smaller and smaller bundles which pass in different
directions to regions varying in position according to the situation of the
ganglion from which they come. The branches are conventionally termed
rami, communicantes or rami viscerales according as they become associated
with spinal nerves or pass directly to visceral structures. Whatever the route
they pursue, it has been shown by histologic and physiologic methods of
investigation that they are ultimately and directly distributed to but two
structures, viz., non-striated muscle and secretor epithelium. Moreover, there
is no evidence to warrant the assumption that these structures ever re-
ceive nerve impulses directly from the spinal or cranial nerves. All nerve
impulses that influence their activities, either in the way of augmentation
or inhibition, emanate directly though not primarily or originally from the
sympathetic ganglion cells. Since non-striated muscle-cells are found in
the walls of the blood-vessels, in the walls of hollow viscera and around
hair follicles, and since secretor epithelium is found in all glands there is
every reason to believe that the ganglia in some way are associated with
vaso-augmentor and vaso-inhibitor, mscero-augmentor and viscera-inhibitor,
secreto-motor and secreto-inhibitor, and pilo-motor phenomena.

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,
as will be stated later, are derived from the spinal nerves, from the fibers of
which, however, they differ in character, being much smaller and finer.
The fibers of the interconnecting cords, as a rule, transmit nerve impulses
from the more centrally to the more peripherally located ganglia, and are
therefore termed rami efferentes. In the vertebral chain some of the cords

cervical ganglion. 25, 26. Anterior branches of the seventh and eighth cervical and the first
dorsal nerves, sending filaments to the inferior cervical ganglion. 27. Middle cervical ganglion.
28. Cord connecting the two ganglia. 29. Inferior cervical ganglion. 30, 31. Filaments connect-
ing this with the middle ganglion. 32. Superior cardiac nerve. 33. Middle cardiac nerve.
34. Inferior cardiac nerve. 35, 35. Cardiac plexus. 36. Ganglion of the cardiac plexus. 37.
Nerve following the right coronary artery. 38, 38. Intercostal nerves, with their two filaments
of communication with the thoracic ganglia. 39, 40, 41. Great splanchnic nerve. 42. Lesser
splanchnic nerve. 43, 43. Solar plexus. 44. Left pneumogastric. 45. Right pneumogastric.
46. Lower end of the phrenic nerve. 47. Section of the right bronchus. 48. Arch of the aorta.
49. Right auricle. 50. Right ventricle. 51, 52. Pulmonary artery. 53. Right half of the
stomach. 54. Section of the diaphragm. — (Sappey.)

THE SYMPATHETIC NERVE SYSTEM.

611

transmit nerve impulses upward, others downward, others again forward, to
the pre-vertebral and peripheral ganglia.

FIG. 284. — LUMBAR AND SACRAL PORTIONS OF THE SYMPATHETIC, i. Section of ihe dia-
phragm. 2. Lower end of the esophagus. 3. Left half of the stomach. 4. Small intestine.
5. Sigmoid flexure of the colon. 6. Rectum. 7. Bladder. 8. Prostate. 9. Lower end of the
left pneumogastric. 10. Lower end of the right pneumogastric. n. Solar plexus. 12. Lower
end of the great splanchnic nerve. -13. Lower end of the lesser splanchnic nerve. 14, 14. Last
two thoracic ganglia. 15, 15. The four lumbar ganglia. 16, 16, 17, 17. Branches from the
lumbar ganglia. 18. Superior mesenteric plexus. IQ, 21, 22, 23. Aortic lumbar plexus. 20.
Inferior mesenteric plexus. 24, 24. Sacral portion of the sympathetic. 25, 25, 26, 26, 27, 27.
Hypogastric plexus. 28, 29, 30. Tenth, eleventh, and twelfth dorsal nerves. 31, 32, 33, 34, 35,
36, 37, 38, 39. Lumbar and sacral nerves. — (Sappey.)

Among the rami efferentes or interconnecting cords, there are some
which possess special interest for the physiologist, viz. :

612 TEXT-BOOK OF PHYSIOLOGY.

1. The cervical, which connects the thoracic ganglia with the superior

cervical ganglion. It is composed mainly of medullated nerve-fibers
which are derived originally from the spinal nerves.

2. The great splanchnic nerve, formed by the union of branches from the fifth

to the tenth thoracic ganglia. It connects these ganglia with the semi-
lunar ganglion.

3. The small splanchnic nerve, formed by the union of branches from the ninth

and tenth thoracic ganglia. It connects these ganglia with the solar
and renal plexuses.

THE ANATOMIC RELATIONS OF THE SYMPATHETIC GANGLIA TO
VISCERAL STRUCTURES.

The Vertebral Ganglia. — Each ganglion of the vertebral chain gives
origin to one or more gray rami communicantes , 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.
In the thoracic, lumbar, and sacral regions, the ganglion sends at least one
gray ramus into the sheath of the corresponding thoracic, lumbar or sacral
nerves.

As previously stated, the gray rami which thus enter the sheath of the
spinal nerve trunks, pass in company with their contained efferent 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. Experimental investigations have made it
apparent that these post-ganglionic fibers may be regarded as having vaso-
motor and secretor functions. 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.

Most if not all the vertebral ganglia give origin, in addition to the gray
rami communicantes just alluded to, other branches known as visceral
branches or rami viscerates which pass to regions near and remote though
their ultimate distribution is not in all instances apparent.

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
corresponding branches from the middle and inferior cervical ganglia. In-

THE SYMPATHETIC NERVE SYSTEM. 613

terlacing in an intricate manner they form the cardiac plexuses. With the
exception of fibers arising in and coming from the inferior cervical ganglion
there is no reason for believing that the branches of the cardiac plexus are
distributed to the heart-muscle. The fibers having this distribution are
derived for the most part from the inferior cervical and in small part from
the first thoracic ganglion. (See page 305.) The thoracic, lumbar, and
sacral ganglia also give off visceral branches which pass for the most part
to neighboring structures, though from the lower lumbar and sacral ganglia
branches pass to viscera in the lower abdominal 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 whatever
organ the visceral branches are distributed they 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 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 cceliac axis and on a level with the
adrenal bodies. From the ganglia an enormous number of visceral branches
are given off which interlace in a very intricate manner forming what is
known as the solar plexus. Subdivisions 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 blood-vessels of
the stomach, kidney, adrenal body, liver, and small intestine; to the muscle-
walls of the stomach and small intestine as well as the sphincter muscles sur-
round the gastro-duodenal, the pyloric and the ileo-colic orifices.

From the inferior mesenteric ganglion situated close to the origin of the
inferior mesenteric artery visceral fibers are given off, which also interlace
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 opthalmic, the spheno-palatine, the otic and the submaxillary
ganglia; the ganglia in the walls of the heart, the respiratory organs, of the
stomach, intestines and base of the bladder (the pelvic). The situation
of the first four of the ganglia, (the cephalic,) and the distribution of their
visceral branches have been considered in connection with the oculo-motor,
facial, and glosso-pharyngeal nerves. The ganglia of the heart and intestines
have been considered in connection with the physiologic action of these
organs.

THE ANATOMIC RELATION OF THE CENTRAL NERVE SYSTEM TO
THE SYMPATHETIC GANGLIA.

The central nerve system is associated with the sympathetic ganglia
through the intermediation of fine medullated nerve-fibers which have
their origin in nerve-cells located in three different regions, viz.: i. in

614 TEXT-BOOK OF PHYSIOLOGY.

the lateral portion of the gray matter of the spinal cord from the level
of the first thoracic nerve to the level of the third or fourth lumbar
nerves; 2. in the gray matter beneath the aqueduct of Sylvius just where it
enlarges to form the cavity of the third ventricle and in the gray matter beneath
the floor of the fourth ventricle; and 3. in the gray matter of the spinal cord
at the levels of origin of the second, third, and fourth sacral nerves. The
nerve-fibers which thus associate the spinal cord with the ganglia are termed
pre-ganglionic fibers in contradistinction to those fibers associating the
ganglia with the tissues and organs which are termed post-ganglionic fibers.

1. The pre-ganglionic fibers that have their origin in nerve-cells in the lateral

portion of the gray matter of the spinal cord emerge from the cord in
the ventral roots of the spinal nerves from and including the first
thoracic to the third lumbar nerves. In association with the large
efferent fibers composing these roots, the fine medullated fibers pass
outward to the point at which the spinal nerve, formed by the union of
the ventral and dorsal roots, separates into an anterior and a posterior
division. At this point the fine medullated fibers leave the spinal nerve,
and after a short course enter the ganglia of the vertebral chain where
some of the fibers terminate in ganglia at the same and different levels
while others pass forward to terminate in the ganglia of the pre-vertebral
chain. On entering the ganglia the peripheral branches of the fibers
arborize around the nerve-cells composing them. The short nerve-
strands that pass from the spinal nerves to the ganglia are termed
from their color white rami communicantes .

In accordance with their distribution, as determined by histologic
and physiologic methods of investigation, these pre-ganglionic fibers
may be divided into seven groups, viz. : i. those passing up the vertebral
chain to the superior cervical ganglion; 2. those passing to the inferior
cervical ganglion; 3. those passing to the first thoracic ganglion;
4. those passing directly to the thoracic and upper lumbar ganglia; 5.
those passing down the vertebral chain to the lower lumbar and sacral
ganglia; 6. those which pass through the thoracic portion of the verte-
bral chain and forward to the ganglia of the pre-vertebral chain,
(the semilunar, renal, and superior mesenteric) which in their course
are known as the splanchnic nerves; 7. those passing through the
lumbar portion of the vertebral chain and forward to the inferior
mesenteric ganglion in which for the most part they terminate. These
nerves are sometimes termed the inferior splanchnics.

2. The pre-ganglionic fibers that arise from nerve-cells in the gray matter

beneath the aqueduct of Sylvius enter the trunk of the third or oculo-
motor nerve, pass forward in it, as far as the interior of the orbit cavity,
where they leave the nerve and terminate around the cells composing
the ciliary ganglion. (See page 580.)

The pre-ganglionic fibers that arise from nerve-cells in the gray matter
beneath the floor of the fourth ventricle, leave by three routes, viz., in
the trunks of the pars intermedia or nerve of Wrisberg, the glosso-
pharyngeal, and the vagus.

The fibers that leave in the pars intermedia enter the facial nerve
and subsequently pass by way of the great superficial petrosal nerve to

THE SYMPATHETIC NERVE SYSTEM. 615

the spheno-palatine ganglion and by way of the chorda tympani to the
submaxillary and sublingual ganglia. The fibers that leave the glosso-
pharyngeal nerve pass into the tympanic branch or nerve of Jacobson
and ultimately terminate around the otic ganglion. The fibers that
leave by the vagus nerve pass to the ganglia in the heart, stomach, and
small intestine.

3. The pre-ganglionic fibers that arise from nerve-cells in the gray matter of
the sacral division of the spinal cord enter the ventral roots of the
second, third, and occasionally fourth sacral nerves. In the pelvis these
fibers leave the sacral nerves, enter the pudendal or pelvic nerve and are
finally distributed to ganglia in the pelvic cavity associated with pelvic
viscera and the external generative organs.

Afferent Sympathetic Fibers. — With the foregoing groups of efferent
fibers, the sympathetic nerves, in the thoracic and lumbar regions more
especially, contain a number of afferent fibers which when stimulated give
rise to sensations of pain or to reflex phenomena. The routes by which these
afferent fibers reach the spinal cord lead through the white rami into the
spinal nerve, thence into the dorsal roots to the spinal ganglia, where they
have their cells of origin. The number of afferent fibers in any trunk in
comparison with the efferent is quite small.

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 spinal cord, is at the present time
very largely discarded. Peripheral structures cease to exhibit their charac-
teristic functions after division of the spinal nerves in connection with
their related ganglia. This does not exclude the possibility of the sym-
pathetic cell-body, in virtue of the interchanges between it and the blood
and lymph by which it is surrounded, maintaining its own nutrition and
exerting a favorable influence over the nutrition of the peripheral tissues
to which its post-ganglionic branches are distributed.

The nerve-tissue in its entirety may be regarded as a single system which
may be functionally divided into a nerve system of animal and a nerve
system of vegetative life, according as the nerve energies originating in and
emanating from the central nervous system are transmitted directly to the
skeletal muscles or indirectly, through the intervention of a sympathetic
neuron, to visceral muscles and glands. In the former system but one neu-
ron, the spino-peripheral, connects the spinal cord proper with the muscle;
in the latter system there are two, the spino-ganglionic and the ganglio-
peripheral.

From the distribution of the post-ganglionic fibers it may be inferred that
the activities of the vascular and visceral muscles, either in the way of aug-
mentation or inhibition, the activities of the muscles of the hair-follicles, and
of the epithelium of glands, are called forth by the ganglia in consequence of
the arrival of nerve impulses coming from the spinal cord through the pre-
ganglionic fibers. Experimental observations show this to be true. The
extent to which these different modes of activity manifest themselves in

616 TEXT-BOOK OF PHYSIOLOGY.

one or more regions of the body will depend to some extent on the portion
of the sympathetic system subjected to experimental procedures.

The Functions of the Cervical Portion. — If the sympathetic cord
central to the superior cervical ganglion be stimulated with the induced
electric current, among the resulting phenomena there will be observed
dilatation of the pupil, retraction of the nictitating membrane in animals
possessing it, contraction of the blood-vessels of the skin and mucous mem-
brane in different parts of the head, neck, and face, contraction of the blood-
vessels of the salivary glands* increase of secretion from the submaxillary
gland, and the perspiratory and mucous glands, erection of hairs in different
localities of the head and neck, and in the dog dilatation of the blood-vessels
of the lips, gums, and hard palate. If the cervical cord be divided, opposite
effects will be observed: viz., contraction of the pupil, dilatation and passive
congestion of the blood-vessels, a rise in temperature, and a loss of the power
of erecting hairs. Stimulation of the peripheral end causes a disappearance
of the latter and a reappearance of the former phenomena. These facts
indicate that the cervical portion is not only efferent in function but that
it transmits both vaso-constrictor and vaeo-dilatator fibers for blood-vessels,
secretor fibers for the salivary and mucous glands, and fibers for the dilatator
muscle of the iris. The fibers composing it are pre-ganglionic medullated
nerve-fibers coming from the spinal cord from the first to the fourth thoracic
nerves. From these several sources the fibers pass by way of the white rami
into the vertebral chain, and thence without interruption, to the superior
cervical ganglion, in and around the cells of which their end-tufts arborize
in their characteristic manner.

That the superior cervical ganglion is the cell station between the spinal
cord and the peripheral organs is shown by the fact discovered and applied
by Langley that the intravenous injection of nicotin or the local application
of it to the ganglion itself, impairs the conductivity of the terminals of pre-
ganglionic fibers, after which their stimulation has no effect on the ganglion
cells, though the latter retain their activity, as shown on direct stimulation.
Of the nerve-centers in the spinal cord which through pre-ganglionic fibers
influence peripheral structures, some appear to be in a state of constant
activity: e.g., the vaso-constrictor centers and the pupillo-dilatator centers.
In how far this action is automatic or autochthonic, or reflex, is uncertain.
The Functions of the Thoracic Portion. — The phenomena which
follow stimulation of this portion of the sympathetic system resemble in a
general way those observed in the head when the cervical portion is stimu-
lated, viz., contraction and at times dilatation of the blood-vessels and a
secretion of sweat and in some animals erection of hairs. The situation of
the resulting phenomena will vary in accordance with the part the subject
of the experiment. For an understanding of the results of experiment the
origin and distribution of the following nerve-branches must be kept in view :
(a) The cardiac nerves which take their origin in part in cells in the first
thoracic or stellate ganglion, and in part in cells in the inferior cervical
ganglion. From this origin they pass downward and forward and
reach the heart by way of the cardiac plexus. Stimulation of these
nerves gives rise to an increased frequency and an augmentation in the
force of the heart-beat. The pre-ganglionic fibers by which these cells

THE SYMPATHETIC NERVE SYSTEM. 617

are excited to activity emerge from the cord by the first and second
thoracic nerves. Stimulation of the white rami of these nerves gives
rise to the same results.

(b) The splanchnic nerves, the roots of which emerge from the fourth to the
tenth or eleventh thoracic ganglia. The fibers composing these nerves
are for the most part pre-ganglionic and derived from the corresponding
spinal nerves. The cell stations of the splanchnic fibers are in the
semilunar, superior mesenteric, and renal ganglia. From these ganglia
non-medullated post-ganglionic fibers pass peripherally to the walls of
the intestines, the blood-vessels of the intestines, liver, kidneys, spleen,
etc. Stimulation of the great splanchnic produces inhibition of the
gastric and intestinal movements and a loss of tone, though occasionally
there is a slight opposite effect, namely an augmentation of the move-
ments, a marked primary contraction of the intestinal blood-vessels and
other viscera, followed by dilatation, coincidently with which there is a
primary rise succeeded by a fall of blood-pressure throughout the body.
Division of the nerve is followed by dilatation of the intestinal vessels
and a fall of blood-pressure. Stimulation of the central end of the divided
nerve excites the activity of the general vaso-motor center, as shown
by the rise of the general blood-pressure. Stimulation of the smaller
splanchnics gives rise to a slight primary contraction of the blood-vessels,
soon followed by a marked dilatation. These facts indicate that the
splanchnic nerves contain visceral nerves which inhibit and at times
augment intestinal movements, vaso-motor fibers both augmentor and
inhibitor, secretor nerves for the intestinal glands and for the adrenal
glands.

(c) The cutaneous nerves for the trunk leave the lateral ganglia by the gray

rami, enter the thoracic spinal nerves, and pass in company with them
to their terminations, to be ultimately distributed to the walls of the
blood-vessels, the arrectores pilorum muscles, and the sweat-glands.
The pre-ganglionic fibers come from the spinal nerves by way of the
white rami. Stimulation of either the white or gray rami gives rise to
contraction of blood-vessels, erection of hairs and a secretion of sweat.
Their functions are therefore vaso-motor, pilo-motor, and secreto-motor.

(d) The cutaneous nerves for the fore-limbs have their origin from cells
in the stellate ganglion (first dorsal). After a short upward course they
enter the trunks of the nerve composing the brachial plexus. The
pre-ganglionic fibers come from the white rami of the fourth to the
ninth thoracic nerves. After entering the lateral chain they take an
upward direction and arborize around the cells of the stellate ganglion.
In the brachial and in the sciatic nerves as well vaso-motor fibers (con-
strictors and dilatators) and secretor fibers are present, as shown by
experimental methods (see page 488).

The Functions of the Lumbo-sacral Portion. — In the lumbar region
the vertebral chain contains a number of pre-ganglionic fibers which have
descended from the thoracic region, as well as fibers which have come into
the chain by the white rami from the lumbar nerves themselves. Some of
these fibers pass through the chain in a manner similar to the splanchnic
nerves in the thoracic region to reach the inferior mesenteric ganglion, in

6i8 TEXT-BOOK OF PHYSIOLOGY.

which they find their cell station. For this reason these nerves are sometimes
termed the injerior splanchnics. Stimulation of these nerves as well as of the
ganglion and its branches gives rise to contraction of the blood-vessels of the
pelvic viscera, contraction of the detrusor muscle of the bladder, contraction
of the muscle-fibers of the uterus and vagina, and inhibition of the circular
and longitudinal fibers of the large intestine.

The cutaneous nerves for the hind-limbs are derived from the lower
lumbar and the upper sacral ganglia. They also enter the spinal nerves
by the gray rami and pass to the blood-vessels and glands of the skin. The
pre-ganglionic fibers come from the twelfth thoracic to the third lumbar
nerves.

The phenomena that follow stimulation of this portion of the vertebral
sympathetic chain resemble in a general way those that follow stimulation
of the thoracic portion for the reason that the post-ganglionic fibers are dis-
tributed to similar structures. Thus from the lumbar and upper sacral
ganglia gray rami enter the lumbar and sacral nerves to be distributed
ultimately to the blood-vessels and sweat glands of the skin of the hip and
leg. Stimulation, therefore, of these nerves gives rise to contraction of the
blood-vessels and a secretion of sweat of the corresponding parts. If the
stimulation with the induced current be slow, dilatation of blood-vessels
may also be observed, a fact that indicates that these nerves also carry
vaso-dilatator fibers. The pre-ganglionic fibers descend the vertebral chain
having entered it mainly from the lumbar nerves. Stimulation, therefore,
of the lumbar chain gives rise to the same effects as stimulation of the
post-ganglionic fibers.

The Functions of the Peripheral 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
nerves that the ganglia of the trunk bear to the spinal nerves. They consist
of ganglion cells from which post-ganglionic fibers pass to glands, blood-
vessels, and non-striated muscles, and to which pre-ganglionic fibers pass
from the cranial nerves. Motor and sensor nerves pass through one or
more ganglia, though they have no anatomic connection with them. In
their structure, distribution, and functions they closely resemble the col-
lateral ganglia of the abdominal sympathetic:

i . The ciliary or ophthalmic ganglion is situated in the orbital cavity posterior
to the eyeball. It is small in size, gray in color, and consists of a 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. Pre-ganglionic fibers of small size, and similar in their ana-
tomic features to the fibers of the white rami of the spinal nerves, leave
the motor oculi by a short root from the inferior division and arborize
around the ganglion cells. Stimulation of the pre-ganglionic fibers
gives rise to contraction of the circular fibers of the iris, with a diminution
in the size of the pupil, and contraction of the ciliary muscle with accom-
modation of the eye for near vision. Division of these fibers is followed
by the opposite results. Post-ganglionic fibers from the superior cervical
ganglion which come through the cavernous plexus pass through

THE SYMPATHETIC NERVE SYSTEM. 619

the ciliary ganglion to the blood-vessels of the iris and retina and are
vaso-constrictor in function. Sensor fibers from the peripheral division
of the fifth nerve pass to the cornea and endow it with sensibility.

2. The spheno-palatine ganglion is situated in the spheno-maxillary fossa.

Its nerve-cells send non-medullated post-ganglionic fibers to the blood-
vessels and glands of the mucous membrane of the nasal and oral
regions. Stimulation of the ganglion gives rise to dilatation of the blood-
vessels and increase of secretion in this entire region. The pre-ganglionic
fibers are derived from the seventh or facial nerve by way of the great
petrosal. Sensor fibers from the superior maxillary division of the
fifth nerve pass through the ganglion to the same regions.

3. The otic ganglion is situated just below the foramen ovale and internal

to the third division of the fifth nerve. The post-ganglionic fibers
pass to the parotid gland by way of the auriculo-temporal division of the
fifth nerve, and to the blood-vessels of the mucous membrane of the
lower lip, cheek, and gums. The pre-ganglionic fibers are derived from
the efferent fibers in the glosso-pharyngeal or ninth nerve, by way of
Jacobson's nerve and the small petrosal. Stimulation of these nerves
in any part of their course gives rise to vascular dilatation and increase
of secretion in the region of the distribution. Motor fibers from the
small or motor root of the fifth nerve pass through this ganglion to the
tensor tympani muscle.

4. The submaxillary and sublingual ganglia are situated close to the corre-

sponding glands. Their post-ganglionic fibers pass to the blood-vessels
and gland-cells. The pre-ganglionic fibers are derived from the seventh
or facial nerve through the chorda tympani branch. Stimulation of the
chorda or of the ganglia themselves gives rise to marked dilatation of
the blood-vessels and an increased flow of saliva. It therefore contains
vaso-dilatator and secretor fibers for these glands. Vaso-constrictor
and a few secretor nerves, it will be recalled, come to these glands from
the superior cervical ganglion.

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. Stimulation of these ganglia inhibit the action of the heart.
The pre-ganglionic fibers for these ganglia are contained in the trunk
of the vagus (pages, 305, 599). Stimulation of the vagus has a
similar inhibitor action on the heart.

6. The pelvic ganglia, lying at the base of the bladder are by reason of their

position and relations somewhat inaccessible to direct experimentation.
The pre-ganglionic fibers in connection with them are contained in part
in the pudendal or pelvic nerve. Stimulation of the post-ganglionic and
of the pre-ganglionic nerves gives rise to a marked dilatation of the blood-
vessels of the penis, clitoris, vulva, contraction of the muscles of the
bladder, rectum, etc.

CHAPTER XXV.

PHONATION; ARTICULATE SPEECH.

Phonation, the emission of vocal sounds, is accomplished by the vibration
of two elastic membranes which cross the lumen of the larynx antero-
posteriorly and which are thrown into vibration by a blast of air from the
lungs.

Articulate speech is a modification of the vocal sounds or the voice
produced by the teeth and the muscles of the lips and tongue and is employed
for the expression of ideas.

The larynx, the organ of the voice, is situated in the 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

620

PHONATION; ARTICULATE SPEECH.

621

18,

of the vocal bands. The mucous membrane lining the walls of this space,
presents on either side, just above the vocal bands, a crescentic fold which
runs from 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. 285.)

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.
286 and 287.)

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-
eral plates, united anteriorly at an angle of
about 90 degrees. Each plate is directed
backward and outward and terminates in a
free border, which is prolonged upward and
downward for some distance, terminating in
two processes, the superior and inferior cornua.
The upper border to the thyroid is deeply
notched in front. The inferior border over-
laps laterally the cricoid.

The epiglottis is a leaf-shaped piece of cartilage attached to the thyroid
at the median notch. It is firmly united by membranes and ligaments to
the thyroid and arytenoid cartilages and to the base of the tongue.

The arytenoid cartilages are two in number and symmetric in shape.
Each cartilage is a triangular pyramid, the apex of which is recurved, and

19

FIG. 285. — LONGITUDINAL SEC-
TION OF THE HUMAN LARYNX,
SHOWING THE VOCAL BANDS, i.
Ventricle of the larynx. 2. Supe-
rior vocal cord. 3. Inferior vocal
cord. 4. Arytenoid cartilage. 5.
Section of the arytenoid muscle. 6,
6. Inferior portion of the cavity of
the larynx. 7. Section of the pos-
terior portion of the cricoid carti-
lage. 8. Section of the anterior
portion of the cricoid cartilage.
9. Superior border of the cricoid
cartilage. 10. Section of the thy-
roid cartilage, n, n. Superior
portion of the cavity of the larynx.
12, 13. Arytenoid gland. 14, 16.
Epiglottis. 15, 17. Adipose tissue.

18. Section of the hyoid bone. 19,

19, 20. Trachea. — (Sappey.)

622

TEXT-BOOK OF PHYSIOLOGY.

directed backward and inward. The base presents three angles — an ante-
rior, an external, and an internal. The anterior angle is long and pointed
and projects forward in a horizontal plane. It serves for the attach-
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.

FIG. 286. FIG. 287.

FIG. 286. — LARYNGEAL CARTILAGES AND LIGAMENTS, ANTERIOR SURFACE, i. Hyoid bone.
2, 2, 3, 3. Greater and lesser cornua. 4. Thyroid cartilage. 5. Thyro-hyoid membrane. 6.
Thyro-hyoid ligaments. 7. Cartilaginous nodule. 8. Cricoid cartilage. 9. The crico-thyroid
membrane. 10. The crico-thyroid ligaments, n. Trachea. — (Sappey.)

FIG. 287. — 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 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.

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

PHONATION; ARTICULATE SPEECH. 623

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
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 injerior 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 thin, free, reduplicated edge of the mucous membrane constitutes
the true vocal band. The surface of the mucous membrane 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

624

TEXT-BOOK OF PHYSIOLOGY.

of origin and insertion: viz., two posterior crico-arytenoids, two lateral crico-
arytenoids, two thyro-arytenoids, one arytenoid, and two crico-thyroids
(Figs. 288 and 289).

The posterior crico-arytenoid muscle lies on the posterior surface of the
quadrate plate of the cricoid cartilage, on either side of the median line,
from which it takes its origin. The fibers of the muscle pass upward and
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

FIG. 288. FIG. 289.

FIG. 288. — POSTERIOR VIEW OF THE MUSCLES OF THE LARYNX, i. Posterior crico-arytenoid
muscle. 2, 3, 4. Different fasciculi of the arytenoid muscle. 5. Aryteno-epiglottidean muscle. —
(Sappey.)

FIG. 289. — LATERAL VIEW OF THE MUSCLES OF THE LARYNX, i. Body of the hyoid bone.
2. Vertical section of the thyroid cartilage. 3. Horizontal section of the thyroid cartilage
turned downward to show the deep attachment of the crico-thyroid muscle. 4. Facet of articula-
tion of the small cornu of the thyroid cartilage with the cricoid cartilage. 5. Facet on the cricoid
cartilage. 6. Superior attachment of the crico-thyroid muscle. 7. Posterior crico-arytenoid
muscle. 8, 10. Arytenoid muscle. 9. Thyro-arytenoid muscle, n. Aryteno-epiglottidean
muscle. 12. Middle thyro-hyoid ligament. 13. Lateral thyro-hyoid ligament. — (Sappey.)

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. 290).
Since the contraction of the crico-arytenoid has this result, it is generally
spoken of as the abductor or respiratory muscle.

The lateral crico-arytenoid muscle arises from the side of the cricoid
cartilage. From this point its fibers are directed upward and backward
to be inserted into the external process of the arytenoid. Its action is to
draw the arytenoid cartilage forward and inward, thus approximating
and relaxing the vocal band.

PHONATION; ARTICULATE SPEECH.

625

The thyro-arytenoid muscle arises from the inferior two-thirds of the
inner surface of the thyroid cartilage just external to the median line. From
this origin the fibers pass backward and outward, to be inserted into the
anterior surface and external angle of the arytenoid cartilage. The inner
portion of the muscle lies close to and supports, if it does not constitute a part
of, the vocal band. The action of the thyro-arytenoid muscle in conjunction
with the lateral crico-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. 291).

The arytenoid muscle consists (i) of transversely arranged fibers which
arise from and are inserted into the outer surface of the opposite arytenoid

FIG. 290. — GLOTTIS WIDELY OPENED
FROM SIMULTANEOUS CONTRACTION OF
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. 291. — 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.)

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 respiratoria, and so direct the
expiratory blast of air toward and through the rima vocalis.

The collective actions of the three foregoing muscles is to close or con-
strict the glottis, and for this reason they are spoken of as the adductor or
phonatory muscles.

The crico-thyroid muscle arises from the side and front of the cricoid
cartilage and is inserted above into the lower border of the thyroid cartilage.
The action of this muscle is to draw up the anterior part of the cricoid car-
tilage toward the thyroid, which remains stationary, and to swing the
quadrate plate of the cricoid and the arytenoid cartilages downward and
backward. This movement has the result of tensing the vocal bands.
The cricoid is at the same time drawn backward by the action of the more
longitudinally disposed fibers.
40

626

TEXT-BOOK OF PHYSIOLOGY.

Nerves of the Larynx. — The nerves which innervate the muscles of the
larynx and endow the mucous membrane with sensibility are derived from
the vagus trunk. The superior laryngeal is for the most part sensor and
distributed to the mucous membrane, though it contains motor fibers for
the crico-thyroid muscle. The inferior laryngeal is purely motor and is
distributed to all the muscles with the exception of the crico-thyroid.

THE MECHANISM OF PHONATION.

Phonation, the production of vocal sounds in the larynx, is the result of
the vibration of the vocal bands caused by an expiratory blast of air from
the lungs. That a sound may arise it is essential that the glottis be 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-thyroid
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-

FIG. 292. — 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. 293. — 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.)

tigators 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
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. 292. When the foregoing conditions in the glottis are realized,
the air stored or collected in the lungs is forced by the contraction of
the expiratory muscles, through the narrow space between the bands.
As a result of the resistance offered by this narrow outlet and the
force of the expiratory muscles the air within the lungs and trachea is
subjected to pressure, and as soon as the pressure attains a certain level
the vocal bands are thrown into vibrations, which in turn impart to the
column of air in the upper air-passages a corresponding series of vibrations by
which the laryngeal vibrations are reinforced. The degree of pressure to

PHONATION; ARTICUL'ATE SPEECH. 627

which the air in the lungs and trachea is subjected was determined by Latour
to vary from 160 mm. of water for sounds of moderate, to 940 mm. of water
for sounds of highest intensity. With the escape of the air or the separation
of the vocal bands the vibration ceases and the sound dies away.

The Characteristics of Vocal Sounds. — In common with the sounds
produced by other music instruments, all vocal sounds are characterized
by intensity, pitch and quality, tone or color.

The intensity or loudness of a sound depends on the extent or amplitude
of the to-and-fro vibration or the extent of the excursion of the vocal
band on either side of the position of equilibrium or rest; and this in turn
depends on the force with which the blast of air strikes the band. The more
forceful the blast of air, the larger, other things
being equal, will be the primary vibrations of
the bands, and hence the secondary vibrations
of the air in the upper air-passages.

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 FlG- 294.— GLOTTIS SEEN
pitch the tension of the vocal bands and the
narrowing of the glottis attain their maximum. PITCHED SOUNDS, i, 2. Base
In the emission of sounds of lowest pitch the re- °f the tongue. 3, 4. Epigiot-
verse conditions obtain. In passing from the ^noid^ar^ge^s. o^eni^g
lowest to the highest pitched sounds in the range between the true vocal cords.
of the voice peculiar to any one individual, 9- Aryteno-epigiottidean folds.

, . -111 I0- Cartilage of Santormi. n.

there is a progressive increase in both the ten- Cuneiform cartilage. 12. Su-
sion of the vocal bands and the narrowing of perior vocal cords. 13. in-
the glottic aperture. In the production of low-
pitched notes of men, those due to vibrations lying between 80 and 240 per
second, the tension is regulated by the crico-thyroid muscle; the aperture of
the glottis during this time being elliptic in shape and relatively wide (Fig.
295). In the production of notes due to vibrations lying between 240 and
512 vibrations per second, the anterior fibers of the crico-thyroid muscle
relax and the thyro-arytenoid muscle comes into play; by its action the vocal
bands are more closely approximated and the vocal aperture reduced to a
linear slit. In the high-pitched notes emitted by soprano singers the vocal
bands are so closely applied to each other that only a very small portion in
front, bounding a small oval aperture, is capable of vibrating (Fig. 294).
The difference in the pitch of the voice in men and women is due largely to
the greater size and development of the vocal bands in the former than in the
latter.

The quality of the voice, the timbre or tone-color, depends on the form
combined with the intensity and pitch of the vibration. As with sounds pro-
duced by music instruments, the primary or fundamental vibration of the
vocal band is complicated by the superposition of secondary or partial vibra-
tions (overtones). The form of the vibration will therefore be a resultant of

628 TEXT-BOOK OF PHYSIOLOGY.

the blending of a number of different vibrations. The quality of the sound
produced in the larynx is, however, modified by the resonance of the mouth
and nasal cavities; certain of the overtones being reinforced by changes in
the shape of the mouth cavity more especially, thus giving to the voice a
somewhat different quality.

The Varieties of Voice. — The region of the music scale, comprising
all vibrations between 32 and 2048 per second, with which laryngeal sounds
are hi accord will vary in the two sexes and in different individuals of the
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, fap
to c', do3, or from 85 to 256 vibrations per second; those of the baritone
from A, lat, to f, fa3, of from 106 to 341 vibrations per second; those of the
tenor from c', do2, to a', Ia3, or from 128 to 427 vibrations per second; those
of the contralto from e, mi2, to c", do4, or from 160 to 512 vibrations per
second; those of the mezzo-soprano from g, so!2, to e", mi4, or from 192 to
640 vibrations per second; those of the soprano from b, si2, to g", so!4, 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, /, 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.

PHONATION; ARTICULATE SPEECH. 629

Laryngoscopic examination of the larynx in the human being shows that
during quiet respiration the vocal bands are widely separated and almost
stationary, moving but slightly during either 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 infraglottic regions, which is about 200 sq. mm. This con-
dition of the glottis is maintained by the steady continuous contraction of the
posterior crico-ary tenoid muscles, the abductors of the vocal bands.

For phonatory purposes it is essential that the respiratory function be
temporarily suspended and the vocal bands closely approximated. This is
accomplished by the contraction of the remaining muscles of the larynx,
with the exception of the crico-thyroid, which are collectively known as the
adductors of the vocal bands. During phonation the adductor muscles over-
come the activity of the abductors. With the cessation of phonation the
abductors immediately restore the vocal bands to their former respiratory
position.

The activities of these two a-ntagonistic groups of muscles are under the
control of the central nerve system. The only pathway for the excitator
nerve impulses is through the fibers of the inferior or recurrent laryngeal
nerve. The relation of these nerve-fibers both centrally and peripherally, as
well as their physiologic action, has been the subject of much experimenta-
tion. The results have not always been in accord, owing to the choice of
animal, the use of anesthetics, strength of stimulus, etc.

As the outcome of many investigations it is believed that each muscle
group is innervated by its own bundle of nerve-fibers, both of which are con-
tained in the inferior laryngeal, though coming from two separate centers in
the medulla oblongata. Russell succeeded in separating the fibers for the
abductors from the fibers for the adductors in the inferior laryngeal, and in
tracing them to their terminations. So completely was this done that it
became possible to produce at will, through stimulation, either abduction
or adduction, without contraction of the muscle of opposite function.

The laryngeal respiratory center was located by Semon and Horsley,
in the cat, in the upper part of the floor of the fourth ventricle. 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. In the
cat especially, stimulation of the border of the olfactory sulcus gives rise to
complete abduction of the vocal bands on both sides. The representation is
therefore bilateral.

The phonatory center was located by the same investigators in the medulla
near the ala cinerea and the upper border of the calamus scriptorius. Stimu-
lation of this area was invariably followed by bilateral adduction of the vocal
bands and closure of the glottis.

630 TEXT-BOOK OF PHYSIOLOGY.

A cortical representation for phonatory movements also was located in
the lower portion of the pre-central convolution, near the anterior border.
Stimulation of 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 ether
anesthetization gives rise to closure of the glottis; the same stimulation,
however, during deeper anesthetization gives rise to opening or dilatation of
the glottis, a fact indicating that either the adductor muscles or their nerve
terminals are depressed by the action of the ether before the muscles and
nerves of opposite function. The superior laryngeal nerves contain motor
fibers for the crico-thyroid muscles. Stimulation of the nerve gives rise to
contraction of the muscle and increased tension of the vocal bands. It is
believed that these fibers are derived originally from the efferent fibers of the
glosso-pharyngeal nerve. The remaining fibers of the superior laryngeal
endow the upper portion of the larynx with extreme sensibility which to a
certain extent protects the air-passages against the entrance of foreign
bodies. Irritation of the terminal filaments of this nerve by particles of
food, solid or liquid, gives rise to marked reflex spasm of the adductor
muscles and closure of the glottis, followed by a strong expiratory blast of air
from the lungs by which the offending particles are removed. Division of
this nerve on both sides is followed by a paralysis of the crico-thyroid muscles,
a lowering of the tension of the vocal bands, and a loss of sensibility of the
laryngeal mucous membrane.

CHAPTER XXVI.
THE SPECIAL SENSES.

It is one of the functions of the nerve system to bring the individual into
conscious relation with the external world. This is accomplished in part
through the intermediation of afferent nerves, connected peripherally with
highly specialized terminal organs, and centrally with specialized 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
sensations. These sensations, differing in character from those Wague ill-
defined sensations — e.g., fatigue, well-being, discomfort, etc. — caused by
material changes occurring within the body, are termed special sensations —
e.g., touch; pressure; pain; temperature; taste; smell; light and its varying
qualities, intensity, hue, and tint; sound and its varying qualities, intensity,
pitch, and timbre.

The terminal organs which receive the impress of the external world are the
skin, tongue, nose, eye, and ear, and collectively constitute the special sense-
organs. The physiologic mechanisms which underlie and develop these
special sensations are known respectively as the tactile, gustatory, olfactory,
optic, and auditory. Each mechanism responds to but a single form of
stimulus and to no other. $?hus, the stimulus for the skin is mechanic pres-
sure; for the tongue, soluble organic and inorganic matter; for the nose,
/volatile or gaseous matter; for the eye, ether vibrations; for the ear, atmos-
pheric undulations. These stimuli alone are adequate to the physiologic
excitation of the different mechanisms.

The factors involved in the production of the sensations include (i) a
special physical stimulus; (2) a specialized terminal organ; (3) an afferent
nerve pathway, and (4) a specialized receptive sensor cell in the cerebral
cortex.

Though the resulting sensations in each instance differ widely in their
characteristics, it is difficult to present a satisfactory explanation for these
differences. If it be assumed that the nerve impulses which ascend the
different nerves of special sense are alike in quality, then it must be ad-
mitted that the character of the sensation is the expression of a specialization
and organization of the cortical area. If, on the other hand, specialization
of the cortex is denied, then there must be admitted a specialization of the
peripheral organ — with a resulting difference in quality or rapidity of the
nerve impulses which would impress or excite the non-specialized cortex in
such a way as to call forth the characteristic sensation. It is possible, how-
ever, that neither supposition is wholly correct, and that the character of the
sensation depends on the construction and adaptation of the entire sense
apparatus to the character of the stimulus.

631

632 TEXT-BOOK OF PHYSIOLOGY.

Whatever the conditions for their origin and whatever their character-
istics, sensations in themselves do not constitute knowledge; they are but
elementary states of consciousness, raw materials out of which the mind
elaborates conceptions and forms judgments as to the character of any given
object in comparison with former experiences.

THE SENSE OF TOUCH.

The physiologic mechanism involved in the sense of touch includes the
skin and the mucous membrane lining the mouth, the afferent nerves, their
cortical connections, and nerve-cells in the cortex of the parietal lobe.

Peripheral excitation of this mechanism develops nerve impulses which,
transmitted to the cortex, evoke sensations of touch and temperature. To
the skin, therefore, is ascribed a touch sense and a temperature sense.
Of the touch sensations two kinds may be distinguished: viz., pressure
sensations and place sensations. With the contact of an external body
there arises the perception not only of the pressure, but also the perception
of the place or locality of the contact. In accordance with this, it is
customary to attribute to the skin a pressure sense and a location sense.

The specific physiologic stimuli to the terminal organs in the skin and
oral mucous membrane are mechanic pressure and thermic vibrations.

The Skin. — The skin, which constitutes the basis for the sense of touch,
covers and closely invests the entire body. It varies in thickness and 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 486), it is only necessary to state here that it is divided into a deep
and a superficial layer. The former, known as the derma, consists of an
inner layer of rather loose connective tissue and an outer layer of condensed
connective tissue. The latter, known as the epidermis, consists of an inner
layer of pigment cells and a thick outer layer of epithelial cells. The
derma is characterized by the presence of elevations (papillae) 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 ether 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:
i. Free Endings. — These are pointed or club-shaped processes, the ultimate
terminations of afferent nerve-fibrils, found in and among epidermic
cells.

THE SENSE OF TOUCH.

633

2. Tactile Cells. — These are oval nucleated bodies found in the deeper

layers of the epidermis. They rest upon or are embraced by a crescentic
shaped body, the tactile meniscus, which in turn is directly connected
with the nerve-fibril and probably a modification of it (Fig. 295).

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 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 pursu-
ing a more or less spiral course becomes con-
nected with the tactile discs (Fig. 296).

4. Hair Wreaths. — Just below the openings of the

sebaceous glands the hair-follicles are sur- e
rounded by naked axis-cylinder fibrils in the
form of a wreath, which in all probability
terminate in the cells of the external root-

FIG. 295. — TACTILE CELLS FROM SNOUT FIG. 296. — TOUCH-COR-

OF PIG. a. Tactile cell. m. Tactile disc. PUSCLE OF MEISSNER AND

n. Nerve-fiber.— (Stirling.) WAGNER, b. Papilla of

cutis. d. Nerve-fiber of
touch-corpuscle, e, /.
Nerve-fiber in touch-cor-
puscles, g. Cells of Mal-
pighian layer. — (From
Stirling.')

sheath. These, too, are to be regarded as part of the touch apparatus.

5. 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 (Fig. 297).
Other forms of peripheral organs are found in special regions of the skin

as well as in different animals.

634

TEXT-BOOK OF PHYSIOLOGY.

Touch Sense. — The area, stimulation of which evokes sensations of
touch is coextensive with the skin and that limited portion of the mucous
membrane lining the mouth. Careful stimulation of the skin by means of a
fine stiff bristle has revealed the fact, however, that the touch area is not
continuous, but discrete, presenting itself under the form of small areas or
spots, separated by relatively large areas insensitive to the same agent.
Stimulation of these spots always calls forth a sensation of touch. For this
reason they are known as " touch spots." The number of such spots in any
given area of skin varies considerably. Thus, in the skin of the calf fifteen
such spots have been counted in a square centimeter. In the palm of the

hand from forty to fifty have been counted in an
area of the same extent. They are also especi-
ally abundant in the immediate neighborhood of
the hair-follicles.

The peripheral end-organ associated with the
touch spots in the neighborhood of a hair- follicle is
in all probability the wreath of nerve-fibrils sur-
rounding the follicle. In regions devoid of hairs
the end-organ is the 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 are present in each square millimeter
of surface. The specific stimulus necessary to
evoke the sensation of touch is a deformation of the
skin; and the greater this is, within physiologic
limits, the more pronounced is the sensation.

Pressure Sense. — The contact of an 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
threshold value, or the degree of liminal intensity.
Since the sensations are the result of pressure, they are termed pressure
sensations, and their intensity may be expressed in terms of pressure.

The sensitivity of the skin as determined by the pressure sense varies in
different 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
millimeters for the skin of the forehead is 0.002 gram; for the flexor aspect
of the forearm, 0.003 gram; and for the hips, thigh, and abdomen, 0.005
gram; for the palmar surface of the finger, 0.019 gram; for the heel, i gram.
The delicacy of the sense of touch is measured by the slight increase or
decrease in the intensity of the stimulus that will produce an appreciable
change in the intensity 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

FIG. 297.— P ACINI AN

CORPUSCLES, c. Capsules.

d. Endotheiial lining sepa-
rating the latter, n. Nerve.

minai fiber; and a. Where
^splits up into finer fibrils.

THE SENSE OF TOUCH. 635

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 discriminative sensibility of any portion of the body is a function of
its mobility. This is shown by the fact that it increases rapidly from the
shoulders to the fingers and from the hips to the toes.

The Temperature Sense. — The sensations of heat and cold which are
experienced from time to time are caused by changes in the temperature of
the skin produced in a variety of ways. As these sensations are specifically
different from those of touch, as well as different from each other, it is highly
probable that for each sensation there are special nerve-endings distributed
throughout the skin. Investigations have shown that all over the skin there

636

TEXT-BOOK OF PHYSIOLOGY.

are innumerable spots of varying size which if stimulated evoke sensations of
heat or cold. Such points are termed heat and cold spots. Each responds
to but one kind of stimulus. A warm object applied to a heat spot will
evoke a sensation of warmth. It will have no effect on the cold spot. The
reverse is also true. Between the cold and heat spots there are areas that
are neutral, insensitive to either heat or cold. The cold spots are more
numerous than the heat spots in almost all regions of the body. (See Fig.
298.)

The sensitivity of the skin to temperature changes is very acute, as shown
by the fact that even 0.05° C. is readily appreciable. This holds true,
however, only when the temperature of the object lies between 27° and 33° C.
This capability varies in different regions of the skin, and depends on the

FIG. 298. — COLD AND HOT SPOTS FROM THE ANTERIOR SURFACE OF THE FOREARM, a. Cold
spots, b. Hot spots. The dark parts are the most sensitive, the hatched the medium, the dotted
the feebly, and the vacant spaces the non-sensitive. — (Landois and Stirling.)

number of heat and cold spots present, the thickness of the epidermis, the
thermal conductivity of the object touching it, and the extent to which it is
habitually exposed or protected.

The physiologic stimulus to the thermic end-organs is the passage of
heat through the skin from the interior of the body to the surrounding air.
If the radiation is continuous and uniform, the end-organs soon adapt them-
selves to the temperature of the surrounding air and the sensation of heat,
under physiologic conditions, is not evoked. If there is a sudden rise in the
external temperature caused by natural or artificial means, which diminishes
the radiation, the temperature of the skin will at once rise, the end-organs
will be stimulated, and a sensation of warmth developed. If, on the other
hand, there is a sudden fall in temperature and an increased radiation, the
temperature of the skin will fall, the end-organs will be stimulated, and a
sensation of cold developed. Experiment also teaches that the intensity of
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 20° C.

THE SENSE OF TOUCH. 637

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 of
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 motor
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 of

specialized end-organs in the muscles and tendons which are connected
through the intermediation of afferent nerves with nerve-cells in the
cerebral cortex.

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

FIG. 299. — A NEURO-MUSCLE SPINDLE OF A CAT. (Ruffini.) c. Capsule, pr. e. Primary
ending, s. e. Secondary ending, pi. e. Plate ending. (All these are probably sensor in function.)
— (Starling's "Physiology.")

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.
and in breadth from 0.15 to 0.4 mm. Each spindle (Fig. 299)' 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

638 TEXT-BOOK OF PHYSIOLOGY.

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
Rufnni's " annulo-spiral ribbon." The motor nerve also penetrates the
capsule and terminates in the polar extremities of the intra-fusal fiber.
Sensor end-organs supposed to be connected with the muscle sense are also
found in the tendons 'of muscles.

Afferent Nerves. — That muscles are abundantly supplied with afferent
nerves has been proved by different methods of investigation. With histo-
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. 252, 253). In a case reported
by Starr the removal of a small tumor in the pia mater situated over the
junction of the superior and inferior parietal lobules was followed by a loss
of the muscle sense and marked ataxia in the right hand for a period of six
weeks, after which recovery took place. These symptoms were attributed
to injury of the cortex from unavoidable surgical procedures.

The muscle sensations, as stated in foregoing paragraphs, form the
basis of the perception not only of the direction and the duration of a body
movement and the resistance experienced, but also of the position and the
tension of the muscle groups. The latter fact more especially makes it
possible for the mind to direct the muscles and to graduate the energy
necessary to the accomplishment of a definite purpose.

Active Touch. — Active touch or the application of the fingers to the
surfaces of external objects implies the cooperation of the skin and the muscles.
The sensations which are evoked are combinations of contact and muscle
sensations. The union of these sensations forms the basis of the perception
of hardness, softness, smoothness, and roughness of bodies.

THE SENSE OF TASTE.

The physiologic mechanism involved in the sense of taste includes the
tongue, the gustatory nerves (the chorda tympani and the glosso-pharyngeal),
their cortical connections and nerve-cells in the gray matter of the fourth
temporal convolutions. The peripheral excitation of this apparatus gives
rise to nerve impulses which transmitted to the brain evoke the sensations of
taste. The specific physiologic stimulus is matter, organic and inorganic,
in a state of solution.

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 (Fig. 300).

THE SENSE OF TASTE.

639

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 con-
sist of a central projection surrounded by a
wall or circumvallation from which they take
their name.

The Peripheral End-organs. The Taste-
buds. — Embedded in the epithelium covering the
mucous membrane not only of the tongue but of
the palate and posterior surface of the epiglottis are
small ovoid bodies which from their relation to the
gustatory nerves are regarded as their peripheral
end-organs and known as taste-buds or taste-beakers.
Each bud is ovoid in shape (Fig. 301). Its base rests
on the tunica propria; its apex comes up to the epi-
thelium, where it presents a narrow funnel-shaped
opening, the taste-pore. The wall of the bud is composed of elongated
curved epithelium. The interior contains narrow spin-
dle 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 sub-
stances on the peripheral apparatus of this area are in so
many instances combinations of taste, touch, temperature,
and smell that they are extremely difficult of classifica-
tion. Nevertheless six primary tastes can be recognized:
bitter, sweet, acid or sour, salt or saline, alkaline and metallic. Though
the contact of any bitter, sweet, acid, salt, etc., substance with any part of

FIG. 30!; — 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 mucous surface.
—(Ajter Pier sol.}

640 TEXT-BOOK OF PHYSIOLOGY.

the tongue will, if the substance be present in sufficient quantity or con-
centration, develop a corresponding sensation, some regions of the tongue
are more sensitive and responsive than others. Thus, the posterior por-
tion is more sensitive to bitter substances than the anterior; the reverse is
true for sweet substances and perhaps for acids and salines.

The intensity of the resulting sensation in any given instance will depend
on the degree of concentration of the substance, while its massiveness will
depend on the area affected.

THE SENSE OF SMELL.

The physiologic mechanism involved in the sense of smell includes the
nasal fossae, the olfactory nerves, the olfactory tracts, and nerve-cells in those
areas of the cortex known as the uncinate convolution and anterior part of
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 fossae are irregularly shaped cavities
separated by a vertical septum formed by the perpendicular plate of the
ethmoid bone, the vomer, and the triangular cartilage. The outer wall
presents three recesses separated by the projection inward of the turbinated
bones. Each fossa opens anteriorly and posteriorly by the anterior and
posterior nares, the latter 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. In the
former, the mucous membrane over the septum and superior turbinated
bone is somewhat thicker than elsewhere and covered with a neuro-epithelium
which constitutes

The Peripheral End-organ. — This consists of a basement membrane
supporting two kinds of cells, the olfactory and the sustentacular. The
olfactory cells are bipolar nerve-cells, the center of which contains a large
spheric nucleus. The peripheral pole is cylindric or conic in shape and
provided at its extremity with several hair-like processes. The central
pole becomes the axon process and passes directly to the olfactory bulb.

The sustentacular cells are epithelial in character and, as their name
implies, support or sustain the olfactory cells.

For the appreciation of odorous particles the air must be drawn through
the nasal fossae with a certain degree of velocity. If the particles are widely
diffused in the air, they must be drawn not only more quickly but more
forcibly into contact with the olfactory hairs, as in the act of sniffing, the
result of short energetic inspirations. To many substances the olfactory
apparatus is extremely sensitive. Thus, it has been shown that a particle
of mercaptan the actual weight of which was calculated to be i-^nnmrinnr
of a milligram gives rise to a distinct sensation.

The Olfactory Sensations. — The sensations which arise in consequence
of the excitation of the olfactory apparatus are very numerous and their
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

THE SENSE OF SMELL. 641

rise to feelings rather than ideas, this sense plays in man a subordinate part
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 XXVII.
THE SENSE OF SIGHT.

The physiologic mechanism involved in the sense of sight includes the
eyeball, the optic nerve, the optic tracts, their cortical connections, and nerve-
cells in the cuneus and adjacent gray matter. Peripheral stimulation of this
mechanism develops nerve impulses which 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 direction of objects may be judged.

The specific physiologic stimulus to the terminal end-organ, the retina,
is the impact of ether vibrations. In general, it may be said that, at least
for the same color, the intensity of the objective vibration determines the
intensity of the sensation.

THE PHYSIOLOGIC ANATOMY OF THE EYEBALL.

The eyeball is situated at the fore part of the orbit cavity, and in such a
position as to permit of an extensive range of vision. It is loosely held in
position by a fibrous membrane, the capsule of Tenon, which is attached,
on the one hand, to the eyeball itself, and, on the other, to the walls of the
orbit cavity. Thus suspended, the eyeball is susceptible of being 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. It is composed
of several concentrically arranged membranes enclosing various refracting
media essential to vision.

The membranes, enumerating them from without inward, are as follows :
the sclera and cornea, the chorioid and iris, and the retina. The refracting
media are the aqueous humor, the crystalline lens, and the vitreous humor.

The Sclera and Cornea. — The sclera is the thick opaque membrane
covering the posterior five-sixths of the ball. It is composed of layers of
connective tissue which are arranged transversely and longitudinally. It
is pierced posteriorly by the optic nerve about 3 or 4 millimeters internal to
the optic axis. By virtue of its firmness and density the sclera gives form to
the eyeball, protects delicate structures enclosed by it, and serves for the
attachment of the muscles by which the ball is moved (Figs. 302, 307). The
cornea is the transparent membrane forming the anterior one-sixth of the ball.
It is nearly circular in shape, measuring in its horizontal meridian 12 mm.,
in its vertical meridian n mm. The curvature is therefore sharper in the

642

THE SENSE OF SIGHT.

643

latter than in the former. The radius of curvature of the anterior surface
at that central portion ordinarily used in vision is 7.829 mm.; that of the
posterior surface about 6 mm.

The substance of the cornea is made up of thin layers of delicate 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 be-
comes continuous 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 lam-
ina, 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 com-
munication with the canal of
Schlemm.

The Chorioid, Iris,

Ciliary Muscle, and Ciliary FIG. 302. — CHORIOID COAT OF THE EYE. i. Optic

nerve. 2, 2, 2, 2, 3, 3, 3, 4. Sclerotic coat divided and
turned back to show the chorioid. 5, 5, 5, 5. The
cornea, divided into four portions and turned back.
6, 6. Canal of Schlemm. 7. External surface of the
chorioid, traversed by the ciliary nerves and one of the
long ciliary arteries. 8. Central vessel into which
open the vasa vorticosa. 9, 9, 10, 10. Chorioid zone,
ii, n. Ciliary nerves. 12. Long ciliary artery. 13,
I3> T3> I3- Anterior ciliary arteries. 14. Iris. 15, 15.
Vascular circle of the iris. 16. Pupil. — (Sappey.}

Processes. — The chorioid is
the dark brown membrane
which extends forward nearly
to the cornea, where it ter-
minates in a series of folds, the
ciliary processes. Posteri-
orly, it is pierced by the optic

nerve. It is composed largely
of blood-vessels, arteries, capillaries, and veins, supported by connective
tissue. Externally it is loosely connected to the sclera ; internally it is lined
by a layer of hexagonal cells containing black pigment which, though usually
described as a part of the chorioid, are now known to belong, embryolog-
ically 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 contained blood-
vessels bears an important relation to the nutrition and function of the eye.
It provides a free supply of lymph and presents a uniform temperature to
the retina in contact with it.

The iris is the circular, variously colored membrane in the anterior part
of the eye just behind the cornea. It presents a little to the nasal side of the

644 TEXT-BOOK OF PHYSIOLOGY

center a circular opening, the pupil. The outer or circumferential border
is united by connective tissue to the cornea, sclera, and ciliary muscle; the
inner border forms the boundary of the pupil. The iris consists of a frame-
work of connective tissue supporting blood-vessels, muscle-fibers, and pig-
mented connective-tissue cells. The anterior surface is covered by a layer
of cells continuous with those covering the posterior surface of the cornea.
The posterior surface is formed by a thin structureless membrane supporting
a layer of pigment cells continuous with those lining the chorioid. The
color which the iris presents in different individuals depends on the relative
amount of pigment in the connective-tissue corpuscles. In blue eyes the
pigment is wanting. In gray, brown, and black eyes the pigment is present
in progressively increasing amounts. The blood-vessels are connected with
those of the chorioid coat.

The muscle-fibers are of the non-striated variety and arranged in two sets,
one circularly, the other radially, disposed.

The circular fibers are found close to the pupil near the posterior surface
of the iris. Contraction of this band of fibers diminishes, relaxation increases,
the size of the pupil. This muscle is known as the sphincter pupilla or
sphincter iridis.

FIG. 303. — SECTION THROUGH THE CILIARY REGION OF THE HUMAN EYE. a. Radiating
bundles of the ciliary muscle. 6. Deeper bundles, c. Circular network, d. Annular muscle of
Muller. e. Tendon of ciliary muscle. /. Muscle-fibers on posterior side of the iris. g. Muscles
on the ciliary border of the same. h. Ligamentum pectinatum. — (After Iwanoff.)

The radial fibers form a more or less continuous layer in the posterior
part of the iris, extending from the margin of the pupil, where they blend
with the circular fibers, to the outer border. Contraction of the fibers in-
creases the size of the pupil. The muscle is known as the dilatator pupilla.

The nerves exciting the sphincter pupillcB to action are the ciliary nerves,
axons of nerve-cells located in the ciliary or ophthalmic ganglion. Stimula-
tion of these fibers gives rise to contraction of the sphincter and diminution in
the size of the pupil. The nerves exciting the dilatator pupill(B to action are
axons of nerve-cells located in the superior cervical ganglion. They reach
the iris by way of the cervical sympathetic, the ophthalmic division of the
fifth, and the long ciliary nerve. Stimulation of these nerves is followed by
contraction of the dilatator and 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 pages 582, 618.)

The ciliary muscle is a gray circular band about two millimeters in
width, consisting of non-striated muscle-fibers. The majority of these

THE SENSE OF SIGHT.

645

fibers pursue a radial or meridional direction. Taking their origin from the
junction of the sclera, cornea, and iris, they pass backward to be inserted
into the chorioid coat opposite the ciliary processes. The inner portion of
the muscle is interrupted by bundles of fibers which pursue a circular
direction. (Fig. 303.) They collectively constitute the annular or ring
muscle of Miiller. The ciliary muscle in common with the circular fibers
of the iris receives its nerve-supply direct from the nerve-cells in the ciliary
ganglion. Contraction of the
ciliary muscle tenses the cho-
rioid coat, and for this reason
it is frequently termed the ten-
sor chorioidecB.

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 or a s errata.
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,
which pierces the optic nerve
near the sclera, runs forward
in its center, to the retina, in
which its terminal branches are distributed. The veins arising from the
capillary plexus leave the retina by the same route.

In the posterior portion of the retina, at a point corresponding with the
axis of vision, there is a small oval area 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
optic nerve.

The retina is remarkably complex in structure, presenting an appearance,
when viewed microscopically, something like that represented in Fig. 304,
indicating that it is composed of different cellular elements arranged in
layers. These have been named, from behind forward, as follows:

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.

i. Pigment-layer (not shown).

2. Layer of rods and cones.

3. External limiting membrane.

4. Outer nuclear layer.

5. Outer molecular layer.

6. Inner nuclear layer.

7. Inner molecular layer.

8. Layer of ganglion cells.

9. Layer of nerve-fibers.

FIG. 304. — VERTICAL SECTION OF HUMAN RETINA.
— (Schaper.)

646

TEXT-BOOK OF PHYSIOLOGY.

8. The layer of ganglion cells.

9. The layer of nerve-fibers.

Modern histologic methods of research have made it possible to reduce the
retina, exclusive of the pigment cells, to three successive layers of nerve-cells,
supported by a highly developed neuroglia, forming what has been termed the
fibers of Miiller. These nerve-cells are as follows:

1. The visual cells.

2. The bipolar cells.

3. The ganglion cells.

The relation of these nerve-cells one to another and to the supporting neuro-
glia tissue and the manner in which they unite to form the above-mentioned
layers are schematically shown in Fig. 305.

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-
sists of a straight elongated cylinder
extending through the entire thickness
of Jacobson's membrane and a fine
fiber containing a nucleus, which,
after piercing the external limiting
membrane, passes into the outer molec-
ular layer, where it terminates in a
spheric enlargement. The outer por-
tion of the rod is clear and homo-
geneous, though containing a pigment
known as visual purple or rhodopsin;
the inner portion of the rod is slightly granular.

The cone-shaped visual cells also consist of two portions, a conic portion
situated in Jacobson's membrane between the rods, and a fine fiber, contain-
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

FIG. 305. — CROSS-SECTION OF THE RETINA
FROM A MAMMAL. A. Layer of rods and
cones. B. Visual cells (outer granules).
C. Outer molecular layer. E. Bipolar cells
(inner granules). F. Inner molecular layer.
G. Ganglion cells. H. Layer of nerve-
fibers, a. Rods. b. Cones, e. Bipolar
rod. f. Bipolar cone. r. Lower ramifica-
tion of a bipolar rod. f. Lower ramification
of a bipolar cone, g, h, i, j, k. Ganglion
cells in various stages, branching from F.
x, z. Bipolar contact of rods and cones, t.
Muller's supporting fibers. S. Centrifugal
nerve-fibers. — (After Ramon y Cajal.}

THE SENSE OF SIGHT.

647

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
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

FIG. 306. — 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")

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.

From the relation of the ganglion cells, in which the optic nerve-fibers
take their origin, to the visual cells and the bipolar cells, the former may
be regarded as the terminal visual organ, the intermediary between the
ether vibrations and the ganglion cell. The visual cells are directed toward
the chorioid, away from the entering light, dipping into the pigment cells.

648 TEXT-BOOK OF PHYSIOLOGY.

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. 306).

It is estimated that the optic nerve contains about 500,000 nerve-fibers,
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 3,500,000 pigment cells. The distance be-
tween the centers of two adjacent cones in the fovea is 4 micromillimeters.

Media. The vitreous humor is the largest of the refracting media and
occupies by far the largest portion of the interior of the eyeball. From its
position it gives support to the retina. Anteriorly it presents a concavity,
in which the crystalline lens is lodged. The vitreous humor consists of

FIG. 307. — HORIZONTAL SECTION OF THE EYEBALL, i. Sclera. 2. Cornea. 3. Chorioid.
4. Iris. 5. Ciliary muscle. 6. Retina. 7. Lens. 8. Suspensory ligament. 9. Canal of Schlemm.
10. Canal of Petit, u. Optic nerve. — (Deaver).

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.

The aqueous humor is small in amount in comparison with the vitreous
and is found in the space bounded by the cornea, the ciliary body, the sus-
pensory ligament, and the lens. The projection of the iris into this space
partially divides into an anterior and posterior portion or chamber. The
aqueous humor is a clear, watery, alkaline fluid derived from or secreted

THE SENSE OF SIGHT. 649

by the capillary blood-vessels of the ciliary body. From this origin it passes
through the pupil into the anterior chamber. It serves to keep the cornea
tense and smooth. The ocular tension depends partly on the presence of
this fluid in the eyeball. There is every reason for believing that there is a
constant stream of fluid from the blood-vessels into the eye and from the
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.

The lens is the transparent biconvex body situated just behind the iris,
in the concavity of the vitreous. The thickness of the lens is 3.6 mm.,
the diameter about 9 mm. It consists of a transparent capsule containing
elongated hexagonal fibers which, having their origin near the anterior
central portion of the lens, pass out toward the margin, where they bend
around to terminate in a triradiate figure on the opposite side. Chemically
the lens consists of water, a globulin body (crystallin) , and salts.

The Suspensory Ligament. — The lens is held in position by the suspensory
ligament, formed in part by the hyaloid membrane and in part by fibers
derived from the ciliary processes. The former becomes attached to the pos-
terior surface, the latter to the anterior surface of the lens near the equator.
The space between the two layers of the ligament is the canal of Petit. The
anterior surface of the ligament presents a series of plications conforming
to corresponding plications on the surface of the ciliary processes.

The relations of all the parts entering into the structure of the eye are
shown in Fig. 307.

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 of
the retina, the rods and cones. The existence of an image on the retina
can be readily seen in the excised eye of an albino rabbit, when placed
between a lighted candle and the eye of an observer. Its presence in the
human eye can be demonstrated with the ophthalmoscope. It is this image,
composed of focal points of luminous rays, 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 Apparatus. — The formation of an image is made pos-
sible by the introduction of a complex refracting apparatus consisting of the
cornea, aqueous humor, lens, and vitreous humor. Without these agencies
the ether vibrations would give rise only to a sensation of diffused luminosity.
Rays of light emanating from any one point — that is, homocentric rays-
arriving at the eye must traverse successively the different refracting
media. In their passage from one to the other, they undergo at the surfaces

650 TEXT-BOOK OF PHYSIOLOGY.

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,
1.3365. The distance from the vertex of the cornea to the lens is 3.6 mm.;
the thickness of the lens, 3.6 mm.; the distance from the posterior surface
of the lens to the retina, 15 mm. As the two surfaces of the cornea are prac-
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
lens.1

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
the lens is congenitally absent or
has been removed by surgical pro-
cedures. Perfect vision, however,

FIG. 308.— REFRACTION OF HOMOCENTRIC rprmirpc tv,,,* tViP rrmvprapnrp nf
RAYS AND THE FORMATION OF AN IMAGE. equires tnat tne convergence

the light must be great enough to

bring the focal point, the image, on the retina. This is accomplished by
the introduction of an additional refracting body, the lens. On entering
the lens the rays are for the same reason — i.e., the passage from a rarer into
a denser medium — again refracted and converged, and if continued would
come to a focus about 6.5 mm. behind the retina. On passing from the lens
into the vitreous — i.e., from a denser into a rarer medium — the rays are
once more converged and to an extent sufficient to focalize them on the
retina (Fig. 308).

While it is thus possible to follow the rays geometrically through these
media by means of the above-mentioned factors, the procedure is attended
with many difficulties. Moreover, as the relations all change when rays

1 Strictly speaking, the posterior surface of the cornea is not parallel to the anterior surface,
and the index of refraction of the cornea is a trifle greater than that of the aqueous humor, viz,.
1.377. But as the increase in the corneal refraction due to the higher index is almost exactly
counteracted by a decrease in refraction due to the higher curvature of the posterior corneal
surface, the usual assumptions furnish quite accurate results.

THE SENSE OF SIGHT. 651

enter the eye from objects situated progressively nearer the eye, a separate
calculation is necessitated for each distance for the determination of the
size of the image.

A method by which these difficulties are much reduced was suggested
by Gauss and developed by Listing. It was demonstrated by Gauss that
in every complicated system of refracting media separated by centered
spheric surfaces there may be assumed certain ideal or cardinal points, to
which the system may be reduced, and which, if their relative position and
properties be known, permit of the determination, either by calculation
or geometric construction, of the path of the refracted ray, and the position
and size of the image in the last medium, if those of the object in the first
medium be known.

Every dioptric system can be replaced, as Gauss showed, by a single
system composed of six cardinal points and six planes perpendicular to the
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.
309, has the property that every ray which before refraction passes through
it, after refraction is parallel to the axis.

The second focal point, F2, has the property that every ray which before
refraction is parallel to the axis, passes after refraction through it.

The second principal point, H2, is the image of the first, Hv" that is,
rays in the first medium which go through the first principal point pass after

Jf,

FIG. 309. — DIAGRAM SHOWING THE POSITION AND RELATION OF THE CARDINAL POINTS.

the last refraction though 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, A7\: 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. 309 let A B represent
the axis. The distance of the first focal point, Fv from the first principal
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, H1F1. It is evident, therefore, that the distance of the corre-

652

TEXT-BOOK OF PHYSIOLOGY.

spending 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
refractive indices of the first and last media. Planes passing through the
focal points vertically to the axis are known as focal planes.

From these properties of the cardinal points the position of an image in
the last medium of a luminous point in the first may be determined, and the

FIG. 310 — DIAGRAM TO FIND THE IMAGE IN LAST MEDIUM OF A LUMINOUS POINT IN

THE FIRST.

course of a refracted ray in the last medium be constructed if its direction in

the first be given according to the following rules :

i. To find the image in the last medium of a luminous point in the first : Let
A (Fig. 310) be this given point. Draw A B parallel to the axis until it
meets the second principal plane in B ; 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 E, intersect, the image of A will be Arl

FIG. 311.— DIAGRAM TO FIND THE REFRACTED RAY IN THE LAST MEDIUM OF A GIVEN
RAY IN THE FIRST MEDIUM.

2. To find the refracted ray in the last medium of a given ray in the first
medium: Let A B (Fig. 311) be the given ray. Continue this ray until
it meets the first principal plane in C. Draw C D parallel to the axis.
Now assume any point, such as £, in the given ray, and find its image E{
by the Rule i. Then D El becomes the course of the refracted ray.

1 If the point A is infinitely far from the eye, all the rays striking the eye will be parallel to
each other. The nodal ray must therefore be drawn, and the point where this nodal ray meets
the second focal plane will be the image of A , or A t where all rays parallel to the nodal ray will
meet.

THE SENSE OF SIGHT.

653

The Schematic Eye. — Accepting the system of cardinal points, Listing,
Bonders, and v. Helmholtz have constructed " schematic" eyes to be sub-
stituted for the refracting system of the natural eye.

For this purpose it is necessary to make use of the various estimates of
the indices of refraction of the different media, of the radii of curvatures
of the different refracting surfaces, and of the distances separating them, to
deduce an average eye as a basis for calculation. The most widely accepted
attempt is that of v. Helmholtz. The data he assumed are as follows : The
refractive index of air = i; of the cornea and aqueous humor, 1.3365; of the
lens, 1.4371; of the vitreous humor, 1.3365; the radius of curvature of the
cornea, 7.829 mm. ; of the anterior surface of the lens, 10 mm. ; of the posterior
surface, 6 mm.; the distance from the apex of the cornea to the anterior
surface of the lens, 3.6 mm.; thickness of lens, 3.6 mm. From the above-

FIG. 312. — 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.

mentioned data v. Helmholtz calculated the position of the cardinal points
for the eye as follows (see Fig. 312) : The first focal point is situated 13.745
mm. before the anterior surface of the cornea; the second focal point is
situated 15.619 mm. behind the posterior surface of the lens; the first
principal point, 1.753 mm. behind the cornea; the second principal point,
2.106 mm. behind the cornea; the first and second nodal points, 6.968 and
7.321 mm. behind the apex of the cornea, respectively. The anterior focal
distance of this schematic eye, the distance between F1 and Hv there-
fore amounts to 15.498 mm., and the posterior focal distance, H2 to F2, to
20.713 mm.

654

TEXT-BOOK OF PHYSIOLOGY.

When the eye, however, is accommodated for near vision, the relations
of the cardinal points are changed and will be as follows, if the point accom-
modated for lies 152 mm. from the cornea: Anterior focal distance, 13.990
mm.; posterior focal distance, 18.689 mm.; distance from cornea of the first
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
nodal point, which latter becomes the center of curvature of the single re-
fracting surface. The dimensions of this " reduced" eye are as follows (see
Fig. 313). From the anterior surface of the cornea, corresponding to the
principal plane H, to the nodal point N, 5.215 mm., from the anterior focal
point Fv to the principal plane H, i.e., the anterior focal distance/, 15.498

FIG. 313. — THE REDUCED EYE.

B

FIG. 314. — THE FORMATION OF AN IMAGE IN THE
REDUCED EYE.

mm.; from the principal plane H to the posterior focal point F2, i.e., the
posterior focal distance /", 20.713 mm.; the index of refraction is 1.3365.
There is thus substituted for the natural eye a single refracting surface with
a radius of curvature, r, of 5.125 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 unite
at the posterior focal point.

By means of this reduced eye the construction of the refracted ray, the
various calculations as to the size of the image, the size of diffusion circles,
etc., are greatly facilitated: e.g.,

In Fig. 314 let A B represent an object. From A a pencil of rays falls
on the single refracting surface. One of the rays, the nodal ray, falling on
the surface perpendicularly, passes unrefracted through the single nodal point,
N, to the posterior focal plane. The remaining rays, partially represented
in the figure, falling on this surface under varying degrees of incidence,
undergo corresponding degrees of refraction, by which they form a converg-
ing cone of rays which unite at a point situated on the nodal ray. These

THE SENSE OF SIGHT. 655

two points, A , a, are known as conjugate foci. The same holds true for
a pencil of rays emanating from B or any other point of the object.

The Size of the Retinal Image. — The size of the retinal image, / (in
Fig. 316 a b), may now be easily calculated, when the size of the object, O
(in Fig. 316 A B ), and its distance, D, from the refracting surface with radius
of curvature, r, are known, by the following formula :

0:I = D+r:f"-r.

For, as the triangles A N B and a N b are similar, we have

A B: a b =f N:N g,orab=A N g; and therefore / - °

Independent of the foregoing method, the size of the retinal image may be
calculated if it is remembered that the eye, like any optic system, has a point
of such a quality that a ray of light which before entering the eye was directed
toward it, after refraction continues as if it came from this point. In other
words, there is in the eye a point which allows a ray of light to pass unre-
fracted as would a pinhole instead of a lens. This point, termed the
nodal point of the eye, determines the size of the image; for if a line be
drawn from both the upper and lower ends of an object through this nodal
point, it is clear that the images of the respective points must lie on these
two rays where they intersect the retina. The 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 *r
by the distance of the object from
the eye.

The Visual An HP _ Thevimial FlG' 3i5-— DRAWING DESIGNED TO SHOW
isuai Angle. e visual HOW THE VISUAL ANGLE ^ SlZE OF RETINAL

angle IS defined as the angle formed IMAGE VARIES WITH THE DISTANCE OF AN

by the intersection of two lines OBJECT OF GIVEN SIZE. For the distant position

i f ,-1 •,• r of A-B the visual ancle is a; for the near

drawn from the extremities of an position (dotted lines) £ (From stewar^
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. 315. 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 or
the smallest interval between two points which permits the eye to distinguish

656

TEXT-BOOK OF PHYSIOLOGY.

them as such, is about 1.454 mm. Lines drawn from the extremities of
such an object or interval, to the nodal point, subtend an angle of 60 seconds.1
Beyond this the two points are indistinguishable. In other words the
emmetropic eye possesses the power of distinguishing the correspondingly
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 655 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
(for the same visual angle) it is evident that the object would have to be one-
fifth of 1.454 mm. or 0.2908 mm. in order to subtend an angle of 60 seconds
at one meter. From this the size for any other distance in meters is found
simply by multiplying 0.2908 mm. by the distance. The standard letters
are so constructed that each is inscribed within a square the sides of which
at a specific distance subtend an angle of 5 minutes and which is again sub-
divided 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. The letter that could be distinctly seen at a dis-

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; CN, the
line which bisects the object,
represents the distance of the
object from the nodal point;
a, the visual angle subtended
and whose value it is desired
to know, and b one-half of the
angle a. By trigonometry the
size of the angle a can be de-
FIG. 316.— FIGURE SHOWING THE METHOD OF OBTAINING termined in the following way:
THE VISUAL ANGLE EXPRESSED IN DEGREES OR FRACTION one-half the size of the object
OF A DEGREE OF AN ARC A B> *? o^ded by its distance

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 of
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.

657

tance of 5 meters, would have, therefore, a vertical and a horizontal dimen-
sion of 5 times 1.454 mm. or 7.27 mm. (Fig. 317 A), and at 10 meters corre-
sponding dimension of 14.54 mm., etc. (Fig. 317 B.)

If with the accommodation suspended, the emmetropic eye could
clearly distinguish at a distance of 5 meters a letter 7.27 mm. in 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.

E T

A.

EL

FIG. 317. — STANDARD TEST LETTERS, FOR TESTING THE ACUITY OF VISION.

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 =-f$- orV=J, etc. The acuity of vision is expressed,
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 in
other words the distance at
which the patient reads di-
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
42

0 ••*«•••••

FIG. 318. — THE REFRACTION OF PARALLEL AND
DIVERGENT RAYS IN THE EMMETROPIC EYE IN THE
PASSIVE AND IN THE ACTIVE OR ACCOMMODATED
CONDITION.

658 TEXT-BOOK OF PHYSIOLOGY.

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. 318, 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. 318, 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 : ,

Distance of the Focal

Distance of Luminous Point. Point behind the Posterior Diameter of the Diffusion-circle.

Surface of the Retina.

o.o ram. o.o mm.

65 m. 0-005 mm- o.oon mm.

25 m. 0.012 mm. 0.0027 mm.

.12 m. 0.025 mm. 0.0050 mm.

6 m. 0.050 mm. 0.0112 mm.

3 m. o.ioomm. 0.0222 mm.

i.5Oom. 0.20 mm. 0.0443 mm.

o.ysom. 0.40 mm. 0.0825 mm.

0.375111. 0.80 mm. o.i6i6mm.

o.i88m. i. 60 mm. 0.3122 mm.

o.094m. 3.20 mm. 0.5768 mm.

o.o88m. 3.42 mm. 0.6484 mm.

From the foregoing table it is evident that between infinity and 65 meters,
the diffusion-circles are so slight that no perceptible accommodative effort
is required to eliminate them. From 65 meters to 6 meters the diffusion-
circles gradually become larger, though they are yet so faint as to require
for their correction an accommodative effort which is scarcely measurable.
From 6 meters up to 6 centimeters, however, a progressive increase in accom-
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 act
of adjustment, however, for near vision the eye passes into an active state,
the result of a muscle effort, the energy of which is proportional to the 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

THE SENSE OF SIGHT.

659

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 three
images in the eye, one at the surface of the cornea and two at the pupillary
margin (Fig. 319). Of the two latter, one is quite large
and situated apparently in front of the third, which is
faint, small, and inverted. The middle image is reflec-
ted from the anterior surface of the lens, the last from
the posterior surface. These images of reflection are
known as catoptric images. If now the individual be
directed to fix the gaze on a near object, the second im-
age changes its position, advances toward the corneal
image and at the same time becomes smaller, a change
which, in accordance with the laws of optics, could only
be due to an increase in the convexity of the anterior
surface of the lens. A slight displacement of the third
image sometimes observed indicates a possible increase
in the convexity of the posterior surface of the lens.

According to Helmholtz, during accommodation the
entire anterior surface of the lens becomes more convex,
while at the same time it slightly advances, possibly as
much as 0.4 mm. in extreme efforts. This change is represented in Fig.
320. According to Tscherning, the increase in convexity of the anterior
surface is confined to the central portion, the remainder of the surface
becoming somewhat flattened. There is, moreover, no evidence that there
is any advance of the surface or any increase in the thickness of the lens.
A series of new and ingenious experiments lend support to Tscherning's

FIG. 319. — CATOP-
TRIC IMAGES IN THE
EYE. a. Upright
image of reflection,
from the cornea, b.
Upright image from
the anterior surface
of the lens. c. In-
verted image, from
the posterior surface
of the lens. — (Helm-
holtz.)

FIG. 320. — THE LEFT HALF REPRESENTS THE EYE IN A STATE OF REST. THE RIGHT
HALF IN STATE OF ACCOMMODATION.

view, though of late Hess has brought forward definite experimental^evi-
dence in favor of the view of Helmholtz. The radius of curvature in either
case approximates 6 mm. in extreme efforts of accommodation. The in-
crease 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

66o TEXT-BOOK OF PHYSIOLOGY.

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
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 and 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
very much hyper trophied, thus reinforcing the meridional fibers. In
myopia, on the contrary, where the accommodative effort is at a minimum,
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 with
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 their
focalization. So long as the luminous point remains between infinity and
65 meters, the eye, directed toward it, remains completely relaxed. The
point at which the object can be distinctly seen without accommodation

THE SENSE OF SIGHT. 661

is termed the far point or the punctum remotum. This for the normal eye
is at a distance of 65 meters or beyond.1 If the luminous point gradually
approaches the eye from a point 65 meters distant, the accommodative
power comes into play and gradually increases until it attains its maximum.
The nearest point up to which the eye is able to form distinct images of
objects is called its near point or punctum proximum. This near point in a
healthy boy of twelve years will lie at 2§ inches or 7 cm. from the eye,
while the same point lies only 8 inches or 20 cm. distant in a man of forty
years. Of objects which lie nearer than the punctum proximum the eye
cannot form distinct images. The distance between the punctum remotum
and the punctum proximum is termed the range of accommodation.

Force of Accommodation. — The increase in curvature of the lens
necessary to focalize rays when the eye is directed from the far to the near
point necessitates the expenditure of energy on the part of the ciliary muscle.
The force expended in the act of accommodation may be measured by
a lens the refracting power of which is such as to enable it to produce the
same result — that is, to give the diverging rays coming from the near point,
e.g., 20 cm., a parallel direction. A lens, therefore, which has a focal dis-
tance of 20 cm. would be a measure of the force expended ; for such a lens
placed in front of the crystalline lens, when in a state of repose, would,
with the assistance of the latter, bring diverging rays coming from the near
point to a focus on the retina. A lens of this character is said to have a
refracting power of 5 dioptrics.

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 refractive power of 2, 5, or 10 dioptrics
respectively, obtained by dividing 100 cm. by the focal distance. The re-
fracting 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 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.
At ten years the increase is 14 dioptrics, as the near point is 7 cm. ; at thirty
years the increase is but 7 dioptrics, as the near point is 14 cm. ; at sixty the
increase is but i dioptry and the near point 100 cm.; at seventy it is zero.

1 In practical ophthalmic work a point six meters distant is taken as the far point for the
reason that the rays at this distance are practically parallel.

662 TEXT-BOOK OF PHYSIOLOGY.

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.

Convergence of the Eyes during Accommodation. — In binocular
vision of near objects the eyes are turned inward and the optic axis of each
— a line passing through the center of the cornea and the center of the eye
— turned toward the median line during accommodation. So long as the
eyes are directed toward the far point, 65 meters or beyond, the optic axes
are parallel. When the eyes are directed to any point within 65 meters the
optic axes are converged, the convergence increasing steadily as the near
point is approached. In this way the fovea of each eye is directed to the
same point and single vision made possible. Were this not the case, double
vision would result.

Functions of the Iris. — For purposes of distinct vision it is essen-
tial that the quantity of light entering the interior of the eye shall be so
adjusted that the formation and subsequent perception of the image shall be
sharp and distinct. This is accomplished by the iris, the circular fibers
of which respectively contract and relax with increasing and decreasing in-
tensities of the light. The size of the pupil, therefore, through which the
light passes, will vary from moment to moment and in accordance with
variation in the light intensity. The quantity of light necessary to distinct
vision is thus regulated.

In the total absence of light the sphincter 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 pupilla
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. The
contraction of the dilatator pupillae 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 iris by
way of the fifth nerve. (See Fig. 270, page 582.) These two muscles
appear to bear an antagonistic relation to each other, for section of the motor

THE SENSE OF SIGHT. 663

oculi is followed by relaxation of the sphincter muscle and dilatation of the
pupil. Stimulation of the sympathetic is followed by a more pronounced
dilatation. The size of the pupil is the resultant of a balancing of these
two forces.

OPTIC DEFECTS.

Presbyopia. — Presbyopia may be defined as a condition of the normal
eye in which the accommodation has become so reduced by age that reading has
become impossible at ordinary distances. As age advances the lens loses
its elasticity and the power to increase in refraction, and vision at the normal
reading distance becomes impossible. The near point, the punctum prox-
imum, therefore, advances toward the far point, or recedes from the indi-
vidual. The range of accommodation is also diminished. At forty years
the near point is about 22 cm.; at forty-five years it has receded to 28 cm.
This would indicate that the lens in these five years has lost i dioptry of refracting
power; at fifty years the near point recedes to 43 cm., and at sixty to 200
cm., indicating a loss in refracting power on the part of the lens of 2 and 4
dioptrics respectively. Convex lenses placed before the eyes having a
refracting power of i, 2, and 4 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
dioptrics would therefore be required by such a man, for clear vision at 10
inches or 22 centimeters.

Myopia. — Myopia may be defined as a condition of the eye characterized
by an increase in the antero-posterior diameter or a hypernormal refracting

FIG. 321. — MYOPIA. Parallel rays FIG. 322. — CORRECTION OF MYOPIA BY
focus at F, cross and form diffusion- A CONCAVE LENS.

circles; divergent rays from A focus
on the retina.

power of the lens. The former is the usual condition. Parallel rays of
light brought to a focus in front of the retina again diverge, giving rise to
diffusion-circles and indistinctness of the image. Divergent rays alone
are capable of being focalized on the retina in its new position. The 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. 321).

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 restored by the use of a biconcave lens with

664 TEXT-BOOK OF PHYSIOLOGY.

a diverging power in the first instance of 0.5 dioptry and the second of 10
dioptrics. (Fig 322.)

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. Parallel rays of light do not, therefore, come to a focus when the
accommodation is suspended. Falling on the retina previous to focalization,
they give rise to diffusion-circles and indistinctness of the image. As no
object can be seen distinctly no matter how remote, there is no positive far

FIG. 323. — 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.

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. 323, 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 that only converging rays

FIG. 324. — HYPERMETROPIA. PAR- FIG. 325. — CORRECTION OF HYPER-

ALLEL RAYS FOCUSED BEHIND THE METROPIA BY A CONVEX LENS.

RETINA.

can be focalized by a passive hypermetropic eye. 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 hypermetropic person attempts to focalize the rays by increasing the
convexity of the lens though an increased accommodative effort which often
gives rise to accomodation fatigue and headache. The convergence of the
rays of light before they enter the hypermetropic eye is accomplished by
the placing before the eye convex lenses the converging power of which is
proportional to the degree of hypermetropia. (Figs. 324, 325).

THE SENSE OF SIGHT.

665

Astigmatism. — Astigmatism may be defined as a condition of the eye
characterized by an inquality 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
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

FIG. 326. — REFRACTION BY AN ASTIGMATIC SURFACE. — (Hansell and Sweet.)

of this cone are shown in Fig. 326, 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
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

666

TEXT-BOOK OF PHYSIOLOGY.

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 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
red and clearly defined, the periphery blue and ill-defined. The reason
for this is clear. The eye more readily accommodates itself for the
red rays, and hence their focal point is distinct. The blue rays, having a
higher degree of refrangibility, come to a focus, cross and diverge, and give
rise to diffusion-circles. If a biconcave glass be placed before the cobalt,
the blue rays can be focalized on the retina, while the red will fall on the retina
without focalization. The image will now be blue and distinct in the center,
the periphery red and ill-defined. 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

FIG. 327. — DIAGRAM SHOWING THE CORNEAL Axis D D, THE OPTIC Axis O A, THE VISUAL

AXIS V L, AND THE LINE OF FIXATION V C\ ALSO THE THREE ANGLES, «, 0, V.

foregoing paragraphs, there is yet another, viz.: an imperfect centering of
the refracting surfaces. In first-class optic instruments the lenses are
centered — that is, their optic centers are situated on the same axis. In
viewing an object through such a system the visual line corresponds with the
axis of the lens system. This is not the case with the refracting system of the
eye. A line passing through the center of the cornea and the center of the
eye, the optic axis (O A in Fig. 327), does not pass exactly through the
center of the lens and does not fall into the point for most distinct vision, the
fovea. This has lead to the recognition of other lines the relations of which
must be kept in mind in all optic discussions, viz. :

i. The visual axis or visual line (V L), the line connecting the point viewed,
the nodal point, and the fovea centralis.

THE SENSE OF SIGHT. 667

2. The line oj fixation or line oj regard (V C), the line connecting the point
viewed with the center of rotation, the latter being situated 6 mm. behind
the nodal point of the eye and 9 mm. before the retina. The relation
of these lines and certain angles connected with them are shown in Fig.
327. The angle included between the line D D (the major axis of the
corneal ellipse) and the visual line is the angle alpha, amounting on the
average to 5°. The angle incuded between the optic axis and the line of
fixation or regard is the angle gamma, while the angle between the optic
axis and the line of vision is the angle beta. In emmetropia the angle alpha
is about 5°. In hypermetropia it is greater, amounting to 7° or 8°,
giving to the eye an appearance of divergence. In myopia it is much
smaller — 2° — or in extreme cases may be abolished, the line of vision
corresponding with the optic axis or even passing beyond it. The
angle gamma is of value in 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

FIG. 328. — DIAGRAM FOR OBSERVING THE SITUATION OF THE BLIND SPOT. —

(HelmhoUz.)

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
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

668 TEXT-BOOK OF PHYSIOLOGY.

of Mariotte, which consists in placing before the eye two objects having the
relation to each other shown in Fig. 328. With the left eye closed and the
right eye directed to the cross, both objects may be visible. But by moving
the figure away from or toward the eye, there will be found a distance,
about 30 cm., when the circle will be invisible. This occurs when the
image falls on the optic nerve at its entrance. The experiment of Purkinje
as described in the following paragraph demonstrates also the fact that the
sensitive portion of the retina is to be found only in the layer of rods and
cones.

It is well known that the blood-vessels of the retina are situated in its
innermost layers a short distance behind the optic-nerve fibers. Owing to
this anatomic arrangement, a portion of the light coming through the pupil
will be intercepted by the vessels and a shadow projected on the layer of
rods and cones. Ordinarily, these shadows are not perceived, for the reason
that the shaded parts are more sensitive, so that the small amount of light
passing through the vessels produces as strong an impression on this part
as does the full amount of light on the unshaded parts of the retina, and
perhaps because the mind has learned to disregard them. But if light be
made to enter the eye obliquely, the position of the shadows will be changed,
when at once they become apparent. This can be shown in the following
way: If in a darkened room a lighted candle be held several inches to the
side and to the front of the eye, and then moved up and down, there will
be perceived, apparently in the field of vision, an arborescent figure 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. — The retina is not equally excitable in all parts
of its extent. The maximum degree of sensibility is found in the macula
lutea, and especially in its central portion, the fovea. In this region the
layers of the retina almost entirely disappear, the layers of rods and cones
alone remaining, and in the fovea only the cones are present. That this
area is the point of most distinct vision is shown by the observation that
when the eye is directed to any given point of light, its image always falls
in the fovea. Any pathologic change in the fovea is attended by marked
indistinctness of vision. The sensibility of the retina gradually but irregu-
larly diminishes from the macula toward the periphery. This diminution
in insensibility holds true for monochromatic as well as white light.

As stated above, the nature of the molecular processes which take place in
the retinal tissue, caused on one hand by the light vibrations, and on the other
hand developing nerve impulses, is entirely unknown. The discovery of
the visual purple in the outer segment of the rods gave promise of some
explanation of the process, especially when it was shown to undergo
changes when exposed to the action of light. But as the pigment is wanting
in the cones, and especially in the fovea, it cannot be considered essential
to distinct vision, although that it plays some important role in the visual
process is highly probable. It was observed by Van Genderen Stort,
that when an animal is kept in darkness some time before death, the cones
are long and filiform; but if the animal has been exposed to light, they are
short and swollen. It was discovered by Boll that if an animal is kept in
darkness an hour or two before death the pigment is massed at the ends of

THE SENSE OF SIGHT.

669

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. 329.

FIG. 329. — SECTION OF THE RETINA OF A FROG. A. In darkness. B. In light. —
(After Van Genderen Start, from T scheming1 s "Physiologic Optics."}

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 simil-
arity 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
varying distances of the object from the lens. The
presence of the visual purple in the rods of the retina
capable of being altered by light makes the compari-
son still more striking.

Kuhne even succeeded in obtaining a fixed image Streak
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,

FIG. 330. — RETINA OF
A RABBIT. OPTOGRAM
OF A WINDOW FOUR
METERS DISTANT a.
Yellow spot, b, b.
of Medullated
nerve-fibers. — (Kuhne.)

670 TEXT-BOOK OF PHYSIOLOGY.

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. 330). 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 corres-
ponding 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
sensation. If the direction of either visual axis be changed by pressure on
the eyeball, there arise two sensations, and the object appears to be doubled.
The reason assigned for this, in the first instance, is that the two images
fall into the foveae, 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 foveae. The parts of the retinae
which correspond physiologically are
shown in Fig. 331. In this figure the
AREAS OF THE retinal area is divided into quadrants
RETINA. by vertical and horizontal lines of sepa-

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
eye, pass through the nodal point, and fall on corresponding parts of the
two retinae and give rise to but single images. All such points lie, for the
horizontal line of separation, on a line termed the horopter. The form of
this line is that of a circle which passes through the fixation point and the
two nodal points. Any object on the horopter will give rise to but a single
image. This is shown in Fig. 332, in which the objects I, II, III project
their rays into both eyes and upon corresponding areas.

In addition to the horopter for the horizontal line of separation, there

THE SENSE OF SIGHT.

671

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
project its rays on parts of the retinae which do not correspond, and hence
give rise to double images. This is evident from examination of Fig. 333.
While the eyes are directed to figure 2, of which there is but a single image,
the objects B and A give rise to double images, for reasons already given.
It the eyes are now directed to B, double images will be formed of 2 and A.

At all times, therefore, double images are formed on the retinae the
existence of which is scarcely noticed unless the attention is directed to them.

FIG. 332. — HOROPTER FOR THE
SECONDARY POSITION, WITH CON-
VERGENCE OF THE VISUAL AXES.
— (Landois.)

FIG. 333. — SCHEME OF IDENTICAL AND
NON-IDENTICAL POINTS OF THE RETINA. —
(Landois.)

This is due to the fact that many of the images fall on the peripheral, less
sensitive parts of the retinae. At the same time, from a want of accommo-
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 (Fig.
334). 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

672

TEXT-BOOK OF PHYSIOLOGY.

which point its tendon passes through a cartilaginous pulley, after which
it is reflected backward to be inserted into the superior surface of the sclera
about 1 6 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 oj fixation or line oj
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
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 ad-
duction (toward the nose up
to 45 degrees) and abduction
(toward the temple up to 42
degrees) occur. The six mus-
cles 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 — name-
ly, with the vertical axis —
thus moving the ball only in-
wardly or outwardly — respec-
tively. The other two pairs,
and their movements of the

FIG. 334. — MUSCLES OF THE EYE AND TENDON
OR LIGAMENT OF ZINN. i. Tendon of Zinn. 2. Ex-
ternal rectus divided. 3. Internal rectus. 4. In-
ferior rectus. 5. Superior rectus. 6. Superior
oblique. 7. Pulley for superior oblique. 8. In-
ferior oblique. 9. Levator palpebrae superioris.
10,10. Its anterior expansion, n. Optic nerve. —
(Sappey.)

however, have their own axes of action,
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

THE SENSE OF SIGHT.

673

(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
Outward

Upward

Downward

Inward and
upward

. Rectus interims.
. Rectus externus.
Rectus superior.
Obliquus inferior
Rectus inferior.
Obliquus superior.
' Rectus internus.
Rectus superior.

Obliquus inferior.

Inward and
downward.

Outward and
ui

Outward and
downward .

Rectus internus.
Rectus inferior.
Obliquus superior.
Rectus externus.
Rectus superior.
Obliquus inferior.
Rectus externus.
Rectus inferior.
Obliquus superior.

If both eyes have their line of vision in the horizontal plane parallel
with each other and with the median plane of the body, they are said to be
in the primary position. All other positions are called secondary 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 (i) 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 prism
is decomposed into a series of colors — red, orange, yellow, green, blue, and
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.

Purple.
Dk.-rose.

Dk.-rose.
Wh.-rose.

Wh.-rose.
White.

White.
Wh.-yellow.

Wh.-yellow.
Yellow.

Gold-yellow.
Yellow.

Orange.

Yellow.

Wh.-rose.

White.

Wh.-green

Wh.-yellow.

Gr.-yellow.

.

Gr.-yellow.

White.

Wh.-green.

Wh.-green.

Green.

Green.

White-blue.

Water-blue.

Bl.-green.

Bluish-green

Water-blue

Water-blue.

Cyan-blue

43

674 TEXT-BOOK OF PHYSIOLOGY.

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 obtained, 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.

Externally 90° 80° 65° 50°

Internally . 60° 55° 50° 40°

Superiorly 45° 40° 35° 3°°

Inferiorly 70° 60° 45° 35°

Theories of Color-perception. — The theory oj v. Helmholtz, originated
by Thomas Young (1807), assumes in its latest form the existence in the
human retina of three different kinds of end-organs, 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
group has its end-organs provided with a green-sensitive substance, which
is mainly excited by the green color; while the third group is provided with
a blue-sensitive substance, this latter being mainly affected and decomposed
by the blue- violet portion of the spectrum. All these three different end-
organs are present in every part of the most sensitive area of the retina, and
are connected by separate nerve-fibers with special parts of the brain, in the
cells of which each calls up its separate sensation of red or green or blue.

Out of these three primary color-sensations all other color-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. But if two of
these photo-chemical substances are stimulated simultaneously, quite differ-
ent sensations arise. Thus simultaneous stimulation of the red and green

THE SENSE OF SIGHT. 675

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 oj 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 assimila-
tion in the same visual substance antagonize each other, and consequently
produce no color-sensation by means of this substance, but only the
sensation of white, by reason of decomposition, by both colors, in the
white-black substance. Thus, 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 sensation
of white, as both yellow and blue decompose the white-black material.
Color-blindness is explained by the assumption of the absence of either the
red-green or the yellow-blue visual substance in the retina.

Accessory Structures. — The eyeball is protected anteriorly by the
eyelids and their associated structures, the Meibomian glands, the lachrymal
glands, and tears.

The eyelids consist of a central framework of connective tissue 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 (Fig. 335). Their ducts open on the free border
of the lid. These glands secrete an oleaginous material resembling sebace-
ous 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.

676

TEXT-BOOK OF PHYSIOLOGY.

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.

FIG. 335. — THE LACRIMAL AND MEIBOMIAN GLANDS, AND ADJACENT ORGANS OF THE EYE.
i, i. Inner wall of orbit. 2, 2. Inner portion of orbicularis palpebrarum. 3, 3. Attachment to
circumference of base of orbit. 4. Orifice for transmission of nasal artery. 5. Muscle of Horner
(tensor tarsi). 6, 6. Meibomian glands. 7, 7. Orbital portion of lacrimal gland. 8, 9, 10.
Palpebral portion, n, n. Mouths of excretory ducts. 12, 13. Lacrimal puncta. — (Sappey).

It eventually passes into the lachrymal ducts and then into the nose. The
lachrymal glands receive secretory fibers by way of the fifth nerve and the
cervical sympathetic. The secretion can be excited reflexly from stimulation
of sensor nerves as well as by emotional states.

CHAPTER XXVIII.
THE SENSE OF HEARING.

The physiologic mechanism involved in the sense of hearing includes the
ear, the auditory nerve, its cortical connections, and nerve-cells in the cortex
of the temporal lobe.

Peripheral excitation of this mechanism develops nerve impulses which,
transmitted to the cortex, evoke the sensation of sound and its varying
qualities — intensity, pitch, and timbre.

The specific physiologic stimulus to the terminal organ, the organ of
Corti, is the impact of atmospheric undulations of varying energy and
rapidity.

THE PHYSIOLOGIC ANATOMY OF THE EAR.

The ear, the organ of hearing, is lodged within the petrous portion of the
temporal bone. It may, for convenience of description, be divided into three
portions: viz., the external, the middle, and the internal portion (Fig. 336).

The external ear consists of the pinna or auricle and the external audi-
tory canal. The pinna is composed of a thin layer of cartilage which presents
a series of elevations and depressions. It is attached by fibrous tissue to the
outer edge of the auditory canal and covered by a layer of skin continuous
with that covering adjacent structures. The general shape of the pinna is
concave. Its anterior surface presents, a little below the center, a deep
depression — the concha.

The external auditory canal extends from the concha inward for a dis-
tance of from 25 to 30 mm. It is directed at first upward, forward, inward,
and then somewhat downward to its termination. It is composed partly
of bone and partly of cartilage and lined by a reflection of the skin covering
the pinna. At the external portion of the canal the skin contains a number
of tubular glands, the ceruminous glands, which resemble in their con-
formation the perspiratory glands. They secrete cerumen or ear-wax.

The middle ear, or tympanum, is an irregularly shaped cavity hollowed
out of the temporal bone and situated between the external auditory canal
and the internal ear. It is narrow from side to side, though wider above than
below. It is relatively long in its antero-posterior and vertical diameters.
The upper portion is known as the attic. The middle ear is in communica-
tion posteriorly with the mastoid cells, anteriorly with the pharynx through
the Eustachian tube.

The Eustachian Tube. — The passageway between the tympanic cavity and
the naso-pharynx is known from its discoverer as the Eustachian tube. It
is composed internally of bone, externally of cartilage, and is lined by mucous
membrane covered with ciliated epithelium. Near the middle of its course
the tube is contracted, though expanded at either extremity (Fig. 336). It

677

678 TEXT-BOOK OF PHYSIOLOGY.

measures about 40 mm. in length. Its general direction from the pharyn-
geal orifice is outward, backward, and upward at an angle of about 45
degrees.

The middle ear cavity is separated from the external ear by a membrane
—the membrana tympani — and from the internal ear by an osseo-mem-
branous partition which forms a common wall for both cavities. The
interior of the cavity is crossed from side to side by a chain of bones and
lined by a mucous membrane continuous with that lining the pharynx.

The membrana tympani is a thin, translucent, nearly circular membrane,
measuring about 10 mm. in diameter, placed at the inner termination of the
external auditory canal. It is inclosed in a ring of bone which in the fetal

FIG. 336. — THE EAR. i. Pinna, or auricle. 2. Concha. 3. External auditory canal.
4. Membrana tympani. 5. Incus. 6. Malleus. 7. Manubrium mallei. 8. Tensor tympani.
9. Tympanic cavity. 10. Eustachian tube. n. Superior semicircular canal. 12. Posterior semi-
circular canal. 13. External semicircular canal. 14. Cochlea. 15. Internal auditory canal.
16. Facial nerve. 17. Large petrosal nerve. 18. Vestibular branch of auditory nerve. 19.
Cochlear branch. — (Sappey.}

condition can be easily removed, but in the adult condition cannot be re-
moved, owing to its consolidation with the surrounding bone. This mem-
brane consists primarily of a layer of fibrous tissue which is covered extern-
ally by a thin layer of skin continuous with that lining the auditory canal,
and internally by a thin mucous membrane. The tympanic membrane is
placed obliquely at the bottom of the auditory canal, inclining from above
and behind downward and forward at an angle of about forty-five degrees.
The external surface of this membrane presents a funnel-shaped depression,
the sides of which are slightly convex.

The Ear-bones. — Running across the tympanic cavity and forming an
irregular line of joined levers is a chain of bones, which articulate one with
another at their extremities. These bones are known as the malleus, incus,

THE SENSE OF HEARING.

679

and stapes. The form and arrangement of these bones are shown in Figs.
337, 338.

The malleus, or hammer bone, consists of a head, neck, and handle,
of which the latter is atttached to the inner surface of the membrana tympani.
The incus or anvil bone presents a concave articular surface which receives
the head of the malleus. The stapes, or stirrup bone, articulates externally
with the long process of the incus, and internally, by its oval base, with the
edges of an oval opening, the foramen ovale. The entire chain is partially
supported by a ligament attached to the short process of the incus and
to the walls of the tympanic cavity.

The Tensor Tympani Muscle. — This is a delicate muscle, about 15 mm.
in length, situated in a narrow groove just above the Eustachian tube (Fig.

, 339). It arises from the car-

7 tilaginous portion of the Eusta-

W h/ / chian tube and the adjacent

portion of the sphenoid bone.
From this origin it passes nearly
horizontally backward to the
tympanic cavity; just opposite

FIG. 337 . — TYMPANIC MEMBRANE AND THE AUDI-
TORY OSSICLES (LEFT) SEEN FROM WITHIN, i. e.,
FROM THE TYMPANIC CAVITY. M. Manubrium
or handle of the malleus. T. Insertion of the
tensor tympani. h. Head. IF. Long process of
the malleus, a. Jncus, with the short (K) and the
long (/) process. 5. Plate of the stapes. Ax,
Ax, is the common axis of rotation of the auditory
ossicles. S1. The pinion-wheel arrangement be-
tween the malleus and incus. — (Landois.)

FIG. 338.— 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.)

the foramen ovale its tendon bends at a right angle over the processus
cochleariformis and then passes outward across the tympanic cavity to be
inserted into the handle of the malleus near the neck.

The stapedius muscle emerges from the cavity of a pyramid of bone which
projects from the posterior wall of the tympanum. Its tendon passes forward
to be inserted into the neck of the stapes bone near its point of articulation
with the incus.

The internal ear, or labyrinth, is located within the petrous portion
of the temporal bone. It consists of an osseous and a membranous portion,
the latter contained within the former.

The osseous labyrinth is subdivided into vestibule, semicircular
canals, and cochlea.

The vestibule is a small, triangular-shaped cavity between the semicir-

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TEXT-BOOK OF PHYSIOLOGY.

cular canals and the cochlea. It is separated from the cavity of the middle
ear by an osseous partition which presents near its center an oval opening,
the foramen ovale. In the living 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 (Fig. 340).

The semicircular canals are three in number,
each at right angles to the other two, a superior
vertical, an inferior vertical, and a horizontal,
each of which opens by two orifices into the
cavity of the vestibule, with the exception of the
two vertical, which unite at one extremity and
then open by a single orifice. Each canal near
its vestibular orifice in enlarged to almost twice
the size of the rest of the canal, forming what
is known as the ampulla.

The cochlea, the anterior portion of the laby-
rinth, is a gradually tapering canal, about 35
mm. in length, wound spirally two and a half
times around a central bony axis, the modiolus. The cavity of the cochlea
is partially subdivided into two cavities by a thin spiral plate of bone which
projects from the inner wall, known as the lamina 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 upper passage or scala is in free communication with the vestibule,
and is known as the scala vestibuli; the lower passage or scala in the dead con-

FIG. 339. — M, THE TENSOR
TYMPANI MUSCLE — THE Eus-
TACHIAN TUBE (LEFT).—
(Landois.)

FIG. 340. — 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. 341. — i. Utricle. 2.
Saccule. 3. Vestibular end of
cochlea. 4. Canalis reuniens.
5. Membranous cochlea. 6.
Membranous semicircular

canals.

dition communicates with the tympanum by means of a round opening, the
foramen rotundum, and is therefore known as the scala tympani. In the liv-
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-

THE SENSE OF HEARING.

681

ing from the central canal, pass through the osseous lamina spiralis and
transmit fibers of the auditory nerve. The interior of the bony labyrinth is
lined by periosteum covered by epithelium and in communication with lymph-
spaces at the base of the skull by means of the aqueduct of the vestibule.

The membranous labyrinth, lying within the osseous labyrinth, cor-
responds with it in form, though it is smaller in size. It may be subdivided
into vestibule, semicircular canals, and cochlea (Fig. 341).

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 communicate with the utricle in the same manner
as the bony canals communicate with the vestibule. The saccule communi-
cates with the membranous cochlea by a short canal, the canalis reuniens.
The walls of the utricle, saccule, and semi-
circular canals are composed of connective-
tissue lined by epithelium. At the points of
entrance of the auditory nerve, the macula
acusticcB, in all three structures, the epithe-
lium undergoes a marked change in appear-
ance and structure. It becomes columnar
in shape and provided with stiff hair-like
processes or threads, which project into the
cavity. In the saccule and utricle the hair-
like processes are covered by a layer of
small crystals of calcium carbonate held
together by a gelatinous material. The
crystals are known as otoliths (Fig. 342).

The fibers of the vestibular nerve, arising
from the cells of the ganglion of Scarpa in
the internal auditory meatus, send their
peripherally directed branches through the
foramina in the inner wall of the vestibule,
through the walls of the utricle and semi-
circular canals near the ampulla. As the fibers approach the maculae
acusticae they subdivide into delicate fibrillae, which ultimately become
histologically and physiologically related to the neuroepithelium. From
the relation of the nerve-fibers to the epithelium, the latter must be re-
garded as the highly specialized terminal organ of the vestibular portion of
the auditory nerve.

The cochlea is a closed membranous tube situated between the osseous
lamina spiralis and the outer bony wall. A transverse section of the entire
cochlea shows the relation of the osseous and membranous portions (Fig. 343).
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 modified and

FIG. 342. — SECTION OF WALL OP
UTRICLE OF 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 (/)
passing toward hair-cells and
becoming non-medullated at base-
ment-membrane.— (After Piersol.)

682

TEXT-BOOK OF PHYSIOLOGY.

specialized along the surface of the basilar membrane, to constitute what is
known as

The Organ of Corti. — In Fig. 343 this organ is represented as it appears
on cross-section of the cochlea. It consists 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 columnar epithelial cells provided with hair-
like processes which rest upon and are supported by the rods both on the
inner and outer aspects of the arch. The space beneath the arch is known
as the tunnel. The inner hair cells are not nearly so numerous as the outer
hair cells. The epithelial cells external to the outer and inner hair cells are
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
estimated at 5600; the number which form the outer limb is estimated at
3850. The outer rods are broader than the inner and at some places articu-
late with two or three inner rods. The
upper edges of the rods are flattened,
elongated, and project outward, form-
ing a reticulated membrane through
the meshes of which the hair-like pro-
cesses of the cells project.

From the connective-tissue thicken-
ing 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 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.

FIG. 343 j — A TRANSVERSE SECTION OF A
TURN OF THE COCHLEA.

THE SENSE OF HEARING. 683

THE PHYSIOLOGY OF HEARING.

The general function of the ear is the reception and transmission of at-
mospheric vibrations from the concha to the percipient elements — the hair
cells — of the organ of Corti. The vibratory excitation of these end-organs
thus caused, is the basis of auditory perceptions. The atmospheric vibra-
tions are collected by the pinna and concha, conveyed by the auditory canal
to the tympanic membrane, transmitted by the chain of bones to the laby-
rinth 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 nerve to the auditory centers
of the cerebrum, where the sensations of sound are evoked. In order to
appreciate the function of the individual structures concerned in this gen-
eral function there must be kept in mind a few of the characteristics of
atmospheric vibrations.

Atmospheric Vibrations. — The vibrations of the atmosphere which are
the objective causes of the sensations of sound are communicated to it by the
vibrations of elastic bodies such as tuning-forks, rods, strings, membranes,
etc. These produce in the air a to-and-fro movement of its particles, re-
sulting 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.

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 sounds which physiologically result from the impact and transmis-
sion of the effects of sound-waves, possess intensity, pitch, and quality or
timbre.

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 u 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.

684 TEXT-BOOK OF PHYSIOLOGY.

The quality of a sound depends on ihejorm 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 it is doubtful if it plays a part at all
necessary for hearing. Nevertheless an individual with defective hearing
may have the perception of sound increased by placing the pinna at an angle
of 90 degrees to the side of the head or by placing the hand behind it. The
external auditory canal transmits the sonorous vibrations to the tympanic
membrane. From the obliquity of this canal it has been supposed that the
vibrations, after passing the concha, undergo a series of reflections on their
way to the tympanic membrane, which, owing to its inclination, would be
struck by them in a much more effective manner.

The Function of the Tympanic Membrane. — The function of the
tympanic membrane is the reception of the atmospheric vibrations which
are transmitted to it. This it does by vibrating In unison with them. The
vibrations which the membrane exhibits correspond in amplitude, in fre-
quency, and in form to those of the atmosphere. That this membrane
actually reproduces all vibrations within the range of audibility has been
experimentally demonstrated. The membrane, not being fixed as far as its
tension is concerned, does not possess a fixed fundamental note, like a station-
ary fixed membrane, and is therefore just as well adapted for the reception of
one set of vibrations as another. This is made possible by variations in its
tension in accordance with the pitch or frequency of the atmospheric vibra-
tions. In the absence of vibration the membrane is in a condition of re-
laxation; with the advent of sound-waves possessing a gradual increase of
pitch, as in the ascent of the music scale, the tension of the membrane in-
creases until its maximum is reached at the upper limit of the range of
audibility. By this change in tension certain tones become perceptible and
distinct, while others become imperceptible and indistinct.

The Function of the Tensor Tympani Muscle. — The function of this
muscle is, as its name indicates, to change and to fix the tension of the tym-
panic membrane, so that it can most readily vibrate in unison with vibrations
of varying degrees of rapidity. The tendon of this muscle playing around
the processus cochleariformis is attached almost at a right angle to the handle
of the malleus. Hence as the muscle contracts it exerts its traction from the
process and draws the handle of the malleus inward, thus increasing the
convexity of the tympanic membrane and at the same time its tension.
With the relaxation of the muscle the handle of the malleus passes outward,
and the convexity and tension diminish.

THE SENSE OF HEARING. 685

In the ascent of the music scale, each note corresponding to an increase
in vibration frequency requires for its perception an increase in tension and
an increase in the force of the contraction of the tensor muscle. In the
descent of the music scale the reverse conditions obtain. The contraction of
the muscle is of the nature of a single twitch, and of just sufficient force and
duration to tense the membrane for a given rate of vibration.

The contraction of the muscle is excited reflexly. The afferent path is
through fibers of the trigeminal nerve distributed to the tympanic mem-
brane; the efferent path is through fibers in the small root of the trigeminal.
The stimulus is sudden pressure on the tympanic membrane. The more
frequently and forcibly the stimulus is applied, the greater is the muscle
response. The tensor tympani muscle may therefore be regarded as an
accommodative apparatus by which the tympanic membrane is adjusted
for the reception of vibrations of varying degrees of frequency.

The Function of the Chain of Bones. — The function of the chain of
bones is to transmit the effects of the atmospheric vibrations to the fluid of the
labyrinth. The manner in which this is accomplished becomes evident
from the relation which the bones of this chain bear to one another and to the
tympanic membrane on the one hand and to the fluid of the labyrinth on
the other.

When pressure is made on the outer surface of the tympanic membrane
it is at once pushed inward, carrying with it the handle of the malleus, the
head at the same time rotating outward around an axis corresponding to its
ligamentous attachments. As the handle moves inward a small ledge of bone
just below the malleo-incudal joint locks with, and hence pushes inward,
the long process of the incus. Since this process is united at almost a
right angle to the stapes bone, the latter is forced toward and into the
foramen ovale, thus producing a pressure on the perilymph. With the
cessation of the pressure the elastic forces of the membrane and of the liga-
ments return the handle of the malleus to its former position; by the unlock-
ing of the malleo-incudal 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 malleus,
and as the movement between them takes place around an axis from before
backward, it follows that the excursion of the incus and stapes will be less
than that of the malleus, while the force will be greater. Hence as the
vibrations are transferred from the tympanic membrane of large area to the
base of the stapes of small area (20 to 1.5), they lose in amplitude but in-
crease is force. Their pressure on the perilymph is therefore 13 . 3 times
greater than on the membrana tympani. In addition to its function as a
transmitter of vibrations, the chain of bones serves as a point of attachment
for muscles which regulate the tension of the tympanic membrane and
the pressure on the labyrinth.

The Function of the Stapedius Muscle.— The function of thestapedius
muscle is a subject of much discussion. According to Henle, its function
is so to adjust the stapes bone that it will be prevented from exerting an undue
pressure on the perilymph during the inward excursions of the incus process.
According to Toynbee, its function is to press the posterior part of the stapes

686 TEXT-BOOK OF PHYSIOLOGY.

inward, make it a fixed point, and place the anterior part in such a position
that it will vibrate freely and accurately.

The Function of the Eustachian Tube. — In order that the tympanic
membrane may vibrate freely it is essential that the air pressure on both
sides shall be equal at all times. This is made possible by the Eustachian
tube. Were it not for this passageway, with each inward swing of the mem-
brane the air in the tympanic cavity would be condensed and its pressure
raised, in consequence of which the movement of the membrane would be
retarded; with each outward swing, the air would be rarefied and its pressure
lowered below that of the atmosphere, and in consequence the movement
outward would be retarded; the maximum response, therefore, of the 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

through the Eustachian tube into the tympanum, thus increasing the
pressure.

2. By closing the mouth and nose and then making an effort of deglutition.

As this act is attended by an opening of the pharyngeal end of the
Eustachian tube, the air in the tympanum is partly withdrawn and the
pressure lowered. In each instance hearing is impaired. After
either experiment the normal condition is restored by swallowing with
the nasal passages open.

The Functions of the Internal Ear. — From the anatomic arrange-
ment of the structures of the internal ear it is evident that if the vibrations
of the stapes bone are to reach the peripheral organs — the hair cells — of
both the vestibular and cochlear nerves, they must traverse successively
the perilymph, the membranous walls, and the endolymph. As the perilymph
is incompressible, the inward movement of the stapes would be prevented
were it not for the elastic character of the membrane closing the foramen
rotundum. The pressure wave occasioned by each inward movement of the
stapes is transmitted through the scala vestibuli, the helicotrema, and the
scala tympani, to this membrane, which by virtue of its elasticity is pressed
into the tympanic cavity. With the outward movement of the stapes,
equilibrium is at once restored.

The Functions of the Cochlea. — The cochlea is the portion of the in-
ternal ear which is concerned in the perception of tones. The arrangement
of the histologic elements of the organ of Corti indicates that they in some
way respond to the vibrations of varying frequency and form, and through
the development of nerve impulses, evoke the sensations of pitch and quality.
The manner in which this is accomplished is largely a matter of speculation.
While many theories have been offered in explanation of the power to distin-
guish the pitch and the quality or timbre of a tone, most physiologists prefer
that of Helmholtz, who regarded the transverse fibers of the basilar mem-
brane as the elements immediately concerned, and compared them, both in

THE SENSE OF HEARING. 687

their arrangement and power of sympathetic vibration, with the strings of a
piano. He said : " If we could so connect every string of a piano with a nerve-
fiber that the nerve-fiber would be excited as often as the string vibrated,
then, as is actually the case in the ear, every musical note which affected the
instrument would excite a series of sensations exactly corresponding to the
pendulum-like vibrations into which the original movements of the air can
be resolved; and thus the existence of each individual overtone would be
exactly perceived, as is actually the case with the ear. The perception of
tones of different pitch would, under these circumstances, depend upon
different nerve-fibers, and hence would occur quite independently of each
other. Microscopic investigation shows that there are somewhat similar
structures in the ear. The free ends of all the nerve-fibers are connected
with small elastic particles which we must assume are set into sympathetic
vibration by sound-waves." (Stirling.)

The mechanism might be regarded, therefore, somewhat as follows:
The sound-waves received by the membrana tympani and transmitted by the
chain of bones to the fenestra ovalis produce variable pressures in the
fluids of the internal ear; these pressures vary in intensity, in number,
and in quality, and correspond with the intensity, pitch, and quality of the
tones. If, therefore, a compound wave of pressure be communicated by
the base of the stapes, it will be resolved into its constituents by the different
transverse fibers of the basilar membrane, each picking out its peculiar
portion of the wave and thus stimulating corresponding nerve filaments.
Thus different nerve impulses are transmitted to the brain, where they are
fused in such a manner as to give rise to a sensation of a particular
quality, but still so imperfectly fused that each constituent, by a strong
effort of attention, may be still recognized. The transverse fibers of the
basilar membrane vary in length from 0.04155 mm. at the base of the
cochlea to 0.495 mm- at tne apex, and, according to Retzius, are about 24,000
in number. As the human ear usually cannot distinguish more than 1 1 ,oco
tones, it is evident that there is a sufficient anatomic basis for this theory.

The functions of the semicircular canals have already been stated
in connection with the chapter relating to the functions of the cerebellum.

CHAPTER XXIX.
REPRODUCTION.

Reproduction is the process by which a new individual is initiated and
developed and the species to which it belongs is preserved. 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 reproductive organs characteristic of the two sexes.

Embryology is a department of anatomic science which has for its
object the investigation of the successive stages that the new being passes
through during its gradual development prior to birth.

THE REPRODUCTIVE ORGANS OF THE FEMALE.

The reproductive organs of the female comprise the ovaries, Fallopian
tubes, uterus, and vagina (Fig. 344).

The Ovaries. — The ovaries are two small, flattened bodies, measuring
about 40 mm. in length and 20 in breadth. They are situated in the cavity
of the pelvis, one on either side, and embedded in a fold of the peritoneum,

FIG. 344. — UTERUS, FALLOPIAN TUBES AND OVARIES; POSTERIOR VIEW, i, i. Ovaries.
2, 2. Fallopian tubes. 3, 3. Fimbriated extremity of the left Fallopian tube seen from its concavity.
4. Opening of the left tube. 5. Fimbriated extremity of the right tube, posterior view. 6, 6.
Fimbriae which attach the extremity of each tube to the ovary. 7, 7. Ligaments of the ovary.
8, 8, 9, 9. Broad ligament. 10. Uterus, n. Cervix uteri 12. Os externum. 13, 13. Vagina.

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,

688

REPRODUCTION.

689

though they are most abundant toward its peripheral portions. It is esti-
mated that each human ovary contains from 20,000 to 40,000 follicles. The
follicles vary considerably in size; while many are visible to the unaided eye,
others require for their detection high powers of the microscope. Although
the follicles are present in the ovary at the time of birth, it is not until the
period of puberty that they assume functional activity.

From this time on to the catamenial period there is a constant growth
and development of these follicles. Each follicle consists of an external
investment of fibrous tissue and blood-vessels, and an internal investment of
cells, the membrane, granulosa. At the lower portion of this membrane there
is an accumulation of cells,
the proligerous disc (Fig. 345).
The cavity of the follicle con-
tains a slightly yellowish,
alkaline, albuminous fluid, a
transudate in all probability
from the blood-vessels. The
Graafian follicle is of especial
interest, for it is in this struc-
ture, and more especially in
the proligerous disc, that the
true germ-cell or ovum is
developed.

The ovum is a spheric
body measuring about 0.3
mm. in diameter. It consists
of a mass of living, proto-
plasmic material, cytoplasm,
a nucleus or germinal vesicle,
and a nucleolus or germinal
spot. The cytoplasm presents
toward its central portion a
quantity of granular material,
partly fatty in character, the
deutoplasm or vitellus. The
peripheral portion of the cyto-
plasm is surrounded by a

Clear thick membrane, the granulosa. e. Discus proligerus. /. Zona pellucida.

g. Vitellus. h. Germinal vesicle, i. Germinal spot.
k. Cavity of liquor folliculi. — (After Piersol.)

FIG. 345.— SECTION OF CORTEX OF CAT'S OVARY,
EXHIBITING LARGE GRAAFIAN FOLLICLES, a. Per-
ipheral zone of condensed stroma. b. Groups of im-
mature follicles, c. Theca of follicle, d. Membrana

icida.
spot.

zona pellucida, external to
which is a layer of radially
placed columnar epithelium, the corona radiata (Fig. 346).

The nucleus consists of a nuclear membrane enclosing contents. The
latter consist of an amorphous material in which is embedded a network,
some of the threads of which have a strong affinity for certain staining
materials, and hence are known as chromatin, in the meshes of which lies a
material that stains less deeply and known as achromatin.

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

44

6go

TEXT-BOOK OF PHYSIOLOGY.

the uterus, the wide portion close to the ovary. The outer extremity of the
tube is expanded and subdivided, and presents a series of processes termed
fimbriae, one of which is attached to the ovary. The tube consists of three
coats — an external or nbro-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

FIG. 346. — OVUM OF A Cow. i. Zona
pellucida. 2. Cytoplasm, vitellus. 3. Nu-
cleus, germinal vesicle. 4. Xucleolus, germ-
inal spot. 5. Corona radiata. The radial
striation of the zona pellucida can not be
seen.— (Stohr.)

FIG. 347. — FRONTAL SEC-
TION OF THE UTERUS, i. Cav-
ity of the body. 2, 3. Lateral
walls. 4,4. Cornua. 5. Cs
internum. 6. Cavity of the
cervix. 7. Arbor vitse of the
cervix. 8. Os externum. 9.
Vagina. — (Sappey.)

shows a central cavity which in the body is triangular in shape, in the neck
oval or fusiform (Fig. 347). 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.

REPRODUCTION. 691

The Vagina. — The vagina is a musculo-membranous canal, from 12 to
1 8 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 chromatin
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
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

692 TEXT-BOOK OF PHYSIOLOGY.

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
Dalton:

CORPUS LUTEUM OF MENSTRUATION. CORPUS LUTEUM OF PREGNANCY.

At the end of three Three-quarters of an inch in diameter; central clot reddish; convoluted
weeks. wall pale.

One month j Smaller; convoluted wall bright | Larger; convoluted wall bright yellow;

yellow; dot still reddish. clot still reddish.

Two months i Reduced to the condition of an

insignificant cicatrix.

Four months . . . i Absent or unnoticeable . . .

Six months Absent.

Nine months . . . Absent .

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.

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. 348) 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.

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

REPRODUCTION.

693

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.

The vas deferens, the excretory duct of the testicle, is about 60 cm. in
length and from 2 to 3 mm. in diameter, and extends upward from the
epididymis to the inguinal canal, through which it passes into the abdominal
cavity and then to the under surface of the base of the bladder, where it
unites with the duct of the vesicula seminalis to form the ejaculatory duct.

The vesiculse seminales are two lobulated pyriform bodies, about 40
mm. in length, situated on the under surface of the bladder. Each vesicula
seminalis consists of an external fibrous coat, a middle, muscular coat, and an
internal mucous coat. The mucous coat contains a number of small
tubular albumin-producing glands which secrete a characteristic fluid.

FIG. 348. — DIAGRAM OF A VER-
TICAL SECTION THROUGH A TES-
TICLE, i. Mediastinum testis. 2,
2. Trabeculae. 3. One of the
lobules. 4, 4. Vas a recta. 5.
Globus major of the epididymis.
6. Globus minor. 7. Vas def-
erens.— (Holden.}

FIG. 349. — VAS DEFERENS, VESICULSE
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 ejaculatory duct, formed by the union of the vas deferens and the
duct of the vesicula seminalis, opens into the prostatic portion of the urethra
(Fig. 349).

The prostate gland is a musculo-glandular mass surrounding the
posterior extremity of the urethra. It contains a large number of tubules,
more or less branched and convoluted, and lined by columnar epithelium.
They secrete a fluid which is poured into the urethra at the time of the ejac-
ulation of semen and impart motility to the spermatozoa or spermia.

The penis consists of three parts: the corpus spongiosum below, through
which passes the urethra, and the two corpora cavernosa, one on either side
and above. The corpus spongiosum terminates anteriorly in a conic-

694

TEXT-BOOK OF PHYSIOLOGY.

shaped structure, the glans penis; the corpora cavernosa consist externally
of a fibrous investment and internally of a fibrous investment and internally
of erectile tissue. These bodies are abundantly supplied with blood, which
after entering their susbtance by the arteries, passes into sinuses or reser-
voirs, from which it is carried away by veins. These vessels pass to the
dorsum of the penis and unite to form a large vein by which the blood is re-
turned to the general circulation. By virtue of the erectile tissue in the cor-
pora cavernosa the penis becomes erect and rigid when the blood supply
is increased. This takes place in response to peripheral stimulation or
emotional states, or both combined. When these conditions are established
nerve impulses pass outward through nerves, the nervi erigentes, which have
their origin in the lumbar segment of the spinal cord, and
bring about an active dilatation of the arteries and a re-
laxation of the non-striated muscle-fibers in the corpora
cavernosa. (See page 619.) With these events there is
a rapid influx of blood and a distention and an erection
of the organ. This condition is furthered and main-
tained by a partial compression of the dorsal vein by the
fibrous capsule.

Semen. — The semen is a complex fluid composed of
the secretions of the testicles, the vesiculae seminales, the
prostatic tubules, and urethral or Cowper's glands. It
is grayish- white in color, mucilaginous in consistence,
characteristic in odor, and somewhat heavier than water.
In response to appropriate stimulation the muscle-fibers
in the walls of the vasa deferentia, vesiculae seminales,
and prostatic tubules contract and discharge their con-
tents into the urethra, from which they are forcibly
ejected by the rhythmic contraction of the ejaculatory
muscles, the ischio- and bulbo-cavernosi. The amount of
semen discharged at each ejaculation varies from i to
5 c.c.

Spermatozoa. — The spermatozoa or spermia are
peculiar morphologic elements which arise within the
seminiferous tubules as a result of complex histologic
changes in the lining epithelium. An adult spermatozoon
consists of a conoid slightly flattened head, from the
posterior part of which there projects a short straight
rod, provided with a long filamentous tail or cilium and an end-piece
(Fig. 350). The head contains a nucleus of chromatin material. The
lotal 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 considerable rapidity. The power
of locomotion depends on the possession of the tail which, by lashing the
surrounding fluid now in this and now in that direction, propels the head
from place to place. The vitality of spermatozoa is such as to enable

V

FIG. 350. — HUMAN
SPERMATOZOON, i.
Front view, 2, side
view, of the head.
k. Head. m. mid-
dle piece. /. Tail.
e. Terminal fila-
ment. — (After Ret-
zius.}

REPRODUCTION. 695

them to retain their physiologic activities in the uterus for more than eight
days.

The development of spermatozoa from testicular cells as observed in lower
animals indicates that each cell gives rise to four embryonic forms — spermatids
—which subsequently 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
chromosomes. The changes by which this condition is brought about are
comparable to the changes exhibited by the ovum, and have for their result
a reduction in the quantity of hereditary substance to be transmitted.

Fecundation. — Fecundation is the union of the spermatozoon (the
sperm-cell) with the ovum (the germ-cell) and takes place in the great
majority of instances in the Fallopian tube. After the introduction of the
spermatozoa into the vagina during the act of copulation, they soon begin to
pass upward, into and through, the uterine cavity and out into the Fallopian
tube, where they accumulate in large numbers and retain their vitality for
some days. The migration is effected by the propelling power of the fila-
mentous 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.

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 having
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

696 TEXT-BOOK OF PHYSIOLOGY.

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. reftexa) , that beneath the ovum the decidua
basilaris (placental d.), while the remainder constitutes the decidua parietalis
(d. vera). As development proceeds the decidua basilaris becomes greater,
ultimately developing into the placenta.

Segmentation of the Ovum. — Immediately after fertilization the ovum
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 amniotic
cavity, limited by the ectoderm; the entodermal cavity, limited by the ento-
derm; and the mesodermal or celomic cavity, limited by the extra-embryonic
mesoderm. Meanwhile cells in various parts of the thickened trophoderm
have disappeared leaving this layer in the form of delicate trophodermal
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 disk. 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-
porary structure that is soon overshadowed by changes in the areas just 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 folds.
The dorsal lips of the groove approach each other in the mid-line and fuse,
separating from the original ectoderm which closes over the ectodermal
tube. This ectodermal tube is the neural tube from which the nerve system
is developed.

In the immediate vicinity of the head end of the primitive streak is seen
a darkened area, Hensen's node that represents the beginning invagination
of the ectoderm in the formation of the embryonic mesoderm and notochord
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 neurenteric 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-

REPRODUCTION.

697

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 sclerotome, 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 primitive 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 of the
amniotic 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
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 inorganic salts. Traces of urea indicate
that some of its constituents are derived 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. 351.)

FIG. 351 . — HUMAN EMBRYO AND ITS EN-
VELOPES AT THE END OF THE THIRD MONTH,
—(Dalton.}

698

TEXT-BOOK OF PHYSIOLOGY.

Placenta Formation. — The chorionic villi increase rapidly in size and
number and usually surround the whole fetal sac, giving it a peculiar shaggy
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 vitli 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

rt f Compact
layer.

•3 Cavernous/
layer. {

Muscularis.

I Chorionic villi.

Intervillous spaces.
Floating villus.

!> Attached villi.
Vein.

Spiral artery.
Gland.

Vein.

FIG. 352. — DIAGRAM OF HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. — (Schaper.)

manner these uterine glands have become converted into blood sinuses. The
chorionic villi either attach themselves to the tunica propria of the mucosa
( fixed villi) 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. 352.)

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

REPRODUCTION. 699

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 with varies in accordance when 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
additional material in the same way. The period during which it does so
is, however, very limited.

The Vitelline Circulation. — The vitelline circulation, which in oviparous
animals, e.g., the chick, is of primary importance because of the large
amount of food stored in the vitellus or yolk, is in mammals of relatively
slight importance because of the limited supply of food in the vitellus. It
is nevertheless present in early stages.

The Allantoic Circulation which in oviparous animals is also of primary
importance in the latter half of the incubation period both as an absorption
and respiratory apparatus is also present in mammals to a slight extent,
but it is merely a transition stage in the development of placental circulation.

The Placental Circulation. — The development of the fetal or placental
circulatory apparatus by which the fetus obtains its food supply and neces-
sary oxygen and frees itself from carbon dioxid has been alluded to in a
foregoing paragraph relating to the formation of the placenta. After the

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TEXT-BOOK OF PHYSIOLOGY.

blood-vessels of the embryo, the umbilical arteries and vein have come into
histologic and physiologic relations with the uterine blood-vessels, the
nutritive materials and the oxygen are derived entirely from the maternal
blood-stream which at the same time receives carbon dioxid and perhaps
other waste products from the fetal blood-stream. The placenta thus
serves as a digestion and respiratory organ. The blood having undergone
these changes now leaves the placenta and returns to the fetus by the um-
bilical vein.^This blood is relatively rich in nutritive material and of a

res

FIG. 353. — THE FETAL CIRCULATION, ao. Aorta, a.pu. Pulmonary artery, au. Umbilical
artery, da. Ductus arteriosus. dv. Ductus venosus. int. Intestine, vci and vcs. Inferior and
superior venae cavae. vh. Hepatic vein. vp. Venaporte v. pu. Pulmonary vein. vu. Umbilical
vein. — (From Kollmann.)

scarlet red color by reason of the presence of an increased amount of oxygen.
As it passes into the abdominal cavity a portion, about one-half, of the blood
is directed by the ductus venosus into the inferior vena cava, while the re-
mainder is emptied into the portal vein, by which it is distributed to the
liver and from which it emerges by the hepatic veins and is poured into the
inferior vena cava. The blood in the vena cava is thus a mixture of venous
blood from the lower extremities and liver, and oxygenated blood from the
placenta. After its discharge into the right auricle the blood is directed by
a fold of the lining membrane, the Eustachian valve, through an opening in

REPRODUCTION. 701

the interauricular septum, the foramen ovale, into the left auricle. It then
flows through the auriculo-ventricular opening into the left ventricle, thence
into the aorta, and by its branches is distributed to all parts of the body.

The blood from the head and upper extremities is emptied by the superior
vena cava into the right auricle, but as it passes in front of the Eustachian
valve, it flows directly into the right ventricle and then into the pulmonary
artery. On account of the unexpanded condition of the lungs and the
almost impervious condition of the pulmonary capillaries, but a small
portion of the blood passes through them, while the larger portion by far
passes into the aorta directly through a duct, the ductus arteriosus, which
enters at a point below the origin of the left carotid and subclavian arteries.
A comparison of the blood distributed to the head and upper extremities,
with that distributed to the lower extremities, will show a larger percentage
of nutritive material and oxygen in the former than in the latter, a fact
which has been offered as an explanation of the more rapid growth of the
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 the child, the placenta, and decidua vera. To
this process in its entirety the term parturition is given. At this time, from
causes not clearly defined, the uterine walls begin to exhibit throughout their
extent a series of slight 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 ammo tic 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
exerted from all sides on the body of the child, the head gradually descends
still further into the vagina and finally emerges through the vulva to be
followed 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

702 TEXT-BOOK OF PHYSIOLOGY.

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 in
size, partly from a deposition of fat and connective tissue and partly from
a multiplication of the secreting acini. The lining epithelial cells at the same
time increase in size, and toward the end of pregnancy begin to exhibit
functional activity. At the time of birth, or within a day or so after birth,
the acini are filled with a fluid which in its qualitative composition 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-
ulates on boiling, and certain inorganic salts which have a laxative effect
on the new-born child. Normal lactation and the phenomena which
accompany it are fully established by the end of the second or third day.

The composition of milk and the mechanism of its production have been
stated in the chapter on Secretion.

Physiologic Activities of the Embryo. — During intrauterine life the
evolution of structure is accompanied by an evolution of function. The
relatively simple and uniform metabolism of the undifferentiated 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.

REPRODUCTION. 703

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 applied to
either tissue, will call forth a contraction of the muscle, or the development of
a nerve impulse in the nerve. The most convenient stimulus is electricity,
for the reason that, with appropriate apparatus, its intensity and duration
can be graduated with the utmost nicety. Moreover, it does not destroy
the tissues, as do many chemic, physical, and mechanic stimuli.

It is therefore necessary that the student should have a practical ac-
quaintance with those appliances by means of which electricity is generated,
applied and controlled.

The electric cell is an apparatus composed of different elements, which,
by virtue of chemic actions taking place among them, generate and conduct
electricity. In its simplest form an electric cell consists of two metals —
zinc and copper, or carbon, or platinum, etc., immersed in an exciting fluid,
usually dilute sulphuric acid (Fig. 354).

The zinc element is the one acted on chemically by the sulphuric acid,
and at the expense of which the electricity is maintained. It is known as the
generating element. The copper is the collecting and conducting element.

With the immersion of these elements in a solution of H2SO4 a chemic
action at once takes place between the zinc and the acid, with the formation
of zinc sulphate and the liberation of hydrogen, as expressed in the following
formula:

Zn + H2S04 = ZnS04 + H2.

The zinc sulphate passes into the solution, while the hydrogen accu-
mulates on the surface of the copper element.

As all chemic action is accompanied by the development of electricity,
it can be shown by appropriate means that this is the case at the surface of
the zinc. Such a combination is the means of establishing a difference of
potential between two points; the point of highest potential being the surface of
the zinc or the positive element, the point of lowest potential being the copper
or the negative element. So long as the elements remain unconnected
there is no movement of electricity, no current.

If the ends of the elements projecting beyond the fluid are connected by
a copper wire, a pathway or circuit is established, and a movement of the
electricity takes place. As electricity flows from the point of high to the
point of low potential, it follows that inside the cell the current flows from
the zinc to the copper, and outside the cell from the copper to the zinc.
Such a current is termed a continuous, a galvanic or a voltaic current. Inas-
much as there is a progressive fall in potential between the Merriest and

704

PHYSIOLOGIC APPARATUS.

70S

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

T-4-

FIG. 354.—AN
ELECTRIC CELL.

FIG. 355. — Two SIMPLE ELECTRIC CELLS JOINED
IN SERIES. C. Copper. Z. Zinc.

current will therefore not depend entirely on the electro-motive force, but,
rather on the ratio between the electro-motive force and the resistance.

For the measurement of electric quantities, a system of units has been
devised. The unit of electro-motive force is the volt; the unit of resistance
is the ohm, i.e., the resistance offered by a column of 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:

C (current strength) =

Electro-motive force (E. M. F.)

Resistance (R)

Volts

or Ampers = ^.

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. 355). 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

45

7o6

TEXT-BOOK OF PHYSIOLOGY.

which its chemic activity ceases. The current, therefore, soon diminishes in
strength.

2. The accumulation of hydrogen bubbles on the surface of the copper
hinders the passage of the electricity. In a short time they develop a current
in the opposite direction, which also tends to weaken the original current.
This action is termed polarization of the elements.

Cells of this character are not suited for physiologic work, in which
constancy in the strength of the current is absolutely necessary. To over-
come these disadvantages, cells have been devised which are less violent in
action, which prevent polarization, and which maintain a current of constant
strength for a long period of time. One of the most generally used for
physiologic purposes is —

The Daniell cell. This consists of a porous cup containing a saturated
solution of CuSO4, 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. 356).

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
the porous cup, to take the place of that used. This cell is remarkably
constant for these reasons, and well adapted for physiologic as well as other
purposes where a current of uniform strength is necessary.

The projecting ends of the copper and zinc plates are termed respectively
the positive pole or anode, and the negative pole or kathode. The electro-
motive force of a Daniell cell is practically i volt; but when the two poles
are connected by a wire of i ohm resistance, the current strength will be less
than i ampere, possibly only 0.7, owing to the resistance offered to the flow of
electricity by the fluids between the zinc and the copper. In all measurements,
the internal resistance of the cell must be taken into consideration.

The Dry Cell. — The commercial dry cell is a convenient source of
electricity for general laboratory work. It consists of a cup of zinc, the inner
surface of which is covered over with a thick layer of a paste of plaster of
Paris, saturated with ammonium chlorid. In the center of the cup there is
a rod of carbon. Surrounding this rod and occupying the space between it
and the plaster-of-Paris paste, is a mixture of manganese dioxid and charcoal.

FIG. 356. — DANIELL CELL.

PHYSIOLOGIC APPARATUS.

707

The upper surface of the cell is sealed to prevent evaporation. The electric-
ity is generated at the surface of the zinc cup by the chemic action of the
chlorin which arises from the dissociation of the ammonium chlorid. When
the plates are united by a conjunctive wire the current within the cell flows
from the zinc (the positive element) to the carbon (the negative element),
and without the cell from the carbon (the positive pole) to the zinc (the
negative pole).

Leads. — By means of insulated wires attached to the poles of a cell, the
electricity may be conducted from the cell and used for exciting or stimulat-
ing purpose. As the wires thus become practically prolongations of the
plates their ends become the corresponding poles. In experimental work
the ends of the wires are provided with special devices, termed —

Non-polarizable electrodes. — The necessity for the employment of
such electrodes arises from the fact that when the ends of the wires from a
cell are placed in direct contact with the tissues chemic changes are produced
in a short time, which lead to their polarization. As a result, a current
opposite in direction to that of the cell is
developed, which tends to weaken or neu-
tralize it. This polarization current vitiates
the result of many experiments made with
highly irritable tissue such as nerve-tissue.
Whether for stimulating purposes or for the
purpose of detecting the existence of electric
currents in living tissues, it is essential that
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. 357. It consists of
a flattened glass tube attached to a universal
joint and supported by an insulated brass
stand. The lower end of the tube
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 camePs-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 experimenta-
tion, as their free ends neither corrode the tissues nor develop electric currents.

G- 357« — NON-POLARIZABLE ELEO
1S TRODES. i. Du Bois-Reymond 's. 2.
Von FleischPs. 3. d' Arson vaPs.

yo8

TEXT-BOOK OF PHYSIOLOGY.

Keys. — Muscle and nerve-tissues are conductors of electricity. When,
therefore, the terminals (the non-polarizable electrodes) of the wires of a cell
are placed in contact with either a muscle or a nerve a circuit is made through
which a current of electricity flows; when one or both are removed, the
circuit is broken and the current ceases. In practical work it is often 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. 358). 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.
The amount of the current which is returned
to the cell through the short circuit will be proportional to the resistance
of the longer circuit. As the latter is usually great in comparison with
the former, practically all the current is short-circuited. When the
bridge is lowered, therefore, the current is short-circuited; when it is
raised, the current flows into the longer circuit through the tissue or
apparatus.

The Mercury Key. — In this form the connection is established by means
of mercury. It consists of a circular block in the center of which there is a
cup containing mercury (Fig. 359). At opposite points there are binding
posts, one of which is provided with a rigid fixed copper rod passing into the
mercury; the other is provided with a movable bent rod which may be made
to dip into or be withdrawn from the mercury by the ebonite handle.

The effect of a constant or galvanic current on a muscle or nerve will

FIG. 358. — Du BOIS-REY-
MOND FRICTION KEY.

PHYSIOLOGIC APPARATUS.

709

depend to some extent on its strength. This may be accurately regulated
by means of an apparatus known as —

The Rheocord. — With this apparatus an electric current may be divided,
one portion continuing through a conductor back to the battery, the other
portion being sent off through the nerve. The strengths of these two currents
are inversely proportional to the resistances of their circuits. A simple form
of rheocord (Fig. 360) 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. 359. — A MERCURY KEY.

FIG. 360. — 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. 361). In each of the
two cups marked i and 2, + and — , is inserted one end of a copper wire

710 TEXT-BOOK OF PHYSIOLOGY.

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-
ment, whereby the opposite ends of the semicircular wires can be made to dip
into cups 3 and 4, and into cups 5 and 6 alternately. Two wires crossed in
the middle of the block serve to connect opposite pairs of cups. When in
use, the cups are filled with clean mercury. The method of using the com-
mutator is as follows:

i. As a Current Reuerser. — The positive and negative poles of the electric
cell are connected by wires with binding posts i and 2 respectively.
A key is interposed in the circuit. Wires are then carried from binding
posts 3 and 4 to the electrodes in connection with the muscle or nerve.
The rocker of the commutator is so turned that the ends of the semicir-
cular wires dip into cups 3 and 4. The direction of the current will be
on the closure of the circuit from i to 3, then from 3 along a wire to and
through the tissue and back to 4, and thence to the cell. If the position of
the rocker be now reversed so that the opposite ends of the semicircular

i i

FIG. 361. — POHL'S COMMUTATOR. A. Arranged as a current reverser; B, as a cur-
rent deflector.

wires dip into cups 5 and 6, the direction of the current through the tissue
will be reversed. The positive current, after entering binding post i,
will flow to 5; then along one of the cross wires to 4; then along a wire
to and through the tissue and back to 3, along the opposite cross wire
to 6, thence to 2 and so back to the cell.

2. As a Current Deflector. — When it is desirable to deflect the current to two
pairs of electrodes differently situated, wires are carried from binding
posts 3 and 4 to one pair, and from 5 and 6 to the other pair. The cross
wires are then removed. According to the position of the rocker the
current will be deflected to one or the other.

The Inductorium. — This is an apparatus designed for the purpose of
obtaining single or rapidly succeeding electric currents by induction. Its
construction is based on facts discovered by Faraday, some of which are the
following:

If two circuits, a primary and a secondary, are placed parallel to each
other, 4he former connected with a galvanic cell, the latter with a galvan-
ometer, it is found that, at the moment the primary circuit is made, and at the

PHYSIOLOGIC APPARATUS.

711

moment it is broken, a current is induced in the secondary circuit, as shown
by a momentary deflection of the galvanometer needle. During the con-
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 induced
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, other
things being equal, the strength of the induced currents will be proportional
to the number of turns in the coils.

If the coils are placed
at varying distances
from each other, the
strength of the induced
current varies, 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 or 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. 362).

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.

FIG. 362. — INDUCTORIUM OF DU BOIS-REYMOND. R', Pri-
mary, R", secondary spiral. B. Board on which R" moves,
i. Scale. + — . Wires from battery. P', P". Pillars. H.
Neef's hammer. B'. Electro-magnet. S'. Binding screw
touching the steel spring (H). S" and S'". Binding screws to
which to attach wires where Neef's hammer is not required.

7i2 TEXT-BOOK OF PHYSIOLOGY.

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 in-
creasing the action of the inducing current.

The secondary coil, R", consists of a much greater number of turns of
a finer copper wire, the ratio being about 40 to i, also wound around a spool,
having a tunnel sufficiently large to enable it to slide over the primary. By
these means the strength of the induced current is increased . As a result of the
construction of the inductorium, the low electro-motive force of the cell is trans-
formed into the high electromotive force characteristic of the induce d current.
As the number of turns of wire in the secondary coil is to the number in the
primary, so are the electro-motive forces in the secondary coil to those in the
primary coil.

The secondary coil slides along a track, B, which permits it to be moved
toward or away from the primary. The distance between the two coils can
be measured and the strength of the induced current again reproduced,
other things being equal, by means of a centimeter-millimeter scale pasted on
the edge of B.

The ends of the wire of the secondary coil are fastened to two binding
posts to which conducting wires provided with hand electrodes can be
attached.

The inductorium may be used for obtaining either a single current or a
series of rapidly repeated induced currents.

The Single Induced Current. — On account of its high electro-motive force,
its penetrative power, and short duration, the single induced current is a
most convenient and suitable form of stimulus for many purposes. In
order to obtain such a current, the positive wire of the cell is carried to
binding post S", and the negative wire either to S'" or P". A key is placed in
the primary circuit. The course of the current will then be on the closure of
the circuit from the cell to S", thence around R' to S'", and so back to
the cell; or if the negative wire is connected with P", the course of the cur-
rent on leaving R' will be through the coils surrounding the two vertical bars
B', thence to P", and so back to the cell. It the secondary coil be placed
close to the primary and the wires of the secondary brought into contact
with a muscle, it will be found that with both the make and the break of
the primary circuit a current is induced in the secondary, as shown by a
short quick pulsation of the muscle; but during the time of closure of the cir-
cuit the induced current is wanting, as shown by the quiescent condition of the
muscle. It will be apparent, however, from the energy of the contraction
that the break-induced current is a more efficient stimulus than the make-
induced current. That this is the case is made evident by removing the
secondary to the end of the slide- way and then gradually bringing it toward the
primary half a centimeter at a time, making and breaking the circuit after each
movement until a pulsation of the muscle occurs. It will be found to occur
first on the break of the circuit. As the secondary approaches the primary a
position will be reached when a pulsation occurs on the make as well as on
the break of the circuit, though it will be less pronounced.

The explanation offered for this difference in the strength of the two induced currents is as
follows: With the make of the circuit and the passage of the battery current through the primary
coil there is induced in the neighboring and parallel turns of the wire and extra current opposite in

PHYSIOLOGIC APPARATUS. 713

direction to the the primary current. This extra or self -induced current antagonizes and prevents
the current from attaining its maximum development as quickly as it otherwise would, and therefore
its efficiency as an inducer of a current in the secondary is diminished. On the break of the cir-
cuit the primary current disappears quickly, and as there is nothing to retard its disappearance its
efficiency as an inducer of a current in the secondary coil is not diminished. It is not infrequently
stated that the disappearance of the primary current induces in the neighboring coils a break extra
current corresponding in direction which assists in the development of the induced current. This
is not the case, however, as no break-extra current is developed in the inductorium as ordinarily
used when actuated by a battery current of moderate strength.

As it is not so much the intensity of the current as it is rapid variations in intensity that produce
effects, it is readily apparent why the induced current developed at the break of the primary is
more effective as a stimulus than the induced current developed at the make of the primary circuit.
The quantity of electricity is, however, the same in both cases.

If the secondary be pushed further along the slideway until it largely
covers the primary coil, a position will be reached when the make -induced
current equals in its efficiency as a stimulus the break-induced current;
and if the secondary be yet further advanced, a position is reached when the
make-induced current becomes more powerful and efficient than the break-
induced current, as shown by the greater contraction of the muscle. This
result is explained by the fact that the make-extra current is now able of
itself to induce a current in the secondary coil, on account of its proximity,
which, added to that induced by the battery current, produces a current,
greater than that induced on the break of the circuit.1

Rapidly Repeated Induced Currents. — As the single induced current is of
extremely short duration, it is inefficient as a stimulus in the conduct of
many experiments. It is necessary, therefore, to develop it with a frequency
that is sufficient to give rise to a summation of effects. The duration of the
stimulation may be thus considerably prolonged. This is accomplished
by introducing in the primary circuit close to the primary coil an automatic
interrupter, usually Neef's modification of Wagner's hammer (Fig. 371).
This consists of a vertical post, P', to the top of which is fastened a metallic
spring carrying at its opposite end a steel or iron hammer, H, which hangs over,
but does not touch, the two vertical bars of soft iron around which the wire
of the primary coil is wound. About the middle of the spring on its upper
surface there is a small plate of platinum which is in contact with an adjust-
able, platinum-tipped screw, S', carried by a plate of brass in connection with
binding post S".

For the purpose of interrupting the primary circuit frequently in a unit
of time, and thus developing induced currents in quick succession, the
apparatus is arranged in the following way : The positive and 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 with
the spring, the current on the closure of the circuit will enter P', pass
along the spring to S', thence into and through the primary coil R', to the
coils surrounding the vertical bars B', then to P", and so back to the cell.

As the current passes around the vertical bars, they are magnetized.
The magnetization draws down the hammer, and, in so doing, breaks the
circuit at the tip of the screw, S'. The vertical bars are at once 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

1 "On certain peculiarities of the inductorium," Prof. Colin C. Stewart, "Univ. Pa. Medical
Bulletin," Feb., 1904.

7*4

TEXT-BOOK OF PHYSIOLOGY.

coils, the bars are again magnetized, the hammer is drawn down, to be
followed by a second break of the circuit.

The number of times the circuit is thus made and broken per second will
vary with the length of the spring.

As each interruption of the primary circuit develops an induced current,
it follows that the latter must succeed each other with a frequency corres-
ponding with the frequency of the former. If while the primary circuit
is thus being interrupted the wires of the secondary coil be placed in 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 posts
P' and S", and in providing binding post P" with an
adjustable screw which can be raised until the spring
comes in contact with it, when the hammer is drawn
down by the electromagnet B'. This latter 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

FIG. 363. — HELMHOLTZ'S MODIFI-
CATION OF NEEF'S HAMMER. As
long as c is not in contact with d,
g h remains magnetic; thus c is at-
tracted to d and a secondary circuit,
a, b, c, d, e, is formed; c then springs
back again, and thus the process
goes on. A new wire is introduced
to connect a with /. K. Battery.

through the primary coil. When the short-circuiting key is closed, most of the current fails to
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— rather
an alternation between a full primary current and a weaker one. The difference in the phases is
thus lessened, the extent of the change on making and breaking is lessened, and correspondingly
the efficiency of the make and break currents induced in the secondary coil is slightly decreased.

" Second, on making the primary current, as in the ordinary coil, the sudden appearance of the
primary current is antagonized by the opposing make extra current, with the result that the make
induced current is still further reduced; while on breaking the current the break extra current can
now flow through the primary coil across the short-circuiting key. This current, trailing behind
the disappearing primary current in the same direction, produces the same effect as if the primary
current itself were to disappear slowly. As a result the disappearance of the primary current loses
its former efficiency as an inducer of secondary currents, and the break induction current is reduced
to about the efficiency of the make.

"This so-called 'equalizing' of the make and break induced currents is never perfect, if for no
other reason, because the make extra current must take the long circuit through the battery, while
the break extra current has an easier path through the short-circuiting 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

PHYSIOLOGIC APPARATUS.

movements accompanying physiologic processes. If these movements
can be translated in one direction, they may be recorded in different

ways :
i

FIG. 364.— A RECEIVING TAMBOUR.

By attaching the moving structure — e.g., heart, muscle, etc. — to a 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

FIG. 365.-A RECORDING TAMBOUR.-(MarO'.) recording tambour

(Figs. 364, 365).

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 482) ;
or (2) a tambour and lever (page 347); or (3) a mercurial manometer
carrying a float and pen (page 333). 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.

7i6

TEXT-BOOK OF PHYSIOLOGY.

The writing point may be (i) some form of pen carrying ink which records
the movement on a white paper surface, or (2) a piece of metal, glass, or
paper which records the movement on smoked paper or glass.

The Recording Surface. — The surface which receives and records the
movements 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 of pulleys in
connection with its axis. As the
tracing is wave-like in form, the
cylinder is frequently spoken of as
a kymograph or wave recorder
(Fig. 366).

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. The
magnification of the lever is readily determined by dividing the distance
between the axis of the lever and its writing point by the distance between
the axis and the point of attachment of the structure, and then dividing
the height of the tracing by this quotient. The final quotient represents
the extent of the movement.

The Time Relations of a Movement. — When recorded in the form of a
curve, the duration of the entire movement, or of any one portion of it, can
be determined by means of a time marking or chronographic apparatus, con-
sisting of (i) a small signal magnet provided with a movable armature, to

FIG. 366. — KYMOGRAPH.
zold, Leipzig.)

(Boruttau's, Pet-

PHYSIOLOGIC APPARATUS. 717

which is attached a writing style; (2) an automatic interrupter; and (3) an
electric cell.

The Signal Magnet. — The magnet (Fig. 367) is actuated by the electric
current made and broken at regular and known intervals by an automatically
acting interrupter placed in the circuit. With each make and break of the cir-
cuit the armature and style move alternately downward and upward. The
excursion of the style can be readily recorded on a traveling surface. The
character and number of the interruptions per second will 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

.6 ^-11 FlG- 367.— SIGNAL MAGNET.

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. 368. 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
FIG. 368 .-PAGE'S VIBRATING REED. (Reichert's ave tracing w{\\ represent TlU- of
modification.) . -L u.u

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. 369). 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.

7i8

TEXT-BOOK OF PHYSIOLOGY.

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 prep-
aration, 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
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. The end of this
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

FIG. 369. — MOIST CHAMBER.

until the instant the contraction of the muscle begins. This screw, the
after-loading screw, also enables the lever to be placed in a horizontal position.
The portion of the lever near the axis is provided with a double hook, the
lower portion of which serves for the attachment of the weight by which the
muscle is counterpoised.

In some experiments, as in the registration of a muscle contraction under
varying conditions, it is necessary to give the lever mass by attaching weights
directly beneath the muscle. This, however, introduces certain errors in
the movements of the lever, which somewhat deform what would otherwise
be the normal curve. If the weight be attached, not opposite to the muscle
attachment, but close to the axis of the lever, the undesirable acceleration
of the lever movement, during both contraction and relaxation, is largely
prevented. The lever may be a straw, a strip of celluloid or aluminium.
It should be as light as possible. The writing point may be made of stiff
paper, a piece of tinsel, glass, or aluminium. It should have sufficient elas-

PHYSIOLOGIC APPARATUS. 719

ticity to keep it in contact with the cylinder during the excursion of the
lever. The writing point should be placed as nearly parallel as possible
to the surface of the cylinder.

Normal Saline Solution. — To prevent drying and a loss of irritability
the tissue under investigation should be kept moist with the normal saline
solution (NaCl, 0.6 per cent.). This solution very largely prevents either
absorption or extraction of water from the tissues and thus retards chemic
changes in their composition.

Ringer's solution, largely used for the same purpose, is made by
saturating 0.65 per cent. NaCl solution with calcium phosphate and then
adding 2 c.c. of a 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 electro-
meter. 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 galvano-
meter, or boussole, as con-
structed by Wiedemann,
is the form most frequently Fia 37o.-WiEDEMANN's BOUSSOLE.

employed in physiologic investigations (Fig. 370). It consists primarily
of ^ 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

720 . TEXT-BOOK OF PHYSIOLOGY.

of air are prevented by inclosing it with a brass cover provided with a glass
window. The coils are placed on each side of the copper chamber, and
supported by a rod, on which they slide. By this arrangement they can be
approximated until they meet and completely conceal the cylinder. By
varying the position of the coils the influence of the current upon the needle
can be increased or diminished. An advantage which this galvanometer
possesses is the damping of the oscillation of the needle, so that it quickly
comes to rest after deflection. This is accomplished by the development of
induction currents in the copper cylinder, the direction of which is opposite
to that of the movement of the needle. The instrument, therefore, is
aperiodic — that is to say, when the needle is influenced by a current it
moves comparatively slowly until the maximum deflection is reached, when
it comes to rest without 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 rend ered 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 for
the same purposes.

The Capillary Electrometer. — Not withstanding the extreme sensi-
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 capil-
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 by hy-
drogen taking place at its surface. If a capillary glass tube be filled with mer-
cury and its lower end inserted into a solution of sulphuric acid, and the former
connected with the positive and the latter with the negative electrode, it
will be observed, upon the passage of the current, that a definite movement
of the mercury takes place, in the direction of the negative electrode, in conse-
quence of the diminution of its capillary constant or the tension of its surface in
contact with the acid. As a reverse movement follows a cessation of the current,
a series of oscillations will follow a rapid making and breaking of the current.
If the direction of the current is reversed, the capillary constant is increased

PHYSIOLOGIC APPARATUS.

721

and the mercury ascends the tube toward the negative pole. From facts
such as these Lippmann constructed the capillary electrometer, a con-
venient modification of which devised by M. v. Frey, is shown in Fig. 371.
This consists of a glass tube, A , forty millimeters in length, three millimeters
in diameter, the lower end of which is drawn out to a fine capillary point.
The tube is filled with mercury and its capillary point immersed in a 10 per
cent, solution of sulphuric acid. The vessel containing the acid is filled to
the extent of several millimeters with mercury also. The mercury in the
tube is put in connection with a platinum wire (a) , and the acid in the vessel
with a second wire (b) . When a constant current passes into the apparatus
in the direction from b to a the mercury is pushed up the tube, and, upon
the breaking of the current, it may or may not return to the zero-point. For

FIG. 371. — VON FREY'S CAPILLARY ELECTROMETER.

FIG. 372. — CAPILLARY
ELECTROMETER. R.
Mercury in tube; capil-
lary tube. s. Sulphuric
ac'id. q. Hg. B. Ob-
server. M . Microscope.

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. 372).
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
potential, according to Lippmann's observation, as slight as the rTrg"g"o of a
Daniell. These rapid oscillations can be recorded by photographic methods.
In using either the galvanometer or the electrometer for detecting the
existance of electric currents or differences of potential in living tissues, it is
46

722 TEXT-BOOK OF PHYSIOLOGY.

absolutely essential that non-polarizable electrodes be employed in con-
nection with it.

DISSECTION OF THE HIND-LEG OF THE FROG.

Much of our knowledge of the physiologic properties of muscles and
nerves has been derived from the study of the muscles and nerves of the
cold-blooded animals, especially of the frog, for the reason that in these
animals the tissues retain their vitality under appropriate conditions for a
considerable period of time after death or removal from the body. The
muscles generally employed for 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 together, 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 viscera and the abdominal walls. Draw the
hind-legs out of the skin. Place the legs on a glass plate, back uppermost,
and moisten them freely with normal saline solution.

Observe on the outer side of the dorsal surface of the thigh the follow-
ing muscles (Fig. 373, 374). The triceps femoris (tr), made up of the
rectus anticus (ra), the vastus externus (ve), and the vastus internus (vi),
not seen from behind; on the inner side, the semimembranosus (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
ileococcygeus (ci), and the pyriformis (p).

In the leg observe the gastrocnemius (g) with its tendon (the tendo
Achillis), the tibialis anticus (ta), and the peroneus (pe).

Turn the frog on its back and note the muscles on the ventral surface
of the thigh, the rectus internus major (ri'), and minor (ri"), the 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
the pelvic cavity, it passes down the thigh between the semimembranosus
and the biceps muscles, in company with the femoral blood-vessels. Below

PHYSIOLOGIC APPARATUS.

723

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

FIG. 373. — LEG MUSCLES OF THE FROG.
VENTRAL SURFACE. — (Ecker.)

pc

FIG. 374. — LEG MUSCLES OF THE FROG.
DORSAL SURFACE. — (Ecker.)

the scissors cut off the vertebral column above the seventh vertebra. Place
the legs on the dorsal surface and then divide the seventh, eighth, and ninth
vertebrae lengthwise. With the forceps lift up one lateral half of the vertebrae
and free the nerve as far as the knee by dividing connective tissue and nerve
branches. Be careful not to injure the nerve with scissors or forceps.

The Nerve-Muscle Preparation. — Divide the tendo Achillis just
below its nbro-cartilaginous thickening at the heel, and detach the gastroc-
nemius up to the knee. Cut through the tibio-fibular bone just below
the knee-joint. Cut the femur transversely near its middle and remove
the muscles from the lower end, carefully avoiding injury to the nerve.
The completed preparation consists of the gastrocnemius muscle, the sci-
atic nerve, with half of the seventh, eighth, and ninth vertebrae and the lower
half of the femur.

724 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. 375- — VENTRAL SURFACE OF THE FROG HEART. (After Howes.) ra. Right auricle.
la. Left auricle, v. Ventricle, ca. Conus arteriosus. p' '. Pulmo-cutaneous trunk, s'. 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-ventric-
ular, is provided with two valves the free edges of which are directed toward
the cavity of the ventricle.

PHYSIOLOGIC APPARATUS. 725

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 large branches, a
right and a left, each of which is subdivided by two longitudinal partitions
into 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 below the
free end of the longitudinal valve. After pursuing a short course these

1C.

FIG. 376. — DORSAL SURFACE OF THE FROG HEART, (after Howes.) ra. Right auricle.
la. Left auricle, sv. Sinus venosus. sv' . Opening of sinus venosus into right auricle.
pv. Pulmonary vein. v. Right anterior caval vein, p' s' c' . The pulmo-cutaneous, aortic and
carotid trunks respectively.

three vessels separate from one another to distribute blood to the various
organs of the body. The two aortic trunks wind around the esophagus
and unite posteriorly to form the dorsal aorta; the pulmo-cutaneous divides
into a pulmonary artery which is distributed to the lung and a cutaneous
branch which is distributed to the skin.

The course of the blood through the heart cavities is therefore as follows :
The venous blood poured by the venae cavse into the sinus venosus passes
through the sino-auricular foramen into the right auricle. While the
right auricle is being filled from this source, the left auricle is being filled
by blood coming through the pulmonary veins. "When the auricles contract,
which they do simultaneously, each passes its blood into the corresponding
part of the ventricle, which then instantly contracts before the venous and
arterial bloods have time to mix. Since the conus arteriosus springs from

726 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 aortae
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
valves aside and makes its way into the bulbus aortae. Here again the
course taken is that of least resistance; owing to the presence of the carotid

CC.

FIG. 377. — FROG HEART WITH VENTRAL SURFACE DISSECTED AWAY TO SHOW ITS STRUC-
TURE. (After Parker and Hasw ell), ra. Right auricle, la. Left auricle, sa. Septum between
auricles, sao. Sino-auricular opening, ca. Conus arteriosus. sv. Semilunar valves, av.
Auriculo-ventricular valves. Iv. Longitudinal valve, sv' . Semilunar valve, p' s' c' . Pulmo-
cutaneous aortic and carotid trunks respectively, cc. Columnae carneae.

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
to the conus arteriosus are continuous, though at the sino-auricular, the

PHYSIOLOGIC APPARATUS. 727

auriculo-ventricular, and the ventriculo-conic junctions the continuity is to
some extent interrupted by bands of circularly disposed fibrous tissue, serving
for the support of the valves, which momentarily interfere with the ready
passage of the contraction wave from one division of the heart to another.
The frog heart receives its nutritive material from 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.
The Heart Beat. — If the heart while beating is lifted up by a ligature
attached to the apex it will be observed that the contraction begins in the
walls of the sinus venosus, then passes to the auricles, thence to the ventricle
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.

INDEX.

Abducent nerve, 589
Aberration, chromatic, 665

spheric, 665
Absorption, 205

by epithelium of villi, 216

of foods, 214
of fat, 209
of proteins, 217
of sugar, 216
of water, 216

of lymph, 214

spectra of blood, 248
Acapnia, 424
Accommodation of the eye, 657

convergence of eyes during, 662

force of, 66 1

mechanism of, 658

range, 660
Acoustic area, 554

nerve, 595

Action currents of muscles, 79
of nerves, 104

reflex, 113

of medulla oblongata, 533
of spinal cord, 505, 508
Adrenal bodies, 463
Agraphia, 558
Albuminoids, 16
Albumins, 15
Alcohol, effects of, 122
Alimentary canal, 133
Animo-acids, 13
Amnion, 697
Amylopsin, 188
Amylase, 149
Amyloses, 7
Animal body, structure of, 2

heat, 429
Ankle clonus, 510

jerk, 510

Anti-dromic nerves, 370
Aphasia, 557

amnesic, 558

ataxic, 557
Apnea, 424

chemica or vera, 424

vagi or inhibitoria, 424
Arterial circulation, 319

pressure, 333

Arteries, structure and properties of, 319
Articulate speech, 620
Asphyxia, 425

Association centers of cerebrum, 558
Astigmatism, 665

Autonomic nerve system, 608

Basal ganglia, 525
Bile, 192

composition of, 192
mode of secretion, 193
physiologic action, 194
pigments, 193
^ salts, 193
Bilirubin, 193
Biliverdin, 193
Bioplasm, 28

physiologic properties, 28
Blind spot, 667
Blood, 227

changes in, during respiration, 403
circulation of, 261
coagulation of, 230
chemistry of, 258
extra vascular, 258
intra vascular, 259
constituents of, 227
corpuscles, 234, 252
defibrinated, 232
general composition of, 257
physical properties of, 228
plates, 256
pressure, 332
arterial, 333

auscultatory method, 348
capillary, 337
causes of, 338

determination of, in man, 343
methods of estimation, 332, 345
variations in, 340
venous, 337
quantity of, 257
serum, 232

velocity of, in arteries, 350
of, in capillaries, 352
of, in veins, 353
viscosity of, 230
Bronchial innervation, 379
Burdach, column of, 500

Calcium salts of the body, 20
Calorimeter, 433
Capillary blood-vessels, 321
functions of, 321

circulation, 361

electrometer, 720
Capsule, internal, 527

functions of, 535
Carbohydrates, 7

729

730

INDEX.

Carbon monoxide hemoglobin, 250
Cardiac cycle, 272

impulse, 272
Catalysis, 136
Cardio-accelerator center, 313

factors which determine its activity, 314
Cardio-inhibitor center, 314

factors which determine its activity, 315
Cardio-pulmonary vessels, 264
Carotid pulse, 358
Caseinogen 16, 19
Catalysis, 136
Caudate nucleus, 525
Cells, structure of, 23

chemic composition, 24

manifestations of life by, 25

reproduction of, 27

Central organs of the nerve system, 491
Cerebellar tract, 499
Cerebellum, 565

functions of, 567

results of experimental lesions, 568
Cerebrum, 537

convolutions of, 539

fissures of, 537

functions of, 544

localization of function in, 546

motor area of the chimpanzee brain, 552

motor area of the human brain, 555

motor area of the monkey's brain, 549

sensor areas of the human brain, 554

sensor areas of the monkey's brain, 548

structure of the gray matter, 541

structure of the white matter, 542
Chemic composition of the body, 6
Cheyne-Stokes respiration, 426
Chimpanzee brain, motor area of, 552
Cholesterin, 193

Chorda tympani nerve, 150, 594
Chorion, 697
Chro mo-proteins, 17
Chyle, 219
Ciliary ganglion, 618

movement, 86

muscle, 644

function of, 660
Circulation of blood, 261

forces concerned, 365

hydrodynamic considerations, 322
Clark's vesicular column, 496
Classification of food principles, 118
Coagulated proteins, 18
Cochlea, 680

functions of, 686
Colostrum, 444
Commutator, 709
Complemental air, 398
Conjugated proteins, 17
Connective tissues, 32

H physical and physiologic properties of, 37
Coronary arteries, 287

vaso-motor nerves of, 288

effects of ligation, 288
Corpora quadrigemina, 524
functions of, 534

stria turn, 525
functions of, 534

Corpus luteum, 692
Cranial nerves, 572
Crura cerebri, 523

functions of, 533
Crystalline lens, 649

Defecation, 202

nerve mechanism of, 203
Deglutition, 156

nerve mechanism of, 162
Demarcation current, 78
Depressor nerve, 316, 375, 602
Dextrin, 8
Dextroses, 8
Diabetes, 452

Diapedesis of leucocytes, 363
Diaphragm, 383
Diffusion, 222
Digestion, 133
Digestive apparatus, 133
Dilatator pupillae muscle, 644
Direct cerebellar tract, 499

pyramidal tract, 498
Ductless glands, 454
Ductus arteriosus, 701

venosus, 700
Dyspnea, 425

Electrodes, non-polarizable, 707
Electrometer capillary, 720
Electrotonic alterations in excitability of nerves,
1 06

current, 105
Electrotonus, 105
Encephalo-spinal fluid, 492
Endocardium, 263
Enterokinase, 200
Enzymes, 135
Epidermis, 487
Epididymis, 693
Epinephrin, 466

Epithelial tissues, functions of, 30, 31
Equilibration, mechanism of, 567
Erepsin, 189, 190

Erlanger's sphygmomanometer, 346
Erythrocytes, 234
Eupnea, 425

Eustachian tube, 677, 686
Excretion, 470

Expiratory forces and. muscles, 391
Expired air, composition of, 401
Eye, cardinal points of, 651

dioptric apparatus of, 649

muscles of, 671

physiologic anatomy of, 642

reduced, 654

schematic, 653

Facial nerve, 590

paralysis of, 591
Fallopian tube, 689
Fat, 10

absorption of, 219

digestion of, 189

emulsification of, 12

saponification of, n
Feces, 201

INDEX.

Fecundation, 695
Fehling's solution, 8
Ferments, 135
Fetal circulation, 679

membranes, 697
Fibrin, 18
Fibrinogen, 233
Fillet, 513, 521, 522
Filtration, 225
Follicle, Graafian, 688
Food, 115

animal, 128

cereal, 130

composition of, 128, 131

disposition of, 119

heat value of, 123

principles, 119

quantities required daily, 116

vegetable, 131
Forced expiration, 372
Forces aiding the movement of lymph and

chyle, 220
Fovea, 648, 668
Frog heart, anatomy of, 724

Galactose, 9

Gall-bladder, 191

Galvanic current, effect of, on nerves, 105

Galvanometer, 719

Ganglia, peripheral, 618

Gaseous exchange in lungs, 411

in tissues, 410

Gases of blood, relation of, 403
tension of, 408

carbon dioxid, 407

nitrogen, 407

oxygen, 405
Gastric digestion, 163

fistulae, 1 66

glands, 165

juice, 1 68

mode of secretion, 170
physiologic action of, 172
Globulins, 15

Glossopharyngeal nerve, 597
Glycogen, 8, 451

Glycogenic function of the liver, 448
Gluco-proteins, 17
Gmelin's test for bile pigments, 193
Goll, column of, 500
Gowers' antero-lateral tract, 499
Graafian follicle, 688
Graphic method, 714
Green vegetables, 132

Hairs, 489

Hearing, sense of, 677

Heart, 261

action of sympathetic nerve on, 307, 310

of vagus nerve on, 308, 311
auriculo- ventricular bundle, 268
beat, nature of the stimulus, 297
actidn of inorganic salts, 298
frequency of, 286
theory of, 300
of the excised heart, 289
blood-supply, 286

Heart, causes of the variations of, 286
course of blood through, 264
cycle of, 272

idio- ventricular rhythm, 294
intracardiac nerve-cells, 303
in tra ventricular pressure curve, 278
mechanics of, 270
modifications of beat due to the action of

drugs, 317

muscle-band of His, 268
muscle-fibers of, 266
negative pressure of, 281
nerve, mechanism of, 303
orifices and valves, 265, 266
origin and distribution of the sympathetic

nerves to, 395
origin and distribution of the vagus nerve

to, 395

physiologic anatomy of, 261

relative function of auricles and ventricles,
276

sounds, 285

synchronism of the two sides, 278

valves, action of, 274

work done by, 366
Heart- muscle, properties of, 289

automaticity, 297

conductivity, 291

irritability, 289

response to action of an artificial stimulus,
300

rhythmicity, 296

tonicity, 296
Heat dissipation, 434, 436

production, 435

relation to work, 437

rigor, 65

Helmholtz's theory of color perception, 674
Hematin, 251
Hemianopsia, 549, 578
Hemoglobin, 243

absorption spectra of, 248

chemic composition of, 244

compounds of, 250

quantity of, 244

Hemoglobinometer, Gowers', 246
Hemometer, v. Fleischl's, 247
Hering's theory of color perception, 675
Histons, 14
Hormone, 171
Horopter, 670
Hypermetropia, 664
Hyperpnea, 423
Hypoglossal nerve, 606

Incus, 679

Indol, 474

Induced currents, 712, 713

Inductorium, 710

Infra-proteins, 18

Insalivation, 143

nerve mechanism of, 149
Inspiration, 395

movements of thorax, 389

muscles, 388
Insula, 541
Intercostal muscles, 384

732

INDEX.

Internal capsule, 527

functions of, 535

secretion, 454
Intestinal digestion, 181

fermentation, 201

juice, 183

physiologic action of, 190

movements, 196

nerve mechanism of, 198

rhythmic segmentation, 196
Intraauricular pressure, 281
Intracranial circulation, 560

mechanism of, 561
Intrapulmonic pressure, 386
Intra thoracic pressure, 386
Intra vascular coagulation, 259
Intraventricular pressure, 278
Invertase, 190
Iris, 643

functions of, 662

nerve mechanism of, 580, 583
Iron of the body, 19
Irritability of muscles, 54

of nerves, 98
Island of Langerhans, 185

of Reil, 541

Isometric myogram, 66
Isotonic myogram, 61
Isthmus of encephalon, 52 1

functions of, 527

Jacobson's nerve, 598
Joints, 45

classification of, 45

Keith-Flack node, 276
Kidney, 473

histology of, 476
Knee-jerk, 509
Kymograph, 716

Labyrinth of ear, 679
Lacrimal glands, 676
Lactation, 702
Lacteals, 219
Lactose, 10
Language, 556
Larynx, 620

nerve mechanism of, 628

structure of, 620

Lateral columns of the spinal cord, 499
Law of contraction, 108
Lecithin, 193
Lemniscus, 513, 521, 522
Lens, crystalline, 649
Lenticular nucleus, 525
Leukocytes, 252

chemic composition of, 250

classification of, 254

functions of, 255

number of, 282

origin of, 255

physiologic properties, 253
Levers, 81
Levulose, 9
Limbic lobe, 540

Liver, 191, 444

elaboration of bile by, 447

formation of urea in, 453

functions of, 447

influence of the nerve system on, 447, 450

production of glycogen, 448
Localization of functions in cerebrum, 546
Lungs, structure of the, 379
Lymph, 210

absorption of, 214

composition of, 211

functions of, 213

movement of, 220

physical properties, 210

production of, 211
Lymph-capillaries, 206
Lymph-glands, 207
Lymph-nodes, 206
Lymph- vessels, 206
Lymphocytes, 209, 254

Macula lutea, 645
Malleus, 679
Maltose, 10
Mammary glands, 441
Mastication, 138
muscles of, 180
nerve mechanism of, 141
Meats, composition of, 128
Medulla oblongata, 519

reflex activities of, 533
Meibomian glands, 676
Membrana tympani, 678

function of, 684
Menstruation, 691
Metabolism on protein diet, 127

on fat and carbohydrate diet, 127
Methemoglobin, 251
Migration of leukocytes, 254, 363
Milk, 442

composition of, 129, 442
mechanism of secretion, 443
Moist chamber, 718
Mosso's plethysmograph, 360

spygmomanometer, 344
Motor area of chimpanzee brain, 552
of human brain, 554
of monkey brain, 549
oculi nerve, 579
Mouth digestion, 138
Movements of the eyeball, 671
of the intestines, 196, 200
of the lower jaw, 140
of the lungs, 392
of the stomach, 177
Muscle, action currents of, 79
contraction, 58

chemic phenomena of, 73

electric phenomena of, 76

graphic representation of, 60

modifying influences of, 63

phenomena following stimulation, 58

physical phenomena of, 58

rigor mortis, 74

summation effects, 68

tetanus, 69

thermic phenomena of, 75

INDEX.

733

Muscles, electric currents from, 76

electric currents, negative variation of, 77

energy, source of, 74

fatigue, 65

groups, special action of, 80

sense, 637

sound, 73

spindle, 637

stimuli, 56

tissue, 48

chemic composition of, 51
elasticity, 53, 59
histology of, 49, 83
irritability, 54
physical properties of, 52
physiologic properties of, 52
tonicity, 54

My enteric plexus, 183, 198
Myopia, 663
Myosinogen, 15, 52
Myxedema, 456

Nerve, abducent, 589

acoustic, 595

facial, 590

glosso-pharyngeal, 597

hypoglossal, 606

irritability, 98

oculo-motor, 579

olfactory, 574

optic, 575

pneumogastric, vagus, 599

spinal accessory, 604

stimuli, 99

trigeminal, 585

trochlear, 584
Nerve impulse, 100
Nerve-muscle preparation, 101, 723
Nerve system, functions of, 493
Nerve tissue, 87

histology of, 87
Nerves, autonomic system of, 608

chemic composition and metabolism of, 92

classification of, 97

degeneration of, 95

development of, 95

effects of galvanic current on, 105

electric currents of, 102

electric currents of, negative variation of,
103

electric excitation of, 101

electric phenomena of, 102
action currents, 104
diphasic action currents, 104

peripheral endings of, 93

physiologic properties of, 98

pilo-motor, 97

polar stimulation of, 108, 109

relation of, to central nerve system, 92

stimuli of, 99
Neuron, 87

Nicotin, actions of, 317, 368
Nucleo-proteins, 17
Nucleus caudatus, 525

cuneatus, 521

gracilis, 521

lenticularis, 525

Nutritive supply of the embryo, 699

Oculo-motor nerve, 579
Ohm's law, 722
Olein, ii

Olfactory nerve, 574
Oncograph, 482
Oncometer, 482
Ophthalmic ganglion, 618
Optic constants, 650

nerve, 575

thalamus, 526

functions of, 534
Optogram, 669
Organ of Corti, 682
Osazones, 10
Osmometer, 224
Osmosis, 222
Osmotic pressure, 223
Ossicles of ear, 679
Otic ganglion, 619
Ovary, 688
Ovulation, 691
Ovum, 689
Oxygen in blood, 405

in tissues, 409

quantity absorbed daily, 401, 413
Oxyhemoglobin, 250

Pacinian corpuscle, 634
Palmitin, 11
Pancreas, 184
Pancreatic juice, 186

mode of secretion, 186

physiologic action of, 188
Parathyroids, 458

effects of removal, 458
Partial pressure of gases, 405
Parturition, 701
Pepsin, 168
Peptones, 174

Peripheral organs of the nerve system, 491
Peristalsis, 158, 196
Perspiration, 486
Petrosal nerves, 91, 593, 594
Pettenkofer-Voit respiration apparatus, 412
Pexin, 169
Phagocytosis, 256
Phloridzin diabetes, 453
Phonation, 620

mechanism of, 626
Phospho- proteins, 16
Physiology of the cell, 23

of movement, 38
Pilo-motor nerves, 97, 489
Pituitary body, 458

effects of total removal, 460
of anterior lobe removal, 461
of posterior lobe removal, 462
of injection of extracts, 463
Placenta, 698

Plasma of blood, composition of, 232
Pleura, 384
Pneumatograph, 399
Pneumogastric nerve, 599
Pneumograph, 396

734

INDEX.

Polar stimulation, 108

of human nerves, 109
Pons varolii, 522

functions of, 533
Portal vein, 216
Postures, 82
Presbyopia, 663
Prosecretin, 187
Protamins, 14
Proteins, n

chemic composition, n

color reactions, 19

physical properties, 13

precipitation tests, 19

structure of, 13
Ptyalin, 149
Pulmonary blood-vessels, 381

vascular apparatus, 364

ventilation, 402
Pulse, 354

frequency, 357

volume, 281

wave, velocity of, 356
Punctum proximum, 66 1

remotum, 66 1
Pyramidal tracts of spinal cord, 498, 499

Reaction of degeneration, 112
Red corpuscles, 234

chemic composition of, 243

effects of reagents, 239

function of, 241

life history of, 242

number of, 237

of vertebrated animals, 240
Reduced hemoglobin, 249
Reflex action, 112

laws of, 508

Refractory period of the heart, 301
Regnault's and Reisset's respiration apparatus,

412

Relation of gases in the blood, 404
Renhin, 169
Reproduction, 688

Reproductive organs of the female, 688
Reproductive organs of the male, 692
Reserve air, 398
Residual air, 398
Respiration, 377

changes in composition of air during, 400

changes in composition of blood, 403

changes in tissues^ 408

chemistry of, 400

expiratory forces and muscles, 391

first inspiration, 418

mechanism of gaseous exchange, 411

nerve mechanism of, 415

number per minute, 396

reflex stimulation of, 418

effects of a change of pressure of the blood
gases on, 422

total respiratory exchange, 412

volumes of air breathed, 397
Respiratory apparatus, 677

movements, 388

muscles, 388

effects of, on arterial pressure, 427

Respiratory muscles, effects of, on the flow of
blood through the thoracic vessels, 426
of upper air passages, 395
pressures, 413
quotient, 401,
rhythm, 396

modification of, 423
Cheyne-Stokes, 426
sounds, 399
. types, 395
Retina, 645

functions of, 667
Retinal image, 649
size of, 655
Rheocord, 709
Rhythmic segmentation, 196
Rigor mortis, 75
Rima glottidis, 620
respiratoria, 625
vocalis, 625

Routes of the absorbed food, 220
Rush peristalsis, 197

Saccharose, 9
Saliva, 145

physiologic action of, 148
Salivary glands, 143

histologic changes in, during secretion.

*47

nerve mechanism of, 149
Sebaceous glands, 490
Sebum, 489
Sclero-proteins, 16
Secretin, 187
Secretion, 438

internal, 454
Semen, 694

Semicircular canals, 680
Sensor areas of human brain, 553

of monkey brain, 547
Serum, 232

Setchenow's center, 512
Sight, sense of, 642
Sino-auricular node, 270
Skatol, 474

Skeleton, physiology of, 44
Skin, 486, 632

nerve endings in, 632

reflexes, 508
Sleep, 563
Smell, sense of, 640
Sodium glycocholate, 192

taurocholate, 192
Special senses, 631
Spectroscope, 248
Speech, 628
Spermatozoa, 694
Spheno-palatine ganglion, 619
Sphygmograph, 357
Sphygmomanometer, 344, 346
Spinal accessory nerve, 604

cord, 494

encephalo-spinal conduction, 516
functions of, 503
as a conductor, 512
as an independent center, 503
nerve-cells, classification of, 496

INDEX.

735

Spinal cord, nerve-fibers of, 498

classification of, 498
reflex actions of, 505
reflex irritability of, 510
relation of spinal nerves to, 501
segmentation of, 502
spinal nerve roots, functions of, 501
spino-encephalic conduction, 512
structure of gray matter, 494
structure of white matter, 498
tracts of, 499
Spirometer, 397
Splanchnic nerves, 617
Spleen, 467

functions of, 467

Stanton's sphygmomanometer, 345
Stapes, 679
Starch, 7

digestion of, 148
Starvation, 123
Stearin, n

Stereognostic area, 555
Stomach, 163

movements of, 177
nerve mechanism of, 179
Suprarenal capsules, 463
Sweat-glands, 487
Sweat, influence of nerve system on production

of, 488
Sympathetic nerve system, 608

functions of the cervical portions, 616
functions of the lumbosacral portions,

617

functions of the thoracic portion, 616
functions of peripheral ganglia, 618
ganglia, relation to visceral structures, 612
relation to the central nerve system, 614

Taste buds, 639

nerve of, 638

sense of, 638
Teeth, 138
Tegmentum, 522
Temperature of body, 429

regulation of, 434

sense, 635

Tendon reflexes, 509
Tension of gases in blood, 408

tissues, 410
Tensor tympani muscle, 679

functions of, 684
Testicles, 692
Tetanus, 69

experimental, 72

pathologic, 72

pharmacologic, 72

physiologic, 71
Thoracic duct, 209
Thorax, 381

dynamic condition of, 387

mechanic movements of, 385

static condition of, 385
Thyroid gland, 455

effects of removal, 445

cretinism, 446
Tidal air, 398
Tissue spaces, 205

Tongue, 638

Total carbon-dioxid exhaled, 413

oxygen absorbed, 413

respiratory exchange, 412
Touch, sense of, 632
Trachea, 378

Traube-Hering waves, 428
Trochlear nerve, 584
Trypsin, 188
Tiirck, column of, 498
Tympanum, 677

Upper air-passages, respiratory movements of,

395
Urea, 453

seat of formation, 453, 472
Uric acid, 473
Urine, 470

composition of, 471
mechanism of secretion, 479

influence of blood composition, 483
influence of nerve system, 483

of blood-pressure, 480
Urination, 484

nerve mechanism of, 485
Uterus, 690

Vagina, 691
Vagus nerve, 606

influence on heart, 308, 311

seat of action,
Valves of heart, 265
Vas deferens, 693
Vascular apparatus, 319

glands, 454

hydrodynamic considerations, 322

stream bed, 328

nerve mechanism of, 366
Vaso-motor center, 371, 372

direct stimulation, 373

nerves, 367

reflex stimulation, 373
Veins, 322

structure and function, 322
Velocity of blood, 349
Venous circulation, 363

pulse, 358

Vertebral column, 47
Vesiculse seminales, 693
Villi, 214

functions of, 216
Visceral muscle, 83

functions of, 84

properties of, 84
Viscosity of blood, 230
Vision, 649

accommodation, 657

astigmatism, 665

binocular, 670

color perception, 673

functions of retina, 667

hypermetropia, 664

myopia, 663

presbyopia, 663
Visual angle, 655
Vital capacity of lungs, 398

736

INDEX.

Vocal bands, 623
sounds, 627
Voice and speech, 628
Volume pulse, 360

Wernicke's pupillary reaction, 583
White blood-corpuscles, 252

function of, 256

migration of, 369

origin of, 255

White physiologic properties, 253

varieties of, 2 54
Wrisberg, nerve of, 592

Yellow spot, 645

Zymogen, 149

pepsinogen, 168
ptyalogen, 149
trypsinogen, 190

DATE DUE SLIP

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THIS BOOK IS DUE ON THE LAST DATE
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