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HISTOLOGY

By the same Juthor.

DIRECTIONS FOR CLASS WORK
IN PRACTICAL PHYSIOLOGY:

Elementary Physiology of Muscle and Nerve and
of the Vascular and Nervous Systems.

With 48 Diagrams to shcnu arrangement of Apparatus.
8vo. 3s. net.

LONDON : LONGMANS. GREEN & CO.

A COURSE OF PRACTICAL HISTOLOGY

Containing plain directions for individual

work in Histology.

8vo. 7s. 6d.

LONDON : SMITH, ELDER & CO.

THE essentials''"'*'^

OF

IT 1 S T O L O G Y

DESCRIPTIVE AND PRACTICAL

FOE THE USE OF STUDE.YTS

BY

E. A. SCHAFER, LL.IX, 8c.D., F.K.t^.

PROFESSOR OF PHTSIOLOGY IN THE UNIVERSITY OF EDINBURGH
FORMERLY .lODRELL PROFESSOR OF PHYSIOLOGY IN UNIVERSITY COLLEGE, LONDON

SEVENTH EDITION

LEA BROTHERS & CO.

PHILADELPHIA AND NEW YORK
1907

Ho7

PREFACE TO THE SEVENTH EDITION.

This Book is written with the object of supplying the student with
directions for the microscopical examination of the tissues. At the
same time it is intended to serve as an Elementary Text-book of
Histology ; comprising the essential facts of the science, liut
omitting less important details.

For conveniently accompanying the work of a class of medical
students, the book is divided into fifty lessons. Each of these
may be supposed to occupy from one to three hours, according to
the relative extent to which the preparations are made beforehand
by the teacher, or during the lesson by the students. A few of
the preparations cannot well be made by a class, but it has been
thought advisable not to injure the completeness of the work by
omitting mention of them.

Only those methods are recommended upon which experience has
proved that full dependence can be placed, but the directions given
are for the most part capable of easy verbal modification in accord-
ance with the ideas or experience of different teachers.

The present edition has been considerably enlarged, partly by
additions to the text — especially that descriptive of the structure

vi PKEFACE.

of the central nervous system, a proper knowledge of which is
essential to students of medicine — partly by the provision of new
illustrations derived from many sources. The author desires to
express his recognition of the readiness with which other authors
have placed illustrations at his disposal. This recognition is
especially due to Professor Sobotta and to Professor Ram6n Cajal,
the latter of whom was good enough to lend many of his original
drawings for reproduction in this work.

A new feature is the printing of many of the illustrations in
colour, which it is believed will give a better idea of the appear-
ance of the stained preparations.

To Dr. P. T. Herring, who has read and corrected the final
proofs ; and to Professor Sherrington, who has looked through the
chapters on the central nervous system, the author begs to offer
his grateful acknowledgments.

CONTENTS.

INTRODUCTORY.

PAGE

Enumeration of the Tissues — General Structure of Animal Cells, 1

LESSON I.
Use of the Microscope — Examination of Common Objects, . . 24

LESSONS II. and III.

Human Blood-Corpuscles — Development of Blood-Corpuscles —

Bone-Marrow, . . ' 28

LESSON IV.
Action of Reagents upon the Human Blood-Corpuscles, . . 41

LESSON V.
Blood-Corpuscles of Amphibia, 45

LESSON VI.
Amceboid Phenomena of the Colourless Blood-Corpuscles, . 48

LESSON VII.
Epithelium, 52

viii CONTENTS.

LESSON VIII.

PAGE

Columnar and Ciliated Epithelium, 60

LESSON IX.

Connective Tissues : Areolar and Adipose Tissue — Retiform

Tissue, 67

LESSON X.

Connective Tissues {continued) : Elastic Tissue — Fibrous Tissue-
Development OF Connective Tissue, ...... 78

LESSON XI.

Connective Tissues {continued) : Articular Cartilage — Synovial

Membranes, 86

LESSON XII.

Connective Tissues {continued) : Costal Cartilage— Fibro-

Cartil.\ge, ........... 92

LESSON XIII.

Connective Tissues {continued) : Structure and Development

of Bone, 96

LESSON XIV.
Striated Muscle, 110

LESSON XV.

Connection of Muscle with Tendon — Blood- Vessels of Muscle —

Cardiac Muscle — Development of Muscle — Plain Muscle, 120

LESSON XVI.
Nerve-Fibres, " 128

CONTENTS. ix

LESSONS XVII. AND XVIII.

PAGE

Nerve-Cells — Neuroglia — Development of Nerve-Fibres and

Nerve-Cells — Degeneration and Regeneration, . . .137

LESSON XIX.
Modes of Termination of Nerve-Fibres, 166

LESSON XX.
The Larger Blood-Vessels, 184

LESSON XXI.

Smaller Blood-Vessels — Ltmph-Vessels — Serous Membranes. —
Microscopic Study of the Circulation — Development of
Blood- and Ltmph-Vessels, 191

LESSON XXII.
Lymph-Glands — Tonsil— Thymus, 203

LESSON XXIII.
Spleen — Suprarenal Capsules — Thyroid Body — Pituitary Body, 213

LESSONS XXIV. and XXV.

Skin, Nails, Hairs, etc. — Mammary Glands, 226

LESSON XXVI.
Heart, 250

LESSON XXVII.
Trachea and Lungs, 254

X CONTENTS.

LESSON XXVIII.

PAGE

Structure and Development of the Teeth, . . . . . 263

LESSON XXIX.

Tongue and Mucous Membrane of the Mouth — Taste-Buds —

Pharynx and Oesophagus, 275

LESSON XXX.
Salivary Glands, 281

LESSON XXXI.
Stomach, 287

LESSONS XXXII. AND XXXIII.

Small and Large Intestine, : . 295

LESSONS XXXIV. and XXXV.

Liver and Pancreas, 309

LESSON XXXVI.
Kidney, 320

LESSON XXXVII.

Ureter, Bladder, and Male Generative Organs, . . . 328

LESSON XXXVIII.

Generative Organs of the Female, 344

LESSON XXXIX. AND XL.
Spinal Cord, 355

CONTENTS. xi

LESSON XLI.

PAOK

Medulla Oblongata, 374

LESSONS XLII. AND XLIIL
Pons Varolii, Mesencephalon, and Thalamencephalon, . . 390

LESSONS XLIV. and XLV.
Cerebellum and Cerebrum, 417

LESSONS XLYI., XLVII. and XLVIII.
Eye, 443

LESSON XLIX.
Olfactory Mucous Membrane— External and Middle Ear, . 467

LESSON L.
Internal Ear, 472

APPENDIX.
Methods, 484

INDEX, 501

THE ESSENTIALS OF HISTOLOGY

INTRODUCTORY.

EX U ME RATION OF THE TISSUES AND THE GENERAL
STRUCTURE OF ANIMAL CELLS.

Animal Histology ^ is the science which treats of the minute struc-
ture of the tissues and organs of the animal body ; it is studied with
the aid of the microscope, and is therefore also termed Microscopic
Anatomy.

Every part or organ of the body, when separated into minute
fragments, or when examined in thin sections, is found to consist
of certain textures or tissues, which differ in their arrangement in
different organs, but each of which exhibits characteristic structural
features.

The following is a list of the principal tissues which compose the
body :—

1. Epithelial.

2. Connective : Areolar, Fibrous, Elastic, Adipose, Lymphoid,
Cartilage, Bone.

3. Muscular : Voluntary, Involuntary or plain. Cardiac.

4. Nervous.

Some organs are formed of several of the above tissues, others
contain only one or two.

It is convenient to include such fluids as the hlood and lymph
amongst the tissues, because they are studied in the same manner
and contain cellular elements similar to those met with in some of
the other tissues.

All the tissues are, prior to differentiation, masses of cells (embryonic
cells). In some tissues other tissue-elements become developed
which take the form of fibres. Thus the epithelial tissues are com-
posed throughout life entirely of cells, only slightly modified in

^ From tffToy, a web or texture.
A

THE ESSENTIALS OF HISTOLOGY,

structure, and the nervous and muscular tissues are formed of cells
which are greatly modified to form the characteristic fibres of those
tissues. On the other hand, in the connective tissues an amorphous
material becomes formed between the cells which is termed intercellular
substance or ground substance, and in this substance fibres make their
appearance, sometimes, as in the fibrous connective tissue, in so large
an amount as to occup}' the whole of the intercellular substance, and
greatly to preponderate over the cells. This ground substance, by
virtue of its containing a certain amount of inorganic chlorides, has
the property of becoming stained brown or black by nitrate of
silver and subsequent exposure to light, in which case the cells,
which remain unstained, look like white spaces (cell-spaces) in the
ground substance. When an epithelial tissue is similarly treated, the
narrow interstices between the cells are also stained, from which it may
be concluded that a similar substance exists in small amount between
the cells of this tissue. It has here been termed cement-substance, but
it is better to apply to it the general term intercellular substance.

The cells of a tissue are not always separate from one another, but
are in many cases connected by bridges of the cell-substance, which
pass across the intercellular spaces. This is especially the case with
the cells of the higher plants, but it has also been found to occur in
animal tissues, as in some varieties of epithelium and in cardiac and
plain muscular tissue. Occasionally the connexion of the cells of a
tissue is even closer, and lines of separation between them are
almost or entirely absent. The term syncytium is given to any such
united mass of cells.

Fig. 1.— Diagram of a cell, highly m.\gnified.
p, protoplasm, consisting of hyaloplasm and a network of spongioplasm ; ex, exoplasm ;
end, endoplasm, with distinct granules and vacuoles ; r, double centrosome ; n,
nucleus ; n', nucleolus.

Cells. — A cell is a minute portion of living substance {cytoplasm)^
which is sometimes inclosed by a cell-membrane and always contains a
specially differentiated part which is known as the nucleus.

The cytoplasm of a cell (fig. \, p) is composed of protoplasm, which
consists chemically of proteid or nucleoproteid substances, with which

STRUCTURE OF THE CELL. 3

lecithin, a combination of fatty acid with glycerophosphoiic acid,
and cholesienn, a monatomic alcohol, having many of the physical
characters of fats, appear always to be associated. The protoplasm

V

w

5

Fig. 2. — Successive changes exhibited by ax amceba. (Verworn.)

tends during life to exhibit movements which are apparently spon-
taneous, and when the cell is uninclosed by a membrane a change
in the shape, or even in the position of the cell, may be thereby
produced. This is characteristically shown in the movements of the
unicellular organism known as the amceba (fig. 2) ; hence the name

Fig. 3. — Protoplasjiic strl'cture in a pseudopodidm of a foraminifer
(miliola). (Verworn, after Biitschli. )

amoeboid movement, bj'^ which it is generally designated.^ The proto-
plasm often, but not always, contains a fine spongework, which
takes under high powers of the microscope the appearance of a
network (figs. 1, 3), the remainder of the protoplasm being a clear

^The amoeboid phenomena of cells will be studied later (in the colourless
corpuscles of blood).

THE ESSENTIALS OF HISTOLOGY.

■•*«-*.-

substance which occupies the interstices of the sponge, and may
also cover the surface or project beyond the rest of the cell. A

granular appearance is often produced by
the knots in the network when imper-
fectly observed looking like separate
granules. The material which forms the
reticulum is termed spongioplasni ; the
A V'*">."' ':^.f'ti^^t'^^ clearer material which occupies its meshes
* ^"^ ;;■* ^''*' ; is hyaloplasm. The protoplasm of some
^ ^: . ,* cells shows a considerable degree of

. *'j,. '- differentiation into fibrils which may be

W"^ - unbranched or may form a network

^ . „ , within the cell. Some cells exhibit a

Fig. 4.— Troi'hosi'()N(jium (canal-
isation) WITHIN A GANGLION fine caualisatiou of their protoplasm
CELL. . omgren.) ^^^^ ^^^ ^^^ according to Holmgren the

canaliculi are in many cases occupied by branching processes of other
(nutrient) cells, which form what he terms a "trophospongium."
Protoplasm often, if not always, includes actual granules of a

,'*^A,'

Fig. 5.— Cells from the testicle of the mouse in process of transfor-
mation INTO spermatozoa. (Benda.)

The "mitochondria" are darkly stained and are seen in the successive stages (a to y) to
be arranging themselves so as to constitute the spiral filament of the spermatozoon
{h).

proteid nature. Some of these granules may be essential con-
stituents of the protoplasm (Altmann) ; others are materials which

STRUCTURE O?^ THE CELL. 5

have been formed by the protoplasm, and which are in a sense

accidental inclusions. That the former are of importance appears

to be evident from the fact that many of the chemical changes of

cells occur in them. Moreover they are closely associated with the

most active part of the protoplasm, the part, namely, in the

neighbourhood of the nucleus, and appear to become formed in

this part, and from it to extend through the cell. When fibrils

are formed in the protoplasm, they are believed to be produced

from the granules in question, to which

the name mitochondria has been given

(Benda), (fig. 5). The mitochondria are

sometimes collected into a spherical mass

near the nucleus which stains more

deeply than the rest of the cytoplasm

(fig. 6). To this body the term pam-

imcleus has been applied. The granules

referred to may be regarded as actual

constituents of the cytoplasm, and formed Fig. 6.— Pancreas cells of frog,

■,• ,1 n .. ,1 i • T , SHOWING PARANUCLEUS AXD

directly from Its protoplasm. As indicat- choxdromitome fibrils formed
ing this close connexion with protoplasm ^^?\ ^"tochoxdria. (Gur-

'^ _ '^ '■ witsch, after Matthews.)

they may conveniently be termed deufo-

plasm. This name has also been used to include materials which
are merely included in the cytoplasm and not factors in its constitu-
tion, such as pigment granules, fat globules, and vacuoles containing
watery fluid, with or without glycogen or other substances in solution.
Materials M'hich are thus included in the protoplasm of a cell are
either stored up for the nutrition of the cell itself, or are converted
into substances which are eventually extruded from the cell in order
to serve some purpose useful to the whole organism, or to be
got rid of from the body. The term ixiraplasm may be employed
to denote any such materials within a cell. Paraplasm is often
present in sufficient quantity to reduce the cytoplasm to a relatively
small amount, the bulk of the cell being occupied by other material, as
when starch becomes collected within vegetable cells or fat within
the cells of adipose tissue. It is frequently the case that the para-
plasm and deutoplasm are confined mainly to the middle of the
cell in the neighbourhood of the nucleus, an external zone of the
protoplasm being left clear. The two portions of protoplasm which
are thus somewhat imperfectly differentiated off from one another
are termed respectively the endoplasm and the exoplasm (fig. 1).
They are exhibited in the amoeba (fig. 2), and also in the white
blood-corpuscle (fig. 8).

6 THE ESSENTIALS OF HISTOLOGY

According to the view advocated by Blitschli the apparent reticulum or
spongioplasm of a cell is the optical effect of a soft honeycomb or froth-like
structure : in other words, the meshes of the reticulum do not communicate
with one another as in a sponge, but are closed cavities as in a honeycomb.
Blitschli finds indications of the same alveolar structure in all cells, including
nerve-fibres and muscle-fibres, and has devised experiments with drops of
froth made up of a mixture of oil and alkaline cai'bonate or sugar solution,
which, when examined in water under the microscope, imitate very closely
not only the structural appearance (fig. 7) but even the so-called spontaneous

Fig. 7. — Comparisox of protoplasm with oil and water emulsiox.

A , Protoplasm of Thalassicola.

B. Froth-like appearance of a mixture .of oil and cane sugar. (Verwora, after Blitschli.)

or amceboid movements of actual protoplasm. It may be stated, however,
that although it is a matter of difficulty to determine whether a microscopic
reticulum is a sponge-work or a honeycomb, it is probable that neither
><tructure is essential to living substance, for the outermost layer of the cell
protoplasm, which is usually the most active in exhibiting movements, often
shows no indication of such .structure. And further, it has been shown bv
Hardy that a colloid solution such as that which exists in protoplasm may,
under some circumstances, appear homogeneous and under others may
separate out into two parts, one more solid the other more fluid, and after
.such separation may exhibit either a granular, a reticular, or a honeycomb
structure, according to circumstances. Xor is a "froth" necessary for the
imitation of amoeboid movements, for similar movements, due to changes in
surface tension, are brought about in a simple oil drop or in a drop of
oil-clad albumen when brought in contact with solution of soap or of any
alkali (Berthold, Quincke). A ch'op of any colloid solution containing
electrolytes is also subject to such changes of surface tension when exposed
to varying electrical influences, so that these amoeboid movements, which
are certainly " vital," are capable of being explained by well-known physical
laws

There are grounds for belie\-ing that a very fine pellicle covers the exterior
of the protoplasm of all free cells, and that this pellicle is composed of a
material which, although not .soluble in water, is permeable to watery fluids,
and may also allow the passage of solids without rupture. Such a material
might be furnished by the lecithin and cholesterin (Overton), which are, as
we have seen, constant constituents of cell-protoplasm. It must, however,
be stated that it has not been proved that these substances are collected at
the surface of jnotoplasm.

Properties of living matter. — Living cells exhibit (1) irritability or the
property of responding to stimuli ; (2) metabolic or chemical changes which
result in assimilation or the taking in of nutrient matter and converting it
into living substance (anabolism), and disassimilation^ the property of break-

STRUCTURE OF THE CELL. 7

ing down or getting rid of such substance (katabolism) ; (3) reprodwition
resulting in the multiplication of cells. Of these properties (2) and (3) are
certainly governed or influenced by the cell-nucleus, and (3) appears to be
usually initiated by the centrosome (see below). The irritability of the cell
ilepends, however, mainly upon the cytoplasm itself. It is in consequence of
this pro|)erty that protoplasm reacts, sometimes by contraction sometimes
by relaxation, to mechanical, chemical, thermal, and electrical stimuli, and in
the case of some cells {e.g. the pigment-cells and cones of the retina) to the
stimulus of light. The amoelwid movements of cells are a manifestation of
irritability, being produced and influenced by various external conditions
and stimuli. Sometimes the result of a stimulus is to cause a cell or organism
to move towards the source of excitation (attraction) ; in other cases the
movement is in the reverse direction (repulsion). The terms positive and
negative chemotaxis, phototaxis, thermotaxis, and the like, are used to indi-
cate the nature of the effects produced by various forms of stimulation.

Attraction-sphere and centrosome. — In some cells, as already indi-
cated, there are line but distinct striae or fibrils (cytomifome) running
in definite directions. These are very commonly met with in fixed

Fig. 8. — An amceboid cell (white Fig. 9. — A cell (white bloodcorpuscle)

CORPUSCLE OF XEWT) VERT HIGHLY

MAG.MFIED.
Showing a double nucleus with recticulum
of chromoplasm, and the protoplasm com-
posed of two iwrtions, a clearer exoplasm,
and a gi-anular-looking eudoplasm.

SHOWING ITS ATTRACTION-SPHERE.

(Wilson, after M. Heidenhain.)

In this, as in most cases, the attraction-sphere, a,

lies near the nucleus, n.

cells, such as various kinds of epithelium-cells, nerve-cells, and muscle-
cells. But besides these special differentiations, which appear to be
related to special functions, there are other fibril-like structures in the
cell-protoplasm, associated with what is known as the centrosome (figs.
1, 9). This is a minute particle (centriole), usually situated near the
nucleus, and staining darkly with iron-haematoxylin, surrounded by
a clear area (attraction-sphere), and from it jadiate into the surround-
ing protoplasm a number of fine fibrils with dot-like enlargements
at intervals. The attraction-sphere, Avith its central particle, was
first noticed in the ovum and was at first supposed to be peculiar

8 THE ESSENTIALS OF HISTOLOGY.

to the egg-cell, but it has now been recognised in very many kinds
of cells, and, in animal cells, is of nearly universal occurrence. The
structures in question are frequently double (fig. 1), the twin spheres
being connected by a spindle-shaped system of delicate fibrils
{acliromatic sjnndle) : this duplication invariably precedes the division
of a cell into two.

In some cells the centrioles are multiple ; this is frequently the case with
leucocytes and always with the giant-cells of bone marrow." The material
which immediately surrounds the centrosome, and of which the radiating
fibres and the fibres of the s])indle are composed, is considered by some to
be distinct in nature from the general protoplasm : it has been termed
the archoplasm. It appears clear tliat in some cells the centrosome and
archoplasm may have an existence independent of one another; thus no
centrosome has been found in the cells of the liigher plants, although the
avchoplasmic fibres are very well marked in them during cell-division.

A cell-membrane is rarely distinct in animal cells. When present,
it is usually formed by transformation of the external layer of the
protoplasm, but its chemical nature has not been suflficiently investi-
gated. In plant cells a cellulose membrane is of common occurrence.

The nucleus of the cell (fig. 1, n) is a small vesicle, spherical,
ovoid, elongated, annular, or irregularly lobulated (figs. 8, 9, 10),
embedded in the protoplasm. Cells have sometimes two or more nuclei.
The nucleus is bounded by a membrane which incloses a clear sub-
stance (miclear hi/aloplasm, karpoplasm), and the whole of this substance
is generally pervaded by an irregular network of fibres, some coarser,
others finer {nuclear reticuhim, hiryomitome). The membrane is the
outermost layer of the nuclear reticulum, and may itself have meshes
like a basket-work, thus allowing direct communication between the
hyaloplasm of the cell and that of the nucleus. The knots of the
nuclear reticulum are sometimes very distinct and give an appearance
of conspicuous granules within the nucleus {pseudonucleoli). The nucleus
usuall)' contains a single distinct highly refracting spherical particle
known as the nucleolus. This is sometimes multiple, and occasionally
has a vacuole-like globule in its interior. The material of the nucleolus
diflPers in its chemical and staining reactions from the nuclear
reticulum, but prior to cell-division it becomes indistinguishable
from the substance of the nuclear fibres. Whether it blends with
them or becomes absorbed and removed is at present uncertain. The
nuclear membrane, intranuclear fibres, and nucleoli all stain deeply
with haematoxylin and with basic dyes generally ; this property distin-
guishes them from the nuclear matrix, and they are accordingly spoken
of as chromatic (containing chromatin, which in the nucleus appears to
be chemically identical with nnclein), the hyaloplasm being achromatic.
Sometimes instead of beine united into a network the intranuclear

STRUCTURE OF THE CELL.

9

fibres take the form of convoluted filaments (rhromosomes), having a
skein-like arrangement (fig. 11). This is always found when a
nucleus is about to divide, but it may also occur in the resting

Fig. 10. — Cell from bone-marrow.
(Carnoy.)

p, protoplasm with fine reticulum; v,
nucleus, long and folded, with intra-
nuclear network.

Fig. 11.— Gland-cell of Chirono-
MU.s. (Flemming.)

condition. These filaments may sometimes be seen with high magni-
fication to be made up of fine juxtaposed particles (chromomeres)
arranged either in single or double rows (fig. 12), which impart a
cross-striated appearance to the filament. The chromomeres are
united to form the chromosomes by a non-staining material to which

4 Wi*i^*^««^

Fig. 12.— Spermatocyte of pkoteus, showing chromosomes of nucleus

formed of particles op chromatin united by acromatic filaments.

(Giirwitsch, after Hermann.)

The nucleolus is distinct from the chromosomes. In the cytoplasm an archoplasmic
mass containing mitochondria is seen on the right.

the term linin has been applied. The nuclear fibres are sometimes
clumped together into a solid mass which comprehends the nucleolus
when present, and has the appearance of an enlarged nucleolus. The

10

THE ESSENTIALS OF HISTOLOGY.

fibres within the nucleus have been observed to undergo spontaneous
changes of form and arrangement, but these become much more
evident during its division. The division of the protoplasm is
always preceded by that of the nucleus, and the nuclear fibres
undergo during division a series of remarkable transformations which
are known collectively by the term hni/okinesis (Schleicher) or mitosis
(Flemming). These changes may easily be studied in the division of
epithelium cells, but exactly similar phenomena have been shown to
occur in cells belonging to other tissues.

Fig. 13. — Cell of bladder epithelium, showing amitotic division of
NUCLEUS. (Nemileff.)

/. cj'toplasm ; //. daughter nuclei ; ///. strand of fibrils uniting daughter nuclei.

The simple division of a nucleus by a process of fission without
karyokinetic changes is termed amitotic division : it occurs in com-
paratively rare instances, and is not usually followed by the division
of the cell, so that it is apt to result in the formation of bi-nucleated
and multi-nucleated cells, as in the superficial layer of the epithelium
of the urinary bladder and in some of the giant cells of bone-marrow.

The nucleus of the cell is not only concerned with its division and
multiplication in the manner shown below, but takes an active part
in the chemical (metabolic) processes which occur in the protoplasm.
Hence^ cells depriA'ed artificially of their nuclei do not assimilate
nourishment, and lose any power of secretion they may have
possessed, although the protoplasm may continue for a time to live
and exhibit amoeboid movements.

DIVISION OF CELLS. 11

Division of cells. — The division of a cell is preceded l)y the division
of its attraction-sphere, and this again appears to determine the
division of the nucleus. The latter, in dividing, passes through a
series of remarkable changes, which may thus be briefly summarised

Fig. 14. — Epithelium-cells op salamandra larva in different phases of
DIVISION BY karyokinesis OR MITOSIS. (Flemming.)

as they occur in typical animal cells such as the epithelium cells of
Salamandra : —

1. The network of chromoplasm-filaments of the resting nucleus
becomes transformed into a sort of skein, formed apparently of one
long convoluted filament, but in reality consisting of a number of
filaments (spirem) ; the nuclear membrane and the nucleoli disappear
or are merged into the skein (fig. 14, a to d).

2. The filament breaks into a number of separate portions, often

12

THE ESSENTIALS OF HISTOLOGY

V-shaped, the chromosomes. The number of chromosomes varies with
the species of animal or plant ; in some animals the dividing nuclei
may contain at this stage only four chromosomes ; in man there are
said to be twenty -four in the ordinary or somatic cells ; a like
number occurs in many animals and plants : others have more
or fewer. As soon as the chromosomes become distinct they are
often arranged radially around the equator of the nucleus like a
star {(ister, fig. 14, e, f, g).

Fig. 15. — The prinx'ipal ph.\ses of the nuclear chromatin filaments in the
PROCESS OF ordinary jiitosis OF THE SOMATIC CELL. (Flemming.)

A, skein or spirem ; B, aster with splitting of chromosomes ; C, separation of the split
chromosomes (metakinesis) ; D, continuation of this process ; JF, diaster ; F, di-
spirem. The cell protoplasm is represented in oiitline in F: it has itself undergone
division at this stage. In this figure the (somatic) cells represented are supposed
to have eight chromosomes.

3. Each of the chromosomes splits longitudinally into two, so that
they are now twice as numerous as before (stage of cleavage, fig. 14,
h, i). This longitudinal cleavage may occur at an earlier stage.

4. The fibres separate into two groups, the ends being for a time
interlocked (fig. 14, /, k).

5. The two groups pass to the opposite poles of the now elongated
nucleus and form a star-shaped figure at either pole {diaster, fig. 14, I).
Each of the stars represents a daughter nucleus.

6. 7, 8. Each star of the diaster goes through the same changes as
the original nucleus, but in the reverse order — viz. a skein, at first
more open and rosette-like (fig. 14, m), then a closer skein (fig. 14,. n).

DIVISION OF CELLS.

13

^W-

E ^^

^«^* ^i}»

Fig 1(5.-Hetero-typical axd homo-typical mitosis of the

GENERATIVE CELL. (Flemmmg.)

The asterisk marks the middle, the cross the end of a chromosome.

The changes are to be compared with tl^ose shown in fig^ 15. ^- ^^^^^^tmrn^^cef /^s
the chromosomes are already arranged in pairs betore f^'",'''" hc prophase,

condition (geminal condition, f, ^Ser'''sU<^rand the daughteV nSc^ei^ the
A longitudinal split is seen in ^f Piaster stgo^ndtUeaag .^^ j^^.^

somatic iiumber of chromosomes 0> in «nsm.^^^^^^ ^^^ separation to

Sthe-^L^ghfel nucfe^ rL^^ultf in^a i^dS to one-half the somatic number.

14

THE ESSENTIALS OF HISTOLOGY.

then a network (fig. 14-, o, p, q) ; passing finally into the typical
reticular condition of a resting nucleus.

Fig. 17.— Spermocttes of salamaxdra showing V-shaped chromosomes
AT THE EQUATOR OF THE SPINDLE. (Wilson, after Drliner. j

A, seen in profile ; four chromosomes only arc represented.

B, seen end-on. All twenty-four chromosomes are represented ; the fibrils of the spindle

are seen in optical section.

The splitting and separation of the chromosomes is often spoken of as the-
onetaphase {metakinesis) ; the stages leading up to this beiug termed the
anaphase and those by which the process is terminated the katapha.se or
telophase.

The mode of division of the nuclear chromatin above described is
frequently spoken of as somatic or orcUna.ry mitosis (fig. 15) to distinguish
it from two modes of division which are only seen normally at certain stages
of multiplication of the generative cells, and which are known as hetero-
typical and homotypical mitosis (fig. 16). In the latter the chromosomes do
not undergo the usual longitudinal S23litting, but one half of the total number
passes into each daughter nucleu.*, so that the number of chromosomes iu
each of these is only one half the usual somatic number. This is termed
the reduction-division} Heterotypical mitosis (which immediately precede*
the homotypical) is characterised by a peculiar arrangement of the
chromosomes, tlie split halves of which, before sejjarating to pass to the
daughter nuclei, tend to adhere together in the forul of loops or rings, or
in the case of short straight chromosomes into small quadrangular masses
(tetrads), all of which are observable in various instances of heterotypical
mitosis, (see fig. 19).

It is further noteworthy that the generative cells which later undergo the
reduction-division above described exhibit either immediately (sperm-cells)
or a long while (germ-cells) before the maiotic divisions a remarkable
series of changes in their nuclear chromatin ; the chi-omosomes becoming first
distinct in place of forming a network, then entangled together at one side
of the nucleus (synaptic condition), and finally becoming again distinct, but
now arranged in pairs (gemini) which later take various forms, such as
double rods, loops, or rings as in heterotypical mitosis, but without forth-
with proceeding to nuclear division.

The protoplasm of the cell divides soon after the formation of the
diaster (fig. 14, m). During division fine lines are seen in the proto-

^ "Maiotic division" or "maiosis" of Farmer, Moore and Walker.

DIVISION OF CELLS.

15

plasm, radiating from tlie cetitrosomes at the poles of the nucleus,
whilst other lines form a spindle-shaped system of achmnatir Hhi-es

; 1 ^

VI.

VII.

VIII.

Fie 18 -Diagram showing the changes which occur in the centro-

■ SOMES and nucleus OF A CELL IN THE PROCESS OF MITOTIC DIVISION.
The nucleus is supposed to have four chromosomes.

within the nucleus, diverging from the poles towards the equator
(fig. 18). These are usually less easily seen than the chromatic fibres

16

THE ESSENTIALS OF HISTOLOGY.

or chromosomes already described, but are not less important, for
they are derived from the attraction-spheres. These, with their
centrosomes, as we have -seen, always initiate the division of the
cell ; indeed they are often found divided in the apparently resting
nucleus, the two particles being united by a small system of fibres
forming a minute spindle at one side of the nucleus (fig. 1). When
mitosis is about to take place this spindle enlarges, and as the
changes in the chromatin of the nucleus which have been above

Fig. 19. — Thkek stages of heterotype mitosis in spermatocyte of
TRITON. (Moore.)

a, geminal condition of ehroniosomes ; h, gemini arranged in quadrate loops or tetrads ;
c, separation of tetrads into the duplex chromosomes of the daughter nuclei.

described occur — which changes involve the disappearance of the
nuclear membrane — the spindle gradually passes into the middle of
the mitotic nucleus, with the two poles of the spindle at the poles of
the nucleus, and with the fibres of the spindle therefore completely
traversing the nucleus (fig. 18). The spindle-fibres appear to form
directing lines, along which the chromosomes pass, after the cleavage,
towards the nuclear poles to form the daughter nuclei.

In some cells, especially in plants, the line of division of the proto-
plasm of the cell becomes marked out by thickenings upon the fibres
of the spindle which occur just in the plane of subsequent division,
and have been termed collectively the cell-plate (fig. 20). But in
most animal cells no cell-plate is formed, the protoplasm simply
becoming constricted into two parts midway between the two

DIVISION OF CELLS.

17

daughter nuclei. Each chuightcr coll so fonued retains one of the
two attraction-particles of the spindle as its centrosome, and when
the daughter cells are in their turn again about to divide this centro
some divides first and forms a new spindle, and the whole process
goes on as before.

^^i_.^'

Fig

20. — Cell-plate ix dividing
spore-cell of lilt.
(Gurwitsch, after Zimmermann.)

Fig. 21. — Dividing cell constricted
to form two daughter cells each

WITH CENTROSOME. (Gebei'g.)
The particle at the junction of the daughtei-
cells represents a rudimentary cell-plate.

Earely the division of a nucleus is into three or more parts instead of
two. In such cases the centrosome becomes correspondingly multiplied and
the achromatic system of fibres takes a more complex form than the simple
spindle.

Division of the Ovum. — Usually the two daughter cells are of
equal size ; but there is a notable exception in the case of the ovum,
which, prior to fertilisation, divides twice (by hetero- and homotypical
mitosis respectively) into two very unequal parts, the larger of which
retains the designation of ovum, while the two small parts which
become detached from it are known as the jyolar bodies (fig. 22).
Further, in the formation of the second polar body a redudion-divimm
occurs, and the nucleus of the ovum, after the polar bodies are
extruded, contains only one half the number of chromosomes that
it had previously (e.g. twelve in place of the normal twenty-four in
man, and two instead of four in Ascaris megalocephala (var. bivalens),
fig. 22, C). Should fertilisation supervene the chromosomes which
are lacking are supplied by the male element (sperm-cell), the nucleus
of which has also undergone, in the final cell-division by which it was
produced, the process of reduction in the number of chromosomes to
one half the normal number. The two reduced nuclei — which are
formed respectively from the remainder of the nucleus of the oocyte

18

THE ESSENTIALS OF HISTOLOGY.

(ovum) after extrusion of the polar bodies, and from the head of
the spermatozoon, which contains the nucleus of the sperm-cell — are
known (within the ovum) as the sperm and germ nuclei or the male and

Fig. 22. —Formation of the polar glob-
ules AND reduction OF THE NUMBER OF

chromosomes in the ovum of ascaris
megalocephala.

A, B, ovum showing division of nucleus to form
first polar globule (Van Gehuchten). m, gelatin-
ous envelope of ovum ; m' , membrane dividing
the polar globule from tiie ovum ; cs (in A),
head of spermatozoon.

C, formation of second polar globule (Carney) ;
(/I, first ; g~, second polar globule : n, nucleus
of ovum, now containing only two chromo-
somes ; ns, nucleus formed from head of sper-
matozoon.

Fig. 23.— Ovum of bat with polar bodies and germ- and
sperm-nuclei. (Van tier Stricht.)

The development of the sperm-nucleus from the head of the spermatozoon is very
evident in this case, because the rest of the spermatozoon happens not to have been
thrown off.

DIVISION ()F THE OVUM.

19

female pronuclei. When these blend, the ovum again contains a nucleus
with the number of chromosomes normal to the species (fig. 24, E).

Fig. 24. — Fertilisation and first division of ovum of ascaris megalo-
CEPHALA (slightly modified from Boveri and Wilson).

A, second polar globule just formed ; the head of the spermatozoon is becoming changed

into a reticular nucleus (^), which, however, shows distinctly two chromosomes;
just above it, its archoplasm is shown : the egg-nucleus (J) also shows two chromo-
somes.

B, both pro-nuclei are now reticular and enlarged ; a double centrosorae (a) is visible

in the archoplasm which lies between them.

C, the chromatin in each nucleus is now converted into two filamentous chromosomes ;

the centrosomes arc separating from one another.

D, the chromosomes arc more distinct and shortened ; the nuclear meml^ranes have dis-

appeared ; the attraction-spheres are distinct.

E, mingling and splitting of the four chromosomes (e) ; the achromatic spindle is fully

formed.

F, separation (towards the poles of the spindle) of the halves of the split chromosomes,

and commencing division of the cytoplasm. Each of the daughter cells now has
four chromosomes ; two of these have been derived from the ovum nucleus, two from
the spermatozoon nucleus.

20

THE ESSENTIALS OF HISTOLOGY.

When it divides after fertilisation each daughter cell is found to
contain the normal or somatic number of chromosomes, derived from
the splitting of both male and female elements, half the number
from the one and half from the other.

Fig. 25. — Human ovum (oocyte) from graafian follicle : examined fresh

IN LIQUOR FOLLICULI WITH VERT HIGH MAGNIFYING POWER. (Waldeyer.)

The cells which are attached to the outside, and which appear to be joined into a syn-
cytium ai'ound the ovum, are follicular cells belonging to the discus proligerus. Within
them is the clear membrane of the ovum (zona pellucida). The cytoplasm of the
ovum shows a distinction between clear ectoplasm and granular endoplasm : the
large granules are yolk particles. The nucleus (germinal vesicle) is clear. The
nucleolus (germinal spot) is distinct.

Formation of the tissues. — It appears to be established beyond
doubt that new cells can only be formed from pre-existing cells.

FORMATION OF THE TISSUES.
A ^ B 0

21

G

FIG. 2fi.-FORMATIOK OF BLASTODERM IN BABBIT BY ^I^I^^Oj;^ J^^^^" '''™

A NUMBER OF CELLS. (Allen Thomson, after E. v. Beneden )
A\c^■R division of ovum and formation of "mulberry mass" : p gl, polar g^o'^i'l^^ ' "V^
^ '"ceVof primar/division which already show a differon^ of -PPe-aBce Th s a 1
differentiation is not, however, accepted by ""^f „^"^^°"^^„"^^- enucida)!*^ subzonal
ovum in subsequent stages, zp, membrane of °^Xiwto the uterSe mucous

SKouId be .b»wn in t esteadrng .U rouDd the inner ceU.m,».

22

THE ESSENTIALS OF HISTOLOGY.

In the early embryo the whole body is an agglomeration of cells.
These have all been formed from the ovum or egg-cell (fig. 25), which,
after fertilisation, divides first into two cells, these again into two,
and so on until a large number of cells (embryonic cells) are pro-
duced. These form at first an outer clear stratum lying at the
surface (fig, 26 sz) and a darker granular mass attached to this
layer at one part, but elsewhere separated from it by clear fluid.
Eventually the cells of the inner mass arrange themselves in the
form of a membrane {blastoderm) which is composed of three layers.
These layers are known respectively as the ectoderm, mesoderm, and
en.toclervi. The ectoderm gives rise to most of the epithelial tissues
and to the tissues of the nervous system ; the entoderm to the
epithelium of the alimentary canal (except the mouth), and the glands
in connection with it ; and the mesoderm to the connective and
muscular tissues.

f7?^f^^^'^M^>7??7?7^'^^

jZ^f^Si

'Ml

Fig. 27.— Section of blastoderm showing the commencing formation of
THE MESODERM. (Kolliker.)

(J), ectoderm ; hy, entoderm ; tm, mesoderm ; ox, axial part of ectoderm with cells under-
going division (t). The mesoderm is growing from this part.

The tissues are formed either by changes which occur in the inter-
cellular substance, or by changes in the cells themselves ; frequently
by both these processes combined amongst the cells which are least
altered from their embryonic condition are the white corpuscles of the
blood, and these may be regarded therefore as typical animal cells.

The histogenetical relation between the three layers of the blasto-
derm and the several tissues and organs of the body is exhibited in the
following table : —

.The epithelium of the skin or epidermis, and its appendages, viz., the hairs, nails,

sebaceous and sweat glands, and mammarj' glands.
The epithelium of the mouth, and the epithelium of the anus and anal canal.
The salivary and other glaud.*) which open into the mouth. The enamel of the

teeth. The gustatorj' organs.
The epithelium of the lower part of the urethra and vagina.
The epithelium of the nasal passages, and of the cavities and glands which open
Ectoderm -' into them.

The epithelium covering the front of the eye. The crystalline lens. The retina.
The epithelium lining the membranous labyrinth of the ear. The epithelium

lining the external auditory meatus.
The epithelium lining the central canal of the spinal cord, the aqueduct of Sylvius,

and the fourth, third, and lateral ventricles of the brain.
The tissues of the nervous system.
^The pituitary body. The pineal gland. The medulla of the suprarenal capsules.

ORIGIN OF THE TISSUES,

23

'The connective tissues.
The blood- and lyiniih-corpuscles.
The spk'cii aiul other vascular and lymphatic glands.

The oi)itbclial lining of the heart, blood-vessels, lymphatics, and serous mcm-
Mesoderni. - brancs (endotlielium).

The epithelium of the urinifcrous tubules.

The epithelium of the internal genenitive organs, and the generative products in
both sexes.
I, The muscular tissues, voluntary, involuntary, and cardiac.

'The epithelium of the alimentary canal (from the pharynx to the lower end of
the rectum) and of all the glands which open into it (including the liver and
paiuTcas).
Entoderm -i '^'^ epithelium of the Eustachian tube and cavity of the tympanum.

■ "^ The epithuliuin of the larynx, trachea, and bronchi, and of all their ramifications.
The e|iithuliinn of the pulmonary alveoli.
The thyroid body. The thymus gland.
,The epithelium of the urinary bladder and ureters, and of part of the urethra.

1 All the connective tissues, the endothelium (mcsotheliurn) of the vascular system, and the
vascular and lymjihatic glands are formed from a special jiart of the mesoderm termed parnhlast
or mesenchyme, which consists of a syncytium of branched cells with a homogeneous intercellular
matrix. Plain muscular tissue is for the most part also formed from mesenchyme, but in
certain situations, as in the sweat glands and muscular tissue of the iris, it is said to be ecto-
dermal in origin.

24

THE ESSENTIALS OF HISTOLOGY.

LESSON I.

USE OF THE MICROSCOPE. EXAMINATION OF
CERTAIN COMMON OBJECTS.

The requisites for practical histology
are a good compound microscope ; slips
of glass technically known as ' slides,'
upon which the preparations are made ;
pieces of thin glass used as covers for
the preparations ; a few instruments,
such as a microtome, a scaljael, scissors,
forceps, and needles mounted in wooden
handles ; and a set of fluid re-agents
for mounting and staining microscopic
preparations.^ A sketch-book and pencil
are also necessary, and must be constantly
employed.

The microscope (fig. 28) consists of a
tube {t t') 160 millimeters (6"4 inches) long
having two systems of lenses, one at the
upper end tei'raed the 'eye-piece' or
'ocular' {oc), the other at the lower end
termed the ' objective ' {ohj). For ordi-
nary work there should be at least two
objectives — a low power working at about
8 millimeters (^ inch) from the object,
and a high power, having a focal dis-
tance of about 3 millimeters (^ inch) ;
it is useful also to have a lower power
(commanding a larger field of view) for
finding objects readily, and two or more
oculars of different power. The focus
is obtained by cautiously bringing the
tube and lenses down towards the
object by the coarse adjustment, which
is usually a rack-and-pinion movement
(adj), and focussing exactly by the fine
adjustment, which is always a finely cut
screw (adj).

The stage (st) upon which the prepar-
tions ai'e placed for examination, the
mirror (?h) which serves to reflect light
up through the central aperture in the
stage and along the tube of the instrument,
and the diaphragm (d) below the stage

Fig

Diagram of mickoscope.

^ The directions for making the principal fluids used in histological work will be
found in the Appendix.

MICROSCOPICAL EXAMINATION OF COMMON OBJECTS. 25

which is used to rej^idate tlie amount of Ijirht thus thrown up, are all
parts the einj)loyuieiit of which is readily understood. A substat^e con-
denser (not shown in the diagram), which serves to concentrate the light
thrown up by the mirror to the centre of the object, is valuable when
higli powers and stained preparations are employed.

The combinations of objectives and oculars above referred to will
generally give a magnifying power of from .W to 400 diameters, and this
is sutticient for most purposes of histology. But to bring out minute
points of detail in the structure of cells and of certain tissues examination
with much higher magnifying powers may be necessary. Objectives of
higli power are usually made as immersion -lenses ; i.e. they are constructed
to form a proper image of the object when the lowermost lens of the
system is immersed in a layer of li(iuid which lies on the cover-glass of
the object and has a refractive index not far removed from that of the
glass itself. For this purpose either water or an essential oil (oil of cedar-
wood) is used. The advantages obtained by the employment of these lenses,
especially those for oil-immersion, are : — increased working distance from
the object, increased angle of aperture with sharper definition of the object,
and increased amount of light traversing the microscope.

The best lenses for histological work are made of the so-called 'apochro-
matic' glass ; specially constructed 'compensating' eye-pieces are used with
these.

A scale for measuring objects should be constructed for each microscope.
To do this, put a stage-micrometer (which is a glass slide ruled in the centre
with lines y\j and too niillimeter apart) under the microscope in such a
manner that the lines run from left to right (the microscope must not be
inclined). Focus them exactly. Put a piece of white card on the table at
the right of the microscope. Look through the instrument with the left
eye, keeping the right eye open. The lines of the micrometer will appear
projected upon the paper. Mark their apparent distance with jieucil upon
the card, and afterwards make a scale of lines in ink, of the same interval
apart. A magnified representation is thus obtained of the micrometer
scale. Mark upon it the number of the eye-piece and of the objective,
and the length of the microscope-tube. This scale-card will serve for the
measurement of any object without the further use of the micrometer.
To measure an object, place the scale-card upon the table to the right of
the microscope and view the object with the left eye, keeping the right
eye open. The object appears projected upon the scale, and its size in ^
or Yw ^f a millimeter can be read off. It is essential that the same
objective and eye-piece should be employed as were used in making the
scale, and that the microscope tube should be of the same length. The
lines on English stage-micrometers are often ruled ^j^ and i-jpf^o inch apart.^

Before beginning the study of histology the student should endeavour
to familiarise himself with the use of the microscope, and at the same
time learn to recognise some of the chief objects which are liable to occur
accidentally in microscopic specimens. On this account it has been con-
sidered desirable to introduce directions for the examination and recogni-
tion of starch-granules, moulds and torulae, air-bubbles, linen, cotton, and
woollen fibres, and tlie usual constituents of the dust of a room, into the
first practical lesson (fig. 29).

1. Examination of starch-granules. Gently scrape the cut surface of a
potato with the point of a knife ; shake the starch-granules so obtained
into a drop of water upon a clean slide and apply a cover-glass.

With the low power the starch granules look like dark specks differing

1 For the method of measuring with an ocular micrometer, and for determining
the magnifying power of a microscope, the reader is referred to the author's
Course of Practical Histology.

26

THE ESSENTIALS OF HISTOLOGY.

m

>\

-•#^'

/-

n

■4^

'.■ \

/ \

. 1

y^y^

"^

-i^

Fig. 29.— Objects frequently present in microscopic preparations.

1, starch granules ; 2, a small air bubble and i)art of a large one ; 3, yeast torulaj ; 4, a
mould (Aspergillus glaucus) ; 5, linen fibres ; 0, cotton fibres ; 7, wool ; 8, hair,
human ; 9, epithelium scales ; 10, micrococci ; 11, bacilli and spores (B. subtilis).
Magnified about 250 diameters.

MICROSCOPICAL EXAMINATION OF COMMON OBJECTS. 27

coiisidenihly in size ; iiiuler tlie liii,'h ])()\ver they are clear, flat, ovoid
particles (lig. 29, 1), with a sharp outline when exactly focussed. Notice
the chanije in appearance of the outline as the microscope is focus-sed up
and down. On close examination fine concentric lines are to be seen in the
granules arranged around a minute s])ot which is generally placed eccen-
trically near the smaller end of the gianule. Sketch two or three starch
granules.

Notice the appearance of air-bubble.s in the water. If comparatively
large they are clear in the middle, with a broad dark border due to
refraction of the light ; if small they may look entii'ely dark.

2. Examine some yeast which has been grown in solution of sugar.
Observe the yeast-particles or torulw, some of them budding. Each torula
contains a clear vacuole, ami has a well-defined outline, due to a membrane.
Sketch two or three toruhe.

3. Examine some mould in water. Notice the long branching filaments
(hyphiv), and also the torula-like particles (spores) from which hyphse may in
some instances be seen sprouting. Sketch part of a hypha.

4. Examine fibres of linen and of cotton in water, using a high power.
Compare the well-defined, rounded, relatively straight or but slightly
twisted linen, with the longer, broader but thinner, and more twisted
cotton fibres. Sketch one of each kind.

5. Mount one or two hairs from the head in water and look at them first
with the low, then with the high power. Examine also fibres from any
woollen material and compare them with the hairs. They have the same
structure, although the wool is finer and is curled ; its structure may be
obscured by the dye. Draw one or two of each.

6. Examine a drop of hay infusion, which has been standing a day or two,
for bacteria and other putrefactive organisms. The active movements which
these exhibit are due to minute cilia or flagella, which can only be made
.visible by special staining methods and a very high magnifying power. Any
minute inactive particles, organic or inorganic, which occur in this or other
fluids may be seen to exhibit a peculiar tremulous dancing movement which
is known as the 'Brownian' movement.

7. Examine some dust of the room in water with a high power. In
addition to groups of black particles of carbon (soot) there will probably
be seen fibres of linen, cotton, or wool, and shed epithelium-cells derived
from the epidermis.

28 THE ESSENTIALS OF HISTOLOGY.

LESSONS II. AND III.
STUDY OF THE HUMAN BLOOD-CORPUSCLES.

1. Having cleaned a slide and cover-glass, prick the finger above the nail
and mount a small drop of blood quickly, so that it has time neither to dry
nor to coagulate. Examine it at once with the higli jjower.

Note {a) the coloured corpuscles mostly in rouleaux and clumps, but some
lying apart seen flat or in profile ; (6) the colourless corpuscles, easily made
out if the cover-glass is touched by a needle, on account of their tendency to
stick to the glass, whilst the coloured corpuscles are driven past by the
currents set up ; (c) in the clear spaces, fibrin-filaments and elementary
particles or blood-platelets.

Sketch a roll of coloured corpuscles and one or two colourless corpuscles.
Count the number of colourless corpuscles in a field of the microscope.

2. To be made as in § 1 , but the drop of blood is to be mixed upon the slide
with an equal amount of isotonic saline solution, so that the red corpuscles
tend to be less massed together, and their peculiar shape is better displayed.

Sketch a red corpuscle seen on the flat and another in profile or (optical
section). Also a crenated corpuscle.

Measure ten red corpuscles, and from the results ascertain the average
diameter of a corpuscle. Measure also the largest and the smallest you can
find.

3. Make a preparation of blood as in § 1 and put it aside to coagulate*
Keep the edges from drying by placing it in a moist chamber or by occasion-
ally breathing upon it. After a few minutes place a drop of 1 p.c. methyl
violet at one edge of the cover and allow this to pass in and mix with the
blood : it may be drawn through the preparation by applying a small piece
of blotting paper to the opposite edge. The dye stains the nuclei of the
white corpuscles, the blood-platelets, the network of fibrin-filaments, and
the membranes of the red blood-corpuscles.

The three preparations just described cannot be kept, but the two follow-
ing will serve as permanent preparations of blood : — ■

4. To fix and stain the coloured corpuscles : — Place upon a slide a drop of
1 p.c. osmic acid mixed with an equal amount of saturated aqueous solution
of eosin. Prick the finger, and mix the blood directly with the coloured
fluid, stirring them together with a needle. Cover the mixture and put
aside for an hour, protected from evaporation ; then place a drop of glycerine
and water at the edge of the cover-glass. When this has passed under fix
the cover-glass with gold size.

5. To study the granules of the colourless corpuscles and their different
reactions to staining reagents, a film of blood is inclosed between two cover-
glasses, which are at once separated and the film on each quickly dried in the
air. A slide may be used instead of a cover-glass ; the drop of blood is placed
close to the ground edge of one slide and this is drawn evenly over the
middle of another. The films are fixed by immersion for one hour or more
in a mixture of alcohol and ether, equal parts of each. They are then stained
by (1) a saturated solution of eosin in 75 p.c. alcohol (three minutes), after
which they are rinsed with water, and are then treated with (2) a 1 p.c.

sTri>Y OF riiK Hr:vrAN hi.ood-c^orpuscles.

29

Fig. 30. — H-EMacytomkikk si, mi:, ullku in stiUAKKs for the knumkiiation

OK BLOOD-CORPUSCLES.

Fig. 31.— Oliver's apparatus for estimating the number of corpuscles
in blood by means of the opacity method.

a pipette for measuring blood ; b, dropper for adding mixing solution ; c, graduated
' tube ; d, mode of observing.

a, b, c, natural size.

30 THE ESSENTIALS OF HISTOLOGY.

solution of metliylene blue (one minute). The film is again rinsed with
water, rapidly dried, and mounted in xylol balsam or dammar.^

6. Mount in xylol balsam or dammar sections of marrow from a long
bone of a rabbit fixed with mercuric chloride or forniol and stained with
eosni and methylene blue. Observe the fat-cells, the supporting reticular
tissue, the proper marrow-cells in this ti.ssue, the myeloplaxes and the
erythroblasts.

7. Tease in salt solution or serum some of the red marrow from the rib of
a recently killed animal. Observe and sketch the proper marrow cells and
look for myeloplaxes and nucleated coloured blood-corpuscles (erythroblasts).

8. Make a film preparation of red marrow by smearing a little upon a
cover-glass or slide, allowing it to dry quickly, and placing it in a mixture of
equal parts of ether and alcohol. After an hour or more in this, the prepara-
tion may be stained with eosin and methylene blue in exactly the same way
as a film preparation of blood (see § 5), and mounted in xylol balsam or
dammar.

9. Enumeration of the blood-corpuscles. This is done by some form of
blood-counter such as the hsemacytometer of Gowers, or the similar apparatus
of Thoma. This instrument consists of a glass slide (fig. 30), the centre of
which is ruled into ^^ millimeter squares and surrounded by a glass ring
j\j mm. thick (in Gowers' instrument, the ruling is into i mm. squares with
a ring I mm. thick). There must also be provided a pipette (fig. 31, a) for
measuring the blood, constructed to hold about .5 cubic millimeters of fluid ; a
dropper (fig. 31, b) to deliver the diluting solution; a small cylindrical mixing-
glass, not shown in the figure, with a mark indicating 100 times the capacity of
the blood pipette ; a small glass stirrer, and a guarded needle. The diluting
solution may either be that of Hayem, viz. distilled water 200 cc, sulphate
of soda 5 grms., common salt 1 grra., corrosive sublimate 0'5 grm., or that of
Mai'cano, viz. 97 cc. of a solution of sulphate of soda in distilled water of sp.
gr. 1020, to which is added chloride of .sodium 1 grm., and formol 3 cc.
A little of the diluting solution is put in the mixing vessel, the finger is
pricked, and the pipette filled exactly with blood (by capillarity). The blood
is then washed out of it with diluting solution, by aid of the dropper, into the
mixing vessel, which is now filled up to the 100 mark with diluting solution,
and the blood and this are thoroughly mixed. A drop of the mixture is next
placed in the centre of the cell, the cover-glass is gently laid on (so as to
touch the drop, which thus forms a layer ^'^y mm. thick between the slide
and cover-glass), and pressed down by two brass springs. In a few minutes
the corpuscles have sunk to the bottom of the layer of fluid and rest on the
squares. The number in ten squares is then counted, and this, multiplied
by 100, gives the number in a cubic millimeter of the mixture, or if again
multiplied by 100 (the amount of dilution) the number in a cubic millimeter
of blood.

For the enumeration of the white corpuscles the blood is diluted only 10
times instead of 100 times. It is also convenient to use one half per cent,
solution of acetic acid just coloured with methyl violet as a diluent (Thoma).
This destroys the coloured corpuscles and stains the nuclei of the white.

A rapid method of estimating the number of colouVed corpuscles is that
devised bj' G. Oliver. The blood is taken up as before in a capillary pipette
(fig. 31, «), and is washed out of this with Hayem's fluid by the dropper, b,
into a flattened graduated glass mixer, c, the diluent being added until the
flame of a small wax candle in a dark room will just show sharply through
the mixture, when the vessel is held close to the eye and about ten feet from
the candle and so that the light traverses the greater thickness of fluid. The

^ Other stains, such as Elu'lich's tri-aoid and the Ehrlich-Biondi, may also be
employed for films.

STUDY OF THE HUMAN BLOOD-CORPUSCLES. 31

graduations are so arranged that for noinial blood (5,000,000 corpuscles per
cub. mm.), the mixture will now stand exactly at the 100 mark : if the blood
contain more or fewei- corpuscles than normal, it will re(iuire a greater or
less dilution to attain the requisite tianslucejicy, and the mark at which the
mixture then stands will indicate the percentage of corpuscles as compared
with the normal.

Another rapid method of estimating the relative number of blood-cor-
puscles is to (ieternnne the corpuscular volume in a known amount of blood.
This is done by the use of the lucmatocrit, in which the blood, suitably
diluted, is centrifugalised and the volume of corpuscles read off on a scale.

The coloured blood-corpuscles. — The coloured corpuscles are com-
posed of a delicate colourless highly elastic (1 protoplasmic) envelojye,
and coloured Jluid contents, consisting mainly of a solution of haemoglobin.

Fig. 32.— Human red blood-corpuscles : Photograph magnified 650 diameters.

The existence of such an envelope is shown by the osmotic effect of
water upon the corpuscle, which passing in through the envelope,
distends, and eventually bursts the corpuscle and sets free the con-
tents. The description which is current in many text-books that
the red corpuscles consist of a porous solid stroma, permeated with
dissolved haemoglobin, is incompatible with this and similar reactions.
Moreover, the envelope can be distinctly seen with the microscope,
especially in the amphibian corpuscle, and can be stained by reagents.
The envelope contains lecithin and cholesterin in considerable amount,
and these substances impart a certain greasiness to the surface of
the corpuscle. It is in all probability due to such greasiness that

32 THE ESSENTIALS OF HISTOLOGY.

the corpuscles run together into rouleaux when the blood comes to
rest (see p. 43).

Under the microscope blood is seen to consist of a clear fluid
{plasma), in which are suspended the blood -corjmsdes (fig. 32). The
latter are of two kinds : the red or coloured (erythrocytes), which are
by far the most numerous, and the white, pale, or colourless {leucocytes).
In addition to these more obvious corpuscles, blood contains a
variable number of minute particles which were termed by Zimmer-
mann the elementary particles of the blood, but which are now more
usually known as the hlood-ptlatelets on account of their flattened form.

Erythrocytes. — When seen singly the coloured corpuscles are not
distinctly red, but appear of a reddish-yellow tinge. In the blood
of man and of all other mammals, except the Camelidiie, they are
biconcave circular disks. Their central part usually has a lightly
shaded aspect under a moderately high po\ver, but this is due to their
biconcave shape, not to the presence of a nucleus. They have, as just
stated, a strong tendency to become aggregated into rouleaux and
clumps when the blood is at re.st, but if it is disturbed they readily
become separated.

If the density of the plasma is increased in any way, as by evapora-
tion, many of the red corpuscles become shrunken and crenated by
the passage of water out of the corpuscle. On the other hand, a
diminution in the density of the plasma tends to cause the red
corpuscles to become cup-shaped, but it is erroneous to describe this as
the normal form of the corpuscle.

The average diameter of the human red corpuscle is 0"0075 milli-
meter^ (about ■3-i>Vo inch), but a few will always be found somewhat
larger (0-0085) and a few somewhat smaller (0'0065 mm.).-

There are from four to five millions of coloured corpuscles in a
cubic millimeter of blood.

Leucocytes. — The colourless corpuscles of human blood are proto-
plasmic cells, averaging O'Ol mm. (^^oo i"ch) in diameter when
spheroidal, but they vary much in size. The}^ are far fewer than
the coloured corpuscles, usually numbering not more than eight to
ten thousand in a cubic millimeter (about 1 to 600 red corpuscles).
Moreover, they are specifically lighter, and tend to come to the
surface of the preparation. If examined immediately the blood is
drawn, they are spherical in shape, but soon become flattened and

^Also expi-essed as 7 "5 fx or micromillinieters ; a micromillimeter being yJ^jj
millimeter.

'- The following list gives the diameter in parts of a millimeter of the red blood-
corpuscles of some of the common domestic animals: — Dog, 0'0073; rabbit,
0-0069 ; cat, 0-0065 ; goat, 0-0041.

LEUCOCYTES.

33

then irregular in form (fig. 33), and their outline continually alters,
owing to the amoeba-like changes to which they are subject. In some
kinds {phagocytes) the protoplasm tends to take in foreign particles with
which the cells come in contact ; in others there seems to be little or no
such tendency. Some of the colourless corpuscles are very pale and
filled with fine granules, others contain coarser and more distinct
granules in their protoplasm ; others again have a hyaline protoplasm
without any apparent granules. In some corpuscles {lymphocytes)
the protoplasm forms only a relatively thin coating to the nucleus.
The corpuscles are classified according to the character and appearance
of the nucleus and the nature and staining qualities of the granules in

Fig. 33. — Three .vmceboid white corpuscles of the newt, killed by
instant.vneous application of steam.

a, eosinophil cell ; h, c, polymorphous cells. The nuclei appear multiple, but are seen to
be connected with fine filaments of niiclear substance traversing the protoplasm.

the protoplasm. Thus some granules are readily stained by basic dyes
such as methylene blue, and such granules are accordingly termed
basophil. Distinct coarse basophil granules are, however, rare in
normal blood, although cells with these granules are normally present
in the marrow and in some connective tissues, and make their appear-
ance in the blood in leucocythsemia. On the other hand, some
granules more readily take up colour from acid dyes, such as eosin,
and these have been termed oxi/phil or eosmophil. Other cells possess
granules (amphophd) which are stained by both acid and basic dyes ; and
others chiefly by neutral dyes {neutrophil). In some cells more than
one kind of granule is met with. The protoplasm may also contain
clear spaces or vacuoles. Each leucocyte has at least one nucleus,
which is difficult to see in a fresh preparation, but is easily seen
after the action of most reagents and after staining. There is also a
centrosome with attraction-sphere, but special methods of staining are
necessary to exhibit these. (See fig. 9, p. 7.)

The following are the chief varieties of leucocytes : — 1. Polymorphs.
Cells with lobed or multipartite nuclei and a relatively large amount of
protoplasm, which is highly amoeboid (fig. 33, h and r). These are often
termed multi-(poly-)nuclear, but the nucleus is rarely if ever multiple,

34 THE ESSENTIALS OF HISTOL(XtY.

its several parts being nearly always joined by threads of nuclear
substance. The cells in question vary in size, but when spherical are
usually not quite 0"01 mm. in diameter. Their protoplasm stains
with eosin, this being due to the presence of fine oxyphil granules
(Kanthack and Hardy). They are highly amoeboid and phagocytic,
and constitute from sixty to seventy per cent, of all the leucocytes
of the blood (&g. .34, a).

Fig. 34. — Variou.s kixds of colourle.ss corpcscle.s, showing the different
CHAR.vcTERS OF THE GRANULES. ( From a film preparation of normal human
blood.) Two of each kind are represented.

2. LymphocyteH. — These are small cells, with a very limited
amount of clear protoplasm around the nucleus, which is simple,
not lobed or divided (fig. 34, hj. The amoiboid phenomena are
less marked in them than in the other varieties of leucocytes.
The protoplasm stains with methylene blue. They are about 0*0065
mm. in diameter, but some are larger and appear to be transitional
between this and the next variety. They constitute from fifteen to
thirty per cent, of the total number of leucocytes in the blood. They
are relatively more numerous in infancy.

3. Macrocyfes. — Large uninucleated cells similar to the last, but
larger, and containing much more protopla-^im (fig. 34, c). Some,
however, are smaller and are regarded as transitional forms from the
last variety. The nucleus may be spherical, oval, or kidney-shaped.
The protoplasm is hyaline : it stains slightly with methylene blue,
perhaps owing to very fine basophil granules. These cells are highly
amoeboid and phagocytic. Including the transitional forms, they
constitute about five per cent, of all the leucocytes in blood.

4. Eosinophils. — These are characterised by their coarse granules,
which stain deeply with acid dyes, such as eosin. Their average

LEUCOCYTES.

35

diameter in the spherical condition is UOl mm. The nucleus may be
simple or lobed (fig. 34, d ; fig. 33, a). They are ameboid, but less
actively so than the finely granular cells. They are more variable in
number than the other varieties, constituting sometimes not more than
one per cent., and at other times as much as ten per cent, of the
total leucocytes of blood.

5. Baiiophih. — These are rarely if ever found in normal blood (adult),
but occur in children and in certain pathological conditions affecting
the bone marrow.

Blood-platelets. — In the clear fiuid in which the blood-corpuscles are
suspended, a network of fine straight intercrossing filaments (fibrin)

Fig. 35.— Network ok fibrin, shown

AFTKR W.^SHINCt AW.\Y THE CORPUSCLES
FROM A PREFAR.\TION OF BLOOD THAT
HAS BEEN ALLOWED TO CLOT ; MANY OF
THE FILAMENTS RADIATE FROM SMALL
CLUMPS OF BLOOD-PL.\TELETS.

Fig. 36. — Blood-corpuscles and elemen-
tary PARTICLES OR BLOOD-PLATELETS,

WITHIN A SMALL VEIN. (From Osier.)

ACi-^v?^;;^, Fig. .38. — Blood-pl.4TELets, highly

magnified, showing the amceboid
forms which they assume when
examined under suitable condi-
tions, and also exhibiting the
chromatic particle which each
platelet contains, and which has

BEEN REGARDED AS A NUCLEUS. (After

Fig. 37. — A mass of blood-platelets, Kopsch.)

FROM HUMAN BLOOD. (Osier.)
A few at the edge ai-c detached from the rest.
The preparation had been kept in salt solu-
tion on the warm stage for some time, thus
causing a partial breaking up of the mass of
platelets. These will be observed to have
filaments attached to them.

soon makes its appearance (fig. 35). These often seem to radiate from
minute round colourless discoid particles less than one-third the diameter
of a red corpuscle, either separate or collected into groups or masses,
of variable, sometimes of considerable, size. These are the elementary
particles, blood-platelets, or thrombocytes. In the blood-vessels they are

36 THE ESSENTIALS OF HISTOLOGY.

discrete but immediately clump together in drawn blood (fig. 37).
If, however, the blood is examined on agar jelly containing certain salts
in definite proportions, the platelets can be kept separate, and may
then be submitted to very high powers of the microscope. The result
of such examination seems to show that the blood-platelets are not
mere inert particles, as has generally been supposed, but that they are
protoplasmic and amoeboid, and that each one contains a nucleus
(fig. 38), that they are in fact minute cells (Deetjen). Blood platelets
vary greatly in number : they are estimated by Brodie and Russell to
amount to from 5 millions to 45 millions in the cubic centimeter of blood.
Fatty particles, derived from the chyle, may also occur in the
plasma.

0^ ^- V<^>,^^

V> ft

m:

u.

Fig. 39. — Development of blood-vessels and blood-corpuscles in the
vascular area op the guinea-pig.

hi, blood-corpuscles becoming fi-ee in the interior of a nucleated protoplasmic mass.

Development of red blood-corpuscles. — In the embryo, the first-formed
coloured hlood-corpusdes are amoeboid nucleated cells, the protoplasm
of which contains haemoglobin. These embryonic blood-corpuscles are
developed within cells of the mesoderm (mesenchyme), which are
united with one another to form a syncytium (fig. 39). The nuclei
of the cells multiply, and around some of them there occurs an
aggregation of coloured protoplasm. Finally the network becomes
hollowed out by an accumulation of fluid in the syncytial protoplasm,
and thus are produced a number of capillary blood-vessels, within
which the coloured nucleated portions of protoplasm are set free
as embryonic blood-corpuscles (eri/throblnsts, fig. 39, bl). Within the
circulation these multiply by mitotic division, and thus become
rapidly more numerous.

In later embryonic life, nucleated coloured corpuscles disappear from
mammalian blood, and are replaced by the usual discoid corpuscles.
Many of these are doubtless derived from the nucleated embryonic

DEVELOPMENT OF BLOOD-CORPUSCLES.

37

blood-cells, the absence of the nucleus being accounted for either by
its atrophy or extrusion from the cell or l>y the separation of a part
of the coloured cell-substance. The foetal liver has been supposed to
be one of the places of formation of red blood-corpuscles. Erythrocytes
are also formed at a somewhat later stage of development within
certain cells of the connective tissue (vafidformative cells), a portion of
the substance of the cell becoming coloured by haemoglobin, and
separated into globular particles (fig. 40, a, b, c), which are gradually
moulded into disk-shaped red corpuscles. In the meantime the cells

Fig 40.— Blood-corpuscles developing within connective-tissue cells.

a, a cell containing diffused haemoglobin ; b, a cell filled with coloured globules ; c, a cell
containing coloured globules in the protoplasm, within which also are numerous
vacuoles ; d, an elongated cell with a cavitj' in its protoplasm occupied by fluid and
blood-corpuscles mostly globular ; e, a hollow cell, the nucleus of which has multi-
plied. The new nuclei are arranged around the wall of the cavity, the corpuscles in
which have now become discoid ; /, shows the mode of union of a 'haemapoietic' cell,
which in this instance contains only one corpuscle, with the prolongation (bl) of a
previously existing vessel.

become hollowed out, and join -with similar neighbouring cells to form

blood-vessels (fig. 40, d, e, f). The process is therefore the same as in

the early embryo, except that cell-nuclei are not included in the

hsemoglobin-holding protoplasm. ^

^It has been suggested by some writers that the vasoformative cells con-
taining coloured corpuscles in various stages of formation are in reality portions
of an already formed vascular network which is undergoing atrophy ; and
that the corpuscles within such cells are not in process of formation but of
disappearance. But since the appearances in question are seen in parts in which
vascular tissues (such as fat) are undergoing not atrophy but formation ; and since,
moreover, the h^matoidin crystals and pigment granules which are character-
istic of the disintegration of erythrocytes within cells are not present, it seems
more reasonable to interpret the appearances as indicative of intracellular de-
velopment of blood-corpuscles by differentiation of part of the protoplasm of the
vasoformative mesenchyme cell, rather than a degeneration of already formed
blood-vessels and blood-corpuscles.

38 THE ESSENTIALS OF HISTOLOGY.

Formation in bone-marrow.— Although no nucleated coloured cor-
puscles (erythroblasts) are as a rule to be seen in the blood in post-
embryonic life, they are found in the marrow of the bones, and
in some animals also found in the spleen. They vary in size, most
measuring about "007 mm. (normoblasts), but some being considerably
larger (megaloblasts), and others considerably smaller (microblasts).
It is probable that the red disks are formed from these nucleated red
corpuscles of the marrow by the nucleus disappeaiing and the coloured

m' I ^ m! e m to

\ M i i i

mt'j III m ^ ni c"

Fig 4L — Red marbow of tocxg BABBrr. Magnified 450 diameters.

e, erythrocytes ; e', erythroblasts ; t", an erythroblast undergoing mitotic division ; /, a
polymorph leucocyte ; m. ordinary myelocytes ; m\ myelocytes undergoing mitotic
division ; 'o, an eosinophil myelocyte ; 6, a basophil myeloc5i^ ; raeg, a giant-cell or
megakaryocyte.

protoplasm becoming moulded into a discoid shape. At what time this

formation of blood-corpuscles in the bone-marrow begins has not been

ascertained, but after it has commenced it continues throughout the

whole of life — the red marrow, especially that of the ribs, being

especially active in this respect. In mammals the formation of

nucleated coloured corpuscles appears to take place within the tissue

of the marrow external to the blood-vessels. It is uncertain to what

extent the capillary vessels of the marrow are limited by a complete

endothelium (see p. 40), but in any case the formed erythroblasts

seem to readily pass into the blood stream.^

^ In birds the erythroblasts are coutined to the large blood-channels of the
marrow, and the transformation into erythrocytes occurs within these channels.

MARROW OF BONE.

39

The marrow of boiu' is of a yellow colour in the shafts of the long
bones of most animals, and is there largely composed of adipose
tissue, but in the shafts of the long bones of some animals, and in the
cancellated tissue of most, it is usually red, the colour being partly
due to the large amount of blood in its vessels. This red marrow
(tig. 41) is chieriy composed of spherical cells — the mijelocijtes or
marrow-cells — which resemble rather large blood-leucocytes, and,
like these, are amoeboid. They also exhibit the same kind of
differences as to the character of the granules which they contain,
some being oxyphil and others amphophil or neutrophil. But while
the blood-leucocytes rarely contain any coarse basophil granules, some

k I m

0 p

^ Q ^^ I Si C) ;fXI «

Fig. 42. — Cklls of the red marrow of the gdinea-pig. (Highly magnified.)

a, a large cell, the nucleus of which apisears to be partly divided into three by constric-
tions ; h, a cell, the enlarged nucleus of which shows an appearance of being con-
stricted into a number of smaller nuclei ; c, a so-called giant-cell or myeloplaxe with
many nuclei ; d, a smaller myeloplaxe with three nuclei ; e-i, proper cells of the
marrow ; j-t, various forms of coloured nucleated cells (erythroblasts), some in
process of division ; in others the nucleus appears to bo undergoing atrophy.

of the marrow-cells contain these in considerable numbers. There
are also to be seen mingled with the marrow-leucocytes a number of
corpuscles somewhat smaller in size, nucleated, and at least some
of them amoeboid, but of a reddish tint (fig. 41, e'). These cells, which
are termed erythroblasts, resemble the nucleated coloured blood-corpuscles
of the embryo, and are believed to be cells from which the coloured
blood-disks become developed. Many of them are in process of
mitotic division. Others are seen with the nucleus in a more or less
atrophied condition (fig. 42, k) ; from this it may perhaps be inferred
that the transformation into a discoid blood-corpuscle is accompanied
by the disappearance of the nucleus (Bizzozero). Lastly, the marrow

40 THE ESSENTIALS OF HISTOLOGY.

contains a number of very large cells, the giant-cells or myeloplaxes
of Robin (fig. 41, meg: fig. 42, a-d). These are especially numerous
wherever bone is becoming absorbed, but are not confined to such
situations, being indeed normal constituents of marrow. Sometimes
they possess several nuclei, but most — the so-called megakari/oci/tes—
contain but one large nucleus, which has usually an annular form.
They are also characterised by possessing a number of centrioles
grouped together near the nucleus. Lastly, the existence of cells
within the marrow containing blood-corpuscles in various stages of
transformation into pigment, similar to those which occur in the
spleen-pulp, has been noted (Osier).

The marrow is very vascular, the capillaries and veins being large
and thin-walled; indeed, according to some authorities, the walls of
the capillaries are imperfect, so that there is an open communication
between them and the interstices of the tissue, and in this way it is
supposed that the coloured blood-disks, which are, it is believed,
produced from the coloured nucleated cells (erythroblasts) of the
marrow, may get into the circulation. There is not, however, an
interstitial circulation of blood in the marrow such as is found in the
spleen, nor does injection material such as carmine gelatine pass
into the interspaces of the tissue, but remains confined to the vessels,
so that the existence of an open communication is doubtful.

Development of white corpuscles. — ^The ivhite blood- and lymph-
corpuscles occur originally as free cells, Avhich are believed to find
their way into the vessels from the circumjacent mesoderm. They
do not occur within the first-formed blood-vessels of the embryo nor
within the vasoformative cells. In later stages of fcetal life and
during the whole of post-embryonic life they become formed in the
bone-marrow as well as in lymph-glands and other organs composed
of lymphoid tissue, and pass from these directly into the lymphatics
and into the blood.

It is probable, but has not been ascertained with certainty, that the
lymphocytes are all produced in lymph-glands and other lymphoid
tissues, and that the macrocytes are formed by enlargement of the
lymphocytes. On the other hand, the polymorphs and the coarsely
granular oxyphil cells are believed to be formed within the bone
marrow, which contains cells of similar character. Cells with well-
marked basophil granules are also met with in bone marrow, and
sometimes, in abnormal conditions, pass in large numbers into the
blood.

HUMAN BLOOD-CORPUSCLES. 41

LESSON IV.

ACTION OF REAGENTS UPON THE HUMAN BLOOD-
CORPUSCLES.

1. Make a preparation of luunan blood, and apply a drop of water, at one
edge of the cover-glass. Examine at a place where the two fluids are
becoming mixed. Notice particularly the first effect of water upon both red
and white corpuscles, as well as the ultimate action.

Sketch both kinds of corpuscles under the action of water.

2. Repeat on another preparation, using very dilute alkali (0'2 per cent,
caustic potash) instead of water. Notice the complete solution first of the
white and then of the coloured corpuscles as the alkali reaches them.

3. Repeat on another preparation, using dilute acetic acid (1 per cent.).
Observe that the effect of the acid upon the coloured corpuscles is similar
to that of water, but that it has a different action upon the colourless
corpuscles.

Sketch two or three of the latter after the action is completed.

4. Make a prepai'ation of blood mixed with salt solution, as in Lesson II. 2,
and investigate the action of tannic acid (1 part tannic acid to 100 of distilled
water) in the same way.

Sketch two or three coloured corpuscles after the action is complete.

5. Examine blood-crystals of rat, guinea-pig, and squirrel. Preparations
of htemoglobin crystals cannot be kept permanently.

6. Prepare hoemin by heating a dry smear of blood on a slide with glacial
acetic acid. The crystals of hyemin are permanent.

Structure of erythrocytes. — The action of reagents upon the
human red blood-corpuscles shows that, although to all appearance
homogeneous, they in reality consist of

an external envelope of colourless material " '' <' '' <"

which forms a thin film inclosing the dis-
solved colouring matter or hcenioglobin. Thus,

when water reaches the corpuscles, it passes yH .^^

through the film and swells the corpuscle, •'"^'' ' '' •

causing it to become globular ; eventually Fig. 43.

the film is burst through, and the colouring «-f, successive effects of water upon

o ' & a red corpuscle ; /, effect of

matter escapes into the serum. The addition solution of salt; V, effect of

I tannic acid.

of hyperisotonic solution of salt, on the

other hand, by increasing the density of the fluid in which the
corpuscles float, causes diffusion of water out of the corpuscle, and
consequent shrinking and corrugation of the surface, the crenated form

42 THE ESSENTIALS OF HISTOLOGY.

(fig. 43,/) being thereby produced. The same change is brought about
by evaporation of water, if the blood is exposed to air. The separation
of hcBmoglobin from the corpuscle can be effected not only by water (fig.
43, a-e), but also b}^ dilute acids, by the action of heat (60° C), the
freezing and thawing of blood, the action of ether or chloroform, and the
passage of electric shocks. Bile and dilute alkalies rapidly cause the red
corpuscles to become spherical and then almost instantly effect their
complete solution (haemolysis). The mixing of blood from one species
of animal with the blood or serum of animals of other species
frequently also has a similar effect. In this case the haemolytic

Fig. 44.— Blood crystals, magnified.
1, from human blood ; 2, from the guinea-pig ; 3, squirrel ; 4, hamster.

action is exerted by some constituent (hsemolysin) of the foreign
blood, which is special for each species and against which the " host "
can render itself immune if, prior to any large quantity of the foreign
blood or serum being injected, successive small injections be made ;
an " antiha^molysin " being gradually produced. This fact is not
only of interest as bearing upon the general doctrine of immunity,
but also serves to detect the source of a given sample of blood.

Tannic acid produces a peculiar effect upon the red corpuscles (fig.
43, g) ; the haemoglobin is discharged from the corpuscle, but is im-
mediately altered and precipitated, remaining adherent to the envelope
in the form of a round or irregular globule of a brownish tinge
(hsematin ?).

Some of these reactions occur by a process of osmosis as iu the case of
water, but in others a solution of the envelope of the corpuscle is produced

HUMAN BLOOD-COEPUSCLES. 43

by the reuneiit, ami the luemo^lobin is thu.s allowed to escape. The lilui or
envelope is probably composed of protoplasm contaiuin<,', besides iiucleo-
proteids, lecitiiin and cholesterin (myelin), and these are substances which
possess many of the physical properties of fats, although of a different
chemical composition. If we assume that .such fatty substances form an
external film to the corpuscle, the running of the red disks into rouleaux can
readily be explained, since it has been shown by Norris that disks of any
material, e.g. cork, suspended in a fluid, tend in the same way to adhere in
rouleaux, provided their surfaces are covered with a layer which is not
wetted by the fluid. We may also explain on the .same hypothesis the fact
that no I'ent is ever seen in the envelopes of the red corpuscles even when
they appear to have burst after imbibition of water, for, if the film which
represents an envelope is myelinic in nature, any rent in it would tend
immeil lately to close up again when the opposed edges come in contact.

It was also shown by Norris that droplets of fluid encoiupassed by myelin
have a tendency to assume a flattened shape.

^ -^ / '^_

L' 1 -^ ^

"*##♦„ ••1^'

Fig. 45.— H^min crystals, jiagnifiep. Fig. 4G.— Hjijiatoidin crystals.

(Prejer.) (Frey.)

The more solid part of the red corpuscle is often termed the stroma., but
this name rests upon an entirely false conception of the structure of the
corpuscle. In adopting the name, it was supposed that the corpuscle is
formed of a homogeneous porous material (stroma — Rollett), in the pores of
which the hfemoglobin is contained, but there is no reasonable foundation
for this belief, which fails to explain, except on the assumption of a still more
complex hypothesis, the well-known osmotic phenomena of the corpuscle ;
whereas the supposition that there exists a delicate external film or envelope
inclosing a coloured fluid is in accordance with all the known facts regarding
the action of reagents u])on these bodies. It is true that in the fresh
mammalian corpuscle the envelope is too delicate to be actually observed in
the optical section of the corpuscle, but in the blood-corpuscles of amphibia
it can be quite distinctly seen, and with any slight increase in density of the
plasma it tends to become wrinkled and the creases in it are plainly visible.
In these corpuscles also the nucleus becomes readily displaced in drawn
blood from its position in the centre of the corpuscle and may lie quite at
the side ; this is a clear indication of the fluid nature of the contents of the
corpuscle, and by analogy we may fairly assume a similar constitution for
the mammalian corpuscle. Lastly, it is possible to stain the envelope of the
red corpuscles of a ditterent colour from the remainder of the corpuscle.

Blood-crystals — Haemoglobin. — In the blood of many animals (fig. 44),
crystals of haemoglobin readily form after its separation from the red

44 THE ESSENTIALS OF HISTOLOGY.

corpuscles. These crystals are rhombic prisms in man and most animals,
e.g. the rat, but tetrahedra in the guinea-pig, and hexagonal plates in
the squirrel. In these animals they at once appear on shaking up the
blood with chloroform or ether, or even on the addition of water, with or
without subsequent evaporation.

Hsemin. — This name has been applied to the minute dark-brown rhombic
crystals of hydrochlorate of hsematin (fig. 45), which are formed when dried
blnod from any source whatever is heated with glacial acetic acid.

Haematoidin. — This occui's in the form of brownish yellow crystals (fig. 46).
It is found in old blood extravasations and in other places where blood-
corpuscles are undergoing disintegi-ation within the tissues.

Action of reagents on leucocytes. — The structure of the colourless
corpuscles is also brought out by the action of some of the reagents
above noticed. As the water reaches them their amceboid movements
cease ; they become swollen out into a globular form by imbibition

Fig. 47.

1, first effect of the action of water upon a white blood-corpuscle ; 2, 3, white corpuscles
treated with dilute acetic acid ; v, nucleus.

of fluid (fig. 47, 1), and the granules within the protoplasm can be
seen to be in active Brownian motion. Their nuclei also become
clear and globular, and are more conspicuous than before. With
the further action of the water, the corpuscle bursts and the granules
are set free.

Acids have an entirely different action upon the white corpuscles.
Their nuclei become somewhat shrunken and very distinct (fig. 47,
2 and 3), and a granular precipitate is formed in the protoplasm
around the nucleus. At the same time, a part of the protoplasm
generally swells out so as to form a clear bleb-like expansion (an
appearance which also often accompanies the death of the corpuscle
from other causes). Dilute caustic alkalies rapidly cause the complete
destruction of the white corpuscles.

BLOOD-COllPUSCLES OF AMriilBlA. 45

LESSON V.
THE BLOOD-CORPUSCLES OF AMPHIBIA.

1. Obtain a drop of frog's, toad's or newt's blood, and mix it witli a very
small quantity of salt solution upon a slide. Examine with the high power.
Notice the shape of the colonized corpuscles both wheu seen flat and edge-
ways, and the nucleus within each.

Measure ten corpuscles (long and short diameters), and from the results
obtain the average dimensions of a corpuscle.

Notice also the colourless corpuscles, smaller than the reil, but larger than
the pale corpuscles of human blood, although otherwise generally resembling
these.

Sketch two or three red corpuscles and as many white.

Be careful not to mistake the rouuded liberated nuclei of crushed red
corpuscles for pale corpuscles.

Enormous cells and nuclei belonging to the cutaneous glands as well as
the granular secretion of those glands may be present in this preparation if
it is obtained from the newt's tail.

2. Apply a drop of water to the edge of the cover-glass of the same
preparation and notice its action upon the corpuscles.

Sketch two or three corpuscles altered b}' the action of the water.

3. Mount another drop of blood, and apply dilute acetic acid (1 per cent.)
instead of water at the edge of the cover-glass. Make sketches showing the
effect of the acid upon both red and white corpuscles.

4. Examine the corpuscles of newt's blood which has been allowed to flow
into boric acid solution (2 per cent.). Notice the effect produced upon the
coloured corpuscles. Sketch one or two.

5. Mount drops of glycerine-jelly containing {a) frog's blood and {h) bird's
blood, previously fixed by Flemming's solution and stained with picro-
carmine.

6. Make a film preparation of amphibian or avian blood as described on
p. 28, § 5.

The coloured blood-corpuscles of amphibia {fig. 48), as well as
of nearly all vertebrates below mammals, are biconvex elliptical disks,
considerably larger than the biconcave circular disks of mammals. ^
In addition to the coloured body of the corpuscle, which consists,
as in mammals, of haemoglobin inclosed within an envelope, there

1 The following are the dimensions in parts of a millimeter of the coloured
coi-puscles of some oviparous vertebrates : —

Pigeon, - - . .

Pi'og,

Newt,

Proteus, ... -

Amphiuma, - - - -

Long diameter.

Short diatnetec

00147

OOO60

0 0223

0 0157

0 0293

00195

0 0580

0-0350

0 0770

0 0460

46 THE ESSENTIALS OF HISTOLOGY.

is a colourless nucleus, also of an elliptical shape, but easily becoming
globular, especially if liberated by any means from the corpuscle.
The nucleus resembles that of other cells in structure, being bounded
by a membrane and having a network of chromatin. It is not very

Fig. 48. — Amphibian erythrocytes. (From photographs.) Magnified 450 diameters.
A, from the frog. B, from the toad.

distinct in the unaltered corpuscle, but is brought clearly into
view by the action of reagents, especially acids. The action of
reagents upon the red corpuscle of amphibia is otherwise similar to
that upon the mammalian corpuscle, water and acetic acid causing it to
swell into a globular form and then to become decolorised ; solution
of salt causing M^rinkling of the envelope, and so on. As a first effect,
water and certain other fluids may cause the hemoglobin to
retire from the envelope at the points where the fluid is passing
through the membrane : a stellate appearance is thereby often pro-
duced (Hiinefeldt, Hensen). Boric acid causes the haemoglobin of the
newt's corpuscle to become partially or wholly collected around the
nucleus, which may then be extruded from the corpuscle (Briicke).

Immediately within the envelope, at the periphery of the amphibian
erythrocyte, is a band of fine fibrils which are stained by gentian
violet (Meves) and can also be seen cut across in sections of the
corpuscles (Bryce).

The colourless corpuscles of amphibia, although larger, are
very similar to those of mammals. Like them, they are either
wholly pale and finely granular or inclose a number of very

ULOOD-COPvPUSCLES OF AMPHIBIA. 47

distinct grannies of similar nature to those met with in mammals.
These corpuscles vary much in size and in the activity of their
amoeboid movements : those which have a nuiltilobular nucleus (fig.
33, b, c) are usually the most active. Reagents have the same eflPect
upon them as on those of mammals. The presence of glycogen may
be demonstrated in them by its reaction with iodine (port-wine
colour).

The blood-platelets (thrombocytes) in the frog are fewer in number
than in mammals. Many are of a spindle shape. They contain
a nucleus-like body and like the blood-platelets of mammals they
show amoeboid changes and tend rapidly to clump together in
drawn blood.

48

THE ESSENTIALS OF HISTOLOGY.

LESSON VI.

THE AMBCEBOID PHENOMENA OF THE COLOURLESS
BLOOD-CORPUSCLES.

1. Make a preparation of blood from the finger in the usual way. Draw a
brush just moistened with ]3erfectly neutral oil around the edge of the cover-
glass to check evaporation. Place the jjreparation upon a ' warm stage,' and
heat this to about the temperature of the body (38° C). Bring a white
corpuscle under observation with the high power, and watch the changes
of shape which it undergoes. 'To become convinced of these alterations in
form, make a series of outline sketches of the same corpuscle at intervals of
a minute.

Fig. 49. — Simple warming app.\k.\tus, complete, shown in operation.

The simplest form of warm stage is a copper plate of about the size of an
ordinary slide, perforated in the centre and with a long tongue of the same
metal projecting from the middle of one edge (fig. 49). The copper plate
rests upon the stage of the microscope, with a piece of cloth or other non-
conducting material between. The preparation is made upon an ordinary
slide or on a large cover-glass, which is placed upon the warm stage and

AMCEBOID PHENOMENA OF COLOURLESS CORPUSCLES. 49

pressed into loiitact witli it by the brass clips. Heat is applied to the copper
ton,i,nie by a small spirit-lamp flame, and a greater or less amount is con-
ducted to the warm stage and the superjacent preparation according to the
point to which the flame is ap))lied. To ascertain that the riglit temperature
is got and maintained, put two pieces of paraffin, one melting at 35" C.
{9;")" F.) and another at 38" C. (100° F.), on either side of the preparation.
The temperature must be such that the first piece is melted and remains so
whilst the second remains unmelted.'

2. Mount a drop of frog's or newt's blood diluted with an equal amount of
salt solution, and examine it in the same manner upon the copper stage, at
first cold, afterwards warm ; the temperature must, however, be kept l)elow
30' C. Observe the effect of heat in acclerating the auKeboid movements of
the pale corpuscles. Sketch one at intervals of a minute (a) in the cold, (b)
whilst warmed.

3. Take some yeast which has been mixed with salt solution, and mix a
very little of the yeast and salt solution with a fresh drop of newt's blood,
.slightly oiling the edge of the cover-glass as before. Endeavour to observe
the inception of the yeast-torulte by the white corpuscles. Sketch one or
two corpuscles containing toruUe.

Milk-globules or particles of carbon or of vermilion may also be used for
this experiment, but the process of inception or 'feeding' is most readily
observed with the yeast particles.

4. At the beginning of the lesson collect a drop of newt's or frog's blood
into a fine capillary tube, seal the ends of the tube, and mount it in a drop
of oil of cedar-wood or dammar varnish (or the clot may be blown out into
a drop of salt solution on a slide and mounted in this solution). Towards the
end of the lesson examine it to see white corpuscles emigrating from the
shrunken clot (see fig. 50).

5. To obtain a specimen with the white corpuscles fixed in amoeboid con-
dition, make a preparation of newt's blood, mixed with salt solution, and
set it aside for ten minutes. By this time the corpuscles will be freely
amoeboid, and will probably show well-marked pseudopodia. To fix them in
this condition let a jet of steam from a flask or kettle play for two or three
seconds upon the covei'-glass. The heat instantly kills the corpuscles, and
they are fixed in the form they presented at the moment the steam was
applied. They may now be stained by passing dilute hsemalum- under the
cover-glass, or by removing the latter and staining with eosin and methylene
blue in the manner recommended on p. 28, § 5. If hsemalum is used,
the stain is followed by dilute glycerine, after which the cover may be
cemented and the preparation kept.

The amoeboid phenomena which are exhibited by the protoplasm
of the colourless blood-corpuscles consist of spontaneous changes of
form, produced by the throwing out of processes or pseiulopodia in
various directions. When first thrown out the pseudopodia are quite
clear ; they appear to be produced by a flowing of the hyaloplasm
(see p. 4). If the corpuscle is stimulated, either mechanically, as by

1 For exact work, an apparatus somewhat more complex than the above is
required. For description of such, see A Course of Practical Histolor/y.

- Delafield's or P]hrlich's hematoxylin can he substituted for htemalum wherever
the latter is mentioned. The water used for the dilution of haematoxylin solu-
tions must always be distilled.

D

50

THE ESSENTIALS OF HISTOLOGY.

tapping the cover-glass, or electricall}', the pseudopodia are retracted,
the corpuscle becoming spherical. A change of form caused by the
protrusion of the pseudopodia maj-, when active, be followed by
changes in place or actual locomotion (migration) of the corpuscle.
When a pseudopodium, or the external surface of the corpuscle,
comes in contact with any foreign particle, the protoplasm tends to
flow round and enwrap the particle, which is then drawn into the
corpuscle ; particles thus incepted may be conveyed by the corpuscle
in its movements from one place to another (fig. 51). This property
plays an important part in many physiological and pathological pro-
cesses ; thus cells in the spleen resembling large leucocytes — the

Fig. .50. — White corplscles of frog's blood iiiGRAXiNG i-rom shrinke.v
CLOT WITHIN A CAPILLARY TUBE. (Fi'om Sanderson's Handbook for the
Ph_ysiological Laboratorj-.)

so-called splenic cells — incept blood-corpuscles, which become broken
down within them ; and pathogenic bacteria become taken into the
protoplasm of certain leucocytes (on this account termed jyhagoci/tes),
there to be destroyed (Metchnikoff). The phagocytic properties of
the leucocytes become especially developed as the result of the action
upon the bacteria of certain chemical substances which are present
to a greater or less extent in blood and which are termed opsonins
(Wright).

It is probable that particles of organic matter which are taken up
by the pale corpuscles may undergo some slow process of intracellular
digestion within their protoplasm.

AM(Ei;c>ll) IMIENUMENA (>F COLUTRLESS CORPUSCLES. 51

The processes of the granular corpuscles are quite clear at first ;
the granules afterwards flow into them.

The migration of the colourless corpuscles from the blood-vessels
into the surrounding tissues (which especially occurs in inflamed
parts), or from a blood-clot into the surrounding serum (fig. 50), is
due to these ama'boid properties.

The conditions which are favourable to this amoeboid activity of
the white corpuscles are (1) the natural slightly alkaline medium,
such as plasma, serum, or lymph, or faintly alkaline normal saline
solution. Any increase of density of the medium produces a diminu-
tion of amreboid activity, whilst, on the other hand, a slight decrease
in its density has the opposite eft'ect ; (2) a certain temperature. In

Fig. 51. — Changes of form of a white blood-corpuscle sketched at
intervals of a few minutes, showing the inception of two small
granules and the changes of position these underwent within
the corpuscle.

warm-blooded animals the phenomena cease below about 10° C. When
gradually warmed the white corpuscles become more and more active
up to a certain point, the maximum being a few degrees above the
natural temperature of the blood. Above this point they become
spheroidal and at a somewhat higher temperature their protoplasm
is coagulated and killed. Acids at once kill the corpuscles and stop
the movements. Xarcotic gases and vapours, such as carbonic acid
gas or chloroform vapour, also arrest the movement, but it recom-
mences after a time if their action is not too prolonged.

52 THE ESSENTIALS OF HISTOLOGY.

LESSON VII.
EPITHELIUM AND SECRETING GLANDS.

1. MorxT a drop of saliva and examine first with a low, afterwards with a
high power. Observe the nucleated epithelium-cells, some single, and others
still adhering together by overlapping edges. Measure three or four, and
also their nuclei. Sketch one or two on the flat and one edgeways. Notice
the salivary corpuscles, which are migrated white blood-corpuscles, swollen
out by imbibition of water. The preparation may be stained with diluted
hsemalum and preserved with glycerine.

2. Put a small shred of human epidermis into a drop of strong caustic
potash solution (35 p.c.) for five minutes. Then break it up in water with
needles, cover and examine. Observe the now isolated swollen cells.

3. Study the arrangement of the cells in a section through some stratified
epithelium, such as that of the mouth, skin, or cornea.^ Notice the changes
in .shape of the cells as they are ti'aced towards the free surface. Measure
the thickness of the epithelium. Count the number of layers of cells.

4. Make a preparation of the epithelium of the urinary bladder, which
may be moderately distended with bichromate of potash solution (1 part to
800 of salt solution), and after an hour or two cut open and placed in moi"e
of the same solution. Take a small scraping of the lining epithelium on the
point of a scalpel, and break it up by tapping it in a drop of very dilute
hsematoxylin on a slide. Put a small hair in the drop and cover. Add a
small drop of glycerine at one edge : allow this to difi'use under. Cement
next day. Observe the large flat superficial cells, and the pear-shaped cells
of the second layer. Sketch one of each kind. The cells will vary
greatly in appearance according to the amount of distension of the organ.

b. Study the minute structure of epithelium-cells and their nuclei, both
at rest and dividing, in sections of the skin of the newt's tail, or in shreds
of peritoneum or of epidermis or in sections of the salamander-tadpole. The
preparation may, for this purpose, be stained either with ha?matoxylin or
iron-hfematoxylin, or with some aniline dye such as saff'ranin.^

Sketch an epithelium-cell with resting nucleus, and others with nuclei in
diflPerent phases of mitosis.

The simple saccular skin-glands of Amphibia may also be studied in these
preparations.

An epithelium is a tissue composed entirely of cells separated by
a very small amount of intercellular substance (cement-substance) and
generally arranged so as to form a membrane covering either an
external or internal free surface.

The structure of epithelium-cells, and the changes which they
undergo in cell-division, are best seen in the epidermis of the newt

1 The methods of preparing and staining sections are given in the Appendix.

EPITHELIUM. 53

or of the salamander-tadpole (fig. 52) ; in the latter especially, the
cells and nuclei are much larger than in mammals.

Structure of the cells. — Each epithelium-cell consists of protoplasm
containing a nucleus. The protoplasm may either look granular, or
it may have a reticulated appearance, or may exhibit fibrils. The
nucleus is spherical or ovoid. Usually there is only one, but there
may be two or more. The cell-substance is often modified in its
chemical nature; its external layer may become hardened to form a

/J? - - - 'H -^

Fig. 52. — Epidermis cells of a lakval salamander.
Magnified 400 diameters. (Wilson.)

Thi-ee of the cells are undergoing division. The intercellular channels are bridged
across by fine fibres. At one place a branched pigment ceU is lying between the
epithelium cells.

sort of membrane, or the whole cell may become horny (keratinised) ;
or there may be a separation of materials (granules) within the cell
which are ultimately used by the organism, as in some secreting
glands.

Classification of epithelia. — Epithelia are somewhat illogically
classified partly according to the shape and arrangement of the cells,
partly according to their function. Thus we speak of scali/ or pavement,
cubical, columnar, glandular, and ciliated epithelium. Most of these
are simple epithelia, with the cells only one layer deep. If forming
several superposed layers, the epithelium is said to be stratified, and
then the .shape of the cells differs in the different layers. Where
there are only three or four layers in an epithelium, it is termed
transitional.

54

THE ESSENTIALS OF HISTOLOGY.

Stratified epithelium covers the anterior surface of the cornea, lines
the mouth, pharnyx (lower part), gullet, anal canal and part of the
urethra, and forms the epidermis which covers the skin. The vocal
cords are also covered b\' stratified epithelium. In the female it
also lines the vagina and covers the os uteri. The cells nearest the

C^

/

O

■^^^

(^

3^

'%-m

J yj

) (

Fig. 53. — Section of the stratified epithelium covering the front
OF the cornea of the eye (man).
c, lowermost columnar cells ; p, polygonal cells above these ; fl, flattened cells near the
surface. Between the cells are seen intercellular channels bridged over by processes
which pass from coll to cell.

surface are always flattened and scale-like (fig. 53,/; fig. 54), whereas
the deeper cells are polyhedral, and those of the deepest layer some-
what columnar in shape (fig. 53, c). Moreover, the deep cells are
soft and protoplasmic, and are separated from one another by a system
of intercellular channels, which are bridged across by numerous fibres
passing from cell to cell ; giving the cells, when separated, the

appearance of being beset with short
spines (prickle-cells of Max Schultze).
These ' bridging fibres ' are not peculiar
to stratified epithelium, but occur in
many if not in all kinds of epithelia.

The deeper cells multiply by mitotic
division, the nuclei first dividing in
Fig. 54. -Epithelium-scales from the manner already described. The
™I ?«^^^,?^ "^"^ mouth. (Mag- newly formed cells tend as they enlarge

nified 260 diameters.) •' -^ °

to push those external to them nearer
to the surface, from which they are eventually thrown off. As they
approach the surface they become hard and horny, and in the case of
the epidermis entirely lose their cellular appearance, which can, however,
be in a measure restored by the action of alkalies (§ 2). The cast-off
superficial cells of the stratified epithelium of the mouth, which are
seen in abundance in the saliva (§ 1), are less altered, and the remains
of a nucleus is still visible in them (fig. 54). The stratified epithelium
of the human skin (epidermis) shows many peculiarities: these will
be considered when the skin itself is treated of.

TBANSITIONAL EPITHELIUM. 55

Transitional epithelium is ;i stratified epithelium consisting of only
three or four layers of cells. It occurs in the urinary bladder, the
ureter, and the pelvis of the kidney. The superficial cells (fig. 55, a)
are large and flattened ; they often have two nuclei. Their free sur-
face is covered with a euticular stratum (Eggeling), and on their
under surface they exhibit depressions, into which fit the larger
ends of pyriform cells, which form the next layer (fig. 55, h).
Between the tapered ends of the pyriform cells one or two layers
of smaller polyhedral cells are found. The epithelium seems to be
renewed by mitotic division of these deeper cells ; it is probable that
the superficial cells also multiply, but in this case by amitosis.

Fig. 5.5. — Epithelial cells from the bladder of the rabbit. (Klein.)
(Magnified 500 diameters.)

a, large flattened cell from the superficial layer, with two uuclei and with strongly
marked ridges and intervening depressions on its under surface ; 6, pear-shaped coll
of the second layer adapted to a depression on one of the superficial cells.

Simple scaly or pavement epithelium is found in the saccules of the
lungs, in the ducts of the mammary glands, in the kidney (in the tubes
of Henle, lining the capsules of the Malpighian bodies, and covering
the glomeruli), and also lining the cavities of serous membranes
(fig. 56), and the interior of the heart, blood-vessels, and lymphatics.
When occurring on internal surfaces, such as those of the serous
membranes, blood-vessels, and lymphatics, it is often spoken of as
endothelium or mesothelium. According to v. Brunn the cells of a
serous epithelium may be provided with a striated border on their
free surface, somewhat like that which is found on columnar cells.

Columnar epithelium and ciliated epithelium are for the most part
found covering the inner surface of mucous membranes ; which are
membranes moistened by mucus and lining passages in communication
with the exterior, such as the alimentary canal and the respiratory and
generative passages. The cells of a columnar epithelium form a single

56 THE ESSENTIALS OF HISTOLOGY.

layer, varying in thickness according to the length of the constituent
cells, and when the cells of a columnar epithelium are short, the
epithelium is spoken of as cubical, such as that which lines the
vesicles of the thyroid gland.

Fig. 56. — Pavement epithelium or endothelicm of a sekous membrane.
Nitrate of silver preparation. Carmine staining of ndclei.

Ciliated epithelium is found in man throughout the whole extent of
the air-passages and their prolongations, but not in the uppermost
part of the nostrils which is supplied by the olfactory nerves,
nor in the lower part of the pharynx, nor in the terminal bronchioles
and pulmonary alveoli. It also occurs in the Fallopian tubes and
the greater part of the uterus ; in the efferent tubes of the testicle ;
and in the ventricles of the brain, and the central canal of the
spinal cord.

GLANDULAR EPITHELIUM AND SECRETING GLANDS.

Glandular epithelium is the essential tissue of all the organs which
are known as secreting glands. Glands are of two chief kinds. Those
which are best known and which are termed secreting glands proper are
furnished with a duct which ramifies in all parts of the gland and by
means of which the products of the secretory activity of the gland-cells
are brought to a free surface. Such glands have been developed as
involutions of the surface upon which they open, and their epithelium
is continuous with that of this surface, and is in some cases, especially
where the surface upon which the gland opens is covered with columnar

GLANDULAR EPITHELIUM.

57

II.

Fig. 57.— Various kinds of glands.

I. Simple saccular gland from amphibian skin (Flemming). II. Simple tubular gland
from intestine (Flemming). III. A small racemose gland with a single duct into
which a number of irregularly tubular acini open (Flemming). IV. Part of a tubulo-
racemose gland with the acini unravelled (Flemming). V. Wax model of a small
tubulo-racemose gland from the epiglottis (Maziarski).

58

THE ESSENTIALS OF HISTOLOGY.

epithelium, of a similar character to the epithelium of the surface ; in
others diiferent in character. In most glands the epithelium alters as
we trace the duct back into the recesses or alveoli of the gland, and it is
in these that the characteristic glandular cells, which are generally poly-
hedral in shape, are found. Every such involution or ingrowth of
epithelium to form a gland is, when first formed, of a simple character,

Fig. 58. — Simple tubulak glands sekn in a section of the mucous membrane

OF THE STOJIACH OF THE KANGAROO.

t, epithelium of general surface ; hm, basement membrane ; n, neck or duct of gland ;
6, base or fundus ; yl, glandular epithelium ; It, lymphoid tissue ; mm, muscular
tissue of the mucous membrane.

shaped like a test-tube or flask and filled with a solid mass of cells, but
it presently becomes hollowed out and the cells are left as a lining to the
connective tissue membrane which bounds the involution. The gland
may remain simple and unbranched (simjyle saccular and simple tubular
glands, fig. 57, I. and 11,), or it may branch again and again until a
complicated structure, in some cases small, in others of considerable size,
is produced (compound tubular and compound saccular (or racemose) gla?ids
(fig. 57, III., IV., v.), instances of which are furnished by the kidneys
and salivary glands respectively). The cells which furnish the secretion
of the gland and which line the secreting parts of the tubules of a tubular

SECRETING GLANDS. 59

gland, or the alveolar enlai'gemonts (acini) at the ends of the ducts of a
racemose gland, are frequently partly or wholly filled with graiuiles
in the intervals of secretory activity, and these granules become
discharged or dissolved and pass into the secretion during activity.
Secreting glands are always abundantly supplied with blood-vessels
and nerves. The former are distributed in the connective tissue which
holds together the acini and groups of acini (lobules) of the gland ;
the latter are supplied partly to the blood-vessels and partly ramify
amongst the secretory epithelium cells.

The liver differs from all other secreting glands in being composed of
solid masses of cells (hepatic lobules) instead of tubular acini lined by
epithelium. It exhibits also other important differences in the nature
of its blood-supply and the relation between the blood and the liver-
cells.

The other kind of secreting glands, known as the internally secreting
glands, are not furnished with ducts and are usually described (along
with the spleen and the lymphoid structures) as ductless glands. The
internally secreting glands are, like the externally secreting organs,
composed of epithelial cells, sometimes grouped in solid masses {e.g.
suprarenal gland), in other cases disposed around hollow vesicles (e.g.
thyroid) which become filled with the material of the secretion. But
as in these glands there is no duct the secretion is carried into the
blood either directly by the blood-vessels of the gland or indirectly
through the lymphatics.

The detailed study of the glands and of other epithelial structures
may be reserved until the organs in which they occur are described,
but columnar and ciliated epithelia will be dealt with in the next
Lesson.

The hairs and nails and the enamel of the teeth are modified
epithelial tissues. They will be described along with the skin and
structures connected with the mouth respectively.

60 THE ESSENTIALS OF HISTOLOGY.

LESSON VIII.

COLUMXAR ASD CILIATED EPITHELIUM: ACTION OF CILIA.

1. Break up in dilute glycerine a shred of epithelium from a minute piece
of the mucous membrane of intestine (frog) that has been treated with 1 per
cent, osmic acid for some hours, and has subsequently macerated in water
for a few days. The cells easily separate on tapping the cover-glass.
Measure and sketch one or two cells.

The cover-glass may be at once fixed by gold size.

2 Prepare ciliated epithelium from a trachea that has been in chromic
acid solution (1 to 2000 normal saline) for a few days, in the same way as
with transitional epithelium (.^ 4, p. 52). Measure in one or two of the
cells (a) the length of the cells, (h) the length of the cilia, (c) the size of the
nucleus. Sketch two or three cells.

3. Mount in sea-water one or two bars of the gill of the marine mussel
(fig. 59). Study the action of the large cilia. Now place the preparation
upon the copper warm stage (see Lesson VI.) and observe the effect of raising
the tempei-ature.

Fig. 59. — Valve of mussel (mttilus edclis) showing Ir, Ir, the expanded

GILLS OB BKAXCHI.E, WHICH, OWING TO THE LITTLE BARS OF WHICH THEY
ARE COMPOSED, PRESENT A STRIATED ASPECT.

ml, mantle ; m, cut adductor muscle ; !, mass of viscera ; the dark projection just above

is the foot.

Keep this preparation until the end of the lesson, by which time many of
the cilia will have become languid. "When this is the case pass a drop of
dilute potash solution (1 part KHO to lO'X) of sea-water) under the cover-
glass and observe the effect.

4. Cement with sealing-wax a piece of small glass tubing to a slide so that
one end of the tube comes nearly to the centre of the slide. To do this
effectually the slide must be heated and some sealing-wax melted on to it
and allowed to cool. The glass tube is then made hot and applied to the
slide, embedding itself as it does so in the sealing-wax. Apply a ring

COLUMNAR AND CILIATED EPITHELIUM.

61

of putty or modelling wax (half an inch in diameter and rising above the
glass tube) so as to include the end of the tube. Make a deep notch in the
ring opposite the tube for the exit of the gas. Place a drop of water
within the ring (fig. 60).

Fig. 60.— Moist chamber adapted for passing a gas or vapour to a
preparation under the microscope.

Put a bar from the gill upon a cover-glass in the least possible quantity of
sea-water ; invert the cover-glass over the putty ring, and press it gently
and evenly down. The preparation hangs in a -moist chamber within which
it can be studied through the cover-glass, and into which gases or vapours
can be passed and their effects observed.

Fig. 61. — Method of subjecting a preparation to a .stream of carbon

DIOXIDE.
6, bottle containing marble and hydrochloric acid ; h', wash-bottle, connected by india-
rubber tube, t, with the moist chamber, a.

Pass C0.2 through the chamber, and after observing the eflfect replace it by
air (see fig. 61). Eepeat with ether and with chloroform vapour.

Columnar epithelium. — The cells of a columnar epithelium (fig. 62)
are prismatic columns, which are set closely side by side, so that when
seen from the surface a mosaic appearance is produced. They often

62

THE ESSENTIALS OF HISTOLOGY.

taper somewhat towards their attached end, which is generally
truncated, and set upon a basement membrane. Their free surface is

Fig. 62.

Fig. 63.

Fig. 62.— a row of columnar cells from the intestine of the rabbit.

Smaller cells are seen between the epithelium-cells ; these are leucocytes.

Fig. 63.— Columnar epithelium-cells of the rabbit's intestine.
The cells have been isolated after maceration in very weak chromic acid. The cells are
much vacuolated, and one of them has a fat-globule adhering to it near its attached
end ; the striated border (.sf;) is well seen, and the bright disk separating it from
the cell-protoplasm ; n, nucleus, with intranuclear network ; a, a thinned-out wing-
like projection of the cell which probably fitted between two adjacent cells.

•■.'t©

&■

Fig. 6o.

Fig. 64.

Fig. 66.

Fig. 64. — A columnar epithelium-cell, showing mass of fibrils (cytomitome)'
within the cytoplasm. (M. Heidenbain.)

Fig. 65. — A goblet or mucus-secreting cell in columnar

epithelium. (M. Heidenbain.)

The centrosome is in the mucigen-mass. An ordinary columnar cell is also shown.

Fig. 66. — Ciliated columnar epithelium, from the trachea of a babbit.

9)ii, m-, m^, mucus-secreting cells in various stages of mucigon formation. The prepara-
tion was treated with dilute chromic acid.

covered by a thick striated border (fig. 63, str.) which may sometimes
become detached in teased preparations. The protoplasm of the cell
is highly vacuolated and reticular, and fine longitudinal stride may be

COLUMNAR EPITHELIUM.

63

seen in it, M'hich appear continuous with the striae of the free bordei'.
Between the striated border and the protoplasm of the cell is a highly
refracting disk which contains fine dumb-bell shaped particles set
vertically, connected below with the fibrils or striie which run through
the cell protoplasm (fig. 64, 65). It has been suggested that these
particles are formed by multiplication of the centrosome, but the
fact cannot be regarded as established. The nucleus is ovoid and
reticular. The lateral borders of the cells
are often somewhat irregular or jagged,
the result of the presence of amosboid
cells, which are generally found between
the columnar cells, at least in the intes-
tine. After a meal containing fat the
epithelium-cells of the small intestine con-
tain fat globules, which become stained
black in osmic preparations.

Columnar epithelium-cells are found
lining the whole of the interior of the
stomach and intestines : they are also
present in the ducts of most glands, and
sometiriies also in their secreting tubes
and saccules. The epithelium which
covers the ovary is also of a modified
columnar shape, but cells having all the
structural peculiarities indicated above
are found only in the alimentary canal
and in its diverticula.

Goblet-cells. — Some of the cells of a
columnar epithelium, and also cells of
glandular, ciliated, and transitional epi-
thelia, contain mucigen, which is laid
down within the cell in the form of
granules (fig. 65, fig. 66, w\ «?- ; fig. 67).
swell up to form globular masses which may run together and greatly
distend the part of the cell nearest the free border. When the
mucigen is extruded as mucus the cell takes the form of an open
cup or chalice (fig. 66, m^), hence the name.

These goblet-cells, or, as they are more appropriately termed, mums-
secreting cells, are probably not mere temporary modifications of the
ordinary columnar and ciliated cells amongst which they are found, but
permanently differentiated cells, which, after having got rid of their
mucus by extrusion, again form a fresh supply in the same way as

Fig. 67.— Three mucus-secret-
ing CELLS FROM THE STOMACH,
FILLED WITH MUCIGEN GRAN-
ULES, SOME OF WHICH ARE IN
PROCESS OF EXTRUSION. (M.

Heidenliain.)

These granules eventually

64

THE ESSENTIALS OF HISTOLOGY.

before. In the gastric mucous membrane all the surface epithelium is
composed of mucus-secreting cells, and they extend also into the mouths
of the glands. In the large intestine also most of the cells both of the
surface and in the glands are goblet-cells. According to the observations
of Carlier those of the gastric mucous membrane are connected together
laterally by protoplasmic fibres.

Ciliated epithelium. — The
y>\- ^ cells of a ciliated epithelium are

usually columnar in shape (figs.
66, 68), but in place of the
striated border of the ordinary
columnar cell the free surface is
surmounted by a bunch of fine
tapering filaments {vibratile cilia),
which, during life, move spon-
taneously to and fro, and serve
to produce a current in the fluid
which covers them. The border
upon which the cilia are set is
bright in the living condition :
after fixation it appears formed
of little juxtaposed knobs or basal
particles, to each of which a
cilium is attached.

In the large ciliated cells which
line the alimentary canal of .some
molluscs (fig.s. 68, 70), and with less
distinctness in the ciliated cells of
vertebrates, the knob may be ob-
served to be prolonged into the
protoplasm of the cell as a fine
varicose filament, termed the rootlet
of the cilium. .Since the axial fibril
in the tail of the spermatozoon (which is commonly regarded as a cilium)
is developed in connection with the centrosome, it has been supposed
that the cilia of an ordinary ciliated cell may also be outgrowths from
the (multiplied) centrosome. But although it may be the case that the
basal i^articles are formed by the division of the centrosome of the cell, in
which case the rootlets may represent the fibrils of archoplasm which
radiate from the centrosome of such a cell as the white corpuscle (fig. 9), it
appears not to be true that the cilia are developed from these basal par-
ticles, for the cilia sometimes appear before the basal particles. In plant
spores, which have no centrosomes, the cilia are developed from amoeboid
processes of the ectoplasm of the cell (Strassburger). Similar basal particles
and longitudinal fibrils are found in columnar cells (pp. 62, 63), and these
are probably homologous with the knobs and rootlets of the ciliated cell,
while the bunch of cilia of the latter is represented by the striated border
of the columnar cell.

Fig. 68.

-Four cili.\ted cells.
(Lenhossek. )

CILIATED EPITHELIUM.

65

Schuberg has described in the cilia of certain infusoria an end-piece whicli
stains ditt'erently from tlie rest of the ciliuni (tig 71).

The action of cilia.— When in motion a cilium is bent quickly over
in one direction with a lashing whip-like movement, immediately
recovering itself. When vigorous the action is so rapid, and the
rhythm so frequent (ten or more times in a second), that it is im-

FiG. 69.— Columnar ciliated

EPITHELIUM-CELLS FROM THE
LOWER PART OF THE HUJIAN
NASAL PASSAGES. EXAMINED

FRESH IN SERUM. (Sharpey.)

s %&

Fig. 71- — Cilia of frontonia leucas.

(A. Schuberg.)

Loffler's flagellum stain, x 22.50.

Fig. 70.— Ciliated cell, from

THE intestine OF A MOLLUSC.
(Engelmann.)

possible to follow the motion with the eye. All the cilia upon a
ciliated surface are not in action at the same instant, but the move-
ment travels in weaves over the surface. If a cell is detached from
the general surface, its cilia continue to act for a while, but their
movement at once ceases if they are detached from the cell. If,
however, a portion of the cell protoplasm is detached with them,
they will continue to move for a time.

The rhythm is slowed by cold, quickened by warmth ; but heat

E

66 THE ESSENTIALS OF HISTOLOGY.

beyond a certain point kills the cells. The movement will continue
for some time in water deprived of oxygen. Both CO., gas and
ether and chloroform vapour arrest the action, but it recommences
on restoring air, if their action is not too prolonged. Dilute alkaline
solutions quicken the activity of cilia, or may even restore it shortly
after it has ceased.

Theories of ciliary action. — Various attempts have been made to explain
the manner in which cilia act. One hypothesis supposes that one side only
of eacli ciliura is contractile, the other side being elastic, or that there is a
more rigid but elastic axis and a contractile covering. This supposition is
negatived by the fact that in heat rigor the cilia are not bent over as they
would be by the contraction whicli always accomjjanies rigor, but stand up
straight. It is moreover impossible to suppose that a soft structure like a
ciliiim could be bent over in a uniform gentle curve by contraction along one
side ; such contraction could only produce shortening and wrinkling of the
cilium, effects which are never observed. Another hypothesis assumes that
the projecting cilia are set in action by rhythmic lateral contractions in the
protoplasm ; which, by moving the rootlets, cause the cilia to bend over as a
whip is bent by movements of the wrist applied to its handle. But this
again implies an amount of rigidity wh'ch cilia do not possess, for it must
be borne in mind that they have to overcome the resistance of fluid, and of
fluid which is in many cases highly viscous.

If in our ignorance of the structure of the individual cilia we are to form
an idea as to the cause of the rhythmical bending over of these minute cell
processes, the simplest hypothesis appears to be to assume that they are
curved flattened hollow filaments, the interior communicating with the cell-
protoplasm.^ If this is the case, then rhythmical variations of pressure
within the cell-protoplasm, which might, as in the case of amoeboid move-
ments, be caused by alterations in surface tension, would be transmitted to
the cilia and wouM cause the curve to open out, and again to assert itself,
according to the degree of tension within the tubular filament. Such action
can be imitated with a fine curved and flattened indiarubber tube attached
to a pressure bag. Any increase of pressure within the tuV>e causes it to
straighten out ; on again decreasing the pressure the tube bends over
exactly in the manner of a cilium. This hypothesis has the advantage over
the others which have been oflered that it explains the movements of cilia
on a theory which is precisely similar to that which gives the most probable
explanation of amieboid movements of protoplasm, viz., that they are due
to variations in surface tension, and it thus brings these two forms
of protoplasmic activity into line with one another. It will presently be
shown that the changes which occur in muscle in contraction are suscep-
tible of a similar explanation.

^All cilia and cilium-like structures (flagella) which are sufficiently large to
show any structural differentiation, exhibit au external membranous covering
and a clear and usually structureless contents, but the minute size of ordinary
cilia prevents one from determining whether this is also the case with them.

THE CONNECTIVE TISSUES. 67

LESSON IX.

THE CONNECTIVE TISSUES.
AREOLAR AND ADIPOSE TISSUE. RETIFORM TISSUE.

1. Take a little of the subcutaneous tissue or of the intermuscular connective
tissue of a rabbit or guinea-pig and spread it out with needles on a dry slide
into a large thin film. Keep the centre moist by occasionally breathing on
it, but allow the edges to dry to the slide. Before commencing put a drop
of salt solution on a cover-glass, and now invert this over the film. Examine
with a high power. Sketch one or two bundles of white fibres and also one
or two elastic fibres, distinguishable from the former by their sharp outline,
isolated course, and by their branching. Sketch also one or more connective-
tissue corpuscles, if any such are visible in the clear interspaces. Look also
for migratory cells (leucocytes). Ne.Kt carefully remove the cover-glass and
replace the salt solution by dilute acetic acid (1 per cent.). Watch its effect
in swelling the white fibres and bringing more clearly into view the elastic
fibres and corpuscles. Look for constricted bundles of white fibres.

2. Make another very thin film in the same way, but allow to dry com-
pletely. Pour over the film a 1 per cent, solution of magenta in 50 per cent,
alcohol, to which 1 drop per cubic centimeter of a 1 per cent, .solution of
gentian violet in alcohol has just been added. After one minute drain this
otf, wipe round the specimen and allow the remainder of the staining solution
to dry on the film. When completely dry mount in dammar. The elastic
fibres are deeply stained ; the cells are also well shown.

3. Prepare another film of the subcutaneous tissue, including a little
adipose tissue. Fix by pouring over it formol (10 p.c.) and leave this in
contact with the film for 20 minutes. Wash with water and stain with
saturated solution of Sudan III. or Scharlach R. in 75 p.c. alcohol ; wash
with 75 p.c. alcohol to remove stain from everything except fat, then wash
with water and stain with dilute hsematoxylin. Mount in glycerine and
water. Examine first with a low and afterwards with a high power. The
fat is well brought out by the Sudan III. or Scharlach R. stain ; if the
preparation is from a young animal, fat-cells will be found in process of
formation. Measure and sketch two or three of the cells.

The fat may also be stained, without prior fixation, by treatment with
1 p.c. osmic acid solution.

4. Spread out another large film of connective tissue, letting its edges dry
to the slide, but keeping the centre moist by the breath. Place on its centre
a large drop of nitrate of silver solution (1 per cent.). After five minutes
wash this away with distilled water, and expose to direct sunlight until
stained brown. Now allow the film to dry completely, and cover it in
dammar varnish or Canada balsam dissolved in xylol. Sketch the outlines
of two or three of the cell-spaces.

5. To display retiform tissue the following method is recommended
(Spalteholz). Place a piece of the organ {e.g. lymphatic gland) for twenty-

68 THE ESSENTIALS OF HISTOLOGY.

four hours or more in alcohol, then overnight at 38° C. in a 1 per cent,
solution of carbonate of soda to which a few drops of a solution containing
trypsin have been added. Cautiously transfer the semi-digested structure
to alcohol again, and leave it for a few hours. Embed in paraffin in the
usual way and stain the sections with iron lipematoxylin. The fibrils of
connective and retiform tissue are the only structures which have remained
undigested and they are deeply coloured by the hasmatoxylin.

The connective tissues include areolar tissue, adipose tissue, elastic
tissue, fibrous tissue, reticular and lymphoid tissue, cartilage and bone.
All these tissues agree in certain microscopical and chemical characters.
They, for the most part, have a large amount of intercellular substance
in which fibres are developed, and these fibres are of two kinds — white
and yellow or elastic. Moreover, there are many points of similarity
between the cells which occur in these tissues ; they are all developed
from the same embryonic formation, and they tend to pass imper-
ceptibly the one into the other. Besides this, the use of these several
tissues is similar; they mostlj^ serve to connect and support the
other tissues, performing thus a passive mechanical function. They
may therefore be grouped together, although differing considerably
in external and even in microscopic characters. Of the connective
tissues, however, there are three which are so intimately allied as
to be naturally considered together, being composed of exactlj^ the
same elements, although differing in the relative development of those
elements : these are the areolar, elastic, and fibrous tissues. Adipose
tissue and reticular tissue may both be looked upon as special modi-
fications of areolar tissue. Areolar tissue being the commonest and,
in a sense, the most typical, its structure may be considered first.

Areolar tissue. — The areolar tissue presents to the naked eye an
appearance of fine transparent threads and laminae which intercross in
every direction with one another, leaving intercommunicating meshes,
or areolae, between them. When examined with the microscope, these
threads and fibres are seen to be principally made up of wavy bundles of
exquisitely fine transparent fibres {tchite fibres, fig. 73, A). The bundles
run in different directions, and may branch and intercommunicate with
one another (fig. 75); but the individual fibres, although they pass from
one bundle to another, never branch or join other fibres. The fibres
are cemented together into the bundles by a clear substance containing
mucin, and the same clear material forms also the basis or ground-
substance of the tissue, in which the bundles themselves course, and in
which also the corpuscles of the tissue lie embedded. This ground-
substance between the bundles can with difficulty be seen in the fresh
tissue on account of its extreme transparency ; but it can be biought to
view by staining with nitrate of silver, as in § 4. The whole of the

AREOLAR TISSUE.

69

tissue is thereby stained of a yellowish brown colour, with the excep-
tion of the spaces which are occupied by the corpuscles (cell-spaces).

Fig. 72. Ground substance of connective tissue stained by silver.
The cell-spaces are unstained. From a photograph. Magnified 250 diameters.

Fig. 73.— White and elastic fibres of areolar tissue.
A, bundles of white fibres partly unravelled. B, elastic fibres.

As Macallum has shown, this reaction is due to the presence of chlorides
in the intercellular substance, whereas the cell-protoplasm contains
none.

Besides the white fibres of connective tissue here described, fibres
of a different kind (fig. 73, B) may be made out in the preparations;
these are the elastic fibres. They are especially well seen after treatment

70 THE ESSENTIALS OF HISTOLOGY.

with acetic acid, and after staining with magenta, or, in sections,
with orcein ; but thej' can be detected also in the fresh preparation.
They are characterised by their distinct outline, their straight course,
the fact that they never run in bundles, but singly, and that they
branch or join neighbouring fibres. If V^roken by the needles in
making the preparation, the elastic recoil causes them to curl up,
especially near the broken ends. Besides these histological differ
ences, the two kinds of fibres differ also in their chemical characters.
Thus the white fibres are formed of a material {collagen) which is dis-
solved by boiling in water yielding gelatin, and by peptic digestion,
but is not dissolved by tryptic digestion ; whereas the substance of
which the elastic fibres are composed (elastin) resists for a long time
the action of boiling water and peptic digestion, although it is dis-
solved by tryptic digestion. Moreover, the white fibres swell and

Fig. 74. — A white bundle swollen by acetic acid. From the scbakachnoid

TI.SSUE AT the BASE OK THE BRAIN. (Toldt. )

become indistinct under the action of acetic acid ; the elastic fibres
are unaltered by this reagent. Elastic fibres appear to have a sheath
which is more resistant to reagents than the rest of the fibre.

The bundles of white fibres which have been swollen out by acid
sometimes exhibit constrictions at irregular intervals (fig. 74). These
are in many instances due to elastic fibres coiling round the white
bundles.

The cells of areolar tissue. — Several varieties of connective tissue
cells are distinguished, viz. : (I) Lamellar cells, which are flattened
and often branched (fig. 75, c, c') and may be united one to the
other by their branches, as in the cornea. Sometimes they are
unbranched and may lie along the fibril-bundles and even themselves
show a fibrillar appearance. Some authors have inferred from this
that these cells are transformed into white fibril-bundles and have
termed them " fibroplasts " ; but the fibrillation which tbey exhibit is
not of the same character as that of the white fibres, and is pro-
bably a form of cytomitome, such as is seen in many protoplasmic
cells. In certain situations the lamellar connective-tissue cells

THE CELLS OF ARKOLAR TISSUE.

71

Fig. 75.— Subcutaneous tissue from a young rabbit, PREPAREn as directed

IN § 1. Highly magnified.

The white fibres are in wavy bundles ; the elastic fibres form an open network, p, p,
plasma-cells ; g, granule-cell ; c, c', lamellar-cells ; /, fibrillated-cell.

Fig. 76. — Epithelioid cells of connective tissue from the surface of
AN aponeurosis. (Nitrate of silver preparation.)

72

THE ESSENTIALS OF HISTOLOGY

are greatly flattened out, especially when they lie upon the surface of
aponeuroses and they are there joined edge to edge like the cells of an
endothelium (fig. 76. The apparent cell-spaces in silver preparations
have of course in all cases a similar arrangement to that of the cells).
(2) Plasma cells (fig. 75, p), which are composed of a soft much-
vacuolated protoplasm, rarely flattened, but otherwise varying
greatly in shape and size. (3) Granular cells (g) {Mast-zellen of Ehrlich,
clasmatocj/tes of Ranvier), usually spheroidal or ovoidal in shape, and
formed, like the plasma-cells, of soft protoplasm, but thickly occupied
with albuminous granules, which are deeply stained by gentian violet

Fig. 77.- -A pew cells from the margin of a fat lobule. Highly magnified.
From a photograph.
f.g. fat-globule distending a fat-cell ; u, nucleus ; m, membranous envelope of the fat-
cell ; c. r. bunch of crystals within a fat-cell ; c, capillary vessel ; )', venule ; c.t. con-
nective-tissue cell ; (I, granular cell ; the connective-tissue fibres are not represented.

and by other basic aniline dyes. (4) Migratory leucocytes may also be
seen here and there in the areolar tissue {wander-cells). (5) In the middle
coat of the eye in mammals, and in some parts of the skin, some of the
connective-tissue cells are filled with granules of pigment (pigment-cells).

These are much more extensively present in lower vertebrates, especially
in amphibia and fishes, where they exhibit amoeboid changes which result in
the pigment being at one time diffused over a considerable area and at
another time restricted to the immediate neighbourhood of the nucleus.
The changes thus produced cause alteration in the general colour and shade
of the integument, where such pigment cells are very numerous, and serve
the purpose of protective adaptation of the animals to their environment.

The cells lie in spaces in the ground-substance, between the bundles
of white fibres. In some parts of the connective tissue the white

THE CELLS OF AREOLAR TISSUE.

bundles are developed to such an extent as to pervade almost the
whole of the ground-substance, and then the connective-tissue corpuscles
become squeezed into the interstices, flattened lamellar expansions of
the cells extending between the bundles, as in tendon (see next Lesson).

A"

nr

Fig. 78. — Deposition of f.\t in connective-tissue cells.

/, a cell with a few isolated fat-droplets in its protoplasm ; f, a cell with a single large
and several minute drops ; /", fusion of two large drops ; g, granular cell, not yet
exhibiting any fat-deposition ; c.t., flat connective-tissue corpuscle ; c, c, network of
capillaries.

The cells and cell-spaces of areolar tissue come into intimate relation
Avith the cells lining the lymphatic vessels and small blood-vessels.
This connection can best be seen in
silvered preparations ; it will be
again referred to in speaking of the
origin of the lymphatics.

Adipose tissue consists of vesicles
filled with fat (figs. 77, 79) and
collected into lobules, or into tracts
which accompany the small blood-
vessels. The vesicles are round
or oval in shape, except where
closely packed, when the}^ become
polyhedral from mutual compres-
sion. The fat-drop is contained with-
in a delicate protoplasmic envelope
(fig. 77, m) which is thickened at
one part, and here includes an oval
flattened nucleus. The fat is
stained black by osmic acid ; a
deep yellow colour by Sudan IIL ;
and an intense red by Scharlach R.

Fig. 79. — Fat-cells from young animal

(Ranvier.) Osmic acid preparation.

Tlie drops of fat are stained of an

intense black, n, nucleus ; g, small

globules of fat.

74

THE ESSENTIALS OF HISTOLOGY.

The vesicles are supported partly by filaments of areolar tissue, but
chiefly by a fine network of capillary bloodvessels.

The fat when first formed in the embryo is deposited within large
granular cells of areolar tissue (fig. 78) similar in general appearance to
the " Mast "-cells of Ehrlich; some authorities regard them as of a specific
nature, for they are in certain situations collected into large gland-

Fig.

-Two STAGES OF FORMATION OF ADIPOSE TISSUE. (H. Batty Shaw.

In A the tissue is formed of a gland-like mass of cells, in some of which the cj'toplasm is
occupied by fat globules (looking white in the sections). In B the fat fills many of
the cells.

like masses (fig. 80) abundantly supplied with blood-vessels, which
gradually become transformed into fat-cells by the deposition of fat in
the cell-protoplasm. Fat is, however, also laid down in ordinary

Fig. 81. — Retifoem tissue from a lymph-gland. Moderately magnified.

tr, a trabeculum of connective tissue ; r, 7-', retiform tissue, with more open meshes at >•

and denser at ■>•'.

branched cells of connective-tissue. The fat appears to be produced
by a transformation of albuminous granules which the cells con-
tain into droplets of fat. As the droplets increase in size they run

adh'osk tissue.

7ri

together into a larger drop, which gradually fills the cell more and
more, swelling it out so that the cell-protoplasm eventually appears
merely as the envelope of the fat-vesicle.

Fig. 82. — Portion of the .\bove, more highly magnified,

showing the continuity of the retiform tissue >■, r, with the connective tissue of a
trabeculum, ()•.

Fat is found most abundantly in subcutaneous areolar tissue, and
under the serous membranes ; especially in some parts, as at the
back of the peritoneum around the kidneys, under the epicardium.

Fig. 83.— Reticulum of bone-m.'^hkow. (Enderlen.)
and in the mesentery and omentum. The yellow marrow of the
bones is also princi[)ally composed of fat. There is no adipose tissue
within the cavity of the cranium.

Eetiform or reticiilar tissue (figs. 81, 82, 83) is a variety of con-
nective tissue in which the intercellular or ground-substance has largely

76

THE ESSENTIALS OF HISTOLOGY.

disappeared or is replaced by fluid. There are very few or no elastic
fibres in it, but a dense network of white fibres, the meshes of which
vary in size, being very small and close in some parts ; more open
and like areolar tissue in other parts. In some places where the
tissue occurs the fibres are enwrapped by flattened branched con-
nective-tissue cells, and until these are removed it is not easy to see
the fibres. Chemical diff'erences between the fibres of retiform tissue
and those of ordinary areolar tissue have been described by Mall, but
microscopically the fibres of the two tissues are indistinguishable
and are found in continuity with one another (see figs. 82, 84). This

Fig. 84.— Lymphoid tissue of a lymph-gland.

tissue forms a fine framework in many organs, supporting the proper
elements and extending into all the interstices between the coarser
connective tissue bundles. It can best be shown by dissolving the
cells of the tissue by tryptic digestion and subsequently staining the
fibres forming the reticulum (p. 67, § 5). In this way it may be demon-
strated in lymph-glands, in the spleen, liver, bone-marrow (fig. 83),
mucous membranes, and many other parts.

Lymphoid or adenoid tissue is reticular tissue in which the meshes
of the network are largely occupied by lymph-corpuscles (fig. 84).
This is by far the most common condition of a retiform tissue, and

BA8KM KNT-MEMBRANES. 77

is met with in the lyniph-i^huids and allied structures, and also in
parts of the alimentary nuicous membrane, and in some other
situations.

Basement-membranes {me inb ranee propriw) are homogeneous-looking
membranes, which are found forming the surface layers of connective-
tissue ex[)ansions in many parts, especially where there is a covering
of epithelium, as on mucous membranes, in secreting glands, and else-
where. They are generally formed of flattened connective-tissue cells
joined together to form a membrane; but in some cases they are
evidently formed not of cells, but of condensed ground-substance, and
in yet other cases they are composed of elastic substance ; the name
basement-membrane is therefore used to denote structures of an
entirely different nature.

Jelly-like connective tissue, although occurring largely in the
embryo, is found only in one situation in the adult — viz. forming
the vitreous humour of the eye. It is composed mainly of soft,
fluid, ground-substance, with cells scattered here and there through
it, and with a few fibres which interlace throughout the tissue and
confine the fluid of the ground-substance within their meshes ; thus
conferring upon the tissue its jelly-like character. All embryonic
connective-tissue is in the first instance of this jelly-like nature
(see p. 82).

THE ESSENTIALS OF HISTOLOGY.

LESSON X.

THE CONNECTIVE TISSUES {contimted).

ELASTIC TISSUE. FIBROUS TISSUE. DEVELOPMENT OF CONNECTIVE

TISSUE.

1. Tease out as finely as possible a small shred of elastic tissue (ligamentum
nuclije of the ox or ligamentum subflavura of man) in glycerine and water,
slightly coloured by magenta. Cover and cement the preparation. Note
the large well-defined fibres constantly branching and uniting with one
another. Sketch a small part of the network. Note the existence of
bundles of white fibres amongst the elastic fibres.

2. Examine a thin transverse section of ligamentum nnchge which has
been hardened in 2 per cent, solution of bichromate of potash. The section
is to be stained with haemalum and mounted in Canada balsam or dammar
by the usual process, or simply in glycerine and water. Observe the
grouping of the fibres and their angular shaj)e.

3. Pinch ofi" the end of the tail of a dead mouse or rat, draw out the long
silk-like tendons and put them into salt solution. Take one of the threads,
which should be nearly three inches long, and stretch it along a slide, letting
the ends dry firndy to the glass but keeping the middle part wet. Put a
piece of hair on either side and cover in salt solution. Observe with a high
power the fine wavy fibrillation of the tendon. Draw. Now run dilute
acetic acid (0'75 per cent.) under the cover-glass, watch the tendon where it
is becoming swollen by the acetic acid. Notice the oblong nucleated cells
comiug into view between the tendon-bundles, ^ketch three or four cells in
a row. Lastly, lift the cover-glass, wash away the acid with distilled water,
place a drop of Ehrlieh's hematoxylin or carmalum solution on the tendons,
and leave the preparation until it is deeply stained ; then wash away the
stain and mount the preparation in faintly acidulated glycerine.

4. Take another long piece of tendon, and after washing it in distilled
water, stretch it upon a slide as before, fixing the ends by allowing them to
dry on to the slide. Put a drop of nitrate of silver solution (1 per cent.)
on the middle of the tendon, and leave it on for five minutes. Then wash
off the silver nitrate with distilled water, and expose the slide to direct
sunlight. In a very few minutes the silvered part of the tendon will be
brown. As soon as this is the case, dehydrate the tendon with absolute
alcohol in situ upon the slide, run ofi" the alcohol, and at once put a drop of
clove oil on the preparation. In a minute or two the clove oil can be
replaced by xylol balsam or dammar and the preparation covered.

5. Stain, with magenta solution, a thin section of a tendon which has been
hardened in 70 per cent, alcohol. Mount in dilute glycerine and cement
at once.

6. For developing connective tissue study sections of the umbilical cord
at different periods. Fix with formol. Stain -with acid fuchsine and
haematoxylin.

ELASTIC TISSUE.

79

Elastic tissue is a variety of connective tissue in which the elastic
fibres preponderate. It is found most characteristically in the liga-
mentum ruicha.' of quadrupeds and the liganienta subHava of the
vertebrae, but the connective tissue of other parts may also have a

Fig. 85. — Ela.stic fibres from the ligamentum nuch^e of the ox,
showing transverse markings on the fibres.

considerable development of elastic fibres. It occurs in an almost
pure form in the walls of the air-tubes, and uniting the cartilages
of the larynx. It also enters largely into the formation of the lungs
and of the walls of the blood-vessels, especially the arteries.

I,. ,

Fig. 86.— Cross-section of elastic fibres from the ligamentum NUCHiE

OP THE ox.

In the ligamentvim nuchae most of the fibres are very large (figs.
85, 86). They often exhibit cross markings or even transverse clefts.
When dragged asunder, they break sharply across. They constantly
branch and unite, so as to form a close network. In transverse section
they are seen to be separated into small groups or bundles (fig. 86) by
intervening septa of areolar tissue.

80 THE ESSENTIALS OF HISTOLOGY.

Elastic tissue does not always take the form of fibres, but may
occur as membranes {e.g. in the blood-vessels). Sometimes the fibres
are very small, but their microscopic and chemical characters are
always very marked (see p. 70).

Fig. 87. — Section of tendon, human. (Sobotta.) x 32.
t, tendon-bundles ; s, septa of areolar tissue ; v, vessels.

Fibrous tissue is almost wholly made up of bundles of white fibres
running in a determinate direction. These again are collected into
larger bundles, w^hich give the fibrous appearance to the tissue. The
bundles are constantly uniting with one another in their course,
although their component fibres remain perfectly distinct.

The interspaces between the larger bundles are occupied by areolar

FiniJULTS TISSUE.

81

tissue (fig. 87, fi ; fig. 88, c, d, e) in which the blood-vessels and
lymphatics of the fibrous tissue are conveyed. The interstices between
the smallest bundles are occupied by rows of lamellar connective-tissue

Fig. 88.— Part of a large tendon in transverse section. More highly magnified.

a, areolar sheath of the tendon, with the fibres for the most part running transversely ;
but with two or three longitudinal bundles, 6 ; I, lymphatic cleft in the sheath ;
Immediately over it a blood-vessel is seen cut across, and on the other side of the
figure a small artery is shown cut longritudinally ; c, large septum of areolar tissue ;
d, smaller septum ; e, still smaller septum. The irregularly stellate bodies are the
tendon-cells in section.

A £.-

Fig. 89. — Tendon of mouse's tail (175 diameters) ; showing chains ok

CELLS BETWEEN THE TENDON-BUNDLES.
A, stained with hsematoxylin. B, staineil with silver nitrate, showing the cell-spaces.

F

82 THE ESSENTIALS OF HISTOLOGY.

corpuscles {tendon-rdh), which, from being squeezed up between three
or more bundles, become flattened out in two or three directions. In
transverse section the cells appear somewhat stellate (fig. 88), but
when seen on the flat they appear lamellar (fig. 89), and from this
aspect their general shape is square or oblong. They lie, as before
said, in rows between the tendon-bundles, and the nuclei of adjacent
cells are placed opposite one another in pairs (fig. 90). The cell-spaces
correspond in general figure and arrangement to the cells which occupy
them.

Fibrous tissue forms the tendons and ligaments, and also certain
membranes, such as the dura mater, the fibrous pericardium, the
fasciae of the limbs, the fibrous covering of certain organs, etc. It is
found wherever great strength, combined with flexibility, is concerned.
It receives a few blood-vessels, disposed longitudinally for the most
part, and contains many lymphatics. Both blood-vessels and lymphatics
run in the areolar tissue which separates and surrounds the tendon-
bundles. Tendons and ligaments also receive nerve-fibres, which, in

^lUg^^^gl^P^^^^P

Fig. 90. — Eight cells i'ROM the same tendon' .\s kepresented in fig. 89.
(425 diameters.)

The dark lines on the surface of the cells are the optical sections of lamellar extensions
directed towards or away from the observer.

some cases, end in localised ramifications within fusiform enlargements
of the tendon-bundles (organs of Golgi), while others terminate in
end-bulbs or in simple Pacinian corpuscles. These will be described
along with the modes of ending of nerve-fibres.

Development of connective tissue. — Connective tissue is developed
in and from the cells of the mesoderm (mesenchyme) of the embryo.
In those parts which are to form connective tissue, there muy
frequently be seen a clear space separating the cell-layers which are
already formed, this clear space being permeated with fibres which
appear to be produced from the cells bounding the space. Presehtly
branching mesenchyme cells, which are derived from the bounding cell-
layers, are found forming a syncytium within the clear space (fig. 91, m).
In the meshes of the reticular syncytium is a muco-albuminous semi-
fluid intercellular substance (ground-substance). The connective-tissue
fibres, both white and elastic, are deposited in this ground-substance,
the elastic substance appearing in the form of granules (fig. 95, g),
which subsequently become connected together into elastic fibres or

DEVELOPMENT OF CONNECTIVE TISSUE.

83

^V

'' « ft

Fig. 91.— Developing connective tissue in heart of chick-embuto of
48 HOURS. (Szili.)
my, cells forming myocardium ; j, jelly formed of reticulum with enclosed fluid ; <■, endo-
thelium (mesothelium) of heart ; m, mesenchyme cells in jelly; bl, blood-corpuscles.

K..:S^

^0

\^^-"''''^^'^-
X^'^^

X>^

Fig. 92.— Developing connective tissue from the umbilical cord of

A HUMAN EMBRYO 21 MM. LONG. (Minot. ) X 540.

84

THE ESSENTIALS OF HISTOLOGY.

0

Al-

i, "^ ^ \ 5^ "^t"^/^^ '^^ Wi^'' ''-■-••••■■"'''' ^"-\

Fig. 93.— Developing connective tissue from the umbilical cord of a

THREE MO.NTHS' HUMAN EMBRYO. (Minot.) X 511.

>

.--/

4

#. «

4.

Fig. 94. — Jelly of vvharton from umbilical cord of new-born child.

(Sobotta.) x280.

/, connective-tissue fibres ; c, cells.

DEVELOPMENT OF CONNECTIVE TISSUE.

86

laminae, as the case may be, the white fibres appearing at first in
the form of very fine bundles, which afterwards become gradually
larger (fig. 93) ; so that in fibrous tissue the whole ground-substance
is eventually pervaded by the bundles, and the cells of the tissue
become squeezed up into the intervals between them. Before any
considerable development of fibres has taken place, the embryonic
connective tissue has a jelly-like appearance ; in this form it occurs
in the umbilical cord, where it is known as the jelly of Wharton
(fig. 94).

There has been always a considerable difference of opinion as to the
origin of the fibres of connective tissue, some histologists holding that
they are formed within the protoplasm of cells, which gradually lose

Fig. 9.5. — Development of elastic tissue by deposition of fine gkandles..

(Kanvier.)

g, fibres being formed of rows of ' elastin ' granules ; p, flat plate-like expansion of
elastic substance formed by the fusion of ' elastin' granules.

their cell-characters as the fibres become developed within them ;
others taking the view that the fibres, both white and elastic, are
extracellular formations. While there is no doubt that they are
produced under the influence of the cells, for they first appear in
close proximity to those structures, it seems on the whole probable
that the fibres are deposited in the ground-substance and not actually
in the cell-protoplasm, so that they are rather to be looked upon, like
the ground-substance itself, as formed by a process of secretion than
by one of direct cell-transformation.

86 THE ESSENTIALS OF HISTOLOGY.

LESSON XL

THE CONNECTIVE TISSUES {continued).

ARTICULAR CARTILAGE. SYNOVIAL MEMBRANES.

1. Cut two or three very thin tangential slices of the fresh cartilage of a
joint, mount them in .salt solution, and examine with the high power.
Observe the form and grouping of the cells. Look at the thin edge of the
section for spaces from which the cells have dropped out. Measure two or
three cells and their nuclei, and sketch one or two groups. Now replace
the salt solution by water and set the preparation aside for a little while.
On again examining it, many of the cartilage cells will be found to have
retracted away from their containing capsules.

2. Make other sections of the cartilage (1) from near the middle, (2) from
near the edge. Place the sections for two or three minutes in acetic acid
(1 per cent.), wash them with water, and stain with dilute hjemalum or
carmalnm solution. When stained mount in dilute glycerine and cement
the cover-glass. In (2) look for branched cartilage cells.

3. Study vertical sections of articular cartilage from an end of bone
which has been fixed and decalcified, and mount the sections in glycerine
and water, or, after staining with ha?malum, in dammar or xylol balsam.
Sketch the ai^rangement of the cells in the different layers.

4. Brush a fresh joint with distilled water ; drop 1 per cent, nitrate of
silver solution over it ; after five minutes wash away the nitrate of silver
and expose in water to direct sunlight. When browned, place in spirit for
half an hour or more, and then with a razor wetted with spirit cut thin
sections from the surface and mount in xylol balsam or dammar after passing
through clove oil. The cells and cell-spaces show white in the brown
ground-substance.

5. To study the structure of the synovial membrane mount other slices
from the silvered preparation of the joint (§ 4) just beyond the limits of
the articular cartilage, and also look for small fringed projections of the
membrane. Snip them off with scissors and mount as before.

6. The superficial flexor tendons of the foot of the ox or sheep run in
grooves formed by the deep flexors, and these grooves are lined, and the
tendons which pass through them are covered by vaginal synovial mem-
branes. To show the structure of these treat one of the superficial flexor
tendons with silver nitrate in the manner recommended for the joint, § 4,
and after hardening in spirit cut sections from the surface and mount them
in balsam or dammar varnish.

Cartilage or gristle is a translucent bluish-white tissue, firm, and at
the same time elastic, and for the most part found in connection with
bones of the skeleton, most of which are in the embryo at first repre-
sented entirely by cartilage. Three chief varieties of cartilage are

CARTILAGE.

87

distinguished. In one, whicli is tei-med liiialinc, the matrix or ground-
substance is almost clear, and free from obvious fibres; in the other
two, which are tcrined Jihro-cartilage, the matrix is everywhere pervaded
by connective-tissue fibres. When these are of the white variety, the
tissue is white Jihro-cartiliuje ; when they are elastic fibres, it is yellow or
elastic fihro-cariihuic

Hyaline cartilage occurs principally in two situations — -namely (1)
covering the ends of the bones in the joints, where it is known as
articular cartilage ; and (2) forming the rib-cartilages, where it is known

■#■

'^

f-^S'

( (m

^::nM0^''''^''^^-''''^^^^^^^^^-^ '

Fig. 96. — Articular cartilage from head of metatarsal bone of ampu-
tated FOOT, HUMAN (OSMIC ACID PREPARATION). THE CELL-BODIES
ENTIRELY FILL THE SPACES IN THE MATRIX. (340 diameters.)

a, group of two cells ; b, group of four cells ; h, protoplasm of cell, with g, fatty
granules ; n, nucleus.

as costal cartilage. It also forms the cartilages of the nose, of the
external auditory meatus (but not the pinna), most of those of the
larynx, and the cartilages of the windpipe ; in these places it serves
to maintain the shape and patency of the orifices and tubes.

Articular cartilage. — The cells of articular cartilage are generally
scattered in groups of two or four throughout the matrix (fig, 96).
The latter is free from obvious fibres, except at the extreme edge
of the cartilage, where the connective-tissue fibres from the synovial

88 THE ESSENTIALS OF HISTOLOGY.

membrane extend into it ; and here also the cartilage-cells are often
branched, and offer transitions to the branched connective-tissue
corpuscles of that membrane {transitional cartilage, fig. 97). By long
maceration in brine, however, evidence of a fibrous structure may be

..'/

(v:

Fig. 97.— Border op articulak cartilage showing transition of cartil-
age CELLS into connective-tissue CORPUSCLES OF SYNOVIAL MEMBRANE.

From head of metatarsal bone, human. (About 340 diameters.)

a, ordinary cartilage-cells ; h, h, with branching processes.

obtained, even in the matrix of true hyaline cartilage. Some his-
tologists also describe fine communications in the matrix uniting the
cartilage-cells with one another, but these are of doubtful occurrence in

vertebrate cartilage, although they
unquestionably exist in the cartilage
of cephalopods.

The matrix immediately around
the cartilage-cells is often marked
off from the rest by a concentric
line or lines, this part of the matrix,
which is the latest formed, being
known as the capsule of the cell
(fig. 98). The cells are bluntly
angular in form, the sides opposite
to one another in the groups being
generally flattened. The proto-
plasm is very clear, but it may
contain droplets of fat; and with a high power fine interlacing
filaments and granules have been observed in it. During life the
protoplasm entirely fills the cavity or cell-space which it occupies in
the matrix ; but after death, and in consequence of the action of water
and other agents, it tends to shrink away from the capsule. The
nucleus is round, and shows the usual intranuclear network.

Fig. 98. — A group of cartilage-cells

SHOWING THE CAPSULAR OUTLINES IN
THE MATRIX SURROUNDING THE GROUP.

(Rauvier. )
«, nucleolus ; h, nucleus ; c, cytoplasm of a
cell ; d, capsular lines in pericellular
matrix ; (, fibrils in cartilage matrix.

ARTICULAR CARTILAGE.

8!>

In vertical section {tig. Di)) the deeper cell groups (c) are seen to be
arranged vertically to the surface, the more superficial ones (a) parallel
to the surface ; whilst in an intermediate zone the groups are irregu-

r

i Gs ©(Siea

ii

Fig. 99.— Vertical section of articular cartilace (ovkkixc; the lower
KND OF THE TIBIA, HUMAN. (Magnified about 30 diameters.)

a, cells and coil-groups flattened conformably with the surface ; b, cell-groups Irregu-
larly arranged ; c, cell-groups disposed perpendicularly to the surface ; d, layer
of calcified cartilage ; e, bone.

Fig. 100.— Plan of the multiplication of cells op cartilage. (Sharpey.)

A, cell in its capsule ; b, divided into two, each with a capsule ; c, primary capsule
disappeared, secondary capsules coherent with matrix ; d, tci'tiary division ; e,
secondary capsules disappeared, tertiary coherent with matrix.

larly disposed (h). In the deepest part of the cartilage, next to the
bone, there is often a deposit of calcareous salts in the matrix (calcified
cartilage, d).

The disposition of the cells of cartilage in groups of two, four, eight,
etc., is apparently due to the fact that these groups have originated

90

THE ESSENTIALS OF HISTOLOGY.

from the division of a single cell first into two, and these again into
two, and so on (fig. 100). The division of the cartilage-cell, like that
of most other cells, is mitotic.

It would seem that the matrix is formed of successive portions,
which are deposited around each cartilage-cell as the so-called
'capsules,' each newly formed portion soon blending in its turn with
the previously formed matrix, whilst a new capsule is formed within it.
The most newly formed portions of matrix stain with haematoxylin
more deeply than the rest, and in some cartilages this gives the

appearance of rounded balls of darkly
stained matter surrounding each cell or
cell-group {clwivirin-haVs, Morner).

Embryonic cartilage is characterised by
the cells being usually more sharply angular
and irregular ; they are even in some cases
markedly branched, like those which occur
at the junction of cartilage and synovial
membrane in the adult. The cells are
also more closely packed, the matrix being
in relative!}' less amount than in later life.
Development. — Cartilage is formed in
the embryo from mesenchyme similar to
that which gives origin to other forms
of connective-tissue. Each cell forms a
capsule around itself, and the blended
capsules compose the first matrix. Cartil-
age sometimes remains in this condition
throughout life ; it is then termed paren-
chymatous cartilage. This can be seen in
the mouse's ear : where also the cartilage
cells become filled with fat. Cartilage
grows at first partly by interstitial expan-
sion (accompanied by cell multiplication
and by formation around and between the cells of intercellular sub-
stance), partly by apposition at the perichondrium, the connective
tissue becoming here transformed into cartilage. At a later period of
growth the increase in size and change in shape of cartilages are due
almost entirely to the agency of the perichondrium.

Fig. 101. — Villus OF STXovi.\L
MEMBRAXE. (Hammar.)

Synovial Membranes.
The synovial membranes are often compared with the serous
membranes. They are indeed, like the latter, connective-tissue

SYNOVIAL MEMBRANES. 91

membranes which bound closed cavities moistened with fluid, but
they are not connected with the lymphatic system, nor is the fluid
(synovia) which moistens them of the nature of lymph. Moreover,
there is either no endothelial lining, or it occurs only in patches,
in place of the continuous lining which we find in the serous
membranes. Long villus-like projections occur in many parts ; they
are often covered by small rounded cells, and probably serve to
extend the surface for the secretion of synovia. The blood-vessels
of synovial membranes are numerous, and approach close to the inner
surface of the membrane. They are well seen in preparations from
an injected limb.

92 THE ESSENTIALS OF HISTOLOGY.

LESSON XII.

THE COS SECT IV E TISSUES (continued).

COSTAL CARTILAGE. FIBRO-CARTILAGE.

L Make transverse and tangential sections of a rib-cartilage, which may
either be fresh, or may have been preserved in spirit or formol. Stain them
with hsenialum or carmalum (if fresh, after treatment with acetic acid as in
Lesson XL § 2, or they may be placed for an hour or two in "5 p.c. osmic
acid), and mount in glycerine. Sketch a part of a transverse section under
a low power and a cell-grou^i from one of the tangential sections under a
high power. Notice especially the arrangement of the cells, somewhat
concentric near the surface but radial near the centre. The co.stal cartilages
tend to become ossified near the middle in most animals, but in man when
ossification occurs it is the superficial layer which is invaded.

2. Make sections of the cartilage of the external ear (pinna), either fresh
or after hardening in alcohol. Mount in dilute glycerine faintly coloured
with magenta, or stain with orcein and mount in balsam. If from the ox,
notice the very large reticulating elastic fibres in the matrix. Notice also
the isolated granules of elastin, and around the cartilage-cells an area of
clear ground-substance. If from the mouse or rat there is very little matrix
and no elastic fibres, and the cells are almost in contact (parenchymatous
cartilage) ; they also contain fat (staining with osmic acid).

3. Mount a section of the epiglottis in the same way. Notice the closer
network of much finer fibres in its cartilage.

4. Cut sections of white fibro-cartilage (intervertebral disk or semilunar
cartilage of knee), which has been hardened in picric acid, followed by spirit,
or in spirit only. Stain the sections with dilute htemalum or carmalum.
Mount in dilute glycerine. Observe the wavy fibres in the matrix and the
cartilage-cells lying in clear areas often concentrically striated. Look for
branched cartilage-cells. Sketch three or four cells and the adjoining
fibrous matrix.

Costal cartilage. — In the costal cartilages the matrix is not
always so clear as in the cartilages of the joints, for it more often
happens that fibres become developed in it. The cells are generally
larger and more angular than those of articular cartilage, and
collected into larger groups (fig. 102). Xear the circumference,
and under the perichondrium or fibrous covering of the cartilage,
they are flattened and parallel to the surface, but in the deeper
parts they have a more irregular or a radiated arrangement. They
frequently contain fat. The cartilages of the larynx and windpipe
and of the nose resemble on the whole the costal cartilages, but the

COSTAL CARTILAGE. 93

study of them may be deterred until the organs where they occur
-are dealt with.

Elastic or yellow fibre -cartilage occurs in only a few situations.
These are, the cartilage of the external ear and that of the Eustachian
tube, and the epiglottis and cartilages of Santorini of the larynx. The
matrix is everywhere pervaded with well-defined branching fibres,
which unite with one another to form a close network (figs. 103, 104).
These fibres resist the action of acetic acid, and are stained deeply

;)

■ ■^'>y-/i^

Fig. 102. — Sectiox of rib-cartilage, showing cells and cell-groups ix
ax indistixctly fibrous matrix.

Two or three empty cell-spaces are seen from which the cells have dropped out in

preparing the section.

by magenta ; they are evidently elastic fibres. In the ox they are
very large, but smaller in man, especially in the cartilage of the
epiglottis. They appear to be developed, as with elastic tissue else-
where (see p. 82), by the deposition of gi'anules of elastin in the
matrix, which at first lie singly, but afterwards become joined to form
the fibres.

White fibro-cartilage is found wherever great strength combined
with a certain amount of rigidity is required : thus we frequently
find fibro-cartilage joining bones together, as in the intervertebral
disks and other symphyses. But in these cases the part in contact
with the bone is always hyaline cartilage, which passes gradually
into ^the fibro-cartilage forming the bulk of the symphysis. Fibro-
cartilage is often found lining grooves in which tendons run,

94

THE ESSENTIALS OF HISTOLOGY.

4.>

zap ; \- ^

f

Fig. 103. — Section of elastic cartilage of ear, hcmax. (Sobotta.) x2S0.

c, cartilage cells ; co.p, their capsules ; in, clear matrix around ceUs and cell-groups ;

r", elastic fibres.

mM

Fig. 104.— Section of the elastic cartilage of the ear.
Highly magnified.

(R. Hertwig.)

Fig. 10.5.— "White fibro-caetilage from an intervertebral disk, human.
Highly magnified.

The concentric lines around the cells indicate the limits of deposit of successive
capsules. One of the cells has a forked process which extends beyond the hyaline
area surrounding the cell, amongst the fibres of the general matrix.

FIBRO-CARTILAGE. 95

and it may be found in the tendons themselves. It is also em-
ployeil to deepen cup-shaped articular surfaces ; and in the case of
the interarticular cartilages, such us those of the knee and lower jaw,
to allow greater freedom of movement whilst diminishing the liability
to dislocation. Under the microscope white fibro -cartilage looks very
like fibrous tissue, but its cells are cartilage-cells, not tendon-cells
(fig. 105). They are rounded or bluntly angular and surrounded
by a concentrically striated area of clear cartilage-matrix. In some
parts of the intervertebral disk many of the cells are branched,
and may be looked upon as ti'ansitional forms to connective-tissue
corpuscles.

m THE ESSENTIALS OF HISTOLOGY.

LESSON XIII.
THE C02sNECTIVE TISSUES {continued).

BONE; STEUCTITRE AND DEVELOPMENT.

1. Ix thin sections of hard bone made by grinding,^ observe the Haversian
canals, lamellae, lacunae, canaliculi, etc. Make a sketch first under a low and
afterwards under a high power.

2. With fine forceps strip off a thin shred from the superficial layers of a
bone which has been decalcified in 5 p.c. commercial sulphurous acid and
afterwards washed with water for 24 houi^s. It may be kept in dilute
alcohol. Mount the shred in water. Observe the fibrous structure of the
lamella;. Look for perforating fibres or the holes from which they have
been dragged out. Sketch a small piece of the thin edge of a lamella.

3. Stain with dilute magenta and hremalum solution, or with methyl-blue
and eosin, very thin sections of compact bone which has been fixed with
10 p.c. formol (1 to 3 days) and then decalcified in sulphurous acid as above.
Mount in dilute glycerine, cementing at once. Look for fibres of Sharpey
piercing the circumferential lamellae. The elastic perforating fibres are more
darkly stained than the others. Notice the stained nuclei of the bone-
corpuscles in the lacunae. In the thinne.st parts of the sections try to make
out the blood-vessels and other structures in the Haversian canals.

4. Mount in xylol balsam or dammar a section of a fcetal lower jaw
which has been stained in bulk and embedded in paraffin. Find the part
where the lower jaw-bone is becoming ossified, and carefully study the
appearance which it presents. The bone is prolonged in the form of
osteogenic fibres which are covered with osteoblasts.

5. Intramembranous ossification may also be studied in the parietal bone
of a foetus which has been preserved in Muller's fiuid. A piece of the
growing edge is scraped or brushed free from its investing membranes, and
from most of the cells which cover and conceal it, and is mounted in
glycerine with or without previous staining with carmalum.

6. Mount in balsam or dammar sections of a fcetal limb (which may have
been stained in bulk). The bones will be found in different stages of
ossification, those of the digits being least developed. Make sketches illus-
trating the three chief stages of endochondral ossification. Notice the
peculiar terminal ossification of the third phalanx.

Bone i.s a connective tissue in which the ground-substance is
impregnated with salts of lime, chiefly phosphate, these salts consti-
tuting about two-thirds of the weight of the bone. When bones are
macerated this earthy matter prevents the putrefaction of the animal
matter. When bones are calcined they lose one-third of their weight,
owing to the destruction of the animal matter ; when steeped in acid
^ It is best to purchase these.

BONE. 97

the earthy salts are dissolved and only the animal matter is left. This,
like areolar and fibrous tissue, is converted into gelatine by boiling.

.>P'''

Fig. 106.— Section of a decalcified human radius. (Sobotta.) x48.

periosteum ; pi, periosteal bony lamellae ; p'V, deeply seated lamellas parallel with
periosteal surface; H, Haversian systems; tr, tr, trabeculae of spongy substance;
rrd, lamellae bounding medullary spaces.

Bony tissue is either compact or cancellated. Compact bone is dense,
like ivory ; cancellated is spongy with obvious interstices. The outer
layers of all bones are compact, and the inner part is generally

G

98

THE ESSENTIALS OF HISTOLOGY.

cancellated, but the shaft of a long bone is almost entirely made up of
compact substance, except along the centre, which is hollow and filled
with marrow. The interstices of cancellated bone are also occupied
by marrow. Externally bones are covered except at the joints by a
vascular fibrous membrane, the jjeriosteum.

True bone is always made up of lamellce, and these again are com-
posed of fine fibres lying in a calefied fjround-mhstance. Between the
lamellae are branched cells, the bone-corpuscles, which lie in cell-spaces
or lacunm. The ramified passages which contain the cell-processes are
termed canaliculi.

^l^iM'^

//

^A^~x*'^yA ' '

Ui£jSk

A-

Fig. 107.

-TRAN.SVERSE SECTION OF COMPACT TISSUE (OF HUMERUS).

Magnified about 1.50 diameters.

(Sharpey.)

Tliree of the Haversian canals are seen, with their concentric rings ; also the lacunae,
with the canaliculi extending from them across the direction of the lamellae. The
Haversian apertures had become filled with air and debris in giinding down the
section, and therefore a]>pear black in the figure, which represents the object as
viewed by transmitted light.

In cancellated bone the blood-vessels run in the interstices supported
by the marrow. In compact bone they are contained in little canals —
the Haversian canals — which everywhere pervade the bone. These
canals are about 0-05 mm. {-g^Q inch) in diameter, but some are
smaller, others larger than this. Their general direction is longi-
tudinal, i.e. parallel to the long axis of the bone, but they are
constantly united by transversely and obliquely running passages.
In a section across the shaft of a long bone they are seen as small
rounded or irregular holes (fig. 106). When the section has been
made by grinding, the holes get filled up with air and debris, and
they then look black by transmitted light, as do also the lacunae

BONE.

99

and canaliculi (fig. 107). Most of the lamelhie in compact bone are
disposed concentrically around the Haversian canals ; they are known
as the Haversian lamellw, and with the included canal form what
is known as a Haversian system. The lacunae of a Haversian system
communicate with one another and with the Haversian canal, but
not as a rule with the lacunae of adjacent Haversian systems. The
angular interstices between the Haversian systems are generally

Fig. 108.— Transverse section of decalcified human tibia, from near the
surface of the shaft.

H, H, Haversian canals, with their systems of concentric lamellas ; in all the rest of the
figure the lamellae are circumferential ; jt, ordinary perforating fibres of Sharpey ;
e, e, elastic perforating fibres. Drawn under a power of about 150 diameters.

occupied by bony substance which is fibrous but not lamellar.
Besides the lamellae of the Haversian systems there is a certain
thickness of bone at the surface, immediately underneath the
periosteum, which is composed of lamellae arranged parallel with
the surface ; these are the circumferential or periosteal lamelhe (fig.
106, pi). They are pierced here and there by simple canals for
blood-vessels, the so-called Volhnann's canals, which are proceeding
from the periosteum to join the system of Haversian canals, and
also by calcified bundles of white fibres and by elastic fibres which
may also be prolonged from the periosteum. These are the per
forating fibres of Sharpey (fig. 108).

100 THE ESSENTIALS OF HISTOLOGY.

The lamellfe of bone are fibrous in structure. This may be seen in
shreds torn off from the superficial layers of a decalcified bone
(fig. 109). The fibres {decussating fibres of Sharpey) often cross one
another in adjacent lamellae, and in the Haversian systems they run
in some lamellae concentrically, in others parallel with the Haversian
canal. In shreds of lamellae which have been peeled off" from the
surface the perforating fibres ma}^ sometimes be seen projecting from
the surface of the shred, having been torn out of the deeper lamellae

Fig. 109.— Lamellae torn off from a decalcified human parietal bone at

SOME DEPTH FROM THE SURFACE. (Sbarpej'.)

a, lamellie, showing decussating fibi-es ; b, i, thicker part, where several lamellae are
superposed ; c, c, jierforating fibres ; the fibrils which compose them are not shown
in the figure. Apertures through which perforating fibres had passed are seen,
especially in the lower part, a, of the figure. Magnitude as seen under a power of
200 diameters, but not drawn to scale. (From a sketch by Allen Thomson.)

(fig. 109, c, c). When tendons or ligaments are inserted into bone,
their bundles of white fibres are prolonged into the bone as perforating
fibres.

The lacunae are occupied by nucleated corpuscles, which send
branches along the canaliculi (fig. 110). They have a special lining
layer different in chemical composition from the rest of the bone, being
much more resistant to the action of strong chemical solvents such as
hydrochloric acid (Neumann). The dentinal tubules of the teeth have
a similar lining layer.

The Haversian canals contain one or two blood-capillaries and
nervous filaments, besides a little connective tissue ; and the larger ones
may also contain a few marrow-cells. There are also cleft-like lym-
phatic spaces running with the vessels, their cells being connected

BONE.

101

through canaliculi with branches from corpuscles within the neigh-
bouring lacunae of the osseous substance (fig. 111).

The periosteum may be studied in torn-off shreds, in preparations
stained in situ Avith silver nitrate, and in stained sections from an
unmacerated bone which has been decalcified. It is a fibrous
membrane composed of two layers, the inner of which contains
many elastic fibres. In the outer layer numerous blood-vessels

Fig. 110.— a bone-cell isol-^ted
and highly magnified.
(Joseph. )
a, proper wall of the lacuna (Neu-
mann's layer), where the corpuscle
has shrunken away from it.

Fig. 111.— Section of a haversian canal,

SHOWING ITS CONTENTS. (Highly

magnified.)
a, small arterial capillary vessel ; v, large venous
capillary ; 71, pale nerve-fibres cut across ;
I, cleft-like lymphatic vessel; one of the cells
forming its wall communicates by fine bi-anches
with the branches of a bone-corpuscle. The
substa,nce in which the vessels run is connec-
tive tissue with ramified cells ; its finely
gi-anular appearance is probably due to the
cross-section of fibrils. The canal is sur-
rounded by several concentric lamellaj.

ramify and send branches to the Haversian canals of the bone. The
periosteum ministers to the nutrition of the bone, partly on account
of the blood-vessels and lymphatics it contains, partly, especially in
young animals, on account of the existence between it and the
bone of a layer of osteoblasts or bone-foj-ming cells, a remainder of
those which originally produced the bone. It also serves to give
attachment to muscular fibres.

The marrow of bone has been already studied (pp. 38, 39).

DEVELOPMENT OF BONE.

True bone is essentially formed in all cases by an ossification of
connective tissue. Sometimes the bone is preceded by cartilage, which
first becomes calcified, and this is then invaded, and for the most part
removed, by an embryonic tissue which re-deposits bony matter in the
interior of the cartilage. This is intracartilaginous or endochondral ossi-
fiadion. At the same time layers of bone are being formed outside the

102

THE ESSENTIALS OF HISTOLOGY.

cartilage underneath the periosteum. The whole bone thus formed is
termed a cartilage-hone. Sometimes the bone is not preceded by carti-
lage, and then the only process which occurs is one corresponding to the
subperiosteal ossification of the cartilage-bone ; the ossification is then
known as intramemhranous, and the bone formed is a membrane-bone.

Fig. 112.— Section of phalangeal bonk of hdman fcetus at the time of
COMMENCING OSSIFICATION. (From a prepaiation bj- F. A. Dixey.) The
preparation was stained in bulk with magenta. The drawing is made from
a photograph, and is magnified about 75 diameters.

The cartilage cells in the centre are enlarged and separated from one another by stained
calcified matrix ; im, layer of bone deposited underneath the periosteum ; o, layer of
osteoblasts by which the layer has been formed. Some of the osteoblasts are already
embedded in the new bone as lacun*. The cartilage-cells ai-e becoming enlarged and
flattened and arranged in rows above and below the calcified centre. At the ends of
the cartilage the cells are small, and the groups are irregularly arranged ; the fibrous
periosteum is not sharply marked off from the cartilage.

Ossification of cartilage.— This may be described as occurring in
three stages. In the _^rs; stage the cells in the middle of the cartilage
become enlarged and arranged in rows radiating from the centre

OSSIFICATION OF CARTILAGE.

103

(fig. 112), and fine granules of calcareous matter are deposited in the
matrix. Simultaneously with this the osteoblasts underneath the
periosteum deposit a layer or layers of fibrous lamella? upon the
surface of the cartilage, and these lamellae also become calcified
(fig. 112, im). As they are formed, some of the osteoblasts (o) are
included l)etween them and become bone-corpuscles.

Fig. 113.— Section of part of
one of the limb-bones of
a fcetal cat, at a more
advanced stage of ossifi-
cation than is represented
in fig. 112, and somewhat
more highly magnified.
Drawn from a photograph.

The calcification of the cartilage-
matrix has advanced from the
centre, and is extending between
the groups of cartilage-cells, which
arc arranged in characteristic
rows. The subperiosteal bony
deposit (im) has extended pari
passu with the calcification of the
cartilage-matrix. The cartilage
cells in the calcified part are
mostly shrunken and stellate ; in
some cases they have dropped out
of the spaces. At ir and in two
other places an irruption of the
subperiosteal tissue, composed of
ramified cells with osteoblasts
and growing blood-vessels, has
penetrated the subperiosteal bony
crust, and has begun to excavate
secondary areolaj or medullary
spaces ; p, fibrous layer of the
periosteum ; o, layer of osteo-
blasts, some of them are em-
bedded in the osseous layer as
bone-corpuscles in lacuna;. The
blood-vessels are occupied by blood-
corpuscles. Beyond the line of
ossific advance the periosteum
may be noticed to be distinctly
incurved. This incurvation is
gradually moved on, the carti-
lage expanding behind it until
the head of the bone is reached,
when it forms the periosteal
notch or groove represented in
figs. 116 and 119.

In the second stage some of the subperiosteal tissue eats its way
through the newly formed layer of bone and into the centre of the
calcified cartilage (fig. 113, ir). This is freely absorbed before it
(fig. 115), so that large spaces are produced which are filled with
osteoblasts, and contain numerous blood-vessels which have grown
in at the same time. These spaces are termed medullary spaces, and
this second stage may be termed the stage of irruption.

104

THE ESSENTIALS OF HISTOLOGY.

In the third stage of endochondral ossification there is a gradual
advance of the ossification towards the extremities of the cartilage, and
at the same time a gradual deposition of fresh bony lamellse and spicules
on the walls of the medullary spaces, and on the surface of the new
bone under the periosteum. The advance into the cartilage always
takes place by a repetition of the same changes, the cartilage-cells first
enlarging and becoming arranged in rows, the matrix between the

Fig. 114. — Part of a longi-
tudinal SECTION OF THE
DEVELOPING FEMDR OF THB

RABBIT. (Klein.) (Drawn un-
der a magnifying power of 350
diameters. )

a, rows of flattened cartilage-cells ;

b, gieatly enlarged cartilage-cells
close to the advancing bone, the
matrix between is partly calcified ;

c, d, already formed bone, the
osseous trabeculte being covered
with osteoblasts (() except here and
there, where an osteoclast (/) is
seen eroding parts of the trabe-
culfe ; g, h, cartilage-cells which
have become shrunken and irre-
gular in shape. From the middle
of the figure downwards the
trabecular, which are formed of
calcified cartilage-matrix, are be-
coming covered with secondary
osseous substance deposited by
the osteoblasts. The vascular
loops at the extreme limit of the
bone are well shown, as well as
the abrupt disappearance of the
cartilage-cells.

rows becoming calcified, and then the calcified cartilage becoming
excavated from behind by the osteoblastic tissue so as to form new
medullary spaces (fig. 114). The walls of these are at first formed only
by remains of the calcified cartilage-matrix (fig. 114, c), but they soon
become thickened by lamellae of fibrous bone which are deposited by
the osteoblasts, and between which bone-corpuscles become included,
as in the case of the subperiosteal bone. The latter advances pan passu
with the endochondral calcification, but beyond this the uncalcified

OSSIFICATION OF CARTILAGE.

105

cartilage grows both in length and breadth, so that the ossification
is always advancing into larger portions of cartilage ; hence the
endochondral bone as it forms assumes the shape of an hour-glass, the
cylindrical shape of the whole bone being maintained by additions of
periosteal bone to the outside (see fig. 116). The absorption of the
calcified cartilage-matrix appears to be effected, as is the case with
absorption of bony matter wherever it occurs, by large multi-nucleated

Fig. 115.— Longitudinal section through part of a phalanx of a six
months' human embryo. (Kolliker. )

The calcified cartilage is completely absorbed almost to the limit of advancing calcifica-
tion. The osseous substance on either side is periosteal bone. The embryonic
marrow has shrunk somewhat away from it.

cells (fig. 11-i, /, /) which are termed osteoclasts. They are cells of the
same nature as the myeloplaxes of the marrow, and are found
on surfaces where absorption of bone is taking place, whereas the
osteoblasts are always found covering surfaces where bony deposit
is proceeding (fig. 117).

The bone which is first formed is more reticular and less regularly
lamellar than that of the adult, and contains no Haversian systems.

106

THE ESSENTIALS OF HISTOLOGY.

The regular lamellae are not deposited until some little time after
birth, and their deposition is generally preceded by a considerable

Fig. 110.— Longitudinal sec-
tion THROUGH THE UPPER HALF
OF THE DECALCIFIED HUMERUS
OF A FCETAL SHEEP, AS SEEN
UNDER A M.\GNIFY1NG POWER
OF ABOUT 30 DIAMETERS.

, the part of the shaft which was
primarily ossified in cartilage ;
what remains of the primary bone
is represented dark, enveloped by
the clear secondary deposit. The
areolae of the bone are occupied by
embryonic marrow with osteo-
blasts, and blood-vessels variously
cut. One long straight vessel (pv)
passes in advance of the line
of ossification far into the cartil-
aginous head, most of the others
loop round close to the cartilage.
At one or two places in the
older parts of the bone elongated
groups of cartilage-cells (er) may
still be seen, which have hitherto
escaped absorption, m, the part
of the bone that has been ossified
in membrane, that is to say, in
tlie osteoblastic tissue under the
periosteum. It is well marked
off from the central portion, and
is bounded, peripherally, by a
jagged edge, the projections of
which are indistinctly seen to be
prolonged by bunches of osteo-
genic fibres. A row of osteoblasts
covei-s the superficial layer of the
bone. The subperiosteal layer is
prolonged above into the thicken-
ing {p) which encroaches upon
the cartilage of the head of the
bone, and in which are seen
amongst numerous osteoblasts and
a few blood-vessels, the straight
longitudinal osteogenic fibres («/'),
and some other fibres (pf) crossing
them, and perhaps representing
fibres of Sharpey. The calcareous
salts having been removed by an
acid, the granular ossific deposit
passing up between the rows of
cartilage-cells is not seen in this
specimen ; it would have extended
as far as a line joining the marks
X X. ObseiTe the general ten-
dency of the osseous trabeculse
and the vascular channels between
them to radiate from the original
centre of ossification. This is
found to prevail more or less in
all bones when they are first
formed, although the direction of
the ti-abeculie may afterwards be-
come modified in relation with
varying physiological conditions,
and especially as the result of
pressure in different directions.

amount of absorption. It is about this time also that the medullary
canal of the long bones is formed by the absorption of the bony
tissue which originally occupies the centre of the shaft.

OSSIFICATION OF CARTILAGE.

107

\\

\

After a time the cartilage in one or both ends of the long bones
begins to ossify independently, and
the epiphi/se.^ are formed. These
are not joined to the shaft until the
growth of the bone is completed.
Cxrowth takes place in lemjth by an
expansion of the cartilage {inter-
mediate cartilage) which intervenes
between the shaft and the epiphj^ses,
and by the gradual extension of
the ossification into it ; in midth
entirely by the deposition of fresh
bony layers under the periosteum.
In the terminal phalanges of the
digits the ossification starts,
from the middle of the cartilage, but
from its distal extremity (Dixey).

For the regeneration of portions
of bone which [have been removed
by disease or operation it is important that the periosteum be left

Y

not Fig. 117.— Boxy trabecul,e from the

DEVELOPING LOWER J.\W OF A CALF,
SHOWING OSTEOCLASTS .\T THE EX-
TREMITIES WHERE ABSORPTION IS PRO-
CEEDING, AND OSTEOBLASTS COVERING
THE SIDES WHERE DEPOSITION OF BONE

IS GOING ON. (Kolliker. )

Fig. 118.— Transverse section of a developing
bone, showing the periosteal layer becom-
ing formed from osteogenic fibres.

cb, cartilage bone ; pb, periosteal bono ; sp, bone spicules
prolonged by osteogenic fibres ; p, periosteum ; hi,
blood-vessels ; c, remains of the calcified cartilage ;
o, osteoblasts forming bone upon this.

Fig. 119.— Section of the ossi-
fication GROOVE IN THE

HEAD OF A LONG BONE.

c, cartilage ; p, periosteal tissue
with osteogenic fibres and osteo-
blasts. This tissue occupies the
"groove."

108

THE ESSENTIALS OF HISTOLOGY.

Intramembranous ossification. — In this A-ariety of ossification (fig.
120), the bone is not preceded by cartilage at all, and therefore no
endochondral bone is formed, but the calcification occurs in a sort
of embryonic fibrous tissue which contains numerous osteoblasts and
blood-vessels. The fibres of this tissue (osteogenic fibres), which,
like those of fibrous tissue, are collected into small bundles, become
inclosed in a calcareous matrix, produced by the deposition of lime
salts in the ground-substance of the connective tissue : and as the
fibres grow, the calcification extends further and further, so that bony

Fig. 120.— Part of the growing edge of the developixg parietal boxe of
A fcetal cat, 1| inch long.

sp, bony spicules, with some of the osteoblasts embedded iu them, producing the
lacunM ; of, osteogenic fibres prolonging the spicules, with osteoblasts {pit) between
them and applied to them ; a, granular calcific deposit occurring in the ground-
substance between the fibres ; c, union of two adjacent spicules.

spicules are formed, which, as they become thickened, run together
to form reticular layers, leaving spaces filled with osteoblasts around
the blood-vessels. The osteogenic fibres are covered with osteoblasts,
and as the bone forms, some of these become left as bone-corpuscles
within lacunae. Thus in every particular the development of these
bones resembles that of the subperiosteal laj-er of endochondral bone ;
which is also to be considered as an instance of intramembranous
ossification, although taking place on the surface of cartilage. More-
over, it is the same subperiosteal tissue which, in endochondral
ossification, deposits the true or secondary bone upon those parts of

OSSIFICATION IN ^lEMBRANE. 109

the calcified cartilage-matrix which have escaped absorption; and this
must also, therefore, be reckoned as developed according to the same
type. In fact, even in intracartilaginous ossification, very little of the
calcified cartilage-matrix eventually remains ; this being almost wholly
absorbed and either replaced by true or fibrous bone which has been
formed by osteoblasts, or swept away to form the medullary and
other cavities.

"With reference to the origin of the osteoblasts, it has been thought by
some authors that they are derived from the blood-vessels, and are in fact
leucocytes whicli have wandered out of the vessels and have taken on the
special osteogenic function. Another and a more proljable view regards
theru as niodi6ed connective tissue cells formed within the periosteum and
merely accompanying the vessels into the interior of the ossifying cartilages.
They have also been thought to be formed by division and alteration of the
cartilacre-cells.

110 THE ESSENTIALS OF HISTOLOGY.

LESSON XIV.

STRUCTURE OF STRIATED MUSCLE.

1. Take a shred of muscle from a recently killed mammal, and on a dry slide
carefully sepai'ate long pieces of muscular fibres (single fibres if possible) and
stretch them out, keejjing them moist during the process by breathing on
the slide. Put a drop of serum on the cover-glass before placing this over
the preparation. Study first with a low, then with a high power. Sketch
all the apj)earances to be seen in a small piece of a fibre, focussing carefully
the most superficial layers. Notice the oval nuclei immediately under the
sarcolemiua. Then allow a little dilute acetic acid to run under the cover-
glass and watch its effect.

2. Prepare some fibres of frog's nmscle in the same way, but mount in
salt solution instead of serum. Notice the muscular substance shrinking
away here and there from the sarcolemma, which then becomes distinctly
visible. Sketch a piece of sarcolemma bridging across an interval thus
produced.

3. Study transverse sections of muscle which has been hardened in alcohol
or formol and stained. Mount in dammar varnish or xylol bals^am. Examine
the section of a fibre first with a low and then with a high power. Sketch
the appearances which are seen.

In each of the above preparations measure the diameter of some of the
fibres.

Sections of muscle-spindles may be searched for in the transverse sections
of muscle.

4. Place in 1 per cent, osmic acid a small shred of mammalian muscular
tissue which has been stretched upon a cork. After '24 hours, when it will
be deeply stained, wash it in water and with needles break the fibres up in
glycerine as finely as possible. Cover and examine with a high power.

5. Cut off the head of a small garden beetle or wasp, and bisect the trunk
with scissors so as to expose the interior. Notice two kinds of muscular
tissue, the one belonging to the legs greyish in colour, the other attached to
the wings yellowish. Preparations of both kinds of muscle are to be made
in the same way as living mammalian muscle (i$ 1), but it is better to mount
them in a drop of white of egg. In both jn-eparations the dark-looking
air-tubes or tra<-he:e form prominent objects ramifying amongst the fibres.
Observe the structure of the two kinds of muscle so far as it can be made
out in the fresh ])reparation. If the preparation is made quickly, waves of
contraction will probably be observed passing along the fibres.

6. Make another preparation of the leg-muscles, mounting the muscle in
vinegar. (Alcohol-hardened muscle of insect or crab may be used for this
purpose.) Notice that the muscular substance swells up somewhat and
becomes clearer, whilst the sarcoplasm-network, with its lines and dots,
comes more distinctly into view. In a well-teased preparation of alcohol-
hardened muscle, the fibres will be frequently found breaking across into
disks. Make careful drawings from this preparation.

7. Rollett's method. Cut off" the head of an insect (wasp, small beetle),
bisect the trunk and- place in 90 per cent, alcohol for from 24 to 48 hours

STRUCTURE OF STRIATED MUSCLE.

Ill

or more. Then take a small piece of each kind of muscle, and place in
strong glycerine for some hours. Wash thoroughly with water and
transfer to I per cent, chloride of gold solution : leave the pieces of muscle
in this from lo to 30 minutes according to their size. From the gold
solution tliey are transferred to formic acid (1 part of the strong acid to
3 of water), and kept in the dark for 24 hours, but they may be kept longer
without disadvantage. The muscle is then teased in glycerine. Some of
the fibres will be found after this method to have their sarcoplasm darkly
stained, and to show, therefore, the appearance of a network both in longi-
tudinal and transverse view : others, on the other hand, liave the sarcous
elements of the fibrils or sarcostyles stained, whilst the sarcoplasm has
remained colourless.

Voluntary muscle is composed of long cylindrical fibres, measuring
on an average about 0-05 mm. in diameter (^i^y inch) in mammalian

Fig. 121.

Fig. 123.

Fig. 121. — S.\rcolemma of mammalian muscle highly magnified.
The fibre is represented at a place whei-e the muscular s\ibstauce has become ruptured
and has shrunk away, leaving the sarcoleninia (with a nucleus adhering to it) clear.
The fibi-e had been treated with serum acidulated with acetic acid.

Fig. 122. — Muscular fibre of a mammal examined fresh in serum, highly

magnified, the surface op the fibre being accurately focussed.
The nuclei are seen on the flat at the surface of the fibre, and in profile towards the edge.

Fig. 123.— Portion of a medium-sized human muscular fibre, showing the

INTERMEDIATE LINE (DOBIE's LINE) MENTIONED IN THE TEXT. (Sliarpey.)

muscles, and often having a length of an inch or more. Each fibre
has an elastic sheath, the sarcolemma, which incloses the contractile
substance. The sarcolemma is seldom distinct, unless the contained
substance becomes broken (fig. 121).

112

THE ESSENTIALS OF HISTOLOGY.

The contractile substance of the fibre is characterised by the alter-
nate dark and light stripes which run across the length of the fibre ;
hence the name, cross-striated or striped muscle. On focussing, it can be
seen that the stripes pass through the whole thickness of the fibre ;
they may therefore be looked upon as representing alternate disks of
dark and light substance. If the fibre be very carefully focussed, rows
of apparent granules are seen lying in or at the boundaries of the light
streaks, and very fine longitudinal lines may, with a good microscope,
be detected uniting the apparent granules (fig. 122). These fine lines,
with their enlarged extremities the granules, are more conspicuous

Fig. 124. — S.mall portion of .\
mdscle fibre of cr.\b split-
TING UP INTO FIBRILS. (FlODl a

photogra])li. )

Magnified 600 diameters.

Fig. 125. — Section of a muscdlar
fibre, showing areas of cohn-

HEIM.
Three nuclei are seen lying close to the
sarcolemma.

in the muscles of insects. They indicate the divisions between the
longitudinal elements {fibrils or sarcosti/k.<) which compose the fibre,
and in preparations treated with dilute acid they appear to form part
of a fine network, which pervades that substance, and serves to unite
the granules both transverseh" and longitudinally. This network,
which is sometimes very distinct in preimrations of muscle treated
with chloride of gold, is, however, a network in appearance only : in
reality it is the optical expression of the interstitial substance which
lies between the fibrils. This substance is termed sarcoplasm.

On examining the transverse section of a fibre with a high power,
it is seen to be subdivided everywhere into small angular fields,
Cohnheim's areas (fig. 125), which are themselves again divided up.
The smallest divisions represent sections of the fibrils of which the
fibres are composed, and into which they may be split after death,
especially after being hardened in certain reagents, e.g. chromic acid

VOLUNTARY MUSCLE. 113

or osmic acid. The larger areas represent groups of fibrils. These
areas of Cohiiheim are usually polyhedral, but they may be elongated,
and disposed either radially, or concentrically with the circumference
of the section. The interstitial substance or sarcoplasm lies between
them and can l)e made visible by treatment with dilute acid or by
staining with chloride of gold (figs. 127, 128, and 129). It is some-
times in relatively large amount, but in most muscular fibres is
I'educed to a very fine interstitium.

An ill-defined clear line is sometimes seen running trans versel}''
across the fibre in the middle of each dark band. This is termed
Hcnsen's line.

If instead of focussing the surface of the fibre it be observed in its
depth, an appearance different from that shown in fig. 122 is frequently
visible, namely, a fine dotted line {Dobie's line), bisecting each clear
stripe (fig. 123); this appearance is often considered to represent a
membrane {Krause's membrane), which subdivides the fibrils at regular
intervals (see p. 116). But the membrane of the individual fibrils or
.sarcostyles is rarel}^, if ever, visible in an intact mammalian fibre, and
it is certain that the appearance of such a line in the middle of the
clear stripe of an intact fibre is in most cases due to interference,
caused by the light being transmitted between disks of diff"erent
refrangibility.

Haycraft ha.s suggested that the cross-striation of voluntary muscle is
due to refractive effects produced by a varicosity of the component fibrils,
basing his view upon the fact that in impressions of the fibres made in soft
collodion all the cross-striations which are observed in the fibre itself are
reproduced. Theie is no doubt that a well-marked cross-striated appearance
can be produced in homogeneous fibrils by regularly-occurring varicosities,
and many of the appearances observed in muscle may, as Haycraft contends,
be referred to this cause. But even when a fibre or fibril is stretched so
that it exhibits no varicosities, the cross-striations are still perfectly distinct.
Moreover, in view of the entirely different manner in which the substance
of the dark and clear stripes behave to many staining reagents, and especially
to chloride of gold when applied as directed in § 7, the fact being that very
definite structural appearances can under these circumstances be made out,
the homogeneity of the muscle-fibinl cannot be admitted. This inference is
strongly confirmed by the microchemical work of A. B. Macallum, who has
shown that the j^otassium salts of the muscle are mainly accumulated in the
sarcous elements.

Nuclei. — Besides the sarcolemma and striated substance, a muscular
fibre also exhibits a number of oval nuclei which have the usual
structure of cell-nuclei: their chromatin often has a spiral aixangement.
Sometimes there is a little graimlar substance (protoplasm) at each
pole of the nucleus ; each nucleus with the adjacent protoplasm has
then been spoken of as a inuscle-corpusde. But the protoplasm which
is adjacent to the nuclei is in all probability continuous with the

114

THE ESSENTIALS OF HISTOLOGY.

sarcoplasm between the fibrils ; both being the remains of the original
undifferentiated protoplasm of the cells from which the muscular fibres
are developed. In mammalian muscle the nuclei usually lie imme-

FiG. 126.— Living muscle of water-beetle
(dttiscus marginalis.) (Highly maguified.)

s, sarcoloiuma ; a, dim stripe ; h, bright stripe ; c, row
of dots in ijright stripe, which seem to be the
enlai-ged ends of rod-shaped paiticles, d, but are
really expansions of the interstitial sarcoplasm
which appear in the living muscles as fine dark
lines with dot-like enlargements upon them.

Fk;. 127. — Portion of leg-muscle
OF insect treated with dilute

ACID.
S, sarcolemma ; V. dot-like enlargement
of sarcoplasm ; K, Krause's membrane.
The sarcous elements are dissolved or
at least rendered invisible by the acid.

liiiir

.Jim iiiiii

llpiSH liliiliiiiiJiB)

^iffiTm\

\\^'

Fig. 128.

Fig. 129.

Fig 128. — Transverse section of leg-muscle fibre of an insect,
stained with gold chloride
The sarcoplasm is here stained, and appears in the foroi of a network, in the meshes of
■which He the sections of the fibrils. Xotice the mottled appearance of the sections
of the sarcostylcs or fibrils, indicating a porous structure, as in the wing fibrils
(see fig. 132). The central protoplasm (with a nucleus) is also evident. (From a
photograph.)

Fig 129. — Leg-muscle fibre of insect treated with dilute acid, showing

a tendency to break across into disks.
The sarcoplasm is in the form of fine lines. The ordinary dark stripes of the fibre have

disapi>eared in the acid. A, a disk seen partly in section and exhibiting the reticular

an-angement of the sarcoplasm ; B, longitudinal view of fibre.

VOLUNTARY MUSCLE. 115

diately under the sarcolemma (figs. 121, 122, 125), in frog's muscle
they are scattered throughout the sul)stance of the fibre ; in insect
muscle they occupy the middle of the fibre, embedded in graiuilar
protoplasm (fig. 128). Some animals, such as the rabbit, have, besides
muscles of the ordinary type of structure, which in this animal are pale
in colour, others of a deep red colour. These red muscles were found by
Kanvier to exhibit certain differences both in structure and function.
One difference of structure is that the nuclei, which are numerous, are
not confined to the surface, but are scattered throughout the substance
of the fibres. The fibres in question also contain more sarcoplasm
than the ordinary fibres, and their blood-

vessels have a peculiarity of structure %

which will be afterwards noticed. Here ! ,^._ |

and there, in all mammals, amongst the )••••:'>•• = .-.^ — K

ordinary fibres are some in which the ' „ j

nuclei are distributed through the thick- ! . ; ..i^-^-

ness of the fibres; this is the case also, \iniTi . . , . . ; ..'

as just remarked, with all the muscular ["['"'^^"['"""""^X
fibres of the fros;. In muscles which are ! ,

in constant activity, such as the dia- s '"i": i : r, -^

phragm and the dorsal fin muscles of

TT- ,1 ^ 1 / Fig. 130.— Leg-muscle fibre of

Hippocampus, the protoplasm (sarco- insect, st-^ined with gold

plasm) of the fibres is present in rela- chloride by rollett's method.

, , . , , . . , A', line formed by membranes of

tively large proportion, and this is also Kiause; S.E., dark stripe formed

, •.■I ^1 ■ -I n by sarcous elements. The sarco-

the case with the wing muscles OI pksm has the appearance of longi-

, tudinal lines.

insects.

The transverse section of a muscle shows the fibres to be nearl}''
cylindrical in figure. Between the fibres there is a certain amount of
areolar tissue, which serves to support the blood-vessels and also
unites the fibres into fasciculi ; the fasciculi are again united together
by a larger amount of this intramuscular connective tissue (endo-
mysium.)

Ordinary or leg-muscles of insects. — In the muscles of insects the
stripes are relatively broad, and their structure can be more readily
seen than in mammals. In the living fibres from the muscles which
move the legs, the sarcoplasm presents a striking appearance of fine
longitudinal lines traversing the muscle, and enlarging within the
light stripes into rows of dots (fig. 126). This is still better seen in
fibres and portions of fibres which have been treated with dilute acid
(fig. 127). In separated disks produced by the breaking across of
muscle-fibres, the surfaces of the disks show a network with poly
hedral meshes in some insects (fig. 129, a), one formed of lines radiating

116 THE ESSENTIALS OF HISTOLOGY.

from the centre of the fibre in others. The nuclei, with some inclosing
protoplasm, lie in the middle of the fibre.

Wing-muscles of insects. — The wing-muscles of insects are easily
broken up into fibrils (sarcostyles), which also show alternate dark
and light striiB (fig. 131).

The sarcostyles are subdivided at regular intervals by thin transverse
disks {membranes of Krause) into successive portions, which may be

Fig. 131. — Fibrils of the wi.ng-muscles of a wasp, PREPAREn by kollet's
METHOD. Highly magnified. (From photographs.)

A, a contracted fibril. B, a stretched fibril, with its sarcous elements separated at
the line of Hensen. C, an uncouti-acted fibril, showing the porous structure of
the sarcous elements.

termed sarcomeres. Each sarcomere is occupied by a portion of the
dark stria of the whole fibre (sarcous element) : the sarcous element
is really double, and in the stretched fibre separates into two at the
line of Hensen (fig. 131, b). At either end of the sarcous element is
a clear substance (probably fluid or semi-fluid) separating it from the
membrane of Krause : this clear substance is more evident the more
the fibril is extended, but diminishes, even to complete disappearance,
in the contracted muscle (fig. 131, a). The cause of this change is

WING-MUSCLES OF INSF.CTS.

ir

explained when we study more minutely the structure of the sarcous
element. For we find that each sarcous element is pervaded with
longitudinal canals or pores, which are open in the direction of Krause's
membranes, but closed at the middle of the sarcous element (fig. 132).
In the contracted muscle, the clear part of the muscle-substance has
disappeared from view, but the sarcous element is swollen and the
sarcomere is thus shortened: in the uncontracted muscle, on the other
hand, the clear part occupies a considerable interval between the
sarcous element and the membrane of Krause, the sarcomere being

/ I

Fig. 132. — Isolated sarcous elements
OP A wing-muscle, showing the

TUBULAR OR POROUS STRUCTURE.

(Magnified 2300 diameters.)
Some are seen in profile ; others on the flat.

S.E.-

Fig. 133. — Diagram of a sarcomere in a
moderately extended condition, a,
and in a contracted condition, b.

K, K, membranes of Krause ; H, line or plane of
Hensen ; S.B., poriferous sarcous element.

lengthened and narrowed. The sarcous element does not lie free
in the middle of the sarcomere, but is attached at either end to
Krause's membrane by very fine lines, which may represent fine septa,
running through the clear substance (fig. 133); on the other hand,
Krause's membrane appears to be attached laterally to a fine membrane
which limits the fibril externally.

The planes of sarcous elements set side by side in a muscle-fibre
form the dark stripe (the so-called principal disk) of the muscle-
substance of ordinary muscle-fibres (fig. 130). But in the wing-muscles
of insects the sarcous elements of the fibrils less constantly lie in
continuous planes, and the whole fibre is therefore very indistinctly
and irregularly cross-striated, although each individual fibril is markedly
so (fig. 131). As already stated, the sarcous elements are remarkable
for containing a large proportion of potassium salts (Macallum).

Sometimes in the ordiiiar\' (leg) muscles of arthropods what look like
detached dot-like portions of the sarcous element are seen within the clear
stripes, lying usually near Krause's membrane. The rows of such dots liave
been termed accessor^/ disks. Most muscles show no accessory disks, but the
sarcoplasmic enlargements between the fibrils (fig. 127, d) are often mistaken
for them.

118

THE ESSENTIALS OF HISTOLOGY.

Muscle in polarised light. — When muscle-fibres are examined with
polarised liglit between crossed Nichol's prisms, the sarcous elements (which
form the dark stripe) are seen to be doubly refracting (anisotropous), while
the clear substance (forming the light stripe) is singly refracting (isotropous).
In contracted parts of the muscle the (anisotropous) sarcous elements are
seen to have increased in bulk, while the isotropous substance of the clear
stripe has correspondingly diminished in amount (fig. 134, b).

Fig 134— Leg-muscle fibre of chbysomela coerulea with (fixed) con-
traction WAVE photographed UNDER POLARISING MICROSCOPE.l

A, with uncrossed Nichols ; B, with crossed Xichols,

F. Merkel described a reversal of the stripes during contraction, i.e. a
transference of the anisotropous substance of the dark stripe from Hensen's
line to Krause's membrane, the place of the dark stripes thus becoming
occupied by clear material, that of the light stripes by dark. He further
described this condition as being preceded by an intermediate stage in which
the fibril shows homogeneity of shading. No doubt in the ordinary muscle-
fibres of arthrojjods, when we observe the so-called ' fixed ' waves of con-
traction, there is an apparent blurring of the cross-striation of the fibre just
where the muscle is passing from extension to contraction, but this appear-
ance is explicable by the unequal pull of the contracted parts of the fibrils
upon those which are not yet contracted. The contraction in each fibre
starts from the nerve-ending, which is at one side of the fibre, and spreads
first across the fibre and then tends to pass as a wave towards either end.
But the one side always has a start in the progress of this wave, and the
fibrils must thus receive an unequal pull, so that they are shifted along one
another and the liue of cross-striping is broken up. That no transference of
anisotropous substance really occurs is at once clear from the appearance

1 1 am indebted to Professor Engelmann for these two photographs.

MUSCLK IN POLARISED LIGHT.

119

of the contracting fibre under jjolaiised linht (fi,t^. 134, h), nnd the study of

the isolated (ibrils of winy;-niuscle gives no support to the tlieory of reversal,

although it is widely held by (xerinan authors. That the apparent reversal

is not real is also illustrated by

fig. 135, which represents a leg

muscle fibre of an insect in process

of contraction. The dark bands of

the ct)ntraction-wave are seen to

be really due to accumulations of

sarcoplasm. These accumulations

appear as dark lines which obscure

the continuity of the fibrils, and

by contrast cause the whole of the

sarcomeres between them to aiipear

light.

"Mechanism of contraction. —
Comparing the structure of the sar-
comere with that of the protoplasm
of an amteboid cell we find in both
a framework (spongiopiasm, sub-'
stance of sarcous element) which
incloses in its meshes or pores a
clear, probably fluid substance
(hyaloplasm, clear substance of
sarcomere). In both instances also
the clear substance or hyaloplasm,
when the tissue is subjected to
stimulation, passes into the pores
of the porous substance or spongio-
piasm (contraction), whilst in the
ab.sence of such stimulation it tends
to pass out from the spongiopiasm
(formation of pseudopodia, resting
condition of muscle). The effect
of stimulation appears in both
structures to be the production of a
change in surface tension (perhaps
between the hyaloplasm and spon-
giopiasm) ; this change being de-
monstrably accompanied in muscle
by a ditference in electric jJotential.
In all probability such an electric
change occurs in all protoplasm. Thus both the movements of cell-protoplasm
and those of muscle seem brought about by like means, although at first
sight the structure of muscle is cjuite dissimilar from that of protoplasm.

We have already noticed that the movements of cilia are susceptible of a
somewhat similar explanation.

Fig. 13.5. — "Wave of contraction passing
over a leg-muscle fibre of dytiscus.
Highly magnified.

120 THE ESSENTIALS OF HISTOLOGY.

LESSON XV.

CONNEXION OF MUSCLE WITH TENDON; BLOOD-VESSELS OF
MUSCLE; CARDIAC MUSCULAR TISSUE; DEVELOPMENT OF
MUSCLE; PLAIN MUSCULAR TISSUE.

1. To study the connexion of muscle with tendon, a frog is killed by destruc-
tion of the brain and spinal cord, and placed in about a litre of water raised
to a temperature of 55° C. It is left in this for 15 minutes, the water
gradually cooling. It is then easy to dissociate the muscular fibres in large
numbers. To obserA^e their attachment to the tendon-bundles a fine longi-
tudinal shred must be snipped off with scissors at the tendinous attachment^
and dissociated upon a slide in a drop of water. It will usually be found
that the muscular substance is retracted from the end of the sarcolemma
tube, which is firmly cemented to the tendon-bundle. The structure may be
brought more distinctly into view by adding to the dissociated fibres a drop
of a weak solution of iodine in salt solution or iu serum (iodised serum).'

2. The blood-vessels of muscle. These are studied in longitudinal and
transverse sections or in flattened-out pieces of injected muscle. It will be
noticed that the capillaries are very numerous, and form a network with
oblong meshes. In the red muscles of the rabbit, small dilatations are seen
on the transverse cords of the network.

3. The muscular tissue of the heart is studied in sections of that organ
(see Lesson XXVII.) and also in teased preparations. To prepare the latter,
place a small piece of heart-muscle in 33 per cent, alcohol for a few days ;
stain in picro-carmine solution for some hours or days ; and tease in dilute
glycerine.

4. Tear off a small shred of the muscular coat of a piece of cat's intestine
which has been for 48 hours or more in J per cent, bichromate of potash
solution or in 33 per cent, alcohol. Hold the shred with forceps in a drop of
water and fray it out with a needle. In this process many cells will be set
free and can be found with a low power. The preparation may then be
covered and examined with a high power. Sketch one of the cells. Then
allow dilute hfematoxylin solution to pass under the cover-glass and lastly a
drop of glycerine. Sketch another cell after staining. Measure two or
three cells and their nuclei.

Ending of muscle in tendon. — A small tendon-bundle passes to
each muscular fibre and becomes firmly united with the sarcolemma,
which extends over the end of the fibre (fig. 136). Besides this
immediate attachment, a further connexion is established by the

'This method is the one given by Kanvier (Traite Technique, p. 395). The
nuiscle-endings may also sometimes be well seen at the extremities of the
tendons which are removed from the mouse's tail in the manner described in
Lesson X.

BLOOD-VESSELS OF MUSCLE.

121

fact that the areohir tissue between the tendon biuuUes is continuous
witli that which lies between the muscular fibres.

Blood-vessels of muscle. — The capillaries of muscular tissue are
very numerous. They run, for the most part, longitudinally, with
transverse branches, so as to form long oblong meshes (fig. 137).
Ho blood-vessels ever penetrate the sarcolemma. In the red muscles
of the rabbit, the transverse capillaries have small dilatations upon

Fig. 136. — Termination of a mus-
cular FIBRE IN TENDON. (Ran-

^'^^^•) Fig. 137.— Capillary vessels of

7)j, sarcolemma ; s, the same membrane MUSCLE,

passing over the end of the fibre ; p, ,
extremity of muscular substance, c,
retracted from the lower end of the
sarcolemma-tube ; t, a tendon-bundle
passing to be fixed to the sarcolemma.

them (fig. 13S). Associated with this and other peculiarities of
structure (see p. 115), it is found that the red muscles have a much
slower rate of contraction, and a much longer period of latency than
the ordinary muscles.

Lymph-vessels, although present in the connective-tissue sheath
(perimysium) of a muscle, do not penetrate between the component
fibres.

The motor nerves of voluntary muscles pierce the sarcolemma

122

THE ESSENTIALS OF HISTOLOGY.

and terminate in ramified expansions knovvn as end-plates or motor
end-organs ; the sensoiy nerves end in groups of specially modified muscle
fibres known as mnsde-spiiulles (see Lesson XIX.).

Development.— Voluntary muscular fibres are developed from em-
bryonic cells of the mesoderm (muscle-plate), which become elongated,
and the nuclei of which become multiplied, so as to produce long
slender multi-nucleated fusiform or cylindrical embryonic fibres.
According to most recent authorities the embryonic fibres are not
formed by the growth of a single cell, but by the joining together

Fig. 138. — Vascular network of a red muscle (semi-texdixosus) ok the
RABBIT. (Ranv'ier. )

o, arteriole ; v, v, venules ; n, dilatation on transverse branch of capillaries.

end to end of a number of cells of the muscle-plate (or even of more
than one muscle-plate), so as to prbduce a syncytium, within which
the striated fibrils make their appearance. These appear at first
along one side of the fibre, the change gradually extending around
the circumference and also penetrating towards the centre ; but
the protoplasm at the middle of the fibre, to which the nuclei are
presently confined, and at the side opposite to that at which the
diSerentiation began, remains for some time unaltered in character
(fig. 139). Eventually the change in structure extends to these parts
also, and the nuclei pass gradually to occupy their ordinary position
under the sarcolemma, which has by this time become formed. The
sarcolemma is "believed to be produced, not by the muscle-fibre itself.

BLOOD-VESSELS OF MUSCLE.

V23

but b}' the mcsench3'iue or connective-tissue cells between the fibres,
since it is directly continuous with the connective-tissue bundles
of the tendon and of the interstitial tissue.

Fig. 139. — Developing muscular fibres.

A, elongated cell with two nuclei. A striation is beginning in the protoplasm along one

.side of the cell ; from fcetal sheep. (Wilson Fox.)

B, from human ffetus of two months. (Ranvier.) p, central protoplasm with several

nuclei, ii, scattered in it ; s, commencing sarcolemma, with striated muscular sub-
.staiice developing immediately beneath it.

C, from human foetus uf three months. (Ranvier.) The contractUe substance, s, f, now

almost incloses the unaltered protoplasm, g ; only one nucleus, n, is represented.

CARDIAC MUSCLE.

The muscular substance of the heart is composed of transversely
striated muscular fibres, which differ from those of voluntary muscle
in the following particulars, viz. :— their striations are less distinct ;
they have no sarcolemma, although there is a thin superficial
layer of non-fibrillated substance ; they branch and unite by their
branches and also at the side with neighbouring fibres, and their

124

THE ESSENTIALS OF HISTOLOGY.

nuclei lie in the substance and often near the centre of the fibres.
In man and many mammals the fibres are marked off" into a series of
short cylindrical cells (figs. 140, 141) joined together end to end and
side to side, each corresponding to one of the nuclei. The junctions of
these cells may be seen in longitudinal sections appropriately stained ;
they come also distinctly into view in sections of the fresh tissue
stained with nitrate of silver. They appear to be bridged across by fine
fibrils, continued into the cells above and below the lines of junction
(fig. 143). These lines have usually been regarded as intercellular
spaces separating the constituent cells of the tissue from one another

f

ii

Fig. 140.— Muscular fibres from the
heart, magnified, showing their
cross-stri.e, divisions, and junc-
TIONS. (Schweigger-Seidel.)

The nuclei and cell-junctions are only repre-
sented on the right-hand side of the figure.

Fig. 141. — Six muscular fibre-cells
FROM THE HEART. (Magnified 425
diameters. )

a, line of junction between two colls; 6, c,
branching of cells. (From a drawing by
J. E. Neale.)

(Schweigger-Seidel). But recent authorities (Przewosky, v. Ebner,
M. Heidenhain) are inclined to regard the cardiac muscular tissue
as forming a syncytium, the cells being all continuous both laterally
and longitudinally, and the apparent intercellular lines being special
diff"erentiations. These, according to v. Ebner, are due to localised con-
tractions, but, according to Heidenhain, represent portions of the fibres
at which growth in length occurs (analogous to the suture-lines
between the flat bones of the cranium). As against this view of the
structure of the heart-muscle, and in favour of that of Schweigger-
Seidel, must be set the silver-staining of the supposed cell-junctions,
and the fact that it is easily possible in some animals to separate the
fibres after maceration into short uninucleated fragments as in fig. 141.

CARDIAC MUSCLE.

125

The short non-nucleated lengths of fibres (fig. 142), which Heidenhain
regards as fatal to the cellular theory, may be parts of cells lying
in other planes of the myocardium, which are inserted between those
belonging to the plane included in the longitudinal section. On
the other hand, the continuity of the muscular fibrils within the
masses of Purkinje's fibres under the endocardium in the sheep, the
fibrils around one cell being freely continued around the neighbouring
cells (see fig. 304, p. 252), is in favour of the syncytial theory. Further,

Fig. 142. — Diagram of segmextatiox
OF HEART JIUSCLE. (M. Heidenhain.)
Parts op the segments coxtaix one

OR TWO nuclei, but SOME ABE QUITE
SMALL AND XON-NUCLEATED.

, iM

fmrnm

$\m^-

Fig. 143. — Portiox of cardiac muscle

EXHIBITING continuity OF FIBBILS
ACROSS JUNXTIONAL LINE. (Przewoskj.)

Highlv ma^uified

in many vertebrates, including some mammals, no cell-territories can
be made out in the myocardium, whilst in others, and especially in
man and some mammals, although definite cell-territories can be shown
to exi.st in the adult condition, they are absent in young animals.
We must therefore conclude that both conditions may occur.

The explanation of these diff'erences appears to lie in the fact that
in all heart-muscle at a certain period of development the cells form a
syncytium within which the contractile fibrils are developed, and only in
mammals is a differentiation of the syncytium into cells produced ; the
lines of junction being even here bridged across by the muscle-fibrils.

126

THE ESSENTIALS OF HISTOLOGY.

Non-striated, Smooth or Plain Muscle.

Involuntary or plain muscular tissue is composed of long, somewhat
flattened, fusiform cells (fig. Hi), which vary much in length. Each
cell has an oval or rod-shaped nucleus, which shows the usual intra-
nuclear network and commonly one or two nucleoli. The cell-substance

A,

Fig. 144.— Muscular fibre-cells from the muscular coat of
the small intestine, highly magnified.

A, a complete cell, showing the nucleus with intra-nuclear network, and
the longitudinal fibrillation of the cell-substance, with finely vacuolated
protoplasm between the fibrils ; B, a cell broken in the process of
isolation ; a delicate external layer projects at the broken end a little
beyond the striated substance of the cell.

Fig. 144.

Fig. 145.— Muscle-cells of intes- Fig. 146.— Plain muscle

TINE. (Szymonowicz.) Magnified
530 diameters.
The fibres are represented in longitudinal
section ; and the interstices between
them are seen to be bridged across by
fine fibrils, i, interstice ; n, nucleus.

FIBRE, SHOWING NU-
CLEUS, centriole, .\Nn

CYTOPLASM WITH FIB-
RILS. (Lenhossek.)

is finely fibrillated, but does not exhibit cross-strise like those of
voluntary muscle. There appears, as in cardiac muscle, to be a
delicate non-striated external layer, probably a stratum of undifferen-
tiated protoplasm, certainly not a true sarcolemma. Next to this, in
some smooth muscle, is a layer containing coarser fibrils (boundary
fibrils of M. Heidenhain). There is a little intercellular substance

PLAIN MUSCLE. 127

which can be stained by nitrate of silver, and which is Imdged across
liy Hlaments passing from cell to cell (fig. 145). Some authorities,
however, deny that the involuntary cells are thus connected, and hold
that the appearance of bridging fibres is due to intercellular connective
tissue. It is however difficult to understand how the contractions are
propagated from cell to cell if there is no sort of continuity between
the cells.

Plain muscular tissue is found chiefly in the walls of hollow viscera ;
thus it forms the muscular coat of the stomach and intestine.s, and
occurs abundantly in the muscular coat of the gullet, although it is
here intermixed with cross-striated muscle ; it is found also in the
mucous membrane of the whole alimentary canal from the oesophagus
downwards ; in the trachea and its ramifications ; in the urinarv l)ladder
and ureters; in the uterus and Fallopian tubes; in the prostate; the
spleen and lymphatic glands ; the muscle of Miiller in the orbit, and
in the ciliary muscle and iris. The walls of gland-ducts also contain
it : and the middle coat of the arteries, veins and lymphatics is largely
composed of this tissue. It occurs in the skin, both in the secreting
part of the sweat glands, and in small bundles attached to the hair-
follicles ; in the scrotum it is found abundantly in the subcutaneous
tissue (dartos), and it also occurs in the areola of the nipple.

Development. — According to the observations of C. M'Gill, the
smooth muscle of the alimentary canal (pig) is developed from the
syncytium of mesenchyme cells which surrounds the entoderm.
Some of these cells become elongated and spindle-shaped while
retaining their inter-connexion. Myofibrils are developed in their
protoplasm. These are not confined to the limits of a single cell, but
extend over two or even a large number of cells. The myofibrils are
of two kinds, coarse and fine, varying in relative number in different
parts. The distinction is seen even in the fully formed muscle, which
retains its syncytial character, and is not formed of completely
separated cells.

In certain situations smooth muscle is formed from epithelium, as
with the muscular tissue of the sweat glands (Ranvier) and that of
the iris (Xussbaum, Szili).

128 THE ESSENTIALS OF HISTOLOGY.

LESSON XVI.

STRUCTURE OF NERVE-FIBRES.

1. Tease a piece of fresh nerve rapidly in salt solution (or by the method of
seniidesiccation, afterwards mounting in salt solution), injuring the fibres as
little and obtaining them as long and straight as possible. Study the medul-
lated fibres, carefully noticing all the structures that are visible — viz., nodes
of Ranvier, nucleus of primitive sheath, double contour of medullary sheath,
medullary segments, etc. Measure the diameter of half a dozen fibres. Draw
a short length of a fibre very exactly.

2. Prepare a piece of sympathetic nerve in the same way. The nerves
passing to the spleen are well adapted for the study of non-medullated
fibres. They may also be found amongst the medullated fibres of the
ordinary nerves. The nuclei may be stained by gentian violet.

3. Separate (in dilute glycerine) into its fibres a small piece of nerve or
nerve-root that has been twenty-four hours in 1 per cent, osmic acid. The
nerve should have been moderately stretched on a piece of cork by means of
glass pins before being placed in the acid. Keep the fibres as straight as
possible and only touch them near their ends with the needles. Sketch two
portions of a fibre under a high power, one showing a node of Ranvier and
the other a nucleus of the primitive sheath. Look for fibres of Remak.
Measure the length of the nerve-segments between the nodes of Ranvier.

4. Mount in xylol balsam or dammar sections of a nerve which has been
hardened in picric acid and alcohol, or fixed with osmic acid and hardened
in alcohol. The sections may be stained with picro-carmine or hsematoxylin.
The nerve should be pinned out straight upon a cork with glass pins before
being placed in the hardening solutions. Examine the sections first with
a low and afterwards with a high power. Notice the lamellar structure of
the j^erineurium, the varying size of the nerve-fibres, the axis cylinder in
the centre of each fibre, etc. Measure the diameter of five or six fibres,
and sketch a small portion of one of the sections.

5. Study sections of splenic nerve placeil as soon as possible after death
in Flemming's solution.

6. Teased })reparations and sections from nerves which, some days pre-
viously, have been cut nearer the spinal cord. The nerves should have been
prepared with osmic acid, as in ^ 3. Notice the breaking up of the myelin
of the medullary sheath, varying in degree according to the length of time
the section has been made previously. In pre])arations from the central
cut end of the nerve prepared by Cajal's reduced silver method ^ new fibres
may be seen budding from near the extremities of the undegenerated fibres
of the stump.

Nerve-fibres are of two kinds, medullated and non-medtdlated. The
cerebro-spinal nerves and the white matter of the nerve-centres are

^ See Appendix.

STRUCrrUKE OF NEUVK-FIBRKS.

129

composed of medullated fibres; the sympathetic nerves near their
peripheral distiibution are largely made up of non-medullated fibres.

Fig. 147. —"White or medullated
nerve-fibbes, showing the sinuous
outline and double contours.

Fig. 148.— Portions of two nerve-
fibres STAINED with OSMIC ACID,
FROM A YOUNG ANIMAL. (Diagram-
matic.)

i?, R, constrictions of Ranvier, with axis-
cylinder passing through, a, neurolemma
of the nerve ; c, opposite the middle of the
segment, indicates the nucleus and proto
plasm Ij-ing between the primitive sheath
and the medullary sheath. In A the nodes
are wider, and the intersegmental sub-
stance more apparent than in B.

mm

Fig. 148.

The medullated or white fibres are characterised, as their name
implies, by the presence of the so-called medullar!/ sheath or white
substance. This is a layer of soft substance, physically of a fatty

130

THE ESSENTIALS OF HISTOLOGY".

nature, which encircles the essential part of a nerve-fibre, viz., the
axis-cylinder. Outside the medullary sheath is a delicate but tough
homogeneous membrane, the primitive sheath or nucleated sheath of
Schicann, but this is not present in all medullated fibres, being absent
in those which are within the nerve-centres. The primitive sheath is
known as the neurolemma?-

ifeStri

ii'i

Fig. 149.— a small part of a ^iedullated
FIBRE. (Highly magnified. )

The fibre looks in optical section like a tube—
heuce the term tubular, formerly applied to
these fibres. Two partial breaches of continuity
(medullary clefts) are seen in the medullary
sheath, which at these places exhibits a tendency
to split into laminfe. The primitive sheath is
here and there apjiarent outside the medullary
sheath, and the delicate stria? which are visible
in the middle of the fibre indicate the fibrilla-
tions of the axis cvlinder.

w.

Fig. I.'jO. — Two portions of medul-
lated NERVE FIBRES, AFTER TREAT-
MENT WITH O.SillC ACID, SHOWING THE
AXIS-CYLINDER ANI; THE MEDULLARY
AND PRIMITIVE SHEATHS. (Kej and

Retzius. )
A, node of Ranvier. B, middle of an inter-
node with nucleus, c, axis-cj'linder pro-
jecting ; p, primitive sheath, within which
the medullary sheath, which is stained
dark by the osmic acid, is broken away
for a short distance.

The medullary sheath is composed of a highly refracting fatty material
(myelin), which gives a characteristic dark contour and tubular appearance
to the nerve-fibres (fig. 147). It afiords a continuous investment to the
axis-cylinder, except that, as was shown by Ranvier, in the peripheral
nerve-fibres it is interrupted at regular intervals. At these places the
neurolemma appears to produce a constriction in the nerve-fibre, and
the interruptions of the medullary sheath are accordingly known as
the constrictions (Ranvier) or nodes (figs. 148, 151), the latter term
being applied from the resemblance which they bear to the nodes
of a bamboo. It is, however, uncertain whether the constriction is

1 Often termed "neurilemma," a name formerly applied also to the sheath
of Henle (see p. 136).

NERVE- FIBRES. i:il

entirely occupied by the neurolemma itself or partly by a special
band (constricting band of Ranvier) of a material which resembles
intercellular substance in its reaction to nitrate of silver (fig. 16"J).
The length of nerve l)etween two successive nodes is termed an inter-
node ; in the middle of each internode is one of the nuclei of the

Fig. 151. — Nerve-fibre prepared with osmic acid. (Szymonowicz.)

6, constriction of Rauvicr. The intervals between the modulliuy sognicnt.s appear as
clear oblique lines, «, a.

neurolemma. Besides these interruptions the medullary sheath shows
a variable number of oblique clefts (Lantermann) (figs. 149, 151), sub-
dividing it into irregular portions, which have been termed medullary
segments ; but there is some reason to believe that the clefts are
artificially produced. At the clefts there is an appearance of spiral
fibres in the medullary sheath, especially after treatment of the nerve

Fig. 152. — Spiral and reticular fibrils ix the sheath of a nerve-
fibre, (fiolgi.)

with certain reagents (Golgi) (fig. 152); it is, however, possible that
this appearance does not represent any pre-existing structure.
A reticular appearance has also been described in the medullary sheath
(neurokei'aiin network of Kiihne), and can be readily seen in nerve fibres
fixed in alcohol and treated with ether, but it varies greatly in
aspect, and is perhaps produced by the action of the reagents

Fig. 153.^Recticular aitearance in the medull.vry she.\th of a
nerve-fibre. (Gedoelst.) (From the guinea-pig. )

employed to show it (figs. 152, 153). By other modes of fixation
{e.g. picric acid) the medullary sheath seems to have a rod-like
structure (fig. 155) ; this again may be due to the manner in which
certain of its constituents are coagulated by the reagent. Osmic acid
stains the medullary sheath black (figs. 151, 154, 156).

132

THE ESSENTIALS OF HISTOLOGY.

The axis-ci/linder, which runs along the middle of the nerve-fibre, is
a soft transparent thread which is continuous from end to end of the
nerve. On account of the peculiar refractive nature of the medullary
sheath it is difficult to see the axis-cylinder in the fresh nerve except at
the nodes, where it may be observed stretching across the interruptions

Fig. 154. — Longitudinal .\xd transverse section of medullatkd nerve-
fibre OF FROG (osMic ACID AND ACID fuchsine). (After Biedermaiin. )

The longitudinal section shows one node of Ranvier and two of Lanterniann's clefts.
The fibi-illar structure of the axis-cylinder is shown in both longitudinal and
transverse section.

/^^>X

in the medullary sheath ; it may also sometimes be seen projecting
from a broken end of a nerve-fibre. It is longitudinally striated, being
made up of exceedingly fine fibrils {nenro-fihrils, fig. 154). They are

readily seen at the terminations
of nerves as in the cornea and are
also visible in the section of a nerve-
fibre as fine dots (fig. lo-t), which
sometimes appear to have a clear
centre (fig. 155), as if the fibrils
were tubular. Staining with nitrate
of silver produces a curious trans-
versel}" striated appearance in the
axis-cylinder (Fromann) (fig. 162, c),
but this is due to the precipita-
tion of chlorides, and does not
indicate a pre-existing structure
(Macallum).

Medullated nerve - fibres vary
greatly in size (figs. 155, 156), but
may be classified as large, inter-
mediate, and small. The largest are
those which are passing to the skin and to the voluntary muscles : the
smallest are those which are distributed to the viscera and blood-
vessels by way of the autonomic nerves. ^ As shown by Gaskell, the

'This term has been introduced by Laugley to include both the nerves of
the sympathetic system and also the analogous nerves which proceed from the
cranial and sacral regions for the innervation of certain involuntary muscles
and secreting glands.

Fig. ICio.

FIBRES.

-Section across five nerve-
(Magnified 1000 diameters. )

The nerve was hardened in picric acid and
stained with picro-carmine. The radial
striation of the medullary sheath is very
apparent. In one fibre the i-aysare broken
by shrinkage of the axis-cylinder. The
fibrils of the axis-cylinder appear tubular.
(From a photogi-aph.)

NERVE-FIBRES.

133

anterior roots of the last one or two cervical nerves, of all the
thoracic, of the first and second lumbar, and of the second and third
sacral nerves contain besides the ordinary large medullated fibres a
bundle of very small medullated fibres which are destined for the

Fig. 1.56. — Section of the sciatic nerve of a c.\t, showing the vakiations
IN SIZE OF ITS constituent FIBRES. (Magnified ;W0 diameters.) The
nerve was fixed with osinic acid.

viscera and blood-vessels, and which for the most part pass to the
sympathetic system. The roots of some of the cranial nerves (the
spinal accessory, vagus, glossopharyngeal, and facial) contain similar
fine medullated fibres.

Non-medullated fibres. — Intermingled with the medullated fibres
there may always, even in the cerebro-spinal nerves, be found a certain

Fig. 157. — Non-medullated nekve-fibres. (Magnified 400 diameters.)

number of pale fibres devoid of the dark double contour which is
characteristic of the presence of a medullary sheath. These are the
grey OT non-medullated fibres, also called, after their discoverer, ^5re5 of
Remak (fig. 157). They frequently branch, Avhich the medullated

134

THE ESSENTIALS OF HISTOLOGY.

fibres rarely do except near their termination, and they are beset
with numerous nuclei which perhaps belong to a delicate sheath,
but this is not certain, and undoubtedly both in longitudinal view and
in cross section the nuclei seem to lie in
the substance of the fibres. The sympathetic
nerves, as they approach their peripheral dis-
tribution, are largely made up of fibres of this
nature, but man}?^ of the fibres contained in
the sympathetic nerves possess a thin medul-
lary sheath, and have the usual structure of
medullated fibres.

Structure of the nerve-trunks. — In their
course through the body the nerve-fibres are
gathered up into bundles or funiculi, and the
funiculi are again united together to form the nerves which we
meet with in dissection. The connective tissue which unites the
funiculi and invests the whole nerve, connecting it to neighbouring

Fig. 158. — Section across
non-medullated fibres
from the splenic nerve
OF THE OX. (Tuckett.)

Fig. 1.59. — Section op part of a nerve-trunk fixed with osmic acid.
(From a photograph.) Magnified 40 diameters.

Three small funiculi and a small part of a larger funiculus are shown. The fat-cells in
the epineurium are stained black by the osmic acid.

parts and conveying to it blood-vessels, lymphatics, and even nerve-
fibres destined for its coats, is termed the cfineurium ; it frequently
contains fat-cells. That which ensheaths the funiculi is known as
the perineurium (figs. L59 to 161). It has a distinctly lamellar
structure (fig. 160), the lamellae being composed of connective
tissue covered by flattened epithelioid cells (fig. 162, a). Between

STRUCTURE OF NERVE-TRUNKS.

135

the lamellae are clefts for the conveyance of lymph to the lymphatics
of the epineurium. The delicate connective tissue which lies between
the nerve-fibres of the funiculus is the endoneurinni. It assists in

r ' <r

^^ 1

Fig. 160. — Section of part of a funiculus of the sciatic nerve of a cat
FIXED WITH Flemming's SOLUTION. Magnified 400 (liametera.

cp, epineuriuru with blood-vessels ; e, section of an end-bulb ; p, perineurium ; m, medul-
lated fibre cut at the level of a nucleus : n, n, bundles of non-medullated fibres.

^^-r:

^.tf^?^

h

^

Fig. 161. — Section of the thoracic sympathetic coed of the cat.
(Fischer.) Osmic preparation.

The nerve is composed in almost equal parts of fine medullated fibres (3, 4) derived from
the thoracic anterior roots, and grey fibres (.5) derived from the syrupathctic
ganglion-cells. The dark bodies in the epineurium (l)are fat-cells ; 2, perineurium.

supporting the longitudinally arranged meshwork of blood-capillaries,
and its interstices communicate with the lymph-clefts of the
perineurium.

All the branches of a nerve, and even sinojle nerve-fibres which

136

THE ESSENTIALS OF HISTOLOGY

are passing to their distribution, are invested with a prolongation of
the perineural sheath, which is then known as the sheath of Henle.

200

I

%ulm^'\

<m\m~\

200

1

4. +

'^

W^^ 1 •■■nip

Fig. 162. — Xerves stalned with silvee yixRAXE. (Ranvier.)

In A, the epithelial-like layer of flattened cells belonging to the sheath of Henle is
stained. In B, the cross-like markings at the nodes are exhibited. In C, a single fibre
is shown more highly magnified, with Fromann's transverse markings of the axis-
cylinder, a, constricting band ; to, medullary sheath ; cy, axis-cylinder.

The nerve-trunks themselves receive nerve-fibres {nervi nenwum)
which ramify chiefly in the epineurium and terminate within this
in end-bulbs (Horsley) (fig. 160, e).

The degenerative processes which occur in cut nerve-fibres as well
as the subsequent reparative processes will be dealt with after the
structure of nerve-cells has been studied (see p. 154).

NERVE-CELLS. 137

LESSONS XVII. AND XYIII.

NERVE-CELLH.

1. Put a small piece of spinal ganglion into 1 per cent, osmic acid for a
few hours. Place in water containing a fragment of thymol for two days
or more. Tease in dilute glycerine. Notice the spheroidal ganglion-cells ;
their large nuclei and distinct nucleoli. Many of the cells may still be seen
within their nucleated membranous sheath. Look for cells which still retain
the axis-cylinder process and for T-shaped junctions of nerve-fibres with
this. Fat-cells may be present in the periganglionic connective tissue.
These will appear intensely black in osmic preparations.

2. Prepare in the same way a spinal ganglion or the Gasserian ganglion
of the skate or cod. Notice the bipolar character of most of the cells.

3. Prepare a piece of sympathetic ganglion as in §§ 1 and 2. If from a
rabbit observe that many of the cells are bi-nucleated.

Measure two or three cells in each of the above preparations.

4. Mount stained sections of ganglia, both spinal and sympathetic. These
■will serve to show the arrangement of the cells and fibres in the ganglion
and the nucleated sheaths aroinid the nerve cells.

The ganglia may be fixed and hardened in saturated solution of corrosive
sublimate or of picric acid or in 10 per cent, formol. They may either be
stained in bulk or sections cut from paraffin and stained on the slide bv
Nissl's method. Ehrlich's methylene blue method, Golgi's silver chromate
method, or Cajal's silver reduction method, especially the last named, are
all useful for showing the cells and their connections with nerve-fibres.
These methods are described in the Appendix.

5. Place a portion of the grey matter from a piece of spinal cord in 33 per
cent, alcohol. After macerating for two days or longer in this fluid, a little of
the grey matter may be shaken up in a test-tube with water so as to bieak it
up into fine fragments. Allow these to subside, decant off the water and
substitute a dilute solution (1 to 500) of methylene blue or solution of
picrocarmine. When it appears sufficiently stained some of the debris is
pipetted off and examined under a low power of the microscope ; at first
without a cover-glass so that the cells may, if necessary, be separated from
the rest of the tissue. Mount in water with a thick hair under the cover-
glass. Notice the large branching cells, some with a mass of 2:>igment near
the nucleus. Observe the fibrillation of the cell-processes. Many axis-
cylinders will be seen in this preparation deprived wholly or partially of
their medullary sheath, and their fibrillar structure can then also be well
seen. C'arefiilly sketch these appearances. To keep the methylene blue
preparation the stain must be fixed with picrate of ammonia, after which
a mixture of glycerine and pici'ate of ammonia may be used for mounting.
If picrocarmine is used the specimen is simply preserved in dilute glycerine.
Similar preparations may be made from the grey matter of the cerebral
cortex and cerebellar cortex.

138 THE ESSENTIALS OF HISTOLOGY.

6. Examine sections of s])iiial cord, medulla oblongata and brain stained
by methylene blue (Nissl's method), to exhibit the angular particles within
the nerve-cells.

7. Examine sections of parts of brain, spinal cord and ganglia jirepared by
Cajal's method, to exhibit the neuro-fibi'ils in the cells and cell-processes.

8. Examine the nerve-cells and neuroglia-cells in sections from the spinal
cord, cerebrum, or cerebelluru of a small animal, e.g. young rat or kitten,
pre|)ared by Golgi's method. The sections must be mounted in thick xylol
balsam or dammar varnish, without a cover-glass, and dried lapidly on a
warm plate.

9. Examine sections of spinal cord (lumbar enlargement) and correspond-
ing spinal ganglia from an animal in which the sciatic nerve had been cut
about three weeks before it was killed. The sections are to be stained by
Nissl's method. Many of the anterior horn nerve-cells and of the ganglion-
cells on the side of the lesion will exhibit the chromatolysis or breaking down
of the Nissl granules, which is characteristic of cells the axons of which have
been severed. They may be compared with the normal cells on the intact side.

Nerve-cells, neurocytes or neurones. — Nerve-cells occur in the grey
matter of the nerve centres, and in little groups on the course of
certain of the peripheral nerves, these groups often causing nodular
enlargements of the nerves, which are knoAvn as (janglia. The most
conspicuous ganglia are those which are found vipon the posterior roots
of the spinal nerves, upon the roots of some of the cranial nerves,
and upon the trunk and principal branches of the sympathetic nerve.
Minute ganglia are also found very numerously in connection with the
nerves which are supplied to glands and involuntaiy muscular tissue,
as in the salivary glands, heart, alimentary canal, bladder, uterus, etc.

Nerve-cells A^ary much in size and shape ; many are large,
some being amongst the largest cells met with in the body, but
others are quite small. All nerve-Cells possess at least one process,
the axon, which becomes either a non-medullated fibre or the axis-
cylinder of a medullated fibre. If other processes are present they
are always branched almost from their commencement at the cell-body,
and they are therefore termed dendrons (dendrites). The nucleus is
generally large, clear, and spherical, with a single large and distinct
nucleolus ; there may also be a network of chromatin, but this is
not always to be seen. The cj^toplasm is tibrillated, the fibrils
passing into the processes; they are known as neuro-fibrils (p. 132),
and are believed to be the actual conductors of nerve-impulses. It
also contains peculiar angular particles {Nisd granidcs) staining deeply
with methylene blue, but the size, number, and arrangement of these in
different cells vary greatly (fig. 163). The granules also vary in number
and size with the physiological condition of the cells ; thus it is found

i

NERVE.CELLS

139

Fig. 163.— Multipolar and unipolar types of nervk-cell.

A, Large pyramidal cell of cerebral cortex, human. Nissl method. (Cajal.) a, axon ;

h, cell-bodj^ ; c, apical dendi-oii ; d, placed between two of the basal dendrons
points to the nucleus of a neuroglia cell.

B, Unipolar cell from spinal ganglion of rabbit. Nissl method. (Cajal.) a, axon ;

b, circumnuclear zone, poor in granules ; c, capsule ; d, network within nucleus ;
e, nucleolus.

140

THE ESSENTIALS OF HISTOLOGY.

that nerve-cells which have been fatigued by prolonged activity (fig.

164), and also those the axis-cylinder process of which has been cut

(fig. 165), show the Nissl granules
becoming disintegrated ; they may
even disappear for a time from
the cell. A similar result is found
to occur after the action of poisons
which especially affect the nervous
system. The Nissl granules of
the nerve-cell appear to consist
chemically mainly of nucleoproteid ;
they contain organically combined
iron (Macallum). Many nerve-cells
have also a clump of pigment-
granules, containing lecithin, at
one side of the nucleus. This is
especially marked in certain locali-
ties (locus coeruleus, locus niger),
and is more frequent in man than
in the lower animals. The pigment
also tends to increase in amount as

I

Fig. 164.

-Two MOTOR NERVE-CELLS
FROM THE DOG.
a, normal ; b, after a period of prolonged
activit}'. (Photographed from preparations age advailCCS.
by Dr. Gustav Mann.) °

As already stated, the body of
every nerve-cell is traversed by fine fibrils (neuro-Jibrils) continuous
with those in the axis-cylinder of the issuing nerve and with similar

Fig. 165. — Chromatolysis of nerve-cells, produced by severance of axon.

(Diagrammatic.)

A, Nissl granules normal ; B, commencing chromatolysis, the cell and nucleus swollen
and the granules beginning to disintegrate (the nucleus is usually close to the
periphery at this stage); C, advanced condition of chromatolysis, the cell and
nucleus shrunken.

NERVE-CELLS. 141

fibrils in their dendrons. They were noticed by Max Schultze, but
their course and connections were first accurately described by Apathy
in the nerve-cells of certain annelids. They can be seen without any
difficulty in the nerve-cells of vertebrates (fig. 166) by the employ-

FiG. 166. — Neeve-cells of kitten (froji the anterior corpora quadrigemina)
SHOWING neuro-fibrils. (Cajal.)

II, axon ; 6, c, d, various parts of the intracellular plexus of fibrils.

ment of the silver reduction method of Cajal. The neuro-fibrils
are said to present variations in thickness according to the condition
of activity of the animal at the time of death.

Most, if not all, nerve-cells show a delicate superficial reticulum
(fig. 167), described by Golgi, which is generally regarded as composed
of neuro-fibrils, but, according to J. Turner, may be an investment
derived from neuroglia-cells. Golgi has also described another network

142

THE ESSENTIALS OF HISTOLOGY.

of fibrils with somewhat larger meshes {deep reticulum of Golgi) (fig. 168)
in the deeper parts of the cell. According to some authorities both

&^

Fig. 167. — Superficial network of golgi .surrounding
two cells from the cerebral cortex of the cat;
ehrlich's method. (Cajal.)
A, large cell ; B, small cell, a, a, folds in the network ; b, a ring-
like condensation of the network at the poles of the larger
cell ; c, spinous projections from the surface.

Fig. 168. — Nerve-cell from spinal ganglion, showing

NETWORK around THE NUCLEUS. (Golgi.)

ill

Fig. 169.— Axis-cylinder

PROCESS of a nerve-
cell FROM THE SPINAL

CORD. (M. Schultze.)
X X , portion of the cell-body,
out of which the fibrils of
the axis-cylinder process, o,
are seen to emerge. At «',
this process acquires a
medullary .sheath. (Highly
magnified.)

NERVE-CELLS.

143

superficial cand deep networks are in continuity throughout the cell,
and receive and are prolonged from the neuro-fibrils of an entering
axon on the one hand, and with those of the axis-cylinder process of
the nerve-cell, and also of the dendrons, on the other hand. Other
authorities regard these networks as distinct from the neuro-fibrils,
which they suppose to run independently through the nerve-cell body,
entei-ing it by way of the dendrons and emerging in the axon.

Trophospongium of NERVE-CELi.s. — Entirely distinct from the fibrils
is a system of fine canaliculi, which has been described by E. Holmgren,
permeating the cytoplasm of the nerve-cell body for the purpose
of subserving its nutrition by conveying plasma into its substance (see
fig. 4, p. 4). These channels are stated by Holmgren to be occupied by
branching processes of other (connective-tissue or neuroglia) cells. In
the very large nerve-cells from which the nerves of the electric organs
of Malapterurus arise blood-vessels penetrate into the cytoplasm.

Fig. 170.— Two bipolar g.-^nglion cells (fish). (Holmgren.)
In B the medullar}- sheath is continued as a thin layer over the oell-hody.

Processes of nerve-cells. — As already intimated the processes are of
two kinds. The first is that known as the axis-cylinder process (Deiters)
or nerve-fibre process, so called because it becomes the axis-cylinder
of a nerve-fibre (fig. 169 a, a); in the case of the non-medullated
fibres, it becomes the nerve-fibre itself. It is also termed the neuraxon
or simply the axon.

Probably no nerve-cell is without this process. The place where it
arises from the body of the nerve-cell {cone of origin) is marked off from
the rest of the cell-substance by absence of Nissl granules (see fig. 163).
The other processes of the nerve-cell are those which were termed
by Deiters the protoplasmic processes ; they are now usually termed the

144 THE ESSENTIALS OF HISTOLOGY.

dendrons or dendrites and are generally multiple, whereas the axon
is generally single. The dendrons are characterised by the fact that
as soon as they leave the cell they begin to branch dendritically,
whereas the axis-cylinder process does not branch until near its

Fig. 171. — Vakiods forms of pericellular ending of entering nerve fibres
IX THE TRAPEZOID NUCLEUS OF THE CAT. (Edinger, after Veratti. )

termination, with the exception of a few fine lateral offshoots, which
are sometimes given off in its course. Dendrons may be absent ;
the cell is then adendric. Most nerve-cells have only one nerve-fibre
process (unipolar), but some have two or more (bipolar, multipolar).
The dendrons contain Nissl's granules, but the axons do not.

PROCESSES OF NERVE-CELLS.

145

The shape of the cell depends largely on the number of processes,
and the manner in which they come off from the cell. If there is but
one chief process the cell is generally nearly spherical. This is the
case with most of the cells of the spinal ganglia (fig. 163, B) ; in these
the single process, after a short course, divides into two fibres, which
pass the one centrally the other peripherally (fig. 178). When there
are two main processes from a nerve-cell they often go off in opposite
directions from the cell, which is thus
rendered somewhat spindle-shaped
(fig. 170), but occasionally they
emerge at the same part. When
there are three or more processes,
the cell becomes irregularly angular,
as in the motor-cells of the spinal
cord and the pyramidal cells of the
cerebral cortex.

In some cases where there appear
to be two fibres connected with a
cell, one of them is derived from
another nerve-cell elsewhere, and is
passing to end in a ramification
which envelops the cell-body. In
certain situations the ramification
is coarse and forms a calyx-like
investment to the cell-body : this
investment may be so intimately
united to the body of the second
cell that it appears to be rooted into
the external layer (fig. 171); in
other places the pericellular fibrils
are very fine and form a felt-work
over the cell-body (fig. 172), the fibrils coming in contact with
the surface of the cell and sometimes ending in small button-like
enlargements or varicosities.

In preparations made by Golgi's chromate of silver method the
nerve cells and their processes are coloured black by a deposit of
reduced silver, so that the processes can be traced for a considerable
distance from the body of the cell, in fact in many instances as far as
their remotest ramifications. It is found by the employment of this
method that the axis-cylinder process is not always an unbranched
process, as was formerly supposed, but that it usually, if not invariably,
1 1 am indebted to Dr. J. Turner for the drawing here reproduced.

Fig. 172. — Pericellular neuro-fibbils
around a large pyramidal cell
of the human cortex cerebri. ^
Methylene blue preparation.

146

THE ESSENTIALS OF HISTOLOGY.

Fig. 173. -a pyramidal cell of the cobtex cerebri of the rabbit. (Cajal.)

a, basal dendrons ; p, apical dendron ramifying near surfeoe ; e, axon ; c, its

collaterals ; 6, fibres of white matter of biam.

PROCESSES OF NERVE CELLS.

147

Fig. 174.

-Cell op type II. of Golgi, with short axon ramifying in thi

ADJACENT GREY MATTER.

Fig. 175. — Synaptic connections of sympathetic cells from the

SUPERIOR CERVICAL GANGLION OF MAN. (Cajal.)

The cells (A, B) show well-marked intracapsular dendrons. C, D, synapses between
dendrons outside the cell-capsules ; E, a fibre, which is itself surrounded by a fine spirally
wound fibril, passing to a cell and forming a synapse with the cell-dendrons within the
capsule, a, a, axons ; b, c, d, e,f, extra-capsular dendrons.

148

THE ESSENTIALS OF HISTOLOGY.

gives off fine lateral branches (collaterals), which themselves tend to
ramify in the adjacent nerve-substance (fig. 173). And although the

main part of the axis-cylinder process
usually passes on and becomes part of a
long medullated nerve-fibre (cell of type
I. of Golgi, fig. 173), this is not always
the case, for in another type of nerve-cell
within the nerve-centres (cell of type II.
of Golgi, fig. 17-i) the axis-cylinder pro-
cess breaks up almost immediately into
an arborescence. Moreover, the long
process of type I. (which becomes the
axis-cylinder of a long nerve-fibre) ulti-
mately ends in a similar manner, that
is to say, in a terminal ramification or
arborescence, as will be seen in study-
ing the endings of nerve-fibres, and
the structure of the central nervous
system.

Neurone theory. — Each nerve-cell is
generally regarded as an anatomically
independent element (nerve-unit, neurone),
and the connection of one nerve-cell
with another is believed to be effected
through the medium of the terminal
arborisations of the dendrons or axons.
Such arborisations from different cells
may interlace with one another (as in
the olfactory glomeruli, in the retina,
and in the sympathetic ganglia) (fig.
175), or a terminal arborisation from one cell may embrace the
body or the cell-processes of another cell; as with the cells of the spinal
cord (fig. 176) and the cells of the trapezoid nucleus of the pons Varolii
(fig. 171) and in many other places. The term neuro-synapse may
be applied to these modes of junction. By them nerve-cells are
linked together into long chains of neurones, the physiological path
being uninterrupted, although the anatomical path is, as above indi-
cated, believed to be interrupted at the synapses.

Fig. 176.— Arborisatiox of col-
laterals FROM THE POSTERIOR
ROOT-FIBRES AROVN'I) CELLS IN
THE POSTERIOR HORX OF GREY

MATTER. (Cajal.)
A, fibres of posterior column derived
from posterior root ; B, collaterals ;
C, D, iiurve-cells in gi-ey matter sur-
rounded by the arborisations of the
collaterals ; E, an arborisation shown
separately.

The doctrine of the anatomical independence of the nerve-cell is known as
the "neurone-theory " (Waldeyer). It is supported by the appearances of
chromate of silver preparations of nerve-cells. In these the reduction of the
silver is strictly confined to single cells, which become stained with all their

NEURONE THEORY.

149

])rocesses ; and these processes, wlien demonstrated by this method, are never
found in continuity either with the processes or with the bodies of other
nerve-cells. Moreover many of the facts relating to nerve-degeneration can
be more readily interpreted by this theory than by one which assumes the
existence of direct continuity between the nerve-units. But it has been
.shown by Apathy that in annelids (the nervous system of which was
formerl}' supposed to offer a typical example of isolated, linked "neurones"),
the fibrils are in fact continuous from cell to cell and are not interrupted at
the synapses ; it is therefore possible that the same may prove true for
vertebrates also, in which case the doctrine of independent units would
require modification. We may at any rate assume the truth of the
hypothesis so far as the nutrition of all the processes of the nerve-cell to
their remotest termination is concerned, independently of the question
whether there is or is not anatomical continuity of nerve-fibrils from one
unit to the other ; for there are many examples in both animal and
plant cells of such interdependence by means of fibrils, combined with
trophic independence.

STRUCTURE OF GANGLIA.

In the ganglia (fig. 177) each nerve-cell has a nucleated sheath which
is continuous with the neurolemma of the nerve-fibre with which the

Fig. 177. — Longitudinal section through the middle of a ganglion on

THE posterior ROOT OF ONE OF THE SACRAL NERVES OF THE DOG, AS
SEEN UNDER A LOW MAGNIFYING POWER.

o, nerve-root entering the ganglion ; h, fibres leaving the ganglion to join the mixed
spinal nerve ; c, connective-tissue coat of the ganglion ; d, principal group of nerve-
cells, with fibres passing down from amongst the cells, to unite with the longitu-
dinally coursing nerve-fibres by T-shaped junctions.

cell is connected. In the spinal ganglia, and in many of the corre-
sponding ganglia on the roots of the cranial nerves of mammals and of
most other vertebrates, the cells have only one issuing process, the
axis-cylinder process, which soon acquires a medullary sheath and then
passes with a somewhat convoluted course to some little distance from
the cell-body, where, still within the ganglion, it divides into two, one
fibre passing to the nerve-centre, and the other towards the periphery.

loO

THE ESSENTIALS OF HISTOLrXxY,

The branching is T-shaped or Y-shaped, and always occurs at a node
of Eanvier (figs. 178, 179). The neuro-fibrils of the central and
peripheral branches retain their individuality in the common trunk
and are traceable into a neuro-fibril network within the cell-body.
These spinal ganglion-cells have, as a rule, no dendrons, but some show

Fig. i:

-Two SPECAL GAXGLIOS-CELLS, SHOWING BIFCECATIOX OF THEIB
XERVE-FIBRE PBOCES.SES. (Ranvier.)

n, nuclens of one of the cells ; n, nuclei of capsules; «", nuclei of Schwann's sheatli;
c, c, e', c', constrictions of Ranvier.

short dendrons terminating in bulbous enlargements (fig. 182) either
within the cell-capsule or immediately outside it (Huber, Cajal).

The origin of the axon is not always simple, but may be multiple,
the several parts forming at first a plexus close to the cell, eventually
joining to produce a single axon. This multiple condition tends to
become accentuated with age (fig. 183). The intracapsular dendrons
also occur in sympathetic ganglia (Cajal) (figs. 175, 185).

Two chief types of cells occur in the spinal ganglia, one large and clear,
the other small and staining almost uniformly dark (fig. 179). As was first
shown bv Dogiel, the cell-body of the spinal gangliou-cell is partially
Invested by the convolut+^.d ramifications of a fine afferent uerve-fibre,
derived either from one of the other cells of the same ganglion or from a
cell in a neighbouring sympathetic ganglion (fig. 180). Similar afferent
^bres forming pericellular plexuses als) occur in the sympathetic ganglia
(fig. 186).

In the sympathetic ganglia the nerA-e-cells usually have several
dendrons and one axon ; this usually becomes a non-medullated nerve-

STliUCTUKE OF GANGLIA.

151

Fig. 179.— TY^^:-^ m- ckkebko-spinal ganglion-cells, from vagus ganglion

OF CAT. (Ehrlich's method.) (Cajal.)

A, B, large cells with much convoluted commenceraent of axon ; C, D, smaller cells ;

E, F, smallest cells, staining darkly and without convolutions.

Fig. 180.— Pericellular arborisations in spinal ganglion-cells. (Cajal.

In A the arborisation extends over the cell-body ; in B it is limited to the axon.

a, b, c, d, afferent fibre.

152

THE ESSENTIALS OF HISTOLOGY.

Fig. 181. — Diagram showing some of the cells of a spinal ganglion

AND THEIR CONNECTION WITH NERVE-FIBRES. (Dogiel.)

a, p, anterior and posterior root of spinal nerve ; n, an issuing nerve bundle ; sy, fibres
from sympathetic ; x , a cell, the axon of which ends in ramifications around the
cell-bodies of the ordinary ganglion-cells.

Fig. 182. — Cerebro-spinal ganglion-cells, man. (Cajal.)
o, 6, intracapsular dendrons, with knobbed extremities.

SYMPATHETIC GANGLIA.

15^

ribre, but is occasionally liiiely medullatcd. In certain animals (rabbit,
hare, guinea-pig) the sympathetic cells have each two nuclei (fig. 184).
In the frog they are unipolar, but sometimes with a second spiral fibre
winding round the issuing axon.

■J -^ •._■;■. ^ , -^ -

Fig. 183. — Senile type of cerebro-spinal ganglion-cell. (Cajal.)
o, issuing axon ; b, part of pericellular plexus ; c, pericellular loops.

Fig. 184. — A sympathetic nerve-cell. (Ranvier.)
nn, nuclei of cell ; /,/, pale fibres issuing from cell ; n', )i", nuclei on fibres.

The cells of ganglia are disposed in aggregations of different size,
separated by the bundles of nerve-fibres which are traversing the

154

THE ESSENTIALS OF HISTOLOGY.

ganglion (fig. 177). The ganglion if large is inclosed by an investing
capsule of connective tissue which is continuous with the epineurium
and perineurium of the entering and issuing nerve-trunks.

Fig. 185.— Two sympathetic cells, man. (Cajal.)
rt, a, axon ; 6, c, intracapsular deudrons ; d, knob-like ending of an intracapsular dendron.

DEGENERATION AND REGENERATION OF NERVE-FIBRES AND
NERVE-CELLS.

Since each nerve-fibre is the process of a nerve-cell, when a nerve is
cut, the separated part degenerates. Its axis-cylinder becomes broken up
and disappears, the nuclei of the neurolemma multiply, and the medullary
sheath undergoes a process of disintegration into droplets of fatty
substance which stain intensely like fat itself in a mixture of bichro-
mate of potash and osmic acid Avhich does not stain the medullary
sheath of normal fibres. The change which results in the fibres was
described by A. Waller in 1850, and is known as Wallerian degeneration
(fig 187, A to c). In man and mammals these changes begin 24 to 48
hours after section of the nerve, and proceed rapidly, so that by the

DEGENERATION OF NERVE-FIBRES.

155

third day the nerve-tibres cease to conduct impulses. When a peri-
pheral nerve is cut, all the nerve-fibres distal to the point of section
must degenerate, because all have grown from and are processes of
nerve-cells in or near the nerve-centre — the afferent fibres from the
cells of the ganglion on the posterior root, the efferent fibres from the
cells of the anterior horn of the spinal cord.

Fig. 186. — Two cells from a sympathetic ganglion of man showing the

TERMINATION OF AFFERENT FIBRES WITHIN THE CELL-CAPSULE. (Cajal.)
A, large ; B, small cell, a, b, afferent fibres surrounding a dendron and passing into capsule.

Waller supposed that no changes are produced centrally to the
injury when a nerve is cut, nor indeed is there any obvious immediate
alteration in the nerve-fibre itself between the injury and the cell-
body, although it is stated that the fibrils of the axis-cylinder dis-
appear for a time. But it was found by Nissl that degenerative changes
occur in the cell-body of every cell, whether motor or sensory, the
axis-cylinder of which has been severed. ^ These changes become

^ But section of the posterior root-fibres central to the ganglia does not entail
<3egeneration of the ganglion cells from which they arise. Nor does section of
a spinal nerve always entail degeneration of the anterior horn cells from which
its motor filires arise (Van Gehuchten). Why these apparent exceptions occur
is not understood.

156

THE ESSENTIALS OF HISTOLOGY

apparent a few days after section of the nerve-fibre and consist in
a disintegration of the chromatin granules, associated at first with
a general swelling of the cell-body and nucleus, which passes to the
periphery of the cell. After a time the disintegrated chromatic
substance becomes in great measure removed and the cell-body and

Fig. 187. — Degexeration and regeneration of nerve-fibres in the rabbit.

(Ranvier.)

A, part of a uerve-fibre in which degeneration has commenced in consequence of the
section, fifty hours previously, of the trunk of the nerve higher up ; ni>/, medullarj-
sheath becoming broken up into drops of myelin ; p, granular jirotoplasmic sub-
stance which is replacing the myelin ; n, nucleus ; g, neurolemma. £, another
fibre in which degeneration is proceeding, the nerve having been cut four days pre-
viously ; p, as before ; cw, axis-cj-linder partly broken up, and the pieces inclosed in
portions of myelin, mu. C, more advanced stage of degeneration, the medullary
sheath having almost disappeared, and being replaced by protoplasm, p, in which,
besides drops of fatty substance, /;<, are numerous nuclei, n", which have resulted
from the division of the single nucleus of the intemode. Z>, commencing regener-
ation of a nerve-fibre. Several small fibres, f , t", have sprouted from the some-
what bulbous cut end, b, of the original fibre, t ; a, an axis-cylinder which has not
yet acquii-ed its medullary sheath ; s, «", neurolemma of the original fibre.
A, C, and D are from o,smic preparations; B, from an alcohol and carmine pre-
paration.

nucleus become shrunken in volume. This process of disintegration
and disappearance of chromatin may be termed AHssl degeneration : it

DEGENERATION OF NERVE-FIBRES. 157

is also known as chromatoli/sis. It is brought about not only by
section of the axon, but also as the result of excessive fatigue of
the intact cell (fig. 164), and of the action of a large number of
drugs and poisons.

The chromatolysis may be persistent or may be recovered from.
Sometimes it is followed by almost complete atrophy of the cell-
body, and when this is marked there may be a secondary Wallerian
degeneration of the part of the nerve-fibre still attached to the cell.
The chromatolysis is accompanied by changes in the neurofibrils
of the cells, which stain differently and become granular (Marinesco).

Regeneration. — After a certain lapse of time, especially if the cut
ends of the nerve are in apposition, continuity between them may
become re-established. But when such regeneration takes place in
the cut nerve, it is effected not by a re-establishment of connection
between the degenerated fibres and the fibres of the central stump,
but by an outgrowth of new fibres from the stump (figs. 187, D; 188),
which endeavour to find their way to the periphery along the course
of the degenerated fibres. If they succeed in doing so, the continuity
and conducting power of the nerve become ultimately restored. This
may not happen for three months or more, according to the length
of nerve cut off and the nature of the severance, although the process
begins within a few days of the injury in man. Some investigators
have attempted to show that regeneration may take place independently
in the peripheral part of the cut nerve, but the evidence oflfered is
not conclusive, although changes occur in the peripheral part pre-
paratory to the down-growth of new fibres into it (Mott, Halliburton
-and Edmunds). There appears, however, to be no union of the down-
growing fibres with regenerated fibres in the peripheral part. The recent
investigations of Cajal have shown conclusively that whenever con-
tinuity is re-established it is invariably due to the growth of fibres from
the central stump of the cut nerve. These down-growing fibres
are usually terminated by a button-like swelling similar to that which
characterises the growing fibres of the embryonic nerves (incremental
■cone), and they may also exhibit numerous lateral ramifications (figs.
188, 189). Even when the cut central stump is turned backwards
and fixed amongst the muscles or under the skin a certain number
of newly-budded fibres may find their way from it into the degenerated
peripheral part of the nerve.

If regeneration fail to establish itself, the central end of the cut
fibre and the cell-body from which it takes origin undergo slow
atrophic changes resulting from disuse. These atrophic changes may
ultimately extend to other links in the cell-chain, so that even

158

THE ESSENTIALS OF HISTOLOGY.

remote cells in the same physiological path may eventually become
atrophied {Guddevh atrophy).

a

e ^1

.ii^y

Fig. 188.— Fibres from the central
CUT end ok sciatic nerve (of

YOUNG KABBIT) CUT 10 DAYS BEFORE

DEATH. (Cajal.)

A, fibi-cs showing down-gi'owth of axis-cylin-
ders (6) which are hivested by flattened
nucleated cells ; <i, intact part still my-
elinated. B, a fibre, the axis-cylinder
of which has not gi-own down with the
rest, but which shows various degenerative
appearances, such as buds from the axis-
cylinder and at (/, a separation of tlie fibrils.

Fig. 189.— From the peripheral end

OF a nerve cut 78 DAYS BEFORE
DEATH. (Oajal.)

n, c, enlai-god growing ends of axis-cylinder
' sjirouts which have grown down from the
central cut end into the old sheaths of the
out nerve-fibres (myelin drops are still
visible within the sheaths). The middle
fibres (h) are interstitial (not in old sheaths),
they show a new formation of a nucleated
sheath. The fibre d has an enlarged end,
n, with sheath h ; e, very fine fibres within
an old sheath ; to the left of it, an old
sheath without nerve-fibres.

REGENERATION.

15^

No regeneration of cut nerve-fibres ever occurs in the brain or
spinal cord, although the process of degeneration of fibres which
are cut off from their cell-bodies occurs in the same manner as
at the periphery, and the Nissl degeneration also takes place in the
cell-bodies. Both in the nerve-centres and in the peripheral nerves (if
regeneration fail to occur), the place of the degenerated nerve-fibres
becomes eventuallv occupied by strands of fine fibres, somewhat similar
to the fibres of cicatricial tissue. These strands stain deeply with
carmine and remani unstained by osmic acid and by the Weigert-Pal
method, and are thus differentiated from the surrounding normal
medullated nerves.

NEUROGLIA.

In the brain and spinal cord the nerve-cells and nerve-fibres are
supported by a peculiar tissue which has been termed the neuroglia.
It is composed of cells and fibres, the latter being prolonged from and

Fig. 190. — Section of spixal cord of embryo chick, showing neuroglia
fibres prolonged from the epithelium op the central canal. (cajal.)

d, dorsal ; v, ventral surface ; c, centi-al canal from which the neviroglia cells and fibres
are seen to radiate to the periphery of the cord. Some detached neui-ogUa cells are
also represented.

through the cells. Of the fibres some are radially disposed. These
start partly from the lining layer of the central canal of the spinal
cord and the ventricles of the brain, where they are originally if
not permanently continuous with the ciliated epithelium cells lining

160

THE ESSENTIALS OF HISTOLOGY

Fig. 191.— Nkuroglia cells of the cerebellum. Golgi method. (G. Retzius.)

a cells with long parallel processes extending to surface ; b, arborescent cells ;
' c, "spider" cells.

nf:uroglta.

161

these cavities. They course in a radial direction, slightly diverging
as they proceed, and constantly branching, towards the surface
of the organ, where they end in enlargements attached to the
pia mater (fig. 191, a). The radial neuroglia cells and fibres are
best seen in the embryo before the
nervous elements are fully developed
(fig. 190); when first distinct they
are termed spongioblasts (His).

Other neuroglia-fibres are prolonga-
tions or cell-processes of branching
ncuroglia-cells (glia-cells). The cells
are stellate in shape (fig. 192), and
their fine processes pass as neuroglia-
fibres between the nerve-cells and
nerve-fibres, which they aid in sup-
porting. There appear to be two
kinds of these neuroglia-cells differing
from one another in the character of
their processes (Andriezen). In the
one kind the processes branch re-
peatedly {arbonscent cells) (fig. 191, b) ; in the other kind they remain
unbranched from their origin in the cell-body to their termination
{spider-cells) (fig. 191, c).

Some authorities {e.g. Weigert) have thought that the fibres of the
neuroglia are inter- not intra-cellular, although it is admitted by all that
they are formed originally by the neuroglia-celLs.

Fig. 192. — Neuroglia cell fkom
SPINAL CORD. (Ranvier.)

Isolated after maceration in 33 p.c. alcohol.

DEVELOPMENT OF NERVE-CELLS AND NERVE-FIBRES.

All nerve-cells in the body are developed from the cells of the neural
groove and neural crest of the early embryo ; the neural groove closing
to form the neural canal (fig. 193), the cells of which form the spinal
cord and brain, and the neural crest giving off" at intervals sprouts
which become the germs of the spinal ganglia. The cells which line
the neural canal are at first all long columnar cells, but amongst these,
and probably produced by cell-division from some of these (fig. 194, g),
rounded cells {neurublads) make their appearance, the remaining
elongated cells forming the spongioblasts. Soon from each neuroblast
a process begins to grow out (fig. 194, n, and fig. 195). This is
the axon, and it is soon characterised by an enlarged extremity
{incremental cone) (fig. 196, h, h ; fig. 197, B, e). As it grows, it may
emerge from the antero-lateral region of the canal and become the
axis-cylinder of a motor nerve or anterioi' root-fibre. The dendrons of

L

162

THE ESSENTIALS OF HISTOLOGY.

the cell appear somewhat later than the axon. The axis-cylinder
processes of some of the neuroblasts remain within the nerve-centre^
and are developed into commissural, association, and intercentral
fibres.

A.

Fig. 193.— Closure of neural c.\nal of human embryo, showing the cells
OF the neural crest becoming separated to form the germs of the:
SPINAL ganglia. (Lenliossek. )

A, canal still opon ; B, canal closed.

Fig. 194.

Fig. 19.5.

Fig. 194.— Section of neural epithelium of early embryo. (His.)
Highly magnified view of jiart of a section, at the time of the first differentiation of
the neuroblasts, .showing, s', spongework formed of the outer ends of columnar
epithelium cells, s ; g, rounded " germinal cells " in process of division (probably
to form neuroblasts) ; n, a neuroblast.

Fig. 195.— Neuroblasts from a pig-embryo, showing three stages of
development. (Gurwitsch, after Scott.) (Highl3' magnified.)

The sprouts from the neural crest contain the neuroblasts from which
the posterior root-fibres are developed. Axons grow out from these
neuroblasts in two directions, so that the cells become bipolar (fig. 198).

DEVELOPMENT OF NERVE-CELLS.

163

One set of processes, forniin^c the posterior root-fibres, grow into the
postero-lateral portion of the spinal cord and ramify in the developing
grey matter ; the other set, containing the afferent fibres of the spinal
nerves, grow towards the developing anterior roots, and eventually
mingle with them to form the mixed nerves. As development pro-
ceeds, the bipolar ganglion cells become gradually transformed in most

Fig. 196.

fa

Fig. 197.

Fig. 196. — Section of spixal cord of chick of third d.\y of incubation.

(Cajal. )

A, anterior root-fibres formed by outgrowths of motor neuroblasts, c, c ; B, posterior
root-fibres formed by ingrowths of bipolar sensory neuroblasts, o, in ganglion rudi-
ment ; a, early neuroblasts ; b, neuroblast giving rise to a commissural nerve-fibre,
d ; h, i, enlarged ends of growing axons ; e, t, neuroblasts of which the dendrons
are beginning to appear.

Fig. 197. — Neuroblasts from the spinal cord of a third-day chick

EMBRYO. (Cajal.)
A, three neuroblasts, stained by Cajal's reduced silver method, showing a network of

neuro-fibrils in the cell-body ; a, a bipolar cell. B, a neuroblast stained by the

method of Golgi, showing the incremental cone, c.

vertebrates, by a shifting of the two axons, into unipolar cells (fig.
198, h, i, j ; fig. 199) ; but in some fishes the cells remain permanently
bipolar (fig. 170). This is also the case with the ganglion-cells of the
eighth cranial nerve (ganglion of Scarpa and ganglion of the cochlea).

The ganglia on the sympathetic and on other peripheral nerves are
developed from small masses of neuroblast-cells which separate off
from the germs of the spinal ganglia and give origin to axons
and dendrons much in the same way as do the neuroblasts within
the central nervous svstem.

164

THE ESSENTIALS OF HISTOLOGY,

The manner in which the medullary sheath and neurolemma of
the nerve-fibres are formed is not well understood. It is usually-
assumed that they are also ectodermic in origin and are developed

Fig 198.— Spinal and sympathetic ganglia and part of spinal cord of

CHICK of seventeenth DAY OF INCUBATION. (Cajal.)

a, autero-lateral part of spinal cord with d, a motor nerve-cell ; the fibres of the anterior
root are seen emerging and passing to B (the connection appears interrupted
in the section) ; C, posterior root formed of fibres which have grown from the
ganglion-cells in D, spinal ganglion ; B, mixed spinal nerve ; F, sympathetic
ganglion ; a, a, axons of sympathetic cells, passing to join the spinal nerve ; b, dcn-
drons of these cells ; c, axons passing to the sj'mpathctic cord ; h, cells of spinal
ganglion still bi]wlar ; i, i, bipolar cells becoming transformed into unipolar ;
V, unipolar cell with T-junction ; /, section of an artery ; </, body of vertebra.

Fig. 199. — Spinal ganglion-cells showing transitions from bipolar to
UNlPOL.\R CONDITION. (Holmgren.)

from ectoderm cells which grow out from the embryonic central
nervous system along the axis-cylinder processes of the neuroblasts.
But this is by no means clear. It is more probable that the medullary

DEVELOPMENT OF NERVE-FIBRES. 166

substance is formed by the axis-cylinder itself, and that the neurolemma
with its nuclei is derived from extrinsic cells, perhaps of mesodermic
orif^in.

The neuroglia-cells appear to be developed from ectoderm cells
(spongioblasts) of the wall of the neural canal, which, in place of
giving ofi" axon and dendrons like the neuroblasts, send out a number
of tine processes in all directions from the cell to form the fibres
of the neuroglia. It is held by some authorities that the neuroglia has
a double origin, some of the cells being developed from ectoderm and
others from mesoderm.

Some neurologists are of opinion that the nerve-fibres do not grow out
from single nerve-cells in tlie manner above described, but are formed of
chains of cells which emerge from the neural ectodeira or from the ganglion-
rudiments, and join end to end into a syncytium, which gradually lengthens
out into the nerve-fibre, the nuclei of the syncytium becoming the nuclei of
the sheath of Schwann, and the protoplasm of the syncytium becoming
differentiated into axis-cylinder,' medullary sheath, and neurolemma as
development advances. Others, whilst agreeing that the axis-cylinders
grow out as cell processes from the neui^oblasts of the neural canal and
ganglia, describe those outgrowing processes as suri^ounded by other neural
ectoderm cells — lemmal cells — which accompany them in their progress
through the tissues, multiplying as they proceed, and forming eventually
the nucleated sheath of Schwann of the medullated nerve.

166 THE ESSENTIALS OF HISTOLOGY.

LESSON XIX.
MODES OF TERMINATION OF NERYE-FIBRES.

1. Shell out a Pacinian coi'jju.scle from a piece of cat's mesentery either fresh
or after having been kept for two or three days in ^X5 P^r cent, chromic acid
or in 5 per cent, formol. Clear it as much as possible of adhering fat, but be
careful not to prick or otherwise injure the corpuscle itself. Mount in water or
saline with a thick hair to pn-event crushing with the cover-glass. Sketch the
corpuscle under a low power, and afterwards draw under a high power the
part of the core where the nerve enters and the part where it terminates.
Notice the fibrous structure of the lamellar tunics of the corpuscle and the
oval nuclei belonging to flattened epithelioid cells which cover the tunics.
The distinct lines, which when seen in the fresh corpuscles are generally
taken for the tunics, are really the optical sections of these flattened cells.

Pacinian corpuscles may be observed in sections of skin ; tactile corpuscles
and end-bulbs may also be seen in certain parts of the integument.

2. Study the corpuscles of Grandry and of Herbst in sections of the skin
covering the duck's bill.

3. Mount in dilute glycerine sections of a rabbit's cornea which has been
stained with chloride of gold by Klein's methocL Notice the arrangement in
plexuses of the darkly-stained nerve-fibres and fibrils, (1) in the connective-
tissue .substance, (2) under the epithelium, and (3) between the epithelial
cells. Make one or two sketches showing the arrangement of the fibrils.

4. Spread out a small piece of muscle which has been stained with chloride
of gold by LiJwit's method, or with htematoxylin by Sihler's method, and
examine it with a low power to find the nerve-fibres cros.sing the muscular
fibres and distributed to them.

The pieces of muscle may advantageously be thinned out for observation
by pressure upon the cover-glass. Search thoroughly for the close terminal
ramifications (end-plates) of the axis-cylinders immediately within the sar-
colemma.

These nerve-endings as well as others elsewhere can also be displayed in
preparations made by Ehrlich's, Golgi's or Cajal's methods (see Appendix).

Modes of ending of sensory nerve-fibres. — Xerve-fibres which are
distributed to sensory parts end either in special organs or in free
terminal ramijications, these last being usually in epithelia. "Within
the special organs the actual nerve-ending is also generally ramified.

Nerve-endings in special connective-tissue organs. — Three chief
kinds of these special organs are usually described, represented in
man by Pacinian corpuscles, tactile corpuscles, and end-hulbs. The type is
the same in all : a lamellated connective-tissue capsule enclosing a core
of a soft material which appears to be composed of nucleated proto-
plasmic cells ; the capsule being an expansion of the perineurium, and
the core of the endoneurium of the nerve. Within the core the axis-

ENDING OF NERVE- FIBRES.

167

cylinder terminates either simply or by a more or less complex
arborescence. The variations which occur are chieHy due to the
complexit}^ of the capsule, which is simplest in the end-bull)s and
most complex in the Pacinian corpuscles. In the tactile corpuscles

Fig. 200.

Fig. 201.
Fig. 200.— Tactile corpuscle within a papilla of the skin of the hand.

STAINED with CHLORIDE OP GOLD. (Ranvier. )
n, two nerve-fibres passing to the corpuscle ; a, a, varicose ramifications of the axis-
cylinders within the corpuscle.

Fig. 201.

-End-bulbs at the termin.\tions of nerves in the human con-
junctiva, AS seen with a lens. (Lougworth.)

Fig. 202.— a medullated fibre terminating in several end-bulbs in the
HUMAN peritoneum. (Dogiel.) Methylene blue preparation. Low power.

168

THE ESSENTIALS OF HISTOLOGY.

and end-bulbs the connective-tissue sheath of the medullated fibre
expands to form a bulbous enlargement, which is cylindrical or
spheroidal in the end-bulbs and ellipsoidal in the tactile corpuscles.
In both Jcinds of end-organ as the nerve-fibre enters (which in the
tactile corpuscle only happens when it has reached the distal part,

Fig. 203. — End-bulbs from the human peritoneum. (Dogiel.) More highl}'

magnified. Methylene blue preparation.

o, medullated fibre ; b, nucleated lamellated capsule of end-bulb ; c, non-taeduUated fibres,

probably destined for the capillaries which surround the end-bulbs.

Fig. 204. — End-bulb from the centr.\l tendon of the diaphr.\gm of the dog.
(Dogiel.) Showing besides the main medullated fibre terminating by an
arhorescence within the core, a second ver3' fine medullated fibre, forming a
more delicate arborescence around the ending of the main fibre in the outer
part of the core. Methylene blue preparation.

after having wound spirally once or twice round the corpuscle) it
loses its sheaths and is prolonged as an axis-cylinder only ; this gene-
rally ramifies and its branches terminate after either a straight or
a convoluted course within the organ ; but it sometimes remains
almost unbranched (see figs. 200 to 205). Tactile corpuscles occur
in some of the papillae of the skin of the hand and foot, in sections
of which they can be studied (see fig. 277). End-bulbs are found
in the conjunctiva of the eye, where in most animals they have a
cylindrical or oblong shape, but in man they are spheroidal (fig. 201).

ENDING OP N KRVE-FIBRES.

169

They have also been t'ouiul in paj)illa' of the lips and tongue, in
serous membranes, in tendons and aponeuroses, and in the epineurium
of the nerve-trunks ; and somewhat similar sensory end-organs
{(jenital rorpasflcs) also occur in the integument of the external genital
organs of both sexes (fig. 205). Similar bodies of larger size are
also met with in the neighbourhood of the joints {articular corpuscles).
In the skin covering the bills of certain birds {e.g. duck), a simple form
of end-organ {corpuscle of Gravdri/, fig. 206) occurs, consisting of two
or more cells arranged in rows within a capsule, with the axis-cylinder
terminating in flattened expansions {tactile dish) between the cells.

Fig. 206. — Tactilk corpuscles from the

duck's tongue. (Izquierdo.)
A, composed of three cells, with two inter-
posed disks, into which the axis-cylinder of
the nerve, n, is observed to pass ; in B there
is but one tactile disk inclosed between two
Fig. 205.— End- bulb from the glans tactile cells.

PENIS, SHOWING ENDING OF AXIS-CYLIN-
DER. Methylene blue preparation.
(Dogiel.)
a, medullated nerve-fibre ; 6, sheath of end-bulb.

The Pacinian corpuscles are larger, and have a more complex
structure, than the tactile corpuscles and end-bulbs (fig. 207). They
are composed of a number of concentric coats arranged like the layers
of an onion, and inclosing the prolonged end of a nerve-fibre. A single
medullated nerve-fibre goes to each Pacinian corpuscle, encircled by
a prolongation of the perineurium {sheath of Henle), and within this by
endoneurium ; when it reaches the corpuscle, of which it appears
to form the stalk, the lamellae of the perineurium expand into the
tunics of the capsule. The nerve passes on, piercing the tunics, sur-
rounded by endoneurium, and still provided with medullary sheath, to
reach the central part of the corpuscle. Here the endoneurium is
prolonged to form a core of cylindrical shape, along the middle of
which the nerve-fibre, now deprived of its medullary and primitive
sheaths, passes in a straight course as a simple axis-cylinder (tigs.

170

THE ESSENTIALS OF HISTOLOGY.

207, n' ; 208, c.f) to terminate at the farther end of the core, either in
an arborisation or in a bulbous enlargement. In its course through
the core it may give off lateral ramifications, which penetrate to all
parts of the core, and themselves end in fine branches.

Fig. 207. — Magnified view of a pacinian body from the cat's mesentery.

(Ranvier.)

n, stalk of corpuscle with nerve-fibre, inclosed in sheath of Henle, passing to the
corpuscle ; n', its continuation through the core, in, as axis-cylinder only ; a, its
terminal arborisation ; c, d, sections of epithelioid cells of tunics, often mistaken for
the tunics themselves ; /, channel through the tunics which expands into the core of
the corpuscle.

PACINIAN CORPUSCLES.

171

Besides the medullated fibre, which is always very conspicuous, it has been
shown that both the Pacinian and Herbst corpuscles receive in addition a
fine non-meilullated nerve-fibre, which arborizes over the outer surface of the
core. A similar arranf^euient also obtains in Granilry's corpuscles, where the
tactile cells are surrounded with such an arborization (Dogiel and others).

The tunics of the capsule are composed of connective tissue, the
fibres of which for the most part run circularly. They are covered

Fig. 209.

Fig. 208.— Part of pacinian body, showing the nerve-fibre entering
THE core. From an osiiic acid preparation.

ms, entering nerve-fibre, the medullary sheath of which is stained darkly, and ends
abruptly at the core, c ; i>s, prolongation of primitive sheath or neurolemma
passing towards the outer part of the core ; c.f, axis-cylinder passing through
the core as the central fibre ; •=, some of the inner tunics of the corpuscle, en-
larged where the\- abut against the canal through which the nerve-fibre passes
— the dots within them are sections of the fibres of which they are composed ;
n, nuclei of the tunics ; »>.', nuclei of the endoneuriuni-cells, continued by others
in the outer part of the core.

Fig. 209.

-Pacinian corpuscle from the cat, stained with

SILVER nitrate.

on both surfaces with a layer of flattened epithelioid cells (fig. 209),
and here and there cleft-like lymph-spaces can be seen between them
like those between the layers of the perineurium.

Pacinian corpuscles occur in many parts, e.g. in the deeper layers of
the skin of the hands and feet, in the periosteum of some bones, in
the neighbourhood of tendons and ligaments, in the connective tissue

172

THE ESSENTIALS OF HISTOLOGY.

Fig. 210. — Section of pacinian corpuscle. (Szymonowicz.)

f , one of the layers of epithelioid colls ; n, nucleus of epithelioid cell. It is seen that the
tunics are very closely packed around the core, in the middle of which the axial-flbro
is cut across.

.^^^^'^^^H,-

•^

Fig. 211.— Herbst corpuscle of duck. (Sobotta.) x380.

11, nieduUated nerve-fibre ; a, its axis-cylinder, terminating in an enlargement at end
of core ; c, nuclei of cells of core ; t, nuclei of cells of outer tunics ; t', inner tunics.

PAC;iNIAN CORPUSCLES.

173

at the back of the abdomen, oiid (in the cat) very numerously in the
mesentery, where they are most easily got for observation,

A simple form of Pacinian corpuscle with fewer tunics and a core formed
of regularly arranged cells occurs iu birds {roi-pusclea of Herhst, fig. 211).

Although most of the nerve endings in connective-tissue structures
are enclosed within lamellated capsules, nerves are found to end in
some situations in arborisations between the bundles of connective-
tissue fibres. This has been shown by Dogiel to occur in intermuscular

Fig. 212.— TERnriNAL arborisation from the intermuscular co.nnective

TISSUE OF THE RECTUS ABDOMINIS OF THE RABBIT. METHYLENE BLUE

preparation. (Dogiel.)

,-j*^^^^

Fig. 213. — Terminal arborisation from the superficial layer of the
peritoneum of the rabbit. Methylene blue preparation. (Dogiel.)

a, medullated fibre ; b, fibre connecting the arborisation with another one
not here represented.

connective-tissue septa (fig. 212); and in serous membranes (fig. 213);
in the latter such arborisations may be quite superficial and placed just
below the endothelium.

Organs of Ruffini. — These, which resemble long cylindrical end-bulbs,
are composed of connective-tissue bundles, within which the axis-
cylinders of the nerves ramify, and end in flattened expansions. They

174 THE ESSENTIALS OF HISTOLOGY.

occur commonly in the subcutaneous tissue of the human finger (fig.
214). Other encl-bulb-like organs, spheroidal, oval, or cylindrical in
form, have been described by Ruffini under the name of Golgi-Mazzoni
corpuscles ; they appear to be varieties of the ordinary end-bulb
of W. Krause. They occur in tendons and in the subcutaneous tissue
of the pulp of the finger.

Fig. 214. — A nerve fibre is shown dividing into seven secondary fibres
TO WHICH are attached FIVE ORGANS OF RUFFINI. (Barker, after RufEni. )

Organs of Golgi. — A special mode of nerve-ending is met with in
many tendons, near the points of attachment of the muscular fibres.
The tendon-bundles become somewhat enlarged and split into a
number of smaller fasciculi, and the nerve-fibres — one, two, or even
more in number — pass to the enlarged part, and penetrating between
the fasciculi of the tendon lose their medullary sheaths, while the axis-
cylinders end in a terminal arborisation, beset with irregular vari-
cosities. The structure (fig. 215) is enclosed within a fibrous capsule
continuous with the areolar tissue covering the bundles of the tendon ;
and between the capsule and the organ proper is a lymph-space,
similar to that which is found in the muscle-spindle (see p. 179).

Free nerve -endings. — When sensory nerve-fibres terminate in epi-
thelium, they generally branch once or twice in the subepithelial
connective tissue on nearing their termination. The sheaths of the
fibres then successively become lost, first the connective tissue or
perineural sheath, then the medullary sheath, and lastly the neuro-
lemma, the axis-cylinder being alone continued as a bundle of primitive
fibrils (fig. 216). This branches, and with the ramifications of the
axis-cylinders of neighbouring nerve-fibres forms a primary plexus.

FREE NERVE-ENDINGS.

175

Fig. 215. — Organ of golgi from the human tendo achillis. Chloride

OF GOLD PREPARATION. (Ciaccio.)

m, muscular fibres ; t, tendon-bundles ; G, Golgi's organ ; n, two nerve-fibres passing to it.

Fig. 216. — Plexus of nerve fibres in the rabbit's cornea :

Methylene blue. (Cajal.)

A, trabecula of primary plexus ; B, secondary plexus ; C, intraepithelial fibrils.

176

THE ESSENTIALS OF HISTOLOGY.

From the primary plexus smaller branches come off, and these form
a secondary plexus nearer the surface, generally immediately under
the epithelium if the ending is in a membrane covered by that tissue.
Finall}', from the secondary plexus nerve-fibrils proceed and form

Fig. 217. — Vertical section of corxe.\ stained with CHLORinE of gold.

(Ranvier. )

n, r, primary plexus In connective tissue of cornea ; o., branch passing to subepithelial
plexus, s ; p, intraepithelial plexus ; b, terminatioiis of fibrils.

terminal ramifications amongst the tissue cells (fig. 217, p, h), the
actual ending being generally in free varicose fibrils (h). This mode
of ending is characteristically seen in the cornea of the eye, but can
also be rendered evident in other epithelia (fig. 218). The fibrillar

Fig. 218. — Intra-efithelial nerve-terminations in the larynx:
GOLGI method. (G. Retzius. )

On the left the epithelium is stratified and on the right ciliated columnar.
/I, nerve-fibres in corium.

Structure of the ramifications of the axis-cylinders is very apparent
in some of the preparations figured.

In some situations the nerve-fibrils within a stratified epithelium
terminate in flattened or crescentic expansions which lie in the inter-
stices of the deeper epithelium cells, to some of which they are applied.

FREE NERVE-ENDINGS.

177

These expansions are known as tactile disks ; they are character-
istically developed in the skin of the pig's snout (fig. 210), and are also
found in the outer root sheath of hairs and in the deeper parts of the
epidermis in various parts. With appropriate treatment it may be
shown that they consist of a fine network of neurofibrils (fig. 220).

cc — =-, ^, —

Fig. 219.— Ending of nerve in tactile disks in the pig's snoct.
( Kanvier. )

71, meduUated fibre ; m, terminal disks or menisci; e, cells of the Malpighian layer of the
epidermis ; o, somewhat modified cell to which a tactile disk is applied.

Fig. 220. — A nerve-fibre ending in a number of tactile menisci
FROM A TACTILE HAIR, RABBIT. (C'ajal and Telle. )

a, point of ramification of axis-cylinder of nerve-fibre; B, isthmus between two menisci ;
C, terminal meniscus ; D, large meniscus at branching of sevei'al divisions of the
nerve-ending.

Sensory nerves of muscles. — The sensory nerves of muscles end
in peculiar organs which were termed by Kiihne muscle-spindles.
Their structure has recently been specially investigated by Eutfini,

M

178

THE ESSENTIALS OF HISTOLOGY.

Huber, and Dogiel ; and also by Sherrington, who has shown
that the large medullated nerves which they receive (about three

H

f*««ii

Fig. 222.

Fig. 221.

Fig. 221.— Nerve-exdixgs upox the intrafusal mi'scle-
fibres of a muscle-spindle of the rabbit; moder.atelt

MAGNIFIED. METHYLENE BLUE PREPARATION. (Dogiel.)
o, large medullated fibre coming off from • ' spindle " nerve and passing
to end in an annulo-spiral termination on and between the intra-
f u.sal fibres ; h, a fine medullated fibre coming off from the same
stem and dividing. Its b\-anches, c, pass towards the ends of the
muscle-fibres and terminate in a number of small localised arborisa-
tions, like end-plates.

Fig. 222.— An annulo-spiral ending of intrafusal fibre;

HIGHLY magnified. METHYLENE BLUE PREPARATION.

(Dogiel.)

MUSCLE-SPINDLES. 179

or four such fibres entering each spindle not far from its equator),
are derived from the posterior root-ganglia.

The muscle-spindle is a fusiform body, from 075 to 4 mm. long,
and from 0-OS to 02 mm. in diameter; it lies parallel with the
general direction of the fibres of a muscle. It consists of a lamellated
connective-tissue sheath externally, within which is a bundle (intrafusal
bundle) of from two to twelve peculiar muscle-fibres. These form an
axial mass with some connective tissue and the nerve-fibres ; between
this axial bundle and the .sheath is a lymphatic periaxial space, bridged

Fig. 223. — .Sensory xekve terminatixg ix arborisatioxs around the
ENDS OF MUSCLE-FIBRES. (Cecchcrelli.)

across by filaments of connective tissue. The intrafusal muscle-fibres
are somewhat like embryonic fibres in appearance, being smaller than
the ordinary fibres of the muscle and having a relatively large number
of nuclei with surrounding protoplasm, as in the red variety of muscle.
At the proximal end of the spindle they are usually only two or three
in number, but they become cleft as they pass through it ; at the distal
end they may terminate in tendon bundles. The nerve-fibres which
pass to the spindle are mostly of large size : they divide after reaching
the intrafusal bundle, but retain their medullary sheath for a time,
although eventually terminating as axis-cylinders merely, which wind
in a spiral manner around the intrafusal muscle fibres (figs. 221, 222),

180

THE ESSENTIALS OF HISTOLOGY.

which they clasp by flattened encircling branches {annulo-spiral endings).
Other, much finer, medullated fibres also pass to the spindle and termi-
nate in neighbouring parts of the intrafusal bundles in flower-like or
plate-like expansions (fig. 221). According to some observers these fine
fibres are prolonged from the annulo-spiral endings of the coarser fibres ;
but Dogiel states that they may run independently to the intrafusal
bundle. No motor nerve-fibres appear to pass into the spindles, unless
the fine fibres above mentioned are to be so regarded, nor do the
muscle-fibres of the spindle undergo atrophy on section of the motor
nerve-roots, as is the case eventually with the ordinary muscle-fibres. It
is not uncommon to find two or three spindles close together or even

B

Fig. 224.— Nerve-ending in muscular fibre of a lizard (Laceita viridis).

(Kiihne.)
A, end-plate seen edgeways ; B, from the surface ; .«, s, sarcoleinma ; p, p, expansion of

axis-cylinder. In B the expansion of the axis-cylinder appears as a clear network

branching from the divisions of the medullated fibres.

inclosed in a common sheath. Muscle-spindles are few in number in
the eye-muscles, and have not yet been found in the muscles of the
tongue, but otherwise their occurrence is general.

In the frog both motor and sensory nerves may teimiiiate in and between
the same muscle-tibres, but at diflerent parts of the fibre. It is not known
whether the muscle-tibres of the spindles also receive motor nerves in
mammals.

Another kind of ending of sensory fibres in muscle has been described

by Ceccherelli, in the form of an arborisation of nerve-fibrils around the
ends of the muscle-fibres which are inserted into tendon (fig. 223).

Ending of motor nerves. — The motor nerves to muscles terminate in
fine ramifications of the axis-cylinder ; in striated (voluntary) muscles
the lamification is localised in special organs termed motor end-organs, or,
less correctly, end-plates.

ENDING OF MOTOR NERVES.

181

/y£^'tH«

Fig. 225. — Motor nerve-endings in the abdominal muscles of a rat.
Gold preparation. Magnified 170 diameters. (Szymoiiowicz. )

Fig. 22G. — Motor end-organ of a lizard, gold preparation. (Kiihne. )

n, nerve-fibre dividing as it approaches the end-organ ; r, ramification of axis-cylinder
upon, 6, granular bed or sole of the end-organ m, clear substance surrounding
the ramifications of the axis-cylinder.

182 THE ESSENTIALS OF HISTOLOGY.

In voluntary muscle, the nerves, which are always meduUated,
terminate, as just stated, in special end-organs (figs. 224 to 226). A
medullated fibre will branch two or three times before ending, and then
each branch passes straight to a muscular fibre. Having reached this,
the neurolemma of the nerve-fibre is continued into the sarcolemma of
the muscle, the medullary sheath stops short, and the axis-cylinder ends
in a close terminal ramification with varicose expansions upon its
branches. This ramification is embedded in a layer of granular
nucleated protoplasm (sole) (fig. 226, b), probably a development of the
sarcoplasm of the muscle. In some cases the ramification is restricted
to a small portion of the muscular fibre, and forms with the granular
bed a slight prominence (eminence of Doyhre). This is the case in insects

Fig. 227.— Ending of motor nerves in rabbit's muscle. Reduced

SILVER METHOD. (Cajal.)

a, axis-cyliiidur of entering nerve ; h, c, d, parts of terminal ramification showing

network of neuro-fibrils.,

aud mammals. In the lizard the ramification is rather more extended
than in mammals, whilst in the frog it is spread over a considerable
length of the fibre. The ramification shows a fibrillar structure
(fig. 227), which is especially evident at the enlargements. In
mammals there appears to be only one end-plate to each fibre, while
in reptiles there may be several. The endplate is covered, externally

INVOLUNTARY MUSCLE.

1^3

to the sarcolemnia, by an expansion of the sheath of Henle of the
nerve-tibre {fclohmnut).

In involuntary muscle, both plain and cardiac (fig. 228), the nerve-
fibres, which near their termination are entirely non-medullated, end in
plexuses. The primary plexuses are generally furnished with ganglion-
cells in abundance. Such gangliated plexuses are best developed in

Fig. 228. — Ending of nervefibkes in CARniAC muscle. (Smirnow.)

connection with the intestine. From the cells of these plexuses other
nerve-fibres pass which form secondary plexuses and terminal ramifica-
tions amongst the contractile fibre-cells, to the surface of which the
endings of the branches, often slightly enlarged, are applied (Huber
and de Witt).

184 THE ESSENTIALS OF HISTOLOGY.

LESSON XX.

STRUCTURE OF THE LARGER BLOOD-VESSELS.

1. Sections of a medium-sized peripheral artery and vein, e.g. popliteal or
radial. In this preparation the limits of the vascular coats can be well seen
and also the differences which they present in the arteries and veins respec-
tively. The sections may be stained with hiemalum and eosiu or with orcein,
and mounted in dammar or xylol balsam.

2. Mount in xylol balsam or dammar a thin slice cut from the inner
surface of a large artery which, after having been cut open longitudinally
and washed with distilled water, has been rinsed with nitrate of silver
solution and then with distilled water and exposed to the sunlight. The
vessel should then be hardened in alcohol, or it may be exposed in this to the
light. This preparation will show the outlines of the epithelium-cells which
line the vessel. A similar preparation may be made from a large vein.

3. A piece of an artery which has been macerated for some days in 33 per
cent, alcohol is to be teased so as to isolate some of the muscular cells of the
middle coat and portions of the elastic layers (networks and fenestrated
membranes) of the inner and middle coats. The tissue may be stained
cautiously with diluted htemalum, and glycerine afterwards added. The
muscular cells are recognisable by their irregular outline and long rod-
shaped nuclei. Sketch one or two and also a piece of the elastic network or
of fenestrated membrane. The fenestrated membrane is best obtained from
one of the arteries of the base of the brain ; it is also well seen in the arteries
within the kidney.

4. Transverse sections of aorta and carotid. Notice the differences in
structure between these and the section of the smaller artery.

5. Transverse section of vena cava inferior. Notice the comparatively
thin layer of circular muscle, and outside this the tliick layer of longitudinal
muscular bundles in the adv^entitia.

Make sketches from 1, 4, and 5 under a low power, from 2 and 3 under a
high power.

An artery is usually described as being composed of three coats, an
innei- or elastic, a middle or muscular, and an external or areolar (fig.
229, h, c.'d). It is, however, more correct to describe the wall of an
artery as being mainly composed of muscular and elastic tissue, lined
internally by a pavement epithelium (endothelium), and strengthened
externally by a layer of connective tissue {adventitia).

The inner coat {tunica intinvx) is lined by a thin layer of pavement
epithelium {endothelium), the cells of which are somewhat elongated
in the direction of the axis of the vessel (fig. 230), and form a
smooth lining to the tube. After death they become easily detached.

STRUCTURE OF ARTERIES.

186

The endothelium is the essential layer in all blood-vessels. It is always
the first part to be developed, and in some it remains as the only layer of
the vessel. This is the case with all true capillaries and with certain veins,

.^S^j^^^Sgr.:^

Fig. 229. — Transverse section of part op the wall ok the posterior
TIBIAL ARTERY. (75 diameters.)

a, epithelial and subepithelial layers of inner coat ; 6, elastic laj-er (fenestrated niom-
braae)of inner coat, appearing as a brifjfht line in section ; c, muscular layer (middle
coat); d, outer coat, consisting of connective tissue bundles. In the interstices of
the bundles are some connective-tissue nuclei, and, esisecially near the muscular
coat, a number of elastic fibres cut across.

and also with the lacunar spaces or sinusoids, which, as Minot has pointed
out, take the place of capillaries in certain ])arts (e.fj. in the liver, the
medulla of the suprarenal capsules and the Wolffian body of the embryo) ;

Fig. 230. —Epithelial layer lining

THE posterior TIBIAL ARTERY.

(250 diameters.)

Fig. 231.— Portion of fenestrated
membrane of henle from an

ARTERY. (Toldt. )

it is also true of the sinuses of erectile tissue, as well as the sihus-Iike
blood-vessels which are met with in invertebrates. In some structures the
endothelial layer of the blood-vessels is imperfect, viz. : in the capillaries and

186

THE ESSENTIALS OF HISTOLOGy.

blood-sinuses of the spleen, the placental mucous membrane of the pregnant
uterus, and probably the sinusoids (capillaries) of the liver : in these places

t

Fig. 232. — Elastic xetwokk of Fig. 233.— McscrLAK fibre-cells from

ARTERT. (Toldt.)

SUPERIOR thyroid ARTERY. (340 dia-
meters.)

Fig. 234. — Section of the llngcal artery. (Griinstein.)
■a, epithelium and subepithelial layer of inner coat; 6, its elastic layer; c, c, d, inner-
most and outermost layers of middle coat, -with elastic fibres passing obliquely to
join the elastic layers which bound that coat ; (, innermost part of outer coat or
adventitia, showing many elastic fibres cut across ; /. outer part of adventitia.

STRUCTURE OF ARTERIES.

187

the blood finds its way into the interstices of the organ and conies in direct
contact with the tissue-cells.

Next to the endothelium comes an elastic layer in the form either of
elastic networks (fig. 232) or of a, fenestrated membrane (fig. 231). In
some arteries there is a layer of fine connective tissue intervening
between the epithelium and the fenestrated membrane {suhepithelial
layer).

The middle coat (tunica media) consists mainly of circularly disposed
plain muscular fibres, but it is also pervaded in most arteries by a
network of elastic fibres which are connected with the fenestrated

4S?yfe:'^

'>-:^^^^^^ ^V"-^ ^'^

Fig. 235.— Section of thoracic aorta as seen under a low power. (Toldt.)

a, the inner coat consisting of three layers, viz. : 1. Epithelium seen as a fine line.
2. Subepithelial layer. 3. Elastic layers. In the outer part of the inner coat, at its
junction with the middle, a layer of longitudinal muscular fibres is represented as
cut across. 6, middle coat with alternating layers of muscle and elastic mem-
branes ; c, outer coat with two vasa vasorum.

membrane of the inner coat and are sometimes almost as much
developed as the muscular tissue itself. This is especially the case
with the larger arteries, such as the aorta and the carotid and its
immediate branches, but in the smaller arteries of the limbs the
middle coat is composed almost purely of muscular tissue. The
muscular fibres are comparatively short, with long rod-shaped nuclei,
and are often irregular in shape (as in fig. 233), especially if the
middle coat contains much elastic tissue.

The Older coat is formed of connective tissue with a good many
elastic fibres, especially next to the middle coat. The strength of an
artery depends largely upon this coat ; it is far less easily cut or torn

188

THE ESSENTIALS OF HISTOLOGY.

than the other coats, and it serves to resist undue expansion of the
vessel. Its outer limit is not sharply marked, for it tends to blend
with the surrounding connective tissue ; hence it has been termed
tunica adventitia.

intima<

adventitia/ _

Fig. 236.— Section of aorta more magnified. (Griinstein.)

a, epithelial and subepithelial layers of inner coat; 6, c, outer layers of inner coat
containing many fine elastic fibres ; d, e, parts of outer coat.

STIUTCTUKE OF AUTERIES. 189

Variations in structure. — The aorta (fis^- 235, 236) differs in some respects
in striuture from an ordinary artery. Its inner coat contaiuvS a consideral)le
thickness of sube})ithelial connective tissue, but the elastic layers of this
coat are chiefly composed of fine fibres, ami are not especially marked off
from those of the middle coat, so that the inner and middle coats appear
blended with one another. On the other hand, there is a very great develop-
ment of elastic tissue in the middle coat, forming membranous layers which
alternate with layers of the muscular tissue. A good deal of connective
tissue also takes part in the formation of the middle coat, making this coat
unusually strong. The inner and middle coats constitute almost the entire
thickness of the wall, the outer coat being relatively thin.

The other variations which occur in the arterial system have reference
chiefly to the development and arrangement of the muscular tissue. Thus in
many of tlie larger arteries there are a few longitudinal muscular fibres at
the inner boundary of the middle coat, and in .some arteries amongst the
circular fibres of the middle coat. This is the case in the aorta. In the
part of the umbilical arteries within the umbilical cord there is a complete
layer of longitudinal fibres internal to the circular fibi'es and another
external to them, wdiilst the amount of elastic tissue is very small. Longi-
tudinal fibres are also present in some other arteries (iliac, superior
mesentei'ic, splenic, renal, etc.), external to the circular fibres, and therefore
in the outer coat of the artery.

\'^,>

V

Fig. 237. — Transverse section of part of the wall op one of the
POSTERIOR TIBIAL VEINS (man). About 200 diameters.

a, epithelial, and 6, subepithelial layers of inner coat ; c, middle coat consisting of
irregular layers of muscular tissue, alternating with connective tissue, and passing
somewhat gradually into the outer connective tissue and elastic coat, d.

The veins (fig. 237) on the whole resemble the arteries in structure,
but they present certain differences. In the internal coat the same
layers may be present, but the elastic tissue is less developed, and
may be quite inconspicuous ; it seldom takes the form of a complete
membrane. The epithelium cells are less elongated than those of the
arteries. The middle coat {c) contains less elastic tissue and also much
less muscular tissue, being partly occupied by bundles of white

190

THE ESSENTIALS OF HISTOLOGY.

connective-tissue fibres. These are continuous with those of the
external coat, which is relatively better developed in the veins than
in the arteries, so that, although thinner, their walls are often
stronger.

Many of the veins are provided with valves, which are crescentic
folds of the internal coat strengthened by a little fibrous tissue :

a few muscular fibres may be
found in the valve near its attach-
ment. The layer of the inner
coat is rather thicker and the
epithelium-cells are more elon-
gated on the side which is subject
to friction from the current of
blood than on that which is turned
towards the wall of the vessel.

Variations in different veins. —

The veins vary in structure more
than do the ai'teries. In many
veins longitudinal muscular fibres
are found in the inner part of the
middle coat, as in the iliac, femoral,
umbilical ; the umbilical vein witliiu
the umbilical cord having three
muscular layers like the correspond-
ing arteries ; it has a well-developed
internal elastic layer. In other
veins, longitudinal fibres occur ex-
ternal to the cii'cularly disposed
fibres, and may be described as
belonging to the outer coat. This
is the case with the abdominal and
especially the hepatic portions of
the inferior vena cava (fig. 238), and
to a less extent with the hepatic veins
and the portal vein and its tribu-
taries. In the superior vena cava, in
the upper part of the inferior vena
cava and in the jugular, subclavian
and innominate veins muscular fibres
are almost entirely absent in the middle coat, and there are but few in the
adventitia. The veins of the pia mater, brain and spinal cord, retina,
and bones, and the venous sinuses of the dura mater and placenta have no
muscular tissue.

It is only the larger veins, especially those of the limbs, that possess
valves. They are wanting in most of the veins of the viscera (although
occurring abundantly in some of the tributaries of the portal vein), in those
within the cranium and vertebral canal, in the veins of the bones, and in
the umbilical vein.

Fig. 2:^.— Transverse section of the
inferior vena cava of the dog.
(Szymonowicz.) Magnified 150 dia-
meters.

a, intima ; b, thin layer of circular muscle ;
c, thick adventitia with longitudinal muscu-
lar bundles ; d, a vas vasis.

SMALLER BLOOD-VESSELS. 191

LESSOX XXI.

SMALLER BLOOD-VESSELS AND LYMPH-VESSELS. SEROUS
MEMBRANES. MICROSCOPIC STUDY OF THE CIRCULA-
TION. DEVELOPMENT OF BLOOD-VESSELS.

1. Take a piece of pia mater which has been fixed with 2 per cent, bichromate
of potassium and stained with hismatoxylin, and separate from it some of the
small blood-vessels of which it is chiefly composed. Mount the .shreds in
dilute glycerine, or after dehydrating with alcohol and passing through clove
oil they can be mounted in dammar or xylol balsam. The structure of the
small arteries can be studied in this preparation, the nuclei of the epithelium
and of the muscular coat being brought distinctly into view by the stain.
The veins of the pia mater possess no muscular tissue. Capillary vessels
which have been dragged out from the brain in removing the jjia mater
may also be seen in this preparation. Sketch two small arteries of different
sizes, giving also their measurements.

2. Mount in dammar or xylol balsam a piece of the omentum of the
rabbit, stained with silver nitrate. The membrane should be stretched over
a cork or a ring of wood or vulcanite, rinsed with distilled water, treated
for five minutes with 1 per cent, nitrate of silver solution, again washed
and exposed to sunlight in spirit. When stained brown, the preparation is
removed from the light and placed in oil of cloves. Pieces may now be cut
off from the membrane and mounted in balsam or dainraar ; they should
include one or more blood-vessels.

This preparation is intended to show the epithelium of the smaller blood-
vessels and accompanying lymphatics, and also the epithelium of the serous
membrane. Sketch a small piece showing the epithelium of the vessels.

3. Mount in balsam or dammar a piece of the central tendon of the rabbit's
diaphragm which has been prepared with silver nitrate, the pleural surface
liaving been first brushed to remove the superficial epithelium and thus
enable the nitrate of silver more readily to penetrate to the network of
underlying lymphatic vessels. Observe the lymphatic plexus under a low
power ; sketch a portion of the network. If the peritoneal surface is
focussed, the epithelium which covers that surface will be seen, and oppo-
site the clefts between the radially disposed tendon-bundles stomata may
be looked for in this epithelium.

4. Examine sections of the thoracic duct. These may be made in the
same way as sections of the blood-vessels.

5. Open the abdomen of a freshly killed frog, preferably a male, and
remove the abdominal viscera, taking care not to injure the membrane or
septum at the back of the abdomen, which lies over and between the kidneys
and separates the peritoneal cavity from the cisterna lympliatira magna, a
large lymph-space in which the aorta and vena cava are contained. Cut
out the kidneys along with as much as possible of the above septum ; rinse
with distilled water ; and place in a watch-glass of 1 per cent, silver
nitrate for 5 minutes. Rinse again in distilled water and expose in tap
water to the light. When slightly browned snip oflF a portion of the mem-
branous septum, float it flat on a slide, drain ofl' the superfluous water and

192

THE ESSENTIALS OF HISTOLOGY.

allow it to dry ; then add a drop of xylol balsam or daTumar and cover the
preparation.

6. Kill a frog by destroying the brain, and study the circulation of the
blood in the mesentery. It can also be studied in the web of the frog's foot,
and in the lung and tongue of the frog or toad, or in the tail of the tadpole
or of any small fish. But for observing the phenomena attending com-
mencing inflammation ami the emigration of leucocytes from the vessels,
the mesentery is the most convenient object. The frog can be immobilised
with curari or by placing it in water in which chloroform or ether has been
shaken up : a lateral incision is made in the abdominal wall, a loop of
intestine drawn out, and laid over a ring of cork which is fixed to a glass
plate and covered with a thin piece of glass. The membrane must be kept
wet with salt solution. ^

The coats of the small arteries and veins are much simpler in
structure than those of the larger vessels, but they contain at lirst all

(■ A h '-(J a b B h

Fig. 239. — Small artery, A, with correspondixg vein, B, treated with
ACETIC ACin. (Kolliker.) (Magnified 350 diameters.)

a, external coat with elongated nuclei ; h, nuclei of the transverse muscular tissue of the
middle coat (when seen endwise, as at the sides of the vessel, their outline is
circular) ; e, nuclei of the epithelium-cells ; <.!, elastic laj'er of the inner coat.

the same elements. Thus there is a lining endothelium and an elastic
layer, the two together forming an inner a at ; a middle coat of circularly
disposed plain muscular tissue ; and a thin adventitia. The same
differences are found between the smaller arteries and veins as with
the larger, the walls of the veins being thinner and containing far less
muscular tissue (fig. 239), and the lining epithelium-cells, much

^ For details of the methods of studying the circulation and also of injecting the
blood-vessels, see A Course of Practical Histology.

SMALLER BLOOD-VESSELS.

193

elongated in both vessels, are far longer and narrower in the small
arteries than in the corresponding veins (fig. 241).

In the smallest vessels it will be found that the elastic layer has
entirely disappeared in the veins, and the muscular tissue is consider-
ably reduced in thickness in both kinds of vessel. Indeed, it is soon
represented by but a single layer of contractile cells, and even these
no longer form a complete layer. By this time also, the outer coat
as well as the elastic layer of the inner coat have disappeared both
from arteries and veins. The vessels are reduced, therefore, to the
condition of a tube formed of pavement-epithelium cells, with a partial
covering of circularly disposed muscular cells.

Fig. 240.

-Transverse section of a small artery and vein.
Magnified 250 diameters.

Even in the smallest vessels, which are not capillaries, the differences
between arteries and veins are still manifested. These differences may
be enumerated as follows : — The veins are larger than the correspond-
ing arteries ; they branch at less acute angles ; their muscular cells are
fewer, and their epithelium-cells less elongated ; the elastic layer of
the inner coat is always less marked, and sooner disappears as the
vessels become smaller.

Capillary vessels. — When traced to their smallest branches the
arteries and veins eventually are seen to be continued into a network
of the smallest blood-vessels or capillaries. The walls of these are
composed only of flattened epithelium-cells (fig. 242) continuous with
those that line the arteries and veins ; these cells can be exhibited by
staining a tissue with nitrate of silver. The cell-outlines are not
shown in developing capillaries; in these, silver nitrate stains the whole

N

194

THE ESSENTIALS OF HISTOLOGY.

wall. This is the case also with the capillaries of the villi, those of
the choroid coat of the eye (Eberth), and those of the kidney-glomeruli
(Eanvier) : in all these places the walls are formed of a syncytium.

Fig. 241. — A small aetkky, A, and vein, V, from the subcutaneous con-
nective TISSUE OF THE RAT, TREATED WITH NITRATE OP SILVER, WITH

subsequent STAINING OF NUCLEI. 175 diameters.

«, o, epithelial cells with b, b their nuclei ; m, m, transverse markings due to staining
of substance between the muscular fibre-cells ; c, c, nuclei of connective-tissue
corpuscles attached to exterior of vessel.

The capillaries vary somewhat in size and in the closeness of their
meshes ; their arrangement in different parts, which is mainly deter-
mined by the disposition of the tissue-elements, may best be studied
in injected preparations, and will be described when the structure
of the several organs is considered.

In the transparent parts of animals, the blood may be seen flowing

c;apillary vessels.

I9r»

through the cinuHary network from the arteries into the veins. The
current is very rapid in the small arteries, somewhat less rapid in the
veins, and slow in the capillaries. The current is fastest in the centre
of the vessels, slowest near the wall (inert layer). In this layer the
leucocytes are carried along by the stream and may be observed —
especially where there is commencing inflammation of the part, as in

Fig. 242. — Capillary vessels from
the bladder of the cat, mag-
NIFIED.

The outlines of the cells are stained by
nitrate of silver.

Si:^*^^ t^^

Fig. 243. — Blood plowing through a

SMALL VEIN OF THE FROG's MESENTERY.
The mesentery had been exposed for a short time,
so that there was commencing inflammation
and many of the white corpuscles are observed
sticking to and even passing through the vas-
cular wall, a, central rapid layer containing
the coloured corpuscles ; b, outer slower layer
(inert layer) containing the white corpuscles.

Fig. 244. — Ending of sensory nerve-fibres in arborescences in the

WALL OF A SMALL ARTERY. (Dogiel. )

The endotheliumcells of the vessel are outlined by dotted lines and the outlines
of the muscular fibres are faintly indicated.

the mesentery in consequence of exposure — to adhere to the inner
surface of the blood-vessels, and here and there to pass through the
coats of the small vessels, and appear as migratory cells in the surround-
ing connective tissue (fig. 243). The blood-platelets are also to be
seen in the inert layer, and show a tendency to adhere to the
wall and to one another in commencing inflammation.

196 THE ESSENTIALS OF HISTOLOGY.

Vessels and nerves of the blood-vessels. — The larger arteries and veins
possess blood-vessels {vdsa vasorum) and lymphatics, both of which ramify
chiefly in the external coat. Nerves are distributed to the raascular tissue
of the middle coat, after forming a plexus in the outer coat. Most of the
nerves ai'e non-raeduUated. But there are a certain number of meduUated
fibres intermingled with the non-niedullated and passing to end in
localised arborescences (fig. 244) partly in the adventitia, partly in the
intima. These nieduUated fibres are doubtless aff"erent ; the majority of
the non-medullated are probably efferent (vaso-motors). In the aorta of
man and in some of the larger trunks Pacinian corpuscles are here and
there met with. The capillary vessels also receive nerve-fibres, which form
a fine plexus of fibrils in close contact with the endotheliura-cells of which
the walls of these vessels are composed. Small cells are found at intervals
in connection with tliese plexuses, but whether they are of the nature of
nerve-cells or not is uncertain.

Development of the blood-vessels. — The blood-vessels are developed
in the connective tissue or in the mesenchyme which precedes it, the
first vessels being formed in the vascular area which surrounds the
early embryo. Their development may be studied in the embryo

Fig. 245. — Isolated capillary network formed by the junction op
a hollowed-out syncytium, containing coloured blood-corpuscles
in a clear fluid.

c, a hollow cell the cavity of which does not yet communicate with the network;
J), p, pointed cell processes, e-vtending in different directions for union with neigh-
bouring capillaries.

chick or mammal, in the omentum of the new-born rabbit, or in
the serous membranes and subcutaneous connective tissue of foetal
animals. They are originally developed from cells (vaso-furmative cells
or angiohlads) which become hollowed out by vacuolation : coloured
blood-corpuscles may be formed within them (see Development of
Blood-corpuscles, Lesson IL). The cells branch and unite with one
another to form a syncytium, and their cavities extend into the
branches. In the meantime their nuclei multiply and become dis-
tributed along the branches, cell-areas being at a later stage
marked out around the nuclei. In this way intercommunicat-
ing vessels — capillaries containing blood — are produced (fig. 245).

DEVELOPMENT OF BLOOD-VESSELS.

197

These presently become connected with previously formed vessels,
which extend themselves by sending out sprouts, at first solid, and
afterwards hollowed out.^ Even the larger blood-vessels appear first
to be developed in the same way as the capillaries, in so far that the
epithelium is first formed and the muscular and other tissues are
subsequently added ; but whether they are formed as clefts in the
mesoblastic tissue, which become bounded by flattened cells, or
whether as a hollowed-out syncytium has not been definitely ascer-
tained.

VCL

Jnt V Ar

Fig. 246. — Diagram to illustrate the development of blood-capillaries
(right side), and sinusoids (left side) respectively. (F. T. Lewis.)

Int, intestinal entoderm -with outgrowth ou the left to form the liver and gall-bladder,
and on the right to form the pancreas. V.C.I. , vena cava inferior ; V.P., vena portse;
v., vein and Ar, artery supplying pancreas. It is seen that the sinusoids or apparent
capillaries of the liver are formed by the breaking up of a large blood-space into
channels by the growth into it of cell-columns derived from the hepatic outgrowth of
the entoderm.

Sinusoids. — These are sinus-like blood-spaces between the cells of a
tissue, which may when fully developed bear a superficial resemblance
to blood-capillaries, but which differ essentially from them both in their
mode of development and in their relationship to the connective tissue,
as well as to the tissue-elements of the organs in which they occur.
Whereas capillary blood-vessels are developed amongst and between
the tissue-elements and are connected with and grow from neighbouring
capillaries which are themselves surrounded by areolar tissue, sinusoids
make their first appearance in the form of comparatively large blood-
spaces connected with the venous (or arterial) system. Into these,
the walls of which are formed only of a single layer of endothelial
cells, the tissue-elements of the developing organ (Wolffian body,
liver, suprarenals, blood-glands, etc.) grow, invaginating the thin wall
and forming cell-trabeculse within the sinus (figs. 246, 247), so that

^ Many authorities consider that new blood-vessels are exclusively formed by
sprouts from pre-existing vessels, and regard the appearances above described
as being due to retrogressive development of an already formed vascular net-
work (see footnote, p. 37).

198

THE ESSENTIALS OF HISTOLOGY.

the cells of the organ are directly in contact with the invaginated
endothelium, and are only separated by this from the blood contained
within the sinus. But the connection may become yet closer than
this, for, as happens in the liver, the invaginated endothelium may

x300

Fig. 247. — Liver of embkyo-chick of eleven days. (C. S. Minot.)
/i.e., hepatic cylinders; Si, sinusoids.

become defective, so that the blood within the sinus comes into
actual contact with the cells of the organ, and runs into the
interstices between them. As development proceeds these interstices
may come to resemble blood-capillaries in general arrangement and
shape ; but the resemblance is only superficial, and the intimate
relationship between the blood and the tissue-elements, which are
both enclosed within the original sinus, is usually maintained. The
distinctive character of sinusoids was first recognised by Minot.

LYMPHATICS OR LYMPH-VESSELS.

To the lymphatic system belong not only the lymphatic vessels and
lymphatic glands, but also the cavities of the serous membranes, which are
moistened with lymph and are in open communication with lymphatic
vessels Avhich run in their parietes.

The larger Ijnnph-vessels somewhat resemble the veins in structure,
except that their coats are much thinner and valves much more
numerous. In lymphatics of smaller size, the wall of the vessel
is formed, first, by a lining of pavement-epithelium cells (lymphatic
endothelium), which are elongated in the direction of the axis of the
vessel ; and, secondly, by a layer of circularly and obliquely disposed
muscular fibres. In the smallest vessels (so-called lymph-capillaries,
which are generally considerably larger than the blood-capillaries).

LYMPH-VESSELS.

199

Fig. 248. — A small part of the lymphatic plexus of the pleural layer

OP the diaphragm. Magnified 110 diameters. (Ranvier.)
I, lymphatics with cliaracteristic epitheUuni : c, cell-spaces of the connective tissue here
and there abutting against the lymphatic.

/

i.

Fig. 249.— Nerves of a lymphatic vessel, shown by methylene blue,

(Uogiel.)

«, a, non-medullated fibres passing to the vessel ; h, part of their terminal ramification.

200

THE ESSENTIALS OF HISTOLOGY.

there is nothing but the epithelium remaining, and the cells of this
are frequently not more elongated in one direction than in another,
but have a characteristic wavy outline (fig. 248).

Fig. 250.— Lymphatic plexus of cextral tendon of diaphragm of rabbit,

PLEURAL SIDE. (Klein.)

a, larger vessels with lauceolate cells and numerous valves ; 6, c, lymph-capillaries

with wavy-bordered cells.

The lymphatics receive numerous nerve-fibres, which are non-
medullated, and which end in a ramification of the finest fibrils, which
are distributed to the coats of the vessel (fig. 249).

LYMPH-VESSELS. 201

Lymphatics begin either in the form of plexuses, as in serous mem-
branes (fig. 250), or of lacunar interstices, as in some of the viscera, and
all transitions occur between the two.

In order to show their structure, it is usual to stain a tissue
vnth. nitrate of silver ; for exhibiting their distribution they may be
injected by sticking the nozzle of an injecting cannula into any tissue
which contains them, and forcing coloured fluid under gentle pressure
into the interstices of the tissue.

In silver preparations it may be observed that the lymphatics
always appear in the form of clear channels in the stained ground-
substance of the connective tissue, and that their walls are in close
connection with the cells and cell-spaces of that tissue (fig. 248). But,
except in the case of the serous membranes, no open communication is
observable between the lymphatic vessels and the interstices of the
connective tissue, although from the readiness with which they can be
injected from the latter there must be a ready means of passage of the
interstitial lymph into the commencing lymphatics. The lymphatic
vessels were originally described by Klein, and more recently by
Retterer, as being developed from hollowed-out cells in the same
manner as the blood-vessels, and by Gulland as becoming formed at
the periphery as clefts in the connective tissue, which later form a
connection with the venous system. But the investigations of Eanvier,
recently confirmed by Lewis and others, tend to show that the
lymphatic trunks grow out from the venous system, and gradually
penetrate into the peripheral parts of the embryo.

Serous Membranes.

The serous membranes, which may be conveniently studied in
connection with the lymphatic system, are delicate membranes of
connective tissue which surround and line the internal cavities of the
body, and are reflected over many of the thoracic and abdominal viscera;
in passing to which they form folds (such as the mesentery), within
which blood-vessels, lymphatics, and nerves are conducted to the viscera.

The inner surface is lined by a continuous layer of 'pavement-
epithelium {endothelium) (fig. 251), which is very distinct in nitrate
of silver preparations. In some places there are apertures in the
epithelium which lead directly into subjacent lymphatic vessels.
These apertures are called stomata, and are sometimes surrounded by
special cells (fig. 251, J?). They are numerous upon the peritoneal
surface of the diaphragm, but are present in most serous membranes.
They are nowhere better studied or more easily seen than in the
peritoneal membrane at the back of the abdominal cavity in the

^02 THE ESSENTIALS OF HISTOLOGY.

frog. This membrane lies between and at the sides of the kidneys,
and serves to separate the peritoneal cavity from the large lymph-
space just behind it. If the membrane is prepared by the nitrate of
silver method the stomata and the cells which surround them on either
side of the membrane are well shown.

The pavement-epithelium of the serous membrane rests upon a
homogeneous basement-membrane, which is especially well marked

[\ 1-

-sh-

y^

IE:-:

V

Fig. 251.

1. Epithelium from the posterior part of the prog's peritoneum, showing

THREE stomata LEADING INTO THE CISTERNA LYMPHATICA MAGNA.

(v. Ebner, after Schweigger Seidel and Dogiel.)

2. A PORTION OF EPITHELIUM FROM THE PERITONEAL SURFACE OF THE RABBIT's

DIAPHRAGM. THREK PORES ARE VISIBLE BETWEEN THE EPITHELIUM CELLS.

(v. Ebner, after Ludwig and Schweigger Seidel.)

in the serous membranes of man. The rest of the thickness of the
membrane is composed of connective tissue, with a network of fine
elastic fibres near the inner surface.

The cavities of the serous membranes are originally formed in the
embryo as a cleft in the mesoderm (pleuro-peritoneal split, coelom)
which becomes lined with epithelium, outside which the coelomic wall
eventually becomes differentiated into the serous membrane.

LYMPH-GLANDS. 203

LESSON XXIT.
LYMPH-GLANDS. TONSILS. THYMUS.

1. Sections of a lymph -inland which has been hardened either in formol
or potassium bichromate, or in chromic acid or picric acid followed by
alcohol, stained in bulk, and embedded in paraffin. Or the sections may be
stained with luematoxylin and eosin. Notice (1) the fibrous and muscular
capsule, with trabecular extending inwards from it through the cortex and
anastomosing with one another in the medulla, (2) the dense lymphoid
tissue (adenoid tissue of some authors) forming large masses in the cortex
(cortical uoilules) and rounded cords in the medulla (medullary cords).
Notice also the clearer channel or lymph-sinus which everywhere intervenes
between the fibrous tissue and the lymphoid tissue. Observe the fine fibres
and branched cells which bridge across this channel.

Make a general sketch under a low power of a portion of the cortex
together with the adjoining part of the medulla, and under a high power
drawings of small portions of cortex and medulla.

The retiform tissue of the lymph-glands has already been studied (p. 75).

2. Sections of a In^mal lymph-gland. These may be readily found in the
neck of the ox, in the neighbourhood of the large blood-vessels. Stain with
eosin and htematoxylin or with eosin and methylene blue. Notice that the
channels around the lymphoid nodules (or some of them) contain blood
instead of lymph.

3. In sections of tonsil prepared similarly to those of the lymphatic
gland, notice the large amount of lymphoid tissue, partly collected into
nodules. Observe also that the stratified epithelium, which covers the
mucous membrane here as elsewhere in the mouth, is infiltrated with lymph-
corpuscles. The tonsil is beset with pit-like recesses, with mucus-secreting
glands opening into the pits.

4. Lymphoid nodules of mucous membranes. In other mucous membranes
besides that of the back of the mouth and pharynx, collections of lymphoid
tissue occur which resemble those of the tonsils ; such nodules form the
solitary glands of the stomach and intestines and the agminated glands of
the small intestine, and are also found in the trachea and bronchial tubes and
in the oesophagus. They may be studied later in sections of those parts.

5. Sections of the thymus gland of an infant or young animal. Notice
that the masses of lymphoid (?) tissue which form the lobules of the gland are
separated by septa of connective tissue, and that the lobules show a distinc-
tion into two parts, cortical and medullary. There are no lymph-paths.
Observe the differences of structure of the cortex and medulla, and especially
notice the concentric corpuscles in the medullary part.

Make a sketch of one of the lobules under a low power and of a small
part of the medulla under a high power, including one or two concentric
corpuscles. Measure the latter.

Lymph-Glands.

Structure of a lymph-gland. —A lymph-gland (lymphatic gland) is
composed of a framework of fibrous and plain muscular tissue, which

204

THE ESSENTIALS OF HISTOLOGY.

incloses and supports the proper glandular substance, but is everywhere
separated from it by a narrow channel, bridged across by cells and fibres,
which is known as the lymph-channel. The frameu-ork consists of an
envelope or capsule (fig. 252, c), and of trabeculce (tr), which pass at
intervals inwards from the capsule, and after traversing the cortex of
the gland, divide and reunite with one another to form a network of
fibrous bands. At one part of the gland there is usually a depression
{hilus), and at the bottom of this the medulla comes to the surface
and its fibrous bands are directly continuous with the capsule.

Fig. 252. — Diagrammatic section of lymph-glaxd. (Sharpey.)

a.l. afiferent, e.l. efferent lymphatics ; C, cortical substance ; M, reticulating cords of
medullary substance; l.h. lymphoid tissue; l.s. lymph-sinus; c, capsule sending
trabecute, tr, into the substance of the gland.

The proper glandular substance {l.h.) is composed of lymphoid tissue,
i.e. a fine reticulum with the meshes thickly occupied by lymph-
coi-puscles. It occupies all the interstices of the gland, forming com-
paratively large rounded masses in the cortex (lymjihoid nodules, C),
which may be two or three deep, and smaller reticulating cord-like
masses (lymphoid cords, M) in the medulla.

The cells which bridge across the lymph-channel in the medulla
(fig. 254, c) are branching nucleated cells which often contain pigment,
so that this part of the gland has a dark colour. Some may contain
disintegrating erythrocytes. The lymph-channel is bridged across not
only by these branched cells, but also by fibres derived from the

LYMPH-GLANDS.

205

capsule and trabeculse, which pass to the lymphoid tissue and become
lost in its reticulum. But the fibres are often completely concealed by
the cells.

Afferent lymph-vessels (fig. 252, a.l.) enter the lymph-channels after
ramifying in the capsule, and the lymph is conveyed slowly along
the channels of the cortical and medullary part towards the hilus,
taking up many Ij'mph-corpuscles in its passage. At the hilus it
is gathered up by an efferent vessel or vessels (e.l.) taking origin in
the lymph-sinuses of the medulla.

Fig. 253. — Sectio.v of .\ ltmph-gl.\nd from the neck of an eight year
OLD CHILD, (v. Ebner.) x 13.

c, capsule; c.n, cortical nodules, some witts germ-centres; l.c, lymphoid cords of
medulla (dark); l.p, lymph-path (light); s, cortical sinus; t, trabeculae; r, vein;
I, eflferent lymph-vessels, accompanying and partly surrounding blood-vessels, 6^

The efferent lymphatics always contain many more lymph-corpuscles
than those which enter the gland, for lymph-corpuscles are constantly
being formed by mitotic division of the pre-existing cells in the
glandular substance, especially in the clearer centre of each cortical
nodule {germ-centre of Flemming) ; they gradually find their way
into the lymph-channel.

The leucocytes of the germ-centres frequently show in sections peculiar
darkly-coloured bodies — the staiaable-bodies of Flemming — the origin of
which has not been determined.

206

THE ESSENTIALS OF HISTOLOGY.

An artery passes into each gland at the hilus ; its bi-anches are
conveyed at first along the fibrous cords, but soon become surrounded
by the lymphoid cords, where they break up into capillaries (fig. 254, d).
The blood is returned by small veins, which are conducted along the
fibrous trabecula3 to the hilus.

Fig. 254. — Section of the medullary substance of a lymph-gland.

300 diameters. (Recklinghausen.)

a, a, a, lymphoid cords ; c, lyniph-sinu.s ; b, b, trabeculas ; d, d, capillary blood-vessels.

In some lymph-glands the fibrous trabeculse are very slightly de-
veloped, so that the gland seems in section to be almost uniformly a
mass of lymphoid tissue, pervaded by lymph-channels and with clearer
rounded nodules (germ-centres) scattered about, especially in the cortex
(fig. 253). This is the case with most of the lymph-glands of man and
some other animals. In other animals, such as the dog and ox, the
trabecule are very well developed and contain much muscular tissue.

Nerve-fibres pass to lymph-glands and appear to be distributed

chiefly as non-medullated fibres to the plain muscular tissue of the

blood-vessels and trabeculse.

Ordinary lymph-glands are confined to mammals, biit Vincent and Harrison
have found hsemal lymph-glands in birds.

Haemal lymph-glands. — In many animals a certain number of lymph-
glands are observable which have a red colour. Some of these on

H^MAL LYMPH-GLANDS. 207

section show that what corresfjonds to the peripheral lymph-channel
in ordinary lymph-glands is in them occupied by blood. Others
have the greater part of the interior occupied by large sinuses filled
with blood ; but some parts have the ordinary structure of a lymph-
gland. The names hcemal glands and hcemal lymph-glands (Robertson)
have been given to these organs. The blood passes into the sinuses
from the arterial capillaries, which probably, as in the spleen, become
incomplete, and open into the tissue interstices, from which at other

Fig. 255. — Section through one of the crypts of the tonsil. (Stohr.)

«, e, stratified epithelium of surface of mucous membrane, continued into cr3'pt ; ./; /,
follicles or nodules of the lymphoid tissue, which is elsewhere diffuse ; some show
clear " germ -centres " ; opposite each nodule numbers of lymph-cells are passing
through the epithelium ; s, masses of cells which have thus escaped from the organ
to mix with the saliva as salivary corpuscles.

parts the small veins in like manner arise. Like the spleen these
haemal glands show cells (phagocytes) which contain red blood-
corpuscles in various stages of transformation into pigment.

Some haemal glands are said by Weidenreich to have no lymph-
channels, but this statement requires confirmation.

The Tonsils.
The tonsils are two masses of lymphoid tissue placed one on each
side of the pharynx, into which they project. They are covered on
the free surface with the stratified epithelium of the mucous membrane.

208

THE ESSENTIALS OF HISTOLOGY.

Fig. 256.— Lymphatics of a peyee'.s patch, injected with silver nitrate;
CAT. (Kolliker.) Magnified 8.5 diameters.

/, a lymphoid nodule or follicle \ j' , its base, resting upon the muscular coat, m ; i.m.,
submucosa ; I, lymph-vessels ; «, sinus-like enlargement of lymph-vessel surrounding
follicle.

Fig. 257. —Developing lymphoid nodcles from the gcineapig's omentum.

(Klein.)

A perilymphatic nodule ; a, lymphatic ; c, its endothelium ; e, lymph-coniuscles ; 6,

' accumulation of lymphoid tissue on one side of it ; d, blood-capillanes withm this.
B endolymphatic nodule consisting of an enlarged lymphatic vessel, d, within which
' is a capillary network c, c, an artery, h, and a vein, a ; «, lymphoid tissue within
the lymphatic, its branched cells being joined to and derived from the lymphatic
endothelium/.

THE TONSILS. 209

and this surface is pitted with apertures which lead into recesses or
crypts in the substance of the organ (fig. 255). These recesses are all
lined by a prolongation of the stratified epithelium, and into them
the ducts of numerous small mucous glands open. The tonsils are
composed almost entirely of lymphoid tissue, which, besides being
diffused over the whole organ, is at intervals aggregated into
nodules, in which the lymph-cells are more closely arranged than
elsewhere. In the clear centre (germ-centre) of some of these nodules
active multiplication of the lymph-cells by mitosis is constantly
proceeding, and is, in fact, the cause of the formation of nodules
in the tissue, as in other organs in which lymphoid tissue occurs.
Even the epithelium which covers the tonsils is infiltrated with
lymph-corpuscles (Stohr), and these mav also wander out on to the
free surface, and become mingled with the saliva as salivary corpuscles.

The lymphoid tissue is highly vascular, and contains many lymphatics.

The mucous membrane of the neighbouring part of the pharynx
and of the back of the tongue and that of the upper part of the
pharynx near the orifices of the Eustachian tubes shows crypts
and masses of lymphoid tissue similar in structure to those of the
tonsils.

Other Ly3iphoid Structures.

Lymphoid tissue occurs in many other parts of the body in
addition to the lymphatic glands and tonsils, although it may not,
as in these structures, constitute the bulk of the organ. Thus it
is found in many mucous membranes, such as those of the intestine
and of the respiratory tract, both in a diffuse form and also collected
into nodular masses which are like the cortical nodules of a lymphatic
gland, and may, like these, be partially surrounded by a lymph-sinus.
In the intestine such nodules constitute the so-called solitary glands and
Peyefs patches. The lymphatics form plexuses of large sinus-like
vessels which to a large extent enclose the nodules (fig. 256). In
the spleen a large amount of lymphoid tissue is found etisheathing
the smaller arteries, and also expanded into nodular masses [Malpighian
corpuscles of the spleen). All these structures will be studied subse-
quently. Lymphoid tissue also occurs in considerable amount in
the serous membranes, especially in young animals ; in the adult it
is here mostly replaced by adipose tissue.

Development of lymphoid tissue. — Lvmph-glauds are developed in
connection with plexuses of lyiuph-vessels, an accumulation of retiform
tissue and lymph-cells taking place either external to and around the
lymphatics {perilymphatic formation) ; or some of the lymphatics are dilated

0

210 THE ESSENTIALS OF HISTOLOGY.

into a sinus or sinuses and the formation of lymphoid tissue occurs within
it {endoli/mph/jtic formation) (fitf. 257, a and b). When there is a develop-
ment of lymphoid tissue outside the lymphatic vessels this may form a
considerable accumulation before the foin)ation of lymph -paths within the
tissue. Blood-vessels are early developed amongst the lymphatic plexuses,
and by these, according to GuUand, the first lymph-corpuscles of the
lymphoid tissue are brought to the gland.

Tlie marginal sinus is produced by the fusion of a number of lymph-
vessels which surround the accumulation of lymphoid tissue, while in the
situation of the future hilus other lymph-vessels grow into the glandular
substance and form channels which subdivide it up into cords and nodules
(Kling). The branched cells of the lymph-path are said to be derived from
the lymphatic endothelium.

The axillary glamls were found by Stiles to increase in number and size
during lactation, diminishing again after lactation has ceased. In the
developing tonsils GuUand occasionally found nests of epithelial cells
detached from the surface epithelium, .somewhat like tho.se found per-
manently in the thymus.

Thymus.
The thymus gland is an organ which in man is found only in
the embryo and during infancy. It is composed of a number
of lobules (fig. 258) varying in .size, which are separated from one
another by septa of connective tissue, along which the blood-vessels
and lymphatics pass to and from the lobules. Each lobule shows
//

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Fig. 2.58. — A lobcle of the thymus ok a child, as seen under a low poweb
c, cortex ; c, conceutric corpuscles within medulla ; 6, blood-vessels ; tr, trabeculae.

plainly, when examined with a low power, a distinction into an outer
cortical and an inner medullary portion. The cortical part of the
lobule is imperfectly divided into nodules by trabeculae of connective
tissue. It is highly vascular, and is superficially similar in structure
to the lymphoid tissue of the lymph-glands and tonsils, with which
it also agrees in exhibiting numerous indications of indirect cell-

THYMUS.

211

division, l)ut without definite germ-centres. Besides leucocytes it
contains a number of peculiar granular cells. The medulla is more
open in its texture, and its reticulum is formed by large transparent,
branched cells (fig. 259), which are sometimes massed together and
then resemble epithelium-cells. The medulla contains fewer lymph-
corpuscles than the cortex and has a clearer aspect. Connective tissue
fibres are not wholly absent from it. Within the medulla, but not

i.m'W '

fiG. 259. — Section of medulla of thv.mcs, showing br.^nched (epithelial)

CELLS OF RETICCLUM AND A CERTAIN NUMBER OF LYMPHOID CELLS IN THE

MESHES. (Hammar.)

in the cortex, are found peculiar concentrically striated bodies (the
concentric corpuscles of Hassal, figs. 258, 260), which are " nests " of
flattened epithelial cells arranged concentrically around one or more
central cells, which have often undergone a degenerative process.
Sometimes these corpuscles are compound, two or three being grouped
together and similarly enclosed by flattened cells. They represent
part of the remains of an epithelial tube, which forms the thymus
rudiment of the early embryo and is derived from certain of the
branchial clefts. According to the observations of Hammar the
reticulum of the gland is also derived from this epithelium, and
Stohr believes that the apparent lymphoid cells of the gland have
a similar origin. The inference drawn by J. Beard from his observ-
ations in Elasmobranchs that the thymus is the original seat of

212

THE ESSENTIALS OF HISTOLOGY.

formation of leucocytes in the embryo, appears from more extended
investigations to be incorrect.

Nucleated red blood- corpuscles (erythroblasts), similar to those found
in red marrow, have also been described in the thymus (J. Schaffer),
and occasionally cysts lined by ciliated epithelium are found. In
some animals islands of striated muscular cells are seen in the
medulla. Multinucleated giant-cells are also found (Watney).

,^ The lobules, the cortex especially, are

abundantly supplied with capillary blood-
vessels. In man the arteries penetrate to
the junction of cortex and medulla, and
give off most of their capillaries radially
into the cortical nodules. Veins pass away
both from the surface of the lobules and
to a less extent directly from the medulla.
The mode of distribution of the lymphatics
has not been definitely ascertained, but none
are seen within the lobules. Nevertheless,
large lymphatic vessels, containing many
lymphocytes, issue from the interstitial
connective tissue of the thymus, but in what way they are connected
with the lobules has not been ascertained.

In the human subject the thymus gland undergoes after childhood
a process of retrogression, its lobules ceasing to grow and becoming
surrounded and concealed by a c|uantity of adipose tissue which
develops in the interstitial connective tissue of the gland. Eventually
the lobules atrophy.

Fia. 200. — Elements of the
THYMUS. 300 diameters.
(Cadiat.)

a, lymph-corpuscles ; b, cou-
centric corpuscle.

STRUCTURE OF THE SPLEEN. 213

LESSON XXIII.

STRUCTURE OF THE SPLEEN, SUPRARENAL CAPSULES,
THYROID BODY, AND PITUITARY BODY.

1. Sections of the spleen hardened in Mliller's fluid or formol and stained
with hjeniatoxylin and eosin. Notice the trabeculse extending into the
substance of the oroan from the capsule. Notice also that the glandular
substance is of two kinds, (1) lymphoid tissue accumulated around the small
arteries and here and there massed to form h/mphoid nodules — the
Malpighian corpuscles — and (2) a tissue — the red pulp — consisting of a
reticulum of fibrils and branching cells : this tissue contains blood in its
interstices.

Sketch part of a section under a low power and a small portion of the pulp
under a high power.

2. Sections across a suprarenal capsule hardened in 2 per cent, bichromate
of potassium. In sections not otherwise stained, notice the deep brown
coloration of the medulla (action of the chromic salt). Stain other sections
with eosin and hematoxylin. Examine first with a low power, noticing
the general arrangement and extent of the cortical and medullary parts of
the orgari, and making a general sketch which shall include both. After-
wards sketch carefully under the high power a group of cells from each
part of the organ.

3. Sections of the thyroid body stained with eosin and lipematoxylin.
Notice the vesicles lined with cubical epithelium and filled wnth a "colloid"
substance which becomes stained with hsematoxylin. Sketch one or two
vesicles. Measure several vesicles. The sections will probably also include
one or more parathyroids.

4. Sections (antero-posterior) through the pituitary body. Notice the
(epithelial) anterior lobe separated by a cleft from the (nervous) posterior
lobe. (The anterior part of the posterior lobe is also covered by an
epithelial layer.)

5. Injected preparations of these organs may also be studied : the spleen
is usually naturally injected with blood.

The Spleen.

The spleen is the largest of the so-called ductless glands. It appears
to be functionally connected with the blood, white blood-corpuscles
being formed and coloured blood-corpuscles being submitted to destruc-
tion within it.

Like the lymph-glands, the spleen is invested with a tibrous and
muscular aipsule (fig. 261), which is however stronger and has far
more plain muscular tissue ; outside the capsule is a covering derived
from the serous membrane. The capsule sends bands of trabecule

Sl4 THE ESSENTIALS OF HISTOLOGY.

into the organ, and these join with a network of similar trabeculae
which pass into the gland at the hilus along with the blood-vessels.
In the interstices of the framework thus constituted lies a soft pulpy
substance containing a large amount of blood, and therefore of a deep
red colour, dotted within which are here and there to be seen small
round bodies, whiter than the pulp in the fresh organ but darker in

Fig. 261. — Vertic.vl section of a portion of the monkey's spleen, as
seen with a low power.

stained sections, the Mulpighian corpuscles of the spleen. These are
composed of lymphoid tissue which is gathered up into masses which
envelop the smaller arteries, whilst the red pulp which everywhere
surrounds them and which forms the bulk of the organ is composed
(Carlier) of a close network of connective tissue fibrils (fig. 262),
partly covered by flattened and branched cells (fig. 263). Passing into
the pulp and communicating with its interstices are capillary blood-
vessels which are connected with the terminations of the arteries ;
whilst in other parts venous channels — characterised in the human
spleen by an encirclement of reticulum-fibres, possibly of an ela.stic
nature (fig. 264), and by the presence of a layer of highly characteristic,

STRUCTURE OF THE SPLEEN.

215

Fig. 262. — Reticulum of spleen, golgi method. (Oppel.)
a, Malpighian coi-p>iscle ; b, part of its reticulum : c, condensed reticulum at its margin ;
d, more open tissue next to this ; e, wall of arteriole ; /, capillaries of Malpighian
corpuscle ; g, reticulum of arteriole expanding into that of the Malpighian corpuscle.

Fig. 203. — Small veins of spleen pulp with reticular tissue. (Hoyer. )

The veins, which are invested by encircling fibres, show gaps in their walls whereby

they communicate with the interstices of the pulp.

216

THE ESSENTIALS OF HISTOLOGY.

comparatively thick and prominent endothelium-cells, ■which exhibit
a longitudinally striated structure — course through the pulp and bring
the blood which has passed into its interstices from the arterial
capillaries towards the larger veins of the organ, which run in the
trabeculse, and are by them conducted to the hilus. The arteries,
which are also at first conducted from the hilus along the trabeculae
into the interior of the organ, presently leave the trabeculse, and their

Fig. 264. — Venous spaces of spleen pulp, showing the encircling fibres

IN THEIR walls. Man. (v. Ebner.)

cv, capillary veins ; p, pulp (the tissue elements are not represented).

external coat becomes gradually converted into a thick sheath of
lymphoid tissue which invests them in the remainder of their course,
and in places becomes swollen into the Malpighian corpuscles already
mentioned. The small arteries distribute a few capillaries to the
Malpighian corpuscles, and then break up into pencils of capillary
vessels which open into the interstices of the pulp.^

The Malpighian corpuscles frequently but not always show a
clearer central nodule or gerin-centre, characterised b)^ the presence of
numerous mitoses ; and the stainable bodies of Flemming (see p. 205)
are also seen in them.

^It is right to state that many authorities hold that the arterial capillaries
open into the venous sinuses and that the blood-sj'stem of the spleen is there-
fore a closed one, the blood-corpuscles passing into the pulp-interstices by
diapedesis.

STRUCTURE OF THE SPLEEN.

217

fl» ^.^1?

9

Fig. 265.— Thin section of spleen-pulp of child, highly magnified,

SHOWING the apparent MODE OF ORIGIN OF A SMALL VEIN IN THE

INTERSTICES OF THE PULP. Magnified 400 diameters.
a, blood in pulp ; a', blood in vein ; 6, phagocyte in vein ; c, branched cell of pulp ; d, splenic cell.

Fig. 266. — A giant cell from the
SPLEEN OF A KITTEN. Magnified
400 diameters.

Fig. 267. — A vertical section
of the suprarenal body
of a fcetus, twice the
natural size, showing
the distinction between

THE medullary AND COR-
TICAL SUBSTANCE. (A.

Thomson.)

V, issuing vein ; r, summit of kidney.

218 THE ESSENTIALS OF HISTOLOGY.

The special cellular elements of the spleen-pulp are of three kinds, viz.
(1) peculiar, large, amoeboid 2>hagocytic cells, (2) megakaryocytes or giant
cells, and (3) branched and generally flattened cells which assist in forming
the spongeworlc. The pulp also contains all the corpuscular elements
of blood. The phagocytic cells are frequently found to contain coloured
blood-corpuscles in their interior in various stages of transformation
into pigment. They occur both in the interstices of the pulp and in
the venous sinuses and veins, where they are often filled with erythro-
cytes (fig. 265). The giant cells are most frequent in young animals
(fig. 266) : their function has not been ascertained. The branched
cells of the spongeworlc are probably of the same nature as the
endothelium cells of the terminal capillaries and veins of the pulp.
They are connected with one another and with the endothelial cells
of the small vessels by branches. The phagocytic spleen cells are
perhaps derived from them.

Nucleated coloured corpuscles are found in the embryo, and
occasionally after birth, in the spleen-pulp. The blood of the
splenic vein is very rich in leucocytes.

The lymphatics of the spleen run partly in the trabeculse and capsule,
and partly in the lymphoid tissue ensheathing the arteries. They join
to form larger vessels which emerge together at the hilus. There are
no lymphatics in the spleen pulp.

The nerves, which are numerous and mostly non-medullated, are
distributed to the muscular tissue of the arteries and to that in the
capsule and trabeculre.

Mall states that tlie distribution of the trabeculcB and of the blood-vessels
within the spleen is such as to indicate a differentiation of the pulp into
divisions which he tenns "spleen lobules," each of which has its own
arteriole and venule, and in which' tlie pulp is arranged in columns or
cords surrounded by venous spaces. It must, however, be understood that
there is nothing of the nature of partitions separating such lobules : to all
appearance the pulp is in continuity throughout the organ.

The Suprarenal Capsules.

The suprarenal capsules (adrenals) belong to the class of bodies
known as ductless glands, but they are entirely different in structure
and function from the spleen and lymphatic glands. A section through
the fresh organ (fig. 267) shows a cortex which is striated verti-
cally to the surface, and of a yellowish colour, and a medidla which is
soft and highly vascular, and of a dark-red colour. The whole organ is
invested by a fibrous capsule which sends fibrous septa inwards through
the cortical substance (fig. 268, a), subdividing this for the most part

THE SUPKAKKNAL CAPSULES.

219

into columnar groups of cells {.:onaf((scicvhif(i, r). Immediately under-
neath the capsule, however, the groups are more rounded, and the cells
tend to assume a columnar form (zotui glomerulosa, b), whilst next to
the medulla they have a reticular arrangement (:on(i rHimlarU, d).

hf (

ivi

Fig. 268. — Vertical .section of cortex of suprarenal of dog. (Bohm

and V. Davidoff). Magnified about 1.50 diameters.

[a, capsule ; b, zona glomerulosa ; c, zona fasciculata ; d, zona reticularis.

The cells which form the cortical substance are, for the most part,
polyhedral in form ; each contains a clear round nucleus, and there
are often yellowish oil-globules in their protoplasm. No arteries and
veins penetrate between these cell.<=!, both these and the lymphatics of
the cortex running in the fibrous septa between the columns of cells,
which they surround with a capillary network. In the zona reticularis
the capillaries widen out and occupy the spaces between the cell--
columns (fig. 268, d). The lymphatics communicate with fine canals
between the cells of the cortex.

220

THE ESSENTIALS OF HISTOLOGY.

The cells of the medulla (fig. 2G9) are more irregularly disposed.
They are supported by a network of elastic fibres. They lie in very
close relation to the large capillary blood-spaces (sinusoids) \vhich

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Fig. 269. — Section showing zona reticularis of cortex, r, and medulla,
m, of suprarenal of dog. (Sz^ymouowicz. ) Magnified 384 diameters.

pervade the medulla and they probably pour a secretion directly into
the blood. Their protoplasm is granular ; in some animals it contains
a brownish pigment, but in man the dark red colour of the medulla is
due to the blood contained in the large sinusoid spaces b}^ which
it is pervaded, and which receive the blood after it has traversed
the capillaries of the cortex. A few arterioles pass straight to the
medulla through the cortex. One large vein usually passes out at

THE Ti[YR011) BODY. 221

the hilus in the anterior surface of the gland. Investing the larger
veins are longitudinal bundles of plain muscular fibres ; but most of
the veins have only an endothelium. Numerous nerves, after traversing
the cortical substance, are distributed throughout the medulla, where
they form a close plexus provided here and there with ganglion-cells.
The cells of the medulla are characterised by staining brown by
chromic acid and its salts, provided the organ is fresh (chromophil
or chromaffin cells).

The medulla of the suprarenal capsule is developed from cells which
become detached from the rudiments of the sympathetic ganglia, and
are therefore of ectodermal origin. The cortex is developed from
mesoderm.

The Thyroid Body.
The thyroid body consists of a framework of connective tissue
inclosing numerous spherical or oval vesicles (fig. 270) which are lined

'■«'::re!

i^^ ■%<^^...j^''^m: i)ffi:i

i^^^±>3^^^JL*%'*J*Jsi^:.s.&^:^Js'.i

Fig. 270. — Srction of human thyroid. (Szymonowicz. ) Magnified about

180 diameters,
a" vesicle occupied by colloid, which has partly shrunk away from the epithelium ;

6, epithelium of a large vesicle ; c, c, epithelium of vesicles which are cut tangen-

tially ; d, interstitial connective tissue.

with cubical epithelium-cells ; these often contain granules of a fatty
character. The cavities of the vesicles are usually occupied by a
peculiar viscid liquid (colloid) which is coagulated by alcohol and
which then becomes stained with hsematoxylin. A similar material

222

THE ESSENTIALS OF HISTOLOGY.

has been found in the lymphatics of the gland, and may often be
detected also in the interstices of the connective tissue.

The bloodvessels of the thyroid are numerous and give a deep
red colour to the organ. The capillaries form close plexuses round
the vesicles (fig. 271), and even extend between the lining epithelium
cells.

Parathyroids. — In close proximity to or embedded in the substance
of the thyroid are always to be found four small glandular organs

:iir*^v

Fig. 271. — Thyroid of dog
injected.

Fig. 272.— P.akathyroid of monkey.
(Vincent and Jolly.)

'I, paratJiyroid tissue; h, blood-vessels; c,
connective tissue ; d, junction of para-
thyroid with thyroid ; c, thyi-oid vesicles ;
«', colloid.

of different structure from the thyroid proper, although somewhat
resembling its embryonic condition (fig. 272). These bodies, one of
which usually lies on the lateral and one on the mesial surface of each
lateral lobe, are formed of columns of granular epithelium-cells, with
a very vascular connective tissue between the columns. If left after
removal of the thyroid, they are stated to undergo hypertrophy, and
to develop a vesicular structure (Vincent and Jolly). Besides these
bodies, there is also frequently to be found in connexion with the
thyroid a small mass of lymphoid tissue which resembles the thymus
tissue in structure, and, like it, contains concentric corpuscles.

CAROTID AND COCCYGEAL GLANDS.

These are minute glandtilar orgaiis without ducts, lying respectively
at the bifurcation of the carotid artery and in front of the apex of the
coccyx. They are composed of polyhedral cells, with numerous blood-
capillaries between them. In the carotid gland the cells are collected

CAROTID AND COCCYGEAL GLANDS.

ft

223

Fig. 273. — A clump or cell-ball from the carotid gland, injected.

(Schaper.)
a, arteriole ; r, venules ; c, sinus-like capillary within nodule ; gl, group of gland

cells ; c, boundary of nodule surrounded by lymph space ; d, inter-nodular connective

tissue of gland.

Fig. 274. — Section of coccygeal gland. (Walker.)
1, blood-spaces; 2, epithelium; 3, connective tissue.

224

THE ESSENTIALS OF HISTOLOGY.

into spheroidal clumps, in the coccygeal gland into irregular nodules.
The blood-vessels, at least in the coccygeal gland, have a sinusoidal
character (Walker). Amongst the cells are some which stain dark
brown with chromic acid like those of the medulla of the suprarenal
capsules (chromophil cells). A certain number of such cells occur
also, according to Kohn, in sympathetic ganglia.

The Piti'itaey Body.

The pituitary body or gland (hypophysis cerebri) is a small reddish
mass which lies in the sella turcica, and is connected with the third
ventricle by the infundibulum. It consists of two lobes, a larger
anterior and a smaller posterior (fig. 275). The anterior lobe is

Jhrs post ![^^.'^.,','^.-%.j;

Pars one '-r - -^Yj^-

Cranium ""

Fig. 275.— Section thbough hypophysis. (Ediuger.)

■4

,1 ■ - - n^'

Fig. 27.5a.— Skctios of akteriob lobe of hypophysis of ox. (Dostoievsky.)

W, blood-sinuses ; c, cell-strands containii g clear cells ; d, strands of darker granular
cells. Other strands contain both kinds of cell. '

THK PITUITARY BODY. ' 225

originally developed as a hollow protrusion of the buccal epithelium.
It consists of a number of tubules, which are lined by epithelium
and united by connective- tissue. In some of the tubes the
epithelium is ciliated, and occasionally a colloid substance is found
in them, but for the most part the lumen of the tubules has become
obliterated in the adult, and they present the appearance of solid
cell-masses between which are numerous large venous capillaries,
perhaps sinusoids. Some of the cells arc clear, others darkly granular
in appearance (fig. 275 a).

The 'posterior lobe of the pituitary body, which is developed from the
infundibulum of the third ventricle, consists chiefly of vascular connec-
tive tissue and neuroglia, but it also includes masses of cells of an
epithelial character {pars intermedia), which are continuous with those
of the anterior lobe. It is partly separated from the 'anterior lobe by
a cleft-like space containing glairy fluid. In man the posterior lobe is
stated to contain no cells in the adult of distinctly nervous character,
but it receives many nerve-fibres which arise from large cells in the
grey matter just behind the optic chiasma, some of which penetrate
into the glandular substance.

226 • THE ESSENTIALS OF HISTOLOGY.

LESSONS XXIV. AND XXV.
THE SKIN.

1. Sections of skin from the palmar surface of the fingers. The skin is
hardened in picric acid or formol, followed by alcohol. The sections are
made vertical to the surface, and should extend down as far as the sub-
cutaneous tissue. Notice the layers of the epidermis and their different
behaviour to staining fluids. Notice also the papilloe projecting from the
corium into the epidermis and look for tactile corpuscles within them. In
very thin jjarts of the sections the fine intercellular channels in the deeper
parts of the epithelium (see Lesson VII.) may be seen with a high power. The
convoluted tubes of the sweat-glands are visible here and there in the deeper
parts of the corium, and in thick sections the corkscrew-like channels by
which the sweat is conducted through the ejjidermis may also be observed.
Make a sketch showing the general structure under a low power, and other
sketches to exhibit the most important details under a high power. Measure
the thickness of the epidermis and the length of the papillse.

2. Sections of the skin of the scalji, vertical to the surface and parallel
to the slope of the hair-follicles, and others parallel to the surface, and
therefore across the hair-follicles. Stain and mount in the same way as in
the last preparation. Examine also the structure of the hairs.

In these preparations the details of structure of the hairs and hair-follicles,
together with the sebaceous glands and the little muscles of the hair-follicles,
are to be made out.

3. Vertical sections of the nail and nail-bed. To cut such hard structures
as the nail it is best, after fixing with picric acid or formol followed by 75
p.c. alcohol, to soak the tissue in strong gum arable for a few days, then place
it in an appropriate position upon a cork or upon the object-carrier of a
microtome, and plunge the whole into 70 per cent, alcohol. This renders the
gum hard, and enables sections to be cut of sufficient fineness. A plane iron
should be used with the microtome, since the hardness of the nail will turn
the edge of a razor. To remove the gum the sections are placed in water for
a few hours ; they may then be stained and mounted. Notice the ridges
(not papillae) of the corium, projecting into the epidermis. Observe also the
distinction of the epidermis into Malpighian layer and nail proper.

4. Mount a section from a portion of skin in which the blood-vessels have
been injected, and notice the distribution of the capillaries to the sweat-
glands, to the hair-follicles, and to the papillary surface of the corium.

5. The cells which compose the nails and hairs can be isolated by warming
a small piece' of nail or hair in strong sidplmric acid ; after this treatment
they are readily separated from one another by piessure upon the cover-
glass.

6. Sections of mammary gland during lactation. The gland may be fixed
in Zenker's fluid (see Appendix) and the sections stained with ha^matoxylin
and eosin.

The skin is composed of two parts, epidermis and cutis vera (fig. 276).

The epidermis, or scarf skin, is a stratified epithelium (fig. 277). It

is composed of a number of layers of cells, the deeper of which are

THE SKIN.

22 ^

soft and protoplasmic, and form the reie mucosurii of Malpighi, whilst
the superficial layers are hard and horny, this horny portion sometimes

^t^^-

)

i

>)

It
\

>-^,-^

stratum
corneuin

. rete mucosum

-cutis vera

fS^^

-

• ',%

^

^ ^d

L-^,' • "

'M- :

%^\

■•: ^/

\>-\ -. -

- " "S*

-'vj/ .

K-: .

sweat glands

adipose tissue

Fig. 276.— Vertical section thkodgh the skin of the sole of the foot.

Magnified about 2-5 diameters.

constituting the greater part of the thickness of the epidermis. The
deepest cells of the rete mueosum, which are set on the surface of the
cutis vera, are columnar in shape. In the coloured races of mankind

228

THE ESSENTIALS OF HISTOLOGY.

these cells contain pigment-granules. In the layers immediately above
them the cells are polyhedral. Between all these cells of the rete
mucosum there are fine intercellular clefts which separate the cells
from one another, but are bridged across by fibres which pass from
cell to cell, and also through the substance of the cells (Ranvier,
Delepine). The intercellular channels serve for the passage of lymph,

!<r*'.-^<:>^-«f:-

stratum
corneum

stratum lucidum

stratum

icranulosum

^ ^ P^-'^

rete mucosum

-*^i -cutis vera

Fig. 277.— Vertical sectiox through the .skix of the p.\lmar side of
the finger, showing two pafill.e (oxe of which coxtaixs a tactile

CORPUSCLE) AXn THE DEEPER LATER OF THE EPIDERMIS. Magnified

about 200 diameters.

and within them occasionally lymph-corpuscles may be found, often
haA^ng a stellate figure from becoming shaped to the interstices.

The superficial layer of the rete mucosum is formed of somewhat
flattened cells filled with granules or droplets of a material (eleidin)
which stains deeply with carmine and haematoxylin (stratum granulosum,
fig. 277; fig. 278). This is not sharply marked off" from the
cells of the rete mucosum which lie next to it, for many of these
show similar granules, although they less completely fill the cell.

THE SKIN. 229

Superficial to the stratum granulosum is a layer in which the cell-
outlines are indistinct and the cells contain flakes or larger droplets of
a hyaline material (kerato-hyalin), which stain less intensely than the
granules in the last layer, and which tend to run together. This
layer has a clear appearance in section, and is known as the stratum
lucidum. Immediately superficial to the stratum lucidum is the horny
part {stratum corneum) of the epidermis. It is composed of a number
of layers of epithelium cells, the nuclei of which are no longer visible.
These cells near the surface take the form of thin horny scales

Fig. 278.— Portiox of epidermis from a sectiox of the skix of the finger,
COLOURED WITH picrocarmixe. (Ranvier.)

a, stratum corneum; b, stratum lucidum with flakes of kerato-hyalin; c, stratum
granulosum, the cells filled with drops of eleidin ; d, prickle-cells ; e, dentate
projections by which the deepest cells of the epidermis are fixed to the cutis vera.

which eventually become detached (fig. 279, .s). In certain parts which
have a thick epidermis and are not covered with hair {e.g. the palms
and soles), the superficial part of the epidermis is a layer mainly
formed by a number of greatly swollen cells {sw), forming collectively
what has been termed the epitrichud layer. In the embryo in the
second and third month of intrauterine life it covers the whole body,
but is thrown off where hairs are developed.

The growth of the epidermis takes place by a multiplication of the
cells of the deeper layers. The newly formed cells, as they grow, push
towards the surface those which were previously formed, and in their
progress the latter undergo a chemical transformation, which converts
their protoplasm into horny material : this change seems to occur just
at and above the stratum granulosum (see fig. 278). The granules
which occupy the cells of the stratum granulosum are composed, as
already stated, of a substance termed eleidin, which according to

230

THE ESSENTIALS OF HI^OLOGY.

Kanvier becomes chemically altered and transformed into the keratin of
the more superficial strata.

No blood-vessels pass into the epidermis, but it receives nerves
which ramify between the cells of the rete mucosum in the form of
fine varicose fibrils (fig. 279). In some parts these are enlarged at
their extremity and along their course, into menisci which lie between

fir (i ^ ^- , -- ■: -

■A

Fig. 279. — Sectiox of epidermis. (Ranvier.)

s, superficial homy scales; sio, swollen li6my cells; $.L, stratum lucidum ; p, prickle-
cells, several rows deep ; c, elongated cells forming a single stratum near the coriuni ;
$.pr, stratum granulosum of Langerhans, just below the stratum lucidum. Part
of a plexus of nerve-fibres is seen in the superficial layer of the cutis vera. From this
plexus fine varicose nerve-fibrils may be traced passing up between the epithelium-
ceUs of the Malpighian layer.

the deeper epidermis cells. Such terminations are seen in the skin
over the pig's snout (fig. 219) and in the root-sheaths of hairs. They
also occur in the skin in the neighbourhood of the entrance of the
sweat-ducts into the epidermis (Ranvier) (fig. 280).

The cutis vera or corium is composed of dense connective tissue,
which becomes more open and reticular in its texture in its deeper
part, where it merges into the subcutaneous tissue. It is thickest over
the posterior aspect of the trunk, whereas the epidermis is thickest on
the palms of the hands and soles of the feet. The superficial or
vascular layer of the corium bears microscopic papillos, which project up

THE SKIN.

231

into the epidermis, which is moulded over them. These papillae for
the most part contain looped capillary vessels, but some, especially
those of the palmar surface of the hand and fingers, and the
corresponding part of the foot, contain tactile corpuscles, to which
medullated nerve-fibres pass (fig. 277).

In some parts of the body (scrotum, penis, nipple, and its areola),
involuntary muscular tissue occurs in the deeper portions of the cutis
vera, and, in addition, wherever hairs occur, small bundles of this tissue
are attached to the hair-follicles.

fi.Kf^FMNSKl

Fig. 280. — Section of the skin of the pulp of the finger of a child,
stained with gold chloride, showing nerves terminating in an
ivt-like arborkscence at the surface of the cutis vera and in the

DEEPEST PART OF THE EPIDERMIS. (Ranvier.)
p, p, outlines of iiapillse ; vi, n', nerve-fibres in cutis vera ; m, tei'minal menisci ;
If, duct of a sweat-gland.

The blood-vessels of the skin are distributed almost entirely to the
surface, where they form a close capillary network, sending up loops
into the papillae (fig. 281). Special branches are also distributed to the
various appendages of the skin, viz. the sweat-glands and hair-follicles,
with their sebaceous glands and little muscles, as well as to the masses
of adipose tissue which may be found in the deeper parts of the cutis.

The lymphatics originate near the surface in a network of vessels,
which is placed a little deeper than the blood-capillary network. They
receive branches from the papillae, and pass into larger vessels, which
are valved, and which run in the deeper or reticular part of the corium.
From these the hmiph is carried away by still larger vessels, which
course in the subcutaneous tissue.

The appendages of the skin are the nails, the haiis, with their

232

THE ESSENTIALS OF HISTOLOGY.

sebaceous glands, and the sweat-glands. They are all developed as thick-
enings and downgrowths of the Malpighian layer of the epidermis.

The Nails.
The nails are thickenings of the deeper part of the stratum corneum
developed over a specially modified portion of the skin (fig. 282), which

r.m.

Fig. 281. — Duct of a sweat-gland passing through the epidermis.
Magnified 200 diameters. (Heitzmann.)

J), papillae with blood-vessels injected; r.m.., rete mucosum Vjotween the papillse ; c, c,
stratum corneum ; s.g., stratum granulosum ; d, d, sweat-duct passing through
epidermis.

is known as the bed of the nail, the depression at the posterior part of
the nail-bed from which the root of the nail grows being known as the
nail-groove. The part of the bed which occupies the inner or central
portion of the groove is termed the nail-matrix, since it is from this part
that the growth of the nail proceeds. The distal part of the nail forms
the/jre border, and is the thickest part of the body of the nail. The
substance of the nail (fig. 283, N) is composed of clear horny cells,
each containing the remains of a nucleus ; it rests immediately upon a
Malpighian layer {B) similar to that which is found in the epidermis
generally, but destitute of a defined stratum granulosum. Never-
theless, in the more superficial cells both of the bed and matrix there
are a large number of granules to be seen, which appear to represent
those of the stratum granulosum of the epidermis. These granules
are, however, not composed of eleidin, but of a material {onychogenic
substance, Kanvier) which stains brown instead of red with carmine ;
a similar material occurs in the cells which form the fibrous
substance and cuticula of the hairs. The corium of the nail-bed

i

THE NAILS.

c d

233

Fig. 282.— Longitudinal section thkough the root ok the nail and its

MATRIX. Magnified about 10 diameters.

o, root of nail ; h, Malpighian layer of matrix ; c, ridges in dermis of nail-bed ; d, epitrichial

layer of epidermis ; e, cponychium ; /, bone (termijial phalanx) of finger.

Fig. 283.

-Section ACiiosb the nail and nail-bed Magnified 100 diameters.
(Heitzmann.)
P, ridges with blood-vessels ; B, rete mucosum ; N, nail.

234

THE ESSENTIALS OF HISTOLOGY.

is beset with longitudinal ridges instead of the papillae which ai'e
present over the rest of the skin ; these, like the rest of the superficial
part of the corium, are extremely vascular.

The nail-bed also receives many nerve-fibres, some of which end in
Pacinian corpuscles whilst others ramify in the ridges of the corium,
and others again penetrate amongst the deeper epithelium cells.

The nails are developed in the foetus at about the third month, the
groove being formed at this time in the corium, and the nail rudiment
appearing in it as a thickening of the stratum lucidum, which lies over
the bed. It becomes free in the sixth month, its free end being at first
thin, but as it grows forward over the bed it receives additions
on its under surface — at least in the posterior part of the bed — so that
after a time the distal end becomes thicker. The epitrichial layer of
the cuticle which originally covered the developing nail becomes
detached after the fifth month, and, after birth, only remains as the
narrow border of cuticle (ejwnychiurn) which overlies the lunula at the
root.

Hairs.

The hairs are growths of the epidermis, developed in little pits —
the hair-follicles — which extend downwards into the deeper part of the

corium, or even into the subcutaneous
tissue. The hair grows from the
bottom of the follicle, the part which
thus lies within the follicle being
known as the root (fig. 285).

The substance of a hair is mainly
composed of a pigmented, horny,
fibrous material (fig.284, /), which can
be separated by the action of sul-
phuric acid into long tapering fibril-
lated cells, the nuclei of which are
still visible. The fibrous substance
of the hair is covered by a layer
of delicate imbricated scales, termed the hair-cuticle (c). In many
hairs, but not in all, the centre is occupied by an axial substance
{medulla, m), formed of angular cells which contain granules of eleidin,
and frequently have a dark appearance from the presence of minute
air-bubbles. The latter may also occur in interstices in the fibrous
substance. When they are present, the hair looks white by reflected
light. The root has the same structure as the body of the hair, except
at its extremity, which is enlarged (fig. 285) ; this enlargement is com-

FiG. 284. — Piece of human hair.
Magnified.

A, seen from the surface ; B, in optical
section, c, cuticle ; /, fibrous sub-
stance ; in, medulla, the air having
been expelled by Canada balsam.

THE HAIRS.

235

posed mainly of soft, growing cells, and fits over a vascular j^o^pi^ld,
which projects up into the bottom of the follicle (fig. 287).

Structure of hair-follicle (figs. 28.5 to 288). — The follicle, like the skin
itself, of which it is a recess, is composed of two parts : one epithelial,
and the other connective-tissue. The epithelial or epidermic part of
the follicle closely invests the hair-root, and is often in great part
dragged out with it ; hence it is known as the root-sheath. It consists
of an outer layer of soft columnar and polyhedral cells, like the

eijidermis

junction of inner and
outer root-sheaths

medulla

fibrous substance

Fig. 285.

papilla

blood-vessels
-Diagram to explain the formation of a hair.

(Maurer.)

Malpighian layer of the epidermis, but without stratum granulosum —
the outer root-sheath ; and of an inner, thinner, horny stratum next
to the hair — the inner root-sheath. The inner root-sheath itself consists
of three layers, the outermost being composed of horny, fibrous, oblong
cells the nuclei of which are obscure and difficult to make out (Henle's
layer), the next of polyhedral nucleated cells containing eleidin {Huxley's
layer), and the third — the cuticle of the root-sheath — a layer of down-
wardly imbricated scales, which fit over the upwardly imbricated scales
of the hair itself. In the more superficial part of the hair-follicle the
layers of Huxley and Henle are indistinguishable, the cells of both

236

THE ESSENTIALS OF HISTOLOGY.

being clear and keratinised ; even lower down where distinguishable
they show a tendency to dovetail into one another. At the bottom
of the follicle no differentiation into layers can be made out in the
root-sheath, which is here formed by a uniform mass of soft cells
surrounding the papilla.

hy. -

//.:■

.--iv

Fig. 286.— Sfx'tions acros.s hair-follicles from the scalp of an infant.

I. Through papilla. II. Just above papilla. III. About middle of follicle. IV. Near
outer part of follicle. In I. :—p, papilla ; c, eiiithelium .surrounding papilla, with
pigment in cells ; A?/, hyaline layer of dermic coat with thin outer root-sheath just
within it. In II., III., IV. :— o, outer root-sheath; i' , layer of Ilenle and i", layer
of Huxley of the inner root-sheath ; c, cuticle of root-sheath ; h, hair.

In the greater extent of the follicle the outer root-sheath is several
layers deep, but as the bottom of the follicle is approached it becomes
thinner and is finally reduced to a single stratum of cells which becomes
flattened out into a very thin layer in the papillary part (fig. 286, I.).

The connective tissue or dermic part of the hair-follicle is composed
internally of a vascular layer, which is separated from the root-sheath

THE HAIES.

237

by a basement-membrane termed the hyaline layer of the follicle. This
inner vascular layer corresponds to the superficial layer of the cutis
vera. Its fibres and cells have a regular circular arrangement around

Fig. 287.— Longitudinal section of a hair-follicle. Magnitied 200 diameters.
0, outer; i, inner root-sheath ; /;, hair ; x, part shown magnified in fig. 28S.

the follicle, the cells being flattened against the hyaline layer. Exter-
nally the dermic coat of the follicle has a more open texture, correspond-
ing to the deeper part of the cutis, and contains the larger branches
of the arteries and veins. In the large tactile hairs of animals, the

238

THE ESSENTIALS OF HISTOLOGY.

Fig. 288. — A small portion of the section shown in fig. 287 enlarged

TO exhibit the structure of the several layers.

h, hair ; c", its cuticle ; <;', cuticle of root-sheath ; i", Huxley's layer ; i', Henle's layer ;

o, outer root-sheath ; hy, hyaline layer ; d, dermic coat ; J] fat-cells.

Fig. 289. — Nerves and nerve-endings in the skin and hair-follicles.
(G. Retzius.)
list, horny stratum ; rm, rete Malpighii ; c, superficial nerve-fibre plexus in the cutis ;
71, cutaneous nerve ; is, inner root-sheath of hair ; as, outer root-sheath ; h, hair ; dr,
sebaceous glands.

THE HAIRS.

239

veins near the bottom of the follicle are dilated into sinuses, so as
to produce a kind of erectile structure.

The hair-follicle receives nerve-tibres which pass into the papilla, and
others which enter the root-sheath. These last are derived from the
supei-ficial nerves of the corium and form ring-like arborisations in the
upper part of the hair follicle. They are especially well developed in
the large tactile hairs (whiskers) of animals (figs. 289, 290, 291).

Fig. 290. — From a section of skin pbbpared by the chromate of silver
method, showing the upper part of two hairs and the terminal
arborisations of nerve-fibees in their root-sheaths. (van
Gehuchten.)

The hair grows from the bottom of the follicle by multiplication
of the soft cells which cover the papilla, these cells becoming elongated
and pigmented to form the fibres of the fibrous substance, and other-
wise modified to produce the medulla and cuticle of the hair and the
several layers of the root-sheath. The cells which form the medulla
of the hair and the inner root-sheath are filled with granules of eleidin,
but those which form the fibrous substance and cuticula of the hair
have granules which stain brown with carmine, and appear similar to
those which are met with in the corresponding cells of the nail-matrix
(Ranvier) (see p. 232).

On the side' to which the hair slopes a small patch of richly innervated
thickened epidermis is usually to be found, developed over an enlarged
papilla of the cutis vera : while on the opposite side of the hair is a flat area
of skin with thickened scale-like ejiiderniis, which may repres^ent a vestige
of the rej3tilian scale (Pinkus).

240

THE ESSENTIALS OF HISTOLOGY.

The hair germs when they first appear (as at a, fig. 293) are singularly
like certain tactile patches which are found in the skin of amphibia and some
reptiles, and it is possible that hairs have become developed phylogenetically
.from these patches. It is well known that the tactile sensibility of many
parts of the skin is intimately associated with the hairs, where these
occur, although parts devoid of hairs mav also have a highly developed
sense of touch.

Besides the hair-follicles already described, which are provided with
a papilla, from the cells on the surftice of which the hair and its inner
root-sheath grow {growing hairs, pupillated hairs, hairs with hollow bulb),

u.

Fig. 291. — Nkrve ending in outer root-she.'^th of t.\ctile hair ov e.vbbit.

(Ranvler.)

n, nerve-fibre ; m, tactile meniscus ; o, outer root-sheath ; i, inner root-sheath ; h, hair ;

hy, hyaline membrane.

there are many hairs which are unprovided with a papilla and the
follicle of which ceases at the level of attachment of the arrector
pili muscle {dub-hairs, non-papillated hairs, hairs with solid bulb). These
are hairs which have lost their papilla and have ceased to grow ; they
are more easily eradicated than the growing hairs, and tend to fall out
spontaneously after a time. In their follicles the whole of the lower
part, including the original papilla and the soft growing cells which
cover it, have entirely disappeared, the hair being now attached at its
sides and below to the root-sheath. A hair which has thus ceased to
grow eventually becomes lost, but its place is presently supplied by a
new hair, which becomes developed in a down-growth from the bottom
of the follicle, a new papilla becoming formed at the extremity of the

THE HAIRS.

241

down-growth (fig. 292). If not previously detached, the old hair is
pushed out from the follicle by the one which replaces it.

The detachment of the non-papillated hairs is preceded by an absorp-
tion of the root of the hair and of the investing inner root-sheath.

] * >j.-<

^ '2 *c,^

Fig. 292.— Loxgitudinal section through the follicle of a hair which

HAS ceased to grow AND THE ROOT OF WHICH IS UNDERGOING ABSORP-
TION. Magnified 200 diameters.

This absorption appears to be effected by the cells of the outer sheath,
which multiply at the expense of the keratinised parts of the hair root
and thus undermine its attachment to the follicle (fig. 292).

The hairs are originally developed in the embryo in the form of
small solid down-growths from the Malpighian layer of the epidermis
(fig. 293). The haii'-germ, as it is called (although it gives rise not
only to the hair proper but to the epithelium-cells of the hair-follicle

242 THE ESSENTIALS OF HISTOLOGY.

also), is at first composed entirely of soft growing cells, the outermost
and deepest having a columnar shape : but presently those in the
centre become differentiated, so as to produce a minute hair invested
by inner root-sheath, its base resting upon a papilla which has become
inclosed by the extremity of the hair-germ and which is continuous
with the connective tissue of the corium (figs. 294, 295). As the minute
hair grows, it pushes its way through the layers of the epidermis,
w^hich it finally perforates, the epitrichial layer being thrown off
(p. 229). At the same time the follicle grows more deeply into the
cutis vera, carrj'ing the papilla down with it.

Fig. 293.— Hair-germs ix a .section of the .scalp of a HUJtAX fcetus.
(Szymonowicz.) Magnified 230 diameters.

a, commencing down-growth of epidermis ; 4, further stajje of down-growth ; c, connec-
tive-tissue cells beginning to accumulate to produce the dermic coat of the follicle ;
d, hair-follicle more advanced in development ; e, section of a blood-vessel.

The hair-rudiments begin to appear at the third or fourth month of
foetal life ; their growth is completed about the fifth or sixth month,
and the fine hairs which they form constitute a complete hairy cover-
ing termed the lanugo. This is entirely shed within a few months of
birth, the neAv hairs being formed in down-growths from the old hair-
follicles in the manner already mentioned.

Hairs grow at the rate of half an inch per month. They are found
all over the surface of the body except on the palms of the hands and
the soles of the feet, and on the distal phalanges of the fingers
and toes. They usually slant, and in the negro the hair-follicles
are even considerably curved. On the scalp they are set in groups,
as is well seen in a horizontal section.

The hairs of animals are often curiously marked by the arrangement of
their medulla, the markings being characteristic of particular species.
In some animals, e.g. the mole, the hairs have a varicose form with alter-
nate enlargements and constrictions. In human hair tlie disappearance of

GROWTH OF THE HAIRS.

243

the papilla is preceded by its <,a-adiial diiiiiimtioii in size, and during this
period the root of the hair is becoming gradually more slender' (Ranvier),
so that when sucli a hair is pulled out it appears to be of least diameter
near the bulb, instead of being largest there, as is the case under ordinary
circumstances.

Muscles of the hairs. — A small muscle composed of bundles of
plain muscular tissue is attached to each hair-follicle {arredoi- pili) ;
it passes from the superficial part of
the corium, on the side to which the
hair slopes, obliquely downwards, to
be attached near the bottom of the
follicle to a projection formed by a
localised hypertrophy of the outer
root-sheath. When the muscle con-
tracts, the hair becomes more erect,
and the follicle is dragged upwards so
as to cause a prominence on the
general surface of the skin, whilst
the part of the corium from which
the little muscle arises is correspond-
ingly depressed ; the roughened con-
dition known as ' goose skin ' being
in this way produced. There is
always a sebaceous gland in the
triangle formed between the arrector
pili, the mouth of the hair-follicle,
and the epidermis, so that the con-
traction of the arrector generally
causes the secretion of the gland to
be extruded.

Glands of the Skin.

Fig. 294.— Developing hair from
humax embryo of 4^ months.
(Ranvier. )

The sebaceous glands (fi^. 285) are P, Papilla,/, hair-rudiment; ^ cells from
.jv^uu.'^v^^/u^ e,xuu.u» \"o- - "/ ""-^ which the inner root-sheath is becom-

small saccular glands, the ducts from

which open into the mouths of the

hair-follicles, but they are also found

in a few situations which are devoid

of hairs (margin of lips, labia minora,

glans, and prepuce). The Meibomian

glands of the eyelid may also be regarded as modified sebaceous

glands. Both the duct and the saccules are lined by epithelium,

which becomes charged with fatty matter. This sebaceous matter is

ing formed ; k, keratiiiised part of inner
root-sheath, uncoloured by carmine ; o,
outer root-sheath ; 6, epithelial projec-
tion for insertion of arrector pili ; s,
sebaceous gland ; t, sebaceous degenera-
tion of cells in the part which will
become the neck of tlie follicle. This
forms a channel for the passage of the
hair-point through the Malpighian
laver.

244

THE ESSENTIALS OF HISTOLOGY.

discharged into the cavity of the saccule, probably owing to the
disintegration of the cells within which it is formed. There may be
more than one sebaceous gland attached to each hair-follicle.

The sebaceous glands are developed as outgrowths from the outer
root-sheath (figs. 294, 295, s).

[:

WM$M

^^h

I ^.-•

Fig. 295.— Loxgitudixal sectiox of a haik with its follicle from a
six-JiON'THs' HCMAX EMBRYO. (Szymonowicz. ) Magnified about 150
diameters.

p, papilla ; h, young hair ; ;', inner root-sheath ; d, dermic coat of follicle ; o, outer root-
sheath ; «, sebaceous gland rudiment ; 6, projection for insertion of arrector pili.

The sweat-glands are abundant over the whole skin, but they are
most numerous on the palm of the hand and on the sole of the
foot. They are composed of coiled tubes, which lie in the deeper
part of the integument and send their ducts up through the cutis

SWEAT-GLANDS.

24r

to open on the surface by corkscrew-like channels in the epidermis
(tigs. 276, 281).

The glandular or secreHng tube is a convoluted tube composed of a
basement-membrane lined by a single layer of cubical or columnar epi-
thelium-cells, and with a layer of longitudinally or obliquely disposed
tibres between the epithelium and basement-membrane (fig. 296). These
fibres are usually regarded as muscular, but the evidence on this point
is not conclusive. The secreting tube is considerably larger than the
efferent tube or duct, which begins within the gland and usually makes
several convolutions before leaving the gland to traverse the cutis vera.

^

Fig. 29G. — Section' of a sweat-glaxd in the skin of man.

a, a, secreting tube in section ; h, a cuil seen from above ; c, c, efferent tube ;
d, intei-tubular connective tissue with blood-vessels. 1, basement-membrane ;
2, muscular fibres cut across ; 3, secreting epithelium of tubule.

The efferent tube has an epithelium consisting of two or three layers of
cells, within which is a well-marked cuticular lining, but there is no
muscular layer. The passage through the epidermis has no proper wall,
but is merely a channel excavated between the epithelium-cells. Very
large sweat-glands occur in the axilla.

The ceruminous cjlavds of the ear (fig. 297; are modified sweat-glands.
The secretion is of a sebaceous nature, instead of being watery like
that of the ordinary sweat-glands.

The sweat-glands are developed, like the hairs, from down-growths
of the Malpighian layer of the epidermis into the corium. They are
distinguishable from the hair-germs by the fact that the cells of the
outermost layer are not columnar in shape, but spheroidal or poly-

246

THE ESSENTIALS OF HISTOLOGY.

hedral. The sweat-gland germs which are thus formed become
eventuall}' coiled up at their extremities and converted into hollow
tubes. The muscular fibres of the tubes as well as the secretin "•
epithelium- cells are ectodermic structures.

The sweat-glands receive nerve-fibres, and each gland has a special
cluster of capillary blood-vessels.

Root-sbeatb of 1
folUcle. /

Root of hair.

Sebaceous glands.

Hair-follicle.

Cenimiijous gland.

Fig. 297.— Section of skin of acditoby meatt.s, ixclcdixg two hair-
follicles WITH THEIK .SEBACEOUS GLANDS ANT) TWO CEKUMINOCS GLANDS.
(Griiber.)

The MAinLVRY Gl.a:nd.s.

The mammary glands are compound racemose glands which open
by numerous ducts upon the apex of the nipple. The ducts are
dilated into small reservoirs just before reaching the nipple. If
traced backwards, they are found to commence in groups of saccular
alveoli (fig. 298). The walls of the ducts and alveoli are formed of

THE MAMMARY GLANDS.

247

Fig. 298.— Section of mammary gland of woman during lactation.
(Testut, after de Sinetv.)

a, lobule of gland ; 6, acini lined by cubical epithelium ; c, duct ; t, connective-tissue

stroma.

.,..f;^^^''^,

Fig. 299.— Section of jiammart gland, human, in full activity.

(v. Ebner. ) x 110.

a, a', a", alveoli variou.sly cut, and distended by secretion ; g, if , commencing ducts ;

i, connective tissue.

248

THE ESSENTIALS OF HISTOLOGY.

a basement-membrane lined by a simple layer of flattened epithelium
(fig. 299). Milk globules may be seen within the alveoli and ducts,
and at the commencement of lactation amoeboid cells containing
fat-particles appear in the secretion {colostrum corj)usdes). These are

N^^

Fig. 300. — Ax alveolus with fat-drops ix cells, (v. Ebner.) x360.

<, cells of alveolus ; k, cells forming basket-like basement membrane, wt ; i, connective

tissue.

Fig. 301.— Sectiox from the same gland as that shown in fig. 299.
(v. Ebner. x 110.

b, connective tissue ; d, undeveloped alveoli ; d', partially developed alveoli ;
g, blood-vessels ; m, portion of larger duct with two-layered epithelium.

MAMMARY GLANDS.

249

probably emigrated leucocytes similar to the salivary corpuscles

of saliva, but some have been looked upon as epithelium-cells or

portions of epithelium-cells which have become detached from the
general linins; of the alveoli.

Fig. 302.— SeCTIOX of DKVELOPING JrAMilART GL.iXD OF HORSE.

(C. Hamburger.)
V, bloo(J-ve8sels ; s, sebaceous glands.

Development. — The mammary glands are developed in the same
manner as the sweat-glands, excepting that the secreting part does not
become convoluted and tubular. In the virgin mamma they show
A'ery few and small groups of alveoli, but as pregnancy advances
the gland ducts bud out extensively, and many more alveoli are formed
and undergo enlargement, until the greater part of the connective tissue
in the mammary region is permeated by them. In sections of the
gland they may be seen in various stages of development (figs. 299,
301). After lactation is over they undergo a process of retrogres.sion.

250 THE ESSENTIALS OF HISTOLOGY.

LESSON XXVI.
STRUCTURE OF THE HEART.

1. In sections through the wall of the auricle note the relative thickness of
the epicardium, myocardium, and endocardium. Observe the blood-vessels
and nerve-fibres under the epicardium, often embedded in fat ; here and
there a ganglion may be seen under this membrane. Notice also the elastic
networks under both the pericardium and endocardium. Make a general
sketch from this section.

2. In sections through the wall of the ventricle the same points are to be
noticed. The muscular fibres are variously cut. In those which are cut
longitudinally, the branching of the filires and their union both laterally
and by their branches may be seen. Notice also that although the fibres
are cross-stiiated this is less distinct than in voluntary muscle, and that the
nuclei lie near the centre of each fibre. Transverse markings may also be
seen passing across the fibres between the nuclei ; this is usually taken as
indicating a division into cells. The endocardium is very thin, especially
over the columnse carnepe.

3. Section through one of the valves of the heart. ^

4. If a portion of endocardium of the sheep's heart is spread out on a slide
and examined in salt solution, a network of large beadetl fibres may be seen
with a low power or even with a lens ; they are also well seen in sections.
These are the fibres of Purkinje ; they are formed of large, square-looking
cells, usually containing two nuclei, and having striated muscular substance
at their periphery. The fibres of Purkinje may also be seen in sections.

5. The lymphatics of the heai't may be injected with Berlin blue by
sticking the nozzle of the injecting syringe into the muscvdar substance, in
the interstices of which the lymphatics arise. These commencing lymphatics
lead to efferent vessels which pass undei' the epicardium towards the base
of the heart.

6. The epithelium which covers the epicardium, and that which lines the
endocardium, may be studied in preparations of the fresh organ which have
been well rinsed with distilled water ; tlien treated with nitrate of silver,
again rinsed, and subsequently exposed to the light and hardened in alcohol.
Surface sections are to l^ie made and mounted in xylol balsam or dammar.

The muscular tissue of the heart {myocardium) forms the main
thickness of the ventricles and also of parts of the auricles. It is
composed of a network of fibres which are formed of uninucleated
transversely striated cells, the structure of which has already been
studied (Lesson XVII. p. 124).

^ The appearances which are to be studied in sections 1, 2, and 3 can all be
obtained in one preparation, viz. a vertical section including a portion of auricle
and ventricle and a flap of the intervening auriculo-ventricular valve.

STRUCTURE OF THE HEART.

251

In the interstices ot" the muscular l)uiulle3 there is a little areolar
tissue in which run the very numerous blood-capillaries and the
lacunar lymphatics.

7C/

h-^'m

Fig. 303 a, b.

Fig. 303 c.

I

Fig. 303 a, b. —Section" of the right auricle.

A, Epicardium and adjacent part of the myocardium, n, serous epithelium in section;

b, connective-tissue layer ; c, elastic network ; d, subserous areolar tissue ; e, fat ;
;■, section of a blood-vessel ; tj, a small ganglion ; k, musculajr fibres of the
myocardium ; i, intermuscular areolar tissue.

B, Endocardium and adjacent layer of the myocardium, a, lining epithelium ; 6, con-

nective tissue with fine elastic fibres ; c, layer with coarser elastic fibres ; d, sub-
endocardial connective tissue continuous with the intermuscular tissue of the
myocardium ; h, muscular fibres of the myocardium ; m, plain muscular tissue in
the endocardium.

Fig. 303 c— Section through one of the flap.s of the .\ortic valve, and

PART of the CORRE-SPONDING SINU.S OF VALSALVA, WITH THE ADJOINING
PART OF THE VENTRICULAR WALL. (Horslej.)

a, endocardium, prolonged over the valve ; 6, sub-cndocardial tissue ; c, fibrous tissue
of the valve, thickened at c' near the free edge ; d, section of the lunula ; e, section
of the fibrous ring ; /', muscular fibres of the ventricle attached to it ; g, loose
areolar tissue at the base of the ventricle ; s. V. sinus of Valsalva ; 1, 3, 3, inner,
middle, and outer coats of the aorta.

252 THE ESSENTIALS OF HISTOLOGY.

The myocardium is covered externall}^ by a layer of serous mem-
brane— the epicardiuin (cardiac pericardium fig. 303,^) — composed,
like other serous membranes, of connective tissue and elastic fibres,
the latter being most numerous in its deeper parts. Underneath the
epicardium run the blood-vessels, nerves, and lymphatic vessels of the
heart embedded in areolar and adipose tissue, this tissue being con-
tinuous with that which lies between the muscular bundles ; the free
surface of the membrane is covered by serous epithelium.

The endocardium (fig. 303, B) has a structure not very unlike the
pericardium. It is lined by a pavement-epithehum (endothelium),
like that of a serous membrane, and consists of connective tissue \y\l\\

Fig. 304.— Fragment of the xetwork of Purkixje's fibres from the

VENTR1CUL.\R EXDOCARDIUJI OF THE SHEEP. (Rauvjer.)
c, clear cell bod}-; n, nuclei; /, striated fibrils.

elastic fibres in its deeper part, between which there may, in some
parts, be found a few plain muscular fibres. Fat is sometimes met with
under the endocardium.

In some animals, e.g. the sheep and ox, large beaded fibres are
found under the endocardium. These are formed of clear cells joined
both end to end and laterally, and generally containing in their centre
two nuclei, Avhilst the peripheral part of the cell is formed of cross-
striated muscular tissue ; the chains of cells form the fibres of Purldnje
(fig. 304). They appear to be cardiac cells which have undergone
differentiation into striated muscle substance only at their periphery,
the non-difterentiated part of the cell having continued to grow until it
has attained a considerable size. In man distinct fibres of Purkinje
are not seen, but the innermost muscular fibres of the ventricles are
larger than those which lie more externally : they also undergo
development somewhat later (J. B. MacCallum).

A muscular bundle which shows less differentiation than the rest of the
cardiac muscle has been described by Stanley Kent, His and others, running

STRUCTURE OF THE HEART. 253

in the septum and affonling a bridging connexion between the muscle of the
auricles and that of "the ventricles. This bundle is commonly believed to
serve to propagate the contractions of the auricles to the ventricles and thus
to maintain their regularity of ihythra ; and it is stated that when the
bundle in question is severed experimentally or by disease this propagation
is no longer possible, and the ventricles in consequence beat with a much
slower rhythiu than the auricles. The accuracy of this statement is, however,
denied by Kronecker, who regards the regularity of the cardiac contractions
as a function, not of the muscular substance of the heart, but of the nerve-
fibres, which are distributed to every part of the myocardium.

The valves of the heart are formed of folds of endocardium
strengthened by fibrous tissue (fig. 303, C). This tissue forms a thicken-
ing near the free edge of the valve (c). At the base of the auriculo-
ventricular valves the muscular tissue of the auricle may be found
passing a short distance into the valve. In the foetus these valves are
at first entirely muscular.

The nerves of the heart are seen in sections underneath the epi-
cardium of both auricles and ventricles ; in the former situation they
are connected at intervals with small ganglia (fig. 303, A, g]. Their
branches pass to the muscular substance, and after dividing into fine
fibrils, these end in enlarged extremities, which are applied directly to
the muscular fibres (Ranvier). Other nerve-fibres, which are probably
afferent, terminate in complex ramifications in the endocardium in
connection with small masses of nucleated cells, forming a kind of end-
plate (Smirnow).

The blood-vessels of the heart are very numerous, and the veins thin-
walled, retaining the capillary structure (endothelium only) in vessels
of as much as 0-25 mm. in diameter. They are accompanied by
numerous lymphatic vessels, which also form plexuses under the cardiac
pericardium and endocardium. The lymphatics appear to be in free
communication with the spaces of the interstitial connective tissue
between the muscle-fibres.

I

254 THE ESSENTIALS OF HISTOLOGY.

LESSON XXVII.
THE TRACHEA AND LUNGS.

1. In sections of the trachea and larynx, notice the epithelium, the basement-
membrane (of some thickness in the human trachea), the lymphoid tissue of
the mucous membrane, the elastic tissue external to this, and, lastly, the
fibrous membrane containing the cartilages. In the mucous membrane and
submucous areolar tissue look for sections of mucous glands, ducts of which
may be seen opening on the surface. At the back of the trachea notice the
plain muscular fibres transversely arranged ; there may be larger mucous
glands external to these.

2. In sections of lung notice the sections of the alveoli collected into groups
(air-sacs). Find sections of bronchial tubes, some cut longitudinally and
passing at their extremities into the alveolar passages, others cut across. In
each tube notice the ciliated epithelium internally. Next to this the mucous
membrane containing numerous elastic fibres and often thrown into folds ;
then the layer of circular muscular fibres, and, outside this, loose fibrous
tissue in which in larger bronchial tubes pieces of cartilage may be seen
embedded. Small mucous glands may also be obsei'ved in the fibrous tissue
sending their ducts through the other layers to open on the inner surface.
Notice that the section of a branch of the pulmonary artery always accom-
panies a section of a bronchial tube.

In the sections of the alveoli observe the capillary vessels passing from one
side to the other of the intervening septa ; and in places where the thin wall
of an alveolus is to be seen in the section, the network of blood-capillaries
upon it. Notice within the alveoli nucleated corpuscles which frequently
contain dark particles in their protoplasm. They are amoeboid cells which
have migrated from the blood-vessels and lymphatics, and have taken in
inhaled particles of carbon. They may pass back into the lung tissue, for
similar cells are seen in this. Make a sketch of part of the wall of one
or more bronchial tubes and of one or two of the alveoli. ;,

3. In sections of a fresh lung the air-cells of which have been filled with a
mixture of gelatine and nitrate of silver solution, the epithelium of the alveoli
may be studied. The sections can be made with the freezing microtome, and
mounted in glycerine, which should be warmed after the cover-glass is applied
in order to melt the gelatine.

4. Mount a section of lung in which the pulmonary vessels have been
injected. Study the general arrangement of the vessels with a low power,
and the network of capillaries of the alveoli with a high power. Observe
that the veins run apart from the arteries. Sketch the capillary netwoi'k of
one or two adjoining alveoli.

The Trachea.

The trachea or windpipe is a fibrous and muscular tube, the wall
of which is rendered somewhat rigid by C-^haped hoops of cartilage

thp: tracihea and lungs.

255

which are embedded in the fibrous tissue. The muscuhir tissue, which
is of the plain variety, forms a flat band, the fibres of which run trans-
versely at the back of the tube. The trachea is lined by a mucous
membrane (fig. 305, a to d), which has ciliated epithelium upon its
inner surface. The epithelium-cells, which have been already described
(Lesson VIII.), rest upon a thick basement-membrane. The corium of

0^

'i0

1^<S>,

^«

a^s^i»#io

^MMoWWMm%i

'90%

Fig. 305.— Longitudinal section of the human trachea, including portions
OF TWO CARTILAGINOUS RINGS. (Klein.) Moderately magnified.

a, ciliated epithelium ; 6, basement-membrane; c, superficial part of the mucous mem-
brane, containing the sections of numerous capillary blood-vessels and much
lymphoid tissue ; d, deeper part of the mucous membrane, consisting mainly of
elastic fibres; f, submucous areolar tissue, containing the larger bIood-ves.sels, small
mucous glands (their ducts and alveoli are seen in section), fat, etc. ; /', fibrous tissue
investing and uniting the cartilages ; g, a small mass of adipose tissue in the fibrous
layer ; h, cartilage.

the mucous membrane consists of areolar and lymphoid tissue, and
contains numerous blood-vessels and lymphatics. In its deepest part
is a well-marked layer of longitudinal elastic fibres {d). Many small
glands — mucous and mixed mucous and serous — are found in the wall
of the trachea. They may lie either within the mucous membrane or
in the submucous areolar tissue {e) or, lastly, at the back of the
trachea, outside the transverse muscular fibres.

The two divisions of the trachea, the hronchi, are precisely similar in
structure to the main tube.

256 THE ESSENTIALS UF HIST(jLOGY.

The larynx is also very like the trachea so far as the structure of
the mucous membrane is concerned. It is lined by ciliated epithelium,
but over the true vocal cords and upon the epiglottis, as well as here
and there in the part above the glottis, stratified epithelium is found :
and taste-buds may occur in this epithelium, except over the vocal
cords. The nerve-endings in the epithelium are shown in fig. 218,
p. 176.

The lymphoid tissue is especially abundant in the mucous membrane
of the ventricle of Morgagni (fig. 306, d). and a large number of mucous
glands open into this ca'vity and into that of the sacculus.

^^ - ■

1

\ '--'-'.^

d

f^

Fig. 306. — LoxGrrcDHfAL SEcnox theocgh the ve>teicle of the laeyxx

OF A CHILD. (Klein.)

a, true vocal cord ; 6, false vocal cord ; c, nodule of cartilage ; d, ventricle of Morgagni ;

I, lymphoid tissue ; m, thyro-arytenoid muscle.

The true vocal cords are composed of fine elastic fibres.

The cartilages of the trachea and the thyroid, cricoid and arytenoid
cartilages of the larynx are hyaline ; all these are liable to ossify as
age advances. The epiglottis and the cartilages of Santorini and of
Wrisberg are composed of elastic fibro-cartilage. This is also
the case with the uppermost part of the arytenoid and the tip of the
vocal process.

The Lungs.

The lungs are formed by the ramifications of the hronchicd tubes and
their terminal expansions, which form groups or lobules of sacculated

THE LUNGS.

257

dilatations (air-sacs, infundibula), beset everywhere with small irregu-
larly hemispherical or cubical bulgings, kno^\^l as the air-cells or
pulmonart/ alveoli.

The bronchial tubes (tigs. 307, 308, 309) are lined (except the

Fig. 307. — Portion" of a traxsverse section of a broxchial tube, hum ax,
6 MM. IX diameter. (F. E. Schultze. ) Magnified 30 diameters.

a, cartilage and fibrous layer with mucous glands, and, in the outer part, a little fat ;
in the middle, the duct of a gland opens on the inner surface of the tube ; h,
annular layer of involuntary muscular fibres ; c, elastic layer, the elastic fibres in
bundles which are seen cut across ; d, columnar ciliated epithelium.

-^r-^^.

Fig. 308. — Section op part of a bronchial tube. Magnified 200 diameters.

a, ciliated epithelium ; 6, basement membrane ; c, superficial part of mucous membrane,
with fine elastic fibres ; d, deeper part with numerous coarser fibres ; e, plain muscle
of bronchus ; /, duct of gland passing through mucous membrane.

R

258

THE ESSENTIALS OF HISTOLOGY.

terminal bronchi) by ciliated epithelium which rests on a basement-
membrane. External to this is the corium of the mucous membrane,
containing a larsje number of longitudinal elastic fibres and some

Fig. 309.-

-Section of a small bronchial tube, human. (Sobotta.)
The elastic fibres of the mucous membrane are stained.

x280.

Fig. 310.— Cast of lobule of dog's lung showing a single infundibuluji

OR AIR-SAC. (W. S. Miller).
A, atrium; r, vestibule or alveolar duct leading to atrium (seen in section); 8, air-sac
(infundibulum) ; P, section of the neck of a second air-sac (cut away). The irregular
projections on the atrium and air-sac are the alveoli.

lymphoid tissue. Outside this again is a complete layer of plain
muscular fibres encircling the tube. Next comes a loose fibrous layer
in which, in the larger tubes (fig. 307), small plates of cartilage
are embedded. Mucous glands are also present in this tissue.

THE LUNGS.

259

The extremities of the bronchial tubes expand into passages, the
respiratory bronchioles, which give off branches, termed alveolar duds or
terminal bronchioles. The walls of these are beset with alveoli. The
terminal bronchioles lead through nearly spherical alveolated dilata-
tions (the atria) into a number of blind and often funnel-shaped

-Section of cat's lung, outlined with camera lucida.
(W. S. Miller.)
The shading indicates the position of plain muscular tissue. V, pulmonary veins ;
A, atria ; A. S, air-sacs ; T. B, terminal bronchioles or alveolar ducts ; B. R, respira-
tory bronchioles ; B, small bronchus.

diverticula completely covered with alveoli ; these are known as the
infunclibula, alveolar sacs or air-sacs (Waters). The arrangement of
these parts, according to the investigations of AV. S. Miller, is as
follows : — Two or more air-sacs, or groups of alveoli, open into a
common chamber (atrium), and three to six atria into an alveolar duct
or tcrrnincd bronchiole. The latter open into the respiratory bronchioles,
which are expanded continuations of the smallest bronchi. All of
these, except the last named, are beset with alveoli.

The epithelium changes in character in the alveolar ducts ; from

260

THE ESSENTIALS OF HISTOLOGY.

Fig. 312.— Diagram of the ending of a bronchial tube. (W. S. Miller.)

B, terminal bronchiole ; V, vestibule ; A, atrium ; S, air-sac (inf undibuluni) ; C, air-cell
(alveolus) ; P, ending of pulmonary arteriole ; T, commencement of pulmonary
venule.

Fig. 313. — Section of part of cat's lung, stained with nitrate of silver.
(Klein.) Highly magnified.

Both the cubical and the large flattened cells of the alveoli are shown. In the middle
is a section of a lobular bronchial tube, with a patch of cubical epithelium cells
at one side.

THE LUNGS. 261

columnar and ciliated it becomes cubical and non-ciliated, and there
are patches of the respiratory epithelium (see below) not only in the
alveoli which beset the ducts, but also elsewhere in their wall. The
plain muscular tissue of the bronchiole is continued on the walls of
the alveolar ducts, but not on those of the atria, although some
occurs round the mouths of the atria and even of the alveoli.

The alveoli are lined by large irregular flattened cells (fig. 313),
which form an extremely delicate layer (respiratory epithelium),
separating the blood-capillaries from the air within the alveoli.
Amongst the flattened cells are here and there groups of smaller and
thicker (cubical) epithelium-cells. The capillary network of the alveoli

Fig. 314. — Section of injected lung of kabbit. including several con-
tiguous ALVEOLI. (Szymonowicz. ) Magnified 300 diameters.

is very close (fig. 314), and the capillary vessels of adjoining alveoli are
in complete continuity, the vessels passing first to one side and then
to the other of the septa which separate the adjacent alveoli. Outside
the epithelium a thin layer of connective tissue (basement membrane 1)
forms the wall of each alveolus. Elastic fibres are numerous around
the mouths of the alveoli, and a certain number course over the wall
of each alveolus.

Blood-vessels. — Branches of the pulmonary artery accompany the
bronchial tubes to be distributed to the capillary networks upon the
alveoli, from which the blood is returned by the pulmonary veins.
An arteriole runs with each terminal bronchiole, and, dividing into as
many branches as there are atria, is distributed to the capillary net-
works of all the air-cells with which the bronchiole is connected

262 THE ESSENTIALS OF HISTOLOGY.

(Miller). From these networks one or two venules collect the blood,
usually coursing (independently of the arteriole) on the outer border
of the group of infundibula, and unite with other venules to form
efferent veins. The venules of the superficial lobules are connected
with a vascular network at the surface of the lung underneath the
pleura. The veins, pursuing a separate course through the tissue of
the lung, join with others to form larger vessels which pass to the
root of the lung. Branches from the bronchial arteries are distributed
to the walls of the bronchial tubes, and to the connective tissue of the
lung. Bronchial veins accompany the bronchial arteries to the larger
tubes, but most of the blood brought to the lungs by the bronchial
arteries is returned by the pulmonary veins. Connective tissue
intervenes everywhere in small quantity between the infundibula
(interstitial tissue), and forms a distinct layer, containing much
elastic tissue, covering the surface of the lung underneath the serous
membrane (subserous tissue). In some animals {e.g. guinea-pig) the
subserous layer contains plain muscular tissue, which is especially
developed near the lung-apex ; it has not been detected in man.

The lymphatics of the lung accompany the bronchial tubes, the
branches of the pulmonary artery, and the branches of the pulmonary
vein ; and they also form a network in the pleura. The atria and
air-sacs have no lymphatics in their walls (Miller). The bronchial
lymphatics are less superficial than the corresponding blood-vessels.
The larger tubes have two plexuses, one within the other outside the
cartilages. The smaller have only one set. The lymphatics of the
bronchi are connected with those of the arteries and veins by lateral
branches curving off at the divarications of the tubes ; at these points
there is usvxally an accumulation of lymphoid tissue. The larger
arteries and veins have two accompanying lvm})hatics, the smaller only
one. All the lymphatics tend towards the hilus, and enter lymphatic
glands at the root of the lung. Those in the pleura have been said to
communicate, by means of stomata between the epithelial cells of the
serous membrane, with the cavity of the pleura, but this connexion is
denied by Miller. The lymphatics of the pleura are furnished with
numerous valves.

The pleura, which covers the surface of the lung, has the usual
structure of a serous membrane. It is provided with a special net-
work of blood-vessels, which is supplied from the pulmonary vessels of
the superficial lobules.

THE TEETH. 263

LESSOX XXYIII.
STRUCTURE AND DEVELOPMEST OF THE TEETH.

1. Study first with the low power and afterwards with the high power a
longitudinal section of a human tooth which has been prepared by grinding.
It is better to purchase this specimen, for the process of preparation is
difficult and tedious without the aid of special apparatus. Examine care-
fully the enamel, the dentine, and the cement. The dark appearance of the
dentinal tubules is due to their containing air in the dried specimen.
Measure the diameter of the enamel prisms and of some of the dentinal
tubules. Make sketches from each of the tissues.

2. Section of a tooth in situ, which has been decalcified after fixation, and
stained. In this section the mode of implantation of a tooth, as well as the
structure of the pulp, can be made out. Make a genei'al sketch under a low
power, and under a high power draw a small piece of the pulp showing the
processes of the odontoblasts extending into the dentinal tubules.

3. The development of the teeth and the formation of their tissues are
studied in sections made across the snout and lower jaw of fcetal and young
animals. The preparations may be stained in bulk or the individual sections
mav be stained.

The Teeth.

A tooth consists in man of three calcified tissues ; the enamel, which
is of epithelial origin, the dentine, and the cement or cruda jiefrosa.
The dentine forms the main substance of a tooth, the enamel covers
the crowni, and the cement is a layer of bone which invests the root
(figs. 315 to 317).

Enamel is formed of elongated hexagonal jmsms (figs. 318, 319),
which are set vertically, or with a slight curvature, upon the surface
of the dentine. They are marked at tolerably regular intervals with
slight transverse shadings producing an indistinct cross-striated appear-
ance. Sometimes coloured lines run through the enamel across the
direction of its prisms. The enamel prisms have when first laid down
a fibrous structure, but this becomes obscured after their calcification
is complete. C. Tomes has shown that the enamel of the fully-formed
tooth contains only an extremely minute proportion of animal matter :
practically it is wholly composed of earthy matter (lime salts).

Dentine is constituted of a hard dense substance like bone, but
containing no Haversian canals or lacunae. It is pierced everywhere
by fine canaliculi {dentinal tubules, fig. 320), radiating outwards from a

264

THE ESSENTIALS OF HISTOLOGY.

central cavity which, during life, contains the pulp. The tubules branch
at acute angles as they pass outwards ; their branches become gradually
finer towards the periphery of the dentine. The dentinal tubules are
occupied by processes of the odontoblasts (p. 268).

Fig. 315. — Vertical section of a tooth in situ. (Waldeyer. )
c, is placed in the pulp-cavitj^, opposite the cervix or neck of the tooth ; the part
above is the crown, that below is the root (fang). 1, enamel with radial and
concentric markings ; £'', dentine with tubules and incremental lines ; 3, cement
or crusta petrosa, with bone corpuscles ; I,, dental j^eriosteum ; ;', bone of lower jaw.

The tubules have a proper wall of their own, which can be isolated
by steeping a section of tooth in strong hydrochloric acid. In the
living tooth they are occupied by protoplasmic fibres (Tomes' fibres),
which are prolonged from the superficial cells of the pulp.

The intertubular substance appears for the most part homogeneous,

THE TEETH.

265

but here and there indications can be seen in it of a globular forma-
tion. This is especially the case near the surface of the dentine,
where tiie globular deposit and the interglobular spaces may produce

X^^N..

/

A '^'

\ 1

// D

Fig. 316.— Section of molar tooth. (Sobotta.) x8.
E, enamel ; D, dentine ; C, cement ; P, pulp cavity.

266

THE ESSENTIALS OF HISTOLOGY.

a granular appearance {granular layer, fig. 317, g), and also in the
course of certain lines or clefts which are seen traversing the dentine
across the direction of the tubules {inter glohular spaces, incremental lines,

Fig. 317.— Cross-sectiox of root of canine tooth, human.

(Sobotta. ) X 2.5.

D, dentine ; G, its granular layer ; C, cement ; P, pulp cavity.

fig. 31.5, shown magnified in fig. 322). After decalcification the
dentine can be separated into lamellae along these incremental lines.

The animal matter of dentine resembles bone and the connective
tissues generally in having its ground-substance pervaded by fibres
which yield gelatine on boiling. These fibres, which have been

THE TEETH.

267

especially investigated by v. Ebner and by Mummery, are difficult of
demonstration in the fully calcified dentine; but in developing
dentine and in tlentine which is attacked by caries they are more
easily shown.

'-'r!i::Hi.l|/|.|:j-p' /, ()) Vil.l\ , , ■

mmiii:.

■I

W^''^'

Fig. 318.— Section through the e.vamkl op a tooth. Magnified 200
diameters. (Rauber.)

a, projection of dentine, showing some of its tubules, b, penetrating into the
enamel; c,c, enamel fibres cut longitudinally; d, d, prisms cut transversely
e, cuticle of the enamel. •' '

Fig. 319.— Examel prisms. Magnified 350 diameters. (Kolliker.)
A, Fragments and single fibres of the enamel, isolated by the action of hydrochloric acid.
a, surtace of a smaU fragment of enamel, showing the hexagonal ends of the fibres.

268

THE ESSENTIALS OF HISTOLOGY.

The pulp (fig. 323) consists of a soft, somewhat jelly-like, connective
tissue, containing many branched cells, a network of blood-vessels, and
many nerve-fibres which pass into the pulp-cavity along with the blood-
vessels by a minute canal at the apex of the fang. The superficial

Fig. 321.

Fig. 320.

Fig. 322.

Fig. 320.~Section of fang, pae.\llel to the dentinal tubules. Magnified

300 diameters. (Waldeyer. )

1, cement, with large bone lacunas and indications of lamellas; 2, granular layer of

Purkinje (interglobular spaces) ; 3, dentinal tubules.

Fig. 321. — Sections of dentinal tubules. Magnified about 300 diameters.

(Fraenckel.)
ft, cut across ; b, cut obliquely.

Fig. 322. — A small portion of dentine with interglobular spaces.

Magnified 350 diameters. (KoUiker.)

c, portion of incremental line formed by the interglobular spaces, which are here filled

up by a transparent material.

cells of the pulp form an almost continuous layer, like an epithelium
(fig. 323, Od, OcV). They are known as odontohlasts, from having been
concerned in the formation of the dentine. The nerve-fibres are said
to pass eventually between the odontoblasts and to end in arborisa-

THE TEETH.

269

tions close to the dentine, but they have not been followed into the
dentinal tubules.

The crusta petrosa (fig. 317, 320) is a layer of laraellated bone,
including lacunie and canaliculi, but without Haversian canals, at least
normally, in the human teeth. It is covered with periosteum (dental
periiisfeum), which also lines the socket, and serves to fix the tooth
securely.

Od'

Fig. .323.— Section across the eoot of a youxg tooth showixg the pulp
IN siTC. (Rose.)

P, pulp ; V, V, veins ; A, A, A, arterioles ; N, nerve bundles ; O'l, columnar odontoblasts
still depositing dentine ; Od', flattened odontoblasts which have ceased to form
dentine.

Formation of the teeth. — The teeth are developed somewhat similarly
to the hairs. A continuous thickening of the epithelium occurs along the
line of the gums, and grows into the corium of the mucous membrane
{common dented germ or (hnfal lamina, fig. 32-1:, a). At regular intervals
there is yet a further thickening and growth from the common germ
into the tissue of the mucous membrane, each of these special rudi-
ments, which are ten in number, swelling out below into a flask-shaped
mass of cells, the special dental germ (fig. 324, b) of a milk tooth. The
intermediate parts of the dental lamina long remain, forming a common
epithelial strand uniting the several special dental germs to one another
and to the epithelium covering the gum (fig. 324, C, D,/). A vascular
papilla is continued from the corium into the bottom of each special

270

THE ESSENTIALS OF HISTOLOGY.

germ (fig. 324, C, D, p) : this papilla has the shape of the crown of the
future tooth. Each special dental germ, with its included papilla,

B
A

6-

/

:WM^^

d'Y

■H

c f

f:im0&

Fig. 324.

A. SECTIOX ACBOSS the CPPEB jaw of a FCETAL sheep, .3 CENTIMETERS LONG.

(Waldeyer. )
1, common dental lamina dipping down into the mucous membrane where it is half sur-
rounded hy a horsc-hoe-shaped more den.se-looking tissue, the germ of the dentine
and dental sac ; 2, palatine process of the maxilla.

B. Section' from fcetal calf similar to that shown in A, but passing through

ONE OF THE SPECIAL DENTAL GERMS HERE BECOMING FLASK-SHAPED. (Rose. )

a, epithelium of mouth, thickened at b, above special dental germ ; c, papilla ; d, special

dental germ ; -:, enamel epithelium ; /", dental sac.

C AND D. Sections at lateb stages than A and B, the papilla having

BECOME FORMED AND HAVING BECOME PARTLY SURROUNDED BY THE

EPITHELI.\L GERM. (Kolliker. )
c, epithelium of gum, sketched in outline ; /, neck of dental germ ; f, enamel organ ; e,
its deeper columnar cells ; e', projections into the corium ; p, papilla ; j*, dental sac
forming. In D, the dental germ (jp) of the corresponding permanent tooth is seen.

FORMATION OF THE TEETH. 271

presently becomes almost entirely cut ofl" from the epithelium of the
mouth, and surrounded by a vascular membrane — the dental sac.
The papilla becomes transformed into the dentine and pulp of the
future tooth, and the enamel is deposited upon its surfece by the
epithelial cells of the dental germ. The root of the tooth, with its
covering of cement, is formed at a later period, when the tooth is
beginning to groAv up through the gum, by a gradual elongation of the
base of the papilla. The shaping of this into the form of the root is

wl

e"

UK

<sv

VK

Fig. 32.5.— Section of a developing incisor tooth of a human embkto.
(Rose.) The section also includes the germ of the adjacent tooth.

DK, dental i)apilla ; od, odontoblasts ; 6, bone of jaw ; e, e', outer and inner layers of
enaiuel-organ ; S.P., enamel pulp; d.f. dental furrow; c, reriialns of common dental
germ or lamina ; n, neck or bridge of cells connecting this with the enamel-organ ;
vi.e., mouth-epithelium; e", enamel organ of adjacent tooth-germ; /•, reserve germ
of permanent tooth.

determined by a growth of the epithelium of the edge of the enamel
germ, which extends in the form of a fold (the epithelial sheath of
V. Brunn) towards the future apex of each fang.

Previously to the deposition of the enamel, the dental germ under-
goes a peculiar transformation of its previously polyhedral epithelium-
cells into three layers of modified cells. One of these is a layer of
columnar cells {ameloblasfs, fig. 326, a), immediately covering the surface
of the dentine. The enamel-prisms are produced by a fibrous forma-
tion (fig. 327, /) followed by a deposition of calcareous salts ; these
changes taking place altogether external to the cells (or, as some hold,
by a direct calcification of their protoplasm). The cells next to the
dental sac form a single layer of cubical epithelium (fig. 325, e), and

272

THE ESSENTIALS OF HISTOLOGY.

nearly all the other cells of the dental germ become transformed into
branching corpuscles (fig. 325, SP ; fig. 326, ;j) communicating by
their processes, and thus forming a continuous network. The dental
germ, after it is thus modified, is known as the enamel organ.

The (h'nfine of the tooth is formed by calcification of the surface of
the papilla. At this surface there is a well-marked layer of odonto-
blasts (fig. 328, od, fig. 329, c), and these produce a layer of dentinal
matrix which forms a sort of cap to the papilla, and which soon

'^'/sy-'

.-ISv-

~3 *t^.

^«

W.u^^i

Fig. 327.

Fig. 326.— Section shovvixg the structure of the part of the enamel

ORGAN WHICH LIES NEXT TO THE DENTINE. (Rose.)

d, dentine ; o, newly formed enamel stained black by osmic acid ; T, Tomes' processes
from the ameloblasts, a; str. int., stratum intermedium of enamel-organ; p,
branched cells of enamel pulp.

Fig. 327. — Developing enamel showing ameloblasts and the fibrous sub-
stance PRODUCED BY THESE CELLS, WHICH FORMS THE BASIS OF THE

ENAMEL PRISMS. (From a photograph by Leon Williams.)
a, portions of the ameloblasts ; /, fibrous basis of enamel jirisms ; e, calcified part of enamel.

becomes calcified by the deposition of globules of calcareous matter.
Processes of the odontoblasts remain in the dentine as it is forming,
and thus the dentinal tubules are produced. Subsequently other layers
of dentine are formed within the first by a repetition of the same
process, and in this way the papilla gradually becomes calcified. A
part, however, remains unaltered in the centre of the tooth, and with
its covering of odontoblasts forms the pulp.

The ten milk-teeth are formed in each jaw in the manner described.
These, however, become lost within a few years after birth, and are
replaced by permanent teeth in much the same way that a new succes-
sion of hair occurs. A small outgrowth takes place at an early period

FORMATION OF THE TEETH.

273

from the dentul germ close to each of the milk-teeth (fig. 324, D, fp),
and this eventually becomes the germ of the corresponding permanent
tooth It gradually enlarges, acquires a papilla, forms an enamel

Fig. 328. — Section op part of a developing tooth. (From a photograph by

Leon Williams.)
d, dentine ; od, odontoblast-s sending their processes into the dentinal tubules ; p,

branched cells of the pulp ; f , developing enamel ; a, ameloblasts ; r, reticulum or

spongework of the enamel organ.

Fig. 329. — Part of section of developing tooth of young rat, showing

THE mode of deposition OF THE DENTINE. Highly magnified.

a, outer layer of fully calcified dentine; 6, uncalcified matrix, with a few nodules of

calcareous matter ; c, odontoblasts with processes extending into the dentine ; d,

pulp. The section being stained, the uncalcified matrix is coloured, but not the
calcified part.

s

274 THE ESSENTIALS OF HISTOLOGY.

organ : in short, passes through the same phases of development as
the germ of the milk-tooth ; and when the milk-tooth drops out of the
jaw in consequence of the absorption of its roots (by osteoclasts) the
permanent tooth grows up into its place.

There are six permanent teeth in each jaw which do not succeed
milk-teeth ; these are the permanent molars. They are developed from
an extension backwards on each side of the jaw of the original epithelial
thickening or common dental germ and by the downgrowth from this
into the corium of three successive special germs at comparatively long
intervals of time. Within these the tissues of the permanent molars
become formed in a manner exactly similar to that in which the
milk-teeth are developed.

THE TONGUE. 275

LESSON XXIX.

THE TONO UE A ND THE G USTA TOR Y ORG A NS. THE M UCO US
MEMBRANE OF THE MOUTH. THE PHARYNX AND
(ESOPHAGUS.

1. Sections of the tongue vertical to the surface, stained with hsematoxylin
and eosin. The sections should be taken from difterent parts and include all
three kinds of papillae.

2. Sections of injected tongue.

3. Sections of the papilla foliata of the rabbit, stained with hsematoxylin
and eosin to show the taste-buds in situ.

The cells composing the taste-buds are studied by teasing osmic prepara-
tions of the papilla foliata ; the nerve-endings are seen in sections of papillae
foliatas which have been treated by Golgi's osmic-bichromate silver method.

4. Sections of the pharynx and of the oesophagus stained with hsema-
toxylin and eosin.

The tongue is mainly composed of striated muscular fibres, running
some longitudinally, and others transversely. It is covered by a mucous
membrane, the epithelium of which, like that of the rest of the mouth,
is thick and stratified, and conceals microscopic papillae (fig. 330) like
those of the skin. Besides these, the upper surface of the organ is
covered with larger papillae, which give it a rough appearance. These,
which are termed the lingual papillce, are of three kinds: (1) About
twelve or thirteen comparatively large circular projections, each of
which is surrounded by a narrow groove (fossa), external to which
the mucous membrane is raised above the general level (vallum)
(fig. 331). These papillae form a V-shaped line towards the back of
the tongue ; they receive filaments of the glosso-pharyngeal nerve,
and have taste-buds in the epithelium which covers their sides, and
in that of the side of the vallum. They are known as the circumvallate
papillce. (2) All the rest of the papillary surface of the tongue is
covered by conical pajnllce, so named from the conical pointed cap of
epithelium which is borne by each ; sometimes this cap is fringed
with fine epithelial filaments, when they are termed filiform (fig. 332).
(3) Scattered here and there amongst the conical papillae are other
larger papillae, the fungiform (fig. 333). These are very vascular, and
lie partly embedded in little depressions of the mucous membrane.

Small tubular glands may be seen between the superficial muscular
fibres sending their ducts to the surface. Most of them secrete mucus,

276

THE ESSENTIALS OF HISTOLOGY.

but those which open into the trenches of the circumvallate papillae,
and a few others elsewhere, yield an albuminous secretion (serous
glands, glands of Elmer).

Fig. 330.— Section of
mucous membraxe
of mouth, showing
three microscopic
papill.e and stra-
tified epithelium.

The BLOOD-VESSELS
HAVE BEEN INJECTED.

(Toldt.)

Fig. 331. — Section of circumvallate papilla,
HUMAN. The figure includes ONE SIDE of the
papilla and the adjoining part of the
VALLUJI. (Magnified 150 diameters. ) (Heitzmann.)

E, epithelium ; G, taste-bud ; C, corium witli injected
blood-vessels ; M, gland with duct.

Fig. 332. — Section of two filiform

PAPILL.E, HUMAN. (Heitzmann.)

E, epithelium ; C, corium ; L, lymphoid

tissue ; 31, muscular fibres of tongue.

Fig. 333. — Section of fungiform papilla,
HUMAN. (Heitzmann.) (Letters as in
previous figure.)

THE TASTE-BUDS.

277

The mucous membrane at the back of the tongue contains a large
amount of lymphoid tissue.

Taste-buds. -The minute gustatory organs which are known as taste-
huds or taste-bulbs may be seen in sections which pass through the
papillae vallata^ or the papillae fungiformes ; they are also present here
and there in the epithelium of the general mucous membrane of the

Fig. 334.

-TONfiUE OF RABBIT, SHOWING THE SITUATION OF THE
PAPILLAE FOLIAT.K. p.

tongue, especially at the back and sides, and occur also upon the under
surface of the soft palate, and on the epiglottis. But they are most
easily studied in the papillae foliatae of the rabbit, two small oval areas
lying on either side of the back of the tongue and marked transversely
with a number of small ridges or laminae with intervening furrows (see

;:^^%4^^^

.,',>J'<ria

^"^^•iOj:- ez-

Fig. .33.5. — Vertical section of papilla foliata of the rabbit, passing
ACROSS THE FOLI^. (Ranvier. )
p. central lamina of the coriuni ; v, section across a vein, which traverses the folium ;
p' , lateral lamina in which the nerve-fibres run ; <?, taste-bud ; n, sections of nerve-
bundles ; a, serous gland.

fig. 334). Sections across the ridges show numerous taste-buds em-
bedded in the thick epithelium which clothes their sides (fig. 335).

The taste-buds are ovoid clusters of epithelium-cells which lie in
cavities in the stratified epithelium (fig. 336). The base of the taste-

278

THE ESSENTIALS OF HISTOLOGY.

bud rests upon the corium of the mucous membrane, and receives a
branch of the glosso-pharyngeal nerve ; the apex is narrow and com-
municates with the cavity of the mouth by a small pore in the superficial
epithelium {gustatory poi'e, fig. 336, p).

V h

ep

Fig. 336.— a taste-bdd within the stratified epithelium of the

TONGUE. (Sobotta. ) X ,W0.

p, gustatory cells ; s, sustentacular cells ; ep, epithelium ; p, gustatory pore ; h, hairlets.

Fig. 337. — Various cells from tastk-bud of rabbit. (Engelmann. )

600 diameters.

rt, four gustatory cells from central part ; 6, two sustentacular cells, and one gustatory

cell, in connection ; c, three sustentacular cells.

The cells which compose the taste-buds are of two kinds, viz. : 1. The
gustoJory cells (fig. 337, a), which are delicate fusiform or bipolar cells
composed of the cell body or nucleated enlargement, and of two pro-
cesses, one distal, the other proximal. The distal process is nearly
straight, and passes towards the apex of the taste-bud, where it
terminates in a small, highly refracting cilium-like appendage, which
projects into the gustatory pore above mentioned, but the cell-body
does not itself quite reach the pore. The proximal process is more
delicate than the other, and is often branched and varicose. The
nerve-fibres (fig. 338) terminate in ramifications amongst the gustatory
cells (Retzius). 2. The sustentacular cells (fig. 337, c), which are
elongated cells, mostly flattened, and pointed at their ends ; they lie

MUCOUS MEMBRANE OF THE MOUTH.

279

between the gustatory cells, which they thus appear to support, and
in addition they form a sort of envelope or covering to the taste-bud.
Between the cells of the taste-bud lymph-corpuscles are often seen,

Fig. 338.

Fig. 339.

Fig. 338.— Nkrve-endings in taste-buds. (G. Retzius.)

-/!, nerve-fibres ; 6, taste-buds in outline ; i, ending of fibrils within taste-bud ; p, ending

in epithelium between taste-buds ; s, sulcus into which the gustatory pores open.

Fig. 339. — Section of the human (esophagus. (Horsley.)
The section is transverse, and from near the middle of the gullet, a, fibrous covering ;
6, divided fibres of the longitudinal muscular coat ; c, transverse muscular fibres ;
d, submucous or areolar layer ; e, muscularis mucosas ; /, mucous membrane with
papillae ; g, laminated epithelial lining ; k, mucous gland ; i, gland duct ; to', striated
muscular fibres in section.

having probably wandered hither from the subjacent mucous membrane.
Connective tissue fibrils penetrate between the taste-bud and the
stratified epithelium in which it is embedded (Drasch).

The mucous membrane of the moutli is lined by a stratified epi-
thelium into which numerous microscopic vascular and, in some parts,
nerve-containing papillae project. The corium is formed of connective
tissue and contains within and beneath it a large number of small
secretory glands (buccal glands). Most of these secrete mucus, but
some are of the mixed type (see under salivary glands, p. 286) : this is
the case, for example, with the glands of the lips. The ducts of the
buccal glands open everywhere upon the surface of the membrane, and
the openings of the large ducts belonging to the salivary glands are
also seen at certain parts.

280 THE ESSENTIALS OF HISTOLOGY.

The pharynx is composed of a fibrous memhrane which is encircled
by striated muscles, the constrictors, and lined by mucous membrane.
The mucous membrane is covered on its inner surface over the
upper part of the pharynx with ciliated epithelium, which is
continuous with that of the nostrils, and through the Eustachian
tube with that of the tympanum. Below the level of the soft palate
the epithelium is stratified like that of the mouth and gullet, into
which it passes. In certain parts the mucous membrane contains a
large amount of lymphoid tissue, and there are numerous glands
opening on its surface.

The cesophagus or gullet, which passes from the pharynx to the
stomach, consists, like the pharynx, of a fibrous covering, a muscular
coat, a lining mucous membrane, and intervening connective tissue {sub-
mucous or areolar coat) (fig. 339). The muscular coat is much more
regularly arranged than that of the pharynx, and is composed of
striated muscle in about its upper third only, the rest being of the
plain variet3\ There are two layers of the muscular coat — an outer
layer, in which the fibres run longitudinally, and an inner, in which
they have a circular arrangement. The mucous membrane is lined by
a stratified epithelium, into which microscopic papillae from the corium
project. The corium is formed of areolar tissue, and its limits are
marked externally by a narrow layer of longitudinally disposed plain
muscular fibres, the viuscularis mvcosce. This is separated from the
proper muscular coat by the areolar coat, which contains the larger
branches of the blood-vessels and lymphatics, and also the mucous
glands of the membrane. The ducts of these glands are large
and usually pass through a nodule of lymphoid tissue, lymph-cells
from which infiltrate the epithelium of the duct and may pass out
into the lumen of the duct.

Besides these mucous glands, there are met with both at the
upper or laryngeal part of the oesophagus and at the lower or
cardiac end a certain number of small tubulo-racemose glands of
a difi'erent character. They are confined to the mucous membrane,
not penetrating the muscularis mucosae, and their ducts open upon
and not between the papilla of the mucous membrane. They closely
resemble the cardiac glands of the stomach (see fig. 353, p. 290), and
it is frequently found that the epithelium of the surface in the
immediate neighbourhood of their ducts is similar to that lining the
stomach.

There are two gangliated nerve-plexuses, one in the muscular coat^
and one in the submucous coat, like those of the intestine (Klein).

THE SALIVARY GLANDS. 281

LESSOK XXX.
TH?: SALIVARY GLANDS.

1. Section's of the submaxillary gland (dog). The gland may be hardened
in alcohol or formol and .>^tained with hsematoxylin-eosin or with iron lijema-
toxylin by Heidenhain's method. Notice the acini filled with clear (mucus-
secreting) cells, the nuclei of which usually lie near the basement-membrane.
Notice here and there, outside the clear cells, demilunes or crescents of small
darkly stained granular-looking (albuminous i cells. Ob.serve also the sections
of the ducts with their striated columnar epithelium. If possible find a
l)lace where one of the ducts is passing into the alveoli. Sketch under a
high power.

2. Study sections of the parotid and sublingual glands prepared in a
similar way, and notice the diflferences between the three glands.

3. Examine small pieces of both submaxillary and parotid gland of the
dog fresh in 2 per cent, salt solution. In the submaxillary gland notice
that the alveolar cells are swollen out with large granules or droplets of
raucigen, which swell up in water to form large clear vacuoles. Dilute
acids and alkalies produce a similar change but more rapidly. The cells of
the parotid gland are also filled with granules, but they are smaller. The
granules are also swollen up and dissolved by these fluids. Make a sketch
from each preparation under a high power.

4. To study the changes which the alveolar cells undergo during secretion,
pilocarpine is administered to an animal in sufficient amount to produce
copious salivation ; after half an hour the animal is killed and its salivary
glands are examined as in preparation 3. The granules are not seen in pre-
parations that have been in alcohol, but osmic acid preserves them moderately
well ; they are well seen in sections stained by Muir's eosin-methyleue blue
method (see Appendix).

The salivary glands may be looked upon as typical of secreting
glands in general. They are composed of a number of lobules bound
together loosely by connective tissue. Each small lobule is formed
of a group of irregularly saccular or tubular alveoli or acini from which
a duct passes, and this, after uniting with other ducts, eventually leaves
the gland to open upon the surface of the mucous membrane of the
mouth.

The alveoli are inclosed by a basement-membrane, which has
flattened branched cells on its inner surface, next to the epithelium
(fig. 340). It may be shown by teasing the fresh gland substance in
water (Langley). This basement-membrane is continued along the
ducts. Within it is the epithelium, which in the alveoli is composed
of polyhedral cells (fig. 341, a), but in the ducts is regularly columnar.

282

THE ESSENTIALS OF HISTOLOGY.

except in that part of the duct which immediately opens into the
alveoli (junctional part) ; in this it is flattened (d'). The columnar
epithelium of the ducts is peculiar, in that the cells show a distinction
into two unequal zones, an outer, larger, striated zone, and an inner,
smaller, granular one (fig. 341, d).

Fig. 340. — Membrana propria op two alveoli, (v. Ebner. ) x600.
The preparation is taken from a mucous gland of the rabbit.

The cells of the alveoli diff"er according to the substance they secrete.
In alveoli which secrete mucus, such as all the alveoli of the dog's
submaxillary (fig. 341), and some of the alveoli of the same gland in

^-s^fe-

Fig. 34 L— Section of the submaxillary gland of the noG, showing the
COMMENCEMENT OF A DUCT IN THE ALVEOLI. (Magnified 425 diameters. )

o, one of the alveoli, several of which are in the section shown grouped around the com-
mencement of the duct d' ; a' , an alveolus, not opened by the section ; b, basement-
membrane in section ; c, interstitial connective tissue of the gland ; d, section of a
duct which has passed away from the alveoli, and is now lined with characteristically
striated columnar cells ; s, semilunar group of darkly stained cells at the periphery
of an alveolus.

man (fig. 344), the cells, if examined in normal saline solution or after
hardening with alcohol, are clear and swollen. But if examined
rapidly in serum, or in solutions of salt of from 2 to 5 per cent.,

THE SALIVARY GLANDS.

283

they are often seen to be occupied hy large and distinct granules
(Langley). These granules can also be rendered visible by certain
methods of staining, when it is apparent that they are not present as
such in all the cells, but have in many cells become clear and swollen,
and converted into a substance which is known as mucigen (fig. 346, a).

Fig. 342.— Section of a dog's submaxillary, after a prolonged period of

REST. (Ranvier.)
I, lumen of alveolus ; (j, mucus-secreting cells ; c, crescent, formed of albiiiwinous cells.

Similar granules are seen also in the cells lining the gland ducts ;
here also they are found to vary in size and number with the condition
of activity of the gland (fig. 348). The mucigen is dissolved out
of the cell and discharged as mucus into the lumen of the alveolus and

',' ^f^

Fig. 343. — Submaxillary of dog, after a period of activity. (Ranvier.)

The mucus-secreting cells, g, have discharged their secretion, and are smaller and stain

better ; the albuminous cells of the crescents, c, are enlarged.

into the ducts, when the gland is stimulated to activity. The cells
are known as mucous cells. But in most alveoli there are some cells
which do not contain mucigen, but small albuminous granules, and
these often form crescentic groups which lie next to the basement-
membrane (figs. 341, s, 342, c). These are the so-called crescents of

284

Fig. 344. —Section of part of the human submaxillary gland.
(Heidenhain.)

To the right of the figure is a grouj) of mucous alveoli ; to the left a group of serous

alveoli.

Fig. 345.— Alveoli of a serous gland. A, at rest. B, after a short period

OF activity. C, after a prolonged period of activity. (Langley.)

In A and B the nuclei are obscured by the granules of zymogen.

Fig. 346. —Mucous cells from fresh submaxillary glands of the dog.

(Linjgley.)

o, from a resting or loaded gland ; b, from a gland which has been secreting for some time ;

a', b', similar cells which have been treated with dilute acid.

THE SALIVARY GLANDS.

285

Gianuzzi ; their constituent cells are known also as nuirginal or serous
cells. Special diverticula pass from the lumen of the alveoli between
the mucous cells to penetrate to the crescents and to branch amongst
and within their constituent cells ; these diverticula are best shown by

i''"£)..^ "•-.*»>''

Fig. 347. — Submaxillary gland of rabbit. (E. Miiller.)
The cells, which are all serous, are in different functional states, as indicated by the
condition and staining of the granules. «, cell filled with darkly staining granules ;
b, clear cell ; c, secretory canaliculi penetrating into the cells.

the Golgi method of staining (figs. 349, 350). They also occur in the
purely serous alveoli (fig. 347), in which none of the cells secrete mucus,
but watery or albuminous saliva. In these when the gland has been long
at rest the cells are filled with granules, which do not swell with water

Fig. 348. — Cells from duct of parotu).
a, prior to secretion ; B, after secretion (Mislawski and Smirnow).

nor form mucin; they appear to be albuminous in nature, and prob-
ably yield to the secretion of the gland its ferment (ptyalin) and its
albumin. The granular substance within the cell is not the ferment,
but the ferment is formed from it when the secretion is poured out.
Hence it has been termed zymogen (mother of ferment). As Langley

286

THE ESSENTIALS OF HISTOLOGY.

showed, the outer part of each cell becomes clear and free from
granules after secretion (fig. 345).

Ill nearly all animals the parotid glands are composed of purely serous
alveoli : in man and most animals the submaxillary and sublingual glands
have both serous and mucous alveoli or "mixed" alveoli, i.e. alveoli con-
taining both serous and mucous cells. The smaller detached anterior parts
of the sublingual gland have purely mucous alveoli.

Fig. 34;). — Alveoli of human scblin-

gttal glakd prepared bv golgi

METHOD. (E. Miiller. )

I, lumen stained, with lateral diverticula

passing between mucus-secreting cells ;

fi, longer diverticula penetrating into the

"crescent " cells.

Fig. 3.50. — Alveoli of the submaxil-
lary GLAXD OF THE DOG. (G.
Retzius. ) Golgi method.
The extensions of the lumen into the
crescents of Gianuzzi are shown, and
also the endings of nerve-fibrils.

The largest ducts have a wall of connective tissue outside the
basement>membrane, and also a few plain muscular cells. The blood-
vessels of the salivary gland form a capillary network around each
alveolus. The lymphatics commence in the form of lacunar vessels
between the alveoli. Lymphoid nodules are occasionally found in
the interstitial connective tissue. The nerve-fibres, which are derived
both from the cerebro-spinal nerves and from the sympathetic, pass
through ganglia before proceeding to their distribution. They
ramify as fine varicose fibrils amongst the alveolar cells (fig. 350), and
many are distributed to the blood-vessels.

The salivary glands are developed as buds from the epithelium of the
buccal cavity, at first solid but becoming gradually hollowed out. To
begin with they are simple, but undergo ramification as they grow into
the mucous membrane and submucous tissue.

THE STOMACH. 287

LESSON XXXI.
THE STOMACH.

1. Vertical longitudinal sections through the cardia, including the lower
end of the oesophagus and the adjacent cardiac portion of the stomach.
These are intended to show the abrupt transition of the sti'atified epithelium
of the oesophagus into the columnar epithelium of the stomach, and also the
character of the gastric and oesophageal glands in the immediate neighbour-
hood of the cardia. The sections may be stained with hsematoxylin and eosin.

2. Sections of the fundus of the stomach, cut perpendicularly to the
surface of the mucous membrane.

In these sections the general arrangement of the coats of the stomach is to
be studied. Sketches are to be made under a low power illustrating this
arrangement, and others under a high power showing the structure of the
glands of the mucous membrane.

Measure the whole thickness of the mucous membrane, the thickness of
the muscular coat, the size of the columnar epithelium-cells of the surface,
and that of the cells in the deeper parts of the glands.

3. Sections of the mucous membrane of the fundus, cut parallel to the
surface.

These sections will show better than the others the arrangement of the
cells in the glands.

4. Vertical sections of the mucous membrane from the pyloric region
of the stomach. If the section is taken longitudinally through the
pylorus, the transition of the gastric glands into the glands of Brunner
of the duodenum will be made manifest. Make a sketch under a low power
of one of the glands in its whole length, filling up some of the details with
the high power.

5. Study the arrangement of the blood-vessels of the stomach in vertical
sections of the wall of an organ the vessels of which have been injected.

I

The wall of the stomach consists of four coats, which, enumerated
from without in, are as follows, viz. : serous, muscular, areolar, or sub-
mucous, and mucous membrane (fig. 351).

The serous coat is a layer which is derived from the peritoneum. It
is deficient only along the lines of the lesser and greater curvatures.

The muscular coat consists of three layers of plain muscular fibres.
Of these the bundles of the outer layer run longitudinally, those of
the middle layer circularly, and those of the inner layer obliquely.
The longitudinal and circular bundles become thicker and stronger
towards the pylorus ; at the pylorus itself the circular layer is greatly
thickened to form a sphincter muscle. The oblique fibres are only
present over the fundus.

The areolar or submucous coat is a layer of areolar tissue, which serves-

288 THE ESSENTIALS OF HISTOLOGY.

to unite the mucous membrane loosely to the muscular coat ; in it

ramify the larger branches of the blood-
vessels and lymphatics.

The mucous membraue is a soft thick
layer, generally somewhat corrugated in
the empty condition of the organ. Its
inner surface is covered by columnar-
shaped epithelium cells, all of which
secrete mucus. They are prolonged into
the ducts of the glands, but when these
divide to form the tubules the cells be-
- "^^ come cubical, and lose their mucus-
secreting character. The thickness of
the mucous membrane is due to the fact
.. ^^ that it is largely made up of long tubular
\ glands, which open upon the inner sur-

^_ face. Between the glands the mucous

-^ membrane is formed of retiform with

fc. y" some lymphoid tissue. Externally it is

^^1 ' : - J bounded by the vuKCuIaris mucosce, which

&:';.- - ' ' consists of an external longitudinal and

S^_ \ \c.n.. an inner circular layer of plain muscular

J^- V '- - - fibres.

Wt-" ' :. Gastric glands. — These are formed of a

J '■'■ basement-membrane lined with epithelium.

%-- ; Each gland consists of secreting tubules from

|i ^-m. one to four in number, opening at the

surface into a larger tube, the duct of the

' '" ' '" "' — -'—^^^=^^- gland. The duct is in all cases lined by

%'hhS^ ^f^c^AxT oT™f"^"C"««ecreting epithelium of the same
STOMACH. (Mall.) character as that which covers the inner

m, mucous membrane ; e, epithelium ; n c ai i -l ^ ii.

rf, orifice of gland-duct ;/».,/(., muscu- surtace ot the mucous membrane, but the

laris mucosa;: sm., submucosa : cm.. -.it i? ^i i.- j. i_ i

circular muscular layer ; Lni./longi- epithehum Ot the SCCretlUg tubulCS IS

tudmai muscular layer; ., serous different from this, and also differs some-

what in the glands of diti'erent regions of
the organ. The following varieties of gastric glands are met with : —

(1) Glands of the cardia. — These are found in man close to the oeso-
phageal opening or cardia ; they are of two kinds : (a) simple tubules,
very similar to the crypts of Lieberkiihn of the intestine and (b) small
tubulo-racemose glands (fig. 353). The secreting tubules of the race-
mose glands are lined by cells which are granular in appearance and of
-a short columnar form, and of the same nature throughout the length

THE STOMACH.

'289

of the tubule, except near the orifice (duct), where they give place to
columnar mucus-secreting cells. Occasionally one or two oxyntic cells
may be present in their tubules.

(2) Ghouls of the fundus (oxi/ntic glands) (figs. 354, .355). — In these
glands the tubules are long and the duct short. The epithelium of the
tubules is composed of two kinds of cells. Those of the one kind,

^^^Hf"

Fig. 352. — Section of the junction of the (esophageal and gastric mucous
MEMBRANE OF THE KANGAROO. (135 diameters.)

S, stratified epithelium of tesophagus abruptly discontinued at S' ; c, columnar epi-
thelium of gastric mucous membrane ; d, orifices or ducts of cardiac glands ; m,
corium of oesophageal mucous membrane sending papillie into the eijithelium;
rn', corium of gastric mucous membrane.

which form a continuous lining to the tubule, are somewhat poly-
hedral in shape, and in stained sections look clearer and smaller
than the others, but in the fresh glands, and with certain methods
of staining, it can be seen that they are filled with granules
(fig. 355). The granules are most numerous at the inner part of
the cell, an outer zone being left clear. After prolonged activity this
outer zone increases in size while the granules diminish in number as

T

290 . THE ESSENTIALS OF HISTOLOGY.

in the analogous cases of the pancreas and parotid glands (Langley).
The cells are believed to form pepsin, and are termed the cMef cells
of the cardiac glands, or from their relative position in the tubule
immediately surrounding the lumen, the central cells. Scattered along
the tubule, and lying between the chief cells and the basement-mem-
brane, are a number of large spheroidal or ovoidal cells. These
are the parietal or ozyntic cells} Each parietal cell is surrounded and

.-t:^^ '

^5

-iscs^ms^--

FiG. 353. — Section of human STO^rACH near the cardia. (v. Ebner, after

J. Schafifer.) x4o.
c, cardiac glands ; d, their ducts ; cr, glands similar to crypts of Licberklihn, with goblet

cells; inm, mucous membrane; ia, muscularis mucosae; !«', muscular tissue within

mucous membrane.

penetrated by a network of minute passages, communicating Avith the
lumen of the gland b}^ a fine canal, which passes between the central
cells (fig. 356) ; but in the neck of the gland the parietal cells abut
against the lumen, being here wedged in betAveen the mucus-secreting
cells (fig. 354, A).

(3) Glands of the pyloric canal (fig. 357). — In the glands of the pjdoric
canal the ducts are much longer than in those of the fundus, and the
secreting tubules possess cells of only one kind.- These correspond to

^ So called because they are believed to produce the acid of the gastric
secretion.

- In man it is only quite near the pylorus that oxyntic cells are altogether
absent.

GASTRIC GLANDS.

291

t

Fia. 354. — Sections of the mucous membrane ov the fundus of the dog's

STOMACH passing ACROSS THE LONG AXIS OF THE GLANDS.

A, Section close to but not quite parallel with the surface, and including on the left

the gland ducts and on the right the commencing gland tubules. Notice the oxyntic
cells beginning to appear between the columnar cells of the ducts.

B, Deeper part of the same section, showing the lumina of the gland tubules sur-

rounded by chief cells, with the oxyntic cells altogether outside them.

292

THE ESSENTIALS OF HISTOLOGY.

the chief cells of the fundus glands, but are not quite identical with
them in appearance, the granules being much less distinct. The cells
are of a columnar or cubical shape, and in the fresh condition of a
granular appearance ; quite unlike the clear columnar epithelium of the

Fig. 355.

Fig. 356.

Fig. 357.

Fig. 355. — A fundus gland of simple form from the bat's stomach.
Osinic acid preparation. (Langlej*.)
c, columnar epithelium of the surface ; n, neck of the gland witli central and parietal
cells ; ./', base, occupied only by principal or central cells, which exhibit the gi'anules
accumulated towards the lumen of the gland.

Fig. 356. —A fundus gland prepared by golgi's method, showing the

MODE OF communication OF THE PARIETAL CELLS WITH THE GLAND-LUMEN.

(E. Muller.)

Fig. 357. — A pyloric gland, from a section of the dog's stomach. (Ebstein.)
in, mouth ; n, neck ; tr, a deep portion of a tubule cut transversely.

surface, which is formed, as elsewhere, of long tapering cells, the outer
part of which is filled with mucigen.

At the pylorus itself the gastric glands become considerably
lengthened and enlarged, and are continued into the submucous
tissue, the muscularis mucosae being here deficient ; they thus

GASTRIC GLANDS.

293

Fig. 358.— Section through the pylorus, including the commencement of
THE duodenum. (Klein.)

r, villi of duodenum ; /), apex of a lymphoid nodule ; c, crypts of Lieberktilin ; s, secreting
tubules of Brunner's glands ; d, ducts of pyloric glands of the stomach ; g, tubes of
these glands in mucous membrane ; (, deeper lying tubes in submucosa, correspond-
ing to secreting tubules of Brunner's glands of duodenum ; m, muscularis mucosae.

I'' if) iYmTt7«rTf4iU,\ ;y, ) h ,

Fig. .359.— Plan of the blood-
vessels OF THE STOMACH.
(Modified from Brinton. )

a, small arteries passing to break up
into the fine capillary network, d,
between the glands ; b, coarser
capillary network around the
mouths of the glands; c, c, veins
passing vertically downwards fron^
the superficial network ; e, larger
vessels in the submucosa.

Fig. 360.— Lymphatics op the human gastric

MUCOUS membrane, injected. (C. Lov^n.)
The tubules are only faintly indicated ; fi, muscularis

mucosas ; b, plexus of fine vessels at base of glands ;

c, plexus of larger valved lymphatics in submucosa.

294 THE ESSENTIALS OF HISTOLOGY.

present transitions to the glands of Brunner, which lie in the sub-
mucous tissue of the duodenum (fig. 358).

Scattered amongst the ordinary secreting cells of the pyloric glands, cells
are seen here and there which stain differently from the rest, and probably
have a different function (Stohr). Occasionally oxyntic cells are met
with in the pyloric glands and even in Bruinier's glands in the duodenum
(Kaufmann).

The blood-vessels of the stomach are very numerous, and pass to the
organ along its curvatures. The arteries traverse the muscular coat,
giving off branches to the capillary network of the muscular tissue,
and ramify in the areolar coat. From this, small tortuous arteries
pierce the muscularis mucosae, and break up into capillaries near the
bases of the glands (fig. 359). The capillary network extends between
the glands to the surface, close to which it terminates in a plexus of
relatively large venous capillaries which encircle the mouths of the
glands. From this plexus straight venous radicles pass through the
mucous membrane, pierce the muscularis mucosae, and join a plexus of
veins in the submucous tissue. From these veins blood is carried
away from the stomach by efferent veins, which accompany the enter-
ing arteries.

The lymphatics (fig. 360) arise in the mucous membrane by a plexus
of large vessels dilated at intervals, and looking in sections like clefts
in the interglandular tissue. From this plexus the lymph is carried
into large valved vessels in the submucous coat, and from these,
efferent vessels run through the muscular coat to reach the serous
membrane, underneath which they pass away from the organ. The
muscular coat has its own network of lymphatic vessels. These lie
between the two principal layers, and their lymph is poured into
the efferent lymphatics of the organ.

The nerves have the same general arrangement and mode of distribu-
tion as those of the intestine (see next Lesson).

THE SMALL AND LARGE INTESTINE. 295

LESSONS XXXII. AND XXXIII.
THE SMALL AND LARGE INTESTINE.

1. Sections of the duodenum, jejunum, and ileum, vertical to the surface.
The three parts of the intestine may be embedded in the same paratfin block,
anil the sections stained and mounted together. Choose a part of the ileum
which includes a Peyer's patch. Observe the nodales of lymphoid tissue
which constitute the patch and which extend into the submucous tissue.
Observe the lymphoid cells in the superjacent epithelium. Notice also the
sinus-like lymphatic or lacteal vessel which encircles the base of each nodule.
In the duodenum study the glands of Eruuner in the submucous tissue.
Make a general sketch of each section under a low power and draw a villus
under the high power. The general arrangement and structure of the
intestinal wall is to be studied in these sections.

2. Sections parallel to the surface of the intestine, and therefore across
the long axis of the villi and glands of the mucous membrane. In order
to keep the sections of the villi together so that they are not lost in the
mounting, it is necessary either to embed in ceiloidin or, if paraffin be used,
to employ an adhesive method of mounting.

In this preparation, sketch the transverse section of a villus and of some
of the crypts of Lieberklihn.

3. To study the process of fat-absorption, kill a fi'og two or three days
after feeding with bacon fat. Put a very small shred of the mucous mem-
brane of the intestine into osmic acid (0'5 per cent.) and another piece into
a mixture of 2 parts Miiller's fluid and 1 part osmic acid solution (1 per
cent.). After foity-eight hours teased preparations may be made from the
osmic acid preparation, in the same manner as directed in Lesson VIII., § 1.
The piece in MLiller and osmic acid may be left for ten days or more in the
fluid. When hardened, sections are made by the freezing method and
mounted in glycerine.

4. Sections of small intestine the blood-vessels of which have been injected.
Notice the arrangement of the vessels in the several layeis. Sketch carefully
the vascular network of a villus.

5. From a piece of intestine which has been stained with chloride of gold
tear ofi" broad strips of the longitudinal muscular coat, and mount them in
glycerine. It will generally be found that portions of the nervous plexus of
Auerbach remain adherent to the strips, and the plexus can in this way easily
be studied.

From the remainder of the piece of intestine tear off with forceps the fibres
of the circular muscular layer on the one side, and the mucous membrane on
the other side, so as to leave only the submucous tissue and the muscularis
mucosae. This tissue is also to be mounted flat in glycerine : it contains the
plexus of Meissner.

Sketch a small portion of each plexus under a high power. The plexuses
can also be studied by the methylene-blue and reduced silver methods (see
Appendix).

6. Sections of the large intestine, perpendicular to the surface. These will

296

THE ESSENTIALS OF HISTOLOGY.

show the general structure and arrangement of the coats. Sketch under a
low power.

7. Sections of the mucous membrane of the large intestine parallel to the
surface, and therefore across the glands. Sketch some of the glands and the
interglandular tissue under a high power.

8. The arrangement of the blood-vessels of the large intestine may be
studied in sections of the injected organ.

The wall of the small intestine consists, like that of the stomach,
of four coats (fig. .361).

The serous coat is complete except over part of the duodenum.

gmy

gls

mm

rm

Fig. 361. — Diagram of sjxtion of aliment-^ry tube. (Sobotta.)
Z, lumen ; rjini, glands of mucous membrane ; tp, epithelium ; gls, glands in subniucosa ;
mm, muscularis mucosse ; sm, submucous coat ; ria, circular muscular layer ; Im, longfi-
tudinal muscular layer ; s, serous coat ; ss, mesentery ; gray, ganglion of plexus
inyentericus ; gsm, ganglion of plexus submucosus.

The muscular coat is composed of two layers of muscular tissue, an
outer longitudinal and an inner circular. Between them lies a network
of lymphatic vessels and also the close gangliated plexus of iion-
medullated nerve-fibres known as the plexus myentericus of Auerbach.
The ganglia of this plexus may usually be seen in vertical sections of
the intestinal wall (in figs. 365, 369), but the plexus, like the one
in the submucous coat immediately to be described, can only be

THE SMALL INTESTINE. 297

properly displayed in preparations made with chloride of gold (fig.
362) or methylene blue or by Golgi's method.

The subimccous coat is like that of the stomach ; in it the blood-ve-ssels
and lacteals ramify before entering or after leaving the mucous mem-
brane, and it contains a gangliated plexus of nerve-fibres — the plexus
of Meissner — which is finer than that of Auerbach and has fewer

Fig. 362. — Auerbach's plexds, from the muscular coat of the intestine.

(Cadiat. )

ganglion cells (fig. 363). Its branches are chiefly supplied to the
muscular fibres of the mucous membrane, but also to the glands and
villi (fig. 364).

The mucous membrane is bounded next to the submucous coat by a
double layer of plain muscular fibres {muscularis mucosce). Bundles
from this pass inwards through the membrane towards the inner
surface and penetrate also into the villi. The mucous membrane
proper is pervaded with simple tubular glands — the cri/pfs of Lieberkichn
(figs. 365, 366, 369) — which are lined throughout by a columnar

298

THE ESSENTIALS OF HISTOLOGY.

Fig. 363. — Meissner's plexus from the submucous coat. (Cadiat.)
a, gauglion ; b, b, nervous cords ; c, a blood-vessel ; d, an entering lymphatic nerve.

Fig. 364. — Nerves of the mucous membrane of the small intestine. (Cajal.

M, part of Meissner's plexus ; n-f, small nerve-cells and nerve-fibres in the tissue of the
mucous membrane and villi.

THE SMALL INTESTINE.

299

epithelium, with scattered goblet cells, like that which covers the
general surface aiul the villi. At the fundus of each crypt are a few
cells containing well-marked granules (Paneth). The cells of the
glands show frequent mitoses, and it is believed that the epithelium of

A'.l^iiii* musculans
muscosa

ubimtcosa

--•---■ -;^

layer of
circular
muscular fibres

;^:

intermuscular
layer

layer of
longitudinal
muscular fibres

Fig. 365. —Section of the small intestine (jejunum) of cat.
(Magnified about 40 diameters.)

the general surface becomes regenerated from them (Bizzozero). The
mucous membrane between these glands is mainly composed of
reticular tissue, which contains here and there nodules of lymphoid
tissue. These nodules constitute when they occur singly the so-called
solitary glands of the intestine (fig. 368), and when aggregated together

300

THE ESSENTIALS OF HISTOLOGY.

form the agminated glands or patches of Peyer (fig. 374).
occur chiefly in the ileum.

The latter

Fig. 367.— Cross-section ok a small
fragment of the mucous membrane
of the intestine, including one entire
crypt of lieberkuhn and parts ok
THREE OTHERS. (Magnified 400 dia-
meters.) (Frey.)

a, cavity of the tubular glands or ci-ypts ; 6, one
of the lining epithelium-cells ; c, the inter-
glandular retifurm tissue ; 0., lymph-cells.

Fig. 366. — A crypt of Lie-

BERKUHN FROM THE HUMAN

intestine. (Flemming.)

Fig. 368. — Section of the ileum through a lymphoid nodule. (Cadiat.)
n, middle of the nodule with the lymphoid tissue jiartly fallen away from the section ;

b, epithelium of the intestine ; c, villi : their ei^ithelium is partly broken away ; d,

crypts of Lieberkiihn ; e, /, muscularis mucosas.

TEE SMALL INTESTINE.

301

The glands of Brumm; which have been already noticed (p. 294),
occur in the duodenum. They are small tubulo-racemose glands of

inds of
unner
sabmucoso.

circular
muscular layer

**,— '-~^'*'''*S.'^~»«

Fig. 369. — iSKCTiox ok duodenum ok cat, showing Beunnee's glands.
(Magnified about 60 diameters.)

serous character, situated in the submucosa (fig. 369) ; they send their
ducts to the inner surface of the mucous membrane either between
the crypts of Lieberkiihn or into them.

302 THE ESSENTIALS OF HISTOLOGY.

The villi with which the whole of the inner surface of the small
intestine is closely beset are clavate or finger-shaped projections of the
mucous membrane, and are composed, like that, of retiform tissue
covered with columnar epithelium (figs. 370 to 372). The characters
of this epithelium have already been described (Lesson VIII.).
Between and at the base of the epithelium-cells many lymph-
corpuscles occur, as well as in the meshes of the retiform tissue.

li

I

?v

Fig. 370. — Longitudixal section of a villus from a kat killed three hours
after feeding with bread and water.

The columnar epithelium shows numerous lymph-corpuscles between the cells ; I, lacteal,
containing lymph-corpuscles, c, some partly disintegrated.

The epithelium rests upon a basement-membrane. In the middle of
the villus is a lymphatic or lacteal vessel which may be somewhat
enlarged near its commencement, but the enlargement is replaced in
some animals by a network of lacteals. Surrounding this vessel are
small bundles of plain muscular tissi;e prolonged from the muscularis
mucosae. The network of blood-capillaries (figs. 370, 373) lies for the
most part near the surface within the basement-membrane; it is
supplied with blood by a small artery which joins the capillary
network at the base of the villus ; the corresponding vein generally
arises near the extremity.

THE SMALL INTESTINE.

303

■/"

— />

Fig. 371.— Transverse section of x villus, man. (v. Ebner.) Magnified
530 diameters.
a, basement-raeinbrane, here somewhat shrunken away from epithelium ; 6, goblet-
cells ; c, cuticula ; «/, lacteal ; c, columnar epithelium ; /, leucocytes in epithelium ;
r, leucocytes below epithelium ; /", large leucocytes ; g, blood-vessels.

Fig. 372.— Part of a section through a villus op the dog, highly
MAGNIFIED. (R. Heidenhain.)
jii, plain muscle ; I, V, I", leucocj'tes, which are seen in large numbers in the interstices
of the reticular tissue ; hi, vessels ; c, connective-tissue cells, covering the fibrils of
the reticulum. The epithelium of the villus is not represented.

304

THE ESSENTIALS OF HISTOLOGY.

The lymphatics (lacteals) of the mucous membrane (fig. 374), after
receiving the central lacteals of the villi, pour their contents into a
plexus of lai'ge valved lymphatics which lie in the submucous tissue
and form sinuses around the bases of the lymphoid nodules (fig. 256,

Fig. 373.— Small intestin'e (vertical transverse section), with the
BLOOD-VESSELS INJECTED. (Heitzmanii. )

F, a villus ; G, glands of Lieberkuhn ; M, muscularis mucosse; A, areolar coat ; R, circular
muscular coat; L, longitudinal muscular coat; P, peritoneal coat.

p. 208). From the submucous tissue eff"erent vessels pass through the
muscular coat, receiving the lymph from an intramuscular plexus of
lymphatics, and are conveyed away between the layers of the mesentery.
Absorption of fat. — In order to study the process of fat transference
in the intestine, it is convenient to stain the fat with osmic acid, which

ABSORPTION OF FAT.

305

^ A

r'

ML

Fig. 374.— Vertical section of a portion of a Peyer's patch, with the
LACTEAL VKS.SELS INJECTED. Magnified 32 diameters. (Frey.)

The specimen is from the lower part of the ileum ; a, villi, with their lacteals left white ;
6, some of the tubular glands ; c, the muscular layer of the mucous membrane ;
d, cupola or projecting part of the nodule ; e, central part ; /, the reticulated lacteal
vessels occupying the lymphoid tissue between the nodules, joined above by the
lacteals from the villi and mucou.s surface, and passing below into .a, the sinus-like
lacteals under the nodules, which again jmss into the large efferent lacteals, (f ; i,
part of the muscular coat.

Fig. 375.— Section of the villu.s of a r.\t killed during fat-absorption.

ep, epithelium ; .s(r, striated border ; c, leucocytes ; d , leucocytes in the epithelium ;
i, central lacteal containing chyle and disintegrating leucocytes.

u

306

THE ESSENTIALS OF HISTOLOGY.

colours it black. It can then be observed that in animals which have
been fed with food containing fat, particles of fat are present (1) in
comparatively large globules within the cohxmnar epithelium-cells ; (2)
in fine granules in the interstitial tissue of the villus, but often confined
to the amoeboid leucocytes, which abound in this tissue ; (3) in fine
granules within the central lacteal of the villus. The leucocytes are
present not only in the reticular tissue of the villus, but also in con-
siderable numbers between and at the base of the epithelium-cells ; and
they can also be seen in thin sections from bichromate-osmic prepara-
tions within the commencing lacteal ; in the last situation they are
undergoing disintegration (figs. 375, 376). These observations are
easily made in the frog.

Fig. 376. — Mucous meiibr.\ne of frog's intesti.ne during fat- absorption.

ep, epithelium ; sir, striated border ; e, leucocytes ; I, lacteal.

Since the leucocytes are amoeboid, it is probable from these facts
that the mechanism of fat-absorption consists of the following processes
— viz. (1) absorption or formation of fat in the columnar epithelium-
cells of the surface ; (2) ejection of fat-granules from the epithelium
into the tissue of the villus; (3) inception of fat by leucocytes,
these taking it up after it has passed out of the epithelium-cells ; (4)
migration of leucocytes carrying fat particles through the tissue
of the villus and into the central lacteal; (5) disintegration and solution
of the immigrated leucocytes, and setting free their contents. Since
fat-particles are never seen in the striated border of the columnar
cell it is probable that the fat first becomes saponified by the action of
the digestive juices, and reaches the epithelium-cell in the form of
dissolved soap ; the fat which is seen and stained by osmic acid within
the cells having become re-formed by a process of synthesis.

In some young animals (puppy, kitten) the fat which is undergoing
absorption is seen not only in the epithelium-cells and leucocytes, but
also in the form of streaks of liquid, stained black by osmic acid, in the
interstices of the reticular tissue of the villi. It has probably passed

THE LARGE INTESTINE.

307

k-.^siS^^ -r:i&«^"»% ^^^- .

~%J

7k,'

Fig. 377.— Glands of the large inte.stine of child. (300 diameters.)
A, in longitudinal section ; B, in transverse section.

308 THE ESSENTIALS OF HISTOLOGY.

out from the epithelium in a dissolved condition by a kind of reversed
secretion.

The migration of leucocytes into the lacteals of the villi is not
a special feature of absorption of fat, but occurs also when absorp-
tion of other matters is proceeding (fig. 370) ; the transference of
fat-particles is merely a part of a more general • phenomenon of
migration of leucocytes which accompanies the process of absorption.

The large intestine has the usual four coats, except near its termina-
tion, where the serous coat is absent. In man the mnscular coat is
peculiar in the fact that along the ca?cum and colon the longitudinal
muscular fibres are gathered up into three thickened bands which
produce puckerings in the wall of the gut.

The mucous membrane of the large intestine is beset with simple
tubular glands somewhat resembling the crypts of Lieberkiihn of the
small intestine, and lined by columnar epithelium similar to that of
the inner surface of the gut, but containing many more mucus-secreting
or goVjJet-cells (fig. 377). The blind extremity of each gland is usually
slightly dilated. These glands of the large intestine are not strictly
homologous with the crypts of the small intestine, for whereas the
latter are developed as depressions in the general surface between the
villi, the glands of the large intestine are formed by the growing
together of villus-like projections of the surface. The interglandular
tissue is a reticular tissue and is beset here and there with solitary
glands, especially in the ca;cum. The mucous membrane of the
vermiform appendix is in great part of its extent packed full of
lymphoid nodules.

The arrangement of the blood-vessels and lymphatics in the large
intestine resembles that in the stomach. The nerves of the large
intestine also resemble those of the stomach and small intestine in
their arrangement.

At the lower end of the rectum the circular muscular fibres of the
gut become thickened a little above the anus to form the internal
sphincter muscle. In the anal region there are a number of compound
racemose mucous glands opening on the surface of the mucous mem-
brane (anal glands). The anus has a lining of stratified epithelium
continuous with that of the skin.

THE LIVER. 309

LESSONS XXXIV. AND XXXV.
THE LIVER AND PANCREAS.

1. Sections of liver are to be studied carefully. They may be stained with
eosiu and hasmatoxylin ; or by eosin and niethyleiie-blue after Muir's
method (see Appendix). Sketch the general arranoement of the cells in a
lobule under the low power; Hud under the high power make very careful
drawings of .some of the hepatic cells and also of a portal canal. If from the
pig, the outlines of the lobules are observed to be very well marked.

Notice that the hepatic cells are in intimate contact with rhe blood-capil-
laries or sinusoids. Some cells may be found to contain red blood-corpuscles ;
many are filled with eosinophil granules. Notice in the sinusoid capillaries
the large partly detached endothelial cells (cells of Kuptfer). These also
frequemly contain erythrocytes, which ajjpear to be in process of destruction.

2. To observe the glycogen within the liver-cells, kill a rabbit or rat (pre-
ferably about six hours after a full meal of carrot), and at once throw a thin
piece of the liver into 9(5 per cent, alcohol. When well hai-dened the piece
may be embedded in paraffin in the usual way, or sections may be cut with
the free hand without embedding. Some of the sections so obtained are to
be treated with a 1 per cent, sulution of iodine in potassium iodide for five
minutes ; they may then be mounted in a nt-arly saturated solution of potas-
sium acetate, the cover-gl;i.ss being cemented with gold size.

3. Presence of iron. Other sections of alcohol-hardened liver are to be
treated first with potassium ferrocyanide solution and then with hydrochloric
acid and alcohol (1 to 10), passed through absolute alcohol into xylol, and
mounted in dammar ; in these many of the pigment granules will be
stained blue (Prussian blue). Or the sections may simply be placed in an
aqueous solution of hsematoxylin (1 to 300), with or without previous treat-
ment with alcohol containing 10 parts per cent, hydrochloric acid (to set
free organically combined iron), after which they are mounted in the ordinary
way (Macallum's method).

■ 4. Study, first with the low power and afterwards with the high power,
a section of the liver in which the blood-vessels have been injected. Almost
invariably the injection will be found to have penetrated into canaliculi
within the liver-cells themselves. Make a general sketch of a lobule under
the low power and liraw a small part of the network of blood-vessels and
intracellular canaliculi under the high power.

5. Take a small piece of liver which has been several weeks in 2 per cent,
bichromate of potassium solution or Mliller's fluid and plunge it in 1 per
cent, nitrate of silver solution, changing the fluid after half an hour. Leave
the piece of liver in the silver solution overnight. It may then be trans-
ferred to alcohol, and after complete dehydration embedded and cut in
parafiin in the usual way and the sections mounted in dammar. In many
parts of such sections the bile-canaliculi are stained.

They can also be brought to view (at the periphery of the lobules) by
injection with solution of Berlin blue from the hepatic duct ; or, throughout
the whole of the lobule, by injecting about 60 c.c. of saturated sulphindi-
gotate of soda solution in three successive portions, at intervals of half an

310

THE ESSENTIALS OF HISTOLOGY.

hour, into the blood-vessels of an anj&sthetized cat or rabbit. Two
hours after the last injection the animal is killed, and the blood-vessels
washed out with saturated solution of potassium chloride. The organ is
then fixed with absolute alcohol. But the chromate of silver method is
easier and surer than the injection methods.

6. Tease a piece of fresh liver in serum or salt solution for the study of
the appearance of the hepatic cells in the recent or living condition.

7. Stained sections of pancreas from a gland which has been hardened in
alcohol, or in formol followed by alcohol. The sections may be double
stained with eosin and hsematoxylin or with eosin and methylene blue.
Notice the islets of Langerhans between the alveoli ; largest and most
evident in animals which have been long fasting and also very well marked
after the gland has been stimulated by secretin.

Make sketches under both low and liigh power.

8. Tease a small piece of fresh pancreas in serum or salt solution. Notice
the granules in the alveolar cells, chiefly accumulated in the half of the cell
which is nejirest the lumen of the alveolus, leaving the outer zone of the
cell clear.

Sketch a small portion of an alveolus under a high power.

9. The ducts of the pancreas, and the termination of nerve-fibres in the
alveoli may be seen in preparations made by the Golgi method.

THE LIVER.
The liver is a solid glandular organ, made up of the hepatic lobules.
These are polyhedral masses (tig. .378) about 1 mm. {^-^ inch) in

Fig. 378. — Diagrammatic representation of two hkpatic lobules.
The left-hand lobule is rejii-esented with the intralobular vein cut across ; in the right-
hand one the section takes the course of the intralobular vein, p, interlobular
branches of the portal vein ; /(, intralobular branches of the hepatic veins ; s, sub-
lobular vein ; c, capillaries of the lobules. The arrows indicate the direction of the
course of the blood. The liver-cells are only represented in one part of each lobule.

diameter, composed of cells, and separated from one another by
connective tissue. In some animals, as in the pig, this separation
is complete, and each lobule is isolated, but in man and mo.st animals
it is incomplete. There is also a layer of connective tissue underneath

THE LIVER.

311

the serous covering of the liver, forming the so-called capsule of the
organ. Each lobule is penetrated by a fine network of reticular
tissue which helps to support the columns of cells within the lobule
(fig. 379).

The afferent blood-vessels of the liver (portal vein and hepatic artery)
enter it on its under surface, where also the bile-duct passes away from
the gland. The branches of these three vessels accompany one another
in their course through the organ, and are inclosed by loose connective

Fig. 379. —Reticulum of a liver-lobule. (Oppel.
V.C.., central vein ; i, interlobular interval.

tissue (capsule of GUssm), in which are lymphatic vessels, the whole
being termed a portal canal (fig. 380). The smaller branches of the
vessels penetrate to the intervals between the hepatic lobules, and
are known as the interlobular branches. The blood leaves the liver
at the back of the organ by the hepatic veins ; the branches of
these run through the gland unaccompanied by other vessels (except
lymphatics) and can also be traced to the lobules, from each of
which they receive a minute branch (central or intralobular vein)
which passes from the centre of the lobule, and opens directly into
the (sublobular) branch of the hepatic vein.

Each lobule is a mass of hepatic cells pierced everywhere with a

312 THE ESSENTIALS OF HISTOLOGY.

network of sinusoid blood-vessels, the so-called hepatic capillaries
(fig. 378), which at the periphery of the lobule receive blood from the
interlobular branches of the portal vein {p), and converge to the centre
of the lobule, where they unite to form the intralobular branch of the
hepatic vein. The interlobular branches of the hepatic arteries join
this network a short distance from the periphery of the lobule. The
blood-capillaries are in direct contact with the liver-cells ; indeed, it
would appear as if the endothelium is deficient, for artificial injections
are seen to be in contact with the cells and even pass between their
interstices and run into canaliculi within their protoplasm. The
endothelium of the blood-vessels (or sinusoids) is in part at least

Fig. 380.— Section of a portal canal.

a, branch of hepatic artery ; v, branch of portal vein ; d, bile-duct ; /, /, lymphatics in
the areolar tissue of Glisson's capsule which incloses the vessels.

represented by certain conspicuous cells which occur at intervals on
the wall of the sinusoids, and lie in contact with the liver cells.
These cells were described by Kupffer. They are phagocytic, like
the endothelial cells of the blood-sinuses of the spleen, and ingest
erythrocytes, which can be seen within them.

The hepatic cells, which everywhere lie between and surround the
capillaries, are polyhedral, somewhat granular-looking cells, each
containing a spherical nucleus. The jDrotoplasm of each cell is
pervaded by an irregular network of fine canaliculi (fig. 383), which
in preparations of injected liver become filled with the injection
material, w^hich has passed into them from the blood-vessels (Herring
and Simpson). They thus form a system of intracellular canals which
probably receive the blood-plasma directlj^ from the vessels. Such
canals were conjectured to exist by Browicz, who showed that under

THE LIVER.

313

certain circumstances not only hsemoglobin but whole red blood-
corpuscles, and even groups of blood-corpuscles, which are in process
of breaking down, are to be found in the interior of the hepatic
cells. In the dog's liver both haemoglobin and bilirubin may be
fouiul in the form of crystals within the nuclei of the liver-cells
(Browicz). It is easy to inject these minute canals from the blood-
vessels, and they are clearly .shown filled with the injection mass
in the preparation of injected liver of rabbit shown in fig. 384.
Besides these plasma-channels, the liver cells may show fine, short

Fig. 381. — Sectiox of babbit's liver with the intercellular network of
BILE-CANALICCLI INJECTED. Highly magnified. (Hering.)

Two or three layers of cells are represented ; 6, blood-capillaries.

canals which communicate with the intercellular bile-ducts (see below)
and generally commence within the cell by a dilatation (secretion-
vacuoles).

After a meal many of the liver cells may contain fat, and masses
of glycogen can also be seen within them (fig. 385) if the liver
be hardened in alcohol and treated in the manner described in
section 2. The cells also contain pigment-granules, many of which
are stained by potassium ferrocyanide and hydrochloric acid, or by
pure hsematoxylin (presence of iron ^).

The ducts commence between the hepatic cells in the form of inter-
cellular bile-canaliculi, which lie between the adjacent sides of the

' The iron which is in organic combination can be set free by treatment for a
short time with alcohol to which 10 p.c. hydrochloric acid has been added.

314

THE ESSENTIALS OF HISTOLOGY.

cells, and receive the contents of the secretion-vacuoles above
mentioned. They form a network, the meshes of which correspond
in size to the cells (fig. 381), and at the periphery of the lobule they
pass into the interlobular bile-ducts (fig. 382). In many animals the
network of bile-canaliculi is incomplete (G. Retzius).

Fig. 382. — Lobule of rabbit's liver : vessels and bile-ducts injected.

(Cadiat.)

a, central vein ; '/, 6, peripheral or Interlobular veins ; c, interlobular bile-duct. The

liver-cells are not represented.

The bile-ducts are lined by clear columnar epithelium (fig. 380, d).
Outside this is a basement-membrane, and in the larger ducts some
fibrous and plain muscular tissue. Many of the large ducts are
beset with small blind diverticula.

The gall-bladder is in its genei-al structure similar to the larger
bile-ducts. It is lined hy columnar epithelium, and its wall is
formed of fibrous and muscular tissue.

THE LIVER.

315

The lymphatics of the liver have been described as commencin<< as
perivascuhir lymphatic spaces inclosing the capillaries of the lobules
(MacGillavry), But this cannot be so, since there is no space between

Fig. 383.

Fig. 384.

Fig. 383. — A cell fkom the human liver, showing intracellular
CANALICULI. (Browicz.)

Fig. 384. — From a section of rabbit's liver injected from the portal

VEIN, showing intracellular CANALICULI COMMUNICATING WITH THE
INTERCELLULAR BLOOD-SINUSOIDS.

Fig. 3S.5.— Liver cells containing glycogen. (Dunham, from Barfurth.)

the liver-cells and the sinusoid capillaries with which they come into
immediate relationship (Herring and Simpson). All that can be posi-
tively asserted is that there are numerous lymphatics accompanying the
branches of the portal vein, and others, less numerous, accompanying

316 THE ESSENTIALS OF HISTOLOGY.

the tributaries of the hepatic veins, but so far as can be ascertained no
direct communication exists between the two sets of lymphatics within
the lobules, although they communicate freely near their exit from
the liver. Most of the lymph passes out by the portal lymphatics.

Nerves are described as distributed both to the blood-vessels and to
the liver-cells.

THE PANCREAS.

The pancreas is a tubulo-racemose gland, resembling the salivary
glands so far as its general structure is concerned, but diifering from

6-;-

Fig. 386. — Section Of human p.^nckeas. (Bohm and v. DavidofF.) ^p.

a, group of cells in interstiti.al tissue (islet of Langerhans) ; 6, connective tissue ; c, larper
duct ; d, d, alveoli with centro-acinar cells ; c, small duct passing into alveoli ; /, inner
granular zoue of alveolus.

them in the fact that the alveoli are longer and more tubular in
character. Moreover, the connective tissue of the gland is somewhat
looser, and there occur in the glandular sub.stance here and there
small groups of epithelium-like cells unfurnished with ducts {islets
of Langerhans) (fig. 386 a ; fig. 387), which are supplied with a
close network of large convoluted capillary vessels (fig. 388). Their
function is unknown, but their presence is very characteristic of the
pancreas. They increase in size during starvation and also as the
result of increasing the activity of the gland by injection of secretin
(Dale), apparently at the expense of the proper glandular alveoli.
The degeneration which they sometimes undergo in cases of diabetes

THE PANCREAS.

317

mcllitus seems to suggest that they are concerned with the influence
exerted by the pancreas on the metabolism of carbohydrates.

Fig. 387— Section of pancreas of armadillo showing several alveoli
AND A large interalveolar CELL-ISLET. (V. D. Harris.)

The cells of the alveoli are shruukeii, but they show markedly the two zones, the outer
or nou-graiuilar stained deeply by haematoxj-lin.

The cells which line the alveoli are columnar or polyhedral in shape.
When examined in the fresh condition, or in sections stained by
certain methods, their protoplasm is seen to be filled in the inner
two-thirds with granules, but the outer third is left clear or; is

Fig. 388. — Injection of blood-vessels of an "islet" of the pancreas.
(Kiibne and Lea.)

Striated (fig. 390, A : fig. 386). After a period of activity the clear
part of the cell becomes larger, and the granular part smaller
(fig. 390, B). In hccmatoxylin-stained sections the outer part is
coloured more deeply than the inner (fig. 387).

318

THE ESSENTIALS OF HISTOLOGY.

Pancreas cells frequently exhibit a rounded mass of granxiles or
fibrils (mitochondria) near the nucleus, which is known as the para-
nucleus (Nebenkern) : this is probably related to the secretory activity
of the cells (see p. 5).

Fig. 389. — From a section of human pancreas, (v. Ebner.) Magnified
530 diameters.
a, a, outer zones of alveolar cells witli striated appearance ; h, inner granular zones ;
))i, rnembrana propria ; c, centro-acinar cells, here occurring in unusuall}' large
amount; d, junctional part of duct; its epithelium is continuous with the centro-
acinar cells.

In the centre of each acinus there may generally be seen some
spindle-shaped cells {centro-acinar cells of Langerhans — fig. 386, (/), the
nature of which has not been definitely determined ; but they appear to
be continued from the cells which line the smallest ducts (fig. 386, e)

Fig. 390. — Part of an alveolus of the rabbit's pancreas. A, at rest;
B, after active secretion. (From Foster, after Kiihne and Lea.)

a, the inner granular zone, which in A is larger and more closely studded with fine
granules than in B, in which the granules are fewer and coarser; h, the outer trans-
parent zone, small in A, larger in B, and in the latter marked witli faint striae ; c, the
lumen, very obvious in B, but indistinct hi A ; d, au indentation at the junction of
two cells, only distinct in B.

Sometimes they are much more conspicuous and fill the parts of the
alveoli which are nearest to the duct (fig. 389) ; in these cases the mass
of cells which they form is liable to be mistaken for a Langerhans' islet.

THE PANCREAS.

319

Diverticula from the lumen of the alveolus penetrate between the
alveolar cells (tii;-. 'M)\), as in the salivary glands (j). 'JSfj). The pancreas
has many nerves, with numerous small nerve-cells distributed upon

Fig. 391. — A duct of thk pancreas with lateral diverticula into the
alveoli; golgi method. (E. Miiller.)

In A the duot i.s shown out longitudinally and giving off ductules, m, to the alveoli,
where they extend between the cells (0- In B the details of their termination are
shown more highly magnified.

their course ; the nerve-fibrils end by ramifying amongst the cells of
the alveoli, as in the salivary glands. In the cat, which has Pacinian
bodies in its mesentery, these terminal organs are also found numer-
ously in the substance of the pancreas (V. D. Harris).

320 THE ESSENTIALS OF HISTOLOGY.

LESSON XXXVI.

STRUCTURE OF THE KIDNEY.

\. Sections passing thiough the whole kidney of a small mammal, such as a
mouse or rat. These sections will show the general arrangement of the organ
and the disposition of the tubules and of tlie Malpighian corpuscles.

2. Thin sections of the kidney of a larger mammal, such as the dog or cat,
may next be studied. In some the direction of the section should be parallel
with the rays of the medulla, and in others acro.ss their direction. The
characters of the epithelium of the sevend parts of the uriniferous tubules
and the structure of the glomeruli are to be made out in these sections.

3. Separate portions of the in-iniferous tubules may be studied in teased
preparations from a kidney which has been macerated in diluted hydro-
chloric acid (1 to .') water). This renders it possible to unravel the
uriniferons tubules for some distance.

4. Thick sections of a kidney in which the blood-vessels have been
injected. Examine these with a low power of the microscope. Follow the
course of the arteries — those to the cortex sending their branches to the
glomeruli, those to the medulla rapidly dividing into pencils of fine vessels
which run between the straight uriniferons tubules of the boundary zone.
Notice also the efferent vessels from the glomeruli breaking up into the
capillaries which are distributed to the tubules of the cortical substance.

The kidney is a compound tubular gland. To the naked eye it
appears formed of two portions — a cortical and a mechdlary. The latter
is subdivided into a number of pyramidal portions {pyramids of
Malpighi), the base (boundary zone) of each being surrounded by
cortical substance, while the apex projects in the form of a papilla into
the dilated commencement of the ureter (pelvis of the kidney).^ Both
cortex and medulla are composed entirely of tubules — the uriniferons
tubules — which have a straight direction in the medulla and a
contorted arrangement in the cortex ; but groups of straight tubules
also pass from the medulla through the thickness of the cortex
(medullary rays).

The uriniferous tubules begin in the cortical part of the organ in
dilatations, each inclosing a tuft or glomerulus of convoluted capillary
blood-vessels (corpuscles of Malpighi), the dilated commencement of the
tubule being known as the capsule (fig. 396, 1). The glomerulus is

1 In many animals (e.g. dog, cat, rabbit, most monkeys) the whole kidney is
formed of only a single pyramid, but in man there are about twelve.

UKINIFEKOUS TUBULES.

321

lobulated (figs. 394, 395) ; the lobules being united by the afferent
and efferent vessels and covered by a syncytium reflected from the

Fig. 392.— Diagram of the course of the tubules in a uxipyramidal

KIDXET, such as THAT OF THE RABBIT. (Toldt.)

'I, Malpighian bodies ; h, first convoluted tubule ; c, d, looped tube of Henle ; e, second
convoluted ; f, collecting tube ; g, ducts of Bellini.

Fig.

393.— Section through part of

DOG'S KIDNEY. (Ludwig. )

p, papillarj', and g, boundary zones of the medulla ;
/-, cortical layer; h, bundles of tubules in the
boundary layer, separated by spaces, 6, containing
bunches of vessels (not here represented), and
prolonged into the cortex as the medullary rays,
III ; c, intervals of cortex, composed chiefly of con-
voluted tubules, with irregular rows of glomeruli,
between the medullary rays.

epithelium lining the capsule. The glomeruli near the medulla are
larger than the rest and have more lobules. The capillary-wall in all
the glomeruli is a syncytium, showing no cell-outlines in silver pre-
parations (Drasch).

L

322

THE ESSENTIALS OF HISTOLOGY.

The tubule leaves the capsule by a neck (2), which is rarely narrower
than the rest of the tubule in mammals, but in some animals (e.r/. frog)

Fig. 394.— a malpighian corpu.scle from the kidney of the momkey.
(Szj-monowicz.) Magnified 3-50 diameters.

ft, a, sections of convoluted tubules ; «', comniencenient of convoluted tube from capsule
b, capsule ; c, afferent and efferent vessels of glomerulus.

Fig. 395. — Model of a glomerulus. (Johnson.
a, afferent ; c, efferent blood-vessel.

URINIFEROUS TUBULES.

323

is long, and has ciliated epithelium ; the tubule is at first convoluted
(first convolaled tubule, 3), but soon becomes nearly straight or slightly
spiral only {spiral tuhuU; 4), and then, rapidly narrowing, passes down

Fig. 39fi.— Diagram of the course of two urixiferous tubules. (Klein.)

A, cortex ; B, boundary zone ; c, papillary zone of the medulla ; a, a', superficial and deep
layers of cortex, free from glomeruli. For the explanation of the numerals, see the
text.

into the medulla towards the dilated commencement of the ureter as
the desrendinrt tubule of Henle (5). It does not at once, however, open
into the pelvis of the kidney, but before reaching the end of the

324 THE ESSENTIALS OF HISTOLOGY.

papilla it turns round in the form of a loop {hop of Henle, 6) and passes
upwards again towards the cortex, parallel to its former course, and at
first somewhat larger than before, but afterwards diminishing in size
{ascending tubule of Henle, 7, 8, 9). Arrived at the cortex it approaches
close to the capsule from which the tubule took origin, but at a point
opposite to the origin, viz. near the afferent and eft'erent vessels of the
glomerulus (Golgi). It then becomes larger and irregularly zigzag
{zigzag or irregular tubule, 10), and may again be somewhat convoluted
(second convoluted tubule, 11), eventually, however, narj-owing into a
small vessel {junctional tubide, 12), which joins a straight or collecting
tubule (13). The last-named unites Avith others to form large collecting

tubes which pass through the medul-
^■---^ lary substance of the kidney (14) to

open at the apex of the papilla as the
ducts of Bellini (15).

The tubules are throughout bounded
k by a basement-membrane, which is

fc_ ■ lined b}' epithelium, but the characters

W:J. of the epithelium cells vary in the

^' • different parts of a tubule. In the

capsule the epithelium is flattened and

"" '■■■*"■'"' — i-^— -^ is reflected over the glomerulus. In

Fig. 397.— Section- of a coNvoLrTED some animals {e.g. mouse) the granular

TUBULE OF THE RABBITS KiDXET, epithclium of the couvoluted tube is

SHOWING THE STRUCTURE OF THE ^

EPITHELIUM. (Szymonowicz.) (Mag- prolonged a little way into the capsule.

nified 1100 diameters.) t^t."!^^ 7jj i ■ i i i i

In the nrst convolutect and spiral tubules
the epithelium is thick, and the cells are markedly granular, with a
tendency for the granules to be arranged in lines perpendicular to the
basement-membrane (rodded or fibrillar appearance, fig. 397). The
granules of the cells are particularh'^ well displaved in sections stained
by Muir's method ; they are eosinophil, like the granules of secreting
cells generally. They often exhibit a brush of cilium-like processes
projecting into the lumen (figs. 397, 400), but these are not vibratile
in mammals. In the narrow descending limb of the looped tubules,
and in the loop itself, the cells are clear and flattened and leave
a relatively large lumen ; in the ascending limb they again acquire a
granular structure and may nearly fill the lumen. The arrangement
of the cell-granules in lines perpendicular to the basement-membrane
is still more marked in the zigzag tubides, and a similar structure
is present also in the second convoluted tubules, into which these pass.
On the other hand, the junctional tidtule has a large lumen and is
lined by clear flattened cells, and the collecting tubes have also a very

URINIFEROUS TUBULES.

325

distinct lumen and are lined by a clear cubical or columnar epithelium
(tiff. 398, a).

Fig. .398.— Section across a papilla of the kidney. (Cadiat.)
a, large collectiug tubes (ducts of Bellini) ; h, c, d, tubules of Henle ; e, .r, blood-capillaries.

The following gives a tabular view of the parts which compose a
uriniferous tubule, and the nature of the epithelium in each part : —

Portion of TuBrLE.

Xatlre of Epithelium.

Position of Tubcle.

Capsule

First convoluted tube .

Spiral tube .

Small or descending i
tube of Henle . . ■

Flattened, reflected over glomerulus,
where its cells form a syncytium.

Cubical, granular, with appearance
of fibrillation ("redded '"), the cells
interlocking .....

Like the last .....

Clear flattened cells ....

Loop of Henle

Larger or ascending

tube of Henle .
Zigzag tube .

Second convoluted tube

Junctional tube .

Straight or collecting
tube ....
Duct of Bellini

Like the last

Cubical, granular : the cells some-
times imbricated ....

Cells strongly '" rodded '"' : varying
height, lumen small

Similar to first convoluted tube, but
cells are longer, with larger nuclei,
and they have a more refractive
aspect ......

Clear flattened and cubical cells

Clear cubical and columnar cells
Clear columnar cells ....

Labyrinth of cortex.^

Labyrinth of cortex.

Medullary ray of
cortex.

Boundary zone and
partly papillary
zone of medulla.

Papillary zone of
medulla.

Medulla, and medul-
lary ray of cortex.

Labyrinth of cortex.

Labyrinth of cortex.

Labyrinth passing to

medullars ray.
Medullary ray and
medulla.
I Opens at apex of
I papilla.

1 The part of the cortex between and surrounding the medullary rays is so named.

326 THE ESSENTIALS OF HISTOLOGY.

Blood-vessels. — The renal artery divides into branches on entering
the organ, and these branches pass towards the cortex, forming
incomplete arches between the cortex and the medulla (fig. 399, a).
The branches of the renal vein form similar but more complete

Fig. 399. — Vascular supply of kidney. ' (Cadiat.) Diagrammatic,
a, part of arterial arch ; 6, interlobular artery ; c, glomerulus ; d, efferent vessel passing
to medulla as false arteria recta ; e, capillaries of cortex ; /, capillaries of medulla ;
g, venous arch ; h, straight veins of medulla ; j, vena stellula ; i, interlobular vein.

arches (g). — From the arterial arches vessels pass through the cortex
(cortical oi- interlobular arteries, b), and give off at intervals small arteri
oles (afferent vessels of the gl'>meruli), each of which enters the dilated
commencement of a uriniferous tubule, within which its capillaries form
a glomerulus. From the glomerulus a somewhat smaller efferent vessel
passes out, and this at once again breaks up into capillaries, which are

BLOOD-VESSELS OF KIDNEY.

827

distributed amongst the tubules of the cortex (e) ; their blood is
collected by veins which run parallel with the cortical arteries but
not in juxtaposition with them : these veins join the venous arches
between the cortex and the medulla ; they receive blood from certain
other veins Avhich arise by radicles having a somewhat stellate arrange-
ment near the capsule {vence dellvlce, j).

The medulla derives its blood-supply from special offsets of the
arterial arches, which almost immediately break up into pencils of
fine straight arterioles running in groups between the straight tubules
of the medulla. These arterioles supply a capillary network with

Fig. 400. — Nerve fibrils ending over capillary blood-vessels and

AMONGST THE EPITHELIUM CELLS OF A CONVOLUTED TUBE OF THE FROG'S

KIDNEY. (Smirnow.)

elongated meshes which pervades the medulla (fig. 399,/), and which
terminates in a plexus of somewhat larger venous capillaries in the
papillce. From these and from the other capillaries the venules of
the medulla collect the blood, and pass, accompanying the straight
arterioles, into the venous arches between the cortex and medulla. The
groups of small arteries and veins (rasn recta) in the part of the medulla
nearest to the cortex alternate with groups of the uriniferous tubules,
and this arrangement confers a striated aspect upon this portion of the
medulla {houndanj zone, fig. 39-3, g).

The efferent vessels of those glomeruli which are situated nearest
to the medulla also break up into pencils of fine vessels {false vasa
recta) which join the capillary network of the medulla (fig. 399, (/).

Between the uriniferous tubules, and supporting the blood-vessels,
is a certain amount of connective tissue (fig. 400), within which are
cleft-like h'mphatics.

Nerve-fibrils are described as ramifying amongst the epithelium-cells
of the tubules (fig. 400), but most of the nerves to the kidneys are
distributed to the blood-vessels.

328 THE ESSENTIALS OF HISTOLOGY.

LESSON XXXVII.

STRUCTURE OF THE URETER, BLADDER, AND
MALE GENERATIVE ORGANS.

1. Section across the lower part of the ureter. Another section may be
taken across the upper part near the pelvis of the kidney.

2. Section of the urinaiy bladder vertical to the surface.

In the sections of the ureter and of the urinary bladder, notice the tran-
sitional epithelium resting on a raucous membrane, which is composed of
areolar tissue without glands (in most animals), and the muscular coat
outside this. In the ureter there is a layer of connective tissue outside the
muscular coat, and at the upper part of the bladder a layer of serous
membrane covering the muscular tissue.

3. Section across the penis (child or monkey). The blood-vessels of the
organ should be injected with the hardening fluid .so as the better to exhibit
the arrangement of the venous spaces which constitute the erectile tissue.
Notice the large venous sinuses of the corpora cavernosa and the smaller
spaces of the corpus spongiosum, in the middle of which is seen the
(flattened) tube of the urethra.

4. Section across urethra and ])rostate gland (child or monkey). Notice
the glandular tubes and the plain muscular tissue of the prostate, and the
character of the urethral epithelium.

5. Section of testis and epididymis. The sections may be made from
a rat's testis which has been hardened in alcohol ; they can be stained
with iron-hsematoxylin. In these sections notice the strong capsule sur-
rounding the gland, the substance of which consi.sts of tubules which are
variously cut ; and the epithelium of the tubules, which is in different
phases of development in different tubules. Observe the strands of poly-
hedral interstitial cells, much more numerous in some animals, lying in the
loose tissue between the tubules ; also the lymphatic clefts in that tissue.
Notice in sections through the epididymis the epithelium of that tube.

Sketch carefully under a high power the contents of some of the semini-
ferous tubules to illustrate the mode of formation of the spermatozoa.

6. Examination of spermatozoa. Spermatozoa may be obtained fresh from
the testicle or seminal vesicles of a recently killed mammal and examined in
saline solution. Their movements may be studied on the warm stage ; to
display their structure a very high power of the microscope is necessary.
They mav be preserved and stained as " fllm " pi'eparations, as with marrow
(P-30). "

The ureter (fig. 401) is a muscular tube litied by mucous membrane.
The muscular coat consists of two layers of plain muscular tissue, an
outer circular, and an inner longitudinal. In the lower part there
are some longitudinal bundles external to the circular. Outside the
muscular coat is a layer of connective tissue in which the blood-vessels
and nerves ramify before entering the muscular layer.

THE BLADDER.

329

The mucous membrane is composed of areolar tissue and is lined by
transitional epithelium (fig. 402).

The urinary bladder has a muscular wall lined by a strong mucous
membrane and covered in part by a serous coat.

*i>..

i'£.

gn-ia&JEK^

i) e"^<iB>'

l^^#%

SF%>

%,

^.i^'>:

^ill^

% ig?--.i

Fig. 401. — Section across the upper part of the oketer. (v. Ebner.)

Magnified 14 diameters.

e, epithelium ; «, mucous membrane ; I, longitudinal muscle ; r, circular mu.scle.

Jio."^"

-.-- d

Fig. 402. —Section of the mucocs jieiibrane of the bladder to show its
epitheliuji. (Sz\-monowicz. )
a, h, superficial epithelium-cells ; c, leucocyte ; d, areolar tissue of mucous membrane.

The muscular coat consists of three layers, but the innermost is
incomplete. The principal fibres run longitudinally and circularly,
and the circular fibres are collected into a layer of some thickness which
immediately surrounds the commencement of the urethra. The mucous
membrane is lined by a transitional stratified epithelium like that of

330

THE ESSENTIALS OF HISTOLOGY.

the ureter. The shape and structure of the cells have already been
studied (p. 5.5). Many of the superficial cells have two nuclei.

The nerves to the bladder form gangliated plexuses, and are dis-
tributed to the muscular tissue and blood-vessels, but some are said to
enter the epithelium.

The penis is mainly composed of cavernous tissue which is collected
into two principal tracts — the corpora cavernosa, one on each side, and
the corpus spongiosum in the middle line inferiorly. All these are
bounded by a strong capsule of fibrous and plain muscular tissue, con-
taining also many elastic fibres and sending in strong septa or trabeculse

Fig. 403.— Section of erectile tissue. (Cadiat.)

a, trabeculse of connective tissue, with elastic fibres, and bundles of plain muscular

tissue, some cut across (c) ; 6, venous spaces.

of the same tissues, which form the boundaries of the cavernous spaces
of the erectile tissue (fig. 403). The arteries of the tissue run in these
trabeculse, and their capillaries open into the cavernous spaces. On
the other hand, the spaces are connected with eflferent veins. The
arteries of the cavernous tissue may sometimes in injected specimens
be observed to form looped or twisted projections into the cavernous
spaces {helicine arteries of MuUer), into which they may open directly.

The integument of the penis and clitoris, especially that of the
glans, contains numerous special nerve end organs of the nature of end-
bulbs (see p. 169), and Pacinian bodies are also found upon the nerves.
Lymphatic vessels are numerous in the integument of the organ and
also in the submucous tissue of the urethra.

THE URETHRA. 331

Urethra.— The cross-section of the urethra appears in the midHle
of the corpus spongiosum in the form of a transverse cleft. It is lined
in the prostatic part by transitional, but elsewhere by columnar
epithelium, except near its orifice, where the epithelium is stratified.
In the female urethra it is stratified throughout. The epithelium rest?
upon a vascular mucous membrane, which contains longitudinally
disposed plain muscular fibres, and in the membranous urethra, cir-
cularly disposed cross-striated fibres. Outside the mucous membrane
is a coating of submucous tissue, with two layers of plain muscular
fibre — an inner longitudinal and an outer circular. Outside this again

'■<£^_

Fig. 404. — Seciiox of prostate. (Heitzmann.)
il, muscular tissue ; E, epithelium ; C, concretions.

is a close plexus of small veins which is connected with, and may be
said to form part of, the corpus spongiosum.

The mucous membrane of the urethra is beset with small mucous
glands, simple and compound (glands of Littre). There are also a
number of oblique recesses termed lacunce. Besides these small glands
and glandular recesses, two compound racemose glands open into the
bulbous portion of the nrethra (Cowpers glands). Their acini are lined
by clear columnar cells which yield a mucus-like secretion.

The prostate, which surrounds the commencement of the urethra, is
a muscular and glandular mass, the glands of which are composed of
tubular alveoli, lined by columnar epithelium, with smaller cells h'ing
between them and the basement-membrane (fig. 404). Their ducts

332 THE ESSENTIALS OF HISTOLOGY.

open upon the floor of the urethra. In old subjects the tubules often
contain colloid or calcareous concretions. The muscular tissue is of
the plain variety.

Blood-vessels and nerves are numerous. The nerves are provided
with small ganglia and are distributed partly to the muscular tissue,
partly to the glands, and others (sensory) to the capsule, and to the
wall of the urethra. The sensory nerves end in plexuses and in peculiar
terminal corpuscles like simple Pacinian bodies (Timofeew).

a h

Fig. 405. — Sectio.\ of humax testis and epididymis, somewhat magnified.
(Bohm and v. Davidoff.)

a, glandular substance divided into lobules by septa of connective tissue ; b, tunica albu-
ginea ; c, head of epididymis ; d, rete testis ; c, middle part or body of epididymis ; /,
mediastinum giving oi-igin to the septa ; rj, sections of the commencing vas deferens.

The testicle is inclosed by a strong fibrous capsule, the tunica
alhug'mea (tig. 405, h). This is covered externally with a layer of serous
epithelium reflected from the tunica vaginalis. From its inner surface
there proceed fibrous processes or tmhecuke, which imperfectly subdivide
the organ into lobules, and posteriorly the capsule is prolonged into the
interior of the gland in the form of a mass of fibrous tissue, which is
known as the mediastimm. testis (fig. 405, /). Attached to the posterior
margin of the body of the gland is a mass (epididymis) which when
investigated is found to consist of a single convoluted tube, receiving at

THE TESTICLE.

333

its upper end the rj/nrnf ducts of the testis and prolonged at its lower
end into a thick-walled nniscidai' tube, the vas deferens, which conducts
the secretion to the urethra.

The glandular substance of the testicle is wholly made up of
canvoluted tubules, which wlien unra\elled are of very considerable
length. Each commences near the tunica albuginea, and after many
windings terminates, usually after joining one or two others, in a

Fig. 406. — Passage of convoluted seminu-'erous tubules into straight

TUBULES AND OF THESE INTO THE RETK TESTIS. (Mihalkowicz. )

(1, seminiferous tubules ; 6, fibrous stroma continued from the mediastinum testis ;

c, rete testis.

straight tubule, which passes into the mediastinum, and there forms,
by uniting with the other straight tubules, a network of intercom-
municating vessels of varying size, which is known as the rete testis
(fig. 406). From the rete a certain number of efferent tubules arise,
and after a few convolutions pass into the tube of the epididymis.

The straight tubules which lead from the convoluted seminiferous
tubes into the rete testis are lined only by a single layer of clear
flattened or cubical epithelium. The tubules of the rete also have a

334 THE ESSENTIALS OF HISTOLOGY.

simple epithelial lining; both in these and in the straight tubules the
basement-membrane is absent, the epithelium being supported directly
by the connective tissue of the mediastinum.

The efferent tubules which pass from the rete to the epididymis are
lined by columnar ciliated epithelium In man their lumen is irregular
in section, and the inner surface pitted with depressions (intra-epithelial
glands) lined by short clear non-ciliated cells (J. Schaffer). The tube
of the epididymis is lined by long columnar cells having at their bases

,'y^

a

5? J I \il\

Fig. 407. — Section of the TinK oi' the epididymis. (Szymonowicz.)

(MagniHed oUO diameters.)

(t, blood-vessel ; b, circular muscular fibres ; r, epithelium.

smaller cubical cells with spherical nuclei (fig. 407). The columnar cells
are provided with what appear to be bunches of cilium-like fibrils pro-
jecting into the lumen of the tube. These apparent cilia are, however,
not vibratile as was formerly supposed, and are therefore not true cilia
(Neumann, Myers-Ward). They appear to vary in development in
different cells, and are probably connected in some way with the
formation of the secretion of the epididymis and its extrusion into
the lumen of the tube. The epididymis cells exhibit canaliculi in
their cytoplasm, which according to Holmgren, communicate with the
exterior at the attached border of the cell (fig. 408). The tube of

THE 'rKSIMCLK.

335

the epididymis has a considerable amount of |)laiii iniiseulai- tissue in
its wall (fig. 407).

The ras deferens (fig. 109) is a thick-walled tube, formed of an outer

A!^A '■'^^mk

Fig. 408.— Cells of epididymis, showing canalization of the cytoplasm.
(E. Holmgren.)

Fig. 409.— Section across the commencement of the vas deferens. (Klein.)
a, epithelium ; b, mucous membrane ; c, d, e, inner, middle, and outer layers of the
muscular coat ; /, bundles of the internal creniaster muscle ; g, section of a blood-
vessel.

layer of longitudinal bundles of plain muscular tissue; within this an
equally thick layer of circular bundles of the same tissue, and within
this again a thinner layer of longitudinal muscle. There is a good deal
of connective and elastic tissue between the muscular bundles. The

336

THE ESSENTIALS OF HISTOLOGY.

tube is lined by a mucous membrane, the inner surface of which is
covered by columnar non-ciliated epithelium.

The ampullcB of the vasa deferentia, and the vesiculce seminales, are in
structure similar to the vas deferens, but their corrugated walls are
much thinner and less muscular.

The connective tissue between the tuljules of the testis is of very
loose texture, and contains numerous lymphatic clefts, which form an
intercommunicating system of commencing lymphatic vessels. Lying
in this intertubular tissue are strands of polyhedral epithelium-like

Fig. 410.

Fig. 411.

Fig. 410. — Section of parts of three semixiferous tubules of the rat.
n, with the spermatozoa lea.st advanced in development ; 6, more advanced ; r, containing

fully developed spermatozoa. Between the tubules are seen strands of interstitial

cells with blood-vessels and lymph-spaces.

Fig. 411.— Human spermatozoa. J-^^. (G. Retzius.)

1, in jirofile ; 2, viewed on the flat ; b, head ; c, middle jiiece ; d, tail; e, end-piece of the
tail, which is described as a distinct part by Retzius.

cells {interstitial cells, see fig. 410) of a yellowish colour; they are much
more abundant in some species of animals (cat, boar) than in others.
They accompan}' the blood-vessels before these break up to form
the capillary networks which cover the walls of the seminiferous
tubules.

The interstitial cells contain in many animals yellowish-brown fat-
globules (staining with osmic acid); and also sometimes needle-shaped
crystals (proteid). Similar fatty globules may occur in the Sertoli
cells of the seminiferous tubules (see above), and have been thought
to be derived from those of the interstitial tissue.

THE SEMINIFEROUS TUBULES.

337

Structure of the tubules. — The seminiferous tubules are formed of a
thick basement-membrane, and contain several layers of epithelium-
cells. Of these Wers, the one next to the basement-membrane is a

Fig. 4r2.— HcjrAX spekmatozoa ox the flat and in profile. (Bramman.)

Those on the right still show protoplasm adhering to them. Only the commencement of
the tail is represeutftd in the two which are shown in profile. Magnified 2500 diameters.

stratum of clear cubical cells {spermatogonia or spermagons, figs. 410, 414, a),
the nuclei of which for the most part exhibit the irregular network
which is characteristic of the resting condition, but in certain tubules
show indications of division. Here and there between the spermatogonia

338

THE ESSENTIALS OF HISTOLOGY.

some of the lining epithelium-cells are enlarged, and project between
the more internal layers, being connected with groups of developing
spermatozoa. The.se enlarged cells are the cells of Sertoli (fig. 414, ((^, a" ;
fig. 417).

Next to this lining epithelium is a zone of larger cells (spermatocytes
or spermocytes, fig. 414, h), the nuclei of which are usualh^ in some
stage of hetero- or homo-typical mitotic division; these cells may be
two or three deep (as in a, fig. 410). Next to them, and most
internal, are to be seen in some tubules (fig. 410, h and c) a large

a

(I

d

S

9

Fig. 413. — Different forms of spermatozoa. (From Verworn.)

a, of bat ; 6, c, of frog ; t?, of finch ; c, of ram ; /, j/, of boar ; A, of a jelly-fish ;
i, of a monkey ; ?, of crab ; k. of round- worm.

number of small protoplasmic cells with simple spherical
nuclei {spermatids or spermids, fig. 414, c). In other tubules
the spermatids are elongated, and the nucleus is at one
end, and in others again these elongated cells are converted
into evident spermatozoa, which lie in groups : their heads
projecting between the deeper cells and connected with one of the
Sertoli cells of the lining epithelium, and their tails emerging into
the lumen of the tubule (fig. 410, b). As they become matured they
gradually shift altogether towards the lumen, where they eventually
become free (c). During the time that this crop of spermatozoa has
been forming, another set of spermocytes has been produced by the
division of the spermogonia, and on the discharge of the spermatozoa
the process is repeated as before (see diagram, fig. 414).

The spermatozoa. — Each spermatozoon or sperm consists of three
parts, a head, a middle part or body, and a long tapering and vibra-
tile tail (figs. 411, 412). In man the head is of a flattened oval
shape, somewhat more flattened anteriorly : in some animals it bears
a small barb-like projection at its extremity, but this appears to be

THE SPERMATOZOA.

339

Fig. 414.— Diagram exhibiting the cycle of phases of spermogenesis

(rat).
o, lining epithelium-cells or spermatogonia, seen dividing in 6 ; a', a", Sertoli cells ;
b, spermatocytes, with skein-like nuclear filaments. These cells are seen actively
dividing in 5. c, spermatids, forming an in-egular column or clump in 6, 7, 8, and 1,
and connected to an enlarged Sertoli cell, a', of the lining epithelium in 2, 3, 4,
and 5. In 6, 7, and 8 advanced spermatozoa of one crop are seen between columns
of spermatids of the next crop. «', parts of the spermatids which disappear when
the spermatozoa are fully formed ; s, seminal granules.

Fig. 41.5.— Spermatozoa from the rat in different stages of develop-
ment. (H. H. Brown.)

1-6, developing spermatozoa from the testicle ; 7, a mature spermatozoon from the vas
deferens. The remains of the protoplasm of the cell, which is seen in 6 still adhering
to the middle piece of the sijermatozoon and containing a number of chromatin
granules, appears to be thrown off as the spermatozoon matures.

340

THE ESSENTIALS OF HISTOLOGY.

absent in the human spermatozoon. The apical part is covered by
a cap of a somewhat different appearance from the rest — the head-cap.
The jiiiddle-jyiece is in man short and cylindrical, and has a spiral fibre
passing round it. An axial fibre, itself fibrillated, passes from a knob
close to the head right through the body and tail. The tail is the
longest part of the spermatozoon, and when examined with the micro-
scope in the fresh condition is seen to be in continual vibratile motion,
the action resembling that of a cilium. The extremity of the tail
{end-piece) forms a distinct part of the spermatozoon, and in some

10^

s

Fig. 416. — Changes in the spermatids in the course of form.\tion ok

THE SPERMATOZOA. (Niessing.)
The tail filament is seen (iu a and e) to extend from the centrosome, which lies close to the

nucleus. The head-cap (.shown in c)is produced by a transformation of part of the

archoplasm which becomes vacuolated (6, c, (0-

animals may split into two or three fibrils ; these can also sometimes
be traced along the whole length of the tail. Human spermatozoa
are about 0*05 mm. (^i^ inch) long, the head and middle-piece each
measuring about yV^^ ^^ ^^^^ amount.

In diff'erent animals the shape of the head and the extent of middle-
piece and tail vary greatly (fig. 413). In the rat (fig. 415, 7) the head
is long, and is recurved anteriorly ; it is set obliquely on the middle-
piece, which is also of considerable extent, and which has a closely
wound spiral filament encircling it (H. H. Brown). In the newt the
head is long and tapering, and the tail has a membranous expansion,
attached in a spiral manner along its whole length. This has also been

SPERMOGENESIS. 341

described in the human spermatozoon, but its existence here is doubt-
ful. In decapods, which possess no cilia, the spermatozoa are stellate
and motionless (fig. 413, /) ; in nematoid worms they are amoelioid
(fig. 413,^-). Sometimes two distinct kinds of spermatozoa are met
with in the same species of animal, one kind being far the larger in size
(giant spermatozoa) but much less numerous. Such giant spermatozoa
have been observed in man.

Although the tail of the spermatozoon is usually classed with cilia,
it is obvious that it exhibits far greater complexity and is a much
more highly differentiated structure. Spermatozoa also difier from
cilia in being highly resistant to putrefaction and to chemical reagents,
even including the strongest acids and alkalies.

Spermogenesis. — The spermatozoa are developed from the small
cells (spermatids) which form the innermost stratum of the seminal
epithelium, and these are themselves produced by the division of
the large spermocytes of the second layer. It is probable that fresh
spermocytes are formed by division of some of the lining epithelium-
cells or spermogons. The cycle of changes therefore which takes place
is as follows: — 1. Division of a lining epithelium-cell or spermogon
into two, one of which grow\s larger ("growing cells " of H. H. Brown),
becomes a spermocyte, and passes into the second layer, while the
other remains in the first layer. 2. Division of the spermocyte.
3. Further division of the daughter-spermocytes thus produced. The
four cells (spermatids) which result from this double division possess only
one-half the somatic number of chromosomes in their nuclei, "reduc-
tion" having been effected in the final cell-divisions by which the
spermatids are produced (see p. 14). 4. Elongation of the spermatids
and their gradual conversion into spermatozoa. As they undergo this
conversion their grouping becomes more evident, and each group is
found to be connected with a cell of Sertoli (figs. 414, a, 417),
which probably ministers to their nutrition. This cell undergoes a
gradual process of elongation so that the spermatozoa by the time
they are fully developed are brought to the lumen of the tube,
in which they then become free. In the meantime other alternate
groups of spermatids from which the next crop of spermatozoa will
be derived are being formed in the same manner, passing through
the same cycle of changes. So that in a longitudinal section even
of the same tubule, different phases of development may be observed,
and in different tubules of the same testicle every phase may be
traced. The accompanying diagram (fig. 414), which is constructed
from drawings by H. H. Brown, illustrates the cj^cle of changes
above, described : it is divided into eight parts, each of which shows

342

THE ESSENTIALS OF HISTOLOGY.

the condition of the epithelium of a seminiferous tubule at a
particular stage.

Each spermatid becomes converted into a spermatozoon in the
following manner (figs. 415, 416, 418). The nucleus forms the
chief part of the head, while the tail develops as an outgrowth of
the centrosome and cytoplasm. The tail-filament appears within the
protoplasm, growing out from the centriole of the cell which lies close

Fig. 418.
Fig. 417. — A cell of Sertoli with which the spermatids (three of which

ARE shown) ARE BEGINNING TO BE CONNECTED : HUMAN. (Bramman.)
The cell contains globules (of nutritive substance) staining with osmic acid, and similar
but smaller globules are also seen in the spermatids. The " ring" formed around the
tail filament by one of the particles uf the centrosome (see text) is shown in each of
these spermatids close to the " head.''

Fig. 418.— Stages of spermogenesis, with transformation of the granules
OR mitochondria of the spermatid into the spiral fibre of the middle
PIECE: MOUSE. (Benda.)

to the nucleus (fig. 416). The centriole is double, and one of its two
particles forms an annular expansion or ring which, as development
proceeds, moves down the tail-filament until it reaches the place where
this leaves the cytoplasm : here it ultimately forms the limit of the
body or middle piece of the spermatozoon. The archoplasm (see p. 8)
assists in forming the head of the spermatozoon ; a portion (the
idiozome of Moves) at an early stage separates from the rest, lying
apically to the nucleus. Within this portion vacuoles form (fig.
416, b, c, d) which presently run together into a clear non-stainable

SPERMOGENESIS. 343

glol)ule wliieh tluttons out over the nucleus and forms (fig. 416, e) the
head-cap of the spermatozoon ; as development proceeds, this may
become indistinguishable from the rest of the head. The spiral fibre
of the middle piece is developed from mitochondria (see p. 5) in the
spermatid (Benda) (fig. 418).

A portion of the protoplasm of each spermatid containing a number
of chromatin-particles (seminal granules) becomes detached and disin-
tegrated before the spermatozoon is fully matured (fig. 414, s, .s').

A few spermocytes undergo incomplete division, and the resulting
spermatids are large (giant spermatids) and contain either one large
nucleus or two or more nuclei which ultimately blend to form the
head of the spermatozoon. In these cases there are a corresponding
number of centrosomes, from each of which a tail-filament may become
developed.

344 THE ESSENTIALS OF HISTOLOGY.

LESSON XXXVIII.

GENERATIVE ORGANS OF THE FEMALE.

1. Sections of the ovary of the non-pregiiaut rabbit or cat. (If from a
pi-egnant animal the organ may be largely occupied by luteal tissue.)
Study the sections with a low power, observing the small and large
Graafian follicles, each inclosing an ovum, scattered through the stroma.
Measure some Graafian follicles of different sizes ; make a general sketch
of a section under the low power. Then sketch carefully two or more of
the follicles with their contents under a high power.

2. Sections across the Fallopian tube. Sketch a section under the low
power.

3. Section across the body of the uterus, or across a cornu of a bicorued
uterus. Observe with the naked eye the thickness of the muscular and
mucous coats respectively. Notice the ciliated columnar epithelium lining
the organ and extending into the glands of the mucous membrane. Draw
a part of the section under the low power.

4. Section of the mucous membrane of the vagina. Notice the stratified
scaly epithelium which lines it and which is continued over the projecting
part of the os uteri.

5. Take the fresh ovary of a recently killed animal aud with a needle or
fine scalpel-point prick one of the largest and most prominent of the Graafian
follicles. The organ must be held just over a slide so that on pricking the
follicle the fluid contents may spui't out on to the glass. E.xamine tlie drop
of liquor folliculi with a low power for the escaped ovum, which will be
surrounded by follicular cells. When found place a piece of hair in the
drop, cover with cover-glass and examine with high ])ower.

THE OVARY.

The ovary is a small solid organ, composed of a stroma of fibrous
tissue, with many spindle-shaped cells, and also containing, near its
attachment to the broad ligament, a large number of plain muscular
fibres. It is covered by a layer of small columnar epithelium-cells
{germinal epithelium, fig. 420, a), between which may here and there
be seen a few larger spheroidal cells, with large round nuclei. In
the young subject the epithelium occasionally dips down into the
subjacent stroma.

The stroma is beset wath vesicles of diflferent sizes, the smallest
being near the surface of the organ, the larger ones placed more
deeply in the stroma, although, as they increase in size, they extend
towards the surface.

THE OVARY.

346

These vesicles ave tlie Graafian foJlides. Each Gruafiaii follicle has
a proper wall (theca follicuU) formed of a layer derived from the stroma,
and a special inner layer containing large cells : both are highly
vascnlar. Each follicle contains an ovum and epitheliiun. In the
smallest follicles the ovum is small, and the epithelium of the follicle
is formed of a single layer of cells, which may be flattened against
the ovum (fig. 421). In somewhat larger follicles the epithelium-cells
are in two layers, and these are columnar in shape (fig. 423, E). In
still larger ones, each of these two layers is formed of several strata of
cells, and fluid has begun t-o collect between the layers at one part. Of
the two layers, the one which lines the cavity of the follicle is termed
the memhrana granulosa, while the mass of cells which more immediately
surrounds the ovum is known as the cumulus or discus proligerus.

Fig. 419. — Section of the ovary of the cat. J. (Schron.)

1, outer covering and free border of the ovary ; 1' , attached border ; 2, tlie central ovariau
stroma, showing a fibrous and vascular structure ; 3, peripheral sti-oma ; U, blood-
vessels ; 5, Graafian follicles in their earliest stages Ij'ing near the surface ; 6, 7, S,
more advanced follicles which are embedded more deeply in the stroma ; 9, an almost
mature follicle containing the ovum in its deepest part ; 9' , a follicle from which the
ovum has fallen out in preparing the section ; 10, coi-pus luteum.

In the largest follicles the fluid has much increased in amount, so
that the follicle has become gradually larger and more tense. Finally
it reaches the surface of the ovary, and projects from that surface,
where it eventually bursts, and the liquor folliculi, with its contained
ovum, is set free. This event is believed to occur usually at some time
during menstruation.

Some of the Graafian follicles do not burst, but, after attaining a
certain stage of maturity, undergo a process of retrograde metamor-
phosis and eventually disappear.

The ovarian ova or ovocytes are large spherical cells (fig. 424),
about 0 2 mm. {^\-g inch) in diameter. When fully formed, as in the

346

THE ESSENTIALS OF HISTOLOGY.

largest Graafian follicles, each ovum is surrounded by a thick trans-
parent membrane {zona pellucida). Within this is the protoplasm of
the ovocyte (vitellus), filled with fatty and albuminous granules (yolk
granules). Lying in the vitellus, generally eccentrically, is the large

Fig. 420.— Section of the ovary of an adult bitch. (Waldever.)

a, germ-epithelium ; h, remains of egg-tubes ; c, small follicles ; d, more advanced follicle ;
e, discus proligerus and ovum ; ./; second ovum in the same follicle (this occurs but
rarely); g, outer tunic of the follicle; h, inner tunic; i, membrana granulosa; k,
collapsed retrograded follicle; I, blood-vessels; in, m, longitudinal and transverse
sections of tubes of the parovarium ; y, involuted portion of the germ-epithelium of
the surface ; z, place of the transition from peritoneal to germinal or ovarian
epithelium.

clear round nucleus {germinal vesicle), which may show an intranuclear
network, and invariably has a well-marked nucleolus {germinal spot),
sometimes more than one.

Oogenesis. — Both the ova and the epithelium of the Graafian follicles

THE OVARY.

347

originate from the germinal epithelium of the embryo. This forms at
first a simple layer covering the stroma, but later becomes thickened
and multiple. After a time rounded cords of epithelium-cells {pjjfj-
tiibes of Pfliiger ; fig. 423, A), i^row down into the stroma, whilst this

TJ- '.jv ij-r\r'i:i^/j i \<='

iyTT^HT'.rr/z/:. ,_ w.<

yM^^^-

Fig. 421. — Section of part of human ovary sHowiNd small Graafian

FOLLICLES IMBEDDED IN A FIBRO-CELLCLAR STROMA. (Sellheim.)

Fig. 422.— a moderately large Graafian follicle from the human ovary,
showing ovum surrounded by '"discus proligerus " and wall of follicle

LINED BY "MEMBRANA GRANULOSA." BETWEEN THEM IS AN ACCUMULATION

OF LIQUOR FOLLICULI. (Sellheim.)

348

THE ESSENTIALS OF HISTOLOGY.

at the same time grows into the epithelium. The cords presently
become broken up by ingrowths of stroma into small isolated nests of

Fig. 423. — Figures showing various stages in the developjient of the
Graafian follicles of the rabbit.

A, from ovary of young rabbit, showing " egg-tubes " of Pfiiiger gi-owing iu from germinal
epithelium ; some of the tubes contain primitive ova ; b, pi-imitive Graafian follicles
formed from the breaking up of an egg-tube ; c, a young Graafian follicle, with a
single layer of follicle-epithelium ; D, a somewhat older follicle, with the second layer
forming within the first ; e, a more advanced follicle, showing two complete layers of
columnar epithelium surrounding the ovum within the follicle.

epithelium-cells, each of which may represent a Graafian follicle. To
form the ova, some of the cells become enlarged (primitive ova),
and usually there is one such enlarged cell in each of the isolated

THE OVARY.

349

nests.^ The remaining cells form the- epithelium of the follicle (see
fig. 42'S, B, g). It is stated that the protoplasm of the ovum remains
connected with the cells of the discus proligerus by fine processes

0\^\\\ 'r

Fig. 424.— Human ovum; highly magnified. (Wakleyer.)
The zona pellucida is surrounded by cells of the discus proligerus, which are adherent to it.

which pass through pores in the zona pellucida, and on the other
hand, the epithelium-cells of the follicle are themselves inter-connected
by protoplasmic bridges, so that the whole forms a syncytium.

1 The nuclei of the primitive ova pass through the pre-inaiotic changes mentioned
on p. 14.

350

THE ESSENTIALS OF HISTOLOGY

The stroma of the ovary contains, besides the spindle-shaped con-
nective-tissue cells and plain muscular fibres already mentioned, a
number of epithelium-like interstitial cells. Some of these are derived
from the germinal epithelium, and appear capable of developing
into ova and follicle epithelium-cells (Lane-Claypon) ; others have
originated from cells of corpora lutea. These last are large yellow

Fig. 425. — Three stages ix the for-
mation' OF THE CORPUS LCTEUM IN
THE MOUSE. (Sobotta.)

A. The follicular epithelium, fe, is hyper-
trophied, and vascular processes, a, of the
theca, th, or wall of the follicle are growing
into it.

The epithelial mass is now subdivided
into loVjule-like masses, /, of luteal cells by
the thecal ingrowths ; e, epithelium of
surface of ovary.
C.' There are now very numerous thecal
septrt. or trabeculaj, and the columns of
luteal cells are much narrower. A central
cavity is still seen.

nodules which are developed out of the Graafian follicles after the
ova have been extruded (figs. 425, 426). They consist of columns of
large yellowish cells {luteal cells), with intervening trabecul^e of vascular
fibrous tissue, which converge to a central strand of connective tissue
occupying the axis of the nodule (fig. 426). The columns of cells
are not unlike those of the cortex of the suprarenal capsule. The
corpus luteum is derived from the wall — probably in the main from
the epithelium — of the follicle, which becomes thickened and folded
by multiplication and hypertrophy of its cells ; between the folds

THE OVARY.

351

connective tissue and blood-vessels grow in from the theca towards
the centre of the follicle ; in this way the columnar arrangement
above mentioned is produced. After persisting for a time the corpus
luteum gradually disappears, its tissue becoming merged in the
surrounding stroma. Corpora lutea grow much larger and remain
much longer persistent in the event of pregnancy supervening.

\

Fig. 426. — Corpus lcteum of jiocse. (Sobotta.)

This figure shows a more advanced stage of development, the luteal tissue being now

vascularized and the central cavity obliterated.

The use of the corpus luteum is not known certainly, but it has recently
been suggested that it may yield an internal secretion, the effect of which
is to produce the fixation of the fertilized ovum in the uterine mucous
membrane (Born). In confirmation of this, experiments seem to indicate
that gestation does not supervene in animals whose corpora lutea have been
destroyed (Fraenkel and Cohn), or from which the ovaries have been removed
during the first stages of pregnancy (Marshall and Jolly).

The blood-vessels of the ovary are very large and numerous, and are
especially distributed to the walls of the Graafian follicles, over -which they
form a close network.

THE FALLOPIAN TTBES AND UTERUS.

The Fallopian tubes are lined by a very vascular mucous membrane
which is covered with ciliated epithelium, and has numerous longi-
tudinal folds (fig. 427). Externally they are covered by a serous
coat, within which is a thin longitudinal stratum of plain muscular

352

THE ESSENTIALS OF HISTOLOGY.

fibres overlying circular fibres of the same tissue, but these layers are
not distinctlj' marked off" from one another.

The human uterus is composed of two parts, the body and cervix.
The body of the uterus is formed of the following layers :

L A seroua layer, derived from the peritoneum, which covers the
greater part of the fundus.

2. A muscular laiier, which is of considerable thickness and is formed
of plain muscular fibres disposed in three, more or less blended, strata.
Of these the outer has its fibres arranged partly longitudinally, partly
circularly. The middle muscular layer, on the other hand, is thick ;

Fio. 427.— Section across the fallopian tcbe. (Diagrammatic.)

its fibres run in different directions, and it contains the ramifications
of the larger blood-vessels. The inner layer, again, is thinner and has
both longitudinal and circular fibres, many of the latter being pro-
longed internally into the deeper part of the mucous membrane ; the
extremities of the uterine glands extend between and amongst its
fibres.

3. A mucous membrane, which is ver}' thick and is composed of soft
connective tissue containing a large number of spindle-shaped cells. It
is lined by ciliated epithelium and contains long, simple, tubular
glands, which take a curved or convoluted course in passing through
the membrane (fig. 429). Their (ciliated) epithelium is continuous with
that which covers the inner surface of the mucous membrane. In the
cervix the mucous membrane is marked by longitudinal and oblique
ridges, and the glands are shorter but more complex than those of the

THE FALLOPIAN TUBES.

353

Fi(

Sii

!•)>

MX'TION OF MUCOUS MEMBRANE OF HUMAN UTERUS DURING MEN-
STRUATION, SHOWING MASSES OF BLOOD WHICH HAVE ESCAPED PROM RUPTURED
CAPILLARIES INTO THE INTEKGLANDULAR TISSUE, AND HAVE AT ONE PLACE (*)
BROKEN THROUGH THE SURFACE EPITHELIUM. (Sellheim.)

Fig. 429. — Section of a cornu of the babbit's uterus.

serous layer; l.ni., longitudinal muscular fibres; cm., circular muscular fibres of the
muscular coat ; (i, areolar tissue with large blood-vessels; m.m., muscularis mucosie ;
iH, mucous membrane.

Z

354

THE ESSENTIALS OF HISTOLOGY.

body of the uterus, and are lined by columnar mucus-secretin
cells. Near the os uteri the epithelium
becomes non-ciliated columnar, and at the
margin of the os uteri this passes into a
stratified epithelium which overlies vascular
papillae of the cerium. The mucous mem-
brane is very vascular, and it also contains a
large number of lymph -vessels.

In many animals the uterus is composed of
two long tubes (cornua uteri) : the arrange-
ment of the muscular tissue in these is simpler
than in the human uterus, which has been
formed by the fusion of two such tubes.
Fig. 429 exhibits the structure of a cornu of
the uterus of the rabbit.

At each menstrual period the mucous
membrane of the uterus undergoes a partial
process of disintegration accompanied by an
escape of blood from the capillaries of the
membrane (fig. 428). This is succeeded by
a rapid renewal of the disintegrated part.
Should gestation supervene, the process of
renewal results in the formation over certain
parts of a greatly thickened mucous mem-
brane, with long convoluted glands, which is
then known as the decidua. The muscular
layer also becomes enormously hypertrophied,
this hypertrophy being produced by the
Fig. 430. — Muscular enlargement of the individual muscle cells

FIBRES (a) FROM NON- . a\

PREGNANT, (6) FROM PREG- (fig. 430).
NANT UTERUS, DRAWN TO
THE SAME SCALE. (Sell-

heim. )

THE SPINAL CORD. 355

LESSONS XXXIX. AND XL.

STRUCTURE OF THE SPINAL CORD.

]. Sectioxs of the spinal cord from the cervical, dorsal, and lumbar regions.
If the human spinal cord cannot be obtained sufficiently fresh, that of a
dog, cat, or monkey may be used. It is to be hardened by suspending it
immediately after removal from the body in a tall jar of formol (10 per cent,
solution). After a few days it may be transferred to alcohol. Sections are
to be made either by the paraffin or celloidin method : the former is prefer-
able for small cords. The sections may be stained by Nissl's method, which
brings to view the nerve-cells and also stains the axis-cylinders of the nerve-
fibres. If it is desired to stain by the Weigert-Pal method, which colours
the medullary sheaths of the nerve-fibres, the pieces of cord should be placed
in 2 per cent, bichromate of potassium solution or Miiller's fluid (either at
once or after formol) and should be left for about a month, after which they
are cut by a freezing microtome. (For the details of these methods see
Appendix.) Carminate of ammonia or thionin may also be employed to
stain the nerve-cells and axis-cylinders.

Notice the relative extent of the grey as compared with the white matter
in the different regions of the cord.

Sketch a section from each region under a low power. Sketch also a
small portion of the white substance, two or three nerve-cells, and the central
canal with its lining epithelium and surrounding neuroglia under the high
power.

Measure the diameter of some of the nerve-fibres in the anterior columns,
in the lateral columns, and in the posterior columns.

2. Tracts in the spinal cord. The conducting tracts of the spinal cord may
be studied in two ways, viz. : (1) by preparing sections of embryonic cords
(from the 5th to the 9th mouth), the sections being stained by the Weigert-Pal
process (Flechsig's method) ; (2) by preparing .sections from the cord of an
animal in which either a complete section or a hemi-section has been performed
about 15 days before the animal is killed, and staining thin pieces of the
cord from below and from above the section by placing them in a solution
consisting of two parts of Miiller's fluid and 1 part of 1 per cent, osmic acid
(Marchi's method). The cord must first be partly hardened by placing it for
a few days in Miiller's fluid.

The spinal cord is composed of grey matter in the centre and of
white matter externally. It is closely invested by a layer of connective-
tissue containing numerous blood-vessels {pia mater), and less closely
by two other membranes (fig. 431). One of these is an areolar mem-
brane, resembling a serous membrane in general structure, but non-
vascular and more delicate in texture {arachnoid). The other, which
lines the vertebral canal, is a strong fibrous membrane known as the
dura mater. At the middle of the anterior and posterior (ventral and

356

THE ESSENTIALS OF HISTOLOGY.

dorsal) surfaces the pia mater dips into the substance of the cord in the
anterior and j^osterior median fissure.^, so as to di\'ide it almost completely
into two lateral halves. These are, however, united by an isthmus or
bridge, which is composed anteriorly of transversely crossing white
fibres (white or anterior amwmsure), posteriorly of grey matter {grey
commissure), in the middle of which is a minute canal lined by ciliated
epithelium (central canal).

Each lateral half of the spinal cord contains a crescent of grey
matter, which is joined to the corresponding crescent of the opposite
side by the grey commissure. Of the two horns of the crescent the
posterior or dorsal is the narrower and comes near the surface of the

3^

Fig. 431.— Section of the
spinal cord within its
IIEMBRANES. (Key and
Retzius. ).

II, duva luater ; b, arachnoid ;
'•, .septum of arachuoid ; d, e,
tr.iliecula; of arachnoid ; g,
ligamciitum denticulatum ;
/, bundles of posterior root ;
h, bundles of anterior root ;
k, I, subarachnoid space.

cord ; close to it the bundles of the posterior nerve-roots enter the
cord. The Ijundles of the anterior nerve-roots emerge from the
anterior horn.

According to Ingbert about 1,300,000 nerve-fibres enter the cord by the
posterior roots, and about one-tliird that number leave it by the anterior
roots.

The posterior root-fibres are derived from the cells of the spinal ganglia,
which lie out.side the cord; the anterior root-fibres from cells v^ithin the
grey matter, cliiefly from cells in the anterior horn, but also from some cells
in tlie middle and posterior parts of the grey matter and (especially in tlie
thoracic region) from cells in the intermedio-lateral tract (lateral horn). The
latter probably furnish the autonomic (sympathetic) fibi'es of the anterior
roots, while the cells of the anterior horn furnish the fibres which are
distributed to the voluntary muscles.

The v}iite matter of each half of the cord is subdivided by the
approach of the posterior horn to the surface into two unequal
columns — antero-lateral and posterior. A distinction is sometimes
drawn between anterior and lateral portions of the antero-lateral

THE SPINAL CORD.

357

pustero-lateral fissui
postcro-inusial column

postero-niediau fissur

postcridi- root-bundl

posterior colmiii

subst. gelat. of

post, liurn ^'■'

tractof Fluclisiu J

lat. pyram. tr _j

form. retic._

lateral liorii _

central canal -

aut. commissun

anterior horn —

ant. median fissure

Fig. 432. — Section of human .spinal cokd from upper cervical region.
(Photograph.) Magnified about 8 diameters.

f.a

• e •

s>^

. ^, .

« •

Fig. 433.— a small portion of a transverse section of the human spinal
coRu in the region of the lateral column, to show the superficial
neuroglia.

a, a, s\iperficial neuroglia ; b, b, transverse section of part of the lateral column of the
cord, in which the dark points are the axis-cylinders, and the clear areas the
medullary substance of the nerve-fibres. The superficial neuroglia is seen to exhibit
the appearance of a fine feltwork in which numerous nuclei and one or two corpora
mw/iacea, c.a., are embedded, and to extend inwards (c, c) among the nerve-fibres.

358 THE ESSENTIALS OF HISTOLOGY.

column, although there is no line of demarcation between them.
In the upper part of the cord the posterior column is subdivided
by a septum of connective tissue into two — the poster o-mesial column or
funiculus gracilis, and the postero-lateral column or funiculus cuneatus.

The white matter is composed of longitudinally coursing medullated
nerve-fibres, which in sections stained with carmine or thionin appear
as clear circular areas with a stained dot, the axis-cylinder, near the
middle (fig. 433) ; while in sections stained by the Weigert-Pal method
they appear as black circles with a clear centre. The nerve-fibres
vary in size in different parts ; on the whole those which are nearest to
the surface of the cord are larger than those nearest to the grey
matter, but there is a bundle of very small fibres (at M, fig. 434)
opposite the tip of the posterior horn.

The medullated fibres are supported by neuroglia, which is com-
posed of fibrillated neuroglia-cells (fig. 192, p. 161). The neuroglia is
accumulated in greater amount at the surface of the cord, underneath
the pia mater (particularly in the human cord, near the entrance of the
posterior roots (fig. 433)), and it extends into the grey matter, in which
it is especially accumulated in the substantia gelatinosa at the apex
(caput) of the posterior horn and around the central canal.

The grey matter, besides neuroglia, contains an interlacement of
nerve-fibres and the arborisations of the nerve-cells which are
embedded in it.

Characters of the spinal cord in the several regions (figs. 434, 439).
— In the cervical region the white matter, especially that of the lateral
columns, occurs in largest proportion. The grey matter in the cervical
enlargement is also in considerable amount, and it encroaches, especi-
ally in the U[)per part of the region, in the form of a network (foimatio
reiimlaris) upon the adjacent part of the lateral white column (fig. 432).
The anterior horns are thick and the posterior slender. The postero-
mesial column is distinctly marked off.

In the dorsal region the grey matter is small in amount, and both
horns are slender. The whole cord is smaller in diameter than either
in the cervical or lumbar region. The columns of nerve-cells known as
Clarke's column and the intermedio lateral tract are well marked.

In the lumba.r region the crescents of grey matter are very thick,
and the white, substance, especially the lateral columns, relatively
small in amount. The isthmus lies nearly in the centre of the
cord, whereas in the cervical and dorsal regions it is nearer the
anterior surface.

In the part of the spinal cord from which the sacral and coccygeal
nerve-roots take origin the grey matter largely preponderates, the

THE SPINAL CORD.

359

/

Fig. 434. — Sections of human
spinal cord from the lower
cervical (a), mid-dorsal (b),
and mid-lumbar (c) regions,
showing the principal groups
of nerve-cells, and on the
right side of each section
the conducting tracts as thky
occur in the several regions.

6, c, groups of cells of the anterior
horn ; (7, cells of the lateral horn ;
(', middle group of cells ; ,/, cells of
Clarke's column ; g, cells of posterior
horn ; e, c, central canal ; a.c. anterior
commissure ; m, marginal bundle of
Lissauer ; p-ra, septomarginal tract.

POSTERIOR
ROOT
BUNDLES

360, THE ESSENTIALS OF HISTOLOGY.

crescents form thick irregular masses, and the grey isthmus is also of
considerable thickness.

TRACTS OF NERVE-FIBRES IN THE WHITE COLUMNS.

The course of the nerve-tracts in the spinal cord, and in other parts of
the central nervous system, can be made out by the method of Flechsig,
which involves the study of sections of the developing cord ; for
it is found that the formation of medullary substance occurs sooner
in some tracts than in others, so that it is easy to make out the
distinction between them. Thus, the peripheral nerves and nerve-
roots become myelinated in the first half of the fifth month of foetal
life. Of the tracts of the spinal cord, those of Burdach and Goll (see
below) are the first to be myelinated, then the tracts of Flechsig and
Gowers, all of these being sensory or centripetally conducting, while
the pyi-amidal tracts, which are motor or centrifugally conducting,
do not receive their myelin sheath until after birth. ^

Another method (that of A. Waller) consists of investigating the
course which is pursued by degeneration of the nerve-fibres in
consequence of lesions produced accidentally or purposely. Those
tracts in which degeneration of fibres occurs below the lesion are
termed "descending" tracts ; those in which it occurs above the lesion
are termed "ascending."

The cells whence the fibres of any tract arise can be identified after a
lesion of the tract by the chromatolysis or degeneration of Nissl which
nerve-cells undergo after section of their axons (see pp. 154 to L59).

Tracts of the posterior column. — 1. Trad of Goll. - The fibres of the
poster o-mesial colnmn belong to a tract which is known as the tract of
Goll (fig. 435, 6). This consists of fibres derived from the posterior
nerve-roots of the sacral, lumbar, and lower dorsal nerves, which,
after having entered the posterolateral columns, pass, as they ascend,
towards the posterior median fissure aud form a distinct tract, which is
marked off from the rest of the posterior column in the cervical region
by a slight furrow and a septum of pia mater (fig. 432). This tract
ends amongst the cells of the nucleus gracilis of the medulla oblongata.

2. Tract of Burdach. — The paste ro-lateral column (tract of Burdach) is
also composed of fibres of the posterior nerve-roots, which all run for
a certain distance in it before entering the grey matter of the cord
or of the medulla oblongata. As each mass of posterior root-bundles

^ Flechsig finds that the fibres of the posterior roots are myelinated in at least
three stages, and that the postero-lateral tract shows a corresponding differentia-
tion into three chief parts: the xx'ntral, middle and dorsal root-zones. He suggests
that this differentiation corresponds with functional differences of the fibres.

THE SPINAL CORD.

361

s^L

enters the column close to the apex of the posterior horn it, so to
speak, pushes the root-fibres which have already entered nearer to
the median fissure ; hence those which are derived from the lowest
nerve-roots are nearest that fissure (in the tract of (tIoII), while those
which are derived from the highest remain near the posterior horn
(in the tract of Burdach). Many of the fibres of both tracts pass
into the grey matter either immediately on entering the cord or
in their course upwards ; the rest are continued into the medulla
oblongata and those of the tract of Burdach end by arborising amongst
the cells of the nucleus cunenhis.

3. Comma trad. — Besides the tracts of Burdach and Goll, which are
wholly composed of long " ascending " fibres having their cells of origin

Fio. 435.— Diagram showing

THE A.SCENniNG (RIGHT SIDE)
AND DESCENDING (LEFT SIDE)
TRACTS IN THE SPINAL CORD.
1 , Crossed pyramidal ; 2, direct
pyramidal ; 3, antero-lateral de-
scending ; 3a, bundle of Helweg ;
4, prepyramidal ; 5, comma ; 6,
postero-mesial; 7, postero-lateral;
8, tract of Lissauer ; !', dorsal cere-
bellar ; 10, antero-latei-al ascend-
ing or ventral cerebellar ; s-m,
septo-inarginal ;s.;)./.,superficiai
postero-lateral fibres (dorsal root
zone of Flechsig) ; a to i>5, groups
of cells in the anterior horn ; (,
intermedio-lateral group or cell-
column in the lateral part of the
grey matter ; p, cells of posterior
horn ; rf, dorsal nucleusof Stilling
or cell-column of Clarke. The
fine dots indicate the situation of
"endogenous" fibres (arising in
grej' matter of cord) having for
the most part a short course.

in the ganglia on the posterior roots, there are a few fibres which have
a shorter "descending" course in the posterior column. These are
believed by some authorities to arise from descending branches of the
posterior root-fibres, by others to arise from cells in the grey matter
of the cord. They form the so-called com ma tract (fig. 435, 5).

Proprio-spinal or endogenous fibres of the posterior column. —
There are a few fibres (septo-marginal), chiefly accumulated near the
median fissure (oval bundle) and near the posterior surface (median
triangle bundle), but also scattered in other parts of the column,
which are derived from cells in the grey matter of the cord itself.
These take a "descending" course in the postei-ior column; while
others which arise in the grey matter and have an " ascending "
course arc especially numerous in the ventral part of the column.

Descending tracts of the antero-lateral column. — 1. Pyramidal or
corticos/iinal tract. — At the posterior part of the lateral column there
is a tract of moderately large "descending" fibres (intermingled with

362

THE ESSENTIALS OF HISTOLOGY.

CEREBELLAR
HEMISPHERE

Fig. 436.— Diagram showing the course, origin, and termination of the

fibres of the principal tracts of the white matter of the spinal

CORD. (The numliers in this diHgram refer to fibies of the tracts showa

with correspondiiig numbers in fig. 435.)

" Descending" tracts :— In, a fibre of the crossed pj-ramidal tract ; lb, an uncrossed fibre of

the pyramidal tract passing to the lateral column of the same side ; S, a fibre of the

dii-ect pyramidal tract; 5, a fibre of the anterolateral descen'iing tract ; i, a fibre of

the prepyramidal tract; 5, fibres of the comma tract. "Ascending" tracts:— 6. a

fibre of the postero-mesial tract ; 7, fibres of the postero lateral tract ; 9, one belonging

to the dorsal cerebellar ; 10, a fibre of the ascending antero-lateral or ventral cerebellar

tract. Also, )k, motor nerves ; s, sensory (afferent) nerves.

THR SPINAL CORD. 363

smaller fibres) which are found to run in the lateral column of the
spinal cord from the opposite side of the brain, after having for the
most part crossed at the decussation of the pyramids of the medulla
oblongata (cro.tsed lateral pyramidal trad, fig. 435, 1 ; fig. 436, la)-
Intermingled with the fibres of the crossed pyramidal tract in the
lateral column are a few fil)res of the pyramid which have not crossed
in the medulla oblongata, and which are therefore derived from the
cerebral cortex of the same side (uncrossed lateral pyramidal fibres,
fig. 436, lb). The large fibres which lie in the anterior columns next
to the anterior median fissure, which are especially numerous in the
upper part of the human coid, also l)elong to a portion of the same
tract which has not undergone decussation {direct pyramidal tract,
figs. 435, 436, 2). The direct pyramidal tract is only found in man
and the anthropoid apes ; in some individuals it is absent, and it
varies considerably in extent.

The pyramidal tracts are composed of " descending " fibres, which
have their cells of origin in the cerebral cortex (ascending frontal
and paracentral gyri) and end by arborisations in the grey matter
at the base of the postei'ior cornua of the spinal cord. In some
mammals (rat, mouse, guinea pig, sheep, kangaroo, squirrel, etc.), the
pyramidal tracts are situated in the posterior columns of the cord,
in others, including the monkey, dog, cat, and rabbit, they run in the
lateral columns The pyramidal tracts are very small in the lower
mammals, and are not found at all in vertebrates below mammals.

It has been calculated that there are about 80,000 fibres of the
pyramidal tract in each half of the human cord. The pyramidal tracts
are generally regarded as the paths along which volitional impulses are
conveyed from the cerebral cortex to the spinal cord. But experiments
have shown that they are not the only cortico-spinal paths nor even
the most important in many animals, for the paralysis which results
from their section is .soon recovered from in most animals, whereas
that resulting from section of the anterior column and adjacent part of
the lateral column may be more marked and permanent. In man it
appears to be the finer and more delicate movements which are
permanently lost when the pyramidal tract is affected by disease.

2. Tract of Loewenthal. — Besides the pyramidal tracts there are
four other "descending" tracts of fibres in the antero-lateral column.
One of these (the anicro-lateral descending tract or tract of Luewentluil,
figs. 435, 436, 3) lies on the side of the anterior median fissure, and
extends along the margin of the cord in the "root" zone, even
reaching the anterior part of the lateral column. These fibres are
continued down, chiefly from the posterior longitudinal bundle (vestibulo-

364

THE ESSENTIALS OF HISTOLOGY.

Fig. 437.— Diagram showing the course of the tr.\cts of Flkchsig and
of gowers in the spinal cord and their continuations to the cere-
bellum, corpora quadrigemina, thalamus and cortex cerebri.

a, posterior root-fibres ; h, tract of Flechsig, passing at b', by the restiform body to the
cerebellar vermis ; c, tract of Gowors ; rf, passage of most of its fibres along the
superior peduncle to the cerebellum ; c, fibres to the corpora quadrigemina, «' ; /,
others to the thalamus ; g, fibres from thalamus to cerebral cortex.

THE SPINAL CORD. 365

^inal fiJyres) of the nicdulhi o])longatti aivd pons Varolii, partly from
other sources which will l)e afterwards referred to. They end by
arborisations in the anterior horn. Similar arborisations pass from
the posterior lotii^itudinal bundle to the nuclei of the motor cranial
nerves. This tract is mainly uncrossed.

3. Rubrospinal trad. — Another "descending" tiact in the antero-
lateral column lies just in front of the crossed pyramidal tract. This
is the prepijramidal or ruhrospinal trad (figs. 435, 436, .4); its fibres
end by arborising in the grey matter of the middle of the crescent ;
the situation of its cells of origin is the red nucleus of the tegmentum
in the mid-brain. This tract is also known as Monukow's trad.
Some of its fibres are stated to be derived from cells in the reticular
formation of the pons and medulla oblongata.

4. Tedu-spimd fibres. — Intermingled with the fibres of the rubro-
spinal tract (but far fewer in number in man) are fibres derived
from the quadrigeminal bodies of the opposite side. These fibres
form a part of the tcdo-spinal trad. Another part of this tract passes
into the anterior column of the cord in the tract of Loewenthal
above mentioned.

5. Olivospinal trad. — Lastly a small triangular group of "descend-
ing" fibres traceable from the neighbourhood of the olive in the
medulla oblongata, and passing down the cervical cord in the
anterior part of the lateral column (fig. 435, Sa), (the exact origin
and destination of the fibres is unknown) is termed the bundle of
Hehceg or olivosjmial trad.

Ascending tracts of the antero-lateral column. — 1. Trad of
Fledisig. — This is a well-marked tract, which is however only distinct
in the cervical and dorsal regions, where it lies external to the
crossed pyramidal tract. It consists of large fibres which are derived
from the cells of Clarke's column (fig. 434, /) and which pass up into
the cerebellar vermis by way of the inferior peduncle of the same side
{dorsal spina-cerebellar bundle or dired cerebellar tract of Flechsig, fig. 434;
figs. 435, 436, 9 ; 437, b, U).

2. Trad of Gowerc. — This is situated more anteriorly, lying in
front of the crossed pyramidal and direct cerebellar tracts in the
luml)ar region ; while in the dorsal and cervical regions it forms a
narrow band of fibres curving round close to the external surface
of the cord, and extending even into the anterior column. It was
termed the aidcro-lutcral ascending trad by Gowers (figs. 43r), 436, 10).
Its fibres are intermingled with those of the antero-lateral descend-
ing tract. Most of the fibres of the tract of Gowers are con-
nected with the vermis of the cerebellum, constituting the ventral

366 THE ESSENTIALS OF HISTOLOGY.

spino-cerebellar hindle, which passes to that organ over and parallel
with the superior cerebellar peduncle (fig. 437). According to Van
Gehuchten, confirmed by Collier and Buzzard, the tract of Gowers
gives oflp a few fibres to enter the opposite cerebellar hemisphere
by the middle peduncle.

Some of the fibres of the antero-lateral ascending tract {spino-tedal
fibres) are continued up to the corpora quadrigemina. Others pass
into the tegmentum of the crus cerebri, where they can be traced as
far as the lower part of the thalamus (spino-thalamic fibres).

The cells from which the fibres of Gowers' tract take origin are
not certainly known, but it is probable that they are cells situated
in the middle and posterior parts of the grey crescent, partly on
the same but chiefiy on the opposite side of the cord. The latter is
almost certainly the case with the cells from which the spino-thalamic
fibres arise.

3. Trad of Lissauer. — Lastly, there is another small tract of fibres
which undergoes degeneration above the point of section. This is
the marginal bundle of Lissauer (marked M in fig. 434). It is formed
by fine fibres from the posterior roots.

Other portions of the antero-lateral columns near the grey matter
which are difterentiated by the method of Flechsig are probably short
tracts uniting adjacent portions of the grey matter of the cord.

Proprio-spinal or endogenous fibres of the antero-lateral column. —
Sherrington has shown that in the dog the lateral column in the dorsal
region of the cord contains a certain number of long fibres which take
origin in the cervical, dorsal and upper lumbar segments and are
traceable down to the lumbo-sacral enlargement. These must serve '
to convey excito-refiex impulses from the upper to the lower parts of
the body. Probably similar fibres arise all along the cord from the
cells of the lateral column and pass upwards as well as downwards.

A tract of endogenous fibres has been observed in man close to the
anterior median fissure lying amongst the fibres of the direct pyramidal
tract. This is known as the anterior sulco-marginal trad of Marie.

The antero-lateral column contains also many endogenous fibres,
both ascending and descending, derived from cells in the grey matter
of the cord, which have only a short course, serving to connect
adjacent segments.

GREY MATTER OF CORD.

The nerve-cells which are scattered through the grey matter are
in part disposed in definite groups. Thus there are several groups
of large multipolar nerve-cells in the anterior horn in the cervical and

THE SPINAL CORD.

367

in.

VII.

VIII.

Fig. 488. — Diagram of sections of the spin.al coed of the monkey show-
ing THE POSITION OF DEGENERATED TRACTS OF NERVE-FIERES AFTER
SPECIFIC LESIONS OF THE CORD ITSELF, THE EFFERENT NERVE-ROOTS AND

OF THE MOTOR REGION OF THE CEREBRAL CORTEX. (The degenerations are
shown by the method of Marchi.) The left side of the cord is at the
reader's left hand.

I. Degenerations resulting from extirpation of the motor area of the cortex of the left

cerebral hemisphere.

II. Degenerations produced by section of the posterior longitudinal bundles in the upper
part of the medulla oblongata.

III. and IV. Result of section of posterior roots of the first, second, and third lumbar
nerves on the right side. Section III. is from the segment of cord between the last
thoracic and first lumbar roots ; section IV. from the same cord in the cervical i-ogion.

V. to VIII. Degenerations re.sulting from (right) semi-section of the cord in the upper
thoracic region. V. is taken a short distance above the level of section ; VI., higher up
the cord (cervical region) ; VII. , a little below the level of section ; VIII. , lumbar region.

368

THE ESSENTIALS OF HISTOLOGY

Sacr.l

Sacr.3

Fig. 439.— Diagram of sections of human spinal cord at different
LEVELS. (Edinger. )
The names refer to the origin of the corresponding nerve roots. The relative shape and
size of the cord and grey matter, and the relative amounts of gi-ey and white matter,
and the principal cell-gi-oups are shown.

THE SPINAL CORD

369

lumbar enlargements (fig. 435), although in other regions of the cord
the number of groups in this situation is reduced to two, a mesial
and a lateral. The larger groups in the enlargements correspond with
segments of the limb (Van (Tehuchten) ; thus there appear to be
groups associated with foot, leg, and thigh, and with hand, arm, and
shoulder movements respectively. The groups from which the motor
nerves to the shouldei' and arm muscles arise appear in somewhat

b a u t ?

Fig. 440.— Diagram showing the probable relations of some of the
CELLS OF the CORD TO THE WHITE COLUMNS. On the left side the col-
laterals from the fibres of the white columns are shown passing into the
grey matter. (Cajal.)
a, b, fibres of posterior column sending collaterals into the grey matter ; c, d, fibres of
posterior root entering posterior column ; c, /", collaterals passing from lateral and
anterior columns into grey matter ; .a, h, i, fibres of white commissure ; j, anterior
root-fibre springing from Ic, cell of anterior lioi-n ; I, m, n, other cells of grey crescent
sending their axons into the white matter ; o, axon of cell of Clarke's column passing
into the dorsal cerebellar tract; p, axon of cell of substantia gclatinosa ; g, fibre of
dorsal cerebellar tract ; r, fibre of posterior root passing to tract of Lissauer ; s, t, cells
of substantia gelatinosa ; u, cell of Clarke's column.

higher segments of the cervical cord than those belonging to the hand
muscles. The same holds good, mutatis mutandis, for the lumbar cord
in relation to the leg and foot. Further, the larger groups show
subdivisions which may be related to particular movements, i.e. to
particular groups of muscles. In the case of the diaphragm there is a
special cell-group or cell-column in the cervical cord (anterior horn)
from which the fibres of the phrenic nerve arise, so that in this case
a cell-group is set apart for a special muscle.

The axis-cylinder processes of the anterior horn cells mostly pass out
into the corresponding anterior nerve-roots (fig. 436, m ; fig. 440, j),
but a few send their axons to the anterior column of the opposite side
through the white commissure (fig. 440, m) or to the anterior or lateral
column of the same side {I, n). It is noteworthy that in birds a few

2a

370

THE ESSENTIALS OF HISTOLOGY.

cells of the anterior horn send their axons into the posterior roots. A
well-marked group of large rounded nerve-cells, best marked in the
thoracic region, lies at the base of the posterior horn (nucleus of Stilling,
Clarke's column, fig. 434, /; fig. 435, d ; fig. 440, u). The cells of
Clarke's column send their axis-cylinder processes into the dorsal
cerebellar tract (Mott), and if this tract V)e cut experimentally, the

Fig 441.— From a LOxcrruDiNAL sec-

TIOX OF SPIXAL CORD, SHOWING THE
ENTRANCE OF POSTERIOR ROOT-FIBRES.

(Cajal.)
A, A, fibres entering the postero-lateral
column, and bifurcating into an ascending
and descending division ; B, C, collaterals
passing from them into the grey matter ;
E, other fibres of the posterior white
columns also giving off collatei-als.

Fig. 442. — Arbori.sation of col-
laterals FROM THE POSTERIOR ROOT-
FIBRES AROUND CELLS IN THE POS-
TERIOR HORN OF GREY MATTER.

(Cajal.)
A, fibres of posterior column derived from
posterior root ; B, collaterals ; C, D, nerve-
cells in grey matter surrounded by the
arborisations of the collaterals ; E, an
aiborisation shown separately.

large cells of Clarke's column on the same side below the section
undergo Xissl degeneration and eventually atrophy. There are, how-
ever, a few small cells with short axons in Clarke's column which do
not undergo this change.

THE SPINAL CORD. 371

Another group is seen on the outer side of the grey matter lying in
a projection which is sometimes known as the lateral horn {lateral
cell-cohuiin, inter media-lateral column, figs. 434, d ; 435, i). This is most
distinct in the dorsal I'ogion (as far up as the second thoracic segment).
The axons from its cells for the most part leave the cord along with
the anterior roots, and probahly furnish the outgoing visceral and
vascular fibres. Another group {middle cell-column) lies in the middle
of the crescent (fig. 434, e). The cells of the posterior horn (cj) are
very numerous hut are not collected into definite groups. Those of
the substantia gelatinosa of Rolando send their nerve-fibre processes
partly into the lateral, partly into the adjacent posterior columns
(fig. 440, s, t).

The cells which seud their axons into the adjacent parts of the white
columns but not into any sjDecial tract are sometimes termed the " cells of
the white columns."

Connection of nerve-roots with spinal cord. — The anterior roots
leave the anterior horn in a nunil)er of bundles. Most of their fil^res
are directly continued from the nerve-cells in the anterior and lateral
horns, and according to Golgi in part also from cells in the posterior
horn. These cells, from which the anterior root-fibres arise, are
surrounded by an interlacement of ramified nerve-endings, which
are derived from various sources, especially the axons of cells of the
posterior horn, from collaterals of the posterior root-fibres (see below),
and from those of the fibres of the adjacent white columns.

The fibres of the posterior roots originate in the cells of the posterior
root ganglia and pass into the posterolateral column (see diagram,
fig. 436), but the smallest fibres enter the
marginal bundle of Lissauer, and some
pass directly into the posterior horn of
grey matter. On entering the spinal
cord the fibres bifurcate (fig. 441), one
branch passing upwards, the other down-
wards. Both from the main fibre and
from its branches collateral fibres pass at
frec^uent intervals into the grey matter,
and end in arborisations of fibrils which ^'^-^if'-^^^Z ?HE™fpZ.
envelop the nerve-cells both of the cord of a child, showing

, „ , ■ y ,n ITS CILIATED EPITHELIUM AND

posterior and of the anterior horn (hg. the surrounding central
442) and in the dorsal region the cells of Su """■ ^^^""^^'^^^^^ "'^'
Clarke's column and those of the inter-

medio-lateral tract. Many of the main fibres also ultimately end in a
similar manner in the grey matter, some after a short course only, but

372

THE ESSENTIALS OF HISTOLOGY.

Fig. 444.— Part of epithelium of central canal of new-born child,

STAINED BY GOLGl'S METHOD. (Sobottfl.) X 120.
ep, epithelium ; ng, neuroglia celLs in adjacent grey matter.

I Fig. 445. — Section of cord of embryo, showing some of the ependyma cells

DETACHED AND BECOMING CONVERTED INTO NEOROGLIA-CELLS. (L^nllOSSek.)

THE SPINAL CORD. 373

others after a longer course. But a considerable number of fibres pass
upwards in the postero-lateral and postero-mesial columns (in the latter
especially those of the lower spinal nerves), until they arrive at the
medulla oblongata, where they end in terminal arborisations around
the cells of the nucleus gracilis and nucleus cuneatus.

The central canal of the spinal cord is lined by columnar ciliated
epithelium-cells {ependyma), which arc surrounded by a quantity of
neuroglia. The cells are best seen in the spinal cord of animals
and in the child (figs. 443, 444) ; in the human adult they have
frequently become proliferated, and their cilia are no longer visible.
In the early embryo their fixed extremities extend through the whole
thickness of the cord to reach the pia mater. This condition is
permanent in the cord of many of the lower vertebrata.

Blood-vessels of the spinal cord. — The blond-supply of the grey matter
is derived mainly from a series of arterioles, which come oil' from the
mesially- situated anterior spinal artery, pass into the anterior median
fissure, and at the bottom of this divide each into two branches, one for the
grey matter of each lateral half of the cord. In the grey matter is a very
close capillary plexus which is supplied not alone by the vessels just
mentioned, but also by small arterioles, which converge from the small
arteries of the pia mater, passing through the white matter, and supplying
this as they pass through it. These arterioles are branches of the above-
mentioned anterior spinal artery and of the posterior spinal arteries (which
run on each side along the line of the posterior roots). The capillary
plexus of the white matter is far less dense than that of the grey matter.
It forms longitudinal meshes.

The veins of the spinal cord accompany the arteries. Two longitudinal
venous vessels, accompanying corresponding anastomotic arterioles, are seen,
one on either side of the central canal, in most transverse sections of the
cord.

374 THE ESSENTIALS OF HISTOLOGY.

LESSON XLI.

THE MEDULLA OBLONGATA.

Sectioxs of the medulla oblongata (made in the same way as with the spinal
cord) : (a) at the level of the decussation of the pyramids, (6) just above the
decussation, (c) opposite the middle of the olivary body, and, (c/) either
through the uppermost part of the olivary body, or just above it.

The brain consists of three great morphological divisions associated
with the three primary cerebral vesicles of the embryo; they are
termed respectively the hind-brain, mid-brain, and fore-brain.

The hind-brain is formed of the parts around the fourth ventricle,
viz., the medulla oblongata or spinal bulb (myelenceplialon), and above
this the pons Varolii with the cerebelhim (metencephalon) : the region
of the corpora quadrigemina forms the mid-brain (mesencephalon) ;
the parts immediately above that region, and centring around the
third ventricle, including the optic thalami, form the thalamencephalon ;
and the corpora striata and cerebral hemispheres constitute the telen-
cephalon.

The structure of the medulla oblongata or spinal bulb can best
be made out by the study of a series of sections taken from below
upwards, and by tracing in these the changes which occur in the
constituent parts of the spinal cord, taking note at the same time
of any parts which may be superadded.

A section through the region of the decussation of the pyramids
(fig. 446) has much the same form as a section through the upper
part of the spinal cord, and most of the structures of the cord can
be easily recognised. A considerable alteration of the grey matter
is, however, produced by the passage of the large bundles of the
crossed pyramidal tract from the lateral column of the spinal cord
on each side through the root of the anterior horn and across the
anterior median fissure to the opposite anterior column of the
medulla oblongata, where, together with the fibres of the direct
pyramidal tract, they constitute the prominent mass of white fibres
which is seen on the front of the bulb, on each side of the middle
line, and which is known as the pyramid. By this passage of fibres
through the grey matter the tip of the anterior horn is cut off from

THE MEDULLA OBLONGATA.

375

the rest and becomes pushed as it were to the side ; part of it
appears as an isolated mass or masses of grey matter, one of which
becomes known as the lateral nucleus. In sections just above the
decussation of the pyramids a wavy mass of gi-ey matter makes its

funiculus gracili.H
post, modiaii fissure -

,/ '^

funiculus cuneatus

nucleus gracilis ^ *;■

7^^^'

_.4

r./ll rj

desc. Vth 4r

bundle from fun. cun. - ' " ' ' —
subst. gelat. Rol.-

tract of Flechsig

pyramidal ti-act

bundle.'

\ :■

*:

decussation of

pyramids

anterior horn

\

ant. median fissure ■.,

pyi-amid

/

Fig. 446.— Section aceoss the lower part of the medulla oblongata in

THE region of THE DECUSSATION OF THE PYRAMIDS. (Magnified 65

diameters.)

appearance on the lateral aspect of each pyramid, corresponding
with a prominence on the surface which is known as the olive. The
wavy or plicated grey matter is termed the olivary nucleus (figs. 447
to 449).

The pyramids (anterior pyramids) of the medulla oblongata are
formed of fibres which originate in the motor region of the cerebral
cortex, and which can be traced from the axons of large cells in the
grey matter of that cortex through the white matter of the hemisphere,
through the middle third or more of the internal capsule and crusta,
through the pyramid bundles of the pons Varolii and into these
structures (pyramids) of the bulb. As we have just seen, they pass
at the lower limit of the bulb chiefly to the opposite or crossed

376

THE ESSENTIALS OF HISTOLOGY.

lateral column of the cord, but partly to the lateral column of the
same side, and, in man and anthropoid apes, partly to the anterior
column. They collectively constitute the p/fvcmiidal trad, which is
smaller in the medulla oblongata than in the pons Varolii, since
many of its fibres have left the main tract whilst within the pons
and have passed across the middle line towards the grey matter on

post, median fissure

nucleus gracilis —
funiculus cuneatus

nucleus cuneatus'

desc. root of otl

central canal

substantia Roland:'

central fibres of Vtl

int. arcuate fibre;

tract of Fleclisig'^

tract of Gowersv

raphe

accessory oliv. nucl.

siliqua oliva;

olivary nucleus'

pyramid

arcuate nucleus^

Fig. 447. — Section taken immediaj'elt above the decussation of the
PYRAMIDS. (Magnified 6g diameters.)

the dorsal aspect of the pons and medulla oblongata. Sometimes
such a bundle of fibres, after passing towards the sensory nuclei in
the lateral part of the medulla oblongata, does not end in them, but
again comes ventral-wards and joins the main or central part of the
tract near its decussation (Imndle of Pick).

It is not a little remarkable that although the fibres of the pyramidal tract
give off numerous collaterals to the grey matter of the cerebral cortex, the
basal ganglia of the cerebrum, the substantia nigra of the mid-brain, the
nuclei pontis of the pons Varolii, and the base of the posterior horn of
the spinal cord, no collaterals are seen to leave them in their course through
the medulla oblongata, except a very few to the olivary nuclei. Various
observers have described collaterals and terminations of the pyramidal fibres

THE MEDULLA OBLONGATA. 377

as passing to the motor nuclei of the cranial nerves as well as to the anterior
horns of the spinal cord, but statements to this effect must be received with
caution for although current in most text-books, they have not been sub-
stantiated by accurate observations. It is certain that most if not all of the
fibres of the pyramidal tract end not in the ventral but in the dorsal part
of the grey matter of the cord.

A change also occurs in the posterior horn in consequence of the
increased development of the posterior column of white matter. This
causes the posterior horns to be pushed towards the side, the V which
they form with one another being thus opened out ; at the .same time
the tip of the hoi'u swells out and causes a prominence upon the surface
of the medulla oblongata, which is known as the tubercle of Rolando.
Its grey matter forms the prolongation of the sensory nucleus of the
fifth nerve. On its outer side and partly embracing it is a bundle of
fibres seen in every section of the medulla oblongata, and traceable up
to the pons Varolii. This is the inferior or descending root of the fifth
nerve — formerly kno^vn as the "ascending" root. Its fibres extend
down as far as the upper cervical region of the spinal cord. Grey
matter also soon becomes formed within the upward prolongations of
the gracile funiculus (postero-mesial column), and of the cuneate
funiculus (postero-lateral column) appearing at first as thin strands in
the middle of the columns, but rapidly increasing in thickness so as
eventually to occupy almost the whole of them, and forming the
nucleus gracilis and the micleus cuneatus respectively.

It is in these nuclei that the fibres of Goll's and Burdach's tracts,
which are continued up from the posterior columns of the spinal cord,
find their ultimate ending in complicated arborisations amongst the
cells of the nuclei. These nuclei do not, however, receive all the
ascending branches of the posterior root fibres, for a considerable
number of these have already disappeared by entering the grey
matter of the cord, in which they also end by arborisation
amongst its cells. The cells of the nucleus gracilis and nucleus
cuneatus are small or of moderate size with long dendrons. Their
axons pass as internal arcuate fibres through the reticular formation
into the inter-olivary layer, cross the median raphe dorsal to the
pyramids (fig. 447), and then turn upwards, constituting the tract of
the fillet. This tract, which in its lowest part is thus formed by the
nerve-fibres which belong to the second relay (or second neurones)
of one of the sensory spinal paths, is reinforced in the higher regions
of the medulla oblongata and in the pons by fibres derived from
cells of the sensory nuclei of the cranial nerves. The majority of
its fibres end in the lateral nucleus of the thalamus, but some pass
to both the anterior and posterior corpora quadrigemina.

378

THE ESSENTIALS OF HISTOLOGY.

According to Van Gehuchten the fibres of the fillet which are derived from
the nucleus cuneatus lie dorsally to those which are derived from the nucleus
gracilis.

The continuation of the central canal of the spinal cord is still seen in
the lower medulla oblongata (figs. 446, 447), but it comes nearer to the
posterior surface and eventually opens out at the point of the calamus
scriptorius of the 4th ventricle (fig. 448). The grey matter which

nucleus gracilis

funiculus cuneatus'
nucleus cuneatus

fasciculus solitarius,
dorsal nucleus of Xth
desc. root of Vth
nucleus of Xlltli,

subst. gelat. Roland

traot of Flechsig.

int. arcuate fibres

rubro-spinal tract

issuing fibres of Xllth,

tract of Gowers

siliqua olivft'
olivary nucleus

ext. arcuate fibres

pyrau 1 id

arcuate nucleus

Fig. 448. — Section across the medulla oblongata at the point of the
CALAMUS SCRIPTORICS OF THE 4th VENTRICLE. (Magnified 65 diameters. )

surrounds it contains two well-marked groups of nerve-cells ; tlie
anterior (ventral) of these is the lower part of the nucleus of the
hypoglossal or twelfth nerve, the posterior (dorsal), with smaller cells,
that of the vago-accessori/ or tenth and eleventh. But most of the grey
matter of the crescent becomes broken up, by the passage of bundles
of nerve-fibres through it. into a reticular formation the production of
which is already foreshadowed in the upper part of the spinal cord.
Instead of the comparatively narrow isthmus which joins the two
halves of the spinal cord, a broad raphe now makes its appearance ;

THE MEDULLA OBLONGATA.

379

this is formed of ol)li([uely and antero-posteriorly coursing fibres,
together with some grey matter containing nerve-cells.

In the section at about the middle of the olive (fig. 449), it will be seen
that a marked change has been produced in the form of the medulla
oblongata and the arrangement of its grey matter, by the opening out
of the central canal into the fourth ventricle. This causes the grey

vestibular nucleus

<lesc. fibres of vestibular
tlorsal nucleus

of Xtl

fasciculus solit.
restiform body
nucl. of Xlltij

subst. gclat.

desc. root of Vth

subst. gelat

int. arc. fibres

and nucl. amb.

issuing fibres of

Xtb

issuing fibres oi
Xllth

raphe

siliqua olivtt;,

hilus olivse,

olivary nucleu

ext. arcuate fibre*

5^r'

arcuate nucleus

m

Fig. 449. — Section across the jiedulla oblongata, .at about the middle
OF THE olivary BODY. (Magnified 6^ diameters.)

matter which lower do^^'Tl surrounded the central canal to be now
spread out at the floor of that ventricle, and the collections of
nerve-cells from which the hypoglossal and A'agus nerves respectively
arise, now, therefore, lie in a corresponding situation near the
ventricular floor. At this level the outer small-celled group which
corresponds with the nucleus of the spinal accessory in the lower part
of the bulb has become the dorsal nucleus of the vagus or tenth nerve,
and yet higher up the dorsal nucleus of the ninth nerve or glosso-

380 THE ESSENTIALS OF HISTOLOGY.

pharyngeal. The nerve -bundles of the roots of these nerves can be
seen in some of the sections (fig. 449) coursing through the thickness
of the bulb and emerging, those of the hypoglossal just outside the
pyramids, those of the vagus at the side of the medulla oblongata.

The posterior part of the section is chiefly occupied by the grey
matter of the floor of the fourth ventricle, and by fibres which are
passing obliquely upwards and outwards towards the cerebellum,
forming its inferior crus (restiform body). The grey matter forming
the nucleus of the funiculus gracilis and of the funiculus cuneatus has
now almost disappeared, but in place of them and near the outer part
of the floor of the fourth ventricle are seen some masses of grey matter
with a number of bundles of nerve-fibres amongst them. The grey
matter is the lower part of the principal nucleus of the vestibular
nerve (see p. 38 7 j, and the white bundles are formed of descending
branches of the fibres of that nen-e. Below these structures is the
descending root of the 5th, with its descending nucleus mesial to it.

The anterior part of the section is occupied in front by the pj-ramid,
and behind this by a reticular formation {reticularis alha), composed of
longitudinally coursing bundles of fibres belonging mainly to the tract
of the fillet and to the dorsal and ventral (posterior and anterior) longi-
tndinal bundles, interlaced with internal arcuate fibres that are passing
across the raphe from the nuclei of the contralateral posterior columns
into the fillet, and from the opposite olive into the restiform body.

The middle portion of the section consists for the most part
of a similar reticular formation, but with more grey matter and
nerve-cells {reticularis grisea). This is a development of the formatio
reticularis of the cervical cord, and the longitudinally coursing
white bundles in it are probably association fibres, derived from
cells in the upper part of the cord. On the other hand the
nerve cells of this grey reticular formation in the medulla oblongata
give origin to fibres which bifurcate and pass both upwards, probably
serving as association fibres for the same area in the pons, and
downwards towards the upper part of the cord. Some also are said
to give origin to fibres, which either after traversing the raphe or
passing directly to the same side as arched fibres, eventually enter
the cerebellum through the inferior peduncle (Van Gehuchten).

Ventro-laterally is the olive, within which is developed a peculiar
wavy lamina of grey matter containing a large number of nerve-cells ;
this is the dentate nudeus of the olive. The lamina is incomplete at its
mesial aspect Onlus olivce), and here a large number of fibres issue, and,
passing through the raphe, course as internal arcuate fibres to the
opposite restiform body, and thus to the cerebellum. Some, however.

THK MEDULLA OBLONGATA. 381

turn sharply round and course helow the dentate nucleus, forming an
investment and capsule to it (siliqiia olivce), and pass towards the
restiform body of the same side : but the main connection of the
olivary nucleus is with the cerebellar hemisphere of the opposite side.
The olives receive numerous collaterals from the neighbouring white
columns, including a few from the pyramids. Just dorsal, or dorso-
lateral to the olive, is the continuation upwards of the ventral spino
cerebelkvr bundle (tract of Gowers) of the spinal cord ; the continuation
of the dorsal spino-cerehellar hundle (tract of Flechsig), just above it,
is now passing into the restiform body.

A tract of fibres which arise within the thalamus passes OA^er the
lateral surface of the nucleus olivse and ends within its grey matter
(thalamo-olmivij trad, central tegmental trad of Bechterew). The cells
of the dentate nucleus have numerous dendrons ; their axons all pass
towards the hilus, where they emerge, and, for the most part, cross
the raphe, pierce the opposite olivary nucleus and pass, as already
mentioned, into the restiform body (lAiro-cerehellar trod).

Nerves arising from the medulla oblongata. — The 12th, 11th, 10th,
9th, and 8th nerves all take origin in the medulla oblongata, and their
fibres may be seen emerging on either side, those of the 12th ventrally
between the pyramid and olive, and those of the other three nerves in
succession at the side of the medulla oblongata between the olive and
restiform body.

The Xllth or hypoglossal nerve arises from a well-marked nucleus
of large cells, similar to those of the anterior horn of the cord. This
nucleus is situated : — in the lower part of the bulb, A'entro-lateral to
the central canal (fig. 447) ; in the upper part, near to the floor of the
4th ventricle, close to the middle line (figs. 448, 449). None of the
fibres cross to the opposite side ; according to Van Gehuchten, this is
true of all the cranial nerves, except a iew fibres of the .3rd nerve and
the whole of the 4th nerve. The hypoglossal nucleus extends through-
out about the lower two-thirds of the bulb (fig. 450, nXII.). It
receives many collaterals from adjacent sensory tracts in the reticular
formation and from the descending sensory nuclei of the 5th, 9th, and
10th nerves, as well as from the posterior longitudinal bundle. These
form a plexus of fine fibrils within the nucleus which is highly
■characteristic. A similar plexus is seen in the oculomotor nucleus.

AJesial to the hypoglossal nucleus, in the open part of the medulla
oblongata, is the nudeus of the fasdculns teres, a column of moderate
sized cells which extends towards the caudal end of the pons and
appears to receive fibres from the cerebellum (Edinger).

The Xlth nerve or spinal accessory begins to take origin from cells

382

THE ESSENTIALS OF HISTOLOGY.

in the lateral part of the grey matter of the spinal cord as low dowrt
as the 5th cervical nerve. Its fibres from the cord (spinal fibres) are
those to the (voluntary) sternomastoid and trapezius muscles. They
pass from the cells of origin in the lateral horn {motor nucleus) at first
dorsalwards and then take a sharp bend outwards through the lateral

Fig. 450.— DiAciRAM.s illustrating theorigin and relations of the root-
fibres OF THE CRANIAL NERVES.

A. Efferent fibres only : profile view.

B. Shows on the left the motor nuclei and efferent fibres, except those of the 4th nerve,

and on the right side the afferent fibres : surface view.

column to emerge at the side of the cord and medulla oblongata. The
bulbar fibres (which join the vagus) take origin in a nucleus of
relatively small cells which lies dorso-laterally to the central canal of
the medulla oblongata and behind the hypoglossal nucleus. This
nucleus is continuous above with the corresponding nucleus of the
vagus, and with it forms the doisal accessm-y-vagus nucleus (figs. 447 to-
450). Below, it extends nearly as far as the first cervical nerve ;:
its upper part (vagal part) is in the floor of the 4th ventricle lateral ta

THE MEDULLA OBLONGATA.

383

the hypoglossal nucleus, ami oxtcnds nearly as far as the lower
bordei' of the pons. Of the whole nucleus about the lower two-thirds,
i.r. as fai- as the lower end of the calamus scriptorius gives origin to
fibres of the accessory. These fibres, as already stated, join the vagus,
to which they supply the motor fibres of the thyroarytenoid muscles
(Van Gehuchten). The 12th and 11th nerves are entirely eti'erent.

a'.^.JT yir/

Fig. 451. — Plan of the origin op the XIIth and Xth nerves.

pyr, pyramid ; n.XII., nucleus of hypoglossal ; XJI., hypoglossal nerve ; d.n.X. Jf A., dorsal
nucleus of vagus and accessory ; n.<i-iub., nucleus ambiguus ; /.«., fasciculus solitarius
(descending root of vagus and glosso-pharyngeal) ; f.s.n., its nucleus; A'., crossing
motor fibre of vagus ; g, cell in ganglion of vagus giving origin to a sensory fibre ;
il. v., descending root of fifth ; c.r., corpus restiforme.

The Xth nerve or vagus (pneumogastric) contains both motor
(efferent) and sensory (afferent) fibres. The efferent fibres arise (1) from
the upper part of the dorsal accessory-vagus nucleus just described,
(2) from a nucleus of grey matter containing large cells situated in
the reticular formation (figs. 449, 451, n.amh.). This nucleus begins
near the loM^er limit of the bulb and extends nearly to the facial
nucleus, which it resembles in general position : it is known as the
nvcleus ambiguus {ventral nucleus of the Xth nerve). The axons of its
cells are directed at first backwards and inwards and then turn sharply
round in the lateral direction to join the rest of the issuing fibres
of the nerve, coursing in the same manner as the spinal fibres of
the accessory ; indeed, this nucleus is continuous below with the
column of cells from which those fibres take origin.

The sensory fibres take origin in the ganglion of the root and the
ganglion of the trunk {jugular and plexiform ganglia) of the nerve, from

384 THE ESSENTIALS OF HISTOLOGY.

unipolar cells like those of the spinal ganglia (fig. 451, g). They enter
the medulla oblongata, and then bifurcate, one branch, a short
(ascending) one, passing at once into the upper sensory or principal
nucleus, the other, a long one, descending. The descending fibres
(with similar fibres of the IXth and those of the pars intermedia of
the Vllth) form the so-called fasciculus solitarius (figs. 448, 449, 451)
{descending root of facial, vagus, and glossopharyngeal), which is traceable
to the lower limit of the medulla oblongata ; they end in grey matter
which lies along its mesial border (descending nucleus of facial, vagus,
and glossopharyngeal). This nucleus approaches the middle line as it
descends, and in some animals terminates by joining its fellow of the
opposite side over the central canal to form the commissural nucleus of
Cajal. The upper sensory nucleus [principal nucleus), in which the
short branches from the sensory root end, lies in grey matter near the
floor of the ventricle, and is continuous with that which accompanies
the fasciculus solitarius.

The IXth or glossopharyngeal nerve also contains both eff'erent and
afferent fibres. The former have their cells of origin in a special
nucleus which occupies a position similar to that of the nucleus
ambiguus, but is mesial to the anterior end of that nucleus, and just
below the nucleus of the facial {motor nucleus of glossopharyngeal). The
afferent fibres of the nerve arise in the upper or jugular and petrosal
ganglia from unipolar cells like those of the spinal ganglia. Their
central axons enter the medulla oblongata, and, like other sensory
fibres, bifurcate into two branches, ascending and descending. Their
course is like those of the vagus, the descending passing down in the
fasciculus solitarius (extending to about one-third of its length, Bruce),
and ending by arborising in the grey matter accompanying it (descend-
ing root and nucleus), while the ascending branches pass nearly
horizontally backwards and inwards to a nucleus (principal nucleus)
beneath the inferior fovea of the floor of the ventricle which is con-
tinuous with the upper end of the nucleus of the descending root. The
arrangement is almost exactly a counterpart of that of the vagus
shown in the diagram given in fig. 451.

According to Edinger the sensory nuclei of these nerves receive fibres from
the cerebellum, constituting a cerebello-bulhar tract, whicli is much better
marked in lower vertebrata than in man and mammals.

The Vlllth nerve. — A section taken through the uppermost jxirt of the
olivary prominence will still show very much the same form and
structural arrangements as that just described. The nucleus of the
hypoglossal (fig. 452, n.XII.) is still visible in the grey matter of the
floor of the ventricle near the middle line, but the nerve which is now

THE MEDULI.A OBLONGATA.

385

seen connected with the Literal part is the eighth or auditory {VI II.),
the bundles of which, as they enter the bulb, embrace the inferior
crus of the cerebellum {corpus restiforme, c.r.), which is now passing
into that organ. The origin of the eighth nerve is thus subdivided
into two principal parts, known respectively as the dorsal or cochlear
and the ventral or vestilndar divisions (fig. 452).

nvm,

njcn

Fig. 452. — Transverse section of the upper part of the medulla
OBLONGATA. |. (Schwalbe.)

Pll, pyramid ; o, olivary nucleus ; V, descending root of the fifth nerve ; VIII. , root of
the auditory nerve, formed of two parts, o, cochlear, and b, vestibular, which inclose
the restiform body, c.r.; n.VIIIp., principal nucleus of the vestibular division;
7). Vlllac, ventral or accessory nucleus of the cochlear division ; </, lateral nucleus of
the cochlear division; n.f.t., nucleus of the funiculus teres; n.XII., nucleus of the
hypoglossal ; r, raphe ; f.r., reticular formation.

The real origin of the nerve fibres in these roots is in the ganglion
of the cochlea and the ganglion of Scarpa respectively. These ganglia,
which are situated at the periphery within and near the internal ear,
are composed of bipolar cells, of which the peripheral axons end by
ramifying amongst the cells of the auditory epithelium, and the
central axons form the cochlear and the vestibular divisions of the
auditory nerve and pass into the medulla oblongata in the manner
here described.

The fibres of the dorsal or coclilear division (cochlear nerve)
bifurcate as they enter the medulla oblongata. Each fibre divides
into a thick and a thin branch. The thicker branches pass partly to
a mass of ganglion cells which is wedged in between the two roots
and the restiform body, and is known as the accessory auditory nucleus

2b

386

THE ESSENTIALS OF HISTOLOGY.

(figs. 452 ; 453, n.acc), applying themselves with a peculiar form of
terminal arborisation to the cells of this nucleus, partly over the
restiform body to terminate in a prominent mass of grey matter which
overlies that body and also extends to the lateral part of the floor of the
fourth ventricle at its widest part {lateral nucleus, tuberculum acusticum).
The cells of the tubercle have a peculiar spindle shape and are set
vertically to the surface. They appear to begin in the root itself, lying
amongst the fibres of the nerve. Here they are sometimes spoken of

FIBRES TO NUCL.LEMNISCI
ftCORPORA QUADRIGEMINA

NERVE-ENDINGS

in organ of corti

Fig. 453. — Plan of the course and connections ov the fibres forming
the cochlear root of the auditory nerve.

r., restifomi body; V., desoendintr root of the fifth nerve; tub.a.c, tuberculum acusticum;
n.acc, accessory nucleus; s.o., superior olive; n.tr., nucleus of trapezium; n.VI.,
nucleus of sixth nerve ; VI., issuing root fibre of sixtli nerve.

as forming the "ganglion of the root." The thinner branches of the
bifurcated cochlear fibres pass downwards for a certain distance and
break up into a plexus of fine fibrils.

These two nuclei, viz., the accessory nucleus and the acoustic
tubercle, are the nuclei of ending of the cochlear fibres. From their
nerve-cells new fibres arise which continue the auditory path centrally
(see fig. 45-3). Those from the accessory nucleus enter the trapezium
— which consists of transverse fibres running behind the pyramid
bundles of the pons Varolii — and pass in it partly to the superior
olive and trapezoid nucleus of the same side of the pons, but mostly
to the corresponding structures on the opposite side. Some end in
those nuclei, but others merely traverse them, giving off" numerous
collaterals to them and to the superior olives and other nuclei close
by (see p. 392), and then turn upwards in the lateral part of the
tract of the fillet to pass ultimately towards the inferior corpora

THE MEDULLA OBLONGATA. 387

(|iuulngomina ; in tending towards these structures at the side of the
mid-brain they form the lateral fillet, or fillet of lieil, which is there
conspicuous. Some of the fibres from the cells of the accessory
nucleus do not pass directly to the trapezium, but first curve round
the restiform body (Held); these form the most dorsally situated fibres
of the trapezium. The fibres which arise in the acoustic tubercle pass
for the most part over the fioor of the fourth ventricle, where they
are seen superficially as the medullary or acoustic strice, and, entering
the raphe, traverse it from behind forwards, and then join the others
from the accessory nucleus in their course to the superior olive and
lateral fillet of which they constitute the deeper layer. A few fibres
are directed into the fillet of the same side as their cells of origin.

Edinger states that, at least in the dog, all the fibres of the
trapezium end in its nucleus or in the superior olivary nucleus, the
central acoustic path being wholly continued, so far as the trapezium
fibres are concerned, by fresh neurones, the cell-bodies of which lie in
those nuclei, and the axons of which pass into the lateral fillet. On
the other hand, from the cells in the tuberculum acusticum, the axons
are said to be continued upwards in the opposite lateral fillet without
the intervention of any corresponding nuclei. The lateral fillet passes
above into the posterior colliculus of the mid-brain.

The accessory nucleus also receives fibres from the trapezium, which
end by ramifying amongst its cells. These are perhaps derived from
the accessory nucleus of the opposite side.

Both sets of fibres (from the accessory nucleus and tuberculum)
give off collaterals near their origin, which terminate within these
nuclei.

The ventral or vestibular division (vestibular nerve), which enters
a little in front of (above) the cochlear division, passes between the
restiform body and the descending root of the fifth (fig. 452), to enter
a mass of grey matter containing for the most part cells of small size,
which is termed the principal w dorsal nucleus of the vestibular
division. Here each of its fibres divides with a Y-shaped division
into an a.scending and a descending branch (fig. 454). The descending
branches are collected into small bundles {descending vestibular root)
which run downwards towards the lower part of the medulla oblongata,
and gradually end by arborising around cells in the adjacent grey
matter {descending vestibular nucleus), which is continued down from the
principal nucleus. The ascending branches pass upwards on the inner
side of the restiform body towards the nucleus tecti of the cerebellum.
In their course they give oft' numerous collaterals which arborise round
the large cells of two nuclei which occur in this part of the medulla

388

THE ESSENTIALS OF HISTOLOGY.

oblongata and pons near the outer part of the floor of the fourth

ventricle. These two nuclei are termed the nvdeus of Deiters and the

nucleus of Bechterew respectively (fig. 454).

Van Gehuchten states that the nucleus of Bechterew aloue recei%'es fibres
from the ascending branches and that all the other nuclei (dorsal, descending,
and nucleus of Deiters) are furnished with fibres from the descending
branches.

TO VERMIS

FIBRES O

VESTIBULA

ROOT

NERVE
ENDINGS
IN MACUL/E
& AMPULL/E

Fig. 454. — Plan of the course and coxxections of the fibkes forming

the vestibular root of the auditory nerve.
r., restiform body; V., descending root of fifth nerve; p., cells of principal nucleus of
vestibular root ; d, fibres of descending vestibular root ; n.d., a cell of the descending
vestibular nucleus ; D., cells of nucleus of Deiters ; B, cells of nucleus of Bechterew ;
n.t., cells of nucleus tecti (fastigii) of the cerebellum ; p.l.b., fibres of posterior longi-
tudinal bundle. Xo attempt lias been made in this diagram to represent the actual
positions of the several nuclei. Thus a large part of Deiters' nucleus lies dorsal to and
in the immediate vicinity of the restiform body.

The nucleus of Deiters is especially characterised by the large size
of its cells and by the manner in which they are enveloped as by a
basket-work by the ramifications of the collaterals in question. From
these cells fibres arise which pass to the posterior longitudinal bundles
of both sides : in these the fibres bifurcate (Cajal), one branch passing
upwards to the oculomotor nucleus and giving off collaterals to the
nucleus of the sixth nerve, and the other downwards, eventually
reaching the anterior column of the spinal cord (antero-lateral
descending tract) and terminating by arborisations amongst the cells of
the anterior horn (see p. 365). By means of the collateral fibres which
supply the sixth and oculomotor nuclei it is probable that the conjugate
movements of the two eyes are brought about. Fibres have also been
described as passing from Deiters' nucleus to the nucleus tecti of the

THE MEDULLA OBLONGATA. 389

cerebellum. Owing to its connections with the semicircular canals,
the cerebellum, the oculomotor nuclei, and the nuclei in the anterior
horn of the spinal cord, this nucleus has important functions in
connection with co-ordination of head and eye movements and
equilibration in general.

The fil)res which originate in the nucleus of Bechterew pass into
the reticular formation and become longitudinal, but their destination
is not certainly known. Some are said to pass into the anterior
column of the cord.

The reticular formation still occupies the .greater part of each lateral
half of the bulb between the grey matter at the floor of the fourth
ventricle and the pyramids, and a small portion of the olivary nucleus
may still be seen, as may also the descending root of the fifth nerve
with its adjacent grey matter.

The restiform body is formed partly of the fibres of the cerebellar
tract of Flechsig of the same side, which are derived below from the
cells of Clarke's column, and pass above into the middle lobe of the
cereV)ellum, partly of fibres from the opposite olivary nucleus, and
partly of fibres from the olivary nucleus of the same side. The olivary
fibres pass mainly to the cerebellar hemisphere. According to some
authorities the restiform body contains fibres derived from the
nucleus gracilis and nucleus cuneatus of the opposite side. It
is said also to receive some fibres from a nucleus which lies just
outside the main mass of grey matter of the funiculus cuneatus, and
is known as the outer cuneate nucleus.

The floor of the fourth ventricle is covered by a layer of ciliated
epithelium-cells, continuous below with those lining the central canal,,
and above, through the Sylvian aqueduct, with the epithelium of the
third and lateral ventricles. The epithelium rests upon, and the pro-
longed extremities of its cells assist in forming, a layer of tissue
known as the ependyma of the ventricle. The fourth ventricle is roofed
over by a layer of pia-mater, with projecting choroid plexuses, the
under surface of which is covered by a thin epithelial layer continuous
at each side with the ciliated epithelium of the floor. The roof becomes
somewhat thickened as it is continued into the ependymal layer of the
floor of the ventricle ; this thickened part (tcenia or ligula, figs. 448,
449, t), is often left attached when the thin epithelial roof is removed
along with the pia-mater which covers it.

390 THE ESSENTIALS OF HISTOLOGY

LESSONS XLIT. and XLIII.

THE PONS VAROLII, MESENCF.PHALON, AND
TEA LA MENCEPHALON.

\. Sections through the lower, middle, and upper parts of the pons Varolii.

2. Sections across the region of the corpora quadrigemina, one at the level
of the inferior, the other at the level of the superior, pair.

3. A section across the posterior part of the third ventricle passing through
the optic thalami.

In all the above sections sketch under a low power the general arrange-
ment of the grey and white matter, inserting the positions of the chief group.s
of nerve-cells.

[The tissue is hardened and the sections are prepared, stained, aud mounted
in the same way as the spinal cord and medulla oblongata.]

THE PONS VAROLII.

Sections through the lower part of the pons (figs. 455, 457, 459)
show much the same arrangement of grey and white matter as that
met with at the upper part of the medulla ol)longata, but the general
appearance of the sections is much modified by the presence of a large
mimber of transversely coursing bundles of nerve-fibres, most if not all
of which are passing to the hemispheres of the cerebellum (fibres of
middle peduncle of cerebellum). ^ Intermingled with these bundles is
a considerable amount of grey matter {nuclei pontis) from the cells of
which the fibres of the middle peduncle (of the opposite side) are
derived.^ Amongst the cells of the nuclei pontis many fibres and
collaterals of the pyramidal tract' end, and the cortico-pontine fibres
also terminate here ; thus forming a connection between the cerebral
hemisphere of the one side and the opposite cerebellar hemisphere
(fig. 479). The continuation of the pyramids of the medulla oblongata
(fig. 455, py) is embedded between these transverse bundles, but the
pyramid bundles of the pons are much larger than the pyramids of the
medulla oblongata, and, in addition to fibres of the pyramidal tract

' Some of the most anterior of these peduncular fibres often form a detached
bundle which is known as the taenia pontia (fig. 461).

-Other of these fibres have been described as arising in the cerebellar hemi-
sphere and crossing the raphe ; some becoming lost amongst the cells of the
opposite nucleus pontis, and some passing to the reticular formation, there
becoming longitudinal. But tliis double origin is denied by Van Gehuchten,
apparently with good reason.

THE PONS VAROLII.

391

proper {corticospinal system), derived from the motor area of' the cortex,
they are largely composed (especially the postero-lateral bundles) of
fibres (cortico pontine si/.'tfcni) connecting other regions of the cortex with
this part of the hind-brain. The pyramid bundles are separated from
the reticular formation by deeper transverse fibres, which belong to

K/l/

-^

J^

Fig. 455.— Tkax.sver.se .section throi gh the lo\\ermosi part ot ihe pons
VAROLII. |. (From a pliotogiaph )

V.I v., fourth ventricle; c. , white matter of cerebellar hemisphere; c.d., corpus dentatum;
fi., flocculus ; c.r., corpus restiforme ; R, bundle of Roller, composed of the descending
branches of the vestibular uerve ; D. nucleus of Deiters ; VIII., issuing root of auditory
nerve; Fill. J., principal or dorsal nucleus of the vestibular nerve; VIII. v., nucleus
of cochlear portion ; ti:, trapezium ; n.tr., its nucleus ; /, fillet ; p. Lb., posterior longi-
tudinal bundle; /.<■., formatio reticularis; n, n', n", various nuclei within it; V.o..,
de.scendiug root of fifth nerve; s.g., sulDstantia gelatinosa ; s.o.. superior olive;
VII., issuing root of facial nerve; n.VII., its nucleus; VI., root -bundles of sixth
nerve ; -py, pyramid bundles ; n.p., nuclei pontis.

a different system from those of the middle peduncle. They form

what has already been referred to as the frape:iuin (figs. 453, 455) ; a

collection of fibres which forms part of the central auditory path, and

some of which appear to 1)e commissural between the auditory nuclei

of the two sides. The fibres of the trapezium traverse a collection of

nerve-cells which lies mesial and ventral to the superior olivary

nucleus, and is known as the nucleus of the trapezium (fig. 453, n.fr.).

This luii-leus is characterised by the peculiar chalice-like synapses which
the enteriucf axons of the larger acoustic fibres form with the ceil- bodies

392 THE ESSENTIALS OF HISTOLOGY.

(Held) (see fig. 171, p. 144). According to Cajal these large fibres are con-
tinued directly from the root-fibres of the cochlear nerve and are not derived
from the cells of its accessory nucleus.

The olivary nucleus is no longer seen, but there are one or two
small collections of grey matter, more conspicuous in some animals
than in man, which lie in the ventral part of the reticular formation, and
are known as the superior olivary nucleus (p.s.), the preolivary nucleus, and
the semilunar nucleus (Cajal). All these, as well as the nucleus of the
trapezium itself, are connected with the fibres of the trapezium which
form the central auditory path, some of these fibres either ending in
the nuclei in question or giving off to them numerous collaterals ;
whilst from the cells of the nuclei axons pass into the trapezium or
into the adjacent lateral part of the fillet (see p. 386). On the other
hand, the superior olive is said to receive fibres from the posterior
colliculi of the corpora quadrigemina. The nucleus of Deiters, which
begins to appear in the upper part of the medulla oblongata, where it
has been already studied (p. 388), extends into the pons Varolii, where
it lies near the floor of the fourth ventricle, a little mesial to the resti-
form body {D, fig. 455). The nerve-fibres connected with its cells pass
towards the middle line and enter the posterior longitudinal bundle.
Here, as already stated, they divide, one branch passing upwards in
the bundle and terminating by arborescence chiefly in the opposite
oculomotor nucleus : the other branch extending downwards in the
medulla oblongata and cord. In the spinal cord they are found in
the antero-lateral descending tract ; fibres from each nucleus of Deiters
occur in both of these tracts (E. H. Fraser). They terminate by
arborescence in the anterior horn of the spinal cord.

The nerves which enter or emerge from the grey matter of this region
of the brain are part of the eighth, the seventh, the sixth, and somewhat
higher up the fifth cranial nerves. Of these the eighth (already
considered) and fifth are connected with groups of nerve-cells which
occupy the grey matter opposite the external border of the floor of
the ventricle ; the sixth with a nucleus which is also placed in the
grey matter of the floor of the ventricle but nearer the middle line,
and the seventh with a special nucleus which lies in the forniatio
reticularis.

The Vllth or facial nerve and the nerve of Wrisberg (pars
intermedia). — The motor fibres of the seventh nerve arise from the facial
nucleus (in the formatio reticularis) which is homologous with
the nucleus ambiguus seen in sections of the medulla oblongata. It has
been shown that the motor fibres to the stapedius arise from the mesial
part of the nucleus and then in succession those of the external ear

THE PONS VAROLII.

39a

muscles, those of the mouth and tuoe, and, tinally, from a group of cells
situated dorsally to the rest, the motor fibres supplied by the superior
branch of the nerve (Marinesco, Van Gehuchten). From the nucleus of
origin the fibres first pass obliquely backwards to the Hoor of the
ventricle, then longitudinally upwards for a short distance (figs. 450, A,
456), and finally bend oblicjuely forwards and downwards to emerge
between the transverse fibres at the side of the pons. None of its
fibres are derived from the micleus of the sixth, as has sometimes been
supposed. As it curves over this luicleus it gives oft' a ])undle of

^//A

//■'•"

Fig. 4.56.— Plan (transverse) op the origin of the sixth and seventh

NERVES.
VJ., sixth nerve ; VII., seventh nerve ; a. VII., ascending part of root of seventh shown
cut across near the floor of the fourth ventricle; <j, genu of seventh; n.VI., chief
nucleus of the sixth nerve; n.VI., accessory nucleus of sixth; n.VIL, nucleus
of seventh ; d. V., descending root of fifth ; pyr, pyramid bundles ; VIII.v., vestibular
root of eighth nerve.

fine fibres which cross the raphe, but their destination is unknown..
The nucleus of the facial receives collaterals from the adjacent sensory
tracts in the formatio reticularis.

The facial is not a purely motor nerve, but has a ganglion upon it of
the spinal type {geniculate ganglion) from which fibres arise (fig. 450, B}
which pass centrally into the pars intermedia of Wrisberg, which enters
the pons between the seventh and eighth nerves, and the fibres of which
bifurcate into ascending and descending branches like other sensory
nerves ; the descending branches pass into the solitary bundle and end
like those of the glossopharyngeal in the upper part of its accompany-
ing grey matter. The peripheral axons of the cells of the geniculate
ganglion pass into the large superficial petrosal and chorda tympani —
to which they probably furnish gustatory fibres. Other (efferent) fibres.

394

THE ESSENTIALS OF HISTOLOGY.

pass into the pars intermedia and ultimately into the chorda tympani
from certain large cells which occur in the dorsal part of the facial
nucleus. These are probably the salivary and vasodilator fibres of the
chorda.

The Vlth nerve (abducens). — The fibres of the sixth nerve (figs. 450,
456), which are purely motor, pass out from the mesial aspect of the
nucleus and turn forwards; traversing the pyramid bundles they

/sup. cerebellar
Dedu

ace. motor root of Vth
motor nucl. of Vth

sensory nucl. of Vth
sen. root fibres of Vth —i.
nucl. in tegmentum
grey matter lateral
to fill(

■white matter of cere
bellar hemisphere

nucl. fun. ter.

fibres of pen

nuclei pontis

central bundle

of Vth

post. long. bund.

ant. long. bund,
rub. -spin, tract

central nucleus
fillet

trapezium

fibres of pons
raphe

pyramid bundles
nuclei pontis

fibres of pons

Fig. 457.

-Sectiox .across the middle of the poxs varolii.
about 4 diameters.

Magnified

emerge at the lower margin of the pons. A few fibres are derived from
a small ventral nucleus lying near the nucleus of the facial ; these run at
first backwards and then turn forwards to join the others (Van
Gehuchten) (fig. 456, n' VI.).

The Vth or trigeminal nerve emerges at the side of the pons in two
roots, a smaller motor and a larger sensory.

The motor root is derived partly from fibres which arise in the upper
part of the pons and lower part of the mesencephalon from large
spherical unipolar nerve-cells lying at the side of the grey matter
bounding the Sylvian aqueduct {accessort/ or superior motor nucleiis of

THE PONS VAROLII.

395

fifth, fig. 450, nVms ; fig. 458, m'n.V.), partly from the motor miclem
propel' (figs. 450, nVm ; 458, mn.V.) which lies in the grey matter at
the lateral edge of the fourth ventricle (fig. 457). As they pass the
motor nucleus proper the fil)res from the superior or accessory nucleus
give off into it a large number of collaterals which ramify l)etween and
around its cells.

Fig. 458. — Plan of the origin of the fibres of the fifth nerve.
G, Gasserian ganglion; a, b, c, three divisions of the nerve; rn'.n.V., superior motor
nucleus; m.n.V., principal motor nucleus; p.s.n.V., principal sensory nucleus;
d.s.n.V., descending sensory nucleus; d.s.V., descending root; cV., c'V., central
sensory tracts composed of fibres emanating from the sensory nuclei ; r, plane of
the raphe.

The fibres of the sensory root are derived from the cells of the
Gasserian ganglion which are homologous with the cells of the spinal
ganglia. These fibres of the sensory root when traced into the pons
are found to bifurcate, the ascending branches ending in a mass of
grey matter {principal sensory nucleus of the fifth, fig. 458, p.s.n.V.)
lying just lateral to the motor nucleus, while the descending
branches trend downwards into the medulla oblongata where they form
the descending or spinal root of the fifth (fig. 458, d.s.V.); some
even reach the upper part of the spinal cord. They lie immediately
lateral to and in close connection with the substantia gelatinosa Kolandi

396 THE ESSENTIALS OF HISTOLOGY.

which forms the inferior sensor// nucleus (d.s.n.V.), and which is con-
tinued above into the principal nucleus. The substantia gelatinosa
which forms the sensory nucleus of the fifth contains numerous
nerve-cells, both small and large ; many of the small cells are grouped
into nest-like clusters (islands of Calleja). The axons of the larger cells
pass for the most part across the raphe to the formatio reticularis of
the opposite side where they reinforce the ascending fibres of the
intermediate fillet, but some ascend in the fillet of the same side, and
others pass to a special ascending bundle of fibres on the opposite side of
the raphe which lies nearer the floor of the fourth ventricle, and in the
tegmentum of the mid-brain lies lateral to the posterior longitudinal
bundle ; hence it is continued upwards into the thalamus. Collaterals
are given off" from these ascending fibres to the adjoining grey matter,
and especially to the nucleus of the facial nerve. Branches also pass
downwards in the formatio reticularis.

Descending tracts in the pons and medulla oblongata. — Besides the
fibres of the pyramids, which are much more numerous in the pons
than in the medulla oblongata, and which send numerous collaterals
into the grey matter of the nuclei pontis, there ai^e several other
descending tracts of fibres in the pons and medulla ol)longata. One of
these, which lies mesial to the fillet (see page 398) consists of fibres
(cortico-bulbar) passing from the motor cortex towards the nuclei of the
facial and hypoglossal. In the crusta of the mid-brain these fibres lie
mesial to the ordinary pyramidal fibres, but they then leave the latter
and pass into the ventral part of the tegmentum and are continued down-
wards in the formatio reticularis into the medulla oblongata. Another
bundle, consisting of both ascending and descending fibres (vestibulo-
inotor), is very distinct, just ventral to the grey matter of the floor of
the fourth ventricle, near the middle line ; this is the dorsal ov posteriw
longitudinal bundle ; as already noticed (pp. 388, 392. See also p. 402)
it connects Deiters' nucleus with the oculomotor nucleus, the nucleus of
the sixth, and the anterior horn cells of the spinal cord ; it probably also
receives fibres from the axons of the large cells of the formatio reticularis.

Other descending tracts in the pons which are not so distinctly
marked in the normal condition, but which can be traced by the
methods of Waller and Flechsig are:— 1. Momdmv's bundle; 2. The
anterior longitudinal bundle ; 3. The ponto-spinal lateral tract ; 4. The
vestibulospinal tract ; 5. The central tract of the tegmentum.

Monakoio's bundle or the rubrospinal tract has already been seen as
the prepyramidal tract of the spinal cord (p. 365. See also p. 403).
Its fibres arise from the cells of the red nucleus of the mid-brain of the
opposite side, crossing the raphe in Forel's decussation (p. 403, foot-

THE PONS VAROLII. 397

note). In the uppei- part of the pons it is dorsal to the mesial fillet,
but lower down luiis in the lateral part of the tegmentum, dorsal to
the lateral fillet.

The anterior longitudinal bundle {tectospinal tract) consists of fibres
which arise in the opposite superior quadrigeminal body, these cross
the raphe in Meynert's decussation (p. 40.3), and run down ventral to
the posterior longitudinal bundle, giving off collaterals to the oculo-
motor nuclei and the nuclei of the fourth and sixth nerves as they
descend. Its fibres eventually mix with those of the posterior
longitudinal ])undle, and i^ass into the anterior column of the cord,
joining the antero-lateral descending tract (p. 3G3).

The ponfo-spinal lateral tract is formed of fibres which arise from the
large cells of the reticular formation, and run down within the lateral
area of this formation in the pons and medulla oblongata to reach the
part of the lateral column of the cord which lies between the grey
matter and the tracts of Monakow and Gowers. It is, however, mixed
here with many fibres of different origin. The destination of its fibres
is similar to those of the posterior and anterior longitudinal bundles,
viz. : the adjacent grey matter of the anterior horn.

The vestibulospinal tract is composed of fibres derived from the cells
of the nuclei of Deiters and Bechterew, and is therefore similar in its
origin to the fibres of the posterior longitudinal bundle. The destina-
tion is in part also similar, for the fibres pass below into the anterior
root zone of the cord and end in the grey matter of the anterior horn,
but in their course downwards they lie in the lateral part of the medulla
oblongata mixed up with those of Monakow's tract and the ponto-spinal
tract, as well as with the ascending fibres of Gowers' tract.

The central tract of the tegmentum (Bechterew) runs in the pons
exactly in the middle of the reticular formation of the tegmentum, but
in the medulla oblongata it lies more ventrally near the olivary
nucleus, beyond which it has not been traced. The origin of its
fibres is not certainly known, but appears to be the thalamus ; their
destination is the olivary body of the same side (see p. 381, thalamo-
olivary tract).

Ascending tracts in the pons and medulla oblongata. — In the
ventral part of the reticular formation is a very well-marked tract of
fibres, somewhat flattened (narrow in section) from above down in the
pons ; this is the tract of the fillet. Its fibres are partly derived from
cells in the nuclei of the opposite fasciculus gracilis and fasciculus
€uneatus of the medulla oblongata which have crossed the raphe as
internal arcuate fibres ; partly from cells in the nuclei which are
connected with the terminations of the sensory cranial nerves.

398 THE ESSENTIALS OF HISTOLOGY.

In the mid-brain the fillet splits up into two distinct bundles of fibres
termed respectively the lateral or lower and the intermediate or upper fillet.
The fibres of the lower fillet are seen at the side of the mesencephalon
{fillet of Reil), and are traceable partly to the grey matter of the
inferior corpora quadrigemina (fig. 465), partly to the mesial geniculate
body, in both of which they terminate ; they are derived from the
sensory nuclei of the medulla oblongata and pons (mainly from the
acoustic nuclei). Those of the upper fillet go to the thalamus (fig.
469) ; they are chiefly the fibres from the cells of the opposite posterior
columns of the medulla oblongata.

Besides the ascending fibres of the tract of the fillet, this bundle
includes a certain number which degenerate below a section of the tract
and are therefore descending (centrifugal) : their cells of origin appear
to lie in the thalamus ; the fibres themselves are situated mesial to the
true fillet of which they were formerly considered to be a part (being
termed "mesial" fillet): they form a thalamo-hulhar tract. Mesial to the
tract just mentioned is a bundle, also consisting of descending fibres,
belonging to the system of the pyramidal tract, and containing fibres
which eventually come into relation with certain of the cranial motor
nuclei (Hoche). This constitutes the cortico-bulbar trad (see page 396).
In the crusta it lies dorso-lateral to the other pyramidal tract fibres.

Many of the fibres which continue the sensory path of the cranial
nerves upwards lie in the formatio reticularis (tegmentum), somewhat
dorsal to the tract of the fillet, forming a homologous but not clearly
defined tract, which runs up through the pons and mid-brain to
terminate in the subthalamic region and in the optic thalamus (central
tract of the sensory cranial nerves). Another ascending tract is the special
bundle of fibres from the sensory nucleus of the 5th to the thalamus
previously referred to (p. 396).

At the upper part of the pons (fig. 459) the fourth ventricle narrows
considerably towards the Sylvian aqueduct, and above and on either
side of it two considerable masses of longitudinal white fibres make
their appearance. These are the superior peduncles of the cerebellum,
and they tend, as they pass forwards, gradually to approach the
middle line ; across which immediately below and in the region of
the posterior colliculi of the corpora quadrigemina they pass,
decussating with one another, to enter the formatio reticularis of the
opposite side.

The fibres of the superior cerebellar peduncles for the most part take
origin in the cerebellum, emerging from its dentate nucleus, from the
cells of which they are derived. They cross the raphe in the mid-brain
and terminate in the red nucleus of the (opposite) tegmentum ; but some

TITK PONS VAROLII.

39£^

of them give oft' a branch within the peihincle before crossing, and these
branches are described by Cajal as forming a deiicending cerehellar bundle
which passes downwards towards the medulla oblongata and spinal
cord on the inner side of the descending root of the fifth, and gives off

. J, intercrossing of

issuing bundle fourth nerves

of IVtli

root bundle nf IVth
ace. motor root of \ I ii — ■--

sup. cerebell. ped

part of lateral fillet ' gggf
post. long, bundle

ant. long, bundl

lateral fillet

decuss. of superior
peduncles

intermediate fillet

substantia nigra
central nucleus

crusta or pes \
pedunculi V

breaking up of crusta into
pyramid bundles

Fig. 459.— Transverse section through the upper part of the pons.

collaterals to the motor nucleus of the fifth, to the facial nucleus,
to the nucleus ambiguus, and to the anterior horn of the spinal cord
(fig. 480). There is also in the superior peduncle a bundle of fibres
which are derived from cells in the thalamus and which pass down-
wards in the peduncle.

The antero-lateral ascending trad of the spinal cord (p. 365) is con-

400

THE ESSENTIALS OF HISTOLOGY.

N ^'/' ;..

^.<=z.^

Fig. 460. — DLiORAii to show the mode of passage of the fibres of the

DORSAL AND VENTRAL SPINO-CEREBELLAR TRACTS RESPECTIVELT INTO THE
CEREBELLAR VERMIS. (F. W. Mott.)

p.c.q., posterior corpora quadrigemina ; s.v., superior vermis of cerebellum ; d.a.c, dorsal
ascending cerebellar tract ; v.a.c, ventral ascending cerebellar tract.

Fig. 461. — The corpora qcadrigemina and neighbouring parts of the
BRAIN. (Edinger f rem G. Retzius. )

Brack, ant. cerebelli, the superior cerebellar peduncles, between them the anterior medul-
lary velum partly covered by the lingula ; Tr. spino-cereb. ventr., tract of Gowers
curving round the peduncle; lemniscus, the lateral fillet; JV. trochl., 4th nerve;
N. v., 5th nerve.

THE PONS VAROLII.

401

tinned up in the lateral column of the medulla oblongata dorso-lateral
to the olive and through the ventral part of the pons Varolii lateral to
the pyramid bundles, but at about the level of the exit of the fifth
nerve many of its fibres begin to pass obliquely towards the dorso-
lateral part of the pons (fig. 460), where the superior cerebellar
peduncle is emerging from the cerebellar hemisphere. The tract in
question {anterior or ventral spino-cerebellar trad) now curves over the
lateral aspect of this peduncle (fig. 461, tr. spino-cereb. venfr.), and then
takes a sharp backward turn, passing over its dorsal aspect to enter
the middle lobe of the cerebellum in the superior medullary velum.

THE MIDBRAIN OR MESENCEPHALON.

In sections across the mesencephalon (figs. 463 to 466), the upward
continuity of the parts which have already been described in the lower
nerve-centres can still in great measure be traced.

s.m.V

^zr" XT

Fig. 462.— Section throcgh the okigix of the fourth nekve. (Scliwalbe.)

A, transverse sectif)ii at the place of emergence of the nerve-fibres. B, oblique section
carried along the course of the bundles from the nucleus of origin to the place of
eniergeuce. Aq, Sylvian aqueduct, with its surrounding grey matter ; JV, the nerve-
bundles emerging; JV, decussation of the nerves of the two sides; /F", a bundle
passing by the side of the aqueduct to emerge a little lower down ; n.IV, nucleus of
the fourth nerve ; /, lateral fillet ; s.c.p., superior cerebellar peduncle; s.iu. r., superior
motor root of the fifth nerve ; pi, posterior longitudinal bundle ; r, raphe.

The Sylvian aqueduct (fig. 464, Sy), with its lining of ciliated
epithelium, represents the central canal of the cord and the fourth
ventricle of the medulla oblongata. In the grey matter Avhich sur-
rounds it [central grey matter) there is seen in all sections of the region
a group (column) of large nerve-cells {oculomotor nucleus) lying ventrally
on each side of the middle line, close to the reticular formation. From
the lower part of this column the root-bundles of the fourth nerve arise
at the lower part of the mesencephalon and pass obliquely backwards
and downwards around the central grey matter, decussating with
those of the opposite side to emerge just above the pons Varolii

2c

402

THE ESSENTIALS OF HISTOLOGY.

(figs. 459, 462). Higher up, in the x^egion of the anterior colliculi, the
bundles of the third nerve spring from a continuation of the same
nucleus (fig. 465, n.IIL), and these pass forwards and downwards with
a curved coui'se through the reticular formation, to emerge at the
mesial side of the crusta. According to Van Gehuchten some of
the fibres of the 3rd nerve cross the middle line and emerge with
the nerve of the opposite side.

Tegmentum. — The reticular formation of the pons is continued up
into the mesencephalon and is here known as the tegmentum. It is com-
posed as before of longitudinal and transverse or arcuate bundles of fibres
with much grey matter intermingled. The transverse fibres include
the decussating fibres of the superior peduncles of the cerebellum {s.c.p.)y

Fig. 463. — Outline of two sections across the mesencephalon.
Xatixral size.
A, througli the middle of the inferior corpora quadrigemina. B, through the region of
the superior corpoi-a quadrigeuiiua. cr, crusta; s.n., substantia nigi-a ; t, teg-
mentum ; s, Sj'lvian aqueduct, with its surrounding grey matter; c.q., grey matter
of the corpora quadrigemina; l.g., lateral gi-oove ; pj., posterior longitudinal bnndle ;
d. v., superior root of the fifth nerve ; s.c.p., superior cerebellar peduncle ; /, lateral
fillet; ///., third nerve; n.IIL, its nucleus. The dotted circle in B indicates the
situation of the tegmental or red nucleus.

which are derived from cells in the dentate nucleus of the cerebel-
lum, and on reaching the opposite side bifurcate. Their ascending
branches become gradually lost amongst a number of nerve-cells which
collectively constitute what is known as the red nucleus or nucleus of
the tegmentum, whilst the descending branches turn downwards in the
reticular formation (Cajal) (see p. 399). But some of the fibres of the
superior peduncle go on past the red nucleus to the ventral part of
the thalamus. The red nucleus also receives fibres in its lateral aspect
which are derived from the lenticular nucleus of the corpus striatum,
and some of which are said to come from the cerebral cortex ; these
fibres form a sort of capsule to the red nucleus before entering it.

Tracts in the tegmentum. — L Vestihulo-motor tract; posterior longi-
tudinal bundle. — This is well marked in the mid-brain, and gives off
many collaterals and terminal filires to the oculomotor nucleus which
is immediately dorsal to it. The bundle largely consists of nerve
fibres derived from the cells of Deiters' nucleus (see p. 388), which

THK MESENCEPHALON. 403

on reaching the situation of the bundle either on the same or on the
opposite side, bifurcate, one branch ascending, the other descending.
But it receives fibres from other sources than Deiters' nucleus, e.g.
from large cells of the sensory nucleus of the oth, and from large cells
in the reticular formation of the medulla oblongata, pons, and mid-
brain. All these fibres, like those from Deiters' nucleus, bifurcate on
joining the bundle, one branch passing upwards, the other downwards.
Some fibres of the bundle are of different origin from the rest, arising
beyond the oculomotor nucleus. These are very fine ; they are
descending filjres, and are traceable from the cells of the nucleus of
the posterior longitudiruil bundle, which lies in front of the Sylvian
aqueduct in the grey matter at the side of the third ventricle.

Some of the fibres of the posterior longitudinal bundle are stated to
be traceable as far up as the thalamus.

The bundle gives collaterals not only to the oculomotor nucleus but
also to the nucleus of the sixth, and probably others to the nuclei of
other cranial motor nerves. Its descending fibres are eventually
continued down the spinal cord in the antero-lateral descending tract,
and give off terminals and collaterals to the anterior horn.

2. Rubrospinal trad ; Monakow's bundle. — The cells of the red nucleus
send their axons downwards and forwards. They form Monakow's
bundle or the rubrospinal tract, which is continued below into the pre-
pyramidal tract of the spinal cord.

3. Tectospinal trad; anterior longitudinal bundle. — Other longitudinal
fibres of the tegmentum are those of the fasciculus retroflexus of Meynert
lying mesially to the red nucleus and passing obliquely downwards
and inwards from the ganglion of the habenula to the interpeduncular
ganglion of the opposite side, and the bundle of Munzer, which passes from
the posterior tubercle downwards into the lateral part of the reticular
formation of the pons. But the longest and most important is the
anterior or ventral longitudinal bundle, which passes lateral to the red
nucleus and partly through it. Although the red nucleus receives
many collaterals from this bundle the fibres of the bundle are derived,
according to Held and Cajal, from cells in the grey matter of the
opposite anterior tubercle of the corpora quadrigemina ; these cells
send their axons sweeping round the central grey matter just central
to the posterior longitudinal bundle to cross in the raphe, where they
form the fountain-like decussation of Meynert (fig. 464, d').''- The down-

^This is not to be confounded with the fountain-like decussation of Forel
(fig. 464, d), which lies nearer the ventral part of the tegmentum, and is partly
formed by the intercrossing of Monakow's bundle and partly by v. Gudden's
bundle coming from the corpora mammillaria to end in the tegmentum.

404

THE ESSENTIALS OF HISTOLOGY.

ward continuation of the tecto-spinal tract has already been studied
(pp. 397 and 365), but it should be stated that the prolongation of its
fibres into the anterior column of the spinal cord is denied by Van
(xehuchten, who traces them only as far as the medulla oblongata.

Pig. 464. — Section across the midbrain through the inferior pair of
CORPORA QUADRIGEMINA. Magnified about '.ih diameters. (From a photo-
graph.)

5»/., aqueduct of Sj'lvius ; c.gr., central grey matter of the aqueduct; n III. IV., group
of colls forming part of the conjoined nucleus of the third and fourth nerves ; c.q.p.,
one of the posterior corpora quadrigehiina ; f/r, median groove separating it from
that of the opposite side; str.L, stratum lemnlsci (layer of the fillet), forming its
superficial laj'er ; /', upper fillet ; ./", lateral fillet; V., accessory tnotor root of fifth
nerve; p. Lb., posterior longitudinal bundle; f.r.t., formatio reticularis tegmenti ;
d, d', decussating fibres of tegmenta (fountain-decussations of Forcl and Meynert) ;
s.c.p., superior cei-ebellar peduncles, decussating; p.p., pes pedunculi (crusta) ; s.n.,
substantia nigra; g.i-p., interpeduncular ganglion.

4r. Trad of the fillet. — The continuation upwards of the fillet is also
apparent in this part of the brain. Some of its fibres are seen passing
in an oblique manner to the side of the mesencephalon, to enter the
grey matter of the prominences of the posterior corpora quadrigemina.

This part is the lateral fillet (see p. 398), which is formed chiefly by
fibres derived from the accessory auditory, the inferior olivary, and the
trapezoid nuclei of the opposite side, forming the central acoustic tract.

THE MESENCEPHALON. 405

Its fibres send minioious collaterals to the i)osterior tubercle (fig. 46'))
and a few to the anterior, and end by ramifying amongst the cells of
the mesial geniculate body (Cajal). In its course it traverses the
nucleus of the fillef, which consists of cells interpolated amongst it.s
fibres (the greater number in the lower part near the superior olive),
amongst which some of the tibres and many collaterals from them end.
The axons of these cells trend inwards towards the raphe. The upper
JiUet is continued upwards in the ventral part of the tegmentum towards
the thalamus (p. 409).

Crusta. — Lateral and ventral to the tegmentum is seen on either side
the white mass known as the crusta or pes pedunculi (figs. 463, cr., 464,
466, p.p.). This is formed by longitudinally coursing bundles of fibres
lying on the ventral aspect of each half of the mesencephalon, and
diverging above into the internal capsule of the cerebral hemisphere.

The fibres of the crusta are continued below into the so-called
"pyramid bundles" of the pons — which contain, as we have seen,
many more fibres than those of the pyramidal tract. This is also the
case with the bundles of the crusta, in which the pyramidal tract
proper — composed of fibres emanating from the ascending frontal and
paracentral gyri — is confined to the middle three-fifths (which, however,
includes many cortico-pontine fibres), whilst the mesial fifth is mainly
occupied by fibres passing from the lower frontal region to the pons,
carrying impulses to the nuclei of the facial and hypoglossal ; and the
lateral fifth by fibres the origin and functions of which are not certainly
known. But it is probable that these last are connected with the
regions of the hemisphere behind the Kolandic fissure, especially,
perhaps, with the temporal and occipital regions ; and are passing
from the pyramidal cells of these parts to end in the nuclei of the pons.

Substantia nigra. — The crusta is separated from the tegmentum by
a layer of grey matter containing a number of very deeply pigmented
nerve-cells (substantia nigra; figs. 464, 466, s.n.). The substantia nigra
receives many collaterals from the adjacent pyramid bundles of the
crusta. The crusta and tegmentum, together with the intervening
substantia nigra, constitute the cerebral peduncle {pes or cms cerebri).

Interpeduncular ganglion. — Between the cerebral peduncles, just
where they diverge from the mass of transverse fibres of the pons, is
seen close to the ventral surface of the brain a small mass of grey
matter containing a large number of small nerve-ceils with large and
irregular dendrons, and axons which are directed dorsally into the
tegmentum. This is the interpediniculur ffnngli&n (fig. 464, g.i.p.). It
receives on either side the ending of the fasciculus retroflexus of Meynert
which comes from the ganglion of the habemda, a collection of nerve

406 THE ESSENTIALS OF HISTOLOGY.

cells near the superior and mesial part of the thalamus, close to the
commencement of the third ventricle (see fig. 472). These ganglia are
both much better marked in many of the lower animals than in man.

Corpora quadrigemina. — The prominences {coUiculi or tubercles) of
the corpora quadrigemina are formed mainly of grey matter.
Connected with each one is a bundle of white fibres forming the
hrachia of the geniculate bodies.

Fig. 465.— Diagram showing the general structure of the posterior

CORPORA QUADRIGEMINA. (Cajal.)

A, principal mass of grey matter ; B, C, cortical layer ; D, grey matter around Sylvian
aqueduct ; K, decussation of superior peduncles of cerebellum ; a, b, c, d, fibres of
central acoustic path from lateral fillet ; c, axons from cells of princij)al nucleus
passing towards l^racbium ; /, fibres fi-om bracliium passing into superficial layer;
CI, fibres from fillet passing into superficial layer ; A, a fibre of fillet passing to centi-al
grey matter of aqueduct ; j, collaterals from posterior longitudinal bundle passing
to oculo-motor nucleus ; I, axons of cells in superomosial part of colliculus curving
round grey matter of aqueduct and forming the deep white layer.

The posterior or inferior colliculi consist of a grey centre which is
enclosed by siiperjicial and deep white layers (figs. 464, 465). The super-
ficial white layer is derived mainly from the brachium. The fibres of the
fillet divide as they approach the colliculus ; one branch enters its
grey matter while the other passes to the mesial geniculate body.
In animals with a highly developed sense of hearing all these parts
are proportionately developed. The deep white layer is derived from
cells of the grey centre, but many of the cells of the latter send their
axons towards the superficial layer. The destination of the fibres of

THE MESENCEPHALON,

407

the deep white layer is not certainly known ; some pass over the
centi'al grey matter of" the afiueduct to the opposite side.

In the anterior or superior coUiculi lour hiyers can be distinguished,
viz. : superficially, a thin white layer containing nerve-fibres and a few
horizontally disposed nerve-cells (fig. 467, A) ; next to this a grey cap

Fig. 466.— Section across the mid-brain through the superior corpora
QUADRIGEMINA. Magnified about 3J diameters. (From a photograph.)

c.p., posterior commissure of brain; gt.pi., pineal gland; c.q.a., grey matter of one of
superior corpora quadrigemina ; c.g.m., mesial geniculate body; e.g. I., lateral geni-
culate body; tr.opt., optic tract; p.p., crusta or pes pedunculi ; p.l.b., posterior
longitudinal bundle; Ji, upper fillet; r.n., red nucleus ; III., issuing fibres of third
nerve ; n.III., its nucleus ; I. 'p.p., locus pei-foratus posticus ; lS>i, Sylvian aqueduct.

^B) containing many and various nerve cells, amongst which the termi-
nations of the optic nerve (h, h) ramify ; below this the optic nerve layer
(C), which is formed of antero-posteriorly running fibres derived from
the optic tract, and ending as just stated for the most part in the grey
layer. This layer also contains some nerve-cells. Lastly there is a
deep white layer, the so-called deep meilulla, of transversely disposed
fibres (D) derived partly from the fillet, but comprising many fibres
which are derived from the cells of the colliculus itself, and a few which

408

THE ESSENTIALS OF HISTOLOGY.

are continued up from the antero-lateral ascending tract of the spinal
cord. This deep layer also contains a number of large dendritic
cells amongst the fibres. The superior corpora quadrigemina receive
through their brachia many of the fibres of the optic tract, which
in mammals enter the grey matter at the middle of its thickness and
traverse it from before back, so that in transverse sections of the mid
brain they appear cut across. In liirds they form a superficial white

Fig. 467. — Di.^gkam showing the ch.^kacters of the cells in the grev

MATTER OF THE ANTERIOR CORPORA QUADRIGEMINA. (Cajal. )
M, portion of dorsal median groove; A, superficial white layer; B, grey cap; C, optic

fibre layer (upper grey-white layer) ; D, layer of the fillet (lower grey-white layer).
«, a', marginal nerve cells : their axons are not represented ; b, V , horizontal spindle-
shaped cells of Golyi's type II. ; c, c', small cells with much branched dendrons and
an axon extending to the optic fibi-e layer ; d. e. e', spindle and stellate cells of the grey
cap, and /, /', cells of the straturu opticum, sending their axons into the stratum
lemnisci; j/, </, cells of the stratum lemnisci ; A, li, fibres of the optic nerve layer
ending in the grey and superficial white layers.

stratum covering the grey matter, but this is not homologous with the
superficial stratum of mammals, for the fibres in the latter are not
derived directly from the optic tract. The optic fibres are derived
from nerve-cells in the retina, and as they traverse the stratum opticum
they pass obliquely into the grey matter (in a ventral direction in
birds, in a dorsal direction in mammals) and end in arborisations
amongst its cells. The cells of the grey matter are very various in
form and size (fig. 467). Most of their axis-cylinder processes pass
ventral wards. The destination of all is not certainly known, but
many appear to join the anterior longitudinal bundle of the opposite
side. Others run down on the same side towards the pons Varolii,
intermingled with the ascending fibres of the fillet. A certain number
of fibres which take oricrin in the cells of the anterior colliculi course

THE MESENCEPHALON. 40&

over the central gvey matter which surruunds the Sylvian aqueduct
and sweep round this towards the fillet-tract of the opposite side.
These commissural fibres are continuous in front with those of the
posterior commissure.

The nerve-fibres of the optic nerve and optic tract do not all enter
the corpora quadrigemina. Many, indeed the majority, pass into the
lateral geniculate bodies and optic thalami to form arborisations there
(fig. 471). On the other hand, axons from the cells of these structures
pass to the cortex of the brain (occipital region).

As has just been stated, many arcuate fibres issue from the grey
matter of the corpora quadrigemina and pass obliquely downwards
into the ventral part of the mesencephalon encircling the central grey
matter. These fibres intercross in the raphe, where they constitute
the fountain-decussation of Mei/ncrf, and after crossing constitute the
main mass of the anterior longitudinal bundles. These are continued
into the anterior columns of the spinal cord ; they give off collaterals
to the motor nuclei of the eye-muscles, and probably to the motor
nuclei generally. Other fibres which appear to belong to the same
{tectospinal) system are traceable as a distinct tract into the lateral
column of the cord (see p. 365).

In the cat, the anterior corpora quadrigemina receive a number
of fibres from the pyramidal tract in the crusta of the same side, a
few crossing over the aqueduct to the opposite corpora quadrigemina
(Boyce, Sutherland Simpson). But in most animals the fibres which
pass from the cortex cerebri to the corpora quadrigemina enter those
bodies through their respective brachia.

No fibres are given off from the cells of the corpora quadrigemina
to the cortex cerebri.

The optic nerves. — The only sensory nerves which are immediately
connected with the mid-brain are the second or optic. Their origin is
from the large nerve-cells of the ganglion of the retina (p. 455).
The nerve leaves the globe of the eye at its posterior aspect, passes
through the optic foramen to the base of the brain, and joins the nerve
of the opposite side to form the optic chiasma (fig. 471). Of the fibres
which enter the chiasma, those from the inner (or nasal) two-thirds
of the retina cross to the optic tract of the opposite side, while the.
remaining third, comprising the fibres from the temporal part of the
retina, pass along the lateral border of the chiasma to the tract of
the same side. In the optic tract they are continued to the parts of
the brain where they have their terminal arborescences, viz., the
external geniculate body and the adjoining posterior part of the
thalamus* (pulvinar) and the anterior corpora quadrigemina. A certain

410 THE ESSENTIALS OF HISTOLOGY.

number of the fibres of the optic nerve bifurcate on reaching the
chiasma, and the branches pass one into each optic tract (Cajal).

The hlires which pass to the anterior corpora quadrigemina are much
finer than those to the corpora geniculata. It is probable that the
former furnish the path for reflex movements of the pupil, etc., and
the latter the i)ath for visual impressions, since the lateral corpora
geniculata and pulvinar thalami are directly connected with the visual
cortex in the occipital lobe, while, as already stated, no such direct
connection obtains between that cortex and the anterior corpora
quadrigemina.

■ A small bundle of fibres (transverse peduncular bundle) leaves the
optic tract as it enters the mid-brain and passes round the cerebral
peduncle to lose itself in the mesial part of the tegmentum near
the fillet. Its destination appears to be a small nucleus siti;ated
near the red nucleus. Its fibres degenerate after enucleation of the
opposite eyeball.

The optic tracts and chiasma also contain the fibres of v. Giidden's
commissure, which connects the posterior corpora quadrigemina, but
these fibres appear to have no relation to the visual function.

There are present in the optic nerve and tract a few fil)res which
originate in the nerve-centres — where is not known — and terminate
in the retina.

Motor nerves. — The motor nerves arising from the mid-brain are the
third and fourth. The position of their nuclei and their mode of exit
have been already described (pp. 401, 402).

Posterior commissure. — Immediately in front of the corpora quadri-
gemina, visible in the roof of this part of the mid-brain, is the posterior
commissure. This consists of fibres which arise in a nucleus at each
side of the Sylvian aqueduct and which pass across the middle line
dorsal to the central grey matter and then turn ventralwards and
caudalwards to pass down in the tegmentum lateral to the posterior
longitudinal bundle, which is partly reinforced by the fibres in question.
The posterior commissure extends into the region of the third
ventricle.

THE THALAMENCEPHALON.

The optic thalamus (fig. 468, th.) which lies at the side of the
third ventricle and forms part of the floor of the lateral ventricle,
is covered externally by a layer of white fibres, most marked next
to the internal capsule. Fibres from the latter pass into the thalamus
and serve to connect it with the hemisphere.

The o'rey matter of the thalamus is partially subdiVided by

THE THALAMENCEPHALON.

411

ail ol)li(iue white luiuiiia into a smaller, mesial, and a larger
lateral itiideus ; these contain a large number of small nerve-cells.
Anteriorly another portion of grey matter (anterior nuclem) is divided
off in a similar way ; this contains comparatively large nerve-cells.
These nuclei are themselves formed of several groups of cells having
difierent connections, many of which still re({uire elucidation.

Fig. 468. — Hokizumal skction through the optic thalamus and corpus

STRIATUM. (Natural size. )
V.I., lateral ventricle, its anterior cornu ; c.c, corpus callosum ; g.l., septum lucidum ;
a.f., anterior pillars of the fornix; v3, third ventricle; th., thalamus opticus;
St., stria medullaris; nc, nr'., nucleus caudatus, and nl., nucleus leuticularis of the
corpus striatum ; i.e., internal capsule ; g, its angle or genu ; nc'., tail of the nucleus
caudatus appearing iu the descending cornu of the lateral ventricle ; cl., claustruiu ;
/, island of Reil.

The thalamus receives the terminal branches of the fibres of the
upper fillet, continued from the cells of the opposite nuclei of Goll and
Burdach (spino-thalamic tract) , of the central path of the fifth cranial
nerve of the opposite side, and some fibres from the superior cerebellar
peduncle of the opposite side ; besides the fibres of the optic tract which
pass to the external geniculate body and pulvinar thalami.

From the cells of the thalamus nerve-fibres pass in every direction
into the white matter of the hemisphere, and eventually to the cortex
(figs. 437, 469). From the outer part they tend especially into the

412 THE ESSENTIALS OF HISTOLOGY.

occipital region, assisting to form the central visual tract which passes
to the visual cortex. From the inner and deeper part they converge
towards the subthalamic region and many are collected into the ansa
lenticularis (see p. 415), by which they pass into the nucleus lenticularis,
while others, as already stated, enter the corona radiata and thus reach
the cortex of the hemisphere. These fibres from the thalamus to the
cortex probably form the third and last link in the chain of sensory

Fig. 469. — Diagram of the coxxectioxs ok the thalamus with the ascexd-
ixg fibbes of the oth xerve, axd of the upper fillet ox the oxe

HAXD, AXD WITH THE CORTEX CEREBRI OX THE OTHER. (Cajal. )

A, B, C, D, E, various nuclei in thalamus ; 1, afferent fibres passing to manimillary body
F; G, tract of upper fillet endirjg in A (at c), and giving collaterals to D (posterior
nucleus) ; H, central tract from sensory nucleus of 5th ; T, cortex cerebri ; V, visual
cortex; II, anterior colliculus ; J, optic chi:isma ; S, optic fibres ; K, hippocampus.

a, fibres from cortex to thalamus, ending at e ; b, fibres from cells in thalamus (d) to
cortex ; /', fibres from lateral geniculate body and thalamus to visual cortex, ending
at g in stria of Gennari.

neurones, the second being formed by the neurones of the fillet and the
first by the neurones of the sensory roots. On the other hand, the
thalamus receives fibres from the cortex and from the corpus striatum,
which end amongst its cells.

Attached to the optic thalamus below and behind are the two
geniculate bodies (fig. 470) which at first sight appear to be both
connected with the optic tract, although only the outer one actually
receives optic fibres. The inner or mesial geniculate body receives
fibres from the central auditory tract through the lateral fillet. Of

THK THALAMENCEPHALON.

413

the geniculate Ixxlies the outer or hiteral lias a lamellated structure
consisting of alternating layers of grey and white matter, the white
layers being composed partly of the entering optic hbres and partly of
fibres emerging from the grey matter and passing to the central optic
path, Avhilc the greysul)stance contains very mimerous nerve-cellsamongst

Fig. 470.~Fi(a'RE showing thk olfactoky tkacts axd their roots : the

OPTIC CHIASMA AND OPTIC TRACTS : THE GENICULATE BODIES AND THE

PULVINAR THALAMI. (Edinger. )
The pons is cat through at the anterior part, and the section shows the Sylvian aqueduct,
the fillet (liiiiiinn laquearis), superior cerebellar peduncles, etc. (see fig. 459). The
corpora mamniillaria are partly concealed by the pons ; between and in front of
them is seen the infundibuhim and pituitarj' body.

•which the fibres of the optic tract end in complex arborisations. From
these cells axons arise and join a bundle of fibres which enters the white
matter of the hemisphere above and along with the internal capsule,
-and passes to the visual area of the cortex (central visual tract) Some
of the fibres from the corpus geniculatum externum, as they enter the
A'isual tract, send branches downwards towards the tegmentum.

414

THE ESSENTIALS OF HISTOLOGY.

The ganglion of the habenula (fig. 472,1'/') is a small collection
of nerve-cells, which lies at the posterior part of the thalamus on each
side, near the roof of the third ventricle. This ganglion receives on
the one hand the fibres of the habenula or stria medullaris, and on the
other hand gives off from its cells the fibres which form the fasciculus
refrqfle.vus (fig. 496), which pass downwards to the interpeduncular
ganglion (p. 405). The two ganglia of the habenulse are joined by
a white commissure.

The corpora mammillaria (fig. 470) are seen at the base of the brain
immediately below the posterior part of the third ventricle. Each is.

Fig. 471.

-Diagram to show the probable course and relations of

THE optic fibres. 1

composed of white matter externally and grey matter internally. It
receives fibres from the anterior pillar of the fornix of the same side ;
these fibres arise from cells in the hippocampus and end in the mammil-
lary body. According to Edinger some fibres from the olfactory
tract pass directly to it. The axons of its cells bifurcate, one branch,
the coarser, passing into the anterior and upper part of the thalamus in

^ Only single fibres are shown emerging from the anterior qiiadrigeminal and
external geniculate bodies, continuing the course of the two fibres from correspond-
ing points in the retinae. This is merely to simplify the diagram and is not
intended to imply that the retinal impressions are fused in those situations.

THE THALAMENCEPHALON.

415

the bundle of Vicq d'Azyr, and the other into the tegmentum of the
mid-brain in v. Gudden's bundle. The cori)ora manimillaria form part
of the central olfactory apparatus (fig. 490).

Subthalamic region. — The tegmentum of the crus cerebri is pro-
longed below the thalamus opticus, and between it and the internal

n^t:

Fig. 472. — Section taken obliquely through the optic thalamus and
internal capsule showing some of the strands of fibres of the
SUBTHALAMUS. (Magnified 2i diameters.)

T/i., thalamus; -0.111., third ventricle; t., taenia, or attachment of epithelial roof of
ventricle; sir., stria medullaris or habenula ; </', ganglion of the habennla ; n.i.,
mesial nucleus of thalamus; o-pt., optic fibres jjassing into pulvinar of thalamus;
zi., zona incerta, from which fibres are seen emerging and sweeping as the ansa
lenticularis, a. I., round the internal capsule, c.i., to pass towards the lenticular
nucleus ; c.s., corpus subthalamicum ; /., anterior pillar of fornix passing backwards
to corpus mammillare ; F.A., bundle of Vicq d'Azyr, passing upwards and foi-wards
from corpus rnammillai e into thalamus ; g, group of nerve cells, probably belonging
to the nucleus of the corpus mammillare ; x, fasciculus retroflexus.

capsule, into a mass of grey substance, with longitudinally and
obliquely crossing white bundles, which is known under the name of
suhthalanms (fig. 472). Its deepest part contains a lens shaped mass of
grey matter prolonged forwards from the substantia nigra, known as
the corpus subthalamicum (Luys). A mass of fibres sweeps round this

416 THE ESSENTIALS OF HISTOLOGY.

and round the internal capsule passing between the thalamus and the
nucleus lenticularis, this is known as the ansa lenticular is.

The pineal gland or epiphysis cerebri (fig. 461), which is developed
in the roof of the third ventricle, but passes backward between the
anterior corpora quadrigemina, is composed of a number of tubes and
saccules lined and sometimes almost filled with epithelium, and con-
taining deposits of earthy salts {brain sand). (Similar deposits may
also occur in other parts of the brain, especially in the pia-mater.)
The follicles are separated from one another by vascular connective
tissue derived from the pia-mater, and along with the vessels are
numerous nerve-fibres of sympathetic type (Cajal). No true nerve-
cells can be seen, although there are a number of cells similar in general
appearance to the "granules" of the cerebellum, but apparently without
axons. In some animals (ox) striated muscular fibres have been met
with.

In the chameleon and some other reptiles, the pineal is better developed,
and is connected by nerve-fibres with a rudimentary median eye of inverte-
brate type, placed upon the upper surface of the head.

The pituitary body or gland {hypophysis cerebri) is connected with the
third ventricle by the infundibulum. It consists of two lobes, a large
anterior and a smaller posterior. The structure of the pituitary body
has already been described (p. 224).

THE CEREBELLUM. 417

LESSONS XLIY. and XLV.
STIU'CTCRI-: OF THE CEREBELLUM AND CEREBRUM.

1. Sections of the cerebellani vertical to the surface, {a) across the direction
of tlie hxniiricTe, (/>) parallel with the laiuinte.

2. Sections across the whole of one hemisphere of the cerebrum of a monkev
passing through the third ventricle.

3. Vertical sections of the cerebral cortex : — one across the ascending
frontal and ascending parietal gyri, another fi'om the occipital lobe (calcarine
region), another across the superior temporal gyrus and island of Eeil, and
one across the hippocampal gyrus and hippocampus.

4. Transvei"se sections of the olfactory tract and bulb.

In all these preparations make sketches under a low power of the general
arrangement of the grey and white matter, and also of the nerve-cells in the
grey matter. Sketch some of the details under a high power.

The preparations are made in the same way as those of the spinal cord.
Other preparations should be made by the Golgi method to exhibit the
relation of the cells to one another. Such preparations have been already
partly studied (Lessons XVIL and XVIII).

The Cerebellum.

The cerebellum is composed of a white centre, and of a grey cortex.
Both e.xtend into all the folds or laminae, so that when the laminae
are cut across, an appearance is presented of a white arborescence
covered superficially by grey matter. The white matter is in largest
amount in the middle of each cerel)ellar hemisphere. There is here
present a peculiar wavy lamina of grey matter, similar to that in the
olivary body, and known as the nucleus dentatus (fig. 473, n.d.). This
receives numerous nerve-fibres from the cells of Purkinje of the cortex,
■which end by arborising around its cells. The latter give off axons
Avhich become the fibres of the superior cerebellar jjeduncles, and
for the most part end in the opposite red nucleus, but some pass beyond
this into the subthalamic region. The dentate nucleus also receives
collaterals from fibres of the inferior peduncle (Cajal).

Other isolated grey nuclei lie in the white matter of the middle lobe
■over the roof of the 4th ventricle and constitute collectively the nuclei
of Stilling. The most important of these appears to be the niideiis tecti
{s. fasfigii) (fig. 473). This receives many of the ascending fibres of the
vestibular nerve (p. 385) and collaterals from the spinocerebellar tracts,
and gives origin to a bundle of fibres which crosses to the opposite side

•2d

418

THE ESSENTIALS OF HISTOLOGY.

and descenfls in the mesial part of the restiform body to the reticular
formation of the medulla oblongata (Risien Russell).

The grey matter of the cerel)elluni appears essentially of similar
structure throughout the whole extent of the cortex. It consists of
two layers. The inner one (that next to the white centre) is composed
of a large number of very small nerve-cells intermingled with a few
larger ones and some neuroglia-cells {granule layer, fig. 474, d). The
outer one is thicker, and is formed chiefly of fine nerve-fibres (fig. 476, A)

Choroid g/^i^,^
jji-ex-us

SI

Fig. 473.— Section across the cerebellum and medulla oblongata

SHOWING the position OF THE NUCLEI IN THE WHITE CENTRE OP THE-
CEREBELLUM. (Stilling. )
n.d., nucleus dentatus cerebelli ; i.c.-p.^ fibres of superior peduncle; com, com', com",
commissural fibres ; A', rootlet of vagus ; XII, rootlet of hypoglossal nerve.

with small nerve cells scattered through it {molecular layer, fig. 474, h).
Into its outer part processes of the pia-mater conveying blood-vessels
pass vertically, and there are also in this part a number of long
tapering neuroglia-cells, somewhat like the Miillerian fibres of the
retina (fig. 479, gP. See also fig. 191, p. 161). Lying between
the two layers of the grey matter is an incomplete stratum of large
flask-shaped cells (fig. 474, c) {cells of Fnrkinje, fig. 475). Each of these
gives off" from its base a fine process (axon), which becomes the axis-
cylinder of one of the medullated fibres of the white centre, while
from the opposite pole of the cell large ramified processes (dendrons)
extend into the superficial layer of the grey matter.

THE CEREBELLUM.

419

The (loiidroiis of the cells of I'nikiiijc spread out in planes trans-
verse to the direction of the lamellae of the organ, so that they
present a different appearance according to whether the section is
taken across the lamella- or along them (compare figs. 476 and 477).
These dendrons are invested at their attachment to the cell, and

Fig. 474. — Section of cortex of cerebellum. (Sankey.)

a, pia-mater ; b, external layer; c, layer of corpuscles of Purkinje ; (', inner or granule
layer ; f, medullary centre.

for some extent along their branchings, by basket-works formed by
the terminal arborisations of certain fibres (climbing or tendril fibres)
of the medullary centre (fig. 479, cl.f.). The body of the cell of
Purkinje is further invested by a felt-work of fibrils formed by the
arborisation of axis-cylinder processes of nerve-cells {basket-cells) in the

420

THE ESSENTIALS OF HISTOLOGY.

outer layer of the grey matter (figs. 478 ; 479, /'). Each cell has
therefore a double investment of this nature, one covering the
dendrons, the other the bod}^ of the cell and extending along the
commencement of the axon.

Fig. 475.— a cell of pcrkinje of the cerebellum, shown by golgi's
METHOD. (Cajal.)

a, axon ; h, collateral from axon ; c, d, arborisation of dendrons.

The granules of the inner layer of grey matter are mostly small
nerve-cells, each with a few dendrons penetrating amongst the other
granules, and an axon which is directed between the cells of Purkinje
into the outer layer. After penetrating a variable distance into this
layer it bifurcates, and its two branches pass in opposite directions at
right angles to the main stem, and parallel to the direction of the
lamella (fig. 476). What ultimately becomes of the branches is not
known. In sections cut across the lamella the cut ends of these fibres
give a finely punctated appearance to the outer layer (fig. 477).

THE CEREBELLUM.

421

Figs. 476 and 477.— Sections of cortex cerebelli stained by golgi's

METHOD. (Oajal. )

Fio. 476.-Section made in the direction of the lamina. Fig. 47r.-Section taken across

A, outer^or'molecular layer ; B, inner or granule layer ; C\ medullary centre

n cormiscles of Purkinie • b, small granules of inner layer ; c, a protoplasmic process

' 7de dron) of a gran le ; -' nerve-fibre process of a granule passing into the molecular

ayer, -^ere it^bifurcates and become^s a longitudinal fib,^ ^"^ f^-^Aj^f^J?T^

tudinal fibres are cut across and appear as dots) ; e, bifurcation of another fibre , g, a

granule lying in the white rentre.

Fig 478 —Basket-work of fibres around two cells of pdrkinje.

(Cajal.)
a axis-cylinder or nerve-fibre process of one of the corpuscles of Purkinje ; b, fibres
' prolonged over the beginning of the axis-cylinder process ; c, branches of the nerve-
fibre processes of cells of the molecular layer, felted together around the bodies of
the coi-puscles of Purkinje.

422

THE ESSENTIALS OF HISTOLOGY.

Some of the cells of the granule layer are far larger than the others,
and send their much-branching axons amongst the smaller granules
(cells of Golgi, fig. 479, G). Besides these, other large "granules" have

Fig. 479.— Diagrammatic section of cerebellum to show the characters

AND relations OF THE CELLS AND FIBRES MET WITH IN THE SEVERAL LAYERS
AS EXHIBITED BY THE CHROMATE OF SILVER METHOD. (After KolHker.)

P, a cell of Purkinje ; G, a cell of Golgi ; h, a basket-cell ; m, m, other cells of the molecular
Ltyer ; gr, granules ; p, a nerve-fibre of the white substance derived from a Purkinje
cell ; m.f., " moss "-fibres ; cl.f., a climbing fibre ; nl^, gl-, gP, types of neurogUa-ceUs.

been noticed by Cajal, occurring both in the granule layer and in the
white centre, with long axons passing into the white matter of the
cerebellum. These are, however, only rarely met with.

Ramifying amongst the cells of the granule layer are peculiar fibres
derived from the white centre, and characterised by having pencils of

THE C'KREI^KLLUM.

423

fine short bmnches at intervals like tufts of moss (fig. 479, iiif). These
have been termed by Cajal the moss-fibre.<f ; they end partly in the
granide layer, iiailly in tlie niolocular layer.

left cortex
cerebr

Rigbt cortex
cerebelli

JXuclcns doiitatus
cerebelli

Kye muscle

Pyramidal tract
fibres

Skeletal muscle

Fig. 480.

-Diagram of the connections of the cerebellum through its
PEDUNCLES. (Cajal.)

The neuroglia of the cerebellum is peculiar in containing, besides the
ordinary branched and unbranched neuroglia cells (fig. 479, gl^, gP),
cells which possess long parallel processes which extend through the
molecular layer to be attached to the surface of the lamellae (gP). (See
also fig. 191.) The cell-bodies of these lie at about the same level as
those of Purkinje's cells.

424 THE ESSENTIALS OF HISTOLOGY.

The peduncles of the cerebellum have been already studied in connec-
tion with the medulla oblongata, pons, and mid-brain. The inferior
pe'luncle (restiform body) is composed of ascending fibres derived from
the dorsal spino-cerebellar tract, from both olivary nuclei — but chiefly
from that of the opposite side : perhaps also from the nuclei of the
gracile and cuneate funiculi, from cells and nuclei of the reticular
formation of the medulla oblongata and from the sensory nuclei of
the cranial nerves, especially of the vestibular nerve. The fibres of the
spinocerebellar tract occupy the outer part of the peduncle. Most
of the fibres of the inferior peduncle pass to the vermis, crossing to
the opposite side over the fourth ventricle, but before doing so they
give off" strong collaterals to the hemisphere of the same side. The
inferior peduncle also contains a small bundle of fibres descending
from the nucleus tecti of the opposite side to the medulla oblongata
(Risien Russell) which bends round the superior peduncle to join the
inferior peduncle, its fibres lying between those of the superior
peduncle and Gowers' bundle. The inferior peduncle contains a very
small nucleus of grey matter (Dejerine) which is almost completely
concealed amongst the mass of white fibres.

The middle peduncle is formed of fibres from the cells of the nuclei
pontis which are passing to the opposite hemisphere of the cerebellum.

The superior peduncle is formed of fibres which mostly take origin
in the corpus dentatum cerebelli, but some are said to arise in the
hemisphere and pass through this. The superior peduncles decussate
in the mid-brain across the raphe, and their fibres then bifurcate into
ascending and descending branches. The ascending branches pass
forwards and end in the red nucleus, but some fibres go past this
into the ventral part of the thalamus. The descending branches are
traceable into the dorsal part of the reticular formation of the pons.
According to Cajal they are connected with the motor iniclei of the
pons, medulla oblongata, and spinal cord (fig. 480). The superior
peduncle, as it issues from the hemisphere, is joined by the bundle
of Gowers, which runs over it, and passes backwards along its mesial
border to the vermis.

STRUCTURE OF THE CEREBRL^I.

The grey matter of the cerebral cortex is described as if. composed
of a number of layers, but they are not sharply marked off" from one
another, and they vary in relative development in diflerent regions of
the cortex. The cells are for the most part of a pyramidal shape
(fig. 481). The following layers are generally distinguishable, but in
some parts of the cortex a larger number can be made out :

THE CEREBRAL CORTEX.

42.'

1. A peripheral stratum (molecular ov plexifffrm layer, figs. 481, 4.S2, 1)
containing scattered nerve-cells and many neuroglia-cells. In the most

Fig. 481.— Ascending parietal coxvolutiox, golgi method. (Cajal.)

1, plexif onn layer ; 2, small pyramids ; 3, medium pyramids ; 4, superficial large pyramids ;

5, granules ; 6, deep large pyramids ; 7, deep medium pyramids.

426 THE ESSENTIALS OF HISTOLOGY.

superficial part of this layer, immediately .under the pia-mater, is a
thin stratum of medullated nerve-fibres, and besides these the layer
contains a large number of fibres, many of which are ramified.
They are largely derived from the deeper nerve-cells of the cortex.
Intermingled with these fibres are a certain number of ramified
nerve-cells, which have several long horizontally disposed dendrons
and a long axon, all of which terminate by arborisation within the
superficial layer {horizontal cells of Cajal) (fig. 482). Besides these,
others of a somewhat similar character but with short axis-cylinder
processes occur in this layer.

2. A layer of closely set small pyramidal nerve-cells, several deep
(layer of small pyramiih, fig. 481, 2). This layer also contains other
kinds of cells (fig. 491, k, f, g).

3. A layer of medium-sized pyramidal cells less closely set, with
small granule-like cells amongst them {layer of medium-sized jxyramids,
fig. 481, 3; fig. 491, J, H, K).

4. A laj^er of larger pyramidal cells {superficial large pijramids,
fig. 481, 4).

5. A layer of small irregular cells {small stellate cells, fig. 481, 5).
The large pyramids may extend down into this layer.

6. A layer of still larger pyramids {deep large pyramids, fig. 481, 6).
In the motor region of the cortex, which in man is confined to the
ascending frontal gyrus and paracentral lobule, pyramidal cells of
very large size (giant cells) occur, and are disposed in small clusters
or " nests " (Betz, Bevan Lewis). The fibres of the pyramidal tract
arise from these giant cells. In some parts of the cortex this layer
is absent or is blended with the next layer.

7. A layer of medium-sized pyramidal cells {deep medium pyramids,
fig. 481, 7).

8. A layer of small scattered cells, many of a fusiform shape
{pjolymorphous layer). This layer lies next to the white centre. In the
island of Eeil it is considerably developed, and is separated from the
rest of the grey matter by a layer of white substance. It is here
known as the claustrum, and on that account the layer is sometimes
termed the daustral layer.

Some authorities describe the cortex as consisting only of three layers,
viz. : the molecular layer, the layer of pyramids, and the layer of polymor-
phous cells ; others of four, five, etc., up to nine. As a matter of fact, the
complexity and the number of distinct layers vary in ditferent regions. The
pyramidal cells of the cortex are so termed from the shape of the cell-body,
which usually gives off several dendrons from tlie base of the pyramid and
one large dendi'on from its apex. This process extends to the plexiform
layer, on approaching which it breaks up into numerous ramifications which
have a general vertical direction and extend almost to the outer surface.

Fig. 482.— Diagram showing the relations of some of the cells in the

CEREBRAL CORTEX. (Barker, after Starr, Strong and Learning.)
1, plexiform layer with cells of Cajal ; 2, small (rf, e) and middle sized (,/) pyramids;

3, large pyramids (f/, </, k) ; also, m, cell with axon passing towards the surface, but
soon ramifying ; n, li, cell of Golgi's second type, with axon ramifying in the adjacent
grey matter : one of these belongs to the kind termed by Cajal "double-brush" cells ;

4, polymorphous cells, of which p ends its axon towards the surface and q its axon
into the medullary centre, 5, which contains also the axons of the pyramids ; r, r,
afferent fibres, ending in the cortex.

428 THE ESSENTIALS OF HISTOLOGY

This apical dendron is beset, both in its undivided part and in its branches,
by minute spinous projections (as seen in specimens prepared by the Golgi
method). Tliese projections are believed In- some authors to be retractile
(amoeboid) and to be the means of effecting (or breaking) nervous connection
with affei'ent fibres ; since they are in some preparations prominent, in others
hardly visible ; or the dendrons are entirely free from them, and have an even
outline or may be slightly moniliform. All the pyramidal cells have a single
axon, which is usually directed towards the medullary centre, of which it
forms one of the fibres ; but the axon sometimes curves back and passes out-
wards again, ending in arborisations in one of the other layers. Intei'mingled
with the pyramids and polymorphous cells are two other kinds of cells,
viz. : (1) cells with axis-cylinder process ramifying near the cell-body ;

these occur in all the layers (fig.
a h 491), .and (2) small cells sending

their axons towards the plexiform
layer (Martinotti), these are found
chiefly in the deeper layers of the
cortex.

From the white centre bundles
of medullated nerve-fibres pass

Fig. 483. -Sections of cerebral con- i" vertical streaks through the
VOLUTIONS. (After Bailiarger.) deeper layers of the grey matter,

(Natural size.) i J o j '

t 4.V. ■ .V, 1 1 f *.i,„„„i^„.i„<. to lose themselves amongst the

a, from the neighbourhood of the calcanne y^ ^ j,

^^^'u^''.Ti^^•°°'^'.°r?'''"'''^'''^'' r"'*''^' pyramidal cells of the more

visible (the hne of German) ; b, ordinary i J

type, with the superficial white layer and superficial layers (figS. 487, 490).

outer and inner lines of Bailiarger shown. i j \ o ' /

Many large fibres however are
seen running not A^ertically but obliquely into the grey centre from the
white matter. Most of the vertically disposed fibres are the nerve-fibre
processes of the pyramidal and polymorphous cells, and therefore take
origin in the cortex ; others, including the oblique fibres just men-
tioned, are passing into the cortex, probably from the thalamus, to end
amongst the cells of the several layers in free arborisations (fig. 484).

Besides these vertical strands of fibres there are others which lie in
planes parallel to the surface of the cortex, and which are derived
partly from the fibres which enter the cortex from the white matter,
partly from the collaterals which are given off" from the axis-cylinder
processes of the cortical cells themselves. The planes in w^hich these
fibres occur are (1) near the surface in the plexiform (molecular)
layer : this superficial stratum of white fibres is best marked in the
hippocampal region ; (2) in the layer of medium-sized pyramids : here
the fibres give the appearance of a whitish line in the section of the
grey matter (outer line of Bailiarger, fig. 483 b). There is a particularly
dense plexus of fibres in this situation in certain regions of the cortex,
especially the occipital lobe (in man in the convolutions bounding the
calcarine fissure), producing a very distinct line, here known as the line
of Gennari (fig. 483, a). This plexus of nerve-fibres is in intimate associa-

THE CEREBRAL CORTEX.

429

tion withj certain large and small stellate cells which are characteristic
of the visual region. (3) In most regions of the l)rain, in the plane of
the layer of large pyramids, another white line is seen ; this is known
as the inner line of BaiUunjer. The planes in which these white lines
are found are characterised especialh' in the occipital and temporal
lobes, by the presence, amongst the pyramids, of great numbers of

Fig. 484.— PrEPARATIOX SHOWIXG SOJIE of the .-iFFERENT FIBRES OF THE

ASCEXniXG FRONTAL GYRUS— HUMAN. (Cajal.)
A, part of second layer ; £, layer of medium-sized pyramids with close terminal plexus ;
C to D, intermediate plexus of horizontal fibres ; £, deep plexus of large oblique
afferent fibres ; n, h, afferent fibres arborising in the layer of middle pyramids,
amongst which they form, along with fibres derived from cells in the cortex itself,
the dense plexus which is shown in the left half of the figure. The efferent fibres
are not shown in this figure.

small nerve-cells, amongst which the white fibres of the layers ramify
and probably terminate.

The axis-cylinder processes of the pyramidal cells pass into the
white centre. Here some of them are continued into the corpus
callosum, and through this to the cortex of the opposite hemisphere
{commissural fibres) ; others form associalion-fibres which eventually pass
again into the grey matter of other parts of the same hemisphere ;

?'|

6

« f f ( I!

I. iH!V

. /•

Fig. 485. Fig. 486. Fig. 487.

Fig. 485. — Section of ascending parietal gyrus of man, stained by
Nissl's method. (Cajal.)
1, plexiform layer ; 2, small pyramids ; 3, medium pyramids ; 4, superficial large
pyramids; 5, small stellate cells (granules); 6, deep large and medium pyramids;
7, fusiform ceUs.
Fig. 486.— Section of ascending frontal gyrus (motor cortex), stained
BY Xissl's method. (Cajal.)
1 to 6 as before ; a, c, small cells amongst the pyramids ; b, a large pyramid ; d, a giant

cell of Betz.

Fig. 487. — Section of one of the motor convolutions (man),
stained by Weigert's method. (Cajal.)

''^ '■') m^G

Fig. 488.

'-'mi'

.I* V2

■•'6 <>■?«■ •"^"^'jr^ri

t<

*!..

Fig. 489.

Dg^)^.

::V / i\ -

/-.''

Fig. 490.

Fig. 488. — Calcarine (visual) cortex of man. (Cajal.) Nissl's method.
1, plexiform layer; 2, small pyramids; 3, medium pyramids; 4, large stellate cells
(characteristic of this part of the cortex) ; .5, small stellate cells; 6, a deep plexiform
layer, containing some small pjramids ; 7, large pyramids ; 8, layer of small and
medium pyramids with bent ascending axons ; H, fusiform cells.

Fig. 489. — Sectio.n' of first temporal gyrus (acou.stic cortex of man),

STAINED BY NiS.SL'S METHOD. (Cajal.)
1, plexiform layer ; 2, layer of small pyramids ; 3, superficial medium pyramids ; 4, large
pyramids ; 5, small stellate cells (granules) ; 6, deep medium pyramids ; 7, fusiform
cells.

Fig.

490.— Section of the fir.st temporal gyrus (man), stained by
"Weigert's method. (Cajal.)

432 THE ESSENTIALS OF HISTOLOGY.

whilst others again, especially those of the largest pyramidal cells,
extend downwards through the corona radiata and internal capsule.
These include the projection-fibres of the pyramidal tract and of the
cortico-pontine tract. As the projection fibres pass through the grey
and white matter of the hemisphere they give off collateral fibres to
the adjacent grey matter, to the corpus callosum, and to the corpus
striatum and optic thalamus, and some probably end in these masses of
grey matter. According to Cajal, in the brain of man as compared
with the lower mammals, there is a marked preponderance of cells of
Golgi's type II. (with short axis-cylinder ramifying near the cell body).
Such cells are most numerous in the layer of stellate cells and in
the laj^er of small pyramids.

Special features of certain parts of the cortex. — There is, as already
stated, a great amount of variation met with in the relative extent of
development of the above layers. This is exemplified in the accom-
panying drawings by Cajal (figs. 48-5 to 488) of certain convolutions
in the human brain. From these it will be seen that smaller-sized
cells prevail in some regions of the cortex (occipital, temporal) ; larger
and fewer cells in others (frontal, parietal, limbic). Nests or groups
of very large "giant" cells are characteristic of the "motor" region
(ascending frontal gyrus and paracentral lobule in man and anthropoid
apes) ; these cells give oingin to the fibres of the pyramidal tract, and
undergo Nissl degeneration when these fibres are severed. The
occipital region (in man, the neighbourhood of the calcarine fissure)
is especiall}^ characterised b}'- the great numbers of small stellate cells
and by the presence in the layer superficial to them of a stratum of
very large stellate cells with long spreading dendrons (fig. 488, 4) :
amongst these stellate cells (small and large) the optic fibres from
the lateral geniculate bodies ramify. A preponderance of small
stellate cells is also seen, but to a less extent, in sections of the
temporal lolje ; to a still less extent in the prefrontal and parietal
regions. The first temporal gyrus is characterised by the presence
in nearly all the layers, but especially the deepest, of special large
cells with widely-spreading dendrons and an axon passing towards
the white substance but giving off" many collaterals in the grey matter.
There are also very many cells of Golgi's type II. with axis cylinder
ramifying in a most complex manner near the cell-body, mainly in a
plane vertical to the surface. The cortex of the insula has special
cells similar to those in the first temporal gyrus, and is further
characterised by the peculiar spindle-shape of many of the large
pyramids.

The size and ruimber of the medullated fibres vary in different

THE CEREBRAL CORTEX. 433

regions. In some they are large and numerous (motor {)art of ascending
frontal, calcarine area, hippocampal area), in others fine and much less
conspicuoiis (g3'rus fornicatus, temporal area, parietal area, prefrontal
area, insula and lobus pyriformis), whilst an intermediate condition

Fig. 4i>l. — .Superficial layers of motor cortex of child, Golgi method. (Cajal.)

A, B, C, cells of Cajal in plexiform laj'er ; D to K, cells of type ii. of Golgi (with
axons ramifying near cell-body); H, J, "double-brush." types of celL

presents itself in the occipital area (except the calcarine region), the
transverse temporal gyri and superior temporal gyrus, the part of the
frontal immediately in front of the motor region and the ascending
frontal. These differences have been employed by Campbell in
attempting to diff'erentiate the functions of the various cerebral
regions by a comparison of their structure.

THE KHINENCEPHALON.

The rhinenceplialon (olfactory region of the telencephalon), on
account of the peculiarities of its structure, its importance in most
a.ninials, and the fact that it has been the part of the telencephalon to

2e

434 THE .ESSENTIALS OF HISTOLOGY

appear first in phylogenetic development (archipallhim of Elliott-Smith)
merits a special description, although in man and primates general!}',
and in some other (microsmatic) mammals, it is reduced to a compara-
tively rudimentary condition. On the other hand, in the so-called
osmatic (macrosmatic) mammals there is a large hollow olfactory bulb
forming the anterior termination of a thick olfactory lobe which
broadens out behind, where it is continuous with the hippocampal
gyrus and hippocampus. The whole forms a pyriform mass, which
is separated from the rest of the cortex {neopallium) by a well-marked
fissure — the limbic fissure — and has special connections through the
anterior commissure and fornix with other parts of the brain on the
same and on the opposite side.

In man the rhinencephalon consists anteriorly of the small olfactmy
bulb from which the thin olfactory tract extends backwards to the
grey matter at the base of the brain and to the hippocampal region.
Posteriorly the cortex of the rhinencephalon is doubled in so as to
form a projection, the hippocatiipus inajor, into the descending cornu
of the lateral ventricle : its edge here thins ofl" and is continued merely
as an epithelial covering to the choroid plexus of the pia-mater, which
is invaginated into the ventricle. At this thin edge the white matter
comes to the surface as the fimbria (which is continuous with the
fornix) ; lying along this is the small and half-concealed dentate gyrus,
which is formed by the sharp bending of the grey matter, and which is
traceable round into the hippocampus major (from this it is separated
by the hippocampal fissure), while this again is directly continuous
externally with the gyrus hippocampi. The olfactory lobe (tract) is-
connected directly with the hippocampal region by its lateral root,
whilst a mesial root passes into the anterior commissure and connects it
with the rhinencephalon of the opposite side. The structure and con-
nections of all these parts as they occur in man may be briefly alluded to.

In the region of the hippocampus major (figs. 492, 493), the cortex
is simpler in structure than elsewhere, and in the hippocampus major
itself, which is an infolded part of the cortex, the pyramids are reduced
to a single layer of large cells lying in the deeper portion and sending
their apical dendrons as long fibres into the plexiform layer. The
plexiform layer and the superficial white stratum which overlies it are
both very strongly marked, the plexiform layer having a distinctly
reticular aspect, due partly to neuroglia cells, partly to the arborescence
of the dendrons of the pyramids : the plexiform layer is here termed
stratum laciniosura ; internal to it near the dentate gyrus is a layer of
closely packed small cells termed stratum granulosum. The pyramidal
cells lie close to the white layer known as the alveus. This is the part

THE RHINENCEPHALON.

4:$-)

of the hippocampus seen within the ventricle, and represents the white
matter of the hemisphere. The alveus is prolonged externally into the
Jiiiihria, in which its fibres become longitudinal in direction and are
continued into part of the fornix.

Fig. 492.— Section across the hippocampus major, dentate fissure,

DENTATE FASCIA AND FIMBRIA. (W. KraUSe.)

D, fascia dentata, or dentate convolution ; F, fimbria, composed of longitudinal fibres
here cut acro.ss ; H, medullary centre of the hippocampal gyrus prolonged around
the hippocampus, as the so-called alveus, into the fimbria; 1, layer of large pyra-
midal cells; -2, their processes (stratum radiatum); 3, stratum granulosum ; 4,
plexiform layer (stratum laciniosum) ; 5, superficial white layer ; 6, nerve-cells of
fascia dentata ; 7, stratum granulosum of fascia dentata ; S, plexiform layer of the
fascia dentata.

In the dentate gyrus (fascia dentata, figs. 492, 493, d) the pyramidal
cells (6) are arranged in an irregularly radiating manner, occupying the
centre of the convolution, and surrounded by a ring of closely packed
small cells {stratum granulosum, fig. 492, 7). External to these is a thick
plexiform layer, occupied by interlacing fibres (stratum laciniosum).

The anterior part of the hippocampal gyrus, which is known as
the lobus pyriformis, and receives the lateral root of the olfactory

436 THE ESSENTIALS OF HISTOLOGY.

tract, is characterised by the presence in the plexiform layer of
peculiar nests of nerve cells. The cells in these nests are of two
types, viz., large polymorphous cells and small pyramidal cells, each
being confined to its own nest. This part of the cortex is regarded
by Cajal as the true olfactory region. In some animals the anterior

Fig. 493. — Hippocampal kegiox, Golgi method. (Cajal.)

A, B, hippocampal gyrus ; C, hippocampus major ; D, dentate gj-rus ; E, fimbria ;
F. white matter of hippocampal gyrus ; G, lateral ventricle ; H, fibres of corpus
callosum.

«, efferent fibres of hippocampal gyrus ; h, afferent fibres of hippocampal gyrus ; c, afiferent
fibres of hippocampus and dentate gyrus ; d, others perforating <rrey matter of hippo-
campal gyrus; c, others cut obliquely (from sphenoidal part of brain); /. fibi-es of
alveus ; p. h, cells of hippocampus major sending their axons into the alveus and
towards the fimbria ; i, k, collaterals from these axons passing to the molecular
layer ; )•, afferent fibres of alveus. The arrows indicate the probable course of the
nerve impulses.

perforated space forms a distinct prominence of the cortex (tuberculum
olfactorium) and this is also characterised by cell-nests (islets of Calleja).
They also occur in the cortex of the hippocampal fissure.

The olfactory tract is an outgrowth of the brain which was
originally hollow, and remains so in many animals ; but in man the

THE OLFACTORY BULB.

437

cavity has become obliterated, and the centre is occupied by neuro-
glia, containing no nerve cells. Outside the central neuroglia lies
the white or medullary substance, consisting of bundles of longi-
tudinal white fibres. Most externally is a thin superficial layer of
neuroglia.

The olfactory bulb (fig. 494) has a more complicated structure.
Dorsally there is a flattened ring of longitudinal white bundles

li

~J ;)

Fig. 494. — Section across a part op the olfactory bulb. (Henle.)

1, 3, bundles of very fine transversely cut nerve-fibres, forming the flattened medullary
ring, inclosing the central neuroglia, 2 : this ring is the anterior continuation of the
olfactory tract ; 5, white layer with numerous small cells (granules) ; 6, mitral-cell
layer ; 7, layer of olfactory glomeruli ; S, layer of olfactory nerve-fibres, bundles of
which are seen at * passing through the cribriform plate of the ethmoid bone.

inclosing neuroglia (1, 2, 3), as in the olfactory tract, but below this
ring several layers are recognised as follows :

1. A lohite or medullary layer (fig. 494, 4, 5), characterised by the
presence of a large number of small cells ("granules") with reticu-
lating bundles of medullated nerve-fibres running longitudinally
between them.

2. A layer of large nerve-cells (6), with smaller ones ("granules")
intermingled, the whole embedded in an interlacement of fibrils which
are mostly derived from the cell-dendrons. From the shape of most

438

THE ESSENTIALS OF HISTOLOGY.

Fig. 495. — Diagram to show. the relations of cells and fibres in the
olfactory bulb.

olf.c, olfactory cells of M. Schiiltze in the olfactory mucous membrane, sending their
basal processes as non-medullated nerve-fibres into the deepest layer of the olfactory
bulb (ol/.n.) ; (/I, olfactoi'y glomeruli containing the terminal arborisations of the
olfactory fibres and of processes from the naitral cells; mc, mitral cells, sending
processes down to the olfactory glomeruli, others laterally to end in free ramifica-
tions in the nerve-cell layer, and their axis-cylinder processes, a, a, upwards, to
turn sharply backwards and become fibres of the olfactory tract (n.tr.). Numerous
collaterals are seen coming off from these fibres ; n', a nerve-fibre of the olfactory
tract ending in a free ramification in the olfactory bulb.

CO

^Mi5su»\E or

Ml

'POCAWPI

^>

K^G.

r^LPON or The

\

A'O HASENOLA

a^^

^

i.

Fig. 496. — Diagram of the olfactory path in the brain. To simplify
the diagram the various divarications of the olfactory path have been
represented by branchings of individual fibres, although in some cases the
divarication is brought about bj' the turning aside of bundles of entire fibres.

THE OLFACTORY BULB. 439

of the large cells of this layer (fig. 495, 7n.c.) it has been termed the
" mitral " layer. These cells send their axons upwards into the next
layer, and they eventually become fibres of the olfactory tract and
pass along this to the base of the brain, giving off numerous collaterals
into the bulb as they run backwards.

3. The lai/er of olfadori/ ghmervU (fig. 494, 7 ; fig. 495, fjl.). This
consists of rounded nest-like interlacements of fibrils which are derived
on the one hand from the terminal arborisations of the non-medullated
olfactory fibres which form the subjacent layer, and on the other hand
from arborisations of dendrons of the large "mitral" cells of the layer
above. There are also a few small nerve-cells immediately external to
and extending within the glomeruli (periglomerular cells). These belong
to Golgi's type II., and appear to connect neighbouring glomeruli.

4. The layer of olfactory nerve-fibres (fig. 494, 8 ; fig. 495, olf.n.).
These are all non-medullated, and are continued from the olfactory
fibres of the olfactory mucous membrane of the nasal fossae. In
this mucous membrane they take origin from the bipolar olfactory
cells which are characteristic of the membrane (see Lesson XLV. fig.
528), and they end in arborisations within the olfactory glomeruli,
where they come in contact with the arborisations of the mitral cells.
The relations of the olfactory cells and fibres to the mitral cells, and
the continuation of the axis-cylinders of the latter upwards and back-
wards in the olfactory tract, are shown in the accompanying diagrams
(figs. 495, 496). Besides these centripetal nerve-fibres there are a
certain number of centrifugal fibres which end by ramifying in the
olfactory bulb amongst the mitral cells.

As is seen in fig. 496, many of the fibres of the olfactory tract pass
to the hippocampal region of the brain, terminating by arborescence in
the grey matter (molecular layer) of the base of the olfactory lobe in
the region of the anterior perforated space, as well as in that of the
uncus and the hippocampal gyrus. Fibres are also given off from the
olfactory tract to the anterior commissure which proceed to the oppo-
site tract and bulb. Besides these the anterior commissure contains
many fibres which are passing from the hippocampal region of one side
to the corresponding region on the opposite side of the brain. From
the p3'ramid -cells of the base of the olfactory lobe and hippocampal
gyrus fibres pass to the grey matter of the hippocampus, and from the
pyramid-cells of the hippocampus others proceed by way of the
fimbria and fornix to the hippocampus of the other side, to the sub-
callosal gyrus and septum lucidum, to the ganglion of the habenula,
and finally by the anterior pillar of the fornix to the corpora
mammillaria.

440 THE ESSENTIALS OF HISTOLOGY.

CORPUS STRIATUM.

Besides the grey matter of the cerebral cortex the cerebral hemi-
spheres conceal in their deeper parts certain other masses of grey
substance (fig. 497). The principal of these are the corpus striatum
{nucleus caudatus, n.c, and nucleus lenticularis, n.l.) and optic thalamus
(th.). Between them run the bundles of white fibres which are
passing downwards to the crus cerebri, forming a white lamina
termed the internal capsule. Above the level of these nuclei the
internal capsule expands into the medullary centre of the hemisphere.
Below the optic thalami are the prominent ganglia known as corpora
albicantia or mammillaria. Of these the optic thalami and corpora
mammillaria have already been described.

The nucleus caudatus of the corpus striatum is composed of a reddish-
grey substance containing cells some with long, others with short axis-
cylinders ; some of the former being very large. It receives fibres
from the part of the internal capsule which separates it from the
nucleus lenticularis, and next to the lateral ventricle it is covered by
a thin layer of neuroglia, and over this by the epithelium of the cavity
(ependyma).

The nucleus lenticularis, which corresponds in position internally
with the island of Reil externally, is divided by two white laminse into
three zones. It is separated from the nucleus caudatus and optic
thalamus by the internal capsule (fig. 497, i.e.), which consists of the
bundles of medullary fibres which are passing between the white
centre of the hemisphere and the crus cerebri ; it receives on its inner
side many white fibres from the capsule, and these impart to it a
radially striated aspect. Many of the nerve-cells of the nucleus
lenticularis contain yellow pigment. The fibres of the ansa lenticularis
appear to arise from some of them, but the exact course and destination
of these fibres is not known.

The internal capsule (fig. 497), which is continued below into the
crusta (pes) of the crus cerebri, consists mainly of projection-fibres,
which are derived from the cortex cerebri, and are passing down to
the thalamus, mid-brain, pons, medulla oblongata, and spinal cord.
A horizontal section across the internal capsule shows it to be
bounded laterally by the lenticular nucleus, mesially by the caudate
nucleus, the stria medullaris, and the optic thalamus. Its section
shows a sharp bend — the genu. The fibres from the motor region of
the cortex (pyramidal tract) pass down in the part of the capsule
extending from the genu as far as the posterior limit of the lenticular
nucleus. In this area the fibres for the head and eyes are massed

INTERNAL CAPSULE.

441

chieHy in the anterior part : those of the lower liml) in the posterior
part, and those of the face, arm, and trunk occupy intermediate posi-
tions from before backward, in the order named (Bcevor and*fHorsley),
but without beino; strictly confined to definite zones.

Fig. 497.— Horizontal section through the optic thalamus and corpus
STRIATUM. (Natural size. )

V.I., lateral ventricle, its anterior cornu ; c.c, corpus callosvim ; s.l., septum lucidum ;
a./., anterior pillars of the fornix; v3, third ventricle; th., thalamus opticus;
St., stria medullaris; nc, nc'., nucleus caudatus, and nl., nucleus lenticularis of the
corpus striatum ; i.e., internal capsule ; g, its angle or genu ; me'., tail of the nucleus
caudatus appearing in the descending cornu of the lateral ventricle ; cl., claustrum ;
/, island of Reil.

The fibres from the cortex to the thalamus lie mainly in the anterior
limb of the capsule, while the afferent fibres from the thalamus to the
cortex occur in the posterior part of the posterior limb, but extend
forwards so as to mingle with the descending fibi-es just referred to
as belonging to the pyramidal tract.

The membranes of the brain are similar in general structure to those
of the spinal cord. The dura mater is, however, more closely adherent
to the inner surface of the bony enclosure than is the case in the
vertebral canal. The arachnoid is in many places close to the dura
mater, and separated by a wide subarachnoid space (which is bridged

442 THE ESSENTIALS OF HISTOLOGY.

across by finely reticulating bands of areolar tissue) from the pia
mater. In the vicinity of the longitudinal sinus, small rounded
elevations (arachnoidal villi, Pacchionian glands) project into the dura
mater, and even Vjecome embedded in the skull itself. The pia mater
is closely adherent to the surface of the brain, and dips into all the
sulci, but without forming actual folds (Tuke). In it the blood-vessels
ramify before passing into the substance of the brain, and they are
accompanied, as they thus enter the cerebral substance, by prolonga-
tions of the pia mater, which do not, however, closely invest them, but
leave a clear space around each vessel, presumably for the passage of
lymph (perivascular space). The capillary network is much closer in
the grey than in the white matter.

THE EYELIDS. 443

LESSOXS XLVI., XLVIL, and XLVIII.

STRUCTURE OF THE EYELIDS AND OF TEE FARTS OF
THE EYEBALL.

1. Sections of the eyelid vertical to its surfaces and transverse to its long
axis.

Notice the long sacculated Meibomian glands lying in dense connective
tissue close to the conjunctival surface, their ducts opening at the margin of
the lid. External to these the small fibres of the orbicularis palpebrarum are
cut across ; a few of the fibres of the muscle lie on the conjunctival side of
the duct. A short distance from the Meibomian gland may be observed a
tolerably large sebaceous gland ; outside this again are the eyelashes. In
the skin covei"ing the outer surface of the eyelid a few small hairs may be
seen. At the attached part of the eyelid are some bundles of involuntary
muscular fibres cut longitudinally in the section, and in the upper eyelid the
fibrous attachment of the elevator muscle may be observed attached to the
dense connective tissue.

Make a general sketch under a low power.

2. Sections through the posterior part of an eyeball. These sections will
show the relative thickness of the several coats and the layers of which each
coat is formed. Sections which pass through the point of entrance of the
optic nerve will also exhibit the manner in which the nerve-fibres pierce the
several coats to reach the inner surface of the retina. The modifications
•which are found in the neighbourhood of the yellow spot may be made out
in sections through that region ; but they must be taken from the human
eye, or from that of the monkey.

3. Sections of the anterior half of an eyeball. Tliese sections should pass
through the middle of the cornea. The lens may be left in situ, but this
renders the preparation of the sections and the mounting of them diflicult
on account of the extreme hardness which is imparted to the lens-tissue by
alcohol.^

In these sections make a general sketch under a low power, showing the
relations of the several parts one with another ; and study carefully, and
sketch in detail, the layers of the cornea, the junction of the cornea and
sclerotic, the ciliary muscle, the muscular tissue of the iris, the mode of
suspension of the lens, and the pars ciliaris retinai.

4. Mount in glycerine thin tangential sections of a cornea stained with
chloride of gold by Cohnheim's method ; if from the frog, the cornea can
be torn with fine forceps into thin lamellte, which are mounted whole.
Sketch three or four of the connective-tissue cells (corneal corpuscles) The
arrangement and distribution of the nerve-fibres and their termination
amongst the epithelium-cells as shown in chloride of gold preparations have
been already studied (Lesson XIX., p. 176).

^The celloidin method of embedding is well adapted for preparations of this
kind.

444 THE ESSENTIALS OF HISTOLOGY.

5. Mount in dammar sections of a cornea wliicli has been stained with
nitrate of silver. Notice the branched cell-spaces corresponding with the
connective-tissue cells of the last preparation.

[This preparation is best made by rubbing the surface of the cornea of a
recently killed animal with lunar caustic, after scraping off the epithelium
with a scalpel. After ten minutes (by which time the nitrate of silver will
have penetrated the thickness of the cornea) the eye is washed with distilled
water, and exposed to the light. When brown, tangential sections may be
made, for which purpose the stained cornea may be hardened in spirit.]

6. Eemove the sclerotic from the anterior part of an eye which has been
preserved in Miillers fluid, and tear off thin shreds from the surface of the
choroid, including amongst them portions of the ciliary muscle. Stain the
shreds with hsematoxylin and mount them in glycerine. Sketch the branched
pigment-cells, the elastic network, the mode of attachment of the fibres of
the ciliary muscle, etc.

7. Injected preparation of choroid and iris. Mount portions of the choroid
coat and iris from an eye (preferably of an albino animal), the blood-vessels
of which have been filled with coloured injection. Make sketches showing
the arrangement of the capillaries and Aeins.

8. Teased preparation of human retina. Break up with needles in a drop
of glycerine a minute fragment of retina which has been placed in 1 per
cent, osmic acid solution for some hours, and has subsequently beeu kept in
dilute glycerine. Complete the separation of the retinal elements by tapping
the cover-glass. Draw carefully under a high power some of the isolated
elements — e.g. the rods and cones with their attached fibres and nuclei, the
inner granules, the ganglion-cells, the fibres of Mliller, hexagonal pigment-
cells, etc. In some of the fragments the arrangement of the elements in the
retinal layers may be made oiit even better than in actual sections.^

Measure the length and diameter of some of the cones, the length of the
cone-fibres, and the diameter of some of the outer and inner nuclei.

9. Teased preparation of frog's retina. To be prepared in the same way as
8. Notice the very large rods, their outer segments breaking up into .disks,
and the relatively small cones. Also the pigment extending between the
rods, the distance varying according as the eye has been kept in the dark or
in the light. A fresh frog-retina should also be teased in salt solution.

10. Sections' of retina of ox or dog, which have been prepared by Golgi's
method. A curled-up piece of fresh retina is placed in osmium-bichromate
mixture and is subsequently treated with nitrate of silver solution. -

IL Teased preparation of lens. Separate in Avater the fibres of a crystalline
lens which has been macerated for some days in bichromate of potassium or
dilute formol solution. Sketch some of the fibres, together and separate.

The eyelids (fig. 498) are covered externally by the skin, and
internally or posteriorly by a mucous membrane, the conjunctiva, which
is reflected from over the globe of the eye. They are composed in
the main of connective tissue, which is dense and fibrous under the
conjunctiva, where it forms Avhat is known as the tarsus.

^ The distribution of the nerve-fibres and cell-processes within the retina can
only be made out satisfactorily by the employment of Golgi's silver chromate
method (see § 10).

^ See Appendix. Cajal's reduced silver method may also be employed.

THE EYELIDS.

445

Embedded in the tarsus is a row of long sebaceous glands (the
Meibomian glands, /), the ducts of which open at the edge of the eyelid.
The rest of the thickness of the eyelid is composed of a somewhat
loose connective tissue, and contains the bundles of the orbicularis

Fig, 498. — Vertical sectiox through the upper eyelid.

(Waldejer. )

a, skin ; i, orbicularis ; h' , ciliary bundle ; c, involuntary muscle of eyelid ; d, con-
junctiva ; (', tarsus vfitli Meibomian gland ; f, duct of the gland ; <i, sebaceous gland
near eyelashes ; A, eyelashes ; (', small hairs in outer skin ; j, sweat-glands ; k, pos-
terior tarsal glands.

muscle ih). In the ui)per eyelid the levator palpebne is inserted into
the tarsus by a fibrous expansion, and some bundles of involuntary
muscle are also present near the attachment of the eyelid. The skin
has the usual structure ; it contains small sweat-glands, and the
follicles of small hairs, and, in addition, at the edge of the eyelid, the

446

THE ESSENTIALS OF HISTOLOGY.

large hair-follicles from which the eyelashes grow. The epithelium
of the conjunctiva palpebrse is columnar, passing at the edge of the
lid into the stratified epithelium of the skin ; it also becomes stratified
in the part which is reflected over the globe of the eye. The nerves of
the conjunctiva terminate for the most part in end-bulbs, which in man
are spheroidal, and formed chiefly of a small mass of polyhedral cells ;
but in the calf and most animals they are elliptical.

The lacrymal gland may be briefly mentioned in connection with
the eyelid. It is a compound racemose gland, yielding a watery
secretion. Its alveoli are lined by columnar cells, which are normally

^io

,^

Fig. 499. — Section of l.4Crtjial gland of dog. A, resting ; B, after

COPIOUS SECRETION PRODUCED BY PILOCARPINE.

filled with granules, but, after profuse secretion, these disappear, and
the cells become much shorter and smaller (fig. 499, A, B). Its ducts,,
of which there are several, open at the upper fold of the conjunctiva,
near its outer extremity.

The globe of the eye (fig. 500) is inclosed by three coats, the cornea-
sclerotic, choroid (with the iris), and retina. It is filled by the
vitreous and aqueous humours and the crystalline lens which lies
between them.

The sclerotic coat is composed of dense fibrous tissue, the bundles
of which are intimately interlaced. It is thickest at the back of the
eyeball. It is covered externally with a lymphatic endothelium, while
internally it is lined by a layer of connective tissue containing pigment-
cells, which give it a brown appearance (luinina fusca). At the entrance-

THE CORNEA.

447

of the optic nerve the sclerotic is prolonged into the sheath of that
nerve, the bundles of which, piercing the coat, give a sieve-like aspect
to the part {hniiina crihrosa, tig. 512, L.).

CANAL OF SCHLEMM

SCLEROTIC

CENTRAL ARTERY OF RETINA

optic nerve
Fig. .500.— Diagram of a section through the (right) human eye passing

HORIZONTALLY NEARLY THROUGH THE MIDDLE. (Magnified about 4

diameters.)

a, h, equator ; ,/■, y, optic axis.

The cornea (fig. 501) consists of the following layers (enumerated
from before back) :

1. A stratified epithelium continuous with the epithelium of the
conjunctiva (1 ).

2. A thin lamina of homogeneous connective tissue {membrane of
Bowman), upon which the deepest cells of the epithelium rest (2).

448

THE ESSENTIALS OF HISTOLOGY.

3. A thick layer of fibrous connective tissue which forms the pr<yper
substance of the cornea (3). This is continuous laterally with the tissue

Fig. 501. — Veetical sectiox of hcmax corxea fbom xear the margin.

(Waldeyer. ) (Magnified.)
1, epithelium ; 2, anterior homogeneous lamina ; 3, substantia propria comese ; 4, pos-
terior homogeneous (elastic) lamina ; 5, epithelium of the anterior chamber ; a, oblique
fibres in the anterior layer of the substantia propria ; h, lamellae, with their fibres
cut across, producing a dotted ajjpearance ; c, corneal corpuscles appearing fu-siform
in section ; d, lamellae with the fibres cut longitudinally ; e, transition to the
sclerotic, with more distinct fibrillation, and surmounted by a thicker epithelium ;
/, small iilood-vessels cut across near the margin of the cornea.

of the sclerotic. It is composed of bundles of white fibres arranged
in regular laminae, the direction of the fibres crossing one another at
right angles in the alternate laminae. Between the laminae lie flattened

THE CORNEA.

449

connective-tissue corpuscles (fig. 502), which are branched and united
by their processes into a continuous network ; there is of course a
corresponding network of cell-spaces. In vertical sections the cells
appear narrow and spindle-shaped (fig. 501, c). In the superficial
lamina there are a few bundles of fibres which run obliquely towards
the surface (a).

4. A homogeneous elastic layer {membrane of Descemet, fig. 501, 4).
This completely covers the back of the cornea, but near the angle
which the cornea forms with the iris it breaks up into separate fibres
{ligamentum pedinatum) which are partly continued into the iris as
the pillars of the iris.

Fig. .502. — Corpuscles of the cornea, isolated. (Waldeyer.)

5. A layer of pavement-epithelium {epithelium of Descemefs membrane)
covering the posterior surface of the elastic lamina, and lining the
front of the anterior chamber of the eye (fig. 501, 5). At the sides
it is continued over the ligamentum pectinatum into a similar epithe-
lium, covering the anterior surface of the iris. The cells of the
epithelium of Descemets membrane are separated from one another
by intercellular spaces, bridged across by bundles of fibrils which pass
through the cells (fig. 503). Each cell has a peculiar basket-like
reticulum close to the nucleus : perhaps a modified centrosome.

The nerves of the cornea pass in from the periphery, losing their
medullary sheath as they enter the corneal substance. They form
a primary plexus in the substantia propria, a secondary or sub
epithelial plexus immediately under the epithelium which covers the
anterior surface, and a terminal plexus of fine fibrils which pass
from the subepithelial plexus in pencil-like tufts and become lost

2f

450

THE ESSENTIALS OF HISTOLOGY

between the epithelium-cells (Fig. 504-). There are no blood-vessels
or lymphatics in the cornea, although they come close up to its margin.

/l\m

M

Fig. 503. — Epithelium-cells of descemet's membrane. (After Sinirnow

and Nutjl.)

— ^— . h

i^^iljii.

•SfSe-r

Fig. 504. — Vertical sectiox through the cornea. (Barker, after Cohnheim.)

The corneal corpuscles and the cells of Descemet's membrane are not represented ; the
anterior epithelium has been drawn in only in part, n, Descemet's membrane ; 6,
parts of nerve plexus in substantia propria ; c, branches going to the epithelium ; rf,
fibres of the subepithelial Layer; e, vertical fibrils with horizontal outrunners.

THE CHOROID COAT.

451

The choroid or vascular coat of the eye is of a black colour in
many animals, hut in the human eye it is dark hrovvn. It is com-
posed of connective tissue, the cells of which are large and filled
with pigment (tigs. 505, 506). It contains in its inner layer a close

CAOIAT.a£,.

Fig. 50.5. — Section of choroid. (Cadiat. )

a, membrane of Bruch : the chorio-capillaris is just above it ; b, vascular layer ; c, vessels

with blood-corpuscles ; (/, lamina suprachoroidea.

Fig. 506. — A small portion of the lamina suprachoroidea. (Highly

magnified.)

The pigment-cells and elastic fibres are well shown ; n, nuclei of endothelial cells (the

outlines of the cells are not indicated) ; I, lymph-cells.

network of blood-vessels, and in its anterior part the involuntary
muscular fibres of the ciliary muscle, which pass backwards from
their origin at the junction of the cornea and sclerotic, to be inserted
into the choroid. The choroid is separable into the following layers
(enumerated from without in):

1. The lamina suprachoroidea (fig. 505, (/). This is a loose membrane

452

THE ESSENTIALS OF HISTOLOGY.

composed of delicate connective tissue pervaded by a network of fine
elastic fibres, and containing many large branched pigment-cells and
lymph-corpuscles (fig. 506). It is covered superficially by a lymphatic
endothelium, and is separated from the lamina fusca of the sclerotic
by a cleft-like lymph-space which is bridged across here and there by
the passage of vessels and nerves, and by bands of connective tissue.

2. The vascular layer of the choroid (fig. 505, h), which resembles
the suprachoroidea in structure, l)ut contains the blood-vessels of the

Fig. 507. — Injected blooij-ve.ssels of the choroid coat. (Sappey.)
1, one of the larger veins; 2, small anastomosing vessels ; 3, branches dividing into the

smallest vessels.

coat. In its outer part are the larger vessels (arteries and veins),
the veins having a peculiar vorticose arrangement; in its inner part
(chorio-capillaris) are the capillaries, Avhich form an extremely close
network with elongated meshes, the capillaries radiating from the
extremities of the small arteries and veins in a highly characteristic
maimer (fig. 507). In the ciliary processes the vessels have for the
most part a longitudinal direction, but there are numerous convo-
luted transversely disposed capillaries uniting the longitudinal vessels
(fig. 510, d).

3. Lining the inner surface of the choroid is a thin transparent
membrane known as the memhrane of Bruch (fig. 505, a).

The ciliary muscle consists of involuntary muscular bundles which
arise at the corneo-sclerotic junction, and pass meridionally backwards

THE IRIS.

453

to 1)0 inserted in the choroid (fig. TiOS, ilf). Many of the deeper-
seated bundles take an o])li(iue direction, and these pass gradually into
others which run circularly around the circumference of the iris, and
on a level with the ciliary processes. This set of circularly arranged
l)undles constitutes the cirmlar ciliary muscle of H. Miiller (Mu.) ; it is
most marked in hypermetropic eyes.

Fig. 50.S.— Section through the ciliary part of the eye, including part
OF the cornea, the ora serrata, the iris and the edge of the lens

WITH its suspensory LIGAMENT. (Fuchs.)
C, cornea ; S, sclerotic ; Ch, choroid ; B, retina ; Fc, its pigmented epithelium ; 0, pars
ciliaiis : this is continued over the choroid processes; p.(., p.c, pigmented and
non-pigmented layer of par.s ciliaris ; L, lens ; 3f, ciliary muscle ; /•, its radiatinsj
(meridional) fibres passing from their origin at the corneo-sclerotic junctiou ; ilu,
circular ciliary muscle; ci, artery of sclerotic ; s, vein (canal of Schlemm) ; :, fibres
of zonula of Zinn pas^sing between choroid jirocesses into the suspensorj- ligament
of the lens (;', ;) I 'i augle of anterior chamber; sp, sphincter pupillas; p, edge of
pupil; A, pigmented epithelium of iris (accidentally detached at this point); c, c,f, /",
creases and folds of anterior surface of iris ; cr, a fissure in this surface (accidental) ;
a, artery at insertion of iris ; k-, capsule of lens.

The iris is that part of the vascular coat of the eye which extends
in front of the lens. It is continuous with the choroid and has a
similar structure, but its pigment-cells often contain variously coloured
pigment. Besides the delicate connective tissue with numerous
elastic fibres and blood-vessels of which it is chiefly composed, it
contains two sets of plain muscular fibres. The- one set forms the
sphincter muscle (figs. 508, sjj., 509, a), which encircles the pupil; the
other set consists of a flattened layer of radiating fibres which

454

THE ESSENTIALS OF HISTOLOGY.

extend from the attachment of the iris nearly to the pupil, lying
close to the posterior surface and constituting the dilatator muscle
(fig. 509, h).

The back of the iris is covered by a thick layer of pigmented
epithelium (uvea) continuous with the epithelium of the pars ciliaris
retinae.

The muscular tissue of the iris is stated to be developed from the epithelium
at the back of the iris (Nussbauni, Szili).

Fig. 509.

Fig. 509. — Segment op the iris, seen from the pos-
terior SURFACE after REMOVAL OF THE UVEAL
PIGMENT. (Iwanoff.)
a, sphincter muscle ; 6, dilatator muscle of the pupil.

Fig. 510.- Vessels of the choroid, ciliary processes
AND IRIS of a child. (Arnold.) (10 diameters.)

a, capillary network of the posterior part of the choioid, ending
at b, the ora serrata ; c, arteries of the corona ciliaris. supjilying
the ciliary processes, d, and passing into the iris, <■ ; /, the
capillary network close to the papillary margin of the iris.

Fig. 510.

The blood-vessels of the iris (fig. 510, e) converge towards the pupil.
Near the pupil the small arteries form an anastomotic circle, from
which capillaries arise and pass still nearer the pnpil, around which
they form a close capillary network.

A large number of nerve-fibres are distributed to the choroid and
iris, probably going chiefly to the muscular tissue of those parts (ciliary
muscle and sphincter and dilatator pupillse).

The retina consists of the eight layers shown in the accompanying
figure (fig. 511), numbered as they occur from within out.

The inner surface of the retina, which is smooth, rests upon the
hyaloid membrane of the vitreous humour. It is formed of the united
bases of the fibres of Midler, which will be afterwards described.

The layer of nerve-fibres is formed by the expansion of the optic nerve
after it has passed through the coats of the eye (fig. 512). At its

THE EETINA.

465

entrance the nerve forms a sliii;lit eminence {roUiculus nervi cptici). The
nerve-fibres lose their medullary sheath on reaching the retina. Most
are connected with (derived from) the cells of the ganglionic or optic
nerve-cell layer (fig. 513), but some (centrifugal) fibres pass through
the ganglionic and molecular layers to form a terminal arborisation in

S. Layer of pigment-cells.

7. Laver of rods and cones.

Membrana limitans externa.

6. Outer nuclear layer.

0 Outer synapse or molecular layer.

4. Inner nuclear or bipolar layer.

3. Inner synapse or molecular layer.

•2. Layer of optic nerve-cells.

1 . Layer of optic nerve-fibres.

. . . Membrana limitans interna.

inner burtLuw

Fig. 511. — Diagrammatic section* of the human retina. (M. Schultze.)

the inner nuclear layer (fig. 51-4 and fig. 513, j). The layer of nerve-
fibres becomes gradually thinner in the anterior part of the retina.

The lai/er of optic n-erve-cells, or ganglionic layer, is composed of nerve
cells somewhat like the cells of Purkinje of the cerebellum but varying
in size, although those of large size are prevalent in most parts of the
retina. In the j-ellow spot, on the other hand, smaller nerve-cells are
met with, and they may here lie several deep. These nerve-cells have

THE ESSENTIALS OF HISTOLOGY.

-1 .0

Fig. 512.— Section thuocgh the coat.s of the eyeball at the point of

ENTRANCE OF THE OPTIC NERVE. (Tolilt.)
Ve, dural sheath ; Vm, araclmoidal sheath, and 17, pia-matral sheath of the optic nerve,
with lymphatic spaces between them; 0, 0.. funiculi of the nerve ; L, lamina cribrosa;
A, central artery ; S, sclerotic ; CA, choroid ; R, retina. The small letters refer to the
various parts of the retina, h being the layer of rods and cones, and i that of nerve-
fibres.

Fig. .513. — Section of dog's retina, golgi method. (Cajal.)
<t, cone-fibre ; h. rod-fibre and nucleus ; c, i/, bipolar cells (inner gi-anides) with vertical
ramifications of their outer processes or dendrons : in the centie of the ramific.ition
lie the enlarged ends of rod-fibres ; t, other bipolars with flattened ramifications
abutting against ramified ends of cone-fibres ; /, large bipolar with flattened ramifica-
tion ; tj, inner granvile-cell sending an axon towards the rod and cone-fibres ; A,
amacrine cell with di£fu.«e arborisation of its processes in inner molecular layer;
i, j, III, nerve-fibrils passing respectively to outer molecular, inner nuclear, and inner
molecular layers ; »>, ganglionic cells, with axons passing into nerve-fibre layer.

THE RETINA.

457

a fino axis-cylinder process proloiii^ed into a fil)re of the layer just
noticed, and a thick branching process, the ramifications of which
terminate in the next layer in flattened arborisations at different levels
(fig. 015, A, B, C).

The inner si/n(q)f<e layer or inner molecular layer is comparatively thick,
and has an appearance very like parts of the grey matter of the nerve-

FiG. 514. — Section through the innek layers op the retina of a bird,

PREPARED BY GOLGl's METHOD. (Cajal. )
A, nerve-fibres of optic nerve la}'cr ; B, some of these fibres passing through the inner
molecuhir layer to end in an arborisation at the junction of the inner molecular
and inner nuclear layers. The layers in this and in the two succeeding cuts are
numbered in correspondence with the layers in fig. 511.

centres. A few nuclei are scattered through it, and it is occupied by
the processes of the nerve-cells and of the inner granules (bipolars and
amacrine cells) which form synapses in it ; it is also traversed by the
centrifugal fibres from the optic nerve layer, as well as by the fibres of
Muller.

Fig. 515. — Section across the molecular and ganglionic layers of bird's

RETINA, prepared BY GOLGl'S METHOD. (Cajal.)

Three or four ganglionic cells, A, B, C, and the terminal arborisations of their dendrons,

(«, b, c, in the molecular layer, are shown.

The inner gramde layer (also termed inner nuclear layer) is mainly
composed of bipolar nerve-cells containing large nuclei. A process
(the axon) of each of these cells (fig. 513) extends inwards into the
inner molecular layer where it spreads out into a terminal arbori-
sation. These arborisations occur at different levels in the layer,
forming synapses with the optic nerve-cells. Another process
(dendron) is directed outwards, and arborises in the outer molecular

458

THE ESSENTIALS OF HISTOLOGY.

layer, where it forms synapses with the terminations of the rod- and
cone-fibres. It has been shown by Ram6n y Cajal that there are two
kinds of bipolars, one kind (rod-hipolars, fig. 513, c.d) being connected
externally with the rods of the retina, and passing inwards to ramify
over the bodies of the nerve-cells, whereas those of the other kind
{cone-hipolars, e) are connected with the cone-fibres, and ramify in the
middle of the inner molecular layer. The outwardly directed pro-
cesses of these cone-bipolars are, in some animals, but not in mammals,

liilliliMiiiiin

Fig. 5L6.— Section of bird's retina, prepared by golgi's method.
(Cajal.)
A, large nerve-cell of inner nuclear layer ; B, C, amacrine cells ; D, small bipolar nerve-
cells with one process, ramifying in the inner molecular layer and the other one
ramifying in the outer molecular layer, and extending (E) as far as the rods and
cones as a fibre of Laudolt ; F, G, rod- and cone-nuclei respectively; H, I, cells with
dendrons ramifying in outer molecular layer ; J, fibre of Miiller.

continued on as far as the external limiting membrane, where each
ends in a free extremity {fibre of Landolt, fig. 516, E). Besides these
bipolar nerve-cells, there are other larger inner granules (spongioblasts
of some authors) which are different in character, having ramified
processes which extend into the inner molecular layer (figs. 513, h ;
516, A, B, c), in which the bodies of these cells are often partly
embedded. The cells in question have been regarded as of the nature
of neuroglia-cells, but according to Cajal they are probably all nerve-
cells. He has termed them amacrine-cells, from the fact that they are
destitute of a long process ; but some have been noticed to give off,
besides the branching processes or dendrons, which ramify in the
molecular layer, an axis-cylinder process which may extend into the

THE RETINA.

459

nerve-fibre layer. There are also some cells in the outer part of the
granule layer which send their processes entirely into the outer molecular
layer (fig. 516, h). These are the horizontal-cells of Kam6n y Cajal
(termed spongioblasts of outer molecular layer by some authors).
The fibres of Miiller have nucleated enlargements (fig. 516, .i) in the
inner nuclear layer.

The Older molecular lai/er is thin, and is composed mainly of the
arborisations of the inner granules, of the rod and coae-fibres, and of
the horizontal cells (figs. 513, 516),
which all form synapses in this layer.

The Older nuclear layer and the lat/er
of rods and cones are composed of
elements which are continuous through
the two layers, and they should pro-
perly, therefore, be described as one.
It has been termed the sensory epi-
thelium of the retina (fig. 517, 6 and 7).
The elements of which this nerve-
epithelium consists are elongated
nerve-cells of two kinds. The most
numerous, which may be termed the
rod-elements, consist of peculiar rod-like
structures {retinal rods) set closely side
by side, each of which is prolonged
internally into a fine varicose fibre
{rod-fibre) Avhich swells out at one part
of its course into a nucleated enlarge-
ment, and ultimately ends (in mam-
mals) in a minute knob within the
outer molecular layer, where it is em-
bedded in the ramifications of the dendrons of the rod-bipolars.
The rod consists of two segments, an outer cylindrical and trans-
versely striated segment, which during life has a purplish-red colour
if the eye has not been recently exposed to light, and an inner
slightly bulged segment which in part of its length is longitudinally
striated. The nucleus of the rod-element in some animals, but accord-
ing to Flemming not in man, has a transversely shaded aspect in the
fresh condition (fig. 517). The cone-elements are formed of a conical
tapering external part, the retinal cone, which is directly prolonged
into a nucleated enlargement, from the farther side of which the
cone-fibre, considerably thicker (in mammals) than the rod-fibre, passes
inwards, to terminate by an expanded arborisation in the outer mole-

FiG. .517. — Diagrammatic repre-

SEXTATION OF THE ROD AND CONE
ELEMENTS OF THE RETINA. (After

Schwalbe. )
The designation of the numbers is the
same as in fiff. oil.

460

THE ESSENTIALS OF HISTOLOGY.

cular layer ; here it comes into relation Avith a similar arborisation
of dendrons of a cone-bipolar. The cone, like the rod, is formed of
two segments, the outer of which, much the smaller, is transversely
striated ; the inner, bulged segment being longitudinally striated.

Fig. .'i19.— PiGMENTEn epithelium of the
HUMAN RETINA. (M. Schultze.) (Highly
magnified.)

a, cells seen from the outer surface with clear
lines of intercellular substance between ; 0,
two cells seen in profile with fine offsets
extending inwards ; c, a cell still in connection
with the outer ends of the rods.

'Sr^

Fig.

518. — Diagram of the connections of the retinal elements with

ONE another and WITH THE CENTRAL NERVOUS SYSTEM. (Cajal.)

o to g, layers of retina ; a, rods and cones ; 6, outer nuclear layer ; c, outer molecular
layer ; U, inner nuclear layer ; i, inner molecular layer ; /, nerve-cells giving origin to
fibres of optic nerve ; p, h, i, a centrifugally conducting fibre, with a terminal
arborescence in the retina ; j, grey matter of corpus geniculatum or corpus quadri-
geminum.

The inner ends of the rod- and cone-fibres, as already stated, form
synapses with the peripheral arborisations of the bipolars, and through
the latter elements and their sjmapses in the inner molecular layer
a connection is brought about with the nerve-cells and nerve-fibres of
the innermost layers. The connection of the retinal elements with
one another and through the optic fibres with the central nervous

THE RETINA.

461

system (aiiterior corpora (luadrigeiniiia and lateral geniculate l)O(lies)
is shown diagrammatically in fig. 518.

In birds, reptiles, and amphibia, a small oil-globule, often lirightly
coloured red, yellow, or green, is found in the inner segment of each
cone. Other variations of structure are met with in different animals.

The cones are most numerous at the back of the retina ; they are
fewer in number, and the rods are proportionally more numerous
towards the anterior part.

J

pff

^'^

^tV*^

pi yi<a->i^

1

'■^HH

Fig. 520. — A. Part of a section of the retina feoji the eye of a fkog

WHICH HAD BEEN KEPT IN THE DARK FOR SOME HOURS BEFORE DEATH.

(v. Genderen-Stort. )
The pigment is collected towards the outer ends of the rods, which were red, except the
outer detached rod, which was green. The cones, which in the frog are much
smaller than the rods, are mostly elongated.

B. A SIMILAR SECTION FROM A FROG WHICH HAD BEEN EXPOSED TO LIGHT.

The pigment is extended between the rods, and is accumulated near their bases. The

rods were colourless. All the cones are contracted.

The pigmentary layer forms the most external part of the retina. It
consists of hexagonal epithelium-cells (fig. 519), which are smooth
externally where they rest against the choroid, but are prolonged
internally into fine filaments which extend between the rods. The
pigment-granules, many of which are in the form of minute crystals,
lie in the inner part of the cell, and after prolonged exposure to light
they are found extending along the cell-processes between the rods
(Kiihne), their function being probably connected with the restoration
of the purple colouring matter which has been bleached by the light.
This extension of the pigment is accompanied by a shortening of the
cones (Engelmann) (fig. 520).

462

THE ESSENTIALS OF HISTOLOGY.

Fibres of 31uller.— The fibres of Miiller (fig. 516, /, and fig. 521) are
long stiff cells which pass through several of the retinal layers.
Commencing at the inner surface of the retina by expanded bases
which unite with one another to form the so-called internal limiting-
membrane (fig. 522), they pass through all the layers in succession,
until they reach the outer granule layer. Here they branch and

m.l.e.

m.l.i.

Fig. .521.

KiG. 522.
Fig. 521. — A fibre of mullek from the dog's

RETINA, GOLGI METHOD. (Cajal.)
1, nerve-fibre layer ; 2, nerve-cell layer ; 3, inner molecular
layer ; 4, inner ftranule lajer ; 5, outer molecular layer; 6,
outer granule layer ; b, nucleus of the fibre ; a, a process
extending into inner molecular layer ; m.l.i,, membrana
limitans interna ; m.l.e., membrana limitans externa.

Fig. 522. — Intern.\l lijiiting membrane of retina
treated with silver nitrate, showing the out-
lines of the b^\ses of the fibres of muller.
(G. Retzius. )

expand into a sort of honeycomb tissue which serves to support the
fibres and nuclei of the rod- and cone-elements. At the bases of the
rods and cones, this sustentacular ti.ssue ceases, being here bounded by
a distinct margin which has been called the external limiting membrane
(fig. 521, m.l.e.), but delicate sheaths pass from it around the bases of
the rods and cones. Each Miillerian fibre, as it passes through the
inner granule layer, has a nucleated enlargement (b), indicating the
cell-nature of the fibre. The fibres of Miiller represent ependyma

THE RETINA.

463

cells or perhaps long neuroglia-cells such as are found in some parts
of the nerve-centres, e.g. the cerebellum (see fig. 479, (jl^).

There are two parts of the retina which call for special description.

The macula lutea (yellow spot), with its central fovea, is the part
of the retina which is immediately concerned in direct vision. It is
characterised firstly by its greater thickness (except at the middle of
the fovea), secondly by the large number of its ganglion-cells, which
are rounded or conical, and thirdly by the large number of cones

Fig. 523. — Section through the central part of the fovea centralis.
''f-^. (From a preparation by C. H. Golding-Bird.)

M, bases of Miillerian fibres; c.h., nuclei of inner granules (bipolars) ; c.n., cone-fibre

nuclei ; c, cones.

it contains as compared with the rods. In the central fovea itself
(fig. 523) there are no rods, and the cones are very long and
slender, measuring not more than 2/^ in diameter ; all the other layers
become gradually thinned down almost to complete disappearance, so
that the middle of the central fovea is the thinnest part of the retina.
Since there are fcAv rods, the outer granule layer loses in great
measure its appearance of being composed of closely packed nuclei,
and the cone-fibres are very distinct, forming the so-called fihrous layer.
The direction of these fibres is for the most part very oblique in this
part of the retina.

464

THE ESSENTIALS OF HISTOLOGY.

Fig. 525. — Section through the mar-
gin OF THE babbit's LENS, SHOWING
THE TRANSITION OF THE EPITHELIUM
OP THE CAPSULE INTO THE LENS-
FIBRES. (Babuchin.)

Fig. 524. — A small portion of the ciliary
PART of the retina. (KolUker. ) 3.50
diameters.

1, pigment-cells ; ~, columnar-cells.

Fig. .526.— Fibres of the crystalline
LENS. (350 diameters.)

A, longitudinal view of the fibres of the lens from
the ox, showing the serrated edges. B, trans-
verse section of the fibres of the lens from the
human eye. C, longitudinal view of a few of
the fibres from the equatorial region of the
human lens. Most of the fibres in C are seen
edgewise, and, towards 1, present the swell-
ings and nuclei of the ' nuclear zone ' ; at 2, the
flattened sides of two fibres are seen. A and
B from KoUiker ; C from Henle.)

THE LENS.

465

The pigmentar}^ layer is thickened over the fovea, and there is also
a thickening in the choroid coat here, due to a large accumulation of
capillary vessels.

The pars ciliaris retinae, which commences at the ora serrafa, where
the retina proper abruptly ends, is composed of two epithelial layers
(fig. 524), and has no nervous structures. Of the two layers, the
external is a thick stratum of pigmented epithelium formed of rounded
cells and continuous with the pigmentary layer of the retina on the
one hand, and with the uvea of the iris on the other ; the inner is a
layer of columnar cells, each containing an oval nucleus. They
probably represent the Mtillerian fibres of the retina.

Fig. 527. — Cells of vitreous. (Schwalbe.)
a, (!, without vacuoles ; h, c, e, jt, g, with vacuoles.

The retina contains but few blood-vessels. The central artery enters
and the vein leaves it in the middle of the optic nerve. The larger
vessels ramify in the nerve-fibre layer, and there are capillary net-
works in this layer and in the inner nuclear layer. There are peri-
vascular (lymph) spaces around the veins and capillaries. The sensory
epithelium receives no blood-vessels, but is nourished from the vessels
of the choroid.

The lens. — The lens is a laminated fibrous body inclosed by a trans-
parent elastic capsule to which, around the circumference, the fibres of
the suspensory ligament are attached (fig. 508). Immediately within the
capsule, in front and at the sides, there is a layer of cubical epithelium
termed the epithelium of the capsule, but at the margin of the lens
the cells become longer and pass by a gradual transition into the
lens-fibres (fig. 525). The fibres which compose the lens are long and
riband-shaped, with finely serrated edges (fig. 526, A) ; in transverse
section they appear prismatic (B). Many of the superficial fibres are

2g

466 THE ESSENTIALS OF HISTOLOGY,

nucleated (c), the lens-fibres having originally been developed by the
elongation of epithelium-cells.

The vitreous humour. — This is composed of soft gelatinous tissue,
apparently structureless when examined in the fresh condition, but
containing fibres and a few scattered cells, the processes of which are
often long and varicose, and the cell-bodies distended by large vacuoles
(fig. 527). The hyaloid memhrane, which invests the vitreous humour,
is homogeneous and structureless except in the region of the ciliary
processes, where it is fibrous in structure, forming the zonule of Zinn
and spreading out into the suspensory ligament of the lens (fig. 508).
This part of the hyaloid membrane is connected with a circular
fibrous portion of the vitreous humour which serves to give addi-
tional firmness to the attachment of the fibres of the suspensory
ligament of the lens (Anderson Stuart).

OLFACTORY MEMBRANE. 467

LESSON XLIX.

STRUCTURE OF THE OLFACTORY MUCOUS MEMBRANE
AND OF THE EXTERNAL AND MIDDLE EAR.

1. Vertical sections of the nasal mucous membrane. The sections may
be carried either across the upper turbinate bone, after decalcification or
across the upper part of the nasal septum. Make a sketch under the low
power. Notice the difference in the character of the epithelium in the
olfactory and respiratory parts of the membrane.

2. Teased preparation of the epithelium of the olfactory mucous membrane.
A piece of the membrane is placed quite fresh in osmic acid (1 per cent.) for
a few hours, and is then macerated for two days or more in water. The
epithelium is broken up in dilute glycerine ; the cells easily separate from
one another on tapping the cover-glass. Notice the two kinds of cells.
Sketch some of the cells under a high power.^

3. Sections of the external ear (these have been already studied for the
cartilage, Lesson XII.).

4. Sections across the cartilaginous part of the Eustachian tube. Sketch
under the low power.

5. Preparation of the membrana tympani. A piece of the membrane,
stained with magenta and gentian violet (see Lesson IX., ^ 2), is mounted
flat in xylol balsam or dammar.

Determine the composition of the membrane — i.e. the several layers com-
posing it — by focussing carefully with the high power.

THE OLFACTORY MUCOUS MEMBRANE.

The olfactory region of the nasal fossae includes the upper and
middle turbinate processes and the upper third of the septum. It
is covered by a soft vascular mucous membrane of a yellow colour
in man.

The epithelium of the olfactory mucous membrane (figs. 528, 529) is
very thick and is composed of long cells, set closely side by side and
bounded superficially by a cuticular lamina, through which the free
ends of the cells project. The cells are of two kinds: 1. Long
narrow spindle-shaped or bipolar nerve-cells consisting of a larger part
or body (h), containing the nucleus, and of two processes or poles, one

(c) straight and cylindrical and extending to the free surface, the other

(d) very delicate and varicose, looking not unlike a nerve-fibril and

^ The connection of the olfactory cells with the olfactory nerve-fibres is displayed
in embryos, the method of Golgi being employed.

468

THE ESSENTIALS OF HISTOLOGY.

extending down towards the corium. The position of the nuclear
enlargement varies, and with it the relative length of the two processes.
The distal or free process terminates in a small clear projection, which
passes beyond the cuticular membrane ; in amphibia, reptiles, and
birds, and perhaps also in mammals, it bears fine stiff hairlike fila-
ments. The proximal or varicose process becomes lost amongst the
plexus of olfactory nerve-fibres at the base of the epithelium ; it is
connected with one of these fibres, and ultimately passes through the

Fig. 528. — Cells and terminal nerve-fibres of the olfactort region.
(Highly magnified.)

1, from the frog ; .' and 3, from man. In 1 and ;.' : -a, epithelial cell, extending deeply
into a ramified process ; b, olfactory cells ; c, their peripheral rods ; e, the extremi-
ties of these, seen in 1 to be prolonged into fine hairs ; d, their central filaments.
In 3 : — k, hairlets ; c, free border of cell ; p, peripheral process ; b, body of cell ;
}(, nerve-fibre. 1 and ;,' from M. Schultze ; .> from v. Brunn.

cribriform plate of the ethmoid to end in an arborisation within one of
the olfactory glomeruli (see diagram, fig. 495, p. 438). These cells
have been termed the olfactory cells. 2. Long columnar epithelium cells
{(i), with comparatively broad cylindrical nucleated cell-bodies placed
next to the free surface, and long, forked, and branching tail-like pro-
cesses extending down to the corium. These are regarded not as
sensory epithelium-cells, but merely as serving to support the proper
olfactory cells. They are the columnar or sustenfacular cells. 3. Taper-
ing cells are present, at least in some animals, in the deeper part of the
epithelium. They rest by their bases upon the corium, and project
between the other cells, which they assist to support.

OLFACTORY MEMBRANE. 469

The corium of the olfuctory mucous membrane is also very thick
(fig. 529). It contains numerous blood-vessels, bundles of the olfactory
nerve-fibres (whioh are non-medullated), and a large number of serous
glands known as Boiiman's glands (h), which open upon the surface by
ducts which i)ass between the epithelium-cells.

Fig. 529. — Section of olfactory mucous membrane. (Cadiat.)
n, epithelium ; b, glands of Bowman ; c, nerve-bundles.

THE EXTERNAL AND MIDDLE EAR.

The external ear proper (pinna) is composed of elastic fibro-cartilage,
invested by a thin closely adherent skin. The skin 'is covered by
small hairs, and connected with these are the usual sebaceous follicles.
In the lobule there is a considerable amount of adipose tissue ; and
voluntary muscular fibres are in places attached to the cartilage of the
pinna, and are seen in sections.

The external auditory meatus is a canal formed partly of cartilage
continuous with that of the pinna, partly of bone. It is lined by a
prolongation of the skin and is closed by the membrana tympani,
over which the skin is prolonged as a very thin layer. Near the
orifice the skin has hairs and sebaceous glands, and the meatus is
also provided throughout the cartilaginous part with small convoluted
tubular glands of a brownish-yellow colour, which yield a waxy
secretion (ceruminous glands). They appear to represent modified
sweat-glands. They are represented in fig. 530.

The tympanum is lined by a mucous membrane which |is continuous
through the Eustachian tube with the mucous membrane of the
pharynx ; it is also prolonged into the mastoid cells. The epithelium

470

THE ESSENTIALS OF HISTOLOGY.

is columnar and ciliated in some parts, but in others — e.g. roof,
promontory, ossicles, and membrana tympani — -it is a pavement-
epithelium.

The membrana tympani is a thin membrane formed of fibrous
bundles which radiate from a central depression (umbo). Within
the radial fibres are a few annular bundles. Coverincr the fibrous

Hair.

Sebaceous glauds.

Root-sheath of 1
follicle. /

Root of hair, -'-.y, <^:

Fig. 530. — Ceruminous glaxds and hairs of the extekxal ear. (Griiber.)

membrane externally is a thin layer continuous with the skin of the
meatus ; covering it internally is another thin layer, derived from the
mucous membrane of the tympanic cavity. Blood-vessels and lym-
phatics are distributed to the membrane chiefly in the cutaneous and
mucous layers.

The Eustachian tube is the canal leading from the tympanum to
the pharynx. It is formed of bone near the tympanum, but below,

THE EUSTACHIAN TUBE.

471

near the pharynx, it is bounded partly by a bent piece of cartilage
(tig. 531, 1, 2), partly by fibrous tissue. The' latter contains numerous

Fig. 531.— Section across the caktilaginous part of the eustachian
TUBE. (Riidinger.)

1, 2, bent cartilaginous plate ; 3, muse, dilatator tubse ; to the left of i, part of the
attachnient of the levator palati muscle ; 5, tissue uniting the tube to the base of the
skull ; e and 7, mucous glands ; S, 10, fat ; 9 to 11, lumen of the tube ; 12, connective
tissue on the lateral aspect of ttie tube.

mucous glands (6, 7), which open into the tube, and on the outer side
a band of muscular tissue (3) which joins the tensor palati. The
epithelium is ciliated.

472 THE ESSENTIALS OF HISTOLOGY.

LESSOX L.

THE IXTERXAL EAR.

1. SECTION'S across one of the membranous semicii-cular canals of a fish
(skate).

2. Longitudinal sections through the ampulla of a semicircular canal
(skate).

1 and 2 may be hardened in chromic and osmic acid (see below under .5)
and embedded in celloidin.

The semicircular canals and their ampulke may also be seen cut across in
sections of the petrosal of the guinea-pig or other mammal.

3. Golgi preparations of the macula of the utricle from the skate.

4. Teased preparations of the auditory epithelium of an ampulla or of the
macula of the utricle, from the skate.

5. Vertical sections through the middle of the cochlea of a mammal
(guinea-pig).

The cochlea is put quite fresh into 0"2 per cent, chromic acid containing
one-fifth its volume of 1 per cent, osmic acid, or into Flemming's solution,
or 10 per cent, formol. The decalcification can be effected by the use of the
phloroglucin-nitric acid fluid, or by sulphurous acid.^ When decalcified, the
preparation is well washed, and then transferred to alcohols of gradually
increasing strength.

In preparing sections of the above three preparations it is advisable, in
order that the epithelium should be kept in position, to embed in celloidin.
If the paraflin method of embedding be used, the sections are fixed to the
slide by an adhesive process. The organ should preferably be stained in
bulk.

6. Teased preparations of the epithelium of the organ of Corti from the
guinea-pig.

Both 4 and 6 are made from osmic preparations.

Make sketches from all these preparations under the high power.^

The labyrinth, which is the essential part of the auditory organ,
consists of a complex membranous tube lined by epithelium and filled
with endolymph, contained within a bony tube — the osseous labyrinth
— of corresponding complexity of shape (figs. 532, 533). The mem-
branous labyrinth does not wholly fill the bony cavity ; the rest of the
space is occupied by perilymph. The membranous labyrinth (fig. 532)
is composed of the utricle (u), and the three semicircular canals (each

^ See Appendi.v.

-For details of the methods of obtaining the various parts of the labyrinth for
microscopical examination, the student is referred to the author's Course of
Practical Histology.

THK LABYRINTH.

47*

with an enlargement or ampulla which opens into it), the saccule (s), and
the canal of the cochlea {ex.).

The branches of the auditory nerve pass to certain parts only of the
membranous labyrinth, viz. the maculae of the utricle and saccule, the
cristjie of the ampulla^ and along the whole length of the canal of
the cochlea (the shaded parts in fig. 532).

At these places the lining epithelium is specially modified to form a
sensory or nerve-epithelium ; elsewhere it is a simple pavement-
epithelium.

The membranous semicircular canals and the utricle and saccule
are composed of fibrous tissue, which is adherent along one side to the

Fig. .^32.— Pl.\n of the bight mem-
branous LABYRINTH VIEWED FROM

THE MESIAL ASPECT. y-

v., utricle, with it.s macula and s.s.c, p.s.c,
and e.s.c, the three semicircular canals
with their ampulla!; s, saccule; aq.v.,
aquseductus vestibuli ; s.e., saccus endo-
lymphaticus; c.r., canalis reuniens ; c.c,
canal of the cochlea.

Fig. .533. —View of the interior of-
the left osseous labyrinth.

The bony wall of the labyrinth is removed
superiorly and externally. 1, fovea hemi-
elliptica ; -2, fovea hemisphasrica ; 8, com-
mon opening of the superior and posterior
semicircular canals ; 4, opening of thfr
aqueduct of the vestibule ; 5, the superior,
0, the posterior, and, 7, the external semi-
circular canals ; S, spiral tube of the coch-
lea ; 9, scala tympani ; 10, scala vestibuli.

endosteum of the bony canal ; from the opposite side bands of fibrous
tissue pass across the perilymph (fig. 534). Within the fibrous mem-
brane is a thick clear tunica propria, which, in the semicircular canals,
may form papilliform elevations in the interior of the tube (fig. 535).

The places of entrance of the nerve-fibres are marked in each
ampulla by a transverse, inwardly projecting ridge (crista), in the
saccule and utricle by a thickening of the tunica propria (macula).
The epithelium at these places is formed of columnar cells (fig. 536),
which are surmounted by long, stiff, tapering hairs {auditm-y hairs,
fig. 536, li). Around these hair-cells the axis-cylinders of the nerve-
fibres ramify (fig. 538) ; they are therefore — like the gustatory cells of
the taste-buds — sensory epithelium cells. Between them are a number

474

TEE ESSENTIALS OF HISTOLOGY

of thin and somewhat rigid nucleated cells {fibre-cells of Retzius), which
rest upon the basement-membrane, and are connected at their free
extremity with a cuticular membrane, through which the auditory
hairs project.

The auditory hairs do not jut freely into the endolymph, but into a
soft mucus-like substance, of a dome-like form in the ampulhie {cupula

cL.

end

Fig. 534.— Section of .v semicircular canal, new-born child. (Sobotta.)

X 55.

c.t., connective tissue strands, between membranous canal and endosteum of bony canal ;
III, membranous canal ; ?<, wall of bony canal ; c, remains of fiKtal cartilage ; end,
endosteum ; r, blood-vessels.

terminals, fig. 536), and which in the saccule and utricle has a mass of
calcareous particles {otoliths) embedded in it.

The cochlea consists of a bony tube coiled spirally around an axis

THE COCHLEA.

475

Fig. 535. — Sectiox of membranous semicircular canal. (Riidinger.)
(More magnified.)
1, outer fibrous layer ; 2, tunica propria ; 3, 6, papilliforin projections with epithelial
covering ; 5, fixed side of the canal, with very thin tunica propria without papilla? ;
7, fibrous bands passing to periosteum.

Fig. 536. — Longitudinal section of an ampulla through the crista

ACU.STICA (diagrammatic).
urap., cavity of the ampulla; sec, semicircular canal opening out of it; c, connective
tissue attached to the wall of the membranous ampulla and traversing the perilymph ;
e, f , flattened epithelium of ampulla ; 7i, auditory hairs projecting from the columnar
cells of the auditory epithelium into the cupula, cap.Urm ; i-, blood-vessels ; n, nerve-
fibres entering the base of the crista and passing into the columnar cells.

476

THE ESSENTIALS OF HISTOLOGY.

which is known as the columella (fig. 539, 540). The tube is divided
longitudinally by a partition which is formed partly by a projecting
lamina of bone {spiral lamina), partly by a flat membrane {basilar

ep

m^^^:^

Fig. 537. — Section of macula acustica, cat. (Sobotta.) x 120.
ep, epithelium ; n, n, fibres of vestibular nerve.

Fig. 538. — Neeve terminations in macula, golgi method.
(Barker, from Lenhossek. )

membrane), into two parts or scalce ; the upper (supposing the cochlea
resting base downwards) being termed the scala 'vestibuli, the lower scala
tympani ; the latter is closed near its larger end by the membrane of
the fenestra rotunda. The scalte are lined by endosteum, and are
filled with perilymph, continuous with that of the rest of the osseous

THK (X)CllLEA.

477

labyrinth at the commencement of the scala vestibuli ; they communi-
•cate at the apex by an opening, the helicotrema.

The scala vestibuli does not occupy the whole of that part of the
bony tube of the cochlea which is above the partition. Its outer and

str.v.

w a

Fig. 539. — Section through the cochlea of the cat. (Sobotta.) x 25.

dc, duct of cochlea ; scv, scala vestibuli ; set, scala tympani ; w, bony wall of cochlea ; C,
organ of Corti on membrana basilaris ; mR, membrane of Reissner ; n, nerve fibres of
cochlear nerve ; gsp, ganglion spirale ; str.v., stria vascularis.

lower third is cut off by a delicate connective-tissue membrane
{membrane of Reissner, fig. 541, E), which springs from near the end
of the spiral lamina, and passes upwards and outwards to the outer
wall, thus separating a canal (d.c.) triangular in section, which is
lined by epithelium, and represents the membranous labyrinth of the
cochlea {dud or canal of the cochlea).

478

THE ESSENTIALS OF HISTOLOGY.

canal of scata membrane of
the cochlea vestibuli Reisstier

gan-
glion

gan- membrane of
glion Reissner

membrana
iectoria

Fig. 540.

scala basilar
tympan i membrane

-Vertical section through the middle of the human cochlea.
(Diagrammatic.)

Fig. 541. — Vertical section of the first turn op the human cochlea.

(G. Retzius.)

s.v, scala vestibuli ; s. t, soala tympani ; d.c, canal or duct of the cochlea ; sp.l, spiral lamina ;

n, nerve-fibres ; <..<!p, spiral ligament ; .s-«)-.r, stria vascularis ; s.s^j, spiral sulcus ; iJ,

section of Reissner's membrane ; I, limbus laminae spiralis ; m.t, membrana tectoria;

tC, tunnel of Corti ; b.m, basilar membrane ; li.i, h.e, internal and external hair-cells.

THE COCHLEA.

479

Canal of the cochlea. — The Hoor of the canal of the cochlea is formed
( I ) of the extremity of the spiral lamina, which is thickened above by
a peculiar kind of connective tissue, forming an overhanging projection
known as the limlms (fig. 541, ^) ; and (2) of the basilar membrane {h.m.),
which stretches across from the end of the bony lamina to the outer
wall, and is attached to this by a projection of reticular connective
tissue termed the spiral ligament (l.sp).

Fig. 542. — A pair of rods of corti, from thk habhit'.s cochlea, in side
VIEW. (Highly magnified.)

6, b, basilar membrane; i.r., inner rod ; e.r., outer rod. The nucleated protoplasmic
masses at the feet are also .shown.

Outer hair-cells.

Inner Blood- Basilar Outer ' — r — '
rod. vessel, membrane. rod. Cells of
Deiters.

Fig. .543. — Section through the organ of corti of the human cochlea.
(G. Retzius.) (Highly magnified.)

The basilar membrane is composed of stiff straight fibres, which extend
from within out, and are embedded in a homogeneous substance. The
membrane is covered below by a layer of connective tissue continuous
with the endosteum of the scala tympani ; above, the modified epi-
thelium which forms the oi-gan of Corti rests upon it. It becomes
gradually broader in the upper turns of the cochlea (rather more than
twice as broad in the uppermost as in the lowermost turn), and its
constituent fibres become therefore gradually longer.

480

THE ESSENTIALS OF HISTOLOGY.

The organ of Corti consists of the following structures :
1. The roiU of Corti, two series (inner and outer) of stiff, striated
structures, of a peculiar shape, the inner somewhat like a human ulna,
the outer like a swan's head and neck (fig. 542). They rest by one
extremity (the foot) on the basilar membrane a short distance apart,
and are inclined towards one another, their larger ends (heads) being
jointed together ; the series of rods thus enclose a sort of tunnel, the
floor of which is formed by a part of the basilar membrane (fig. 544).
Close to their feet may usually be seen the remains of the cells from
which they have been formed. The inner rods are narrower and

Fig. 544.— Semi-diagrammatic view of part of the basilar membrane
and tunnel of corti of the babbit, from above and the side.
(Much magnified. )

I, lirabus ; Cr., extremity or crest of limbus with tooth-like projections ; b, basilar mem-
brane ; sp.l., spiral lamina with, p, perforations for transmission of nerve-fibres;
i.r., fifteen of the inner rods of Corti; h.i., their flattened heads seen from above;
e.r., nine outer rods of Corti; h.e., their heads, with the phalangeal processes extend-
ing outward from them and forming, with the two rows of phalanges, the lamina
reticularis, I.r.

rather more numerous than the outer. The head of each outer rod
has a process which extends outwards and is known as the phalangeal
process. This forms part of —

2. A reticular lamina (fig. 544, I.r.), which is a cuticular structure
extending like a wire-net over the outer epithelium-cells of the organ
of Corti, and is composed of two or three series of stiff" fiddle-shaped
rings {phalanges) cemented together in such a manner as to leave
square or oblong apertures through which the hair-cells (see below)
project.

3. The outer hair-cells placed external to the rods of Corti. These
are epithelium-cells of columnar shape, arranged in three or four series

THE COCHLEA.

481

ffig. 543). The tree extremity of the cell is surmounted by a bundle
of short (ludifwi/ hairs, and projects through one of the apertures in
the reticular lamina ; the Hxed extremity is prolonged into a stiff
cuticular process, which is attached to the basilar membrane. Between
them are other supporting cells which are tapered in the same manner,
but rest by their larger end upon the basilar membrane, and are
prolonged above into a cuticular process which is attached to the
reticular lamina (cells of Deitcrs, figs. 543, 545).

4. The inner hair-celU (fig. 543), placed internal to the rods of Corti.
They form a single series of columnar cells surmounted by auditory
hairs, lying in close apposition to the inner rods.

Fig. 545.

Fig. 545. — Four cells of peiters from the rabbit. (After G. Retzius. )
(Highly magnified.)

The varicose lines are nerve-fibrils. The phalangeal processes are attached above
to a portion of the lamina reticularis.

Fig. 546.— General view of the mode of distribution of the cochlear

NERVE, ALL THE OTHER PARTS HAVING BEEN REMOVED. (Aruold. )

The remaining epithelium-cells have no important characteristics.
They are long and columnar next to the outer hair-cells, but soon
diminish in size, becoming cubical, and in this form they are con-
tinued over the outer wall of the cochlear canal. Here thev cover
a very vascular membrane {stria vasndaris, fig. 541, str.r.), which is
frequently pigmented ; its capillary blood-vessels penetrate between
the epithelium-cells. Internal to the inner hair-cells the epithelium
also soon becomes cubical ; it is prolonged in this form over the limbus
of the spiral lamina. The epithelium of Reissner's membrane is of

the pavement variety.

2h

482

THE ESSENTIALS OF HISTOLOGY.

The memhrana tedoria (figs. 541, 543) is a soft, fibrillated structure,
which is attached along the upper surface of the limbus, and lies
like a pad over the organ of Corti. It thins out towards the distal
margin, here becoming somewhat reticular, and, according to Retzius,

Fig. .547. — Ending of some of the fibres of the cochlear nerve amongst

THE H.\IR-CELLS, (G. EetziuS. )

This preparation i.s made by Golgi's method, and is viewed from above, p, a cell
belonging to the spiral ganglion.

it is attached to the lamina reticularis. In sections it usually appears
raised a short distance above the auditory hairs, but it is probable
that it always rests upon them during life.

The fibres of the cochlear branch of the auditory nerve enter the
base of the columella, and run in canals through its substance (figs.
539, 5 40), being gradually deflected outwards as they pass upwards
into the spiral lamina, at the base of which they swell out into a
ganglionic cord {spiral ganglion). The fibres take origin from the cells
of this ganglion.

THE COCHLEA. 483

After traversing the spiral lamina they emerge in bundles, and
the fibres then, having lost their medullary sheath, pass into the
epithelium of the inner hair-cell region. Here some of them course at
right angles and are directly applied to the irmer hair-cells, whilst
others cross the tunnel of Corti, to become applied in like manner
to the outer hair-cells and the cells of Deiters (figs. 543, 545). They
apparently lie between and in close contact with those cells, but there
does not appear to be any direct continuity between the nerve-fibrils
and the cell-substance.

484 THE ESSENTIALS OF HISTOLOGY.

APPENDIX.

Mounting solutions ;— 1. Normal saline solution.— A 0'6 to 0"9 per cent,
solution of common salt is used in place of serum for mounting fresh
tissues for immediate examination. The' lower percentage is used for
frog's tissues, the higher for mammals'. Preparations mounted in salt
solution cannot be preserved permanently.

2. Glycerine, diluted with an equal quantity of water. The cover-glass
may be fixed by gold size.

3. Canada balsam, from which the volatile oils have been driven oflf by
heat, dissolved in xylol.

4. Dammar varnish, made by dissolving dammar resin in xylol. The
solution is filtered through paper wetted with chloroform. This is used for
the same purposes as xylol balsam and has the advantage of remaining
colourless, whereas Canada balsam becomes yellow after long keeping.

5. Acetate of potassium, a nearly saturated solution. This is the best
medium for osmic preparations and for iodine-stained liver, to show
glycogen within the cells. The cover-glass may be fixed by soluble glass
or by gold size.

General methods of preserving and hardening tissues and organs.^—
The following fluids may be used : — Alcohol (75 per cent, to absolute) ;
acetone ; Carnoy's fluid (absolute alcohol 60 c.c, chloroform 30 c.c, glacial
acetic acid 10 c.c.) ; formol (diluted with from 9 to 99 parts of water) ;
corrosive sublimate (saturated solution in water or spirit) ; chromic acid
solution (1 in 200 to 1 in 500, to which glacial acetic acid may advantageously
be added in the proportion of 2 parts acetic acid to 1000 chromic solution) ;
picric acid solution (saturated, either alone or containing 2 parts of nitric or
sulphuric acid to 1000) ; Mann's fluid (a mixture of equal parts of saturated
aqueous solutions of mercuric chloiide and picric acid) ; osmic acid solution
(1 per cent.) ; bichromate of potassium solution (3 per cent.), to which for
more rapid hardening glacial acetic acid may be added to the extent of 5 per
cent, or less ; Muller's fluid (bichromate of potash 2| parts, sulphate of soda
1 part, water 100 parts) ; Zenker's fluid (which is Miiller's fluid containing
5 parts per cent, of mercuric chloride, to which 5 c.c. of acetic acid is added
at the time of using); and mixtures of Muller's fluid and osmic acid 1 per
cent, in varying proportions.

1 Details of the methods of preparing fixing and staining solutions as well as a
discussion of the theory of their action will be found in Mann's Physiological Histology,
Oxford, 1902.

APPENDIX. 485

It is best, if possible, to inject the fluid used for hanleninc,' into the blood-
vessels after washing them out with warm normal saline ; if this is not
passible, very small pieces of tissue should be taken, and always a consider-
able amount of tlie hardening fluid.

The fluid of most univ^ei'sal application is formol. This is a 40 per cent.
solution of formaldehyde. Mi.xed in the proportion of 10 parts formol to
90 water, it penetrates readily and hardens quickly. The tissue may remain
in formol a few days and should then be transferreil to alcohol. For rapi<l
rt\ation a very small piece of the tissue is placed in 10 per cent, formol
and warmed to a temperature of about 40' or .50" ; it will be sufficiently
liardened in half an Iiour, and may then be transferred first to weak and
then gradually to absolute alcohol, so that it is ready for the preparation of
sections in about an hour.

Pure acetone is also of utility for rapid fi.xatiou and hardening. Small
pieces of the tissue are dropped into a large amount of acetone, which not
only fixes and hardens but also dehydrates, so that the tissue can be trans-
ferred in an hour or so direct to molten paraffin for embedding. But much
better results are got by placing in 10 per cent, formol for thirty minutes
before transferring to acetone.

For preserving the structure of cells and nuclei, one of the best fixing
fluids is that recommended by Flemming. This consists of 15 vols, of 1 per
cent, chromic acid, 4 vols, of 2 per cent, osmic acid, and 1 vol. glacial acetic
acid. It must be freshly pi-epared. It is sometimes diluted with from two
to five times its bulk of water before use. One or two days is sufficient for
fixaticm and hardening. The tissue should be washed for several hours in
running water after removal from the mixture, and then placed in dilute
alcohol. Carnoy's fluid is in many cases excellent for cell-structure and
mitotic changes, and very rapid in its action.

Tissues to be hardened in alcohol are usually placed at once in absolute
alcohol, but for some tissues it is best to begin with 50 per cent, alcohol,
and pass the pieces through successive grades of 75 per cent., 95 per cent.,
into absolute alcohol, leaving them a few hours in each. They are ready for
cutting as soon as they are dehydrated, but as a rule they may be left
a long time in alcohol without deterioration. Organs which contain much
fllirous tissue, such as the skin and tendons, should not go into stronger
alcohol than about 80 or 90 per cent. ; otherwise they become too hard
to cut. Alcohol is generally used after the other fixing reagents, partly
to complete the hardening, partly on account of its dehydrating property,
since previous to embedding in paraffin all trace of water must be eliminated
from the tissue. If mercuric chloride be used for hardening, tincture of
iodine must be added to the alcohols subseijuently used (except tlie final
alcohol), to get rid of the excess of sublimate. Mercuric chloride in alcohol
(saturated solution) is one of the most rapid fixing and hardening reagents,
and may be used if sections are desired within a very short time. It can also
be used in place of alcohol and ether mixture for fixing blood films (Lesson II.
>; 5), in which case it may be saturated with eosin, and used for fixing and
staining at the same time. An immersion of 5 minutes is sufficient.

486 THE ESSENTIALS OF HISTOLOGY.

Many tissues can be instantly hardened by being plunged for a minute
into boiling water and then placed in alcohol : this is not, however, a good
method for glandular organs.

For tissues that are to be hardened in chromic acid an immersion of from
seven to fourteen days is generally necessary ; they may then, after washing
for some hours or days in tap-water, be placed in alcohol for preservation
and to complete the process of hardening. The alcohol should be changeil
once or twice.

Organs placed in bichromate of potassium or Midler's fluid are ready for
section in a fortniglit or three weeks ; they may, however, be left for a
much longer time in those fluids without deterioration.

With picric acid the hardening process is generall} complete in two
or three days ; the organs may tlien be transferred to alcohol, which
ought to be frequently changed.

The hardening of the brain and spinal cord in Mliller's fluid takes fiom
three weeks to as many months. It can be hastened by warmth, and -by
the addition of acetic acid, or by placing small pieces in Marchi's solution
(see below), after they have been a week or ten days in Midler's fluid.

Tissues containing calcai'eous matter, e.g. bone and tooth, may be rapidly
decalcified in a .solution made by dissolving, with the aid of heat, 1 grm.
phloroglucin in 10 c.c. nitric acid, and filling up to 100 c.c. with water,
to which more nitric acid may be added if desired. Another rapid decalci-
fying fluid is commercial sulphurous acid solution. If it is desired to
preserve the soft parts within the bone, it should first be placed for a
few hours in 10 per cent, formol. For decalcifying more slowly a 1 to
5 per cent, solution of nitric acid in water or alcohol, a saturated solution
of picric acid containing a superabundance of crystals, or a 1 per cent,
solution of chromic acid are employed.

Embedding of hardened tissues, and preparation of sections.—
Sections are most advantageously made with some form of microtome. It
is generally needful to support the hardened tissue whilst it is being cut,
and with this object it is embedded in some substance which is applied
to it in the fluid condition and becomes solid on standing. The embedding
substance can either simply inclose the tissue, or the tissue may be soaked
in it ; the latter method is the one commonly employed.

The embedding substance chiefly used is paraffin of about 50" C. melting
point.

Embedding in paraffin. — Before being soaked in melted paraffin, the
piece of tissue may be stained in bulk ; it is then dehydrated b\' ;i,
.series of alcohols (50 per cent., 75 per cent., 95 per cent.), finishing up
with absolute alcohol; after which it is soaked in cedar-wood oil, xylol,
or chloroform. It is now transferred to molten paraffin, which should
not be too hot, and is soaked in this for one or several hours, according
to thickness. Very delicate objects are sometimes passed thi^ough a
solution of paraffin in chloroform. When thoroughly impregnated with
the paraffin the object is placed in a mould or glass which has been
smeared with glycerine, and is covered with molten paraffin which is

APPENDIX. 487

allowed to (.ool quickly. A .s(iuare block of the paiattiu containing the
tissue is then fixed in the desired position on the microtome, thin
sections are cut and fixed to a slide (see below), the parafKn dissolved
out by turpentine or xylol, and the sections mounted in Canada balsam
or dammar.

If it be desired to cut a ril)and of successive sections, and the paraffin
used prove too hard for them to stick to one another at the edges, a
})arafiin of lower melting point (40° C.) is smeared over the opposite sides
of the block ; the sections then adhere together as they are cut.

Preparatinn of frozen sections. — The bichromate solutions and formol are
the best fluids to use for preserving tissues which are to be frozen in
place of being embedded. The tissue requires to be soaked in gum-water
before being placed upon the freezing microtome. A thin syrup of
either gum arable or dextrin may be used.

Embedding in ceUoidin— The piece to be embedded is dehydrated by
alcohol, and is then placed over night in a solution of celloidin in alcohol
and ether similar to ordinary collodion, and afterwards in collodion of
double strength. After twenty-four hours more it is removed from the
celloidiu (collodion) and placed upon a wood or metal holder. When tlie
celloidin is set by evaporation of its ether the holder is plunged in
alcohol (85 per cent), and after a few hours sections may be cut with a
knife wetted with spirit of the same strength. The sections are placed
in 95 per cent, alcohol ; then passed through cedar-wood oil or bergamot
oil into xylol balsam. They must not go into clove-oil, nor into absolute
alcohol. The advantage of the method is that the celloidin, which is
quite transparent, need not be got rid of in mounting the sections, and
serves to keep the parts of a section together ; it is thus very useful for
friable tissues or for large sections. The tissue may either be stained in
Itulk liefore embedding, or the sections may be stained.

Microtomes. — A section-cutting apjsaratus or microtome is essential for
histological work. Useful instruments for students are the Cathcart micro-
tome for freezing and the tripod microtome for objects which have been
embedded in paraffin.

The tripod microtome is a simple and efficient little instrument, and has
the advantage of being inexjjensive. It consists of a metal frame (fig. 548)
in which the razor is fixed, provided with a micrometer screw by which
the height of the razor-edge is adjusted. The paraffin block containing the
tissue is fixed by the aid of heat on a fiat piece of glass over which the
tripod slides. The razor-edge is lowered after each successive section.

In the Cathcart freezing microtome (fig. 549) the tissue, after being soaked
in gum- water, is placed on a metal ])late and frozen by playing an ether-
spray on the under surface of the plate. The plate is moved upwards by a
finely-cut screw, and the knife or plane used to cut the sections is guided
over the plate by passing over glass slips. In the Williams microtome the
freezing agent is ice and salt mixture. In using any freezing microtome,
especially for the nervous system, it is important not to freeze the tissue
too hard, or the section will roll up.

488

THE ESSENTIALS OF HISTOLOGY.

Somewhat more expensive and complicated, but also more efficient, instru-
ments are tlie rocking microtome of the Cambridge Scientific Instrument
Company and the microtomes designed by C. S. Minot and by Delcpine. The

Fig. .548.— Tripod microtome. (Birch's pattern.)

Fig. 549. — Cathcart freezing microtome.

APPENDIX.

489

actimi of all of these is automat if. For example, with the rocker microtome
every to-and-fro movement of the handle, ii, not only cuts a section of the
tissue of delinito thickness, Imt also moves tlie parallin block forwards in
readiness for the next section. And by employing a rectangular block of

Fig. 550. — Rocking microtome.

Fig. 551.— Minots automatic kotary mickotome.

490

THE ESSENTIALS OF HISTOLOGY.

paraitiii of the proper consistency, a long series of sections of the same
object, of equal thickness, can be obtained and made to adhere together in
a riband (as shown in fig. 550). The sections can be kept in series upon

Fig. 552.— Minot's pkecfsion microtome. This is especiall\- adapted for large sections.

Fig. 553. — Inclined plane microtome.

APPENDIX. 491

the slide bv the eniplo>'inent of some adhesive method of mounting tlie
riband.

For eelloidin-embedded prepaiations it is necessary to cut the sections
with a knife kept wetted with sjnrit. For this purpose a sliding micro-
tome, in whicli tlie knife or razor is moved liorizontally over the tissue,
with the edge obliquely inclined to the direction of movement, is the most
useful. The best instrument for this purpose, especially for large sections
of brain, is one in which the celloidin-soaked object is imniersetl in spirit
during the actual process of making the sections. It is all-important for
every kind of microtome that the knife should be in perfect order.

Methods of mounting in xylol balsam or dammar.— Individual paraffin-
cut sections or ribands of sections, such as are cut with the rocking and
other microtomes, are fixed to a slide or cover-glass — preliminary to being
treated with stains and other fluids— in the following way : — The slide (or
cover-glass), after having been carefully cleaned, is smeared very thinly with
fresh white of egg : this can be done with the finger or with a clean rag,
and the slides may be })ut aside to dry, protected from dust. It is con-
venient to prepare a large number of slides at a time in this way, and to
keep them at hand in a suitable receptacle. When required for use a little
water is poured on to the slide and the riband of sections is placed on the
water, which is then warmed on a hot plate or over a small flame until the
paraffin becomes flattened out, without actually melting. The water is then
di'ained oft', the slide put in a warm place for the remainder of the water to
evaporate (this will take from half an hour to an hour according to the
size of the section and the temperature at which it is kept), and then heated
sufficiently to melt the paraffin. It is next immersed in xylol to remove
the paraffin, aftei' which the sections may, if already stained, be mounted at
once in xylol balsam or dammar ; if not stained, treat, after xylol, first with
absolute and then witli gradually lower grades of alcohol, then water, and
then stain and finall}' pass through water, alcohol (in grades), clove-oil,
and xylol, into xylol balsam or damniar. For many sections some of the
grades of alcohol can be omitted, but it is always best to jalace in 50 per
cent, between water and absolute alcohol.

A simpler method, but one which, in most cases, answers tlie purpose
peifectly well, is to place the riband or the individual sections cut from
paraffin on the surface of water in a basin, just sufficiently warm to flatten
out the paraffin, but not to melt it, then pass a perfectly clean slide under
the surface of the water and float the sections on to it ; remove, drain off
the water, and put the slide and sections aside for one or more hours until
all the water has evaporated. The sections ai'e found to have adhered
firmly to the slide (they may, if desired, be yet more firmly fixed by drawing
a brush moistened with solution of celloidin in oil of cloves over them).
The paraffin can now be removed by washing the slide with xylol or
immersing it in xylol. If not previously stained they can then be passed
through alcohols and stained and mounted as just described. It is con-
venient to keep the several solutions which are required in cylindrical
tubes or grooved glass receptacles in a regular row upon the working table,

492 THE ESSENTIALS OF HISTOLOGY.

and transfer the slide from one to the other in succession. Tluis such a
series wouhl be (1) xylol ; (2) absolute alcohol ; (3) 75 per cent, alcohol ;
(4) 50 per cent, alcohol ; (5) distilled water ; (6) staining solution ; (7) tap
water ; (8) distilled water ; (9) 50 per cent, alcohol ; (10) 75 per cent,
alcohol ; (11) absolute alcohol ; (12) clove oil or xylol. The changes can
also be effected by pouring the solutions over the sections and draining off,
but this is less satisfactory.

The following table shows the methods which may be adopted fcjr the
treatment of paraftin-cut sections or libands of sections :

1. Place on a slide or cover-glass in a drop of tap-water :

the glass may previously have been smeared with
egg-white : warm gently.

2. Drain off water and allow to dry completely.

i
.3. Warm until paraffin is just melted.

I
4. Dissolve paraffin away with xylol.

If tissue is already stained in bulk. If tissue is not already stained.

I ~ i

Mount in xylol balsam or dammar. 5. Absolute alcohol.

6. Descending grades of alcohol.

7. Stain.
For sections out by tiie J'reezi?ig \

or celloidin methods, if the 8. Water.

tissue has already been stained |

in bulk, the sections need only 9. Ascendirg grades of alcohol.

be put through the ascending |

grades of alcohol and bergamot 10. Xylol or bergamot oil or

oil, and then mounted in creosote or clove-oil.

dammar. If the tissue has |

not already been stained, begin Mount in xylol balsam or

at No. 7. dammar.

Staining of sections. — The fluids most commonly employed for the
staining of sections are : — (1) Solutions of ha^matoxylin and alum ; (2)
solutions of carmine with or without alum ; (3) certain aniline dyes. The
time of immersion in the staining fluid varies according to the strength
of the fluid and the mode by which the tissue has been hardened. The
necessity of staining sections may be avoided if the tissue is stained in
bulk before embedding. For this purpose a piece of tissue is left to stain
for twenty-four hours or more in a moderately diluted hsematoxylin solution
or in carmalum or borax carmine, and is then embedded and cut into sections
by the paraffin or freezing methods. If the latter be employed the sections
are thoroughly washed with tap-water, dehydrated by alcohol, and passed
through clove-oil or xylol into xylol balsam or dammar. For some purposes
(e.g. the study of ossifying cartilage) an alcoholic solution of magenta is
useful for staining in bulk ; from this the tissue goes direct into a small
quantity of oil of cloves, and after being soaked with this it is passed into
molten paraffin. Sections may also be stained whilst still infiltrated with
paraffin by floating them on to the surface of the staining solution, which

APPENDIX. 493

limy be geiitlv warmed (but not enough to melt tlie paraffin). They
generally require far longer exjjosure to the stain. The subsequent treat-
ment is quite sini|)le, for they neeil only be transferred to warm water,
floated on to a slide and allowed to dry. The parattin is then melted,
dissolved away with xylol, and the sections are mounted in dammar.

The following are some of the principal staining solutions and methods of
staining for special purposes :

1. Delafield's hcematoxi/lin. — To 150 ciil)ic centimetres of a saturated
solution of potash alum in water add 4 cubic centimetres of a saturated
solution of hsematoxylin in alcohol. Let the mixture stand eight days,
then decant, and add 25 cubic centimetres of glycerine, and 25 cubic
centimetres of methylic alcohol. The solution must stand a few days
before it is ready for use.

To stain sections add a few drops of this solution to a watch-glassful
of distilled water. If overstained the excess of colour can be removed by
alcohol containing 1 per cent, nitric or hydrochloric acid. With long
keeping this solution becomes red instead of blue ; a trace of ammonia
will restore the blue colour.

2. ElviiicKs hoimato.vylin. — Dissolve 2 grammes luematoxylin (or, better,
hfematein) in 100 cubic centimeti'es alcohol ; add 100 cubic centimetres
water, 100 cubic centimetres glycerine, and 10 cubic centimetres glacial
acetic acid : add potash alum to saturation. This solution will keep
almost indefinitely : it is valuable for staining in bulk, as it tloes not
easily overstain. For staining sections it is best to dilute the solution
either with distilled water or with 30 per cent, alcohol. After the sections
have been stained they must be thoroughly washed with tap-water. This
develops the blue colour of the hjematoxylin.

3. Kultschitzky^s hceinatoxylin. — Dissolve 1 gramme hfematoxylin in a little
alcohol, and add to it 100 cubic centimetres of a 2 jjer cent, solution of acetic
acid. This solution is valuable foi- staining sections of the nervous system
(see Weigert-Pal process).

4. Ucemalum. — Hsematoxylin-alum solutions acquire their colouring
properties only as the hsematoxylin on keeping becomes converted into
hseraatein. The latter substance may, therefore, as recommended by Mayer,
be used advantageously in place of hpematoxylin if the stain is required
immediately. The following mode of preparing the solution may be
adopted : — Dissolve 50 grammes of ammonia alum in 1 litre of water, and
1 gramme of hsematein in 100 c.c. of rectified spirit. Add the hsematei'n
solution gradually to the alum. The mixture is ready for staining at once,
either as it is or diluted with distilled water. A small piece of thymol or a
little carbolic acid should be added to prevent the growth of moulds.

5. R. Heidenhain's method. — After hardening in alcohol, or in saturated
solution of picric acid and then in alcohol, place the tissue from twelve
to fourteen hours in a ^ per cent, watery solution of hematoxylin, and then
from twelve to twenty-four hours more in a h per cent, solution of yellow
chromate of potash, which may be changed more than once. Then wash in
water, place in alcohol, pass through xylol, and embed in paraffin.

494 THE ESSENTIALS OF HISTOLOGY.

6. J/. HeidenhaMs method. — Harden in sublimate, followed by alcohol ;
fix sections to slide by water method ; treat with iodised alcohol, transfer to
2"5 per cent, iron-alum (solution of sulphate (or tartrate) of iron and
ammonia) and leave a quarter of an hour or longer ; rinse with distilled
water; place in 1 to 0'5 per cent, pure hoematoxylin in water containing
10 per cent, alcohol for a few minutes ; wash with water ; differentiate in
the iron and ammonia solution until nearly decolorised ; wash for fifteen
minutes in tap- water ; dehydrate and mount in the usual way. This
method is especially adapted for exhibiting the centrosomes of cells. It is
also useful for retiform tissue and glands.

Both the process of mordanting with iron-alum and the subsequent
staining with luematoxylin may be considerably prolonged with advantage
for some tissues.

7. Carmalum (Mayer). — Useful either for sections or bulk-staining. If
the sections are subsequently passed through alcohol containing picric acid
in solution a double stain is produced.

Carminic acid, 1 gramme.

Ammonia alum, - - - - - - - 10 grammes.

Distilled water, 200 c.c.

Boil together, allow to cool and filter. Add thymnl or a little carljolic acid
to prevent the growth of moulds.

8. Canninate of ammonia. — Prepared by dissolving carmine in ammonia,
and allowing the excess of ammonia to escape by slow evaporation. The
salt should be allowed to dry and be dissolved in water as required.

9. Picric acid. — A saturated solution of picric acid in spirit may be used
as a second stain after htematoxylin or carmine. Any excess of picric acid
is dissolved o\it by rinsing with strong spirit. This form of double stain is
valuable for exhibiting keratinised tissues and muscle-fibres.

10. Picro-carminate of ammonia, a double stain. a. Ranvier's picro-
carmine. — To a saturated solution of picric acid add a strong solution of
carmine in ammonia, until a precipitate begins to form. Evaporate on
the water bath (or, better, allow it to evaporate spontaneously) to one half
its bulk ; add a little carbolic acid to pievent the growth of moulds ;
filter from the sediment.

/3. Bourne's picro-carmine. — " Add 5 c.c. of ammonia to 2 grammes carmine
in a bottle capable of containing about 250 c.c. Stopper, shake, and put
aside till next day. Add slowly, shaking the while, 200 c.c. of a saturated
solution of picric acid in distilled water. Put aside till next day. Add
slowly, constantly stirring, 1 1 c.c. of 5 per cent, acetic acid. Put aside till
next day. Filter ; to the filtrate add four drops of ammonia, put back in
the stoppered bottle " (Langley).

11. Bora.v-ca)')7une.—D\ssolve 4 grammes borax and 3 grammes carmine
in 100 cubic centimeti-es of warm water. After three days add 100 cubic
centimetres of 70 per cent, alcohol, let stand two days and filter. This
solution im])roves on keeping. It is useful for staining in bulk.

After staining with borax-carmine, the tissue should be placed in 70 per

APPENDIX. 495

cent, alcohol coiitaiuini;' T) drops of hvdrocliloric acid to 100 cubic centi-
metres.

12. Aniline dyes. — These are used either in aqueous solution (which may
contain O'Ol per cent, of caustic potash) or in water shaken up with aniline
oil, and it is usual to overstain a tissue with them, and subsequently to
decolorise with absolute alcohol containing \ its bulk of aniline oil (from
which the sections can pass directly into xylol) or with acid-alcohol (1 to
10 per 1000 hydrochloric acid) followed by absolute alcohol and this by
xylol. Those most employed are the " basic " dyes — methyl-blue, methylene-
blue, gentian-violet, toluidin-blue, thionin, saffranin, and vesuvin ; and the
"acid" dyes — eosin, erythrosin, acid magenta or acid fuchsin, and orange G.
A double stain is obtained by combining eosin with methylene blue or
toluidin blue, the sections being first stained for ten minutes in I per cent,
aqueous eosin and then, after rinsing with water, for twenty minutes in
1 per cent, of the blue solution, after which they are decolorised by absolute
alcohol or absolute alcohol and aniline oil. The decolorisation is arrested
by xylol. Other good double stains are the eosin-methyl blue mixture
devised by G. Mann ' and Jenner's stain, which is made by dissolving
in pm-e methyl alcohol the precipitate which is produced w^hen eosin
solution is added to methylene blue solution. Jenner's stain is valuable
for blood films. For the same purpose Ehrlich's triple stain is also used.
This is formed by mixing together aqueous solution of orange G., acid-
fuchsin, and methyl-green in certain proportions.-

13. Eosin. — A 1 per cent, solution in water may be used. The sections
are first stained deeply with lipematoxylin and rinsed with water. They are
then stained with the eosin solution, passed through 75 per cent, alcohol,
and then through strong spirit — which is allowed to dissolve out some
but not all of the eosin stain — into clove-oil : tliey are finally mounted in
xylol balsam or dammar.

Eosin stains hajmogiobin of an orange red colour ; so that the blood
corpuscles are well shown by it when fixing fluids have been employed
which do not remove the haemoglobin from them (such as mercuric chloride,
bichromate of potassium, and formol.

14. Muir's method of douhle-stainiiig ivith eosin and methylene blue. — For
staining haemoglobin and oxyphil granules in cells the method devised by
Richard Muir will be found valuable. It consists in staining the sections of
formol-hardened tissue (which are fixed on a slide) with saturated solution
of alcohol-soluble eosin crystals dissolved in rectified spirit. This solution is
poured over the section.s, and evaporated over a flame until the alcohol is
driven off, leaving only a watery solution. Rinse with water, place in
saturated solution of potash alum for three minutes, and wash again.
Decolorise with alcohol rendered very faintly cdkaline with ammonia. Wash.
Stain with saturated water-solution of methylene blue ; wash with water;
dehydrate and mount in usual way.

15. Acid fxichsin. — A 1 per cent, solution in 50 per cent, alcohol (to which

> See Methods of PhiisioJogical Histology, p. 216.
- It is best to purchase this solution ready-made.

496 THE ESSENTIALS OF HISTOLOGr.

1 drop of 1 per cent, alcohol-solution of gentian-violet may be added
per cubic centimetre), is an excellent stain for connective tissue (see p. 67).
It may also advantageously be used for developing bone and tooth and
for lymph-glands. The piece of tissue is left for several days in a 1 per
cent, solution in 95 per cent, alcohol and is then placed direct in a small
quantity of clove-oil for a few hours, after which it is transferred to molten
paraffin.

16. Orcein. —DissoWe 1 gi'amme orcein in 100 c.c. absolute alcohol con-
taining 1 c.c. hydrochloric acid. Place the sections in some of this solution
in a watch-glass and warm slightly, allowing the fluid to nearly evaporate
to dryness. Dehydrate in alcohol, which removes the excess of stain ; pass
through xylol into dammar. Orcein stains especially the elastic fibres.

17. Flemming's method for karyokinetie nuclei. — This is especially valuable
for staining cell-nuclei in mitosis. The tissue having been appropriately
fixed, small shreds or thin sections are placed for two days in saturated
alcoholic solution of saffranin, mixed with an equal amount of aniline-
water. They are then washed with distilled water and decolorised
in aniline-alcohol or in alcohol containing 1 per 1000 hydrochloric acid
until the colour is washed out from everything except the nuclei. They
are then again rinsed in water and placed in saturated aqueous solution
of gentian-violet for two hours, washed again in distilled water, decolorised
with aniline-alcohol until only the nuclei are left stained, then transferred
to berganiot oil or xylol, and from this are mounted in xylol balsam or
dammar. Gentian-violet and several other aniline colours may be employed
in place of saff'ianin from the first. Delafield's hsmatoxjdin (followed by
acid), or Ehrlich's h^ematoxylin also stain the mitotic figures well.

18. Staining with nitrate of silver (Recklinghausen). — Wash the fresh tissue
with distilled water ; immerse in ^ to 1 per cent, nitrate of silver solution
for from one to five minutes ; rinse with distilled water and expose to
bright sunlight either in water, 70 per cent, alcohol, or glycerine. The
tissue, which is generally a thin membrane, may either be mounted in
glycerine, or it may be spread out flat in water on a slide, the water
draiued off, the tissue allowed to dry completely, and then dammar
added. This method is used to exhibit endothelium, and generally to stain
intercellular substance. It depends upon the fact that the chlorides of the
tissues are almost exclusively confined to the intercellular sub.stance.

The following methods are esjiecially useful in investigations relating to
the nervous system :

19. Marchi's solution. — This is a mixture of Miillers fluid (2 parts) with
1 per cent, osmic acid (1 part). It is of value for staining nerve-fibres in
the earlier stages of degeneration, before sclerosis sets in (especially a few
days after the establishment of a lesion). All the degenerated medullated
fibres are stained black, whilst the rest of the section remains almost
unstained. It is best to put thin pieces of the brain or cord to be investi-
gated singly into a large quantity of the solution (after previously hardening
for ten days in MUllei-'s fluid), and to leave them in it for a week or more ;
but if necessary sections can be stained ; in tins case the process is more

APPENDIX. 497

complicated.' In either case they are fixed on a slide and mounted by the
usual process in xylol balsam or dammar.

20. Weigert-Pal method. — This method is chiefly used for the central
nervous system. By it all medullated nerve-fibres are stained dark, while
the grey matter and any sclero.sed tracts of white matter are left uncolnured.
The following modification of the original method can be recommended :
Pieces which have been hardened in Miiller's fluid and afterwards kept
a short time in alcohol (without washing in water) are embedded in cel-
loidin, and sections are cut as thin as possible. Or sections may be made
by the freezing method direct from Miiller's fluid, if the tissue is first
soaked in gum-water for a few hours. In either case they are placed in
water, and from this are transferred to Marchi's fluid (see above, § 19),
in which they are left for a few hours. They are then again washed in
water and transferred to Kulschitzky's hsematoxylin (see above, § 3). In
this they are left overnight, by which time they will be completely black.
After again washing in water they are ready to be bleached. This is
accomplished by Pal's method as follows : Place the overstained sections,
first in J per cent, solution of potassium permanganate for five minutes (or
for a longer time in a weaker solution) ; rinse with water and transfer to
Pal's solution (sulphite of soda 1 gramme, oxalic acid 1 gramme, distilled
water 200 cubic centimetres), in which the actual bleaching takes place.^
They are usually sufiiciently diff"erentiated in a few minutes: if not, they
can be left longer in the solution without detriment. If after half an hour
they are not differentiated enough, they must be put again (after washing)
into the permanganate for some minutes, and then again into Pal's solution.
After difi'erentiation they are passed through water, alcohol (with or without
eosin), and oil of bergamot (or xylol), to be mounted in xylol balsam or
dammar. The advantages which this modification has over the original
method are (1) even the finest medullated fibres are brought to view with
great surety ; (2) the staining of the fibres is jet black, and offers a strong
contrast to the colourless grey matter ; (3) the sections are easily seen and
lifted out of the acid hsematoxylin, which has very little colour ; (4) it is
diflScult to overbleach the sections ; (5) the stain is remarkably per-
manent.

Asa modification of the above, Bolton recommends to harden with formol,
place the sections for a few minutes in 1 per cent, osmic acid, stain for
two hours in Kulschitzky's hsematoxylin at 40° C, and then proceed with
the bleaching process.

21. Staining loith chloride of gold. — a. Cohnheim's method. — Place the fresh
tissue for from thirty to sixty minutes in a i per cent, solution of chloride
of gold ; then wash and transfer to a large quantity of water faintly acidu-,
lated with acetic acid. Keep for two or three days in the bght in a warm
place. This answers very well for the cornea. If it be principally desired
to stain the nerve-fibrils within the epithelium, the cornea may be trans-
ferred after twenty-four hours (the outlines of the larger nerves should be

1 See Hamilton, Brain, 1897, p. 180.

2 Diluted sulphurous acid solution may be employed instead of Pal's solution.

2l

498 THE ESSENTIALS OF HISTOLOGY.

just apparent to the naked eye) to a mixture of glycerine (1 part) and water
(2 parts), and left in this for twenty-four hours raore (Klein).

/?. Loicifs method. — Place small pieces of the fresh tissue iu a mixture of
1 part of formic acid to 2 to 4 parts of water for one- half to one minute ;
then in 1 per cent, chloride of gold solution for ten to fifteen minutes ; then
back again into the formic acid mixture for twenty-four hours, and into
pure formic acid for twenty-four hours more. After removal from the gold,
and whilst in the acid, the tissue must be kept in the dark. This method is
especially good for motor nerve endings iu skeletal muscle.

y. Rayiviei^s method. — Immerse in lemon-juice for five to ten minutes,
then wash with water and jilace in 1 per cent, gold-chloride solution for
twenty minutes. Then treat either as in Cohnheim's or as in Lowit's
method.

22. Golgi's ehromate of silver methods. — These are chiefly employed for
investigating the relations of cells and fibres in the central nervous system.
Two methods are mostly used, as follows :

a. Very small pieces of the tissue which has been hardened for some
weeks in 3 per cent, bichromate of potassium or Miiller's fluid are placed for
half an hour in the dark in 075 per cent, nitrate of silver solution, and
are then transferred for twenty-four hours or more to a fresh quantity
of the same solution (to which a trace of formic acid may be added). They
may then be placed in 96 per cent, alcohol (half an hour), and sections,
which need not be thin, are cut either from celloidin with a microtome
or with the free hand after embedding (but not soaking) with parafliu.
The sections are mounted in xylol balsam, which is allowed to dry on the
slide : they must not be covered with a cover-glass, but the balsam must
remain exposed to the air.

/3. Instead of being slowly hardened in bichromate, the tissue is placed
at once in very small pieces in a mixture of bichromate and osmic (3 parts
of 3 per cent, bichromate of potash or of Miillers fluid to 1 of osmic acid).
In this it remains from one to eight days, a piece being transferred each
day to 0'75 per cent, silver nitrate. The subsequent procedure is the same
aa described under a. For some organs it is found advantageous to repeat
the process, replacing them for a day or two in the osmic-bichromate
mixture after silver nitrate and then putting them back into silver nitrate
(Cajal's double method). This method is not only more rapid than that in
which bichi'omate of potassium alone is used, but is more sure in its results.

23. Ehrlich's methylene-blue method. — This method is one of great value
for exhibiting nerve-terminations, and in some cases the relations of nerve-
cells and fibres in the central nervous system. For its application the
tissue must be living : it is therefore best applied by injecting a solution
of methylene-blue (1 part to 100 of warm saline solution) into a vein in
an anaesthetised mammal, until the whole blood is of a bluish colour ; or
the injection may be made through the vessels of the part to be investi-
gated, immediately after killing an animal. But fairly good results can
also be obtained by immersing small pieces of freshly-excised living tissue
in a less concentrated solution (0"1 -pQV cent.), or, in the case of the central

APPENDIX. 499

nervous syetem, by dusting the raethylene-blue powder over a freshly-cut
surface, allowing some time for it to penetrate, and then treating it with
picrate of ammonia and Bethe's solution. In either case the tissue should
be freely exposed to air; the blue colour then appears in the nerve-
cells and axis-cylinders, even to their finest ramifications. It does not
however remain, but after a time fades from them while other tissues
become coloured. To fix the stain the tissue is taken at the moment that
the nerve-fibres are most distinctly seen and is placed for an hour or two
in saturated solution of picrate of ammonia, after which the preparation
can be mounted in glycerine containing picrate of ammonia. But to allow
of sections being made from it for mounting in balsam or dammar, it must,
subsequently to the treatment with picrate of ammonia, be placed for some
hours in Bethe's fluid, viz. :

Molybdate of ammonia, 1 gramme.

Chromic acid 2 per cent, solution, - - - - 10 c.c.

Distilled water, 10 c.c.

Hydrochloric acid, --.-.-- 1 drop.

This renders the colour insoluble in alcohol.

24. Sihler's method of stainmg nerve-endings in muscle and blood-vessels. —
Macerate the tissue for eighteen hours in the following solution :

Ordinary acetic acid, - 1 part.

Glycerine, 1 part.

1 per cent, chloral hydrate solution, . - - - ti parts.

From this transfer to glycerine for from one to two hours ; then unravel
somewhat with needles and place for from three to ten days in the
following :

Ehrlich's hsematoxylin, - 1 part.

Glycerine, - . 1 part.

I per cent, chloral hydrate solution, ... - 6 parts.

It may then be kept for any desired time in glycerine, which should be
changed several times.

Preparations are made by careful dissociation with needles. If over-
stained they may be differentiated by acetic acid until the dark-blue colour
is changed to violet. The muscle spindles and the end-plates are well
shown by this method.

25. Nissl's method of staining the chromatic granules in nerve-cells. — This
is a method of overstaining with methylene blue and subsequent differ-
entiation with alcohol (see § 12). Nissl recommended 90 per cent, alcohol
as the hardening agent, but both formol and corrosive sublimate followed
by alcohol may be employed also. Toluidin-blue (Mann) may be used in
place of methylene-blue. The sections may first be stained with 1 per cent,
aqueous solution of eosin, and then, after rinsing in water, with 1 per
cent, methylene-blue solution : they are best differentiated in aniline-alcohol.
The effect of heating the solutions to about 70° C. is to accelerate and
accentuate the staining, which will then take only a few minutes,

2i2

500 THE ESSENTIALS OF HISTOLOGY.

A Nissl , stain may also be obtained by placing thin pieces of the fixed
and hardened. nervous tissue in 1 per cent, solution of thionin for several
days ; after which the tissue is dehydrated and embedded in paraffin.

26. CajaVs reduced silver method for exhibiting netirofibrih within nerve-cells
and -fibres. — A small piece of the tissue (brain, spinal cord, ganglion, etc.),
not more than 4 mm. thick, and preferably from a young animal, is placed
in 50 CO. of rectified spirit to which 5 drops of ammonia are added. After
twenty-four hours in this, rinse with distilled water and place in a large
quantity of 1 per cent.- solution of silver nitrate, which is maintained at a
temperature of about 30° C. After being five or six days in this solution,
the piece is removed, mixed for a few seconds in distilled water, and
transferred for twenty-four hours to the following solution :

Hydrokinone (or pyrogallic acid), - - - - 1 to 1*5 grammes.

Distilled water, 100 cub. cent.

Formol, 5 to 10 cub. cent.

Rectified spirit, 10 to 15 cub. cent.

The addition of alcohol to the above is not indispensable, but favours
penetration. The piece is then washed in water for some minutes, trans-
ferred to alcohol, embedded in celloidin, and sections are prepared and
mounted in the ordinary way.

INDEX.

601

INDEX.

ACH

Achromatic spindle, 7, 31.

Achromatic substance, 8.

Adenoid tissue, 76.

Adipose tissue, 73.

Adrenals, See suprarenal capsules.

Air-bubbles, 27.

Ameloblasts, 271.

Amreba, 3.

Angioblasts, 196.

Ansa lenticularis, 415.

Aorta, structure of, 189.

Appendix, •IS-i. (See also verniiforni.)

Archoplasni, 8.

Areolar tissue, 68.

cells of, 70.

fibres of, 68.

Arrector pili, 243.
Arteries, nerves of, 196.

— structure of, 184.

— variation in structure of, 189.

— and veins, smaller, structure of, 192.
Articular cartilage, 87.

— corpuscles, 169.
Attraction sphere, 7.
Auerbach, plexus of, 296.
Autonomic nerves, 132.
Axon, 138, 143.

Bacteria, 27.
Baillarger, lines of, 428.
Basilar membrane, 479.
Basement membranes, 77.
Bechterew, nucleus of. See nucleus.
Bellini, ducts of, 324.
Bile-ducts, 313.
Bladder, 329.
Blastoderm, 22.

Blood-corpuscles, action of reagents
upon, 41, 44.

— of amphibia, 45.

— coloured, 31, 32, 41.

— colourless, 32.

amoeboid phenomena of, 48.

granules of, 33.

migration from blood-vessels,

51, 72.
varieties of, 33.

— development of, 36, 40.

— enumeration of, 30.

— structure of, 31.

CEL

Blood-crystals, 43.
Blood-film, 28.
Blood-platelets, 35, 47.
Blood-vessels, development of, 36, 196.

— structure of, 184, 192.
Bone, 96.

— development of, 101.

— lacuna; and canaliculi of, 98.

— lamella of, 98.

— marrow of, 38.

Bowman, glands of. See glands.

— membrane of, 447.
Bronchi, 255.
Bronchial tubes, 257.

Brain. See cerebrum, cerebellum,
medulla oblongata, mesenceph-
alon, pons Varolii.

— divisions of, 374.

— membranes of, 441.
Brunner, glands of. See glands.
Burdach, tract of. See tracts.
Bundle. See tracts.

Calleja, islands of, 396.
Capillaries, 193.

— circulation in, 194,
Carotid gland, 222.
Cartilage, 86.

— articular, 87.

— costal, 87, 92.

— development of, 90.

— embrj'onic, 90.

— hyaline, 87.

— ossification of, 101.

— parenchymatous, 90.

— transitional, 88.

— varieties of, 86.
Cartilage-cells, 87.

— capsules of, 88.

Cajal's method of staining neuro-
fibrils, 499.
Celloidin for embedding, 487.
Cell-plate, 16.
Cells, division of, 10.

amitotic, 10.

reduction, 14.

— embryonic, 1.

— membrane of, 8.

— nucleus of, 8.

— structure of, 2.

502

INDEX.

CEM

FIB

Cement. See crusta petrosa.
Central fovea of retina, 463.

— tendon of diaphragm, 191.
Centriole, 7.
Centrosome, 7.
Cerebellum, 417.

— peduncles of, 398, 402, 424.

superior, 398, 411, 424.

inferior. See restiform body.

middle, 390, 424.

Cerebrum, 424.

— basal ganglia of, 440.

— cortex of, 424.

— — structure of different parts, 432.

— peduncle of, 405.
Chondrin-balls, 90.
Choroid coat of eye, 451.
Chromatic substance, 8.
Chromatolysis, 140. [224.
Chromaffin or chromophil cells, 221,
Chromosomes, 9, 12.

Cilia, 64.

— action of, 65.

theories regarding, 66.

Ciliary muscle, 452.

Clarke, column of, 370.

Claustrum, 426.

Coccygeal gland. See glands.

Cochlea, 474.

Cohnheim, areas of, 112.

— method of staining nerve-endings,

497.
Collaterals, 148.
Colostrum-corpuscles, 248.
Comma tract, ^ee tracts.
Commissures of cerebrum, anterior,

439.
posterior, 410.

— of spinal cord, 356.
Conjunctiva, 444.
Connective tissue, cells of, 70.

development of, 82.

fibres of, 68.

jelly-like, 77.

— tissues, 68.
Cornea, 447.

— nerve endings in, 176, 449.
Corpora albicantia {mammillaria),414.

— geniculata, 412.

— quadrigemina, 406.
Corpus luteum, 350.

— striatum, 440.

— subthalamicum, 415.
Corti, organ of, 479.
Cotton fibres, 27.

Cowper, glands of. See glands.
Crusta, 405.

— petrosa, 269.
Cutis vera, 230.
Cytomitome, 7.
Cytoplasm, 2.

Deiteks, cells of, 481.

— nucleus of. See nucleus.
Dendrons, 138.

Dentine, 263.

— formation of, 272.
Descemet, membrane of, 449.
Deutoplasm, 5.

Dilatator pupillaj, 454.
Dobie, line of, 1 13.
Doyere, eminence of, 182.
Dust, 27.

Ear, 469.

Ebner, glands of. See glands.

Ehrlich's methjdene-blue method, 498.

Elastic tissue, 79.

Eleideu, 228.

Embedding, methods of, 486.

Enamel, 263.

— formation of, 271.

— organ, 272.
End-bulbs, 168.
Endocardium, 252.
Endomysium, 115.
Kndoneurium, 135.
Endoplasm, 5.
Endotheliu!)!. 55.
End-plates, 180.
Ependyma, 373, 389.
Epicardium, 252.
Epidermis, 226.
Epididymis, 334.
Epineurium, 134.

Epiphysis cerebri. See pineal gland.
Epithelium, 52.

— ciliated, 55, 64.

— classification of, 53.

— columnar, 55, 61.

— glandular, 56.

— nerve endings in, 174.

— pavement. 55.

— stratified, 54.

— transitional, 55.
Epitrichial layer, 229.
Erectile tissue, 330.
Erythroblasts, 36.
Erythrocytes. See blood-corpuscles,

coloured.
Eustachian tube, 470.
Exoplasm, 5.
Eye, 443.
Eyelids, 444.
Eye-piece, 24.

Fallopian tubes, 351.
Fat. See adipose tissue.

— absorption of, 304.

— in cartilage cells, 90, 92.
Fenestrated membrane, 187.
Fibres. See connective tissue, muscle,

nerve, etc.
Fibrin, 35.

INDEX.

503

KIB

Fibro-cartilcige, elastic, 9.S.

— white, 93.
Fibrous tissue, 80.
Fillet. See tract of tillet.
Fimbria, 434. ■
Flechsig, method of, 360.

— tract of. See tracts.
Flemming, germ-cent reof,20o,201),216.

— stainable bodies of, 'JOi), 216.

— method of staining nuclei, 4Sn.
Forel, decussation of, 4U3 (footnote).
Freezing method for preparation of

sections, 487.

GAr.L-lSL.VDDKK, 'M4.

Ganglia, 138.

— cells of, 14<».

— development of, 162.
Ganglion of cochlea, 38.5.

of glossopharyngeal, 384.

Scarpa, SSo.

— — of vagus, 383.

— Gasserian, 395.

— geniculate, 393.

— of habenula, 405, 414.

— interpeduncular, 405.
Gas-chamber, 60.
Genital corpuscles, 169.
Gennari, line of, 428.
Germ-centre, 205, 209, 216.
Germ-nuclei, 18.
Gianuzzi, crescents of, 283.
Gland or glands.

— agminated, 209, 300.

— anal, 308.

— of Bowman, 469.

— of Brunner, 294, 3(il.

— carotid, 222.

— ceruminous, 245, 469.

— classification of, 56.

— coccygeal, 222.

— of Cowper, 331.

— ductless, 59.

— of Ebner, 276.

— gastric, 288.

— haemal, 207.

— internallj' secreting, 59.

— lacrymal, 446.

— of Lieberkiihn, 279.

— of Littre, 331.

— lymph, 203.

— mammary, 246.

— Meibomian, 445.

— Pacchionian, 442.

— pineal, 415.

— pituitary, 224

— racemose, 58.

— saccular, 58.

— salivarj', 281.

— sebaceous, 243.

— secreting, 56.

HEN

Gland or glands, secreting, varieties
of, 58.

— serous, 276.

— solitary, 209, 299.

— sweat, 244.

— thymus, 210.

— tubular, 58.
(ilisson, i:apsule of, 311.
(Jlomeruli of kidney, 320.

— olfactory, 439.

Glycogen in colourless blood-cor-
puscles, 47.

— in liver cells, 313.
Goblet-cells, 63.

Gold-methods of staining nerve-
endings, 497, 498.
Golgi, organs of, 174.

— cells of, 418.

— methods of preparing the nervous

system, 498.

— reticulum of, 141, 142.

— types of nerve-cells, 148.
GoU, tract of. 6'ee tracts.
Gowers, tract of. See tracts.
Graafian follicles, 345.
Grandry, corpuscles of, 169.
Grannies of protoplasm, 4.

— of colourless blood-corpuscles, 33.
Ground-substance of connective tissue,

2, 68.
Gudden, atrophy of, 158.

— bundle of. See tracts.

— commissure of, 410.
Gullet. See oesophagus.
Gustatory cells, 278.

— pore, 278.

H.tMAL glands. See glands.

Hpematoidin, 44.

Hctmin, 44.

Ha;moglobin, 43.

Hsemolysis, 42.

Hair-cells of internal ear, 473, 480, 481 .

Hair-follicle, structure of, 235.

Hairs, 27, 234.

— development of, 241.

— muscles of, 243.

Hassal, concentric corpuscles of, 211.
Haversian canals, 98.

— systems, 99.

Haycraft, views of, on muscle struc-
ture, 113.
Heart, 250.

— muscle of, 123.

— nerves of, 253.

— valves of, 253.

Helweg, bundle of. See tracts.
Henle, fenestrated membrane of, 187.

— looped tubules of, 324.

— sheath of, 136.
Hensen, line of, 113.

^04

INDEX.

HEP

Hepatic lobules, 310.

— cells, 312.

Herbst, corpuscles of, 173.
Hippocampus major, 434.
His, bundle of, 252.
Histogenesis, 20.
Histologj', meaning of term, 1.
Hyaloplasm, 4.

Hj'pophysis cerebri. See pituitary
body.

Idiozome, 342.
Internal capsule, 440.
Intestine, large, 308.

— small, 296.
Iris, 453.

Jelly of Wharton, 85.

Karyokine.sis, 10.
Kei'ato-hj'aline, 229.
Kidaey, 320.

— bldod-vessels of, 32(i.
Krause, membi'ane of, 113, 116.

Labyrinth of ear, 472.

of kidney, 325.
Lacteals, 304.
Langerhans, islets of, 316.

— centro-acinar cells of, 318.
Lanugo, 242.

Larjnx, 256.

Lens, 465.

Leucocytes. {Set blood-corpuscles,

colourless).
Lieberkiihn, crypts of, 297.
Linen fibres, 27.

Lissauer, bundle of. See tracts.
Littre, glands of. See glands.
Liver, 310.

— blood-vessels of, 198, 311.

— cells of, 257.

— ducts of, 313.

— lobules of, 310.

— lymphatics of, 315.
Loewenthal, tract of. See tracts.
Lung, 256.

— alveoli of, 261.

— blood-vessels of, 261.

— Ijmph-vessels of, 262.
Lymph-glands or lymphatic glands,

203.

haemal, 206.

Lymph-vessels or lymphatics, 198.

— connection with cells of connective

tissue, 73, 201.

— development of, 201.

— nerves of, 20<J.

Lymph-corpuscles. See blood-cor-
puscles, colourless.

Lymphocytes, 34.

MUS

Lymphoid tissue, 76, 209.

— — development of, 209.

Macula lutea of retina, 463.
Malpighi, rete mucosnm of, 227.

— pyramids of, 320.

Malpighian corpuscles of kidney, 320.

of spleen, 2u9, 214.

Mammary glands, 246.
Marchi's method of staining degen-
erated nerve-fibres, 496.
Marrow, 38.
Measuring objects, 25.
Medulla oblongata, 374.
Megakaryocytes, 40, 218.
Meissuer, plexus of, 297.
Membrana tectoria, 482.

— tympani, 469.
Mesencephalon, 401.
Mesothelium, 55.
Methods of embedding, 486.

— of measuring microscopic objects,

25,

— of mounting sections, 491.

— of preparing sections, 486.

— of preserving and hardening, 484.

— of staining, 492.

Meynert, bundle of. See tracts.

— decussation of, 403, 409.
Micrometer, 25.
Microscope, 24.

Microscopic work, requisites for, 24.
Microtomes, 487.

Migration of colourless blood-corpus-
cles, 51, 195.
Mitochondria, 5.
Mitosis, 10.
Moist chamber, 61.
Mouakow, bundle of. See tracts.
Mould, 27.

Mounting solutions, 484.
Mucus-secreting cells, 63.
Mliller, fibres of, 462.

— helicine arteries of, 330.

— muscle of, 453.

Muscle, accessory disks of, 117.

— blood-vessels and lymphatics of,

121.

— cardiac, 123.

— changes in contraction, 116, 119.

— corpuscles, 113.

— development of, 122.

— ending of, in tendon, 120.

— involuntary or plain, 126.

development of, 127.

of arteries, 187.

— nerves of, 121, 177, 180.

— nuclei of, 113.

— of heart, 123.

— of insects, 115, 116.

— in polarized light, 118.

INDEX.

506

MUS

Muscle, principal disk of, 117.

— red, llo.

— spindles, 122, 177.

— structure of, compared with proto-

plasm, 119.

— voluntary or cross-striated, 11 1.
Myelopla.xes, 40.
Myocardium, 250.

Nails, 232.

— development of, 234.
Nerve-cells, 138.

— development of, 161.

— processes of, 138, 143.

— reticulum of, 141.

— trophospongium of, 143.

— types of, 148.
Nerve-fibres, axis cylinder of, 132.

— degeneration of, 154.

— ilevelopmeiit of, 161.

— meduUated, 129.

— medullary segments of, 131.

— motor, terminations of, 180.

— non-medullated, 133.

— regeneration of, 157-

— sensory, modes of termination of,

166, 177.

— sheaths of, 130.

— varieties of, 128.
Nerve-trunks, structure of, 134.
Nervi nervorum, 136.
Neuroblasts, 161.
Neurofibrils, l;s2, 140, 177.
Neuroglia, 159.
Neurokeratin, 131,
Neurolemma, 130.

Neurone, 138.

— theory, 148.
Neuro-synapse, 148.

Nissl, granules of, in nerve-cells, 138.
Nissl's degeneration of nerve-cells,
140, 155.

— method of staining nerve-cells, 499.
Nucleolus, 8.

Nucleus of cell.
Nucleus or nuclei.

— of abducens, 394.

— of accessory, 378, 381.
"— of Bechterew, 389, 397.

— caudatus, 440.

— of cochlear nerve, 385.

— cuneatus, 377.

— of Deiters, 388, 392, 397, 402.

— dentatus cerebelli, 417-

— of facial, 392.

— of glossopharyngeal, 379, 384.

— gracilis, 377.

— of hypoglossal, 378, 381.

— lenticularis, 440.

— oculomotor, 401.

— of olive, 380.

PIT

Nucleus or nuclei.

— of pons, 390.

— of posterior longitudinal bundle,

403.
~ preolivary, 392.

— red, of tcgumentum, 402.

— semilunar, 392.

— of Stilling, 417.

— superior olivary, 392.

— tecti (s. fastigii), 417.

— of thalamus, 411. •

— of trapezium, 391.

— of trigeminal, 394.

— of vagus, 379, 383.

— of vestibular nerve, 387.

Objective, 24.
Ocular, 24.
Odontoblasts, 268.
CEsophagus, 280.
Olfactory bulb, 436.

— cells, 439, 468.

— mucous membrane, 467.

— path, 439.

— tract, 436.
Olive, 375.

— superior, 392.
Omentum, 191.
Onychogenic substance, 232.
Opsonins, 50.

Optic chiasma, 409.

— nerves, 409.

— thalamus, 410.

— tract, 409.
Ossification in cartilage, 102.

— in membrane, 108.
Osteoblasts, 100, 103.
Osteoclasts, 105.
Osteogenic fibres, 108.
Ovary, 314.

Ovum, 22, 343.

— division of, 17.

Pacinian corpuscles, 169.
Pancreas, 316.
Papillae of tongue, 275.

— of skin, 230.
Paranucleus, 5, 318.
Paraplasm, 5.
Parathyroids, 222.
Penis, 330.
Pericardium, 252.
Perineurium, 134.
Periosteum, 100.

Peyer, patches of, 209, 300.
Phasocytes, 33, 50, 218.
Pharynx, 280.
Pick, bundle of, 376.
Pigment-cells, 72.
Pineal gland, 415.
Pituitary body, 224.

506

INDEX.

PLE

Pleura, 2(j2.
Polar bodies, 17.
Pons Varolii, 390.
Portal canal, 311.

Posterior longitudinal bundle. See
tracts.

— commissure. See commissures.
Prickle-cells, 54.

Pronuclei, 18.

Proprio-spinal tibres of cord, 361, 306.

Prostate, 331.

Protoplasm, 2.

Purkinje, cells of, 41S.

— fibres of, 125, 252.

Pyramids of medulla oblongata, 375.

Ranvier, constrictions of, 130.
Recklinghausen, method of staining

with silver nitrate, 496.
Reil, fillet of, 398.

— island of, 420.
Reissner, membrane of, 477.
Remak, fibres of, 133.
Restiform body, 380, 389, 424.
Reticular or retiform tissue, 75.
Retina, 454.

— macula lutea of, 463.

— pars ciliaris of, 465.
Rhinencephalon, 4.33.
Rolando, tubercle of, 377.
RoUett's method of staining muscle,

110.

Rouleaux (of blood-corpuscles), causa-
tion of, 43.

Ruffini, organs of, 173.

Saccule, 473.
Salivary corpuscles, 52.

— glands, 281.
Sarcolemma, 111.
Sarcomeres, 116.
Sarcoplasm, 112.
Sarcostyles, 112.
Sarcous elements, 116.
Schwann, sheath of, 130.
Sclerotic coat of eye, 446.
Sebaceous glands. .SV^ glands.
Sections, preparation of, 480.
Semicircular canals, 473.
Seminiferous tubules, 337.
Serous membranes, 201.
Sertoli, cells of, 341.
Sharpey, fibres of, 99, 100.

Sihler's method of staining nerve-
endings, 499.
Silver-methods, 496, 499.
Sinusoids, 185, 197.
Skin, 226.

Spermatogenesis, 341.
Spermatozoa, 338.

TRA

Sphincter ani, internal, 308.

Sphincter piipilhe, 453.

Spinal bulb. See medulla oblongata.

Spinal cord, 355.

— • — blood-vessels of, 373.

central canal of, 356, 373, 378.

characters in different parts,

.358.

— — connection of nerve roots with,

371.

gi'ey mattei' of, 366.

membranes of, 355.

nerve-cells of, 366.

— ■ — tracts in, 360.

Spinal ganglia, 149.

Spleen, 213.

Spongioblasts, 161.

Spongioplasm, 4.

Staining of sections, 493.

Stanley-Kent, bundle of, 252.

Starch granules, 25.

Stilling, nucleus of, in cord, 370.

Stomach, 287.

— blood-vessels of, 294.

— glands of, 288.

-— lymphatics of, 294.
Stomata, 201.

Stroma (of blood corpuscle), 43.
Substantia nigra, 405.
Subthalamus, 415.
Suprarenal capsules, 218.
Sweat-glands, 244.
Sylvian aqueduct, 401.
Sympathetic ganglia, 150.

— nerves, 133.
Synapse, 148.
Syncytium, 2, 125, 194.
Synovial membranes, 90.

Tactile corpuscles, 167.

— disks, 169, 177.
Taste-buds, 277.
Teeth, structure of, 263.

— formation of, 269.

— pulp of, 268.
Tegmentum, 402.
Tendon, 82.

— nerve-endings in, 174.
Testicle, 332.
Thalamencephalon, 410.
Thalamus, 410.
Thrombocytes, 35.
Thymus gland, 210.
Thyroid body, 221.
Tissues, enumeration of, 1.

— formation from blastodermic layers,

22.
Tongue, 275.
Tonsils, 207.
Tooth. See teeth.
Trachea, 254.

INDEX.

507

TUA

Tract or tracts or bundles.

— anterior longitiuiinal. See tecto-

spijKil tract.

— anterolateral ascending. See tract

of Gowers.

descending. See tract of Loew-

enthal.

— bulbo thalamic. See tract of fillet.

— of Burdach, 360, 377.

— central, of cranial nerves, 398.

— central, of tegmentum, 3S1, 397.

— cerebello-bulhar, 384.

— comma, 361.

— of cord, 360.

— cortico-bulbar, 396, 398.

— cortico-spinal. (SVe pyramidal tract.

— crossed pyramidal. iS'ee pyramidal

tract.

— descending cerebellar, 399.

— direct pyramidal. See pyramidal

tract.

— of (ioll, 360, 377.

— myelination of, 360.

— of fillet, 377, 387, 397, 404, 411.

— of Flechsig, 365, 381.

— of (iowers, 365, 381, 399.

— of Gudden, 403 (footnote), 414.

— of Helweg, 365.

— of Lissauer, 366, 371.

— of Loewenthal, 363.

— of Marie, 366.

— of Meynert, 405.

— of Monakow, 365, 396, 403.

— of Miinzer, 403.

— of Pick, 376.

— olfactory, 436.

— olivo-spinal. See tract of Helweg.

— optic, 409.

— ponto-spinal, 397.

— posterior longitudinal, 363, 388,

392, 396, 402.

— prepyramidal. See tract of Mona-

kow.

— pyramidal, 361, 376, 390, 396, 405,

409.

— of Risien-Russell, 418.

— rubro-spinal. See tract of Mona-

kow.

YEA

Tract or tracts or bundles.

— spino-cerebellar, 365, 370.

— spino-tectal, 366.

— spino-thalaniic, 366.

— tecto-,spinal, 36.5, .397, 403.

— thalamo-bulbar, 398.

— thalamo-olivary, 397.

— transverse peduncular, 410.

— ventral longitudinal. See tecto-

spinal tract.

— vestibulo-motor. *S^ee posterior

longitudinal tract.

— vestibulo-spinal, 363, 397.

— of Vicq d'Azyr, 414.
Trai^ezium, 391.
Trophospongium, 4, 143.
Tympanum, 469.

Uretkr, 328.
Urethra, 231.
Urinary bladder, 329.
Uriniferous tubules, course of, 322.
Uterus, 352.
Utricle, 473.

Vas deferens, 335.
Vasa vasorum, 196.
Vasoformative cells, 36, 196.
Veins, structure of, 189.

— valves of, 190.

— variations in, 190.
Vermiform appendix, 308.
Vesiculaj seminales, 336.
Villi, arachnoidal, 442.

— of intestine, 302.

— of synovial membrane, 91.
Vitreous humoiir, 466.
Volkmann, canals of, 99.

Wallerian degeneration, 154, 360.
Warming apparatus, 48.
Weigert-Pal method for staining sec-
tions of the nervous system, 496.
Woollen fibres, 27.

Yeast, 27.

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