NORMAL HISTOLOGY

BY

EDWARD K. DUNHAM, Ph.B., M.D.,

PROFESSOR OF GENERAL PATHOLOGY, BACTERIOLOGY, AND HYGIENE IN THE UNI\ liRSITY
AND BELLEVUE HOSP.TAL MEDICAL COLLEGE, NEW YORK.

THIRD EDITION, REVISED AND ENLARGED.

ILLUSTRATED WITH 260 ENGRAVINGS.

LEA BROTHERS & CO.,

NEW YORK AND PHILADELPHIA.

1904.

fpfflTME-NT -OF SNUNGV

LIBRARY

Entered accordiug to Act of Congress in the year 1904, by

LEA BROTHERS & CO.,

in the Office of the Librarian of Congress, at Washington. All rights reserved.

882019

ELCCTROTYPEO BY
WESTCOTT & THOMSON. PHILADA.

PRESS OF
WILLIAM J DOHNAN. PHILADA.

PREFACE TO THE THIRD EDITION.

The general plan and scope of this outline of histology are the
result of experience in teaching this subject to students of medicine
under conditions which require economy of time. In order to
accomplish the greatest amount of instruction under these circum-
stances it seemed necessary to present, early in the course, certain
generalizations which might be kept constantly in mind and assist
the memory in retaining facts by showing their logical correlation.
The broad ideas of almost universal applicability which were
chosen for this purpose were : first, the conception of the cell as the
active constituent of tissues ; second, the general rule that the ele-
mentary tissues are composed of cells and intercellular substances,
and that their differences depend upon the proportions or characters
of those constituents ; third, that the structural details of the dif-
ferent tissues are intimately correlated to their usefulness — i. e.,
that function and structure are mutually dependent upon each
other, being but two aspects of a single device or arrangement ;
and, fourth, that the ability of tissues to perform active functions
is, in the main, roughly proportional to the number or size of the
cells entering into their structure.

The author believes that his experience has shown that the early
introduction of these fundamental ideas has been a distinct aid to
the student ; and it is his belief that such would also be the case no
matter to what limits the course of instruction in this subject might
be extended.

In the elaboration of these ideas the author has been in the habit
of arbitrarily dividing the activities of the cells of the body into
four theoretical groups : the reproductive activities, leading to the
production of new cells; the formative activities, through which the
structural modifications resulting in the formation of different varie-
ties of tissue were brought about ; the nutritive activities, maintain-
ing the integrity of the tissues already formed ; and the functional

3

4 PREFACE.

activities, through which the tissues serve the whole organism of
which they are constituent parts.

Assuming that histology should invariably be taught largely by
practical work and experience in the laboratory, and recognizing
the fact that it must have a place in the beginning of a medical
curriculum when the student's technical facility is least developed,
the writer believes that the best way to teach the subject is by
means of demonstrations at which the students have ample oppor-
tunities to become familiar with the structural details of the best
attainable specimens so prepared as to reveal the presence and
arrangement of the tissue-elements. In addition to such a course
of demonstrations he should receive practical instruction in histo-
logical technique, during which he may become versed in the
methods employed in the preparation of the specimens he uses in
the demonstration course. This work may be so arranged as to
permit his preparing a collection of specimens for himself.

With such related courses of instruction in mind, the writer has
added a section on Histological Technique to the present volume.
In this part of the book he has endeavored to make clear those
various methods of preparing tissues for microscopical study which
he has found to yield excellent results, not only in his own hands,
but also when employed by those who have had no previous experi-
ence in such work. The more complicated methods, requiring such
experience, have been omitted.

The student occasionally desires to consult the original sources of
information which has been published. To encourage this prac-
tice, the author refers him to The Journal of the Royal Microscopical
Society, Arc/iiv fur mikroskopische Anatomie, the Zeitschrift fur uris-
senschaftlichi Mikroskopie, Merkel and Bonnet's Ergsbuisse der Anat-
omie und Enturicklungsgeschichte, and Anatomische TIefte, in which
abstracts of the current literature will be found. Admirable text-
books upon technique are Lee's Microtomists} Vade Mecum, of which
a more recent revised edition is Lee and Mayer's Grundzuge der
mikroskopisehen Technik, and Mai lory and Wright's Pathological
hnique.

E. K. D.
- Twenty-sixth St.,
New York, May, 1<i00.

CONTENTS.

PAGE

INTRODUCTION . 17

CHAPTER I.
THE CELL 27

CHAPTER II.

THE ELEMENTARY TISSUES 43

CHAPTER III.

THE EPITHELIAL TISSUES 47

CHAPTER IV.
THE CONNECTIVE TISSUES 65

CHAPTER V.
TISSUES OF SPECIAL FUNCTION . 86

CHAPTER VI.

TISSUES OF SPECIAL FUNCTION (Continued) 100

CHAPTER VII.

THE ORGANS 112

CHAPTER VIII.
THE CIRCULATORY SYSTEM 114

CHAPTER IX.

THE BLOOD AND LYMPH 128

G CONTENTS.

CHAPTER X.

PAGE

THE DIGESTIVE ORGANS 136

CHAPTER XI.
THE LIVEE 154

CHAPTER XII.
THE URINARY ORGANS 161

CHAPTER XIII.
THE RESPIRATORY ORGANS 170

CHAPTER XIV.
THE SPLEEN 184

CHAPTER XV.
THE DUCTLESS GLANDS 188

CHAPTER XVI.
THE SKIN 204

CHAPTER XVII.
THE REPRODUCTIVE ORGANS 215

CHAPTER XVIII.
THE CENTRAL NERVOUS SYSTEM 248

CHAPTER XIX.
THE ORGANS OF THE SPECIAL SENSES 200

HISTOLOGICAL TECHNIQUE.

CHAPTER XX.

PRACTICAL SUGGESTIONS FOR THE CARE \XD USE OF THE

MICROSCOPE MICROSCOPICAL TECHNIQUE 279

NORMAL HISTOLOGY.

INTRODUCTION.

During life all parts of the human body are the seat of constant
activity. This is a fact too readily overlooked by the student who
gains his knowledge of the structures of the body by a study of the
tissues after death. To make that study of use to him in his medi-
cal thinking he should constantly bear in mind that he is viewing
the mechanism of the body while it is at rest, and, furthermore,
that the methods employed in the study of the minute structure of
the parts not only arrest the normal activities of those parts, but
expose them to mutilation. He must, therefore, constantly supple-
ment the knowledge of structure he gains by his histological studies
by recalling to mind and applying that which he has acquired by
a study of physiology, habitually associating his ideas of structure
and functional activity, until he can hardly think of what a struct-
ure is without at once recalling what it does. This he cannot do
till he has mastered at least the general outlines of systematic
anatomy and of physiology. Those two fundamental subjects are
brought together by an intelligent study of the minute structure of
the body, histology, which, for this reason, has also and appro-
priately been called physiological anatomy.

But the student of medicine must go beyond this. To the con-
ception of the body during health, which he has formed by this
thoughtful method, he must then add a conception of the influence
exerted, both on the structure and activities of the body, by ab-
normal conditions which disturb or thwart the usual working of
that complex mechanism. The more closely he can make those
conceptions agree with observed facts, the more perfect will become
his ability to interpret the physical signs and symptoms of disease,

2 17

18 INTRODUCTION.

and the clearer will grow liis insight into the causes and tendencies
of the processes of which they arc an expression.

In all his studies he must seek not merely to train his powers
of observation ; he must endeavor to cultivate his ability to inter-
pret what he sees ; to deduce the processes and causes that have
wrought the results he perceives, and to compare those deductions
with the conceptions of living things he has already formed, so that
his idea- may remain in perfect accord with one another as his grasp
of the subject enlarges. By so doing he may hope to create a life-
like mental picture of the body both in health and during disease.

The activities of the body involve changes in the substances of
which it is composed. Some of these changes are always destruc-
tive in character — that is, they result in chemical rearrangements
which convert more complex combinations of less stable nature into
simpler combinations of greater stability. Such chemical changes,
whether they take place within the body or in external nature,
among organic or inorganic substances, are always accompanied by
a liberation of energy hitherto locked up or stored in latent or
potential form in the compounds of higher complexity. It is this
liberated or kinetic energy which is utilized by the bodily mechan-
ism for the performance of internal or external work. AVhen
directed in various ways and operating through different structures,
this energy occasions visible movement, appears as heat, etc., or
passes again into the latent form in the elaboration of more com-
plex chemical substances from those of simpler constitution.

These associated transformations of matter and energy involve a
continual loss to the bodily economy. The stock of energy is dim-
inished during the execution of external work and by the dissipa-
tion of heat. The store of useful chemical substances is reduced
by their progressive conversion into compounds that are insuscep-
tible of further utilization, and which, in many cases, may act injuri-
ously upon the structures of the body. Under normal conditions
such substances are eliminated from the bodv.

It i- evident, then, that the body is constantly suffering a loss of
both energy and matter. This loss must be made good if the
activities of the body are to be maintained, and this is accomplished,
during health, through the absorption of fresh material, containing
latent energy, from the food taken into the body.

The activities of the body are not the same in all its parts. They
are all alike in one particular — namely, that each part must main-

INTB OD UCTION. 1 9

tain its own nutrition, incorporating the food-materials that are
accessible to it and using them in such a way as to keep its struct-
ure in a normal condition. But, aside from this duty which is com-
mon to all, each part has a duty to perform for the good of the
whole organism ; and, as we shall see, this duty often appears to be
paramount, the activities which it necessitates being carried on
even if they involve a sacrifice in the nutrition or structure of the
individual part.

Each part of the body has some particular kinds of work assigned
to it, which constitute its functions, and which it performs for the
benefit of the whole body. The development and life-history of
each part has direct reference to those functions, through which it
co-operates with all the other parts in maintaining the integrity and
normal activities of the whole body, all the parts being interde-
pendent upon each other and subservient to the general needs.

The foregoing considerations prepare us for the fact that the
structure of the various parts of the body differs in its details.
The study of those finer details can only be pursued with the aid
of the microscope, for the microscopical constituents of the tissues
are the elements which confer upon them their particular properties
and powers. This study is called histology.

Investigation has shown that there is one form of tissue-element
which is always present in all parts of the body. This is the cell.
It does not always possess the same form or internal structure, but
in all its variations the same general plan of construction is adhered
to. These cells are the essentially active constituents of the tissues.
It is within them that the transformations of matter and energy are
chiefly carried on, and it is due to their activities that the tissues
forming the body are elaborated and enabled to perform their sev-
eral functions. These marvellous powers possessed by the cell
have created our conception of life, and, in spite of eager study,
remain inscrutable. We do not know why a living cell differs from
a dead cell, but we do know that the mysterious vital powers are
only derived from pre-existent living cells and are not antagonistic
to the chemical and physical laws governing unorganized matter.

All the cells of the body are descendants of a single cell, the egg,
from which they arise by successive divisions, and throughout the
existence of the body they retain some of the characters of the
original cell. But as the body develops the cells of the different
parts display divergent tendencies, which finally result in the for-
mation of a considerable variety of tissues, grouped in various ways

20 INTRODUCTION.

to form organs or systems of very different kinds of utility to the
whole organism. This divergent development is known as differen-
tiation and results in a specialization of the different parts of the
body. Its study constitute- embryology, but it will make the com-
prehension of histology easier if some of the simpler and broader facts
derived from a study of development are first briefly summarized.

A new individual arises through the detachment of a single cell,
the ovum (Fig. 1), from the parent organism. This cell,' after
iinderffoine certain modifications and union with another analo-
gously modified cell (see Maturation of the Ovum), divides

Fig. 1.

.- -^ 9

/,_...

-■'..

Section of human ovum and its immediate surroundings within the ovary. (Nagel.) a, zona
pellucida; b, cytoplasm of the ovum ; c, granules and globules of stored food materials
within the cytoplasm, collectively known as the metaplasm or deutoplasm; d, germinal
vesicle or nucleus of the ovum containing, in this case, two germinal spots or nucleoli ;
e, zone of epithelial culls immediately surrounding the ovum ; /, cells of the discus pro-
ligerus; g, perivitelline spaces separating the zona pellucida from the cytoplasm of the
ovum.

into two cells, which, even at this stage of development, differ

slightly from each other. These daughter-cells in turn divide in
two, and this process of division i- continued, each cell giving rise
t « » two new cells, until a considerable aggregate of cells has resulted
(Fig. 2). Then the cell- assume a definite arrangement into layers.
Some become disposed in a superficial layer enclosing the rest of
the cells and a body of fluid. This layer is called the primitive
ectoderm. The remaining cells accumulate in an irregular laminar
mass beneath the primitive ectoderm at the site of the future em-
bryo. This mass of cell- i- the primitive entoderm. Thus, at

INTRODUCTION.

21

this stage of development, there is a eellular sac, containing fluid,
with a reinforcement of its wall at the region occupied by the primi-
tive entoderm (Fig. 3).

Fig. 2.

Segmented egg of Petromyzon Planeri : Surface view of the collection of cells. The nuclei

are invisible. (Kupffer.)

Subsequent to these events a third layer of cells becomes inter-
posed between the primitive ectoderm and entoderm. Most of its

Fig. 3.

Ovum of rabbit : a, primitive ectoderm in section ; b, primitive ectoderm, surface view ;
e, primitive entoderm ; d, dividing cell of the ectoderm, (van Beneden.)

cells are derived from those of the primitive ectoderm, but the

22 INTRODUCTION.

primitive entoderm may also participate in its formation. This
third layer is called the mesoderm. Soon after its formation, the
mesoderm divides ;it the sides of the embryo into two layers — a
parietal, which joins the under surface of the ectoderm, and a vis-
ceral, attached to the upper surface of the entoderm. The space
between these two layers is occupied by fluid, and is destined to
form the future body -cavities. In the axis of the embryo the three
earlier layers remain in continuity, forming a cellular mass around
the site of the future spinal column (Fig. 4).

Fig. 4.

a

' mend

Embryo of Necterus in cross-section. (Piatt.) eel., ectoderm : mend., mesoderm ; end., ento-
derm ; a, neural groove ; ch, site of future spinal column.

From these three embryonic layers of cells the body of the foetus
is developed. The entoderm, with the visceral or lower layer of the
mesoderm, turns downward and inward to meet its fellow of the
opposite side and form the alimentary tract. The ectoderm and
parietal or upper layer of the mesoderm also turn downward and in-
ward, outside of the alimentary tube, and join those of the other side
to form the walls of the body.

Meanwhile, the upper surface of the ectoderm over the axis of the
embryo becomes furrowed. The edges of this furrow grow upward,
deepening the groove between them, and finally arch over it and
coalesce, forming a canal around which the central nervous system
i- developed (Fig. 5). Traces of this canal persist through life as
the central canal of the spinal cord and the ventricles of the brain.

The embryonic layers have a deeper significance than the mere
furnishing of the architectural materials from which the body is
built tip. They are evidence- of a distinct differentiation in the
development of the cells of which they are composed. The ecto-
derm gives rise to the functional part of the nervous system and the
special Benses and to the epithelial structures of the skin and its
appendages. The cell- of the mesoderm elaborate the muscular

INTRODUCTION.

23

tissues and that great group known as the connective tissues, the
vascular system, etc., and the entoderm contains the cells that
build up the linings of the digestive tract, including its glands,
and of the respiratory organs. It appears, then, that this divi-
sion of the cells of the embryo into three layers marks a distinct
difference in the destinies of the cells composing those layers.
This distinction persists through life, the tissues arising from a given
layer showing, in general, a closer relationship to each other than
the tissues arising from different layers. But this relationship is
not always revealed by a similarity in structure, for the latter is
determined by the functions the tissues are destined to perform,
and tissues of like function acquire a similarity in structure. Thus,
for example, the neuroglia in the central nervous system resembles

Fig. 5.

Cross-section of fish embryo. (Ziegler.) a, neural canal, cells enclosing it not represented;
b, chorda dorsalis, site of future spinal column ; ao, aorta ; Bf, external layer of meso-
derm ; c, c, body -cavity ; d, alimentary canal, not yet completely closed * *, passes
through the external layer of the mesoderm to its inner surface : e. deutoplasm, or yolk
of egg.

some of the connective tissues, although one develops from the
ectoderm and the other from the mesoderm ; and the ganglion cells
of the central nervous system differ greatly in structure from the
epithelium of the skin, nails, etc., and the cells of the neuroglia,
notwithstanding the fact that they all spring from the cells of the
ectoderm. The explanation is to be sought in the similarity of the
usefulness of neuroglia and connective tissue and the difference in
the functions of ganglion cells and those of the other tissues email-
ating from the ectoderm.

During the early stages of development the cells of the germinal
layers are very similar in character, although, as we have seen,

24 INTRODUCTION.

their potential qualities are quite diverse. As growth proceeds,
they begin to vary in size, shape, and internal structure in the dif-
ferent parts of the foetus. Their relative positions become modi-
fied. The primitive organs are defined and the tissues of which
they are composed become elaborated. In this elaboration of the
bodily organs, cells from the different embryonic layers enter into
very intimate cooperation, nearly all the organs of the body con-
taining structures derived from all three layers; thus, the lungs,
for example, contain epithelium which is descended from the ento-
derm ; fibrous tissue, cartilage, and bloodvessels from the mesoderm,
and nerves and ganglia arising from the ectoderm.

The elaboration of the tissues is wrought by the cells, which dis-
play what is called their formative powers in the production of
materials of various sorts which lie between them, and are called
the intercellular substances. The amount and kind of intercellular
substance vary, each form of tissue having its own peculiarities in
this respect, dependent upon the role it is to play in the general
economy. Some of the tissues perform functions which require the
active processes that can be carried on only in cells, and in these
the intercellular substances are either small in amount and appar-
ently structureless, as in epithelium, or their place is taken by a
tissue of separate origin, while the cells, relieved of the necessity
for exercising their formative powers in this direction, become
highly specialized to meet the functional demands imposed upon
them. This development is met with in the muscular and nervous

tiSBUeS.

Other tissues of the body are of use mainly because of their
physical properties, such as rigidity, elasticity, tensile strength, plia-
bility, etc. These tissues, collectively called the connective tissues,
are essentially passive. They require little or no cellular activity
for the performance of their functions, and it is in the elaboration
of these tissues thai the cells exercise their most marked formative
power- during the development of the body, causing the deposition
of intercellular substances which possess the requisite physical char-
acter — rigidity and elasticity in the case of bone, pliability and ten-
sile strength in the case of ligamentous structures, etc. As these
substances are perfected, the cells decrease in activity, until they
merely preside over the integrity of the intercellular substances they
have already produced.

INTRODUCTION. 25

It may be well to point out here a distinction that divides the
tissues of active cellular function into two groups. The first group,
including the various modifications of epithelium, displays its ac-
tivity in the elaboration of material products, taking the form of
either new cells which are continually being produced, or of certain
chemical substances which appear as a secretion. The second
group, comprising the muscular and nervous tissues, exercises its
functional activities in the storage of latent energy in such sub-
stances of unstable chemical nature and in such a manner that it
can be liberated when required and directed toward the accomplish-
ment of some definite purpose. The functions of both groups
require an active intracellular metabolism, resulting in the forma-
tion of particular chemical substances. In this they are alike.
But in the first group the production of those substances is, in
itself, the functional purpose of the process, while in the second
group those substances are merely a means for holding energy in
the latent condition. If we may so express ourselves, the first
group utilizes energy for the elaboration of material, the second
group elaborates material for the utilization of energy.

In the adult, under normal conditions, each kind of cell, if it
reproduce at all, gives rise to cells only of its own kind. But when
the conditions are morbid, this is not always the case, the progeny
of a given cell under abnormal conditions may appear less highly
specialized than the parent stock or may differentiate in a some-
what different way. These departures from the normal appear
never to transcend the degree of specialization that is marked by
the formation of the three embryonic layers in the early history
of development ; for example, epithelium which springs from
either the entoderm or ectoderm never gives rise to bone or any
other variety of connective tissue normally developed from the
mesoderm.

The cells of the body differ greatly in their individual perma-
nency. Some are apparently destined to endure throughout the life
of the individual. Among these are the nerve-cells and those form-
ing muscular tissue. Other cells have a comparatively short life-
history. This is true of many of the epithelial cells, which undergo
transformations, ultimately leading to their destruction — e. g., the
superficial cells of the epidermis and those cells which yield the
secretion of sebaceous glands ; or they are destined to proliferate,

26 INTRODUCTION.

Losing their identity in a multitude of descendants that are subse-
quently destroyed in the performance of some useful function.
Finally, some of the embryonic cells form structures of only tran-
Bient utility — e. //., the thymus and Wolffian body — which partially
disappear as the individual becomes older. It follows, then, that we
must regard the tissues of the body as more or less plastic and capa-
ble of some degree of adaptation to the conditions presented by the
whole organism.

CHAPTER I.
THE CELL.

As has been stated in the introductory chapter, the cells of the
body are not all alike. Most of them have undergone modifications
fitting them for the performance of some definite function, and the
majority of them are in consequence not appropriate objects for a
study of the general characters of a cell. The extent to which this
modification has affected the visible structure of the cell is, how-
ever, very different in the different tissues, and in some of them the
cells retain so much of their original embryonic appearance as to
closely resemble the unspecialized cell.

This is true of the cells of some varieties of epithelium. But,
though in appearance they give little evidence of specialization, in
their functional activities they display very marked modifications of
the powers of the primitive cell. Some of those powers, perhaps
the nutritive, perhaps the secretory, have become exaggerated, while
others, e. g., the locomotor y, or reproductive, have fallen into abey-
ance, or suffered almost total extinction.

On the other hand, it is obvious that such cells as constitute the
whole body of unicellular animals must retain all the powers essen-
tial to a living cell in relatively equal states of development. No
one of them can be extinguished or thrown out of its proper bal-
ance with respect to the others if the cell is to remain normal.
And yet, even among the unicellular organisms, certain parts of the
cell may be very evidently specialized for the performance of par-
ticular functions. For example, the cilia of infusoria have the
power of executing much more rapid movements than the other
parts of the same cell. And it is probable that all protozoa, i. e.
unicellular animals, possess similar, though less obvious and in-
ternal, heterogeneity of constitution.

The less the degree of specialization or differentiation in the
structure of an organism, the less highly developed is the functional
activity of which it is capable, and the less perfect its ability to
cope with possible unfavorable environment. The value to the

27

28

NORMAL HISTOLOCY

whole organism of a diversity in its parts is, therefore, unquestion-
able, and the higher we go in the animal kingdom, the greater we
find the development of this diversity, coupled with a more and
more perfectly adjusted co-operative interdependence of the differ-
ent parts of the body.

In the protozoa the single cell does all the work of the whole
organism. In the multicellular animals, the metazoa, this work is
distributed among the component cells of the body, each of which
has developed an efficiency for performing its special work that
would be incompatible with a wider range of duties.

It is quite impossible to find in nature any example of a cell
devoid of all individual peculiarities attributable to differentiation
or specialization. We must, therefore, study several varieties of

Fig. 6.

Ama-ba pellucida. (Frenzel.) a, ectoplasm; b, endoplasm ; c, nucleus : '', nucleolus ; e, large-
contractile v f, incorporated foreign body; g,g, pseudopodia. (It is impossible in

a line-drawing to reproduce the great translucency of the hyaloplasm and the smaller
pseudopodia. The outlines of the latter are so faint that it frequently requires close
attention to detect them.)

cell in order to gain an ideal conception of such a cell. This accom-
plished, we may consider those cells which occur in nature as special
modifications of thai type.

Perhaps the simplest cell leading an independent existence is the
protozoon, amoeba (Fig. 6). There are several species of amcebse
which differ in their morphology, showing that even these simple
organisms display considerable differences in structure. These ani-
mals are widely distributed in moist earth, upon the surfaces of
aquatic plants, and in the soil at the margins of ponds and sluggish,
stream-.

THE CELL. 29

The body of the amoeba consists of a gelatinoid substance which
has received the name protoplasm, or, more definitely, cytoplasm.
Within this cytoplasm and sharply defined from it is a round or
oval, vesicular body, called the nucleus, which in turn contains one
or more particularly conspicuous granules, the nucleoli.

The most superficial layer of the cytoplasm appears perfectly
clear, colorless, and homogeneous. It envelops the rest of the cyto-
plasm, which has a granular appearance. The clear peripheral
portion is distinguished as the " hyaloplasm," or " ectoplasm ;" the
granular internal portion as " spongioplasm," or " endoplasm."
The terms hyaloplasm and spongioplasm are also used in a different
and more restricted sense, as will presently appear.

When viewed under the microscope, the granules of the cyto-
plasm are seen to possess a constant, slight, vibratile motion, the
Brownian movement, to which is added now and then a flowing
movement from one part of the cell to another. At intervals there
is a protrusion of the ectoplasm at some point, extending for some
distance from the body of the cell, a pseud opodium. This may soon
be retracted again, merging with the rest of the ectoplasm, or some
of the endoplasm may flow into the central portion of the pseudo-
podium, converting it into a broad extension of the cell-body. This
may subsequently be withdrawn, or the whole mass of cytoplasm,
with the nucleus, may flow into the pseudopodium, gradually in-
creasing its size, until the whole cell occupies the original site of the
pseudopodium. In this way the animal executes a slow, creeping
locomotion.

These pseudopodial movements and the locomotion occasionally
incident to them appear to be wholly spontaneous, i. e. dependent
upon internal conditions of which we have no knowledge, but they
are manifested only when the amoeba is in contact with some other
bodv, not when it is floating freely in a liquid. They may, how-
ever, be influenced by external circumstances. Certain substances
evidently attract the amoeba, others are either matters of indifference
to it or repel it. The chemical nature of the substance or soluble
materials emanating from it induce these reactions on the part of the
cell. Where this is the case the phenomenon is called chemotropism
or chemotaxis, and may be either " positive" or " negative" accord-
ing to whether the cell is attracted or repelled. Similar reactions
induced by light are included in the term heliotropism, which may
be positive or negative. The effect of gravity in its relations to eel-

30 NORMAL HISTOLOGY.

hilar movements is called geotropism and may have either sign. Cells
may also either seek or be repelled by moisture (hydrotropism), or by
electric currents (galvanotropism). If a pseudopodium comes in con-
tad with some particle in the surrounding medium, it may retreat from
it, appeal- indifferent to it, or he attracted and proceed to incorporate
it. This is accomplished by the cytoplasm flowing around the for-
eign body and coalescing on its further side so as to enclose it. It
is then conveyed to the body of the cell, either by cytoplasmic cur-
rents, by the withdrawal of the pseudopodium containing it, or
by the streaming of the cell-body into that protrusion. The
fate of the particle thus incorporated depends upon its nature,
if it be serviceable as food, it is gradually digested and ab-
sorbed, or such parts of it as are digestible are so utilized, and
the remainder, no longer of use to the amoeba, is extruded from
its body.

These phenomena reveal powers of perception and selection on
the part of this cell which are very closely akin to the intelligence
of more complex organisms. They also demonstrate its power of
assimilating material from without, to serve as nourishment and the
source of the energy which it expends in executing its movements
and in carrying on the chemical processes pertaining to its internal
economy.

At intervals, there appears within the endoplasm a small, clear,
spherical spot. This gradually increases in size and constitutes a
little drop of fluid, sharply defined from the surrounding cytoplasm.
After it has attained a certain size, it suddenly disappears, the cyto-
plasm around it coalescing and leaving no trace of its existence.
Such a clear space, rilled with fluid, within the body of a cell is
called a vacuole, and those which are suddenly obliterated, contrac-
tile vacuoles. Their purpose is not clearly understood, but prob-
ably has to do with a primitive circulatory or respiratory function,
since contractile vacuoles are not observed in the cells of higher
organisms where those functions are carried on by more elaborate
mechanisms. Many unicellular organisms possess contractile vacu-
ole.-, l)in it i- by no means essential or characteristic of the protozoa;
for the majority of species composing that group of organisms are
devoid of such vacuoles.

Eventually the amoeba reproduces its kind by dividing into two
similar cells, each of which grows into a likeness to the parent
individual.

THE CELL. • 31

Let us now compare the amoeba with some other varieties of cell,
in order to learn what they all have in common.

The amoeba has an outer, soft, transparent layer of cytoplasm,
the ectoplasm. This is not present in all cells. In many the
granular cytoplasm has no envelope, but appears to be quite naked.
In other varieties it is enclosed in a distinct membrane.

In the great majority of cells the active streaming of the cyto-
plasm and the pseudopodial protrusions described in the amoeba are
wanting, but the Brownian movement of the granules is more con-
stantly present. The cells have fixed positions and their food is
brought to them, usually in solution, so that the more active move-
ments so essential to the welfare of the amoeba would be superfluous.
For a similar reason, as already stated, they can dispense with the
contractile vacuole.

We learn, then, that when we reduce the cell to its simplest
terms, it consists of a mass of cytoplasm enclosing a nucleus. To
these we must probably add a third essential constituent, the centro-
some, which is a minute granule situated in the cytoplasm. It is so
small that its presence has not been established in all cells, its detec-
tion in many cells being extremely difficult because of the general
granular appearance of the cytoplasm in which it lies. It plays such
an important part, however, in the division of those cells in which
it has been studied, that the inference that it is an essential part of
all cells appears justified.

These three constituents, the cytoplasm, nucleus, and centrosome,
appear to be the essential organs of a cell among which its activities
are distributed (Fig. 7). We do not know how they do their work,
but we have a general conception of the distribution of the work
performed by the whole cell among these three organs.

1. The cytoplasm, which usually makes up the chief bulk of the
cell, especially in those varieties which have active metabolic functions,
appears to be the part of the cell in which the assimilated food is utilized
in the production of chemical substances, either fresh cytoplasm or
some other product, or in the execution of movements or the libe-
ration of energy in other forms. Most of the active processes that
are obvious seem to be carried on in the cytoplasm during the greater
part of the life-history of the cell. Many of these processes involve
highly complex chemical transformations. Some of these are syn-
theses, or the building up of more complex molecules from those of
simpler constitution, and are called anabolic processes or anabolism.

32

NORMAL UlSTOLOCY.

( Others consist in the cleavage of larger molecules into smaller atomic
groups which may variously unite with each other, chemical changes
designated as catabolic (catabolism). Similar cleavages may be
occasioned outside the body by various procedures employed in
chemical manipulations, but usually require powerful reagents in
concentration not met with in the body or the application of heat
greatly exceeding that compatible with life. It is believed, upon
experimental evidence, that these chemical changes within the cell

Fig. 7.

Schematic diagram of a cell: a, ectoplasm composed of hyaloplasm; b, spongioplasm ; c,
chromosome, composed of "chromatin," and forming a part of the intranuclear reticu-
lum ; between these chromatic fibres is the achromatin ; d, hyaloplasm in the meshes of
the spongioplasm ; e, one of the two nucleoli represented in the diagram ; /, one of eight
bodies constituting the metaplasm represented; g, centrosome, with radiate arrangement
hi' die surrounding spongioplasm ; //. nuclear membrane.

are wrought with the aid of ferments or enzymes, chemical sub-
stance- produced by and present in the cell, which have the power
of facilitating chemical reactions and modifying the conditions under
which they take place, so that the plane upon which chemical equi-
librium is estal lished is shifted to a different level in the presence
cf ferments from that which it occupies when the enzymes are
absent, if one may be permitted to use such an illustration. A study
of such transformations constitutes a special field of chemistry, bio-
chemistry, in which great advances are being made.

THE CELL. 33

2. The nucleus appears to preside over the assimilative processes
within the cell. If a cell be subdivided so that the uninjured nu-
cleus is retained in one of the portions, that portion may grow and
become a perfect cell. But the portions that are deprived of a nu-
cleus do not grow, and while they may retain life for a considerable
time, utilizing the assimilated food they retain, eventually perish.

Aside from this assimilative function, the nucleus appears to be
the carrier of hereditary characters from the parent cell to its prog-
eny during the division of the cell. This will become clearer when
the process of cell-division is described.

3. The centrosome appears to be the organ presiding over the
division of the cell. It inaugurates those activities in nucleus and
cytoplasm which result in the production of new cells, and seems to
guide them, at least during the greater part of the whole process.

It is evident, from these statements, that the cell has an exceed-
ingly complex organization, which a simple microscopical study can-
not wholly reveal. Notwithstanding this fact, obvious microscopical
differences are presented by cells which have become specialized in
different directions, and we must know something of the visible
structure of the primitive cell before we can appreciate these depart-
ures from it.

The cytoplasm is not a simple substance. Its constitution is so com-
plex that our present means of research are not adequate to reveal
its structure. We know that its solid constituents are chiefly pro-
teids, together with relatively small quantities of carbohydrates, fats,
and salts. To these is added a large proportion of water which,
while not entering into a definite chemical union with the other
constituents, is so intimately associated with them as to form an
integral part of the cytoplasm.

The visible structure of cytoplasm differs somewhat in different
cells, even among those that appear to be comparatively unspecial-
ized. In the fixed cells of the higher animals and man it appears
to consist of a very delicate network or reticulum of minute fibres,
termed the spongioplasm. The points of junction of these fibres
and their optical cross-sections give a finely granular appearance to
the cytoplasm.

In the meshes of the spongioplasm is a clear, homogeneous sub-
stance, the hyaloplasm. This may also contain some granules, but
they are probably not constituent parts of the cytoplasm and are
grouped under the term metaplasm. Some of them are composed

34 FORMAL HISTOLOGY.

of material taken from without, either in their original form or
slightly modified ; others have been produced within the cell by
chemical transformations, and are either useful products, to be sub-
sequently turned to account by the cell itself or to be discharged as
a secretion, or they are waste matter destined for elimination from
the body.

The relative proportions of the hyaloplasm and the spongioplasm
and the arrangement of the fibres of the latter both vary in differ-
ent cells.1

When seen under the microscope the structure of the nucleus,
except during the division of the cell, closely resembles that of the
cytoplasm. It is traversed by a number of delicate fibres, which
branch and give the nucleus a reticulated appearance. At its sur-
face these filaments unite to form a delicate membranous envelope,
sharply defining the nucleus from the surrounding cytoplasm, but
it is a question whether this membrane is continuous, or whether it
is an exceedingly close meshwork with minute apertures permitting
a direct communication between the cytoplasm and the interior of
the nucleus.

The intranuclear reticulum varies in character in different kinds
of cell. In some its filaments are delicate and the meshes relatively
large. In the other extreme the filaments are so thick and closely
set that the nucleus appears to contain little if anything else.
Hit ween these extremes there is a great variety of intermediate
variations in the relative amount of reticulum.

The spaces between the nuclear filaments are occupied by a clear,
homogeneous substance (karyoplasm or nuclear sap), which may be
identical and continuous with the hyaloplasm of the rest of the cell.

One or more highly refracting bodies, the nucleoli, may be pres-
ent in the nucleus, lying freely in the clear substance between the
filaments <>r attached to the latter. Their purpose is not known,
but it is thought that they are not essential parts of the cell but
correspond more or less closely to the metaplasm in the cell-body.

Owing t<> their affinity for certain coloring matters, the substances

composing the nuclear filaments are called chromatin, or chromo-

1 Tlit- reticulated appearance of the cytoplasm may also be explained by assum-
ing it to have an alveolar structure, and the theory that such is its actual structure
possesses much plausibility. In that case the visible reticulum would be formed by
the walls of the alveoli and their lines and points of intersection, all of which
would be included in the spongioplasm, while the contents of the alveoli would
constitute the hyaloplasm. A line emulsion presents such an apparent structure.

THE CELL. 35

plasm. The hyaline substances making up the rest of the nucleus
do not receive those coloring matters, and for this reason and in
this situation are called achromatin. These terms are used only in
a morphological sense and do not specify any definite chemical com-
pounds, though they are distinguished from each other by staining
affinities which are based on chemical differences. The chromatin
has been further resolved into a ground-substance, giving form to the
reticulum of the nucleus and called linin, and to granules of chromatin
(the term being used in a more restricted sense), and this chromatin has
in turn been subdivided according to staining characteristics, into oxy-
chromatin (or lanthanin) and basichromatin, embedded in the linin
in the form of minute granules. The achromatin has also been
observed to contain granules, not stained with the usual nuclear dyes,
which have been called oedematin granules. The behavior of the
nucleoli toward dyes is somewhat different from that of the chromo-
plasm, which leads to the inference that they are of a different
chemical nature.

Except during cell-division, the nucleus usually lies quiescent
within the cytoplasm, but some observers have seen it execute ap-
parently spontaneous movements, and it is evidently possible for its
position in the cell to vary from time to time.

In marked contrast to this apparently dormant state, as far as
visible alterations of structure are concerned, is the role played by
the nucleus during the reproduction of the cell.

There are two modes of cell-division, the " indirect " and the
" direct," but they are by no means equivalent to each other. The
former, also termed karyokinesis because of the active changes in
the nucleus, appears to be the only truly reproductive process.
Direct cell-division results in the formation of new cells, but they
seem to lack that perfection of organization which would be required
for the complete and indefinite transmission of all the characters of
the parent cells.

Before entering into a description of karyokinesis, a few words
must be said concerning the centrosome. This is an extremely min-
ute granule which is usually situated in the cytoplasm not far from
the nucleus. It is often surrounded by a thin zone of hyaloplasm
which facilitates its recognition among the fibres and nodal points
of union of the spongioplasm. The fibres of the latter are also fre-
quently arranged in a radial manner for a short distance around the
centrosome. But in many instances it is extremely difficult to dis-

30 NORMAL HISTOLOGY.

tinguish the centrosome, and its constant presence in cells is largely
a matter of inference. Sometimes the centrosome is double, the
two granules lying close to each other and often being surrounded
by a common clear zone of hyaloplasm.

The first step in the process of cell-division by the indirect method,
or karyokinesis, is a division of the centrosome into halves (Fig.
15), which separate and pass to opposite points in the cytoplasm.
These points are called the poles of the cell, and when the new cen-
trosomes reach them they are called the polar bodies. In these situa-
tions they are surrounded by a more distinct zone of hyaloplasm
than that which enclosed the original parent centrosome, and beyond
this the spongioplasm is frequently arranged in radiations of unusu-
ally thick fibres. The polar bodies with their clear envelopes and
the prominent radiations about them are collectively known as the
attraction-spheres (Fig. 8).

Fig. 8.

Dividing cell from ovum of ascaris megalocepkahis. (Kostanecki and Siedlecki.) a, polar body,
centrosome, surrounded by a clear zone ; b, chromosomes of the dividing nucleus. Be-
tween the polar bodies is the achromatic spindle, and radiating from each attraction-
sphere are delicate lilaments of spongioplasm. The cytoplasm presents indications of
vacuolation.

While the polar bodies are separating, or after they have passed
into the polar regions of the cell, the nucleus begins to show those
changes in structure which constitute karyokinesis. This process
may be divided into a number of phases, as follows:

1. The Formation of the Spirem (Fig. 9 ).— This consists in a con-
densation of the chromoplasm. The branches of the nuclear fila-
ments are withdrawn into the substance of the main fibres, into
which the unclear membrane or peripheral network bounding the

THE CELL. 37

nucleus is also absorbed. The vesicular character of the nucleus is
Lost during these changes in the arrangement of the chromoplasm,
which appears as a loose tangle or skein of one or more threads
of uniform diameter lying freely in the body of the cell. This
skein is called the spirem. The chromoplasm in this condensed con-
dition stains more deeply with nuclear dyes than in the resting con-
dition of the nucleus. The nucleoli in the meantime become faint
and seem to ultimately disappear. They play no part in the process
of cell-division, unless they participate in the formation of the
achromatic spindle.

2. The Monaster Phase (Fig. 10). — The threads of the spirem
suffer a rearrangement, resulting in the formation of a sort of
wreath, situated midway between the poles, in the equatorial plane,
i. e., the plane perpendicular to and passing through the centre
of a line joining the two polar bodies. This wreath is called the
monaster, because of its star-like configuration when seen from
above. When viewed in profile it appears as a band of fibres lying
in the equator. It is at first made up of a single thread or only a
few threads, but subsequently breaks into a number of similar frag-
ments, called chromosomes. The exact number of these varies in
different species of animal, but is constant for each species and is
always divisible by two. In man it is thought to be sixteen.

The chromosomes are all of nearly, if not quite, the same size,
and, in the same kind of cell, closely resemble each other in shape.
The most common form appears to be a V-shaped rod lying with
its angle directed toward the centre of the wreath or monaster.

3. Metakinesis (Figs. 11, 12, 16). — In this phase of karyokinesis
the chromosomes split along their axes into two exactly equal parts
of similar shape, and these halves separate, each passing toward
one of the attraction-spheres.

Meanwhile, the structure known as the achromatic spindle has
been formed. This is a system of fibres resembling those that have
already been described as radiating from the polar bodies, but of
even greater prominence. They are arranged to form a spindle
with its apices at the polar bodies and its equator coincident with
that of the cell and the plane of the monaster.

It is along the lines of this spindle that the chromosomes travel
toward the centres of the attraction-spheres occupied by the polar
bodies.

NORMAL HISTOLOGY.
Fig. 9. Fig. 10.

Fig. 11.

Fig. 12.

Fig. 14.

Fig. 13.

Diagrams illustrating the phases of karyokinesis. (Flemniiiig.)
Fig. 9. — Spirem.
i ig. 10. — Monaster.
Fig. 1 1.— Metakinesis, early stage.
Pig. 12.— Metakinesis, late stage.
Fig. 18.— Diaster.
Fig. 11. — Dispirem.

The achromatic spindle is represented, but not the centrosomes (polar bodies'). The cell-
body is also omitted.

THE CELL.

39

The phases of karyokinesis that follow metakinesis are similar to
those that preceded it, but occur in inverse order.

4. The Diaster Phase (Fig. 13). — The chromosomes, having
reached the attraction-spheres, group themselves around the polar
body to form a wreath on a plane perpendicular to the axis joining
the poles. These wreaths, with the achromatic spindle, have an
appearance somewhat resembling the letter H, with a long cross-

Fig. 15.

Fro. 16.

<^£'s Y .' ' ■, '•' .V-'

Karyokinetic figures in epithelial cells. From a carcinoma removed by operation. (Lustig

and Galleotti.)

Fig. 15.— The centrosome has divided, but the nucleus is still in the resting condition.
Five nucleoli are represented within the nucleus.

Fig. 16.— Metakinesis. The polar bodies have divided.

piece, formed by the spindle, remaining uncolored or only faintly
tinged by nuclear dyes, while the uprights, made up of the chromo-
somes, are deeply stained.

The ends of the chromosomes now unite to form a thread, and
the wreath-like arrangement gradually passes into that of the
dispirem.

5. Dispirem (Figs. 14 and 17). — The halves of the original chro-
moplasm of the nucleus are now arranged in two skeins about the
poles. From these the two daughter-nuclei of the future cells are
formed (Fig. 18).

During metakinesis the cytoplasm of the cell begins to show
signs of division. This may be accomplished through a constric-
tion of the body of the cell, which gradually becomes deeper and
finally severs the two portions ; or a series of punctiform or short
linear enlargements of the lines of the achromatic spindle appear
in its equator, and through these a plane of cleavage, dividing the
two new cells from each other, is finally established.

It is rarely that any biological process assumes such mathemat-
ical precision as is displayed in karyokinesis. The purpose of that

40 NORMAL HISTOLOGY,

mode of cell-division appears to be an exactly equal partition of
all parts of the chromoplasm between the young cells. Whether
the amount of cytoplasm given to the daughter-cells is the same
or different, the division of the chromoplasm is exactly equal, not
only in its whole bulk, but each chromosome, which appears to be
the morphological unit of the chromoplasm, is split into exactly
equivalent halves, one of which is contributed to the formation of
each daughter-nucleus. It is for this reason that the chromoplasm
is looked upon as the carrier of hereditary peculiarities.

Fig. 17. Fig. 18.

Fig. 17.— Dispirem. In this case the polar bodies have not divided (compare Fig. 16).

Fig. 18.— Daughter-nuclei which have nearly reached their full development. Centrosomes

present in the cytoplasm.
In these figures the structure of the cytoplasm is not given.

After the formation of the daughter-nuclei, the centrosome
usually passes from it into the cytoplasm. It may divide earlier
than has been described, the division taking place while it exists
as the polar body, or even earlier (Fig. 16).

A cell nearly always divides to form two new cells, but some-
time- three or more cells may be produced, the chromosomes being
distributed among them (Fig. 19). Such cases are probably
always morbid, and the resulting cells are not wholly the equiv-
alents of the parent cell.

It occasionally happens that the cytoplasm fails to divide after
the formation of the daughter-nuclei, and cells with two or more
nuclei result. When the nuclei continue to multiply and the
cytoplasm increases in amount, but does not suffer division, large
multinucleated cells are produced, which have been called "giant-
cells." They occur normally in the marrow of bone and are pro-
duced in many of the inflammatory processes.

The direct or amitotic method of cell-division is inaugurated by

THE CELL.

41

an active change in the shape of the nucleus, which may have pre-
viously increased in size and become richer in chromoplasm. The
nucleus becomes constricted and finally separated into two portions,

Fig. 19.

Epithelial cell from a carcinoma. (Galeotti.) The centre-some has divided into four portions,,
and the chromosomes are arranged with reference to these. The figure represents the meta-
kinetic phase of karyokinesis, which will result in the formation of four imperfect
nuclei.

which are not necessarily equally rich in chromoplasm. The cyto-
plasm, either at the same time or later, becomes similarly con-
stricted until it is divided into .two parts, each containing one of
the nuclear divisions (Figs. 20, 21, 22).

Fig. 20.

Fig. 21.

Fig. 22.

Amitotic cell-division. (Flemming.) Epithelial cells from the bladder of a salamander.
Figs. 20 and 21 contain nuclei with constrictions dividing them into nearly equal portions.
Fig. 22.— Contiguous cells, each containing a nucleus about half the size of those prevailing-
in the tissue, and, therefore, probably the result of cell-division by the direct process.

It is believed that this mode of division does not result in the
formation of cells that have the complete character of the parent-
cell, and that their descendants form a degenerate race that is
destined to extinction. It is quite obvious that no such precise
.partition of the chromatic substance is likely to take place as that

42 NORMAL HISTOLOGY.

which is characteristic of karyokinesis, and if the chromosomes are
really the carriers of hereditary peculiarities, this mode of division
can hardly favor their perfect transmission.

The minute structures of the nucleus and cytoplasm described
have been studied in cells that were rapidly killed and then
stained. In the living cells these details cannot be detected, and it
is still uncertain whether they may not be in part due to the methods
employed in their study. If the superficial layer of a frog's cornea
be carefully stripped off and placed upon a slide in a drop of the
aqueous humor of the eye, covered with a cover-glass and then
quickly observed, the nuclei of the cells may be seen as round and
somewhat more highly refracting bodies than the surrounding cyto-
plasm. The nucleoli may also be made out, but the outlines of the
individual cells and the internal differentiation of the nucleus cannot
be seen. If the specimen be observed for a few minutes, there will
come a moment at which the whole picture changes. The nuclei
suddenly become distinct ; an intranuclear network appears and the
cell outlines are more sharply defined. These changes may be has-
tened by placing a drop of acetic acid at the edge of the cover-glass.
From this point the changes start and rapidly occur in the adjacent
cells until the whole specimen is affected. It is as though the definite
structures described in the foregoing parts of this chapter abruptly
crystallized out of a nearly homogeneous medium. These changes
probably mark the instant of death. The chromosomes present in
cells dividing by karyokinesis are visible in the living cells. The
intranuclear network of resting cells is not visible during the life of
the cell. It is not defmitelv known whether it and the minute
structures of the cytoplasm, as seen in dead cells, existed in the liv-
ing cells, or whether they wTere formed by a species of coagulation at
the moment of death or were merely rendered more dense and there-
lore clearlv visible.

CHAPTER II.

THE ELEMENTARY TISSUES.

The various parts of the body are composed of a small number of
" elementary tissues." Each of these elementary tissues has a definite
structure, but the details of that structure may vary within certain lim-
its in different parts of the same mass or in different situations within
the body. Such variations can usually be referred to differences in the
functional activity assigned to the tissue, which is not always exactly
the same throughout the body. For example, epithelium is an ele-
mentary tissue consisting of cells which are nearly always rich in
cytoplasm and are separated from each other by a very small amount
of homogeneous intercellular substance. Wherever epithelium is
found it has these general peculiarities of structure. But the func-
tions demanded of epithelium are of widely diverse character in
different situations, and its structure shows a corresponding diversity
in its details. The fact that it is made up almost exclusively of
cells leads to the natural inference that the usefulness of epithelium
depends upon cellular activities. Inasmuch as these may be of
very different character, we should expect the tissue to vary chiefly
in the structure and arrangement of its component cells according
to the particular activity which was needed and the manner in
which it was utilized. Such, as a matter of fact, is the case. These
considerations will be made clearer if we follow a little more closely
the example offered by epithelium.

In some situations epithelium serves to protect the underlying
tissues from injury. But the usual injurious influences which
threaten the tissues differ in different parts of the body, and
must, therefore, be averted by different means. Upon the sur-
face of the skin they are chiefly of a mechanical or chemical
nature, and to resist them the cells of the epithelium forming the
epidermis undergo a modification in structure, resulting in the
formation of a superficial horny layer which is highly resistant to
abrasion and chemical change. Upon the inner surfaces of the

43

44 NORMAL HISTOLOGY.

respiratory passages the conditions are different. Here the tissues
require protection from particles of dust that may be inhaled. For
this purpose the epithelial cells lining those passages are provided
with minute, hair-like processes, "cilia," which execute lashing move-
ments toward the outlets of the passages and occasion the transpor-
tation of substances coming into contact with them toward the
outer world. In the digestive tract the conditions are again differ-
ent. The tissues underlying the epithelial lining need protec-
tion from the chemical action of the fluids in the stomach and intes-
tine, as well as from friction with their solid contents. The cells
of the epithelium meet these needs by a secretion of mucus, which
is discharged upon the inner surfaces of the digestive organs, where
it serves as a protective layer and as a lubricant.

In other situations epithelium has an excretory function, which is
less clearly of value in protecting its immediate surroundings, but
is essential for the protection of the whole organism from substances
which would exert an injurious effect if they were permitted to ac-
cumulate in the circulating fluids of the body. These substances
are absorbed from those fluids by epithelial cells, from which they
are discharged from the body either unchanged or after transforma-
tion into other chemical compounds. Here the most obvious prod-
ucts of cellular activity are of no use in the economy, and are elim-
inated from it; but it is not improbable that the cells which separate
them or their antecedents from the circulating fluids may also
discharge useful substances into those fluids (" internal secretion ").
We must not assume that the most obvious function exercised by a
tissue is the only service it does to the organism.

The epithelium which carries on this eliminative function is nearly
always associated with other elementary tissues to form an organ,
called a "gland," in which the epithelium is the functionally active
tissue, the other tissues being subservient to it. The glands of the
body differ considerably in both structure and function, but in all
of them it is epithelium which elaborates the materials essential to
the formation of their normal secretions. Mention has already
been made of those glands which furnish secretions charged with
waste materials to be eliminated from the body. Such glands are
called excretory glands, and are exemplified by the kidney. Other
glands, distinguished as secretory in a restricted sense, furnish secre-
tions which are of service to the organism. Examples of such
glands are those which discharge their secretions into the alimentary

THE ELEMENTARY TISSUES. 45

iract where, by virtue of the ferments they contain, they prepare
the food for absorption. Another example of a secretory gland is
furnished by the sebaceous glands of the skin, which produce an
oily substance serving to keep the epidermis upon which it is
discharged soft and pliable.

In the secretory glands the cells of the functional epithelium
elaborate within their bodies the substances necessary to give the
glandular secretion its peculiar and useful characters. These sub-
stances accumulate within the cells, where they are stored until
required, when they are discharged into the secretion. While in
the stored condition within the cells these substances may have a
different chemical constitution from that which they acquire when
they are discharged from the cells. A simple example of this
chemical transformation is furnished by the liver, in the epithelial
cells of which carbohydrates are stored as glycogen, to be liberated
as a closely related chemical substance, glucose. In like manner
the ferments stored in the epithelial cells of the digestive glands
are not fully formed while in that situation, but exist in states
known as " zymogens," from which the potent ferment appears to
be readily formed when the cells are called upon to furnish it.

It is apparent, then, that the elementary tissue, epithelium, can-
not have the same microscopical structure in all the situations in
which it is found ; but, notwithstanding these variations, wherever
epithelium occurs it presents certain general structural peculiarities
which are constant and which distinguish it from the other element-
ary tissues. Similarly, each of the other elementary tissues pre-
sents variations in the details of its structure in different situations,
but always retains certain general structural characteristics dis-
tinguishing it from all the other elementary tissues. It is the first
task of the student of histology to learn to recognize and identify
these elementary tissues wherever they occur and however they may
vary from the type which is first presented to him for study.

In the following chapters an attempt is made to give the student
an idea of the essential structure of the elementary tissues, so that
he may recognize them in specimens which he examines with the
microscope. For this purpose they have been arranged in the
order of their structural simplicity.

When examining a specimen under the microscope with a view
to recognizing the elementary tissues it contains, the student should
habitually ask himself the following questions : (1) What are the

40

NORMAL HISTOLOGY.

general characters of the cells entering into the structure of the
tis-ue? (2) What kind of intercellular substances separates those
cells? (3) How arc the cells arranged with reference to each other
and the intercellular substances? Correct answers to these three
questions will enable him to quickly determine the nature of the
tissue he is observing, even if it should vary considerably in struct-
ural details from examples of the same tissue with which he has
already become familiar.

CHAPTER III.
THE EPITHELIAL TISSUES.1

I. ENDOTHELIUM.

General Characters. — (1) The cells possess thin membranous bodies,
except at the site of the nucleus, to enclose which the cell-body is
thickened. (2) The intercellular substance is minimal in amount ;
clear and homogeneous in character. (3) The cells are arranged,
edge to edge, in a single layer. The wavy or denticulate edges of
neighboring cells fit into each other, being separated by a mere line
of the intercellular substance which in this tissue has received the
name of " cement-substance " (Fig. 23). It appears probable that the
connection between neighboring cells is really much more intimate,
and that what appears to be a homogeneous cement-substance is in
reality a fluid derived from the lymph, and that the cells are con-
nected with each other by exceedingly delicate cytoplasmic projec-
tions which join each other, the tissue fluid lying in the spaces between
these cytoplasmic bridges. This arrangement is analogous to that
described in connection with the prickle-cells which have for a long
time been known to exist in the epidermis (see Stratified Epithelium).

Endothelium forms a thin membranous tissue composed almost
exclusively of cells. It occurs in its most isolated form in the cap-
illary bloodvessels, the walls of which are simply tubes of endo-
thelium, supported externally by the surrounding tissues and fluids
and internally by the enclosed blood. It also covers the tissues
surrounding the serous cavities of the body, where it serves both as
a lining to the cavities and a smooth covering to the organs, dimin-
1 The term " epithelial " is used here in its most inclusive sense to designate
those tissues which cover surfaces, whether those surfaces are exposed to the outer
world, as, for example, the skin and the mucous membranes, or are wholly enclosed,
as are the inner surfaces of the bloodvessels, lymphatics, and serous surfaces. Some-
times all these tissues are called epithelium and the term endothelium is discarded.
Other authors use endothelium to designate only the cells lining the bloodvessels
and lymphatics and similar cells occurring in connective tissue. The term endothe-
lium is retained here to distinguish cells derived from the mesoblast, from the epi-
thelium arising from the epiblast and hypoblast, and because there are morpho-
logical differences between these groups of tissues in the adult.

47

is

NORMAL HISTOLOGY.

ishing the friction resulting from their movements against each
• it her. It does not occur in any situation where it would be exposed
directly to the air.

The eells of endothelium vary somewhat in size and shape. They
mav be polygonal, rhomboid, or stellate in form, and during life are

soft and extensible so that their
sizes may be modified by stretch-
ing; or tension in one or more
directions. The cell-bodies, or
cytoplasm, are usually clear and
apparently structureless or only
slightly granular, but occasion-
ally some of the cells are smaller
and more granular than the
majority. This is especially
marked in the cells surroundino;
minute apertures that are found
here and there in the endothelial
lining of the serous cavities (Fig.
24). These stomata furnish a
direct communication between
the serous cavities and the lym-
phatic spaces in the tissues sur-
rounding them. These openings
virtually convert the serous
cavities into enormous lymph-
spaces forming a part of the
general lymphatic system.

The edges of contiguous endo-
thelial cells are not everywhere
in equally close approximation
to each other (Fig. 25). The
points where they are more
widely separated than usual are
occupied by an increased amount
of the cement-substance, or
processes from cells in the
underlying tissues are here intercalated between the endothelial cells,
reaching the surface of the serous membrane. In either case these
points of separation of the endothelial cells are not openings through
the tissue, though they are spots where the tissue is relatively more

Mesentery of frog treated with silver nitrate.
The mesentery is covered on both surfaces
with a layer of endothelium. Between
these is areolar connective tissue contain-
ing bloodvessels, lymphatics, ami nerves.
In this figure only the two endothelial
layers and a capillary bloodvessel are
represented: ", nucleus of endothelial
cell belonging to uppermost layer; 6, nu-
- of cell belonging to deep layer form-
ing the lower surface of the specimen;
c, intracellular cement between cells of
upper layer of endothelium ; </. '/. nuclei
of endothelial cells, forming a capillary
bloodvessel, aeen in profile. The bodies
of these cells are not reproduced in the
figure. 'lie- cement in the deep layer of
endothelium is represented by finer lines
to distinguish it from that belonging to
tie- upper layer.

THE EPITHELIAL TISSUES.

49

pervious than elsewhere. They are called pseudostomata, to dis-
tinguish them from the stomata already mentioned.

Fig. 24.

Endothelium on a serous surface of the frog. (Klein.) o, stoma bounded by endothelial cells
with granular cytoplasm; 6, pseudostoma. The nuclei of the cells are not represented.

The intercellular substance in endothelium is so small in amount
and so homogeneous and transparent that it escapes observation

Fig. 25.

Endothelial lining of a small vein treated with silver nitrate ; dog. (Engelmann.) The fig-
ure represents a tube formed of endothelium the cells of which vary in size and shape.
The whole wall of a capillary has essentially the same structure as this venous lining
but its calibre is smaller. The upper branch in this figure may represent a capillary
opening into the vein, a, a, pseudostomata occupied by cement-substance.

under the microscope unless special means are employed for its dem-
onstration. The simplest of these consists in treating the fresh

4

.-.(I

yon MA L H1ST0L0G Y

tissue with a 1 per cent, solution of nitrate of silver for a few mo-
ments, washing with distilled water, and then exposing it to the
rays of the sun. During this treatment the intercellular substance
enters into combination with the silver. Upon exposure to strong
light this compound is destroyed, leaving an insoluble black precipi-
tate of silver oxide. When the specimen is examined under the
microscope, the site of the cement-substance is marked by the
presence of this precipitate. Endothelium so treated shows a net-
work of fine dark lines, the meshes of which are occupied by the
cells of the tissue. When no such method has been employed to
render the intercellular substance conspicuous, the outlines of the
cells cannot be distinguished, and the tissue appears as a continuous,
nearly homogeneous membrane containing nuclei at more or less
regular intervals. When seen in profile or vertical section, endo-
thelium appears as a delicate line, expanded at intervals to enclose a
nucleus (Fig. 26). The nuclei of the endothelial cells are round or

Fig. 26.

Diagram of vertical section through a serous membrane : a, nucleus of endothelial cell : ft,.
body of cell : c, line of junction between two cells occupied by cement-substance ; rf, pro-
i ess of connective-tissue cell occupying a portion of the intercellular space between two
endothelial cells, one variety of pseudostoma; e, areolar tissue with fusiform and stel-
late cells. The vessels and nerves in the areolar tissue have been omitted.

oval, and each cell usually possesses but a single nucleus situated
near its centre, but occasionally cells with two nuclei are observed.
Functionally, endothelium appears to play only a passive role in
ni"-t situations in which it is found. It furnishes a smooth cover-
ing for those internal surfaces of the body which are exposed to
friction, as, for example, in the serous cavities and the inner sur-
faces of the vascular systems. In the capillary bloodvessels and
lymphatics endothelium forms the entire wall of the vessels, and
it- thinness permits the passage of fluids through those walls. The
fact that the lymph in different parts of the body varies somewhat

THE EPITHELIAL TISSUES. 51

in composition has led to the inference that the endothelium of the
capillary walls exercises an active function in determining what
shall pass through it ; that the lymph is a sort of endothelial secre-
tion. It is difficult, however, to reconcile this view with the fact
that the endothelial cells are so poor in cytoplasm. (See Lymph,
p. 120.)

Endothelium is developed from the mesoderm.

II. EPITHELIUM.

General Characters. — (1) The cells are nearly always large and
rich in granular cytoplasm. They contain distinct round or oval,
vesicular nuclei, of which there is usually only one in each cell.
(2) The intercellular substance is very small in amount and is clear
and homogeneous. (3) The arrangement of the cells and their size
and shape all vary greatly, giving rise to a number of varieties of
epithelium, which are classified according to the shape and arrange-
ment of the cells. In pavement-epithelium the cells are thin and
arranged in a single layer, not unlike endothelium. In cubical
epithelium the cells are thicker and also usually arranged in but a
single layer. In columnar epithelium the cells are prismatic in form
and rest with their bases upon the surface of the tissues beneath.
They are usually separated at their bases by pyramidal cells, so
that the layer of epithelium cannot be said to consist strictly of but
one layer of cells, and in some situations there are several distinct
layers. In stratified epithelium the cells are superimposed upon
each other to form a layer of cells, the thickness of which is several
times the diameter of a single cell. The cells of the variety of epi-
thelium called ciliated epithelium differ from those of the other
varieties in possessing delicate, hair-like processes which project
from the free surface of the tissue.

Epithelium resembles endothelium in being composed almost
exclusively of cells separated by a minimal amount of intercellular
substance. Like endothelium, it is nearly always found covering
other tissues and having one free surface. The two tissues differ
greatly in the character of their cells, with one notable exception.
This exception is found in the epithelial lining of the pulmonary
alveoli, where the pavement-epithelium contains cells that closely
resemble those of endothelium. These cells are, however, directly
exposed to the inspired air, while endothelium is only found in situa-
tions where it is protected from all contact with the exterior.

1. Cubical Epithelium. — The cells of this variety of epithelium

52

NORMAL HISTOLOGY.

arc approximately of the same diameter in all directions. They
may be almost strictly cubical or spherical, but arc usually polyhed-
ral as the result of mutual compression, their contiguous surfaces
being flattened. They are usually disposed in a single layer upon
a surface furnished by the underlying tissues, as, for example, in
tubular or racemose glands, but they may be aggregated to form a
solid mass of cells filling a sac, as in the sebaceous glands of the
skin, or in strands or columns, variously disposed, as in the liver
and suprarenal bodies.

It is this form of epithelium that is chiefly concerned in perform-
ing the functions of secretion, and, for this reason, it is frequently
designated as "glandular epithelium."

The appearance of the individual cells varies considerably accord-
ing to the functions that they perform and the stage of functional
activity which obtained at the time cellular changes were arrested
when the particular specimen was prepared for study. It will suf-
fice for present purposes of description to call attention to the fact
that the cytoplasm is usually highly granular, partly because of its
own structure, partly because many of the substances elabo-
rated and stored within the cells as the result of their functions
appear in the form of granules (metaplasm). The nature of these
granules varies. They may be albuminoid, zymogenic granules, or
minute drops of fatty substances, which may coalesce to form dis-
tinct oily globules, or they may consist of carbohydrates, e.g., gly-
cogen. The granular condition of the cytoplasm may be so marked

Fig. 27.

Fig. 28.

Fig. 29.

I

,..-"-

■ •'■ is'

^■W--.:

■<¥>'

?b

Cubical epithelium.

Fig. 27.— Sis cells from the sublingual gland of a man who was executed. (Schiefferdecker.)
Fig. 28.— Three isolated cells fr«>m the gastric tubules of the dog and cat. (Trinkler.)
Fig. 29.— Cell with highly granular cytoplasm, the result of stored metaplasm, chiefly gly-
cogen. (Barfurth.)

as to render the detection of the nucleus difficult in unstained speci-
mens (Figs. 27, 28, and L'i)).

In this form of epithelium the presence of two nuclei in a single
cell is mote frequent than in the other varieties.

THE EPITHELIAL TISSUES.

53

2. Pavement-epithelium. — This variety of epithelium consists of
thin cells arranged edge to edge to form a single layer. With the
exception of certain regions on the surfaces of the pulmonary
alveoli, the cells are more cytoplasmic and granular than are those
of endothelium which this tissue in other respects closely resembles.
During feetal life the smaller air-passages and alveoli of the lung
are lined by a pavement-epithelium, the cells of which are nearly
as thick as those of some varieties of cubical epithelium. When,
however, the lung is expanded by the respiratory acts following
birth, many of the cells lining the alveoli become greatly extended
and flattened until their bodies are thin and membranous and their
nuclei inconspicuous or even destroyed (Fig. 30). These greatly
flattened epithelial cells are found covering those portions of the

Fig. 30.

Pavement-epithelium. Surface view of the lining of a pulmonary alveolus ; man. (Kolliker.)
«, membranous cell without a nucleus ; 6, nucleated granular cell ; c, cut surface of the
vertical wall of the alveolus, the structure of which is not represented.

alveolar walls in which the capillary bloodvessels are situated and
permit a ready interchange of gases between the air in the alveolar
cavities and the blood circulating in their walls. Many of the
epithelial cells covering the tissues in the meshes between the
capillaries retain the cytoplasmic and granular character possessed
before birth and appear capable of multiplying and, perhaps,
replacing such of the thinner cells as may be thrown off or
destroyed.

It will be evident, from the foregoing descriptions, that there

54

NORMAL HISTOLOGY.

is do sharp structural Line separating cubical from pavement-epithe-
lium. Functionally, pavement-epithelium is a much less active

tissue than the cubical variety.

:>. Columnar Epithelium (Figs. 31, 32, 33). — The cells of this

Fig. 31

Columnar epithelium. From tongue of pseudopits. (Seiler.) a, three cells with intact cyto-
plasm, except the central one, which contains a vacuole ; b, three cells of which the dis-
tal ends contain drops of fluid (vacuoles) or of metaplasm.

form of epithelium are of a general columnar or prismatic shape
and possess a single nucleus and a cytoplasm that is usually dis-
tinctly granular. They are arranged with their long axes parallel
to each other, so that their free ends form the surface of the epithe-

Fig. 32.

Fig. 33.

f% '■"'•;5-;

Ppfe

«» "■ * ';

y

Columnar epithelium.
' 8.— From small intestine of the mouse. (Paueth.) a. pyramidal reserve cell, nucleus not

included in section ; b, "goblet" cell, enclosing a large drop of secretion.
Fig. 33.— From small intestine of the mouse. (Paneth.) Columnar epithelial cells seen from

above : b, goblet-cell, the mucous contents darkened by the hardening process; s, s, highly

granular cells which have recentlj discharged their secretion.

Hum, while their deeper cuds either rest upon the tissues beneath
the epithelium or upon other epithelial cells of different shape
which form one or more layers between the columnar cells and the
underlying tissue-. When they rest directly upon the tissues
beneath there are usually other epithelial cells of a pyramidal or
oval shape which may be regarded as immature cells ready to take
the place of such fully developed cells a> may become detached or
destroyed. The presence of these cells occasions a narrowing of

THE EPITHELIAL TISSUES.

55

the deep ends of the columnar cells, so that they are not strictly
prismatic in form. In cross-section, or when viewed in a direction
parallel to their long axes, the cells have a polygonal form due to
the lateral pressure they exert upon each other (Fig. 33).

The nuclei of the columnar cells are oval, situated nearer the
base of the cell than its superficial end with their long axes parallel
to those of the cells themselves, and are vesicular in structure with
a distinctly reticular arrangement of the chromatin filaments.

Columnar epithelium is found chiefly upon the free surfaces of
mucous membranes, but also occurs in some of the secreting glands.
The minute structure of the cells varies somewhat in different situ-
ations, but the consideration of these minutiae must be deferred
until a description of the structure of the different organs is under-
taken in a subsequent chapter.

4. Ciliated Epithelium (Figs. 34, 35, 36).— Ciliated epithelium

Fig. 34.

Fig. 35.

Fig. 36.

f»

Wm

Ciliated epithelium. (Frenzel.)

Fig. 34.— Cubical cells with long cilia (hb). The nuclei of the cells are obscured by the gran-
ular cytoplasm.

Fig. 35.— Columnar ceils. The rodded margin, /s, corresponds to the cuticle in Fig. 37.

Fig. 30.— Diagram illustrating variations in the structure of the ciliated ends of cells. The
rodded portion, ok to uk, corresponds to the cuticle of other varieties of epithelium,
though the latter do not possess the knobbed ends of the rods represented in this figure ;
hb, cilia.

is merely a variety of either columnar or cubical epithelium in
which the free ends of the cells are beset with delicate hair-like
processes, which execute lashing movements in some one direction.
It is found lining the trachea and bronchi, the cilia here serving to
propel toward the larynx such particles of dust as are brought into
the respiratory passages by the currents of air during respiration.
Ciliated epithelium also occurs on the lining membranes of the nose

56 NORMAL HISTOLOGY.

and the adjoining bony cavities, the mucous membrane of the uterus
and the Fallopian tubes, the vasa efferentia of the testis and a part
of the epididymus, the ventricles of the brain (except the fifth), the
(•(Mitral canal of the spinal cord, and the ducts of some glands.

The possession of cilia, which are very motile organs, presents
a marked departure in specialization from the usual metabolic func-
tions of epithelium. Ciliated epithelium rarely exercises a secretory
function, its stock of energy being utilized to produce motion instead
of chemical change. But there are secreting varieties of epithelium
possessing a " cuticle " which appears to be morphologically anal-
ogous to the cilia, but in which the fibrils are less highly developed,
probably not motile, and, therefore, functionally not the equiva-

Fig. 37.

mmmmmiBimwmwwm

Cuticularized epithelium, intestine of dog. (Paneth.) Rodded cuticle of the free ends of
columnar cells. In most specimens of ciliated epithelium from human tissues, where no
special care has been taken to preserve the cilia, the ciliated border presents the appear-
ances shown in Fig. 37.

lents of cilia. This cuticle is highly developed in the cells cover-
ing the mucous membrane of the intestine (Fig. 37).

o. Stratified Epithelium. — In the varieties of epithelium hitherto
considered the cells are, in the main, disposed upon some surface
in a single layer, some, at least, of the cells usually extending from
the bottom of the layer to its surface.

Stratified epithelium is distinguished from these by being of
greater depth and consisting of several layers of cells. The epithe-
lium lining the cheek or the (esophagus may be taken as a typical
example of this variety.

The most deeply situated cells are small and nearly filled by the
round or oval nucleus. They undergo frecjuent division, and as
they multiply -one of them are crowded toward the surface. For
a time these increase in size through a growth of their cytoplasm.
But as they are pushed nearer to the surface and farther from the
sources of nutrition in the vascular tissues underlying the epithe-
lium, they become flattened and their bodies lose their cytoplasmic
character, being converted into a dry, horny substance, keratin.

THE EPITHELIAL TISSUES. 57

Upon the free surface they are reduced to thin scales, closely
adhering to each other and their subjacent neighbors, but entirely
devoid of both cytoplasm and nucleus (Fig. 38).

Stratified epithelium is found upon surfaces exposed to friction,
which it serves to protect against mechanical injury, and, in some

Fig. 38.

Stratified epithelium, oesophagus of the rabbit: a, karyokinetic figure in a cell of the deep
layer, demonstrating the fact that the cells multiply in this region ; 6, larger flattened
cell nearer the surface ; c, horny layer made up of cells that have undergone keratoid
degeneration ; rf, underlying fibrous tissue. In one place, near the centre of the figure,
six blood-corpuscles reveal the presence of a small vessel ; e, tangential section of a small
fibrous papilla extending into the epithelium and surrounded by young epithelial cells.

cases, against desiccation. It forms the epidermis of the skin,
and lines the mouth, oesophagus, rectum, and vagina. In these situ-
ations the scaly or squamous cells of the surface are constantly
being removed by the attrition to which they are exposed, but are
as constantly replaced by fresh cells from the deeper layers of the
epithelium. Pressure and moderate friction stimulate the multi-
plication of the cells in the deepest layers of the tissue, so that
parts, e. g. of the skin which are especially subjected to such influ-
ences acquire a thicker epidermis (callus).

Where the stratified epithelium consists of many layers of cells,
as is the case, for instance, upon the skin, there is a provision for
the nourishment of the growing cells which are somewhat removed
from the vascularized subjacent tissues. The cells of the deeper
layers are somewhat separated from each other, leaving a space
between them through which nutrient fluids can circulate. Across
this space numerous minute projections or "prickles," springing
from neighboring cells, join each other, forming connecting bridges
between the cells. When isolated, such cells appear covered with
these small spicules (" prickle-cells "), and their presence probably

58 NORMAL HISTOLOGY.

increases the tenacity with which the cell-remains adhere to each
other when they become hardened and toughened on the surface of
the epithelial layer (Fig. 39).

These delicate bridges connecting neighboring cells are not pecu-
liar to stratified epithelium, though they are more conspicuous in
that tissue than elsewhere. They have been observed between the
cells of the columnar epithelium of the intestinal mucous mem-
brane, and also between the cells of other elementary tissues; e.g.,
smooth muscular tissue.

6. Transitional Epithelium (Figs. 40 and 41). — This variety re-
sembles stratified epithelium in forming layers several cells in thick-

Fig. 39.

W

Prickle cells from human stratified epithelium. (Rabl.) Four cells with delicate processes unit-
ing across an intervening space are represented. The lower right-hand cell is just below
the upper surface of the section, so that its surface is seen. This is covered with minute
spots, which are end views of the prickles directed toward the observer. The nucleus
of this cell is not in sharp focus, a fact indicated by the fainter outline in the figure.

ness, but differs in the character of its superficial cells. These do not
undergo the horny change peculiar to stratified epithelium, but con-
tinue to increase in size, forming a covering; of verv large cells lving
upon those beneath. Under these largest superficial cells are pyri-
forra cells lying with their larger, rounded ends next to the topmost
layer, while their deeper and more attenuated ends lie between the
oval or round cells that form the one or two deepest layers of the
epithelium and rest upon the underlying tissues.

Transitional epithelium is found lining the renal pelves, ureter-,
and bladder. Its structure permits of a considerable stretching of
the tissues beneath without rupture of the epithelial layer over
them, the cells of which become flattened to cover the increased
surface, to return to their first condition when the viscus which they
line is emptied. This is notably the case in the bladder, the epi-

THE EPITHELIAL TISSUES.

59

thelial lining of which may he taken as a type of this variety of
tissue.

The functional activities of epithelium are in marked contrast to
the comparatively inert character of endothelium. The cytoplasmic

Fig. 40.

- m m

— H-

d

Transitional epithelium from bladder of the mouse. (Dogiel.) 1, 2, 3, and 4 indicate the layers
of cells, not everywhere equally well defined, a, hyaloplasmic surface, and, 6, cyto-
plasmic body of large superficial cell ; c, leucocyte— i. e., white blood-corpuscle that has
wandered into the epithelium by virtue of its amoeboid movements ; d, karyokinetic
figure in a cell belonging to the deepest layer. Beneath this layer is the fibrous tissue,
which is covered by the epithelium and forms a part of the wall of the bladder. The
superficial cell, which is fully represented, contains two nuclei, a not very infrequent
occurrence in these cells.

nature of the epithelial cell, when contrasted with the poverty in
cytoplasm of the cell in endothelium, would lead us to expect this
difference in the cellular activities of the two tissues. At the begin-
ning of this chapter a sketch of the manifold functions of epithe-

Fio. 41.

Transitional epithelium. Isolated cells from the bladder of the frog. (List.)

lium was given. It is a fair general statement of its usefulness to
say that epithelium is chiefly concerned in bringing about chemical
changes in substances brought to it. Sometimes these substances
are elaborated into fresh cell-constituents, and the activity of the

60 NORMAL HISTOLOGY.

tissue is displayed chiefly in an active multiplication and growth of
its cells. This is especially true in the stratified variety, where pro-
tection is provided by a constantly renewed supply of cells. In
other cases the substances received by the cells are elaborated into
definite compounds destined to form the essential constituents of a
secretion. This secretory function of epithelium is an extremely
important one, and for its performance that tissue is usually ar-
ranged in a special structure or organ, called a gland. A brief state-
ment of the general characters and classification of these organs
may here appropriately find a place.

Secreting Glands. — The simplest type of secreting structure con-
sists of a surface covered with a layer of epithelium, the cells of which
are endowed with the power of elaborating a secretion and discharg-
ing it upon their free surfaces (Fig. 32, b). The tissues supporting
the epithelium belong to the connective tissues, and are fibrous in
character and well provided with bloodvessels, lymphatics, and
nerves. These bring to the epithelium the substances necessary for
its nourishment and work, and place its activities under the control
of the nervous system. Between the epithelium and the fibrous
tissue supporting it there is frequently a thin membranous layer of
tissue that often appears quite homogeneous, evidently belongs to the
connective tissues, and has received the name of "basement-mem-
brane." This appears to offer a smooth surface for the attachment
of the epithelial cells, which receive their nourishing fluids through it.

The epithelial surfaces of many of the mucous membranes are
examples of the foregoing simple secreting structure. The secretory
function is here of use as an adjunct to the protective function
assigned to the epithelial covering, and the quantity of secretion is
but slight under normal conditions. Where the volume of secre-
tion required i- considerable some provision for an increase in the
extent of secreting surface is necessary. This maybe accomplished
by an invagination of that surface, which then forms the lining of
one or more tubes or sacs, into which the secretion furnished by the
epithelial cells is discharged. Such an arrangement of the tissues
constitutes a gland, and it is evident that these may be arranged
into groups or classes according to whether the secreting surface
forms a single tube or sac, or several such tubes or sacs, uniting to
forma single gland. Thus, there may be simple or compound tubular
glands, or simple or compound saccular glands. Whether the deeper
portion^ of the -land have a tubular or saccular structure, the secre-

THE EPITHELIAL TISSUES.

61

tion of the gland is discharged upon some free surface through a
tubular outlet, called the duct. This is frequently lined with a non-
secreting layer of epithelial cells differing in character from the
actively secreting epithelium in the deeper portions of tire glandular
passages (Figs. 42-47).

Fig. 42.

Fig. 43.

Fin. 44.

Fig. 45.

Diagrams representing various types of gland.

Fig. 42.— Simple tubular gland : a, epithelium covering the surface on which the secretion is
discharged; b, mouth of gland; c, epithelium lining the duct. This gradually passes
into the secreting epithelium. Some simple tubular glands have no such distinction
between the cells near the mouth and those nearer the fundus, but all the cells are of the
secreting variety — i.e., exercise that function, e, secretory epithelium ; d, lumen. The
sweat-glands are simple tubular glands which are coiled in their lower part to form a
globular mass.

Fig. 43. — Compound tubular gland : /, duct ; g, acinus.

Fig. 44.— Racemose tubular gland : /,/,/, ducts ; g, g, acini.

Fig. 45.— Simple saccular gland : /, duct ; g, acinus.

(>2 NORMAL HISTOLOGY.

Fig. 46. Fig. 47.

Diagrams representing various types of gland.

Fig. 46.— Racemose saccular gland : /,/, ducts; g, acinus.

Fig. 47.— Compound tubular gland, with a marked distinction in the character of the epi-
thelium in the duct and acini: c, duct epithelium; /, duct; d, lumen of the acinus;
e, secreting epithelium. This type of gland is common. This figure is introduced to show
how difficult it might be to detect the lumen of the acinus in sections of such a gland.
The lumen is of very small diameter (its size is exaggerated in this diagram) and runs
such a tortuous course among the epithelial cells that even perfect cross-sections of the
acinus might fail to reveal it if it happened at that point to run obliquely to the axis
of the acinus. It would then appear merely as a small clear spot upon the granular
cytoplasm of the cell that lay immediately beneath it. ,«, s', represent the way in which
two such sections would contain portions of the acinus. The lumen in .s' would be more
easily detected than in s, because its general direction is more rectilinear and more
nearly coincident with the line of vision.

It is rarely possible to trace the connection between the ducts
and other portions of a gland in sections, for the axes of these dif-
ferent parts seldom lie in one plane. As a result of this circum-
stance, sections of glands usually present a collection of round or
oval sections of tubes or sacs, which are lined with a single layer of
epithelial cells, surrounding a lumen. The cells in the deeper por-
tions arc usually granular and cubical ; those lining the ducts are
generally more columnar in shape and less granular in character.
The deeper portions are called the alveoli or acini of the gland, to dis-
tinguish them from the duets, and the character of the epithelium they
contain differs according to the function of the gland. Sometimes
the cells are so large that they nearly fill the acini, leaving a scarcely
perceptible lumen. In other glands the cells are less voluminous
and the lumen of each acinus is distinct. It occasionally happens,
e.g., in the submaxillary glands, that the acini contain two sorts of
cells which secrete different materials. Both kinds of cell may be
present in the Bame acinus, or each kind may be confined to differ-
ent acini. In studying sections of glands it must be borne in mind
that the tangential section of an acinus would appear as a group of

THE EPITHELIAL TISSUES.

63

cells surrounded by fibrous tissue, with no trace of a lumen among
the epithelial cells (Fig. 48).

Glands develop from surfaces which are covered by epithelium.

Fia. 48.

Section of gland from human lip. (Nadler.) a, duct, cut in slightly oblique direction (lumen
oval), and probably near a branch, which would account for the apparent thickness of
its epithelial lining in the lower half; b, cross-section of acinus secreting mucus ; c, tan-
gential section of a similar acinus near its extremity and beyond the end of the lumen.
Cross-sections of the cells at the fundus occupy the centre, d, cross-section of an acinus
secreting a serous fluid, revealing a small lumen ; d', a similar acinus with a larger lumen,
probably cut near its junction with a duct; e, acinus with crescentic group of cells with
granular cytoplasm (e')> and other cells like those in b. The granular cells of small size
are considered to be cells which have discharged their secretion and are accumulating
material for a fresh supply. /, nearly axial longitudinal section of a portion of a mucous
acinus ; g, tangential section of a serous acinus ; h, fibrous connective tissue between the
acini; ?', capillary bloodvessel in the fibrous tissue.

The cells of this epithelium multiply and penetrate into the under-
lying tissues, forming little solid tongues or columns of cells (Fig. 181).
If the gland is destined to be of the simple tubular variety, this col-
umn of cells then becomes hollowed to form the lumen, the cells being

64 NORMAL HISTOLOGY.

arranged in a single layer lining the tubule. If the gland is to be
compound, the solid column of cells branches within the tissues, and
then the luminaof the different portions are formed, the epithelium
in the different parts becoming differentiated as specialization of
function develops.

The foregoing general description of the structure of secreting
glands applies to those glands which have a purely secretory func-
tion, discharging the products of their activities upon some free
surface, such as the skin or a mucous membrane. There are other
glandular organs which perform more complicated functions and
the structure of which deviates from that of .the simpler glands.
Examples of these are furnished by the liver and kidney, the struct-
ures of which must be deferred to a subsequent chapter. Other
exceptions are exemplified in the thyroid body and other "duct-
less" glands, which discharge no secretion into a viscus or upon a
free surface, but which have an alveolar structure similar to an
ordinary secreting gland. These alveoli do not communicate with
ducts, which are wanting ; but whatever products they may con-
tribute to the whole organism are apparently discharged into the
circulating fluids of the body by a process of absorption similar to
that through which the glandular epithelium obtains its materials
from those fluids, or by a direct discharge into the lymphatics. (See
chapter on Ductless glands.) This process is indicated by the term
" internal secretion," and is probably of commoner occurrence than
is usually supposed. In fact, it but represents a special interpretation
of the phenomena of interchange of material that is constantly going
on between all the cells of the body and its circulating fluids.

Epithelium is developed from the epiderm or hypoderni ; never
from the mesoderm. In this respect, as well as in its functional
rule, it differs from endothelium.

CHAPTER IV.
THE CONNECTIVE TISSUES.

The two varieties of elementary tissue that have just been con-
sidered— namely, endothelium and epithelium — owe their qualities
directly to the characters of the cells that enter into their composi-
tion. The intercellular substances are insignificant in amount and
subordinate in function.

In marked contrast to these are the tissues composing the group
known as the " connective tissues." Here the usefulness of the
tissues depends upon the character of the intercellular substances
which confer upon the tissues their physical properties. The
activities of the cells entering into the composition of these tissues
appear to be confined to the production of those important inter-
cellular substances and the maintenance of their integrity. The
(•ells may, therefore, be considered as of secondary importance in
determining the immediate usefulness of the tissues, the first place
being1 given to the intercellular substances. As was stated in the
introductory chapter, these connective tissues are essentially passive
— i. e., they are useful because of their physical characters rather
than because of any ability to transform either matter or energy.
Where the ability to accomplish those transformations is of
importance the tissues are found to be essentially cellular in char-
acter, as we have already seen to be the case in the epithelial tis-
sues.

The connective tissues may be divided into three main groups :
the cartilages, bone, and the fibrous tissues. Each of these groups
has certain general structural characters that distinguish it from
the other elementary tissues, but within each group there are
varieties which differ considerably in the detailed character of their
intercellular substances and in the arrangement of these with re-
spect to the cells.

All the elementary tissues belonging to the connective-tissue
group are developed from the mesoderm.

5 65

66 NORMAL HISTOLOGY.

I. THE CARTILAGES.

General Characters. — (1) The typical cell of cartilage is round or
oval in shape, rich in cytoplasm, and possesses one (rarely two)
nucleus of oval form and vesicular and reticulated structure.
Within the cytoplasm there are frequently one or more clear spots,
which are drops of homogeneous fluid, "vacuoles." The cells fre-
quently depart somewhat from this type. Where the tissue is
growing they are usually flattened on the sides turned toward their
nearest neighbors. This is because they are the offspring of a cell
that has recently divided, and are as yet separated by only a small
amount of intercellular substance. Under these circumstances each
cell is frequently surrounded by a thin layer of intercellular sub-
stance, probably of relatively recent formation, which differs a little
from that further from the cell and gives an appearance as though
the cell were enclosed in a capsule. With some dyes this recently
formed intercellular substance receives a somewhat different color
from that of the older intercellular substance. In older cartilage this
difference is no longer evident. Where cartilage is being replaced by

Fig. 49.

6

Hyaline cartilage. Section of human costal cartilage: a, nearly spherical cell containing
two vacuoles ; b, recently formed intercellular substance (" matrix "), separating two cells
that have been produced by the division of a single cell. There are several other
examples of a similar grouping of cells, due to the same cause, in the figure. Between
the cells is the hyaline, nearly structureless " matrix."

bone, " ossification," the cells are arranged in columns, with only a
-mall amount of intervening intercellular substance, and have a
general cubical form.

THE CONNECTIVE TISSUES.

67

(2) The intercellular substance is abundant in amount and has
received the special designation " matrix." According to the char-
acter of this matrix, the cartilages have been divided into three
varieties : hyaline cartilage, fibro-cartilage, and elastic cartilage.
In hyaline cartilage the matrix is clear and homogeneous and has
the consistency of gristle. In fibro-cartilage it is traversed by or
nearly wholly composed of delicate fibres similar to those of
white fibrous tissue, which will be described presently. In elastic
cartilage the matrix contains coarse, branching, and anastomosing
fibres similar to those of elastic fibrous tissue (vide infra).

(3) The arrangement of the cells and intercellular substances
varies considerably. Sometimes the cells are pretty uniformly
distributed throughout the intercellular substance. Sometimes they

Fig. 50.

Hyaline cartilage and perichondrium. Human costal cartilage. Same specimen as Fig. 49 .
a, group of cells formed by division, but not yet separated by matrix ; b, matrix ; c, cells
with a comparatively slight amount of cytoplasm, marking the transition from cartilage
to fibrous tissue ; d, perichondrium, composed of fibrous tissue (spindle-shaped cells with
a fibrous intercellular substance).

are arranged in groups of from two to four or even six cells. To-
ward the surface of a piece of cartilage the cells are apt to be smaller
than those nearer the centre, and are frequently flattened. Here,
also, they often lose the characters that distinguish them in the
body of the tissue, and more and more closely resemble the cells of
the fibrous tissue surrounding the cartilage. This fibrous tissue
is called the " perichondrium," and is usually not sharply defined
from the cartilage itself, the matrix of the latter becoming more
and more fibrous in character and the cells less distinctly like those

68

NOJi M. I L HIS TO LOG Y.

Fig. 51.

l**>v

Hyaline cartilage. Section fmra
human thyroid cartilage.
I Wolters.) a, perichondrium :
b, peripheral /one of cartilage
with flattened cells. In the
deeper portions of the car-
tilage the cells are larger, are
arranged in groups, and are
surrounded by recently
formed matrix. The cells in
the deepest portions of the
cartilage are vacuolated, and
about the groups of cells arc
fine granules of lime salt-.
In the matrix are numerous
anastomosing lines, which are
interpreted as fine canals, serv-
ing to carry nourishment to
the ceils in the cartilage.

tion some of the cells

typical of cartilage until the distinction
between the two tissues is lost. The peri-
chondrium is wanting over the free surfaces
of the articular cartilages.

1. Hyaline Cartilage (Figs. 49, 50, and
51). — Although under ordinary powers of
the microscope and in specimens which
have not been specially prepared the
matrix of hyaline cartilage appears clear
and almost, if not quite, homogeneous,
closer study reveals the presence of a fine
network within the clear intercellular sub-
stance. This network is thought to be a
system of minute channels through which
the nutrient fluids permeate the tissue
and reach its cells. It may be, however,
that this reticulum is of fibrous character
in which case the fibres might be more
pervious than the surrounding matrix, and
bear the same relations to the nutrition of
the tissue as a system of minute channels.
In sections stained with haematoxylin the
matrix of hyaline cartilage often acquires a
faint bluish tinge, the cytoplasm of the
cells a deeper shade of the same color,
and the nuclear chromatin a very dark
blue.

Hyaline cartilage forms the costal car-
tilages, the thyroid cartilage, the ensiform
process of the sternum, the cartilages of
the trachea and bronchi, and the tem-
porary cartilages which are subsequently
replaced by bone.

2. Fibro-cartilage (Fig. 52). — This va-
riety of cartilage is found in only a few
situations: in the interarticular cartilages
of joints, in some of the synchondroses,
in one region in the heart, and in the
intervertebral disks. In the latter situa-

possess branching processes, extending for

THE CONNECTIVE TISSUES.

69

some distance between the fibres of the intercellular substance,
and giving the whole tissue a character closely resembling that of

Fig. 52.

Fibrocartilage. Section from human intervertebral disk. (Schkfer.) The cell to the left
presents a branching process extending into the intercellular substance.

white fibrous tissue. The cells are, however, more cytoplasmic
than those of ordinary fibrous tissue.

3. Elastic Cartilage (Figs. 53 and 60). — This form of cartilage

Fig. 53.

Elastic cartilage. Section from cartilage of human external ear. (Bohm and Davidoff.)
a, cartilage-cell; b, c, network of elastic fibres in the intercellular substance; b, with large
meshes ; c, fine-meshed. Opposite a is a cell showing indications of a division of the
cyptoplasm following division of the nucleus.

is found in the epiglottis, the cornicula of the larynx, the ear, and
the Eustachian tube. The coarseness of the anastomosing fibrous
network of the matrix varies in different situations and in different

70 NORMAL HISTOLOGY.

parts of the same piece of cartilage. The reticulum is usually
more open and composed of larger fibres toward the centre of the
tissue than at the periphery, where it becomes more delicate and
finally blends with the fibrous intercellular substance of the peri-
chondrium.

It is evident, both from the structure of the cartilages and from
the situations in which they are found, that they constitute elastic
tissues suitable for diminishing the effects of mechanical shock.
This is obviously the case in the joints, where both the hyaline
and the fibrous varieties are found. Their elasticity and moderately
firm consistency are also of obvious utility in the larynx and other
air-passages and in the ear, nose, and synchondroses.

II. BONE.

General Characters. — (1) The cells of bone, called " bone-corpus-
cles," have an oval vesicular nucleus, surrounded by a moderate
amount of cytoplasm, which is prolonged into delicate branching
processes that join those of neighboring cells. (2) The intercellular
substance is composed of an intimate association of an organic
substance and salts of the earthy metals. (3) The arrangement
of these constituents is as follows : the organic basis of the inter-
cellular substance is arranged in laminae, which are closely applied
to each other except at certain points where there are cavities, called
" lacunae," giving lodgement to the bone-corpuscles. Joining these
lacunse with each other are minute channels in the intercellular
substances, " canaliculi," which are occupied by the fine processes
of the corpuscles. In the compact portions of the long bones, and
wherever the osseous tissue is abundant, the laminae are arranged
concentrically around nutrient canals, the " Haversian canals,"
which traverse the bone, anastomosing with each other and contain-
ing the nutrient bloodvessels of the tissue. In cancellated bone
these Haversian canals are absent, and the thin plates of bone are
made up of parallel laminae of intercellular substance, between
which are the lacunse, connected with each other by canaliculi. The
bone-corpuscles are nourished from the fluids circulating in the
marrow, which occupies the large spaces of this spongy variety of
bone.

It is not possible in a single preparation to study even these gen-
eral characters of bone. The earthy salts in the intercellular sub-

THE CONNECTIVE TISSUES. 71

stance prevent the preparation of sections by means of the knife,
and, unless they be removed, specimens of bone must be made by
grinding. This can best be accomplished after the bone has been
dried. But drying the bone destroys the corpuscles, which appear
as little desiccated masses, devoid of structure, within the lacuna?.
Ground sections of bone can, therefore, give only an idea of the in-
tercellular substance and the arrangement of the lacuna?, canaliculi,
Haversian canals, etc. (Figs. 54 and 55). Sections may be cut if

Fig. 54.

Ground section of dried bone. Human femur, a, Haversian canal in cross-section; a', Ha-
versian canal occupied by debris ; a", anastomosing branch from a', in nearly longitud-
inal section; b, lacuna belonging to the Haversian system, of which a' occupies the
centre ; c, lacuna in excentric laminae of bone between the Haversian systems. The
delicate lines connecting the lacunse are the canaliculi.

the bone be first decalcified — i. e., if the earthy salts be dissolved
through the action of acids. This treatment not only removes the
earthy constituents of the intercellular substance, rendering it soft
and pliable, but causes the organic constituents to swell. The
effect of this swelling upon the appearance of the bone is very
marked. The fine canaliculi are closed and the lacuna? diminished
in size, so that the structure of the bone appears much simplified,
being reduced to a nearly homogeneous mass of intercellular sub-
stance in which there are spaces arranged in definite order and
enclosing the somewhat compressed bone-corpuscles. The delicate
processes of the latter are not discernible within the canaliculi, but
blend with the swollen intercellular substance forming the walls of
those minute channels. It is important that the student should
learn to recognize these mutilated preparations of bone, since it is

72

NORMA L 1IIST0L OG Y.

in this form that the tissue will most frequently come under his
observation ( Fig. 56).

Minute study of the structure of the intercellular substance of
bone makes it appear that the organic basis is not homogeneous,
but is composed of minute interlacing fibres, held together by

From a section through the bone of a roebuck. The bone cavities (lacuna) are seen from

the side. X 850.

a cement or "ground" substance, containing the deposit of earthy
salts. To these salts, which are chiefly phosphate and carbonate
of calcium, the bone owes its hardness, while the fibres contribute
toughness and elasticity to the tissue. The general arrangement
of the fibres in the intercellular substance is in lamina1, which have
a general parallel direction ; but there are occasional fibres of some
size which pierce these laminae in a perpendicular direction and
appear to bind them together, very much as a nail would hold
a series of thin boards in place, " Sharpey's fibres."

Bone occurs in two forms, the compact and the cancellated.
These do not differ in the nature of the tissue itself, but merely

THE CONNECTIVE TISSUES. 73

in the arrangement of that tissue with respect to its sources of
nourishment. Where the bone is massed in compact form, as in
the shafts of the long bones, special means for supplying it with
nourishment is provided by a series of channels, the Haversian

Fig. 56.

Section of decalcified bone, parallel to axis of human femur, a, longitudinal section of
Haversian canal giving off transverse branch to the left; 6, tangential section of a trans-
verse branch ; c, lacuna occupied by bone-corpuscle ; d, intercellular substance deprived
of its earthy salts and so swollen that the canaliculi are obliterated.

canals, which contain the nutrient bloodvessels, and which anasto-
mose with each other throughout the whole substance of the tissue.
The nourishing lymph, derived from the blood, reaches the cells
through the canaliculi and lacunae, which connect with each other
to form a network of minute channels and spaces pervading the
bone, and not only opening into the Haversian canals, but also upon
the external and internal surfaces of the tissue.

In the shafts of the long bones the Haversian canals lie for the
most part parallel with the axis of the bone, with short transverse
branches connecting them with each other. It is around these lon-
gitudinal Haversian canals that the lamina? of bone are arranged
in concentric tubular layers. Each Haversian canal, with the
laminae surrounding it, is known as an Haversian system. Between
these Haversian systems there are excentric laminae of bone, which
do not conform to the concentric arrangement of the Haversian
systems.

74 NORMAL HISTOLOGY.

In the spongy or cancellated variety of hone the thin plates of
that tissue derive their nourishment from the lymph of the con-
tiguous marrow rilling the spaces between them, and there is no
occasion for Haversian canals. The concentric arrangement of the
laminae is, therefore, absent.

Except where bounded by cartilage at the joints, the external
surfaces of the bones are covered by a fibrous investment, the
periosteum, in which the bloodvessels supplying the bone ramify
and subdivide before sending their small twigs into the Haversian
canals of the compact bone. A few nerve-fibres also penetrate into
the bone from the periosteum. The deep surface of the periosteum
contains connective-tissue cells, " osteoblasts," capable of assuming
the functions of bone-corpuscles and producing bone. In places
where the bone is developing, these osteoblasts are large and cyto-
plasmic, somewhat resembling the cells of cuboidal epithelium.
These facts explain the importance of the periosteum for the nutrition
and growth of bone. The tendons and ligaments attached to the
bones merge with the periosteum, which has a similar fibrous struct-
ure and serves to connect them firmly with the surface of the
bone.

The central cavities of the long bones and the spaces of cancel-
lated bone are occupied by marrow, which may be of two kinds, the
" red " or the " yellow." A description of the structure of marrow
must be deferred until the other varieties of the connective tissues
have been considered.

In the embryo the parts which are destined to become bony first
consist of some other variety of connective tissue, either cartilage
or fibrous tissue. This subsequently "ossifies," during which proc-
ess it is not really converted into bone, but is gradually absorbed
as that tissue develops and replaces it.

Because of its hardness and compact structure it is, perhaps, a
little difficult to realize the plastic character of bone, and to under-
stand the growth and gradual changes that take place in a tissue that
would seem to be so permanent. These changes are wrought by the
absorption of bone already formed and the deposition of new bone.
The long bones increase in length as long as the epiphyseal cartilage
persists; this cartilage grows and is constantly being replaced by
bone formed at its junction with the shaft. Bones increase in diame-
ter by the deposition of fresh osseous tissue produced by the osteo-
blasts forming the deep layer of the periosteum. As the osteoblasts

THE CONNECTIVE TISSUES. 75

niultiplv some of them become bone-corpuscles and through their
action new intercellular substances are produced. The marrow cavity
also enlarges. This is brought about by large multinuclear cells,
" osteoclasts," which cause the absorption of the intercellular sub-
stances of the bone with which they lie in contact, producing little
excavations, the Howship's foveolse or lacunae (Fig. 57). Through

Fia 57.

Lacuna (.jmit ^
/ osteoclast)

Vr^ ' s

•?*—Bone

Marrow cells

*■«.. ---\

«• • • • •»** • ••■>••

G?'«»/ (W/ <&_

(osteoclast) .*• ' '•*

From a longitudinal section of the femur of a rabbit's embryo. X 335.

the agency of the osteoblasts and osteoclasts the bone is gradually
moulded to meet the demands upon it. In muscular individuals the
ridges and spines to which the tendons or muscles are attached
become larger and more prominent and the arrangements of the bony
plates in cancellous tissue are adapted to withstand the strains result-
ing from pressure bearing on the surfaces of the bones. The
eccentric larninse of bone already mentioned as existing between the
Haversian systems of compact bone are probably remains of former
complete systems which have been partially absorbed and replaced
by the existing systems of more recent production.

76 NORMAL HISTOLOGY.

III. THE FIBROUS TISSUES.

General Characters. — This group of elementary tissues, which
may he said to constitute the connective tissues par excellence,
includes a number of varieties which are not very sharply defined,
because of transitional modifications which bridge over the differ-
ences between the more distinct types. It will, therefore, be best
to describe these well-marked types of structure, and then to indi-
cate the direction in which they are modified in particular cases so
as to simulate in greater or less degree other typical varieties of the
same group.

(1) The cells of the fibrous tissues vary considerably in character,
three more or less distinct forms being distinguishable. First, flat-
tened, almost membranous cells with oval nuclei and nearly clear
and homogeneous bodies, possibly identical with the cells that form
endothelium ; second, granular cells, rich in cytoplasm and usually
ovoid or cubical in shape, though sometimes elongated; third, elon-
gated or fusiform cells, with oval nuclei surrounded by a moderate
amount of cytoplasm which is frequently prolonged into processes
of greater or less length and delicacy, and sometimes dividing into
branches. These three sorts of cell are present in varying relative
proportions in the different tissues belonging to this group. (2)
The intercellular substance is composed of distinct fibres, asso-
ciated with a homogeneous cement- or "ground-substance," lying
between the fibres. The fibres are of two kinds : the " white,"
non-elastic, and the elastic or "yellow." The relative abundance
of these and of the ground-substance associated with them, and
also their arrangement, vary greatly in the different members of
the group. (3) The arrangement of the constituents of the fibrous
tissues in the different varieties is so diverse that a statement of the
variations would amount to a description of the tissues themselves.
The general characters already enumerated will serve to distinguish
the whole group from all the other elementary tissues, and enable
the student to recognize the fact that a given form of the tissue
which he may have under observation belongs to this group.

Before entering upon a description of the varieties of fibrous
tissue, it will be of advantage to note the peculiarities of the two
kinds of fibres that are found among their intercellular substance.

The white, non-elastic fibres (Fig. 58) are exceedingly delicate,

THE CONNECTIVE TISSUES.

77

Fig. 58.

and appear, even under high powers of the microscope, as fine, trans
parent, homogeneous lines. They are usu-
ally aggregated into bundles of greater or less
thickness, being held together by a small
amount of the cement-substance already re-
ferred to. In these bundles the fibres run a
somewhat wavy course from one end of the
bundle to the other, but lie parallel to each
other and never branch. When treated with
dilute acetic acid, without previous hardening,
they swell and become almost invisible. They
are converted into gelatin when boiled in water.
The yellow, or elastic, fibres (Figs. 59-61) are
coarser than the white and more highly refracting,
appearing more conspicuous when viewed under
the microscope. They may be nearly straight, but
more usually run a sinuous course. At intervals they divide, and the
branches anastomose with each other to form a fibrous network, the

Fibres of white fibrous
tissue teased apart to
show the individual
fibrils.

Fig. 59.

Fig. 60.

4*

<P,)ii4t

fca

mmsmm

Elastic fibres.

Pig. r>9.— From the subcutaneous areolar tissue of the rabbit. (Schafer.)

Fig. 60.— Section of ear. (Hertwig.) The intercellular substance contains a reticulum of

coarse anastomosing elastic fibres. (See Fig. 53.)
Fig. 61.— Fenestrated membrane from a branch of human carotid artery. (Triepel.)

meshes of which may be large, as is the case in areolar tissue, or so
small and bounded by such broad fibres that the network resembles

78

NORMA L HISTOLOG Y.

a membrane pierced by somewhat elongated apertures, as is exem-
plified in the fenestrated membranes of the arteries. The forma-
tion of such a network is, however, not an essential characteristic
of these fibres, for they appear as isolated wavy fibres in some of
the fibrous tissues of open and loose structures. Elastic fibres are
not affected by acetic acid, nor do they yield gelatin on boiling in
water. According to Schwalbe, they have a tubular structure, con-
sisting of a membrane enclosing a substance called "elastin."

We may now turn our attention to the different varieties of the
fibrous tissues.

Fig. 62.

Mucous tissue. (Ranvier.) a, stellate cells with long and branching processes ; b, elastic fibres
in the homogeneous, mucoid, intercellular substance, which is not visible under the
microscope unless artificially colored. Three of the cells are represented in cross-section.

1. Mucous Tissue (Fig. 62). — The cells of this elementary tissue
are chiefly of the third variety mentioned above. They are spindle-
shaped or stellate in form, and many of them possess processes
that extend far into the intercellular substance, Avhere they may
branch and unite with the processes of neighboring cells. The
predominant constituent of the intercellular substance is a gelatinous
ground-substance, which contains a variable amount of mucin and
appears nearly, if not quite, homogeneous under the microscope.
It is this which gives the whole tissue its soft and gelatinous con-
sistency. A variable number of fibres of both the kinds already
described run through this ground-substance. The white fibres are
arranged in fine bundles, but the elastic fibres appear to be isolated
and, though they may branch, do not appear to form a network.
The manner in which the fibres in the intercellular substances of this
and other forms of connective tissue are formed is not understood.

THE CONNECTIVE TISSUES.

79

According to one view, they are produced from and by the cytoplasm
of connective-tissue cells, which, in consequence are often called
fibroblasts (Fig. 65). It is difficult to understand how this can be
true in all cases, and mucous tissue offers an example of one of
these. Another view is that the fibres are formed within the inter-
cellular " ground substance " — i. e., the apparently homogeneous
material lying between the cells and fibres — as a sort of coagulum or
segregation of the substances constituting the fibres.

Mucous tissue of a rather highly cellular character is abundant
in the embryo, where it constitutes an early stage in the development
of the fibrous tissues (Fig. 63). A variety less rich in cells forms

Ftg. 63.

Embryonic connective tissue (mesenchymatous tissue). (Bohm and Davidoff.) a, nucleus
of stellate cell ; b, cytoplasmic process. The intercellular substance is of gelatinous con-
sistency and optically bomogeneous.

the Whartonian jelly of the umbilical cord. It does not occur in
the adult under normal conditions, except, perhaps, in the vitreous
humor of the eye.

2. Reticular Tissue (Fig. 64). — The fibres of this variety of ele-
mentary tissue are disposed in extremely delicate bundles, which
anastomose with each other to form a fine mesh work. The spaces
between the fibrous bundles are filled with lymph, which is usually
so crowded with cells similar to the white blood-corpuscles that the
structure of the tissue is masked by their presence. The cells of

80

XORM. I L IflSTOL OG Y.

this tissue are flattened and closely applied to the surfaces of the
bundles of fibres, which arc SO fine that they simulate delicate
branching processes emanating from the cells. The cement- or
ground-suhstancc is reduced to a minimum, only a small amount
lyine between the fibres and the cells of the reticulum. The tissue is
bounded by denser forms of fibrous tissue, with the fibrous bundles
of which the reticulum is continuous. It is possible that reticular
tissue contains stellate cells of the third variety mentioned as occur-

Fig. G4.

Reticular tissue. Section through a lymph-sinus in a lymph-node of the rabbit. (Ribbert.)
a, nuclei of stellate cells of the reticulum; b, endothelial cells which are closely applied
to the reticulum. The lymphoid cells, or leucocytes, have been removed from the
meshes of the reticulum.

ring in fibrous tissues, as well as the thin cells already described,
which belong to the first variety. Where this is the case it is
probable that the branching processes of those cells take part in the
formation of the reticulum.

Where the meshes of the reticulum are crowded with lymphoid
cells — i. c, cells identical with some of the white corpuscles of the
blood — the tissue has received the name " lymphadenoid tissue."
This tissue is the chief constituent of lymph-glands and follicles,
and i- also found in a more diffuse arrangement in many of the
mucous membranes (Fig. 114, L).

:'.. Areolar Tissue. — This is the most widely distributed variety
of fibrous tissue. It contains all three kinds of cells mentioned at
the beginning of this section, though not always in the same relative
abundance. The intercellular substance consists chiefly of bundles
and laminae of fibres, which interlace in all directions. The white
fibres predominate over the elastic, but there are always some of the

THE CONNECTIVE TISSUES.

81

latter which either form a wide-meshed reticulum, interlacing with
the bundles of white fibres, or are applied to the latter in a sort of
open spiral, binding them together. In the developing tissue the
cement- or ground-snbstance at first fills all the interspaces between
the cells and the fibres ; but as development proceeds spaces appear
in the tissue, which are occupied by lymph and intercommunicate
throughout the tissue. The ground-substance is then restricted to
a mere cement uniting the fibres within the bundles and laminae.
The flat or endothelial cells of the tissue lie within these bundles or
are applied to their surfaces, forming a more or less perfect lining
to the lymph-spaces within the tissue and becoming continuous with
the endothelial walls of the lymphatic vessels. It is within these
spaces that the lymph accumulates after its passage through the
walls of the smaller bloodvessels, to find its way into the lymphatic
circulation. The spindle-shaped and cuboidal cells of the tissue lie
between or within the bundles of fibres embedded in the cement-
substance (Figs. 65 and G6).

Fig. 65.

Areolar tissue. Preparation from the subcutaneous tissue of a young rabbit. (Schafer.)
c\ endothelioid cell; p,p, cells with granular cytoplasm ; c, c, /, cells of the fusiform or
stellate variety not yet fully developed. The white fibres are in bundles pursuing a wavy
course; the elastic fibres are delicate and forma very open network; g, leucocyte of a
coarsely granular variety.

Areolar tissue varies greatly in different situations in the density
of its structure — i.e., in the size of the fibrous bundles and their
relative abundance, as compared with the number and size of the

6

82

NORMAL HISTOLOGY.

spaces separating them. The name is derived from that form in
which the structure is open and the courses of the fibrous bundles
very diverse, so that thev interlace, leaving relatively large spaces
betweeu them. In this form it occurs in the subcutaneous tissues,
between the muscles, forming the loose fascia? in that situation, and
in many other parts of the body where adjacent structures are
loosely connected with each other. The sinuous course of the in-
terwoven fibrous bundles renders the tissue easily distensible in all
directions and permits considerable freedom of motion between the
parts which it unites.

In other situations the spaces in the tissue are smaller and the

fibrous bundles closer together
and less tortuous in their arrange-
\\ ment, so that the parts connected

with each other are more firmly
held in place. This form of the

Fig. 66.

-

\ \ M

fb' V-

-/»

\

■
■

tissue occurs in all the glandular
organs of the body, supporting
and holding in place the func-
tional] v active tissues of the or-
gans and constituting the chief
constituents of their interstitia
(see Chapter VII.). To distin-
guish this form of fibrous tissue
from the areolar or more open
form it may be designated as con-
nective tissue in a more restricted
use of that term than has hitherto
been employed or as interstitial
connective tissue (Fig. 67, b, b').

A still denser form of the tis-
sue occurs in the fascia? and apo-
neuroses, in which the fibres are
aggregated in thick bundles and
layers that run a comparatively
straight course and are firmly held
together. Ligaments and tendons differ from these only in the
greater density of the fibrous bundles and in their parallel arrange-
ment These denser varieties of the tissues may be designated by
a restricted use of the term, fibrous tissue.

•

Cell from subcutaneous tissue of human
embryo. (Spuler.) c, centrosomc ; fb,
fibrilke in the cytoplasm of the cell : fV,
fibril detached from the cell, but evi-
dently derived from it. This cell corre-
sponds to c, C, and/, in Fig. 63. They are
sometimes called fibroblasts because of
their activity in the formation of fibres.

THE CONNECTIVE TISSUES.

83

4. Adipose Tissue (Fig. 67). — Fat or adipose tissue is a modifica-
tion of the more open or loosely-textured areolar tissue, caused by
the accumulation within the cytoplasm of the cuboidal cells of drops
of oil or fat. The cells which have become the seat of this fatty
infiltration are enlarged, and their cytoplasm, with the enclosed
nucleus, is pressed to one side, the great bulk of the cell being occu-
pied by a single large globule of fat. This globule, together with
the cytoplasm, is enclosed in a delicate cell-membrane. The fatty
cells may occur singly in the midst of an apparently normal areolar
tissue of the usual type, but they are more frequently grouped to-
form " lobules," held in position within the tissue by bands and
layers of unaltered areolar tissue.

In sections of adipose tissue prepared after hardening the tissue
in alcohol the fatty globules can no longer be seen, since the alco-
hol dissolves the fat from the tissues. The partially collapsed

Fig. 67.

Section from the tongue of a rabbit : a,a,a, groups of fat-cells forming small masses of adipose
tissue in the connective tissue ; b, b', fibrous tissue, b in longitudinal, and b' in
cross-section ; c, small vein containing a few red blood-corpuscles. Near the centre of
the figure is another bloodvessel filled with corpuscles. The remainder of the figure
represents striated muscle-fibres in nearly longitudinal section. In the upper left hand
corner these show a tendency to split into longitudinal fibres (sarcostyles).

membranes of the cells, with the cytoplasm and contained nucleus
forming an apparent thickening at one side, are all that remain to
distinguish the tissue (Figs. 67, a, and 68).

Adipose tissue is widely distributed in the body. It serves as a
store of fatty materials which can be drawn upon as a reserve

%4

NORMA L JIISTOL 0 G V.

stock of food when the nutrient supply of the body falls below its
needs.

The usefulness of the fibrous tissues can be readily inferred
from their structure. The more open varieties of areolar tissue
serve to (jive support to the structures they unite and to the blood-
vessels, lymphatics, and nerves supplied to them. They also atford
spaces and channels for the return of the lymph, which transudes
through the walls of the capillary bloodvessels, carries nourishment
to the tissue-elements it bathes, and then returns to the blood in
the veins through the interstices and lymphatic vessels contained in
the areolar tissue. In pursuance of these functions, areolar tissue
pervades nearly all parts of the body. Wherever bloodvessels
are found, there more or less areolar tissue is present, surrounding
them, giving them support, and furnishing channels for the lym-
phatic circulation. As has already been stated, this areolar tissue
varies in the closeness of its texture in different parts of the body.

The fibrous tissues of tendons and ligaments form inextensible

Fig. 68.

Fat crystals

Blood capillary'

Connective tissue
ffibrils

\_Fal cell,
in- face view

\_Fat cell,

■ lateral vieiv

Fat from the subcutaneous layer of the skin of a white mouse X 200.

bands or cords highly resistant to tensile stress, but very pliable.
They consist of bundles of fibres lying parallel to each other and to
the direction in which they are to resist pulling forces. Layers of
loose areolar tissue penetrate the ligaments and tendons, dividing
them into fasciculi, which in turn are united into larger bundles by

THE CONNECTIVE TISSUES.

85

thick layers of areolar tissue (Fig. G9). These sheaths of areolar
tissue support the vessels and nerves supplied to the denser forms
of the fibrous tissue making up the ligaments or tendons. The
thicker aponeuroses of the body may be regarded as broad and flat

Portion of a large tendon in transverse section. (Sehafer.) a, sheath of areolar tissue sur-
rounding the tendon ; 6, longitudinal fasciculus of fibres within that sheath ; I, lymphatic
space ; c, section of a broad extension of the ensheathing areolar tissue, dividing the
tendon into larger bundles ; d, e, more delicate layers of areolar tissue subdividing the
larger bundles of fibres. Between these areolar septa are the bundles of fibres constitut-
ing the tendon. The cells which lie between the smallest fasciculi of fibres appear in
stellate form ; the cross-sections of the individual fibres, among which these cells lie,
are not represented. They would appear as minute dots.

ligaments, in which the bundles of fibres run in various directions.
They present a structural transition between the fibrous arrange-
ment in ligaments and tendons and that in the more open varieties
of areolar tissue. The fibres of these tissues are mostly of the
white variety, but in some situations, notably in the ligamentum
nucha?, they are chiefly of the elastic variety.

Reticular tissue may be regarded as a special modification of
areolar tissue, in which the main bulk of the tissue consists of a
series of freely intercommunicating lymph-spaces. These are often
densely crowded with lymphoid cells, among which the lymph
slowly circulates, thereby being subjected to the modifying influ-
ences of their activities.

CHAPTER V.
TISSUES OF SPECIAL FUNCTION.

The elementary tissues included in this group are highly differ-
entiated in structure so as to adapt them for the performance of
some special function of a high order. The constituent of the
tissues which appears most highly specialized is the cell, which is
often so greatly modified in structure as to have lost many of the
general characters of the cells hitherto studied. Thus, for example,
the cells of striated muscle are multinucleated, and the cytoplasm
has become transformed into a substance known as contractile sub-
stance, which occupies nearly the whole bulk of the cell, leaving
only a small amount of relatively undifferentiated cytoplasm imme-
diately surrounding the nuclei.

In like manner the intercellular substances of some of these
tissues show a complexity of structure in great contrast to those
with which we have become familiar in the preceding tissues. In
fact, it is stretching a point to regard the tissues lying between the
cells of striated muscle as forming an intercellular substance
belonging to that tissue. In this case those tissues are identical
in structure with the loose areolar tissue that was described in the
preceding chapter. We may, therefore, with propriety, regard the
striated muscles as organs in which the muscle-cells constitute the
parenchyma and this areolar tissue the interstitium (see Chapter
VII.). But in other tissues of the group there is either an inter-
cellular substance resembling those of the preceding tissues, or
some special form of sustentacular tissue — e. g., the neuroglia of
the central nervous system.

The tissues of special function are arranged in two groups : the
muscular tissues and the nervous tissues. As is implied in the
title, these tissues are grouped together because of their functional
powers, and not with regard to peculiarities of structure, so that it
is impossible to give concise statements of any common general
structural characters possessed by all the members of each of these

86

TISSUES OF SPECIAL FUNCTION. 87

two groups. Thus, the individual muscular tissues differ consider-
ably from each other in structure, but are closely related in function,
each variety being specialized so as to execute a particular kind of
contraction when functionally active. We must also assume that
the variations in structure met with in the nervous system have
reference to the translation of various impressions into nervous
impulses, or the liberation of such impulses under different condi-
tions, as well as to their transmission and application to the func-
tional activities of other tissues.

The complex functions exercised by the nervous system appear
to necessitate a great variety of nervous structures, and it would
be a matter for surprise to find the visible structure of the nervous
system as simple as it is, were it not for the fact, already learned,
that cells apparently similar in structure may have widely different,
though related, functional powers.

I. THE MUSCULAR TISSUES.

There are three varieties of muscular tissue, which differ from
each other both in structure and in the character of their functional
activities. One variety is that found in the walls of the hollow
viscera and larger bloodvessels. Its activities are not under the
control of the will, and the cells are devoid of marked cross-stri-
ation of the contractile substance. It has, therefore, received the
names, " involuntary " or " smooth " muscular tissue. The other two
varieties present distinct and rather coarse cross-striation of the
contractile substance, but differ in other structural details. One of
these is called "voluntary" or " striated" muscle; the other is found
only in the heart, is not under the control of the will except in
rare instances, and is known as "cardiac" muscular tissue.

1. Smooth Muscular Tissue. — This elementary tissue is composed
of elongated or fusiform cells, which gradually taper to a sharp
point. The body of the cell, except close to the ends of the nucleus,
consists of a modified cytoplasm, called "contractile substance,"
which stains a coppery red with eosin, and presents fine, indistinct,
longitudinal and transverse markings, possibly the optical expression
of certain ridges that are in contact with similar ridges on neigh-
boring cells. Each cell has a single, greatly elongated, rod-shaped
nucleus situated in its centre, with the long axis coincident with
that of the cell (Fig. 70). The nuclei are vesicular and possess

NX

NORMAL HISTOLOGY.

Fig. 70.

a distinct intranuclear reticulum of chromatin. The intercellular
substance is a mere cement of homogeneous character.
The cells are arranged with their long axes parallel to
each other and with the tops of their minute ridges in
contact, So that fine channels exist between the contiguous
cells. This is apparently a provision for the circulation
of nutrient fluids between the cells (Fig. 71).

Recent studies of the intercellular regions of smooth
muscular tissue cast some doubt upon the existence of the
minute ridges described above. The appearances taken
for such ridges may, in reality, be produced by a fine
mesh work of delicate fibrils which encircle the muscle-
cells and bind them together. In the spaces between these
fibrils there is opportunity for circulation of the tissue-
fluids. This view also does away with the idea of a cement-
substance holding the cells together. The reactions which
led to the belief in a cement may be due to proteids in the
tissue-fluids present in this situation. This conception
would make the structure of smooth muscular tissue, as far

Fig. 71.

Smooth muscular tissue.

Pig. 70.— An isolated fibre from the muscular coat of the small intestine. (Schiifer.) The
nucleus is somewhat contracted, so as to appear broader and shorter than when in the
extended state.

Fig. 71. -Cross-section of smooth muscular tissue : human sigmoid flexure. (Barfurth.) Two
of the muscle-cells have been cut in the region occupied by the nucleus, which appears
in round cross-section. The other cells have been cut between the site of the nucleus
and the end of the cell. The structural details of the cytoplasm or contractile substance
are not represented, but the connecting ridges of the cells, with the channels between
them, are shown. These minute ridges can, however, only be seen when the tissue has
been exceptionally well preserved ami is studied under a high power of the microscope.

as the intercellular substances or structures are concerned, analogous
to those of striated and cardiac muscles.

TISSUES OF SPECIAL FUNCTION. 89

Smooth muscular tissue occurs in the form of bundles or layers,
in each of which the cells or fibres run in the same direction. The
tapering ends of the individual cells interdigitate with each other,
masking the intercellular substance, so that the tissue appears as
though wholly composed of cells. Surrounding the muscular
bundles or between the layers of that tissue is vascularized areolar
tissue, giving it support and containing its nerve-supply.

The microscopical appearances of sections of smooth muscular
tissue depend upon the direction in which the individual cells have
been cut. A brief analysis of the different appearances that may
result will be useful as an illustration of the way in which micro-
scopical appearances must be interpreted in order to gain a correct
conception of the structure of an object under observation. It is
rarely that sections happen to be made in such a direction that they
reveal the complete structure of an object. It is nearly always
necessary to study the appearances presented by the section, and to
infer what the structure of the object must be in order to yield the
appearances seen. This is sometimes a matter of considerable
difficulty.

If the plane of the section lie parallel with the long axes of
the cells, the nuclei of the latter will appear as rod-like or long,
oval bodies lying parallel to each other and distributed at regular
intervals throughout the tissue. The outlines of the cells will be
distinctly visible in some places, but in most of the section the
boundaries of the deeper cells will be obscured by the bodies of
the cells at the surface of the section, and the borders of the latter
will be difficult of detection, because in many places the knife has
left only a portion of the cell with a very thin and transparent
edge (Figs. 72 and 73). For the practical recognition of the tissue,
when cut in this direction, we must, therefore, in many cases,
depend solely upon the shape and distribution of the nuclei and
the color of the material between them after the section has been
treated with certain stains (e. g., eosin).

If the cells of the tissue have been cut perpendicular to their
long axes, the section will contain true cross-sections of the indi-
vidual fibres. These appear as round, oval, or, more usually,
polygonal areas of various size, according to the part of the cell
included in the section. If the cell has been cut near one of its
ends, the cross-section will be small ; if near the middle, it will be
large, and will contain a cross-section of the nucleus, situated near

90

NORMAL HISTOLOGY.

Fig. 73.

Diagrams illustrating the appearance of a longitudinal section of smooth muscular tissue.
The distance between the lines A A and B B in the upper figure represents the thickness of
the section, the line A A being in the plane of its upper surface. The line rein the lower
figure is in the plane of the transverse section represented in the upper figure.

It will he noticed that only portions of the cells, h, a. <l. <• and ./". will he contained in the
Longitudinal section (lower figure). The upper cut surfaces of those cells will appear as
..: ana- when Been from above, h', a', ii',,', f. Where the edges of those sections are
thin— e. g., o— the outlines of the corresponding oval fa') will be difficult of detection.
At the same time those portions of cells which lie at the top of the section will obscure
the outlines of the cell- beneath. Tlius. at the point b the outlines of the cells t and e
will he difficult of detection because covered hv the cells I: and <<. and also because the
cell i overlaps the cell e. II the plane of junction were perpendicular to the surface of
the -.it ion. the outlines of / and e would 'he much more clearly defined.

This brief analysis will serve to show that the outlines of the cells will rarely be seen with
distinctness in longitudinal section- of smooth muscular tissue. On the other hand, the
nuclei of the cells will he prominently visible in stained sections because of the color
they have received. Pot the recognition of this tissue, when so cut, we must, therefore,
depend chiefly upon the character and distribution of the nuclei. _

In order to avoid an unnecessarily complicated diagram, many of the cells represented in
the upper cut have been omitted from the lower figure.

TISSUES OF SPECIAL FUNCTION.

91

its centre and appearing as a round dot (Fig. 74). It is in
such sections that one may sometimes see the minute prickles
or ridges, already referred to, projecting from the cell-bodies
and joining with those of the contiguous cells to form delicate

Fig. 74.

Fig. 75.

bed

I ' i

I I

&&6

v

Diagrams of smooth muscular fibres cut in various directions.

Fig. 74.— Fibres cut exactly perpendicular to their long axes. The lines A A and B B in the
upper figure indicate the portions of the fibres included in the section, which is viewed
from above in the lower figure. The cross-sections of the fibres a and 6 contain cross-
sections of the nuclei ; those of c and d are smaller and devoid of nuclei.

Fig. 75 Fibres cut obliquely to their long axes. When the upper surface of the section,

marked by the line A A in the upper figure, is in sharp focus, the sections of fibres appear
as in o, 6, c, and d of the lower figure. When the bottom of the section, indicated by the
line B B in the upper diagram, is in clear view, the sections of the fibres appear as shown
at a', b', c', and d'. It will be noticed that the optical section of fibre a in the upper dia-
gram has moved from a to a' in the lower diagram. As the focus was changed, the nucleus
in fibre a was constantly present and its optical section appeared of uniform size. This
could only be the case when the nucleus was rod-shaped and of the same diameter
throughout that portion contained in the section. In the fibre d the nucleus was visible
when the upper surface of the section was in focus, but disappeared when the focal plane
was depressed.

02

NORMAL HISTOLOGY.

bridges across the narrow intercellular spaces. The only tissue
with which this aspect of smooth muscular tissue is liable to be
confounded is dense fibrous tissue, as seen in the cross-sections of
tendons or ligaments (Fig. 69). There we also see polygonal areas
of various sizes, separated for the most part by only a thin layer of
cement. But these areas never contain nuclei, because they are
composed, not of cell-bodies, but of intercellular substance. The
nuclei of the flattened connective-tissue cells may be seen here and
there apparently lying within the cement, the body of the cell being
so thin as often to escape observation. These differences render it
easy to distinguish the two kinds of tissue in spite of their general
similarity when seen in cross-section.

It is, of course, rarely that sections contain smooth muscular

Fig. 76.

Diagrams of smooth muscular fibres cut very obliquely. The explanation of Fig. 72, already
given, will make this one clear. In this case the outlines of the fibres in section will be
less sharply defined than in the preceding case, because, for instance, at the point a the
fibre a' is cut so as to leave only a thin edge, difficult of detection, and the fibre b has had
such a thin slice removed from it that the loss would be hardly perceptible. The appear-
ance of the section would, therefore, be much less easy of interpretation than is repre-
sented in the lower figure, where the outlines of the sections are equally distinct
throughout.

tissue cut exactly in either of the directions just considered. In the
majority of sections that come under observation the muscle-fibres
are cut obliquely, and the oval or polygonal areas which result are,
therefore, elongated. The nuclei of the cells lie at an angle with
the line of vision, and, in consequence, appear foreshortened. If
now we focus the instrument so as to get a sharp image of the upper
surface of the section, and then rapidly turn the fine adjustment so
a- to bring the lower surface into focus, we shall notice an apparent
lateral motion of the nuclei and cell-sections. This apparent lateral
movement, due to change of focus, is an evidence of the elongated
shape and oblique position of the objects exhibiting it; and a little
reflection will convince the student that such oblique sections, when

TISSUES OF SPECIAL FUNCTION.

93

-carefully studied, are better calculated to reveal the shapes and rela-
tive positions of the tissue-elements than either perfect longitudinal
or cross-sections. He should seek to train his powers of observa-
tion so that he may readily interpret the instructive, though at first
confusing, images presented by such sections (Figs. 75 and 76).

Smooth muscular tissue is not under the direct control of the will.
For this reason it is frequently called " involuntary muscle." It is
also sometimes designated as non-striated or unstriped muscle, in
contradistinction to the other two varieties of muscular tissue, the
fibres of which present distinct cross-striations.

The functional contractions of smooth muscular fibres are slug-
gish. The fibres are slow in responding to stimulation, contract

Fig. 77.

Diagrams of cardiac muscular tissue.

./I, longitudinal section: a, nucleus of muscle-cell ; 6, unmodified cytoplasm; c, contractile
substance with longitudinal and transverse striations ; d, cement-substance uniting con-
tiguous cells; e, areolar tissue (vessels omitted) between the muscle-fibres formed by the
union of the individual cells ; /, small bloodvessel within the areolar tissue. If the lines
of junction between the cells were not visible, the tissue would appear as though com-
posed of interlacing and anastomosing fibres, none of which could be traced for any con-
siderable distance. Such is the usual appearance of longitudinal sections of cardiac
muscle.

B, transverse section : a, section of a cell, including the nucleus ; c, section above the nucleus
and just below a crotch formed by the divergence of a branch; 6, section above the
nucleus and the point where the branch 6' is given off.

leisurely, maintain the contracted condition for a relatively long
time, and then gradually relax. These properties render the tissue
of value in conferring " tone " to certain structures in which it is
found, notably the walls of the arteries and veins. They also
render it of service in producing the vermicular movements that are

94

NORMA L IIISTOL 0 G Y.

essential for the functional activity of such organs as the stomach
and intestine.

Smooth muscular tissue has a wide distribution in the body. It
is found in greatest bulk in the uterus, middle coats of the arteries
and veins, the muscular coats of the stomach and intestine, and the
wall of the bladder ; but we shall find it present in greater or less
amount in many of the organs, the structure of which we shall
presently have to study.

2. Cardiac Muscular Tissue (Figs. 77 and 78). — The heart-muscle
is composed of cells having a general cylindrical form and contain-
ing a single (occasionally, two) nucleus. The nucleus is vesicular,
has a distinct reticulum of chromatin, and is usually oval. It is

Fro. 78.

ffer*ftv-.^-< r. \^--i' ".vf^:. ■■■' "2-. - .v ■ -v-dSlo^

•-. t\>z

$Ml$gi&Q&-?.m1 :^i£6?l

'"•i-f.T

'0S^

t'V'1

Section of human heart. The direction of the section is such thatthe muscular cells are cut
exactly perpendicular to their long axes, a, intermuscular areolar tissue. From this, more
delicate fibrous tissue penetrates between the muscle-fibres forming the muscular bundles,
which are imperfectly separated from each other by the broader septa of fibrous tissue.
b, muscle-cell cut beyond the nucleus; c, cell cut so as to include the nucleus; d, cell cut
jus! below a branch. The index line d points to that part of the cell which passes into
the branch. The granular character of the contractile substance when seen in cross-
section has been omitted from the figure. At the lower edge of the figure the section
has been torn, but a small amount of the subpericardial areolar tissue is represented.

situated near the centre of the cell, and is surrounded by a small
amount of cytoplasm, which is a little more abundant at the ends
of the nucleus. The rest of the cell-body is composed of contractile

TISSUES OF SPECIAL FUNCTION.

95

substance, a modification of the cytoplasm of which the cell was
first composed, which presents a tine longitudinal and a somewhat
coarser transverse striation due to the arrangement of the " sarco-
plasmic disks " of which the contractile substance or sarcoplasm is
composed. The proper intercellular substance was formerly regarded
as a homogeneous cement, which lay between the ends of the cells,
but there appear to be delicate protoplasmic bridges connecting con-
tiguous cells (Figs. 78 and 79). The cells are arranged end to

Fig. 79.

Fig. 80.

Longitudinal section of adult human heart
muscle, showing the junction of two

cells. (MacCallum.)

Longitudinal section of heart muscle Irom
an adult dog, showing protoplasmic
bridges between two cells. (MacCallum.)

end so as to form fibres, the lines of junction between them being
usually invisible. The cells give off branches which unite with
each other in such a way as to convert the heart-muscle into a
reticulum of muscular fibres. The meshes of this reticulum are
occupied by areolar tissue, in which the vascular and nervous supply
of the tissue is situated. Where this tissue is abundant it may also
contain a few fat-cells. The cardiac muscle-cells are destitute of a
cell-membrane, in which respect they differ from the fibres of
voluntary striated muscle.

When seen in longitudinal section it is difficult to trace a given
muscle-fibre for any considerable distance, because the occasional
anastomosing branches of the cells cause a blending of the neigh-
boring fibres with each other. In cross-section the cells have a

96

NORMAL HISTOLOGY.

round, oval, or polygonal shape, and vary considerably in size,
owing to the branching. Their cut surfaces are dotted with the
minute polygonal cross-sections of the elements of the contractile
substance, which give the cell its appearance of longitudinal stri-
ation. These elements are called the " sarcostyles."

Cardiac muscle occurs only in the heart. It is not under the
control of the will, but differs from the other involuntary muscles
in the force and rapidity of its contractions, which resemble those
of the voluntary muscles.

3. Striated Muscular Tissue (Figs. 81-84). — The voluntary muscles
have for their characteristic tissue-element greatly elongated, multi-

Fig. 81.

Fig. 82.

r^;:::::::;::::;::::::;:::::::-,

<"""

u '

tn*1 ..','. ' *••'"-

V r. .'... "•■';

;■:;:::--'■; K::"£

-", ' 4,,, *.'

I,;::::::::: ■■;: :::--::::::.:;'

fti; ' ,'.'....' '■■■'■'■

t....:::::;::.v.:::.:::::;:::;:;;;..:.,. ;

iili.'!:::.-::::::"5fe:"i:;::::::;;:;;..i*i

^;..,:::::::::::::::::;::::::::,..^;
|ii;:;::;:;::4::::;;:;::::::i 3

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- t

;:::::::::::::;::^::::::::':::::::>-;

:,.• :;:. „;; '

•!• .„i

s!:.-::::::::::::::::::;;::::::;;;...,.^

Striated muscular tissue.

Fig. 81. — Portion of a muscle-fibre from a mammal. (Schafer.) This figure represents the
appearances of the fibre when the surface is in sharp focus.

Pig. 82.— Termination of a muscle-fibre in tendon. (Ranvier.) c, contractile substance: p,
retracted end of contractile substance, separated from the sarcolemma during the prep-
aration of the specimen; in, sarcolemma, slightly wrinkled; 8, sarcolemma in contact
with fibrous tissue of tendon; t, tendon.

nucleated, cylindrical cells. The body of these cells is almost ex-
clusively composed of a very complex, contractile substance which pre-
sents both longitudinal and transverse striatums, the latter much coarser

TISSUES OF SPECIAL FUNCTION.

97

Fig. 83.

7. •♦•♦•••••»•«••••«»,»

wwmwnmm

j

jh life

••«•«••••< ••»••♦«

ilill

■••!••■ ■• ■•••••

I II •

11 IHIIM it < On

'»

♦z

5

Striated muscular tissue.

Fig. 83.— Diagrams of the structure of the contractile substance. (Rollet.) Q, sarcous elements,
appearing dark in A, light in B; Zand J, sarcoplasm. The sarcoplasm also lies between
the sarcous elements in Q, appearing as light bands in A and as dark lines in B. A is the
appearance of the fibre when the focal plane is deep ; B, the appearance when the focal
plane is superficial (see Fig. 81). The dots Z in A and Jin B are optical expressions of
differences in the refraction of the sarcoplasm and sarcous elements, and do not repre-
sent actual structures. A complete explanation of the way in which a microscopical im-
age may contain apparent objects which have no actual existence cannot be entered into
here. It is due to the fact that regularly alternating structures of different powers of re-
fraction affect rays of light very much as they are affected by a fine grating, producing
diffraction spectra. These spectra may interfere with each other, occasioning an alter-
nation of light and dark bands or areas above the specimen. When the focal plane is
changed the light areas become dark and the dark areas light, but sometimes with an
alteration in their outline and relative sizes, as exemplified in the cuts.
Fig. 84.— Cross-section of a muscle-fibre. (Rollet.) The fine reticulum, collected into larger
masses at a few points in the midst of the contractile substance, is composed of sarco-
plasm. The clear areas within this reticulum are the cross-sections of the sarcous ele-
ments. These cross-sections are sometimes called "Cohnheim's areas." Immediately
beneath the sarcolemma are cross-sections of two nuclei.

and prominent than the former. It must suffice us to consider this
contractile substance as made up of a number of prismatic bodies,

7

98 NORMAL HISTOLOGY.

" sarcous elements," which are arranged end to end to form eol-
umns, sarcostyles, extending parallel to eaeh other, from one end
of the cell to the other. The sarcous elements of all the sarcostyles
lie in planes perpendicular to the long axis of the cell. It isr
therefore, possible to separate the contractile substance into a
number of fibre-like columns (sarcostyles, Fig. 67), made up of sar-
cous elements attached at their ends, or to split it transversely into
disks composed of sarcous elements lying side by side. Between
the sarcous elements is a substance which has received the name
" sarcoplasm."

The contractile substance is enclosed in a thin, homogeneous mem-
branous envelope, called the " sarcolemma." The nuclei of the cell lie
immediately beneath the sarcolemma, between it and the contractile
substance, and are surrounded by a small amount of unmodified
cytoplasm.

The muscle-fibres lie parallel to each other and to the general
direction of the muscle which they compose, and are separated by
loose areolar tissue, containing their vascular and nervous supplies.
The areolar tissue between the individual muscle-fibres is called the
endomysium and supports an abundant network of capillary blood-
vessels lying parallel to the fibres with numerous transverse anas-
tomoses. Broader septa of areolar tissue divide the whole muscle
into fasciculi and support the larger bloodvessels and nerves. These
septa are collectively known as the perimysium. Surrounding the
whole muscle and continuous with the perimysium and endomysium
is a thicker sheath of fibrous tissue, the epimysium. When seen in
noss-section the muscle-fibres are circular or polygonal in form, and
the cut surface of the contractile substance appears crowded with
small polygonal areas, the sections of the sarcous elements, between
which is the sarcoplasm. Where the nuclei are included in the sec-
tion they appear somewhat flattened and lie at the edge of the con-
tractile substance, where a thin zone of cytoplasm may sometimes
be detected around them. The sarcolemma which lies outside of these
constituents of the cell is so thin that it can rarely be distinctly seen.

The muscle-fibres are in close contact at both ends with the dense
fibrous tissue of the tendons attached to the muscle, or with the peri-
osteal fibrous tissue when the muscle appears to be attached directly
to bone — e. g., the scapula. In most of the voluntary muscles
there are occasional groups of small fibres which differ in structure
from the majority and appear to be subordinate to and certainly

TISSUES OF SPECIAL FUNCTION. 99

form a part of the " muscle-spindles," structures that contain nerve-
endings which have to do with the muscular sense. These muscle-
spindles have not been observed in the pharynx and oesophagus or
in the muscles of the eye, larynx, and penis. In other voluntary
muscles they appear to be present in the proportion of about one
spindle to one hundred muscle-fibres. The muscle-fibres connected
with these spindles are grouped in collections of from three to twenty
individual fibres. They are much smaller than the ordinary muscle-
fibre and their cross-striations much coarser. They possess a sarco-
lemma and nuclei situated immediately beneath it, but in addition
contain oval or round nuclei situated in the contractile substance-
itself. These fibres, sometimes called the fibres of Weismann, pass,
through the muscle-spindles, so that one may distinguish an intrafusal

Fig. 85.

- - -y >I4i ■ ~

Cross-section of muscle-spindle from plantar muscle of cat : c, capsule ; a. s., axial sheath ;
i. /., intrafusal fibre with central nucleus ; p. a. s., periaxial space ; s. n., medullated
nerve ; s. m., ordinary muscle-fibres. (After Huber.)

portion and extrafusal parts. Toward the centre of the intrafusal
part the nuclei within the contractile substance are more abundant
than elsewhere and may entirely displace the contractile substance,
so that the fibre in this region consists of a sarcolemma so crowded
with nuclei that they flatten each other where in contact. The
muscle-spindle consists of a capsule, continuous with the general
perimysium, which here becomes laminated and much thicker, resem-
bling in structure the outer layers of a Pacinian corpuscle. The
sheaths of the nerves which pass into the spindle blend with its
capsule, and the nerve-fibres then branch and form a spiral about
the muscle-fibres, often ending in flattened expansions in close con-
nection with the latter. Besides these sensory nerves there are motor
nerves ending in motor plates similar to those terminating the motor
nerves distributed to the ordinary muscle-fibres (Fig. 85).

CHAPTER VI.

TISSUES OF SPECIAL FUNCTION (continued).

II. THE NERVOUS TISSUES.

The nervous tissues, like the muscles, are tissues of special func-
tion, and are composed of highly specialized structures. Of these,
only the ganglion-cells, the nerve-fibres, the neuroglia, and a few of

Fig. 86.

Nerve- and neuroglia-cells from gray matter of spinal cord ; calf. (Lavdowsky.) The figure
represents two isolated ganglion-cells, with branching protoplasmic processes, and each
with a single axis-cylinder process, en. The axis-cylinder process of the lower cell gives
off a branch a short distance from the cell. Between the ganglion-cells are those of the
neuroglia. The protoplasmic processes of the nerve-cells subdivide into very delicate
fibres, which lie among those of the neuroglia-cells.

the modes of terminal distribution of the nerves will be considered

here.

100

TISSUES OF SPECIAL FUNCTION. 101

1. Ganglion- or Nerve-cells (Figs. 86 and 87). — Nerve-cells vary
greatly both in shape and size. They are rich in cytoplasm, and con-
tain an unusually large nucleus, generally spherical in shape, within
the reticulum of which there is nearly always at least one conspicu-

Fig. 87.

*»

i /

V'. ¥■
1

k ■ &
m

i ■ :;>:■■

Section of unipolar nerve-cell from gray matter of spinal cord. (Flemming.) This figure
shows the fibrillation of the axis-cylinder process and the cytoplasm of the cell, as well
as the prominent chromophilic granules in the latter.

ous nucleolus. The cell-bodies may be spherical, ovoid, polyhedral,
or stellate in form, and are prolonged into one or more long pro-
cesses. Some of these taper and branch repeatedly, the ultimate
delicate fibrils terminating in free extremities lying in the inter-
cellular substance, " dendritic processes." At least one of the
processes emanating from each cell is coarser than these dendritic
processes, and is prolonged into a nerve-fibre, forming the essen-
tial constituent of that structure. This process is called the
"axis-cylinder process." It does not branch as freely as the
other processes, but may give off one or more lateral twigs near
its origin.

It is customary to divide the nerve-cells into unipolar, bipolar,
and multipolar cells, according to the number of processes proceed-
ing from them. The unipolar cells are connected by their single
processes with nerve-fibres, and many of the bipolar cells, which
have a fusiform shape, lie in the course of a fibre with which
the two processes are continuous. In such cases one of the

102 NORMAL HISTOLOGY.

processes is an axis-cylinder process. The multipolar cells have
one axis-cylinder process, the rest being of the dendritic type
already mentioned, which are distinguished as "protoplasmic"
processes.

Nerve-cells are, as a rule, larger than the other cytoplasmic cells
of the body, with the exception of the larger epithelial cells. Their
cytoplasm is so finely granular that the cells look much more trans-
parent than those of epithelium. With a high power the cytoplasm
frequently exhibits fine striations, which are prolonged into the
processes, giving them an appearance of longitudinal fibrillation.
These appearances are due to the arrangement of the fibrils of
spongioplasm. Considerable attention has of late been given to
certain granules, which become evident in the cytoplasm when
nerve-cells have been fixed in alcohol or in acid solutions. These
granules have an affinity for dyes, " chromophilic granules," and
usually occur in groups in the neighborhood of the nucleus. Their
significance is not yet understood.

The protoplasmic processes of the nerve-cells diminish in diam-
eter as they branch, and they also present occasional varicosities,
which give them an irregular contour. They terminate either in
fine-pointed extremities or in little, knobbed ends, and do not unite
with those of neighboring cells, but form with them an intricate
interlacement of delicate nervous twigs.

The axis-cylinder processes arise in conical extensions of the cell,
and then become uniform in diameter and of a smooth contour
without varicosities. When they branch the two divisions retain
their size throughout their course until they enter into the forma-
tion of some terminal structure.

The average size of the nuclei of nerve-cells is greater than that
of the other nuclei in the body, but they appear to contain less
chromatin, and therefore stain less deeply and present a less distinct
intranuclear reticulum.

Nerve- or ganglion-cells are found in the gray matter of the
(•■iitral nervous system, in the ganglia, and sometimes in the course
of nerves and in their peripheral terminations (Fig. 88).

2. Nerve-fibres. — There are two varieties of nerve-fibres : the
white, or medullated, and the gray, or non-medullated. These
differ both in their appearance when seen by the unaided eye and
in their microscopical structure.

(a) Medullated nerve-fibres eonsist of a central cylindrical struct-

TISSUES OF SPECIAL FUNCTION.

103

ure running a continuous course from the cell giving it origin to
the peripheral termination of the nerve, called the "axis-cylin-
der" ; an external membranous envelope, the " neurilemma" ; and
a semisolid material, the " myelin," " white substance of Schwann,"
or " medullary sheath," lying within the neurilemma and surround-
ing the axis-cylinder.

The axis-cylinder is a greatly elongated process (axis-cylinder
process) springing from a nerve-cell. It is marked by longitudinal

Fic4. 88.

Small ganglion in the tongue of a rabbit: a, a', ganglion-cells ; a', cell, with the beginning
of its axis-cylinder process ; b, medullated nerve-fibre in cross-section ; c, fibrous tissue
within the ganglion (part of this fibrous structure may be composed of non-medullated
nerve-fibres) ; d, areolar tissue surrounding the ganglion and containing adipose tissue in
the upper and lower parts of the figure. To the left is a striated muscle-fibre. The gan-
glion is seen in cross-section, so that its connection with the nerves, in the course of which
it lies, is not visible.

striatums, which appear to represent exceedingly delicate fibrils
composing the axis-cylinder. These fibrils frequently separate at
the distal extremity of the nerve and take part in the construction
of the various forms of nerve-endings. A more minute study of
the axis-cylinder leads to the inference that it is composed of
spongioplasm, continuous with that of the body of the cell, and
that the appearance of longitudinal striation is due to the elongated
shape of the spongioplasmic meshwork and the greater thickness
of its longitudinal threads, the transverse threads uniting them
being much less conspicuous.

104

NORMAL HISTOLOGY.

Fig. 89.

der; p, neurilemma, rendered
distinct by the retraction of
the myelin of the medullary

The neurilemma, or external in-
vestment of the nerve-fibre, called
also the " primitive sheath," or
" sheath of Schwann," is a thin,
homogeneous membrane enclosing
the medullary substance or myelin.
At regular intervals, upon the in-
ner surface of the neurilemma, and
surrounded by a small amount of
cytoplasm, are flattened, oval nu-
clei, which appear to belong to the
neurilemma. About midway be-
c tween these nuclei the nerve-fibre

Meduiiated nerve-fibre. (Key jg COnstricted, forming the " nodes "

andRetzius.) A, node of Ran- m r °

vier: b, nucleus belonging to of Ranvier. The neurilemma ap-

the neurilemma ; c, axis-cvlin- ,1 .1 ,1 „ i

pears to pass through these nodes

without interruption, so that the

sheath, in the left-hand fig- neurilemma ol one internode is

ure the clefts of Lanterman continilOUS with that of the adja-

are shown as white lines in the ^

dark myelin. These figures are cent illternodes. At the nodes,

taken from specimens treated i ,i -.i • ,1

with osmic acid, which colors and apparently within the neun-
the fatty constituent of the lemma, is a disk, perforated for the

myelin a dark brown or black. . .

passage 01 the axis-cylinder, called
the " constricting band " of Ranvier. It may be that this
band is of the nature of a cement-substance, joining the
neurilemma of neighboring internodes ; for the latter ap-
pear to be developed from cells, probably of mesoblastic
origin, which surround the nerve-fibres after their for-
mation, becoming flattened to form membranous invest-
ments of the nerve-fibre. If this view be correct, the
neurilemma of each internode, with its single nucleus, is
to be regarded as a single, specialized cell, derived from
the surrounding connective tissues, and serving to protect
the nerve-fibre. In perfect harmony with this conception
of its nature are the facts that the nerves within the brain
and spinal cord are destitute of neurilemma, and that when
a nerve-fibre branches in its course the point of division
is always at one of the nodes of Ranvier (Fig. 89).

The medullary sheath, or myelin, is a soft material inter-

TISSUES OF SPECIAL FUNCTION.

105

posed between the neurilemma and axis-cylinder. It is not a
simple substance, but contains at least one constituent closely re-
sembling fat or oil in its chemical nature ; also a substance chemi-
cally allied to the keratin of horns and the superficial cells of the
epidermis, called neurokeratin ; and a homogeneous, clear fluid.
The way in which these constituents are combined is a matter of
doubt, the apparent structure of the medullary sheath varying
greatly when different modes of preparing the nerve for micro-
scopical study have been employed. But the neurokeratin appears
to exist as a delicate reticulum pervading the medullary substance.
The medullary sheath appears to be interrupted at irregular inter-
vals by oblique clefts, which surround the axis-cylinder like the
flaring portion of a funnel. These " Lanterman's " clefts are
occupied by a soft material, probably similar to that composing the
constricting bands (Figs. 90 and 91).

Fig. 90. Fig. 91.

^l<f

i

Y

Ax. a

Fig. 90.— Longitudinal view of portion of nerve-fibre from sciatic of dog. (Schiefferdecker.)

S, neurilemma ; T, stained substance within the clefts of Lanterman.
Fig. 91.— Cross-section from sciatic nerve of frog. (Botam and Davidoff.) A, axis-cylinder,

showing punctate sections of the fibrillfe ; B, medullary sheath stained with osmic acid ;

«, b, apparent duplication of the medullary sheath, due to the presence of a Lanterman

cleft ; C, areolar tissue between the fibres.

The medullary sheath is developed after the formation of the
axis-cylinder, and is, at first, continuous along the course of the
latter. Subsequently it becomes interrupted at the nodes of
Ranvier by the constricting disk. It seems to be derived from

106

NORMAL HISTOLOGY,

Fig. 92.

the axis-cylinder, and may, therefore, be regarded as a product of
that greatly extended arm of the cytoplasm of the nerve-cell.

The amount of medullary substance present in different nerves
varies greatly. Sometimes it is so slight as to be hardly distin-
guishable. In other cases its thickness considerably exceeds the
diameter of the axis-cylinder. It is present within the spinal cord
and brain, although not enclosed in neurilemma in those situations.

At the peripheral ends of the nerves, on
the contrary, it usually disappears before
the neurilemma.

The individual nerve-fibres are isolated
only at their extremities. Throughout
most of their course they are collected
into bundles, forming the "nerves" of
the body. Within these bundles the nerve-
fibres are held together by fibrous tissue
in the following manner : a delicate areolar
tissue containing their vascular supply lies
between the individual fibres. This fibrous
tissue is called the " endoneurium." The
nerve-fibres, thus held together, are aggre-
gated into bundles, called "funiculi," which
are surrounded by sheaths of still denser
fibrous tissue, rich in lymphatic spaces,
which are called the " perineurium." This
perineurium on its inner surface becomes
continuous with the endoneurium just de-
scribed. The funiculi, enclosed by their
perineurium, are, in turn, held together
by an areolar sheath, which has received
the name, "epineurium," and forms the
outer covering of the nerve.

The funiculi do not run a distinct course
throughout the length of the nerve, but
give off nerve-bundles, enclosed in peri-
neurium, which join other funiculi ; the
nerve-fibres themselves do not, however,
anastomose with each other.
(h) The gray, or non-medullated, nerve-fibres are, as their name
implies, destitute of medullary substance. They consist of an axis-

NVrve-fibres from the sympa-
thetic system. (Key and Ret-
zius.) All the fibres except
that marked m are non-med-
ullated. The fibre m has an
incomplete medullary sheath,
n, n, nuclei of the neurilemma.
These are surrounded by a
small amount of cytoplasm,
which is not clearly repre-
sented in the figure.

TISSUES OF SPECIAL FUNCTION. 107

cylinder, which at intervals appears to be nucleated. These nu-
clei are presumably constituents of a membranous investment or
neurilemma ; but the latter is difficult of demonstration because of
its thinness and transparency, and its constant presence is not defi-
nitely established (Fig. 92).

Unlike the medullated variety, the gray nerve-fibres frequently give
off branches, which join other fibres and constitute true anastomoses.

Non-medullated fibres are most abundant in the sympathetic
nervous system, but occur also in the nerves derived directly from
the brain and spinal cord.

3. Neuroglia. — The nerve-cells and fibres of the central nervous
system are surrounded and supported by a tissue which is derived
from the epiderm, and is called the " neuroglia." It must be re-
garded as a variety of elementary tissue having functions similar to
the connective tissues, although its origin makes its relations to the
epithelial tissues very close.

Neuroglia consists of cells, the " glia-cells," which vary consider-

Fig. 93.

^.^^^ — a

Fro. 94.

Glia-cells from the neuroglia of the human spinal cord. (Retzius.)
Fig. 93.— Three cells from the anterior portion of the white matter : n, processes extending to

the surface of the cord ; b, cell-body ; c. long, delicate process extending far into the white

matter.
Fig. 94— Two cells from the deep portion of the white matter.

ably in character, and an intercellular substance, which is for the
most part soft and homogeneous, resembling in this respect the
cement-substance found in epithelium, but which may, here and
there, contain a few delicate fibres, possibly derived from the pro-
cesses of some of the cells, or possibly of mesodermic origin, and,
in consequence, belonging to the connective tissues.

108

NORMAL JIISTOLOdV.

The glia-cells possess delicate processes, which lie in the cement-
or ground-substance and form a felt-like mass of interlacing fila-
ments, but do not unite with each other. Two types of cell may be
distinguished, but they are not sharply defined, because intermediate
forms are met with. In the first type the cells have relatively large

Fig 95.

Fig. 96.

Glia-cells from the human spinal cord. (Retzius.)
Fig. 95— Cells from the substantia gelatinosa Rolandi of the posterior horn. The cell to the

right has a long process beset with fine branches.
Fig. 96.— Four cells from the gray matter.
Figs. 93-96 are taken from specimens stained by Golgi's method, which fails to reveal the

internal structure of the cells, but is extremely well adapted to show the shapes of the

cells and their extension into fine processes.

bodies, beset with a multitude of comparatively short, very fine, and
frequently branching processes (Figs. 95 and 96). This type is most
frequently met with in the gray matter. The second type of glia-
cell is represented by cells with smaller bodies and longer and some-
what coarser processes that branch much less freely (Figs. 93 and 94).
They also often possess one particularly large and prominent proc-
ess <>f greater length than the others. The small bodies of these
cells serve to distinguish them from nerve-cells, with which they
might otherwise be easily confounded. This type predominates in
the white matter.

Aside from the processes of the glia-cells already mentioned, the

TISSUES OF SPECIAL FUNCTION.

109

central nervous system contains fibrous prolongations of the epi

thelial cells of the ependyma and central canal of

the spinal cord (Fig. 97). Fibrous constituents are

also derived from the areolar tissue which extends

into the organs of the central nervous system from

their fibrous investments, the pia mater, in company

with the vascular supply.

The central nervous system, then, consists of a
small amount of a ground-substance and a great
number of cells, most of which possess numerous
delicate fibrillar processes which interlace in all
directions. Some of these cells are the function-
ally active elements of the organs, the nerve-cells.
Others belong to the sustentacular tissue, and are
probably functionally passive, constituting the in-
terstitium. Both kinds of cell are developed from
the epiderm, and are therefore genetically closely
related to each other.

4. Nerve-endings. — Nerve-fibres terminate in two
ways : first, in free ends lying among the elements
of the tissues to which the nerve is distributed ;
second, in terminal organs, containing not only
nerve-filaments, but cells which are associated with
them to form a special structure. The simplest
mode of termination consists in a separation of the minute fibrillae

Fig. 98.

Ependyma and glia-
cells from the spi-
nal cord. (Retzius.)
a, ependyma cell in
the wall of the cen-
tral canal ; b, neuro-
glia-cell near the
anterior fissure of
the cord.

Termination of nerves by free ends. (Retzius.) Nerve-endings among the ciliated columnar
epithelium on the frog's tongue. Two goblet-cells, the whole bodies of which are colored
black, are represented. The other cells are merely indicated.

composing the axis-cylinders of the medullated fibres, or the chief
bulk of the non-medullated fibres, into a number of delicate fila-

110

NORMAL HISTOLOGY.

Fig. 99.

Fig. 100.

Termination of nerves by free ends. (Retzius.)
Fig. 99.— Two nerves terminating in the stratified epithelium covering the vocal cords of

the cat.
Fig. 100.— Nerve-fibres distributed among the cells lining the bladder of the rabbit; o, super-
ficial layer of the transitional epithelium ; bg, fibrous tissue underlying the epithelium.

merits, which branch and finally end among the tissue-elements to
which the nerve is supplied. The filaments often present small vari-
cosities, and sometimes end in slight enlargements corresponding to
one of those swellings. In other cases the terminations are filiform
(Figs. 98-100).

A more complex mode of termination is that exemplified in the
" motor-plates " of the striated muscle-fibre. Here the axis-cylinder

TISSUES OF SPECIAL FVXCTIOX.

Ill

divides into coarse extensions, which form a network of broad vari-
cose fibres, lying in a finely granular material containing two sorts
of nuclei. This whole structure lies in close relations to the con-
tractile substance of the muscle-fibre, but whether it is covered by
the sarcolemma or not is a matter of doubt. The nuclei in the
motor-plate are derived in part from the muscle-fibre, from the
cytoplasm of which the granular material surrounding the nerve-

Fig.101.

b c

Motor-plate. Tail of a squirrel. (Galeotti and LeviJ a, two branches of axis-cylinder ter-
minating in a plexus of varicose filaments; b, muscle-nucleus; c, nucleus derived from
neurilemma. The finely granular substance surrounding these structures has been
omitted.

endings appears to be derived, in part from cells similar to those
forming the neurilemma, which participate in the production of the
motor-plate (Figs. 101 and 102).

Fig. 102.

Granular bed

of end organ L

Nerve fib

End arborisation

Muscle fibre

Motor nerve-endings in the fibres of a rat's abdominal muscle. In the upper fibre two end-

plates are to be seen.

The nerves of sensation, like those supplying the striated muscles,
end in bodies in which the nervous terminations are associated with
cellular structures of peculiar form. Their consideration will be
postponed until the structure of the nervous system is described.

CHAPTER VII.
THE ORGANS.

In the lowest order of animals, the protozoa, the single cell,
which constitutes the whole individual, performs all the functions
necessary to the life of the animal; but in the higher multicellular
animals, the metazoa, those functions are distributed among a num-
ber of different but definite structures, called organs, each of which
is composed of certain of the elementary tissues arranged according
to a definite and characteristic plan peculiar to the organ.

Within each organ certain of the elementary tissues are charged
with the immediate performance of the function assigned to that
organ. These tissues are collectively termed the parenchyma of
the organ. Thus, for example, the epithelium entering into the
composition of the liver and doing the work peculiar to that organ,
constitutes its parenchyma. The parenchyma of the heart is its
muscular tissue, through the activity of which it is enabled to con-
tract upon its contents.

Functionally ancillary to its parenchyma, each organ possesses a
variety of elementary tissues, some of which belong to the connec-
tive-tissue group, which serve to hold the tissue-elements of the
parenchyma in position, to bring to them the nutrient fluids neces-
sary for their work, and to convey to them the nervous stimuli
which excite and control their functional activities. These sub-
sidiary tissues are collectively known as the interstitium of the
organ. For example, the fibrous tissue and the elementary tissues
forming the bloodvessels, lymphatics, and nerves of the liver, or of
the heart, form the interstitia of those organs.

Two sets of structures entering into the formation of the inter-
stitia of the organs — namely, the nerves and the vessels, including
those which convey blood and those through which the lymph cir-
culates— have a similar general structure in all the organs, and are
connected with each other throughout the body, forming " systems."
These systems serve to bring the various parts of the body, so
diverse in structure and function and yet so interdependent upon

112

THE ORGANS. 113

each other, into that intimate correlation that makes them subordi-
nate parts of a single organism.

Through the medium of the circulatory system the exchanges of
material essential to the well-being of each organ and of the whole
body are made possible, and through the nervous system the activ-
ities of the different parts of the body are so regulated that they
work in harmony with each other and respond to their collective
needs.

Because of their wide distribution throughout the body, we can
hardly study any structures which are not in intimate relations with
both vessels and nerves. It will, therefore, be well to consider the
structure of the circulatory system before proceeding to a study of
other organs. The study of the nervous system must, because of
its complexity, be deferred.

CHAPTER VIII.
THE CIRCULATORY SYSTEM.

The circulatory system is made up of organs which serve to pro-
pel and convey to the various parts of the body the fluids through
the medium of which those parts make the exchanges of material
incident to their nutrition and functional activities.

For some of these exchanges it appears necessary for the circu-
latins: fluids to come into the most intimate contact with the tissue-
elements ; to penetrate the interstices of the tissues and bathe their
structures. For mechanical reasons these fluids must circulate
slowly and consume a considerable time in traversing a relatively
short distance. Such a sluggish current could not avail for the
transportation of oxygen from the lungs to the tissues, and we
find that the circulatory system is divided into two closely related
portions : the haematic circulation and the lymphatic circulation.
The former is rapid, and the circulating fluid is the blood, the red
corpuscles of which serve as carriers of oxygen. The latter is slow,
and the circulating fluid, called " lymph," is derived from the liquid
portion of the blood (" the plasma "). The blood is confined within a
system of closed tubes, the bloodvessels ; but the lymph, when first
produced by transudation through the walls of the bloodvessels, is not
enclosed within vessels, but permeates the tissues or enters minute
interstices between the tissue-elements surrounding the bloodvessels.
Thence it gradually makes its way into larger spaces — lymph-spaces
— which open into the thin-walled vessels constituting the radicles
of the lymphatic vascular system. These smaller lymphatic vessels
join each other to form larger tubes, which finally open into the
venous portion of the haematic circulation, thus returning to the
blood the lymph which has made its way through the tissues.

The circulating fluids are kept in motion chiefly by the pumping
action of the heart, which forces blood into the arteries, whence it
passes through the capillaries into the veins, and thence back to the
heart. During its passage through the smaller arteries, the capil-

114

THE CIRCULATORY SYSTEM. 115

laries, and the smaller veins, a part of the plasma of the blood,
somewhat modified in composition, makes its way through the vas-
cular walls, partly by osmosis, partly by a sort of filtration, and
becomes the nutrient lymph of the tissues. The composition of
this lymph varies a little in the different parts of the body, and
this variation is attributed to some kind of activity, allied to secre-
tion, on the part of the cells lining the vessels.

The larger veins are provided with pocket-like valves, which
collapse when the blood-current is toward the heart, but which fill
and occlude the veins when, for any reason, the current is reversed.
When, therefore, the muscles contiguous to the larger veins thicken
during contraction and press upon the veins the effect is to urge
the blood within them in the direction of the heart. This accessory
mode of propulsion materially aids the heart, especially during
active exercise, when the muscles are in need of an abundant supply
of oxygen.

The large lymphatic vessels are similarly provided with valves,
and valves guard the orifices by which the lymphatic trunks open
into the veins. But the chief reason for the flow of the lymph
appears to be the continuous formation of fresh lymph, which
drives the older fluid before it — the so-called vis a tergo.

For convenient description we may divide the vascular organs
into the heart, arteries, veins, capillaries, and lymphatics.

1. The heart is covered externally by a nearly complete invest-
ment of serous membrane, the epicardium, which is a part of the
wall of the pericardial serous cavity. Its free surface is covered
with a layer of endothelium resting upon areolar fibrous tissue, and
containing a variable amount of fat.

The substance of the heart is made up of a series of interlacing
and connected layers of cardiac muscular tissue, separated by layers
of areolar tissue, which extends into the meshes of the muscle, form-
ing the interstitial tissue of the heart. The fibres in the different
layers of muscle run in different directions, so that sections of the
wall of the heart show the individual muscle-cells cut in various
ways.

The areolar tissue is more abundant and denser near the orifices
of the heart, and at the bases of the valves merges into dense
fibrous rings, which send extensions into the curtains of the valves,
increasing their strength and giving them a firm connection with
the substance of the organ. In the centre of the heart, between

1 16 NORMAL HISTOLOGY.

the auriculo-ventricular orifices and the aortic orifice, this fibrous
tissue is reinforced by a mass of fibro-cartilage.

The cavities of the heart are lined by the endocardium, consisting
of endothelium resting on areolar tissue. The deeper portions of
the epi- and endocardium merge with the areolar tissue of the body
of the heart. Smooth muscle-fibres are of occasional occurrence in
the deeper layers of the endocardium.

The auricles and the basal third of the ventricles contain ganglia,
connected on the one hand with the nerves received by the heart
from the cerebro-spin'al and sympathetic systems, and on the other
hand with a nervous plexus which penetrates the substance of the
heart and gives oif minute nervous fibrillar to the individual cells
of the cardiac muscle. These fibrillae end in minute enlargements
connected with the surfaces of the muscle-cells. Many of the gan-
glia lie beneath the epicardium or in the areolar or adipose tissue
situated in its deeper portions.

The valves of the heart are composed of fibrous tissue, con-
tinuous with that forming the rings around the orifices. Their
surfaces are covered by extensions of the endocardium, except the
outer surfaces of the pulmonary and aortic valves, which are cov-
ered by extensions of the not dissimilar inner coats of the pul-
monary artery or aorta. The fibrous substance of the valvular
pockets of those two valves are further strengthened by tendinous
strips of fibrous tissue at their lines of contact when the valves are
closed. The curtains of the auriculo-ventricular valves are also
reinforced by fibrous tissue derived from fan-like expansions of the
chordae tendineae.

2. The Arteries. — It will be best to consider first the structure of
the smaller arteries, because the individual coats are less complex in
these than in the larger arteries.

The arterial wall consists of three coats : the intima, or internal
coat ; the media ; and the adventitia, or external coat (Fig. 103).

The intima consists of three more or less well-defined layers.
These are, from within outward : 1, a single layer of endothelium ;
2, a layer of delicate fibrous tissue containing branching cells ; 3, a
layer of elastic fibrous tissue. The endothelial layer consists of
cells, usually of a general diamond shape, with their long diago-
nals parallel to the axis of the vessel they line. "When the vessel
expands these cells broaden somewhat and appear very thin. When

THE CIRCULATORY SYSTEM.

117

the vessel is contracted they are thicker and the portion containing
the nucleus projects slightly into the lumen of the vessel.

The subendothelial fibrous tissue forming the second layer of the
intima is composed of very delicate fibrils, closely packed together,
with a little cement between them, and enclosing irregular spaces in
which the branching cells of the tissue lie. Elastic fibres, spring-

Fig. 103.

>^Si

KX

mm

Branch of splenic artery of a rabbit : o, internal endothelial surface of the intima ; b,
elastic lamina of the intima (fenestrated membrane, see Fig. 61) ; c, media composed
of smooth muscular tissue encircling the vessel and therefore appearing in longitudinal
section with elongated nuclei ; d, adventitia of fibrous tissue blending above and to the
left with the surrounding areolar tissue ; e. adipose tissue, between the cells of which a
few lines of red corpuscles reveal the presence of capillary bloodvessels; /, small nerve,
containing both medullated and pale or non-medullated nerve-fibres. There are other
similar sections of nerves in the figure. To the left of the artery the section is slightly
torn, the adipose tissue being separated from the adventitia of the artery. A few red
blood-corpuscles have been extravasated near the nerve at the upper left corner of the
figure. There are also a few corpuscles within the lumen of the artery.

ing from the external layer of the intima, may here and there,
especially in the larger arteries, make their way into the subendo-
thelial layer.

The elastic lamina of the intima is formed by a network of anas-
tomosing elastic fibres, having a general longitudinal disposition with
respect to the axis of the vessel. The spaces left between the fibres
of this network vary considerably in size. Where they are small
and the fibres between them are correspondingly broad this layer
has the appearance of a perforated membrane (the fenestrated mem-
brane of Henle). Even where this membranous character of the
elastic layer is well developed, elastic fibres are given off from its

118

NORMAL HISTOLOGY.

surfaces and enter the subendothelial layer on the one side and the
median coat of the artery on the other.

The tunica media, or middle coat of the arteries, consists essen-
tially of smooth muscular tissue, with the cells arranged trans-
versely to the long axis of the vessels, so that by their contraction
they serve to diminish the calibre of the arteries.

The adventitia is an external sheath or layer of fibrous tissue

*>p

Portion of a transverse section of a human lingual artery from an adult. (Griinstein.)
o, intima; 6, media; c, adventitia; d, endothelium; e, subendothelial stratum (delicate
areolar tissue);/, tunica elastica interna (fenestrated membrane belonging to the intima) :
g, stratum subelasticum containing elastic fibres (fi) that pass from the fenestrated mem-
brane into the media; i, concentric elastic fibres within the media; .;', smooth muscular
fibres of media with elongated nuclei; fc, white fibrous tissue in media ; I, elastic fibres
radiating from the media into the external clastic tunic ; m, stratum submusculare (are-
olar fibrous tissue) ; n, tunica elastica externa ; o, stratum elasticum longitudinale (fibrous
tissue containing elastic fibres running parallel with the axis of the vessel) ; p, stratum
elasticum concentricum (fibrous tissue containing elastic fibres encircling the vessel).
The vasa vasorum supplying the tissues of the vascular wall are not represented.

which merges with the areolar tissue of the parts surrounding the
arteries and serves to support the latter without restricting the
mobility necessary for their functional activity.

THE CIRCULATORY SYSTEM. 119

In the larger arteries the muscle-fibres of the media are grouped
in bundles, which are separated by white and elastic fibrous tissue
(Fig. 104). The muscle-fibres themselves are less highly developed
than in the smaller arteries, so that the vessels are less capable of
contracting, but are more highly elastic, because of the greater
abundance of elastic fibres. In these larger arteries the boundary
between the media and the intima is less sharply defined than
in the smaller arteries, the elastic tissues of the two coats being
more or less continuous. In cross-sections of the smaller arteries
this boundary is very clearly seen, the elastic lamina of the intima
appearing as a prominent line of highly refracting material, which
assumes a wavy course around the artery when the latter is in a
contracted state. In such sections the nuclei of the endothelial
layer of the intima appear as dots at the very surface of the intima.

3. "the Capillaries (Fig. 25). — As the arteries divide into progres-
sively smaller branches the walls of the latter and their individual
coats become thinner. In the smallest arterioles the elastic tissue
of the wall entirely disappears, and the muscular coat becomes so
attenuated that it is represented by only a few transverse fibres
partially encircling the vessel. These in turn disappear, and the
branches of the vessel then consist of a single layer of endothelium
continuous with that lining the intima of the larger vessels. These
thinnest and smallest vessels are the capillaries. They form a net-
work or plexus within the tissues, and finally discharge into the
smallest veins the blood they have received from the arteries. It
is chiefly through the walls of the capillaries that the transudation
giving rise to the lymph takes place, but some transudation prob-
ably also occurs through the walls of the smaller arteries and
veins.

4. The veins closely resemble the arteries in the structure of their
walls, but relative to the size of the vessel the wall of a vein is
thinner than that of an artery. This is chiefly because the media
is less highly developed. The elastic lamina of the intima is also
thinner in veins than in arteries of the same diameter.

The valves of the veins are transverse, semilunar, pocket-like
folds of the intima, which are strengthened by bands of white
fibrous tissue lying between the two layers of intima that form the
surfaces of the valves. The valves usually occur in pairs, the
edges of the two coming into contact with each other when the
valvular pockets are filled by a reversal of the blood-current.

120 NORMAL HISTOLOGY.

Behind each valve the wall of the vein bulges slightly. Single
valves of similar structure not infrequently guard the orifiees by
which the smaller veins discharge into those of larger size.

5. The Lymphatics. — The lymph at first lies in the minute inter-
stices of the tissues surrounding the bloodvessels from which it has
transuded. In most parts of the body those tissues are varieties
of fibrous connective tissue, and contain not only the small crevices
between their tissue-elements, but larger spaces also, which have
a more or less complete lining of flat endothelial cells, but permit
the access of lymph to the intercellular interstices of neighboring
tissues. The lymph finds its way into these " lymph-spaces," and
thence into the lymphatic vessels. These begin either as a network
of tubes with endothelial walls, or as vessels with blind ends, and
have a structure similar to that of the blood-capillaries. They are
larger, however, and are provided with valves. By their union
larger vessels are formed, resembling large veins with very thin and
transparent walls, consisting of intima, media, and adventitia.
These finally unite into two main trunks, the thoracic duct and the
right lymphatic trunk, which open into the subclavian veins.
Valves are of much more frequent occurrence in the lymphatic
vessels than in the veins, but their structure is the same.

In its passage through the lymphatic circulatory system the
lymph has occasionally to traverse masses of reticular tissue con-
taining large numbers of lymphoid cells, called " lymph-glands."

That portion of the lymphatic system which has its origin in the
walls of the intestine not only receives the lymph which transudes
through the bloodvessels supplying that organ, but takes up also a
considerable part of the fluids absorbed from the contents of the in-
testine during digestion. Mixed with this fluid is a variable amount
of fat, in the form of minute globules. These globules give the con-
tents of these lymphatics a milky appearance, and the vessels of
this part of the lymphatic system have, therefore, received the name
" lacteals." They do not differ essentially from the lymphatics in
other parts of the body.

Lymph-glands. — It is a misnomer to call these structures glands,
for they produce no secretion. A better term is " lymph-nodes."

The lymph-nodes are bodies interposed in the course of the
lymphatic vessels through which the lymph-current passes. Their
essential constituent is lymphadenoid tissue.

Each node has a spherical, ovoid, or reniform shape, with a de-

THE CIRCULATORY SYSTEM. 121

pression at one point, called the " hilns." It is invested by a fibrous
capsule, which is of areolar character externally, where it connects
the node with surrounding structures, but is denser, and frequently
reinforced by a few smooth muscular fibres internally. Extensions
from this capsule penetrate into the substance of the node, forming
" trabecular," which support the structures making up the body of
the node.

The lymphadenoid tissue occurs in two forms : first, as spherical
masses, " follicles," lying toward the periphery of the node, except at
the hilus, and constituting the "cortex" (Fig. 105); second, in the

Fig. 105.

— i

'i 'i ■ .

-»-«:_ ' -lb

k>

•

Single lymph-follicle from a mesenteric node of the ox. (Flemming.) lb, wide-meshed
lymphatic sinus at periphery of the follicle. Between this and the peripheral zone of
the follicle z, and within the follicle, the reticulum of the sinus and that supporting the
cells and vessels of the follicle are not represented. The cells are merely indicated by
their nuclei, the cytoplasm being romitted. z, peripheral zone of the follicle, marked by
a close aggregation of small lymphoid cells; p, more scattered cells outside of the
peripheral zone and at the edge of the lymph-sinus. Within the zone z is the germinal
centre of the follicle, in which numerous karyokinetic figures are'present, demonstrating
the active proliferation of the cells in that region. Two such figures are also represented
within the lymph-sinus at the upper left corner. 6, bloodvessels.

form of anastomosing strands, which makes a coarse meshwork of
lymphadenoid tissue in the medullary portion of the node (Fig. 106).
The trabecular springing from the capsule penetrate the substance
of the node between the follicles in the cortex, and then form a net-
work of fibrous tissue lying in the meshes of the medullary lymph-
adenoid tissue, after which they become continuous with the mass

122

NORMA L HISTOL OGY.

of fibrous tissue at the hilus and, through it, with the capsule at that
point.

The lymphatic vessel connected with the node divides into a
number of branches, the "afferent vessels," which penetrate the
capsule at the periphery and open into a wide-meshed reticular
tissue lying between the trabecular and the lymphadenoid tissue of
the follicles and the medullary strands. This more open reticular
tissue, through which the lymph circulates most freely, forms the

Fig. 106.

Portion of the medulla of a lymph-node. (Recklinghausen.) a, a, a, anastomosing columns
of lymphadenoid tissue ; 6, anastomosing extensions of the cortical trabeculse ; c, lymph-
sinus ; d, capillary bloodvessels. The lymphoid cells in the sinus are not shown.

a

lymph-sinuses" of the node, and is less densely crowded with
lymphoid cells than the reticular tissue of the follicles and medul-
lary lymphoid tissue. The walls of these sinuses, which are turned
toward the fibrous tissue of the trabecular and their extensions in
the medulla, are lined with endothelium, and a somewhat similar, but
probably much less complete, lining may partially separate the
sinuses from the lymphadenoid tissue. However this may be, it is
certain that lymphoid cells can freely pass from the lymphoid tissue
into the sinuses, or in the reverse direction, and that there is a ready
interchange of fluids between the two.

From the sinuses the lymph passes into a single vessel, the "effe-
rent vessel," through which it is conveyed from the node at the hilus.

The arteries supplied to the lymph-node may be divided into two

THE CIRCULATORY SYSTEM.

L23

groups: first, small twigs which enter at the periphery and are dis-
tributed in the capsule and fibrous tissues of the trabeculse and the
medulla ; and, second, arteries which enter at the hilus, pass through
the sinuses, and are distributed in the lymphadenoid tissue of the
medulla and cortex. The veins follow the courses of the corre-
sponding arteries. The nerve-supply is meagre, and consists of both
medullated and non-medullated fibres. Their mode of termination
is not known.

In the centre of the follicles the reticular tissue is more open and
the lymphoid cells less abundant than toward the periphery. Mitotic
figures are of frequent occurrence in lymphoid cells in this region,
and it is evidently a situation in which those cells actively multiply.
Further toward the periphery
the reticular tissue is closer
and very densely packed with
small lymphoid cells, to be-
come more open again and
freer of cells as it passes into
the reticulum of the sinus
(Fig. 107). This last reticu-

Fig. 108.

Fig. 10'

'.-5

0

xmr: r-a^- 'fi'i^ a -*-•><-•

Fig. 107.— Portion of lymph-follicle from mesentery of ox. (Flemming.) z, peripheral zone
of small, closely aggregated lymphoid cells. To the right of these is a portion of the
germinal centre of the follicle, with larger cells, many of which are dividing. Opposite
Msa cell executing amoeboid locomotion, pz, pigmented cell, which has taken upcolored
granules from outside; tk, dark chromophilic body, the nature of which has not been
determined. Such bodies occasionally occur in lymph-nodes, but their origin and sig-
nificance are unknown.

Fig. 108.— Section of a small portion of the reticulum of the sinus in a human mesenteric
node. (Saxer.) 6, b, diagrammatic representation of a portion of the neighboring
trabecula.

lum becomes continuous with delicate fibres given off from the
tissues of the capsule and trabeculse (Fig. 108). The distribution

12-1

NORMA L HISTOLOG Y.

of the lymphoid cells gives the follicles a general concentric appear-
ance.

The lymph-follicles of the cortex not infrequently blend with
each other, and the activity of the cellular reproduction in their
centres varies considerably and is sometimes entirely wanting, when
the concentric arrangement of the cells disappears.

The structure of the lymph-nodes causes the lymph entering them
to traverse a series of channels, the " sinuses," which, in the aggre-
gate, are much larger than the combined lumina of the vessels sup-
plying them. The velocity of its current is, therefore, greatly re-
duced, and it remains for a considerable time subjected to the action
of the lymphoid cells in and near the sinuses. Small particles which
may have gained access to the lymph in its course through the tis-
sues are arrested in the lymph-nodes, and are either consumed by
phagocytes — i. e., cells possessing the power of amoeboid move-
ment and capable of incorporating foreign substances — or are con-

Fig. 109.

Section of red marrow; human. (Bohm and Davidoff.) a, a, ery throblasts ; b, b, myelocytes ;
6'. myelocyte undergoing division : c, giant-cell with a single nucleus ; c', giant-cell with
dividing nucleus; d, reticulum ; < , space occupied by a fat-cell (not represented);/, gran-
ules in a portion of an acidophilie cell.

veyed into the marginal portions of the follicles, where, if insus-
ceptible of destruction, they remain. It is in consequence of this
process that the lymph-nodes connected with the bronchial system

THE CIRCULATORY SYSTEM. 125

of lymphatics are blackened as the result of an accumulation of
particles of carbon that have been inhaled and then absorbed into
the lymphatics.

The lymph-nodes may, therefore, be considered as filters which
remove suspended foreign particles from the lymph ; but it is
probable that the dissolved substances in the lymph are also
affected in its passage through the nodes, and that a purification
of that fluid is thereby occasioned. A fresh access of leucocytes
further alters the character of the lymph during its transit through
the lymph-nodes.

Bone-marrow (Fig. 109). — In early life the medullary cavities of
the long bones, as well as the cancellae of the spongy bones, are all
occupied by that form of marrow known as " red " bone-marrow.
This is functionally the most important variety. In after-life the
marrow in the medullary cavities of the long bones becomes fatty
through infiltration of its cells with fat, which converts them
into cells quite similar to those of adipose tissue. Marrow so modi-
fied is called "yellow" marrow. It may subsequently undergo a
species of atrophy, during which the fat is absorbed from the cells
and the marrow becomes serous, fluid taking the place of the mate-
rials that have been removed. This process results in the produc-
tion of a " mucoid " marrow.

The marrow of bones possesses a supporting network of reticular
tissue not unlike that of the lymph-nodes. In the meshes of this
tissue are five different varieties of cell (Fig. 110) : First, myelo-
cytes, cells resembling the leucocytes of the blood, but somewhat
larger in size and possessing distinctly vesicular nuclei. They are
capable of amoeboid movements, and not infrequently contain gran-
ules of pigment which they have taken into their cytoplasm.
Second, erythroblasts, or nucleated red blood-corpuscles, which
divide by karyokinesis and eventually lose their nuclei, becoming
converted into the red corpuscles of the circulating blood. Third,
acidophilic cells, containing relatively coarse granules having an
affinity for " acid " anilin-dyes, such as eosin. These cells are
larger than the majority of the leucocytes circulating in the blood.
Their nuclei are spherical or polymorphic and vesicular. Fourth,
giant-cells with unusually large bodies and generally several nuclei,
though occasionally only one nucleus is present. They possess the
power of executing amoeboid movements and appear to act as phago-
cytes. Where absorption of bone is taking place they are found

L26

NORMAL HISTOLOGY.

closely applied to the bone that is being removed, and have in this
situation been called " osteoclasts." Fifth, basophilic cells, or plasma-
cells, the cytoplasm of which contains granules having an affinity

Fm. 110.

Cells from bone-marrow: a, small leucocyte from circulating blood, with highly chromatic
nucleus and slight amount of cytoplasm, a " lymphocyte " probably derived from a lymph-
node; b,b, myelocytes, larger than «, with vesicular nuclei; c, c, c, erythroblasts, with
nuclei in karyokinesis ; c', mature red corpuscle (erythrocyte) ; tf,acidophile (eosinophile)
leucocyte. The basophilic leucocytes, or plasma cells, resemble this, but have smaller
and less abundant granules of different chemical nature : e, giant-cell (myeloplax) with
three nuclei ; a, b, r, and d, from the marrow of the fowl (Bizzozero), the red corpuscles of
which are oval and nucleated, c' ; c, from the marrow of the guinea-pig. (Schafer.)

for " basic " anilin-dyes, such as dahlia. These cells are relatively
large, and possess vesicular and frequently polymorphic nuclei.
Aside from these cells, which may be regarded as forming a part
of the marrow, it contains red blood-corpuscles and leucocytes,
either formed within the marrow or brought to it by the circulating
blood.

The functions of the various cells in bone-marrow have not been
finally determined, but it is certain that the erythroblasts, by their
multiplication and transformation, maintain the supply of red cor-
puscles circulating in the blood.

The arteries supplied to the marrow divide freely and open into
small capillaries, which appear subsequently to dilate, and either to
blend with the endothelial elements of the reticular tissue or to
become pervious through a separation of the cells forming their
walls. In either case 1 lie blood passes into the meshes of the retic-
ular tissue, where it slowly circulates among the constituents of the
marrow. It then passes into venous radicles devoid of valves, and
is thence conveyed from the bone. In some animals — e.g., birds —
the production of red corpuscles appears to be confined to the venous

THE CIRCULATORY SYSTEM.

127

radicles (Fig. 111). The veins leaving the bones are abundantly
supplied with valves.

Section of small venous radicle in marrow of the fowl. (Bizzozero.) Just within the vascular
wall is a zone of leucocytes, one of which contains a karyokinetic figure. Within this
zone is a second zone of erythroblasts, four undergoing division, and in the centre of the
lumen are a number of matured red blood-corpuscles (containing nuclei in the case of
birds). The cytoplasm of the leucocytes contains no haemoglobin, while that of the
erythroblasts does. In birds and, probably, in other classes of animals the marrow
of the bones is one of the sites for the production of leucocytes as well as red corpuscles.
The latter are not produced from the former, but only from the erythroblasts, which con-
stitute a distinct variety of cell.

Throughout life the cancellated portions of the flat bones and of
the bodies of the vertebras contain red marrow, but the shafts of
the long bones are occupied by the yellow variety, which has lost
its power of producing red blood-corpuscles and leucocytes, and
has, therefore, become functionally passive.

CHAPTER IX.

THE BLOOD AND LYMPH.

The blood consists of a fluid, the plasma, in which three sorts
of bodies are suspended : the red corpuscles, the leucocytes or
white corpuscles, and the blood-plates.

The plasma is a solution in water of albuminous and other sub-
stances. Some of these are of nutritive value to the tissues of the
body. Others have been received from those tissues, and are on
their way toward elimination from the body. Still other con-
stituents have passed into the blood from one part of the body, and
are destined to be of use to other parts.

In the smaller vessels, while on its course through the circulatory
system, portions of the plasma make their way through the vascular
walls and form the fluid of the lymph. This passage appears to be,
in part, a simple filtration through the walls of the vessel, or the
result of osmosis ; in part, the result of a species of secretion

Fig. 112.

a 6 c

Red corpuscles from human blood. (Bohin and Davidoff.) a, optical section of a red blood-
corpuscle, seen from the edge; b, surface view. (The bounds of the central depression
are made a little too distinct in this figure, as is evident from an inspection of a.) c,
rouleau of red corpuscles. When undiluted blood has remained quiescent for a few
moments the red corpuscles arrange themselves in such rows, probably because of the
attraction which they, in common with other bodies suspended in a fluid having a nearly
identical specific gravity, have for each other.

effected by the endothelial cells lining the bloodvessels, these cells

promoting the escape of certain constituents of the plasma and

restraining or preventing that of others. In the exercise of this

secretory function the endothelia in different parts of the vascular

system appear to act differently, the composition of the fluid passing

through the walls of the vessels not being exactly the same in all

parts of the body. It is still a question, however, in what degree
128

THE BLOOD AND LYMPH. 129

the endothelial cells are active in bringing about these differences.
Their character is not such as would be expected of cells carrying
on active processes. Since the lymph is a solution separated from
the blood by membranous capillary walls, and also coming into
most intimate relations to the cells and intercellular substances of the
tissues, the purely physical processes which might affect its composi-
tion are by no means simple. They may be grouped under three
heads : filtration, osmosis, and diffusion. When fluids containing
proteids are filtered under pressure, the filtrate contains a smaller
percentage of proteids than the original fluid. The element of filtra-
tion may, therefore, account for the smaller proteid content of the
lymph than that of the blood-plasma. Osmosis is the flow of water
through a membrane from a solution of lower molecular concen-
tration to one of higher molecular concentration, and diffusion is the
passage of dissolved solids from a solution of greater concentration with
respect to a given substance to a solution which contains that sub-
stance in less concentration. By the term " molecular concentration "
is meant the relative number of molecules, free atoms, or radicles
•contained in a given bulk of liquid. The molecular concentration
of the body fluids is usually measured by the depression of the
freezing-point, as compared with that of pure water. For example,
a solution of urea containing 60.12 grams in a litre and a solution
of glucose in which 180.12 grams of the sugar are dissolved in
enough water to make 1 litre of solution both freeze at a temperature
1.85 degrees C. below that of pure water — i. e., they depress the
freezing-point 1.85 degrees C. If these solutions were placed on
opposite sides of a membrane, there would be no osmosis — i. e., no
water would pass from one solution into the other ; or if water did
pass, equal quantities would pass in both directions, so that the
interchange would cause neither concentration nor dilution of either
solution. Both the urea and salt would diffuse through the mem-
brane, so that eventually both solutions would have the same com-
position ; each containing half of the urea and half of the sugar.
Since the molecular weights of urea and dextrose are, respectively,
00.12 and 180.12, these solutions are called equimolecular solutions,
and are said to have the same molecular concentration and to exert
the same osmotic pressure. The depression of the freezing-point is,
then, a measure of molecular concentration, and therefore of osmotic

pressure, or of what may be considered as the attraction for water
9

130 NORMAL HISTOLOGY.

on the part of a solution when separated from pure water "by a mem-
brane. Now, although the solids in lymph are less than those in
equal bulks of the blood-plasma, the lymph freezes at a slightly
lower point (about 0.02 degree C.) than the plasma, showing that
its molecular concentration is a trifle higher, and that there is a
tendency for water to pass from the bloodvessels into the lymph-
channels. This higher molecular concentration of the lymph is
attributed to cell metabolism, during which larger molecules are
abstracted from the lymph by the cells and smaller molecules —
i. e., molecules of less molecular weight — are returned to it. The
character of the lymph, then, will be affected by the metabolism of
the cells with which it comes into relation. These effects and differ-
ences in the capillary blood-pressure may be sufficient to account
for the differences in composition of lymph in various parts of the
body, so that secretory activity on the part of the capillary endo-
thelium is not necessarily a factor.

The red corpuscles are soft, elastic discs, with a concave impres-
sion in both surfaces (Fig. 112). They are slightly colored by a
solution of haemoglobin, and are so abundant that their presence
gives the blood an intense red color ; but when viewed singly under
the microscope each corpuscle has but a moderately pronounced red-
dish-yellow tinge. The haemoglobin solution is either intimately
associated with the substance composing the body of the corpuscles,
called the " stroma," or it occupies the centre of the corpuscle and
is surrounded by a pellicle of stroma.

Under normal conditions the red corpuscles, in man and most
of the mammalia, are not cells, for they possess no nuclei, nor are
they capable of spontaneous movement or multiplication. They
are, rather, cell-products, being formed either within the cytoplasm
of cells of mesoblastic origin, or by the division of cells derived
from the mesoblast, and called erythroblasts, the descendants of
which become converted into red corpuscles through an atrophy and
disappearance (probably expulsion) of the nuclei and a transforma-
tion of the cytoplasm into the stroma, which take place after the
elaboration of the fuemoglobin within the cell. The former, or
intracellular, mode of production occurs in the embyro, even before
the complete development of the bloodvessels; the latter mode of
production seems to be the only one occurring in the adult, the chief
location of the erythroblasts appearing to be in the red marrow of the

THE BLOOD AND LYMPH. 131

bones, where they are situated either in the tissues of the marrow
itself, whence their descendants, while still cellular, pass into the
vessels, or in the large venous channels of the marrow, where the
blood-current is sluggish and the erythroblasts remain close to the
vascular walls. In some anaemic conditions the erythroblasts ap-
pear in the circulating blood, where they may be distinguished from
the normal red corpuscles by the presence of their nuclei and, fre-
quently, also by a difference in size (see Fig. 110, c).

In the reptilia and birds the red corpuscles are normally nu-
cleated ; but, though morphologically resembling cells, they are
incapable of multiplication or spontaneous movement, and have
undergone such modifications that they are not cells in a physiolog-
ical sense.

The functional value of the red corpuscles is dependent upon the
haemoglobin they contain, which is said to constitute 90 per cent, of
their solid matter. It is readily oxidized and reduced again, and
serves to carry the oxygen of the air, obtained during the passage
of the blood through the pulmonary capillaries, to all parts of
the body. The red corpuscles, therefore, subserve the respiratory
function of the blood, as the plasma subserves its nutritive func-
tion.

The leucocytes, or white blood-corpuscles, are cellular elements
closely resembling the amoeba in their structure, which are present
in the blood in much smaller number than the red corpuscles, the
usual proportion being about one to six hundred. They vary some-
what in size and structure, either because of differences in their origin,
or because they are in different stages of development. The majority
of them are capable of amoeboid movements ; but while they are cir-
culating in the more rapid currents of the blood the constant shocks
they receive through contact with other corpuscles or with the vascu-
lar walls keep their cytoplasm in a contracted state and they maintain
a globular form. If, however, through any chance they remain for
some time in contact with the wall of a vessel, they are able to make
their way between the endothelial cells and pass out of the circulation
into the surrounding tissues. Here they creep about, and for this rea-
son have been called the migratory or wandering cells of the tissues.
They ultimately either suffer degenerative changes and disappear,
or find their way back into the circulation through the lymphatic
channels. During these excursions they may incorporate stray

132

NORMAL HISTOLOGY.

particles in the tissues, and thus act as scavengers. This activity
has been called their phagocytic function, and may play an impor-
tant part in the removal of material that should be absorbed or of
particles that would otherwise be injurious to the tissues ; e. </.,
bacteria. (See statements regarding the nature of colostrum-cor-
puscles.)

The emigration of leucocytes from the bloodvessels is pronounced
in many of the inflammatory processes, and their phagocytic func-
tion may have a marked influence on the result.

The leucocytes are produced in the lymphadenoid tissues of the
body, the lymphatic glands, thymus, spleen, and the more diffusely
arranged tissues of like structure, but probably most abundantly in
the red marrow of the bones.

A close study of the leucocytes has resulted in their subdivision
into a number of groups according to their morphological differences

Fig. 113.

Leucocytes from normal human blood. (Biihm and Davidoff.) a, red blood-corpuscle, intro-
duced for comparison; b, small mononuclear leucocyte (lymphocyte); c, large mono-
nuclear leucocyte ; g, polynuclear leucocyte. These differ in the character of the granules
they contain (not represented in the figure). In normal blood those granules are neutro-
philic in the vast majority of the polynucleated leucocytes. Occasionally they are acido-
philic, " esinophile leucocytes"; sometimes basophilic, " mast-cells " or "plasma-cells."
d, e,f, intermediate and probably transitional forms between the large mononuclear leu-
cocytes c, and the polynucleated leucocytes, or leucocytes with polymorphic nuclei, g.

or to peculiarities in their behavior toward coloring-matters. The
best defined of these groups are :

1. The polynuclear neutrophilic leucocytes, in which the nucleus
has a very irregular form, often presenting the appearance of two
or more nuclei, and the cytoplasm contains granules that have an

THE BLOOD AND LYMPH. 133

affinity for neutral anilin-dyes (Fig. 113, / and g). This variety
constitutes about 72 per cent, of the total number of leucocytes, and
is probably produced chiefly in the red marrow of the bones. They
possess the power of executing amoeboid movements and incor-
porating foreign particles.

2. The lymphocytes, with a single round nucleus and a little clear
cytoplasm around it. These leucocytes are of about the same size
as the red blood-corpuscles (Fig. 113, b). They are derived from
the lymphadenoid tissue in the lymph-nodes and other situations,
and appear to be capable of only feeble amoeboid movement. They
constitute about 23 per cent, of the total number of leucocytes in
normal blood.

3. The large mononuclear leucocytes, which are larger than the
red corpuscles and have oval nuclei surrounded by clear cytoplasm
(Fig. 113, c). This variety has also received the name " myelocyte,"
on the probably correct assumption that they are derived from the
red marrow of the bones. They are capable of passing through
transitional forms until they acquire the characters of the polynuclear
neutrophilic leucocytes described above. The large mononuclear
leucocytes, together with the transitional forms, make up about 3
per cent, of the normal number of leucocytes.

4. The eosinophilic leucocytes (Fig. 110, d), also larger than the red
corpuscles, with irregular, polymorphic nuclei, and a cytoplasm
containing relatively large granules which have an affinity for acid
dyes ; e. (/., eosin. These are frequently seen in unusual numbers
around inflammatory foci or in tissues undergoing involution ; e. g.y
in the connective tissue of the breast when lactation is suspended.
Their significance is not understood, but they appear to be derived
from the red bone-marrow. They constitute from 1 to 2 per cent,
of the total number of leucocytes.

5. Basophilic leucocytes, occasionally met with, which are charac-
terized by the presence of granules in the cytoplasm having a
special affinity for basic anilin-colors. These cells have also
received the names " mast-cells " and plasma-cells, but the latter
term is indefinite, having been applied to a number of cells of
different nature.

The blood-plates are colorless round or oval discs, about one-
fourth the diameter of the red corpuscles. Their function has not
been definitely determined, but it is thought that they may play a role

134 NORMAL HISTOLOGY.

in the production of fibrin, perhaps by the liberation of fibrin-
ferment.

Minute globules of fat are occasionally present in the blood,
especially during digestion.

The lymph, like the blood, consists of a fluid portion, the plasma,
and corpuscles held in suspension.

The plasma, as would be anticipated from its origin, is very
similar in composition to that of the blood.

The corpuscles are, for the most part, identical with the small
leucocytes (lymphocytes) of the blood, which derives its supply of
those cells from the lymph flowing into it.

The chyle is the lymph found in the lacteal lymphatics during
digestion. When absorption of the products of digestion is in
progress this lymph contains a great number of globules of fat,
some so minute as to be barely visible under the microscope. In
the intervals between absorption this lymph does not differ from
that found in the other lymphatics of the body.

Fibrin may present the appearance of a delicate network of
extremely fine fibres, somewhat resembling a cobweb, or these
fibrils may be aggregated into larger threads variously interwoven,
or they may be still further condensed to form masses of a hyaline
character. The fibres may undergo a disintegration into granules,
when their fibrinous nature is not readily revealed. Fibrin is not
found in the body under normal conditions, but separates from the
blood if the circulation be arrested for any considerable length of
time. It appears to be the result of the interaction of four sub-
stances : fibrinogen, fibrinoplastin, fibrin-ferment, and salts of lime.
The latter are always present in the tissues ; fibrinogen exists in
the plasma of the blood and lymph, and is, therefore, very widely
distributed. The fibrinoplastin is believed to be derived from the
bodies of cells that have undergone some destructive change; and
the ferment may be derived from the same source. These four
substances are present when the flow of blood through the ves-
sels has been seriously checked for a considerable period ; fibrin is
then formed, causing a coagulation of the blood. Such a clot,
within a vessel during life, is called a "thrombus." Coagulation
takes place more rapidly if there be a destruction of tissue; e. 7.,
a break in the wall of the vessel. It may also be occasioned by a
roughness on the internal surface of the vessel, if the flow of blood
over that obstruction is seriously retarded. In such a case the

THE BLOOD AND LYMPH. 135

fibrin-forming elements may be liberated from the bodies of leuco-
cytes that find lodgement behind the obstruction and suffer injury,
or they may be derived from blood-plates that have been arrested
and undergone similar changes. In a like manner, fibrin may be
formed in the lymphatic vessels or the interstices of the tissues.1

1 An explanation of fibrin-formation, offered by Lilienfeld, would serve to
elucidate many cases of coagulation under morbid circumstances. According to
this observer, fibrin is formed by the union of "thrombosin" with calcium, and is,
therefore, a calcium-thrombosin compound. The thrombosin is produced from
fibrinogen by the action of nuclein, which in turn is formed from the nucleohiston
contained in the nuclei of cells. Coagulation, then, would be the result of the
following process : the nucleohiston in the nuclei, during " karyolysis " or disintegra-
tion of the nucleus, is decomposed into "histon" and nuclein. The latter, acting
on fibrinogen, produces thrombosin, which unites with calcium to produce fibrin.

CHAPTER X.
THE DIGESTIVE ORGANS.

The digestive tract consists of six hollow, and for the most part,
tubular organs, which successively open into each other and extend
from the pharynx to the anus. The food, after mastication and
admixture with saliva in the mouth, passes through (1) the eesoph-
agus into (2) the stomach. Here it undergoes digestive changes
under the influence of the gastric secretions. Thence it passes into
(3) the duodenum, where the secretions of the liver and pancreas and
other glands are mixed with it and still further fit it for absorption.
From the duodenum it enters (4) the small intestine, the walls of
which take up the available products of digestion, and thence
passes into (5) the colon. In the latter the fluid portions are
gradually absorbed and the relatively dry residue, the faeces, passes-
out of the body through (6) the rectum and the anal orifice.

The walls of the digestive organs have a general similarity through-
out the whole of the digestive tract. They consist of four coats : 1,
an internal mucous membrane ; 2, a submucous coat ; 3, a muscular
coat ; and, 4, either a serous or a fibrous external coat. These coats
are, respectively, continuous with each other throughout the whole
tract. The internal coat, or mucous membrane, varies in both
structure and function in the different organs, and will, therefore, re-
quire closer study than the other coats. The latter have nearly the
same structure in all the organs. The submucous coat is made up
of areolar fibrous tissue, which permits some freedom of motion
between the mucous and muscular coats, and contains the larger
bloodvessels and lymphatics that supply all the coats. The mus-
cular coat consists, in general, of two layers of smooth muscular
tissue: an internal circular layer and an external longitudinal layer.
Its function is to produce those vermicular or peristaltic move-
ments which mix and gradually propel the food along the digestive
tract. The external coat is smooth and serous over those portions
of the tract which require the greatest freedom of motion. It is
nowhere complete, but, where present, is really a portion of the

136

THE DIGESTIVE ORGANS.

137

peritoneum which partially envelops the organs that are contained
in the abdominal cavity. Where this serous covering is wanting
the external coat consists of areolar fibrous tissue, which serves to
connect the organs of the digestive tract with neighboring struct-
ures, and thus becomes continuous with the areolar-tissue system
pervading the whole body. It supports the vessels and nerves
which make their way through it to the different organs.

In addition to the organs above enumerated, it is appropriate to
consider here the structure of the tongue, pharynx, salivary glands,
and pancreas.

1. The tongue consists chiefly of voluntary muscles, the fibres of
which are grouped in bundles running in various directions through
the substance of the organ. Between the individual striated mus-
cle-fibres, and also between the bundles into which they are col-
lected, there is a variable amount of areolar fibrous tissue contain-
ing fat, nerves, and bloodvessels (Fig. 67). This areolar tissue

Fig. 114. Fig. 115.

Sections of papillae of tongue.

Fig. 114.— Filiform papillae ; human. Heitzmann.)

Fig. 115.— Fungiform papilla ; human. (Heitzmann.)

E, stratified epithelium ; C, injected capillaries within the fibrous tissue of the papilla; ;

L, lymphadenoid tissue in lower portion of mucous membrane ; M, muscular tissue of

the tongue.

is more abundant near the surface of the tongue, and is covered
with a layer of stratified epithelium, thicker at the sides and on the
dorsum of the tongue than on its under surface, where it becomes

138

NORMAL HISTOLOGY.

continuous with the stratified epithelium covering the gums and
lining the buccal cavity.

Fig. llii.

...

■SS^5^^ IPSfc^S^e^S^ i. . $S !

Twocircumvallatepapillfe ; rabbit. (Ranvier.) p, p', fibrous tissue extending into the papilla ;
p', that containing the nerves passing to the taste-buds ; g, taste-buds ; v, small vein ; n, n,
nerves ; a, acini of a serous gland.

The upper surface and the edges of the tongue are covered with
papillae, some of which are pointed (filiform papillae), others rounded

Fig. 117.

Portion of a section of a mucous gland in the human tongue. (Benda and Guenther's Atlas.)
a, duct; 6, acinus opening into a duet-radicle; c, acinus lined with mucigenous cells, sim-
ilar to b. Between and below a and c. cross-section of a small artery, recognizable by the
elongated nuclei of its muscular coat.

(fungiform papillae), and still others surrounded by a sulcus (cir-
cumvallate papillae) (Figs. 114-1 1G). Within the epithelium lining

THE DIGESTIVE ORGANS. 139

this sulcus arc peculiar groups of cells, called taste-buds, which will
be described in a subsequent chapter. At the junction of the

middle and posterior thirds of the upper surface of the tongue
there are several of these circum vallate papillae which are of
unusual size.

Within the subepithelial areolar tissue, and often extending for
some distance between the muscles, there are, here and there, small
racemose glands, which secrete a serous or mucous fluid (Figs. 116, a
and 117). They are most abundant on the back and sides of the pos-
terior part of the tongue, and their ducts frequently open into the sulci
of the circumvallate papillae. Within the subepithelial areolar tissue
small collections of lymphadenoid tissue (lymph-follicles) are also of
not infrequent occurrence. The papilla? covering these are low and
inconspicuous, so that the surface of the tongue appears unusually
smooth at those points.

2. The salivary glands belong to the racemose variety of secreting
glands. The secretions which they furnish are of two kinds: 1, a
thin, serous fluid, containing albuminoid materials, among which are
the specific ferments elaborated by the gland ; and, 2, a viscid fluid
containing mucin. These two secretions are furnished by acini lined
with different varieties of epithelium. The parotid gland secretes
only the serous fluid, and is composed of serous alveoli. The sub-
lingual gland secretes only the mucous fluid ; but the submaxillary
gland secretes both, and, therefore, contains both serous- and
mucous-secreting cells.

The cells which line the mucous acini have clear bodies, as the
result of a storage of transparent globules of mucin or naucigen
within the cytoplasm. Where these globules are abundant the
nuclei of the cells are crowded toward the attached ends of the
cells. When the mucin is discharged from the cells they become
smaller, less clear, and more granular in appearance.

At the periphery of the acini, and especially well marked at or
near their blind extremities, are, here and there, crescentic, granular
epithelial cells, which may reach the lumen of the acinus or be
crowded back by the enlarged cells adjoining them. These cells
form the " crescents of Gianuzzi." In the submaxillary gland, at
least, many of these crescents secrete the serous or albuminoid fluid
mentioned above. This secretion reaches the lumen of the gland
through minute intracellular channels (Fig. 118).

The serous alveoli of the salivary glands are lined with cells that,

140

NORMAL HISTOLOGY.

at certain stages of their activity, are so crowded with granules that
the nuclei are obscured. These granules are the accumulated mate-
rial from which the secretion is formed, and when the gland has
been functionally active for some time they diminish in number,

Fig. US.

Section of an acinus of the human submaxillary gland. (Krause.) The lumen is surrounded
by mucous cells, containing globules of mucigen. Two groups of Gianuzzi's crescents are
represented, with the intracellular channels conveying the serous secretion to the lumen.

and the nuclei then come into view. At the same time the cells
become smaller, and the lumen within the acinus, which at first was
barely distinguishable, becomes more obvious.

The epithelium lining the acini of all the salivary glands rests

Fig. 119.

Diagrammatic representation of a portion of a human submaxillary gland. (Krause.) a, duct,
lined with columnar cells, striated at their bases and passing into a more cubical epithe-
lium without such striation ; b, mucous cells; c, serous cells; d, crescent; abasement.
membrane. In this figure the convoluted course of the ducts and tubular acini has been
ignored, and they have been represented as though lying in a single plane.

upon a modified connective tissue, called the " basement-membrane,"
which consists of flattened cells arranged to form a broad, mem-
branous reticulum, the meshes of which are filled with cement.
Outside of this basement-membrane there is a small amount of

THE DIGESTIVE ORGANS.

141

vascular areolar tissue, and broader bands of that tissue divide the
whole gland into small lobes and these again into still smaller
lobules (Fig. 25).

The ducts of the salivary glands are lined with columnar or pyram-
idal epithelial cells, the attached ends of which often show a stria-

Fig. 120.

Part of a cross-section of the oesophagus of a dog. (Bohm and Davidoff.) a, mucous mem-
brane ; b, submucous coat ; c, muscular coat ; d, fibrous coat ; e, stratified epithelium ; /,
subepithelial areolar tissue (sometimes called the " tunica propria " of the mucous mem-
brane) ; g, muscularis mucosa? ; h, areolar tissue of the submucosa, containing the chief
branches of the arterial and venous vessels ; i, internal, encircling layer of the muscular
coat. It is the contraction of this coat that has caused a longitudinal wrinkling of the
mucous membrane. One of those folds is completely and two are partially shown, j,
external, longitudinal layer of the muscular coat ; k, areolar tissue forming the external
coat and connecting the oesophagus with neighboring structures. A few large vessels
entering the oesophagus are represented in this coat.

tion perpendicular to the surface of the basement-membrane (Fig.
119).

The nerves ramify in the interlobular areolar tissue and send
delicate, non-medullated fibres through the basement-membrane to
be distributed upon and between the epithelial cells. Occasionally
small ganglia are seen upon the larger nerves.

142 NORMAL HISTOLOGY.

3. The (Esophagus (Fig. L20). — The mucous membrane of the
oesophagus is composed of three layers. The innermost layer is
made up of stratified epithelium. Beneath this is a layer of fibrous
tissue, with small papillae extending into the deeper portions of the
epithelium (see Fig. 38). Outside of this is a layer of longitudinal
smooth muscular tissue, the "muscularis mucosa?." This is but
imperfectly represented at the upper part of the (esophagus, but at the
lower end forms a continuous layer separating the " tunica propria "
(Fig. 120,/) of the mucous membrane from the submucous coat, and
becoming continuous with a similar layer of smooth muscular tissue
in the mucous membrane of the stomach and intestine. Occasion-
ally small, imperfectly defined lymph-follicles are met with in the
mucous membrane.

The submucous coat of loose areolar tissue contains small race-
mose glands sparsely distributed through it, the ducts of which
penetrate the mucous membrane and open upon the internal surface
of the oesophagus.

The muscular coat consists of an internal circular and an external
longitudinal layer, which at the upper end of the oesophagus are
composed of striated muscle. This is gradually replaced by smooth
muscular tissue further down the oesophagus, and at its lower end
only the latter tissue is found.

The external coat of the oesophagus is represented by a variable
amount of areolar tissue which loosely connects it with the sur-
rounding structures.

4. The Stomach. — Nearly the whole thickness of the mucous
membrane of the stomach is made up of straight tubular glands
(gastric tubules), which lie perpendicular to the surface, and are
separated from each other by only a small amount of a delicate,
highly vascular areolar tissue. This is a little denser and more
abundant below the deep ends of the glands, where it separates
them from the muscularis mucosas forming the deepest layer of the
mucous membrane.

The mouths of the gastric tubules open into shallow, polygonal
depressions or crypts on the surface of the mucous membrane,
several glands opening into each depression. These depressions
give the internal surface of the stomach a reticular appearance when
viewed with a low power. They, and the ridges which separate
them, are covered with a rather tall columnar epithelium. The
glands which open into them are of two kinds: the "pyloric"

THE DIGESTIVE ORGANS.

143

Fig. 122.

Fig. 121.

/,— -f ■

d--^L

Fig. 121.— Vertical section through mucous
membrane of pyloric end of stomach ;
human. (BOhm and PavidofF.) a, co-
lumnar epithelium covering surface of
mucous membrane; b, crypt lined with
somewhat lower columnar epithelium ;
c, gastric tubules; d, tunica propria,
somewhat lymphoid in character; e, mus-
cularis mucosae, of smooth muscular
tissue.

Fig. 122.— Nearly vertical section of the mucous membrane near the cardiac end of the stom-
ach ; rabbit: a, columnar epithelium covering the surface of the mucous membrane ;
h, that lining a crypt ; c, duct ; d, parietal cell extending to the lumen of the gland:
c, lumen, readily traced for only a short distance;/, central or chief cells; g, small
artery, to the left and above it, a small vein ; /(, muscularis mucosa-, consisting of three
thin layers of smooth muscular tissue, the middle layer in transverse, the others in
longitudinal section ; i, portion of submucosa. The specimen was taken from an animal
some time after the ingestion of food, and the chief cells are, in consequence, relatively
small in comparison with the size of the parietal cells.

144

NORMA L HISTOLOG Y.

variety, so-called because more abundant at the pyloric end of the
stomach, and the " cardiac" variety, which preponderate near the
eardiac end.

The pyloric glands (Fig. 121) have the simpler structure. They
possess a comparatively deep and open mouth, lined with columnar
epithelial cells similar to and continuous with those lining the de-
pressions already mentioned, and, like them, mucigenons. Into
these mouths one or more straight tubular glands, lined with low,
granular columnar cells, discharge their secretion.

The cardiac glands (Fig. 122) have shallower mouths than the
pyloric glands, and the tubes that open into them contain two sorts
of epithelial cells : 1, the "chief" or central cells, which line and
nearly fill the whole tubule, leaving only a very small and some-
what tortuous lumen in the centre ; and, 2, the parietal cells, lying
at intervals between the central cells and the surrounding connective
tissue, but sometimes projecting between two central cells nearly
or quite to the lumen of the gland. Very fine channels run from
that lumen to and around these parietal cells, which are believed to
produce the free acid of the gastric juice (Figs. 123-125).

Fig. 123.

Fig. 124.

Fig. 125.

Cross-sections of gastric glands ; dog. (Hamburger.)
Figs. 123 and 124.— From the cardiac end of the stomach, showing the chief or central cells
and the parietal cells. 123, from a dog killed during the second hour of digestion. The
central cells are relatively large, and the lumen is reduced to a mere line, appearing as
a dot in the centre of the cross-section. 124, from a dog killed during the seventh hour of
digestion. The parietal cells are relatively large, and the lumen more distinct than in
123, owing to loss of material on the part of the central cells and a gain on the part of the
parietal cells. One of the latter is in communication with the lumen through a small
channel between the central cells.
Fig. 125— From the pyloric end of the stomach during the fifth hour of digestion. The cells
b have parted with their secretion and are compressed by the cells a, which still retain
the materials stored for secretion. The lumen of the gland is much larger than that of
the glands at the cardiac end of the stomach.

Besides the secreting glands, the mucous membrane of the stom-
ach sometimes contains small lymph-follicles. Its blood- and
lymph-supplies arc abundant, and nerves are distributed to its
various tissue-elements.

THE DIGESTIVE ORGANS. 145

The deepest layer of the mucous membrane is the museularis
mucosae, made up of two or three strata of smooth muscular tissue
in which the fibres run in different directions.

The submucous coat of the stomach consists of loose areolar tissue,
which allows considerable freedom of motion between the mucous
membrane and the muscular coat. When, therefore, the organ is
empty the contraction of the muscular coat throws the mucous
membrane into coarse folds (rugse). The large arteries, veins, and
lymphatics course in this submucous tissue, and thence send branches
into both the mucous and muscular coats. The nerves also form a
ganglionated plexus in this coat.

The muscular coat consists of an external longitudinal layer,
inside of which is another layer encircling the organ. The external
layer is continuous with the outer muscular layer of the oesophagus.
The internal muscular layer of the latter organ is continued into the
wall of the stomach as a scattered set of oblique fibres lying internal
to the encircling fibres already mentioned. The muscular coat of the
stomach may, therefore, be considered as composed of three layers,
the innermost of which is incomplete. At the pylorus the encircling
muscular layer is thickened.

Aside from the fibrous tissue that more or less completely sepa-
rates its layers, the muscular coat contains ganglionated nerve-
plexuses.

The external surface of the stomach is covered with a serous in-
vestment of peritoneum, except along the curvatures, where the
peritoneum is reflected from the organ, permitting the passage of

its vessels and nerves.

5. The Duodenum. — The structures characteristic of the small

intestine first make their appearance in the duodenum. We shall
first consider those features which are found throughout the small
intestine, and then describe those which are peculiar to the duod-
enum (Fig. 126).

The mucous membrane presents thin, transverse folds, the val-
vular conniventes, which are not obliterated when the intestinal wall
is stretched. They are made up of a thin layer of areolar tissue,
extending from the submucous coat of the intestine, which is cov-
ered on both surfaces with mucous membrane. This arrangement
serves greatly to increase the surface of mucous membrane coming
in contact with the contents of the intestine, a provision facilitating

absorption of the products of digestion,
in

14(i

NORMA L HISTOL OGY.

The valvulae conniventes begin a short distance below the pylorus,
and are very numerous and prominent in the duodenum, but become
progressively less frequent and pronounced in the lower portions of
the small intestine.

The absorbent surface of the small intestine is still further in-

Fig. 126.

Diagram representing the structure of the human small intestine. (Bohru, Davidoff, and
Mall, slightly modified.) Two villi are represented. In the one on the left the blood-
vessels are shown ; in the one on the right, the lymphatics. The line S indicates the sur-
face of the mucous membrane between the villi, a, central lacteal vessel; 6, smooth
muscular fibres extending into the villus from the muscularis mucosae: c, lymphadenoid
tissue beneath the epithelial covering of the villus: d, crypt of Lieberkuhn; e, tunica
propria of lymphadenoid tissue, and continuous with that of the villus;/, muscularis
mucosa-, formingthe deepest portion of the mucous membrane: g,submucosa containing
the larger bloodvessels and the lymphatic plexus ft; i. encircling layer of the muscular
coat ; j, longitudinal layer: k, lymphatic plexus within the muscular coat; I, serous coat;
m, vein. The crypts arc lined, and the villi covered, with columnar epithelium.

creased by the presence of innumerable minute, finger-like projec-
tions from the surface of the mucous membrane, the villi. These
are jusl discernible by the unaided eye, and give the internal surface
of the intestine a velvety appearance.

THE DIGESTIVE ORGANS. 147

Between the attached ends of the villi, and opening npon the sur-
face of the mucous membrane, are tubular depressions extending
nearly to the muscularis mucosas. These are the " crypts of Lieber-
kuhn," and have the appearance of simple tubular glands ; but it is
doubtful if they elaborate any peculiar secretion. These crypts are
present, not only in the whole extent of the small intestine, but
also throughout that of the colon.

The crypts of Lieberkuhn are lined with columnar epithelium,
which also covers the surface of the mucous membrane and the villi
springing from it. The cells composing this epithelium multiply in
the crypts, and, as they mature, are gradually moved toward their
orifies, whence they replace those that have been destroyed upon the
surfaces of the villi. The cells possess a granular cytoplasm, which
becomes infiltrated with fat during digestion ; an oval, vesicular
nucleus ; and a delicate cell-membrane. The free ends of the cells
are formed by a well-marked cuticle, which may be either homo-
geneous in appearance, or present very fine vertical striations (Fig.
37). Many of the cells are mucigenous and contain globules of mucus
near their free ends, or appear as goblet-cells after the discharge of
that secretion. These cells are more abundant on the villi, where
they are older, than in the crypts lined with less mature cells.

The epithelium rests upon a basement-membrane, which contains
nuclei, and is therefore composed, in part at least, of cells. Beneath
this basement-membrane is a layer of reticular and areolar tissues,
containing a variable number of lymphoid cells and numerous
capillary bloodvessels. The rest of the mucous membrane, down
to the muscularis mucosa?, and the axes of the villi are occupied by
areolar fibrous tissue.

The thin muscularis mucosa?, which forms the deepest layer of
the mucous membrane, is made up of two layers of smooth muscular
tissue : an internal layer, in which the fibres run transversely to the
axis of the intestine, and an external longitudinal layer. From the
upper surface of this muscular layer of the mucous membrane
muscular fibres extend into the villi, in the areolar tissue in their
axes, and serve to shorten the villi by their contraction, so that the
villi are moved about in the intestinal contents during the process
of absorption. In the centre of each villus is a capillary lymphatic
vessel arising in a blind extremity near the apex of the villus.
These lymphatics open into a lymphatic plexus situated between the
muscularis mucosa? and the ends of the crypts of Lieberkuhn, and
thence discharge their contents into the lymphatics in the sub-

148

NO n MA L JILSTOL 0 0 Y.

mucosa. The muscular fibres in the villi probably aid iu the pro-
pulsion of the chyle in these lymphatics (Fig. 127).

Fig. 127

■ K

y —

- - - - g» . — e

Axial section of villus of the dog. (Kultschitzky.) a, epithelial covering with cuticle ; b,
goblet-cell ; c.'space between tapering ends of the epithelial cells ; d, cell of the base-
ment-membrane; e, smooth muscular fibres; /, reticulum of the tunica propria (the
lymphoid cells have been, for the most part, removed); g, lumen of the central lymphatic
The bloodvessels are not represented.

The submucous coat of the intestine is composed of areolar fibrous
tissue. Outside of this coat is the muscular coat, divisible into two
layers, which is covered throughout the whole circumference of the
intestine, except at the line of mesenteric attachment, with a serous
investment of the peritoneum.

In the duodenum the submucous coat contains compound tubular
glands, the glands of Brunner, the ducts from which penetrate the
miiscularis mucosae and open upon the surface of the mucous mem-
brane, between the crypts of Lieberkiihn. Here and there, in the
duodenum, are little collections <>f lymphadenoid tissue, occupying'
an enlarged villus and often extending through the muscularis
mucosae into the submucous areolar tissue (Fig. 128). These
lymph-follicles may be regarded as the result of an increase in the
amount of reticular tissue of the villus, which has replaced the
other structures usually present. In the lower portions of the

THE DIGESTIVE ORGANS.

149

small intestine there are collections of these solitary follicles, which
have received the name " Peyer's patches."

6. The small intestine below the duodenum resembles the latter

Fig. 128.

Section of solitary follicle from the ileum. (Cadiat. I a, space left by the disintegration of
the central, delicate lymphadenoid tissue of the follicle during the preparation of the sec-
tion; b, columnar epithelium of intestinal surface; c, r, villi, partially denuded of epithe-
lium ; d, crypt; e, /, muscularis mucosae; above /, the point where the vessels enter the
follicle. The Peyer's patches are collections of such solitary follicles, placed side by side
and destitute of villi at their upper surfaces.

in structure, with a few modifications, which become progressively
more marked as the distance from the stomach increases.

The glands of Brunner are most abundant near the upper part
of the duodenum, more sparsely distributed further down, and
usually disappear entirely before the beginning of the jejunum.

The valvulae conniventes, which are most highly developed a
little below the entrance of the gall and pancreatic ducts, also
become lower and less frequent along the course of the intestine,
and finally disappear about the middle of the ileum.

The crypts of Lieberkiihn are deepest in the upper part of the
intestinal tract, but persist in shallower form throughout its whole
extent, as well as along the whole length of the colon.

The Peyer's patches are most abundant in the lower part of the
ileum, where they lie in the intestinal wall opposite the line of
mesenteric attachment, and form oval areas with their long axes
parallel to the axis of the intestine.

7. The Colon. — The mucous membrane of the colon is destitute

ISO

NORMAL HISTOLOGY

of villi, but contains crypts of Lieberkiihn closely arranged side by
side and lined with columnar epithelium rich in mucigenous cells.
The muscularis mucosae is similar to that of the small intestine, and
gives off occasional fibres that penetrate between the crypt-.

The submucous coat resembles that of the small intestine, and, in
common with the mucous membrane, contains solitary lymph-fol-
licles, most abundantly in the caecum and vermiform appendix.

The muscular coat has its outer or longitudinal layer most highly
developed in three bands, which are situated about equidistantly
around the circumference of the bowel and occasion a pouching of
the intervening wall.

Fig. 129.

©

&l

©■

&0 o _ $ ®

Section of human pancreas. (Bohra and Ravidoff.) a, larger duct ; b, beginning of duct ; c, d,
acini with cells belonging to the corresponding duct-radicles in their centers ; c. acinus,
cut just beyond the lumen ; /, interalveolar cell-group (?) ; g, fibrous connective tissue,
forming the interstitial tissue of the organ.

The serous coat is similar to that of the small intestine, but is
occasionally extended over small pendulous projections of the
subserous fibrous tissue, which contain adipose tissue, appendices
epiploic*.

8. The rectum resembles the colon in its structure, except that the
three muscular bands present in the latter are wanting. The mucous
membrane as it passes into the anal canal loses its tubular glands,
and subsequently become- covered, not with columnar, but with
stratified epithelium, continuous with the epidermis of the skin
around the anus.

9. The pancreas (Fig. 129) has a structure similar to that of the

THE DIGESTIVE ORGANS.

151

salivary glands, but its lobules are separated and held in plaee by
a rather more considerable amount of loose areolar tissue, in which
there are occasional groups of cells of uncertain nature, but cer-
tainly distinct from those lining the glandular acini. They are
called the " interalveolar cell-islets," and may, perhaps, be of the
nature of ductless glands (q. v.). These structures, also known as
the islands of Langerhans, have no secretory ducts, and are com-
posed of cells of epithelial nature which have the same origin as the
ordinary secreting cells of the pancreas. They are not grouped so
as to form acini, but are arranged in anastomosing columns, like the
cells of the liver, with an abundant capillary network situated in the
spaces between these columns of cells. The cells are devoid of
zymogen granules, but are of two kinds : 1, smaller cells with faintly
staining cytoplasm and a vesicular nucleus in which the chromatin

Fig. 130.

< V X ■

175

Island of Langerhans, guinea-pis- (Schulze.) a and b, isolated cells; c, injection of blood-
vessels showing the abundant, capillary network within the island.

is more or less massed into granules ; and, 2, larger cells with a
cytoplasm that stains more deeply (Fig. 130). The islands of
Langerhans have of late received much attention as possibly being
the structures elaborating an internal secretion of importance in the
general carbohydrate metabolism of the bodv.

As the pancreas exercises its secretory function the granules
within its cells move toward the lumina of the acini and successively
disappear, the attached ends of the cells becoming clearer and the
whole cell diminishing somewhat in size during the process.

The nerves of the stomach and intestinal tract form two gan-
glionated plexuses, the plexus of Auerbach, which lies between the
two layers of the muscular coat, and the plexus of Meissner, situ-
ated in the submucous coat. From these plexuses fibres are dis-

152

NORMAL HISTOLOGY.

tributed to the muscles and other structural elements. These fibres
are of the non-medullated variety.

The nerves of the panereas are also non-medullated, possess a
few ganglia within the organ, and are finally distributed among the
epithelial cells.

The Tonsils, Lymph-follicles, and Peyer's Patches. — These collee-
tions of lymphadenoid tissue in the alimentary tract have special

Fig. LSI.

I

%■■ . ■

4

\s%

Section through one of the crypts of the tonsil. (Stohr.) e, stratified epithelium of the gen-
eral surface, continued into the crypt:/, follicles containing germinal foci. Between
the follicles is a more diffusely arranged lymphadenoid tissue, s, material within the
crypt, composed in part of lymphoid corpuscles that have wandered through the strati-
tied epithelium.

interest to the physician as being points particularly liable to infec-
tion. The solitary follicles of the stomach and of the small and
large intestine, and the collections of such follicles forming the
patches of Peyer, are the sites which are most vulnerable to
invasion by pathogenic bacteria in the digestive tract, though they
are probably protected to a considerable extent by the germicidal
powers of the acid gastric juice. This is not always capable of
guarding them from infection by the typhoid and tubercle bacilli,,

THE DIGESTIVE ORGANS. 153

and in the diseases of* the intestinal canal occasioned by those bac-
teria the follicles and Peyer's patches are the seat of the earliest
and most extensive ulcerations. The tonsils, which have the same
general structure, are still more prone to infection of various kinds,
for they are more directly exposed to the action of bacteria that may
train access to the month.

The reason for this vulnerability appears to lie in the close prox-
imity of the lymphatics to the surface and their meagre protection
by a thin layer of epithelium liable to abrasion or destruction. The
solitary follicles of the intestine, for example, are covered with a
single layer of columnar epithelium (Fig. 128).

The lymphadenoid tissue of the tonsil, it is true, is protected by
a layer of stratified epithelium ; but the surface of the tonsil is invag-
inated to form the crypts of that organ, and within those crypts it

Section through the fundus of a erypt. (Benda ami Guenther's Atlas.) a, stratified epithe-
lium, desquamating at its surface ; 6, deep portion of the lymphadenoid tissue, in which
proliferation of lymphoid cells takes place as well as in the follicles represented in Fig. 131.

is possible for bacteria to multiply and produce such an accumula-
tion of poisonous products as to destroy the integrity of the epithe-
lium and so permit an invasion of the lymphadenoid tissue beneath.
We therefore find the tonsils specially prone to such inflammatory
processes as tonsillitis and diphtheritic inflammation (Figs. 131 and
132).

CHAPTER XI.
THE LIVER.

That portion of the liver which is exposed in the abdominal
cavity is covered by a reflection of the peritoneum, closely attached
to the organ, because its deeper side is continuous with the fibrous
structures or interstitial tissue of the liver itself. This serous cover-
ing is so thin that the substance of the liver can be readily seen
through it.

At the portal fissure, the serous coat having been reflected from
it, the liver is covered with a loose areolar tissue in which the main
trunks of all but one of the vessels connected with it are situated :
namely, the portal vein, hepatic artery, gall-duct, and lymphatics.
These vessels enter the liver together at this place, and are closely
associated with each other in all their ramifications, being supported
throughout by areolar tissue, which is continuous with that at the
portal fissure and with the interstitial tissue of the liver.

These vessels, with their supporting fibrous investment, called
Glisson's capsule, ramify in the liver in such a way as to resemble
a tree with a multitude of branches and twigs, each composed of
divisions of all the vessels named.

The hepatic vein enters the liver at a different place, and also
suffers a tree-like subdivision; but its branches are surrounded by a
very much smaller amount of fibrous tissue, which may be regarded
as but a slightly reinforced portion of the interstitial tissue of the
organ.

Sections of the liver (Fig. 133) will reveal portions of these two
trees, cut in various directions with respect to their axes. It will
be observed that tin1 twigs and larger branches of the trees are
nowhere in close relations to each other, showing that the hepatic
vein, in all its ramifications, is separated from the other vessels by
the parenchyma of the organ. If we select some part of a section
which contains one of the smallest branches of the hepatic vein, and
cut across its axis so that its lumen appears round, we shall notice
that at about equal distance- from it there are sections of two,

154

THE LIVER.

155

three, or four twigs of the compound tree. In these the gall-duct
can be identified by its distinct lining of columnar or cubical epi-
thelium, and the hepatic artery distinguished from the portal vein
by its relatively thick wall as compared with the size of its lumen.
These vessels are collectively known as the interlobular vessels.
Between and around them is the areolar fibrous tissue, which forms
a part of Glisson's capsule, and which is abundantly supplied with

Fig. 133.

Diagrammatic sketch of a section of liver: a, central vein (radicle of the hepatic vein) ; 6, b,
branches of the portal vein ; c, c, branches of the hepatic artery ; d, d, small bile-ducts ;
e, lymphatic vessel; b, c, d, e are enclosed in areolar tissue, which is continuous with
Glisson's capsule; /, liver-cells; g, line indicating the junction and blending] of two
neighboring lobules.

lymphatic spaces and vessels in the fibrous tissue. The lymphatics
appear as clear spaces with smooth walls, some of them with dis-
tinct endothelial linings, but almost devoid of any other wall.

The parenchyma may be subdivided into portions which surround
the smallest branches of the hepatic vein, and are bounded by
imaginary lines connecting the groups of interlobular vessels.
These subdivisions are called " lobules " of the liver. In the
human liver they blend at their peripheries, between the masses of
connective tissue enclosing the interlobular vessel; but in the liver
of the pig these lobules are veritable subdivisions of the liver, and

156

NORMAL HISTOLOGY.

are separated by septa of fibrous tissue, the interlobular vessels
lying in the lines formed by the junction of three such septa.

Connecting the branches of the portal vein with the hepatic vein
is a plexus of capillaries, called the intralobular vessels, through
which the blood passes from the portal vessels to the radicles of the
hepatic vein and thence into the general circulation. These intra-
lobular vessels also receive blood from the hepatic artery, the
capillaries from which join them at a little distance from the
periphery of the lobule. The radicles of the hepatic vein are
called the central veins, from their situation in the axes of the
lobules, which are conceived as having a somewhat cylindrical
shape (Fig. 134).

Vessels and bile-ducts of a lobule of a rabbit's liver in transverse section. (Cadiat.) a, cen-
tral vein : b, b, interlobular veins (branches of the portal vein) ; c, interlobular bile-duct,
receiving capillary bile ducts from the lobule. Between a and b is the capillary plexus
caned the intralobular vessels. The biliary radicles are not represented throughout the
figure, and the branches of the hepatic artery have been wholly omitted.

Between the interlobular capillaries are rows of epithelial cells,
which constitute the functional part of the liver, its parenchyma.
They appear to touch the walls of the capillaries, but are, in reality,
separated from them by a narrow lymph-space (Fig. 135). In the

THE LIVER.

107

human liver the epithelial cells of the parenchyma form a plexus
lying in the meshes of the capillary network of the interlobular
vessels.

It requires an effort of the imagination to conceive of a third plexus
within the lobule, but such a plexus exists, being formed of the
radicles of the gall-duct. These are minute channels situated
between contiguous epithelial cells, each of which is grooved upon
its surface to form half of the tiny canal. The cells themselves
have fine channels running from the bile-capillaries into their cyto-
plasm and ending there in little rounded expansions. It is difficult
to detect these bile-capillaries in ordinary sections of the liver,
unless they have been previously injected through the main duct ;
but with a high power their cross-sections may sometimes be clearly
seen, appearing as little round or oval spaces at the junction of two

Fig. 135.

Fig. 136.

si

.*

Fig. 135.— Perivascular lymphatic of the human liver. (Disse.) c, capillary in longitudinal
section; a, lymphatic space between the capillary and row of epithelial cells; b, wall
of the lymphatic space, slightly separated from the liver-cells and drawn a little em-
phatically; ?, liver-cells; d, bile-capillaries in cross-section, with their intracellular
ramifications.

Fig. 136.— Bile-capillaries between the liver-cells, with minute channels penetrating the cells
and communicating with secretory vacuoles within the cytoplasm. Injected liver of the
rabbit. (Pfeiffer.)

epithelial cells, midway between the nearest capillary bloodvessels.
Throughout their whole course they appear to be separated from
the nearest bloodvessels by a distance approximately equal to half
the diameter of one of the epithelial cells. It is this fact that makes
it so difficult to frame a mental picture of their distribution in the
lobule (Fig. 136).

The nerves supplying the liver ramify in extremely delicate, non-

1 58

NOW. 1 L 1IIST0L0G Y.

medullated fibrils, which ramify throughout the substance of the
liver and terminate in minute twigs among its epithelial cells.

The epithelial cells of the liver have a cubical shape, the grooved
and other surfaces that come in contact with neighboring cells being
flat, while the remaining surfaces may be somewhat rounded. The
cytoplasm is granular, and, except after a considerable period of
starvation, more or less abundantly infiltrated with irregular gran-
ules and masses of glycogen and globules of fat (Fig. 137). The

Portion of hepatic lobule of the rabbit : cells infiltrated with glycogen. (Barfurth.) The-
animal had been fed for twenty-four hours on wheat-bread, to promote the storage of gly-
cogen within the liver-cells. The cells in close proximity to the central vein contain
the largest amount of glycogen, which appears to fill the cytoplasm. Further from the
central vein the cells contain less glycogen, which is most abundant in that portion of
the cell turned toward the centre of the lobule. Fat-globules are most abundant in the
cells at the periphery of the lobule. No fat-globules are represented in this figure.

glycogen dissolves out of the cells during the ordinary processes of
fixation and hardening preparatory to the preparation of sections,
leaving spaces in the cytoplasm, which cause it to have a coarsely
reticulated appearance in cases where the glycogen was abundant.
This reticulation would render it impossible to distinguish the
minute intracellular bile-passages. Each cell has a round vesicular
nucleus near its centre. In rare instances two nuclei may be found
in a single cell. The functions of the liver are so varied and in-
volve such complex chemical transformations that the liver-cell must
be considered as a structure of marvellous activity. It stores carbo-
hydrate food-stuffs, transforming sugars into glycogen, and converts

THE LIVER. 159

this substance into dextrose as occasion requires. It also stores fats,
reserving- them for the needs of the organism. It elaborates the
bile-salts and pigment, deriving material for the latter from the
haemoglobin of the blood. In addition to these functions, many
synthetical processes occur in the liver. The nitrogenous waste-
products of metabolism in other parts of the body are here changed
into forms suitable for excretion by the kidney. Most of the urea
appearing in the urine is formed in the liver. Injurious substances
absorbed from the intestine are, at least in many instances, combined
in the liver with chemical radicals to form less toxic compounds
which can be eliminated. It therefore exerts an important detoxi-
cating influence highly beneficial to the other organs and cells of the
body. In view of these numerous functions and the great versatility
displayed by the liver-cells, it is not surprising to find that by
special methods of preparation very diverse structural appearances
are presented by these cells, and that these appearances vary with
the condition of the liver at the time of its removal from the
body. Some observers believe that the intercellular bile-radicles
described by others are artefacts, and not permanent structures.
Some investigators have ascribed amoeboid movements of limited
range to these cells, and have noted the presence of red blood-
corpuscles within the cytoplasm, apparently incorporated by active
movements of the cell-body. Injections have even been made to
penetrate the nucleus. Diversities in apparent structure, of which
these are merely a few examples, render it impossible to give a brief
adequate description of the typical liver-cell or to connect definitely
the observed structures with any particular function.

It will, perhaps, make the structure of the liver a little more
comprehensible if it is stated that the liver of some of the lower
animals is a tubular gland, the tubes of which are lined with a layer
of epithelium. In the human liver this tubular structure is dis-
guised by the facts that the tubules anastomose with each other, and
that their lumina are very minute and bounded by only two cells
when seen in cross-section. So inconspicuous are these lumina that
a casual glance at a section of a liver would not reveal the fact that
it was a glandular organ.

The interstitial tissue of the liver consists of a few sparsely
distributed fibres continuous with those of Glisson's capsule.

The intricate structure of the liver prepares us for the fact that
its function is an extremely complex one. It is a secreting gland,

1G0 NORMAL HISTOLOGY.

elaborating the bile and discharging it into the duodenum. But.
the bile has more than one purpose. It aids in the digestion and
absorption of food, and it also contains excrenientitious matters
destined to leave the body through the alimentary tract. Even the
secretory function of the liver, therefore, serves a double purpose :
the supply of substances useful to the organism and the elimina-
tion of products that would be detrimental if retained.

But the function of the liver is not confined to the elaboration
of the bile. It also acts as a reservoir for the storage of nourish-
ment, which can be drawn upon as needed by the organism. This
is the meaning of the glycogen and fat which have infiltrated the
cells.

The food-materials that are absorbed from the digestive tract pass
into the system through two channels : the lymphatic and the portal
circulations. The latter carries them to the liver, where some of
the fat, probably after desaponifieation, is taken up by the epithelial
cells, which also appropriate a portion of the sugar in the portal
blood, transforming it into glycogen and holding it in that form
until a relative deficiency of glucose in the blood reveals its need
by the system.

The blood comes into such close relations with the epithelial cells
of the liver that an interchange of soluble substances between them
appears to be about as easy a matter as the interchange of gases
between the blood and the air in the lungs; and, as in the latter
case, this interchange is mutual : some matter passing from the
blood to the liver-cells and some from the cells to the blood. In
the lung there is a gaseous regeneration of the blood ; in the liver,
a renovation as to certain of its soluble constituents.

The Gall-bladder. — The bile is secreted continuously by the liver,
for it is an excrement ; but it is discharged intermittently into the
alimentary tract, as required by the digestive processes. In the
interval it is stored in the gall-bladder.

The gall-bladder is lined with columnar epithelium, capable of
secreting mucus. Beneath this is a layer of fibrous tissue, which
becomes areolar and supports the chief bloodvessels and lymphatics.
A few glands opening into the gall-bladder are occasionally present
in this tissue. Beneath this is the wall of the organ, composed of
interlacing bands of fibrous and smooth muscular tissues. The sur-
face is invested by a portion of the peritoneum. The excretory bile-
duct has a similar structure.

CHAPTER XII.

THE URINARY ORGANS.

The urine is secreted by the kidney, whence it passes succes-
sively through the renal pelvis, ureter, bladder, and urethra into
the outer world.

1. The kidney is made up of homologous parts or lobes, which
are readily distinguished in early life by the superficial furrows
marking their lines of junction. In later years these depressions
on the surface of the kidney disappear. Each of the lobes corre-
sponds to one of the papillae of the kidney and the pelvic calix that
embraces it. In some of the lower animals — e. g., the rabbit — the
kidney has but one papilla, so that the whole renal pelvis in those
animals corresponds to a single calix in man.

The kidney is a compound tubular gland of peculiar construc-
tion, the tubules taking origin from little spherical bodies, called
Malpighian bodies, instead of from simple blind extremities, and,
after running a definite and somewhat complicated course, uniting
successively with several others to form the excretory ducts, called
the " collecting tubules," which open into the calices near the tips
of the papilla?.

If a section of the organ be made through its convexity down to
the pelvis, the papilla? will be seen projecting into the calices of the
pelvis, and it will be noticed that each papilla forms the apex of a
pyramidal portion of tissue having a different tint and texture from
the rest of the kidney. These pyramids form the " medulla " of
the organ (Fig. 138).

The bloodvessels supplying nearly all its substance enter the

kidney near the bases of the pyramids, having approached the

organ through the fat that lies around the calices. Within the

kidney they break up into branches that run along the base of each

pyramid in that portion of the organ which is called the "boundary

zone." Between that zone and the convex surface of the kidney

the tissue is known as the "cortex."

The arrangement of the renal tubules, which make up the chief
n 161

162

NORMA L HISTOLOG Y.

bulk of the kidney, can be most easily understood if they are
traced back from their openings at the apex of the pyramid to their

Fig. 138.

Whe

Lobu/e

nal Pelvis

Diagrammatic sketch of a section of the kidney: a, columnar epithelium covering the
external surface of the pyramid and continuous on the one hand with the columnar
epithelium lining the collecting tubules within the pyramid, and on the other hand with
the transitional epithelium lining the calices and renal pelvis. This transitional epi-
thelium is indicated at 6. It rests upon the fibrous tissue of the calices and pelvis, which
becomes continuous with the fibrous capsule of the kidney at the junction of the calices
with that organ. Outside of this capsule is the perinephric fat, indicated in the figure
between the calices. The vessels approach the kidney through this fat, entering its sub-
stance near the bases of the pyramids and forming the vascular arcades (e, arterial arcade).
From these arcades the interlobular vessels proceed, between the medullary rays and in
the labyrinth, toward the convex surface of the kidney, d, interlobular artery, giving
off branches, the afferent vessels, to the Malpighian bodies. The extensions of the cor-
tical substance between tbe pyramids, c, «xe known as the columns of Bertini. During
infancy the lobes of the kidney are marked by sulci upon the surface of the organ. With
the growth of the organ these lobes blend with each other, and the sulci between them
become indistinct ot are wholly obliterated. The columns of Btrtini are made up of the
blended lateral portions of the cortex of two contiguous lobes.

origins in the Malpighian bodies. The different portions of the
tubules present somewhat different characters, and have received
special names.

THE URINARY ORGANS. 163

The collecting tubes, which open into the calix at the apex of the
pyramid, are straight, and lie nearly parallel to each other and to
the axis of the pyramid, and, therefore, nearly perpendicular to the
base of the pyramid. As they are followed from the apex, in a
direction the reverse of that taken by the urine in flowing through
them, they branch dichotomously, and the branches become pro-
gressively smaller. At the base of the pyramid these straight
tubules are collected into bundles that radiate toward the convex
surface of the kidney, and are called the " medullary rays." In
these, and in the part of the pyramid that is near the boundary-
zone, the collecting tubes are associated with other straight portions
of the tubules, " Henle's tubes," which will be described pres-
ently. From the medullary rays the tubules pass into the region
between those rays in the cortical portion of the kidney. This
region of the cortex is known as the " labyrinth." Here the tub-
ules lose their straight character and become much contorted, form-
ing the "second convoluted tubules." They then re-enter the
medullary rays, which they descend for a variable distance into the
pyramid, constituting the "ascending branches of Henle's tubes,"
which make a sharp turn, " Henle's loop," and then retrace their
course up the medullary rays into the cortical portion of the kidney,
"descending branches of Henle's tube." They then pass again
into the labyrinth and form the "first convoluted tubules," which
finally merge into the structure of the Malpighian bodies, also
situated in the labyrinth. In consequence of the passage of tubules
from them into the surrounding labyrinth the medullary rays become
smaller as they are followed from the base of the pyramid, and
eventually disappear before the capsule of the kidney is reached.
They are completely surrounded by the labyrinth.

If we now follow the course of the urine in its way from the
Malpighian body to the outlet of the tubule, we shall find that it
passes through the following divisions of the tubule : 1, the "first
convoluted tubule;" 2, the "descending branch of Henle's tube;"
3, "Henle's loop;" 4, the "ascending branch of Henle's tube;"
5, the "second convoluted tubule;" 6, the "collecting tube." Of
these, the two convoluted tubules are situated in the labyrinth ; all
the rest in the medullary rays and pyramid. All of the portions,
with the exception of the convoluted tubules and the loop, are
straight and lie parallel to each other (Fig. 139).

Before entering more particularly into the structure of the renal

164

NORMA L HISTOL 00 Y.

Fig. 139.

Diagram showing the course of the renal tubules within the kidney. (Klein.) A, cortex : a,
subcapsular portion destitute of Malpighian bodies ; a', inner portion, also devoid of Mal-
pighian bodies. B, boundary. C, portion of the medulla at the base of the pyramid.
1, Bowman's capsule surrounding the glomerulus ; 2, neck of the capsule and beginning
of the uriniferous tubule: 3, first convoluted tubule; 4, spiral portion of the first con-
voluted tubule in the medullary ray; 5, descending limb of Henle's tube; 6, Henle's
loop ; 7, 8, 9, ascending limb of Henle's tube ; 10, irregular transition to the second con-
voluted tubule; 11, second convoluted tubule; 12, transition from second convoluted
tubule to the collecting tubule ; 13, 14, collecting tubule, joined below by others to form
the excretory duct, which opens at the apex of the pyramid.

tubule, it will be best to complete this general sketch by considering
the course of the bloodvessels.

As has already been said, the vessels enter the kidney between
the calices and pyramids and are distributed in branches that lie

THE URINARY ORGANS.

165

Fig. 140.

parallel to the bases of the latter, and, therefore, to the convex
surface of the organ, and are situated in the boundary-zone. The

arterial branches in this location form the
"arterial arcade." From this arcade per-
pendicular branches, the " interlobular arte-
ries," pass toward the capsule, taking a
straight course through the labyrinth be-
tween the medullary rays. In this course
they give off branches, the "afferent ves-
sels," which go to the Malpighian bodies.

Fig. 141.

Fig. 140.— Diagram showing the course of the bloodvessels within the kidney. (Ludwig.) a,
interlobular artery ; b, interlobular vein ; c, Malpighian body, with the afferent vessel
entering it from the interlobular artery, and the efferent vessel leaving it to take part in
the formation of the capillary plexus between the renal tubules; d, vena stellata; e,
arterise rectse;/, vense rectse; g, capillary plexus around the mouths of the excretory
ducts.

Fig. 141.— Injected glomerulus from the horse. (Kcilliker, after Bowman.) a, interlobular
artery ; af, afferent vessel ; m, m, capillary loops forming the glomerulus ; ef, efferent
vessel ; 6, capillary network in -ihe labyrinth an5 medullary rays.

The main artery becomes smaller in giving off these branches, and
finally ends in terminal afferent vessels (Fig. 140J.

166

NORMAL HISTOLOG V.

Within the Malpighian body the afferent vessel divides abruptly
into a number of capillary loops, which are compacted together to
form a globular mass, called the "glomerulus" (Fig. 141). These
loops rejoin to form the "efferent" vessel, which is somewhat
smaller than the afferent vessel, and leaves the Malpighian body
at a point close to that at which the afferent vessel enters it.

Fig. 142.

Sketch of a Malpighian body from kidney of a rabbit : a, interlobular artery: b, afferent
vessel ; c, capillary springing from afferent vessel ; d, Bowman's capsule, with epithelial
lining reflected upon the surface of the glomerulus ; e, cavity of the capsule into which
the watery constituents of the urine are first discharged ; /, beginning of a uriniferous
tubule; g, convoluted tubules of the labyrinth. Between these tubules and the capsule
are capillary bloodvessels derived from the efferent vessel (which is not shown, but
emerges from the capsule near the afferent vessel, on a different level from that repre-
sented). These and other structures are held in place by an areolar tissue, containing
lymphatic spaces, some of which are represented.

Soon after leaving the Malpighian body the efferent vessel breaks
up into a second set of capillaries, which lie among the convoluted
tubules of the labyrinth and also penetrate into the medullary rays,
to be distributed between the tubules composing them. This capil-
lary network extends also into the pyramid, in which the capilla-

THE URINARY ORGANS.

167

ries run, for the most part, parallel to the renal tubules, with com-
paratively few transverse anastomosing branches. For this reason
they have been called the "vasa recta." They also receive blood
from little twigs given oft" from the arterial arcade.

The blood from the intertubular capillaries is collected in veins,
which run a course parallel to that of the arteries and lie in close
proximity to them. They have received names similar to those of
the corresponding arteries : " interlobular veins," " venae recta?,"
and " venous arcade." Relatively large veins also leave the kidney
from beneath the capsule on the convex surface of the organ. They
are called the " stellate veins."

The Malpighian body is enclosed by a thin fibrous capsule
(Bowman's capsule), which is perforated at two opposite points to
permit the passage on the one hand of the afferent and efferent
vessels, and on the other hand to allow of a communication between
its cavity and the beginning of the uriniferous tubule. When dis-
tended with blood the glomerulus nearly fills this capsule, but when
collapsed it is retracted toward the attachment formed by the ves-
sels that pierce the capsule. It is covered by a single layer of epi-

Fig. 143.

Fig. 144.

mwmmmm

Cross-sections of convoluted tubules lined with cells in different states of activity. (Disse.)

Fig. 135.— From a criminal directly after execution. Cells in a state of rest. The cells are
low and granular, and present a striation of their free ends resembling cilia.

Fig. 136. — From a cat. The cells are enlarged, because charged with material to be excreted,
and the striated border is nearly obliterated. Similar appearances have been observed
in the human kidney. In one of the lower cells in this figure a faint striation of the
attached end is just discernible. This increases in distinctness as the cell becomes sur-
charged with excretory material, when the more central portion of the cytoplasm
becomes hyaline and contains the nucleus.

thelial cells, which is reflected at that attachment and forms a lining
for the inner surface of the capsule to the point where its cavity
opens into the lumen of the renal tubule. Here the epithelial lining
becomes continuous with that of the tubule (Fig. 142).

The different portions of the uriniferous tubule differ in their

168 NORMAL HISTOLOGY.

external diameters, the diameters of their lamina, and the character
of their epithelial linings. The appearance of the epithelial cells
differs, however, in accordance with their state of functional activity
(Figs. 143 and 144).

The first convoluted tubule is relatively large, and is lined with
large epithelial cells, which project into the tubule about one-third
of its diameter. The cells have round nuclei situated near their
centres, and are granular, with an appearance of radiate striation
in their deeper halves when charged with secretion.

The descending branch of Henle's tube has a smaller diameter,
but its lumen is wide in consequence of the thinness of the clear
epithelial cells lining it. In the ascending branch the lumen is
again smaller, although the diameter of the tube is larger, because
the lining cells are thicker, somewhat resembling those of the first
convoluted tubule. The transition from the character of the de-
scending to that of the ascending branch does not always take place
exactly at the loop.

The second convoluted tubule is a little smaller than the first, and
is lined with cells that are not quite so granular and a little more
highly refracting.

The collecting tubules are lined with columnar epithelium, the
cells of which become longer as the diameter of the tube increases
in its progress toward the apex of the pyramid.

The epithelial lining throughout the course of the renal tubule
is said to rest upon a thin, homogeneous basement-membrane inter-
posed between it and the interstitial fibrous tissue. The latter is
present in small amount, and partakes of the character of an areolar
tissue, holding the tubules and bloodvessels in place. It is rather
abundantly supplied with lymphatics.

For the study of the uriniferous tubules sections made trans-
verse to the course of the straight tubules will be found very use-
ful. In the cortex the medullary rays, with their descending and
ascending branches of Henle's tubes and their collecting tubules,
will appear surrounded by the labyrinth, made up of the con-
voluted tubules, Malpighian bodies, and larger vessels, the latter in
cross-section. Near the apex of the pyramid cross-sections of the
larger collecting tubes and of the vasa recta will be seen ; and near
its base the smaller collecting tubes and the two limbs of Henle's
tube, with, possibly, here and there a "loop" in nearly longitudinal
section, will appear. Among all these sections of the tubules the

THE URINARY ORGANS.

169

interstitial tissue with its capillaries and lymphatics will complete
the picture (Figs. 145 and 146).

Fig. 145.

Fig. 140.

Sections from a rabbit's kidney, made perpendicular to the course of the straight tubules.

Fig. 14").— Through a portion of the pyramid : a, lower portions of the collecting tubules
(excretory ducts) ; 6, Henle's loop in tangential section ; c, capillary bloodvessels ; d,
lymphatic ; e, descending limb of Henle's tube.

Fig. 146.— Through part of a medullary ray and the adjoining labyrinth : a, a, a,a, convoluted
tubules in the labyrinth ; b, spiral tubule ; c, descending limb of Henle's tube ; d, ascend-
ing limb of Henle's tube: e, irregular tubule; /, collecting tubule; g, capillary blood-
vessel.

The nerves of the kidney are small and apparently not abundant.
Their larger branches follow the courses of the arteries.

170

NORM. 1 L 1UHTOLOG Y.

The external surface of the kidney is covered with a capsule of
fibrous tissue, which on its deeper surface becomes continuous with
the interstitial tissue, so that its vascular supply communicates with
the capillaries in the superficial portions of the kidney.

The fibrous capsule of the kidney becomes continuous at the
hilum of that organ with the fibrous coats of the calices and pelvis,
and, through these, with those of the ureter and bladder.

The columnar epithelium lining the collecting tubes is continuous
with a layer of similar cells covering the papillae.

The watery constituent of the urine is secreted in the Malpighian
body, where it passes from the blood through the capillary walls of
the glomerulus into the cavity of Bowman's capsule. Under nor-
mal conditions it is free from albumin, and, therefore, is unlike the
serum that passes through the walls of the capillaries in other parts
of the body. It has been thought that this difference was attrib-

Fig. 147.

-Zz.

KEr

Capillary loop from the glomerulus of the frog. (Nussbaum.) Ez, endothelial wall of the
capillary bloodvessel; Ek, nucleus of one of the endothelial cells (only three such
nuclei are shown in the figure) : KE, nucleus of one of the epithelial cells investing the
capillary. The boundaries of these cells are not reproduced in the figure. At the left
of the cut three epithelial cells have been partially reflected away from the capillary
wall.

utable to the functional action of the endothelium in the odomerulus,
though morphologically it is similar to that throughout the body.
It is more probable that the epithelium covering the glomerulus has

THE URINARY ORGANS.

171

something to do with the prevention of a loss of albumin (Fig. 147).
In disease of the kidney, alterations in the glomerulus and, per-
haps, in other parts of the kidney permit albumin to pass into the
secretion.

The epithelium lining the uriniferous tubules discharges its
secretion into the lumen of the tubules, whence it is carried by
the stream flowing from the Malpighian bodies. The epithelial
cells lining the convoluted tubules and the ascending branches of
Henle's tubes appear to be those most active in carrying on the
eliminative function of the kidney.

2. The pelvis of the kidney and its calices are lined with trans-
itional epithelium. It consists of only three or four layers of
epithelial cells of different shapes. The most superficial layer is
composed of rather large flattened cells, having ridges upon their
lower surfaces, which fill the spaces between the tops of the next
layer. This is made up of pear-shaped or caudate cells, the hemi-
spherical tops of which fit into the cavities between the ridges on
the layer above, while their slender processes penetrate between

Fig. 14S.

Epithelial cells from the pelvis of a human kidney. (Kieder.)

the oval or round cells that make up the deepest layers of the
epithelial covering (Fig. 148).

Beneath the epithelium is a coat of fibrous tissue, denser near the
epithelium and more areolar in its deeper portions. Here it is

172

NORMAL HISTOL OG Y.

interlaced with smooth muscular fibres, outside of which is the
external coat of fibrous tissue.

3. The ureters closely resemble in structure the pelvis of the
kidney ; but the muscular fibres have a somewhat more definite
arrangement, being disposed in an inner imperfect coat of longi-
tudinal and an external layer of circular fibres, outside of which
a few supplementary longitudinal fibres are, here and there, added
(Fig. 149).

4. The bladder also has a lining of transitional epithelium (Fig.

Ftg. 149.

Epithelial cells from the human ureter. (Rieder.)

40), beneath which is a layer of fibrous tissue resembling that of
the renal pelvis, but of greater thickness. The muscular coat,
which comes next, is thick and composed of bundles of smooth
muscular fibres, interlacing in various directions or disposed in
more or less well-defined strata. External to the muscular coat
is a fibrous coat, which is covered by a reflection of the peritoneum
for a part of its extent, and in other situations passes into the sur-
rounding areolar tissue.

The spear-shaped cells of the transitional epithelium of the blad-
der have thicker processes than those of the pelvis or ureter; but
when detached and macerated in the urine it is often very difficult
to determine from their appearance from what part of the urinary
tract such cells were derived (Figs. 150 and 151).

THE URINARY ORGANS.

173

5. The urethra differs in structure in the two sexes. In the
male the prostatic portion is lined with epithelium resembling that

\

\

V

/*->-"- ■

Fig. 151.

\ ys

v . ■'.-■ '■"'- '

Epithelial cells from the mucous membrane of the human bladder. (Rieder.)

Fig. 150. — From the urinary sediment from a case of cystitis. The cells are somewhat

swollen after maceration in the altered urine.
Fig. 151. — Removed from the internal surface of a normal bladder.

of the bladder. Further forward, it gradually passes into cylin-
drical epithelium, at first more than one layer thick ; but in the

174

NORMAL HISTOLOGY.

cavernous portion of the urethra it consists of but a single layer..
The stratified epithelium covering the glans extends for a short
distance from the meatus into the urethra (Fig. 152). The epithe-

Fi«. 152.

Epithelium from the human male urethra. (Rieder.)

lial lining rests upon fibrous tissue containing a number of elastic
fibres, and this is bounded externally by a muscular coat. In the
prostatic portion the muscular coat consists of an inner longitudinal
and an outer circular layer of fibres, which become less well marked
as the course of the urethra is followed, the circular coat disappear-
ing in the bulbous portion and the longitudinal fibres becoming
scattered toward the anterior part of the cavernous portion. The
mucous membrane contains little tubular glands, " Littre's glands,"
some of which are simple, while others are compounded. In the
collapsed condition the urethral mucous membrane is thrown into
niie or more longitudinal folds.

In the female the epithelial lining of the urethra is either strati-
tied or composed of a single layer of columnar cells. The glands
are more sparsely distributed than in the male, except for a group
situated near the meatus. On the other hand, the muscular coat
is thicker and consists throughout the course of the urethra of a
well-defined internal longitudinal and external circular layer of
fibres.

From the pelvis of the kidney to the stratified epithelium of the

THE URINARY ORGANS. 175

meatus the mucous membranes are capable of secreting mucus, which
is much increased in amount under the influence of irritating sub-
stances, such as concentrated urine or the various causes of inflam-
mation. The bloodvessels are most numerous and of largest size
in the areolar tissue beneath the epithelium, and are accompanied
by the lymphatics. The nerves are distributed chiefly to the mus-
cular coats, but also extend into the fibrous tissue, up to and into
the epithelium. The cells of the latter are connected by little
protoplasmic bridges, as in the case of the epidermis, leaving minute
channels between the cells for the passage of nutrient fluids.

CHAPTER XIII.

THE RESPIRATORY ORGANS.

The respiratory tract consists of the larynx, trachea, bronchi,
and lungs.

1. The Larynx. — The interior of the larynx is lined with ciliated
columnar epithelium, which extends over the false vocal cords and
about half-way up the epiglottis above, and is continuous below
with a similar lining throughout the trachea and bronchi. This
lining is interrupted over the true vocal cords by a covering of
stratified epithelium, and at its upper limits passes into the stratified
epithelium lining the buccal cavity and pharynx and covering the
tongue. Opening upon this epithelial surface, except upon the true
vocal cords and in the smallest bronchi, are mucous glands, varying
in number in different situations. Some of the columnar cells upon
the surface are also mucigenous, discharging their secretion upon
the free surface of the mucous membrane.

The thyroid, cricoid, and most of the arytenoid cartilages are
composed of the hyaline variety of that tissue : the epiglottis,
cornicula laryngis, and the apices of the arytenoids, of elastic car-
tilage.

Beneath the epithelium lining the laryngeal ventricle is a con-
siderable layer of lymphadenoid tissue. In other situations the
epithelium rests upon fibrous tissue.

2. The Trachea. — The tracheal wall may be divided into four
coats : a, the mucous membrane ; b, the submucous coat ; c, the
cartilage ; <1, the fibrous coat (Fig. 153).

a. The mucous membrane is covered with ciliated columnar epi-
thelium resting upon a nearly homogeneous basement-membrane,
beneath which is a layer of fibrous tissue. This may be divided
into two portions : an outer one, next to the basement-membrane,
which is areolar in character, with a large admixture of elastic
fibres and lymphadenoid tissue, and an abundant supply of blood-
vessels ; and an inner one, less highly vascularized, and composed
chiefly of elastic fibres running a longitudinal course.

176

THE RESPIRATORY ORGANS.

177

b. The submucous coat is of areolar fibrous tissue, supporting the
mucous glands that open into the trachea, and the bloodvessels,
lymphatics, and nerves, and also little masses of adipose tissue. In
the neighborhood of the cartilages this fibrous tissue becomes con-
densed to form the perichondrium.

c. The cartilages are composed of the hyaline variety of that

Fig. 153.

emJi

^mmmmm

m

*ft*

From a longitudinal section through the trachea of a child. (Klein.) a, the stratified
columnar ciliated epithelium of the internal free surface; b, the basement-membrane;
c, the mucosa (tunica propria) ; d, the network of longitudinal elastic fibres (the oval nuclei
between them indicate connective-tissue corpuscles) ; e, the submucous tissue, con-
taining mucous glands; /, large bloodvessels; g, fat-cells; //, hyaline cartilage of the
tracheal rings. (Only a part of the tracheal wall is given in the figure.)

tissue, and are incomplete rings, interrupted behind, where the two
ends are united by a band of smooth muscular tissue.

d. The fibrous coat is of areolar tissue beyond the bounds of the
perichondrium, and serves to connect the trachea with its sur-
roundings.

3. The Bronchi. — The main bronchi branching from the trachea
have a structure similar to that organ, but the cartilaginous rings
become more delicate as the tubes diminish in size.

12

178

NORMAL HISTOLOGY.

The smaller bronchi (litter in structure from the trachea in
possessing a muscularis mucosa?, with its fibres disposed in a
circular direction, and having irregular cartilaginous plates in their
walls, instead of C-shaped, imperfect rings. The four coats may
be enumerated as follows :

a. Mucous membrane, covered with ciliated columnar epithelium
resting upon a basement-membrane, beneath which is a fibrous
tissue containing numerous elastic fibres lying parallel to the axis
of the bronchus. Under this are the circular fibres of the mus-
cularis mucosae.

b. Submucous coat, similar to that of the trachea and larger
bronchi.

c. Cartilaginous coat, containing the plates of cartilage that sup-
port the walls.

d. Fibrous coat of areolar tissue, containing a little adipose tissue
and passing into the areolar tissue of neighboring structures.

As the bronchi subdivide and become smaller the coats get
thinner, and first the cartilaginous and then the muscular coat dis-
appears. Those air-passages which are without cartilage, but have

Fk;. 154.

Portion of a cross-section of a bronchiole from the lung of a pig. (Schultze.) a, areolar
external coat ; b, muscularis mucosa' ; c. subepithelial areolar tissue, containing numerous
longitudinal clastic fibres, represented here in cross-section ; d, ciliated epithelium, form-
ing the most superficial layer of the mucous membrane ; /. walls of the neighboring pul-
monary alveoli. In these walls branching and anastomosing elastic fibres are shown:
the capillary plexus has been omitted.

a muscularis mucosa?, are called "bronchioles" (Fig. 154). The
still smaller branches, which have lost their muscular tissue, are
known as the "alveolar passages." In the latter the columnar

THE RESPIRATORY ORGANS.

179

epithelium lining- the bronchi gives place to a pavement-epithelium,
composed of small flattened cells disposed in a single laver. The
elastic tissue of the mucous membrane is continued through all the
divisions of the air-passages, and becomes a constituent part of the
alveolar walls of the lung itself.

The alveolar passages open into spaces, called the " infundibula,"
in the sides of which are the openings into the alveoli of the
lung, the ultimate destination of the inspired air. Here and there

Fig. 155.

Section of lung of the dog, showing a transverse section of a bronchiole: a, bronchiole (a
little mucus covers the epithelial lining) ; b, muscular layer of the mucous membrane ; c,
c, radicles of the pulmonary vein ; d, alveolar passage, just at its division to form infun-
dibula. An infundibulum extends from this passage toward the bronchiole. The wall
of the alveolar passage at this point is similar in structure to that of the pulmonary
alveoli, e, alveolar passage in oblique section. This passage is cut at a point further
from its opening into the infundibula, and has a somewhat thicker wall than d. The rest
of the section is made up of infundibula (the larger spaces) and pulmonary alveoli.

stray alveoli open directly into the alveolar passages (Figs. 155, 156,
and 157).

4. The pulmonary alveoli and the smaller air-passages are so
arranged that there are no vacant spaces ; and neighboring alveoli,
whether they belong to a group of infundibula springing from
the same alveolar passages or to separate groups, are so closely
situated that they have but one common wall dividing their cavities

180

NORMAL HISTOLOGY.

from each other. Notwithstanding this general compactness of
arrangement, the lungs are divided by delicate septa of fibrous
tissue into more or less well-defined lobules, corresponding to the
smallest bronchi or the bronchioles.

The alveolar walls are made up of a delicate, loose areolar tissue,
containing numerous elastic fibres and supporting the abundant
capillary plexus in which the blood suffers the gaseous exchanges
with the air that constitute the function of respiration (Fig. 158).

Fig. 156.

•"*. STv £.&£? 'Vi 5*i:5i a _--""■-- War'.. •:1B '•

Si- -i2^"J "^

■r

My*

^&*h£e% r^W-?'»C** ^A&-~£Su L*«§2£Vv ft^'-r'1'

Section of lung of the dog : a, alveolar passage opening into an infnndibulum and also into
a solitary alveolus ; b, cross-section of an infnndibulum. The dotted line indicates the
limits of the infundibular space. Opening into it are a number of alveoli. Were the
dotted line removed, the infundibular cross-section and the alveoli around it would form
a stellate space in the section, ('.junction of two radicles of the pulmonary \ein. At
the top of the section, to the right, is an oblique section of a bronchiole.

Covering the two surfaces of the alveolar wall is a layer of very
thin cellular plates (pavement-epithelium, see Fig. 30), among
which are scattered a few cells resembling those lining the alveolar
passages. This cellular investment is continuous with the lining
of the infnndibulum, which is of similar character, and thence with
the epithelium covering the inner surface of the alveolar passage.
It is to be regarded as a special modification of epithelium, fitting
it for usefulness in this situation.

THE RESPIRATORY ORGANS.

181

The lung receives blood from two sources: 1, venous blood,
through the pulmonary artery, which is oxygenated in the walls of
the alveoli ; 2, arterial blood, through the bronchial arteries. This
arterial blood serves for the nourishment of the tissues of the lung
and is distributed to the bronchi, interlobular connective tissue,
lymph-glands, and walls of the vessels. Part of this blood returns
through the pulmonary veins; the rest through the bronchial veins.

Fig. 157.

a-

Seetion of lung of the dog: a, oblique section of a bronchiole ; b, its muscular coat ; c, longi-
tudinal section of an infundibulum, communicating to the right with an alveolar passage
(the wall of the latter is torn further to the right) ; d, one of the alveoli opening into c.

The lymphatics arise in the walls of the alveoli and bronchi and
pass to the bronchial lymph -glands.

The nerves supplying the lung may be traced along the bronchi,
where they occasionally connect with groups of ganglion-cells, and
along the vessels. They are of both the medullated and the non-
medullated varieties.

The surface of the lung is covered with serous membrane, a por-
tion of the pleura.

Little need be said about the functional activity of the lung.
The cilia, belonging to the columnar epithelium lining nearly the

182

NORMAL HISTOLOGY.

whole of the air-passages, possess a motion that urges particles
lodging in the mucus covering them toward the larynx, whence
they are either coughed out or are swallowed. Such solid particles
as pass beyond the regions guarded by ciliated epithelium are taken
up by leucocytes, which frequently migrate into the alveoli and the
air-passages, and are conveyed by them into the lymphatic vessels
or glands. Because of this the lymphatics and bronchial lymphatic
nodes are apt to be blackened by the deposition of carbon, except
in young individuals. The flow of air into the lung is the result of
atmospheric pressure, which tends to fill the thoracic cavity when the

Fig. 158.

Section of the lung of a dog, killed by ether-narcosis. The lung was hyperaemic at the time
of death, and the capillaries retain their blood in the section, o, alveolus in cross-sec-
tion, communicating with the infundibulum, b. A portion of the wall of the alveolus is
seen, in surface-view, at c. d, e, other alveoli opening into the same infundibulum : /.
cross-section of an infundibulum with alveoli opening into it; g, surface-aspect of an
alveolar wall, showing capillary plexus filled with red blood-corpuselrv

chest is expanded through the action of the muscles of respiration.
The air is expelled from the lungs when those muscles relax, partly
because of the pressure exerted by the thoracic walls, but ehieflv
because of the contraction of the elastic fibres in the alveolar walls.

THE RESPIRATORY ORGANS. 183

Because of their presence the lungs retract when the chest is
opened.

When sections of the lung are examined under the microscope
it is difficult, at first, to identify the different portions, which are
cut in all directions. The smaller bronchi may be recognized
by the presence of cartilage in their walls. The bronchioles pos-
sess no cartilage, but are surrounded by a band of smooth mus-
cular tissue, the muscularis mucosae. This becomes thinner, then
incomplete, and finally disappears as the infundibula are reached.
The infundibulum, it will be remembered, is the space into which
the alveoli open. When seen in section it will appear as a round,
oval, or elongated space, according to the direction in which it
has been cut, bounded by scallops, each of which is the cavity of
an alveolus. In every section there will be many alveoli which
have been so cut that their openings into the infundibulum will not
be included in the section. These alveoli have a continuous wall
surrounding their cavities. Still other alveoli will have been cut
in such a way that a portion of their walls will lie in the plane of
the section and parallel to it, so that the flat surface of the alveolar
wall will be visible, surrounded by an oblique or cross-section, where
the wall meets the surface of the section. Those alveolar walls which
have been cut perpendicular to their surfaces will appear thinner
than those which have been cut obliquely. With these considera-
tions in his mind, the student can have little difficulty in identify-
ing the different portions of the section (see Figs. 155-158).

CHAPTER XIV.
THE SPLEEN.

Nearly the whole surface of the spleen is invested with a cov-
ering of peritoneum similar to that which partially covers the
liver. Beneath this is the true capsule of the spleen, which com-
pletely surrounds it. This capsule is composed of dense fibrous
tissue, containing a large number of elastic fibres and a few of
smooth muscular tissue. From its inner surface bands of the same
tissue, called the " trabecular," penetrate into the substance of the
organ, where they branch, and the branches join each other to form
a coarse meshwork occupied by the parenchyma of the organ, the
" pulp."

The bloodvessels of the spleen enter at the hilum and pass into
the large trabecular, which start from the capsule at that point
and enclose the vessels until they divide into small branches. The
vessels then leave the trabecular and penetrate the pulp, where they
break up into capillaries, which do not anastomose with each other.
There is some doubt as to the way in which these capillaries end.
According to one view, they unite to form the venous radicles, so
that the blood is confined within vessels throughout its course in the
spleen. Another view is that the walls of the capillaries become
incomplete, clefts appearing between their endothelial cells, which
finally change their form and become similar to those of the reticu-
lum of the pulp. The veins, according to this view, arise in a
manner similar to the endings of the arteries. The result of this
would be that the blood is discharged, from the capillary termina-
tions of the arteries, directly into the meshes of the pulp, after which
it is taken up by the capillary origins of the veins (Figs. 159 and 160).
Studies of the vascular structures in the spleen made with the aid of
injections under varying conditions of pressure, in collapsed and in
distended spleens and with different injection-materials, have led to
a third view, which, if correct, would explain the divergent opinions
just mentioned. The delicate reticulum which pervades the organ is
elastic and forms a fine-meshed layer over the small vessels, support-

184

THE SPLEEN.

185

ing the endothelium of the capillaries. This epithelium is composed
of more spindle-shaped cells than are usual in capillary vessels

Fig. 159.

Section from the spleen of the cat. (Bannwarth.) Termination of an arterial capillary in

the pulp.

and, when the vessels are distended, separate, leaving clefts between
them. This separation may also be effected by distention of the

Fig. lbu.

• • AJSt

••IVVV'V

,®*^

4 G»

1

&

Section from the spleen of the eat. (Bannwarth.) Beginning of a capillary venous radicle.

pulp-spaces which produces tension on the reticular fibres, and this
opens the clefts between the endothelia of the capillaries. When the

186 FORMAL HISTOLOGY.

capillary hlood-pressure is increased or the pulp distended or the tra-
becular contract, these clefts open ; under other circumstances they
are closed and the circulating blood is confined to the vessels. The
vascular connection between what may be called the arterial capil-
laries and venous capillaries is a dilated capillary, or "ampulla,"
larger than other capillaries, and it is in these ampullae that the endo-
thelium is composed of the narrowest spindle-shaped cells, so that
the clefts between them would be most pronounced in this part of
the vascular system of the organ. During life the spleen displays
slow rhythmic contractions at intervals of about a minute, and from
what has been said of its structure it will be obvious that these con-
tractions would favor the circulation of blood through its vessels and
pulp. It appears, indeed, that these contractions are essential, for
if the muscular tissues be thrown out of function by severing the
nerves, the circulation is so interfered with that the pulp becomes
engorged with blood. No lymphatic vessels have been discovered
in the spleen.

The pulp consists of a fine reticulum of delicate fibres and cells,
with branching and communicating processes, in the meshes of
which there are red blood-corpuscles, leucocytes in greater number
than normally present in the blood, and free amoeboid cells consid-
erably larger than leucocytes, called the " splenic cells."

The adventitia of the arteries contains considerable lymphadenoid
tissue, which after the exit of the vessels from the trabecular is
expanded at intervals to form spherical bodies, about 1 mm. in diam-
eter, called the " Malpighian bodies " or " corpuscles." These are
like little lymph-follicles, through which the artery takes its course.
The reticulum in these Malpighian corpuscles is scanty and incon-
spicuous near their centres, so that the lymphoid cells it contains
appear densely crowded ; but toward their peripheries the reticulum
is more pronounced and the cells a trifle more separated. At the
surface of the Malpighian body its reticulum becomes continuous
with that of the pulp surrounding it (Fig. 161).

The relations between the spleen and the blood flowing through
it appear to be very similar to those between the lymphatic glands
and the lymph passing through them. It seems to act as a species
of filter, in which foreign particles or damaged red blood-corpuscles
are arrested and destroyed. In many infectious diseases the splenic
pulp is increased in amount and highly charged with granules of
pigment that appear to be derived from the coloring-matter of the

THE SPLEEN.

187

blood. Some observations suggest the conclusion that the cause of
splenic enlargement is the destruction of red corpuscles (or possibly
leucocytes also), haemolysis, which is frequent and pronounced in
many infectious diseases. This is notably the case in malaria, in
which the red corpuscles are destroyed by the plasmodium occasion-
ing the disease. When bacteria gain access to the blood, they are
apt to be especially abundant in the splenic pulp, and it is said that
monkeys, which are normally immune against relapsing fever,
may acquire the disease if the spleen be removed before inoculation

Fig. 161.

Section from human spleen. (Kolliker.) A, capsule ; 6, 6, trabecule ; c, c, Malpighian bodies
(lymph-follicles), traversed by arterial twigs. In the follicle to the left, part of the
arterial twig is seen in longitudinal section ; in that to the right, it appears in cross-
section to the right of the centre of the follicle, rf, arterial branches; e, splenic pulp.
The section is taken from an injected spleen.

with the spirillum which is the cause of that disease. Removal of
the spleen leads to a diminution of the number of red corpuscles in
the peripheral circulation and to a still greater diminution in the
haemoglobin of the blood, which lasts for many weeks, but eventually
normal conditions are usually re-established, perhaps because of
changes in the red marrow of the bones. These observations all
tend to confirm the view that the function of the spleen is to assist
in maintaining the functional integrity of the blood. The lymph-
adenoid tissue within the spleen also enriches the blood with an addi-
tional number of leucocytes.

CHAPTER XV.

THE DUCTLESS GLANDS.

The organs included in this group possess, at some stage of their
development or in the adult, a structure analogous to that of the
secreting glands. Those which retain this structure after complete
development diifer from the other glandular organs in being devoid
of ducts, through which the materials elaborated by their paren-
chyma could be discharged. Of these organs the thyroid is the most
striking example. Other members of this group, notably the thy-
mus, become greatly modified as development advances, and after a

Fig. 162.

Section of human thyroid gland : a, alveolus filled with colloid ; b, alveolus containing a
serous fluid : c, interalveolar areolar tissue ; d, tangential section of an alveolus, giving
a superficial view of the epithelial cells.

while retain mere vestiges of their original epithelial character;
the chief bulk of the organ being composed of lymphadenoid
tissue.

The following organs and structures will be considered as belong-
ing to the general group of ductless glands : the thyroid gland, the

lvs

THE DUCTLESS GLANDS. 189

parathyroids, the adrenal bodies, the pituitary body, the thymus,
and the carotid and coccygeal bodies.

1. The Thyroid Gland (Fig. 162). — This consists of a number
■of alveoli or closed vesicles, lined with cubical epithelial cells ar-
ranged in a single layer upon the delicate, vascularized areolar tissue
which forms their walls and separates the neighboring alveoli from
each other. This fibrous tissue is more abundant in places, where
it serves to divide the gland into a number of imperfectly defined
lobes. At the periphery of the organ its connective tissue becomes
continuous with a thin but moderately dense fibrous capsule.

The individual alveoli differ both in respect to their size and their
contents. Many are more or less completely filled with a nearly
homogeneous, glairy substance, of a slight yellowish tint, called
"colloid," while others appear to be occupied by a serous fluid.

Fig. 163. Fig. 164.

Sections of thyroid gland. (Schmid.)
Tig. 163.— From a dog : a, colloid or secreting cells ; b, reserve cells (these differ only in their

states of activity) ; c, cells containing less colloid than a.
Fig. 164.— From a cat : a, daughter-cells arising from the division of an epithelial cell.

The elaboration of this colloid material seems to be the function
of the organ, though it may have other less obvious duties.

The cells lining the alveoli may be divided into two classes,
which differ in appearance (Fig. 163): first, those engaged in the
production of colloid, secreting cells ; and, second, those in which
no colloid is present, and which are regarded as reserve cells.
The latter are capable of multiplication, thereby replacing such of

100

NORMAL HISTOLOGY.

the secreting cells as may be destroyed (Fig. 164). The colloid
material is produced within the cytoplasm of the secreting cells,

Fig. 165.

Section from thyroid of dog, illustrating the egress of colloid from the alveoli. (Bozzi.)
a, epithelial cells lining the alveolus, seen in section. The internal ends of similar cells
are seen in superficial aspect below, b, colloid within the alveolus; c, exit of colloid
between two epithelial cells ; e, lymphatic vessel ; d, end of a colloid or secreting cell in
the epithelial lining of the alveolus. •

whence it is either expelled into the lumen of the alveolus, or the
whole cell becomes detached from the alveolar wall and suffers col-

Fig. 166.

N

•

Section from thyroid of dog, illustrating the egress of colloid from the alveoli. (Bozzi.)
a epithelial lining of the alveolus ; /), colloid ; c, escape of colloid through a defect in the
wall occasioned by the colloid metamorphosis of some of the epithelial cells, the nuclei
of which are discernible within the colloid near c.

loid degeneration, with destruction of the nucleus, within the
alveolar cavity.

THE DUCTLESS GLANDS. 191

The colloid material subsequently finds its way into the general
circulation, either by passing between the intact cells of the alveolus
l Fig. 165), or after a passage has been prepared for it through altera-
tions in certain of those cells (Fig. 166). The colloid is then taken
up by the lymphatics, through which it reaches the general circula-
tion. This is an example of internal secretion which presents much
of interest. It is probable that a similar, but much less obvious,
process takes place in some of the ordinary secreting glands of the
body, certain elaborated materials being returned to the circulation
by the cells of the gland, while others are utilized for their nourish-
ment and for the elaboration of the more obvious secretion.

That the secretion of the thyroid gland is of importance to the
general organism is shown by the effects of disease or removal of
the gland upon the general nutrition. Total extirpation of the
thyroid, together with the parathyroids, occasions the death of an
animal within a few days, after symptoms of grave disturbances in
the central nervous system, among which are tetanic convulsions.
A partial removal of the gland, or its removal without that of the
parathyroids, causes profound disturbances of nutrition, grouped
under the title "cachexia strumipriva." The animal becomes weak,
drowsy, and emaciated ; the skin dry and scaly, with a loosening
of the hairs. In young animals the growth is retarded, especially
the development of the bones, through degenerative changes in the
epiphysial cartilages. In these, the intercellular substance becomes
swollen and disintegrated ; the cells atrophied or destroyed. Marked
changes, designated as myxcedema, also appear in the subcutaneous
tissue, which is converted into a species of mucoid tissue, probably
as the result of an altered metabolism within the pre-existent cells
of the tissue. The functional activity of the kidney is modified ;
after a while, albuminuria results. Exactly similar disturbances have
been observed in people suffering from disease of the thyroid gland.

The foregoing facts are cited here in order to emphasize by a
striking example the statement previously made, that the organs of
the body are mutually dependent upon each other.

Experimentation and clinical study have further shown that the
symptoms of myxcedema may be moderated or perhaps entirely
arrested by feeding with thyroid extracts, or still more markedly
by injecting extracts from thyroid glands beneath the skin, where
they would speedily pass into the lymphatics and thence into the
general circulation.

1!»2 NORMAL HISTOLOGY.

Chemical examination has revealed the presence of a substance
called " thyroiodin " in the alveoli of the thyroid gland. This is a
proteid containing a large amount of iodine. Its production by the
thyroid gland may be increased by feeding with substances contain-
ing considerable iodine or by administering iodide of potassium.
Injections of thyroiodin serve to mitigate the effects of thyroidec-
tomy, very much as do injections of thyroid extracts. It is by no
means clear, however, that the thyroiodin is the only substance
elaborated by the thyroid gland which may be of use to the tissues

Fig. 1G7.

p-ia

f

%> >

Section of the thyroid gland of a kitten two months old. fKohn.) Showing the positions
of the outer and inner parathyroid bodies and a thymus follicle: t, thyroid gland; p,
inner parathyroid; p\ outer parathyroid: th, thymus follicle; a, portion of the section
showing the intimate relations between the thyroid and the inner parathyroid; b, por-
tion demonstrating a similar intimate relation between the thyroid and the tissues of the
thymus follicle.

of other organs, or that the thyroid may not also remove injurious
substances from the circulation and thus indirectly benefit other
structures in the body.1

1 Attention is also called to the possibility that an excessive or morbid thyroid
secretion may cause symptoms of disease attributable to disturbances in the functions
of other organs and may also occasion disturbances in nutrition.

THE DUCTLESS GLANDS. 193

The bloodvessels of the thyroid are abundant, and form a rich
plexus in the areolar tissue between the alveoli. The lymphatics
are also abundant and large, forming a network of rather large ves-
sels in the same situation. The nerves accompany the vessels, are
destitute of ganglia, and have been traced to the bases of the epi-
thelial cells, whence they may occasionally send minute terminal
twigs with enlarged ends between the epithelial cells.

2. The Parathyroids (Figs. 167, 168, 169).— These are two bodies

Fig. 1GS.

m

-

■■■:'■■ .

J P ■ i

■■■ .• 4 ■■
:1 • "■■€ » V& ]^ ft-

m

Section of a portion of the external parathyroid of a kitten two months old. (Kohn.) Show-
ing the columns of epithelial cells separated by a delicate, vascular areolar tissue. The
nuclei between the columns of epithelium belong chiefly to capillary bloodvessels, m,
in, nuclei exhibiting karyokinetic figures.

of identical structure, which are developed in conjunction with
the thyroid gland ; but, while the latter progresses in its devel-
opment until it attains the structure already described, the para-
thyroids retain a structure similar to that of the embryonic thyroid.
They are composed of solid columns of epithelial cells, which anas-
tomose with each other, but are elsewhere separated by a small
amount of vascular areolar tissue. They are enclosed in a very
thin capsule of areolar tissue, but are in very close relation to the
neighboring tissues of the thyroid gland (Figs. 167 and 169), and
frequently also with isolated follicles of thymus tissue.

Different observers vary in their opinions respecting the para-
thyroids. Some regard them as reserve thyroid tissue, remaining
dormant while the thyroid is functionally competent, but developing

13

194 NORMAL HISTOLOGY.

into thyroid tissue when the gland furnishes an insufficient supply of
secretion. Other observers deny this and regard the parathyroids
as embryonic rudiments, nearly, if not quite, devoid of function. It
is certain that in some cases of thyroidectomy the parathyroids
become enlarged, and that the cachexia strumipriva is not certain to
develop after the removal of the thyroid gland unless the parathy-

Sch

Section of the inner parathyroid of a kitten two months old. (Kohn.) Showing its close
connection with the tissues of the thyroid gland: Sch, alveoli of the thyroid; P, epithelial
columns of the parathyroid ; K, capsule separating the two.

roids are also removed. Histological studies of the parathyroids in
such cases have, however, failed to reveal a tendency on their part
to develop into true thyroid tissue. Their relations to the thyroid,
therefore, still remain undetermined.

In some animals — e. //., the eat — there are four parathyroid bodies,
two associated with each lobe of the thyroid.

3. The Adrenal Bodies (Fig. 170). — The adrenal bodies, or supra-
renal capsules, possess a fibrous capsule, which is more areolar
externally, where it frequently merges into the perinephric fat,

THE DUCTLESS GLANDS.

195

Fig. 170.

and denser internally, where it is reinforced in some animals by
smooth muscular fibres. From
this capsule septa of areolar tissue
penetrate into the substance of the
organ and constitute its interstitial
tissue. The parenchyma of the
organ consists of columns of epi-
thelial cells, which are differently
arranged and have a somewhat dif-
ferent appearance in different parts.
As the result of these differences the
organ has been divided into a cor-
tical and a medullary portion.

In the cortical portion the cells
are arranged in solid columns hav-
ing their long axes perpendicular to
the surface of the organ. Toward
the capsule these columns lose their
parallel arrangement and appear in
vertical sections as islets of cells
surrounded by areolar tissue, the
" zona glomerulosa." In the deep
portion of the cortex the cellular
columns form a meshwork and com-
pletely lose^ their fascicular arrange- Vert x section of ,

ment. This region is called the (Eberth.) 1, cortex; 2, medulla; a,
., ,.,.,, mi •,! !• 1 capsule; 6, zona glomerulosa ; c, zona

zona reticularis." The epithelial fasciculata; a, zona reticularis; e,
cells in the cortical portion are poly- £rouPs of medullary ceils; /, partial

... , _ . . . , section of a large vein.

hedral, and are frequently infiltrated

with numerous globules of oil or fat, which give that part of the

organ a yellow color.

In the medulla the interstitial tissue of the organ encloses groups
of epithelial cells, which differ from those of the cortex in being
free from fat. They are also larger than those cortical cells which
contain no fat (Fig. 171).

The arteries of the adrenal bodies enter as numerous small twigs
at the surface of the organ and divide into capillaries within its
fibrous septa. These open into a venous plexus in the medulla,
which communicates with a single vein leaving the organ.

The nervous supply of the adrenal bodies is very abundant. The

> o

19(5

NORM A I. HISTOLOGY.

Fig. 171.

'foffM^ fated1

--'•> --4. ..1 ' ' ■ •'-. ^.AiW

in

Section through the boundarj' between cortex and medulla in the adrenal body of the horse.
(Dostoiewsky.) /,/,/, cells of the cortex, infiltrated with fat-globules ; </, ganglion-cells ;
m, epithelial cells of the medulla.

nerve-fibres are chiefly of the medullated variety, and their bundles
contain numerous ganglia before entering the organ. Here the
fibres ramify abundantly in the cortex, whence they penetrate into

Fig. 172.

Injected lymphatics in an adrenal body of the ox. (Stilling.) L, injection-mass within the
lymphatic vessels; N, cross-section of a nerve: V, longitudinal section of a vein.
Lymphatic radicles are seen among the epithelial cells (cortical variety free from fat)
to tlic right of the figure.

the medulla. At the junction of the medulla and cortex the
nerve-fibres are connected with ganglion-cells. The nerve-termi-

THE DUCTLESS GLANDS. 197

nations are distributed to the walls of the vessels and penetrate
between the epithelial cells of the parenchyma.

As in the case of the thyroid gland, the relations of the epithelial
cells of the adrenal bodies to the lymphatics appear of special
interest. The lymphatic vessels are abundant and large, and accom-
pany the bloodvessels lying in the areolar tissue of the septa.
Here they come into close relations with the columns of epithelial
cells, and, at least in the cortex, send minute terminal branches
into those columns, where they end among the epithelial cells (Fig.
172). This arrangement of the lymphatics appears to point to the
elaboration of an internal secretion as the function of the adrenal
bodies. Small masses of lymphadenoid tissue are occasionally
observed in the cortical portion of the adrenal body.

4. The Pituitary Body. — The pituitary body (hypophysis cerebri)
is divisible into two portions, which differ both in their structure
and in their embryonic origins. The posterior, or nervous, lobe is
derived from a prolongation of the third cerebral ventricle. The
anterior, or glandular, lobe develops from a tubular prolongation,
lined with epithelial cells, from the buccal cavity of the embryo.
This partially or completely invests the nervous portion of the
body, but its chief bulk is situated in front. The connection with
the buccal cavity is obliterated, and, in the further development of
the detached portion, a number of anastomosing columns of epi-
thelial cells are formed, which are separated from each other by
septa of vascular areolar tissue. These septa become continuous
at the periphery Avith a thin fibrous capsule furnished by the pia
mater.

The cells of the epithelial strands in the glandular lobe appear
to be of two sorts, which, like those in the thyroid gland, probably
represent different stages of functional activity. The darker sort
of cell yields microchemical reactions resembling those of colloid ;
and little masses of colloid, presumably derived from those cells,
are of not infrequent occurrence within or at the margins of the
epithelial columns (Figs. 173 and 174).

The glandular lobe is richly supplied with capillary bloodvessels
in intimate relations with the epithelium, from which they often
appear to be separated by only a thin basement-membrane, and the
existence of this is doubtful in some situations (Fig. 175).

The above description shows that the structure of the hypophysis
is similar to that of the other ductless glands already considered.

11)8

NORMAL HISTOLOGY.
Fig. 173.

Section from the hypophysis of the ox. (Dostoiewsky.) v, veins ; a, alveoli or cell-columns,
with pale, relatively clear cytoplasm ; b, alveoli or columns of darker granular cells.
Other cell-groups contain both varieties of cell.

Fig. 174.

Section from the glandular lobe of the hypophysis; horse. (Lothringer.) Showing the
darker cells at the periphery of the epithelial strands, and the clearer cells, for the most
part, in their centres.

THE DUCTLESS GLANDS.

199

Its function is still very obscure ; but it appears, in cases of experi-
mental thyroidectomy and in disease of the thyroid in the human
subject, to enlarge when the function of the thyroid gland is abol-
ished and to assume vicariously the duties of that organ. In how
far this points to a normal similarity in function of the two organs
must, at present, be left undetermined. In cases of enlargement
of the pituitary body profound changes in nutrition, characterized
chiefly by overgrowth, frequently take place in the bones of the
skeleton (acromegaly).

The nervous supply of the anterior lobe consists of non-medul-

Fir;. 175.

Section from the glandular lobe of the hypophysis ; child six months old. (Lothringer.)
The close relations between the epithelial cells and the capillary bloodvessels, and the
differences in the cells, are indicated in this figure. The red blood-corpuscles within the
capillaries have been stained dark.

lated fibres, destitute of ganglion-cells, which ramify about the
vessels and send some of their terminal twigs between the epithelial
cells.

The posterior lobe consists of tissues resembling those of the
central nervous system : ganglion-cells, non-medullated fibrils, and
neuroglia-cells. Within its substance there are also peculiar oval
bodies surrounded by nervous terminations, to which sensory func-
tions have been attributed, and small follicles, lined with cubical
epithelium.

200

NORMAL HISTOLOGY.

5. The Thymus. — This organ readies its fullest development at
about the second year of life, after which retrograde changes, end-
ing in the substitution of fibrous and adipose tissues for its proper
structure, take place. Its development begins as an ingrowth of
epithelium from the branchial clefts. This epithelium forms a

Fig. 176.

Two concentric corpuscles of Hassall, from the foetal thymus. (Klein.)

branching, solid column of cells surrounded by embryonic connec-
tive tissue, which develops into lymphadenoid tissue. In the
meantime the epithelial strands are broken up and the whole organ
becomes converted into a structure resembling a collection of lymph-
follicles, but with this difference : that remnants of the epithelial
strands remain in the centres of many of the follicles, where their

Fig. 17

Lobule from the thymus of a child. (SchSffer.) tr, trabecula; o, nodule of denser lymph-
adenoid tissue at periphery ("cortex"); b, b, sections of vessels within the less dense
lymphadenoid tissue in the centre (" medulla") ; c, c, concentric corpuscles of Hassall.

cells become flattened and imbricated. These epithelial masses
are known as the concentric corpuscles of Hassall (Fig. 176).

The thymus is enclosed in a fibrous capsule, which penetrates its
substance, dividing it into lobes and lobules. Each of these lobules
closely resembles a lymph-follicle, but it is doubtful whether lymph-

THE DUCTLESS GLANDS.

201

sinuses, corresponding to those in the lymphatic nodes, are present
in the thymus (Fig. 177).

The function of the thymus is still a matter of doubt. It has
been regarded as one of the sites in which red blood-corpuscles are
formed, and also as a temporary lymphadenoid organ playing the
part of the lymph-nodes until these have become fully developed in
other parts of the bod) .

The thymus is connected with the thyroid by a strand of thymus-
tissue, and isolated thymus-lobules are found embedded in the
edges of the thyroid, near the parathyroid body (see Fig. 167).

The bloodvessels ramify in the septa of the organ and send
branches into the lymphoid follicles. The lymphatic vessels accom-
pany the bloodvessels and surround the lobules, but do not appear

Fig. 178.

z

y

^ ...

' •

ff

Section of the carotid gland and carotid arteries near their origin. (Marchand.) ci, internal
carotid ; ce, external carotid ; glc, carotid gland ; I, I, groups of epithelial cells ; i, fibrous
tissue between the epithelial groups ; g, bloodvessel. Numerous vessels are also seen
within the gland.

to penetrate into the lymphadenoid tissue. The nerves are small
and not numerous. They accompany the bloodvessels, but nervous
terminations have not been traced as distributed to the lymphade-
noid tissue.

The involution of the gland appears to be accomplished through

202

NOUMA L HISTOLOG Y.

Portion of the same gland as Fig. 170, more highly magnified : p, epithelial cells ; g, capillary
bloodvessels; e, endothelium forming the capillary wall.

a proliferation of the fibrous tissue around the lobules, which en-
croaches upon the lymphadenoid tissue and gradually replaces it.
This fibrous tissue subsequently becomes, in great measure, con-
verted into adipose tissue. It appears as though the endothelium
of the bloodvessels also proliferated, giving rise, to masses of imbri-

Fig. 180.

Itllipiitfc :

Section of the coccygeal gland. (Sertoli.) The group of cells, apparently of epithelial
nature, is traversed by small bloodvessels and enclosed by fibrous tissue.

cated cells within the follicles and leading to an obliteration of the
vascular lumen.

6. The Carotid Glands. — These consist of groups or islets of epithe-
lial cells, surrounded by fibrous tissue from which numerous capil-

THE DUCTLESS GLANDS. 20.1}

lary bloodvessels arc distributed in close relation with the epithelial
cells (Figs. 178 and 179). Their function is unknown.

7. The Coccygeal Gland. — This body is made up of groups and
strands of cells, probably of epithelial nature, closely applied to the
walls of capillary bloodvessels and surrounded by fibrous tissue.
Its function and mode of origin are both unknown (Fig. 180).

CHAPTER XVI.

THE SKIN.

The skin consists of a deeper, fibrous portion, the corium, or true
skin, and a superficial, epithelial layer, the epidermis. As a part
of the latter, and developing from it, the skin contains two sorts
of glands, the sebaceous and the sweat-glands, and two kinds of
appendages, the hairs and nails.

The corium is composed of vascularized fibrous tissue, which is.

Fig. 181.

Section of skin perpendicular to the surface. (Arloing.) a, horny layer of the epidermis ;
6, rete mucosum ; c, surface of the corium ; d, sebaceous gland ; e, areolar tissue of the
corium;/, hair-shaft within the hair-follicle; g, lobule of adipose tissue in the subcu-
taneous tissue ; h, sweat-gland ; mh, arrector pili : p, papilla of the corium extending into
the rete mucosum. The lower limit of the corium is not marked by a plane parallel to
that of the surface of the skin. The corium may be said to end where the fat of the sub-
cutaneous tissue begins.

made up of bundles loosely arranged in its deeper portions, where it
becomes continuous with the subcutaneous areolar tissue, and contains

204

THE SKIN.

2< I.",

a variable amount of fat, but more compactly disposed in the super-
ficial portions, where it comes in contact with the epidermis, into
which it projects in the form of papillae. Some of these papillae
contain loops of capillary bloodvessels, while others are occupied
in their centres by peculiar nerve-endings, called " tactile corpus-
cles." In some situations, notably upon the palms and soles, the
papillae of the coriuni are arranged in rows. In most parts of the
skin they are irregularly scattered over the surface of the corium
(Fig. 181).

The epidermis (Fig. 182) is a layer of stratified epithelium in

Fig. 182.

ftn

Vertical section of the epidermis of the finger. (Ranvier.) a, stratum corneum, or horny
layer; b, stratum lucidum ; c, stratum granulosum; d, rete mucosum ; e, "prickles" on
the cells bordering on the corium, which is not represented.

which the cells multiply, where they are situated near the corium,
and gradually suffer a conversion into horny scales as they are
pushed toward the surface, where they are eventually desquamated.
The changes the cells undergo in their journey from the deeper
layers of the epidermis to its surface cause variations in their
appearances which have occasioned a division of the epidermis into
a number of more or less well-defined strata. The deepest stratum,
where the cells multiply and grow, is called the "rete mucosum."
It is composed of cells which gradually enlarge, becoming rich in
cytoplasm, and are connected with each other by minute cytoplas-
mic " prickles," between which there is a space affording a channel
for the circulation of nutrient fluids (Fig. 39). Above the rete
mucosum the cells appear more granular, owing to the formation

206 NORMAL HISTOLOGY.

of a substance, called "eleidin," within the cytoplasm (Fig. 183).
These cells form the "stratum granulosum." The eleidin appears
to be produced at the expense of the cytoplasm, the process being
a form of degeneration, so that after a while the whole cell is con-
verted into a homogeneous material in which the nucleus persists
in a form deprived of chromatin, and therefore insusceptible of
staining. The presence of these cells gives rise to the formation of
the "stratum lucidum " immediately above the stratum granulosum.
Within this stratum the eleidin appears to pass into a closely related
substance of a horny nature, keratin, and the cells become con-

Fig. 183.

Cell from the stratum granulosum of the epidermis of the scalp. (Rabl.) The cytoplasm of
the cell has been in great measure converted into granules of eleidin ; the chromatin of
the nucleus has retracted into a compact mass in the centre of the nuclear region, and is
destined to disappear. This cell is from a section made parallel to the surface of the
epidermis, which accounts for its shape and apparent size.

verted into firmly compacted scales, which make up the most super-
ficial or horny layer of the epidermis.

The sweat-glands are simple tubular glands, the deep ends of
which are irregularly coiled to form a globular mass situated in
the deeper portion of the corium or at various depths in the sub-
cutaneous tissue. From these coils the excretory duct passes
through the corium to the epidermis, where it opens into a spiral
channel between the epidermal cells, ending in an orifice at the sur-
face of the skin.

The epithelial lining of the sweat-gland is a continuation of the
stratum mucosum, from which it is derived, and consists of two or
more layers of cubical cells in the duct and of a single layer of more
columnar cells in the deeper, secreting portion of the gland. In
the duct these cells rest upon a homogeneous basement-membrane,
but in the secreting portion there is a more or less complete layer
of elongated cells, similar in appearance to those of smooth muscular
tissue, which lie between the epithelial cells and the basement-mem-
brane (Fig. 184). It is doubtful whether these are really muscle-
cells. The loops of the glandular coil are surrounded by fibrous
tissue, which contains the bloodvessels supplied to the gland and
serves to support it in its globular form.

THE SKIN.

207

The sebaceous glands can best be described in connection with
the hairs and their follicles.

The bulbous attachment, or " root," of the hair, and the adjacent
portion of its shaft, are contained in an invagination of the corium
and epidermis, called the "hair-follicle" (Fig. 181,/). This is sur-
rounded by fibrous tissue, forming its external coat, which may be
imperfectly distinguished into an outer layer, containing relatively
abundant longitudinal fibres, and an inner layer, in which encircling

Fig. 184.

Muscle cells in, a diagonally
cut tubule

Gland celL

Muscle cells

Forked
muscle cell

Muscle cells

Nucleus of
gland cell

■p — - Y ® Jj[

Basal

membrane-

Duct

Fat cell
colored
by osmic
acid

Some tubules of sweat-glands from the skin of a human finger. To the left of the two lower
fat-cells are muscle-fibres and a few gland-cells belonging to a tangential section of a
sweat-gland tubule. (X 350.)

fibres predominate. At the bottom of the follicle this fibrous tissue
becomes continuous with that of a vascularized papilla, similar to
those existing on the surface of the corium, which projects into the
root of the hair.

The fibrous sac constituting the outer part of the hair-follicle is
lined with a continuation of the epidermis, leaving a cylindrical
cavity occupied by the hair. This layer of epithelium is reflected

208 NORMAL HISTOLOGY.

upon the surface of the papilla, where it forms the root of the hair,

Fig. 185.

Pi

6

- c

e.

Hair-follicle from the human scalp. (Mertsching.) Longitudinal axial section through the
fundus: a, b, longitudinal and encircling layers of the fibrous coat; c, hyaline layer,
formed of an outer faintly fibrillated and an inner more homogeneous lamina; d,
papilla; e, outer root-sheath, continuous with rete mucosum of epidermis ; c', its outer
layer, continuous with deepest cells of rete and with columnar cells covering the papilla;
e", its inner layer, continuous with the cortical cells of hair ; /, Henle's sheath ; g, Hux-
ley's layer; h, cuticle of root-sheath ; k, cuticle of hair; /, cortical cells of the hair; m,
medulla.

and then passes into the shaft, which is made up of cells, derived
from those of the root, that have suffered keratoid degeneration.
The epithelium lining the follicle, as well as that which composes

THE SKIN. 209

the hair, is not of uniform character throughout, and lias been divided
into a number of layers, to which different observers have given
special names. The group of cells surrounding the papilla are the
seat of the multiplication which results in the growth of the hair.
Upon the surface of the shaft these cells become transformed into
thin scales, each of which overlaps that above it. This very thin
layer is called the "cuticle" of the hair. Beneath the cuticle the
cells are crowded together into fusiform or fibrous elements, which
make up the chief mass of the hair-shaft. In the centre of this
mass there is sometimes a line of more loosely aggregated cells,
forming the " medulla " of the hair. When this is present the sur-
rounding part of the shaft, between it and the cuticle, is known as
the "cortex" (Figs. 185 and 186).

The sebaceous glands (Fig. 181, d) are sacculations in the corium
near the hair- follicles, which are tilled with epithelial cells. The
cells at the periphery divide, and, as they increase in size, push

Fig. 186.

Hair-follicle from the human scalp. (Mertsching.) Cross-section from middle third of
the follicle: 6, longitudinal and encircling layers of the fibrous coat; c, hyaline layer,
formed of an outer faintly fibrillated and an inner more homogeneous lamina, & ; e, outer
root-sheath, continuous with rcte mucosum of epidermis ;/, Henle's sheath; <?, Huxley's
layer ; h, cuticle of root-sheath ; fc, cuticle of hair ; I, cortical cells of the hair ; m,
medulla.

each other toward the centres of the sacs. Here they undergo a
fatty degeneration, ending in destruction of the cells and the forma-
tion of an oily secretion, the sebum, which is discharged into the
hair-follicle a short distance below its opening on the surface of the
skin. The sebum is a lubricant for both the hair and the epi-
dermis (Fig. 187).

The color of the epidermis and of the hair is due to a pigmenta-

14

210

NORMAL HISTOLOGY.

Sebaceous gland from the external auditory canal. (Benda and Guenther's Atlas.) a, epi-
thelium continuous with that lining the hair-follicle ; b, layer of proliferating epithelium
lining the sac of the gland ; c, enlarged cell beginning to undergo fatty metamorphosis
of the cytoplasm ; d, mass of sebum derived from a single epithelial cell.

tion of the cells in the deeper layers of the rete mucosum and those

Fig. 188.

b : • ••-^.;^.. ;_.--•/-- •/•••s.„;C

Section through the root of the nail of a sixth-months fcetus. (Ernst.) a, matrix of the nail
formed by an invagination of the rete mucosum. Near the point indicated by the letter
the epithelial cells have begun to change into keratoid material, b, loosened scales of the
surface of the nail; c, remains of the foetal cuticle which have not become keratoid.
The letter a and line proceeding from it both lie in the corium.

composing the hair. The whiteness of the hair which comes with
years is due to little spaces which appear in unusual numbers

THE SKIN. 211

between the cells of the cortex, and are filled with air, reflecting the
light and masking the pigmentation of the cells.

The nails are especially thick and condensed masses of epithelial
cells which have undergone keratoid degeneration and are closely
compacted. They are produced at the root of the nail, and as they
accumulate push the body of the nail forward. They, therefore,
correspond to the horny layer of the epidermis, which has become
modified to form these special structures (Fig. 188).

The skin contains little muscular bands, the arrectores pili (Fig.
181, mh), composed of smooth muscular fibres, which are attached
to the fibrous coat of the hair- follicles near their deep extremities
and to the superficial layer of the corium on the side of the fol-
licle toward which the hair leans. The action of these mus-
cles is to cause the hair to assume a more vertical position, and
to raise it and the follicle, producing the effect known as " goose
flesh." By their contraction they may also aid in the discharge
of sebum, since their fibres often partially invest the sebaceous
glands.

The functions of the skin have reference to its being the organ
coming in contact with the external world. The epidermis protects
the underlying tissues from mechanical and chemical injury and
from desiccation. The keratin in its horny layer forms an imper-
vious and tough investment of the body, which is highly resistant
toward chemical action and mechanical abrasion, and is constantly
renewed from the layers that lie beneath it. It is kept in a pliable
condition by the sebum discharged upon its surface and by the
moisture proceeding from the sweat-glands, the " insensible perspi-
ration." The skin also plays a prominent role in the regulation of
the bodily temperature. When its vessels are contracted the amount
of heat given off from the surface of the body is reduced ; when
they are dilated, it is increased. A further loss of heat is occa-
sioned by an increased secretion of sweat, which bathes the surface
of the skin and abstracts from the body the heat required to con-
vert it into vapor. Under the influence* of sudden and marked
cold the vessels of the skin become much contracted and the
arrectores pili shorten, occasioning the production of a roughness
of the skin, goose-flesh, and probably also a discharge of se-
bum, which reduce the evaporation from the skin. At the same
time a reflex rhythmical contraction and relaxation of the volun-
tary muscles is brought about — shivering, which increases the

212

NORMA L HISTOLO G Y.

liberation of stored energy within the body, and causes it to appear
as heat. In conjunction with these functions the skin is also an
organ of tactile and thermal sensation, functions which are not

Fig. 189.

Hair-rudiment from an embryo of six weeks. (Kolliker.) a, horny layer of epidermis ; b,
Malpighian layer, rete mucosum ; i, limiting membrane; m, m, cells extending from the
rete mucosum to rill the future hair-follicles. The elongated cells near the base of the
sac are those from which hair is developed. The secreting glands of the body arise from
some epithelial layer in a similar manner.

merely beneficial in themselves, but are useful auxiliaries in the
furthering of the other functions exercised by the skin. It is a
common experience that the sensation of cold stimulates the desire

Fig. 190.

Section of developing tooth. From embryo of sheep. (Bohm and Davidoff.) a, epi-
thelium of the gum; b, its deepest layer; c, superficial cells of the enamel-pulp; d,
enamel-pulp formed of modified epithelial cells ; s, cells of the enamel-pulp destined to
produce the enamel (" adamantoblasts"); p, dental papilla.

for muscular exercise, of which the liberation of heat is a result.
The sensation of pain often gives timely warning of exposure to an
injury sufficiently great to overcome the usual protective powersof

THE SKIN. 213

the epidermis. Thus vvc see that when the automatic action of the
skin is inadequate for the performance of its functions it calls forth
an auxiliary activity of other organs through the medium of the
nervous system.

The hair-follicles are developed from the rete mucosum of the epi-
dermis, and first appear as little masses of cells growing into the
underlying connective tissues (Fig. 189). The sebaceous glands
arise as offshoots from these cellular masses.

Fig. 191.

L

-B

--Sr

• '

Section of developing tooth. From embryo of rabbit. (Freuiid.) ep, epithelium of gum:
sit, epithelial cells forming outer layer of the enamel-pulp of the temporary tooth ; L, sim-
ilar layer belonging to the rudiment of the permanent tooth ; Sr, enamel-pulp; p, dental
pulp of the tooth-cavity ; d, dentin ; v, bloodvessels ; B, rudiment of second or permanent
tooth ; a, embryonic connective tissue of the alveolar process.

The Teeth. — The development of the teeth presents close anal-
ogies to that of the hairs. They also first appear as little masses
of cells, growing into the connective tissues of the alveolar proc-
esses from the stratified epithelium covering them. Into the bases
of these masses connective-tissue papilla? are developed, which
eventually become differentiated into the pulp of the tooth-cavities.
The epithelial cells which immediately surround these papilla? be-
come elongated to a columnar form and then become converted
into or elaborate the tissue of the enamel. The superficial cells

214

NORMA L HISTOLOG Y.

Fig. 192.

CROWN

of the papillae likewise elongate and produce the dentin. The
cement which constitutes the outer layer of the root of the tooth
is bone, and is developed from the foetal connective tissue in that
region (Figs. 190 and 191).

Only a brief description of the structures entering into the forma-
tion of the fully developed tooth can be given here. For a more
detailed account of them the student is referred to special works on
the subject.

The centre of the tooth is hollow, and the cavity opens oy a
small orifice at the tip of the root. This cavity is filled with a

highly vascular delicate areolar tissue,
richly supplied with nerves. Where
this pulp is in contact with the tooth
its outer layer is made up of modified
connective-tissue cells, odontoblasts,
which are capable of elaborating den-
tin. The body of the tooth is com-
posed of dentin. This contains minute
canals, analogous to the canaliculi in
bone, but much longer. They extend
from the pulp-cavity nearly, if not quite,
to the outer boundary of the dentin,
and, toward their terminations, give off
branches. These canals are occupied
by long fibrous processes of the odonto-
blasts already mentioned.

The crown of the tooth, down to its
neck, is covered with enamel. This
is a tissue derived from epithelium,
and is composed of long, prismatic ele-
ments extending from the surface of
the tooth to the dentin. These prisms
have a polygonal cross-section and are
held together by a hard cement-sub-
stance. They are not perfectly recti-
linear, but pursue a wavy course, being
disposed in lamina? or bundles, in which
the prisms have not quite the same direction.

The root of the tooth, below the point where the enamel ends, is
covered with cement, which has the structure of ordinary bone, but
is usually devoid of Haversian canals (Fig. 192).

NECK-

ROOT—)

Axial section of a human tooth
having but one root: o, enamel;
b, dentin; c, cement.

CHAPTER XVII.

THE REPRODUCTIVE ORGANS.

I. IN THE FEMALE.

The female reproductive organs are : (1) the ovary, in which the
egg is produced ; (2) the Fallopian tube, through which it is con-
veyed to (3) the uterus, where it develops into the foetus, and from
which the child at maturity passes through (4) the vagina and (5)
external genitals into the external world.

1. The Ovary (Fig. 193). — The free surface of the ovary is cov-
ered with a single layer of columnar epithelium, called the "germinal
epithelium." Beneath this the substance of the organ is composed
of a vascularized fibrous tissue, the "stroma," which is slightly dif-
ferent in the details of its structure in different parts of the organ.
Immediately beneath the germinal epithelium it is slightly richer
in intercellular substance than in the subjacent parts, so that the
organ appears to have a proper fibrous coat. This coat is not dis-
tinct, however, and gradually passes into a highly cellular form of
fibrous tissue, in which the spindle-shaped cells are separated by
only a small amount of a delicate fibrous intercellular substance.
Toward the hilum of the ovary this connective tissue passes into a
more distinctly fibrous tissue, containing a larger amount of inter-
cellular substance and cells that are less prominent. In this portion
of the stroma the larger vessels supplying the organ are situated,
and from it they send smaller branches throughout the stroma of
the organ. Within the more cellular regions of the stroma are the
structures known as the Graafian follicles, each of which contains
an ovum. In order to understand the structure of these Graafian
follicles it will be well to trace the history of their development.

The Graafian follicles and ova are derived during foetal life from
the germinal epithelium covering the ovary. From this layer of
cells little columns of epithelium make their way into the stroma,
where they become broken up into small isolated groups, in each of
which one of the cells develops into an ovum, while the rest con-
tribute to the formation of the Graafian follicle. This mode of origin

215

21 G

XOHMAL HISTOLOGY.

may serve to explain the fact that the younger Graafian follicles
are most abundant in the peripheral portion of the stroma. At
first the Graafian follicle consists of a large central cell, the ovum,

Fig. 193.

Section from the ovary of an adult bitch. (Waldeyer.) a, germinal epithelium : b, b, columns
of germinal epithelium within the stroma; c, c. small follicles; d, much more advanced
follicle ; e, discus proligerus and ovum ; /, second ovum in same follicle (a rare occur-
rence); g, fibrous coat of the follicle: h, basement-membrane; i, membrana granulosa
of epithelium; d, liquor folliculi: k, old follicle from which the ovum has been dis-
charged (corpus album) ; I, bloodvessels ; m, m, sections of the parovarium ; y, ingrowth
from the germinal epithelium ; z, transition from the germinal epithelium to the peri-
toneal endothelium.

surrounded by an envelope of somewhat flattened epithelial cells,
which arc in direct contact externally with the unmodified, highly
cellular tissue of the stroma (Fig. 194).

As the Graafian follicle develops, its position in the ovary becomes
more central, and the cells around the ovum lose their flattened
shape and divide, forming a double layer of cubical or columnar
cells. These two layers then become separated by a clear fluid.

THE REPRODUCTIVE ORGANS.

217

the liquor folliculi, so that the outer layer forms the wall of a sac,,
while the inner layer remains as a close investment of the ovum.
The cells of these two layers multiply : those surrounding the
ovum forming the "discus proligerus," and those lining the sac the
" tunica granulosa " ; but they blend with each other at one point on
the wall of the follicle, so that the ovum retains a fixed position
(Fig. 195).

Meanwhile the tissue of the stroma undergoes modifications which

Fig. 194.

•'■■' --.■■
■■•■''. .

w&mk

. _ • ■

f.SS*!

-1=

■

i
-----

Graafian follicle and stroma in ovary of adult sow. (Plato.) The ovum occupies the centre
of the follicle, appearing as a very large cell with a large vesicular nucleus ("germinal
vesicle"), within which is a large nucleolus ("germinal spot"), exceeding in size the
whole nucleus of the surrounding epithelial cells of the follicle. The cells of the stroma
are arranged about the follicle as though to form the fibrous coat of the latter. In the
lower portion of the figure are three large cytoplasmic cells, containing globules of fat
and granules of pigment. These cells are analogous to those found in the interstitial
tissue of the testis. The epithelium of the Graafian follicle, and the ovum, also contain
globules of fat of various sizes, stained black by the osmic acid used in the preparation
of the specimen.

contribute a clear basement-membrane and a fibrous envelope, the
" membrana propria," to the structure of the follicle.

The follicle now enlarges, as the result of an increase in the
amount of the liquor folliculi, eventually approaches the surface of
the ovary at some point, and then ruptures, discharging the ovum.

After the rupture of the Graafian follicle and the escape of
its contents a slight hemorrhage usually takes place into its
cavity, which then appears filled with remains of the liquor fol-
liculi mixed with coagulated blood. The cells of the tunica granu-

218

NORM. 1 L HISTOLOG Y.
Fro. 195.

Theca

funiculi
(fibrillin coai

~'- .* vj-

i ir ii iii with
zona pellu-
cida, germi-
nal vesicle,
and germi-
nal spot

Section through a Graafian follicle from an ape's ovary, (x 90.)

Fig. 196.

Blood-vessel

Lumen of tube

Spaces between
folds of mucosa

'■'olds of the
mucosa

■■■ s'Y

Transverse section through the ampulla of the Fallopian tube of a young woman. (X 25.)

THE REPRODUCTIVE ORGANS.

219

losa, epithelial in nature, now enlarge and probably also proliferate
so that this layer of cells becomes greatly thickened. The cells of
the connective tissue upon which the tunica granulosa rests also pro-
liferate and columns of them penetrate among the epithelial cells,
which they very closely resemble, and later these ingrowths of con-
nective-tissue cells become vascularized with loops of capillary blood-

Fig. 197.

Lutein cells

Connective-
tissue septa

Blood-vessel

Part of a corpus luteum of a bitch. (X 300.)

vessels sprouting from the older vessels of the stroma. While this
growth of connective tissue is taking place, the epithelial cells
undergo retrograde changes which are characterized by the appear-
ance of fat droplets in the cytoplasm, giving the tissue a yellow
color when they are abundant. For this reason the cells are called
lutein cells. This process gradually leads to the ultimate destruction
of the epithelium, which is replaced by connective tissue. Fibres

"220 NOBMAL HISTOLOGY.

develop in the latter, and eventually the follicle is reduced to a
small mass of dense fibrous tissue (corpus album, Fig. 193, k). If
pregnancy occurs during the progress of these changes, they are
much slower than if pregnancy is absent. In the latter case the
epithelium does not become so prominent and the follicle is not so
yellow, but passes from the hemorrhagic to the fibrous stage more
rapidly (corpus hsemorrhagicum). When pregnancy exists, the
lutein cells are more prominent in the composition of the mass and
the follicle is more yellow in color (corpus luteum, Figs. 196 and
197). The corpus album formed from the corpus luteum is also
larger than that derived from the corpus hrcmorrhagicum.

Fjg. 198.

thi C

Section from rabbit's ovary, illustrating the formation of the corpus luteum. (Sobotta.)
Recently ruptured Graafian follicle, ke, germinal epithelium; beneath it, the ovarian
stroma. Bounding the follicle externally is the fibrous capsule of the follicle. Within
this, thi, is a layer of proliferating fibrous tissue, composed of polyhedral cells with round
nuclei. Among these are elongated nuclei belonging to endothelial cells springing from
the capillaries, and destined to form the walls of future bloodvessels; e, epithelium of
the membrana granulosa. Within this are the viscid remains of the liquor folliculi,

containing a few red l>l L-corpuscles and some epithelial cells detached from the mem-

brana granulosa, bl, red hi l-corpuscles. This section was prepared from an ovary

about twenty four hours after coitus, and the development of the layer thi probably
took place within that time.

2. The Fallopian Tube. — The free surface of the Fallopian tube
is covered by a serous membrane, continuous with the rest of the
peritoneum. This rests upon fibrous tissue, in which the longi-
tudinal bundles of smooth muscular tissue constituting the external

THE REPRODUCTIVE ORGANS. 221

Fig. 199.

Section of corpus luteum four days after coitus. Rabbit. The proliferating connective
tissue has nearly filled the cavity of the follicle, only a small mass of fibrin remaining
in its centre. The young connective tissue is highly vascularized, the blood in some of
the capillaries being represented, g. ke, germinal epithelium. Below is the margin of a
Graafian follicle, with its membrana granulosa.

muscular coat are situated. This is followed by an internal mus-
cular coat of encircling bundles of smooth muscular tissue, inside
of which is the submucous coat of areolar tissue, containing a few
scattered ganglion-cells.

The mucous membrane consists of a highly cellular connective
tissue covered with ciliated columnar epithelium. During life these
cilia propel toward the uterine cavity substances coming into con-
tact with them. Toward and at the fimbriated extremity of the
tube the mucous membrane is thrown into deep longitudinal folds
(Fig. 200), upon which are numerous secondary and tertiary folds,
but further toward the uterus these folds give place to branching
villous projections into the lumen (Fig. 201). Toward the uterine
end of the tube these complicated folds and villi disappear and the
lumen of the tube becomes round or stellate.

3. The Uterus. — The external surface of the uterus, throughout
most of its extent, is covered by a reflection of the peritoneum.
Beneath this are three distinct coats of smooth muscular tissue, the

222 NORMAL HISTOLOGY.

outer two in close contact with each other; the two inner separated
by a thin layer of areolar fibrous tissue, supporting large blood-
vessels. This separation of the innermost layer from the middle
layer leads to the inference that the former is analogous to the mus-
cularis mucosae found in other hollow viscera, although in the uterus
it forms the chief mass of the muscular tissue of the organ. The
outer layer is made up of bundles of fibres that have a general
longitudinal position ; the two inner layers have a general circular

Fig. 200.

-Cilia

**!$* r" © * fcs^ Xm-lei of
. S l&i** S, /ciliated cells

Connective-tissue

v^,- '^ <j^g, V^>crf/s of stratum

proprium

From a section through a fold of the mucous membrane of a human Fallopian tube. (X 480.)

disposition of their bundles, though the latter interlace with each
other in various directions within the muscularis mucosa?, leaving
masses of areolar tissue containing the larger bloodvessels between
them.

Covering the surface of the muscularis mucosae is a highly cellu-
lar connective tissue. It is composed of round and fusiform cells,
lying in a small amount of intercellular substance, in which fibres
can be distinguished only with difficultv. The surface of the mucous
membrane is covered with a layer of ciliated columnar epithelium,
which is continued into long tubular glands penetrating the super-
ficial ]>< unions of the muscularis mucosa?, where they frequently
branch before terminating in blind extremities. It should be borne

THE REPRODUCTIVE ORGANS.

223

in mind that at the extremities of these glands the whole tubule is
often filled with epithelial cells, so that no lumen is visible. In
their course into the mucous membrane these glands are usually
straight at first, but in their deeper portions become tortuous (Figs.
202 and 203).

Fig. 201.

-cr«v

Transverse section of the Fallopian tube near its free end. (Orthmann.) Numerous branch-
ing villous projections of the wall, covered by ciliated colum r epithelium, extend into
the lumen. The open spaces in these villous projections are sections of the bloodvessels.

During the childbearing period of life the portion of the mucous
membrane resting upon the muscularis mucosa? is the seat of active
changes which pass through a cycle corresponding to each men-
strual period, but interrupted by a special series of changes during
pregnancy. These changes are of importance in their bearing upon
the pathology of the organ, and must be briefly described.

At the menstrual period the superficial portion of the mucous
membrane, down to its muscular coat, suffers a degeneration, which
results in its disintegration and discharge, along with some blood
derived from the exposed and damaged vessels of small size within
its tissues. After this degeneration the membrane is restored by a

224

NORMAL HISTOLOGY.

Fig. 202.

Jf J3ec/leT.t&
Normal endometrium in a patient twenty-six years of age. (X 25.)
The mucosa is slightly thickened, its surface is wavy, and its epithelial covering a is intact.
In this section it is possible to trace the glands in their continuity almost from the sur-
face to the muscle. A few of them are practically cylindrical throughout, but the major-
ity have a wavy contour presenting a well-defined corkscrew arrangement. Quite a num-
ber, cut just along their margin, can be recognized as little masses of epithelial cells ;
c, is cut longitudinally ; '/, almost transversely. At first sight one would think that there
was a great excess of glands in the section, whereas in reality, at most, there are not
more than twelve, the distances between any neighboring two being about the same.
The gland epithelium is intact throughout. The stroma in the superficial portions is
rather lax, in the deeper portions more compact, b indicates the line of junction between
the muscle and mucosa. Its irregularity is especially noticeable. (T. S. Cullen, Cancer
of the Uterus, Now York, l'.n.Mi.)

THE REPRODUCTIVE ORGANS.

225

proliferation of the elements contained between the bundles of the
muscularis mucosa?, the glands being reformed from the remnants
of their deep extremities. The mucous membrane slowly continues

Section of the human uterine mucous membrane parallel to its surface. (Henle.) 1, 2, 3,
uterine glands in cross-section. In 2, the basement-membrane alone is represented, the
epithelium having fallen out of the section. 4, bloodvessel in longitudinal section. Be-
tween these structures is the highly cellular stroma of the mucous membrane, only the
nuclei of its cells being represented.

to increase in thickness and the glands in tortuousness until the
next menstruation, when the same process is repeated. It will be
noticed that the connective tissue of the mucous membrane, in the
absence of pregnancy, is subject to periodical degeneration and re-
generation, which probably prevent its development into a mature
fibrous tissue with an abundance of fibrillated intercellular substance.
If an ovum, discharged from the ovary, becomes fertilized, the
menstrual cycle of changes in the superficial portion of the mucous
membrane of the uterus is interrupted. That portion of the mu-
cous membrane then undergoes extensive modifications in structure
during the early months of the ensuing pregnancy. The tissue
between the uterine glands becomes more hyperplastic than during
the intervals separating the menstrual periods, and at the same time
some of the cells composing it become hypertrophied until they
closely resemble large epithelial cells. These cells have been called
" decidual cells," and occasionally contain more than one nucleus.
When there are several nuclei in a single cell, it is called a giant
cell. The bloodvessels and glands in the mucous membrane also

15

22b' NORMAL HISTOLOGY.

become enlarged, so that the whole mucosa is greatly thickened.
Later in the course of pregnancy compression by the growing foetus
and increased amount of amniotic fluid cause a condensation of
these tissues, which become more fibrous with flattened spaces repre-
senting the uterine glands, the lining epithelium of which becomes
diminished in size and flattened. These changes occur in those por-
tions of the mucous membrane, the decidua vera, which do not take
part in the formation of the placenta. The ovum, when it reaehes
the cavity of the uterus, becomes embedded in this tissue, which
grows around and encloses it, after which it is differentiated into
three portions. The part beneath the ovum is called the decidua
serotina ; that which invests the ovum, the decidua reflexa ; and
that lining the rest of the uterine cavity, the decidua vera. As the
ovum enlarges, the decidua reflexa comes in contact with the decidua
vera, and the two layers exert a mutual pressure upon each other,
which flattens the spaces they contain and may obliterate many of
them. These atrophic changes are most marked in the decidua
reflexa, which, according to some authors, entirely disappears or is
reduced to a thin layer blending with and indistinguishable from the
decidua vera. The disappearance of the superficial epithelium of the
mucosa removes what might otherwise be a certain means of distin-
guishing these layers. The decidual tissue now consists of a number
of flattened spaces which are separated from each other by thin walls
of fibrous tissue produced by the further development of the decidual
tissue. The decidua reflexa and the decidua vera blend with each
other to form a part of the membranes that are expelled from the
uterus, along with the placenta, after the birth of the child. The
placenta is formed in the decidua serotina, and the changes in this
portion of the uterine mucosa during pregnancy differ from those in
the deciduse vera and reflexa. Most of the tissues which constitute
the afterbirth at parturition are of fetal origin. The outer surface
of the ovum, when it reaches the uterus, is composed of a single
layer of ectodermal cells ; beneath this is a layer of embryonic
connective tissue, derived from the mesoderm and resembling mucous
tissue, with few if any fibres. These two layers of tissue form
the outer envelope of the ovum, the chorion. Through a prolifera-
tion of the ectodermal cells, little projections, or villi, are formed on
the surface of the chorion, and subsequently the mesodermal con-
nective tissue grows into these villi, so that it forms the central part
of each villus, covered with a single layer of ectodermal epithelium.

THE REPRODUCTIVE ORGANS.

227

Still later, vascular loops from the allantois penetrate into the villi,
and the latter subdivide dichotomously to form the cotyledons of
the placenta. During this growth of the villi the ectodermal cells
covering them divide in such a way that the covering consists of a
double layer. The cells of the more superficial layer are not distin-
guishable from each other, but blend so as to form a layer of cyto-
plasm containing many nuclei, a structure called a syncytium (Fig.
204). The ectodermal cells beneath this syncytium are at first

Fjg. 204.

Connective tissue— -,'^7,\ )~

Blood-vessel-

Ectoderm
of villus

Proliferation island

Blood-vessel with
nucleated
blood cells

■ n

Syncytium _JJfl§ -
of villus''

/■&>■

$> „*> Proliferation island

"ft* ^f c"' tangentially

Transverse section of a human chorionic villus at the fifth month of pregnancy. (X 300.)

distinct, but during the later months of pregnancy they disappear,
leaving only the syncytium, in which there are occasional thickenings,
called proliferation islands (Fig. 204) ; still later, the syncytium also
becomes altered, its nuclei disappear, and the whole layer is con-
verted into a hyaline substance, " canalized fibrin " (Fig. 205).

The placental villi come to occupy large spaces in the placenta
through which the maternal blood circulates, interchange of nutrient
and waste materials and of gases taking place between the maternal
and fcetal blood through the tissues of the villus. There is still
some doubt as to the way in which these cavernous blood-spaces are
formed. According to one view, they are dilatations between the
uterine mucosa and the foetal parts of the placenta, into which the
bloodvessels open. Another view is that the villi penetrate into the
dilated vessels of the decidua serotina. Those who hold this view

22S

yORMA L HISTOL OGY.

also believe that the villous syncytium is not derived from the
chorionic ectoderm, but from the uterine epithelium. In sections
of these tissues, while the placenta is developing, it is extremely
difficult, often impossible, to decide whether given cells are of foetal
or maternal origin (Fig. 20G). After the birth of the child and the
expulsion of the membranes the uterine mucous membrane is regen-
erated from the tissues remaining in the superficial layers of the

muscularis mucosae.

Fig. 205.

fs »s — -

. -' . .*

1

.' '. % '^PM^^ Decidual

■J y cells

51 :

o ,- '.;?" • ■' -' * Z' n. - £ iSP"^
Syncytium---** *«a * * ',»!»■>• »■ .. ■=»- _"-;...-

Canalized

fibrin

r ■.«■ „

c » »

*g$V fc ,1 NyM"

tTmV. • ■* * jT-v ■■■-■ •' *••»<. o
a* - ■ .* - it;-1 _

Connective

tissue

J?.. I Oblique section

,?{*• £*- of syncytium

*** of villus

I

From a

section through a human placenta at the fifth month of pregnancy. (X 80.)

The mucous membrane of the cervical portion of the uterus does
not participate in these changes incident to menstruation and preg-
nancy, and the connective tissue underlying its epithelial lining is
more fibrous in character than that in the corresponding part of the
uterine body. About the middle of the cervical canal the ciliated
epithelium, which is continuous with that of the body, passes into
a stratified epithelium, which extends over the cervix uteri, the
portio vaginalis, and the inner surface of the vagina to join that of
the epidermis upon the labia minora. The fibrous tissue beneath
this stratified epithelium possesses papilla? similar to those upon the
skin, and contains mucigenous glands, which secrete a tenacious

THE REPRODUCTIVE ORGANS.

229

mucus serving to close the cervical canal during pregnancy. The
orifices of these glands sometimes become occluded, causing a cystic
dilatation of the acini, due to accumulated secretion, " ovula Nabothi."

Fig. 206.

Placenta
uterina

■•;•'•-• */< - .".' •'^V.'V.-.. ••'••..:-. ■:'■''; ,-./.•• •:/'' 0-"'5" ,v-.'.'\'«i Glandof
: .'^■•v'.'v.'*"- -'••'•; i .. '.*'." ..-.v's- ■•- ' -V.'-*H

r

(J Epithelium
villus cut
tangentially

i

\ - ■

V2 i^**-— -;v'- -.-;

__N I Proliferation

1 ,~i island

Placenta j
foetalis*

x J ,,;y

"'■-.

-'

t : - Vi ■

Intervillous
spaces filled
with mater-
nal blood

^-Villus

^.: \

V \

x<

A

Membrana
chorii

Transverse section through a human placenta at the second month of pregnancy.
(Alter a preparation by Prof. Mars.) (X 50.)

The muscular and other tissues of the uterine wall undergo
hypertrophy during pregnancy, the individual muscular fibres be-
coming as much as thirty times their original bulk in the non-preg-

230 NORMAL HISTOLOGY.

nant uterus. The bloodvessels also enlarge and acquire thicker
walls. These retain much of this increase of size, even after the
involution of the uterus following parturition, but the muscular
fibres suffer a partial fatty degeneration, which restores them to
nearly their original condition.

4. The Vagina (Fig. 207). — The subepithelial fibrous coat of the
vagina is covered with small papillae, which project into the epithe-

Fig. 207.

Portion of a longitudinal section of the vaginal wall. (Benda and Guenther's Atlas.) a,
stratified epithelium ; 6, subepithelial areolar tissue ; c, muscularis mucosae ; d, areolar
submucosa containing vascular trunks ; e, muscular coat. Outside of the latter is the
ill-defined fibrous coat, not represented in the figure.

Hum. Outside of this coat is one of smooth muscular tissue, which
is not clearly divisible into layers, but in which the inner fibres are
chiefly circular, forming an imperfectly defined muscularis mucosa?,
while the outer have a longitudinal direction, and may be regarded
as the true muscular coat of the vagina. Outside of the muscular
coat is a layer of areolar tissue connecting the vagina with the
neighboring parts, except at its posterior and upper part, where it
is covered with a serous membrane, forming part of the peritoneum.
5. The External Genitals. — The hymen is a fold of the mucous
membrane, and consists of fibrous tissue with a covering of strati-
fied epithelium. The same general structure obtains also in the labia

THE REPRODUCTIVE ORGANS. 231

minora, prepuce, and labia majora ; hut the labia minora and prepuce
are destitute of fat, while the labia majora contain considerable adipose
tissue. All three organs are supplied with sebaceous glands, which
are numerous beneath the prepuce and are associated with hairs only
on the labia majora. The latter also contain fibres of smooth mus-
cular tissue, corresponding to the analogous dartos of the scrotum.
The bulbi vestibuli, crura of the clitoris, and the body and glans of
that organ are composed of erectile tissue. The glands of Bartholin
are compound racemose glands, in which the alveoli are lined with
a columnar epithelium resembling in structure that of the mucous
glands in other parts of the body. The epithelium lining their
ducts is of the cubical variety.

The parovarium is a remnant of the Wolffian body of the foetus,
consisting of a series of blind tubules lined with epithelium (Fig.
193). It is situated between the Fallopian tube and the ovary. The
remains of the Wolffian duct and of the duct of Midler, having a sim-
ilar structure to the tubules of the parovarium, are sometimes per-
sistent, the one connected with the parovarium, the other with the
extremity of the Fallopian tube. These structures are of interest
because tumors occasionally arise from them.

The Maturation of the Ovum. — Before the ovarian ovum is ready
for fertilization it must undergo two divisions, during which the
amount of chromatin left in the mature egg is reduced one-half.
The first division results in the formation of two cells, which differ
enormously in the amount of cytoplasm they possess, but which
have equal shares of the chromatin in the original nucleus. The
smaller of these two cells is known as the " first polar body." After
its separation from the larger cell both cells divide again, without
an intermediate growth of the chromatin. In this second division
of the larger cell the two resulting cells are again very unequal
in size, the smaller being the " second polar body." The first polar
body having also divided, there result from these successive divis-
ions one mature egg and three polar bodies, each with only half
as many chromosomes in its nucleus as are commonly found in the
general or " somatic " cells of the body (Fig. 208). The polar bodies
perish, as does also the ovum, unless fertilized by the introduction
of a spermatozoon. The latter, as we shall see, also contains half
the number of chromosomes contained in the somatic cells ; so that
after its entrance into the mature ovum the latter acquires its full
complement of chromosomes and is ready for development.

232

NORMAL HISTOL OG Y.

The Mammary Gland. — Each mamma consists of a group of about
twenty similar compound racemose glands, opening by distinct orifices
at the tip of the nipple, and separated and enclosed by fibrous tissue,
in which there is a variable amount of fat. At the edges of the
mamma this fibrous stroma becomes continuous with the tissues of
the superficial fascia in which the breast is situated.

Each of the glands entering into the composition of the breast
possesses a single main duct, the " galactiferous duct," which is lined
with columnar epithelium, except near its orifice, where the strati-

Fig. 208.

\

Maturing ovum of physa (fresh-water snail). (Kostanecki and Wierzejski.) Above are the
two small cells resulting from the division of the first polar body. Below is the ovum,
the nucleus of which is dividing to form the second polar body. Near the centre of the
ovum is the nucleus of the spermatozoon, just above which is its (divided) centrosome
with surrounding radiations in the cytoplasm. When the second polar body has been
formed the chromosomes remaining in the ovum will be ready to participate with those
of the spermatozoon in the further development of the then fertilized egg.

fied epithelium of the epidermis extends for a short distance into
its lumen. A little below the base of the nipple the duct presents
a fusiform dilatation, called the " ampulla," which serves as a reser-
voir for the comparatively small amount of milk secreted in the
intervals between nursings.

The main duct branches in its course from the nipple into the

THE REPRODUCTIVE ORGANS. 233

deeper portions of the gland, and these branches give off twigs,
which terminate in the tubular alveoli of the gland. The columnar
epithelium lining the main duct gradually passes into a cubical variety
in the branches, and this becomes continuous with the epithelial
lining of the alveoli. The terminal branches of the ducts are short,
so that the alveoli opening into them lie close together and are col-
lectively known as a " lobule" of the gland. These lobules are, in
turn, grouped into lobes, each of which corresponds to one of the
main ducts of the breast.

The individual alveoli and the lobules are surrounded by fibrous
tissue, which may be subdivided into an intralobular and an inter-
lobular portion, the latter more abundant than the former. This
fibrous tissue supports the vessels and nerves supplied to the gland.

The character of the epithelium lining the alveoli varies with the
functional activity of the gland.

Before puberty the secreting acini are only slightly, if at all,
developed, the mamma consisting of a little fibrous tissue and the
ducts of the gland, which possess slightly enlarged extremities.

When the gland has become fully developed, at or about puberty,
the epithelial cells lining the acini are small and granular and nearly
fill the diminutive lumina. The fibrous stroma is, at this period,
abundant and makes up the chief bulk of the breast.

When the gland assumes functional activity the cells enlarge
and multiply (Fig. 209), and the lumina of the acini become dis-
tinct and filled with a serous fluid. Into this fluid a few fat-globules
are discharged from the epithelial lining, forming an imperfect milk,
very poor in cream and differing in the proportions of the dissolved
constituents from the milk that is produced after the function of
the gland is fully established. This secretion is called " colostrum."
Besides the scant supply of fat-globules which it contains, it is fur-
ther characterized by the presence of so-called colostrum-corpuscles.
These are leucocytes which have wandered into the acini of the
gland from the bloodvessels in the interstitial tissue, and have taken
some of the fat-globules of the secretion into their cytoplasm. This
process results in an enlargement of the leucocyte, and, in extreme
cases, to an obscuring of the nucleus and cytoplasm by fat-globules,
so that the whole appears as though composed of an agglutination
of numerous drops of fat (Fig. 210).

As the functional activity of the gland matures the epithelial

234

NORMA L HISTOLOG Y.

cells lining its acini produce drops of fat in the cytoplasm bor-
dering on the lumen, and these are subsequently discharged into
the lumen, forming the fat or cream of the milk. The casein of
the milk appears to be produced in the following manner : it has
been observed that during lactation the nuclei of some of the cells
present changes in form that lead to the inference that they undergo
division by the direct mode — i. e., without passing through the
phases of karyokinesis. It thus happens that some of the epi-
thelial cells contain two nuclei. These cells, after a while, project
into the lumen of the acinus, the two nuclei lying in a line perpen-

Fig. 210.

©

O

©

«&;

o

Fig. 209. — Dividing epithelial cells from the mammary gland of the guinea-pig. (Michaelis.)
The figure represents the proliferation of the cells by the indirect mode before lactation
has been established — i. e., during the maturation of the gland.

Fig. 210. — Colostrum-corpuscles and leucocytes from the colostrum of a guinea-pig.
(Michaelis.)

dicular to its wall (Fig. 211). It is supposed that the nuclei nearest
the lumen become detached, together with some of the cytoplasm,
and that the chemical constituents of the nucleus and cytoplasm enter
into the formation of the casein. Such free nuclei have been observed
in the lumina of the acini, and it is known that the chromatin which
they contain disintegrates and eventually disappears (chromolysis),
so that it is not found in the secreted milk. Whether this is,
in detail, the exact process by which casein is formed is still an
open question, but that there is an increase in the nuclear materials
of the secreting cells appears certain, and that these nuclei furnish
the nucleic acid entering into combination with proteid substances to
form the nucleo-albumin, casein, is most probable. The site of the
origin of milk-sugar is not known; it appears to be elaborated
within the secreting cells.

THE REPRODUCTIVE ORGANS.

2:55

When lactation is suspended the breast at first secretes a fluid in
-every way resembling colostrum, and eventually returns to the dor-
mant state, in which the cells are again small and granular and the
stroma is relatively abundant.

As the glandular portion of the breast enlarges during lactation,
the whole breast becomes increased in size, but this increase is not
proportional to the development of the alveoli, for the stroma is
reduced in amount, so that the lobules of the gland are closer to
each other. After the period of lactation is passed the alveoli
return almost to their original size, but the stroma is not repro-

Fig. 211.

"Section from the mamillary gland of a guinea-pig during lactation. (Michaelis.) The figure
represents sections of two acini and 'the margin of a third, separated by richly vas-
cularized areolar tissue, a, fat-globule, separated from the lumen by a mere film of
cytoplasm ; 6, projecting cell with two nuclei ; c, two nuclei which appear to have been
produced by constriction of a single pre-existent nucleus.

duced in fibrous form, but its place is taken by adipose tissue, the
amount of which depends upon the individual, being great in
those that are fat, and slight in those that are lean. In the
latter, therefore, the breast becomes soft and pendulous after
lactation has ceased.

It is important to bear the above changes in the normal gland in
mind when examining the mamma for evidences of a tumor. When,
for example, the stroma is abundant and the glandular structures
undeveloped, as is the case before puberty, sections of the gland
may be mistaken for those of a mammary fibroma.

236 NORMAL HISTOLOGY.

The nipple is composed of fibrous tissue, with a considerable
admixture of elastic fibres, in which there are scattered bundles of
smooth muscular tissue lying parallel to the axis of the nipple. A
circular bundle of the same tissue is found at the base of the nipple,
and by its compression on the bloodvessels may be the cause of the
erection of the nipple. The skin at the base of the nipple and in
the areola surrounding it contains large sebaceous glands.

The mammary gland in the male is functionless, and, while it
contains the same structures as in the female, it remains in a com-
paratively undeveloped condition.

II. IN THE MALE.

The male organs of generation include the penis, prostate, vesic-
ulse seminales, vasa deferentia, epididymis, and testes, together with
certain accessory glands.

1. The Penis. — This is formed by three parallel structures: the
corpora cavernosa, lying side by side and partially blending in the
median line, and the corpus spongiosum, situated beneath their line
of junction and containing the urethra. At its anterior end the
corpus spongiosum expands about the ends of the corpora cavernosa
to form the glans penis. These three bodies, except over the glans,
are firmly held together by fibrous tissue, which is condensed at
their surfaces to form compact sheaths or external coats enveloping
the erectile tissue of which each is composed. The sheaths of the
corpora cavernosa are incomplete where they are in contact, permit-
ting the erectile tissue to blend in the median line. This inter-
communication is freer toward the anterior end of the penis than
near its root, where the corpora cavernosa are more distinctly sepa-
rated, preparatory to their divergence to form the crura.

The sheaths of the corpora cavernosa are composed of fibrous
tissue containing an abundance of elastic fibres. From its inner
surface each sheath gives off a number of fibrous bands, called
" trabecuhe," which divide and anastomose with each other, forming
the chief constituent of the erectile tissue. Within these trabecular
are numerous bundles of smooth muscular tissue.

The erectile tissue is made up of these trabecular, which give it a
spongy character and are covered with endothelial cells, converting the
spaces between them into cavernous venous channels. These become
engorged with blood during erection. The vessels supplying this blood
are situated in the trabecula?, and give off capillary branches, which

THE REPRODUCTIVE ORGANS.

2:37

Fig. 212.

open into the intertrabecular spaces, discharging blood into those

enormously dilated venous radicles. Here
and there arterial twigs, surrounded by an
investment of fibrous tissue, project from the
trabeculse into the venous spaces. These,
because of their twisted forms, have received
the name helicine arteries (Figs. 212 and 218).
The structure of the corpus spongiosum is

Fig. 213.

Fig. 212.— Section of injected corpus cavernosum. (Henle.) a, fibrous capsule ; b, trabeculse ;

c, section of the arteria profunda penis. All the spaces are filled with the material used

for injection.
Fig. 213.— Helicine arteries. A, B, C, from the corpus cavernosum; D, from the corpus

spongiosum; * *, fibrous bands forming a part of the trabecular network.

similiar to that of the corpora cavernosa, but the trabeculse are
more delicate and the spaces between them of more uniform size.
Its sheath is studded with papilla? where it covers the glans, at the
edge of which they are unusually large. They are covered with a
layer of stratified epithelium, which conceals them over the surface
of the glans, where they are comparatively small, but merely invests
the larger ones at the corona. This layer of epithelium is continu-
ous with that of the skin covering the rest of the penis, which is
elsewhere loosely connected with the underlying structures by

2:5S

NORMAL HISTOLOGY.

areolar tissue devoid of fat. The skin is without hairs on the ante-
rior two-thirds of the penis, but contains sebaceous glands, which
are especially numerous in the fold of the prepuce, where it is
attached near the corona of the glans, glands of Tyson.

2. The Prostate. — This body is regarded as the analogue of the
uterus, its utricle corresponding to the cavity of that organ. It has
a fibrous investment, which merges into the areolar tissue connect-
ing the prostate with the surrounding structures and, in its deeper
portions, contains smooth muscular tissue, which accompanies it in
forming the stroma of the organ. Within this stroma are the
prostatic glands, composed of acini, lined with epithelium of the
columnar variety, and opening into a series of ducts having their
orifices in the floor of the urethra. The gland ular alveoli frequently
contain little concretions of a substance closely resembling amyloid,
corpora amylacea, which often display a marked concentric lamina-
tion (Fig. 214).

Fig. 214.

-iif

9H ^^mf:W¥m^mmm

gg3 •■'", w&^$&m

m

m§&

Section of the prostate. (Heitzmann.) Sections of one acinus and portions of three others
are included in the figure. These arc surrounded by fibrous tissue traversed by bundles
of smooth muscular fibres. E, epithelial lining of the acini : M, M, smooth muscular
tissue; C, concretions of amyloid material, showing concentric lamination.

The two ejaculatory ducts pass through the prostate to open into
the urethra in its course within that organ. A little behind their
orifices is the verumontanum, containing erectile tissue, which is

THE REPRODUCTIVE ORGANS. 239

supposed, during erection, to serve as a dam, preventing the entrance
of semen into the bladder.

The ejaculatory ducts divide behind the prostate, one branch
forming the duct of the seminal vesicle, while the other becomes
continuous with the vas deferens.

3. The Seminal Vesicles. — These are tubular sacs ending in blind
extremities, with occasional saccular branches given off from their
sides. They are lined with a mucous membrane covered with columnar
epithelium, resting upon areolar fibrous tissue. Outside of this is
a muscular coat containing internal circular and external longi-
tudinal fibres, and surrounded by an ill-defined fibrous coat that
passes into the general areolar tissue of the region. The seminal
vesicles sometimes contain semen, for which they may serve as a
temporary reservoir, but they also secrete a fluid that is mixed with
the semen at the time of ejaculation.

4. The Vasa Deferentia. — The vas deferens of each side resembles
the seminal vesicle in structure. It is lined with columnar epi-
thelium, beneath which is a layer of areolar fibrous tissue, resting
upon the muscular coat. This is surrounded by fibrous tissue,
becoming areolar as it blends with that of the neighboring parts.
The muscular coat is thicker than that of the seminal vesicle, and
is divisible into an inner layer of circular and an outer layer of
longitudinal fibres. The mucous membrane, like that of the sem-
inal vesicle, is thrown into folds, which are longitudinal throughout
most of the course of the vas deferens, but are irregular in the
sacculated distal portions of the tube, giving the surface a reticu-
lated or alveolar appearance.

5. The Epididymis. — The vas deferens of each side becomes con-
tinuous with the canal of the epididymis, which is an enormously
long tube, twenty feet, so convoluted and packed together as to
occupy but little space. It is lined throughout with columnar epi-
thelium, continuous with that of the vas deferens; but, except for a
short distance from the junction with the vas, the cells possess cilia
of considerable length, which induce currents toward the vas deferens.
The muscular coat of the latter is continued in the epididymis, but
is very thin. Opening into the canal of the epididymis are the vasa
efferentia of the testis.

6. The Testis. — The testis is a compound tubular gland, of which
the secretion contains the spermatozoa. The latter are derived from
certain of the cells lining the tubules, and contain writhin their

240 NORMAL HISTOLOGY.

structure a definite amount of chromatin and a centrosome. During

the fertilization of the ovum this chromatin unites with a similar
amount present in the egg-cell, and thus forms a complete cell, the
nucleus of which contains equal amounts of chromatin from the
male and female parents of the future offspring. We have seen
(Chapter I.) that the nuclei of the cells throughout the body break
up, during karyokinesis, into a definite and constant number of
fragments, called "chromosomes," which split during metakinesis;
one-half of each chromosome going to each of the daughter-nuclei.
These chromosome-halves form a reticulum within the daughter-
nuclei, and while in that form the chromatin appears to increase in
amount, so that by the time the cell divides again the full supply
of chromatin is present in its nucleus. During the two cell-divis-
ions which immediately precede and result in the formation of the
spermatozoa and the matured egg this growth of the chromatin
does not take place, and, as we shall presently see, each spermato-
zoon or matured ovum contains but half of the chromosomes that
are normally present in the somatic or general cells of the body. This
" reduction of the chromatin " has been a matter of much study
within the last few years, because of its probable bearing upon the
problems of heredity. The fact of its occurrence is strongly con-
firmatory of the idea that the chromatin is the carrier of hereditary
characteristics, the fertilized ovum receiving equal shares from both
parents.

The tubular glands of the testis are enclosed in a strong fibrous cap-
sule, made up of interlacing bands of fibrous tissue. This becomes con-
tinuous, behind, with a mass of areolar tissue containing the vascular
supply of the organ and the epididymis, with the vasa eff'erentia open-
ing into it. The fibrous capsule is called the " tunica albuginea." It
is covered, except posteriorly, by the visceral portion of a serous mem-
brane, the "tunica vaginalis." From the inner surface of the capsule
numerous bands and strands of fibrous tissue, trabecular traverse the
glandular part of the organ, imperfectly dividing it into lobes, each
of which contains several of the glandular or seminiferous tubes.

Upon the surfaces of the trabecular and upon the inner surface
of the capsule the dense fibrous tissue of those structures passes
into a delicate areolar tissue, which gives support to the numerous
small bloodvessels and abundant lymphatics distributed within the
organ. This vascular areolar tissue also penetrates between the
seminiferous tubules, giving them support. In this region the

THE REPRODUCTIVE ORGANS.

241

interstitial tissue just mentioned contains large cytoplasmic cells
of connective-tissue origin, which frequently contain globules of
fat or granules of pigment, and in many instances, in man, have
been observed to contain crystalloids of proteid nature. It has
been surmised that these cells may serve for the storage of nutri-
ment required by the active proliferation of the cells that produce
the spermatozoa within the seminiferous tubes (Fig. 215).

Fig. 215.

A '■■"■<- Ht ft fw.wc)4W y

mat . ,v^-,iV \

7^ •-- et, .,.'

-5> : - .- ■ ■ '

- '

® fv'<

Interstitial tissue in the testis of the cat. (Plato.) Three bloodvessels are shown in either
complete or partial section. Portions of two seminiferous tubules are represented at the
upper corners. Between these structures is the interstitial tissue, containing large cyto-
plasmic cells. This tissue is rather more abundant in this instance than in the human
subject.

Each seminiferous tube is provided with a basement-membrane,
upon the inner surface of which are epithelial cells. These are di-
visible into three groups : first, a parietal layer of cells, the " sper-
matogonia," lying next to the basement-membrane; second, a layer
of cells, often two or three deep, called the " spermatocytes," lying
upon and derived from the spermatogonia; third, the "spermatids,"
lying most centrally. The spermatids are derived from the spermato-
cytes, and are the elements from which the spermatozoa develop,
one spermatozoon being formed from each spermatid.

The cells of the parietal layer, that containing the spermatogonia,
are not all alike. At intervals certain cells, called " sustentacular "

16

242

NORMAL HISTOLOGY.

cells, or the "cells of Sertoli," are differentiated from the others (Figs.
216-228). These sustentaeular cells rest with a broad base, the

Fig. 216.

Superficial aspect of the parietal cells of the seminiferous tube: rat. (Ebner.) /, basal
plates of the sustentaeular cells (cells of Sertoli), each containing a large vesicular
nucleus, poor in chromatin, and a distinct nucleolus of considerable size; w, spermato-
gonia resting upon the basal plates of the cells of Sertoli. Only a few of the spermato-
gonia are represented.

Fig. 217.

Fig. 218.

s28

s 29

<<• 1 J " / w 2 f

Sections from the testis of the rat. illustrating spermatogenesis. (Ebner.)

Figs. 217-228.— u\ spermatogonia ; /, sustentaeular cells, or cells of Sertoli ; h, spermatocytes ;
s, spermatids ; sp, spermatids becoming transformed into spermatozoa : uil to wlO traces
the history of the spermatogonia from the resting condition to that in which they have
grown to become primary spermatocytes. During this process they move from the parietal
layer into that covering it. All, a recently formed spermatocyte; ftlS to ft20, growth of
the spermatocyte; A<21, beginning of the division to form secondary spermatocytes : /<22,
its end; 623, secondary spermatocyte, with chromatin in open spirem; A24, division of
the secondary spermatocyte to form two spermatids; s25, recently formed spermatid ; s26
to 829, growth of the spermatid. (By this time the preceding crop of spermatozoa is fully
developed and lias been discharged into the lumen of the seminiferous tube.) s30 and
s31, beginning transformation of the spermatids into spermatozoa. Their cytoplasm
blends with that of the sustentaeular cell. .^32 to 8p39, stages in the differentiation of
the spermatozoa; 10, completed spermatozoon ready to pass into the lumen of the tube.
ill (Fig. 227) and toll (Fig. 228) illustrate the division of the spermatogonia before they
begin to develop into spermatocytes. It is supposed that the sustentaeular cells aid in
the nourishment of the spermatids during their transformation into spermatozoa, and
that after the discharge of the latter the cytoplasmic process is retracted toward the base-
ment-membrane, bringing with it the globules of fat and cytoplasmic fragments of the
spermatids represented by dark spots and small round bodies in nearly all the figures.
This retraction is taking place at /, Fig. 219. The cells of Sertoli do not appear to mul-
tiply ; at least no karyokinetic figures have been observed in their nuclei.

THE REPRODUCTIVE ORGANS.
Fig. 219. Fig. 220.

\ml u&

s31

243

8 30

Fig. 221.

Fig. 222.

w o

h 19

Fig. 224.

fe20

w w 7 J

f ir 8 w

244

NORMAL HISTOLOGY.

Fig. 225.

// 22

h 20

m

%'V'iV lv \ ;V--i':m lli '\A"i

»Jgip m

• h 24

/

w

w 9 to

Fig. 227.

sp 38

s2G

h 11

- — 7-/i 23

w to 10 / w

Fig. 228.

■A 12

THE REPRODUCTIVE ORGANS.

245

"basal plate," directly upon the basement-membrane, where the
edges of the basal plates are in contact, forming a sort of bed
with depressions in its upper surface, in which the spermatogonia
find lodgement. The cells of Sertoli possess a thick cytoplasmic
process, which extends toward the lumen of the tubule, and to
which those spermatids which are developing into spermatozoa
become attached. For this reason they are called sustentacular
cells. Their nuclei differ from those of the neighboring spermato-
gonia in being less rich in chromatin and
in possessing a single and prominent nu-
cleolus.

The appearances of the various cells
enumerated depend upon the stage in
their activity which happens to be under
observation. The general course of de-
velopment, ending in the formation of
the spermatozoa, is as follows : the
spermatogonia, between the cells of Ser-

Fig. 229.

toli, multiply until quite a collection of
such cells is produced. Each division is
followed by a period of rest, during which
the chromatin increases in amount. When
the final stage of rest is at an end and the
cells have attained their maturity, they
constitute what are called the primary
spermatocytes. These now divide, each
forming two secondary spermatocytes,
which in turn divide, without an inter-
mediate distinct resting-stage, to form
two spermatids. Each primary spermato-
cyte, therefore, gives rise to four sperm-
atids. It is during the division of the
secondary spermatocytes that the reduc-
tion in chromatin, which was mentioned
above, takes place (Figs. 217-228). Each
spermatid receives, in addition to its por-
tion of chromatin, a single centrosome.

The spermatozoon, then, is derived from a corpuscle, the spermatid,
which contains all the essential organs of a cell, differing from the gen-
eral cells of the body, the somatic cells, only in possessing half the

Human spermatozoa. (Bohmand
Davidoff, after Retzius and
Jensen.) The left figure repre-
sents the side view and the
middle figure surface-view of
a spermatozoon, a, head (nu-
cleus) ; b, end-knob (centro-
some?); c, middle piece; d,
tail of flagella ; e, end-piece.
The thickness of d may be
owing to the presence of a
sheath surrounding the actual
flagella, which projects from
the sheath at e.

24<5 NORMAL HISTOLOGY.

usual number of chromosomes in its nucleus. It is unnecessary to
pursue the chain of events through which the spermatid gives rise to
the spermatozoon. It may suffice to state that the body of the latter
consists of the chromatin of the nucleus ; that the long cilium con-
stituting the tail of the spermatozoon is developed from the cyto-
plasm ; and that the centrosome of the spermatid is probably con-
tained in the middle piece of the spermatozoon (Fig. 229). Even
these conclusions are inferences from studies of spermatogenesis
in the lower animals, and not from direct studies of that process in
man. The latter undoubtedly conforms very closely to the former
in all essential details.

To return to the histology of the testis : the epithelial cells of the
seminiferous tubules rest upon a basement-membrane, which is divis-

Fio. 230.

■

*y

->■-. v.-.

r~

_

Basement-membrane from seminiferous tube of the rat. (Ebner.) m, endothelial cells com-
posing the external layer ; I, cells, presumably leucocytes, intercalated between the endo-
thelial cells. The faint striations upon the endothelial cells represent wrinkles in the
homogeneous membrane forming the inner surface of the basement-membrane ; the
wrinkling is probably due to a slight shrinkage of the endothelium.

ible into two layers : first, an internal, extremely delicate, homoge-
neous membrane, upon which the epithelial cells rest ; and, second,
a layer of endothelial cells (Fig. 230). The latter may bound, at
least in places, the lymphatic spaces, which are abundant in the
interstitial tissue of the testis.

Toward the back of the testis the seminiferous tubules unite

THE REPRODUCTIVE ORGANS.

247

with each other and open into a number of straight ducts of
smaller diameter, called the " vasa recta." These are lined with
a cubical epithelium resting upon an extension of the basement-
membrane of the seminiferous tubes, and, in turn, open into a
reticulum of tubules of larger diameter, situated in the mass of
areolar tissue at the posterior aspect of the testis. This reticulum
is called the " rete vasculosum," and the tubules composing it are
lined with a low epithelium, apparently resting upon the surround-
ing fibrous tissue, without an intermediate basement-membrane.
These tubes permit an accumulation of semen before it enters the
vasa efferentia.

The vasa efferentia have a peculiar epithelial lining, which may
be regarded as transitional between the cubical epithelium of the
vasa recta and rete and the ciliated columnar variety lining the
epididymis. It consists of alternating groups of cubical and
ciliated columnar epithelial cells (Fig. 231).

Fig. 231.

Section of vasa efferentia from human testis. (B6hm and Davidoff.) a, cubical or secretory
epithelium ; 6, columnar ciliated epithelium, with deeper pyramidal cells beneath those
that bear the cilia. This form of ciliated epithelium corresponds to that found in the
epididymis where the cubical epithelium is absent.

The vasa efferentia, as already stated, open into the canal of the
epididymis, through which their contents reach the vas deferens.
The walls of the efferent tubes possess a layer of encircling smooth
muscular fibres, which are reinforced in the epididymis by an addi-
tional external layer of longitudinal fibres.

The nerves supplied to the testis are destitute of ganglia, and are
distributed to the vessels and surfaces of the seminiferous tubules.
No terminations have been traced to the epithelial lining of those
tubules.

CHAPTER XVIII.

THE CENTRAL NERVOUS SYSTEM.

The functional part, or parenchyma, of the central nervous
system is composed of ganglion-cells with their processes. Some
of these processes are of cytoplasmic nature, and, as explained in
the chapter on the elementary tissues, are called the protoplasmic
processes. From each ganglion-cell at least one process is given
off which differs from the protoplasmic processes, and is called the
"axis-cylinder- process." This in most cases becomes the axis-
cylinder of a nerve-fibre, and may be invested with a medullary
sheath and neurilemma at some point near or at some distance from
its exit from the cell.

It will be convenient, for the brief description of the central
nervous system to which this chapter must be restricted, to adopt a
special terminology for the different portions of the ganglion-cell
and its processes, as follows : the term (/ancjlion-cell will be restricted
to the nucleus and the cytoplasm surrounding it ; the protoplasmic
processes will be called the dendrites, and their terminations
the teledend rites. The axis-cylinder process will be termed the
neurite; the delicate branches it may give off in its course, the
collaterals; and the terminal filaments of the main trunk, col-
lectively the teleneurites. The cell, with its processes and their
terminations, will collectively constitute a neuron.

A complete neuron, then, consists of (1) certain teledendrites, which
unite to form one or more dendrites connecting them with the gan-
glion-cell ; (2) the cell itself; and (3) one or more neurites, which may
give off collaterals and finally terminate in teleneurites (Fig. 232).

At the present time these neurons are believed to be without
actual connection with each other, but to convey nervous stimuli by
contact. The course of the nervous impulses is from the teleden-
drites to the nerve-cell, and thence, by way of the neurite, to the
teleneurites, whence it is communicated, without a direct structural
union, to the next tissue-element in the chain of nervous transmis-
sion. Those neurites which carry stimuli from the nerve-centres

248

THE CENTRAL NERVOUS SYSTEM:

249

to the periphery, centrifugal impulses, form the axis-cylinders of
some of the nerves. The axis-cylinders of those nerves which
convey impulses from the periphery toward the nervous centres,

Fig. 232.

Sketch illustrating the composition of neurons. I, a neuron transmitting centrifugal
impulses. II, a neuron receiving and transmitting centripetal impulses. Ill, a neuron,
the function of which is supposed to be the distribution of impulses within the nerve-
centre in which it is situated, a, ganglion-cell ; 6, dendrite ; c, teledendrites ; d, neurite ;
e, collaterals ; /, teleneurites. In II the body c represents some sensory organ imparting
nervous impulses to the teledendrites of a sensory nerve. The nervous filament g is a
neurite, presumably derived from the sympathetic nervous system, leading to teleneu-
rites applied to a ganglion-cell, a, of a posterior spinal ganglion. The portion h of the
" nerve " springing from that cell is regarded as a portion of the cell itself. In the
embryonic condition the dendrite and neurite both spring directly and separately from
the body of the cell, the portion h being a subsequent development, i, endothelial
envelope surrounding the ganglion-cell. Ill represents a ganglion-cell, apparently devoid
of distinct dendrites, but having numerous processes that at first appear protoplasmic,
but soon assume the characters of neurites. These cells are found in the retina and
olfactory bulb, and have been termed spongioblasts, cellulas amacrinas, and parareticu-
lar cells. It is thought that nervous stimuli are received directly by the cytoplasm of
the cell, without the intermediation of dendrites, x represents the omission of a portion
of a fibre. The arrows indicate the directions taken by nervous impulses.

centripetal stimuli, may be the dendrites connected with ganglion-
cells in or near those centres; e. g., in the posterior root-ganglia of
the spinal nerves, or they may be the neurites springing from

250

NORMA L HISTOL OGY.

peripheral ganglion-cells, as is exemplified in many, if not all, of
the organs of special sense.

I. THE SPINAL CORD.

The axis of the spinal cord is composed of a column of gray
matter containing numerous ganglion-cells and nervous filaments
held in position by a cement-substance, neuroglia-cells, the fibrous
prolongation of the ependyma cells lining the central canal, and a little
fibrous tissue accompanying the vessels derived from the pia mater.

Fig. 233.

Fig. 234.

Figs. 233 and 234.— Transverse sections of human spinal cord. (Schiifer.)
Fig. 233, from the lower cervical region : Fig. 234, from the middle dorsal region, a, b, c, groups
of ganglion-cells in the anterior horn; d, cells of the lateral horn: e, middle group of
cells ; /, cells of Clarke's column : g, cells of posterior horn ; c, c, central canal , a, c, an-
terior commissure of white matter

THE CENTRAL NERVOUS SYSTEM.

251

Fig. 2:15.

POSTERIOR
ROOT

Transverse section of human spinal cord, from the middle lumbar region. (Schafer.) a b, c,
groups of ganglion-cells in the anterior horn ; d, cells of the lateral horn ; e, middle
group of cells ; /, cells of Clarke's column ; g, cells of posterior horn ; c. c, central canal ;
a, c, anterior commissure of white matter.

In cross-section this column of gray matter presents a transverse
central portion, the gray commissure, near the middle of which is
the central canal. At each side this gray commissure blends with
masses of gray matter, occupying nearly the centre of each lateral
half of the cord and having a general crescentic form. The ends
of these crescentic masses form the anterior and posterior cornua
of the gray matter, from which the anterior and posterior roots of
the spinal nerves proceed. The anterior cornua are larger than the
posterior and contain larger ganglion-cells.

Surrounding the column of gray matter everywhere, except at
the bottom of the posterior median fissure of the cord, and the
interruptions formed by the nerve-roots in their exit from the gray
matter, is a laver of white matter, formed of medullated nerve-
fibres running parallel with the axis of the cord and held together
by neuroglia and delicate vascularized fibrous bands proceeding
from the deep surface of the pia mater.

The white matter of the cord has been divided into a number of
•columns, for the most part indistinguishable through structural dif-
ferences, but each containing fibres that play similar functional roles.
These columns, with their names, are indicated in Figs. 233, 234,
and 235. The columns of Goll and Burdach, forming the posterior

252

NORMAL HISTOLOGY.

column of the white matter, between the posterior cornua and the
posterior median fissure, conduct, for the most part, centripetal
impulses. Impulses having the same upward direction are also
conveyed by the direct cerebellar tract and the tract of (lowers in
the lateral column of the white matter. Centrifugal impulses,
motor stimuli, are conveyed by the fibres in the direct pyramidal
tract of the anterior column and by those of the crossed pyramidal

Diagram of spinal cord, illustrating the associations of its various nervous elements. (R. y
Cajal.) a, collateral from Goll's tract, entering into the formation of the posterior com-
missure ; b, collateral to the posterior horn ; c, collateral to the formatio reticularis and
the anterior horn ; d, posterior nerve neurite, with its collaterals ; e, collaterals from the
lateral column ; /, collaterals to the anterior commissure ; g, central canal ; h, neurite in
the crossed pyramidal tract from the commissure-cell of the opposite side ; ?', its course
in the commissure ; j, neurite from a large motor cell in the anterior horn k ; I, cell of
the anterior horn, giving off a neurite dividing into an ascending and a descending
branch (compare Fig. 239, D) ; m, commissure-cell ; n, cell giving off a collateral within
the gray matter ; o, neurite of the cell u, in Clarke's column ; p, neurite from the mar-
ginal cell 8, of the substance of Rolando ; q, cross-section of an axis-cylinder (neurite)
in the white substance of the cord ; r, division of a posterior nerve-fibre (neurite) into
ascending and descending branches; t, small cell in the substance of Rolando. Aside
from the cells indicated in the figure, the gray matter contains some that give off neurites
which divide into two or three branches while in the gray matter, the branches going
to different columns of white matter. There are also cells with very short neurites,
which terminate in teleneurites within the gray matter, and probably distribute nervous
impulses for short longitudinal distances.

tract in the lateral column. The tracts hitherto considered contain
fibres that are continued into the higher nerve-centres of the brain
and cerebellum, to or from which they convey nervous impulses.
But the spinal cord is not merely a collection of such transmitting

THE CENTRAL NERVOUS SYSTEM. 253

fibres. It is also a nerve-centre of complex constitution, in which
neurons terminate in teleneurites or arise in teledendrites.

Some of the neurons within the cord are confined to its substance,
and constitute nervous connections between the different parts at
various levels. These may be termed longitudinal commissural
neurons, or association-fibres. Portions of such neurons are repre-
sented in the diagram of a cross-section of the cord (Fig. 236),
which also contains representations of some of the neurites in the
posterior spinal nerve-roots, with their collaterals ending in tele-
neurites within the gray matter (d). On the right side of the figure,
the nerve-cells, with their dendrites and the beginning of the neu-
rites, are shown. On the left side the neurites connected with cells
at another level are shown, re-entering the gray matter, where they
terminate in teleneurites. In studying this figure it must be borne
in mind that the teledendrites of the neurons on the right are in
close relations with the teleneurites of other neurons, and that the
teleneurites represented on the left are in close relations with the
teledendrites of other neurons. These association-neurons are,
therefore, merely links in chains of communicating neurons. They
are again represented in Fig. 239, I) and E.

Aside from these association-neurites, the gray matter of the
cord receives innumerable collaterals from the neurites forming the
axis-cylinders of the nerves in the various columns of the white
matter. These collaterals terminate in teleneurites, which are in
close relations with the teledendrites of the neurons arising in the
cord. The distribution of these collaterals is represented in Fig.
237. The collaterals from the anterior column enter the anterior
horn of the gray matter, where they are chiefly distributed about
the large ganglion-cells in the antero-lateral portion of its substance
(Fig. 233, b; Fig. 236,,/), but may also extend to other parts of
the gray matter. The collaterals from the fibres in the lateral
columns of the white matter are most numerous near the pos-
terior horn, which they enter, many of them passing through the
gray matter behind the central canal and forming a part of the
posterior or gray commissure of the cord (Fig. 237, I). The col-
laterals from the posterior column are divisible into four groups :
first, those which are given off in the lateral portion of that column
(Fig. 237, G), and are distributed in the outer portion of the pos-
terior horn and in the substance of Rolando (Fig. 237, I) ; second,
those which end in Clarke's column (Fig. 237, J) ; third, those

254

NORMAL HISTOLOGY.

which arise chiefly in the column of Goll, pass through the sub-
stance of Rolando, and then form an expanding bundle distributed
in the anterior horn of the gray matter, where they are in associa-
tion with the dendrites of the motor cells in that region (these
fibres form the reflex bundle of Kolliker, Fig. 237, H) ; fourth,
collaterals springing from fibres in the posterior column, passing

Cross-section of the spinal cord of a newborn child, showing the distribution within the gray
matter of the collaterals from the neurites of the white matter. (R. y Cajal.) a, anterior
fissure ; B, pericellular branches of the collaterals from the anterior column ; C, collaterals
of the anterior commissure; D, posterior bundle of collaterals in the posterior commis-
sure ; E, middle bundle of the posterior commissure ; /, anterior bundle ; G, collaterals
from the posterior column ; H, senso-motory collaterals from the posterior column ;
I, pericellular terminations of collaterals in the posterior horn ; J, collateral terminations
in the column of Clarke.

through the posterior commissure of gray matter and ending in the
substance of Rolando of the opposite side (Fig. 237, D).

The reflex collaterals arising in the posterior column are shown
in Fig. 238, where their teleneurites are in close relations with the
teledendrites of the motor cells e.

The centripetal or sensory neurites of the posterior spinal nerve-
nuits spring from the ganglion-cells of the spinal ganglia. When
they have entered the white matter of the spinal cord they divide

THE CENTRAL NERVOUS SYSTEM.

255

into two branches (Fig. 236, r). One of these ascends in the white
substance and the other descends. Both branches give off numer-
ous collaterals, which penetrate the gray matter, ending in teleneu-
rites associated with the teledendrites of the cells in both the ante-
rior and the posterior horns, and the column of Clarke. The main
branches of the sensory neurite also enter the gray matter, after

Fig. 238.

Fig. 239.

Fig. 238— Diagram of the senso-rnotory reflex collaterals in the cord. (R. y Cajai.) a, gan-
glion-cell of the posterior nerve-root ; 6, division of its neurite into ascending and de-
scending branches ; c, collaterals to anterior horn ; d, terminal teleneurites in the pos-
terior horn ; e, motor cell of the anterior horn, with its processes.

Fig. 239.— Longitudinal section of a part of the spinal cord, including a posterior nerve-root.
Semidiagrammatic. (R. y Cajal.) A, posterior nerve-root; S, white substance of the
cord ; 0, gray matter ; B, collateral teleneurites in the gray matter ; C, cell with a single
ascending neurite ; D, cell with bifurcating neurite, terminating at Fand /: E, cell with
a single descending neurite; F, G, terminal teleneurites ; a', collateral from a branch
of the posterior root-neurite ; b', collateral from the main neurite before its bifurcation.

following the posterior column for a short distance, and end in tele-
neurites among the cells of the posterior horn and the substance of
Rolando. The collaterals which pass to the anterior horns (Fig.
237, H, and Fig. 238, c) have to do with the origin of reflex cen-

256

NORMAL HISTOLOGY.

trifugal impulses emanating from the motor cells in that region
(Fig. 238, e, and Fig. 236, j). The further transmission of these
centripetal stimuli toward the higher nerve-centres of the brain
probably takes place : first, through the cells in the posterior horns,
the neurites from which pass into the lateral columns and there
ascend the cord ; second, through the cells of Clarke's column,
which also send neurites into the lateral column, where they enter
the direct cerebellar tract (Fig. 236, o ; see also Fig. 239). In
addition to these centripetal or sensory neurites, the posterior nerve-
roots contain a few centrifugal neurites.

Fig. 240.

Diagram of a sensory and a motor tract. (R. yCajal.) A, psycho-motor region in cerebral
cortex ; B, spinal cord ; C, voluntary muscle ; D, spinal ganglion ; D', skin ; a, axis-cylin-
der of a neuron extending from the cerebral cortex to the anterior horn of the spinal
cord, where the terminal teleneurites are in relations with the teledendrites of the motor
cell at b. The sensory stimulus arising in the skin, U , is transmitted by the neuron
dDce to /, where it is communicated to the neuron fy. The point / may be in the cord
or in the medulla oblongata.

In order to understand the origin of the anterior spinal nerve-
roots we must first consider the course of the centrifugal neurites in
the pyramidal tracts (Figs. 233, 234, 235). These enter the gray
matter and end in teleneurites, which are associated with the tele-

THE CENTRAL NERVOUS SYSTEM. 2">7

dendrites of the cells in the anterior horn, especially those which
give off neurites to the anterior roots of the spinal nerves (Fig.
236,./).

The foregoing details may be summarized by means of the accom-
panying diagram (Fig. 240), in which the course of a nervous stim-
ulus is traced from the organ of sense in, e. g., the skin, to the
cortex of the cerebrum, where it is translated into a nervous im-
pulse, the course of which is traced to the motor plates of the vol-
untary muscles. The reflex mechanism which might at the same time
be set into operation is not represented in the diagram, but will be
sufficiently obvious from an inspection of Fig. 238. It will be
noticed in Fig. 240 that both the sensory stimulus and the motor
impulse are obliged to pass through at least two neurons before they
reach the ends of their journeys. But the nervous currents are by
no means entirely confined to the course marked by the arrows.
Impulses may be transmitted in an incalculable number of delicate
tracts through the collaterals given off from the neurites within the
central nervous system, some of which are indicated in the diagram,
and all of which end in teleneurites associated with the teledendrites
of, perhaps, several neurons. One of these collateral tracts has
already been considered, namely the senso-motory reflexes illus-
trated in Fig. 238.

II. THE CEREBELLUM.

The cerebellum is subdivided into a number of laminae by deep
primary and shallow secondary fissures. The gray matter of the
organ occupies the surfaces of these lamina?, while their central por-
tions are composed of white matter. The gray matter may be
divided into two layers : an external or superficial " molecular
layer" and an inner "granular layer" (Figs. 241 and 242).

The molecular layer contains two forms of nerve-cells : first, the
large cells of Purkinje ; second, small stellate cells.

The cells of Purkinje have large, oval, or pear-shaped bodies lying
at the deep margin of the molecular layer. Their dendrites form
an intricate arborescent system of branches extending peripherally
to the surface of the gray matter, and give off innumerable small
teledendrites throughout their course. All these branches lie in one
place, perpendicular to the long axis of the lamina in which they
are situated, and the teledendrites come into relations with certain
longitudinal neurites springing from the cells of the granular layer,

17

258

NORMAL HISTOLOdY.

to be presently described. The neurites of the cells of Purkinje
extend through the granular layer into the white matter and soon
acquire medullary sheaths (Fig. 241, 6); but before they leave the
granular layer they give off collaterals, which re-ascend into the
molecular layer, where their teleneurites are in relations with the

Fig. 241.

Section of a cerebellar lamina perpendicular to its axis. (R. y Cajal.) A, molecular layer
of the gray matter ; B, granular layer ; C, white substance ; a, cell of Purkinje ; o, its
neurite, giving off two recurrent collaterals ; b, b, stellate cells of the molecular layer; d,
basket-like distribution of the teleneurites of one of their collaterals around the body
of a cell of Purkinje ; e, superficial stellate cell, which does not appear to come into rela-
tions with the bodies of the cells of Purkinje, but must lie close to their dendrites; /,
large stellate cell of the granular layer; g, small stellate cell of the granular layer; h,
centripetal neurite of a " moss " fibre ; n, centripetal neurite distributed in the molecular
layer; j, to, neuroglia-cells. The arborescent dendrites of only one of the cells of Pur-
kinje are represented in the figure. Were those of the neighboring cells also represented,
the molecular layer of the gray matter would display an enormously complex interdigi-
tation of such filaments.

teledendrites of neighboring cells of Purkinje. These collaterals are
believed to occasion a certain co-ordination in the action of those
cells of Purkinje which are near each other.

The stellate cells of the molecular layer (Fig. 241, b, e) pos-

THE CENTRAL NERVOUS SYSTEM.

259

sess neurites, which lie in the same plane with the arborescent
dendrites of the cells of Purkinje, and send collaterals to end in a
basket-work of teleneiirites applied to the bodies of the cells of
Purkinje. The terminal teleneiirites of these stellate cells also end
in the same situation. Other smaller collaterals extend toward the
surface of the cerebellar lamina.

The granular layer of the gray matter also contains two varieties
of nerve-cells : the " small stellate cells," which are most numerous,
and the " large stellate cells."

Fig. 242.

Section of a cerebellar lamina parallel to its axis. (R. y Cajal.) A, molecular layer of the
gray matter; B, granular layer; C, white substance; a, small stellate cell of the granular
layer, from which a neurite enters the molecular layer, where it bifurcates, sending
branches throughout the length of the lamina; 6, bifurcation of one of these neurites;
e, slightly bulbous termination of one of the neuritic branches ; d, body of a cell of Pur-
kinje seen in profile ; /, neurite of a cell of Purkinje.

The small stellate cells (Fig. 241, g, and Fig. 242, a) are scat-
tered throughout the granular layer, and it is owing to the abun-
dance of their nuclei that this layer has received that name. Their
dendrites are few in number and short, but their neurites are very
long. They extend perpendicularly into the molecular layer, where
they bifurcate, the branches lying parallel with the axis of the
cerebellar lamina and its surface. These fibres appear to run the
whole length of the lamina, and to come in contact with the tele-
dendrites of the cells of Purkinje, to the planes of which they run
perpendicularly. They are thought to coordinate the action of a
long series of the cells of Purkinje.

260 NORMAL HISTOLOGY.

The large stellate cells of the granular layer lie near its external
margin, whence they send their dendrites into a large area of the
molecular layer, while their neurites are distributed in the granular
layer, where they must come into relations with the dendrites of the
small stellate cells (Fig. 241,/).

The distribution of the cells and their processes in the cerebellum
indicates a very complex interchange of nervous impulses and an
extraordinary coordination in the action of the various neurons.

This complication is still further increased by the presence of
centripetal neurites, which enter the cerebellum through the white
matter and are distributed in the gray matter. These are of two
sorts : first, neurites which penetrate the granular layer and are
distributed among the proximal dendrites of the cells of Purkinje
(Fig. 241, n) ; second, neurites, called " moss " fibres, which are dis-
tributed among the cells of the granular layer. The teleneurites of
these fibres have a mossy appearance, whence the name (Fig. 241, h).
The origin of these centripetal neurites is not known, but it is sur-
mised that the " moss " fibres may enter the cerebellum through the
direct cerebellar tracts of the cord.

III. THE CEREBRUM.

The gray matter of the cerebral cortex has been divided into four
layers : first, an external molecular layer ; second, the layer of
small pyramidal cells ; third, the layer of large pyramidal cells ;
and, fourth, an internal layer of irregular or stellate cells. Of
these layers, the second and third are not clearly distinguishable
from each other (Fig. 243).

The molecular layer contains three sorts of nerve-cells, two of
which are closely related to each other, differing onlv in the form
of the cell-bodies, which are small in both varieties (Fig. 244, Ay
B, and C) ; while the cell-bodies of the third variety are large and
polygonal (Fig. 244, D). The small cells (A, B, C, Fig. 244) pos-
sess two or three tapering processes, which at first resemble proto-
plasmic processes, but soon assume the characters of neurites or axis-
cylinders. These neurons, then, resemble the type depicted in Fig.
232, III. Their neurites run parallel to the surface of the convo-
lution in which they are situated, sending off numerous perpen-
dicular collaterals, and finally end in teleneurites within the molec-
ular layer. The collateral and terminal teleneurites are probably
in relations with the dendrites of the pyramidal cells of the under-

THE CENTRAL NERVOUS SYSTEM.

261

Fig. 243.

lying layers, which form arborescent expansions in the molecular
layer, similar to those of the cells of Purkinje in the cerebellum,
extending to the surface of the gray matter.

The large stellate cells of the molecular layer (Fig. 244, D) send
their dendrites in various directions into
the molecular layer and the layer of
small pyramidal cells lying beneath it.
The neurite is distributed in the molec-
ular and upper portions of the under-
lying layers, but is never extended into
the white matter. The dendrites of these
cells come into relations with the neurites
of the other cells of this layer and with
those that proceed upward from some of
the cells in the deeper layers.

The small spindle- and stellate cells
(A, B, C, Fig. 244) are considered to be
the autochthonous cells of the cerebral
cortex — i. e., the cells of the brain in
which the highest order of nervous im-
pulses find their origin. The small
spindle-shaped cells, with their peculiar
neurites, are extremely abundant and
fill the molecular layer with a mass
of interwoven filaments.

The second and third layers of the
cerebral gray matter are characterized
by the presence of pyramidal nerve-
cells of various sizes, the smaller being
relatively more abundant in the second
layer and the larger in the third layer. From the apex of the pyram-
idal cell a stout, " primordial " dendrite passes vertically into the
molecular layer, where, as well as during its course to the molecular
layer, it gives off numerous branches, and finally ends in a brush of
teledendrites extending to the surface of the gray matter (Fig. 245,
A, B). Other and shorter dendrites are given off from the body of the
cell, which ramify and end in the second, third, or fourth layer of the
gray matter. The neurites from the bases of the pyramidal cells pass
vertically downward into the white substance, where they may
bifurcate, giving axis-cylinders to two nerve-fibres. While within

j s

Vertical section of the cerebral cor-
tex, showing its layers. (R. y
Cajal.) 1, molecular layer; t,
layer of the small pyramidal
cells ; 3, layer of the large pyram-
idal cells ; 4, layer of polymor-
phic cells ; 5, white matter.

2(V2

NORMAL HISTOLOGY.
Fig. 244.

Cells of the molecular layer of the cerebral cortex. (R. y Cajal.) A, C, small spindle-shaped
cells ; B, small stellate cell ; D, large stellate cell. The branches marked c are neurites.

Fig. 245.

Fig. 240.

Fig. 245.— Diagrammatic section through the cerebral cortex. (R. y Cajal.) A, small pyram-
idal cell in the second layer ; B, two large pyramidal cells in the third layer ; C, D, poly-
morphic cells in the fourth layer : E, centripetal neurite from distant nerve-centres ;
F, collaterals from the white substance ; 6, bifurcation of a neurite in the white sub-
stance. The arrows indicate the centripetal and centrifugal courses of nerve-impulses,
but it is probable that centripetal impulses have to pass through other neurons (perhaps
the spindle-cells of the molecular layer) before they are translated into centrifugal im-
pulses.

Fig. 240.— Cells with short neurites in the cerebral cortex. (R. y Cajal.) A, molecular layer ;
B, white substance ; a, cells with neurites, which speedily divide into numerous tele-
neurites in the neighborhood of the cell belonging to the same neuron ; b, cell with a
neurite extending vertically toward, but not entering, the molecular layer: c, cell with
a neurite distributed within the molecular layer; d, small pyramidal cell.

THE CENTRAL NERVOUS SYSTEM. 263

the gray matter, and after their entrance into the white matter,
these neurites give off collaterals, which branch and end in terminal
bulbous expansions without breaking up into a set of teleneurites.

The irregular cells of the fourth layer (Fig. 245, C, D) do not
send their dendrites into the molecular layer, but distribute them
within the deeper layers of the gray matter. Their neurities, like
those of the pyramidal cells, enter the white matter, where they
may or may not bifurcate.

Besides the cells in the deeper layers of the gray matter hitherto
described, those layers contain cells with short neurites, which are
divisible into two classes : first, spindle-shaped or stellate cells,
sending their neurites into the molecular layer (Fig. 246, c) or into
the second layer of tlie gray matter (Fig. 246, 6) ; second, poly-
morphic cells with radiating dendrites and copiously branching
neurites, both of which are distributed within a short distance of the
cell. These cells are believed to distribute nervous impulses to the
neurons in their vicinity.

The gray matter of the cortex also receives centripetal neurites
from the white matter, which give oif numerous collaterals and ter-
minate in the molecular layer.

The white matter of the cerebrum contains fibres that may be
divided into four groups : first, centrifugal or " projection " fibres ;
second, " commissure-fibres," which bring the two sides of the brain
into coordination (these lie in the corpus callosum and in the ante-
rior commissure); third, "association-fibres," which coordinate the
different regions of the cerebral cortex on the same side ; fourth,
centripetal fibres, reaching the cortex from the peripheral nervous
system or cord.

The centrifugal or projection-fibres arise from all parts of the
cortex, springing from the pyramidal and, perhaps, also from the
irregular cells. Many of these fibres give off a collateral, which
passes into the corpus callosum, to be distributed in the cortex of
the opposite side, commissural collaterals, and then pass on to the
corpus striatum, to the gray matter of which further collaterals
may be given off, after which the main neurite probably passes into
the pyramidal tracts of the cord through the cerebral crus (Fig.
247, a).

The commissure-fibres (Fig. 247, 6, c) also arise from the pyram-
idal cells of the cortex, mostly from the smaller variety, and pass
into the corpus callosum or the anterior commissure, to be dis-

264 NORMAL HISTOLOGY.

tributed in the gray matter of the eortex of the opposite hemisphere,
but not necessarily to the corresponding region. These commissural

Fig. 247.

Centrifugal and commissural fibres of the cerebrum. (R. y Cajal.) A, corpus callosum ; B,
anterior commissure ; C, pyramidal tract ; «, large pyramidal cell, with a neurite sending
a large collateral into the corpus callosum and then entering the pyramidal tract.
Between a and 6 is a second similar cell, the neurite from which contributes no branch
to the corpus callosum. ft, small pyramidal cell giving rise to a commissural neurite ; c, a
similar cell, the neurite of which divides into a commissural and an association branch ;
d, collateral entering the gray matter of the opposite hemisphere; e, terminal teleneu-
rites of a commissural fibre.

fibres give off collaterals, which also end in the gray matter, and
are accompanied by collaterals from the centrifugal fibres, which
likewise end in, and send collaterals to, the gray matter.

Fig. 248.

Association-fibres of the cerebrum. (R. y Cajal.) The figure represents, diagrammatically,
a sagittal section through one of the cerebral hemispheres, a, pyramidal cell, with neu-
rite giving off collaterals to, and ending in, the gray matter of the same side ; ft, a similar
cell ; c, cell with a branching neurite passing to different parts of the hemisphere; d,
teleneurites ; e, terminal collateral twigs.

The origin, course, and general distribution of the association-
fibres are indicated in Fig. 248. They are so numerous that they

THE CENTRAL NERVOUS SYSTEM. 265

form the great bulk of the white substance, where they are inex-
tricably interwoven with the other fibres there present.

Besides the centripetal neurites of the association and commissural
neurons, their collaterals and those of the projection-fibres, the gray
matter of the cortex receives terminal neurites from larger fibres
that are probably derived from the cerebellum and cord (Fig. 245,
E). These give off numerous collaterals and teleneurites, which are
distributed to the small pyramidal cells of the second layer, and
probably also penetrate into the molecular layer, where they end in
numerous teleneurites among the cells of that layer.

In the diagrammatic figure 245 the probable course of nervous
stimuli to and from the cerebral cortex is indicated. The possi-
bilities of transmission within a structure of such marvellous com-
plexity are incalculable.

The above structural details of the central nervous system are
chiefly taken from the publications of Ramon y Cajal. They are
the result of researches carried on by the application of the methods
devised by Golgi to the nervous structures of the lower vertebrates
and embryos. Such details cannot be observed when specimens
have been hardened and stained by methods used for the study of
other structures. In such specimens the nuclei of the nerve-cells
and those of the neuroglia are stained and become prominent. But
the multitude of nervous filaments lying between the cells and the
processes of the neuroglia-cells are not differentiated, but appear
as an indefinite, finely granular material, in which the cell-bodies
apparently lie. Where the cells are sparse or small, as in the first
layer of the cerebral gray matter, the tissue appears finely molecu-
lar. Where the cells are numerous but small, their stained nuclei
give the tissue a granular appearance, as, for example, in the second
layer of the cerebellar cortex.

The brain and spinal cord are invested by a membrane of areolar
tissue, called the "pia mater." Extensions of this areolar tissue
penetrate the substance of the cord and brain, giving support to
bloodvessels and their accompanying lymphatics. This areolar
tissue also extends into the ventricles of the brain, where it receives
an external covering of epithelium continuous with that lining the
ventricles, which is ciliated. Externally, the areolar tissue is con-
densed to form a thin superficial layer.

CHAPTER XIX.

THE ORGANS OF THE SPECIAL SENSES.

1. Touch. — The nervous filaments distributed among the cells
of stratified epithelium have already been depicted in Fig. 99.
Similar filaments occur in the human epidermis, and it is probable
that some of them are the teledendrites of spinal ganglion-cells,
while others are centrifugal teleneurites subserving the functions

Fig. 249.

Fig. 250.

a

Tactile corpuscles.

Fig. 249.— Meissner's corpuscle, from the human corium. iBohm and Davidoff.) a, upper
portion, in which the epithelial cells alone are represented. The nuclei of those cells
are in the broader peripheral portion of the cytoplasm ; 6, nerve-dendrite coiled about
the epithelial cells ; c, nerve-fibre.

Fig. 250.— Krause's corpuscle, from the human conjunctiva. (Dogiel.) a, endothelial
envelope; b, nucleus of connective-tissue cell within the fibrous capsule ; c, nerve-fibre.

of nutrition, etc., or the teledendrons of neurons belonging to other
than the spinal system of nerves.

Besides these nervous terminations the skin possesses certain
bodies, which are called " tactile corpuscles" and "Pacinian bodies."

266

THE ORGANS OF THE SPECIAL SENSES. 267

These arc situated in the corium, the former lying in some of the
papillae projecting into the rete mucosnm.

The tactile corpuscles are of two forms, differing slightly from
each other in structure : first, those of Meissner, and, second, those
of Krause.

The tactile corpuscles of Meissner (Fig. 249) consist of a group
of epithelial cells closely associated with the teledendrites of a
nerve-fibre. The cells are closely compacted together to form an
ellipsoid body. The nervous dendrite, with its medullary sheath,
enters this body at one of its ends, and, after making one or two
spiral turns around the mass of epithelial cells, loses its medullary
sheath and breaks up into a number of teledendrons, which are dis-
tributed among the epithelial cells. The neurilemma and the
endoneurium of the fibre are continued over the corpuscle, consti-
tuting a species of capsule.

The tactile corpuscles of Krause (Fig. 250) possess a capsule
composed of delicate fibrous tissue, covered and lined with endo-
thelial cells. The dendrite of the nerve-fibre loses its medullary
sheath upon penetrating this capsule, and then breaks up into tele-
dendrites, that form a complex tangle within the cavity of the cor-
puscle. There appear to be no cells among the teledendrites, the
interstices being occupied by lymph. These corpuscles are espe-
cially abundant in the conjunctivae and the edges of the eyelids,
but occur also in the lip, large intestine, posterior surface of the
epiglottis, and the glans penis and clitoris. They may receive
dendrites from more than one nerve. Those of Meissner are found
throughout the skin, being most abundant where the tactile sense is
most acute.

The Pacinian corpuscles (Fig. 251) are large oval bodies, com-
posed of a number of concentric cellular lamellae, surrounding a
central, almost cylindrical cavity, and covered externally with a
layer of endothelioid cells, which appear to be continuous with the
delicate endoneurium of the fibre. The latter enters the corpuscle
at one of its ends, soon loses its medullary sheath, and is finally
subdivided into a number of teledendrites within the central cavity.

The "genital corpuscles" which are found in the glans of the
penis and that of the clitoris are similar in structure to the Pacinian
corpuscles, but the lamellar envelope of the latter is here reduced to
one or two ill-developed lamellae.

The nervous impulses inaugurated in the tactile and Pacinian

268

NORMAL HISTOLOGY.

corpuscles are probably transmitted to the sensorium in the manner
indicated in Fig. 240.

Pacinian corpuscles are found in the palms and soles, on the
nerves of the joints and periosteum, in the pericardium, and in the
pancreas.

2. Taste. — The special organs of taste appear to be the taste-

Fig. 251.

Pacinian corpuscle, from the mesentery of the cat. (Klein.) a, nerve-fibre; b, concentric
capsule. The nature of the cells in this capsule is a matter of doubt ; analogy would
suggest their epithelial nature.

buds, situated in the walls of the sulci surrounding the circum-
vallate papilla? of the tongue (see Fig. 116).

The taste-buds are bulb-shaped groups of epithelial and nervous
cells, situated within the stratified epithelium lining the sulci. The
cells composing these buds are spindle-shaped or tapering, and their
ends are grouped together at the base of the bud and converge at
its apex, where they occupy a " pore " in the stratified epithelium.
The epithelial cells do not appear to be active in the inauguration
of nervous impulses, but the more spindle-shaped cells lying among
them seem to be endowed with nervous functions. They may, pos-
sibly, be regarded as peculiar neurons ; their distal processes, which
receive stimuli at the pore, being the dendrite, while the proximal
process is the neurite. The latter divides into a number of minute
branches, which, from this point of view, might be regarded as tele-
neurites. Be this as it may, these branches come into close relations

THE ORGANS OF THE SPECIAL SENSES. 209

with the teledendrites of nerve-fibres supplied to the taste-bud
(Fig. 252). The stratified epithelium surrounding the taste-buds,
as elsewhere, contains teledendrites from sensory nerves.

3. Smell. — The olfactory organ occupies a small area at the top
of the nasal vault, and extends for a short distance upon the sep-
tum and external wall. Its exposed surface is about equal to that

Fig. 252.

Diagram of a taste-bud and its nervous supply. (Dogiel.) a, radicle of the gustatory nerve ;
b, radicle of a sensory nerve ; c, epithelial cell ; d, nerve-cell. The shaded part of the
figure represents the stratified epithelium lining the sulcus of the circumvallate papilla.
Only one of the epithelial or supporting cells of the upper bud is represented in the
figure ; the others are omitted. The structure of the lower bud is not shown.

of a five-cent piece. It is a modified portion of the mucous mem-
brane of the nose, which may be divided into this, the olfactory
portion, and the general or respiratory portion.

The respiratory portion of the nasal mucous membrane is covered
with a stratified, columnar, ciliated epithelium, with occasional
mucigenous goblet-cells, resting upon a basement-membrane. Be-
neath this is the membrana propria, resembling that of the small
intestine in being rich in lymphadenoid tissue, which may, here and
there, be condensed into solitary follicles. Beneath the membrana
propria is a richly vascularized submucous areolar tissue, containing
compound tubular glands, the glands of Bowman, which open upon
the surface of the mucous membrane. These glands secrete both
mucus and a serous fluid.

In the olfactory region the columnar epithelial cells are devoid of
cilia, but possess a thin cuticle, and the epithelium rests directly
upon the lymphadenoid tissue, without the intermediation of a base-
ment-membrane (Fig. 253). Between these epithelial cells are the

270

NORMAL HISTOLOGY.

nervous colls, which constitute the receptive elements of the olfac-
tory nervous tract. These are cells with large nuclei and cylin-
drical distal bodies, which terminate at the surface of the epithelial
layer in several delicate hairs projecting from the surface (Figs. 254
and 255). The proximal ends of the cells rapidly taper to a delicate

Fig. 253.
Ba

rz-

Iffiift 'Wftiiin

rin ■»»»«!»»

i M'

Bb-JJH£ -

Bt

Vertical section through the olfactory mucous membrane of the human nose. (Brunn.)
ez, nuclei of the columnar epithelial cells ; rz, nuclei of the nervous or olfactory cells
lying among those of the epithelium ; bz, nuclei of basal pyramidal epithelial cells lying
among the branching proximal ends of the columnar epithelial cells and tapering ends of
the nervous cells; pz, pigmented cell in the layer of lymphadenoid tissues beneath the
epithelium; Ba, duct of a gland of Bowman; Bb, dilated subepithelial portion of the
duct, receiving several of the tubular acini, Bt. The connection between the duct and
tubes is not shown, n, it, branches of the olfactory nerve ; rz*, atypical nervous cell.

filament, which extends through the subepithelial tissue and becomes
associated with others to form the olfactory nerve. The distal ends
of the nerve-cells represent the dendrites of neurons, the neurites of
which form the axis-cylinders in the olfactory nerve.

The neurites in the olfactory nerve pass through the cribriform
plate of the ethmoid bone to the olfactory bulb of the brain, where

THE ORGANS OF THE SPECIAL SENSES.

Fig. 254.

Ill

271

Epithelial layer of the human olfactory mucous membrane. (Brunn.) Isolated elements.
Three epithelial cells, with forked proximal ends, are represented, together with a ner-
vous cell bent out of position and the distal end of a second nervous cell. M.l, cuticle
of the columnar epithelium, which is not continued over the end of the nervous cell.
The cuticle of neighboring cells unites at the edges to form a species of membrane, which
appears to be perforated for the exit of the distal ends of the nervous cells. A similar
cuticle is found in the retina, where it has received the name " limiting membrane."

Fig. 255.

Vertical section of the epithelium, showing the arrangements of its elements. The nervous

cells, with their neurites, are black.

they terminate in teleneurites within little globular structures, called
the "glomeruli of the bulb."

272 NORMAL HISTOLOGY.

The olfactory bull) may be divided into five layers: first, the
layer of peripheral nerves, containing the neurites of the olfactory
nerve; second, the layer containing the olfactory glomeruli; third,
the molecular layer ; fourth, the layer of the mitral cells ; fifth, the
granular layer.

The first layer is, as already stated, occupied by the neurites from
the nervous cells in the olfactory mucous membrane. These neurites
constitute the axis-cylinders of the olfactory nerve.

The glomeruli of the second layer are small globular masses
formed by the closely associated teleneurites of the olfactory nerves
and teledendrites from the mitral cells of the fourth layer, the den-
drites from which pass through the third or molecular layer. A
few cells of neurogliar nature may be associated with these nervous
terminations, but the chief mass of each glomerulus is composed of
interwoven teleneurites and teledendrites.

The third, or molecular, layer contains small spindle-shaped
nerve-cells, which send dendrites to the glomeruli of the second
layer and neurites into the granular (fifth) layer, where they turn
and take a centripetal direction toward the cerebrum.

The fourth layer is characterized by the presence of large tri-
angular nerve-cells, the mitral cells, the dendrites from which pass
through the molecular layer, to end in teledendrites within the
glomeruli. A single mitral cell sends dendrites to more than one
glomerulus. The neurites from these cells pass, centripetally, to
the olfactory centre of the cerebrum.

The fifth, granular, layer contains the centripetal neurites of the
mitral cells, and also centrifugal neurites from the cerebrum. The
latter are distributed in teleneurites within the granular layer
itself. This layer also contains small polygonal nerve-cells of two
sorts : first, cells resembling those of the third type represented in
Fig. 232, the processes from which are distributed in the granular
and molecular layers. They are probably association-cells. Second,
cells (Fig. 25(5) with dendrites in the granular layer and teleneurites
in the molecular layer. These cells would distribute impulses re-
ceived from the centrifugal fibres, which end in the granular layer,
among the teledendrites in the molecular layer.

The sense of smell, then, is aroused by stimulations of the distal
ends of the nervous cells in the olfactory mucous membrane (Fig.
256), which are transmitted to the glomeruli, where they leave the
first neuron, being communicated to the second, represented by the

THE ORGANS OF THE SPECIAL SENSES.

273

mitral cells and their processes, by which they are conveyed to the
cerebral cortex. In its passage through this tract numerous collat-

Fig. 256.

Diagram of the nervous mechanism of the olfactory apparatus. (R. y Cajal.) a, olfactory
portion of the nasal mucous membrane ; b, second or glomerular layer of the olfactory
bulb j, at the right edge of the molecular layer, which is dotted. The cells of this layer
are omitted, c, fourth layer of the bulb, the layer of the mitral cells, two of which are
represented ; e, iiti, cells of the fifth or granular layer ; d, olfactory tract ; g, cerebral cor-
tex ; h, neurite from a mitral cell, giving off a collateral to the dendrites of a pyramidal
cell in the gray matter of the brain ; /, pyramidal cells of the olfactory tract ; j, collateral
from a mitral neurite passing, recurrently, into the molecular layer ; I, centrifugal neurite
from the cerebrum.

eral and association-tracts may be influenced in a manner too com-
plicated to be readily followed.

Fig. 257.

Diagram of the distribution of the auditory nerve within the mucous membrane of the crista
acustica. (Niemack.) The bodies of the hair-cells are dotted. Between them are the
cells of Deiters, the nuclei of which are shown below the hair-cells. The nervous fila-
ments are distributed between these cells.

4. Hearing. — The acoustic nervous apparatus resembles somewhat
that which subserves the sense of touch. The receptive portion consists

18

274 NORMAL HISTOLOGY.

of a layer of epithelium containing two sorts of cells: first, ciliated
cells, which are somewhat flask-shaped and are called "hair-cells";
second, epithelial cells, the " cells of Deiters," which surround and
enclose the hair-cells, except at their free ends, and reach the sur-
face of the mucous membrane, where their ends are cuticularized.
These cells of Deiters extend from the surface of the membrane to
the basement-membrane, while the hair-cells extend only for a por-
tion of that distance.

The dendrites of the auditory nerve are distributed among these
cells, but are not in organic union with them (Fig. 257). In this
respect the auditory apparatus differs from the olfactory and resem-
bles the tactile. The nervous dendrites are processes of bipolar
ganglion-cells situated in the ganglia on the branches of the auditory
nerve. The neurites from those cells presumably carry the nervous
stimuli to the cerebrum. The bipolar cells are, therefore, analogous
to the posterior root ganglion-cells of the spinal nerves. Whether
this single neuron carries the nervous stimulus directly to the cere-
bral cortex cannot be stated, but it is probable that there is an inter-
mediate neuron in the tract of transmission, perhaps in the medulla
oblongata.

The external auditory meatus is lined with skin containing compara-
tively few papilla;. The ceruminous glands which secrete the wax
resemble the sweat-glands of the skin. The deep portions of these
coiled tubular glands have a rather wider lumen than the sweat-glands
and are lined with a single layer of cubical epithelium containing gran-
ules of pigmented material and fat destined to become constituents
of the cerumen. The ducts of the glands are lined with a double
layer of epithelial cells. Muscular fibres are present in the walls
of the glands, the arrangement being similar to that in the sweat-
glands. The skin extends over the outer surface of the tympanic
membrane, but here the corium is very thin and devoid of papillae.
The rete Malpighii is also thin. Beneath this skin, but separated by
a thin vascularized layer of areolar tissue, is the tympanic mem-
brane, consisting of two layers of fibrous tissue, an outer in which
the fibres run radially, and an inner composed of circular fibres.
The inner covering of the tympanic membrane is a single layer of
flattened epithelium resting on a small amount of vascularized areo-
lar tissue. The tympanic cavity (middle ear) is also lined with
flattened epithelium in a single layer which passes into ciliated epi-
thelium near the opening into the Eustachian tube, likewise lined

THE ORGANS OF THE SPECIAL SENSES. 275

with ciliated epithelium continuous with that of the nasal cavity into
which its other extremity opens.

5. Sight. — The receptive nervous organ of vision is the retina.
This has an extremely complicated structure, which may be divided
into the following nine layers :

1. The layer of pigmented epithelium, which lies next to the
choroid coat of the eye, and is, therefore, the most deeply situated
coat of the retina ; 2, the layer of rods and cones ; 3, the external
limiting membrane ; 4, the outer granular layer ; 5, the outer molec-
ular layer; 6, the inner granular layer; 7, the inner molecular
layer ; 8, the ganglionic layer ; 9, the layer of nerve-fibres.

Internal to the ninth layer is the internal limiting membrane,
which separates the retinal structures from the vitreous humor
occupying the cavity of the eyeball. The general character and
associations of these layers are shown in Fig. 258.

1. The layer of pigmented epithelium is made up of hexagonal
cells, which are separated from each other by a homogeneous
cement and form a single continuous layer upon the external sur-
face of the retina. They are in contact with the rods and cones
of the next layer, and send filamentous prolongations between those
structures. The pigment lies within these filamentous processes
and the portion of cytoplasm continuous with them, but its position
varies with the functional activities of the organ. When the eye
has been exposed to light the pigment is found lying deeply between
the rods. When the eye has been at rest for some time the pigment
is retracted in greater or less degree within the body of the cell.

2. The rods and cones are the terminal structures of cells which
extend from the fifth layer to the first. The nuclei of these cells
lie within the fourth layer, to which they give a granular appear-
ance (Fig. 258).

3. The external limiting membrane is formed by the cuticularized
outer ends of certain sustentacular epithelial cells, the " cells of
Muller" (Fig. 258, x), which extend from this layer to the in-
ternal limiting membrane and serve to support the various elements
of the retina. The nuclei of these cells lie in the seventh layer, to
the granular character of which they contribute. The portion of
the cell which lies in the fourth layer of the retina is indented
with numerous oval depressions receiving the nuclei of the cells
carrying the rods and cones, which they both support and isolate
from each other. The filamentous cell-bodies of those elements

276

NORMA L HISTOLOG Y.

are also separated by the cells of Miiller. In the sixth and seventh
layers delicate processes from these cells serve a similar purpose,
and in the eighth layer their deep extremities fork to give support
to the ganglion-cells. Beyond the ninth layer the ends of these
forks expand and come in contact with each other at their edges
to form the " internal limiting membrane."

Fig. 253.

VIII.

Diagram of the retina. (Kallius.) I., pigmented epithelial layer; II., layer of the rods and
cones; III., external limiting membrane; IV., outer granular layer ; V., outer molecular
layer; VI., inner granular layer; VII., inner molecular layer; VIII., ganglionic layer;
IX., layer of nerve-fibres, z, pigmented epithelial cells; a, at the bottom of the external
limiting membrane, rods ; 6, cone cells ; c-h, ganglion-cells of the sixth layer connecting
the fourth layer with the eighth; i, horizontal cell sending a process into the seventh
layer; k-q, "spongioblasts," or neurons of the third type (Fig. 232); r-w, ganglion-cells
of the eighth layer; x, sustentacular cell of Miiller, with striated upper end forming
a part of the external limiting membrane; y, y, neuroglia-cells. It should be borne in
mind that in sections of the retina numerous elements of the various sorts here rep-
resented are crowded together to form a compact tissue. The centrifugal fibres which
reach the retina from the cerebrum are omitted from this diagram. They are distributed
in the inner granular or sixth layer. The light entering the eye passes through the layers
represented in the lower part of this figure before it can affect the rods and cones.

4. The fourth, or outer granular layer contains, as already stated,
the nuclei and elongated bodies of the cells that carry the rods and
cones of the second layer. The bodies of the former are almost
filamentous in character, but expand to enclose the oval nucleus,

THE ORGANS OF THE SPECIAL SENSES.

277

which lies at various depths in different cells. The cell-body
expands again near the external limiting membrane, through which
it passes to form the rod. At the other end the filamentous cell-
body terminates in a minute knob in the fifth layer of the retina.
The cells which form the cones have nuclei lying near the external
limiting membrane and cylindrical bodies terminating in a brush
of filaments in the fifth layer.

5. The outer molecular layer, also called the " outer plexiform
layer," owes its appearance to a multitude of filaments, part of which
have been described as the terminations of the cells bearing the rods
and cones, the rest being the terminations of nerve-processes spring-
ing from the cells of the sixth layer.

6. The sixth layer has a granular appearance, because of the
presence within it of the cells of a great number of short neurons.
These are of two sorts : first, those belonging to the first type, rep-
resented in Fig. 232, which have dendrites in relation in the fifth
layer with the filaments of the cells bearing the rods and cones, and
neurites that come into relation in the seventh laver with the den-
drites of ganglion-cells lying in the eighth layer ; second, neurons
of the third type, shown in Fig. 232, which, in this situation
have been called " spongioblasts." These, which we may regard
as association-neurons, form two groups : first, those which send

Fig. 259.

Diagram of the nervous mechanism of vision. (R. y Cajal.) A, retina; B, optic nerve; C,
corpus geniculatum. a, cone ; 6, rod ; c, d, bipolar nerve-cells of the outer granular layer ;
e, ganglion-cell ; /, centrifugal teleneurites ; g, " spongioblast " ; h, teleneurites from optic
nerve ; j, neuron receiving and further transmitting the nervous impulse ; r, cell trans-
mitting the centrifugal impression. The courses of nervous impressions are indicated
by the arrows.

processes into the fifth layer; and, second, these which send their
processes into the seventh layer ; but, aside from the neurons in-

278 NORMAL HISTOLOGY.

eluded in these two groups, there are certain cells (Fig. 258, i)
which send processes into both the fifth and the seventh layers.

7. The seventh, inner molecular or " inner plexiform " layer owes
its delicate structure to the fact that it is here that the teleneurites of
the cells in the sixth layer come into relations with the teledendrites
of the ganglion-cells of the eighth layer.

8. The eighth layer contains those ganglion-cells whose teleden-
drites receive impressions from the teleneurites derived from the
sixth layer, and send their neurites into the optic nerve. These
lieu rites form the chief constituent of the ninth layer of the retina.

It will be observed in Fig. 258 that the basal expansions of the
cells bearing the cones are mostly in relation with the teledendrites
of a single neuron of the sixth layer, and that this neuron is, again,
in close relations with the teledendrites of but one ganglion-cell of
the eighth layer. This arrangement would not favor a diffusion of
the impressions inaugurated in the cones. The arrangement is quite
different in the case of the cells bearing the rods.

The probable course of nervous impressions to and from the
retinal elements is represented in Fig. 259.

HISTOLOGICAL TECHNIQUE.

CHAPTER XX.

PRACTICAL SUGGESTIONS FOR THE CARE AND USE OF
THE MICROSCOPE.— MICROSCOPICAL TECHNIQUE.

In selecting a microscope the following considerations are of
importance :

The stand should be supported on three points and rest firmly on
the table; have a rack-and-pinion coarse adjustment, and a fine
adjustment free from all loss of motion. It is rarely used in an
inclined position, and a jointed stand is unnecessary. A triple
nose-piece, or revolver, is a great convenience, and an Abbe con-
denser with iris-diaphragm is almost indispensable.

Three objectives are needed : a Leitz No. 3 or No. 4, No. 7, and
Yoth or yV^1 °^ immersion, or their equivalents of other manu-
facture, are suitable powers for general use. Two oculars, No. 2
and No. 4, will answer.

The microscope should be protected from direct sunlight and acid
fumes, and be kept in a dry, moderately cool place. When not in
use it should be covered or placed in its case, to protect it from
dust. If the lenses become dirty, they may be wiped with a soft,
clean cloth or Japanese paper, cither dry or moistened with water,
and followed by a dry cloth or paper. Balsam or cedar oil may be
removed with a cloth or soft paper moistened with xylol, after which
the parts should be carefully wiped dry.

In making microchemical tests special care should be taken not
to let the reagents come in contact with the objectives.

Objects should always be examined in a liquid, unless there is
some special reason for examining them in a dry state ; and should
be covered with a cover-glass, unless a cursory inspection with a
very low power is all that is required.

279

280 HISTOLOGICAL TECHNIQUE.

In studying a specimen always use the lowest power that will
reveal the structures it is desired to see; and, in any event, use
a low power first, to get a general idea of the topography of the
specimen. In this way the portions for more minute study can be
readily selected, with a great saving of time.

The proper illumination of the specimen is just as important as-
careful focussing. If the Abbe condenser is in use, employ the
plane surface of the mirror during the day ; either the plane or the
concave surface when artificial light is used, selecting the surface
which causes less glare. The iris-diaphragm should be kept ad-
justed so as to give the best definition of the specimen under exam-
ination when the latter is in focus. It will be found that when
colorless objects are examined a small opening gives the clearest
picture, while with colored objects a larger opening is preferable.
A small diaphragm serves to bring out the "structure-picture" ; a
large diaphragm, the " color-picture " (see p. 284).

A bottle of oil of cedar-wood, having approximately the same
refractive index as the glass from which the cover-glasses are made,
is furnished with the immersion-objectives. When these are used
a drop of this oil is placed on the cover, and the end of the objec-
tive immersed in this drop. This arrangement permits the light to
pass from the object to the bottom lens of the objective without sen-
sible refraction, increasing the amount of light entering the objec-
tive, the sharpness of definition, and the purity of the color-picture.
When the lens has been used the oil should be removed with a soft
cloth or Japanese paper. The oil on the cover may be wiped off at
once, or it may be allowed to dry and then removed with a cloth
moistened with xylol.

Microscopical Measurements. — These may be made, with a fair
degree of accuracy, by means of an eye-piece micrometer-scale.
This is a ruled disc of glass that can be placed upon the diaphragm
within the ocular, where its scale should be well defined when seen
through the upper lens of the eye-piece. Special micrometer ocu-
lars are made which permit of focussing the scale, but these are
unnecessary if the diaphragms of the ordinary oculars are in the
right places within the eye-piece tubes. The value of the divisions
of the eye-piece micrometer-scale must be determined by comparing
it with the scale of a micrometer-slide which is placed upon the
stage of the microscope. These scales usually consist of 1 mm.
divided into hundredths, and the eye-piece scale will have dif-

MICROSCOPICAL TECHNIQUE. 281

ferent values for each combination of lenses used and for every
variation in the length of the microscope-tube. The unit for micro-
scopical measurements is one-thousandth of a millimeter, or one-
millionth of a meter; it is called a "micrometer," and is desig-
nated by the Greek letter [i. One division of the micrometer-slide
mentioned above would, therefore, equal 10 fi. From these data it
is possible to calculate the value of each division of the eye-piece
micrometer-scale in terms of fi for each combination of lenses, the
length of the microscope-tube being fixed. (Most Continental
stands and many American stands have graduated tubes, and the
objectives are constructed for a standard tube-length of 160 milli-
meters.)

It is well for the student to get into the habit of estimating the
sizes of the objects he examines. A good standard for mental com-
parison is the diameter of the unaltered red blood-corpuscle, which,
is about 7.5 //.

MICROSCOPICAL TECHNIQUE.

Useful preparations for study under the microscope may be pre-
pared from tissues in one of three ways : 1, simple scrapings of the
tissues may be mounted on a slide in the fluids derived from the
tissues themselves, or in a neutral solution — e. g., 0.75 per cent, salt
solution ; 2, the tissue-elements, cells, and intercellular fibres, etc.,
may be separated from each other by treatment with some macerat-
ing-rluid — e. c/., very weak chromic acid (1 : 10,000), 36 per cent,
caustic potash, ^ alcohol ; 3, sections of the tissue may be prepared
either while they are fresh, with a razor or a freezing-microtome, or
after hardening.

The first method has a limited application. It is serviceable
onlv when the tissue-elements are so looselv held together that thev
readily separate from each other and can be examined in an isolated
condition. This is the case with a considerable number of tumors,
the superficial tissues of mucous membranes, the spleen, etc. If
the inside of the cheek be scraped with the finger-nail, and the
material thus removed be diluted with saliva, placed upon a slide,
and covered with a cover-glass, the squamous epithelial cells lining
the cavity of the mouth will be readily seen in an isolated state.
An appropriate dye may now be introduced under the cover, and by
its means the nuclei of the cells stained, thus bringing them into
clearer view.

282 HISTOLOGICAL TECHNIQUE.

When a simple scraping of the natural or freshly cut surface does
not yield useful preparations, showing isolated tissue-elements, some
process of maceration may be employed. Bits of the tissue are
soaked for a time in some solution that serves to soften the cement-
substances lying between the elements of the tissues, so that they
may be easily separated with needles (teasing). Such specimens
are usually best examined when mounted on a slide in some of the
macerating-fluid. Many of the macerating-solutions not only favor
the separation of the constituents of tissues, but also preserve them,
so that a fair idea of their natural size and shape may be obtained
from such preparations. It is evident, however, that with this
method very little can be learned of their arrangement in the tis-
sues before they were separated, and a knowledge of that arrange-
ment is often of greater importance in the determination of the
character of the tissue than a knowledge of the exact shape and
size of the tissue-elements.

The third method, that of preparing sections of the tissues, is the
one most commonly employed, because it yields the most useful
results. The structural elements composing the tissues are seen in
their natural relative positions, and can be distinguished from each
other and identified by the use of dyes and other reagents that
affect them in some characteristic manner. But in order to ob-
tain useful sections the tissues must almost always undergo some
preliminary treatment with reagents, to give them a proper consist-
ency for cutting and to hold the tissue-elements together so that
the sections shall not fall apart after they have been cut. This may
be accomplished by freezing the tissue before cutting it ; but more
satisfactory results are obtained by causing a coagulation of the
albuminous substances and subsequently extracting some or all of
the water contained in the tissues. These changes in the tissues
give them a firmness which favors the preparation of very thin sec-
tions ; but sometimes even they are inadequate, and then the tissues
are usually impregnated with some substance, like paraffin or col-
lodion, which fills the interstices of the tissues and can then be
hardened, when it serves to hold the tissue-elements together and
retain them in their natural positions. The paraffin or hardened
collodion is cut with the tissues and keeps the sections from disinte-
grating. Before mounting the section, the substance used for im-
pregnation may be removed from the section, or it may be retained

MICROSCOPICAL TECHNIQUE. 283

permanently, since it is usually easily recognized in the specimen
and does not interfere with its study under the microscope.

The study of tissues by means of sections has the disadvantage
that the elements of the tissues are cut, and the sections contain the
resulting portions as well as complete elements. The incomplete
portions lie near and at the surfaces of the sections, where they are
in clearest view, while the uncut elements are situated in the body
of the section, more or less obscured by the overlying portions that
have been cut by the knife. Moreover, the tissue-elements may lie
obliquely to the plane of the section, so that only a portion of them
can be seen at a time, the rest being brought into clear view only
when the focal plane is raised or lowered. These circumstances and
the fact that the tissue-elements are frequently closely crowded
together make the interpretation of sections a matter of some dif-
ficulty in many cases. These difficulties are in a measure overcome
by examining sections of different thicknesses, but a more satis-
factory way of studying the structure of a tissue is to examine por-
tions after maceration as well as in section.

The processes of coagulation and dehydration, which have already
been mentioned as usual preliminaries to the cutting of sections,
deserve a few words in explanation of their purposes.

The coagulation of the albuminous substances in the tissues has
for its chief aim the preservation of the minute structure of the
tissue-elements, so that a lapse of time or the subsequent manipula-
tions of the tissues shall not cause an alteration in the details which
it is desired to study. If this precaution be omitted, the tissues
undergo post-mortem changes which seriously alter the appear-
ance of the elements of which they are composed. Coagulation
brought about for this purpose is called " fixation " of the tissues.
It may be induced in a variety of ways : the tissues may be sub-
jected to heat for a few moments, thus rendering the albumins they
contain both solid and insoluble ; but the more usual procedure is
to immerse the tissues in a solution of some substance that causes
rapid death with coagulation. These solutions are called fixing-
solutions, and not infrequently the substances they contain not only
cause death and coagulation, but also form a union with some of
the structural materials of the tissues which may facilitate their
subsequent recognition.

The number of formulae that have been devised for the prepara-
tion of fixing-solutions is very great, and some of the solutions are

284 HISTOLOGICAL TECHNIQUE.

better for the fixation of some tissues than for others. As a ruley
those solutions that most perfectly preserve the finer intracellular
details of structure have very little power of penetrating masses
of tissue. They can, therefore, only be employed when very small
bits of tissue are to be fixed. Other fixing-solutions penetrate
much better, but fail to fix the most delicate structures, which may
undergo changes before they are preserved. It follows that the
choice of the method of fixation must in each case depend upon
the object to be attained.

The removal of water from the fixed tissues is accomplished by
means of alcohol. The fixing-agents are nearly all aqueous solu-
tions, and while they increase the consistency of the tissues to a
certain extent, they do not usually render them sufficiently firm for
the preparation of thin and uniform sections. If the water in the
tissues be replaced by alcohol, a greater and more uniform con-
sistency is obtained, and the tissues are also partly prepared for
impregnation with an embedding-material (collodion or paraffin)
should that be necessary for section-cutting.

After sections of fixed tissues have been obtained they usually
require staining before they can be profitably studied. The chief
reason for this will appear in the following explanation :

When a specimen is examined under the microscope differences
in structure among the colorless elements of the specimen may be
seen, or differences in color between the different elements may be
perceptible. We may, then, distinguish between a "structure-
picture," due to differences that are not those of color, and a " color-
picture," due solely to such differences. The manner in which the
latter is produced is, perhaps, self-evident. The structure-picture
is the result mainly of differences in refraction due to the various
densities of different parts of the specimen. But the processes of
fixation and hardening have for their purpose the rendering of the
tissues of a relatively uniform density. They must, in consequence,
tend to obliterate the details of the structure-picture which the
sections yield when viewed under the microscope. For this reason
the sections are stained, which converts the structure-picture into a
color-picture.

The substances composing the tissues have various affinities for
dyes, and it is possible to take advantage of this in staining sec-
tion-, so that structures of the same nature shall receive one color,
while those of different composition shall be dyed of a different

METHODS OF FIX. I HON. 285

hue or an entirely different color. The coloring-matters, when so
•employed, not only bring out the structure of the tissue by creating
a color-picture, but they also serve as valuable reagents in revealing
the nature of the substances to which they impart a color. Again,
it is often necessary that a certain method of fixation or other pre-
liminary treatment should be used before the particular dye selected
can display its greatest selective power for a particular substance.
These facts explain the great number of formulae for stains and the
preparation of specimens that are found in the technical text-books
and journals. The subject has become so expanded within recent
years that it has almost created a distinct branch of learning ; but
it will only be necessary for the student of medicine to acquire a
knowledge of a few methods that will serve to reveal the general
structure of cells and the characters of the intercellular substances.
The general outline of the procedures in common use for this pur-
pose are as follows: 1, fixation; 2, hardening; 3, impregnation;
4, embedding ; 5, cutting ; 6, staining ; 7, dehydration ; 8, clearing ;
9, mounting.

Some methods of preparation combine one or more of these steps
in a single manipulation, thus considerably reducing the time requi-
site for the completion of the process. Other methods necessitate
the intercalation of still other manipulations, or the subdivision
•of those already enumerated.

Methods of Fixation.
1. Miiller's Fluid. — This classic fixing- and hardening-solution con-
sists of potassium bichromate, 2.5 per cent., and sodium sulphate,
1 per cent., dissolved in water (preferably distilled water). It is
slow in action, requiring from six to eight weeks for the preservation
of an average specimen, but with proper care can be made to yield
excellent results when the finer details of structure are not to be
studied. It is important to use large quantities of the fluid, at
least ten times the volume of the tissues immersed in it, and to
renew the fluid so frequently that its strength shall be constantly
maintained. When fresh tissues are placed in Miiller's fluid they
speedily render it cloudy. This is a sign that the fluid should be
renewed, even if only an hour has elapsed since the tissues were
placed in it. When cloudiness no longer appears the fluid should
he renewed once a dav for the first two weeks : after that, two or
three times a week till the process is completed.

286 HISTOLOGICAL TECHNIQUE.

After fixation in Mailer's fluid specimens should be washed in
running water over night, or for twenty-four hours, and then hard-
ened in alcohols of progressively greater strengths. While in the
weaker alcohols specimens should be kept in the dark, to avoid
the formation of precipitates, which occur under the influence of
light. Pieces of tissue placed in Midler's fluid should not be more
than 1 cm. in thickness.

Two excellent modifications of Midler's fluid have been devised
by Zenker and Orth for the purpose of hastening the fixation and
of securing a more faithful preservation of structural detail.

2. Zenker's Fluid. —

Potassium bichromate,

2.5 grams

Sodium sulphate,

1 gram.

Mercuric chloride,

5 grams

Distilled water,

100 cc.

To this stock solution 5 per cent, of glacial acetic acid is to be
added just before use of the fluid.

Zenker's fluid fixes tissues in from three to twenty-four hours.
The pieces should not be more than 5 mm. thick, and after fixa-
tion should be washed for several hours in running water and then
hardened in alcohol.

This solution possesses the disadvantage that a precipitation of
mercury or some mercurial compound is likely to take place within
the tissues. This deposit may be, at least in great measure, removed
from the tissues by adding a little tincture of iodine to the harden-
ing-alcohols. The iodine combines with the mercury and produces
a soluble compound, which is dissolved out by the alcohol. As the
iodine disappears from the alcohol the latter becomes bleached, and
fresh tincture must be added until the alcohol remains permanently
tinged. If, after sections of the tissue have been prepared, they
are found to contain a mercurial deposit, this can be removed by
treatment with dilute iodine tincture or with Lugol's solution.

3. Orth's Fluid.—

Potassium bichromate, 2.5 grams.

Sodium sulphate, 1 gram.

Distilled water, 100 cc.

This stock solution is Miiller's fluid. Before use, 10 cc. of for-

METHODS OF FIXATION. 287

maldehyde (4CT per cent.) is to be added to every 100 cc. of the
Miiller's fluid.

Orth's fluid fixes in three or four days. The pieces of tissue
should not be more than 1 cm. thick. The time for fixation can
be shortened if smaller pieces are used and the process is carried
on at a slightly elevated temperature ; e. (/., in an incubator kept at
37° C. (98.6° F.). After fixation the specimens should be washed
in running water, as in the previous methods.

4. Mercuric Chloride Solution. — A saturated solution of corrosive
sublimate in 0.5 per cent, salt solution is prepared by heating an
excess of sublimate crystals in the salt solution and allowing the
mixture to cool. The clear fluid is decanted from the crystals when
desired for use. The penetration and action of the solution are
favored by the addition of 5 per cent, of glacial acetic acid at the
time of using. The thickness of the pieces of tissue should not
exceed 5 mm., and much thinner pieces are better. Fixation takes
place within six hours, after which the tissues may be washed in
running water, or placed at once in 70 per cent, alcohol. If acetic
acid has been used, it is best to wash in water before immersing in
alcohol. Tincture of iodine should be added to the alcohol for the
reasons given in the description of Zenker's fluid.

5. Formaldehyde. — This gas is capable of being absorbed by
water to form a 40 per cent, solution, but its volatility renders such
a solution liable to deterioration. The strength employed for fixa-
tion is usually 4 per cent., and may be prepared by adding 10 cc.
of 40 per cent, formaldehyde to 90 cc. of distilled water. A 0.75
per cent, solution of common salt may be substituted for the distilled
water with possible advantage and the addition of about 2 per cent,
of acetic acid is also advantageous.

Formaldehyde penetrates deeply and quickly into the tissues,
which may be 1 cm. in thickness, and accomplishes fixation within
twenty-four hours, but the preservation of structural detail is not
very perfect. The solution is useful where the general characters
of the tissues are to be determined and the details of the cells are
of comparatively little consequence. After fixation the tissues may
be washed in water, or placed directly in 70 per cent, alcohol ; or
frozen sections may be at once prepared. Satisfactory sections may
be obtained from small pieces of tissue if they are put in the for-
maldehyde solution for an hour or two and then cut with the
freezing- microtome. After they have been washed for a short time
in water they may be stained by any of the more usual methods.

288 HISTOLOGICAL TECHNIQUE.

(j. Flemming's Solution. — Tins is a solution containing osniic acid,
chromic acid, and acetic acid. It does not keep well, and it is best
to prepare it just before it is to be used. For this purpose the
following stock solutions may be kept on hand :

A. 2 per cent, solution of osmic acid in 1 per cent, chromic acid.

B. 1 per cent, solution of chromic acid in distilled water.
Osmic acid mav be bought in sealed tubes containing either i or 1

gram. To prepare the stock solution " A," the tube should be washed
on the outside and a deep file-scratch made near its centre. It should
then be broken into a bottle containing 50 cc. of a 1 per cent, solution
of chromic aid in distilled water. The halves of the tube should be
dropped into the bottle and its contents shaken at intervals until
solution is effected. This solution had best be kept in the dark
to avoid decomposition of the osmic acid. When required for use,
prepare the Flemming's solution by mixing :

Solution " A," 4 cc.

Solution " B," 15 "

Glacial acetic acid, 1 "

Flemming's solution is especially useful for fixing the finer details
of structure within the cell. It was devised for the preservation
of the mitotic figures formed during karyokinesis, but its range of
usefulness far exceeds that limited application. Its power of pene-
tration is very slight and the pieces of tissue selected for fixation
must be small. They should not exceed 2 mm. in their least
measurement, and thinner pieces are apt to give better results.
Owing to the presence of osmic acid, Flemming's solution stains
fat a dark-brown or black color, and may be used as a reagent for
the identification of fatty substances.

Tissues should be left in Flemming's solution for about twenty-
four hours, though twice that length of time would cause little if
any harm. They must then be thoroughly washed in running water
for twenty-four hours or longer, and hardened in alcohol. Since
Flemming's solution is usually employed for the study of the
individual cells, it is desirable to prepare very thin sections of the
tissues that have been hardened in it. For this purpose embedding
in paraffin is the best method.

The foregoing fixing solutions will meet most of the requirements
of the practitioner of medicine, but it frequently happens that he

METHODS OF FIXATION. 289

would like to obtain speedy results from a microscopical examina-
tion without running the risk of loss of material or of poor results.
When this is the ease he may use absolute alcohol as a fixing-agent,
thus taking advantage also of its ability to harden tissues and tit
them for rapid embedding in eollodion or paraffin.

7. Absolute Alcohol. — If fresh tissues are placed in strong alcohol,
say 95 per cent., they are hardened ; but during the process there
is an opportunity for the albuminous fluids in the tissues to escape
to a certain extent, and for shrinkage to take place in consequence.
If absolute alcohol be employed, it causes such rapid coagulation
that this leaching of the tissues does not take place. It is neces-
sary, however, that the alcohol should remain of nearly its original
strength, otherwise the water in the tissues will dilute it sufficiently
to destroy this coagulating action.

An excellent means for maintaining the strength of the alcohol
is to keep a layer of anhydrous sulphate of copper in the bottom of
the vessel. Crystals of cupric sulphate have a deep-blue color and
contain 36 per cent, of water of crystallization which can be removed
by heat ; the anhydrous salt has a dirty white color. Both the
anhydrous and the hydrated salt are insoluble in alcohol. If, then,
crystals of cupric sulphate are heated in an oven until the blue color
disappears, and the resulting anhydrous salt is brought in contact
with alcohol, any water which the latter may contain will be removed.
When the sulphate has become hydrated, its deep-blue color will be
restored. It may then be removed and the water again expelled by
heat. Care should be taken to allow any finely divided sulphate to
settle completely to the bottom of the jar before the supernatant
alcohol is used, lest particles come in contact with the tissues im-
mersed in the alcohol. Take a small jar with a tightly fitting cover
(a museum jar, holding six or eight ounces, will answer). Place the
anhydrous cupric sulphate in the bottom and then nearly fill with
absolute alcohol. When all the sulphate has settled, a few pieces of
crumpled filter-paper are placed upon it and overlaid with a smooth
piece placed so as to slant a little. The latter should lie near the
surface of the alcohol, but be wholly submerged. Small pieces of
the tissue to be fixed are placed upon the filter-paper where they
will be covered by the alcohol. The alcohol immediately coagulates
the albuminous substances on the surface of the pieces and then
gradually replaces the water in the specimen, coagulating the deeper-
seated albumins as it penetrates the tissues. The expelled water

19

290 NORMAL HISTOLOGY.

sinks to the bottom of the jar owing to its greater specific gravity,
and is at once taken up by the copper sulphate.1

It will be seen that this method not only fixes the tissues, but also
quickly dehydrates them. The real dehydrating-agent is, however,
the cupric sulphate, the alcohol serving merely as a vehicle for
conveying the water from the specimen to that salt. If the pieces
of tissue are small, not over 5 mm. thick, they will be hardened by
remaining in the absolute alcohol over night, and mounted sections
may be ready for examination by the next afternoon.

8. Fixation by Boiling. — Throw small pieces of the tissue, not
larger than 1 cm., into boiling 0.75 per cent, salt solution. Keep
them at the temperature of boiling for two minutes. Then throw
them into cold water. They may then be cut with the freezing-
microtome, or may be placed in 70 per cent, alcohol for hardening.
This method is excellent for the detection of albuminous exudates
within the tissues, but it causes so much shrinkage that it is not
useful for general purposes.

Reference to the literature reveals a great number of formulae for
fixing solutions. These formulae are nearly all empirical, and not
based on any clearly defined knowledge of the utility of the various
ingredients. Of late years there has been a growing tendency to use
acetic acid in the preparation of these fluids, and the mode of action
of their different constituents has received careful attention. It
appears that acetic acid may be useful in two wrays : first, by virtue
of its rapid penetration ; and, second, because it renders the proteid
substances in the tissues coagulable by other constituents of the fix-
ing solution — e.g., potassium bichromate, which, in neutral mixtures,
does not of itself coagulate these substances. If these conclusions be
correct, the addition of acetic acid to Muller's fluid should make it a
more valuable fixing solution, and its action would then be as follows :
the acetic acid penetrates rapidly and kills the cells ; it also renders
the proteids coagulable by the bichromate of potassium. The latter
salt penetrates but slowly, and at first acts in very slight concentra-
tion, not sufficient to cause gross coagulation, but nevertheless enters
into a combination with the proteids which, as the process goes on,

1 A jar of absolute alcohol, prepared as above, may be used for the purposes of
fixing or hardening until the sulphate of copper has become of a deep-blue color,
or the alcohol so impregnated with dissolved fat that the latter interferes with
embedding in celloidin. When the latter is the case, the hardened celloidin is
opaque or opalescent.

METHODS OF HARDENING. 291

renders them insoluble. Similar events occur when Flemming's
solution is used for fixation, but here the osmic acid also enters into
some form of combination with the proteids and fats. It is not
unlikely that Orth's fluid owes its value in part to the formic acid
which formalin contains as the result of oxidation of the formalde-
hyde ; this acid having an action similar to that of acetic acid.
These recent analyses of the action of fixing solutions tend to render
many of the old formulae superfluous. Probably, all that is desirable
may be obtained by the use of three fixing solutions : first, Flem-
ming's solution, which fixes admirably, but penetrates with difficulty
owing to the slow diffusion of the osmic acid, and renders the tissues
relatively refractory to dyes ; second, a formula containing potassium
bichromate or corrosive sublimate and acetic acid, such as Zenker's
fluid, which penetrates well and leaves the tissues in a condition
favorable for staining ; third, diluted formalin to which about 5 per
cent, of acetic acid has been added. This solution penetrates rela-
tively larger pieces of tissue readily and does not weaken their
affinity for dyes, but the fixation of the tissue elements is less
perfect owing to the comparatively slight coagulating action of the
formalin.

Methods of Hardening-.

Solutions of chromates, as Muller's fluid, will, after a time, con-
fer a pretty firm consistency upon tissues, and even render them
brittle. Tissues fixed in corrosive sublimate are also very much
hardened. But the usual practice is to harden specimens in alcohol
after fixation. To obtain the best results this hardening should be
done gradually, since immersion in strong alcohol is apt to produce
undesirable shrinkage, affecting the various tissue-elements in dif-
ferent degree.

Seventy per cent, alcohol (736 cc. 95 per cent, alcohol to 264 cc.
water) is weak enough to begin with. After the tissues have been in
alcohol of that strength for twenty-four to forty-eight hours, accord-
ing to the size of the pieces, they are placed in 80 per cent, alcohol
(842 cc. 95 per cent, alcohol to 158 cc. water) for an equal length
of time, and then in 95 per cent, alcohol. From the 95 per cent,
alcohol thev are placed in absolute alcohol, if it be desired to embed
them in either collodion or paraffin. If they are not intended for
immediate use, they may be kept indefinitely in 80 per cent, alcohol.

During the hardening it is best not to allow the tissues to rest
on the bottom of the vessel containing the alcohol, as they are

292 NORMAL HISTOLOGY.

liable to slight maceration in the alcohol, diluted with water from
the specimen. They can be kept off the bottom by means of a
little crumpled filter-paper. Specimens that have been fixed in a
chromatic solution should be kept in the dark while being hard-
ened ; those that have been fixed in corrosive sublimate should be
hardened in alcohols to which a little tincture of iodine (sufficient to
give them a sherry color) has been added. When absolute alcohol
is used, its strength should be maintained by contact with quick-
lime (see directions for fixing tissues in absolute alcohol).

Methods of Impregnation.

When tissues are so porous or friable that sections are likely to
tear or disintegrate it is desirable to impregnate them with some
embedding-material. The most useful substances for this purpose
are collodion, or celloidin, and paraffin. Whichever of these is
used, it is necessary to remove the water from the specimen before
the impregnation can be accomplished, for both collodion and
paraffin are insoluble in water. Tissues that have been hardened
in alcohol are to a certain extent already dehydrated. The residual
water may be removed or reduced to a trace by treatment with
absolute alcohol, in which collodion is soluble.

The "celloidin" manufactured by Sobering is an excellent prep-
aration of gun-cotton, but almost equally good results may be
obtained by using the more economical soluble cottons employed by
photographers. Two solutions in a mixture of equal volumes of
ether and absolute alcohol (both, if possible, of Squibb's prepara-
tion) should be kept in stock : one, a weaker solution, having about
the consistency of thin mucilage ; the other, a stronger solution,
resembling a syrup.

Collodion is soluble in absolute alcohol, so that tissues containing
only that fluid are ready for impregnation without further prelim-
inary treatment. When thorough impregnation is desired the tissues
should be immersed in equal parts of ether and absolute alcohol for
a few days, and then in the weaker solution of celloidin or collodion
for a number of days or weeks — the longer the better;1 but such
complete impregnation is often unnecessary, and soaking for a day
or two will often suffice if the sections to be made need not be very
thin. It is not possible, in any event, to make very thin sections

1 Impregnation may be greatly hastened if done at the body-temperature in a
hermetically closed vessel.

METHODS OF IMPREGNATION. 293

from tissues embedded in colloidin ; but sections of large area may
be obtained, which is often of greater importance. For very thin
sections it is better to use paraffin for the embedding-material,
although the resulting sections will have to be smaller.

Paraffin is insoluble in alcohol of all strengths. It is therefore
necessary to remove the absolute alcohol from the tissues before
they can be impregnated with paraffin. This may be done by
immersing the tissues in some liquid that is a solvent for paraffin
and is also miscible with alcohol. For this purpose, xylol, chloro-
form, or oil of cedar-wood may be used. Xylol yields the most
rapid results, but its use is contraindicated when it is desired to
retain fatty substances that have been colored with osmic acid, as
the xylol extracts them. If their preservation within the tissues
is important, chloroform should be used ; but the sojourn even in
that liquid should be as short as possible. Oil of cedar-wood prob-
ably causes less change in tissues than chloroform, but the method
is more protracted, and, requiring longer treatment in the paraffin-
oven, probably has little ultimate advantage over chloroform for
general purposes.

If xylol is used, the tissues are transferred from the absolute
alcohol to xylol, on which they at first float. Subsequently they
sink, and are gradually rendered transparent as the alcohol is
expelled by the xylol. When there are no opacities left the speci-
men is ready for the paraffin-oven. These changes take from two
to twenty-four hours.

The treatment with chloroform is similar to that with xylol, but
after the tissues have been cleared in chloroform (six to twenty-four
hours) they are immersed in a saturated solution of paraffin in
chloroform for about the same length of time. They are then ready
for the paraffin-oven.

When oil of cedar-wood is used the pieces should be soaked in
two successive portions of the oil, about twelve hours in each, to
insure removal of the alcohol.

The foregoing steps are all preliminary to the actual impregnation
with paraffin.

It is important that the paraffin used for impregnation and
embedding should have a wax-like, and not a crystalline, texture,
and that its melting-point should be such that its consistency will
be favorable for cutting at the average temperature of the labora-
tory. Griibler, of Leipzig, furnishes excellent qualities of paraffin.

294 NORMAL HISTOLOGY.

For a room-temperature of 20° C. (68° Fah.) a variety melting at
50° C. (122° Fah.) will give good results. If the laboratory is
warmer, a paraffin of* higher melting-point should be used.

Impregnation is accomplished by placing the bits of tissue in a
bath of melted paraffin maintained at a temperature only slightly
above that of fusion, say 52° C. (125.6° Fah.), if the paraffin melts
at 50° C. (122° Fah.). This may be accomplished in a water-
jacketed oven provided with a thermoregulatory or upon a plate of
brass or copper, resting on a tripod and heated at one end by a
burner. When the latter method is employed the paraffin is melted
in a little glass dish, which is moved along the plate until a point
is found at which the paraffin remains melted at the bottom, but is
covered at the edges of the surface with a thin layer of congealed
paraffin.

The length of time that the specimens should remain in the
melted paraffin will vary with the character of the tissues and the
method of getting rid of the alcohol which has been employed. It
should not be protracted longer than necessary for complete impreg-
nation, as heat is injurious to the tissues. When xylol has been
used two hours will usually suffice, especially if the pieces of tissue
are transferred to a fresh paraffin-bath after about an hour. This
renewal of the paraffin is still more important if oil of cedar- wood
has been used. Chloroform requires about the same time as xylol,
and the tissues should be transferred to fresh paraffin once or
twice.

When impregnation has taken place and the final bath of paraffin
no longer has the slightest odor of the clearing-agent, the pieces of
tissue are removed from the bath with warmed forceps and are ready
for immediate embedding (vide infra), or may be placed on smooth
writing-paper, to which they adhere. A designation of the specimen
may be written on these papers, and the tissues kept in this condition
until required for cutting. They then are embedded.

Methods of Embedding1.

The object of embedding is to surround the piece of tissue from
which sections are to be cut with a mass of the embedding-sub-
stance, which then not only supports the tissue when it comes in
contact with the knife, but also affixes it to a block or other support
which can be fitted into the clamp of the microtome.

Microtomes designed for cutting paraffin usually have special

METHODS OF EMBEDDING. 295

supports for the embedded specimen, but blocks of hard wood may
be used in their place.

For the support of tissues embedded in collodion blocks of plate-
glass are probably both better and cheaper than those made of other
materials. They may be easily prepared from waste pieces of plate-
glass, about a quarter of an inch thick, and " obscured " or ground on
one surface. The glass may be cut into blocks of any desired size by
scoring the smooth side with a diamond and then splitting the pieces
apart with a sharp blow from a wedge-shaped hammer. The em-
bedded specimen is affixed to the rough surface of these blocks by
means of collodion, and the blocks may be numbered with a lead
pencil upon the rough surface. The writing will be preserved from
obliteration by the specimen subsequently placed upon it, and can
be read through the glass.

1. Embedding in Collodion (or Celloidin). — Tissues of firm con-
sistency and moderately uniform structure, such as liver, kid-
ney, and the majority of tumors which have been hardened,
may be embedded without previous impregnation. Before this
can be done, however, they must be either dehydrated with abso-
lute alcohol, or soaked for a few hours in a mixture of equal
volumes of ether and alcohol (95 per cent, alcohol will answer,
if absolute alcohol is not to be had). For this rapid method the
bottom of the piece of tissue must be flat and parallel to the plane of
the desired sections. When the necessary trimming of the speci-
men is completed moisten it with absolute alcohol or the ether-
alcohol mixture, then dip it in the thick solution of gun-cotton and
place it at once upon the ground surface of the glass block (pre-
viously labelled). In a few minutes the collodion will have evap-
orated sufficiently for the formation of a distinct pellicle upon its
surface. When this has become firm enough to withstand gentle
pressure immerse the block and specimen in several times their vol-
ume of 80 per cent, alcohol. This will harden the collodion, and in
the course of a few hours the specimen will be ready for cutting.

Tissues impregnated with collodion had best be embedded by a
slower process than the foregoing, although that method will answer
where only a slight support of the tissue-elements within the speci-
men is needed. A gradual concentration of the collodion within
the tissues may be brought about in the following manner :

Smear the inside of a small, straight-sided glass dish with a trace
of glycerin and then fill it with enough moderately thick collodion

296 NORMAL HISTOLOGY.

to cover the pieces of tissue with a layer about one-quarter of an
inch deep. Now place the specimens that have been in thin collo-
dion in the dish, with the surfaces from which sections are to be cut
resting on the bottom. Place the dish in a larger vessel with higher
sides and loosely cover the latter. The ether and alcohol in the
collodion will gradually evaporate, and their vapors will first fill the
outer vessel and then overflow its sides. The depth of the outer
vessel keeps these vapors in contact with the surface of the collo-
dion, preventing the formation of a pellicle, which would retard
evaporation and also favor the formation of bubbles in the collo-
dion. After an interval of one or more days the collodion will have
a gelatinous consistency. It should be allowed to become so hard
that it has considerable firmness, but is still soft enough to receive
an impression of the ridges in the skin when pressed with the
finger. The outer vessel is then partly filled with 80 per cent,
alcohol so that the whole of the inner dish is submerged.

By the next day the collodion will be hard enough for removal
from the dish. With a small scalpel, held vertically, divide the
hardened mass of collodion into portions, each of which contains
one of the pieces of tissue (for several pieces may be embedded in
the same dish, provided care be taken to preserve their identity).
Remove the pieces and trim down the collodion around the speci-
mens, leaving a margin of about an eighth of an inch. Trim the
top surfaces of the collodion parallel with the bottom surfaces, then
dip the trimmed surface into a little absolute alcohol contained in
a watch-glass, in order to dehydrate it. This will take about two
minutes. Label glass blocks with lead-pencil, place a drop of
thick collodion on the writing, and transfer the embedded specimens
immediately from the absolute alcohol to the drop of collodion,
pressing it into contact with the glass. When a good pellicle has
formed on the collodion drop the whole block into 80 per cent,
alcohol. If the block of hardened collodion containing the speci-
men be sufficiently dehydrated on the surfaces coming in contact
with the drop of collodion, and the latter have not time for the
formation of a pellicle before it receives the block, there will be no
difficulty in cementing the embedded specimen to the roughened
surface of the glass. It is best not to cut sections until the day
after the specimen has been affixed to the glass block. These
blocks, with attached specimens, may be preserved indefinitely in
80 per cent, alcohol.

METHODS OF EMBEDDING.

297

The thin coating of glycerin on the inside of the embedding-dish
serves the purpose of preventing the collodion from sticking to the
glass.

2. Embedding in Paraffin. — The specimen should first be trimmed
so as to have one surface parallel to the plane of the future sections.
This trimming had best be done before the specimen is placed in the
first paraffin bath. When specimens are to be embedded directly
from the paraffin bath, the simplest method is to employ a small box
made from rather heavy writing-paper by folding it in the follow-
ing way (Fig. 260) : Suppose the box is to be 2 inches long, 1 inch

Fig. 260.

(3)

r ■

I

\

\
\

\
\

N
\
X
\
\
N
\

(4)
A B

y
/
/

/
/
/
/

/
/
/
/
/

/

/
/
/
/

/
/

/

s

/

/
/
/

\

\

\
\
\
\
\
\
\
\
\
\
\
\

\,

\

/

/
s

/
/
/
/

/

(3)

C D

The upper half of the diagram represents the creases in the paper which are produced by
folding; the lower half shows the last fold on the right and the next to the last on the
left. The bottom of the box is the rectangle A, B, C, D.

wide and £ inch deep. Cut a piece of paper 2^- inches wide
(= width of box + twice its depth) and 4^- inches long (= length
of box + twice its depth + 1 inch to allow ^ inch fold at each end).
Crease the paper, longitudinally, f inch from and parallel to the edge
by folding and pressure (folds 1-C and 1-D). Then open these

298 NORMAL HISTOLOGY.

folds and flatten the paper again. Next, make a transverse fold 11
inch from one end of the paper (folds 2-2, also partly represented
by C-D), and then fold in the corners so that the creases make angles
of 45 degrees with the edges of the paper (folds 3-C and 3-D).
When this has been done, make the fold 4 by turning down the free
edge of the paper so as to keep the corners in place. Treat the other
end of the paper in the same way. The folded edge 4 and its
counterpart (4) can now be drawn apart and, after a little manipula-
tion, the box will assume its final shape. This box is nearly filled
with melted paraffin kept fluid by placing the box on top of the
paraffin oven. The bits of tissue are placed in the paraffin with the
surfaces from which sections are to be made resting upon the bottom
of the box. The pieces of tissue should not touch the sides of the
box. During these manipulations the forceps should be kept warm
enough to keep the paraffin melted. When the tissues are in proper
position the box is floated on cold water, which rapidly chills the
paraffin. When cold, the paper can usually be readily stripped from
the block of paraffin. If there is any difficulty due to a softening
of the paper by water, drying will restore a sufficient firmness to the
paper. It is important that the paraffin should cool rapidly, and
that enough should be originally placed in the box more than to
cover the specimens completely, as some shrinkage takes place on
cooling. If more than one specimen is embedded in the same box,
they should be well separated from each other, so that irregular
cleavages of the paraffin when it is subdivided may not bring the
surfaces of the individual blocks too near the embedded specimen.
A little study of the method of folding paper to make boxes will
enable the reader to make a box of any desired dimensions.

If the specimens to be embedded have previously been impreg-
nated with paraffin and then preserved on pieces of paper as described
above, any excess of paraffin upon the specimen may be removed
by placing it on a piece of filter-paper and warming it gently until
the superfluous paraffin is absorbed by the paper. The trimmed
surface is then laid upon a small glass plate that has been smeared
with a mere trace of glycerin, and metallic right-angles, similarly
smeared on the inside, are placed around the specimen in such a way
as to form a box with a clear space at least an eighth of an inch broad
between its sides and the specimen. Melted paraffin, at a temperature
only slightly exceeding that necessary to keep it fluid, is then poured
into the box, filling it. The paraffin should now be made to cool

METHODS OF CUTTING. 299

as rapidly as possible, in order to prevent its becoming crystalline.
For this reason it is well to prepare the box formed by the plate of
glass and the metallic right-angles in the bottom of a deep soup-
plate or some similar vessel. After the box has been filled with
melted paraffin cold water may be poured into the plate until its
surface is nearly on a level with the top of the box, and when the
top of the paraffin has congealed the plate may be filled with cold
water. After a few minutes the box may be taken apart and the
block of paraffin left in the water to become cold.

These paraffin-blocks may be labelled with a needle and kept
indefinitely in the dry condition, at a temperature below that at
which the paraffin softens. When they are to be used the bottom
of the block should be trimmed parallel with the top, sufficient
paraffin being removed to obliterate the hollow which formed when
the paraffin solidified. This trimmed surface is then made to ad-
here to the paraffin-support of the microtome, or a block of hard
wood, by means of a heated scalpel.

It often happens that little air-bubbles are present in the paraffin
close to the specimen, or that cracks exist between the specimen
and the surrounding paraffin, owing to the retention of a little air
at the time of embedding. These defects can be remedied by melt-
ing the paraffin with a heated needle. It is important that the
paraffin should everywhere be in perfect contact with the specimen.
When this repairing, if necessary, has been done and the paraffin
has become cold again, the block should be trimmed so that the
specimen, or at least its upper part, is contained in a little cubical
mass resting on the main block, with a margin of paraffin, about
1 mm. thick at the places where the edges of the cube are nearest
to the specimen. Those edges should be straight and at right angles
to each other, and the sides of the trimmed cube should be vertical.
In trimming the block only thin slices should be removed at a time,
in order to avoid cracking the paraffin forming the small cubical
mass enclosing the specimen.

These manipulations prepare the specimen for cutting.

Methods of Cutting.

It is possible to obtain useful sections from fresh or hardened
tissues by free-hand cutting with a sharp razor; for this purpose the
razor should either be very hollow ground, so as to have a thin
blade, or the lower surface should be ground flat. In stropping

300 NORMAL HISTOLOGY.

the razor, or microtome-knife, the stroke should be from point to
heel during both the forward and return motions. In cutting,
the edge should be used from heel to point, and this same motion
should be used in honing. A wire arrangement is usually furnished
with microtome-knives, which is intended for use while honing or
stropping. It serves to raise the back of the knife when the flat
side is sharpened, and should always be employed. Care must be
taken not to press the knife against the strop, as this is liable to
turn or blunt the edge. A few light strokes on the strop immedi-
ately after each day's use will keep the knife sharp and coat it with
a little grease, protecting it from rust. A microtome-knife should
never be allowed to rest with its edge on any hard surface ; the mere
weight of the knife is sufficient to spoil its edge.

In cutting free-hand sections of fresh tissues the upper surface
of the razor should be kept flooded with normal (0.75 per cent.)
salt solution. The sections float in this fluid and are kept from
tearing. Each section should be removed by a single stroke of the
razor. When hardened specimens are cut, 80 per cent, alcohol
should be used instead of salt solution.

Free-hand sections cannot be made either so thin or uniform as-
sertions prepared with a microtome, and these instruments are now
so cheap that they are universally used. There are three principal
forms: 1, freezing-microtomes ; 2, paraffin-microtomes; 3, micro-
tomes for cutting sections of tissues embedded in collodion. The
last are often fitted with attachments intended for use in cutting
frozen sections, and can also be used for paraffin. But the best
results are obtained by using instruments especially designed for
each purpose.

1. Frozen Sections. — Freezing is usually employed when sections of
fresh tissues are to be made, but hardened tissues may be cut with
a freezing-microtome if the alcohol be first removed by soaking for
a considerable time in water. The tissue may be placed upon the
plate of the microtome in a little water or neutral salt solution ; but
a better method is first to soak the tissue in a syrupy solution of
gum-arabic, and to moisten the plate with the same before freezing.
This solution freezes in less coarsely crystalline form than water or
salt solution.

When the tissues are frozen, thin sections are removed Avith a
quick forward and slightly oblique stroke of the knife. The motion
is intermediate between that of a plane and a single stroke of a saw.

METHODS OF CUTTING. .'JOl

The sections are floated from the knife in a dish of water or normal
salt solution ; or they may be fixed in a 4 per cent, solution of for-
maldehyde. The frozen tissue must not be too hard. Should that
be the case, the upper surface may be moistened by means of a
camel's-hair brush, dipped in water or salt solution, or warmed
with the breath.

2. Collodion-sections. — The block upon which the embedded speci-
men is fastened is secured in the clamp of the microtome in such
a position that the sections will be made in the desired plane.
The knife is then adjusted on its carrier in an oblique position, so
that the greatest possible length of its edge will be utilized in cut-
ting. The upper surface of the knife is flooded with 80 per cent,
alcohol, and slices are removed with the knife until the desired
level of the specimen has been reached. Sections are then made
as thin as is compatible with obtaining complete sections from the
whole surface. The sections float in the 80 per cent, alcohol, with
which the knife should be kept flooded, and may be removed with
a camel's-hair brush. At no time should either the knife or the
specimen be allowed to dry. The sections may be kept indefinitely
in 80 per cent, alcohol, or they may be dropped into water if they
are to be used within a short time.

After use, the knife should be carefully wiped, stropped, and placed
in its case. The microtome should be dried and the tracks moistened
"with a little oil of sweet almonds or paraffin oil, to prevent rusting.

3. Paraffin-sections. — The knife should be fixed perpendicular to
the direction of cutting, its edge acting like that of a plane. Its
surfaces must be clean and dry ; adherent paraffin can be removed
with a cloth moistened with xylol.

The paraffin-block containing the specimen to be cut is firmly
clamped with one of its narrow edges parallel to the edge of the
knife. The block is now raised and moderately thick slices re-
moved until the desired level is reached, when the thin sections
desired may be cut. It not infrequently happens that the sections
roll before the edge of the knife. This is probably due to the
paraffin being too hard. In that case the cutting should be done in
a warmer room, or a lamp or Bunsen burner may be placed within a
foot or two of the face of the paraffin block. This rolling will,
however, cause little trouble in the use of the sections unless it be
desired to have them adhere to each other at the edges to form rib-
bons, in which the succession of the sections is preserved. If the

302 NORMAL HISTOLOGY.

sections are compressed into tiny folds or ridges, the paraffin is too
soft and the block must be somewhat chilled.

Before paraffin-sections can be stained it is necessary to remove
the paraffin. If the tissues are sufficiently coherent, this can be
done by dropping the sections into xylol or chloroform ; but if this
would cause a disintegration of the sections, they must be affixed to
slides or cover-glasses by means of a cement which shall hold the dif-
ferent parts of the tissues in their proper relative positions after the
paraffin has been removed. The simplest cement for this purpose
is Mayer's albumin mixture, prepared as follows : beat up the white
of an egg and allow the froth to liquefy. Then add an equal bulk
of glycerin and a few pieces of camphor (for the preservation of
the mixture). This cement is applied to the clean surface of a
slide, or cover-glass in a very thin layer with the side of a camel's-
hair brush, care being taken to leave no air-bubbles. The paraffin-
sections are removed from the knife with a fine camel's-hair
brush or a small, but rather stiff feather inserted into a handle,
and placed upon the coating of cement. They are then flat-
tened out with the brush or feather and pressed against the glass to
remove superfluous cement. If the sections have rolled, unrolling will
be facilitated by Avarming the sections with the breath. The cover-
glasses are set aside to dry a little, and are then heated to render
the albumin insoluble. This requires some practice. The manipu-
lation is intended to accomplish the following results : the paraffin
melts at a lower temperature than that at which the albumin is
coagulated, and this fact is utilized to remove all excess of the
cement, which is washed away from the tissues by the flow of melted
paraffin. The residual albumin is sufficient to make the section
adhere to the glass when subjected to a high enough temperature to
cause its coagulation. The albumin should be dried to a consider-
able extent before it is converted by the heat into its insoluble form,
otherwise it will coagulate in opaque masses. To bring about the
desired results the cover-glass, held in a pair of forceps, is waved
over a flame until the paraffin is seen to melt. That tempera-
ture is maintained for a few moments, and then the cover-glass
is heated until vapors are distinctly seen to rise from its surface.
Great care must be taken not to scorch the sections. When the
sections have been cemented to them the cover-glasses are placed in
absolute alcohol to dehydrate them, and are then treated with xylol,
chloroform, or some other solvent of paraffin. The solvent is then

METHODS OF STAINING. 303

removed by another bath of absolute alcohol, and the alcohol
removed by water, when the sections are ready for staining.

It is, however, much more convenient to handle paraffin sections-
when they adhere to each other at their edges to form ribbons..
With a paraffin microtome a little practice will enable one to secure
these ribbons without difficulty and of almost any desired length.
The ribbon may be carefully laid on the surface of perfectly clean
water having a temperature of 40° to 43° C, where the paraffin
will become softened and the ribbon will then flatten out upon the
water. While it is floating on the water the ribbon may be cut into
lengths with scissors, slides coated with Mayer's albumin mixture
slipped under them, and then slide and ribbon removed together
and drained. The slide may then be dried upon or within the
paraffin oven, when the sections will adhere to the glass.

When the sections do not require affixing to slides or cover-glasses
they may be dropped into the solvent for the paraffin, and the latter
removed with absolute alcohol, for which water is then substituted,
preparing the sections for staining. It sometimes happens that
when sections are transferred from absolute alcohol to water the
diffusion-currents are so strong that the sections are destroyed..
When this is the case the transition must be made more gradually,
baths of 80 per cent., 50 per cent., and 30 per cent, alcohol being^
interposed between the absolute alcohol and the water.

Methods of Staining.

A large number of methods have been devised for bringing out
the structure of tissues. Many of the methods are of almost uni-
versal application, while others require special methods of fixa-
tion or other preliminary treatment of the tissues. Some are calcu-
lated to render the general features of structure more evident than
they would be if the tissues were not stained ; others stain certain
elements some characteristic color, and, to that extent, serve the
purpose of microchemical reagents. Only a few of the more useful
methods can be described here ; for others the reader is referred to
the larger text-books and the technical journals.

Sections cut in paraffin and affixed to slides may be stained by
flooding the slide with the filtered dye, but it is preferable to use
tumblers of small diameter or Coplin jars into which the dye has
been filtered. The slides are then placed upright in the stain.
Similar vessels can also be used for the alcohol, xylol, etc., used in

304 NORMAL HISTOLOGY.

preparing the sections for mounting. Loose sections can be stained,
etc., in porcelain butter dishes, being transferred with a bit of
platinum wire fused at one end into a glass rod. Such needles can
be readily bent to any desired shape and may be cleansed by passing
through a flame.

1. Hematoxylin and Eosin. — Hematoxylin, the coloring-principle
of logwood, has proved a very useful stain for the nuclei of cells. It
is not a pure nuclear stain, but also tints the cytoplasm of cells and
the intercellular substances. It is most commonly employed in
combination with alum. Such combinations of coloring-matter
with a base are called " lakes."

A hsematoxylin-lake may be used alone, or its use maybe preceded
or followed by the employment of a counterstain with some diffuse
color not affecting the nuclei. For counterstaining, eosin or neutral
carmine is usually employed. Both stain the tissues a diffuse red,
varying in depth according to the nature of the tissue-elements in
the section.

There are several formulae for the preparation of alum-hsema-
toxylin, but that devised by Bohmer will answer all purposes, and
is very simple :

1. Hematoxylin crystals, 1 gram.
Absolute alcohol, 10 cc.

2. Alum, 20 grams.
Distilled water, 200 cc.

>

Cover the solutions and allow them to stand over night. The
next day mix them and allow the mixture to stand for one week
in a wide-mouthed bottle lightly plugged with cotton. Then filter
into a bottle provided with a good cork. The solution is then
ready for use. Nearly all solutions of alum-haematoxylin require
an interval of time for "ripening," and their stain ing-powers
improve with age.

Alum-hsematoxylin is intended for staining sections from tissues
that have been fixed and hardened. It is especially useful when
the fixing-solution employed contained chromates, but may be used
after almost any method of fixation, if the time of staining is of
the right length and the sections are previously freed from acidity
bv thorough washing.

If the following directions are closely adhered to, the student

METHODS OF STAINING. 305

•can hardly fail to obtain good results with the use of Bohmer's
hsematoxylin :

Transfer the sections from the 80 per eent. alcohol in which they
have been kept to a dish of distilled water. At first they will float
on the surface of the water ; this is a favorable moment for removing
all folds and wrinkles. The sections should be manipulated with
platinum needles, prepared by fusing a bit of platinum wire into
the end of a glass rod. Such needles can be cleaned by heating
the wire red in a flame.

When the sections sink to the bottom of the dish of water, and
remain there, it may be assumed that they are free from alcohol.

Filter about 5 cc. of the hematoxylin into a watch-glass or butter-
dish and transfer the sections from the water to the dye.

Let the sections stain for three minutes by the watch, and then
transfer them to a dish of distilled water. At first the sections will
have a reddish tint, but as the washing proceeds the color will turn
to a pure blue. During the washing the water should be renewed,
until finally it acquires no color from the sections and the latter
have lost all traces of a red tint. This washing may take several
minutes, or even a few hours ; but if good, permanent stains are
desired, it is of great importance that it be thorough. This wash-
ing completes the actual staining with hsematoxylin, and the sections
are then ready for counterstaining with eosin or for dehydration.

The eosin solution used for diffuse staining is prepared by dis-
solving 1 gram of eosin in 60 cc. of 50 per cent, alcohol. Of this
solution, about ten drops are added to 5 cc. of distilled water in a
small dish ; the sections are stained for about five minutes and then
washed in distilled water. They are then ready for dehydration
and mounting. The diluted eosin should be thrown away after use,
but the hsematoxylin can be filtered back into the stock-bottle.

Since the hsematoxylin solution improves with age, no exact
directions can be given as to the length of time sections should
remain in a particular solution. Three minutes will usually yield
good results ; but if it is found that the color is too dark, a shorter
time should be employed, and vice versa. One soon becomes famil-
iar with the staining-powers of the particular solution used. The
dishes that have contained hsematoxylin should be washed soon
after use, or may be subsequently cleaned with a little hydrochloric
acid, all traces of which should then be removed by thorough wash-
ing in water.

20

306 HISTOLOGICAL TECHNIQUE.

The above method for staining with hematoxylin and eosin is
highly recommended for general routine work.
2. Neutral Carmine. —

Carmine, "No. 40,"

1 gram.

Distilled water,

50 cc.

Ammonia,

5 "

The solution is allowed to remain exposed to the air until the
odor of ammonia is no longer perceptible. It is then filtered into
a bottle, where it is kept till needed.

Neutral carmine gives a diffuse stain, resembling that of eosin,
but rather clearer in character. It is employed in a greatly diluted
form, according to the following directions :

One drop of the neutral carmine is mixed with about 20 cc. of
distilled water. A trace of acetic acid is then added by dipping a
platinum needle into the acid and stirring the diluted dye with the
acidulated needle. A piece of filter-paper is then placed upon the
bottom of the dish, and the sections to be stained are transferred
from distilled water to the dye and distributed upon the paper in
such a way that they do not lie over each other. The dye acts
very slowly, twenty-four hours being none too long for good results.
If the staining be hastened by using a stronger solution, it suffers
in sharpness. After staining, the sections are thoroughly washed
in distilled water, and may then be subjected to a nuclear dye, such
as hematoxylin. The proper aeidulation of the diluted dye is of
importance for the success of this method. If the solution is not
sufficiently neutralized, the sections will not be stained ; if it is too
acid, precipitation of the carmine will take place.

3. Alum-carmine. —

Alum,

5 grams

Distilled water,

100 cc.

Carmine, " No. 40,"

2 grams

The alum is dissolved in the water with the aid of heat, the
carmine then added, and the mixture kept at the boiling-point for
about half an hour. It is then allowed to cool and filtered into the
stock-bottle. Two or three drops of deliquesced carbolic acid may
be added to prevent the development of fungi.

Sections are stained in the undiluted, but filtered, dye for at least

METHODS OF STAIN TNG. 307

five minutes. There is no danger of over-staining. It is a pure
nuclear stain, coloring the chromatin red. After staining, the sec-
tions are either washed, and are then ready for dehydration, or
they may receive a counterstain with picric acid, coloring the tissues
a diffuse yellow. This may be most readily accomplished by adding
a few small crystals of picric acid to the first dish of dehydrating
alcohol (see p. 313).
4. Borax-carmine. —

Borax,

4 grams

Distilled water,

100 cc.

Carmine, " No. 40,"

3 grams

Alcohol, 70 per cent.,

100 cc.

The borax is dissolved in the water by warming, and the solution
allowed to cool ; the carmine is then stirred in and the alcohol added.
After standing twenty-four hours the solution is filtered into the
stock-bottle, a process that is exceedingly slow.

Borax-carmine is used for the staining of little masses of tissue
before they are embedded. It is a nuclear dye, giving the chromatin
a red color. It is useful when paraffin-embedding is to be employed
and it is desirable to restrict the manipulation of the sections to a
minimum.

Small pieces of hardened tissues, not over 5 mm. thick, are trans-
ferred from distilled water to the undiluted dye and allowed to stain
for twenty-four hours, or longer. After staining they are immedi-
ately placed in an acid alcohol, prepared by adding 5 drops of con-
centrated hydrochloric acid to 100 cc. of 70 per cent, alcohol. The
tissue should not rest on the bottom of the vessel containing the
alcohol, but upon crumpled filter-paper, so that the extracted excess
of coloring-matter may sink to the bottom. If the acid alcohol
around the specimen becomes colored, fresh portions of alcohol
should be used. The treatment with acid alcohol is continued until
no more color is given off from the specimen. It is then transferred
to 90 per cent, alcohol, in which it should remain for twenty-
four hours, after which it can be subjected to the dehydration neces-
sary for embedding.

5. Orth's Lithio-carmine. —

Carmine, " No. 40," 3 grams.

Lithium carbonate, saturated aqueous solution, 100 cc.

308 HISTOLOGICAL TECHNIQUE.

The solution of lithium carbonate is prepared by occasionally
shaking a mixture of distilled water and an excess of lithium car-
bonate. Twenty-four hours will suffice for the production of a
strong enough solution. The supernatant liquid is then filtered.
Carmine readily dissolves in this solution. For preservation a
crystal of thymol may be added.

Lithio-carmine stains sections in about five minutes, and there is
no danger of overstating. Like borax-carmine, it requires after-
treatment with acid alcohol. The sections should be transferred,
without intermediate washing, to 70 per cent, alcohol containing 1
per cent, of concentrated hydrochloric acid ; they may then be de-
hydrated, and, if desired, counterstained with picric acid during the
dehvdration.

6. Unna's Methylene -blue. —

Methylene-blue, 1 gram.

Potassium carbonate, 1 "

Distilled water, 100 cc.

When required for use, this solution should be diluted with dis-
tilled water to one-tenth or one-twentieth of its strength. It gives
excellent results when used to stain paraffin sections of tissues
hardened in Zenker's fluid, especially when preceded by eosin.
Stain half an hour in a 5 per cent, aqueous solution of eosin
(Griibler's bluish) ; wash in running water ; stain in the dilute
methylene-blue until the red color is no longer visible ; wash ;
differentiate in absolute alcohol till the sections appear pink (or
violet, if there be many nuclei in the tissue) ; clear in xylol and
mount in damar or Canada balsam dissolved in xylol.

Upon long standing, this solution of methylene-blue undergoes
chemical changes which reduce its staining value when used as
above, but it will keep for a number of months.

7. Aqueous Methylene-blue. — This is usually prepared at the time
when needed by mixing one part of a saturated solution of the ani-
lin-color in 95 per cent, alcohol with nine parts of distilled water.
It is frequently employed as a general stain for bacteria.

Other anilin-colors, such as fuchsin, gentian-violet, methyl-violet,
and Bismarck-brown, may be kept in concentrated alcoholic solu-
tion, to be diluted in a similar manner just before use. When these
solutions are used for staining sections or cover-glass preparations
the adherent dye is washed off with water, after which the intensity

METHODS OF STAINING. 309

of the stain is reduced by the use of alcohol, 95 per cent, or abso-
lute, which bleaches the portions of the specimen which retain the
color with the least tenacity. If" the action of the alcohol be main-
tained for too long a time, the color may be discharged from all
parts of the specimen. The method of overstaining a specimen,
and then discharging the color from those parts where it is not de-
sired, is a common one. The process of discharging the color is
called the " differentiation " of the stain, because it serves to dis-
tinguish those elements which hold the color strongly from those-
which part with it easily.

8. Carbol-fuchsin. —

Saturated alcoholic solution of fuchsin, 10 cc.

Aqueous solution of carbolic acid crystals, 5 per cent., 90 cc.

This solution should always be carefully filtered before use.

9. Anilin-gentian-violet. — A. Ehrlich's formula :

Saturated alcoholic solution of gentian-violet, 1.5 cc.

Freshly prepared anilin-water, 8.5 cc.

The anilin-water is prepared by shaking a few drops of anilin
with distilled water, allowing the mixture to stand for about ten
minutes, and then filtering through well-moistened filter-paper.
The filtrate should contain no globules of the anilin. In order to
avoid this the filtration should be stopped before all the watery
part of the mixture has run through the paper, otherwise oily drops
of anilin will follow.

Precipitates are likely to occur in this gentian-violet solution
when it is first prepared. After twenty-four hours these are less
abundant. The solution deteriorates soon after that time, and
should not be used more than a week after its preparation.

B. Stirling's formula :

Gentian-violet, 5 grams.

Alcohol, 10 cc.
Anilin, 2 cc.

Distilled water, 88 cc.

This solution keeps better than the preceding. Both must be
filtered carefully through moistened filter-paper immediately before
being used.

310 HISTOLOGICAL TECHNIQUE.

10. Gram's Solution. — This is a differentiating agent used in con-
nection with anilin-gentian-violet :

Iodine, 1 gram.

Potassium iodide, 2 grams.

Distilled water, 300 ec.

The specimens are first overstained with the gentian-violet solu-
tion. They are then washed in water and placed in Gram's solution
for from three to five minutes. While in this solution they turn a
brown color, and the combination between the coloring-matter and
some of the elements of the specimen is loosened. The specimen
is then transferred to 95 per cent, alcohol, in which it remains until
no more color is given oif. If sufficient color has not been removed,
the treatment with Gram's solution and alcohol may be repeated.
After this differentiation the specimen may be dehydrated, cleared,
and mounted ; or a contrast-stain may be used before those manipu-
lations. This is a useful method for staining bacteria in sections
of tissue when the species of bacteria are such as resist the decolor-
izing action of the iodine. In this respect different species of bac-
teria differ greatly, and the method is commonly employed in bac-
teriological work to distinguish those species which retain the stain,
or are " positive to Gram," from those which are decolorized or
" negative to Gram."

11. Van Giesen's Picric Acid and Acid Fuchsin Stain. —

Aqueous solution of acid fuchsin, 1 per cent., 5 cc.

Saturated aqueous solution of picric acid, 100 "

This solution serves to stain fibrous intercellular substances. It
is used in the following manner :

1. Slightly overstain with alum hematoxylin j e. <j., Bohmer's
hsematoxylin.

2. Wash thoroughly in distilled water.

3. Stain in Van Giesen's dye for five minutes.

4. Wash in water.

5. Dehydrate in 95 per cent, alcohol.

6. Clear in oil of origanum.

7. Mount in xylol-balsam or xylol-dammar.

The tissues should have been fixed in a corrosive-sublimate solu-
tion or one containing eliminates ; e. g., Midler's fluid, Zenker's

METHODS OF STAINING. 311

fluid, or sublimate solution. The connective-tissue fibres are stained
red bv the acid fuchsin. The reason for overstaining with hsema-
toxylin is that subsequent treatment with picric acid discharges
some of that color.

12. Benda's Iron-haematoxylin Stain. — This is a powerful stain
well adapted to the staining of paraffin-sections that have been
affixed to cover-glasses. It stains nuclei and intercellular sub-
stances, as well as the protoplasm of cells, various shades of gray,
and the color is very permanent. The outline of the method is as
follows :

1. Mordant the sections (after affixing to cover-glasses, if that
method is used) in a mixture of equal parts of liquor ferri sul-
furici oxydati of the German Pharmacopoeia and distilled water for
twenty-four hours.

2. Rinse in distilled water, and then wash in three changes of
tap-water.

3. Stain in aqueous solution of hematoxylin, prepared by mix-
ing 10 drops of a concentrated alcoholic solution of the crystals
with 10 cc. of distilled water. Stain for from one-half to twenty-
four hours.

4. Rinse in distilled water.

5. Differentiate in equal parts of glacial acetic acid and distilled
water.

6. Wash thoroughly in distilled water.

7. Dehydrate in absolute alcohol.

8. Clear in xylol, carbol-xylol, or some essential oil.

9. Mount in balsam.

13. Pal's Modification of Weigert's Stain for the Medullary Sheath
of Nerves. — This method is useful for the study of the central ner-
vous system, and may, with advantage, be preceded by staining
with neutral carmine. The tissues should have been fixed in a
chromate solution ; e.g., Muller's fluid.

1. Soak sections several hours in 1 per cent, chromic acid solu-
tion in water.

2. Stain twenty-four to forty-eight hours in :

Hematoxylin crystals, 1 gram,

Absolute alcohol, 10 cc.

Lithium carbonate (saturated aqueous solution), 7 "
Distilled water, 90 "

312 HISTOLOGICAL TECHNIQUE.

The hematoxylin crystals may be dissolved in the alcohol and
the solution kept in stock, the proper proportions of lithium carbon-
ate solution and water being; added at the time of use.

3. Wash in water to which a little lithium carbonate solution has
been added (about 2 cc. to each 100 cc. of water). The sections
should acquire a deep-blue color.

4. Differentiate in 0.25 per cent, solution of potassium perman-
ganate in distilled water, till the gray matter — e. g., of the spinal
cord — becomes brownish-yellow (one-half to five minutes).

5. Decolorize the gray matter in the following solution :

Oxalic acid, 1 gram,

Potassium sulphite, 1

Distilled water, 200 cc.

6. Wash thoroughly in distilled water.

7. Dehydrate in 95 per cent, alcohol.

8. Clear in carbol-xylol, oil of bergamot, or oil of origanum.

9. Mount in xylol-balsam or xylol-dammar.

This method stains the myelin-sheaths of the medullated nerve-
fibres a dark blue, nearly black, color. If it has been preceded by
a stain with neutral carmine, the axis-cylinders of the nerve-fibres-
will be stained red, and the nuclei of the nerve-cells will also
appear red.

14. Golgi's Methods. — These methods have yielded most excel-
lent results in the study of the central nervous system, the dis-
tribution of the peripheral nerves, and the delicate terminations-
of the ducts of glands; e.g., the bile-capillaries. The methods
must be regarded as special procedures in such studies, and can
but be referred to here. They all depend upon hardening in some
chromium salt, with or without the addition of osmic acid, and the
subsequent impregnation with silver nitrate. A precipitate is thus
produced on or within certain of the elements in the specimen, giving
them a dark-brown or black color. The methods are capricious,
and not all of the tissue-elements of like character in the specimen
are rendered prominent. This is an advantage, but necessitates
a degree of care in the interpretation of the results. Furthermore,
irrelevant precipitates may form in the tissues which have no
definite relations to any structure. Considerable practice is, there-
fore, required for the successful employment of all these methods,.

METHODS OF DEHYDRATION. 313

not only for a satisfactory execution of the manipulations, but also
in the study of the results. The methods have no value for the
study of cell-structure, since the whole cell is either covered or
filled with the precipitates formed during- the impregnation with
silver.

Golgi has divided his methods into three groups : the slow, the
rapid, and the mixed. For the details of these methods and of
the various modifications introduced by different investigators the
student is referred to the journals on microscopy. It must suffice
to state here that the slow method begins with a hardening of the
tissues in a 2 per cent, solution of potassium bichromate, which is
gradually raised to 5 per cent. This hardening takes from fifteen
days to three months. In the rapid method the tissues are first
hardened in a mixture of 4 parts of a 2 per cent, solution of potas-
sium bichromate and 1 part of a 1 per cent, solution of osmic acid.
The tissues remain in this mixture for from two to six days, when
they are ready for impregnation. For either method the pieces
of tissue should not be thicker than 1.5 cm.

Methods of Dehydration.

The final manipulation in nearly all the methods for staining
described above is a washing of the sections in water. This water
must be removed before permanent mounts can be made. Dehy-
dration is accomplished by treating the sections with alcohol. If
they are impregnated, or have been embedded in collodion or cel-
loidin, they must not be dehydrated in absolute alcohol, as that dis-
solves the collodion. In such cases 95 per cent, alcohol is employed,
the sections being treated with two baths of alcohol. When sections
have been stained with carmine a contrast-stain may be obtained by
adding a few small crystals of picric acid to the first dish of dehy-
drating alcohol. The excess of picric acid is then removed by the
alcohol in the second dish. Absolute alcohol may be used for dehy-
dration when the sections have not been embedded in collodion or
celloidin.

When anilin-dyes have been used to stain sections it must be
borne in mind that alcohol not merely dehydrates, but also differ-
entiates the stain. If the sections are left too long in the alcohol,
they may lose more color than is desired.

Sections that are to be mounted in glycerin or glycerin-jelly
require no dehydration, but can be mounted directly from water.

314 HISTOLOGICAL TECHNIQUE.

Methods of Clearing.

Clearing is necessary when specimens are to be permanently
mounted in Canada balsam or dammar. Its object is to impreg-
nate the section with some liquid that will drive out alcohol and
also be miscible with the resin used for mounting. Of these clear-
ing-agents there is a large number, from which a choice must be
made according to the method of embedding that has been employed
and the nature of the dve with which the tissues have been stained.
Clearing-agents also differ in their miscibility with water, some
requiring dehydration with absolute alcohol, others clearing well
when 95 per cent, alcohol has been used for dehydration.

1. Xylol. — This is an excellent clearing-agent when the sections
have been well dehydrated with absolute alcohol. It does not
injure anilin-dyes. It is, perhaps, the best clearing-agent for
sections of tissue stained with borax-carmine before cutting, when
no counter-stain is employed. Xylol then both removes the paraffin
in the section and clears it.

2. Carbol-xylol. —

Carbolic acid crystals (melted), 1 vol.

Xylol, 3 vols.

This mixture is much more tolerant of water than pure xylol.
Sections dehydrated in 95 per cent, alcohol may be cleared with
this reagent, which does not dissolve collodion. The carbolic acid
used should be pure, but need not be the more expensive synthetic
product.

3. Oil of Bergamot. — This light-green essential oil clears well and
does not dissolve collodion. It may be used when 95 per cent,
alcohol has been employed for dehydrating.

4. Oil of Origanum. — The oleum origani cretici should be used.
It is of light-brown color and clears sections dehydrated in 95 per
cent, alcohol or stronger. It slowly discharges anilin-colors.

5. Oil of Cloves. — This clearing-agent dissolves collodion and
discharges anilin-colors. It may be used when it is desired to get
rid of the collodion used for embedding after the sections have been
stained. This removal is favored by dehydration in absolute alcohol
before clearing.

6. Oil of Cedar-wood. — This, when pure, has a very light-yellow
color and smells like cedar-wood. It should be free from the more

METHODS OF MOUNTING. 315

pungent odor of the oil derived from the leaves. This essential
oil does not discharge anilin-colors, and is, therefore, useful when
those dyes have been employed. It clears slowly, but well, and
may be used after dehydration with 95 per cent, alcohol.

Methods of Mounting-.

Sections that have been treated by the foregoing methods of
preparation are fitted for mounting in a solution of some resin.
The most commonly employed are Canada balsam and dammar.
The best solvent for these resins is xylol, though chloroform and
benzol are sometimes used for this purpose. All traces of turpen-
tine should be removed from the balsam before its solution, to avoid
the discharge of stains with hematoxylin or anilin-dyes which tur-
pentine occasions.

When sections are transferred from alcohol to a clearing-agent
they float upon the surface of the latter, and can then be flattened
and all folds removed. As the alcohol is extracted the sections
sink in the clearing -agent. In order to transfer them from the
clearing-agent to a slide, the first step in mounting, a good method
is to slip a strip of cigarette-paper under the section, withdraw it
along with the section (using a platinum needle as aid, if necessary),
drain oft' the superfluous fluid, and then lay the cigarette-paper on
the slide, section side down. Light pressure will now squeeze out
-considerable of the clearing-agent, when the paper can be stripped
from the section and slide, leaving the section nearly dry and with-
out folds or wrinkles. With a little care, this method of transferring
the section to the slide rarely fails. When such is the case the
manipulations must be repeated.

A drop of the mounting-medium is now placed upon the section
and a cover-glass laid on and gently pressed down until it comes in
contact with the section and the excess of balsam or dammar is
expelled from beneath the cover. If the sections tend to raise the
<;over, the latter may be weighted with a bullet placed in its centre.
Freshly mounted specimens are not so favorable for examination
with high powers as those that have been mounted for a few hours
or days. This is because the refractive indices of the clearing-agent
and mounting-medium are not identical. When these have become
thoroughlv mixed, or the former has evaporated, the specimen is
impregnated with and surrounded by a homogeneous medium that
does not scatter the light passing through it.

310 HISTOLOGICAL TECHNIQUE.

Canada balsam lias a somewhat higher refractive index than
dammar. It therefore renders the sections a little more transparent
and more completely obliterates the structure-picture. When it is
desired to retain as much of the structure-picture as possible,
which is usually the case, dammar should be chosen for the mount-
ing-medium. It dries a little more slowly than balsam, but soon is
sufficiently dry at the edges of the coyer-glass to preserve the sec-
tion from injury. If the slides are kept in a horizontal position,
in a warm place (40° to 50° C. ; 104° to 122° F.), for a couple of
days, they will be dry enough for storage, but for several weeks
must be handled with care.

Stained sections may be examined in glycerin, having been
mounted by the same manipulations as those used for mounting
in balsam, without previous dehydration or clearing. Such mounts
are, however, difficult of preservation. The various cements that
have been recommended for fastening the edges of the cover-glass
to the slide are usually inefficient, as the changes of temperature
that are inevitable cause the glycerin to make its way between the
glass and cement, loosening the latter.

A better medium than glycerin for sections that cannot be sub-
jected to the action of alcohol for the purpose of dehydration is
glycerin-jelly. This is prepared by soaking the best French gelatin
in cold water until it has imbibed all it will readily take up, then
melting the gelatin, after pouring off the excess of water, and
adding an equal bulk of glycerin. A little carbolic acid may be
added to the mixture to preserve it. The manipulations for mount-
ing are similar to those given above, the sections being transferred
from water to the slide. The glycerin-jelly may be melted and a
drop placed upon the section, or a little lump of the solid jelly may
be placed upon a cover-glass, melted by gentle heat, and the cover-
glass then inverted over the section on the slide. After the jelly
has dried at the edges of the cover-glass they may be painted with
xylol balsam, dammar, or some cement.

The Rapid Preparation of Sections for Diagnosis.

The most expeditious means of obtaining sections of fresh tis-
sues is to cut them without preliminary treatment with reagents,
either free hand with a razor, or with the aid of a freezing mi-
crotome. Such sections may be stained with methylene-blue, or they
may be examined in neutral salt solution, if they are to be stained,

SPECIAL METHODS. 317

spread them on a slide, pour a few drops of the methylene-blue
solution over them, and after a few moments wash off the dye with
water and cover the section. If such rapid work is not necessary,
the sections can be fixed in formalin (page 287), and, after washing
out that reagent, stained. Such sections may be hardened and de-
hydrated by placing them in dishes of increasingly strong alcohols,
and finally mounted in dammar ; but the results are by no means so
good as when fixation and hardening are done before sections are cut.
When time is not pressing, the following method will give good
results :

1. Fix and harden pieces not over \ inch thick in absolute alcohol
kept absolute with anhydrous cupric sulphate. This will take from
four to twelve hours, according to the sizes of the specimens.

2. Dip the specimen in thick collodion and embed it on a glass
block by the rapid method. When the block has been in 80 per
■cent, alcohol for three or four hours it may be cut ; but it is better
to let the collodion harden for twenty-four hours. If a paraffin
microtome is available, remove the alcohol with chloroform, renewed
at least once. If the specimens are small, two to four hours will
suffice for this step in the process. Half an hour each in chloroform
saturated with paraffin and in fresh melted paraffin in the oven will
accomplish sufficient impregnation with paraffin, and the specimen
can then be embedded by the box method.

3. Stain with hematoxylin and eosin, cutting short the time of
washing after the hematoxylin, if in a hurry.

4. Dehydrate in 95 per cent, alcohol ; two successive baths.

5. Clear in carbol-xylol.

6. Mount in xylol-dammar.

When paraffin has been used, follow the directions given on
pages 294 and 297.

Very serviceable sections can be prepared in less than twenty-
four hours by these methods, and the specimens, though not of the
best quality, will be permanent, and may be kept for future refer-
ence. The paraffin method may yield excellent permanent mounts
in eight to ten hours if the pieces are small.

Special Methods.

The foregoing methods are for the preparation of sections of soft
tissues. It is sometimes desirable to separate the tissue-elements by
.some process of maceration, or to prepare sections of calcified tissues,

318 HISTOLOGICAL TECHNIQUE.

such as bone. Furthermore, some microscopical objects are obtained
suspended in fluids. In these cases special methods of preparation
must be used.

1. Examination of Sediments. — For the collection of sediments
vessels — e. (/., test-tubes — with vertical walls should be used, or a
centrifuge may be employed, the sediment being removed from
the bottom of the tube with a clean pipette. Place the finger over
one end of the pipette before introducing it into the fluid, to retain
the air ; then bring the other end in contact with the sediment and
allow the air to escape slowly. Close the upper end of the pipette
and withdraw it. Carefully wipe the outside of the pipette, and
transfer the sediment to a slide and cover it with a cover-glass.
Many sediments may be stained with alum-carmine, methylene-blue,.
or Gram's solution, drained, and mounted in glycerin jelly.

2. Microchemical Reactions. — These reactions are resorted to to
determine the chemical nature of objects under the microscope.
Every stain is the result of a microchemical reaction, but as yet
the knowledge obtained by staining tissues cannot always be ex-
pressed in chemical language.

The manipulations are usually so conducted that the reaction can.
be directly observed under the microscope. The object to be studied
is placed in the middle of the field. The reagent used is then
placed at one edge of the cover-glass, whence some of it will flow
beneath the latter. To facilitate the entrance of the reagent a nar-
row strip of filter-paper may be brought in contact with the oppo-
site edge of the cover, withdrawing some of the fluid from beneath
it. It is best to sharpen the end of the strip which comes in con-
tact with the cover-glass, so that the absorption of fluid shall be
slow ; otherwise the currents induced will be likely to wash the
object from the field of vision. The following tests, applied in this
way, may be of use :

a. Urates. Insoluble in 1 per cent, acetic acid ; soluble, on the
application of heat, in water (or urine). The slide must be removed
from the microscope when heat is applied to it.

b. Earthy phosphates. Dissolve on the addition of 1 per cent,
acetic acid. Are not dissolved by heat.

c. Calcium oxalate. Insoluble in 1 per cent, acetic acid ; soluble
in 1 per cent, hydrochloric acid.

(1. Carbonates. Soluble in 1 per cent, acetic acid or hydrochloric
acid, with evolution of gas-bubbles.

SPECIAL METHODS. 319

e. Albuminoid granules. Become indistinct, and finally invisible,
on the addition of 1 per cent, acetic acid or 1 per cent, potassium
hydrate ; not blackened by osmic acid.

/. Fatty granules. Not affected by 1 per cent, acetic acid or 1
per cent, potassium hydrate. Stained black or dark brown by osmic
acid.

g. Starch. Stained dark blue to black by iodine solutions. Use
Gram's solution.

h. Cellulose. Stained yellow by iodine solutions. If the water
be then removed and concentrated sulphuric acid introduced, the
color becomes blue. The wTalls of most vegetable cells are composed
of cellulose.

i. Teichmann's test for haemoglobin. This test depends upon the
conversion of the haemoglobin or its derivatives into haemin, which
crystallizes in rhombic plates of a reddish-brown color. The haemin
is produced by heating with a little salt and strong acetic acid.
Evaporate a drop of neutral salt solution to dryness on a slide.
Place the substance to be tested upon it and cover. Fill the space
between cover and slide with glacial acetic acid and heat over a
flame till bubbles begin to form. Maintain that heat for a few
minutes, replacing loss by fresh additions of acetic acid. Let the
slide cool slowly, and, when cold, examine. If the results are nega-
tive, repeat the heating with acetic acid. The acid should not
actually boil, but should be kept at the point of incipient ebullition.

j. Tests for amyloid substance. Sections of fresh tissue may be
soaked for some time in Gram's solution, then washed and examined
in water. Amyloid substance is stained reddish-brown, the tissues
yellow. Sections of tissues fixed in alcohol, corrosive sublimate, or
formaldehyde, may be stained in a solution of 1 per cent, methyl-
violet dissolved in distilled water, without the addition of alcohol.
The sections are then washed in 1 per cent, hydrochloric acid for
the purpose of differentiating the stain. After thorough washing
in several changes of water they may be mounted in glycerin-jelly.
The amyloid substance is stained reddish-violet, the other tissues
blue.

k. Test for iron in pigmentations. The iron from the haemo-
globin of the blood is sometimes present in the pigmentation result-
ing from old extravasations, in the form of haemosiderin. The
same compound is also sometimes found in the tissues in cases of

320 HISTOLOGICAL TECHNIQUE.

pernicious anaemia. The presence of iron in this pigmentation may
be demonstrated by the following method :

(a) The tissues should be fixed in alcohol.

(6) Soak the section in a 2 per cent, solution of potassium ferro-
cyanide for ten minutes.

(c) Transfer to Orth's acid alcohol (page 290) for five or ten
minutes.

The sections may now be examined in a glycerin-mount with a
wide diaphragm, or they may be counterstained, for which purpose
treat as follows :

(d) Wash with water.

(e) Stain with Orth's lithio-carmine.

(/) Dehydrate and mount in xylol-dammar.

The iron in the section is converted into Prussian blue ; the nuclei
of the cells, when the counterstain has been employed, are red.

/. Examination of sputa for elastic fibres. In pulmonary disease
involving a destruction of pulmonary tissue and the appearance of
fragments in the expectoration, elastic fibres from the alveolar walls
may frequently be found in the sputa :

Fill a test-tube one-third full of sputa, add five or six drops of
36 per cent, potassium hydrate solution, and boil the mixture for
three or four minutes. Add an equal bulk of distilled water.
Divide the contents of the tube between the two tubes of the cen-
trifuge and precipitate their contents. If elastic fibres were pres-
ent, they will be found either in the sediment or in the scum on the
top of the fluid.

3. Methods of Maceration. —

a. One-third alcohol.

95 per cent, alcohol, 35 cc.

Distilled water, 65 "

This dilute alcohol is excellent for the separation of epithelium
from the surfaces of mucous membranes. The fresh tissues are
placed in the alcohol for a day or two, after which the cells can
easily be detached and separated by shaking. The cells are well
preserved, and may be stained with methylene-blue or alum-car-
mine.

b. Potassium hydrate.

Potassium hydrate, pure by alcohol, 36 grams.

Distilled water, 64 cc.

SPECIAL METHODS. 321

The solution should be cold before use. It cannot be filtered
through paper ; but if not clear, should be decanted from any sedi-
ment, or a fresh solution prepared. Maceration takes place very
quickly in this solution. The tissues can usually be teased apart
within fifteen to thirty minutes. They must be examined in the
potash solution without dilution, as the addition of water quickly
destroys the tissue-elements. For this reason the specimens to be
macerated should be placed in several times their bulk of the pot-
ash solution ; otherwise the water they contain will dilute the pot-
ash. Permanent mounts cannot be made.

c. Chromic acid. A solution of 1 part of the acid in 10,000
parts of distilled water will facilitate the teasing apart of tissue-
elements which have macerated in it for one to several days. After
careful washing on the slide alum-carmine alone, or followed by
picric acid, may be used for staining.

4. Methods of Decalcification. — Tissues which contain calcified
nodules or bone must be freed from lime-salts before they can be
cut. It is difficult to do this rapidly without injury to the softer
tissue-elements. When good results are desired, and the necessary
time can be afforded, the tissues should first be fixed and hardened,
small pieces being selected. Zenker's fluid fixes well for this pur-
pose, but Orth's fluid or alcohol may be used. If Zenker's or Orth's
fluid is used, the tissues must be washed in water and hardened in
alcohol for at least a day before they are decalcified.

Decalcification is accomplished by treatment with acids. Five
per cent, nitric acid will decalcify small pieces of bone in from one
to five days. The progress of the decalcification may be deter-
mined by pricking the tissue with a needle, but after it appears
to be soft it is well to continue the action of the acid for a day or
two, lest some undissolved particles should remain and injure the
edge of the microtome-knife. A saturated aqueous solution of
picric acid is sometimes used for decalcifying. Its action is very
slow, though not injurious to the tissues, which require no prelimi-
nary treatment, the picric acid acting as a fixing and decalcifying
agent.

After decalcifying in nitric acid the tissues should be thoroughly
washed in running water for twenty-four hours and then rehardened
in alcohol, after which they may be embedded. After decalcifying
in picric acid the tissues are placed in 70 per cent, alcohol and hard-
ened without previous washing in water.

21

322 HISTOLOGICAL TECHNIQUE.

When rapid decalcification is necessary nitric acid and phloro-
gluein, which restrains the destructive action of the acid, may be
used. The solution is prepared by dissolving 1 gram of phloro-
glucin in 10 cc. of pure nitric acid. To this 100 cc. of 10 per cent.
nitric acid are added. Decalcification takes place within a few hours
in this solution, which contains about 20 per cent, of nitric acid.
The tissues should then be washed and hardened.

Another rapid method which combines decalcification with hard-
ening is to place the fresh tissues in a large bulk of 5 per cent,
nitric acid in 80 per cent, alcohol. After decalcification has taken
place the tissues are hardened in alcohols of increasing strength,
large quantities being used in order to remove the acid. Before
staining, the sections should be washed thoroughly in water to get
rid of any residual traces of acid.

INDEX.

ABBE condenser, 281
Absorption, 128
Achromatic spindle, 38
Achromatin, 35
Acidophilic cells, 125

leucocytes, 125
Acini, glandular, 02, 63
Acromegaly, 199
Adipose tissue, 83

functions of, 84
Adrenal bodies, 188, 194
zona granulosa of, 195
reticularis of, 195
Adventitia, 116, 118
Afferent vessels of kidney, 165

of lymph-glands, 122
Air passages, 176
Albumin, egg, Mayer's, 302
Albuminoid granules, tests for, 319
Albuminuria, 170, 191
Alcohol, absolute, 289

methods of maceration, 320
Alum-carmine, 306

methods of staining, 306
Alveolar passages, 178
Alveoli, glandular, 62

pulmonary, 179
Amacrinas, cellulas, 249 (Fig. 232)
Amoeba, 28

comparison with other cells, 30
Amoeboid movement, 29
Ampulla of breast, 232
Amvlacea, corpora, 238
Amyloid, 319
Anabolism, 31
Anilin-water, 309
Animals, multicellular, 28

unicellular, 27, 28
Aponeuroses, 85

Arcades, vascular, of kidney, 165
Areolar tissue, 80

development of, 80
"ground-substance" of, 80
Arrectores pili, 211
Arterial arcade of kidney, 165
Arteries, 116

helicine, 237
Artery, hepatic, 154, 156

Articular cartilages, 70
Arytenoid cartilage, 178
Assimilation, nucleus in, 33
Association-cells,see Spongioblasts, 249,

277 _
Association-fibres of cerebrum, 263

of spinal cord, 253
Attraction-spheres, 36
Axis-cylinder, 100, 103, 248

BAND, constricting, of Ranvier, 104
Basement-membrane, 60, 140,
147, 246
Basophilic leucocytes, 126, 133
Bergamot, oil of, 314

methods of clearing, 314
Bile capillaries, 157, 158

duct, 154, 160
Bioplasm, see Cytoplasm, 29, 32
Bipolar nerve-cells, 101
Bladder, 172

lining of, 172
Blood, 128

functions of, 114, 130
Blood-plates, 128, 133
Bodies, adrenal, 194

carotid, 189, 202

Malpighian, of kidney, 162
of spleen, 185

Pacinian, 266

polar, 36, 231
Body, coccygeal, 189, 203

pituitary, 197
Bone, 70

canal iculi of, 70

cement in, 72

classification of, 72

corpuscles, 70

decalcified, 71

development of, 73

effects of decalcification on, 71

general character of, 70

"ground substance " of, 72

Haversian canals of, 70

intercellular substances in, 72

lacunae of, 70

nutrition of, 73
Bone-marrow, 73, 125

323

324

INDEX.

Bone-marrow, red, 125

yellow, 125, 127
Borax-carmine, 307

methods of staining, 307
Boundary zone of kidney, 161
Bowman, glands of, 269
Bowman's capsule, 169
Breast, see Mammary gland, 232

ampulla of, 232
Bridges, intercellular, 58, 88, 89
Bronchi, 177
Bronchial cartilages, 178
Bronchioles, 178
Brownian movement, 29
Brunner, glands of, 148, 149
Bulb, olfactory, 272

glomeruli of, 272

CACHEXIA strumipriva, 191
Calcium oxalate, tests for, 318
Callus, 57
Canada-balsam, 315, 316

methods of mounting, 315, 316
Canal, central, 22

neural, 22, 23
Canaliculi of bone, 70
Canals, Haversian, 72. 73
Cancellated bone, 70
Capillaries, 47, 50, 119

bile, 157, 158
Capsule, Bowman's, 167

Glisson's, 124. 155

of kidney. 170

of spleen, 134
Capsules, suprarenal, 194
Carbohydrates, in cytoplasm, 32
Carbol-fuchsin, 309*

methods of staining, 309
Carbol-xylol, 314

methods of clearing, 314
Carbon in lymph-nodes, 125
Carbonates, tests for, 318
Cardiac glands, 144

muscles, 94
Carmine, alum-, 306

methods of staining, 306

borax-, 307

methods of staining, 307

lithio-, 307

methods of staining, 307

neutral, 306

methods of staining, 306
Carotid bodies, 189, 202

glands, 202
Cartilage, 66

articular, 68

classification of, 67

costal, 68

cricoid, 176

Cartilage, elastic, 69

ensiform, 00

of Eustachian tube, 69

fibro-, 68

functions of, 70

general character of, 66

in heart, 68

hyaline, 68

interarticular, 68

intercellular substance in, 67

matrix of, 67

nutrition of, 68

ossification of, 66, 74

temporary. 68

thyroid, 176
Cartilages, arytenoid, 176

bronchial, 178

tracheal, 177
Catabolism, 32
Cavities, serous, 47
Cedar-wood, oil of, 280, 314

methods of clearing, 314
Cell, or cells, 27

acidophilic, 127

of connective tissue, 76, 81

constituents of, structural, 31

death of, 42

decidual, 225

of Deiters, 274

ditferentiacion of, 27

-division, 35, 40
amitotic, 40
centrosome in, 35
direct, 35, 40
indirect, 35

ectoplasm of, 29, 30

endothelial, 47

epithelial, 51

of fibrous tissue, 76

fixed, 30

functions of, 32

ganglion-, 100, 248

giant-, 42, 127

glia-, 107

goblet-, 54, 139

hair-. 207, 274

lutein, 219

membrane, 30

migratory, 128

mitral, 272

modification of, related to function,
27

of Muller, 275

muscular, 87, 94, 96

nerve-, 100
bipolar, 100
multipolar, 100
unipolar, 100

organization of, 32

INDEX.

325

Cell or cells, organs of, 31, 32

plasma-, 126

polar bodies of, 36

poles of, 34

prickle-, 57, 205

primitive, 27, 31

of Purkinje, 257

pyriform, 58

reproduction of, 35, 40

of Sertoli, 242

simplest, 28

specialization of, 27

splenic, 186

stellate, 259
large, 259
small, 259

structure of, 27, 31

sustentacular, of retina, 276
of testis, 245

unspecialized, 27

wandering, 132
Celloidin, see Collodion, 292, 295
Cellulas amacrinas, 249 (Fig. 232)
Cellulose, tests for, 319
Cement of teeth, 213, 214

substance, 47
in bone, 78
in fibrous tissue, 80
in muscular tissue, 92
Central nervous system, 111

veins, 156
Centrosome, 31, 34, 35

division of, 34, 39

function of, 33
Cerebellum, 257
Cerebrum, 260

association-fibres of, 264

commissure-fibres of, 263

projection-fibres of, 263
Cheek, lining of 56
Chemotaxis, 29
Chemotropism, 29
Chromatin, 34

reduction of, 240, 245
Chromic acid, 321

methods of maceration, 320
Chromolysis, 234
Chromophilic granules of nerve-cells,

102
Chromoplasm, 34, 40

and heredity, 40

during karyokinesis, 35
Chromosomes, 37 (Fig. 8), 39
Chyle, 134
Cilia, 27, 55

movement of, 27
Ciliated epithelium, 44, 55
Circulation, intracellular, 30
Circulatory system, 114

Clarke, columns of, 253, 255
Classification of bone, 72
of bone-marrow, 73, 125
of cartilage, 67
of connective tissues, 65
of epithelium, 51, et *eq
of fibrous tissue, 74, 76
of glands, 60, et scq.
of muscular tissues, 87
of nerve-cells, 102. 248
of nerve-fibres, 102
Cleaning objectives, 279
Clearing, methods of, 314
bergamot, oil of, 314
carbol-xylol, 314
cedar-wood, oil of, 314
cloves, oil of, 314
origanum, oil of, 314
xylol, 314
Clefts of Lanterman, 105
Cloves, oil of, 314

methods of clearing, 314
Coagulation, explanation of, 134
Coccygeal body, 189, 203

gland, 203
Collateral fibres of spinal cord, 253
Collecting tubules of kidney, 103, 168
Collodion, 292, 295
embedding, 295
methods of impregnation, 292
Colloid degeneration in thyroid, 189
in hypophysis, 197
in thyroid body, 189
Colon, 149
Colostrum, 233

-corpuscles, 233
Columnar epithelium, 54
Columns of Clarke, 253, 255
Commissure-fibres of cerebrum, 263
Condenser, Abbe, 280
Connective tissue, 65
cells of, 76, 81
classification of, 65, 74
development of, 65. 77
fibres, staining of, 310
functions of, 65, 79, 81
intercellular substance in, 65
Contractile substance, 87

vacuoles, 30
Convoluted tubules of kidney, 163
Cord, spinal, 250
umbilical, 79
vocal, 176
Corium, 204

Cornicula of larynx, 69, 176
Corpora amylacea, 244

cavernosa, 242
Corpus album, 220
cavernosum, 242

:V26

INDEX.

Corpus, hsemorrhagicum, 220

luteum, 22o

spongiosum, 242
Corpuscles, colostrum-, 233

genital, 267

of Hassall, 200

of Krause, 267

Malpighian, of spleeu, 186

of Meissner, 267

red, 130

diameter of, 281
nucleated, 125, 130
stroma of, 130

tactile, 266

white, 131
Cortex of kidney, 161

of lymph-glands, 121
Costal cartilages, 68
Crescents of Gianuzzi, 139
Cricoid cartilage, 176
Crypts of Lieberkiihn, 147

of stomach, 142
Cubical epithelium, 51
Cuticle of epithelium, 52

of hair, 209
Cuticularized epithelium, 56
Cutting, methods of, 299
celloidin sections, 301
collodion sections, 301
free-hand, 299
frozen sections, 300
paraffine sections, 301
Cytoplasm. 29, 32, 33

carbohydrates in, 32

division of, in cell reproduction, 38

fats in, 32

functions of, 32

proteids in, 32

salts in, 32

water in, 32

DAMMAR, methods of mounting,
315
Daughter nucleus, 38
Decalcification, effects of, on bone, 71

methods of, 321
Decidua, 22-r>
Decidual cells, 22-1
Degeneration, keratoid, 211
Dehydration, methods of, 313
Deiters' cells, 274
Dendrite, 248

Dendritic processes, 101, 248
Dentin, 213, 214
Deutoplasm, 23
Development :

of areolar tissue, 81

of bone, 73

of connective tissue, 65, 78

Development of endothelium, 51

of epithelium, 64

of fibrous tissue, 78

of glands, 63
Diaphragm iris, 280

of microscope, use of, 280
Diaster-phase of karyokinesis, 39
Differentiation in structure, 28
Diffusion, 129
Digestion, intracellular, 30
Digestive function, 30

organs, 136
Direct cell division, 35, 40
Discus proligerus. 217
Disks, intervertebral, 68
Dispirem-phase of karvokinesis, 39
Duct, bile-, 154, 160

thoracic. 122
Ductless glands, 64, 188
Ducts of glands, 61
Duodenum, 145

EAR, 69, 273
Ectoderm, 20
Ectoplasm, '29
Efferent vessels of the kidney, 166

of lymph-glands, 122
E°-f 20 '231

albumin, 302
Elastic cartilage, 69
fibres, 76. 77, 320
Elastin, 78
Eleidin, 206
Elementary tissues, 43
recognition of, 45
structural variations in, 45
Elements, sarcous, 98
Elimination, 44
Embedding, methods of, 294
celloidin, 295
collodion, 295
paraffin. 297
tissues, 283
Embryonic layers, 22
Enamel of teeth, 213, 214
Endocardium, 116
Endoderm, 2o
Endomysium, 98
Endoneurium, 106
Endoplasm, 29
Endothelial cells, 47
Endothelium, 47
development of, 51
functions of, 50
general characters of, 47
intercellular substance in, 47, 49
Energy, kinetic, 18

potential, 18
Ensiform cartilage, 06

INDEX.

327

Enzymes, 32
Eosiu, 305

methods of staining, 305
Eosinophilic leucocytes, 125
Ependyma, 109
Epicardium, 115
Epidermis, 57, 204, 205
Epididymis, 239
Epiglottis, 69, 170
Epimysium, 98
Epineurium, 106
Epithelial cells, 51
pyriform, 58

tissues, 47
Epithelium, 43, 51

activities of, 59

ciliated, 55

classification of, 51, et seq.

columnar, 54

cubical, 51

cuticle of, 52

cuticularized, 56

development of, 64

excretory, 44, 52

functions of, 43, 59

general characters of, 51

germinal, 215

glandular, 44, 52

horny, 43, 56

intercellular substance in, 51

mucigenous, 44

pavement-, 53

secretory, 44

stratified, 56

transitional, 58
Erectile tissue, 236
Erythroblasts, 125, 130
Erythrocytes, see Red corpuscles, 130
Eustachian tube, cartilage of, 69
Excretion, 44
External genitals, 230, 236
Eye, 275

vitreous humor of, 79

17ULLOPIAN tubes, 220
1 Fascia;, 70, 82
Fat, 83

infiltration with, 82
Fats in cytoplasm, 32
Fatty degeneration, 209. 230

granules, tests for, 319
Female organs of generation, 215
Fenestrated membrane of Henle, 77,

117
Ferments, 32, 45
Fibres, association-, of cerebrum, 264

in bone, 72

collateral, of cord, 253

commissure-, of cerebrum, 263

Fibres, connective-tissue, staining, 310
of cord, 2~>'4
elastic, 77
moss-, 246

nerve-, 102

classification of, 102
-staining, 311
non-elastic, 76
non-medullated, 102
projection-, of cerebrum, 263
Sharpey's, 72
Weismann's, 99
white, 76
yellow, 77
Fibrin, 134

formation, 134
Fibroblasts, 79
Fibro-cartilage, 68
Fibrous tissues, 76
cells of, 76
cement in, 76
classification of, 76, 78
development of, 77
general character of, 76
ground-substance of, 76
Figures, mitotic, preservation of, 288
Filtration in lymph formation, 129
Fixation of tissues, 283, 290
methods of, 285
alcohol, absolute, 289
boiling, 290

Flemming's solution, 288
formaldehyde, 287
mercuric chloride solution, 287
Midler's fluid, 285
Orth's fluid, 286
Zenker's fluid, 286
Flemming's solution, 288
Follicles, Graafian, 215
hair-, 207

lymph-, 80, 148, 151
Formaldehyde, 287
Foveolre, Howship's, 75
Freezing-point, depression of, 129
Fresh specimens, examination of, 286,

299
Frozen sections, 300
Fuchsin, carbol-, methods of staining,

309
Function and structure, 27, 28, 32, 43,

59, 62, 65, 70, 83, 84
Function, or functions,
of adipose tissue, 84
of blood, 111, 130
of cardiac muscle, %
of cartilage, 70
of centrosome, 32
of connective-tissue, 65, 84
of cytoplasm, 32

328

INDEX.

Function or functions, digestive, 30

of endothelium, 50

of epithelium, 48, 62, 53-55, 57, 59

of hypophysis, 197

of liver, 159

of lung, 181

of lymph, 114

of lymph-nodes, 125

of nervous tissue, 87

of nuclei, 32

nutritive, of cells, 65

perceptive, 30

of smooth muscles, 93

of spleen, 186

of thyroid body, 189, 191
Funiculi of nerves, 106

GALACTI FERGUS ducts, 232
Gall-bladder, 160
Gall-duct, 157, 150
Gelvanotropism, 30
Ganglia, 105

of heart, 118
Ganglion-cells, 100, 248
Generation, female organs of, 215

male organs of, 236
Genital corpuscles, 267
Genitals, external, 230
Gentian violet, 309

methods of staining, 309
Geotropism, 30
Germinal epithelium, 215

of ovary, 215
Giant cells, 42, 125
Gianuzzi, crescents of, 139
Gland, mammary, 232

thymus, 188, 200

thyroid, 189
Glands, 44, 52, 55, 60

of Bowman, 269

of Brunner, 148

cardiac, of stomach, 144

carotid, 202

classification of, 61

coccygeal, 203

compound, 61

development of, 63

ductless, 64, 188

ducts of, 61

lymphatic. 122

mammary, 232

parotid. 139

pyloric, of stomach, 142

racemose, 61, 232

saccular, 61

salivary, 139

sebaceous, 209

secreting. 60, 232

simple, 61

Glands, sublingual, 139

submaxillary, 139

sweat-, 206

thyroid, 188, 189

tubular, 61

of Tyson, 237
Glandular alveoli, 62

epithelium. 52
Glans penis, 236, 237
Glia-cells, 107
Glisson's capsule, 154, 155
Glomeruli of kidney, 166

olfactory, 272
Glomerulus, 166
Glucose, 45, 159
Glycerin, 316

jelly, 316
Glycogen, 45, 52, 158, 160

methods of mounting, 316
Goblet-cells, 54, 147
Golgi's method of staining, 312
"Goose-flesh," 211
Graafian follicles, 215

development of, 217
Gram's solution, 310
Granules, chromophilic, of nerve-cells,

102
Gray matter, of spinal cord, 250

nerve-fibres, 106
" Ground-substance " :

of areolar tissue, 78

of bone, 72

of central nervous system, 108

of fibrous tissue, 76

of mucous tissue, 78

of reticular tissue, 80

HEMATOXYLIN, 304
methods of staining with, 304
Hemoglobin, 130, 131

tests for, 319
Hair, 207

color of, 209

cortex of, 209

cuticle of, 209
Hair-cells, 274
Hair-follicles, 207, 213
development of, 212
medulla of, 209
Hardening, methods of, 291
Hassall, corpuscles of, 200
Haversian canals, 70, 73, 74

system, 7'!
Hearing, 273
Heart, 1 15

cartilage in, 68, 116

valves of, 116
Heat-regulation, 212
Helicine arteries, 237

INDKX.

329

Heliotropisrn, 29

Henle, fenestrated membrane of, 77,
117

loop of, 163

tubes of, 163
Hepatic artery, 154, 156

vein, 154, 156
Heredity, nuclear function in, 32, 231,

232, 240
Horny epithelium, 44
Howship's foveolae, 75
Hyaline cartilage, 68
Hyaloplasm, 29, 33
Hydrotropism, 30
Hymen, 230
Hypophysis cerebri, 197

ILLUMINATION, microscopical,
281
Image, microscopical, 280, 284
Immersion-objectives, 280
Impregnation, methods of, 292

celloidin, 292

collodion, 292

paraffin, 292
Indirect cell-division, 35
Infiltration with fat, 82
Infundibula of lung, 179
Infusion, 27

Interalveolar cell-islets, 151
Interarticular cartilages, 68
Intercellular bridges, 58, 88
substances, 24, 44

in bone, 72

in cartilage, 67

in connective-tissue, 65

in endothelium, 47, 49

in epithelium, 51

in muscle, 86, 88, 94
Interlobular vessels of kidney, 165

of liver, 155
Internal secretion, 44, 64, 151
Interpretation of microscopical ap-
pearances, 89, 183, 278, 281
Interstitial tissue, 80
Interstitium, 112
Intervertebral disks, 68
Intestine, large, 149
lining cells of, 56
muscularis mucosae of, 147
small, 145, 149

villi of, 146, 147
Intima, 116
Intracellular circulation, 30

digestion, 30
Involuntary muscles, 87
Iodothyrin, see Thyroidin, 192
Iris, diaphragm, 280
Iron, hematoxylin stain, 311

Iron, tests for, 319
Islands of Langerhans, 151

KARYOKINKSIS, 35
chromoplasm during, 36
di aster-phase, 39
dispirem-phase, 39
monaster-phase, 37
significance of, 39
spirem-phase, 36
Karyolysis, 220
Karyoplasm, 34
Keratin, 56, 206
Keratoid degeneration, 208
Kidney, afferent vessels of, 165
boundary zone of, 161
capsule of, 170

collecting tubules of, 163, 168
cortex of, 161
efferent vessels of, 166
interlobular vessels of, 165
labyrinth of, 163
lobes of, 161

Malpighian bodies of, 162
medulla of, 161
papillae of, 161, 162, 168
pelvis of, 171
stellate veins of, 147
vascular arcades of, 165
Kidneys, 161
Kinetic energy, 18
Krause, corpuscles of, 267

LABYRINTH of kidney, 163
Lacteals, 120
Lacunae of bone, 70
Langerhans, islands of, 151
Lanterman, clefts of, 105
Lanthanin, 35
Larynx, 176
Layers, embryonic, 22
Lenses, cleaning, 279
Leucocytes, 126, 128, 131

acidophilic, 125, 133

basophilic, 126, 133

eosinophilic, 1 25, 133

large mononuclear, 125, 133

polynuclear neutrophilic, 132
Lieberkuhn, crypts of, 147
Ligaments, 82, 84
Ligamentum nucha?, 85
Linin, 35

Liquor folliculi, 217
Lithio-carmine, 307

methods of staining, 307
Littre's glands, 174
Liver, 154

functions of, 159

interlobular vessels of, 155

330

INDEX.

Liver, intralobular vessels of, 156

lobules of, 155
Lobes of kidney, 161
Lobules of kidney, 162
of liver, 155
of lung, 180
Lung, circulation of, 181
foetal 53

functions of, 181
infundibula of, 179
lobules of, 180
structure of, 177, 183
Lymph, 120, 129, 134
circulation of, 115
-follicles, 80, 121, 139, 148, 151, 185
functions of, 114
formation of, 129
-glands, 12d

afferent vessels of, 122
cortex of, 121
efferent vessels of, 122
medulla of, 121
-nodes, 120
carbon in, 125
function of, 125
-sinus, 122 ,

trabecular of, 121
spaces, 48, 81, 114, 120, 156
Lympbadenoid tissue, 78
Lymphatic glands, 78, 120
valves, 115, 120
vessels, 120
Lymphocytes, 133

MACERATION, methods of, 320
alcohol, 320
chromic acid, 321
potassium hydrate, 320
Malaria, 184

Male organs of generation, 236
Malpighian bodies of kidney, 161, 162

of spleen, 186
Mammary gland, 232
Marrow, 74, 125

mucoid, 125

red, 125

yellow, 12.">, 127
Matrix of cartilage, 67
Maturation of the ovum, 231
Mayer's albumin, 302
Measurements, microscopical, 280
Media, 116, 118,119
Medulla of bone, see Marrow, 125

of kidney, 161

of lymph-glands, 121
Medullary rays, 161

sheath, 103
Medullated nerve-fibres, 102
Meissner, corpuscles of, 267

Membrane, basement, 60, 140, 147, 246

fenestrated, 77, 117
Membranes, mucous, 55, 60
Menstruation, 223
Mercuric chloride solution, 287
Mesentery, 48
Mesoderm, 22
Metakinesis, 37
Metaplasm, 33, 52
Metazoa, 28, 112
Methylene-blue, aqueous, 308

methods of staining, 308

Unna's 308
Microchemical reactions, 318
Micrometer, 280, 281

scales, 280
Micromillimeter, see Micrometer, 280
Microscope, care of, 279

selection of, 279
Microscopical appearances, interpre-
tation of, 89, 183, 278, 285

measurements, 280

technique, 279
Migratory cells, 132
Milk, 234
Mitral cells, 272
Mitosis, see KaryoMnesis, 35
Molecular concentration, 129
Monaster-phase of karyokinesis, 37
Mononuclear leucocvtes, large, 133
Moss-fibres, 260
Motor plates, 110
Mounting, methods of. 315
Canada-balsam, 315, 316
dammar, 315, 316
glycerin, 316
glycerin-jelly, 316
Mouth, lining of, 57
Movement, amoeboid, 29

Brownian, 29

of cilia, 27
Mucin, 78

Mucoid marrow, 125
Mucosum, rete, 205
Mucous membranes, 55, 60
covering of, 55

tissue, 78

" ground-substance " of, 78
Mailer, cells of, 275

fluid of, 285
Multicellular animals, 28, 112
Multipolar nerve-cells, 100
Muscle, cardiac, 94

intercellular substance in, 86

involuntary, 87

nutrition of, 88

smooth, 87

function of, 93

-spindle, 99

INDEX.

331

Muscle, striated, 96
unstriped, 89
voluntary, 96
Muscular cells, 86, 87, 94, 96
sense, 99
tissues, 87
cement in, 88
Muscularis mucosae of intestine, 147
of oesophagus, 142
of stomach, 142, 145
of uterus, 222
Myelin, 103, 104
Myelocytes, 125
Myxoedema, 191

NAILS, 211
Nerve-cells, 101
bipolar, 101

chromophiiic granules of, 102
multipolar, 101
protoplasmic processes of, 103
unipolar, 101
Nerve-fibres, 102
classification of, 102
gray, 102, 106
non-medullated, 102, 106
Nerve-terminations, 109
Nerves, 106

funiculi of, 106
Nervous system, 248

central, ground-substance of, 110
tissues, 100

functions of, 87
Neurilemma, 103, 104
Neurite, 248
Neuroglia, 107
Neurokeratin, 105
Neurons, 248
Neutral carmine, 306
Nipple, 242
Nodes, lymph-, 120

of lianvier, 104
Non-elastic fibres, 76
Non-medullated nerve-fibres, 102, 106
Normal salt solution, see Neutral solu-
tion, 281
Nose, lining of, 55, 269
Nucleated red corpuscles, 119, 123
Nuclei, functions of, 33
Nucleolus, 29, 34
Nucleus, 29, 31, 33

multiple division of, 39
structure of, 34, 39

during cell division, 35, 40
Nutrition :
of bone, 73
of cartilage, 68
of muscle, 84
of tissues, 81, 82, 114, 117

OBJECTIVES, 279
cleaning, 270, 280
immersion, 27!', 280
Ocular micrometer, 280
Oculars, 270
Odontoblasts, 214
CEdematin, 35
GEsophagus, 142

lining of, 54

muscularis mucosae of, 142

"tunica propria" of, 142
Olfactory bulb, 272
layers of, 272

glomeruli, 272
Oil of bergamot, 314

of cedar-wood, 280, 314

of cloves, 314

of origanum, 314
Organs, 112

of generation, female, 215
male, 236
Origanum, oil of, 314

methods of cleaning, 314
Orth's fluid, 286
Osmosis, 117, 120
Ossification of cartilage, 66, 72
Osteoblasts, 74
Osteoclasts, 75, 126
Ovary, 215

germinal epithelium of, 215

stroma of, 215
Ovula Nabothi, 229
Ovum, 20, 215, 217

maturation of, 231

maturing, polar bodies of, 231

PACINIAN bodies, 266
Pal's method of staining, 311
Pancreas, 150
Papilla? of kidney, 161, 162, 168

of skin, 205

of tongue, 13S
Paraffin, 282, 293, 297

embedding, 297

methods of impregnation, 292
Parathyroids, 193
Parenchyma, 112
Parotid gland, 139
Parovarium, 231
Passages, alveolar, 178

respiratory, 53, 176
Pavement-epithelium, 53
Pelvis of kidney, 171

renal, 171
lining of, 58
Penis, 236

trabecular of, 236
Perceptive function, 3C
Pericardium, 115

332

INDEX.

Perichondrium, 67

Perimysium, 98
Perineurium, lOtJ
Periosteum, 74
Peristalsis, 88, 136
Peritoneum, 137
Perspiration, 211
Pever's patches, 149
Phagocytes, 124, 125, 132
Phosphates, earthy, tests for, 318
Pia mater, 109
Picture, color-, 284

structure, 284
Pituitary body, 197
Placenta, 226
Plasma, 114, 129
Plasma-cells, 126
Plates, blood-, 127
Polar bodies of dividing cell, 36

of maturing ovum, 231
Polynuclear neutrophilic leucocytes,

133
Portal vein, 154, 159
Potassium hydrate, 281, 320
Potential energy, 18
Pregnancv, 224
Prickle-cells, 57, 205
Primitive cells, 27, 31

sheath, 104
Processes, axis-cylinder, 100

dendritic, 101

protoplasmic, 100
Projection-fibres of cerebrum, 263
Prostate, 238
Proteids in cytoplasm, 32
Protoplasm, 29

movements in, 29
Protoplasmic processes of nerve-cells,

103
Protozoa, 28, 112
Pseudopodium, 29
Pseudo-stomata, 49
Pulmonary alveoli, 179

lining of, 53
Pulp of spleen, 184

of teeth, 213, 214
Purkinje, cells of, 257
Pyloric glands, 142

RACEMOSE glands, 61, 137
Ranvier, constricting bands of,
L04
nodes of, 104
Rays, medullary, 161
Razor, stropping, 299
Reactions, microchemical, 318
Rectum, 150

lining of, .">;
Red corpuscles, 130

Red corpuscles, nucleated, 125

stroma of, 130
Red marrow of bone, 125
Reduction of chromatin, 240, 245
Renal pelvis, 171
lining of, 58

tubules, 161, et seq.
Reproduction, 20, 30
Reproductive organs, 215
Respiration, 30, 53, 55, 132, 176, 178
Respiratory organs, 176

passages, 53, 55
Rete mucosum, 250

vasculosum, 247
Reticular tissue, 79, 85

" ground substance " of, 80
Retina, 275

sustentacular cells of, 275
Ruga? of stomach, 145

SACCULAR glands, 61
Salivary glands, 139
Salt solution, normal, 281
Salts in cytoplasm, 32
Sarcolemma, -98
Sarcoplasm, 98
Sarcostyles, 96, 98
Sarcous elements, 98
Scales, micrometer, 280
Schwann, sheath of, 104

white substance of, 103
Sebaceous glands, 209
Sebum, 209

Secreting glands, 44, 60
Secretion, 44, 45, 60

internal, 44, 64, 191, 207
Secretory epithelium, 44
Sections of tissues, 282

interpretation of, 89, 183, 278, 281

rapid preparation of, 316

staining of, 303
Sediments, examination of, 318
Seminal vesicles, 239
Seminiferous tubules, 240, 241
Serous cavities, 47, 48
stomata in, 154

membranes, 50
Sertoli, cells of, 242
Sharpey's fibres, 72
Sheath, medullary, 103

primitive, 104

of Schwann, 104
Shiverinsr, 211
Sight, 275

Silver nitrate as reagent, 50
Skin, 204

functions of, 211

papillae of, 205
Slide, micrometer, 280

INDEX.

333

Smell, 269
Smooth muscles, 87
functions of, 93
in heart, 116
Spaces, lymph-, 48, 84, 116
Specialization and structure, 28, 82.

See Functions and Structure.
Special senses, organs of, 266
Specimens, study of, 92, 183, 278, 281
Spermatids, 241, 245
Spermatocytes, 241, 245
Spermatogonia, 241, 245
Spermatozoa, 240, 245
Spinal cord, 250

association-fibres of, 253
collateral fibres of, 253
gray matter of, 250
white matter of, 251
Spindle, achromatic, 38
Spirem, formation of, 36
Spirem-phase of karyokinesis, 36
Spleen, 184
capsule of, 184
function of, 186
Malpighian bodies of, 186
corpuscles of, 186
pulp of, 186
trabecular of, 184
Splenic cells, 186
Spongioblasts, 249 (Fig. 232), 277
Spongioplasm, 29, 33
Squamous epithelium, 379. See Strati-
fied epithelium.
Staining, methods of, 803
carmine, alum-, 306
borax, 307
lithio-, 307
neutral, 306
eosin, 305

fuschin, carbol-, 309
gentian-violet, 309
Golgi's methods, 312
Gram's solution, 310
haematoxylin, 304
iron-haematoxylin, 311
methvlene-blue, 308
Pal's method, 311
Unna's methylene-blue, 308
Van Giesen's stain, 310
Starch, tests for, 319
Stellate cells, 259

veins of kidnev, 167
Stomach, 142

cardiac glands of, 144
crypts of, 142

muscularis mucosae of, 142, 145
rugae of, 145
Stomata, 48
pseudo-, 49

Stratified epithelium, 56, 134, 142, 222
Stratum granulosum, 206

lucidum, 20(i
Striated muscles, 96
Stroma of ovary, 215

of red corpuscles, 131
Stropping, method of, 299
Strumipriva, cachexia, 191
Subcutaneous tissues, 83, 204
Sublingual glands, 139
Submaxillary glands, 139
Substance, contractile, 87, 94, 96
Sudoriparous glands, see Sweat-glands.
Suprarenal capsules, 194
Sustentacular cells of retina, 275

of testis, 231
Sweat-glands, 206
Synchondroses, 68
Syncytium, 227
System, Haversian, 73

nervous, 109, 112, 248

vascular, 114

TACTILE corpuscles, 205, 266
Taste-buds, 268
Teasing, 282

Technique, microscopical, 279
Teeth, 213
Teledendrites, 248
Teleueurites, 248
Temporary cartilages, 68
Tendon, 82, 84
Terminations, nerve-, 111
Testis, 239

sustentacular cells of, 241

trabecular of, 240
Tests, calcium oxalate, 318

carbonates, 318

cellulose, 318

granules, albuminoid, 318
fatty, 318

haemoglobin, 318

iron, 319

phosphates, earthy, 318

starch, 318

urates, 318
Thoracic duct, 122
Thrombus, 127
Thymus, 188, 200
Thyroid body, functions of, 189, 191

cartilage, 68, 176

gland, 188, 189
Thyroiodin, 192
Tissue, adipose, 78

areolar, 80

development of, 80

connective, 65

development of, 65, 78

elementary, 43

334

INDEX.

] issue, elementary, recognition of, 45
embedding, 294
erectile, 236
fibrous, 76

cells of, 7<>
development of, 78

fixation of, 283, 290

interstitial, 82

lympbadenoid, 80

mucoid, 1!»1

mucous, 78

muscular, 87
cement in, 88

nervous, 94

nutrition of. 81, 82, 114, 117

reticular, 79
Tissues, cardiac muscular, 89

connective, 65

elementary, 4.'!

epithelial, 47

fixation of, 283, 290

preparation of, 181

reticular, 77

sections of, 268

interpretation of, 89, 183, 278, 281

smooth muscular, 88

staining of, 303

striated muscular, 96

subcutaneous, 62
Tone, vascular, 93
Tongue, 137

papilla? of, 138
Tonsils, 1~>2
Touch, 266
Trabeculae of lymph-nodes, 121

of penis, 236

of spleen, 184

of testis, 240
Trachea, 176
Tracheal cartilages, 177
Transitional epithelium, 58
Tubes, Fallopian, 220

of Henle, 163
Tubular glands, 61
Tubules, renal, course of, 163
lining of, 168

seminiferous, 240, 241
Tunica adventitia, 118, 120, J 21

albuginea, - 10

granulosa, 21 7

intima, 1 18, 121

media, lis, 120, 121
"Tunica propria" of oesophagus, 142

vaginalis, 240
Tyson, glands of, 237

UMBILICAL cord, 79
Unipolar nerve-cells, 103
Unna's methylene-blue, 308

ETnstriped muscle, 96
Urates, tests for, 318
Ureter, 172

lining of, 58
Urethra, 173
Urinary organs, 171
Urine, secretion of, 190
Uterus, 221

muscularis mucosae of, 222

VACUOLES, 30, 66
contractile, 30
Vagina, 230

lining of, 57
Valves of heart, 115

of lymphatics, 115, 120

of veins, 115, 119
Valvuhe conniventes, 146, 147
Van Giesen's stain, 310
Vasa efferentia, 247

recta, 167, 247
Vascular arcades of kidney, 165

system, 114
Vas deferens, 239
Veins. 119

hepatic, 154, 156

portal, 159

stellate, of kidney, 167

valves of, 115, 119
Ventricles, cerebral, 22
Vermicular movement, 93
Vesicles, seminal, 239
Villi of intestine, 146, 147
Vitreous humor of eye, 79
Vocal cords, 176
Voluntary muscle, 96

analogous to organs, 86

WANDERING cells. 132
Water in cytoplasm, 32
Weismann, fibres of, 99
Whartonian jelly, 79
White corpuscles, 132
fibres, 76

matter of spinal cord, 251
nerve-fibres, 102
substance of Schwann, 103

XYLOL, 314
methods of clearing, 314

YELLOW fibres, 77
marrow, 125

ZENKER'S fluid, 286
Zona irranulosa of adrenal bodvr
1 95
reticularis of adrenal body, 195>
Zymogens, 45, 52, 140

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